Abstract 4902: Mitochondrial and cytosolic folylpolyglutamate synthetase in one-carbon metabolism and anti-tumor efficacy of mitochondrial-targeted antifolates
Authors: O’Connor Carrie , Katinas Jade , Schneider Mathew , Nayeen Md. Junayed , Bao Xun , Li Jing , Dann Charles , Gangjee Aleem , Matherly Larry H. , Hou Zhanjun 
Published: April 2023

Abstract
Folates mediate one-carbon (C1) transfers, which are essential for cellular homeostasis and survival. C1 metabolism encompasses distinct cytosol (Cyto) and mitochondria (Mito) pathways connected by an interchange between serine, glycine and formate. Mito C1 metabolism provides cellular glycine and C1 units (as formate) for de novo synthesis of thymidylate and purine nucleotides in the Cyto. Polyglutamyl folates are the predominant folate forms in cells and are generally preferred for C1 transfers. Thus, folate polyglutamylation is essential for cellular homeostasis. Folate polyglutamylation is catalyzed by folypolyglutamate synthetase (FPGS), including Cyto (cFPGS) and Mito (mFPGS) isoforms. C1 metabolism is critical for tumor growth and thus offers a plethora of therapeutic targets for cancer. Of particular interest are Mito C1 inhibitors including “non-classical” pyrazolopyran compounds SHIN1 and SHIN2, and “classical” pyrrolo[3,2-d]pyrimidine antifolate compounds typified by AGF347, all targeting serine hydroxymethyltransferase 2 (SHMT2). Both SHIN1 and AGF347 effected in vitro anti-tumor efficacy, and SHIN2 and AGF347 showed in vivo efficacy toward tumor xenografts. We systematically explored the roles of cFPGS and mFPGS levels as critical determinants of C1 inhibitor target engagement and net Cyto versus Mito C1 flux, resulting in anti-tumor efficacy. We found that FPGS transcript levels significantly correlated with the effects of AGF347 toward a panel of human pancreatic cancer cell lines including MIA PaCa-2. We engineered MIA PaCa-2 cells with FPGS gene knockout to stably express inducible mFPGS or cFPGS form. FPGS in mFPGS-transfected cells (mFPGS#32) was expressed mainly in Mito over Cyto, whereas FPGS in cFPGS-transfected cells (cFPGS#3) was expressed exclusively in Cyto. Accumulation of radiolabeled folate and AGF347 increased with increasing FPGS in both Cyto and Mito for mFPGS#32, but only in Cyto for cFPGS#3. Metabolomics with [2,3,3-2H]serine in cFPGS#3 and mFPGS#32 established FPGS levels in Mito versus Cyto as important determinants of C1 fluxes. Increased FPGS levels in mFPGS#32 enhanced C1 flux in Mito far greater than in Cyto, and greater than for the increased FPGS in cFPGS#3. As a result, Cyto-targeted antifolates pemetrexed and AGF94 were only modestly impacted by increasing FPGS levels in both mFPGS#32 and cFPGS#3 (~3x). In contrast, increasing FPGS levels dramatically enhanced the inhibitory effects of Mito-targeted antifolates such as AGF347 in mFPGS#32 (~8-25x), and to a greater extent than in cFPGS#3 (~6-8x). Conversely, increasing FPGS levels substantially attenuated the cytotoxic effect of SHIN1 in mFPGS#32 (~19x). In summary, FPGS levels are an important determinant of C1 fluxes, particularly in Mito where they contribute to cytotoxic potencies of Mito-targeted C1 inhibitors at SHMT2.
Citation Format: Carrie O’Connor, Jade Katinas, Mathew Schneider, Md. Junayed Nayeen, Xun Bao, Jing Li, Charles Dann, Aleem Gangjee, Larry H. Matherly, Zhanjun Hou. Mitochondrial and cytosolic folylpolyglutamate synthetase in one-carbon metabolism and anti-tumor efficacy of mitochondrial-targeted antifolates. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4902.