Dual Roles of Nicotinamide Phosphoribosyltransferase as a Promising Target for Cancer Radiotherapy

Nicotinamide phosphoribosyltransferase (NAMPT) is the key rate-limiting enzyme in the regulation of nicotinamide adenine dinucleotide (NAD) biosynthesis, and its activity is critical for the replenishment of NAD level as well as cell survival or death. As one of the most important components in the electron transport chain of complex I in mitochondrion, sustained supply of NAD is essential to the maintenance of energy metabolism both in normal and cancer cells. Recent research showed that X-ray radiation sharply downregulated the expression of NAMPT, which may be the main cause of radiation damage in salivary gland. Consistently, upregulation of NAMPT by phenylephrine restored the function and tissue structure of salivary gland, indicating the cytoprotective role of NAMPT in preventing radiation damage in normal tissues of patients with head and neck cancer during radiotherapy. On the other hand, NAMPT downregulation and NAD depletion could induce cell death in oral squamous cell cancer, suggesting that a combination of NAMPT inhibitor and radiotherapy presents a promising therapeutic strategy for cancer treatment. Based on our and other's studies, NAMPT may have dual roles in cancer radiotherapy: the upregulation of NAMPT could serve to suppress radiotherapy complications such as radiation sialadenitis, and combination regimens that involve NAMPT inhibitors may enhance efficacy of radiotherapy for cancer treatment.