INSULIN STIMULATES VITAMIN-C RECYCLING AND ASCORBATE ACCUMULATION IN OSTEOBLASTIC CELLS

Insulin modulates the differentiation and synthetic activity of osteob lasts, but its mechanisms of action are not fully understood. Because ascorbate also influences osteoblast differentiation and is a cofactor for collagen synthesis, we examined the effects of insulin on the tra nsport and metabolism of vitamin C in osteoblastic cells. UMR-106 rat osteoblast-like cells accumulated ascorbate intracellularly when incub ated with dehydroascorbic acid (DHAA; oxidized vitamin C). Insulin inc reased the intracellular concentration of ascorbate derived from DHAA and also increased the initial rates of uptake of DHAA and 2-deoxygluc ose, but not that of ascorbate. A half-maximal effect on DHAA uptake w as observed with approximately 100 pM insulin, whereas insulin-like gr owth factor I (IGF-I) was less potent. Preincubation with insulin for 6-12 h was required for stimulation, similar to the period needed for increased expression of facilitative hexose transporters (GLUT). DHAA uptake was inhibited by the GLUT antagonist cytochalasin B as well as by the GLUT substrates D-glucose and 2-deoxyglucose, whereas L-glucose and fructose had no effect. We conclude that insulin and IGF-I stimul ate osteoblastic uptake of DHAA through facilitative hexose transporte rs. The relative potency of insulin in stimulating DHAA uptake is cons istent with mediation by insulin receptors. DHAA is reduced to ascorba te within osteoblasts, maintaining a high intracellular concentration of ascorbate available for collagen synthesis. Impaired uptake of DHAA may contribute to the osteopenia associated with type I diabetes. In addition, cytotoxic levels of DHAA may accumulate in the extracellular fluid due to decreased transport activity and competitive inhibition by elevated concentrations of glucose.