Abstract 4173: Compensatory interactions between tryptophan catabolism and serine metabolism highlight potential vulnerabilities in TNBC cells with dysregulated <i>de novo</i> serine synthesis

Abstract Background: Despite the major advances people have achieved in treating breast cancer, triple-negative breast cancer (TNBC) remains a difficult cancer to conquer. TNBC is one of the most heterogeneous and aggressive forms of breast cancer and is characterized by the loss of ER, PR, and HER expression. Due to the lack of these receptors, TNBCs are especially hard to treat with traditional methods that target these receptors, necessitating a more targeted approach to tackling these cancers. Serine and tryptophan are both metabolites that play crucial roles in the production and balance of nucleotides, energy and redox. These overlaps in metabolic functions make it likely that these two metabolites play compensatory roles under certain conditions. Different TNBCs have been shown to have vastly varying levels of de novo serine synthesis capacity and IDO1 expression. In our study, we attempt to uncover the role of tryptophan catabolism, in relation to serine metabolism in TNBC cells. Methods: LC-MS with [U-13C11]-Tryptophan and [U-13C3]-Serine were used to analyze their contributions to nucleotide synthesis, and energy metabolism via the tryptophan catabolism pathway and the folate/methionine cycle, respectively. [U-13C6]-Glucose was used to analyze its contribution to de novo serine synthesis under disrupted serine and tryptophan metabolism conditions. IDO1 inhibitor (Epacadostat), PHGDH inhibitor, and SHMT inhibitor (SHIN1) were used to inhibit tryptophan catabolism, de novo serine synthesis, and folate cycle, respectively. Cell growth was assessed via cell count imaging and changes in protein expression in relevant pathways were evaluated via western blot. Results: IDO1 inhibition showed varying levels of efficacy in different TNBC cells relating to their ability to synthesize serine and the availability of extracellular serine. Growth was most affected in cell lines with dysfunctional de novo serine synthesis indicating a compensatory effect between tryptophan catabolism and serine synthesis. This was further corroborated with the changes observed in IDO1 and PHGDH expression under enzymatic inhibition and metabolic deprivation. LC-MS analysis on IDO1, PHGDH, and SHMT inhibition showed different methods of compensation in different cell lines, with some exhibiting larger changes in nucleotide synthesis, while others in energy (NAD/H and ATP) and redox metabolism (NAD(P)/H). Conclusion: Our findings show serine and tryptophan metabolism are tightly co-regulated in TNBC cells. It also highlights the heterogeneity and complexity of how these cells utilize varying metabolic pathways to achieve these compensatory interactions. Our data supports that certain TNBC cells with dysfunctional de novo serine synthesis may be vulnerable to strategies that leverage their limited capacity to compensate between these pathways. Citation Format: Jin Heon Jeon, Abhinav Achreja, Mark Slayton, Olamide Animasahun, Minal Nenwani, Fulei Wuchu, Sofia D. Merajver, Deepak Nagrath. Compensatory interactions between tryptophan catabolism and serine metabolism highlight potential vulnerabilities in TNBC cells with dysregulated de novo serine synthesis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 4173.