Investigator: Karen H. Almeida, Rhode Island College

Scientific Theme: Cancer

Abstract: NAD+ is essential for life and therefore must be strictly regulated. It is the cofactor used in cell energy production and metabolism by providing electrons required for redox reactions. However, NAD+ is also consumed by enzymes such as poly(ADP-ribose)polymerases (PARP) and Sirtuins. Imbalance in NAD+ metabolism has been linked to human diseases such as cancer and diabetes. To maintain sufficient levels of NAD+, cells can use multiple pathways, each involving a phosphoribosyltransferase type II enzyme. Nicotinamide can be converted to nicotinamide mononucleotide (NMN) by the enzyme Nicotinamide phosphoribosyltransferase (NAMPT, PBEF or Vistafin). Alternatively, the dietary supplement nicotinic acid can be converted to nicotinic acid mononucleotide (NaMN) by nicotinic acid phosphoribosyltransferase (NAPRT) or the denovo pathway can convert tryptophan to quinolinic acid which is transformed to NaMN by quinolinic acid phosphoribosyl transferase (QPRT). Both NMN and NaMN are further processed by nicotinamide mononucleotide adenyltransferase (NMNAT) to yield NAD+ or nicotinic acid adenine dinucleotide (NaAD), respectively. NaAD is then amidated to NAD+ by NAD synthase. Although there is little sequence homology between these phosphoribosyltransferase enzymes, there is significant similarity in structure and mechanism. QPRT, NAPRT and NAMPT are all homodimers, QPRT and NAPRT use these dimers to generate hexameric ring structures. The active site of each enzyme is located at the dimer interface. NAMPT and NAPRT undergo autophosphorylation in the active site and QPRT has a likely site of modification but autophosphorylation has yet to be confirmed. Previous work on the NAMPT enzyme has identified two small molecule activators. We propose to expand the NAMPT activation data by testing QPRT and NAPRT for activation. The overall goal will establish a common mechanism for the regulation of phosphoribosyltransferase enzymes by 1) Monitoring the binding dynamics of small molecules activators of NAMPT and 2) Measuring the effect of activators on NAMPT paralogs, QPRT and NAPRT. This information will then direct the screening of drug libraries for novel effectors of NAD+ biosynthetic enzymes.

Human Health Relevance: The connection between nicotinamide adenine dinucleotide (NAD+) metabolism and human diseases, such as cancer and diabetes, is under intense investigation. Dysregulation of the NAD-consuming and NAD­ synthesizing enzymes is thought to play a central role in disease progression. Thus, a thorough understanding of the cellular mechanisms used to regulate NAD metabolism will provide critical information and drive drug development to combat these disorders.