FOXO3a Expression Regulated by ERK Signaling is Inversely Correlated With Y‐Box Binding Protein‐1 Expression in Prostate Cancer

BACKGROUND FOXO3a is a member of the forkhead O transcription factors. FOXO3a induces the factors that contribute to cell cycle arrest and is considered a tumor suppressor in several malignant tumors. Y‐box binding protein‐1 (YB‐1) is a multifunctional protein whose high expression is correlated with poor prognoses in various malignant tumors. In the current study, we investigated the relationship between FOXO3a and YB‐1 to validate their functional roles in prostate cancer. METHODS Western blotting and cytotoxicity assays were conducted in prostate cancer cells, LNCaP, and 22Rv1 cells. We also evaluated the protein expressions of FOXO3a and YB‐1 in human prostate cancer tissues, using radical prostatectomy specimens. Then, we investigated the correlations between protein expressions and clinicopathologic parameters. RESULTS We found that both FOXO3a and YB‐1 proteins were phosphorylated by ERK signaling, resulting in FOXO3a inactivation and YB‐1 activation in LNCaP and 22Rv1 cells. Inversely, inhibition of MEK or treatment with metformin activated FOXO3a through inactivation of ERK signaling and suppressed the viability of LNCaP and 22Rv1 cells in a dose‐dependent manner. In immunohistochemical analysis, FOXO3a nuclear expression was inversely correlated with YB‐1 nuclear expression (P