Oncogenic ras-induced downregulation of a pro-apoptotic protease caspase-2 is required for malignant transformation of intestinal epithelial cells

Abstract

Resistance of carcinoma cells to anoikis, apoptosis that is normally induced by loss of cell-to-extracellular matrix (ECM) adhesion, is thought to be essential for the ability of these cells to form primary tumors, invade adjacent tissues and metastasize to distant organs. The current knowledge about the mechanisms by which cancer cells evade anoikis is far from complete. In an effort to understand these mechanisms we found that ras, a major oncogene, downregulates protease caspase-2 (that initiates certain steps of the cellular apoptotic program) in malignant human and rat intestinal epithelial cells. This downregulation could be reversed by inhibition of a protein kinase Mek, a mediator of Ras signaling. We also found that enforced downregulation of caspase-2 in non-malignant intestinal epithelial cells by RNA interference protected them from anoikis. Furthermore, the reversal of the effect of Ras on caspase-2 achieved by the expression of exogenous caspase-2 in detached ras-transformed intestinal epithelial cells promoted well established apoptotic events, such as the release of the pro-apoptotic mitochondrial factors cytochome c and HtrA2/Omi into the cytoplasm of these cells, significantly enhanced their anoikis susceptibility and blocked their long-term growth in the absence of adhesion to the ECM. Finally, the blockade of the effect of Ras on caspase-2 substantially suppressed growth of tumors formed by the ras-transformed cells in mice. We conclude that ras-induced downregulation of caspase-2 represents a novel mechanism by which oncogenic Ras protects malignant intestinal epithelial cells from anoikis, promotes their anchorage-independent growth and allows them to form tumors in vivo.