Leptin biosynthetic pathway in white adipocytes

The Role of Short-Chain Fatty Acids in Microbiota-Gut-Brain Cross-Talk with a Focus on Amyotrophic Lateral Sclerosis: A Systematic Review

Amyotrophic lateral sclerosis is a devastating neurodegenerative disease characterized by the gradual loss of motor neurons in the brain and spinal cord, leading to progressive motor function decline. Unfortunately, there is no effective treatment, and its increasing prevalence is linked to an aging population, improved diagnostics, heightened awareness, and changing lifestyles. In the gastrointestinal system, the gut microbiota plays a vital role in producing metabolites, neurotransmitters, and immune molecules. Short-chain fatty acids, of interest for their potential health benefits, are influenced by a fiber- and plant-based diet, promoting a diverse and balanced gut microbiome. These fatty acids impact the body by binding to receptors on enteroendocrine cells, influencing hormones like glucagon-like peptide-1 and peptide YY, which regulate appetite and insulin sensitivity. Furthermore, these fatty acids impact the blood-brain barrier, neurotransmitter levels, and neurotrophic factors, and directly stimulate vagal afferent nerves, affecting gut-brain communication. The vagus nerve is a crucial link between the gut and the brain, transmitting signals related to appetite, inflammation, and various processes. Dysregulation of this pathway can contribute to conditions like obesity and irritable bowel syndrome. Emerging evidence suggests the complex interplay among these fatty acids, the gut microbiota, and environmental factors influences neurodegenerative processes via interconnected pathways, including immune function, anti-inflammation, gut barrier, and energy metabolism. Embracing a balanced, fiber-rich diet may foster a diverse gut microbiome, potentially impacting neurodegenerative disease risk. Comprehensive understanding requires further research into interventions targeting the gut microbiome and fatty acid production and their potential therapeutic role in neurodegeneration.

Leptin and fractalkine: novel subcutaneous cytokines in burn injury

Burn injury is a pathology underpinned by progressive and aberrant inflammation. It is a major clinical challenge to survival and quality of life. Although the complex local and disseminating pathological processes of a burn injury ultimately stem from local tissue damage, to date relatively few studies have attempted to characterise the local inflammatory mediator profile. Here, cytokine content and associated transcriptional changes were measured in rat skin for three hours immediately following induction of a scald-type (60°C, 2 min) burn injury model. Leptin (P=0.0002) and fractalkine (P=0.0478) concentrations were significantly elevated post-burn above pre-burn and control site values, coinciding with the development of burn site oedema and differential expression of leptin mRNA (P=0.0004). Further, gene sequencing enrichment analysis indicated cytokine-cytokine receptor interaction (P=1.45×10-6). Subsequent behavioural studies demonstrated that, following subcutaneous injection into the dorsum of the paw, both leptin and fractalkine induced mechanical allodynia, heat hyperalgesia and the recruitment of macrophages. This is the first report of leptin elevation specifically at the burn site, and the first report of fractalkine elevation in any tissue post-burn which, together with the functional findings, calls for exploration of the influence of these cytokines on pain, inflammation and burn wound progression. In addition, targeting these signalling molecules represents a therapeutic potential as early formative mediators of these pathological processes.

Leptin resensitisation: a reversion of leptin-resistant states

Leptin resistance refers to states in which leptin fails to promote its anticipated effects, frequently coexisting with hyperleptinaemia. Leptin resistance is closely associated with obesity and also observed in physiological situations such as pregnancy and in seasonal animals. Leptin resensitisation refers to the reversion of leptin-resistant states and is associated with improvement in endocrine and metabolic disturbances commonly observed in obesity and a sustained decrease of plasma leptin levels, possibly below a critical threshold level. In obesity, leptin resensitisation can be achieved with treatments that reduce body adiposity and leptinaemia, or with some pharmacological compounds, while physiological leptin resistance reverts spontaneously. The restoration of leptin sensitivity could be a useful strategy to treat obesity, maintain weight loss and/or reduce the recidivism rate for weight regain after dieting. This review provides an update and discussion about reversion of leptin-resistant states and modulation of the molecular mechanisms involved in each situation.

Leptin-induced cardiomyocyte hypertrophy is associated with enhanced mitochondrial fission

Cardiac pathology including hypertrophy has been associated with an imbalance between mitochondrial fission and fusion. Generally, well-balanced mitochondrial fission and fusion are essential for proper functions of mitochondria. Leptin is a 16-kDa appetite-suppressing protein which has been shown to induce cardiomyocyte hypertrophy. In the present study, we determined whether leptin can influence mitochondrial fission or fusion and whether this can be related to its hypertrophic effect. Cardiomyocytes treated for 24 h with 3.1 nM leptin (50 ng/ml), a concentration representing plasma levels in obese individuals, demonstrated an increase in surface area and a significant 1.6-fold increase in the expression of the β-myosin heavy chain. Mitochondrial staining with MitoTracker Green dye showed elongated structures in control cells with an average length of 4.5 µm. Leptin produced a time-dependent increase in mitochondrial fragmentation with decreasing mitochondrial length. The hypertrophic response to leptin was also associated with increased protein levels of the mitochondrial fission protein dynamin-related protein1 (Drp1) although gene expression of Drp1 was unaffected possibly suggesting post-translational modifications of Drp1. Indeed, leptin treatment was associated with decreased levels of phosphorylated Drp1 and increased translocation of Drp1 to the mitochondria thereby demonstrating a pro-fission effect of leptin. As calcineurin may dephosphorylate Drp1, we determined the effect of a calcineurin inhibitor, FK506, which prevented leptin-induced hypertrophy as well as mitochondrial fission and mitochondrial dysfunction. In conclusion, our data show that leptin-induced cardiomyocyte hypertrophy is associated with enhanced mitochondrial fission via a calcineurin-mediated pathway. The ability of leptin to stimulate mitochondrial fission may be important in understanding the role of this protein in cardiac pathology especially that related to mitochondrial dysfunction.

Insulin as a hormone regulator of the synthesis and release of leptin by white adipose tissue

Leptin and its receptor are widely distributed in several tissues, mainly in white adipose tissue. The serum leptin is highly correlated with body mass index in rodents and humans, being documented that leptin levels reduces in the fasting state and increase during refeeding, similarly to insulin release by pancreatic islets. Insulin appears to increase leptin mRNA and protein expression and its release by adipocytes. Some studies have suggested that insulin acts through the activation of the transcription factors: sterol regulatory element binding protein 1 (SREBP1), CCAAT enhancer binding protein-α (C/EBP-α) and specificity protein 1 (Sp1). Insulin stimulates the release of preformed and newly synthesized leptin by adipocytes through its signaling cascade. Its effects are blocked by inhibitors of the insulin signaling pathway, as well as by inhibitors of protein synthesis and agents that increase the intracellular cAMP. The literature data suggest that chronic hyperinsulinemia increases serum leptin levels in humans and rodents. In this review, we summarized the most updated knowledge on the effects of insulin on serum leptin levels, presenting the cell mechanisms that control leptin synthesis and release by the white adipose tissue.

Involvement of leptin in the molecular physiology of the placenta

Leptin is a homeostatic regulator in the placenta where it promotes proliferation, protein synthesis and the expression of tolerogenic maternal response molecules such as HLA-G. Leptin also exerts an anti-apoptotic action in placenta controlling the expression of p53 master cell cycle regulator under different stress conditions. On the other hand, leptin is an integrative target of different placental stimuli. The expression of leptin in placenta is regulated by hCG, insulin, steroids, hypoxia and many other growth hormones, suggesting that it might have an important endocrine function in the trophoblastic cells. The leptin expression is induced involving the cAMP/PKA or cAMP/Epac pathways which have profound actions upon human trophoblast function. The activation of PI3K and MAPK pathways also participates in the leptin expression. Estrogens play a central role during pregnancy, particularly 17β-estradiol upregulates the leptin expression in placental cells through genomic and non-genomic actions. The leptin promoter analysis reveals specific elements that are active in placental cells. The transcription factors CREB, AP1, Sp1, NFκB and the coactivator CBP are involved in the placental leptin expression. Moreover, placental leptin promoter is a target of epigenetic marks such as DNA methylation and histone acetylation that regulates not only the leptin expression in placenta during pregnancy but also determines the predisposition of acquiring adult metabolism diseases. Taken together, all these results allow a better understanding of leptin function and regulatory mechanisms of leptin expression in human placental trophoblasts, and support the importance of leptin during pregnancy and in programming adult health.

Cannabinoid CB1 /CB2 receptor agonists attenuate hyperactivity and body weight loss in a rat model of activity-based anorexia

BACKGROUND AND PURPOSE

Anorexia nervosa (AN) is a serious psychiatric condition characterized by excessive body weight loss and disturbed perceptions of body shape and size, often associated with excessive physical activity. There is currently no effective drug-related therapy of this disease and this leads to high relapse rate. Clinical data suggest that a promising therapy to treat and reduce reoccurrence of AN may be based on the use of drugs that target the endocannabinoid (EC) system, which appears dysregulated in AN patients.

EXPERIMENTAL APPROACH

The activity-based anorexia (ABA) rodent model mimics the severe body weight loss and increased physical activity, as well as the neuroendocrine disturbances (i.e. hypoleptinaemia and hypercortisolaemia) in AN. This study investigated whether cannabinoid agonists can effectively modify anorexic-like behaviours and neuroendocrine changes in rats subjected to a repeated ABA regime that mimics the human condition in which patients repeatedly undergo a recovery and illness cycle.

KEY RESULTS

Our data show that subchronic treatment with both the natural CB₁ /CB₂ receptor agonist Δ⁹ -tetrahydrocannabinol and the synthetic CB₁ /CB₂ receptor agonist CP-55,940 significantly reduced body weight loss and running wheel activity in ABA rats. These behavioural effects were accompanied by an increase in leptin signalling and a decrease in plasma levels of corticosterone.

CONCLUSION AND IMPLICATIONS

Taken together, our results further demonstrate the involvement of the EC system in AN pathophysiology and that strategies which modulate EC signalling are useful to treat this disorder, specifically in patients where physical hyperactivity plays a central role in its progression and maintenance.

Determination of the half-life of circulating leptin in the mouse

Background

The adipokine hormone, leptin, is a major component of body weight homeostasis. Numerous studies have been performed administering recombinant mouse leptin as an experimental reagent; however, the half life of circulating leptin following exogenous administration of recombinant mouse leptin has not been carefully evaluated.

Methods

Exogenous leptin was administered (3 mg leptin/kg body weight) to ten week old fasted non-obese male mice and plasma was serially collected at seven time points; plasma leptin concentration was measured by ELISA at each time point to estimate the circulating half life of mouse leptin.

Results

Under the physiological circumstances tested, the half life of mouse leptin was 40.2 (+/− 2.2) minutes. Circulating leptin concentrations up to one hour following exogenous leptin administration were 170-fold higher than endogenous levels at fasting.

Conclusions

The half life of mouse leptin was determined to be 40.2 minutes. These results should be useful in planning and interpreting experiments employing exogenous leptin. The unphysiological elevations in circulating leptin resulting from widely used dosing regimens for exogenous leptin are likely to confound inferences regarding some aspects of the hormone’s clinical biology.

Counterregulation of insulin by leptin as key component of autonomic regulation of body weight

A re-examination of the mechanism controlling eating, locomotion, and metabolism prompts formulation of a new explanatory model containing five features: a coordinating joint role of the (1) autonomic nervous system (ANS); (2) the suprachiasmatic (SCN) master clock in counterbalancing parasympathetic digestive and absorptive functions and feeding with sympathetic locomotor and thermogenic energy expenditure within a circadian framework; (3) interaction of the ANS/SCN command with brain substrates of reward encompassing dopaminergic projections to ventral striatum and limbic and cortical forebrain. These drive the nonhomeostatic feeding and locomotor motivated behaviors in interaction with circulating ghrelin and lateral hypothalamic neurons signaling through melanin concentrating hormone and orexin-hypocretin peptides; (4) counterregulation of insulin by leptin of both gastric and adipose tissue origin through: potentiation by leptin of cholecystokinin-mediated satiation, inhibition of insulin secretion, suppression of insulin lipogenesis by leptin lipolysis, and modulation of peripheral tissue and brain sensitivity to insulin action. Thus weight-loss induced hypoleptimia raises insulin sensitivity and promotes its parasympathetic anabolic actions while obesity-induced hyperleptinemia supresses insulin lipogenic action; and (5) inhibition by leptin of bone mineral accrual suggesting that leptin may contribute to the maintenance of stability of skeletal, lean-body, as well as adipose tissue masses.

Adipocyte dysfunction in rats with streptozotocin-nicotinamide-induced diabetes

Administration of streptozotocin (STZ) and nicotinamide (NA) to adult rats allows for the induction of mild diabetes. However, this experimental model has not been fully characterized. This study was undertaken to determine the metabolic and secretory activity of adipose tissue in rats with STZ-NA-induced diabetes. Experiments were performed using epididymal adipocytes isolated from control and mildly diabetic rats. Lipogenesis, glucose transport as well as glucose and alanine oxidation, lipolysis, anti-lipolysis, cAMP levels and adipokine secretion were compared in cells isolated from the control and diabetic rats. Lipogenesis, glucose transport and oxidation were diminished in the adipocytes of diabetic rats compared with the fat cells of control animals. However, alanine oxidation appeared to be similar in the cells of non-diabetic and diabetic animals. Lipolytic response to low epinephrine concentrations was slightly increased in the adipocytes of diabetic rats; however, at higher concentrations of the hormone, lipolysis was similar in both groups of cells. The epinephrine-induced rise in cAMP levels was higher in the adipocytes of STZ-NA-induced diabetic rats, even in the presence of insulin. Lipolysis stimulated by dibutyryl-cAMP did not significantly differ, whereas anti-lipolytic effects of insulin were mildly decreased in the cells of diabetic rats. Secretion of adiponectin and leptin was substantially diminished in the adipocytes of diabetic rats compared with the cells of control animals. Our studies demonstrated that the balance between lipogenesis and lipolysis in the adipose tissue of rats with mild diabetes induced by STZ and NA is slightly shifted towards reduced lipid accumulation. Simultaneously, adiponectin and leptin secretion is significantly impaired.

Prognostic value of serum leptin in advanced lung adenocarcinoma patients with cisplatin/pemetrexed chemotherapy

Cisplatin/pemetrexed chemotherapy has been established as a standard treatment in lung adenocarcinoma. However, the response to the cisplatin/pemetrexed combination varies considerably among patients due to individual variations. Thus, novel biomarkers are required to aid the prediction of the response to the cisplatin/pemetrexed combination. We hypothesized that leptin expression may be a determinant for prognosis in lung adenocarcinoma patients with cisplatin/pemetrexed chemotherapy. Serum from consenting patients with lung adenocarcinoma were obtained for the measurement of leptin and associated tumor biomarkers. Leptin expression was measured by radioimmunoassay. Carcinoembryonic antigen (CEA), carbohydrate antigen 19-9 (CA19-9), CA15-3, CA125, CA72-4, cytokeratin 19 fragment (CYFRA21-1) and neuron-specific enolase (NSE) expression were determined by electrochemiluminescence immunoassays. Serum squamous cell carcinoma antigen levels were measured using a microparticle enzyme immunoassay. The associations between serum leptin and tumor biomarker expression were evaluated by Spearman's correlation analysis. Serum CEA, CA19-9, CA15-3, CA125, CA72-4, CYFRA21-1 and NSE levels showed no obvious difference among patients. However, a trend towards an improved prognosis was observed in patients with lower serum leptin at diagnosis and an increase during cisplatin/pemetrexed chemotherapy. The results indicated that the serum leptin level has prognostic indications in patients with advanced lung adenocarcinoma during cisplatin/pemetrexed chemotherapy, which indicates that it may be a useful marker for the prognosis of cancer patients undergoing chemotherapy treatment.

Insulin-stimulated leptin secretion requires calcium and PI3K/Akt activation

Insulin stimulates leptin secretion through the PI3K/Akt, but not the MAPK, pathway. Although Ca2+ alone does not trigger leptin secretion, it is required for robust Akt phosphorylation and leptin secretion.

Leptin signaling and hyperparathyroidism: clinical and genetic associations

BACKGROUND

The role of leptin in mediating calcium-related metabolic processes is not well understood.

STUDY DESIGN

We enrolled patients with hyperparathyroidism undergoing parathyroidectomy in a prospective study to assess postoperative changes to serum leptin and parathyroid hormone levels and to determine the presence of LEPR (leptin receptor) polymorphisms. Patients undergoing hemithyroidectomy under identical surgical conditions were enrolled as controls. Wilcoxon signed-rank test was used to analyze changes in leptin. Pearson correlations and Bland-Altman methods were used to examine the between-subject and within-subject correlations in changes in leptin and parathyroid hormone levels. Five single-nucleotide polymorphisms in the LEPR gene were genotyped, and linear regression analysis was performed for each polymorphism.

RESULTS

Among the 71 patients included in the clinical study, after-surgery leptin levels decreased significantly in the parathyroid adenoma (p < 0.001) and parathyroid hyperplasia subgroups (p = 0.002) and increased in the control group (p = 0.007). On multivariate analysis, parathyroid disease subtype, baseline leptin levels, age, body mass index, and calcium at diagnosis was associated with changes in leptin. Among the 132 patients included in the genotyping analysis, under a recessive model of inheritance, single-nucleotide polymorphism rs1137101 had a significant association with the largest parathyroid gland and total mass of parathyroid tissue removed (p = 0.045 and p = 0.040, respectively). When analyzing obese patients only, rs1137100 and rs1137101 were significantly associated with total parathyroid size (p = 0.0343 and p = 0.0259, respectively).

CONCLUSIONS

Our results suggest a role for the parathyroid gland in regulating leptin production. Genetic contributions from the leptin pathway might predispose to hyperparathyroidism.

Stimulation of leptin secretion by insulin

Leptin has a crucial role in regulating food intake and maintaining metabolic homeostasis. Although little is known about the process of leptin secretion, insulin, which has an important role in the metabolism of glucose and lipids, is believed to regulate leptin secretion through a posttranscriptional mechanism in the short term, and via glucose metabolism in the long term. The gastric mucosa secretes leptin, but this mechanism has not been completely elucidated. Understanding the mechanism of insulin-regulated leptin secretion could lead to the development of new treatment methods for obesity and its comorbidities, which are serious public health concerns.

New roles of carboxypeptidase E in endocrine and neural function and cancer

Carboxypeptidase E (CPE) or carboxypeptidase H was first discovered in 1982 as an enkephalin-convertase that cleaved a C-terminal basic residue from enkephalin precursors to generate enkephalin. Since then, CPE has been shown to be a multifunctional protein that subserves many essential nonenzymatic roles in the endocrine and nervous systems. Here, we review the phylogeny, structure, and function of CPE in hormone and neuropeptide sorting and vesicle transport for secretion, alternative splicing of the CPE transcript, and single nucleotide polymorphisms in humans. With this and the analysis of mutant and knockout mice, the data collectively support important roles for CPE in the modulation of metabolic and glucose homeostasis, bone remodeling, obesity, fertility, neuroprotection, stress, sexual behavior, mood and emotional responses, learning, and memory. Recently, a splice variant form of CPE has been found to be an inducer of tumor growth and metastasis and a prognostic biomarker for metastasis in endocrine and nonendocrine tumors.

A review on gastric leptin: the exocrine secretion of a gastric hormone

A major advance in the understanding of the regulation of food intake has been the discovery of the adipokine leptin a hormone secreted by the adipose tissue. After crossing the blood-brain barrier, leptin reaches its main site of action at the level of the hypothalamic cells where it plays fundamental roles in the control of appetite and in the regulation of energy expenditure. At first considered as a hormone specific to the white adipose tissue, it was rapidly found to be expressed by other tissues. Among these, the gastric mucosa has been demonstrated to secrete large amounts of leptin. Secretion of leptin by the gastric chief cells was found to be an exocrine secretion. Leptin is secreted towards the gastric lumen into the gastric juice. We found that while secretion of leptin by the white adipose tissue is constitutive, secretion by the gastric cells is a regulated one responding very rapidly to secretory stimuli such as food intake. Exocrine-secreted leptin survives the hydrolytic conditions of the gastric juice by forming a complex with its soluble receptor. This soluble receptor is synthesized by the gastric cells and the leptin-leptin receptor complex gets formed at the level of the gastric chief cell secretory granules before being released into the gastric lumen. The leptin-leptin receptor upon resisting the hydrolytic conditions of the gastric juice is channelled, to the duodenum. Transmembrane leptin receptors expressed at the luminal membrane of the duodenal enterocytes interact with the luminal leptin. Leptin is actively transcytosed by the duodenal enterocytes. From the apical membrane it is transferred to the Golgi apparatus where it binds again its soluble receptor. The newly formed leptin-leptin receptor complex is then secreted baso-laterally into the intestinal mucosa to reach the blood capillaries and circulation thus reaching the hypothalamus where its action regulates food intake. Exocrine-secreted gastric leptin participates in the short term regulation of food intake independently from that secreted by the adipose tissue. Adipose tissue leptin on the other hand, regulates in the long term energy storage. Both tissues work in tandem to ensure management of food intake and energy expenditure.

Genetic polymorphisms in adipokine genes and the risk of obesity: a systematic review and meta-analysis

Polymorphisms in adipokine genes, such as leptin (LEP), leptin receptor (LEPR), resistin (RETN), adiponectin (ADIPOQ), interleukin-1β (IL-1β), IL-6 (IL-6), and tumor necrosis factor-α (TNF-α) may be involved in the development of obesity. We conducted a systematic review of published evidence on the association between different adipokine genes and the risk of obesity. Librarian-designed searches of PubMed and HuGeNet, review of reference lists from published reviews and content expert advice identified potentially eligible studies. The genotyping information and polymorphisms of different adipokine genes, numbers of genotyped cases and controls and frequencies of genotypes were extracted from 48 eligible studies included in this review. Twenty-one polymorphisms each associated with obesity in at least one study were identified. Polymorphisms in the adipokine genes, LEP, LEPR, and RETN were not associated with obesity susceptibility, whereas ADIPOQ G276T (T vs. G: odds ratio (OR), 1.59; 95% confidence interval (CI), 1.39-1.81), IL-1β C3953T (CC vs. CT+TT: OR, 1.61; 95% CI, 1.18-2.20), and TNF-α G308A (GG vs. GA+AA: OR, 1.19; 95% CI, 1.02-1.39) polymorphisms were associated with an increased risk of obesity. The IL-6 G174C polymorphism was also associated obesity when using allelic comparisons, the recessive genetic model and the dominant genetic model with OR (95% CI) of 1.95 (1.37-2.77), 1.44 (1.15-1.80), and 1.36 (1.16-1.59), respectively. No significant evidence of publication bias was present. However, these "null" results were underpowered due to a small pooled sample size, and analysis of additional case-control studies with larger sample sizes should provide further clarifications.

Cross-talk between adipose and gastric leptins for the control of food intake and energy metabolism

The understanding of the regulation of food intake has become increasingly complex. More than 20 hormones, both orexigenic and anorexigenic, have been identified. After crossing the blood-brain barrier, they reach their main site of action located in several hypothalamic areas and interact to balance satiety and hunger. One of the most significant advances in this matter has been the discovery of leptin. This hormone plays fundamental roles in the control of appetite and in regulating energy expenditure. In accordance with the lipostatic theory stated by Kennedy in 1953, leptin was originally discovered in white adipose tissue. Its expression by other tissues was later established. Among them, the gastric mucosa has been shown to secrete large amounts of leptin. Both the adipose and the gastric tissues share similar characteristics in the synthesis and storage of leptin in granules, in the formation of a complex with the soluble receptor and a secretion modulated by hormones and energy substrates. However while adipose tissue secretes leptin in a slow constitutive endocrine way, the gastric mucosa releases leptin in a rapid regulated exocrine fashion into the gastric juice. Exocrine-secreted leptin survives the extreme hydrolytic conditions of the gastric juice and reach the duodenal lumen in an intact active form. Scrutiny into transport mechanisms revealed that a significant amount of the exocrine leptin crosses the intestinal wall by active transcytosis. Leptin receptors, expressed on the luminal and basal membrane of intestinal epithelial cells, are involved in the control of nutrient absorption by enterocytes, mucus secretion by goblet cells and motility, among other processes, and this control is indeed different depending upon luminal or basal stimulus. Gastric leptin after transcytosis reaches the central nervous system, to control food intake. Studies using the Caco-2, the human intestinal cell line, in vitro allowed analysis of the mechanisms of leptin actions on the intestinal mucosa, identification of the mechanisms of leptin transcytosis and understanding the modulation of leptin receptors by nutrients and hormones. Exocrine-secreted gastric leptin thus participates in a physiological axis independent in terms of time and regulation from that of adipose tissue to rapidly control food intake and nutrient absorption. Adipocytes and gastric epithelial cells are two cell types the metabolism of which is closely linked to food intake and energy storage. The coordinated secretion of adipose and gastric leptins ensures proper management of food processing and energy storage.

Insulin regulates leptin secretion from 3T3-L1 adipocytes by a PI 3 kinase independent mechanism

To better define the molecular mechanisms underlying leptin release from adipocytes, we developed a novel protocol that maximizes leptin production from 3T3-L1 adipocytes. The addition of a PPARgamma agonist to the Isobutylmethylxanthine/Dexamethasone/Insulin differentiation cocktail increased leptin mRNA levels by 5-fold, maintained insulin sensitivity, and yielded mature phenotype in cultured adipocytes. Under these conditions, acute insulin stimulation for 2 h induced a two-fold increase in leptin secretion, which was independent of new protein synthesis, and was not due to alterations in glucose metabolism. Stimulation with insulin for 15 min induced the same level of leptin release and was blocked by Brefeldin A. Inhibiting PI 3-kinase with wortmannin had no effect on insulin stimulation of leptin secretion. These studies show that insulin can stimulate leptin release via a PI3K independent mechanism and provide a cellular system for studying the effect of insulin and potentially other mediators on leptin secretion.

Resistin Is a Key Mediator of Glucose-dependent Insulinotropic Polypeptide (GIP) Stimulation of Lipoprotein Lipase (LPL) Activity in Adipocytes

Studies on the physiological roles of the incretin hormone, glucose-dependent insulinotropic polypeptide (GIP) have largely focused on its insulinotropic action and ability to regulate beta-cell mass. In previous studies on the stimulatory effect of GIP on adipocyte lipoprotein lipase (LPL), a pathway was identified involving increased phosphorylation of protein kinase B (PKB) and reduced phosphorylation of LKB1 and AMP-activated protein kinase (AMPK). The slow time of onset of the responses suggested that GIP may have induced release of an intermediary molecule, and the current studies focused on the possible contribution of the adipokine resistin. In differentiated 3T3-L1 adipocytes, GIP, in the presence of insulin, increased resistin secretion through a pathway involving p38 mitogen-activated protein kinase (p38 MAPK) and the stress-activated protein kinase/Jun amino-terminal kinase (SAPK/JNK). The other major incretin hormone, glucagon-like peptide-1 (GLP-1), exhibited no significant effects. Chronic elevation of circulating GIP levels in the Vancouver Diabetic Fatty (VDF) Zucker rat resulted in increases in circulating resistin levels and activation of p38 MAPK or SAPK/JNK in epididymal fat tissue, suggesting the existence of identical pathways in vivo as well as in vitro. Administration of resistin to 3T3-L1 adipocytes mimicked the effects of GIP on the PKB/LKB1/AMPK/LPL pathway: increasing phosphorylation of PKB, reducing levels of phosphorylated LKB1 and AMPK, and increasing LPL activity. Knockdown of resistin using RNA interference attenuated the effect of GIP on the PKB/LKB1/AMPK/LPL pathway in 3T3-L1 adipocytes, supporting a role for resistin as a mediator.

Transcytosis of gastric leptin through the rat duodenal mucosa

Leptin is secreted into the gastric juice by epithelial Chief cells and reaches the duodenum in a biologically intact active form. We assessed the possibility that this gastric leptin crosses the intestinal mucosa by transcytosis through enterocytes to reach blood circulation. Endogenous gastric leptin secretion was triggered by cholinergic stimulation. In another set of experiments, recombinant leptin was inserted in vivo into the duodenal lumen. Plasma levels of leptin were assessed by enzyme immunoassay and Western blot, and duodenal tissue was processed for immunocytochemistry. We first observed that leptin was found inside duodenal enterocytes from fed rats but not inside those from fasted ones. Stimulation of gastric secretion by a cholinergic agent led to rapid increases in plasma leptin levels (202 +/- 39%) except when the pylorus was clamped. Insertion of recombinant leptin into the duodenal lumen raised plasma leptin concentrations (558 +/- 34%) quite rapidly, whereas carrier solution without leptin had no effect. The use of FITC-tagged leptin reinforced these results. Light and electron microscopy revealed the cellular compartments involved in its transcytosis, namely, the enterocyte microvilli, the endocytotic vesicles, the Golgi complex, and the basolateral interdigitations. Leptin was also present in the lamina propria, in capillary endothelial cell plasmalemmal vesicles, and in capillary lumina. These results demonstrate that gastric exocrine leptin is internalized by duodenal enterocytes and delivered to the lamina propria and blood circulation. It may thus be able to play important paracrine and endocrine functions for the control of gastric emptying and nutrient absorption.

Polymorphism of the porcine leptin gene promoter and analysis of its association with gene expression and fatness traits

We describe for the first time a 245 bp fragment of the porcine leptin gene promoter in the proximity of the transcription start site. Altogether, 720 pigs were screened with the PCR-SSCP technique for polymorphism in this region. Four SNPs, segregating as two haplotypes, have been identified, one of them (C113G) in the putative consensus site for the AP-2 transcription factor. This polymorphism was evenly distributed in the Duroc breed (n=21) and was absent in the Polish Landrace (n=248) and Pietrain breed (n=12). In the Polish Large White (n=191) and synthetic line 990 (n=243), allele G occurred with a very low frequency. The investigation was performed to test if the C113G SNP affects leptin mRNA level in subcutaneous fat and leptin protein concentration in serum. Additionally, the effect of this polymorphism on fatness traits was statistically analyzed. Although there was a trend toward decreased expression in GG animals, the differences were not significant between genotypes. We also found no evidence for an association of the LEP promoter genotype with the analyzed fatness traits.