INSULIN INDUCES THE PHOSPHORYLATION OF NUCLEOLIN - A POSSIBLE MECHANISM OF INSULIN-INDUCED RNA EFFLUX FROM NUCLEI

Insulin induces the serine phosphorylation of the nucleolar protein nu cleolin at subnanomolar concentrations in differentiated 3T3-442A cell s. The stimulation is biphasic with phosphorylation reaching a maximum at 10 pM insulin and then declining to only 40% of basal levels at in sulin concentrations of 1 muM. These changes are rapid, reaching half- maximal after 4 min and maximal after 15 min of incubation. The cell-p ermeable casein kinase II inhibitor 5,6-dichlorobenzimidazole-riboside prevents the insulin-stimulated phosphorylation of nucleolin suggesti ng that casein kinase II may mediate this effect of the hormone. Insul in-like growth factor 1 mimics the action of insulin on dephosphorylat ion of nucleolin at nanomolar concentrations suggesting that the latte r effect may be mediated by insulin-like growth factor 1 receptors. In sulin treatment of 3T3-442A cells also results in a stimulation of RNA efflux from isolated, intact cell nuclei. The dose dependence of insu lin-induced nucleolin phosphorylation and insulin-stimulated RNA efflu x from intact cell nuclei are almost identical. Insulin induces an inc rease in the RNA efflux at subnanomolar concentrations in 3T3-442A adi pocytes, while high (micromolar) concentrations of insulin inhibited t he efflux of RNA. These data indicate that insulin regulates the phosp horylation/dephosphorylation of nucleolin, possibly via stimulation of casein kinase II, and this may play a role in regulation of the RNA e fflux from nuclei.