Endocrine Regulation of Fetal Adipose Tissue Metabolism in the Pig: Interaction of Porcine Growth Hormone and Thyroxine

HAUSMAN, D.B., G.J. HAUSMAN, AND R.J. MARTIN. Endocrine regulation of fetal adipose tissue metabolism in the pig: interaction of porcine growth hormone and thyroxine. Obes Res. 1999;7:76–82. Objective : This study tested the hypothesis that combined treatment of thyroxine (T₄) and growth hormone (GH) could normalize cellular and metabolic aspects of adipose tissue development of hypophysectomized fetal pigs. Research Methods and Procedures : On day 70 of gestation, pig fetuses were hypophysectomized by microcauterization or remained intact. Hypophysectomized fetuses remained untreated or were treated from day 90 to day 105 of gestation with T₄, GH, or a combination of both hormones. Results : Body weights were unaffected by hypophysectomy or hormone treatment. De novo lipogenesis in subcutaneous adipose tissue was increased 10-fold by hypophysectomy, consistent with our previous results. This increase was abolished by GH treatment in the hypophysectomized fetuses. In contrast, T₄ treatment of the hypophysectomized fetuses resulted in a 12-fold further increase in adipose tissue lipogenesis, an effect that was negated by concomitant administration of GH. Lipolytic response to isoproterenol was decreased by hypophysectomy, unaffected by GH treatment, and restored to intact values by T₄ or by T₄+GH treatment in the hypophysectomized fetuses. Discussion : In contrast to T₄, GH does not influence serum insulin-like growth factor-I or adipose tissue lipolysis, but decreases lipogenesis in the fetal pig. However, replacing both T₄ and GH normalized hypophysectomized fetuses to a greater extent than either GH or T₄ alone. Thus, any influence of thyroid hormones on stimulating adipose tissue lipogenesis in the developing fetal pig may be normally counterregulated by pituitary-derived growth hormone.     

Adrenalectomy Reduces Adiposity by Decreasing Feed Efficiency,Not Direct Effects on White Adipose Tissue

EDENS, N. K., A. MOSHIRFAR, G. M. POTTER, S. K. FRIED, AND T. W. CASTONGUAY. Adrenalectomy reduces adiposity by decreasing feed efficiency, not direct effects on white adipose tissue. Obes Res. Objective: This study was conducted to establish the effects of adrenalectomy (ADX) on adipose tissue metabolism in male Sprague—Dawley rats fed a standard chow diet. Research Methods and Procedures: The effects of adrenalectomy on adipose cell size, lipoprotein lipase activity, and basal and insulin-stimulated glucose conversion to lipid and lipolysis were measured. Results: ADX decreased body weight gain during the postoperative period in the absence of changes in food intake; feed efficiency was decreased significantly. ADX decreased adipocyte size by 30%. ADX increased adipocyte response to the effect of submaximal concentrations of insulin on lipid synthesis and lipolysis. ADX decreased maximally insulin-stimulated lipid synthesis, but this effect was accounted for by decreased adipocyte size. In contrast, ADX had no effect on maximally insulin-inhibited lipolysis. ADX did not affect heparin-releasable LPL. The small effect of ADX on residual extractable adipose tissue LPL activity was accounted for by decreased fat cell size. Discussion: ADX decreased adiposity in the absence of changes in food intake, lipoprotein lipase activity, and adipocyte lipid metabolism. The effect is best attributed to decreased feed efficiency.     

The Influence of Hydrocortisone (HC) on Differentiation of Adipose Tissue is Dependent on Fetal Age

Elevated serum hydrocortisone (HC) levels are associated with larger fat cells and elevated levels of Iipogenic and associated enzymes in late term pig fetuses from genetically obese dams. We have investigated the influence of HC status per se on these and other adipose tissue traits by chronically treating pig fetuses hypophysectomized (hypox) on day 70 with HC between either day 70 and 90 or 90 and 105 of gestation. Treatment with HC during both periods increased serum HC levels (P<.05) and increased fat cell size (P<.05) in the perirenal (PERI) and subcutaneous (SQ) depots, but failed to influence body weights, insulin-like growth factor-1 and insulin levels. Quantitative analysis of sections of PERI and SQ adipose tissue indicated that HC increased lipoprotein lipase (LPL), esterase and glucose-6-phosphate dehydrogenase (G6PDH) activities. The degree of esterase and G6PDH, but not LPL response to HC, was greater during the 90- to 105-day period than during the earlier period. HC significantly increased lipid accretion only in the SQ depot between 90 and 105 days. Overall, HC significantly augmented hypox-induced alterations in cellular and metabolic traits of developing adipose tissue. The general increase in fat cell size (21%) with moderate (SQ-105d) or no (PERI-90, 105d; SQ-90d) increase in lipid accretion indicates that HC either did not influence or decreased apparent fat cell number. Regardless, these data indicate that changes in serum HC per se may account for adipose tissue traits that characterize fetuses from genetically obese dams.     

Effect of Carbohydrate Overfeeding on Whole Body and Adipose Tissue Metabolism in Humans

Objective: To evaluate the effect of a 4‐day carbohydrate overfeeding on whole body net de novo lipogenesis and on markers of de novo lipogenesis in subcutaneous adipose tissue of healthy lean humans. Research Methods and Procedures: Nine healthy lean volunteers (five men and four women) were studied after 4 days of either isocaloric feeding or carbohydrate overfeeding. On each occasion, they underwent a metabolic study during which their energy expenditure and net substrate oxidation rates (indirect calorimetry), and the fractional activity of the pentose‐phosphate pathway in subcutaneous adipose tissue (subcutaneous microdialysis with 1, 6¹³C₂, 6, 6²H₂ glucose) were assessed before and after administration of glucose. Adipose tissue biopsies were obtained at the end of the experiments to monitor mRNAs of key lipogenic enzymes. Results: Carbohydrate overfeeding increased basal and postglucose energy expenditure and net carbohydrate oxidation. Whole body net de novo lipogenesis after glucose loading was markedly increased at the expense of glycogen synthesis. Carbohydrate overfeeding also increased mRNA levels for the key lipogenic enzymes sterol regulatory element‐binding protein‐1c, acetyl‐CoA carboxylase, and fatty acid synthase. The fractional activity of adipose tissue pentose‐phosphate pathway was 17% to 22% and was not altered by carbohydrate overfeeding. Discussion: Carbohydrate overfeeding markedly increased net de novo lipogenesis at the expense of glycogen synthesis. An increase in mRNAs coding for key lipogenic enzymes suggests that de novo lipogenesis occurred, at least in part, in adipose tissue. The pentose‐phosphate pathway is active in adipose tissue of healthy humans, consistent with an active role of this tissue in de novo lipogenesis.     

ATGL调节细胞内脂质代谢和代谢相关疾病

脂肪组织甘油三酯水解酶(adipose triglyceride lipase, ATGL)是一种催化甘油三酯第一步水解的重要脂肪酶,在机体能量代谢调节中发挥重要作用.本文介绍了ATGL的基因和蛋白质结构,并详细综述了ATGL的功能调控和与其相关联疾病的研究进展,最后通过与激素敏感脂肪酶(HSL)比较,对ATGL的特征进行总结.     

Daily rhythms of lipid metabolic gene expression in zebra fish liver: Response to light/dark and feeding cycles

Despite numerous studies about fish nutrition and lipid metabolism, very little is known about the daily rhythm expression of lipogenesis and lipolysis genes. This research aimed to investigate the existence of daily rhythm expressions of the genes involved in lipid metabolism and their synchronization to different light/dark (LD) and feeding cycles in zebra fish liver. For this purpose, three groups of zebra fish were submitted to a 12:12?h LD cycle. A single daily meal was provided to each group at various times: in the middle of the light phase (ML); in the middle of the dark phase (MD); at random times. After 20 days of acclimation to these experimental conditions, liver samples were collected every 4?h in one 24-h cycle. The results revealed that most genes displayed a significant daily rhythm with an acrophase of expression in the dark phase. The acrophase of lipolytic genes (lipoprotein lipase – lpl, peroxisome proliferator-activated receptor – pparα and hydroxyacil CoA dehydrogenase – hadh) was displayed between ZT 02:17?h and ZT 18:31?h. That of lipogenic genes (leptin-a – lepa, peroxisome proliferator-activated receptor – pparγ, liver X receptor – lxr, insulin-like growth factor – igf1, sterol regulatory element-binding protein – srebp and fatty acid synthase – fas) was displayed between ZT 15:25?h and 20:06?h (dark phase). Feeding time barely influenced daily expression rhythms, except for lxr in the MD group, whose acrophase shifted by about 14?h compared with the ML group (ZT 04:31?h versus ZT 18:29?h, respectively). These results evidence a strong synchronization to the LD cycle, but not to feeding time, and most genes showed a nocturnal acrophase. These findings highlight the importance of considering light and feeding time to optimize lipid metabolism and feeding protocols in fish farming.     

Metabolic and Hemodynamic Response of Adipose Tissue to Angiotensin II

Objective: Recent studies have revealed the presence of a local renin‐angiotensin system in adipose tissue. To examine the possible role of this system in adipose tissue, we performed microdialysis studies on the effect of angiotensin II (Ang II) on blood flow and metabolism in abdominal subcutaneous adipose tissue (aSAT) and femoral subcutaneous adipose tissue (fSAT) in young healthy men. Research Methods and Procedures: Using the microdialysis technique, two different protocols were run perfusion with Ringer's solution + 50 mM ethanol with the subsequent addition of 125, 250, and 500 μg/liter Ang II (n = 8) and Ringers's solution + 50 mM ethanol with the subsequent addition of isoproterenol (1 μM) alone and in combination with 500 μg/liter Ang II (n = 6). Dialysate concentrations of ethanol, glycerol, glucose, and lactate were measured for estimating blood flow (ethanol dilution technique), lipolysis, and glycolysis, respectively. Results: Perfusion with Ang II resulted in a dose‐dependent decrease in blood flow (fSAT > aSAT), lipolysis (fSAT > aSAT), and glucose uptake (fSAT = aSAT). Isoproterenol increased blood flow and lipolysis at both sites and those effects could be returned to baseline values by the addition of Ang II in aSAT but not fSAT. Discussion: In conclusion, our data indicate that in addition to its well‐known vasoconstricting effect, Ang II inhibits lipolysis in adipose tissue, whereby femoral fat depots seem to be more sensitive to this effect than abdominal depots.     

脂肪甘油三酯脂肪酶（ATGL）的生物学功能及调控机制

脂肪甘油三酯脂肪酶(ATGL)是近年来研究发现的启动脂肪动员的又一关键脂肪酶. ATGL能特异性地水解甘油三酯(TAG)的第一酯键,被认为是TAG水解过程的限速酶. ATGL在脂肪组织和非脂肪组织脂代谢过程中都发挥着重要作用,其活性和表达在细胞内受到转录水平、翻译后水平等调控.ATGL介导的脂解过程可能与肥胖、糖尿病、脂肪肝等代谢疾病存在关联.本文主要就ATGL的结构特征、生物学功能及其调控机制进行综述,并对今后的研究方向和应用进行了展望.     

棕色脂肪的产热及其调控机制

棕色脂肪组织是小型哺乳动物重要的兼性产热器官,位于棕色脂肪细胞线粒体内膜的解偶联蛋白是此种产热机制的关键物质.产热刺激激活解偶联蛋白构成的质子通道,在线粒体内膜形成质子短路,从而解除ATP合成偶联对氧化呼吸速率的控制,使产热高速进行.去甲肾上腺素、甲状腺激素、胰岛素、pH值以及食物、环境温度等因素综合调控着棕色脂肪组织的结构与功能状态.     

Fragments of Functional Proteins: Role in Endocrine Regulation

Systematic analysis of structures, localization, formation and biological activities of endogenous peptides derived from functional proteins, such as hemoglobin, myelin basic protein, immunoglobulins, etc., allowed establishing the basic features of that group of compounds. The sets of these peptides in mammalian tissues, or tissue-specific peptide pools are: (i) tissue specific; (ii) stable at normal conditions; (iii) conservative in the same tissues of different mammalian species; (iv) dependent on the general state of homeostasis of tissue or the whole organism. Formation of such peptides has features of both conformation and site specificity and also involves the action of carboxy- and amino-peptidases. As a result, the families of structurally related families of peptides are generated. The fragments of functional proteins exhibit a wide range of the biological effects, characteristic both for hormones and parahormones, from hormone-releasing to growth-regulatory activity. At the same time, the molecular mechanisms of action of the majority of such peptides are unknown. On the basis of the data obtained the components of tissue-specific peptide pools are considered to form a novel regulatory system, complementary to other peptidergic systems such as hormonal, nervous, immune, etc. The biological role of the fragments of functional proteins in vivo and the patterns of interaction with other regulatory systems are suggested.     

Upper and Lower Body Adipose Tissue Function: A Direct Comparison of Fat Mobilization in Humans

Objectives: Fat in the lower body is not associated with the same risk of cardiovascular disease as fat in the upper body. Is this explained by differences in the physiological functioning of the two depots? This study had two objectives: 1) to determine whether fat mobilization and blood flow differ between gluteal and abdominal adipose tissues in humans, and 2) to develop a new technique to assess gluteal adipose tissue function directly. Research Methods and Procedures: We performed detailed in vivo studies of adipose tissue function involving the assessment of fat mobilization by measurement of adipose tissue blood flows, arterio‐venous differences of metabolites across each depot, and gene expression in tissue biopsies in a small‐scale physiological study. Results: Gluteal adipose tissue has a lower blood flow (67% lower, p < 0.05) and lower hormone‐sensitive lipase rate of action (87% lower, p < 0.05) than abdominal adipose tissue. Lipoprotein lipase rate of action and mRNA expression are not different between the depots. This is the first demonstration of a novel technique to directly investigate gluteal adipose tissue metabolism. Discussion: Direct assessment of fasting adipose tissue metabolism in defined depots show that the buttock is metabolically “silent” in terms of fatty acid release compared with the abdomen.     

The Adipose Tissue Phenotype of Hormone‐Sensitive Lipase Deficiency in Mice

Objective: To directly ascertain the physiological roles in adipocytes of hormone‐sensitive lipase (HSL; E.C. 3.1.1.3), a multifunctional hydrolase that can mediate triacylglycerol cleavage in adipocytes. Research Methods and Procedures: We performed constitutive gene targeting of the mouse HSL gene (Lipe), subsequently studied the adipose tissue phenotype clinically and histologically, and measured lipolysis in isolated adipocytes. Results: Homozygous HSL^?/? mice have no detectable HSL peptide or cholesteryl esterase activity in adipose tissue, and heterozygous mice have intermediate levels with respect to wild‐type and deficient littermates. HSL‐deficient mice have normal body weight but reduced abdominal fat mass compared with normal littermates. Histologically, both white and brown adipose tissues in HSL^?/? mice show marked heterogeneity in cell size, with markedly enlarged adipocytes juxtaposed to cells of normal morphology. In isolated HSL^?/? adipocytes, lipolysis is not significantly increased by β₃‐adrenergic stimulation, but under basal conditions in the absence of added catecholamines, the lipolytic rate of isolated HSL^?/? adipocytes is at least as high as that of cells from normal controls. Cold tolerance during a 48‐hour period at 4 °C was similar in HSL^?/? mice and controls. Overnight fasting was well‐tolerated clinically by HSL^?/? mice, but after fasting, liver triglyceride content was significantly lower in HSL^?/? mice compared with wild‐type controls. Conclusions: In isolated fat cells, the lipolytic rate after β‐adrenergic stimulation is mainly dependent on HSL. However, the observation of a normal rate of lipolysis in unstimulated HSL^?/? adipocytes suggests that HSL‐independent lipolytic pathway(s) exist in fat. Physiologically, HSL deficiency in mice has a modest effect under normal fed conditions and is compatible with normal maintenance of core body temperature during cold stress. However, the lipolytic response to overnight fasting is subnormal.     

Subcutaneous Abdominal Adipose Tissue Subcompartments: Potential Role in Rosiglitazone Effects

Abdominal visceral tissue (VAT) and subcutaneous adipose tissue (SAT), comprised of superficial‐SAT (sSAT) and deep‐SAT (dSAT), are metabolically distinct. The antidiabetic agents thiazolidinediones (TZDs), in addition to their insulin‐sensitizing effects, redistribute SAT suggesting that TZD action involves adipose tissue depot‐specific regulation. We investigated the expression of proteins key to adipocyte metabolism on differentiated first passage (P1) preadipocytes treated with rosiglitazone, to establish a role for the diverse depots of abdominal adipose tissue in the insulin‐sensitizing effects of TZDs. Adipocytes and preadipocytes were isolated from sSAT, dSAT, and VAT samples obtained from eight normal subjects. Preadipocytes (P1) left untreated (U) or treated with a classic differentiation cocktail (DI) including rosiglitazone (DIR) for 9 days were evaluated for strata‐specific differences in differentiation including peroxisome proliferator‐activated receptor‐γ (PPAR‐γ) and lipoprotein lipase (LPL) expression, insulin sensitivity via adiponectin and glucose transport‐4 (GLUT4), glucocorticoid metabolism with 11β‐hydroxysteroid dehydrogenase type‐1 (11βHSD1), and alterations in the adipokine leptin. While depot‐specific differences were absent with the classic differentiation cocktail, with rosiglitazone sSAT had the most potent response followed by dSAT, whereas VAT was resistant to differentiation. With rosiglitazone, universal strata effects were observed for PPAR‐γ, LPL, and leptin, with VAT in all cases expressing significantly lower basal expression levels. Clear dSAT‐specific changes were observed with decreased intracellular GLUT4. Specific sSAT alterations included decreased 11βHSD1 whereas secreted adiponectin was potently upregulated in sSAT with respect to dSAT and VAT. Overall, the subcompartments of SAT, sSAT, and dSAT, appear to participate in the metabolic changes that arise with rosiglitazone administration.     

Expression and Regulation of Soluble Epoxide Hydrolase in Adipose Tissue

Obesity is an increasingly important public health issue reaching epidemic proportions. Visceral obesity has been defined as an important element of the metabolic syndrome and expansion of the visceral fat mass has been shown to contribute to the development of insulin resistance and cardiovascular disease. To identify novel contributors to cardiovascular and metabolic abnormalities in obesity, we analyzed the adipose proteome and identified soluble epoxide hydrolase (sEH) in the epididymal fat pad from C57BL/6J mice that received either a regular diet or a “western diet.” sEH was synthesized in adipocytes and expression levels increased upon differentiation of 3T3‐L1 preadipocytes. Although normalized sEH mRNA and protein levels did not differ in the fat pads from mice receiving a regular or a “western diet,” total adipose sEH activity was higher in the obese mice, even after normalization for body weight. Furthermore, peroxisome proliferator–activated receptor γ (PPARγ) agonists increased the expression of sEH in mature 3T3‐L1 adipocytes in vitro and in adipose tissue in vivo. Considering the established role for sEH in inflammation, cardiovascular diseases, and lipid metabolism, and the suggested involvement of sEH in the development of type 2 diabetes, our study has identified adipose sEH as a potential novel therapeutic target that might affect the development of metabolic and cardiovascular abnormalities in obesity.     

Metabolic Dysregulation and Adipose Tissue Fibrosis: Role of Collagen VI

Adipocytes are embedded in a unique extracellular matrix whose main function is to provide mechanical support, in addition to participating in a variety of signaling events. During adipose tissue expansion, the extracellular matrix requires remodeling to accommodate adipocyte growth. Here, we demonstrate a general upregulation of several extracellular matrix components in adipose tissue in the diabetic state, therefore implicating “adipose tissue fibrosis” as a hallmark of metabolically challenged adipocytes. Collagen VI is a highly enriched extracellular matrix component of adipose tissue. The absence of collagen VI results in the uninhibited expansion of individual adipocytes and is paradoxically associated with substantial improvements in whole-body energy homeostasis, both with high-fat diet exposure and in the ob/ob background. Collectively, our data suggest that weakening the extracellular scaffold of adipocytes enables their stress-free expansion during states of positive energy balance, which is consequently associated with an improved inflammatory profile. Therefore, the disproportionate accumulation of extracellular matrix components in adipose tissue may not be merely an epiphenomenon of metabolically challenging conditions but may also directly contribute to a failure to expand adipose tissue mass during states of excess caloric intake.Adipose tissue is a key regulator of systemic energy homeostasis. The physiological state of adipose tissue is driven by cell-autonomous processes within the adipocyte. In addition to this, the adipocyte itself is subject to major modifications by other cell types that infiltrate adipose tissue, such as macrophages and vascular cells; moreover, adipocytes can be markedly influenced by several hormones and cytokines that circulate systemically.Although all these cellular interactions have been the subject of extensive studies in numerous laboratories, the extracellular matrix of adipose tissue has received limited attention to date, despite evidence suggesting that it is a functionally relevant constituent of adipose tissue physiology.It is currently unknown what consequential effects metabolic stress exerts on the extracellular matrix and vice versa. In other words, what is the impact of dysregulation of the extracellular constituents of adipose tissue on the systemic metabolic state? Here, we approach this subject from two different perspectives. We first assessed the overall level of extracellular matrix components under different metabolic conditions and established that the extracellular constituents are globally upregulated during metabolically challenging conditions. We then selected a specific member of the collagen family, collagen VI (exhibiting predominant expression in adipose tissue), and utilized a genetic model of collagen VI disruption to investigate the effects of disruption of the extracellular matrix of adipose tissue. Remarkably, our studies demonstrated that such weakening of adipose tissue extracellular matrix results in considerable improvement of the metabolic phenotype in the context of both a high-fat diet and a challenge with the ob/ob mutation.Our observations highlight the extracellular matrix of adipose tissue as an important and novel site of modulation of systemic metabolism. Obese adipose tissue displays hallmarks similar to other fibrotic tissues, such as the liver; this suggests that specific constituents of this normally rather rigid extracellular matrix environment may provide possible targets for pharmacological intervention for the treatment of metabolic disorders.     

Genetic Regulation of SN-Glycerol-3-Phosphate Dehydrogenase in Brown Adipose Tissue

The levels of sn-glycerol-3-phosphate dehydrogenase (GPDH) were determined in the brown adipose tissue (BAT) of different inbred strains of mice. The BAT of the BALB/cJ strain contains twice as much enzyme activity per milligram protein as do other strains. The appearance of this difference is developmentally dependent, since it is not detected in BAT until 25-30 days postpartum. Genetic analysis of this strain difference has shown that the mechanism of inheritance involves at least two genes, one of which is linked to the Gdc-1 structural locus on chromosome 15. Determinations of GPDH synthesis by immunoprecipitation of GPDH protein labeled in vivo with [3H]leucine, and of GPDH mRNA by Northern blot analysis, establish that in BALB/cJ mice higher rates of enzyme synthesis are determined by elevated levels of GPDH mRNA. It was also found that cold stress increases GPDH mRNA levels in all the strains examined.