Expression of FOXM1 and related proteins in breast cancer molecular subtypes

Forkhead box (FOX) proteins constitute an extended family of transcriptional regulators. FOXM1 is ubiquitously expressed in cells undergoing proliferation, and overexpression of FOXM1 is associated with poor prognosis in various malignant tumours. FOXM1 and FOXO3a are often transcriptionally antagonistic. FOXO3a plays a critical tumour-suppressive role in breast cancer. FOXO activity is modulated by its acetylation status, which is regulated by class III histone deacetylases (sirtuins; also known as SIRTs). This study evaluated the role of FOX proteins and their regulators in each molecular subtype of breast cancer. Immunohistochemical expressions of FOXM1, FOXO3a, SIRT1 and SIRT6 were evaluated in tissue microarray blocks containing 688 consecutive breast cancer samples. Mean expression levels were used to categorize tumours according to the expression of each protein (high or low). High expression of FOXM1 was significantly correlated with high SIRT1 and SIRT6 expression, higher histologic grade and triple-negative breast cancer (TNBC). High expression of nuclear FOXO3a and nuclear SIRT1 was correlated with a lower histologic grade and the hormone receptor-positive/HER2-negative subtype. In survival analysis, FOXM1 was an independent adverse prognostic factor for disease-free and overall survival in the hormone receptor-positive/HER2-negative subtype but not in the HER2-positive subtype or TNBC. In conclusion, although high FOXM1 expression was noted in the TNBC subtype, it had no prognostic impact in TNBC. However, it had prognostic significance in the hormone receptor-positive/HER2-negative subtype.