Prognostic implications and oncogenic roles of MYBL2 protein expression in esophageal squamous-cell carcinoma

Background: The MYBL2 gene, a highly conserved member of the Myb transcription-factor family, has been implicated in the genesis and progression of many types of tumors. Methods: We analyzed the expression of MYBL2 and Ki67 in tissue samples of esophageal squamous-cell carcinoma (ESCC) patients by immunohistochemistry. We further analyzed the effect of MYBL2 on cell proliferation and DNA replication using a CCK8 assay, 5-ethynyl-2'-deoxyuridine-retention assay, flow-cytometry analysis, real-time quantitative PCR, Western blot, and a xenograft model of ESCC cells in nude mice. Results: MYBL2 expression was significantly higher in ESCC tissue when compared to the adjacent normal tissue (P=0.007). MYBL2 was found to be positively correlated with Ki67 (gamma=0.286, P=0.003). Furthermore, Kaplan-Meier curves indicated that MYBL2 expression in ESCC tissue was associated with poor patient outcome (P<0.001), with MYBL2-positive patients who exhibited high Ki67 expression in ESCC tissue showing the worst prognosis for overall survival (P=0.003). Our in vitro results showed that downregulation of MYBL2 in ESCC cell lines inhibited cell proliferation and DNA replication (P<0.05 for both). We also found that loss of MYBL2 caused a reduction in levels of cell cycle-related G2/M proteins CDK1 and cyclin B1 in ESCC cells. In contrast, overexpression of MYBL2 caused an increase in these proteins. In vivo, we found that in nude mice that received cells knocked down for MYBL2, tumor growth was inhibited in comparison to the group that received control cells (P<0.05). Conclusion: MYBL2 overexpression induces tumor proliferation in ESCC cells by regulating cell-cycle at the S and G2/M phase. Therefore, MYBL2 may serve as a novel prognostic biomarker in ESCC patients.