Anorexigenic hormones leptin, insulin, and alpha-melanocyte-stimulating hormone directly induce neurotensin (NT) gene expression in novel NT-expressing cell models

mTOR-neuropeptide Y signaling sensitizes nociceptors to drive neuropathic pain

Neuropathic pain is a refractory condition that involves de novo protein synthesis in the nociceptive pathway. The mTOR is a master regulator of protein translation; however, mechanisms underlying its role in neuropathic pain remain elusive. Using the spared nerve injury-induced neuropathic pain model, we found that mTOR was preferentially activated in large-diameter dorsal root ganglion (DRG) neurons and spinal microglia. However, selective ablation of mTOR in DRG neurons, rather than microglia, alleviated acute neuropathic pain in mice. We show that injury-induced mTOR activation promoted the transcriptional induction of neuropeptide Y (Npy), likely via signal transducer and activator of transcription 3 phosphorylation. NPY further acted primarily on Y2 receptors (Y2R) to enhance neuronal excitability. Peripheral replenishment of NPY reversed pain alleviation upon mTOR removal, whereas Y2R antagonists prevented pain restoration. Our findings reveal an unexpected link between mTOR and NPY/Y2R in promoting nociceptor sensitization and neuropathic pain.

The Neurotensinergic System: A Target for Cancer Treatment

BACKGROUND

The scientific interest regarding the involvement of peptides in cancer has increased in the last years. In tumor cells the overexpression of peptides and their receptors is known and new therapeutic targets for the treatment of cancer have been suggested. The overexpression of the neurotensinergic system has been associated with poor prognosis, tumor size, higher tumor aggressiveness, increased relapse risk and worse sensitivity to chemotherapy agents.

OBJECTIVE

The aim of this review is to update the findings regarding the involvement of the neurotensinergic system in cancer to suggest anticancer therapeutic strategies targeting this system. The neurotensin (NT) precursor, NT and its receptors (NTR) and the involvement of the neurotensinergic system in lung, breast, prostate, gastric, colon, liver and pancreatic cancers, glioblastoma, neuroendocrine tumors and B-cell leukemia will be mentioned and discussed as well as the signaling pathways mediated by NT. Some research lines to be developed in the future will be suggested such as: molecules regulating the expression of the NT precursor, influence of the diet in the development of tumors, molecules and signaling pathways activated by NT and antitumor therapeutic strategies targeting the neurotensinergic system.

CONCLUSION

NT, via the NTR, exerts oncogenic (tumor cell proliferation, invasion, migration, angiogenesis) and antiapoptotic effects, whereas NTR antagonists inhibit these effects. NTR expression can be used as a diagnostic tool/therapeutic target and the administration of NTR antagonists as antitumor drugs could be a therapeutic strategy to treat tumors overexpressing NTR.

Daidzein Pro-cognitive Effects Coincided with Changes of Brain Neurotensin1 Receptor and Interleukin-10 Expression Levels in Obese Hamsters

At present, concerns are pointing to "tasteful" high-fat diets as a cause of conditioning physical-social states that through alterations of some key emotional- and nutritional-related limbic circuits such as hypothalamic and amygdalar areas lead to obesity states. Feeding and energetic homeostatic molecular mechanisms are part of a complex neuronal circuit accounting for this metabolic disorder. In an attempt to exclude conventional drugs for treating obesity, daidzein, a natural glycosidic isoflavone, which mimics estrogenic neuroprotective properties against increased body weight, is beginning to be preferred. In this study, evident anxiolytic-like behaviors were detected following treatment of high-fat diet hamsters with daidzein as shown by extremely evident (p < 0.001) exploration tendencies in novel object recognition test and a notably greater amount of time spent (p < 0.01) in open arms of elevated plus maze. Moreover, the isoflavone promoted a protective role against neurodegeneration processes as shown by few, if any, amino cupric silver granules in amygdalar, hypothalamic and hippocampal neuronal fields when compared with obese hamsters. Interestingly, elevated expression levels of the anorexic neuropeptide receptor neurotensin1 in the above limbic areas of obese hamsters were extremely reduced by daidzein, especially during recovery of cognitive events. Contextually, such effects were strongly paralleled by increased levels of the anti-neuroinflammatory cytokine, interleukin-10. Our results corroborate a neuroprotective ability of this natural glycosidic isoflavone, which through its interaction with the receptor neurotensin1 and interleukin-10 pathways is correlated not only to improved feeding states, and subsequently obesity conditions, but above all to cognitive performances.

New Actors Driving the Epithelial-Mesenchymal Transition in Cancer: The Role of Leptin

Leptin is a hormone secreted mainly by adipocytes; physiologically, it participates in the control of appetite and energy expenditure. However, it has also been linked to tumor progression in different epithelial cancers. In this review, we describe the effect of leptin on epithelial-mesenchymal transition (EMT) markers in different study models, including in vitro, in vivo, and patient studies and in various types of cancer, including breast, prostate, lung, and ovarian cancer. The different studies report that leptin promotes the expression of mesenchymal markers and a decrease in epithelial markers, in addition to promoting EMT-related processes such as cell migration and invasion and poor prognosis in patients with cancer. Finally, we report that leptin has the greatest biological relevance in EMT and tumor progression in breast, lung, prostate, esophageal, and ovarian cancer. This relationship could be due to the key role played by the enriched tumor microenvironment in adipose tissue. Together, these findings demonstrate that leptin is a key biomolecule that drives EMT and metastasis in cancer.

Short-term fasting promotes insulin expression in rat hypothalamus

In the hypothalamus, insulin takes on many roles involved in energy homoeostasis. Therefore, the aim of this study was to examine hypothalamic insulin expression during the initial phase of the metabolic response to fasting. Hypothalamic insulin content was assessed by both radioimmunoassay and Western blot. The relative expression of insulin mRNA was examined by qPCR. Immunofluorescence and immunohistochemistry were used to determine the distribution of insulin immunopositivity in the hypothalamus. After 6-h fasting, both glucose and insulin levels were decreased in serum but not in the cerebrospinal fluid. Our study showed for the first time that, while the concentration of circulating glucose and insulin decreased, both insulin mRNA expression and insulin content in the hypothalamic parenchyma were increased after short-term fasting. Increased insulin immunopositivity was detected specifically in the neurons of the hypothalamic periventricular nucleus and in the ependymal cells of fasting animals. These novel findings point to the complexity of mechanisms regulating insulin expression in the CNS in general and in the hypothalamus in particular.

Xenin-induced feeding suppression is not mediated through the activation of central extracellular signal-regulated kinase signaling in mice

Xenin is a gut hormone that reduces food intake by partly acting through the hypothalamus via neurotensin receptor 1 (Ntsr1). However, specific signaling pathways that mediate xenin-induced feeding suppression are not fully understood. Activation of Ntsr1 leads to the activation of the extracellular signal-regulated kinase (ERK). Hypothalamic ERK participates in the regulation of food intake by mediating the effect of hormonal signals. Therefore, we hypothesized that the anorectic effect of xenin is mediated by hypothalamic ERK signaling. To address this hypothesis, we compared levels of phosphorylation of ERK1/2 (pERK1/2) in the hypothalamus of both control and xenin-treated mice. The effect of xenin on ERK1/2 phosphorylation was also examined in mouse hypothalamic neuronal cell lines with or without Ntsr1. We also examined the effect of the blockade of central ERK signaling on xenin-induced feeding suppression in mice. The intraperitoneal (i.p.) injection of xenin caused a significant increase in the number of pERK1/2-immunoreactive cells in the hypothalamic arcuate nucleus. The majority of pERK1/2-positive cells expressed neuronal nuclei (NeuN), a marker for neurons. Xenin treatment increased pERK1/2 levels in one cell line expressing Ntsr1 but not another line without Ntsr1 expression. Both i.p. and intracerebroventricular (i.c.v.) injections of xenin reduced food intake in mice. The i.c.v. pre-treatment with U0126, a selective inhibitor of ERK1/2 upstream kinases, did not affect xenin-induced reduction in food intake. These findings suggest that although xenin activates ERK signaling in subpopulations of hypothalamic neurons, xenin does not require the activation of hypothalamic ERK signaling pathway to elicit feeding suppression.

The effects of graded levels of calorie restriction: VI. Impact of short-term graded calorie restriction on transcriptomic responses of the hypothalamic hunger and circadian signaling pathways

Food intake and circadian rhythms are regulated by hypothalamic neuropeptides and circulating hormones, which could mediate the anti-ageing effect of calorie restriction (CR). We tested whether these two signaling pathways mediate CR by quantifying hypothalamic transcripts of male C57BL/6 mice exposed to graded levels of CR (10 % to 40 %) for 3 months. We found that the graded CR manipulation resulted in upregulation of core circadian rhythm genes, which correlated negatively with circulating levels of leptin, insulin-like growth factor 1 (IGF-1), insulin, and tumor necrosis factor alpha (TNF-α). In addition, key components in the hunger signaling pathway were expressed in a manner reflecting elevated hunger at greater levels of restriction, and which also correlated negatively with circulating levels of insulin, TNF-α, leptin and IGF-1. Lastly, phenotypes, such as food anticipatory activity and body temperature, were associated with expression levels of both hunger genes and core clock genes. Our results suggest modulation of the hunger and circadian signaling pathways in response to altered levels of circulating hormones, that are themselves downstream of morphological changes resulting from CR treatment, may be important elements in the response to CR, driving some of the key phenotypic outcomes.

Hypothalamic Phosphodiesterase 3B Pathway Mediates Anorectic and Body Weight-Reducing Effects of Insulin in Male Mice

BACKGROUND

Insulin action in the hypothalamus plays a critical role in the regulation of energy homeostasis, yet the intracellular signaling mechanisms mediating insulin action are incompletely understood. Although phosphodiesterase 3B (PDE3B) mediates insulin action in the adipose tissue and it is highly expressed in the hypothalamic areas implicated in energy homeostasis, its role, if any, in mediating insulin action in the hypothalamus is unknown. We tested the hypothesis that insulin action in the hypothalamus is mediated by PDE3B.

METHODS

Using enzymatic assay, we examined the effects of peripheral or central administration of insulin on hypothalamic PDE3B activity in adult mice. Western blotting and immunohistochemistry also examined p-Akt and p-STAT3 levels in the hypothalamus. Effects of leptin on these parameters were also compared. We injected cilostamide, a PDE3 inhibitor, prior to central injection of insulin and examined the 12- to 24-hour food intake and 24-hour body weight. Finally, we examined the effect of cilostamide on insulin-induced proopiomelanocortin (Pomc), neurotensin (Nt), neuropeptide Y (Npy) and agouti-related peptide (Agrp) gene expression in the hypothalamus by qPCR.

RESULTS

Peripheral or central injection of insulin significantly increased PDE3B activity in the hypothalamus in association with increased p-Akt levels but without any change in p-STAT3 levels. However, leptin-induced increase in PDE3B activity was associated with an increase in both p-Akt and p-STAT3 levels in the hypothalamus. Prior administration of cilostamide reversed the anorectic and body weight-reducing effects as well as stimulatory effect of insulin on hypothalamic Pomc mRNA levels. Insulin did not alter Nt, Npy and Agrp mRNA levels.

CONCLUSION

Insulin induction of hypothalamic PDE3B activity and the reversal of the anorectic and body weight-reducing effects and stimulatory effect of insulin on hypothalamic Pomc gene expression by cilostamide suggest that activation of PDE3B is a novel mechanism of insulin signaling in the hypothalamus.

Establishment of Leptin-Responsive Cell Lines from Adult Mouse Hypothalamus

Leptin resistance is considered to be the primary cause of obesity. However, the cause of leptin resistance remains incompletely understood, and there is currently no cure for the leptin-resistant state. In order to identify novel drug-target molecules that could overcome leptin resistance, it would be useful to develop in vitro assay systems for evaluating leptin resistance. In this study, we established immortalized adult mouse hypothalamus—derived cell lines, termed adult mouse hypothalamus (AMH) cells, by developing transgenic mice in which SV40 Tag was overexpressed in chromogranin A—positive cells in a tamoxifen-dependent manner. In order to obtain leptin-responsive clones, we selected clones based on the phosphorylation levels of STAT3 induced by leptin. The selected clones were fairly responsive to leptin in terms of STAT3, ERK, and Akt phosphorylation and induction of c-Fos mRNA induction. Pretreatment with leptin, insulin, and palmitate attenuated the c-Fos mRNA response to leptin, suggesting that certain aspects of leptin resistance might be reconstituted in this cellular model. These cell lines are useful tools for understanding the molecular nature of the signal disturbance in the leptin-resistant state and for identifying potential target molecules for drugs that relieve leptin resistance, although they have drawbacks including de-differentiated nature and lack of long-time stability.

MADS-Box Genes and Gibberellins Regulate Bolting in Lettuce (Lactuca sativa L.)

Bolting in lettuce is promoted by high temperature and bolting resistance is of great economic importance for lettuce production. But how bolting is regulated at the molecular level remains elusive. Here, a bolting resistant line S24 and a bolting sensitive line S39 were selected for morphological, physiological, transcriptomic and proteomic comparisons. A total of 12204 genes were differentially expressed in S39 vs. S24. Line S39 was featured with larger leaves, higher levels of chlorophyll, soluble sugar, anthocyanin and auxin, consistent with its up-regulation of genes implicated in photosynthesis, oxidation-reduction and auxin actions. Proteomic analysis identified 30 differentially accumulated proteins in lines S39 and S24 upon heat treatment, and 19 out of the 30 genes showed differential expression in the RNA-Seq data. Exogenous gibberellins (GA) treatment promoted bolting in both S39 and S24, while 12 flowering promoting MADS-box genes were specifically induced in line S39, suggesting that although GA regulates bolting in lettuce, it may be the MADS-box genes, not GA, that plays a major role in differing the bolting resistance between these two lettuce lines.

Delineating the regulation of energy homeostasis using hypothalamic cell models

Attesting to its intimate peripheral connections, hypothalamic neurons integrate nutritional and hormonal cues to effectively manage energy homeostasis according to the overall status of the system. Extensive progress in the identification of essential transcriptional and post-translational mechanisms regulating the controlled expression and actions of hypothalamic neuropeptides has been identified through the use of animal and cell models. This review will introduce the basic techniques of hypothalamic investigation both in vivo and in vitro and will briefly highlight the key advantages and challenges of their use. Further emphasis will be place on the use of immortalized models of hypothalamic neurons for in vitro study of feeding regulation, with a particular focus on cell lines proving themselves most fruitful in deciphering fundamental basics of NPY/AgRP, Proglucagon, and POMC neuropeptide function.

Endogenous CNS expression of neurotensin and neurotensin receptors is altered during the postpartum period in outbred mice

Neurotensin (NT) is a neuropeptide identical in mice and humans that is produced and released in many CNS regions associated with maternal behavior. NT has been linked to aspects of maternal care and previous studies have indirectly suggested that endogenous NT signaling is altered in the postpartum period. In the present study, we directly examine whether NT and its receptors exhibit altered gene expression in maternal relative to virgin outbred mice using real time quantitative PCR (qPCR) across multiple brain regions. We also examine NT protein levels using anti-NT antibodies and immunohistochemistry in specific brain regions. In the medial preoptic area (MPOA), which is critical for maternal behaviors, mRNA of NT and NT receptor 3 (Sort1) were significantly up-regulated in postpartum mice compared to virgins. NT mRNA was also elevated in postpartum females in the bed nucleus of the stria terminalis dorsal. However, in the lateral septum, NT mRNA was down-regulated in postpartum females. In the paraventricular nucleus of the hypothalamus (PVN), Ntsr1 expression was down-regulated in postpartum females. Neurotensin receptor 2 (Ntsr2) expression was not altered in any brain region tested. In terms of protein expression, NT immunohistochemistry results indicated that NT labeling was elevated in the postpartum brain in the MPOA, lateral hypothalamus, and two subregions of PVN. Together, these findings indicate that endogenous changes occur in NT and its receptors across multiple brain regions, and these likely support the emergence of some maternal behaviors.

The cytokine ciliary neurotrophic factor (CNTF) activates hypothalamic urocortin-expressing neurons both in vitro and in vivo

Ciliary neurotrophic factor (CNTF) induces neurogenesis, reduces feeding, and induces weight loss. However, the central mechanisms by which CNTF acts are vague. We employed the mHypoE-20/2 line that endogenously expresses the CNTF receptor to examine the direct effects of CNTF on mRNA levels of urocortin-1, urocortin-2, agouti-related peptide, brain-derived neurotrophic factor, and neurotensin. We found that treatment of 10 ng/ml CNTF significantly increased only urocortin-1 mRNA by 1.84-fold at 48 h. We then performed intracerebroventricular injections of 0.5 mg/mL CNTF into mice, and examined its effects on urocortin-1 neurons post-exposure. Through double-label immunohistochemistry using specific antibodies against c-Fos and urocortin-1, we showed that central CNTF administration significantly activated urocortin-1 neurons in specific areas of the hypothalamus. Taken together, our studies point to a potential role for CNTF in regulating hypothalamic urocortin-1-expressing neurons to mediate its recognized effects on energy homeostasis, neuronal proliferaton/survival, and/or neurogenesis.

Neuroanatomy of melanocortin-4 receptor pathway in the lateral hypothalamic area

The central melanocortin system regulates body energy homeostasis including the melanocortin-4 receptor (MC4R). The lateral hypothalamic area (LHA) receives dense melanocortinergic inputs from the arcuate nucleus of the hypothalamus and regulates multiple processes including food intake, reward behaviors, and autonomic function. By using a mouse line in which green fluorescent protein (GFP) is expressed under control of the MC4R gene promoter, we systemically investigated MC4R signaling in the LHA by combining double immunohistochemistry, electrophysiology, and retrograde tracing techniques. We found that LHA MC4R-GFP neurons coexpress neurotensin as well as the leptin receptor but do not coexpress other peptide neurotransmitters found in the LHA including orexin, melanin-concentrating hormone, and nesfatin-1. Furthermore, electrophysiological recording demonstrated that leptin, but not the MC4R agonist melanotan II, hyperpolarizes the majority of LHA MC4R-GFP neurons in an ATP- sensitive potassium channel-dependent manner. Retrograde tracing revealed that LHA MC4R-GFP neurons do not project to the ventral tegmental area, dorsal raphe nucleus, nucleus accumbens, and spinal cord, and only limited number of neurons project to the nucleus of the solitary tract and parabrachial nucleus. Our findings provide new insights into MC4R signaling in the LHA and its potential implications in homeostatic regulation of body energy balance.

Diverse roles of neurotensin agonists in the central nervous system

Neurotensin (NT) is a tridecapeptide that is found in the central nervous system (CNS) and the gastrointestinal tract. NT behaves as a neurotransmitter in the brain and as a hormone in the gut. Additionally, NT acts as a neuromodulator to several neurotransmitter systems including dopaminergic, sertonergic, GABAergic, glutamatergic, and cholinergic systems. Due to its association with such a wide variety of neurotransmitters, NT has been implicated in the pathophysiology of several CNS disorders such as schizophrenia, drug abuse, Parkinson's disease (PD), pain, central control of blood pressure, eating disorders, as well as, cancer and inflammation. The present review will focus on the role that NT and its analogs play in schizophrenia, endocrine function, pain, psychostimulant abuse, and PD.

The regulation of food intake by the gut-brain axis: implications for obesity

Our understanding of the regulation of appetite has improved considerably over the last few decades. Recent work, stimulated by efforts aimed at curbing the current obesity epidemic, has unravelled some of the complex pathways regulating energy homeostasis. Key factors to this progress have been the discovery of leptin and the neuronal circuitry involved in mediating its effects, as well as the identification of gut hormones that have important physiological roles relating to energy homeostasis. Despite these advances in research, there are currently no effective treatments for the growing problem of obesity. In this article, we summarise the regulatory pathways controlling appetite with a special focus on gut hormones. We detail how recent findings have contributed to our knowledge regarding the pathogenesis and treatment of common obesity. A number of barriers still need to be overcome to develop safe and effective anti-obesity treatments. We outline problems highlighted by historical failures and discuss the potential of augmenting natural satiety signals, such as gut hormones, to treat obesity.

Direct regulation of the proglucagon gene by insulin, leptin, and cAMP in embryonic versus adult hypothalamic neurons

The proglucagon gene is expressed not only in the pancreas and intestine but also in the hypothalamus. Proglucagon-derived peptides have emerged as potential regulators of energy homeostasis. Whether leptin, insulin, or cAMP activation controls proglucagon gene expression in the hypothalamus is not known. A key reason for this has been the inaccessibility of hypothalamic proglucagon-expressing neurons and the lack of suitable neuronal cell lines. Herein we describe the mechanisms involved in the direct regulation of the proglucagon gene by insulin, leptin, and cAMP in hypothalamic cell models. Insulin, through an Akt-dependent manner, significantly induced proglucagon mRNA expression by 70% in adult-derived mHypoA-2/10 neurons and significantly suppressed it by 45% in embryonic-derived mHypoE-39 neurons. Leptin, via the Janus kinase-2/ signal transducer and activator of transcription-3 pathway, caused an initial increase by 66 and 43% at 1 h followed by a decrease by 45 and 34% at 12 h in mHypoA-2/10 and mHypoE-39 cells, respectively. Furthermore, cAMP activation by forskolin up-regulated proglucagon expression by 87% in mHypoE-39 neurons and increased proglucagon mRNA, through Epac activation, in the mHypoE-20/2 neurons. Specific regions of the proglucagon promoter were regulated by cAMP signaling, as determined by transient transfections, whereas mRNA stability assays demonstrate that insulin and leptin increase proglucagon mRNA stability in the adult cells. These findings suggest that insulin, leptin, and cAMP act directly, but differentially, on specific hypothalamic neurons to regulate proglucagon gene expression. Because proglucagon-derived peptides are potential regulators of energy homeostasis, an understanding of hypothalamic proglucagon neurons is important to further expand our knowledge of alternative feeding circuits.

Glucagon-like peptide-2 directly regulates hypothalamic neurons expressing neuropeptides linked to appetite control in vivo and in vitro

Glucagon-like peptide-2 (GLP-2), a proglucagon-derived peptide, has been postulated to affect appetite at the level of the hypothalamus. To gain better insight into this process, a degradation-resistant GLP-2 analog, human (Gly(2))GLP-2(1-33) [h(Gly(2))GLP-2] was intracerebroventricularly injected into mice to examine its action on food and water intake and also activation of hypothalamic anorexigenic α-melanocyte-stimulating hormone/proopiomelanocortin, neurotensin, and orexigenic neuropeptide Y, and ghrelin neurons. Central h(Gly(2))GLP-2 administration significantly suppressed food and water intake with acute weight loss at 2 h. Further, central h(Gly(2))GLP-2 robustly induced c-Fos activation in the hypothalamic arcuate, dorsomedial, ventromedial, paraventricular, and the lateral hypothalamic nuclei. We found differential colocalization of neuropeptides with c-Fos in specific regions of the hypothalamus. To assess whether hypothalamic neuropeptides are directly regulated by GLP-2 in vitro, we used an adult-derived clonal, immortalized hypothalamic cell line, mHypoA-2/30, that endogenously expresses functional GLP-2 receptors (GLP-2R) and two of the feeding-related neuropeptides linked to GLP-2R activation in vivo: neurotensin and ghrelin. Treatment with h(Gly(2))GLP-2 stimulated c-Fos expression and phosphorylation of cAMP response element-binding protein/activating transcription factor-1. In addition, treatment with h(Gly(2))GLP-2 significantly increased neurotensin and ghrelin mRNA transcript levels by 50 and 95%, respectively, at 24 h after treatment in protein kinase A-dependent manner. Taken together, these findings implicate the protein kinase A pathway as the means by which GLP-2 can up-regulate hypothalamic neuropeptide mRNA levels and provide evidence for a link between central GLP-2R activation and specific hypothalamic neuropeptides involved in appetite regulation.

Glucagon-like peptide-1 receptor agonist, exendin-4, regulates feeding-associated neuropeptides in hypothalamic neurons in vivo and in vitro

Exendin-4, a long-acting glucagon-like peptide-1 receptor (GLP-1R) agonist, is a potential regulator of feeding behavior through its ability to inhibit gastric emptying, reduce food intake, and induce satiety. GLP-1R activation by exendin-4 induces anorexia; however, the specific populations of neuropeptidergic neurons activated by exendin-4 within the hypothalamus, the central regulator of energy homeostasis, remain unclear. This study determines whether exendin-4 regulates hypothalamic neuropeptide expression and explores the signaling mechanisms involved. The distribution and quantity of exendin-4-induced c-Fos immunoreactivity were evaluated to determine activation of α-melanocyte-stimulating hormone/proopiomelanocortin, neuropeptide Y, neurotensin (NT), and ghrelin neurons in hypothalamic nuclei during exendin-4-induced anorexia in mice. Additionally, exendin-4 action on NT and ghrelin transcript regulation was examined in immortalized hypothalamic neurons. With anorexia induced by intracerebroventricular exendin-4, α-melanocyte-stimulating hormone/proopiomelanocortin and neuropeptide Y neurons were activated in the arcuate nucleus, with simultaneous activation of NT-expressing neurons in the paraventricular nucleus, and ghrelin-expressing neurons in the arcuate nucleus, paraventricular nucleus, and periventricular hypothalamus, suggesting that neurons in one or more of these areas mediate the anorexic action of exendin-4. In the hypothalamic neuronal cell models, exendin-4 increased cAMP, cAMP response element-binding protein/activating transcription factor-1 and c-Fos activation, and via a protein kinase A-dependent mechanism regulated NT and ghrelin mRNA expression, indicating that these neuropeptides may serve as downstream mediators of exendin-4 action. These findings provide a previously unrecognized link between central GLP-1R activation by exendin-4 and the regulation of hypothalamic NT and ghrelin. Further understanding of this central GLP-1R activation may lead to safe and effective therapeutics for the treatment of metabolic disorders.

Cellular leptin resistance impairs the leptin-mediated suppression of neuropeptide Y secretion in hypothalamic neurons

Evidence shows that neuropeptide Y (NPY) neurons are involved in mediating the anorexigenic action of leptin via neuronal circuits in the hypothalamus. However, studies have produced limited data on the cellular processes involved and whether hypothalamic NPY neurons are susceptible to cellular leptin resistance. To investigate the direct regulation of NPY secretion by leptin, we used novel NPY-synthesizing, immortalized mHypoA-NPY/green fluorescent protein and mHypoA-59 hypothalamic cell lines derived from adult hypothalamic primary cultures. We report that leptin treatment significantly suppressed NPY secretion in the cells by approximately 20%. We found a decrease in c-fos expression upon leptin exposure, indicating deactivation or hyperpolarization of the neurons. Protein analysis indicated that leptin inhibits AMP-activated protein kinase (AMPK) activity and activates acetyl-coenzyme A carboxylase in NPY neurons, supporting the hypothesis of an AMPK-dependent mechanism. Inhibiting both AMPK with Compound C or phosphatidylinositol 3 kinase (PI3K) with 2-(4-morpholinyl)-8-phenyl-1(4H)-1-benzopyran-4-one hydrochloride prevented the leptin-mediated decrease in NPY secretion, indicating both AMPK- and PI3K-mediated mechanisms. Further, NPY secretion was stimulated by 30% by the AMPK activator, aminoimidazole carboxamide ribonucleotide. Importantly, prolonged leptin exposure in the mHypoA-NPY/green fluorescent protein cells prevented leptin-induced changes in AMPK phosphorylation and suppression of NPY secretion, indicating that NPY neurons are susceptible to leptin resistance. Our studies indicate that AMPK and PI3K pathways are involved in leptin action in NPY neurons and that leptin resistance blocks the feedback response likely required to maintain energy homeostasis.

The role of neurotensin in physiologic and pathologic processes

PURPOSE OF REVIEW

Neurotensin is a 13-amino acid peptide found in the central nervous system central nervous system and the gastrointestinal tract. Since its initial discovery in 1973, neurotensin has been shown to play a role in a wide range of physiologic and pathologic processes throughout the body. Ongoing research efforts continue to clarify the role of neurotensin in various central nervous system and gastrointestinal processes, as well as how disruption of these normal mechanisms may lead to diseases ranging from schizophrenia to colorectal cancer. The goal of this review is to provide an overview of the most recent advances in the field of neurotensin research, in the context of what has been previously published.

RECENT FINDINGS

Because of the seemingly unrelated functions of neurotensin in the central nervous system and the periphery, the scope of the articles reviewed is rather broad. Contributions continue to be made to our understanding of the downstream effects of neurotensin signaling and the complex feedback loops between neurotensin and other signaling molecules. By selective targeting or blockade of specific neurotensin receptors, investigators have identified potential drugs for use in the treatment of schizophrenia, alcoholism, chronic pain, or cancer. Neurotensin-based pharmacologic agents are being used successfully in animal models for a number of these conditions.

SUMMARY

The review highlights the wide array of biological processes in which neurotensin has a role, and summarizes the most recent advances in various fields of neurotensin research. The knowledge gained through this research has led to the development of first-in-class drugs for the treatment of various medical conditions, and it is clear that in the coming years some of these agents will be ready to move from the bench to the bedside in clinical trials.

Immortalized neurons for the study of hypothalamic function

The hypothalamus is a vital part of the central nervous system: it harbors control systems implicated in regulation of a wide range of homeostatic processes, including energy balance and reproduction. Structurally, the hypothalamus is a complex neuroendocrine tissue composed of a multitude of unique neuronal cell types that express a number of neuromodulators, including hormones, classical neurotransmitters, and specific neuropeptides that play a critical role in mediating hypothalamic function. However, neuropeptide and receptor gene expression, second messenger activation, and electrophysiological and secretory properties of these hypothalamic neurons are not yet fully defined, primarily because the heterogeneity and complex neuronal architecture of the neuroendocrine hypothalamus make such studies challenging to perform in vivo. To circumvent this problem, our research group recently generated embryonic- and adult-derived hypothalamic neuronal cell models by utilizing the novel molecular techniques of ciliary neurotrophic factor-induced neurogenesis and SV40 T antigen transfer to primary hypothalamic neuronal cell cultures. Significant research with these cell lines has demonstrated their value as a potential tool for use in molecular genetic analysis of hypothalamic neuronal function. Insights gained from hypothalamic immortalized cells used in conjunction with in vivo models will enhance our understanding of hypothalamic functions such as neurogenesis, neuronal plasticity, glucose sensing, energy homeostasis, circadian rhythms, and reproduction. This review discusses the generation and use of hypothalamic cell models to study mechanisms underlying the function of individual hypothalamic neurons and to gain a more complete understanding of the overall physiology of the hypothalamus.

Rhythmic clock and neuropeptide gene expression in hypothalamic mHypoE-44 neurons

The rhythmic expression of specific clock genes: Bmal1, Per2, Clock and Rev-Erbalpha; and specific hypothalamic neuropeptides: NPY, Crh, AgRP, neurotensin and preproghrelin, expressed in clonal hypothalamic neuronal cell lines, was assayed and analyzed using a novel non-linear least squares statistical analysis to determine rhythmicity in an in vitro milieu. In silico analysis of the neuropeptide promoter regions identified putative E-box motifs and a motif in the NPY promoter is bound to in an oscillatory fashion. Within the mHypoE-44 neurons, we demonstrate that mRNA of four core circadian components: Bmal1, Clock, Per2 and Rev-Erbalpha, oscillate with approximate 24h rhythms. NPY and NT demonstrated significant 24h gene expression. However, CRH and preproghrelin mRNA cycled significantly in an ultradian fashion, oscillating approximately every 18h. AgRP mRNA did not show a significant rhythm. We speculate that endogenous rhythmic neuropeptide expression contributes to neuroendocrine homeostasis, which may include energy balance.

A role of phosphodiesterase-3B pathway in mediating leptin action on proopiomelanocortin and neurotensin neurons in the hypothalamus

Leptin signaling in the hypothalamus is required for normal food intake and body weight homeostasis. Recent evidence suggests that besides the signal transducer and activator of transcription-3 (STAT3) pathway, several non-STAT3 pathways mediate leptin signaling in the hypothalamus. We have previously demonstrated that leptin stimulates phosphodiesterase-3B (PDE3B) activity in the hypothalamus, and PDE3 inhibitor cilostamide reverses anorectic and body weight reducing effects of leptin. To establish the physiological role of PDE3B signaling in the hypothalamus, we examined if leptin signaling through the PDE3B pathway is responsible for the activation of proopiomelanocortin (POMC) and neurotensin (NT) neurons, which are known to play a critical role in energy homeostasis. To this end, we assessed the effect of cilostamide on leptin-induced POMC and NT gene expression in the rat hypothalamus. Results showed that while central injection of leptin significantly increased both POMC and NT mRNA levels in the medial basal hypothalamus, cilostamide completely reversed this effect of leptin suggesting a PDE3B-activation dependent induction of POMC and NT gene expression by leptin. This result further suggests that the PDE3B pathway plays an important role in mediating leptin action in the hypothalamus.

Palmitate attenuates insulin signaling and induces endoplasmic reticulum stress and apoptosis in hypothalamic neurons: rescue of resistance and apoptosis through adenosine 5' monophosphate-activated protein kinase activation

Hypothalamic insulin signaling is essential to the maintenance of glucose and energy homeostasis. During pathological states, such as obesity and type 2 diabetes mellitus, insulin signaling is impaired. One key mechanism involved in the development of insulin resistance is lipotoxicity, through increased circulating saturated fatty acids. Although many studies have begun to determine the underlying mechanisms of lipotoxicity in peripheral tissues, little is known about the effects of excess lipids in the brain. We used a hypothalamic, neuronal cell model, mHypoE-44, to understand how the highly prevalent nonesterified fatty acid, palmitate, affects neuronal insulin signaling. Through Western blot analysis, we discerned that prolonged exposure to palmitate impairs insulin activation, as assessed by phosphorylation of Akt. We investigated the role of endoplasmic reticulum (ER) stress, which is known to promote cellular insulin resistance and apoptosis in peripheral tissues. Palmitate treatment induced ER stress through a c-Jun N-terminal kinase (JNK)-dependent pathway because a selective JNK inhibitor blocked palmitate activation of the ER stress pathways eIF2 alpha and X-box binding protein-1. Interestingly, JNK inhibition did not prevent the palmitate-mediated cleaved caspase-3 increase, an apoptotic marker, or insulin signaling attenuation. However, pretreatment with the AMP kinase activator, aminoimidazole carboxamide ribonucleotide, blocked JNK phosphorylation and importantly prevented caspase-3 cleavage and restored insulin signaling during short-term exposure to palmitate. Thus, activation of AMP kinase prevents the deleterious effects of palmitate on hypothalamic neurons by inhibiting the onset of insulin resistance and apoptosis.

Effect of intracerebroventricular infusion of neurotensin in glucose-dependent insulinotropic peptide secretion in dogs

GIP is a major physiological component of the enteroinsular axis. Several researchers have pointed to a neural regulation of GIP secretion. We have previously studied the effect of intracerebroventricular (icv) infusion of insulin, NPY and bombesin in the regulation of GIP secretion. The aim of the present study is to evaluate a possible role of neurotensin in neural regulation of GIP secretion. Thirty-two adult dogs were used in this study. In a dose-response study (experiment 1) we used 3 different doses of neurotensin (25, 50 and 100microg) in a bolus icv infusion. In experiment 2 the animals received a bolus icv infusion of 50microg neurotensin and an equivalent amount of artificial cerebrospinal fluid (aCSF) at 1-week interval. In experiment 3 the animals received a continuous icv infusion of neurotensin at a constant rate of 1microg/kg/h and aCSF over a 3-h period, at 1-week interval. In experiment 4 the experiment of group 3 was repeated with a simultaneous intraduodenal infusion of a glucose load through the Mann-Bollman fistula. Plasma levels of glucose, insulin and GIP were assayed.

RESULTS

Bolus and continuous icv infusion of neurotensin produced a significant increase in glucose, GIP and insulin levels. In the 4th experiment icv infusion of neurotensin produced a more prominent increase of glucose and insulin levels compare to infusion of aCSF. GIP levels were lower after infusion of neurotensin compared to aCSF.

CONCLUSIONS

Our data suggest a differential effect of neurotensin on GIP secretion, dependent on the energy load.

Hypothalamic cell lines to investigate neuroendocrine control mechanisms

The hypothalamus is the control center for most physiological processes; yet has been difficult to study due to the inherent heterogeneity of this brain region. For this reason, researchers have turned towards cell models. Primary hypothalamic cultures are difficult to maintain, are heterogeneous neuronal and glial cell populations and often contain a minimal number of viable peptide-secreting neurons. In contrast, immortalized, clonal cell lines represent an unlimited, homogeneous population of neurons that can be manipulated using a number of elegant molecular techniques. Cell line studies and in vivo experimentation are complementary and together provide a powerful tool to drive scientific discovery. This review focuses on three key neuroendocrine systems: energy homeostasis, reproduction, and circadian rhythms; and the use of hypothalamic cell lines to dissect the complex pathways utilized by individual neurons in these systems.

Lipopolysaccharide (LPS) stimulates adipokine and socs3 gene expression in mouse brain and pituitary gland in vivo, and in N-1 hypothalamic neurons in vitro

Adipokines that modulate metabolic and inflammatory responses, such as resistin (rstn) and fasting-induced adipose factor (fiaf), are also expressed in mouse brain and pituitary gland. Since lipopolysaccharide (LPS)-induced endotoxinemia provokes an anorectic response via a hypothalamic-dependent mechanism we hypothesized that LPS would also modify hypothalamic adipokine expression. Challenging male CD-1 mice with LPS (5 mg/kg; s.c.) significantly reduced bodyweight (24 h) and realtime RT-PCR revealed time- and tissue-dependent increases in rstn, fiaf and suppressor of cytokine signaling-3 (socs-3) mRNA in hypothalamic, pituitary, cortical and adipose tissues. Gene expression was rapidly increased (3-6 h) in the hypothalamus and pituitary, but returned to normal within 24 h. In contrast, with the exception of rstn in fat, the expression of target genes remained elevated in cortex and visceral fat at 24 h post-injection. In order to more specifically examine the hypothalamic response to LPS we investigated its effects directly on N-1 hypothalamic neurons in vitro. LPS (25 microg/mL; 3 h) had no effect on rstn mRNA, but significantly stimulated fiaf and socs-3 expression. Although various toll-like receptor 4 (TLR4) antagonists (parthenolide, PD098059, and SB202190) did not prevent the LPS-induced increases in fiaf and socs-3, they did partially attenuate its stimulatory effects. We conclude that LPS treatment increases the expression of central, and possibly neuronal, adipokine genes which may influence local tissue repair and function, but could also have downstream consequences on the hypothalamic control of appetite and energy metabolism following an inflammatory insult.

Estrogen facilitates both phosphatidylinositol 3-kinase/Akt and ERK1/2 mitogen-activated protein kinase membrane signaling required for long-term neuropeptide Y transcriptional regulation in clonal, immortalized neurons

It is established that increases in neuropeptide Y (NPY) expression are associated with hyperphagia and obesity. These effects can be reversed by estrogen, a recognized anorexigen. We found that 17beta-estradiol (E(2)) regulates biphasic NPY gene expression in a clonal, immortalized hypothalamic cell line, N-38, through estrogen receptor (ER) action at the level of the NPY promoter. However, rapid, nongenomic actions of estrogen, linked to the phosphatidylinositol 3-kinase (PI3-K)/Akt and ERK1/2 mitogen-activated protein kinase (MAPK) pathways, may also play a role. We therefore examined the changes in the phosphorylation status of Akt, ERK1/2, and cAMP response element-binding protein (CREB) after treatment with 10 nm E(2) in the N-38 neurons and found activation of these signaling proteins within 5-30 min. We also demonstrated possible cross talk between the estrogen-activated PI3-K/Akt and MAPK/extracellular signal-regulated kinase pathways using pharmacological inhibitors. We find that only ERalpha is involved in the early signaling events using the ERalpha agonist 4,4',4''-(4-propyl-[1H]-pyrazole-1,3,5-triyl)trisphenol and the ERbeta agonist 2,3-bis(4-hydroxyphenyl)-propionitrile. Furthermore, we can detect colocalization of ERalpha and caveolin-1, a membrane-associated signaling protein. Remarkably, we find that the membrane-mediated events are critical for the long-term estrogen-mediated repression of NPY gene expression that can be mapped to within -97 bp of the NPY promoter. To link the early signaling events to downstream effectors, we detected induction of c-fos and inactivation of MSK-1 by estrogen and binding of CREB to this minimal promoter region. These observations suggest that rapid estrogen-mediated signaling is mediated by ERalpha, and the signal transduction events potentiate the genomic actions of estrogen on NPY gene expression in the N-38 NPY neurons.

Hormonal regulation of clonal, immortalized hypothalamic neurons expressing neuropeptides involved in reproduction and feeding

The hypothalamus has been particularly difficult to study at the molecular level because of the inherent cellular heterogeneity and complexity of neuronal circuits within. We have generated a large number of immortalized, clonal cell lines through retroviral gene transfer of the oncogene SV40 T-Ag into primary murine hypothalamic neuronal cell cultures. A number of these neuronal cell lines express neuropeptides linked to the control of feeding behavior and reproduction, including neuropeptide Y (NPY) and neurotensin (NT). We review recent studies on the direct regulation of NPY gene expression by estrogen, and the leptin-mediated control of signal transduction pathways and NT transcription. These studies provide new insights into the direct control of neuropeptide synthesis by hormones and nutrients at a mechanistic level in the individual neuron, not yet possible in the whole brain. Using these novel cell models, we expect to contribute substantially to the understanding of how individual neuronal cell types control overall endocrine function, especially with regard to two of the most well-known roles of distinct peptidergic neurons; these being the control of reproduction and energy homeostasis.

Hypothalamic dysfunction and neuroendocrine and metabolic alterations in Huntington's disease: clinical consequences and therapeutic implications

Huntington's disease (HD) is a hereditary neurodegenerative disorder characterized by cognitive, psychiatric, behavioural and motor disturbances. Although the course of HD is also frequently complicated by unintended weight loss, sleep disturbances and autonomic nervous system dysfunction, the aetiology of these signs and symptoms remains largely unknown. In recent years, many novel findings from both animal and human studies have emerged that indicate considerable hypothalamic, endocrine and metabolic alterations in HD. However, a comprehensive overview of these findings is lacking and their precise clinical significance is far from clear. Therefore, in this review we attempt to put these recent developments in the field into perspective by integrating them with previous findings in a comprehensible manner, and by discussing their clinical relevance, with a special focus on body weight, sleep and autonomic functions in HD, which will also allow for the identification of future lines of research in this area.

Adipokine gene expression in a novel hypothalamic neuronal cell line: resistin-dependent regulation of fasting-induced adipose factor and SOCS-3

Adipokines such as leptin, resistin, and fasting-induced adipose factor (FIAF) are secreted by adipocytes, but their expression is also detectable in the brain and pituitary. The role of central adipokines remains elusive, but we speculate that they may modulate those hypothalamic signaling pathways that control energy homeostasis. Here we describe experiments to test this in which we exploited a novel hypothalamic neuronal cell line (N-1) that expresses a variety of neuropeptides and receptors that are known to be implicated in appetite regulation. Using real-time RT-PCR, we confirmed that N-1 neurons express resistin (rstn) and fiaf, as well as suppressor of cytokine signaling-3 (socs-3), a feedback inhibitor of leptin signaling. Treating N-1 cells with recombinant resistin (200 ng/ml, 30 min) reduced both fiaf (25%, p < 0.005) and socs-3 (29%, p < 0.005) mRNA levels, and similar reductions in fiaf (40%, p < 0.001) and socs-3 (25%, p < 0.001) resulted following the overexpression of resistin. Conversely, when RNA interference (RNAi) was used to reduce endogenous rstn levels (-60%, p < 0.005), fiaf and socs-3 expression was increased (46 and 65% respectively, p < 0.005). A similar reduction in rstn mRNA was achieved using RNAi in differentiated 3T3-L1 adipocytes, and this manipulation also reduced fiaf and socs-3 expression (-53, -21 and -20% respectively, p < 0.005). In contrast, although RNAi successfully reduced fiaf mRNA by 50% (p < 0.001) in N-1 cells and 40% (p < 0.001) in 3T3-L1 cells, there was no effect on rstn or socs-3 mRNA. These data suggest that resistin exerts a novel autocrine/paracrine control over fiaf and socs-3 expression in both 3T3-L1 adipocytes and N-1 neurons. Such a mechanism could be part of the central feedback system that modulates the effects of adipokines, and other adiposity signals, implicated in hypothalamic energy homeostasis. However, it remains to be determined whether these in vitro results can be translated to the control of adipokine expression in brain and adipose tissue.

Role of neuropeptides in appetite regulation and obesity--a review

Obesity represents the most prevalent nutritional problem worldwide which in the long run predisposes to development of diabetes mellitus, hypertension, endometrial carcinoma, osteoarthritis, gall stones and cardiovascular diseases. Despite significant reductions in dietary fat consumption, the prevalence of obesity is on a rise and is taking on pandemic proportions. Obesity develops when energy intake exceeds energy expenditure over time. Recently, a close evolutionary relationship between the peripheral and hypothalamic neuropeptides has become apparent. The hypothalamus being the central feeding organ mediates regulation of short-term and long-term dietary intake via synthesis of various orexigenic and anorectic neuropeptides. The structure and function of many hypothalamic peptides (neuropeptide Y (NPY), melanocortins, agouti-related peptide (AGRP), cocaine and amphetamine regulated transcript (CART), melanin concentrating hormone (MCH), orexins have been characterized in rodent models The peripheral neuropeptides such as cholecystokinin (CCK), ghrelin, peptide YY (PYY3-36), amylin, bombesin regulate important gastrointestinal functions such as motility, secretion, absorption, provide feedback to the central nervous system on availability of nutrients and may play a part in regulating food intake. The pharmacological potential of several endogenous peripheral peptides released prior to, during and/or after feeding are being explored. Long-term regulation is provided by the main circulating hormones leptin and insulin. These systems implicated in hypothalamic appetite regulation provide potential targets for treatment of obesity which could potentially pass into clinical development in the next 5 years. This review summarizes various effects and interrelationship of these central and peripheral neuropeptides in metabolism, obesity and their potential role as targets for treatment of obesity.

Leptin signaling in neurotensin neurons involves STAT, MAP kinases ERK1/2, and p38 through c‐Fos and ATF1

The adipokine leptin signals energy status to the hypothalamus, which triggers a network of neuropeptide responses. Each hypothalamic cell type expresses a unique complement of neuropeptides, receptors, and second messengers; thus each likely responds specifically to peripheral hormones. We describe here the analysis of leptin signaling in a clonal population of mouse neurotensin (NT) -expressing hypothalamic neurons, N-39. Leptin induced phosphorylation of STAT3 and MAPK ERK1/2, but not the downstream effector of PI3K, Akt, and also induced c-Fos protein. We found activation of p38 MAPK by leptin, accompanied by phosphorylation of its downstream effector ATF-1. Phosphorylation of ATF-1 is blocked by the p38 MAPK inhibitor SB 203580. We linked this signaling directly to NT transcription. Protein binding analysis indicates that both ATF-1 and c-Fos are capable of binding to the mouse NT/N gene predominantly at physiological or high concentrations of leptin. The evidence indicates activation of distinct leptin signal transduction pathways that directly result in changes in NT gene expression and links these specific neurons to the control of energy homeostasis.

The role of neurotensin in central nervous system pathophysiology: what is the evidence?

The peptide neurotensin has been studied for more than 30 years. Although it is widely distributed in the central and peripheral nervous systems, neurotensin has been more intensely studied with regard to its interactions with the central dopamine system. A number of claims have been made regarding its possible implication in many diseases of the central nervous system, including schizophrenia. In this review, we describe briefly the basic biology of this neuropeptide, and then we consider the strengths and the weaknesses of the data that suggest a role for neurotensin in schizophrenia, drug abuse, Parkinson's disease, pain, central control of blood pressure, eating disorders, cancer, neurodegenerative disorders and inflammation.