Glycogen metabolism links glucose homeostasis to thermogenesis in adipocytes.

Adipocytes increase energy expenditure in response to prolonged sympathetic activation via persistent expression of uncoupling protein 1 (UCP1)1,2. Here we report that the regulation of glycogen metabolism by catecholamines is critical for UCP1 expression. Chronic ß-adrenergic activation leads to increased glycogen accumulation in adipocytes expressing UCP1. Adipocyte-specific deletion of a scaffolding protein, protein targeting to glycogen (PTG), reduces glycogen levels in beige adipocytes, attenuating UCP1 expression and responsiveness to cold or ß-adrenergic receptor-stimulated weight loss in obese mice. Unexpectedly, we observed that glycogen synthesis and degradation are increased in response to catecholamines, and that glycogen turnover is required to produce reactive oxygen species leading to the activation of p38 MAPK, which drives UCP1 expression. Thus, glycogen has a key regulatory role in adipocytes, linking glucose metabolism to thermogenesis.

Culture and Differentiation of Primary Preadipocytes from Mouse Subcutaneous White Adipose Tissue.

Adipocytes are terminally differentiated cells derived from fibroblastic preadipocyte precursors. Here, we describe a method for the isolation and proliferation of preadipocytes from murine subcutaneous white adipose tissue, followed by differentiation in culture to mature adipocytes; we refer to these cells as primary preadipocytes differentiated in vitro (PPDIVs). Compared to adipogenic cell lines, PPDIV metabolism and adipokine secretion more closely resemble in vivo adipocyte biology. While primary mature adipocytes have the greatest in vivo relevance, their fragility and buoyancy make them unsuitable for many cell culture-based methods. PPDIVs can also take advantage of transgenic and knockout mouse models to produce genetically modified adipocytes. Thus, PPDIVs are a valuable resource for studying adipocyte biology in cell culture.

Adipocyte-Secreted IL-6 Sensitizes Macrophages to IL-4 Signaling.

Complex bidirectional cross talk between adipocytes and adipose tissue immune cells plays an important role in regulating adipose function, inflammation, and insulin responsiveness. Adipocytes secrete the pleiotropic cytokine IL-6 in response to both inflammatory and catabolic stimuli. Previous studies have suggested that IL-6 secretion from adipocytes in obesity may promote adipose tissue inflammation. Here, we investigated catabolic stimulation of adipocyte IL-6 secretion and its impact on adipose tissue immune cells. In obesity, catecholamine resistance reduces cAMP-driven adipocyte IL-6 secretion in response to catabolic signals. By restoring adipocyte catecholamine sensitivity in obese adipocytes, amlexanox stimulates adipocyte-specific IL-6 secretion. We report that in this context, adipocyte-secreted IL-6 activates local macrophage STAT3 to promote Il4ra expression, thereby sensitizing them to IL-4 signaling and promoting an anti-inflammatory gene expression pattern. Supporting a paracrine adipocyte to macrophage mechanism, these effects could be recapitulated using adipocyte conditioned media to pretreat bone marrow-derived macrophages prior to polarization with IL-4. The effects of IL-6 signaling in adipose tissue are complex and context specific. These results suggest that cAMP-driven IL-6 secretion from adipocytes sensitizes adipose tissue macrophages to IL-4 signaling.

FGF21 is required for the metabolic benefits of IKKε/TBK1 inhibition.

The protein kinases IKKε and TBK1 are activated in liver and fat in mouse models of obesity. We have previously demonstrated that treatment with the IKKε/TBK1 inhibitor amlexanox produces weight loss and relieves insulin resistance in obese animals and patients. While amlexanox treatment caused a transient reduction in food intake, long-term weight loss was attributable to increased energy expenditure via FGF21-dependent beiging of white adipose tissue (WAT). Amlexanox increased FGF21 synthesis and secretion in several tissues. Interestingly, although hepatic secretion determined circulating levels, it was dispensable for regulating energy expenditure. In contrast, adipocyte-secreted FGF21 may have acted as an autocrine factor that led to adipose tissue browning and weight loss in obese mice. Moreover, increased energy expenditure was an important determinant of improved insulin sensitivity by amlexanox. Conversely, the immediate reductions in fasting blood glucose observed with acute amlexanox treatment were mediated by the suppression of hepatic glucose production via activation of STAT3 by adipocyte-secreted IL-6. These findings demonstrate that amlexanox improved metabolic health via FGF21 action in adipocytes to increase energy expenditure via WAT beiging and that adipocyte-derived IL-6 has an endocrine role in decreasing gluconeogenesis via hepatic STAT3 activation, thereby producing a coordinated improvement in metabolic parameters.

Catecholamines suppress fatty acid re-esterification and increase oxidation in white adipocytes via STAT3.

Catecholamines stimulate the mobilization of stored triglycerides in adipocytes to provide fatty acids (FAs) for other tissues. However, a large proportion is taken back up and either oxidized or re-esterified. What controls the disposition of these FAs in adipocytes remains unknown. Here, we report that catecholamines redirect FAs for oxidation through the phosphorylation of signal transducer and activator of transcription 3 (STAT3). Adipocyte STAT3 is phosphorylated upon activation of ß-adrenergic receptors, and in turn suppresses FA re-esterification to promote FA oxidation. Adipocyte-specific Stat3 KO mice exhibit normal rates of lipolysis, but exhibit defective lipolysis-driven oxidative metabolism, resulting in reduced energy expenditure and increased adiposity when they are on a high-fat diet. This previously unappreciated, non-genomic role of STAT3 explains how sympathetic activation can increase both lipolysis and FA oxidation in adipocytes, revealing a new regulatory axis in metabolism.