Transgenic complementation of leptin receptor deficiency. II. Increased leptin receptor transgene dose effects on obesity/diabetes and fertility/lactation in lepr-db/db mice

Conditional knockout of leptin receptor in the female reproductive tract reduces fertility due to parturition defects in mice

Leptin is required for fertility, including initiation of estrous cycles. It is therefore challenging to assess the role of leptin signaling during pregnancy. While neuron-specific transgene approaches suggest that leptin signaling in the central nervous system is most important, experiments with pharmacologic inhibition of leptin in the uterus or global replacement of leptin during pregnancy suggest leptin signaling in the reproductive tract may be required. Here, conditional leptin receptor knockout (Lepr cKO) with a progesterone receptor-driven Cre recombinase was used to examine the importance of leptin signaling in pregnancy. Lepr cKO mice have almost no leptin receptor in uterus or cervix, and slightly reduced leptin receptor levels in corpus luteum. Estrous cycles and progesterone concentrations were not affected by Lepr cKO. Numbers of viable embryos did not differ between primiparous control and Lepr cKO dams on days 6.5 and 17.5 of pregnancy, despite a slight reduction in the ratio of embryos to corpora lutea, showing that uterine leptin receptor signaling is not required for embryo implantation. Placentas of Lepr cKO dams had normal weight and structure. However, over four parities, Lepr cKO mice produced 22% fewer live pups than controls, and took more time from pairing to delivery by their fourth parity. Abnormal birth outcomes of either dystocia or dead pups occurred in 33% of Lepr cKO deliveries but zero control deliveries, and the average time to deliver each pup after crouching was significantly increased. Thus, leptin receptor signaling in the reproductive tract is required for normal labor and delivery. Summary sentence. Mice lacking leptin receptor in the reproductive tract produce fewer live pups and have more adverse labor outcomes than controls, but normal numbers of embryos near term, showing that leptin receptor signaling is required for normal parturition.

Essential Role for Hypothalamic Calcitonin Receptor‒Expressing Neurons in the Control of Food Intake by Leptin

The adipocyte-derived hormone leptin acts via its receptor (LepRb) on central nervous system neurons to communicate the repletion of long-term energy stores, to decrease food intake, and to promote energy expenditure. We generated mice that express Cre recombinase from the calcitonin receptor (Calcr) locus (Calcrcre mice) to study Calcr-expressing LepRb (LepRbCalcr) neurons, which reside predominantly in the arcuate nucleus (ARC). Calcrcre-mediated ablation of LepRb in LepRbCalcrknockout (KO) mice caused hyperphagic obesity. Because LepRb-mediated transcriptional control plays a crucial role in leptin action, we used translating ribosome affinity purification followed by RNA sequencing to define the transcriptome of hypothalamic Calcr neurons, along with its alteration in LepRbCalcrKO mice. We found that ARC LepRbCalcr cells include neuropeptide Y (NPY)/agouti-related peptide (AgRP)/γ-aminobutyric acid (GABA) ("NAG") cells as well as non-NAG cells that are distinct from pro-opiomelanocortin cells. Furthermore, although LepRbCalcrKO mice exhibited dysregulated expression of several genes involved in energy balance, neither the expression of Agrp and Npy nor the activity of NAG cells was altered in vivo. Thus, although direct leptin action via LepRbCalcr cells plays an important role in leptin action, our data also suggest that leptin indirectly, as well as directly, regulates these cells.

Role of the adipocyte-derived hormone leptin in reproductive control

Achievement of sexual maturation and maintenance of fertility in adulthood are functions that are sensitive to the metabolic status of the organism, particularly the magnitude of fat reserves. In this sense, the adipocyte-derived hormone, leptin, plays a major role in linking metabolic cues and the control of multiple neuroendocrine axes. The hypothalamus is a key site mediating leptin actions, including those involved in the modulation of the hypothalamus-pituitary-gonads (HPG) axis at different stages of development and in different environmental conditions. In the present review, we provide an update of the role of leptin in reproduction and discuss its interactions with neurons, neurotransmitters and downstream targets of the reproductive axis, with a special emphasis on the actions of leptin in the central nervous system. We hope this review will contribute to the understanding of the mechanisms whereby metabolic signals, especially leptin, influence the reproductive neuroendocrine axis modulating its activity in different nutritional states. Special attention will be given to recent advances in the identification of key hypothalamic sites and signaling pathways relevant to leptin's action in reproductive control.

Distinct networks of leptin- and insulin-sensing neurons regulate thermogenic responses to nutritional and cold challenges

Defense of core body temperature (Tc) can be energetically costly; thus, it is critical that thermoregulatory circuits are modulated by signals of energy availability. Hypothalamic leptin and insulin signals relay information about energy status and are reported to promote thermogenesis, raising the possibility that they interact to direct an appropriate response to nutritional and thermal challenges. To test this idea, we used an Nkx2.1-Cre driver to generate conditional knockouts (KOs) in mice of leptin receptor (L(2.1)KO), insulin receptor (I(2.1)KO), and double KOs of both receptors (D(2.1)KO). L(2.1)KOs are hyperphagic and obese, whereas I(2.1)KOs are similar to controls. D(2.1)KOs exhibit higher body weight and adiposity than L(2.1)KOs, solely due to reduced energy expenditure. At 20-22°C, fed L(2.1)KOs maintain a lower baseline Tc than controls, which is further decreased in D(2.1)KOs. After an overnight fast, some L(2.1)KOs dramatically suppress energy expenditure and enter a torpor-like state; this behavior is markedly enhanced in D(2.1)KOs. When fasted mice are exposed to 4°C, L(2.1)KOs and D(2.1)KOs both mount a robust thermogenic response and rapidly increase Tc. These observations support the idea that neuronal populations that integrate information about energy stores to regulate the defense of Tc set points are distinct from those required to respond to a cold challenge.

Multifaceted leptin network: the molecular connection between obesity and breast cancer

High plasma levels of leptin, a major adipocytokine produced by adipocytes, are correlated with increased fat mass in obese state. Leptin is emerging as a key candidate molecule linking obesity with breast cancer. Acting via endocrine, paracrine, and autocrine manner, leptin impacts various stages of breast tumorigenesis from initiation and primary tumor growth to metastatic progression. Leptin also modulates the tumor microenvironment mainly through supporting migration of endothelial cells, neo-angiogenesis and sustaining recruitment of macrophage and monocytes. Various studies have shown that hyperactive leptin-signaling network leads to concurrent activation of multiple oncogenic pathways resulting in enhanced proliferation, decreased apoptosis, acquisition of mesenchymal phenotype, potentiated migration and enhanced invasion potential of tumor cells. Furthermore, the capability of leptin to interact with other molecular effectors of obese state including, estrogen, IGF-1, insulin, VEGF and inflammatory cytokines further increases its impact on breast tumor progression in obese state. This article presents an overview of the studies investigating the involvement of leptin in breast cancer.

Integral role of PTP1B in adiponectin-mediated inhibition of oncogenic actions of leptin in breast carcinogenesis

The molecular effects of obesity are mediated by alterations in the levels of adipocytokines. High leptin level associated with obese state is a major cause of breast cancer progression and metastasis, whereas adiponectin is considered a "guardian angel adipocytokine" for its protective role against various obesity-related pathogenesis including breast cancer. In the present study, investigating the role of adiponectin as a potential inhibitor of leptin, we show that adiponectin treatment inhibits leptin-induced clonogenicity and anchorage-independent growth. Leptin-stimulated migration and invasion of breast cancer cells is also effectively inhibited by adiponectin. Analyses of the underlying molecular mechanisms reveal that adiponectin suppresses activation of two canonical signaling molecules of leptin signaling axis: extracellular signal-regulated kinase (ERK) and Akt. Pretreatment of breast cancer cells with adiponectin protects against leptin-induced activation of ERK and Akt. Adiponectin increases expression and activity of the physiological inhibitor of leptin signaling, protein tyrosine phosphatase 1B (PTP1B), which is found to be integral to leptin-antagonist function of adiponectin. Inhibition of PTP1B blocks adiponectin-mediated inhibition of leptin-induced breast cancer growth. Our in vivo studies show that adenovirus-mediated adiponectin treatment substantially reduces leptin-induced mammary tumorigenesis in nude mice. Exploring therapeutic strategies, we demonstrate that treatment of breast cancer cells with rosiglitazone results in increased adiponectin expression and inhibition of migration and invasion. Rosiglitazone treatment also inhibits leptin-induced growth of breast cancer cells. Taken together, these data show that adiponectin treatment can inhibit the oncogenic actions of leptin through blocking its downstream signaling molecules and raising adiponectin levels could be a rational therapeutic strategy for breast carcinoma in obese patients with high leptin levels.

Diminished leptin signaling can alter circadian rhythm of metabolic activity and feeding

Leptin, a hormone mainly produced by fat cells, shows cell-specific effects to regulate feeding and metabolic activities. We propose that an important feature of metabolic dysregulation resulting in obesity is the loss of the circadian rhythm of biopotentials. This was tested in the pan-leptin receptor knockout (POKO) mice newly generated in our laboratory. In the POKO mice, leptin no longer induced pSTAT-3 signaling after intracerebroventricular injection. Three basic phenotypes were observed: the heterozygotes had similar weight and adiposity as the wild-type (WT) mice (>60% of the mice); the homozygotes were either fatter (∼30%), or rarely leaner (<5%) than the WT mice. By early adulthood, the POKO mice had higher average body weight and adiposity than their respective same-sex WT littermate controls, and this was consistent among different batches. The homozygote fat POKO showed significant reduction of midline estimating statistic of rhythm of circadian parameters, and shifts of ultradian rhythms. The blunted circadian rhythm of these extremely obese POKO mice was also seen in their physical inactivity, longer feeding bouts, and higher food intake. The extent of obesity correlated with the blunted circadian amplitude, accumulative metabolic and locomotor activities, and the severity of hyperphagia. This contrasts with the heterozygote POKO mice which showed little obesity and metabolic disturbance, and only subtle changes of the circadian rhythm of metabolic activity without alterations in feeding behavior. The results provide a novel aspect of leptin resistance, almost manifesting as an "all or none" phenomenon.

Eicosanoids and adipokines in breast cancer: from molecular mechanisms to clinical considerations

Chronic inflammation is one of the foremost risk factors for different types of malignancies, including breast cancer. Additional risk factors of this pathology in postmenopausal women are weight gain, obesity, estrogen secretion, and an imbalance in the production of adipokines, such as leptin and adiponectin. Various signaling products of transcription factor, nuclear factor-kappaB, in particular inflammatory eicosanoids, reactive oxygen species (ROS), and cytokines, are thought to be involved in chronic inflammation-induced cancer. Together, these key components have an influence on inflammatory reactions in malignant tissue damage when their levels are deregulated endogenously. Prostaglandins (PGs) are well recognized in inflammation and cancer, and they are solely biosynthesized through cyclooxygenases (COXs) from arachidonic acid. Concurrently, ROS give rise to bioactive isoprostanes from arachidonic acid precursors that are also involved in acute and chronic inflammation, but their specific characteristics in breast cancer are less demonstrated. Higher aromatase activity, a cytochrome P-450 enzyme, is intimately connected to tumor growth in the breast through estrogen synthesis, and is interrelated to COXs that catalyze the formation of both inflammatory and anti-inflammatory PGs such as PGE(2), PGF(2α), PGD(2), and PGJ(2) synchronously under the influence of specific mediators and downstream enzymes. Some of the latter compounds upsurge the intracellular cyclic adenosine monophosphate concentration and appear to be associated with estrogen synthesis. This review discusses the role of COX- and ROS-catalyzed eicosanoids and adipokines in breast cancer, and therefore ranges from their molecular mechanisms to clinical aspects to understand the impact of inflammation.

Effects of leptin and melanocortin signaling interactions on pubertal development and reproduction

Leptin and melanocortin signaling control ingestive behavior, energy balance, and substrate utilization, but only leptin signaling defects cause hypothalamic hypogonadism and infertility. Although GnRH neurons do not express leptin receptors, leptin influences GnRH neuron activity via regulation of immediate downstream mediators including the neuropeptides neuropeptide Y and the melanocortin agonist and antagonist, α-MSH, agouti-related peptide, respectively. Here we show that modulation of melanocortin signaling in female db/db mice through ablation of agouti-related peptide, or heterozygosity of melanocortin 4 receptor, restores the timing of pubertal onset, fertility, and lactation. Additionally, melanocortin 4 receptor activation increases action potential firing and induces c-Fos expression in GnRH neurons, providing further evidence that melanocortin signaling influences GnRH neuron activity. These studies thus establish melanocortin signaling as an important component in the leptin-mediated regulation of GnRH neuron activity, initiation of puberty and fertility.

Leptin-induced epithelial-mesenchymal transition in breast cancer cells requires β-catenin activation via Akt/GSK3- and MTA1/Wnt1 protein-dependent pathways

Perturbations in the adipocytokine profile, especially higher levels of leptin, are a major cause of breast tumor progression and metastasis; the underlying mechanisms, however, are not well understood. In particular, it remains elusive whether leptin is involved in epithelial-mesenchymal transition (EMT). Here, we provide molecular evidence that leptin induces breast cancer cells to undergo a transition from epithelial to spindle-like mesenchymal morphology. Investigating the downstream mediator(s) that may direct leptin-induced EMT, we found functional interactions between leptin, metastasis-associated protein 1 (MTA1), and Wnt1 signaling components. Leptin increases accumulation and nuclear translocation of β-catenin leading to increased promoter recruitment. Silencing of β-catenin or treatment with the small molecule inhibitor, ICG-001, inhibits leptin-induced EMT, invasion, and tumorsphere formation. Mechanistically, leptin stimulates phosphorylation of glycogen synthase kinase 3β (GSK3β) via Akt activation resulting in a substantial decrease in the formation of the GSK3β-LKB1-Axin complex that leads to increased accumulation of β-catenin. Leptin treatment also increases Wnt1 expression that contributes to GSK3β phosphorylation. Inhibition of Wnt1 abrogates leptin-stimulated GSK3β phosphorylation. We also discovered that leptin increases the expression of an important modifier of Wnt1 signaling, MTA1, which is integral to leptin-mediated regulation of the Wnt/β-catenin pathway as silencing of MTA1 inhibits leptin-induced Wnt1 expression, GSK3β phosphorylation, and β-catenin activation. Furthermore, analysis of leptin-treated breast tumors shows increased expression of Wnt1, pGSK3β, and vimentin along with higher nuclear accumulation of β-catenin and reduced E-cadherin expression providing in vivo evidence for a previously unrecognized cross-talk between leptin and MTA1/Wnt signaling in epithelial-mesenchymal transition of breast cancer cells.

Obesity and weight loss result in increased adipose tissue ABCG1 expression in db/db mice

The prevalence of obesity has reached epidemic proportions and is associated with several co-morbid conditions including diabetes, dyslipidemia, cancer, atherosclerosis and gallstones. Obesity is associated with low systemic inflammation and an accumulation of adipose tissue macrophages (ATMs) that are thought to modulate insulin resistance. ATMs may also modulate adipocyte metabolism and take up lipids released during adipocyte lipolysis and cell death. We suggest that high levels of free cholesterol residing in adipocytes are released during these processes and contribute to ATM activation and accumulation during obesity and caloric restriction. Db/db mice were studied for extent of adipose tissue inflammation under feeding conditions of ad libitum (AL) and caloric restriction (CR). The major finding was a marked elevation in epididymal adipose ABCG1 mRNA levels with obesity and CR (6-fold and 16-fold, respectively) over that seen for lean wild-type mice. ABCG1 protein was also elevated for CR as compared to AL adipose tissue. ABCG1 is likely produced by cholesterol loaded ATMs since this gene is not highly expressed in adipocytes and ABCG1 expression is sterol mediated. Our data supports the concept that metabolic changes in adipocytes due to demand lipolysis and cell death lead to cholesterol loading of ATMs. Based on finding cholesterol-loaded peritoneal leukocytes with elevated levels of ABCG1 in CR as compared to AL mice, we suggest that pathways for cholesterol trafficking out of adipose tissue involve ATM egress as well as ABCG1 mediated cholesterol efflux. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Central nervous control of energy and glucose balance: focus on the central melanocortin system

Studies have suggested that manipulations of the central melanocortin circuitry by pharmacological agents produce robust effects on the regulation of body weight and glucose homeostasis. In this review, we discuss recent findings from genetic mouse models that have further established the physiological relevance of this circuitry in the context of glucose and energy balance. In addition, we will discuss distinct neuronal populations that respond to central melanocortins to regulate food intake, energy expenditure, insulin sensitivity, and insulin secretion, respectively. Finally, multiple hormonal and neural cues (e.g., leptin, estrogen, and serotonin) that use the melanocortin systems to regulate energy and glucose homeostasis will be reviewed. These findings suggest that targeting the specific branches of melanocortin circuits may be potential avenues to combat the current obesity and diabetes epidemics.

Leptin and cancer: from cancer stem cells to metastasis

There is growing evidence that obesity is a risk factor of cancer incidence and mortality. Hence, the identification of the mechanistic links between obesity and cancer progression is emerging as a topic of widespread interest. Recently, several groups have addressed the functional roles of leptin, an adipocyte-derived adipokine, for mammary tumor progression. In this issue of Endocrine-Related Cancer, Zheng et al. study the role of leptin on tumor growth in a xenograft model of MMTV-Wnt1-derived cancer cells. They study growth of these cancer cells in the context of obese animals, such as ob/ob mice (lacking leptin) and db/db mice (lacking functional leptin receptors (LEPR)) and find that leptin triggers LEPR-positive cancer stem cell differentiation, thereby promoting tumor cell survival. These findings highlight the therapeutic potential for leptin and leptin signaling in the context of mammary tumor growth.

Molecular mapping of mouse brain regions innervated by leptin receptor-expressing cells

Leptin acts via the long form of the leptin receptor (LepRb) on specialized sets of neurons in the brain to modulate diverse functions in concert with energy stores. Previous studies have revealed the distribution of LepRb-expressing neurons in the brain but not the regions to which LepRb neurons project to mediate downstream leptin actions. We utilized LepRb-cre in combination with cre-inducible enhanced green fluorescent protein (EGFP) and farnesylated EGFP (EGFPf) mouse reporter strains to visualize LepRb neurons and their projections, respectively, throughout the brain. The areas containing LepRb soma and projections were relatively circumscribed, as many brain regions contained no detectable EGFP or EGFPf. The highest concentrations of LepRb neurons and LepRb projections were found in the hypothalamus, where the ventral premamillary (PMv), dorsomedial (DMH), and arcuate (ARC) nuclei contained the greatest number of cell bodies, in addition to substantial EGFPf-reactivity. Furthermore, both LepRb soma and projections were present in a few midbrain and brainstem nuclei. Several brain regions including the hypothalamic paraventricular nucleus (PVH), the anteroventral periventricular nucleus (AVPe), and the central nucleus of the amygdala (CeA) contained few LepRb neurons but substantial EGFPf, suggesting that these regions represent targets of LepRb neurons that lie elsewhere in the brain. In some nuclei that contained both soma and projections, the distribution of soma and projections differed, suggesting that these areas transmit leptin-encoded information in a neuroanatomically directional manner.

Leptin receptor signaling supports cancer cell metabolism through suppression of mitochondrial respiration in vivo

Obesity represents a risk factor for certain types of cancer. Leptin, a hormone predominantly produced by adipocytes, is elevated in the obese state. In the context of breast cancer, leptin derived from local adipocytes is present at high concentrations within the mammary gland. A direct physiological role of peripheral leptin action in the tumor microenvironment in vivo has not yet been examined. Here, we report that mice deficient in the peripheral leptin receptor, while harboring an intact central leptin signaling pathway, develop a fully mature ductal epithelium, a phenomenon not observed in db/db mice to date. In the context of the MMTV-PyMT mammary tumor model, the lack of peripheral leptin receptors attenuated tumor progression and metastasis through a reduction of the ERK1/2 and Jak2/STAT3 pathways. These are tumor cell-autonomous properties, independent of the metabolic state of the host. In the absence of leptin receptor signaling, the metabolic phenotype is less reliant on aerobic glycolysis and displays an enhanced capacity for β-oxidation, in contrast to nontransformed cells. Leptin receptor-free tumor cells display reduced STAT3 tyrosine phosphorylation on residue Y705 but have increased serine phosphorylation on residue S727, consistent with preserved mitochondrial function in the absence of the leptin receptor. Therefore, local leptin action within the mammary gland is a critical mediator, linking obesity and dysfunctional adipose tissue with aggressive tumor growth.

CNS leptin action modulates immune response and survival in sepsis

Sepsis describes a complex clinical syndrome that results from an infection, setting off a cascade of systemic inflammatory responses that can lead to multiple organ failure and death. Leptin is a 16 kDa adipokine that, among its multiple known effects, is involved in regulating immune function. Here we demonstrate that leptin deficiency in ob/ob mice leads to higher mortality and more severe organ damage in a standard model of sepsis in mice [cecal ligation and puncture (CLP)]. Moreover, systemic leptin replacement improved the immune response to CLP. Based on the molecular mechanisms of leptin regulation of energy metabolism and reproductive function, we hypothesized that leptin acts in the CNS to efficiently coordinate peripheral immune defense in sepsis. We now report that leptin signaling in the brain increases survival during sepsis in leptin-deficient as well as in wild-type mice and that endogenous CNS leptin action is required for an adequate systemic immune response. These findings reveal the existence of a relevant neuroendocrine control of systemic immune defense and suggest a possible therapeutic potential for leptin analogs in infectious disease.

Functional consequences of the human leptin receptor (LEPR) Q223R transversion

Perturbations in the functional integrity of the leptin axis are obvious candidates for mediation of altered adiposity. In a large number of genetic association studies in humans, the nonconservative LEPR Q223R allele has been inconsistently associated with adiposity. Subtle, long-term effects of such genetic variants can be obscured by effects of the environment and other confounders that render definitive inferences difficult to reach. We directly assessed the biological effects of this variant in 129P3/J mice segregating for the humanized Lepr allele at codon 223. No effects of this allele were detected on body weight, composition, or energy expenditure in animals fed diets of varying fat content over periods as long as 235 days. In vitro, Q223R did not affect leptin signaling as reflected by activation of STAT3. We conclude that Q223R is unlikely to play a significant role in regulation of human adiposity. This approach to vetting of human allelic variation might be more widely used.

The role of leptin in the regulation of neuroendocrine function and CNS development

Leptin, a hormone produced by adipocytes in proportion to fat stores, signals the sufficiency of energy reserves to the brain to control feeding and metabolism. Leptin represents a vital link between metabolic and neuroendocrine pathways, and adequate circulating leptin levels are required to permit the expenditure of energy on reproduction, growth, and other energy-intensive endocrine outputs. Leptin mediates its effects by acting upon a distributed network of CNS neurons that express the signaling form of the leptin receptor (LRb). Nutritional status early in development influences a lifelong metabolic program that modulates risk for diabetes, obesity and other elements of the metabolic syndrome. Recent evidence has demonstrated a number of important roles for leptin in the regulation of neural development and metabolic programming. In this review, we discuss leptin action, the neural circuits on which leptin acts, and our nascent understanding of how early leptin exposure may influence neural development and the predisposition to metabolic diseases.

Hind Limb Ischemia-Reperfusion in the Leptin Receptor Deficient (db/db) Mouse

BACKGROUND

Diabetic patients have high incidence of peripheral vascular disease and limb loss after acute extremity injury. Experiments were designed to test the hypothesis that acute tissue injury in leptin receptor deficient (Db) diabetic (type2) mice would be more severe than in non-diabetic mice.

METHODS

Db and wild type (Wt) mice were subjected to 3 h of ischemia followed by either 4 or 24 h of reperfusion (3/4 IR, 3/24 IR). Muscle analyzed for tissue viability (mitochondrial activity), cytokines (KC-murine equivalent of human IL-8, TNFalpha, IL-6), growth factor, and histological evaluation (neutrophils/uninjured muscle fibers). Tissue perfusion was detected during basal and reperfusion conditions using laser Doppler imaging.

RESULTS

Mitochondrial activity and histological evaluation for tissue injury did not differ in the Db versus Wt mice at the time intervals studied. When compared with their respective sham animals, both Db and Wt mice had similarly increased levels of KC, IL-6, and VEGF after 3/24 IR. TNFalpha levels increased in Db but not Wt mice after IR. Although absolute increases in TNFalpha and KC were higher in Db mice, VEGF levels were actually lower in the Db mice.

CONCLUSION

The patterns of tissue perfusion, cytokines, and growth factors were different in Db versus Wt mice. At the acute time intervals studied, these differences did not correlate with an expected greater degree of acute muscle injury in Db mice.

Complete rescue of obesity, diabetes, and infertility in db/db mice by neuron-specific LEPR-B transgenes

We have generated mice that carry a neuron-specific leptin receptor (LEPR) transgene whose expression is driven by the rat synapsin I promoter synapsin-LEPR B (SYN-LEPR-B). We have also generated mice that are compound hemizygotes for the transgenes SYN-LEPR-B and neuron-specific enolase-LEPR B (NSE-LEPR-B). We observed a degree of correction in db/db mice that are hemizygous (Syn db/db) and homozygous (Syn/Syn db/db) for the SYN-LEPR-B transgene similar to that previously reported for the NSE-LEPR-B transgene. We also show complete correction of the obesity and related phenotypes of db/db mice that are hemizygous for both NSE-LEPR-B and SYN-LEPR-B transgenes (Nse+Syn db/db). Body composition, insulin sensitivity, and cold tolerance were completely normalized in Nse+Syn db/db mice at 12 weeks of age compared with lean controls. In situ hybridization for LEPR B isoform expression in Nse+Syn db/db mice showed robust expression in the energy homeostasis-relevant regions of the hypothalamus. Expression of 3 neuropeptide genes, agouti-related peptide (Agrp), neuropeptide Y (Npy), and proopiomelanocortin (Pomc), was fully normalized in dual transgenic db/db mice. The 2 transgenes in concert conferred normal fertility to male and female db/db mice. Male mice with partial peripheral deletion of Lepr, induced in the periweaning phase, did not show alterations in body composition or mass. In summary, we show that brain-specific leptin signaling is sufficient to reverse the obesity, diabetes, and infertility of db/db mice.

Suppression of leptin receptor messenger ribonucleic acid and leptin responsiveness in the ventromedial nucleus of the hypothalamus during pregnancy in the rat

Pregnancy in the rat is a state of leptin resistance associated with impaired leptin signal transduction in the hypothalamus. The aim of this study was to determine whether this leptin-resistant state is mediated by a change in the level of leptin receptors in the hypothalamus. Real-time RT-PCR was used to determine levels of mRNA for the various leptin receptor isoforms in a number of microdissected hypothalamic nuclei and the choroid plexus. To investigate the functional activation of the leptin receptor, immunohistochemistry for phosphorylated signal transducer and activator of transcription 3 (pSTAT3) was examined in the arcuate nucleus and the ventromedial nucleus of the hypothalamus (VMH) of fasted diestrous and d-14 pregnant rats after an intracerebroventricular (i.c.v.) injection of either leptin (4 mug) or vehicle. A significant reduction of Ob-Rb mRNA levels was observed in the VMH during pregnancy compared with the nonpregnant controls. Furthermore, in pregnant rats the number of cells positive for leptin-induced pSTAT3 in the VMH was greatly reduced during pregnancy compared with nonpregnant rats. There were no differences in the level of Ob-Rb mRNA or in the number of leptin-induced pSTAT3-positive cells in the arcuate nucleus of nonpregnant and pregnant rats. These data implicate the VMH as a key hypothalamic site involved in pregnancy-induced leptin resistance. There were also reduced levels of mRNA for Ob-Ra, a proposed leptin transporter molecule, in the choroid plexus on d 7 and 21 of pregnancy. Hence, diminished transport of leptin into the brain may also contribute to pregnancy-induced leptin resistance.

Roles for leptin receptor/STAT3-dependent and -independent signals in the regulation of glucose homeostasis

Leptin activates the long form of the leptin receptor (LRb) to control feeding and neuroendocrine function and thus regulate adiposity. While adiposity influences insulin sensitivity, leptin also regulates glucose homeostasis independently of energy balance. Disruption of the LRb/STAT3 signal in s/s mice results in hyperphagia, neuroendocrine dysfunction, and obesity similar to LRb null db/db mice. Insulin resistance and glucose intolerance are improved in s/s compared to db/db animals, however, suggesting that LRb/STAT3-independent signals may contribute to the regulation of glucose homeostasis by leptin. Indeed, caloric restriction normalized glycemic control in s/s animals, but db/db mice of similar weight and adiposity remained hyperglycemic. These differences in glucose homeostasis were not attributable to differences in insulin production between s/s and db/db animals but rather to decreased insulin resistance in s/s mice. Thus, in addition to LRb/STAT3-mediated adiposity signals, non-LRb/STAT3 leptin signals mediate an important adiposity-independent role in promoting glycemic control.

Role of leptin in reproduction

PURPOSE OF REVIEW

This article focuses on recently gained knowledge concerning the different emerging aspects of the role of leptin in reproduction, through both its central hypothalamus-mediated and peripheral actions.

RECENT FINDINGS

As delineated in murine models, STAT3-independent signals triggered by the leptin receptor are clearly important in fertility, and candidate pathways such as those via phosphatidylinositol-3 kinase and extracellular signal-related kinase are implicated in leptin-regulated cascades. Another aspect whose importance has recently been revealed is that of the bioavailability of leptin in general, and the fate and action of carrier-bound versus free leptin at central and peripheral sites in particular.

SUMMARY

Besides the well-established role of leptin in the control of appetite and energy expenditure in humans and animals, evidence for a major involvement of the hormone in the function of the reproductive system is rapidly accumulating through physiological and molecular genetic approaches. Powerful animal models that facilitate the dissection of increasingly complex pathways, together with detailed studies in man, will soon delineate in detail the diverse roles of leptin in biological regulation. The development of therapeutic agents primarily directed against obesity must therefore take into consideration the consequences of treatment not only on the amelioration of leptin resistance, but also on the bioactivity of leptin in the context of growth, glucose homeostasis, and last but not least, fertility.