c-Abl-independent p73 stabilization during gemcitabine- or 4′-thio-β-d-arabinofuranosylcytosine–induced apoptosis in wild-type and p53-null colorectal cancer cells

• Published in: Molecular cancer therapeutics 2006-02, Vol.5 (2), p.400-410
• Main Authors: Thottassery, Jaideep V, Westbrook, Louise, Someya, Hitoshi, Parker, William B
• Format: Article
• Language: English
• Subjects: Apoptosis; ara-C and gemcitabine; Arabinonucleosides - therapeutic use; Bax; c-Abl; Carcinoma - drug therapy; Carcinoma - genetics; Carcinoma - metabolism; Caspase 3; Caspases - metabolism; Colorectal Neoplasms - drug therapy; Colorectal Neoplasms - genetics; Colorectal Neoplasms - metabolism; deoxycytidine; Deoxycytidine - analogs & derivatives; Deoxycytidine - therapeutic use; DNA-Binding Proteins - metabolism; Gene Deletion; Genes, Tumor Suppressor; Humans; Nuclear Proteins - metabolism; p53; p73; Poly(ADP-ribose) Polymerases - metabolism; Proto-Oncogene Proteins c-abl - genetics; Proto-Oncogene Proteins c-abl - metabolism; PUMA; pyrimidine nucleosides; Tumor Protein p73; Tumor Suppressor Protein p53 - genetics; Tumor Suppressor Protein p53 - metabolism; Tumor Suppressor Proteins; Up-Regulation
• ISSN: 1535-7163; 1538-8514
• DOI: 10.1158/1535-7163.MCT-05-0409

Nucleoside anticancer drugs like gemcitabine (2′-deoxy-2′,2′-difluorocytidine) are potent inducers of p53, and ectopic expression of wild-type p53 sensitizes cells to these agents. However, it is also known that nucleosides are efficient activators of apoptosis in tumor cells that do not express a functional p53. To clarify this issue, we examined the effects of gemcitabine and 4′-thio-β- d -arabinofuranosylcytosine (T-ara-C) on p73, a structural and functional homologue of p53, whose activation could also account for nucleoside-induced apoptosis because no functionally significant mutations of p73 have been reported in cancers. Acute treatment of HCT 116 colon carcinoma cells with gemcitabine or T-ara-C induced marked cytotoxicity and cleavage of caspase-3 and poly(ADP-ribose) polymerase. T-ara-C and gemcitabine markedly induced p53 accumulation as well as increased levels of phospho-p53 (Ser15/Ser20/Ser46) and induced its binding to a consensus p53 response element. Despite robust activation of p53 by T-ara-C and gemcitabine, we found that wild-type and p53−/− HCT 116 cells exhibited almost equivalent sensitivity towards these nucleosides. Examination of p73 revealed that T-ara-C and gemcitabine markedly increased p73 protein levels and p73 DNA-binding activities in both p53−/− and wild-type cells. Furthermore, T-ara-C- and gemcitabine-induced increases in p73 levels occur due to a decrease in p73 protein turnover. RNA interference studies show that nucleoside-induced p73 increases are independent of c-Abl, a nucleoside-activated kinase recently implicated in p73 stabilization. HCT 116 lines, wherein the downstream p53/p73 targets Bax and PUMA (p53 up-regulated modulator of apoptosis) were deleted, were less sensitive to T-ara-C and gemcitabine. Together, these studies indicate that c-Abl-independent p73 stabilization pathways could account for the p53-independent mechanisms in nucleoside-induced apoptosis. [Mol Cancer Ther 2006;5(2):400–10]