Dietary Methionine Restriction Reverses Liver Cancer Driving Metabotype in GNMT KO Mice

Glycine N-methyltransferase (GNMT) is a major liver enzyme that catalyzes the S-adenosyl-methionine (SAM) dependent synthesis of sarcosine from glycine and regulates the availability of SAM for cellular methylation reactions, as well as for detoxification pathways in liver cells. The whole-body enzyme knockout in mice results in dysregulated methionine metabolism, including massive elevations of SAM. As a result, Gnmt−/− mice develop fatty livers and subsequently, hepatocellular carcinomas (HCC). We hypothesize that since the HCC development in Gnmt−/− mice is associated with the deregulation of metabolic processes, alteration of nutrients supply may delay/prevent cancer development in animals with genetic deficiency of GNMT. Wild type and Gnmt−/− littermates of both sexes were randomized into dietary treatment groups. Experimental diets contained defined mixtures of purified amino acids instead of protein, and identical amounts and sources of carbohydrates, fat and vitamins. A standard amino acid defined diet (AAD, Envigo) was used as a control. Methionine adjusted diet (MA, same levels of amino acids as standard diet, except methionine was 8-fold lower) was tested for the effect on liver metabolome. Mice were kept on respective diets for 8 months after weaning, and blood and tissue samples were collected at endpoint. Aliquots of frozen liver samples (∼60–70 mg) were subjected to untargeted metabolomic analysis through Metabolon®. Metabolomic data underscore dramatic elevation of polyamines, TCA metabolites, bile acids and glycogen degradation products in Gnmt−/- livers on standard diet compared to WT controls. On the MA diet however, metabotypes of Gnmt−/- mice were similar to those of their wildtype littermates, with reversals in elevations of SAM and almost all the aforementioned metabolites. There were no appreciable differences between diets in WT mice metabotypes. Our data demonstrate that diet alteration can reverse the deregulation of metabolic processes associated with HCC development in Gnmt−/- mice. These results also suggest that diet modification should be investigated for the potential of improving liver cancer treatments. UNC NRI 2018 Pilot Project.