PtdIns(3,4,5)P₃-dependent Rac exchanger 1 (P-Rex1) promotes mammary tumor initiation and metastasis.

Abstract

The Rac-GEF, P-Rex1, activates Rac1 signaling downstream of G protein-coupled receptors and PI3K. Increased P-Rex1 expression promotes melanoma progression; however, its role in breast cancer is complex, with differing reports of the effect of its expression on disease outcome. To address this we analyzed human databases, undertook gene array expression analysis, and generated unique murine models of P-Rex1 gain or loss of function. Analysis of <i>PREX1</i> mRNA expression in breast cancer cDNA arrays and a METABRIC cohort revealed that higher <i>PREX1</i> mRNA in ER^+ve/luminal tumors was associated with poor outcome in luminal B cancers. <i>Prex1</i> deletion in MMTV-<i>neu</i> or MMTV-<i>PyMT</i> mice reduced Rac1 activation in vivo and improved survival. High level MMTV<i>-</i>driven transgenic <i>PREX1</i> expression resulted in apicobasal polarity defects and increased mammary epithelial cell proliferation associated with hyperplasia and development of de novo mammary tumors. MMTV-<i>PREX1</i> expression in MMTV-<i>neu</i> mice increased tumor initiation and enhanced metastasis in vivo, but had no effect on primary tumor growth. Pharmacological inhibition of Rac1 or MEK1/2 reduced P-Rex1-driven tumoroid formation and cell invasion. Therefore, P-Rex1 can act as an oncogene and cooperate with HER2/neu to enhance breast cancer initiation and metastasis, despite having no effect on primary tumor growth.