Transmembrane Ca2+ signaling and a new class of inhibitors

KN-62, a selective inhibitor of Ca2+/calmodulin-dependent protein kinase II, inhibits the lysozyme pre-mRNA splicing in myelomonocytic HD11 cells

The lysozyme primary transcript has been shown to be slowly spliced, particularly in LPS-activated myelomonocytic HD11 cells. In this study, Northern blot analysis shows that the splicing of lysozyme pre-mRNA in LPS-activated cells is significantly delayed by treatment with KN-62, a selective inhibitor of the Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), but not with Gö 6976 and herbimycin A, inhibitors of Ca(2+)-dependent PKC and PTK, respectively. In vitro kinase assay using autocamtide 2 as specific substrate for CaMKII demonstrates that KN-62, when added to the extract from HD11 cells, inhibits selectively CaMKII activity. Treatment of HD11 cells with cycloheximide, a potent inhibitor of protein synthesis, results in a transient increase in lysozyme pre-mRNA levels, whilst the mature mRNA levels are not increased. Moreover, neither cycloheximide nor KN-62 has any effect on the glyceraldehyde-3-phosphate dehydrogenase pre-mRNA splicing. Together, our results indicate that phosphorylation by CaMKII, and probably new protein synthesis may be required for the lysozyme pre-mRNA processing.

Mechanisms in histamine-mediated secretion from adrenal chromaffin cells

The great majority of the sustained secretory response of adrenal chromaffin cells to histamine is due to extracellular Ca(2+) influx through voltage-operated Ca(2+) channels (VOCCs). This is likely to be true also for other G protein-coupled receptor (GPCR) agonists that evoke catecholamine secretion from these cells. However, the mechanism by which these GPCRs activate VOCCs is not yet clear. A substantial amount of data have established that histamine acts on H(1) receptors to activate phospholipase C via a Pertussis toxin-resistant G protein, causing the production of inositol 1,4,5-trisphosphate and the mobilisation of store Ca(2+); however, the molecular events that lead to the activation of the VOCCs remain undefined. This review will summarise the known actions of histamine on cellular signalling pathways in adrenal chromaffin cells and relate them to the activation of extracellular Ca(2+) influx through voltage-operated channels, which evokes catecholamine secretion. These actions provide insight into how other GPCRs might activate Ca(2+) influx in many excitable and non-excitable cells.

Assembly of gap junction channels: mechanism, effects of calmodulin antagonists and identification of connexin oligomerization determinants

The assembly of connexins (Cxs) into gap junction intercellular communication channels was studied. An in vitro cell-free synthesis system showed that formation of the hexameric connexon hemichannels involved dimeric and tetrameric connexin intermediates. Cx32 contains two putative cytoplasmic calmodulin-binding sites, and their role in gap junction channel assembly was investigated. The oligomerization of Cx32 into connexons was reversibly inhibited by a calmodulin-binding synthetic peptide, and by W7, a naphthalene sulfonamide calmodulin antagonist. Removing the calmodulin-binding site located at the carboxyl tail of Cx32 limited connexon formation and resulted in an accumulation of intermediate connexin oligomers. This truncation mutant, Cx32Delta215, when transiently expressed in COS-7 cells, accumulated intracellularly and had failed to target to gap junctions. Immunoprecipitation studies suggested that a C-terminal sequence of Cx32 incorporating the calmodulin-binding site was required for the formation of hetero-oligomers of Cx26 and Cx32 but not for Cx32 homomeric association. A chimera, Cx32TM3CFTR, in which the third transmembrane and proposed channel lining sequence of Cx32 was substituted by a transmembrane sequence of the cystic fibrosis transmembrane conductance regulator, did not oligomerize in vitro and it accumulated intracellularly when expressed in COS-7 cells. The results indicate that amino-acid sequences in the third transmembrane domain and a calmodulin-binding domain in the cytoplasmic tail of Cx32 are likely candidates for regulating connexin oligomerization.

Nuclear accumulation of c-myc mRNA in phytohaemagglutinin-activated T lymphocytes treated with anti-HLA class I monoclonal antibody

Anti-HLA class I monoclonal antibody 01.65 inhibits the proliferative response of PHA-activated human T lymphocytes from peripheral blood mononuclear cells. The recruitment rate in the cell cycle is slack and the G1 and S phases are prolonged. Among the early events after PHA activation, only the calcium-dependent PKC activity appears to be modified: particulate PKC is completely depleted while cytosolic residual PKC is reduced by 80% after MAb 01.65 treatment. We have carried out in greater detail the study of c-myc gene regulation by MAb 01.65 and the results are as follows: (i) c-myc RNA transcription is normally initiated and finished, suggesting a post-transcriptional regulation of c-myc gene expression; (ii) no alteration in c-myc mRNA stability has been documented; (iii) steady-state levels of c-myc mRNA expression by Northern blot analysis and PCR amplification are decreased in the cytoplasmic compartment, while in the nuclear compartment they appear to be increased. Nuclear accumulation of mature mRNA after MAb 01.65 and PKC inhibitor (H7 and StSp) treatment appears to be the most probable mechanism involved. The possible implications of this are discussed.

Terminal complement complexes induce cell cycle entry in oligodendrocytes through mitogen activated protein kinase pathway

Sublytic complement attack through C5b-9 assembly induces oligodendrocytes (OLG) to express proto-oncogenes and to enter the cell cycle from resting G0/G1 phase to S phase. We have investigated whether cell cycle induction by C5b-9 is mediated by mitogen activated protein kinase (MAPK) pathway in OLG. C5b-9 but not C5b6 induced activation of both ERK1 and c-jun NH2 terminal kinases 1 (JNK1) in OLG. The increased ERK1 and JNK1 activities are transient, reaching a maximum around 20 min following exposure to C5b-9. Activation of Raf-1 and MEK1, upstream kinases of ERK1, was shown by increased Raf-1 kinase activity in anti-Raf-1 immunoprecipitates of OLG treated with C5b-9 and ERK1 activity that can be inhibited by PD098,059, a specific MEK1 inhibitor. Requirement for the ERK1 pathway in DNA synthesis was then evaluated using PD098,059. Enhanced DNA synthesis induced by serum complement was completely abolished when OLG were pretreated with PD098,059. On the other hand, c-fos mRNA expression induced by complement was inhibited only 50% by PD098,059, while the c-jun mRNA level was not affected by this MEK1 inhibitor. Interestingly, p70 S6 kinase, an important ribosomal kinase in mitogenesis, was also activated by C5b-9. These findings indicated that the MAPK pathways appears to play a major role in inducing OLG to enter the S phase of the cell cycle from the resting G1/G0 phase.

Glucose-independent inhibition of yeast plasma-membrane H+-ATPase by calmodulin antagonists

Glucose metabolism causes activation of the yeast plasma-membrane H+-ATPase. The molecular mechanism of this regulation is not known, but it is probably mediated by phosphorylation of the enzyme. The involvement in this process of several kinases has been suggested but their actual role has not been proved. The physiological role of a calmodulin-dependent protein kinase in glucose-induced activation was investigated by studying the effect of specific calmodulin antagonists on the glucose-induced ATPase kinetic changes in wild-type and two mutant strains affected in the glucose regulation of the enzyme. Preincubation of the cells with calmidazolium or compound 48/80 impeded the increase in ATPase activity by reducing the Vmax of the enzyme without modifying the apparent affinity for ATP in the three strains. In one mutant, pma1-T912A, the putative calmodulin-dependent protein kinase-phosphorylatable Thr-912 was eliminated, and in the other, pma1-P536L, H+-ATPase was constitutively activated, suggesting that the antagonistic effect was not mediated by a calmodulin-dependent protein kinase and not related to glucose regulation. This was corroborated when the in vitro effect of the calmodulin antagonists on H+-ATPase activity was tested. Purified plasma membranes from glucose-starved or glucose-fermenting cells from both pma1-P890X, another constitutively activated ATPase mutant, and wild-type strains were preincubated with calmidazolium or melittin. In all cases, ATP hydrolysis was inhibited with an IC50 of approximately 1 microM. This inhibition was reversed by calmodulin. Analysis of the calmodulin-binding protein pattern in the plasma-membrane fraction eliminates ATPase as the calmodulin target protein. We conclude that H+-ATPase inhibition by calmodulin antagonists is mediated by an as yet unidentified calmodulin-dependent membrane protein.

Binding of a protein to an AU-rich domain of tumour necrosis factor alpha mRNA as a 35 kDa complex and its regulation in primary rat astrocytes

Newcastle disease virus (NDV) induces tumour necrosis factor alpha (TNF alpha) gene transcription and increases the mRNA stability. NDV stabilizes TNF alpha mRNA by preventing poly(A) shortening in a protein kinase C-dependent manner. TNF alpha 3'-untranslated region (UTR) contains an AU-rich domain (ARD) with seven AUUUA pentamers, a motif implicated in poly(A) removal and mRNA degradation. In this report, protein binding to TNF alpha ARD and the effects of NDV and kinases on ARD-binding activity were investigated in primary rat astrocytes. Both nuclear and cytoplasmic extracts contained proteins binding to centrally located 27 nt AUUUAUUAUUUAUUUAUUAUUUAUUUA, within TNF alpha ARD. Portions of ARD with a single AUUUA did not show ARD-binding activity. The ARD-protein complexes migrated as two bands on electrophoretic mobility-shift assay. The slower moving complexes appeared either as a broader band or doublets. The UV cross-linked ARD-protein complexes, however, migrated as a single 35 kDa band on SDS/PAGE. In cytoplasmic extracts treated with alkaline phosphatase there was a decrease in the faster moving complex and an increase in the slower moving complex, whereas NDV infection produced the reverse effect. In addition, the faster moving complex was decreased when cytoplasmic extracts from NDV-infected cells were treated with protein phosphatase 1 or 2A. Neither NDV infection nor phosphatase treatment affected the mobility pattern of nuclear extracts. The data indicate that a protein of molecular mass less than 35 kDa binds to a segment of TNF alpha ARD containing primarily UUAUUUAUU motifs, and the ARD-binding activity in cytoplasmic compartment is post-transcriptionally modified.

Biosynthesis of large dense-core vesicles in PC12 cells: effects of depolarization and second messengers on the mRNA levels of their constituents

mRNA levels of various constituents of large dense-core vesicles were determined in PC12 cells during depolarization and/or in the presence of BayK 8644, forskolin or phorbolester. For the soluble (secretory) proteins of the vesicles the mRNAs of chromogranin A and B, secretogranin II, neuropeptide Y and VGF were analyzed. Depolarization in the presence of BayK induced a strong up-regulation of the messages for chromogranin B, neuropeptide Y and VGF. Addition of forskolin enhanced this response for neuropeptide Y and VGF, phorbolester did the same only for VGF. Partly membrane-bound and membrane-spanning components analyzed were carboxypeptidase H, dopamine beta-hydroxylase and glycoprotein III (clusterin), peptidylglycine alpha-amidating mono-oxygenase and cytochrome b-561, respectively. Changes of mRNAs for these components were in general smaller and delayed. Six days of depolarization caused an up-regulation of glycoprotein III, peptidylglycine alpha-amidating mono-oxygenase and carboxypeptidase H mRNA levels which were not further increased by cyclic AMP and phorbolester. The dopamine beta-hydroxylase message increased after 6 days of depolarization, however, addition of phorbolester reduced this effect. For cytochrome b-561 there was no change after any of the conditions employed. These in vitro results are compared with those obtained for the biosynthesis regulation of large dense-core vesicles under in vivo conditions. It is suggested that in vivo acetylcholine and vasoactive intestinal polypeptide released from splanchnic nerve induce a differential change in the biosynthesis of large dense-core vesicles by acting via calcium and protein kinase A and C.

Differentiation-inducing factor of D. discoideum raises intracellular calcium concentration and suppresses cell growth in rat pancreatic AR42J cells

DIF (differentiation-inducing factor) is a putative morphogen that induces stalk cell differentiation in the lower eukaryote, Dictyostelium discoideum. In this study, we have examined the effects of DIF on growth and the intracellular calcium concentration ([Ca2+]i) in rat pancreatic acinar AR42J cells. Growth of AR42J cells was inhibited when DIF was present in the media, and approximately 50% growth inhibition was attained with 20 microM DIF. DIF was also found to raise [Ca2+]i in a dose-dependent manner (1-40 microM), both in the presence and absence of extracellular Ca2+. These results suggest that DIF elicits both calcium influx from the extracellular space and calcium release from intracellular pool(s), thereby inhibiting cell growth in AR42J.

Delayed activation of plasma membrane Ca2+ pump in human neutrophils

The interplay between Ca2+ efflux mechanisms of the plasma membrane (PM) and transient changes of the cytosolic concentration of ionized calcium ([Ca2+]i) was studied in suspensions of human neutrophils loaded with the [Ca2+]i indicator, Fura-2. To reveal Ca2+ efflux through PM the interference of intracellular Ca stores was prevented by preincubating the cells in the presence of EGTA, thapsigargin, and ionomycin. Addition of econazole prevented varying entry of divalent cations regulated by the filling state of Ca stores. The preincubation seemed to empty and permeabilize virtually all Ca stores, ensuring that the monitored changes of [Ca2+]i were caused exclusively by PM Ca2+ transporters. Following preincubation, the addition of CaCl2 induced, mediated by ionomycin, a transient rise of [Ca2+]i, a spike, eventually decreasing to an intermediary [Ca2+]i level. The ATP-dependent decrease of [Ca2+]i terminating the spike was abolished by the calmodulin antagonist, N-(6-aminohexyl)-1-naphthalenesulfonamide (W-7), but not by the protein kinase C inhibitor, staurosporine, nor by Na(+)-free medium, suggesting that neither activity of protein kinase C nor Na+/Ca2+ exchange was necessary for generation of the Ca2+ spike. In conclusion, the PM Ca2+ pump was responsible for the Ca2+ spike by responding to the rapid rise of [Ca2+]i by a delayed activation, possibly involving calmodulin. This characteristic feature of the PM pump may be important for the generation of cellular [Ca2+]i spikes in general.

Effect of substance P and protein kinase inhibitors on beta-amyloid peptide-induced proliferation of cultured brain cells

The present study investigated the effect of substance P (SP) and protein kinase inhibitors (H7 and HA1004) on beta-amyloid peptide-induced proliferation of neonatal rat brain cells in primary cultures. The beta-amyloid peptide1-28 (designated as beta AP28), at nanomolar concentrations (10(-9) M), significantly (P < or = 0.05) increased the proliferation of brain cells (presumably non-neuronal) as measured by [3H]thymidine uptake into DNA (mitogenesis). The effect was dependent on time of culture, concentration of beta AP28, and presence of fetal calf serum. The supplementation of SP into cell cultures at time zero reversed the proliferative response of beta AP28. Moreover, the beta AP28-induced proliferation was inhibited by protein kinase inhibitor H7, but not by HA1004. Since H7 is a selective protein kinase C (PKC) inhibitor and SP action involves PKC, we conclude that beta AP28 induces normal brain cell proliferation through PKC pathway of cell signaling.

Isolation of the bile canalicular actin-myosin II motor

Cytoskeleton-rich canalicular membranes (CCMs) with preserved cytoskeleton and demembranated CCMs, consisting only of cytoskeletal elements, were used to examine the relationship of pericanalicular microfilaments, myosin II phosphorylation, and canalicular contraction. The components of CCMs were visualized by fluorescence microscopy using the filamentous actin probe rhodamine-phalloidin and by electron microscopy, before and after incubation in 1 microM Ca2+/1 mM ATP (contraction solution). Canalicular contraction (luminal closure) was evaluated by morphometric analysis. Myosin II was extracted from CCMs, purified by immunoprecipitation, and analyzed on Western blots. In sequential experiments, autoradiographs of gels from [gamma-32P]-ATP-treated CCMs in the presence or absence of Ca2+ were examined after 0.25, 0.50, 1, 2, 3, 5, and 10 min, and the effects of W7 (a calmodulin antagonist) and ML9 (a myosin light chain kinase inhibitor) were evaluated. The results showed that phosphorylation of the 20-kDa protein was low in controls but enhanced beginning 0.25-0.50 min after addition of contraction solution. Both W7 and ML9 significantly inhibited this reaction and inhibited canalicular contraction. The results indicate that phosphorylation of the regulatory 20-kDa myosin light chain of canaliculus-associated myosin II coincides with or precedes contraction of the canaliculus. We conclude that the canalicular contractile apparatus is composed of actin filaments and a myosin II motor.

In vivo and in vitro study of the effects of chlorpromazine on tooth mineralization in rats and mice

The effects of chlorpromazine (CPZ) on tooth mineralization were examined using incisor dentine in adult rats and cultured tooth germs of mandibular first molars dissected from mouse embryos. CPZ (10, 50 and 250 mg/kg, s.c.) substantially inhibited dentine mineralization as evaluated by contact microradiographs. Plasma calcium and phosphorus concentrations were not decreased by CPZ (10 and 50 mg/kg). Physicochemical effects were not involved in the action of CPZ on the mineralization. In vitro experiments showed that CPZ (1 and 10 microM) inhibited mineralization and alkaline phosphatase (ALP) activity in the tooth germs. As CPZ has the properties of a calmodulin antagonist, the calmodulin antagonists W-7 and W-5 were also examined. Both inhibited mineralization and ALP activity in tooth germs; W-5 had less effect than W-7. These in vivo and in vitro findings suggest that CPZ inhibited cell-mediated mineralization in dentine without affecting the calcium-regulating system and physicochemical mineral deposition. In addition, calmodulin could be involved in cell-mediated mineralization.

Cellular mechanism of acetylcholine-induced response in dissociated outer hair cells of guinea-pig cochlea

1. The acetylcholine (ACh)-induced currents (IACh) in dissociated outer hair cells (OHCs) of guinea-pig cochlea were investigated using the whole-cell patch-clamp technique, in both conventional and nystatin perforated-patch configurations. 2. ACh and carbamylcholine (CCh) induced outward currents at a holding potential (VH) of -60 mV in the perforated-patch configuration. The IACh increased in a sigmoidal fashion over the concentration range between 3 x 10(-6) and 10(-3) M. The dissociation constant (KD) was 1.7 x 10(-5) M and the Hill coefficient (n) was 2.7. The KD and n for CCh were 8.7 x 10(-5) M and 2.2, respectively. Neither nicotine nor muscarine induced any detectable current up to a concentration of 10(-3) M. 3. Various muscarinic agonists such as oxotremorine-M, McN-A-343 and oxotremorine could also induce the outward currents, although these current amplitudes were about one-third that of ACh, indicating that they were partial agonists. 4. The muscarinic antagonists atropine, 4-DAMP, AF-DX 116 and pirenzepine inhibited the IACh in a concentration-dependent manner. The half-inhibitory concentrations (IC50) for atropine, 4-DAMP, AF-DX 116 and pirenzepine were 4.8 x 10(-6), 6.2 x 10(-6), 2.1 x 10(-5) and 2.9 x 10(-4) M, respectively. 5. When the extracellular Ca2+ concentration ([Ca2+])o) was reduced to lower than 1 mM, the amplitude of IACh, abruptly decreased. In a nominally Ca(2+)-free external solution ACh did not induce any current. The increase of [Ca2+]o beyond 1 mM did not change the IACh. 6. When OHCs were perfused intracellularly with a pipette solution containing 10 mM BAPTA in the conventional whole-cell mode, ACh could not induce outward K+ currents. The Ca2+ ionophore A23187 induced an outward current. These results indicate that intracellular Ca2+ is involved in the ACh response. 7. Calmodulin inhibitors such as chlorpromazine, W-7 and trifluoperazine inhibited the IACh in a concentration-dependent manner. 8. When OHCs were dialysed with either 100 microM GDP beta S or 1 micrograms/ml pertussis toxin (PTX) through the patch pipette at a VH of -60 mV, the IACh diminished within 10 min, whereas the IACh of the control remained steady for over 20 min, suggesting that a PTX-sensitive G-protein is involved in the ACh response.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium/calmodulin transduces thrombin-stimulated secretion: studies in intact and minimally permeabilized human umbilical vein endothelial cells

Thrombin stimulates cultured endothelial cells (EC) to secrete stored von Willebrand factor (vWF), but the signal transduction pathways are poorly defined. Thrombin is known to elevate the concentration of intracellular calcium ([Ca2+]i) and to activate protein kinase C (PKC) in EC. Since both calcium ionophores and phorbol esters release vWF, both second messenger pathways have been postulated to participate in vWF secretion in response to naturally occurring agonists. We find that in intact human EC, vWF secretion stimulated by either thrombin or by a thrombin receptor activating peptide, TR(42-55), can be correlated with agonist-induced elevations of [Ca2+]i. Further evidence implicating calcium in the signal transduction pathway is suggested by the finding that MAPTAM, a cell-permeant calcium chelator, in combination with the extracellular calcium chelator EGTA, can inhibit thrombin-stimulated secretion. In contrast, the observation that staurosporine (a pharmacological inhibitor of PKC) blocks phorbol ester- but not thrombin-stimulated secretion provides evidence against PKC-mediated signal transduction. To examine further the signal transduction pathway initiated by thrombin, we developed novel conditions for minimal permeabilization of EC with saponin (4-8 micrograms/ml for 5-15 min at 37 degrees C) which allow the introduction of small extracellular molecules without the loss of large intracellular proteins and which retain thrombin-stimulated secretion. These minimally permeabilized cells secrete vWF in response to exogenous calcium, and EGTA blocks thrombin-induced secretion. Moreover, in these cells, thrombin-stimulated secretion is blocked by a calmodulin-binding inhibitory peptide but not by a PKC inhibitory peptide. Taken together, these findings demonstrate that thrombin-stimulated vWF secretion is transduced by a rise in [Ca2+]i and provide the first evidence for the role of calmodulin in this process.

Molecular pharmacology of calmodulin pathways in the cell functions

In this paper we summarize much of the pharmacological evidence that has led to our current understanding of calmodulin-regulated cell function, with emphasis on aspects that may be relevant to drug design. These newly developed compounds are one of the most powerful tools as molecular probes for pharmacological approach, and will shed light on the physiological significance and molecular mechanisms of calmodulin-dependent pathways in various cell functions.

The newly synthesized selective Ca2+/calmodulin dependent protein kinase II inhibitor KN-93 reduces dopamine contents in PC12h cells

We reported that one of the isoquinolinesulfonamide derivatives, KN-62, is a potent and specific inhibitor of Ca2+/calmodulin-dependent protein kinase II (CaMKII) (Tokumitsu, H., Chijiwa, T., Hagiwara, M., Mizutani, A., Terasawa, M. and Hidaka, H. (1990) J. Biol. Chem. 265, 4315-4320). We have now investigated the inhibitory property of a newly synthesized methoxybenzenesulfonamide, KN-93, on CaMKII activity in situ and in vitro. KN-93 elicited potent inhibitory effects on CaMKII phosphorylating activity with an inhibition constant of 0.37 microM but this compound had no significant effects on the catalytic activity of cAMP-dependent protein kinase, Ca2+/phospholipid dependent protein kinase, myosin light chain kinase and Ca(2+)-phosphodiesterase. KN-93 also inhibited the autophosphorylation of both the alpha- and beta-subunits of CaMKII. Kinetic analysis indicated that KN-93 inhibits CaMKII, in a competitive fashion against calmodulin. To evaluate the regulatory role of CaMKII on catecholamine metabolism, we examined the effect of KN-93 on dopamine (DA) levels in PC12h cells. The DA levels decreased in the presence of KN-93. Further, the tyrosine hydroxylase (TH) phosphorylation induced by KCl or acetylcholine was significantly suppressed by KN-93 in PC12h cells while events induced by forskolin or 8-Br-cAMP were not affected. These results suggest that KN-93 inhibits DA formation by modulating the reaction rate of TH to reduce the Ca(2+)-mediated phosphorylation levels of the TH molecule.

Differential localization of protein kinase C isozymes in retinal neurons

We report the immunohistochemical localization of protein kinase C isozymes (types I, II, and III) in the rabbit retina using the monospecific monoclonal antibodies MC-1a, MC-2a, and MC-3a. Using immunoblot analysis of partially purified protein kinase C preparations of rabbit retina, types II and III isozymes alone were detected. The activity of type III was the stronger. By light microscopic immunohistochemical analysis, retinal neurons were negative for type I and positive for type II and type III isozymes. Type II was more diffusely distributed through the retinal layers, but was distinctive in ganglion cells, bipolar cells, and outer segments. The immunoreactivity was stronger for type III isozyme, and it was observed in mop (rod) bipolar cells and amacrine cells. By using immunoelectron microscopy, the cytoplasm of the cell body, the axon, and dendrites of the mop bipolar cells were strongly immunoreactive for type III. The so-called rod bipolar cells were for the first time seen to form synapses with rod photoreceptor cells. These differential localizations of respective isozymes in retinal neurons suggest that each isozyme has a different site of function in each neuron.

Regulatory functions of calmodulin

Calmodulin is a Ca2+ binding protein present in all eukaryotic cells that serves as the primary intracellular receptor for Ca2+. This 148 amino acid protein is involved in activation of more than 20 enzymes which mediate a wide variety of physiological processes. Many of these enzymes are inhibited in an intramolecular manner and the Ca(2+)-calmodulin complex relieves this inhibition. Calmodulin is essential for life as disruption of the gene in genetically tractable organisms is lethal. This protein plays important regulatory roles in cell proliferation and is required at multiple points in the cell cycle. The mechanism of enzyme activation by calmodulin and its importance in cell growth regulation are reviewed.

Expression of protein kinase C isozyme in epidermal Langerhans cells of the mouse

Protein kinase C (PKC) is encoded by a complex of a gene family, and its multiple isozymes are expressed in various mammalian tissues. We examined whether PKC is expressed in epidermal Langerhans cells (LC) of the mouse by using monoclonal antibodies specific to PKC I, PKC II, and PKC III isozymes (respective products of PKC genes gamma, beta, and alpha). Immunohistochemical and immunoblotting studies revealed that LC of adult C57BL/6 mice express PKC II, while PKC I and PKC III were not detected. In keratinocytes, none of the isozymes were detected. These results suggest that PKC II is a novel marker of LC and that it possibly plays a regulatory role in epidermal LC of the mouse in vivo.

The effect of N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7) on muscarinic receptor-induced Ca2+ mobilization in a human salivary epithelial cell line

We have investigated the effect of W-7, a calmodulin (CaM) antagonist, on Ca2+ mobilization in a human salivary epithelial cell line, HSG-PA, after muscarinic receptor stimulation. In a medium containing 1.5 mmol/l Ca2+, W-7 reduced both the maximum peak increase in cytosolic Ca2+ [( Ca2+]i) which follows stimulation by carbachol (Cch, 100 mumol/l) and the sustained nature of the response. Using an experimental approach which allows separate visualization of the intracellular Ca2+ release and extracellular Ca2+ entry phases, W-7 was shown preferentially to inhibit Ca2+ release. At 100 mumol/l W-7, Cch-induced Ca2+ release was completely inhibited, but Cch-induced Ca2+ entry was partially (approximately 40%) maintained. This W-7 residual Ca2+ entry response was abolished when cells were depolarized with high K+ or gramicidin D. W-7 also substantially inhibited Cch-induced inositol trisphosphate (IP3) production (approximately 5%). W-5, a less potent CaM antagonist than W-7, had markedly smaller effects on Cch-induced Ca2+ mobilization and IP3 formation. W-7 (100 mumol/l) completely blocked (comparable to 10 mumol/l atropine) the binding of the muscarinic antagonist [3H] quinuclidinyl benzilate (QNB) to muscarinic receptors on cell membranes, whereas Cch (at 100 mumol/l) had minimal effects on ligand binding. W-7 and W-5 were equipotent in their ability to inhibit [3H] QNB binding. These results suggest that W-7 reduces Ca2+ mobilization in HSG-PA cells by a mechanism which likely involves the antagonism of a CaM regulatory step(s) but may also involve at least a partial blockade of the muscarinic receptor.

Type-specific expression of protein kinase C isozymes in CNS tumor cells

We examined specific expression of protein kinase C (PK-C) isozymes in cultured human glial and neuronal cell lines, using type-specific monoclonal antibodies MC-1a, -2a, and -3a (Hidaka H. et al., J. Biol. Chem., 263 (1988) 4523-4526). Immunoblotting experiments revealed that a 80 kDa band of three kinds of glioblastoma cells (A-172, SK-MG-1, SK-MG-4) was stained with MC-3a, whereas that of neuroblastoma cells (SK-N-MC) reacted with MC-2a. Immunoenzymetric assay showed that glioblastoma cells (A-172, SK-MG-1, SK-MG-4) contained 127.6 +/- 14.4, 248.8 +/- and 148.5 +/- 35.8 ng/mg protein of type III. respectively, while neuroblastoma cells (SK-N-MC) contained 389.5 +/- 20.7 ng/mg protein of type II. These results suggest that PK-C isozymes may be specifically expressed, depending on types of central nervous system (CNS) tumor cells.

Biosynthesis of endothelium-derived relaxing factor: a cytosolic enzyme in porcine aortic endothelial cells Ca2+-dependently converts L-arginine into an activator of soluble guanylyl cyclase

In the presence of porcine aortic endothelial cytosol, soluble guanylyl cyclase purified from bovine lung was activated by L-arginine up to 2.5-fold, with an EC50 of about 6 microM. This activation was dependent on NADPH and Ca2+. The EC50 for Ca2+ was about 60 nM. No effect of L-arginine on guanylyl cyclase was observed when the cytosolic proteins were heat-denaturated. The effect of L-arginine was inhibited by NG-monomethyl-L-arginine and hemoglobin. These results indicate that endothelial cells contain a cytosolic enzyme which is directly or indirectly regulated by Ca2+ and converts L-arginine into a compound which in stimulating soluble guanylyl cyclase behaves similar to endothelium-derived relaxing factor.

Ca2+-dependent formation of an L-arginine-derived activator of soluble guanylyl cyclase in bovine lung

In a fraction of cytosolic proteins from bovine lung, soluble guanylyl cyclase was concentration-dependently stimulated by L-arginine but not by D-arginine. Stimulation was up to 20-fold with an EC50 of about 3 x 10(-5) M. Activation of guanylyl cyclase by L-arginine was dependent on NADPH (EC50 about 5 x 10(-7) M) and Ca2+ (EC50 about 1.4 x 10(-6) M). The activation by L-arginine was inhibited by NG-monomethyl-L-arginine and hemoglobin. The effect of L-arginine was dependent on the protein concentration and was not observed in preparations of purified gyanylyl cyclase. These results suggest that bovine lung contains a Ca2+-regulated enzyme or enzyme system which converts L-arginine into an activator of soluble guanylyl cyclase.

Specific inhibition of 40 K protein phosphorylation in platelet by a new antithrombotic agent, d-indobufen

Indobufen, (+)2[p-oxo-2-isoindolinyl)-phenyl] butyric acid exhibited unique inhibitory pattern of platelet function with suppression of protein phosphorylation. In collagen-induced activation, d-indobufen potently inhibited aggregation and release reaction in a dose-dependent manner (IC50:0.49 microM). This compound delayed the onset of aggregation and release reaction induced by arachidonic acid without reducing the magnitude of final reactions. When platelets were stimulated with thrombin, indobufen did not inhibit the aggregation significantly up to 100 microM. d-Indobufen suppressed platelet 40 kDa protein phosphorylation in collagen-stimulated platelets and delayed the onset of the phosphorylation in arachidonic acid-stimulated platelets. However, d-indobufen did not directly inhibited protein kinases in platelets, in vitro. These findings suggest that the inhibitory actions of d-indobufen may be due to its effects on the initiation process of 40 kDa protein phosphorylation.

Immunocytochemical evidence for translocation of protein kinase C in human megakaryoblastic leukemic cells: synergistic effects of Ca2+ and activators of protein kinase C on the plasma membrane association

Immunological analysis using monoclonal antibodies against subspecies of protein kinase C revealed the predominant expression of the isozyme, type II, in human megakaryoblastic leukemic cells. We investigated the effects of phorbol diester 12-O-tetradecanoyl phorbol-13-acetate (TPA), the Ca2+ ionophore ionomycin and synthetic diacylglycerol 1-oleoyl-2-acetylglycerol (OAG) on the immunocytochemical localization of protein kinase C in these cells. Indirect immunofluorescence techniques revealed the enzyme to be located in a diffuse cytosolic pattern, in the intact cells. When the cells were exposed to 100 nM TPA, the immunofluorescent staining was translocated from the cytoplasm to the plasma membrane. The translocation was protracted and staining on the membrane decreased in parallel with the Ca2+, phospholipid-dependent protein kinase activity. Treatment of the cells with 500 nM ionomycin caused an apparent translocation comparable with that seen with TPA, however, this translocation was transient and most of the cytosolic staining was within 60 min. We also found that 30 micrograms/ml OAG did not have significant effects on distribution of the staining, but rather acted synergistically on the translocation with the suboptimal concentration of 100 nM ionomycin. A similar synergism was also observed with 10 nM TPA and 100 nM ionomycin. These results obtained in situ provide evidence that intracellular Ca2+ and diacylglycerol regulate membrane binding of the enzyme in vivo.

Thyroid hormones inhibit the Ca2+ calmodulin-induced activation of myosin light chain kinase

L-Thyroxine (T4) and L-triiodothyronine (T3) specifically, inhibited myosin light chain kinase (MLC-kinase) from various tissues whereas inhibitory effects of T4 and T3 on other protein kinases such as protein kinase C, cAMP-dependent protein kinase, casein kinase I, casein kinase II and calmodulin kinase II were much weaker. T4 was a more potent inhibitor of MLC-kinase than T3. Kinetic studies showed that T4 behaved as a competitive inhibitor of MLC-kinase toward calmodulin (CaM) and that Ki value was 2.5 microM. The activity of the catalytic fragment of MLC-kinase, which is active without CaM, was not inhibited by T4. 125I-T4 gel overlay revealed that CaM did not bind T4 but MLC-kinase had 125I-T4 binding activity. These observations suggest that T4 binds at or near CaM binding domain of MLC-kinase and inhibits CaM-induced activation of MLC-kinase.