SUMMARY While fatty acids (FAs) released by white adipose tissue (WAT) provide energy for other organs, lipolysis is also critical in brown adipose tissue (BAT), generating FAs for oxidation and UCP-1 activation for thermogenesis. Here, we show that adipose-specific ablation of desnutrin/ATGL in mice converts BAT to a WAT-like tissue. These mice exhibit severely impaired thermogenesis with increased expression of WAT-enriched genes but decreased BAT genes including UCP-1 with lower PPARα binding to its promoter, revealing the requirement of desnutrin-catalyzed lipolysis for maintaining BAT phenotype. We also show that desnutrin is phosphorylated by AMPK at S406, increasing TAG hydrolase activity, and provide evidence for increased lipolysis by AMPK phosphorylation of desnutrin in adipocytes and in vivo. Despite adiposity and impaired BAT function, desnutrin-ASKO mice have improved hepatic insulin sensitivity with lower DAG levels. Overall, desnutrin is phosphorylated/activated by AMPK to increase lipolysis and brings FA oxidation and UCP-1 induction for thermogenesis.

The role of AMP-activated protein kinase (AMPK) in promoting fatty acid (FA) oxidation in various tissues, such as liver and muscle, has been well understood. However, the role of AMPK in lipolysis and FA metabolism in adipose tissue has been controversial. To investigate the role of AMPK in the regulation of adipose lipolysis in vivo, we generated mice with adipose-tissuespecific knockout of both the ␣1 and ␣2 catalytic subunits of AMPK (AMPK-ASKO mice) by using aP2-Cre and adiponectin-Cre. Both models of AMPK-ASKO ablation show no changes in desnutrin/ATGL levels but have defective phosphorylation of desnutrin/ATGL at S406 to decrease its triacylglycerol (TAG) hydrolase activity, lowering basal lipolysis in adipose tissue. These mice also show defective phosphorylation of hormone-sensitive lipase (HSL) at S565, with higher phosphorylation at protein kinase A sites S563 and S660, increasing its hydrolase activity and isoproterenol-stimulated lipolysis. With higher overall adipose lipolysis, both models of AMPK-ASKO mice are lean, having smaller adipocytes with lower TAG and higher intracellular free-FA levels. Moreover, FAs from higher lipolysis activate peroxisome proliferator-activated receptor delta to induce FA oxidative genes and increase FA oxidation and energy expenditure. Overall, for the first time, we provide in vivo evidence of the role of AMPK in the phosphorylation and regulation of desnutrin/ATGL and HSL and thus adipose lipolysis.A dipose tissue plays a key role in whole-body energy homeostasis by storing triacylglycerol (TAG) during excess energy intake, which is hydrolyzed (so-called lipolysis) to release fatty acids (FAs) into the circulation for use by other tissues during energy shortage. Thus, adipose TAG metabolism, especially the unique function of adipose lipolysis for FA release, must be exquisitely regulated according to nutritional conditions. For example, during fasting, lipolysis is stimulated upon the release of catecholamines to activate the ␤-adrenergic receptor-adenylyl cyclase-cyclic AMP (cAMP)-protein kinase A (PKA) pathway in adipocytes. In contrast, in the fed state, insulin released from pancreatic islet ␤ cells activates phosphodiesterase 3B for degradation of cAMP in adipocytes, resulting in suppression of lipolysis (1). In addition to hormonal regulation, adipose lipolysis may also be regulated by the intracellular energy state for the maintenance of cellular TAG homeostasis.AMP-activated protein kinase (AMPK) is a widely expressed multisubstrate serine/threonine kinase and a well-known sensor of the intracellular energy state that responds to metabolic stresses and other regulatory signals. AMPK is a heterotrimeric complex with a catalytic ␣ subunit and two regulatory subunits, ␤ and ␥ (2). There are several isoforms of each of the AMPK subunits, including two isoforms of the catalytic subunit, ␣1 and ␣2. AMPK is known to be activated allosterically by AMP but also is activated by phosphorylation of its catalytic subunit at T172 by the upstream kinases liver kinase B1 a...

Multiple signals have been shown to be involved in regulation of fatty acid (FA) and glucose metabolism in contracting skeletal muscle. This study aimed to determine whether a Ca(2+)-stimulated kinase, CaMKK, is involved in regulation of contraction-induced substrate metabolism and whether it does so in an AMP-activated protein kinase (AMPK)-dependent manner. Rat hindlimbs were perfused at rest (n = 16), with 3 mM caffeine (n = 15), with 2 mM 5-aminoimidazole-4-carboxamide 1-beta-d-ribofuranoside (AICAR; n = 16), or during moderate-intensity muscle contraction (MC; n = 14) and with or without 5 microM STO-609, a CaMKK inhibitor. FA uptake and oxidation increased (P < 0.05) 64% and 71% by caffeine, 42% and 93% by AICAR, and 65% and 143% by MC. STO-609 abolished (P < 0.05) caffeine- and MC-induced FA uptake and oxidation but had no effect with AICAR treatment. Glucose uptake increased (P < 0.05) 104% by caffeine, 85% by AICAR, and 130% by MC, and STO-609 prevented the increase in glucose uptake in caffeine and muscle contraction groups. CaMKKbeta activity increased (P < 0.05) 113% by caffeine treatment and 145% by MC but was not affected by AICAR treatment. STO-609 prevented the caffeine- and MC-induced increase in CaMKKbeta activity. Caffeine, AICAR, and MC increased (P < 0.05) AMPKalpha2 activity by 295%, 11-fold, and 7-fold but did not affect AMPKalpha1 activity. STO-609 decreased (P < 0.05) AMPKalpha2 activity induced by caffeine treatment and MC by 60% and 61% but did not affect AICAR-induced activity. Plasma membrane transport protein content of CD36 and glucose transporter 4 (GLUT4) increased (P < 0.05) with caffeine, AICAR, and MC, and STO-609 prevented caffeine- and MC-induced increases in protein content. These results show the importance of Ca(2+)-dependent signaling via CaMKK activation in the regulation of substrate uptake and FA oxidation in contracting rat skeletal muscle and agree with the notion that CaMKK is an upstream kinase of AMPK in the regulation of substrate metabolism in skeletal muscle.

Excessive caloric intake leading to obesity is associated with insulin resistance and dysfuntion of islet β cells. High fat feeding decreases desnutrin (also called ATGL/PNPLA2) levels in islets. Here we show that desnutrin ablation via RIP-Cre (βKO) or RIP-CreER results in hyperglycemia with impaired glucose-stimulated insulin secretion (GSIS). Due to decreased lipolysis, islets have higher TAG content but lower free FA levels. βKO islets exhibit impaired mitochondrial respiration and lower production of ATP required for GSIS, along with decreased expression of PPARδ target genes involved in mitochondrial oxidation. Furthermore, synthetic PPARδ, but not PPARα, agonist restores GSIS and expression of mitochondrial oxidative genes in βKO mice, revealing desnutrin-catalyzed lipolysis generates PPARδ ligands. Finally, adenoviral expression of desnutrin in βKO islets restores all defects of βKO islet phenotype and function including GSIS and mitochondrial defects, demonstrating the critical role of the desnutrin-PPARδ-mitochondrial oxidation axis in regulating islet β cell GSIS.

Myokines are specialized cytokines that are secreted from skeletal muscle (SKM) in response to metabolic stimuli, such as exercise. Interleukin-15 (IL-15) is a myokine with potential to reduce obesity and increase lean mass through induction of metabolic processes. It has been previously shown that IL-15 acts to increase glucose uptake in SKM cells. However, the downstream signals orchestrating the link between IL-15 signaling and glucose uptake have not been fully explored. Here we employed the mouse SKM C2C12 cell line to examine potential downstream targets of IL-15-induced alterations in glucose uptake. Following differentiation, C2C12 cells were treated overnight with 100 ng/ml of IL-15. Activation of factors associated with glucose metabolism (Akt and AMPK) and known downstream targets of IL-15 (Jak1, Jak3, STAT3, and STAT5) were assessed with IL-15 stimulation. IL-15 stimulated glucose uptake and GLUT4 translocation to the plasma membrane. IL-15 treatment had no effect on phospho-Akt, phospho-Akt substrates, phospho-AMPK, phospho-Jak1, or phospho-STAT5. However, with IL-15, phospho-Jak3 and phospho-STAT3 levels were increased along with increased interaction of Jak3 and STAT3. Additionally, IL-15 induced a translocation of phospho-STAT3 from the cytoplasm to the nucleus. We have evidence that a mediator of glucose uptake, HIF1α, expression was dependent on IL-15 induced STAT3 activation. Finally, upon inhibition of STAT3 the positive effects of IL-15 on glucose uptake and GLUT4 translocation were abolished. Taken together, we provide evidence for a novel signaling pathway for IL-15 acting through Jak3/STAT3 to regulate glucose metabolism.