TAp73/ΔNp73 influences apoptotic response, chemosensitivity and prognosis in hepatocellular carcinoma

p73, a p53 family protein, has significant homology to p53. Unlike p53, the expression of p73 is regulated by two different promoters resulting in proteins with opposite functions: the full-length transcriptionally active TAp73 and the dominant negative p73 (ΔNp73). We investigated the downstream mechanisms by which TAp73ß and ΔNp73 regulate apoptosis. TAp73ß transactivated the CD95 gene via the p53-binding site in the first intron. In addition, TAp73ß induced expression of pro-apoptotic Bcl–2 family members and led to apoptosis via the mitochondrial pathway. Furthermore, endogenous p73 was upregulated in response to DNA damage by bleomycin and doxorubicin. TAp73ß and bleomycin synergized in the induction of cell death. On the contrary, ΔNp73, conferred resistance to chemotherapy. Inhibition of CD95 gene transactivation was one mechanism by which ΔNp73 functionally inactivated the tumor suppressor action of p53 and TAp73ß. Concomitantly, ΔNp73 inhibited apoptosis emanating from mitochondria. Thus, ΔNp73 expression in tumors selects against both the death receptor and the mitochondrial apoptosis activity of TAp73ß. Western Blot analysis of tumor tissue from patients with hepatocellular carcinoma revealed that TAp73 and ΔNp73 are specifically expressed in tumor tissue, but not in non-neoplastic liver tissue. Hereby, TAp73 and ΔNp73 seemed to be inversely regulated, i.e. high expression of TAp73 corresponded to low expression of ΔNp73 and vice versa. By immunostaining, our antibody raised specifically against ΔNp73, identified 31 out of 84 (37%) of the HCCs to overexpress ΔNp73. The clinical relevance of these data is evidenced by our finding that upregulation of ΔNp73 in hepatocellular carcinoma patients correlates with reduced survival. The survival analysis took into account ΔNp73 positivity (defined by immunohistochemistry), UICC tumor stage and Edmondson grade. Preferential upregulation of ΔNp73 in HCCs might impose oncogenic activity that specifically interferes with the tumor suppressor function of wild-type p53 and TAp73, as we have shown in vitro. In conclusion, our data indicate that ΔNp73 is an important gene in hepatocarcinogenesis and a relevant prognostic factor.

Keywords: Apoptose, HCC, chemosensitivity, p73, ΔNp73