Targeting Arginine Auxotrophy in Colorectal Cancer

Arginine is a semi essential amino acid with numerous functions in cellular metabolism. Loss-of-expression of urea cycle enzymes in several malignancies results in arginine auxotrophy and sensitivity to arginine starvation. Lack of expression of the urea cycle enzymes argininosuccinate synthase 1 (ASS1) and ornithine transcarbamylase (OTC) identifies patients bearing urea-cycle defective cancers and therefore likely to benefit from arginine deprivation therapies. PEGylated formulations of the arginine-degrading enzymes arginine deiminase (ADI) and arginase are currently being tested in numerous clinical trials to ascertain the antitumour efficacy of pharmacological arginine deprivation. Here, we report remarkable arginine auxotrophy in colorectal cancer (CRC). We show that CRC cell lines are unable to grow in arginine-free medium and, in vivo, dietary withdrawal of arginine decreases the growth of xenograft tumours in mice subcutaneously implanted with ASS1-deficient CRC cells. In addition, CRC cell lines with low or undetectable expression of ASS1 are sensitive to pharmacological arginine depletion mediated by ADI treatment. Notably, ASS1-expressing, ADI-resistant cell lines remain auxotrophic for arginine and are susceptible to arginase activity in vitro and in vivo. Arginase degrades arginine in urea and ornithine. Cells with fully functional urea cycle can reutilize ornithine through the enzyme OTC. All CRC cell lines analysed show lack of expression of OTC and analysis of tissue microarrays unveils a common lack of OTC expression in CRC patients, suggesting that defective expression of OTC could contribute the observed arginine auxotrophy. Finally, in drug combination studies, we demonstrate that ADI and arginase synergise with the chemotherapeutics 5-Fluorouracil and Oxaliplatin in at least 2 of the cell lines tested. In summary, we demonstrate that reduced expression of ASS1 and OTC enzymes results in arginine auxotrophy in CRC, a metabolic vulnerability that is amenable to arginine depleting strategies.