Rewiring of proline metabolism in colorectal carcinogenesis: causes and consequences

Colorectal cancer (CRC) is the fourth most common cancer in the UK. Recently, the metabolism of non-essential amino acids in cancer has been identified as an area of interest for novel therapeutic targets. Of particular interest here, the metabolism of proline has been linked with cancer progression in a number of malignancies. The final step of proline biosynthesis is catalysed by pyrroline-5-carboxylate reductases (PYCRs) which utilise NAD(P)H as a cofactor. PYCR1 and PYCR2 are mitochondrial isoforms whilst PYCR3 is localised to the cytoplasm. PYCR1 has been reported to be upregulated in a number of cancers including CRC. Here, evidence is provided to confirm the role of PYCR1 in CRC using both RNA interference and CRISPR/cas9 technologies. siRNA knockdown resulted in decreased cell growth with increased cell cycle arrest and apoptosis. Supplementation with proline and nucleotides indicate that neither biosynthesis of proline, nor the production of nucleotides are limiting in PYCR1-depleted cells. Evidence is also provided to indicate an increased role of PYCR2 than previously thought with analysis of online datasets showing a correlation between PYCR2 expression and reduced survival in stage IV CRC. This suggests a role for PYCR2 in metastatic cancers. Overall, this thesis indicates that the proline metabolic pathway is a suitable target for novel CRC therapies. In particular, this thesis provides a novel role for PYCR2 in CRC which should be investigated further.