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 o f transforming growth factor-beta (TGF-beta 1), a prosclerotic cytokine cau sally 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 : fructos e-6-phosphateamidotransferase (GFAT), the rate-limiting enzyme of this path way. Furthermore, stable overexpression of GFAT increased levels of TGF-be ta 1 protein, mRNA, and promoter activity. Inasmuch as stimulation or inhib ition of GFAT increased or decreased high glucose stimulated activity of pr otein kinase C (PKC), respectively, the observed effects appear to be trans duced by PKC. In similar experiments, involvement of the hexosamine pathway in hyperglycemia-induced production of cytokines (TGF-alpha and basic fibr oblast growth factor [bFGF]) was demonstrated in vascular smooth muscle cel ls. These studies also revealed a rapid increase in GFAT activity by treatm ent with agents that elevated levels of cyclic adenosine 3',5' monophosphat e (cAMP), thus indicating that GFAT activity is tightly regulated by cAMP-d ependent phosphorylation. Using immunohistochemistry and in situ hybridizat ion, 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 stai ning occurred in glomerular cells of patients with diabetic nephropathy. Cu rrent 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.