On the action of hormones which accelerate the rate of oxygen consumption and fatty acid release in rat adipose tissue in vitro

Futile cycles: Emerging utility from apparent futility

Futile cycles are biological phenomena where two opposing biochemical reactions run simultaneously, resulting in a net energy loss without appreciable productivity. Such a state was presumed to be a biological aberration and thus deemed an energy-wasting "futile" cycle. However, multiple pieces of evidence suggest that biological utilities emerge from futile cycles. A few established functions of futile cycles are to control metabolic sensitivity, modulate energy homeostasis, and drive adaptive thermogenesis. Yet, the physiological regulation, implication, and pathological relevance of most futile cycles remain poorly studied. In this review, we highlight the abundance and versatility of futile cycles and propose a classification scheme. We further discuss the energetic implications of various futile cycles and their impact on basal metabolic rate, their bona fide and tentative pathophysiological implications, and putative drug interactions.

Futile lipid cycling: from biochemistry to physiology

In the healthy state, the fat stored in our body isn't just inert. Rather, it is dynamically mobilized to maintain an adequate concentration of fatty acids (FAs) in our bloodstream. Our body tends to produce excess FAs to ensure that the FA availability is not limiting. The surplus FAs are actively re-esterified into glycerides, initiating a cycle of breakdown and resynthesis of glycerides. This cycle consumes energy without generating a new product and is commonly referred to as the 'futile lipid cycle' or the glyceride/FA cycle. Contrary to the notion that it's a wasteful process, it turns out this cycle is crucial for systemic metabolic homeostasis. It acts as a control point in intra-adipocyte and inter-organ cross-talk, a metabolic rheostat, an energy sensor and a lipid diversifying mechanism. In this Review, we discuss the metabolic regulation and physiological implications of the glyceride/FA cycle and its mechanistic underpinnings.

Parallel control of cold-triggered adipocyte thermogenesis by UCP1 and CKB

That uncoupling protein 1 (UCP1) is the sole mediator of adipocyte thermogenesis is a conventional viewpoint that has primarily been inferred from the attenuation of the thermogenic output of mice genetically lacking Ucp1 from birth (germline Ucp1-/-). However, germline Ucp1-/- mice harbor secondary changes within brown adipose tissue. To mitigate these potentially confounding ancillary changes, we constructed mice with inducible adipocyte-selective Ucp1 disruption. We find that, although germline Ucp1-/- mice succumb to cold-induced hypothermia with complete penetrance, most mice with the inducible deletion of Ucp1 maintain homeothermy in the cold. However, inducible adipocyte-selective co-deletion of Ucp1 and creatine kinase b (Ckb, an effector of UCP1-independent thermogenesis) exacerbates cold intolerance. Following UCP1 deletion or UCP1/CKB co-deletion from mature adipocytes, moderate cold exposure triggers the regeneration of mature brown adipocytes that coordinately restore UCP1 and CKB expression. Our findings suggest that thermogenic adipocytes utilize non-paralogous protein redundancy-through UCP1 and CKB-to promote cold-induced energy dissipation.

Promoting metabolic inefficiency for metabolic disease

Recent advances in pharmacotherapies that promote appetite suppression have shown remarkable weight loss. Therapies targeting energy expenditure lag behind, and as such none have yet been identified to be safe and efficacious for sustaining negative energy balance toward weight loss. Multiple energy dissipating pathways have been identified in adipose tissue and muscle. The molecular effectors of some of these pathways have been identified, but much is still left to be learned about their regulation. Understanding the molecular underpinnings of metabolic inefficiency in adipose tissue and muscle is required if these pathways are to be therapeutically targeted in the context of obesity and obesity-accelerated diseases.

β-Adrenergic Receptors and Adipose Tissue Metabolism: Evolution of an Old Story

The role of β-adrenergic receptors (βARs) in adipose tissue to promote lipolysis and the release of fatty acids and nonshivering thermogenesis in brown fat has been studied for so many decades that one would think there is nothing left to discover. With the rediscovery of brown fat in humans and renewed interest in UCP1 and uncoupled mitochondrial respiration, it seems that a review of adipose tissue as an organ, pivotal observations, and the investigators who made them would be instructive to understanding where the field stands now. The discovery of the β₃-adrenergic receptor was important for accurately defining the pharmacology of the adipocyte, while the clinical targeting of this receptor for obesity and metabolic disease has had its highs and lows. Many questions still remain about how βARs regulate adipocyte metabolism and the signaling molecules through which they do it.

Brown adipocyte glucose metabolism: a heated subject

The energy expending and glucose sink properties of brown adipose tissue (BAT) make it an attractive target for new obesity and diabetes treatments. Despite decades of research, only recently have mechanistic studies started to provide a more complete and consistent picture of how activated brown adipocytes handle glucose. Here, we discuss the importance of intracellular glycolysis, lactate production, lipogenesis, lipolysis, and beta-oxidation for BAT thermogenesis in response to natural (temperature) and artificial (pharmacological and optogenetic) forms of sympathetic nervous system stimulation. It is now clear that together, these metabolic processes in series and in parallel flexibly power ATP-dependent and independent futile cycles in brown adipocytes to impact on whole-body thermal, energy, and glucose balance.

Phosphorylation of Beta-3 adrenergic receptor at serine 247 by ERK MAP kinase drives lipolysis in obese adipocytes

OBJECTIVE

The inappropriate release of free fatty acids from obese adipose tissue stores has detrimental effects on metabolism, but key molecular mechanisms controlling FFA release from adipocytes remain undefined. Although obesity promotes systemic inflammation, we find activation of the inflammation-associated Mitogen Activated Protein kinase ERK occurs specifically in adipose tissues of obese mice, and provide evidence that adipocyte ERK activation may explain exaggerated adipose tissue lipolysis observed in obesity.

METHODS AND RESULTS

We provide genetic and pharmacological evidence that inhibition of the MEK/ERK pathway in human adipose tissue, mice, and flies all effectively limit adipocyte lipolysis. In complementary findings, we show that genetic and obesity-mediated activation of ERK enhances lipolysis, whereas adipose tissue specific knock-out of ERK2, the exclusive ERK1/2 protein in adipocytes, dramatically impairs lipolysis in explanted mouse adipose tissue. In addition, acute inhibition of MEK/ERK signaling also decreases lipolysis in adipose tissue and improves insulin sensitivity in obese mice. Mice with decreased rates of adipose tissue lipolysis in vivo caused by either MEK or ATGL pharmacological inhibition were unable to liberate sufficient White Adipose Tissue (WAT) energy stores to fuel thermogenesis from brown fat during a cold temperature challenge. To identify a molecular mechanism controlling these actions, we performed unbiased phosphoproteomic analysis of obese adipose tissue at different time points following acute pharmacological MEK/ERK inhibition. MEK/ERK inhibition decreased levels of adrenergic signaling and caused de-phosphorylation of the β3-adrenergic receptor (β3AR) on serine 247. To define the functional implications of this phosphorylation, we showed that CRISPR/Cas9 engineered cells expressing wild type β3AR exhibited β3AR phosphorylation by ERK2 and enhanced lipolysis, but this was not seen when serine 247 of β3AR was mutated to alanine.

CONCLUSION

Taken together, these data suggest that ERK activation in adipocytes and subsequent phosphorylation of the β3AR on S247 are critical regulatory steps in the enhanced adipocyte lipolysis of obesity.

Heart hormones fueling a fire in fat

Our view of how adipose tissue metabolism is regulated recently experienced a change in focus and breadth, meaning that some of the key controlling factors were not fully in the picture. The catecholamines of the sympathetic nervous system are well-known activators of β-adrenergic receptors in adipocytes to increase lipolysis. They also drive energy expenditure in brown adipose tissue and, importantly, the “browning” of cells in white adipose depots. However, this is clearly not the whole story. In earlier work, we established a pathway from β-adrenergic receptors to p38 MAP kinase to drive the transcription of brown adipocyte genes and respiratory uncoupling. Now we recently discovered that cardiac natriuretic peptides (NPs) stimulate a similar “browning” of human and mouse adipocytes. NPs activate the guanylyl cyclase coupled NP receptor A and activation of protein kinase G. Importantly, this pathway also depends upon p38 MAPK. These two pathways work together, additively increasing expression of brown adipocyte marker genes, as well as reflexively controlling each other’s components. We discuss these findings and how the control of body fat by these cardiac hormones, in conjunction with the sympathetic nervous system, has implications for obesity as well as cardiovascular disease, including hypertension and heart failure.

Acute stimulation of white adipocyte respiration by PKA-induced lipolysis

OBJECTIVE

We examined the effect of β-adrenergic receptor (βAR) activation and cAMP-elevating agents on respiration and mitochondrial uncoupling in human adipocytes and probed the underlying molecular mechanisms.

RESEARCH DESIGN AND METHODS

Oxygen consumption rate (OCR, aerobic respiration) and extracellular acidification rate (ECAR, anaerobic respiration) were examined in response to isoproterenol (ISO), forskolin (FSK), and dibutyryl-cAMP (DB), coupled with measurements of mitochondrial depolarization, lipolysis, kinase activities, and gene targeting or knock-down approaches.

RESULTS

ISO, FSK, or DB rapidly increased oxidative and glycolytic respiration together with mitochondrial depolarization in human and mouse white adipocytes. The increase in OCR was oligomycin-insensitive and contingent on cAMP-dependent protein kinase A (PKA)-induced lipolysis. This increased respiration and the uncoupling were blocked by inhibiting the mitochondrial permeability transition pore (PTP) and its regulator, BAX. Interestingly, compared with lean individuals, adipocytes from obese subjects exhibited reduced OCR and uncoupling capacity in response to ISO.

CONCLUSIONS

Lipolysis stimulated by βAR activation or other maneuvers that increase cAMP levels in white adipocytes acutely induces mitochondrial uncoupling and cellular energetics, which are amplified in the absence of scavenging BSA. The increase in OCR is dependent on PKA-induced lipolysis and is mediated by the PTP and BAX. Because this effect is reduced with obesity, further exploration of this uncoupling mechanism will be needed to determine its cause and consequences.

Adrenoceptors, uncoupling proteins, and energy expenditure

Interest in the biology of adipose tissue has undergone a revival in recent years with the discovery of a host of genes that contribute to the regulation of satiety and metabolic rate. The catecholamines have long been known to be key modulators of adipose tissue lipolysis and the hydrolysis of triglyceride energy stores. However, more recent efforts to understand the role of individual adrenergic receptor subtypes expressed in adipocytes and their signal transduction pathways have revealed a complexity not previously appreciated. Combined with this interest in the modulation of adipocyte metabolism is a renewed focus upon brown adipose tissue and the mechanisms of whole body thermogenesis in general. The discovery of novel homologs of the brown fat uncoupling protein (UCP) such as UCP2 and UCP3 has provoked intensive study of these mitochondrial proteins and the role that they play in fuel metabolism. The story of the novel UCPs has proven to be intriguing and still incompletely understood. Here, we review the status of adipose tissue from inert storage depot to endocrine organ, interesting signal transduction pathways triggered by beta-adrenergic receptors in adipocytes, the potential of these receptors for discriminating and coordinated metabolic regulation, and current views on the role of UCP2 and UCP3 based on physiological studies and gene knockout models.

Immunohistochemical evidence for the existence of a dopamine- and cyclic AMP-regulated phosphoprotein (DARPP-32) in brown adipose tissue of pigs

The indirect immunofluorescence technique was used to study the cellular localization of DARPP-32, a dopamine- and cyclic AMP-regulated phosphoprotein, in brown adipose tissue of newborn piglets. Clusters of strongly DARPP-32-immunoreactive cells were found in brown adipose tissue from the interscapular area and around lymph nodes close to the kidneys, adrenal glands, descending aorta, and great veins in the neck. The DARPP-32-immunoreactive cells contained multilocular lipid droplets, had round, centrally located nuclei, and were polygonal in shape, thus possessing characteristics and location sites typical for brown fat cells. The results indicate that brown adipose tissue from the newborn pig contains DARPP-32, an intracellular third messenger for dopamine. Together with recent functional data, these results strongly suggest that dopaminergic D1 mechanisms--i.e., activation of adenylate cyclase and formation of cyclic AMP--may be involved in cold-induced, nonshivering, and/or diet-induced thermogenesis.

Neural and vascular provisions of rat interscapular brown adipose tissue

The innervation of rat interscapular brown adipose tissue has been studied by light and fluorescence microscopy and electron microscopy after treatment with "false" adrenergic neurotransmitters 5- and 6-hydroxydopamine. The vascular markers neoprene latex and thioflavin S were used to define the blood vascular arrangements within the around the tissue. Catecholaminergic innervation was revealed by fluorescence microscopy at both parenchymal and vasomotor sites. In animals injected with 6-hydroxydopamine, this catecholaminergic fluorescence was extinguished in the parenchymal nerve distribution and markedly reduced in the vasomotor plexus. Identification of an extensive network of noradrenergic vasomotor and parenchymal nerve terminals was established by electron microscopy after 5- and 6-hydroxydopamine administration, but unmarked terminals were also observed in both distributions. These unmarked terminals might represent an additional nonnoradrenergic nerve supply to interscapular brown adipose tissue. The thoracodorsal veins draining the fat pads are directly tributary to a large median perforating vein, which joins the azygos vein, and are also continuous with the axillary vein. In addition to the recognized vascular distribution pattern of lobular arteries supplying an abundant capillary plexus drained by lobular veins, direct arteriovenous anastomoses were observed within the interscapular brown fat pad. It is postulated that these additional vascular arrangements are determinant in the phenomenal increase in blood flow through brown adipose tissue during metabolic stimulation.

Stereological assessment of age-related changes in lipid droplet surface area and vascular volume in rat interscapular brown adipose tissue

The surface area of stored triglyceride and the volume of capillaries in the interscapular brown fat pad of the rat have been adopted as morphological indices of the overall thermogenic capability of the tissue. The present study examines the relationship between the chronological profiles of these parameters and reported senile changes in the biochemical and physiological characteristics of brown adipose tissue. Lipid droplet surface density and vascular volume density were estimated by computer-assisted planimetry with electron micrographs prepared from interscapular brown adipose tissue samples obtained from rats of various ages. The volumes of the fat pad at these ages were also determined and used to calculate droplet surface areas and vascular volumes. Triglyceride surface area showed a 20-fold enhancement in the early postnatal period (0.44 X 10(2) cm2 at birth; 9.43 X 10(2) cm2 at 4 weeks). A further but less remarkable increase occurred in adulthood (11.67 X 10(2) cm2 at 6 months) and, in old age, only a slight fall was noted (9.66 X 10(2) cm2 at 2 years). Intralobular capillary volume also rose sharply early during the first week after birth, reaching a peak at 4 weeks (2.40 X 10(-2) cm3). The values recorded in adulthood and old age (1.90 X 10(-2) cm3 at 6 months; 1.76 X 10(-2) cm3 at 2 years) were not significantly different from that obtained at 4 weeks of age. These results show that the attainment of maximum values of lipid droplet surface area and vascular volume in rat interscapular brown adipose tissue coincides with the period of the tissue's peak metabolic capacity.(ABSTRACT TRUNCATED AT 250 WORDS)

Brown adipose tissue. Its in vivo immunology and involvement in neuroimmunomodulation

The material presented here summarizes the bulk of the presently available immunologic data bearing upon the in vivo relationship between brown adipose tissue and the immune system. The experiments were carried out in rats adipectomized (by surgical excision of the interscapular brown adipose tissue at birth), thymectomized (by neonatal removal of the thymus), adipectomized and thymectomized, and corresponding sham-operated controls. The following immune phenomena were studied: antibody production to soluble and corpuscular antigens; Arthus and delayed hypersensitivity skin reactions to bovine serum albumin; rejection of allogeneic skin and thyroid grafts; lymph node enlargement in a host-versus-graft reaction; experimental allergic encephalomyelitis and thyroiditis; immune response in normal animals treated with extracts from brown adipose tissue; allergic encephalomyelitis in thymoadipectomized animals; plaque-forming cell response and hemagglutinating antibody titers in animals injected with met-enkephalin and leu-enkephalin; and survival rate of adipectomized mice inoculated with Sarcoma-I cells. The results indicated that the cell-mediated immune reactions were potentiated in adipectomized rats. Antibody production was not significantly changed by neonatal adipectomy. Adipectomized mice, inoculated with Sa-I tumor cells, survived longer than controls, thus indicating that adipectomy made possible the recognition of discrete histocompatible differences between Sa-I cells and A/JAX mice. Adipectomy increased the ability of rats to develop autoimmune diseases. Saline extracts from brown adipose tissue of newborn rats suppressed hypersensitivity skin reactions in normal adult rats. Thymoadipectomized rats showed an almost normal ability to develop allergic encephalomyelitis, a finding that suggested that the potentiating influence of adipectomy on encephalomyelitis was neutralized by thymectomy. It appears that brown adipose tissue functions as a natural antagonist of the thymus. Enkephalins were found to be more effective immunosuppressors in adipectomized than in normal animals. The last finding establishes a functional link between brown adipose tissue and neuropeptides. It seems that the potentiation of immune response in adipectomized animals is effected by altered release of yet unidentified mediators and modulators. The evidence indicates that brown adipose tissue, in which neurohumoral activity occurs, may be an important component of an integrated immunoneuroendocrine system.