Effects of IGF-I and Glucose on Protein and Proteoglycan Synthesis by Human Fetal Mesangial Cells in Culture

Abstract

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 × 10⁻⁹ M IGF-I. In the presence of physiological glucose concentrations, IGF-I stimulated incorporation of [³H]leucine into protein and [³⁵S]sulfate or [³H]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-³⁵SO₄ and chondroitin/dermatan-³⁵SO₄ proteoglycans in all experimental media were the same. However, high glucose concentrations decreased the iduronic acid content of dermatan-³⁵SO₄. 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 isosmolar 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 [³⁵S]sulfate to [³H]glucosamine in proteoglycans and their glycosaminoglycan side chains. This difference persisted in disaccharides derived by chondroitin ABC lyase digestion of dermatan-³⁵SO₄. A higher specific activity of [³H]glucosamine without a change in [³⁵S]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.