Nicotinamide N-Methyltransferase (NNMT) is an Anti-Cancer Target in Cultured Human Oral Squamous Cell Carcinoma (OSCC).

Background: Oral squamous cell carcinoma makes up 90% of oral malignancies. With a poor five-year survival, a targeted approach may prove a useful adjunct to established cancer therapeutics. The enzyme, N-Nicotinamide Methyltransferase (NNMT) is overexpressed in a variety of cancers and has been investigated as an anti-cancer target. We recently identified two inhibitors of NNMT based on a pharmacophore of the in-silico nicotinamide binding site. The effect of these modulators of NNMT on OSCC cell viability and metabolism was evaluated.

Methods: Cells were cultured as per ATCC recommendations. Detection of NNMT was assessed by immunoblot from cell lysates. Cell viability was determined by alamar blue assay. Cellular respiration and extracellular acidification rate were determined by Seahorse XF analyser. Mitochondria were isolated from rat liver and NADH-dependent oxygen consumption in the presence/absence of inhibitors was determined using a Rank Oxygen Electrode. An Inhibitor Screening Assay kit was used to determine the potency of inhibitors on the isolated enzyme.

Results: NNMT was expressed in SCC-4 and DOK (dysplastic) cells. Both inhibitors resulted in cell death when treated of SCC-4 cells. The IC50 values for each compound were subsequently calculated. Significantly, our data has shown that both inhibitors reduce oxidative phosphorylation and increase glycolysis in DOK and SCC-4 cells. Neither inhibitor showed a significant effect on NADH-dependant oxygen consumption in the coupled and uncoupled rat liver mitochondria.

Conclusions: Our data suggests that NNMT is expressed in both SCC-4 and DOK cells. Both inhibitors had a cytotoxic effect on the SCC-4 cells. These compounds inhibit oxygen consumption in intact SCC4 and DOK cells, resulting in increased glycolytic flux. The NNMT inhibitors do not directly inhibit mitochondrial electron transport chain function. It is hypothesized that these inhibitors cause cell death of SCC-4 cells indirectly, via NNMT, through a mechanism that is yet to be determined.