Id-1 negatively regulates PTEN transcription through p53 downregulation and activates Akt pathway in human breast cancer

Abstract

Id-1 has been thought to be an oncogene, which is overexpressed in various human cancers and has been implicated in cell proliferation, survival, invasion, angiogenesis, and metastasis. In this study, we examined the mechanism of Akt regulation by Id-1. Firstly, we explored the effect of Id-1 on Akt activity and its downstream signals in human breast cancer. Upon Id-1 overexpression in MCF7 cells, Akt phosphorylation was augmented and consequently, its downstreams, which related with Wnt pathway, GSK-3b phosphorylation, b-catenin and cyclin D1 expressions were also increased. Moreover, another Akt downstream, p27, was phosphorylated. These effects were abrogated by Id-1 antisense expression or PI3K inhibitor LY294002 treatment. Consistently, upon Id-1 knockdown by antisense expression in MDA-MB-231 cells, Akt phosphorylation was downregulated and GSK-3b and p27 phosphorylations, b-catenin and cyclin D1 expressions were also decreased. Moreover, we show that Id-1 inhibits transcription of PTEN, a negative regulator of PI3K/Akt pathway, through p53 downregulation. PTEN expression was decreased at the transcriptional level and p53, which can act as PTEN transactivator, was also downreguated by Id-1 overexpression in MCF7 cells. In promoter assay with serial deletion, PTEN promoter activity was dependent on p53 binding in MCF7 cells. The binding of p53 to the PTEN promoter was reduced by Id-1 as assessed by ChIP assay. Moreover, inhibition of PTEN expression by Id-1 was abrogated by constitutive overexpression of p53. Downregulation of PTEN transcription resulted in activation of the Akt-mediated Wnt/TCF signaling pathway. Enhanced GSK-3b phosphorylation by Id-1 overexpression led to inductions of nuclear b-catenin expression and TCF/LEF transcription activity. Moreover, enhancement of Akt-mediated p27Kip1 phosphorylation by Id-1 resulted in cytosolic retention of p27 Kip1 that is implicated in inhibition of p27 function. Taken together, these results suggest Id-1 as a negative regulator of PTEN and the novel action of the mechanism of Id-1 through the PTEN/Akt pathway in human breast carcinogenesis.