Abstract 704: RALB GTPase: A critical regulator of DR5 cell surface expression and TRAIL sensitivity in RAS mutant colorectal cancer

Abstract

Abstract RAS mutant (MT) metastatic colorectal cancer (mCRC) remains a difficult-to-treat group. RALA and RALB GTPases function downstream of RAS and have been found to be key regulators of several cell functions and implicated in KRAS-driven tumourigenesis. However, their role as regulators of the apoptotic machinery remains to be elucidated. Here, we found that inhibition of RALB expression, but not RALA, resulted in caspase 8-dependent cell death in KRASMT cells, that was not further increased following MEK1/2 inhibition. Proteomics analysis and mechanistic studies revealed that RALB inhibition induces marked upregulation of the pro-apoptotic cell-surface TRAIL Death Receptor 5 (DR5) receptor in KRASMT CRC. Moreover, DR5 knockout also attenuated siRALB-induced apoptosis, confirming the role of the extrinsic apoptotic pathway as regulator of siRALB-induced cell death. Importantly, TRAIL treatment resulted in acute association of RALB with the death-inducing signalling complex (DISC) and targeting RALB using pharmacologic inhibition or RNAi approaches, resulted in a potent increase in TRAIL-induced cell death in RASMT CRC. Significantly, high RALB mRNA levels were found in the poor prognostic CRIS-B CRC subgroup. Collectively, this study provides the first evidence for a role of RALB in apoptotic priming and that RALB inhibition may be a promising strategy to improve response to TRAIL treatment in poor prognostic RASMT CRC. Citation Format: Hajrah Khawaja, Jamie Z. Roberts, Arman Javadi, Paul O'Reilly, Darragh McArt, Wendy L. Allen, Markus Morrison, Richard Kennedy, Nicolas Vitale, Patrick G. Johnston, Tim Harrison, Daniel B. Longley, Emma Evergren, Sandra Van Schaeybroeck. RALB GTPase: A critical regulator of DR5 cell surface expression and TRAIL sensitivity in RAS mutant colorectal cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 704.