Identification of Insulin Receptor Substrate 1 (IRS-1) and IRS-2 as Signaling Intermediates in the α6β4 Integrin-Dependent Activation of Phosphoinositide 3-OH Kinase and Promotion of Invasion

Expression of the α6β4 integrin increases the invasive potential of carcinoma cells by a mechanism that involves activation of phosphoinositide 3-OH kinase (PI3K). In the present study, we investigated the signaling pathway by which the α6β4 integrin activates PI3K. Neither the α6 nor the β4 cytoplasmic domain contains the consensus binding motif for PI3K, pYMXM, indicating that additional proteins are likely to be involved in the activation of this lipid kinase by the α6β4 integrin. We identified insulin receptor substrate 1 (IRS-1) and IRS-2 as signaling intermediates in the activation of PI3K by the α6β4 integrin. IRS-1 and IRS-2 are cytoplasmic adapter proteins that do not contain intrinsic kinase activity but rather function by recruiting proteins to surface receptors, where they organize signaling complexes. Ligation of the α6β4 receptor promotes tyrosine phosphorylation of IRS-1 and IRS-2 and increases their association with PI3K, as determined by coimmunoprecipitation. Moreover, we identified a tyrosine residue in the cytoplasmic domain of the β4 subunit, Y1494, that is required for α6β4-dependent phosphorylation of IRS-2 and activation of PI3K in response to receptor ligation. Most importantly, Y1494 is essential for the ability of the α6β4 integrin to promote carcinoma invasion. Taken together, these results imply a key role for the IRS proteins in the α6β4-dependent promotion of carcinoma invasion.