The adipose organ: white-brown adipocyte plasticity and metabolic inflammation

RIP140 represses the "brown-in-white" adipocyte program including a futile cycle of triacylglycerol breakdown and synthesis

Receptor-interacting protein 140 (RIP140) is a corepressor of nuclear receptors that is highly expressed in adipose tissues. We investigated the role of RIP140 in conditionally immortal preadipocyte cell lines prepared from white or brown fat depots. In white adipocytes, a large set of brown fat-associated genes was up-regulated in the absence of RIP140. In contrast, a relatively minor role can be ascribed to RIP140 in the control of basal gene expression in differentiated brown adipocytes because significant changes were observed only in Ptgds and Fabp3. The minor role of RIP140 in brown adipocytes correlates with the similar histology and uncoupling protein 1 and CIDEA staining in knockout compared with wild-type brown adipose tissue (BAT). In contrast, RIP140 knockout sc white adipose tissue (WAT) shows increased numbers of multilocular adipocytes with elevated staining for uncoupling protein 1 and CIDEA. Furthermore in a white adipocyte cell line, the markers of BRITE adipocytes, Tbx1, CD137, Tmem26, Cited1, and Epsti1 were repressed in the presence of RIP140 as was Prdm16. Microarray analysis of wild-type and RIP140-knockout white fat revealed elevated expression of genes associated with cold-induced expression or high expression in BAT. A set of genes associated with a futile cycle of triacylglycerol breakdown and resynthesis and functional assays revealed that glycerol kinase and glycerol-3-phosphate dehydrogenase activity as well as [³H]glycerol incorporation were elevated in the absence of RIP140. Thus, RIP140 blocks the BRITE program in WAT, preventing the expression of brown fat genes and inhibiting a triacylglycerol futile cycle, with important implications for energy homeostasis.

Translational issues in targeting brown adipose tissue thermogenesis for human obesity management

The recent advancements in unraveling novel mechanisms that control the induction, (trans)differentiation, proliferation, and thermogenic activity and capacity of brown adipose tissue (BAT), together with the application of imaging techniques for human BAT visualization, have generated optimism that these advances will provide novel strategies for targeting BAT thermogenesis, leading to efficacious and safe obesity therapies. This paper first provides an overview of landmark events of the past few decades that have been driving the search for pharmaceutical and nutraceutical compounds that would increase BAT thermogenesis for obesity management. It then addresses issues about what could be expected from an ideal thermogenic antiobesity approach, in particular to what extent daily energy expenditure will need to increase in order to achieve long-term weight loss currently achievable only through bariatric surgery, and whether the human body will have enough thermogenic capacity to reach this target weight loss by future therapies focused on BAT.

Cellular reprogramming into a brown adipose tissue-like phenotype by co-expression of HB-EGF and ADAM 12S

Abnormal adipogenesis leads to excessive fat accumulation and several health disorders. Mouse fibroblasts (MLC) transfected with ADAM 12S and HB-EGF promoted lipid accumulation. Addition of KBR-7785, an ADAM 12S inhibitor, to HB-EGF/ADAM 12S expressing cells suppressed adipogenesis. BrdU incorporation was attenuated and enhanced mitotracker staining was observed in HB-EGF/ADAM 12S cells. Quantitative real time RT-PCR resulted in elevated levels of expression of three brown adipose tissue (BAT) genes (PRDM16, PGC-1α, and UCP-1), while expression levels of the three white adipose tissue (WAT) genes (PPARγ, C/EBPα, and AKT-1) were unaltered in HB-EGF/ADAM 12S cells. Amino- or carboxy-terminal deletions of HB-EGF (HB-EGFΔN and HB-EGFΔC) co-expressed with ADAM 12S stimulated lipid accumulation. Human epidermoid carcinoma cells (A431) also exhibited lipid accumulation by HB-EGF/ADAM 12S co-expression. These studies suggest ADAM 12S and HB-EGF are involved in cellular plasticity resulting in the production of BAT-like cells and offers insight into novel therapeutic approaches for fighting obesity.

BMP7 activates brown adipose tissue and reduces diet-induced obesity only at subthermoneutrality

Background/Aims

Brown adipose tissue (BAT) dissipates energy stored in triglycerides as heat via the uncoupling protein UCP-1 and is a promising target to combat hyperlipidemia and obesity. BAT is densely innervated by the sympathetic nervous system, which increases BAT differentiation and activity upon cold exposure. Recently, Bone Morphogenetic Protein 7 (BMP7) was identified as an inducer of BAT differentiation. We aimed to elucidate the role of sympathetic activation in the effect of BMP7 on BAT by treating mice with BMP7 at varying ambient temperature, and assessed the therapeutic potential of BMP7 in combating obesity.

Methods and Results

High-fat diet fed lean C57Bl6/J mice were treated with BMP7 via subcutaneous osmotic minipumps for 4 weeks at 21°C or 28°C, the latter being a thermoneutral temperature in which sympathetic activation of BAT is largely diminished. At 21°C, BMP7 increased BAT weight, increased the expression of Ucp1, Cd36 and hormone-sensitive lipase in BAT, and increased total energy expenditure. BMP7 treatment markedly increased food intake without affecting physical activity. Despite that, BMP7 diminished white adipose tissue (WAT) mass, accompanied by increased expression of genes related to intracellular lipolysis in WAT. All these effects were blunted at 28°C. Additionally, BMP7 resulted in extensive ‘browning’ of WAT, as evidenced by increased expression of BAT markers and the appearance of whole clusters of brown adipocytes via immunohistochemistry, independent of environmental temperature. Treatment of diet-induced obese C57Bl6/J mice with BMP7 led to an improved metabolic phenotype, consisting of a decreased fat mass and liver lipids as well as attenuated dyslipidemia and hyperglycemia.

Conclusion

Together, these data show that BMP7-mediated recruitment and activation of BAT only occurs at subthermoneutral temperature, and is thus likely dependent on sympathetic activation of BAT, and that BMP7 may be a promising tool to combat obesity and associated disorders.

Interleukin-6 and obesity: the crosstalk between intestine, pancreas and liver

PURPOSE OF REVIEW

The concept of IL-6 as a deleterious interleukin was challenged by its anti-inflammatory actions.

RECENT FINDINGS

The beneficial health effects of exercise and the crosstalk between insulin-sensitive tissues and insulin-producing cells are mediated by IL-6.

SUMMARY

IL-6 displays pleiotropic functions in a tissue-specific and physiological context-dependent manner. There is evidence suggesting that IL-6 worsens insulin resistance in the liver and adipose tissue, while improving insulin sensitivity in the muscle. The effects of this cytokine are influenced by its acute or chronical presence, the latter being associated with insulin resistance. IL-6 has anti-inflammatory effects and a compensatory role in obesity by increasing islet GLP-1 production. The therapeutic approach of blocking IL-6 signal can be diabetogenic.

Obese adipocytes show ultrastructural features of stressed cells and die of pyroptosis

We previously suggested that, in obese animals and humans, white adipose tissue inflammation results from the death of hypertrophic adipocytes; these are then cleared by macrophages, giving rise to distinctive structures we denominated crown-like structures. Here we present evidence that subcutaneous and visceral hypertrophic adipocytes of leptin-deficient (ob/ob and db/db) obese mice exhibit ultrastructural abnormalities (including calcium accumulation and cholesterol crystals), many of which are more common in hyperglycemic db/db versus normoglycemic ob/ob mice and in visceral versus subcutaneous depots. Degenerating adipocytes whose intracellular content disperses in the extracellular space were also noted in obese mice; in addition, increased anti-reactive oxygen species enzyme expression in obese fat pads, documented by RT-PCR and immunohistochemistry, suggests that ultrastructural changes are accompanied by oxidative stress. RT-PCR showed NLRP3 inflammasome activation in the fat pads of both leptin-deficient and high-fat diet obese mice, in which formation of active caspase-1 was documented by immunohistochemistry in the cytoplasm of several hypertrophic adipocytes. Notably, caspase-1 was not detected in FAT-ATTAC transgenic mice, where adipocytes die of apoptosis. Thus, white adipocyte overexpansion induces a stress state that ultimately leads to death. NLRP3-dependent caspase-1 activation in hypertrophic adipocytes likely induces obese adipocyte death by pyroptosis, a proinflammatory programmed cell death.

A novel automated image analysis method for accurate adipocyte quantification

Increased adipocyte size and number are associated with many of the adverse effects observed in metabolic disease states. While methods to quantify such changes in the adipocyte are of scientific and clinical interest, manual methods to determine adipocyte size are both laborious and intractable to large scale investigations. Moreover, existing computational methods are not fully automated. We, therefore, developed a novel automatic method to provide accurate measurements of the cross-sectional area of adipocytes in histological sections, allowing rapid high-throughput quantification of fat cell size and number. Photomicrographs of H&E-stained paraffin sections of murine gonadal adipose were transformed using standard image processing/analysis algorithms to reduce background and enhance edge-detection. This allowed the isolation of individual adipocytes from which their area could be calculated. Performance was compared with manual measurements made from the same images, in which adipocyte area was calculated from estimates of the major and minor axes of individual adipocytes. Both methods identified an increase in mean adipocyte size in a murine model of obesity, with good concordance, although the calculation used to identify cell area from manual measurements was found to consistently over-estimate cell size. Here we report an accurate method to determine adipocyte area in histological sections that provides a considerable time saving over manual methods.

Distinct populations of adipogenic and myogenic Myf5-lineage progenitors in white adipose tissues

Brown adipose tissues (BAT) are derived from a myogenic factor 5 (Myf5)-expressing cell lineage and white adipose tissues (WAT) predominantly arise from non-Myf5 lineages, although a subpopulation of adipocytes in some WAT depots can be derived from the Myf5 lineage. However, the functional implication of the Myf5- and non-Myf5-lineage cells in WAT is unclear. We found that the Myf5-lineage constitution in subcutaneous WAT depots is negatively correlated to the expression of classical BAT and newly defined beige/brite adipocyte-specific genes. Consistently, fluorescent-activated cell sorting (FACS)-purified Myf5-lineage adipo-progenitors give rise to adipocytes expressing lower levels of BAT-specific Ucp1, Prdm16, Cidea, and Ppargc1a genes and beige adipocyte-specific CD137, Tmem26, and Tbx1 genes compared with the non-Myf5-lineage adipocytes from the same depots. Ablation of the Myf5-lineage progenitors in WAT stromal vascular cell (SVC) cultures leads to increased expression of BAT and beige cell signature genes. Strikingly, the Myf5-lineage cells in WAT are heterogeneous and contain distinct adipogenic [stem cell antigen 1(Sca1)-positive] and myogenic (Sca1-negative) progenitors. The latter differentiate robustly into myofibers in vitro and in vivo, and they restore dystrophin expression after transplantation into mdx mouse, a model for Duchenne muscular dystrophy. These results demonstrate the heterogeneity and functional differences of the Myf5- and non-Myf5-lineage cells in the white adipose tissue.

Role of adipose specific lipid droplet proteins in maintaining whole body energy homeostasis

Excess or insufficient lipid storage in white adipose tissue lipid droplets is associated with dyslipidemia, insulin resistance and increased risk for diabetes type 2. Thus, maintenance of adipose lipid droplet growth and function is critical to preserve whole body insulin sensitivity and energy homeostasis. Progress in understanding biology of lipid droplets has underscored the role of proteins that interact with lipid droplets. Here, we review the current knowledge of adipose specific lipid droplet proteins, which share unique functions controlling adipocyte lipid storage, limiting lipid spill-over and lipotoxic effects thought to contribute to disease. This article is part of a Special Issue entitled: Modulation of Adipose Tissue in Health and Disease.

Endogenous Acute Phase Serum Amyloid A Lacks Pro-Inflammatory Activity, Contrasting the Two Recombinant Variants That Activate Human Neutrophils through Different Receptors

Most notable among the acute phase proteins is serum amyloid A (SAA), levels of which can increase 1000-fold during infections, aseptic inflammation, and/or trauma. Chronically elevated SAA levels are associated with a wide variety of pathological conditions, including obesity and rheumatic diseases. Using a recombinant hybrid of the two human SAA isoforms (SAA1 and 2) that does not exist in vivo, numerous in vitro studies have given rise to the notion that acute phase SAA is a pro-inflammatory molecule with cytokine-like properties. It is however unclear whether endogenous acute phase SAA per se mediates pro-inflammatory effects. We tested this in samples from patients with inflammatory arthritis and in a transgenic mouse model that expresses human SAA1. Endogenous human SAA did not drive production of pro-inflammatory IL-8/KC in either of these settings. Human neutrophils derived from arthritis patients displayed no signs of activation, despite being exposed to severely elevated SAA levels in circulation, and SAA-rich sera also failed to activate cells in vitro. In contrast, two recombinant SAA variants (the hybrid SAA and SAA1) both activated human neutrophils, inducing L-selectin shedding, production of reactive oxygen species, and production of IL-8. The hybrid SAA was approximately 100-fold more potent than recombinant SAA1. Recombinant hybrid SAA and SAA1 activated neutrophils through different receptors, with recombinant SAA1 being a ligand for formyl peptide receptor 2 (FPR2). We conclude that even though recombinant SAAs can be valuable tools for studying neutrophil activation, they do not reflect the nature of the endogenous protein.

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.

Molecular and functional characterization of human bone marrow adipocytes

Adipocytes are a cell population largely located in the human bone marrow cavity. In this specific microenvironment where adipocytes can interact with a variety of different cells, the role of fat is mainly unknown. To our knowledge, this report is the first to characterize mature adipocytes isolated from human bone marrow (BM-A) molecularly and functionally to better understand their roles into the hematopoietic microenvironment. Healthy BM-A were isolated after collagenase digestion and filtration. We studied the morphology of BM-A, their gene expression and immunophenotypic profile and their functional ability in the hematopoietic microenvironment, comparing them with adipocytes derived from adipose tissue (AT-A). BM-A showed a unilocular lipid morphology similar to AT-A and did not lose their morphology in culture; they showed a comparable pattern of stem cell-surface antigens to AT-A. In line with these observations, molecular data showed that BM-A expressed some embryonic stem cells genes, such as Oct4, KLf4, c-myc, Gata4, Tbx1, and Sox17, whereas they did not express the stem cell markers Sox2 and Nanog. Moreover, BM-A had long telomeres that were similar to bone marrow mesenchymal stem cells. Notably, BM-A supported the survival and differentiation of hematopoietic stem cells in long-term cultures. These results showed that BM-A are stromal cells with a gene expression pattern that distinguished them from AT-A. BM-A showed stem cell properties through their hematopoietic supporting function, which was certainly linked to their role in the maintenance of the bone marrow microenvironment. Depending on specific demands, BM-A may acquire different functions based on their local environment.

The circadian clock in cancer development and therapy

Most aspects of mammalian function display circadian rhythms driven by an endogenous clock. The circadian clock is operated by genes and comprises a central clock in the brain that responds to environmental cues and controls subordinate clocks in peripheral tissues via circadian output pathways. The central and peripheral clocks coordinately generate rhythmic gene expression in a tissue-specific manner in vivo to couple diverse physiological and behavioral processes to periodic changes in the environment. However, with the industrialization of the world, activities that disrupt endogenous homeostasis with external circadian cues have increased. This change in lifestyle has been linked to an increased risk of diseases in all aspects of human health, including cancer. Studies in humans and animal models have revealed that cancer development in vivo is closely associated with the loss of circadian homeostasis in energy balance, immune function, and aging, which are supported by cellular functions important for tumor suppression including cell proliferation, senescence, metabolism, and DNA damage response. The clock controls these cellular functions both locally in cells of peripheral tissues and at the organismal level via extracellular signaling. Thus, the hierarchical mammalian circadian clock provides a unique system to study carcinogenesis as a deregulated physiological process in vivo. The asynchrony between host and malignant tissues in cell proliferation and metabolism also provides new and exciting options for novel anticancer therapies.

Endocrine and metabolic signaling in retroperitoneal white adipose tissue remodeling during cold acclimation

The expression profiles of adiponectin, resistin, 5'-AMP-activated protein kinase α (AMPK α ), hypoxia-inducible factor-1 α (HIF-1 α ), and key enzymes of glucose and fatty acid metabolism and oxidative phosphorylation in rat retroperitoneal white adipose tissue (RpWAT) during 45-day cold acclimation were examined. After transient suppression on day 1, adiponectin protein level increased following sustained cold exposure. In parallel, on day 1, the protein level of HIF-1 α was strongly induced and AMPK α suppressed, while afterwards the reverse was seen. What is more, after an initial decrease on day 1, a sequential increase in pyruvate dehydrogenase, acyl-CoA dehydrogenase, cytochrome c oxidase, and ATP synthase and a decrease in acetyl-CoA carboxylase (from day 3) were observed. Similar to adiponectin, protein level of resistin showed a biphasic profile: it increased after days 1, 3, and 7 and decreased below the control after 21 days of cold-acclimation. In summary, the data suggest that adiponectin and resistin are important integrators of RpWAT metabolic response and roles it plays during cold acclimation. It seems that AMPK α mediate adiponectin effects on metabolic remodeling RpWAT during cold acclimation.