cise analysis and understanding of the pathogenetic mechanisms involved. 0 1. 0.Stutman, E.J. Yunis and R.A. Good: Deficient Immunologic Functions of NZB Mice. Proc. SOC. Exp. Biol. Med. 127: 1204-1207, 1968 2. R.C. Mellors: Wild-Type Gross Leukemia Virus and Heritable Autoimmune Disease of New Zealand Mice. Am. J. Clin. Path. 56: 270-279, 1971 3. D.R. Barthold, S. Kysela and A.D. Steinberg: Decline in Suppressor T Cell Function with Age in Female NZB Mice. J. Immunol. 112: 9-16, 1974 4. 0. Stutman: Lymphocyte Subpopulations in NZB Mice: Deficit of Thymus-Dependent Lymphocytes. J. Immunol. 109: 602-61 1, 1972 5. G. Fernandes, E.J. Yunis and R.A. Good: Influence of Protein Restriction on Immune Functions in NZB Mice. J . Immunol. 116: 782-790, 1976 6. N.S. Scrimshaw, C.E. Taylor and J.E. Gordon: Interactions of Nutrition and Infection. World Health Organization, Geneva, 1968 7. A. Tannenbaum: The Dependence of Tumor Formation on the Degree of Caloric Restriction. Cancer Res. 5609-615, 1945
8. M.H. Ross and G. Bras: Tumor Incidence Patterns and Nutrition in the Rat. J . Nutrition 87: 245-260, 1965 9. R.K. Chandra: Antibody Formation in First and Second Generation Offspring of Nutritionally Deprived Rats. Science 190: 289-290, 1975 10. B.M. Gebhardt and P.M. Newberne: Nutrition and Immunological Responsiveness. T-cell Function in the Offspring of Lipotrope and Protein-Deficient Rats. Immunology 26: 489-495, 1974 11. L.C. Robson and M.R. Schwarz: Vitamin Be Deficiency and the Lymphoid System. ll. Effects of Vitamin BS Deficiency In Utero on the Immunological Competence of the Offspring. Cell. Immunol. 16: 145-152, 1975 12. M. Mathur, V. Ramalingaswami and M.G. Deo: Influence of Protein Deficiency on 19s Antibody-Forming Cells in Rats and Mice. J. Nutrition 102: 841-846, 1972 13. D.J. Jose and R.A. Good: Quantitative Effects of Nutritional Essential Amino Acid Deficiency upon Immune Responses to Tumors in Mice. J . €xp. Med. 137: 1-9, 1973
EFFECTS OF DEXAMETHASONE ON GLUCOSE METABOLISM Adipocytes exposed to dexamethasone have a reduced ability to oxidize glucose due to inhibition of glucose transport. This effect is not mediated by a reduction in insulin binding capacity. Key Words: dexamethasone, insulin, glucose metabolism
Altered glucose metabolism on exposure to glucocorticoids has been demonstrated both in vivo and in vitro. In man prolonged glucocorticoid administration produces insulin resistance and an inability to handle a glucose load,’ while the oxidation of glucose is inhibited in isolated adipocytes and muscle tissue exposed to glucocorticoids. Since the inhibition of glucose oxidation in vitro can be overcome by insulin, it has been suggested that the transport of glucose is accomplished by two separate systems, one insulin sensitive and the other a basal system which is unresponsive to insulin. Of the two systems, it is proposed that 293
only the basal component is affected by glucocorticoids. More recent work by Czech and Fain,4 however, using a range of glucose and insulin concentrations, demonstrated that under some conditions dexamethasone could inhibit glucose metabolism in the presence of insulin. These workers proposed that dexamethasone exerted its effect only when the transport of glucose was rate limiting, that is when the action of insulin on glucose metabolism was at the level of the transport system. A possible mechanism of action for dexamethasone in these circumstances is suggested by the work of Olefsky and co-workers5 who demonstrated that the number of insdin receptors is decreased ‘in dexamethasonetreated hepatocytes and adipocytes. NUTRITION REVIEWS / VOL. 34, NO. 6 i JUNE 1976
185
A recent study by Olefsky6sought to examine intracellular action was confirmed by further further the nature of the glucose transport sysexperiments in which transport and oxidation tem and to elucidate the mechanism of action were measured in the presence and absence of dexamethasone. Furthermore, the relationof cytochalasin B. These showed that while ship between the interaction of insulin at the both basal and insulin-stimulated facilitated membrane and its intracellular action was intransport were inhibited by cytochalasin B, vestigated. The experiments were carried out insulin still retained the power to increase the with normal and dexamethasone-treated rat rate of oxidation of that glucose entering the adipocytes. cell by simple diffusion. Initial studies of glucose oxidation, as meaWhen transport was measured in the pressured by the conversion of (l-14C) glucose to ence of dexamethasone, a 30 to 40 percent 14C02showed that dexamethasone in the ab- inhibition was found, both in the absence of sence of insulin lowered oxidation by 30 perinsulin and at all levels of insulin stimulation. cent and that oxidation remained depressed at Expressed as a percent of the basal transport submaximal plasma insulin levels. When plasrate, however, the stimulatory effects of insulin ma insulin was high, however (more than were found to be the same in both groups of 100 pU per milliliter), control and dexamethacells. Since at higher substrate concentrations, sone-treated cells had identical abilities to oxithe dexamethasone-treated cells were capdize glucose. This result is compatible with able of taking up much larger amounts of subthe hypothesis that dexamethasone is only efstrate than that seen in these experiments, the fective where transport is rate limiting. Thus, lower uptake in the presence of dexamethaafter insulin concentration increases above a sone appeared not to be a result of a decreased certain level, the intracellular oxidation mechcapacity of the cells for substrate uptake. Studanism itself is saturated, so that any decrease ies of uptake over a range of substrate conin membrane transport cannot be reflected in centrations revealed that, like insulin, dexathe ability of the cell to oxidize glucose. methasone exerts its effect on the saturable In order to measure the transport process intransport component, leaving diffusion undependently of oxidation, the uptake of the nonchanged. Also, similar to the insulin effect was metabolizable sugar, 2-deoxyglucose was exthe observation that the K m of the cell remained amined. Preliminary studies were first carried the same as for control cells and only,,V, was out to characterize the transport of this particualtered, in this case being significantly delar sugar. Two components of uptake were creased. demonstrated: below a concentration of 5mM Another means of examining transport is 2-deoxyglucose, uptake was a saturable funcby studying the uptake of 3-O-methylglucose. tion of concentration, while at higher concenUnlike 2-deoxyglucose, this sugar is not phostrations, a nonsaturable component was seen, phorylated, so its concentration rapidly builds which increased with increasing 2-deoxygluup inside the cell, and an equilibrium position cose levels. As suggested by other w o r k e r ~ , ~ is reached between inward and outward movethese two components probably represent a ment. This occurs after approximately 45 seconds in the basal state and is shortened to saturable facilitated transport system, together with uptake by simple diffusion. This was supabout 20 seconds in the presence of insulin. ported by the inhibition of the saturable comSince the uptake is nonlinear, use of this sugar ponent alone with cytochalasin B. Insulin markfor transport studies is somewhat limited, but edly increased the rate of the facilitated transits uptake does give an indication solely of port, the effect being on the maximum velocity transport. Dexamethasone was found in these (Vmax) with the Michaelis constant (K,) reexperiments to decrease the uptake of 3-0maining unaltered. The diffusion component methylglucose in both control and insulinwas the same as for control cells. It is generstimulated cells. ally thought that insulin acts primarily at the Any direct effect of dexamethasone on the site of transport,8 but these experiments demoxidation mechanism seems unlikely, since onstrated a greater effect on oxidation. A major further experiments showed no difference in 186 NUTRITION R E V E W S
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I JUNE
1976
total hexokinase activity between control and dexamethasone-treated cells. This result, together with the 2-deoxyglucose and the 3-0methylglucose studies, strongly suggests that dexamethasone acts at the site of transport alone. Since insulin still retains the same stimulatory effect on transport, it seems that dexamethasone inhibits a fixed proportion of this system, and that the basal and insulin-stimulated components are part of the same system. As the actions of dexamethasone might be mediated via a decreased capacity of the cells to bind insulin, binding studies were carried out in presence and absence of the glucocorticoid. In contrast to observations made in vitro, where dexamethasone was found to decrease binding,5 these experiments revealed identical capacities for insulin binding of the two groups of cells. A possible explanation for these differing results is that in vivo, dexamethasone increases plasma insulin, which in turn decreases the number of insulin receptors. Such an effect of insulin on the receptor population has recently been reviewed.9 In these in vitro studies, for a given degree of insulin stimulation, dexamethasone-treated cells could transport less glucose than normal controls. Whatever the nature of the interaction, it seems unlikely that the relationship between insulin receptors and the transport system is affected. This follows from the effect of dexamethasone on the uptake rate of glucose, which is the same both in the presence and absence of insulin. The inhibition of glucose transport appears to be mediated through the binding of dexamethasone to a specific glucocorticoid receptor, rather than by a nonspecific steroid-membrane interaction. Thus, experiments showed that another steroid, progesterone, had no effect on glucose oxidation but acted as a glucocorticoid receptor agonist and abolished the effects of dexamethasone. The decreased ability of dexamethasonetreated cells to oxidize glucose therefore seems to arise from an inhibition of the transport system alone. In these in vitro studies,
this effect does not appear to be due to an altered binding capacity for insulin, which is the same in both dexamethasone treated and control cells. It is likely that transport is accomplished by a single system. Thus the effects of dexamethasone are only observed when this is rate limiting. The demonstration of a stimulatory effect of insulin on the glucose oxidation itself is interesting, and further studies are needed to investigate the significance of this. 0 ~
~~
1. J.W. Conn and S.S. Fajans: Influence of Adrenal Cortical Steroids on Carbohydrate Metabolism in Man. Metabolism 5: 114-127, 1956 2. R.E. Yorke: The Influence of Dexamethasone on Adipose Tissue Metabolism In Vitro. J. Endocrinol. 39: 329-343, 19.67 3. F.A. Riddick, Jr., D.M. Reisler and D.M. Kipnix: The Sugar Transport System in Striated Muscle. Effect of Growth Hormone, Hydrocortisone and Alloxan Diabetes. Diabetes 11: 171178, 1962 4. M.P. Czech and J.N. Fain: Antagonism of Insulin Action on Glucose Metabolism in White Fat Cells by Dexamethasone. Endocrinology 91 : 518-522, 1972 5. J.M. Olefsky, J. Johnson, F. Lui, P. Jen and G.M. Reaven: The Effects of Acute and Chronic Dexamethasone Administration on Insulin Binding to Isolated Rat Hepatocytes and Adipocytes. Metabolism 24: 517-527, 1975 6. J.M. Olefsky: Effect of Dexamethasone on Insulin Binding, Glucose Transport, and Glucose Oxidation of Isolated Rat Adipocytes. J. CIin. Invest. 56: 1499-1508, 1975 7. M.P. Czech, J.C. Lawrence, Jr. and W.S. Lynn: Hexose Transport in Isolated Brown Fat Cells. A Model System for Investigating Insulin Action on Membrane Transport. J. Biol. Chem. 249: 5421-5427, 1974 8. K.J. Chang and P. Cuatrecasas: Adenosine Triphosphate-Dependent Inhibition of InsulinStimulated Glucose Transport in Fat Cells. Possible Role of Membrane Phosphorylation. J. Clin. Invest. 53: 3170-3180, 1974 9. Insulin Receptors, Insulin Resistance Diabetes and Obesity. Nutrition Reviews 34: 145-148, 1976
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8. M.H. Ross and G. Bras: Tumor Incidence Patterns and Nutrition in the Rat. J . Nutrition 87: 245-260, 1965 9. R.K. Chandra: Antibody Formation in First and Second Generation Offspring of Nutritionally Deprived Rats. Science 190: 289-290, 1975 10. B.M. Gebhardt and P.M. Newberne: Nutrition and Immunological Responsiveness. T-cell Function in the Offspring of Lipotrope and Protein-Deficient Rats. Immunology 26: 489-495, 1974 11. L.C. Robson and M.R. Schwarz: Vitamin Be Deficiency and the Lymphoid System. ll. Effects of Vitamin BS Deficiency In Utero on the Immunological Competence of the Offspring. Cell. Immunol. 16: 145-152, 1975 12. M. Mathur, V. Ramalingaswami and M.G. Deo: Influence of Protein Deficiency on 19s Antibody-Forming Cells in Rats and Mice. J. Nutrition 102: 841-846, 1972 13. D.J. Jose and R.A. Good: Quantitative Effects of Nutritional Essential Amino Acid Deficiency upon Immune Responses to Tumors in Mice. J . €xp. Med. 137: 1-9, 1973
EFFECTS OF DEXAMETHASONE ON GLUCOSE METABOLISM Adipocytes exposed to dexamethasone have a reduced ability to oxidize glucose due to inhibition of glucose transport. This effect is not mediated by a reduction in insulin binding capacity. Key Words: dexamethasone, insulin, glucose metabolism
Altered glucose metabolism on exposure to glucocorticoids has been demonstrated both in vivo and in vitro. In man prolonged glucocorticoid administration produces insulin resistance and an inability to handle a glucose load,’ while the oxidation of glucose is inhibited in isolated adipocytes and muscle tissue exposed to glucocorticoids. Since the inhibition of glucose oxidation in vitro can be overcome by insulin, it has been suggested that the transport of glucose is accomplished by two separate systems, one insulin sensitive and the other a basal system which is unresponsive to insulin. Of the two systems, it is proposed that 293
only the basal component is affected by glucocorticoids. More recent work by Czech and Fain,4 however, using a range of glucose and insulin concentrations, demonstrated that under some conditions dexamethasone could inhibit glucose metabolism in the presence of insulin. These workers proposed that dexamethasone exerted its effect only when the transport of glucose was rate limiting, that is when the action of insulin on glucose metabolism was at the level of the transport system. A possible mechanism of action for dexamethasone in these circumstances is suggested by the work of Olefsky and co-workers5 who demonstrated that the number of insdin receptors is decreased ‘in dexamethasonetreated hepatocytes and adipocytes. NUTRITION REVIEWS / VOL. 34, NO. 6 i JUNE 1976
185
A recent study by Olefsky6sought to examine intracellular action was confirmed by further further the nature of the glucose transport sysexperiments in which transport and oxidation tem and to elucidate the mechanism of action were measured in the presence and absence of dexamethasone. Furthermore, the relationof cytochalasin B. These showed that while ship between the interaction of insulin at the both basal and insulin-stimulated facilitated membrane and its intracellular action was intransport were inhibited by cytochalasin B, vestigated. The experiments were carried out insulin still retained the power to increase the with normal and dexamethasone-treated rat rate of oxidation of that glucose entering the adipocytes. cell by simple diffusion. Initial studies of glucose oxidation, as meaWhen transport was measured in the pressured by the conversion of (l-14C) glucose to ence of dexamethasone, a 30 to 40 percent 14C02showed that dexamethasone in the ab- inhibition was found, both in the absence of sence of insulin lowered oxidation by 30 perinsulin and at all levels of insulin stimulation. cent and that oxidation remained depressed at Expressed as a percent of the basal transport submaximal plasma insulin levels. When plasrate, however, the stimulatory effects of insulin ma insulin was high, however (more than were found to be the same in both groups of 100 pU per milliliter), control and dexamethacells. Since at higher substrate concentrations, sone-treated cells had identical abilities to oxithe dexamethasone-treated cells were capdize glucose. This result is compatible with able of taking up much larger amounts of subthe hypothesis that dexamethasone is only efstrate than that seen in these experiments, the fective where transport is rate limiting. Thus, lower uptake in the presence of dexamethaafter insulin concentration increases above a sone appeared not to be a result of a decreased certain level, the intracellular oxidation mechcapacity of the cells for substrate uptake. Studanism itself is saturated, so that any decrease ies of uptake over a range of substrate conin membrane transport cannot be reflected in centrations revealed that, like insulin, dexathe ability of the cell to oxidize glucose. methasone exerts its effect on the saturable In order to measure the transport process intransport component, leaving diffusion undependently of oxidation, the uptake of the nonchanged. Also, similar to the insulin effect was metabolizable sugar, 2-deoxyglucose was exthe observation that the K m of the cell remained amined. Preliminary studies were first carried the same as for control cells and only,,V, was out to characterize the transport of this particualtered, in this case being significantly delar sugar. Two components of uptake were creased. demonstrated: below a concentration of 5mM Another means of examining transport is 2-deoxyglucose, uptake was a saturable funcby studying the uptake of 3-O-methylglucose. tion of concentration, while at higher concenUnlike 2-deoxyglucose, this sugar is not phostrations, a nonsaturable component was seen, phorylated, so its concentration rapidly builds which increased with increasing 2-deoxygluup inside the cell, and an equilibrium position cose levels. As suggested by other w o r k e r ~ , ~ is reached between inward and outward movethese two components probably represent a ment. This occurs after approximately 45 seconds in the basal state and is shortened to saturable facilitated transport system, together with uptake by simple diffusion. This was supabout 20 seconds in the presence of insulin. ported by the inhibition of the saturable comSince the uptake is nonlinear, use of this sugar ponent alone with cytochalasin B. Insulin markfor transport studies is somewhat limited, but edly increased the rate of the facilitated transits uptake does give an indication solely of port, the effect being on the maximum velocity transport. Dexamethasone was found in these (Vmax) with the Michaelis constant (K,) reexperiments to decrease the uptake of 3-0maining unaltered. The diffusion component methylglucose in both control and insulinwas the same as for control cells. It is generstimulated cells. ally thought that insulin acts primarily at the Any direct effect of dexamethasone on the site of transport,8 but these experiments demoxidation mechanism seems unlikely, since onstrated a greater effect on oxidation. A major further experiments showed no difference in 186 NUTRITION R E V E W S
I VOL.
34, NO. 6
I JUNE
1976
total hexokinase activity between control and dexamethasone-treated cells. This result, together with the 2-deoxyglucose and the 3-0methylglucose studies, strongly suggests that dexamethasone acts at the site of transport alone. Since insulin still retains the same stimulatory effect on transport, it seems that dexamethasone inhibits a fixed proportion of this system, and that the basal and insulin-stimulated components are part of the same system. As the actions of dexamethasone might be mediated via a decreased capacity of the cells to bind insulin, binding studies were carried out in presence and absence of the glucocorticoid. In contrast to observations made in vitro, where dexamethasone was found to decrease binding,5 these experiments revealed identical capacities for insulin binding of the two groups of cells. A possible explanation for these differing results is that in vivo, dexamethasone increases plasma insulin, which in turn decreases the number of insulin receptors. Such an effect of insulin on the receptor population has recently been reviewed.9 In these in vitro studies, for a given degree of insulin stimulation, dexamethasone-treated cells could transport less glucose than normal controls. Whatever the nature of the interaction, it seems unlikely that the relationship between insulin receptors and the transport system is affected. This follows from the effect of dexamethasone on the uptake rate of glucose, which is the same both in the presence and absence of insulin. The inhibition of glucose transport appears to be mediated through the binding of dexamethasone to a specific glucocorticoid receptor, rather than by a nonspecific steroid-membrane interaction. Thus, experiments showed that another steroid, progesterone, had no effect on glucose oxidation but acted as a glucocorticoid receptor agonist and abolished the effects of dexamethasone. The decreased ability of dexamethasonetreated cells to oxidize glucose therefore seems to arise from an inhibition of the transport system alone. In these in vitro studies,
this effect does not appear to be due to an altered binding capacity for insulin, which is the same in both dexamethasone treated and control cells. It is likely that transport is accomplished by a single system. Thus the effects of dexamethasone are only observed when this is rate limiting. The demonstration of a stimulatory effect of insulin on the glucose oxidation itself is interesting, and further studies are needed to investigate the significance of this. 0 ~
~~
1. J.W. Conn and S.S. Fajans: Influence of Adrenal Cortical Steroids on Carbohydrate Metabolism in Man. Metabolism 5: 114-127, 1956 2. R.E. Yorke: The Influence of Dexamethasone on Adipose Tissue Metabolism In Vitro. J. Endocrinol. 39: 329-343, 19.67 3. F.A. Riddick, Jr., D.M. Reisler and D.M. Kipnix: The Sugar Transport System in Striated Muscle. Effect of Growth Hormone, Hydrocortisone and Alloxan Diabetes. Diabetes 11: 171178, 1962 4. M.P. Czech and J.N. Fain: Antagonism of Insulin Action on Glucose Metabolism in White Fat Cells by Dexamethasone. Endocrinology 91 : 518-522, 1972 5. J.M. Olefsky, J. Johnson, F. Lui, P. Jen and G.M. Reaven: The Effects of Acute and Chronic Dexamethasone Administration on Insulin Binding to Isolated Rat Hepatocytes and Adipocytes. Metabolism 24: 517-527, 1975 6. J.M. Olefsky: Effect of Dexamethasone on Insulin Binding, Glucose Transport, and Glucose Oxidation of Isolated Rat Adipocytes. J. CIin. Invest. 56: 1499-1508, 1975 7. M.P. Czech, J.C. Lawrence, Jr. and W.S. Lynn: Hexose Transport in Isolated Brown Fat Cells. A Model System for Investigating Insulin Action on Membrane Transport. J. Biol. Chem. 249: 5421-5427, 1974 8. K.J. Chang and P. Cuatrecasas: Adenosine Triphosphate-Dependent Inhibition of InsulinStimulated Glucose Transport in Fat Cells. Possible Role of Membrane Phosphorylation. J. Clin. Invest. 53: 3170-3180, 1974 9. Insulin Receptors, Insulin Resistance Diabetes and Obesity. Nutrition Reviews 34: 145-148, 1976
NUTRITION REVIEWS I VOL. 34, NO. 6 1 JUNE 1976
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