Adipose Physiology Research Articles

Postprandial fatty acid uptake and adipocyte remodeling in angiotensin type 2 receptor-deficient mice fed a high-fat/high-fructose diet

ABSTRACTThe role of the angiotensin type-2 receptor in adipose physiology remains controversial. The aim of the present study was to demonstrate whether genetic angiotensin type-2 receptor-deficiency prevents or worsens metabolic and adipose tissue morphometric changes observed following a 6-week high-fat/high-fructose diet with injection of a small dose of streptozotocin. We compared tissue uptake of nonesterified fatty acid and dietary fatty acid in wild-type and angiotensin type-2 receptor-deficient mice by using the radiotracer 14(R,S)-[18F]-fluoro-6-thia-heptadecanoic acid in mice fed a standard or high-fat diet. Postprandial fatty acid uptake in the heart, liver, skeletal muscle, kidney and adipose tissue was increased in wild-type mice after a high-fat diet and in angiotensin type-2 receptor-deficient mice on both standard and high-fat diets. Compared to the wild-type mice, angiotensin type-2 receptor-deficient mice had a lower body weight, an increase in fasting blood glucose and a decrease in plasma insulin and leptin levels. Mice fed a high-fat diet exhibited increased adipocyte size that was prevented by angiotensin type-2 receptor-deficiency. Angiotensin type-2 receptor-deficiency abolished the early hypertrophic adipocyte remodeling induced by a high-fat diet. The small size of adipocytes in the angiotensin type-2 receptor-deficient mice reflects their inability to store lipids and explains the increase in fatty acid uptake in non-adipose tissues. In conclusion, a genetic deletion of the angiotensin type-2 receptor is associated with metabolic dysfunction of white adipose depots, and indicates that adipocyte remodeling occurs before the onset of insulin resistance in the high-fat fed mouse model.

Metabolic pathways linking glucoregulatory dysfunction and inflammatory physiology in ethnically distinct populations

In spite of publicity and policy efforts, industrialized countries continue to struggle with the increased prevalence of obesity, metabolic syndrome and cardiovascular disease. All conditions have been associated with inflammatory processes. However, the complex interactions between adipose, lipid, glucoregulatory, and inflammatory physiology challenge our thorough understanding of the causal and underlying pathways involved in metabolic disease progression. Furthermore, it is known that metabolic interactions vary by age, race and gender. Pathway analyses of complex multifactorial systems are not feasible in small population studies. We took advantage of the biomarkers of inflammation, adiposity, lipid balance and glucoregulation, assessed in a large sample of adults in the Midlife in the United States (MIDUS) and the Midlife in Japan (MIDJA) Projects. Structural equation models were employed to test the fit of a hypothetical model based on known physiological and biochemical linkages within and across these physiological systems. We also tested how well the primary model captured national differences between Japanese and Americans, and distinct racial differences between white and black Americans, and ultimately derived better models to fit each ethnic group. In our a priori model, we hypothesized causal pathways leading from glucoregulatory dysfunction to lipid abnormalities associated with inflammation, as indexed by proinflammatory cytokines (IL-6, CRP, FBG, ICAM-1, E-SEL). Our findings confirm the policy and clinical concerns about the extent of obesity and inflammation among American adults.

Interaction of circadian and stress systems in the regulation of adipose physiology.

Endogenous circadian clocks facilitate the adaptation of physiology and behavior to recurring environmental changes brought about by the Earth's rotation around its axis. Adipose tissues harbor intrinsic circadian oscillators based on interlocked transcriptional-translational feedback loops built from a set of clock genes that regulate important aspects of lipid metabolism and adipose endocrine function. These adipocyte clocks are reset via neuronal and endocrine pathways originating from a master circadian pacemaker residing in the hypothalamic suprachiasmatic nucleus. One important mediator of circadian output is the stress hormone cortisol, which, at the same time, is one of the major regulators of adipose physiology. In this review we summarize recent findings on the interaction between circadian and stress systems in the regulation of adipose physiology and discuss the implications of this crosstalk for the development of metabolic disorders associated with circadian disruption and/or chronic stress, for example in shift workers.

Recent advance in brown adipose physiology and its therapeutic potential

Brown adipose tissue (BAT) is a specialized thermoregulatory organ that has a critical role in the regulation of energy metabolism. Specifically, energy expenditure can be enhanced by the activation of BAT function and the induction of a BAT-like catabolic phenotype in white adipose tissue (WAT). Since the recent recognition of metabolically active BAT in adult humans, BAT has been extensively studied as one of the most promising targets identified for treating obesity and its related disorders. In this review, we summarize information on the developmental origin of BAT and the progenitors of brown adipocytes in WAT. We explore the transcriptional control of brown adipocyte differentiation during classical BAT development and in WAT browning. We also discuss the neuronal control of BAT activity and summarize the recently identified non-canonical stimulators of BAT that can act independently of β-adrenergic stimulation. Finally, we review new findings on the beneficial effects of BAT activation and development with respect to improving metabolic profiles. We highlight the therapeutic potential of BAT and its future prospects, including pharmacological intervention and cell-based therapies designed to enhance BAT activity and development.

Peripheral tumor necrosis factor α regulation of adipose tissue metabolism and adipokine gene expression in neonatal pigs

The neonatal pig is susceptible to stress and infection, conditions which favor tumor necrosis factor α (TNFα) secretion. This study examined whether TNFα can alter metabolic activity and cytokine gene expression within neonatal pig adipose tissue. Cell cultures were prepared from neonatal subcutaneous adipose tissue using standard procedures. Cultures (5 experiments) were incubated with medium containing (14)C-glucose for 4 h to measure glucose conversion to lipid in the presence of combinations of TNFα (10 ng), insulin (10 nM) and an anti-pig TNFα antibody (5 μg). Basal lipogenesis was not affected by TNFα treatment (P > 0.05). However, insulin stimulated lipogenesis was reduced by TNFα (P < 0.02). For gene expression studies, cultures were incubated with 0, 2.5, 5.0 or 10 ng TNFα for 2, 4 or 24 h (n = 4 experiments). Interleukin 6 and TNFα gene expression were acutely (2-4 h) stimulated by exogenous TNFα treatment (P < 0.05), as analyzed by real-time PCR. Adiponectin mRNA abundance was reduced (P < 0.001) while monocyte chemotactic gene expression was increased by TNFα treatment at all time points (P < 0.001). Chronic treatment (24 h) was required to increase monocyte multiplication inhibitory factor or suppress lipoprotein lipase gene expression (P < 0.02). These data suggest conditions which increase serum TNFα, like sepsis, could suppress lipid accumulation within adipose tissue at a time of critical need in the neonate and induce a variety of adipose derived cytokines which may function to alter adipose physiology.

Natural compounds involved in adipose tissue mass control in in vitro studies

The World Health Organization (WHO) has recognized obesity as an epidemic of the 21st century. Obesity is pathological fat accumulation in the body influenced by many factors: metabolic, endocrine, genetic, environmental, psychological and behavioral. The quality and quantity of food intake to a considerable degree determine excessive fat accumulation in the body. The strategy in obesity prevention includes, among other things, a proper diet. It is widely known that a diet rich in fruits and vegetables reduces body weight. Adipocytes are not only cells serving as storage depots for "energy", but are also specialized cells influenced by various hormones, cytokines and nutrients, which have pleiotropic effects on the body. Knowledge of adipocyte biology is crucial for our understanding of the pathophysiological basis of obesity and metabolic diseases, such as type 2 diabetes. Furthermore, rational manipulation of adipose physiology is a promising avenue for therapy of these conditions. Adipose tissue mass can be reduced through elimination of adipocytes by apoptosis, inhibition of adipogenesis and increased lipolysis in adipocytes. Natural products have a potential to induce apoptosis, inhibit adipogenesis and stimulate lipolysis in adipocytes. Various dietary bioactive compounds target different stages of the adipocyte life cycle and may be useful as natural therapeutic agents in obesity prevention.

Smad3 Deficiency in Mice Protects Against Insulin Resistance and Obesity Induced by a High-Fat Diet

OBJECTIVEObesity and associated pathologies are major global health problems. Transforming growth factor-β/Smad3 signaling has been implicated in various metabolic processes, including adipogenesis, insulin expression, and pancreatic β-cell function. However, the systemic effects of Smad3 deficiency on adiposity and insulin resistance in vivo remain elusive. This study investigated the effects of Smad3 deficiency on whole-body glucose and lipid homeostasis and its contribution to the development of obesity and type 2 diabetes.RESEARCH DESIGN AND METHODSWe compared various metabolic profiles of Smad3-knockout and wild-type mice. We also determined the mechanism by which Smad3 deficiency affects the expression of genes involved in adipogenesis and metabolism. Mice were then challenged with a high-fat diet to study the impact of Smad3 deficiency on the development of obesity and insulin resistance.RESULTSSmad3-knockout mice exhibited diminished adiposity with improved glucose tolerance and insulin sensitivity. Chromatin immunoprecipitation assay revealed that Smad3 deficiency increased CCAAT/enhancer-binding protein β-C/EBP homologous protein 10 interaction and exerted a differential regulation on proliferator-activated receptor β/δ and proliferator-activated receptor γ expression in adipocytes. Focused gene expression profiling revealed an altered expression of genes involved in adipogenesis, lipid accumulation, and fatty acid β-oxidation, indicative of altered adipose physiology. Despite reduced physical activity with no modification in food intake, these mutant mice were resistant to obesity and insulin resistance induced by a high-fat diet.CONCLUSIONSSmad3 is a multifaceted regulator in adipose physiology and the pathogenesis of obesity and type 2 diabetes, suggesting that Smad3 may be a potential target for the treatment of obesity and its associated disorders.

CIRCADIAN RHYTHMICITY IN MURINE PRE-ADIPOCYTE AND ADIPOCYTE CELLS

Adipose tissue is central to metabolic homeostasis, signaling in part through the secretion of molecules termed adipokines. Circadian rhythms play an important role in adipose physiology, with plasma adipokine concentration and ∼20 % of the murine adipose transcriptome undergoing 24 h variation. However, due to the heterogeneity of adipose tissue and rhythmical input from both neuronal and humoral signals, the cellular basis of adipose rhythms is unclear. We tested the hypothesis that adipocyte cells contain a circadian clock that drives rhythmic mRNA expression and adipokine secretion. From the murine pre-adipocyte 3T3-L1 cell line, we generated populations of both pre-adipocytes and differentiated adipocytes. Cells were then treated with a 2 h serum pulse and sampled every 4 h over a 48 h period. Expression of clock gene, ‘metabolic’ gene (PPARα, PPARγ, SREBP1), and adipokine mRNA was analyzed by quantitative real-time PCR, and secretion of the adipokines leptin and adiponectin was measured in culture medium from differentiated adipocytes. In pre-adipocytes, we observed robust rhythms of clock genes Per2, Rev-erbα, and Dbp, but not of Per1, Cry1, Bmal1, or any of the ‘metabolic’ genes. Adipocytes produced similar temporal profiles of mRNA expression, albeit with a markedly reduced amplitude of Per2 and Dbp rhythms. Despite no circadian rhythm of adipokine mRNA expression, leptin accumulation in the culture medium suggested circadian control of leptin secretion from adipocytes. Adiponectin secretion showed temporal variation, but without any apparent circadian rhythmicity. Our data, therefore, suggest that an endogenous adipocyte clock controls the rhythmic expression of only a subset of genes that are reported to exhibit 24 h rhythmicity in murine adipose tissue. Moreover, secretion of leptin may also be regulated by the adipocyte clock. (Author correspondence: j.johnston@surrey.ac.uk).

Do calorie restriction or alternate-day fasting regimens modulate adipose tissue physiology in a way that reduces chronic disease risk?

Adipose tissue physiology plays an important role in the development of several obesity-related disorders. Dietary restriction regimens, i.e., daily calorie restriction (CR) or alternate-day fasting (ADF), have been shown to decrease the risk of these disorders. Whether changes in adipose mass or physiology are required for the beneficial effects of CR or ADF is an important question. Accordingly, this review summarizes the effects of CR and ADF regimens on parameters of adipose physiology, i.e., adipose tissue morphology, triglyceride metabolism, and adipokine release, and attempts to link these changes to indicators of chronic disease risk.

Adipocytes as regulators of energy balance and glucose homeostasis

Adipocytes have been studied with increasing intensity as a result of the emergence of obesity as a serious public health problem and the realization that adipose tissue serves as an integrator of various physiological pathways. In particular, their role in calorie storage makes adipocytes well suited to the regulation of energy balance. Adipose tissue also serves as a crucial integrator of glucose homeostasis. Knowledge of adipocyte biology is therefore crucial for understanding the pathophysiological basis of obesity and metabolic diseases such as type 2 diabetes. Furthermore, the rational manipulation of adipose physiology is a promising avenue for therapy of these conditions.

Endothelin-1 induces lipolysis in 3T3-L1 adipocytes

Endothelin-1 (ET-1) affects glucose uptake in adipocytes and may play an important role in adipose physiology. One of the principal functions of adipose tissue is the provision of energy substrate through lipolysis. In the present study, we investigated the effects of ET-1 on lipolysis in 3T3-L1 adipocytes. When glycerol release in the culture medium was measured as an index of lipolysis, the results showed that ET-1 caused a significant increase that was time and dose dependent. With a concentration of 10 nM ET-1, stimulation of glycerol release plateaued after 4 h of exposure. This effect was inhibited by the ETA receptor antagonist BQ-610 (10 microM) but not by the ETB receptor antagonist BQ-788 (10 microM). To further explore the underlying mechanisms of ET-1 action, we examined the involvement of the cAMP-dependent protein kinase A-mediated, phospholipase A2 (PLA2)-mediated, protein kinase C (PKC)-mediated, phosphatidylinositol 3 (PI 3)-kinase-mediated, and the mitogen-activated protein kinase (MAPK)-mediated pathways. Inhibition of adenylyl cyclase activation by SQ-22536 (100 microM) did not block ET-1-induced lipolysis. Pretreatment of adipocytes with the PLA2 inhibitor dexamethasone (100 nM), the PKC inhibitor H-7 (6 microM), or the PI 3-kinase inhibitor wortmannin (100 nM) also had no effect. ET-1-induced lipolysis was blocked by inhibition of extracellular signal-regulated kinase (ERK) activation using PD-98059 (75 microM), whereas a p38 MAPK inhibitor (SB-203580; 20 microM) had no effect. Results of Western blot further demonstrated that ET-1 induced ERK phosphorylation. These data show that ET-1 induces lipolysis in 3T3-L1 adipocytes via a pathway that is different from the conventional cAMP-dependent pathway used by isoproterenol and that involves ERK activation.

Surgical implantation of adipose tissue reverses diabetes in lipoatrophic mice.

In lipoatrophic diabetes, a lack of fat is associated with insulin resistance and hyperglycemia. This is in striking contrast to the usual association of diabetes with obesity. To understand the underlying mechanisms, we transplanted adipose tissue into A-ZIP/F-1 mice, which have a severe form of lipoatrophic diabetes. Transplantation of wild-type fat reversed the hyperglycemia, dramatically lowered insulin levels, and improved muscle insulin sensitivity, demonstrating that the diabetes in A-ZIP/F-1 mice is caused by the lack of adipose tissue. All aspects of the A-ZIP/F-1 phenotype including hyperphagia, hepatic steatosis, and somatomegaly were either partially or completely reversed. However, the improvement in triglyceride and FFA levels was modest. Donor fat taken from parametrial and subcutaneous sites was equally effective in reversing the phenotype. The beneficial effects of transplantation were dose dependent and required near-physiological amounts of transplanted fat. Transplantation of genetically modified fat into A-ZIP/F-1 mice is a new and powerful technique for studying adipose physiology and the metabolic and endocrine communication between adipose tissue and the rest of the body.

Measurement and seasonal variations of black bear adipose lipoprotein lipase activity

The black bear ( Ursus americanus) provides a unique model for the study of adipose physiology because it exhibits seasonal periods of rapid weight gain and weight loss without marked changes in its metabolic rate. To better understand fat cycling in this model, we obtained plasma and gluteal adipose tissue from five captive adult bears at approximately 20-day intervals from October 1 1992 through March 31 1993. The study included a predenning and denning period for each animal. Sampling during the predenning period followed a 12-h fast. Bears were anorectic while denning. Adipose LPL activities and plasma insulin concentrations were determined for each time point. Predenning LPL activities (4.83 ± 0.64 μmol/h/g) were significantly greater than those seen during the denning period (1.82 ± 0.65, p < 0.001). A biphasic pattern of fasting LPL activity was seen in four of the five bears during the predenning period. Fasting insulin concentrations showed no such pattern of variation during the study period (mean = 25.1 ± 1.36 pmol/l; range 1.1–6.0). We found no evidence of a linear relationship between LPL activity and insulin levels ( p = 0.139). Neither LPL activity nor insulin concentrations were related to changes in weight ( p = 0.257 and p = 0.7104, respecitively). We conclude that LPL activity can be measured in black bear adipose tissue and that fall (predenning) activities are significantly higher than those seen during the winter (denning period). Furthermore, the seasonal regulation of LPL involves some factor(s) in addition to insulin.

Biosynthetic regulation of monobutyrin, an adipocyte-secreted lipid with angiogenic activity.

1-Butyrylglycerol (monobutyrin) is a novel angiogenic compound that is synthesized and secreted during the differentiation of 3T3-F442A preadipocytes into adipocytes. To study the regulation of monobutyrin biosynthesis we have developed an assay utilizing the lgycerol kinase enzyme from Cellulomonas to quantitate the levels of this compound in cell-conditioned medium. Analysis of several cultured cell types, including tumor cell lines, indicated that monobutyrin production is detectable only from adipocytes, reaching a steady-state concentration of approximately 1.0 microM in conditioned medium. Monobutyrin synthesis was demonstrated in vitro using [14C]butyryl-CoA with total homogenate or particulate fractions from adipocytes. Similar fractions from non-adipocyte cell lines failed to synthesize monobutyrin. This biosynthetic activity was shown to be distinct by substrate competition studies from the microsomal sn-glycerol-3-phosphate acyltransferase, whose activity is known to increase during adipocyte differentiation. The production of monobutyrin was hormonally regulated, as the addition of epinephrine to adipocytes caused a 10-fold increase in the amount of monobutyrin secreted. These results indicate that monobutyrin synthesis is adipocyte specific, occurs through an apparently novel particulate enzyme system, and is regulated in a hormone-dependent manner. The implications of these results for adipose physiology and angiogenesis are discussed.