A Novel Mechanism for UDCA-Induced Growth Suppression

Bile acids have been studied for many years for their role in either promoting (Deoxycholic Acid) or suppressing (Ursodeoxycholic Acid) colon tumor development in animal models. However, the molecular mechanisms of both DCA's and UDCA's biological effects in colon tumorigenesis is still unclear. The cholesterol-like composition of bile acids and evidence of deregulating signal transduction pathways, such as the p42/44 MAP kinase cascade, led us to identify the plasma membrane as a target for bile acid-mediated effects. Specifically, plasma membrane microdomains such as lipid rafts and caveolae are particularly capable of altering mitogenic signaling due to have their role as platforms to concentrate receptors and assemble signal transduction machinery. In this study I tested the hypothesis that the growth suppressive effects of UDCA are mediated by stimulating membrane microdomains to activate protein degradation machinery to facilitate the down-regulation of receptor tyrosine kinase activity. We found that UDCA suppresses EGF-induced ERK activation, promotes interactions between EGFR and Caveolin-1 membrane fractions, whereas DCA causes redistribution. EGFR proteins that are localized to membrane fractions in the UDCA treated cells are extensively ubiquitinylated and we present evidence that this yields recruitment of the ubiquitin ligase c-Cbl to membrane fractions. UDCA increases the rate of EGFR degradation, whereas DCA sustains its' stability. I present evidence that UDCA's growth inhibitory effects on colon cancer cells may be mediated by recruitment of protein degradation machinery to membrane domains that are enriched with signaling receptors, a mechanism which has not been previously described. Importantly, I demonstrate for the first time a novel mechanism by which UDCA promotes growth inhibition, through increasing the rates of degradation of EGFR, thereby down-regulating mitogenic signaling in the cell. These experiments show exciting insights into the mechanism of bile acids and represent potential mechanisms for other chemopreventive agents