In mammals, nicotinamide phosphoribosyltransferase (NAMPT) is responsible for the first and rate-limiting step in the conversion of nicotinamide to nicotinamide adenine dinucleotide (NAD+). NAD+ is an obligate cosubstrate for mammalian sirtuin-1 (SIRT1), a deacetylase that activates peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), which in turn can activate mitochondrial biogenesis. Given that mitochondrial biogenesis is activated by exercise, we hypothesized that exercise would increase NAMPT expression, as a potential mechanism leading to increased mitochondrial content in muscle. A cross-sectional analysis of human subjects showed that athletes had about a twofold higher skeletal muscle NAMPT protein expression compared with sedentary obese, nonobese, and type 2 diabetic subjects ( P < 0.05). NAMPT protein correlated with mitochondrial content as estimated by complex III protein content ( R 2 = 0.28, P < 0.01), MRS-measured maximal ATP synthesis ( R 2 = 0.37, P = 0.002), and V̇o2max ( R 2 = 0.63, P < 0.0001). In an exercise intervention study, NAMPT protein increased by 127% in sedentary nonobese subjects after 3 wk of exercise training ( P < 0.01). Treatment of primary human myotubes with forskolin, a cAMP signaling pathway activator, resulted in an ∼2.5-fold increase in NAMPT protein expression, whereas treatment with ionomycin had no effect. Activation of AMPK via AICAR resulted in an ∼3.4-fold increase in NAMPT mRNA ( P < 0.05) as well as modest increases in NAMPT protein ( P < 0.05) and mitochondrial content ( P < 0.05). These results demonstrate that exercise increases skeletal muscle NAMPT expression and that NAMPT correlates with mitochondrial content. Further studies are necessary to elucidate the pathways regulating NAMPT as well as its downstream effects.

Background: Elevated saturated fats during obesity activate proinflammatory pathways in macrophages, contributing to insulin resistance. Results: The monounsaturated fatty acid cis-palmitoleate antagonizes saturated fat-induced proinflammatory macrophage polarization through an AMPK-dependent mechanism. Conclusion: Palmitoleate is a lipid mediator that confers an anti-inflammatory macrophage phenotype. Significance: Understanding lipid-mediated macrophage polarization is critical to develop nutritional or cell-based strategies to combat insulin resistance.

Objective: In obesity, immune cells infiltrate adipose tissue. Skeletal muscle is the major tissue of insulindependent glucose disposal, and indices of muscle inflammation arise during obesity, but whether and which immune cells increase in muscle remain unclear. Methods: Immune cell presence in quadriceps muscle of wild type mice fed high-fat diet (HFD) was studied for 3 days to 10 weeks, in CCL2-KO mice fed HFD for 1 week, and in human muscle. Leukocyte presence was assessed by gene expression of lineage markers, cyto/chemokines and receptors; immunohistochemistry; and flow cytometry. Results: After 1 week HFD, concomitantly with glucose intolerance, muscle gene expression of Ly6b, Emr1 (F4/80), Tnf, Ccl2, and Ccr2 rose, as did pro-and anti-inflammatory markers Itgax (CD11c) and Mgl2. CD11c 1 proinflammatory macrophages in muscle increased by 76%. After 10 weeks HFD, macrophages in muscle increased by 47%. Quadriceps from CCL2-KO mice on HFD did not gain macrophages and maintained insulin sensitivity. Muscle of obese, glucose-intolerant humans showed elevated CD68 (macrophage marker) and ITGAX, correlating with poor glucose disposal and adiposity. Conclusion: Mouse and human skeletal muscles gain a distinct population of inflammatory macrophages upon HFD or obesity, linked to insulin resistance in humans and CCL2 availability in mice.

BackgroundAMP-activated protein kinase (AMPK) is a heterotrimeric enzyme that is evolutionarily conserved from yeast to mammals and functions to maintain cellular and whole body energy homeostasis. Studies in experimental animals demonstrate that activation of AMPK in skeletal muscle protects against insulin resistance, type 2 diabetes and obesity. The regulatory γ3 subunit of AMPK is expressed exclusively in skeletal muscle; however, its importance in controlling overall AMPK activity is unknown. While evidence is emerging that gamma subunit mutations interfere specifically with AMP activation, there remains some controversy regarding the impact of gamma subunit mutations [1]–[3]. Here we report the first gain-of-function mutation in the muscle-specific regulatory γ3 subunit in humans.Methods and FindingsWe sequenced the exons and splice junctions of the AMPK γ3 gene (PRKAG3) in 761 obese and 759 lean individuals, identifying 87 sequence variants including a novel R225W mutation in subjects from two unrelated families. The γ3 R225W mutation is homologous in location to the γ2R302Q mutation in patients with Wolf-Parkinson-White syndrome and to the γ3R225Q mutation originally linked to an increase in muscle glycogen content in purebred Hampshire Rendement Napole (RN-) pigs. We demonstrate in differentiated muscle satellite cells obtained from the vastus lateralis of R225W carriers that the mutation is associated with an approximate doubling of both basal and AMP-activated AMPK activities. Moreover, subjects bearing the R225W mutation exhibit a ∼90% increase of skeletal muscle glycogen content and a ∼30% decrease in intramuscular triglyceride (IMTG).ConclusionsWe have identified for the first time a mutation in the skeletal muscle-specific regulatory γ3 subunit of AMPK in humans. The γ3R225W mutation has significant functional effects as demonstrated by increases in basal and AMP-activated AMPK activities, increased muscle glycogen and decreased IMTG. Overall, these findings are consistent with an important regulatory role for AMPK γ3 in human muscle energy metabolism.