Phosphorylated mTOR Expression is Associated with Poor Prognosis for Patients with Esophageal Squamous Cell Carcinoma

Background. The mammalian target of rapamycin (mTOR) plays central roles in the regulation of cell growth and proliferation by monitoring nutrient availability, cellular energy level, oxygen level, and mitogenic signals. The aberrant activation of mTOR in relation to clinical outcome has been reported in several types of cancers. mTOR is increasingly important as a potential target for anticancer therapy. Nonetheless, a prognostic feature of mTOR activation in esophageal squamous cell carcinoma (ESCC) remains uncertain. Materials and Methods. First, in order to validate phospho-specific mTOR antibody (Ser2448), phosphorylated mTOR (p-mTOR) expression levels in five ESCC cell lines under cultural conditions with or without everolimus (mTOR inhibitor, also known as RAD001) were evaluated by in vitro immunohistochemistry and immunoblotting. Second, we examined p-mTOR expression by immunohistochemistry using 143 resected ESCC specimens. Prognostic significance of p-mTOR expression was examined by Cox regression and Kaplan-Meier analyses. Results. Among 143 patients, 71(49.7%) were classified into p-mTOR-positive and 72 (50.3%) were classified into p-mTOR-negative. Compared with p-mTOR-negative patients, p-mTOR-positive patients experienced high overall mortality [hazard ratio (HR) 2.44; 95% confidence interval (CI), 1.24-4.83; P = 0.008], which persisted in multivariate analysis (multivariate HR 2.92; 95% CI, 1.48-5.78; P = 0.002). A similar finding was observed for esophageal cancer-specific mortality. p-mTOR expression was not related with any clinical or pathologic variables including age, sex, tumor location, histological grading, operative procedure, T classification (tumor invasion), or lymph-node metastasis. Conclusions. p-mTOR overexpression was independently associated with poor prognosis in ESCC, supporting the potential for mTOR as a therapeutic target for ESCC.