INSULIN INHIBITS NUCLEAR PHOSPHATASE-ACTIVITY - REQUIREMENT FOR THE C-TERMINAL DOMAIN OF THE INSULIN-RECEPTOR

Insulin's interaction with its receptor initiates a multitude of cellu lar effects on metabolism, growth, and differentiation. We recently de scribed an insulin-mediated inhibition of nuclear protein phosphatase 2A (PP-2A), which is associated with an increase in phosphorylation of the transcription factor cAMP response element-binding protein. To cl arify the role of nuclear PP-2A inhibition in the insulin signaling ca scade, we examined the regulation of this phosphatase activity by insu lin in Rat-1 fibroblasts overexpressing normal (HIRc) or mutant human insulin receptors (Delta CT cells, deletion of a 43-amino acid C-termi nal domain). The Delta CT cells represent an excellent model of impair ed metabolic and intact mitogenic action of insulin. Insulin inhibited nuclear PP-2A activity and enhanced cAMP response element-binding pro tein phosphorylation in HIRc cells, but not in Delta CT cells. The Del ta CT cells exhibited normal ras activation and blunted mitogen-activa ting protein kinase phosphorylation and activation in response to insu lin (16-fold in HIRc cells vs. 3-fold in Delta CT cells), indicating t hat the mitogen-activating protein kinase pathway is important for the regulation of nuclear PP-2A activity by insulin. We conclude that ins ulin inhibits nuclear PP-2A activity, and that the carboxy-terminal do main of the insulin receptor is important for this effect.