Structure-function relationships in the adipose cell. I. Ultrastructure of the isolated adipose cell

Adipose tissue plasticity mediated by the counterregulatory axis of the renin-angiotensin system: Role of Mas and MrgD receptors

The renin-angiotensin system (RAS) is an endocrine system composed of two main axes: the classical and the counterregulatory, very often displaying opposing effects. The classical axis, primarily mediated by angiotensin receptors type 1 (AT1R), is linked to obesity-associated metabolic effects. On the other hand, the counterregulatory axis appears to exert antiobesity effects through the activation of two receptors, the G protein-coupled receptor (MasR) and Mas-related receptor type D (MrgD). The local RAS in adipose organ has prompted extensive research into white adipose tissue and brown adipose tissue (BAT), with a key role in regulating the cellular and metabolic plasticity of these tissues. The MasR activation favors the brown plasticity signature in the adipose organ by improve the thermogenesis, adipogenesis, and lipolysis, decrease the inflammatory state, and overall energy homeostasis. The MrgD metabolic effects are related to the maintenance of BAT functionality, but the signaling remains unexplored. This review provides a summary of RAS counterregulatory actions triggered by Mas and MrgD receptors on adipose tissue plasticity. Focus on the effects related to the morphology and function of adipose tissue, especially from animal studies, will be given targeting new avenues for treatment of obesity-associated metabolic effects.

Mitochondria isolated from lipid droplets of white adipose tissue reveal functional differences based on lipid droplet size

Recent studies in brown adipose tissue (BAT) described a unique subpopulation of mitochondria bound to lipid droplets (LDs), which were termed PeriDroplet Mitochondria (PDM). PDM can be isolated from BAT by differential centrifugation and salt washes. Contrary to BAT, this approach has so far not led to the successful isolation of PDM from white adipose tissue (WAT). Here, we developed a method to isolate PDM from WAT with high yield and purity by an optimized proteolytic treatment that preserves the respiratory function of mitochondria. Using this approach, we show that, contrary to BAT, WAT PDM have lower respiratory and ATP synthesis capacities compared with WAT cytoplasmic mitochondria (CM). Furthermore, by isolating PDM from LDs of different sizes, we found a negative correlation between LD size and the respiratory capacity of their PDM in WAT. Thus, our new isolation method reveals tissue-specific characteristics of PDM and establishes the existence of heterogeneity in PDM function determined by LD size.

Lipid Droplets Specific Fluorophore for Demarcation of Normal and Diseased Tissues

Using high-fidelity, permeable, lipophilic, and bright fluorophores for imaging lipid droplets (LDs) in tissues holds immense potential in diagnosing conditions such as diabetic or alcoholic fatty liver disease. In this work, we utilized linear and Λ-shaped polarity-sensitive fluorescent probes for imaging LDs in both cellular and tissue environments, specifically in rats with diabetic and alcoholic fatty liver disease. The fluorescent probes possess several key characteristics, including high permeability, lipophilicity, and brightness, which make them well-suited for efficient LD imaging. Notably, the probes exhibit a substantial Stokes shift, with 143 nm for DCS and 201 nm for DCN with selective targeting of the lipid droplets. Our experimental investigations successfully differentiated morphological variations between diseased and normal tissues in three distinct tissue types: liver, adipose, and small intestine. They could help provide pointers for improved detection and understanding of LD-related pathologies.

High Resolution Proteome of Lipid Droplets Isolated from the Pennate Diatom Phaeodactylum tricornutum (Bacillariophyceae) Strain pt4 provides mechanistic insights into complex intracellular coordination during nitrogen deprivation

Lipid droplets (LDs) are an organelle conserved amongst all eukaryotes, consisting of a neutral lipid core surrounded by a polar lipid monolayer. Many species of microalgae accumulate LDs in response to stress conditions, such as nitrogen starvation. Here, we report the isolation and proteomic profiling of LD proteins from the model oleaginous pennate diatom Phaeodactylum tricornutum, strain Pt4 (UTEX 646). We also provide a quantitative description of LD morphological ontogeny, and fatty acid content. Novel cell disruption and LD isolation methods, combined with suspension-trapping and nanoflow liquid chromatography coupled to high resolution mass spectrometry, yielded an unprecedented number of LD proteins. Predictive annotation of the LD proteome suggests a broad assemblage of proteins with diverse functions, including lipid metabolism and vesicle trafficking, as well as ribosomal and proteasomal machinery. These proteins provide mechanistic insights into LD processes, and evidence for interactions between LDs and other organelles. We identify for the first time several key steps in diatom LD-associated triacylglycerol biosynthesis. Bioinformatic analyses of the LD proteome suggests multiple protein targeting mechanisms, including amphipathic helices, post-translational modifications, and translocation machinery. This work corroborates recent findings from other strains of P. tricornutum, other diatoms, and other eukaryotic organisms, suggesting that the fundamental proteins orchestrating LDs are conserved, and represent an ancient component of the eukaryotic endomembrane system. We postulate a comprehensive model of nitrogen starvation-induced diatom LDs on a molecular scale, and provide a wealth of candidates for metabolic engineering, with the potential to eventually customize LD contents.

Electro-physiology Models of Cells with Spherical Geometry with Non-conducting Center

We study the flow of electrical currents in spherical cells with a non-conducting core, so that current flow is restricted to a thin shell below the cell's membrane. Examples of such cells are fat storing cells (adipocytes). We derive the relation between current and voltage in the passive regime and examine the conditions under which the cell is electro-tonically compact. We compare our results to the well-studied case of electrical current flow in cylinder structures, such as neurons, described by the cable equation. In contrast to the cable, we find that for the sphere geometry (1) the voltage profile across the cell depends critically on the electrode geometry, and (2) the charging and discharging can be much faster than the membrane time constant; however, (3) voltage clamp experiments will incur similar distortion as in the cable case. We discuss the relevance for adipocyte function and experimental electro-physiology.

Biology, strategies, and fresh meat consequences of manipulating the fatty acid composition of meat

The utility and attractiveness of adipose tissue within meat products vary based on species, cut, and consumer preference. In beef, producers are rewarded for producing carcasses with greater visual marbling at the 12th and 13th rib juncture, while pork producers are either not rewarded or penalized for producing carcasses with too much adipose tissue. Some consumers prefer to purchase leaner meat cuts, while other consumers pay premiums to consume products with elevated fat content. While no clear consumer adipose tissue preference standard exists, advances in beef and swine nutrition have enabled producers to target markets that enable them to maximize profits. One niche market that has increased in popularity over the last decade is manipulating the fatty acid profile, specifically increasing omega-3 fatty acid content, of beef and pork products to increase their appeal in a healthy diet. While much research has documented the ability of preharvest diet to alter the fatty acid profile of beef and pork, the same studies have indicated both the color and palatability of these products were negatively affected if preharvest diets were not managed properly. The following review discusses the biology of adipose tissue and lipid accumulation, altering the omega-3 fatty acid profile of beef and pork, negative fresh meat color and palatability associated with these studies, and strategies to mitigate the negative effects of increased omega-3 fatty acid content.

Adipocytes spectrum - From homeostasia to obesity and its associated pathology

Firstly identified by anatomists, the fat tissue is nowadays an area of intense research due to increased global prevalence of obesity and its associated diseases. Histologically, there are four types of fat tissue cells which are currently recognized (white, brown, beige, and perivascular adipocytes). Therefore, in this study we are reviewing the most recent data regarding the origin, structure, and molecular mechanisms involved in the development of adipocytes. White adipocytes can store triglycerides as a consequence of lipogenesis, under the regulation of growth hormone or leptin and adiponectin, and release fatty acids resulted from lipolysis, under the regulation of the sympathetic nervous system, glucocorticoids, TNF-α, insulin, and natriuretic peptides. Brown adipocytes possess a mitochondrial transmembrane protein thermogenin or UCP1 which allows heat generation. Recently, thermogenic, UCP positive adipocytes have been identified in the subcutaneous white adipose tissue and have been named beige adipocytes. The nature of these cells is still controversial, as current theories are suggesting their origin either by transdifferentiation of white adipocytes, or by differentiation from an own precursor cell. Perivascular adipocytes surround most of the arteries, exhibiting a supportive role and being involved in the maintenance of intravascular temperature. Thoracic perivascular adipocytes resemble brown adipocytes, while abdominal ones are more similar to white adipocytes and, consequently, are involved in obesity-induced inflammatory reactions. The factors involved in the regulation of adipose stem cells differentiation may represent potential pathways to inhibit or to divert adipogenesis. Several molecules, such as pro-adipogenic factors (FGF21, BMP7, BMP8b, and Cox-2), cell surface proteins or receptors (Asc-1, PAT2, P2RX5), and hypothalamic receptors (MC4R) have been identified as the most promising targets for the development of future therapies. Further investigations are necessary to complete the knowledge about adipose tissue and the development of a new generation of therapeutic tools based on molecular targets.

Adipose morphology and metabolic disease

Adipose morphology is defined as the number and size distribution of adipocytes (fat cells) within adipose tissue. Adipose tissue with fewer but larger adipocytes is said to have a 'hypertrophic' morphology, whereas adipose with many adipocytes of a smaller size is said to have a 'hyperplastic' morphology. Hypertrophic adipose morphology is positively associated with insulin resistance, diabetes and cardiovascular disease. By contrast, hyperplastic morphology is associated with improved metabolic parameters. These phenotypic associations suggest that adipose morphology influences risk of cardiometabolic disease. Intriguingly, monozygotic twin studies have determined that adipose morphology is in part determined genetically. Therefore, identifying the genetic regulation of adipose morphology may help us to predict, prevent and ameliorate insulin resistance and associated metabolic diseases. Here, we review the current literature regarding adipose morphology in relation to: (1) metabolic and medical implications; (2) the methods used to assess adipose morphology; and (3) transcriptional differences between morphologies. We further highlight three mechanisms that have been hypothesized to promote adipocyte hypertrophy and thus to regulate adipose morphology.

Renal perivascular adipose tissue: Form and function

Renal sympathetic activity affects blood pressure in part by increasing renovascular resistance via release of norepinephrine (NE) from sympathetic nerves onto renal arteries. Here we test the idea that adipose tissue adjacent to renal blood vessels, i.e. renal perivascular adipose tissue (RPVAT), contains a pool of NE which can be released to alter renal vascular function. RPVAT was obtained from around the main renal artery/vein of the male Sprague Dawley rats. Thoracic aortic PVAT and mesenteric PVAT also were studied as brown-like and white fat comparators respectively. RPVAT was identified as a mix of white and brown adipocytes, because of expression of both brown-like (e.g. uncoupling protein 1) and white adipogenic genes. All PVATs contained NE (ng/g tissue, RPVAT:524 ± 68, TAPVAT:740 ± 16, MPVAT:96 ± 24). NE was visualized specifically in RPVAT adipocytes by immunohistochemistry. The presence of RPVAT (+RPVAT) did not alter the response of isolated renal arteries to NE compared to responses of arteries without RPVAT (-RPVAT). By contrast, the maximum contraction to the sympathomimetic tyramine was ~2× greater in the renal artery +PVAT versus -PVAT. Tyramine-induced contraction in +RPVAT renal arteries was reduced by the α₁-adrenoceptor antagonist prazosin and the NE transporter inhibitor nisoxetine. These results suggest that tyramine caused release of NE from RPVAT. Renal denervation significantly (>50%) reduced NE content of RPVAT but did not modify tyramine-induced contraction of +RPVAT renal arteries. Collectively, these data support the existence of a releasable pool of NE in RPVAT that is independent of renal sympathetic innervation and has the potential to change renal arterial function.

How lipid droplets "TAG" along: Glycerolipid synthetic enzymes and lipid storage

Triacylglycerols (TAG) serve as the predominant form of energy storage in mammalian cells, and TAG synthesis influences conditions such as obesity, fatty liver, and insulin resistance. In most tissues, the glycerol 3-phosphate pathway enzymes are responsible for TAG synthesis, and the regulation and function of these enzymes is therefore important for metabolic homeostasis. Here we review the sites and regulation of glycerol-3-phosphate acyltransferase (GPAT), acylglycerol-3-phosphate acyltransferase (AGPAT), lipin phosphatidic acid phosphatase (PAP), and diacylglycerol acyltransferase (DGAT) enzyme action. We highlight the critical roles that these enzymes play in human health by reviewing Mendelian disorders that result from mutation in the corresponding genes. We also summarize the valuable insights that genetically engineered mouse models have provided into the cellular and physiological roles of GPATs, AGPATs, lipins and DGATs. Finally, we comment on the status and feasibility of therapeutic approaches to metabolic disease that target enzymes of the glycerol 3-phosphate pathway. This article is part of a Special Issue entitled: Recent Advances in Lipid Droplet Biology edited by Rosalind Coleman and Matthijs Hesselink.

Mfn2 is critical for brown adipose tissue thermogenic function

Mitochondrial fusion and fission events, collectively known as mitochondrial dynamics, act as quality control mechanisms to ensure mitochondrial function and fine‐tune cellular bioenergetics. Defective mitofusin 2 (Mfn2) expression and enhanced mitochondrial fission in skeletal muscle are hallmarks of insulin‐resistant states. Interestingly, Mfn2 is highly expressed in brown adipose tissue (BAT), yet its role remains unexplored. Using adipose‐specific Mfn2 knockout (Mfn2‐adKO) mice, we demonstrate that Mfn2, but not Mfn1, deficiency in BAT leads to a profound BAT dysfunction, associated with impaired respiratory capacity and a blunted response to adrenergic stimuli. Importantly, Mfn2 directly interacts with perilipin 1, facilitating the interaction between the mitochondria and the lipid droplet in response to adrenergic stimulation. Surprisingly, Mfn2‐adKO mice were protected from high‐fat diet‐induced insulin resistance and hepatic steatosis. Altogether, these results demonstrate that Mfn2 is a mediator of mitochondria to lipid droplet interactions, influencing lipolytic processes and whole‐body energy homeostasis.

The why, when and how of lipid droplet diversity

Lipid droplets are the universal cellular organelles for the transient or long-term storage of lipids. The number, size and composition of lipid droplets vary greatly within cells in a homogenous population as well as in different cell types. The variability of intracellular lipid-storage organelles reflects the diversification of lipid droplet composition and function. Lipid droplet diversification results, for example, in two cellular lipid droplet populations that are prone to diminish and grow, respectively. The aberrant accumulation or depletion of lipids are hallmarks or causes of various human pathologies. Thus, a better understanding of the origins of lipid droplet diversification is not only a fascinating cell biology question but also potentially serves to improve comprehension of pathologies that entail the accumulation of lipids. This Commentary covers the lipid droplet life cycle and highlights the early steps during lipid droplet biogenesis, which we propose to be the potential driving forces of lipid droplet diversification.

Microscopy tools for the investigation of intracellular lipid storage and dynamics

Background

Excess storage of lipids in ectopic tissues, such as skeletal muscle, liver, and heart, seems to associate closely with metabolic abnormalities and cardiac disease. Intracellular lipid storage occurs in lipid droplets, which have gained attention as active organelles in cellular metabolism. Recent developments in high-resolution microscopy and microscopic spectroscopy have opened up new avenues to examine the physiology and biochemistry of intracellular lipids.

Scope of review

The aim of this review is to give an overview of recent technical advances in microscopy, and its application for the visualization, identification, and quantification of intracellular lipids, with special focus to lipid droplets. In addition, we attempt to summarize the probes currently available for the visualization of lipids.

Major conclusions

The continuous development of lipid probes in combination with the rapid development of microscopic techniques can provide new insights in the role and dynamics of intracellular lipids. Moreover, in situ identification of intracellular lipids is now possible and promises to add a new dimensionality to analysis of lipid biochemistry, and its relation to (patho)physiology.

Spatiotemporal dynamics of triglyceride storage in unilocular adipocytes

Real-time fluorescence microscopy is used to investigate the trafficking of metabolizable fluorescent fatty acid in unilocular adipocytes from adipose tissue of nonhuman primates. The study reveals novel cell biological features that may contribute to the mechanism of adipocyte hypertrophy.

The spatiotemporal dynamics of triglyceride (TG) storage in unilocular adipocytes are not well understood. Here we applied ex vivo technology to study trafficking and metabolism of fluorescent fatty acids in adipose tissue explants. Live imaging revealed multiple cytoplasmic nodules surrounding the large central lipid droplet (cLD) of unilocular adipocytes. Each cytoplasmic nodule harbors a series of closely associated cellular organelles, including micro–lipid droplets (mLDs), mitochondria, and the endoplasmic reticulum. Exogenously added free fatty acids are rapidly adsorbed by mLDs and concurrently get esterified to TG. This process is greatly accelerated by insulin. mLDs transfer their content to the cLD, serving as intermediates that mediate packaging of newly synthesized TG in the large interior of a unilocular adipocyte. This study reveals novel cell biological features that may contribute to the mechanism of adipocyte hypertrophy.

Endoplasmic reticulum stress in adipose tissue augments lipolysis

The endoplasmic reticulum (ER) is an organelle important for protein synthesis and folding, lipid synthesis and Ca²⁺ homoeostasis. Consequently, ER stress or dysfunction affects numerous cellular processes and has been implicated as a contributing factor in several pathophysiological conditions. Tunicamycin induces ER stress in various cell types in vitro as well as in vivo. In mice, a hallmark of tunicamycin administration is the development of fatty livers within 24–48 hrs accompanied by hepatic ER stress. We hypothesized that tunicamycin would induce ER stress in adipose tissue that would lead to increased lipolysis and subsequently to fatty infiltration of the liver and hepatomegaly. Our results show that intraperitoneal administration of tunicamycin rapidly induced an ER stress response in adipose tissue that correlated with increased circulating free fatty acids (FFAs) and glycerol along with decreased adipose tissue mass and lipid droplet size. Furthermore, we found that in addition to fatty infiltration of the liver as well as hepatomegaly, lipid accumulation was also present in the heart, skeletal muscle and kidney. To corroborate our findings to a clinical setting, we examined adipose tissue from burned patients where increases in lipolysis and the development of fatty livers have been well documented. We found that burned patients displayed significant ER stress within adipose tissue and that ER stress augments lipolysis in cultured human adipocytes. Our results indicate a possible role for ER stress induced lipolysis in adipose tissue as an underlying mechanism contributing to increases in circulating FFAs and fatty infiltration into other organs.

Trypanosoma cruzi infection and host lipid metabolism

Trypanosoma cruzi is the causative agent of Chagas disease. Approximately 8 million people are thought to be affected worldwide. Several players in host lipid metabolism have been implicated in T. cruzi-host interactions in recent research, including macrophages, adipocytes, low density lipoprotein (LDL), low density lipoprotein receptor (LDLR), and high density lipoprotein (HDL). All of these factors are required to maintain host lipid homeostasis and are intricately connected via several metabolic pathways. We reviewed the interaction of T. cruzi with each of the relevant host components, in order to further understand the roles of host lipid metabolism in T. cruzi infection. This review sheds light on the potential impact of T. cruzi infection on the status of host lipid homeostasis.

Etiology of the membrane potential of rat white fat adipocytes

The plasma membrane potential (Vm) is key to many physiological processes; however, its ionic etiology in white fat adipocytes is poorly characterized. To address this question, we employed the perforated patch current clamp and cell-attached patch clamp methods in isolated primary white fat adipocytes and their cellular model 3T3-L1. The resting Vm of primary and 3T3-L1 adipocytes were -32.1 ± 1.2 mV (n = 95) and -28.8 ± 1.2 mV (n = 87), respectively. Vm was independent of cell size and fat content. Elevation of extracellular K(+) to 50 mM by equimolar substitution of bath Na(+) did not affect Vm, whereas substitution of bath Na(+) with the membrane-impermeant cation N-methyl-D-glucamine(+)-hyperpolarized Vm by 16 mV, data indicative of a nonselective cation permeability. Substitution of 133 mM extracellular Cl(-) with gluconate-depolarized Vm by 25 mV, whereas Cl(-) substitution with I(-) caused a -9 mV hyperpolarization. Isoprenaline (10 μM), but not insulin (100 nM), significantly depolarized Vm. Single-channel ion activity was voltage independent; currents were indicative for Cl(-) with an inward slope conductance of 16 ± 1.3 pS (n = 11) and a reversal potential close to the Cl(-) equilibrium potential, -29 ± 1.6 mV. Although the reduction of extracellular Cl(-) elevated the intracellular Ca(2+) of adipocytes, this was not as large as that produced by elevation of extracellular K(+). In conclusion, the Vm of white fat adipocytes is well described by the Goldman-Hodgkin-Katz equation with a predominant permeability to Cl(-), where its biophysical and single-channel properties suggest a volume-sensitive anion channel identity. Consequently, changes in serum Cl(-) homeostasis or the adipocyte's permeability to this anion via drugs will affect its Vm, intracellular Ca(2+), and ultimately its function and its role in metabolic control.

Perivascular adipose tissue contains functional catecholamines

The sympathetic nervous system and its neurotransmitter effectors are undeniably important to blood pressure control. We made the novel discovery that perivascular adipose tissue (PVAT) contains significant concentrations of catecholamines. We hypothesized that PVAT contains sufficient releasable catecholamines to affect vascular function. High-pressure liquid chromatography, isometric contractility, immunohistochemistry, whole animal approaches, and pharmacology were used to test this hypothesis. In normal rat thoracic aorta and superior mesenteric artery, the indirect sympathomimetic tyramine caused a concentration-dependent contraction that was dependent on the presence of PVAT. Tyramine stimulated release of norepinephrine (NA), dopamine (DA) and the tryptamine serotonin (5-hydroxytryptamine [5-HT]) from PVAT isolated from both arteries. In both arteries, tyramine-induced concentration-dependent contraction was rightward-shifted and reduced by the noradrenaline transporter inhibitor nisoxetine (1 μmol/L), the vesicular monoamine transporter inhibitor tetrabenazine (10 μmol/L), and abolished by the α adrenoreceptor antagonist prazosin (100 nmol/L). Inhibitors of the DA and 5-HT transporter did not alter tyramine-induced, PVAT-dependent contraction. Removal of the celiac ganglion as a neuronal source of catecholamines for superior mesenteric artery PVAT did not significantly reduce the maximum or shift the concentration-dependent contraction to tyramine. Electrical field stimulation of the isolated aorta was not affected by the presence of PVAT. These data suggest that PVAT components that are independent of sympathetic nerves can release NA in a tyramine-sensitive manner to result in arterial contraction. Because PVAT is intimately apposed to the artery, this raises the possibility of local control of arterial function by PVAT catecholamines.

Transgenic overexpression of hexose-6-phosphate dehydrogenase in adipose tissue causes local glucocorticoid amplification and lipolysis in male mice

The prereceptor activation of glucocorticoid production in adipose tissue by NADPH-dependent 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) has emerged as a potential mechanism in the pathogenesis of visceral obesity and metabolic syndrome. Hexose-6-phosphate dehydrogenase (H6PDH) is an endoplasmic reticulum lumen-resident enzyme that generates cofactor NADPH and thus mediates 11β-HSD1 activity. To determine the role of adipose H6PDH in the prereceptor modulation of 11β-HSD1 and metabolic phenotypes, we generated a transgenic (Tg) mouse model overexpressing H6PDH under the control of the enhancer-promoter region of the adipocyte fatty acid-binding protein (aP2) gene (aP2/H6PDH Tg mice). Transgenic aP2/H6PDH mice exhibited relatively high expression of H6PDH and elevated corticosterone production with induction of 11β-HSD1 activity in adipose tissue. This increase in corticosterone production in aP2-H6PDH Tg mice resulted in mild abdominal fat accumulation with induction of C/EBP mRNA expression and slight weight gain. Transgenic aP2/H6PDH mice also exhibited fasting hyperglycemia and glucose intolerance with insulin resistance. In addition, the aP2/H6PDH Tg mice have elevated circulating free fatty acid levels with a concomitant increased adipose lipolytic action associated with elevated HSL mRNA and Ser(660) HSL phosphorylation within abdominal fat. These results suggest that increased H6PDH expression specifically in adipose tissue is sufficient to cause intra-adipose glucocorticoid production and adverse metabolic phenotypes. These findings suggest that the aP2/H6PDH Tg mice may provide a favorable model for studying the potential impact of H6PDH in the pathogenesis of human metabolic syndrome.

Pleiotropic effects of cavin-1 deficiency on lipid metabolism

Mice and humans lacking caveolae due to gene knock-out or inactivating mutations of cavin-1/PTRF have numerous pathologies including markedly aberrant fuel metabolism, lipodystrophy, and muscular dystrophy. We characterized the physiologic/metabolic profile of cavin-1 knock-out mice and determined that they were lean because of reduced white adipose depots. The knock-out mice were resistant to diet-induced obesity and had abnormal lipid metabolism in the major metabolic organs of white and brown fat and liver. Epididymal white fat cells from cavin-1-null mice were small and insensitive to insulin and β-adrenergic agonists resulting in reduced adipocyte lipid storage and impaired lipid tolerance. At the molecular level, the lipolytic defects in white fat were caused by impaired perilipin phosphorylation, and the reduced triglyceride accumulation was caused by decreased fatty acid uptake and incorporation as well as the virtual absence of insulin-stimulated glucose transport. The livers of cavin-1-null mice were mildly steatotic and did not accumulate more lipid after high-fat feeding. The brown adipose tissues of cavin-1-null mice exhibited decreased mitochondria protein expression, which was restored upon high fat feeding. Taken together, these data suggest that dysfunction in fat, muscle, and liver metabolism in cavin-1-null mice causes a pleiotropic phenotype, one apparently identical to that of humans lacking caveolae in all tissues.

Small GTPase Rab40c associates with lipid droplets and modulates the biogenesis of lipid droplets

The subcellular location and cell biological function of small GTPase Rab40c in mammalian cells have not been investigated in detail. In this study, we demonstrated that the exogenously expressed GFP-Rab40c associates with lipid droplets marked by neutral lipid specific dye Oil red or Nile red, but not with the Golgi or endosomal markers. Further examination demonstrated that Rab40c is also associated with ERGIC-53 containing structures, especially under the serum starvation condition. Rab40c is increasingly recruited to the surface of lipid droplets during lipid droplets formation and maturation in HepG2 cells. Rab40c knockdown moderately decreases the size of lipid droplets, suggesting that Rab40c is involved in the biogenesis of lipid droplets. Stimulation for adipocyte differentiation increases the expression of Rab40c in 3T3-L1 cells. Rab40c interacts with TIP47, and is appositionally associated with TIP47-labeled lipid droplets. In addition, over-expression of Rab40c causes the clustering of lipid droplets independent of its GTPase activity, but completely dependent of the intact SOCS box domain of Rab40c. In addition, Rab40c displayed self-interaction as well as interaction with TIP47 and the SOCS box is essential for its ability to induce clustering of lipid droplets. Our results suggest that Rab40c is a novel Rab protein associated with lipid droplets, and is likely involved in modulating the biogenesis of lipid droplets.

Packaging of fat: an evolving model of lipid droplet assembly and expansion

Lipid droplets (LDs) are organelles found in most types of cells in the tissues of vertebrates, invertebrates, and plants, as well as in bacteria and yeast. They differ from other organelles in binding a unique complement of proteins and lacking an aqueous core but share aspects of protein trafficking with secretory membrane compartments. In this minireview, we focus on recent evidence supporting an endoplasmic reticulum origin for LD formation and discuss recent findings regarding LD maturation and fusion.

Historical perspectives in fat cell biology: the fat cell as a model for the investigation of hormonal and metabolic pathways

For many years, there was little interest in the biochemistry or physiology of adipose tissue. It is now well recognized that adipocytes play an important dynamic role in metabolic regulation. They are able to sense metabolic states via their ability to perceive a large number of nervous and hormonal signals. They are also able to produce hormones, called adipokines, that affect nutrient intake, metabolism and energy expenditure. The report by Rodbell in 1964 that intact fat cells can be obtained by collagenase digestion of adipose tissue revolutionized studies on the hormonal regulation and metabolism of the fat cell. In the context of the advent of systems biology in the field of cell biology, the present seems an appropriate time to look back at the global contribution of the fat cell to cell biology knowledge. This review focuses on the very early approaches that used the fat cell as a tool to discover and understand various cellular mechanisms. Attention essentially focuses on the early investigations revealing the major contribution of mature fat cells and also fat cells originating from adipose cell lines to the discovery of major events related to hormone action (hormone receptors and transduction pathways involved in hormonal signaling) and mechanisms involved in metabolite processing (hexose uptake and uptake, storage, and efflux of fatty acids). Dormant preadipocytes exist in the stroma-vascular fraction of the adipose tissue of rodents and humans; cell culture systems have proven to be valuable models for the study of the processes involved in the formation of new fat cells. Finally, more recent insights into adipocyte secretion, a completely new role with major metabolic impact, are also briefly summarized.

Urban planning of the endoplasmic reticulum (ER): how diverse mechanisms segregate the many functions of the ER

The endoplasmic reticulum (ER) is the biggest organelle in most cell types, but its characterization as an organelle with a continuous membrane belies the fact that the ER is actually an assembly of several, distinct membrane domains that execute diverse functions. Almost 20 years ago, an essay by Sitia and Meldolesi first listed what was known at the time about domain formation within the ER. In the time that has passed since, additional ER domains have been discovered and characterized. These include the mitochondria-associated membrane (MAM), the ER quality control compartment (ERQC), where ER-associated degradation (ERAD) occurs, and the plasma membrane-associated membrane (PAM). Insight has been gained into the separation of nuclear envelope proteins from the remainder of the ER. Research has also shown that the biogenesis of peroxisomes and lipid droplets occurs on specialized membranes of the ER. Several studies have shown the existence of specific marker proteins found on all these domains and how they are targeted there. Moreover, a first set of cytosolic ER-associated sorting proteins, including phosphofurin acidic cluster sorting protein 2 (PACS-2) and Rab32 have been identified. Intra-ER targeting mechanisms appear to be superimposed onto ER retention mechanisms and rely on transmembrane and cytosolic sequences. The crucial roles of ER domain formation for cell physiology are highlighted with the specific targeting of the tumor metastasis regulator gp78 to ERAD-mediating membranes or of the promyelocytic leukemia protein to the MAM.

Retinyl ester storage particles (retinosomes) from the retinal pigmented epithelium resemble lipid droplets in other tissues

Levels of many hydrophobic cellular substances are tightly regulated because of their potential cytotoxicity. These compounds tend to self-aggregate in cytoplasmic storage depots termed lipid droplets/bodies that have well defined structures that contain additional components, including cholesterol and various proteins. Hydrophobic substances in these structures become mobilized in a specific and regulated manner as dictated by cellular requirements. Retinal pigmented epithelial cells in the eye produce retinyl ester-containing lipid droplets named retinosomes. These esters are mobilized to replenish the visual chromophore, 11-cis-retinal, and their storage ensures proper visual function despite fluctuations in dietary vitamin A intake. But it remains unclear whether retinosomes are structures specific to the eye or similar to lipid droplets in other organs/tissues that contain substances other than retinyl esters. Thus, we initially investigated the production of these lipid droplets in experimental cell lines expressing lecithin:retinol acyltransferase, a key enzyme involved in formation of retinyl ester-containing retinosomes from all-trans-retinol. We found that retinosomes and oleate-derived lipid droplets form and co-localize concomitantly, indicating their intrinsic structural similarities. Next, we isolated native retinosomes from bovine retinal pigmented epithelium and found that their protein and hydrophobic small molecular constituents were similar to those of lipid droplets reported for other experimental cell lines and tissues. These unexpected findings suggest a common mechanism for lipid droplet formation that exhibits broad chemical specificity for the hydrophobic substances being stored.

Caveolae and lipid trafficking in adipocytes

The abundance of caveolae in adipocytes suggests a possible cell-specific role for these structures, and because these cells take up and release fatty acids as their quantitatively most robust activity, modulation of fatty acid movement is one such role that is supported by substantial in vitro and in vivo data. In addition, caveolae are particularly rich in cholesterol and sphingolipids, and indeed, fat cells harbor more cholesterol than any other tissue. In this article, we review the role of adipocyte caveolae with regard to these important lipid classes.

Apelin decreases lipolysis via G(q), G(i), and AMPK-Dependent Mechanisms

The release of free fatty acids (FFAs) from adipocytes (i.e. lipolysis) is increased in obesity and is a contributory factor to the development of insulin resistance. A recently identified adipokine, apelin, is up-regulated in states of obesity. Although apelin is secreted by adipocytes, its functions in them remain largely unknown. To determine whether apelin affects lipolysis, FFA, glycerol, and leptin levels, as well as abdominal adiposity, were measured at baseline and after reintroduction of exogenous apelin in apelin-null mice. To examine apelin's effects in vitro, isoproterenol-induced FFA/glycerol release, and hormone-sensitive lipase (HSL) and acetyl CoA carboxylase phosphorylation were investigated in 3T3-L1 cells and isolated wild-type adipocytes. Serum FFA, glycerol, and leptin concentrations, as well as abdominal adiposity, were significantly increased in apelin-null vs. wild-type mice; these changes were ameliorated in response to exogenous apelin. Apelin also reduced isoproterenol-induced FFA release in adipocytes isolated from wild-type but not APJ-null mice. In 3T3-L1 cells and isolated adipocytes, apelin attenuated isoproterenol-induced FFA/glycerol release. Apelin's inhibition was reversed by pertussis toxin, the G(q) inhibitor glycoprotein antagonist 2A, and the AMP-activated protein kinase inhibitors compound C and dorsomorphin. Apelin increased HSL phosphorylation at Ser-565 and also abrogated isoproterenol-induced HSL phosphorylation at Ser-563. Notably, apelin increased acetyl CoA carboxylase phosphorylation, suggesting AMPK activation. In conclusion, apelin negatively regulates lipolysis. Its actions may be mediated by pathways involving G(q), G(i), and AMP-activated protein kinase.

Lipid droplets: a dynamic organelle moves into focus

Lipid droplets (LDs) were perceived as static storage deposits, which passively participate in the energy homeostasis of both cells and entire organisms. However, this view has changed recently after the realization of a complex and highly dynamic LD proteome. The proteome contains key components of the fat mobilization system and proteins that suggest LD interactions with a variety of cell organelles, including the endoplasmic reticulum, mitochondria and peroxisomes. The study of LD cell biology, including cross-talk with other organelles, the trafficking of LDs in the cell and regulatory events involving the LD coat proteins is now on the verge of leaving its infancy and unfolds that LDs are highly dynamic cellular organelles.

Diacylglycerol enrichment of endoplasmic reticulum or lipid droplets recruits perilipin 3/TIP47 during lipid storage and mobilization

Fatty acid-induced triacylglycerol synthesis produces triacylglycerol droplets with a protein coat that includes perilipin 3/TIP47 and perilipin 4/S3-12. This study addresses the following two questions. Where do lipid droplets emerge, and how are their coat proteins recruited? We show that perilipin 3- and perilipin 4-coated lipid droplets emerge along the endoplasmic reticulum (ER). Blocking membrane trafficking with AlF(4)(-) during fatty acid-induced triacylglycerol synthesis drove perilipin 3 to the tubular ER. Forskolin, which like AlF(4)(-) activates adenylate cyclase, did not redistribute perilipin 3, but when added together with AlF(4)(-) perilipin 3 was recruited to lipid droplets rather than the ER. Thus inhibiting trafficking with AlF(4)(-) redistributed perilipin 3 differently under conditions of triacylglycerol synthesis (fatty acid addition) versus hydrolysis (forskolin) suggesting a shared acylglycerol-mediated mechanism. We tested whether diacylglycerol (DG), the immediate precursor of triacylglycerol and its first hydrolytic product, affects the distribution of perilipin 3. Stabilizing DG with the DG lipase inhibitor RHC80267 enhanced the perilipin 3 recruited to lipid droplets and raised DG levels in this fraction. Treating cells with a membrane-permeable DG recruited perilipin 3 to the ER. Stabilizing DG, by blocking its hydrolysis with RHC80267 or its acylation with triacsin C, enhanced recruitment of perilipin 3 to the ER. Expressing the ER enzyme DGAT1, which removes DG by converting it to triacylglycerol, attenuated perilipin 3 DG-induced ER recruitment. Membrane-permeable DG also drove perilipin 4 and 5 onto the ER. Together the data suggest that these lipid droplet proteins are recruited to DG-enriched membranes thereby linking lipid coat proteins to the metabolic state of the cell.

Cell based in vitro and ex vivo models in metabolic disease drug discovery: nice to have or critical path?

BACKGROUND

The use of cellular models as tools in drug discovery is almost universal. However, in disease areas such as metabolic diseases, are they relevant to the process and do they add value?

OBJECTIVE

In this article, we explore the variety of cellular models now used in drug discovery in metabolic diseases as revealed by publication. We have tried to make some connections between drug phenotypes in these models with clinical parallels. We also ask the question as to whether such models add value in the drug discovery process. This overview is not about recombinant cell systems used in target-based screening; rather, we focus on in vitro, including ex vivo, models as physiological systems in drug discovery in obesity and diabetes.

CONCLUSION

In terms of building target confidence, in vitro models are often the only mechanistic link to human systems early in a projects life. Many of the current targets in metabolic diseases in the early discovery phase are not yet clinically supported, let alone validated. In this respect, therefore, in vitro models warrant a place in the critical path in early discovery. In terms of any predictive role for decision-making today, this is much more difficult and is more likely pushed to a supporting role as part of a wider package. However, there is a rapid rate of advancement in this field and future developments hold much promise.

Lipid droplet-organelle interactions; sharing the fats

Lipid droplets (LDs) are key cellular organelles involved in lipid storage and mobilisation. While the major signalling cascades and many of the regulators of lipolysis have been identified, the cellular interactions involved in lipid mobilisation and release remain largely undefined. In non-adipocytes, LDs are small, mobile and interact with other cellular compartments. In contrast, adipocytes primarily contain very large, immotile LDs. The striking morphological differences between LDs in adipocytes and non-adipocytes suggest that key differences must exist in the manner in which LDs in different cell types interact with other organelles. Recent studies have highlighted the complexity of LD interactions, which can be both homotypic, with each other, and heterotypic, with other organelles. The molecules involved in these interactions are also now emerging, including Rab proteins, key regulators of membrane traffic, and caveolin, an integral membrane protein providing a functional link between the cell surface and LDs. Here we summarise recent insights into the cell biology of the LD particularly focussing on the homotypic and heterotypic interactions in both adipocytes and non-adipocytes. We speculate that these interactions may involve inter-organelle membrane contact sites or a hemi-fusion type mechanism to facilitate lipid transfer.

GLUT4 translocation: the last 200 nanometers

Insulin regulates circulating glucose levels by suppressing hepatic glucose production and increasing glucose transport into muscle and adipose tissues. Defects in these processes are associated with elevated vascular glucose levels and can lead to increased risk for the development of Type 2 diabetes mellitus and its associated disease complications. At the cellular level, insulin stimulates glucose uptake by inducing the translocation of the glucose transporter 4 (GLUT4) from intracellular storage sites to the plasma membrane, where the transporter facilitates the diffusion of glucose into striated muscle and adipocytes. Although the immediate downstream molecules that function proximal to the activated insulin receptor have been relatively well-characterized, it remains unknown how the distal insulin-signaling cascade interfaces with and recruits GLUT4 to the cell surface. New biochemical assays and imaging techniques, however, have focused attention on the plasma membrane as a potential target of insulin action leading to GLUT4 translocation. Indeed, it now appears that insulin specifically regulates the docking and/or fusion of GLUT4-vesicles with the plasma membrane. Future work will focus on identifying the key insulin targets that regulate the GLUT4 docking/fusion processes.

Adrenaline is a critical mediator of acute exercise-induced AMP-activated protein kinase activation in adipocytes

Exercise increases AMPK (AMP-activated protein kinase) activity in human and rat adipocytes, but the underlying molecular mechanisms and functional consequences of this activation are not known. Since adrenaline (epinephrine) concentrations increase with exercise, in the present study we hypothesized that adrenaline activates AMPK in adipocytes. We show that a single bout of exercise increases AMPKalpha1 and alpha2 activities and ACC (acetyl-CoA carboxylase) Ser79 phosphorylation in rat adipocytes. Similarly to exercise, adrenaline treatment in vivo increased AMPK activities and ACC phosphorylation. Pre-treatment of rats with the beta-blocker propranolol fully blocked exercise-induced AMPK activation. Increased AMPK activity with exercise and adrenaline treatment in vivo was accompanied by an increased AMP/ATP ratio. Adrenaline incubation of isolated adipocytes also increased the AMP/ATP ratio and AMPK activities, an effect blocked by propranolol. Adrenaline incubation increased lipolysis in isolated adipocytes, and Compound C, an AMPK inhibitor, attenuated this effect. Finally, a potential role for AMPK in the decreased adiposity associated with chronic exercise was suggested by marked increases in AMPKalpha1 and alpha2 activities in adipocytes from rats trained for 6 weeks. In conclusion, both acute and chronic exercise are significant regulators of AMPK activity in rat adipocytes. Our findings suggest that adrenaline plays a critical role in exercise-stimulated AMPKalpha1 and alpha2 activities in adipocytes, and that AMPK can function in the regulation of lipolysis.

A proposed model of fat packaging by exchangeable lipid droplet proteins

Humans have evolved mechanisms of efficient fat storage to survive famine, but these mechanisms contribute to obesity in our current environment of plentiful food and reduced activity. Little is known about how animals package fat within cells. Five related structural proteins serve roles in packaging fat into lipid droplets. The proteins TIP47, S3-12, and OXPAT/MLDP/PAT-1 move from the cytosol to coat nascent lipid droplets during rapid fat storage. In contrast, perilipin and adipophilin constitutively associate with lipid droplets and play roles in sustained fat storage and regulation of lipolysis. Different tissues express different complements of these lipid droplet proteins. Thus, the tissue-specific complement of these proteins determines how tissues manage lipid stores.

Insulin stimulates the halting, tethering, and fusion of mobile GLUT4 vesicles in rat adipose cells

Glucose transport in adipose cells is regulated by changing the distribution of glucose transporter 4 (GLUT4) between the cell interior and the plasma membrane (PM). Insulin shifts this distribution by augmenting the rate of exocytosis of specialized GLUT4 vesicles. We applied time-lapse total internal reflection fluorescence microscopy to dissect intermediates of this GLUT4 translocation in rat adipose cells in primary culture. Without insulin, GLUT4 vesicles rapidly moved along a microtubule network covering the entire PM, periodically stopping, most often just briefly, by loosely tethering to the PM. Insulin halted this traffic by tightly tethering vesicles to the PM where they formed clusters and slowly fused to the PM. This slow release of GLUT4 determined the overall increase of the PM GLUT4. Thus, insulin initially recruits GLUT4 sequestered in mobile vesicles near the PM. It is likely that the primary mechanism of insulin action in GLUT4 translocation is to stimulate tethering and fusion of trafficking vesicles to specific fusion sites in the PM.

S3-12, Adipophilin, and TIP47 Package Lipid in Adipocytes

Animals have evolved mechanisms to maintain circulating nutrient levels when energy demands exceed feeding opportunities. Mammals store most of their energy as triacylglycerol in the perilipin-coated lipid droplets of adipocytes. How newly synthesized triacylglycerol is delivered to perilipin-coated lipid droplets is poorly understood. Perilipin is a member of the evolutionarily related family of PAT proteins (Perilipin, Adipophilin, TIP47), which is defined by sequence similarity and association with lipid droplets. We previously showed that S3-12, which is also a member of this family, associates with a separate pool of lipid droplets that emerge when triacylglycerol storage is driven by adding oleate to the culture medium of adipocytes. Our current data extend these findings to demonstrate that nascent lipid droplets emerge with a coat composed of S3-12, TIP47, and adipophilin. After 100 min of oleate treatment, the nascent lipid droplets are more heterogeneous: S3-12 and TIP47 coat smaller, peripheral droplets and adipophilin coats a more medial population of droplets. Fractionation of untreated and oleate-treated adipocytes shows oleate-dependent redistribution of TIP47 and adipophilin from cytosolic fractions to the lipid droplet fraction. Inhibition of protein synthesis with cycloheximide does not block the oleate-induced formation of the nascent lipid droplets, nor does it prevent TAG accumulation. We suggest that the non-lipid droplet pools of S3-12, adipophilin, and TIP47 constitute a ready reservoir of coat proteins to permit rapid packaging of newly synthesized triacylglycerol and to maximize energy storage during nutrient excess.

Functions of pancreatic β cells and adipocytes in bombesin receptor subtype-3-deficient mice

We previously reported that mice lacking bombesin receptor subtype-3 (BRS-3) exhibit mild late-onset obesity and glucose intolerance [Nature 390 (1997) 160]. To examine the mechanism by which glucose intolerance is developed in these mice, we studied insulin release and proinsulin biosynthesis in isolated pancreatic islets and glucose uptake and facilitative glucose transporter (GLUT)-4 translocation in adipose tissues. Although islet insulin contents and the size and number of islets of Langerhans in BRS-3-deficient mice decreased, there was no difference in glucose-stimulated insulin release and proinsulin biosynthesis between BRS-3-deficient and wild-type control mice. In contrast, adipose tissues exhibited a marked difference: the uptake of [(14)C]2-deoxy-D-glucose by adipocytes isolated from BRS-3-deficient mice was not stimulated by 10(-7)M insulin addition, and membrane fractionation analysis showed that GLUT4 was barely detected in the fraction of plasma membrane in BRS-3-deficient mice in the presence of 10(-7)M insulin. Quantitative reverse transcription-PCR (RT-PCR) showed that mRNA levels of GLUT4, insulin receptor, insulin receptor substrate (IRS)-1 and IRS-2, syntaxin 4, SNAP23, and VAMP-2 in adipose tissues of BRS-3-deficient mice were unchanged compared with those in wild-type control mice. We concluded that impaired glucose metabolism observed in BRS-3-deficient mice was mainly caused by impaired GLUT4 translocation in adipocytes.

β-ADRENERGICRECEPTORS ANDREGULATION OFENERGYEXPENDITURE: A Family Affair

The family of adrenergic receptors (ARs) expressed in adipocytes includes three sibling betaARs and two alphaAR cousins. Together they profoundly influence the mobilization of stored fatty acids, secretion of fat-cell derived hormones, and the specialized process of nonshivering thermogenesis in brown adipose tissue. The two types of fat cells that compose adipose tissue, brown and white, are structurally and functionally distinct. Studies on the mechanisms by which individual betaAR regulates these cell-specific functions have recently uncovered new signal transduction cascades involved in processes traditionally ascribed to adenylyl cyclase/cAMP/protein kinase A system. They illustrate how betaAR signaling can orchestrate a coordinated set of intracellular responses for fine control of metabolic balance.

Synthesis and β-blocking activity of ( R , S )-( E )-oximeethers of 2,3-dihydro-1,8-naphthyridine and 2,3-dihydrothiopyrano[2,3- b ]pyridine: identification of β 3 -antagonists

Drugs acting on beta(1)- and beta(2)-adrenergic receptors are widely used for the clinical management of a large number of cardiovascular and respiratory pathologies. In the last decade, the discovery of the third subtype of beta receptors, the beta(3)-adrenoceptor, gave a further pharmacological target for the development of new selective drugs. Initially, a potential therapeutic use of beta(3)-selective agents seemed to be restricted to agonists, for the treatment of metabolic diseases, such as obesity, non-insulin-dependent diabetes, urinary frequency and incontinence. More recently, some interesting theories about a negative role played by the cardio-depressant activity of myocardial beta(3)-adrenoceptors in heart failure, seemed to justify a clinical use of beta(3)-antagonists in the last phases of this cardiac disease. Following the indications deriving from previous experimental work, the beta-antagonist properties of newly-synthesised (R,S)-(E)-oximeethers of 2,3-dihydro-1,8-naphthyridine and of 2,3-dihydrothiopyrano[2,3-b]pyridine were evaluated, in order to identify some useful structure-activity relationships, which might account for selectivity towards the three beta-subtypes and, in particular, the beta(3)-adrenoceptor. Among the various observations regarding possible structure-activity relationships, able to explain the pharmacodynamic patterns of the synthesised compounds on the three subtypes of beta-adrenoceptors, the most significant data derived from the evaluation of the beta(3)-blocking properties of some oximeethers of 1,8-naphthyridine derivatives. In these molecules, although the presence of the large substituents in position 7, such as 4-chloro-phenoxy- or 4-t-butyl-phenoxy groups determined a dramatic decline in both the beta(1)- and beta(2)-activities, this structural characteristic had a modest influence on the beta(3)-affinity, which was only slightly lower. Hence, this last structural requirement of oximeethers of 1,8-naphthyridine derivatives seems to represent a useful expedient to induce an appreciable selectivity towards the beta(3)-receptor, through a markedly negative effect on the beta(1)- and beta(2)-activities rather than an increase in the beta(3)-affinity.

Cell surface orifices of caveolae and localization of caveolin to the necks of caveolae in adipocytes

Caveolae are noncoated invaginations of the plasma membrane that form in the presence of the protein caveolin. Caveolae are found in most cells, but are especially abundant in adipocytes. By high-resolution electron microscopy of plasma membrane sheets the detailed structure of individual caveolae of primary rat adipocytes was examined. Caveolin-1 and -2 binding was restricted to the membrane proximal region, such as the ducts or necks attaching the caveolar bulb to the membrane. This was confirmed by transfection with myc-tagged caveolin-1 and -2. Essentially the same results were obtained with human fibroblasts. Hence caveolin does not form the caveolar bulb in these cells, but rather the neck and may thus act to retain the caveolar constituents, indicating how caveolin participates in the formation of caveolae. Caveolae, randomly distributed over the plasma membrane, were very heterogeneous, varying in size between 25 and 150 nm. There was about one million caveolae in an adipocyte, which increased the surface area of the plasma membrane by 50%. Half of the caveolae, those larger than 50 nm, had access to the outside of the cell via ducts and 20-nm orifices at the cell surface. The rest of the caveolae, those smaller than 50 nm, were not open to the cell exterior. Cholesterol depletion destroyed both caveolae and the cell surface orifices.

Regulation of Wnt signaling during adipogenesis

We have identified Wnt10b as a potent inhibitor of adipogenesis that must be suppressed for preadipocytes to differentiate in vitro. Here, we demonstrate that a specific inhibitor of glycogen synthase kinase 3, CHIR 99021, mimics Wnt signaling in preadipocytes. CHIR 99021 stabilizes free cytosolic beta-catenin and inhibits adipogenesis by blocking induction of CCAAT/enhancer-binding protein alpha and peroxisome proliferator-activated receptor gamma. Preadipocyte differentiation is inhibited when 3T3-L1 cells are exposed to CHIR 99021 for any 24 h period during the first 3 days of adipogenesis. Consistent with this time frame of inhibition, expression of Wnt10b mRNA is suppressed upon induction of differentiation, with a 50% decline by 6 h and complete inhibition by 36 h. Of the agents used to induce differentiation, exposure of 3T3-L1 cells to methyl-isobutylxanthine or cAMP is sufficient to suppress expression of Wnt10b mRNA. Inhibition of adipogenesis by Wnt10b is likely mediated by Wnt receptors, Frizzled 1, 2, and/or 5, and co-receptors low density lipoprotein receptor-related proteins 5 and 6. These receptors, like Wnt10b, are highly expressed in preadipocytes and stromal vascular cells. Finally, we demonstrate that disruption of extracellular Wnt signaling by expression of secreted Frizzled related proteins causes spontaneous adipocyte conversion.

Impaired PI 3-kinase activation in adipocytes from early growth-restricted male rats

Epidemiological studies have established a relationship between early growth restriction and subsequent development of type 2 diabetes. Animal studies have shown that offspring of protein-restricted rats undergo a greater age-related loss of glucose tolerance than controls. The aim of this study was to investigate the possibility that this deterioration of glucose tolerance is associated with changes in adipocyte insulin action. Adipocytes from low-protein offspring had higher basal levels of glucose uptake than controls. Insulin stimulated glucose uptake into control adipocytes but had little effect on low-protein adipocytes. Both groups had similar levels of basal and isoproterenol-stimulated lipolysis. Insulin inhibited lipolysis in control adipocytes but had a reduced effect on low-protein adipocytes. These changes in insulin action were not related to altered expression of insulin receptors or insulin receptor tyrosine phosphorylation; however, they were associated with reduced phosphatidylinositol 3-kinase and protein kinase B activation. These results demonstrate that reduced glucose tolerance observed in late adult life after early growth restriction is associated with adipocyte insulin resistance.

Catecholamine levels and receptor expression in low protein rat offspring

AIMS

Low birthweight in humans has been shown to lead to increased resting pulse rate in adult life, suggesting possible increased sympathoadrenal activity. The hypothesis that early growth restriction is associated with permanent alterations in catecholamine metabolism was tested.

METHODS

Circulating catecholamine concentrations (by radioimmunoassay) and adipocyte adrenoceptor expression from different fat depots (by Western blot) were estimated in 12-week-old male offspring of rat dams fed a reduced protein diet during pregnancy and lactation.

RESULTS

In the fed state, median (interquartile range) plasma adrenaline concentrations for male control and low protein offspring rats were: 0.65 (0.48-0.86) vs. 1.42 (0.89-1.87) nmol/l (P < 0.005), respectively. Equivalent noradrenaline concentrations were: 2.71 (2.16-3.46) vs. 3.45 (3.00-4.28) nmol/l (P < 0.05). After 24 h starvation, plasma adrenaline concentrations of controls rose to become similar to those of low protein offspring: 1.03 (0.95-1.31) vs. 1.41 (0.69-1.62) nmol/l (P = 0.3), respectively. Noradrenaline concentrations rose in both groups to become similar: 3.84 (3.33-4.54) vs. 4.32 (3.70-6.54) nmol/l (P = 0.3). In epididymal adipocytes adrenoceptor expression (relative to that of controls) was: alpha2A 0.79 (0.66-0.94) (P = 0.08), beta1 2.60 (2.27-3.07) (P = 0.04), beta3 1.37 (1.27-1.46) (P = 0.02). Similar-pattern differences in adrenoceptor expression were observed in subcutaneous and intra-abdominal adipocytes.

CONCLUSIONS

These results are consistent with the suggestion that long-term alterations in catecholamine metabolism are present in adult offspring of rats fed a reduced protein diet during pregnancy and lactation.

Synthesis and beta-blocking activity of (R,S)-(E)-oximeethers of 2, 3-dihydro-1,8-naphthyridine and 2,3-dihydrothiopyrano[2, 3-b]pyridine:potential antihypertensive agents - part IX

The synthesis of oximeethers of 2,3-dihydro-1,8-naphthyridine and 2, 3-dihydrothiopyrano[2,3-b]pyridine is described. These compounds exhibit a selective beta-blocking activity, with a selectivity towards beta(2)-receptors. Groups in the N(1) position giving rise to a considerable steric hindrance led to a higher beta(2)-blocking selectivity, whereas groups creating a moderate hindrance caused a weak but significant decrease in beta(2)-antagonist potency. Substitution of the N(1)-R group with a sulfur atom led to compounds possessing beta(1)-, beta(2)- and beta(3)-blocking properties. Compounds 9c(1) and 10a(1) showed a beta(3)-antagonist activity slightly lower than that of propranolol.

Altered adipocyte properties in the offspring of protein malnourished rats

It is becoming well established that poor fetal and early postnatal growth can have long-term effects on adult health, including susceptibility to non-insulin-dependent diabetes mellitus, cardiovascular disease and hypertension. It is suggested that this results from poor nutrition during early life having permanent effects on the structure and metabolism of certain organs and tissues. In the present study we investigated the effect of a low-protein diet during pregnancy and lactation on adipocyte properties and glucose tolerance. Rat dams were fed on a diet containing either 200 (control) or 80 (low protein) g protein/kg during pregnancy and lactation. In addition cross-fostering techniques were employed to enable a separate evaluation of the prenatal and postnatal periods. All offspring were weaned onto a 200 g protein/kg diet at 21 d of age and then studied at 6 weeks of age. The mothers' protein supply during lactation appeared to be the most critical time window for long-term growth. In contrast, the offspring of mothers fed on a low-protein diet during pregnancy or lactation were significantly more glucose tolerant than controls, suggesting that both time windows can have long-term effects on glucose tolerance. In addition offspring of mothers fed on a low-protein diet during pregnancy or lactation had significantly smaller adipocytes than controls. However the largest reduction in adipocyte size was observed when there was a low-protein diet during both pregnancy and lactation. The amount of insulin receptor present in adipocyte membranes was increased in the three animal groups that had been exposed to the low-protein diets while levels of the insulin responsive glucose transporter (GLUT 4) were similar in adipocyte membranes from all groups.

Protein kinase C isoforms epsilon, eta, delta and zeta in murine adipocytes: expression, subcellular localization and tissue-specific regulation in insulin-resistant states

The Ca(2+)-insensitive protein kinase C (PKC) isoforms epsilon, eta, delta and zeta are possible direct downstream targets of phosphatidylinositol 3-kinase (P13-K), and might therefore be involved in insulin signalling. Although isoform-specific changes in PKC expression have been reported for skeletal muscle and liver in insulin-resistant states, little is known about these isoforms in adipocytes. Therefore we studied (1) expression and subcellular localization of these isoforms in murine adipocytes, (2) translocation of specific isoforms to membranes in response to treatment with insulin and phorbol 12-myristate 13-acetate (PMA) and (3) regulation of expression in insulin-resistant states. The PKC isoforms epsilon, eta, delta and zeta are expressed in adipocytes. Immunoreactivity for all isoforms is higher in the membranes than in the cytosol, but subcellular fractionation by differential centrifugation shows an isoform-specific distribution within the membrane fractions. PMA treatment of adipocytes induces translocation of PKC-epsilon and -delta from the cytosol to the membrane fractions. Insulin treatment does not alter the subcellular distribution of any of the isoforms. 3T3-L1 adipocytes express PKC-epsilon and -zeta, and PKC-epsilon expression increases with differentiation from preadipocytes to adipocytes. PKC-epsilon expression decreases in an adipose-specific and age/obesity-dependent manner in two insulin-resistant models, the brown-adipose-tissue-deficient mouse and db/db mouse compared with control mice. We conclude that, although none of the isoforms investigated seems to be activated by insulin, the decrease in PKC-epsilon expression might contribute to metabolic alterations in adipocytes associated with insulin resistance and obesity.

Overexpression of Ha-ras selectively in adipose tissue of transgenic mice. Evidence for enhanced sensitivity to insulin

To determine the role of Ras-dependent signaling pathways in adipocyte function, we created transgenic mice that overexpress Ha-ras in adipocytes using the aP2 fatty acid-binding protein promoter/enhancer ligated to the human genomic ras sequence. ras mRNA was increased 8-17-fold and Ras protein 4-5-fold in white and brown fat, with no overexpression in other tissues. The subcellular distribution of overexpressed Ras paralleled that of endogenous Ras. [U-14C]Glucose uptake into isolated adipocytes was increased approximately 2-fold in the absence of insulin, and the ED50 for insulin was reduced 70%, with minimal effect on maximally stimulated glucose transport. Expression of Glut4 protein was unaltered in transgenic adipocytes, but photoaffinity labeling of transporters in intact cells with [3H]2-N-[4-(1-azi-Z,Z,Z-trifluoroethyl)benzoyl]-1,3-bis-(D-mann os-4- yloxy)-2-propylamine revealed 1.7-2.6-fold more cell-surface Glut 4 in the absence of insulin and at half-maximal insulin concentration (0.3 nM) compared with nontransgenic adipocytes. With maximal insulin concentration (80 nM), cell-surface Glut4 in nontransgenic and transgenic adipocytes was similar. Glut1 expression and basal cell-surface Glut1 were increased 2-2.9-fold in adipocytes of transgenic mice. However, Glut1 was much less abundant than Glut4, making its contribution to transport negligible. These in vitro changes were accompanied by in vivo alterations including increased glucose tolerance, decreased plasma insulin levels, and decreased adipose mass. We conclude that ras overexpression in adipocytes leads to a partial translocation of Glut4 in the absence of insulin and enhanced Glut4 translocation at physiological insulin concentration, but no effect with maximally stimulating insulin concentrations.