Oct4 Connects Akt to Cancerous Stem Cells

Science Signaling  04 Dec 2012:
Vol. 5, Issue 253, pp. ec310
DOI: 10.1126/scisignal.2003837

Embryonal carcinoma cells (ECCs) are stem cells derived from teratocarcinomas and as such are the malignant counterparts to embryonic stem cells (ESCs). Oct4 is a transcription factor that plays a role in pluripotency and self-renewal in both ESCs and ECCs. Lin et al. investigated the regulation of Oct4 in ECCs. Bioinformatic analysis identified a potential phosphorylation site in Oct4 for the kinase Akt. Akt phosphorylated Oct4 in an in vitro kinase assay. In human ECCs, endogenous Oct4 colocalized with Akt in the nucleus, and Akt coimmunoprecipitated with Oct4 from human or mouse ECC lysates. Western blotting of lysates of human ECCs treated with cycloheximide and expressing wild-type, a phosphorylation-mimicking mutant, or a phosphorylation-impaired mutant showed that phosphorylation stabilized Oct4. The degradation rate of Oct4 was decreased by pretreatment of cells with proteasome inhibitor MG132 and increased by pretreatment with an Akt inhibitor. Wild-type Oct4 coimmunoprecipitated with the pluripotency factor Sox2 in mouse ECCs but not in mouse ECCs pretreated with an Akt inhibitor. Electrophoretic mobility shift assays (EMSAs) and chromatin immunoprecipitation (ChiP) showed that, in the presence of Sox2, pOct4 preferentially bound to the promoter regions of its own gene and the genes encoding pluripotency factors Sox2 and Nanog. However, unphosphorylated Oct4 preferentially bound to the Akt promoter. Knockdown of Oct4 in human ECCs reduced the abundance of Nanog and Oct4 mRNA but increased the abundance of Akt mRNA, suggesting that Oct4 functioned as a promoter-specific transcriptional enhancer or repressor. Human ECCs transfected with Oct4-T235D, the phosphorylation mimic, did not undergo apoptosis induced by ultraviolet (UV) radiation as readily as did human ECCs transfected with wild-type Oct4, and pretreatment with an Akt inhibitor increased the number of wild-type cells that underwent UV-induced apoptosis. Xenografts of mice inoculated with mouse ECC cell lines that had a large abundance of pOct4 had larger tumors than mice inoculated with ECCs that had a low abundance of pOct4. Together these results support a model in which Akt phosphorylation of Oct4 prevents Oct4 degradation; promotes transcription of stem cell factors, including Oct4; and reduces Oct4-mediated repression of Akt expression.

Y. Lin, Y. Yang, W. Li, Q. Chen, J. Li, X. Pan, L. Zhou, C. Liu, C. Chen, J. He, H. Cao, H. Yao, L. Zheng, X. Xu, Z. Xia, J. Ren, L. Xiao, L. Li, B. Shen, H. Zhou, Y.-J. Wang, Reciprocal regulation of Akt and Oct4 promotes the self-renewal and survival of embryonal carcinoma cells. Mol. Cell 48, 627–640 (2012). [PubMed]