horylation in isolated rat liver- nship to mRNA metabolism

The direct addition of insulin to highly purified nuclear envelopes prepared from the livers of diabetic rats re- sulted in a decrease in the incorporation of 32p into trichloroacetic acid-precipitable proteins. Autoradiography of 32P-labeled enve- lopes, solubilized in sodium dodecyl sulfate and subjected to elec- trophoresis, revealed that insulin decreased the phosphorylation of all major protein bands. Insulin produced detectable effects at concentrations between 0.1 and 1 pM, maximal effects at 10 pM, and progressively diminished effects at higher concentrations. Two insulin analogs, desdipeptide proinsulin and desoctapeptide in- sulin, had approximately 10% and 1%, respectively, the activity of native insulin. When nuclear envelopes were first phosphorylated with (y-32P)ATP and insulin was then added with an excess of un- labeled ATP, dephosphorylation was enhanced, suggesting that in- sulin was regulating nuclear envelope phosphatase activity. The direct addition of insulin to isolated rat liver nuclei in the presence of ATP stimulated the release of previously "4C-labeled trichlo- roacetic acid-precipitable mRNA-like material, and the direct ad- dition of insulin to nuclear envelopes stimulated the activity of nu- cleoside triphosphatase, the enzyme that participates in mRNA nucleocytoplasmic transport. Moreover, the dose-response curves for these functions mirrored insulin's inhibition of nuclear enve- lope phosphorylation. These data suggest, therefore, a mechanism whereby insulin directly inhibits the phosphorylation of the nu- clear envelope, leading in turn to the regulation of mRNA me- tabolism.