ATP and beta-adrenergic stimulation enhance voltage-gated K current inactivation in brown adipocytes

A key role for P2RX5 in brown adipocyte differentiation and energy homeostasis

Brown adipocytes are defined based on a distinct morphology and genetic signature that includes, amongst others, the expression of the Purinergic 2 Receptor X5 (P2RX5). However, the role of P2RX5 in brown adipocyte and brown adipose tissue function is poorly characterized. In the present study, we conducted a metabolic characterization of P2RX5 knockout male mice; next, we characterized this purinergic pathway in a cell-autonomous context in brown adipocytes. We then tested the role of the P2RX5 receptor agonism in metabolic responses in vivo in conditions of minimal adaptive thermogenesis requirements. Our data show that loss of P2RX5 causes reduced brown adipocyte differentiation in vitro, and browning in vivo. Lastly, we unravel a previously unappreciated role for P2RX5 agonism to exert an anti-obesity effect in the presence of enhanced brown adipose tissue recruitment in male mice housed at thermoneutrality. Altogether, our data support a role for P2RX5 in mediating brown adipocyte differentiation and function that could be further targeted for benefits in the context of adipose tissue pathology and metabolic diseases.

Molecular and Pharmacological Evidence for the Expression of Multiple Functional P2 Purinergic Receptors in Human Adipocytes

Extracellular ATP exerts important functions as an extracellular signaling molecule via the activation of specific P2 purinergic receptors (P2X and P2Y). We investigated the expression of the different P2 receptors and their possible functional activation in human adipocytes in primary culture. We performed molecular expression analysis of the P2 receptors in human mature adipocytes; examined their functional activation by different nucleotides evaluating [Ca²⁺]_i modifications and IL-6 secretion, and determined the ability of adipocytes to release ATP in the extracellular medium. Human adipocytes express different P2X and P2Y receptors. Extracellular ATP elicited a rise in [Ca²⁺]_i via the activation of P2X and P2Y receptor subtypes. Human adipocytes spontaneously released ATP in the extracellular medium and secreted IL-6 both at rest and after stimulation with ATP. This stimulatory effect of ATP on IL-6 secretion was inhibited by pre-incubation with apyrase, an ATP metabolizing enzyme. These results demonstrate that human adipocytes express different P2X and P2Y receptors that are functionally activated by extracellular nucleotides. Furthermore, human adipocytes spontaneously release ATP, which can act in an autocrine/paracrine fashion on adipocytes, possibly participating in the regulation of inflammatory cytokine release. Thus, P2 purinergic receptors could be a potential therapeutic target to contrast the inflammatory and metabolic complications characterizing obesity.

Caveolar targeting links Kv1.3 with the insulin-dependent adipocyte physiology

The voltage-dependent potassium channel Kv1.3 participates in peripheral insulin sensitivity. Genetic ablation of Kv1.3 triggers resistance to diet-induced weight gain, thereby pointing to this protein as a pharmacological target for obesity and associated type II diabetes. However, this role is under intense debate because Kv1.3 expression in adipose tissue raises controversy. We demonstrated that Kv1.3 is expressed in white adipose tissue from humans and rodents. Moreover, other channels, such as Kv1.1, Kv1.2, Kv1.4 and especially Kv1.5, from the same Shaker family are also present. Although elevated insulin levels and adipogenesis remodel the Kv phenotype, which could lead to multiple heteromeric complexes, Kv1.3 markedly participates in the insulin-dependent regulation of glucose uptake in mature adipocytes. Adipocyte differentiation increased the expression of Kv1.3, which is targeted to caveolae by molecular interactions with caveolin 1. Using a caveolin 1-deficient 3T3-L1 adipocyte cell line, we demonstrated that the localization of Kv1.3 in caveolar raft structures is important for proper insulin signaling. Insulin-dependent phosphorylation of the channel occurs at the onset of insulin-mediated signaling. However, when Kv1.3 was spatially outside of these lipid microdomains, impaired phosphorylation was exhibited. Our data shed light on the putative role of Kv1.3 in weight gain and insulin-dependent responses contributing to knowledge about adipocyte physiology.

Systems genetic analysis of brown adipose tissue function

Brown adipose tissue (BAT) has been suggested to play an important role in lipid and glucose metabolism in rodents and possibly also in humans. In the current study, we used genetic and correlation analyses in the BXH/HXB recombinant inbred (RI) strains, derived from Brown Norway (BN) and spontaneously hypertensive rats (SHR), to identify genetic determinants of BAT function. Linkage analyses revealed a quantitative trait locus (QTL) associated with interscapular BAT mass on chromosome 4 and two closely linked QTLs associated with glucose oxidation and glucose incorporation into BAT lipids on chromosome 2. Using weighted gene coexpression network analysis (WGCNA) we identified 1,147 gene coexpression modules in the BAT from BXH/HXB rats and mapped their module eigengene QTLs. Through an unsupervised analysis, we identified modules related to BAT relative mass and function. The Coral4.1 coexpression module is associated with BAT relative mass (includes Cd36 highly connected gene), and the Darkseagreen coexpression module is associated with glucose incorporation into BAT lipids (includes Hiat1, Fmo5, and Sort1 highly connected transcripts). Because multiple statistical criteria were used to identify candidate modules, significance thresholds for individual tests were not adjusted for multiple comparisons across modules. In summary, a systems genetic analysis using genomic and quantitative transcriptomic and physiological information has produced confirmation of several known genetic factors and significant insight into novel genetic components functioning in BAT and possibly contributing to traits characteristic of the metabolic syndrome.

Actions and Mechanisms of Polyunsaturated Fatty Acids on Voltage-Gated Ion Channels

Polyunsaturated fatty acids (PUFAs) act on most ion channels, thereby having significant physiological and pharmacological effects. In this review we summarize data from numerous PUFAs on voltage-gated ion channels containing one or several voltage-sensor domains, such as voltage-gated sodium (Na_V), potassium (K_V), calcium (Ca_V), and proton (H_V) channels, as well as calcium-activated potassium (K_Ca), and transient receptor potential (TRP) channels. Some effects of fatty acids appear to be channel specific, whereas others seem to be more general. Common features for the fatty acids to act on the ion channels are at least two double bonds in cis geometry and a charged carboxyl group. In total we identify and label five different sites for the PUFAs. PUFA site 1: The intracellular cavity. Binding of PUFA reduces the current, sometimes as a time-dependent block, inducing an apparent inactivation. PUFA site 2: The extracellular entrance to the pore. Binding leads to a block of the channel. PUFA site 3: The intracellular gate. Binding to this site can bend the gate open and increase the current. PUFA site 4: The interface between the extracellular leaflet of the lipid bilayer and the voltage-sensor domain. Binding to this site leads to an opening of the channel via an electrostatic attraction between the negatively charged PUFA and the positively charged voltage sensor. PUFA site 5: The interface between the extracellular leaflet of the lipid bilayer and the pore domain. Binding to this site affects slow inactivation. This mapping of functional PUFA sites can form the basis for physiological and pharmacological modifications of voltage-gated ion channels.

Inhibitory effects of neochamaejasmin B on P-glycoprotein in MDCK-hMDR1 cells and molecular docking of NCB binding in P-glycoprotein

Stellera chamaejasme L. (Thymelaeaceae) is widely distributed in Mongolia, Tibet and the northern parts of China. Its roots are commonly used as “Langdu”, which is embodied in the Pharmacopoeia of the P.R. China (2010) as a toxic Traditional Chinese Medicine. It is claimed to have antivirus, antitumor and antibacterial properties in China and other Asian countries. Studies were carried out to characterize the inhibition of neochamaejasmin B (NCB) on P-glycoprotein (P-gp, ABCB1, MDR1). Rhodamine-123 (R-123) transport and accumulation studies were performed in MDCK-hMDR1 cells. ABCB1 (MDR1) mRNA gene expression and P-gp protein expression were analyzed. Binding selectivity studies based on molecular docking were explored. R-123 transport and accumulation studies in MDCK-hMDR1 cells indicated that NCB inhibited the P-gp-mediated efflux in a concentration-dependent manner. RT-PCR and Western blot demonstrated that the P-gp expression was suppressed by NCB. To investigate the inhibition type of NCB on P-gp, K_i and K_i’ values were determined by double-reciprocal plots in R-123 accumulation studies. Since K_i was greater than K_i’, the inhibition of NCB on P-gp was likely a mixed type of competitive and non-competitive inhibition. The results were confirmed by molecular docking in our current work. The docking data indicated that NCB had higher affinity to P-gp than to Lig1 ((S)-5,7-dihydroxy-2-(4-hydroxyphenyl)chroman-4-one).

Selective Kv1.3 channel blocker as therapeutic for obesity and insulin resistance

Obesity is an epidemic, calling for innovative and reliable pharmacological strategies. Here, we show that ShK-186, a selective and potent blocker of the voltage-gated Kv1.3 channel, counteracts the negative effects of increased caloric intake in mice fed a diet rich in fat and fructose. ShK-186 reduced weight gain, adiposity, and fatty liver; decreased blood levels of cholesterol, sugar, HbA1c, insulin, and leptin; and enhanced peripheral insulin sensitivity. These changes mimic the effects of Kv1.3 gene deletion. ShK-186 did not alter weight gain in mice on a chow diet, suggesting that the obesity-inducing diet enhances sensitivity to Kv1.3 blockade. Several mechanisms may contribute to the therapeutic benefits of ShK-186. ShK-186 therapy activated brown adipose tissue as evidenced by a doubling of glucose uptake, and increased β-oxidation of fatty acids, glycolysis, fatty acid synthesis, and uncoupling protein 1 expression. Activation of brown adipose tissue manifested as augmented oxygen consumption and energy expenditure, with no change in caloric intake, locomotor activity, or thyroid hormone levels. The obesity diet induced Kv1.3 expression in the liver, and ShK-186 caused profound alterations in energy and lipid metabolism in the liver. This action on the liver may underlie the differential effectiveness of ShK-186 in mice fed a chow vs. an obesity diet. Our results highlight the potential use of Kv1.3 blockers for the treatment of obesity and insulin resistance.

Effect of electrical stimulation on lipolysis of human white adipocytes

The goal of the present study was to investigate the effect of 30 min of electrical stimulation on the activation of lipolysis in human white adipocytes. Two stimulation protocols (S1, S2) were conducted in vitro on isolated human white adipocytes. Subcutaneous adipose tissue was obtained from female subjects undergoing abdominal adipose tissue reduction. Adipose tissue of 10 female subjects (mean age, 38.7 ± 9.1 years) and 6 female subjects (mean age, 37.2 ± 11.3 years) was obtained for S1 and S2, respectively. All subjects fasted overnight before tissue removal. The control conditions were a basal and a β-adrenergic stimulation (isoproterenol (ISO), 10(-6) mol·L(-1)) of lipolysis. For S1, the 3 electrostimulation conditions consisted of a monopolar square-wave pulse current for 30 min at intensities of 4, 8, and 20 mA, respectively. In S2, the 2 electrostimulation conditions consisted of a bipolar square-wave alternating current for 30 min at intensities of 4 and 6 mA, respectively. Fat cell lipolysis was measured by quantifying the release of glycerol from adipocytes for 3 trials in each experimental condition. For S1, 4 mA significantly increased lipolysis 1.5 times over basal values (p ≤ 0.01), 8 mA and 20 mA did not increase lipolysis significantly, and no significant difference (p > 0.05) was found between ISO and 4 mA. For S2, 4 mA (p ≤ 0.05) and 6 mA (p ≤ 0.01) significantly increased lipolysis by 1.8 and 2.3 times above basal, respectively. Our results demonstrate that both monopolar (4 mA) and bipolar (4 and 6 mA) electrical stimulations significantly activated in vitro lipolysis. Our findings suggest the existence of a new lipolytic pathway that may involve K(v) channels shown to exist in human white adipocytes.

Caffeine inhibits InsP3 responses and capacitative calcium entry in canine pulmonary arterial smooth muscle cells

Caffeine is a well described and characterized ryanodine receptor (RyR) activator. Previous evidence from independent research studies also indicate caffeine inhibits InsP3 receptor functionality, which is important to activation of capacitative Ca2+ entry (CCE) in some cell types. In addition, RyR activation elicits excitatory-coupled Ca2+ entry (ECCE) in skeletal muscle myotubes. Recent studies by our group show that canine pulmonary arterial smooth muscle cells (PASMCs) have functional InsP3 receptors as well as RyRs, and that CCE is dependent on InsP3 receptor activity. The potential for caffeine to activate ECCE as well as inhibit InsP3 receptor function and CCE was examined using fura-2 fluorescent imaging in canine PASMCs. The data show caffeine causes transient as well as sustained cytosolic Ca2+ increases, though this is not due to CCE or ECCE activity as evidenced by a lack of an increase in Mn2+ quench of fura-2. The experiments also show caffeine reversibly inhibits 5-HT elicited-InsP3 mediated Ca2+ responses with an IC50 of 6.87x10(-4) M and 10 mM caffeine fully inhibits CCE. These studies provide the first evidence that caffeine is an inhibitor of InsP3 generated Ca2+ signals and CCE in PASMCs.

Functional roles of 5-hydroxytryptamine 3/4 receptors in neurons of rat dorsal motor nucleus of the vagus

In neurons of dorsal motor nucleus of the vagus that is involved in the gastric motility and possibly emesis, application of 5-hydroxytryptamine produces membrane depolarization, and suppresses spike-repolarization and spike-afterhyperpolarization, suggesting divergent effects of 5-hydroxytryptamine through activating multiple subtypes of 5-hydroxytryptamine receptors. However, only the role of 5-hydroxytryptamine 2A receptors has been established to be responsible for the depolarization, and the mechanisms underlying the modulation of spikes remain unknown although a role of 5-hydroxytryptamine 4 receptors was implicated in modulations of spikes. There is now increasing evidence for the role of 5-hydroxytryptamine receptors in neurons involved in generating emesis following administration of anticancer drug. Since antagonists of 5-hydroxytryptamine 3/4 receptors are widely used as anti-emetic drugs, we have reevaluated the functional roles of 5-hydroxytryptamine 3/4 receptors of dorsal motor nucleus of the vagus neurons, especially in modulating transient outward currents that are presumed to be involved in spike-repolarization and spike-afterhyperpolarization. Whole-cell patch-clamp recordings were made from the dorsal motor nucleus of the vagus neurons, which were identified by a retrograde tracing method with dextran-tetramethylrhodamine-lysine injected into a bundle of abdominal vagus nerves. Under a voltage-clamp condition, dorsal motor nucleus of the vagus neurons expressed a prominent A-like current. The activation of 5-hydroxytryptamine 3 receptors reversibly increased the resting membrane conductance while the activation of 5-hydroxytryptamine 4 receptors led to an almost irreversible decrease in the A-like current. A long-lasting suppression of A-like current by transient activation of 5-hydroxytryptamine 4 receptors would result in a long-lasting increase in the excitability of dorsal motor nucleus of the vagus neurons, which might be involved in generation of the long-lasting facilitation of gastric motility or in generation of the long-lasting gastric relaxation through the activation of enteric non-adrenergic non-cholinergic neurons as implicated in the delayed emesis induced by anticancer drugs.

Stimulation of lipogenesis in rat adipocytes by ATP, a ligand for P2-receptors

The activation of P2-receptors has a wide range of diverse effects in many tissues. Here we show that extracellular ATP stimulates lipogenesis in adipocytes derived from the epididymal fat pads of male Wistar rats. The lipogenic effect of ATP is not susceptible to treatment of adipocytes with adenosine deaminase or an adenosine receptor antagonist. Degradation of ATP in adipocyte suspension by ectonucleotidases is slow and remaining ATP concentrations are sufficient to activate P2-receptors. ATP does not affect basal or insulin stimulated glucose transport, or basal or isoproterenol stimulated lipolysis, respectively. The lipogenic effect of ATP is mimicked by the adenine compounds, ADP, AMP, and beta,gamma-methylene-ATP, but not by other nucleotides (UTP, UDP, CTP, GTP, ITP, and diadenosine tetraphosphate), indicating that extracellular nucleotides stimulate lipogenesis via a P2-receptor. ATP and its receptor may define a signalling system in adipocytes, which regulates fat stores independently from established hormones.

Visceral Periadventitial Adipose Tissue Regulates Arterial Tone of Mesenteric Arteries

Periadventitial adipose tissue produces vasoactive substances that influence vascular contraction. Earlier studies addressed this issue in aorta, a vessel that does not contribute to peripheral vascular resistance. We tested the hypothesis that periadventitial adipose tissue modulates contraction of smaller arteries more relevant to blood pressure regulation. We studied mesenteric artery rings surrounded by periadventitial adipose tissue from adult male Sprague-Dawley rats. The contractile response to serotonin, phenylephrine, and endothelin I was markedly reduced in intact vessels compared with vessels without periadventitial fat. The contractile response to U46619 or depolarizing high K + -containing solutions (60 mmol/L) was similar in vessels with and without periadventitial fat. The K + channel opener cromakalim induced relaxation of vessels precontracted by serotonin but not by U46619 or high K + -containing solutions (60 mmol/L), suggesting that K + channels are involved. The intracellular membrane potential of smooth muscle cells was more hyperpolarized in intact vessels than in vessels without periadventitial fat. Both the anticontractile effect and membrane hyperpolarization of periadventitial fat were abolished by inhibition of delayed-rectifier K + (K v ) channels with 4-aminopyridine (2 mmol/L) or 3,4-diaminopyridine (1 mmol/L). Blocking other K + channels with glibenclamide (3 μmol/L), apamin (1 μmol/L), iberiotoxin (100 nmol/L), tetraethylammonium ions (1 mmol/L), tetrapentylammonium ions (10 μmol/L), or Ba 2+ (3 μmol/L) had no effect. Longitudinal removal of half the perivascular tissue reduced the anticontractile effect of fat by almost 50%, whereas removal of the endothelium had no effect. We suggest that visceral periadventitial adipose tissue controls mesenteric arterial tone by inducing vasorelaxation via K v channel activation in vascular smooth muscle cells.

Human adipose cells have voltage-dependent potassium currents

The whole-cell patch-clamp method was used to study the membrane electrical properties of human adipocyte cells obtained by differentiating from precursors of human abdominal and mammary tissues. All differentiated cells exhibited outward currents with sigmoidal activation kinetics. The outward currents showed activation thresholds between -20 to -30 mV and slow inactivation. The ionic channels underlying the macroscopic current were highly selective for K(+). Their selectivity was for typical K(+) channels with relative permeabilities of K(+)>NH4+>Cs(+)>Na(+). No evidence of any other type of voltage-gated channel was found. The potassium currents ( I(KV)) were blocked reversibly by tetraethylammonium and barium. The IC(50) value and Hill coefficient of tetraethylammonium inhibition of I(KV) were 0.56 m M and 1.17 respectively. These results demonstrate that human adipose cells have voltage-dependent potassium currents.