Brown adipocytes postnatally arise through both differentiation from progenitors and conversion from white adipocytes in Syrian hamster

Regulation of mitochondrial metabolism by hibernating bear serum: Insights into seasonal metabolic adaptations

Hibernating animals undergo a unique and reversible decrease in their whole-body metabolism, which is often accompanied by a suppression of mitochondrial respiration. However, the precise mechanisms underlying these seasonal shifts in mitochondrial metabolism remain unclear. In this study, the effect of the serum from active and hibernating Japanese black bears on mitochondrial respiration was assessed. Stromal-vascular cells were obtained from bear white adipose tissue and cultured with or without an adipocyte differentiation cocktail. When the oxygen consumption was measured in the presence of bear serum, the hibernating bear serum reduced maximal respiration by 15.5 % (p < 0.05) and spare respiratory capacity by 46.0 % (p < 0.01) in the differentiated adipocytes in comparison to the active bear serum. Similar reductions of 23.4 % (p = 0.06) and 40.6 % (p < 0.05) respectively were observed in undifferentiated cells, indicating the effect is cell type-independent. Blue native PAGE analysis revealed that hibernating bear serum suppressed cellular metabolism independently of the assembly of mitochondrial respiratory chain complexes. RNA-seq analysis identified 1094 differentially expressed genes (fold change>1.5, FDR<0.05) related to insulin signaling and glucose metabolism pathways. These findings suggest that the rapid alterations in mitochondrial metabolism during hibernation are likely induced by a combination of reduced insulin signaling and suppressed mitochondrial function, rather than changes in respiratory complex assembly.

Adipocytes and Stromal Cells Regulate Brown Adipogenesis Through Secretory Factors During the Postnatal White-to-Brown Conversion of Adipose Tissue in Syrian Hamsters

Brown adipose tissue (BAT) is a specialized tissue that regulates non-shivering thermogenesis. In Syrian hamsters, interscapular adipose tissue is composed primarily of white adipocytes at birth, which is converted to BAT through the proliferation and differentiation of brown adipocyte progenitors and the simultaneous disappearance of white adipocytes. In this study, we investigated the regulatory mechanism of brown adipogenesis during postnatal BAT formation in hamsters. Interscapular adipose tissue of a 10-day-old hamster, which primarily consists of brown adipocyte progenitors and white adipocytes, was digested with collagenase and fractioned into stromal–vascular (SV) cells and white adipocytes. SV cells spontaneously differentiated into brown adipocytes that contained multilocular lipid droplets and expressed uncoupling protein 1 (Ucp1), a marker of brown adipocytes, without treatment of adipogenic cocktail such as dexamethasone and insulin. The spontaneous differentiation of SV cells was suppressed by co-culture with adipocytes or by the addition of white adipocyte-conditioned medium. Conversely, the addition of SV cell-conditioned medium increased the expression of Ucp1. These results indicate that adipocytes secrete factors that suppress brown adipogenesis, whereas SV cells secrete factors that promote brown adipogenesis. Transcriptome analysis was conducted; however, no candidate suppressing factors secreted from adipocytes were identified. In contrast, 19 genes that encode secretory factors, including bone morphogenetic protein (BMP) family members, BMP3B, BMP5, and BMP7, were highly expressed in SV cells compared with adipocytes. Furthermore, the SMAD and MAPK signaling pathways, which represent the major BMP signaling pathways, were activated in SV cells, suggesting that BMPs secreted from SV cells induce brown adipogenesis in an autocrine manner through the SMAD/MAPK signaling pathways. Treatment of 5-day-old hamsters with type I BMP receptor inhibitor, LDN-193189, for 5 days reduced p38 MAPK phosphorylation and drastically suppressed BAT formation of interscapular adipose tissue. In conclusion, adipocytes and stromal cells regulate brown adipogenesis through secretory factors during the postnatal white-to-brown conversion of adipose tissue in Syrian hamsters.

Lactate Fluxes and Plasticity of Adipose Tissues: A Redox Perspective

Lactate, a metabolite produced when the glycolytic flux exceeds mitochondrial oxidative capacities, is now viewed as a critical regulator of metabolism by acting as both a carbon and electron carrier and a signaling molecule between cells and tissues. In recent years, increasing evidence report its key role in white, beige, and brown adipose tissue biology, and highlights new mechanisms by which lactate participates in the maintenance of whole-body energy homeostasis. Lactate displays a wide range of biological effects in adipose cells not only through its binding to the membrane receptor but also through its transport and the subsequent effect on intracellular metabolism notably on redox balance. This study explores how lactate regulates adipocyte metabolism and plasticity by balancing intracellular redox state and by regulating specific signaling pathways. We also emphasized the contribution of adipose tissues to the regulation of systemic lactate metabolism, their roles in redox homeostasis, and related putative physiopathological repercussions associated with their decline in metabolic diseases and aging.

Changes in liver microRNA expression and their possible regulatory role in energy metabolism-related genes in hibernating black bears

Hibernating bears survive up to 6 months without feeding while yet maintaining metabolic homeostasis. We previously reported expression changes in energy metabolism-related genes in the liver of hibernating Japanese black bears. The present study examined the role of microRNAs in the regulation of hepatic gene expression during hibernation. The quantitative analyses revealed significant increases in the expression of 4 microRNAs (miR-221-3p, miR-222-3p, miR-455-3p, and miR-195a-5p) and decreases of 2 microRNAs (miR-122-5p and miR-7a-1-5p) during hibernation. RNA sequencing and in silico target prediction regarding 3 upregulated microRNAs (miR-221-3p, miR-222-3p and miR-455-3p) found 13 target mRNAs with significantly decreased expression during hibernation. The transfection of microRNA mimics into cells showed that miR-222 and miR-455 reduced solute carrier family 16 member 4 (SLC16A4) and fatty acid synthase (FASN) mRNA expression, respectively. Our results suggest that the increased levels of hepatic miRNA during hibernation (miR-222-3p and miR-455-3p) negatively regulate the expression of targeted genes predicted to be involved in the transport of energy source and de novo fatty acid synthesis, is consistent with a regulatory role of these miRNAs in energy metabolism in hibernating black bears.

Lactate fluxes mediated by the monocarboxylate transporter-1 are key determinants of the metabolic activity of beige adipocytes

Activation of energy-dissipating brown/beige adipocytes represents an attractive therapeutic strategy against metabolic disorders. While lactate is known to induce beiging through the regulation of Ucp1 gene expression, the role of lactate transporters on beige adipocytes' ongoing metabolic activity remains poorly understood. To explore the function of the lactate-transporting monocarboxylate transporters (MCTs), we used a combination of primary cell culture studies, 13C isotopic tracing, laser microdissection experiments, and in situ immunofluorescence of murine adipose fat pads. Dissecting white adipose tissue heterogeneity revealed that the MCT1 is expressed in inducible beige adipocytes as the emergence of uncoupling protein 1 after cold exposure was restricted to a subpopulation of MCT1-expressing adipocytes suggesting MCT1 as a marker of inducible beige adipocytes. We also observed that MCT1 mediates bidirectional and simultaneous inward and outward lactate fluxes, which were required for efficient utilization of glucose by beige adipocytes activated by the canonical β3-adrenergic signaling pathway. Finally, we demonstrated that significant lactate import through MCT1 occurs even when glucose is not limiting, which feeds the oxidative metabolism of beige adipocytes. These data highlight the key role of lactate fluxes in finely tuning the metabolic activity of beige adipocytes according to extracellular metabolic conditions and reinforce the emerging role of lactate metabolism in the control of energy homeostasis.

Hibernating bear serum hinders osteoclastogenesis in-vitro

Bears do not suffer from osteoporosis during hibernation, which is associated with long-term inactivity, lack of food intake, and cold exposure. However, the mechanisms involved in bone loss prevention have scarcely been elucidated in bears. We investigated the effect of serum from hibernating Japanese black bears (Ursus thibetanus japonicus) on differentiation of peripheral blood mononuclear cells (PBMCs) to osteoclasts (OCs). PBMCs collected from 3 bears were separately cultured with 10% serum of 4 active and 4 hibernating bears (each individual serum type was assessed separately by a bear PBMCs), and differentiation were induced by treatment with macrophage colony stimulating factor (M-CSF) and receptor activator of NF-kB ligand (RANKL). PBMCs that were cultured with the active bear serum containing medium (ABSM) differentiated to multi-nucleated OCs, and were positive for TRAP stain. However, cells supplemented with hibernating bear serum containing medium (HBSM) failed to form OCs, and showed significantly lower TRAP stain (p < 0.001). On the other hand, HBSM induced proliferation of adipose derived mesenchymal stem cells (ADSCs) similarly to ABSM (p > 0.05), indicating no difference on cell growth. It was revealed that osteoclastogenesis of PBMCs is hindered by HBSM, implying an underlying mechanism for the suppressed bone resorption during hibernation in bears. In addition, this study for the first time showed the formation of bears’ OCs in-vitro.

The emerging roles of lactate as a redox substrate and signaling molecule in adipose tissues

Thermogenic (brown and beige) adipose tissues improve glucose and lipid homeostasis and therefore represent putative targets to cure obesity and related metabolic diseases including type II diabetes. Beside decades of research and the very well-described role of noradrenergic signaling, mechanisms underlying adipocytes plasticity and activation of thermogenic adipose tissues remain incompletely understood. Recent studies show that metabolites such as lactate control the oxidative capacity of thermogenic adipose tissues. Long time viewed as a metabolic waste product, lactate is now considered as an important metabolic substrate largely feeding the oxidative metabolism of many tissues, acting as a signaling molecule and as an inter-cellular and inter-tissular redox carrier. In this review, we provide an overview of the recent findings highlighting the importance of lactate in adipose tissues, from its production to its role as a browning inducer and its metabolic links with brown adipose tissue. We also discuss additional function(s) than thermogenesis ensured by brown and beige adipose tissues, i.e., their ability to dissipate high redox pressure and oxidative stress thanks to the activity of the uncoupling protein-1, helping to maintain tissue and whole organism redox homeostasis and integrity.

Role of brown adipose tissue in body temperature control during the early postnatal period in Syrian hamsters and mice

Brown adipose tissue (BAT) contributes to non-shivering thermogenesis and plays an important role in body temperature control. The contribution of BAT thermogenesis to body temperature control in a non-cold environment was evaluated using developing hamsters. Immunostaining for uncoupling protein 1 (UCP1), a mitochondrial protein responsible for BAT thermogenesis, indicated that interscapular fat tissue had matured as BAT at day 14. When pups were placed on a thermal plate kept at 23°C, the body surface temperature decreased in day 7- and 10-day-old pups but was maintained at least for 15 min in 14-day-old pups, indicating that hamsters are unable to maintain their body temperature until around day 14 even in a non-cold environment. Body temperature maintenance was also evaluated in UCP1-deficient mice. BAT analysis showed that the UCP1 protein level in Ucp1^+/− Hetero mice was 61.3 ± 1.4% of that in wild-type (WT) mice and was undetected in Ucp1^−/− knockout (KO) mice. When 12-day-old pups were place on a thermal plate at 23°C, body surface temperature was maintained for at least 15 min in WT and Hetero mice but gradually dropped by 2.4 ± 0.2°C in 15 min in KO mice. It is concluded that BAT thermogenesis is indispensable for body temperature maintenance in pups of hamsters and mice, even in the non-cold circumstances. The early life poikilothermy and the later acquirement of homeothermy in hamsters may be because of the postnatal development of BAT.

Brown and Brite: The Fat Soldiers in the Anti-obesity Fight

Brown adipose tissue (BAT) is proposed to maintain thermal homeostasis through dissipation of chemical energy as heat by the uncoupling proteins (UCPs) present in their mitochondria. The recent demonstration of the presence of BAT in humans has invigorated research in this area. The research has provided many new insights into the biology and functioning of this tissue and the biological implications of its altered activities. Another finding of interest is browning of white adipose tissue (WAT) resulting in what is known as beige/brite cells, which have increased mitochondrial proteins and UCPs. In general, it has been observed that the activation of BAT is associated with various physiological improvements such as a reduction in blood glucose levels increased resting energy expenditure and reduced weight. Given the similar physiological functions of BAT and beige/ brite cells and the higher mass of WAT compared to BAT, it is likely that increasing the brite/beige cells in WATs may also lead to greater metabolic benefits. However, development of treatments targeting brown fat or WAT browning would require not only a substantial understanding of the biology of these tissues but also the effect of altering their activity levels on whole body metabolism and physiology. In this review, we present evidence from recent literature on the substrates utilized by BAT, regulation of BAT activity and browning by circulating molecules. We also present dietary and pharmacological activators of brown and beige/brite adipose tissue and the effect of physical exercise on BAT activity and browning.

Effect of ambient temperature on the proliferation of brown adipocyte progenitors and endothelial cells during postnatal BAT development in Syrian hamsters

In Syrian hamsters, brown adipose tissue (BAT) develops postnatally through the proliferation and differentiation of brown adipocyte progenitors. In the study reported here, we investigated how ambient temperature influenced BAT formation in neonatal hamsters. In both hamsters raised at 23 or 30 °C, the interscapular fat changed from white to brown coloration in an age-dependent manner and acquired the typical morphological features of BAT by day 16. However, the expression of uncoupling protein 1, a brown adipocyte marker, and of vascular endothelial growth factor α were lower in the group raised at 30 °C than in that raised at 23 °C. Immunofluorescent staining revealed that the proportion of Ki67-expressing progenitors and endothelial cells was lower in the 30 °C group than in the 23 °C group. These results indicate that warm ambient temperature suppresses the proliferation of brown adipocyte progenitors and endothelial cells and negatively affects the postnatal development of BAT in Syrian hamsters.