ERK/MAPK Signaling Drives Overexpression of the Rac-GEF, PREX1, in BRAF- and NRAS-Mutant Melanoma

Abstract

Recently we identified that PREX1 overexpression is critical for metastatic but not tumorigenic growth in a mouse model of NRAS-driven melanoma. In addition, a PREX1 gene signature correlated with and was dependent on ERK mitogen-activated protein kinase (MAPK) activation in human melanoma cell lines. In the current study, the underlying mechanism of PREX1 overexpression in human melanoma was assessed. PREX1 protein levels were increased in melanoma tumor tissues and cell lines compared with benign nevi and normal melanocytes, respectively. Suppression of PREX1 by siRNA impaired invasion but not proliferation in vitro. PREX1-dependent invasion was attributable to PREX1-mediated activation of the small GTPase RAC1 but not the related small GTPase CDC42. Pharmacologic inhibition of ERK signaling reduced PREX1 gene transcription and additionally regulated PREX1 protein stability. This ERK-dependent upregulation of PREX1 in melanoma, due to both increased gene transcription and protein stability, contrasts with the mechanisms identified in breast and prostate cancers, where PREX1 overexpression was driven by gene amplification and HDAC-mediated gene transcription, respectively. Thus, although PREX1 expression is aberrantly upregulated and regulates RAC1 activity and invasion in these three different tumor types, the mechanisms of its upregulation are distinct and context-dependent.

P-Rex1 and P-Rex2 RacGEFs and cancer

“…Active Rac regulates many essential cellular responses including actin cytoskeletal rearrangement, cell migration, adhesion, and the production of reactive oxygen species (ROS) [6,7]. Recent evidence links both Rac and RacGEFs, including P-Rex1 and P-Rex2, to increased cell migration and proliferation in various human cancers [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23].…”

“…In addition, an association between P-Rex1 expression levels and prostate cancer patient survival, disease progression, or stage has not been reported. P-Rex1 protein up-regulation has been shown in benign melanocyte nevi [14], primary melanoma, and lymph node metastases, and the relative expression of P-Rex1 increases with disease progression [13]. The majority of human cell lines derived from NRAS-or BRAF-mutated melanomas exhibit up-regulation of P-Rex1 protein compared with normal melanocytes or NRAS-/BRAF-wild-type cell lines [13,14], suggesting subset-specific involvement of P-Rex1 in melanoma.…”

“…P-Rex1 protein up-regulation has been shown in benign melanocyte nevi [14], primary melanoma, and lymph node metastases, and the relative expression of P-Rex1 increases with disease progression [13]. The majority of human cell lines derived from NRAS-or BRAF-mutated melanomas exhibit up-regulation of P-Rex1 protein compared with normal melanocytes or NRAS-/BRAF-wild-type cell lines [13,14], suggesting subset-specific involvement of P-Rex1 in melanoma. Interestingly, the in vivo role P-Rex1 plays in melanoma metastasis has been examined experimentally by crossing Prex1 −/− mice with an Nras Q61K -driven mouse model of melanoma (Tyr::NrasQ61K/°transgenic mice) [13].…”

“…In melanoma, P-Rex1 mutations are mostly located in the C-terminal domains. In contrast, P-Rex2 mutations are distributed throughout the protein [14]. Sequence analysis has identified non-synonymous P-Rex2 mutations in 14% of 107 human melanoma samples, and 13 nonsynonymous mutations spanning the length of P-Rex2 were detected in 25 human metastatic melanomas [17].…”

“…P-Rex1 depletion by shRNA or siRNA in oestrogen receptor-positive (ER+) breast cancer cell lines (MCF-7 and T-47D) and in a metastatic prostate cancer cell line (PC3) impaired cell migration and invasion [8,9,15]. P-Rex1 knockdown via siRNA in multiple melanoma cell lines (CHL1, Wm793, Mel224, WM2664, Wm852) resulted in reduced invasive phenotypes [13,14,46]. P-Rex1 overexpression in breast and prostate cancer cell lines with negligible endogenous P-Rex1 expression (MDA-MB-231 and 22Rv1, respectively) increased migratory and invasive phenotypes [15,53].…”