Questioning the oncogenic role of ΔNp73α in different cell lines expressing p53 or not

The recent finding that the 1p36.3 locus gene encodes an array of different p73 isoforms with apparently distinct and sometimes opposing cellular functions, might explain the difficulty in establishing the protein's role as tumor suppressor. Therefore we need to investigate the roles of each of these splicing variants in cellular functions when expressed alone or in combination with other family members, as well as the genetic background on which the proteins are expressed. We investigated, in two p53 null cell lines, the human SCLC line H1299 and a subline derived from the human colon carcinoma cell line HCT116 (HCT116/379.2), the effects of Delta Np73 alpha overexpression on cell growth and the response to anticancer treatment. We generated three different clones overexpressing Delta Np73 alpha under a tetracycline inducible promoter. Immunofluorescent staining and luciferase reporter assays confirmed that clones HCT116/Delta NA and H1299/Delta N7 and H1299/Delta N11 did express a functional, nuclear localized Delta Np73 alpha protein. The stable overexpression of Delta Np73 alpha protein did not confer any cell growth advantage. Doubling time of clones overexpressing Delta Np73 alpha were comparable to counterparts not expressing it. Clonogenic assays showed that the cytotoxic activity of different DNA damaging agents, such as cDDP, UV light and doxorubicin, were comparable in clones expressing Delta Np73 or not. The overall data argue against an oncogenic role for this isoform. These findings are independent of the p53 status since they overlap with those previously obtained by our group in HCT116 cell lines, wild type for p53.