GSK3 is a negative regulator of the thermogenic program in brown adipocytes

www.nature.com/scientificreports OPEN Received: 16 January 2017 Accepted: 9 February 2018 Published: xx xx xxxx GSK3 is a negative regulator of the thermogenic program in brown adipocytes Lasse K. Markussen1, Sally Winther1, Barton Wicksteed2 & Jacob B. Hansen 1 Brown adipose tissue is a promising therapeutic target in metabolic disorders due to its ability to dissipate energy and improve systemic insulin sensitivity and glucose homeostasis. β-Adrenergic stimulation of brown adipocytes leads to an increase in oxygen consumption and induction of a thermogenic gene program that includes uncoupling protein 1 (Ucp1) and fibroblast growth factor 21 (Fgf21). In kinase inhibitor screens, we have identified glycogen synthase kinase 3 (GSK3) as a negative regulator of basal and β-adrenergically stimulated Fgf21 expression in cultured brown adipocytes. In addition, inhibition of GSK3 also caused increased Ucp1 expression and oxygen consumption. β-Adrenergic stimulation triggered an inhibitory phosphorylation of GSK3 in a protein kinase A (PKA)dependent manner. Mechanistically, inhibition of GSK3 activated the mitogen activated protein kinase (MAPK) kinase 3/6-p38 MAPK-activating transcription factor 2 signaling module. In summary, our data describe GSK3 as a novel negative regulator of β-adrenergic signaling in brown adipocytes. The discovery of active brown adipose tissue (BAT) in healthy adults has revitalized the concept of combating metabolic dysfunction via recruitment and activation of brown adipocytes1–3. BAT can dissipate energy by uncoupled respiration, a process called adaptive thermogenesis1–3. BAT is classically activated by cold, which through sympathetic nervous system-mediated release of norepinephrine at the surface of brown adipocytes activates β-adrenergic receptors. This activation results in augmented lipolysis, mitochondrial uncoupling, oxygen consumption and thermogenesis. In mice, activated BAT promotes glucose and triacylglycerol clearance, improves insulin sensitivity and glucose tolerance, and counteracts obesity1–3. Most of these effects of brown adipocytes depend on their high mitochondrial density, the unique presence of uncoupling protein 1 (UCP1) in the inner mitochondrial membrane and a high oxidative capacity1–3. In humans, BAT activity correlates with cold-induced energy expenditure and BAT activity is recruited after regular cold exposures4,5. In addition, brown-like (also called beige, brite or inducible brown) adipocytes appear in certain white adipose tissues in response to prolonged cold exposure or treatment with β-adrenergic agonists1–3. Recent data suggest that BAT has beneficial metabolic functions beyond thermogenesis which might involve an endocrine role6,7. Several signaling molecules with hormonal properties have been found to be released by BAT, particularly under conditions of cold-induced BAT activation7. Additionally, the improved glucose tolerance, enhanced insulin sensitivity and decreased adiposity observed with BAT transplantation have also been associated with the endocrine properties of BAT8,9. Recent reports have established fibroblast growth factor 21 (FGF21) as a bona fide BAT-released factor, secreted by brown adipocytes following cold or β-adrenergic stimulation10–12. Fgf21 expression is controlled by activating transcription factor 2 (ATF2), which in turn is activated by β-adrenergic stimulation in a cAMP-protein kinase A (PKA)-mitogen activated protein kinase (MAPK) kinase 3/6 (MKK3/6)-p38 MAPK-dependent manner10. The same intracellular signaling pathway that controls Fgf21 expression in brown adipocytes is required for induction of a broader thermogenic gene expression program that also includes uncoupling protein 1 (Ucp1), type II iodothyronine deiodinase (Dio2) and peroxisome proliferator-activated receptor γ co-activator-1α (Ppargc1α)10,13–16. Pharmacological administration of FGF21 has been ascribed a number of beneficial metabolic effects, including lowering of adiposity and increased glucose tolerance, and some of these effects have been associated with a direct effect of FGF21 on adipocytes17–19. Recently, autocrine and/or endocrine actions of FGF21 were shown to induce BAT differentiation and WAT browning 1 Department of Biology, University of Copenhagen, DK-2100, Copenhagen, Denmark. 2Division of Endocrinology, Diabetes and Metabolism, Department of Medicine, University of Illinois at Chicago, Chicago, IL, USA. Lasse K. Markussen and Sally Winther contributed equally to this work. Correspondence and requests for materials should be addressed to J.B.H. (email: ) SciEntific Reports | (2018) 8:3469 | DOI:10.1038/s41598-018-21795-y 1 www.nature.com/scientificreports/ Figure 1. FGF21 expression and secretion is under β-adrenergic control in brown adipocytes. Ten weeks old male C57BL/6JBomTac mice were housed at 29 °C (thermoneutrality, TN) or 4 °C (cold) for 4 days. Relative mRNA levels of Ucp1 (a) and Fgf21 (b) in various tissues. Cold exposure effect on average body weight gain during the 4-day period (c), iWAT wet weight (d) iBAT wet weight (e) and plasma concentration of FGF21 (f). (g) Expression of Fgf21 in immortalized or primary brown adipocytes treated with 0.1 μM isoproterenol (ISO) for 6 h. (h) Secreted FGF21 in cell culture medium from immortalized or primary brown adipocytes treated with 0.1 μM ISO for 24 h. Data represent mean +SEM (n = 6, mice) or mean of means +SEM (n = 3, cell culture). Unpaired two-tailed Student’s t-test was applied in all panels. *p < 0.05 versus TN/H2O control. in response to activation of G protein-coupled receptor 12020. Glycogen synthase kinase 3 (GSK3) is a Ser/Thr kinase implicated in the insulin signaling pathway to control glycogen metabolism, but is now also recognized as a multifunctional kinase regulating an array of additional cellular functions21. GSK3 exists as two paralogs: GSK3α and GSK3β. Small-molecule inhibitors of GSK3 have favourable metabolic effects in rodents, including prevention of diet-induced obesity and improved glucose tolerance22–25. Thus, GSK3 inhibitors exert some of the same metabolic effects as FGF21 administration. In a search for novel kinases regulating the thermogenic program of brown adipocytes, we carried out screens with a kinase inhibitor library. We identified GSK3 to be a novel negative regulator of Fgf21 and thermogenic gene expression in brown adipocytes. Following thermogenic activation, GSK3 becomes inactivated by phosphorylation in a PKA-dependent manner, which in turn leads to increased activity of the MKK3/6-p38 MAPK-ATF2 signaling module. Thus, inhibition of GSK3 unleashes thermogenic signaling in brown adipocytes, an observation pointing to GSK3 as a potentially interesting target in metabolic diseases. Results FGF21 is under β-adrenergic control in adipose tissue and cultured brown adipocytes. Unbiased kinase inhibitor screens have been applied to successfully identify novel roles for kinases in regulating t (...truncated)