The vascular endothelium of the adipose tissue gives rise to both white and brown fat cells

Adipose tissue expansion involves the enlargement of existing adipocytes, the formation of new cells from committed preadipocytes, and the coordinated development of the tissue vascular network. Here we find that murine endothelial cells (ECs) of classic white and brown fat depots share ultrastructural characteristics with pericytes, which are pluripotent and can potentially give rise to preadipocytes. Lineage tracing experiments using the VE-cadherin promoter reveal localization of reporter genes in ECs and also in preadipocytes and adipocytes of white and brown fat depots. Furthermore, capillary sprouts from human adipose tissue, which have predominantly EC characteristics, are found to express Zfp423, a recently identified marker of preadipocyte determination. In response to PPARγ activation, endothelial characteristics of sprouting cells are progressively lost, and cells form structurally and biochemically defined adipocytes. Together these data support an endothelial origin of murine and human adipocytes, suggesting a model for how adipogenesis and angiogenesis are coordinated during adipose tissue expansion.     

Cellular changes during cold acclimatation in adipose tissues

Cold exposure is a well-known physiological stimulus that activates the sympathetic nervous system and induces brown adipose tissue (BAT) hyperplasia. The effects of cold exposure or cold acclimatation have been extensively studied in interscapular BAT (IBAT). However, it has been recently shown that brown adipocytes are present in adipose deposits considered as white fat such as periovarian (PO) fat pad. We have investigated the kinetic of brown precursor recruitment in adipose tissues using DNA measurement and specific marker expression. In IBAT, cold exposure induces proliferation of precursor cells and differentiation into preadipocytes characterized by the expression of A2COL6, a marker specific to early steps of the differentiation process. A chronic stimulation of the tissue is necessary to observe the full effect. In PO fat pad, no proliferation can be detected, whereas differentiation of brown preadipocytes and maybe phenotypic conversion of white adipocytes seems to be promoted. In conclusion, these data demonstrated that 1) the same stimulus (cold exposure) does not induce the same response in terms of preadipocyte proliferation and differentiation in periovarian and brown adipose tissues, although both contain brown adipocytes, and 2) preadipocyte recruitment in adipose tissues after cold exposure depends on the predominant type of fat cells. © 1996 Wiley-Liss, Inc.     

Development of brown fat cells in monolayer culture. II. Ultrastructural characterization of precursors, differentiating adipocytes and their mitochondria

The stroma of mature brown fat has been shown to contain cells which can proliferate and accumulate fat in monolayer cultures, and which have inherent characteristics distinct from those of white fat precursor cells. The purpose of the present investigation was to characterize by electron microscopic analysis these brown fat cells and their subsequent development when they were grown in vitro. By comparison with the existing ultrastructural data on brown fat in situ, it could thus be determined whether or not the precursor cells have the capacity to differentiate in culture. The stromal-vascular fraction isolated from the brown fat of weaned rats was identified as containing adipocyte stem cells, preadipocytes, endothelial cells and a few mature adipocytes. During the first week in culture (i.e., growth phase to confluence), when multilocular fat accumulation occurred, the mitochondria of the preadipocytes developed cristae and matrix granules, as they do in differentiating brown fat in situ. Such granules have been shown to be a sign of intense inner membrane synthetic activity. After confluence, the mitochondria regressed in internal structure and became morphologically more similar to white fat mitochondria. It was concluded that mature brown fat contains precursor cells which can differentiate in vitro. However, this differentiation was incomplete, and the necessity of specific factors for a full mitochondrial development in brown fat is discussed.     

Conditionally immortalized white preadipocytes: a novel adipocyte model

This study describes a novel approach to generate conditionally immortalized preadipocyte cell lines from white adipose tissue (IMWAT) that can be induced to differentiate into white adipocytes even after expansion in culture. Such adipocytes express markers of white fat such as peroxisome proliferator-activated receptor gamma and aP2 but not brown fat markers, have an intact insulin signaling pathway, and express proinflammatory cytokines. They can be readily transduced with adenoviral vectors, allowing them to be used to investigate the consequences of the depletion of specific adipocyte factors using short hairpin RNA. This approach has been used to study the effect of reduced expression of the nuclear receptor corepressor receptor interacting protein 140 (RIP140), a regulator of adipocyte function. The depletion of RIP140 results in changes in metabolic gene expression that resemble those in adipose tissue of the RIP140 null mouse. Thus, IMWAT cells provide a novel model for adipocytes that are derived from preadipocytes rather than fibroblasts and provide an alternative system to primary preadipocytes for the investigation of adipocyte function.     

Differentiation of ovine brown adipocyte precursor cells in a chemically defined serum-free medium. Importance of glucocorticoids and age of animals

The rapid apparent conversion of brown adipose tissue into white adipose tissue in newborn offspring of large mammals, such as sheep and cattle is not explained at the cellular level. To study the differentiation of lamb brown adipocyte, a genomic fragment corresponding to the uncoupling protein was cloned from an ovine DNA library. Stromal vascular fibroblasts isolated from the perirenal adipose tissue of newborn lambs completely differentiated into brown adipocytes expressing the uncoupling protein gene, in a chemically defined serum-free medium. Dexamethasone was necessary for the expression of the uncoupling protein gene. When stromal vascular fibroblasts were isolated from 3-week-old lambs, the glucocorticoid analog still promoted in vitro differentiation of adipocytes. However those adipocytes were unable to express uncoupling mRNA and could be considered as white adipocytes. The data indicate that dexamethasone is necessary but not sufficient clone for the complete differentiation of brown adipocytes, and that the preadipocytes are committed to differentiation into brown or white adipocytes before culture.     

Tejido adiposo: heterogeneidad celular y diversidad funcional

There are two types of adipose tissue in the body whose function appears to be clearly differentiated. White adipose tissue stores energy reserves as fat, whereas the metabolic function of brown adipose tissue is lipid oxidation to produce heat. A good balance between them is important to maintain energy homeostasis. The concept of white adipose tissue has radically changed in the past decades, and is now considered as an endocrine organ that secretes many factors with autocrine, paracrine, and endocrine functions. In addition, we can no longer consider white adipose tissue as a single tissue, because it shows different metabolic profiles in its different locations, with also different implications. Although the characteristic cell of adipose tissue is the adipocyte, this is not the only cell type present in adipose tissue, neither the most abundant. Other cell types in adipose tissue described include stem cells, preadipocytes, macrophages, neutrophils, lymphocytes, and endothelial cells. The balance between these different cell types and their expression profile is closely related to maintenance of energy homeostasis. Increases in adipocyte size, number and type of lymphocytes, and infiltrated macrophages are closely related to the metabolic syndrome diseases. The study of regulation of proliferation and differentiation of preadipocytes and stem cells, and understanding of the interrelationship between the different cell types will provide new targets for action against these diseases.     

Exercise ameliorates high-fat diet-induced metabolic and vascular dysfunction, and increases adipocyte progenitor cell population in brown adipose tissue

A high-fat diet (HFD) is associated with adipose inflammation, which contributes to key components of metabolic syndrome, including obesity and insulin resistance. The increased visceral adipose tissue mass associated with obesity is the result of hyperplasia and hypertrophy of adipocytes. To investigate the effects of exercise on HFD-induced metabolic disorders, male C57BL/6 mice were divided into four groups: SED (sedentary)-ND (normal diet), EX (exercise)-ND, SED-HFD, and EX-HFD. Exercise was performed on a motorized treadmill at 15 m/min, 40 min/day, and 5 day/wk for 8 wk. Exercise resulted in a decrease in abdominal fat contents and inflammation, improvements in glucose tolerance and insulin resistance, and enhancement of vascular constriction and relaxation responses. Exercise with or without HFD increased putative brown adipocyte progenitor cells in brown adipose tissue compared with groups with the same diet, with an increase in brown adipocyte-specific gene expression in brown and white adipose tissue. Exercise training enhanced in vitro differentiation of the preadipocytes from brown adipose depots into brown adipocytes and enhanced the expression of uncoupling protein 1. These findings suggest that exercise ameliorates high-fat diet-induced metabolic disorders and vascular dysfunction, and increases adipose progenitor cell population in brown adipose tissue, which might thereby contribute to enhanced functional brown adipose.     

Identification of depot-specific human fat cell progenitors through distinct expression profiles and developmental gene patterns

Anatomically separate fat depots differ in size, function, and contribution to pathological states, such as the metabolic syndrome. We isolated preadipocytes from different human fat depots to determine whether the basis for this variation is partly attributable to differences in inherent properties of fat cell progenitors. We found that genome-wide expression profiles of primary preadipocytes cultured in parallel from abdominal subcutaneous, mesenteric, and omental fat depots were distinct. Interestingly, visceral fat was not homogeneous. Preadipocytes from one of the two main visceral depots, mesenteric fat, had an expression profile closer to that of subcutaneous than omental preadipocytes, the other main visceral depot. Expression of genes that regulate early development, including homeotic genes, differed extensively among undifferentiated preadipocytes isolated from different fat depots. These profiles were confirmed by real-time PCR analysis of preadipocytes from additional lean and obese male and female subjects. We made preadipocyte strains from single abdominal subcutaneous and omental preadipocytes by expressing telomerase. Depot-specific developmental gene expression profiles persisted for 40 population doublings in these strains. Thus, human fat cell progenitors from different regions are effectively distinct, consistent with different fat depots being separate mini-organs.     

Beta 3-adrenoceptor knockout in C57BL/6J mice depresses the occurrence of brown adipocytes in white fat.

White and brown adipocytes are usually located in distinct depots; however, in response to cold, brown adipocytes appear in white fat. This response is mediated by beta-adrenoceptors but there is a controversy about the subtype(s) involved. In the present study, we exposed to cold beta 3-adrenoceptor knockout mice (beta 3KO) on a C57BL/6J genetic background and measured in white adipose tissue the density of multilocular cells and the expression of the brown adipocyte marker uncoupling protein-1 (UCP1). In brown fat of beta 3KO mice, UCP1 expression levels were normal at 24 degrees C as well as after a 10-day cold exposure. Strikingly, under both conditions, in the white fat of beta 3KO mice the levels of UCP1 mRNA and protein as well as the density of multilocular cells were decreased. These results indicate that beta 3-adrenoceptors play a major role in the appearance of brown adipocytes in white fat and suggest that the brown adipocytes present in white fat differ from those in brown fat.     

过表达鸡Gata2或Gata3基因抑制Pparγ基因的转录

脂肪细胞分化是脂肪组织发育的一个重要过程. 目前,鸡脂肪细胞分化的分子调控机制还不十分清楚. 哺乳动物的研究结果表明,转录因子GATA结合蛋白2(Gata2)和GATA结合蛋白3(Gata3)具有抑制脂肪细胞分化的功能,它们在白色脂肪组织和棕色脂肪组织中的表达模式不同. 鸡没有棕色脂肪组织,目前还没有关于Gata2 和Gata3作用于鸡脂肪细胞分化的研究报道. 本研究利用半定量RT PCR的方法分析了Gata2和Gata3基因在鸡腹部脂肪组织和前脂肪细胞中的表达规律,发现鸡腹部脂肪组织中高水平表达Gata2 基因,低水平表达Gata3基因|鸡前脂肪细胞中Gata2 基因的表达水平远高于Gata3基因的表达水平,油酸诱导分化后的鸡前脂肪细胞Gata2基因的表达水平明显下调.此外,鸡过氧化物酶体增殖体激活受体γ(Pparγ)启动子（－1985/－89）报告基因荧光素活性分析和半定量RT PCP发现,在DF1细胞中过表达Gata2 或Gata3抑制鸡PPARγ基因的转录. 本研究结果为进一步研究鸡脂肪细胞分化的分子调控机制和Gata2和Gata3基因的生物学功能提供了参考.     

Beige adipocytes are a distinct type of thermogenic fat cell in mouse and human

Brown fat generates heat via the mitochondrial uncoupling protein UCP1, defending against hypothermia and obesity. Recent data suggest that there are two distinct types of brown fat: classical brown fat derived from a myf-5 cellular lineage and UCP1-positive cells that emerge in white fat from a non-myf-5 lineage. Here, we report the isolation of "beige" cells from murine white fat depots. Beige cells resemble white fat cells in having extremely low basal expression of UCP1, but, like classical brown fat, they respond to cyclic AMP stimulation with high UCP1 expression and respiration rates. Beige cells have a gene expression pattern distinct from either white or brown fat and are preferentially sensitive to the polypeptide hormone irisin. Finally, we provide evidence that previously identified brown fat deposits in adult humans are composed of beige adipocytes. These data provide a foundation for studying this mammalian cell type with therapeutic potential. PAPERCLIP:     

Measuring committed preadipocytes in human adipose tissue from severely obese patients by using adipocyte fatty acid binding protein

To understand the significance of the reported depot differences in preadipocyte dynamics, we developed a procedure to identify committed preadipocytes in the stromovascular fraction of fresh human adipose tissue. We documented that adipocyte fatty acid binding protein (aP2) is expressed in human preadipocyte clones capable of replication, indicating that can be used as a marker of committed preadipocytes. Because aP2 expression can be induced in macrophages, stromovascular cells were also stained for the macrophage marker CD68. We found aP2+CD68- cells (designated as committed preadipocytes) that did not have lipid droplets (true preadipocytes) and that did have lipid droplets < 6.5 microm in diameter (very immature adipocytes). Adipose tissue from subcutaneous, omental, and mesenteric depots was obtained from nine patients undergoing bariatric surgery for measurement of stromovascular cell number, the number of committed preadipocytes (aP2+CD68-), aP2+ macrophages (aP2+CD68+), and aP2- macrophages (aP2-CD68+). The number of committed preadipocytes did not differ significantly between depots but varied >20-fold among individuals. Total cell number, stromovascular cell number, and the number of aP2- macrophages was less (P < 0.05) in subcutaneous than in omental fat (means +/- SE, in millions: subcutaneous, 2.3 +/- 0.3, 1.4 +/- 0.3, and 0.17 +/- 0.08; and omental, 4.8 +/- 0.7, 3.8 +/- 0.5, and 0.34 +/- 0.06); mesenteric depot was intermediate. These data indicate that the cellular composition of adipose tissue varies between depots and between individuals. The ability to quantify committed preadipocytes in fresh adipose tissue should facilitate study of adipose tissue biology.     

Irisin influences the in vitro differentiation of human mesenchymal stromal cells,promoting a tendency toward beiging adipogenesis

Mammals exhibit two distinct types of adipose depots: white adipose tissue (WAT) and brown adipose tissue (BAT). While WAT primarily functions as a site for energy storage, BAT serves as a thermogenic tissue that utilizes energy and glucose consumption to regulate core body temperature. Under specific stimuli such as exercise, cold exposure, and drug treatment, white adipocytes possess a remarkable ability to undergo transdifferentiation into brown-like cells known as beige adipocytes. This transformation process, known as the “browning of WAT,” leads to the acquisition of new morphological and physiological characteristics by white adipocytes. We investigated the potential role of Irisin, a 12 kDa myokine that is secreted in mice and humans by skeletal muscle after physical activity, in inducing the browning process in mesenchymal stromal cells (MSCs). A subset of the MSCs possesses the remarkable capability to differentiate into different cell types such as adipocytes, osteocytes, and chondrocytes. Consequently, comprehending the effects of Irisin on MSC biology becomes a crucial factor in investigating antiobesity medications. In our study, the primary objective is to evaluate the impact of Irisin on various cell types engaged in distinct stages of the differentiation process, including stem cells, committed precursors, and preadipocytes. By analyzing the effects of Irisin on these specific cell populations, our aim is to gain a comprehensive understanding of its influence throughout the entire differentiation process, rather than solely concentrating on the final differentiated cells. This approach enables us to obtain insights into the broader effects of Irisin on the cellular dynamics and mechanisms involved in adipogenesis.     

Induction of uncoupling protein-2 (UCP2) gene expression on the differentiation of rat preadipocytes to adipocytes in primary culture

We have examined uncoupling protein-2 (UCP2) gene expression in the adipose tissue of obese and normal rats and mice, and also in differentiated rat adipocytes in primary culture. Expression of the UCP2 gene was examined in rat and mouse adipose tissues using both RT-PCR and Northern blotting. Although the RT-PCR was not quantitative, the band corresponding to the UCP2 mRNA was stronger in white adipose tissue than in brown fat, regardless of the body weight of the rats. In agreement with the RT-PCR data, there was a higher level of UCP2 mRNA in the white adipocytes than in brown adipocytes, the level being greater in obese mice. Fibroblastic preadipocytes were obtained from the inguinal fat pad of suckling rats. Lipid droplets developed inside the cells upon differentiation and adipsin and UCP2 mRNAs were detected by Northern blotting. Both mRNAs were evident in the adipocytes at 4, 6, and 10 d after the induction of differentiation. There was no indication that the expression of UCP2 was markedly affected by the addition of leptin, dexamethasone or isoprenaline.     

Invited review: Pre- and postnatal adipose tissue development in farm animals: from stem cells to adipocyte physiology

Both white and brown adipose tissues are recognized to be differently involved in energy metabolism and are also able to secrete a variety of factors called adipokines that are involved in a wide range of physiological and metabolic functions. Brown adipose tissue is predominant around birth, except in pigs. Irrespective of species, white adipose tissue has a large capacity to expand postnatally and is able to adapt to a variety of factors. The aim of this review is to update the cellular and molecular mechanisms associated with pre- and postnatal adipose tissue development with a special focus on pigs and ruminants. In contrast to other tissues, the embryonic origin of adipose cells remains the subject of debate. Adipose cells arise from the recruitment of specific multipotent stem cells/progenitors named adipose tissue-derived stromal cells. Recent studies have highlighted the existence of a variety of those cells being able to differentiate into white, brown or brown-like/beige adipocytes. After commitment to the adipocyte lineage, progenitors undergo large changes in the expression of many genes involved in cell cycle arrest, lipid accumulation and secretory functions. Early nutrition can affect these processes during fetal and perinatal periods and can also influence or pre-determinate later growth of adipose tissue. How these changes may be related to adipose tissue functional maturity around birth and can influence newborn survival is discussed. Altogether, a better knowledge of fetal and postnatal adipose tissue development is important for various aspects of animal production, including neonatal survival, postnatal growth efficiency and health.     

Ontogenesis of muscle and adipose tissues and their interactions in ruminants and other species

The lean-to-fat ratio, that is, the relative masses of muscle and adipose tissue, is a criterion for the yield and quality of bovine carcasses and meat. This review describes the interactions between muscle and adipose tissue (AT) that may regulate the dynamic balance between the number and size of muscle v. adipose cells. Muscle and adipose tissue in cattle grow by an increase in the number of cells (hyperplasia), mainly during foetal life. The total number of muscle fibres is set by the end of the second trimester of gestation. By contrast, the number of adipocytes is never set. Number of adipocytes increases mainly before birth until 1 year of age, depending on the anatomical location of the adipose tissue. Hyperplasia concerns brown pre-adipocytes during foetal life and white pre-adipocytes from a few weeks after birth. A decrease in the number of secondary myofibres and an increase in adiposity in lambs born from mothers severely underfed during early pregnancy suggest a balance in the commitment of a common progenitor into the myogenic or adipogenic lineages, or a reciprocal regulation of the commitment of two distinct progenitors. The developmental origin of white adipocytes is a subject of debate. Molecular and histological data suggested a possible transdifferentiation of brown into white adipocytes, but this hypothesis has now been challenged by the characterization of distinct precursor cells for brown and white adipocytes in mice. Increased nutrient storage in fully differentiated muscle fibres and adipocytes, resulting in cell enlargement (hypertrophy), is thought to be the main mechanism, whereby muscle and fat masses increase in growing cattle. Competition or prioritization between adipose and muscle cells for the uptake and metabolism of nutrients is suggested, besides the successive waves of growth of muscle v. adipose tissue, by the inhibited or delayed adipose tissue growth in bovine genotypes exhibiting strong muscular development. This competition or prioritization occurs through cellular signalling pathways and the secretion of proteins by adipose tissue (adipokines) and muscle (myokines), putatively regulating their hypertrophy in a reciprocal manner. Further work on the mechanisms underlying cross-talk between brown or white adipocytes and muscle fibres will help to achieve better understanding as a prerequisite to improving the control of body growth and composition in cattle.     

Exploring the activated adipogenic niche: Interactions of macrophages and adipocyte progenitors

Adult adipose tissue contains a large supply of progenitors that can renew fat cells for homeostatic tissue maintenance and adaptive growth or regeneration in response to external challenges. However, the in vivo mechanisms that control adipocyte progenitor behavior are poorly characterized. We recently demonstrated that recruitment of adipocyte progenitors by macrophages is a central feature of adipose tissue remodeling under various adipogenic conditions. Catabolic remodeling of white adipose tissue by β3-adrenergic receptor stimulation requires anti-inflammatory M2-polarized macrophages to clear dying adipocytes and to recruit new brown adipocytes from progenitors. In this Extra Views article, we discuss in greater detail the cellular elements of adipogenic niches and report a strategy to isolate and characterize the subpopulations of macrophages and adipocyte progenitors that actively participate in adrenergic tissue remodeling. Further characterization of these subpopulations may facilitate identification of new cellular targets to improve metabolic and immune function of adipose tissue.