Abstract 3921: Malic enzyme 2 identified as metabolic target in triple-negative breast cancers with increased serine biosynthesis
Authors: Slayton Mark D. , Jeon Jin Heon , Achreja Abhinav , Farah Zackariah A. , Nilaj Brisilda , Eu Yi-Hsien , Lui Alisa , Collard Mason , Bao Liwei , Kleer Celina , Nagrath Deepak , Merajver Sofia D. 
Published: April 2023

Abstract
Background & Hypothesis: Triple-negative breast cancer (TNBC) is a difficult to treat and deadly form of breast cancer. Pyruvate is an essential intermediate component of cellular metabolism which cancerous cells, including breast cancer cells, rely upon. Importantly, pyruvate and phosphoenolpyruvate are important nodes in the glycolytic pathway that support serine synthesis, lactate production and the TCA cycle. As a result of this, inhibition of the metabolic enzymes which regulate pyruvate production is currently used for cancer therapy. Apart from the glycolytic pathway, malic enzyme 2 (ME2) contributes to pyruvate and NADPH production through the oxidation of malate. Therefore, ME2 is a critical pathway supporting multiple metabolic needs of TNBC. Here, we investigate TNBC cell lines and assess their reliance on ME2 to drive accelerated growth, and thus potentially making ME2 a therapeutic target in TNBC.
Methods: We investigated four TNBC lines (HCC1806, Hs578T, BT20, MDAMB468) with distinct serine synthesis activity assessed via western blotting of serine metabolism enzymes. To knockdown ME2 in TNBC cell lines, we used siRNA transient transfection and shRNA-encoding lentiviral vectors. The growth rates of TNBC cells were determined by cell counting in the BioTek BioSpa 8. Lastly, we used immunohistology to analyze the expression of ME2 and serine metabolic genes in a TNBC tumor microarray.
Results: Modulation of the serine synthesis pathway revealed an unexpected synergy between serine metabolism and ME2. Knockdown of ME2 reduced the expression of serine metabolism genes but did not affect the expression of ME1 or ME3. The proliferative rate of cells with elevated serine synthesis, BT20 and MDAMB468 - was significantly reduced under ME2 knockdown, while Hs578T and HCC1806 experienced a modest reduction. Immunohistologic staining of TNBC tumors corroborated the link between ME2 and serine metabolism.
Conclusions: We observed that TNBC cell lines with elevated, but not reduced, expression of serine synthesis genes were growth-inhibited in the absence of ME2. Our results indicate that ME2 is required for optimal growth in some forms of TNBC and may serve as a therapeutic target for the treatment of cancers with elevated serine biosynthesis. Ongoing studies are aimed at discerning additional biomarkers of sensitivity to ME2 inhibition in TNBC and other aggressive cancers.
Citation Format: Mark D. Slayton, Jin Heon Jeon, Abhinav Achreja, Zackariah A. Farah, Brisilda Nilaj, Yi-Hsien Eu, Alisa Lui, Mason Collard, Liwei Bao, Celina Kleer, Deepak Nagrath, Sofia D. Merajver. Malic enzyme 2 identified as metabolic target in triple-negative breast cancers with increased serine biosynthesis. [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 3921.