Abstract
Phosphatidylinositol-3,4,5-Trisphosphate Dependent Rac Exchange Factor 1 (P-Rex1) is a key mediator of growth factor-induced activation of Rac1, a small GTP-binding protein widely implicated in actin cytoskeleton reorganization. This Guanine nucleotide Exchange Factor (GEF) is overexpressed in human luminal breast cancer, and its expression associates with disease progression, metastatic dissemination and poor outcome. Despite the established contribution of P-Rex1 to Rac activation and cell locomotion, whether this Rac-GEF has any relevant role in mitogenesis has been a subject of controversy. To tackle the discrepancies among various reports, we carried out an exhaustive analysis of the potential involvement of P-Rex1 on the activation of the mitogenic Erk pathway. Using a range of luminal breast cancer cellular models, we unequivocally showed that silencing P-Rex1 (transiently, stably, using multiple siRNA sequences) had no effect on the phospho-Erk response upon stimulation with growth factors (EGF, heregulin, IGF-I) or a GPCR ligand (SDF-1). The lack of involvement of P-Rex1 in Erk activation was confirmed at the single cell level using a fluorescent biosensor of Erk kinase activity. Depletion of P-Rex1 from breast cancer cells failed to affect cell cycle progression, cyclin D1 induction, Akt activation and apoptotic responses. In addition, mammary-specific P-Rex1 transgenic mice (MMTV-P-Rex1) did not show any obvious hyperproliferative phenotype. Therefore, despite its crucial role in Rac1 activation and cell motility, P-Rex1 is dispensable for mitogenic or survival responses in breast cancer cells.
Highlights
Rac small G-proteins (Rac1, Rac2, Rac3 and RhoG) play essential roles in the regulation of actin cytoskeleton dynamics and cell motility, and control other cellular functions such as adhesion, polarization, survival, cell cycle progression and gene expression
We have previously reported that P-Rex1, which is highly expressed in luminal breast cancer cells, is the main Rac-Guanine nucleotide Exchange Factor (GEF) mediating Rac1 activation in response to ErbB receptor ligands [18]
When Rac1-GTP levels in response to the ErbB3 receptor ligand Heregulin β1 (HRG) were measured using a p21-binding domain (PBD)-pull-down assay, a substantial reduction in Rac1 activation was observed in P-Rex1 knocked-down cells compared to parental MCF-7 cells or cells transfected with a non-target control (NTC) small interfering ribonucleic acid (siRNA) duplex pool (Figure 1A)
Summary
Rac small G-proteins (Rac, Rac, Rac and RhoG) play essential roles in the regulation of actin cytoskeleton dynamics and cell motility, and control other cellular functions such as adhesion, polarization, survival, cell cycle progression and gene expression. Like most Rho. GTPases, Rac proteins act as molecular switches that cycle between GDP-bound (inactive) and GTP-bound (active) states. Once in the inactive conformation, Guanine nucleotide-Dissociation Inhibitors (GDIs) bind to and stabilize Rac, and preclude it from getting activated [1,2,3]. It has been well established over the last two decades that Rac and their regulators play fundamental roles in cancer progression [3,4,5,6,7]. Changes in abundance or mutational status of Rac and their regulators, contribute to tumor growth and metastatic dissemination of cancer cells [3, 8,9,10,11]
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call