DNA elements and protein factors involved in the transcription of the beta 2-adrenergic receptor gene in rat liver. The negative regulatory role of C/EBP alpha

Identification of a Novel Function of Resveratrol and Genistein as a Regulator of β2 -Adrenergic Receptor Expression in Skeletal Muscle Cells and Characterization of Promoter Elements Required for Promoter Activation

SCOPE

Modulating β₂ -adrenergic receptor (β₂ -AR) expression and activation is important for maintaining skeletal muscle function. In this study, two food factors, resveratrol (RSV) and genistein (GEN), that are able to regulate β₂ -AR promoter activity and may improve skeletal muscle function are identified.

METHODS AND RESULTS

Using luciferase reporter assay, 357 functional food factors as candidates for β₂ -AR promoter activity have been screened and subsequently RSV and GEN increase β₂ -AR promoter activity and β₂ -AR mRNA expression. Using promoter sequence analysis, it is shown that the CCAAT box and the GC box on the β₂ -AR promoter are required for the regulation of β₂ -AR expression by RSV or GEN. It is also ascertained that transcription factor NF-YA binds to the CCAAT box on the β₂ -AR promoter and that the amount of NF-YA bound to the CCAAT box is unchanged by RSV or GEN treatment. Finally, it is confirmed that a GEN-containing diet increases β₂ -AR expression in mouse skeletal muscle and increased skeletal muscle mass.

CONCLUSIONS

The findings show that food-derived molecules have the potential to influence skeletal muscle mass and function by regulating G protein-coupled receptor expression.

Identification and Functional Characterization of Cis-Regulatory Elements Controlling Expression of the Porcine ADRB2 Gene

The beta-2 adrenergic receptor (beta-2 AR) modulates metabolic processes in skeletal muscle, liver, and adipose tissue in response to catecholamine stimulation. We showed previously that expression of the porcine beta-2 AR gene (ADRB2) is affected by cis-regulatory polymorphisms. These are most likely responsible for the association of ADRB2 with economically relevant muscle-related traits in pigs. The present study focused on characterization of promoter elements involved in basal transcriptional regulation of the porcine ADRB2 in different cell types to aid identification of its cis-regulatory polymorphisms. Based on in silico analysis, luciferase reporter gene assays and gel shift assays were performed using COS-7, HepG2, C2C12, and 3T3-L1 cells. Deletion mapping of the 5´ flanking region (-1324 to +33) of ADRB2 revealed the region between -307 and -269 to be the minimal promoter, including regulatory elements essential for the basal transcriptional activity in all four tested cell types. Directly upstream (-400 to -323) we identified an important enhancer element required for maximal promoter activity. In silico analysis and gel shift assays revealed that this GC-rich element harbors two evolutionarily conserved binding sites of Sp1, a constitutive transcriptional activator. Significant transcriptional activation of the porcine ADRB2 promoter was demonstrated by overexpression of Sp1. Our results demonstrate, for the first time, an important role of Sp1 and of the responsive enhancer element in the regulation of ADRB2 expression. Polymorphisms located in this domain of the porcine ADRB2 promoter represent candidate causal cis-regulatory variants.

Expression variation of the porcine ADRB2 has a complex genetic background

Porcine adrenergic receptor beta 2 (ADRB2) gene exhibits differential allelic expression in skeletal muscle, and its genetic variation has been associated with muscle pH. Exploring the molecular-genetic background of expression variation for porcine ADRB2 will provide insight into the mechanisms driving its regulatory divergence and may also contribute to unraveling the genetic basis of muscle-related traits in pigs. In the present study, we therefore examined haplotype effects on the expression of porcine ADRB2 in four tissues: longissimus dorsi muscle, liver, subcutaneous fat, and spleen. The diversity and structure of haplotypes of the proximal gene region segregating in German commercial breeds were characterized. Seven haplotypes falling into three clades were identified. Two clades including five haplotypes most likely originated from introgression of Asian genetics during formation of modern breeds. Expression analyses revealed that the Asian-derived haplotypes increase expression of the porcine ADRB2 compared to the major, wild-type haplotype independently of tissue type. In addition, several tissue-specific differences in the expression of the Asian-derived haplotypes were found. Inspection of haplotype sequences showed that differentially expressed haplotypes exhibit polymorphisms in a polyguanine tract located in the core promoter region. These findings demonstrate that expression variation of the porcine ADRB2 has a complex genetic basis and suggest that the promoter polyguanine tract is causally involved. This study highlights the challenges of finding causal genetic variants underlying complex traits.

A 3D wavelet fusion approach for the reconstruction of isotropic-resolution MR images from orthogonal anisotropic-resolution scans

Hardware constraints, scanning time limitations, patient movement, and signal-to-noise ratio (SNR) considerations, restrict the slice-selection and the in-plane resolutions of MRI differently, generally resulting in anisotropic voxels. This nonuniform sampling can be problematic, especially in image segmentation and clinical examination. To alleviate this, the acquisition is divided into (two or) three separate scans, with higher in-plane resolutions and thick slices, yet orthogonal slice-selection directions. In this work, a noniterative wavelet-based approach for combining the three orthogonal scans is adopted, and its advantages compared with other existing methods, such as Fourier techniques, are discussed, including the consideration of the actual pulse response of the MRI scanner, and its lower computational complexity. Experimental results are shown on simulated and real 7 T MRI data.

Epidermal growth factor upregulates beta-adrenergic receptor signaling in a human salivary cell line

The effects of epidermal growth factor (EGF) on the beta-adrenergic receptor-coupled adenylyl cyclase system were studied in a human salivary cell line (HSY). The beta-adrenergic agonist isoproterenol (10(-5) M) stimulated adenylyl cyclase activity by approximately 2-fold, and the isoproterenol response was increased 1.8-fold after prolonged (48 h) exposure to EGF (5 x 10(-10) M). In contrast, enzyme activation via stimulatory prostaglandin receptors and by agents acting on nonreceptor components of the adenylyl cyclase system was not enhanced by EGF. beta-Adrenergic receptor density, assessed by binding of the beta-adrenergic receptor antagonist (-)-[(125)I]iodopindolol, was increased threefold after EGF treatment. Competition binding studies with unlabeled antagonists selective for beta(1)- and beta(2)-adrenergic receptor subtypes indicated that the increase in (-)-[(125)I]iodopindolol binding sites induced by EGF reflected an increased number of beta(2)-adrenergic receptors. Likewise, Northern blot analysis of RNA from EGF-treated cells revealed selective induction of beta(2)-adrenergic receptor mRNA, which was blocked by the RNA synthesis inhibitor actinomycin D. The increase in beta-adrenergic receptor density produced by EGF was unaltered after phorbol ester-induced downregulation of protein kinase C (PKC). Enhancement of isoproterenol-responsive adenylyl cyclase activity and phosphorylation of mitogen-activated protein kinase (MAPK) by EGF were both blocked by the MAPK pathway inhibitor PD-98059. The results suggest that in HSY cells EGF enhances beta-adrenergic responsiveness by upregulating beta(2)-adrenergic receptor expression at the transcriptional level. Moreover, the stimulatory effect of EGF on beta(2)-adrenergic receptor signaling appears to be mediated by the MAPK pathway and independent of PKC activation.

Nuclear factor-kappa B augments beta(2)-adrenergic receptor expression in human airway epithelial cells

Interleukin (IL)-1 beta increases beta(2)-adrenergic receptor (beta(2)-AR) mRNA and density by protein kinase C (PKC)-dependent mechanisms in human airway epithelial cells. The present study examined the role of several nuclear transcription factors in the PKC-activated upregulation of beta(2)-AR expression. BEAS-2B cells were exposed to the PKC activator phorbol 12-myristate 13-acetate (PMA; 0.1 microM for 2-18 h). PMA had no effect on activator protein (AP)-2 or cAMP response element binding protein DNA binding activity but markedly increased nuclear factor (NF)-kappa B and AP-1 binding as assessed by electrophoretic gel mobility shift assay. PMA also increased the activity of a beta(2)-AR promoter-luciferase reporter construct in transiently transfected cells. These effects were inhibited by the PKC inhibitors Ro-31-8220 and calphostin C. Furthermore, with increasing Ro-31-8220, beta(2)-AR promoter-reporter activity correlated closely with both NF-kappa B and AP-1 activities (r > 0.89 for both). Finally, the selective NF-kappa B inhibitor MG-132 dose dependently reduced NF-kappa B binding and beta(2)-AR promoter activity but increased AP-1 binding. We conclude that PKC-induced upregulation of beta(2)-AR expression in human airway epithelial cells appears to be mediated, at least in part, by increases in NF-kappa B activity.

As the proliferation promoter noradrenaline induces expression of ICER (induced cAMP early repressor) in proliferative brown adipocytes, ICER may not be a universal tumour suppressor

The CREM (cAMP-response-element modulator) gene product ICER (induced cAMP early repressor) has been proposed to function as a tumour (cell proliferation) suppressor. To investigate the generality of this concept, the expression pattern of ICER in brown adipocytes was followed; this was critical because brown adipocytes are one of few cell types in which cAMP is associated positively with cell proliferation but negatively with apoptosis. In response to the physiological stimulus of cold (which induces cell proliferation), ICER mRNA levels were increased in brown adipose tissue in vivo. In brown adipocytes in primary culture, ICER gene expression was induced by noradrenaline (norepinephrine) not only in the mature state (where noradrenaline potentiates differentiation), but also in the proliferative state of the cell cultures (where noradrenaline enhances cell proliferation). The induction was mediated via beta-receptors and the cAMP/protein kinase A pathway. The induced ICER appeared to repress its own expression and that of the beta2-adrenoceptor. It is thus evident that also in cell types in which cAMP induces proliferation, and even when these cells are in the proliferative state, ICER expression is induced by the same agents that stimulate proliferation. This can either mean that ICER is not a general tumour suppressor, or that brown adipocytes temporally or spatially avoid this role of ICER.

As the proliferation promoter noradrenaline induces expression of ICER (induced cAMP early repressor) in proliferative brown adipocytes, ICER may not be a universal tumour suppressor

The CREM (cAMP-response-element modulator) gene product ICER (induced cAMP early repressor) has been proposed to function as a tumour (cell proliferation) suppressor. To investigate the generality of this concept, the expression pattern of ICER in brown adipocytes was followed; this was critical because brown adipocytes are one of few cell types in which cAMP is associated positively with cell proliferation but negatively with apoptosis. In response to the physiological stimulus of cold (which induces cell proliferation), ICER mRNA levels were increased in brown adipose tissue in vivo. In brown adipocytes in primary culture, ICER gene expression was induced by noradrenaline (norepinephrine) not only in the mature state (where noradrenaline potentiates differentiation), but also in the proliferative state of the cell cultures (where noradrenaline enhances cell proliferation). The induction was mediated via beta-receptors and the cAMP/protein kinase A pathway. The induced ICER appeared to repress its own expression and that of the beta2-adrenoceptor. It is thus evident that also in cell types in which cAMP induces proliferation, and even when these cells are in the proliferative state, ICER expression is induced by the same agents that stimulate proliferation. This can either mean that ICER is not a general tumour suppressor, or that brown adipocytes temporally or spatially avoid this role of ICER.

CCAAT/enhancer-binding protein-alpha is a component of the growth hormone-regulated network of liver transcription factors

GH regulates gene expression by modulating the concentration or activity of transcription factors. To identify transcription factors that mediate the effects of GH in liver we analyzed the promoter of the gene coding for hepatocyte nuclear factor-6 (HNF-6), whose expression in liver is stimulated by GH. In protein-DNA interaction studies and in transfection experiments, we found that the liver-enriched transcription factor CCAAT/enhancer-binding protein-alpha (C/EBPalpha) binds to the hnf6 gene and inhibits its expression. This inhibitory effect involved an N-terminal subdomain of C/EBPalpha and two sites in the hnf6 gene promoter. Using liver nuclear extracts from GH-treated hypophysectomized rats, we found that GH induces a rapid, transient decrease in the amount of C/EBPalpha protein. This GH-induced change is concomitant with the transient stimulatory effect of GH on the hnf6 gene. Stimulation of the hnf6 gene by GH therefore involves lifting of the repression exerted by C/EBPalpha in addition to the known GH-induced stimulatory effects of STAT5 (signal transducer and activator of transcription-5) and HNF-4 on that gene. Our data provide further evidence that GH controls a network of liver transcription factors and show that C/EBPalpha participates in this process.

Transcriptional regulation of the gonadotropin-releasing hormone receptor gene is mediated in part by a putative repressor element and by the cyclic adenosine 3',5'-monophosphate response element

The levels of the GnRH receptor (GnRHR) and its messenger RNA depend on the pattern of administration of GnRH. In this study, internal deletion mutants in a luciferase reporter gene vector (GnRHR-pXP2) containing a 1226-bp promoter fragment of mouse GnRHR gene were used to examine the regulation of GnRHR gene transcription in GGH3 cells. Our results indicate that the mouse GnRHR promoter contains one putative repressor element located at position -343/-335. When this sequence was deleted, the GnRHR promoter activity was significantly increased in both basal and GnRH agonist (Buserelin)-, phorbol ester-, and forskolin-stimulated cells. Gel mobility shift assay showed that the sequence -343/-335 is capable of binding GGH3 nuclear proteins. With deletion of the cAMP response element (-107/-100), basal and Buserelin-stimulated transcription was decreased. The same response was observed after stimulation with forskolin. Stimulation with (Bu)2cAMP did not alter transcription above basal levels. The stimulation with phorbol ester resulted in an attenuated increase in transcriptional activity, suggesting that this sequence of the GnRHR promoter is a cAMP response element. These results suggest that the transcriptional activity of the GnRHR gene is mediated in part by a putative repressor element and by the cAMP response element.

Transcriptional and posttranscriptional regulation of beta(2)-adrenergic receptor gene in rat liver during sepsis

Changes in beta(2)-adrenergic receptor (beta(2)-AR) gene expression in the rat liver during different phases of sepsis were studied. Sepsis was induced by cecal ligation and puncture (CLP). Septic rats exhibit two metabolically distinct phases: an initial hyperglycemic (9 h after CLP; early sepsis) followed by a hypoglycemic phase (18 h after CLP; late sepsis). The [(3)H]dihydroalprenolol binding studies show that the density of beta(2)-AR was decreased by 12 and 35% during the early and late phases of sepsis, respectively. Western blot analyses depict that the beta(2)-AR protein level was reduced by 37 and 72% during early and late sepsis, respectively. The reverse transcription polymerase chain reaction and Southern blot analyses reveal that the steady-state level of beta(2)-AR mRNA was decreased by 37% during early phase and 77% during late phase of sepsis. Nuclear run-off assays show that the rate of transcription of beta(2)-AR mRNA was reduced by 36% during early sepsis and 64% during late sepsis. The stability assays indicate that the half-life of beta(2)-AR mRNA was shortened by 21 and 50% during the early and late phases of sepsis, respectively, indicating that the rate of degradation of beta(2)-AR mRNA was progressively enhanced during sepsis. These findings demonstrate that the beta(2)-AR gene was underexpressed in the liver during the progression of sepsis, and, furthermore, the underexpression of the beta(2)-AR gene was the result of a reduction in the rate of transcription coupled with an enhancement in the rate of degradation of beta(2)-AR gene transcripts. Thus our findings that the transcriptional and posttranscriptional regulation of beta(2)-AR gene associated with decreases in beta(2)-AR number and its protein expression may provide a molecular mechanistic explanation for the development of hypoglycemia during the late stage of sepsis.

Identification of a glucocorticoid response element in the rat beta2-adrenergic receptor gene

Regulation of beta2-adrenergic receptor (beta2AR) levels by glucocorticoids is a physiologically important mechanism for altering beta2AR responsiveness. Glucocorticoids increase beta2AR density by increasing the rate of beta2AR gene transcription, but the cis-elements involved have not been well characterized. We now show that one of six potential glucocorticoid response elements (GREs) in the 5'-flanking region of the rat beta2AR gene is necessary for glucocorticoid-dependent stimulation of receptor gene expression. Using a nested set of deletion fragments of the rat beta2AR gene 5'-flanking region fused to a luciferase reporter gene, glucocorticoid-dependent induction of reporter gene expression in HepG2 cells was localized to a region between positions -643 and -152, relative to the transcription initiation site. In electrophoretic mobility shift assays, a double-stranded oligonucleotide incorporating a near-consensus GRE from this region (positions -379 to -365) formed complexes with the human recombinant glucocorticoid receptor, as well as with nuclear protein from dexamethasone-treated HepG2 cells. Mutation of a single base within this GRE sequence greatly diminished interaction of the mutated oligonucleotide with the human recombinant glucocorticoid receptor. The functional activity of the GRE was characterized using a luciferase reporter construct driven by a minimal thymidine kinase promoter. In HepG2 cells transfected with constructs containing the GRE, dexamethasone increased reporter gene expression approximately 3-fold, whereas a dexamethasone effect was not observed with constructs lacking the GRE. Taken together, these findings show that a GRE located at positions -379 to -365 in the 5'-flanking region of the rat beta2AR gene mediates glucocorticoid stimulation of beta2AR gene transcription.

Phosphorylation of the cAMP response element-binding protein and activation of transcription by alpha1 adrenergic receptors

Activation of alpha1 adrenergic receptors not only stimulates smooth muscle contraction but also modifies gene expression. We wondered if alpha1 adrenergic receptors could activate transcription of genes regulated by the cAMP response element-binding protein (CREB). Using Rat1 cells stably transfected with each of the three cloned human alpha1 adrenergic receptor subtypes, norepinephrine strongly stimulated CREB phosphorylation in alpha1A and alpha1B but more weakly in alpha1D-transfected cells. Norepinephrine increased the activity of a somatostatin cAMP-regulated enhancer-chloramphenicol acetyltransferase reporter in these cells. alpha1 adrenergic receptors are known to activate protein kinase C (PKC) and increase [Ca2+ ]i. Nonetheless, neither GF109203X, a PKC inhibitor, nor BAPTA-AM, a calcium chelator, blocked phosphorylation of CREB induced by norepinephrine. In addition, alpha1 adrenergic receptor-induced CREB phosphorylation was not mediated via the mitogen-activated protein kinase pathway because norepinephrine did not stimulate mitogen-activated protein kinase activity in these cells. Activation of alpha1 adrenergic receptors increased cAMP accumulation in these cells. Norepinephrine-induced cAMP-regulated enhancer-chloramphenicol acetyltransferase activity was inhibited either by expression of the PKA inhibitory peptide or a dominant negative PKA regulatory subunit mutant. These results demonstrate that alpha1 adrenergic receptors activate the transcription factor CREB by a PKA-dependent pathway.

Transcription of the beta2-adrenergic receptor gene in rat liver is regulated during early postnatal development by an upstream repressor element

As early postnatal development of the male rat proceeds, there is a decline in transcription of the beta2-adrenergic receptor gene in liver which is associated with a decline in beta2-adrenergic receptor mediated glucose mobilization. In this study, primary cultures of rat hepatocytes transiently transfected with fusion genes containing various segments of beta2-adrenergic receptor gene 5'-flanking DNA fused to a promoterless luciferase reporter gene were used to identify genetic elements that might control beta2-adrenergic receptor gene expression during the first 10 days of postnatal life. We found that 261 bp of beta2-adrenergic receptor gene 5'-flanking region (-372 to -95, start of translation is +1) was sufficient to direct high luciferase expression in fetal day 18 hepatocytes and therefore included the beta2-adrenergic receptor gene promoter. Luciferase activities in fetal day 18 hepatocytes transfected with pbeta2AR(-372/-95), pbeta2AR(-1,335/-95) and pbeta2AR(-3,349/-95) were fourfold greater than that in either postnatal day 5 or postnatal day 10 hepatocytes transfected with the same fusion genes. By use of gel mobility shift assays, we observed increased protein binding to a 50 bp segment (-372 to -323) of the beta2-adrenergic receptor gene 5'-flanking region with nuclear extracts prepared from postnatal day 5 and postnatal day 10 hepatocytes compared to fetal day 18 hepatocytes. These findings suggest the presence of a regulatory element in the 5'-flanking region of the beta2-adrenergic receptor gene that appears to be involved in suppression of transcription of the beta2-adrenergic receptor gene in liver during early postnatal development.

Histone H1 isoforms purified from rat liver bind nonspecifically to the nuclear factor 1 recognition sequence and serve as generalized transcriptional repressors

Two polypeptides with molecular masses of 34 and 30 kDa were copurified from rat liver during DNA affinity purification of a sequence-specific transcription factor binding to the footprint II sequence within the P2 promoter of the rat alpha1B adrenergic receptor (alpha1B AR) gene, and were identified by microsequencing their endoproteinase Lys-C-derived peptides as histone H1d and histone H1c, respectively. Histone H1 was previously reported to bind to the nuclear factor 1 (NF1) recognition sequence, although the specificity of this binding has been controversial. Here, DNA mobility shift and supershift assays, DNase I footprinting and mutational analyses indicated that the binding of histone H1 to the NF1 sites located within footprint II of the alpha1B AR gene P2 promoter is nonspecific. Transient cotransfections into Hep3B cells of histone H1d cDNA with CAT constructs containing promoter regions of different genes resulted in generalized and non-specific suppression of CAT activity. The histone H1d-mediated repression of the activities of the alpha1B AR gene P2/CAT or beta2 AR gene P(-186/1307)/CAT constructs was reversed by the cotransfection of a cDNA encoding the sequence-specific transcription factor NF1/X, and the fold increase in CAT activities was similar to that obtained in the absence of histone H1d. These results suggest that sequence-specific transcription factors counteract the histone H1-mediated transcriptional repression in vivo by a true activation, which is different from the in vitro antirepression in histone H1-repressed chromatin templates (Laybourn and Kadonaga, (1991) Science 254: 238-245).

Both the cyclic AMP response element and the activator protein 2 binding site mediate basal and cyclic AMP-induced transcription from the dominant promoter of the rat alpha 1B-adrenergic receptor gene in DDT1MF-2 cells

cAMP markedly increases alpha 1B adrenergic receptor (alpha 1B-AR) expression in FRTL-5 and PC C13 rat thyroid cells, DDT1MF-2 smooth muscle cells, primary rat hepatocytes, and K9 rat liver cells. Here, we used DDT1MF-2 cells to evaluate further the mechanisms by which cAMP stimulates alpha 1B-AR expression. Receptor binding assays, Northern blotting, and nuclear run-on analyses demonstrated that forskolin (1 microM) in the presence of isobutylmethylxanthine (0.25 mM) increased alpha 1B-AR numbers, mRNA level, and gene transcription rate by 2.3 +/- 0.2-, 2.5 +/- 0.3-, and 3.5 +/- 0.2-fold over control, respectively. Dibutyryl cAMP (1 mM) plus isobutylmethylxanthine (0.25 mM) also enhanced alpha 1B-AR density by 2.7 +/- 0.1-fold over control. Further experiments demonstrated that the induction of alpha 1B-AR by forskolin requires new protein synthesis and is protein kinase A dependent. In DDT1MF-2 cells transfected with alpha 1B-AR gene P2 promoter/CAT constructs, both forskolin and dibutyryl cAMP significantly increased P2 promoter activity. The P2 promoter region of the rat alpha 1B-AR gene (-813 to -432) contains a cAMP response element (CRE) (-444 to -437) and an AP2 binding site (-647 to -638). Mutations in either one of these elements alone led to a decrease in both basal and cAMP-induced P2 promoter activity. Mutations in both elements caused a further inhibition of basal transcription and a complete block of cAMP-induced P2 promoter activity. Direct binding of purified activator protein 2 (AP2) to the AP2 element in the P2 promoter was reported previously. Gel mobility shift and super-shift assays using liver nuclear extracts from either rat liver or DDT1MF-2 cells demonstrated that the CRE in the alpha 1B-AR gene bound CRE binding protein. These data indicate that both the CRE and the AP2 element in the P2 promoter contribute to basal as well as cAMP-induced transcription of the alpha 1B-AR gene in DDT1MF-2 cells.

C/EBP binds over the TATA box and can activate the M promoter of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase

The gene A coding for 6-phosphofructo-2-kinase gives rise to three mRNA that originate from distinct promoters called F (fetal), M (muscle) and L (liver). The regulation of the M promoter is ill-understood and its TATA box region binds an unidentified factor. The aim of this work was to identify this factor and to investigate its activity. In vitro protein-DNA binding assays and transfection experiments showed that this factor is C/EBP and that C/EBP can stimulate the M promoter despite its potential interference with TFIID binding. The effect of C/EBP was abolished by a dominant negative variant of C/EBP. These data and other work lead to the conclusion that C/EBP may participate to the regulation of promoter switching in 6-phosphofructo-2-kinase gene A.