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SJ Kaczmarczyk, S Andrikopoulos, J Favaloro, AA Domenighetti, A Dunn, M Ernst, D Grail, M Fodero-Tavoletti, CE Huggins, LM Delbridge, JD Zajac and J Proietto

The aim of this study was to investigate the metabolic and structural consequences of a decrease in glucose transporter-4 (GLUT4) levels on the heart. The CreLoxP system was utilised to delete GLUT4 in muscle tIssue including heart. The presence of the PGK-neoR cassette in the GLUT4-Lox mice resulted in reduced expression in all tIssues to levels 15-30% of wild-type control mice. In mice expressing Cre recombinase, there was a further reduction of GLUT4 in cardiac tIssue to almost undetectable levels. Cardiac glucose uptake was measured basally and during a euglycaemic/hyperinsulinaemic clamp using 2-deoxy-[1-(14)C]glucose. Insulin-stimulated glucose uptake was normal in hearts expressing 15% of normal GLUT4 levels but markedly reduced in mice with more profound reduction in GLUT4. Cardiac enlargement occurred only when GLUT4 levels were less than 5% of normal values. In heart there is a threshold level of GLUT4 above which insulin-stimulated glucose uptake is maintained. As little as 5% of normal GLUT4 levels expressed in heart is sufficient to prevent the development of cardiac hypertrophy.

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Xueyao Yin, Fenping Zheng, Qianqian Pan, Saifei Zhang, Dan Yu, Zhiye Xu and Hong Li

Oxidative stress is considered to be an important factor in producing lethal hepatocyte injury associated with nonalcoholic fatty liver disease (NAFLD). Glucose fluctuation, more pronounced in patients with diabetes, has been recognized as an even stronger oxidative stress inducer than the sustained hyperglycemia. Here, we investigated the role of glucose variability in the development of the NAFLD based on hepatocyte apoptosis and possible mechanisms. To achieve this goal we studied C57BL/6J mice that were maintained on a high fat diet (HFD) and injected with glucose (3 g/kg) twice daily to induce intermittent high glucose (IHG). We also studied hepatic L02 cells incubated with palmitic acid (PA) to induce steatosis. The following experimental groups were compared: normal glucose (NG), sustained high glucose (SHG) and IHG with or without PA. We found that, although hepatic enzyme levels and liver lipid deposition were comparable between HFD mice injected with glucose or saline, the glucose injected mice displayed marked hepatocyte apoptosis and inflammation, accompanied by increased lipid peroxide in liver. In vitro, in the presence of PA, IHG increased L02 cell apoptosis and oxidative stress and produced pronounced mitochondrial dysfunction relative to the NG and SHG groups. Furthermore, treatment with the mitochondrial permeability transition (MPT) inhibitor, cyclosporin A (1.5 μmol/l), prevented mitochondrial dysfunction, oxidative stress and hepatocyte apoptosis. Our data suggests that IHG under lipotoxicity might contribute to the development of NAFLD by increasing oxidative stress and hepatocyte apoptosis via MPT and its related mitochondrial dysfunction.

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Boris Thurisch, Shermi Y Liang, Nanette Sarioglu, Lutz Schomburg, Jörg Bungert and Christof Dame

Homozygous deficiency of the transcription factor Gata4 in mice causes lethality due to defects in ventral morphogenesis and heart tube formation. There is increasing evidence demonstrating that GATA4 function is also relevant for normal developed organ systems, including the heart and endocrinum. To analyze the implication of Gata4 beyond development, we generated transgenic mice expressing inducible small interfering RNA against Gata4. In longitudinal analysis, efficient suppression of Gata4 mRNA (down to 80% of wild-type levels) and protein expression in the heart was detected 38 days after induction of Gata4 short hairpin RNA. Decreased Gata4 expression was associated with reduction in the expression of known cardiac target genes, but the function of the heart remained unperturbed at 20–30% of normal Gata4 levels. Interestingly, Gata4 expression was almost abolished in the ovary and testis. This was accompanied in the testis by a significant reduction of GATA4 downstream target genes, such as the genes encoding Mullerian inhibiting substance (MIS) and steroidogenic acute regulatory (StAR) protein. By contrast, expression levels of Mis and Star were only slightly modified in the ovary, and concentrations of circulating FSH and LH were normal in female transgenic mice after induction of Gata4 short hairpin RNA. However, inhibition of Gata4 expression led to the formation of ovarian teratoma in 10% of females. Histology of the teratomas showed predominantly ectodermal and mesodermal structures. Our data demonstrate that Gata4 is critically involved in the function and integrity of the gonads in vivo.

Free access

A Kawakoshi, S Hyodo, A Yasuda and Y Takei

In teleost fish and tetrapods, the natriuretic peptide (NP) family consists of ANP (atrial natriuretic peptide), BNP (brain natriuretic peptide) and VNP (ventricular natriuretic peptide) that are secreted from the heart, and C-type natriuretic peptide (CNP) that is found in the brain. However, CNP is the only NP identified in the heart and brain of elasmobranchs, suggesting that it is the ancestral type of the NP family and that ANP, BNP and VNP appeared later in the vertebrate phylogeny. To delineate more clearly the molecular evolution of this hormone family, we determined the sequence of NP molecule(s) in evolutionarily the oldest vertebrate group, the cyclostomes. We have cloned a novel NP cDNA from the heart and brain of hagfish, Eptatretus burgeri, using the RACE method and degenerate primers that amplify all known types of NP cDNAs. The novel NP, named EbuNP after the scientific name of this hagfish, appears to be the only NP in the heart and brain, as no other NP cDNAs were amplified even after specific removal of the cloned EbuNP mRNA from the mRNA pool, except for a minor alternatively spliced EbuNP cDNA with a truncated 3'-untranslated sequence. The EbuNP was equally similar to known NPs but was not considered to be a CNP because of the presence of a C-terminal tail sequence. The EbuNP gene was abundantly expressed in the cardiac atrium, ventricle, portal heart and brain but scarcely in the intestine; no expression was observed in the gill and kidney. Mass spectrometry of affinity-purified EbuNP in plasma, heart and brain revealed a 68 amino acid peptide circulating in the blood and stored in the heart, which is cleaved at the typical cleavage signal of a processing enzyme, furin, as observed in mammalian BNP. The C-terminal Gly residue was used for amidation as is the case in eel ANP. The immunoreactive EbuNP was not detected in the brain, suggesting the presence of a different processing form in the brain. These results show that the molecular evolution of the NP family in vertebrates is more complex than previously thought.

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Rebecca Roy, Caitlyn Nguyen-Ngo and Martha Lappas

Gestational diabetes mellitus (GDM) affects up to 16% of pregnant women and is associated with significant long-term health detriments for mum and her offspring. Two central features of GDM are low-grade inflammation and maternal peripheral insulin resistance, therefore therapeutics which target these may be most effective at preventing the development of GDM. Short-chain fatty acids (SCFAs), such as butyrate and propionate, are metabolites produced from the fermentation of dietary fibre by intestinal microbiota. SCFAs possess anti-inflammatory, anti-obesity and anti-diabetic properties. Therefore, this study aimed to investigate the effect of SCFAs on inflammation and insulin signalling defects in an in vitro model of GDM. Human placenta, visceral adipose tissue (VAT) and subcutaneous adipose tissue (SAT) were stimulated with either the pro-inflammatory cytokine TNF or bacterial product lipopolysaccharide (LPS). The SCFAs butyrate and propionate blocked TNF- and LPS-induced mRNA expression and secretion of pro-inflammatory cytokines and chemokines in placenta, VAT and SAT. Primary human cells isolated from skeletal muscle were stimulated with TNF to assess the effect of SCFAs on inflammation-induced defects in the insulin signalling pathway. Butyrate and propionate were found to reverse TNF-induced increases in IRS-1 serine phosphorylation and decreases in glucose uptake. Butyrate and propionate exerted these effects by preventing ERK activation. Taken together, these results suggest that the SCFAs may be able to improve insulin sensitivity and prevent inflammation induced by sterile or bacterial inflammation. Future in vivo studies are warranted to investigate the efficacy and safety of SCFAs in preventing insulin resistance and inflammation associated with GDM.

Free access

A Kawakoshi, S Hyodo, K Inoue, Y Kobayashi and Y Takei

The natriuretic peptide (NP) family is composed of three members: atrial, brain/ventricular and C-type NPs (ANP, BNP/VNP and CNP respectively) in tetrapods and teleostean fish, but only CNP in elasmobranch fish. In order to trace the process of divergence of the NP family in early vertebrate evolution, we attempted to detect NPs in the primitive ray-finned fish, the sturgeon (Acipenser transmontanus). Unexpectedly, we isolated four distinct NP cDNAs from the heart and brain of this chondrostean fish. The single NP from the brain was CNP, as judged from the lack of C-terminal 'tail' sequence extending from the intramolecular ring. Two of the three cardiac NPs were ANP and VNP, as judged by the presence of an amidation signal at its C-terminus (ANP) and a long and conserved C-terminal tail sequence (VNP) respectively. The third cardiac NP was most probably BNP because it possessed all the features characteristic of BNP including: (1) the presence of dibasic amino acids within the intramolecular ring; (2) the presence of AUUUA repeats in the 3'-untranslated region of its mRNA; (3) equivalent expression of its mRNA in the atrium and ventricle and appreciable expression in the brain. Based on the sturgeon BNP sequence, we further isolated BNP cDNA from the heart of tilapia and pufferfish for the first time in teleostean fish. Phylogenetic analysis of the precursors showed that newly identified NPs belong to each group of the four NPs. The current identification of both VNP and BNP in the sturgeon clearly showed that BNP and VNP are coded by distinct genes, and that the NP family consists of at least four members in the ray-finned fish. VNP has not been molecularly identified in mammals but its presence is suggested from physiological studies; heterologous fish VNP exhibited more potent vasorelaxant activity than homologous mammalian ANP in the isolated coronary artery of dogs.

Free access

Cassandra Roeske, Amy Martinuk, Asna Choudhry, Geoffrey N Hendy, Michael Gollob, Qiuji Li, Tina Georgalis and Adolfo J de Bold

Expression of the G protein subunit Goα has been shown to be prominent in the atria of the rat heart and to be significantly associated with atrial natriuretic factor (ANF)-containing atrial-specific secretory granules by immunocytochemistry. In addition, differential expression profile analysis using oligonucleotide arrays has shown that the Goα isoform 1 (Goα1) is 2.3-fold more abundant in the atria than it is in the ventricles. In the present report, we show protein–protein interaction between Goα and ANF by yeast two-hybrid and by immunoprecipitation. A cardiac conditional Goα knockout model developed for the present study showed a 90% decrease in Goα expression and decreased atrial expression and ANF and brain natriuretic peptides (BNP) content. Expression of chromogranin A, a specific atrial granule core constituent, was not affected. Morphometric assessment of atrial tissue showed a very significant decrease in atrial-specific granule density as well as granule core electron density. Atrial electrical activity was not affected. The results obtained are compatible with the suggestion that Goα plays a role in ANF sorting during intracellular vectorial transport and with the presence of a mechanism that preserves the molar relationship between cellular ANF and BNP stores in the face of the decreased production of these hormones.

Free access

Manuela Cabiati, Serena Raucci, Chiara Caselli, Maria Angela Guzzardi, Andrea D'Amico, Tommaso Prescimone, Daniela Giannessi and Silvia Del Ry

Obesity is a complex pathology with interacting and confounding causes due to the environment, hormonal signaling patterns, and genetic predisposition. At present, the Zucker rat is an eligible genetic model for research on obesity and metabolic syndrome, allowing scrutiny of gene expression profiles. Real-time PCR is the benchmark method for measuring mRNA expressions, but the accuracy and reproducibility of its data greatly depend on appropriate normalization strategies. In the Zucker rat model, no specific reference genes have been identified in myocardium, kidney, and lung, the main organs involved in this syndrome. The aim of this study was to select among ten candidates (Actb, Gapdh, Polr2a, Ywhag, Rpl13a, Sdha, Ppia, Tbp, Hprt1 and Tfrc) a set of reference genes that can be used for the normalization of mRNA expression data obtained by real-time PCR in obese and lean Zucker rats both at fasting and during acute hyperglycemia. The most stable genes in the heart were Sdha, Tbp, and Hprt1; in kidney, Tbp , Actb, and Gapdh were chosen, while Actb , Ywhag, and Sdha were selected as the most stably expressed set for pulmonary tissue. The normalization strategy was used to analyze mRNA expression of tumor necrosis factor α, the main inflammatory mediator in obesity, whose variations were more significant when normalized with the appropriately selected reference genes. The findings obtained in this study underline the importance of having three stably expressed reference gene sets for use in the cardiac, renal, and pulmonary tissues of an experimental model of obese and hyperglycemic Zucker rats.

Free access

P White and MJ Dauncey

The genomic actions of thyroid hormones (THs) are mediated by receptors (TRs) that are encoded by two protooncogenes, c-erbA-alpha and c-erbA-beta. The precise functions of the TR isoforms are unclear and this study focuses on the potential roles of the TRalpha and TRbeta isoforms in mammalian striated muscles postnatally. The porcine TRalpha1, TRalpha2 and TRbeta1 cDNAs were first cloned, sequenced and characterised by Northern blotting. A quantitative analysis of TR isoform expression was then undertaken, using RNase protection analysis with novel riboprobes designed to detect relative expression levels of TRalpha1, TRalpha2, TRbeta1 and TRbeta2, in functionally distinct muscles from 7-week-old pigs kept under controlled conditions of nutrition and thermal environment. We found a striking muscle-specific pattern of TRalpha isoform distribution: in heart the mRNA level of TRalpha2 (non-TH binding) was markedly greater (P<0.01) than that of TRalpha1 (TH binding); in longissimus dorsi the opposite pattern of expression occurred (TRalpha1>TRalpha2, P<0.001); in soleus, diaphragm and rhomboideus there were no differences between the two isoforms. The overall abundance of TRbeta was very much lower than that of TRalpha, and TRbeta1 was expressed at a higher level than TRbeta2 in all muscles. Together with recent data from TR gene inactivation studies and the established role of TH in determining myosin heavy chain isoform expression and muscle phenotype, these results suggest a role for differential expression of TR isoforms in acquisition and maintenance of optimal cardiac and skeletal muscle function.

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C. L. Broadhead, U. T. O'Sullivan, C. F. Deacon and I. W. Henderson

ABSTRACT

The presence of atrial natriuretic peptide (ANP) and the nature of its binding sites were studied in freshwater (FW)- and seawater (SW)-adapted eels using a heterologous analogue, that of the rat (rANP). Rat ANP-like immunoreactivity was demonstrated in the cardiac atria and ventricles of both FW and SW eels, and electron-dense ANP-like granules were observed. The atria and ventricles of FW eels contained significantly more granules than those of SW animals and, in both types, the atria were more granular than the ventricles.

Specific binding sites for rANP were demonstrated by displacement and uptake experiments using labelled rANP in dispersed eel branchial cell preparations, enriched in chloride cells. The concentration of rANP required to produce a 50% inhibition of binding in FW cells was significantly lower than that in SW cells. Scatchard analyses revealed the presence of two classes of binding site in SW eel branchial cells but only a single class of receptor in FW cells. The affinity of the FW receptor was not significantly different from that of the SW high affinity site.

Rat ANP stimulated the production of cyclic GMP (cGMP) in a dose-dependent manner, and both basal and stimulated levels of cGMP were significantly greater in SW branchial cells.

These studies suggest that ANP is involved in the adaptation of the euryhaline eel to differing environmental salinities; the levels of the peptide in the heart alter with changing salinity, and the nature of the receptors in the sodium chloride-transporting epithelium of the gill changes in response to the need either to eliminate or to absorb sodium chloride.