Abstract 17971: Overexpression of Nicotinamide Phosphoribosyltransferase (NAMPT) Negatively Regulates Cardiac Function in Response to Pressure Overload in Mice

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Nicotinamide phosphoribosyltransferase (NAMPT) is a rate-limiting enzyme in the salvage pathway of NAD+ synthesis. Besides respiration and oxidative phosphorylation, NAD+ is required as a cofactor for NAD+-dependent enzymes, such as sirtuins. We have reported previously that NAMPT is downregulated during pressure overload (PO) in the heart. However, the role of the downregulation of endogenous NAMPT during PO-induced hypertrophy and heart failure still needs to be elucidated. To address this issue, cardiac-specific NAMPT overexpression (Tg-NAMPT) and non-transgenic (Non-Tg) mice were subjected to transverse aortic constriction (TAC) for 4 weeks. mRNA expression of Nampt in the heart was increased in Tg-NAMPT mice at basal level (210% vs. Sham in non-Tg, p<0.05). Tg-NAMPT showed a decrease in Nampt mRNA expression after TAC but the level of Nampt mRNA was similar to that in Non-Tg mice at baseline. Both Tg-NAMPT and Non-Tg mice developed similar levels of cardiac hypertrophy (left ventricular (LV) weight/tibia length ratio: 7.19±0.33, 6.99±0.31) in response to TAC. LV cardiomyocyte cross sectional area (CSA) was similarly increased by TAC in Tg-NAMPT and Non-Tg (relative CSA ratio: 2.56±0.12, 2.48±0.22) compared to sham mice (1.30±0.12, 1.10±0.16). Although Tg-NAMPT exhibited similar baseline LV systolic function as Non-Tg (ejection fraction (EF): 80.5, 84.9%), Tg-NAMPT exhibited more severely decreased LV function after TAC than Non-Tg mice (EF: 54.2%, 80.2%, p<0.01). LV end diastolic pressure (LVEDP, mmHg) was significantly increased in Tg-NAMPT than in Non-Tg mice after TAC (16.2±3.3, 7.0±0.8, p<0.05). Cardiac interstitial fibrosis was significantly increased in Tg-NAMPT compared to Non-Tg mice (5.86±0.22 vs. 2.95±0.99%, p<0.05). mRNA expression of monocyte chemoattractant protein-1 (MCP-1), as evaluated with qRT-PCR analyses, was elevated in both Non-Tg and Tg-NAMPT after TAC (relative ratio/GAPDH, 2.12±0.39, p<0.05; 4.89±1.18, P<0.05) but the level was significantly higher in Tg-NAMPT than in Non-Tg mice. Taken together, NAMPT appears to promote cardiac dysfunction during PO, possibly by promoting myocardial inflammation and fibrosis. PO-induced downregulation of endogenous NAMPT may be an adaptive mechanism during PO.

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