Abstract
Nicotinamide adenine dinucleotide (NAD+) is an important redox cofactor and substrate for key enzymes and is involved in several cellular processes such as, energy metabolism, cell signaling, and DNA repair. Disturbance in NAD+ homeostasis is associated with multiple human diseases. NAD+ boosting agents, such as, nicotinamide mononucleotide (NMN) is under investigation in clinical trials and is a subject of active research in various pathological conditions associated with declining NAD+ levels including, Alzheimer's disease, diabetes, heart failure and overall health in aging. NMN administration in mice improved age-associated changes in body weight, energy metabolism, insulin resistance, mitochondrial respiration, and reversed gene expression changes. In human trials, NMN improved insulin sensitivity in pre-diabetic women, aerobic capacity in amateur runners and physical performance in older people. Although many diseases are negatively impacted due to a decline in NAD+ levels, cancer cells are known to upregulate NAD+ biosynthesis to support their growth and progression. For example, NAMPT, a rate limiting enzyme of NAD+ salvage pathway, is frequently upregulated in cancer and is associated with cancer progression and stemness. As safety of prolonged NMN supplementation in humans are being evaluated, it is equally important to explore its potential impact, adverse or beneficial, in relation to cancer pre-disposition conditions. To evaluate the impact of NMN in skin tumorigenesis, SKH-1 mice (∼11-17 weeks old) were supplemented with NMN in drinking water (vehicle) at 300mg/Kg/day. Mice were either sham treated or treated with UV at 18Kj/m2 five times a week for spontaneous development of cutaneous squamous cell carcinoma (cSCC). Number of tumors per mice (tumor multiplicity) was evaluated once a week starting at week 23 post UV irradiation. Tumors from both groups, vehicle UV and NMN UV, were harvested on week 37 and were subjected to exome sequencing, bulk RNA-sequencing, and Ki67 immunofluorescence for assessment of differences in mutation burden, gene expression and proliferation, respectively. Effect of NMN and nicotinamide (NAM) on cell proliferation and cell migration of cSCC cells, Colo-16, and SRB12, was assessed in vitro via MTT assay and scratch assay, respectively. NMN supplemented group exhibited increase in tumor multiplicity upon UV treatment in male and female SKH-1 mice. NMN supplemented tumors had relatively high mutation burden and Ki67 expression. Additionally, increase in extracellular matrix organization, epithelial mesenchymal transition, and angiogenic gene signatures were also elevated in NMN UV group. Colo-16 and SRB12 exhibited increased proliferation and migration in the presence of NMN or NAM. Overall, this study indicates that continuous NMN supplementation in cancer pre-disposition conditions might enhance development and progression of skin cancer.