Iridals are a novel class of ligands for phorbol ester receptors with modest selectivity for the RasGRP receptor subfamily.

Since 1990, the National Cancer Institute has performed extensive in vitro screening of compounds for anticancer activity. To date, more than 70 000 compounds have been screened for their antiproliferation activities against a panel of 60 human cancer cell lines. We probed this database to identify novel structural classes with a pattern of biological activity on these cell lines similar to that of the phorbol esters. The iridals form such a structural class. Using the program Autodock, we show that the iridals dock to the same position on the C1b domain of protein kinase C delta as do the phorbol esters, with the primary hydroxyl group of the iridal at the C3 position forming two hydrogen bonds with the amide group of Thr12 and with the carbonyl group of Leu 21 and the aldehyde oxygen of the iridal forming a hydrogen bond with the amide group of Gly23. Biological analysis of two iridals, NSC 631939 and NSC 631941, revealed that they bound to protein kinase C alpha with K(i) values of 75.6 +/- 1.3 and 83.6 +/- 1.5 nM, respectively. Protein kinase C is now recognized to represent only one of five families of proteins with C1 domains capable of high-affinity binding of diacylglycerol and the phorbol esters. NSC 631939 and NSC 631941 bound to RasGRP3, a phorbol ester receptor that directly links diacylglycerol/phorbol ester signaling with Ras activation, with K(i) values of 15.5 +/- 2.3 and 41.7 +/- 6.5 nM, respectively. Relative to phorbol 12,13-dibutyrate, they showed 15- and 6-fold selectivity for RasGRP3. Both compounds caused translocation of green fluorescent protein tagged RasGRP3 expressed in HEK293 cells, and both compounds induced phosphorylation of ERK1/2, a downstream indicator of Ras activation, in a RasGRP3-dependent fashion. We conclude that the iridals represent a promising structural motif for design of ligands for phorbol ester receptor family members.

Protein kinase Calpha and protein kinase Cdelta play opposite roles in the proliferation and apoptosis of glioma cells.

Protein kinase C (PKC) has been implicated in the proliferation and apoptosis of glial tumors, but the role of specific PKC isoforms remains unresolved. Comparing brain tumors differing in degree of malignancy, we found that malignant gliomas expressed higher levels of PKCalpha and lower levels of PKCdelta as compared with low-grade astrocytomas. Consistent with a mechanistic role for these differences, overexpression of PKCalpha in the human U87 glioma cell line resulted in enhanced cell proliferation and decreased glial fibrillary acidic protein (GFAP) expression as compared with controls. Reciprocally, overexpression of PKCdelta inhibited cell proliferation and enhanced GFAP expression. Using PKC chimeras, we found that the regulatory domains of PKCalpha and PKCdelta mediated their effects on cell proliferation and GFAP expression. PKCalpha and delta have been implicated as potential signaling molecules in apoptosis. Therefore, we examined the role of these isoforms in the resistance of glioma cells to apoptotic stimuli. In U87 cells, manipulation of PKCalpha levels had little effect on apoptosis in response to etoposide. In contrast, overexpression of PKCdelta rendered the U87 cells more sensitive to the apoptotic effect of etoposide, and PKCdelta was cleaved in these cells by a caspase-dependent process. Furthermore, the glioma cell line U373, which expresses endogenous PKCdelta, underwent apoptosis in response to etoposide, and the apoptotic response was blocked by the PKCdelta inhibitor rottlerin. Our results suggest that PKCalpha and PKCdelta play opposite roles in the proliferation and apoptosis of glioma cells.

Structural basis of binding of high-affinity ligands to protein kinase C: prediction of the binding modes through a new molecular dynamics method and evaluation by site-directed mutagenesis.

The structural basis of protein kinase C (PKC) binding to several classes of high-affinity ligands has been investigated through complementary computational and experimental methods. Employing a recently developed q-jumping molecular dynamics (MD) simulation method, which allows us to consider the flexibility of both the ligands and the receptor in docking studies, we predicted the binding models of phorbol-13-acetate, phorbol-12,13-dibutyrate (PDBu), indolactam V (ILV), ingenol-3-benzoate, and thymeleatoxin to PKC. The "predicted" binding model for phorbol-13-acetate is virtually identical to the experimentally determined binding model for this ligand. The predicted binding model for PDBU is the same as that for phorbol-13-acetate in terms of the hydrogen-bonding network and hydrophobic contacts. The predicted binding model for ILV is the same as that obtained in a previous docking study using a Monte Carlo method and is consistent with the structure-activity relationships for this class of ligands. Together with the X-ray structure of phorbol-13-acetate in complex with PKCdelta C1b, the predicted binding models of PDBu, ILV, ingenol-3-benzoate, and thymeleatoxin in complex with PKC showed that the binding of these ligands to PKC is governed by a combination of several highly specific and optimal hydrogen bonds and hydrophobic contacts. However, the hydrogen-bonding network for each class of ligand is somewhat different and the number of hydrogen bonds formed between PKC and these ligands has no correlation with their binding affinities. To provide a direct and quantitative assessment of the contributions of several conserved residues around the binding site to PKC-ligand binding, we have made 11 mutations and measured the binding affinities of the high-affinity PKC ligands to these mutants. The results obtained through site-directed mutagenic analysis support our predicted binding models for these ligands and provide new insights into PKC-ligand binding. Although all the ligands have high affinity for the wild-type PKCdelta C1b, our site-directed mutagenic results showed that ILV is the ligand most sensitive to structural perturbations of the binding site while ingenol-3-benzoate is the least sensitive among the four classes of ligands examined here. Finally, we have employed conventional MD simulations to investigate the structural perturbations caused by each mutation to further examine the role played by each individual residue in PKC-ligand binding. MD simulations revealed that several mutations, including Pro11 --> Gly, Leu21 --> Gly, Leu24 --> Gly, and Gln27 --> Gly, cause a rather large conformational alteration to the PKC binding site and, in some cases, to the overall structure of the protein. The complete abolishment or the significant reduction in PKC-ligand binding observed for these mutants thus reflects the loss of certain direct contacts between the side chain of the mutated residue in PKC and ligands as well as the large conformational alteration to the binding site caused by the mutation.

Phorbol esters modulate the Ras exchange factor RasGRP3.

RasGRP represents the prototype of a new class of guanine nucleotide exchange factors that activate small GTPases. The guanyl nucleotide-releasing protein (GRP) family members contain catalytic domains related to CDC25, the Ras exchange factor of Saccharomyces cerevisiae. They also contain a motif resembling a pair of calcium-binding EF-hands and a C1 domain similar to the diacylglycerol interaction domain of protein kinase C. The sequence of KIAA0846, identified in a human brain cDNA library, encodes a member of the GRP family that we refer to as RasGRP3. We show here that RasGRP3 bound phorbol esters with high affinity. This binding depended on anionic phospholipids, which is characteristic of phorbol ester binding to C1 domain proteins. In addition, phorbol esters also caused activation of the RasGRP3 exchange activity in intact cells, as determined by an increase in RasGTP and phosphorylation of the extracellular-regulated kinases. Finally, both phorbol 12-myristate 13-acetate and the diacylglycerol analogue 1,2-dioctanoyl-sn-glycerol induced redistribution of RasGRP3 to the plasma membrane and/or perinuclear area in HEK-293 cells, as demonstrated using a green fluorescent fusion protein. We conclude that RasGRP3 serves as a PKC-independent pathway to link the tumor-promoting phorbol esters with activation of Ras GTPases.

Phosphorylation of protein kinase Cdelta on distinct tyrosine residues regulates specific cellular functions.

Protein kinase Cdelta (PKCdelta) inhibits proliferation and decreases expression of the differentiation marker glutamine synthetase (GS) in C6 glioma cells. Here, we report that distinct, specific tyrosine residues on PKCdelta are involved in these two responses. Transfection of cells with PKCdelta mutated at tyrosine 155 to phenylalanine caused enhanced proliferation in response to 12-phorbol 12-myristate 13-acetate, whereas GS expression resembled that for the PKCdelta wild-type transfectant. Conversely, transfection with PKCdelta mutated at tyrosine 187 to phenylalanine resulted in increased expression of GS, whereas the rate of proliferation resembled that of the PKCdelta wild-type transfectant. The tyrosine phosphorylation of PKCdelta and the decrease in GS expression induced by platelet-derived growth factor (PDGF) were abolished by the Src kinase inhibitors PP1 and PP2. In response to PDGF, Fyn associated with PKCdelta via tyrosine 187. Finally, overexpression of dominant negative Fyn abrogated the decrease in GS expression and reduced the tyrosine phosphorylation of PKCdelta induced by PDGF. We conclude that the tyrosine phosphorylation of PKCdelta and its association with tyrosine kinases may be an important point of divergence in PKC signaling.

The guanine nucleotide exchange factor RasGRP is a high -affinity target for diacylglycerol and phorbol esters.

RasGRP is a recently described guanine nucleotide exchange factor (GEF) that possesses a single C1 domain homologous to that of protein kinase C (PKC). The phorbol ester [(3)H]phorbol 12, 13-dibutyrate ([(3)H]PDBu) bound to this C1 domain (C1-RasGRP) with a dissociation constant of 0.58 +/- 0.08 nM, similar to that observed previously for PKC. Likewise, the potent PKC activator bryostatin 1, a compound currently in clinical trials, showed high affinity binding for C1-RasGRP. Structure activity analysis using several phorbol ester analogs showed both similarities and differences in ligand selectivity compared with PKC; the differences were comparable in magnitude to those between different PKC isoforms. Similarly, the potency of the PKC inhibitor calphostin C to inhibit [(3)H]PDBu binding to C1-RasGRP was similar to that observed for PKC. In contrast to the relative similarities in ligand recognition, the lipid cofactor requirements differed between RasGRP and PKC. The C1 domain plus the EF-hand motif of RasGRP (C1EF-RasGRP) was markedly less dependent on acidic phospholipids than was PKCalpha. The differences in lipid requirements were reflected in differential ligand selectivity under conditions of limiting lipid. Despite the presence of twin EF-hand like motifs, calcium did not affect the binding of [(3)H]PDBu to C1EF-RasGRP. We conclude that RasGRP is a high affinity receptor for phorbol esters and diacylglycerol. RasGRP thus provides a direct link between diacylglycerol generation or phorbol ester/bryostatin treatment and Ras activation.

Protein kinase Cdelta targets mitochondria, alters mitochondrial membrane potential, and induces apoptosis in normal and neoplastic keratinocytes when overexpressed by an adenoviral vector.

Inactivation of protein kinase Cdelta (PKCdelta) is associated with resistance to terminal cell death in epidermal tumor cells, suggesting that activation of PKCdelta in normal epidermis may be a component of a cell death pathway. To test this hypothesis, we constructed an adenovirus vector carrying an epitope-tagged PKCdelta under a cytomegalovirus promoter to overexpress PKCdelta in normal and neoplastic keratinocytes. While PKCdelta overexpression was detected by immunoblotting in keratinocytes, the expression level of other PKC isozymes, including PKCalpha, PKCepsilon, PKCzeta, and PKCeta, did not change. Calcium-independent PKC-specific kinase activity increased after infection of keratinocytes with the PKCdelta adenovirus. Activation of PKCdelta by 12-O-tetradecanoylphorbol-13-acetate (TPA) at a nanomolar concentration was lethal to normal and neoplastic mouse and human keratinocytes overexpressing PKCdelta. Lethality was inhibited by PKC selective inhibitors, GF109203X and Ro-32-0432. TPA-induced cell death was apoptotic as evidenced by morphological criteria, TUNEL (terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling) assay, DNA fragmentation, and increased caspase activity. Subcellular fractionation indicated that PKCdelta translocated to a mitochondrial enriched fraction after TPA activation, and this finding was confirmed by confocal microscopy of cells expressing a transfected PKCdelta-green fluorescent protein fusion protein. Furthermore, activation of PKCdelta in keratinocytes altered mitochondrial membrane potential, as indicated by rhodamine-123 fluorescence. Mitochondrial inhibitors, rotenone and antimycin A, reduced TPA-induced cell death in PKCdelta-overexpressing keratinocytes. These results indicate that PKCdelta can initiate a death pathway in keratinocytes that involves direct interaction with mitochondria and alterations of mitochondrial function.

Probing the binding of indolactam-V to protein kinase C through site-directed mutagenesis and computational docking simulations.

Protein kinase C (PKC) comprises a family of ubiquitous enzymes transducing signals by the lipophilic second messenger sn-1, 2-diacylglycerol (DAG). Teleocidin and its structurally simpler congener indolactam-V (ILV) bind to PKC with high affinity. In this paper, we report our computational docking studies on ILV binding to PKC using an automatic docking computer program, MCDOCK. In addition, we used site-directed mutagenesis to assess the quantitative contribution of crucial residues around the binding site of PKC to the binding affinity of ILV to PKC. On the basis of the docking studies, ILV binds to PKC in its cis-twist conformation and forms a number of optimal hydrogen bond interactions. In addition, the hydrophobic groups in ILV form "specific" hydrophobic interactions with side chains of a number of conserved hydrophobic residues in PKC. The predicted binding mode for ILV is entirely consistent with known structure-activity relationships and with our mutational analysis. Our mutational analysis establishes the quantitative contributions of a number of conserved residues to the binding of PKC to ILV. Taken together, our computational docking simulations and analysis by site-directed mutagenesis provide a clear understanding of the interaction between ILV and PKC and the structural basis for design of novel, high-affinity, and isozyme-selective PKC ligands.

Comparison of the roles of the C1a and C1b domains of protein kinase C alpha in ligand induced translocation in NIH 3T3 cells.

To explore the relative roles of the two C1 domains of protein kinase C alpha (PKC alpha) in the response to phorbol esters and related analogs, we mutated the individual C1 domains, expressed the mutated PKC alpha in NIH 3T3 cells, and then examined the ability of ligands to induce its translocation to the membrane. The C1a and C1b domains play equivalent roles for translocation in response to phorbol 12-myristate 13-acetate, mezerein, and (-)octylindolactam V. These results contrast with those previously reported for PKC delta, suggesting that the domains play different roles in different PKC isoforms.

Differential roles of the tandem C1 domains of protein kinase C delta in the biphasic down-regulation induced by bryostatin 1.

Bryostatin 1 (Bryo), currently in clinical trials, has been shown to induce a biphasic concentration-response curve for down-regulating protein kinase C (PKC) delta, with protection of the enzyme from down-regulation at high Bryo doses. In our ongoing studies to identify the basis for this unique behavior of PKCdelta, we examined the participation of the two ligand binding sites (C1a and C1b) in the regulatory domain of the enzyme. Three mutants of PKCdelta prepared by introducing a point mutation in either C1a or Clb or both C1a and Clb were overexpressed in NIH 3T3 cells. All of the constructs retained a biphasic response to down-regulation assessed after 24-h treatment with Bryo. However, the roles of the individual C1 domains were different for the two phases of the response. For down-regulation, both the C1a and the C1b mutants displayed equivalent 3-4-fold reductions in their affinities for the ligand. For protection from down-regulation, a reduced protection was observed for the C1a mutant, which showed a broader biphasic curve compared with those for wild-type PKCdelta and the Clb mutant. Like wild-type PKCdelta, all of the mutants showed the same subcellular partitioning of the protected enzyme to the particulate fraction of the cells, arguing against changes in sensitivity to Bryo due to differences in localization. Likewise, relatively similar patterns of localization were observed using green fluorescent protein-PKCdelta constructs. We conclude that the C1 domains of PKCdelta do not have equivalent roles in inducing protection against Bryo-induced down-regulation. The C1a domain plays a critical role in conferring the degree of protection at high concentrations of Bryo. Elucidation of the differential effect of Bryo on PKCdelta may suggest strategies for the design of novel ligands with Bryo-like activities.

Protein kinase C delta (PKCdelta) inhibits the expression of glutamine synthetase in glial cells via the PKCdelta regulatory domain and its tyrosine phosphorylation.

Protein kinase C (PKC) plays an important role in the proliferation and differentiation of glial cells. In a recent study we found that overexpression of PKCdelta reduced the expression of the astrocytic marker glutamine synthetase (GS). In this study we explored the mechanisms involved in the inhibitory effect of PKCdelta on the expression of glutamine synthetase. Using PKC chimeras we first examined the role of the catalytic and regulatory domains of PKCdelta on the expression of glutamine synthetase. We found that cells stably transfected with chimeras between the regulatory domain of PKCdelta and the catalytic domains of PKCalpha or epsilon inhibited the expression of GS, similar to the inhibition exerted by overexpression of PKCdelta itself. In contrast, no significant effects were observed in cells transfected with the reciprocal PKC chimeras between the regulatory domains of PKCalpha or epsilon and the catalytic domain of PKCdelta. PKCdelta has been shown to undergo tyrosine phosphorylation in response to various activators. Tyrosine phosphorylation of PKCdelta in response to phorbol 12-myristate 13-acetate and platelet-derived growth factor occurred only in chimeras which contained the PKCdelta regulatory domain. Cells transfected with a PKCdelta mutant (PKCdelta5), in which the five putative tyrosine phosphorylation sites were mutated to phenylalanine, showed markedly diminished tyrosine phosphorylation in response to phorbol 12-myristate 13-acetate and platelet-derived growth factor and normal levels of GS. Our results indicate that the regulatory domain of PKCdelta mediates the inhibitory effect of this isoform on the expression of GS. Phosphorylation of PKCdelta on tyrosine residues in the regulatory domain is implicated in this inhibitory effect.

Differential selectivity of ligands for the C1a and C1b phorbol ester binding domains of protein kinase Cdelta: possible correlation with tumor-promoting activity.

Protein kinase C (PKC) represents the major, high-affinity receptor for the phorbol esters as well as for a series of structurally diverse natural products. The phorbol esters function by binding to the tandem C1a and C1b domains in PKC, leading to enzyme activation. Although the typical phorbol esters represent the paradigm for tumor promoters in mouse skin, it is now clear that different high affinity ligands for PKC have distinct biological effects. Thus, the daphnane analogue mezerein is a second-stage promoter, the macrolide bryostatin 1 is a partial antagonist, and certain 12-deoxyphorbol 13-monoesters also function as partial antagonists but with a different pattern of activity. The biochemical basis for these differences is an area of active investigation. In this report, we have examined the relative interaction of ligands differing in structure and pattern of biological response with the C1a and C1b domains of PKCdelta. We mutated either or both of the C1 domains of PKCdelta, expressed the constructs in NIH 3T3 cells, and monitored the interaction of the ligands by their ability to induce translocation of the mutated PKCdelta from the cytosol to the particulate fraction. We found that different ligands showed different dependence on the C1a and C1b domains for translocation. Whereas phorbol 12-myristate 13-acetate and the indole alkaloids indolactam and octylindolactam were selectively dependent on the C1b domain, selectivity was not observed for mezerein, for the 12-deoxyphorbol 13-monoesters prostratin or 12-deoxyphorbol 13-phenylacetate, or for the macrocyclic lactone bryostatin 1. Provocatively, the pattern of response corresponds with the activity of the compounds as complete tumor promoters.

Both the catalytic and regulatory domains of protein kinase C chimeras modulate the proliferative properties of NIH 3T3 cells.

Protein kinase C (PKC) isozymes exhibit important differences in terms of their regulation and biological functions. Not only may some PKC isoforms be active and others not for a given response, but the actions of different isoforms may even be antagonistic. In NIH 3T3 cells, for example, PKCdelta arrests cell growth whereas PKCepsilon stimulates it. To probe the contribution of the regulatory and the catalytic domains of PKC isozymes to isozyme-specific responses, we prepared chimeras between the regulatory and the catalytic domains of PKCalpha, -delta, and -epsilon. These chimeras, which preserve the overall structure of the native PKC enzymes, were stably expressed in mouse fibroblasts. A major objective was to characterize the growth properties of the cells that overexpress the various PKC constructs. Our data demonstrate that both the regulatory and the catalytic domains play roles in cell proliferation. The regulatory domain of PKCepsilon enhanced cell growth in the absence or presence of phorbol 12-myristate 13-acetate (PMA), and, in the presence of PMA, all chimeras with the PKCepsilon regulatory domain also gave rise to colonies in soft agar; the role of the catalytic domain of PKCepsilon was evident in the PMA-treated cells that overexpressed the PKC chimera containing the delta regulatory and the epsilon catalytic domains (PKCdelta/epsilon). The important contribution of the PKCepsilon catalytic domain to the growth of PKCdelta/epsilon-expressing cells was also evident in terms of a significantly increased saturation density in the presence of PMA, their formation of foci upon PMA treatment, and the induction of anchorage-independent growth. Aside from the growth-promoting effect of PKCepsilon, we have shown that most chimeras with PKCalpha and -delta regulatory domains inhibit cell growth. These results underscore the complex contributions of the regulatory and catalytic domains to the overall behavior of PKC.

The catalytic domain of protein kinase C chimeras modulates the affinity and targeting of phorbol ester-induced translocation.

Emerging evidence suggests important differences among protein kinase C (PKC) isozymes in terms of their regulation and biological functions. PKC is regulated by multiple interdependent mechanisms, including enzymatic activation, translocation of the enzyme in response to activation, phosphorylation, and proteolysis. As part of our ongoing studies to define the factors contributing to the specificity of PKC isozymes, we prepared chimeras between the catalytic and regulatory domains of PKCalpha, -delta, and -epsilon. These chimeras, which preserve the overall structure of the native PKC enzymes, were stably expressed in NIH 3T3 fibroblasts. Their intracellular distribution was similar to that of the endogenous enzymes, and they responded with translocation upon treatment with phorbol 12-myristate 13-acetate (PMA). We found that the potency of PMA for translocation of the PKCalpha/x chimeras from the soluble fraction was influenced by the catalytic domain. The ED50 for translocation of PKCalpha/alpha was 26 nM, in marked contrast to the ED50 of 0.9 nM in the case of the PKCalpha/epsilon chimera. In addition to this increase in potency, the site of translocation was also changed; the PKCalpha/epsilon chimera translocated mainly into the cytoskeletal fraction. PKCx/epsilon chimeras displayed twin isoforms with different mobilities on Western blots. PMA treatment increased the proportion of the higher mobility isoform. The two PKCx/epsilon isoforms differed in their localization; moreover, their localization pattern depended on the regulatory domain. Our results emphasize the complex contributions of the regulatory and catalytic domains to the overall behavior of PKC.

Possible participation of nitric oxide in the increase of norepinephrine release caused by angiotensin peptides in rat atria.

In rat atria isolated with their cardioaccelerans nerves and labeled with [3H]norepinephrine, exposure to 1 x 10(-7) mol/L angiotensin II (Ang II) and 1 x 10(-7) mol/L Ang-(1-7) increased the release of radioactivity elicited by nerve stimulation (0.5-millisecond-long square-wave pulses at 2 Hz during 2 minutes) by 90% and 60%, respectively. The facilitatory effect on noradrenergic neurotransmission caused by both peptides was stereospecifically prevented by N omega-nitro-L-arginine methyl ester (1 x 10(-4) mol/L), an inhibitor of nitric oxide synthase that catalyzes the conversion of L-arginine to nitric oxide, as well as by 1 x 10(-5) mol/L methylene blue, a substance that inhibits the guanylate cyclase considered as the final target of nitric oxide action. On the other hand, the precursor of nitric oxide synthesis. L-arginine (1 x 10(-3) mol/L), reversed the prevention produced by N omega-nitro-L-arginine methyl ester on the increased release of norepinephrine caused by Ang II and Ang-(1-7). The present results suggest that nitric oxide could be involved in the neuromodulatory function elicited by both Ang II and Ang-(1-7) in rat atria. The physiological role of this observation is still under study.

The catalytic domain of protein kinase Cdelta confers protection from down-regulation induced by bryostatin 1.

Bryostatin 1 (Bryo) has been shown to induce biphasic dose-response curves for down-regulating protein kinase Cdelta (PKCdelta) as well as for protecting PKCdelta from down-regulation induced by phorbol 12-myristate 13-acetate (PMA). To identify regions within PKCdelta that confer these responses to Bryo, we utilized reciprocal PKCalpha and PKCdelta chimeras (PKCalpha/delta and PKCdelta/alpha) constructed by exchanging the regulatory and catalytic domains of these PKCs. These chimeras and wild-type PKCalpha/alpha and PKCdelta/delta constructed in the same way were stably expressed in NIH 3T3 fibroblasts. Twenty-four h of treatment with Bryo induced a biphasic dose-response curve for down-regulating both wild-type PKCdelta/delta and the PKCalpha/delta chimera. In contrast, Bryo led to a nearly complete down-regulation of both PKCalpha/alpha and PKCdelta/alpha and also produced a faster mobility form of these species on SDS-polyacrylamide gel electrophoresis. The nature of both the regulatory and, to a lesser extent, the catalytic domains affected the potency of Bryo to down-regulate the chimeric PKC proteins as well as to protect PKCalpha/delta and PKCdelta/delta from down-regulation. Bryo at high concentrations also inhibited the down-regulation of PKCdelta/delta and PKCalpha/delta induced by 1 microM PMA when co-applied. The portion of PKC protected by Bryo from down-regulation by either Bryo or PMA was localized in the particulate fraction of the cells. We conclude that the catalytic domain of PKCdelta confers protection from down-regulation induced by Bryo or Bryo plus PMA, suggesting that this domain contains the isotype-specific determinants involved in the unique effect of Bryo on PKCdelta.

Involvement of monoamine oxidase and noradrenaline uptake in the positive chronotropic effects of apigenin in rat atria.

In rat isolated atria spontaneously beating and labelled with [3H]noradrenaline, exposure to the flavonoid apigenin increased the atrial rate in a concentration-dependent manner (0.01-30 microM). This increase was accompanied by a reduction of 60% in the uptake of [3H]noradrenaline as well as by a modification in the pattern of [3H]noradrenaline and metabolites spontaneously released. Sixty minutes after exposure to 30 microM apigenin, the proportion of unmetabolized [3H]noradrenaline increased from 11% to 45% of the total products collected in the organ bath whereas the tritiated O-methylated deaminated metabolites decreased from 33% to 14% of the total efflux. A small but significant decrease in the outflow of [3H]3,4-dihydroxymandelic acid as well as a tendency to a decrease in the efflux of [3H]3,4-dihydroxyphenylglycol was also observed. Furthermore, apigenin inhibited in a concentration-dependent manner the activity of monoamine oxidase in the rat atrial homogenates. The calculated IC50 (7.7 microM) was within the range that produced 50% of the maximal increase in atrial rate. It is concluded that apigenin possesses the property to increase the atrial rate, probably as a result of a reduction in noradrenaline uptake as well as in monoamine oxidase activity.

Effects of the in vitro treatment with gangliosides on the release of endogenous amino acids from rat hypoxic atria.

1. In the rat isolated atria the in vitro exposure to 60 min of hypoxia in the absence of glucose followed by 30 min of reoxygenation increased the release of the amino acids glutamate (Glu) and taurine (Tau). The efflux of the remaining amino acids assayed (aspartate, glycine and alanine) did not change throughout the period studied. 2. The increase in Tau release started 45 min after the onset of the hypoxic period whereas that of Glu started during the reoxygenation phase. These increases were not observed when glucose was present during the hypoxic period. 3. The in vitro pretreatment for 2 h with 50 microM bovine brain gangliosides (BBG) prevented the increases in the release of Tau and Glu induced by the hypoxia reoxygenation. 4. These results constitute a further example where BBG appears to exert a protective role in cardiac tissues submitted to injuries.

Prostanoid production in hypoxic rat isolated atria: influence of acute diabetes.

The effects of hypoxia on prostanoid production were studied in atria from normal, acute diabetic and insulin-treated diabetic rats. Diabetes was induced by intravenous administration of 65 mg/kg of streptozotocin, the rats were killed 5 days later. Hypoxia was performed by incubation of the atria during 60 min in nitrogen-equilibrated glucose free Krebs' solution followed by 15 min of reoxygenation. The prostanoids 6-keto prostaglandin F1 alpha (6-keto PGF1 alpha) and thromboxane B2 (TXB2), stable metabolites of prostacyclin and TXA2, respectively, as well as PGF2, were measured by reversed phase HPLC-UV. In control atria, the production of 6-keto PGF1 alpha was equivalent to that of PGE2, whereas TXB2 was released in a much smaller amount. In diabetic atria, 6-keto PGF1 alpha production was reduced by 65%, whereas TXB2 release was increased by 158% compared to the controls. When the normal atria were exposed to 60 min of hypoxia, the release of 6-keto PGF1 alpha increased by 142% compared to basal values and remained elevated after 15 min of reoxygenation whereas in diabetic and insulin-treated diabetic tissues the 6-keto PGF1 alpha production was not modified by the hypoxia-reoxygenation period. The release of TXB2 was increased after 60 min hypoxia in normal as well as in diabetic and insulin-treated diabetic tissues and remained elevated during the reoxygenation. The PGE2 output increased only after the onset of the reoxygenation in the three groups studied.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of L-glutamate on the responses to nerve stimulation in rat isolated atria.

In rat atria isolated with their sympathetic fibres the chronotropic responses to nerve stimulation with pulses of 2 ms duration were reduced in a concentration-dependent manner by 10 microM to 1 mM L-glutamate (Glu) and by 0.01 to 1.00 microM (R,S)-3-hydroxy-5-methoxyloxasole-4-propionic acid (AMPA), whereas they were unaffected by other agonists of Glu receptors such as 1 microM to 1 mM N-methyl-D-aspartic acid (NMDA), 10 microM to 1 mM kainate and 1 to 100 microM (+/-)-2-amino-4-phosphonobutyric acid (AP4). The reductions in the atrial responses to nerve stimulation caused by Glu were not accompanied by alterations in either the basal efflux of [3H]noradrenaline or its overflow in response to the stimulation. The sensitivity of the atria to exogenous noradrenaline was not modified by either Glu or AMPA. The decreases in the chronotropic responses caused by Glu and by AMPA were prevented by both the non-selective Glu receptor antagonist, 100 microM kynurenic acid, and the selective AMPA receptor antagonist, 10 to 50 microM 6,7-dinitroquinoxaline-2,3-dione (DNQX). In addition, the adenosine receptor antagonist, 8-phenyltheophylline (10 microM), as well as the muscarinic acetylcholine receptor antagonist, atropine (3 microM), prevented the inhibitory effects of both Glu and AMPA on the chronotropic responses of rat isolated atria. Since both adenosine and acetylcholine are known to exert negative inotropic and chronotropic effects in cardiac tissues, it is proposed that Glu could contribute, through the interaction with receptors of the AMPA type, to facilitate the release of adenosine and acetylcholine from the atria.(ABSTRACT TRUNCATED AT 250 WORDS)