Homozygous deletion of glycogen synthase kinase 3β bypasses senescence allowing Ras transformation of primary murine fibroblasts

In primary mammalian cells, expression of oncogenes such as activated Ras induces premature senescence rather than transformation. We show that homozygous deletion of glycogen synthase kinase (GSK) 3 beta (GSK3 beta(-/-)) bypasses senescence induced by mutant Ras(V12) allowing primary mouse embryo fibroblasts (MEFs) as well as immortalized MEFs to exhibit a transformed phenotype in vitro and in vivo. Both catalytic activity and Axin-binding of GSK3 beta are required to optimally suppress Ras transformation. The expression of Ras(V12) in GSK3 beta(-/-), but not in GSK3 beta(+/+) MEFs results in translocation of beta-catenin to the nucleus with concomitant up-regulation of cyclin D1. siRNA-mediated knockdown of beta-catenin decreases both cyclin D1 expression and anchorage-independent growth of transformed cells indicating a causal role for beta-catenin. Thus Ras(V12) and the lack of GSK3 beta act in concert to activate the beta-catenin pathway, which may underlie the bypass of senescence and tumorigenic transformation by Ras.