Metabolic pathway analyses identify proline biosynthesis as a promoter of liver carcinogenesis

Cancer cells undergo dramatic metabolic reprogramming, largely to meet the requirements of sustained proliferation. These metabolic pathways are tightly regulated by various oncogenes, such as Myc and Ras. However, it has been difficult to directly and effectively inhibit these oncogenic signaling cascades with pharmacological compounds. Therefore, focusing on the downstream metabolic pathways that enable indefinite growth may provide a therapeutic opportunity. To identify metabolic enzymes required for hepatocellular carcinoma (HCC) tumorigenesis, we compared gene expression profiles of normal liver tissue to the Morris Hepatoma and DEN (Diethylnitrosamine)-induced HCC models as well as a liver regeneration model. PYCR1 (Pyrroline-5-Carboxylate Reductase 1), an enzyme in the proline biosynthesis pathway, was identified as one of the top up-regulated genes in the HCC models. An increase in PYCR1 protein levels in tumor samples versus normal liver tissue was confirmed by Western blot. shRNA-mediated knockdown of PYCR1 in HCC cell lines (Huh-7, Hep3B, HepG2 SNU-398, and MH3924a) potently reduced cell proliferation in vitro and tumor growth in vivo. Conversely, overexpression of PYCR1 enhanced the proliferation of Hep3B and AML12 cells. Importantly, PYCR1 expression was not elevated in regenerating tissues, and knockdown in a non-tumorigenic cell line did not influence proliferation. Additional enzymes in the proline biosynthesis pathway were also altered in tumors. For instance, ALDH18A1 was up-regulated and knockdown by shRNA decreased proliferation of human HCC cell lines. Metabolomic analyses suggested that the metabolism of glucose was disrupted in PYCR1 knockdown cells, with reduced fluxes through glycolytic and pentose phosphate pathways. Clinical data demonstrated that PYCR1 expression was increased in HCC, and expression levels correlated with increasing tumor grades and were independent predictors of clinical outcome. Overall, our data suggest that enhanced expression of proline biosynthetic enzymes promotes cell proliferation, and that inhibiting PYCR1 or ALDH18A1 may be a novel therapeutic strategy to target HCC. Citation Format: Zhaobing Ding, Russell E. Ericksen, Michael Steckel, Andrea Haegebarth, Sylvia Gruenewald, Weiping Han. Metabolic pathway analyses identify proline biosynthesis as a promoter of liver carcinogenesis [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 3477.