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Am J Physiol Heart Circ Physiol 280: H2264-H2270, 2001;
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Vol. 280, Issue 5, H2264-H2270, May 2001

PKC-beta is not necessary for cardiac hypertrophy

Brian B. Roman1, David L. Geenen1, Michael Leitges2, and Peter M. Buttrick1

1 Section of Cardiology, Department of Medicine, University of Illinois at Chicago, Illinois 60612; and 2 Max Planck Institut für Immunbiologie, Freiburg D-79011, Germany


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Studies in human and rodent models have shown that activation of protein kinase C-beta (PKC-beta ) is associated with the development of pathological hypertrophy, suggesting that ablation of the PKC-beta pathway might prevent or reverse cardiac hypertrophy. To explore this, we studied mice with targeted disruption of the PKC-beta gene (knockout, KO). There were no detectable differences in expression or distribution of other PKC isoforms between the KO and control hearts as determined by Western blot analysis. Baseline hemodynamics were measured using a closed-chest preparation and there were no differences in heart rate and arterial or left ventricular pressure. Mice were subjected to two independent hypertrophic stimuli: phenylephrine (Phe) at 20 mg · kg-1 · day-1 sq infusion for 3 days, and aortic banding (AoB) for 7 days. KO animals demonstrated an increase in heart weight-to-body weight ratio (Phe, 4.3 ± 0.6 to 6.1 ± 0.4; AoB, 4.0 ± 0.1 to 5.8 ± 0.7) as well as ventricular upregulation of atrial natriuretic factor mRNA analogous to those seen in control animals. These results demonstrate that PKC-beta expression is not necessary for the development of cardiac hypertrophy nor does its absence attenuate the hypertrophic response.

transgenic mouse; LacZ; cell signaling


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

CARDIAC HYPERTROPHY represents a complex adaptation of the terminally differentiated adult cardiocyte to an imposed mechanical or neurohumoral stimulus. Despite decades of study, the precise cellular signals that are responsible for the initiation and maintenance of this adaptation are poorly understood. Recent attention has focused on the permissive role of chronic activation of regulatory kinases including pathways linked to G protein activation (8, 18, 26) and involving calcineurin, calcium/calmodulin-dependent protein kinase (CaMK), and protein kinase C (PKC), among others. Activation of many of these processes has been shown to be sufficient to affect features of the hypertrophic phenotype in adult cardiocytes, and cardiac-restricted gain-of-function studies using transgenic mouse models have further established these links (8).

Activation of PKC and in particular PKC-beta in the cardiocyte has been associated with transcriptional transactivation of atrial natriuretic factor (ANF) and beta -myosin heavy chains (beta -MHC), altered calcium transients (4), impaired unloaded cell shortening, and gradual onset of left ventricular hypertrophy in the intact murine heart (7, 23). The hearts of humans and animals with end-stage heart failure are characterized by activation of this PKC isoform (3, 21). Moreover, chemical blockade of PKC-beta in isolated cells tends to restore normal rates of cell shortening and calcium cycling. Taken together, these studies strongly suggest that chronic activation of PKC-beta in the heart is sufficient to induce a pathological phenotype, but the studies do not establish whether or not activation of PKC-beta is necessary for such an adaptation to occur. This is a critical issue that must be resolved especially because agents that block PKC-beta have been suggested as possible therapies for chronic cardiac diseases (3, 15, 25). To approach this question, we have determined whether hearts from mice with targeted disruption (knockout, KO) of the PKC-beta gene (16) are able to manifest a cardiac hypertrophic response to mechanical and pharmacological stimuli.

Our data demonstrate that PKC-beta is not necessary for the cardiac hypertrophic response and suggest that redundant signaling pathways are responsible for cardiac adaptation and remodeling.


    METHODS
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Animal Care

All animals were bred and maintained in a temperature-controlled room. The mice were provided with food and water ad libitum. All animals used were males, ages 8-12 wk.

Generation of PKC-beta KO mice. Production of mice with a null mutation for PKC-beta was described previously (16). These animals are homozygous for the PKC-beta KO and are a mixed background of strains 129/Ola and C57Bl/6 mice. There was no significant difference in body weights between wild-type (C57Bl/6) and transgenic animals.

Induction of cardiac hypertrophy. Hypertrophy was induced in all animals by two different mechanisms: pharmacology and pressure overload. Pharmacological hypertrophy was induced by subcutaneous injections of phenylephrine (Phe) (22). Control and transgenic mice were given subcutaneous injections of Phe for 3 days (22). This level of Phe is toxic to rats but not to mice. Pressure-overloaded hearts were established by constriction of the transverse aorta as previously described (20). Once the animals were intubated and ventilated, a thoracotomy at the second intercostal space and the ascending aorta was dissected free of surrounding tissue. The transverse aorta was banded with 4-0 silk suture placed around extruded polyethylene-50 (PE-50) tubing next to the aorta; when the suture was tied, the tubing was removed, leaving constriction of ~30% of the original lumen diameter. The chest cavity was sutured closed in three layers and negative pressure was reestablished, allowing the animal to recover and breathe on its own. The animals were maintained for 7-10 days at which point they were killed by cervical dislocation. Their hearts were removed to measure the extent of hypertrophy and then frozen in liquid nitrogen for subsequent biochemical analyses. Histologic sections were obtained from each group that demonstrate an increase in myocyte size as a result of Phe stimulation and aortic banding (data not shown).

Biochemical Analysis

Western analysis. Tissue samples were homogenized in ice-cold sucrose buffer (in mM: 320 sucrose, 20 Tris · HCl, 2 EDTA, 10 EGTA, 10 µl/ml beta -mercaptoethanol, 0.3 phenylmethylsulfonyl fluoride; and 20 µg/ml of leupeptin) with protease inhibitors. This total homogenate was assayed spectrophotometrically (Bradford assay) for protein content. Equal amounts of protein were isolated and tricarboxylic acid was extracted and then resuspended in SDS sample buffer. In addition to this total PKC sample, the homogenate was fractionated by differential centrifugation into cytosolic and particulate fractions. Protein (60-80 µg) was loaded onto 10% acrylamide gels to separate the constituent proteins. PKC-purified enzymes (positive control) and molecular weight markers were run in parallel. The proteins were transferred to nitrocellulose using a semidry blot apparatus (Bio-Rad). The membranes were blocked overnight at 4°C in 5% dry milk and the primary antibody was applied for 1-2 h at room temperature. PKC polyclonal antibodies for the alpha -, beta -, delta - and epsilon -isoforms (1:3,000 dilution, Santa Cruz; and 1:1,000 dilution, Transduction Laboratories) were used. After a 30-min wash, the membrane was probed with secondary antibody (1:10,000-1:20,000 dilution) conjugated to horseradish peroxidase for 1 h at room temperature. The membrane was then washed for 30 min and exposed to chemiluminesence reagents (Pierce) for 10 min and developed on radiographic film. Semiquantitative densitometric analysis was performed using the Bio-Rad Gel Doc system.

Northern blot analysis. ANF mRNA was used as a marker of cardiac hypertrophy. Atrial and ventricular tissues were isolated from freshly excised hearts of both control and hypertrophied wild-type and transgenic mice. RNA was isolated using TRIzol reagent (GIBCO-BRL) according to the protocol supplied by the manufacturer. The cDNA probes for ANF and GAPDH have been previously published and 32P labeling was done using the Stratagene primer kit. A GAPDH probe was used for normalization and was prepared in the same manner.

PKC-beta beta -galactosidase fusion protein expression. Generation of the PKC-beta KO mice is described elsewhere in detail (16). In brief, the second exon of the PKC-beta gene was disrupted by homologous recombination with a LacZ-neo cassette. This insertion provides an endogenous marker of PKC-beta expression by quantification of LacZ expression. Because LacZ is not expressed in nontransgenic tissue, the background signal is low and therefore is very sensitive to defining PKC-beta expression. Hearts and brains harvested from animals subjected to hypertrophic stimuli were infused with paraformaldehyde, isolated, and frozen. The tissue was cryosectioned (5-µm sections were acquired). Tissue sections were stained for beta -galactosidase activity using standard techniques. LacZ mRNA levels were measured by standard RT-PCR protocols (GeneAMP, Perkin-Elmer) using primers designed to specifically amplify the LacZ cassette used for gene disruption.

In situ hemodynamics. For in situ measurements, the animals were anesthetized with an initial dose of methoxyflurane (Metofane) and sedation was maintained after intubation with 1% isoflurane using a Harvard small-animal ventilator (respiratory rate 2 Hz, respiratory volume, 0.1-0.03 ml). Body temperature was monitored using a rectal thermistor and maintained at 39°C using a warming blanket placed under the animal. An incision lateral to the right carotid was made and the carotid artery was exposed. The vessel was opened and a 1.4-Fr Millar pressure transducer was passed through the carotid artery and into the left ventricle. Arterial pressure and waveform was measured before the heart was entered; after this the ventricular waveform was obtained, which indicated the location of the transducer. Baseline hemodynamic measurements of wild-type and transgenic mice were obtained in this manner. All data were acquired and analyzed through the Gould/Ponemah system (Gould, Columbus, OH).

Statistics. The are the sample means ± SD. The results were compared by an unpaired Student's t-test with the significance established at P < 0.05.


    RESULTS
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Characterization of PKC-beta KO hearts: PKC expression and compensation. Figure 1 shows a composite Western blot from extracts of normal and transgenic hearts. Each lane has similar amounts of total protein loaded. The level of expression was measured to investigate the effect of the null mutation of PKC-beta on the expression of the other PKC isoforms. There was no significant difference between wild-type and transgenic mice in the expression of PKC-alpha or PKC-epsilon (the major isoforms expressed in the heart) as a result of the absence of PKC-beta in the transgenic hearts. Although expressed in lower amounts, PKC-delta was also examined and there was no change in expression level compared with the wild-type heart.


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Fig. 1.   Protein kinase C (PKC) isoform expression in mouse hearts. Western analysis of total heart homogenates from wild-type (Wt) and PKC-beta knockout (KO) mice. Owing to low expression of PKC in murine hearts, 70 µg of protein per lane were loaded. Purified PKC enzymes were loaded as positive controls for specificity and as molecular weight markers.

In situ hemodynamics. Pooled hemodynamic data from several animals are shown in Table 1. There was no significant difference in either carotid or intraventricular pressure between wild-type and transgenic animals. The pressures measured were expected and similar to those seen in other studies involving mice (4, 20). The wild-type mouse had a carotid pressure of 91 ± 8 mmHg and an intraventricular pressure of 109 ± 2 mmHg (n = 6) compared with the transgenic mice, which had a carotid pressure of 99 ± 6 mmHg and an intraventricular pressure of 103 ± 5 mmHg (n = 6).

                              
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Table 1.   In situ baseline hemodynamic measurements

Development of cardiac hypertrophy. The mice used in this study were subjected to hypertrophic stimuli to examine the effect of targeted PKC-beta disruption on cardiac hypertrophy. The wild-type heart weight-to-body weight ratio (HW/BW) was 4.1 ± 0.2 and the PKC-beta KO HW/BW was 4.3 ± 0.6; this indicated no baseline hypertrophy due to the PKC-beta deficiency (Table 2). Cardiac hypertrophy was induced by two different mechanisms: alpha -adrenergic stimulation via Phe injection and pressure overload via aortic banding. The hearts were removed and the wet weights were obtained. Both groups of animals had significant increases in heart weight (Table 2).

                              
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Table 2.   Hypertrophic response to pharmacological stimuli and pressure overload

The ability of Phe to activate PKC, as demonstrated by PKC-epsilon translocation, is illustrated in Fig. 2. PKC-epsilon expression in the membrane fraction increased 35% compared with baseline saline-treated hearts. Acute physiological response to this dose of Phe is shown in Fig. 3. There is an overall stimulation of cardiac performance with heart rate returning to near-baseline levels at 15 min postinjection; however, the left ventricular pressure and contractility indexes remain elevated for a prolonged period. Hearts were examined for gross necrosis and edema and none was observed. Gross edema was also ruled out by measuring dry tissue weights, which showed conservation of an increase in HW/BW independent of being derived by wet or dry weight. Cellular hypertrophy was confirmed by histological assessment (data not shown).


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Fig. 2.   PKC activation via phenylephrine (Phe) stimulation in PKC-beta KO hearts. Western blot of cardiac homogenate fractions from transgenic mice treated with saline or Phe subcutaneous injections. Stimulation of PKC-epsilon translocation from cytosolic to membrane fractions (35% increase over baseline membrane fraction) indicates that PKC activation is maintained in hypertrophied PKC-beta -deficient hearts.



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Fig. 3.   Acute cardiac response to subcutaneous Phe dose. In vivo cardiac performance indicators (isoflurane anesthesia) obtained from PKC-beta KO mice using a 1.4-Fr Millar pressure transducer: heart rate and developed pressure responses to Phe (top) and effect on contractility (bottom). Values are means ± SD; n = 4 for each group.

Pressure overload-induced cardiac hypertrophy. Banding of the aorta in control and PKC-beta KO mice was performed to induce a hypertrophic response (Table 2). After 7 days of coarctation, there was a measurable increase in cardiac mass in both wild-type (HW/BW, 4.1 ± 0.2 at baseline and 5.1 ± 0.1 banded) and PKC-beta KO mice (HW/BW, 4.0 ± 0.1 at baseline and 5.8 ± 0.6 banded). There was no significant change in body weight for any of the animals in either group. The animals that survived the banding represent ~65% of those banded. Those animals that did not survive the surgery died 3-4 days postbanding.

As a control for the extent of pressure overload, carotid pressures were measured on a sample of banded animals. A pressure gradient of 50-70 mmHg was measured across the coarctation, indicating that the constriction was intact (Fig. 4).


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Fig. 4.   Aortic pressure gradient across coarctation: representative pressure measurements from right and left carotid arteries before (top) and after (bottom) aortic banding are shown. Banding procedure used produced on average a 50- to 70-mmHg pressure gradient.

PKC-beta expression via beta -galactosidase and RT-PCR. Brain and cardiac ventricular tissue sections were obtained from hypertrophic PKC-beta KO animals. beta -Galactosidase staining was readily apparent in the brain sections and was primarily localized to the hippocampus (Fig. 5). In comparison, there was no detectable beta -galactosidase staining in the heart sections (data not shown). However, the RT-PCR results in Fig. 5 indicate expression of LacZ mRNA in hearts and brains from PKC-beta KO and control animals. Lanes 1-3 are from wild-type animals and show the absence of LacZ message in both brain and heart tissue. In contrast, lanes 4-7 are from PKC-beta KO brain and heart tissue and show positive expression of the LacZ message, thereby confirming the expression of PKC-beta in the adult murine myocardium. Lane 8 is a positive control for the RT-PCR method and the results from the same PKC-beta KO heart sample minus the reverse transcriptase used in lane 6 are shown in lane 9. The presence of signal in lane 8 and the absence of signal in lane 9 demonstrate that the results are due to RT of mRNA and not contaminating DNA. The high sensitivity of this technique provided a positive signal in the ventricle and brain tissue of PKC-beta KO animals, but no signal was seen in wild-type tissue.


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Fig. 5.   LacZ as an indicator of PKC-beta expression. Histological slices of brain obtained from a PKC-beta KO mouse injected with Phe stained positive with beta -galactosidase in the region of the hippocampus (top). RT-PCR of LacZ expression in cardiac and brain tissue (bottom) illustrates the absence of mRNA in nontransgenic tissue (heart, lanes 1 and 2, and brain, lane 3) and the presence of mRNA in transgenic tissue (brain, lanes 4 and 5, and heart, lanes 6 and 7). Lane 8 is a positive control for the RT reaction and lane 9 is the transgenic heart from lane 6 minus the reverse transcriptase.

Hypertrophic marker: ANF. Figure 6 illustrates the expression of ANF in the ventricle of the mouse heart in response to isoproterenol and Phe. Figure 6 (left) illustrates the ANF expression from the wild-type ventricle. The left lane demonstrates that there is a very low level of ANF expression in the ventricle under normal conditions. When the ventricle is stimulated to hypertrophy, there is an increase in ANF expression due to both isoflurane (14 ± 2%) and Phe (12 ± 3%) stimulation. Figure 6 (right) shows that the increase in ANF expression in the ventricle of the PKC-beta KO heart recapitulates the wild-type response to isoflurane (15 ± 2%) and Phe (16 ± 5%). These results indicate that the correlation of cardiac hypertrophy and ANF mRNA is conserved in mice lacking PKC-beta expression.


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Fig. 6.   Atrial natriuretic factor (ANF) mRNA expression in response to hypertrophy in wild-type and PKC-beta KO hearts exposed to either saline (C), isoproterenol (Iso), or Phe. Ventricular apex samples were used to minimize signal from atrial tissue. GAPDH was probed as a loading control. ANF mRNA expression increased with pharmacological stimulation in the PKC-beta KO hearts similar to the wild-type hearts.


    DISCUSSION
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

The development and persistence of cardiac hypertrophy under a multiplicity of conditions involves a cascade of cellular events including G proteins, protein kinases, and transcription factors, many of which interact in a temporal and spatial manner resulting in a complex regulatory network. The importance of PKC in the modulation of cellular processes has been clearly delineated in several model systems (1-5) and specifically in the heart, the regulation of PKC expression has been shown to be involved in protection from ischemia (24) as well as in the induction of cardiac hypertrophy (4). Only recently has the specific role of particular PKC isoforms in the heart been investigated (4, 12, 24). Attention has centered on PKC-epsilon , which has been implicated in the preservation of function during ischemic preconditioning in the adult heart and in phosphorylation of troponin I. The role of PKC-beta is less well defined (especially in the adult heart) although its expression has been linked to cardiac hypertrophy in animals (4) and to heart failure in humans (3). The goal of the present work was twofold: to ascertain the extent of PKC-beta expression in the adult murine myocardium and to establish whether expression of PKC-beta is necessary for the cardiac hypertrophic response. The major results of the study were the following: 1) using a PKC-beta KO mouse model, we have established that expression of PKC-beta is not necessary for the development of both quantitative and qualitative features of cardiac hypertrophy; and 2) PKC-beta is expressed in the adult mouse heart albeit at lower levels than in other species.

Characterization of PKC-beta KO hearts. For these studies we chose to use a PKC-beta gene KO transgenic mouse model. This approach avoids the question of chemical inhibitor specificity and allows the investigation of specific PKC isoform function. The integrity of the mutated PKC-beta transgene was checked for its ability to be activated by endogenous transcription signals. The brain has an abundant amount of PKC-beta (14, 17), and therefore the brains of the PKC-beta KO mice should express a high level of LacZ and be amenable to beta -galactosidase staining. Figure 5 illustrates beta -galactosidase staining of brain tissue from a PKC-beta KO mouse. The longitudinal section clearly shows specific expression of the LacZ gene in the hippocampus of the brain. This indicates that the transcription factors for the PKC-beta gene are functional, and therefore LacZ is expressed in the same manner as PKC-beta in the brain. This is the same in the myocardium as demonstrated by the RT-PCR results in Fig. 5. These data are significant because PKC-beta expression in the adult myocardium has been linked to heart failure (3), although the positive LacZ results indicate that it is also present under basal conditions. The baseline hemodynamics of these mice were measured to rule out any predisposition toward the development of cardiac hypertrophy and/or any compensatory physiological adaptation due to the absence of the gene. Because PKC-beta has been shown to be expressed in arterial smooth muscle, it was conceivable that gene ablation might have altered arterial resistance. As shown in Table 1, there was no overt baseline hemodynamic phenotype present in these mice. There was no difference between the wild-type and PKC-beta KO control hearts in terms of heart rate, aortic pressure, or left ventricular pressure and contractility. These data preclude the hypothesis that there is an alteration in baseline hemodynamics that would predispose these hearts to develop heart failure. It does not, however, exclude a predisposition to an enhanced response to hypertrophic stimuli.

One possible explanation of the lack of phenotype at baseline might be the compensatory change in the expression of other isoforms as has been described for structural and contractile proteins in the heart. It has been observed that with overexpression of structural/contractile proteins there is usually downregulation of the wild-type protein as a mechanism of compensation. In addition to PKC-beta , the other classical calcium-dependent PKC expressed in the heart is PKC-alpha . Figure 1 illustrates that this isoform is expressed in both wild-type and PKC-beta KO murine hearts, and there appears to be equivalent expression of this isoform. Additionally two members of the novel class of PKCs, PKC-delta and PKC-epsilon , were identified in hearts. Similar to the results for PKC-alpha , there was no change in the expression level of either PKC-delta or PKC-epsilon in the KO hearts. A lack of expression adaptation is not indicative of a defect in the PKC system as a whole. The PKC system in these mice is still active and will respond to activation via the classic mechanism of protein translocation. When activated, PKC (PKC-epsilon in particular) translocated from the cytosolic to the membrane fraction as illustrated in Fig. 2. When the amount of each isoform was quantitated by reference to a known purified protein standard, it was apparent that small amounts of PKC-beta were expressed relative to the other two isoforms, so it is conceivable that small changes might have been masked. Although not all of the PKC isoforms were measured in these hearts, it is apparent from the data represented here that there was no overt compensatory response to the absence of PKC-beta in the heart. We focused on those PKC isoforms of known cardiac importance, and it is apparent that adult murine hearts haves a similar PKC-isoform profile yet not at the same level of expression that is seen in other species.

The reports that PKC-beta is upregulated in human heart failure as well as indications that PKC-beta overexpression causes cardiac hypertrophy in transgenic models suggest that activation of PKC-beta may be sufficient to effect the hypertrophic phenotype; however, the question of necessity remains unanswered. Because the PKC-beta gene has been knocked out in the mice used in these studies, this can be addressed by subjecting the animals to common and robust hypertrophic stimuli such as pharmacological and surgical interventions. This strategy was used because these two different stimuli initiate hypertrophic response via different mechanisms. The data clearly demonstrate no qualitative difference between wild-type and PKC-beta KO mice in response to these stimuli (Table 2). This reveals that PKC-beta expression is not necessary for developing cardiac hypertrophy.

This result does not exclude the presence of other, subtler phenotypic changes. Although morphologically similar, it is conceivable that the PKC-beta KO hypertrophic myocytes may differ in function from wild-type myocytes, particularly as relates to the suggested substrates of PKC-beta (ion channels, gene expression, and contraction) (6, 10, 11, 13, 19) and their role in the hypertrophic heart. To begin to address these issues, we looked at markers of gene expression and contractility in hypertrophic wild-type and PKC-beta KO hearts. ANF, a standard marker of hypertrophy, message was detected in the ventricle of hypertrophied hearts but not in the untreated hearts. This is similar to results seen in wild-type hearts and demonstrates that the classic transcriptional response to a hypertrophic stimuli was conserved in the PKC-beta KO hearts.

A final issue relates to the level of PKC expression and isoform distribution in the heart. This has been controversial, as some investigators have been able to detect PKC-beta in adult cardiocytes, whereas others have not. The discordant data may reflect technical issues related to specific antibodies and/or real species differences in isoform expression. Our lab and others have shown that in mice the isoform is present albeit at low levels. This discrepancy may reflect technical aspects and/or real species differences. We and others have established that the PKC-beta protein is present although at low levels because it is necessary to load 65-80 ug from a total-heart homogenate to define a clear signal via Western analysis. This is in contrast to data derived from other species including rat, rabbit, and dog. The low concentration of PKC in general in murine hearts raises questions about species differences, and it is conceivable that larger animals are more reliant on this signal messenger cascade to effect phenotypic changes than are smaller animals. It is not yet clear due to the limited number of studies currently in the literature, but it is clear that PKC has a role in the developmental process (9) and this may speak to the hierarchical role of the PKC signaling system in different species. Pasquet (19a) has demonstrated that when murine hearts are separated into myocyte and fibroblast fractions at various stages of development, the apparent decrease in PKC expression is actually an artifact that is corrected by loading an equivalent number of cells (as opposed to total protein). It is clear in these cell-fractionation studies that PKC-beta is expressed in the myocyte as well as in the fibroblasts, a result that is similar to that represented here (Fig. 1).

In summary, our data indicate that PKC-beta is not a necessary factor for the development of cardiac hypertrophy either in response to pharmacological stimuli or to pressure-overload methods. Although more "key" hypertrophic pathways and factors are being continuously elucidated, understanding the complexity of the cardiac hypertrophic response remains elusive and it appears to be the case that no single factor is necessary. This is an important observation especially as pharmacological techniques are proposed that target only unique points of regulation.


    ACKNOWLEDGEMENTS

The authors thank Paul H. Goldspink for help and discussions of the RT-PCR experiments and Ron D. McKinney for technical assistance.


    FOOTNOTES

This work is supported by National Institutes of Health Grant R01-15498 (to P. M. Buttrick).

Address for reprint requests and other correspondence: B. B. Roman, Dept. of Medicine, Section of Cardiology, Univ. of Illinois at Chicago, 840 S. Wood St. (M/C 787), Chicago, IL 60612 (E-mail: broman{at}uic.edu).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Received 30 May 2000; accepted in final form 8 December 2000.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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Am J Physiol Heart Circ Physiol 280(5):H2264-H2270
0363-6135/01 $5.00 Copyright © 2001 the American Physiological Society



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