Hzf, a p53-responsive gene, regulates maintenance of the G(2) phase checkpoint induced by DNA damage

The hematopoietic zinc finger protein, Hzf, is induced in response to genotoxic and oncogenic stress. The Hzf protein is encoded by a p53-responsive gene, and its overexpression, either in cells retaining or lacking functional 53, halts their proliferation. Enforced expression of Hzf led to the appearance of tetraploid cells with supernumerary centrosomes and, ultimately, to cell death. Eliminating Hzf mRNA expression by use of short hairpin (sh) RNAs had no overt effect on unstressed cells but inhibited the maintenance of G, phase arrest following ionizing radiation (IR), thereby sensitizing cells to DNA damage. Canonical p53-responsive gene products such as p21(Cip1) and Mdm2 were induced by IR in cells treated with Hzf shRNA. However, the reduction in the level of Hzf protein was accompanied by increased polyubiquitination and turnover of p21(Cip1), an inhibitor of cyclin-dependent kinases whose expression contributes to maintaining the duration of the G, checkpoint in cells that have sustained DNA damage. Thus, two p53-inducible gene products, Hzf and p21(Cip1), act concomitantly to enforce the G, checkpoint.