High expression of cytoplasmic FOXO3 protein associated with poor prognosis of rectal cancer patients: A study from Swedish clinical trial of preoperative radiotherapy to big database analysis

Introduction: Accumulating evidence has implicated a pivotal role for FOXO3, FOXM1 and SIRT6 in cancer progression. The majority of researches focused on the functions of these proteins in drug resistance, but their relationships with radiotherapy (RT) response remain unclear. In this study, we examined protein expression of FOXO3, FOXM1 and SIRT6 and their clinical significance in a Swedish rectal cancer trial of preoperative RT. Methods: Expression of FOXO3, FOXM1 and SIRT6 protein was examined by immunohistochemistry in patient samples. Genetic analysis of FOXO3, FOXM1 and SIRT6 were performed by cBioportal and MEXPRESS database. Gene-gene network analysis was conducted using GeneMANIA. Functional enrichment analysis was performed based on LinkedOmics and Metascape online software. Results: FOXO3 and FOXM1were mainly expressed in the cytoplasm in both normal and tumour tissues, and SIRT6 in both the cytoplasm and nucleus in normal and tumour tissues. FOXO3 and FOXM1 expression increased from normal mucosa to primary cancer (P < 0.001), while SIRT6 expression decreased from normal mucosa to primary cancer (P < 0.001). High FOXO3 expression correlated with late TNM stage (P = 0.040), distant metastasis (P = 0.032) and independently with disease free survival (DFS) in the RT patients (HR = 7.948; P = 0.049; 95% CI = 1.002–63.032) but not in non-RT patients (P > 0.05). Genetic analysis indicated that DNA methylation status contributed to FOXO3 overexpression. Functional enrichment analysis demonstrated that FOXO3 was closely related to metabolism-related signalling pathway which in turn associated with cancer radioresistance. Moreover, there were strong gene–gene interactions between FOXO3 and metabolism-related signalling. Conclusions: Our findings suggest that FOXO3 may be a prognostic factor in rectal cancer patients with RT.