Pharmacologic modulation of glycogen synthase kinase-3β promotes p53-dependent apoptosis through a direct bax-mediated mitochondrial pathway in colorectal cancer cells

Abstract

Activation of p53 tumor suppressor induces either cell cycle arrest or apoptosis through transcription-dependent and independent pathways; however, their relative roles in apoptosis induction and how these pathways are regulated remains elusive. Here, we report a unique role for glycogen synthesis kinase-3β (GSK-3β) in regulating p53 functions in human colorectal cancer cells. Pharmacologic modulation of GSK-3β markedly impaired p53-dependent transactivation of targets including p21 and Puma but promoted p53-dependent conformational activation of Rax, resulting in cytochrome c release, loss of mitochondrial membrane potential, and caspase-9 processing. Thus, p53-mediated damage response is converted from cell cycle arrest to apoptosis following exposure to a variety of chemotherapeutic agents. We found that this effect is associated with the modulation of inhibitory Ser9 phosphorylation of GSK-3β but not with the activating tyrosine phosphorylation. We further show that the induction of apoptosis is through a direct mitochondrial pathway that requires Bax but not Puma. Our results underscore the importance of transcription-independent mechanism in p53-induced apoptosis and indicate that GSK-3β plays distinct dual roles in regulating p53 pathways: promoting p53 transcriptional activity in the nucleus but suppressing p53-mediated direct apoptotic function at the mitochondria. Importantly, our data suggest that small-molecule inhibition of GSK-3β might represent a novel approach for modulating chemotherapy. ©2005 American Association for Cancer Research.