Regulation of leptin release by insulin, glucocorticoids, G(i)-coupled receptor agonists, and pertussis toxin in adipocytes and adipose tissue explants from obese humans in primary culture

Moderate differences in plasma leptin in mares have no effect on either the amino acid or the fatty acid composition of the uterine fluid

Female mammalian reproductive functions are closely linked to body condition and metabolic status. Energy homeostasis is regulated by endocrine hormones such as insulin, IGF-I, leptin and adiponectin via the hypothalamic-pituitary-adrenal axis. These metabolic hormones and their receptors are also expressed in reproductive tissues and the embryo. We investigated the relationship between circulating leptin and the fatty acid (FA) and amino acid (AA) composition of the equine uterine fluid (UF) and peripheral blood plasma (BP) by using a mass spectrometry-based approach. UF and BP were collected from ten broodmares on days 6 and 7 post ovulation, respectively. The mares were retrospectively assigned to two groups according to their BP leptin concentrations (high leptin (> 1.6 ng/ml) versus low leptin (<0.8 ng/ml)). Specific AA and FA compositions for BP and UF were found with different levels of respective metabolite abundances. The main FAs in BP were stearic, palmitic and linoleic acid. In UF, the three most abundant FAs were eicosapentaenoic, arachidonic and stearic acid. The AA profile of BP was dominated by glycine, glutamine, serine and alanine, which were likewise among the highly abundant AAs in UF. In UF, glutamic acid had by far the highest concentration. Therefore, BP leptin concentration within a physiological range do not seem to affect the specific FA nor the AA composition of the UF. The composition of the UF may therefore be mediated by local rather than by peripheral metabolic hormones.

Temporal relationship between diet-induced steatosis and onset of insulin/leptin resistance in male Wistar rats

Rats fed with high-fat-high-sucrose (HFHS) diet are known to manifest metabolic syndrome including hyperinsulinemia, hyperleptinemia, hyperglycemia, diabetic dyslipidemia, and hepatic steatosis. The aim of the current study is to determine the temporal relationships between the development of hepatic steatosis and the onset of insulin and leptin resistance in hypothalamus and liver in male Wistar rats (six weeks of age) fed chow or HFHS diet for up to 8 weeks. Fasting plasma glucose, lipids/lipoproteins, insulin and leptin levels were quantified, histopathologic score of hepatic steatosis and inflammation were assessed, and the responses of common checkpoints of insulin and leptin signalling responsible for lipogenesis and gluconeogenesis were analyzed. In addition, acute insulin or leptin administration was performed at different stages of HFHS dieting to determine the responsiveness of the respective signalling pathways. Hyperinsulinemia, hyperglycemia, dyslipidemia, and increased homeostasis model assessment of basal insulin resistance occurred 1-week after HFHS dieting, coinciding with upregulation of suppressor of cytokine signalling 3 in both hypothalamus and liver. However, hepatosteatosis, accompanied with increased expression of sterol regulatory element binding protein 1c and phosphoenolpyruvate carboxykinase, did not manifest until 4- to 8-week after HFHS dieting. Lowered insulin sensitivity (shown by decreased insulin receptor substrate 1 and protein kinase B phosphorylation) occurred approximately 2 weeks prior to leptin resistance (shown by impaired signal transducer and activator of transcription 3 activation) in both the liver and hypothalamus. Acute insulin/leptin administration also demonstrated the impaired insulin or leptin signalling transduction. These data suggest that lowered insulin sensitivity and leptin resistance occurred at least 2–3 weeks earlier than the manifestation of hepatosteatosis in rats fed HFHS diet.

Clock genes and clock-controlled genes in the regulation of metabolic rhythms

Daily rotation of the Earth on its axis and yearly revolution around the Sun impose to living organisms adaptation to nyctohemeral and seasonal periodicity. Terrestrial life forms have developed endogenous molecular circadian clocks to synchronize their behavioral, biological, and metabolic rhythms to environmental cues, with the aim to perform at their best over a 24-h span. The coordinated circadian regulation of sleep/wake, rest/activity, fasting/feeding, and catabolic/anabolic cycles is crucial for optimal health. Circadian rhythms in gene expression synchronize biochemical processes and metabolic fluxes with the external environment, allowing the organism to function effectively in response to predictable physiological challenges. In mammals, this daily timekeeping is driven by the biological clocks of the circadian timing system, composed of master molecular oscillators within the suprachiasmatic nuclei of the hypothalamus, pacing self-sustained and cell-autonomous molecular oscillators in peripheral tissues through neural and humoral signals. Nutritional status is sensed by nuclear receptors and coreceptors, transcriptional regulatory proteins, and protein kinases, which synchronize metabolic gene expression and epigenetic modification, as well as energy production and expenditure, with behavioral and light-dark alternance. Physiological rhythmicity characterizes these biological processes and body functions, and multiple rhythms coexist presenting different phases, which may determine different ways of coordination among the circadian patterns, at both the cellular and whole-body levels. A complete loss of rhythmicity or a change of phase may alter the physiological array of rhythms, with the onset of chronodisruption or internal desynchronization, leading to metabolic derangement and disease, i.e., chronopathology.

Multiple effects of a short-term dexamethasone treatment in human skeletal muscle and adipose tissue

Glucocorticoids are frequently prescribed drugs with important side-effects such as glucose intolerance and tissue remodeling. The goal was to explore the molecular basis of the response of skeletal muscle and adipose tissue during a short-term dexamethasone treatment to better understand the induction of side-effects of glucocorticoids on these metabolic tissues. Fifteen healthy male subjects were assigned to a 4-day treatment with dexamethasone at 4 mg/day. The primary outcome measures were changes in gene expression profiling of subcutaneous skeletal muscle and adipose tissue. Urinary cortisol, plasma, and metabolic biochemistry were also assessed. In both tissues the prominent observation was a response to stress and increased inflammatory responses. An upregulation of the serum amyloid A was detected in skeletal muscle, adipose tissue, and plasma, whereas circulating levels of C reactive protein, another acute phase protein, decreased along with a worsened insulin sensitivity index. As tissue-specific features, tissue remodeling was shown in skeletal muscle while the adipose tissue exhibited a decreased energy metabolism. Several limitations might be raised due to the small number of subjects investigated: a possible cross talk with the mineralocorticoid receptor, and a single time point may not identify regulations occurring during longitudinal treatment. In line with the known physiological effect of glucocorticoids the early modulation of stress response genes was observed. An unexpected feature was the upregulation of the inflammatory and immune pathways. The identification of novel impact on two glucocorticoid target tissues provides a molecular basis for the design of more specific glucocorticoids devoid of adverse effects.

Dexamethasone and the inflammatory response in explants of human omental adipose tissue

Dexamethasone is a synthetic glucocorticoid that is a potent anti-inflammatory agent. The present studies examined the changes in gene expression of 64 proteins in human omental adipose tissue explants incubated for 48h both in the absence and presence of dexamethasone as well as the release of 8 of these proteins that are putative adipokines. The proteins were chosen because they are inflammatory response proteins in other cells, are key regulatory proteins or are proteins with known functions. About 50% were significantly up-regulated while about 10% were unchanged and the remaining 40% were down-regulated. Dexamethasone significantly up-regulated the expression of about 33% of the proteins but down-regulated the expression of about 12% of the proteins. We conclude that dexamethasone is a selective anti-inflammatory agent since it inhibits only about one-fourth of the proteins up-regulated during in vitro incubation of human omental adipose tissue.

Hormonal regulation of leptin and leptin receptor expression in porcine subcutaneous adipose tissue

The current study was performed to examine the response of the leptin gene to hormonal stimuli in porcine adipose tissue from finishing pigs. Yorkshire gilts (approximately 150 kg BW) were used in this study. Tissue from four to six pigs was used per experiment. Dorsal subcutaneous adipose tissue samples were acquired, and adipose tissue explants (approximately 100 mg) were prepared using sterile technique. Tissue slices were transferred to 12-well tissue culture plates containing 1 mL of Media 199 with 25 mM HEPES, 0.5% BSA, pH 7.4, and various hormone supplements. Triplicate tissue slices were incubated with either basal medium or hormone-supplemented media in a tissue culture incubator at 37 degrees C with 95% air:5% CO2. Hormones included insulin (100 nM), dexamethasone (1 microM), porcine GH, 100 ng/mL), triiodothyronine (T3, 10 nM), porcine leptin (100 ng/mL), or IGF-I (250 ng/mL). Following incubation for 24 h, tissue samples from the incubations were blotted and transferred to microfuge tubes, frozen in liquid N, and stored at -80 degrees C before analysis for gene mRNA abundance by reverse-transcription PCR and subsequent quantification of transcripts by capillary electrophoresis with laser-induced fluorescence detection. Media from the incubations were collected in microfuge vials and stored at -20 degrees C before analysis for leptin content by RIA. Insulin was required to maintain tissue and mRNA integrity; therefore, insulin was included in all incubations. The combination of insulin and dexamethasone stimulated leptin secretion into the medium by 60% (P < 0.05; n = 6). Porcine GH inhibited insulin induced leptin secretion by 25% (P < 0.05; n = 6). Dexamethasone in combination with insulin produced a 22% increase in leptin mRNA abundance relative to insulin (P < 0.05; n = 4), and T3 stimulated a 28% increase in insulin-induced leptin mRNA abundance (P < 0.05; n = 4). Leptin receptor mRNA abundance was decreased by 25% with the combination of insulin and dexamethasone, relative to insulin-treated adipose tissue slices (P < 0.05; n = 4). Porcine GH decreased leptin receptor mRNA abundance by 17% (P < 0.05; n = 6). These data suggest that leptin secretion is a regulated phenomenon and that posttranslational processing may be significant. Alternatively, transport and exocytosis of leptin containing vesicles in the pig adipocyte may be quite complicated, which could account for the differences in observed mRNA abundance and protein secretion.

Intermittent and rhythmic exposure to melatonin in primary cultured adipocytes enhances the insulin and dexamethasone effects on leptin expression

Considering the cyclic characteristic of production and secretion of pineal melatonin, it is reasonable to assume that this oscillation might be important in determining the variety of its circadian and seasonal effects. To simulate this physiological condition in vitro, isolated adipocytes were exposed to melatonin in a circadian-like pattern by adding the hormone to the incubating medium during 12 hr (mimicking the night), followed by an equal period without melatonin (mimicking the day). This intermittent procedure was interrupted when three cycles with melatonin were fulfilled (60-hr incubation). Here, we report the effects of melatonin (1 nM) added intermittently or continuously to the incubating medium alone or in combination with insulin (5 nM) and/or dexamethasone (7 nM) on leptin release and expression by rat adipocytes. After acute 12-hr incubation neither melatonin nor insulin alone affected leptin expression, but together they increased it by 105%. Dexamethasone increased leptin mRNA content and release (70%) but this effect was not enhanced by melatonin. Nevertheless, after 60 hr under intermittent melatonin, we observed a synergism between melatonin and dexamethasone. This interaction promoted an increment (75% compared with dexamethasone alone) in leptin release and expression. Our results suggest that circadian-like exposure to melatonin potentiates the dexamethasone action and is important to the effects promoted by insulin on leptin expression. Based on an in vitro approach, this work helps to clarify the physiological relevance and the repercussions of the in vivo circadian pattern of melatonin secretion.

Transforming growth factor beta1 release by human adipose tissue is enhanced in obesity

The present studies examined the effect of obesity in humans on the release of transforming growth factor beta1 (TGF-beta1) by human adipose tissue. The regulation of TGF-beta1 release by adipose tissue as well as the question of whether its release is due to the adipocytes or the nonfat cells in adipose tissue was also examined. There was a statistically significant (r=0.50) correlation between the body mass index of the fat donors and the subsequent release of TGF-beta1 release by subcutaneous adipose tissue. There was also a positive correlation between total TGF-beta1 release by adipose tissue explants and body fat content (r=0.69). The question of whether tumor necrosis factor alpha (TNF-alpha) and/or interleukin 1 beta (IL-1 beta) regulate the release of TGF-beta1 was investigated by incubation of adipose tissue explants with a soluble human TNF-alpha receptor (etanercept) and a neutralizing antihuman IL-1 beta antibody. The release of TGF-beta1 over 48 hours by adipose tissue explants was significantly enhanced in the presence of both the inhibitor of TNF-alpha and of IL-1 beta. It is of interest, in view of the elevated circulating insulin in blood of morbidly obese women, that the release of TGF-beta1 by adipose tissue was enhanced in the presence of insulin. The question of whether the release of TGF-beta1 by human adipose tissue explants was primarily due to adipocytes, as is the case for leptin, or the nonfat cells present in human adipose tissue, as is the case for IL-8 and prostaglandin E(2), was examined. The release of TGF-beta1 was primarily by the nonfat cells of human adipose tissue because release by adipocytes was less than 10% of that by the nonfat cells of adipose tissue.

Melatonin enhances leptin expression by rat adipocytes in the presence of insulin

Leptin and melatonin play an important role in the regulation of body mass and energy balance. Both hormones show a circadian rhythm, with increasing values at night. In addition, melatonin receptors were recently described in adipocytes, where leptin is synthesized. Here, we investigated the influence of melatonin and its interaction with insulin and dexamethasone on leptin expression. Isolated rat adipocytes were incubated with melatonin (1 nM) alone or in combination with insulin (5 nM) and/or dexamethasone (7 nM) for 6 h. Melatonin or insulin alone did not affect leptin expression, but together they increased it by 120%. Dexamethasone increased leptin mRNA content (105%), and this effect was not enhanced by melatonin. Simultaneous treatment with the three hormones provoked a further increase in leptin release (250%) and leptin mRNA (100%). Melatonin prevented the forskolin-induced inhibition (95%) of leptin expression. In addition, melatonin's ability to stimulate leptin release (in the presence of insulin) was completely blocked by pertussis toxin and luzindole. To gain further insight into the molecular basis of melatonin and insulin synergism, the insulin-signaling pathway was investigated. Melatonin increased the insulin-induced insulin receptor-beta tyrosine phosphorylation, which led to an increased serine phosphorylation of the downstream convergent protein Akt. We concluded that melatonin interacts with insulin and upregulates insulin-stimulated leptin expression. These effects are caused by melatonin binding to the pertussis toxin-sensitive G(i) protein-coupled membrane receptor (MT1 subtype) and the cross talk with insulin, since insulin receptor and its convergent target Akt are coactivated by melatonin.

Insulin enhances vascular endothelial growth factor, interleukin-8, and plasminogen activator inhibitor 1 but not interleukin-6 release by human adipocytes

The present studies were designed to investigate the hormonal regulation of vascular endothelial growth factor (VEGF) release by human subcutaneous adipose tissue explants and adipocytes incubated in primary culture for 48 hours. Vascular endothelial growth factor and IL-8 release by adipocytes were less than 10% of that by tissue explants, whereas that of leptin in adipocytes was comparable to that by tissue. Dexamethasone inhibited VEGF formation by both adipose tissue explants and isolated adipocytes, whereas insulin stimulated VEGF release only in isolated adipocytes. Insulin also enhanced the formation of IL-8 and plasminogen activation inhibitor 1 (PAI-1), but not that of IL-6 by adipocytes although having little effect on that of IL-6 or PAI-1 by adipose tissue explants. Pertussis toxin stimulated lipolysis and inhibited leptin release by human adipose tissue or adipocytes but did not affect release of IL-8 or VEGF. Isoproterenol also stimulated lipolysis by human adipocytes, but this was not accompanied by any significant changes in VEGF, IL-8, IL-6, or PAI-1 release. In contrast, insulin stimulated VEGF release by human adipocytes, and this stimulation was enhanced in the presence of isoproterenol. Insulin stimulated VEGF formation as well as that of PAI-1 by human adipocytes, but not by explants under conditions where it had little effect on that of IL-6. The ability of insulin to stimulate VEGF formation by adipocytes suggests that the elevated circulating levels of insulin in obesity promote angiogenesis in adipose tissue as well as the enhanced accumulation of fat in human adipocytes.

Glucocorticoids down-regulate glucose uptake capacity and insulin-signaling proteins in omental but not subcutaneous human adipocytes

Visceral adiposity is associated with insulin resistance and type 2 diabetes. This study explores the metabolic differences between s.c. and visceral fat depots with respect to effects in vitro of glucocorticoids and insulin on glucose uptake. Adipocytes from human s.c. and omental fat depots were obtained during abdominal surgery in 18 nondiabetic subjects. Cells were isolated, and metabolic studies were performed directly after the biopsies and after a culture period of 24 h with or without dexamethasone. After washing, basal and insulin-stimulated [14C]glucose uptake as well as cellular content of insulin signaling proteins and glucose transporter 4 (GLUT4) was assessed. Omental adipocytes had an approximately 2-fold higher rate of insulin-stimulated glucose uptake compared with s.c. adipocytes (P < 0.01). Dexamethasone treatment markedly inhibited (by approximately 50%; P < 0.05) both basal and insulin-stimulated glucose uptake in omental adipocytes but had no consistent effect in s.c. adipocytes. The cellular content of insulin receptor substrate 1 and phosphatidylinositol 3-kinase did not differ significantly between the depots, but the expression of protein kinase B (PKB) tended to be increased in omental compared with s.c. adipocytes (P = 0.09). Dexamethasone treatment decreased the expression of insulin receptor substrate 1 (by approximately 40%; P < 0.05) and PKB (by approximately 20%; P < 0.05) in omental but not in s.c. adipocytes. In contrast, dexamethasone pretreatment had no effect on insulin-stimulated Ser473 phosphorylation of PKB. GLUT4 expression was approximately 4-fold higher in omental than s.c. adipocytes (P < 0.05). Dexamethasone treatment did not alter the expression of GLUT4. In conclusion, human omental adipocytes display approximately 2-fold higher glucose uptake rate compared with s.c. adipocytes, and this could be explained by a higher GLUT4 expression. A marked suppression is exerted by glucocorticoids on glucose uptake and on the expression of insulin signaling proteins in omental but not in s.c. adipocytes. These findings may be of relevance for the interaction between endogenous glucocorticoids and visceral fat in the development of insulin resistance.

Pinealectomy alters adipose tissue adaptability to fasting in rats

This study investigated the effects of pinealectomy and fasting on rat adipose tissue metabolism, as well as on profiles of the hormones directly involved in its regulation (insulin, leptin, and corticosterone). Pinealectomized (PINX) and sham-operated (CONTROL) adult male Wistar rats were killed 6 weeks after surgery, in either fed or fasted (12 and 36 hours) states. Blood samples (for glucose and hormone determinations) and peri-epididymal adipocytes (for in vitro insulin-stimulated glucose uptake, oxidation, and incorporation into lipids) were collected. Pineal ablation decreased insulin-stimulated glucose uptake in adipocytes of both fed and fasted animals without affecting insulin-binding capacity. Pinealectomy attenuated the reduction in the ability to oxidize glucose in both basal and insulin-stimulated states during fasting. This alteration in the ability of adipocytes to oxidize glucose appeared together with a decrease in insulin-induced glucose incorporation into lipids in PINX animals. Additionally, pinealectomized rats showed higher corticosterone levels in both fed and fasted states, and a lower leptinemia with 36 hours of fasting, in comparison to CONTROLs. In conclusion, our data reinforce the hypothesis that the pineal gland has a role in the modulation of adipocyte metabolism, and its absence alters metabolic adaptation to fasting in rats.

Resistin release by human adipose tissue explants in primary culture

Resistin, also known as Fizz3 or ADSF, is a protein found in murine adipose tissue and inflammatory lung exudates. The present studies found that resistin was released by explants of human adipose tissue but the release was quite variable ranging from 3 to 158 ng/g over 48 h. The release of resistin was 250% greater by explants of omental than by explants of human subcutaneous abdominal adipose tissue. Resistin release by adipocytes was negligible as compared to that by the non-fat cells of adipose tissue. Leptin formation by adipocytes was 8-fold greater than its formation by the non-fat cells, while the formation of PAI-1 by adipocytes was 38% of that by the non-fat cells. The conversion of glucose to lactate as well as the formation of PGE(2) and IL-8 was approximately 15% of that by the non-fat cells. In contrast the release of IL-6 and IL-1beta by adipocytes was 4-7% of that by the non-fat cells while the formation of resistin and IL-10 by adipocytes was 2% of that by non-fat cells. The release of adiponectin by explants ranged from 1000 to 5000 ng/g over 48 h but did not correlate with that of resistin. The present data suggest that resistin release by explants of human adipose tissue in primary culture is largely derived from the non-fat cells present in the explants.