EFFECTS OF IGF-I AND GLUCOSE ON PROTEIN AND PROTEOGLYCAN SYNTHESIS BY HUMAN FETAL MESANGIAL CELLS IN CULTURE

Abnormalities in proteoglycan metabolism have been implicated in the pathogenesis of diabetic nephropathy. Whether hyperglycemia plays a direct role in these events is unknown. To evaluate the effects of high glucose concentrations and insulinlike growth factor I (IGF-I) on kidney proteoglycan and protein metabolism, we incubated quiescent, subconfluent human fetal mesangial cells for 24 h in serum-free media containing either physiological (5.6-mM) or elevated (25-mM) glucose concentrations with or without 1.3 x 10(-9) M IGF-I. In the presence of physiological glucose concentrations, IGF-I stimulated incorporation of [H-3]leucine into protein and [S-35]sulfate or [H-3]glucosamine into proteoglycans. High glucose concentrations significantly amplified IGF-I-mediated stimulation of protein synthesis but totally abolished IGF-I-induced proteoglycan synthesis. The hydrodynamic size and proportions of heparan-(SO4)-S-35 and chondroitin/dermatan-(SO4)-S-35 proteoglycans in all experimental media were the same. However, high glucose concentrations decreased the iduronic acid content of dermatan-(SO4)-S-35. In separate experiments, quiescent cells were cultured for 7 days in media supplemented with 2% fetal calf serum. IGF-I had no effect on mesangial cell proliferation, but as cells reached confluence, high glucose concentrations significantly inhibited cell proliferation. This inhibition was not mimicked by isoosmolar concentrations of mannitol. After 7 days, uptake of radioactive precursors into proteoglycans and proteins over 24 h was similar under all culture conditions. However, IGF-I decreased the ratio of [S-35]sulfate to [H-3]glucosamine in proteoglycans and their glycosaminoglycan side chains. This difference persisted in disaccharides derived by chondroitin ABC lyase digestion of dermatan-(SO4)-S-35. A higher specific activity of [H-3]glucosamine without a change in [S-35]sulfate uptake or proteoglycan structure indicated that IGF-I treatment of confluent mesangial cells altered the glucosamine pool size. Thus, in subconfluent human mesangial cells, high concentrations of glucose amplified IGF-I-stimulated protein synthesis but abrogated IGF-I-stimulated proteoglycan synthesis, resulting in a relatively proteoglycan-depleted state. This effect was not seen in cells that had been confluent for several days.