Insulin receptor substrate is a mediator of phosphoinositide 3-kinase activation in quiescent pancreatic cancer cells

Phosphoinositide 3-kinase (PI3K) is activated in pancreatic cancer cells and plays a central role in their proliferation, survival, and drug resistance. Although the mechanism is unclear, PI3K activation in these cells could be due to physical interaction between its regulatory subunit (p85) and specific tyrosine kinases or their mediators. Consistent with this possibility, PI3K was precipitated with anti-phosphotyrosine antibodies and Akt phosphorylation was blocked by the tyrosine kinase inhibitors SU6656 and PD158780 in quiescent pancreatic cancer cells. Pull-down assays with a fusion protein (GST-p85NC-SH2), and coimmunoprecipitation studies, indicated that the insulin receptor substrate (IRS), and not the epidermal growth factor and insulin-like growth factor receptors or the Src tyrosine kinase, was physically associated with PI3K in these cells. Our data also indicated that SU6656 and PD158780 inhibited Akt activation in pancreatic cancer cells by interfering with the ability of IRS-1 to recruit PI3K. Furthermore, IRS-1 was phosphorylated on a p85-binding site (Y-612), and IRS-specific small interfering RNA potently inhibited activation of PI3K and Akt in transfected cells. Taken together, these observations indicate that IRS is a mediator of PI3K activation in quiescent pancreatic cancer cells.