Circulating Levels of the Adipokines Monocyte Chemotactic Protein-4 (MCP-4), Macrophage Inflammatory Protein-1ß (MIP-1ß), and Eotaxin-3 in Severe Obesity and Following Bariatric Surgery.

The aim of the study was to investigate the involvement of the adipokines eotaxin-3, MIP-1ß, and MCP-4 in obesity and related comorbidities and the modification of their circulating levels after bariatric surgery. Eighty severely obese subjects and 20 normal-weight controls were included in the study. Circulating levels of MCP-4, MIP-1ß, and eotaxin-3, and the main clinical, biochemical, and instrumental parameters for the evaluation of cardiovascular and metabolic profile were determined in controls and in obese subjects at baseline and 10 months after surgery. Within the obese group at baseline, eotaxin-3 levels were higher in males than females and in smokers than non-smokers and showed a positive correlation with LDL-cholesterol, apolipoprotein B, and leptin. MIP-1ß showed a positive correlation with age and leptin and a negative correlation with adiponectin and was an independent predictor of increased carotid artery intima-media thickness. MCP-4 levels were higher in obese subjects than controls and showed a positive correlation with body mass index, eotaxin-3, and MIP-1ß. Bariatric surgery induced a marked decrease in all the 3 adipokines. MCP-4 is a novel biomarker of severe obesity and could have an indirect role in favoring sub-clinical atherosclerosis in obese patients by influencing the circulating levels of eotaxin-3 and MIP-1ß, which are directly related to the main atherosclerosis markers and risk factors. The reduction of circulating levels of MCP-4, eotaxin-3, and MIP-1ß could be one of the mechanisms by which bariatric surgery contributes to the reduction of cardiovascular risk in these patients.

Semaphorin 3C is a novel adipokine linked to extracellular matrix composition.

AIMS/HYPOTHESIS: Alterations in white adipose tissue (WAT) function, including changes in protein (adipokine) secretion and extracellular matrix (ECM) composition, promote an insulin-resistant state. We set out to identify novel adipokines regulated by body fat mass in human subcutaneous WAT with potential roles in adipose function. METHODS: Adipose transcriptome data and secretome profiles from conditions with increased/decreased WAT mass were combined. WAT donors were predominantly women. In vitro effects were assessed using recombinant protein. Results were confirmed by quantitative PCR/ELISA, metabolic assays and immunochemistry in human WAT and adipocytes. RESULTS: We identified a hitherto uncharacterised adipokine, semaphorin 3C (SEMA3C), the expression of which correlated significantly with body weight, insulin resistance (HOMA of insulin resistance [HOMAIR], and the rate constant for the insulin tolerance test [KITT]) and adipose tissue morphology (hypertrophy vs hyperplasia). SEMA3C was primarily found in mature adipocytes and had no direct effect on human adipocyte differentiation, lipolysis, glucose transport or the expression of ß-oxidation genes. This could in part be explained by the significant downregulation of its cognate receptors during adipogenesis. In contrast, in pre-adipocytes, SEMA3C increased the production/secretion of several ECM components (fibronectin, elastin and collagen I) and matricellular factors (connective tissue growth factor, IL6 and transforming growth factor-ß1). Furthermore, the expression of SEMA3C in human WAT correlated positively with the degree of fibrosis in WAT. CONCLUSIONS/INTERPRETATION: SEMA3C is a novel adipokine regulated by weight changes. The correlation with WAT hypertrophy and fibrosis in vivo, as well as its effects on ECM production in human pre-adipocytes in vitro, together suggest that SEMA3C constitutes an adipocyte-derived paracrine signal that influences ECM composition and may play a pathophysiological role in human WAT.

1,25-Dihydroxyvitamin D3 inhibits the cytokine-induced secretion of MCP-1 and reduces monocyte recruitment by human preadipocytes.

BACKGROUND: Adipose tissue expansion during obesity is associated with a state of low-grade inflammation and an increase in macrophage infiltration, which predisposes to insulin resistance and vascular malfunction. Growing evidence suggests that vitamin D3 has immunoregulatory effects and adipose tissue could be a target for vitamin D3 action. Preadipocytes, one of the major cell types in adipose tissue, are actively involved in inflammatory processes. OBJECTIVES: This study investigated whether the active form of vitamin D3 (1,25(OH)2D3) affects the production of proinflammatory chemokines/cytokines and the monocyte recruitment by human preadipocytes. METHODS/RESULTS: The secretion levels of monocyte chemoattractant proteint-1 (MCP-1), IL-8 and IL-6 were significantly higher in preadipocytes than in differentiated adipocytes, suggesting that preadipocytes could be a major source of proinflammatory mediators. Cytokine profile analysis revealed that 1,25(OH)2D3 (10 nM) markedly reduced the release of MCP-1, IL-6 and IL-8 by preadipocytes. The involvement of NFκB signalling was shown by the upregulation of IκBα protein abundance by 1,25(OH)2D3 in preadipocytes. In addition, 1,25(OH)2D3 was able to decrease the migration of THP-1 monocytes. Treatment with proinflammatory stimuli, including macrophage-conditioned (MC) medium, TNFα and IL-1ß, led to a marked increase in protein release of MCP-1 and IL-6 by preadipocytes. Pretreatment with 1,25(OH)2D3 (10 nM and 100 nM) significantly decreased the stimulatory effects of MC medium, TNFα and IL-1ß on MCP-1 expression and protein release, although the effect on stimulated release of IL-6 was less potent. CONCLUSIONS: These results demonstrate that 1,25(OH)2D3 decreases the production of MCP-1 and other proinflammatory mediators by preadipocytes and reduces monocyte migration. Thus, vitamin D3 may protect against adipose tissue inflammation by disrupting the deleterious cycle of macrophage recruitment.

Enhanced ZAG production by subcutaneous adipose tissue is linked to weight loss in gastrointestinal cancer patients.

BACKGROUND: Profound loss of adipose tissue is a hallmark of cancer cachexia. Zinc-α2-glycoprotein (ZAG), a recently identified adipokine, is suggested as a candidate in lipid catabolism. METHODS: In the first study, eight weight-stable and 17 cachectic cancer patients (weight loss 5% in previous 6 months) were recruited. Zinc-α2-glycoprotein mRNA and protein expression were assessed in subcutaneous adipose tissue (SAT), subcutaneous adipose tissue morphology was examined and serum ZAG concentrations were quantified. In the second cohort, ZAG release by SAT was determined in 18 weight-stable and 15 cachectic cancer patients. The effect of ZAG on lipolysis was evaluated in vitro. RESULTS: Subcutaneous adipose tissue remodelling in cancer cachexia was evident through shrunken adipocytes with increased fibrosis. In cachectic cancer patients, ZAG mRNA was upregulated (2.7-fold, P=0.028) while leptin mRNA decreased (2.2-fold, P=0.018); serum ZAG levels were found to be unaffected. Zinc-α2-glycoprotein mRNA correlated positively with weight loss (r=0.51, P=0.01) and serum glycerol levels (r=0.57, P=0.003). Zinc-α2-glycoprotein release by SAT was also elevated in cachectic patients (1.5-fold, P=0.024) and correlated with weight loss (r=0.50, P=0.003). Recombinant ZAG stimulated lipolysis in human adipocytes. CONCLUSIONS: Zinc-α2-glycoprotein expression and secretion by adipose tissue is enhanced in cachectic cancer patients. Given its lipid-mobilising effect, ZAG may contribute to adipose atrophy associated with cancer cachexia in human beings.

Zinc-α2-glycoprotein: an adipokine modulator of body fat mass?

The importance of white adipose tissue in the control of energy balance is now firmly recognized. In addition to fuel storage, adipocytes secrete an array of proteins factors (adipokines), which regulate multiple physiological and metabolic processes as well as influence body fat accumulation. Zinc-α2-glycoprotein (ZAG), a lipid mobilizing factor initially characterized as a tumor product associated with cachexia, has recently been identified as a novel adipokine. Although the exact role of ZAG in adipose tissue remains to be clarified, there is evidence that ZAG expression appears to be inversely related to adiposity, being upregulated in cachexia whereas reduced in obesity. Investigations on the regulation of ZAG give insights into its potential function in adipose tissue with a link to lipid mobilization and an anti-inflammatory action. Recent work shows that ZAG stimulates adiponectin secretion by human adipocytes. Data from genetic studies suggest that ZAG may be a candidate gene for body weight regulation; this is supported by the demonstration that ZAG-knockout mice are susceptible to weight gain, whereas transgenic mice overexpressing ZAG exhibit weight loss. The present review summarizes these new perspectives of ZAG and the potential mechanisms by which it might modulate adipose tissue mass and function.

The adipokine zinc-alpha2-glycoprotein (ZAG) is downregulated with fat mass expansion in obesity.

INTRODUCTION: Zinc-alpha2-glycoprotein (ZAG) is a novel adipokine, which may act locally to influence adipocyte metabolism. This study assessed the effect of increased adiposity on ZAG expression in adipose tissue in human subjects. The study also examined the association between ZAG and adiponectin expression in human adipose tissue, and whether ZAG modulates adiponectin secretion by human adipocytes. METHODS: Adipose tissue (visceral and subcutaneous) was collected from human subjects with a wide range of BMIs. Human Simpson-Golabi-Behmel syndrome (SGBS) adipocytes were used for in vitro studies. ZAG mRNA levels were quantified by real-time PCR and protein by Western blotting. RESULTS: In human subjects, ZAG mRNA level was negatively correlated with BMI (r = -0.61, P < 0.001, n = 23, visceral; r = -0.6, P < 0.05, n = 14, subcutaneous) and fat mass (r = -0.62, P < 0.01, visceral; r = -0.6, P < 0.05, subcutaneous). Negative associations were also found between ZAG mRNA and insulin resistance parameters including plasma insulin (r = -0.65, P < 0.001, visceral; r = -0.55, P < 0.05, subcutaneous) and homeostasis model of insulin resistance (HOMA-IR) (r = -0.65, P < 0.001, visceral; r = -0.52, P = 0.055, subcutaneous), and C reactive protein (CRP) (r = -0.46, P < 0.05, visceral; r = -0.53, P < 0.05, subcutaneous). However, ZAG mRNA was positively correlated with adiponectin (r = 0.5, P < 0.05, visceral; r = 0.82, P < 0.001, subcutaneous) but negatively associated with leptin mRNA (r = -0.42, P < 0.05, visceral; r = -0.54, P < 0.05, subcutaneous). ZAG secretion by differentiated human adipocytes was abundant. Addition of recombinant ZAG stimulated adiponectin release from human adipocytes. CONCLUSION: ZAG gene expression in adipose tissue is downregulated with increased adiposity and circulating insulin. ZAG mRNA is positively correlated with adiponectin mRNA, and ZAG enhances adiponectin production by human adipocytes. We suggest that ZAG is linked to obesity and obesity-related insulin resistance.

Upregulation of the expression of inflammatory and angiogenic markers in human adipocytes by a synthetic cannabinoid, JTE-907.

Inflammation in adipose tissue is a characteristic of obesity and the metabolic syndrome. It is suggested that the endocannabinoid system is involved in the regulation of inflammatory and angiogenic processes within the tissue. Human subcutaneous preadipocytes (Zen Bio) were used as the source of human preadipocytes or adipocytes. Gene expression was examined by RT-PCR and real-time PCR. The secretion of inflammation-related proteins was determined by an ELISA array. In experiments on adipocytes treated at day 14 post-differentiation, JTE-907, a synthetic cannabinoid, upregulated the expression of key inflammatory markers - IL-6, MCP-1 and IL-1 beta - and angiogenic factors - VEGF and ANGPTL4 - at 10 microM after 20 h of treatment, having also increased the expression of TRPV1 at 10 microM. JTE-907 showed no effect after 4 h. The ELISA array showed a 2.6-fold increase in IL-6 protein release. The effect of JTE-907 was inhibited by AM251 (CB1 antagonist), and partially by arachidonyl serotonin (TRPV1 and FAAH antagonist). The CB2 antagonist, AM630, partially upregulated the effect of JTE-907. Preadipocytes fed 14 days after 100% confluence exhibited downregulation of CB1, MCP-1, and IL-1 beta, 20 h after having been exposed to JTE-907. CB1 and TRPV1 receptors participate in the regulation of several inflammatory and angiogenic factors in human adipocytes, indicating their potential value as targets for the treatment of disorders related to obesity.

Fatty acid synthesis in brown adipose tissue in relation to whole body synthesis in the cold-acclimated golden hamster (Mesocricetus auratus).

The synthesis of fatty acids has been measured in vivo with 3H2O in brown adipose tissue, the liver, white adipose tissue and the 'carcass' of cold-acclimated (4 degrees C) golden hamsters (Mesocricetus auratus), and the results compared with those of warm-acclimated (30 degrees C) animals. In warm-acclimated hamsters the highest rate of synthesis was found in the liver, which accounted for more than a quarter of the total body synthesis. Cold-acclimation led to an almost 3-fold increase in whole-body fatty acid synthesis, compared to warm-acclimated animals, and this resulted from increases in all the individual tissues examined, particularly in brown adipose tissue. In cold-acclimated hamsters the rate of synthesis appeared to be similar in brown adipose tissue and the liver. However, studies with Triton WR 1339 suggested that at least one-half of the apparent synthesis in brown adipose tissue resulted from the rapid incorporation into the tissue of fatty acids synthesised elsewhere. On a whole-body basis, the liver made a much greater contribution than brown adipose tissue to total fatty acid synthesis in cold-acclimated hamsters; this is in marked contrast to the relative importance of these two tissues in cold-acclimated rats and mice. It is concluded that in the hamster, an animal widely used for studying the mechanisms of non-shivering thermogenesis in brown adipose tissue, the fatty acids utilised for generating thermoregulatory heat are synthesised principally in tissues other than brown adipose tissue.

Fatty acid synthesis in mouse brown adipose tissue. The influence of environmental temperature on the proportion of whole-body fatty acid synthesis in brown adipose tissue and the liver.

Fatty acid synthesis has been measured in vivo with 3H2O in mice acclimated at different environmental temperatures (33, 22, 4 degrees C), and the importance of brown adipose tissue and the liver to whole-body fatty acid synthesis at each temperature assessed. At 33 degrees C, when non-shivering thermogenesis is minimal, the rate of fatty acid synthesis in interscapular brown adipose tissue was lower than in the liver, but higher than in white adipose tissue and the carcass. At 4 degrees C, when non-shivering thermogenesis is maximal, the fatty acid synthesis rate in interscapular brown adipose tissue was many times greater than in any other tissue. High fatty acid synthesis rates were also found in other brown adipose tissue depots--subscapular, dorsocervical and axillary--of cold-acclimated mice. In mice maintained at 22 degrees C the rate of fatty acid synthesis was also higher in brown adipose tissue than in other tissues. Overall, the relative importance of brown adipose tissue as a site of fatty acid synthesis increased with lower environmental temperatures, while that of the liver decreased. It was calculated that brown adipose tissue in total accounted for approx. 5% of whole-body fatty acid synthesis at 33 degrees C, 10% at 22 degrees C and 30% at 4 degrees C. In contrast, hepatic synthesis amounted to 32% of whole-body fatty acid synthesis at 33 degrees C, 16% at 22 degrees C and only 11% at 4 degrees C. An estimate of the contribution that de novo synthesis makes to total fatty acid utilization by interscapular brown adipose tissue suggests that fatty acid synthesis and breakdown constitutes a significant heat-dissipating 'cycle' in brown adipose tissue of cold-acclimated mice. Such a cycle is not evident in suckling animals since fatty acid synthesis in brown adipose tissue is very low during early development.

Lipogenesis in genetically diabetic (db/db) mice: developmental changes in brown adipose tissue, white adipose tissue and the liver.

Developmental changes in lipogenesis have been examined in interscapular brown adipose tissue (BAT), epididymal white adipose tissue and the liver of genetically diabetic (db/db) mice and their normal siblings. Lipogenesis was measured in vivo with 3H2O, from weaning (21 days of age) until 20 weeks of age. Hyperinsulinaemia was evident in db/db mice at all ages. Low rates of lipogenesis were observed at weaning in tissues of both groups of mice, but the rate rose rapidly in the first few days post-weaning. In normal mice, peak lipogenesis was obtained in each tissue at 4-5 weeks of age, and there were no major changes (on a whole-tissue basis) thereafter. A different developmental pattern was apparent in db/db mice. The rate of lipogenesis in BAT rose sharply after weaning, reaching a peak at 26 days of age (several times higher than normal mice), and then falling rapidly such that by 45 days of age it was lower than in normal mice; at age 20 weeks lipogenesis in BAT of the diabetic animals was negligible. In white adipose tissue of the db/db mutants lipogenesis (per tissue) reached a maximum at 5 weeks of age, and fell substantially between 10 and 20 weeks of age. Hepatic lipogenesis in the db/db mice rose progressively from weaning until 8 weeks of age, and then decreased. Except at weaning, hepatic lipogenesis (per tissue) was much greater in db/db mice than in normal mice, and the liver was a more important site of lipogenesis in diabetic mice than in normals, accounting for up to 60% of the whole-body total. In contrast, BAT accounted for a considerably smaller proportion of whole-body lipogenesis in db/db mice than in normal mice. It is concluded that there are major developmental differences in lipogenesis between tissues of db/db mice, and between diabetic and normal animals. The data suggest that there is an early and preferential development of insulin resistance in BAT of the db/db mutant.

Studies on the activity of brown adipose tissue in suckling, pre-obese, ob/ob mice.

The properties and activity of brown adipose tissue have been investigated in suckling, pre-obese, ob/ob mice in order to determine whether decreased thermogenesis in the tissue precedes the development of obesity in this mutant. At 14 days of age there was no difference between the ob/ob and normal animals in the total amount of interscapular brown adipose tissue, and the DNA content, protein content, and cytochrome oxidase activity of the tissue were similar in the two groups of mice. Respiration rates of brown adipose tissue mitochondria in the presence of albumin were, however, greater in the normal than the ob/ob animals, although after the addition of GDP to recouple the mitochondria there was no difference between the two groups. The mitochondrial membrane potential, measured with [3H]methyltriphenylphosphonium, was less affected by exogenous GDP in ob/ob mice than in normal animals. GDP binding to brown adipose tissue mitochondria, an index of the proton conductance pathway, was much greater in normal than in ob/ob mice at both 10 and 14 days of age; the decreased GDP binding in the mutant animals was found to result from a reduction in the number of binding sites. It is concluded that brown adipose tissue mitochondria of pre-obese ob/ob mice are more tightly coupled than those of normal siblings, and that the activity of the 'thermogenic' proton conductance pathway is lower in the mutant animals. A decrease in thermogenesis in brown adipose tissue is therefore an early event in the development of the ob/ob mouse and precedes the appearance of obesity.

Suppression of fatty acids synthesis in brown adipose tissue of mice fed diets rich in long chain fatty acids.

The influence of dietary lipid on fatty acid synthesis in brown adipose tissue has been investigated by feeding different high-fat diets to cold-acclimated mice for a period of 2 weeks. Fatty acid synthesis was measured in vivo with 3H2O, and the fats used in the study were maize oil, beef tallow and medium chain triacylglycerol oil. In the mice fed the maize oil and the beef tallow diets fatty acids synthesis was inhibited in all tissue examined--interscapular brown adipose tissue, epididymal white adipose tissue, the liver and the carcass. Synthesis was more inhibited, however, in brown adipose tissue than in other tissues, and the inhibition was greater on the maize oil diet than on the beef tallow. The medium chain triacylglycerol oil had no inhibitory effect on fatty acid synthesis in any tissue, and hepatic synthesis was even elevated on this diet. It is concluded that fatty acid synthesis in brown adipose tissue, as in other lipogenic tissues, is subject to strong suppression by dietary long chain fatty acids, and particularly by linoleic acid.

Purification and characterization of fatty acid-binding proteins from brown adipose tissue of the rat.

Fatty acid-binding proteins (FABPs) have been identified and purified from interscapular brown adipose tissue of the rat. The proteins were characterized and their properties compared with the FABP present in white adipose tissue. FABP was purified to electrophoretic homogeneity from brown adipose tissue by a procedure involving precipitation with 70% ammonium sulphate, followed sequentially by ion-exchange chromatography and gel filtration chromatography. The purified fraction migrated as a single band on SDS-PAGE with an apparent molecular mass of 14,200. Scatchard analysis of [14C]oleate-binding to purified FABP gave a Kd value of approx. 0.80 +/- 0.02 microM and a maximal binding of 0.65 +/- 0.03 mol per mol of protein; these values were similar to that found with the FABP purified from white fat. The FABP concentration in brown adipose tissue was almost twice that of FABP in white adipose tissue. Fatty acid analysis of FABP from brown adipose tissue revealed that the intrinsic arachidonic acid content was proportionately higher than that present in FABP of white adipose tissue. Isoelectric focusing of delipidated FABP indicated that it existed with two charge isoforms (pI 6.85 and 7.35). The purified FABP additionally emerged in two peaks (FABP-I and FABP-II) from a reverse phase HPLC column. Amino acid analysis showed that Gly, Thr, and Ser residues in FABP-I were almost twice as high as in FABP-II. The N-terminals of both FABP-I and -II were not blocked. These components have been partially sequenced and showed a sequence homology only between 25-31 residues from the N-terminal. Further studies are required to elucidate the precise function of the two different isoforms of FABP in brown adipose tissue.

Interactions between leptin and hypothalamic neuropeptide Y neurons in the control of food intake and energy homeostasis in the rat.

Leptin acts on the brain to inhibit feeding, increase thermogenesis, and decrease body weight. Neuropeptide Y (NPY)-ergic neurons of the hypothalamic arcuate nucleus (ARC) that project to the paraventricular nuclei (PVN) and dorsomedial nuclei (DMH) are postulated to control energy balance by stimulating feeding and inhibiting thermogenesis, especially under conditions of energy deficit. We investigated whether leptin's short-term effects on energy balance are mediated by inhibition of the NPY neurons. Recombinant murine leptin (11 microg) injected into the lateral ventricle of fasted adult Wistar rats inhibited food intake by 20-25% between 2 and 6 h after administration, compared with saline-treated controls (P < 0.05). Uncoupling protein mRNA levels in brown adipose tissue (BAT) rose by 70% (P < 0.01). Leptin treatment significantly reduced NPY concentrations by 20-50% (P < 0.05) in the ARC, PVN, and DMH and significantly decreased hypothalamic NPY mRNA levels (0.61 +/- 0.02 vs. 0.78 +/- 0.03 arbitrary units; P < 0.01). A second study examined changes in leptin during 5 days' intracerebroventricular NPY administration (10 microg/day), which induced sustained hyperphagia and excessive weight gain. In NPY-treated rats, leptin mRNA levels in epididymal fat were comparable to those in saline-treated controls (0.94 +/- 0.17 vs. 1.0 +/- 0.28 arbitrary units; P > 0.1), but plasma leptin levels were significantly higher (4.88 +/- 0.66 vs. 2.85 +/- 0.20 ng/ml; P < 0.01). Leptin therefore acts centrally to decrease NPY synthesis and NPY levels in the ARC-PVN projection; reduced NPY release in the PVN may mediate leptin's hypophagic and thermogenic actions. Conversely, NPY-induced obesity results in raised circulating leptin concentrations. Leptin and the NPY-ergic ARC-PVN neurons may interact in a homeostatic loop to regulate body fat mass and energy balance.

Regulation of leptin production: sympathetic nervous system interactions.

Leptin is secreted primarily from white adipose tissue and stimulates long-form OB-Rb receptors in the hypothalamus to decrease food intake and increase energy expenditure. A variety of neuropeptides are involved in these responses, including neuropeptide Y, agouti-related protein, the prepro-melanocortin system and cocaine- and amphetamine-regulated transcript. OB-Rb receptors (and other receptor isoforms) are also found in peripheral tissues. Leptin is now known to have a wide range of peripheral actions and is involved in activating the immune system, haematopoiesis, angiogenesis and as a growth factor, as well as being a regulator of many cellular functions. The identification of leptin has led to reappraisal of the role of white adipose tissue from being an organ concerned primarily with energy storage as fat to an understanding that it is also a major endocrine and secretory organ. While the importance of the sympathetic nervous system in mobilising fatty acids from adipose tissue has long been known, it has become apparent that the sympathetic system is a key regulator of leptin production in white adipose tissue as well. Sympathomimetic amines and cold exposure or fasting (which lead to sympathetic stimulation of white fat), decrease leptin gene expression in the tissue and leptin production. On the other hand, sympathetic blockade often increases circulating leptin and leptin gene expression, and it is possible that the sympathetic system has a tonic inhibitory action on leptin synthesis. Apart from the few instances where leptin is absent, leptin levels are increased in obesity, while the sympathetic sensitivity of adipose tissue is reduced, consistent with the high leptin levels that are seen. The dysregulation of energy balance leading to obesity may partly involve a decrease in leptin sensitivity, or the leptin system may be set to have maximal effects at low leptin levels.

Suppression of leptin during lactation: contribution of the suckling stimulus versus milk production.

Lactation in the rat is characterized by the suppression of pulsatile LH secretion, a large increase in food intake, and changes in energy balance due to the metabolic drain of milk production. The change in energy balance may be a major component in altering reproductive function. A number of factors may contribute to changing energy balance of a lactating animal; one is leptin, the product of adipose tissue, which is known to act partly as a satiety factor to decrease food intake. The aims of the present study were to determine whether there are changes in leptin levels during lactation, a state of high energy demand, and during periods of acute suckling in the presence or absence of changes in energy demand. Our goals were to determine whether lactation and the suckling stimulus influenced serum leptin levels and whether there was a potential role for leptin in the suppression of LH secretion during lactation. The first experiment was performed during diestrus of the estrous cycle, and chronic lactation, (day 9 post partum) in animals suckling 8 pups. The results showed that leptin levels were significantly decreased in both ovarian intact or ovariectomized lactators; this decrease parallels the suppression of pulsatile LH secretion. Serum insulin levels were not altered in the lactating animals. The second experiment was performed in ovariectomized lactators whose 8 pup litters were removed for 48 h, starting on day 9. On day 11, mothers received no pups or pups that were either nonfostered (resulting in no milk production) or fostered (resulting in milk production). The pups were allowed to suckle for 24 h. Following 24 h of acute suckling, serum leptin, and insulin levels correlated with the energy drain on the mother. The levels of leptin were normal and of insulin were elevated in mothers producing no milk. Conversely, leptin levels were suppressed and insulin levels normal in mothers producing milk. The third experiment used the same groups as described for the second experiment except that serial blood samples were collected for measurement of pulsatile LH secretion following 24 h of acute suckling. The results showed that regardless of whether leptin levels remained normal or were suppressed in response to acute suckling, pulsatile LH secretion was significantly inhibited compared with the nonsuckled control animals. In summary, these data suggest that the metabolic drain of milk production, and not the suckling stimulus itself, is the most likely factor responsible for the suppression of leptin secretion during lactation. Furthermore, although the decreased levels of leptin may be causally related to the inhibition of pulsatile LH secretion during chronic lactation, changes in leptin are not a prerequisite for the suppression of LH secretion in response to suckling.

Regulation of the level of uncoupling protein in brown adipose tissue by insulin requires the mediation of the sympathetic nervous system.

The role of the sympathetic nervous system in the regulation by insulin of the level of uncoupling protein in brown adipose tissue has been examined. The amount of uncoupling protein was substantially reduced in streptozotocin-diabetic rats, while insulin replacement to diabetic animals induced a partial restoration. Unilateral denervation of the interscapular brown fat pads also lowered the amount of uncoupling protein, and in diabetic animals inhibited the stimulation of the level of the protein by insulin replacement. Maintenance of normal uncoupling protein levels requires both insulin and the sympathetic system; regulation of the protein by insulin involves sympathetic mediation.

Effects of ciglitazone on insulin resistance and thermogenic responsiveness to acute cold in brown adipose tissue of genetically obese (ob/ob) mice.

Genetically obese (ob/ob) mice develop a marked insulin resistance in brown adipose tissue soon after weaning, and this is paralleled by a fall in the acute activation of the mitochondrial proton conductance pathway in the tissue on cold exposure. Treatment of ob/ob mice with ciglitazone, a new oral hypoglycaemic, led to a restoration of insulin sensitivity in brown adipose tissue. The amelioration of insulin resistance was accompanied by a normalization of the acute, cold-induced increase in mitochondrial GDP binding. These results support the hypothesis that the development of insulin resistance in brown adipose tissue is an important factor in the impaired thermogenic responsiveness of obese mice.

Effect of acclimation temperature on the concentration of the mitochondrial 'uncoupling' protein measured by radioimmunoassay in mouse brown adipose tissue.

The effect of acclimation temperature on the concentration of the mitochondrial 'uncoupling' protein (Mr 32000) from brown adipose tissue of mice has been investigated. The uncoupling protein was measured by a specific radioimmunoassay. Between 33 degrees C (thermoneutrality) and -2 degrees C there was a progressive increase with decreasing environmental temperature in the amount of uncoupling protein. For mice at -2 degrees C the mitochondrial concentration of the protein was 9-times higher than at 33 degrees C, while the total amount of the protein in interscapular brown adipose tissue was estimated to be nearly 80-times greater at -2 degrees C compared to 33 degrees C.

Hyperleptinaemia in mice induced by administration of the tyrosine hydroxylase inhibitor alpha-methyl-p-tyrosine.

Alpha-methyl-p-tyrosine (alphaMPT), an inhibitor of tyrosine hydroxylase, was administered to mice to block noradrenaline synthesis. Ten hours after injection of alphaMPT there was a 6-fold increase in plasma leptin. The level of ob mRNA in epididymal white adipose tissue was also increased, but UCP1 mRNA in brown fat fell. In contrast to lean mice, ob mRNA in white fat of ob/ob mice was not increased by alphaMPT. AlphaMPT raised plasma leptin in fasted as well as fed mice. Hyperleptinaemia was attenuated by treatment with a beta3-adrenoceptor agonist. Inhibition of noradrenaline synthesis leads to the rapid induction of hyperleptinaemia; it is suggested that sympathetic tone plays a pivotal role in regulating leptin production.