Transgenic mice lacking white fat: models for understanding human lipoatrophic diabetes

Adipocyte biology: It is time to upgrade to a new model

Globally, the obesity pandemic is profoundly affecting quality of life and economic productivity, but efforts to address this, especially on a pharmacological level, have generally proven unsuccessful to date, serving as a stark demonstration that our understanding of adipocyte biology and pathophysiology is incomplete. To deliver better insight into adipocyte function and obesity, we need improved adipocyte models with a high degree of fidelity in representing the in vivo state and with a diverse range of experimental applications. Adipocyte cell lines, especially 3T3-L1 cells, have been used extensively over many years, but these are limited in terms of relevance and versatility. In this review, I propose that primary adipose-derived stromal/stem cells (ASCs) present a superior model with which to study adipocyte biology ex vivo. In particular, ASCs afford us the opportunity to study adipocytes from different, functionally distinct, adipose depots and to investigate, by means of in vivo/ex vivo studies, the effects of many different physiological and pathophysiological factors, such as age, body weight, hormonal status, diet and nutraceuticals, as well as disease and pharmacological treatments, on the biology of adipocytes and their precursors. This study will give an overview of the characteristics of ASCs and published studies utilizing ASCs, to highlight the areas where our knowledge is lacking. More comprehensive studies in primary ASCs will contribute to an improved understanding of adipose tissue, in healthy and dysfunctional states, which will enhance our efforts to more successfully manage and treat obesity.

Hexarelin, a Growth Hormone Secretagogue, Improves Lipid Metabolic Aberrations in Nonobese Insulin-Resistant Male MKR Mice

Despite the occurrence of dyslipidemia and its contribution to the development of insulin resistance in obese subjects, a growing number of studies have described abnormal lipid profiles among leaner persons. For example, individuals with an abnormal paucity or distribution of fat (lipodystrophy) develop severe insulin resistance, dyslipidemia, and hepatic steatosis. Deranged adipocyte metabolism and differentiation contribute to ectopic fat deposition and consequent development of insulin resistance. Growth hormone (GH) therapy has been shown to correct body composition abnormalities in some lipodystrophy patients. However, little is known about the effects of GH-releasing peptides in this regard. Hexarelin, a GH secretagogue, has recently been shown to have beneficial effects on fat metabolism via the CD36 receptor. In this study, the effects of twice daily intraperitoneal injections of hexarelin (200 μg/kg body weight) were examined in nonobese insulin-resistant MKR mice and corresponding wild-type FVB mice for 12 days. Hexarelin treatment significantly improved glucose and insulin intolerance and decreased plasma and liver triglycerides in MKR mice. These beneficial metabolic effects could be due to the improved lipid metabolism and enhanced adipocyte differentiation of white adipose tissue with hexarelin treatment. Interestingly, although food intake of hexarelin-treated MKR mice was significantly increased, this did not change total body weight. Moreover, hexarelin treatment corrected the abnormal body composition of MKR mice, as demonstrated by a decrease in fat mass and an increase in lean mass. Our results suggest a possible application of hexarelin in treatment of lipid disorders associated with the metabolic syndrome.

MECHANISMS IN ENDOCRINOLOGY: Skeletal muscle lipotoxicity in insulin resistance and type 2 diabetes: a causal mechanism or an innocent bystander?

Dysfunctional adipose tissue is associated with an increased risk of developing type 2 diabetes (T2D). One characteristic of a dysfunctional adipose tissue is the reduced expandability of the subcutaneous adipose tissue leading to ectopic storage of fat in organs and/or tissues involved in the pathogenesis of T2D that can cause lipotoxicity. Accumulation of lipids in the skeletal muscle is associated with insulin resistance, but the majority of previous studies do not prove any causality. Most studies agree that it is not the intramuscular lipids per se that causes insulin resistance, but rather lipid intermediates such as diacylglycerols, fatty acyl-CoAs and ceramides and that it is the localization, composition and turnover of these intermediates that play an important role in the development of insulin resistance and T2D. Adipose tissue is a more active tissue than previously thought, and future research should thus aim at examining the exact role of lipid composition, cellular localization and the dynamics of lipid turnover on the development of insulin resistance. In addition, ectopic storage of fat has differential impact on various organs in different phenotypes at risk of developing T2D; thus, understanding how adipogenesis is regulated, the interference with metabolic outcomes and what determines the capacity of adipose tissue expandability in distinct population groups is necessary. This study is a review of the current literature on the adipose tissue expandability hypothesis and how the following ectopic lipid accumulation as a consequence of a limited adipose tissue expandability may be associated with insulin resistance in muscle and liver.

Adipose Tissue as an Endocrine Organ: An Update on Pro-inflammatory and Anti-inflammatory Microenvironment

Adipose tissue is recognized as an active endocrine organ that produces a number of endocrine substances referred to as "adipokines" including leptin, adiponectin, adipolin, visfatin, omentin, tumour necrosis factor-alpha (TNF-α), interleukin-6 (IL-6), resistin, pigment epithelium-derived factor (PEDF), and progranulin (PGRN) which play an important role in the food intake regulation and significantly influence insulin sensitivity and in some cases directly affect insulin resistance in skeletal muscle, liver, and adipose tissue. The review summarizes current knowledge about adipose tissue-derived hormones and their influence on energy homeostasis regulation. The possible therapeutic potential of these adipokines in the treatment of insulin resistance, endothelial dysfunction, a pro-inflammatory response, obesity, eating disorders, progression of atherosclerosis, type 1 diabetes, and type 2 diabetes is discussed.

Chlorinated persistent organic pollutants, obesity, and type 2 diabetes

Persistent organic pollutants (POPs) are lipophilic compounds that travel with lipids and accumulate mainly in adipose tissue. Recent human evidence links low-dose POPs to an increased risk of type 2 diabetes (T2D). Because humans are contaminated by POP mixtures and POPs possibly have nonmonotonic dose-response relations with T2D, critical methodological issues arise in evaluating human findings. This review summarizes epidemiological results on chlorinated POPs and T2D, and relevant experimental evidence. It also discusses how features of POPs can affect inferences in humans. The evidence as a whole suggests that, rather than a few individual POPs, background exposure to POP mixtures-including organochlorine pesticides and polychlorinated biphenyls-can increase T2D risk in humans. Inconsistent statistical significance for individual POPs may arise due to distributional differences in POP mixtures among populations. Differences in the observed shape of the dose-response curves among human studies may reflect an inverted U-shaped association secondary to mitochondrial dysfunction or endocrine disruption. Finally, we examine the relationship between POPs and obesity. There is evidence in animal studies that low-dose POP mixtures are obesogenic. However, relationships between POPs and obesity in humans have been inconsistent. Adipose tissue plays a dual role of promoting T2D and providing a relatively safe place to store POPs. Large prospective studies with serial measurements of a broad range of POPs, adiposity, and clinically relevant biomarkers are needed to disentangle the interrelationships among POPs, obesity, and the development of T2D. Also needed are laboratory experiments that more closely mimic real-world POP doses, mixtures, and exposure duration in humans.

Central sympathetic innervations to visceral and subcutaneous white adipose tissue

There is a link between visceral white adipose tissue (WAT) and the metabolic syndrome in humans, with health improvements produced with small visceral WAT reduction. By contrast, subcutaneous WAT provides a site for lipid storage that is rather innocuous relative to ectopic lipid storage in muscle or liver. The sympathetic nervous system (SNS) is the principal initiator for lipolysis in WAT by mammals. Nothing is known, however, about the central origins of the SNS circuitry innervating the only true visceral WAT in rodents, mesenteric WAT (MWAT), which drains into the hepatic portal vein. We tested whether the central sympathetic circuits to subcutaneous [inguinal WAT (IWAT)] and visceral WAT (MWAT) are separate or shared and whether they possess differential sympathetic drives with food deprivation in Siberian hamsters. Using two isogenic strains of pseudorabies virus, a retrograde transneuronal viral tract tracer within the same hamsters, we found some overlap (∼20-55% doubly infected neurons) between the two circuitries across the neural axis with lesser overlap proximal to the depots (spinal cord and sympathetic chain) and with more neurons involved in the innervation of IWAT than MWAT in some brain regions. Food deprivation triggered a greater sympathetic drive to subcutaneous (IWAT) than visceral (MWAT) depots. Collectively, we demonstrated both shared and separate populations of brain, spinal cord, and sympathetic chain neurons ultimately project to a subcutaneous WAT depot (IWAT) and the only visceral WAT depot in rodents (MWAT). In addition, the lipolytic stimulus of food deprivation only increased SNS drive to subcutaneous fat (IWAT).

Fyn deficiency promotes a preferential increase in subcutaneous adipose tissue mass and decreased visceral adipose tissue inflammation

Previous studies have demonstrated that Fyn knockout (FynKO) mice on a standard chow diet display increased glucose clearance and whole-body insulin sensitivity associated with decreased adiposity resulting from increased fatty acid use and energy expenditure. Surprisingly, however, despite a similar extent of adipose tissue (AT) mass accumulation on a high-fat diet, the FynKO mice remained fully glucose tolerant and insulin sensitive. Physiologic analyses demonstrated that the FynKO mice had a combination of skewed AT expansion into the subcutaneous compartment rather than to the visceral depot, reduced AT inflammation associated with reduced T-cell and macrophage infiltration, and increased proportion of anti-inflammatory M2 macrophages. These data demonstrate that Fyn is an important regulator of whole-body integrative metabolism that coordinates AT expansion, inflammation, and insulin sensitivity in states of nutrient excess. These data further suggest that inhibition of Fyn function may provide a novel target to prevent AT inflammation, insulin resistance, and the dyslipidemia components of the metabolic syndrome.

Pathophysiology of human visceral obesity: an update

Excess intra-abdominal adipose tissue accumulation, often termed visceral obesity, is part of a phenotype including dysfunctional subcutaneous adipose tissue expansion and ectopic triglyceride storage closely related to clustering cardiometabolic risk factors. Hypertriglyceridemia; increased free fatty acid availability; adipose tissue release of proinflammatory cytokines; liver insulin resistance and inflammation; increased liver VLDL synthesis and secretion; reduced clearance of triglyceride-rich lipoproteins; presence of small, dense LDL particles; and reduced HDL cholesterol levels are among the many metabolic alterations closely related to this condition. Age, gender, genetics, and ethnicity are broad etiological factors contributing to variation in visceral adipose tissue accumulation. Specific mechanisms responsible for proportionally increased visceral fat storage when facing positive energy balance and weight gain may involve sex hormones, local cortisol production in abdominal adipose tissues, endocannabinoids, growth hormone, and dietary fructose. Physiological characteristics of abdominal adipose tissues such as adipocyte size and number, lipolytic responsiveness, lipid storage capacity, and inflammatory cytokine production are significant correlates and even possible determinants of the increased cardiometabolic risk associated with visceral obesity. Thiazolidinediones, estrogen replacement in postmenopausal women, and testosterone replacement in androgen-deficient men have been shown to favorably modulate body fat distribution and cardiometabolic risk to various degrees. However, some of these therapies must now be considered in the context of their serious side effects. Lifestyle interventions leading to weight loss generally induce preferential mobilization of visceral fat. In clinical practice, measuring waist circumference in addition to the body mass index could be helpful for the identification and management of a subgroup of overweight or obese patients at high cardiometabolic risk.

Increased Susceptibility to Insulin Resistance Associated with Abdominal Obesity in Aging Rats**

OBJECTIVE

Recent data have suggested that the insulin resistance observed with aging may be more related to adiposity than aging per se. We asked whether the insulin resistance observed in aged rats was comparable (both in magnitude and location) to that of fat-fed rats.

RESEARCH METHODS AND PROCEDURES

We performed hyperinsulinemic (5 mU/min per kg) euglycemic clamps with tracer in conscious, 6-hour fasted young (YL), fat-fed young (YF), fat-fed old (OF), and calorically restricted old (OL) rats.

RESULTS

Intraabdominal fat measurements showed that OF and YF rats were more obese than YL (p<or=0.001; YF>OF>YL). Caloric restriction not only prevented age-related obesity but also reduced the ratio of intraabdominal fat to lean body mass (LBM) compared with YL (OL: 0.59+/-0.05 vs. YL: 1.07+/-0.04; p=0.017). Despite similar incremental insulin, YF and OF rats required 40% less infused glucose to maintain euglycemia than YL and OL rats (p<0.001). Insulin-stimulated glucose uptake (Si(Rd): DeltaRd/(DeltaInsulin x Glucose(SS)) was impaired in OF rats (OF: 14.03+/-1.79 vs. YL: 23.08+/-1.87x10(3) dL/min x kg LBM per pM; p=0.004) and improved in OL rats (29.41+/-1.84x10(3) dL/min x kg LBM per pM; p=0.031) compared with YL. Despite greater obesity, YF rats did not exhibit lower SiRd compared with OF rats (p=0.58). In contrast, the ability of insulin to suppress endogenous glucose production (EGP; Si(EGP): DeltaEGP/(DeltaInsulin x GlucoseSS) was not impaired in OF rats (OF vs. YL; p=0.61) but was markedly impaired in YF rats by approximately 75% (1.72+/-0.66x10(3) dL/min x kg per pM; p=0.013). Surprisingly, separate regression analysis for old and young animals revealed that old rats exhibited a significantly steeper regression between Si (Rd and EGP) and adiposity than young rats (p<0.05). Thus, older rats showed a proportionately greater decrement in insulin sensitivity with an equivalent increase in adiposity.

DISCUSSION

These data suggest that, in rodents, youth affords significant protection against obesity-induced insulin resistance.

Fat maintenance is a predictor of the murine lifespan response to dietary restriction

Dietary restriction (DR), one of the most robust life-extending manipulations, is usually associated with reduced adiposity. This reduction is hypothesized to be important in the life-extending effect of DR, because excess adiposity is associated with metabolic and age-related disease. Previously, we described remarkable variation in the lifespan response of 41 recombinant inbred strains of mice to DR, ranging from life extension to life shortening. Here, we used this variation to determine the relationship of lifespan modulation under DR to fat loss. Across strains, DR life extension correlated inversely with fat reduction, measured at midlife (males, r= -0.41, P<0.05, n=38 strains; females, r= -0.63, P<0.001, n=33 strains) and later ages. Thus, strains with the least reduction in fat were more likely to show life extension, and those with the greatest reduction were more likely to have shortened lifespan. We identified two significant quantitative trait loci (QTLs) affecting fat mass under DR in males but none for lifespan, precluding the confirmation of these loci as coordinate modulators of adiposity and longevity. Our data also provide evidence for a QTL previously shown to affect fuel efficiency under DR. In summary, the data do not support an important role for fat reduction in life extension by DR. They suggest instead that factors associated with maintaining adiposity are important for survival and life extension under DR.

Tissue-autonomous function of Drosophila seipin in preventing ectopic lipid droplet formation

Obesity is characterized by accumulation of excess body fat, while lipodystrophy is characterized by loss or absence of body fat. Despite their opposite phenotypes, these two conditions both cause ectopic lipid storage in non-adipose tissues, leading to lipotoxicity, which has health-threatening consequences. The exact mechanisms underlying ectopic lipid storage remain elusive. Here we report the analysis of a Drosophila model of the most severe form of human lipodystrophy, Berardinelli-Seip Congenital Lipodystrophy 2, which is caused by mutations in the BSCL2/Seipin gene. In addition to reduced lipid storage in the fat body, dSeipin mutant flies accumulate ectopic lipid droplets in the salivary gland, a non-adipose tissue. This phenotype was suppressed by expressing dSeipin specifically within the salivary gland. dSeipin mutants display synergistic genetic interactions with lipogenic genes in the formation of ectopic lipid droplets. Our data suggest that dSeipin may participate in phosphatidic acid metabolism and subsequently down-regulate lipogenesis to prevent ectopic lipid droplet formation. In summary, we have demonstrated a tissue-autonomous role of dSeipin in ectopic lipid storage in lipodystrophy.

Obesity and lipodystrophy are medical conditions characterized by excess body fat or too little body fat, respectively. Interestingly, a common feature of both conditions is ectopic accumulation of lipids (fat) in cells where fat is not normally stored. This can cause tissue damage with health-threatening consequences. We are trying to understand how these two very different diseases lead to lipid storage in non-fat tissues. In this study, we used fruit flies (Drosophila melanogaster) with a mutation in the dSeipin gene as a lipodystrophy model to explore the mechanism of ectopic lipid storage. In dSeipin mutant flies, we found numerous lipid droplets in the salivary gland, a non-fat storage tissue, and reduced lipid storage in the fat body, an adipose tissue. Furthermore, we proved that dSeipin functions within salivary gland cells to prevent the formation of ectopic lipid droplets. We also found that dSeipin genetically interacts with other fat synthesis and metabolism genes in the formation of ectopic lipid droplets. The fruit fly dSeipin mutant provides an excellent model system for dissecting the mechanisms that regulate the storage of excess lipids.

Improved glucose control and reduced body fat mass in free fatty acid receptor 2-deficient mice fed a high-fat diet

Free fatty acid receptor 2 (Ffar2), also known as GPR43, is activated by short-chain fatty acids (SCFA) and expressed in intestine, adipocytes, and immune cells, suggesting involvement in lipid and immune regulation. In the present study, Ffar2-deficient mice (Ffar2-KO) were given a high-fat diet (HFD) or chow diet and studied with respect to lipid and energy metabolism. On a HFD, Ffar2-KO mice had lower body fat mass and increased lean body mass. The changed body composition was accompanied by improved glucose control and lower HOMA index, indicating improved insulin sensitivity in Ffar2-KO mice. Moreover, the Ffar2-KO mice had higher energy expenditure accompanied by higher core body temperature and increased food intake. The liver weight and content of triglycerides as well as plasma levels of cholesterol were lower in the Ffar2-KO mice fed a HFD. A histological examination unveiled decreased lipid interspersed in brown adipose tissue of the Ffar2-KO mice. Interestingly, no significant differences in white adipose tissue (WAT) cell size were observed, but significantly lower macrophage content was detected in WAT from HFD-fed Ffar2-KO compared with wild-type mice. In conclusion, Ffar2 deficiency protects from HFD-induced obesity and dyslipidemia at least partly via increased energy expenditure.

[Adipose tissue as a therapeutic target in obesity]

Obesity is characterized by an increase of adipose tissue as a result of a positive imbalance between food intake and energy expenditure. Recent studies have indicated that adipocyte function is more complex than expected, since these cells have multiple functions and are integrated in a homeostatic network to optimize energy resources. As metabolic sensors in the body, adipocytes and the surrounding stromal vascular cells produce and secrete autocrine, paracrine and endocrine factors, able to regulate aspects involved in the development of adipocytes, as well as effects in peripheral organs important for metabolism. Regulation of these endocrine factors could lead to new therapeutic approaches targeted at aspects related to adipogenesis, preadipocyte proliferation and differentiation, inflammatory cytokine release and secretion, adipose tissue vascularization, and regulation of lipid metabolism or, alternatively, regulation of energy dissipation in mitochondria. In the study of the mechanisms of adipogenesis and remodulation of adipose tissue with respect to adipocyte size and function, an alternative and unorthodox strategy to improve obesity-associated metabolic complications could consist of increasing the storage capacity of adipose tissue to prevent a toxic response known as lipotoxicity.

From chronic overnutrition to insulin resistance: the role of fat-storing capacity and inflammation

AIMS

We analyze how the inflammatory state in adipose tissue caused by a condition of chronically positive energy balance can lead to insulin resistance first in adipose tissue, then in all insulin-sensitive tissues.

DATA SYNTHESIS

Chronic nutrient overload causes an increase in adipose depots that, if adipose tissue expandability is low, are characterized by an increased presence of hypertrophic adipocytes. This adipocyte hypertrophy is a possible stress condition for the endoplasmic reticulum (ER) that would lead to a proinflammatory state in adipose tissue. In this condition, ER stress would activate metabolic pathways that trigger insulin resistance, release of macrophage chemoattractant proteins, and in chronic inflammation, the death of the hypertrophic adipocyte. The infiltrated macrophages in turn release inflammatory proteins causing further recruitment of macrophages to adipose tissue and the release of inflammatory cytokines. Following these events, insulin resistance becomes extended to all adipose tissue. Insulin-resistant adipocytes, characterized by low liposynthetic capacity and high lipolytic capacity, cause increased release of free fatty acids (FFA). FFA released by lipolitic adipocytes may also activate Toll-like receptors 4 and then chemokines and cytokines release amplifying insulin resistance, lipolysis and inflammation in all adipose tissue. Moreover, increased circulating FFA levels, reduced circulating adiponectin levels and leptin resistance lead to decreased lipid oxidation in non-adipose tissues, thereby triggering ectopic accumulation of lipids, lipotoxicity and insulin resistance.

CONCLUSION

All the conditions that increase circulating fatty acids and cause lipid overloading (obesity, lipoatrophy, lipodystrophy, catabolic states, etc.) induce a lipotoxic state in non-adipose tissues that gives rise to insulin resistance.

It's not how fat you are, it's what you do with it that counts

Mechanisms underlying obesity-related metabolic disorders are poorly understood. Samuel Virtue and Antonio Vidal-Puig explore the evidence for an emerging hypothesis that attributes metabolic complications not to obesity per se, but to an individual's capacity for adipose tissue expandability.

The secretory function of adipocytes in the physiology of white adipose tissue

White adipose tissue, previously regarded as a passive lipid storage site, is now viewed as a dynamic tissue. It has the capacity to actively communicate by sending and receiving different types of signals. An overview of these signals, the external modulators that affect adipose tissue and the secreted signaling molecules, the adipokines, is presented. The secretory function is highlighted in relation to energy metabolism, inflammation and the extracellular matrix and placed in the context of adipose tissue biology. We observe that the endocrine function of adipocytes receives much attention, while its paracrine and autocrine functions are underestimated. Also, we provide examples that species specificity should not be neglected. We conclude that adipose tissue primarily is an energy storage organ, well supported by its secretory function.

Amelioration of steatohepatitis with pentoxifylline in a novel nonalcoholic steatohepatitis model induced by high-fat diet

We sought to evaluate the effects of pentoxifylline (PTX) on steatohepatitis in a novel experimental nonalcoholic steatohepatitis (NASH) model induced by a high-fat diet (HFD). Thirty-three male Sprague-Dawley rats were randomly divided into 3 groups. The first group received only standard rat diet (control group); groups 2 (placebo group) and 3 were given HFD, ad libitum. After week 4, 0.5 mL of physiologic serum was injected subcutaneously to the placebo group and 50 mg/kg/d PTX was given intraperitoneally to the third group (group PTX). After 6 weeks all rats were humanely killed. Serum biochemistry, tumor necrosis factor-alpha (TNF-alpha), plasma, and liver tissue malondialdehyde (MDA) were analyzed. Histopathologically, steatosis, ballooning degeneration, inflammation, and fibrosis were determined. Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) levels, plasma and liver tissue MDA, and plasma TNF-alpha levels were significantly higher in placebo group than in the control group. Tumor growth factor-beta levels, however, were comparable in the placebo and control groups. On histopathologic examination, steatosis, inflammatory cells per square millimeter, and ballooning degeneration were significantly higher in the placebo group than in the control group. No fibrosis or Mallory bodies were found in the placebo group. AST, ALT, plasma and liver tissue MDA, and plasma TNF-alpha levels were significantly lower in PTX group compared to the placebo group. Histopathologically, steatosis, mean number of inflammatory cells/mm(2) and ballooning degeneration in PTX group were also significantly lower than in the placebo group. In conclusion, PTX strikingly ameliorates steatohepatitis in this novel NASH model not only by inhibiting the TNF-alpha but also suppressing the oxidative stress markers.

Visceral adipocytes and the metabolic syndrome

Recently, the high responsiveness of omental adipocytes to positive lipolytic stimuli has been clearly demonstrated in women. We conclude that adipose tissue fatty acid release, storage capacity, and secreted cytokines may all be involved in the etiology of the metabolic syndrome. The anatomical location of visceral adipocytes close to the liver, combined with possible depot-specific alterations in various adipocyte or adipose tissue features likely play critical roles in this process. This highly complex etiology is concordant with the heterogeneous clinical manifestations of the metabolic syndrome, and suggests possible interindividual variability in the extent to which each pathophysiological mechanism is involved.

Genomic DNA damage in mouse transgenesis

Creating transgenic mammals is currently a very inefficient process. In addition to problems with transgene integration and unpredictable expression patterns of the inserted gene, embryo loss occurs at various developmental stages. In the present study, we demonstrate that this loss is due to chromosomal damage. We examined the integrity of chromosomes in embryos produced by microinjection of pronuclei, intracytoplasmic sperm injection (ICSI), and in vitro fertilization (IVF)-mediated transgenesis, and correlated these findings with the abilities of embryos to develop in vitro and yield transgenic morulas/blastocysts. Chromosomal analysis was performed after microinjection of the pronuclei in zygotes, as well as in parthenogenetic and androgenetic embryos. In all the pronuclei injection groups, significant oocyte arrest and increased incidence of chromosome breaks were observed after both transgenic DNA injection and sham injection. This indicates that the DNA damage is a transgene-independent effect. In ICSI-mediated transgenesis, there was no significant oocyte arrest. The observed chromosomal damage was lower than that after pronuclei microinjection in zygotes and was dependent upon the presence of exogenous DNA. The occurrence of DNA breaks, as measured by comet assay performed on the sperm prior to ICSI, showed that DNA damage was present in the sperm before fertilization. Embryonic development in vitro and transgene expression at the morula/blastocyst stage were higher in ICSI-mediated transgenesis than after microinjection of pronuclei into zygotes. Sperm-mediated gene transfer via IVF did not affect chromosome integrity, allowed good embryo development, but did not yield any transgenic embryos. The present study demonstrates that DNA damage occurs after both the microinjection of pronuclei and ICSI-mediated transgenesis, albeit through different mechanisms.

Searching for genetic factors of fatty liver in SMXA-5 mice by quantitative trait loci analysis under a high-fat diet

Fatty liver is strongly associated with the metabolic syndrome characterized by obesity, insulin resistance, and type 2 diabetes, but the genetic basis and functional mechanisms linking fatty liver with the metabolic syndrome are largely unknown. The SMXA-5 mouse is one of the SMXA recombinant inbred substrains established from SM/J and A/J strains and is a model for polygenic type 2 diabetes, characterized by moderately impaired glucose tolerance, hyperinsulinemia, and mild obesity. SMXA-5 mice also developed fatty liver, and a high-fat diet markedly worsened this trait, although SM/J and A/J mice are resistant to fatty liver development under a high-fat diet. To dissect loci for fatty liver in the A/J regions of the SMXA-5 genome, we attempted quantitative trait loci (QTLs) analysis in (SM/JxSMXA-5)F2 intercross mice fed a high-fat diet. We mapped a major QTL for relative liver weight and liver lipid content near D12Mit270 on chromosome 12 and designated this QTL Fl1sa. The A/J allele at this locus contributes to the increase in these traits. We confirmed the effect of Fl1sa on lipid accumulation in liver using the A/J-Chr12(SM) consomic strain, which showed significantly less accumulation than A/J mice. This suggests that the SM/J and A/J strains, neither of which develops fatty liver, possess loci causing fatty liver and that the coexistence of these loci causes fatty liver in SMXA-5 mice.

Control of fatty acid metabolism by leptin in L6 rat myoblasts is regulated by hyperinsulinemia

The development of hypothalamic leptin resistance plays a role in the development of obesity, yet whether peripheral leptin resistance occurs in obesity and diabetes is controversial. Here we investigate whether hyperinsulinemia, as observed during the development of Type 2 diabetes, modifies the effects of leptin on long chain fatty acid metabolism in skeletal muscle cells. We used boron dipyrromethene difluoride (BODIPY)-labeled palmitate to show that leptin (60 nM) caused a time-dependent (0-60 min) increase in fatty acid uptake in L6 myoblasts. Quantitative analysis using 3H-palmitate showed that pre-incubation with insulin (100 nM, 24 h) prevented stimulation of fatty acid uptake by leptin. Insulin pre-treatment also attenuated the ability of leptin to phosphorylate acetyl Co-A carboxylase and increase palmitate oxidation. Suppressor of cytokine-3 (SOCS-3) has been proposed as a possible mediator of insulin-induced leptin resistance. Here we show that treatment of L6 cells with insulin elicited a time-dependent increase in both SOCS-3 mRNA and protein content. In summary, hyperinsulinemia can induce leptin resistance in L6 myoblasts and this may be mediated via a SOCS-3-dependent mechanism.

Fatty acid transport protein 1 is required for nonshivering thermogenesis in brown adipose tissue

Nonshivering thermogenesis in brown adipose tissue (BAT) generates heat through the uncoupling of mitochondrial beta-oxidation from ATP production. The principal energy source for this process is fatty acids that are either synthesized de novo in BAT or are imported from circulation. How uptake of fatty acids is mediated and regulated has remained unclear. Here, we show that fatty acid transport protein (FATP)1 is expressed on the plasma membrane of BAT and is upregulated in response to cold stimuli, concomitant with an increase in the rate of fatty acid uptake. In FATP1-null animals, basal fatty acid uptake is reduced and remains unchanged following cold exposure. As a consequence, FATP1 knockout (KO) animals display smaller lipid droplets in BAT and fail to defend their core body temperature at 4 degrees C, despite elevated serum free fatty acid levels. Similarly, FATP1 is expressed by the BAT-derived cell line HIB-1B upon differentiation, and both fatty acid uptake and FATP1 protein levels are rapidly elevated following isoproterenol stimulation. Stimulation of fatty uptake by isoproterenol required both protein kinase A and mitogen-activated kinase signaling and is completely dependent on FATP1 expression, as small-hairpin RNA-mediated knock down of FATP1 abrogated the effect.

Decreased expression of adipogenic genes in obese subjects with type 2 diabetes

OBJECTIVE

Our objective was to delineate the potential role of adipogenesis in insulin resistance and type 2 diabetes. Obesity is characterized by an increase in adipose tissue mass resulting from enlargement of existing fat cells (hypertrophy) and/or from increased number of adipocytes (hyperplasia). The inability of the adipose tissue to recruit new fat cells may cause ectopic fat deposition and insulin resistance.

RESEARCH METHODS AND PROCEDURES

We examined the expression of candidate genes involved in adipocyte proliferation and/or differentiation [CCAAT/enhancer-binding protein (C/EBP) alpha, C/EBPdelta, GATA domain-binding protein 3 (GATA3), C/EBPbeta, peroxisome proliferator-activated receptor (PPAR) gamma2, signal transducer and activator of transcription 5A (STAT5A), Wnt-10b, tumor necrosis factor alpha, sterol regulatory element-binding protein 1c (SREBP1c), 11 beta-hydroxysteroid dehydrogenase, PPARG angiopoietin-related protein (PGAR), insulin-like growth factor 1, PPARgamma coactivator 1alpha, PPARgamma coactivator 1beta, and PPARdelta] in subcutaneous adipose tissue from 42 obese individuals with type 2 diabetes and 25 non-diabetic subjects matched for age and obesity.

RESULTS

Insulin sensitivity was measured by a 3-hour 80 mU/m2 per minute hyperinsulinemic glucose clamp (100 mg/dL). As expected, subjects with type 2 diabetes had lower glucose disposal (4.9 +/- 1.9 vs. 7.5 +/- 2.8 mg/min per kilogram fat-free mass; p < 0.001) and larger fat cells (0.90 +/- 0.26 vs. 0.78 +/- 0.17 microm; p = 0.04) as compared with obese control subjects. Three genes (SREBP1c, p < 0.01; STAT5A, p = 0.02; and PPARgamma2, p = 0.02) had significantly lower expression in obese type 2 diabetics, whereas C/EBPbeta only tended to be lower (p = 0.07).

DISCUSSION

This cross-sectional study supports the hypothesis that impaired expression of adipogenic genes may result in impaired adipogenesis, potentially leading to larger fat cells in subcutaneous adipose tissue and insulin resistance.

Psammomys obesus (sand rat), a new animal model of non-alcoholic fatty liver disease

BACKGROUND/AIMS

Non-alcoholic fatty liver disease (NAFLD) is an increasingly recognized condition that includes a spectrum of clinicopathologic conditions ranging from steatotosis to cirrhosis and liver failure. NAFLD is usually associated with features of the metabolic syndrome. No established therapies can be offered to patients with NAFLD. An appropriate animal model of NAFLD would be of help in understanding the mechanisms of the disease and in testing novel therapeutic modalities. Available animal models, such as ob/ob and db/db mice, are unsatisfactory since they show only partial resemblance to human NAFLD. Psammomys obesus (sand rat) is a well-established model of type-2 diabetes and obesity, which shares most metabolic parameters of the human metabolic syndrome. In the present study, we hypothesized that P. obesus will also show features of non-alcoholic fatty liver disease.

METHODS

Experimental and control animals were fed normal rat chow or either chow to which fiber (30% wheat straw) was added for 6-18 weeks. Body weight and capillary glucose were measured regularly. At sacrifice blood samples, liver and epididymal fat were obtained. Histology of the liver was blindly determined by a pathologist.

RESULTS

The experimental group showed increased body weight, liver and abdominal fat pad mass, raised plasma glucose, insulin and lipids. Also, alanine-aminotransferase (189+/-76 IU versus 86+/-26 IU; p<0.0001) was significantly higher in the experimental than the control group. Microscopic examination of liver tissue demonstrated marked macrovesicular fat infiltration in the experimental group while it was histologicaly normal in the control animals (liver fat score 1.7+/-1.0 and 0.2+/-0.4; p<0.0001, respectively).

CONCLUSIONS

Fed a calorie-rich diet P. obesus develops a syndrome, which shares metabolic, laboratory and histopathologic characteristics compatible with human NAFLD.

Failure of fat cell proliferation, mitochondrial function and fat oxidation results in ectopic fat storage, insulin resistance and type II diabetes mellitus

BACKGROUND

It is widely accepted that increasing adiposity is associated with insulin resistance and increased risk of type II diabetes. The predominant paradigm used to explain this link is the portal/visceral hypothesis. This hypothesis proposes that increased adiposity, particularly in the visceral depots, leads to increased free-fatty acid flux and inhibition of insulin-action via Randle's effect in insulin-sensitive tissues.

OBJECTIVES

In this review, limitations of this paradigm will be discussed and two other paradigms that may explain established links between adiposity and insulin resistance/diabetes will be presented. (A) Ectopic fat storage syndrome. Three lines of evidence support this concept. Firstly, failure to develop adequate adipose tissue mass (also known as 'lipodystrophy') results in severe insulin resistance and diabetes. This is thought to be the result of ectopic storage of lipid into liver, skeletal muscle and the pancreatic insulin-secreting beta cell. Secondly, most obese patients also shunt lipid into the skeletal muscle, the liver and probably the beta cell. The importance of this finding is exemplified by several studies demonstrating that the degree of lipid infiltration into skeletal muscle and liver highly correlates with insulin resistance. Thirdly, increased fat cell size is highly associated with insulin resistance and the development of diabetes. Increased fat cell size may represent the failure of the adipose tissue mass to expand and therefore to accommodate an increased energy influx. Taken together, these observations support the 'acquired lipodystrophy' hypothesis as a link between adiposity and insulin resistance. Ectopic fat deposition is therefore the result of additive or synergistic effects including increased dietary intake, decreased fat oxidation and impaired adipogenesis. (B) Endocrine paradigm. This concept was developed in parallel with the 'ectopic fat storage syndrome' hypothesis. Adipose tissue secretes a variety of endocrine hormones such as leptin, interleukin-6, angiotensin II, adiponectin and resistin. From this viewpoint, adipose tissue plays a critical role as an endocrine gland, secreting numerous factors with potent effects on the metabolism of distant tissues.

CONCLUSIONS

The novel paradigms of ectopic fat and fat cell as an endocrine organ probably will constitute a new framework for the study of the links between our obesigenic environment and the risk of developing diabetes.

Relationship between visceral adiposity and intramyocellular lipid content in two rat models of insulin resistance

High visceral adiposity and intramyocellular lipid levels (IMCL) are both associated with the development of type 2 diabetes. The relationship between visceral adiposity and IMCL levels was explored in diet- and glucocorticoid-induced models of insulin resistance. In the diet-induced model, lean and fa/fa Zucker rats were fed either normal or high-fat (HF) chow over 4 wk. Fat distribution, IMCL content in the tibialis anterior (TA) muscle (IMCL(TA)), and whole body insulin resistance were measured before and after the 4-wk period. The HF diet-induced increase in IMCL(TA) was strongly correlated with visceral fat accumulation and greater glucose intolerance in both groups. The increase in IMCL(TA) to visceral fat accumulation was threefold greater for fa/fa rats. In the glucocorticoid-induced model, insulin sensitivity was impaired with dexamethasone. In vivo adiposity and IMCL(TA) content measurements were combined with ex vivo analysis of plasma and muscle tissue. Dexamethasone treatment had minimal effects on visceral fat accumulation while increasing IMCL(TA) levels approximately 30% (P < 0.05) compared with controls. Dexamethasone increased plasma glucose by twofold and increased the saturated fatty acid content of plasma lipids [fatty acid (CH2)n/omegaCH3 ratio +15%, P < 0.05]. The lipid composition of the TA muscle was unchanged by dexamethasone treatment, indicating that the relative increase in IMCL(TA) observed in vivo resulted from a decrease in lipid oxidation. Visceral adiposity may influence IMCL accumulation in the context of dietary manipulations; however, a "causal" relationship still remains to be determined. Dexamethasone-induced insulin resistance likely operates under a different mechanism, i.e., independently of visceral adiposity.

Genetic kininogen deficiency contributes to aortic aneurysm formation but not to atherosclerosis

Brown Norway (BN) and BN Katholiek (BN/Ka) rat strains are both susceptible to develop lesions in the internal elastic lamina (IEL) of the aorta. BN/Ka rats are characterized by a single point mutation in the kininogen gene leading to deficiency in high- and low-molecular-weight kininogen. Recently, a suggestive quantitative trait locus for lesions in the IEL of the abdominal aorta was identified in an F2 intercross between Dahl salt-sensitive (SS) and BN rats, implicating kininogen as a positional candidate gene. Therefore, BN and BN/Ka rat strains represent ideal model organisms with which to study the contribution of kininogen to the genetic predisposition to IEL lesion formation and to characterize the early events underlying vascular remodeling. Here we present data demonstrating that genetic kininogen deficiency promotes the formation of aneurysms in the abdominal aorta but not the development of atherosclerosis upon 12-wk treatment with an atherogenic diet. Aneurysm formation was associated with an enhanced elastolysis, increased expression of MMP-2 and MMP-3, downregulation of TIMP-4, and with FasL- and caspase-3-mediated apoptosis. Kininogen-deficient animals also featured changes in plasma cytokines compatible with apoptotic vascular damage, i.e., upregulation of IFN-gamma and downregulation of GM-CSF and IL-1beta. Finally, in response to atherogenic diet, kininogen-deficient animals developed an increase in HDL/total cholesterol index, pronounced fatty liver and heart degeneration, and lipid depositions in aortic media without atherosclerotic plaque formation. These findings suggest that genetic kininogen deficiency renders vascular tissue prone to aneurysmatic but not to atherosclerotic lesions.

Resistin is expressed in pancreatic islets

Resistin, a recently described adipocyte factor, is regulated by peroxisome proliferator-activated receptor gamma (PPARgamma) agonists. While resistin has been proposed to mediate insulin resistance in rodents, little is known about human resistin and its expression in pancreatic islets has not been tested. The goal of the present study was therefore to analyze whether resistin, like PPARgamma, is expressed in islets. Human islets from seven donors were analyzed by quantitative RT-PCR revealing resistin expression in all samples. Immunohistochemistry using a resistin-specific antibody on human pancreatic sections localized resistin protein to the islets. Mouse resistin was also detected in the Min6 beta cell line. Interestingly, we found a 4-fold increase in islet resistin expression in insulin resistant A-ZIP transgenic compared to wild-type mice. Our results demonstrate that resistin is expressed in islets and up-regulated in insulin resistance and thereby shed new light on the role of resistin in mice and humans.

Variation in type 2 diabetes--related traits in mouse strains susceptible to diet-induced obesity

C57BL/6J (B6) and AKR/J (AKR) inbred strains of mice develop a comparable degree of obesity when fed a high-fat diet. However, although obese B6 mice are more glucose intolerant, obese AKR mice are more insulin resistant. To understand the basis for these strain differences, we characterized features of adiposity and glucose homeostasis in mice fed a high-fat diet for 8 weeks. The results indicated that despite hyperglycemia and impaired glucose tolerance, B6 mice have lower plasma insulin and are more insulin sensitive than AKR mice. Compared with adipose tissue of AKR mice, adipose tissue of B6 mice contained about threefold higher levels of total membrane-bound GLUT4 protein, whereas in skeletal muscle the levels were similar. Uptake of 2-[(14)C]deoxyglucose in vivo was reduced by a high-fat diet in adipose tissue, but not in skeletal muscle. Surprisingly, no significant differences in uptake occurred between the strains, despite the differences in GLUT4; however, glucose flux was calculated to be slightly higher in B6 mice. Higher expression of PEPCK in the liver of B6 mice, under both standard-diet and high-fat-diet conditions, suggests a plausible mechanism for elevated glycemia in these mice. In conclusion, phenotypic variation in insulin resistance and glucose production in the B6 and AKR strains could provide a genetic system for the identification of genes controlling glucose homeostasis.

Perturbations in adiponectin, leptin and resistin levels in acromegaly: lack of correlation with insulin resistance

BACKGROUND

Insulin resistance, impaired glucose tolerance and type 2 diabetes are common in acromegalic subjects. The mechanism underlying this insulin resistance is unclear.

DESIGN

We investigated the levels of the adipocytokines, resistin, adiponectin and leptin in a group of 18 acromegalic subjects and 18 control subjects matched for age, gender and body mass index.

RESULTS

Here we demonstrate for the first time significant elevation in adiponectin levels in acromegalic subjects compared to control subjects 12.5 +/- 1.2 vs. 8.97 +/- 1.1 mg/l, P = 0.029. The resistin levels were similar in acromegalic subjects and controls; 20.65 +/- 2.99 vs. 19.03 +/- 4.72 micro g/l. No evidence of a correlation between adiponectin and insulin resistance as calculated from HOMA-R was found. No correlation was observed either between adiponectin or resistin levels and GH levels, total IGF-I or free IGF-I levels. Leptin levels were significantly reduced in acromegalic subjects, 8.22 +/- 2.26 vs. 18.3 +/- 4.1 micro g/l, P = 0.004. In control subjects, significant correlations between leptin levels and HOMA-R and between resistin levels and HOMA-R were observed. These relationships were not apparent in acromegalic subjects.

CONCLUSION

From these data we conclude that changes in resistin and adiponectin levels are unlikely to account for the insulin resistance of acromegaly.

The making of "transgenic spermatozoa"

The processes of making transgenic animals by microinjecting DNA into the pronucleus of a fertilized oocyte or after the transfection of embryonic stem cells are now well established. However, attempts have also been made, with varying degrees of success, to use spermatozoa as a vector for transgenesis in mammals and other vertebrates during the last decade. A number of different approaches for making transgenic spermatozoa have been developed. These include directly incubating mature, isolated spermatozoa with DNA or pretreating mature, isolated spermatozoa before assisted fertilization. Microinjection procedures have also been established to transfect male germ cells directly in vivo within the seminiferous tubules or to reimplant previously isolated male germ cells submitted to in vitro transfection into a recipient testis. The latter two techniques present the advantage of being able to create transgenic progeny simply by mating with wild-type females, which avoids the possibility of interference or damage as a result of assisted fertilization or the manipulation of embryos. The different aspects of sperm-mediated transgenesis are presented.

Dietary and lifestyle interventions in the management of the metabolic syndrome: present status and future perspective

OBJECTIVE

To review the mechanisms underlying the metabolic syndrome, or syndrome X, in humans, and to delineate dietary and environmental strategies for its prevention.

DESIGN

Review of selected papers of the literature.

RESULTS

Hyperinsulinemia and insulin resistance play a key role in the development of the metabolic syndrome. Strategies aimed at reducing insulin resistance may be effective in improving the metabolic syndrome. They include low saturated fat intake, consumption of low-glycemic-index foods, physical exercise and prevention of obesity.

CONCLUSIONS

Future research, in particular the genetic basis of the metabolic syndrome and the interorgan interactions responsible for insulin resistance, is needed to improve therapeutic strategies for the metabolic syndrome.

Animal models of steatohepatitis

Animal models of hepatic steatosis and steatohepatitis have improved our understanding of the pathogenesis of non-alcoholic fatty liver disease (NAFLD). Three models, genetically obese ob/ob mice, lipoatrophic mice and normal rats fed choline-deficient, methionine-restricted diets, have been particularly informative. All support the multiple 'hit' hypothesis for NAFLD pathogenesis that suggests that fatty livers are unusually vulnerable to oxidants and develop steatohepatitis when secondary insults generate sufficient oxidants to cause liver cell death and inflammation. Steatohepatitis, in turn, increases sensitivity to other insults that induce hepatic fibrosis, promoting the evolution of cirrhosis. Early during NAFLD pathogenesis, inhibitor kappa kinase beta (IKKbeta), an enzyme that induces tumour necrosis factor alpha (TNFalpha) and other proinflammatory cytokines, is activated and this causes insulin resistance. Inhibition of IKKbeta or TNFalpha improves insulin sensitivity, steatosis and steatohepatitis in animals, suggesting novel strategies to prevent and treat early NAFLD in humans.

Hepatic steatosis and type 2 diabetes mellitus

Type 2 diabetes is strongly associated with nonalcoholic fatty liver disease (NAFLD), a spectrum of liver damage that ranges from relatively benign hepatic steatosis to potentially fatal cirrhosis. The severities of insulin resistance and liver damage parallel each other, with the greatest prevalence of cirrhosis occurring in cirrhotics. However, it is unknown whether one of these conditions causes the other, or if both are consequences of another process. Experimental evidence suggests that both insulin resistance and NAFLD result from a chronic inflammatory state. The mechanisms driving this chronic inflammation are unknown but might include the egress of products from intestinal bacteria into the portal blood, liver, and systemic circulation to trigger a sustained inflammatory cytokine response in genetically susceptible individuals. More research is needed to evaluate this hypothesis and to determine the benefits of treatments that interrupt this pathogenic cascade.

Increased fat intake, impaired fat oxidation, and failure of fat cell proliferation result in ectopic fat storage, insulin resistance, and type 2 diabetes mellitus

It is widely accepted that increasing adiposity is associated with insulin resistance and increased risk of type 2 diabetes. The predominant paradigm used to explain this link is the portal/visceral hypothesis. This hypothesis proposes that increased adiposity, particularly in the visceral depots, leads to increased free fatty acid flux and inhibition of insulin action via Randle's effect in insulin-sensitive tissues. Recent data do not entirely support this hypothesis. As such, two new paradigms have emerged that may explain the established links between adiposity and disease. (A) Three lines of evidence support the ectopic fat storage syndrome. First, failure to develop adequate adipose tissue mass in either mice or humans, also known as lipodystrophy, results in severe insulin resistance and diabetes. This is thought to be the result of ectopic storage of lipid into liver, skeletal muscle, and the pancreatic insulin-secreting beta cell. Second, most obese patients also shunt lipid into the skeletal muscle, the liver, and probably the beta cell. The importance of this finding is exemplified by several studies demonstrating that the degree of lipid infiltration into skeletal muscle and liver correlates highly with insulin resistance. Third, increased fat cell size is highly associated with insulin resistance and the development of diabetes. Increased fat cell size may represent the failure of the adipose tissue mass to expand and thus to accommodate an increased energy influx. Taken together, these three observations support the acquired lipodystrophy hypothesis as a link between adiposity and insulin resistance. (B) The endocrine paradigm developed in parallel with the ectopic fat storage syndrome hypothesis. Adipose tissue secretes a variety of endocrine hormones, such as leptin, interleukin-6, angiotensin II, adiponectin (also called ACRP30 and adipoQ), and resistin. From this viewpoint, adipose tissue plays a critical role as an endocrine gland, secreting numerous factors with potent effects on the metabolism of distant tissues. These two new paradigms provide a framework to advance our understanding of the pathophysiology of the insulin-resistance syndrome.

Emerging paradigms for understanding fatness and diabetes risk

It is widely accepted that increasing adiposity is associated with insulin resistance and increased risk of type 2 diabetes. The predominant paradigm used to explain this link is the portal/visceral hypothesis. This hypothesis proposes that increased adiposity, particularly in the visceral depots, leads to increased free fatty acid flux and inhibition of insulin action via Randle's effect in insulin-sensitive tissues. Recent data do not entirely support this hypothesis. As such, two new paradigms have emerged that may explain the established links between adiposity and disease.

Uncoupling proteins and thermoregulation

Energy balance in animals is a metabolic state that exists when total body energy expenditure equals dietary energy intake. Energy expenditure, or thermogenesis, can be subcategorized into groups of obligatory and facultative metabolic processes. Brown adipose tissue (BAT), through the activity of uncoupling protein 1 (UCP1), is responsible for nonshivering thermogenesis, a major component of facultative thermogenesis in newborn humans and in small mammals. UCP1, found in the mitochondrial inner membrane in BAT, uncouples energy substrate oxidation from mitochondrial ATP production and hence results in the loss of potential energy as heat. Mice that do not express UCP1 (UCP1 knockouts) are markedly cold sensitive. The recent identification of four new homologs to UCP1 expressed in BAT, muscle, white adipose tissue, brain, and other tissues has been met by tremendous scientific interest. The hypothesis that the novel UCPs may regulate thermogenesis and/or fatty acid metabolism guides investigations worldwide. Despite several hundred publications on the new UCPs, there are a number of significant controversies, and only a limited understanding of their physiological and biochemical properties has emerged. The discovery of UCP orthologs in fish, birds, insects, and even plants suggests the widespread importance of their metabolic functions. Answers to fundamental questions regarding the metabolic functions of the new UCPs are thus pending and more research is needed to elucidate their physiological functions. In this review, we discuss recent findings from mammalian studies in an effort to identify potential patterns of function for the UCPs.

Mitochondrial Proliferation, Dna Depletion and Adipocyte Differentiation in Subcutaneous Adipose Tissue of HIV-Positive Haart Recipients

Objectives

To examine the in vivo effects of highly active antiretroviral therapy (HAART) regimens on adipose tissue mitochondrial DNA (mtDNA) depletion, mitochondrial organellar proliferation, and markers of adipocyte differentiation and phenotype.

Design and methods

DNA and mRNA quantification using real-time PCR methods was performed on adipose tissue samples from 31 HIV-infected individuals, of whom 11 were treatment-naive and 20 were receiving HAART. mtDNA depletion was measured as mtDNA copies/cell, and mitochondrial proliferation by quantification of mitochondrial protein mass. Regulation of mitochondrial biogenesis was assessed by NRF-1 and mtTFA mRNA. PPAR γ, UCP2 and UCP1 mRNA expression was used to assess adipocyte differentiation and phenotype.

Results

Stavudine-based HAART recipients ( n=10) displayed significant mtDNA depletion (12.8% of control, P<0.001), mildly increased mitochondrial protein mass (2.6-fold of control, P=0.032) and decreased expression of PPARγ (53.9% of control, P=0.021), UCP2 (62.2% of control, P=0.024) and UCP3 (51.8% of control, P=0.047) mRNA compared with controls. Zidovudine-based HAART recipients ( n=7) also displayed significant mtDNA depletion (34.45% of control, P=0.031), increased mitochondrial protein mass (5.7-fold of control, P=0.009), and markedly increased UCP1 (18-fold of control, P=0.009) mRNA. Elevated UCP1 mRNA expression was found to be associated with non-stavudine (zidovudine or abacavir), protease inhibitor (PI)-containing HAART (95-fold of non-stavudine, non-PI-containing HAART, P=0.006).

Conclusion

Differential effects of stavudine and zidovudine therapy on mtDNA depletion and expression of adipocyte differentiation markers PPARγ and UCP2 were observed, consistent with increased adipose tissue toxicity associated with stavudine therapy. Increased UCP1 mRNA, a marker of brown adipose tissue phenotype, was associated with non-stavudine, PI-containing HAART, and may represent an adaptive response to the increased fatty acid flux associated with PI therapy, and may contribute to the increased resting energy expenditure reported in such patients.

ACRP30, a new hormone controlling fat and glucose metabolism

Adipocyte complement-related protein of 30 kDa (ACRP30) is a secreted serum protein expressed exclusively in differentiated adipocytes. Recent studies have indicated that its expression and serum levels are reduced in humans and animals with obesity and insulin resistance. Metabolic studies have demonstrated a role for ACRP30 in the regulation of glucose and lipid homeostasis. This review will describe the current literature on the biochemistry of ACRP30 and its physiological functions. We will also discuss issues that are relevant to the directions of future research.

Control of energy homeostasis and insulin action by adipocyte hormones: leptin, acylation stimulating protein, and adiponectin

Adipose tissue performs complex metabolic and endocrine functions. This review will focus on the recent literature on the biology and actions of three adipocyte hormones involved in the control of energy homeostasis and insulin action, leptin, acylation-stimulating protein, and adiponectin, and mechanisms regulating their production. Results from studies of individuals with absolute leptin deficiency (or receptor defects), and more recently partial leptin deficiency, reveal leptin's critical role in the normal regulation of appetite and body adiposity in humans. The primary biological role of leptin appears to be adaptation to low energy intake rather than a brake on overconsumption and obesity. Leptin production is mainly regulated by insulin-induced changes of adipocyte metabolism. Consumption of fat and fructose, which do not initiate insulin secretion, results in lower circulating leptin levels, a consequence which may lead to overeating and weight gain in individuals or populations consuming diets high in energy derived from these macronutrients. Acylation-stimulating protein acts as a paracrine signal to increase the efficiency of triacylglycerol synthesis in adipocytes, an action that results in more rapid postprandial lipid clearance. Genetic knockout of acylation-stimulating protein leads to reduced body fat, obesity resistance and improved insulin sensitivity in mice. The primary regulator of acylation-stimulating protein production appears to be circulating dietary lipid packaged as chylomicrons. Adiponectin increases insulin sensitivity, perhaps by increasing tissue fat oxidation resulting in reduced circulating fatty acid levels and reduced intramyocellular or liver triglyceride content. Adiponectin and leptin together normalize insulin action in severely insulin-resistant animals that have very low levels of adiponectin and leptin due to lipoatrophy. Leptin also improves insulin resistance and reduces hyperlipidemia in lipoatrophic humans. Adiponectin production is stimulated by agonists of peroxisome proliferator-activated receptor-gamma; an action may contribute to the insulin-sensitizing effects of this class of compounds. The production of all three hormones is influenced by nutritional status. These adipocyte hormones, the pathways controlling their production, and their receptors represent promising targets for managing obesity, hyperlipidemia, and insulin resistance.