A novel CYR61-triggered ‘CYR61-αvβ3 integrin loop’ regulates breast cancer cell survival and chemosensitivity through activation of ERK1/ERK2 MAPK signaling pathway

The angiogenic inducer CYR61 is differentially overexpressed 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 αvβ3 (>200 times). Small peptidomimetic αvβ3 integrin antagonists dramatically decreased cell viability of CYR61-overexpressing MCF-7 cells, whereas control MCF-7/V remained insensitive. Mechanistically, functional blockade of αvβ3 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, αvβ3 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 αvβ3 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 αvβ3, 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 αvβ3 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 αvβ3-activated ERK1/ERK2 MAPK signaling. The identification of a ‘CYR61-αvβ3 autocrine loop’ in the epithelial compartment of breast carcinoma strongly suggests that targeting αvβ3 may simultaneously prevent breast cancer angiogenesis, growth, and chemoresistance.