A BAT-Centric Approach to the Treatment of Diabetes: Turn on the Brain Mohammed K. Hankir, Michael A. Cowley, Wiebke K. Fenske  Cell Metabolism  Volume 24, Issue 1, Pages 31-40 (July 2016) DOI: 10.1016/j.cmet.2016.05.003 Copyright © 2016 Elsevier Inc. Terms and Conditions

Cell Metabolism 2016 24, 31-40DOI: (10.1016/j.cmet.2016.05.003) Copyright © 2016 Elsevier Inc. Terms and Conditions

Figure 1 Fate of Glucose in Brown Adipocytes This model is largely derived from Labbe et al. (2015) and Hao et al. (2015). During cold acclimation, glucose is efficiently converted into fatty acids (FAs) in brown adipocytes that are then esterified to form triglycerides (TGs) required for thermogenesis. After binding of noradrenaline (NA) to the β3-adrenergic receptor and glucose uptake by the brown adipocyte, cytosolic hexokinase (HK) (1) phosphorylates glucose in preparation for glycolysis during which phosphofructokinase 1 (PFK1) contributes to the formation of acetyl co-enzyme A (Acetyl CoA) that then (2) is converted into free fatty acids de novo by acetyl CoA carboxylase (ACC) and fatty acid synthase (FASN) in the ER. (3) Cytosolic glycerol-3-phosphate dehydrogenase 1 (GPD1) contributes to the synthesis of glycerol from glucose. (4) Glycerol-3-phosphate acyltransferase (GPAT3) catalyzes the production of monoacylglycerol, and (5) diacylglycerol acyltransferases 1 and 2 (DGAT1 and 2) finally catalyze the formation of TGs in the ER that are then transported to and stored in lipid vacuoles. Sites of modulation of expression of these enzymes by chronic cold exposure are indicated. Notably, glucose is also converted into glycogen in BAT by uridine diphosphate-glucose pyrophosphorylase (UDPG) that is hydrolyzed by glycogen phosphorylase (PYGL). Binding of NA to the β3-adrenergic receptor also increases the catalytic activity of PKA that simulates lipolysis (through phosphorylation of adipose tissue triglyceride lipase [ATGL] and hormone sensitive lipase [HSL]). The liberated free fatty acids (mainly from that catalytic action of HSL) then bind to UCP1 in the inner mitochondrial membrane resulting in proton transport, heat generation, and oxygen consumption (Li et al., 2014). Cell Metabolism 2016 24, 31-40DOI: (10.1016/j.cmet.2016.05.003) Copyright © 2016 Elsevier Inc. Terms and Conditions

Figure 2 Intracellular Signaling Cascade Governing β3-Adrenergic Receptor Stimulated Glucose Uptake by Brown Adipocytes This model is largely derived from Olsen et al. (2014) and Albert et al. (2016). In mouse brown adipocytes, noradrenaline (NA) binds to β3-adrenergic receptors increasing the catalytic activity of protein kinase A (PKA), which phosphorylates the transcription factor cyclic response element binding protein (CREB). CREB then translocates to the nucleus and binds to the Glut1 promoter, resulting in increased transcription of Glut1. In parallel, mammalian target of rapamycin complex 2 (mTORC2) is phosphorylated at Ser2481 (downstream of the guanosine exchange factor Epac1 and phosphatidylinositol 3 kinase [PI3K]), allowing for GLUT1 incorporation into the cell membrane. It has been shown in HEK cells that the downstream target of Epac1, the Ras GTPase Rap1, increases the catalytic activity of the p110-α subunit of PI3K (possibly through a Raf kinase) (Rodriguez-Viciana et al., 2004). It has further been shown in MEF cells that mTORC2 phosphorylates protein kinase Cζ (PKC) at Thr560 thereby fully activating the enzyme. This leads to the translocation of Rho GTPases (Rac1, Cdc42, and RhoA) to the cell membrane where they organize the actin cytoskeleton (Li and Gao, 2014). It should be noted that acute stimuli increase GLUT1 expression (Dallner et al., 2006; Labbe et al., 2015; Olsen et al., 2014), whereas chronic stimuli increase GLUT4 expression (Dallner et al., 2006; Labbe et al., 2015; Shimizu et al., 1993). It should further be noted that in mature human brown adipocytes the GLUT4 isoform predominates over the GLUT1 isoform (Lee et al., 2016). Cell Metabolism 2016 24, 31-40DOI: (10.1016/j.cmet.2016.05.003) Copyright © 2016 Elsevier Inc. Terms and Conditions

Figure 3 A Glucoregulatory Central Nervous System Circuit Mediating Glucose Uptake by BAT This circuit is derived largely from the thermoregulatory one described by Morrison and Madden (2014). Cold stimulates cutaneous primary sensory afferents terminating in the dorsal horn (DH) of the spinal cord. Second-order glutamatergic (green axons) spinothalamic neurons send collaterals to the parabrachial nucleus (PBN), which contains third-order neurons that project to an inhibitory network of neurons in the preoptic area (POA) of the hypothalamus starting with the median preoptic (MnPO) subnucleus. From the medial preoptic area (MPO) of the POA, inhibitory GABAergic neurons (red axons) project to the dorsomedial hypothalamus (DMH), which sends (disinhibited) descending excitatory glutamatergic efferents to premotor neurons in the rostral raphe pallidus (rRPa). Neurons in the rRPA (glutamatergic and serotonergic) project to and stimulate preganglionic cholinergic sympathetic neurons (gray axons) in the intermediolateral (IML) cell column of the spinal cord. Postganglionic noradrenergic neurons (blue neurons) arising from the stellate ganglion (SG) release noradrenaline, which binds to adrenergic receptors in brown adipocytes causing glucose uptake. Potential sites of hormonal regulation of this putative glucoregulatory circuit by leptin, insulin, glucagon-like peptide-1 (GLP-1), and fibroblast growth factor 21 (FGF21) are indicated. The effects of leptin are presumed to be inhibitory on GABAergic RIP-Cre neurons (Kong et al., 2012), whereas the effects of GLP-1 on hypothalamic neurons are presumed to be excitatory (Parkinson et al., 2009). The effects of α-MSH (black axons) on orexinergic neurons (purple axons) of the lateral hypothalamic area (LHA) (Cui et al., 2012) and preganglionic neurons of the IML (Sohn et al., 2013) are presumed to be excitatory by pre- and post-synaptic mechanisms, respectively. Orexin stimulates premotor neurons in the rRPA potentially by multiple mechanisms (Morrison et al., 2012). Insulin and cooling hyperpolarizes inhibitory GABAergic POA neurons (Sanchez-Alavez et al., 2010). For FGF21, an excitatory effect on glutamatergic paraventricular nucleus (PVN) neurons (von Holstein-Rathlou et al., 2016) that project to the IML and an inhibitory effect on GABAergic neurons that project to the rRPa are presumed based on the circuitry described by Morrison and Madden (2014). Cell Metabolism 2016 24, 31-40DOI: (10.1016/j.cmet.2016.05.003) Copyright © 2016 Elsevier Inc. Terms and Conditions