Role of IRS-4 in PI3-K activation by insulin in HepG2 cells, modulation by Angiotensin II.

Insulin receptor substrate-4 (IRS-4) has a limited tissue expression and its modulation by tyr-phosphorylation is still controversial. We evaluated the participation of IRS-4 in the cross-talk between Angiotensin II (Ang II) and Insulin (Ins) receptors in HepG2 cells. Ins (10(-7)M) induced tyr-phosphorylation of IRS-4 (maximal at 5 min), an effect potentiated by Ang II AT(1) receptors. Phosphatydilinositol-3 kinase (PI3-K) inhibitors Wortmanin or LY294002 reduced Ang II effect on tyr-phosphorylation of IRS-4 to a level comparable to that of Ins alone. Physical association between IRS-4 substrate and PI3-K was demonstrated by co-immunoprecipitation. Recruitment of PI3-K by IRS-4 was induced by Ins (10(-7)M, 5 min) not by Ang II (10(-7)M) and this was inhibited by Wortmanin and LY294002. Ang II did not modify either the association or activation of PI3-K in immunocomplexes. The present data provide novel evidence of IRS-4 phosphorylation mediated by Ins, an effect modulated by Ang II. We report also Ins-induced PI3-K activation mediated by IRS-4. Our findings suggest a role for IRS-4 as a docking protein in the Ins signaling pathway that involves PI3-K association and activation. The present data suggest a possible participation of IRS-4 in cell proliferation Ins-induced.