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 [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 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 [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.