Protective effect of D-alpha-tocopherol on the function of human mesangial cells exposed to high glucose concentrations

The Role of Diacylglycerol Kinase in the Amelioration of Diabetic Nephropathy

The drastic increase in the number of patients with diabetes and its complications is a global issue. Diabetic nephropathy, the leading cause of chronic kidney disease, significantly affects patients’ quality of life and medical expenses. Furthermore, there are limited drugs for treating diabetic nephropathy patients. Impaired lipid signaling, especially abnormal protein kinase C (PKC) activation by de novo-synthesized diacylglycerol (DG) under high blood glucose, is one of the causes of diabetic nephropathy. DG kinase (DGK) is an enzyme that phosphorylates DG and generates phosphatidic acid, i.e., DGK can inhibit PKC activation under diabetic conditions. Indeed, it has been proven that DGK activation ameliorates diabetic nephropathy. In this review, we summarize the involvement of PKC and DGK in diabetic nephropathy as therapeutic targets, and its mechanisms, by referring to our recent study.

α-Tocopherol and Hippocampal Neural Plasticity in Physiological and Pathological Conditions

Neuroplasticity is an "umbrella term" referring to the complex, multifaceted physiological processes that mediate the ongoing structural and functional modifications occurring, at various time- and size-scales, in the ever-changing immature and adult brain, and that represent the basis for fundamental neurocognitive behavioral functions; in addition, maladaptive neuroplasticity plays a role in the pathophysiology of neuropsychiatric dysfunctions. Experiential cues and several endogenous and exogenous factors can regulate neuroplasticity; among these, vitamin E, and in particular α-tocopherol (α-T), the isoform with highest bioactivity, exerts potent effects on many plasticity-related events in both the physiological and pathological brain. In this review, the role of vitamin E/α-T in regulating diverse aspects of neuroplasticity is analyzed and discussed, focusing on the hippocampus, a brain structure that remains highly plastic throughout the lifespan and is involved in cognitive functions. Vitamin E-mediated influences on hippocampal synaptic plasticity and related cognitive behavior, on post-natal development and adult hippocampal neurogenesis, as well as on cellular and molecular disruptions in kainate-induced temporal seizures are described. Besides underscoring the relevance of its antioxidant properties, non-antioxidant functions of vitamin E/α-T, mainly involving regulation of cell signaling molecules and their target proteins, have been highlighted to help interpret the possible mechanisms underlying the effects on neuroplasticity.

Non-antioxidant properties of α-tocopherol reduce the anticancer activity of several protein kinase inhibitors in vitro

The antioxidant properties of α-tocopherol have been proposed to play a beneficial chemopreventive role against cancer. However, emerging data also indicate that it may exert contrasting effects on the efficacy of chemotherapeutic treatments when given as dietary supplement, being in that case harmful for patients. This dual role of α-tocopherol and, in particular, its effects on the efficacy of anticancer drugs remains poorly documented. For this purpose, we studied here, using high throughput flow cytometry, the direct impact of α-tocopherol on apoptosis and cell cycle arrest induced by different cytotoxic agents on various models of cancer cell lines in vitro. Our results indicate that physiologically relevant concentrations of α-tocopherol strongly compromise the cytotoxic and cytostatic action of various protein kinase inhibitors (KI), while other classes of chemotherapeutic agents or apoptosis inducers are unaffected by this vitamin. Interestingly, these anti-chemotherapeutic effects of α-tocopherol appear to be unrelated to its antioxidant properties since a variety of other antioxidants were completely neutral toward KI-induced cell cycle arrest and cell death. In conclusion, our data suggest that dietary α-tocopherol could limit KI effects on tumour cells, and, by extent, that this could result in a reduction of the clinical efficacy of anti-cancer treatments based on KI molecules.

Long-term dietary antioxidant cocktail supplementation effectively reduces renal inflammation in diabetic mice

Diabetic nephropathy is a serious complication for diabetic patients, yet the precise mechanism that underlies the development of diabetic complications remains unknown. We hypothesised that dietary antioxidant supplementation with single N-acetylcysteine (NAC) or vitamin C combined with either vitamin E or vitamin E and NAC improves diabetic renal inflammation through the modulation of blood glucose levels, oxidative stress and inflammatory response. Experimental animals were treated with alloxan monohydrate to induce diabetes. Mice were divided into five groups and supplemented with single or a combination of antioxidants. Body weights and blood glucose levels were measured once a week. After 8 weeks of dietary antioxidant supplementation, mice were killed and blood urea N (BUN) and plasma creatinine levels were measured to evaluate renal function. NF-κB protein was indirectly demonstrated by the phosphorylated IκBα (pIκBα) level, and the expressions of oxidative stress- and inflammatory response-related proteins were also determined. We demonstrated that dietary antioxidant supplementation decreased lipid peroxidation levels demonstrated by thiobarbituric acid-reacting substances, BUN and plasma creatinine levels in diabetic kidneys. Moreover, dietary antioxidant cocktail supplementation improved blood glucose levels and selectively regulated the expressions of Cu-Zn superoxide dismutase, haeme oxygenase-1, pIκBα, inducible NO synthase, cyclo-oxygenase-2 and C-reactive protein in diabetic kidneys effectively. These findings demonstrated that diabetic renal failure was associated with inflammatory responses induced by hyperglycaemia. In addition, results in the study suggest that antioxidant cocktail supplementation may have beneficial effects on diabetic nephropathy through selective reduction of blood glucose levels and inflammatory response.

Significant differences in the electrochemical behavior of the alpha-, beta-, gamma-, and delta-tocopherols (vitamin E)

Alpha-, beta-, gamma-, and delta-tocopherols can be oxidized in dry CH2Cl2 or CH3CN by one electron to form cation radicals that deprotonate to form the neutral phenoxyl radicals, which are then immediately further oxidized by one electron to the phenoxonium cations (an ECE electrochemical mechanism, where E signifies an electron transfer and C represents a chemical step, with the electrochemical mechanism having been determined by in situ spectroscopic analysis). The principal difference in the electrochemical behavior of the tocopherols relates to the stability of their associated phenoxonium cations. The phenoxonium cation of alpha-tocopherol is stable in solution for at least several hours, the phenoxonium cation of beta-tocopherol is stable for several minutes, and the phenoxonium cations of gamma- and delta-tocopherol are stable for <1 s. In dry CH2Cl2 containing >0.75 M acid (CF3COOH), the deprotonation reaction of the cation radicals can be completely inhibited resulting in the cyclic voltammetric behavior of the tocopherols appearing as chemically reversible one-electron oxidation processes (an E mechanism). In dry acid conditions, the cation radicals can be further oxidized by one electron to form the dications, which are unstable and immediately deprotonate. The high stability of the phenoxonium cation of alpha-tocopherol compared to the other tocopherols (and most other phenols) is a chemically important feature that may shed new light on understanding alpha-tocopherol's unique biological properties.

Modulation of signal transduction by vitamin E

The ability of vitamin E to modulate signal transduction and gene expression has been observed in numerous studies; however, the detailed molecular mechanisms involved are often not clear. The eight natural vitamin E analogues and synthetic derivatives affect signal transduction with different potency, possibly reflecting their different ability to interact with specific proteins. Vitamin E modulates the activity of several enzymes involved in signal transduction, such as protein kinase C, protein kinase B, protein tyrosine kinases, 5-, 12-, and 15-lipoxygenases, cyclooxygenase-2, phospholipase A2, protein phosphatase 2A, protein tyrosine phosphatase, and diacylglycerol kinase. Activation of some these enzymes after stimulation of cell surface receptors with growth factors or cytokines can be normalized by vitamin E. At the molecular level, the translocation of several of these enzymes to the plasma membrane is affected by vitamin E, suggesting that the modulation of protein-membrane interactions may be a common theme for vitamin E action. In this review the main effects of vitamin E on enzymes involved in signal transduction are summarized and the possible mechanisms leading to enzyme modulation evaluated. The elucidation of the molecular and cellular events affected by vitamin E could reveal novel strategies and molecular targets for developing similarly acting compounds.

Acute effect of high glucose on long-term cell growth: a role for transient glucose increase in proximal tubule cell injury

Although chronic exposure of renal cells to high glucose has been shown to cause cell injury, the effect of acute exposure has not been elucidated. In this study, we demonstrate that acute (10 min) exposure of human proximal tubule epithelial cells (hPTEC) to high glucose (25 mM) induces a time-dependent dual effect consisting of an early proliferation and a late apoptosis. Acute exposure of hPTEC to high glucose induced a twofold increase in DNA synthesis and cell number at 12 h. However, after 36 h, a significant decrease in cell growth is observed, followed by apoptosis. On glucose treatment, both p42/p44 mitogen-activated protein (MAP) kinases and the downstream signaling intermediate NF-κB were phosphorylated and translocated to the nucleus. Pretreatment of cells with MAP kinase and NF-κB-specific inhibitors abolished glucose-induced proliferation. However, these inhibitors were ineffective in preventing glucose-induced apoptosis. Interestingly, conditioned medium from cells exposed to high-glucose concentrations inhibited proliferation and concomitantly induced apoptosis in normal cells, suggesting that the inhibitory effect of glucose occurs through secretion of a secondary factor(s). In parallel to apoptosis, we observed an increased production of reactive oxygen species (ROS). Pretreatment of cells with the antioxidant N-acetyl cysteine reversed glucose-mediated ROS production and apoptosis, suggesting that ROS is involved in apoptosis. Our study demonstrates for the first time that a single high-glucose exposure for 10 min alone is sufficient to elicit proliferation and apoptosis in hPTEC and suggests that episodes of transient increase in glucose may contribute to cell damage leading to epithelial cell dysfunction.

Adjuvant strategies for prevention of glomerulosclerosis

The glomerulosclerosis which frequently complicates diabetes and severe hypertension is mediated primarily by increased mesangial production and activation of transforming growth factor-beta (TGF-beta), which acts on mesangial cells to boost their production of matrix proteins while suppressing extracellular proteolytic activity. Hyperglycemia and glomerular hypertension work in various complementary ways to stimulate superoxide production via NADPH oxidase in mesangial cells; the resulting oxidant stress results in the induction and activation of TFG-beta. Nitric oxide, generated by glomerular capillaries and by mesangial cells themselves, functions physiologically to oppose mesangial TGF-beta overproduction; however, NO bioactivity is compromised by oxidant stress. In addition to low-protein diets and drugs that suppress angiotensin II activity, a variety of other agents and measures may have potential for impeding the process of glomerulosclerosis. These include vitamin E, which blunts the rise in mesangial diacylglycerol levels induced by hyperglycemia; statins and (possibly) policosanol, which down-regulate NADPH oxidase activity by diminishing isoprenylation of Rac1; lipoic acid, whose potent antioxidant activity antagonizes the impact of oxidant stress on TGF-beta expression; pyridoxamine, which inhibits production of advanced glycation endproducts; arginine, high-dose folate, vitamin C, and salt restriction, which may support glomerular production of nitric oxide; and estrogen and soy isoflavones, which may induce nitric oxide synthase in glomerular capillaries while also interfering with TGF-beta signaling. Further research along these lines may enable the development of complex nutraceuticals which have important clinical utility for controlling and preventing glomerulosclerosis and renal failure. Most of these measures may likewise reduce risk for left ventricular hypertrophy in hypertensives, inasmuch as the signaling mechanisms which mediate this disorder appear similar to those involved in glomerulosclerosis.

Antioxidants and endothelium protection

The endothelium is a complex organ system that controls the homeostasis of the vasculature by integrating signals between the vascular wall and the vessel lumen. Under physiological conditions, it maintains a normal vascular tone and blood fluidity by elaborating a variety of factors, such as nitric oxide, prostacyclin and endothelin. However, in pathological situations the endothelium can also modify its phenotype facilitating vasoconstriction, inflammation, and thrombotic events. These abnormal responses manifest in different clinical settings, such as hypercholesterolemia, hypertension, diabetes mellitus, and occur in the absence of any morphological change of the vessel. The etiology of these altered endothelial functions is multi-factorial, and the mechanisms underlying them are complex and not yet fully elucidated. Today, there is substantial evidence that many endothelial functions are sensitive to the presence of reactive oxygen species and subsequent oxidative stress. Here, I will review the increasing number of studies showing that exogenous antioxidants can modulate the endothelium-dependent vasodilation responses, the homeostatic endothelium-leukocyte interactions, the balance between pro- and anti-thrombotic properties, and the vascular apoptotic responses. Finally, the non-antioxidant activities of some antioxidant will also be described.

Combination treatment of vitamin C and desferrioxamine suppresses glomerular superoxide and prostaglandin E production in diabetic rats

AIMS

Increased oxidative stress may contribute to the development of diabetic nephropathy. Conversely, it has been proposed that enhanced glomerular production of prostaglandin E(2) (PGE(2)) may be the cause of glomerular hyperfiltration in streptozotocin (STZ)-induced diabetic rats. As the role of superoxide anion (O(2-)) production in early diabetic nephropathy is not fully understood, we investigated the effect of vitamin C and desferrioxamine treatment on glomerular O(2-) and PGE(2) production in diabetic rats.

METHODS

STZ-induced diabetic rats were given drinking water containing 1 g/l of vitamin C and desferrioxamine for 10 days, and glomerular O(2-) production, glomerular PGE(2) synthesis and creatinine clearance were examined.

RESULTS

Glomerular O(2-) production increased in untreated diabetic rats compared to non-diabetic controls (142.2 +/- 12.4 vs. 65.4 +/- 3.6 counts/mg protein/min). Treatment with vitamin C and desferrioxamine significantly decreased glomerular O(2-) production (93.7 +/- 6.7 counts/mg protein/min). Glomerular PGE(2) synthesis and creatinine clearance were significantly increased in untreated diabetic rats compared to controls and PGE(2) synthesis was reduced and creatinine clearance tended to decrease by the treatment.

CONCLUSIONS

Our results demonstrated that vitamin C and desferrioxamine suppressed the enhanced glomerular O(2-) production with subsequent decrease in PGE(2) production. Antioxidant therapy may be beneficial in preventing the development of diabetic nephropathy.

Anti-atherosclerotic effects of vitamin E--myth or reality?

Atherosclerosis and its complications such as coronary heart disease, myocardial infarction and stroke are the leading causes of death in the developed world. High blood pressure, diabetes, smoking and a diet high in cholesterol and lipids clearly increase the likelihood of premature atherosclerosis, albeit other factors, such as the individual genetic makeup, may play an additional role. Several epidemiological studies and intervention trials have been performed with vitamin E, and some of them showed that it prevents atherosclerosis. For a long time, vitamin E was assumed to act by decreasing the oxidation of LDL, a key step in atherosclerosis initiation. However, at the cellular level, vitamin E acts by inhibition of smooth muscle cell proliferation, platelet aggregation, monocyte adhesion, oxLDL uptake and cytokine production, all reactions implied in the progression of atherosclerosis. Recent research revealed that these effects are not the result of the antioxidant activity of vitamin E, but rather of precise molecular actions of this compound. It is assumed that specific interactions of vitamin E with enzymes and proteins are at the basis of its non-antioxidant effects. Vitamin E influences the activity of several enzymes (e.g. PKC, PP2A, COX-2, 5-lipooxygenase, nitric oxide synthase, NADPH-oxidase, superoxide dismutase, phopholipase A2) and modulates the expression of genes that are involved in atherosclerosis (e.g. scavenger receptors, integrins, selectins, cytokines, cyclins). These interactions promise to reveal the biological properties of vitamin E and allow designing better strategies for the protection against atherosclerosis progression.

Role of protein kinase C-angiotensin II pathway for extracellular matrix production in cultured human mesangial cells exposed to high glucose levels

The intrarenal renin-angiotensin system has been implicated in the pathogenesis of diabetic nephropathy. This study investigates the mechanisms for glucose-induced increase in angiotensin II (AII) production by human mesangial cells (MCs) in relation to protein kinase C (PKC). We also examine whether locally produced AII mediates extracellular matrix protein production in high-glucose conditions. Human MCs were cultured in 5 or 33 mmol/l glucose for 8 days, and were incubated with or without 5 mmol/l GFX, a PKC inhibitor, 0.1 micromol/l candesartan cilexetil (CC), a specific type 1 AII receptor antagonist, for another 24 h. In addition, MCs grown in 5 mmol/l glucose were incubated with 0.1 micromol/l phorbol-12,13-dibutyrate (PDBu) for 24 h. AII, TGF-beta1, fibronectin and type IV collagen in the culture media were measured by ELISA. The amount of AII secreted from MCs exposed to high-glucose levels was significantly greater (P<0.01) than that in normal glucose levels. The increase in AII production was completely prevented by GFX. The addition of PDBu mimicked the effect of glucose on AII production. The glucose-induced increases in the production of TGF-beta1, fibronectin and type IV collagen were partially, but significantly restored (P<0.01) by CC, while GFX totally abolished these effects of glucose. These results suggest that elevated glucose levels stimulate AII production via mechanisms dependent on glucose-induced PKC activation in human MCs, and that locally produced AII partly mediates the increase in mesangial matrix synthesis in high-glucose conditions.

Regulation of cell signalling by vitamin E

Vitamin E, the most important lipid-soluble antioxidant, was discovered at the University of California at Berkeley in 1922. Since its discovery, studies of the constituent tocopherols and tocotrienols have focused mainly on their antioxidant properties. In 1991 Angelo Azzi's group (Boscoboinik et al. 1991a,b) first described non-antioxidant cell signalling functions for alpha-tocopherol, demonstrating that vitamin E regulates protein kinase C activity in smooth muscle cells. At the transcriptional level, alpha-tocopherol modulates the expression of the hepatic alpha-tocopherol transfer protein, as well as the expression of liver collagen alphal gene, collagenase gene and alpha-tropomyosin gene. Recently, a tocopherol-dependent transcription factor (tocopherol-associated protein) has been discovered. In cultured cells it has been demonstrated that vitamin E inhibits inflammation, cell adhesion, platelet aggregation and smooth muscle cell proliferation. Recent advances in molecular biology and genomic techniques have led to the discovery of novel vitamin E-sensitive genes and signal transduction pathways.

Role of oxidative stress in advanced glycation end product-induced mesangial cell activation

BACKGROUND

Levels of advanced glycation end products (AGE) are elevated in individuals with advancing age, renal failure, and diabetes, and accumulation of these molecules may contribute to disease progression. The mechanism by which AGE proteins alter glomerular mesangial cell function, however, is not completely understood. The present study assessed the involvement of oxidative stress in AGE-dependent mesangial cell signaling events.

METHODS

Primary cultures of rat renal mesangial cells were exposed to in vitro AGE-BSA and H2O2. Nuclear factor-kappaB (NF-kappaB) and protein kinase C (PKC) isoform activation were studied using confocal microscopy and Western blotting. Quantitative polymerase chain reaction (PCR) was used to measure transforming growth factor-beta1 (TGF-beta1) levels. The involvement of oxidative stress was assessed by supplementing or compromising cellular antioxidant capacity.

RESULTS

NF-kappaB was dose-dependently activated by AGE. PKC activation was not involved in this response, but analysis of PKC-beta1 activation showed a stimulatory effect of AGE proteins on this isoform. Transcription of TGF-beta1 was stimulated by AGE and was prevented by PKC inhibition. Challenge with H2O2 had similar downstream effects on mesangial cell signaling. Antioxidants, vitamin E and nitecapone, prevented AGE-dependent NF-kappaB activation and normalized PKC activity and associated TGF-beta1 transcription. Depletion of the intracellular antioxidant, glutathione, effectively lowered the AGE concentration needed for mesangial cell activation of NF-kappaB and PKC-beta1. Treatment with a suboptimal AGE dose, under glutathione-depleted conditions, revealed a synergistic effect on both parameters.

CONCLUSION

The results support a central role for oxidative stress in AGE-dependent mesangial cell signaling and emphasize the importance of ROS in determining cell responsiveness.

Non-antioxidant molecular functions of alpha-tocopherol (vitamin E)

alpha-Tocopherol (the major vitamin E component) regulates key cellular events by mechanisms unrelated with its antioxidant function. Inhibition of protein kinase C (PKC) activity and vascular smooth muscle cell growth by alpha-tocopherol was first described by our group. Later, alpha-tocopherol was shown to inhibit PKC in various cell types with consequent inhibition of aggregation in platelets, of nitric oxide production in endothelial cells and of superoxide production in neutrophils and macrophages. alpha-Tocopherol diminishes adhesion molecule, collagenase and scavenger receptor (SR-A and CD36) expression and increases connective tissue growth factor expression.

The 80th anniversary of vitamin E: beyond its antioxidant properties

Molecules provided with an antioxidant function may have additional properties, the latter being sometimes of greater importance than the former. In the last ten years, alpha-tocopherol has revealed precise cellular functions, some of which are independent of its antioxidant/radical scavenging ability. At the posttranslational level, alpha-tocopherol inhibits protein kinase C and 5-lipoxygenase and activates protein phosphatase 2A and diacylglycerol kinase. Some genes (CD36, alpha-TTP, alpha-tropomyosin, and collagenase) are affected by alpha-tocopherol at the transcriptional level. alpha-Tocopherol also induces inhibition of cell proliferation, platelet aggregation and monocyte adhesion. These effects are unrelated to the antioxidant activity of vitamin E, but rather are believed to be a result of specific interactions of vitamin E with components of the cell, e. g. proteins, enzymes and membranes. This review focuses on novel non-antioxidant functions of alpha-tocopherol and discusses the possibility that many of the effects previously attributed to the antioxidant functions can also be explained by non-antioxidant mechanisms.

Vitamin E: protective role of a Janus molecule

Since the discovery of vitamin E in 1922, its deficiency has been associated with various disorders, particularly atherosclerosis, ischemic heart disease, and the development of different types of cancer. A neurological syndrome associated with vitamin E deficiency resembling Friedreich ataxia has also been described. Whereas epidemiological studies have indicated the role of vitamin E in preventing the progression of atherosclerosis and cancer, intervention trials have produced contradictory results, indicating strong protection in some cases and no significant effect in others. Although it is commonly believed that phenolic compounds like vitamin E exert only a protective role against free radical damage, antioxidant molecules can exert other biological functions. For instance, the antioxidant activity of 17-beta-estradiol is not related to its role in determining secondary sexual characters, and the antioxidant capacity of all-trans-retinal is distinguished from its role in rhodopsin and vision. Thus, it is not unusual that alpha-tocopherol (the most active form of vitamin E) has properties independent of its antioxidant/radical scavenging ability. The Roman god Janus, shown in ancient coins as having two faces in one body, inspired the designation of 'Janus molecules' for these substances. The new biochemical face of vitamin E was first described in 1991, with an inhibitory effect on cell proliferation and protein kinase C activity. After a decade, this nonantioxidant role of vitamin E is well established, as confirmed by authoritative studies of signal transduction and gene regulation. More recently, a tocopherol binding protein with possible receptor function has been discovered. Despite such important developments in understanding the molecular mechanism and the targets of vitamin E, its new Janus face is not fully elucidated. Greater knowledge of the molecular events related to vitamin E will help in selecting the parameters for clinical intervention studies such as population type, dose response effects, and possible synergism with other compounds.

Glucose alters the susceptibility of mesangial cells to contrast media

RATIONALE AND OBJECTIVES

Diabetic patients frequently suffer contrast media-induced nephropathy. Hyperglycemia in diabetes mellitus causes gradual deterioration of glomerular mesangial cells (MCs) in the kidney. In this study, the authors investigated the response of rat MCs cultured in high-glucose medium to diatrizoate and iohexol, high- and low-osmolar contrast media, respectively.

METHODS

Isolated rat MCs were precultured under basal-glucose (5.5 mmol/L) and high-glucose (30 and 55 mmol/L) conditions for 24 hours to mimic hyperglycemia in diabetes mellitus and then were exposed to diatrizoate (40 and 80 mg I/mL) and iohexol (80, 120, 160 mg I/mL) for 2 hours. The cytotoxic effects of diatrizoate and iohexol were monitored by neutral red uptake in MCs. The protective effects of an antioxidant, d-alpha-tocopherol (Toc), on cytotoxicity of the contrast media were determined when MCs were precultured with Toc under high-glucose conditions or were exposed to the contrast media together with Toc. Peroxide levels in the cells exposed to the contrast media were analyzed by flow cytometry using dichlorofluorescin diacetate.

RESULTS

Exposure to both contrast media (diatrizoate and iohexol) induced a concentration-dependent decrease in viability of the cells precultured under basal-glucose conditions (5.5 mmol/L). Preculture under high-glucose conditions (30 and 55 mmol/L) augmented the cytotoxic effects of both contrast media. An increase in the intracellular peroxide level was detected after exposure to both contrast media. Preculture with Toc prevented augmentation of the cytotoxic effects of diatrizoate by the higher glucose concentration (55 mmol/L). The exposure to diatrizoate together with Toc also attenuated its cytotoxic effects. Toc showed no such protective effects against iohexol exposure.

CONCLUSIONS

These findings suggest that high-glucose conditions enhance the susceptibility of MCs to the cytotoxic effects of both contrast media; the enhanced susceptibility was in part attributable to oxidative stress caused by high-glucose conditions; diatrizoate exerted the cytotoxic effects by means of oxidative stress; and iohexol appeared to exert its cytotoxicity in a manner different from diatrizoate.

High glucose levels enhance TGF-beta1-thrombospondin-1 pathway in cultured human mesangial cells via mechanisms dependent on glucose-induced PKC activation

We have previously demonstrated that thrombospondin-1 (TSP-1) promotes activation of latent TGF-beta1 in cultured human mesangial cells (MCs) [Nephron 79 (1998) 38.]. This study was performed to clarify the relationship between protein kinase C (PKC) activity and activation of TGF-beta1 and TSP-1 production in cultured human MCs exposed to high glucose levels. MCs grown in 33 mmol/l glucose demonstrated a significant (P< .01) increase in activation of TGF-beta1 compared with those in 5 mmol/l glucose, which were evaluated by both an ELISA and a bioassay. High glucose-induced increase in active TGF-beta1 was completely inhibited by coincubation with 5 micromol/l GFX, a PKC inhibitor, and was mimicked by the addition of 0.1 micromol/l phorbol 12,13-dibutyrate (PDBu). On the other hand, increased TSP-1 production from MCs stimulated by 0.1 nmol/l recombinant TGF-beta1 and 0.1 micromol/l PDBu were significantly (P< .01) reduced by the addition of 10 nmol/l latency-associated peptide (LAP), a specific inhibitor of TGF-beta1 activity. The amount of TSP-1 secreted from MCs increased by a high ambient glucose. The glucose-induced increase in TSP-1 production was markedly attenuated by the treatment with GFX and LAP, while those agents did not affect TSP-1 production in low-glucose concentrations. Taken together, our results suggest that glucose-induced activation of TGF-beta1 is dependent on PKC activity, leading to a sequential increase in TSP-1 synthesis in cultured human MCs. Thus, we propose that high glucose conditions induce an increase in PKC-TGF-beta1 activity-TSP-1 pathway, and that glucose-induced increase in TSP-1 may synergistically facilitate TGF-beta1 activation in an autocrine manner in MCs.

Nonantioxidant functions of alpha-tocopherol in smooth muscle cells

Most tocopherols and tocotrienols, with the exception of alpha-tocopherol, are not retained by humans. This suggests that alpha-tocopherol is recognized uniquely; therefore, it may exert an exclusive function. alpha-Tocopherol possesses distinct properties that are independent of its prooxidant, antioxidant or radical-scavenging ability. alpha-Tocopherol specifically inhibits protein kinase C, the growth of certain cells and the transcription of the CD36 and collagenase genes. Activation events have also been seen on the protein phosphatase 2A (PP(2)A) and on the expression of other genes (alpha-tropomyosin and connective tissue growth factor). Neither ss-tocopherol nor probucol possessed the same specialty functions as alpha-tocopherol. Recently, we isolated a new ubiquitous cytosolic alpha-tocopherol binding protein (TAP). Its motifs suggest that it is a member of the hydrophobic ligand-binding protein family (CRAL-TRIO). TAP may also be involved in the regulation of cellular alpha-tocopherol concentration and alpha-tocopherol-mediated signaling.

F(2)-isoprostanes mediate high glucose-induced TGF-beta synthesis and glomerular proteinuria in experimental type I diabetes

BACKGROUND

The recently discovered arachidonic acid derivatives, isoprostanes, are increased in pathological conditions associated with oxidative stress, such as diabetes. No role has yet been described for isoprostanes during the development of diabetic nephropathy. Cell culture in high ambient glucose has been used as a model in elucidating cellular mechanisms underlying diabetic nephropathy. Among the growth factors involved in the effect of high glucose, transforming growth factor-beta (TGF-beta) has been described as playing a key role in the development of nephropathy.

METHODS

Streptozotocin-induced diabetic rats were supplemented in their diet with the antioxidant vitamin E (1000 U/kg diet). Blood and urine samples were taken to determine renal function and isoprostane concentration, as determined by gas chromatography/mass spectrometry. Glomerular mesangial and endothelial cells were cultured in high ambient glucose to determine the synthesis of isoprostanes and the role of isoprostanes in high glucose-induced synthesis of TGF-beta.

RESULTS

Streptozotocin-induced diabetic rats had marked increases in plasma levels and urinary excretion rates of F(2)-isoprostanes. Dietary supplementation with vitamin E normalized (plasma) and reduced (urine) isoprostane levels and, surprisingly, improved proteinuria and blood urea nitrogen (BUN) levels. High ambient glucose increased F(2)-isoprostane synthesis in glomerular endothelial and mesangial cells in culture. Incubation of glomerular cells with F(2)-isoprostanes stimulated the production of TGF-beta.

CONCLUSIONS

Increased F(2)-isoprostane synthesis during diabetes appears to be responsible in part for the increase in renal TGF-beta, a well-known mediator of diabetic nephropathy.

Specific cellular responses to alpha-tocopherol

In the last 10 years precise cellular functions of alpha-tocopherol, some of which are independent of its antioxidant/radical-scavenging ability, have been revealed. Absorption of alpha-tocopherol from the gut is a selective process. Other tocopherols are not absorbed or are absorbed to a lesser extent. At the post-translational level, alpha-tocopherol inhibits protein kinase C and 5-lipoxygenase and activates protein phosphatase 2A and diacylglycerol kinase. Some genes [platelet glycoprotein IV/thrombospondin receptor/class B scavenger receptor (CD36), alpha-tocopherol transfer protein (alpha-TTP), alpha-tropomyosin, connective tissue growth factor and collagenase] are affected by alpha-tocopherol at the transcriptional level. alpha-Tocopherol also inhibits cell proliferation, platelet aggregation, monocyte adhesion and the oxygen burst in neutrophils. Other antioxidants, such as beta-tocopherol and probucol, do not mimic these effects, suggesting a nonantioxidant, alpha-tocopherol-specific molecular mechanism.

Vitamin E: non-antioxidant roles

Vitamin E was originally considered a dietary factor of animal nutrition especially important for normal reproduction. The significance of vitamin E has been subsequently proven as a radical chain breaking antioxidant that can protect the integrity of tissues and play an important role in life processes. More recently alpha-tocopherol has been found to possess functions that are independent of its antioxidant/radical scavenging ability. Absorption in the body is alpha-tocopherol selective and other tocopherols are not absorbed or are absorbed to a lesser extent. Furthermore, pro-oxidant effects have been attributed to tocopherols as well as an anti-nitrating action. Non-antioxidant and non-pro-oxidant molecular mechanisms of tocopherols have been also described that are produced by alpha-tocopherol and not by beta-tocopherol. alpha-Tocopherol specific inhibitory effects have been seen on protein kinase C, on the growth of certain cells and on the transcription of some genes (CD36, and collagenase). Activation events have been seen on the protein phosphatase PP2A and on the expression of other genes (alpha-tropomyosin and Connective Tissue Growth Factor). Non-antioxidant molecular mechanisms have been also described for gamma-tocopherol, delta-tocopherol and tocotrienols.