TP63, an important epithelial developmental gene, has significant homology to p53. Unlike p53, the expression of p63 is regulated by two different promoters resulting in proteins with opposite functions: the full-length transcriptionally active TAp63 and the dominant-negative DNp63. We investigated the downstream mechanisms by which TAp63a elicits apoptosis. TAp63a directly transactivates the CD95 gene via the p53 binding site in the first intron resulting in upregulation of a functional CD95 death receptor. Stimulation and blocking experiments of the CD95, TNF-R and TRAIL-R death receptor systems revealed that TAp63a can trigger expression of each of these death receptors. Furthermore, our findings demonstrate a link between TAp63a and the mitochondrial apoptosis pathway. TAp63a upregulates expression of proapoptotic Bcl-2 family members like Bax and BCL2L11 and the expression of RAD9, DAP3 and APAF1. Of clinical relevance is the fact that TAp63a is induced by many chemotherapeutic drugs and that inhibiting TAp63 function leads to chemoresistance. Thus, beyond its importance in development and differentiation, we describe an important role for TAp63a in the induction of apoptosis and chemosensitivity.

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We investigated the downstream mechanisms by which chemotherapeutic drugs elicit apoptosis in hepatocellular carcinoma (HCC). Genomic signatures of HCC cell lines treated with different chemotherapeutic drugs were obtained. Analyses of apoptosis pathways were performed and RNA interference was used to evaluate the role of the p53 family. Endogenous p53, p63 and p73 were upregulated in response to DNA damage by chemotherapeutic drugs. Blocking p53 family function led to chemoresistance in HCC. Stimulation and blocking experiments of the CD95-, the TNF-and the TRAIL-receptor systems revealed that cytotoxic drugs, via the p53 family members as transactivators, can trigger expression of each of these death receptors and consequently sensitize HCC cells toward apoptosis. Furthermore, our findings demonstrate a link between chemotherapy, the p53 family and the mitochondrial apoptosis pathway in HCC. Chemotherapeutic treatment induces expression of proapoptotic Bcl-2 family members like Bax and BCL2L11 and the expression of Apaf1, BNIP1, Pdcd8 and RAD. Thus, upon DNA damage, p53, p63 and p73 promote apoptosis via the extrinsic and the intrinsic signaling pathway. In addition, not only proapoptotic genes were upregulated, but also genes known to exert antiapoptotic functions. Bleomycininduced upregulation of BCL-XL/BCLXL1 and MDM2 suggests that it is the ratio of proapoptotic and antiapoptotic proteins that regulates the apoptosis response of HCC cells toward chemotherapy, thereby playing a decisive role between treatment sensitivity vs. drug resistance. The clinical importance of these data is evidenced by our finding that the bleomycin target gene signature can predict the prognosis of patients suffering from HCC.Inactivation of tumor suppressor genes or activation of protooncogenes can lead to clonal outgrowth and tumor progression. These oncogenic events evolve as important determinants in the response of human tumors to commonly used DNA damaging agents. 1 Many anticancer agents induce DNA damage as part of their mechanism of tumor cytotoxicity. DNA damage activates p53, which in turn induces the expression of proteins that halt the cell-division cycle to allow for DNA repair. 2,3 Activation of p53 can also initiate programs of cell death (apoptosis) or permanent growth arrest (senescence) if the DNA damage is severe. [4][5][6][7][8][9][10][11][12][13] Furthermore, the recent identification and characterization of the p53/p63/p73 network provides evidence of a tight link between developmental processes and tumorigenesis. 14-16 p63 and p73 are not only important for normal development and differentiation, but are also implicated in tumorigenesis and the response to chemo-or radiotherapies. [17][18][19][20] The p53 family genes, p53, p63 and p73, produce multiple isoforms that vary in composition of the NH 2 -and C-termini. Isoforms of the p53 family can interact with each other and form a complicated network. The dominant-negative (DN) isoforms can oppose the transactivation capabilities of the full length (TA) prot...