Activated ras prevents downregulation of Bcl-X-L triggered by detachment from the extracellular matrix: A mechanism of ras-induced resistance to anoikis in intestinal epithelial cells

Detachment of epithelial cells from the extracellular matrix (ECM) results in a form of apoptosis often referred to as anoikis, Transformation of inte stinal epithelial cells by oncogenic ras leads to resistance to anoikis, an d this resistance is required for the full manifestation of the malignant p henotype, Previously, we demonstrated that ms-induced inhibition of anoikis in intestinal epithelial cells results, in part, from the ras-induced cons titutive downregulation of Bak, a pro-apoptotic member of the Bcl-2 family. Since exogenous Ball could only partially restore susceptibility to anoiki s in the ms-transformed cells, the existence of at least another component of the apoptotic machinery mediating the effect of activated ms on anoikis was suggested. Indeed, here we show that, in nonmalignant rat and human int estinal epithelial cells, detachment from the ECM or disruption of the cyto skeleton results in a significant downregulation of the antiapoptotic effec tor BcL-X-L, and that activated H- or K-ras oncogenes completely abrogate t his downregulation. In addition, we found that enforced downregulation of B cl-X-L in the ms-transformed cells promotes anoikis and significantly inhib its tumorigenicity, indicating that disruption of the adhesion-dependent re gulation of Bcl-X-L is an essential part of the molecular changes associate d with transformation by ms. While the ms-induced downregulation of Bak cou ld be reversed by pharmacological inhibition of phosphatidylinositol 3 kina se (PI3-kinase), the effect of ras on Bcl-X-L was PI 3-kinase- and mitogen- activated protein kinase (MAP kinase)-independent. We conclude that uas-ind uced resistance to anoikis in intestinal epithelial cells is mediated by at least two distinct mechanisms: one that triggers downregulation of Bak and another that stabilizes Bcl-X-L expression in the absence of the ECM.