Role of calcium in the secretion of leptin from white adipocytes

Metabolic complications of psychotropic medications in psychiatric disorders: Emerging role of de novo lipogenesis and therapeutic consideration

Although significant advances have been made in understanding the patho-physiology of psychiatric disorders (PDs), therapeutic advances have not been very convincing. While psychotropic medications can reduce classical symptoms in patients with PDs, their long-term use has been reported to induce or exaggerate various pre-existing metabolic abnormalities including diabetes, obesity and non-alcoholic fatty liver disease (NAFLD). The mechanism(s) underlying these metabolic abnormalities is not clear; however, lipid/fatty acid accumulation due to enhanced de novo lipogenesis (DNL) has been shown to reduce membrane fluidity, increase oxidative stress and inflammation leading to the development of the aforementioned metabolic abnormalities. Intriguingly, emerging evidence suggest that DNL dysregulation and fatty acid accumulation could be the major mechanisms associated with the development of obesity, diabetes and NAFLD after long-term treatment with psychotropic medications in patients with PDs. In support of this, several adjunctive drugs comprising of anti-oxidants and anti-inflammatory agents, that are used in treating PDs in combination with psychotropic medications, have been shown to reduce insulin resistance and development of NAFLD. In conclusion, the above evidence suggests that DNL could be a potential pathological factor associated with various metabolic abnormalities, and a new avenue for translational research and therapeutic drug designing in PDs.

Adipocyte-specific ablation of the Ca2+ pump SERCA2 impairs whole-body metabolic function and reveals the diverse metabolic flexibility of white and brown adipose tissue

OBJECTIVE

Sarco/endoplasmic reticulum Ca²⁺-ATPase (SERCA) transports Ca²⁺ from the cytosol into the ER and is essential for appropriate regulation of intracellular Ca²⁺ homeostasis. The objective of this study was to test the hypothesis that SERCA pumps are involved in the regulation of white adipocyte hormone secretion and other aspects of adipose tissue function and that this control is disturbed in obesity-induced type-2 diabetes.

METHODS

SERCA expression was measured in isolated human and mouse adipocytes as well as in whole mouse adipose tissue by Western blot and RT-qPCR. To test the significance of SERCA2 in adipocyte functionality and whole-body metabolism, we generated adipocyte-specific SERCA2 knockout mice. The mice were metabolically phenotyped by glucose tolerance and tracer studies, histological analyses, measurements of glucose-stimulated insulin release in isolated islets, and gene/protein expression analyses. We also tested the effect of pharmacological SERCA inhibition and genetic SERCA2 ablation in cultured adipocytes. Intracellular and mitochondrial Ca²⁺ levels were recorded with dual-wavelength ratio imaging and mitochondrial function was assessed by Seahorse technology.

RESULTS

We demonstrate that SERCA2 is downregulated in white adipocytes from patients with obesity and type-2 diabetes as well as in adipocytes from diet-induced obese mice. SERCA2-ablated adipocytes display disturbed Ca²⁺ homeostasis associated with upregulated ER stress markers and impaired hormone release. These adipocyte alterations are linked to mild lipodystrophy, reduced adiponectin levels, and impaired glucose tolerance. Interestingly, adipocyte-specific SERCA2 ablation leads to increased glucose uptake in white adipose tissue while glucose uptake is reduced in brown adipose tissue. This dichotomous effect on glucose uptake is due to differently regulated mitochondrial function. In white adipocytes, SERCA2 deficiency triggers an adaptive increase in FGF21, increased mitochondrial UCP1 levels, and increased oxygen consumption rate (OCR). In contrast, brown SERCA2 null adipocytes display reduced OCR despite increased mitochondrial content and UCP1 levels compared to wild type controls.

CONCLUSIONS

Our data suggest causal links between reduced white adipocyte SERCA2 levels, deranged adipocyte Ca²⁺ homeostasis, adipose tissue dysfunction and type-2 diabetes.

Disrupted leptin-fatty acid biosynthesis is an early manifestation of metabolic abnormalities in schizophrenia

BACKGROUND

Insulin resistance (IR) and impaired energy expenditure (IEE) are irreparable metabolic comorbidities in schizophrenia. Although mechanism(s) underlying IR and IEE remains unclear, leptin and fatty acid signaling, which has profound influence on insulin secretion/sensitivity, glucose metabolism and energy expenditure, could be disrupted. However, no association of plasma leptin with erythrocyte membrane fatty acids, body mass index (BMI), and psychotic symptoms in the same cohort of untreated patients with first-episode psychosis (FEP) or medicated patients with chronic schizophrenia (CSZ) is presented before. These studies are crucial for deciphering the role of leptin and fatty acids in the development of IR and IEE in schizophrenia.

AIM

To determine the association between plasma leptin, erythrocyte membrane fatty acids, particularly, saturated fatty acids (SFAs), BMI and psychotic symptoms in patients with FEP and CSZ.

METHODS

In this study, twenty-two drug naive patients with FEP, twenty-one CSZ patients treated with atypical antipsychotic drugs, and fourteen healthy control (CNT) subjects were analyzed. Plasma leptin was measured using sandwich mode enzyme-linked immunosorbent assay. Erythrocyte membrane SFAs were measured using ultrathin capillary gas chromatography. BMI was calculated by using the formula: weight (kg)/height (m²). Psychiatric symptoms were evaluated at baseline using brief psychiatric rating scale (BPRS), and positive and negative syndrome scale (PANSS). The total BPRS scores, positive and negative symptom scores (PANSS-PSS and PANSS-NSS, respectively) were recorded. Pearson correlation coefficient (r) analyses were performed to find the nature and strength of association between plasma leptin, PANSS scores, BMI and SFAs, particularly, palmitic acid (PA).

RESULTS

In patients with FEP, plasma leptin not BMI was significantly lower (P = 0.034), whereas, erythrocyte membrane SFAs were significantly higher (P < 0.005) compared to the CNT subjects. Further, plasma leptin showed negative correlation with erythrocyte membrane SFAs-PA (r = −0.4972, P = 0.001), PANSS-PSS (r = −0.4034, P = 0.028), and PANSS-NSS (r = −0.3487, P = 0.048). However, erythrocyte membrane SFAs-PA showed positive correlation with PANSS-PSS (r = 0.5844, P = 0.0034) and PANSS-NSS (r = 0.5380, P = 0.008). In CSZ patients, plasma leptin, BMI, and erythrocyte membrane SFAs, all were significantly higher (P < 0.05) compared to the CNT subjects. Plasma leptin showed positive correlation with BMI (r = 0.312, P = 0.032) but not with PANSS scores or erythrocyte membrane SFAs-PA. However, erythrocyte membrane SFAs-PA showed positive correlation with PANSS-NSS only (r = 0.4729, P = 0.031). Similar changes in the plasma leptin and erythrocyte membrane SFAs have also been reported in individuals at ultra-high risk of developing psychosis; therefore, the above findings suggest that leptin-fatty acid biosynthesis could be disrupted before the onset of psychosis in schizophrenia.

CONCLUSION

Disrupted leptin-fatty acid biosynthesis/signaling could be an early manifestation of metabolic comorbidities in schizophrenia. Large-scale studies are warranted to validate the above findings.

Protective Effects of Melatonin against Obesity-Induced by Leptin Resistance

Consumption of an exceedingly high-fat diet with irregular eating and sleeping habits is typical in the current sedentary lifestyle, leading to chronic diseases like obesity and diabetes mellitus. Leptin is a primary appetite-regulating hormone that binds to its receptors in the hypothalamic cell membrane and regulates downstream appetite-regulating neurons NPY/AgRp and POMC in the hypothalamus. Based on the fat content of the adipose tissue, leptin is secreted, and excess accumulation of fat in adipose tissue stimulates the abnormal secretion of leptin. The secreted leptin circulating in the bloodstream uses its transporters to cross the blood-brain barrier (BBB) and reach the CSF. There is a saturation limit for leptin bound to its transporters to cross the BBB, and increased leptin secretion in adipose tissue has a defect in its transport across the BBB. Leptin resistance is due to excess leptin, a saturation of its transporters, and deficiency in either the receptor level or signalling in the hypothalamus. Leptin resistance leads to obesity due to excess food intake and less energy expenditure. Normal leptin secretion follows a rhythm, and alteration in the lifestyle leads to hormonal imbalances and increases ROS generation leading to oxidative stress. The sleep disturbance causes obesity with increased lipid accumulation in adipose tissue. Melatonin is the master regulator of the sleep-wake cycle secreted by the pineal gland during the night. It is a potent antioxidant with anti-inflammatory properties. Melatonin is secreted in a pattern called the circadian rhythm in humans as well. Research indicates that melatonin plays a vital role in hormonal regulation and energy metabolism, including leptin signalling and secretion. Studying the role of melatonin in leptin regulation will help us combat the pathologies of obesity caused by leptin resistance.

Role of TRPV4 Channel in Human White Adipocytes Metabolic Activity

BACKGROUND

Intracellular calcium (Ca2+) homeostasis plays an essential role in adipocyte metabolism and its alteration is associated with obesity and related disorders. Transient receptor potential vanilloid 4 (TRPV4) channels are an important Ca2+ pathway in adipocytes and their activity is regulated by metabolic mediators such as insulin. In this study, we evaluated the role of TRPV4 channels in metabolic activity and adipokine secretion in human white adipocytes.

METHODS

Human white adipocytes were freshly cultured and the effects of the activation and inhibition of TRPV4 channels on lipolysis, glucose uptake, lactate production, and leptin and adiponectin secretion were evaluated.

RESULTS

Under basal and isoproterenol-stimulated conditions, TRPV4 activation by GSK1016709A decreased lipolysis whereas HC067047, an antagonist, increased lipolysis. The activation of TRPV4 resulted in increased glucose uptake and lactate production under both basal conditions and insulin-stimulated conditions; in contrast HC067047 decreased both parameters. Leptin production was increased, and adiponectin production was diminished by TRPV4 activation and its inhibition had the opposite effect.

CONCLUSION

Our results suggested that TRPV4 channels are metabolic mediators involved in proadipogenic processes and glucose metabolism in adipocyte biology. TRPV4 channels could be a potential pharmacological target to treat metabolic disorders.

Thapsigargin suppresses alpha 1-acid glycoprotein secretion independently of N-glycosylation and ER stress

Alpha-1 acid glycoprotein (AGP) is a major acute-phase protein that is involved in drug/ligand binding and regulation of immune response. In response to inflammation, AGP secretion from the liver increases, resulting in elevated concentration of plasma AGP. AGP exhibits multiple N-glycosylation sites, and thus, is highly glycosylated. Although AGP glycosylation is considered to affect its functions, the significance of AGP glycosylation for its secretion is unclear. In this study, we investigated the effects of AGP glycosylation using glycosylation-deficient mouse AGP mutants lacking one, four, or all five N-glycosylation sites. Furthermore, we examined the effects of endoplasmic reticulum (ER) stress-inducing reagents, including tunicamycin and thapsigargin, which induce ER stress in an N-glycosylation-dependent and -independent manner, respectively. Here, we found that glycosylation deficiency and ER stress induce a little or no effect on AGP secretion. Conversely, thapsigargin significantly suppressed AGP secretion in glycosylation-independent manner. These findings indicate that AGP secretion is regulated via thapsigargin-sensitive pathway that might be further controlled by the intracellular calcium concentrations.

An Update on the Role of Leptin in the Immuno-Metabolism of Cartilage

Since its discovery in 1994, leptin has been considered as an adipokine with pleiotropic effects. In this review, we summarize the actual information about the impact of this hormone on cartilage metabolism and pathology. Leptin signalling depends on the interaction with leptin receptor LEPR, being the long isoform of the receptor (LEPRb) the one with more efficient intracellular signalling. Chondrocytes express the long isoform of the leptin receptor and in these cells, leptin signalling, alone or in combination with other molecules, induces the expression of pro-inflammatory molecules and cartilage degenerative enzymes. Leptin has been shown to increase the proliferation and activation of immune cells, increasing the severity of immune degenerative cartilage diseases. Leptin expression in serum and synovial fluid are related to degenerative diseases such as osteoarthritis (OA), rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE). Inhibition of leptin signalling showed to have protective effects in these diseases showing the key role of leptin in cartilage degeneration.

Resistin is co-secreted with adiponectin in white mouse adipocytes

In the current work we have investigated the cellular and molecular regulation of resistin secretion in cultured and primary mouse adipocytes. Resistin is an adipose tissue hormone proposed to contribute to metabolic disease. In rodents, resistin is secreted from white adipocytes whereas it is in humans synthesised and released from other cell types within white adipose tissue. The metabolic importance of resistin has been studied in both mouse and man, but the regulation of its release remains poorly investigated. Here we define that, in mouse adipocytes, resistin secretion is triggered by an intracellular elevation of cAMP and/or Ca²⁺. Resistin release is stimulated via activation of beta 3 adrenergic receptors (β₃ARs) and the downstream signalling protein exchange protein activated by cAMP (Epac). The secretion of resistin is markedly abrogated in adipocytes isolated from obese and diabetic mice. Immunocytochemical staining demonstrates a significant overlap between signals for resistin and the adipocyte hormone adiponectin. Our data propose that resistin and adiponectin are contained within the same vesicles in mouse adipocytes and that the two hormones are co-secreted in response to the same exocytosis-triggering signals.

A Transcriptomic Analysis Reveals Novel Patterns of Gene Expression During 3T3-L1 Adipocyte Differentiation

Background

Obesity is characterized by increased adipose tissue mass that results from increased fat cell size (hypertrophy) and number (hyperplasia). The molecular mechanisms that govern the regulation and differentiation of adipocytes play a critical role for better understanding of the pathological mechanism of obesity. However, the mechanism of adipocyte differentiation is still unclear.

Objective

The present study aims to compare the gene expression changes during adipocyte differentiation in the transcriptomic level, which may help to better understand the mechanism of adipocyte differentiation.

Methods

RNA sequencing (RNA-seq) technology, GO and KEGG analysis, quantitative RT-PCR, and oil red O staining methods were used in this study.

Results

A lot of genes were up- or down-regulated between each two differentiation stages of 3T3-L1 cells. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis revealed that lipid metabolism and oxidation–reduction reaction were mainly involved in the whole process of adipocyte differentiation. Decreased immune response and cell cycle adhesion occurred in the late phase of adipocyte differentiation, which was demonstrated by divergent expression pattern analysis. Moreover, quantitative RT-PCR results showed that the mRNA expression levels of Trpv4, Trpm4, Trpm5, and Trpm7 were significantly decreased in the differentiated adipocytes. On the other hand, the mRNA expression levels of Trpv1, Trpv2, Trpv6, and Trpc1 were significantly increased in the differentiated adipocytes. Besides, the mRNA expressions of TRPV2 and TRPM7 were also significantly increased in subcutaneous white adipose tissue from diet-induced mice. In addition, the activation of TRPM7, TRPV1, and TRPV2 suppressed the differentiation of adipocytes.

Conclusion

These data present the description of transcription profile changes during adipocyte differentiation and provides an in-depth analysis of the possible mechanisms of adipocyte differentiation. These data offer new insight into the understanding of the mechanisms of adipocyte differentiation.

Functional expression of TRPM7 as a Ca2+ influx pathway in adipocytes

In adipocytes, intracellular Ca²⁺ and Mg²⁺ modulates physiological functions, such as insulin action and the secretion of adipokines. TRPM7 is a Ca²⁺/Mg²⁺‐permeable non‐selective cation channel. TRPM7 mRNA is highly expressed in adipose tissue, however, its functional expression in adipocytes remains to be elucidated. In this study, we demonstrated for the first time that TRPM7 was functionally expressed in both freshly isolated white adipocytes and in 3T3‐L1 adipocytes differentiated from a 3T3‐L1 pre‐adipocyte cell line by whole‐cell patch‐clamp recordings. Consistent with known properties of TRPM7 current, the current in adipocytes was activated by the elimination of extracellular divalent cations and the reduction of intracellular free Mg²⁺ concentrations, and was inhibited by the TRPM7 inhibitors, 2‐aminoethyl diphenylborinate (2‐APB), hydrogen peroxide (H₂O₂), N‐methyl maleimide (NMM), NS8593, and 2‐amino‐2‐[2‐(4‐octylphenyl)ethyl]‐1,3‐propanediol (FTY720). Treatment with small‐interfering (si) RNA targeting TRPM7 resulted in a reduction in the current to 23 ± 7% of nontargeting siRNA‐treated adipocytes. Moreover a TRPM7 activator, naltriben, increased the TRPM7‐like current and [Ca²⁺]_i in 3T3‐L1 adipocytes but not in TRPM7‐knockdown adipocytes. These findings indicate that TRPM7 is functionally expressed, and plays a role as a Ca²⁺ influx pathway in adipocytes.

Cold Exposure Distinctively Modulates Parathyroid and Thyroid Hormones in Cold-Acclimatized and Non-Acclimatized Humans

Cold-induced activation of thermogenesis modulates energy metabolism, but the role of humoral mediators is not completely understood. We aimed to investigate the role of parathyroid and thyroid hormones in acute and adaptive response to cold in humans. Examinations were performed before/after 15 minutes of ice-water swimming (n = 15) or 120 to 150 minutes of cold-induced nonshivering thermogenesis (NST) applied to cold-acclimatized (n = 6) or non-acclimatized (n = 11) individuals. Deep-neck brown adipose tissue (BAT) was collected from non-acclimatized patients undergoing elective neck surgery (n = 36). Seasonal variations in metabolic/hormonal parameters of ice-water swimmers were evaluated. We found that in ice-water swimmers, PTH and TSH increased and free T3, T4 decreased after a 15-minute winter swim, whereas NST-inducing cold exposure failed to regulate PTH and free T4 and lowered TSH and free T3. Ice-water swimming-induced increase in PTH correlated negatively with systemic calcium and positively with phosphorus. In non-acclimatized men, NST-inducing cold decreased PTH and TSH. Positive correlation between systemic levels of PTH and whole-body metabolic preference for lipids as well as BAT volume was found across the 2 populations. Moreover, NST-cooling protocol-induced changes in metabolic preference for lipids correlated positively with changes in PTH. Finally, variability in circulating PTH correlated positively with UCP1/UCP1, PPARGC1A, and DIO2 in BAT from neck surgery patients. Our data suggest that regulation of PTH and thyroid hormones during cold exposure in humans varies by cold acclimatization level and/or cold stimulus intensity. Possible role of PTH in NST is indicated by its positive relationships with whole-body metabolic preference for lipids, BAT volume, and UCP1 content.

Transient receptor potential (TRP) channels: a metabolic TR(i)P to obesity prevention and therapy

Cellular transport of ions, especially by ion channels, regulates physiological function. The transient receptor potential (TRP) channels, with 30 identified so far, are cation channels with high calcium permeability. These ion channels are present in metabolically active tissues including adipose tissue, liver, gastrointestinal tract, brain (hypothalamus), pancreas and skeletal muscle, which suggests a potential role in metabolic disorders including obesity. TRP channels have potentially important roles in adipogenesis, obesity development and its prevention and therapy because of their physiological properties including calcium permeability, thermosensation and taste perception, involvement in cell metabolic signalling and hormone release. This wide range of actions means that organ-specific actions are unlikely, thus increasing the possibility of adverse effects. Delineation of responses to TRP channels has been limited by the poor selectivity of available agonists and antagonists. Food constituents that can modulate TRP channels are of interest in controlling metabolic status. TRP vanilloid 1 channels modulated by capsaicin have been the most studied, suggesting that this may be the first target for effective pharmacological modulation in obesity. This review shows that most of the TRP channels are potential targets to reduce metabolic disorders through a range of mechanisms.

A mechanically activated TRPC1-like current in white adipocytes

Ca²⁺ impacts a large array of cellular processes in every known cell type. In the white adipocyte, Ca²⁺ is involved in regulation of metabolic processes such as lipolysis, glucose uptake and hormone secretion. Although the importance of Ca²⁺ in control of white adipocyte function is clear, knowledge is still lacking regarding the control of dynamic Ca²⁺ alterations within adipocytes and mechanisms inducing intracellular Ca²⁺ changes remain elusive. Own work has recently demonstrated the existence of store-operated Ca²⁺ entry (SOCE) in lipid filled adipocytes. We defined stromal interaction molecule 1 (STIM1) and the calcium release-activated calcium channel protein 1 (ORAI1) as the key players involved in this process and we showed that the transient receptor potential (TRP) channel TRPC1 contributed to SOCE. Here we have aimed to further characterised SOCE in the white adipocyte by use of single cell whole-cell patch clamp recordings. The electrophysiological measurements show the existence of a seemingly constitutively active current that is inhibited by known store-operated Ca²⁺ channel (SOCC) blockers. We demonstrate that the mechanical force applied to the plasma membrane upon patching leads to an elevation of the cytoplasmic Ca²⁺ concentration and that this elevation can be reversed by SOCC antagonists. We conclude that a mechanically activated current with properties similar to TRPC1 is present in white adipocytes. Activation of TRPC1 by membrane tension/stretch may be specifically important for the function of this cell type, since adipocytes can rapidly increase or decrease in size.

Extracellular ATP activates store-operated Ca2+ entry in white adipocytes: functional evidence for STIM1 and ORAI1

In the present study, we have applied ratiometric measurements of intracellular Ca²⁺ concentrations ([Ca²⁺]_i) to show that extracellularly applied ATP (adenosine triphosphate) (100 µM) stimulates store-operated Ca²⁺ entry (SOCE) in 3T3-L1 adipocytes. ATP produced a rapid increase in [Ca²⁺]_i consisting of an initial transient elevation followed by a sustained elevated phase that could be observed only in the presence of extracellular Ca²⁺. Gene expression data and [Ca²⁺]_i recordings with uridine-5′-triphosphate or with the phospholipase C (PLC) inhibitor U73122 demonstrated the involvement of purinergic P2Y2 receptors and the PLC/inositol trisphosphate pathway. The [Ca²⁺]_i elevation produced by reintroduction of a Ca²⁺-containing intracellular solution to adipocytes exposed to ATP in the absence of Ca²⁺ was diminished by known SOCE antagonists. The chief molecular components of SOCE, the stromal interaction molecule 1 (STIM1) and the calcium release-activated calcium channel protein 1 (ORAI1), were detected at the mRNA and protein level. Moreover, SOCE was largely diminished in cells where STIM1 and/or ORAI1 had been silenced by small interfering (si)RNA. We conclude that extracellular ATP activates SOCE in white adipocytes, an effect predominantly mediated by STIM1 and ORAI1.

TRPV4 Channels in Human White Adipocytes: Electrophysiological Characterization and Regulation by Insulin

Intracellular calcium homeostasis in adipocytes is important for the regulation of several functions and is involved in pathological changes in obesity and other associated diseases. Transient Receptor Potential Vanilloid 4 (TRPV4) channels are an important route for calcium entry that operates in a variety of cells and intervenes in a number of functions. In this study, the expression and operation of TRPV4 channels in human cultured adipocytes was evaluated using RT-PCR, Western blotting, the whole-cell patch-clamp technique and fluorescence measurements to characterize these channels and determine intracellular calcium responses. Both the hypoosmolarity and 4alpha-phorbol-didecanoate (4αPDD), a specific TRPV4 agonist, induced a similar HC-067047-sensitive current, which was predominantly inward, and an intracellular Ca(2+) concentration increase, which was exclusively dependent on extracellular calcium, and membrane depolarization. The current had a reverse potential of +31 ± 6 mV and exhibited preferential permeability to Ca(2+) . Insulin, which regulates metabolic homeostasis in adipocytes, attenuated the TRPV4-mediated effects. These results confirm the function of TRPV4 in human cultured adipocytes and its regulation by insulin. J. Cell. Physiol. 231: 954-963, 2016. © 2015 Wiley Periodicals, Inc.

Activation of transient receptor potential vanilloid 3 channel suppresses adipogenesis

The present study shows that activation of the transient receptor potential vanilloid 3 channel (TRPV3) suppresses adipocyte differentiation. We also found that a major functional catechin compound in green tea and cocoa, (-)-epicatechin, exerts antiadipogenic effects in the adipocytes through direct activation of TRPV3. TRPV3 was detected in the 3T3-L1 adipocytes using immunohistochemistry and semiquantitative PCR. TRPV3 activation by activators (-)-epicatechin and diphenylborinic anhydride was determined using live cell fluorescent Ca(2+) imaging and patch-clamp electrophysiology. Using RNA interference, immunoblotting, and Oil red O staining, we found that the TRPV3 agonists prevented adipogenesis by inhibiting the phosphorylation of insulin receptor substrate 1, the downstream phosphoinositide 3-kinase/Akt/forkhead box protein O1 axis, and the expression of the adipogenic genes peroxisome proliferator-activated receptor γ and CCAAT/enhancer-binding protein α. TRPV3 overexpression hindered adipogenesis in the 3T3-L1 cells. In vivo studies showed that chronic treatment with the TRPV3 activators prevented adipogenesis and weight gain in the mice fed on high-fat diets. Moreover, TRPV3 expression was reduced in the visceral adipose tissue from mice fed on high-fat diets and obese (ob/ob) and diabetic (db/m(+)) mice. In conclusion, our study illustrates the antiadipogenic role of TRPV3 in the adipocytes.

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.

Role of transient receptor potential channels in adipocyte biology

The transient receptor potential (TRP) channel superfamily is a family of 28 nonselective cation channels expressed on the plasma membrane with a high permeability to calcium. Role of TRP channels, especially TRP vanilloid 1, TRP ankyrin 1 and TRP melastatin 8, is widely documented in nociception. During the last few years, there has been a consistent increase in reports indicating the presence and significance of these channels in different tissues including bladder, skin, respiratory system and brain. Based on calcium permeability of these channels and the much published role of calcium and its signaling in adipogenesis, there is a potential for importance of these ion channels in adipocyte biology. This review provides insight into the involvement of TRP channels in adipocyte differentiation, obesity and associated complications. Furthermore, the authors will focus to evaluate these channels as potential therapeutic targets for the management of obesity.

Constitutively active TRPC channels of adipocytes confer a mechanism for sensing dietary fatty acids and regulating adiponectin

RATIONALE

Calcium entry is pivotal in the heart and blood vessels, but its significance and mechanisms in adipose tissue are largely unknown. An important factor produced by adipocytes is adiponectin, which confers myocardial protection, insulin-sensitization, and antiatherosclerotic effects.

OBJECTIVE

To investigate the relevance of calcium channels to adipocytes and the production of adiponectin.

METHODS AND RESULTS

Microarray analysis led to identification of transient receptor potential canonical (TRPC)1 and TRPC5 as channel subunits that are induced when adipocytes mature. Both subunits were found in perivascular fat of patients with atherosclerosis. Intracellular calcium and patch-clamp measurements showed that adipocytes exhibit constitutively active calcium-permeable nonselective cationic channels that depend on TRPC1 and TRPC5. The activity could be enhanced by lanthanum or rosiglitazone, known stimulators of TRPC5 and TRPC5-containing channels. Screening identified lipid modulators of the channels that are relevant to adipose biology. Dietary ω-3 fatty acids (eg, α-linolenic acid) were inhibitory at concentrations that are achieved by ingestion. The adipocyte TRPC1/TRPC5-containing channel was functionally negative for the generation of adiponectin because channel blockade by antibodies, knock-down of TRPC1-TRPC5 in vitro, or conditional disruption of calcium permeability in TRPC5-incorporating channels in vivo increased the generation of adiponectin. The previously recognized capability of α-linolenic acid to stimulate the generation of adiponectin was lost when calcium permeability in the channels was disrupted.

CONCLUSIONS

The data suggest that TRPC1 and TRPC5 contribute a constitutively active heteromultimeric channel of adipocytes that negatively regulates adiponectin and through which ω-3 fatty acids enhance the anti-inflammatory adipokine, adiponectin.

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.

The purinergic P2Y1 receptor supports leptin secretion in adipose tissue

Extracellular nucleotides have been shown to trigger intracellular calcium release and influence leptin secretion in differentiated white and brown adipocytes through activation of various but not clearly identified P2 receptors. In the present study, we wished to assess whether or not the P2Y1 ADP receptor is functional in white adipocytes and whether it could affect the secretion of adipocyte-derived hormones. Stromal cells and mature adipocytes were isolated from epididymal adipose tissue from wild-type and P2Y1 knockout (KO) C57-black/six male mice. The expression of the P2Y1 receptor in adipocytes was confirmed by RT-PCR and intracellular calcium measurements with fura 2-AM. KO of P2Y1 receptors did not affect the cell size and lipid content of mature adipocytes or the differentiation of the stromal cell fraction, but the leptin production of mature adipocytes was decreased under basal and insulin-stimulated conditions. A selective P2Y1 antagonist, MRS2500, reduced leptin release in isolated adipocytes. The plasma and adipose tissue mRNA levels of leptin were also lower in P2Y1 KO mice as compared with wild-type animals. However, in mice fed a high-fat diet, the plasma leptin levels were greatly enhanced and the inhibitory effect of P2Y1 KO was not observed. These results show that the P2Y1 receptor supports leptin production in isolated white adipocytes through a transcriptional mechanism. This function of the receptor may regulate plasma leptin in lean mice but is overcome in obese animals.

Calcium-sensing beyond neurotransmitters: functions of synaptotagmins in neuroendocrine and endocrine secretion

Neurotransmitters, neuropeptides and hormones are released through the regulated exocytosis of SVs (synaptic vesicles) and LDCVs (large dense-core vesicles), a process that is controlled by calcium. Synaptotagmins are a family of type 1 membrane proteins that share a common domain structure. Most synaptotagmins are located in brain and endocrine cells, and some of these synaptotagmins bind to phospholipids and calcium at levels that trigger regulated exocytosis of SVs and LDCVs. This led to the proposed synaptotagmin-calcium-sensor paradigm, that is, members of the synaptotagmin family function as calcium sensors for the regulated exocytosis of neurotransmitters, neuropeptides and hormones. Here, we provide an overview of the synaptotagmin family, and review the recent mouse genetic studies aimed at understanding the functions of synaptotagmins in neurotransmission and endocrine-hormone secretion. Also, we discuss potential roles of synaptotagmins in non-traditional endocrine systems.

Leptin biosynthetic pathway in white adipocytes

The aim of this study was to determine through morphological and biochemical means the biosynthetic and secretory pathway followed by leptin in adipocytes. Immunocytochemistry revealed the presence of leptin in the rough endoplasmic reticulum, the Golgi apparatus, and in numerous small vesicles along the plasma membrane of white adipo cytes. In vitro, isolated adipocytes under nonstimulated conditions (basal) continuously secreted leptin while their intra cellular content remained unchanged. When adipocytes were stimulated with insulin, leptin cellular content and secretion increased in parallel and were significantly different from basal secretion only after 45 min. L-leucine and L-glutamate also strongly stimulated leptin synthesis and secretion. These stimulating effects were abolished by cycloheximide and brefeldin A. The transcriptional inhibitor actinomycin D did not have any effects in either basal or stimulated conditions. Leptin mRNA levels were not affected by any stimulating or inhibiting agents. Finally, norepinephrine, isoproterenol, CL316243, and palmitate inhibited the effects of insulin, L-leucine, and L-glutamate on leptin synthesis. We thus conclude that (i) adipocytes continuously synthesize and secrete leptin along a rough endoplasmic reticulum-Golgi secretory vesicles pathway, (ii) an increase in leptin secretion requires increased de novo synthesis, and (iii) short-term leptin secretion does not involve changes in mRNA levels.

Endocrine and exocrine secretion of leptin by the gastric mucosa

Leptin is a hormone that plays important roles in nutritional status and in obesity. By means of immunocytochemistry, two populations of leptin-secreting cells were found in the lower half of the gastric mucosa. One consists of numerous large cells located around the gastric pits, the Chief epithelial cells, whereas the second refers to much smaller cells, strongly stained, few in number, and scattered between the gastric pits, the endocrine cells. By double immunostaining, leptin and pepsinogen were colocalized in the Chief cells, whereas the endocrine cells were positive only for leptin. Immunoelectron microscopy showed that leptin is present along the rough endoplasmic reticulum-Golgi-granules secretory pathways of the Chief and endocrine cells. On the other hand, leptin-receptor (long and short forms) immunolabelings, although absent in the gastric epithelial cell plasma membranes, were present in enterocytes at the level of their apical and basolateral membranes. Duodenal, jejunal, and ileal enterocytes displayed similar labelings for the leptin receptor. Thus, exocrine and endocrine secretions of leptin together with the presence of leptin receptors on enterocyte plasma membranes constitute a gastroenteric axis that coordinates the role played by leptin in the digestive tract.

Regulation of leptin secretion from white adipocytes by insulin, glycolytic substrates, and amino acids

The aim of the present study was to determine the respective roles of energy substrates and insulin on leptin secretion from white adipocytes. Cells secreted leptin in the absence of glucose or other substrates, and addition of glucose (5 mM) increased this secretion. Insulin doubled leptin secretion in the presence of glucose (5 mM), but not in its absence. High concentrations of glucose (up to 25 mM) did not significantly enhance leptin secretion over that elicited by 5 mM glucose. Similar results were obtained when glucose was replaced by pyruvate or fructose (both 5 mM). L-Glycine or L-alanine mimicked the effect of glucose on basal leptin secretion but completely prevented stimulation by insulin. On the other hand, insulin stimulated leptin secretion when glucose was replaced by L-aspartate, L-valine, L-methionine, or L-phenylalanine, but not by L-leucine (all 5 mM). Interestingly, these five amino acids potently increased basal and insulin-stimulated leptin secretion in the presence of glucose. Unexpectedly, L-glutamate acutely stimulated leptin secretion in the absence of glucose or insulin. Finally, nonmetabolizable analogs of glucose or amino acids were without effects on leptin secretion. These results suggest that 1) energy substrates are necessary to maintain basal leptin secretion constant, 2) high availability of glycolysis substrates is not sufficient to enhance leptin secretion but is necessary for its stimulation by insulin, 3) amino acid precursors of tricarboxylic acid cycle intermediates potently stimulate basal leptin secretion per se, with insulin having an additive effect, and 4) substrates need to be metabolized to increase leptin secretion.