Abstract 6146: Tumor suppressor p73 primes extrinsic apoptosis via transcriptional activation of DR5 and c-FLIP

Abstract

Abstract The tumor suppressor p73 is a member of p53 family. p73 transcriptionally activates p53-targeted genes involved in cell cycle regulation and apoptosis. It is well known that p73 induces cellular apoptosis via the intrinsic apoptotic pathway upon cellular stresses. However, the role of p73 in extrinsic apoptosis is poorly studied. We observed cell cycle arrested at the G1 phase in cells infected with a p73-expressing recombinant adenovirus (Ad-p73). We observed that overexpression of p73 upregulated c-FLIP at the protein level, in addition to DR5 and p21. c-FLIP is an anti-apoptotic factor that inhibits caspase 8/10 activation, thus blocking extrinsic pathway apoptosis. We demonstrated that c-FLIP is a transcriptional target of p73 based on our observations that: 1) p73 increases c-FLIP at mRNA level, 2) p73 binds to the c-FLIP promoter and 3) p73 enhances c-FLIP promoter-driven luciferase expression. We sought to explore whether the upregulation of c-FLIP prevents extrinsic apoptosis in cancer cells following p73 activation. To address this question, p73 was overexpressed in cells by adenovirus infection and c-FLIP was knocked down by siRNA. Overexpression of p73 induced cellular apoptosis in the cells with knockdown of c-FLIP and this effect was blocked by knockdown of caspase 8. Our results suggest that p73 induces extrinsic apoptosis when c-FLIP is depleted in cancer cells. We further found that the knockdown of DR5 inhibited p73-induced apoptosis in c-FLIP-deficient cancer cells. By contrast, PUMA knockdown did not block p73-induced apoptosis. Our results suggest that DR5 is required for cell extrinsic apoptosis in c-FLIP-deficient cancer cells upon p73 activation. p21 is the cyclin dependent kinase inhibitor that leads to cell cycle arrest at the G1/S phase. Knockdown of p21 enhanced cleaved-caspase 8 and cleaved-PARP in c-FLIP-depleted cancer cells treated with Ad-p73. Our results, taken together, suggest that the p73 primes extrinsic apoptosis via DR5 and the priming level appears to be titrated at the level of c-FLIP. Upregulation of c-FLIP decreases the priming level, resulting in cells converting to cell cycle arrest and survival via p21 whereas reduction of c-FLIP leads the primed cancer cells to undergo extrinsic apoptosis upon p73 treatment in cancer cells. Our discovery of p73 transcriptional upregulation of c-FLIP provides a rationale for depleting c-FLIP to improve antitumor efficacy of p73-targeting cancer therapy. Citation Format: Shengliang Zhang, Wafik S. El-Deiry. Tumor suppressor p73 primes extrinsic apoptosis via transcriptional activation of DR5 and c-FLIP [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6146.