Renal cell carcinoma (RCC) is a cancer that is hard to treat because of its evasive nature, and its resistance to chemotherapy, radiotherapy, and immunotherapy. There is only a 10% survival rate in humans if RCC is not caught at early stages. L-Arginine metabolism is a highly regulated process that can produce global effects in tumors and in the immune system. Arginase 1 (ARG1) and ARG2 can metabolize L-arginine to induce the synthesis of polyamines necessary for tumor growth, whereas inducible nitric oxide synthase (NOS2) can metabolize L-arginine to produce nitric oxide (NO) which has been shown to possess antitumor activity. In addition, L-citrulline, another metabolite of NOS2, may play an important role in cancer, since it acts as a substrate for the de novo synthesis of L-arginine. Previous data have shown that certain RCC cell lines respond to IFNy or IFNa treatments by activating the NOS2 protein to produce NO, which inhibits tumor growth. However, in several other RCC cell lines there is a lack of NOS2 protein induction, suggesting that these RCC cells have developed a mechanism that blocks NOS2 expression. Therefore, treatments that could activate NOS2 expression to induce tumor regression are much needed. We believe that the lack of NOS2 is due to L-arginine deprivation, which is highly depleted in ARG2-RCC tumors. It is possible that decreased extracellular L-arginine limits its intracellular availability, deactivating downstream translational proteins, then blocking NOS2 expression. We hypothesize that in some RCC, the competition of ARG and NOS2 for L-arginine regulates the expression of NOS2 as a mechanism to increase tumor growth. To test our hypothesis, increasing concentrations of L-arginine of 1,000, 2,000 and 4,000 uM were added to RPMI media and cells were cultured for 24 and 48 hours. At each time point cell lysates were tested for ARG2, NOS2, GCN2 and eIF2a proteins, as well as for intracellular levels of L-arginine. Supernatants were tested for L-arginine, L-glutamine, L-citrulline and nitrite production. Our results show that after 48 hours in culture, there was a decrease in ARG activity as compared to untreated controls (p=0.004) in cells treated with 4,000 uM of L-arginine; a slight increase in NOS2 protein was observed with no significant increases in nitrites. More experiments using closer L-arginine concentration increments are under way in lieu to determine the expression of GNC2 and eIF2a. The levels of L-citrulline were significantly higher at 48 hours in cells cultured in 4,000 uM of L-arginine. One striking observation was that the cells consumed L-glutamine at higher rates as early as 24 hours. This finding is very important because these cells are possibly using L-glutamine as a source for polyamine synthesis, bypassing the L-arginine-ARG pathway. Understanding the mechanisms by which RCC tumors grow could help us to develop new and more effective immune-therapeutic strategies to overcome RCC growth and resistance. Citation Format: Charity F. Sylvester, Paula Datri, Arnold Zea. L-Arginine in the regulation of NOS2 expression to overcome renal cell carcinoma tumor growth [abstract]. In: Proceedings of the Eleventh AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2018 Nov 2-5; New Orleans, LA. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2020;29(6 Suppl):Abstract nr B067.