Overexpression of 14-3-3ζ in cancer cells activates PI3K via binding the p85 regulatory subunit

The ubiquitously expressed 14-3-3 proteins regulate many pathways involved in transformation. Previously, we found that 14-3-3 zeta overexpression increased Akt phosphorylation in human mammary epithelial cells. Here, we investigated the clinical relevance and molecular mechanism of 14-3-3 zeta-overexpression-mediated Akt phosphorylation, and its potential impact on breast cancer progression. We found that 14-3-3 zeta overexpression was significantly (P = 0.005) associated with increased Akt phosphorylation in human breast tumors. Additionally, 14-3-3 zeta overexpression combined with strong Akt phosphorylation was significantly (P = 0.01) associated with increased cancer recurrence in patients. In contrast, knockdown of 14-3-3 zeta expression by small interfering RNA in cancer cell lines and tumor xenografts reduced Akt phosphorylation. Furthermore, 14-3-3 zeta enhanced Akt phosphorylation through activation of phosphoinositide 3-kinase (PI3K). Mechanistically, 14-3-3 zeta bound to the p85 regulatory subunit of PI3K and increased PI3K translocation to the cell membrane. A single 14-3-3-binding motif encompassing serine 83 on p85 is largely responsible for 14-3-3 zeta-mediated p85 binding and PI3K/Akt activation. Mutation of serine 83 to alanine on p85 inhibited 14-3-3 zeta binding to the p85 subunit of PI3K, reduced PI3K membrane localization and activation, impeded anchorage-independent growth and enhanced stress-induced apoptosis. These findings revealed a novel mechanism by which 14-3-3 zeta overexpression activates PI3K, a key node in the mitogenic signaling network known to promote malignancies in many cell types. Oncogene (2012) 31, 897-906; doi: 10.1038/onc.2011.284; published online 11 July 2011