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  • Abstract: Leptin x
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N T Lam, S D Covey, J T Lewis, S Oosman, T Webber, E C Hsu, A T Cheung and T J Kieffer

Obesity is typically associated with resistance to leptin, yet the mechanism by which leptin signaling becomes impaired is poorly understood. Here we sought to determine if the development of obesity and leptin resistance correlates with increased expression of protein tyrosine phosphatase 1B (PTP1B) in peripheral tissues and whether over-expression of this phosphatase, specifically in liver, could alter the leptin-mediated effects on feeding and glucose metabolism. Obesity was induced in mice through a high-fat diet that resulted in hyperglycemia, hyperinsulinemia and hyperleptinemia. Resistance to leptin was confirmed as exogenous leptin administration reduced food intake in animals on low-fat, but not high-fat diets. Diet-induced resistance to leptin and insulin was associated with increased hepatic levels of PTP1B. Intriguingly, hepatic adenoviral over-expression of PTP1B in ob/ob mice attenuated the ability of exogenous leptin to reduce both plasma glucose levels and food intake. These findings suggest that leptin reduces both plasma glucose and food intake in part through actions on the liver, and hepatic leptin resistance resulting from over-expression of PTP1B may contribute to the development of both diabetes and obesity.

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Salman Azhar, Dachuan Dong, Wen-Jun Shen, Zhigang Hu and Fredric B Kraemer

miRNAs are endogenous noncoding single-stranded small RNAs of ~22 nucleotides in length that post-transcriptionally repress the expression of their various target genes. They contribute to the regulation of a variety of physiologic processes including embryonic development, differentiation and proliferation, apoptosis, metabolism, hemostasis and inflammation. In addition, aberrant miRNA expression is implicated in the pathogenesis of numerous diseases including cancer, hepatitis, cardiovascular diseases and metabolic diseases. Steroid hormones regulate virtually every aspect of metabolism, and acute and chronic steroid hormone biosynthesis is primarily regulated by tissue-specific trophic hormones involving transcriptional and translational events. In addition, it is becoming increasingly clear that steroidogenic pathways are also subject to post-transcriptional and post-translational regulations including processes such as phosphorylation/dephosphorylation, protein‒protein interactions and regulation by specific miRNAs, although the latter is in its infancy state. Here, we summarize the recent advances in miRNA-mediated regulation of steroidogenesis with emphasis on adrenal and gonadal steroidogenesis.

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Jessica A Deis, Hong Guo, Yingjie Wu, Chengyu Liu, David A Bernlohr and Xiaoli Chen

Lipocalin-2 (LCN2) has been previously characterized as an adipokine regulating thermogenic activation of brown adipose tissue and retinoic acid (RA)-induced thermogenesis in mice. The objective of this study was to explore the role and mechanism for LCN2 in the recruitment and retinoic acid-induced activation of brown-like or ‘beige’ adipocytes. We found LCN2 deficiency reduces key markers of thermogenesis including uncoupling protein-1 (UCP1) and peroxisome proliferator-activated receptor gamma coactivator 1α (PGC-1α) in inguinal white adipose tissue (iWAT) and inguinal adipocytes derived from Lcn2 −/− mice. Lcn2 −/− inguinal adipocytes have attenuated insulin-induced upregulation of thermogenic gene expression and p38 mitogen-activated protein kinase (p38MAPK) signaling pathway activation. This is accompanied by a lower basal and maximal oxidative capacity in Lcn2 −/− inguinal adipocytes, indicating mitochondrial dysfunction. Recombinant Lcn2 was able to restore insulin-induced p38MAPK phosphorylation in both WT and Lcn2 −/− inguinal adipocytes. Rosiglitazone treatment during differentiation of Lcn2 −/− adipocytes is able to recruit beige adipocytes at a normal level, however, further activation of beige adipocytes by insulin and RA is impaired in the absence of LCN2. Further, the synergistic effect of insulin and RA on UCP1 and PGC-1α expression is markedly reduced in Lcn2 −/− inguinal adipocytes. Most intriguingly, LCN2 and the retinoic acid receptor-alpha (RAR-α) are concurrently translocated to the plasma membrane of adipocytes in response to insulin, and this insulin-induced RAR-α translocation is absent in adipocytes deficient in LCN2. Our data suggest a novel LCN2-mediated pathway by which RA and insulin synergistically regulates activation of beige adipocytes via a non-genomic pathway of RA action.

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Ciro Menale, Maria Teresa Piccolo, Grazia Cirillo, Raffaele A Calogero, Alfonso Papparella, Luigi Mita, Emanuele Miraglia Del Giudice, Nadia Diano, Stefania Crispi and Damiano Gustavo Mita

Bisphenol A (BPA) is a xenobiotic endocrine-disrupting chemical. In vitro and in vivo studies have indicated that BPA alters endocrine-metabolic pathways in adipose tissue, which increases the risk of metabolic disorders and obesity. BPA can affect adipose tissue and increase fat cell numbers or sizes by regulating the expression of the genes that are directly involved in metabolic homeostasis and obesity. Several studies performed in animal models have accounted for an obesogen role of BPA, but its effects on human adipocytes – especially in children – have been poorly investigated. The aim of this study is to understand the molecular mechanisms by which environmentally relevant doses of BPA can interfere with the canonical endocrine function that regulates metabolism in mature human adipocytes from prepubertal, non-obese children. BPA can act as an estrogen agonist or antagonist depending on the physiological context. To identify the molecular signatures associated with metabolism, transcriptional modifications of mature adipocytes from prepubertal children exposed to estrogen were evaluated by means of microarray analysis. The analysis of deregulated genes associated with metabolic disorders allowed us to identify a small group of genes that are expressed in an opposite manner from that of adipocytes treated with BPA. In particular, we found that BPA increases the expression of pro-inflammatory cytokines and the expression of FABP4 and CD36, two genes involved in lipid metabolism. In addition, BPA decreases the expression of PCSK1, a gene involved in insulin production. These results indicate that exposure to BPA may be an important risk factor for developing metabolic disorders that are involved in childhood metabolism dysregulation.

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S Viengchareun, H Bouzinba-Segard, J-P Laigneau, M-C Zennaro, P A Kelly, A Bado, M Lombès and N Binart

The pituitary hormone prolactin (PRL) exerts pleiotropic effects, which are mediated by a membrane receptor (PRLR) present in numerous cell types including adipocytes. Brown adipose tissue (BAT) expresses uncoupling proteins (UCPs), involved in thermogenesis, but also secretes leptin, a key hormone involved in the control of body weight. To investigate PRL effects on BAT, we used the T37i brown adipose cell line, and demonstrated that PRLRs are expressed as a function of cell differentiation. Addition of PRL leads to activation of the JAK/STAT and MAP kinase signaling pathways, demonstrating that PRLRs are functional in these cells. Basal and catecholamine-induced UCP1 expression were not affected by PRL. However, PRL combined with insulin significantly increases leptin expression and release, indicating that PRL potentiates the stimulatory effect of insulin as revealed by the recruitment of insulin receptor substrates and the activation of phosphatidylinositol 3-kinase. To explore the in vivo physiological relevance of PRL action in BAT, we showed that leptin content was significantly increased in BAT of PRLR-null mice compared with wild-type mice, highlighting the involvement of PRL in the leptin secretion process. This study provides the first evidence for a functional link between PRL and energy balance via a cross-talk between insulin and PRL signaling pathways in brown adipocytes.

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Chung Thong Lim, Blerina Kola and Márta Korbonits

AMP-activated protein kinase (AMPK) is a key molecular player in energy homeostasis at both cellular and whole-body levels. AMPK has been shown to mediate the metabolic effects of hormones such as leptin, ghrelin, adiponectin, glucocorticoids and insulin as well as cannabinoids. Generally, activated AMPK stimulates catabolic pathways (glycolysis, fatty acid oxidation and mitochondrial biogenesis) and inhibits anabolic pathways (gluconeogenesis, glycogen, fatty acid and protein synthesis), and has a direct appetite-regulating effect in the hypothalamus. Drugs that activate AMPK, namely metformin and thiazolidinediones, are often used to treat metabolic disorders. Thus, AMPK is now recognised as a potential target for the treatment of obesity and associated co-morbidities.

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Inge Seim, Amy A Lubik, Melanie L Lehman, Nadine Tomlinson, Eliza J Whiteside, Adrian C Herington, Colleen C Nelson and Lisa K Chopin

Ghrelin is a multifunctional hormone, with roles in stimulating appetite and regulating energy balance, insulin secretion and glucose homoeostasis. The ghrelin gene locus (GHRL) is highly complex and gives rise to a range of novel transcripts derived from alternative first exons and internally spliced exons. The wild-type transcript encodes a 117 amino acid preprohormone that is processed to yield the 28 amino acid peptide ghrelin. Here, we identified insulin-responsive transcription corresponding to cryptic exons in intron 2 of the human ghrelin gene. A transcript, termed in2c-ghrelin (intron 2-cryptic), was cloned from the testis and the LNCaP prostate cancer cell line. This transcript may encode an 83 amino acid preproghrelin isoform that codes for ghrelin, but not obestatin. It is expressed in a limited number of normal tissues and in tumours of the prostate, testis, breast and ovary. Finally, we confirmed that in2c-ghrelin transcript expression, as well as the recently described in1-ghrelin transcript, is significantly upregulated by insulin in cultured prostate cancer cells. Metabolic syndrome and hyperinsulinaemia have been associated with prostate cancer risk and progression. This may be particularly significant after androgen deprivation therapy for prostate cancer, which induces hyperinsulinaemia, and this could contribute to castrate-resistant prostate cancer growth. We have previously demonstrated that ghrelin stimulates prostate cancer cell line proliferation in vitro. This study is the first description of insulin regulation of a ghrelin transcript in cancer and should provide further impetus for studies into the expression, regulation and function of ghrelin gene products.

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David García-Galiano, Victor M Navarro, Francisco Gaytan and Manuel Tena-Sempere

Nesfatin-1 was originally identified as a hypothalamic neuropeptide, derived from the precursor NEFA (for DNA binding/EF-hand/acidic protein)/nucleobindin 2 (NUCB2), with the ability to suppress food intake, acting in a leptin-independent manner. Departing from this seminal finding, the patterns of expression of NUCB2/nesfatin-1 have been thoroughly characterized in different hypothalamic nuclei and brain areas with proven roles in energy homeostasis, and its potential interactions with other key neuropeptide regulators of appetite have been documented. Intriguingly, recent experimental evidence suggests that NUCB2/nesfatin-1 is also expressed in peripheral tissues with relevant metabolic functions, such as the pancreas, the adipose, and the gut. In addition, evidence is mounting that nesfatin signaling may participate in adaptative responses and in the control of body functions gated by the state of energy reserves, such as puberty onset. Altogether, these observations have broadened our perception of the biological profile of nesfatin-1 that, rather than a simple anorectic signal in the hypothalamus, might operate at different tissues as an integral regulator of energy homeostasis and closely related neuroendocrine functions.

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Rhonda D Kineman, Manuel D Gahete and Raul M Luque

The mouse ghrelin gene contains 5 exons (Ex), with Ex2–Ex5 encoding a 117 amino acid preproprotein that is processed to yield a 28 amino acid mature peptide. The current study examined if pituitary (PIT) and hypothalamus (HPT) ghrelin expression is up-regulated in response to fasting and down-regulated in obesity, as previously reported in the stomach. In the process of establishing a quantitative real-time RT-PCR system to accurately assess the changes in PIT and HPT ghrelin mRNA levels, we observed that primer sets located in Ex2 and Ex3 amplified a ghrelin transcript that contained the entire intron 2 (In2). Size and sequence analysis of RT-PCR products using multiple primer sets located throughout the ghrelin gene suggested that the In2-ghrelin variant contains Ex2 and Ex3, but lacks Ex1, Ex4, and Ex5. In2-ghrelin variant mRNA was not detected in stomach extracts, while expression levels were 10- and 50-fold greater than that of the native ghrelin transcript in the PIT and HPT respectively. In2-ghrelin variant mRNA levels increased in the PIT after 24 h fasting and decreased in the HPT and PIT of diet-induced obese mice. These changes may be due to the changes in circulating insulin or IGF-I, since both decreased In2-ghrelin variant expression in a mouse HPT cell line (N6) and in primary mouse PIT cell cultures. The fact that In2-ghrelin variant mRNA levels are dependent on energy intake in the PIT and HPT suggests that this transcript may encode a peptide important in coordinating the neuroendocrine response to metabolic stress.

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Xinwang Chen, Xiao Jia, Jie Qiao, Youfei Guan and Jihong Kang

Polycystic ovary syndrome (PCOS) is the most common endocrinopathy associated with infertility and metabolic disorder in women of reproductive age. Dysfunction of adipose tissue has been implicated in the pathophysiology of PCOS. Increasing evidence shows that the dysregulated expression of adipokines, the secreted products of adipose tissue, plays an important role in the pathology of PCOS. Here, we review the role of several identified adipokines that may act as a link between obesity and PCOS. PCOS also reciprocally influences the profile of adipokines. Insight into the underlying mechanisms will help better understand the pathology of PCOS and identify new therapeutic targets of this syndrome.