Transcriptional regulation of leptin gene promoter in rat

Regulatory Mechanisms of Somatostatin Expression

Somatostatin is a peptide hormone, which most commonly is produced by endocrine cells and the central nervous system. In mammals, somatostatin originates from pre-prosomatostatin and is processed to a shorter form, i.e., somatostatin-14, and a longer form, i.e., somatostatin-28. The two peptides repress growth hormone secretion and are involved in the regulation of glucagon and insulin synthesis in the pancreas. In recent years, the processing and secretion of somatostatin have been studied intensively. However, little attention has been paid to the regulatory mechanisms that control its expression. This review provides an up-to-date overview of these mechanisms. In particular, it focuses on the role of enhancers and silencers within the promoter region as well as on the binding of modulatory transcription factors to these elements. Moreover, it addresses extracellular factors, which trigger key signaling pathways, leading to an enhanced somatostatin expression in health and disease.

Effect of zinc and vitamin E supplementation on hormones and blood biochemicals in peri-partum Sahiwal cows

Thirty-two advanced pregnant multiparous Sahiwal cows were used to study the effect of additional zinc (Zn) and vitamin E (VE) supplementation on hormonal and biochemical changes. Cows were randomly assigned to four groups and fed a basal diet of compounded concentrate, berseem fodder, and wheat straw in a ratio of 60:20:20. The groups were: (1) the basal diet with no supplement (control treatment); (2) the basal diet supplemented with 60 mg/kg DM/cow daily of Zn (Zn treatment); (3) the basal diet supplemented with 1000 IU/cow daily of vitamin E (VE treatment); and (4) the basal diet supplemented with a combination of 60 mg Zn/kg DM/cow and 1000 IU vitamin E/cow/d (Zn + VE treatment). Blood samples were collected on -60, -45, -30, -15, -7, -3, 0, 3, 7, 15, 30, 45, 60, 90, and 120 d in relation to expected date of calving and were analyzed for endocrine variables and biochemical changes. Plasma concentrations of leptin, insulin, insulin like growth factor-1 (IGF-1), triidothyronine (T3), and tetraiodothyronine (T4) were decreased toward calving and observed lowest (P < 0.05) on 3 d post-partum. However, plasma levels of growth hormone (GH) and cortisol increased toward calving and were found highest (P < 0.05) on 3 d post-partum. Pre-partum concentrations of leptin and IGF-1 were higher (P < 0.05) than its respective concentration observed during post-partum. Post-partum concentrations of GH and cortisol were higher (P < 0.05) than its respective pre-partum concentration. Pre-partum concentrations of urea, triglycerides, Zn, and VE were higher (P < 0.05) and total cholesterol and HDL cholesterol were lower than its values observed in post-partum among all the groups. Treatments had significant (P < 0.05) effect on plasma hormonal levels and levels of Zn and VE but no effect on biochemical attributes. Cows fed on diet supplemented with Zn + VE had highest (P < 0.05) pre as well as post-calving concentrations of leptin (6.38 vs 5.01 ng/ml), insulin (1.39 vs 1.33 ng/ml), GH (9.29 vs 13.72 ng/ml), IGF-1 (14.55 vs 12.59 nmol/l), T3 (1.45 vs 1.40 ng/ml), T4 (32.44 vs 31.79 ng/ml) whereas as lowest concentration of cortisol hormone (3.05 vs 3.44 ng/ml). Cows supplemented with combination of Zn and VE showed minimum decline in plasma concentration of leptin, insulin, GH, IGF-1, T3, and T4, and minimum increase in cortisol concentration. In conclusion, dairy cows around parturition faces various endocrine and biochemical alterations and supplementation of Zn in combination with VE can ameliorate adverse effect of calving stress by maintaining circulatory concentration of hormone and biochemicals towards the basal levels.

Emerging role of leptin in rheumatoid arthritis

Numerous studies have suggested the importance of leptin against autoimmune diseases such as systemic lupus erythematosus (SLE), multiple sclerosis (MS) and psoriasis. To summarize our current understanding of the role of leptin in inflammatory responses and rheumatoid arthritis (RA), a systematic review was conducted to assess the discrepancy of leptin in RA and its effect on immunity according to different studies. Recently, emerging data have indicated that leptin is involved in the pathological function of RA, which is common in autoimmune disorders. This review discusses the possible consequences of leptin levels in RA. Blocking the key signal pathways of leptin and inhibiting the leptin activity-like leptin antagonist may be a promising way for potential therapeutic treatment of RA at risk of detrimental effects. However, leptin was increased in patients with RA and may also regulate joint damage. Thus, more understanding of the mechanism of leptin in RA would be advantageous in the future.

Gene structure and expression of leptin in Chinese perch

Leptin is an important hormone involved in regulation of food intake, energy expenditure and reproduction in mammals, but its role in acanthomorph fishes remains scant. In the present study, we characterized leptin gene structure and its tissue expression in Chinese perch (Siniperca chuatsi). In contrast to typical leptin gene organization of 3 exons and 2 introns in other vertebrates, Chinese perch leptin gene consisted of 2 exons and 1 intron. This is the first leptin gene characterized in Perciformes, and is also the first leptin gene lacking an intron reported in Perciformes. The unique gene structure, the conservation of both cysteines that form the single disulfide bridge in leptin, and stable clustering in phylogenetic analyses substantiate the unambiguous orthology of mammalian and fish leptins, despite low amino acid identity. Polymorphism of leptin gene was examined in wild and cultivated populations of Chinese perch by direct sequencing of 120 fish. No SNP was found in leptin gene. Leptin mRNA of Chinese perch was highly expressed in liver, and expressed at low levels in brain, visceral adipose tissue, intestine, spleen and muscle.

Insulin enhances leptin expression in human trophoblastic cells

Leptin, one of the adipokines that controls energy metabolism via the central nervous system, also has pleiotropic peripheral effects, acting as a proinflammatory cytokine. Leptin is also produced by trophoblastic cells in the placenta, where leptin seems to function as a trophic autocrine hormone. Leptin expression is regulated by various tissue-specific factors, such as insulin, in the adipocyte. However, the complete regulation of leptin production in the placenta is still poorly understood. That is why we investigated the regulation of leptin expression by insulin in JEG-3 trophoblastic cells and human placental explants from normal pregnancies. Western blot analysis and quantitative real time RT-PCR was performed to determine the leptin expression level after treatment of cells or trophoblast explants with different concentrations of insulin (0.1-100 nM). Leptin promoter activity was evaluated by transient transfection with a plasmid construct containing different promoter regions and the reporter luciferase gene. We found a stimulatory, dose-dependent effect of insulin on endogenous leptin expression in human placental explants. Maximal effect was achieved at 10 nM insulin, and this effect can be totally prevented both by blocking phosphatidylinositol 3 kinase (PI3K) pathways and mitogen-activated protein kinase (MAPK). Moreover, insulin treatment significantly enhanced leptin promoter activity up to 40% in JEG-3 trophoblastic cells. Deletion analysis demonstrated that a minimal promoter region between -1951 and -1546 bp is necessary to achieve insulin effects. In conclusion, we provide evidence suggesting that insulin induces leptin expression in trophoblastic cells, enhancing the activity of leptin promoter region between -1951 and -1546 bp, via both PI3K- and MAPK-signaling pathways.

Modulation of leptin levels by oxidized linoleic acid: a connection to atherosclerosis?

The objective of the study was to determine the effects of oxidized linoleic acid (Ox-LA) on plasma leptin and to determine the relationship between plasma leptin levels and atherosclerosis in animals treated with Ox-LA. Low-density lipoprotein (LDL) receptor knockout (LDL r(-/-)) mice were fed a high fat diet with or without Ox-LA for 11 weeks. Plasma leptin levels in the high fat group consuming Ox-LA were significantly higher (14,052 ± 601 pg/mL vs. 10,950 ± 541 pg/mL; P < .01) compared to the group receiving the high fat diet alone. There was a highly significant correlation between the plasma leptin levels and aortic atherosclerotic lesions. From this we conclude that chronic exposure to dietary Ox-LA increases the plasma levels of leptin in LDL r(-/-) mice on a high fat diet. Considering our previous finding that dietary Ox-LA increased atherosclerosis, the current findings emphasize the need to reduce dietary intake of oxidized fat.

KLF15 Is a transcriptional regulator of the human 17beta-hydroxysteroid dehydrogenase type 5 gene. A potential link between regulation of testosterone production and fat stores in women

CONTEXT

Kruppel-like factor 15 (KLF15) is a newly discovered transcription factor that plays an important role in glucose homeostasis and lipid accumulation in cells. We present evidence for KLF15 as a transcriptional regulator of the human 17beta-hydroxysteroid dehydrogenase type 5 gene (HSD17B5) and its potential role in the pathogenesis of hyperandrogenism.

OBJECTIVE

The aim was to investigate the molecular mechanism of HSD17B5 regulation.

METHODS

Diverse molecular biology techniques were used.

DESIGN AND RESULTS

We identified a KLF15 binding site in the HSD17B5 promoter by using luciferase promoter constructs, EMSA, and chromatin immunoprecipitation assays. Overexpression of KLF15 increased HSD17B5 promoter activity and testosterone formation at least 3-fold in cultured H295R cells. Insulin increased KLF15 mRNA expression according to real-time RT-PCR and increased HSD17B5 promoter activity according to luciferase assays. KLF15 overexpression in combination with insulin, glucocorticoid, and cAMP stimulated adipogenesis in H295R cells. In silico and RT-PCR analyses showed that the KLF15 gene promoter undergoes alternative splicing in a tissue-specific manner. Comparison of the HSD17B5 promoter in seven different species revealed that the KLF15 binding site has no human homolog in species other than orangutans.

CONCLUSIONS

KLF15 is potentially a novel link between the regulation of testosterone production and fat stores by insulin in humans.

A critical role of Sp1 transcription factor in regulating gene expression in response to insulin and other hormones

Specificity protein 1 (Sp1) belongs to a family of ubiquitously expressed, C(2)H(2)-type zinc finger-containing DNA binding proteins that activate or repress transcription of many genes in response to physiological and pathological stimuli. There is emerging evidence to indicate that in addition to functioning as 'housekeeping' transcription factors, members of Sp family may be key mediators of gene expression induced by insulin and other hormones. The founding member of the family, Sp1, by virtue of its multi-domain organization, potential for posttranslational modifications and interactions with numerous transcription factors, represents an ideal mediator of nuclear signaling in response to hormones. Insulin regulates the sub-cellular localization, stability and trans-activation potential of Sp1 by dynamically modulating its post-translational modification by O-linked beta-N-acetylglucosamine (O-GlcNAc) or phosphate residues. We briefly review the recent literature demonstrating that an involvement of Sp-family of transcription factors in the regulation of differential gene expression in response to hormones is more common than previously appreciated and may represent a key regulatory mechanism.

Sp1 is required for glucose-induced transcriptional regulation of mouse vesicular glutamate transporter 2 gene

BACKGROUND & AIMS

Vesicular glutamate transporter (VGLUT) has been reported to be involved in glucose-induced insulin secretion. It has been shown that glucose stimulates the expression of VGLUT isoform 2 (VGLUT2) in beta cells via transcriptional mechanism. In this study, we identified the mouse VGLUT2 (mVGLUT2) promoter and characterized the transcriptional mechanism of glucose-stimulated mVGLUT2 expression in beta-cells.

METHODS

A promoter region of mVGLUT2 was cloned by genomic polymerase chain reaction. The mechanism of Sp1 in glucose-induced transactivation of mVGLUT2 was investigated by luciferase assay, electrophoretic mobility shift assay, chromatin immunoprecipitation assay, and Western blot analysis.

RESULTS

A promoter containing 2133 base pairs of upstream sequence of the 5'-flanking region of mVGLUT2 complementary DNA was cloned. Transient transfection of various 5'-end deletion constructs of the mVGLUT2 promoter/luciferase reporter indicated that the region between -96 to +68 base pair contains the basal promoter for mVGLUT2. Mutational analysis and electromobility shift assay showed an important role for the transcription factor Sp1 in both basal and glucose-induced mVGLUT2 transcription. The interaction between Sp1 and mVGLUT2 was confirmed by chromatin immunoprecipitation assays. Glucose stimulates the phosphorylation of Sp1 via mitogen-activated protein kinase P38 and P44/42. This leads to increased binding activity of Sp1 to the mVGLUT2 promoter and results in activation of the gene.

CONCLUSIONS

We cloned the mouse VGLUT2 promoter and showed a novel molecular mechanism of glucose-induced mVGLUT2 transcription.

The adipocyte as an active participant in energy balance and metabolism

Obesity is responsible for the mounting incidence of metabolic disease in adult and pediatric populations. Understanding of the pathogenesis and maintenance of the obese state has advanced rapidly over the past 10 years. Bodily energy reserves are managed actively by complex systems that regulate food intake, substrate partitioning, and energy expenditure. An underlying assumption that circulating factors released from storage organs were able to signal bodily energy reserves was confirmed with the discovery of the leptin system. This proof of concept has spurred on the discovery of a multitude of other adipocyte-generated factors. These circulating factors signal to the brain and other organs of metabolic importance, including adipose tissue, liver, muscle, and the immune system. Adipose-derived factors have numerous implications for the basic biology of obesity and provide prospective targets for the amelioration of obesity and its adverse metabolic consequences. In this review we detail the current understanding of leptin as a prototypical adipose tissue-derived hormone related to appetite and obesity. We also describe other important adipose-derived factors in relation to their metabolic effect.

Transcriptional regulation by insulin: from the receptor to the gene

Insulin, after binding to its receptor, regulates many cellular processes and the expression of several genes. For a subset of genes, insulin exerts a negative effect on transcription; for others, the effect is positive. Insulin controls gene transcription by modifying the binding of transcription factors on insulin-response elements or by regulating their transcriptional activities. Different insulin-signaling cascades have been characterized as mediating the insulin effect on gene transcription. In this review, we analyze recent data on the molecular mechanisms, mostly in the liver, through which insulin exerts its effect. We first focus on the key transcription factors (viz. Foxo, sterol-response-element-binding protein family (SREBP), and Sp1) involved in the regulation of gene transcription by insulin. We then present current information on the way insulin downregulates and upregulates gene transcription, using as examples of downregulation phosphoenolpyruvate carboxykinase (PEPCK) and insulin-like growth factor binding protein 1 (IGFBP-1) genes and of upregulation the fatty acid synthase and malic enzyme genes. The last part of the paper focuses on the signaling cascades activated by insulin in the liver, leading to the modulation of gene transcription.

Regulation and function of collagenous repeat containing sequence of 26-kDa protein gene product "cartonectin"

OBJECTIVE

Collagenous repeat containing sequence of 26-kDa protein (CORS-26) was identified as a new gene transcript expressed in cartilage with unknown function. It was the aim of this study to investigate expression, regulation, and function of CORS-26 in adipocytes.

RESEARCH METHODS AND PROCEDURES

CORS-26 mRNA and protein expression was studied by reverse transcriptase-polymerase chain reaction, Western blot analysis, and quantitative real-time polymerase chain reaction. Transcriptional regulation was studied by electrophoretic mobility shift assay and luciferase reporter gene assay. The adipocytic secretion of adiponectin and resistin was measured by enzyme-linked immunosorbent assay.

RESULTS

CORS-26 mRNA is absent in 3T3-L1 preadipocytes and adipocytes after 48 hours of differentiation. CORS-26 mRNA was induced from Day 4 to Day 9 of adipocyte differentiation. CORS-26 protein was induced in mature adipocytes. Peroxisome proliferator-activated receptor (PPAR) gamma (but not PPARalpha) in nuclear extracts prepared from adipocytes was shown to bind specifically to a putative peroxisome proliferator response element-one-half-site located at -641/-596 bp. Increasing doses of the ligands troglitazone (1, 10, 20 microM) and fenofibrate (50, 100, 200 microM) but not 15-deoxy-prostaglandin (J(2)) (0.5, 1.0, 2.5 microM) resulted in a significant reduction of both promoter activity and the amount of mRNA expression. Recombinant CORS-26 significantly stimulated the adipocytic secretion of adiponectin and resistin in a dose-dependent manner.

DISCUSSION

The mRNA and protein expression profile puts CORS-26 in the adipocytokine family. Cartonectin is negatively regulated by exogenous, but not endogenous, PPARgamma ligands. Because CORS-26 up-regulates adipokine secretion, it might be involved in metabolic and immunologic pathways.

Leptin secretion is related to glucose-derived lipogenesis in isolated adipocytes

OBJECTIVE

Leptin secretion in rats is regulated acutely by nutritional state. Insulin plays an important role in this acute nutritional regulation both directly and indirectly through effects on glucose metabolism. The aim of this study was to investigate if the fasting-induced suppression of leptin secretion was reversed by incubation under conditions mimicking nutritional repletion.

DESIGN

Leptin secretion and glucose metabolism were measured following incubation with glucose and insulin in adipocytes isolated from fed and fasted rats.

RESULTS

Leptin secretion was stimulated by incubation with glucose and insulin in adipocytes isolated from fed but not from fasted rats as was glucose flux through oxidative and lipogenic pathways. Ob expression and intracellular leptin content were decreased in adipocytes isolated from fasted rats throughout the whole incubation period. Suppression of glucose metabolism with cytochalasin B was accompanied by suppression of leptin secretion. The amount of leptin secretion correlated with the glucose incorporated into lipid under insulin-stimulated conditions.

CONCLUSIONS

It is proposed that glucose incorporation into lipid, at least during insulin-stimulated conditions, reflects the metabolic status of the adipocyte and may be a more important regulator of leptin production and secretion than circulating glucose or insulin levels.

Activation of the nuclear transcription factor SP-1 by insulin rapidly increases the expression of protein kinase C delta in skeletal muscle

SP-1, a ubiquitous transcription factor involved in regulation of target genes participating in specific signaling pathways, is utilized by insulin for induction of gene transcription. Transcriptional activation generally occurs only after several (14-24) hours. A major element rapidly activated by insulin in skeletal muscle is PKCdelta, which plays a positive regulatory role in insulin signaling. We recently reported that insulin stimulation of skeletal muscle increases PKCdelta RNA expression and PKCdelta protein levels within 5 min. These effects were blocked by inhibitors of either translation or transcription. In this study, we investigated the possibility that SP-1 may participate in this unusually rapid effect. Studies were performed on myoblasts and myotubes of the L6 skeletal muscle cell line. Insulin rapidly increased SP-1 levels and stimulated SP-1 phosphorylation in the nuclear fraction of L6 myotubes. The increase in nuclear SP-1 was blocked by inhibition of nuclear import. Inhibition of SP-1, either pharmacologically or by suppression of SP-1 by RNAi, nearly completely abrogated insulin-induced increase in PKCdelta promoter activity. Insulin induced a rapid association of SP-1 with the PKCalpha promoter. In addition, SP-1 inhibition blocked insulin-induced increases in both PKCdelta RNA expression and PKCdelta protein levels. We conclude that insulin rapidly stimulates SP-1, which mediates the ability of this hormone to induce the rapid transcription of a major target gene utilized in the insulin signaling cascade.

Genomic structure and an alternative transcript of bovine mitochondrial glycerol-3-phosphate acyltransferase gene (GPAM)

GPAM maps in BTA26q22, where several QTLs affecting milk production, milk fat and protein content have been mapped. On the basis of the QTL location, the GPAM gene could be considered a good candidate gene for the mentioned traits. Glycerol-3-phosphate acyltransferase mitochondrial (GPAM) is the enzyme that catalyses the initial and committed step of glycerolipid synthesis and, therefore, it is a potential site for triacylglycerol synthesis regulation. In this study, the structure of the cDNA and the genomic DNA of the bovine GPAM gene were determined and the expression of its mRNA was studied. The cDNA of the gene was cloned by RT-PCR, 5' and 3' rapid amplification of cDNA ends. The GPAM mRNA sequence contains a 2,475-bp coding region and a 3,689-bp 3' UTR. Its ORF encoded for an 825-amino acid protein and has an 89% homology with the coding regions of previously characterized mouse and human GPAM genes. The predicted amino acid sequence had an 89 and 93% similarity with mouse and human GPAM proteins, respectively. Using a 5' RACE strategy, two different 5' UTRs were cloned. Northern blot analysis confirmed the presence of two different transcripts. Adipose tissues and lung had the highest levels of GPAM mRNA expression, whereas it was barely detectable in liver. This expression pattern differs with those of non-ruminant animals where liver is one of the tissues with higher GPAM mRNA expression level.

Regulation of the ClC-2 lung epithelial chloride channel by glycosylation of SP1

Chloride channel-2 (ClC-2) is a pH- and voltage-activated chloride channel that is highly expressed in mammalian fetal airway epithelia during the period of maximal fluid secretion. A high level of luminal ClC-2 protein expression is maintained by the SP1 transcription factor until SP1 and ClC-2 decline rapidly at birth. Using fetal (preII-19) and adult (L2) rat lung Type 2 cell lines, we demonstrate that the active higher-molecular-weight 105-kD isoform of SP1 is phosphorylated and glycosylated. Exposure of either cell line to high-dose glutamine is sufficient to induce glycosylation of SP1 and to induce and maintain ClC-2. Exposure to tunicamycin to inhibit SP1 glycosylation reduces ClC-2 expression. We also demonstrate that in vivo ClC-2 expression is similarly regulated. SP1 from 6-wk-old murine lung (high ClC-2 expression) is hyperphosphorylated and hyperglycosylated compared with SP1 from 16-wk-old lung (low ClC-2 expression). Our results support the hypothesis that glycosylation of SP1 produces the 105-kD isoform of SP1 and is involved in regulating ClC-2 gene expression.

Mutation in the Sp1 motif of the bovine leptin gene affects its expression

Leptin is expressed mainly by adipocytes and plays a crucial role in the regulation of energy expenditure, food intake, and adiposity. Using PCR-heteroduplex analysis and sequencing, we investigated a C/G substitution in the promoter region of the bovine leptin gene. Application of the electrophoretic mobility shift assay showed that the C-->G transversion decreased the leptin gene promoter binding capacity for nuclear proteins. With real-time PCR and Western blotting, we showed that the leptin expression level was higher in cattle with the CC than with the GG genotype.

Leptin Gene Polymorphisms and Their Phenotypic Associations

In an era of rapidly increasing prevalence of human obesity and associated health problems, leptin gene polymorphisms have drawn much attention in biomedical research. Leptin gene polymorphisms have furthermore drawn much attention from animal scientists for their possible roles in economically important production and reproduction traits. Of the polymorphisms reported for exonic, intronic, and promoter regions of the leptin gene, 16 have been included in association studies in humans, 19 in cattle, and 6 (all exonic or intronic) in pigs. In humans, associations have been found with overweight or (early-onset) obesity, non-insulin-dependent diabetes mellitus, prostate cancer, and non-Hodgkin's lymphoma. In cattle, associations have been found with feed intake, milk yield traits, carcass traits, and reproduction-related traits, and in pigs with feed intake, average daily gain, carcass traits (backfat/leanness), and reproduction performance traits. Many of the polymorphisms were only included in a limited number of association studies, or the phenotypes studied varied largely for a given polymorphism between studies. Therefore, many of the associations found for these polymorphisms need to be confirmed in future studies before firm conclusions can be drawn.

Chronic leptin administration increases serum NEFA in the pig and differentially regulates PPAR expression in adipose tissue11Purdue University Agriculture Research Program #16748

Two in vivo studies were conducted with pigs to determine the effects of exogenous leptin on the expression of peroxisome proliferator activated receptors (PPAR), and on serum concentrations of selected metabolites and hormones. Initially, leptin was administered i.m. to young pigs for 15 days at 0 (control), 0.003 (low), 0.01 (medium) and 0.03 (high) mg. kg(-1). day(-1). There was no leptin effect on serum glucose (P > 0.84), triglycerides (P > 0.69), non-esterified fatty acids (NEFA, P > 0.53), or glycerol (P > 0.33). Leptin at the intermediate and high doses depressed adipose expression of both PPARgamma1 (P < 0.06) and PPARgamma2 (P < 0.01). In a second study, we used a paired-feeding experimental design to determine the effects of a higher dose of leptin (0.05 mg. kg(-1). day(-1)) on serum metabolites and PPAR expression in selected tissues. At this dose, leptin increased (P < 0.0001) serum NEFA concentrations relative to both the ad libitum and pair-fed control groups. However, in this study, there was no difference in the expression of PPARgamma1 in adipose tissue, but PPARgamma2 mRNA was upregulated by leptin (P < 0.08). In contrast, leptin had no impact on the expression of PPARalpha in liver, skeletal muscle or adipose tissue. Adipose tissue explants were also incubated with leptin to assess the effect on PPARgamma expression, in vitro. The abundance of PPARgamma1 mRNA (P < 0.05) was increased after 24 hr of exposure, but the effect of leptin on gamma2 was not significant (P > 0.24). The lipolytic effect of leptin was also evaluated in vitro using isolated adipocytes. In keeping with the increase in serum NEFA concentrations in vivo, leptin stimulated lipolysis in vitro, increasing glycerol concentrations in the medium to about 219% of that in basal (non-treated) culture medium after 8 hr of incubation. Collectively, the data presented herein indicate that leptin modulates lipid metabolism in the pig, but that PPARalpha expression is not a parallel target of leptin as it is in rodent models. The regulation of PPARgamma by leptin seems complex in that it varied in relation to dose in vivo, and may be impacted by in vitro vs. in vivo circumstances.

Effects of arachidonic acid on leptin secretion and expression in primary cultured rat adipocytes

Leptin, a hormone produced in adipocytes, is a key signal in the regulation of food intake and energy expenditure. Several studies have suggested that leptin can be regulated by macronutrients intake. Arachidonic acid is a dietary fatty acid known to affect cell metabolism. Controversial effects of this fatty acid on leptin have been reported. The aim of this experimental trial was to evaluate the effect of the arachidonic acid on basal and insulin-stimulated leptin secretion and expression in isolated rat adipocytes. Because insulin-stimulated glucose metabolism is an important regulator of leptin expression and secretion by the adipocytes, the effects of the arachidonic acid on indices of adipocyte metabolism were also examined. Isolated adipocytes were incubated with arachidonic acid (1-200 microM) in the absence and presence of insulin (1.6 nM). Leptin secretion and expression, glucose utilization and lactate production were determined at 96 h. The arachidonic acid (200 microM) inhibited both the basal and insulin stimulated leptin secretion and expression. Glucose utilization was not affected by the acid. Basal lactate production was increased by the fatty acid at the highest concentration used (200 microM), however lactate production in presence of insulin was not modified. Finally, the percentage of glucose carbon released as lactate was significantly increased (200 microM). These results suggest that the inhibitory effect of the arachidonic acid on leptin secretion and expression may be due, al least in part, to the increase in the anaerobic utilization of glucose.

Effects of inhibiting transcription and protein synthesis on basal and insulin-stimulated leptin gene expression and leptin secretion in cultured rat adipocytes

We have previously reported that glucose metabolism mediates the effects of insulin to increase leptin gene expression and leptin secretion by isolated adipocytes. The aim of the present study was to investigate the role of transcription and translation in the regulation of basal and insulin-stimulated leptin production. The short-term (4 h) and long-term (24-48 h) effects of actinomycin D, a transcriptional inhibitor, and cycloheximide, an inhibitor of protein synthesis, on leptin gene expression and leptin secretion by isolated adipocytes were determined. Actinomycin D (5 microg/ml) increased both basal and insulin-stimulated (1.6 nM) leptin secretion at 4 and 24h (193+/-14.9% and 153.8+/-10.4% of respective controls at 24h, both p<0.001). Similar effects of actinomycin D were observed on basal and insulin-stimulated leptin mRNA levels. 5,6-dichlororibofuranosyl benzimidazole (DRB), another inhibitor of transcription, also increased basal (175.4+/-18.2% of control; p<0.01) and insulin-stimulated leptin secretion (141.0+/-11.1% of insulin-treated cells; p<0.05) at 24 h. The effect of actinomycin D and DRB to increase basal leptin secretion observed at 4 and 24 h was not present at 48 h when actinomycin D and DRB both markedly inhibited insulin-stimulated leptin secretion (to 36+/-16%, p<0.05 and 21.9+/-5.6% of control, for actinomycin D and DRB, respectively, both p<0.001). Neither actinomycin D nor DRB had any effect on adipocyte glucose utilization between 24 and 48 h. The observed effects of inhibitors of transcription on leptin gene expression and leptin secretion are consistent with a long-term transcriptional mechanism for insulin-stimulated glucose metabolism to increase leptin production. Cycloheximide treatment (10 microg/ml) abolished the effects of insulin to stimulate leptin secretion (29+/-11% of control, p<0.01) during the first 4 h of treatment and at all later time points, which indicate that de novo protein synthesis is required for insulin-mediated glucose metabolism to increase leptin secretion.

Synergistic effects of hypoxia and insulin are regulated by different transcriptional elements of the human leptin promoter

Leptin is a regulator on placenta and conceptus during pregnancy. Hyperinsulinism and hypoxia induce partially overlapping pathophysiological disturbances during pregnancy. As insulin and hypoxia are known inducers of leptin secretion, we asked whether these two stimuli have synergistic effects. By analyzing mRNA levels of leptin after stimulation of BeWo cells with insulin in the presence or absence of oxygen, we found a supraadditive effect when incubating hypoxic cells with insulin. As shown by Western-blot of hypoxia-inducible-factor-1 alpha (HIF-1 alpha), the additive effects of these stimuli were not mediated by an increased stabilization of the HIF-complex. We therefore asked what elements of the leptin promoter are responsible for these effects. When deleting a 0.6 kb fragment of the cloned leptin promoter, a so far unknown loss of insulin-dependent activation of transcription, as well as a loss of the supraadditive effect of insulin and hypoxia could be observed. These results provide strong evidence that insulin and hypoxia act as agonists on the human leptin transcription but on two different regulatory elements.

Adiponutrin mRNA expression in white adipose tissue is rapidly induced by meal-feeding a high-sucrose diet

Adiponutrin is a recently identified gene of unknown function that is expressed exclusively in adipose tissues. To provide information about its physiological regulation and possible function, the effect of meal-feeding rats on the expression of adiponutrin mRNA in white adipose tissue was studied. A high-sucrose meal increased adiponutrin mRNA levels by at least 5-fold within 3 h. Post-meal levels returned to pre-meal levels with a half-life of about 5 h. The induction was prevented by injection of actinomycin-D prior to the meal. This pattern of expression was very similar to that seen for leptin mRNA. There were only minimal, or no, effects on acrp30/adiponectin, resistin, adipsin, or stearoyl-CoA desaturase 1. Adiponutrin appears more like leptin with respect to its acute regulation by meal-feeding than to any of the other adipokines or to enzymes directly involved in lipogenesis. This suggests that adiponutrin could be involved in overall energy homeostasis, as is leptin.

High glucose and insulin promote O-GlcNAc modification of proteins, including alpha-tubulin

Increased flux through the hexosamine biosynthesis pathway has been implicated in the development of glucose-induced insulin resistance and may promote the modification of certain proteins with O-linked N-acetylglucosamine (O-GlcNAc). L6 myotubes (a model of skeletal muscle) were incubated for 18 h in 5 or 25 mM glucose with or without 10 nM insulin. As assessed by immunoblotting with an O-GlcNAc-specific antibody, high glucose and/or insulin enhanced O-GlcNAcylation of numerous proteins, including the transcription factor Sp1, a known substrate for this modification. To identify novel proteins that may be O-GlcNAc modified in a glucose concentration/insulin-responsive manner, total cell membranes were separated by one- or two-dimensional gel electrophoresis. Selected O-GlcNAcylated proteins were identified by mass spectrometry (MS) analysis. MS sequencing of tryptic peptides identified member(s) of the heat shock protein 70 (HSP70) family and rat alpha-tubulin. Immunoprecipitation/immunoblot studies demonstrated several HSP70 isoforms and/or posttranslational modifications, some with selectively enhanced O-GlcNAcylation following exposure to high glucose plus insulin. In conclusion, in L6 myotubes, Sp1, membrane-associated HSP70, and alpha-tubulin are O-GlcNAcylated; the modification is markedly enhanced by sustained increased glucose flux.

Effects of Trecadrine, a beta3-adrenergic agonist, on leptin secretion, glucose and lipid metabolism in isolated rat adipocytes

OBJECTIVE

Leptin, a hormone produced in adipocytes, is a key signal in the regulation of food intake and energy expenditure. Beta-adrenergic agonists have been shown to inhibit leptin gene expression and leptin secretion. The mechanisms underlying the inhibitory effects of beta-adrenergic agonists have not been established. In this study, we examined the effects of Trecadrine, a novel beta3-adrenergic agonist, on basal and insulin-stimulated leptin secretion in isolated rat adipocytes. Because insulin-stimulated glucose metabolism is an important regulator of leptin expression and secretion by the adipocytes, the effects of Trecadrine on indices of adipocyte metabolism were also examined.

MEASUREMENTS

Isolated adipocytes were incubated with Trecadrine (10(-8)-10(-4) M) in the absence or presence of insulin (1.6 nM). Leptin secretion, glucose utilization, lactate production, glucose incorporation into CO(2) and triglyceride, as well as lipolysis (glycerol release) were determined.

RESULTS

Trecadrine induced a concentration-dependent inhibition of basal leptin secretion. Trecadrine also decreased insulin-stimulated leptin secretion; however, the effect was not as pronounced as in the absence of insulin. Treatment of adipocytes with Trecadrine increased basal glucose utilization and produced a further increase in insulin-stimulated glucose utilization. Basal lactate production was also increased by Trecadrine; however, the proportion (percentage) of glucose carbon released as lactate was unaffected. In the presence of insulin, absolute lactate production was unaffected by Trecadrine at 96 h. However, the percentage of glucose carbon released as lactate was significantly decreased by insulin treatment, and was further decreased by the co-treatment with Trecadrine. Trecadrine induced a dose-dependent increase of the absolute amount of glucose incorporated into triglyceride. However, the percentage of glucose utilized that was incorporated into triglyceride was unaffected by Trecadrine. Trecadrine did not modify the proportion of glucose utilized that was oxidized to CO(2). Trecadrine increased glycerol release after 96 h of treatment. Glycerol release was negatively correlated with leptin secretion.

CONCLUSIONS

These results suggest that alterations of glucose metabolism are not directly involved in the effects of beta3-adrenergic agonists to inhibit leptin expression and secretion. The inverse relationship between leptin secretion and the increase of glycerol levels, which is an index of the activation of cAMP-dependent protein kinases, suggests that activation of the cAMP signaling pathway mediates the inhibitory effects of Trecadrine on leptin gene expression and secretion.

Glucocorticoid-induced osteopenia in the mouse as assessed by histomorphometry, microcomputed tomography, and biochemical markers

Glucocorticoids are potent anti-inflammatory molecules used in the treatment of asthma, rheumatoid arthritis, inflammatory bowel disease, and other inflammatory and dermatological diseases, as well as in posttransplantation immunotherapy. Although glucocorticoids have been prescribed for many years, their potential side effects, when administered orally, can prevent their long-term use. The most serious side effect observed in the clinic is glucocorticoid-induced osteoporosis (GIOP). To develop a small animal model to characterize glucocorticoid-induced bone loss, we carried out a series of experiments using BALB/c mice given daily intraperitoneal doses of the synthetic glucocorticoid, dexamethasone. Following dexamethasone treatment, the mice became osteopenic, with highly significant decreases in bone formation rate and mineral apposition rate, as assessed by standard histomorphometry. Moreover, 3 week treatment with dexamethasone resulted in a decrease in trabecular thickness and trabecular number with an increase in surface-to-volume ratio of trabeculae in the distal femur, as measured using microcomputed tomography (micro-CT). The serum bone formation marker, osteocalcin, was dose-dependently decreased in all mice treated with dexamethasone and showed a parallel extent of regulation to the bone formation rate changes. In addition, serum levels of leptin, recently identified as playing a role in the regulation of bone mass, increased following dexamethasone treatment. BALB/c mice therefore represent a useful model system in which the detrimental effects of glucocorticoids on bone can be studied.

Transcriptional regulation of adipocyte hormone-sensitive lipase by glucose

Hormone-sensitive lipase (HSL) catalyzes the rate-limiting step in the mobilization of fatty acids from adipose tissue, thus determining the supply of energy substrates in the body. HSL mRNA was positively regulated by glucose in human adipocytes. Pools of stably transfected 3T3-F442A adipocytes were generated with human adipocyte HSL promoter fragments from -2,400/+38 to -31/+38 bp linked to the luciferase gene. A glucose-responsive region was mapped within the proximal promoter (-137 bp). Electromobility shift assays showed that upstream stimulatory factor (USF)-1 and USF2 and Sp1 and Sp3 bound to a consensus E-box and two GC-boxes in the -137-bp region. Cotransfection of the -137/+38 construct with USF1 and USF2 expression vectors produced enhanced luciferase activity. Moreover, HSL mRNA levels were decreased in USF1- and USF2-deficient mice. Site-directed mutagenesis of the HSL promoter showed that the GC-boxes, although contributing to basal promoter activity, were dispensable for glucose responsiveness. Mutation of the E-box led to decreased promoter activity and suppression of the glucose response. Analogs and metabolites were used to determine the signal metabolite of the glucose response. The signal is generated downstream of glucose-6-phosphate in the glycolytic pathway before the triose phosphate step.

Insulin and glucocorticoids differentially regulate leptin transcription and secretion in brown adipocytes

Leptin, the ob gene product, is produced by adipose tissue and is submitted to a complex hormonal and metabolic regulation. Leptin plays a critical role in the balance of body weight. Here we report on secretion and hormonal regulation of leptin by brown adipocytes. Using the recently established T37i cell line, we show that leptin expression and secretion occurred as a function of cell differentiation. In differentiated T37i cells, insulin induced leptin release ( approximately 0.25 ng/10(6) cells/h) in a concentration-dependent manner (EC50=0.1 nM), and this was totally suppressed by beta3-adrenergic ligand, thiazolidinedione, cycloheximide, or actinomycin D. Insulin induced a strong, rapid (within 2 h) but transient fivefold increase in leptin mRNA levels. This transcriptional control of ob gene expression by insulin involved both phosphatidylinositol 3-kinase- and MAP kinase-dependent pathways. Glucocorticoids inhibited both insulin-stimulated leptin secretion and ob gene expression without affecting leptin mRNA stability (t(1/2)=3h05). Altogether, our results demonstrate that brown adipocytes express and secrete leptin, whose hormonal regulation clearly differs from that described in white adipose tissue. These findings point to tissue-specific molecular mechanisms and suggest that leptin might exert direct effects on energy homeostasis through an autocrine mechanism.

Transcriptional regulation of the leptin gene promoter in rat GH3 pituitary and C6 glioma cells

Leptin was originally believed to be an exclusively adipocyte-derived hormone regulating appetite and energy balance. It has recently become apparent that leptin is actively expressed in a number of other tissues including the CNS and pituitary, as well as brain- and pituitary-derived cell lines. However, the factors controlling leptin expression in cells of neuroectodermal origin are unknown. The mouse leptin gene 5'-flanking DNA contains multiple AP-1 and SRF-1 binding sites as well as a consensus CRE site at -491 to -482 bp. In addition, a number of potential PIT1 and Oct-1 binding sites may contribute to leptin gene transcription in pituitary and brain. We have used leptin promoter-luciferase reporter constructs to examine the regulation of the leptin promoter in 3T3-L1 preadipocytes, C6 glioma cells, and GH3 pituitary cells in response to serum and hormonal stimuli. Cells were transiently transfected with reporter constructs containing either the proximal 500 bp of the leptin promoter (-500-luc) or 6000 bp of the leptin gene 5' flanking region (-6000-luc). Functional analysis indicates that the leptin promoter is constitutively active in all 3 cell lines. Transcriptional activity was significantly higher with a -500 to +9 promoter than with a construct containing -6000 to +9 bp of 5' flanking DNA, indicating the presence of repressor elements which may contribute to the tissue-specific regulation of leptin expression. However, qualitatively similar results were observed with both constructs in response to serum and hormonal manipulation. Leptin promoter activity was significantly stimulated by serum in all cell lines, although to varying extents. In contrast, the response of the leptin promoter to insulin, IGF-1 and dibutyryl cAMP was cell-type specific and dependent on the presence or absence of FBS in the culture medium. Insulin, IGF-1 and dibutyryl cAMP each caused an approximately two-fold stimulation of leptin promoter activity in 3T3-L1 cells under serum-free conditions, but had no significant effect in the presence of 10% FBS. In contrast, dibutyryl cAMP markedly stimulated leptin promoter activity (5-8-fold) in C6 or GH3 cells in the presence or absence of FBS, whereas insulin or IGF-1 had minimal effects. These findings support our previous studies on the regulation of leptin steady state mRNA levels in C6 cells and demonstrate tissue-specific differences in the regulation of leptin gene transcription in adipose vs. neuroectodermal tissues.

Transcriptional regulation of the leptin promoter by insulin-stimulated glucose metabolism in 3t3-l1 adipocytes

Insulin-stimulated glucose metabolism plays a key role in the regulation of leptin mRNA expression and protein secretion. However, it is not known whether stimulation of leptin production by glucose metabolism is regulated at the level of promoter activation or at a step distal to the promoter. Therefore, in order to investigate the transcriptional regulation of the leptin promoter by insulin-stimulated glucose metabolism, 3T3-L1 cells were transfected with a plasmid containing the leptin promoter driving a luciferase reporter gene. Leptin promoter activity was increased after 48 hours of treatment by 219 +/- 64 (p = 0.028) and 225 +/- 69% (p = 0.046) at insulin concentrations of 16 and 160 nM, respectively. The activation of the leptin promoter induced by insulin (16 nM) was markedly inhibited by 2-deoxy-D-glucose (2-DG, 50 mg/dl), a competitive inhibitor of glucose metabolism. The increment of insulin-stimulated leptin promoter activation was reduced by 52 +/- 11% (p = 0.028 vs insulin alone). The activity of a control plasmid (pGL2-Control) was unaffected by insulin or 2-DG. These results provide strong evidence that insulin-stimulated glucose metabolism, and not insulin per se, mediates the effects of insulin to increase the transcriptional activity of the leptin promoter.

Regulation of metabolism and body fat mass by leptin

The relative stability of body weight over the long term and under a variety of environmental conditions that alter short-term energy intake and expenditure provides strong evidence for the regulation of body energy content. The lipostatic theory of energy balance regulation proposed 40 years ago that circulating factors, generated in proportion to body fat stores, acted as signals to the brain, eliciting changes in energy intake and expenditure. The discovery of leptin and its receptors has now provided a molecular basis for this theory. Leptin functions as much more than an adipocyte-derived signal of lipid stores, however. Although suppression of food intake is an important centrally mediated effect of leptin, considerable evidence indicates that leptin also functions both directly and indirectly, via the brain, to orchestrate complex metabolic changes in a number of organs and tissues, altering nutrient flux to favor energy expenditure over energy storage.

Transcriptional regulation of insulin receptor gene promoter in rat hepatocytes

To investigate the DNA regulatory sequences -618 to -593 (initiator ATG is +1) of the insulin receptor (IR) promoter required for IR gene expression, primary cultured hepatocytes of rats were transfected with plasmids containing the 5'-flanking sequences of the rat IR gene fused to the luciferase gene. When three copies of the nucleotides -618 to -593 of the IR promoter were transfected, the reporter activity was significantly increased in the presence of glucose and more increased in the presence of glucose/insulin. The glucose/insulin stimulation was inhibited by the addition of polyunsaturated fatty acids. These results were similar to those found earlier for the transcriptions of the fatty acid synthase, FAS(-57/-35), ATP citrate-lyase, ACL(-64/-41), and leptin(-101/-83) genes, which promoter sequences have a high similarity to IR(-618/-593). However, similarly to the leptin gene, the IR gene was more responsive to glucose stimulation than the FAS and ACL genes. Mutation of either one of the Sp1 binding sites (-618/-593) did not significantly affect the transcription, whereas mutation of three or four Sp1 binding sites resulted in a loss of responsiveness to glucose/insulin. Gel mobility shift assays revealed that nuclear factor(s) from rat liver specifically formed complexes with the sequence of IR(-618/-593). By antibody supershift assays, the transcription factors Sp1and Sp3 were found to bind with the IR(-618/-593). The IR, leptin, ACL and FAS genes contain common DNA-sequences responsible for the glucose/insulin-stimulation, suggesting that these genes are similarly regulated.

Effects of polyunsaturated fatty acids and clofibrate on chicken stearoyl-coA desaturase 1 gene expression

In chicken, adiposity is influenced by hepatic stearoyl-CoA desaturase (SCD) 1. This gene is up-regulated by low-fat high-carbohydrate diet and down-regulated by addition of polyunsaturated fatty acids (PUFA). In this study, we present evidence for an inhibition of chicken SCD1 expression by PUFA using reporter gene constructs in transient transfection assays. This inhibition does not involve the peroxisome proliferator-activated receptor pathway, in contrast with what has been observed in rodents. We were able to localise a PUFA as well as an insulin response element within the -372/+125 bp region of the promoter. Sequence analyses of this region allowed identification of several cis-regulatory elements: A sterol regulatory element (SRE) and a juxtaposed NF-Y element which have been shown to be involved in the regulation of mouse SCD genes by PUFA. In addition, we identified an overlapping Sp1/USF motif, which was described to play a role in insulin/glucose and PUFA regulation of fatty synthase, ATP-citrate-lyase, and leptin genes. These data provide the first characterisation of the chicken SCD1 promoter and putative cis-sequences involved in the regulation of this gene by PUFA and insulin.

Nutritional and insulin regulation of leptin gene expression

The leptin and lipogenic enzyme genes contain the common DNA sequences of binding sites for Sp1 proteins. These sites appear to be responsible for glucose/insulin stimulation and polyunsaturated fatty acid suppression. In rat adipose tissue leptin and lipogenic gene expression is similarly regulated by nutritional manipulation. Interestingly, leptin has the ability to down-regulate lipogenic enzyme expression.

Gene expressions of leptin, insulin receptors and lipogenic enzymes are coordinately regulated by insulin and dietary fat in rats

Regulation of the gene expressions of leptin, insulin receptors and lipogenic enzymes was investigated after refeeding a fat-free diet or a 10 g/100 g corn oil diet to food-deprived rats. Plasma glucose and insulin concentrations began to increase 30 min after the feeding and further increased up until 8 h. In these rats, the expression of leptin mRNA in adipose tissue began to increase significantly only 30 min after feeding, and reached a maximum at 8-16 h. However, plasma leptin levels did not increase until 4 h after refeeding, then markedly increased and reached the maximal level after 8 h. The expression of leptin mRNA and plasma leptin concentrations generally were greater in rats fed the corn oil diet compared to those fed the fat-free diet. Insulin receptor mRNA concentrations in the liver and adipose tissue began to decrease 30 min after the refeeding, in contrast to the plasma insulin increase, and continued to decrease until 8 h. The expression of acetyl-CoA carboxylase and fatty acid synthase mRNA began to increase 4-8 h after feeding and reached maximal levels at 16-24 h. Leptin treatment suppressed the expression of lipogenic enzyme mRNA in rats fed the fat-free diet but not in corn oil-fed rats, in which the expression was suppressed by polyunsaturated fatty acids and leptin expression was higher. Thus, we suggest that the glucose and insulin-dependent expressions of leptin, insulin receptors and lipogenic enzymes are coordinately and/or mutually regulated by dietary manipulation.