Trivalent chromium inhibits TSP-1 expression, proliferation, and O-GlcNAc signaling in vascular smooth muscle cells in response to high glucose in vitro

Trivalent chromium (Cr3+) is a mineral nutrient reported to have beneficial effects in glycemic and cardiovascular health. In vitro and in vivo studies suggest that Cr3+ supplementation reduces the atherogenic potential and lowers the risk of vascular inflammation in diabetes. However, effects of Cr3+ in vascular cells under conditions of hyperglycemia, characteristic of diabetes, remain unknown. In the present study we show that a therapeutically relevant concentration of Cr3+ (100 nM) significantly downregulates a potent proatherogenic matricellular protein, thrombospondin-1 (TSP-1), in human aortic smooth muscle cells (HASMC) stimulated with high glucose in vitro. Promoter-reporter assays reveal that this downregulation of TSP-1 expression by Cr3+ occurs at the level of transcription. The inhibitory effects of Cr3+ on TSP-1 were accompanied by significant reductions in O-glycosylation of cytoplasmic and nuclear proteins. Using Western blotting and immunofluorescence studies, we demonstrate that reduced protein O-glycosylation by Cr3+ is mediated via inhibition of glutamine: fructose 6-phosphate amidotransferase, a rate-limiting enzyme of the hexosamine pathway, and O-linked N-acetylglucosamine (O-GlcNAc) transferase, a distal enzyme in the pathway that controls intracellular protein O-glycosylation. Additionally, we found that Cr3+ attenuates reactive oxygen species formation in glucose-stimulated HASMC, suggesting an antioxidant effect. Finally, we report an antiproliferative effect of Cr3+ that is specific for high glucose and conditions triggering elevated protein O-glycosylation. Taken together, these findings provide the first cellular evidence for a novel role of Cr3+ to modulate aberrant vascular smooth muscle cell function associated with hyperglycemia-induced vascular complications.