Hyperglycemia down-regulates apolipoprotein M expression in vivo and in vitro

Diabetes is associated with low concentrations of apoM in plasma. In db/db mice, ob/ob mice as well as in the alloxan-induced diabetic mouse, the low apoM levels are paralleled by decreased expression of the apoM gene. In the latter model, insulin substitution tended to reverse the low apoM expression. It is not known whether the impairment in apoM expression can be ascribed to hyperglycemia, insulin deficiency or insulin resistance. In the present study, we investigated apoM levels and expression in rats rendered hyperglycemic by short-term glucose infusion. As expected, serum insulin concentrations rose moderately during the infusions. Serum apoM concentrations and hepatic apoM mRNA levels were significantly reduced in the hyperglycemic rats, indicating that the low expression of apoM in these diabetic models can be ascribed to hyperglycemia rather than to insulin deficiency or insulin resistance. However, in HepG2 cells both glucose and insulin markedly inhibited apoM expression these effects were additive. Thus, the possible effects of insulin in vivo seem to be mediated indirectly.