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

Abstract Background Aim Under the regulation of various oncogenic pathways, cancer cells undergo adaptive metabolic programming to maintain specific metabolic states that support their uncontrolled proliferation. As it has been difficult to directly and effectively inhibit oncogenic signaling cascades with pharmaceutical compounds, focusing on the downstream metabolic pathways that enable indefinite growth may provide therapeutic opportunities. Thus, we sought to characterize metabolic changes in hepatocellular carcinoma (HCC) development and identify metabolic targets required for tumorigenesis. Methods We compared gene expression profiles of Morris Hepatoma (MH3924a) and DEN (Diethylnitrosamine)-induced HCC models to those of liver tissues from normal and rapidly regenerating liver models, and performed gain- and loss-of-function studies of the identified gene targets for their roles in cancer cell proliferation in vitro and in vivo. Results The proline biosynthetic PYCR1 (Pyrroline-5-Carboxylate Reductase 1) was identified as a top up-regulated gene in the HCC models. Knockdown (KD) of PYCR1 potently reduced cell proliferation of multiple HCC cell lines in vitro and tumor growth in vivo. Conversely, overexpression of PYCR1 enhanced the proliferation of the HCC cell lines. Importantly, PYCR1 expression was not elevated in the regenerating liver, and KD or overexpression of PYCR1 had no effect on proliferation of non-cancerous cells. Besides PYCR1, we found that additional proline biosynthetic enzymes, such as ALDH18A1, were upregulated in HCC models and also regulated HCC cell proliferation. Clinical data demonstrated that PYCR1 expression was increased in HCC, correlated with tumor grade, and was an independent predictor of clinical outcome. Conclusion Enhanced expression of proline biosynthetic enzymes promotes HCC cell proliferation. Inhibition of PYCR1 or ALDH18A1 may be a novel therapeutic strategy to target HCC.