Full-length p73alpha represses drug-induced apoptosis in small cell lung carcinoma cells

The p73 gene, a member of the p53 family, encodes several variants through differential splicing and use of alternative promoters. At the NH2 terminus, two different promoters generate the full-length and the DeltaN isoforms, with or without the transactivating domain. At the COOH terminus, seven isoforms generated through alternative splicing have been cloned. Previous studies have demonstrated that DeltaNp73 isoforms exert a dominant-negative effect on p73 by blocking their transactivation activity and hence the ability to induce apoptosis. Considerable efforts are made to identify the functional diversity of the COOH-terminal p73 variants. In this study, we found that p73alpha inhibited drug-induced apoptosis in small cell lung carcinoma cells, whereas p73beta promoted it. p73alpha prevented Bax activation, mitochondrial dysfunction, and caspase activation. In addition, p73alpha was also able to reduce apoptosis induced by the BH3-only protein PUMA (p53 up-regulated modulator of apoptosis). Furthermore, we discovered that p73alpha is able to inhibit the pro-apoptotic effect of p73beta, demonstrating the existence of equilibrium between these two p73 isoforms. In conclusion, the reported overexpression of p73alpha in certain tumor types, and our findings that p73alpha exerts anti-apoptotic functions, indicate a potential oncogenic activity for p73.     

Identification of protein kinase A catalytic subunit beta as a novel binding partner of p73 and regulation of p73 function

Post-translational modifications play a crucial role in regulation of the protein stability and pro-apoptotic function of p53 as well as its close relative p73. Using a yeast two-hybrid screening based on the Sos recruitment system, we identified protein kinase A catalytic subunit beta (PKA-Cbeta) as a novel binding partner of p73. Co-immunoprecipitation and glutathione S-transferase pull-down assays revealed that p73alpha associated with PKA-Cbeta in mammalian cells and that their interaction was mediated by both the N- and C-terminal regions of p73alpha. In contrast, p53 failed to bind to PKA-Cbeta. In vitro phosphorylation assay demonstrated that glutathione S-transferase-p73alpha-(1-130), which has one putative PKA phosphorylation site, was phosphorylated by PKA. Enforced expression of PKA-Cbeta resulted in significant inhibition of the transactivation function and pro-apoptotic activity of p73alpha, whereas a kinase-deficient mutant of PKA-Cbeta had no detectable effect. Consistent with this notion, treatment with H-89 (an ATP analog that functions as a PKA inhibitor) reversed the dibutyryl cAMP-mediated inhibition of p73alpha. Of particular interest, PKA-Cbeta facilitated the intramolecular interaction of p73alpha, thereby masking the N-terminal transactivation domain with the C-terminal inhibitory domain. Thus, our findings indicate a PKA-Cbeta-mediated inhibitory mechanism of p73 function.     

p73 induces apoptosis by different mechanisms

p73, like its homologue, the tumor suppressor p53, is able to induce apoptosis in several cell types. This property is important for the involvement of p73 in cancer development and therapy. However, in contrast with p53, the TAp73 gene has two distinct promoters coding for two protein isoforms with opposite effects: while the transactivation proficient TAp73 shows pro-apoptotic effects, the amino-terminal-deleted DeltaNp73 has an anti-apoptotic function. Indeed, the relative expression of these two proteins is related to the prognosis of several cancers. Here we discuss recent developments in the control of p73-induced apoptosis. First, TAp73 induces ER stress via the direct transactivation of Scotin. Second, TAp73 induces the mitochondrial pathway by directly transactivating both Bax and the BH3 only protein PUMA promoters. While the first transactivation is weak, and not sufficient to trigger apoptosis (at least in the in vitro cellular models so far evaluated), the induction of PUMA is strong and lethal. Third, the promoter of the death receptor CD95 contains a p53 responsive element and preliminary experiments suggest that TAp73 also activates the death receptor pathway. In addition, TAp73 is able to transactivate its own second promoter, thus inducing the expression of the anti-apoptotic DeltaNp73 isoform. Therefore, the balance between TAp73 and DeltaNp73 finely regulates cellular sensitivity to death.