Discovery of Small Molecules Targeting the Synergy of Cardiac Transcription Factors GATA4 and NKX2-5.

Transcription factors are pivotal regulators of gene transcription, and many diseases are associated with the deregulation of transcriptional networks. In the heart, the transcription factors GATA4 and NKX2-5 are required for cardiogenesis. GATA4 and NKX2-5 interact physically, and the activation of GATA4, in cooperation with NKX2-5, is essential for stretch-induced cardiomyocyte hypertrophy. Here, we report the identification of four small molecule families that either inhibit or enhance the GATA4-NKX2-5 transcriptional synergy. A fragment-based screening, reporter gene assay, and pharmacophore search were utilized for the small molecule screening, identification, and optimization. The compounds modulated the hypertrophic agonist-induced cardiac gene expression. The most potent hit compound, N-[4-(diethylamino)phenyl]-5-methyl-3-phenylisoxazole-4-carboxamide (3, IC50 = 3 µM), exhibited no activity on the protein kinases involved in the regulation of GATA4 phosphorylation. The identified and chemically and biologically characterized active compound, and its derivatives may provide a novel class of small molecules for modulating heart regeneration.

Mitogen-activated protein kinase p38 target regenerating islet-derived 3γ expression is upregulated in cardiac inflammatory response in the rat heart.

Regenerating islet-derived 3γ (Reg3γ) is a multifunctional protein, associated with various tissue injuries and inflammatory states. Since chronic inflammation is characteristics also for heart failure, the aim of this study was to characterize Reg3γ expression in cardiac inflammatory conditions. Reg3γ expression was studied in experimental rat models of myocardial infarction (MI) and pressure overload in vivo. For cell culture studies neonatal rat cardiac myocytes (NRCMs) were used. In addition, adenovirus-mediated gene transfer of upstream mitogen-activated protein kinase (MAPK) kinase 3b and p38α MAPK in vivo and in vitro was performed. Reg3γ mRNA (12.8-fold, P < 0.01) and protein (5.8-fold, P < 0.001) levels were upregulated during the postinfarction remodeling at day 1 after MI, and angiotensin II (Ang II) markedly increased Reg3γ mRNA levels from 6 h to 2 weeks. Immunohistochemistry revealed that the Ang II-induced expression of Reg3γ was localized into the cardiac fibroblasts and myofibroblasts of the proliferating connective tissue in the heart. Stretching and treatments with endothelin-1, lipopolysaccharide (LPS), and fibroblast growth factor-1 increased Reg3γ mRNA levels in NRCMs. SB203580, a selective p38 MAPK inhibitor, markedly attenuated LPS and mechanical stretch-induced upregulation of Reg3γ gene expression. Moreover, combined overexpression of MKK3bE and WT p38α increased Reg3γ gene expression in cultured cardiomyocytes in vitro and in the rat heart in vivo. Our study shows that cardiac stress activates Reg3γ expression and p38 MAPK is an upstream regulator of Reg3γ gene expression in heart. Altogether our data suggest Reg3γ is associated with cardiac inflammatory signaling.

TSC-22 up-regulates collagen 3a1 gene expression in the rat heart.

BACKGROUND: The transforming growth factor (TGF)-ß is one of the key mediators in cardiac remodelling occurring after myocardial infarction (MI) and in hypertensive heart disease. The TGF-ß-stimulated clone 22 (TSC-22) is a leucine zipper protein expressed in many tissues and possessing various transcription-modulating activities. However, its function in the heart remains unknown. METHODS: The aim of the present study was to characterize cardiac TSC-22 expression in vivo in cardiac remodelling and in myocytes in vitro. In addition, we used TSC-22 gene transfer in order to examine the effects of TSC-22 on cardiac gene expression and function. RESULTS: We found that TSC-22 is rapidly up-regulated by multiple hypertrophic stimuli, and in post-MI remodelling both TSC-22 mRNA and protein levels were up-regulated (4.1-fold, P <0.001 and 3.0-fold, P <0.05, respectively) already on day 1. We observed that both losartan and metoprolol treatments reduced left ventricular TSC-22 gene expression. Finally, TSC-22 overexpression by local intramyocardial adenovirus-mediated gene delivery showed that TSC-22 appears to have a role in regulating collagen type IIIα1 gene expression in the heart. CONCLUSIONS: These results demonstrate that TSC-22 expression is induced in response to cardiac overload. Moreover, our data suggests that, by regulating collagen expression in the heart in vivo, TSC-22 could be a potential target for fibrosis-preventing therapies.

Bone morphogenetic protein-2--a potential autocrine/paracrine factor in mediating the stretch activated B-type and atrial natriuretic peptide expression in cardiac myocytes.

Hemodynamic overload exposes the heart to variety of neural, humoral and mechanical stresses. Even without the neurohumoral control of the entire organism cardiac myocytes have the ability to sense mechanical stretch and convert it into adaptive intracellular signals. This process is controlled by several growth factors. Here we show that mechanical stretch in vitro and hemodynamic overload in vivo activated the expression of bone morphogenetic protein-2 (BMP-2), while expression of BMP-4 was temporarily attenuated by stretch. BMP-2 and BMP-4 alone stimulated B-type and atrial natriuretic peptide (BNP and ANP) expression and protein synthesis, and activated transcription factor GATA-4 resembling the effects of mechanical stretch of cultured cardiac myocytes. Further, BMP antagonist Noggin was able to inhibit stretch and hypertrophic agonist induced BNP and ANP expression. Together these data provide evidence for BMP-2 as a new autocrine/paracrine factor that regulates cardiomyocyte mechanotransduction and adaptation to increased mechanical stretch.

Characterization of the regulatory mechanisms of activating transcription factor 3 by hypertrophic stimuli in rat cardiomyocytes.

AIMS: Activating transcription factor 3 (ATF3) is a stress-activated immediate early gene suggested to have both detrimental and cardioprotective role in the heart. Here we studied the mechanisms of ATF3 activation by hypertrophic stimuli and ATF3 downstream targets in rat cardiomyocytes. METHODS AND RESULTS: When neonatal rat cardiomyocytes were exposed to endothelin-1 (ET-1, 100 nM) and mechanical stretching in vitro, maximal increase in ATF3 expression occurred at 1 hour. Inhibition of extracellular signal-regulated kinase (ERK) by PD98059 decreased ET-1- and stretch-induced increase of ATF3 protein but not ATF3 mRNA levels, whereas protein kinase A (PKA) inhibitor H89 attenuated both ATF3 mRNA transcription and protein expression in response to ET-1 and stretch. To characterize further the regulatory mechanisms upstream of ATF3, p38 mitogen-activated protein kinase (MAPK) signaling was investigated using a gain-of-function approach. Adenoviral overexpression of p38α, but not p38ß, increased ATF3 mRNA and protein levels as well as DNA binding activity. To investigate the role of ATF3 in hypertrophic process, we overexpressed ATF3 by adenovirus-mediated gene transfer. In vitro, ATF3 gene delivery attenuated the mRNA transcription of interleukin-6 (IL-6) and plasminogen activator inhibitor-1 (PAI-1), and enhanced nuclear factor-κB (NF-κB) and Nkx-2.5 DNA binding activities. Reduced PAI-1 expression was also detected in vivo in adult rat heart by direct intramyocardial adenovirus-mediated ATF3 gene delivery. CONCLUSIONS: These data demonstrate that ATF3 activation by ET-1 and mechanical stretch is partly mediated through ERK and cAMP-PKA pathways, whereas p38 MAPK pathway is involved in ATF3 activation exclusively through p38α isoform. ATF3 activation caused induction of modulators of the inflammatory response NF-κB and Nkx-2.5, as well as attenuation of pro-fibrotic and pro-inflammatory proteins IL-6 and PAI-1, suggesting cardioprotective role for ATF3 in the heart.

p38α regulates SERCA2a function.

cAMP-dependent protein kinase (PKA) regulates the L-type calcium channel, the ryanodine receptor, and phospholamban (PLB) thereby increasing inotropy. Cardiac contractility is also regulated by p38 MAPK, which is a negative regulator of cardiac contractile function. The aim of this study was to identify the mechanism mediating the positive inotropic effect of p38 inhibition. Isolated adult and neonatal cardiomyocytes and perfused rat hearts were utilized to investigate the molecular mechanisms regulated by p38. PLB phosphorylation was enhanced in cardiomyocytes by chemical p38 inhibition, by overexpression of dominant negative p38α and by p38α RNAi, but not with dominant negative p38ß. Treatment of cardiomyocytes with dominant negative p38α significantly decreased Ca(2+)-transient decay time indicating enhanced sarco/endoplasmic reticulum Ca(2+)-ATPase function and increased cardiomyocyte contractility. Analysis of signaling mechanisms involved showed that inhibition of p38 decreased the activity of protein phosphatase 2A, which renders protein phosphatase inhibitor-1 phosphorylated and thereby inhibits PP1. In conclusion, inhibition of p38α enhances PLB phosphorylation and diastolic Ca(2+) uptake. Our findings provide evidence for novel mechanism regulating cardiac contractility upon p38 inhibition.

(Pro)renin receptor triggers distinct angiotensin II-independent extracellular matrix remodeling and deterioration of cardiac function.

BACKGROUND: Activation of the renin-angiotensin-system (RAS) plays a key pathophysiological role in heart failure in patients with hypertension and myocardial infarction. However, the function of (pro)renin receptor ((P)RR) is not yet solved. We determined here the direct functional and structural effects of (P)RR in the heart. METHODOLOGY/PRINCIPAL FINDINGS: (P)RR was overexpressed by using adenovirus-mediated gene delivery in normal adult rat hearts up to 2 weeks. (P)RR gene delivery into the anterior wall of the left ventricle decreased ejection fraction (P<0.01), fractional shortening (P<0.01), and intraventricular septum diastolic and systolic thickness, associated with approximately 2-fold increase in left ventricular (P)RR protein levels at 2 weeks. To test whether the worsening of cardiac function and structure by (P)RR gene overexpression was mediated by angiotensin II (Ang II), we infused an AT(1) receptor blocker losartan via osmotic minipumps. Remarkably, cardiac function deteriorated in losartan-treated (P)RR overexpressing animals as well. Intramyocardial (P)RR gene delivery also resulted in Ang II-independent activation of extracellular-signal-regulated kinase1/2 phosphorylation and myocardial fibrosis, and the expression of transforming growth factor-ß1 and connective tissue growth factor genes. In contrast, activation of heat shock protein 27 phosphorylation and apoptotic cell death by (P)RR gene delivery was Ang II-dependent. Finally, (P)RR overexpression significantly increased direct protein-protein interaction between (P)RR and promyelocytic zinc-finger protein. CONCLUSIONS/SIGNIFICANCE: These results indicate for the first time that (P)RR triggers distinct Ang II-independent myocardial fibrosis and deterioration of cardiac function in normal adult heart and identify (P)RR as a novel therapeutic target to optimize RAS blockade in failing hearts.

Increased thrombospondin-2 in human fibrosclerotic and stenotic aortic valves.

BACKGROUND: Active involvement of extracellular matrix (ECM) and its composition regulating factors may have a central role in the pathogenesis of calcific aortic valve disease (CAVD). Thrombospondins (TSPs) are highly conserved matricellular proteins regulating inflammation, angiogenesis and ECM remodeling. These processes are strongly associated with progression of aortic valve stenosis (AS). However, the expression of TSPs in CAVD is not known. METHODS: We characterized the expression of TSPs 1-4 in human aortic valves by real-time quantitative reverse transcriptase polymerase chain reaction and immunohistochemistry. Control valves (n=8), thickened and stiffened fibro(sclero)tic valves (n=8), and calcified AS valves (n=24) were compared. Furthermore, potential factors regulating TSP-2 expression was studied by western blotting and gel mobility shift assay in another set of control (n=10) and AS (n=20) valves. RESULTS: TSP-2 mRNA levels were increased 4.9-fold (P=0.037) and 4.8-fold (P=0.001) in fibro(sclero)tic and stenotic valves, respectively, whereas the expression of other TSPs did not change significantly. All TSPs 1-4 were detected from aortic valves by immunohistochemistry. Positive TSP-2 immunostaining was seen in the valvular myofibroblasts and patchily in endothelial cells. Semiquantitative analysis of TSP-2 staining indicated increased immunoreactivity for TSP-2 in neo vessels of fibro(sclero)tic and calcified aortic valves. Finally, when compared to controls, AS was associated with significant down regulation of Akt-pathway and diminished binding activity of nuclear factor-κB (NF-κB). CONCLUSIONS: We report for the first time that TSPs 1-4 are expressed in human aortic valves. CAVD is characterized by myofibroblastic proliferation and neovascularization associated upregulation of TSP-2 expression, as well as inactivation of Akt and NF-κB.

M-CAT element mediates mechanical stretch-activated transcription of B-type natriuretic peptide via ERK activation.

The muscle-CAT (M-CAT) promoter element is found on promoters of most muscle-specific cardiac genes, but its role in cardiac pathology is poorly understood. Here we studied whether the M-CAT element is involved in hypertrophic process activated by mechanical stretch, and identified the intracellular pathways mediating the response. When an in vitro stretch model of cultured neonatal rat cardiomyocytes and luciferase reporter construct driven by rat B-type natriuretic peptide (BNP) promoter were used, mutation of M-CAT element inhibited not only the basal reporter activity (88%), but also the stretch-activated BNP transcription (58%, p < 0.001). Stretch-induced BNP promoter activation was associated with an increase in transcriptional enhancer factor-1 (TEF-1) binding activity after 24 h mechanical stretch (p < 0.05). Inhibition of mitogen-activated protein kinases ERK, JNK, or p38 attenuated stretch-induced BNP activation. Interestingly, as opposed to p38 and JNK, inhibition of ERK had no additional effect on transcriptional activity of BNP promoter harboring the M-CAT mutation, suggesting a pivotal role for ERK in regulating stretch-induced BNP transcription via M-CAT binding site. Finally, immunoprecipitation studies showed that mechanical stretch induced myocyte enhancer factor-2 (MEF-2) binding to TEF-1. These data suggest a central role for M-CAT element in regulation of mechanical stretch-induced hypertrophic response via ERK activation.

Intramyocardial BNP gene delivery improves cardiac function through distinct context-dependent mechanisms.

BACKGROUND: B-type natriuretic peptide (BNP) is an endogenous peptide produced under physiological and pathological conditions mainly by ventricular myocytes. It has natriuretic, diuretic, blood pressure-lowering, and antifibrotic actions that could mediate cardiorenal protection in cardiovascular diseases. In the present study, we used BNP gene transfer to examine functional and structural effects of BNP on left ventricular (LV) remodeling. METHODS AND RESULTS: Human BNP was overexpressed by using adenovirus-mediated gene delivery in normal rat hearts and in hearts during the remodeling process after infarction and in an experimental model of angiotensin II-mediated hypertension. In healthy hearts, BNP gene delivery into the anterior wall of the LV decreased myocardial fibrosis (P<0.01, n=7 to 8) and increased capillary density (P<0.05, n=7 to 8) associated with a 7.3-fold increase in LV BNP peptide levels. Overexpression of BNP improved LV fractional shortening by 22% (P<0.05, n=6 to 7) and ejection fraction by 19% (P<0.05, n=6 to 7) after infarction. The favorable effect of BNP gene delivery on cardiac function after infarction was associated with normalization of cardiac sarcoplasmic reticulum Ca(2+)-ATPase expression and phospholamban Thr17-phosphorylation. BNP gene delivery also improved fractional shortening and ejection fraction in angiotensin II-mediated hypertension as well as decreased myocardial fibrosis and LV collagen III mRNA levels but had no effect on angiogenesis or Ca(2+)-ATPase expression and phospholamban phosphorylation. CONCLUSIONS: Local intramyocardial BNP gene delivery improves cardiac function and attenuates adverse postinfarction and angiotensin II-induced remodeling. These results also indicate that myocardial BNP has pleiotropic, context-dependent, favorable actions on cardiac function and suggest that BNP acts locally as a key mechanical load-activated regulator of angiogenesis and fibrosis.

Distinct regulation of B-type natriuretic peptide transcription by p38 MAPK isoforms.

Persistent controversy underlies the functional roles of specific p38 MAPK isoforms in cardiac biology and regulation of hypertrophy-associated genes. Here we show that adenoviral gene transfer of p38ß but not p38α increased B-type natriuretic peptide (BNP) mRNA levels in vitro as well as atrial natriuretic peptide mRNA levels both in vitro and in vivo. Overexpression of p38α, in turn, augmented the expression fibrosis-related genes connective tissue growth factor, basic fibroblast growth factor and matrix metalloproteinase-9 both in vitro and in vivo. p38ß-induced BNP transcription was diminished by mutation of GATA-4 binding site, whereas overexpression of MKK6b, an upstream regulator of p38α and p38ß, activated BNP transcription through both GATA-4 and AP-1. Overexpression of MKK3, upstream regulator of p38α, induced BNP transcription independently from AP-1 and GATA-4. These data provide new evidence for diversity in downstream targets and functional roles of p38 pathway kinases in regulation of hypertrophy-associated cardiac genes.

(Pro)renin receptors and angiotensin converting enzyme 2/angiotensin-(1-7)/Mas receptor axis in human aortic valve stenosis.

BACKGROUND: There is increasing evidence that renin-angiotensin system (RAS) may play a major role in the actively regulated fibrocalcific process in aortic valve stenosis (AS), but the gene expression or function of (pro)renin receptor ((P)RR), prorenin and renin or angiotensin converting enzyme 2(ACE2)/angiotensin-(1-7)/Mas receptor axis in calcific aortic valve disease is not known. METHODS AND RESULTS: We characterized expression of (P)RR, ACE2 and Mas receptor as well as renin, prorenin and angiotensin II type 2 (AT(2)) receptors in human aortic valves, and compared normal control valves (n = 11) with valves obtained from patients with aortic regurgitation (AR, n = 14), AR with fibrosis (n = 20) and AS (n = 61). By immunohistochemistry (P)RR positive staining was seen in the valvular endothelial cells of control and in the neovessels of stenotic valves. By RT-PCR, renin mRNA levels were 72% (P = 0.001) and prorenin mRNA levels 64% lower (P = 0.002) in stenotic aortic valves compared to control valves. ACE2, Mas receptor and AT(2)-receptor mRNA levels were 69% (P < 0.001), 58% (P = 0.008) and 75% (P = 0.001) lower, respectively, in stenotic valves. ACE2 positive staining, existing to lesser extent in stenotic aortic valves, was localized mainly to stromal area in spongiosa layer in control valves. CONCLUSIONS: (P)RR, prorenin and renin are expressed in human aortic valves. We also report for the first time expression of ACE2/angiotensin-(1-7)/-Mas receptor axis in human aortic valve cusps. The downregulation of ACE2/angiotensin-(1-7)/-Mas receptor axis as well as AT(2)-receptors may promote fibrosis, proliferation and inflammation in patients with AS.

Parthenolide inhibits STAT3 signaling and attenuates angiotensin II-induced left ventricular hypertrophy via modulation of fibroblast activity.

Parthenolide has shown promise in treatment of various cancers via inhibition of the transcription factor signal transducer and activator of transcription 3 (STAT3). Activation of STAT3 has been observed in left ventricular hypertrophy (LVH); however, its exact role is not known. The aim of the study was to examine the effects of parthenolide on pressure overload-induced LVH in rats. Pressure overload was induced by angiotensin II (Ang II) infusion (33 µg/kg/h) for 1 week in the presence or absence of parthenolide (0.5mg/kg/day, i.p.). Ang II infusion resulted in LVH associated with increased phosphorylation of STAT3 at Tyr705 and Ser727. Parthenolide treatment had no effect on ejection fraction, but abolished the activation of STAT3 and reduced the Ang II-induced LVH (LV posterior wall thickness in end-diastole: 2.28 ± 0.12 mm vs. 1.80 ± 0.06 mm, P<0.001). Importantly, parthenolide treatment had no effect on heart rate or blood pressure. Parthenolide treatment almost completely abolished the Ang II-induced increase in the number of cells positive for prolyl-4-hydroxylase, a marker for collagen-synthesizing cells, as well as Ang II-induced interstitial fibrosis in the left ventricles. This was associated with significant attenuation of Ang II-induced increase in mRNA levels of type 1 collagen and fibronectin. Moreover, parthenolide attenuated the Ang II-induced expression of interleukin-6, a potent pro-hypertrophic fibroblast-derived factor. We conclude that pharmacological inhibition of STAT3 signaling by parthenolide has favorable effects on pressure overload-induced LVH through attenuation of fibroblast activation. Therefore parthenolide may prove as a useful therapy for certain cardiovascular disease.

Metoprolol treatment lowers thrombospondin-4 expression in rats with myocardial infarction and left ventricular hypertrophy.

Thrombospondins are matrix proteins linked to extracellular matrix remodelling but their precise role in the heart is not known. In this study, we characterised left ventricular thrombospondin-1 and -4 expression in rats treated with a beta-blocker metoprolol during the remodelling process in response to pressure overload and acute myocardial infarction. Left ventricular thrombospondin-1 and thrombospondin-4 mRNA levels increased 8.4-fold (p < 0.001) and 7.3-fold (p < 0.001) post-infarction, respectively. Metoprolol infusion by osmotic minipumps (1.5 mg/kg/hr) for 2 weeks after myocardial infarction decreased thrombospondin-1 and thrombospondin-4 mRNA levels (55% and 50%, respectively), improved left ventricular function, and attenuated left ventricular remodelling with reduction of left ventricular atrial natriuretic peptide and brain natriuretic peptide gene expression. Thrombospondin-1 and -4 mRNA levels correlated positively with echocardiographic parameters of left ventricular remodelling as well as with atrial natriuretic peptide and brain natriuretic peptide gene expression. Moreover, there was a negative correlation between left ventricular ejection fraction and thrombospondin-1 mRNA levels. In 12-month-old spontaneously hypertensive rats with left ventricular hypertrophy, metoprolol decreased left ventricular thrombospondin-4 levels and attenuated remodelling while thrombospondin-1, atrial natriuretic peptide and brain natriuretic peptide mRNA levels as well as left ventricular function remained unchanged. In metoprolol-treated spontaneously hypertensive rats, thrombospondin-4 gene expression correlated with parameters of left ventricular remodelling, while no correlations between thrombospondins and natriuretic peptides were observed. These results indicate that thrombospondin-1 expression is linked exclusively to left ventricular remodelling process post-infarction while thrombospondin-4 associates with myocardial remodelling both after myocardial infarction and in hypertensive heart disease suggesting that thrombospondins may have unique roles in extracellular matrix remodelling process.

A novel p38 MAPK target dyxin is rapidly induced by mechanical load in the heart.

Dyxin is a novel LIM domain protein acting as a transcriptional cofactor with GATA transcription factors. Here, we characterized dyxin as a p38 mitogen-activated protein kinase (MAPK) regulated gene, since combined upstream MAPK kinase 3b and wild-type p38 alpha MAPK gene transfer increased left ventricular dyxin mRNA and protein levels in vivo. We also studied cardiac dyxin expression in experimental models of pressure overload and myocardial infarction (MI) in vivo. Angiotensin II infusion increased left ventricular dyxin mRNA levels (9.4-fold, p<0.001) rapidly at 6 h followed by induction of protein levels. Furthermore, simultaneous administration of p38 MAPK inhibitor SB203580 abolished angiotensin II-induced activation of dyxin gene expression. During the post-infarction remodeling process, increased dyxin mRNA levels (7.7-fold, p<0.01) were noted at day 1 followed by the increase in proteins levels at 2 weeks after MI (1.5-fold, p<0.05). Moreover, direct wall stretch by using isolated rat heart preparation as well as direct mechanical stretch of cardiomyocytes in vitro activated dyxin gene expression within 1 h. Our results indicate that dyxin expression is rapidly upregulated in response to mechanical load, this increase being at least partly mediated by p38 MAPK. These results suggest that dyxin may play an important role in regulating hypertrophic process.

Upregulation of cardiac matrix Gla protein expression in response to hypertrophic stimuli.

Matrix Gla protein (MGP) expression is increased in cardiac hypertrophy, but the precise mechanisms regulating its expression are unknown. Here we characterized the effect of pressure overload and myocardial infarction in vivo as well as mechanical stretch and hypertrophic agonists in vitro on MGP expression. When angiotensin II (Ang II) was administered by osmotic minipumps, left ventricular (LV) MGP mRNA levels increased significantly from 6 h to 2 weeks, whereas intravenous arginine(8)-vasopressin increased LV MGP mRNA levels within 4 h. During post-infarction remodeling process, MGP mRNA levels were elevated at 24 h (1.3-fold, p<0.05) and the maximal increase was observed at 4 weeks (2.8-fold, p<0.01). Ang II increased MGP mRNA levels 20% (p<0.05) in neonatal rat cardiac myocytes and 40% (p<0.05) in cardiac fibroblasts, whereas endothelin-1 decreased MGP mRNA levels 30% (p<0.01) in myocytes and had no effect in fibroblasts. Cyclic mechanical stretch resulted in reduction of MGP gene expression in both cardiac myocytes and fibroblasts. These results demonstrate that MGP is rapidly upregulated in response to cardiac overload well before the development of LV hypertrophy and post-infarction remodeling process. Our results also suggest that Ang II may be involved in mediating load-induced activation of MGP expression.

Thrombospondin-4 expression is rapidly upregulated by cardiac overload.

The precise mechanisms regulating gene expression of thrombospondins (TSPs) in the heart remain incompletely understood. Here we characterized cardiac TSP-4 expression in response to pressure overload and myocardial infarction in vivo. Arginine(8)-vasopressin (AVP) infusion increased left ventricular (LV) TSP-4 mRNA levels within 30 min. Also angiotensin II infusion rapidly activated LV TSP-4 expression, TSP-4 mRNA levels being highest at 6h and protein at 72 h and 2 weeks. During remodeling process following myocardial infarction, LV TSP-4 mRNA levels increased at day one, as studied by quantitative RT-PCR. TSP-4 immunostaining was localized to endothelial cells in hypertrophied hearts of spontaneously hypertensive rats. AVP-infusion increased LV TSP-1 mRNA levels similarly to TSP-4 within 30 min showing that rapid induction of gene expression, well before the development of cardiac hypertrophy, is typical for the thrombospondin family. These results further suggest that TSP-4 may be an endothelial specific marker of cardiac overload.

Involvement of endogenous ouabain-like compound in the cardiac hypertrophic process in vivo.

The Na(+)/K(+)-ATPase inhibitor ouabain has been shown to trigger hypertrophic growth of cultured cardiomyocytes; however, the significance of endogenous ouabain-like compound (OLC) in the hypertrophic process in vivo is unknown. Here we characterized the involvement of OLC in left ventricular (LV) hypertrophy induced by norepinephrine (NE) and angiotensin II (Ang II) infusions in rats. Administration of NE (300 microg/kg/h) via subcutanously implanted osmotic minipumps for 72 h resulted in a significant increase in left ventricular weight to body weight (LVW/BW) ratio (P<0.001) and a substantial up-regulation of atrial natriuretic peptide (ANP) gene expression (13.2-fold, P<0.001). NE infusion induced a transient increase in plasma OLC levels at 12 h (P<0.05), which returned to control levels by 72 h. Adrenalectomy markedly reduced both basal and NE-induced increase in plasma OLC levels. LVW/BW ratio was not modulated by adrenalectomy; however, ANP gene expression was blunted by 44% (P<0.01) and 47% (P<0.05) at 12 and 72 h, respectively. In agreement, adrenalectomy reduced up-regulation of ANP without affecting LV mass in rats infused with Ang II (33 microg/kg/h). Administration of exogenous ouabain (1 nM to 100 microM) for 24 h had no effect on ANP gene expression in cultured neonatal rat ventricular myocytes. However, the up-regulation of ANP mRNA levels induced by the alpha-adrenergic agonist phenylephrine (1 microM) was markedly enhanced by ouabain (100 microM) (5.6-fold vs. 9.6-fold, P<0.01). These data show that OLC as an adrenal-derived factor may be required for the induction LV ANP gene expression during the hypertrophic process.

Early left ventricular gene expression profile in response to increase in blood pressure.

The heart adapts to increased pressure overload by hypertrophic growth of terminally differentiated cardiomyocytes. At the genetic level, the hypertrophic response is characterized by the reprogramming of gene expression, i.e. upregulation of immediate early genes, natriuretic peptide genes and genes encoding structural proteins. In the present study, we characterized the early changes in gene expression with cDNA expression arrays in response to increase in blood pressure produced by arginine8-vasopressin infusion (0.05 microg/kg/min, i.v.) for 30 min and 4 h in conscious normotensive rats. Expression profiling revealed differential expression of 14 genes in the left ventricle, and several novel factors of immediate early genetic response to pressure overload were identified, such as growth arrest and DNA damage inducible protein 45 (GADD45alpha), epidermal fatty acid-binding protein (E-FABP) and Bcl-X. Administration of angiotensin II (Ang II) for 6 h by osmotic minipumps also increased left ventricular GADD45alpha, E-FABP and Bcl-X gene expression. Furthermore, the induction of GADD45alpha and Bcl-X gene expression by Ang II was blocked by angiotensin II type 1 receptor antagonist losartan. In summary, our analysis provided new insights into the pathogenesis of pressure overload-induced hypertrophy by suggesting the existence of novel regulators of the immediate early gene expression program.