Transforming Growth Factor-α Prevents Detachment-induced Inhibition of c-Src Kinase Activity, Bcl-XLDown-regulation, and Apoptosis of Intestinal Epithelial Cells

Abstract

Detachment of epithelial cells from the extracellular matrix (ECM) results in apoptosis, a phenomenon often referred to as anoikis. Acquisition of anoikis resistance is now thought to be a prerequisite for the progression of carcinomas. Colorectal cancer cells frequently secrete epidermal growth factor receptor (EGFR) ligands, which are known to have anti-apoptotic activity. However, whether these ligands have the ability to inhibit anoikis of intestinal epithelial cells is unclear, since at least in some cell types efficient EGFR signaling requires cell-ECM adhesion. Here we report that transforming growth factor-alpha (TGF-alpha), an EGFR ligand that is frequently secreted by colorectal cancer cells, strongly inhibits anoikis of the non-malignant rat intestinal epithelial cell lines, IEC-18 and RIE-1. TGF-alpha exerts its anti-anoikis effect by preventing detachment-induced inhibition of c-Src kinase activity. We also show that Fas activation, a molecular event known to play a critical role in anoikis, is not suppressed by TGF-alpha. On the other hand, this growth factor strongly inhibits the detachment-induced down-regulation of Bcl-X(L), another change that is involved in the induction of anoikis. We further demonstrate that this inhibition occurs in a c-Src-dependent manner. We conclude that TGF-alpha has the ability to suppress anoikis of intestinal epithelial cells, at least in part, by reverting the loss of c-Src activity and Bcl-X(L) expression induced by detachment from the ECM.