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Wiley, The Journal of Physiology, 20(591), p. 5207-5220, 2013

DOI: 10.1113/jphysiol.2013.259515

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AMP-activated protein kinase regulates nicotinamide phosphoribosyl transferase expression in skeletal muscle

This paper is made freely available by the publisher.
This paper is made freely available by the publisher.

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Abstract

Deacetylases such as sirtuins convert nicotinamide adenine dinucleotide (NAD) to nicotinamide (NAM). Nicotinamide phosphoribosyl transferase (Nampt) is the rate-limiting enzyme in the NAD salvage pathway responsible for converting NAM to NAD to maintain cellular redox state. Activation of AMP-activated protein kinase (AMPK) increases sirtuin activity by elevating NAD levels. As NAM directly inhibits sirtuins, increased Nampt activation or expression could be a metabolic stress response. Evidence suggests that AMPK regulates Nampt mRNA content, but whether repeated AMPK activation is necessary for increasing Nampt protein levels is unknown. To this end, we assessed whether exercise training- or 5-amino-1-β-D-ribofuranosyl-imidazole-4-carboxamide (AICAR)-mediated increases in skeletal muscle Nampt abundance are AMPK dependant. One-legged knee-extensor exercise training in humans increased Nampt protein by 16% (p<0.05) in the trained, but not the untrained leg. Moreover, increases in Nampt mRNA following acute exercise or AICAR treatment (p<0.05 for both) were maintained in mouse skeletal muscle lacking a functional AMPK α2 subunit. Despite a reduction in Nampt protein in skeletal muscle of sedentary AMPK α2 kinase dead (KD), 6.5 weeks of endurance exercise training increased skeletal muscle Nampt protein to a similar extent in both wild-type (WT) (24%) and AMPK α2 KD (18%) mice. In contrast, four weeks of daily AICAR treatment increased Nampt protein in skeletal muscle in WT mice (27%), but this effect was abolished in AMPK α2 KD mice. In conclusion, functional α2-containing AMPK heterotrimers are required for elevation of skeletal muscle Nampt protein, but not mRNA induction. These findings suggest AMPK plays a post-translational role in the regulation of skeletal muscle Nampt protein abundance and further indicate that the regulation of cellular energy charge and nutrient sensing is mechanistically related.