Mechanism of insulin action on hepatic messenger RNA activity

Liver poly(A) RNA, isolated from adrenalectomized rats after insulin treatment, was translated in a nuclease-treated lysate of rabbit reticulocytes and quantitated for both total activity and the capacity to synthesize the insulin inducible enzyme tyrosine aminotransferase. Analysis of the translated products from poly(A) RNA isolated 1h after insulin treatment showed a 2.7-fold increase in activity of tyrosine aminotransferase mRNA. During the same interval, the capacity of poly(A) RNA to direct the synthesis of total protein in lysates is also changed. showing a 30 to 40% increase in translational activity per unit RNA. Characterization of the in vitro translation of poly(A) RNA from control and insulin-treated rats showed no detectable differences in their translation products, salt requirements, or RNA saturating concentrations. The increase in translational activity was indeed an increase in the rate of translation of the poly(A) RNA from insulin-treated animals; this was due to an enhanced capacity to bind reticulocyte ribosomes and initiate protein synthesis. The aminotransferase mRNA levels were measured in different poly(A) RNA preparations using purified DNA complementary to tyrosine aminotransferase mRNA. Hybridizational analyses revealed no significant increase in the concentration of aminotransferase mRNA in poly(A) RNA prepared after insulin treatment. Insulin thus mediates a generalized change in mRNAs leading to an increased efficiency for translation; induction of tyrosine aminotransferase reflects an unusual sensitivity to this effect.