Glucose inhibits replication of cultured human endothelial cells

Human Decidua Basalis mesenchymal stem/stromal cells reverse the damaging effects of high level of glucose on endothelial cells in vitro

Recently, we reported the therapeutic potential of mesenchymal stem/stromal cells (MSCs) from the maternal decidua basalis tissue of human term placenta (DBMSCs) to treat inflammatory diseases, such as atherosclerosis and cancer. DMSCs protect endothelial cell functions from the negative effects of oxidative stress mediators including hydrogen peroxide (H₂O₂) and monocytes. In addition, DBMSCs induce the generation of anti‐cancer immune cells known as M1 macrophages. Diabetes is another inflammatory disease where endothelial cells are injured by H₂O₂ produced by high level of glucose (hyperglycaemia), which is associated with development of thrombosis. Here, we investigated the ability of DBMSCs to reverse the damaging effects of high levels of glucose on endothelial cells. DBMSCs and endothelial cells were isolated from human placental and umbilical cord tissues, respectively. Endothelial cells were incubated with glucose in presence of DBMSCs, and their functions were evaluated. The effect of DBMSCs on glucose‐ treated endothelial cell expression of genes was also determined. DBMSCs reversed the effects of glucose on endothelial cell functions including proliferation, migration, angiogenesis and permeability. In addition, DBMSCs modified the expression of several genes mediating essential endothelial cell functions including survival, apoptosis, permeability and angiogenesis. We report the first evidence that DBMSCs protect the functions of endothelial cells from the damaging effects of glucose. Based on these results, we establish that DBMSCs are promising therapeutic agents to repair glucose‐induced endothelial cell injury in diabetes. However, these finding must be investigated further to determine the pathways underlying the protective role of DBMSCs on glucose‐stimulated endothelial cell Injury.

Compatibility of Peritoneal Dialysis Fluids Containing Alternative Osmotic Agents with Cells Present in the Peritoneal Cavity

The success of continuous ambulatory peritoneal dialysis (CAPD) lies in preserving the peritoneum as a dialyzing membrane. Repeated infusions of nonphysiological fluids are potentially detrimental to the peritoneal membrane and its host defense. The disadvantages of the currently used peritoneal dialysis fluids (PDPs) containing glucose as an osmotic agent (short ultrafiltration profile, systemic carbohydrate load, nonphysiological composition) have stimulated the search for alternative, less toxic osmotic agents devoid of metabolic side effects and capable of sustaining ultrafiltration. PDFs containing glycerol, amino acids, or glucose polymers have had clinical usage in CAPD patients and were reviewed with regard to their compatibility with cells present in the peritoneal cavity.

Overall, glycerol appears to have no advantage over glucose-based PDFs, although it is less inhibitory for mesothelial cell proliferationin vitro. The optimum formulation of amino acid-based PDFs has not yet been established; its lactate and specific amino acid content may limit their biocompatibility. The virtually iso-osmolar glucose polymer (icodextrin)-containing PDFs were associated with improved biocompatibility compared to glucose monomer-based solutions.

Modifications of PDFs towards a more balanced salt solution with a neutral pH may further increase their compatibility with peritoneal host defense as well as with the integrity of the mesothelial membrane. Such improvement in PDF biocompatibility may result in clinical benefit, that is, enhanced resistance to infection and preservation of peritoneal ultrafiltration capacity.

Detrimental Effects of Peritoneal Dialysis Solutions upon In Vivo and In Situ Exposed Mesothelium

The constancy of the internal environment is in itself evidence that physiological agencies are acting, or ready to act, to maintain this constancy. W.B. Cannon (1)

Human chorionic villous mesenchymal stem/stromal cells protect endothelial cells from injury induced by high level of glucose

BACKGROUND

Mesenchymal stem/stromal cells derived from chorionic villi of human term placentae (pMSCs) protect human endothelial cells from injury induced by hydrogen peroxide (H₂O₂). In diabetes, elevated levels of glucose (hyperglycaemia) induce H₂O₂ production, which causes the endothelial dysfunction that underlies the enhanced immune responses and adverse complications associated with diabetes, which leads to thrombosis and atherosclerosis. In this study, we examined the ability of pMSCs to protect endothelial cell functions from the negative impact of high level of glucose.

METHODS

pMSCs isolated from the chorionic villi of human term placentae were cultured with endothelial cells isolated from human umbilical cord veins in the presence of glucose. Endothelial cell functions were then determined. The effect of pMSCs on gene expression in glucose-treated endothelial cells was also determined.

RESULTS

pMSCs reversed the effect of glucose on key endothelial cell functions including proliferation, migration, angiogenesis, and permeability. In addition, pMSCs altered the expression of many genes that mediate important endothelial cell functions including survival, apoptosis, adhesion, permeability, and angiogenesis.

CONCLUSIONS

This is the first comprehensive study to provide evidence that pMSCs protect endothelial cells from glucose-induced damage. Therefore, pMSCs have potential therapeutic value as a stem cell-based therapy to repair glucose-induced vascular injury and prevent the adverse complications associated with diabetes and cardiovascular disease. However, further studies are necessary to reveal more detailed aspects of the mechanism of action of pMSCs on glucose-induced endothelial damage in vitro and in vivo.

Assessment of Different Transfection Parameters in Efficiency Optimization

Achieving optimal transfection efficiency is the most critical step in overcoming the primary obstacle to success in nonviral-mediated gene therapy. Several transfection parameters were being examined including the effects of different types of transfection media, glucose concentration, reporter DNA concentration, and incubation time in lipotransfectant. Efficiency of transfection obtained was highest for Opti-MEM I (29 ± 2.28%; p = 0.001) followed by M199 (24 ± 1.54%; p = 0.009), both of which performed significantly better than DMEM (14 ± 0.28%) as a transfection medium. The rate of transfection was affected by glucose levels in only DMEM with higher efficiency achieved using low glucose containing DMEM (17 ± 0.38%) than its counterpart. Furthermore, transfection rate and cell viability were severely hampered by lengthened exposure to transfection complexes, leading to an overall mean efficiency of 5 ± 0.87%. However, doubling the DNA content in the transfection mixture did not significantly change the mean rate of transfection in M199 medium (24 ± 1.54% to 27 ± 1.54%; p = 0.273). The overall range of mean efficiency acquired with our protocol under different transfection conditions was between 14% and 29%. Hopefully results from this study will further potential success in nonviral-mediated gene transfer.

Deregulation of microRNA-503 contributes to diabetes mellitus-induced impairment of endothelial function and reparative angiogenesis after limb ischemia

BACKGROUND

Diabetes mellitus impairs endothelial cell (EC) function and postischemic reparative neovascularization by molecular mechanisms that are not fully understood. microRNAs negatively regulate the expression of target genes mainly by interaction in their 3' untranslated region.

METHODS AND RESULTS

We found that microRNA-503 (miR-503) expression in ECs is upregulated in culture conditions mimicking diabetes mellitus (high D-glucose) and ischemia-associated starvation (low growth factors). Under normal culture conditions, lentivirus-mediated miR-503-forced expression inhibited EC proliferation, migration, and network formation on Matrigel (comparisons versus lentivirus.GFP control). Conversely, blocking miR-503 activity by either adenovirus-mediated transfer of a miR-503 decoy (Ad.decoymiR-503) or by antimiR-503 (antisense oligonucleotide) improved the functional capacities of ECs cultured under high D-glucose/low growth factors. We identified CCNE1 and cdc25A as direct miR-503 targets which are downregulated by high glucose/low growth factors in ECs. Next, we obtained evidence that miR-503 expression is increased in ischemic limb muscles of streptozotocin-diabetic mice and in ECs enriched from these muscles. Moreover, Ad.decoymiR-503 delivery to the ischemic adductor of diabetic mice corrected diabetes mellitus-induced impairment of postischemic angiogenesis and blood flow recovery. We finally investigated miR-503 and target gene expression in muscular specimens from the amputated ischemic legs of diabetic patients. As controls, calf biopsies of nondiabetic and nonischemic patients undergoing saphenous vein stripping were used. In diabetic muscles, miR-503 expression was remarkably higher, and it inversely correlated with cdc25 protein expression. Plasma miR-503 levels were also elevated in the diabetic individuals.

CONCLUSIONS

Our data suggest miR-503 as a possible therapeutic target in diabetic patients with critical limb ischemia.

Distinct Effects of High-Glucose Conditions on Endothelial Cells of Macrovascular and Microvascular Origins

Recent studies implicate hyperglycemia as an important cause of macrovascular and ocular complications in diabetes mellitus. In this study, the authors examined the effect of high glucose on macrovascular and microvascular endothelial cell viability and apoptosis in culture. Human aortic endothelial cells (HAECs) and human retinal endothelial cells (HRECs) were exposed to normal-glucose conditions (NG) and high-glucose conditions (NG supplemented with 25 mM D-glucose) for 72 h in vitro. D-Mannitol was used as an osmotic control. Cell viability was assessed by methlythiazolydiphenyltetrazolium bromide (MTT) assay, and induction of apoptosis was assessed by Hoechst staining. Statistics were analyzed by paired t tests. In HAECs, cell viability was decreased by 12.9% in high-glucose conditions, and apoptotic cells were significantly increased by 77%. However, in HRECs, cell viability was increased by 14.9% in high-glucose conditions, and apoptotic cells were significantly decreased by 33.3%. Mannitol did not show any effect on cell survival or apoptosis ruling out an osmotic effect. High-glucose conditions reduce cell viability and induce apoptosis in HAECs, which may contribute to macrovascular complications associated with diabetes. In contrast, high-glucose increases viability in HRECs and inhibits apoptosis, which may contribute to the development of diabetic retinopathy.

Green Light Emitting Diode Irradiation Enhances Fibroblast Growth Impaired by High Glucose Level

BACKGROUND AND OBJECTIVE

The chronic metabolic disorder diabetes mellitus is an important cause of morbidity and mortality due to a series of common secondary metabolic complications, such as the development of severe, often slow healing skin lesions. In view of promoting the wound-healing process in diabetic patients, this preliminary in vitro study investigated the efficacy of green light emitting diode (LED) irradiation on fibroblast proliferation and viability under hyperglycemic circumstances.

MATERIALS AND METHODS

To achieve hyperglycemic circumstances, embryonic chicken fibroblasts were cultured in Hanks' culture medium supplemented with 30 g/L glucose. LED irradiation was performed on 3 consecutive days with a probe emitting green light (570 nm) and a power output of 10 mW. Each treatment lasted 3 min, resulting in a radiation exposure of 0.1 J/cm2.

RESULTS

A Mann-Whitney U test revealed a higher proliferation rate (p = 0.001) in all irradiated cultures in comparison with the controls.

CONCLUSION

According to these results, the effectiveness of green LED irradiation on fibroblasts in hyperglycemic circumstances is established. Future in vivo investigation would be worthwhile to investigate whether there are equivalent positive results in diabetic patients.

Superoxide anions and endothelial cell proliferation in normoglycemia and hyperglycemia

-Oxygen free radicals are believed to play a key role in cellular proliferation, and increased concentrations of these molecules have been implicated in the pathogenesis of endothelial dysfunction in diabetes mellitus. Our aim was to study the role of superoxide anions in endothelial cell proliferation under conditions of normoglycemia and hyperglycemia. Human aortic endothelial cells (HAECs) and human umbilical vein endothelial cells (HUVECs) exposed to adenoviral vectors encoding CuZnSOD (AdCuZnSOD), ss-galactosidase (Adssgal), or diluent (control) were cultured in normal glucose (NG, 5.5 mmol/L) or high glucose (HG, 28 mmol/L) medium. Cell proliferation was compared by use of [(3)H]thymidine incorporation and cell count in transduced and control cells in the setting of NG and HG. Transgene expression was detected in transduced cells by X-gal staining and by Western analysis and SOD activity assay in AdCuZnSOD-transduced cells. Superoxide production was significantly (P:<0.05) decreased in AdCuZnSOD-transduced cells cultured in both NG and HG medium. In NG, AdCuZnSOD-transduced endothelial cells had decreased proliferation compared with control cells. After 48 hours in HG, superoxide levels were increased and DNA synthesis was decreased (P:<0.05) in control and Adssgal-transduced but were not affected in AdCuZnSOD-transduced cells. In addition, after 7 days in HG, cell counts were reduced (P:<0.05) in control (73+/-2.5%) and Adssgal-transduced (75+/-3.4%) but not in AdCuZnSOD-transduced cells (89+/-3.4%). These results suggest that either a deficiency or an excess of superoxide anions inhibits endothelial cell proliferation, and the inhibitory effect of increased superoxide due to hyperglycemia can be reversed by CuZnSOD overexpression.

High glucose accelerates the life cycle of the in vivo exposed mesothelium

BACKGROUND

Mouse mesothelium exposed in vivo for 30 days to high glucose solutions develop morphological changes that characterize a population of cells near the end of their life span.

METHODS

The present study was designed to explore, in mesothelial cell imprints, whether these changes could derive from an early acceleration of the cell population life cycle in mice exposed for periods of up to 30 days to a 4.25% glucose fluid (236 mmol/L/L) prepared in Hank's balanced salt solution (HBSS). Three critical points of the cell's life cycle were evaluated: the G1 checkpoint [proliferating cell nuclear antigen (PCNA) expression], DNA synthesis ((3)H-thymidine incorporation), and the prevalence of mitosis.

RESULTS

Cell populations exposed to a high glucose concentration showed an initial acceleration of their life cycle, as sustained by a peak of mitosis at two hours, an early increase of DNA incorporation sustained during the first 24 hours, as well as a top level of PCNA expression after three to four hours. These significantly higher values, compared with the control animals treated with HBSS, collapsed after 24 hours and were nil after 30 days of exposure.

CONCLUSIONS

Exposure to a high glucose concentration induced an early and short-lived acceleration of the mesothelial cell cycle, and with a longer exposure this was followed by a depletion of the growth capabilities of the exposed monolayer.

Antiproliferative effect of elevated glucose in human microvascular endothelial cells

Diabetic microangiopathy has been implicated as a fundamental feature of the pathological complications of diabetes including retinopathy, neuropathy, and diabetic foot ulceration. However, previous studies devoted to examining the deleterious effects of elevated glucose on the endothelium have been performed largely in primary cultured cells of macrovessel origin. Difficulty in the harvesting and maintenance of microvascular endothelial cells in culture have hindered the study of this relevant population. Therefore, the objective of this study was to characterize the effect of elevated glucose on the proliferation and involved signaling pathways of an immortalized human dermal microvascular endothelial cell line (HMEC-1) that possess similar characteristics to their in vivo counterparts. Human dermal microvascular endothelial cells (HMEC-1) were grown in the presence of normal (5 mM) or high D-glucose (20 mM) for 14 days. The proliferative response of HMEC-1 was compared under these conditions as well as the cAMP and PKC pathways by in vitro assays. Elevated glucose significantly inhibited (P < 0.05) HMEC-1 proliferation after 7, 10, and 14 days. This effect was not mimicked by 20 mM mannitol. The antiproliferative effect was more pronounced with longer exposure (1-14 days) to elevated glucose and was irreversible 4 days after a 10-day exposure. The antiproliferative effect was partially reversed in the presence of a PKA inhibitor, Rp-cAMP (10-50 microM), and/or a PKC inhibitor, Calphostin C (10 nM). HMEC-1 exposed to elevated glucose (20 mM) for 14 days caused an increase in cyclic AMP accumulation, PKA, and PKC activity but was not associated with the activation of downstream events such as CRE and AP-1 binding activity. These data support the hypothesis that HMEC-1 is a suitable model to study the deleterious effects of elevated glucose on microvascular endothelial cells. Continued studies with HMEC-1 may prove advantageous in delineation of the molecular pathophysiology associated with diabetic microangiopathy.

Blood coagulability and fibrinolysis in streptozotocin-induced diabetic rats

Changes in coagulation and fibrinolysis in the plasma (in vivo) and hepatocytes (ex vivo) were studied using hyperglycemic rats. Hyperglycemia was induced by intravenous injection of 50 mg/kg streptozotocin (STZ). Eight weeks after the injection, we observed increases in thrombin-antithrombin III complex and tissue type plasminogen activator activity, decreases in plasma levels of antithrombin III, plasminogen and alpha2-plasmin inhibitor, and significant shortening of activated partial thromboplastin time. In freshly isolated or cultured hepatocytes from STZ-induced hyperglycemic rats, concentrations of proteins related to coagulation were increased. An increase in alanine-aminotransferase leakage and decreases in the levels of amylase, triglycerides and phospholipids were observed in the culture medium of hepatocytes from STZ treated rats. In vivo study revealed that STZ-induced subchronic diabetes induced imbalance between coagulation and fibrinolysis, and ex vivo study in hepatocytes from STZ-treated rats showed membrane degeneration and reduction in amylase synthesis, while protein synthesis related to coagulation was not inhibited. These results suggest that, despite vulnerability of liver cells from STZ treated rats, coagulation activity in the liver is retained and rather enhanced in STZ-induced hyperglycemic rats, which may contribute to the promotion of atherosclerosis.

High glucose and hyperosmolarity increase secretion of interleukin-1 beta in cultured human aortic endothelial cells

Interleukin-1 (IL-1) is secreted by endothelial cells (ECs) and smooth-muscle cells (SMCs), which are two major component cells of vessels and detected in atherosclerotic lesions. To evaluate the effect of hyperglycemia on the secretion of IL-1 beta in endothelial cells in diabetic patients, we investigated the effects of high glucose and hyperosmolar conditions on the secretion of IL-1 beta from cultured human aortic endothelial cells (HAECs). HAECs were treated with high concentration of glucose or hyperosmolar condition for 3 days. IL-1 beta in the supernatant was measured by high sensitive enzyme-linked immunosorbent assay (ELISA). Under high concentration of glucose (16.6 mmol/L) and hyperosmolar condition (glucose 5.5 mmol/L + mannitol 11.1 mmol/L), the secretion of IL-1 beta was significantly increased (41.0 +/- 2.8 and 26.3 +/- 5.9% increase, respectively, compared with that of 5.5 mmol/L glucose). In conclusion, high glucose and hyperosmolar condition increase the secretion of IL-1 beta in HAECs. The results suggest that diabetic macroangiopathies might be accelerated partly through the increase of IL-1 beta secretion in HAECs.

Effect of glucose and pyruvate in acidic and non-acidic peritoneal dialysis fluids on leukocytes cell functions

A new peritoneal dialysate containing pyruvate anions has been tested for its effects on cell functions and compared with conventional lactate and bicarbonate based solutions. The dialysate has a final pH of 5.4 to 5.6 and is composed of 1.36 to 3.86% glucose-monohydrate, 132 mmol/liter sodium, 1.75 mmol/liter calcium, 0.75 mmol/liter magnesium, 102 mmol/liter chloride and 35 mmol/liter pyruvate. For cytotoxicity testing peritoneal macrophages and peripheral blood mononuclear cells (PBMC) were exposed to conventional lactate dialysate, pyruvate dialysate, bicarbonate dialysate and a control medium RPMI 1640 (Biochrom KG, Berlin, Germany), followed by activation with different bacterial stimuli. In addition, the study further investigated the effect of varying glucose concentration in the different dialysates ranging from 0 to 3.86% and pH changes between 5.2 and 7.4 on the cytotoxicity effect on the selected cells. Mononuclear cells exposed to pyruvate-based dialysate before stimulation with endotoxin exhibited a tumor necrosis factor (TNF)-mRNA signal comparable to those of cells exposed to RPMI. In contrast, exposure to lactate-based dialysate completely inhibited TNF-mRNA synthesis. In addition, cytokine synthesis in macrophages and PBMCs after exposure to pyruvate was less inhibited when compared to the corresponding levels measured after exposure to lactate. The chemotactic response of polymorphonuclear cells and O-2 generation in all tested cell types after exposure to pyruvate was found not to be inhibited, whereas a complete inhibition was observed after exposure to lactate. The results demonstrate that cytotoxicity effects of peritoneal dialysate on cell lines can be minimized by using a new dialysate formulation containing pyruvate anions instead of lactate.

Early morphologic changes of atherosclerosis induced by ventromedial hypothalamic lesion in the spontaneously diabetic Goto-Kakizaki rat

It is generally thought that typical atherosclerotic lesions do not develop in the rodent. The Goto-Kakizaki (GK) rat is a nonobese strain in which a spontaneous type of non-insulin-dependent diabetes mellitus develops without apparent macroangiopathy. In our previous study, making ventromedial hypothalamic (VMH) lesions in GK rats induced hyperphagia and a further deterioration in glucose metabolism. In the current study, male GK rats in which VMH lesions were made were examined for vascular changes, with special reference to atherosclerotic lesions. Marked hyperglycemia in GK rats with VMH lesions (hereafter referred to as VMH lesion rats) was revealed over an observation period (plasma glucose levels 16 weeks after the operation: VMH lesion GK rats, 19.3 +/- 2.0 mmol/L, vs sham-operated GK rats, 10.1 +/- 1.3 mmol/L; p < 0.0001). Light microscopic observation of the descending aorta in VMH lesion GK rats 16 weeks after the surgery revealed that the intimal thickening and the number of infiltrating cells into the intima were significantly increased as compared with sham-operated GK rats (17531 +/- 3747 microm2 vs 3072 +/- 1192 microm2, p < 0.0001; 15.6 +/- 3.1 per one transverse section vs 6.8 +/- 2.5 per one transverse section, p < 0.0005). Electron microscopic observations demonstrated an increased number of microvilli and lysosomes in endothelial cells, infiltration of macrophages and lymphocytes into the intima, and migration of medial smooth muscle cells into the intima that are considered to be early events in atherosclerosis. These morphologic changes could be induced by a deterioration in glucose metabolism. This rat may thus be useful for studying the process of the initiation of atherosclerosis in diabetes mellitus.

Restoration of nitric oxide production by aldose reductase inhibitor in human endothelial cells cultured in high-glucose medium

The effects of elevated glucose and aldose reductase inhibitor (ARI:ONO-2235) on nitric oxide (NO) production in cultured human umbilical endothelial cells (HUVEC) were evaluated. Aldose reductase and nitric oxide synthase(NOS) share NADPH as an obligate cofactor, therefore it is suggested that the enhanced of glucose flux (27.5 mM) by aldose reductase inhibited NO production by blunting NOS activity. However, the addition of ONO-2235 (100 microM) prevented the inhibition of [NO2-] production. Since ARI decreases glucose-mediated inhibition of NO production in HUVEC. this agent might ameliorate endothelial function associated with diabetes.

Effect of elevated glucose concentrations on cellular lipid peroxidation and growth of cultured human kidney proximal tubule cells

This study has examined whether elevated glucose can induce lipid peroxidation and contribute to the inhibition of cell growth in human kidney proximal tubule(HPT) cells. HPT cells were cultured in media containing glucose concentrations of 8 mM (control), 25 mM, and 50 mM. Lipid peroxidation was assessed by the thiobarbituric acid reactivity and cell growth was assessed by 3H-thymidine uptake. Results show decreased (59%, p < 0.01) growth of HPT cells cultured in 50 mM glucose. Cells cultured in 50 mM mannitol did not show any growth inhibition, suggesting that the decreased cell growth associated with glucose is not due to osmolarity changes. There was an increase (108%, p < 0.02) in lipid peroxidation in cells cultured with high levels of glucose (50 mM) compared with controls and cells cultured with 50 mM mannitol. To examine if membrane lipid peroxidation or malondialdehyde (MDA, an end product of lipid peroxidation) has any role in the inhibition of cell growth, we examined the effect of tertiary butylhydroperoxide (TBH, known to cause lipid peroxidation and generate MDA) on the growth of HPT cells. TBH decreased cell growth (49, 17 and 3% of controls at 0.1, 0.25, and 0.5 mumole TBH/ml medium). Similarly, a marked reduction in the growth was observed with exogenous MDA (72, 69 and 34% of controls at 0.1, 0.25, and 0.5 mumole MDA/ml medium). This suggests that elevated glucose can induce membrane lipid peroxidation and accumulation of MDA, which in turn can inhibit cellular growth and contribute to the altered structure and function of HPT cells in diabetes.

High glucose and hyperosmolarity increase heparin-binding epidermal growth factor-like growth factor (HB-EGF) production in cultured human aortic endothelial cells

Heparin-binding epidermal growth factor-like growth factor (HB-EGF) has been shown to be a potent smooth muscle cell (SMC) mitogen and chemoattractant, and might be a candidate factor for the progression of atherosclerosis. We have investigated the effects of high glucose and hyperosmolarity on HB-EGF production in cultured human aortic endothelial cells. Following the culture of the cells for 2 days with high concentrations of glucose or in the hyperosmolar conditions, we measured the content of HB-EGF and the rate of production in the cells using a semi-quantitative immunofluorescent technique and a metabolic radiolabelling method. With high glucose (16.6 mmol) and hyperosmolar conditions (glucose 5.5 mmol + mannitol 11.1 mmol or glucose 5.5 mmol + raffinose 11.1 mmol), the content of HB-EGF was significantly increased and the metabolic rate was also significantly increased (more than a twofold increase, compared to that of 5.5 mmol glucose). In conclusion, conditions of high glucose or hyperosmolarity increase HB-EGF production in human aortic endothelial cells. These results suggest that diabetic macroangiopathy might be attributed at least in part to HB-EGF-related vascular changes which may be induced by glucose.

Effect of an aldose reductase inhibitor on type IV collagen production by human endothelial cells cultured in high glucose

Diabetic microangiopathy is characterized by a thickening of capillary basement membranes associated with type IV collagen accumulation. An increase in type IV collagen content of the aortic wall is also observed in macroangiopathy. In order to analyse the importance of the polyol pathway in the development of the collagen metabolism alterations seen in diabetic angiopathy and their prevention by aldose reductase inhibitors, we have studied the effects of sorbinil on the high glucose-induced stimulation of type IV collagen biosynthesis in human umbilical vein endothelial cells. Primary cultures were exposed to high glucose (16.7 mmol/l), with and without 0.11 mmol/l sorbinil, for 3 or 6 days after beginning of confluence. We measured the soluble type IV collagen secreted into the culture medium and the insoluble type IV collagen accumulated in the extracellular matrix and cells, by ELISA. We also studied [14C]proline incorporation into the newly synthesized collagenous and total proteins in the culture supernatant and in the extracellular matrix and cell fraction. High glucose decreased the number of cells and increased the amount of type IV collagen in the culture supernatant and in the extracellular matrix and cell fraction. It also increased proline incorporation into the newly synthesized collagenous and total proteins in the culture supernatant and in the extracellular matrix and cell fraction. Sorbinil corrected all these high glucose-induced alterations. The corrective effects of sorbinil on the proliferation and on type IV collagen metabolism of endothelial cells cultured in high glucose may be attributed to prevention of polyol pathway dysregulation.

Nutritional modulation of the final outcome of hepatotoxic injury by energy substrates: an hypothesis for the mechanism

Survival after hepatocellular injury and necrosis may depend on the ability of the remaining hepatocytes to divide and restore an adequate population of functioning cells. Although adequate nutritional support is necessary for liver regeneration after severe liver damage, much is yet to be discovered concerning which nutritional factors are critical for liver regeneration. Clinically, nutritional substances are administered only from the energy aspect, without regard to whether or how these substrates may facilitate or impede liver tissue repair processes. Glucose is used as principal source of energy in liver damage because of accompanying marked hypoglycemia. But the contribution of glucose to compensatory liver regeneration and/or survival is unclear. This paper advances the hypotheses that: (1) glucose increases the toxicity of centrilobular hepatotoxicants by inhibiting hepatic cell division and tissue repair allowing unrestrained progression of injury; (2) fatty acids facilitate hepatic-cell division permitting hepatolobular restoration to occur thus preventing death from even a lethal dose. If hepatic tissue repair can be stimulated by some therapeutically compatible mechanism, then it might be possible to prevent death from even massive hepatocellular injury. In addition to nutritional manipulation, it should be possible to exploit molecular mechanisms that regulate organized cell division (tissue repair) to increase survival rates of patients suffering from fulminant hepatic failure. These findings have significant impact on tissue repair in a variety of other organs and tissues, particularly in diabetes-like conditions.

Production of type IV collagen and 72-kDa gelatinase by human endothelial cells cultured in high glucose. Effects of a protein kinase C inhibitor, GF 109203X

Since diabetic microangiopathy and macroangiopathy are characterized by type IV collagen accumulation in vascular basement membranes, it was of interest to study type IV collagen production and type IV collagenase secretion by endothelial cells (EC) cultured in high glucose and to evaluate the role of protein kinase C (PKC) activation in the alterations induced by high glucose. Primary cultures of human umbilical vein EC were exposed to high glucose concentration for 3 days at the beginning of confluence. The number of EC decreased with glucose concentration from 5 to 50 mM. At 16.7 mM glucose concentration, the amount of type IV collagen, determined by a two-step ELISA, increased in the culture supernatant and in the insoluble fraction associated with the extracellular matrix and cells; proline incorporation was more markedly elevated in the collagenous than in the total proteins of the culture supernatant and of the extracellular matrix and cell extracts. Gelatin zymography of the culture supernatant showed that EC mainly produce a 72-kDa gelatinase known to degrade type IV collagen. At 16.7 mM glucose concentration, total gelatinase activity per millilitre of culture supernatant was reduced and the 72-kDa gelatinase activity measured on the zymogram scan was lowered. When EC were exposed to 16.7 mM glucose, the specific PKC inhibitor GF 109203X corrected the increases in type IV collagen concentration and in proline incorporation into the collagenous or total proteins present in he culture supernatant or in the extract of the insoluble fraction, including the extracellular matrix and cells. Our results show that soluble and insoluble type IV collagen accumulation by EC cultured at high glucose concentration is not only associated with increased synthesis of the collagenous and total proteins but also with decreased total 72-kDa gelatinase activity in the extracellular fluid. The observed effects of GF 109203X are in favor of the involvement of PKC activation in the type IV collagen accumulation.

Pyruvate neutralizes peritoneal dialysate cytotoxicity: maintained integrity and proliferation of cultured human mesothelial cells

Toxic effects of commercially available peritoneal dialysate (PD) fluid include damage to mesothelial cells (MC), causing a severely disturbed proliferation of cultured MC. We investigated the injury to the cell membrane (by release of lactate dehydrogenase, LDH), the proliferation (by cell counts and by 3H-thymidine incorporation), and optional the cytokine generation (by IL-1 receptor-antagonist production, IL-1 ra) of cultured human MC during the 48 hours after a 30 minute exposure to PD containing either 35 mmol/liter sodium lactate or sodium pyruvate. All solutions had a pH of 5.2 to 5.6 and were composed as standard PD. Glucose contents of 1.36 and 3.86 mmol/liter were tested. After exposure to the lactate-PD containing 1.36% glucose, LDH activity was increased by more than 30%, proliferation of MC was inhibited by more than 30%, and IL-1 ra production was reduced significantly when compared to pyruvate-PD and the control solution. After preincubation with 3.86% glucose containing PD, all negative effects became even more pronounced in the lactate group whereas the MC maintained their integrity, rate of proliferation and IL-1 ra release after pre-exposure to pyruvate containing PD. These results suggest that the acute toxic effects of commercially available PD on the integrity, proliferation and IL-1 ra production of MC can be avoided by the use of sodium pyruvate instead of sodium lactate.

High glucose increases platelet-derived growth factor production in cultured human vascular endothelial cells and preventive effects of eicosapentaenoic acids

The regulation of the production of platelet-derived growth factor (PDGF) and the influence of high glucose concentration, eicosapentaenoic acid (EPA) were studied in cultured human umbilical vein endothelial cells (HUE). The PDGF production of HUE increased markedly depending on glucose concentration. However, EPA (3 x 10(-4)M) markedly inhibited PDGF production [27.5 mM glucose group: 123 +/- 3% of control (5.5 mM glucose group), 27.5 mM glucose+EPA group: 104 +/- 5% of control]. These results suggested that a high glucose concentration and a high osmotic pressure-induced increase in PDGF production is involved in the development and progression of diabetic macroangiopathy. As eicosapentaenoic acid inhibits the PDGF production induced by high glucose concentration in HUE, use of this agent may exhibit anti-arteriosclerotic effects.

Hypoglycemia stimulates thrombin-induced PGI2 production by cultured human umbilical vein endothelial cells

The effect of a low concentration of glucose on prostacyclin (PGI2) production by human umbilical vein endothelial cells in culture (HUVEC) was examined. When HUVEC were cultured for 24 h in a medium containing various concentrations of glucose (0.26, 0.63, or 1.29 mg/ml), thrombin-induced PGI2 production was increased in the cells exposed to low concentrations of glucose; the increase was inversely proportional to the glucose concentration and was seen at all concentrations of thrombin tested. Thus, hypoglycemia may increase the production of PGI2 by endothelial cells in vivo, leading to a compensatory vasodilation with an increase in blood flow to increase the delivery of glucose.

Increased aortic endothelial death and enhanced transendothelial macromolecular transport in streptozotocin-diabetic rats

Hypertension, cigarette smoking and diabetes mellitus are well-known risk factors for atherosclerosis and coronary heart disease. Repeated endothelial cell injury and increased lipid entry have been suggested as initiating events in atherogenesis. Our previous studies have demonstrated that the frequency of endothelial cell death and associated endothelial permeability were significantly increased in the aorta of spontaneously hypertensive rats and chronic oral nicotine-treated rats. In the present investigation, we examined the hypothesis that diabetes also increases the frequency of arterial endothelial cell death and hence transendothelial macromolecular transport, which may have some implications in increasing lipid entry and thus accelerating atherogenesis. Diabetes was induced in 15 male Sprague-Dawley rats by intraperitoneal injection of 60 mg streptozotocin per kg body weight. The duration of diabetes was 6 weeks. A group of 15 age-matched rats, injected only with the buffer and maintained over the same time period, served as the controls. In en face preparations of the thoracic aorta, IgG-containing dead endothelial cells were identified by an indirect immunoperoxidase method, and endothelial leakage to Evans blue-albumin complexes was quantified by fluorescence microscopy. Diabetic rats, compared to control rats, had significantly higher values for the frequency of endothelial cell death (0.77 +/- 0.10% vs 0.38 +/- 0.04%; p < 0.005 by two-tailed, unpaired Student's t-test) and the number density of Evans blue-albumin leaky foci (4.33 +/- 0.48/mm2 vs 2.99 +/- 0.38/mm2; p < 0.05 by two-tailed, unpaired t-test) in the aorta.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential modulation of mitogenic and metabolic actions of insulin-like growth factor I in rat glomerular mesangial cells in high glucose culture

In order to explore the possible contribution of insulin-like growth factor I to the development of diabetic nephropathy, the effect of glucose on the mitogenic and metabolic actions of insulin-like growth factor I in cultured rat glomerular mesangial cells was examined. The stimulation of [3H]-thymidine incorporation by insulin-like growth factor I in the cells exposed to high concentrations (55 mmol/l) of glucose (4.6 +/- 1.3 fold stimulation) was significantly suppressed as compared with that in the cells cultured in 11 mmol/l glucose (17.5 +/- 0.8 fold). In contrast, [3H]-amino-isobutylic acid uptake into the mesangial cells was significantly enhanced by glucose (2.03 +/- 0.03 nmol.mg protein-1. 15 min-1 at 55 mmol/l glucose vs 0.59 +/- 0.01 at 11 mmol/l glucose), while 2-deoxyglucose uptake remained unchanged. [125I]-insulin-like growth factor I binding was slightly but significantly increased in the cells exposed to high concentrations of glucose. Thus, glucose may modulate the mitogenic and metabolic actions of insulin-like growth factor I differently in cultured mesangial cells probably at the post-insulin-like growth factor I receptor level. These results may indicate that the differential modulation of the actions of insulin-like growth factor I by glucose could result in the increase in amino acid uptake and decrease in the cell proliferation in the mesangial cells, possibly leading to enhanced mesangial matrix synthesis with a relatively small increase in mesangial cell volume as seen in diabetic nephropathy.

The role of endothelium in the pathogenesis of diabetic microangiopathy

Damage caused to the vessel wall by diverse mechanisms may lead to diabetic microangiopathy. Consequently, research work is more and more focusing on the pathophysiology of vascular cells, with particular emphasis on endothelium. This paper reviews the present knowledge on the alterations of small vessel endothelium in diabetes. The most important risk factors for diabetic microangiopathy are the duration of disease and the degree of metabolic control maintained throughout the years. However, genetic factors may also contribute. These are examined first, followed by the presumed roles played by increased protein glycation and the production of Advanced Glycosylation End Products, the "polyol pathway" and free radical generation. Endothelium is a widespread, extremely active organ which regulates complex physiologic functions and its structure and function are discussed in the second section of this review. The third part deals with how diabetes can affect endothelium and describes observations on endothelial metabolism in vitro as well as morphologic and functional alterations in the patients. Unfortunately, the mechanisms leading to progressive degeneration of the microcirculation and organ damage in diabetic patients remain largely unaccounted for.

An oxidative mechanism is involved in high glucose-induced serum protein modification causing inhibition of endothelial cell proliferation

In order to investigate the role of hyperglycemia on the development of atherosclerosis in diabetics, the effects of high glucose-induced modification of serum factors on the proliferation of bovine carotid artery endothelial cells were studied. Dialysates of high glucose-treated serum inhibit cell growth in a time- and glucose concentration-dependent manner. With 6 weeks of pretreatment, 16.7 mM glucose causes a 47.2% inhibition in cell growth compared to 5.6 mM glucose (P < 0.001). Pretreatment of serum in the presence of reduced glutathione (0.5-1.0 mM), an antioxidant, significantly prevents the high glucose-induced inhibition without inhibiting the formation of early non-enzymatic glycosylation products. Dithiothreitol (7.5 mM) treatment after preincubation with glucose fully restores the glucose-induced inhibition. When the dialysates are fractionated according to molecular mass, the high glucose-induced inhibition is maximal in the MW fraction above 100 kDa. These data suggest that high glucose conditions facilitate the susceptibility of serum proteins to sulfhydryl oxidation forming disulfide crosslinks and this oxidative process may contribute to the inhibition of endothelial cell proliferation.

The effect of diabetes on the proliferation of aortic endothelial cells

Diabetic vascular disease is more severe, diffuse, and accelerated when compared to nondiabetic vascular disease. Endothelial cell injury or alteration in endothelial cell function is hypothesized to be the initial cellular event in the pathophysiology of diabetic vascular disease. We examined the effect of insulin-treated and untreated alloxan diabetes on the proliferation of rabbit aortic endothelial cells in vitro by growing thoracic aortic endothelial cells from alloxan diabetic rabbits in serum obtained from alloxan diabetic rabbits. Diabetes adversely affected the proliferation of aortic endothelial cells; the most significant decrease in cell proliferation was noted in untreated diabetic cells. Crossover studies between endothelial cells and serum from different groups revealed that diabetes slows endothelial proliferation by not only a serum effect but also an intrinsic cellular effect. These observations suggest that diabetes adversely affects the proliferation of aortic endothelial cells by changing cell and serum functions.

Toxicity of osmotic solutes on human mesothelial cells in vitro

We evaluated the effect of the various osmotic solutes on the growth rate of human mesothelial cells (HMC) in an in vitro culture. Glucose inhibited proliferation of HMC in a dose dependent way. At high glucose concentrations (60 mM, 90 mM) the effect was instant but at lower concentration (30 mM) decrease in the mesothelial cell proliferation was significant only after five days of incubation. Reversibility of the glucose effect was inversely proportional to exposure time to this solute. Mannitol and glycerol studied in similar concentrations as glucose decreased proliferation of the mesothelial cells less than glucose, whereas amino acid glycine had a similar effect to glucose. However, all osmotic solutes caused similar injury to mesothelial cells membrane as measured by release of LDH. These results suggest that the toxic effect of the osmotic solutes on proliferation of the mesothelial cells depends not only on the hyperosmolality but also on some metabolic effect(s). In an in vitro culture, HMC may provide a suitable model for the study of the toxic effect of dialysis fluid on peritoneal mesothelium.

Effects of glucose on migration, proliferation and tube formation by vascular endothelial cells

In order to elucidate the association between hyperglycemia and the vascular complications of diabetes, the effects of high glucose concentrations on the migration, proliferation and tube formation of bovine carotid artery endothelial cells were investigated. Cells treated with 16.7 and 33.3 mM glucose for 6 days showed 1.69- and 1.75-fold increase in serum-induced migration compared with cells treated with 5.6 mM glucose (p less than 0.05). The effect of glucose on cell proliferation was affected by serum concentration. When this was below 0.5%, a high glucose concentration stimulated cell growth to a maximum of 1.73 times that at a serum concentration of 0.05% (p less than 0.01) whereas at a serum concentration of 10%, growth was inhibited (p less than 0.05). Tube formation was studied by culturing the cells between two layers of collagen gel. Ultrastructurally, tubular structures were composed of one to several endothelial cells containing pinocytotic vesicles and cytoplasmic projections, and linked by junctional complexes. A basal lamina-like structure surrounded the abluminal surface. Treatment of the cells with 16.7 and 27.8 mM glucose for 4 days stimulated tubular elongation 1.85 and 1.71 times, respectively (p less than 0.01). Other osmogenic molecules such as mannitol and sucrose did not affect tube formation. These data imply that high glucose concentrations mimicking diabetic hyperglycemia may not inhibit the repair of endothelial injury and could act as a stimulator of neovascularization.

Insulin as a mitogenic factor: role in the pathogenesis of cardiovascular disease

Evidence has been accumulating that insulin has actions that may promote the development of atherosclerosis. Research has involved three broad areas: actions of insulin on cultured arterial cells, the effect of insulin on isolated artery preparations, and the development of lipid-containing lesions in the arteries of experimental animals. Insulin, in concentrations similar to those found in physiologic conditions, stimulates proliferation of cultured arterial smooth muscle cells from a number of species, including humans. Insulin also stimulates migration of smooth muscle cells. Cholesterol synthesis and low-density lipoprotein interaction with its receptor in smooth muscle cells are stimulated by insulin. Insulin's mitogenic action appears to be mediated by the insulin-like growth factor receptor. Endothelial cells cultured from large vessels are resistant to the actions of insulin, but hyperglycemia inhibits their proliferation. Insulin deficiency protects animals from experimental atherosclerosis; this protection is lost with insulin treatment. Insulin administration results in lipid-containing lesions in chickens and rats fed a normal diet, and in increased lipid synthesis in the arteries of pigs and dogs. Isolated artery preparations from insulin-deficient or insulin-treated animals undergo lipid metabolism at a rate that correlates with the insulin concentrations in the donor animals. The biological actions of insulin (and glucose) on arterial tissue suggest that hyperglycemia and hyperinsulinemia may promote the development of atherosclerosis.

Effects of glucose and insulin on cultured human microvascular endothelial cells

The prolonged effects of glucose and insulin on cultured human microvascular endothelial cells from omental tissue (HOMEC) were observed to identify the contribution of sustained hyperglycemia and/or hyperinsulinemia to the pathogenesis of diabetic microangiopathy. When the cells were cultured for 10 days in Medium 199 with 100 or 500 mg/dl glucose, the number of cells was reduced to 78% in the culture of 500 mg/dl glucose as opposed to that of 100 mg/dl glucose. The difference in the number of cells between these two groups became obvious between the 5th and the 7th culture days. The replacement of D-glucose with L-glucose did not show any reduction in the number of cells, indicating the impertinence of high osmolarity, induced by high glucose (305 mOsm/kg) to the number of cells. This reduction resulted from the cellular damage during the culture period rather than the retardation of growth, according to the experiments of [3H]thymidine uptake and the 51Cr release assay. Since the uptake of glucose, measured as the uptake of 3-O-methyl-alpha-D-glucose, was higher and the Na+/K+ pump activity decreased in high glucose condition, it is suggested that the excessive intracellular accumulation of glucose caused the damage of cells through the disturbance of ion exchange. Insulin augmented the reduction in the number of cells induced by high glucose when supplemented together for 10 days at concentrations of 10(-6)-10(-12)M. The uptake of glucose increased further to 154% by the addition of insulin to high glucose as compared to that of high glucose alone, however, the decreased Na+/K+ pump activity by high glucose was restored to the control level by insulin. The aggravating effect of insulin to the cellular damage induced by high glucose seems to be mediation via the mechanism other than the decreased Na+/K+ pump activity. In conclusion, HOMEC were gradually damaged by high glucose and by insulin, and hyperglycemia and hyperinsulinemia would be of pathogenetic importance for diabetic microangiopathy.

Effect of diabetes on the healing process of synthetic grafts implanted in dogs: a preliminary study

The deleterious effects of diabetes in peripheral vascular surgery and blood access are generally considered to be associated with the greater failure rate of vascular prostheses. To understand better these phenomena, we investigated the healing characteristics of an ePTFE graft in dogs. The animals were made diabetic after total pancreatectomy and the grafts implanted for scheduled periods of 24 h, 48 h, 1 wk and 1 month, either as thoracic (series I) or abdominal (series II) aortic substitutes. The same implantations were performed in non-diabetic dogs used as controls. The luminal surfaces of grafts implanted in diabetic dogs, either as thoracic or abdominal bypasses, proved to be more invaded by platelets as a result of the blood's increased platelet aggregation properties. The presence of endothelial-like cells spreading over the pannus after 1 month implantation was strikingly different in diabetic versus non-diabetic dogs. In the first group, the cells were fragile and less abundant, whereas in the second they were more adhesive, elongated and orientated in the direction of the blood flow.

Haemostatic variables, serum lipid abnormalities and vascular complications in diabetes mellitus: a 5 year follow-up study

The relationship among blood lipids, haemostatic and fibrinolytic parameters have been evaluated, during a follow-up study, in 27 non-insulin dependent (type II) diabetic patients. Upon recruitment, and in periodical controls, we observed that plasma triglycerides and VLDL levels correlated inversely, and HDL directly, with the fibrinolytic activity of plasma and euglobulin precipitate. Furthermore triglycerides and VLDL correlated directly with Factor VIII antigen (vWFAg). After 5 years in the study, 12 patients (44%) had macroangiopathic complications, and 9 of these subjects showed persistently high levels of triglycerides (above 2.36 mmol/l). These haemostatic and lipid components, however, do not influence the progression of diabetic retinopathy and nephropathy. The alterations of lipid, haemostatic and fibrinolytic parameters and their possible relationships seem to play an important role in the occurrence of diabetic macroangiopathy.

High glucose concentrations inhibit protein synthesis in retinal pigment epithelium in vitro

There is mounting evidence suggesting functional and structural alterations in the retinal pigment epithelium (RPE) in experimental and clinical diabetes. In this study we examined the effect of high glucose concentrations on human RPE cells in vitro. After 24-hr incubation in media supplemented with glucose (19.5 mM, 25.5 mM, and 45.5 mM) and prepared both with and without osmotic adjustment, there was no significant effect on [3H]thymidine or [3H]uridine incorporation into TCA-precipitable material. There was, however, a significant decrease in [35S]methionine incorporation which became more marked with increasing glucose concentrations. This could not be attributed to increased osmolarity caused by the additional glucose as it occurred in isosmolar high glucose media. 3-O-methyl glucose, a non-metabolized glucose analog, did not have the same effect, suggesting that metabolism of glucose may be important. Resolution of newly synthesized proteins by gel electrophoresis and autoradiography suggests a generalized decrease in protein synthesis. These data suggest that elevated glucose levels cause a significant metabolic alteration in RPE cells in vitro.

Diabetes and atherosclerosis: an epidemiologic view

Diabetes is associated with changes in plasma lipids and lipoproteins into atherogenic direction. In IDDM these changes are small or absent if good metabolic control can be maintained. Diabetic nephropathy is, however, associated with the appearance of dyslipoproteinemia. In NIDDM plasma total and VLDL triglyceride levels are elevated, and HDL-cholesterol level is decreased, and this pattern of dyslipoproteinemia does not always respond to improved control of hyperglycemia. Abnormalities of lipoprotein metabolism, not reflected in conventional plasma lipid and lipoprotein level measurements, and glucosylation of lipoproteins and resulting alterations in lipoprotein catabolism may be of importance in the enhanced atherogenesis in diabetes. Both IDDM and NIDDM are associated with an increased frequency of hypertension, but the underlying mechanisms appear to be different. In IDDM hypertension is usually associated with the development of diabetic nephropathy and thus with a long duration of the disease. In NIDDM hypertension is often present already at the time of diagnosis, and also in IGT, the precursor stage of NIDDM, the prevalence of hypertension is already increased. Obesity explains only in part the high prevalence of hypertension in patients with NIDDM. Diabetes is known to be associated with multiple abnormalities in hemostatic factors and, although these abnormalities may contribute importantly to the increased risk of ASVD in diabetic patients, information about their real role is scanty and conflicting. The impact of general major risk factors for ASVD, elevated plasma cholesterol, elevated blood pressure, and smoking, on the risk of ASVD appears to be similar in diabetics and nondiabetics. Only a relatively small proportion of the excessive occurrence of ASVD in diabetics can, however, be explained by the effects of diabetes on the levels of general risk factors for ASVD. This proportion mediated through the effects of diabetes on risk factors is larger in female diabetics than in male diabetics. The major proportion of the excess of ASVD in diabetics remains, however, unexplained and must be due to effects of diabetes itself through mechanisms that are incompletely understood.

Atherosclerosis: an insulin-dependent disease?

Evidence is reviewed that dietary habits in industrially developed countries, especially an increased frequency of ingestion of foods of high energy density, may contribute to excessive hepatic cholesterol synthesis and to a preponderance of lipogenic versus lipolytic effects on the arterial intima, thereby favoring the formation and progression of atheroma. These effects are mediated by the rise and fall of circulating insulin levels. The evidence is suggestive of the possibility that frequent and prolonged exposure of the arterial wall to high circulating levels of insulin may favor the development of atherosclerotic lesions. Research on diet-atherosclerosis relationships should take into account not only overall diet composition but individual meal composition and size and their effects on serum insulin levels, as well as meal spacing and the relative durations of absorptive and postabsorptive periods during the 24-hour daily cycle.

Overview of the association between insulin and atherosclerosis

The suggestion that insulin is associated with atherosclerosis is based on clinical, epidemiologic, and experimental evidence. In general, atherosclerosis of the coronary, cerebral, and peripheral arteries is associated with abnormally high insulin responses to oral glucose. This hyperinsulinemia is not related to acute injury or to tissue necrosis, does not occur in response to intravenous glucose or tolbutamide, and is independent of other cardiovascular risk factors. Populations who are at high risk for cardiovascular disease have higher insulin responses to oral glucose than those at lower risk. Prospective studies carried out in Australia, Finland, and France have shown that elevated insulin levels, either fasting or in response to oral glucose, have a predictive role in the development of cardiovascular disease. This association is independent of the effects of other cardiovascular risk factors. In experimental animals, insulin deficiency retards the development of diet-induced arterial disease, whereas administration of insulin promotes lesion development and prevents lesion regression. Insulin stimulates lipid synthesis in isolated arteries and stimulates proliferation and lipid accumulation in cultured arterial smooth muscle cells. The evidence linking insulin with atherosclerosis has been gathered from nondiabetic subjects; this evidence is unavailable in diabetics. As it is clear that hyperinsulinemia is often present in diabetes, either in relation to mild glucose intolerance or obesity (in noninsulin-dependent diabetes mellitus) or because of insulin therapy (in insulin-dependent diabetes mellitus), it is essential that further consideration should be given to the possibility that hyperinsulinemia may have harmful effects on the arterial wall.