Class II Phosphoinositide 3-Kinases Are Downstream Targets of Activated Polypeptide Growth Factor Receptors

Abstract

The class II phosphoinositide 3-kinases (PI3K) PI3K-C2α and PI3K-C2β are two recently identified members of the large PI3K family. Both enzymes are characterized by the presence of a C2 domain at the carboxy terminus and, in vitro, preferentially utilize phosphatidylinositol and phosphatidylinositol 4-monophosphate as lipid substrates. Little is understood about how the catalytic activity of either enzyme is regulated in vivo. In this study, we demonstrate that PI3K-C2α and PI3K-C2β represent two downstream targets of the activated epidermal growth factor (EGF) receptor in human carcinoma-derived A431 cells. Stimulation of quiescent cultures with EGF resulted in the rapid recruitment of both enzymes to a phosphotyrosine signaling complex that contained the EGF receptor and Erb-B2. Ligand addition also induced the appearance of a second, more slowly migrating band of PI3K-C2α and PI3K-C2β immunoreactivity on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Since both PI3K enzymes can utilize Ca²⁺ as an essential divalent cation in lipid kinase assays and since the catalytic activity of PI3K-C2α is refractory to the inhibitor wortmannin, these properties were used to confirm the recruitment of each PI3K isozyme to the activated EGF receptor complex. To examine this interaction in greater detail, PI3K-C2β was chosen for further investigation. EGF and platelet-derived growth factor also stimulated the association of PI3K-C2β with their respective receptors in other cells, including epithelial cells and fibroblasts. The use of EGF receptor mutants and phosphopeptides derived from the EGF receptor and Erb-B2 demonstrated that the interaction with recombinant PI3K-C2β occurs through E(p)YL/I phosphotyrosine motifs. The N-terminal region of PI3K-C2β was found to selectively interact with the EGF receptor in vitro, suggesting that it mediates the association of this PI3K with the receptor. However, the mechanism of this interaction remains unclear. We conclude that class II PI3K enzymes may contribute to the generation of 3′ phosphoinositides following the activation of polypeptide growth factor receptors in vivo and thus mediate certain aspects of their biological activity.

ACKNOWLEDGMENTS

We thank Christopher Odell for help with automated DNA sequencing, Shun-Cheng Li (Samuel Lunenfield Research Institute, Toronto, Ontario, Canada) for EGFR and ErbB-2 phosphopeptides, Julian Downward and Peter Parker (ICRF, London, United Kingdom) for purified monoclonal anti-Glu and anti-myc tag antibodies, and Parmjit Jat for the pBabeNeo vector and the BOSC 23 cell line.

A.A. was supported by a grant from the Swiss National Science Foundation.