Diversity of the protein kinase C gene family Implications for cardiovascular disease

Crosstalk between PKC and MAPK pathway activation in cardiac fibroblasts in a rat model of atrial fibrillation

OBJECTIVE

Atrial fibrillation (AF) is the most frequent form of cardiac arrhythmia and major cause of cardiac ischemia. Defective calcium homeostasis due to anomalous expression of ryanodine receptor type 2 (RyR2) or its hyperactivation by phosphorylation by serine threonine kinases has been implicated as a central mechanism of AF pathogenesis. Given the role of protein kinase C (PKC) isoforms in cardiac function we investigated role of PKC in AF using a rat model.

RESULTS

PMA induced global increase in protein synthesis in cardiac fibroblasts isolated from AF rats, but not healthy controls, and the increase was inhibited by PKC inhibition. PMA mediated activation of both PKC and ERK and either inhibition of PKC by Go6983 or ERK by the MEK inhibitor Trametinib attenuated both P-ERK and P-PKC in both cardiac fibroblasts isolated from AF rats or from healthy rats but transduced with PKC-delta. The PKC and ERK mediated induction of global protein synthesis was found to be mediated by increased phosphorylation of the ribosomal protein S6.

CONCLUSION

Our findings provide a foundation for future testing of PKC and MEK inhibitors to treat AF in pre-clinical models. It also needs to be determined if PKC and MAPK pathway activation is functioning via RyR2 or some yet undefined substrates.

Protein kinase C and cardiac dysfunction: a review

Heart failure (HF) is a physiological state in which cardiac output is insufficient to meet the needs of the body. It is a clinical syndrome characterized by impaired ability of the left ventricle to either fill or eject blood efficiently. HF is a disease of multiple aetiologies leading to progressive cardiac dysfunction and it is the leading cause of deaths in both developed and developing countries. HF is responsible for about 73,000 deaths in the UK each year. In the USA, HF affects 5.8 million people and 550,000 new cases are diagnosed annually. Cardiac remodelling (CD), which plays an important role in pathogenesis of HF, is viewed as stress response to an index event such as myocardial ischaemia or imposition of mechanical load leading to a series of structural and functional changes in the viable myocardium. Protein kinase C (PKC) isozymes are a family of serine/threonine kinases. PKC is a central enzyme in the regulation of growth, hypertrophy, and mediators of signal transduction pathways. In response to circulating hormones, activation of PKC triggers a multitude of intracellular events influencing multiple physiological processes in the heart, including heart rate, contraction, and relaxation. Recent research implicates PKC activation in the pathophysiology of a number of cardiovascular disease states. Few reports are available that examine PKC in normal and diseased human hearts. This review describes the structure, functions, and distribution of PKCs in the healthy and diseased heart with emphasis on the human heart and, also importantly, their regulation in heart failure.

Role of PKC in autocrine regulation of rat ventricular K+ currents by angiotensin and endothelin

Transient and sustained K(+) currents were measured in isolated rat ventricular myocytes obtained from control, steptozotocin-induced (Type 1) diabetic, and hypothyroid rats. Both currents, attenuated by the endocrine abnormalities, were significantly augmented by in vitro incubation (>6 h) with the angiotensin-converting enzyme inhibitor quinapril or the angiotensin II (ANG II) receptor blocker saralasin. Western blots indicated a parallel increase in Kv4.2 and Kv1.2, channel proteins that underlie the transient and (part of the) sustained currents. Under diabetic and hypothyroid conditions, both currents were also augmented by an endothelin receptor blocker (PD142893) or by an endothelin-converting enzyme inhibitor. Kv4.2 density was also enhanced by PD142893. Incubation (>5 h) with the PKC inhibitor bis-indolylmaleimide augmented both currents, whereas the PKC activator dioctanoyl-rac-glycerol (DiC8) prevented the augmentation of currents by quinapril. DiC8 also prevented the augmentation of Kv4.2 density by quinapril. Specific peptides that activate PKC translocation indicated that PKC-epsilon and not PKC-delta is involved in ANG II action on these currents. In control myocytes, quinapril and PD142893 augmented the sustained late current but had no effect on peak current. It is concluded that an autocrine release of angiotensin and endothelin in diabetic and hypothyroid conditions attenuates K(+) currents by suppressing the synthesis of some K(+) channel proteins, with the effects mediated at least partially by PKC-epsilon.

Transmural differences in rat ventricular protein kinase C epsilon correlate with its functional regulation of a transient cardiac K+ current

The effects of PKC activation on a transient (It) and a sustained (Iss) cardiac K+ current and the subcellular distribution of the epsilon isoform of PKC (PKC(epsilon)) were compared in epicardial and endocardial regions of the rat ventricle. Activation of PKC(epsilon) with a diacylglycerol analogue (di-octanoyl-glycerol (DiC8), 20 (mu)M) leads to differential effects in epicardial and endocardial cells. In epicardial cells (n = 20) It and Iss are attenuated by 17.7 +/- 2.1 % and 11.9 +/- 3.1 %, respectively (means +/- S.E.M.). In endocardial cells It attenuation was significantly smaller (4.6 +/- 1.6 %, n = 14, P < 0.0005). Iss attenuation was similar to that in epicardial cells (10.5 +/- 3.8 %). PKC[epsilon] expression was measured by Western blotting. Calculated endocardial/epicardial ratios showed no regional differences in total protein extracts (1.04 +/- 0.11, mean +/- S.E.M, n = 4), but PKC[epsilon] distribution in the cytosolic fraction showed a marked difference, with significantly (P < 0.05) higher levels in endocardial extracts. The cytosolic endocardial/epicardial PKC[epsilon] ratio was 2.64 +/- 0.24 (n = 4), indicating a reduced amount of PKC[epsilon] in the membrane fraction of the endocardium. This could account for the reduced effect of DiC8 on It in endocardial myocytes. Under both hypothyroid and streptozotocin-induced diabetic conditions the difference in endocardial and epicardial cytosolic PKC[epsilon] levels was absent (ratios of 0.86 +/- 0.21 (n = 4) and 1.09 +/- 0.16 (n = 3), respectively; means +/- S.E.M.). Ratios in the total protein extracts were not significantly different from those in control conditions. The results show transmural differences in the functional effects of PKC(epsilon) activation on a cardiac K+ current, and in the subcellular distribution of PKC(epsilon). These differences are absent in diabetic and hypothyroid conditions.

Altered cardiac endothelin receptors and protein kinase C in deoxycorticosterone-salt hypertensive rats

The aim of the present study was to assess the status of ET-1 receptor subtypes (ET(A)and ET(B)) in ventricular myocytes and fibroblasts and to determine the role of PKC-dependent pathways in ET-1-stimulated cardiac cells in deoxycorticosterone acetate (DOCA)-salt hypertensive rats. Systolic blood pressure and relative heart to body weight were significantly increased in DOCA-salt rats. In unilaterally nephrectomized (Uni-Nx) control rats, more than 90% of cardiomyocyte ET receptors were of the ET(A)subtype, whereas in fibroblasts ET(A)and ET(B)receptors were present in a 1:3 ratio. In DOCA-salt rats, the density of the ET(A)receptor subtype was reduced by 31% in cardiomyocytes and in cardiac fibroblasts only ET(B)receptor density was decreased by 29%. Affinity was unchanged. The relative expression of immunoreactive PKC alpha, gamma and epsilon was significantly increased, whereas PKC delta was not altered in cardiac extracts of DOCA-salt rats. In cardiac fibroblasts from DOCA-salt rats PKC delta was significantly increased and PKC epsilon was not translocated after ET-1 stimulation. The hearts of DOCA-salt hypertensive rats are thus characterized by: (1) decreased density of cardiomyocyte ET(A)receptors and fibroblast ET(B)receptors; (2) cell-specific enhanced expression of some PKC isoenzymes (alpha, gamma, delta and epsilon); and (3) unresponsiveness of PKC epsilon to translocate in the presence of ET-1. Together with alterations of ET-1-induced Ca(2+)handling in cardiac myocytes and fibroblasts, which we previously reported, results from the present study indicate a marked modification of the cardiac ET-1 system of DOCA-salt hypertensive rats.

Constitutively active mutants of the alpha(1a)- and the alpha(1b)-adrenergic receptor subtypes reveal coupling to different signaling pathways and physiological responses in rat cardiac myocytes

Activation of alpha(1)-adrenergic receptors influences both the contractile activity and the growth potential of cardiac myocytes. However, the signaling pathways linking activation of specific alpha(1)-adrenergic receptor (AR) subtypes to these physiological responses remain controversial. In the present study, a molecular approach was used to identify conclusively the signaling pathways activated in response to the individual alpha(1A)- and alpha(1B)-AR subtypes in cardiac myocytes. For this purpose, a mutant alpha(1a)-AR subtype (alpha(1a)-S(290/293)-AR) was constructed based on analogy to the previously described constitutively active mutant alpha(1b)-AR subtype (alpha(1b)-S(288-294)-AR). The mutant alpha(1a)-S(290/293)-AR subtype displayed constitutive activity based on four criteria. To introduce the constitutively active alpha(1)-AR subtypes into cardiac myocytes, recombinant Sindbis viruses encoding either the alpha(1a)-S(290/293)-AR or alpha(1b)-S(288-294)-AR subtype were used to infect the whole cell population with >90% efficiency, thereby allowing the biochemical activities of the various signaling pathways to be measured. When expressed at comparable levels, the alpha(1a)-S(290/293)-AR subtype exhibited a significantly elevated basal level as well as agonist-stimulated level of inositol phosphate accumulation, coincident with activation of atrial natriuretic factor-luciferase gene expression. By contrast, the alpha(1b)-S(288-294)-AR subtype displayed a markedly increased serum response element-luciferase gene expression but no activation of atrial natriuretic factor-luciferase gene expression. Taken together, this study provides the first molecular evidence for coupling of the alpha(1a)-AR and the alpha(1b)-AR subtypes to different signaling pathways in cardiac myocytes.

Protein kinase C expression and subcellular distribution in chronic myocardial ischemia. Comparison of two different canine models

We studied protein kinase C (PKC) isozyme expression and activity distribution in two models of chronically ischemic canine myocardium: (1) single vessel obstruction (SVO), produced by tight stenosis of LAD followed by preconditioning and acute ischemia (40 min); (2) three vessel obstruction (3VO), produced by LAD-stenosis and gradual occlusion of right coronary artery and left circumflex. In both models after 8 weeks of chronic ischemia the dogs were either sacrificed or had PTCA of the LAD with a follow up of another 4 weeks. Control dogs were sham operated. PKC activity was measured in subcellular fractions of tissue samples from anterior and posterior regions in the presence of histone and gamma-[32P]-ATP. PKC isozymes were detected by Western blotting. All regions perfused by the obstructed coronaries were dysfunctional at 8 weeks when compared to baseline, with improvement of anterior wall function after PTCA of LAD. PKC activity was elevated in the membrane fraction of SVO, but unchanged in the 3VO model. PKCs alpha, epsilon, and zeta prevailed in cytosol fraction of the controls (cytosol/membrane ratios were +/- 3.34, 1.38 and 4.56 for alpha, epsilon and zeta, respectively), consistent with PKC activity distribution, while delta was not detected. There was no significant difference between the groups concerning the relative membrane amount of the isozymes. PKCs alpha and epsilon were decreased in the cytosol fraction of both models at 8 weeks (for anterior region, by 56 and 57% in SVO and by 49 and 46% in 3VO, respectively) without there being any differences between anterior and posterior regions, and were low also in the PTCA group. PKC zeta distribution however varied between the two models. The amount of PKC zeta isozyme was downregulated by 45% after 8 weeks of chronic ischemia and returned towards the control values after PTCA in the anterior region of SVO, while it did not change in anterior wall after 8 weeks in 3VO but was significantly decreased (by 47%) in posterior region after PTCA. In conclusion, our results suggest modified PKC signalling in chronically ischemic canine myocardium.

Protein kinase C regulation of K+ currents in rat ventricular myocytes and its modification by hormonal status

1. The effects of protein kinase C (PKC) activation on cardiac K+ currents were studied in rat ventricular myocytes, using whole-cell voltage clamp methods. Control rats were compared to hypothyroid or diabetic rats, in which PKC expression and activity were enhanced. 2. In control myocytes, two calcium-independent outward K+ currents, the transient It and the sustained Iss, were attenuated by 18.9 +/- 2.0 and 16.8 +/- 3.5 %, respectively (mean +/- s.e.m.), following addition of a synthetic analogue of diacylglycerol, DiC8 (20 microM). In myocytes from hypothyroid or diabetic rats, It and Iss were not affected by DiC8. 3. The effects of DiC8 were restored in myocytes from thyroidectomized rats by injection of physiological doses of tri-iodothyronine (T3; 10 microg kg-1 for 6-8 days). Incubating cells from diabetic rats with 100 nM insulin for 5-9 h also restored the ability of DiC8 to attenuate It and Iss. 4. The attenuation of K+ currents by DiC8 in control cells was absent in the presence of a peptide known to inhibit the translocation of the isoform PKCepsilon (EAVSKPLT, 24 microM introduced through the recording pipette). A scrambled peptide (LSETKPAV) was without effect. 5. Under hypothyroid conditions the inhibitory peptide restored the effects of DiC8 on It and Iss. These currents were attenuated by 11.9 +/- 1. 5 and 9.8 +/- 1.5 %, respectively, which was significantly (P < 0. 001) more than without the peptide or with the scrambled peptide. 6. These results show that the PKC-mediated suppression of cardiac K+ currents is normally mediated by PKCepsilon translocation. This effect is absent under hypothyroid and diabetic conditions, presumably due to prior PKC activation and translocation. A PKCepsilon translocation inhibitor restores the ability of DiC8 to attenuate K+ currents under hypothyroid conditions. This presumably reflects a (partial) reversal of a chronic translocation and a shift in the balance between PKC and its anchoring proteins.

Differential coupling of alpha1-adrenoreceptor subtypes to phospholipase C and mitogen activated protein kinase in neonatal rat cardiac myocytes

Activation of cardiac alpha1-adrenoreceptors has a number of physiological effects. Ascribing these effects to a specific alpha1-adrenoreceptor subtype first requires the elucidation of the subtypes that are present in the tissue of interest. In the present study, mRNA transcripts for the alpha1A, alpha1B and alpha1D-adrenoreceptor subtypes were detected in cultured neonatal rat cardiac myocytes, using reverse transcriptase-polymerase chain reaction analysis. However, binding sites for only the alpha1A and alpha1B-adrenoreceptor subtypes were detected in cultured neonatal rat cardiac myocytes, using competition binding analysis with a variety of alpha1 selective receptor antagonists. Phenylephrine-stimulated phosphatidylinositol hydrolysis was inhibited by alpha1 selective receptor antagonists with affinities consistent with the alpha1A-adrenoreceptor subtype, whereas phenylephrine-induced activation of the mitogen activated protein kinase cascade was inhibited by these same antagonists with affinities more closely resembling the alpha1B-adrenoreceptor subtype. In the case of both signaling pathways, the alpha1D selective receptor antagonist, BMY 7378, exhibited affinities suggestive of the relative absence of a alpha1D-adrenoreceptor subtype. Thus, despite the presence of mRNA transcripts for all three alpha1-adrenoreceptor subtypes, only the alpha1A and alpha1B-adrenoreceptor subtypes were expressed and functionally coupled at detectable levels in neonatal rat cardiac myocytes. Of particular interest, phenylephrine-induced activation of the mitogen activated protein kinase cascade appears to be mediated by a subtype resembling most closely the pharmacological profile of the alpha1B-adrenoreceptor subtype.

Left ventricular stretch stimulates angiotensin II--mediated phosphatidylinositol hydrolysis and protein kinase C epsilon isoform translocation in adult guinea pig hearts

Stretch of neonatal cardiomyocytes activates phospholipase C with production of inositol trisphosphate and diacylglycerol in part by formation of angiotensin II (Ang II). However, the response of this pathway to physical stimuli in the adult heart is poorly understood. Thus, in isovolumic perfused guinea pig hearts, we characterized stretch-mediated phosphatidylinositol (PI) hydrolysis and protein kinase C (PKC) isoform translocation using elevated diastolic pressure. Balloon dilatation (minimum diastolic pressure, 25 mm Hg) of the left ventricle (LV) stimulated PI hydrolysis. Pretreatment of stretched hearts with the specific angiotensin (AT1) receptor antagonist losartan abolished stretch-mediated accumulation of inositol phosphates. To examine PKC isoform expression and activation under these conditions, whole-heart extracts were examined by immunoblot analysis. Ang II translocated PKC epsilon to the particulate fraction. 4 beta-Phorbol 12-myristate 13-acetate but not an inactive congener translocated PKC epsilon to the particulate fraction and produced a decrease in myocardial contractile function. Mechanical stretch also translocated PKC epsilon to the particulate fraction; however, this was attenuated but not abolished by losartan. We conclude that in the adult heart, LV dilation produced stretch-mediated activation of phospholipase C, which resulted in PI hydrolysis and PKC epsilon activation in part by stimulation of the local renin angiotensin system. In contrast to stretch-mediated inositol phosphate accumulation, PKC epsilon translocation is not prevented by AT1 receptor blockade, indicating that this PKC isoform can be activated in response to mechanical deformation by an Ang II-independent mechanism in the adult myocardium.

Differential desensitization of thromboxane A2 receptor subtypes

Two subtypes of the thromboxane A2 (TxA2) receptor (TxA2R-E and TxA2R-P), which differ in their alternatively spliced cytoplasmic tails, have been identified. The initial concentration of the TxA2 mimetic IBOP required to reduce peak intracellular Ca2+ concentration ([Ca2+]i) induced by a second addition of IBOP (100 nmol/L) was similar (IC50 for TxA2R-E and TxA2R-P, 0.46 +/- 0.16 and 0.40 +/- 0.07 nmol/L) in fibroblasts overexpressing either the TxA2R-E or -P subtype. Although the number of TxA2 binding sites decreased in TxA2R-P cells after prolonged stimulation with a TxA2 mimetic, those in the TxA2R-E cells increased markedly. To determine whether the mechanism for desensitization differs between subtypes, the effect of activation of protein kinase C (PKC) or cAMP-dependent kinase on TxA2-induced [Ca2+]i mobilization was measured. Forskolin reduced the IBOP-induced peak [Ca2+]i in neither TxA2R-E nor TxA2R-P cells; however, treatment with phorbol esters (IC50, 0.57 +/- 0.70 nmol/L) strongly prevented IBOP-mediated [Ca2+]i rise in TxA2R-E but not in TxA2R-P cells. Desensitization of TxA2R-E by phorbol esters was prevented by the PKC inhibitor calphostin C or by downregulation of PKC-alpha. Thus, the response of TxA2R-E to prolonged stimulation differs from that of TxA2R-P in both the regulation of the number of binding sites and the mechanism for desensitization; agonists that activate PKC-alpha might interfere with TxA2R-E-mediated signaling.

Does protein kinase C play a pivotal role in the mechanisms of ischemic preconditioning?

This communication reviews the evidence for the pivotal role of protein kinase C in ischemic myocardial preconditioning. It is believed that several intracellular signalling pathways via receptor-coupled phospholipase C and its "cross-talk" with phospholipase D converge to activation of protein kinase C isotypes which is followed by phosphorylation of until now (a number of) unknown target proteins which produce the protective state of ischemic preconditioning. After briefly introducing the general biochemical properties of protein kinase C, its isotypes and the limitations of the methodology used to investigate the role of protein kinase C, studies are discussed in which pharmacological inhibition and activation and (immunore) activity and/or isotypes measurements of protein kinase C isotypes were applied to assess the role of activation of protein kinase C in ischemic myocardial preconditioning. It is concluded that definitive proof for the involvement of protein kinase C in preconditioning requires future studies which must focus on the isotype(s) of protein kinase C that are activated, the duration of action, cellular translocation sites and the identity and stability (of covalently bound phosphate) of phosphorylated substrate proteins.