Inactive membrane protein kinase Cs: a possible target for receptor signalling

Robustness and evolvability of the human signaling network

Biological systems are known to be both robust and evolvable to internal and external perturbations, but what causes these apparently contradictory properties? We used Boolean network modeling and attractor landscape analysis to investigate the evolvability and robustness of the human signaling network. Our results show that the human signaling network can be divided into an evolvable core where perturbations change the attractor landscape in state space, and a robust neighbor where perturbations have no effect on the attractor landscape. Using chemical inhibition and overexpression of nodes, we validated that perturbations affect the evolvable core more strongly than the robust neighbor. We also found that the evolvable core has a distinct network structure, which is enriched in feedback loops, and features a higher degree of scale-freeness and longer path lengths connecting the nodes. In addition, the genes with high evolvability scores are associated with evolvability-related properties such as rapid evolvability, low species broadness, and immunity whereas the genes with high robustness scores are associated with robustness-related properties such as slow evolvability, high species broadness, and oncogenes. Intriguingly, US Food and Drug Administration-approved drug targets have high evolvability scores whereas experimental drug targets have high robustness scores.

Biological systems are known to be robust and evolvable to internal mutations and external environmental changes. What causes these apparently contradictory properties? This study shows that the human signaling network can be decomposed into two structurally distinct subgroups of links that provide both evolvability to environmental changes and robustness against internal mutations. The decomposition of the human signaling network reveals an evolutionary design principle of the network, and also facilitates the identification of potential drug targets.

Regulation of basal lateral membrane mobility and permeability to divalent cations by membrane associated-protein kinase C

Biological membrane stabilization is essential for maintenance of cellular homeostasis, functionality and appropriate response to various stimuli. Previous studies have showed that accumulation of PKCs in the cell membrane significantly downregulates the membrane fluidity and Ca²⁺ influxes through the membranes in activated cells. In addition, membrane-inserted form of PKCs has been found in a variety of resting mammalian cells and tissues. This study is aimed to investigate possible role of the endogenous membrane-associated PKCs in the modulation of basal membrane fluidity. Here, we showed that interfering PKC expression by chronic activation of PKC with phorbol myristate acetate (PMA) or shRNA targeting at PKCα lowered the levels of PKCα in cytosol, peripheral membrane and integral membrane pools, while short-term activation of PKC with PMA induced accumulation of PKCα in the membrane pool accompanied by a dramatic decrease in the cytosol fraction. The lateral membrane mobility increased or decreased in accordance with the abundance alterations in the membrane-associated PKCα by these treatments. In addition, membrane permeability to divalent cations including Ca²⁺, Mn²⁺ and Ba²⁺ were also potentiated or abrogated along with the changes in PKC expression on the plasma membrane. Membrane stabilizer ursodeoxycholate abolished both of the enhanced lateral membrane mobility and permeability to divalent cations due to PKCα deficiency, whereas Gö6983, a PKC antagonist, or Gd³⁺ and 2-aminoethyoxydipheyl borne, two Ca²⁺ channels blockers, showed no effect, suggesting that this PKC-related regulation is independent of PKC activation or a modulation of specific divalent cation channel. Thus, these data demonstrate that the native membrane-associated PKCα is involved in the maintenance of basal membrane stabilization in resting cells.

Metabolic programming during lactation stimulates renal Na+ transport in the adult offspring due to an early impact on local angiotensin II pathways

Background

Several studies have correlated perinatal malnutrition with diseases in adulthood, giving support to the programming hypothesis. In this study, the effects of maternal undernutrition during lactation on renal Na⁺-transporters and on the local angiotensin II (Ang II) signaling cascade in rats were investigated.

Methodology/Principal Findings

Female rats received a hypoproteic diet (8% protein) throughout lactation. Control and programmed offspring consumed a diet containing 20% protein after weaning. Programming caused a decrease in the number of nephrons (35%), in the area of the Bowman's capsule (30%) and the capillary tuft (30%), and increased collagen deposition in the cortex and medulla (by 175% and 700%, respectively). In programmed rats the expression of (Na⁺+K⁺)ATPase in proximal tubules increased by 40%, but its activity was doubled owing to a threefold increase in affinity for K⁺. Programming doubled the ouabain-insensitive Na⁺-ATPase activity with loss of its physiological response to Ang II, increased the expression of AT₁ and decreased the expression of AT₂ receptors), and caused a pronounced inhibition (90%) of protein kinase C activity with decrease in the expression of the α (24%) and ε (13%) isoforms. Activity and expression of cyclic AMP-dependent protein kinase decreased in the same proportion as the AT₂ receptors (30%). In vivo studies at 60 days revealed an increased glomerular filtration rate (GFR) (70%), increased Na⁺ excretion (80%) and intense proteinuria (increase of 400% in protein excretion). Programmed rats, which had normal arterial pressure at 60 days, became hypertensive by 150 days.

Conclusions/Significance

Maternal protein restriction during lactation results in alterations in GFR, renal Na⁺ handling and in components of the Ang II-linked regulatory pathway of renal Na⁺ reabsorption. At the molecular level, they provide a framework for understanding how metabolic programming of renal mechanisms contributes to the onset of hypertension in adulthood.

Involvement of the Gi/o/cGMP/PKG pathway in the AT2-mediated inhibition of outer cortex proximal tubule Na+-ATPase by Ang-(1-7)

The molecular mechanisms involved in the Ang-(1-7) [angiotensin-(1-7)] effect on sodium renal excretion remain to be determined. In a previous study, we showed that Ang-(1-7) has a biphasic effect on the proximal tubule Na+-ATPase activity, with the stimulatory effect mediated by the AT1 receptor. In the present study, we investigated the molecular mechanisms involved in the inhibition of the Na+-ATPase by Ang-(1-7). All experiments were carried out in the presence of 0.1 nM losartan to block the AT1 receptor-mediated stimulation. In this condition, Ang-(1-7) at 0.1 nM inhibited the Na+-ATPase activity of the proximal tubule by 54%. This effect was reversed by 10 nM PD123319, a specific antagonist of the AT2 receptor, and by 1 muM GDP[beta-S] (guanosine 5'-[beta-thio]diphosphate), an inhibitor of G protein. Ang-(1-7) at 0.1 M induced [35S]GTP[S] (guanosine 5'-[gamma-[35S]thio]triphosphate) binding and 1 mug/ml pertussis toxin, an inhibitor of G(i/o) protein, reversed the Ang-(1-7) effect. Furthermore, it was observed that the inhibitory effect of Ang-(1-7) on the Na+-ATPase activity was completely reversed by 0.1 microM LY83583, an inhibitor of guanylate cyclase, and by 2 muM KT5823, a PKG (protein kinase G) inhibitor, and was mimicked by 10 nM d-cGMP (dibutyryl cGMP). Ang-(1-7) increased the PKG activity by 152% and this effect was abolished by 10 nM PD123319 and 0.1 microM LY83583. Taken together, these data indicate that Ang-(1-7) inhibits the proximal tubule Na+-ATPase by interaction with the AT2 receptor that subsequently activates the G(i/o) protein/cGMP/PKG pathway.

Oxidant-mediated activation of cytosolic phospholipase a(2) in pulmonary endothelium: role of protein kinase C alpha and a pertussis toxin-sensitive protein

The authors have previously demonstrated that the oxidant t-buOOH stimulates phospholipase A(2) (PLA(2)) activity in bovine pulmonary artery endothelial cells (S. Chakraborti et al. American Journal of Physiology, 257, L430-L437, 1989). Herein, the authors sought to investigate the mechanism by which t-buOOH stimulates PLA(2) activity and the role of protein kinase C (PKC) in this scenario. Treatment of bovine pulmonary artery endothelial cells with t-buOOH stimulated an aprotinin-sensitive protease activity, PKC activity, and PLA(2) activity in the cell membrane. Pretreatment with intracellular Ca(2+) chelator (BAPTA-AM), PKCalpha inhibitor (Go6976), cPLA(2) inhibitor (AACOCF(3)), and pertussis toxin prevented t-buOOH-stimulated PLA(2) activity. Immunoblot studies with aprotinin, cPLA(2), PKCalpha, and Gialpha antibodies revealed their presence in the endothelial membrane. Immunoblot studies of the cell membrane isolated from t-buOOH-stimulated cells with cPLA(2) and PKCalpha antibodies elicited an apparent increase in their immunoreactive protein profiles along with an additional 47-kDa immunoreactive fragment in the membrane. t-buOOH caused Gialpha phosphorylation in the membrane and pretreatment with Go6976 prevented the phosphorylation. Overall, these results suggest that t-buOOH stimulates an aprotinin-sensitive protease activity that proteolytically activates PKCalpha and that subsequently phosphorylates a pertussis toxin-sensitive protein, resulting in the stimulation of cPLA(2) activity in the cell membrane.

Role of MMP-2 in PKCδ-mediated inhibition of Na+ dependent Ca2+ uptake in microsomes of pulmonary smooth muscle: Involvement of a pertussis toxin sensitive protein

Treatment of bovine pulmonary artery smooth muscle with the O2 *- generating system hypoxanthine plus xanthine oxidase stimulated MMP-2 activity and PKC activity; and inhibited Na+ dependent Ca2+ uptake in the microsomes. Pretreatment of the smooth muscle with SOD (the O2 *- scavenger) and TIMP-2 (MMP-2 inhibitor) prevented the increase in MMP-2 activity and PKC activity, and reversed the inhibition of Na+ dependent Ca2+ uptake in the microsomes. Pretreatment with calphostin C (a general PKC inhibitor) and rottlerin (a PKCdelta inhibitor) prevented the increase in PKC activity and reversed O2 *- caused inhibition of Na+ dependent Ca2+ uptake without causing any change in MMP-2 activity in the microsomes of the smooth muscle. Treatment of the smooth muscle with the O2 *- generating system revealed, respectively, 36 kDa RACK-1 and 78 kDa PKCdelta immunoreactive protein profile along with an additional 38 kDa immunoreactive fragment in the microsomes. The 38 kDa band appeared to be the proteolytic fragment of the 78 kDa PKCdelta since pretreatment with TIMP-2 abolished the increase in the 38 kDa immunoreactive fragment. Co-immunoprecipitation of PKCdelta and RACK-1 demonstrated O2 *- dependent increase in PKCdelta-RACK-1 interaction in the microsomes. Immunoblot assay elicited an immunoreactive band of 41 kDa G(i)alpha in the microsomes. Treatment of the smooth muscle tissue with the O2 *- generating system causes phosphorylation of G(i)alpha in the microsomes and pretreatment with TIMP-2 and rottlerin prevented the phosphorylation. Pretreatment of the smooth muscle tissue with pertussis toxin reversed O2 *- caused inhibition of Na+ dependent Ca2+ uptake without affecting the protease activity and PKC activity in the microsomes. We suggest the existence of a pertussis toxin sensitive G protein mediated mechanism for inhibition of Na+ dependent Ca2+ uptake in microsomes of bovine pulmonary artery smooth muscle under O2 *- triggered condition, which is regulated by PKCdelta dependent phosphorylation and sensitive to TIMP-2 for its inhibition.

Protein kinase C-mediated inhibition of renal Ca2+ ATPase by physiological concentrations of angiotensin II is reversed by AT1- and AT2-receptor antagonists

Angiotensin II (Ang II) increases the cytosolic Ca2+ concentration in different cell types. In this study, we investigate the effect of Ang II on the Ca2+ ATPase of purified basolateral membranes of kidney proximal tubules. This enzyme pumps Ca2+ out of the cytosol in a reaction coupled to ATP hydrolysis, and it is responsible for the fine-tuned regulation of cytosolic Ca2+ activity. Ca2+-ATPase activity is inhibited by picomolar concentrations of Ang II, with maximal inhibition being attained at approximately 50% of the control values. The presence of raising concentrations (10(-11) to 10(-7) M) of losartan (an AT1-receptor antagonist) or PD123319 (an AT2-receptor antagonist) gradually reverts inhibition by Ang II. Both the phospholipase C (PLC) inhibitor U-73122 (10(-6) M) and the inhibitor of protein kinase C (PKC) staurosporine (10(-7) M) prevent inhibition of the Ca2+ pump by Ang II. Incubation of the previously isolated membranes with a PKC activator-the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (10(-8) M)-mimics the inhibition found with Ang II, and the effects of the compounds are not additive. Taken as a whole, these results indicate the Ang II inhibits Ca2+-ATPase by activation of a PKC system present in primed state in these membranes after binding of the hormone to losartan- and PD123319-sensitive receptors coupled to a PLC. Therefore, inhibition of the basolateral membrane Ca2+-ATPase by kinase-mediated phosphorylation appears to be one of the pathways by which Ang II promotes an increase in the cytosolic Ca2+ concentration of proximal tubule cells.

Proteolytic activation of protein kinase Calpha by peroxynitrite in stimulating cytosolic phospholipase A2 in pulmonary endothelium: involvement of a pertussis toxin sensitive protein

We sought to determine the roles of PKCalpha and G(i)alpha in regulating cPLA(2) activity in bovine pulmonary artery endothelial cell membrane under peroxynitrite (ONOO(-)) stimulation. Treatment of bovine pulmonary artery endothelial cells with ONOO(-) markedly stimulates the cell membrane associated protease activity, protein kinase C (PKC) activity, phospholipase A(2) (PLA(2)) activity, and arachidonic acid (AA) release from the cells. ONOO(-) significantly increases (Ca(2+))(i) in the cells, and pretreatment with the intracellular Ca(2+) chelator BAPTA-AM prevents the increase in (Ca(2+))(i), protease activity, PKC activity, and cPLA(2) activity in the cell membrane and AA release from the cells. Pretreatment of the cells with arachidonyl trifluoromethyl ketone (AACOCF(3)) (a cPLA(2) inhibitor) prevents ONOO(-)-stimulated cPLA(2) activity and AA release without producing a significant alteration of the protease activity. Pretreatment with vitamin E and aprotinin prevents ONOO(-)-induced increase in the protease activity, PKC activity, and cPLA(2) activity in the cell membrane and AA release from the cells. Pretreatment with the PKC inhibitor calphostin C prevents ONOO(-)-caused increase in PKC activity and cPLA(2) activity in the cell membrane and AA release from the cells. An immunoblot study of the cell membrane isolated from the ONOO(-)-treated cells with polyclonal PKCalpha antibody elicited an increase in the 80 kDa immunoreactive protein band along with an additional 47 kDa immunoreactive fragment. An immunoblot study with anti-nitrotyrosine antibody revealed that ONOO(-) induces nitration of tyrosine residues in PKCalpha. Pretreatment of the cells with aprotinin abolished the 47 kDa immunoreactive fragment in the immunoblot. An immunoblot study of the endothelial cell membrane with polyclonal cPLA(2) antibody revealed that treatment of the cells with ONOO(-) markedly increases the cPLA(2) immunoreactive protein profile in the membrane. Pretreatment of the endothelial cells with Go6976, a PKCalpha inhibitor, prevents the increase in PKC activity and cPLA(2) activity in the cell membrane under ONOO(-)-triggered condition. It, therefore, appears from the present study that treatment of the cells with ONOO(-) causes an increase in the protease activity, and that plays an important role in activating PKCalpha, which subsequently stimulates cPLA(2) activity in the cell membrane and AA release from the cells. An immunoblot assay with polyclonal G(i)alpha antibody elicited an immunoreactive band having a molecular mass of 41 kDa. Pretreatment of the cells with pertussis toxin markedly inhibits ONOO(-)-induced increase in cPLA(2) activity and AA release without significantly altering (Ca(2+))(i), protease activity, and PKC activity in the cell membrane. Treatment of the cells with ONOO(-) causes phosphorylation of G(i)alpha in the cell membrane, and pretreatment with Go6976 prevents its phosphorylation. We suggest the existence of a pertusssis toxin sensitive G protein-mediated mechanism for activation of cPLA(2) by ONOO(-) in bovine pulmonary artery endothelial cell membrane, which is regulated by PKCalpha-dependent phosphorylation and sensitive to aprotinin for its inhibition.

Role of an aprotinin-sensitive protease in protein kinase Calpha-mediated activation of cytosolic phospholipase A2 by calcium ionophore (A23187) in pulmonary endothelium

Treatment of bovine pulmonary artery endothelial cells with the calcium ionophore, A23187, stimulates the cell membrane associated protease activity, phospholipase A2 (PLA2) activity, and arachidonic acid (AA) release from the cells. Pretreatment of the cells with arachidonyl-trifluomethylketone (AACOCF3), a cPLA2 inhibitor, but not bromoenollactone (BEL), a iPLA2 inhibitor, prevents A23187 stimulated PLA2 activity and AA release without producing an appreciable alteration of the protease activity. Pretreatment of the cells with aprotinin, an ambient protease inhibitor, prevents the increase in the protease activity and cPLA2 activity in the membrane and AA release from the cells caused by both low and high doses of A23187, and also inhibits protein kinase C (PKC) activity caused by high doses of A23187. Immunoblot study of the endothelial cell membrane isolated from A23187 (10 microM)-treated cells with polyclonal PKCalpha antibody elicited an increase in the 80 kDa immunoreactive protein band along with an additional 47 kDa immunoreactive fragment. Pretreatment of the cells with aprotinin abolished the 47 kDa immunoreactive fragment in the immunoblot. Immunoblot study of the endothelial membrane with polyclonal cPLA2 antibody revealed that treatment of the cells with A23187 dose-dependently increases cPLA2 immunoreactive protein profile in the membrane. It therefore appears from the present study that treatment of the cells with a low dose of A23187 (1 microM) causes a small increase in an aprotinin-sensitive protease activity and that stimulates cPLA2 activity in the cell membrane without an involvement of PKC. By contrast, treatment of the cells with a high dose of 10 microM of A23187 causes optimum increase in the protease activity and that plays an important role in activating PKCalpha, which subsequently stimulates cPLA2 activity in the cell membrane. Although pretreatment of the cells with pertussis toxin caused ADP ribosylation of a 41 kDa protein in the cell membrane, it did not inhibit the cPLA2 activity and AA release caused by both low and high doses of A23187.

Regulation of membrane-bound PKC in adult cardiac ventricular myocytes

Activation of protein kinase C (PKC) is thought to involve translocation to the particulate fraction. The present study demonstrates a membrane-associated, inactive pool of PKC in adult rat ventricular myocytes. Membranes were isolated from stimulated (phorbol 12-myristate 13-acetate (PMA), endothelin-1 (ET-1)) or control myocytes and PKC activity determined in the absence (active PKC) or presence (total PKC) of PMA. An inactive, PMA-responsive, pool of PKC was detected. In intact myocytes, PMA or ET-1 induced a translocation of PKC epsilon from the cytosol into the particulate fraction. In contrast, ET-1 decreased both total and active PKC in the membranes: this decrease was associated with a loss of PKC epsilon immunoreactivity. PMA increased the amount of membrane-associated, inactive PKC. Our results demonstrate the presence of a membrane-associated pool of PKC in cardiac myocytes that is differentially modulated by ET-1 or PMA.

Angiotensin II stimulates renal proximal tubule Na(+)-ATPase activity through the activation of protein kinase C

Recently, our group described an AT(1)-mediated direct stimulatory effect of angiotensin II (Ang II) on the Na(+)-ATPase activity of proximal tubules basolateral membranes (BLM) [Am. J. Physiol. 248 (1985) F621]. Data in the present report suggest the participation of a protein kinase C (PKC) in the molecular mechanism of Ang II-mediated stimulation of the Na(+)-ATPase activity due to the following observations: (i) the stimulation of protein phosphorylation in BLM, induced by Ang II, is mimicked by the PKC activator TPA, and is completely reversed by the specific PKC inhibitor, calphostin C; (ii) the Na(+)-ATPase activity is stimulated by Ang II and TPA in the same magnitude, being these effects abolished by the use of the PKC inhibitors, calphostin C and sphingosine; (iii) the Na(+)-ATPase activity is activated by catalytic subunit of PKC (PKC-M), in a similar and nonadditive manner to Ang II; and (iv) Ang II stimulates the phosphorylation of MARCKS, a specific substrate for PKC.

Characterization of cyclin D1 expression in a rat global model of cerebral ischemia

During normal development of the central nervous system there is expression of cyclins that regulate the progression of cells through various stages of mitosis. Cyclins have also been implicated in neuronal degeneration and apoptosis in adult brain, especially cyclin D1 as it is permissive for the transition from growth phase to synthesis phase in mitotic cell division. There is controversy as to whether cyclin D1 expression increases in both in vitro and in vivo models of cerebral ischemia. In this study we use immunohistochemistry and Western blot analysis to characterize cyclin D1 expression in an in vivo rat global model of cerebral ischemia to address the hypothesis that cyclin D1 alterations are involved in ischemic neuronal death. Although there was no change in cyclin D1 expression in either the vulnerable CA1 or resistant CA3 regions of the hippocampus prior to neuronal cell death (<3 days reperfusion), concomitant with the death of CA1 neurons and the loss of cyclin D1 in these cells, there was an increase in non-neuronal cyclin D1 positive cells. Some of the non-neuronal cyclin D1 expressing cells were identified to be activated microglia. In contrast to the cytoplasmic expression of cyclin D1 in neurons, the cyclin D1 expression in the microglia and other non-neuronal cells in CA1 was both nuclear and cytosolic. This study suggests that cyclin D1 does not play a role in the death of vulnerable CA1 neurons in global ischemia.

Stimulation of protein kinase C activity in cells expressing human parathyroid hormone receptors by C- and N-terminally truncated fragments of parathyroid hormone 1-34

The parathyroid hormone (PTH) fragment PTH(1-34) stimulates adenylyl cyclase, phospholipase C (PLC), and protein kinase C's (PKCs) in cells that express human, opossum, or rodent type 1 PTH/PTH-related protein (PTHrP) receptors (PTHR1s). Certain carboxyl (C)-terminally truncated fragments of PTH(1-34), such as human PTH(1-31) [hPTH-(1-31)NH2], stimulate adenylyl cyclase but not PKCs in rat osteoblasts or PLC and PKCs in mouse kidney cells. The hPTH(1-31)NH2 peptide does fully stimulate PLC in HKRK B7 porcine renal epithelial cells that express 950,000 transfected hPTHR1s per cell. Amino (N)-terminally truncated fragments, such as bovine PTH(3-34) [bPTH(3-34)], hPTH(3-34)NH2, and hPTH(13-34), stimulate PKCs in Chinese hamster ovary (CHO) cells expressing transfected rat receptors, opossum kidney cells, and rat osteoblasts, but an intact N terminus is needed to stimulate PLC via human PTHR1s in HKRK B7 cells. We now report that the N-terminally truncated analogs bPTH(3-34)NH2 and hPTH(13-34)OH do activate PKC via human PTHR1s in HKRK B7 cells, although less effectively than hPTH(1-34)NH2 and hPTH(1-31)NH2. Moreover, in a homologous human cell system (normal foreskin fibroblasts), these N-terminally truncated fragments stimulate PKC activity as strongly as hPTH(1-34)NH2 and hPTH(1-31)NH2. Thus, it appears that unlike their opossum and rodent equivalents, hPTHR1s can stimulate both PLC and PKCs when activated by C-terminally truncated fragments of PTH(1-34). Furthermore, hPTHR1s, like the PTHR1s in rat osteoblasts, opossum kidney cells, and rat PTHR1-transfected CHO cells also can stimulate PKC activity by a mechanism that is independent of PLC. The efficiency with which the N-terminally truncated PTH peptides stimulate PKC activity depends on the cellular context in which the PTHR1s are expressed.

Transient NMDA receptor inactivation provides long-term protection to cultured cortical neurons from a variety of death signals

NMDA receptor antagonists, such as (+)-5-methyl-10, 11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate (MK-801), potently block glutamate-induced neuronal death in myriad in vitro cell models and effectively attenuate ischemic damage in vivo. In this report, a novel role for MK-801 and other NMDA receptor antagonists in preconditioning neurons to withstand a wide range of subsequent lethal insults is described. A brief 30 min exposure to 0.1 microM MK-801, applied up to 96 hr before a "lethal" insult, protected primary cortical neurons from a diverse group of neurotoxic agents, including NMDA, beta-amyloid, staurosporine, etoposide, and oxygen-glucose deprivation. This neuroprotective preconditioning by MK-801 arose from transient NMDA receptor inactivation, because the noncompetitive NMDA receptor antagonists memantine and nylindin and the competitive antagonist AP-5 gave similar effects. MK-801 protection was dependent on new protein synthesis during the first 2 hr, but not from 2 to 5 hr, after MK-801 exposure. The MK-801 transient did not alter the ability of NMDA to trigger normally lethal [Ca(2+)](i) influx 48 hr later, but it did block early downstream signaling events coupled to NMDA neurotoxicity, including PKC inactivation and the activation of calpain. Moreover, MK-801 protected neurons from staurosporine-induced apoptosis, although caspase activation in these cells was unimpeded. It is likely that the stress associated with transient inactivation of NMDA receptors triggered a rapid compensatory survival response that provided long-term protection from a spectrum of insults, inducing apoptotic and nonapoptotic death. The possibility that MK-801 preconditioning blocks an event common to seemingly diverse death mechanisms suggests it will be an important tool for obtaining a clearer understanding of the salient molecular events at work in neuronal death and survival pathways.

Involvement of protein kinase C in the presynaptic nicotinic modulation of [(3)H]-dopamine release from rat striatal synaptosomes

1. Presynaptic nicotinic ACh receptors modulate transmitter release in the brain. Here we report their interactions with protein kinase C (PKC) with respect to [(3)H]-dopamine release from rat striatal synaptosomes, monitored by superfusion. 2. Two specific PKC inhibitors, Ro 31-8220 (1 microM) and D-erythro-sphingosine (10 microM) significantly reduced (by 51 and 26% respectively) [(3)H]-dopamine release stimulated by anatoxin-a (AnTx), a potent and selective agonist of nicotinic ACh receptors. The inactive structural analogue of Ro 31-8220, bisindolylmaleimide V (1 microM) had no effect. 3. Two phorbol esters, PDBu (1 microM) and PMA (1 microM) potentiated AnTx-evoked [(3)H]-dopamine release by 50 - 80%. This was Ca(2+)-dependent and prevented by PKC inhibitors. In the absence of nicotinic agonist, phorbol esters enhanced basal release through a PKC-independent mechanism. 4. A (86)Rb(+) efflux assay of nicotinic ACh receptor function confirmed that Ro 31-8220 has no nonspecific effect on presynaptic nicotinic ACh receptors. 5. These results suggest that PKC is activated by nicotinic ACh receptor stimulation and mediates a component of AnTx-evoked [(3)H]-dopamine release. In addition, independent activation of PKC can further amplify the response, offering a potential mechanism for receptor crosstalk.

Phosphorylation cascades control the actions of ethanol on cell cAMP signalling

Our studies indicate that, in the presence of particular isoforms of adenylyl cyclase (i.e., type 7 AC), moderately intoxicating concentrations of ethanol will significantly potentiate transmitter-mediated activation of the cAMP signaling cascade. Activation of this signaling cascade may have important implications for the mechanisms by which ethanol produces intoxication, and/or for the mechanisms of neuroadaptation leading to tolerance to, and physical dependence on, ethanol. We initiated a series of studies to investigate the phosphorylation of AC7 by PKC, the role of this phosphorylation in modulating the sensitivity of AC7 to activation by Gsalpha, and the PKC isotype(s) involved in the phosphorylation of AC7. The T7 epitope-tagged AC7 expressed in Sf9 and HEK293 cells was found to be phosphorylated in vitro by the catalytic subunit of PKC. Treatment of AC7-transfected HEK293 cells with phorbol dibutyrate (PDBu) or ethanol increased the phosphorylation of AC7 and its responsiveness to Gsalpha. In human erythroleukemia (HEL) cells, which endogeneously express AC7, ethanol and PDBu increased AC activity stimulated by PGE(1). The potentiation by both PDBu and ethanol was found to be sensitive to the PKC delta-selective inhibitor, rottlerin. The potentiation of AC activity by ethanol in HEL cells was also selectively attenuated by the RACK inhibitory peptide specific for PKC delta, and by expression of the dominant negative, catalytically inactive, form of PKC delta. These data demonstrate that AC7 can be phosphorylated by PKC, leading to an increase in functional activity, and ethanol can potentiate AC7 activity through a PKC delta-mediated phosphorylation of AC7.

Transient NMDA receptor inactivation provides long-term protection to cultured cortical neurons from a variety of death signals

NMDA receptor antagonists, such as (+)-5-methyl-10, 11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate (MK-801), potently block glutamate-induced neuronal death in myriad in vitro cell models and effectively attenuate ischemic damage in vivo. In this report, a novel role for MK-801 and other NMDA receptor antagonists in preconditioning neurons to withstand a wide range of subsequent lethal insults is described. A brief 30 min exposure to 0.1 microM MK-801, applied up to 96 hr before a "lethal" insult, protected primary cortical neurons from a diverse group of neurotoxic agents, including NMDA, beta-amyloid, staurosporine, etoposide, and oxygen-glucose deprivation. This neuroprotective preconditioning by MK-801 arose from transient NMDA receptor inactivation, because the noncompetitive NMDA receptor antagonists memantine and nylindin and the competitive antagonist AP-5 gave similar effects. MK-801 protection was dependent on new protein synthesis during the first 2 hr, but not from 2 to 5 hr, after MK-801 exposure. The MK-801 transient did not alter the ability of NMDA to trigger normally lethal [Ca(2+)](i) influx 48 hr later, but it did block early downstream signaling events coupled to NMDA neurotoxicity, including PKC inactivation and the activation of calpain. Moreover, MK-801 protected neurons from staurosporine-induced apoptosis, although caspase activation in these cells was unimpeded. It is likely that the stress associated with transient inactivation of NMDA receptors triggered a rapid compensatory survival response that provided long-term protection from a spectrum of insults, inducing apoptotic and nonapoptotic death. The possibility that MK-801 preconditioning blocks an event common to seemingly diverse death mechanisms suggests it will be an important tool for obtaining a clearer understanding of the salient molecular events at work in neuronal death and survival pathways.

Loss of cyclin D1 in necrotic and apoptotic models of cortical neuronal degeneration

Recent evidence suggests that apoptosis in post-mitotic neurons involves an aborted attempt of cells to re-enter the cell cycle and it is characterized by increased expression of cyclins, such as cyclin D1, prior to death. Cyclin D1 increases to permit transition from growth phase (G0/G1) to synthesis phase (S) during normal development but there is controversy as to which of the cyclins are activated prior to apoptotic cell death. We looked at the expression of cyclin D1 in cortical neuronal cultures treated with either staurosporine to produce apoptotic death, or with glutamate, to produce a non-apoptotic death. Cyclin D1 immunoreactivity was observed in the cytoplasm and nucleus of virtually all neurons under control conditions. Following the addition of either staurosporine or glutamate, cyclin D1 immunoreactivity did not change within 4 h. The cyclin D1 immunoreactivity was lost by 6 h with the appearance of either staurosporine-induced fragmented nuclei or glutamate-induced pyknotic nuclei. These immunocytochemical observations were confirmed with immunoblot analysis. Therefore, cyclin D1 is not a reliable indicator of apoptosis in cortical neuronal cultures and should not be used as an indicator of apoptotic cell death.

Adenosine inhibits the renal plasma-membrane (Ca2+ + Mg2+)-ATPase through a pathway sensitive to cholera toxin and sphingosine

Adenosine, a potent autacoid produced and released in kidneys, affects nearly all aspects of renal function, and an increase in cytosolic calcium has been implicated in adenosine effects. The aim of this work was to investigate whether adenosine modifies the calcium pump present in basolateral membranes of kidney proximal tubule cells. Adenosine exerts a biphasic influence on (Ca2+ + Mg2+)-ATPase activity. Inhibition occurs up to 0.1 microM and then gradually disappears as the adenosine concentration increases to 100 microM, an effect mimicked by the adenosine analog N6-cyclohexyladenosine, which preferentially binds to A1-type receptors. In contrast, the A2 receptor agonist 5', N-ethylcarboxamideadenosine is ineffective. The A1 receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine blocks the inhibitory effect of 0.1 microM adenosine and stimulates (Ca2+ + Mg2+)-ATPase activity in the presence of 1 mM adenosine, a concentration high enough to occupy the low-affinity A2 receptors. Inhibition by adenosine increases as medium ATP is lowered to micromolar concentrations, is maintained in the presence of pertussis toxin, and is completely abolished with 0.1 microM cholera toxin or 1 microM sphingosine. The inhibitory effect of adenosine can be reproduced by guanosine 5'-[gamma-thio]triphosphate, inositol 1,4, 5-trisphosphate or the diacylglycerol analog 12-O-tetradecanoylphorbol 13-acetate. In conjunction with the selectivity for its analogs and for its receptor agonist, the concentration profile of adenosine effects indicates that both inhibitory (A1) and stimulatory (A2) receptors are involved. The results obtained with the toxins indicate that a pathway that is modulated by G-proteins, involves a phospholipase C and a protein kinase C, and is affected by local variations in adenosine concentrations participates in the regulation of the (Ca2+ + Mg2+)-ATPase resident in basolateral membranes of kidney proximal tubules.

Changes in nuclear protein kinase C-delta holoenzyme, its catalytic fragments, and its activity in polyomavirus-transformed pyF111 rat fibroblasts while proliferating and following exposure to apoptogenic topoisomerase-II inhibitors

Protein kinase C-delta (PKC-delta) appears to be variously involved in proliferation and apoptosis. To compare the changes of this enzyme in these two processes, we have determined the levels and activities of the 79-kDa PKC-delta holoenzyme and its catalytically active 47- and 40-kDa C-terminal fragments in the nuclei of proliferating untreated polyomavirus-transformed pyF111 rat fibroblasts and pyF111 cells treated with the apoptogenic topoisomerase-II inhibitors VP-16 (etoposide), VM-26 (teniposide), and doxorubicin. PyF111 cells were chosen because they hyperexpress PKC-delta and they are hypersusceptible to apoptosis because they do not express the antiapoptotic proteins Bcl-2 and Bcl-XL. The highest PKC-delta activity in cells before they started proliferating or were exposed to one of the inhibitors was in the NM (nuclear envelope-containing) fraction, which contained the holoenzyme and both C-terminal fragments, while only the two fragments were in the nucleoplasmic (NP) fraction where they were tightly associated with chromatin. When the cells began proliferating the amounts of the PKC-delta holoenzyme and the two fragments increased in the NM and the NP fractions and the already high PKC-delta activity either increased or stayed the same in these fractions until the end of the 72-h incubation. And there was no leakage of cytochrome c from the mitochondria into the cytoplasm. VP-16 exposure caused a prompt release of cytochrome c from the mitochondria into the cytosol and at the same time triggered a sharp drop (35% by 3 h and 60% by 6 h) in the PKC-delta activity in the NM fraction without changing the actual amounts of the holoenzyme or its fragments. This prompt inactivation of PKC-delta and its fragments during the first 6 h of exposure to the drug was not due to their dephosphorylation and could not be reversed by phosphatidylserine and/or 12-O-tetradecanoylphorbol 13-acetate (TPA). Between 6 and 24 h the PKC-delta activity in the NM fraction dropped a further 20%, the kinase's activity transiently surged in the NP fraction, and cytoplasmic CPP-32-like (DEVD-specific caspase) activity increased without an increase in the proteolysis of nuclear PKC-delta or PARP. Between 24 and 72 h nuclear CPP-32-like activity increased along with a massive proteolysis of PKC-delta, an accumulation of various PKC-delta fragments, and the cleavage of PARP. But despite this proteolysis, the cells were still able to maintain or even increase the amounts of holoenzyme and 40- and 47-kDa fragments in the NM and NP fractions before dying. VM-26 and doxorubicin caused the same prompt release of cytochrome c from the mitochondria and dramatic drop of NM PKC-delta activity as did VP-16. Thus, high levels of activity of nuclear PKC-delta, particularly PKC-delta in the nuclear membrane, might have a role driving the cell cycle of pyF111 cells. On the other hand, the prompt and sustained large drop in the activity of PKC-delta at this site that precedes the onset of the caspase-mediated proteolysis of the isoform may be involved in starting and driving apoptogenesis in pyF111 fibroblasts exposed to topoisomerase-II inhibitors.

Evidence from cultured rat cortical neurons of differences in the mechanism of ischemic preconditioning of brain and heart

Ca2+ influx and activation of protein kinase C (PKC) and mitogen-activated protein kinase (MAPK) during nonlethal ischemic preconditioning have been implicated in the protection of the heart against subsequent lethal ischemic injury. Thus, we determined if Ca2+ influx, PKC and MAPK also mediate ischemic preconditioning-induced protection in neurons. Preconditioning by exposure of E18 rat cortical cultures to 90 min of nonlethal oxygen-glucose deprivation (OGD) 24 h prior to 180-240 min of lethal OGD was neuroprotective. Exposure to nominally free Ca2+, or blockade of the alpha-amino-hydroxy-5-methyl-isoxazolepropionate (AMPA) receptor with CNQX did not eliminate protection. MAPK activity did not change and PKC activity decreased by 50% relative to normal baseline levels at 0 and 24 h following preconditioning. The sustained decrease in PKC activity was not due to a loss of enzyme as determined from immunoblots using pan and epsilon-, beta- and zeta-specific PKC antibodies. Neuroprotection was maintained with pharmacological inhibition of PKC activity by staurosporine, chelerythrine and calphostin C and MAPK activity by PD 98059 during preconditioning, indicating that activation of these enzymes during preconditioning was not necessary for protection. Therefore, in contrast to cardiac tissue, ischemic preconditioning of neurons does not require activation of PKC and MAP kinase, and protection is maintained with substantial removal of extracellular Ca2+ or blockade of the AMPA receptor.

Activation of DNA-dependent protein kinase may play a role in apoptosis of human neuroblastoma cells

Treating SH-SY5Y human neuroblastoma cells with 1 microM staurosporine resulted in a three- to fourfold higher DNA-dependent protein kinase (DNA-PK) activity compared with untreated cells. Time course studies revealed a biphasic effect of staurosporine on DNA-PK activity: an initial increase that peaked by 4 h and a rapid decline that reached approximately 5-10% that of untreated cells by 24 h of treatment. Staurosporine induced apoptosis in these cells as determined by the appearance of internucleosomal DNA fragmentation and punctate nuclear morphology. The maximal stimulation of DNA-PK activity preceded significant morphological changes that occurred between 4 and 8 h (40% of total number of cells) and increased with time, reaching 70% by 48 h. Staurosporine had no effect on caspase-1 activity but stimulated caspase-3 activity by 10-15-fold in a time-dependent manner, similar to morphological changes. Similar time-dependent changes in DNA-PK activity, morphology, and DNA fragmentation occurred when the cells were exposed to either 100 microM ceramide or UV radiation. In all these cases the increase in DNA-PK activity preceded the appearance of apoptotic markers, whereas the loss in activity was coincident with cell death. A cell-permeable inhibitor of DNA-PK, OK-1035, significantly reduced staurosporine-induced punctate nuclear morphology and DNA fragmentation. Collectively, these results suggest an intriguing possibility that activation of DNA-PK may be involved with the induction of apoptotic cell death.

Stimulation of membrane-associated protein kinase-C activity in spleen lymphocytes by hPTH-(1-31)NH2, its lactam derivative, [Leu27]-cyclo(Glu22-Lys26)-hPTH-(1-31)NH2, and hPTH-(1-30)NH2

Human parathyroid hormone, hPTH-(1-34), stimulates adenylyl cyclase and phosphatidylinositol-bisphosphate-specific phospholipase-C (PIP2-PLC), as indicated by increased membrane-associated protein kinase C (PKC) activity in ROS 17/2 rat osteosarcoma cells. The C-terminally truncated hPTH-(1-31)NH2 stimulates adenylyl cyclase as strongly as hPTH-(1-34) in these cells, but it does not stimulate PKC activity. Even [Leu27]-cyclo(Glu22-Lys26)-hPTH-(1-31)NH2, a 6-fold stronger adenylyl cyclase stimulator than hPTH-(1-34), cannot stimulate PKC activity in ROS cells. Therefore PTH required its 32-34 region to stimulate PIP2-PLC/PKCs in this osteosarcoma line. In contrast, hPTH-(1-31)NH2 [Leu27]-cyclo(Glu22-Lys26)-hPTH-(1-31)NH2 and even hPTH-(1-30)NH2 can stimulate PKC activity in freshly isolated rat spleen lymphocytes as strongly as hPTH-(1-34)NH2. The difference in the ability of membrane-associated PKC activity in spleen lymphocytes, but not in ROS cells, to be stimulated by C-terminally truncated PTH fragments might be due to different receptor densities or to the lymphocyte's atypical PTH/PTHrP receptor.

Evidence that brain-derived neurotrophic factor neuroprotection is linked to its ability to reverse the NMDA-induced inactivation of protein kinase C in cortical neurons

Several lines of evidence indicate that a rapid loss of neuronal protein kinase C (PKC) activity is a characteristic feature of cerebral ischemia and is a necessary step in the NMDA-induced death of cultured neurons. Exposing embryonic day 18 primary rat cortical neurons to 50 microM NMDA or 50 microM glutamate for 10 min caused approximately 80% cell death over the next 24 h, but excitotoxic death was largely averted, i.e., by 70-80%, in cells pretreated with brain-derived neurotrophic factor (BDNF). An 8-h preexposure to BDNF (50-100 ng/ml) maximally protected cortical cells from the effects of NMDA and glutamate, although the transient application of BDNF between 8 and 4 h before NMDA was equally protective. These effects of BDNF were abolished at supralethal, i.e., >100 microM, NMDA concentrations. It is significant that BDNF pretreatment prevented the inactivation of PKC in cortical cells normally seen 30 min to 2 h following lethal NMDA or glutamate exposure. This BDNF effect did not arise from changes in NMDA channel activity because neither whole-cell NMDA current amplitudes nor increases in intracellular free Ca2+ concentration were altered by the 8-h BDNF pretreatment. Furthermore, BDNF offered no neuroprotection to cells treated with the PKC inhibitors staurosporine (10-20 nM), calphostin C (1-2.5 microM), or GF-109203X (100 nM) at the time of NMDA addition. These results underscore the importance of PKC inactivation in glutamate-induced neuronal death. They also suggest that BDNF neuroprotection arises, at least in part, via its ability to block the mechanism by which pathophysiological Ca2+ influx through the NMDA receptor causes membrane PKC inactivation.

Brain derived neurotrophic factor induction of N-methyl-D-aspartate receptor subunit NR2A expression in cultured rat cortical neurons

N-methyl-D-aspartate (NMDA) receptor subunit expression changes during development and following injury in several brain regions. These changes may be mediated by neurotrophic factors, such as brain derived neurotrophic factor (BDNF). Exposure of cultured cortical neurons to BDNF (100 ng/ml) for 24 h produced a significant decrease in the NMDA-induced whole-cell currents sensitive to the NR2B subunit selective NMDA receptor antagonist, CP-101,606, suggesting a relative decrease in NR2B subunit expression. There was a significant increase in NR2A by Western blot analysis. Consistent with the electrophysiology and Western blot analysis, reverse transcriptase-polymerase chain reaction (RT-PCR) amplification revealed that BDNF caused a significant increase in relative NR2A subunit expression, a significant decrease in relative NR2B subunit expression and no change in relative NR2C subunit expression. These results suggest that BDNF enhances NMDA receptor maturation, warranting further study of the mechanism of BDNF effects on NMDA receptor subunit expression and the role these effects play in development and neuronal injury.

Comparison of the changes in protein kinase C induced by glutamate in primary cortical neurons and by in vivo cerebral ischaemia

Changes in protein kinase C (PKC) were compared in primary cortical neurons exposed to glutamate and in the CA-1 hippocampal region of rats subjected to transient cerebral ischaemia. After a 15-min exposure of cortical neurons to excitotoxic levels of glutamate, a 50-60% loss of membrane PKC activity but only about a 20% loss in the amount of enzyme was observed, suggesting that in addition to enzyme loss other mechanisms also contributed to the overall loss of membrane PKC activity. Glutamate induced a 25-40% decrease in immunodetectable levels of PKC alpha, beta, gamma, and lambda but no detectable changes in PCK epsilon and zeta. The loss of PKC activity coincided with a shift in electrophoretic mobility of PKC gamma, epsilon, and lambda, but not of PKC alpha, beta, or zeta, suggesting post-translational modification of some PKC isoforms. By comparison, in rats subjected to transient (15-min) global ischaemia, a similar 50-60% decrease in membrane PKC activity, a 20-25% loss in the amount of PKC, and a shift in PKC mobility were observed in CA-1 neurons 6 h post-reperfusion. In both the in vivo and the in vitro "ischaemic" models, administration of the AMPA receptor antagonist NBQX prevented the loss of PKC activity. These results indicate that the loss of PKC observed in in vivo ischaemia is likely to be due to excitotoxic damage and that this event can be closely mirrored in primary neuronal cultures damaged by glutamate.

The presence of an unusual PKC isozyme profile in rat liver cells

We have previously shown that protein kinase C (PKC) is involved in the mitogenic response of T51B cells to epidermal growth factor. In fact, epidermal growth factor was an excellent mitogen, even after prolonged pretreatment of cells with TPA, suggesting that the PKC isoform implicated in proliferation is not down-regulated by 12-O-tetradecanoyl phorbol-13-acetate (TPA). We have now determined that the PKC isozymes -α, -βI, -δ, -ε, and -ζ are present in T51B cells. All five isoforms are associated with the plasma membrane and the cytoplasm and are either in or around the nucleus. PKC-βI has a slightly different subcellular profile from that of the other isoforms in that it is clearly and strongly associated with the nuclear membrane. Also, a unique and novel pattern is obtained from immunoblots with anti-PKC-βI. PKC-βI is detected as a single band of 70 kDa in the cytosolic fraction and as a doublet of 65 and 77 kDa in the membrane fraction. PKC-α, -δ, and -ε were down-regulated by pretreatment of cells with TPA, while PKC-ζ was unaffected. Of particular interest was the fact that TPA did not down-regulate PKC-βI. In fact, the amount of this isoform associated with the plasma membrane increased. These findings indicate that it is probably PKC-βI that is involved in the mitogenic response of T51B cells to epidermal growth factor. Since PKC-ζ is also not down-regulated by TPA, the possible involvement of this isoform needs to be resolved.Key words: protein kinase C, intracellular localization, cell proliferation, liver.

Receptor-mediated activation of protein kinase C in hippocampal long-term potentiation: facts, problems and implications

During the last decade hippocampal long-term potentiation has become one of the most frequently used models to study cellular mechanisms of learning and memory. Receptor-mediated activation of protein kinase C is thought to be involved in LTP stabilisation. In the present review, 1. the molecular structure and activation mechanisms of PKC isoenzymes, 2. the biochemical evidences for PKC activation after induction of LTP using different stimulation paradigms as well as 3. the involvement of metabotropic glutamate receptors in PKC activation after induction of LTP are critically discussed.

Activation of metabotropic glutamate receptors increases endogenous protein kinase C substrate phosphorylation in adult hippocampal slices

We previously reported (Staak, S., Behnisch, T. and Angenstein, F., Hippocampal long-term potentiation: transient increase but no persistent translocation of protein kinase C (PKC) isoenzymes alpha and beta, Brain Res., 682 (1995) 55-62) that Ca(2+)-dependent PKC isoenzymes alpha/beta and gamma are not translocated between subcellular compartments after stimulation of glutamate receptor subtypes in hippocampal slices. Extending our previous work in this study in situ phosphorylation of endogenous PKC substrates and the translocation of novel PKC isoenzymes delta and epsilon was analysed to detect PKC activation. Two proteins of approximately 94 kDa and 18 kDa were first characterised to be specific PKC substrates. As control of the technique carbachol was shown to increase in situ phosphorylation of the two substrates without any measurable translocation of PKC protein. Activation of metabotropic glutamate receptors by 50 microM DHPG also increased the situ-phosphorylation by 43.9% (94 kDa) and 32.8% (18 kDa) compared to controls but did not induce a measurable subcellular redistribution of conventional and novel PKC isoenzymes. Stimulation by 50 microM trans-ACPD or 0.1 mM quisqualate enhanced the situ phosphorylation in the same range, whereas 0.1 mM NMDA was ineffective. To our knowledge this is the first report showing a direct link between metabotropic glutamate receptor activation and increased endogenous PKC substrate phosphorylation in adult hippocampal slices. This PKC activation was not detectable by a redistribution of enzyme protein between subcellular compartments. We, therefore, conclude, that the failure to detect PKC translocation in physiological experiments is not an indicator for unchanged enzyme activity.

Protein kinase C in IL-2 signal transduction

Interleukin-2 (IL-2), secreted principally by activated helper T-cells, plays a pivotal role in the generation and regulation of the immune response. The various biologic functions of IL-2 have been the focus of intensive study over the years and have been well worked out. By contrast, an understanding of the intracellular signals coupled to the IL-2 receptor and responsible for mediating IL-2 effects in T-cells is far less developed, and the role that protein kinase C (PKC) may play in the various cellular responses to IL-2 receptor activation is unclear. In this article we will discuss IL-2, its receptors, and IL-2 signal transduction in relation to the physiological roles PKC activation may play in IL-2-mediated activation of T-cells and other hematopoietic cells.

Pulse treatment with the tumor promoter TPA delays the onset of desensitization response and prolongs the inhibitory effect on gap junctional intercellular communication of a rat liver epithelial cell line WB F-344

The tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) is an inhibitor of gap junctional intercellular communication (GJIC) of the rat liver epithelial cell line, WB F-344. We have previously reported that prolonged treatment of the WB cells with TPA (10 ng/ml) caused a reversal of the inhibition of GJIC that was initially induced (Oh, S.Y., et al. (1988) Carcinogenesis, 9, 135-139). Under this condition, addition of fresh TPA did not inhibit GJIC of these cells. In the present investigation we examined whether pulse exposure to TPA delays the onset of this desensitization response. Cultures were treated for 5 or 15 min with TPA and shifted to normal medium. Intercellular communication was measured at 15 min, 1 h and 6 h after the 5 or 15 min pulse treatments. Under these pulse treatment conditions, GJIC of the cells was markedly inhibited for up to 4 h and gradually reverted to near control levels by 6-8 h. At every sixth hour of pulse treatment the cells were given an additional pulse treatment (5 or 15 min) and the inhibitory effect of TPA on the GJIC of the cells was assayed 15 min after each such treatment. The results clearly showed that, when the cells were treated with 10 ng/ml TPA for 5 or 15 min every 6 h they maintained their sensitivity to the inhibitory effect of TPA on GJIC. This response to TPA was sustained for a considerably longer time when the duration of the pulse treatment was 5 min. Our data suggested that pulse exposure to TPA delays the desensitization response normally observed in prolonged treatment regimens and that this delay is possibly due to maintenance of the TPA activatable pool of protein kinase C under these conditions.

C-terminal fragment of parathyroid hormone-related protein, PTHrP-(107-111), stimulates membrane-associated protein kinase C activity and modulates the proliferation of human and murine skin keratinocytes

Low concentrations of the C-terminal parathyroid hormone-related protein (PTHrP) fragments, PTHrP-(107-111) and PTHrP-(107-139), stimulated membrane-associated protein kinase Cs (PKCs), but not adenylyl cyclase or an internal Ca2+ surge, in early passage human skin keratinocytes and BALB/MK-2 murine skin keratinocytes. The fragment maximally stimulated membrane-associated PKCs in BALB/MK-2 cells at 5 x 10(-9) to 10(-8) M. The maximally PKC-stimulating concentrations of PTHrP-(107-111) also stopped or stimulated BALB/MK-2 keratinocyte proliferation depending on whether the cells were, respectively, cycling or quiescent at the time of exposure. Thus, just one brief (30-minute) pulse of 10(-8) M PTHrP-(107-111) stopped the proliferation of BALB/MK-2 keratinocytes for at least 5 days. On the other hand, daily 30-minute pulses of 10(-8) M PTHrP-(107-111) started and then maintained the proliferation of initially quiescent BALB/MK-2 cells. Similarly PTHrP-(107-111) inhibited DNA synthesis by cycling primary adult human keratinocytes, but it stimulated DNA synthesis by quiescent human keratinocytes.

Mechanisms of 1S,3R-ACPD-induced neuroprotection in rat hippocampal slices subjected to oxygen and glucose deprivation

The efficacy and mechanisms of 1-amino-cyclopentyl-1S,3R-dicarboxylate (1S,3R-ACPD)-induced neuroprotection were investigated in rat hippocampal slices subjected to 10 min of oxygen and glucose deprivation. Neuronal viability was assessed by measuring both the amplitude of evoked population spike in the CA1 pyramidale and by imaging CA1 neurons using a live/dead fluorescence assay with confocal microscopy. CA1 pyramidal neurons in oxygen-glucose deprived slices remained viable for up to 120 min following the insult but were dead by 240 min. Pretreatment with 1S,3R-ACPD significantly protected the oxygen-glucose deprived slices in a concentration-dependent fashion. Oxygen-glucose deprived slices pretreated for the same period with the protein kinase C (PKC) activation phorbol 12-myristate 13-acetate (PMA; 1 microM) were significantly protected whereas oxygen-glucose deprived slices treated with the adenylyl cyclase activator, forskolin (30 microM) were not. Oxygen-glucose deprivation induced a rapid and persistent decrease (approximately 50%) in PKC activity and a > 6 fold increase in cyclic adenosine monophosphate (cAMP) levels in whole hippocampal slices. While 1S,3R-ACPD did not stimulate PKC activity and had no effect on basal cAMP in whole slices, it significantly enhanced the rate of return of cAMP to basal levels following reperfusion. Consistent with this observation, the 1S,3R-ACPD-induced neuroprotection was inhibited by forskolin (30 microM). These results suggest that in vitro neuroprotection of CA1 neurons by 1S,3R-ACPD involves metabotropic glutamate receptors negatively linked to cAMP and possibly those which increase PKC activity.

Protein kinase C mediation of Ca(2+)-independent contractions of vascular smooth muscle

Tumour-promoting phorbol esters induce slow, sustained contractions of vascular smooth muscle, suggesting that protein kinase C (PKC) may play a role in the regulation of smooth muscle contractility. In some cases, e.g., ferret aortic smooth muscle, phorbol ester induced contractions occur without a change in [Ca2+]i or myosin phosphorylation. Direct evidence for the involvement of PKC came from the use of single saponin-permeabilized ferret aortic cells. A constitutively active catalytic fragment of PKC induced a slow, sustained contraction similar to that triggered by phenylephrine. Both responses were abolished by a peptide inhibitor of PKC. Contractions of similar magnitude occurred even when the [Ca2+] was reduced to close to zero, implicating a Ca(2+)-independent isoenzyme of PKC. Of the two Ca(2+)-independent PKC isoenzymes, epsilon and zeta, identified in ferret aorta, PKC epsilon is more likely to mediate the contractile response because (i) PKC epsilon, but not PKC zeta, is responsive to phorbol esters; (ii) upon stimulation with phenylephrine, PKC epsilon translocates from the sarcoplasm to the sarcolemma, whereas PKC zeta, translocates from a perinuclear localization to the interior of the nucleus; and (iii) when added to permeabilized single cells of the ferret aorta at pCa 9, PKC epsilon, but not PKC zeta, induced a contractile response similar to that induced by phenylephrine. A possible substrate of PKC epsilon is the smooth muscle specific, thin filament associated protein, calponin. Calponin is phosphorylated in intact smooth muscle strips in response to carbachol, endothelin-1, phorbol esters, or okadaic acid. Phosphorylation of calponin in vitro by PKC (a mixture of alpha, beta, and gamma isoenzymes) dramatically reduces its affinity for F-actin and alleviates its inhibition of the cross-bridge cycling rate. Calponin is phosphorylated in vitro by PKC epsilon but is a very poor substrate of PKC zeta. A signal transduction pathway is proposed to explain Ca(2+)-independent contraction of ferret aorta whereby extracellular signals trigger diacylglycerol production without a Ca2+ transient. The consequent activation of PKC epsilon would result in calponin phosphorylation, its release from the thin filaments, and alleviation of inhibition of cross-bridge cycling. Slow, sustained contraction then results from a slow rate of cross-bridge cycling because of the basal level of myosin light chain phosphorylation (approximately 0.1 mol Pi/mol light chain). We also suggest that signal transduction through PKC epsilon is a component of contractile responses triggered by agonists that activate phosphoinositide turnover; this may explain why smooth muscles often develop more force in response, e.g., to alpha 1-adrenergic agonists than to K+.

Phosphatidic acid activation of protein kinase C in LA-N-1 neuroblastoma cells

Phosphatidic acid (PA), a hydrolytic product of phospholipase D activity, stimulated cytosolic protein kinase C (PKC) activity when LA-N-1 neuroblastoma cells in culture were treated with PA, without translocating the enzyme to the membrane. Treatment of cells with 12-O-tetradecanoylphorbol-13-acetate (TPA) translocated and activated PKC in a dogmatic manner. Partially purified PKC activity derived from LA-N-1 neuroblastoma cells was stimulated by PA alone or in the presence of phosphatidylserine or TPA, without affecting [3H]phorbol dibutyrate binding, indicating that the site of action of PA was different from the phorbol ester or diacylglycerol binding site. These results suggest an unorthodox pattern of PKC stimulation mediated by PA which appears to be yet another mode of PA signal transduction.

Cellular distribution of protein kinase C isozymes in CD3-mediated stimulation of human T lymphocytes with aging

Protein kinase C (PKC) is involved in a variety of cellular responses, such as the expression and secretion of IL-2, the regulation of cytotoxic killing and cell proliferation. It is known that these immune functions are altered with aging. Here, we show that anti-CD3-triggered T cell proliferation is significantly decreased with aging and that H7, an inhibitor of PKC, impairs the anti-CD3-induced T cell proliferation in a differential manner, lymphocytes of healthy young subjects being more sensitive to the PKC inhibitor than those of elderly subjects. We examined (Western blot) the presence and the cellular distribution of PKC isozymes in T lymphocytes of healthy young and elderly subjects in the resting state and after anti-CD3 mAb stimulation using antibodies directed against PKC alpha, beta, delta, epsilon and zeta isoforms in the cytosol and the plasma membrane fractions. These five PKC isotypes were present in human T cells of young and elderly subjects. However, their distribution between the cytosolic and membrane fractions varied according to the isozymes and the age of the subjects. In resting lymphocytes of young subjects, all the PKC isozymes were found in the cytosolic fraction, except PKC-zeta. In resting lymphocytes of elderly subjects PKC-zeta and -epsilon were almost equally distributed between the cytosolic and the membrane fractions, whereas PKC-alpha and -zeta were mainly found in the membrane fraction and PKC-beta was almost exclusively located in the cytosolic fraction. The translocation of PKC-alpha, -beta, -delta and -epsilon could be observed under anti-CD3 mAb stimulation in lymphocytes of young subjects, while in the case of elderly subjects only the PKC beta isoform was translocated. Our results suggest tha the decreased availability of cytosolic PKC may contribute to the diminished PKC-dependent responses to CD3-triggered stimulation of human T lymphocytes with aging.

35H, a sequence isolated as a protein kinase C binding protein, is a novel member of the adducin family

We recently cloned a partial cDNA (35H) for a protein kinase C (PKC) binding protein from a rat kidney cDNA library and demonstrated that it is a PKC substrate in vitro (Chapline, C., Ramsay, K., Klauck, T., and Jaken, S. (1993) J. Biol. Chem. 268, 6858-6861). Additional library screening and 5' rapid amplification of cDNA ends were used to obtain the complete open reading frame. Amino acid sequence analysis, DNA sequence analysis, and Northern analysis indicate that 35H is a unique cDNA related to alpha-and beta-adducins. Antisera prepared to the 35H bacterial fusion protein recognized two polypeptides of 80 and 90 kDa on immunoblots of kidney homogenates and cultured renal proximal tubule epithelial cell extracts. The 35H-related proteins were similar to alpha- and beta-adducins in that they were preferentially recovered in the Triton X-100-insoluble (cytoskeletal, CSK) fraction of cell extracts and were predominantly localized to cell borders. Phorbol esters stimulated phosphorylation of CSK 35H proteins, thus emphasizing that sequences isolated according to PKC binding activity in vitro are also PKC substrates in vivo. The phosphorylated forms of the 35H proteins were preferentially recovered in the soluble fraction, thus demonstrating that phosphorylation regulates their CSK association and, thereby, their function in regulating cytoskeletal assemblies. We have isolated another PKC binding protein partial cDNA (clone 45) from a rat fibroblast library with substantial homology to alpha-adducin. Antisera raised against this expressed sequence recognized a protein of 120 kDa, the reported size of alpha-adducin, on immunoblots of renal proximal tubule epithelial cell extracts. A 120-kDa protein that cross-reacts with the clone 45 (alpha-adducin) antisera coprecipitated with 35H immunecomplexes, indicating that alpha-adducin associates with 35H proteins in vivo. Taken together, these results indicate that 35H is a new, widely expressed form of adducin capable of forming heterodimers with alpha-adducin. We propose naming this adducin homologue gamma-adducin.

Ca2+ x calmodulin prevents myristoylated alanine-rich kinase C substrate protein phosphorylation by protein kinase Cs in C6 rat glioma cells

Ionomycin stimulated membrane-associated protein kinase Cs (PKCs) activity in C6 rat glioma cells as much as the potent PKCs stimulator 12-O-tetradecanoyl phorbol 13-acetate (TPA). However, while TPA, as expected, powerfully stimulated the phosphorylation of the PKCs' 85-kDa myristoylated alanine-rich protein kinase C substrate (MARCKS) protein, ionomycin unexpectedly did not. Instead, ionomycin reduced the basal MARCKS phosphorylation. Pretreating the glioma cells with ionomycin prevented TPA-stimulated PKCs from phosphorylating the MARCKS protein. The stimulation of membrane PKCs activity and the prevention of MARCKS phosphorylation by ionomycin required external Ca2+ because they were both abolished by adding 5 mM EGTA to the culture medium. Recently (Chakravarthy, B. R., Isaacs, R. J., Morley, P., Durkin, J. P., and Whitfield, J. F. (1995) J. Biol. Chem. 270, 1362-1368), we proposed that Ca2+ x calmodulin complexes block MARCKS phosphorylation by the activated PKCs in keratinocytes stimulated by raising the external Ca2+ concentration. In the present experiments calmodulin prevented MARCKS phosphorylation by TPA-stimulated PKCs in glioma cell lysates, and this blockade was lifted by a calmodulin antagonist, the calmodulin-binding domain peptide. But, physiologically more significant, pretreating intact glioma cells with a cell-permeable calmodulin antagonist, calmidazolium, prevented ionomycin from blocking MARCKS phosphorylation by PKCs in unstimulated and TPA-stimulated cells. The effect of ionomycin on MARCKS phosphorylation was not due to the stimulation of Ca2+ x calmodulin-dependent phosphoprotein phosphatase, calcineurin, because cyclosporin A, a potent inhibitor of this phosphatase, did not stop ionomycin from preventing MARCKS phosphorylation. The ability of ionomycin to prevent TPA-stimulated PKCs from phosphorylating MARCKS depended on whether ionomycin was added before, with, or after TPA. Maximum blockade occurred when ionomycin was added before TPA but was less effective when added with or after TPA. These results indicate that Ca2+ x calmodulin can profoundly affect PKCs' signaling at the substrate level.

Distribution and translocation of isoforms of protein kinase C in rat submandibular acinar cells

The distribution of six isoforms of protein kinase C (PKC) in seromucous acinar cells of rat submandibular gland was examined and their translocation from the cytosolic- to the membrane fraction after different stimuli investigated. Western blotting, immunostaining with isoform-specific antibodies and scanning densitometry showed that PKC-alpha and epsilon were distributed fairly evenly between the cytosol and membranes in resting cells, while isoforms- beta, delta and zeta were all predominantly localized (over 80%) in membranes. PKC-gamma was not detected. PKC-alpha was mobilized to the membrane fraction by the phorbol ester, TPA, but not by the phosphoinositide-coupled agonists carbachol, methoxamine and substance P (SP). PKC-epsilon was translocated by TPA and carbachol but not by SP or methoxamine. Biochemical assay of total PKC confirmed that cytosolic enzyme activity was significantly reduced by TPA and carbachol to 29% and 75% respectively of control levels. These results suggest that muscarinic regulation of the mucosecretory response in the rat submandibular gland may be mediated by the PKC-epsilon isoform.