Large-scale preparation of biologically active mouse and rat leptins and their L39A/D40A/F41A muteins which act as potent antagonists

Leptin: A Heavyweight Player in Obesity-Related Cancers

Obesity, defined as the abnormal or excessive expansion of white adipose tissue, has reached pandemic proportions and is recognized as an important health concern since it is a common root for several comorbidities, including malignancies. Indeed, the current knowledge of the white adipose tissue, which shifts its role from an energy storage tissue to an important endocrine and metabolic organ, has opened up new avenues for the discovery of obesity's effects on tumor biology. In this review, we will report the epidemiological studies concerning the strong impact of obesity in several types of cancer and describe the mechanisms underlying the heterotypic signals between cancer cell lines and adipocytes, with particular emphasis on inflammation, the insulin/IGF-1 axis, and adipokines. Among the adipokines, we will further describe the in vitro, in vivo, and clinical data concerning the role of leptin, recognized as one of the most important mediators of obesity-associated cancers. In fact, leptin physiologically regulates energy metabolism, appetite, and reproduction, and several studies have also described the role of leptin in affecting cancer development and progression. Finally, we will summarize the newest pharmacological strategies aimed at mitigating the protumorigenic effects of leptin, underlining their mechanisms of action.

Leptin-Activity Modulators and Their Potential Pharmaceutical Applications

Leptin, a multifunctional hormone primarily, but not exclusively, secreted in adipose tissue, is implicated in a wide range of biological functions that control different processes, such as the regulation of body weight and energy expenditure, reproductive function, immune response, and bone metabolism. In addition, leptin can exert angiogenic and mitogenic actions in peripheral organs. Leptin biological activities are greatly related to its interaction with the leptin receptor. Both leptin excess and leptin deficiency, as well as leptin resistance, are correlated with different human pathologies, such as autoimmune diseases and cancers, making leptin and leptin receptor important drug targets. The development of leptin signaling modulators represents a promising strategy for the treatment of cancers and other leptin-related diseases. In the present manuscript, we provide an update review about leptin-activity modulators, comprising leptin mutants, peptide-based leptin modulators, as well as leptin and leptin receptor specific monoclonal antibodies and nanobodies.

The Postnatal Leptin Surge Supports Immune Cell Function in Rats

Background: Leptin plays an important role in the regulation of the immune response. There is a physiological surge of leptin in rodents during the neonatal period, which has mainly been studied in the context of brain development. However, little is known about the effects of this neonatal leptin surge on immunity. Therefore, we investigated whether blocking this leptin surge could affect several immune functions.Methods: Male and female rats were injected subcutaneously with 5 mg/Kg/day of rat pegylated super leptin antagonist during the neonatal period (PND5-9). On the peripubertal period, relevant functions as well as cytokine release by spleen leukocytes were studied in these animals.Results: The results showed that the animals significantly display an impaired anti-tumor NK activity and chemotactic and proliferation capacity of lymphocytes in response to mitogens. In addition, several cytokine concentrations, released under mitogen-stimulated conditions, were also altered.Conclusion: In conclusion, the neonatal leptin surge seems to be involved in the establishment of an adequate immune response and cytokine profile, which are crucial for the maintenance of a healthy life.

Leptin-induced signaling pathways in cancer cell migration and invasion

BACKGROUND

Increasing evidence indicates that obesity is associated with tumor development and progression. Leptin is an adipocyte-related hormone with a key role in energy metabolism and whose circulating levels are elevated in obesity. The effect of leptin on cancer progression and metastasis and its underlying mechanisms are still unclear. Leptin can impact various steps in tumor metastasis, including epithelial-mesenchymal transition, cell adhesion to the extracellular matrix (ECM), and proteolysis of ECM components. To do so, leptin binds to its receptor (OB-Rb) to activate signaling pathways and downstream effectors that participate in tumor cell invasion as well as distant metastasis.

CONCLUSIONS

In this review, we describe metastasis steps in detail and characterize metastasis-related molecules activated by leptin, which may help to develop a roadmap that guides future work. In addition, we conclude that a profound understanding of the fundamental molecular processes that contribute to leptin-induced metastasis may pave the way for the development of new prognostic molecules and appropriate approaches to the treatment of obesity-related cancers.

Adipokines as therapeutic targets in breast cancer treatment

INTRODUCTION

Adipocytes, which represent a substantial part of the tumor microenvironment in breast cancer, secrete several adipokines that affect tumorigenesis, cancer progression, metastasis, and treatment resistance via multiple signaling pathways. Areas covered: In this review, we focus on the role of leptin, adiponectin, autotaxin, and interleukin-6 in breast cancer initiation, progression, metastasis, and drug response. Furthermore, we investigated adipokines as potential targets of breast cancer-specific drugs. Expert opinion: Adipokines and adipokine receptors are deregulated in breast cancer. Adipokines play various roles in breast cancer initiation, progression, metastasis, and drug response, hence, adipokine signaling could be an effective drug target. Several clinical trials are in progress to test the efficacy of adipokine targeting agents. However, adipokines also affect metabolic homeostasis; hence, the adverse effects of the targeted drug should be investigated and addressed.

Neonatal overfeeding induces early decline of the ovarian reserve: Implications for the role of leptin

Early life nutrition is crucial for reproduction. Overweight and obese girls are more likely to experience early menarche, increasing the risk of adult disease. We have previously demonstrated neonatal overfeeding in the rat leads to accelerated growth, early puberty and increased circulating levels of leptin, an adipocyte-derived hormone that regulates puberty. However, the long-term consequences of accelerated puberty and metabolic dysfunction on ovarian reserve are unknown. Here we show that neonatal overfeeding reduced the number of ovarian follicles in adult rats; specifically, the primordial follicle pool was reduced compared to controls. The reduction of ovarian reserve coincided with a diminished release of pituitary gonadotropins at ovulation and altered expression of ovarian markers important for follicular recruitment and survival. These changes were associated with increased levels of ovarian leptin and its receptor. Postnatal administration of leptin antagonist did not reverse the weight gain induced by early life overfeeding, but rescued the decline in the primordial follicle pool and abolished the differences in circulating leptin and gonadotropins. Our findings suggest that the acute effects of elevated circulating leptin may be responsible for the long-term reproductive outcomes after neonatal overfeeding, leading to premature ovarian ageing and changes in reproductive efficiency.

Leptin suppresses sweet taste responses of enteroendocrine STC-1 cells

Leptin is an important hormone that regulates food intake and energy homeostasis by acting on central and peripheral targets. In the gustatory system, leptin is known to selectively suppress sweet responses by inhibiting the activation of sweet sensitive taste cells. Sweet taste receptor (T1R2+T1R3) is also expressed in gut enteroendocrine cells and contributes to nutrient sensing, hormone release and glucose absorption. Because of the similarities in expression patterns between enteroendocrine and taste receptor cells, we hypothesized that they may also share similar mechanisms used to modify/regulate the sweet responsiveness of these cells by leptin. Here, we used mouse enteroendocrine cell line STC-1 and examined potential effect of leptin on Ca(2+) responses of STC-1 cells to various taste compounds. Ca(2+) responses to sweet compounds in STC-1 cells were suppressed by a rodent T1R3 inhibitor gurmarin, suggesting the involvement of T1R3-dependent receptors in detection of sweet compounds. Responses to sweet substances were suppressed by ⩾1ng/ml leptin without affecting responses to bitter, umami and salty compounds. This effect was inhibited by a leptin antagonist (mutant L39A/D40A/F41A) and by ATP gated K(+) (KATP) channel closer glibenclamide, suggesting that leptin affects sweet taste responses of enteroendocrine cells via activation of leptin receptor and KATP channel expressed in these cells. Moreover, leptin selectively inhibited sweet-induced but not bitter-induced glucagon-like peptide-1 (GLP-1) secretion from STC-1 cells. These results suggest that leptin modulates sweet taste responses of enteroendocrine cells to regulate nutrient sensing, hormone release and glucose absorption in the gut.

Leptin Suppresses Mouse Taste Cell Responses to Sweet Compounds

Leptin is known to selectively suppress neural and behavioral responses to sweet-tasting compounds. However, the molecular basis for the effect of leptin on sweet taste is not known. Here, we report that leptin suppresses sweet taste via leptin receptors (Ob-Rb) and KATP channels expressed selectively in sweet-sensitive taste cells. Ob-Rb was more often expressed in taste cells that expressed T1R3 (a sweet receptor component) than in those that expressed glutamate-aspartate transporter (a marker for Type I taste cells) or GAD67 (a marker for Type III taste cells). Systemically administered leptin suppressed taste cell responses to sweet but not to bitter or sour compounds. This effect was blocked by a leptin antagonist and was absent in leptin receptor-deficient db/db mice and mice with diet-induced obesity. Blocking the KATP channel subunit sulfonylurea receptor 1, which was frequently coexpressed with Ob-Rb in T1R3-expressing taste cells, eliminated the effect of leptin on sweet taste. In contrast, activating the KATP channel with diazoxide mimicked the sweet-suppressing effect of leptin. These results indicate that leptin acts via Ob-Rb and KATP channels that are present in T1R3-expressing taste cells to selectively suppress their responses to sweet compounds.

Modulation of sweet taste sensitivities by endogenous leptin and endocannabinoids in mice

KEY POINTS

Potential roles of endogenous leptin and endocannabinoids in sweet taste were examined by using pharmacological antagonists and mouse models including leptin receptor deficient (db/db) and diet-induced obese (DIO) mice. Chorda tympani (CT) nerve responses of lean mice to sweet compounds were increased after administration of leptin antagonist (LA) but not affected by administration of cannabinoid receptor antagonist (AM251). db/db mice showed clear suppression of CT responses to sweet compounds after AM251, increased endocannabinoid levels in the taste organ, and enhanced expression of a biosynthesizing enzyme of endocannabinoids in taste cells. The effect of LA was gradually decreased and that of AM251 was increased during the course of obesity in DIO mice. These findings suggest that circulating leptin, but not local endocannabinoids, is a dominant modulator for sweet taste in lean mice and endocannabinoids become more effective modulators of sweet taste under conditions of deficient leptin signalling.

ABSTRACT

Leptin is an anorexigenic mediator that reduces food intake by acting on hypothalamic receptor Ob-Rb. In contrast, endocannabinoids are orexigenic mediators that act via cannabinoid CB1 receptors in hypothalamus, limbic forebrain, and brainstem. In the peripheral taste system, leptin administration selectively inhibits behavioural, taste nerve and taste cell responses to sweet compounds. Opposing the action of leptin, endocannabinoids enhance sweet taste responses. However, potential roles of endogenous leptin and endocannabinoids in sweet taste remain unclear. Here, we used pharmacological antagonists (Ob-Rb: L39A/D40A/F41A (LA), CB1 : AM251) and examined the effects of their blocking activation of endogenous leptin and endocannabinoid signalling on taste responses in lean control, leptin receptor deficient db/db, and diet-induced obese (DIO) mice. Lean mice exhibited significant increases in chorda tympani (CT) nerve responses to sweet compounds after LA administration, while they showed no significant changes in CT responses after AM251. In contrast, db/db mice showed clear suppression of CT responses to sweet compounds after AM251, increased endocannabinoid (2-arachidonoyl-sn-glycerol (2-AG)) levels in the taste organ, and enhanced expression of a biosynthesizing enzyme (diacylglycerol lipase α (DAGLα)) of 2-AG in taste cells. In DIO mice, the LA effect was gradually decreased and the AM251 effect was increased during the course of obesity. Taken together, our results suggest that circulating leptin, but not local endocannabinoids, may be a dominant modulator for sweet taste in lean mice; however, endocannabinoids may become more effective modulators of sweet taste under conditions of deficient leptin signalling, possibly due to increased production of endocannabinoids in taste tissue.

A novel leptin antagonist peptide inhibits breast cancer growth in vitro and in vivo

The role of the obesity cytokine leptin in breast cancer progression has raised interest in interfering with leptin's actions as a valuable therapeutic strategy. Leptin interacts with its receptor through three different binding sites: I–III. Site I is crucial for the formation of an active leptin–leptin receptor complex and in its subsequent activation. Amino acids 39-42 (Leu-Asp-Phe-Ile- LDFI) were shown to contribute to leptin binding site I and their mutations in alanine resulted in muteins acting as typical antagonists. We synthesized a small peptide based on the wild-type sequence of leptin binding site I (LDFI) and evaluated its efficacy in antagonizing leptin actions in breast cancer using in vitro and in vivo experimental models. The peptide LDFI abolished the leptin-induced anchorage-dependent and -independent growth as well as the migration of ERα-positive (MCF-7) and -negative (SKBR3) breast cancer cells. These results were well correlated with a reduction in the phosphorylation levels of leptin downstream effectors, as JAK2/STAT3/AKT/MAPK. Importantly, the peptide LDFI reversed the leptin-mediated up-regulation of its gene expression, as an additional mechanism able to enhance the peptide antagonistic activity. The described effects were specific for leptin signalling, since the developed peptide was not able to antagonize the other growth factors' actions on signalling activation, proliferation and migration. Finally, we showed that the LDFI pegylated peptide markedly reduced breast tumour growth in xenograft models. The unmodified peptide LDFI acting as a full leptin antagonist could become an attractive option for breast cancer treatment, especially in obese women.

The Multifaceted Mechanism of Leptin Signaling within Tumor Microenvironment in Driving Breast Cancer Growth and Progression

Adipokines represent likely candidates to mediate the increased breast cancer risk and the enhanced progression associated with obesity. Other contributors to obesity-related cancer progression are insulin/IGF-1 pathways and hormones. Among these, the adipokine leptin is the most intensively studied in both metabolism in general and in cancer due to the fact that leptin levels increase in proportion of fat mass. Leptin is primarily synthesized from adipocytes but it is also produced by other cells including fibroblasts. In this latter case, it has been well demonstrated how cancer-associated fibroblasts express leptin receptor and secrete leptin, which sustains a short autocrine loop and is able to target tumor epithelial cells enhancing breast cancer cell motility and invasiveness. In addition, it has been reported that leptin may induce breast cancer to undergo a transition from epithelial to spindle-like mesenchymal morphology, activating the signaling pathways devoted to the EMT. Thus, it emerges how leptin may play a crucial role in mediating malignant cell and tumor microenvironment interactions. Here, we present an overview of the role of leptin in breast cancer, covering the following topics: (1) leptin as an amplifier of estrogen signaling in tumor epithelial cells contributing to the promotion of carcinogenesis; (2) leptin as a crucial player in mediating tumor-stroma interaction and influencing EMT-linked mechanisms, that may sustain breast cancer growth and progression; (3) leptin and leptin receptor targeting as novel therapeutic strategies for breast cancer treatment.

Pluronic modified leptin with increased systemic circulation, brain uptake and efficacy for treatment of obesity

Modification of hydrophilic proteins with amphiphilic block copolymers capable of crossing cell membranes is a new strategy to improve protein delivery to the brain. Leptin, a candidate for the treatment of epidemic obesity, has failed in part because of impairment in its transport across the blood-brain barrier (BBB) that develops with obesity. We posit that modification of leptin with poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide), Pluronic P85 (P85) might permit this protein to penetrate the BBB independently of its transporter, thereby overcoming peripheral leptin resistance. Here we report that peripherally administered leptin-P85 conjugates exhibit biological activity by reducing food intake in mouse models of obesity (ob/ob, and diet-induced obese mouse). We further generated two new leptin-P85 conjugates: one, Lep(ss)-P85(L), containing one P85 chain and another, Lep(ss)-P85(H), containing multiple P85 chains. We report data on their purification, analytical characterization, peripheral and brain pharmacokinetics (PK). Lep(ss)-P85(L) crosses the BBB using the leptin transporter, and exhibits improved peripheral PK along with increased accumulation in the brain compared to unmodified leptin. Lep(ss)-P85(H) also has improved peripheral PK but in a striking difference to the first conjugate penetrates the BBB independently of the leptin transporter via a non-saturable mechanism. The results demonstrate that leptin analogs can be developed through chemical modification of the native leptin with P85 to overcome leptin resistance at the level of the BBB, thus improving the potential for the treatment of obesity.

Leptin-induced endothelium-dependent vasorelaxation of peripheral arteries in lean and obese rats: role of nitric oxide and hydrogen sulfide

Adipose tissue hormone leptin induces endothelium-dependent vasorelaxation mediated by nitric oxide (NO) and endothelium-derived hyperpolarizing factors (EDHF). Previously it has been demonstrated that in short-term obesity the NO-dependent and the EDHF-dependent components of vascular effect of leptin are impaired and up-regulated, respectively. Herein we examined the mechanism of the EDHF-dependent vasodilatory effect of leptin and tested the hypothesis that alterations of acute vascular effects of leptin in obesity are accounted for by chronic hyperleptinemia. The study was performed in 5 groups of rats: (1) control, (2) treated with exogenous leptin for 1 week to induce hyperleptinemia, (3) obese, fed highly-palatable diet for 4 weeks, (4) obese treated with pegylated superactive rat leptin receptor antagonist (PEG-SRLA) for 1 week, (5) fed standard chow and treated with PEG-SRLA. Acute effect of leptin on isometric tension of mesenteric artery segments was measured ex vivo. Leptin relaxed phenylephrine-preconstricted vascular segments in NO- and EDHF-dependent manner. The NO-dependent component was impaired and the EDHF-dependent component was increased in the leptin-treated and obese groups and in the latter group both these effects were abolished by PEG-SRLA. The EDHF-dependent vasodilatory effect of leptin was blocked by either the inhibitor of cystathionine γ-lyase, propargylglycine, or a hydrogen sulfide (H2S) scavenger, bismuth (III) subsalicylate. The results indicate that NO deficiency is compensated by the up-regulation of EDHF in obese rats and both effects are accounted for by chronic hyperleptinemia. The EDHF-dependent component of leptin-induced vasorelaxation is mediated, at least partially, by H2S.

Effect of peripherally administered leptin antagonist on whole body metabolism and bone microarchitecture and biomechanical properties in the mouse

Leptin's in vivo effect on the rodent skeleton depends on the model used and the mode of administration. Superactive mouse leptin antagonist (SMLA) was produced and then pegylated (PEG) to prolong and enhance its in vivo activity. We blocked leptin signaling by injecting this antagonist peripherally into normal mice at various time points and studied their metabolic and skeletal phenotypes. Subcutaneous PEG-SMLA injections into 4-wk-old female C57BL/6J mice increased weight gain and food consumption significantly after only 1 mo, and the effect lasted for the 3 mo of the experiment, proving its central inhibiting activity. Mice showed a significant increase in serum glucose, cholesterol, triglycerides, insulin, and HOMA-IR throughout the experiment. Quantification of gene expression in "metabolic" tissues also indicated the development of insulin resistance. Bone analyses revealed a significant increase in trabecular and cortical parameters measured in both the lumbar vertebrae and tibiae in PEG-SMLA-treated mice in the 1st and 3rd months as well as a significant increase in tibia biomechanical parameters. Interestingly, 30 days of treatment with the antagonist in older mice (aged 3 and 6 mo) affected body weight and eating behavior, just as they had in the 1-mo-old mice, but had no effect on bone parameters, suggesting that leptin's effect on bones, either directly or through its obesogenic effect, is dependent upon stage of skeletal development. This potent and reversible antagonist enabled us to study leptin's in vivo role in whole body and bone metabolism and holds potential for future therapeutic use in diseases involving leptin signaling.

The emerging role of leptin antagonist as potential therapeutic option for inflammatory bowel disease

Inflammatory bowel disease (IBD) is a chronic relapsing immune-mediated inflammatory disorder that affects millions of people around the world. Leptin is a satiety hormone produced primarily by adipose tissue and acts both centrally and peripherally. Leptin has been shown to play a major role in regulating metabolism, which increases during IBD progression. Leptin mediates several physiological functions including elevated blood pressure, tumorogenesis, cardiovascular pathologies and enhanced immune response in many autoimmune diseases. Recent development of a leptin mutant antagonist that blocks leptin activity raises great hope and opens up new possibilities for therapy in many autoimmune diseases including IBD. To this end, preliminary data from an ongoing study in our laboratory on pegylated leptin antagonist mutant L39A/D40A/F41A (PEG-MLA) treatment shows an inhibition of chronic colitis in IL-10-/- mice. PEG-MLA effectively attenuates the overall clinical scores, reverses colitis-associated pathogenesis including a decrease in body weight, and decreases systemic leptin level. PEG-MLA induces both central and peripheral leptin deficiency by mediating the cellular immune response. In summary, after blocking leptin activity, the correlative outcome between leptin-mediated cellular immune response, systemic leptin levels, and amount of adipose tissue together may provide new strategies for therapeutic intervention in autoimmune diseases, especially for intestinal inflammation.

Early leptin blockade predisposes fat-fed rats to overweight and modifies hypothalamic microRNAs

Perinatal leptin impairment has long-term consequences on energy homeostasis leading to body weight gain. The underlying mechanisms are still not clearly established. We aimed to analyze the long-term effects of early leptin blockade. In this study, newborn rats received daily injection of a pegylated rat leptin antagonist (pRLA) or saline from day 2 (d2) to d13 and then body weight gain, insulin/leptin sensitivity, and expression profile of microRNAs (miRNAs) at the hypothalamic level were determined at d28, d90, or d153 (following 1 month of high-fat diet (HFD) challenge). We show that pRLA treatment predisposes rats to overweight and promotes leptin/insulin resistance in both hypothalamus and liver at adulthood. pRLA treatment also modifies the hypothalamic miRNA expression profile at d28 leading to the upregulation of 34 miRNAs and the downregulation of four miRNAs. For quantitative RT-PCR confirmation, we show the upregulation of rno-miR-10a at d28 and rno-miR-200a, rno-miR-409-5p, and rno-miR-125a-3p following HFD challenge. Finally, pRLA treatment modifies the expression of genes involved in energy homeostasis control such as UCPs and AdipoRs. In pRLA rat muscle, Ucp2/3 and Adipor1/r2 are upregulated at d90. In liver, pRLA treatment upregulates Adipor1/r2 following HFD challenge. These genes are known to be involved in insulin resistance and type 2 diabetes. In conclusion, we demonstrate that the impairment of leptin action in early life promotes insulin/leptin resistance and modifies the hypothalamic miRNA expression pattern in adulthood, and finally, this study highlights the potential link between hypothalamic miRNA expression pattern and insulin/leptin responsiveness.

Leptin-activity blockers: development and potential use in experimental biology and medicine

The first adipokine, leptin, discovered almost 20 years ago, is secreted into circulation mainly from adipose tissue and acts both centrally and peripherally. Leptin regulates energy metabolism, reproductive function, bone metabolism, and immune response. However in some physiological or pathological situations such as enhancement of undesired immune responses in autoimmune diseases, tumorigenesis, elevated blood pressure, and certain cardiovascular pathologies, leptin activity may be harmful. In this review we screen different approaches to blocking leptin action, in vitro and in vivo. The recent development of superactive leptin muteins exhibiting antagonistic properties, and other leptin-action-blocking peptides, proteins, monoclonal antibodies, and nanobodies, opens new perspectives for their use in research, and eventually, therapy for cachexia, autoimmune disease, cancer, and other pathologies.

Cloning and characterization of leptin in a Perciform fish, the striped bass (Morone saxatilis): control of feeding and regulation by nutritional state

In mammals, leptin is an anorexigenic peptide hormone that regulates energy homeostasis. It is produced predominantly by white adipose tissue and circulates as an endocrine indicator of energy reserves. Teleost leptin has been characterized in a few fish species, but its regulation is not well understood, particularly in response to nutritional status. In this study, we cloned a putative leptin in striped bass (Morone saxatilis) and report the first characterization of leptin in a Perciforme, the largest and most diverse order of fish. The striped bass leptin coding sequence was 65% homologous with pufferfish, 52% with Atlantic salmon, and 46% with human. PCR showed that leptin mRNA was exclusively expressed in the liver, and not adipose or other tissues. The leptin coding sequence of striped bass and the more widely cultured hybrid striped bass variety (HSB; Morone chrysops, white bass×M. saxatilis) were identical. We then evaluated whether the metabolic status of HSB might alter leptin gene expression. Juvenile HSB were subjected to 3weeks feed deprivation followed by 3weeks of refeeding. Quantitative PCR showed that fasting for 3weeks reduced hepatic leptin mRNA levels relative to fed controls. Leptin mRNA levels then increased upon refeeding, albeit levels were not completely restored to those seen in control fish fed throughout the experiment. Intraperitoneal injection of human leptin suppressed appetite in HSB. In as much as hepatic HSB leptin mRNA is regulated by nutritional state and has a corresponding anorexigenic effect, our results suggest that leptin may play a role in energy homeostasis in these advanced Perciformes.

Leptin action in the dorsomedial hypothalamus increases sympathetic tone to brown adipose tissue in spite of systemic leptin resistance

Leptin regulates body weight in mice by decreasing appetite and increasing sympathetic nerve activity (SNA), which increases energy expenditure in interscapular brown adipose tissue (iBAT). Diet-induced obese mice (DIO) are resistant to the anorectic actions of leptin. We evaluated whether leptin still stimulated sympathetic outflow in DIO mice. We measured iBAT temperature as a marker of SNA. We found that obese hyperleptinemic mice have higher iBAT temperature than mice on regular diet. Conversely, obese leptin-deficient ob/ob mice have lower iBAT temperature. Additionally, leptin increased SNA in obese (DIO and ob/ob) and control mice, despite DIO mice being resistant to anorectic action of leptin. We demonstrated that neurons in the dorsomedial hypothalamus (DMH) of DIO mice mediate the thermogenic responses to hyperleptinemia in obese mammals because blockade of leptin receptors in the DMH prevented the thermogenic effects of leptin. Peripheral Melotan II (MTII) injection increased iBAT temperature, but it was blunted by blockade of DMH melanocortin receptors (MC4Rs) by injecting agouti-related peptide (AgRP) directly into the DMH, suggesting a physiological role of the DMH on temperature regulation in animals with normal body weight. Nevertheless, obese mice without a functional melanocortin system (MC4R KO mice) have an increased sympathetic outflow to iBAT compared with their littermates, suggesting that higher leptin levels drive sympathoexcitation to iBAT by a melanocortin-independent pathway. Because the sympathetic nervous system contributes in regulating blood pressure, heart rate, and hepatic glucose production, selective leptin resistance may be a crucial mechanism linking adiposity and metabolic syndrome.

Nonsynonymous natural genetic polymorphisms in the bovine leptin gene affect biochemical and biological characteristics of the mature hormone

Leptin (LEP) is a cytokine-like hormone proven to be involved in diverse biological processes. In livestock, it regulates feed intake, BW homeostasis, and energy balance, among other traits. Natural nonsynonymous genetic polymorphisms in the ovine leptin (oLEP) alter the biochemical and physiological characteristics of its gene products. Here we studied in vitro and in vivo the biochemical and physiological characteristics of recombinant hormones representing the oLEP and bovine leptin (bLEP) reference sequences of wild-type (WT) leptins (GenBank accession No. U84247 and U50365, respectively), oLEP and bLEP recombinant muteins carrying the R4C mutation, and oLEP recombinant hormones carrying the A59V and Q62R mutations, which were detected in bLEP. All proteins were purified to homogeneity as monomers and formed 1:1 molar ratio complexes with the chicken leptin-binding domain (LBD). Surface plasmon resonance experiments revealed that all protein variants exhibit reduced (P < 0.05) affinity to chicken (ch) and human (h) LBD compared with the WT oLEP and bLEP recombinant proteins. The ovine and bovine R4C muteins exhibited significantly (P < 0.05) greater induction of cell proliferation in a Baf/3 cell line bioassay, despite lower affinity toward both hLBD and chLBD. Intra-third cerebral ventricle infusion of oLEP and its 3 muteins in sheep resulted in reduced feed intake. However, the 3 tested muteins had a decreased (P < 0.05) inhibitory effect than the WT LEP. It was concluded that natural genetic polymorphisms in the bLEP are associated with variation in the biochemical and physiological properties of the protein.

Acute disruption of leptin signaling in vivo leads to increased insulin levels and insulin resistance

Leptin, an adipocyte-derived hormone, plays an essential role in the maintenance of normal body weight and energy expenditure, as well as glucose homeostasis. Indeed, leptin-deficient ob/ob mice are obese with profound hyperinsulinemia, insulin resistance, and often hyperglycemia. Interestingly, low doses of exogenous leptin can reverse the hyperinsulinemia and hyperglycemia in these animals without altering body weight. The hyperinsulinemia in ob/ob mice may result directly from the absence of leptin signaling in pancreatic β-cells and, in turn, contribute to both obesity and insulin resistance. Here, we acutely attenuated endogenous leptin signaling in normal mice with a polyethylene glycol (PEG)ylated mouse leptin antagonist (PEG-MLA) to determine the contribution of leptin signaling in the regulation of glucose homeostasis. PEG-MLA was either injected or continuously administered via osmotic minipumps for several days, and various metabolic parameters were assessed. PEG-MLA-treated mice had increased fasting and glucose-stimulated plasma insulin levels, decreased whole-body insulin sensitivity, elevated hepatic glucose production, and impaired insulin-mediated suppression of hepatic glucose production. Moreover, PEG-MLA treatment resulted in increased food intake and increased respiratory quotient without significantly altering energy expenditure or body composition as assessed by the lean:lipid ratio. Our findings indicate that alterations in insulin sensitivity occur before changes in the lean:lipid ratio and energy expenditure during the acute disruption of endogenous leptin signaling.

Biochemical and in vitro biological significance of natural sequence variation in the ovine leptin gene

The hormone leptin is involved in diverse biological processes, including regulation of food intake, body-weight homeostasis and energy balance. Sequence variation in the bovine leptin gene has been found to be associated with variations in carcass fat content and average daily gain, as well as in milk yield, milk somatic cell count and several traits governing reproduction. We sequenced genomic DNA and cDNA samples of individuals from three divergent sheep breeds and revealed synonymous as well as novel non-synonymous allelic variation at the third exon of the ovine leptin gene (oLEP) as compared to the sequence published at Accession No. U84247 (reference sequence). In addition, two alternatively spliced oLEP transcripts were found in the abdominal fat tissue. The biochemical and the in vitro biological significance of the sequence variation in the oLEP was examined by generating recombinant oLEP-protein variants namely: p.Q28del, p.N78S, p.R84Q, p.P99Q, p.V123L and p.R138Q, carrying the corresponding sequence variation. Surface plasmon resonance experiments revealed, in most cases, reduced affinity of the oLEP protein variants examined, to human leptin-binding domain (hLBD), relative to the reference variant, being 0.75, 0.60, 0.60, 0.89, 0.92 and 1.03, respectively. In competitive binding assays between biotinylated oLEP and the recombinant leptin protein variants, p.N78S and p.R84Q variants exhibited the lowest affinity to hLBD (0.18 and 0.41, respectively) as compared to the reference hormone. We then tested the protein variants' ability to induce proliferation in Baf-3 cells stably expressing the long form of the human leptin receptor: significant differences in proliferative activity were only found for p.N78S (1.8-fold higher) and p.R138Q (4.2-fold lower) relative to the reference oLEP variant.

Efficient Expression of Bioactive Human Leptin in Escherichia coli in Soluble Fusion Form

Leptin, a 16 kDa nonglycosylated hormone, is produced by mature adipocytes and functions primarily in the hypothalamus to reduce food intake and body weight. To explore a new approach for high-level expression of human Leptin in Escherichia coli, the human Leptin gene, synthesized according to the published sequence, was cloned into the vector pET32a to construct a fusion expression plasmid: Trx-Leptin/pET32a. Our data showed that more than 40% of the fusion protein Trx-Leptin was expressed in soluble form. After purified by Ni-IDA affinity chromatography, cleaved by enterokinase and applied Ni-IDA affinity chromatography again, purified Leptin with homogeneity over 96% was achieved. The bio-functional experiments of purified Leptin showed a significant reduction in food intake and body weight of female mice treated with Leptin by comparing with control mice, and it indicated that the purified Leptin has full biological activity. In addition, our expression system was a very low-cost and efficient prokaryotic expression system. So taken together, our results demonstrated that our expression system of bio-active Leptin provided a new method for producing Leptin in big scale and would be widely applied in commercial Leptin producing industries.

Chronic central leptin infusion differently modulates brain and liver insulin signaling

Recent studies reported the impact of leptin on peripheral insulin sensitivity and glucose utilization. However, little is known concerning the effect of central leptin on hypothalamic and hepatic insulin efficiency. This study aimed to determine the consequence of chronic intra-cerebroventricular (ICV) leptin or murine leptin antagonist (MLA) infusion on hypothalamic and hepatic insulin signaling pathways, in rats. A 2-week central leptin infusion enhanced insulin-dependent Akt phosphorylation in the liver without changing PTP-1B protein expression, associated to insulin receptor (IR) upregulation and reduced IRS-1 phosphorylation on Ser302 residue. In the hypothalamus, a chronic ICV leptin infusion induced PTP-1B associated with a specific decrease in insulin-dependent Akt phosphorylation. In contrast, a chronic MLA infusion did not alter IR and PTP-1B expressions in hypothalamus and liver. Our results underline a brain leptin-dependent increase in hepatic insulin efficiency as mirrored by IR up-regulation, increased insulin-dependent Akt phosphorylation and reduced IRS-1 phosphorylation on Ser302 residue.

Unexpected long-term protection of adult offspring born to high-fat fed dams against obesity induced by a sucrose-rich diet

Background

Metabolic and endocrine environment during early life is crucial for metabolic imprinting. When dams were fed a high fat diet (HF diet), rat offspring developed hypothalamic leptin resistance with lean phenotype when weaned on a normal diet. Interestingly, when grown on the HF diet, they appeared to be protected against the effects of HF diet as compared to offspring of normally fed dams. The mechanisms involved in the protective effect of maternal HF diet are unclear.

Methodology/Principal Findings

We thus investigated the impact of maternal high fat diet on offspring subjected to normal or high palatable diet (P diet) on metabolic and endocrine parameters. We compared offspring born to dams fed P or HF diet. Offspring born to dams fed control or P diet, when fed P diet exhibited a higher body weight, altered hypothalamic leptin sensitivity and metabolic parameters suggesting that maternal P diet has no protective effect on offspring. Whereas, maternal HF diet reduces body weight gain and circulating triglycerides, and ameliorates corpulence index of offspring, even when subjected to P diet. Interestingly, this protective effect is differently expressed in male and female offspring. Male offspring exhibited higher energy expenditure as mirrored by increased hypothalamic UCP-2 and liver AdipoR1/R2 expression, and a profound change in the arcuate nucleus astrocytic organization. In female offspring, the most striking impact of maternal HF diet is the reduced hypothalamic expression of NPY and POMC.

Conclusions/Significance

HF diet given during gestation and lactation protects, at least partially, offspring from excessive weight gain through several mechanisms depending upon gender including changes in arcuate nucleus astrocytic organization and increased hypothalamic UCP-2 and liver AdipoR1/2 expression in males and reduced hypothalamic expression of NPY and POMC in females. Taken together our results reveal new mechanisms involved in the protective effect of maternal HF diet.

Basal, endogenous leptin is metabolically active in newborn rat pups

OBJECTIVE

The adipocyte-derived hormone leptin regulates food intake and body weight via the activation of JAK-STAT pathway in mammalian adult hypothalamic neurons. To investigate whether endogenous leptin is metabolically active in newborn rat pups, the JAK-STAT leptin signaling pathway was analyzed following leptin antagonist challenge.

METHODS

One day old male control pups were injected with either (i) saline, (ii) leptin (10 μg/g, s.c; n=4), (iii) pegylated leptin antagonist (PEG-MLA, 20 μg/g, s.c, n=4), or (iv) leptin plus PEG-MLA. Hypothalamus was dissected from individual pups at 30, 45, and 60 min. Protein expression of ObR, STAT3, pSTAT3, and SOCS3 was analyzed by Western blot.

RESULTS

Leptin, but not PEG-MLA, produced a significant increase in hypothalamic pSTAT3 relative to saline treatment. Systemically administered PEG-MLA effectively blocks leptin signal induction of hypothalamic JAK-STAT signaling. The presence of PEG-MLA in combination with leptin attenuated the leptin-induced increase in pSTAT3.

CONCLUSIONS

Thus, basal leptin levels are metabolically active in the newborn rats. These results brings new insights in considering the importance of endogenous leptin at birth, especially in low birth weight offspring who may be predisposed to altered neurogenesis and later obesity, and provide potential therapeutic strategies for programmed or diet-induced obesity.

Development and characterization of high affinity leptins and leptin antagonists

Leptin is a pleiotropic hormone acting both centrally and peripherally. It participates in a variety of biological processes, including energy metabolism, reproduction, and modulation of the immune response. So far, structural elements affecting leptin binding to its receptor remain unknown. We employed random mutagenesis of leptin, followed by selection of high affinity mutants by yeast surface display and discovered that replacing residue Asp-23 with a non-negatively charged amino acid leads to dramatically enhanced affinity of leptin for its soluble receptor. Rational mutagenesis of Asp-23 revealed the D23L substitution to be most effective. Coupling the Asp-23 mutation with alanine mutagenesis of three amino acids (L39A/D40A/F41A) previously reported to convert leptin into antagonist resulted in potent antagonistic activity. These novel superactive mouse and human leptin antagonists (D23L/L39A/D40A/F41A), termed SMLA and SHLA, respectively, exhibited over 60-fold increased binding to leptin receptor and 14-fold higher antagonistic activity in vitro relative to the L39A/D40A/F41A mutants. To prolong and enhance in vivo activity, SMLA and SHLA were monopegylated mainly at the N terminus. Administration of the pegylated SMLA to mice resulted in a remarkably rapid, significant, and reversible 27-fold more potent increase in body weight (as compared with pegylated mouse leptin antagonist), because of increased food consumption. Thus, recognition and mutagenesis of Asp-23 enabled construction of novel compounds that induce potent and reversible central and peripheral leptin deficiency. In addition to enhancing our understanding of leptin interactions with its receptor, these antagonists enable in vivo study of the role of leptin in metabolic and immune processes and hold potential for future therapeutic use in disease pathologies involving leptin.

Leptin-based therapeutics

Leptin, a pluripotent adipokine, has been discovered as a hormone controlling energy balance in hypothalamic neuroendocrine centers. In addition, recent studies provided ample evidence that leptin can be produced by cells other than adipocytes, and that the hormone can regulate many physiological processes other than energy balance and appetite. In this context, it is not surprising that both leptin excess as well as leptin insufficiency have been implicated in various pathologies. Consequently, despite initially disappointing results with recombinant leptin as the drug for obesity management, new leptin receptor modifiers have been developed and emerged as potential treatment modalities for numerous metabolic, immunological and neoplastic diseases. The major focus of this paper is a systematic review of current experimental leptin-based therapies, including pharmacological advantages and limitations of each prodrug category.

Leptin up-regulates MUC5B expression in human airway epithelial cells via mitogen-activated protein kinase pathway

Leptin, an adipocyte-secreted hormone that regulates food intake and metabolic response, has been recently reported to increase in the serum during inflammatory airway disease associated with mucus-hypersecretion. We investigated the effects of leptin on mucin expression in human airway epithelial cells and the signaling pathways. The expression of the leptin receptor was evaluated in human nasal mucosa and NCI-H292 cells. Leptin-induced expression of major respiratory mucins in NCI-H292 cells was analyzed. Mutant leptin, which acts as a receptor antagonist, and specific inhibitors of extracellular signal-regulated kinase (ERK1/2), p38 and Janus kinase-2 (JAK2)/signal transducer and activator of transcription-3 (STAT3) were used. Leptin receptors were expressed in the nasal mucosa and NCI-H292 cells. Treatment with leptin significantly increased the expression of MUC5AC and MUC5B in NCI-H292 cells; these effects were blocked by mutant leptin. The cells activated by leptin showed increased ERK1/2, p38, and STAT3 phosphorylation. Leptin-induced MUC5B expression was blocked by the ERK1/2 and p38 pathway inhibitors, but not by the JAK2/STAT3 pathway inhibitor. Leptin might significantly contribute to the production of major gel-forming mucins by direct stimulation of airway epithelial cells and the activation of leptin receptors coupled with the activation of ERK1/2 or p38, but not the JAK2/STAT3 pathway.

Structural and functional studies of leptins from hibernating and non-hibernating bats

Leptin, a 16-kDa hormone produced by mature adipocytes, has been shown to regulate the hibernation of mammals. In this study, the leptin gene sequences of both hibernating (Rhinolophus ferrumequinum) and non-hibernating (Rousettus leschenaultii) bats were determined, and the leptin proteins from these two different species of bats were expressed in Escherichia coli for the first time. Results showed that the amino acid sequence of the leptin protein from hibernating bats had a lower degree of identity than that from non-hibernating bats to those of several non-hibernating mammals. The leptin protein of hibernating bats had a stronger growth inhibitory effect on 3T3-L1 cells than that of non-hibernating bats. Structural modeling revealed that the structures of the receptor binding site III, which is critical for signal transduction, of the two bat leptins were very different. Similar to the human leptin, the leptin protein of non-hibernating bats was predicted to have a random loop, whereas that of hibernating bats had a helical structure in this region. This observation provided a clue as to the differential effects of the two different leptins on 3T3-L1 cells.

Luminal leptin inhibits L-glutamine transport in rat small intestine: involvement of ASCT2 and B0AT1

L-glutamine is the primary metabolic fuel for enterocytes. Glutamine from the diet is transported into the absorptive cells by two sodium-dependent neutral amino acid transporters present at the apical membrane: ASCT2/SLC1A5 and B(0)AT1/SLC6A19. We have demonstrated that leptin is secreted into the stomach lumen after a meal and modulates the transport of sugars after binding to its receptors located at the brush border of the enterocytes. The present study was designed to address the effect of luminal leptin on Na(+)-dependent glutamine (Gln) transport in rat intestine and identify the transporters involved. We found that 0.2 nM leptin inhibited uptake of Gln and phenylalanine (Phe) (substrate of B(0)AT1) using everted intestinal rings. In Ussing chambers, 10 mM Gln absorption followed as Na(+)-induced short-circuit current was inhibited by leptin in a dose-dependent manner (maximum inhibition at 10 nM; I(C50) = approximately 0.1 nM). Phe absorption was also decreased by leptin. Western blot analysis after 3-min incubation of the intestinal loops with 10 mM Gln, showed marked increase of ASCT2 and B(0)AT1 protein in the brush-border membrane that was reduced by rapid preincubation of the intestinal lumen with 1 nM leptin. Similarly, the increase in ASCT2 and B(0)AT1 gene expression induced by 60-min incubation of the intestine with 10 mM Gln was strongly reduced after a short preincubation period with leptin. Altogether these data demonstrate that, in rat, leptin controls the active Gln entry through reduction of both B(0)AT1 and ASCT2 proteins traffic to the apical plasma membrane and modulation of their gene expression.

Leptin as a potential therapeutic target for breast cancer prevention and treatment

IMPORTANCE OF THE FIELD

Obesity is considered to be an important risk factor for postmenopausal breast cancer. Elevated estrogen levels are thought to be a growth factor associated with this relationship. However, there is increasing evidence that factors produced directly in adipose tissue, adipokines, can also affect breast cancer development. Leptin is one of the adipokines that is measured in serum/plasma in increasing amounts as body weight/body fat increases.

AREAS COVERED IN THIS REVIEW

We highlight important aspects of leptin in relationship to mammary/breast tumor development. This includes findings from human, in vitro and animal studies. Information on leptin-related compounds which may have therapeutic use is presented. Additionally strategies to alter serum leptin levels by dietary and pharmacological interventions are discussed.

WHAT THE READER WILL GAIN

The reader will gain insights into the relationship of an adipose tissue protein and its potential role in breast cancer development as well as ways to intervene in leptin's actions.

TAKE HOME MESSAGE

Continued research will determine if interfering with the action of leptin has preventive or therapeutic applications in breast cancer.

The obese phenotype-inducing N82K mutation in human leptin disrupts receptor-binding and biological activity

A novel homozygous mutation of the leptin gene was recently reported in an Egyptian child and his sister with severe early onset obesity. This mutation results from the substitution of asparagine (AAC) by lysine (AAA) at codon 103 of a non-mature (signal peptide-containing) leptin and corresponds to the N82K mutation in the mature protein. The patient had very low serum leptin levels, raising the question of whether the obese phenotype resulted from low leptin levels or from its lower intrinsic activity. To answer this question, we characterized the functional consequences of the N82K mutation. Wild-type (WT) human leptin was mutated accordingly, expressed in Escherichia coli at high yield, purified to homogeneity as a monomer and compared to WT human leptin prepared by the same methodology. Circular dichroism analysis of the mutated leptin indicated proper refolding and a secondary structure identical to that of the WT human leptin. In contrast to WT human leptin, the N82K mutant did not form a detectable complex with human leptin-binding domain (hLBD) and its binding capacity to hLBD assessed in a nonradioactive receptor-binding assay was at least 500-fold lower than that of WT human leptin. The biological activity of the N82K mutant, tested in two cell bioassays, was reduced by more than three orders of magnitude relative to WT human leptin. Therefore, though the present report does not explain the reason for the low circulating leptin levels it definitely documents that the reported obese phenotype originates not only from low serum leptin levels but also from the N82K mutant's almost total lack of intrinsic leptin activity.

Expression of leptin receptor in nasal polyps: leptin as a mucosecretagogue

OBJECTIVES/HYPOTHESIS

Leptin is a pleiotropic hormone that regulates food intake and metabolic and endocrine functions. Serum leptin levels have been reported to be increased in patients with allergic rhinitis and nasal polyposis; however, the explanation for this is unclear. We aimed to demonstrate the differential expression of leptin receptors in normal human nasal mucosa and nasal polyps, and to elucidate the effects of leptin on mucin gene expression in human nasal polyp epithelial cells.

STUDY DESIGN

Case-control and in vitro study.

METHODS

Normal ethmoid sinus mucosa was obtained from 10 subjects and used as a control; nasal polyps were obtained from 10 patients. Leptin receptor expression was analyzed using reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blot analysis. Leptin-induced expression of major respiratory mucins (MUC5AC and MUC5B) in the human nasal polyp epithelial cells was determined using RT-PCR and enzyme-linked immunosorbent assay.

RESULTS

The leptin receptor expression was stronger in the nasal polyps than in the normal nasal mucosa. In human nasal polyp epithelial cells, leptin increased the expression of MUC5AC and MUC5B, in a dose- and time-dependent manner, at the gene and protein levels. Leptin-induced mucin expression was inhibited by the leptin receptor antagonist.

CONCLUSIONS

The increased expression of leptin receptors in nasal polyps implies leptin has a certain role in nasal polyposis. In addition, leptin appears to induce the expression of MUC5AC and MUC5B through leptin receptors in the human nasal polyp epithelial cells.

Positive regulatory control loop between gut leptin and intestinal GLUT2/GLUT5 transporters links to hepatic metabolic functions in rodents

Background and Aims

The small intestine is the major site of absorption of dietary sugars. The rate at which they enter and exit the intestine has a major effect on blood glucose homeostasis. In this study, we determine the effects of luminal leptin on activity/expression of GLUT2 and GLUT5 transporters in response to sugars intake and analyse their physiological consequences.

Methodology

Wistar rats, wild type and AMPKα₂^−/− mice were used. In vitro and in vivo isolated jejunal loops were used to quantify transport of fructose and galactose in the absence and the presence of leptin. The effects of fructose and galactose on gastric leptin release were determined. The effects of leptin given orally without or with fructose were determined on the expression of GLUT2/5, on some gluconeogenesis and lipogenic enzymes in the intestine and the liver.

Principal Findings

First, in vitro luminal leptin activating its receptors coupled to PKCβII and AMPKα, increased insertion of GLUT2/5 into the brush-border membrane leading to enhanced galactose and fructose transport. Second in vivo, oral fructose but not galactose induced in mice a rapid and potent release of gastric leptin in gastric juice without significant changes in plasma leptin levels. Moreover, leptin given orally at a dose reproducing comparable levels to those induced by fructose, stimulated GLUT5-fructose transport, and potentiated fructose-induced: i) increase in blood glucose and mRNA levels of key gluconeogenesis enzymes; ii) increase in blood triglycerides and reduction of mRNA levels of intestinal and hepatic Fasting-induced adipocyte factor (Fiaf) and iii) increase in SREBP-1c, ACC-1, FAS mRNA levels and dephosphorylation/activation of ACC-1 in liver.

Conclusion/Significance

These data identify for the first time a positive regulatory control loop between gut leptin and fructose in which fructose triggers release of gastric leptin which, in turn, up-regulates GLUT5 and concurrently modulates metabolic functions in the liver. This loop appears to be a new mechanism (possibly pathogenic) by which fructose consumption rapidly becomes highly lipogenic and deleterious.

Competitive inhibition of leptin signaling results in amelioration of liver fibrosis through modulation of stellate cell function

Leptin signaling is involved in T-cell polarization and is required for profibrotic function of hepatic stellate cells (HSCs). Leptin-deficient ob/ob mice do not develop liver fibrosis despite the presence of severe long-standing steatohepatitis. Here, we blocked leptin signaling with our recently generated mouse leptin antagonist (MLA), and examined the effects on chronic liver fibrosis in vivo using the chronic thioacetamide (TAA) fibrosis model, and in vitro using freshly-isolated primary HSCs. In the chronic TAA fibrosis model, leptin administration was associated with significantly enhanced liver disease and a 100% 5-week to 8-week mortality rate, while administration or coadministration of MLA markedly improved survival, attenuated liver fibrosis, and reduced interferon gamma (IFN-gamma) levels. No significant changes in weight, serum cholesterol, or triglycerides were noted. In vitro administration of rat leptin antagonist (RLA), either alone or with leptin, to rat primary HSCs reduced leptin-stimulated effects such as increased expression of alpha-smooth muscle actin (alpha-SMA), and activation of alpha1 procollagen promoter.

CONCLUSION

Inhibition of leptin-enhanced hepatic fibrosis may hold promise as a future antifibrotic therapeutic modality.

A simple novel method for the preparation of noncovalent homodimeric, biologically active human interleukin 10 in Escherichia coli-enhancing protein expression by degenerate PCR of 5' DNA in the open reading frame

DNA inserts encoding human interleukin 10 (hIL-10), optimized for codon usage and secondary RNA structure, were purchased from several commercial sources and subcloned into a pMon vector. Despite the optimization, protein expression was nil. We therefore subjected the 5' segment of the cDNA encoding N-terminal amino acids 2-11 to degenerate PCR in order to create a small library of 130K theoretical cDNA combinations that would not change the respective amino acid sequence and tested their expression. After screening over 320 colonies 10 hIL-10 clones encoding the original amino acid sequence were identified. Three nucleotide substitutions were sufficient to ensure reasonable protein expression. Subsequently, hIL-10 was expressed in Escherichia coli, refolded and purified to homogeneity, yielding over 95% electrophoretically pure noncovalent homodimeric protein, which was biologically active in MC/9 cells. The yield of recombinant hIL-10 from 10L of fermentation culture was 60mg and a protocol for its long-term storage as a carrier-free lyophilized powder at -20 degrees was developed.

Purification and characterization of recombinant pufferfish (Takifugu rubripes) leptin

Synthetic cDNA encoding pufferfish (Takifugu rubripes) leptin (pfLEP) was prepared according to the published sequence. The pfLEP, transformed into Escherichia coli and expressed upon induction with nalidixic acid, was found almost entirely in the insoluble inclusion bodies (IBs). The proteins were solubilized, refolded and purified to homogeneity by anion-exchange chromatography and gel-filtration. The respective yield of dimers and monomers was 50-100mg from 5L of fermentation culture. Circular dichroism analyses revealed similarity of the purified pfLEP secondary structure to that of mammalian leptins. The purified monomers and dimers showed a single band of approximately 15 kDa following SDS-PAGE in the presence of reducing agent, whereas the dimer showed one band of approximately 30 kDa in the absence of reducing agent, indicating its formation by S-S bonds. The purified product also showed a single peak following gel-filtration under nondenaturating conditions and reverse-phase chromatography. Monomeric and dimeric pfLEPs were stable for at least 6 months in sterile solution frozen at -20 degrees C or as lyophilized powder. Both pfLEPs were biologically active in promoting proliferation of BAF/3 cells stably transfected with the long form of human leptin (hLEP) receptor, but their activity was four to five orders of magnitude lower than that of hLEP. The specificity of this activity was further evidenced by its complete inhibition by hLEP antagonist. In contrast to mammalian leptins, neither form of pfLEP bound to or formed 1:1 complex with chicken leptin-binding domain, likely due to low affinity. No specific binding of either ovine or pufferfish leptins to tilapia liver membranes was detected. This work is the first report on the purification of leptin from any fish species.

Leptin directly controls proliferation, apoptosis and secretory activity of cultured chicken ovarian cells

The aim of our in-vitro experiments was to examine, whether leptin can directly control functions of avian ovarian cells and to outline potential intracellular mediators of its effects. Granulosa cells or fragments of ovarian follicular wall were cultured with leptin (0, 1, 10 or 100 ng/mL medium). The expression of peptides involved in apoptosis (TdT, bax, its binding protein, bcl-2, ASK-1 and p53), cell cycle-related peptides (PCNA and cyclin B1), release of hormones (progesterone, testosterone, estradiol, arginine-vasotocin), as well as the expression of protein kinases (PKA, MAPK/ERK1,2 and CDK/p34) in the ovarian cells were examined by using immunocytochemistry, TUNEL, SDS-PAGE-Western immunoblotting, EIA and RIA. It was found that leptin inhibited expression of all markers of cytoplasmic apoptosis (bax, ASK-1 and p53), stimulated expression of anti-apoptotic peptide bcl-2, but did not affect nuclear DNA fragmentation (TdT). Furthermore, leptin inhibited expression of PCNA (marker of S-phase of mitosis), but not of cyclin B1 (marker of G phase of cell cycle). Moreover, it promoted release of progesterone and estradiol, suppressed release of testosterone, but did not affect arginine-vasotocin. Finally, leptin inhibited expression of MAPK/ERK1,2 and CDK/p34 and stimulated expression of PKA. The present observations demonstrate that leptin can directly control basic chicken ovarian functions - inhibit cytoplasmic apoptosis and proliferation (S-phase, but not G-phases of mitosis), regulate secretory activity (release of steroids, but not nonapeptide hormone) and expression of MAPK, PKA and CDC2, which might be potential intracellular mediators of leptin action.

Leptin analog antagonizes leptin effects on food intake and body weight but mimics leptin-induced vagal afferent activation

A recombinantly produced murine leptin analog (MLA) antagonizes leptin-induced signaling in cell lines that express the long form of the leptin receptor. However, the effects of MLA on the activity of leptin-sensitive neurons and on central neural controls of food intake have not been reported. Here we report effects of MLA on food intake and body weight in adult rats and on the activity of cultured rat vagal afferent neurons. Daily intracerebroventricular coinjection of MLA with exogenous leptin significantly attenuated leptin-induced reduction of 48-h food intake and body weight. Coinjection of MLA with leptin also reduced leptin-induced phosphorylation of signal transducer and activator of transcription 3 (STAT3) in the hypothalamus. In addition, chronic intracerebroventricular MLA infusion over 14 d via osmotic minipumps significantly increased daily food intake, rate of body weight gain, fat-pad mass, and circulating plasma leptin concentrations. Surprisingly, however, MLA did not antagonize leptin-evoked increases in cytosolic calcium concentrations in vagal afferent neurons in primary culture. Rather, MLA itself produced acute activation selectively in leptin-responsive vagal afferent neurons. These data suggest that MLA is an antagonist for the central effects of leptin on food intake and body weight but an agonist at sites where leptin induces acute neuronal activation. This mixed antagonist/agonist action suggests either 1) that the coupling of a single leptin receptor (ObRb) to acute activation of neurons occurs by a signaling mechanism different from those that mediate centrally evoked reductions in food intake and body weight or 2) that acute neuronal activation and centrally induced reductions of food intake and body weight are mediated by different leptin receptor subtypes.

Preparation of Leptin Antagonists by Site-Directed Mutagenesis of Human, Ovine, Rat, and Mouse Leptin's Site III

Six muteins of human, ovine, rat, and mouse leptins mutated to Ala in amino acids 39-41 or 39-42 were prepared by site-directed mutagenesis of the putative site III, which does not affect binding but is necessary for receptor activation, then expressed, solubilized in 4.5 M urea, at pH 11.3 in presence of cysteine, refolded and purified to homogeneity by anion-exchange chromatography on Q-Sepharose or combination of anion-exchange chromatography followed by gel filtration. The overall yields were 400-800 mg from 5 L of fermentation. All proteins were >98% pure as evidenced by SDS-PAGE and contained at least 95% monomers as documented by gel-filtration chromatography under nondenaturing conditions. Circular dichroism analysis revealed that all six muteins have identical secondary structure characteristic of nonmutated leptins, namely 52-63% of alpha helix content. All muteins formed a 1:1 complex with chicken leptin binding domain, (chLBD) and bound chLBD or membrane-embedded leptin receptor with affinity identical to WT leptins. Muteins were devoid of any biological activity in several bioassays but were potent competitive antagonists. Some muteins were pegylated using 40 kDa PEG. Although pegylation decreased the in vitro activity, increasing circulation half-life can recompensate this deficit, so pegylated antagonists are expected to be more potent in vivo.

Development of leptin antagonists and their potential use in experimental biology and medicine

Leptin is a pleotropic hormone that acts both centrally and peripherally. Whereas leptin exhibits positive effects on several physiological functions, such as regulation of energy metabolism, reproductive function and immune responses, negative actions, such as enhancement of undesired immune responses in autoimmune diseases, tumorigenesis, elevated blood pressure and cardiovascular pathologies, have also been documented. The recent development of leptin muteins with antagonistic properties and other proteins that block leptin activity opens up new possibilities for their use in research and, eventually, therapy.