The new insight into extracellular NAD(+) degradation‐the contribution of CD38 and CD73 in calcific aortic valve disease

Nicotinamide adenine dinucleotide (NAD(+)) is crucial for cell energy metabolism and many signalling processes. Recently, we proved the role of ecto‐enzymes in controlling adenine nucleotide–dependent pathways during calcific aortic valve disease (CAVD). This study aimed to investigate extracellular hydrolysis of NAD(+) and mononucleotide nicotinamide (NMN) in aortic valves and aorta fragments of CAVD patients and on the inner aortic surface of ecto‐5′‐nucleotidase knockout mice (CD73−/−). Human non‐stenotic valves (n = 10) actively converted NAD(+) and NMN via both CD73 and NAD(+)‐glycohydrolase (CD38) according to our analysis with RP‐HPLC and immunofluorescence. In stenotic valves (n = 50), due to reduced CD73 activity, NAD(+) was degraded predominantly by CD38 and additionally by ALP and eNPP1. CAVD patients had significantly higher hydrolytic rates of NAD(+) (0.81 ± 0.07 vs 0.56 ± 0.10) and NMN (1.12 ± 0.10 vs 0.71 ± 0.08 nmol/min/cm(2)) compared with controls. CD38 was also primarily engaged in human vascular NAD(+) metabolism. Studies using specific ecto‐enzyme inhibitors and CD73−/− mice confirmed that CD73 is not the only enzyme involved in NAD(+) and NMN hydrolysis and that CD38 had a significant contribution to these pathways. Modifications of extracellular NAD(+) and NMN metabolism in aortic valve cells may be particularly important in valve pathology and could be a potential therapeutic target.