Siah-1L, a novel transcript variant belonging to the human Siah family of proteins, regulates β-catenin activity in a p53-dependent manner

β -Catenin is a potent oncogenic protein whose cytoplasmic accumulation is a frequent event in cancer cells. The level of β -catenin is regulated by two mechanisms: the adenomatous polyposis coli/Axin/glycogen synthase kinase 3 β -dependent degradation pathway and the Siah-1/Siah interacting protein/Ebi-mediated degradation pathway. In this study, we have investigated the functional significance of p53-inducible human Siah-family protein expression in the regulation of β -catenin activity. We show here by reverse-transcriptase polymerase chain reaction that two mRNA transcripts, designated human Siah-1 and Siah-1L, are generated from the human Siah-1 locus. Interestingly, the expression of Siah-1L was upregulated by p53, whereas human Siah-1 expression was constant. Furthermore, introduction of exogenous Siah-1L protein downregulated β -catenin protein and promoted apoptosis induced by anticancer drugs in cancer cells that lack endogenous p53. Thus, Siah-1L represents a new member of the human Siah family that is induced in response to p53 and plays an important role in the regulation of β -catenin activity in tumor cells. These findings also suggest new strategies for restoring tumor suppressive pathways lost in cancer cells that have suffered p53 inactivation.