Investigation of PI3Kγ signaling downstream of IGF-1R-CXCR4 transactivation in metastatic MDA-MB-231 breast cancer cells.

Abstract

Breast cancer metastasis is a multi-step process regulated by a number of homeostatic factors. The insulin-like growth factor 1 tyrosine kinase receptor (IGF-1R) and the chemokine G-protein coupled receptor, CXCR4 have been shown to play an important role in breast cancer metastasis. More recently, accumulating evidence suggest that these two distinct receptors may regulate breast cancer cell migration through receptor transactivation. However, the underlying molecular mechanisms by which IGF-1RCXCR4 transactivation regulates breast cancer cell metastasis remain unclear. Since phosphoinositide 3 kinases (PI3Ks) are known to be key signaling molecules governing cell migration, PI3K signaling downstream of IGF-1R-CXCR4 transactivation was investigated. In the present study the expression of class I PI3K isoforms was investigated in metastatic MDA-MB-231 breast cancer cells compared to that in non-metastatic MCF-7 cells. The data show that high levels of class IB PI3K catalytic subunit, p110γ are restricted to the highly metastatic cell types, correlating with the metastatic potential of the cell lines. Moreover, PI3Kγ is the major PI3K isoform regulating cell migration and activation of Akt downstream of IGF-1R-CXCR4 transactivation in metastatic MDA-MB-231 cells. Finally, several downstream targets that are dependent on PI3Kγ were identified using 2-D Fluorescence Difference Gel Electrophoresis (DIGE) and mass spectrometry analysis, including eukaryotic elongation factor 2 (eEF2), pyruvate kinase isozymes M1/M2 (PKM1/M2) and phosphoglycerate kinase 1 (PGK1) with PI3Kγ being shown to regulate phosphorylation of eEF2. In summary, the data in this study demonstrate a novel role for PI3Ky in regulating cell migration downstream of IGF-1R-CXCR4 transactivation, potentially by attenuating cell proliferation via inhibition of eEF2 activation. The understanding of molecular mechanisms underlying receptor transactivation, including PI3K signaling transduction pathways in the progression of breast cancer metastasis and invasion may lead to development of more effective diagnostic and therapeutic strategies.