Abstract P1-07-02: Significance of RB-ZEB axis in EMT phenotype of breast cancer and inhibition of ZEB by CDK4/6 inhibitor

Abstract

Abstract The retinoblastoma protein (pRB) is a key tumor suppressor that regulates the progression of cell cycles from G1 to S phase. pRB is often inactivated due to mutation, very low level expression or hyperphosphorylation by cyclin-dependent kinases (CDKs) in many cancers. We found that knockdown of pRB induced the expression of ZEB1 (zinc finger E-box binding homeobox 1), which is one of the transcriptional repressors of the E-cadherin gene, and led to epithelial-mesenchymal transition (EMT) through the ZEB-dependent pathways in breast cancer cells. EMT is defined by the loss of epithelial characteristics and the acquisition of a mesenchymal phenotype, and is associated with an invasive and metastatic potential in cancers. We found that siRNA-mediated depletion of ZEB in pRB-inactive cells suppressed cell invasiveness and proliferation and induced epithelial-cell marker expressions. We also found that knockdown of ZEB1 reduced the dissemination of breast cancer cells in bone marrow from primary tumors growing in mammary fat pads of nude mice. We then conducted a screen with an existing drug library for inhibitors of ZEB1 expressions and identified several hits, including CDK inhibitors. We confirmed that the CDK4/6 inhibitor, Palbociclib (PD0332991), suppressed breast cancer cell proliferation and invasiveness in vitro, and the treatment with the PD0332991 suppressed tumor growth in vivo using an orthotopic xenograft breast cancer model. In terms of cancer stemness, we found that the treatment of PD0332991 decreased the CD44+/CD24Low/- breast cancer stem cell population. We also determined that CDK4 inhibitors blocked tumorsphere formations, which are a characteristic of stem-like cancer cells. p16INK4a (p16) is an endogenous CDK4/6 inhibitor, and we found that knockdown of p16 increased the CD44+/CD24Low/- breast cancer stem cell population in estrogen receptor-negative breast cancer. CDK4/6 inhibitors appear to substitute for the p16 functions which suppress cancer stemness. Our findings indicate that the CDK4/6 inhibitor affects tumor progression, not only through cell cycle control, but also through the suppression of EMT and stem-like properties of cancer cells. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P1-07-02.