Abstract
p27 is a cell cycle regulator and an atypical tumor suppressor. However, p27 gene deletions or mutations are rarely observed in human cancers. Instead, p27 is frequently degraded or mislocalized to the cytoplasm in aggressive human cancers. Phosphorylation at T157 or T198 by different PI3K effector kinases leads to p27 cytoplasmic accumulation. However, the functional contribution of phosphorylated and cytoplasmic p27 to cancer progression remains unknown. By introducing a cell cycle defective (CK-) and double phosphomimetic p27 mutant (T157D/T198D or DD), we found that phosphorylation of p27 at T157 or T198 by PI3K/mTOR directly regulated tumor cell migration and invasion. Targeted inhibition of PI3K/mTOR impaired tumor cell motility and metastasis via modulation of p27. Furthermore, we uncovered a novel oncogenic function of p27 in regulating tumor progression. p27CK-DD induced epithelial-mesenchymal transition (EMT) and transformation of human mammary epithelial cells and enhanced the mesenchymal characteristics and metastatic potential of cancer cell lines. Knockdown of p27 in mesenchymal cell lines with enriched p27T157/T198 reverted EMT and impaired metastatic potential. Mechanistically, p27CK-DD activated STAT3 and facilitated its transactivation of TWIST1 to induce EMT. Pharmacological inhibition of STAT3 or dominant negative STAT3 (STAT3DN) decreased TWIST1 and reversed p27CK-DD-mediated EMT and tumor progression, while constitutive active STAT3 (STAT3CA) rescued EMT phenotype and metastatic potential in p27 knockdown cells. We also identified a potential signaling feedforward loop containing AKT activation, p27 phosphorylation, STAT3 activation and further AKT activation that might contribute to tumor progression. These findings reveal a novel oncogenic function of p27 to regulate tumor progression through EMT that involves STAT3-mediated TWIST1 regulation. Combined inhibition of both AKT and STAT3 in PI3K/mTOR activated, p27pT157/pT198 enriched human cancers may ultimately have therapeutic potential to limit p27-mediated EMT and cancer metastasis.