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

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-3 (GSK-3) in regulating p53 functions in human colorectal cancer cells. Pharmacologic modulation of GSK-3 beta markedly impaired p53-dependent transactivation of targets including p21. and Puma but promoted p53-dependent conformational activation of Bax, 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 Ser(9) phosphorylation of GSK-beta 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-3p 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-3 beta might represent a novel approach for modulating chemotherapy.