Constitutive CXCL12 Expression Induces Anoikis in Colorectal Carcinoma Cells

Abstract

CXCL12 and CXCR4 signaling plays critical roles in development, homeostasis, and tumor metastasis. Previously, we have shown that epigenetic silencing of CXCL12 in colorectal and mammary carcinomas promotes metastasis. Anoikis is an essential process of colonic epithelial turnover and limits the metastatic progression of carcinoma. We sought to determine the role for anoikis in limiting tumor metastasis following reexpression of CXCL12 in human colorectal carcinoma cells. Tumor formation and metastasis of colonic carcinoma cells was monitored using in vivo bioluminescence imaging. Anoikis was defined by using caspase-3/7, focal adhesion kinase (FAK) and p130Cas cleavage, DNA fragmentation, and cell survival assays. Phosphorylation of extracellular-regulated kinase-1/2 (ERK1/2) was monitored by immunoblot and immunohistochemistry, and activity was inhibited by using U0126. Constitutive expression of CXCL12 in human colorectal carcinoma cells reduced orthotopic tumor formation and inhibited metastasis in severe combined immunodeficient mice. Further, CXCL12 expression induced apoptosis specifically in nonadherent colorectal carcinoma cells. Apoptotic cell death was preceded by hypophosphorylation and cleavage of FAK and p130Cas, leading to increased cellular detachment in culture, and depended on alterations in the extracellular matrix. Similar to in vivo colonic epithelium, CXCL12-induced anoikis of carcinoma cells depended on basal ERK1/2 activation. These data significantly expand the current paradigm of chemokine signaling in carcinogenesis by showing that endogenous CXCL12, in marked contrast to exogenous ligand, inhibits tumor metastasis through increased anoikis. Altered ERK1/2 signaling provides a mechanism for the dichotomy between the physiologic and pathophysiologic roles of CXCL12-CXCR4 signaling in the intestinal epithelium.