NB1011 induces Ser15 phosphorylation of p53 and activates the G2/M checkpoint.

NB1011, a phosphoramidate derivative of (E)-5-(2-bromovinyl)-2'-deoxyuridine, is a novel anti-cancer agent that selectively targets tumor cells expressing high levels of thymidylate synthase (TS), an enzyme required for DNA biosynthesis. NB1011 treatment of high-TS-expressing breast carcinoma cells (MCF7TDX) results in the induction of p53 and p21 protein levels, whereas no p53 or p21 induction is observed in the low-TS-expressing MCF7 tumor cells. Furthermore, MCF7TDX cells accumulate in the G(2)/M phase of the cell cycle in response to NB1011. In this study, the effect of NB1011 on the phosphorylation status of p53 was analyzed. We demonstrate that NB1011 treatment of various tumor cell lines expressing high TS results in the phosphorylation of p53 on Ser15, whereas this p53 phosphorylation is not observed in low-TS-expressing tumor cells. Also, we examined the role of several key cell cycle regulators in the growth inhibition observed in response to NB1011. Our results show that the mRNA and protein levels of the G(2)/M regulators cdc2, cyclin B1 and cdc25C are down-regulated in MCF7TDX cells, while unaffected in MCF7 cells. The mRNA and protein levels of 14-3-3sigma, also a direct transcriptional target of p53, are up-regulated in MCF7TDX cells following NB1011 treatment, while unchanged in MCF7 cells. Taken together, our data indicate that the growth inhibition caused by NB1011 in MCF7TDX cells is mediated through phosphorylation of p53 and activation of the G(2)/M checkpoint.