Differential signaling imposed by BRMS1 as a potential mechanism for differential growth of breast cancer cells Free

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Metastatic cells grow at ectopic sites; whereas, normal cells are spatially growth restricted. The breast cancer metastasis suppressor-1, BRMS1, allows orthotopic tumor growth but prevents growth at ectopic sites. This observation suggests microenvironment-dependent signaling in BRMS1-expressing cells. For this study, MDA-MB-231/435 (231/435) breast carcinoma cells (lacking endogenous BRMS1) and their BRMS1 transfectants (231BRMS1/435BRMS1) were exposed to epidermal growth factor (EGF), platelet derived growth factor (PDGF), or hepatocyte growth factor (HGF) and downstream activation of AKT(pSer473), p42/p44 MAPK was measured by immunoblotting, and calcium mobilization was measured by ratiometric analysis. 231BRMS1/435BRMS1 cells showed decreased EGF-receptor (EGFR) expression (mRNA and protein) and reduced signaling through PI3K-AKT, but not MAPK. Receptor phosphorylation studies showed that EGFR was equally functional in the vector-control and BRMS1-expressors suggesting receptor downregulation to be a cause of decreased signaling. Calcium signaling through EGFR was reduced concomitant with decreased EGFR expression. Further, downstream activation of both AKT and MAPK through PDGFR was reduced in the BRMS1-expressors, although receptor levels were unchanged. In contrast, no reduction in AKT or MAPK signaling was detected upon HGF treatment and surprisingly, c-Met was slightly elevated in BRMS1-expressing cells. Globally, AKT phosphorylation was decreased in response to selected growth factors, suggesting compensatory signaling through HGFR. BRMS1-expressors also showed reduced PI3K-AKT signaling in response to insulin or phorbol ester. Taken together, these data demonstrate that BRMS1 regulates AKT and MAPK differentially depending upon the stimulus. Such differential signaling responses imposed by BRMS1 may ultimately determine which microenvironments become permissive/restrictive for cellular survival and growth, perhaps explaining why BRMS1-expressing cells grow at orthotopic, but not ectopic sites.

Support: CA87728, Susan G. Komen Breast Cancer Foundation Post-doctoral Fellowship PDF1122006, National Foundation for Cancer Research.