Insulin and a putative insulin metabolic mediator fraction from liver and muscle stimulate p33 messenger ribonucleic acid accumulation by apparently different mechanisms.

Abstract

We have previously shown that in rat H4 hepatoma cells insulin enhances the nuclear transcription of p33 mRNA in a dose- and time-dependent manner, with no alteration in mRNA half-time (t1/2). Presumably, this effect is mediated by the cell surface receptor. In this report, we have investigated the effect of putative insulin mediator fractions which act to control metabolic events on p33 mRNA accumulation in these cells. Initial experiments originally demonstrated an insulin-like effect of an added putative metabolic fraction to enhance p33 mRNA concentrations. However, when the fetal calf serum supply was changed, the effect of insulin remained, but that of added mediator was no longer observed. After a series of experimental approaches designed to alter the permeability of the cell membrane, it was found that in the presence of increased Ca2+, the effect of mediator could again be observed. The present data demonstrate that the partially purified cAMP-dependent protein kinase/adenylate cyclase inhibitory putative mediator fractions from liver and muscle enhance p33 mRNA accumulation in intact H4 hepatoma cells by a mechanism that is differentiated from that of insulin. The action of the putative mediator is inhibited by cycloheximide, while the action of insulin itself is not. These results suggest that insulin may control nuclear transcription by multiple signaling mechanisms. Alternatively, the added putative metabolic mediator may not enter the cell in the presence of cycloheximide or is inactive as such within the cell and must first be converted to an active species by a step requiring protein synthesis.