Insulin action in cultured HTC and H35 rat hepatoma cells: receptor binding and hormone sensitivity.

Insulin receptors and several biological effects of insulin were measured in H35 and HTC cells, two rat hepatoma lines in permanent cell culture that have been employed previously to study glucocorticoid action. In H35 cells, insulin at concentrations as low as 1 pM both enhanced tyrosine amino-transferase activity and stimulated [3H]uridine incorporation into RNA. In this cell line, the binding of approximately 10 molecules of insulin/cell was sufficient to elicit these two biological responses. In contrast, in HTC cells, much higher concentrations of insulin (3 nM or greater) were needed to stimulate both tyrosine aminotransferase activity and other biological functions. When insulin receptors were measured, both cell types had a major insulin-binding site with a Kd of approximately 20 nM. H35 cells, however, had approximately 6 times more receptors per cell than HTC cells. In both cell types, insulin degradation was minimal. These findings indicate that several biological functions in H35 cells are extremely sensitive to insulin in vitro. In contrast, HTC cells are much less sensitive to the hormone. This lack of sensitivity in HTC cells most likely reflects a decrease in the number of insulin receptors present. In addition, the very large differences in responsiveness suggest that postreceptor alterations are also operative.