RNA Interference–Mediated Depletion of Phosphoinositide 3-Kinase Activates Forkhead Box Class O Transcription Factors and Induces Cell Cycle Arrest and Apoptosis in Breast Carcinoma Cells

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Abstract
Breast cancer is one of the most common malignancies affecting women in the Western world and one in seven women is predicted to develop invasive breast cancer in their lifetime. Breast cancer arises following the accumulation of a series of somatic changes often including deregulation of key signal transduction pathways. The phosphoinositide 3-kinase (PI3K) pathway has been shown to be activated in breast cancer and overexpression of PI3K is sufficient to confer a malignant phenotype. Activation of the PI3K pathway serves to repress forkhead box class O (FoxO) transcription factor–mediated growth arrest and apoptosis. In this study, we used small interfering RNA (siRNA) to knockdown PI3K in three breast cancer cell lines representing different stages of cancer development. Transfection of PI3K siRNA in breast cancer cells resulted in a significant decrease in cell viability and induction of apoptosis irrespective of their estrogen receptor α (ERα) or ErbB2 status. PI3K depletion also resulted in a significant G1 phase cell cycle arrest in ERα-positive breast cancer cells. Further, our data showed that PI3K knockdown resulted in a significant activation of FoxO; interestingly, a simultaneous knockdown of FoxO1a rescued the cells from apoptosis. Furthermore, the downstream effects of FoxO activation were found to be inhibition of cyclin-dependent kinase 4, cyclin-dependent kinase 6, and cyclin D1, and accumulation of p27/Kip1. Thus, we suggest that (a) PI3K plays a critical role in breast cancer development and (b) gene therapeutic approaches aimed at PI3K or the pharmacologic inhibitors of PI3K could be developed for the management of breast cancer. (Cancer Res 2006; 66(2): 1062-9)