Perspective Sodium/Glucose Cotransporter 2 Inhibitors and Prevention of Diabetic Nephropathy: Targeting the Renal Tubule in Diabetes Luca De Nicola, MD, PhD,1 Francis B. Gabbai, MD,2 Maria Elena Liberti, MD,1 Adelia Sagliocca, MD,1 Giuseppe Conte, MD,1 and Roberto Minutolo, MD, PhD1 Optimal prevention and treatment of chronic kidney disease in diabetes requires implementing therapies that specifically interfere with the pathogenesis of diabetic nephropathy. In this regard, significant attention has been given to alterations of the proximal tubule and resulting changes in glomerular filtration rate. At the onset of diabetes mellitus, hyperglycemia causes increases in proximal tubular reabsorption secondary to induction of tubular growth with associated increases in sodium/glucose cotransport. The increase in proximal reabsorption leads to a decrease in solute load to the macula densa, deactivation of the tubuloglomerular feedback, and increases in glomerular filtration rate. Because glomerular hyperfiltration currently is recognized as a risk factor for progression of kidney disease in diabetic patients, limiting proximal tubular reabsorption constitutes a potential target to reduce hyperfiltration. The recent introduction of sodium/glucose cotransporter 2 (SGLT2) inhibitors opens new therapeutic perspectives for this high-risk patient population. Experimental studies have shown that these new agents attenuate the progressive nature of diabetic nephropathy by blood glucose– dependent and –independent mechanisms. SGLT2 inhibition may prevent glomerular hyperfiltration independent of the effect of lowering blood glucose levels while limiting kidney growth, inflammation, and albuminuria through reductions in blood glucose levels. Clinical data for the potential role of the proximal tubule in the pathophysiology of diabetic nephropathy and the nephroprotective effects of SGLT2 inhibitors currently are limited compared to the more extensive experimental literature. We review the evidence supporting this working hypothesis by integrating the experimental findings with the available clinical data. Am J Kidney Dis. -(-):---. ª 2014 by the National Kidney Foundation, Inc. INDEX WORDS: Diabetes mellitus (DM); diabetic nephropathy (DN); hyperfiltration; glomerular filtration rate (GFR); proximal tubule; sodium/glucose cotransport; sodium/glucose cotransporter 2 (SGLT2); SGLT2 inhibitor; canagliflozin; dapagliflozin.
D
iabetes mellitus (DM) is a major public health problem worldwide. The prevalence of type 2 DM is increasing exponentially; it is estimated that more than 400 million people will have DM by 2030.1 Among the various complications of DM, diabetic nephropathy (DN) is considered a major threat because it affects approximately one-third of all diabetic individuals and is a main cause of mortality and a leading cause of end-stage renal disease.2 DN is characterized by persistent albuminuria, decline in glomerular filtration rate (GFR), increasing blood pressure (BP), and high cardiovascular (CV) risk. Patients with non–dialysis-dependent type 2 DN, a condition frequently encountered in endocrine/ diabetes and nephrology clinics, benefit from strict cooperation between the 2 specialties to improve prognosis.3,4 Optimal prevention and treatment of chronic kidney disease (CKD) in patients with DM requires implementing therapies that specifically interfere with the pathogenesis of DN.5 In this regard, significant attention has been given to the role of the proximal tubule.6,7 It is well known that the dimensions and function of the proximal tubule increase in response to higher glucose load. These changes have been linked to the increase in GFR, or so-called diabetic hyperfiltration. The development of new antidiabetic agents, such as inhibitors of sodium/glucose Am J Kidney Dis. 2014;-(-):---
cotransporter 2 (SGLT2; encoded by the SLC5A2 gene), has raised the intriguing possibility of new tools to prevent DN. Data for the role of increased tubular reabsorption in the pathophysiology of human DN are limited compared to the more robust experimental literature. We review the information supporting this hypothesis by integrating the experimental findings with the available clinical data.
EARLY CHANGES IN THE DIABETIC KIDNEY Glomerular Hyperfiltration In the setting of DM, hyperglycemia is accompanied by an increase in GFR. Investigators have proposed several mechanisms to explain diabetic hyperfiltration, From the 1Nephrology Division, Second University of NaplesMed School, Naples, Italy; and 2Department of Medicine, Veterans Administration San Diego Healthcare System-University of California at San Diego Medical School, San Diego, CA. Received October 25, 2013. Accepted in revised form February 5, 2014. Address correspondence to Luca De Nicola, MD, PhD, Nephrology Division, Second University of Naples-Med School, P.O. S.M.D.P Incurabili, Via Maria Longo 50, 80138 Napoli, Italy. E-mail: [email protected] 2014 by the National Kidney Foundation, Inc. 0272-6386/$36.00 http://dx.doi.org/10.1053/j.ajkd.2014.02.010 1
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including impaired constriction of the afferent arteriole.6-8 These hemodynamic changes have been associated with activation of neurogenic, hormonal, and myogenic factors (known as the hemodynamic hypothesis). However, increased proximal tubular reabsorption also has been shown within the spectrum of the early changes observed in diabetic kidneys, suggesting an alternative hypothesis to explain the increase in GFR (known as the tubular hypothesis). Under normal and pathophysiologic conditions, glomeruli and tubules are functionally linked to ensure maintenance of adequate extracellular volume and consequently systemic perfusion.9 Two mechanisms are involved in this process. Glomerulotubular balance modulates proximal tubular reabsorption according to changes in GFR in order to guarantee a constant percentage of tubular reabsorption of filtered solutes. Any increase in GFR determines an increase in proximal tubular reabsorption, and vice versa. The mechanism mainly depends on parallel changes in peritubular capillary oncotic pressure that accompany changes in filtration fraction. However, glomerulotubular balance is not fully efficient in maintaining fractional reabsorption constant. Due to the limitation of glomerulotubular balance, part of any change in GFR will pass along the nephron, eliciting the tubuloglomerular feedback response, which will counteract some of the original disturbance. Tubuloglomerular feedback is driven by signals directly generated at the level of the macula densa whereby changes in sodium chloride concentration in tubular fluid cause opposite changes in single-nephron GFR (Fig 1). When a primary reduction in proximal tubular reabsorption ensues, the amount of sodium chloride in the thick ascending limb of Henle and the first part of the distal tubule, the so-called distal delivery of sodium chloride, increases, which in turn leads to a decrease in GFR. Conversely, a reduction in distal delivery driven by increased reabsorption in the proximal tubule induces an increase in single-nephron GFR. Tubuloglomerular feedback is continuously active and modulates the tone of glomerular afferent resistance through the release of adenosine, a vasoconstrictive substance. Tubuloglomerular feedback and glomerulotubular balance maintain relatively constant distal salt delivery and sodium excretion when GFR increases. The efficiency of the 2 mechanisms ensures the optimal defense against volume depletion because if the primum movens is of a vascular nature, there is a tubular adaptation (glomerulotubular balance), whereas if the primum movens is of tubular origin, a glomerular adaptation (tubuloglomerular feedback) ensues. In the early stages of diabetes, tubuloglomerular feedback has a major role. In diabetic rats, hyperglycemia 2
Figure 1. Physiology of tubuloglomerular feedback. Abbreviations: NaCl, sodium chloride; GFR, glomerular filtration rate.
induces an increase in proximal tubular reabsorption due to proximal tubular hypertrophy and a consequent increase in sodium/glucose cotransport. The reduction in solute delivery to the macula densa thereby decreases tubuloglomerular feedback activity with a consequent increase in single-nephron GFR.7 According to the hemodynamic hypothesis of hyperfiltration, an excess of total-body sodium in patients with DM was proposed as a contributor to the renal vasodilatation possibly by suppressing vasoconstricting neurohormonal systems. Such a hypothesis was not supported by results of a randomized trial in patients with uncomplicated insulin-dependent DM.10 In that study, investigators measured GFR and renal plasma flow in patients with DM and in ageand sex-matched controls consuming a normal(200 mmol/d) and low- (20 mmol/d) sodium diet. A normal-sodium diet was associated with lower renal vascular resistance and higher GFR in patients. Unexpectedly, salt restriction did not correct kidney hyperperfusion, but exacerbated it. These alterations were specific to DM; no significant changes in response to variations in dietary salt intake were observed in controls. Experimental studies have explained the potential mechanism of this “salt paradox” in DM by demonstrating that proximal tubular reabsorption is more sensitive to variations in dietary salt in this model, with an abnormal tendency for GFR to vary inversely with sodium intake.6,7,11 Am J Kidney Dis. 2014;-(-):---
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Enhanced Proximal Tubular Reabsorption The increase in proximal tubular reabsorption and consequent reduction in distal delivery of sodium chloride in patients with types 1 and 2 DM originally was described more than 20 years ago in studies using lithium clearance.12,13 Based on these observations, Hannedouche et al14 hypothesized that a decrease in distal sodium delivery could contribute to diabetic glomerular hyperfiltration through a reduction in tubuloglomerular feedback–mediated vasoconstriction. Subsequently, micropuncture studies in diabetic rats confirmed that increased proximal tubular reabsorption has a major role in the development of glomerular hyperfiltration in the early stages of DM.15,16 In these studies, increased proximal tubular reabsorption of chloride anions and sodium and potassium cations led to a 20%-28% reduction in the concentration of these ions in the distal tubule compared with nondiabetic rats. These concepts were confirmed recently in a clinical study showing a higher prevalence of glomerular hyperfiltration and associated enhanced proximal sodium reabsorption in African individuals with impaired fasting glucose or type 2 DM.17 Glucose is not bound to proteins or macromolecules and is filtered freely by glomeruli. In healthy adults, glomeruli filter w180 g of glucose daily. Most of the filtered glucose is recovered by tubular reabsorption to protect the individual against wide variations in glucose supply and demand, a process essential for life. Glucose is removed from the tubular lumen by active transport at the level of the proximal segments. The amount of glucose reabsorbed is proportional to the filtered load and hence depends on plasma glucose level times the GFR value, up to the maximal transport level. The ability of the proximal tubule to reabsorb glucose increases with increments in the filtered load secondary to higher GFR or plasma glucose levels. When the filtered load reaches the maximal transport level, any additional increase results in glycosuria. Under normal conditions, ,0.5 (range, 0.03-0.3) g/d of glucose is excreted in urine. However, glycosuria with normal plasma glucose levels may appear during pregnancy, in individuals with one kidney, or in the presence of primary renal glycosuria, a benign condition that does not require a specific therapy.18 In the early 1980s, experimental studies in animal models described 2 different glucose transporters located in the proximal tubule19: a “high-flux lowaffinity” transporter (SGLT2, located in the S1 segment) and a “low-flux high-affinity” transporter (SGLT1, located in the S3 segment). The S1 segment reabsorbs the largest proportion of the filtered glucose and sodium load, while the remaining 10% is Am J Kidney Dis. 2014;-(-):---
reabsorbed by the S3 segment.20-22 In diabetic patients, glycosuria is limited by upregulation of tubular glucose reabsorption.23,24 Early experiments in human kidney tubular cells harvested from urine of patients with type 2 DM demonstrate an increase in the expression, protein concentration, and a-methylglucose transport capacity of SGLT2 compared with nondiabetic individuals.25 More recent insights have been provided by studies in nondiabetic mice lacking expression of SGLT1 (Sglt2/2). These studies have demonstrated that SGLT1, while critical for intestinal absorption of glucose, also is required for complete reabsorption of filtered glucose in the kidney.26 Furthermore, during SGLT2 inhibition, the glucose load to the SGLT1-expressing S2/S3 segments of the proximal tubule is enhanced and a compensatory increase in SGLT1-mediated transport occurs.27 The contribution of SGLT1 may explain, at least in part, why SGLT2 inhibition is associated with excretion of only 50%-60% of filtered glucose. These results are also consistent with the observation that combined SGLT1 and SGLT2 knockout is associated with higher glucose excretion and improved glycemia control compared to inhibition of SGLT2 alone.28 Enlargement of the Diabetic Kidney A peculiar feature of the diabetic kidney is its enlargement at the onset of DM. As discussed in detail in recent reviews by Vallon and Thomson,29,30 experimental results suggest that structural tubular changes explain most early functional changes. The enlargement of the diabetic kidney caused by the diabetic milieu is attributed primarily to the proximal tubule, in which a period of hyperplasia precedes hypertrophy.31 Early in the course of hyperglycemia, hyperplasia is followed by G1 cell-cycle arrest, development of hypertrophy, and a senescence-like phenotype.31,32 This growth switch is mediated by mainly TGFb1 (transforming growth factor b1).33 The molecular pathways involved in senescence are linked to tubulointerstitial fibrosis and inflammation and thus may contribute to the progression to endstage renal disease.32,34 Hypertrophy and senescence also may alter tubular function, providing a potential explanation for the salt paradox of the diabetic kidney.6,30 Notably, acceleration of the senescent phenotype in tubule cells has been confirmed in humans by means of kidney biopsy studies in patients with type 2 DM.35 Hyperplasia is mediated by several growth factors, such as insulin-like growth factor 1 (IGF-1), plateletderived growth factor (PDGF), vascular endothelial growth factor (VEGF), and epidermal growth factor (EGF).36 High levels of glucose activate endogenous tubular cell renin and angiotensin II (Ang II), which 3
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binds to its receptors and induces synthesis of VEGF.37 Glucose and Ang II activate several intracellular signal transduction systems, including ornithine decarboxylase, which leads to hyperplasia and most likely also to hypertrophy of the proximal tubule in the early stages of DM.7,16 The growth of the proximal tubule is strictly linked to and precedes the increased capacity of tubular reabsorption and hyperfiltration.38 Difluoromethylornithine, an ornithine decarboxylase inhibitor, attenuates kidney growth in early experimental DM.39 Although difluoromethylornithine does not modify tubular dimensions and GFR in nondiabetic rats, it significantly limits both tubular growth and hyperfiltration in diabetic rats. Altogether, these data support the claim that hyperglycemia-related tubular growth reasonably can be considered as the primary event in the pathophysiology of diabetic hyperfiltration (Fig 2). Increased Activity of the Renin-Angiotensin System It is well known that the intrarenal renin-angiotensin system (RAS) is activated by hyperglycemia leading
to synthesis of kidney Ang II.40 Besides the role of Ang II in the pathophysiology of tubular growth previously described, suppressing RAS activity in patients with DM using either an angiotensin-converting enzyme (ACE) inhibitor or an angiotensin receptor blocker (ARB) slows the decline in kidney function, mainly by means of their antialbuminuric effect. ARBs have been found to be effective throughout the entire spectrum of type 2 DN, with greater efficacy in terms of nephroprotection when therapy is started early in the course of the disease.41-44 A significant role of the RAS in the early stages of DM (10-15 days after inducing DM by streptozotocin administration) also can be inferred based on the effects of ACE inhibitors and ARBs on renal functional reserve, as shown by measurements of glomerular hemodynamics and proximal tubular reabsorption at baseline and during intravenous glycine infusion (to measure renal functional reserve) in control and diabetic rats.45 Glycine was observed to increase single-nephron GFR (presence of renal functional reserve) in controls, whereas no change in GFR or single-nephron GFR was elicited in diabetic rats (absence of renal functional reserve). Interestingly, the absence of renal functional reserve in diabetic rats was associated with a decrease in absolute proximal reabsorption due to the abnormal increase in Ang II and consequent suppression of tubular reabsorption. Restoring normal tubular reabsorption with Ang II blockade elicited the physiologic increase in singlenephron GFR and GFR during glycine administration in diabetic rats (renal functional reserve restoration). Overall, these data indicate that Ang II makes a significant contribution to the pathophysiology of DN that extends from the induction of glomerular damage in the late stages (albuminuric) of diabetes to the control of proximal tubular growth and reabsorption in the early stages of disease (prior to development of albuminuria).
EARLY RENAL ALTERATIONS AND PROGNOSIS
Figure 2. Tubular hypothesis of diabetic hyperfiltration. Abbreviations: NaCl, sodium chloride; SNGFR, single-nephron glomerular filtration rate. 4
From the clinical perspective, it is essential to establish whether the early alterations detected in DM determine the prognosis. Until recently, there have been conflicting data about the prognostic role of diabetic hyperfiltration. Whereas some studies suggested that diabetic individuals with glomerular hyperfiltration have a higher risk of more rapid progression of CKD,46,47 others did not confirm these findings.48,49 The discrepancies likely are related to the heterogeneity and small number of patients studied. Ruggenenti et al50 recently tested the link between glomerular hyperfiltration and progression of DN by pooling data from 2 prospective randomized clinical trials (RCTs), thereby obtaining an adequate sample size (600 hypertensive patients with type Am J Kidney Dis. 2014;-(-):---
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2 DM with normo- or microalbuminuria, followed up for a median of 4.0 years). Fifteen percent of participants were hyperfiltering at enrollment. Long-term GFR decline and progression to micro- or macroalbuminuria were faster in individuals with persistent hyperfiltration compared with nonhyperfiltering individuals and those for whom hyperfiltration at enrollment was ameliorated by intensified BP and metabolic control. The authors concluded that hyperfiltration has a central role in the pathogenesis of DN and that correcting it is nephroprotective. Similar to hyperfiltration, renal hypertrophy originally was found to be associated with greater risk of progression of DN.51-53 This concept later was emphasized by Zerbini et al,54 who demonstrated that in patients with type 1 DM and normoalbuminuria, larger kidney size was associated with a 3-times higher incidence of microalbuminuria and 3-fold greater decline in kidney function over a 9-year follow-up. An additional critical question is whether kidney size also predicts disease progression in patients with overt DN. To explore this specific issue, Rigalleu et al55 followed up, for 5 years, 75 patients (20 with type 1 and 55 with type 2) with DN defined by the presence of albuminuria with albumin excretion . 30 mg/24 h and/or GFR , 60 mL/min/1.73 m2. A kidney ultrasound was used to determine whether their outcome differed according to kidney size at baseline. Patients who were identified as having “large kidneys” at the time of enrollment had greater basal GFRs compared with those with “small kidneys” and constituted the majority of patients who had started dialysis therapy by the end of follow-up. The authors concluded that kidney hypertrophy predicts disease progression in patients with overt diabetic CKD as well.
NEW PERSPECTIVES: THE SGLT2 INHIBITORS Overview Various classes of agents (oral hypoglycemic agents, insulin, anti-RAS, and diuretics) currently are available for the prevention and treatment of DN; however, use of these agents can be limited by side effects. More important, there has been no substantial improvement in the prognosis of DM despite increasing awareness and screening of DM and implementation of more intensive and personalized therapies in these patients.5,56,57 Thus, new therapeutic strategies are desperately needed to limit the burden of DM-induced CKD and associated CV morbidity and mortality. The latter point is critical, considering that recent evidence has indicated that lifestyle interventions, unexpectedly and unfortunately, do not reduce CV risk in diabetic patients.58,59 SGLT2 inhibitors constitute a novel class of oral hypoglycemic agents with peculiar pharmacokinetic Am J Kidney Dis. 2014;-(-):---
and pharmacodynamic characteristics. Phlorizin, a natural nonspecific inhibitor of SGLT1 and SGLT2, was the first agent to demonstrate beneficial effects in diabetic rats. However, the presence of serious gastrointestinal effects due to SGLT1 inhibition precluded the use of this agent in clinical trials. From the phlorizin chemical structure, selective inhibitors of SGLT2 have been developed.60 Nine SGLT2 inhibitors currently are in clinical development61; among them, the first registered agents have been canagliflozin (in the United States) and dapagliflozin (in Europe). Studies of these agents in experimental diabetes demonstrate that SGLT2 is a main determinant of hyperglycemia and glomerular hyperfiltration. In particular, short- and long-term blockade of SGLT2 lowers proximal reabsorption of glucose, leading to increased distal delivery of glucose and sodium and a decrease in GFR of 20% in acute and 15% in chronic blockade due to tubuloglomerular feedback activation.62 The effects on kidney growth and injury are less clear, with some studies demonstrating an important dissociation between the correction of hyperfiltration and persistence of hypertrophy and development of inflammation.63 Vallon et al64 recently obtained more insights into this dissociation. These investigators evaluated the effects of the SGLT2 inhibitor empagliflozin on blood glucose levels and early kidney changes in a nonobese insulindependent model of type 1 diabetes (Akita mice). In contrast to what is seen in animals made diabetic by streptozotocin,63 kidney SGLT2 expression is upregulated in the Akita model system, making it more similar to type 2 DM in humans.25 SGLT2 inhibition in Akita mice lowered blood glucose levels by w60%, fully prevented hyperfiltration, and attenuated, to the same extent that it mitigated hyperglycemia, the increase in albuminuria, kidney weight, and several markers of inflammation. Overall, the findings of this and previous studies suggest that inhibiting SGLT2 suppresses diabetic hyperfiltration independent of its effects on blood glucose levels, whereas the beneficial effect on kidney growth and injury is dependent on its glycemiclowering effect. Another potential mechanism is the anti-inflammatory effects demonstrated by empagliflozin in vitro. This SGLT2 inhibitor reduces the inflammatory and fibrotic responses of human proximal tubular cells to high glucose levels, most likely by blocking glucose entry into the cell.65 Clinical Studies A study by De Fronzo et al66 in 12 patients with type 2 DM evaluated the effects of dapagliflozin on kidney function. The investigators found that compared with nondiabetic controls, diabetic individuals had elevated 5
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glucose maximal transport levels (maximum renal glucose reabsorptive capacity), splay (the curvilinear relationship between plasma glucose level and urinary glucose excretion rate), and threshold (plasma glucose concentration at which glucose first appears in urine that corresponds to the beginning of the splay). One week of dapagliflozin treatment reduced the maximal transport level and splay in diabetic participants and controls. However, the most significant effect was the reduction of the threshold for glucose excretion in both groups. Tubular effects of dapagliflozin were associated with a 14% decrease in GFR measured by the gold standard of plasma clearance of iohexol. Similarly, a recent small RCT by Lambers Heerspink et al67 demonstrated that in patients with type 2 DM with normal kidney function, administration of dapagliflozin for 12 weeks decreased iohexol-measured GFR by 11%, an effect partially dependent on a moderate reduction in 24-hour systolic BP and circulating volume. These findings provide evidence in humans of the complex interaction between the proximal tubule and GFR underlying the tubular hypothesis of hyperfiltration in diabetes. Interestingly, in the latter study, GFR was unchanged when estimated by the MDRD (Modification of Diet in Renal Disease) Study equation, thus supporting the observation of a recent reanalysis of RCTs that GFR-estimating formulas are of questionable valuable when used to identify and follow up hyperfiltering patients with DM.68
Several large RCTs, most of which were included in 2 recent meta-analyses,69,70 have been conducted to evaluate glycemic control with SGLT2 inhibitors in large cohorts of patients with DM. Results have consistently shown that these agents, either as monotherapy or add-on treatment, induce a reduction in hemoglobin A1c levels of 0.6%-0.7% associated with a decrease in body weight (ranging from 1.0-3.0 kg) due to osmotic diuresis (urinary volume increased by 200-600 mL/d) and loss of calories related to glycosuria (50-80 g/d). Not trivial are the secondary findings that SGLT2 inhibitors decrease systolic/ diastolic BP (by w4/2 mm Hg) and that these agents allow for lower doses of insulin, thus possibly reducing the CV risk associated with increased body weight, hypoglycemia, water retention, and congestive heart failure. The major side effect of these agents is the modest and expected increase in risk of urogenital infections. Unfortunately, these large trials do not offer insights into the potential nephroprotective effect of these agents due to the short-term follow-up. To our knowledge, there are only 4 trials evaluating kidney function, all by means of estimated GFR (eGFR) only, after treatment with SGLT2 inhibitors in large cohorts of patients with DM.71-74 In 3 of them, a slight decrease in eGFR was observed (Table 1). Interestingly, in the largest one,73 a slight decrease in urinary albumin-creatinine ratio also was observed by the end
Table 1. Large Randomized Clinical Trials on SGLT2 Inhibitors in Type 2 Diabetes Evaluating Kidney Function
Study
N
Active Arm
Control Arm
Follow-up (wk)
Kidney Function
List et al71 (2009)
389
Dapaglifozin (2.5-50 mg/d)
Placebo or metformin
12
eGFR unchanged
Wilding et al72 (2012)
808
Dapagliflozin (2.5, 5, 10 mg/d)
Placebo
48
eGFR decreased by 2.0, 2.0, and 1.1 mL/min/1.73 m2 in 2.5-, 5-, and 10-mg groups vs 0.7 in placebo
Canagliflozin (100, 300 mg/d)
Glimepiride
52
At wk 4, greater eGFR decrease in active arm (23.0 and 25.6 mL/min/1.73 m2 in 100- and 300-mg groups) than in control group (21.3 mL/min/1.3 m2); from wk 12 to 52, eGFR stable in canagliflozin groups while progressively decreased with glimepiride
Dapagliflozin (5, 10 mg/d)
Placebo
24-104
At wk 24, greater eGFR decrease in active arm (22.4 and 24.8 mL/min/ 1.73 m2 in 5- and 10-mg groups) than in placebo (20.3 mL/min/1.73 m2); from wk 24 to 104, eGFR stable in active arm but progressively decreased in placebo group (vs baseline, 21.7, 23.5, 22.4 mL/min/ 1.73 m2 in 5-mg dapagliflozin, 10-mg dapagliflozen, and placebo groups)
Cefalu et al73 (2013)
1,450
Kohan et al74 (2013)
252
Abbreviations: eGFR, estimated glomerular filtration rate; SGLT2, sodium/glucose cotransporter 2. 6
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of the 1-year follow-up in the 100- and 300-mg canagliflozin groups (20.1 6 4.7 and 20.9 6 6.7 g/mol), whereas albuminuria increased in the control group (10.7 6 15.3 g/mol). This finding has been confirmed in the most recent trial, which was carried out in 252 patients with inadequately controlled type 2 DM and moderately decreased kidney function (basal eGFR, 45.6 6 10.0 mL/min/1.73 m2).74 In this study, patients receiving dapagliflozin were more likely to regress to a lower albumin excretion category than patients receiving placebo. Results for albuminuria and eGFR support the potential nephroprotective effect of SGLT2 inhibition.
CONCLUSIONS During the last decade, there has been a relentless increase in the number of diabetic individuals with CKD reaching chronic kidney failure. More burdensome is the observation that the prognosis has not changed significantly despite implementation of new drugs to treat diabetes and associated nephropathy. Two questions therefore arise: Should we explore new avenues? Should we move to intensify preventive strategies at earlier stages rather than therapy of overt disease? The tubular hypothesis of diabetic hyperfiltration and characterization of the early kidney alterations in diabetes are not novel; however, relatively few studies have verified in humans the information provided by a large set of animal studies. At this time, it is essential to persevere in improving our knowledge of the pathophysiologic role of the proximal tubule in humans. From this perspective, the recent introduction of SGLT2 inhibitors appears as an exciting opportunity. In particular, from the nephrologist’s point of view, we need studies with these new agents that specifically address: (1) kidney function over shortand long-term treatment (changes in measured GFR and albuminuria and incidence of end-stage renal disease), (2) the prognostic value of lower BP (potentially contributing to better renal and CV outcomes), and (3) the impact on kidney function in patients at risk of renal hypoperfusion (severe proteinuria and dual RAS blockade).
ACKNOWLEDGEMENTS Support: None. Financial Disclosure: The authors declare that they have no relevant financial interests.
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3. De Nicola L, Chiodini P, Zoccali C, et al. Prognosis of CKD patients receiving outpatient nephrology care in Italy. Clin J Am Soc Nephrol. 2011;6(10):2421-2428. 4. Sasso FC, Chiodini P, Carbonara O, et al. High cardiovascular risk in patients with type 2 diabetic nephropathy: the predictive role of albuminuria and glomerular filtration rate. The NID-2 Prospective Cohort Study. Nephrol Dial Transplant. 2012;27(6):2269-2274. 5. Rossing P, de Zeeuw D. Need for better diabetes treatment for improved renal outcome. Kidney Int Suppl. 2011;120:S28-S32. 6. Vallon V, Komers R. Pathophysiology of the diabetic kidney. Compr Physiol. 2011;1(3):1175-1232. 7. Thomson SC, Vallon V, Blantz RC. Kidney function in early diabetes: the tubular hypothesis of glomerular filtration. Am J Physiol Renal Physiol. 2004;286(1):F8-F15. 8. Kanwar YS, Sun L, Xie P, Liu FY, Chen S. A glimpse of various pathogenetic mechanism of diabetic nephropaty. Annu Rev Pathol. 2011;6:395-423. 9. Thomson SC, Blantz RC. Glomerulotubular balance, tubuloglomerular feedback, and salt homeostasis. J Am Soc Nephrol. 2008;19(12):2272-2275. 10. Miller IA. Renal responses to sodium restriction in patients with early diabetes mellitus. J Am Soc Nephrol. 1997;8(5):749-755. 11. Miracle CM, Rieg T, Mansoury H, Vallon V, Thomson SC. Ornithine decarboxylase inhibitor eliminates hyperresponsiveness of the early diabetic proximal tubule to dietary salt. Am J Physiol Renal Physiol. 2008;295(4):F995-F1002. 12. Ditzel J, Lervang HH, Brochner-Mortensen J. Renal sodium metabolism in relation to hypertension in diabetes. Diabetes Metab. 1989;15(5, pt 2):292-295. 13. Mbanya JC, Thomas TH, Taylor R, Alberti KG, Wilkinson R. Increased proximal tubular sodium reabsorption in hypertensive patients with type 2 diabetes. Diabet Med. 1989;6(7):614-620. 14. Hannedouche TP, Delgado AG, Gnionsahe DA, Boitard C, Lacour B, Grunfeld JP. Renal hemodynamics and segmental tubular reabsorption in early type 1 diabetes. Kidney Int. 1990;37(4):1126-1133. 15. Vallon V, Richter K, Blantz RC, Thomson S, Osswald H. Glomerular hyperfiltration in experimental diabetes mellitus: potential role of tubular reabsorption. J Am Soc Nephrol. 1999;10(12):2569-2576. 16. Thomson SC, Deng A, Bao D, Satriano J, Blantz RC, Vallon V. Ornithine decarboxylase, kidney size, and the tubular hypothesis of glomerular hyperfiltration in experimental diabetes. J Clin Invest. 2001;107(2):217-224. 17. Pruijm M, Wuerzner G, Maillard M, et al. Glomerular hyperfiltration and increased proximal sodium reabsorption in subjects with type 2 diabetes or impaired fasting glucose in a population of the African region. Nephrol Dial Transplant. 2010;25(7):2225-2231. 18. Scholl-Bürgi S, Santer R, Ehrich JH. Long-term outcome of renal glucosuria type 0: the original patient and his natural history. Nephrol Dial Transplant. 2004;19(9):2394-2396. 19. Barfuss DW, Schafer JA. Differences in active and passive glucose transport along the proximal nephron. Am J Physiol. 1981;241(3):F322-F332. 20. Lee YJ, Han HJ. Regulatory mechanisms of Na(1)/glucose cotransporters in renal proximal tubule cells. Kidney Int Suppl. 2007;106:S27-S35. 21. Vallon V, Platt KA, Cunard R, et al. SGLT2 mediates glucose reabsorption in the early proximal tubule. J Am Soc Nephrol. 2011;22(1):104-112. 22. Quamme GA, Freeman HJ. Evidence for a high-affinity sodium-dependent D-glucose transport system in the kidney. Am J Physiol. 1987;253(1, pt 2):F151-F157.
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De Nicola et al 23. Mogensen CE. Maximum tubular reabsorption capacity for glucose and renal hemodynamics during rapid hypertonic glucose infusion in normal and diabetic subjects. Scand J Clin Lab Invest. 1971;28(1):101-109. 24. Bakris GL, Fonseca VA, Sharma K, et al. Renal sodiumglucose transport: role in diabetes mellitus and potential clinical implications. Kidney Int. 2009;75(12):1272-1277. 25. Rahmoune H, Thompson PW, Ward JM, Smith CD, Hong G, Brown J. Glucose transporters in human renal proximal tubular cells isolated from the urine of patients with non-insulindependent diabetes. Diabetes. 2005;54(12):3427-3434. 26. Gorboulev V, Schurmann A, Vallon V, et al. Na(1)D-glucose cotransporter SGLT1 is pivotal for intestinal glucose absorption and glucose-dependent incretin secretion. Diabetes. 2012;61(1):187-196. 27. Rieg T, Masuda T, Gerasimova M, et al. Increase in SGLT1-mediated transport explains renal glucose reabsorption during genetic and pharmacologic SGLT2 inhibition in euglycemia. Am J Physiol Renal Physiol. 2014;306(2):F188-F193. 28. Powell DR, DaCosta CM, Gay J, et al. Improved glycemic control in mice lacking SGLT1 and SGLT2. Am J Physiol Endocrinol Metab. 2013;304(2):E117-E130. 29. Vallon V. The proximal tubule in the pathophysiology of the diabetic kidney. Am J Physiol Regul Integr Comp Physiol. 2011;300(5):1009-1022. 30. Vallon V, Thomson SC. Renal function in diabetic disease models: the tubular system in the pathophysiology of the diabetic kidney. Annu Rev Physiol. 2012;74:351-375. 31. Huang HC, Preisig PA. G1 kinases and transforming growth factor-beta signaling are associated with a growth pattern switch in diabetes-induced renal growth. Kidney Int. 2000;58(1): 162-172. 32. Satriano J, Mansoury H, Deng A, et al. Transition of kidney tubule cells to a senescent phenotype in early experimental diabetes. Am J Physiol Cell Physiol. 2010;299:374-380. 33. Han HJ, Lee YJ, Park SH, Lee JH, Taub M. High glucoseinduced oxidative stress inhibits Na/glucose cotransporter activity in renal proximal tubule cells. Am J Physiol Renal Physiol. 2005;288:F988-F996. 34. Kuilman T, Michaloglou C, Vredeveld LC, et al. Oncogeneinduced senescence relayed by an interleukin-dependent inflammatory network. Cell. 2008;133(6):1019-1031. 35. Verzola D, Gandolfo MT, Gaetani G, et al. Accelerated senescence in the kidneys of patients with type 2 diabetic nephropathy. Am J Physiol Renal Physiol. 2008;295:F1563-F1573. 36. Wolf G, Ziyadeh FN. Molecular mechanisms of diabetic renal hypertrophy. Kidney Int. 1999;56(2):393-405. 37. Feliers D, Kasinath BS. Mechanism of VEGF expression by high glucose in proximal tubule epithelial cells. Mol Cell Endocrinol. 2010;314(1):136-142. 38. Bak M, Thomsen K, Christiansen T, Flyvbjerg A. Renal enlargement precedes renal hyperfiltration in early experimental diabetes in rats. J Am Soc Nephrol. 2000;11(7):1287-2192. 39. Pedersen SB, Flyvbjerg A, Richelsen B. Inhibition of renal ornithine decarboxylase activity prevents kidney hypertrophy in experimental diabetes. Am J Physiol Cell Physiol. 1993;264(2, pt 1): C453-C456. 40. Burns KD. Angiotensin II and its receptors in the diabetic kidney. Am J Kidney Dis. 2000;36(3):449-467. 41. Lewis EJ, Hunsicker LG, Clarke WR, et al; for the Collaborative Study Group. Renoprotective effect of the angiotensin-receptor antagonist irbesartan in patients with nephropathy due to type 2 diabetes. N Engl J Med. 2001;345(12):851-860.
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42. Brenner BM, Cooper ME, de Zeeuw D, et al; for the RENAAL Study Investigators. Effects of losartan on renal and cardiovascular outcomes in patients with type 2 diabetes and nephropathy. N Engl J Med. 2001;345(12):861-869. 43. Parving HH, Lehnert H, Bröchner-Mortensen J, Gomis R, Andersen S, Arner P; for the Irbesartan in Patients With Type 2 Diabetes and Microalbuminuria Study Group. The effect of irbesartan on the development of diabetic nephropathy in patients with type 2 diabetes. N Engl J Med. 2001;345(12):870-878. 44. Sasso FC, Carbonara O, Persico M, et al. Irbesartan reduces the albumin excretion rate in microalbuminuric type 2 diabetic patients independently of hypertension. Diabetes Care. 2002;25(11):1909-1913. 45. De Nicola L, Blantz RC, Gabbai FB. Renal functional reserve in the early stage of experimental diabetes. Diabetes. 1992;41(3):267-273. 46. Magee GM, Bilous RW, Cardwell CR, Hunter SJ, Kee F, Fogarty DG. Is hyperfiltration associated with the future risk of developing diabetic nephropathy? A meta analysis. Diabetologia. 2009;52(4):691-697. 47. Moriya T, Tsuchiya A, Okizaki S, Hayashi A, Tanaka K, Schichiri M. Glomerular hyperfiltration and increased glomerular filtration surface are associated with renal function decline in normo and microalbuminuric type 2 diabetes. Kidney Int. 2012;81(5):486-493. 48. Chaiken RL, Eckert-Norton M, Bard M, et al. Hyperfiltration in African-American patients with type 2 diabetes. Cross-sectional and longitudinal data. Diabetes Care. 1998;21(12):2129-2134. 49. Nelson RG, Bennet PH, Beck GJ, et al; Diabetic Renal Disease Study Group. Development and progression of renal disease in Prima Indians with non-insulin-dependent diabetes mellitus. N Engl J Med. 1996;335(22):1636-1642. 50. Ruggenenti P, Porrini EL, Gaspari F, et al. Glomerular hyperfiltration and renal disease progression in type 2 diabetes. Diabetes Care. 2012;35(10):2061-2068. 51. Kleinman KS, Fine LG. Prognostic implications of renal hypertrophy in diabetes mellitus. Diabetes Metab Rev. 1988;4(2): 179-189. 52. Mogensen CE, Andersen MJF. Increased kidney size and glomerular filtration rate in early juvenile diabetes. Diabetes. 1973;22(9):706-712. 53. Mogensen CE, Christiansen CK. Predicting diabetic nephropathy in insulin-dependent patients. N Engl J Med. 1984;311(2):89-93. 54. Zerbini G, Bonfanti RN, Meschi F, et al. Persistent renal hypertrophy and faster decline of glomerular filtration rate precede the development of microalbuminuria in type 1 diabetes. Diabetes. 2006;55(9):2620-2625. 55. Rigalleau V, Garcia M, Lasseur C, et al. Large kidneys predict poor renal outcome in subjects with diabetes and chronic kidney disease. BMC Nephrol. 2010;11:3. 56. Couchoud C, Villar E. End-stage renal disease epidemic in diabetics: is there light at the end of the tunnel? Nephrol Dial Transplant. 2013;28(5):1073-1076. 57. Ali MK, Bullard KM, Saaddine JB, Cowie CC, Imperatore G, Gregg EW. Achievement of goals in U.S. diabetes care, 1999-2010. N Engl J Med. 2013;368(17):1613-1624. 58. Wing RR, Bolin P, Brancati FL, et al; for the Look AHEAD Research Group. Cardiovascular effects of intensive lifestyle intervention in type 2 diabetes. N Engl J Med. 2013;369(2):145-154. 59. Schellenberg ES, Dryden DM, Vandermeer B, Ha C, Korownyk C. Lifestyle interventions for patients with and at risk
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Proximal Tubule in Diabetes Mellitus for type 2 diabetes: a systematic review and meta-analysis. Ann Intern Med. 2013;159(8):543-551. 60. Hanefeld M, Forst T. Dapagliflozin, an SGLT2 inhibitor, for diabetes. Lancet. 2010;375(9733):2196-2198. 61. Washburn WN, Poucher SM. Differentiating sodiumglucose co-transporter-2 inhibitors in development for the treatment of type 2 diabetes mellitus. Expert Opin Investig Drugs. 2013;22(4):463-486. 62. Thomson SC, Rieg T, Miracle C, et al. Acute and chronic effects of SGLT2 blockade on glomerular and tubular function in the early diabetic rat. Am J Physiol Regul Integr Comp Physiol. 2012;302(1):R75-R83. 63. Vallon V, Rose M, Gerasimova M, et al. Knockout of Naglucose transporter SGLT2 attenuates hyperglycemia and glomerular hyperfiltration but not kidney growth or injury in diabetes mellitus. Am J Physiol Renal Physiol. 2013;304(2):F156-F167. 64. Vallon V, Gerasimova M, Rose M, et al. SGLT2 inhibitor empagliflozin reduces renal growth and albuminuria in proportion to hyperglycemia and prevents glomerular hyperfiltration in diabetic Akita mice. Am J Physiol Renal Physiol. 2014;306(2): F194-F204. 65. Panchapakesan U, Pegg K, Gross S, et al. Effects of SGLT2 inhibition in human kidney proximal tubular cells. Renoprotection in diabetic nephropathy? PLoS One. 2013;8(2):e54442. 66. Defronzo RA, Hompesch M, Kasichayanula S, et al. Characterization of renal glucose reabsorption in response to dapagliflozin in healthy subjects and subjects with type 2 diabetes. Diabetes Care. 2013;36(10):3169-3176. 67. Lambers Heerspink HJ, de Zeeuw D, Wie L, Leslie B, List J. Dapagliflozin a glucose-regulating drug with diuretic properties in
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subjects with type 2 diabetes. Diabetes Obes Metab. 2013;15(9): 853-862. 68. Gaspari F, Ruggenenti P, Porrini E, et al. The GFR and GFR decline cannot be accurately estimated in type 2 diabetics. Kidney Int. 2013;84(1):164-173. 69. Musso G, Gambino R, Cassader M, Pagano G. A novel approach to control hyperglycemia in type 2 diabetes: sodium glucose co-transport (SGLT) inhibitors: systematic review and meta-analysis of randomized trials. Ann Med. 2012;44(4): 375-393. 70. Vasilakou D, Karagiannis T, Athanasiadou E, et al. Sodium– glucose cotransporter 2 inhibitors for type 2 diabetes: a systematic review and meta-analysis. Ann Intern Med. 2013;159(4):262-274. 71. List JF, Woo V, Morales E, Tang W, Fiedorek FT. Sodiumglucose cotransport inhibition with dapagliflozin in type 2 diabetes. Diabetes Care. 2009;32(4):650-657. 72. Wilding J, Woo V, Soler NG, et al. Long-term efficacy of dapagliflozin in patients with type 2 diabetes mellitus receiving high doses of insulin. Ann Intern Med. 2012;156(6):405-415. 73. Cefalu WT, Leiter LA, Yoon KH, et al. Efficacy and safety of canagliflozin versus glimepiride in patients with type 2 diabetes inadequately controlled with metformin (CANTATA-SU): 52 week results from a randomised, double-blind, phase 3 noninferiority trial. Lancet. 2013;382(9896):941-950. 74. Kohan DE, Fioretto P, Tang W, List JF. Long-term study of patients with type 2 diabetes and moderate renal impairment shows that dapagliflozin reduces weight and blood pressure but does not improve glycemic control [published online ahead of print September 25, 2013]. Kidney Int. http://dx.doi.org/10.1 038/ki.2013.356.
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D
iabetes mellitus (DM) is a major public health problem worldwide. The prevalence of type 2 DM is increasing exponentially; it is estimated that more than 400 million people will have DM by 2030.1 Among the various complications of DM, diabetic nephropathy (DN) is considered a major threat because it affects approximately one-third of all diabetic individuals and is a main cause of mortality and a leading cause of end-stage renal disease.2 DN is characterized by persistent albuminuria, decline in glomerular filtration rate (GFR), increasing blood pressure (BP), and high cardiovascular (CV) risk. Patients with non–dialysis-dependent type 2 DN, a condition frequently encountered in endocrine/ diabetes and nephrology clinics, benefit from strict cooperation between the 2 specialties to improve prognosis.3,4 Optimal prevention and treatment of chronic kidney disease (CKD) in patients with DM requires implementing therapies that specifically interfere with the pathogenesis of DN.5 In this regard, significant attention has been given to the role of the proximal tubule.6,7 It is well known that the dimensions and function of the proximal tubule increase in response to higher glucose load. These changes have been linked to the increase in GFR, or so-called diabetic hyperfiltration. The development of new antidiabetic agents, such as inhibitors of sodium/glucose Am J Kidney Dis. 2014;-(-):---
cotransporter 2 (SGLT2; encoded by the SLC5A2 gene), has raised the intriguing possibility of new tools to prevent DN. Data for the role of increased tubular reabsorption in the pathophysiology of human DN are limited compared to the more robust experimental literature. We review the information supporting this hypothesis by integrating the experimental findings with the available clinical data.
EARLY CHANGES IN THE DIABETIC KIDNEY Glomerular Hyperfiltration In the setting of DM, hyperglycemia is accompanied by an increase in GFR. Investigators have proposed several mechanisms to explain diabetic hyperfiltration, From the 1Nephrology Division, Second University of NaplesMed School, Naples, Italy; and 2Department of Medicine, Veterans Administration San Diego Healthcare System-University of California at San Diego Medical School, San Diego, CA. Received October 25, 2013. Accepted in revised form February 5, 2014. Address correspondence to Luca De Nicola, MD, PhD, Nephrology Division, Second University of Naples-Med School, P.O. S.M.D.P Incurabili, Via Maria Longo 50, 80138 Napoli, Italy. E-mail: [email protected] 2014 by the National Kidney Foundation, Inc. 0272-6386/$36.00 http://dx.doi.org/10.1053/j.ajkd.2014.02.010 1
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including impaired constriction of the afferent arteriole.6-8 These hemodynamic changes have been associated with activation of neurogenic, hormonal, and myogenic factors (known as the hemodynamic hypothesis). However, increased proximal tubular reabsorption also has been shown within the spectrum of the early changes observed in diabetic kidneys, suggesting an alternative hypothesis to explain the increase in GFR (known as the tubular hypothesis). Under normal and pathophysiologic conditions, glomeruli and tubules are functionally linked to ensure maintenance of adequate extracellular volume and consequently systemic perfusion.9 Two mechanisms are involved in this process. Glomerulotubular balance modulates proximal tubular reabsorption according to changes in GFR in order to guarantee a constant percentage of tubular reabsorption of filtered solutes. Any increase in GFR determines an increase in proximal tubular reabsorption, and vice versa. The mechanism mainly depends on parallel changes in peritubular capillary oncotic pressure that accompany changes in filtration fraction. However, glomerulotubular balance is not fully efficient in maintaining fractional reabsorption constant. Due to the limitation of glomerulotubular balance, part of any change in GFR will pass along the nephron, eliciting the tubuloglomerular feedback response, which will counteract some of the original disturbance. Tubuloglomerular feedback is driven by signals directly generated at the level of the macula densa whereby changes in sodium chloride concentration in tubular fluid cause opposite changes in single-nephron GFR (Fig 1). When a primary reduction in proximal tubular reabsorption ensues, the amount of sodium chloride in the thick ascending limb of Henle and the first part of the distal tubule, the so-called distal delivery of sodium chloride, increases, which in turn leads to a decrease in GFR. Conversely, a reduction in distal delivery driven by increased reabsorption in the proximal tubule induces an increase in single-nephron GFR. Tubuloglomerular feedback is continuously active and modulates the tone of glomerular afferent resistance through the release of adenosine, a vasoconstrictive substance. Tubuloglomerular feedback and glomerulotubular balance maintain relatively constant distal salt delivery and sodium excretion when GFR increases. The efficiency of the 2 mechanisms ensures the optimal defense against volume depletion because if the primum movens is of a vascular nature, there is a tubular adaptation (glomerulotubular balance), whereas if the primum movens is of tubular origin, a glomerular adaptation (tubuloglomerular feedback) ensues. In the early stages of diabetes, tubuloglomerular feedback has a major role. In diabetic rats, hyperglycemia 2
Figure 1. Physiology of tubuloglomerular feedback. Abbreviations: NaCl, sodium chloride; GFR, glomerular filtration rate.
induces an increase in proximal tubular reabsorption due to proximal tubular hypertrophy and a consequent increase in sodium/glucose cotransport. The reduction in solute delivery to the macula densa thereby decreases tubuloglomerular feedback activity with a consequent increase in single-nephron GFR.7 According to the hemodynamic hypothesis of hyperfiltration, an excess of total-body sodium in patients with DM was proposed as a contributor to the renal vasodilatation possibly by suppressing vasoconstricting neurohormonal systems. Such a hypothesis was not supported by results of a randomized trial in patients with uncomplicated insulin-dependent DM.10 In that study, investigators measured GFR and renal plasma flow in patients with DM and in ageand sex-matched controls consuming a normal(200 mmol/d) and low- (20 mmol/d) sodium diet. A normal-sodium diet was associated with lower renal vascular resistance and higher GFR in patients. Unexpectedly, salt restriction did not correct kidney hyperperfusion, but exacerbated it. These alterations were specific to DM; no significant changes in response to variations in dietary salt intake were observed in controls. Experimental studies have explained the potential mechanism of this “salt paradox” in DM by demonstrating that proximal tubular reabsorption is more sensitive to variations in dietary salt in this model, with an abnormal tendency for GFR to vary inversely with sodium intake.6,7,11 Am J Kidney Dis. 2014;-(-):---
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Enhanced Proximal Tubular Reabsorption The increase in proximal tubular reabsorption and consequent reduction in distal delivery of sodium chloride in patients with types 1 and 2 DM originally was described more than 20 years ago in studies using lithium clearance.12,13 Based on these observations, Hannedouche et al14 hypothesized that a decrease in distal sodium delivery could contribute to diabetic glomerular hyperfiltration through a reduction in tubuloglomerular feedback–mediated vasoconstriction. Subsequently, micropuncture studies in diabetic rats confirmed that increased proximal tubular reabsorption has a major role in the development of glomerular hyperfiltration in the early stages of DM.15,16 In these studies, increased proximal tubular reabsorption of chloride anions and sodium and potassium cations led to a 20%-28% reduction in the concentration of these ions in the distal tubule compared with nondiabetic rats. These concepts were confirmed recently in a clinical study showing a higher prevalence of glomerular hyperfiltration and associated enhanced proximal sodium reabsorption in African individuals with impaired fasting glucose or type 2 DM.17 Glucose is not bound to proteins or macromolecules and is filtered freely by glomeruli. In healthy adults, glomeruli filter w180 g of glucose daily. Most of the filtered glucose is recovered by tubular reabsorption to protect the individual against wide variations in glucose supply and demand, a process essential for life. Glucose is removed from the tubular lumen by active transport at the level of the proximal segments. The amount of glucose reabsorbed is proportional to the filtered load and hence depends on plasma glucose level times the GFR value, up to the maximal transport level. The ability of the proximal tubule to reabsorb glucose increases with increments in the filtered load secondary to higher GFR or plasma glucose levels. When the filtered load reaches the maximal transport level, any additional increase results in glycosuria. Under normal conditions, ,0.5 (range, 0.03-0.3) g/d of glucose is excreted in urine. However, glycosuria with normal plasma glucose levels may appear during pregnancy, in individuals with one kidney, or in the presence of primary renal glycosuria, a benign condition that does not require a specific therapy.18 In the early 1980s, experimental studies in animal models described 2 different glucose transporters located in the proximal tubule19: a “high-flux lowaffinity” transporter (SGLT2, located in the S1 segment) and a “low-flux high-affinity” transporter (SGLT1, located in the S3 segment). The S1 segment reabsorbs the largest proportion of the filtered glucose and sodium load, while the remaining 10% is Am J Kidney Dis. 2014;-(-):---
reabsorbed by the S3 segment.20-22 In diabetic patients, glycosuria is limited by upregulation of tubular glucose reabsorption.23,24 Early experiments in human kidney tubular cells harvested from urine of patients with type 2 DM demonstrate an increase in the expression, protein concentration, and a-methylglucose transport capacity of SGLT2 compared with nondiabetic individuals.25 More recent insights have been provided by studies in nondiabetic mice lacking expression of SGLT1 (Sglt2/2). These studies have demonstrated that SGLT1, while critical for intestinal absorption of glucose, also is required for complete reabsorption of filtered glucose in the kidney.26 Furthermore, during SGLT2 inhibition, the glucose load to the SGLT1-expressing S2/S3 segments of the proximal tubule is enhanced and a compensatory increase in SGLT1-mediated transport occurs.27 The contribution of SGLT1 may explain, at least in part, why SGLT2 inhibition is associated with excretion of only 50%-60% of filtered glucose. These results are also consistent with the observation that combined SGLT1 and SGLT2 knockout is associated with higher glucose excretion and improved glycemia control compared to inhibition of SGLT2 alone.28 Enlargement of the Diabetic Kidney A peculiar feature of the diabetic kidney is its enlargement at the onset of DM. As discussed in detail in recent reviews by Vallon and Thomson,29,30 experimental results suggest that structural tubular changes explain most early functional changes. The enlargement of the diabetic kidney caused by the diabetic milieu is attributed primarily to the proximal tubule, in which a period of hyperplasia precedes hypertrophy.31 Early in the course of hyperglycemia, hyperplasia is followed by G1 cell-cycle arrest, development of hypertrophy, and a senescence-like phenotype.31,32 This growth switch is mediated by mainly TGFb1 (transforming growth factor b1).33 The molecular pathways involved in senescence are linked to tubulointerstitial fibrosis and inflammation and thus may contribute to the progression to endstage renal disease.32,34 Hypertrophy and senescence also may alter tubular function, providing a potential explanation for the salt paradox of the diabetic kidney.6,30 Notably, acceleration of the senescent phenotype in tubule cells has been confirmed in humans by means of kidney biopsy studies in patients with type 2 DM.35 Hyperplasia is mediated by several growth factors, such as insulin-like growth factor 1 (IGF-1), plateletderived growth factor (PDGF), vascular endothelial growth factor (VEGF), and epidermal growth factor (EGF).36 High levels of glucose activate endogenous tubular cell renin and angiotensin II (Ang II), which 3
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binds to its receptors and induces synthesis of VEGF.37 Glucose and Ang II activate several intracellular signal transduction systems, including ornithine decarboxylase, which leads to hyperplasia and most likely also to hypertrophy of the proximal tubule in the early stages of DM.7,16 The growth of the proximal tubule is strictly linked to and precedes the increased capacity of tubular reabsorption and hyperfiltration.38 Difluoromethylornithine, an ornithine decarboxylase inhibitor, attenuates kidney growth in early experimental DM.39 Although difluoromethylornithine does not modify tubular dimensions and GFR in nondiabetic rats, it significantly limits both tubular growth and hyperfiltration in diabetic rats. Altogether, these data support the claim that hyperglycemia-related tubular growth reasonably can be considered as the primary event in the pathophysiology of diabetic hyperfiltration (Fig 2). Increased Activity of the Renin-Angiotensin System It is well known that the intrarenal renin-angiotensin system (RAS) is activated by hyperglycemia leading
to synthesis of kidney Ang II.40 Besides the role of Ang II in the pathophysiology of tubular growth previously described, suppressing RAS activity in patients with DM using either an angiotensin-converting enzyme (ACE) inhibitor or an angiotensin receptor blocker (ARB) slows the decline in kidney function, mainly by means of their antialbuminuric effect. ARBs have been found to be effective throughout the entire spectrum of type 2 DN, with greater efficacy in terms of nephroprotection when therapy is started early in the course of the disease.41-44 A significant role of the RAS in the early stages of DM (10-15 days after inducing DM by streptozotocin administration) also can be inferred based on the effects of ACE inhibitors and ARBs on renal functional reserve, as shown by measurements of glomerular hemodynamics and proximal tubular reabsorption at baseline and during intravenous glycine infusion (to measure renal functional reserve) in control and diabetic rats.45 Glycine was observed to increase single-nephron GFR (presence of renal functional reserve) in controls, whereas no change in GFR or single-nephron GFR was elicited in diabetic rats (absence of renal functional reserve). Interestingly, the absence of renal functional reserve in diabetic rats was associated with a decrease in absolute proximal reabsorption due to the abnormal increase in Ang II and consequent suppression of tubular reabsorption. Restoring normal tubular reabsorption with Ang II blockade elicited the physiologic increase in singlenephron GFR and GFR during glycine administration in diabetic rats (renal functional reserve restoration). Overall, these data indicate that Ang II makes a significant contribution to the pathophysiology of DN that extends from the induction of glomerular damage in the late stages (albuminuric) of diabetes to the control of proximal tubular growth and reabsorption in the early stages of disease (prior to development of albuminuria).
EARLY RENAL ALTERATIONS AND PROGNOSIS
Figure 2. Tubular hypothesis of diabetic hyperfiltration. Abbreviations: NaCl, sodium chloride; SNGFR, single-nephron glomerular filtration rate. 4
From the clinical perspective, it is essential to establish whether the early alterations detected in DM determine the prognosis. Until recently, there have been conflicting data about the prognostic role of diabetic hyperfiltration. Whereas some studies suggested that diabetic individuals with glomerular hyperfiltration have a higher risk of more rapid progression of CKD,46,47 others did not confirm these findings.48,49 The discrepancies likely are related to the heterogeneity and small number of patients studied. Ruggenenti et al50 recently tested the link between glomerular hyperfiltration and progression of DN by pooling data from 2 prospective randomized clinical trials (RCTs), thereby obtaining an adequate sample size (600 hypertensive patients with type Am J Kidney Dis. 2014;-(-):---
Proximal Tubule in Diabetes Mellitus
2 DM with normo- or microalbuminuria, followed up for a median of 4.0 years). Fifteen percent of participants were hyperfiltering at enrollment. Long-term GFR decline and progression to micro- or macroalbuminuria were faster in individuals with persistent hyperfiltration compared with nonhyperfiltering individuals and those for whom hyperfiltration at enrollment was ameliorated by intensified BP and metabolic control. The authors concluded that hyperfiltration has a central role in the pathogenesis of DN and that correcting it is nephroprotective. Similar to hyperfiltration, renal hypertrophy originally was found to be associated with greater risk of progression of DN.51-53 This concept later was emphasized by Zerbini et al,54 who demonstrated that in patients with type 1 DM and normoalbuminuria, larger kidney size was associated with a 3-times higher incidence of microalbuminuria and 3-fold greater decline in kidney function over a 9-year follow-up. An additional critical question is whether kidney size also predicts disease progression in patients with overt DN. To explore this specific issue, Rigalleu et al55 followed up, for 5 years, 75 patients (20 with type 1 and 55 with type 2) with DN defined by the presence of albuminuria with albumin excretion . 30 mg/24 h and/or GFR , 60 mL/min/1.73 m2. A kidney ultrasound was used to determine whether their outcome differed according to kidney size at baseline. Patients who were identified as having “large kidneys” at the time of enrollment had greater basal GFRs compared with those with “small kidneys” and constituted the majority of patients who had started dialysis therapy by the end of follow-up. The authors concluded that kidney hypertrophy predicts disease progression in patients with overt diabetic CKD as well.
NEW PERSPECTIVES: THE SGLT2 INHIBITORS Overview Various classes of agents (oral hypoglycemic agents, insulin, anti-RAS, and diuretics) currently are available for the prevention and treatment of DN; however, use of these agents can be limited by side effects. More important, there has been no substantial improvement in the prognosis of DM despite increasing awareness and screening of DM and implementation of more intensive and personalized therapies in these patients.5,56,57 Thus, new therapeutic strategies are desperately needed to limit the burden of DM-induced CKD and associated CV morbidity and mortality. The latter point is critical, considering that recent evidence has indicated that lifestyle interventions, unexpectedly and unfortunately, do not reduce CV risk in diabetic patients.58,59 SGLT2 inhibitors constitute a novel class of oral hypoglycemic agents with peculiar pharmacokinetic Am J Kidney Dis. 2014;-(-):---
and pharmacodynamic characteristics. Phlorizin, a natural nonspecific inhibitor of SGLT1 and SGLT2, was the first agent to demonstrate beneficial effects in diabetic rats. However, the presence of serious gastrointestinal effects due to SGLT1 inhibition precluded the use of this agent in clinical trials. From the phlorizin chemical structure, selective inhibitors of SGLT2 have been developed.60 Nine SGLT2 inhibitors currently are in clinical development61; among them, the first registered agents have been canagliflozin (in the United States) and dapagliflozin (in Europe). Studies of these agents in experimental diabetes demonstrate that SGLT2 is a main determinant of hyperglycemia and glomerular hyperfiltration. In particular, short- and long-term blockade of SGLT2 lowers proximal reabsorption of glucose, leading to increased distal delivery of glucose and sodium and a decrease in GFR of 20% in acute and 15% in chronic blockade due to tubuloglomerular feedback activation.62 The effects on kidney growth and injury are less clear, with some studies demonstrating an important dissociation between the correction of hyperfiltration and persistence of hypertrophy and development of inflammation.63 Vallon et al64 recently obtained more insights into this dissociation. These investigators evaluated the effects of the SGLT2 inhibitor empagliflozin on blood glucose levels and early kidney changes in a nonobese insulindependent model of type 1 diabetes (Akita mice). In contrast to what is seen in animals made diabetic by streptozotocin,63 kidney SGLT2 expression is upregulated in the Akita model system, making it more similar to type 2 DM in humans.25 SGLT2 inhibition in Akita mice lowered blood glucose levels by w60%, fully prevented hyperfiltration, and attenuated, to the same extent that it mitigated hyperglycemia, the increase in albuminuria, kidney weight, and several markers of inflammation. Overall, the findings of this and previous studies suggest that inhibiting SGLT2 suppresses diabetic hyperfiltration independent of its effects on blood glucose levels, whereas the beneficial effect on kidney growth and injury is dependent on its glycemiclowering effect. Another potential mechanism is the anti-inflammatory effects demonstrated by empagliflozin in vitro. This SGLT2 inhibitor reduces the inflammatory and fibrotic responses of human proximal tubular cells to high glucose levels, most likely by blocking glucose entry into the cell.65 Clinical Studies A study by De Fronzo et al66 in 12 patients with type 2 DM evaluated the effects of dapagliflozin on kidney function. The investigators found that compared with nondiabetic controls, diabetic individuals had elevated 5
De Nicola et al
glucose maximal transport levels (maximum renal glucose reabsorptive capacity), splay (the curvilinear relationship between plasma glucose level and urinary glucose excretion rate), and threshold (plasma glucose concentration at which glucose first appears in urine that corresponds to the beginning of the splay). One week of dapagliflozin treatment reduced the maximal transport level and splay in diabetic participants and controls. However, the most significant effect was the reduction of the threshold for glucose excretion in both groups. Tubular effects of dapagliflozin were associated with a 14% decrease in GFR measured by the gold standard of plasma clearance of iohexol. Similarly, a recent small RCT by Lambers Heerspink et al67 demonstrated that in patients with type 2 DM with normal kidney function, administration of dapagliflozin for 12 weeks decreased iohexol-measured GFR by 11%, an effect partially dependent on a moderate reduction in 24-hour systolic BP and circulating volume. These findings provide evidence in humans of the complex interaction between the proximal tubule and GFR underlying the tubular hypothesis of hyperfiltration in diabetes. Interestingly, in the latter study, GFR was unchanged when estimated by the MDRD (Modification of Diet in Renal Disease) Study equation, thus supporting the observation of a recent reanalysis of RCTs that GFR-estimating formulas are of questionable valuable when used to identify and follow up hyperfiltering patients with DM.68
Several large RCTs, most of which were included in 2 recent meta-analyses,69,70 have been conducted to evaluate glycemic control with SGLT2 inhibitors in large cohorts of patients with DM. Results have consistently shown that these agents, either as monotherapy or add-on treatment, induce a reduction in hemoglobin A1c levels of 0.6%-0.7% associated with a decrease in body weight (ranging from 1.0-3.0 kg) due to osmotic diuresis (urinary volume increased by 200-600 mL/d) and loss of calories related to glycosuria (50-80 g/d). Not trivial are the secondary findings that SGLT2 inhibitors decrease systolic/ diastolic BP (by w4/2 mm Hg) and that these agents allow for lower doses of insulin, thus possibly reducing the CV risk associated with increased body weight, hypoglycemia, water retention, and congestive heart failure. The major side effect of these agents is the modest and expected increase in risk of urogenital infections. Unfortunately, these large trials do not offer insights into the potential nephroprotective effect of these agents due to the short-term follow-up. To our knowledge, there are only 4 trials evaluating kidney function, all by means of estimated GFR (eGFR) only, after treatment with SGLT2 inhibitors in large cohorts of patients with DM.71-74 In 3 of them, a slight decrease in eGFR was observed (Table 1). Interestingly, in the largest one,73 a slight decrease in urinary albumin-creatinine ratio also was observed by the end
Table 1. Large Randomized Clinical Trials on SGLT2 Inhibitors in Type 2 Diabetes Evaluating Kidney Function
Study
N
Active Arm
Control Arm
Follow-up (wk)
Kidney Function
List et al71 (2009)
389
Dapaglifozin (2.5-50 mg/d)
Placebo or metformin
12
eGFR unchanged
Wilding et al72 (2012)
808
Dapagliflozin (2.5, 5, 10 mg/d)
Placebo
48
eGFR decreased by 2.0, 2.0, and 1.1 mL/min/1.73 m2 in 2.5-, 5-, and 10-mg groups vs 0.7 in placebo
Canagliflozin (100, 300 mg/d)
Glimepiride
52
At wk 4, greater eGFR decrease in active arm (23.0 and 25.6 mL/min/1.73 m2 in 100- and 300-mg groups) than in control group (21.3 mL/min/1.3 m2); from wk 12 to 52, eGFR stable in canagliflozin groups while progressively decreased with glimepiride
Dapagliflozin (5, 10 mg/d)
Placebo
24-104
At wk 24, greater eGFR decrease in active arm (22.4 and 24.8 mL/min/ 1.73 m2 in 5- and 10-mg groups) than in placebo (20.3 mL/min/1.73 m2); from wk 24 to 104, eGFR stable in active arm but progressively decreased in placebo group (vs baseline, 21.7, 23.5, 22.4 mL/min/ 1.73 m2 in 5-mg dapagliflozin, 10-mg dapagliflozen, and placebo groups)
Cefalu et al73 (2013)
1,450
Kohan et al74 (2013)
252
Abbreviations: eGFR, estimated glomerular filtration rate; SGLT2, sodium/glucose cotransporter 2. 6
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Proximal Tubule in Diabetes Mellitus
of the 1-year follow-up in the 100- and 300-mg canagliflozin groups (20.1 6 4.7 and 20.9 6 6.7 g/mol), whereas albuminuria increased in the control group (10.7 6 15.3 g/mol). This finding has been confirmed in the most recent trial, which was carried out in 252 patients with inadequately controlled type 2 DM and moderately decreased kidney function (basal eGFR, 45.6 6 10.0 mL/min/1.73 m2).74 In this study, patients receiving dapagliflozin were more likely to regress to a lower albumin excretion category than patients receiving placebo. Results for albuminuria and eGFR support the potential nephroprotective effect of SGLT2 inhibition.
CONCLUSIONS During the last decade, there has been a relentless increase in the number of diabetic individuals with CKD reaching chronic kidney failure. More burdensome is the observation that the prognosis has not changed significantly despite implementation of new drugs to treat diabetes and associated nephropathy. Two questions therefore arise: Should we explore new avenues? Should we move to intensify preventive strategies at earlier stages rather than therapy of overt disease? The tubular hypothesis of diabetic hyperfiltration and characterization of the early kidney alterations in diabetes are not novel; however, relatively few studies have verified in humans the information provided by a large set of animal studies. At this time, it is essential to persevere in improving our knowledge of the pathophysiologic role of the proximal tubule in humans. From this perspective, the recent introduction of SGLT2 inhibitors appears as an exciting opportunity. In particular, from the nephrologist’s point of view, we need studies with these new agents that specifically address: (1) kidney function over shortand long-term treatment (changes in measured GFR and albuminuria and incidence of end-stage renal disease), (2) the prognostic value of lower BP (potentially contributing to better renal and CV outcomes), and (3) the impact on kidney function in patients at risk of renal hypoperfusion (severe proteinuria and dual RAS blockade).
ACKNOWLEDGEMENTS Support: None. Financial Disclosure: The authors declare that they have no relevant financial interests.
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