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) 3β (GSK3β−/−) bypasses senescence induced by mutant RasV12 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 GSK3β are required to optimally suppress Ras transformation. The expression of RasV12 in GSK3β−/−, but not in GSK3β+/+ MEFs results in translocation of β-catenin to the nucleus with concomitant up-regulation of cyclin D1. siRNA-mediated knockdown of β-catenin decreases both cyclin D1 expression and anchorage-independent growth of transformed cells indicating a causal role for β-catenin. Thus RasV12 and the lack of GSK3β act in concert to activate the β-catenin pathway, which may underlie the bypass of senescence and tumorigenic transformation by Ras.