Efficacious Anti-Cancer Drugs Targeting Nicotinamide N-Methyltransferase (NNMT) in Cultured Human Oral Squamous Cell Carcinoma (OSCC)

Background/Objectives: Oral squamous cell carcinoma (OSCC) is a major cause of human cancer. The enzyme, nicotinamide N-methyltransferase (NNMT), is overexpressed in a variety of human cancers, including OSCC. Our objective was to target NNMT with novel inhibitors and determine their anti-cancer efficacy while shedding light on their possible mechanism of action. Methods: We identified two small molecule inhibitors of NNMT (AG-670 and AO-022) based on a pharmacophore of the in silico nicotinamide binding site. These inhibitors were investigated for (i) potency to inhibit the activity of the isolated NNMT enzyme (EC50 values), (ii) cytotoxicity (IC50 values) against the human OSCC cell line, SCC-4, and (iii) ability to affect cellular energy metabolism, as measured by oxygen consumption, in SCC-4 cells (plus dysplastic oral keratinocytes (DOK) cells and breast cancer MCF-7 cells). Immunoblotting was used to determine whether NNMT was expressed in the aforementioned cells. Results: NNMT is expressed in SCC-4 and DOK cells (and primary human oral keratinocytes) but not MCF 7 cells. The NNMT inhibitors inhibit isolated NNMT enzyme activity and were cytotoxic to SCC-4 cells (EC50 and IC50 values in the micromolar range). Sublethal doses of the inhibitors were demonstrated to inhibit in situ mitochondrial oxygen consumption in SCC-4 and DOK cells but not in MCF-7 cells. It was demonstrated that the NNMT inhibitors do not directly inhibit mitochondrial electron transport chain activity. Thus, we deduce that the NNMT inhibitors affect mitochondrial activity indirectly via NNMT. Conclusions: It is concluded that NNMT is a potential drug target for oral cancer.