Adipocyte hypertrophy is associated with lysosomal permeability both in vivo and in vitro: role in adipose tissue inflammation

Obesity in both humans and rodents is characterized by adipocyte hypertrophy and the presence of death adipocytes surrounded by macrophages forming "crown-like structures." However, the biochemical pathways involved in triggering adipocyte death as well as the role of death adipocytes in adipose tissue remodeling and macrophage infiltration remain poorly understood. We now show that induction of adipocyte hypertrophy by incubation of mature adipocytes with saturated fatty acids results in lysosomal destabilization and cathepsin B (ctsb), a key lysosomal cysteine protease, activation and redistribution into the cytosol. ctsb activation was required for the lysosomal permeabilization, and its inhibition protected cells against mitochondrial dysfunction. With the use of a dietary murine model of obesity, ctsb activation was detected in adipose tissue of these mice. This is an early event during weight gain that correlates with the presence of death adipocytes, and precedes macrophage infiltration of adipose tissue. Moreover, ctsb-deficient mice showed decreased lysosomal permeabilization in adipocytes and were protected against adipocyte cell death and macrophage infiltration to adipose tissue independent of body weight. These data strongly suggest that ctsb activation and lysosomal permeabilization in adipocytes are key initial events that contribute to the adipocyte cell death and macrophage infiltration into adipose tissue associated with obesity. Inhibition of ctsb activation may be a new therapeutic strategy for the treatment of obesity-associated metabolic complications.     

Very Low Density Lipoprotein Receptor (VLDLR) Expression Is a Determinant Factor in Adipose Tissue Inflammation and Adipocyte-Macrophage Interaction

Obesity is associated with adipose tissue remodeling, characterized by adipocyte hypertrophy and macrophage infiltration. Previously, we have shown that very low density lipoprotein receptor (VLDLR) is virtually absent in preadipocytes but is strongly induced during adipogenesis and actively participates in adipocyte hypertrophy. In this study, we investigated the role of VLDLR in adipose tissue inflammation and adipocyte-macrophage interactions in wild type and VLDLR-deficient mice fed a high fat diet. The results show that VLDLR deficiency reduced high fat diet-induced inflammation and endoplasmic reticulum (ER) stress in adipose tissue in conjunction with reduced macrophage infiltration, especially those expressing pro-inflammatory markers. In adipocyte culture, VLDLR deficiency prevented adipocyte hypertrophy and strongly reduced VLDL-induced ER stress and inflammation. Likewise, cultures of primary peritoneal macrophages show that VLDLR deficiency reduced lipid accumulation and inflammation but did not alter chemotactic response of macrophages to adipocyte signals. Moreover, VLDLR deficiency tempered the synergistic inflammatory interactions between adipocytes and macrophages in a co-culture system. Collectively, these results show that VLDLR contributes to adipose tissue inflammation and mediates VLDL-induced lipid accumulation and induction of inflammation and ER stress in adipocytes and macrophages.     

Cytological and functional characteristics of fascia adipocytes in rats: A unique population of adipocytes

The dermal adipocytes, superficial fascia and subcutaneous adipose tissue (SAT) exist in the interspaces between the dermis and muscular fascia. They are adjacent to each other and traditionally recognized as one SAT. Recently, the dermal adipocyte was redefined as a unique population independent from the SAT. Also, we identified a novel type of adipogenic progenitors in rat superficial fascia. This study aimed to examine cytological and functional characteristics of fascial adipocytes in rats. Superficial fascia had no adipocytes in neonatal rats but gradually appeared numbers of adipocytes in growing rats. Adipogenic progenitors were found to reside in fascia and had strong ability in spontaneous and induced adipogenic differentiation in vitro. Differentiated fascial adipocytes versus subcutaneous or visceral adipocytes expressed increased adipose triglyceride lipase but decreased beta-adrenoreceptor, perilipin-1 and hormone-sensitive lipase (HSL), thus having high basal lipolysis but low lipolysis response to catecholamines. Phosphorylation of perilipin-1 and HSL and translocation of HSL to lipid droplets were attenuated in response to catecholamines rather than post-adrenoreceptoral lipolytic stimulators. The results suggested that superficial fascia was an origin of adipocytes with distinct developmental, cytological and functional characteristics. We proposed that fascial adipocytes could be considered as a unique population of adipocytes in the body. The fascia origin of adipocytes as an adipogenic model might logically explain fat neogenesis occurred at anatomical locations where originally exist no adipose tissues and thereby no adipose-derived stromal precursors. Also, the special histoanatomical relations and overlaps between the dermis, superficial fascia, SAT, and their adipocytes were discussed.     

Adipose tissue plasticity from WAT to BAT and in between

Adipose tissue plays an essential role in regulating energy balance through its metabolic, cellular and endocrine functions. Adipose tissue has been historically classified into anabolic white adipose tissue and catabolic brown adipose tissue. An explosion of new data, however, points to the remarkable heterogeneity among the cells types that can become adipocytes, as well as the inherent metabolic plasticity of mature cells. These data indicate that targeting cellular and metabolic plasticity of adipose tissue might provide new avenues for treatment of obesity-related diseases. This review will discuss the developmental origins of adipose tissue, the cellular complexity of adipose tissues, and the identification of progenitors that contribute to adipogenesis throughout development. We will touch upon the pathological remodeling of adipose tissue and discuss how our understanding of adipose tissue remodeling can uncover new therapeutic targets. This article is part of a Special Issue entitled: Modulation of Adipose Tissue in Health and Disease.     

脂肪细胞分化及其调控的研究进展

肥胖症等多种代谢疾病在全世界范围内的流行使得人们高度关注脂肪沉积调控的机制研究。在细胞水平上,脂肪组织的沉积是脂肪细胞数目增加和单个细胞体积增大的结果。其中,脂肪细胞数目由多潜能干细胞定向分化为前体脂肪细胞的程度决定,而单个细胞体积则与其分化程度和甘油三酯积累量相关。因此,揭示脂肪细胞分化的细胞和分子机制,将为上述代谢性疾病预防和治疗提供重要的理论基础。本文对脂肪细胞的起源、脂肪细胞分化的体外研究模型、脂肪细胞分化的规律和调控以及脂肪细胞分化研究中关键的问题等方面的研究成果进行总结,综述了近年来关于脂肪细胞分化及其调控的研究进展。     

An essential role for Dicer in adipocyte differentiation

Dicer is a cellular enzyme required for the processing of pre‐miRNA molecules into mature miRNA, and Dicer and miRNA biogenesis have been found to play important roles in a variety of physiologic processes. Recently, reports of alterations in miRNA expression levels in cultured pre‐adipogenic cell lines during differentiation and findings of differences between the miRNA expression signatures of white and brown adipose have suggested that miRNA molecules might regulate adipocyte differentiation and the formation of adipose tissue. However, direct evidence that miRNAs regulate adipogenesis is lacking. To determine if Dicer and mature miRNA govern adipocyte differentiation, we utilized primary cells isolated from mice bearing Dicer‐conditional alleles to study adipogenesis in the presence or absence of miRNA biogenesis. Our results reveal that Dicer is required for adipogenic differentiation of mouse embryonic fibroblasts and primary cultures of pre‐adipocytes. Furthermore, the requirement for Dicer in adipocyte differentiation is not due to miRNA‐mediated alterations in cell proliferation, as deletion of the Ink4a locus and the prevention of premature cellular senescence normally induced in primary cells upon Dicer ablation fails to rescue adipogenic differentiation in fibroblasts and pre‐adipocytes. J. Cell. Biochem. 110: 812–816, 2010. © 2010 Wiley‐Liss, Inc.     

In vivo identification of bipotential adipocyte progenitors recruited by β3-adrenoceptor activation and high-fat feeding

Nutritional and pharmacological stimuli can dramatically alter the cellular phenotypes in white adipose tissue (WAT). Utilizing genetic lineage tracing techniques, we demonstrate that brown adipocytes (BA) that are induced by β3-adrenergic receptor activation in abdominal WAT arise from the proliferation and differentiation of cells expressing platelet-derived growth factor receptor alpha (PDGFRα), CD34, and Sca-1 (PDGFRα(+) cells). PDGFRα(+) cells have a unique morphology in which extended processes contact multiple cells in the tissue microenvironment. Surprisingly, these cells also give rise to white adipocytes (WA) that can comprise up to 25% of total fat cells in abdominal fat pads following 8?weeks of high-fat feeding. Isolated PDGFRα(+) cells differentiated into both BA and WA in?vitro and generated WA after transplantation in?vivo. The identification of PDGFRα(+) cells as bipotential adipocyte progenitors will enable further investigation of mechanisms that promote therapeutic cellular remodeling in adult WAT.     

Follistatin promotes adipocyte differentiation,browning, and energy metabolism

Follistatin (Fst) functions to bind and neutralize the activity of members of the transforming growth factor-β superfamily. Fst has a well-established role in skeletal muscle, but we detected significant Fst expression levels in interscapular brown and subcutaneous white adipose tissue, and further investigated its role in adipocyte biology. Fst expression was induced during adipogenic differentiation of mouse brown preadipocytes and mouse embryonic fibroblasts (MEFs) as well as in cold-induced brown adipose tissue from mice. In differentiated MEFs from Fst KO mice, the induction of brown adipocyte proteins including uncoupling protein 1, PR domain containing 16, and PPAR gamma coactivator-1α was attenuated, but could be rescued by treatment with recombinant FST. Furthermore, Fst enhanced thermogenic gene expression in differentiated mouse brown adipocytes and MEF cultures from both WT and Fst KO groups, suggesting that Fst produced by adipocytes may act in a paracrine manner. Our microarray gene expression profiling of WT and Fst KO MEFs during adipogenic differentiation identified several genes implicated in lipid and energy metabolism that were significantly downregulated in Fst KO MEFs. Furthermore, Fst treatment significantly increases cellular respiration in Fst-deficient cells. Our results implicate a novel role of Fst in the induction of brown adipocyte character and regulation of energy metabolism.     

Antiobesity effects of coumestrol through expansion and activation of brown adipose tissue metabolism

Coumestrol is a dietary phytoestrogen with estrogen-mimicking characteristics. This study investigated the molecular mechanisms of antiobesity effects of coumestrol. Two weeks of coumestrol treatment reduced body weight and improved glucose tolerance of high-fat diet (HFD)-fed mice. Notably, coumestrol treatment reduced adiposity but expanded brown adipose tissue mass. In addition, coumestrol treatment induced up-regulation of brown adipocyte markers and lipolytic gene expression in adipose tissue. Mechanistically, coumestrol induced an increase in mitochondrial contents of brown adipose tissue, which was associated with up-regulation of adenosine monophosphate-activated protein kinase and sirtuin 1. In vitro knockdown of estrogen receptor 1 inhibited the effect of coumestrol on brown adipogenic marker expression, increase in mitochondrial contents and oxygen consumption rate in brown adipocytes. Furthermore, lineage tracing of platelet-derived growth factor receptor A-positive (PDGFRA+) adipocyte progenitors confirmed increased levels of de novo brown adipogenesis from PDGFRA+ cells by coumestrol treatment. In conclusion, our results indicate that coumestrol has antiobesity effects through the expansion and activation of brown adipose tissue metabolism.     

A pericellular collagenase directs the 3-dimensional development of white adipose tissue

White adipose tissue (WAT) serves as the primary energy depot in the body by storing fat. During development, fat cell precursors (i.e., preadipocytes) undergo a hypertrophic response as they mature into lipid-laden adipocytes. However, the mechanisms that regulate adipocyte size and mass remain undefined. Herein, we demonstrate that the membrane-anchored metalloproteinase, MT1-MMP, coordinates adipocyte differentiation in vivo. In the absence of the protease, WAT development is aborted, leaving tissues populated by mini-adipocytes which render null mice lipodystrophic. While MT1-MMP preadipocytes display a cell autonomous defect in vivo, null progenitors retain the ability to differentiate into functional adipocytes during 2-dimensional (2-D) culture. By contrast, within the context of the 3-dimensional (3-D) ECM, normal adipocyte maturation requires a burst in MT1-MMP-mediated proteolysis that modulates pericellular collagen rigidity in a fashion that controls adipogenesis. Hence, MT1-MMP acts as a 3-D-specific adipogenic factor that directs the dynamic adipocyte-ECM interactions critical to WAT development.     

Lactoferrin gene knockdown leads to similar effects to iron chelation in human adipocytes

In human and mice adipose tissue, lactoferrin (LTF) has been found to be associated with increased adipogenesis and decreased inflammatory markers. Here, we aimed to investigate the effects of LTF knockdown (KD) in human adipocyte differentiation. In addition, the effects of exogenous LTF administration and iron chelation [using deferoxamine (DFO, 10 μM)] were tested. In both subcutaneous and visceral pre‐adipocytes, LTF KD led to decrease significantly adipogenic, lipogenic and insulin signalling‐related gene expression and a significant increase in the gene expression of inflammatory mediators. Human lactoferrin (hLf, 1 μM) administration led to recover adipocyte differentiation in LTF KD pre‐adipocytes. Interestingly, iron chelation triggered similar effects to LTF KD, decreasing significantly adipogenic gene expressions. Of note, DFO (10 μM) and hLf (1 and 10 μM) co‐administration led to a dose‐dependent recovery of adipocyte differentiation. These new data reveal that endogenous LTF biosynthesis during human adipocyte differentiation is essential to achieve this process, possibly, modulating adipocyte iron homoeostasis. hLf administration might be a useful therapeutic target in obesity‐associated adipose tissue dysfunction.     

The regulation of adipogenesis through GPR120

Recently, it has been found that long-chain fatty acids activate the G protein-coupled receptors (GPRs), GPR120 and GPR40. However, there have been no reports to date on the possible physiological roles of these GPRs in adipose tissue development and adipocyte differentiation. GPR120 mRNA was highly expressed in the four different adipose tissues, and the amount of mRNA was elevated in adipose tissues of mice fed a high fat diet. However, GPR40 mRNA was not detected in any of the adipose tissues. The expression of GPR120 mRNA was higher in adipocytes compared to stromal-vascular (S-V) cells. The level of GPR120 mRNA increased during adipocyte differentiation in 3T3-L1 cells. Similar results were observed in human adipose tissue, human preadipocytes, and cultured adipocytes. Moreover, use of a small interference RNA (siRNA) to down-regulate GPR120 expression resulted in inhibition of adipocyte differentiation. Our results suggest that GPR120 regulates adipogenic processes such as adipocyte development and differentiation.     

Larger anti-adipogenic effect of angiotensin II on omental preadipose cells of obese humans

Objective: The ability to form new adipose cells is important to adipose tissue physiology; however, the mechanisms controlling the recruitment of adipocyte progenitors are poorly understood. A role for locally generated angiotensin II in this process is currently proposed. Given that visceral adipose tissue reportedly expresses higher levels of angiotensinogen compared with other depots and the strong association of augmented visceral fat mass with the adverse consequences of obesity, we studied the role of angiotensin II in regulating adipogenic differentiation in omental fat of obese and non‐obese humans. Research Methods and Procedures: The angiotensin II effect on adipose cell formation was evaluated in human omental adipocyte progenitor cells that were stimulated to adipogenic differentiation in vitro. The adipogenic response was measured by the activity of the differentiation marker glycerol‐3‐phosphate dehydrogenase. Results: Angiotensin II reduced the adipogenic response of adipocyte progenitor cells, and the extent of the decrease correlated directly with the subjects’ BMI (p = 0.01, R² = 0.30). A 56.3 ± 3.4% and 44.5 ± 2.7% reduction of adipogenesis was found in obese and non‐obese donors’ cells, respectively (p < 0.01). The effect of angiotensin II was reversed by type 1 angiotensin receptor antagonist losartan. Discussion: A greater anti‐adipogenic response to angiotensin II in omental adipose progenitor cells from obese subjects opens a venue to understand the deregulation of visceral fat tissue cellularity that has been associated with severe functional abnormalities of the obese condition.     

The Effect of the Interleukin‐1 Cytokine Family Members IL‐1F6 and IL‐1F8 on Adipocyte Differentiation

Obesity is characterized by chronic low‐grade inflammation originating from expanding adipose tissue. In the present study, we examined the adipogenic expression levels of IL‐1F6 and IL‐1F8, both members of the IL‐1 family of cytokines, and their effects on adipose tissue gene expression. Although IL‐1F6 is primarily present in adipose tissue resident macrophages and induced by inflammation, IL‐1F8 is absent. IL‐1F6, but not IL‐1F8, reduces adipocyte differentiation, as shown by a significant decrease in PPARγ gene expression. Finally, both IL‐1F6 and IL‐1F8 are able to induce inflammatory gene expression in mature adipocytes. In conclusion, we demonstrate for the first time that IL‐1F6 is present in adipose tissue and that IL‐1F6 and IL‐1F8 are involved in the regulation of adipose tissue gene expression. Importantly, IL‐1F6 inhibits PPARγ expression which may lead to reduced adipocyte differentiation suggesting metabolic effects of this cytokine.     

Peroxisome proliferator-activated receptor gamma activation promotes infiltration of alternatively activated macrophages into adipose tissue

Obesity is associated with infiltration of macrophages into adipose tissue. Adipose macrophages may contribute to an elevated inflammatory status by secreting a variety of proinflammatory mediators, including tumor necrosis factor alpha and interleukin-6 (IL-6). Recent data suggest that during diet-induced obesity the phenotype of adipose-resident macrophages changes from alternatively activated macrophages toward a more classical and pro-inflammatory phenotype. Here, we explore the effect of peroxisome proliferator-activated receptor gamma activation on obesity-induced inflammation in 129SV mice fed a high fat diet for 20 weeks. High fat feeding increased bodyweight gain, adipose tissue mass, and liver triglycerides. Rosiglitazone treatment further increased adipose mass, reduced liver triglycerides, and changed adipose tissue morphology toward smaller adipocytes. Surprisingly, rosiglitazone markedly increased the number of macrophages in adipose tissue, as shown by immunohistochemical analysis and quantification of macrophage marker genes CD68 and F4/80+. In adipose tissue, markers for classically activated macrophages including IL-18 were down-regulated, whereas markers characteristic for alternatively activated macrophages (arginase 1, IL-10) were up-regulated by rosiglitazone. Importantly, conditioned media from rosiglitazone-treated alternatively activated macrophages neutralized the inhibitory effect of macrophages on 3T3-L1 adipocyte differentiation, suggesting that alternatively activated macrophages may be involved in mediating the effects of rosiglitazone on adipose tissue morphology and mass. Our results suggest that short term rosiglitazone treatment increases infiltration of alternatively activated macrophages in adipose tissue. The alternatively activated macrophages might play a role in peroxisome proliferator-activated receptor gamma-dependent expansion and remodeling of adipose tissue.     

OSBP-Related Proteins (ORPs) in Human Adipose Depots and Cultured Adipocytes: Evidence for Impacts on the Adipocyte Phenotype

Oxysterol-binding protein (OSBP) homologues, ORPs, are implicated in lipid homeostatic control, vesicle transport, and cell signaling. We analyzed here the quantity of ORP mRNAs in human subcutaneous (s.c.) and visceral adipose depots, as well as in the Simpson-Golabi-Behmel syndrome (SGBS) adipocyte cell model. All of the ORP mRNAs were present in the s.c and visceral adipose tissues, and the two depots shared an almost identical ORP mRNA expression pattern. SGBS adipocytes displayed a similar pattern, suggesting that the adipose tissue ORP expression pattern mainly derives from adipocytes. During SGBS cell adipogenic differentiation, ORP2, ORP3, ORP4, ORP7, and ORP8 mRNAs were down-regulated, while ORP11 was induced. To assess the impacts of ORPs on adipocyte differentiation, ORP3 and ORP8, proteins down-regulated during adipogenesis, were overexpressed in differentiating SGBS adipocytes, while ORP11, a protein induced during adipogenesis, was silenced. ORP8 overexpression resulted in reduced expression of the aP2 mRNA, while down-regulation of adiponectin and aP2 was observed in ORP11 silenced cells. Furthermore, ORP8 overexpression or silencing of ORP11 markedly decreased cellular triglyceride storage. These data identify the patterns of ORP expression in human adipose depots and SGBS adipocytes, and provide the first evidence for a functional impact of ORPs on the adipocyte phenotype.