Death by protein kinase C inhibitor: a stressful event

Structural basis of protein kinase C isoform function

Protein kinase C (PKC) isoforms comprise a family of lipid-activated enzymes that have been implicated in a wide range of cellular functions. PKCs are modular enzymes comprised of a regulatory domain (that contains the membrane-targeting motifs that respond to lipid cofactors, and in the case of some PKCs calcium) and a relatively conserved catalytic domain that binds ATP and substrates. These enzymes are coexpressed and respond to similar stimulatory agonists in many cell types. However, there is growing evidence that individual PKC isoforms subserve unique (and in some cases opposing) functions in cells, at least in part as a result of isoform-specific subcellular compartmentalization patterns, protein-protein interactions, and posttranslational modifications that influence catalytic function. This review focuses on the structural basis for differences in lipid cofactor responsiveness for individual PKC isoforms, the regulatory phosphorylations that control the normal maturation, activation, signaling function, and downregulation of these enzymes, and the intra-/intermolecular interactions that control PKC isoform activation and subcellular targeting in cells. A detailed understanding of the unique molecular features that underlie isoform-specific posttranslational modification patterns, protein-protein interactions, and subcellular targeting (i.e., that impart functional specificity) should provide the basis for the design of novel PKC isoform-specific activator or inhibitor compounds that can achieve therapeutically useful changes in PKC signaling in cells.

Adenosine A2A and beta-adrenergic calcium transient and contractile responses in rat ventricular myocytes

The adenosine A2A receptor (A2AR) enhances cardiac contractility, and the adenosine A1R receptor (A1R) is antiadrenergic by reducing the adrenergic beta1 receptor (beta1R)-elicited increase in contractility. In this study we compared the A2AR-, A1R-, and beta1R-elicited actions on isolated rat ventricular myocytes in terms of Ca transient and contractile responses involving PKA and PKC. Stimulation of A2AR with 2 microM (approximately EC50) CGS-21680 (CGS) produced a 17-28% increase in the Ca transient ratio (CTR) and maximum velocities (Vmax) of transient ratio increase (+MVT) and recovery (-MVT) but no change in the time-to-50% recovery (TTR). CGS increased myocyte sarcomere shortening (MSS) and the maximum velocities of shortening (+MVS) and relaxation (-MVS) by 31-34% with no change in time-to-50% relengthening (TTL). beta1R stimulation using 2 nM (approximately EC50) isoproterenol (Iso) increased CTR, +MVT, and -MVT by 67-162% and decreased TTR by 43%. Iso increased MSS, +MVS, and -MVS by 153-174% and decreased TTL by 31%. The A2AR and beta1R Ca transient and contractile responses were not additive. The PKA inhibitor Rp-adenosine 3',5'-cyclic monophosphorothioate triethylamonium salt prevented both the CGS- and Iso-elicited contractile responses. The PKC inhibitors chelerythrine and KIE1-1 peptide (PKCepsilon specific) prevented the antiadrenergic action of A1R but did not influence A2AR-mediated increases in contractile variables. The findings suggest that cardiac A2AR utilize cAMP/PKA like beta1R, but the Ca transient and contractile responses are less in magnitude and not equally affected. Although PKC is important in the A1R antiadrenergic action, it does not seem to play a role in A2AR-elicited Ca transient and contractile events.

Gö 6983: A Fast Acting Protein Kinase C Inhibitor that Attenuates Myocardial Ischemia/Reperfusion Injury

Reperfusion injury is characterized by a decrease in endothelial release of nitric oxide within 5 min after reperfusion, increased leukocyte-endothelium interaction, and transmigration of leukocytes into the myocardium, producing cardiac contractile dysfunction. Gö 6983 is a fast acting, lipid soluble, broad spectrum protein kinase C inhibitor. When administered at the beginning of reperfusion, it can restore cardiac function within 5 min and attenuate the deleterious effects associated with acute ischemia/reperfusion. Gö 6983 may offer greater cardioprotection than other broad-spectrum PKC inhibitors in postischemic reperfusion injury because it inhibits PKC(zeta) as well as four other isoforms. The cardioprotection is associated with decreased leukocyte superoxide release and increased endothelial derived nitric oxide from vascular tissue. In vitro studies of human tissue showed that Gö 6983 significantly inhibited antigen-induced superoxide release from leukocytes of patients previously sensitized to tree pollen. In human vascular tissue, Gö 6983 inhibited intracellular Ca(2+) accumulation, suggesting a mechanism for its vasodilator properties. These studies suggest that Gö 6983 would be an effective compound to use in a clinical ischemia/reperfusion setting of organ transplantation and/or cerebral ischemia where inhibiting superoxide release and vasoconstriction in postischemic tissues would allow for better restoration of organ function during reperfusion. However, given the broad-spectrum action of Gö 6983, careful titration of the dose regimen would be recommended to ensure a successful outcome in the setting of organ transplantation and/or cerebral ischemia.

Distinctive activation mechanisms and functions for protein kinase Cdelta

PKCdelta (protein kinase Cdelta) is a serine/threonine kinase that plays a key role in growth regulation and tissue remodelling. Traditional models of PKC activation have focused on lipid cofactors and anchoring proteins that localize the active conformation of PKCdelta to membranes, in close proximity with its target substrates. However, recent studies identify a distinct mode for PKCdelta activation involving tyrosine phosphorylation by Src family kinases. The tyrosine-phosphorylated form of PKCdelta (which accumulates in the soluble fraction of cells exposed to oxidant stress) displays lipid-independent kinase activity and is uniquely positioned to phosphorylate target substrates throughout the cell (not just on lipid membranes). This review summarizes (1) recent progress towards understanding structure-activity relationships for PKCdelta, with a particular focus on the stimuli that induce (and the distinct functional consequences that result from) tyrosine phosphorylation events in PKCdelta's regulatory, hinge and catalytic domains; (2) current concepts regarding the role of tyrosine phosphorylation as a mechanism to regulate PKCdelta localization and actions in mitochondrial and nuclear compartments; and (3) recent literature delineating distinct roles for PKCdelta (relative to other PKC isoforms) in transcriptional regulation, cell cycle progression and programmed cell death (including studies in PKCdelta-/- mice that implicate PKCdelta in immune function and cardiovascular remodelling). Collectively, these studies argue that the conventional model for PKCdelta activation must be broadened to allow for stimulus-specific differences in PKCdelta signalling during growth factor stimulation and oxidant stress.

Specificity of action of bisindolylmaleimide protein kinase C inhibitors: do they inhibit the 70kDa ribosomal S6 kinase in cardiac myocytes?

Bisindolylmaleimide protein kinase C (PKC) inhibitors, such as GF109203X and Ro31-8220, are used as pharmacological tools in many cellular systems. However, in vitro, GF109203X and Ro31-8220 also inhibit the 70kDa ribosomal S6 kinase (p70(S6K)) with similar potency. We determined whether GF109203X and Ro31-8220 inhibit p70(S6K) activity in intact adult rat ventricular myocytes (ARVM). First, we confirmed that increased phosphorylation of the 40S ribosomal S6 protein (a cellular substrate for both p70(S6K) and the 90 kDa ribosomal S6 kinase) in response to stimulation of ARVM by insulin-like growth factor-1 (300 ng/mL; 10 min) occurs specifically through rapamycin-sensitive activation of p70(S6K). Then, using this response as the index of cellular p70(S6K) activity, we determined the effects of GF109203X and Ro31-8220 (1, 3 or 10 microM) on such activity. At these concentrations, neither GF109203X nor Ro31-8220 inhibited cellular p70(S6K) activity. In contrast, even at 1 microM, cellular PKC activity (stimulated by a 3 min exposure to 30 nM phorbol 12-myristate 13-acetate) was significantly inhibited by each agent. We conclude that; (1) data obtained in vitro may not necessarily be extrapolated to intact cells and (2) inhibition of p70(S6K) is unlikely to contribute to the actions of GF109203X and Ro31-8220 in ARVM.

Gö 6983 Exerts Cardioprotective Effects in Myocardial Ischemia/Reperfusion

Ischemia followed by reperfusion (I/R) in the presence of polymorphonuclear leukocytes (PMNs) results in cardiac contractile dysfunction. Inhibiting protein kinase C (PKC) inhibits the release of superoxide from PMNs. The compound Gö 6983 is an inhibitor of all five PKC isoforms present in PMNs. Therefore, we hypothesized that Gö 6983 could attenuate PMN-induced cardiac dysfunction by suppression of superoxide production from PMNs. We studied isolated rat hearts following ischemia (20 minutes) and reperfusion (45 minutes) infused with activated PMNs. In hearts reperfused with PMNs and Gö 6983 (100 nM, n = 7), left ventricular developed pressure (LVDP) and the rate of LVDP (+dP/dt max) recovered to 89 +/- 7% and 74 +/- 2% of baseline values, respectively, at 45 minutes postreperfusion compared with I/R hearts (n = 9) receiving PMNs alone, which only recovered to 55 +/- 3% and 45 +/- 5% of baseline values for LVDP and +dP/dtmax, respectively (P < 0.01). Gö 6983 (100 nM) significantly reduced PMN adherence to the endothelium and infiltration into the myocardium compared with I/R + PMN hearts (P < 0.01), and significantly inhibited superoxide release from PMNs by 90 +/- 2% (P < 0.01). In the presence of PMNs, Gö 6983 attenuated post-I/R cardiac contractile dysfunction, which may be related in part to decreased superoxide production.

Isoenzyme-selective regulation of SERCA2 gene expression by protein kinase C in neonatal rat ventricular myocytes

Patients with cardiac hypertrophy and heart failure display abnormally slowed myocardial relaxation, which is associated with downregulation of sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA2) gene expression. We previously showed that SERCA2 downregulation can be simulated in cultured neonatal rat ventricular myocytes (NRVM) by treatment with the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA). However, NRVM express three different PMA-sensitive PKC isoenzymes (PKCalpha, PKCepsilon, and PKCdelta), which may be differentially regulated and have specific functions in the cardiomyocyte. Therefore, in this study we used adenoviral vectors encoding wild-type (wt) and kinase-defective, dominant negative (dn) mutant forms of PKCalpha, PKCepsilon, and PKCdelta to analyze their individual effects in regulating SERCA2 gene expression in NRVM. Overexpression of wtPKCepsilon and wtPKCdelta, but not wtPKCalpha, was sufficient to downregulate SERCA2 mRNA levels, as assessed by Northern blotting and quantitative, real-time RT-PCR (69 +/- 7 and 61 +/- 9% of control levels for wtPKCepsilon and wtPKCdelta, respectively; P < 0.05 for each adenovirus; n = 8 experiments). Conversely, overexpression of all three dnPKCs appeared to significantly increase SERCA2 mRNA levels (dnPKCdelta > dnPKCepsilon > dnPKCalpha). dnPKCdelta overexpression produced the largest increase (2.8 +/- 1.0-fold; n = 11 experiments). However, PMA treatment was still sufficient to downregulate SERCA2 mRNA levels despite overexpression of each dominant negative mutant. These data indicate that the novel PKC isoenzymes PKCepsilon and PKCdelta selectively regulate SERCA2 gene expression in cardiomyocytes but that neither PKC alone is necessary for this effect if the other novel PKC can be activated.

Regulation of cardiac myocyte cell death

Cardiac myocyte death, whether through necrotic or apoptotic mechanisms, is a contributing factor to many cardiac pathologies. Although necrosis and apoptosis are the widely accepted forms of cell death, they may utilize the same cell death machinery. The environment within the cell probably dictates the final outcome, producing a spectrum of response between the two extremes. This review examines the probable mechanisms involved in myocyte death. Caspases, the generally accepted executioners of apoptosis, are significant in executing cardiac myocyte death, but other proteases (e.g., calpains, cathepsins) also promote cell death, and these are discussed. The two principal cell death pathways (death receptor- and mitochondrial-mediated) are described in relation to the emerging structural information for the principal proteins, and they are discussed relative to current understanding of myocyte cell death mechanisms. Whereas the mitochondrial pathway is probably a significant factor in myocyte death in both acute and chronic phases of myocardial diseases, the death receptor pathway may prove significant in the longer term. The Bcl-2 family of proteins are key regulators of the mitochondrial death pathway. These proteins are described and their possible functions are discussed. The commitment to cell death is also influenced by protein kinase cascades that are activated in the cell. Whereas certain pathways are cytoprotective (e.g., phosphatidylinositol 3'-kinase), the roles of other kinases are less clear. Since myocyte death is implicated in a number of cardiac pathologies, attenuation of the death pathways may prove important in ameliorating such disease states, and possible therapeutic strategies are explored.

Factors contributing to neurotrophin-independent survival of adult sensory neurons

Dorsal root ganglion (DRG) sensory neurons become less dependent upon neurotrophins for their survival as they mature. DRG neurons from young adult rats were dissociated and cultured in vitro in serum-free defined medium. We show that adult DRG sensory neurons are able to survive for at least 2 weeks in culture in the absence of nerve growth factor (NGF). We then investigated potential mechanisms contributing to this apparent neurotrophin-independent survival in these neurons through the use of inhibitors of cellular signaling pathways. The phosphoinositide kinase-3 (PI 3-K) inhibitor LY294002, and a protein kinase C (PKC) inhibitor, chelerythrine resulted in significant decreases in neuronal survival. Neither the mitogen activated protein kinase kinase (MEK) inhibitor U0126 nor two other PKC inhibitors (bisindolylmaleimide and rottlerin) had any significant effect on survival. Our results point to the importance of PI 3-K and PKC signaling in the neurotrophin-independent survival of adult DRG neurons.

Cardioprotection mediated by sphingosine-1-phosphate and ganglioside GM-1 in wild-type and PKC epsilon knockout mouse hearts

Sphingosine-1-phosphate (S1P) protects neonatal rat cardiac myocytes from hypoxic damage through unknown signaling pathways. We tested the hypothesis that S1P-induced cardioprotection requires activation by the epsilon-isoform of protein kinase C (PKC epsilon) by subjecting hearts isolated from PKC epsilon knockout mice and wild-type mice to 20 min of global ischemia and 30 min of reperfusion. Pretreatment with a 2-min infusion of 10 nM S1P improved recovery of left ventricular developed pressure (LVDP) in both wild-type and PKC epsilon knockout hearts and reduced the rise in LV end-diastolic pressure (LVEDP) and creatine kinase (CK) release. Pretreatment for 2 min with 10 nM of the ganglioside GM-1 also improved recovery of LVDP and suppressed CK release in wild-type hearts but not in PKC epsilon knockout hearts. Importantly, GM-1 but not S1P, increased the proportion of PKC epsilon localized to particulate fractions. Our results suggest that GM-1, which enhances endogenous S1P production, reduces cardiac injury through PKC epsilon-dependent intracellular pathways. In contrast, extracellular S1P induces equivalent cardioprotection through PKC epsilon-independent signaling pathways.

Dual actions of the Galpha(q) agonist Pasteurella multocida toxin to promote cardiomyocyte hypertrophy and enhance apoptosis susceptibility

Previous attempts to delineate the consequences of Galpha (q) activation in cardiomyocytes relied largely on molecular strategies in cultures or transgenic mice. Modest levels of wild-type Galpha(q) overexpression induce stable cardiac hypertrophy, whereas intense Galpha(q) stimulation induces cardiomyocyte apoptosis. The precise mechanism(s) whereby traditional targets of Galpha (q) subunits that induce hypertrophy also trigger cardiomyocyte apoptosis is not obvious and is explored with recombinant Pasteurella multocida toxin (rPMT, a Galpha(q) agonist). Cells cultured with rPMT display cardiomyocyte enlargement, sarcomeric organization, and increased atrial natriuretic factor expression in association with activation of phospholipase C, novel protein kinase C (PKC) isoforms, extracellular signal-regulated protein kinase (ERK), and (to a lesser extent) JNK/p38-MAPK. rPMT stimulates the ERK cascade via epidermal growth factor (EGF) receptor transactivation in cardiac fibroblasts, but EGF receptor transactivation plays no role in ERK activation in cardiomyocytes. Surprisingly, rPMT (or novel PKC isoform activation by PMA) decreases basal Akt phosphorylation; rPMT prevents Akt phosphorylation by EGF or IGF-1 and functionally augments cardiomyocyte apoptosis in response to H2O2. These results identify a Galpha(q)-PKC pathway that represses basal Akt phosphorylation and impairs Akt stimulation by survival factors. Because inhibition of Akt enhances cardiomyocyte susceptibility to apoptosis, this pathway is predicted to contribute to the transition from hypertrophy to cardiac decompensation and could be targeted for therapy in heart failure.

Role of protein kinase C-epsilon in the development of kappa-opioid receptor tolerance to U50,488H in rat ventricular myocytes

The role of protein kinase C-epsilon (PKC-epsilon) in the development of kappa-opioid receptor (kappa-OR) tolerance to the effects of trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]cyclohexyl) (U50,488H), the selective agonist of kappa-OR, was determined in rat ventricular myocytes. Incubation of ventricular myocytes with 1 microM U50,488H for 24 h significantly attenuated the inhibitory effects of 30 microM U50,488H on the electrically-induced [Ca(2+)](i) transient and forskolin-stimulated cyclic AMP accumulation, indicating the development of tolerance to the kappa-OR agonist. Chronic treatment of ventricular myocytes with U50,488H also induced translocation of PKC-epsilon to the particulate fraction. On the other hand, administration of 30 microM U50,488H for 10 min induced translocation of PKC-alpha to the particulate fraction in naïve ventricular myocytes, but not in cells pretreated with 1 microM U50,488H for 24 h. In ventricular myocytes incubated for 24 h with 1 microM U50,488H together with 1 microM chelerythrine or 1 microM GF109203X, PKC inhibitors, or 0.1 microM epsilonV1-2 peptide, a selective inhibitor of PKC-epsilon, 30 microM U50,488H still produced the inhibitory effect on the electrically-induced [Ca(2+)](i) transient as it did in naïve ventricular myocytes. Chronic treatment of ventricular myocytes with U50,488H and chelerythrine also attenuated the development of tolerance to acute U50,488H on cyclic AMP accumulation. Cells exposed to chelerythrine, GF109203X, or epsilonV1-2 peptide alone did not show an altered [Ca(2+)](i) response to U50,488H. These results indicate that activation of PKC-epsilon is a critical step in the development of tolerance in the kappa-OR.