Rewiring of proline metabolism through increased expression of mitochondrial pyrroline-5-carboxylate reductase sustains colorectal cancer.

Proline is non-essential amino acid and its metabolism has been linked to cancer progression. Pyrroline-5- carboxylate reductase (PYCR) enzymes catalyze the NAD(P)H-dependent conversion of the precursor pyrroline-5-carboxylate to proline. There are three PYCR isoforms mitochondrial PYCR1 and PYCR2, and cytosolic PYCRL. Analysis of colorectal cancer (CRC) datasets indicated that PYCR1is overexpressed in cancer compared to normal tissue and we could confirm robust expression of PYCR1 in numerous CRC cell lines. In addition, analysis of primary cells isolated from surgically resected cancers and scoring of tissue microarray (TMA) confirmed the robust increase in PYCR1 protein expression in both adenoma and cancer lesions. To investigate the functional meaning of the PYCR1 expression, we used siRNA technology. Knockdown of PYCR1 enzyme decreased cell number by reducing both cell proliferation and survival in a variety of CRC cell lines. Indeed, loss of PYCR1 protein enhanced expression of the cell cycle inhibitor p21 and substantially reduced expression of cyclin D1 and D3. Additionally, depletion of PYCR1 protein enhanced apoptosis that measured by AnnexinV/PI staining. Notably, the addition of proline was unable to compensate for PYCR1-deficiency, indicating that production of proline is not responsible for the pro-tumorigenic activity of PYCR1. In summary, our findings indicate that proline metabolism is necessary to sustain colorectal carcinogenesis and that PYCR1 is a potential target for anti-cancer therapies.