Tryptophan metabolism plays a significant role in multiple myeloma survival

Abstract Introduction Multiple myeloma (MM) is a hematological malignancy caused by abnormally proliferating plasma cells in the bone marrow. It is considered incurable, as patients almost always go through cycles of treatment, remission and treatment-resistant relapse. Early-stage MM depends on the Bone Marrow Microenvironment (BMME) for survival. We have previously shown that MM interacts with BMME Dendritic Cells (DC) which induces DC production of Indolamine Dioxygenase 1 (IDO1), which catabolizes tryptophan (TRP) to kynurenine (KYN). Depletion of TRP suppresses T effector cell activation and the production of KYN activates pro-survival pathways in MM through the activation of the transcription factor Aryl Hydrocarbon Receptor (AHR). We now show that MM cells can produce KYN independent of DCs through expression of the TRP metabolizing enzyme TDO, indicating a novel mechanism by which MM can become independent of the BMME. Methods Patient RNA expression data were taken from the CoMMpass database. We measured MM cell lines U266, 8226, MM1S and KMS11 for expression of TRP metabolizing enzymes with western blot and qPCR. TRP metabolizing enzyme Tryptophan 2,3-Dioxygenase (TDO) was knocked down with shRNA or inhibited with TDO-specific inhibitor 680C91. In some experiments MM cell lines were co-cultured with monocyte-derived DC. AHR activation was measured through qPCR of CYP1a1, a transcriptional downstream target. KYN production was measured by ELISA. KMS11 cells were treated with 680C91, then underwent bulk mRNAseq and were analyzed using gene set enrichment analysis. Results CoMMpass patients with the highest quartile of TDO expression have significantly lower rates of progression-free and overall survival. Three of our cell lines 8226, MM1S and KMS11 express TDO, while U266 does not. Inhibiting TDO with the non-competitive inhibitor 680C91 or knocking down TDO significantly reduced MM cell survival in TDO+ MM cell lines. Inhibiting TDO reduced KYN production. Inhibiting TDO also reduced AHR activation as shown by expression of downstream target CYP1a1 in TDO+ MM cell lines, but did not affect a TDO- MM cell line. Co-culture with IDO1+ DCs or treatment with AHR ligand TCDD rescued MM cell viability from TDO inhibitor-induced cell death, indicating that TRP metabolism to KYN is important to MM survival. Total mRNA seq with KMS11 cells revealed that cells treated with TDO inhibitor 680C91 had significantly reduced expression of genes involved in MYC signaling, AHR signaling, cholesterol homeostasis and fatty acid metabolism; 680C91-treated KMS11 cells had upregulated expression of genes involved in TNFa signaling through NFkB, hypoxia and apoptosis. Conclusions MM depends on TRP metabolism for survival both in the BMME and as it becomes independent of it. MM cells express TDO, which supports MM survival through the activation of AHR and could repress T effector activation through the depletion of TRP. We now show evidence that TRP metabolism in MM also supports survival through MYC signaling and through upregulation of metabolic pathways. TRP metabolism is a novel treatment target in MM and could lead to more effective cell killing and immunotherapy, especially in relapse/refractory disease.