The direct effect of leptin on skeletal muscle thermogenesis is mediated by substrate cycling between de novo lipogenesis and lipid oxidation

Abstract

We report here studies that integrate data of respiration rate from mouse skeletal muscle in response to leptin and pharmacological interference with intermediary metabolism, together with assays for phosphatidylinositol 3-kinase (PI3K) and AMP-activated protein kinase (AMPK). Our results suggest that the direct effect of leptin in stimulating thermogenesis in skeletal muscle is mediated by substrate cycling between de novo lipogenesis and lipid oxidation, and that this cycle requires both PI3K and AMPK signaling. This substrate cycling linking glucose and lipid metabolism to thermogenesis provides a novel thermogenic mechanism by which leptin protects skeletal muscle from excessive fat storage and lipotoxicity. Keywords: Obesity; Diabetes; Lipotoxicity; Gluco-lipotoxicity; Insulin resistance; Phosphatidylinositol 3-kinase; AMP-activated protein kinase; Sterol regulatory element binding protein-1c IntroductionSkeletal muscle, which accounts for 30-40% of body mass in mammals, is an important site for glucose disposal, lipid oxidation and thermogenesis whose impairments contribute to the pathogenesis of obesity and type 2 diabetes. It has long been suspected that these metabolic events are often interdependent in normal and disease states [1,2], but a mechanistic link between glucose and lipid metabolism to skeletal muscle thermogenesis is still ill-defined. Leptin, an adipocyte-derived hormone which is well known for its role in weight regulation, has also been shown to protect insulin-sensitive tissues like skeletal muscle against excessive fat storage that can lead to functional impairments known as lipotoxicity [3]. The demonstrations that leptin can act directly on skeletal muscle, specifically via the long form of the leptin receptor (ObRb), to stimulate glucose utilization [4], lipid oxidation through AMPactivated protein kinase (AMPK) [5,6] or thermogenesis in a phosphatidylinositol 3-kinase (PI3K)-dependent manner [7], have provided the impetus to investigate the mechanisms by which muscle substrate metabolism and thermogenesis are interdependent. Although the mechanisms leading to increased fatty acid oxidation in skeletal muscle in response to leptin have been described in molecular details [6], those underlying its effects on thermogenesis are still unknown, amid continuing controversies concerning the role of novel uncoupling proteins, UCP2 and UCP3, as effectors of skeletal muscle thermogenesis [3,8,9]. Furthermore, the mechanism by which glucose and lipid metabolism are linked to thermogenesis in response to leptin's direct effect on skeletal muscle is unknown. With the objective of elucidating the mechanisms by which leptin exerts its direct effect on skeletal muscle thermogenesis, we report here a study that integrates data of respiration rate from intact mouse skeletal muscle ex vivo in response to leptin and pharmacological interference with key control points of intermediary metabolism, together with biochemical measurements for PI3K and AMPK signaling. Materials and methods Mice and ...

Thrifty energy metabolism in catch-up growth trajectories to insulin and leptin resistance

“…Indeed, ex-vivo studies of microcalorimetry in intact skeletal muscle have shown that PI3K and AMPK signalling -which are well known to influence insulin sensitivity in this tissue -are both required for hormonal stimulation of muscle thermogenesis, since the direct thermogenic effects of leptin or corticotropin-releasing hormone in skeletal muscle can be inhibited by selective inhibitors of either AMPK or PI3K. [60][61][62] On the basis of these findings, it follows that either diminished PI3K and/or AMPK signalling in skeletal muscle could constitute impairments that are common to pathways that lead to both suppressed thermogenesis and to resistance to the actions of insulin and leptin during catch-up growth (Figure 4). This notion is indeed supported by recent data from our laboratory 63 indicating diminished basal and insulin-stimulated PI3K activity in skeletal muscle of rats showing catch-up fat driven by suppressed thermogenesis during re-feeding on a low-fat diet, with additional impairments in leptin-induced activation of AMPK during isocaloric re-feeding on diets high in saturated fats.…”

“…This thermogenic effector has recently been implicated in hormonal control of skeletal muscle thermogenesis, with PI3K being required in controlling the flux of substrates through de-novo lipogenesis, and AMPK being required in controlling the flux of de-novo-synthesized fatty acids through mitochondrial b-oxidation. 61,62,66 Overall, therefore, there is emerging evidence for insulin and leptin resistance in the early phase of catch-up fat, and for impairments in PI3K and AMPK signalling in skeletal muscle in the mechanisms by which suppressed thermogenesis might crosslink with insulin and leptin resistance during catch-up growth. Consequently, impairments in these signalling pathways in skeletal muscle may constitute the molecular basis by which the thrifty metabolism (suppressed thermogenesis) that accelerates fat recovery confers enhanced susceptibility to the development of insulin resistance and hyperinsulinaemia during catch-up growth.…”

“…Schematic diagram depicting mechanisms by which the thrifty catch-up fat phenotype, driven by the adipose-specific suppression of thermogenesis, may cross-link with early development of insulin and leptin resistance. The model is built upon emerging evidence that an important thermogenic effector system in skeletal muscle is mediated by substrate cycling between de novo lipogenesis and lipid oxidation 61,62 , and it is orchestrated by phosphatidyloinositol 3-kinase (PI3K) and AMP-activated protein kinase (AMPK) under the influence of insulin, leptin, adiponectin and perhaps other hormones (e.g. urocortins).…”

“…urocortins). 61,62,66 These hormones can interact to stimulate thermogenesis via the enhancement of AMPK and PI3K signalling either by acting directly on skeletal muscle or centrally through the sympathetic (SNS)/thyroid axis via norepinephrine (NE) and tri-iodothyronine (T3). By interfering with PI3K and/or AMPK signalling in skeletal muscle, the actions of the adipose-specific signal(s) that sense the state of depletion (or delayed expansion) of the adipose tissue fat stores will suppress skeletal muscle thermogenesis, which during re-feeding will lead to concomitant insulin and leptin resistance.…”

Substrate cycling between de novo lipogenesis and lipid oxidation: a thermogenic mechanism against skeletal muscle lipotoxicity and glucolipotoxicity

“…63 In other experiments that involved interference with key control points of intermediary metabolism in order to investigate the nature of an 'apparent' link between leptin's direct effects on skeletal muscle to stimulate glucose metabolism, 58-60 lipid oxidation [57][58][59][60][61] and thermogenesis, 62 evidence emerged of a requirement for de novo lipogenesis in the mechanism by which leptin stimulates muscle thermogenesis. 63 Specifically, the direct thermogenic effect of leptin on skeletal muscle O 2 consumption was found to be completely inhibited by the addition of any one of the following compounds that would inhibit the synthesis of lipids from glucose, namely (a) 2-deoxyglucose, which interferes with glucose metabolism, (b) hydroxy-citrate, which inhibits citrate lyase and hence the conversion of citrate to acetyl-CoA and (c) cerulenin, which inhibits fatty acid synthase and hence the conversion of malonyl-CoA to fatty acids. Taken together, these studies suggest that the direct effect of leptin in stimulating thermogenesis in skeletal muscle could be mediated by substrate cycling between de novo lipogenesis and lipid oxidation, and that the orchestration of this substrate cycling requires both PI3K and AMPK signaling.…”