Endocrine Abstracts

JOINT1935

Medullary Thyroid Carcinoma (MTC) is an aggressive cancer associated with immunosuppression mediated by indoleamine 2, 3-dioxygenase 1 (IDO1), which IFN-gamma induces. IDO1 converts tryptophan (TRP) into kynurenine (KYN), promoting an immunosuppressive microenvironment. IDO1-producing tumor cells ensure their survival during TRP depletion by producing WARS, which binds TRP to tRNA. This mechanism has been described in some tumors but not yet in MTC. Thus, WARS inhibition could exert a cytotoxic effect on tumor cells. We aimed to: (1) evaluate IDO1 and WARS expression in MTC cells; (2) verify if IDO1 expression is associated with IDO1 activity; (3) assess the effect of WARS inhibition on cell viability.

Methods: TT cells (C634W) were cultured and incubated with IFN-gamma (50ng/mL) for 24h. The relative quantification of IDO1 and WARS expression was determined by qPCR with SYBR_Green, using TBP for normalization and ΔΔCt. IDO1 activity was assessed by measuring TRP and KYN levels in the supernatant (KYN/TRP ratio) using HPLC. For WARS inhibition, tryptamine was applied at varying concentrations (0. 0001-100mM) to evaluate cell toxicity (MTT). Linear regression and ANOVA were used.

Results: TT cells constitutively express WARS but not IDO1. Both genes were induced by IFN-gamma (10-fold increase in WARS; CQ of 24. 8 for IDO1 vs. undetectable in Control). IDO1 was positively correlated with WARS (r = 0. 898, P < 0. 05). Interestingly, the KYN/TRP ratio was not altered by IFN-gamma (ratio 1. 8±0. 1 vs. 1. 5±0. 2 in Control), indicating that enzyme activity did not increase despite mRNA expression. Tryptamine reduced cell viability (IC50=5. 8mM), independently of IFN-gamma (IC50=5. 8mM). In conclusion, IFN-gamma induces IDO1 and WARS; however, this does not necessarily lead to increased IDO1 activity. The cytotoxic effect of WARS inhibition may be enhanced in cases where IDO1 activity is present. This study highlights the importance of confirming IDO1 activity beyond its expression when designing treatment strategies that combine IDO1 and WARS inhibitors for MTC.