Therapeutic value of brown adipose tissue: Correcting metabolic disease through generating healthy fat

Beyond the Cold: Activating Brown Adipose Tissue as an Approach to Combat Obesity

With a dramatic increase in the number of obese and overweight people, there is a great need for new anti-obesity therapies. With the discovery of the functionality of brown adipose tissue in adults and the observation of beige fat cells among white fat cells, scientists are looking for substances and methods to increase the activity of these cells. We aimed to describe how scientists have concluded that brown adipose tissue is also present and active in adults, to describe where in the human body these deposits of brown adipose tissue are, to summarize the origin of both brown fat cells and beige fat cells, and, last but not least, to list some of the substances and methods classified as BAT promotion agents with their benefits and side effects. We summarized these findings based on the original literature and reviews in the field, emphasizing the discovery, function, and origins of brown adipose tissue, BAT promotion agents, and batokines. Only studies written in English and with a satisfying rating were identified from electronic searches of PubMed.

The endocrine role of brown adipose tissue: An update on actors and actions

In recent years, brown adipose tissue (BAT) has been recognized not only as a main site of non-shivering thermogenesis in mammals, but also as an endocrine organ. BAT secretes a myriad of regulatory factors. These so-called batokines exert local autocrine and paracrine effects, as well as endocrine actions targeting tissues and organs at a distance. The endocrine batokines include peptide factors, such as fibroblast growth factor-21 (FGF21), neuregulin-4 (NRG4), phospholipid transfer protein (PLTP), interleukin-6, adiponectin and myostatin, and also lipids (lipokines; e.g., 12,13-dihydroxy-9Z-octadecenoic acid [12,13-diHOME]) and miRNAs (e.g., miR-99b). The liver, heart, and skeletal muscle are the most commonly reported targets of batokines. In response to BAT thermogenic activation, batokines such as NRG4 and PLTP are released and act to reduce hepatic steatosis and improve insulin sensitivity. Stress-induced interleukin-6-mediated signaling from BAT to liver favors hepatic glucose production through enhanced gluconeogenesis. Batokines may act on liver to induce the secretion of regulatory hepatokines (e.g. FGF21 and bile acids in response to miR-99b and PLTP, respectively), thereby resulting in a systemic expansion of BAT-originating signals. Batokines also target extrahepatic tissues: FGF21 and 12,13-diHOME are cardioprotective, whereas BAT-secreted myostatin and 12,13-diHOME influence skeletal muscle development and performance. Further research is needed to ascertain in humans the role of batokines, which have been identified mostly in experimental models. The endocrine role of BAT may explain the association between active BAT and a healthy metabolism in the human system, which is characterized by small amounts of BAT and a likely moderate BAT-mediated energy expenditure.

Emerging Roles for Browning of White Adipose Tissue in Prostate Cancer Malignant Behaviour

In addition to its well-known role as an energy repository, adipose tissue is one of the largest endocrine organs in the organism due to its ability to synthesize and release different bioactive molecules. Two main types of adipose tissue have been described, namely white adipose tissue (WAT) with a classical energy storage function, and brown adipose tissue (BAT) with thermogenic activity. The prostate, an exocrine gland present in the reproductive system of most mammals, is surrounded by periprostatic adipose tissue (PPAT) that contributes to maintaining glandular homeostasis in conjunction with other cell types of the microenvironment. In pathological conditions such as the development and progression of prostate cancer, adipose tissue plays a key role through paracrine and endocrine signaling. In this context, the role of WAT has been thoroughly studied. However, the influence of BAT on prostate tumor development and progression is unclear and has received much less attention. This review tries to bring an update on the role of different factors released by WAT which may participate in the initiation, progression and metastasis, as well as to compile the available information on BAT to discuss and open a new field of knowledge about the possible protective role of BAT in prostate cancer.

Comprehensive proteomics and functional annotation of mouse brown adipose tissue

Knowledge about the mouse brown adipose tissue (BAT) proteome can provide a deeper understanding of the function of mammalian BAT. Herein, a comprehensive analysis of interscapular BAT from C57BL/6J female mice was conducted by 2DLC and high-resolution mass spectrometry to construct a comprehensive proteome dataset of mouse BAT proteins. A total of 4949 nonredundant proteins were identified, and 4495 were quantified using the iBAQ method. According to the iBAQ values, the BAT proteome was divided into high-, middle- and low-abundance proteins. The functions of the high-abundance proteins were mainly related to glucose and fatty acid oxidation to produce heat for thermoregulation, while the functions of the middle- and low-abundance proteins were mainly related to protein synthesis and apoptosis, respectively. Additionally, 497 proteins were predicted to have signal peptides using SignalP4 software, and 75 were confirmed in previous studies. This study, for the first time, comprehensively profiled and functionally annotated the BAT proteome. This study will be helpful for future studies focused on biomarker identification and BAT molecular mechanisms.

Imaging adipose tissue browning using the TSPO-18kDa tracer [18F]FEPPA

Objectives

The browning of white adipose tissue (WAT) into beige has been proposed as a strategy to enhance energy expenditure to combat the growing epidemic of obesity. Research into browning strategies are hampered by the lack of sensitive, translatable, imaging tools capable of detecting beige fat mass non-invasively. [¹⁸F]FDG is able to detect activated beige fat but provides little information on unstimulated beige fat mass. We have assessed the use of [¹⁸F]FEPPA, a tracer for the TSPO-18KDa found on the outer mitochondrial membrane, as an alternative imaging agent capable of detecting unstimulated brown fat (BAT) and beige fat.

Methods

Female Balb/c mice (n = 5) were treated for 7 days with the β3 adrenergic agonist CL-316,243 to induce the browning of inguinal WAT (beige fat). Animals were imaged longitudinally with [¹⁸F]FDG and [¹⁸F]FEPPA and uptake in interscapular BAT and inguinal WAT assessed. The browning of inguinal WAT was confirmed using H&E and immunohistochemical detection of UCP-1 and TSPO.

Results

Repeated dosing with β3-adrenergic agonist CL-316,243 caused a significant increase in [¹⁸F]FDG uptake in both interscapular BAT and inguinal WAT associated with the increased metabolic activity of brown and beige adipocytes respectively. [¹⁸F]FEPPA uptake was likewise increased in inguinal WAT but showed no increase in BAT uptake due to stimulation over the same time course. Furthermore, inguinal WAT uptake was unaffected by pharmacological blockade, indicating that [¹⁸F]FEPPA uptake is associated with the expression of mitochondria in BAT and beige adipocytes and independent of activation.

Conclusion

These data show that [¹⁸F]FEPPA can detect BAT and newly formed beige fat under non-stimulated, thermoneutral conditions and that uptake after stimulation is linked to mitochondrial expression as opposed to activation.

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TSPO-18kDa tracers can detect BAT under non-stimulated, thermoneutral conditions.

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TSPO-18kDa tracers can detect the formation of beige adipocytes in white adipose tissue.

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TSPO-18kDa tracers may aid in the development of new approaches to treat obesity.

Fluorescence Imaging of Interscapular Brown Adipose Tissue in Living Mice

Brown adipose tissue (BAT) plays a key role in energy expenditure and heat generation and is a promising target for diagnosing and treating obesity, diabetes and related metabolism disorders. While several nuclear and magnetic resonance imaging methods are established for detecting human BAT, there are no convenient protocols for high throughput imaging of BAT in small animal models. Here we disclose a simple but effective method for non-invasive optical imaging of interscapular BAT in mice using a micellar formulation of the commercially available deep-red fluorescent probe, SRFluor680. Whole-body fluorescence imaging of living mice shows extensive accumulation of the fluorescent probe in the interscapular BAT and ex vivo analysis shows 3.5-fold selectivity for interscapular BAT over interscapular WAT. Additional imaging studies indicate that SRFluor680 uptake is independent of mouse species and BAT metabolic state. The results are consistent with an unusual pharmacokinetic process that involves irreversible translocation of the lipophilic SRFluor680 from the micelle nanocarrier into the adipocytes within the BAT. Multimodal PET/CT and planar fluorescence/X-ray imaging of the same living animal shows co-localization of BAT mass signal reported by the fluorescent probe and BAT metabolism signal reported by the PET agent, ¹⁸F-FDG. The results indicate a path towards a new, dual probe molecular imaging paradigm that allows separate and independent non-invasive visualization of BAT mass and BAT metabolism in a living subject.

BMP7 drives human adipogenic stem cells into metabolically active beige adipocytes

Adult humans have a substantial amount of inducible-brown (or beige) fat, which is associated with increased energy expenditure and reduced weight gain via thermogenesis. Despite the identification of key regulators of beige adipogenesis, impacts of dietary factors on adaptive thermogenesis are largely unknown, partly due to a lack of validated human cell models. Bone morphogenetic protein 7 (BMP7) is known to promote brown adipogenesis in rodent and human progenitor cells. However, controversy still surrounds the cellular identity in BMP7-mediated transition of white to brown adipocytes. The aim of this study was to confirm BMP7-derived human adipocytes as a relevant in vitro model of human beige adipocyte by verifying the cellular lineage and metabolic activity. In this study, we hypothesized that pre-exposure of the stromal vascular (SV) fraction of primary human adipogenic precursor cells (hASC) to BMP7 would convert metabolically active brown adipocytes. Our results showed that exposure of hASC to human BMP7 was associated with significant escalation of (1) UCP1 gene expression, a signature gene of brown adipocytes, (2) beige specific marker gene expression (i.e., CD137 and TMEM26), (3) glucose and fatty acid uptake, and (4) basal and cAMP-stimulated oxygen consumption rate compared to white adipocyte control. Taken together, we demonstrated that BMP7 mediates conversion of hASC into metabolically active beige adipocytes. By confirming the cellular identity and metabolic activity, this BMP7-induced human beige adipocytes from hASC should aid in the discovery and assessment of bioactive molecules to promote adaptive thermogenesis.

Metabolically active human brown adipose tissue derived stem cells

Brown adipose tissue (BAT) plays a key role in the evolutionarily conserved mechanisms underlying energy homeostasis in mammals. It is characterized by fat vacuoles 5-10 µm in diameter and expression of uncoupling protein one, central to the regulation of thermogenesis. In the human newborn, BAT depots are typically grouped around the vasculature and solid organs. These depots maintain body temperature during cold exposure by warming the blood before its distribution to the periphery. They also ensure an optimal temperature for biochemical reactions within solid organs. BAT had been thought to involute throughout childhood and adolescence. Recent studies, however, have confirmed the presence of active BAT in adult humans with depots residing in cervical, supraclavicular, mediastinal, paravertebral, and suprarenal regions. While human pluripotent stem cells have been differentiated into functional brown adipocytes in vitro and brown adipocyte progenitor cells have been identified in murine skeletal muscle and white adipose tissue, multipotent metabolically active BAT-derived stem cells from a single depot have not been identified in adult humans to date. Here, we demonstrate a clonogenic population of metabolically active BAT stem cells residing in adult humans that can: (a) be expanded in vitro; (b) exhibit multilineage differentiation potential; and (c) functionally differentiate into metabolically active brown adipocytes. Our study defines a new target stem cell population that can be activated to restore energy homeostasis in vivo for the treatment of obesity and related metabolic disorders.