Regulation of HIF-1{alpha} activity in adipose tissue by obesity-associated factors: adipogenesis, insulin, and hypoxia

Systems biology of adipose tissue metabolism: regulation of growth, signaling and inflammation

Adipose tissue (AT) depots actively regulate whole body energy homeostasis by orchestrating complex communications with other physiological systems as well as within the tissue. Adipocytes readily respond to hormonal and nutritional inputs to store excess nutrients as intracellular lipids or mobilize the stored fat for utilization. Co-ordinated regulation of metabolic pathways balancing uptake, esterification, and hydrolysis of lipids is accomplished through positive and negative feedback interactions of regulatory hubs comprising several pleiotropic protein kinases and nuclear receptors. Metabolic regulation in adipocytes encompasses biogenesis and remodeling of uniquely large lipid droplets (LDs). The regulatory hubs also function as energy and nutrient sensors, and integrate metabolic regulation with intercellular signaling. Over-nutrition causes hypertrophic expansion of adipocytes, which, through incompletely understood mechanisms, initiates a cascade of metabolic and signaling events leading to tissue remodeling and immune cell recruitment. Macrophage activation and polarization toward a pro-inflammatory phenotype drives a self-reinforcing cycle of pro-inflammatory signals in the AT, establishing an inflammatory state. Sustained inflammation accelerates lipolysis and elevates free fatty acids in circulation, which robustly correlates with development of obesity-related diseases. The adipose regulatory network coupling metabolism, growth, and signaling of multiple cell types is exceedingly complex. While components of the regulatory network have been individually studied in exquisite detail, systems approaches have rarely been utilized to comprehensively assess the relative engagements of the components. Thus, need and opportunity exist to develop quantitative models of metabolic and signaling networks to achieve a more complete understanding of AT biology in both health and disease.

Nutrient restriction and radiation therapy for cancer treatment: when less is more

Calorie restriction (CR), or a diet modification aiming to reduce the total intake of calories by 20%-40%, has been shown to increase longevity across multiple species. Recently, there has been growing interest in investigating the potential role of CR as a treatment intervention for age-related diseases, such as cancer, because an increasing body of literature has demonstrated a metabolic component to both carcinogenesis and tumor progression. In fact, many of the molecular pathways that are altered with CR are also known to be altered in cancer. Therefore, manipulation of these pathways using CR can render cancer cells, and most notably breast cancer cells, more susceptible to standard cytotoxic treatment with radiation and chemotherapy. In this review article we demonstrate the laboratory and clinical evidence that exists for CR and show compelling evidence through the molecular pathways CR induces about how it may be used as a treatment in tandem with radiation therapy to improve our rates of disease control.

Insulin promotes iron uptake in human hepatic cell by regulating transferrin receptor-1 transcription mediated by hypoxia inducible factor-1

Hepatic iron is known to regulate insulin signaling pathways and to influence insulin sensitivity in insulin resistance (IR) patients. However, the role of insulin on hepatic iron homeostasis remains unexplored. Here, we report that insulin promotes transferrin-bound iron uptake but shows no influence on non transferrin-bound iron uptake in human hepatic HepG2 cells. As a mechanism we detected increased transferrin receptor-1 (TfR1) expression both at protein and mRNA levels. Unaltered stability of protein and transcript of TfR1 suggested the regulation at transcriptional level that was confirmed by promoter activity. Involvement of transcription factor hypoxia inducible factor-1 (HIF-1) was shown by mutational analyses of the TfR1 promoter region and by electrophoretic mobility shift assay. When HepG2 cells were transfected with specific siRNA targeted to 3'UTR of HIF-1α, the regulatory subunit of HIF-1; insulin-induced TfR1 expression and iron uptake were inhibited. Transfection of cDNA expressing stable form of HIF-1α reversed the increased TfR1 expression and iron uptake. These results suggest a novel role of insulin in hepatic iron uptake by a HIF-1 dependent transcriptional regulation of TfR1.

Adipose tissue oxygen tension: implications for chronic metabolic and inflammatory diseases

PURPOSE OF REVIEW

The present review aims to address the role of adipose tissue oxygen partial pressure (PO2) in the metabolic and endocrine derangements in conditions characterized by insulin resistance.

RECENT FINDINGS

The balance between adipose tissue oxygen supply and its metabolic rate seems to determine adipose tissue PO2. Studies in ob/ob and dietary-induced obese mice have provided evidence for adipose tissue hypoxia in obesity, which has been explained by insufficient adipose tissue angiogenesis during the massive and rapid weight gain in these animals. However, conflicting data have been reported in humans, showing both increased and decreased adipose tissue PO2 in obese compared with lean individuals. Both low and high adipose tissue PO2 may induce a proinflammatory phenotype in (pre)adipocytes, but most studies have been performed under rather extreme PO2 levels, not reflecting human adipose tissue physiology. Furthermore, adipose tissue PO2 may affect glucose and lipid metabolism as well as adipogenic differentiation, but many issues still need to be addressed.

SUMMARY

Adipose tissue hypoxia has been demonstrated in animal models of obesity, but findings in humans are controversial and require further investigation. Although adipose tissue PO2 seems to be involved in metabolic and endocrine derangements in human adipose tissue, future studies should investigate how low and high adipose tissue PO2 within the human physiological range (3-11% O2) relates to adipose tissue blood flow and oxygen consumption, cellular metabolic responses, and the inflammatory phenotype.

NADPH oxidase 4 mediates insulin-stimulated HIF-1α and VEGF expression, and angiogenesis in vitro

Acute intensive insulin therapy causes a transient worsening of diabetic retinopathy in type 1 diabetes patients and is related to VEGF expression. Reactive oxygen species (ROS) have been shown to be involved in HIF-1α and VEGF expression induced by insulin, but the role of specific ROS sources has not been fully elucidated. In this study we examined the role of NADPH oxidase subunit 4 (Nox4) in insulin-stimulated HIF-1α and VEGF expression, and angiogenic responses in human microvascular endothelial cells (HMVECs). Here we demonstrate that knockdown of Nox4 by siRNA reduced insulin-stimulated ROS generation, the tyrosine phosphorylation of IR-β and IRS-1, but did not change the serine phosphorylation of IRS-1. Nox4 gene silencing had a much greater inhibitory effect on insulin-induced AKT activation than ERK1/2 activation, whereas it had little effect on the expression of the phosphatases such as MKP-1 and SHIP. Inhibition of Nox4 expression inhibited the transcriptional activity of VEGF through HIF-1. Overexpression of wild-type Nox4 was sufficient to increase VEGF transcriptional activity, and further enhanced insulin-stimulated the activation of VEGF. Downregulation of Nox4 expression decreased insulin-stimulated mRNA and protein expression of HIF-1α, but did not change the rate of HIF-1α degradation. Inhibition of Nox4 impaired insulin-stimulated VEGF expression, cell migration, cell proliferation, and tube formation in HMVECs. Our data indicate that Nox4-derived ROS are essential for HIF-1α-dependent VEGF expression, and angiogenesis in vitro induced by insulin. Nox4 may be an attractive therapeutic target for diabetic retinopathy caused by intensive insulin treatment.

Hypoxia and estrogen are functionally equivalent in breast cancer-endothelial cell interdependence

Background

Rapid breast tumor development relies on formation of new vasculature to supply the growing malignancy with oxygenated blood. Previously we found that estrogen aided in this neovasculogenesis via recruitment of bone marrow derived endothelial progenitor cells (BM-EPCs), leading to increased vessel formation and vascular endothelial growth factor (VEGF) production in vivo. However, the cellular mechanism of this induction and the signaling pathways involved need elucidation.

Results

Using the murine mammary cell line TG1-1 we observed estrogen (E₂) lead to an up regulation of hypoxia inducible factor-1 (HIF-1), an effect abrogated by the anti-estrogen Fulvestrant and the HIF-1 inhibitor YC-1 (3-(5’-hydroxymethyl-2’-furyl)-1-benzylindazole) suggesting the interchangeability of hypoxia and estrogen mediated effects. Estrogen modulation of HIF-1 and subsequent effects on endothelial cells is dependent on the Akt/PI3K pathway and protein synthesis as validated by the use of the inhibitors wortmannin and cycloheximide which abrogated estrogen’s effects respectively. Estrogen treated TG1-1 cells secreted higher levels of VEGF which were comparable to secreted levels from cells grown under hypoxic conditions. Soluble factors in conditioned media from E₂ treated breast cancer cells also lead to migration and tube formation of human umbilical vein endothelial cells (HUVEC) in vitro.

Conclusions

Our data provide evidence that estrogen signaling mediates the tumor vasculogenic process required for breast cancer progression and involves a key regulator of the hypoxia signaling pathway. Further, hypoxia and estrogen are interchangeable as both similarly modulate epithelial-endothelial cell interaction.

Early chronotype and tissue-specific alterations of circadian clock function in spontaneously hypertensive rats

Malfunction of the circadian timing system may result in cardiovascular and metabolic diseases, and conversely, these diseases can impair the circadian system. The aim of this study was to reveal whether the functional state of the circadian system of spontaneously hypertensive rats (SHR) differs from that of control Wistar rat. This study is the first to analyze the function of the circadian system of SHR in its complexity, i.e., of the central clock in the suprachiasmatic nuclei (SCN) as well as of the peripheral clocks. The functional properties of the SCN clock were estimated by behavioral output rhythm in locomotor activity and daily profiles of clock gene expression in the SCN determined by in situ hybridization. The function of the peripheral clocks was assessed by daily profiles of clock gene expression in the liver and colon by RT-PCR and in vitro using real time recording of Bmal1-dLuc reporter. The potential impact of the SHR phenotype on circadian control of the metabolic pathways was estimated by daily profiles of metabolism-relevant gene expression in the liver and colon. The results revealed that SHR exhibited an early chronotype, because the central SCN clock was phase advanced relative to light/dark cycle and the SCN driven output rhythm ran faster compared to Wistar rats. Moreover, the output rhythm was dampened. The SHR peripheral clock reacted to the dampened SCN output with tissue-specific consequences. In the colon of SHR the clock function was severely altered, whereas the differences are only marginal in the liver. These changes may likely result in a mutual desynchrony of circadian oscillators within the circadian system of SHR, thereby potentially contributing to metabolic pathology of the strain. The SHR may thus serve as a valuable model of human circadian disorders originating in poor synchrony of the circadian system with external light/dark regime.

Negative regulation of human growth hormone gene expression by insulin is dependent on hypoxia-inducible factor binding in primary non-tumor pituitary cells

Insulin controls growth hormone (GH) production at multiple levels, including via a direct effect on pituitary somatotrophs. There are no data, however, on the regulation of the intact human (h) GH gene (hGH1) by insulin in non-tumor pituitary cells, but the proximal promoter region (nucleotides -496/+1) responds negatively to insulin in transfected pituitary tumor cells. A DNA-protein interaction was also induced by insulin at nucleotides -308/-235. Here, we confirmed the presence of a hypoxia-inducible factor 1 (HIF-1) binding site within these sequences (-264/-259) and investigated whether HIF-1 is associated with insulin regulation of "endogenous" hGH1. In the absence of primary human pituitary cells, transgenic mice expressing the intact hGH locus in a somatotroph-specific manner were generated. A significant and dose-dependent decrease in hGH and mouse GH RNA levels was detected in primary pituitary cell cultures from these mice with insulin treatment. Increasing HIF-1α availability with a hypoxia mimetic significantly decreased hGH RNA levels and was accompanied by recruitment of HIF-1α to the hGH1 promoter in situ as seen with insulin. Both inhibition of HIF-1 DNA binding by echinomycin and RNA interference of HIF-1α synthesis blunted the negative effect of insulin on hGH1 but not mGH. The insulin response is also sensitive to histone deacetylase inhibition/trichostatin A and associated with a decrease in H3/H4 hyperacetylation in the proximal hGH1 promoter region. These data are consistent with HIF-1-dependent down-regulation of hGH1 by insulin via chromatin remodeling specifically in the proximal promoter region.

Adipose tissue inflammation and adiponectin resistance in patients with advanced heart failure: correction after ventricular assist device implantation

BACKGROUND

Heart failure (HF) is characterized by inflammation, insulin resistance, and progressive catabolism. We hypothesized that patients with advanced HF also develop adipose tissue inflammation associated with impaired adipokine signaling and that hemodynamic correction through implantation of ventricular assist devices (VADs) would reverse adipocyte activation and correct adipokine signaling in advanced HF.

METHODS AND RESULTS

Circulating insulin, adiponectin, leptin, and resistin levels were measured in 36 patients with advanced HF before and after VAD implantation and 10 healthy control subjects. Serum adiponectin was higher in HF patients before VAD implantation compared with control subjects (13.3±4.9 versus 6.4±2.1 μg/mL, P=0.02). VAD implantation (mean, 129±99 days) reduced serum adiponectin (7.4±3.4 μg/mL, P<0.05) and improved insulin resistance (Homeostasis Assessment Model of insulin resistance: 7.6±7.7-4.5±3.6; P=0.012). [corrected] Adiponectin expression in adipose tissue decreased after VAD implantation (-65%; P<0.03). Adiponectin receptor expression was suppressed in the failing myocardium compared with control subjects and increased after mechanical unloading. Histomorphometric analysis of adipose tissue specimens revealed reduced adipocyte size in patients with advanced HF compared with control subjects (2105±585 μm(2) [corrected] versus 5583±142 μm(2) in control subjects; P<0.05), which increased after VAD placement. Of note, macrophage infiltration in adipose tissue was higher in advanced HF patients compared with control subjects (+25%; P<0.01), which normalized after VAD implantation.

CONCLUSIONS

Adipose tissue inflammation and adiponectin resistance develop in advanced HF. Mechanical unloading of the failing myocardium reverses adipose tissue macrophage infiltration, inflammation, and adiponectin resistance in patients with advanced HF.

Hypoxia-inducible factor 1 activation from adipose protein 2-cre mediated knockout of von Hippel-Lindau gene leads to embryonic lethality

von Hippel-Lindau protein, an E3 ubiquitin ligase from the von Hippel-Lindau (Vhl) gene, inhibits the transcriptional activity of hypoxia-inducible factor 1α in cells. To gain insight into the hypoxia-inducible factor 1α signalling pathway in adipose tissue, a study was conducted to generate fat-specific Vhl knockout mice. Cre-recombinase (Cre)/locus of crossover in P1(loxP) technology was used in the knockout study. The mice carrying floxed-Vhl alleles were crossed with adipose protein 2 (aP2)-Cre mice, in which the Cre gene is driven by the aP2 (fatty acid binding protein 4) gene promoter. The homozygous knockout mice exhibited embryonic lethality at E14.5-E18.5. The homozygous embryos suffered from haemorrhages in the brain and liver. Hypoxia-inducible factor 1α protein and its target gene protein, vascular endothelial growth factor, increased in the brain and liver. Endothelial proliferation and capillary leakage were observed in the tissues. Heterozygous knockout mice appeared normal in development, growth and reproductivity. β-galactosidase reporter mice were used in the analysis of tissue-specificity of Cre in aP2-Cre mice. Strong Cre activity was observed in the dorsal hindbrain region and vertebrae of E12.5 embryos. These results suggest that in the aP2-Cre mice, the recombinase activity is expressed in the central nervous system of the embryos. Central and peripheral haemorrhages are responsible for the embryonic lethality in the homozygous knockout mice.

Increased angiogenesis protects against adipose hypoxia and fibrosis in metabolic disease-resistant 11β-hydroxysteroid dehydrogenase type 1 (HSD1)-deficient mice

In obesity, rapidly expanding adipose tissue becomes hypoxic, precipitating inflammation, fibrosis, and insulin resistance. Compensatory angiogenesis may prevent these events. Mice lacking the intracellular glucocorticoid-amplifying enzyme 11β-hydroxysteroid dehydrogenase type 1 (11βHSD1^−/−) have “healthier” adipose tissue distribution and resist metabolic disease with diet-induced obesity. Here we show that adipose tissues of 11βHSD1^−/− mice exhibit attenuated hypoxia, induction of hypoxia-inducible factor (HIF-1α) activation of the TGF-β/Smad3/α-smooth muscle actin (α-SMA) signaling pathway, and fibrogenesis despite similar fat accretion with diet-induced obesity. Moreover, augmented 11βHSD1^−/− adipose tissue angiogenesis is associated with enhanced peroxisome proliferator-activated receptor γ (PPARγ)-inducible expression of the potent angiogenic factors VEGF-A, apelin, and angiopoietin-like protein 4. Improved adipose angiogenesis and reduced fibrosis provide a novel mechanism whereby suppression of intracellular glucocorticoid regeneration promotes safer fat expansion with weight gain.

Apelin, diabetes, and obesity

Apelin is a peptide known as the ligand of the G-protein-coupled receptor APJ. Several active apelin forms exist such as apelin-36, apelin-17, apelin-13, and the pyroglutamated form of apelin-13. Apelin and APJ are expressed in the central nervous system, particularly in the hypothalamus and in many peripheral tissues. Apelin has been shown to be involved in the regulation of cardiovascular and fluid homeostasis, food intake, cell proliferation, and angiogenesis. In addition to be an ubiquitous peptide, apelin is also produced and secreted by adipocytes and thus considered as an adipokine. This has opened a new field of investigation establishing a link between apelin and metabolic disorders (obesity, type 2 diabetes, etc.) which is the focus of the present review. Several studies, but not all, have reported an increase of plasma apelin concentrations in humans and in animal models with different metabolic pathologies. Moreover, important roles for apelin both in glucose and lipid metabolism have been highlighted as well as the associated signaling pathways. Apelin appears as a beneficial adipokine with anti-obesity and anti-diabetic properties and thus as a promising therapeutic target in metabolic disorders.