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

Glucose promotes membrane cholesterol crystalline domain formation by lipid peroxidation

Oxidative damage to vascular cell membrane phospholipids causes physicochemical changes in membrane structure and lipid organization, contributing to atherogenesis. Oxidative stress combined with hyperglycemia has been shown to further increase the risk of vascular and metabolic diseases. In this study, the effects of glucose on oxidative stress-induced cholesterol domain formation were tested in model membranes containing polyunsaturated fatty acids and physiologic levels of cholesterol. Membrane structural changes, including cholesterol domain formation, were characterized by small angle X-ray scattering (SAXS) analysis and correlated with spectrophotometrically-determined lipid hydroperoxide levels. Glucose treatment resulted in a concentration-dependent increase in lipid hydroperoxide formation, which correlated with the formation of highly-ordered cholesterol crystalline domains (unit cell periodicity of 34 A) as well as a decrease in overall membrane bilayer width. The effect of glucose on lipid peroxidation was further enhanced by increased levels of cholesterol. Treatment with free radical-scavenging agents inhibited the biochemical and structural effects of glucose, even at elevated cholesterol levels. These data demonstrate that glucose promotes changes in membrane organization, including cholesterol crystal formation, through lipid peroxidation.

Hemoglobin induces cytotoxic damage of glycine-preserved renal tubules

In isolated tubular segments (ITS) of rat kidney cortex, we studied the effect of hemoglobin (Hb) on reoxygenation damage. All tubules were suspended in Ringer's solution containing 5-mm glycine and oxygenated for 30 min with 95% O(2):5% CO(2), followed by a 30-min period with 95% N(2):5% CO(2), and final reoxygenation for 60 min. Untreated tubules served as controls. Different concentrations of free Hb and equivalent amounts of intact erythrocytes were added to the incubation medium. Secondly, we added deferoxamine (DFO) to Hb and erythrocytes. Membrane leakage and lipid peroxidation were measured by lactate dehydrogenase and glutamate dehydrogenase and the development of thiobarbituric acid reactive substances. Cell function was quantified by gluconeogenesis and intracellular potassium accumulation. Hb exerted concentration-dependent cytotoxic effects indicated by significantly increased enzyme leakage rates, lipid peroxidation and a significantly decreased cell function (P < 0.05), in ITS during hypoxia, and subsequent reoxygenation. Moreover, we found that toxicity of both Fe(2+) and Fe(3+) ions increased with rising concentration. However, Fe(2+) showed a higher tissue toxicity than Fe(3+). DFO reduced significantly the reoxygenation damage of free Hb and iron ions. Our data clearly demonstrate a pronounced cytotoxic effect of free Hb in ITS, which critically depended on the reduction state of the iron ions.

Hyperketonemia (ketosis), oxidative stress and type 1 diabetes

The long-term complications of diabetes are the leading causes of morbidity and mortality in the type 1 diabetic population and remain a major public health issue. Hyperglycemia is one of the major risk factors in the development of vascular complications. A growing body of evidence indicates that hyperglycemia leads to increased oxidative stress and monocyte and endothelial cell dysfunction. In addition to hyperglycemia, type 1 diabetic patients frequently experience ketosis (hyperketonemia). The blood concentration of ketone bodies reaches higher than 25mM in diabetics with severe ketosis. Traditionally, clinical practice has considered hypertketonemia to be present only in type 1 diabetic patients. Newer data indicate that diabetic ketoaciosis or hyperketonemia co-exists with hyperglycemia among older type 2 diabetic patients and in African Americans and other minority groups with type 2 diabetes. This review will focus on the role of hyperketonemia in the etiology of oxidative stress in diabetic patients. The data presented here illustrate that the ketone body acetoacetate (AA) can generate superoxide radicals and cause increases in oxidative stress and cellular dysfunction. The data included in this review demonstrate that blood levels of markers of oxidative stress are elevated in hyperketonemic patients compared with those of normoketonemic diabetic patients. Thus, both in vitro and in vivo research indicate that ketosis can generate oxygen radicals and result in excess cellular oxidative stress in type 1 diabetic patients. Elevated oxidative stress levels in ketotic patients can play a significant role in the development of vascular inflammation and contribute to the increased incidence of vascular disease and complications associated with type 1 diabetes.

Pyridoxine and pyridoxamine inhibits superoxide radicals and prevents lipid peroxidation, protein glycosylation, and (Na+ + K+)-ATPase activity reduction in high glucose-treated human erythrocytes

Vitamin B(6) (pyridoxine) supplementation has been found beneficial in preventing diabetic neuropathy and retinopathy, and the glycosylation of proteins. Oxygen radicals and oxidative damage have been implicated in the cellular dysfunction and complications of diabetes. This study was undertaken to test the hypothesis that pyridoxine (P) and pyridoxamine (PM) inhibit superoxide radical production, reduce lipid peroxidation and glycosylation, and increase the (Na+ + K+)-ATPase activity in high glucose-exposed red blood cells (RBC). Superoxide radical production was assessed by the reduction of cytochrome C by glucose in the presence and absence of P or PM in a cell-free buffered solution. To examine cellular effects, washed normal human RBC were treated with control and high glucose concentrations with and without P or PM. Both P and PM significantly lowered lipid peroxidation and glycated hemoglobin (HbA(1)) formation in high glucose-exposed RBC. P and PM significantly prevented the reduction in (Na+ + K+)-ATPase activity in high glucose-treated RBC. Thus, P or PM can inhibit oxygen radical production, which in turn prevents the lipid peroxidation, protein glycosylation, and (Na+ + K+)-ATPase activity reduction induced by the hyperglycemia. This study describes a new biochemical mechanism by which P or PM supplementation may delay or inhibit the development of complications in diabetes.

Molecular mechanisms of diabetic renal hypertrophy

Altered growth of renal cells is one of the early abnormalities detected after the onset of diabetes. Cell culture studies whereby renal cells are exposed to high glucose concentrations have provided a considerable amount of insight into mechanisms of growth. In the glomerular compartment, there is a very early and self-limited proliferation of mesangial cells with subsequent hypertrophy, whereas proximal tubular cells primarily undergo hypertrophy. There is overwhelming evidence from in vivo and cell culture studies that induction of the transforming growth factor-beta (TGF-beta) system mediates the actions of high ambient glucose and that this system is pivotal for the hypertrophy of mesangial and tubular cells. Other factors such as hemodynamic forces, protein glycation products, and several mediators (for example, angiotensin II, endothelin-1, thromboxane, and platelet-derived growth factor) may further amplify the synthesis of TGF-beta and/or the expression of its receptors in the diabetic state. Cellular hypertrophy can be characterized by cell cycle arrest in the G1 phase. The molecular mechanism arresting mesangial cells in the G1 phase of the cell cycle is the induction of cyclin-dependent kinase (CdK) inhibitors such as p27Kip1 and p21, which bind to and inactivate cyclin-CdK complexes responsible for G1-phase exit. High-glucose-induced activation of protein kinase C and stimulated TGF-beta expression appear to be essential for stimulated expression of p27Kip1. In addition, a decreased turnover of protein caused by the inhibition of proteases contributes to hypertrophy. The development of irreversible renal changes in diabetes mellitus such as glomerulosclerosis and tubulointerstitial fibrosis is always preceded by the early hypertrophic processes in the glomerular and the tubular compartments. It may still be debated whether diabetic renal hypertrophy will inevitably lead to irreversible fibrotic changes in the absence of other factors such as altered intraglomerular hemodynamics and genetic predisposition. Nevertheless, understanding cellular growth on a molecular level may help design a novel therapeutic approach to prevent or treat diabetic nephropathy effectively.

Effect of hyperketonemia on blood monocytes in type-I diabetic patients and apoptosis in cultured U937 monocytes

Recent studies have proposed a role for a reduced number of circulating monocytes in the development of atherosclerosis and circulatory diseases in diabetes. Ketosis is frequently encountered in type-I diabetics. This study was undertaken to test the hypothesis that hyperketonemia can lower blood monocyte count in type-I diabetic patients. Blood monocyte count was significantly lower (p < 0.05) in diabetics (n = 27) compared with age-matched normal siblings (n = 22). Blood levels of acetoacetate (AA) and triglycerides were significantly higher in diabetics compared with normals. To examine whether hyperketonemia can directly alter the monocyte count in diabetics, we studied the effect of ketone bodies AA and beta-hydroxybutyrate (BHB) on U937 cells, a human-derived promonocytic cell line as an in vitro model. The cell culture studies showed a dose-dependent growth inhibition and induction of apoptosis as a result of treatment with AA in U937 cells. Furthermore, there was a significant decrease in GSH and increase in lipid peroxidation products in AA-treated U937 cells. Taken together, this study suggests that elevated levels of ketone body AA can result in oxidative damage, accelerated apoptosis, and inhibition of cell growth in monocytes, which in turn can lower monocyte count in the blood of type-I diabetic patients.

Ketosis (acetoacetate) can generate oxygen radicals and cause increased lipid peroxidation and growth inhibition in human endothelial cells

Elevated level of cellular lipid peroxidation can increase the incidence of vascular disease. The mechanism by which ketosis causes accelerated cellular damage and vascular disease in diabetes is not known. This study was undertaken to test the hypothesis that elevated levels of ketone bodies increase lipid peroxidation in endothelial cells. Human umbilical venous endothelial cells (HUVEC) were cultured for 24 h at 37 degrees C with ketone bodies (acetoacetate, beta-hydroxybutyrate). Acetoacetate, but not beta-hydroxybutyrate, caused an increase in lipid peroxidation and growth inhibition in cultured HUVEC. To determine whether ketone bodies generate oxygen radicals, studies using cell-free buffered solution were performed. They showed a significant superoxide dismutase (SOD) inhibitable reduction of cytochrome C by acetoacetate, but not by beta-hydroxybutyrate, suggesting the generation of superoxide anion radicals by acetoacetate. Additional studies show that Fe2+ potentiates oxygen radical generation by acetoacetate. Thus, elevated levels of ketone body acetoacetate can generate oxygen radicals and cause lipid peroxidation in endothelial cells, providing a possible mechanism for the increased incidence of vascular disease in diabetes.

The effect of oxygen radicals metabolites and vitamin E on glycosylation of proteins

Glycosylation (glycation) of proteins is a major complication of hyperglycemia in diabetes. This study has examined the effect of hydrogen peroxide (H2O2) and tertiary-butylhydroperoxide (TBH) and vitamin E (E) on glycation of hemoglobin (GHb). The RBC (15%) in phosphate-buffered saline were treated with 5-50 mM glucose (G) with and without H2O2 or TBH at 37 degrees C for 1-3 d. Glycation of hemoglobin was assessed by GHb formation using Glyc-affinity columns. There was an increase in the GHb formation with increasing G concentrations. GHb formation increased significantly in the presence of H2O2 at all G concentrations. The increase in GHb was blocked when RBC were pretreated with E. E also inhibited formation of malondialdehyde (MDA), an end product of lipid peroxidation, as assessed by the thiobarbituric acid reactivity. Similar increase in the GHb formation was observed when TBH was used instead of H2O2 to induce oxidant stress to the RBC. To examine any role of MDA per se in increased glycation, RBC were treated ex vivo with and without exogenous standard MDA. GHb formation was significantly higher with G-MDA in contrast to G alone. Thus, increased oxygen radicals activity can initiate per-oxidation of lipids and MDA accumulation, which in turn, can stimulate glycation of proteins in diabetes. E can block the glycation of proteins by inhibiting MDA formation.