commentary
5.
6.
7.
Fell LH, Zawada AM, Rogacev KS et al. Distinct immunologic effects of different intravenous iron preparations on monocytes. Nephrol Dial Transplant 2014; 29: 809–822. Weiss G, Schett G. Anaemia in inflammatory rheumatic diseases. Nat Rev Rheumatol 2013; 9: 205–215. Ganz T, Nemeth E. Hepcidin and iron homeostasis. Biochim Biophys Acta 2012; 1823: 1434–1443.
8.
9.
Theurl M, Nairz M, Schroll A et al. Hepcidin as a predictive factor and therapeutic target in erythropoiesis-stimulating agent treatment for anemia of chronic disease in rats. Haematologica 2014; 99: 1516–1524. Nairz M, Schroll A, Moschen AR et al. Erythropoietin contrastingly affects bacterial infection and experimental colitis by inhibiting nuclear factor-kappaB-inducible immune pathways. Immunity 2011; 34: 61–74.
see basic research on page 74
Revisiting inflammation in diabetic nephropathy: the role of the Nlrp3 inflammasome in glomerular resident cells Norihiko Sakai1 and Takashi Wada2 Inflammation is a protective response to remove dangerous stimuli from the body. The multiprotein complex called the inflammasome regulates inflammation through the cleavage of proinflammatory cytokines into mature forms. Shahzad et al. unravel the importance of the Nlrp3 inflammasome, especially activated in glomerular resident cells, in diabetic nephropathy. They also show the role of mitochondrial reactive oxygen species as a mediator to activate the inflammasome in diabetic conditions. Targeting the inflammasome could be a promising approach for diabetic nephropathy. Kidney International (2015) 87, 12–14. doi:10.1038/ki.2014.322
Diabetic nephropathy (DN) is the leading cause of end-stage renal diseases; however, the precise mechanisms by which it develops remain to be determined. Previous studies reported that renal accumulation of inflammatory cells is observed in the progressive course of DN, and the intensity of this accumulation is closely associated with the deterioration of renal function.1 The inhibition of inflammatory-cell recruitment into the kidney has also been 1 Division of Blood Purification, Kanazawa University Hospital, Kanazawa, Japan and 2 Department of Laboratory Medicine, Institute of Medical, Pharmaceutical and Health Sciences, Faculty of Medicine, Kanazawa University, Kanazawa, Japan Correspondence: Takashi Wada, Department of Laboratory Medicine, 13-1 Takara-machi, Kanazawa, 920-8641, Japan. E-mail: [email protected]
12
shown to be protective in experimental DN models.1 Therefore, inflammation has been thought to play a central role in the pathogenesis of DN. Recent studies have demonstrated that inflammation by the aberrant activation of the innate immune system significantly contributes to metabolic disorders such as insulin resistance in type 2 diabetes patients, in which myeloid-lineage innate immune cells drive a proinflammatory state.2 The innate immune system is known to be activated to produce the proinflammatory cytokines interleukin (IL)-1b and IL-18 by a series of germ-lineencoded pattern recognition receptors (PRRs) that recognize conserved pathogen-associated molecular patterns (PAMPs) on invading organisms or damage-associated molecular patterns (DAMPs) released from damaged cells.3
Currently, at least four different PRR families have been identified: Toll-like receptors (TLRs), C-type lectin receptors (CLRs), retinoic acid-inducible gene (RIG)-1 receptors (RLRs), and nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs). TLRs and CLRs are transmembrane receptors and bind to extracellular PAMPs and DAMPs, whereas RLRs and NLRs are cytoplasmic proteins activated by intracellular molecular patterns. Cytosolic NLRs assemble with apoptosisassociated speck-like protein containing a caspase recruitment domain (ASC) adaptor protein and pro-caspase-1 to form the large multiprotein complex called the inflammasome, leading to autocatalytic activation of caspase-1. Several NLRs, including Nlrp1 (NLR family, pyrin domain containing 1), Nlrp3, Nlrp6, Nlrp12, and Nlrc4 (NLR family, caspase recruitment domain containing 4), form an inflammasome complex resulting in the cleavage of proIL-1b and pro-IL-18 into mature forms of IL-1b and IL-18 (Figure 1).3,4 Serum and urinary levels of IL-18 have been reported to be higher in patients with DN than in control subjects. Serum and urinary levels of IL-18 also showed significant positive correlations with urinary albumin excretion rate in DN patients.5 In addition, polymorphism in IL-1b and IL-1b receptor antagonist genes is associated with renal outcome in patients with type 2 diabetes.6 These findings suggest that the inflammasome contributes to the pathogenesis of DN through the production of the proinflammatory cytokines IL-1b and IL-18. Shahzad et al.7 (this issue) now clarify the importance of the Nlrp3 inflammasome in the pathogenesis of DN. The authors show that the levels of circulating IL-1b and IL-18 as well as renal expression of Nlrp3 increased in mouse DN models, and those increases preceded albuminuria and glomerular extracellular matrix accumulation, suggesting that inflammasome activation triggers the onset of DN. Importantly, the inactivation of the Nlrp3 inflammasome by deficiency of Nlrp3 or caspase-1 protected diabetic mice from albuminuria and mesangial Kidney International (2015) 87
commentary
Glucose
AGE
RAGE
Stimuli (diabetic condition?) TLRs
? Interaction
GLUT?
NF-κB shc
Pro-IL-1β
Nlrp3
p66
Pro-IL-18 NUCLEUS ETC Mitochondria
ROS Inflammasome
Nlrp3 ASC Pro-caspase-1 Caspase-1
Glomerular cells (podocytes, endothelial cells)
Pro-IL-1β Pro-IL-18
Cleavage IL-1β IL-18
Diabetic nephropathy
Figure 1 | The involvement of glomerular Nlrp3 inflammasome in diabetic conditions through mitochondrial reactive oxygen species. In diabetic glomerular resident cells, glucose and advanced glycation endproducts (AGEs) enhance Nlrp3 expression to interact and assemble with apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC) and pro-caspase-1, leading to autocatalytic activation of caspase-1 through mitochondrial reactive oxygen species (ROS) dependent on p66shc. In addition, the production of pro-interleukin-1b (pro-IL-1b) and pro-IL-18 could be regulated by nuclear factor-kB transcription factor in response to activation of various pattern recognition receptors such as Toll-like receptors (TLRs). Finally, caspase-1 cleaves pro-IL-1b and pro-IL-18 to form mature forms of IL-1b and IL-18 to drive inflammation leading to glomerular damage. ETC, electron transport chain; GLUT, glucose transporter; Nlrp3, NOD-like receptor family, pyrin domain containing 1; RAGE; receptor for AGE.
expansion without affecting the body weight or blood glucose levels. Furthermore, pharmacological inhibition of IL-1 receptor signaling showed the reduction of albuminuria and mesangial expansion in diabetic mice. These data strongly support the concept that the Nlrp3 inflammasome has a fundamental role in the development of DN. As described above, myeloid-lineage innate immune cells produce proinflammatory cytokines in diabetic conditions.2 Not only innate immune cells but also nonimmune cells such as intrinsic renal cells (podocytes, endothelial cells, epithelial cells, and mesangial cells) are capable of secreting cytokines.8 Likewise, the Nlrp3 inflammasome is expressed primarily by innate immune cells, whereas a recent study has shown that podocytes also express the Nlrp3 inflammasome, resulting in caspase-1 activation in response to L-homocysteine.8 However, it has not been well clarified how myeloid-lineage immune cells and intrinsic renal cells contribute to the development of DN through the Nlrp3 inflammasome. To address the pathogenic relevance of inflammaKidney International (2015) 87
some activation in myeloid-lineage cells, Shahzad et al.7 performed bone marrow transplantation experiments using Nlrp3 / or caspase-1 / bone marrow and db/db mice. Albuminuria and mesangial expansion in db/db mice transplanted with Nlrp3 / or caspase-1 / bone marrow increased to the same extent as those in control db/db mice, suggesting that the Nlrp3 inflammasome in myeloid-lineage immune cells has less impact on the development of DN. Next, the authors evaluated the importance of the Nlrp3 inflammasome in renal resident cells by transplantation of wild-type bone marrow into Nlrp3 / mice in a streptozotocininduced DN model. Nlrp3 deficiency in the kidney protected mice from DN despite transplantation of wild-type bone marrow, indicating that the Nlrp3 inflammasome in renal resident cells significantly contributes to the pathogenesis of DN. However, renal infiltration of immune cells such as macrophages increases in accordance with the progression of DN, and the inflammasome of these cells might be activated during the course of DN.1 Therefore, this study
cannot exclude the hypothesis that inflammasome activation in immune cells actively recruited from the bone marrow into the kidney contributes to disease progression, especially at later stages of DN. The next question is the cell source of the Nlrp inflammasome and proinflammatory cytokines in diabetic mice. This study demonstrated partial colocalization of Nlrp3 or cleaved caspase-1 with podocytes and glomerular endothelial cells in diabetic mice and DN patients, indicating that the Nlrp3 inflammasome in glomerular resident cells greatly contributes to DN. In terms of renal function, tubulointerstitial lesions such as interstitial fibrosis and tubular atrophy have been recognized as a critical determinant of renal prognosis regardless of etiology. To date, tubular epithelial cells have also been reported to express a high amount of Nlrp3 inflammasome, and the Nlrp3 inflammasome/IL-1b/IL-18 axis is involved in the formation of tubulointerstitial lesions.4 The role of the Nlrp3 inflammasome in the progression of tubulointerstitial lesions in diabetic mice remains to be determined; however, further studies using cell-specific deletion of the Nlrp3 inflammasome will be required to address the role of inflammasome activation in glomerular and tubular epithelial cells. To elucidate the mechanism(s) by which glomerular Nlrp3 inflammasome is activated is important in order to discover new molecular targets to treat DN. The authors showed that glucose upregulated the expression of Nlrp3 and cleaved IL-1b in glomerular endothelial cells and podocytes, and glucose-induced production of cleaved IL-1b was abolished by caspase-1 deficiency, indicating that glucose is capable of controlling Nlrp3 inflammasome/caspase-1 axis activation in glomerular resident cells. Next, the authors focused on mitochondrial reactive oxygen species (ROS) as a mediator of glucose-induced Nlrp3 inflammasome/ caspase-1 axis activation, since enhanced glycolytic flux is sufficient to increase mitochondrial ROS at the mitochondrial electron transport chain 13
commentary
located on the inner mitochondrial membrane. Recent studies have revealed that mitochondrial ROS can activate the Nlrp3 inflammasome.9 This study demonstrates that the mitochondria-targeted antioxidant MitoTempo, a superoxide dismutase mimetic, reduced mitochondrial ROS and the expression of Nlrp3 and cleaved IL-1b in glucose-stimulated podocytes. In addition to glucose, glucose-modified proteins such as advanced glycation endproducts (AGEs) are known to induce mitochondrial ROS by binding to receptor for AGEs (RAGE).10 The authors show that the AGE–RAGE system in podocytes also enhanced the expression of Nlrp3 and cleaved IL-1b. Furthermore, in vivo treatment of diabetic mice with MitoTempo or the deficiency of p66shc inhibited renal expression of Nlrp3 and cleaved IL-1b as well as the extent of albuminuria and mesangial expansion. Taken together, these results show that mitochondrial ROS in glomeruli are involved in the activation of the Nlrp3 inflammasome in diabetic conditions and could be a therapeutic target for combating the development of DN. However, the precise molecular interactions of how diabetic conditions modulate Nlrp3 inflammasome activation through mitochondrial ROS are still unclear; therefore, further investigation will be needed to address it. Despite obvious contribution by the Nlrp3 inflammasome to the pathogenesis of DN as shown in this study, there seem to be several issues to be clarified. First, PRRs consist of various molecules, including Nlrp3, and may have cross-talk among them; PRRs are activated simultaneously in response to inflammatory stimuli. The expression of pro-IL-1b and IL-18 is regulated by nuclear factor-kB, which is controlled by various PRRs such as TLRs; therefore, the cooperation of Nlrp3 and other PRRs could be critical for the secretion of IL-1b and IL-18 (Figure 1). To clarify it, comprehensive analyses of PRRs could be important. Second, it is not clear how mature forms of IL-1b and IL-18 are working during the course of DN. Third, mechanistic study to show the role of the Nlrp3 inflam14
masome in glomerular endothelial cells is lacking, as these cells have been known to be involved in the induction of albuminuria. More detailed study to address these issues would provide better understanding of the meanings of the Nlrp3 inflammasome in DN. Inflammation associated with inflammasome activation is a two-faced biological event, leading to cure or damage. Therefore, the appropriate regulation of the inflammasome is needed to apply to patients with inflammatory disorders. Further investigations to clarify the precise mechanisms of inflammasome activation dependent on etiology would open novel therapeutic strategies combating inflammatory diseases including DN.
2.
3. 4.
5.
6.
7.
8.
DISCLOSURE
The authors declared no competing interests. REFERENCES 1.
Awad AS, Kinsey GR, Khutsishvili K et al. Monocyte/macrophage chemokine receptor CCR2 mediates diabetic renal injury. Am J Physiol Renal Physiol 2011; 301: F1358–F1366.
9.
10.
Chawla A, Nguyen KD, Goh YP. Macrophagemediated inflammation in metabolic disease. Nat Rev Immunol 2011; 11: 738–749. Takeuchi O, Akira S. Pattern recognition receptors and inflammation. Cell 2010; 140: 805–820. Turner CM, Arulkumaran N, Singer M et al. Is the inflammasome a potential therapeutic target in renal disease? BMC Nephrol [online] 2014; 15: 21. Nakamura A, Shikata K, Hiramatsu M et al. Serum interleukin-18 levels are associated with nephropathy and atherosclerosis in Japanese patients with type 2 diabetes. Diabetes Care 2005; 28: 2890–2895. Lee SH, Ihm CG, Sohn SD et al. Polymorphisms in interleukin-1 beta and interleukin-1 receptor antagonist genes are associated with kidney failure in Korean patients with type 2 diabetes mellitus. Am J Nephrol 2004; 24: 410–414. Shahzad K, Bock F, Dong W et al. Nlrp3inflammasome activation in non-myeloidderived cells aggravates diabetic nephropathy. Kidney Int 2015; 87: 74–84. Zhang C, Boini KM, Xia M et al. Activation of Nod-like receptor protein 3 inflammasomes turns on podocyte injury and glomerular sclerosis in hyperhomocysteinemia. Hypertension 2012; 60: 154–162. Zhou R, Yazdi AS, Menu P, Tschopp J. A role for mitochondria in NLRP3 inflammasome activation. Nature 2011; 469: 221–225. Coughlan MT, Thorburn DR, Penfold SA et al. RAGE-induced cytosolic ROS promote mitochondrial superoxide generation in diabetes. J Am Soc Nephrol 2009; 20: 742–752.
see clinical investigation on pages 169 and 176
Apolipoprotein L1: from obscurity to consistency to controversy Michael S. Lipkowitz1 Apolipoprotein L1 (APOL1) is associated with increased incidence of chronic kidney disease (CKD) and end-stage renal disease, and with faster progression of CKD, in African Americans. APOL1 is expressed in intra- and extrarenal vascular tissue, making it a candidate to explain the increased incidence of cardiovascular disease in CKD. This Commentary discusses the disparate results from three studies showing that APOL1 renal risk genotypes are either harmful, neutral, or protective in the context of cardiovascular disease. Kidney International (2015) 87, 14–17. doi:10.1038/ki.2014.319 1 Nephrology and Hypertension Division, Georgetown University Medical Center, Washington, DC, USA Correspondence: Michael S. Lipkowitz, Nephrology and Hypertension Division, Georgetown University Medical Center, 3800 Reservoir Road NW, Washington, DC 20007, USA. E-mail: [email protected]
African Americans have a well-documented, severalfold greater risk of developing kidney disease than the Caucasian population, even after controlling for clinical and sociodemographic factors.1 This excess risk Kidney International (2015) 87
5.
6.
7.
Fell LH, Zawada AM, Rogacev KS et al. Distinct immunologic effects of different intravenous iron preparations on monocytes. Nephrol Dial Transplant 2014; 29: 809–822. Weiss G, Schett G. Anaemia in inflammatory rheumatic diseases. Nat Rev Rheumatol 2013; 9: 205–215. Ganz T, Nemeth E. Hepcidin and iron homeostasis. Biochim Biophys Acta 2012; 1823: 1434–1443.
8.
9.
Theurl M, Nairz M, Schroll A et al. Hepcidin as a predictive factor and therapeutic target in erythropoiesis-stimulating agent treatment for anemia of chronic disease in rats. Haematologica 2014; 99: 1516–1524. Nairz M, Schroll A, Moschen AR et al. Erythropoietin contrastingly affects bacterial infection and experimental colitis by inhibiting nuclear factor-kappaB-inducible immune pathways. Immunity 2011; 34: 61–74.
see basic research on page 74
Revisiting inflammation in diabetic nephropathy: the role of the Nlrp3 inflammasome in glomerular resident cells Norihiko Sakai1 and Takashi Wada2 Inflammation is a protective response to remove dangerous stimuli from the body. The multiprotein complex called the inflammasome regulates inflammation through the cleavage of proinflammatory cytokines into mature forms. Shahzad et al. unravel the importance of the Nlrp3 inflammasome, especially activated in glomerular resident cells, in diabetic nephropathy. They also show the role of mitochondrial reactive oxygen species as a mediator to activate the inflammasome in diabetic conditions. Targeting the inflammasome could be a promising approach for diabetic nephropathy. Kidney International (2015) 87, 12–14. doi:10.1038/ki.2014.322
Diabetic nephropathy (DN) is the leading cause of end-stage renal diseases; however, the precise mechanisms by which it develops remain to be determined. Previous studies reported that renal accumulation of inflammatory cells is observed in the progressive course of DN, and the intensity of this accumulation is closely associated with the deterioration of renal function.1 The inhibition of inflammatory-cell recruitment into the kidney has also been 1 Division of Blood Purification, Kanazawa University Hospital, Kanazawa, Japan and 2 Department of Laboratory Medicine, Institute of Medical, Pharmaceutical and Health Sciences, Faculty of Medicine, Kanazawa University, Kanazawa, Japan Correspondence: Takashi Wada, Department of Laboratory Medicine, 13-1 Takara-machi, Kanazawa, 920-8641, Japan. E-mail: [email protected]
12
shown to be protective in experimental DN models.1 Therefore, inflammation has been thought to play a central role in the pathogenesis of DN. Recent studies have demonstrated that inflammation by the aberrant activation of the innate immune system significantly contributes to metabolic disorders such as insulin resistance in type 2 diabetes patients, in which myeloid-lineage innate immune cells drive a proinflammatory state.2 The innate immune system is known to be activated to produce the proinflammatory cytokines interleukin (IL)-1b and IL-18 by a series of germ-lineencoded pattern recognition receptors (PRRs) that recognize conserved pathogen-associated molecular patterns (PAMPs) on invading organisms or damage-associated molecular patterns (DAMPs) released from damaged cells.3
Currently, at least four different PRR families have been identified: Toll-like receptors (TLRs), C-type lectin receptors (CLRs), retinoic acid-inducible gene (RIG)-1 receptors (RLRs), and nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs). TLRs and CLRs are transmembrane receptors and bind to extracellular PAMPs and DAMPs, whereas RLRs and NLRs are cytoplasmic proteins activated by intracellular molecular patterns. Cytosolic NLRs assemble with apoptosisassociated speck-like protein containing a caspase recruitment domain (ASC) adaptor protein and pro-caspase-1 to form the large multiprotein complex called the inflammasome, leading to autocatalytic activation of caspase-1. Several NLRs, including Nlrp1 (NLR family, pyrin domain containing 1), Nlrp3, Nlrp6, Nlrp12, and Nlrc4 (NLR family, caspase recruitment domain containing 4), form an inflammasome complex resulting in the cleavage of proIL-1b and pro-IL-18 into mature forms of IL-1b and IL-18 (Figure 1).3,4 Serum and urinary levels of IL-18 have been reported to be higher in patients with DN than in control subjects. Serum and urinary levels of IL-18 also showed significant positive correlations with urinary albumin excretion rate in DN patients.5 In addition, polymorphism in IL-1b and IL-1b receptor antagonist genes is associated with renal outcome in patients with type 2 diabetes.6 These findings suggest that the inflammasome contributes to the pathogenesis of DN through the production of the proinflammatory cytokines IL-1b and IL-18. Shahzad et al.7 (this issue) now clarify the importance of the Nlrp3 inflammasome in the pathogenesis of DN. The authors show that the levels of circulating IL-1b and IL-18 as well as renal expression of Nlrp3 increased in mouse DN models, and those increases preceded albuminuria and glomerular extracellular matrix accumulation, suggesting that inflammasome activation triggers the onset of DN. Importantly, the inactivation of the Nlrp3 inflammasome by deficiency of Nlrp3 or caspase-1 protected diabetic mice from albuminuria and mesangial Kidney International (2015) 87
commentary
Glucose
AGE
RAGE
Stimuli (diabetic condition?) TLRs
? Interaction
GLUT?
NF-κB shc
Pro-IL-1β
Nlrp3
p66
Pro-IL-18 NUCLEUS ETC Mitochondria
ROS Inflammasome
Nlrp3 ASC Pro-caspase-1 Caspase-1
Glomerular cells (podocytes, endothelial cells)
Pro-IL-1β Pro-IL-18
Cleavage IL-1β IL-18
Diabetic nephropathy
Figure 1 | The involvement of glomerular Nlrp3 inflammasome in diabetic conditions through mitochondrial reactive oxygen species. In diabetic glomerular resident cells, glucose and advanced glycation endproducts (AGEs) enhance Nlrp3 expression to interact and assemble with apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC) and pro-caspase-1, leading to autocatalytic activation of caspase-1 through mitochondrial reactive oxygen species (ROS) dependent on p66shc. In addition, the production of pro-interleukin-1b (pro-IL-1b) and pro-IL-18 could be regulated by nuclear factor-kB transcription factor in response to activation of various pattern recognition receptors such as Toll-like receptors (TLRs). Finally, caspase-1 cleaves pro-IL-1b and pro-IL-18 to form mature forms of IL-1b and IL-18 to drive inflammation leading to glomerular damage. ETC, electron transport chain; GLUT, glucose transporter; Nlrp3, NOD-like receptor family, pyrin domain containing 1; RAGE; receptor for AGE.
expansion without affecting the body weight or blood glucose levels. Furthermore, pharmacological inhibition of IL-1 receptor signaling showed the reduction of albuminuria and mesangial expansion in diabetic mice. These data strongly support the concept that the Nlrp3 inflammasome has a fundamental role in the development of DN. As described above, myeloid-lineage innate immune cells produce proinflammatory cytokines in diabetic conditions.2 Not only innate immune cells but also nonimmune cells such as intrinsic renal cells (podocytes, endothelial cells, epithelial cells, and mesangial cells) are capable of secreting cytokines.8 Likewise, the Nlrp3 inflammasome is expressed primarily by innate immune cells, whereas a recent study has shown that podocytes also express the Nlrp3 inflammasome, resulting in caspase-1 activation in response to L-homocysteine.8 However, it has not been well clarified how myeloid-lineage immune cells and intrinsic renal cells contribute to the development of DN through the Nlrp3 inflammasome. To address the pathogenic relevance of inflammaKidney International (2015) 87
some activation in myeloid-lineage cells, Shahzad et al.7 performed bone marrow transplantation experiments using Nlrp3 / or caspase-1 / bone marrow and db/db mice. Albuminuria and mesangial expansion in db/db mice transplanted with Nlrp3 / or caspase-1 / bone marrow increased to the same extent as those in control db/db mice, suggesting that the Nlrp3 inflammasome in myeloid-lineage immune cells has less impact on the development of DN. Next, the authors evaluated the importance of the Nlrp3 inflammasome in renal resident cells by transplantation of wild-type bone marrow into Nlrp3 / mice in a streptozotocininduced DN model. Nlrp3 deficiency in the kidney protected mice from DN despite transplantation of wild-type bone marrow, indicating that the Nlrp3 inflammasome in renal resident cells significantly contributes to the pathogenesis of DN. However, renal infiltration of immune cells such as macrophages increases in accordance with the progression of DN, and the inflammasome of these cells might be activated during the course of DN.1 Therefore, this study
cannot exclude the hypothesis that inflammasome activation in immune cells actively recruited from the bone marrow into the kidney contributes to disease progression, especially at later stages of DN. The next question is the cell source of the Nlrp inflammasome and proinflammatory cytokines in diabetic mice. This study demonstrated partial colocalization of Nlrp3 or cleaved caspase-1 with podocytes and glomerular endothelial cells in diabetic mice and DN patients, indicating that the Nlrp3 inflammasome in glomerular resident cells greatly contributes to DN. In terms of renal function, tubulointerstitial lesions such as interstitial fibrosis and tubular atrophy have been recognized as a critical determinant of renal prognosis regardless of etiology. To date, tubular epithelial cells have also been reported to express a high amount of Nlrp3 inflammasome, and the Nlrp3 inflammasome/IL-1b/IL-18 axis is involved in the formation of tubulointerstitial lesions.4 The role of the Nlrp3 inflammasome in the progression of tubulointerstitial lesions in diabetic mice remains to be determined; however, further studies using cell-specific deletion of the Nlrp3 inflammasome will be required to address the role of inflammasome activation in glomerular and tubular epithelial cells. To elucidate the mechanism(s) by which glomerular Nlrp3 inflammasome is activated is important in order to discover new molecular targets to treat DN. The authors showed that glucose upregulated the expression of Nlrp3 and cleaved IL-1b in glomerular endothelial cells and podocytes, and glucose-induced production of cleaved IL-1b was abolished by caspase-1 deficiency, indicating that glucose is capable of controlling Nlrp3 inflammasome/caspase-1 axis activation in glomerular resident cells. Next, the authors focused on mitochondrial reactive oxygen species (ROS) as a mediator of glucose-induced Nlrp3 inflammasome/ caspase-1 axis activation, since enhanced glycolytic flux is sufficient to increase mitochondrial ROS at the mitochondrial electron transport chain 13
commentary
located on the inner mitochondrial membrane. Recent studies have revealed that mitochondrial ROS can activate the Nlrp3 inflammasome.9 This study demonstrates that the mitochondria-targeted antioxidant MitoTempo, a superoxide dismutase mimetic, reduced mitochondrial ROS and the expression of Nlrp3 and cleaved IL-1b in glucose-stimulated podocytes. In addition to glucose, glucose-modified proteins such as advanced glycation endproducts (AGEs) are known to induce mitochondrial ROS by binding to receptor for AGEs (RAGE).10 The authors show that the AGE–RAGE system in podocytes also enhanced the expression of Nlrp3 and cleaved IL-1b. Furthermore, in vivo treatment of diabetic mice with MitoTempo or the deficiency of p66shc inhibited renal expression of Nlrp3 and cleaved IL-1b as well as the extent of albuminuria and mesangial expansion. Taken together, these results show that mitochondrial ROS in glomeruli are involved in the activation of the Nlrp3 inflammasome in diabetic conditions and could be a therapeutic target for combating the development of DN. However, the precise molecular interactions of how diabetic conditions modulate Nlrp3 inflammasome activation through mitochondrial ROS are still unclear; therefore, further investigation will be needed to address it. Despite obvious contribution by the Nlrp3 inflammasome to the pathogenesis of DN as shown in this study, there seem to be several issues to be clarified. First, PRRs consist of various molecules, including Nlrp3, and may have cross-talk among them; PRRs are activated simultaneously in response to inflammatory stimuli. The expression of pro-IL-1b and IL-18 is regulated by nuclear factor-kB, which is controlled by various PRRs such as TLRs; therefore, the cooperation of Nlrp3 and other PRRs could be critical for the secretion of IL-1b and IL-18 (Figure 1). To clarify it, comprehensive analyses of PRRs could be important. Second, it is not clear how mature forms of IL-1b and IL-18 are working during the course of DN. Third, mechanistic study to show the role of the Nlrp3 inflam14
masome in glomerular endothelial cells is lacking, as these cells have been known to be involved in the induction of albuminuria. More detailed study to address these issues would provide better understanding of the meanings of the Nlrp3 inflammasome in DN. Inflammation associated with inflammasome activation is a two-faced biological event, leading to cure or damage. Therefore, the appropriate regulation of the inflammasome is needed to apply to patients with inflammatory disorders. Further investigations to clarify the precise mechanisms of inflammasome activation dependent on etiology would open novel therapeutic strategies combating inflammatory diseases including DN.
2.
3. 4.
5.
6.
7.
8.
DISCLOSURE
The authors declared no competing interests. REFERENCES 1.
Awad AS, Kinsey GR, Khutsishvili K et al. Monocyte/macrophage chemokine receptor CCR2 mediates diabetic renal injury. Am J Physiol Renal Physiol 2011; 301: F1358–F1366.
9.
10.
Chawla A, Nguyen KD, Goh YP. Macrophagemediated inflammation in metabolic disease. Nat Rev Immunol 2011; 11: 738–749. Takeuchi O, Akira S. Pattern recognition receptors and inflammation. Cell 2010; 140: 805–820. Turner CM, Arulkumaran N, Singer M et al. Is the inflammasome a potential therapeutic target in renal disease? BMC Nephrol [online] 2014; 15: 21. Nakamura A, Shikata K, Hiramatsu M et al. Serum interleukin-18 levels are associated with nephropathy and atherosclerosis in Japanese patients with type 2 diabetes. Diabetes Care 2005; 28: 2890–2895. Lee SH, Ihm CG, Sohn SD et al. Polymorphisms in interleukin-1 beta and interleukin-1 receptor antagonist genes are associated with kidney failure in Korean patients with type 2 diabetes mellitus. Am J Nephrol 2004; 24: 410–414. Shahzad K, Bock F, Dong W et al. Nlrp3inflammasome activation in non-myeloidderived cells aggravates diabetic nephropathy. Kidney Int 2015; 87: 74–84. Zhang C, Boini KM, Xia M et al. Activation of Nod-like receptor protein 3 inflammasomes turns on podocyte injury and glomerular sclerosis in hyperhomocysteinemia. Hypertension 2012; 60: 154–162. Zhou R, Yazdi AS, Menu P, Tschopp J. A role for mitochondria in NLRP3 inflammasome activation. Nature 2011; 469: 221–225. Coughlan MT, Thorburn DR, Penfold SA et al. RAGE-induced cytosolic ROS promote mitochondrial superoxide generation in diabetes. J Am Soc Nephrol 2009; 20: 742–752.
see clinical investigation on pages 169 and 176
Apolipoprotein L1: from obscurity to consistency to controversy Michael S. Lipkowitz1 Apolipoprotein L1 (APOL1) is associated with increased incidence of chronic kidney disease (CKD) and end-stage renal disease, and with faster progression of CKD, in African Americans. APOL1 is expressed in intra- and extrarenal vascular tissue, making it a candidate to explain the increased incidence of cardiovascular disease in CKD. This Commentary discusses the disparate results from three studies showing that APOL1 renal risk genotypes are either harmful, neutral, or protective in the context of cardiovascular disease. Kidney International (2015) 87, 14–17. doi:10.1038/ki.2014.319 1 Nephrology and Hypertension Division, Georgetown University Medical Center, Washington, DC, USA Correspondence: Michael S. Lipkowitz, Nephrology and Hypertension Division, Georgetown University Medical Center, 3800 Reservoir Road NW, Washington, DC 20007, USA. E-mail: [email protected]
African Americans have a well-documented, severalfold greater risk of developing kidney disease than the Caucasian population, even after controlling for clinical and sociodemographic factors.1 This excess risk Kidney International (2015) 87
