Regulation of cardiac gene expression during myocardial growth and hypertrophy: molecular studies of an adaptive physiologic response

Increased production of endothelin-1 in the hypertrophied rat heart due to pressure overload

Endothelin-1 (ET-1) has been demonstrated to induce hypertrophy in cultured cardiac myocytes. We investigated the production of ET-1 in the heart of aorta-banded rats in vivo. Seven days after the banding of the abdominal aorta, rats developed a significant left ventricular hypertrophy. The tissue content of mature ET-1 and the level of expression of prepro ET-1 mRNA were higher in the left ventricle of aorta-banded rats than in those of sham-operated rats. The expression of prepro ET-1 mRNA in the right ventricle was not different between the two groups. These findings indicate that the production of ET-1 increased in the hypertrophied left ventricle, thereby suggesting the possible involvement of endogenous ET-1 in the development of cardiac hypertrophy due to pressure overload.

Contractile activity and cell-cell contact regulate myofibrillar organization in cultured cardiac myocytes

Adult feline ventricular myocytes cultured on a laminin-coated substratum reestablish intercellular junctions, yet disassemble their myofibrils. Immunofluorescence microscopy reveals that these non-beating heart cells lack vinculin-positive focal adhesions; moreover, intercellular junctions are also devoid of vinculin. When these quiescent myocytes are stimulated to contract with the beta-adrenergic agonist, isoproterenol, extensive vinculin-positive focal adhesions and intercellular junctions emerge. If solitary myocytes are stimulated to beat, an elaborate series of vinculin-positive focal adhesions develop which appear to parallel the reassembly of myofibrils. In cultures where neighboring myocytes reestablish cell-cell contact, myofibrils appear to reassemble from the fascia adherens rather than focal contacts. Activation of beating is accompanied by a significant reduction in the rate of total and cytoskeletal protein synthesis; in fact, myofibrillar reassembly, redevelopment of focal adhesions and fascia adherens junctions require no protein synthesis for at least 24 h, implying the existence of an assembly competent pool of cytoskeletal proteins. Maturation of the fasciae adherens and the appearance of vinculin within Z-line/costameres, does require de novo synthesis of new cytoskeletal proteins. Changes in cytoskeletal protein turnover appear dependent on beta agonist-induced cAMP production, but myofibrillar reassembly is a cAMP-independent event. Such observations suggest that mechanical forces, in the guise of contractile activity, regulate vinculin distribution and myofibrillar order in cultured adult feline heart cells.

Targeting gene expression to specific cardiovascular cell types in transgenic mice

Transgenic techniques, which allow the introduction of exogenous genes into the genome of experimental animals, promise to bridge the gap between the in vitro observations made by molecular and cellular biologists on cardiac and vascular cells in tissue culture and the physiology and pathology of the whole organ system. One such application of these techniques is tissue targeting: by genetic manipulation to direct expression of a protein--such as a signaling peptide, a growth factor receptor, or an oncogene involved in cell growth--to a tissue where it normally would not be expressed (or where expression is tightly controlled) by fusing it to the transcriptional control sequences of another gene normally expressed in that tissue. In the cardiovascular system, regulatory sequences for cardiomyocyte-specific proteins, vascular endothelium-specific proteins, and smooth muscle-specific proteins can be used to target heterologous genes to their respective tissues in transgenic animals. The effects that such perturbations have on organ physiology and intracellular and intercellular communication can be observed by applying established physiological and molecular approaches. In this review, we highlight some tissue-specific genes from cardiac and vascular cell types whose regulatory sequences may be used to target heterologous proteins; we discuss neutral "reporter" proteins and signal transduction components as paradigms for the application of this technique; and we briefly touch on the potentials and pitfalls of transgenic approaches to molecular physiology.

Behaviour of human atrial myocytes in culture is donor age dependent

The characteristics of cultured myocardial cells isolated from small mammals are well documented, but there is a dearth of data on cultured human cardiocytes. The aim of this study was to determine the main features of myocytes isolated from human atria and maintained in culture in the presence of 10% fetal calf serum (FCS), according to the age of the donor. The following characteristics were analysed: (1) yield and viability; (2) adhesive properties; and (3) changes in cell morphology. Myocytes preferentially adhered to laminin-coated dishes and could be maintained in culture for at least 2 weeks, whatever the age of the donor (which was from 6 days to 85 yr). Maintenance in culture induced morphologic changes characterized by myocyte spreading and changes in myofibrillar organization. Interestingly, the time of onset of these changes depended on the age of the donor: they occurred earlier in young atrial myocytes (< 1 yr) than in older cells (> 13 yr).

Molecular characterization of angiotensin II--induced hypertrophy of cardiac myocytes and hyperplasia of cardiac fibroblasts. Critical role of the AT1 receptor subtype

Increasing evidence suggests that angiotensin II (Ang II) may act as a growth factor for the heart. However, direct effects of Ang II on mammalian cardiac cells (myocytes and nonmyocytes), independent of secondary hemodynamic and neurohumoral effects, have not been well characterized. Therefore, we analyzed the molecular phenotype of cultured cardiac cells from neonatal rats in response to Ang II. In addition, we examined the effects of selective Ang II receptor subtype antagonists in mediating the biological effects of Ang II. In myocyte culture, Ang II caused an increase in protein synthesis without changing the rate of DNA synthesis. In contrast, Ang II induced increases in protein synthesis, DNA synthesis, and cell number in nonmyocyte cultures (mostly cardiac fibroblasts). The Ang II-induced hypertrophic response of myocytes and mitogenic response of fibroblasts were mediated primarily by the AT1 receptor. Ang II caused a rapid induction of many immediate-early genes (c-fos, c-jun, jun B, Egr-1, and c-myc) in myocyte and nonmyocyte cultures. Ang II induced "late" markers for cardiac hypertrophy, skeletal alpha-actin and atrial natriuretic factor expression, within 6 hours in myocytes. Ang II also caused upregulation of the angiotensinogen gene and transforming growth factor-beta 1 gene within 6 hours. Induction of immediate-early genes, late genes, and growth factor genes by Ang II was fully blocked by an AT1 receptor antagonist but not by an AT2 receptor antagonist. These results indicate that: (1) Ang II causes hypertrophy of cardiac myocytes and mitogenesis of cardiac fibroblasts, (2) the phenotypic changes of cardiac cells in response to Ang II in vitro closely mimic those of growth factor response in vitro and of load-induced hypertrophy in vivo, (3) all biological effects of Ang II examined here are mediated primarily by the AT1 receptor subtype, and (4) Ang II may initiate a positive-feedback regulation of cardiac hypertrophic response by inducing the angiotensinogen gene and transforming growth factor-beta 1 gene.

Signal transduction pathways of angiotensin II--induced c-fos gene expression in cardiac myocytes in vitro. Roles of phospholipid-derived second messengers

Angiotensin II (Ang II) causes a rapid induction of immediate-early genes and hypertrophy in the cardiac myocyte. However, the signaling mechanism of Ang II-induced immediate-early gene expression in cardiac myocytes has not been characterized. Therefore, we examined signal transduction of Ang II in neonatal rat cardiac myocytes, using c-fos gene expression as a model system. Transient transfection of c-fos reporter gene constructs indicated that the serum response element is not only required but also sufficient for Ang II-induced activation of the c-fos promoter. Ang II is known to cause an increase in [Ca2+]i. We found that Ang II also causes a small increase in cAMP in cardiac myocytes. However, the Ca2+/cAMP response element of the c-fos gene was not sufficient to confer Ang II responsiveness to the c-fos promoter, and inhibitors of protein kinase A had no effects on Ang II-induced c-fos expression. On the other hand, chelating intracellular Ca2+ with BAPTA-AM inhibited Ang II-induced c-fos expression in a dose-dependent manner, suggesting that Ca2+ is required for Ang II-induced signaling. Measurements of phospholipid-derived second messengers revealed that Ang II increased production of inositol trisphosphate, diacylglycerol, phosphatidic acid, and arachidonic acids, resulting in a sustained increase in protein kinase C activity. This and other evidence suggest that Ang II activates phospholipase C, phospholipase D, and possibly phospholipase A2. All of these second-messenger systems are activated through the AT1 receptor. Pharmacological inhibition of phospholipase C or downregulation of protein kinase C significantly suppressed Ang II-induced c-fos expression. In conclusion, Ang II activates multiple phospholipid-derived second-messenger systems via the AT1 receptor in cardiac myocytes. Among these second-messenger systems, phospholipase C and protein kinase C seem essential for Ang II-induced c-fos gene expression, whereas Ca2+ may play a permissive role. Finally, the "Ang II response element" of the c-fos gene maps to the protein kinase C-dependent portion of the serum response element.

Enhanced functional expression of transient outward current in hypertrophied feline myocytes

Cardiac hypertrophy can decrease myocardial contractility and alter the electrophysiological activity of the heart. It is well documented that action potentials recorded from hypertrophied feline ventricular cells can exhibit depressed plateau voltages and prolonged durations. Similar findings have been made by others in rabbit, rat, guinea pig, and human heart. Whole-cell patch voltage-clamp studies designed to explain these changes in the action potential suggest that the only component of the membrane current recorded from feline right ventricular (RV) myocytes found to be substantially different from normal is the 4-amino-pyridine-sensitive transient outward current (I(to)). However, it was not clear if the change in I(to) could explain the changes in the action potential of hypertrophied cardiocytes, nor was it clear if these changes reflect an alteration in the electrophysiological character of the channels underlying I(to). A kinetic comparison of I(to) elicited by hypertrophied RV myocytes with that elicited by comparable normal RV myocytes previously revealed no differences, suggesting that the increased magnitude of the peak I(to) recorded from hypertrophied myocytes arises because the current density increases and not because of any alteration in the kinetic parameters governing the current. This finding suggests that in hypertrophy additional normal channels are expressed rather than a kinetically different channel subtype emerging. Investigations designed to determine if enhancement of I(to) could explain the hypertrophy-induced changes in plateau voltage and action potential duration suggest that a change in I(to) density can indeed explain the entire effect of hypertrophy on RV action potentials. If this notion is correct, the likelihood of "sudden death" in patients with myocardial hypertrophy might be decreased by a blocker selective for cardiac I(to).

Chronic ethanol ingestion modifies the renin-aldosterone axis independent of alterations in the regulation of atrial natriuretic peptide

Using an animal model, we have investigated the effects of chronic ethanol ingestion on the regulation of atrial natriuretic peptide (ANP) synthesis and release. Male Sprague-Dawley rats were maintained for 6 weeks on a liquid diet of ethanol (up to 20% v/v) as part of a 2% solution of calf milk replacer. Weight-matched controls received an equal volume of ethanol-free solution, and normal animals drank ad libitum. All animals received rat chow throughout the experiment. This model produced physiologically relevant levels of blood ethanol, as concentrations at the time of sacrifice were 171.98 +/- 39.26 mg/dl. Plasma renin activity was significantly elevated in response to ethanol treatment, whereas circulating aldosterone concentration was reduced. No alterations in the plasma or atrial tissue levels of ANP were evident, although we did observe a significant increase in the ventricular tissue levels of ANP from 45.1 to 71.8 ng/g as a consequence of ethanol treatment. Levels of both atrial and ventricular ANP mRNA were not different between alcohol-treated and liquid-restricted control animals, although both groups showed significant increases in the amount of transcript in comparison with rats drinking ad libitum. No significant increases in either arterial blood pressure or heart/body weight ratio were observed for ethanol-treated rats. These results suggest that modifications in the renin-aldosterone axis can occur independently of alterations in the regulation of ANP under the influence of chronic ethanol ingestion.

Positional specification of ventricular myosin light chain 2 expression in the primitive murine heart tube

To study the process of ventricular specification during cardiogenesis, we examined the in situ expression of cardiac ventricular myosin light chain 2 (MLC-2v) mRNA during murine embryogenesis. As assessed by hybridization with a specific MLC-2v riboprobe, mRNA expression can be found in the ventricular region at day 8.0 postcoitum (pc). MLC-2v expression is high in the ventricular portion of the heart tube, with no detectable expression in the atrial or sinus venosus regions. The proximal outflow tract of the heart tube also expresses MLC-2v mRNA at minimally detectable levels at this time but then displays a temporally and spatially distinct pattern with expression well established in the proximal out-flow tract region adjacent to the ventricular segment by days 9-10 pc, eventually reaching levels comparable to the trabeculated ventricular myocardium. By day 11 pc, prior to the completion of septation, expression then becomes restricted to the ventricular region at and below the level of the atrioventricular cushion. Transgenic mice harboring a 250-base-pair MLC-2v promoter fragment fused to a luciferase reporter gene demonstrate reporter gene activity from at least day 9 pc. Ventricular region-restricted expression of the luciferase reporter in the embryonic heart, as assessed by immunofluorescence and direct assay of reporter activity in microdissected atrial and ventricular muscle specimens, was confirmed from at least day 15 pc on. Taken together, this provides evidence for early positional specification of MLC-2v gene expression in the primitive heart tube and indicates regional specification of part of the ventricular muscle gene program can precede ventricular septation during mammalian cardiogenesis. Since the 250-base-pair promoter fragment is active developmentally in transgenic mice, this establishes it as a molecular target for the process of ventricular specification in the developing heart tube.

Antithetical accumulation of myosin heavy chain but not alpha-actin mRNA isoforms during early stages of pressure-overload-induced rat cardiac hypertrophy

Myocardial response to a hemodynamic overload involves changes in the expression of isogenes encoding myosin heavy chain (MHC) and actin: beta-MHC/alpha-MHC and skeletal/cardiac alpha-actin mRNA isoform ratios are increased. It is not known whether these changes are due to increased accumulations of the two neosynthesized transcripts, beta-MHC and skeletal alpha-actin, or whether the mRNA isoforms normally present, alpha-MHC and cardiac alpha-actin, are concomitantly decreased. To answer these questions, using dot-blot hybridizations, primer extension, and exonuclease VII mapping assays, we have analyzed the content of sarcomeric MHC and actin mRNAs in the poly(A+) RNA in left ventricles of 23-24-day-old rats 18 and 24 hours after a pressure overload induced by stenosis of the thoracic aorta. The results showed a 1.9-fold increase in poly(A+) RNA after the stenosis. Skeletal/cardiac alpha-actin mRNA isoforms were already increased fivefold (from 0.19 to 0.99) at 18 hours, and this was exclusively due to a 5.5-fold increase in skeletal alpha-actin mRNA. At 24 hours, this ratio was increased ninefold (from 0.14 to 1.22), and this was due to a 4.3-fold increase in the level of skeletal alpha-actin mRNAs (p < 0.001) and a 1.9-fold decrease of cardiac alpha-actin mRNA (p < 0.001), restoring the same proportion of sarcomeric actin mRNA in sham-operated and operated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of protein kinase C isotype expression in adult rat heart. Protein kinase C-epsilon is a major isotype present, and it is activated by phorbol esters, epinephrine, and endothelin

The pattern of protein kinase C (PKC) isotype expression in whole extracts of dispersed, freshly isolated adult rat ventricular myocytes and adult rat heart ventricle was examined by immunoblot analysis using antisera specific for PKC-alpha, -beta 1, -gamma, -delta, -epsilon, -zeta, or -eta isotypes. This analysis revealed significant levels of expression of the Ca(2+)-independent isotype PKC-epsilon, which was detected as band of 97-kd molecular mass. PKC-zeta was detected principally as a 66-kd band that probably represented a proteolytic product of the holoenzyme. PKC-eta was detected only in whole ventricle as a doublet at 75 and 81 kd and was therefore probably present in nonmyocytic cells. PKC-alpha, -beta 1, -gamma, and -delta could not be detected. Because of our inability to detect PKC-alpha, -beta 1, -gamma, and -delta in whole extracts, PKC isotypes were partially purified from whole heart by DEAE Sepharose chromatography. PKC-alpha, -beta 1, -gamma, and -delta could still not be detected in the appropriate fractions. All PKC isotypes were detectable in appropriate positive control extracts (brain or certain cultured cell lines). In unstimulated isolated cardiomyocytes, the majority (80-95%) of the PKC-epsilon immunoreactivity was present in the soluble fraction of the extract. On exposure of the cardiomyocytes to 1 microM phorbol 12-myristate 13-acetate (PMA), PKC-epsilon undergoes a rapid (< 30 seconds), sustained (at least 60 minutes), and virtually complete association with the Triton X-100-soluble membrane fraction. There was an associated loss of PKC-epsilon from the soluble fraction. The EC50 for PMA of the translocation event was 15-37 nM. Exposure of cardiomyocytes to 1 microM 4 beta-phorbol 12,13-didecanoate or 1 microM phorbol 12,13-dibutyrate also resulted in translocation of PKC-epsilon to the membrane fraction, whereas exposure to 1 microM 4 alpha-phorbol 12,13-didecanoate was without effect. PKC-epsilon also translocated on exposure of cardiomyocytes to 50 microM epinephrine or 100 nM endothelin-1. However, in both cases, the extent of translocation was significantly less than that after exposure to PMA. We conclude that interventions that lead to hypertrophy of cardiomyocytes (phorbol esters, epinephrine, and endothelin-1) activate PKC-epsilon.

Stimulation of adult rat ventricular myocyte protein synthesis and phosphoinositide hydrolysis by the endothelins

The effects of endothelin-1 (ET-1) on protein synthesis and phosphoinositide (PI) hydrolysis were investigated in ventricular myocytes isolated by collagenase digestion of adult rat hearts. The maximum stimulation of protein synthesis by ET-1 was about 35% and the EC50 value was about 0.3 nM. The stimulation was exerted at the translational stage since it was insensitive to inhibition by actinomycin D. The maximum stimulation of PI hydrolysis by ET-1 as measured by the formation of [3H]inositol phosphates was about 11-fold and the EC50 value was about 0.7 nM. The ET-1 analogue sarafotoxin-6b stimulated protein synthesis by a maximum of 27% and stimulated PI hydrolysis about 8- to 9-fold. The EC50 values were 1.6 nM and 0.6 nM, respectively. Other endothelins stimulated protein synthesis and PI hydrolysis in the following order of potency: ET-1 approximately ET-2 > ET-3. This order of potency suggests that the stimulation of both protein synthesis and PI hydrolysis is mediated through the ETA receptor. Although both angiotensin II and [Arg]vasopressin stimulated PI hydrolysis significantly, the stimulation was less than 60%, i.e., much less than the stimulation by ET-1 and its analogues. Neither insulin nor substance P stimulated PI hydrolysis. Stimulation of protein synthesis by ET-1 and its analogues correlated strongly with the stimulation of PI hydrolysis and we suggest that the stimulation of protein synthesis may be dependent on the stimulation of PI hydrolysis. We hypothesize that the mechanism may involve a protein kinase C-mediated increase in intracellular pH.

Endothelin-1, phorbol esters and phenylephrine stimulate MAP kinase activities in ventricular cardiomyocytes

ET-1 stimulated MBP kinase activity in cultured cardiomyocytes. Maximal activation (3.5-fold) was at 5 min. EC50 was 0.2 nM. PMA or PE also increased MBP kinase (4- or 2.5-fold, respectively). Pre-treatment with PMA down-regulated the subsequent response to ET-1 or PMA. ET-1- or PMA-stimulated MBP kinase was resolved into 2 major (peaks II and IV) and 2 minor peaks by FPLC on Mono Q. Peaks II and IV were inactivated by either LAR or PP2A. Renatured MBP kinase activities following SDS-PAGE in MBP-containing gels and immunoblot analysis showed that peak II was a p42 MAP kinase and peak IV was a p44 MAP kinase.

Altered left ventricular remodeling with beta-adrenergic blockade and exercise after coronary reperfusion in rats

BACKGROUND

beta-Adrenergic blockade is known to improve the survival of patients after acute myocardial infarction and to reduce myocardial infarct size in experimental coronary occlusion. However, the effects of beta-blockade on global and regional left ventricular (LV) remodeling have not been characterized after coronary occlusion with reperfusion. In rats subjected to coronary occlusion and reperfusion, we have demonstrated beneficial effects of reperfusion in sedentary rats with a hypertrophic response to exercise in the surviving outer wall of the infarcted zone, and in this study we investigated whether such remodeling is modified by beta-blockade in both sedentary and exercised rats.

METHODS AND RESULTS

Female Sprague-Dawley rats were randomized into groups at 5 days after 45 minutes of left coronary artery occlusion followed by reperfusion to produce nontransmural infarction. Animals completing the experiment included a propranolol-treated sedentary group (750 mg/l drinking water) (n = 20) and a propranolol-treated exercised group (n = 19), and these groups were compared with two groups of simultaneously randomized rats (which were also part of a separate study): an untreated sedentary group (n = 21) and an untreated exercised group (n = 20). After 3 weeks of intervention, global and regional LV morphological changes were analyzed in 78 completed experiments from midventricular transverse slices of hearts after perfusion fixation. Compared with sedentary untreated rats, propranolol-treated sedentary rats showed significantly increased LV cavity area (41.6 versus 31.5 mm2, p < 0.001) and reduced wall thicknesses in the noninfarcted (1.77 versus 1.95 mm, p < 0.01) and infarcted regions (1.29 versus 1.36 mm, p < 0.01) (two-way ANOVA). In the propranolol-treated rats, exercise further increased the LV cavity area (44.6 versus 39.1 mm2, p < 0.001), reduced the noninfarcted wall thickness (1.62 versus 1.81 mm, p < 0.01), and increased the LV dimension/wall thickness ratio in the noninfarcted region (4.7 versus 3.95 mm, p < 0.001). Whereas exercise in the reperfused group in the absence of beta-blockade significantly increased the viable subepicardial area of the infarcted zone, as reported elsewhere, propranolol treatment prevented this exercise-induced increase of subepicardial area (7.8 rest versus 7.7 mm2 exercise, NS), and there was a significant reduction of total myocardial area in the propranolol-treated exercised group compared with the untreated exercised group (p < 0.05). Corresponding trends in LV weights were not statistically significant.

CONCLUSIONS

These findings provide evidence for altered LV remodeling, including LV cavity dilation and reduced wall thickness, after 3 weeks of propranolol treatment in sedentary rats after coronary artery occlusion and reperfusion to produce nontransmural infarction. In addition, after 3 weeks of exercise together with propranolol treatment, morphological changes were consistent with suppression of exercise-induced myocardial hypertrophy globally and in the viable outer wall of the infarcted region, accompanied by further LV cavity dilation.

Myocardial cell hypertrophy after myocardial infarction with reperfusion in dogs

BACKGROUND

The potential role of myocardial cell hypertrophy in the ischemic zone in the mechanism of late recovery of regional contractile function after myocardial infarction followed by reperfusion has not been examined.

METHODS AND RESULTS

Eight chronically instrumented, conscious dogs were subjected to 90-120 minutes of circumflex coronary artery occlusion followed by reperfusion. The thickness and function of the anterior (AT) and posterior (PT) walls was measured by ultrasonic gauges at control, during occlusion, and after reperfusion. After 3 weeks, cross-sectional areas of surviving cells were determined from subepicardial (epi), midwall (mid), and subendocardial (endo) regions in six dogs and compared with those from six animals without infarction, including three sham-operated control dogs. PT systolic wall thickening showed dyskinesia during occlusion but recovered after reperfusion to 48% of control at 1 week and 67% at 3 weeks. End-diastolic thickness of the PT wall increased markedly after reperfusion, but AT and PT walls were only slightly thicker (p = NS) than in control dogs at 3 weeks. Cross-sectional areas of reperfused dogs in the infarct region averaged 279 (PTepi), 291 (PTmid), and 317 microns 2 (PTendo) and were significantly larger than in control animals (237 [PTepi], 241 [PTmid], and 233 microns 2 [PTendo]). PT cell areas were significantly larger than AT cells, ENDO cell areas were larger than EPI cells (both p < 0.05), and ENDO cells of the AT wall were larger than those of noninfarcted dogs (p < 0.05).

CONCLUSIONS

In dogs with myocardial infarction followed by reperfusion, the cross-sectional areas of cells in the infarcted PT wall were larger than those in the noninfarcted AT wall, and within both the infarcted and noninfarcted zones, cell areas were larger in the endocardial than the epicardial region. In all regions of the infarcted wall and in the ENDO region of the noninfarcted wall, cell areas were generally larger than those of control dogs without infarction, and the control dogs showed no transmural differences in cell areas. The mechanisms responsible for this significant remodeling of the reperfused infarcted zone, which involves myocardial cellular hypertrophy, are unknown, but it is possible that hypertrophy of surviving regions of the infarcted wall played a role in the late recovery of regional function that accompanied this hypertrophic response.

Contractile arrest accelerates myosin heavy chain degradation in neonatal rat heart cells

Mechanical forces influence the growth and metabolism of a variety of cells, including cultured neonatal rat ventricular myocytes. To determine whether mechanical activity affected the synthesis and turnover of myosin heavy chain (MHC) in these striated muscle cells, MHC fractional degradative rates were measured in spontaneously beating cells and in arrested myocytes in which contractile activity was prevented by L-channel blockade (with verapamil, nifedipine, nisoldipine, and diltiazem) or K+ depolarization. MHC degradative rates were measured as the difference between rates of MHC synthesis and accumulation and in pulse-chase biosynthetic labeling experiments. Both methods indicated that contractile arrest markedly increased MHC degradation. Contractile arrest produced by L-channel blockade accelerated MHC degradation to a greater extent than K+ depolarization. The signal transduction pathway linking contractile activity to alterations in MHC degradation did not involve protein kinase C (PKC), because MHC degradation was unaffected by activating PKC in arrested cells or inhibiting PKC in spontaneously beating cells. Chloroquine and E-64 did not suppress the accelerated MHC degradation, suggesting that the rate-limiting step in MHC turnover occurred before degradative processing by cellular proteinases. Using a computer simulation, we hypothesize that the rate-limiting step in MHC turnover preceded (or was coincident with) MHC release from thick filaments. Thus mechanical forces may influence MHC half-life by regulating the rate of myosin disassembly.

Microinjection of antibodies and expression vectors into living myocardial cells. Development of a novel approach to identify candidate genes that regulate cardiac growth and hypertrophy

BACKGROUND

Microinjection approaches in the cardiac cell context have allowed delivery of various calcium dyes and monitoring of short-term physiological responses. However, unlike other cell types, it has proved difficult to microinject myocardial cells without the concomitant loss of long-term cell viability.

METHODS AND RESULTS

An analysis of experimental variables was conducted to adapt microinjection techniques to the neonatal rat ventricular cell context. Among the variables optimized were the selection of culture dishes, plating substrate, microinjection parameters, and a variety of maneuvers to inhibit myocyte hypercontracture, injury, and consequent death after micropuncture. With the modified technique, the percentage of injected cells that maintained long-term viability (48 hours) increased from less than 1% to 30%. Similarly, an increased efficiency of gene transfer and expression (measured as the percentage of injected cells that express the delivered gene) was obtained after either cytoplasmic or nuclear injection of a beta-galactosidase expression vector into cardiac myocytes. Microinjection of marker immunoglobulin G does not interfere with the induction of the hypertrophic response or the expression of a coinjected atrial natriuretic factor promoter-luciferase reporter fusion gene construct.

CONCLUSIONS

To the best of our knowledge, this study provides the first description of the efficient microinjection of neonatal cardiac muscle cells with maintenance of long-term cell viability. The microinjection technique is now a viable approach to examine cause-and-effect relations between specific gene products and any defined feature or response of cardiac myocytes that can be assayed at a single-cell level.

A ubiquitous factor (HF-1a) and a distinct muscle factor (HF-1b/MEF-2) form an E-box-independent pathway for cardiac muscle gene expression

Recent studies have identified a conserved 28-bp element (HF-1) within the rat cardiac MLC-2 gene which confers cardiac muscle-specific and inducible expression during myocardial cell hypertrophy. Utilizing a combination of independent experimental approaches, this study characterizes two cardiac nuclear factors which bind to HF-1, a ubiquitous factor (HF-1a), and an A + T-rich binding factor (HF-1b) which is preferentially expressed in differentiated cardiac and skeletal muscle cells. The HF-1a binding site is located in a core region of the 28-bp conserved element, immediately upstream from the A + T-rich HF-1b site, which is homologous to the MEF-2 site found in a number of muscle genes. By a number of separate criteria (gel mobility shift, competition, and mutagenesis studies), HF-1b and MEF-2 appear to be indistinguishable and thus are either identical or closely related muscle factors. Transient assays of luciferase reporter genes containing point mutations throughout the 28-bp HF-1 regulatory element document the importance of both the HF-1a and HF-1b sites in transient assays in ventricular muscle cells. In the native 250-bp MLC-2 promoter fragment, mutations in the single E box had little effect on cardiac muscle specificity, while point mutations in either the HF-1a or HF-1b binding site significantly reduced promoter activity, underscoring the importance of both the HF-1a and HF-1b sites in the transcriptional activation of this cardiac muscle gene. Thus, this study provides evidence that a novel, ubiquitous factor (HF-1a) and a muscle factor (HF-1b/MEF-2) can form a novel, E-box-independent pathway for muscle-specific expression in ventricular cardiac muscle cells.

A ubiquitous factor (HF-1a) and a distinct muscle factor (HF-1b/MEF-2) form an E-box-independent pathway for cardiac muscle gene expression

Recent studies have identified a conserved 28-bp element (HF-1) within the rat cardiac MLC-2 gene which confers cardiac muscle-specific and inducible expression during myocardial cell hypertrophy. Utilizing a combination of independent experimental approaches, this study characterizes two cardiac nuclear factors which bind to HF-1, a ubiquitous factor (HF-1a), and an A + T-rich binding factor (HF-1b) which is preferentially expressed in differentiated cardiac and skeletal muscle cells. The HF-1a binding site is located in a core region of the 28-bp conserved element, immediately upstream from the A + T-rich HF-1b site, which is homologous to the MEF-2 site found in a number of muscle genes. By a number of separate criteria (gel mobility shift, competition, and mutagenesis studies), HF-1b and MEF-2 appear to be indistinguishable and thus are either identical or closely related muscle factors. Transient assays of luciferase reporter genes containing point mutations throughout the 28-bp HF-1 regulatory element document the importance of both the HF-1a and HF-1b sites in transient assays in ventricular muscle cells. In the native 250-bp MLC-2 promoter fragment, mutations in the single E box had little effect on cardiac muscle specificity, while point mutations in either the HF-1a or HF-1b binding site significantly reduced promoter activity, underscoring the importance of both the HF-1a and HF-1b sites in the transcriptional activation of this cardiac muscle gene. Thus, this study provides evidence that a novel, ubiquitous factor (HF-1a) and a muscle factor (HF-1b/MEF-2) can form a novel, E-box-independent pathway for muscle-specific expression in ventricular cardiac muscle cells.

Transcriptional activation of the cardiac myosin light chain 2 and atrial natriuretic factor genes by protein kinase C in neonatal rat ventricular myocytes

A cultured myocardial cell model was used to examine the role of protein kinase C-dependent pathways in the transcriptional activation of two cardiac muscle genes [myosin light chain 2 (MLC-2) and atrial natriuretic factor (ANF)] during alpha-adrenergic receptor-mediated hypertrophy. Phorbol ester (phorbol 12-myristate 13-acetate) and the alpha-adrenergic agonist phenylephrine both activate protein kinase C (PKC) and induce 4- to 5-fold increases in the expression of MLC-2 and ANF promoter/luciferase reporter genes with little effect on Rous sarcoma virus/luciferase or minimal prolactin promoter/luciferase genes. To further assess the role of PKC in cardiac gene regulation, PKC expression vectors encoding constitutively activated PKC-alpha or PKC-beta, or a catalytically inactive PKC, were transiently cotransfected with the cardiac promoter/luciferase constructs. Cotransfection of either activated PKC-alpha or PKC-beta cDNA induces the expression of MLC-2 and ANF promoter/luciferase genes and of a reporter gene responsive to the transcription factor AP-1. The Rous sarcoma virus/luciferase and minimal prolactin promoter/luciferase genes are not concomitantly induced by cotransfectin with the PKC genes, indicating specificity of the transcriptional effect. The finding that activated PKC increases cardiac gene transcription suggests that activation of this enzyme may be a proximal signal for coregulation of two cardiac genes, MLC-2 and ANF, during the course of myocardial cell hypertrophy.

Signaling mechanisms for the activation of an embryonic gene program during the hypertrophy of cardiac ventricular muscle

To study the signaling mechanisms which mediate ventricular hypertrophy, we utilized the induction of the ANF gene as a marker of the hypertrophic response. The induction of the atrial natriuretic factor gene (ANF) is one of the most conserved features of ventricular hypertrophy, occurring in multiple species (mouse, rat, hamster, canine, and human) in response to diverse stimuli (hormonal, mechanical, pressure/volume overload, genetic, IHSS, hypertension, etc.). The ANF gene is expressed in both the atrial and ventricular compartments during embryonic development, but shortly after birth ANF expression is down-regulated to negligible levels in the adult myocardium. Since the reactivation of ANF gene expression in the hypertrophied ventricle is a hallmark of the activation of an embryonic gene program, it has also become of interest to determine if similar mechanisms activate ANF expression during hypertrophy and the initial stages of cardiogenesis. A combination of cotransfection, microinjection, and transgenic approaches has been coupled to well characterized cultured cell systems and in vivo murine models employing normal and transgenic mice. The microinjection of oncogenic RAS proteins into living myocardial cells does not lead to the activation of cell proliferation, but activates ANF gene expression, as assessed by immunofluorescence. Co-transfection of mutant and wild-type RAS expression vectors with a ANF-luciferase fusion gene supports a direct effect of activated RAS on ANF gene transcription. Co-transfection of a dominant negative RAS vector effectively inhibits the induction of the ANF gene during alpha adrenergic mediated hypertrophy of ventricular muscle cells, thereby establishing that a RAS-mediated pathway is required for ANF induction.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of myocardial sarcoplasmic reticulum Ca(++)-ATPase in cardiac hypertrophy by angiotensin converting enzyme?

Myocardial hypertrophy in response to hemodynamic overload is an established risk factor for cardiovascular morbidity and mortality. Partially, this may be due to alterations in cardiac gene expression, resulting in a more fetal-like myocyte phenotype with a fragile Ca(++)-homeostasis. Depressed expression of the sarcoplasmic reticulum Ca(++)-ATPase is the hallmark of this overload phenotype, contributing to prolonged cytosolic Ca(++)-transients, disturbed diastolic relaxation, altered force-frequency relation, and probably, electrophysiologic instability with susceptibility to malignant arrhythmias. Since angiotensin II is a growth-promoting factor in several cellular systems, the local formation of angiotensin II within the myocardium might contribute to the trophic response and the phenotype shift of overloaded myocardium. Several observations are consistent with this hypothesis: the cardiac expression of ACE and angiotensinogen is enhanced in experimental myocardial overload and in human endstage congestive heart failure; prolonged observations of experimental cardiac overload with hypertrophy-induced putative normalisation of myocardial systolic wall stress demonstrated a renormalization of ventricular tissue ACE activity and of ventricular sarcoplasmic Ca(++)-ATPase expression and activity; normalizing ventricular tissue ACE activity in experimental cardiac overload by chronic nonhypotensive ACE inhibitor therapy caused a parallel partial normalization of hypertrophy and underexpression of sarcoplasmic CA(++)-ATPase. This partial normalization of myocyte Ca(++)-homeostasis in overload hypertrophy by non-hypotensive chronic ACE-inhibition is attenuated by concomitant chronic application of bradykinin-2 receptor blockade, indicating an involvement of altered bradykinin metabolism in the phenotype modulation due to chronic ACE inhibition. While these observations are consistent with a direct influence of local ACE activity on the sarcoplasmic reticulum, the cell type contributing to the enhanced ACE expression in overload and the specific mechanism of this influence are unknown.

Cardiac troponin T in the diagnosis of myocardial injury

In the last several decades serum levels of cardiac enzymes and isoenzymes have become the final arbiters by which myocardial damage is diagnosed or excluded. Because conventionally used enzymes are neither perfectly sensitive nor specific, there is need for a new sensitive and cardiospecific marker of myocardial damage. Cardiac troponin T (TnT) is a contractile protein unique to cardiac muscle and can be differentiated by immunologic methods from its skeletal-muscle isoform. An enzyme immunoassay specific for cardiac TnT is now available in a commercial kit for routine use. The biggest advantage of this assay is its cardiospecificity. TnT measurements, however, are also highly sensitive in diagnosis of myocardial injury and accurately discern even small amounts of myocardial necrosis. TnT measurements are, therefore, particularly useful in patients with borderline CK-MB and in clinical settings in which traditional enzymes fail to diagnose myocardial damage efficiently because of lack of specificity--for example, perioperative myocardial infarction or blunt heart trauma. TnT release kinetics reveal characteristics of both soluble, cytoplasmic, and structurally bound molecules. It starts to increase a few hours after the onset of myocardial damage and remains increased for several days. TnT allows late diagnosis of myocardial infarction. The diagnostic efficiency remains at 98% until 6 d after the onset of infarct-related symptoms. TnT is also useful in monitoring the effectiveness of thrombolytic therapy in myocardial infarction patients. The ratio of peak TnT concentration on day 1 to TnT concentration at day 4 discriminates between patients with successful (greater than 1) and failed (less than or equal to 1) reperfusion. TnT measurements are very sensitive and specific for the early and late diagnosis of myocardial damage and could, therefore, provide a new criterion in laboratory diagnosis of the occurrence of myocardial damage.