Integrin Activates Receptor-Like Protein Tyrosine Phosphatase α, Src, and Rho to Increase Prolactin Gene Expression through a Final Phosphatidylinositol 3-Kinase/Cytoskeletal Pathway that Is Additive with Insulin

We previously showed that receptor-like protein tyrosine phosphatase (RPTP)-α inhibited insulin-increased prolactin gene transcription. Others suggested that RPTPα was a key intermediary between integrins and activation of Src. We present evidence that inhibition of insulin-increased prolactin gene transcription was secondary to RPTPα activation of Src, reflecting its role as mediator of integrin responses. Src kinase activity was increased in GH4 cells transiently or stably expressing RPTPα and cells plated on the integrin-α5β1 ligand fibronectin. C-terminal Src kinase inactivated Src and blocked RPTPα inhibition of insulin-increased prolactin gene transcription. Expression of dominant-negative Src also prevented the RPTPα-mediated inhibition of insulin-increased prolactin gene expression. Low levels of a constitutively active Src mutant (SrcY/F) stimulated whereas higher expression levels of Src Y/F inhibited prolactin gene expression. Src-increased prolactin gene transcription was inhibited by expression of a blocking Rho-mutant (RhoN19), suggesting that Src acted through or required active Rho. Experiments with an activated Rho-mutant (RhoL63) demonstrated a biphasic activation/repression of prolactin gene transcription that was similar to the effect of Src. The effects of both Src and Rho were phosphatidylinositol 3-kinase dependent. Expression of SrcY/F or RhoL63 altered the actin cytoskeleton and morphology of GH4 cells. Taken together, these data suggest a physiological pathway from the cell matrix to increased prolactin gene transcription mediated by RPTPα/Src/Rho/phosphatidylinositol 3-kinase and cytoskeletal change that is additive with effects of insulin. Over activation of this pathway, however, caused extreme alteration of the cytoskeleton that blocked activation of the prolactin gene.