Cancer cells have an altered metabolic state to meet the requirements of rapid proliferation, and altered amino acid metabolism is known to be significant for cancer cell growth. Amino acids consist of L- and D-amino acids, and humans are known to metabolize L-amino acids. D-amino acids have been considered not to exist in human body except in intestinal flora. Recently, D-serine was reported to be synthesized and catabolized by serine racemase (SRR) and D-amino acid oxidase, respectively, and it has an important function as co-activator of the N-methyl-D-aspartate receptor in the human brain. However, whether D-amino acids function in other human organs has not been elucidated. Moreover, the role of enzymes metabolizing D-amino acids in the context of cancer metabolism has not been studied. Here, we present a novel role for SRR in colon cancer. First, we analyzed SRR expression levels in colon adenoma, colon adenocarcinoma, and normal colon mucosa in ONCOMINE datasets. We found that SRR expression levels were significantly elevated in colon adenoma and colon adenocarcinoma compared to normal colon mucosa in several datasets, including that of the Cancer Genome Atlas (TCGA). To determine whether SRR has a functional role in colon cancer cells, we disrupted its expression using the Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/CRISPR-associated 9 (Cas9) system in two colon cancer cell lines, HCT116 and DLD-1, using two distinct targets of short guide RNAs and examined the effects on cell proliferation. We found that SRR knockout resulted in reduced cell proliferation in both HCT116 and DLD-1 cells, which was recovered by exogenous SRR expression. In addition, a positive correlation was found between SRR expression levels and cell proliferation rate in four colon cancer cell lines, HCT116, DLD-1, WiDr, and Lovo. Moreover, phenazine methosulfate and L-aspartic acid β-hydroxamate, which have been previously reported to inhibit SRR, suppressed in vitro colon cancer cell proliferation. In conclusion, SRR is highly expressed in colon adenoma and colon adenocarcinoma compared to normal colon mucosa, and it enhances colon cancer cell proliferation. SRR is expected to be a potential new target for the development of novel therapies for colon cancer.

Citation Format: Kenji Ohshima, Satoshi Nojima, Naoki Wada, Yumiko Hori, Shinichiro Tahara, Masako Kurashige, Keisuke Kawasaki, Junichiro Ikeda, Eiichi Morii. Serine racemase is a new therapeutic target for colon cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5488.