Chromium activates glucose transporter 4 trafficking and enhances insulin-stimulated glucose transport in 3T3-L1 adipocytes via a cholesterol-dependent mechanism

Evidence suggests that chromium supplementation may alleviate symptoms associated with diabetes, such as high blood glucose and lipid abnormalities, yet a molecular mechanism remains unclear. Here, we report that trivalent chromium in the chloride (CrCl3) or picolinate (CrPic) salt forms mobilize the glucose transporter, GLUT4, to the plasma membrane in 3T3-L1 adipocytes. Concomitant with an increase in GLUT4 at the plasma membrane, insulin-stimulated glucose transport was enhanced by chromium treatment. In contrast, the chromium-mobilized pool of transporters was not active in the absence of insulin. Microscopic analysis of an exofacially Myc-tagged enhanced green fluorescent protein-GLUT4 construct revealed that the chromium-induced accumulation of GLUT4-containing vesicles occurred adjacent to the inner cell surface membrane. With insulin these transporters physically incorporated into the plasma membrane. Regulation of GLUT4 translocation by chromium did not involve known insulin signaling proteins such as the insulin receptor, insulin receptor substrate-1, phosphatidylinositol 3-kinase, and Akt. Consistent with a reported effect of chromium on increasing membrane fluidity, we found that chromium treatment decreased plasma membrane cholesterol. Interestingly, cholesterol add-back to the plasma membrane prevented the beneficial effect of chromium on both GLUT4 mobilization and insulin-stimulated glucose transport. Furthermore, chromium action was absent in methyl-beta-cyclodextrin-pretreated cells already displaying reduced plasma membrane cholesterol and increased GLUT4 translocation. Together, these data reveal a novel mechanism by which chromium may enhance GLUT4 trafficking and insulin-stimulated glucose transport. Moreover, these findings at the level of the cell are consistent with in vivo observations of improved glucose tolerance and decreased circulating cholesterol levels after chromium supplementation.     

Chromium Improves Protein Deposition Through Regulating the mRNA Levels of IGF-1, IGF-1R,and Ub in Rat Skeletal Muscle Cells

The aim of this study was to evaluate the impact of three different chromium forms—chromic chloride (CrCl₃), chromium picolinate (CrPic), and a newly synthesized complex of chromium chelated with small peptides (CrSP)—on protein metabolism in vitro. In cultured skeletal muscle cells, CrSP was able to increase the basal and insulin-stimulated levels of protein deposition in skeletal muscles cells. CrCl₃ and CrPic augmented insulin-stimulated protein synthesis. At the molecular level, insulin significantly increased the mRNA levels of insulin-like growth factor 1 and insulin-like growth factor 1 receptor. These impacts could be enhanced by the addition of chromium, especially CrSP. The mRNA levels of ubiquitin were significantly reduced when cells were cultured with chromium or/and insulin. Assuming that the mRNA level increase or decrease results in increased or decreased levels of these proteins, chromium would improve protein anabolism and reduce protein catabolism and then prove protein deposition in rat skeletal muscle cells.     

Comparing metabolic effects of six different commercial trivalent chromium compounds

Recent reports provide cogent evidence that the average individual becomes chromium deficient with age. Unfortunately, chromium deficiency is strongly associated with many aspects of the Metabolic Syndrome, including insulin resistance and type 2 diabetes. Since replacement of chromium, per os, often ameliorates many deleterious manifestations associated with insulin resistance and diabetes, it is not surprising that many different, commercial trivalent chromium compounds are available on the market. However, previous reports have shown that the form of trivalent chromium (negative charges) can influence effectiveness markedly. We compared various commercial forms of trivalent chromium commonly used alone or in formulations, to examine whether they are equally effective and non-toxic. In the first study, five different chromium products were examined - citrate, amino acid chelate (AAC), chelavite, polynicotinate (NBC), and nicotinate. In the second study, effects of NBC and picolinate were assessed. Results demonstrated that only chelavite and NBC improved insulin sensitivity, and only NBC decreased systolic blood pressure (SBP) significantly. In the second study, both picolinate and NBC significantly decreased SBP compared to control. NBC and picolinate decreased malonyldialdehyde concentrations (free radical formation) and DNA fragmentation in hepatic and renal tissues. No evidence of adverse effects was noted with any of the compounds tested. In conclusion, while all the trivalent chromium compounds tested seem safe, only three enhanced insulin sensitivity (NBC, chelavite, and picolinate) and only two decreased SBP significantly (NBC and picolinate). Furthermore, both NBC and picolinate were protective in lessening free radical formation and DNA damage in the liver and kidneys.     

Insulin sensitising action of chromium picolinate in various experimental models of diabetes mellitus.

Although chromium is an essential element for carbohydrate and lipid metabolism, its effects in diabetic patients are still debated. We have studied the effect of 6 week treatment with chromium picolinate (8 microg/ml in drinking water) in streptozotocin (STZ)-induced type 1 and type 2 diabetic rat models. The mechanism of anti-diabetic action of chromium picolinate was studied using C2C12 myoblasts and 3T3-L1 adipocytes. Chromium picolinate significantly decreased the area under the curve over 120 min for glucose of both STZ-induced type 1 (40mg/kg, i.v. in adult rats) and type 2 (90 mg/kg, i.p. in 2 day old rat neonates) diabetic rats without any significant change in area under the curve over 120 min for insulin as compared to controls. The composite insulin sensitivity index and insulin sensitivity index (KITT) values of both type 1 and type 2 diabetic rats were increased significantly by chromium picolinate. Treatment with chromium picolinate produced a significant decrease in elevated cholesterol and triglyceride levels in both types of diabetic rats. In 3T3-L1 adipocytes, chromium picolinate (0-10 micromol) per se did not produce any effect, however, when co-incubated with insulin it significantly increased the intracellular triglyceride synthesis (EC50 = 363.7nmol/1). Similarly in C2C12 myoblasts, chromium picolinate alone did not produce any effect, however, it significantly increased insulin-induced transport of 14C-glucose. In conclusion, chromium picolinate significantly improves deranged carbohydrate and lipid metabolism of experimental chemically induced diabetes in rats. The mechanism of in vivo anti-diabetic action appears to be peripheral (skeletal muscle and adipose tissue) insulin enhancing action of chromium.     

Chromium Improves Glucose Uptake and Metabolism Through Upregulating the mRNA Levels of IR,GLUT4, GS,and UCP3 in Skeletal Muscle Cells

The aim of this study was to evaluate the impact of three different chromium forms as chromic chloride (CrCl), chromium picolinate (CrPic), and a newly synthesized complex of chromium chelated with small peptides (CrSP) on glucose uptake and metabolism in vitro. In cultured skeletal muscle cells, chromium augmented insulin-stimulated glucose uptake and metabolism as assessed by a reduced glucose concentration of culture medium. At the molecular level, insulin significantly increased the mRNA levels of insulin receptor (IR), glucose transporter 4 (GLUT4), glycogen synthase (GS), and uncoupling protein-3 (UCP3), and these impacts can be enhanced by the addition of chromium, especially in the form of CrSP. Collectively, results of this study demonstrate that chromium improves glucose uptake and metabolism through upregulating the mRNA levels of IR, GLUT4, GS, and UCP3 in skeletal muscle cells, and CrSP has higher efficacy on glucose uptake and metabolism compared to the forms of CrCl and CrPic.     

Type 2 Diabetic Rats on Diet Supplemented With Chromium Malate Show Improved Glycometabolism,Glycometabolism-Related Enzyme Levels and Lipid Metabolism

Our previous study showed that chromium malate improved the regulation of blood glucose in mice with alloxan-induced diabetes. The present study was designed to evaluate the effect of chromium malate on glycometabolism, glycometabolism-related enzymes and lipid metabolism in type 2 diabetic rats. Our results showed that fasting blood glucose, serum insulin level, insulin resistance index and C-peptide level in the high dose group had a significant downward trend when compared with the model group, chromium picolinate group and chromium trichloride group. The hepatic glycogen, glucose-6-phosphate dehydrogenase, glucokinase, Glut4, phosphor-AMPKβ1 and Akt levels in the high dose group were significantly higher than those of the model, chromium picolinate and chromium trichloride groups. Chromium malate in a high dose group can significantly increase high density lipoprotein cholesterol level while decreasing the total cholesterol, low density lipoprotein cholesterol and triglyceride levels when compared with chromium picolinate and chromium trichloride. The serum chromium content in chromium malate and chromium picolinate group is significantly higher than that of the chromium trichloride group. The results indicated that the curative effects of chromium malate on glycometabolism, glycometabolism-related enzymes and lipid metabolism changes are better than those of chromium picolinate and chromium trichloride. Chromium malate contributes to glucose uptake and transport in order to improved glycometabolism and glycometabolism-related enzymes.     

Effect of different doses of chromium picolinate on protein metabolism in infant rats.

This 12-day study was conducted to evaluate the effects of three different levels of dietary chromium (100, 200, and 500 microg/day) in the form of chromium picolinate (CrPic) on growth and protein use in weaned rats. No significant effect of CrPic on body weight gain, food intake, or food conversion rate was observed. Elevated doses of CrPic seemed to increase muscle mass, either by stimulating protein anabolism by activation of insulin by chromium or by lowering protein degradation. However, these effects had no repercussions on overall growth, suggesting that any anabolic effect of chromium due to the action of insulin was probably marginal.     

Kinetic and equilibrium studies on the interaction of reduced flavoprotein D-amino acid oxidase with pyridine carboxylates

The equilibrium constants and the rate constants (binding and dissociation constants) between reduced D-amino acid oxidase and pyridine carboxylates were obtained at various pH values (from pH 6.0 to 8.3). The pH dependence of the constants is consistent with the previous conclusion from a resonance Raman study that pyridine carboxylates in the form of a cation protonated at the N atom can bind to the reduced enzyme, but those in the neutral form cannot bind, showing that the positive charge of cationic pyridine carboxylates interacts with the negative charge of the anionic reduced flavin in the reduced enzyme. The binding rate constants of picolinate and nicotinate in the cationic form for the reduced enzyme were quite similar to each other, but the dissociation rate constant of picolinate is several times smaller than that of nicotinate. Thus, it is concluded that the difference in affinity of picolinate and nicotinate for the reduced enzyme is derived from the difference of the dissociation rate constants.     

Down-regulation and recycling of insulin receptors. Effect of monensin on IM-9 lymphocytes and U-937 monocyte-like cells

Receptor down-regulation is the result of various cellular processes including receptor internalization, new synthesis, and recycling. Monensin, a monocarboxylic acid ionophore, has been used to characterize the role of recycling in the metabolism of insulin receptors on two cultured human cell lines, U-937 and IM-9, which have different rates of internalization. The U-937 monocyte-like cell internalizes insulin receptors readily. Incubation with monensin at low doses (10(-6) to 10(-7) M) for 2 h did not affect subsequent surface insulin binding. However, the drug markedly enhanced insulin-induced down-regulation. Monensin had little effect on ligand internalization in this cell line as demonstrated by quantitative morphometric analysis. The IM-9 lymphocyte, a slow internalizer, was less sensitive to monensin exposure. Prolonged exposure (12 h) to this compound of either cell line resulted in apparent inhibition of insertion into the surface membrane of both newly synthesized and recycled receptors. When solubilization was used to quantitate total cell receptors, there was essentially no difference in intact cell binding (i.e. surface receptors) and total cell binding in IM-9 cells when insulin-induced down regulation alone was compared to insulin and monensin. By contrast for the U-937 cells there was only a small further decrease in binding when monensin was added to insulin in the solubilized cells compared to the marked augmentation of down-regulation when monensin was added to insulin in intact cells. These data demonstrate that cells with a rapid internalization rate have an associated active recycling process. By contrast cells with a slow internalization rate have a similarly slow recycling rate. This is consistent with relatively equal rates of receptor biosynthesis and plasma membrane insertion in both cell types.     

Comparison of acute absorption of commercially available chromium supplements

Chromium (Cr) supplements are available as picolinate, nicotinate or chloride (the latter primarily in multivitamin-mineral supplements). The picolinate form has been reported to be the best absorbed and most efficacious, but some reports question which form has superior absorption. The present study examined acute Cr absorption, based on 24h urinary Cr values, for picolinate, two types of nicotinate, and chloride in young adult, non-overweight females. College-aged women were given 200 microg of Cr as each of the four supplement types in random order accompanied by a small standardized meal, separated by at least a week washout. Cr picolinate produced significantly higher 24h urinary Cr than either of two nicotinate supplements or Cr chloride given in a multivitamin-mineral supplement. This difference was seen for absolute values of the urinary Cr and for percent increases. In conclusion, based on an indirect measure of acute absorption, Cr picolinate was superior to three other Cr complexes commonly sold as supplements.     

Ligand-independent internalization and recycling of the insulin receptor. Effects of chronic treatment of 3T3-C2 fibroblasts with insulin and dexamethasone.

Upon binding insulin at the plasma membrane, the insulin receptor internalizes into the endosomal compartment of the cell with a half-time of approximately 10 min. Our earlier work demonstrated that receptor inactivation (loss of insulin binding capacity) is a regulated process. Long term treatment of cultured cells with insulin or the glucocorticoid dexamethasone increases or decreases, respectively, the rate constant for insulin receptor inactivation (Knutson, V. P., Ronnett, G. V., and Lane, M. D. (1982) Proc. Natl. Acad. Sci. U.S.A. 79, 2822-2826). In these studies, monolayer cultures of 3T3-C2 fibroblasts were chronically treated with insulin or dexamethasone. Subsequently, the surface receptors were labeled with the photoactivatable cross-linking agent 125I-labeled 2-(p-azidosalicylamido)ethyl-1,3'- dithiopropionate -insulin. Following equilibration of the radiolabeled receptor between the plasma membrane and internal pools, the steady-state rate constant for receptor recycling was determined by quantitating the rate at which internal radiolabeled receptor was inserted into the plasma membrane. The steady-state rate constant for this recycling process was the same in control, insulin-treated, or steroid-treated cells (t1/2 = 2h). In contrast, the rate constant for receptor internalization was regulated; the half-times were 10 h for control cells, 5 h for insulin-treated cells, and 19 h for dexamethasone-treated cells. These changes in rate constants for internalization and inactivation lead to changes in the relative numbers of receptor molecules undergoing recycling versus inactivation. Therefore, whereas the recycling of the insulin receptor is not a regulated process, the internalization of surface receptor in the absence of bound ligand is a metabolically controlled step in receptor processing.     

Effect of chromium picolinate on histopathological alterations in STZ and neonatal STZ diabetic rats

Earlier studies from our laboratory have indicated insulin sensitizing action of chromium picolinate as the mechanism of its anti-diabetic activity in experimental models of type I and type II diabetes. In the present investigation, we have evaluated the effects of chronic administration of chromium picolinate on the functional and histological alterations of streptozotocin (STZ)-induced diabetes in rats. Type I diabetes was induced by intravenous injection of STZ (40 mg/kg) in adult rats, whereas, type II diabetes was induced by intraperitoneal injection of STZ (90 mg/kg) in 2-day old rat pups which in adulthood develop abnormalities resembling type II diabetes. Chromium picolinate was administered at 8 μg/ml in drinking water for 6 weeks and was found to improve glucose tolerance and increase insulin sensitivity of STZ-diabetic rats. This treatment decrease elevated serum creatinine and urea levels as well as elevated serum levels of hepatic enzymes of both groups of diabetic rats. Histopathological studies of kidney and liver show decrease in the intensity and incidence of vacuolations, cellular infiltration and hypertrophy of STZ and nSTZ (neonatal STZ) diabetic rats. Chronic treatment with chromium picolinate however, did not alter the normal function or morphology of control rats. Chronic chromium picolinate at the therapeutic doses that improved glucose tolerance, was observed to have no hepatotoxic or nephrotoxic potential. It was rather found to improve renal and hepatic function and to reduce abnormalities associated with STZ-diabetes. Chromium picolinate could play an important role in the long term management of diabetes mellitus.     

Elevation of apparent membrane viscosity in ovarian granulosa cells by follicle-stimulating hormone

Continued exposure of cultured granulosa cells to follicle-stimulating hormone (FSH) induced: (i) a rise in apparent membrane microviscosity, as reflected by an increase in fluorescence polarization of the lipid-soluble probe, 1,6-diphenyl-1,3,5,-hexatriene; and (ii) a progressive decline in the cyclic AMP response to renewed challenge with the same hormone. Both changes were reduced or prevented by pretreatment of the cells with oleic or linoleic acid, agents which reduce membrane viscosity, but not by elaidic or palmitic acid which increase the rigidity of membrane lipids. Other agents that inhibited FSH-induced changes in membrane fluidity (gonadotropin-releasing hormone, actinomycin D and cycloheximide) also prevented desensitization to FSH. Cyclic AMP and cyclic GMP derivatives did not mimic the effects of FSH on apparent membrane viscosity or desensitization. Changes in membrane fluidity are unlikely to be the sole cause of desensitization since (i) pretreatment of the cells with fatty acids that increase lipid viscosity did not induce desensitization to FSH, and (ii) desensitization of granulosa cells to lutropin and prostaglandin E2 by exposure to the homologous hormone was not attended by increased membrane viscosity. The experiments described provide the first example of a hormonally induced increase in the target cell apparent membrane viscosity.     

Chromium in metabolic and cardiovascular disease.

Chromium is an essential mineral that appears to have a beneficial role in the regulation of insulin action, metabolic syndrome, and cardiovascular disease. There is growing evidence that chromium may facilitate insulin signaling and chromium supplementation therefore may improve systemic insulin sensitivity. Tissue chromium levels of subjects with diabetes are lower than those of normal control subjects, and a correlation exists between low circulating levels of chromium and the incidence of type 2 diabetes. Controversy still exists as to the need for chromium supplementation. However, supplementation with chromium picolinate, a stable and highly bioavailable form of chromium, has been shown to reduce insulin resistance and to help reduce the risk of cardiovascular disease and type 2 diabetes. Since chromium supplementation is a safe treatment, further research is necessary to resolve the confounding data. The existing data suggest to concentrate future studies on certain forms as chromium picolinate and doses as at least 200 mcg per day.     

Insulin binding and vesicular ingestion in capillary endothelium

Capillary endothelium can actively regulate vascular permeability of various serum proteins. Hormones such as insulin must interact with this capillary barrier in order to reach their respective target tissues. We have studied the binding and subsequent internalization of 125I-insulin in both native (freshly isolated) and primary cultured capillary endothelium derived from rat epididymal fat pads. Insulin association with the endothelium, internalization and degradation differed between freshly isolated and primary cultured capillaries. Specific binding in freshly isolated and cultured capillaries was temperature dependent, and was competitively inhibited in the presence of unlabelled insulin. Primary cultures of capillaries grown to confluence did not exhibit specific binding of insulin. Despite the lack of specific receptors for insulin, cultured cells vesicularly internalized insulin. Greater than 50% of the total associated insulin was not degraded by cultured endothelium. Morphological examinations using ferritin labelled insulin localized insulin associated to the capillary endothelial cell membrane and sequestered within pinocytotic vesicles. Incubation of freshly isolated capillaries with insulin stimulated the fluid phase endocytosis of 14C-sucrose; however, insulin had no effect on fluid phase endocytosis in cultured capillaries. These results indicate that capillary endothelium, isolated from rat epididymal fat, exhibit specific receptors for insulin. Binding of insulin to the capillary membrane is followed by internalization into cytoplasmic vesicles and partial degradation.     

Regulation of insulin receptor internalization in vascular endothelial cells by insulin and phorbol ester

Phorbol 12-myristate 13-acetate (PMA) was used to examine the role of insulin receptor phosphorylation in the regulation of insulin receptor internalization in vascular endothelial cells. Association of 125I-insulin in rat capillary and bovine aortic endothelial cells preincubated with PMA was increased by 80 and 64% over control, respectively. The increase was due to enhanced 125I-insulin internalization as opposed to an effect on surface-bound hormone. PMA had no significant effect on 125I-insulin degradation or on release of internalized insulin from the cells. Internalization of 125I-labeled insulin receptor was determined by the resistance of labeled receptor to trypsinization. At 10 degrees C, nearly all of the labeled receptor was sensitive to removal by trypsin, indicating that it was exposed on the cell surface. Exposure of labeled cells to insulin (100 nM) at 37 degrees C resulted in the rapid appearance of trypsin-resistant insulin receptor, indicating receptor internalization. Steady state for receptor internalization was attained at 10-15 min. When surfaced-labeled cells were preincubated with PMA at 37 degrees C, the rate of insulin receptor internalization was increased by 3.6 +/- 0.2-fold and 2.1 +/- 0.5-fold at 1 and 5 min of insulin exposure, respectively (ED50 at 16 nM PMA). This effect of PMA was associated with an increase in serine phosphorylation of the insulin receptor. Thus, PMA increased insulin internalization in the endothelial cells by modulating the insulin-induced internalization of the receptor. The additive effects of PMA and insulin on insulin receptor phosphorylation suggest that the phorbol ester and insulin act via independent signaling mechanisms.     

Chromium supplementation in non-obese non-diabetic subjects is associated with a decline in insulin sensitivity

The use of chromium supplements is widespread for the prevention and treatment of diabetes mellitus but there are conflicting reports on efficacy, possibly reflecting discrepant effects across different populations. In the present studies, we test the hypothesis that chromium supplementation raises serum chromium levels and correspondingly improves insulin sensitivity. A double blind placebo-controlled randomized trial was conducted on 31 non-obese, normoglycemic subjects. After baseline studies, the subjects were randomized to placebo or chromium picolinate 500 μg twice a day. The primary endpoint was change in insulin sensitivity as measured by euglycemic hyperinsulinemic clamp. Pre-specified secondary endpoints included fasting lipids, blood pressure, weight, body composition measured by DXA scan. After 16 weeks of chromium picolinate therapy there was no significant change in insulin sensitivity between groups (p=0.83). There was, however, a strong association between serum chromium and change in insulin resistance (β = -0.83, p=0.01), where subjects with the highest serum chromium had a worsening of insulin sensitivity. This effect could not be explained by changes in physiological parameters such as body weight, truncal fat and serum lipids with chromium therapy. Chromium therapy did not improve insulin sensitivity in non-obese normoglycemic individuals. Further, subjects who have high serum chromium levels paradoxically had a decline in insulin sensitivity. Caution therefore should be exercised in recommending the use of this supplement. The study was registered on the NIH registry (clinicaltrials.gov) and the identifier is NCT00846248     

Reversible reduction of insulin receptor affinity by ATP depletion in rat adipocytes.

The effects of the metabolic inhibitor NaN3 on insulin receptors in isolated rat fat-cells were investigated. The agent reduced insulin binding in parallel to a decrease of the ATP content of cells. Both effects were observed in the same concentration range of NaN3, and were fully reversible. According to the binding curves the affinity rather than the number of receptors was reduced. Kinetic experiments revealed an increased dissociation rate of the insulin-receptor complex. The effects outlasted cell disruption, since the receptor affinity was still lowered in plasma membranes obtained from NaN3-treated cells. Thus an inhibition of insulin internalization could not account for the observed effects. It is suggested that the observed ATP-dependence of insulin receptor affinity reflects a reversible structural alteration of the receptor, or of some closely related membrane protein.     

Chromium-induced glucose uptake, superoxide anion production, and phagocytosis in cultured pulmonary alveolar macrophages of weanling pigs

The dose-dependent effects of chromium chloride (CrCl₃) and chromium picolinate (CrPic) were evaluated for their glucose uptake, superoxide anion (O ₂ ⁻ ) production, activity of glucose-6-phosphate dehydrogenase, and phagocytosis of incubated pulmonary alveolar macrophages in medium containing no or 5 × 10⁻⁸ M insulin. Glucose uptake was found to increase in cells treated with 20 μg/L CrCl₃. Incubation with 20 μg/L of CrPic enhanced glucose uptake and O ₂ ⁻ production in an insulin-dependent manner. However, the inclusion of CrPic to 100 μg/L in the medium absent of insulin also increased O ₂ ⁻ production. The activity of glucose-6-phosphate dehydrogenase was not affected by either the addition of Cr or insulin. The phagocytosis of Escherichia coli by macrophages was enhanced significantly (p<0.05) in medium containing 10–100 μg/L CrCl₃ or 20–100 μg/L CrPic in the presence of insulin. These results suggest that the addition of 10–20 μg/L CrCl₃ enhances directly the cellular activity of macrophages, whereas the effect of CrPic requires the cooperative action of insulin in enhancing their glucose uptake and phagocytosis.     

Chromium chloride inhibits oxidative stress and TNF-alpha secretion caused by exposure to high glucose in cultured U937 monocytes.

Chromium supplementation has been proposed to promote the action of insulin and the lowering of blood glucose levels in diabetic patients. However, the molecular mechanism by which chromium increases insulin sensitivity is not known. Using U937 monocytes as a cell culture model, this study demonstrates that chromium inhibits the secretion of TNF-alpha, a cytokine known to inhibit the sensitivity and action of insulin. U937 cells were cultured with high levels of glucose (mimicking diabetes) in the presence or absence of chromium chloride in the medium at 37 degrees C for 24 h. This study demonstrates that chromium supplementation prevents the increase in TNF-alpha levels and oxidative stress caused by the high levels of glucose in cultured U937 monocytic cells. Similarly, chromium supplementation prevented elevated TNF-alpha secretion and lipid peroxidation levels in H(2)O(2)-treated U937 cells. This study demonstrates for the first time that chromium supplementation inhibits TNF-alpha secretion in U937 monocytes cultured in high-glucose medium, which appears to be mediated by its antioxidative effect. This provides evidence for a novel molecular mechanism by which chromium supplementation may increase insulin sensitivity and glycemic control in diabetic patients.