Intense FDG activity in the brown adipose tissue in omental and mesenteric regions in a patient with malignant pheochromocytoma

Main mechanisms and clinical implications of alterations in energy expenditure state among patients with pheochromocytoma and paraganglioma: A review

Pheochromocytoma and paraganglioma (PPGL) are rare neuroendocrine tumors with diverse clinical presentations. Alterations in energy expenditure state are commonly observed in patients with PPGL. However, the reported prevalence of hypermetabolism varies significantly and the underlying mechanisms and implications of this presentation have not been well elucidated. This review discusses and analyzes the factors that contribute to energy consumption. Elevated catecholamine levels in patients can significantly affect substance and energy metabolism. Additionally, changes in the activation of brown adipose tissue (BAT), inflammation, and the inherent energy demands of the tumor can contribute to increased resting energy expenditure (REE) and other energy metabolism indicators. The PPGL biomarker, chromogranin A (CgA), and its fragments also influence energy metabolism. Chronic hypermetabolic states may be detrimental to these patients, with surgical tumor removal remaining the primary therapeutic intervention. The high energy expenditure of PPGL has not received the attention it deserves, and an accurate assessment of energy metabolism is the cornerstone for an adequate understanding and treatment of the disease.

Browning of Mesenteric White Adipose Tissue in Crohn's Disease: A New Pathological Change and Therapeutic Target

BACKROUND

Hypertrophic mesenteric adipose tissue [htMAT] is a distinctive hallmark of Crohn's disease [CD], and it affects enteritis via inflammatory adipokine secretion by dysfunctional white adipocytes. White adipocytes can become beige adipocytes, which are characterized by active lipid consumption and favourable endocrine function, via white adipocyte browning. Our study aimed to determine whether white adipocyte browning occurs in htMAT and its role in CD.

METHODS

White adipocyte browning was examined in MAT samples from CD patients and controls. Human MAT explants and primary mesenteric adipocytes were cultured for in vitro experiments. Mice with 2,4,6-trinitrobenzenesulphonic acid solution [TNBS]-induced colitis were used for in vivo studies. A β3-adrenergic receptor agonist [CL316,243] was used to induce white adipocyte browning, and IL-4/STAT6 signalling was analysed to explore the mechanism underlying the anti-inflammatory activity of beige adipocytes.

RESULTS

White adipocyte browning was observed in htMAT from CD patients, as shown by the appearance of uncoupling protein 1 [UCP1]-positive multilocular [beige] adipocytes with lipid-depleting activity and anti-inflammatory endocrine profiles. Both human MAT and primary mesenteric adipocytes from CD patients and controls could be induced to undergo browning, which increased their lipid-depleting and anti-inflammatory activities in vitro. Inducing MAT browning ameliorated mesenteric hypertrophy and inflammation as well as colitis in TNBS-treated mice in vivo. The anti-inflammatory activity of beige adipocytes was at least partially related to STAT6 signalling activation via the autocrine and paracrine effects of IL-4.

CONCLUSION

White adipocyte browning is a newly identified pathological change in htMAT of CD patients and a possible therapeutic target.

Metabolic associated fatty liver disease is a disease related to sympathetic nervous system activation

Strong evidence from animal and human studies shows that sympathetic nervous system (SNS) activation is a key factor in the development of metabolic associated fatty liver disease (MAFLD). Activation of the sympathetic nervous system plays an important role in the pathogenesis of obesity, metabolic syndrome, diabetes, hypertension, and MAFLD. When genetically susceptible subjects are exposed to a variety of epigenetic changes, their liver damage may develop into MAFLD. Thus, the pathogenesis of MAFLD is complex, involving the complex interaction of insulin resistance, abnormal hormone secretion, obesity, diet, genetic factors, immune activation, gut microbiota, and other factors. In these processes, the role of sympathetic nerves cannot be underestimated. Notably, SNS has been proposed as a therapeutic target for MAFLD by inhibiting sympathetic nerves. It is worthy of further discussion and research.

HYPOTHesizing about central comBAT against obesity

The hypothalamus is a brain region in charge of many vital functions. Among them, BAT thermogenesis represents an essential physiological function to maintain body temperature. In the metabolic context, it has now been established that energy expenditure attributed to BAT function can contribute to the energy balance in a substantial extent. Thus, therapeutic interest in this regard has increased in the last years and some studies have shown that BAT function in humans can make a real contribution to improve diabetes and obesity-associated diseases. Nevertheless, how the hypothalamus controls BAT activity is still not fully understood. Despite the fact that much has been known about the mechanisms that regulate BAT activity in recent years, and that the central regulation of thermogenesis offers a very promising target, many questions remain still unsolved. Among them, the possible human application of knowledge obtained from rodent studies, and drug administration strategies able to specifically target the hypothalamus. Here, we review the current knowledge of homeostatic regulation of BAT, including the molecular insights of brown adipocytes, its central control, and its implication in the development of obesity.

Mechanistic Links Between Obesity, Diabetes, and Blood Pressure: Role of Perivascular Adipose Tissue

Obesity is increasingly prevalent and is associated with substantial cardiovascular risk. Adipose tissue distribution and morphology play a key role in determining the degree of adverse effects, and a key factor in the disease process appears to be the inflammatory cell population in adipose tissue. Healthy adipose tissue secretes a number of vasoactive adipokines and anti-inflammatory cytokines, and changes to this secretory profile will contribute to pathogenesis in obesity. In this review, we discuss the links between adipokine dysregulation and the development of hypertension and diabetes and explore the potential for manipulating adipose tissue morphology and its immune cell population to improve cardiovascular health in obesity.

Obesity phenotypes: depot-differences in adipose tissue and their clinical implications

Obesity, defined as excess fat mass, increases risks for multiple chronic diseases, such as type 2 diabetes, cardiovascular disease, and several types of cancer. Beyond adiposity per se, the pattern of fat distribution, android or truncal as compared to gynoid or peripheral, has a profound influence on systemic metabolism and hence risk for obesity complications. Not only factors as genetics, environment, gender, and age account for the apparent compartmentalization of white adipose tissue (WAT) in the body. Indeed, the heterogeneity among different anatomical depots also appears to stem from their intrinsic diversity, including cellular developmental origin, proliferative capacity, glucose and lipid metabolism, insulin sensitivity, cytokine pattern, thermogenic ability, and vascularization. Under the obese condition, these depot-specific differences translate into specific WAT distribution patterns, giving rise to different cardiometabolic consequences. This review summarizes the clinical and mechanistic evidence for the depot-specific differences and the phenotypic characteristics of different WAT depots that link their depot-specific biology to obesity-specific complications.

Adipocyte Liver Kinase b1 Suppresses Beige Adipocyte Renaissance Through Class IIa Histone Deacetylase 4

Uncoupling protein 1+ beige adipocytes are dynamically regulated by environment in rodents and humans; cold induces formation of beige adipocytes, whereas warm temperature and nutrient excess lead to their disappearance. Beige adipocytes can form through de novo adipogenesis; however, how "beiging" characteristics are maintained afterward is largely unknown. In this study, we show that beige adipocytes formed postnatally in subcutaneous inguinal white adipose tissue lost thermogenic gene expression and multilocular morphology at the adult stage, but cold restored their beiging characteristics, a phenomenon termed beige adipocyte renaissance. Ablation of these postnatal beige adipocytes inhibited cold-induced beige adipocyte formation in adult mice. Furthermore, we demonstrated that beige adipocyte renaissance was governed by liver kinase b1 and histone deacetylase 4 in white adipocytes. Although neither presence nor thermogenic function of uncoupling protein 1+ beige adipocytes contributed to metabolic fitness in adipocyte liver kinase b1-deficient mice, our results reveal an unexpected role of white adipocytes in maintaining properties of preexisting beige adipocytes.

The Role of the Autonomic Nervous System in the Pathophysiology of Obesity

Obesity is reaching epidemic proportions globally and represents a major cause of comorbidities, mostly related to cardiovascular disease. The autonomic nervous system (ANS) dysfunction has a two-way relationship with obesity. Indeed, alterations of the ANS might be involved in the pathogenesis of obesity, acting on different pathways. On the other hand, the excess weight induces ANS dysfunction, which may be involved in the haemodynamic and metabolic alterations that increase the cardiovascular risk of obese individuals, i.e., hypertension, insulin resistance and dyslipidemia. This article will review current evidence about the role of the ANS in short-term and long-term regulation of energy homeostasis. Furthermore, an increased sympathetic activity has been demonstrated in obese patients, particularly in the muscle vasculature and in the kidneys, possibily contributing to increased cardiovascular risk. Selective leptin resistance, obstructive sleep apnea syndrome, hyperinsulinemia and low ghrelin levels are possible mechanisms underlying sympathetic activation in obesity. Weight loss is able to reverse metabolic and autonomic alterations associated with obesity. Given the crucial role of autonomic dysfunction in the pathophysiology of obesity and its cardiovascular complications, vagal nerve modulation and sympathetic inhibition may serve as therapeutic targets in this condition.

18F-FDG PET/CT of Brown Adipose Tissue Hyperactivation Associated With Pregnancy and Paraganglioma

This 32-year-old woman presented with pregnancy-related arterial hypertension unresponsive to antihypertensive therapy. During cesarean delivery, a lobulated retroperitoneal mass was discovered. F-FDG PET/CT performed 18 days postpartum demonstrated the hyperactive retroperitoneal mass and extensive hyperactivated brown adipose tissue. The mass was surgically removed, revealing a para-aortic multicentric paraganglioma. After surgery, blood pressure normalized, and serum chromogranin A and urinary metanephrines normalized. Brown adipose tissue hypermetabolism disappeared on follow-up FDG PET. Her initial FDG PET demonstrated brown adipose tissue hyperactivation, which may have been caused by the combination of hormonal changes in pregnancy and the paraganglioma.

Convertible visceral fat as a therapeutic target to curb obesity

New therapeutic and preventative strategies are needed to address the growing obesity epidemic. In animal models, brown adipose tissue activation and the associated heat produced contribute to countering obesity and the accompanying metabolic abnormalities. Adult humans also have functional brown fat. Here, we present and discuss the concepts of murine and human white adipose tissue plasticity and the transdifferentiation of white adipocytes into brown adipocytes. Human visceral adipocytes - which are crucial contributors to the burden of obesity and its complications - are particularly susceptible to such transdifferentiation. Therefore, we propose that this process should be a focus of anti-obesity research. Approved drugs that have browning properties as well as future drugs that target molecular pathways involved in white-to-brown visceral adipocyte transdifferentiation may provide new avenues for obesity therapy.

Glucagon increases energy expenditure independently of brown adipose tissue activation in humans

AIMS

To investigate, for a given energy expenditure (EE) rise, the differential effects of glucagon infusion and cold exposure on brown adipose tissue (BAT) activation in humans.

METHODS

Indirect calorimetry and supraclavicular thermography was performed in 11 healthy male volunteers before and after: cold exposure; glucagon infusion (at 23 °C); and vehicle infusion (at 23 °C). All volunteers underwent (18)F-fluorodeoxyglucose ((18)F-FDG) positron emission tomography (PET)/CT scanning with cold exposure. Subjects with cold-induced BAT activation on (18)F-FDG PET/CT (n = 8) underwent a randomly allocated second (18)F-FDG PET/CT scan (at 23 °C), either with glucagon infusion (n = 4) or vehicle infusion (n = 4).

RESULTS

We observed that EE increased by 14% after cold exposure and by 15% after glucagon infusion (50 ng/kg/min; p < 0.05 vs control for both). Cold exposure produced an increase in neck temperature (+0.44 °C; p < 0.001 vs control), but glucagon infusion did not alter neck temperature. In subjects with a cold-induced increase in the metabolic activity of supraclavicular BAT on (18)F-FDG PET/CT, a significant rise in the metabolic activity of BAT after glucagon infusion was not detected. Cold exposure increased sympathetic activation, as measured by circulating norepinephrine levels, but glucagon infusion did not.

CONCLUSIONS

Glucagon increases EE by a similar magnitude compared with cold activation, but independently of BAT thermogenesis. This finding is of importance for the development of safe treatments for obesity through upregulation of EE.

Thermogenic brown and beige/brite adipogenesis in humans

Evidence from rodents established an important role of brown adipose tissue (BAT) in energy expenditure. Moreover, to sustain thermogenesis, BAT has been shown to be a powerful sink for draining and oxidation of glucose and triglycerides from blood. The potential of BAT activity in protection against obesity and metabolic syndrome is recognized. Recently, an unexpected presence and activity of BAT has been found in adult humans. Here we review the most recent research in this field and, specifically, how new findings apply to humans. Moreover, we seek to clarify the underlying biological processes occurring beyond the burst of new nomenclature in the field. The cell type responsible for thermogenesis, the brown adipocyte, arises from complex developmental processes. In addition to 'classical' brown adipocytes, present in developmentally programmed BAT depots, there are brown adipocytes, named 'brite' (from 'brown-in-white') or 'beige', which appear in response to thermogenic stimuli in white fat due to the so-called 'browning' process. Beige/brite cells appear to be important components of BAT depots in adult humans. In addition to the known control of BAT activity by the sympathetic nervous system, metabolic and hormonal signals originating in muscle or liver (e.g. irisin, FGF21) are recognized as activators of BAT and beige/brite adipocytes.

Insights into Brown Adipose Tissue Physiology as Revealed by Imaging Studies

There has been resurgence in interest in brown adipose tissue (BAT) following radiological and histological identification of metabolically active BAT in adult humans. Imaging enables BAT to be studied non-invasively and therefore imaging studies have contributed a significant amount to what is known about BAT function in humans. In this review the current knowledge (derived from imaging studies) about the prevalence, function, activity and regulation of BAT in humans (as well as relevant rodent studies), will be summarized.

Activating brown adipose tissue for weight loss and lowering of blood glucose levels: a microPET study using obese and diabetic model mice

Purpose

This study aims at using ¹⁸F-FDG microPET to monitor the brown adipose tissue (BAT) glucose metabolism in obese and diabetic mouse models under different interventions, and study the therapeutic potential of BAT activation for weight loss and lowering of blood glucose in these models.

Methods

Obese mice were established by a high-fat diet for eight weeks, and diabetes mellitus(DM) models were induced with Streptozocin in obese mice. ¹⁸F-FDG microPET was used to monitor BAT function during obese and DM modeling, and also after BRL37344 (a β₃-adrenergic receptor agonist) or levothyroxine treatment. The BAT function was correlated with the body weight and blood glucose levels.

Results

Compared with the controls, the obese mice and DM mice showed successively lower ¹⁸F-FDG uptake in the interscapular BAT (P = 0.036 and <0.001, respectively). After two-week BRL37344 treatment, the BAT uptake was significantly elevated in both obese mice (P = 0.010) and DM mice (P = 0.004), accompanied with significantly decreased blood glucose levels (P = 0.023 and 0.036, respectively). The BAT uptake was negatively correlated with the blood glucose levels in both obese mice (r = −0.71, P = 0.003) and DM mice (r = −0.74, P = 0.010). BRL37344 treatment also caused significant weight loss in the obese mice (P = 0.001). Levothyroxine treatment increased the BAT uptake in the control mice (P = 0.025) and obese mice (P = 0.013), but not in the DM mice (P = 0.45).

Conclusion

The inhibited BAT function in obese and DM mice can be re-activated by β₃-adrenergic receptor agonist or thyroid hormone, and effective BAT activation may lead to weight loss and blood glucose lowering. Activating BAT can provide a new treatment strategy for obesity and DM.

Hypermetabolic mesenteric brown adipose tissue on dual-time point FDG PET/CT in a patient with benign retroperitoneal pheochromocytoma

Intense FDG uptake of the mesenteric brown adipose tissue (BAT) is rare. We present a case of benign retroperitoneal pheochromocytoma showing multiple hypermetabolic regions corresponding with common locations of BAT. Furthermore, the mesenteric region showed intense FDG uptake, with SUVmax of 10.6. FDG PET/CT findings of this case supports the previous theories that the incidence of BAT may be higher in patients with pheochromocytoma than in patients without, and the intra-abdominal fat of human adults, including the omental fat, which is brown adipose tissue in infancy, becomes reactivated in the presence of high circulating noradrenaline concentrations.

Brown adipose tissue: endocrine determinants of function and therapeutic manipulation as a novel treatment strategy for obesity

Introduction

Recent observation of brown adipose tissue (BAT) being functional in adult humans provides a rationale for its stimulation to increase energy expenditure through ‘adaptive thermogenesis’ for an anti-obesity strategy. Many endocrine dysfunctions are associated with changes in metabolic rate that over time may result in changes in body weight. It is likely that human BAT plays a role in such processes.

Review

In this brief review article, we explore the endocrine determinants of BAT activity, and discuss how these insights may provide a basis for future developments of novel therapeutic strategies for obesity management.

A review of electronic and print data comprising original and review articles retrieved from PubMed search up to December 2013 was conducted (Search terms: brown adipose tissue, brown fat, obesity, hormone). In addition, relevant references from the articles were screened for papers containing original data.

Conclusion

There is promising data to suggest that targeting endocrine hormones for BAT modulation can yield a cellular bioenergetics answer for successful prevention and management of human obesity. Further understanding of the physiological link between various endocrine hormones and BAT is necessary for the development of new therapeutic options.

Electronic supplementary material

The online version of this article (doi:10.1186/s40608-014-0013-5) contains supplementary material, which is available to authorized users.

Extensive hypermetabolic pattern of brown adipose tissue activation on 18F-FDG PET/CT in a patient diagnosed of catecholamine-secreting para-vesical paraganglioma

The widespread use of (18)F-FDG PET-CT scanning in oncological patients has allowed to demonstrate the existence of metabolically active brown fat, also called brown adipose tissue (BAT), in adult humans, and specifying its anatomical distribution in vivo. As physiological determinants to BAT (18)F-FDG uptake has been identified gender, age, temperature, and body mass index. We have observed extensive activation of the BAT, including the mesenteric region, in a patient with a catecholamine-secreting para-vesical paranganglioma. The extensive BAT activation could be secondary to adrenergic stimulation due to excess of circulating norepinephrine concentration.

White-to-brown transdifferentiation of omental adipocytes in patients affected by pheochromocytoma

In all mammals, white adipose tissue (WAT) and brown adipose tissue (BAT) are found together in several fat depots, forming a multi-depot organ. Adrenergic stimulation induces an increase in BAT usually referred to as "browning". This phenomenon is important because of its potential use in curbing obesity and related disorders; thus, understanding its cellular mechanisms in humans may be useful for the development of new therapeutic strategies. Data in rodents have supported the direct transformation of white into brown adipocytes. Biopsies of pure white omental fat were collected from 12 patients affected by the catecholamine-secreting tumor pheochromocytoma (pheo-patients) and compared with biopsies from controls. Half of the omental fat samples from pheo-patients contained uncoupling protein 1 (UCP1)-immunoreactive-(ir) multilocular cells that were often arranged in a BAT-like pattern endowed with noradrenergic fibers and dense capillary network. Many UCP1-ir adipocytes showed the characteristic morphology of paucilocular cells, which we have been described as cytological marker of transdifferentiation. Electron microscopy showed increased mitochondrial density in multi- and paucilocular cells and disclosed the presence of perivascular brown adipocyte precursors. Brown fat genes, such as UCP1, PR domain containing 16 (PRDM16) and β3-adrenoreceptor, were highly expressed in the omentum of pheo-patients and in those cases without visible morphologic re-arrangement. Of note, the brown determinant PRDM16 was detected by immunohistochemistry only in nuclei of multi- and paucilocular adipocytes. Quantitative electron microscopy and immunohistochemistry for Ki67 suggest an unlikely contribution of proliferative events to the phenomenon. The data support the idea that, in adult humans, white adipocytes of pure white fat that are subjected to adrenergic stimulation are able to undergo a process of direct transformation into brown adipocytes. This article is part of a Special Issue entitled Brown and White Fat: From Signaling to Disease.

[Positron emission tomography-computed tomography in tumors of the locomotor apparatus]

Positron emission tomography/computed tomography (PET/CT) is a hybrid imaging technique that combines the anatomic information from CT with the metabolic information acquired from PET after the administration of specific radiotracers, the most commonly used of which is F18-fluorodeoxyglucose (FDG). In oncology, this technique is based on the increased uptake of FDG by malignant lesions. In the locomotor apparatus, some uptake by bones and soft tissues is physiological or benign and this uptake must be differentiated from uptake by malignancies, whether primary or secondary. The most important limitations are active inflammatory or infectious processes, which are positive on PET images, and malignant lesions that are smaller than 1cm, cystic, necrotic, or low-grade, which are negative on PET images. PET/CT in the locomotor apparatus is especially useful for the detection of metastases from the most common tumors. It is also used for staging and monitoring the response to treatment of some hematological tumors like lymphoma, where it is fundamental to determine whether the bone marrow has been infiltrated, or myeloma. Lastly, although it is not yet an established indication, PET/CT is being increasingly used to study sarcomas, because it can provide additional information that can be useful for the characterization and grading of tumors, for guiding biopsies, for staging and re-staging, and for evaluating the response to neoadjuvant therapy as well as for evaluating new drugs in clinical trials.