Increased protein kinase C and isozyme redistribution in pressure-overload cardiac hypertrophy in the rat

The Raf-MEK-ERK cascade represents a common pathway for alteration of intracellular calcium by Ras and protein kinase C in cardiac myocytes

Ras and protein kinase C (PKC), which regulate the Raf-MEK-ERK cascade, may participate in the development of cardiac hypertrophy, a condition characterized by diminished and prolonged contractile calcium transients. To directly examine the influence of this pathway on intracellular calcium ([Ca2+]i), cardiac myocytes were cotransfected with effectors of this pathway and with green fluorescent protein, which allowed the living transfected myocytes to be identified and examined for [Ca2+]i via indo-1. Transfection with constitutively active Ras (Ha-RasV12) increased cell size, decreased expression of the myofibrils and the calcium-regulatory enzyme SERCA2, and reduced the magnitude and prolonged the decay phase of the contractile [Ca2+]i transients. Similar effects on [Ca2+]i were obtained with Ha-RasV12S35, a Ras mutant that selectively couples to Raf, and with constitutively active Raf. In contrast, Ha-RasV12C40, a Ras mutant that activates the phosphatidylinositol 3-kinase pathway, had a lesser effect. The PKC-activating phorbol ester, phorbol 12-myristate 13-acetate, also prolonged the contractile [Ca2+]i transients. Cotransfection with dnMEK inhibited the effects of Ha-RasV12, Raf, and phorbol 12-myristate 13-acetate on [Ca2+]i. The effects of Ha-RasV12 and Raf on [Ca2+]i were also counteracted by SERCA2 overexpression. Both Ras and PKC may thus regulate cardiac [Ca2+]i via the Raf-MEK-ERK cascade, and this pathway may represent a critical determinant of cardiac physiological function.

In vivo phosphorylation of cardiac troponin I by protein kinase Cbeta2 decreases cardiomyocyte calcium responsiveness and contractility in transgenic mouse hearts

Recently, it has been reported that the protein kinase C (PKC) beta isoform plays a critical role in the development of hypertrophy and heart failure. The purpose of the present study was to clarify the mechanism by which activation of PKCbeta led to depressed cardiac function. Thus, we used a PKCbeta2 overexpressing mouse, an animal model of heart failure, to examine mechanical properties and Ca2+ signals of isolated left ventricular cardiomyocytes. The percentage of shortening, rate of shortening, and rate of relengthening of cardiomyocytes were markedly reduced in PKCbeta2 overexpression mice compared to wild-type control mice, although the baseline level and amplitude of Ca2+ signals were similar. These findings suggested a decreased myofilament responsiveness to Ca2+ in transgenic hearts. Therefore, the incorporation of [32P] inorganic phosphate into cardiac myofibrillar proteins was studied in Langendorff-perfused hearts. There was a significant increase in the degree of phosphorylation of troponin I in PKCbeta2-overexpressing transgenic mice. The depressed cardiomyocyte function improved after the superfusion of a PKCbeta selective inhibitor. These findings indicate that in vivo PKCbeta2-mediated phosphorylation of troponin I may decrease myofilament Ca2+ responsiveness, and thus causes cardiomyocyte dysfunction. Since chronic and excess activation of PKCbeta2 plays a direct and contributory role in the progression of cardiac dysfunction, the PKCbeta selective inhibitor may provide a new therapeutic modality in the setting of heart failure.

Troponin I phosphorylation and myofilament calcium sensitivity during decompensated cardiac hypertrophy

We have measured myocyte cell shortening, troponin-I (Tn-I) phosphorylation, Ca2+ dependence of actomyosin adenosinetriphosphatase (ATPase) activity, adenosine 3',5'-cyclic monophosphate (cAMP) levels, and myofibrillar isoform expression in the spontaneously hypertensive rat (SHR) during decompensated cardiac hypertrophy (76 wk old) and in age-matched Wistar-Kyoto rat (WKY) controls. The decreased inotropic response to beta-adrenergic stimulation previously observed in myocytes from 26-wk-old SHR was further reduced at 76 wk of age. In response to beta-adrenergic stimulation, Tn-I phosphorylation was greater in the 76-wk-old SHR than in the WKY, although cAMP-dependent protein kinase A (PKA)-dependent Tn-I phosphorylation in the SHR did not increase with progression from compensated (26 wk) to decompensated (76 wk) hypertrophy. We also observed a dissociation between the increased PKA-dependent Tn-I phosphorylation and decreased cAMP levels in the 76-wk-old SHR versus WKY during beta-adrenergic stimulation. Baseline Tn-I phosphorylation was significantly reduced in 76-wk-old SHR versus WKY and was associated with decreased basal cAMP levels and increased Ca2+ sensitivity of actomyosin ATPase activity. The change in myofilament Ca2+ sensitivity during beta-adrenergic stimulation in the 76-wk-old SHR (0.65 pCa units) was over twofold greater than in the 76-wk-old WKY (0.30 pCa units). We also determined whether embryonic troponin T isoforms were reexpressed in decompensated hypertrophy and observed significant reexpression of the embryonic cardiac troponin T isoforms in the 76-wk-old SHR. The significant decrease in Ca2+ sensitivity with beta-adrenergic stimulation in 76-wk-old SHR may contribute to the severely impaired inotropic response during decompensated hypertrophy in the SHR.

Changes in protein kinase C in early cardiomyopathy and in gracilis muscle in the BB/Wor diabetic rat

Hyperglycemia can upregulate protein kinase C (PKC), which may be an important mediator of the progression from normal heart and muscle function to diabetic myopathy in the myocardium and skeletal muscle in type 1 insulin-dependent diabetes mellitus (IDM). We evaluated this possibility during the early stage of IDM in BB/Wor diabetic (D) rats and age-matched BB/Wor diabetes-resistant (DR) rats. Interventricular septal thickness, E wave peak velocity of tricuspid inflow (both minimum and maximum), and left ventricular (LV) weight index were increased, and the rate of change in LV pressure (LV dP/dt) decreased in D rats subjected to M-mode and two-dimensional echocardiography and hemodynamic recording of heart rate, LV pressure (LVP), + LV dP/dt, -LV dP/dt, and LV end-diastolic pressure (LVEDP) in vivo and in vitro 41 days after the onset of hyperglycemia. Whole ventricle basal PKC activity was increased by 44.4 and 18.4% in the particulate and soluble fractions, respectively, from D rats compared with that from DR rats using r-32P phosphorylation of appropriate peptide substrates. When measured by Western blot gel densitometry, particulate PKC-alpha and PKC-delta content increased by 89 and 24%, respectively, but soluble PKC-beta and soluble and particulate PKC-epsilon were unchanged compared with that of DR rats. Similarly, gracilis muscle PKC activity and PKC-alpha and PKC-delta were elevated in the gracilis muscle, whereas that of the circulating neutrophil did not differ between the D and DR rats. Thus, in vivo, the early diabetic cardiomyopathy of the D rat is characterized by a restrictive LV with increased septal thickness and is associated with elevated PKC activity and increased amounts of myocardial particulate PKC-alpha and PKC-delta, which are also seen in the skeletal muscle. We conclude that increased PKC isozymes may play a pivotal role during IDM in the development of diabetic cardiomyopathy and skeletal muscle myopathy.

Expression of protein kinase C beta in the heart causes hypertrophy in adult mice and sudden death in neonates

Protein kinase C (PKC) activation in the heart has been linked to a hypertrophic phenotype and to processes that influence contractile function. To establish whether PKC activation is sufficient to induce an abnormal phenotype, PKCbeta was conditionally expressed in cardiomyocytes of transgenic mice. Transgene expression in adults caused mild and progressive ventricular hypertrophy associated with impaired diastolic relaxation, whereas expression in newborns caused sudden death associated with marked abnormalities in the regulation of intracellular calcium. Thus, the PKC signaling pathway in cardiocytes has different effects depending on the timing of expression and, in the adult, is sufficient to induce pathologic hypertrophy.

Effects of adrenergic and muscarinic agonist stimulation on IP3 and cyclic nucleotide levels in the pressure overloaded rat heart

In this study, the dynamic interrelationships between myocardial functional state and changes in the second messenger content in pressure-overloaded hypertrophied hearts were investigated. Forty-three rat hearts were used after partial clamping of the abdominal aorta. The isolated hearts were perfused with Krebs-Henseleit buffer and allocated to perfusion for 20 s or 40 min as controls (n = 12); or with noradrenaline (10(-6) mol l-1, n = 11); carbachol (3 x 10(-7) mol l-1, n = 9); or noradrenaline plus carbachol (10(-6) mol l-1 + 3 x 10(-7) mol l-1, respectively, n = 11). maxdP/dt increased more than 2-fold already after 20 s on noradrenaline stimulation, followed by a significant increase in cAMP. After 40 min, maxdP/dt was lower than the maximal value, although higher than controls. cAMP was also decreased, but still significantly higher than controls. Perfusion with noradrenaline plus carbachol produced the same changes in maxdP/dt as those seen after noradrenaline stimulation alone, but failed to increase cAMP content after both 20 s and 40 min. The inositol trisphosphate (IP3) content was increased 40 min of control perfusion (p < 0.05). Noradrenaline and carbachol, separately, produced an increase in IP3 content already after 20 s (p < 0.05). The combination of noradrenaline plus carbachol also produced an increase of IP3 (p < 0.05; compared to controls), but to a lesser extent when compared either to noradrenaline or carbachol (p < 0.05). After 40 min of perfusion, IP3 was in the same range regardless of added agonist(s) and still slightly above control level (p < 0.05). The early increase in maxdP/dt induced by noradrenaline or the combination of noradrenaline plus carbachol was not paralleled by a decrease in ATP content. This was also the case upon addition of carbachol alone. However, after 40 min of agonistic perfusion, ATP levels were substantially decreased. In conclusion, myocardial IP3 content in pressure-overloaded hypertrophied hearts was not different from that of sham-operated hearts. After agonistic stimulation, an early increase in IP3 formation was seen. Attenuation of the IP3 response by combined stimulation with noradrenaline and carbachol was initially present in pressure-overloaded hypertrophied hearts. After 40 min no attenuation was found for either IP3 or for cAMP content, suggestive of induction of a desensitization.

Transgenic Galphaq overexpression induces cardiac contractile failure in mice

The critical cell signals that trigger cardiac hypertrophy and regulate the transition to heart failure are not known. To determine the role of Galphaq-mediated signaling pathways in these events, transgenic mice were constructed that overexpressed wild-type Galphaq in the heart using the alpha-myosin heavy chain promoter. Two-fold overexpression of Galphaq showed no detectable effects, whereas 4-fold overexpression resulted in increased heart weight and myocyte size along with marked increases in atrial naturietic factor ( approximately 55-fold), beta-myosin heavy chain ( approximately 8-fold), and alpha-skeletal actin ( approximately 8-fold) expression, and decreased ( approximately 3-fold) beta-adrenergic receptor-stimulated adenylyl cyclase activity. All of these signals have been considered markers of hypertrophy or failure in other experimental systems or human heart failure. Echocardiography and in vivo cardiac hemodynamic studies indeed revealed impaired intrinsic contractility manifested as decreased fractional shortening (19 +/- 2% vs. 41 +/- 3%), dP/dt max, a negative force-frequency response, an altered Starling relationship, and blunted contractile responses to the beta-adrenergic agonist dobutamine. At higher levels of Galphaq overexpression, frank cardiac decompensation occurred in 3 of 6 animals with development of biventricular failure, pulmonary congestion, and death. The element within the pathway that appeared to be critical for these events was activation of protein kinase Cepsilon. Interestingly, mitogen-activated protein kinase, which is postulated by some to be important in the hypertrophy program, was not activated. The Galphaq overexpressor exhibits a biochemical and physiologic phenotype resembling both the compensated and decompensated phases of human cardiac hypertrophy and suggests a common mechanism for their pathogenesis.

Translocation of protein kinase C-alpha, delta and epsilon isoforms in ischemic rat heart

To explore the spatial and temporal localization of PKC isoforms during ischemia, we quantified PKC isoforms in the subcellular fractions in perfused rat heart by immunoblotting using specific antibodies against PKC isoforms. PKCs-alpha and epsilon translocated from the 100000 x g supernatant (S, cytosolic) fraction to the 1000 x g pellet (PI, nucleus-myofibril) and the 1000-100000 x g pellet (P2, membrane) fractions during 5-40 min of ischemia. PKC-delta redistributed from the P2 to the S fraction. A 50-kDa fragment of PKC-alpha appeared during ischemia possibly through calpain action. Immunohistochemical observations showed the different localizations of PKC-alpha, delta, and epsilon in the myocytes. The PKC assay displayed high basal levels of Ca(2+)-independent PKC, the activation of Ca(2+)-dependent PKC in the P1 and P2 fractions, and the activation of Ca(2+)-independent PKC in the P1 fraction after 20 min of ischemia. These observations show that ischemia induces different patterns of translocation of the three PKC isoforms, suggesting differences in their roles.

Stress-induced cardioadaptation reveals a code linking hormone receptors and spatial redistribution of PKC isoforms

Extracellular agents, including growth factors, cytokines and hormones, transmit their information into cells utilizing a balanced mosaic of intracellular phosphatases and kinases. How do these agonists select the correct substrates and modify them in order to produce defined physiological responses? Our studies have centered on the mechanisms of stress-induced cardioprotection (preconditioning) against postischemic dysfunction. In several species, the ischemia-reperfusion resistant phenotype appears to be induced by metabotropic-receptor pathways linked to PKC. Our results on the isolated rat heart show that each protective stimulus involves a characteristic mosaic of PKC isoforms, translocating into distinct cellular compartments. The distinct receptor-stimulated PKC isoform profile engaged by each extracellular metabotropic agent could allow the heart several overlapping modes of phenotypic adaptation to ischemia.

Myocardial contractile response and IP3, cAMP and cGMP interrelationships

An experimental study in the perfused working normal and pressure overloaded rat heart. A mini review based on a doctoral thesis.

Effects of endothelin on mitogen-activated protein kinase activity and protein synthesis in isolated adult feline cardiac myocytes

The growth-promoting effects of endothelin-1 (ET-1) were examined in adult heart cells. The activity of mitogen-activated protein kinases (MAPKs) was measured in cytosolic extracts of isolated adult feline cardiac myocytes incubated with and without ET-1. Kinase activity was assessed by phosphorylation of the exogenous substrate, myelin basic protein. ET-1 stimulated the activity of MAPK up to 4-fold, with peak activation occurring between five and ten minutes after addition of ET-1. Polyclonal antisera raised against a 14-amino acid sequence of the erk-2 gene product, a MAPK isoform, identified two major bands in cytosolic extracts of the cardiac myocytes. Partial purification of kinase activities using Mono Q anion-exchange chromatography demonstrated two major peaks of myelin basic protein kinase activity. Subsequent immunoblots of the eluted fractions demonstrated that the immunoreactive bands observed in the cytosolic extracts eluted in those fractions possessing kinase activity. Overnight pretreatment of the cardiac myocytes with 100 ng/ml pertussis toxin inhibited the ET-1 stimulated increase in MAPK activity by 50 - 70%, but did not alter stimulation by 100 nM phorbol 12-myristate 13-acetate (PMA). These data suggest that stimulation of MAPK by ET-1 may be mediated by more than one pathway. MAPK has been shown to be activated in the intracellular transmission of growth factor signals. Indicative of a growth effect in this adult heart cell model, myocytes exposed to increasing concentrations of ET-1 demonstrated a dose dependent increase in [3H]-phenylalanine incorporation into cellular protein. This response was blocked by staurosporine and partially inhibited by pretreatment with pertussis toxin, again suggesting the possible involvement of multiple early signals. These data from isolated adult cardiac myocytes further support the hypothesis that ET-1 has a role in the regulation of cardiac growth.