Role of the hexosamine biosynthetic pathway in diabetic nephropathy

The hexosamine biosynthetic pathway has been hypothesized to be involved in the development of insulin resistance and diabetic vascular complications. In particular, it was demonstrated that hyperglycemia-induced production of transforming growth factor-beta (TGF-beta 1), a prosclerotic cytokine causally involved in the development of diabetic nephropathy. Several lines of evidence indicate that TGF-beta 1 induction is mediated by the hexosamine pathway. In cultured mesangial cells, high glucose levels induce TGF-beta 1 production. This effect is eliminated by inhibition of glutamine : fructose-6-phosphateamidotransferase (GFAT), the rate-limiting enzyme of this path way. Furthermore, stable overexpression of GFAT increased levels of TGF-beta 1 protein, mRNA, and promoter activity. Inasmuch as stimulation or inhibition of GFAT increased or decreased high glucose stimulated activity of protein kinase C (PKC), respectively, the observed effects appear to be transduced by PKC. In similar experiments, involvement of the hexosamine pathway in hyperglycemia-induced production of cytokines (TGF-alpha and basic fibroblast growth factor [bFGF]) was demonstrated in vascular smooth muscle cells. These studies also revealed a rapid increase in GFAT activity by treatment with agents that elevated levels of cyclic adenosine 3',5' monophosphate (cAMP), thus indicating that GFAT activity is tightly regulated by cAMP-dependent phosphorylation. Using immunohistochemistry and in situ hybridization, high expression of GFAT was found in human adipocytes, skeletal muscle, vascular smooth muscle cells, and renal tubular epithelial cells, whereas glomerular cells remained essentially unstained. However, significant staining occurred in glomerular cells of patients with diabetic nephropathy. Current data indicate that the flux through the hexosamine pathway, regulated by GFAT, may be causally involved in the development of diabetic vascular disease, particularly diabetic nephropathy.