Nicotinamide phosphoribosyltransferase (NAMPT) is over-expressed and abundantly released from malignant pleural mesothelioma cells becoming a potential biomarker.

Introduction Malignant pleural mesothelioma (MPM) is an aggressive and incurable cancer of the pleural surface. Chronic inflammation, oxidative stress, and persistent aberrant signaling due to asbestos exposure led to mesothelial cells transformation over years. Despite increasing studies on MPM biology, continue efforts to identify novel biomarkers and tumor vulnerabilities to be targeted are needed. Nicotinamide adenine dinucleotide (NAD) biosynthesis is essential to support tumor energetic needs, as well as to regulate NADPH-mediated detoxification system. Activation of NAD metabolism through its rate-limiting biosynthetic enzyme nicotinamide phosphoribosyltransferase (NAMPT) has been identified as key event to support cancer metabolic rewiring. NAMPT, in addition to possess a key function in NAD generation, can be secreted in the extracellular space (eNAMPT), where it behaves as a mediator of inflammation, regulating tumor-host interactions. NAD/NAMPT axis emerges deregulated in several tumors; however, no data are available in MPM. Methods NAMPT-NAD axis (NAD-biosynthetic enzymes expression, activities, and metabolites) in MPM cell lines and primary samples was studied using biochemical, enzymatic, immunochemical assays. eNAMPT was evaluated exploiting commercial ELISA assay in sera and pleural effusions for more than 100 MPM patients with different histotype derived from AOU-Alessandria Biobank. In silico analysis using TCGA database was performed to correlate NAMPT expression and biological processes analyzing the cohort of MPM patients. Results Bioinformatics analysis on TCGA database showed that NAMPT is the main expressed NAD- biosynthetic enzymes in MPM, and its expression correlates with hallmark gene sets related to inflammation, metabolic and signaling pathways. RT-PCR and western blot analysis on MPM cell lines and mesothelioma primary cells vs normal mesothelium (Met-5A, or primary mesothelial cells) confirmed that NAMPT is overexpressed in MPM. Data showed that NAD levels are similar in the comparison between tumor and normal tissue, while NADP levels are increased in MPM cell lines, suggesting an impact on cellular redox state and metabolic homeostasis that will further evaluate. Within the extracellular space, we revealed significantly increased serum eNAMPT levels from a cohort of 115 MPM patients compared to healthy donors. eNAMPT is strongly released in pleural effusions from the same MPM patients, mainly in MPM with the most aggressive sarcomatoid phenotype. Moreover, we found also a second biosynthetic enzyme nicotinate phosphoribosyltransferase (NAPRT) released by MPM cells. Lastly, preliminary data showed that MPM cells are uniquely sensitive to NAMPT inhibition. Conclusions Overall, these data support the hypothesis of an impact of NAD/NAMPT axis in MPM biology and highlight a potential role of eNAMPT as biomarker with a functional activity like a damage- associated molecular pattern (DAMPs)/cytokine in MPM that will be further investigated.