Patients with hyperinsulinemia, defined by increased concentrations of IRI in plasma, experience increased cardiovascular mortality. In type II diabetic patients, the increase in IRI may reflect, in part, not only insulin but also proinsulinemia as a result of impaired conversion of proinsulin to insulin by pancreatic beta-cells. High IRI is accompanied by attenuation of endogenous fibrinolytic activity and increased plasma PAI-1, the primary physiological inhibitor of t-PA. Concordant increases of plasma PAI-1 and plasma IRI appear to reflect direct effects of insulin and proinsulin on the synthesis and secretion of PAI-1 by endothelial and liver cells as judged from results of studies in vitro. Because attenuated fibrinolysis may predispose to thrombosis, the increased exposure of luminal surfaces of vessels to atherogenic, clot-associated mitogens and chemoattractants may activate macrophages and potentiate proliferation of vascular smooth muscle cells. Accordingly, increased concentrations of plasma IRI may contribute to macrovascular disease in diabetic patients by impairing endogenous fibrinolysis.

Immunoisolation is a potentially important approach to transplanting islets without need for immunosuppressive drugs. Immunoisolation systems have been conceived in which the transplanted tissue is separated from the immune system of the host by an artificial barrier. These systems offer a solution to the problem of human islet procurement by permitting use of islets isolated from animal pancreases. The devices used are referred to as biohybrid artificial organs because they combine synthetic, selectively permeable membranes that block immune rejection with living transplants. Three major types of biohybrid pancreas devices have been studied. These include devices anastomosed to the vascular system as AV shunts, diffusion chambers, and microcapsules. Results in diabetic rodents and dogs indicate that biohybrid pancreas devices significantly improve glucose homeostasis and can function for more than a year. Recent progress made with this approach is discussed, and some of the remaining problems that must be resolved to bring this technology to clinical reality are addressed.

Glucokinase, the major enzyme that phosphorylates glucose upon entry into liver and islet beta-cells, has been considered a prime candidate for inherited defects predisposing to NIDDM. Now that the human gene has been isolated, this question has been addressed directly. Polymorphic markers flanking the gene were identified. These markers (microsatellites) are composed of variable numbers of dinucleotide repeats that vary in size, resulting in different alleles. Variably sized alleles can be typed rapidly from genomic DNA of individuals by the PCR. Studies of inheritance of glucokinase genes have revealed significant linkage in families with early-onset NIDDM, or MODY, and mutations have been identified within the coding region of the gene in some families. These studies are extremely encouraging, as they indicate that genes can be identified even in this heterogeneous genetic disorder. This study considers the phenotypes that result from glucokinase defects and the relationship of MODY to NIDDM, and it estimates the role of glucokinase defects in NIDDM in general.

Insulin resistance contributes to the pathogenesis of NIDDM. We have investigated the molecular mechanisms of insulin resistance in patients with genetic syndromes caused by mutations in the insulin-receptor gene. In general, patients with two mutant alleles of the insulin-receptor gene are more severely insulin-resistant than are patients who are heterozygous for a single mutant allele. These mutations can be put into five classes, depending upon the mechanisms by which they impair receptor function. Some mutations lead to a decrease in the number of insulin receptors on the cell surface. For example, some mutations decrease the level of insulin receptor mRNA or impair receptor biosynthesis by introducing a premature chain termination codon (class 1). Class 2 mutations impair the transport of receptors through the endoplasmic reticulum and Golgi apparatus to the plasma membrane. Mutations that accelerate the rate of receptor degradation (class 5) also decrease the number of receptors on the cell surface. Other mutations cause insulin resistance by impairing receptor function--either by decreasing the affinity to bind insulin (class 3) or by impairing receptor tyrosine kinase activity (class 4). The prevalence of mutations in the insulin receptor gene is not known. However, theoretical calculations suggest that approximately 0.1-1% of the general population are heterozygous for a mutation in the insulin-receptor gene; the prevalence is likely to be higher among people with NIDDM. Accordingly, it is likely that mutations in the insulin-receptor gene may be a contributory cause of insulin resistance in a subpopulation with NIDDM.

Plasma lipid levels are elevated in people with diabetes, and a direct relationship can be demonstrated between indices of diabetic control and plasma lipid levels. Many observations suggest that diabetes may be associated with enhanced cytokine production, raising the possibility that some of the metabolic abnormalities associated with diabetes may be due to or exacerbated by cytokine overproduction. Tumor necrosis factor induces a rapid increase in serum triglyceride levels caused by an increase in VLDL of normal composition. Although in vitro studies showed that TNF decreases adipose tissue lipoprotein lipase activity, recent studies with intact animals demonstrated that TNF increases serum triglyceride levels by stimulating hepatic lipid secretion, not by affecting clearance. The increase in hepatic VLDL triglyceride secretion induced by TNF is due to both the stimulation of hepatic de novo fatty acid synthesis and an increase in lipolysis. Other cytokines including IL-1, IL-6, and alpha-interferon increase hepatic de novo fatty acid synthesis. Similarly, cytokines such as IL-1 and alpha-, beta-, and gamma-interferon also increase lipolysis. Thus, a variety of cytokines acting at different receptors can affect multiple processes that can alter lipid metabolism and increase serum lipid levels. These cytokine-induced increases in serum lipoprotein levels may be a beneficial response for the host. Studies show that lipoproteins, including VLDL, bind endotoxin and can protect against the toxic effects of endotoxin. Moreover, lipoproteins bind a variety of viruses, reducing their infectivity. Lipoproteins also bind urate crystals, which reduces the inflammatory response induced by these crystals.(ABSTRACT TRUNCATED AT 250 WORDS)

It was recently proposed that the increased levels of modified lipoproteins in diabetic patients may be responsible for the accelerated development of macrovascular complications associated with the disease. Modified lipoproteins are believed to induce the transformation of macrophages into foam cells and, in some cases, to induce endothelial cell damage. In addition, modified lipoproteins trigger an immune response leading to the formation of antibodies and then to the formation of LDL-containing immune complexes. In this review, we summarize the evidence linking LDL glycation and oxidation with intracellular accumulation of cholesterol esters and foam-cell formation, and we discuss their potential for inducing an autoimmune response and the formation of lipoprotein-containing immune complexes. The formation of LDL-ICs seems particularly significant, because these ICs are avidly taken up by macrophages through their Fc receptors and induce not only massive intracellular accumulation of CE but also a paradoxical increase in LDL-receptor expression. Our experimental data suggest that the uptake of LDL-IC is facilitated by RBC adsorption, in agreement with the role of RBC in the adsorption of circulating IC and their delivery to phagocytic cells. In addition, macrophages are activated when ingesting LDL-IC and release IL-1 beta and TNF-alpha, which can contribute to the initiation and progression of an atheromatous lesion by several mechanisms. Although it is difficult to envisage how LDL-IC could initiate an endothelial lesion, it is easy to speculate about their role as cofactors in the initiation and progression of the atherosclerotic process.

Because the accumulation of lipid in macrophages is a characteristic feature of atherosclerosis, the mechanisms by which this lipid accumulation occurs have been intensively studied. This paper reviews the receptor- and non-receptor-mediated pathways that promote lipid accumulation in macrophages. Particular emphasis is placed on the contributions of two secretory products of macrophages, lipoprotein lipase and apolipoprotein E, to both receptor- and non-receptor-mediated uptake of triglyceride-rich lipoproteins by macrophages. The hormonal, lipid, and immunological factors that regulate the secretion of LpL and apoE by macrophages are discussed, as are how changes in the secretion of apoE and LpL that can modulate the uptake of triglyceride-rich lipoproteins by macrophages might influence the atherosclerotic process in people with diabetes.

Studies from several laboratories suggest that oxidized LDL may play an important role in atherogenesis. Our group previously showed that treatment of aortic endothelial cells with low levels of MM-LDL caused increased expression of MCP-1, M-CSF, tissue factor, and a monocyte-binding protein. In these studies MM-LDL was produced by storage of native LDL. We now show that cocultures of endothelial and smooth muscle cells can also produce MM-LDL from native LDL. This production of MM-LDL by cells is prevented by preincubating the LDL with probucol or vitamin E. However, addition of antioxidants to MM-LDL did not block its action. In past studies we also showed that endothelial cells exhibit differential sensitivity to the effects of MM-LDL. We report herein that in resistant cells there is no elevation of catalase, glutathione peroxidase, or copper-zinc-dependent SOD. However, manganese-dependent SOD is elevated in resistant cells. Ways in which MM-LDL production may be elevated in poorly controlled diabetics subjects are discussed.

In people with diabetes, glycation of apolipoproteins correlates with other indices of recent glycemic control, including HbA1. For several reasons, increased glycation of apolipoproteins may play a role in the accelerated development of atherosclerosis in diabetic patients. Recognition of glycated LDL by the classical LDL receptor is impaired, whereas its uptake by human monocyte-macrophages is enhanced. These alterations may contribute to hyperlipidemia and accelerated foam-cell formation, respectively. Glycation of LDL also enhances its capacity to stimulate platelet aggregation. The uptake of VLDL from diabetic patients by human monocyte-macrophages is enhanced. This enhancement may be due, at least in part, to increased glycation of its lipoproteins. Glycation of HDL impairs its recognition by cells and reduces its effectiveness in reverse cholesterol transport. Glycation of apolipoproteins may also generate free radicals, increasing oxidative damage to the apolipoproteins themselves, the lipids in the particle core, and any neighboring macromolecules. This effect may be most significant in extravasated lipoproteins. In these, increased glycation promotes covalent binding to vascular structural proteins, and oxidative reactions may cause direct damage to the vessel wall. Glycoxidation, or browning, of sequestered lipoproteins may further enhance their atherogenicity. Finally, glycated or glycoxidized lipoproteins may be immunogenic, and lipoprotein-immune complexes are potent stimulators of foam-cell formation.

There is ample evidence that oxidized lipoproteins exist in vivo, not only in atherosclerotic lesions, but also associated with some experimental models of diabetes. Whether the lipoprotein oxidation is an epiphenomenon of other atherogenic or diabetogenic agents or processes or whether it is causally related to lesion formation in atherosclerosis or other forms of tissue damage in people with diabetes is unresolved. Intense interest in testing these ideas derives from in vitro observations of the ways in which oxidized lipoproteins interact with cells that are unlike the interactions with native lipoproteins. Many of these altered interactions suggest known features of atherosclerotic lesions, and recent data show that antioxidant treatment reduces the progression of vascular lesions. There are reasons to believe that hyperglycemia may worsen lipid and lipoprotein oxidation. If this observation is the case in vivo, and if it is ultimately proved that lipoprotein oxidation facilitates lesion development, these events may help explain the accelerated atherosclerosis suffered by diabetic patients. The multiple pathways for which there is evidence that hyperglycemia may contribute to oxidative events--for example, by enhancing free radical production in stimulated inflammatory cells or by forming glycation products that can propagate free radical events--suggest avenues for further research and may ultimately indicate points for intervention in the various manifestations of the disease.

Diabetes increases the risk of developing atherosclerotic arterial disease significantly. Although elevated glycohemoglobin was shown to be an independent risk factor in older women in the Framingham Heart Study, the relationship between hyperglycemia and macrovascular disease is complicated by the many other factors that influence atherogenesis in nondiabetic people. Studies in vitro suggest that chronic hyperglycemia may accelerate the atherogenic process through excessive glycation of various components of the arterial wall. These data are reviewed critically, and the biochemistry and pharmacological potential of the glycation-inhibitor aminoguanidine is discussed.

Laminin and type IV collagen are two major basement membrane glycoproteins; they are large multidomain macromolecules that are involved in two types of functions. First, they provide the structural framework of all basement membranes, and second, they interact with cell-surface molecules and are key to adhesion, spreading, and proliferation of cells. We summarize experimental evidence that nonenzymatic glucosylation of these two macromolecules in vitro alters their structure, their ability to polymerize, and their ability to promote cell adhesion. Additional studies are needed to document these changes in situ and therefore extend these conclusions to intact basement membranes.

Recent progress in structure elucidation of products of the advanced Maillard reaction now allows probing specifically for the role of this reaction in the pathogenesis of age- and diabetes-related complications. Pyrraline is a glucose-derived advanced glycation end product against which polyclonal and monoclonal antibodies have been raised. Immunohistochemical localization studies revealed that pyrraline is found predominantly in the sclerosed extracellular matrix of glomerular and arteriolar renal tissues from both diabetic and aged nondiabetic individuals. Pentosidine and carboxymethyllysine are Maillard end products derived from both glucose and ascorbate. In addition, pentosidine can be formed from several other sugars under oxidative conditions, and in vitro studies suggest that a common intermediate involving a pentose is a necessary precursor molecule. The highest levels of these advanced Maillard products are generally found in the extracellular matrix, but these products are also present in lens proteins and in proteins with a fast turnover such as plasma proteins. Diabetes, and especially uremia, greatly catalyzes pentosidine formation. Both conditions are characterized by accelerated cataractogenesis, atherosclerosis, and neuropathy, suggesting that molecular damage by advanced Maillard reaction products may be a common mechanism in their development.

Although platelets can contribute to atherosclerosis and its thromboembolic complications in the nondiabetic population, the role of platelets in enhanced vascular disease in the diabetic population remains unclear. Most studies indicate that platelet function in vitro is enhanced in platelets from people and animals with diabetes, and the mechanisms are being identified. There remains some controversy about whether platelet changes occur before, and therefore could contribute to, vascular complications or whether they are secondary to vascular disease. It is possible that only intervention trials to determine if inhibiting platelet function limits the progression of vascular disease in diabetic patients will definitively answer this question. The earlier premise that enhanced activity of the arachidonate pathway is responsible for the hypersensitivity of platelets from diabetic humans needs to be modified to recognize that additional mechanisms are involved in platelet activation and are modified in people with diabetes and also that altered activity of the arachidonate pathway may reflect changes in earlier pathways involved in platelet activation. Clearly, alterations in these nonarachidonate pathways need to be taken into account when considering the appropriate antiplatelet agents to use in intervention trials. Information about whether hypersensitivity of platelets from people with diabetes persists in vivo and, if so, how this influences platelet-vessel wall interactions and thrombotic tendencies needs to be pursued more intensely in suitable animal models so that the theories developed from studies in vitro can be tested in the more complex environment in vivo. These are important areas for research in the future.

In people with diabetes, the concentration of an individual lipoprotein or apolipoprotein can be highly variable and is totally different in the two major forms of the disease. Alterations in the concentrations of major lipids and lipoproteins are well characterized in both IDDM and NIDDM. In general, the lipoprotein pattern is antiatherogenic in individuals with IDDM who are treated and have optimal glycemic control. In contrast, NIDDM is associated with atherogenic changes of serum lipids and lipoproteins regardless of the mode of treatment. In people with both types of diabetes, the distribution of apoE phenotype seems to be similar to that in nondiabetic populations. IDDM patients with microalbuminuria show atherogenic changes of lipoproteins and have elevated levels of Lp(a), which is a risk factor of coronary artery disease. Whether glycemic control influences the concentration of Lp(a) is still an open question. An important issue is that the concentration of a lipoprotein can be normal without excluding compositional abnormalities that are potentially atherogenic. Such alterations are present in people with both IDDM and NIDDM. Consequently, it has been questioned whether the target values to start treatment should be lower in diabetic than in nondiabetic populations.

Patients with diabetes mellitus are at increased risk of morbidity and mortality from macrovascular disease manifesting as coronary heart disease, cerebrovascular accidents, and peripheral vascular disease. Increased frequency of dyslipidemia, hyperglycemia, obesity, hypertension, and associated nephropathy may contribute to accelerated atherogenesis in diabetic patients. Therefore, besides intensive control of hyperglycemia, management of dyslipidemia, hypertension, and obesity should also be emphasized in diabetic patients. Those who smoke should be strongly encouraged to quit smoking. Besides attempts to achieve normal levels of plasma lipoproteins, consideration also should be given to normalization of compositional abnormalities of various lipoproteins in patients with diabetes mellitus. The therapeutic goals for cholesterol reduction should be lower in diabetic patients than nondiabetic subjects. The first step is to achieve good metabolic control of diabetes mellitus by diet, exercise, and weight reduction and, if needed, with sulfonylureas or insulin therapy. Because most of the patients with insulin-dependent diabetes mellitus achieve normal levels of plasma lipoproteins with intensive insulin therapy, lipid-lowering medications are rarely needed. In patients with non-insulin-dependent diabetes mellitus, however, dyslipidemia often persists despite good glycemic control. Lipid-lowering medications should be considered in such patients. Because nicotinic acid can cause marked deterioration in glycemic control, and bile acid-binding resins may accentuate hypertriglyceridemia, these agents are less desirable for use by diabetic patients. Inhibitors of hydroxymethylglutaryl coenzyme A reductase may be preferred in patients with elevated LDL cholesterol and mld hypertriglyceridemia. For diabetic patients with marked hypertriglyceridemia, however, fibric acid derivatives should be the drug of choice.

Abnormal lipoprotein metabolism contributes to the increased risk of premature atherosclerosis in people with insulin-dependent (type I) diabetes. Although hypertriglyceridemia is common in those with untreated IDDM, treatment with conventional insulin therapy usually restores fasting lipoprotein profiles to nondiabetic levels. Intensive insulin therapy improves glycemic control and lipoprotein concentrations, but does not ameliorate the changes in lipoprotein composition described in people with IDDM. Some of these persistent changes in lipoprotein composition have been attributed to peripheral hyperinsulinemia associated with s.c. insulin therapy. The recent availability of implantable insulin-infusion pumps for treatment of IDDM has allowed the study of the effect of i.p. insulin delivery on lipoprotein metabolism. i.p. insulin therapy is capable of maintaining near normal plasma glucose levels while reducing the peripheral hyperinsulinemia. Although results have been contradictory, studies of i.p. insulin therapy may eventually help to determine whether some of the observed changes in lipoprotein metabolism and composition in people with IDDM are due to the peripheral hyperinsulinemia associated with s.c. insulin therapy.

A review of the putative risk factors associated with the development of coronary heart disease in diabetes is presented. Emphasis is given to the effect of nephropathy (persistent proteinuria) and hypertension on cardiovascular mortality in IDDM. Risk factors associated with CHD in NIDDM are also reviewed. Finally, possible reasons to explain the increased incidence of CHD associated with proteinuria in IDDM patients, including lipoprotein abnormalities, increased fibrinogen levels, increased platelet adhesiveness, and altered hemostatic variables, are discussed.

Herein, we review the applicability to human beta-cells of an electrophysiologically based hypothesis of the coupling of glucose metabolism to insulin secretion. According to this hypothesis, glucose metabolism leads to the generation of intracellular intermediates (including ATP), which leads to closure of ATP-sensitive K+ channels. Channel closure results in membrane depolarization, the onset of electrical activity, and voltage-dependent Ca2+ entry. The resultant rise in cytosolic Ca2+ leads to Ca(2+)-dependent exocytosis of insulin granules. We found that most of the published experimental evidence for human beta-cells supports this hypothesis. In addition, we present three other emerging lines of evidence in support of this hypothesis for human islet beta-cells: 1) the effects of pHi-altering maneuvers on insulin secretion and electrical activity; 2) preliminary identification of LVA and HVA single Ca2+ channel currents; and 3) validation of the feasibility of Cm measurements to track insulin granule exocytosis. On the basis of this last new line of evidence, we suggest that combinations of Cm measurements and electrical activity/membrane current measurements may help define the roles of diverse electrical activity patterns, displayed by human beta-cells, in stimulus-induced insulin secretion.