Abstract 2336: Eradicating melanoma cells using combination of nutritional stress and checkpoint inhibitors
Authors: Feun Lynn G. , Wu Chunjing , Li Ying-Ying , You Min , Wangpaichitr Medhi , Suarez Miguel , Savaraj Niramol G. 
Published: July 2021

Abstract
Arginine (Arg) deprivation can inhibit tumors which do not express argininosuccinate synthetase (ASS), a key enzyme for synthesis of Arg. Arg deiminase (ADIPEG20) has shown antitumor activity in such tumors, including melanoma and mesothelioma. Resistance to this treatment occurs due to re-expression of ASS. We have shown that this is mediated via cMyc (positive regulator) and HIF1α ( negative regulator ). Arg starvation rapidly induces chromatin remodeling complex P300-HDAC2-Sin3A which epigenetically deacetylates H3K14ac and H3K27ac at the ASS promoter. Following the PHD2-derived HIF1α-degrading system, the promoter-bound HIF1α is degraded (Sci Rep 7:10814). This allows cMyc (an E-Box binder), to turn on ASS. Arg starvation rapidly triggers externalization of Gas6 to interact with its receptor tyrosine kinase (RTK) Axl. This activates the downstream Ras-PI3K/AKT/GSK3β pathway, resulting in stabilization of cMyc. However, this activation also turns on PDL1 gene expression. This is not surprising since cMyc has been shown to bind to PDL1 promoter and increase its expression. Thus, re-expression of ASS also leads to PDL1 expression. We studied these findings in two other cell lines, Mel114 and Skmel2, and obtained similar results, Interestingly, in three cell lines with high levels of antiapoptotic protein Bcl-2, Arg starvation only leads to growth inhibition, with only less than 2 fold increase in ASS and no PDL1 expression. These results suggest that increased Bcl-2 expression negates the necessity to increase PDL1 and ASS to prevent cell death from nutritional stress. In contrast, transfection of ASS in A2058, A375, Mel1220 and SkMel2 will only confer resistance to Arg starvation, and does not affect PDL1, Bcl2 expression or other alterations in PI3K/AKT pathway. Furthermore, in samples from patients who failed ADIPEG20 due to increased ASS expression, the primary cultures or re-biopsy tumor also exhibited high levels of PDL1 expression, which most likely will make them susceptible to anti PDL1 treatment. On the other hand, six primary cultures derived from patients who failed BRAF/MEK inhibitor (BMR) also showed low levels of ASS and increase in PDL1 expression (1.5-4 fold increase both in mRNA and proteins level compared to their parental cells). These results also applied to 5 pairs of BMR cell lines (A375/BMR, A2058/BMR, Skmel2/BMR, UACC62/BMR, MelRR/BMR). These cells also are very susceptible to combination of ADIPEG20 and PD-L1 treatment. Overall, our data suggest that combination of Arg deprivation and check point inhibitors is effective in patients whose tumors do not express ASS and as salvage therapy for BRAF/MEK inhibitor resistant patients. Supported by the VA Merit Review Award(1BX003328)
Citation Format: Lynn G. Feun, Chunjing Wu, Ying-Ying Li, Min You, Medhi Wangpaichitr, Miguel Suarez, Niramol G. Savaraj. Eradicating melanoma cells using combination of nutritional stress and checkpoint inhibitors [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2336.