- Email: [email protected]
Effect of the postprandial state on nontraditional risk factors
Effect of the Postprandial State on Nontraditional Risk Factors Harold E. Lebovitz,
MD
Hyperglycemia is the cause of chronic microvascular complications in diabetes. Increased macrovascular disease occurring in diabetes is multifactorial, with hyperglycemia being only 1 of the factors responsible. A major deficiency in our understanding of the role of hyperglycemia in the pathogenesis of chronic diabetic complications is the contribution made by exaggerated postprandial glycemic excursions, in contrast to that made by chronic fasting hyperglycemia. Hyperglycemia is thought to cause chronic diabetic complications through >1 of the following biochemical mechanisms: (1) an exaggerated polyol pathway, (2) protein glyca-
tion and the formation of advanced glycation end products, (3) excessive activation of protein kinase C (PKC) isoenzymes in vascular and mesangial cells, and (4) greater oxidative stress. The focus of this article is how postprandial hyperglycemia and glucose excursions might specifically activate >1 of these mechanisms and how they might contribute to the development of the chronic complications of diabetes by causing endothelial dysfunction, a procoagulant state, carbonyl stress, and/or vascular and oxidative effects secondary to PKC activation. 䊚2001 by Excerpta Medica, Inc. Am J Cardiol 2001;88(suppl):20H–25H
ther articles in this symposium have focused on whether the postprandial excursions of plasma O glucose and plasma triglycerides are independent risk
membrane and circulating proteins is increased. Cells in which glucose is freely permeable (eg, nerve, retinal, and glomerulus cells) are exposed to very high intracellular concentrations, which can lead to increased activation of the aldose reductase pathway, or excessive activation of the protein kinase C (PKC) or other pathways, and cause injury to the cell. Insulin-sensitive cells, such as those in muscle and adipose tissue, can protect themselves by becoming insulin resistant and decreasing their flux of glucose. Inappropriate regulation of essential nutrients can lead to many diverse pathophysiologic processes. The postprandial state exaggerates the detrimental effects of impaired nutrient regulation because the flux of nutrients is the greatest and concentrations are the highest at this time.
factors for the development of macrovascular disease. The clinical data suggest that increases in postprandial plasma glucose levels within the normal or impaired glucose tolerance ranges correlate with an increase in coronary artery disease events and/or death.1,2 Microvascular disease, on the other hand, occurs only with postprandial plasma glucose levels within the diabetic range.3 The mechanisms by which postprandial elevations of glucose, triglycerides, and other nutrients can contribute to macrovascular and/or microvascular disease are the subject of this review. The postprandial state is a highly integrated physiologic process, the primary function of which is to deliver nutrients to the tissues at concentrations that are adequate, while not causing detrimental effects. Gastrointestinal and endocrine regulatory systems operate together to achieve this goal. The rate of emptying of the stomach and the rate of digestive processes control the flux of nutrients into the extracellular compartment. The uptake of nutrients by the liver and peripheral tissues determines the flux of nutrients from the extracellular compartment. The balance between these processes determines the extracellular concentrations of the nutrients. Renal excretion serves as a safety valve if concentrations become very excessive. The extracellular concentration of a nutrient can alter intracellular function by modifying cellular function at the level of the plasma membrane or by increasing the flux and concentration of the nutrient inside the cell. Hyperglycemia is an excellent example of such processes. When the extracellular concentration of glucose is excessive, nonenzymatic glycosylation of plasma From the State University of New York Health Sciences Center, Brooklyn, New York, USA. Address for reprints: Harold E. Lebovitz, MD, State University of New York Health Sciences Center, 450 Clarkson Avenue, Box 50, Brooklyn, New York 11203. E-mail: [email protected].
20H
©2001 by Excerpta Medica, Inc. All rights reserved.
MECHANISMS OF HYPERGLYCEMIC DAMAGE The mechanisms by which hyperglycemia can cause the tissue damage that ultimately leads to vascular disease have not been absolutely defined. However, there is considerable evidence to support the hypothesis that several different mechanisms are operative4 and that they may act in concert (Table 1). Mechanisms related to increases in intracellular glucose concentrations: In tissues that are insulin indepen-
dent, an increase in the plasma glucose level is followed by a comparable increase in intracellular glucose. Tissues that have high concentrations of aldose reductase enzymes5–7 will convert a significant quan-
TABLE 1 Proposed Mechanisms for Hyperglycemia-mediated Tissue Damage ● ● ● ●
Activation of the polyol pathway Excessive glycation of proteins and lipids Activation of the diacylglycerol-PKC second-messenger system Increase in “oxidative stress” PKC ⫽ protein kinase C.
0002-9149/01/$ – see front matter PII S0002-9149(01)01833-1
FIGURE 1. The polyol pathway. Glucose is reduced by aldose reductase to sorbitol by oxidation of nicotinamide adenine dinucleotide phosphate (NADPH to NADP). Sorbitol is then oxidized to fructose by reduction of nicotinamide adenine dinucleotide (NAD to NADH). The aldose reductase reaction predominates, and during hyperglycemia, sorbitol and NADP accumulate within the cell.
FIGURE 2. Hyperglycemia: aldose reductase activity. Exaggerated polyol pathway activity causes a decrease in myoinositol and sodium/ potassium (Na/K)–adenosine-triphosphatase (ATPase) activity and an alteration in inositol phospholipid metabolism, which increases protein kinase C (PKC) activity. Additionally, the increase in intracellular sorbitol causes an increase in intracellular osmolality, and the increase in nicotinamide adenine dinucleotide phosphate (NADP) is associated with an increase in oxidative stress. NADPH ⴝ reduced NADP.
tity of that glucose into sorbitol (Figure 1). The sorbitol is slowly metabolized to fructose. In the presence of high glucose concentrations, sorbitol is generated more rapidly than it is metabolized to fructose, leading to an increase in intracellular sorbitol. The consequences of the increase in intracellular sorbitol are an increase in intracellular osmolality and a reduction in intracellular myoinositol content. Inositol phospholipid metabolism and membrane function are altered, leading to additional changes such as decreased Na⫹/ K⫹-adenosine-triphosphatase activity and increased PKC activity (Figure 2). This polyol pathway is thought to play a major role in the development of diabetic cataracts and diabetic neuropathy.6,7 Another, and perhaps even more important pathway by which increases in intracellular glucose can cause detrimental changes in cells, is through excessive activation of PKC.8 –10 PKC is a serine and threonine kinase that phosphorylates those residues in protein substrates. In so doing, it changes the function of many intracellular proteins.9,10 In the PKC family, there are at least 12 isoenzymes with both tissue and functional specificity.9,10 The PKCs are ordinarily activated by calcium ions and phospholipid metabolites (Figure 3) and are part of a
second messenger–signaling pathway that relies on diacylglycerol as an activator. Hyperglycemia can cause an activation of PKC through the de novo pathway of diacylglycerol synthesis, as shown in Figure 3,8 bypassing the normal regulatory system and causing disturbances in many functions of the cell, particularly in the case of vascular and mesangial cells.11–13 Vascular cells contain the specific PKC isoenzyme PKC 2. The consequences of excessive activation of this isoenzyme in vascular cells are depicted in Figure 4. A specific inhibitor of this PKC isoenzyme8,9 has been developed (LY333531), and it has been shown, in vitro and in vivo, to reverse many of the abnormalities described in Figure 4.14 It is currently being evaluated in clinical trials. Mechanisms related to extracellular hyperglycemia:
Glucose is a highly reactive molecule that contains an aldehyde group. When an aldehyde group comes into close proximity with an amino group, it forms a complex known as a Schiff base (Figure 5). The Schiff base undergoes a rearrangement and forms a stable complex known as an Amadori product, which is essentially a glycosylated protein. Glycosylated proteins with a slow turnover rate have a tendency to undergo dehydrations, oxidations, and rearrangements
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FIGURE 3. Mechanisms of protein kinase C (PKC) activation in diabetes mellitus. PKC enzyme activation is normally regulated by an agonist stimulating the production of phosphoinositides and phosphatidic acid and generates 1,2-diacylglycerol (DAG), which activates PKC. In the presence of hyperglycemia, DAG is synthesized by the de novo pathway from glycolysis-generated triose phosphates, dihydroxyacetone phosphate (DHAP), and glycerol 3-phosphate (G3P). This is a mechanism by which hyperglycemia can directly increase PKC activation. Acyl-CoA ⴝ acyl-coenzyme A; CoA ⴝ coenzyme A; FDP ⴝ fructose 1,6-diphosphate; F6P ⴝ fructose 6-phosphate; GAP ⴝ glyceraldehyde 3-phosphate; G6P ⴝ glucose 6-phosphate; LysoPA ⴝ lysophosphatidic acid.
FIGURE 4. Cellular consequences of 1,2-diacylglycerol (DAG)-protein kinase C (PKC) activation induced by hyperglycemia. An increase in PKC 2 by hyperglycemia-generated DAG causes many changes in the metabolism of endothelial, vascular, and mesangial cells. Some of these effects and the known mechanisms by which they are mediated are shown. ANP ⴝ atrial naturetic peptide; C-fos ⴝ transcription factor; eNOS ⴝ endothelial nitric oxide synthase; ET-1 ⴝ endothelin-1; ICAMs ⴝ intercellular adhesion molecules; LY333531 ⴝ specific inhibitor for PKC  isoforms; Naⴙ-Kⴙ-ATPase ⴝ sodium-potassium–adenosine triphosphate; PAI-1 ⴝ plasminogen activator inhibitor 1; PGE2 ⴝ prostaglandin E2; PLA2 ⴝ phospholipase A2; TGF- ⴝ transforming growth factor–; VEGF ⴝ vascular endothelial growth factor. (Reproduced with permission from Vasc Med.14)
to form large polymers15,16 known as advanced glycation end products (AGEs). Glycation products are being continuously formed and removed by AGE receptors ([RAGE] and scavenger receptor types I and 22H THE AMERICAN JOURNAL OF CARDIOLOGY姞
II) in all persons.15,16 Glycosylation product formation is a function of the ambient glucose concentrations. The bulk of the in vivo advanced glycation products involve circulating, basement membrane, or matrix
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FIGURE 5. Nonenzymatic glycation: pathway of oxidative production of advanced glycation end products (see text for further description). K ⴝ potassium; NH ⴝ sodium-hydrogen.
FIGURE 6. Biologic effects of advanced glycation end products (AGEs). The effects are the consequence of binding of the AGEs to receptors for AGEs (RAGEs) on the specific tissues. EGF ⴝ epidermal growth factor; EGF-R ⴝ epidermal growth factor receptor.
proteins. Intracellular and intranuclear protein glycosylation can occur. Receptors for AGEs are present on monocytes, macrophages, endothelial cells, pericytes, podocytes, and fibroblasts as well as on mesangial cells, astrocytes, and microglia.17 Cell activation in response to AGE-modified proteins is associated with increased expression of extracellular matrix proteins, vascular adhesion molecules, cytokines, and growth factors. These result in chemotaxis, angiogenesis, oxidative stress, cell proliferation, or programmed cell death, depending on the tissue and other factors influencing the cell’s function (Figure 6). Initial studies of the interactions of glucose with proteins focused on oxidative formation of AGEs derived from Amadori products. More recent studies have described nonoxidative pathways in which molecules derived from the glycolysis of glucose (methylglyoxal and 3-deoxyglucosone) can interact with arginine and lysine residues and generate imidazolone and imidazolium AGEs.18,19 Thus, AGEs can be derived from both oxidative and nonoxidative pathways. Oxidative stress: A popular theory to explain the mechanism by which hyperglycemia ultimately causes diabetic complications is the formation of reactive oxygen intermediates and the presence of elevated oxidative stress.20 –22 This theory is appealing because
it can account for the increase in glycoxidation and lipoxidation products in plasma and tissue proteins. Intracellular oxidative stress leads to activation of the redox-sensitive transcription factor, nuclear factor-B, and tissue factor expression.23 An exaggerated polyol pathway and interaction of AGEs with the RAGE receptor can be viewed as processes that generate oxidative stress.20,23 Baynes and Thorpe22 reviewed the evidence for the role of oxidative stress in diabetic complications, and they concluded that the interaction of glucose with proteins is both oxidative and nonoxidative. They suggested that inadequate detoxification of these modified AGE proteins, rather than excessive oxidation, leads to the increase in tissue levels. They proposed that treatments to reduce the increased carbonyl stress of hyperglycemia are more important in preventing diabetic complications than attempts to reduce oxidative stress.
SPECIFIC PATHOPHYSIOLOGIC ABNORMALITIES ASSOCIATED WITH POSTPRANDIAL HYPERGLYCEMIA
Several responses to postprandial hyperglycemia have been described that may have a significant relation to the development of the chronic complications of type 2 diabetes. These include acute hyperglycemia–induced changes in endothelial function, in co-
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TABLE 2 Functions of the Endothelium ● Regulates vascular tone —Nitric oxide: vasodilation —Endothelin: vasoconstriction ● Controls matrix protein synthesis ● Stimulates cell growth and migration ● Controls permeability ● Regulates thrombogenesis —Increases plasminogen activator inhibitor–1 synthesis —Modulates platelet adhesion ● Modulates inflammatory responses —Cytokine production —Adhesion molecule synthesis
agulation, and in the formation of nonoxidative glycation products. Endothelial function: The endothelium influences multiple functions in the vascular system (Table 2), including (1) regulating vascular tone, (2) controlling cell growth, (3) regulating thrombogenesis, (4) modulating inflammatory responses, (5) determining matrix protein synthesis, and (6) controlling permeability. In diabetes, endothelial function is abnormal and its regulatory functions are disturbed. These abnormalities are thought to be major factors in the development of both the microvascular and macrovascular complications of diabetes. Metabolic abnormalities in type 2 diabetes that cause endothelial dysfunction are (1) insulin resistance, (2) hypertriglyceridemia, (3) increased levels of low-density lipoprotein cholesterol, (4) hypertension, and (5) hyperglycemia. Several recent studies indicate that acute hyperglycemia, such as occurs in postprandial hyperglycemia, causes endothelial dysfunction.24 Kawano et al25 examined flow-mediated endothelium-dependent vasodilation with ultrasound techniques during oral glucose tolerance testing in 17 nondiabetic subjects, 24 subjects with impaired glucose tolerance, and 24 subjects with type 2 diabetes.25 Flow-mediated vasodilation at baseline was decreased in type 2 diabetes patients. All groups had a striking decrease in flowmediated vasodilation 1 and 2 hours after the administration of glucose. They also measured the formation of oxidation products during the oral glucose tolerance test and found them to be elevated 1 and 2 hours after the glucose administration. They concluded that acute hyperglycemia suppresses endothelium-dependent vasodilation, probably through increased production of oxygen-derived free radicals. In a similar study, Akbari et al26 measured endothelium-dependent vasodilation in the brachial artery (macrocirculation) and erythrocyte flux after acetylcholine iontophoresis (microcirculation) in 20 healthy subjects during fasting conditions and 1 hour after ingestion of 75 g of glucose. They found that the ingestion of glucose impaired the endothelium-dependent vasodilation of both the microcirculation and macrocirculation. Acute hyperglycemia in nondiabetic individuals has been shown to reduce methacholine-mediated vasodilation, and the effect was increased if the secretion of insulin was simultaneously inhibited.27 Insulin increases endothelium-dependent vasodilation by increasing nitric oxide production. Hypergly24H THE AMERICAN JOURNAL OF CARDIOLOGY姞
cemia decreases endothelium-dependent function by suppressing nitric oxide production.28 In healthy individuals, the hyperglycemia after glucose administration appears to predominate over the increased insulin secretion in impairing endothelium-dependent vasodilation. The addition of insulin resistance and relative insulin deficiency to the metabolic environment makes the effect of hyperglycemia more severe. It is not clear which other aspects of endothelial cell function are altered by acute hyperglycemia in vivo. The promoter region of the plasminogen activator inhibitor–1 (PAI-1) gene has a region just upstream from the transcriptional start site, which can be activated by hyperglycemia.29 Whether acute hyperglycemia in vivo increases plasma PAI-1 levels is difficult to determine because of the concomitant effects of insulin, proinsulin, and triglycerides. For example, a recent investigation showed that intravenous administration of glucose plus intralipid for 6 hours in healthy subjects doubled plasma PAI-1 levels for several hours. In that study, euglycemic hyperinsulinemia had no effect on PAI-1 levels.30 Postprandial coagulation effects: Several investigations have shown that postprandial hyperglycemia is accompanied by a series of alterations of the coagulation system.31–33 An oral glucose load in both healthy and type 2 diabetes patients causes a shortening in the half-life of fibrinogen and an increase in plasma fibrinopeptide A and the fragments of prothrombin and factor VII. In patients with diabetes, meal-mediated hyperglycemia also causes hyperproduction of thrombin proportional to the blood glucose increase.31 Amelioration of the meal-mediated hyperglycemia by the ␣-glucosidase inhibitor, acarbose, reduces the coagulation activation.32 Other studies have reported that acute hyperglycemia in healthy subjects increases platelet aggregation to adenosine diphosphate and blood viscosity.33 These effects can be prevented by increasing nitric oxide production through a simultaneous L-arginine infusion; they can be reproduced by blocking endogenous nitric oxide production by infusion of the inhibitor NG-monomethyl-L-arginine. The conclusion drawn from these various studies is that postprandial hyperglycemia can promote coagulation activation. Postprandial hyperglycemia and AGE formation: As discussed in the preceding section, the activated carbonyl compounds, methylglyoxal and 3-deoxyglucosone (␣-oxoaldehydes), are generated from triose intermediates during glycolysis. These ␣-oxoaldehydes are early glycation products that respond to fluctuations in plasma glucose and are important precursors of advanced glycation addicts.18,19 Because short periods of hyperglycemia (such as those that occur in postprandial hyperglycemia) may be sufficient to increase the concentrations of ␣-oxoaldehydes in vivo, this is a mechanism whereby glycemic excursions can play a significant role in the development of chronic diabetic complications.18,19 Postprandial hyperglycemia activation of PKC and oxidative stress: Fragmentary studies in humans point
to the possibility that fluctuations in the plasma glucose levels of healthy humans as well as patients with diabetes may directly activate PKC in some tissues
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and thus directly contribute to oxidative stress.13 Healthy controls and type 2 diabetes patients infused with glucose and insulin had a significant increase in thrombocyte PKC 2 when the isoenzyme was measured at 60 minutes and 150 minutes postinfusion. Several studies have measured the oxidation-reduction state of healthy subjects and subjects with type 2 diabetes in the fasting state and after meal ingestion. After the meal, plasma malondialdehyde and vitamin C levels increased, whereas protein sulfhydryl groups, uric acid, vitamin E, and the total plasma radical-trapping parameter decreased more significantly in the subjects with diabetes than in the control subjects.21 Ceriello et al32 concluded from their studies that hyperglycemia could play an important role in the generation of postprandial oxidative stress in diabetic patients.
CONCLUSION
Hyperglycemia is a major factor in the development of microvascular disease in the diabetic person. There is controversy about the level of glycemia that confers cardiovascular risk and the extent of the role of hyperglycemia relative to the other components of the insulin resistance syndrome in the pathogenesis of the macrovascular complications. The relative importance of postprandial versus fasting hyperglycemia in the pathogenesis of the chronic complications remains an unanswered question. Until recently, the predominant concept was that fasting hyperglycemia is the primary factor in the generation of glycosylated proteins and the main cause of the chronic vascular disease associated with diabetes. Newer information, however, relates postprandial hyperglycemia to the development of coronary artery disease. Data indicate that changes in glycosylated hemoglobin correlate better with postprandial than with fasting plasma glucose. Good glycemic control is not achievable unless treatment addresses postprandial glycemic goals. The data reviewed here provide a basis for understanding why exaggerated glycemic excursions may have detrimental effects. Changes in postprandial nutrient and hormone levels can (1) exaggerate the polyol pathway, (2) activate PKC, (3) elevate methylglyoxal and 3-deoxyglucosone, and (4) cause increased oxidative stress. The consequences of these biochemical effects are changes in endothelial, mesangial, pericyte, smooth muscle, and macrophage cell functions in such a manner as to cause microvascular and macrovascular disease. The recognition of these consequences of postprandial hyperglycemia and hyperlipidemia demands that we change our treatment strategies to focus on postprandial as well as fasting metabolic control. The availability of newer pharmacologic agents designed specifically to target postprandial control will allow us to do this. The consequence of this refocused treatment approach should be a great reduction in diabetic vascular complications.
1. Haffner SM. The importance of hyperglycemia in the nonfasting state to the
development of cardiovascular disease. Endocr Rev 1998;19:583–592. 2. Laakso M. Hyperglycemia and cardiovascular disease in type 2 diabetes.
Diabetes 1999;48:937–942. 3. The Expert Committee on the Diagnosis and Classification of Diabetes Mel-
litus. Report of the Expert Committee on the Diagnosis and Classification of Diabetes Mellitus. Diabetes Care 1999;22(suppl 1):S5–S19. 4. Haller H. The clinical importance of postprandial glucose. Diabetes Res Clin Pract 1998;40(suppl):S43–S49. 5. Frank RN. The aldose reductase controversy. Diabetes 1994;43:169 –172. 6. Tomlinson DR, Stevens EJ, Diemel LT. Aldose reductase inhibitors and their potential for the treatment of diabetic complications. Trends Pharmacol Sci 1994;15:293–297. 7. Yabe-Nishimura C. Aldose reductase in glucose toxicity: a potential target for the prevention of diabetic complications. Pharmacol Rev 1998;50:21–33. 8. King GL, Wakasaki H. Theoretical mechanisms by which hyperglycemia and insulin resistance could cause cardiovascular diseases in diabetes. Diabetes Care 1999;22(suppl 3):C31–C37. 9. Way KJ, Chou E, King GL. Identification of PKC-isoform-specific biological actions using pharmacological approaches. Trends Pharmacol Sci 2000;21:181–187. 10. Goekjian PG, Jirousek MR. Protein kinase C in the treatment of disease: signal transduction pathways, inhibitors, and agents in development. Curr Med Chem 1999;6:877–903. 11. Kim NH, Jung HH, Cha DR, Choi DS. Expression of vascular endothelial growth factor in response to high glucose in rat mesangial cells. J Endocrinol 2000;165:617– 624. 12. Williams B, Gallacher B, Patel H, Orme C. Glucose-induced protein kinase C activation regulates vascular permeability factor mRNA expression and peptide production by human vascular smooth muscle cells in vitro. Diabetes 1997;46: 1497–1503. 13. Pirags V, Assert R, Haupt K, Schatz H, Pfeiffer A. Activation of human platelet protein kinase C-beta 2 in vivo in response to acute hyperglycemia. Exp Clin Endocrinol Diabetes 1996;104:431– 440. 14. Meier M, King GL. Protein kinase C activation and its pharmacological inhibition in vascular disease. Vasc Med 2000;5:173–185. 15. Vlassara H. Advanced glycation end-products and atherosclerosis. Ann Med 1996;28:419 – 426. 16. Brownlee M. Negative consequences of glycation. Metabolism 2000;49(suppl 1):9 –13. 17. Vlassara H, Brownlee M, Cerami A. Macrophage receptor-mediated processing and regulation of advanced glycosylation endproduct (AGE)-modified proteins: role in diabetes and aging. Prog Clin Biol Res 1989;304:205–218. 18. Thornalley PJ, Langborg A, Minhas HS. Formation of glyoxal, methylglyoxyal and 3-deoxyglucosone in the glycation of proteins by glucose. Biochem J 1999;344:109 –116. 19. Takeuchi M, Makita Z, Bucala R, Suzuki T, Koike T, Kameda Y. Immunological evidence that non-carboxymethyllysine advanced glycation end-products are produced from short chain sugars and dicarbonyl compounds in vivo. Mol Med 2000;6:114 –125. 20. Ceriello A. Hyperglycemia: the bridge between non-enzymatic glycation and oxidative stress in the pathogenesis of diabetic complications. Diabetes Nutr Metab 1999;12:42– 46. 21. Ceriello A, Lizzio S, Bortolotti N, Russo A, Motz E, Tonutti L, Crescentini A, Taboga C. Meal-generated oxidative stress in type 2 diabetic patients. Diabetes Care 1998;21:1529 –1533. 22. Baynes JW, Thorpe SR. Role of oxidative stress in diabetic complications: a new perspective on an old paradigm. Diabetes 1999;48:1–9. 23. Mohamed AK, Bierhaus A, Schiekofer S, Tritschler H, Ziegler R, Nawroth PP. The role of oxidative stress and NF- B activation in late diabetic complications. Biofactors 1999;10:157–167. 24. Shige H, Ishikawa T, Suzukawa M, Ito T, Nakajima K, Higashi K, Ayaori M, Tabata S, Ohsuzu F, Nakamura H. Endothelium-dependent flow-mediated vasodilation in the postprandial state in type 2 diabetes mellitus. Am J Cardiol 1999;84:1272–1274. 25. Kawano H, Motoyama T, Hirashima O, Hirai N, Miyao Y, Sakamoto T, Kugiyama K, Ogawa H, Yasue H. Hyperglycemia rapidly suppresses flowmediated endothelium-dependent vasodilation of brachial artery. J Am Coll Cardiol 1999;34:146 –154. 26. Akbari CM, Saouaf R, Barnhill DF, Newman PA, LoGerfo FW, Veves A. Endothelium-dependent vasodilatation is impaired in both micro circulation and macro circulation during acute hyperglycemia. J Vasc Surg 1998;28:687– 694. 27. Williams SB, Goldfine AB, Timimi FK, Ting HH, Roddy MA, Simonson DC, Creager MA. Acute hyperglycemia attenuates endothelium-dependent vasodilation in humans in vivo. Circulation 1998;97:1695–1701. 28. Giugliano D, Marfella R, Coppola L, Verrazzo G, Acampora R, Giunta R, Nappo F, Lucarelli C, D’Onofrio F. Vascular effects of acute hyperglycemia in humans are reversed by L-arginine: evidence for reduced availability of nitric oxide during hyperglycemia. Circulation 1997;95:1783–1790. 29. Kohler HP, Grant PJ. Plasminogen-activator inhibitor type 1 and coronary artery disease. N Engl J Med 2000;342:1792–1801. 30. Calles-Escandon J, Mirza SA, Sobel BE, Schneider DJ. Induction of hyperinsulinemia combined with hyperglycemia and hypertriglyceridemia increases plasminogen activator inhibitor 1 in blood in normal human subjects. Diabetes 1998;47:290 –293. 31. Ceriello A. Coagulation activation in diabetes mellitus: the role of hyperglycaemia and therapeutic prospects. Diabetologia 1993;36:1119 –1125. 32. Ceriello A, Taboga C, Tonutti L, Giacomello R, Stel L, Motz E, Pirisi M. Post-meal coagulation activation in diabetes mellitus: the effect of acarbose. Diabetologia 1996;39:469 – 473. 33. Yamada T, Sato A, Nishimori T, Mitsuhashi T, Terao A, Sagai H, Komatsu M, Aizawa T, Hashizume K. Importance of hypercoagulability over hyperglycemia for vascular complication in type 2 diabetes. Diabetes Res Clin Pract 2000;49:23–31.
A SYMPOSIUM: MANAGING THE ATHEROGENIC POTENTIAL OF THE POSTPRANDIAL STATE
25H
MD
Hyperglycemia is the cause of chronic microvascular complications in diabetes. Increased macrovascular disease occurring in diabetes is multifactorial, with hyperglycemia being only 1 of the factors responsible. A major deficiency in our understanding of the role of hyperglycemia in the pathogenesis of chronic diabetic complications is the contribution made by exaggerated postprandial glycemic excursions, in contrast to that made by chronic fasting hyperglycemia. Hyperglycemia is thought to cause chronic diabetic complications through >1 of the following biochemical mechanisms: (1) an exaggerated polyol pathway, (2) protein glyca-
tion and the formation of advanced glycation end products, (3) excessive activation of protein kinase C (PKC) isoenzymes in vascular and mesangial cells, and (4) greater oxidative stress. The focus of this article is how postprandial hyperglycemia and glucose excursions might specifically activate >1 of these mechanisms and how they might contribute to the development of the chronic complications of diabetes by causing endothelial dysfunction, a procoagulant state, carbonyl stress, and/or vascular and oxidative effects secondary to PKC activation. 䊚2001 by Excerpta Medica, Inc. Am J Cardiol 2001;88(suppl):20H–25H
ther articles in this symposium have focused on whether the postprandial excursions of plasma O glucose and plasma triglycerides are independent risk
membrane and circulating proteins is increased. Cells in which glucose is freely permeable (eg, nerve, retinal, and glomerulus cells) are exposed to very high intracellular concentrations, which can lead to increased activation of the aldose reductase pathway, or excessive activation of the protein kinase C (PKC) or other pathways, and cause injury to the cell. Insulin-sensitive cells, such as those in muscle and adipose tissue, can protect themselves by becoming insulin resistant and decreasing their flux of glucose. Inappropriate regulation of essential nutrients can lead to many diverse pathophysiologic processes. The postprandial state exaggerates the detrimental effects of impaired nutrient regulation because the flux of nutrients is the greatest and concentrations are the highest at this time.
factors for the development of macrovascular disease. The clinical data suggest that increases in postprandial plasma glucose levels within the normal or impaired glucose tolerance ranges correlate with an increase in coronary artery disease events and/or death.1,2 Microvascular disease, on the other hand, occurs only with postprandial plasma glucose levels within the diabetic range.3 The mechanisms by which postprandial elevations of glucose, triglycerides, and other nutrients can contribute to macrovascular and/or microvascular disease are the subject of this review. The postprandial state is a highly integrated physiologic process, the primary function of which is to deliver nutrients to the tissues at concentrations that are adequate, while not causing detrimental effects. Gastrointestinal and endocrine regulatory systems operate together to achieve this goal. The rate of emptying of the stomach and the rate of digestive processes control the flux of nutrients into the extracellular compartment. The uptake of nutrients by the liver and peripheral tissues determines the flux of nutrients from the extracellular compartment. The balance between these processes determines the extracellular concentrations of the nutrients. Renal excretion serves as a safety valve if concentrations become very excessive. The extracellular concentration of a nutrient can alter intracellular function by modifying cellular function at the level of the plasma membrane or by increasing the flux and concentration of the nutrient inside the cell. Hyperglycemia is an excellent example of such processes. When the extracellular concentration of glucose is excessive, nonenzymatic glycosylation of plasma From the State University of New York Health Sciences Center, Brooklyn, New York, USA. Address for reprints: Harold E. Lebovitz, MD, State University of New York Health Sciences Center, 450 Clarkson Avenue, Box 50, Brooklyn, New York 11203. E-mail: [email protected].
20H
©2001 by Excerpta Medica, Inc. All rights reserved.
MECHANISMS OF HYPERGLYCEMIC DAMAGE The mechanisms by which hyperglycemia can cause the tissue damage that ultimately leads to vascular disease have not been absolutely defined. However, there is considerable evidence to support the hypothesis that several different mechanisms are operative4 and that they may act in concert (Table 1). Mechanisms related to increases in intracellular glucose concentrations: In tissues that are insulin indepen-
dent, an increase in the plasma glucose level is followed by a comparable increase in intracellular glucose. Tissues that have high concentrations of aldose reductase enzymes5–7 will convert a significant quan-
TABLE 1 Proposed Mechanisms for Hyperglycemia-mediated Tissue Damage ● ● ● ●
Activation of the polyol pathway Excessive glycation of proteins and lipids Activation of the diacylglycerol-PKC second-messenger system Increase in “oxidative stress” PKC ⫽ protein kinase C.
0002-9149/01/$ – see front matter PII S0002-9149(01)01833-1
FIGURE 1. The polyol pathway. Glucose is reduced by aldose reductase to sorbitol by oxidation of nicotinamide adenine dinucleotide phosphate (NADPH to NADP). Sorbitol is then oxidized to fructose by reduction of nicotinamide adenine dinucleotide (NAD to NADH). The aldose reductase reaction predominates, and during hyperglycemia, sorbitol and NADP accumulate within the cell.
FIGURE 2. Hyperglycemia: aldose reductase activity. Exaggerated polyol pathway activity causes a decrease in myoinositol and sodium/ potassium (Na/K)–adenosine-triphosphatase (ATPase) activity and an alteration in inositol phospholipid metabolism, which increases protein kinase C (PKC) activity. Additionally, the increase in intracellular sorbitol causes an increase in intracellular osmolality, and the increase in nicotinamide adenine dinucleotide phosphate (NADP) is associated with an increase in oxidative stress. NADPH ⴝ reduced NADP.
tity of that glucose into sorbitol (Figure 1). The sorbitol is slowly metabolized to fructose. In the presence of high glucose concentrations, sorbitol is generated more rapidly than it is metabolized to fructose, leading to an increase in intracellular sorbitol. The consequences of the increase in intracellular sorbitol are an increase in intracellular osmolality and a reduction in intracellular myoinositol content. Inositol phospholipid metabolism and membrane function are altered, leading to additional changes such as decreased Na⫹/ K⫹-adenosine-triphosphatase activity and increased PKC activity (Figure 2). This polyol pathway is thought to play a major role in the development of diabetic cataracts and diabetic neuropathy.6,7 Another, and perhaps even more important pathway by which increases in intracellular glucose can cause detrimental changes in cells, is through excessive activation of PKC.8 –10 PKC is a serine and threonine kinase that phosphorylates those residues in protein substrates. In so doing, it changes the function of many intracellular proteins.9,10 In the PKC family, there are at least 12 isoenzymes with both tissue and functional specificity.9,10 The PKCs are ordinarily activated by calcium ions and phospholipid metabolites (Figure 3) and are part of a
second messenger–signaling pathway that relies on diacylglycerol as an activator. Hyperglycemia can cause an activation of PKC through the de novo pathway of diacylglycerol synthesis, as shown in Figure 3,8 bypassing the normal regulatory system and causing disturbances in many functions of the cell, particularly in the case of vascular and mesangial cells.11–13 Vascular cells contain the specific PKC isoenzyme PKC 2. The consequences of excessive activation of this isoenzyme in vascular cells are depicted in Figure 4. A specific inhibitor of this PKC isoenzyme8,9 has been developed (LY333531), and it has been shown, in vitro and in vivo, to reverse many of the abnormalities described in Figure 4.14 It is currently being evaluated in clinical trials. Mechanisms related to extracellular hyperglycemia:
Glucose is a highly reactive molecule that contains an aldehyde group. When an aldehyde group comes into close proximity with an amino group, it forms a complex known as a Schiff base (Figure 5). The Schiff base undergoes a rearrangement and forms a stable complex known as an Amadori product, which is essentially a glycosylated protein. Glycosylated proteins with a slow turnover rate have a tendency to undergo dehydrations, oxidations, and rearrangements
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FIGURE 3. Mechanisms of protein kinase C (PKC) activation in diabetes mellitus. PKC enzyme activation is normally regulated by an agonist stimulating the production of phosphoinositides and phosphatidic acid and generates 1,2-diacylglycerol (DAG), which activates PKC. In the presence of hyperglycemia, DAG is synthesized by the de novo pathway from glycolysis-generated triose phosphates, dihydroxyacetone phosphate (DHAP), and glycerol 3-phosphate (G3P). This is a mechanism by which hyperglycemia can directly increase PKC activation. Acyl-CoA ⴝ acyl-coenzyme A; CoA ⴝ coenzyme A; FDP ⴝ fructose 1,6-diphosphate; F6P ⴝ fructose 6-phosphate; GAP ⴝ glyceraldehyde 3-phosphate; G6P ⴝ glucose 6-phosphate; LysoPA ⴝ lysophosphatidic acid.
FIGURE 4. Cellular consequences of 1,2-diacylglycerol (DAG)-protein kinase C (PKC) activation induced by hyperglycemia. An increase in PKC 2 by hyperglycemia-generated DAG causes many changes in the metabolism of endothelial, vascular, and mesangial cells. Some of these effects and the known mechanisms by which they are mediated are shown. ANP ⴝ atrial naturetic peptide; C-fos ⴝ transcription factor; eNOS ⴝ endothelial nitric oxide synthase; ET-1 ⴝ endothelin-1; ICAMs ⴝ intercellular adhesion molecules; LY333531 ⴝ specific inhibitor for PKC  isoforms; Naⴙ-Kⴙ-ATPase ⴝ sodium-potassium–adenosine triphosphate; PAI-1 ⴝ plasminogen activator inhibitor 1; PGE2 ⴝ prostaglandin E2; PLA2 ⴝ phospholipase A2; TGF- ⴝ transforming growth factor–; VEGF ⴝ vascular endothelial growth factor. (Reproduced with permission from Vasc Med.14)
to form large polymers15,16 known as advanced glycation end products (AGEs). Glycation products are being continuously formed and removed by AGE receptors ([RAGE] and scavenger receptor types I and 22H THE AMERICAN JOURNAL OF CARDIOLOGY姞
II) in all persons.15,16 Glycosylation product formation is a function of the ambient glucose concentrations. The bulk of the in vivo advanced glycation products involve circulating, basement membrane, or matrix
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FIGURE 5. Nonenzymatic glycation: pathway of oxidative production of advanced glycation end products (see text for further description). K ⴝ potassium; NH ⴝ sodium-hydrogen.
FIGURE 6. Biologic effects of advanced glycation end products (AGEs). The effects are the consequence of binding of the AGEs to receptors for AGEs (RAGEs) on the specific tissues. EGF ⴝ epidermal growth factor; EGF-R ⴝ epidermal growth factor receptor.
proteins. Intracellular and intranuclear protein glycosylation can occur. Receptors for AGEs are present on monocytes, macrophages, endothelial cells, pericytes, podocytes, and fibroblasts as well as on mesangial cells, astrocytes, and microglia.17 Cell activation in response to AGE-modified proteins is associated with increased expression of extracellular matrix proteins, vascular adhesion molecules, cytokines, and growth factors. These result in chemotaxis, angiogenesis, oxidative stress, cell proliferation, or programmed cell death, depending on the tissue and other factors influencing the cell’s function (Figure 6). Initial studies of the interactions of glucose with proteins focused on oxidative formation of AGEs derived from Amadori products. More recent studies have described nonoxidative pathways in which molecules derived from the glycolysis of glucose (methylglyoxal and 3-deoxyglucosone) can interact with arginine and lysine residues and generate imidazolone and imidazolium AGEs.18,19 Thus, AGEs can be derived from both oxidative and nonoxidative pathways. Oxidative stress: A popular theory to explain the mechanism by which hyperglycemia ultimately causes diabetic complications is the formation of reactive oxygen intermediates and the presence of elevated oxidative stress.20 –22 This theory is appealing because
it can account for the increase in glycoxidation and lipoxidation products in plasma and tissue proteins. Intracellular oxidative stress leads to activation of the redox-sensitive transcription factor, nuclear factor-B, and tissue factor expression.23 An exaggerated polyol pathway and interaction of AGEs with the RAGE receptor can be viewed as processes that generate oxidative stress.20,23 Baynes and Thorpe22 reviewed the evidence for the role of oxidative stress in diabetic complications, and they concluded that the interaction of glucose with proteins is both oxidative and nonoxidative. They suggested that inadequate detoxification of these modified AGE proteins, rather than excessive oxidation, leads to the increase in tissue levels. They proposed that treatments to reduce the increased carbonyl stress of hyperglycemia are more important in preventing diabetic complications than attempts to reduce oxidative stress.
SPECIFIC PATHOPHYSIOLOGIC ABNORMALITIES ASSOCIATED WITH POSTPRANDIAL HYPERGLYCEMIA
Several responses to postprandial hyperglycemia have been described that may have a significant relation to the development of the chronic complications of type 2 diabetes. These include acute hyperglycemia–induced changes in endothelial function, in co-
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TABLE 2 Functions of the Endothelium ● Regulates vascular tone —Nitric oxide: vasodilation —Endothelin: vasoconstriction ● Controls matrix protein synthesis ● Stimulates cell growth and migration ● Controls permeability ● Regulates thrombogenesis —Increases plasminogen activator inhibitor–1 synthesis —Modulates platelet adhesion ● Modulates inflammatory responses —Cytokine production —Adhesion molecule synthesis
agulation, and in the formation of nonoxidative glycation products. Endothelial function: The endothelium influences multiple functions in the vascular system (Table 2), including (1) regulating vascular tone, (2) controlling cell growth, (3) regulating thrombogenesis, (4) modulating inflammatory responses, (5) determining matrix protein synthesis, and (6) controlling permeability. In diabetes, endothelial function is abnormal and its regulatory functions are disturbed. These abnormalities are thought to be major factors in the development of both the microvascular and macrovascular complications of diabetes. Metabolic abnormalities in type 2 diabetes that cause endothelial dysfunction are (1) insulin resistance, (2) hypertriglyceridemia, (3) increased levels of low-density lipoprotein cholesterol, (4) hypertension, and (5) hyperglycemia. Several recent studies indicate that acute hyperglycemia, such as occurs in postprandial hyperglycemia, causes endothelial dysfunction.24 Kawano et al25 examined flow-mediated endothelium-dependent vasodilation with ultrasound techniques during oral glucose tolerance testing in 17 nondiabetic subjects, 24 subjects with impaired glucose tolerance, and 24 subjects with type 2 diabetes.25 Flow-mediated vasodilation at baseline was decreased in type 2 diabetes patients. All groups had a striking decrease in flowmediated vasodilation 1 and 2 hours after the administration of glucose. They also measured the formation of oxidation products during the oral glucose tolerance test and found them to be elevated 1 and 2 hours after the glucose administration. They concluded that acute hyperglycemia suppresses endothelium-dependent vasodilation, probably through increased production of oxygen-derived free radicals. In a similar study, Akbari et al26 measured endothelium-dependent vasodilation in the brachial artery (macrocirculation) and erythrocyte flux after acetylcholine iontophoresis (microcirculation) in 20 healthy subjects during fasting conditions and 1 hour after ingestion of 75 g of glucose. They found that the ingestion of glucose impaired the endothelium-dependent vasodilation of both the microcirculation and macrocirculation. Acute hyperglycemia in nondiabetic individuals has been shown to reduce methacholine-mediated vasodilation, and the effect was increased if the secretion of insulin was simultaneously inhibited.27 Insulin increases endothelium-dependent vasodilation by increasing nitric oxide production. Hypergly24H THE AMERICAN JOURNAL OF CARDIOLOGY姞
cemia decreases endothelium-dependent function by suppressing nitric oxide production.28 In healthy individuals, the hyperglycemia after glucose administration appears to predominate over the increased insulin secretion in impairing endothelium-dependent vasodilation. The addition of insulin resistance and relative insulin deficiency to the metabolic environment makes the effect of hyperglycemia more severe. It is not clear which other aspects of endothelial cell function are altered by acute hyperglycemia in vivo. The promoter region of the plasminogen activator inhibitor–1 (PAI-1) gene has a region just upstream from the transcriptional start site, which can be activated by hyperglycemia.29 Whether acute hyperglycemia in vivo increases plasma PAI-1 levels is difficult to determine because of the concomitant effects of insulin, proinsulin, and triglycerides. For example, a recent investigation showed that intravenous administration of glucose plus intralipid for 6 hours in healthy subjects doubled plasma PAI-1 levels for several hours. In that study, euglycemic hyperinsulinemia had no effect on PAI-1 levels.30 Postprandial coagulation effects: Several investigations have shown that postprandial hyperglycemia is accompanied by a series of alterations of the coagulation system.31–33 An oral glucose load in both healthy and type 2 diabetes patients causes a shortening in the half-life of fibrinogen and an increase in plasma fibrinopeptide A and the fragments of prothrombin and factor VII. In patients with diabetes, meal-mediated hyperglycemia also causes hyperproduction of thrombin proportional to the blood glucose increase.31 Amelioration of the meal-mediated hyperglycemia by the ␣-glucosidase inhibitor, acarbose, reduces the coagulation activation.32 Other studies have reported that acute hyperglycemia in healthy subjects increases platelet aggregation to adenosine diphosphate and blood viscosity.33 These effects can be prevented by increasing nitric oxide production through a simultaneous L-arginine infusion; they can be reproduced by blocking endogenous nitric oxide production by infusion of the inhibitor NG-monomethyl-L-arginine. The conclusion drawn from these various studies is that postprandial hyperglycemia can promote coagulation activation. Postprandial hyperglycemia and AGE formation: As discussed in the preceding section, the activated carbonyl compounds, methylglyoxal and 3-deoxyglucosone (␣-oxoaldehydes), are generated from triose intermediates during glycolysis. These ␣-oxoaldehydes are early glycation products that respond to fluctuations in plasma glucose and are important precursors of advanced glycation addicts.18,19 Because short periods of hyperglycemia (such as those that occur in postprandial hyperglycemia) may be sufficient to increase the concentrations of ␣-oxoaldehydes in vivo, this is a mechanism whereby glycemic excursions can play a significant role in the development of chronic diabetic complications.18,19 Postprandial hyperglycemia activation of PKC and oxidative stress: Fragmentary studies in humans point
to the possibility that fluctuations in the plasma glucose levels of healthy humans as well as patients with diabetes may directly activate PKC in some tissues
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and thus directly contribute to oxidative stress.13 Healthy controls and type 2 diabetes patients infused with glucose and insulin had a significant increase in thrombocyte PKC 2 when the isoenzyme was measured at 60 minutes and 150 minutes postinfusion. Several studies have measured the oxidation-reduction state of healthy subjects and subjects with type 2 diabetes in the fasting state and after meal ingestion. After the meal, plasma malondialdehyde and vitamin C levels increased, whereas protein sulfhydryl groups, uric acid, vitamin E, and the total plasma radical-trapping parameter decreased more significantly in the subjects with diabetes than in the control subjects.21 Ceriello et al32 concluded from their studies that hyperglycemia could play an important role in the generation of postprandial oxidative stress in diabetic patients.
CONCLUSION
Hyperglycemia is a major factor in the development of microvascular disease in the diabetic person. There is controversy about the level of glycemia that confers cardiovascular risk and the extent of the role of hyperglycemia relative to the other components of the insulin resistance syndrome in the pathogenesis of the macrovascular complications. The relative importance of postprandial versus fasting hyperglycemia in the pathogenesis of the chronic complications remains an unanswered question. Until recently, the predominant concept was that fasting hyperglycemia is the primary factor in the generation of glycosylated proteins and the main cause of the chronic vascular disease associated with diabetes. Newer information, however, relates postprandial hyperglycemia to the development of coronary artery disease. Data indicate that changes in glycosylated hemoglobin correlate better with postprandial than with fasting plasma glucose. Good glycemic control is not achievable unless treatment addresses postprandial glycemic goals. The data reviewed here provide a basis for understanding why exaggerated glycemic excursions may have detrimental effects. Changes in postprandial nutrient and hormone levels can (1) exaggerate the polyol pathway, (2) activate PKC, (3) elevate methylglyoxal and 3-deoxyglucosone, and (4) cause increased oxidative stress. The consequences of these biochemical effects are changes in endothelial, mesangial, pericyte, smooth muscle, and macrophage cell functions in such a manner as to cause microvascular and macrovascular disease. The recognition of these consequences of postprandial hyperglycemia and hyperlipidemia demands that we change our treatment strategies to focus on postprandial as well as fasting metabolic control. The availability of newer pharmacologic agents designed specifically to target postprandial control will allow us to do this. The consequence of this refocused treatment approach should be a great reduction in diabetic vascular complications.
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