Adipose tissue-derived tumor necrosis factor activity correlates with fat cell size but not insulin action in aging rats

Adipose tissue aging: mechanisms and therapeutic implications

Adipose tissue, which is the crucial energy reservoir and endocrine organ for the maintenance of systemic glucose, lipid, and energy homeostasis, undergoes significant changes during aging. These changes cause physiological declines and age-related disease in the elderly population. Here, we review the age-related changes in adipose tissue at multiple levels and highlight the underlying mechanisms regulating the aging process. We also discuss the pathogenic pathways of age-related fat dysfunctions and their systemic negative consequences, such as dyslipidemia, chronic general inflammation, insulin resistance, and type 2 diabetes (T2D). Age-related changes in adipose tissue involve redistribution of deposits and composition, in parallel with the functional decline of adipocyte progenitors and accumulation of senescent cells. Multiple pathogenic pathways induce defective adipogenesis, inflammation, aberrant adipocytokine production, and insulin resistance, leading to adipose tissue dysfunction. Changes in gene expression and extracellular signaling molecules regulate the aging process of adipose tissue through various pathways. In addition, adipose tissue aging impacts other organs that are infiltrated by lipids, which leads to systemic inflammation, metabolic system disruption, and aging process acceleration. Moreover, studies have indicated that adipose aging is an early onset event in aging and a potential target to extend lifespan. Together, we suggest that adipose tissue plays a key role in the aging process and is a therapeutic target for the treatment of age-related disease, which deserves further study to advance relevant knowledge.

Prenatal exposure to ambient fine particulate matter induces dysregulations of lipid metabolism in adipose tissue in male offspring

Prenatal exposure to ambient fine particles (diameter < 0.25 μm, PM_2.5) has been found to be associated with abnormal growth and development in offspring. However, the effects of PM_2.5 on the lipid metabolism of adipose tissue in offspring are unclear. In the present study, we established a mouse model of prenatal exposure to PM_2.5 by intratracheal instillation to pregnant C57BL/6 female mice with PM_2.5 suspension or normal saline. We found that prenatal exposure to PM_2.5 of a mouse model reduced body weight in adult male offspring after 6 weeks old. Histological analysis showed that the adipocyte size was significantly reduced in epididymal adipose tissue (eWAT) in male offspring, but not in brown adipose tissue. The expression levels of genes related to fatty acid synthesis (ACC1, ACSL1) and oxidation (PPARα) in eWAT were also significantly decreased. In addition, downregulation of pro-inflammatory cytokines (TNFα, IL-1β, IL-6) was also observed. Lipidomics analysis of eWAT demonstrated that prenatal exposure of PM_2.5 reduced lysophosphatidylcholines (LPC), phosphatidylcholines (PC), phosphatidylethanolamines (PE), sphingomyelins (SM), and ceramides (Cer), indicating that metabolic pathways, including SM-Cer signaling and glycerophospholipids remodeling, were disrupted. In summary, prenatal exposure to PM_2.5 was associated with the dysregulations in lipid metabolism of eWAT and pro-inflammatory response in male offspring.

Ageing, adipose tissue, fatty acids and inflammation

A common feature of ageing is the alteration in tissue distribution and composition, with a shift in fat away from lower body and subcutaneous depots to visceral and ectopic sites. Redistribution of adipose tissue towards an ectopic site can have dramatic effects on metabolic function. In skeletal muscle, increased ectopic adiposity is linked to insulin resistance through lipid mediators such as ceramide or DAG, inhibiting the insulin receptor signalling pathway. Additionally, the risk of developing cardiovascular disease is increased with elevated visceral adipose distribution. In ageing, adipose tissue becomes dysfunctional, with the pathway of differentiation of preadipocytes to mature adipocytes becoming impaired; this results in dysfunctional adipocytes less able to store fat and subsequent fat redistribution to ectopic sites. Low grade systemic inflammation is commonly observed in ageing, and may drive the adipose tissue dysfunction, as proinflammatory cytokines are capable of inhibiting adipocyte differentiation. Beyond increased ectopic adiposity, the effect of impaired adipose tissue function is an elevation in systemic free fatty acids (FFA), a common feature of many metabolic disorders. Saturated fatty acids can be regarded as the most detrimental of FFA, being capable of inducing insulin resistance and inflammation through lipid mediators such as ceramide, which can increase risk of developing atherosclerosis. Elevated FFA, in particular saturated fatty acids, maybe a driving factor for both the increased insulin resistance, cardiovascular disease risk and inflammation in older adults.

Cancer cachexia prevention via physical exercise: molecular mechanisms

Cancer cachexia is a debilitating consequence of disease progression, characterised by the significant weight loss through the catabolism of both skeletal muscle and adipose tissue, leading to a reduced mobility and muscle function, fatigue, impaired quality of life and ultimately death occurring with 25-30 % total body weight loss. Degradation of proteins and decreased protein synthesis contributes to catabolism of skeletal muscle, while the loss of adipose tissue results mainly from enhanced lipolysis. These mechanisms appear to be at least, in part, mediated by systemic inflammation. Exercise, by virtue of its anti-inflammatory effect, is shown to be effective at counteracting the muscle catabolism by increasing protein synthesis and reducing protein degradation, thus successfully improving muscle strength, physical function and quality of life in patients with non-cancer-related cachexia. Therefore, by implementing appropriate exercise interventions upon diagnosis and at various stages of treatment, it may be possible to reverse protein degradation, while increasing protein synthesis and lean body mass, thus counteracting the wasting seen in cachexia.

Racial Differences in Adipocyte Size and Relationship to the Metabolic Syndrome in Obese Women

OBJECTIVE

To determine whether racial differences exist in the relationship of the abnormalities defining the metabolic syndrome (MS) to regional adiposity and fat cell size (FCS) in obese postmenopausal women.

RESEARCH METHODS AND PROCEDURES

We determined the relationship of metabolic variables associated with the MS to regional body composition and abdominal (ABD) and gluteal (GLT) FCS in 25 white (CAU) and 25 African-American (AF-AMER) older women matched for age (58 +/- 5 years; mean +/- SD) and BMI (35 +/- 4 kg/m2).

RESULTS

MS was present in 36% of the AF-AMER and 57% of the CAU women. There were no differences in total body, trunk, gluteofemoral fat mass or regional FCS, but AF-AMER women had 22% lower visceral fat, 24% higher insulin, and 31% lower triglyceride levels than CAU women (p < 0.05). Multiple regression analysis with body fat, visceral ABD fat area, and FCS as independent variables showed that GLT FCS was independently correlated with 2-hour insulin (r = 0.56), triglyceride (r = 0.62), and high-density lipoprotein cholesterol (r = -0.72) levels in AF-AMER women but not in CAU women, where only systolic blood pressure correlated with subcutaneous ABD fat area (r = 0.57) (p < 0.05).

DISCUSSION

The associations between GLT FCS and metabolic dysfunction in obese AF-AMER but not CAU women suggest that central obesity is a less valid predictor of the MS in obese postmenopausal AF-AMER women than in CAU women and that GLT FCS may be a more sensitive indicator of risk for the MS in AF-AMER women.

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.

Adipose-specific disruption of autotaxin enhances nutritional fattening and reduces plasma lysophosphatidic acid

Autotaxin (ATX) is a secreted lysophospholipase D that generates the lipid mediator lysophosphatidic acid (LPA). ATX is secreted by adipose tissue and its expression is enhanced in obese/insulin-resistant individuals. Here, we analyzed the specific contribution of adipose-ATX to fat expansion associated with nutritional obesity and its consequences on plasma LPA levels. We established ATX(F/F)/aP2-Cre (FATX-KO) transgenic mice carrying a null ATX allele specifically in adipose tissue. FATX-KO mice and their control littermates were fed either a normal or a high-fat diet (HFD) (45% fat) for 13 weeks. FATX-KO mice showed a strong decrease (up to 90%) in ATX expression in white and brown adipose tissue, but not in other ATX-expressing organs. This was associated with a 38% reduction in plasma LPA levels. When fed an HFD, FATX-KO mice showed a higher fat mass and a higher adipocyte size than control mice although food intake was unchanged. This was associated with increased expression of peroxisome proliferator-activated receptor (PPAR)γ2 and of PPAR-sensitive genes (aP2, adiponectin, leptin, glut-1) in subcutaneous white adipose tissue, as well as in an increased tolerance to glucose. These results show that adipose-ATX is a negative regulator of fat mass expansion in response to an HFD and contributes to plasma LPA levels.

Review article: lymphatic system and associated adipose tissue in the development of inflammatory bowel disease

BACKGROUND

The lymphatic system plays critical roles in tissue fluid homoeostasis, immune defence and metabolic maintenance. Lymphatic vessels transport lymph, proteins, immune cells and digested lipids, allowing fluid and proteins to be returned to the blood stream, lipids to be stored and metabolized and antigens to be sampled in lymph nodes. Lymphatic drainage is mainly driven by rhythmic constrictions intrinsic to the vessels and critically modulated by fluid pressure and inflammatory mediators.

AIM

To collect and discuss the compelling available information linking the lymphatic system, adiposity and inflammation.

METHODS

A literature search was performed through PubMed focusing on lymphatic system, inflammation, immune cells and fat transport and function in the context of IBD.

RESULTS

Evidence collected allows us to propose the following working model. Compromised lymph drainage, reported in IBD, leads to oedema, lymphangiogenesis, impaired immune cell trafficking and lymph leakage. Lymph factor(s) stimulate adipose tissue to proliferate and produce cytokines, which affect immune cell functions and exacerbate inflammation.

CONCLUSIONS

Understanding the lymphatic system's role in immune cell trafficking and immune responses, contribution to fat transport, distribution, metabolism and implication in the pathogenesis of chronic intestinal inflammation may provide the basis for new therapeutic strategies and improved quality-of life.

EGFL6 is increasingly expressed in human obesity and promotes proliferation of adipose tissue-derived stromal vascular cells

With increasing rates of obesity driving the incidence of type 2 diabetes and cardiovascular diseases to epidemic levels, understanding of the biology of adipose tissue expansion is a focus of current research. Identification and characterization of secreted proteins of the adipose tissue could provide further insights into the function of adipose tissue and might help to therapeutically influence the development of obesity and associated metabolic disorders. In the present study, we identified human epidermal growth factor-like domain multiple-6 (EGFL6) as an adipose tissue-secreted protein. EGFL6 expression in human subcutaneous adipose tissue significantly increased with obesity and decreased after weight loss. Further, expression and secretion of EGFL6 increased with in vitro differentiation of human preadipocytes, suggesting that mature adipocytes are the main source of EGFL6. Containing epidermal growth factor (EGF)-like repeats, an Arg-Gly-Asp (RGD) integrin binding motif and a mephrin, A5 protein and receptor protein-tyrosine phosphatase mu (MAM) domain, EGFL6 was suggested to be an extra-cellular matrix protein. Recombinant human EGFL6 protein mediated cell adhesion of human adipose tissue-derived stromal vascular cells (AD-SVC) in an RGD-dependent manner. FACS analyses revealed specific binding of the protein to the cell surface of AD-SVC with the binding being predominantly mediated by the EGF-like repeats. Recombinant EGFL6 enhanced proliferation of human AD-SVC as measured by MTS assay and [(14)C]-thymidine incorporation. These results indicate that human EGFL6 is a paracrine/autocrine growth factor of adipose tissue up-regulated in obesity and potentially involved in the process of adipose tissue expansion and the development of obesity.

Weight regain after sustained weight reduction is accompanied by suppressed oxidation of dietary fat and adipocyte hyperplasia

A dual-tracer approach (dietary14C-palmitate and intraperitoneal3H-H2O) was used to assess the trafficking of dietary fat and net retention of carbon in triglyceride depots during the first 24 h of weight regain. Obesity-prone male Wistar rats were allowed to mature under obesogenic conditions for 16 wk. One group was switched to ad libitum feeding of a low-fat diet for 10 wk (Obese group). The remaining rats were switched to an energy-restricted, low-fat diet for 10 wk that reduced body weight by 14% and were then assessed in energy balance (Reduced group), with free access to the low-fat diet (Relapse-Day1 group), or with a provision that induced a minor imbalance (+10 kcal) equivalent to that observed in obese rats (Gap-Matched group). Fat oxidation remained at a high, steady rate throughout the day in Obese rats, but was suppressed in Reduced, Gap-Matched, and Relapse-Day1 rats though 9, 18, and 24 h, respectively. The same caloric excess in Obese and Gap-Matched rats led to less fat oxidation over the day and greater trafficking of dietary fat to visceral depots in the latter. In addition to trafficking nutrients to storage, Relapse-Day1 rats had more small, presumably new, adipocytes at the end of 24 h. Dietary fat oxidation at 24 h was related to the phosphorylation of skeletal muscle acetyl-CoA carboxylase and fatty acid availability. These observations provide evidence of adaptations in the oxidation and trafficking of dietary fat that extend beyond the energy imbalance, which facilitate rapid, efficient regain during the relapse to obesity.

Increased TNFalpha and CCAAT/enhancer-binding protein homologous protein with aging predispose preadipocytes to resist adipogenesis

Fat depot sizes peak in middle age but decrease by advanced old age. This phenomenon is associated with ectopic fat deposition, decreased adipocyte size, impaired differentiation of preadipocytes into fat cells, decreased adipogenic transcription factor expression, and increased fat tissue inflammatory cytokine generation. To define the mechanisms contributing to impaired adipogenesis with aging, we examined the release of TNFalpha, which inhibits adipogenesis, and the expression of CCAAT/enhancer-binding protein (C/EBP) homologous protein (CHOP), which blocks activity of adipogenic C/EBP family members, in preadipocytes cultured from young, middle-aged, and old rats. Medium conditioned by fat tissue, as well as preadipocytes, from old rats impeded lipid accumulation by preadipocytes from young animals. More TNFalpha was released by preadipocytes from old than young rats. Differences in TNFalpha-converting enzyme, TNFalpha degradation, or the presence of macrophages in cultures were not responsible. TNFalpha induced rat preadipocyte CHOP expression. CHOP was higher in undifferentiated preadipocytes from old than younger animals. Overexpression of CHOP in young rat preadipocytes inhibited lipid accumulation. TNFalpha short interference RNA reduced CHOP and partially restored lipid accumulation in old rat preadipocytes. CHOP normally increases during late differentiation, potentially modulating the process. This late increase in CHOP was not affected substantially by aging: CHOP was similar in differentiating preadipocytes and fat tissue from old and young animals. Hypoglycemia, which normally causes an adaptive increase in CHOP, was less effective in inducing CHOP in preadipocytes from old than younger animals. Thus increased TNFalpha release by undifferentiated preadipocytes with elevated basal CHOP contributes to impaired adipogenesis with aging.

Activation of diacylglycerol O-acyltransferase 1 gene results in increased tumor necrosis factor-alpha gene expression in 3T3-L1 adipocytes

The tumor necrosis factor-alpha (TNF-alpha) expression has been reported to be largely dependent on the size of adipocytes. We herein investigated the gene regulation of diacylglycerol O-acyltransferase (DGAT) in order to clarify the mechanism of TNF-alpha expression induced in large adipocytes. 3T3-L1 cells were cultured in the presence of 5 mM or 25 mM glucose to generate adipocytes from which the triglyceride content differs. The expression of TNF-alpha, DGAT1, and DGAT2 were upregulated in adipocytes cultured with 25 mM glucose. Furthermore, knockdown of DGAT1 gene significantly inhibited the TNF-alpha expression. Finally, the DGAT1 expression levels were closely related to the TNF-alpha level in 3T3-L1 adipocytes.

Peripheral metabolic responses to prolonged weight reduction that promote rapid, efficient regain in obesity-prone rats

Weight regain after weight loss is the most significant impediment to long-term weight reduction. We have developed a rodent paradigm that models the process of regain after weight loss, and we have employed both prospective and cross-sectional analyses to characterize the compensatory adaptations to weight reduction that may contribute to the propensity to regain lost weight. Obese rats were fed an energy-restricted (50–60% kcal) low-fat diet that reduced body weight by 14%. This reduced weight was maintained for up to 16 wk with limited provisions of the low-fat diet. Intake restriction was then removed, and the rats were followed for 56 days as they relapsed to the obese state. Prolonged weight reduction was accompanied by 1) a persistent energy gap resulting from an increased drive to eat and a reduced expenditure of energy, 2) a higher caloric efficiency of regain that may be linked with suppressed lipid utilization early in the relapse process, 3) preferential lipid accumulation in adipose tissue accompanied by adipocyte hyperplasia, and 4) humoral adiposity signals that underestimate the level of peripheral adiposity and likely influence the neural pathways controlling energy balance. Taken together, long-term weight reduction in this rodent paradigm is accompanied by a number of interrelated compensatory adjustments in the periphery that work together to promote rapid and efficient weight regain. These metabolic adjustments to weight reduction are discussed in the context of a homeostatic feedback system that controls body weight.

The adipocyte life cycle hypothesis

The adipocyte life cycle hypothesis states that the metabolic properties of an adipocyte vary predictably during its life cycle: that as an adipocyte matures, it accumulates triacylglycerol (triglyceride) and becomes larger; that the rates of triacylglycerol synthesis and lipolysis are matched within adipocytes and that larger adipocytes, in general, have greater rates of triacylglycerol synthesis and, concurrently, greater rates of lipolysis and, therefore, larger adipocytes have greater rates of transmembrane fatty acid flux; and that the secretion of cytokines can also be related to adipocyte size with larger adipocytes having a more unfavourable profile of cytokine secretion than smaller adipocytes. Adipocyte location is an important modifier of this relationship and the favoured sites of adipocyte proliferation are a function of gender and the position within the life cycle of the organism at which proliferation occurs. The adipocyte life cycle hypothesis posits that the metabolic consequences of obesity depend on whether expansion of adipose tissue is achieved primarily by an increase in adipocyte number or adipocyte size. This hypothesis may explain a variety of previously unanswered clinical puzzles such as the vulnerability of many peoples from South East Asia to the adverse metabolic consequences of obesity.

Lower weight gain and higher expression and blood levels of adiponectin in rats fed medium-chain TAG compared with long-chain TAG

Previous studies demonstrated that, compared with long-chain TAG (LCT), dietary medium-chain TAG (MCT) could improve glucose tolerance in rats and humans. It has been well established that adiponectin acts to increase insulin sensitivity. The effects of dietary MCT on adiponectin serum concentration and mRNA levels in adipose tissue were studied in rats. Male Sprague-Dawley rats were fed a diet containing 20% MCT or LCT for 8 wk. After 6 wk of dietary treatment, an oral glucose tolerance test was performed. Rats fed the MCT diet had less body fat accumulation than those fed the LCT diet (P < 0.01). The cell diameter of the perirenal adipose tissue, one of the abdominal adipose tissues, was smaller (P < 0.01) in the MCT diet group. The serum adiponectin concentration was higher (P < 0.01) in the MCT diet group than in the LCT diet group. The adiponectin content in the perirenal adipose tissue was higher (P < 0.01) in the MCT diet group. The MCT-fed group had a higher adiponectin mRNA level in their perirenal adipose tissue (P < 0.05). The increase of the plasma glucose concentration after glucose administration (area under the curve) was smaller (P < 0.01) in the MCT diet group than in the LCT diet group. These findings suggest that dietary MCT, compared with LCT, results in a higher serum adiponectin level with transcriptional activation of the adiponectin gene in rats. We speculate that improved glucose tolerance in rats fed an MCT diet may be, at least in part, ascribed to this higher serum adiponectin level.

The role of TNF-alpha in insulin resistance

Insulin resistance is an important component of the metabolic syndrome associated with obesity. Early-stage insulin-resistance and related mild glucose intolerance may be compensated by increased insulin secretion. When combined with impaired insulin secretion, insulin resistance plays an important role in type 2 diabetes (1). Insulin-resistance is also associated with a variety of pathological conditions, including trauma, infection, and cancer. Obesity and type 2 diabetes are the most common metabolic diseases in Western societies, together affecting as much as half of the adult population (2). The prevalence of these conditions is not only high, but continues to increase. We have only recently come to appreciate the role of fat, especially visceral fat, as an endocrine organ. Visceral fat is the source of a number of substances which might play a role in the development of insulin resistance. Among the latter are tumor necrosis factor-alpha (TNF-alpha), adiponectin, IL-6, resistin and free fatty acids. This review will discuss the regulation of insulin responses by TNF-alpha and evidence supporting the hypothesis that over expression of TNF-alpha plays a role in the pathophysiology of insulin resistance.

Tumor necrosis factor alpha inhibits insulin-like growth factor I-induced hematopoietic cell survival and proliferation

Proinflammatory cytokines, such as TNFalpha and IL-1beta, are both cytostatic and cytotoxic. In contrast, IGF-I promotes proliferation and survival of hematopoietic progenitor cells. In this report, we establish that both the cytostatic and cytotoxic activity of TNFalpha on murine myeloid progenitor cells is only evident in the presence of IGF-I. We first confirmed that IGF-I (100 ng/ml) increases DNA synthesis and reduces apoptosis in murine myeloid progenitor cells induced to die by growth factor withdrawal. TNFalpha inhibits, in a dose-dependent fashion from 0.1 to 10 ng/ml, both activities of IGF-I. TNFalpha activity was not detected in the absence of IGF-I. Another proinflammatory cytokine, IL-1beta, did not inhibit IGF-I-induced activity in murine factor-dependent cell progenitor-1/Mac-1 cells. However, the ability of TNFalpha to impair IGF-I-induced DNA synthesis in human promyeloid cells extends to IL-1beta. Statistically significant inhibition of all these events occurs at very low concentrations of 1 ng/ml or less. These results support the general concept that proinflammatory cytokines impair the actions of hormones on hematopoietic cells, leading to IGF-I receptor resistance.

The role of TNF-alpha in insulin resistance

Insulin resistance is an important component of the metabolic syndrome associated with obesity. Early-stage insulin-resistance and related mild glucose intolerance may be compensated by increased insulin secretion. When combined with impaired insulin secretion, insulin resistance plays an important role in type 2 diabetes (1). Insulin-resistance is also associated with a variety of pathological conditions, including trauma, infection, and cancer. Obesity and type 2 diabetes are the most common metabolic diseases in Western societies, together affecting as much as half of the adult population (2). The prevalence of these conditions is not only high, but continues to increase. We have only recently come to appreciate the role of fat, especially visceral fat, as an endocrine organ. Visceral fat is the source of a number of substances which might play a role in the development of insulin resistance. Among the latter are tumor necrosis factor-alpha (TNF-alpha), adiponectin, IL-6, resistin and free fatty acids. This review will discuss the regulation of insulin responses by TNF-alpha and evidence supporting the hypothesis that over expression of TNF-alpha plays a role in the pathophysiology of insulin resistance.

Growth hormone-mediated alteration of fuel metabolism in the aged rat as determined from transcript profiles

Age-related changes in body composition and serum lipids resemble symptoms of adult-onset growth hormone (GH) deficiency. GH treatment has been shown to normalize these changes in both GH-deficient adult patients and elderly subjects. The aim of this study was to identify GH-responsive genes that might mediate positive effects of GH treatment on fuel metabolism and body composition. cDNA microarrays were used to analyze age- and GH-induced changes in gene expression patterns in male rats. Tissues analyzed were liver, adipose tissue, and skeletal muscle from animals on or off GH treatment. A value of 1.5 was chosen to denote differences (increased or decreased expression) in the level of mRNA expression. In the liver, 7.3% of the expressed genes were affected by age and 6.5% by GH. Similar values for the other tissues were 8.3% and 5.3% (fat), and 7.9% and 9.6% (muscle), respectively. Among the differentially expressed genes, we identified several that encode proteins involved in fuel metabolism. Old rats were shown to have induced expression of genes involved in hepatic glucose oxidation and lipid synthesis, whereas these pathways were reduced in adipose tissue. GH treatment induced the expression of genes for lipid oxidation in liver and for glucose oxidation in skeletal muscle. In adipose tissue, GH reduced the expression of genes involved in lipogenesis even further. Changes in transcript levels were reflected in serum in terms of altered lipid profiles. Serum levels of triglycerides, high-density lipoprotein (HDL) cholesterol, and total cholesterol were higher in the old animals than in the young and normalized by GH treatment.

Comparison of the effects of glimepiride and glibenclamide on adipose tissue tumour necrosis factor-alpha mRNA expression and cellularity

AIM

The aim of this study was to compare the effects of glimepiride and glibenclamide on tumour necrosis factor (TNF)-alpha expression and adipocyte cellularity in spontaneously diabetic, obese rats.

METHODS

Eight-week-old male Otsuka Long-Evans Tokushima Fatty (OLETF) rats were randomized to receive glimepiride, glibenclamide or control treatment for 12 weeks, after which TNF-alpha mRNA levels, adipose tissue cellularity and physiological parameters were measured.

RESULTS

TNF-alpha mRNA expression in retroperitoneal fat tissue was significantly greater in the glibenclamide group (2.2 +/- 1.1), compared with the control group (0.9 +/- 0.4; p<0.05) or the glimepiride group (0.9 +/- 0.2; p<0.01). The mean fat-cell area in retroperitoneal fat tissue was increased at study endpoint in the glibenclamide group (13,764 +/- 7036 microm2), compared with both the control and glimepiride groups (10,755 +/- 6193 microm2 and 11,317 +/- 5646 microm2 respectively; p<0.05). Investigation of cellularity revealed a decrease in the frequency of small fat cells and an increase in the frequency of large fat cells in both the glibenclamide and glimepiride groups compared with the control group, with a greater increase in large fat cells in the glibenclamide group. At study endpoint, insulin and triglyceride values were significantly higher in the active treatment groups compared with the control group; however, insulin levels were significantly greater in glibenclamide-treated animals compared with glimepiride-treated animals. An oral glucose tolerance test performed at the end of the study showed that there were no significant differences among the three groups in terms of plasma glucose and insulin concentrations.

CONCLUSIONS

This study suggests that although both glibenclamide and glimepiride may have a hypertrophisizing effect on fat cells in adipose tissues, this effect is greater with glibenclamide, leading to augmentation of TNF-alpha mRNA expression and possible exacerbation of insulin resistance.

Adipogenesis and aging: does aging make fat go MAD?

In advanced old age, fat depot size declines while lipid is redistributed to muscle, bone marrow, and other tissues. Decreased fat depot size is related to reduced fat cell size and function and impaired differentiation of preadipocytes into fat cells. Reduced differentiation-dependent gene expression results from decreased abundance of the adipogenic transcription factors, CCAAT/enhancer binding alpha (C/EBPalpha) and peroxisome proliferator activated receptor gamma (PPARgamma). Increased expression of anti-adipogenic C/EBP family members contributes, perhaps due to cellular stress response pathway activation with aging. Hence, dysfunctional adipocyte-like cells appear in adipose tissue that are smaller and less insulin responsive than fully differentiated fat cells. Adipogenesis can be restored by overexpressing adipogenic transcription factors in preadipocytes from old animals. Redistribution of lipid to extra-adipose sites with aging could result from loss of lipid storage capacity in fat depots, altered fatty acid handling resulting in lipid accumulation, dysdifferentiation of mesenchymal precursors, such as muscle satellite cells and osteoblast precursors, into a partial adipocyte phenotype, or a combination of these mechanisms. Thus, accumulation of mesenchymal adipocyte-like default (MAD) cells in fat depots, muscle, bone marrow, and elsewhere is a potentially reversible process that could contribute to maldistribution of fat in old age.

Determinants of leptin gene expression in fat depots of lean mice

The relationship of leptin gene expression to adipocyte volume was investigated in lean 10-wk-old male C57BL/6J mice. mRNA levels for leptin, insulin receptor, glucocorticoid receptor, and tumor necrosis factor (TNF)-alpha in inguinal, epididymal, and retroperitoneal adipose tissues were quantified and related to adipocyte volume. Leptin mRNA levels were highly correlated with adipocyte volume within each fat depot. Multiple regression analysis of pooled data from the three depots showed that leptin mRNA levels were strongly correlated with adipocyte volumes (beta = 0.84, P < 0.001) and, to a smaller degree, with glucocorticoid receptor mRNA levels (beta = 0.36, P < 0.001). Depot of origin had no effect (P > 0.9). Rates of leptin secretion in vitro were strongly correlated with leptin mRNA levels (r = 0.89, P < 0.001). mRNA levels for TNF-alpha, insulin receptor, and glucocorticoid receptor showed no significant correlation with adipocyte volume. These results demonstrate that depot-specific differences in leptin gene expression are mainly related to the volumes of the constituent adipocytes. The strong correlation between leptin gene expression and adipocyte volume supports leptin's physiological role as a humoral signal of fat mass.

The biology of white adipocyte proliferation

Expanded adipose tissue mass increases the risk for many clinical conditions including diabetes, hypertension, coronary atherosclerotic heart disease, and some forms of cancer. Therefore, it is imperative that we understand the mechanisms by which fat pads expand. The enlargement of fat cells during the development of obesity has been previously hypothesized to be a triggering factor for the proliferation of new fat cells. There is now a preponderance of evidence that adipose tissue is a source of growth factors such as IGF-I, IGF binding proteins, TNF alpha, angiotensin II, and MCSF that are capable of stimulating proliferation. The relative importance of these autocrine/paracrine factors in the normal control of preadipocyte proliferation is unknown. In addition, the proliferative response of preadipocytes to the paracrine milieu is undoubtedly modulated by neural inputs to fat tissue and/or serum factors. Together, these multiple regulatory controls orchestrate overall and region-specific adipose tissue cellularity responses associated with the development of hyperplastic obesity. Both in vivo and in vitro studies are needed to understand the complex, interacting physiological mechanisms by which growth of this important organ is regulated.

Tumor necrosis factor-alpha regulates secretion of the adipocyte-derived cytokine, leptin

The seminal observation that secretion of the adipocyte-derived hormone leptin was induced by inflammatory challenge has been expanded upon to demonstrate the importance of the pro-inflammatory cytokines, especially tumor necrosis factor (TNF)-alpha, in inflammatory hyperleptinemia. Initially, it was thought that cytokine-induced hyperleptinemia might somehow be involved in the anorexia and cachexia that often accompany chronic infectious, neoplastic, and autoimmune disease. While the role of leptin in disease-associated anorexia and cachexia appears tenuous in light of recent findings, there is evidence that the hyperleptinemia induced by cytokines is an integral part of the acute phase response and necessary for comprehensive immunocompetence. This hints at the existence of an integrated communication network, wherein the energy status of the animal impacts its ability to fight pathogens.

Tumor necrosis factor-alpha regulates secretion of the adipocyte-derived cytokine, leptin

The seminal observation that secretion of the adipocyte-derived hormone leptin was induced by inflammatory challenge has been expanded upon to demonstrate the importance of the pro-inflammatory cytokines, especially tumor necrosis factor (TNF)-alpha, in inflammatory hyperleptinemia. Initially, it was thought that cytokine-induced hyperleptinemia might somehow be involved in the anorexia and cachexia that often accompany chronic infectious, neoplastic, and autoimmune disease. While the role of leptin in disease-associated anorexia and cachexia appears tenuous in light of recent findings, there is evidence that the hyperleptinemia induced by cytokines is an integral part of the acute phase response and necessary for comprehensive immunocompetence. This hints at the existence of an integrated communication network, wherein the energy status of the animal impacts its ability to fight pathogens.

Developmental stage modifies diet-induced peripheral insulin resistance in rats

In the present study, the effects of age and diet on glucose disappearance and tissue-specific glucose uptake (R'g) were examined under basal or hyperinsulinemic, euglycemic conditions in male Sprague-Dawley rats. Rats were equicalorically fed either a high-starch diet (68% of kcal), high-fat diet (HFD; 45% of kcal), or high-sucrose diet (68% of kcal), beginning at either 5 (W; weanling), 10 (Y; young), 18 (M; mature), or 58 wk (O; older) of age for 5 wks (n = 6-9. group(-1) x diet(-1)). Body weight gain was not significantly different among dietary groups within a given age. Significant (P< 0.05) age effects were observed on basal and clamp free fatty acid concentrations. Significant diet effects were observed on basal and clamp triglyceride concentrations. There were significant diet and age effects on basal skeletal muscle R'g. This interaction was primarily due to an age-associated increase in basal R'g microg x g(-1). min(-1)) in HFD (gastrocnemius R'g: 0.9+/-0.2 in W, 1.1+/-0.2 in Y, 1.8+/-0.2 in M, 2.5+/-0.2 in O). Both age and diet significantly decreased insulin-stimulated muscle R'g. However, whereas age-associated reductions in both glucose-6-phosphate concentration and glycogen synthase activity were observed, significant diet effects were observed on glucose-6-phosphate concentrations only. Age significantly reduced basal and clamp adipose tissue R'g when expressed per gram of tissue but significantly increased R'g when expressed per total fat pad mass. These data suggest that diet-induced changes in peripheral glucose metabolism are modulated by age.