Essential Role for Rac in Heregulin β1 Mitogenic Signaling: a Mechanism That Involves Epidermal Growth Factor Receptor and Is Independent of ErbB4

Abstract

Heregulins are a family of ligands for the ErbB3/ErbB4 receptors that play important roles in breast cancer cell proliferation and tumorigenesis. Limited information is available on the contribution of Rho GTPases to heregulin-mediated signaling. In breast cancer cells, heregulin β1 (HRG) causes a strong activation of Rac; however, it does so with striking differences in kinetics compared to epidermal growth factor, which signals through ErbB1 (epidermal growth factor receptor [EGFR]). Using specific ErbB receptor inhibitors and depletion of receptors by RNA interference (RNAi), we established that, surprisingly, activation of Rac by HRG is mediated not only by ErbB3 and ErbB2 but also by transactivation of EGFR, and it is independent of ErbB4. Similar receptor requirements are observed for HRG-induced actin cytoskeleton reorganization and mitogenic activity via extracellular signal-regulated kinase (ERK). HRG-induced Rac activation was phosphatidylinositol 3-kinase dependent and Src independent. Furthermore, inactivation of Rac by expression of the Rac GTPase-activating protein β2-chimerin inhibited HRG-induced ERK activation, mitogenicity, and migration in breast cancer cells. HRG mitogenic activity was also impaired by depletion of Rac1 using RNAi. Our studies established that Rac is a critical mediator of HRG mitogenic signaling in breast cancer cells and highlight additional levels of complexity for ErbB receptor coupling to downstream effectors that control aberrant proliferation and transformation.

This work was supported by grants RO1-CA74197 (NIH) and RPG-97-092-06-CNE (American Cancer Society) to M.G.K. and RO1-CA096768 (NIH) to M.A.L. C.Y. is supported by a postdoctoral fellowship from the Department of Defense (DAMD17-03-1-0469).

We thank Richard K. Assoian and Margaret M. Chou (University of Pennsylvania) for their generous gifts of plasmids, Kathryn M. Ferguson (University of Pennsylvania) for her kind gift of cetuximab, and Marc Symons (North Shore-LIJ Research Institute, NY) for providing Rac1 RNAi sequence.