Abstract B18: Loss of caspase-8 renders head and neck squamous cell carcinomas susceptible to necroptosis
Authors: Uzunparmak Burak , Myers Jeffrey N. , Pickering Curtis R. 
Published: September 2020

Abstract
Head and neck squamous cell carcinoma (HNSCC) is the sixth most common cancer in the world, with 500,000 new diagnoses and 300,000 cancer-related deaths worldwide annually. Despite 50 years of basic, clinical, and translational research, survival rates for patients with HNSCC have not remarkably improved, indicating a need for identification of new targets and biomarkers to guide therapy. Recent sequencing data from our group and others have identified Caspase-8 (CASP8) as one of the most frequently mutated genes in HNSCCs, with CASP8 mutations occurring in ~10% of the primary tumors. Genomics analysis using The Cancer Genome Atlas (TCGA) HNSCC dataset revealed that CASP8 mutations are associated with poorer overall patient survival. CASP8 belongs to the caspase family of proteases and has important roles in both apoptotic and necroptotic death. Our goal was to understand CASP8 signaling in HNSCC and examine whether those pathways could be exploited therapeutically. CASP8 serves as the apical-most caspase for the extrinsic apoptosis pathway, initiating programmed cell death following death receptor ligation. Cells deficient in Caspase-8 show resistance to death-receptor mediated killing in vitro and in vivo. Intriguingly, however, knockout of CASP8 in some tissues promotes cell death through induction of necroptosis, a.k.a programmed necrosis. Necroptosis is mediated by receptor-interacting protein kinases RIP1, RIP3 and their substrate mixed lineage kinase-like (MLKL) and often occurs following activation of tumor necrosis factor alpha (TNF-alpha) signaling. CASP8 has been shown to cleave and inhibit RIP1 and RIP3, thereby suppressing necroptosis. We hypothesized that mutation or loss of CASP8 could promote necroptosis and modulation of the pathway with small-molecule agents might have the potential for clinical utility. We found that deletion of CASP8 rendered HNSCCs susceptible to necroptosis induced by second mitochondria-derived activator of caspase (SMAC) mimetic treatment (TL-32711) when combined with TNF-alpha and Z-VAD-FMK, a pancaspase inhibitor, in vitro. Inhibition of Caspase-8 function was also associated with enhanced necroptotic killing by ionizing radiation in combination with SMAC mimetic treatment in vitro. We further demonstrated that the level of RIP3 expression determines susceptibility to SMAC mimetic-induced necroptosis in HNSCCs. In conclusion, CASP8 is an important regulator of cell death pathways in HNSCC. Modulation of these pathways can enhance the efficacy of radiotherapy and has the potential for clinical translation. Ongoing studies will give further insight into how CASP8 regulates necroptosis in HNSCCs and provide preclinical justification for use of these pathways as a therapeutic target in HNSCC patients.
Citation Format: Burak Uzunparmak, Jeffrey N. Myers, Curtis R. Pickering. Loss of caspase-8 renders head and neck squamous cell carcinomas susceptible to necroptosis [abstract]. In: Proceedings of the AACR-AHNS Head and Neck Cancer Conference: Optimizing Survival and Quality of Life through Basic, Clinical, and Translational Research; 2019 Apr 29-30; Austin, TX. Philadelphia (PA): AACR; Clin Cancer Res 2020;26(12_Suppl_2):Abstract nr B18.