Divergent regulation of the human atrial natriuretic peptide gene by c-jun and c-fos

Cardiac deficiency of P21-activated kinase 1 promotes atrial arrhythmogenesis in mice following adrenergic challenge

P21-activated kinase 1 (Pak1) signalling plays a vital and overall protective role in the heart. However, the phenotypes of Pak1 deficiency in the cardiac atria have not been well explored. In this study, Pak1 cardiac-conditional knock-out (cKO) mice were studied under baseline and adrenergic challenge conditions. Pak1 cKO mice show atrial arrhythmias including atrial fibrillation (AF) in vivo, detected during anaesthetized electrocardiography without evidence of interstitial fibrosis upon Masson's trichrome staining. Optical mapping of left atrial preparations from Pak1 cKO mice revealed a higher incidence of Ca2+ and action potential alternans under isoprenaline challenge and differences in baseline action potential and calcium transient characteristics. Type-2 ryanodine receptor (RyR2) channels from Pak1 cKO hearts had a higher open probability than those from wild-type. Reverse transcription-quantitative polymerase chain reaction and Western blotting indicated that pCamkIIδ and RyR2 are highly phosphorylated at baseline in the atria of Pak1 cKO mice, while the expression of Slc8a2 and Slc8a3 as a Na+-Ca2+ exchanger, controlling the influx of Ca2+ from outside of the cell and efflux of Na+ from the cytoplasm, are augmented. Chromatin immunoprecipitation study showed that pCreb1 interacts with Slc8a2 and Slc8a3. Our study thus demonstrates that deficiency of Pak1 promotes atrial arrhythmogenesis under adrenergic stress, probably through post-translational and transcriptional modifications of key molecules that are critical to Ca2+ homeostasis. This article is part of the theme issue 'The heartbeat: its molecular basis and physiological mechanisms'.

Cytochrome P450 2J2: distribution, function, regulation, genetic polymorphisms and clinical significance

Cytochrome P450 2J2 (CYP2J2) is an enzyme mainly found in human extrahepatic tissues, with predominant expression in the cardiovascular systems and lower levels in the intestine, kidney, lung, pancreas, brain, liver, etc. During the past 15 years, CYP2J2 has attracted much attention for its epoxygenase activity in arachidonic acid (AA) metabolism. It converts AA to four epoxyeicosatrienoic acids (EETs) that have various biological effects, especially in the cardiovascular systems. In recent publications, CYP2J2 is shown highly expressed in various human tumor cells, and its EET metabolites are demonstrated to implicate in the pathologic development of human cancers. CYP2J2 is also a human CYP that involved in phase I xenobiotics metabolism. Antihistamine drugs and many other compounds were identified as the substrates of CYP2J2, and studies have demonstrated that these substrates have a broad structural diversity. CYP2J2 is found not readily induced by known P450 inducers; however, its expression could be regulated in some pathological conditions, might through the activator protein-1(AP-1), the AP-1-like element and microRNA let-7b. Several genetic mutations in the CYP2J2 gene have been identified in humans, and some of them have been shown to have potential associations with some diseases. With the increasing awareness of its roles in cancer disease and drug metabolism, studies about CYP2J2 are still going on, and various inhibitors of CYP2J2 have been determined. Further studies are needed to delineate the roles of CYP2J2 in disease pathology, drug development and clinical practice.

The AP-1 transcription factor c-Jun prevents stress-imposed maladaptive remodeling of the heart

Systemic hypertension increases cardiac workload and subsequently induces signaling networks in heart that underlie myocyte growth (hypertrophic response) through expansion of sarcomeres with the aim to increase contractility. However, conditions of increased workload can induce both adaptive and maladaptive growth of heart muscle. Previous studies implicate two members of the AP-1 transcription factor family, junD and fra-1, in regulation of heart growth during hypertrophic response. In this study, we investigate the function of the AP-1 transcription factors, c-jun and c-fos, in heart growth. Using pressure overload-induced cardiac hypertrophy in mice and targeted deletion of Jun or Fos in cardiomyocytes, we show that c-jun is required for adaptive cardiac hypertrophy, while c-fos is dispensable in this context. c-jun promotes expression of sarcomere proteins and suppresses expression of extracellular matrix proteins. Capacity of cardiac muscle to contract depends on organization of principal thick and thin filaments, myosin and actin, within the sarcomere. In line with decreased expression of sarcomere-associated proteins, Jun-deficient cardiomyocytes present disarrangement of filaments in sarcomeres and actin cytoskeleton disorganization. Moreover, Jun-deficient hearts subjected to pressure overload display pronounced fibrosis and increased myocyte apoptosis finally resulting in dilated cardiomyopathy. In conclusion, c-jun but not c-fos is required to induce a transcriptional program aimed at adapting heart growth upon increased workload.

Activator protein-1 regulation of murine aldehyde dehydrogenase 1a1

Previously we demonstrated that aldehyde dehydrogenase (ALDH) 1a1 is the major ALDH expressed in mouse liver and is an effective catalyst in metabolism of lipid aldehydes. Quantitative real-time polymerase chain reaction analysis revealed a ≈2.5- to 3-fold induction of the hepatic ALDH1A1 mRNA in mice administered either acrolein (5 mg/kg acrolein p.o.) or butylated hydroxylanisole (BHA) (0.45% in the diet) and of cytosolic NAD⁺-dependent ALDH activity. We observed ≈2-fold increases in ALDH1A1 mRNA levels in both Nrf2⁺/⁺ and Nrf2⁻/⁻ mice treated with BHA compared with controls, suggesting that BHA-induced expression is independent of nuclear factor E2-related factor 2 (Nrf2). The levels of activator protein-1 (AP-1) mRNA and protein, as well as the amount of phosphorylated c-Jun were significantly increased in mouse liver or Hepa1c1c7 cells treated with either BHA or acrolein. With use of luciferase reporters containing the 5'-flanking sequence of Aldh1a1 (-1963/+27), overexpression of c-Jun resulted in an ≈4-fold induction in luciferase activity, suggesting that c-Jun transactivates the Aldh1a1 promoter as a homodimer and not as a c-Jun/c-Fos heterodimer. Promoter deletion and mutagenesis analyses demonstrated that the AP-1 site at position -758 and possibly -1069 relative to the transcription start site was responsible for c-Jun-mediated transactivation. Electrophoretic mobility shift assay analysis with antibodies against c-Jun and c-Fos showed that c-Jun binds to the proximal AP-1 site at position -758 but not at -1069. Recruitment of c-Jun to this proximal AP-1 site by BHA was confirmed by chromatin immunoprecipitation analysis, indicating that recruitment of c-Jun to the mouse Aldh1a1 gene promoter results in increased transcription. This mode of regulation of an ALDH has not been described before.

c-Jun binding site identification in K562 cells

Determining the binding sites of the transcription factor is important for understanding of transcriptional regulation. Transcription factor c-Jun plays an important role in cell growth, differentiation and development, but the binding sites and the target genes are not clearly defined in the whole human genome. In this study, we performed a ChIP-Seq experiment to identify c-Jun binding site in the human genome. Forty-eight binding sites were selected to process further evaluation by dsDNA microarray assay. We identified 283 c-Jun binding sites in K562 cells. Data analysis showed that 48.8% binding sites located within 100 kb of the upstream of the annotated genes, 28.6% binding sites comprised consensus TRE/CRE motif (5'-TGAC/GTCA-3', 5'-TGACGTCA-3') and variant sequences. Forty-two out of the selected 48 binding sites were found to bind the c-Jun homodimer in dsDNA microarray analysis. Data analysis also showed that 1569 genes are located in the neighborhood of the 283 binding sites and 191 genes in the neighborhood of the 42 binding sites validated by dsDNA microarray. We consulted 38 c-Jun target genes in previous studies and 16 among these 38 genes were also detected in this study. The identification of c-Jun binding sites and potential target genes in the genome scale may improve our fundamental understanding in the molecular mechanisms underlying the transcription regulation related to c-Jun.

Atrial natriuretic peptide suppresses endothelin gene expression and proliferation in cardiac fibroblasts through a GATA4-dependent mechanism

AIMS

Atrial natriuretic peptide (ANP) is a hormone that has both antihypertrophic and antifibrotic properties in the heart. We hypothesized that myocyte-derived ANP inhibits endothelin (ET) gene expression in fibroblasts.

METHODS AND RESULTS

We have investigated the mechanism(s) involved in the antiproliferative effect of ANP on cardiac fibroblasts in a cell culture model. We found that cardiac myocytes inhibited DNA synthesis in co-cultured cardiac fibroblasts as did treatment with the ET-1 antagonist BQ610. The effect of co-culture was reversed by antibody directed against ANP or the ANP receptor antagonist HS-142-1. ANP inhibited the expression of the ET-1 gene and ET-1 gene promoter activity in cultured fibroblasts. The site of the inhibition was localized to a GATA-binding site positioned between -132 and -135 upstream from the transcription start site. GATA4 expression was demonstrated in cardiac fibroblasts, GATA4 bound the ET-1 promoter both in vitro and in vivo, and siRNA-mediated knockdown of GATA4 inhibited ET-1 expression. ET-1 treatment resulted in increased levels of phospho-serine(105) GATA4 in cardiac fibroblasts and this induction was partially suppressed by co-treatment with ANP.

CONCLUSION

Collectively, these findings suggest that locally produced ET-1 serves as an autocrine stimulator of fibroblast proliferation, that ANP produced in neighbouring myocytes serves as a paracrine inhibitor of this proliferation, and that the latter effect operates through a reduction in GATA4 phosphorylation and coincident reduction in GATA4-dependent transcriptional activity.

Multiprotein bridging factor 1 cooperates with c-Jun and is necessary for cardiac hypertrophy in vitro

Cardiac hypertrophy is induced by a number of stimuli and can lead to cardiomyopathy and heart failure. Cardiomyocyte hypertrophy is characterized by increased cell size and altered gene expression. By differential-display polymerase chain reaction and Western blotting we found that the transcriptional coactivator MBF1 was upregulated during hypertrophy in cardiomyocyte cultures. Furthermore, MBF1 protein level increased in two animal models of hypertrophy, angiotensin II treatment and aortic banding. MBF1 antisense oligodeoxynuclotides blocked phenylephrine-induced hypertrophy, suggesting MBF1 plays a key role in hypertrophic growth. In contrast, overexpression of MBF1 potentiated the hormone-induced response of the atrial natriuretic peptide promoter. MBF1 overexpressed by transient transfection cooperated with the transcription factor c-Jun in activation of transcription but not with GATA4. MBF1 and c-Jun induced the activity of a transiently transfected atrial natriuretic peptide promoter, whereas neither MBF1 nor c-Jun could induce the promoter alone. Moreover, MBF1 bound to c-Jun in vitro. These data suggest that MBF1 is a transcriptional coactivator of c-Jun regulating hypertrophic gene expression. Inhibitor studies suggested that MBF1 activates the atrial natriuretic peptide promoter independently of the calcineurin and CaMK signaling pathways. Our results indicate that MBF1 participates in hormone-induced cardiomyocyte hypertrophy and activates hypertrophic gene expression as a coactivator of c-Jun.

Peroxisome proliferator-activated receptor gamma plays a critical role in inhibition of cardiac hypertrophy in vitro and in vivo

BACKGROUND

Peroxisome proliferator-activated receptors (PPARs) are transcription factors of the nuclear receptor superfamily. It has been reported that the thiazolidinediones, which are antidiabetic agents and high-affinity ligands for PPARgamma, regulate growth of vascular cells. In the present study, we examined the role of PPARgamma in angiotensin II (Ang II)-induced hypertrophy of neonatal rat cardiac myocytes and in pressure overload-induced cardiac hypertrophy of mice.

METHODS AND RESULTS

Treatment of cultured cardiac myocytes with PPARgamma ligands such as troglitazone, pioglitazone, and rosiglitazone inhibited Ang II-induced upregulation of skeletal alpha-actin and atrial natriuretic peptide genes and an increase in cell surface area. Treatment of mice with a PPARgamma ligand, pioglitazone, inhibited pressure overload-induced increases in the heart weight-to-body weight ratio, wall thickness, and myocyte diameter in wild-type mice and an increase in the heart weight-to-body weight ratio in heterozygous PPARgamma-deficient mice. In contrast, pressure overload-induced increases in the heart weight-to-body weight ratio and wall thickness were more prominent in heterozygous PPARgamma-deficient mice than in wild-type mice.

CONCLUSIONS

These results suggest that the PPARgamma-dependent pathway is critically involved in the inhibition of cardiac hypertrophy.

Focal adhesion kinase and p130Cas mediate both sarcomeric organization and activation of genes associated with cardiac myocyte hypertrophy

Hypertrophic terminally differentiated cardiac myocytes show increased sarcomeric organization and altered gene expression. Previously, we established a role for the nonreceptor tyrosine kinase Src in signaling cardiac myocyte hypertrophy. Here we report evidence that p130Cas (Cas) and focal adhesion kinase (FAK) regulate this process. In neonatal cardiac myocytes, tyrosine phosphorylation of Cas and FAK increased upon endothelin (ET) stimulation. FAK, Cas, and paxillin were localized in sarcomeric Z-lines, suggesting that the Z-line is an important signaling locus in these cells. Cas, alone or in cooperation with Src, modulated basal and ET-stimulated atrial natriuretic peptide (ANP) gene promoter activity, a marker of cardiac hypertrophy. Expression of the C-terminal focal adhesion-targeting domain of FAK interfered with localization of endogenous FAK to Z-lines. Expression of the Cas-binding proline-rich region 1 of FAK hindered association of Cas with FAK and impaired the structural stability of sarcomeres. Collectively, these results suggest that interaction of Cas with FAK, together with their localization to Z-lines, is critical to assembly of sarcomeric units in cardiac myocytes in culture. Moreover, expression of the focal adhesion-targeting and/or the Cas-binding proline-rich regions of FAK inhibited ANP promoter activity and suppressed ET-induced ANP and brain natriuretic peptide gene expression. In summary, assembly of signaling complexes that include the focal adhesion proteins Cas, FAK, and paxillin at Z-lines in the cardiac myocyte may regulate, either directly or indirectly, both cytoskeletal organization and gene expression associated with cardiac myocyte hypertrophy.

Rapid effects of stretched myocardial and vascular cells on gene expression of neonatal rat cardiomyocytes with emphasis on autocrine and paracrine mechanisms

Passive stretch of the heart has a direct effect on cardiomyocytes and other cell types including cardiac fibroblasts, endothelial cells, and vascular smooth muscle cells (VSMCs). Cardiomyocytes are targets for the action of peptide growth factors found in myocardium, suggesting an autocrine or paracrine model of the hypertrophic process. In this study we examined stretch-dependent cellular communication between cardiomyocytes, cardiac fibroblasts, endothelial cells, and VSMCs. Stationary cardiomyocytes were incubated with stretch-conditioned medium (CM0-CM60) derived from stretched (for 0-60 min) cardiomyocytes, cardiac fibroblasts, endothelial cells, and VSMCs. The expression levels of protooncogenes (as c-fos, c-jun, and fra-1) were measured, and as an indication of a hypertrophic response the expression of atrial natriuretic peptide (ANP) was measured. Stationary cardiomyocytes that have been incubated for 30 min with CM from stretched (for 0-60 min) cardiomyocytes, cardiac fibroblasts, endothelial cells, and VSMCs showed distinct gene expression patterns that were time-dependent and cell-type specific. In stationary cardiomyocytes, CM derived from stretched cardiomyocytes caused decreased c-fos and fra-1 expression by 37 and 20%, respectively (CM30), elevated c-jun expression by 20% (CM45-CM60), and increased ANP expression by 106% (CM45). CM derived from stretched cardiac fibroblasts caused increased c-fos expression by 41% (CM60), no significant changes in c-jun expression, and increased fra-1 and ANP expression by 39 and 20%, respectively (CM45). CM derived from stretched VSMCs induced an initial decrease in c-fos expression followed by an increase of 13% (CM45) and induced increased c-jun, fra-1, and ANP expression by 39, 24, and 22%, respectively. CM15-CM60 derived from stretched endothelial cells caused decreased c-fos, c-jun and fra-1 expression by 20, 25, and 25%, respectively, and increased ANP expression by 18%. Our data indicate that gene expression of cardiomyocytes in stretched myocardium is regulated by mediators released by cardiomyocytes, cardiac fibroblasts, endothelial cells, and VSMCs. This observation emphasizes the involvement of nonmyocyte cells in the early stages of cardiomyocyte hypertrophy caused by cardiac stretch.

Mechanical strain activates BNP gene transcription through a p38/NF-kappaB-dependent mechanism

Application of mechanical strain to neonatal rat ventricular myocytes in culture evokes changes in gene expression reminiscent of those that occur with hypertrophy in vivo, such as stimulation of brain natriuretic peptide (BNP) gene expression. Here, we show that a major component of strain-dependent BNP promoter activation results from stimulation of p38 mitogen-activated protein kinase (MAPK) in the cardiac myocyte. Strain increased p38 activity in a time-dependent fashion. The p38 inhibitor SB203580 led to a reduction of approximately 60% in strain-activated human BNP (hBNP) promoter activity. Cotransfection of wild-type p38 increased both basal and strain-dependent promoter activity, while cotransfection with MKK6AL, a dominant-negative inhibitor of p38 MAPK kinase, resulted in partial inhibition of either p38- or strain-activated hBNP promoter activity. p38 MAPK increased hBNP promoter activity through activation of the transcription factor NF-kappaB. Activation of the hBNP promoter by either p38 or strain was mediated by DNA elements present in the 5' flanking sequence of the gene. Mechanical strain promoted assembly of NF-kappaB components on these DNA elements in vitro. Thus, induction of the hBNP promoter by mechanical strain depends, at least in part, on stimulation of p38 and subsequent activation of NF-kappaB. This activation may play an important role in signaling the increased BNP gene expression that accompanies hemodynamic overload and cardiac hypertrophy in vivo.

Emotional stress induces immediate-early gene expression in rat heart via activation of alpha- and beta-adrenoceptors

We have studied the adrenergic mechanisms of immediate-early gene (IEG) induction in the discrete types of cardiac cells with the use of in situ hybridization histochemistry in an immobilization-stress model in conscious rats. Expression of c-fos, fos B, c-jun, jun B, NGFI-A, and NGFI-B mRNA was rapidly upregulated in the endothelial, myocardial, and smooth muscle cells of coronary vessels by 15-45 min after the onset of immobilization. Simultaneous blockade of both alpha- and beta-adrenoceptors completely abolished expression of IEGs in these cardiac cells. Application of an alpha-agonist or beta-agonist alone to the perfused rat heart under constant pressure elicited the upregulation of IEGs in a fashion similar to that of emotional stress. These data suggest that activation of either alpha- or beta-adrenoceptor is sufficient to evoke expression of these genes and that there may be cross talk in signal transduction downstream from alpha- and beta-adrenoceptors in cardiac cells.

Angiotensin II-induced transactivation of epidermal growth factor receptor regulates fibronectin and transforming growth factor-beta synthesis via transcriptional and posttranscriptional mechanisms

The signaling cascade elicited by angiotensin II (Ang II) resembles that characteristic of a growth factor, and recent evidence indicates transactivation of epidermal growth factor receptor (EGF-R) by G protein-coupled receptors. Here, we report the involvement of EGF-R in Ang II-induced synthesis of fibronectin and transforming growth factor-beta (TGF-beta) in cardiac fibroblasts. Ang II stimulated fibronectin mRNA levels dose dependently, with a maximal increase (approximately 5-fold) observed after 12 hours of incubation. Fibronectin synthesis induced by Ang II or calcium ionophore was completely abolished by tyrosine kinase inhibitors and intracellular Ca2+ chelating agents. Ang II-induced fibronectin mRNA was not affected by protein kinase C inhibitors or protein kinase C depletion, whereas specific inhibition of EGF-R function by a dominant negative EGF-R mutant and tyrphostin AG1478 abolished induction of fibronectin mRNA. We isolated the rat fibronectin gene, including the 5'-flanking region, and found that the activator protein-1 (AP-1) binding site present in the promoter region was responsible for the Ang II responsiveness of this gene. A gel retardation assay revealed the binding of nuclear protein to the AP-1 site, which was supershifted with anti-c-fos and anti-c-jun but not anti-activating transcription factor (ATF)-2 antibodies. Conditioned medium from Ang II-treated cells contained TGF-beta bioactivity, and addition of neutralizing TGF-beta antibody modestly (46%) inhibited induction of fibronectin. Ang II-induced synthesis of TGF-beta was also abolished by inhibition of EGF-R function. The effect of TGF-beta was exerted by stabilizing fibronectin mRNA without affecting the promoter activity and required de novo protein synthesis. We concluded that Ang II-induced expression of fibronectin and TGF-beta is mediated by downstream signaling of EGF-R transactivated by Ca2+-dependent tyrosine kinase and that Ang II-induced fibronectin mRNA expression is regulated by 2 different mechanisms, which are transcriptional control by binding of the c-fos/c-jun complex to the AP-1 site and posttranscriptional control by mRNA stabilization due to autocrine or paracrine effects of TGF-beta. Thus, this study suggests that the action of Ang II on extracellular matrix formation should be interpreted in association with the EGF-R signaling cascade.

DT-diaphorase activity in NSCLC and SCLC cell lines: a role for fos/jun regulation

To assess the potential differential lung tumour expression of NAD(P)H:quinone reductase (NQO1), the human (h) NQO1 promoter was characterized in gene transfer studies. A deletion panel of 5' flanking hNQO1 promoter constructs was made and tested in transient transfection assays in NSCLC and SCLC cell lines. The largest hNQO1 construct (-1539/+115) containing the antioxidant response element (ARE), exhibited robust levels of reporter activity in the NSCLC (H460, H520, and A549) cell lines and expression was over 12 to 77-fold higher than the minimal (-259/+115) promoter construct. In contrast, there was little difference in promoter activity between the largest and minimal promoter construct in the SCLC (H146, H82 and H187) cell lines. Deletion of the sites for NFkappaB and AP-2 and the XRE did not significantly affect hNQO1 promoter activity in either the NSCLC or SCLC cell lines. Robust promoter activity in NSCLC lines was mediated by a 359 bp segment of the proximal promoter that contained a canonical AP-1 binding site, TGACTCAG, within the ARE. Gel supershift assays with various specific Fos/Jun antibodies identified Fra1, Fra2 and Jun B binding activity in NSCLC cells to a promoter fragment (-477 to -438) spanning the AP-1 site, whereas SCLC do not appear to express functional Fra or Jun B. These results suggest a possible role for AP-1 activity in the differential expression of hNQO1 in NSCLC.

c-Src activation plays a role in endothelin-dependent hypertrophy of the cardiac myocyte

Activation of the atrial natriuretic peptide (ANP) gene is regarded as one of the earliest and most reliable markers of hypertrophy in the ventricular cardiac myocyte. We have examined the role of the nonreceptor tyrosine kinases in the signaling mechanism(s) leading to hypertrophy using human ANP gene promoter activity as a marker. Endothelin (ET), a well known hypertrophic agonist, increased activity of c-Src, c-Yes, and Fyn within minutes and promoted a selective redistribution of each of these kinases within the cell. Overexpression of c-Src effected a significant increase in activity of a cotransfected human ANP promoter-driven chloramphenicol acetyl transferase reporter, while expression of either c-Yes or Fyn was considerably less effective in this regard. ET-dependent stimulation of the human ANP gene promoter was partially inhibited by co-transfection with dominant negative Ras or dominant negative Src or Csk or by treatment with the potent Src family-selective tyrosine kinase inhibitor PP1, suggesting that the Src family kinases are involved in signaling ET-dependent activation of this promoter. Both ET- and Src-dependent activation of the ANP promoter required the presence of a CArG motif in a serum response element-like structure between -422 and -413 but did not appear to require assembly of a ternary complex for full activity. These findings support a role for Src in the activation of ANP gene expression and suggest that this kinase may contribute in an important way to the signaling mechanisms that activate hypertrophy in the cardiac myocyte.

Angiotensin II type 1 receptor-induced extracellular signal-regulated protein kinase activation is mediated by Ca2+/calmodulin-dependent transactivation of epidermal growth factor receptor

The signaling cascade elicited by angiotensin II (Ang II) resembles that characteristic of growth factor stimulation, and recent evidence suggests that G protein-coupled receptors transactivate growth factor receptors to transmit mitogenic effects. In the present study, we report the involvement of epidermal growth factor receptor (EGF-R) in Ang II-induced extracellular signal-regulated kinase (ERK) activation, c-fos gene expression, and DNA synthesis in cardiac fibroblasts. Ang II induced a rapid tyrosine phosphorylation of EGF-R in association with phosphorylation of Shc protein and ERK activation. Specific inhibition of EGF-R function by either a dominant-negative EGF-R mutant or selective tyrphostin AG1478 completely abolished Ang II-induced ERK activation. Induction of c-fos gene expression and DNA synthesis were also abolished by the inhibition of EGF-R function. Calmodulin or tyrosine kinase inhibitors, but not protein kinase C (PKC) inhibitors or downregulation of PKC, completely abolished transactivation of EGF-R by Ang II or the Ca2+ ionophore A23187. Epidermal growth factor (EGF) activity in concentrated supernatant from Ang II-treated cells was not detected, and saturation of culture media with anti-EGF antibody did not affect the Ang II-induced transactivation of EGF-R. Conditioned media in which cells were incubated with Ang II could not induce phosphorylation of EGF-R on recipient cells. Platelet-derived growth factor-beta receptor was not phosphorylated on Ang II stimulation, and Ang II-induced c-jun gene expression was not affected by tyrphostin AG1478. Our results demonstrated that in cardiac fibroblasts Ang II-induced ERK activation and its mitogenic signals are dominantly mediated by EGF-R transactivated in a Ca2+/calmodulin-dependent manner and suggested that the effects of Ang II on cardiac fibroblasts should be interpreted in association with the signaling pathways regulating cellular proliferation and/or differentiation by growth factors.

Autocrine/paracrine determinants of strain-activated brain natriuretic peptide gene expression in cultured cardiac myocytes

The application of mechanical strain leads to activation of human brain natriuretic peptide gene promoter activity, a marker of hypertrophy, in cultured neonatal rat ventricular myocytes. We have used a combination of transient transfection analysis and reverse transcriptase-polymerase chain reaction to examine the role of locally produced factors in contributing to this activation. Conditioned media from strained, but not static, cultures led to a dose-dependent increase in human brain natriuretic peptide gene promoter activity. This increase was completely blocked by losartan or BQ-123, implying a role for angiotensin and endothelin as autocrine/paracrine mediators of the response to strain. Inclusion of the same antagonists in the cultures themselves led to only partial inhibition (approximately 60%), whereas inclusion of exogenous endothelin or angiotensin II resulted in amplification of the strain response. Angiotensin II and endothelin appear to be arrayed in series in the regulatory circuitry; the angiotensin response was blocked by BQ-123, whereas the endothelin response was unaffected by losartan. Mechanical strain was also shown to stimulate expression of the endogenous angiotensinogen, angiotensin-converting enzyme, and endothelin genes in this system. Collectively, these data indicate that locally generated angiotensin II and endothelin, acting in series, play an important autocrine/paracrine role in mediating strain-dependent activation of cardiac-specific gene expression.

Biphasic regulation of the preproendothelin-1 gene by c-myc

Endothelin-1 (ET-1), a potent vasoconstrictive/mitogenic peptide originally isolated from vascular endothelium, stimulates the expression of immediate early response genes such as c-myc. The c-myc protooncogene participates in regulating the cascade of events that follow mitogenic stimulation of quiescent cells. Using a panel of isogenic fibroblast cell lines with differential c-myc expression levels (obtained by disrupting one c-myc gene copy with targeted homologous recombination and subsequently stably transfecting the heterozygous cells with an exogenous c-myc transgene), we demonstrate that c-Myc protein regulates ET-1 gene transcription in a biphasic fashion: as an activator at low concentrations and as a repressor at high concentrations. Using rat endothelial cells treated with antisense c-myc oligodeoxynucleotides, we also show that c-myc regulates ET-1 synthesis and secretion in a biphasic manner. The present report, therefore, demonstrates the existence of a signal transduction pathway that regulates the synthesis and secretion of ET-1 via the immediate early transcription factor, c-Myc.

Involvement of transcriptional and posttranscriptional mechanisms in cardiac overload-induced increase of B-type natriuretic peptide gene expression

The induction of atrial and ventricular B-type natriuretic peptide (BNP) gene expression is one of the earliest events occurring during hemodynamic overload. To examine the molecular mechanisms for increased BNP gene expression during cardiac overload, we studied the induction of the BNP gene expression compared with that of atrial natriuretic peptide (ANP) in a modified perfused rat heart preparation. An increase in right atrial pressure of 5 mm Hg resulted in a 1.4-fold (P < .05) and 2.2-fold (P < .01) increase in BNP mRNA levels after 1 and 2 hours, respectively, whereas ANP mRNA levels remained unchanged. Stretching for up to 2 hours also significantly increased right atrial immunoreactive BNP (ir-BNP) levels (from 15.8 +/- 2.2 to 20.1 +/- 1.2 ng/mg, P < .05). Actinomycin D (10 micrograms/mL), a transcriptional inhibitor, completely inhibited the stretch-induced increase in atrial BNP mRNA levels at 1 hour (P < .05) and 2 hours (P < .001), whereas a protein synthesis inhibitor, cycloheximide (90 micrograms/mL), had no effect on basal or direct mechanical stretch-induced increase in right atrial BNP mRNA levels. Furthermore, we examined the role of tyrosine kinase and protein kinase C activities in acute mechanical stretch-induced increase in BNP synthesis. Tyrosine kinase inhibitors lavendustin A (1 mumol/L) and tyrphostin A25 (3 mumol/L) and protein kinase C inhibitors staurosporine (30 nmol/L) and chelerythrine (1 mumol/L) prevented the stretch-induced increase in right atrial ir-BNP concentrations at 2 hours. In addition, chelerythrine inhibited the increase of right atrial BNP mRNA levels stimulated by cardiac overload. These resuls demonstrate that the early increase of BNP mRNA levels by mechanical stretch results from increased transcriptional activation and is independent of protein synthesis. Our results also suggest that protein kinase C and tyrosine kinases activities may be involved in coupling cardiac overload to alterations in atrial BNP synthesis.

Stress-induced expression of immediate early genes in the brain and peripheral organs of the rat

Stress causes rapid and transient expression of immediate early genes (IEGs) in the brain, and the monitoring of IEGs has enabled the visualization of the neurocircuitry of stress. Previous studies have postulated that stressors can be divided into two categories; processive and systemic. The neural circuits of brain activation differ between the two kinds of stressors. For example, processive stressors, such as immobilization (IMO), induce c-fos mRNA first in the cortical and limbic areas and then in the paraventricular hypothalamic nucleus (PVH), while c-fos expression in the PVH precedes that in other areas in animals subjected to systemic stressors. We further show that prior exposure to IMO stress for 6 days, or implantation of corticosterone pellets suppresses the induction of c-fos, fos B, jun B and NGFI-B, but not that of NGFI-A in the rat PVH. Plasma glucocorticoid may be an important factor regulating stress-induced IEG expression. It is well known that AP-1 and glucocorticoid receptors (GR) interact and suppress each other. Thus, decreased AP-1 levels in chronically stressed animals may help enhance the negative feedback effects of GR and prevent hypersecretion of glucocorticoid, which is implicated in the pathogenesis of stress-related diseases. IMO stress induces rapid expression of c-fos, c-jun and NGFI-A mRNAs in the heart and stomach. These were observed in the ventricular myocardium and coronary arteries, and in the epithelium, smooth muscles and arteries of the stomach after 30 min of IMO. IEG expression in the peripheral organs may provide a molecular basis for stress-induced psychosomatic disorders.

Mechanical strain increases expression of the brain natriuretic peptide gene in rat cardiac myocytes

Using a device that applies cyclical strain (1 Hz) to ventricular cardiocytes cultured on collagen-coated silicone elastomer surfaces, we have demonstrated strain-dependent increases in brain natriuretic peptide (BNP) secretion, BNP mRNA levels, and expression of a transiently transfected -1595 human BNP-luciferase reporter. When actinomycin D (10 microM) was introduced concomitantly with the strain stimulus, the strain-induced increase in BNP mRNA was eliminated, and the decay of transcripts was identical in the control and strained cells, indicating the lack of independent effects on transcript stability. Strain-dependent -1595 human BNP-luciferase activity was completely inhibited by chelerythrine, 2-aminopurine, genistein, and W-7 and only partially or not at all by KN-62, wortmannin, and H-89. The effects of these individual agents paralleled their effects on mitogen-activated protein kinase (MAPK) activity, but not c-Jun N-terminal kinase (JNK) activity, in the cells. Overexpression of wild-type MAPK and, to a lesser extent, JNK increased strain-dependent BNP promoter activity, whereas dominant-negative mutants of MAPK kinase, JNK kinase, or Ras completely blocked strain-dependent reporter activity. These findings provide the first demonstration that mechanical strain can increase myocardial gene expression through a transcriptional mechanism and suggest important roles for MAPK and JNK in mediating this effect.

Collaborative roles for c-Jun N-terminal kinase, c-Jun, serum response factor, and Sp1 in calcium-regulated myocardial gene expression

Electrical stimulation of contractions (pacing) of primary neonatal rat ventricular myocytes increases intracellular calcium and activates a hypertrophic growth program that includes expression of the cardiac-specific gene, atrial natriuretic factor (ANF). To investigate the mechanism whereby pacing increases ANF, pacing was tested for its ability to regulate mitogen-activated protein kinase family members, ANF promoter activity, and the trans-activation domain of the transcription factor, Sp1. Pacing and the calcium channel agonist BAYK 8644 activated c-Jun N-terminal kinase (JNK) but not extracellular signal-regulated kinase. Pacing stimulated ANF-promoter activity approximately 10-fold. Furthermore, transfection with an expression vector for c-Jun, a substrate for JNK, also activated the ANF promoter, and the combination of pacing and c-Jun was synergystic, consistent with roles for JNK and c-Jun in calcium-activated ANF expression. Proximal serum response factor and Sp1 binding sites were required for the effects of pacing or c-Jun on the ANF promoter. Pacing and c-Jun activated a GAL4-Sp1 fusion protein by 3- and 12-fold, respectively, whereas the two stimuli together activated GAL4-Sp1 synergistically, similar to their effect on the ANF promoter. Transfection with an expression vector for c-Fos inhibited the effects of c-Jun, suggesting that c-Jun acts independently of AP-1. These results demonstrate an interaction between c-Jun and Sp1 and are consistent with a novel mechanism of calcium-mediated transcriptional activation involving the collaborative actions of JNK, c-Jun, serum response factor, and Sp1.

The AT4 receptor agonist [Nle1]-angiotensin IV reduces mechanically induced immediate-early gene expression in the isolated rabbit heart

Angiotensin II (ANG II), acting principally at the AT1 receptor, modulates mechanically-induced cardiac growth. The ANG II metabolite Angiotensin IV (ANG IV) has been shown to inhibit ANG II-induced mRNA and protein synthesis in chick cardiomyocytes. This effect did not involve the AT1 receptor, but was likely an action at the AT4 receptor. To determine if ANG IV also modulates a mechanically-induced cardiac growth response, we studied the effects of two AT4 receptor ligands, [Nle1]-ANG IV and [divalinal]-ANG IV, on mechanically-induced immediate-early gene expression (c-fos, egr-1, and c-jun) in the buffer perfused (30 degrees C), ejecting, isolated rabbit heart. Mechanical load alone (high systolic pressure and high end-diastolic volume) induced approximately 23-, 49- and 5-fold increases in c-fos, egr-1 and c-jun mRNA (in comparison to control hearts). Perfusion with [Nle1]-ANG IV (10[-10] mol/l) reduced the mechanically-induced expression of c-/fos and egr-1 by 42% and 48%, respectively (P < 0.05). Mechanically-induced c-jun expression was not significantly reduced. Perfusion with [divalinal]-ANG IV (10[-8] mol/l) had no effect on mechanically-induced immediate-early gene expression. We conclude that AT4 receptor agonism influences mechanical immediate-early gene expression, and propose the hypothesis that AT1 and AT4 receptors initiate opposing effects on mechanically-induced immediate-early gene expression in the isolated rabbit left ventricle.

Differential expression of c-jun and junD in end-stage human cardiomyopathy

The proto-oncogenes c-jun and junD are closely related transcriptional factors with opposing actions on cell growth and division. Expression of c-jun rapidly increases as cells enter the cell cycle. Levels of c-jun are also increased in the early stages of experimental cardiac hypertrophy and failure but expression decreases with time. In contrast, junD accumulates in quiescent cells. Expression in end-stage cardiomyopathy has not been studied. Steady-state levels of c-jun and junD mRNA were determined in failing human myocardium (obtained at the time of cardiac transplantation) and in control myocardium from patients who died of noncardiac causes. Relative expression was normalized for glyceraldehyde-3-phosphate dehydrogenase expression. Levels of junD were almost four-fold depressed in myocardium from myopathic hearts (2.1 +/- 0.27, x +/- SE; n = 20) vs. the controls (7.7 +/- 1.1; n = 3). Levels of c-jun were similar in both myopathic and control hearts. Relative expression of beta-myosin heavy chain was the same in both myopathic and control hearts. Levels of junD were still found to be depressed in the myopathic hearts after normalization for myosin heavy chain gene expression. We conclude that c-jun and junD are differentially regulated in end-stage human cardiomyopathy with expression of junD being decreased while relative levels of c-jun mRNA remain unchanged. Further studies are needed to determine the role of junD down-regulation in the development and/or maintenance of the abnormalities present in end-stage heart disease.

Differential effects of protein kinase C, Ras, and Raf-1 kinase on the induction of the cardiac B-type natriuretic peptide gene through a critical promoter-proximal M-CAT element

The cardiac genes for the A- and B-type natriuretic peptides (ANP and BNP) are coordinately induced by growth promoters, such as alpha1-adrenergic receptor agonists (e.g. phenylephrine (PE)). Although inducible elements in the ANP gene have been identified, responsible elements in the BNP gene are unknown. In this study, reporter constructs transfected into neonatal rat ventricular myocytes showed that in the context of 2.5 kilobase pairs of native BNP 5'-flanking sequences, a 2-base pair mutation in a promoter-proximal M-CAT site (CATTCT) disrupted basal and PE-inducible transcription by more than 98%. Expression of constitutively active forms of Ras, Raf-1 kinase, and protein kinase C, all of which are activated by PE in cardiac myocytes, strongly stimulated BNP reporter expression. Isolated M-CAT elements conferred PE, protein kinase C, and Ras inducibility to a minimal BNP promoter, however, they did not confer Raf-1 inducibility. These results show that M-CAT elements can serve as targets for Ras-dependent, Raf-1-independent pathways, implying the involvement of c-Jun N-terminal kinase and/or p38 mitogen-activated protein kinases, but not extracellular signal-regulated protein kinase/mitogen-activated protein kinase. Moreover, the essential M-CAT element distinguishes the BNP gene from the ANP gene, which utilizes serum response elements and an Sp1-like sequence.

An effector-like function of Ras GTPase-activating protein predominates in cardiac muscle cells

In contrast to familiar role for Ras in proliferation, we and others previously suggested that Ras also mediates hypertrophy, the increase in cell mass characteristic of post-natal ventricular muscle. We showed that activated (G12R) and dominant-negative (S17N) Ha-Ras regulate "constitutive" and growth factor-responsive genes equivalently, in both cardiac myocytes and non-cardiac, Mv1Lu cells. Here, we attempt to delineate pathways by which Ras exerts this global effect. The E63K mutation, which impairs binding of guanine nucleotide releasing factor to Ras, alleviated suppression by S17N, consistent with sequestration of exchange factors as the mechanism for inhibition. To compare potential Ras effector proteins, we first engineered G12R/D38N, to abolish binding of Raf and phosphatidylinositol-3-kinase and established that this site was indispensable for augmenting gene expression. To distinguish between inhibition of Ras by Ras GTPase-activating protein (GAP) versus a potential effector function of GAP, we tested the effector domain substitution P34R: this mutation, which abolishes GAP binding, enhanced Ras-dependent transcription in Mv1Lu cells, yet interfered with Ras-dependent expression in ventricular myocytes. To examine the dichotomous role of Ras-GAP predicted from these P34R results, we transfected both cell types with full-length GAP, the C-terminal catalytic domain (cGAP), or N-terminal Src homology domains (nGAP). In Mv1Lu cells, cGAP markedly inhibited both reporter genes, whereas GAP and nGAP had little effect. Antithetically, in ventricular myocytes, GAP and nGAP activated gene expression, whereas cGAP was ineffective. Thus, Ras activates gene expression through differing effectors contingent on cell type, and an effector-like function of GAP predominates in ventricular muscle.

Angiotensin II stimulates c-Jun NH2-terminal kinase in cultured cardiac myocytes of neonatal rats

Many lines of evidence have suggested that angiotensin II (Ang II)plays an important role in cardiac hypertrophy. Ang II not only increases protein synthesis but also induces the reprogramming of gene expression in cultured cardiac myocytes. In the present study, to elucidate the mechanism by which Ang II regulates gene expression in cardiac myocytes, we examined whether Ang II activates c-Jun NH2-terminal kinase (JNK), which is a member of the mitogen-activated protein kinase family and activates the transcription factor, activator protein-1 (AP-1). The activity of JNK increased 5 minutes after the addition of Ang II, peaked at 20 minutes, and gradually decreased thereafter. Examination of the Ang II dose-response relation revealed detectable JNK activation at 10(-9) mol/L and maximal activation at 10(-6) mol/L. Ang II activated JNK through the AT1 receptor, and the activation was attenuated by the downregulation of protein kinase C or the chelation of intracellular Ca2+. Although the addition of either Ca2+ ionophore or phorbol ester resulted in little or no activation of JNK, simultaneous addition of both Ca2+ ionophore and phorbol ester markedly activated JNK. Slight expressions of the c-jun gene were observed in unstimulated cardiac myocytes, and Ang II increased expressions of the c-jun gene as well as the c-fos gene. Ang II increased transcription of the endothelin-1 gene through the AP-1 binding site. In conclusion, Ang II may activate JNK in cultured cardiac myocytes through an increase in intracellular Ca2+ and activation of protein kinase C, and the activated JNK may regulate gene expression by activating AP-1 during Ang II-induced cardiac hypertrophy.

Immobilization stress induces c-fos and c-jun immediate early genes expression in the heart

Emotional stress is considered to be one of the etiological factors in ischemic heart disease (IHD) and sudden cardiac death (SCD), mechanisms of which are poorly understood. Immediate early genes (IEGs), such as c-fos and c-jun are used as tools for detection of cellular activation. Male Wistar rats were exposed to acute immobilization (IMO). IMO stress for 30 min induced c-fos and c-jun mRNAs expression in the myocardium and the smooth muscle layer of the coronary arteries. IMO stress for 2 h also induced Fos and Jun like-immunoreactivities in the same regions. Distribution of IEG mRNAs and their protein products in the myocardium was not uniform but rather localized. These data provided histological evidence for an early cellular event in the stress response whose consequences could result in activation of tissues in the myocardium and coronary arterial smooth muscle cells which play a role in the pathophysiological changes in IHD and SCD.

Serum response factor mediates AP-1-dependent induction of the skeletal alpha-actin promoter in ventricular myocytes

"Fetal" gene transcription, including activation of the skeletal alpha-actin (SkA) promoter, is provoked in cardiac myocytes by mechanical stress and trophic ligands. Induction of the promoter by transforming growth factor beta or norepinephrine requires serum response factor (SRF) and TEF-1; expression is inhibited by YY1. We and others postulated that immediate-early transcription factors might couple trophic signals to this fetal program. However, multiple Fos/Jun proteins exist, and the exact relationship between control by Fos/Jun versus SRF, TEF-1, and YY1 is unexplained. We therefore cotransfected ventricular myocytes with Fos, Jun, or JunB, and SkA reporter genes. SkA transcription was augmented by Jun, Fos/Jun, Fos/JunB, and Jun/JunB; Fos and JunB alone were neutral or inhibitory. Mutation of the SRF site, SRE1, impaired activation by Jun; YY1, TEF-1, and Sp1 sites were dispensable. SRE1 conferred Jun activation to a heterologous promoter, as did the c-fos SRE. Deletions of DNA binding, dimerization, or trans-activation domains of Jun and SRF abolished activation by Jun and synergy with SRF. Neither direct binding of Fos/Jun to SREs, nor physical interaction between Fos/Jun and SRF, was detected in mobility-shift assays. Thus, AP-1 factors activate a hypertrophy-associated gene via SRF, without detectable binding to the promoter or to SRF.

Partial inhibition of Na+/K+-ATPase by ouabain induces the Ca2+-dependent expressions of early-response genes in cardiac myocytes

Exposure of neonatal rat cardiac myocytes to ouabain concentrations that caused partial inhibition of Na+/K+-ATPase but no loss of viability, increased c-fos and c-jun mRNAs and the transcription factor AP-1. The increased mRNAs were proportional to the extent of inhibition of Na+/K+-ATPase and the resulting rise in steady state intracellular Ca2+ concentration. The rapid and sustained increase of c-fos mRNA was shown to be due to increased transcriptional rate. Induction of c-fos by ouabain was prevented when either extracellular or intracellular Ca2+ was lowered and was attenuated by pretreatment of myocytes with a phorbol ester under conditions known to down-regulate protein kinase C. Exposure to ouabain for 24-48 h also increased total transcriptional activity and protein content of myocytes. The findings suggest that the same signal responsible for the positive inotropic action of ouabain, i.e. net influx of Ca2+ caused by partial inhibition of Na+/K+-ATPase, also initiates the rapid protein kinase C-dependent inductions of the early-response genes, the subsequent regulations of other cardiac genes by the resulting transcription factors, and stimulation of myocyte growth. Whether these hitherto unrecognized effects of cardiac glycosides are obtained in the intact heart and their relevance to the therapeutic uses of these drugs remain to be determined.

Involvement of multiple cis elements in basal- and alpha-adrenergic agonist-inducible atrial natriuretic factor transcription. Roles for serum response elements and an SP-1-like element

In the present study, cis elements in the 5'-flanking sequence (FS) of the rat atrial natriuretic factor (ANF) gene involved in regulating basal and alpha 1-adrenergic-inducible transcription were investigated. Truncation analyses using ANF-luciferase reporter constructs transfected into primary neonatal rat cardiac myocytes showed that an A/T-rich serum response element (SRE) at -114 bp of the ANF 5'-FS, which bound serum response factor (SRF), was required for basal and inducible transcription. In constructs composed of 134 bp of rat ANF 5'-FS driving luciferase (ANF-134Luc), mutations in the SRE at -114 bp disrupted SRF binding and ANF promoter activity. However, the same mutations in ANF-638Luc had little effect, suggesting a collaborating role for more distal sequences, such as the other SRE in ANF-638 at -406 bp. In ANF-638Luc, mutations in the SRE at -406 bp that disrupted SRF binding to that site decreased ANF reporter activity by only 25%; however, mutating both of the SREs completely blocked alpha 1-adrenergic-inducible activity. Mutation analyses showed that an ... (SP-1)-like site at -69 bp, shown previously to confer inducibility in reporters with 134 bp of ANF 5'-FS, was not required in ANF-638Luc. However, double mutants in the SP-1-like region and either SRE completely blocked alpha 1-adrenergic-inducible ANF promoter activity. These findings emphasize that no single element is responsible for alpha 1-adrenergic agonist-regulated ANF transcription but that the SREs at -114 and -406 bp and the SP-1-like sequence at -69 bp mediate the effect in collaboration.

Differential localization of atrial natriuretic peptide and skeletal alpha-actin messenger RNAs in left ventricular myocytes of patients with dilated cardiomyopathy

OBJECTIVES

This study was designed to determine whether atrial natriuretic peptide and skeletal alpha-actin messenger RNAs (mRNAs) are co-localized in ventricular myocytes of patients with dilated cardiomyopathy.

BACKGROUND

Atrial natriuretic peptide and skeletal alpha-actin are known as augmented genes with cardiac hypertrophy. However, the expression and localization of both genes in chronic failing heart remain unclear.

METHODS

Left ventricular biopsy specimens were obtained from 14 patients with dilated cardiomyopathy. Atrial natriuretic peptide and skeletal alpha-actin mRNAs were detected by in situ hybridization with specific sulfur-35 uridine triphosphate-labeled RNA probes in the serial sections.

RESULTS

Atrial natriuretic peptide mRNA was detected in 10 patients, and intense signals were localized in the myocytes located in the subendocardium and around the interstitial fibrous area. By contrast, skeletal alpha-actin mRNA was homogeneously detected in all myocytes in seven patients. By left ventriculography, patients with skeletal alpha-actin-positive findings had a lower ejection fraction (37.1 +/- 6.0%) than those with negative findings (46.3 +/- 5.8%, p < 0.05), but atrial natriuretic peptide mRNA expression was not related to left ventricular function.

CONCLUSIONS

These results indicate that the expression of atrial natriuretic peptide and skeletal alpha-actin mRNAs are not always co-localized in the left ventricle of patients with dilated cardiomyopathy and suggest that the mechanisms of the regulation of these two genes in the chronic failing heart are different.

Fra-1, a Fos gene family member that activates atrial natriuretic peptide gene transcription

Previous studies suggested that individual components of the activator protein 1 (AP-1) complex behave in a highly idiosyncratic fashion at the level of the human atrial natriuretic peptide (ANP) gene promoter. ANP gene transcription is activated by c-jun and is generally suppressed by c-fos. In the present study, fra-1, a close relative of the c-fos gene product in terms of its structure and functional activity, behaved like fos in cardiac atriocytes, effecting an approximately 50% reduction in c-jun-activatable expression of a human ANP chloramphenicol acetyltransferase (CAT) reporter. In cardiac ventriculocytes, however, fra-1 effected a synergistic amplification of the c-jun response (a 2.5-fold increase over c-jun alone). In atrial cells, fos-like proteins were not uniformly inhibitory in that a carboxy terminal deletion mutant of c-fos activated a human ANP-CAT reporter in the atriocyte cultures. Finally, using a series of domain-swap mutations in the fos/fra structural sequences, we showed that sequences at both the amino and the carboxy termini are required to realize the full fra-1-dependent stimulatory effect as well as the c-fos-dependent inhibition of ANP gene transcription. These findings suggest considerable heterogeneity in the response of the ANP promoter to different components of the AP-1 complex. Such heterogeneity may serve to broaden the range of biological responses available to this promoter as the cardiac cell attempts to adapt to perturbations in the extracellular environment.

Noradrenergic regulation of immediate early gene expression in rat forebrain: differential effects of alpha 1- and alpha 2-adrenoceptor drugs

Noradrenergic (NAergic) transmission in the rat cerebral cortex has recently been shown to be involved in the regulation of the basal expression of NGFI-A, an immediate early gene (IEG) which encodes a zinc-finger transcription factor. The present study further investigated the role of the NAergic system in mediating cortical IEG expression and possible topographical changes in expression of NGFI-A mRNA in rat forebrain after alpha 1- and alpha 2-adrenoceptor (AR) agonist and antagonist treatment. Expression of c-fos and c-jun, which encode leucine-zipper class transcription factors, was also studied. Male Sprague-Dawley rats were injected intraperitoneally with either an alpha 1-AR agonist (methoxamine, 5 or 10 mg/kg); an alpha 1-AR antagonist (prazosin, 5 mg/kg); an alpha 2-AR agonist (clonidine, 0.5 mg/kg); or an alpha 2-AR antagonist (methoxyidazoxan, 5 mg/kg) and killed after 1 h. IEG mRNA levels were detected by quantitative in situ hybridization histochemistry using 35S-labelled oligonucleotides. High basal levels of NGFI-A mRNA were present in cortical layers IV and VI, hippocampal CA1, piriform cortex, amygdala and caudate putamen. alpha 1-AR agonist and antagonist treatment had essentially no effect on IEG mRNA, despite producing characteristic behavioral and peripheral effects at the doses used. Methoxyidazoxan significantly increased (mean%) NGFI-A mRNA in: cerebral cortex (44); caudate putamen (82); amygdala (92); and CA1 of hippocampus (48), while clonidine significantly decreased NGFI-A mRNA in the various cortical layers to a similar extent (27-37%). Basal c-fos mRNA expression was lower than that for NGFI-A in forebrain areas including cortex, caudate putamen and hippocampus.(ABSTRACT TRUNCATED AT 250 WORDS)

c-fos expression in hypothalamic neurosecretory and brainstem catecholamine cells following noxious somatic stimuli

Noxious somatic stimuli elicit vasopressin secretion, an effect thought to result from activation of a facilitatory input from A1 catecholamine cells of the medulla oblongata. To better characterize the A1 cell response and effects on other neuroendocrine A1 projection targets, particularly within the paraventricular nucleus, we have now mapped c-fos expression in neurochemically identified catecholamine and neurosecretory cells following a noxious somatic stimulus. Unilateral hind paw pinch significantly increased c-fos expression in contralateral A1 cells whereas other brainstem catecholamine cell groups were unaffected. Expression of c-fos was also increased in the supraoptic nucleus, this effect being more pronounced for vasopressin than oxytocin neurosecretory cells and, as with A1 cells, primarily on the side contralateral to the stimulated paw. In contrast, the increase in the paraventricular nucleus was greater in oxytocin rather than in vasopressin cells. Additionally there was a significant rise in c-fos expression in medial parvocellular paraventricular nucleus cells of noxiously stimulated animals. Notably, the majority of tuberoinfundibular corticotropin-releasing factor cells are located in this medial parvocellular zone. These results are consistent with and expand on those previously reported from electrophysiological and anatomical studies. The finding of differing neurosecretory cell responses between supraoptic and paraventricular nuclei has interesting implications with regard to the afferent control of neurosecretory cell activity. For example, the substantially greater activation of supraoptic versus paraventricular nucleus vasopressin cells, despite being innervated by the same medullary noradrenergic cell group, raises the possibility of a differential input or differences in responsiveness. Furthermore, the activation of paraventricular nucleus parvocellular cells is consistent with suggestions that the A1 cell group provides an excitatory input to this population.

Transforming growth factor-beta in cardiac ontogeny and adaptation

The transforming growth factor-beta (TGF-beta) superfamily comprises a set of regulatory peptides with multiple effects on cell growth and differentiation. The elaborate regulation of TGF-beta s during embryonic development of the heart, the upregulation of TGF-beta after hemodynamic stress, and the impact of TGF-beta on cardiac gene expression together imply a prominent functional role for this family of growth factors in cardiac organogenesis and hypertrophy. Basal and TGF-beta-induced expression of skeletal alpha-actin, one of several genes specifically associated with developing or hypertrophied myocardium, each are contingent on transcriptional activation by serum response factor. A truncated form of the type II TGF-beta receptor, created by deletion of the cytoplasmic kinase domain, acts as a dominant suppressor of TGF-beta signal transduction in cultured cardiac muscle cells and may provide a suitable means to establish the functions of TGF-beta in vivo.

Structure, expression, and genomic mapping of the mouse natriuretic peptide type-B gene

The structure of the mouse natriuretic peptide type-B (BNP) gene was determined by isolating and sequencing genomic clones. The mouse BNP gene was structurally similar to other natriuretic peptide genes and comprised three exons and two introns. Expression of the mouse BNP gene was found only in cardiac tissue as determined by ribonuclease protection analyses. Initiation of transcription was 31 bp downstream from a consensus TATA box as determined by primer extension analysis of cardiac RNA. Comparative DNA sequence analysis identified several DNA elements with potential transcriptional regulatory function. Comparative amino acid sequence analysis showed that the N-terminal portion of the mouse and rat BNP precursors was more conserved than the C-terminal 45-amino-acid sequence that constitute the bioactive BNP-45 peptide. The proteolytic processing site (RXXR-S) generating bioactive BNPs was highly conserved among all BNP precursors and was identical to the consensus site of furin, a calcium-dependent serine endoprotease. Finally, the BNP gene was mapped using recombinant inbred DNA and a polymerase chain reaction-based restriction fragment-length polymorphism assay to mouse chromosome 4 near the atrial natriuretic factor (Anf) locus. No recombination event between Bnp and Anf was evident in the 39 recombinant inbred and inbred strains examined. This physical linkage between the two natriuretic peptide genes expressed in cardiac tissue may be important for their transcriptional regulation.

Proto-oncogene expression in porcine myocardium subjected to ischemia and reperfusion

The molecular basis of myocardial adaptation to ischemia and reperfusion is poorly understood. It is thought that nuclear proto-oncogenes act as third messengers, converting cytoplasmic signal transduction into long-term changes of gene expression. We studied the expression of six nuclear proto-oncogenes (Egr-1, c-fos, fosB, c-jun, junB, and c-myc) in myocardium subjected to ischemia and reperfusion in anesthetized pigs. Stunning was achieved by two 10-minute left anterior descending coronary artery occlusions separated by 30 minutes of reperfusion. Hearts were excised after the first occlusion, after the first reperfusion, and at 30, 120, 150, and 210 minutes of reperfusion after the second occlusion. Total RNA was prepared from stunned as well as normally perfused myocardial tissue and subjected to Northern blotting. The response of the six nuclear proto-oncogenes varied.fosB gene expression was never detected. The c-myc gene was expressed, but its level was unchanged by ischemia. c-jun expression was slightly increased by ischemia (3.1 +/- 0.6-fold). The c-fos, Egr-1, and junB genes were highly induced, being fivefold to sevenfold higher in experimental than in control tissue. In three animals pretreated with the beta 1-antagonist metoprolol and then subjected to the above experimental protocol, the induction of proto-oncogenes was similar to that in nonblocked controls. Our results show that the myocardial adaptive response to ischemic stress includes the induction of at least four transcription factors that may be further operative in repair processes and angiogenesis.

Neurotransmitter-stimulated immediate-early gene responses are organized through differential post-synaptic receptor mechanisms

The products of the cellular immediate-early genes (IEGs) are thought to act as messengers in the coupling of trans-synaptic stimuli with altered neuronal gene expression. However, the manner in which neurotransmission specifies particular responses through the IEGs is undefined. In this report, mRNA and transcription analysis of a precisely-timed, physiological IEG response illustrates how an IEG signal may be organized through differential neurotransmitter receptor activation. The nocturnal pattern of IEG expression in the rat pineal gland has been shown to be differentially regulated through post-synaptic adrenergic receptors. Induction of the c-fos gene is primarily mediated through alpha 1-receptors, whereas the coordinately regulated jun-B gene exhibits dual regulation through alpha 1- and beta-receptors. A simultaneous repression of c-jun expression is partly mediated through a beta-receptor mechanism. In vitro analysis of IEGs in cultured pineal glands has confirmed the receptor-specific link between adrenergic neurotransmission and IEG induction. The pineal is a unique neuroendocrine model in which the characteristics and function of the IEG third messenger system may be defined.