A novel CYR61-triggered 'CYR61-α_vβ₃ integrin loop' regulates breast cancer cell survival and chemosensitivity through activation of ERK1/ERK2 MAPK signaling pathway

The angiogenic inducer CYR61 is differentially over-expressed in breast cancer cells exhibiting high levels of Heregulin (HRG), a growth factor closely associated with a metastatic breast cancer phenotype. Here, we examined whether CYR61, independently of HRG, actively regulates breast cancer cell survival and chemosensitivity, and the pathways involved. Forced expression of CYR61 in HRG-negative MCF-7 cells notably upregulated the expression of its own integrin receptor α_vβ₃ (> 200 times). Small peptidomimetic α_vβ₃ integrin antagonists dramatically decreased cell viability of CYR61-overexpressing MCF-7 cells, whereas control MCF-7/V remained insensitive. Mechanistically, functional blockade of α_vβ₃ specifically abolished CYR6-induced hyperactivation of ERK1/ERK2 MAPK, whereas the activation status of AKT did not decrease. Moreover, CYR61 overexpression rendered MCF-7 cells significantly resistant (> 10-fold) to Taxol-induced cytotoxicity. Remarkably, α_vβ₃ inhibition converted the CYR61-induced Taxol-resistant phenotype into a hypersensitive one. Thus, the augmentation of Taxol-induced apoptotic cell death in the presence of α_vβ₃ antagonists demonstrated a strong synergism as verified by the terminal transferase-mediated dUTP nick-end labeling (TUNEL) assay and by flow cytometric analysis for DNA content. Indeed, functional blockade of α_vβ₃, similarly to the pharmacological MAPK inhibitor U0126, synergistically increased both the proportion of CYR61-overexpressing breast cancer cells in the G₂ phase of the cell cycle and the appearance of sub-G₁ hypodiploid (apoptotic) cells caused by Taxol. Strikingly, CYR61 overexpression impaired the accumulation of wild-type p53 following Taxol exposure, while inhibition of α_vβ₃ or ERK1/ERK2 MAPK signalings completely restored Taxol-induced upregulation of p53. Moreover, antisense downregulation of CYR61 expression abolished the anchorage-independent growth of breast cancer cells engineered to overexpress HRG, and significantly increased their sensitivity to Taxol. Our data provide evidence that CYR61 is sufficient to promote breast cancer cell proliferation, cell survival, and Taxol resistance through a α^vβ₃-activated ERK1/ERK2 MAPK signaling. The identification of a 'CYR61-α_vβ₃ autocrine loop' in the epithelial compartment of breast carcinoma strongly suggests that targeting α_vβ₃ may simultaneously prevent breast cancer angiogenesis, growth, and chemo resistance.