RB silencing compromises the DNA damage-induced G2/M checkpoint and causes deregulated expression of the ECT2 oncogene

As alterations in retinoblastoma (RB)/E2F pathway are commonly found in human cancers, the molecular mechanism underlying cell cycle deregulation caused by the mutations in the RB/E2F pathway needs to be investigated extensively. Compared with good understanding of RB/E2F functions in G1-S cell cycle progression, it is not fully understood how an abrogated RB pathway affects the G2-M phase of the cell cycle. Here, we report that disruption of RB accelerated G2-M progression in the presence of DNA damage by elevating the expression of a set of mitotic regulatory genes. We generated RB(+)- and (-)-matched cells using short hairpin RNA. In the RB(-) cells, the G2/M checkpoint mediated by a DNA-damaging agent was over-ridden. With microarray analysis, we found that the expression of key G2-M regulatory genes was upregulated in RB(-) cells. In particular, we demonstrated that the proto-oncogene ECT2 was directly regulated by E2Fs. Furthermore, suppression of ECT2 expression by small interfering RNA in RB(-) cells resulted in cytokinesis arrest, suggesting that RB(-) cells lack the regulation of E2F-mediated cytokinesis. These results indicate that aberrant ECT2 expression, observed in various human tumors, could be the direct result of RB/E2F pathway deficiency, thereby contributing to cell division in cancers.