A factor that initiates myocardial hypertrophy in hypertension

MMP-2 mediates angiotensin II-induced hypertension under the transcriptional control of MMP-7 and TACE

Development of cardiovascular disease induced by excessive Gq protein-coupled receptor agonist stimulation depends on signaling networks involving multiple matrix metalloproteinases (MMPs) and metalloproteinase disintegrins (ADAMs). Here, we hypothesized that MMP-2, being a major gelatinase in cardiac and vascular tissue, was likely to play a key role in cardiovascular homeostasis. We targeted MMP-2 using complementary and overlapping approaches involving pharmacological inhibition and RNA interference in mice treated with angiotensin II (1.4 mg/kg per day) for 12 days. We studied the development of hypertension (by tail cuff plethysmography), cardiac hypertrophy (by M-mode echocardiography, cardiomyocyte cross-sectional area, and quantitative real-time polymerase chain reaction (qRT-PCR) analysis of hypertrophy marker genes), and fibrosis (by picrosirius red collagen staining and qRT-PCR analysis of fibrosis marker genes) in mice receiving angiotensin II. We found that angiotensin II infusion upregulated MMP-2 concurrent with the development of hypertension, hypertrophy, and fibrosis. This upregulation of MMP-2 depended on MMP-7 and TACE (tumor necrosis factor-α convertase, ADAM-17). RNA interference targeting MMP-7 and TACE attenuated the angiotensin II-induced upregulation of MMP-2 and prevented the development of hypertension, as well as development of cardiac hypertrophy and fibrosis. In contrast, pharmacological inhibition and RNA interference of MMP-2 attenuated angiotensin II-induced hypertension, without influencing development of cardiac hypertrophy or fibrosis. Downstream of MMP-7 and TACE, MMP-2 mediated angiotensin II-induced hypertension, but did not mediate cardiac hypertrophy or fibrosis. This suggests a functional specialization of MMP-2 in agonist-induced cardiovascular disease development that has potential implications for the design of metalloproteinase-based therapeutic strategies.

Role of myofibrillogenesis regulator-1 in myocardial hypertrophy

Myofibrillogenesis regulator-1 (MR-1) is a novel homologous gene, identified from a human skeletal muscle cDNA library, that interacts with contractile proteins and exists in human myocardial myofibrils. The present study investigated MR-1 protein expression in hypertrophied myocardium and MR-1 involvement in cardiac hypertrophy. Cardiac hypertrophy was induced by abdominal aortic stenosis (AAS) in Sprague-Dawley rats. Left ventricular (LV) hypertrophy was assessed by the ratio of LV wet weight to whole heart weight (LV/HW) or LV weight to body weight (LV/BW). Rat MR-1 (rMR-1) expression in the myocardium was detected by immunohistochemical and Western blotting analysis. Hypertrophy was induced by ANG II incubation in cultured neonatal rat cardiomyocytes. The effect of rMR-1 RNA interference on ANG II-induced hypertrophy was studied by transfection of cardiomyocytes with an RNA interference plasmid, pSi-1, which targets rMR-1. Hypertrophy in cardiomyocytes was assessed by [3H]Leu incorporation and myocyte size. rMR-1 protein expression in cardiomyocytes was detected by Western blotting. We found that AAS resulted in a significant increase in LV/HW and LV/BW: 89% and 86%, respectively (P < 0.01). Immunohistochemistry and Western blot analysis demonstrated upregulated rMR-1 protein expression in hypertrophic myocardium. ANG II induced a 24% increase in [3H]Leu incorporation and a 65.8% increase in cell size compared with control cardiomyocytes (P < 0.01), which was prevented by treatment with losartan, an angiotensin (AT1) receptor inhibitor, or transfection with pSi-1. rMR-1 expression increased in ANG II-induced hypertrophied cardiomyocytes, and pSi-1 transfection abolished the upregulation. These findings suggest that MR-1 is associated with cardiac hypertrophy in rats in vivo and in vitro.

Myotrophin-kappaB DNA interaction in the initiation process of cardiac hypertrophy

To investigate how cardiac hypertrophy and heart failure develop, we isolated and characterized a candidate initiator, the soluble 12-kDa protein myotrophin, from rat and human hearts. Myotrophin stimulates protein synthesis and myocardial cell growth associated with increased levels of hypertrophy marker genes. Recombinant myotrophin from the cloned gene showed structural/functional motifs, including ankyrin repeats and putative phosphorylation sites for protein kinase C (PKC) and casein kinase II. One repeat, homologous with I kappaB, interacts with rel/NF-kappaB in vitro. We analyzed the interaction of recombinant myotrophin and nuclear extracts prepared from neonatal and adult cardiomyocytes; gel mobility shift assay showed that myotrophin bound to kappaB DNA. To define PKC's role in myotrophin-induced myocyte growth, we incubated neonatal rat myocytes (normal and stretch) with specific inhibitors and found that myotrophin inhibits [3H]leucine incorporation into myocytes and different hypertrophic gene expression in neonatal myocytes. Using confocal microscopy, we observed that a basal level of myotrophin was present in both cytoplasm and nucleus under normal conditions, but under cyclic stretch, myotrophin levels became elevated in the nucleus. Myotrophin gene levels were upregulated when myocytes underwent cyclic stretch or were treated with tumor necrosis factor-alpha (TNF-alpha) or interleukin-1beta and also when excised beating hearts were exposed to high pressure. Our data showed that the myotrophin-kappaB interaction was increased with age in spontaneously hypertensive rats (SHRs) only. Our data provide evidence that myotrophin-kappaB DNA interaction may be an important step in initiating cardiac hypertrophy.

Increased protein kinase C activity in myotrophin-induced myocyte growth

Myotrophin, a novel protein that has been shown to stimulate myocyte growth, has been isolated, purified, and sequenced from the hearts of spontaneously hypertensive rats and dilated cardiomyopathic human tissue. Recently, the cDNA clones encoding myotrophin have been isolated and expressed in Escherichia coli, and the recombinant myotrophin was found to be as biologically and immunologically active as natural myotrophin. The mechanism by which myotrophin stimulates protein synthesis and initiates myocardial hypertrophy is not known. To evaluate the involvement of protein kinase C (PKC) in myotrophin-induced hypertrophy, PKC activity and its distribution in the subcellular fraction were determined in cultured neonatal and adult myocytes. PKC activity was determined by measuring the incorporation of 32P into histone type III-S and PKCepsilon substrate peptide (epsilon(pep)) from [gamma-32P]ATP in neonatal myocytes. Myotrophin significantly stimulated PKC activity in neonatal myocytes and was associated with a significant increase in protein synthesis. The effect of myotrophin on the stimulation of PKC activity and [3H]leucine incorporation was abolished by pretreatment with either staurosporine or H-7, two selective, pharmacological PKC inhibitors. Pretreatment of myocytes with staurosporine also reduced the myotrophin-induced mRNA levels of c-fos and beta-myosin heavy chain. To evaluate the subcellular events whose occurrence was due to myotrophin and translocation of PKC, we studied the effect of genistein, a tyrosine kinase inhibitor, on myotrophin-induced neonatal myocyte growth. Genistein attenuated the [3H]leucine incorporation induced by myotrophin. To define the specificity of the PKC isoform(s) involved in myotrophin-stimulated myocyte growth, both neonatal and adult myocytes were treated with myotrophin, and Western blot analyses were performed by using the antibodies of different PKC isoforms. Results showed that both PKCalpha and PKCepsilon isoforms participated in the myotrophin-induced neonatal myocyte growth, whereas only the PKCepsilon isoform was involved in myotrophin-induced adult myocyte hypertrophy. PKCdelta and PKCzeta do not seem to participate in either neonatal or adult myocyte growth induced by myotrophin. Treatment with antisense oligonucleotides specific for PKCalpha and PKCepsilon isoforms further supported this result. PKCalpha is the major PKC isoform in neonatal myocytes and needs Ca2+ and phospholipids for its activation, and PKCepsilon (the Ca2+-independent PKC isoform) is present in both neonatal and adult myocytes; the 15-mer antisense oligodeoxynucleotides of each were used for this study. Treatment of neonatal myocytes with the PKCalpha and PKCepsilon antisense oligodeoxynucleotides for 5 days significantly reduced Ca2+-dependent and Ca2+-independent PKC activity, respectively, as well as the [3H]leucine incorporation induced by myotrophin. Furthermore, myotrophin-induced PKC activity was primarily located in the particulate fraction and did not result in a concomitant decrease in the cytosolic fraction. Myotrophin does not change PKC isoform expression (both Ca2+ dependent and independent PKC isoforms used in this study) in rat neonatal cardiac fibroblasts. Our data suggest that myotrophin exerts its action on protein synthesis, possibly through a tyrosine kinase-coupled pathway and translocation of PKC from the cytosol to the cell membrane.

Nuclear magnetic resonance assignment and secondary structure of an ankyrin-like repeat-bearing protein: myotrophin

Multidimensional heteronuclear NMR has been applied to the structural analysis of myotrophin, a novel protein identified from spontaneously hypertensive rat hearts and hypertrophic human hearts. Myotrophin has been shown to stimulate protein synthesis in myocytes and likely plays an important role in the initiation of cardiac hypertrophy, a major cause of mortality in humans. Recent cDNA cloning revealed that myotrophin has 11B amino acids containing 2.5 contiguous ANK repeats, a motif known to be involved in a wide range of macromolecular recognition. A series of two- and three-dimensional heteronuclear bond correlation NMR experiments have been performed on uniformly 15N-labeled or uniformly 15N/13C-labeled protein to obtain the 1H, 15N, and 13C chemical shift assignments. The secondary structure of myotrophin has been determined by a combination of NOEs, NH exchange data, 3JHN alpha coupling constants, and chemical shifts of 1H alpha, 13C alpha, and 13 C beta. The protein has been found to consist of seven helices, all connected by turns or loops. Six of the seven helices (all but the C-terminal helix) form three separate helix-turn-helix motifs. The two full ANK repeats in myotrophin are characteristic of multiple turns followed by a helix-turn-helix motif. A hairpin-like turn involving L32-R36 in ANK repeat #1 exhibits slow conformational averaging on the NMR time scale and appears dynamically different from the corresponding region (D65-169) of ANK repeat #2.

Quantification of myotrophin from spontaneously hypertensive and normal rat hearts

A novel peptide, myotrophin, has been isolated, purified, cloned, and sequenced from the hearts of spontaneously hypertensive rats (SHR) and from dilated cardiomyopathic human heart tissue. Myotrophin accelerates myocyte growth by stimulating protein synthesis (not by altering myocardial cell division). Our successive studies were conducted to evaluate the pathophysiological significance of myotrophin; a solid-phase radioimmunoassay technique was developed for quantifying the protein in hypertrophied and normal hearts. Specific antipeptide antibody was raised in rabbits against a peptide that represents a selected amino acid sequence of a 17-amino acid myotrophin segment by using the multiple antigenic peptide technique. The specificity of the antibody was evaluated by determining the affinity constant after constructing the Scatchard plot obtained from the ratio of bound to free myotrophin against bound myotrophin. The value obtained was 2.61 x 10(7) L/mol. The specificity was further demonstrated by Western blot analysis, in which a single protein band was obtained in the region of 12 kD. Pretreatment of the antibody with myotrophin completely blocked the binding sites, because no protein band was detected on the immunoblot. The antibody prevented the myocardial protein synthesis induced by myotrophin as revealed by the blockage of the stimulation of [3H]leucine incorporation into myocyte protein. Quantification of myotrophin from different heart tissues was achieved by Western blot and dot blot analyses. Amounts of myotrophin present in different dots were determined by using a video image analyzer. The level of myotrophin in the embryonic tissue was found to be similar in male normal and SHR hearts.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects on left ventricular hypertrophy of long-term nonpharmacological treatment with sodium restriction in mild-to-moderate essential hypertension

BACKGROUND

Cross-sectional studies on human hypertension have suggested an association between sodium intake and left ventricular hypertrophy (LVH).

METHODS AND RESULTS

The effects on LVH of a nonpharmacological treatment program based mainly on sodium restriction were examined by serial echocardiography in a 12-month controlled, randomized study that included 76 previously untreated subjects with uncomplicated mild-to-moderate hypertension. The mean daily sodium excretion of 38 subjects randomized into the treatment group decreased from 195 +/- 95 to 94 +/- 73 mmol (P < .001) at 6 months and to 109 +/- 74 mmol (P < .001) at 12 months. This was accompanied by a weight decrease from 81.4 +/- 18.0 to 79.2 +/- 17.4 kg (P < .001) at 6 months and to 80.6 +/- 17.5 kg (NS) at 12 months. The net blood pressure decrease (difference in change from baseline between the treatment and control groups) was 8.9 mm Hg (P < .001) in systolic blood pressure and 6.5 mm Hg (P < .001) in diastolic blood pressure during the first 6 months and 6.7 mm Hg (P < .01) in systolic blood pressure and 3.8 mm Hg (P < .01) in diastolic blood pressure during the last 6 months. After 12 months of sodium restriction, left ventricular mass (LVM) had decreased by 5.4% (from 238 +/- 63 to 225 +/- 51 g, P < .01), and LVM index (LVMI) had decreased by 4.7% (from 123 +/- 26 to 117 +/- 22 g/m2, P < .05), whereas no changes occurred in these parameters in the control group. In treated subjects with baseline LVMI of more than the median value of 133 g/m2 in men and 107 g/m2 in women, LVM decreased by 8.6% (from 272 +/- 62 to 249 +/- 51 g, P < .01), and LVMI decreased by 7.1% (from 140 +/- 23 to 130 +/- 22 g/m2, P < .01). LVM and LVMI remained unchanged in treated subjects with LVMI values equal to or less than the median.

CONCLUSIONS

Our data suggest that long-term nonpharmacological treatment with moderate sodium restriction decreases LVH.

Myotrophin induces early response genes and enhances cardiac gene expression

We have identified and partially sequenced a soluble factor, myotrophin, from spontaneously hypertensive rat hearts and hypertrophic human hearts that enhances myocyte protein synthesis and stimulates myocardial cell growth. Our studies suggest that myotrophin may be a biochemical link between hemodynamic stress and myocardial cellular hypertrophy. When rat neonatal cardiac myocytes maintained in culture were incubated with myotrophin for 30 minutes, they showed a marked increase in c-myc, c-fos, and c-jun messenger RNA levels. Cardiac myocytes treated for 24 hours with myotrophin showed a fourfold increase in connexin 43 (gap junction protein), a sixfold increase in atrial natriuretic factor, a threefold increase in skeletal alpha-actin, and a threefold increase in total myosin transcript levels. Studies on myosin isoforms showed a selective increase in the beta-myosin heavy chain transcript levels but no reciprocal decrease in alpha-myosin heavy chain transcript levels. Our data suggested that myotrophin appears to be a primary modulator for myocardial cell growth and differentiation and may play an important role in the pathogenesis of cardiac hypertrophy. Myotrophin may be involved in the upregulation of myofibrillar protein and the activation of cardiac gene transcription during growth and hypertrophy of the myocardium, and the induction of early response gene expression may be linked to this response.

Enalapril reduces the enhanced 1,2-diacylglycerol content and RNA synthesis in spontaneously hypertensive rat hearts before established hypertension

There is evidence that cardiac hypertrophy in spontaneously hypertensive rats (SHR) occurs before the development of hypertension. 1,2-Diacylglycerol, which is thought to be a second messenger activating protein kinase C, is also produced in excess in SHR hearts at 4 weeks of age, before established hypertension. We determined myocardial 1,2-diacylglycerol content in SHR with and without prazosin and enalapril from 3 to 4 weeks of age. Hearts from untreated SHR had greater RNA and DNA synthesis and greater relative weights at 4 weeks of age than those from Wistar-Kyoto (WKY) rats. There was no difference in triglyceride content or phospholipid species between WKY rats and untreated SHR, except for a higher cholesterol content in SHR. Treatment of SHR with enalapril, but not prazosin, lowered not only 1,2-diacylglycerol content but also RNA synthesis to the levels of WKY rats. Moreover, fatty acids involved in 1,2-diacylglycerol were altered by enalapril despite the lack of a difference between WKY rats and untreated SHR. Prazosin did not have any effect on 1,2-diacylglycerol fatty acid composition. Enalapril may decrease cardiac hypertrophy in SHR by lowering myocardial 1,2-diacylglycerol production.

Left ventricular insulin-like growth factor I increases in early renal hypertension

Increasing interest has been directed toward the possible role of trophically acting molecules as modulators or initiators, or both, of myocardial hypertrophy. The aim of the present study was to investigate the possible role of one such molecule, namely, insulin-like growth factor I, in myocardial hypertrophy developed in response to renal artery stenosis. Two-kidney, one clip Goldblatt hypertension was induced in Wistar rats weighing 180 g, and sham-operated animals were used as controls. Blood pressure was increased as early as 2 days after clipping (133 +/- 4 versus 116 +/- 4 mm Hg, p less than 0.05), and the increase persisted 4 and 7 days after clipping (148 +/- 6 versus 129 +/- 3 mm Hg, p less than 0.01 and 171 +/- 5 versus 139 +/- 3 mm Hg, p less than 0.01, respectively). Left ventricular weight followed a similar pattern (373 +/- 7 versus 350 +/- 8 mg, NS, 415 +/- 11 versus 386 +/- 9 mg, p less than 0.01, and 466 +/- 11 versus 391 +/- 10 mg, p less than 0.01 at 2, 4, and 7 days after clipping, respectively), but no changes in body weight between the groups were observed. Insulin-like growth factor I messenger RNA (mRNA) was quantified using a solution hybridization assay. After 4 days of renal hypertension, there was a significant increase in left ventricular insulin-like growth factor I mRNA (2.0 x 10(-18) +/- 0.48 x 10(-18) versus 0.4 x 10(-18) +/- 0.07 x 10(-18) mol.microgram DNA-1), which was no longer detectable 7 days after clipping.(ABSTRACT TRUNCATED AT 250 WORDS)

1,2-diacylglycerol content in myocardium from spontaneously hypertensive rats during the development of hypertension

1,2-Diacylglycerol (DAG) has been considered to play an important role as an activator of protein kinase C in the signal transduction of inositol phospholipid metabolism. To examine the relation of 1,2-DAG in heart tissues to cardiac hypertrophy associated with hypertension, we measured the amount of 1,2-DAG in spontaneously hypertensive rat (SHR) hearts at 4, 10 and 20 weeks of age, and in age-matched normotensive Wistar-Kyoto (WKY) rat hearts using thin-layer chromatography with flame ionization detection (TLC-FID). Significant cardiac hypertrophy was found in 4-week-old SHR, while SHR did not yet have significant hypertension. Major phospholipids such as phosphatidylcholine and phosphatidylethanolamine increased from 4 to 20 weeks in the myocardium, but there was no difference between the two strains. The cholesterol levels of 4- and 20-week-old SHR were significantly higher than WKY rats. The 1,2-DAG contents of SHR hearts were significantly higher than WKY rats at 4 weeks. An increase in the RNA content was also observed in 4-week-old SHR hearts. However, analysis of the fatty acid composition of 1,2-DAG revealed no difference between the two strains. However, there was no significant difference in the 1,2-DAG content or in its fatty acid composition between SHR and WKY rat hearts at 10 and 20 weeks of age. It is suggested that an increase in the 1,2-DAG content of SHR hearts during the early stages appears related to the initiation of cardiac hypertrophy in SHR hearts before developed hypertension.

Neutrophil adherence to isolated adult canine myocytes. Evidence for a CD18-dependent mechanism

Cardiac myocytes were isolated from adult dogs and incubated with isolated canine neutrophils (PMN). Intercellular adhesion was low and unchanged by stimulation of the PMN with zymosan activated serum or platelet activating factor (PAF) at concentrations that significantly enhance PMN adhesion to protein-coated glass and canine endothelial cell monolayers. Intercellular adhesion was significantly increased only when both myocytes and PMN were stimulated (e.g., myocytes incubated with IL-1, tumor necrosis factor, or phorbol myristate acetate, and PMN were chemotactically stimulated). Inhibitors of protein synthesis diminished the IL-1 beta-induced effect by greater than 80%. The IL-1 beta, PAF-stimulated PMN-myocyte adhesion was associated with substantial H2O2 production. Under conditions with low PMN-myocyte adhesion (i.e., IL-1 beta alone, PAF alone, or no stimulus) H2O2 production was generally less than 5% of that occurring with high adhesion. An anti-CD18 monoclonal antibody (R15.7) inhibited stimulated PMN-myocyte adhesion by greater than 95% and reduced H2O2 production by greater than 90%. Control isotype-matched, binding, and nonbinding antibodies were without effect on adherence or H2O2 production. The results indicate that cytokine stimulation of adult myocytes induces expression of a ligand involved in CD18-dependent adherence of canine neutrophils.

Ventricular myosin pattern of spontaneously hypertensive turkeys is unaffected by labetalol treatment

In most animal species, left ventricular hypertrophy due to pressure overload is associated with an advantageous increase of the "slow" V3 isomyosin. In contrast, in spontaneously hypertensive turkeys, the development of left ventricular hypertrophy is associated with the synthesis of a "fast" V1-like isomyosin, with high incidence of cardiac failure. This could be related to the high catecholamine levels found in these animals. This is why we studied the ventricular myosin pattern after lowering of blood pressure and regression of cardiac hypertrophy obtained by means of labetalol, and alpha- and beta-blocking drug which inhibits the effects of catecholamines. From the 2nd to the 32nd week of age, 22 turkeys were treated with increasing doses of p.o. labetalol (from 20 to 35 mg/kg body weight daily) and 16 other turkeys were given daily p.o. placebo. Blood pressure and heart rate were periodically measured by an indirect method. After sacrifice, the degree of cardiac hypertrophy was evaluated by the biventricular weight to body weight ratio, ventricular myosin was purified, Ca++-activated ATPase activity assessed, and ventricular myosin pattern was determined by two-dimensional gel electrophoresis of myosin heavy chains. Plasma and cardiac catecholamines were measured by high performance liquid chromatography. Throughout the study period, blood pressure and heart rate were significantly reduced in the labetalol-treated animals as compared to the untreated ones. At the end of the study period, the ventricular mass was significantly lower in the labetalol group. Nevertheless, no differences were observed in ventricular myosin pattern and Ca++-activated ATPase activity levels between the two groups. In the labetalol group, an increase in plasma catecholamines and only a slight, but not significant, increase in cardiac catecholamines was found. These data indicate that in spontaneously hypertensive turkeys, the synthesis of the "fast" V1-like isomyosin is not influenced by known pathophysiological stimuli like blood pressure, cardiac hypertrophy and catecholamines.

Myocardial interstitial fibrosis in experimental uremia--implications for cardiac compliance

Experimental uremia is known to cause cardiac hypertrophy. In the present study we examined the effect of uremia with or without concomitant treatment of hypertension by the converting enzyme Ramipril (125 micrograms/day) on micromorphometric indices of cardiac interstitium at the light microscopical and ultrastructural level. In male SD rats, 21 days of uremia caused an increase of total heart weight (1040 +/- 73 mg wet wt vs. 871 +/- 81 in controls, P less than 0.05) with an increase of both right and left ventricular weight. This was accompanied by reduction of capillary cross-sectional area despite unchanged capillary length. The volume density (cm3/cm3) of cardiomyocytes was unchanged (0.881 +/- 0.01 vs. 0.871 +/- 0.016 in controls), but volume density of interstitial tissue (excluding capillary lumen) was significantly increased (0.042 +/- 0.011 cm3 interstitial tissue/cm3 total heart tissue vs. 0.019 +/- 0.007 in controls). This was associated with signs of activation of interstitial cells, that is, increased volume of interstitial cell nuclei and interstitial cell cytoplasm. Concomitantly, a significant increase of volume density of non-cellular interstitial ground substance was found which was not normalized by antihypertensive treatment using Ramipril. After three months of uremia, electron microscopy showed collagen fiber deposition in the interstitium. Comparable interstitial fibrosis was not observed in hearts of rats with renovascular (one clip-two kidney) hypertension. It is concluded that uremia increases myocardial interstitial ground substance by mechanisms independent of hypertension. The data may be relevant for recent findings of diastolic heart malfunction secondary to impaired compliance in uremic patients.