Knockdown of FLOT1 Impairs Cell Proliferation and Tumorigenicity in Breast Cancer through Upregulation of FOXO3a

Lipid rafts, specialized domains in cell membranes, function as physical platforms for various molecules to coordinate a variety of signal transduction processes. Flotinllin-1 (FLOT1), a marker of lipid rafts, is involved in the progression of cancer, but the precise mechanism remains unclear. The aim of the present study was to examine the role of FLOT1 on the tumorigenesis of breast cancer cells and its clinical significance in progression of the disease. FLOT1 expression was analyzed in 212 paraffin-embedded, archived clinical breast cancer samples by using immunohistochemistry (IHC). The effect of FLOT1 on cell proliferation and tumorigenesis was examined in vitro and in vivo. Western blotting and luciferase reporter analyses were carried out to identify the effects of downregulating FLOT1 on expression of cell cycle regulators and transcriptional activity of FOXO3a. IHC analysis revealed high expression of FLOT1 in 129 of the 212 (60.8%) paraffin-embedded archived breast cancer specimens. The overall expression level of FLOT1 significantly correlated with clinical staging and poor patient survival of breast cancer. Strikingly, we found that silencing FLOT1 inhibited proliferation and tumorigenicity of breast cancer cells both in vitro and in vivo, which was further shown to be mechanistically associated with suppression of Akt activity, enhanced transcriptional activity of FOXO3a, upregulation of cyclin-dependent kinase inhibitor p21(Cip1) and p27(Kip1), and downregulation of the CDK regulator cyclin D1. FLOT1 plays an important role in promoting proliferation and tumorigenesis of human breast cancer and may represent a novel prognostic biomarker and therapeutic target for the disease.