Amino Acid-Metabolizing Enzymes in Advanced High-Grade Serous Ovarian Cancer Patients: Value of Ascites as Biomarker Source and Role for IL4I1 and IDO1

SIMPLE SUMMARY: Ovarian cancer is the most lethal gynecological malignancy in the United States. Despite the success of immunotherapy for treatment of various cancer types, its impact on ovarian cancer is restrained by a highly immunosuppressive tumor microenvironment. We aimed to evaluate the contribution of several amino acid-metabolizing enzymes to this environment by measuring the levels of amino acids and corresponding metabolites in liquid biopsies of high-grade serous ovarian cancer patients. The levels of different amino acid-derived metabolites were higher in ascites compared to plasma samples, demonstrating the value of utilizing ascites for biomarker identification. Moreover, the enzymes IDO1 and IL4I1 were identified as active players in high-grade serous ovarian cancer, and a correlation between IL4I1 metabolite levels and disease stage was revealed. Further exploration of the implications of enhanced IL4I1 activity in ovarian cancer is warranted to pave the way for new immunotherapeutic strategies in the treatment of this disease. ABSTRACT: The molecular mechanisms contributing to immune suppression in ovarian cancer are not well understood, hampering the successful application of immunotherapy. Amino acid-metabolizing enzymes are known to contribute to the immune-hostile environment of various tumors through depletion of amino acids and production of immunosuppressive metabolites. We aimed to collectively evaluate the activity of these enzymes in high-grade serous ovarian cancer patients by performing targeted metabolomics on plasma and ascites samples. Whereas no indication was found for enhanced l-arginine or l-glutamine metabolism by immunosuppressive enzymes in ovarian cancer patients, metabolism of l-tryptophan by indoleamine 2,3-dioxygenase 1 (IDO1) was significantly elevated compared to healthy controls. Moreover, high levels of l-phenylalanine- and l-tyrosine-derived metabolites associated with interleukin 4 induced 1 (IL4I1) activity were found in ovarian cancer ascites samples. While l-tryptophan is a major substrate of both IDO1 and IL4I1, only its enhanced conversion into l-kynurenine by IDO1 could be detected, despite the observed activity of IL4I1 on its other substrates. In ascites of ovarian cancer patients, metabolite levels were higher compared to those in plasma, demonstrating the value of utilizing this fluid for biomarker identification. Finally, elevated metabolism of l-phenylalanine and l-tyrosine by IL4I1 correlated with disease stage, pointing towards a potential role for IL4I1 in ovarian cancer progression.