Activation of p53-Dependent Growth Suppression in Human Cells by Mutations in PTEN or PIK3CA

Abstract

In an effort to identify genes whose expression is regulated by activated phosphatidylinositol 3-kinase (PI3K) signaling, we performed microarray analysis and subsequent quantitative reverse transcription-PCR on an isogenic set of PTEN gene-targeted human cancer cells. Numerous p53 effectors were upregulated following PTEN deletion, including p21, GDF15, PIG3, NOXA, and PLK2. Stable depletion of p53 led to reversion of the gene expression program. Western blots revealed that p53 was stabilized in HCT116 PTEN^−/− cells via an Akt1-dependent and p14^ARF-independent mechanism. Stable depletion of PTEN in untransformed human fibroblasts and epithelial cells also led to upregulation of p53 and senescence-like growth arrest. Simultaneous depletion of p53 rescued this phenotype, enabling PTEN-depleted cells to continue proliferating. Next, we tested whether oncogenic PIK3CA, like inactivated PTEN, could activate p53. Retroviral expression of oncogenic human PIK3CA in MCF10A cells led to activation of p53 and upregulation of p53-regulated genes. Stable depletion of p53 reversed these PIK3CA-induced expression changes and synergized with oncogenic PIK3CA in inducing anchorage-independent growth. Finally, targeted deletion of an endogenous allele of oncogenic, but not wild-type, PIK3CA in a human cancer cell line led to a reduction in p53 levels and a decrease in the expression of p53-regulated genes. These studies demonstrate that activation of PI3K signaling by mutations in PTEN or PIK3CA can lead to activation of p53-mediated growth suppression in human cells, indicating that p53 can function as a brake on phosphatidylinositol (3, 4, 5)-triphosphate-induced mitogenesis during human cancer pathogenesis.

We thank Susette Mueller for assistance with microscopy, Karen Cresswell and Michelle Lombard for assistance with flow cytometry, Xiaojun Zou for assistance with microarray processing, Michael Johnson for advice on performing soft-agar assays, and Annabell Oh for advice on qRT-PCR.

This work was supported by National Institutes of Health grants K01 CA087828 and R01 CA115699 to T.W., American Cancer Society RPG MGO-112078 to T.W., and the Lombardi Comprehensive Cancer Center Support grant P30 CA051008. C.L.B. and C.L. were supported in part by NIH training grant T32 CA009686.