Positive and negative control of the skeletal alpha-actin promoter in cardiac muscle. A proximal serum response element is sufficient for induction by basic fibroblast growth factor (FGF) but not for inhibition by acidic FGF

Desmoglein 2 mutation provokes skeletal muscle actin expression and accumulation at intercalated discs in murine hearts

Arrhythmogenic cardiomyopathy (AC) is an incurable progressive disease that is linked to mutations in genes coding for components of desmosomal adhesions that are localized to the intercalated disc region, which electromechanically couples adjacent cardiomyocytes. To date, the underlying molecular dysfunctions are not well characterized. In two murine AC models, we find an upregulation of the skeletal muscle actin gene (Acta1), which is known to be a compensatory reaction to compromised heart function. Expression of this gene is elevated prior to visible morphological alterations and clinical symptoms, and persists throughout pathogenesis with an additional major rise during the chronic disease stage. We provide evidence that the increased Acta1 transcription is initiated through nuclear activation of the serum response transcription factor (SRF) by its transcriptional co-activator megakaryoblastic leukemia 1 protein (MKL1, also known as MRTFA). Our data further suggest that perturbed desmosomal adhesion causes Acta1 overexpression during the early stages of the disease, which is amplified by transforming growth factor β (TGFβ) release from fibrotic lesions and surrounding cardiomyocytes during later disease stages. These observations highlight a hitherto unknown molecular AC pathomechanism.

Inhibition of the cardiac alpha-actin gene in embryonic cardiac myocytes by dominant-negative serum response factor

Serum response factor (SRF), a transcription factor ubiquitously involved in the processes of cellular proliferation and differentiation, has been implicated in cardiac and skeletal muscle development because of its strong expression in embryonic muscle lineages, and its necessity for the transcription of transiently transfected muscle genes that contain SRF binding sites. This study was designed to ascertain whether SRF is required for the expression of an endogenous SRF-dependent gene during differentiation of early embryonic cardiac myocytes by introducing a dominant-negative SRF construct via retroviral delivery. Although no effect on overt cellular differentiation was detected, semi-quantitative RT-PCR revealed that expression of the SRF-dependent gene cardiac alpha-actin was inhibited, whereas expression of the non-SRF-dependent genes glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and cardiac troponin-C was unaffected. No effect on myocyte proliferation was detected. Curiously, immunohistochemical localization of SRF protein suggested that whereas endogenous SRF was homogeneously dispersed throughout the cytoplasm and nucleus, the dominant-negative SRF protein was concentrated in the nucleus. These results extend previous findings using transiently transfected genes to the endogenous level, indicating that SRF is required for the full expression of muscle genes that contain SRF binding sites during cardiac myocyte differentiation.

Transforming growth factor beta-1 attenuates endothelin-1-induced functions in neonatal cardiac myocytes

In the present study we characterized a "crosstalk" mechanism between transforming growth factor beta-1 (TGF beta-1) and endothelin-1 (ET1) signaling pathways in neonatal cardiac myocytes. A 5 minute pretreatment with 1 ng/ml concentrations of TGF beta-1 attenuated ET1-induced negative chronotropic effects and translocation of the alpha, delta and varepsilonPKC isozymes to the particulate cell fraction. We found no effect of TGF beta-1 on responses induced by the P(2) purinergic agonist ATP or phorbol ester. Treatment of cardiac myocytes with acidic fibroblast growth factor (aFGF) did not alter ET1- or ATP-mediated effects on contraction rate or translocation of PKC isozymes to the particulate fraction. Our studies suggest that TGF beta-1 may act as a negative modulator of ET1- but not ATP- or phorbol ester-induced PKC isozyme signaling events in neonatal cardiac myocytes. A better understanding of the complex ET1 and TGF beta-1 signaling mechanisms in neonatal heart cells should enhance our knowledge regarding the interplay between these pathways.

Dilated cardiomyopathy and impaired cardiac hypertrophic response to angiotensin II in mice lacking FGF-2

FGF-2 has been implicated in the cardiac response to hypertrophic stimuli. Angiotensin II (Ang II) contributes to maintain elevated blood pressure in hypertensive individuals and exerts direct trophic effects on cardiac cells. However, the role of FGF-2 in Ang II-induced cardiac hypertrophy has not been established. Therefore, mice deficient in FGF-2 expression were studied using a model of Ang II-dependent hypertension and cardiac hypertrophy. Echocardiographic measurements show the presence of dilated cardiomyopathy in normotensive mice lacking FGF-2. Moreover, hypertensive mice without FGF-2 developed no compensatory cardiac hypertrophy. In wild-type mice, hypertrophy was associated with a stimulation of the c-Jun N-terminal kinase, the extracellular signal regulated kinase, and the p38 kinase pathways. In contrast, mitogen-activated protein kinase (MAPK) activation was markedly attenuated in FGF-2-deficient mice. In vitro, FGF-2 of fibroblast origin was demonstrated to be essential in the paracrine stimulation of MAPK activation in cardiomyocytes. Indeed, fibroblasts lacking FGF-2 expression have a defective capacity for releasing growth factors to induce hypertrophic responses in cardiomyocytes. Therefore, these results identify the cardiac fibroblast population as a primary integrator of hypertrophic stimuli in the heart, and suggest that FGF-2 is a crucial mediator of cardiac hypertrophy via autocrine/paracrine actions on cardiac cells.

Binding of serum response factor to CArG box sequences is necessary but not sufficient to restrict gene expression to arterial smooth muscle cells

Serum response factor (SRF) plays an important role in regulating smooth muscle cell (SMC) development and differentiation. To understand the molecular mechanisms underlying the activity of SRF in SMCs, the two CArG box-containing elements in the arterial SMC-specific SM22alpha promoter, SME-1 and SME-4, were functionally and biochemically characterized. Mutations that abolish binding of SRF to the SM22alpha promoter totally abolish promoter activity in transgenic mice. Moreover, a multimerized copy of either SME-1 or SME-4 subcloned 5' of the minimal SM22alpha promoter (base pairs -90 to +41) is necessary and sufficient to restrict transgene expression to arterial SMCs in transgenic mice. In contrast, a multimerized copy of the c-fos SRE is totally inactive in arterial SMCs and substitution of the c-fos SRE for the CArG motifs within the SM22alpha promoter inactivates the 441-base pair SM22alpha promoter in transgenic mice. Deletion analysis revealed that the SME-4 CArG box alone is insufficient to activate transcription in SMCs and additional 5'-flanking nucleotides are required. Nuclear protein binding assays revealed that SME-4 binds SRF, YY1, and four additional SMC nuclear proteins. Taken together, these data demonstrate that binding of SRF to specific CArG boxes is necessary, but not sufficient, to restrict transgene expression to SMCs in vivo.

beta1 integrin and organized actin filaments facilitate cardiomyocyte-specific RhoA-dependent activation of the skeletal alpha-actin promoter

Activation of RhoA GTPase causes actin filament bundling into stress fibers, integrin clustering, and focal adhesion formation through its action on actin cytoskeleton organization. RhoA also regulates transcriptional activity of serum response factor (SRF). Recent studies in NIH 3T3 fibroblasts have shown that SRF activation by RhoA does not require an organized cytoskeleton and may be regulated by G-actin level. In cardiac myocytes, the organization of actin fibers into myofibrils is one of the primary characteristics of cardiac differentiation and hypertrophy. The primary purpose of this study was to examine if RhoA regulates SRF-dependent gene expression in neonatal cardiomyocytes in a manner different from that observed in fibroblasts. Our results show that RhoA-dependent skeletal alpha-actin promoter activation requires beta1 integrin and a functional cytoskeleton in cardiomyocytes but not in NIH 3T3 fibroblasts. Activation of the alpha-actin promoter by RhoA is greatly potentiated (up to 15-fold) by co-expression of the integrin beta1A or beta1D isoform but is significantly reduced by 70% with a co-expressed dominant negative mutant of beta1 integrin. Furthermore, clustering of beta1 integrin with anti-beta1 integrin antibodies potentiates synergistic RhoA and beta1 integrin activation of the alpha-actin promoter. Cytochalasin D and latrunculin B, inhibitors of actin polymerization, significantly reduced RhoA-induced activation of the alpha-actin promoter. Jasplakinolide, an actin polymerizing agent, mimics the synergistic effect of RhoA and beta1 integrin on the actin promoter. These observations support the concept that RhoA regulates SRF-dependent cardiac gene expression through cross-talk with beta1 integrin signal pathway via an organized actin cytoskeleton.

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.

A Ras-dependent pathway regulates RNA polymerase II phosphorylation in cardiac myocytes: implications for cardiac hypertrophy

Despite extensive evidence implicating Ras in cardiac muscle hypertrophy, the mechanisms involved are unclear. We previously reported that Ras, through an effector-like function of Ras GTPase-activating protein (GAP) in neonatal cardiac myocytes (M. Abdellatif et al., J. Biol. Chem. 269:15423-15426, 1994; M. Abdellatif and M. D. Schneider, J. Biol. Chem. 272:527-533, 1997), can up-regulate expression from a comprehensive set of promoters, including both cardiac cell-specific and constitutive ones. To investigate the mechanism(s) underlying these earlier findings, we have used recombinant adenoviruses harboring a dominant negative Ras (17N Ras) allele or the N-terminal domain of GAP (nGAP), responsible for the Ras-like effector function. Inhibition of endogenous Ras reduced basal levels of [3H]uridine and [3H]phenylalanine incorporation into total RNA, mRNA, and protein, with parallel changes in apparent cell size. In addition, 17N Ras markedly inhibited phosphorylation of the C-terminal domain (CTD) of RNA polymerase II (pol II), known to regulate transcript elongation, accompanied by down-regulation of its principal kinase, cyclin-dependent kinase 7 (Cdk7). In contrast, nGAP elicited the opposite effects on each of these parameters. Furthermore, cotransfection of constitutively active Ras (12R Ras) with wild-type pol II, rather than a truncated mutant lacking the CTD, demonstrated that Ras activation of transcription was dependent on the pol II CTD. Consistent with a potential role for this pathway in the development of cardiac myocyte hypertrophy, alpha1-adrenergic stimulation similarly enhanced pol II phosphorylation and Cdk7 expression, where both effects were inhibited by dominant negative Ras, while pressure overload hypertrophy led to an increase in both hyperphosphorylated and hypophosphorylated pol II in addition to Cdk7.

Oncogenic src, raf, and ras stimulate a hypertrophic pattern of gene expression and increase cell size in neonatal rat ventricular myocytes

In response to hormones and growth factors, cultured neonatal ventricular myocytes increase in profile, exhibit myofibrillogenesis, and re-express genes whose expression is normally restricted to the fetal stage of ventricular development. These include atrial natriuretic factor (ANF), beta-myosin heavy chain (beta-MHC), and skeletal muscle (SkM)-alpha-actin. By using luciferase reporter plasmids, we examined whether oncogenes that activate the extracellular signal-regulated kinase cascade (srcF527, Ha-rasV12, and v-raf) increased expression of "fetal" genes. Transfection of myocytes with srcF527 stimulated expression of ANF, SkM-alpha-actin, and beta-MHC by 62-, 6.7-, and 50-fold, respectively, but did not induce DNA synthesis. Stimulation of ANF expression by srcF527 was greater than by Ha-rasV12, which in turn was greater than by v-raf. General gene expression was also increased but to a lesser extent. The response to srcF527 was inhibited by dominant-negative Ha-rasN17. Myocyte area was increased by srcF527, Ha-rasV12, and v-raf, and although it altered myocyte morphology by causing a pseudopodial appearance, srcF527 did not detectably increase myofibrillogenesis either alone or in combination with Ha-rasV12. A kinase-dead src mutant increased myocyte size to a much lesser extent than srcF527 and also did not inhibit ANF-luciferase expression in response to phenylephrine. We conclude that members of the Src family of tyrosine kinases may be important in mediating the transcriptional changes occurring during cardiac myocyte hypertrophy and that Ras and Raf may be downstream effectors.

S100beta inhibits alpha1-adrenergic induction of the hypertrophic phenotype in cardiac myocytes

In an experimental rat model of myocardial infarction, surviving cardiac myocytes undergo hypertrophy in response to trophic effectors. This response involves gene reprogramming manifested by the re-expression of fetal genes, such as the previously reported isoform switch from adult alpha- to embryonic beta-myosin heavy chain. We now report the transient re-expression of a second fetal gene, skeletal alpha-actin in rat myocardium at 7 days post-infarction, and its subsequent down-regulation coincident with the delayed induction of S100beta, a protein normally expressed in brain. In cultured neonatal rat cardiac myocytes, co-transfection with an S100beta-expression vector inhibits a pathway associated with hypertrophy, namely, alpha1-adrenergic induction of beta-myosin heavy chain and skeletal alpha-actin promoters mediated by beta-protein kinase C. The induction of beta-myosin heavy chain by hypoxia was similarly blocked by forced expression of S100beta. Our results suggest that S100beta may be an intrinsic negative regulator of the hypertrophic response of surviving cardiac myocytes post-infarction. Such negative regulators may be important in limiting the adverse consequences of unchecked hypertrophy leading to ventricular remodeling and dysfunction.

Urokinase and tissue-type plasminogen activator are required for the mitogenic and chemotactic effects of bovine fibroblast growth factor and platelet-derived growth factor-BB for vascular smooth muscle cells

The present study was undertaken to evaluate in vitro the relative importance of tissue-type plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA) in the mitogenic and chemotactic potential of bovine fibroblast growth factor (bFGF) and platelet-derived growth factor (PDGF)-BB for smooth muscle cells (SMC). Aortic SMC were isolated from transgenic mice showing single inactivations of the t-PA, u-PA, plasminogen activator inhibitor-1, or urokinase-type plasminogen activator receptor (u-PAR) genes. With regard to serum-induced proliferation, all cell types showed similar responses. However, SMC isolated from t-PA-deficient mice did not proliferate or migrate in response to PDGF, whereas SMC isolated from u-PA-deficient animals appeared to be much less sensitive to bFGF than the cells isolated from the other animals. Supplementation of cells from deficient animals with exogenous murine t-PA or u-PA restored the normal response of the growth factors with regard to both migration and proliferation. The mitogenic and chemotactic responses of bFGF were specifically inhibited in u-PAR-deficient cells or in wild-type SMC, cultured in the presence of antibodies to u-PAR. The role of u-PA and t-PA in bFGF and PDGF-induced growth and migration of SMC was not dependent on plasmin generation and activity as demonstrated by the inactivity of epsilon-aminocaproic acid and aprotinin. A 4-5-fold increase in the steady-state levels of u-PA and t-PA mRNA and proteins were observed after 24 h of incubation of the cell cultures with bFGF and PDGF-BB, respectively. These results therefore indicate that, at least in vitro, t-PA is an important element of the activity of PDGF-BB with regard to the proliferation and migration of SMC whereas u-PA is a key factor in the effect of bFGF on SMC.

The bcl-2 gene product prevents programmed cell death of ventricular myocytes

BACKGROUND

To formally test whether the antiapoptotic protein bcl-2 would prevent programmed cell death in cardiac muscle cells provoked by p53, a known trigger of apoptosis in a variety of different cell types, we used replication defective adenovirus encoding either the bcl-2 and p53 genes to deliver bcl-2 and p53 to ventricular myocytes with high efficiency and uniformity.

METHODS AND RESULTS

Vital staining of ventricular myocytes revealed a significant (7-fold, P<.05) increase in myocyte cell death in the presence of p53 in contrast to uninfected cells or those infected with a control virus. In addition, in the presence of p53, nucleosomal DNA fragmentation observed by Hoescht 33258 staining and terminal transferase deoxynucleotide end labeling indicated a significant increase in apoptotic cardiac nuclei compared with control cells, confirming the hypothesis that p53 alone is sufficient to trigger apoptosis of ventricular myocytes. Moreover, a significant increase in transcription of the bax promoter was seen in the presence but not in the absence of p53 compared with control cells. Expression of the antiapoptotic gene bcl-2 in ventricular myocytes was sufficient to prevent ventricular myocyte death and apoptosis provoked by p53. Importantly, the antiapoptotic effects of bcl-2 were independent of altered p53 expression or localization of p53 to cardiac nuclei. However, p53 dependent transcription of bax was repressed 4-fold (P<.05) by bcl-2, suggesting a tentative link between p53-mediated apoptosis and the protective properties conferred by bcl-2 in ventricular myocytes.

CONCLUSIONS

To our knowledge, the data provide the first indication for the operation of bcl-2 in ventricular myocytes as an antiapoptotic factor.

Epidermal growth factor induces CD44 gene expression through a novel regulatory element in mouse fibroblasts

Growth factors coordinately regulate a variety of genes associated with pathological states including tumor invasion and metastasis. Overexpressed epidermal growth factor receptor (EGFR) on tumor cell surfaces is associated with enhanced cell attachment and migration into extracellular matrices, which promotes tumor aggressiveness. We have demonstrated that epidermal growth factor (EGF) up-regulates the cell surface adhesion molecule CD44 at both the mRNA and protein levels on mouse fibroblasts expressing full-length wild-type EGFR (NR6-WT) but not on EGFR-deficient cells (NR6-P). This increases cell attachment to hyaluronic acid. In this investigation, transcriptional regulation of CD44 by EGF was confirmed by defining an EGF-regulatory element. By employing human CD44 gene promoter-chloramphenicol acetyltransferase (CAT) constructs transfected into NR6-WT cells, EGF inducibility was observed within a 120-base pair (bp) DNA fragment located 450 bp upstream of the RNA initiation site. Differential EGF inducibility was found among different cell lines chosen, indicating a 3.2- and 1.8-fold enhancement in DU145 cells carrying exogenous wild-type EGFR and in MCF-7 cells, respectively, while minimal EGF induction was found in cervical cancer HeLa cells. Utilizing gel shift assays, a time-dependent increase of DNA-protein complex formation was found upon EGF stimulation in NR6-WT cells but not in NR6-P cells. Based upon these observations, a novel 22-bp EGF regulatory element (ERE) (5'--604CCCTCTCTCCAGCTCCTCTCCC-583-3') was isolated from the CD44 gene promoter. This ERE conferred DNA-protein binding ability in vitro, as well as the full functional recovery of EGF inducibility of CAT activity when linked to a homologous CD44 promoter or a SV40 promoter driving a CAT reporter gene. A two-base mutation of the ERE completely eliminated its binding activity as well as its EGF inducibility of CAT expression. Our studies indicate that EGF induces CD44 gene expression through an interaction between a specific ERE and putative novel transcriptional factor so as to regulate cell attachment to extracellular matrix.

Differential regulation of insulin-like growth factor-I (IGF-I) receptor gene expression by IGF-I and basic fibroblastic growth factor

Insulin-like growth factor-I receptor (IGF-IR) gene expression is regulated by various stimuli, including hormones, growth factors, and nutritional status. We have investigated the molecular mechanism by which two growth factors, insulin-like growth factor-I (IGF-I) and basic fibroblast growth factor (bFGF) regulate IGF-IR gene expression. bFGF increases the endogenous IGF-IR mRNA levels and IGF-IR promoter activity. This effect is mediated by a region of the IGF-IR promoter located between nucleotides -476 and -188 in the 5'-flanking region. In contrast, IGF-I decreases the IGF-IR mRNA levels. IGF-I down-regulates IGF-IR transcriptional activity as deduced from experiments in which the levels of pre-mRNA and mRNA were measured. IGF-I reduced pre-mRNA and mRNA levels in parallel, while the mRNA stability was found to be unchanged by IGF-I treatment. While these results strongly suggest an effect of IGF-I on IGF-IR transcriptional activity, no specific IGF-I response element was demonstrated in the 5'-untranslated region or 5'-flanking region studied. Thus, bFGF and IGF-I have differential effects on IGF-IR gene transcription, with the IGF-I response region as yet unidentified.

Interleukin-1beta is a negative transcriptional regulator of alpha1-adrenergic induced gene expression in cultured cardiac myocytes

We recently reported that interleukin-1beta (IL-1beta) induces a novel form of cardiac myocyte hypertrophy characterized by an increase in protein content but an absence of the fetal program of skeletal alpha-actin or beta-myosin heavy chain (beta-MHC) gene expression (Palmer, J. N., Hartogensis, W. E., Patten, M., Fortuin, F. D., and Long, C. S. (1995) J. Clin. Invest. 95, 2555-2564). Because of the apparent disparity between this myocardial phenotype and that seen with other hypertrophic agents in culture, such as catecholamines, we investigated the effect of IL-1beta on alpha1-induced cardiomyocyte hypertrophy. Although there was no augmentation in total protein when IL-1beta and phenylephrine were given simultaneously, IL-1beta attenuated the increase in contractile protein mRNAs (skeletal alpha-actin and beta-MHC) in response to phenylephrine. Transient transfection studies with skeletal alpha-actin and beta-MHC promoter constructs linked to the chloramphenicol acetyltransferase (CAT)-reporter gene indicate that repression occurred at the level of gene transcription. In view of the previously reported activity of the zinc finger protein YY1 in the negative regulation of the skeletal alpha-actin promoter in cardiomyocytes (MacLellan, W. R., Lee, T. C., Schwartz, R. J., and Schneider, M. D. (1994) J. Biol. Chem. 269, 16754-16760), we investigated the potential role of this factor in the IL-1beta-mediated effects. Using transient transfection, we found that a mutation in the YY1 binding site of the skeletal alpha-actin promoter abolished the inhibitory effect of IL-1beta. We further found that the 127-base pair fragment of the skeletal alpha-actin promoter required for the IL-1beta effect is also required for inhibition by the overexpression of YY1 in the myocytes. Furthermore, increased levels of YY1 protein are found in IL-1beta treated myocytes. Taken together these results suggest that the repression of contractile protein gene transcription by IL-1beta may be due, at least in part, to activation of the negative transcription factor YY1.

Atrial chamber-specific expression of the slow myosin heavy chain 3 gene in the embryonic heart

The quail slow myosin heavy chain 3 (slow MyHC 3) gene is expressed in the developing heart and in slow muscles of the developing limb. It is first expressed in the pulsatile cardiac tube in the embryo, and as the heart chamberizes its expression becomes restricted to the atria. To identify regulatory elements responsible for atrial-specific expression, the 5' upstream region of slow MyHC 3 gene was investigated. An atrial regulatory domain (ARD1) between -840 and -680 acts as an atrial cell-specific enhancer in primary cardiocyte cultures. ARD1 also specifies atrial-specific expression in vivo when the ARD1/heterologous promoter was introduced into developing chick embryos by a replication-competent retroviral vector. ARD1 is the first atrial cell-specific enhancer to be identified. Fine deletion and mutation analysis within ARD1 defined a 40-base pair vitamin D3 receptor-like element that controls atrial cell-specific expression of the slow MyHC 3 gene by inhibiting its expression in ventricular cardiocytes.

SRF and TEF-1 control of chicken skeletal alpha-actin gene during slow-muscle hypertrophy

The purpose of this study was to delineate the alpha-actin regulatory elements and transcription factors that are responsible for conferring stretch-overload responsiveness during hypertrophy of the anterior latissimus dorsi (ALD) muscle of young chickens by weighting one wing. Minimal promoter constructs were evaluated by direct injection into the ALD, which demonstrated that both serum response element 1 (SRE1) and the transcriptional enhancer factor 1 (TEF-1) elements were sufficient for increased expression during stretch overload. A mutated SRE1 prevented expression in both basal and stretched ALD muscles, whereas a mutated TEF-1 element reduced actin promoter function in both control and stretched muscles. The serum response factor (SRF)-SRE1 binding complex demonstrated faster migration in mobility shift assays from day 3-and day 6-stretched ALD nuclear extracts relative to their control. TEF-1 binding was qualitatively increased in stretched extracts at day 3 but not day 6 of stretch overload. Skeletal alpha-actin mRNA accumulated from day 3 to day 6 of stretch overload. These data demonstrate that SRE1 is necessary and sufficient for stretch-overload responsiveness from the skeletal alpha-actin promoter and that the SRF-SRE1 binding complex migrates faster in stretched nuclear extracts of hypertrophied relative to control extracts from intact ALD muscles of chickens.

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.

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.

The mitogen-activated protein kinase kinase MEK1 stimulates a pattern of gene expression typical of the hypertrophic phenotype in rat ventricular cardiomyocytes

Adult mammalian ventricular cardiomyocytes are terminally differentiated cells that enlarge adaptively by hypertrophy. In this situation, genes normally expressed in the fetal ventricular cardiomyocyte (e.g. atrial natriuretic factor (ANF), beta-myosin heavy chain (beta-MHC), and skeletal muscle (SkM) alpha-actin) are re-expressed, and there is transient expression of immediate early genes (e.g. c-fos). Using appropriate reporter plasmids, we studied the effects of transfection of the constitutively active or dominant negative mitogen-activated protein kinase kinase MEK1 on ANF, beta-MHC, and SkM alpha-actin promoter activities in cultured ventricular cardiomyocytes. ANF expression was stimulated (maximally 75-fold) by the hypertrophic agonist phenylephrine in a dose-dependent manner (EC50, 10 microM), and this stimulation was inhibited by dominant negative MEK1. Cotransfection of dominant negative MEK1 with a dominant negative mitogen-activated protein kinase (extracellular signal-regulated protein kinase (ERK2)) increased this inhibition. Transfection with constitutively active MEK1 constructs doubled ANF promoter activity. The additional cotransfection of wild-type ERK2 stimulated ANF promoter activity by about 5-fold. Expression of beta-MHC and SkM alpha-actin was also stimulated. Promoter activity regulated by activator protein-1 or c-fos serum response element consensus sequences was also increased. We conclude that the MEK1/ERK2 cascade may play a role in regulating gene expression during hypertrophy.

Induction of tissue inhibitor and matrix metalloproteinase by serum in human heart-derived fibroblast and endomyocardial endothelial cells

To understand the regulatory mechanisms of extracellular matrix (ECM) turnover and proteinase expression in human cardiovascular tissue, we have isolated and characterized human heart fibroblast (HHF) and human heart endothelial (HHE) cells from endomyocardial biopsy specimens. HHE cell in culture exhibited the typical cobblestone growth pattern and positive immunofluorescent staining for factor VIII related antigen. HHF demonstrated the typical spindle shape during culture and were positive for vimentin. Both cell types were negative for alpha-actin, indicating that these cells were of nonmuscle origin. Cell growth studies revealed significant growth when maintained in limiting serum concentration, suggesting mitogenic activity of these cells, and demonstrated growth inhibitory activity when grown in serum-free medium. Serum-dependent matrix metalloproteinases (MMPs) and tissue inhibitor of metalloproteinases (TIMPs) expression was measured by zymography, immunoblot, and Northern blot analysis. Results indicated that serum induces both the MMP and TIMP expression at the mRNA and protein levels in a dose-dependent manner. This induction was inhibited by actinomycin D and cycloheximide, suggesting transcriptional and translational regulation of MMP and TIMP. Indirect immunofluorescence labeling indicated expression of MMP and TIMP in HHF and HHE cells. These results suggested that the serum induces proliferation as well as expression of MMP and TIMP in HHE and HHF cells. The growth inhibitory activity of these cell cultures will enable us to explore further the nature of this response and compare this phenomenon with other growth inhibitors and growth promoters identified in other normal and transformed cells.

Transcriptional and posttranscriptional regulation of urokinase-type plasminogen activator expression in endothelial cells by basic fibroblast growth factor

The mechanism of induction of urokinase-type plasminogen activator (uPA) by basic fibroblast growth factor (bFGF) was explored in fetal bovine aortic endothelial GM 7373 cells. A three- to four-fold increase in the steady-state levels of uPA mRNA was observed after 6 h of incubation of the cell cultures with bFGF. Accordingly, nuclear run-on experiments showed a 2-2.4-fold increase in the rate of uPA gene transcription during the first 4 h of treatment with the growth factor. bFGF did not affect uPA mRNA stability, as evaluated by chase experiments with the mRNA synthesis inhibitor actinomycin D. Upregulation of uPA mRNA was followed by a delayed increase in uPA protein synthesis paralleled by an increase in secreted and cell-associated uPA activity. Twelve h were required before accumulated uPA mRNA was translated into the corresponding protein. During this time interval, the continuous presence of biologically active bFGF in the extracellular environment represented an absolute requirement for uPA mRNA translation. Substitution of residues Lys-27, Lys-30, and Arg-31 to glutamine residues in the bFGF molecule resulted in a mutant (M1Q-bFGF) that caused uPA mRNA accumulation in the absence of a significant increase in cell-associated uPA activity. M1Q-bFGF also induced an increase in cell-associated uPA activity only when added to the cell cultures in the presence of soluble heparin. These results provide evidence that bFGF can affect uPA expression in endothelial GM 7373 cells both at transcriptional and posttranscriptional translational levels. They also show the possibility to dissociate upregulation of uPA mRNA from upregulation of uPA activity by mutagenesis of the bFGF molecule.

M-CAT, CArG, and Sp1 elements are required for alpha 1-adrenergic induction of the skeletal alpha-actin promoter during cardiac myocyte hypertrophy. Transcriptional enhancer factor-1 and protein kinase C as conserved transducers of the fetal program in cardiac growth

Induction of the fetal isogenes skeletal alpha-actin (skACT) and beta-myosin heavy chain (beta-MHC) is characteristic of cardiac growth in many models, suggesting a conserved signaling pathway. However, divergent regulation has also been observed. beta-Protein kinase C (PKC) and transcriptional enhancer factor-1 (TEF-1) are involved in induction of beta-MHC in alpha 1-adrenergic-stimulated hypertrophy of cultured cardiac myocytes (Kariya, K., Farrance, I.K. G., and Simpson, P.C. (1993) J. Biol. Chem. 268, 26658-26662; Kariya, K., Karns, L. R., and Simpson, P.C. (1994) J. Biol. Chem. 269, 3775-3782). In the present study, we asked whether the skACT promoter used the same mechanism. A mouse skACT promoter fragment (-113/-46) was induced by both alpha 1-adrenergic stimulation and co-transfection of activated beta-PKC, and contained three required DNA sequence elements: M-CAT, CArG, and Sp1. The skACT M-CAT element bound TEF-1 in cardiac myocytes. Thus the skACT and beta-MHC promoters both require a TEF-1 binding site for activation by alpha 1-adrenergic stimulation, but differ in that skACT also requires a CArG box. These results provide a potential molecular basis for divergent regulation of the fetal program, and also imply that PKC and TEF-1 are conserved transducers for this program during cardiac growth.

Nuclear accumulation of exogenous basic fibroblast growth factor in endothelial, fibroblast, and myoblast cell lines results in diverse biological responses

During studies comparing 125I-bFGF internalization between endothelial cells and other cell types, we found, unexpectedly, internalization and nuclear translocation of exogenously added 125I-bFGF in two cell lines: Chinese hamster ovary cells (CHO) and rat L6 myoblasts. These cell lines were previously reported to be devoid of FGF receptors. Furthermore, CHO cells showed a weak mitogenic response to added bFGF, while L6 cells were mitogenically unresponsive. By comparison, coronary venular endothelial cells (CVEC), BALB/c 3T3 fibroblasts, and BHK-21 cells, demonstrated internalization and nuclear translocation of added 125I-bFGF, and mitogenic responsiveness to the growth factor. Insulin alone stimulated DNA synthesis in all cell types, yet augmented bFGF-dependent DNA synthesis only in CVEC, 3T3, and BHK. All five cell types expressed FGF receptors as assessed by covalent crosslinking with 125I-bFGF and immunoblotting with anti-FGF receptor antibodies. Differing rates of cytoplasmic and nuclear accumulation of 125I-bFGF and partial inhibition of internalization by pretreatment of CVEC with chlorate support a recent model that bFGF can internalize by two mechanisms. Insulin did not significantly affect 125I-bFGF internalization or metabolism in any cell type. bFGF treatment resulted in weak inhibition of RNA synthesis in L6 cells. bFGF appears firmly bound to the nuclear matrix as little nuclear-bound 125I-bFGF in CVEC is released by DNAse I or RNAse A digestion, while washes with 0.5 M NaCl result in partial release. Nuclear bFGF may thus be involved in regulation of nuclear events (e.g., gene transcription and/or DNA replication).

Control of cardiac gene transcription by fibroblast growth factors

Skeletal alpha-actin (SkA) is representative of the cardiac genes that are expressed at high levels in embryonic myocardium, downregulated after birth, and reactivated by tropic signals including basic fibroblast growth factor (FGF-2) and type beta transforming growth factors (TGF beta). To investigate the molecular basis for cardiac-restricted and growth factor-induced SkA transcription, we have undertaken a mutational analysis of the SkA promoter in neonatal ventricular myocytes, with emphasis on the role of three nominal serum response elements. Serum response factor (SRF) and the bifunctional factor YY1 are the predominant cardiac proteins contacting the proximal SRE (SRE1). Mutations of SRE1 that prevent recognition by SRF and YY1. or SRF alone, virtually abolish SkA transcription; mutation of distal SREs was ineffective. A mutation which selectively abrogates YY1 binding increases expression, substantiating the predicted role of YY1 as an inhibitor of SRF effects. SkA transcription requires combinational action of SRE1 with consensus sites for Sp1 and the SV40 enhancer binding protein, TEF-1. As an isolated motif, SRE1 can confer responsiveness to both FGF-2 and TGF beta to a heterologous promoter. Whether TEF-1 binding sites likewise can function as FGF response elements is unknown. Molecular dissection of mechanisms that govern the differentiated cardiac phenotype has largely been undertaken to date in neonatal ventricular myocytes, as the adult ventricular myocyte has been refractory to conventional procedures for gene transfer. To circumvent expected limitations of other methods, we have used replication-deficient adenovirus to achieve efficient gene transfer to adult cardiac cells in culture.(ABSTRACT TRUNCATED AT 250 WORDS)

Involvement of basic fibroblast growth factor NH2 terminus in nuclear accumulation

The human basic Fibroblast Growth Factor (bFGF) gene was shown to encode four polypeptides by an alternative use of initiation codons (three CUG and one AUG). In this report, we present a comparative study of the fate and intracellular localization of individual bFGF isoforms. For this purpose, we have produced the various bFGF isoforms in E. coli and purified them to homogeneity: the 210 amino acid form initiated at CUG1 that contains a nuclear localization sequence (NLS), the 155 amino acid form (AUG-mediated initiation) and the 146 amino acid form (processed form extracted from tissues). While the different bFGFs were taken up by the cell with equal efficiency, more of the 210 amino acid form accumulated in the nucleus and represented 36% of the internalized bFGF compared with 15% in the others. A chimeric protein containing the minimal SV40 Large T NLS (SV40NLS) fused to the 155 amino acid bFGF form (SVbFGF) behaves like the native 155 amino acid form, indicating that nuclear accumulation of exogenous bFGF is not mediated by the NLS-associated function. These results suggest that the amino-terminal part of the 210 amino acid bFGF contains a sequence responsible for its nuclear retention. Bioactivities of the different forms were tested on adult bovine aortic endothelial (ABAE) cells. The bFGF degradation pathways, mitogenic activity and stimulation of rRNA synthesis appeared to be the same for all bFGFs but the stimulation of plasminogen activator was enhanced by the 210 amino acid form and correlated with nuclear accumulation.

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.

Regulation of muscle transcription by the MyoD family. The heart of the matter

The two striated muscle cell types, skeletal and cardiac muscle, express overlapping sets of muscle-specific genes. Activation of muscle-specific transcription in skeletal muscle is controlled by the MyoD family of regulatory factors, which are expressed exclusively in skeletal muscle. Members of the MyoD family share homology within a basic helix-loop-helix (HLH) motif that mediates DNA binding and dimerization and form heterodimers with widely expressed HLH proteins, referred to as E proteins. Although many of the genes that are regulated by members of the MyoD family are also expressed in cardiac muscle, known members of the MyoD family have never been detected in cardiac muscle, suggesting that cardiac myocytes either express unique cell type-specific HLH proteins or rely on a distinct regulatory strategy for activation of cardiac muscle transcription. This review will summarize current knowledge of the mechanisms through which the MyoD family activates skeletal muscle transcription and will consider potential mechanisms that may regulate gene expression in the heart.

Growth factors, growth factor response elements, and the cardiac phenotype

Fibroblast growth factors (FGF) and type beta-1 transforming growth factor (TGF beta 1) are pleiotropic regulatory peptides which are expressed in myocardium in a precise developmental and spatial program and are up-regulated, in the adult heart, by ischemia or a hemodynamic burden. The accumulation of trophic factors after aortic banding supports the hypothesis that autocrine or paracrine pathways might function to mediate, in part, the consequences of mechanical load. Our laboratory has demonstrated that cardiac muscle cells are targets for the action of peptide growth factors and, more specifically, that modulation of the cardiac phenotype by basic FGF (bFGF) and TGF beta 1 strongly resembles the induction of fetal cardiac genes--including skeletal alpha-actin (SkA), beta-myosin heavy chain, and atrial natriuretic factor--which are characteristic of pressure-overload hypertrophy. Unexpectedly, and despite effects like those of bFGF on five other cardiac genes, acidic FGF (aFGF) was found to repress, rather than stimulate, SkA transcription in neonatal cardiac muscle cells. The proximal 200 nucleotides of a heterologous SkA promoter were sufficient for basal tissue-specific transcription, for induction by bFGF, and for inhibition by aFGF. Thus, both positive and negative regulation by peptide growth factors can be localized to the proximal SkA promoter. Full promoter activity required each of three CC[A/T]6GG motifs similar to the serum response element (SRE) for activation of the c-fos proto-oncogene, as previously shown for SkA transcription in a skeletal muscle background. The most proximal SRE, SRE1, was sufficient in the absence of other SkA promoter sequences for efficient tissue-specific expression in cardiac myocytes (versus cardiac fibroblasts), and was stimulated by bFGF to the same extent as the full-length promoter and endogenous gene. Despite its ability to repress the SkA promoter, aFGF had no significant effect on SRE1. Both FGFs up-regulated the canonical fos SRE, to a comparable degree. Thus, SRE1 can discriminate between signals generated in cardiac myocytes by bFGF and aFGF. In cardiac myocyte extracts, two predominant proteins contact SRE1: serum response factor (SRF) and a second protein, F-ACT-1. Thus, serum response factor and F-ACT-1 are candidate trans-acting factors for basal transcription of the SkA gene in cardiac muscle cells and for induction of SkA by bFGF and, potentially, other trophic signals.