Differential activation of protein kinase C alpha is associated with arachidonate release in Madin-Darby canine kidney cells

Translocation of PKC isoforms in bovine aortic smooth muscle cells exposed to strain

Our laboratory has previously reported that the exposure of smooth muscle cells (SMC) to the cyclic strain results in significant stimulation of protein kinase C (PKC) activity by translocating the enzyme from the cytosol to the particulate fraction. We now sought to examine the strain-induced translocation of individual PKC isoforms in SMC. Confluent bovine aortic SMC grown on collagen type I-coated plates were exposed to cyclic strain for up to 100 s at average 10% strain with 60 cycles/min. Immunoblotting analysis demonstrates that SMC express PKC-alpha, -beta and -zeta in both cytosolic and particulate fractions. Especially, PKC-alpha and -zeta were predominantly expressed in the cytosolic fraction. However, cyclic strain significantly (P < 0.05) increased PKC-alpha and -zeta in the particulate fraction and decreased in the cytosolic fraction. Thus, the cyclic strain-mediated stimulation of PKC activity in SMC may be due to the translocation of PKC-alpha and -zeta from the cytosolic to the particulate fraction. These results demonstrate that mechanical deformation causes rapid translocation of PKC isoforms, which may initiate a cascade of proliferation responses of SMC since NF-kappaB, which is involved in the cellular proliferation has been known to be activated by these PKC isoforms.

Potassium transport in the mammalian collecting duct

The mammalian collecting duct plays a dominant role in regulating K(+) excretion by the nephron. The collecting duct exhibits axial and intrasegmental cell heterogeneity and is composed of at least two cell types: collecting duct cells (principal cells) and intercalated cells. Under normal circumstances, the collecting duct cell in the cortical collecting duct secretes K(+), whereas under K(+) depletion, the intercalated cell reabsorbs K(+). Assessment of the electrochemical driving forces and of membrane conductances for transcellular and paracellular electrolyte movement, the characterization of several ATPases, patch-clamp investigation, and cloning of the K(+) channel have provided important insights into the role of pumps and channels in those tubule cells that regulate K(+) secretion and reabsorption. This review summarizes K(+) transport properties in the mammalian collecting duct. Special emphasis is given to the mechanisms of how K(+) transport is regulated in the collecting duct.

Keloids in rural black South Africans. Part 3: a lipid model for the prevention and treatment of keloid formations

In the third part of this study a basic lipid model (regarding phospholipids, triglycerides, cholesterol esters and free fatty acids) for keloids (n=20), compared with normal skin of keloid prone and non-keloid prone patients (n=20 of each), was constructed according to standard methods, to serve as a sound foundation for essential fatty acid supplementation strategies in the prevention and treatment of keloid formations. Essential fatty acid deficiency (EFAD) of the omega-6 series (linoleic acid (LA), g-linolenic acid (GLA), and dihomo-g-linolenic acid (DGLA)) and the omega-3 series (a-linolenic acid (ALA) and eicosapentaenoic acid (EPA)), but enhanced arachidonic acid (AA) levels, were prevalent in keloid formations. Enhanced AA, but a deficiency of AA precursors (LA, GLA and DGLA) and inflammatory competitors (DGLA and EPA), are inevitably responsible for the overproduction of pro-inflammatory metabolites (prostaglandin E(2)(PGE(2))) participating in the pathogenesis of inflammation. Of particular interest was the extremely high free oleic acid (OA) levels present, apart from the high free AA levels, in the keloid formations. OA stimulates PKC activity which, in turn, activates PLA(2)activity for the release or further release of AA from membrane pools. Interactions between EFAs, eicosanoids, cytokines, growth factors and free radicals can modulate the immune response and the immune system in undoubtedly involved in keloid formation. The histopathology of keloids can be adequately explained by: persistence of inflammatory- and cytokine-mediated reactions in the keloid/dermal interface and peripheral areas, where fibroblast proliferation and continuous depletion of membrane linoleic acid occur; microvascular regeneration and circulation of sufficient EFAs in the interface and peripheral areas, where maintenance of metabolic active fibroblasts for collagen production occur; microvessel occlusion and hypoxia in the central areas, where deprivation of EFAs and oxygen with consequent fibroblast apoptosis occur, while excessive collagen remain. All these factors contribute to different fibroblast populations present in: the keloid / dermal interface and peripheral areas where increases in fibroblast proliferation and endogenous TGF-b occur, and these metabolic active fibroblast populations are responsible for enhanced collagen production: the central areas where fibroblast populations under hypoxic conditions occur, and these fibroblasts are responsible for excessive collagen production. It was concluded that: fibroblast membrane EFAD of AA precursors and inflammatory competitors, but prevailing enhanced AA levels, can contribute to a chain of reactions eventually responsible for keloid formations.

High glucose-induced inhibition of alpha-methyl-D-glucopyranoside uptake is mediated by protein kinase C-dependent activation of arachidonic acid release in primary cultured rabbit renal proximal tubule cells

Abnormal glucose handling in the proximal tubule may play an important role in the development of diabetic nephropathy. Thus, the present study was designed to examine the effect of high glucose on alpha-methyl-D-glucopyranoside (alpha-MG) uptake and its signaling pathways in the primary cultured rabbit renal proximal tubule cells (PTCs). When PTCs were preincubated with 25 or 50 mM glucose for 4 h, 25 or 50 mM glucose significantly inhibited alpha-MG uptake, while 25 or 50 mM mannitol and L-glucose did not affect. Actinomycin D and cycloheximide did not block the effect of high glucose on alpha-MG uptake. Twenty-five millimoles glucose-induced inhibition of alpha-MG uptake was blocked by mepacrine and AACOCF(3), phospholipase A(2) (PLA(2)) inhibitors. Twenty-five millimoles of glucose, not mannitol or L-glucose, significantly increased the [(3)H]-arachidonic acid (AA) release compared to control. In addition, the 25 mM glucose-induced [(3)H]-AA release was completely blocked by mepacrine or AACOCF(3). Indomethacin, a cyclooxygenase inhibitor, blocked the high glucose-induced inhibition of alpha-MG uptake, although econazole, cytochrome P-450 a epoxygenase inhibitor, and nordihydroguaiaretic acid (NDGA), a lipoxygenase inhibitor, did not. On the other hand, staurosporine and bisindolylmaleimide I, protein kinase C (PKC) inhibitors, blocked 25 mM glucose-induced increase of [(3)H]-AA release and inhibition of alpha-MG uptake. However, neomycin, U 73122, and phospholipase c(PLC) inhibitors did not block the effect of 25 mM glucose on [(3)H]-AA release and alpha-MG uptake. Pretreatment of methoxyverapamil, an L-type Ca(2+) channel blocker, abolished 25 mM glucose-induced increase of [(3)H]-AA release. Indeed, 25 mM glucose increased translocation of cPLA(2) from cytosolic fraction to membrane fraction. In conclusion, the present results demonstrate that high glucose inhibits alpha-MG uptake by the increase of AA release via the activation of PKC.

Involvement of protein kinase C in Rickettsia rickettsii-induced transcriptional activation of the host endothelial cell

Our laboratory has reported on a biphasic pattern of nuclear factor kappaB (NF-kappaB) activation in cultured human umbilical vein endothelial cells during infection with Rickettsia rickettsii, an obligate, intracellular bacterium, and the etiologic agent of Rocky Mountain spotted fever. Transcriptional activation of the tissue factor (TF) gene during this infection has been shown to involve NF-kappaB. To further understand the signal transduction events underlying these phenomena, we studied the role of protein kinase C (PKC), a ubiquitous family of phospholipid-dependent enzymes implicated in the regulation of a variety of cell signaling pathways. Two inhibitors of PKC, namely, bisindolylmaleimide I hydrochloride (BM-1) and calphostin C, which exhibit different inhibitory properties towards various isozymes of PKC, were used. Infection of cells with R. rickettsii in the presence of BM-1 (50 nM) did not significantly affect NF-kappaB, whereas calphostin C (2.5 microM) completely blocked the early phase of NF-kappaB activation. The late, sustained phase also was not affected by treatment with BM-1. Downregulation of phorbol ester-sensitive PKCs by long-term treatment with phorbol 12-myristate 13-acetate (PMA) did not inhibit NF-kappaB activation. Likewise, this downregulation had no effect on induction of TF activity. The activity of TF was, however, sensitive to BM-1 and calphostin C, whereas expression of TF mRNA was inhibited only by calphostin C. Overall, these results suggest the lack of involvement of classical PKC pathways in R. rickettsii-induced NF-kappaB activation but the possible involvement of a non-PMA-responsive PKC isoform in the posttranscriptional control of TF expression.

Comparison of the fatty acid compositions in intraepithelial and infiltrating lesions of the cervix: part II, free fatty acid profiles

In the second part of this study, the emphasis is on the free fatty acids during cervical carginogenesis, since they may reflect active cell metabolism during this disease process. Lipids were extracted from biopsies of normal epithelial tissue (N) (n=36), cervical intraepithelial lesions (CIL) (n=47), and infiltrating lesions (Ca) (n=47) of the cervix. Samples, from which the free fatty acid compositions were determined, were saponified, methylated and analysed by GLC. In accordance with results obtained on total fatty acid compositions, essential fatty acid deficiency (EFAD) in the intraepithelial lesions, compared with normal tissue (linoleic acid, P< 0.01), and infiltrating lesions compared with intraepithelial lesions (linoleic acid and arachidonic acid, P< 0.01) were observed. High levels of oleic acid were also observed when infiltrating lesions were compared with normal tissue (P < 0.01). As previously mentioned by us in part I of this study, with regard to possible disturbances in metabolic pathways based on the total fatty acid profiles during stages of cervical cancer, EFAD is prevalent during cervical carcinogenesis. This EFAD in cancer cells may result in many defective cell mechanisms, since fatty acids are associated with biochemical events such as lipid peroxidation, signal transduction and immune responses. The high level of oleic acid in cancer cells is known to activate PKC and thus contribute to the continous growth stimulus thought to exist in malignant cells. From a therapeutic viewpoint, substantial changes in the fatty acid composition of the membranes can be produced in cancer cells by selective fatty acid supplementation strategies. At present, modifications of the fatty acid compositions of cell membranes represent an experimental model that has promoted increased understanding of lipid transportation, membrane remodelling, and the relationship between membrane lipids and membrane function. By addressing factors responsible for insufficient essential fatty acid levels, carginogenesis may be prevented or treated. The clinical feasibility of using modification of fatty acids in tumours or cancer by diet or perfusion as an adjunct to standard therapies should be tested.

Role of protein kinase C alpha in endothelin-1 stimulation of cytosolic phospholipase A2 and arachidonic acid release in cultured cat iris sphincter smooth muscle cells

We have investigated the role and mechanism of protein kinase C (PKC) isoforms in endothelin-1 (ET-1)-induced arachidonic acid (AA) release in cat iris sphincter smooth muscle (CISM) cells. ET-1 increased AA release in a concentration (EC50=8 nM) and time-dependent (t1/2=1.2 min) manner. Cytosolic phospholipase A2 (cPLA2), but not phospholipase C (PLC), is involved in the liberation of AA in the stimulated cells. This conclusion is supported by the findings that ET-1-induced AA release is inhibited by AACOCF3, quinacrine and manoalide, PLA2 inhibitors, but not by U-73122, a PLC inhibitor, or by RHC-80267, a diacylglycerol lipase inhibitor. A role for PKC in ET-1-induced AA release is supported by the findings that the phorbol ester, PDBu, increased AA release by 96%, that prolonged treatment of the cells with PDBu resulted in the selective down regulation of PKCalpha and the complete inhibition of ET-1-induced AA release, and that pretreatment of the cells with staurosporine or RO 31-8220, PKC inhibitors, blocked the ET-1-induced AA release. Gö-6976, a compound that inhibits PKCalpha and beta specifically, blocked ET-1-induced AA release in a concentration-dependent manner with an IC50 value of 8 nM. Thymeatoxin (0.1 microM), a specific activator of PKCalpha, beta, and gamma induced a 150% increase in AA release. Treatment of the cells with ET-1 caused significant translocation of PKCalpha, but not PKCbeta, from cytosol to the particulate fraction. These results suggest that PKCalpha plays a critical role in ET-1-induced AA release in these cells. Immunochemical analysis revealed the presence of cPLA2, p42mapk and p44mapk in the CISM cells. The data presented are consistent with a role for PKCalpha, but not for p42/p44 mitogen-activated protein kinase (MAPK), in cPLA2 activation and AA release in ET-1-stimulated CISM cells since: (i) the PKC inhibitor, RO 31-8220, inhibited ET-1-induced AA release, cPLA2 phosphorylation and cPLA2 activity, but had no inhibitory effect on p42/p44 MAPK activation, (ii) genistein, a tyrosine kinase inhibitor, inhibited ET-1-stimulated MAPK activity but had no inhibitory effect on AA release in the ET-1-stimulated cells. We conclude that in CISM cells, ET-1 activates PKCalpha, which activates cPLA2, which liberates AA for prostaglandin synthesis.

Novel aspect on metal fume fever: zinc stimulates oxygen radical formation in human neutrophils

Exposure to zinc fume may cause metal fume fever, an acute reaction characterized by an invasion of neutrophils into the airways. This investigation was conducted to examine the possibility that Zn2+ and ZnO might stimulate the formation of oxygen radicals by human neutrophils. Luminol-amplified chemiluminescence (CL) was monitored during 2 h from human neutrophils exposed to Zn2+ or ZnO. The response was compared to that of other metal ions and to that of endotoxin and phorbol myristate acetate (PMA). Zn2+ (6-50 microM) gradually caused a 2-6-fold increase of CL that reached an optimum after 70- 80 min. By contrast, Cd2+, Cr2+, Cr3+, Fe2+, Fe3+, Ni2+ or Co2+ in corresponding concentrations did not increase the CL. Similar to Zn2+, endotoxin (40-640 micrograms/ml) caused a 2-5-fold increase of CL with an optimum after 70 min, and endotoxin (40 micrograms/ml) together with Zn2+ (50 microM) synergistically increased the CL. ZnO (12-100 micrograms/ml) also augmented CL, with a 1.5-5-fold increase at 25-100 micrograms/ml ZnO but with a time response similar to that found after PMA stimulation, in which CL peaked after 20-40 min incubation. Both Zn(2+)- and ZnO-induced CL was inhibited by manoalide, a phospholipase A2 inhibitor, with IC50 of 0.25 microM and 0.66 microM respectively. These results indicate that Zn2+ and ZnO both stimulates oxygen radical formation in human neutrophils and that this might contribute to the pathogenesis of zinc fume fever.

Isoform-specific redistribution of protein kinase C in living cells

We have used confocal laser scanning microscopy to determine the dynamics of distribution of activated protein kinase C (PKC) in living Madin Darby canine kidney (MDCK) cells. Using fluorescently tagged phorbol myristate acetate (PMA) as a probe for PKC we have demonstrated its distribution in association with the cell periphery and with the nucleus. Dual labeling experiments using PKC alpha and PKC beta II specific antisera indicate that activated PKC alpha is found in association with the periphery whereas activated PKC beta II is translocated to the nucleus. We have demonstrated increased activity of PKC in nuclear fractions isolated from cells treated with PMA and other PKC activators. These data indicate that upon activation individual isoforms of PKC translocate to different subcellular locations where they are likely to mediate different actions.

NO2-induced expression of specific protein kinase C isoforms and generation of phosphatidylcholine-derived diacylglycerol in cultured pulmonary artery endothelial cells

The present study examines whether nitrogen dioxide (NO2)-induced activation of protein kinase C (PKC) is associated with increased expression of specific PKC isoforms and/or with enhanced generation of phosphatidylcholine(PC)-derived diacylglycerol (DAG) in pulmonary artery endothelial cells (PAEC). Western blot analysis revealed that exposure to 5 ppm NO2 resulted in increased expression of PKC alpha and epsilon isoforms in both cytosol and membrane fractions in a time-dependent fashion compared with controls. A time-dependent elevated expression of PKC isoform beta was observed in the cytosol fraction only of N02-exposed cells. PKC isoform gamma was not detectable in either the cytosolic or membrane fractions from control or N02-exposed cells. Scatchard analysis of [3h]phorbol 12,13-dibutyrate (PDBu) binding showed that exposure to N02 for 24 h increased the maximal number of binding sites (Bmax) from 15.2 +/- 2.3 pmol/mg (control) to 42.3 +/- 5.3 pmol/mg (p < 0.01, n = 4) (NO2-exposed). Exposure to NO2 significantly increased PC specific-phospholipase C and phospholipase D activities in the plasma membrane of PAEC (p < 0.05 and p < 0.001, respectively). When [3H]-myristic acid-labeled cells were exposed to NO2, significantly increased radioactivity was associated with cellular DAG. These results show for the first time that exposure of PAEC to NO2 results in elevated expression of specific PKC isoforms and in enhanced generation of cellular DAG, and the latter appears to arise largely from the hydrolysis of plasma membrane PC.

Paradoxical effect of 3-isobutyl-1-methylxanthine on cytochrome P450 cholesterol side-chain cleavage mRNA accumulation in porcine granulosa cells

Earlier studies in immature porcine granulosa cells cultured in serum-free medium showed dual actions of the protein kinase C activator 12-O-tetradecanoylphorbol-13-acetate (TPA). In cells incubated for 24 h, TPA inhibited follicle-stimulating hormone (FSH)-stimulated cytochrome P450 cholesterol side-chain cleavage (P450scc) mRNA accumulation. In contrast, at 4 h, TPA increased P450scc mRNA concentration in the absence and presence of FSH or 8-bromo-cAMP; in addition, TPA augmented FSH-stimulated cAMP accumulation. The actions of TPA were then examined in the presence of the phosphodiesterase (PDE) inhibitor, 3-isobutyl-1-methylxanthine (IBMX). With IBMX present, TPA caused a smaller relative augmentation of cAMP accumulation during a 4-h incubation period, suggesting that TPA may both increase cAMP synthesis and inhibit its degradation. The stimulatory effect of FSH or 8-bromo-cAMP on P450scc mRNA concentration was not modified by IBMX. However, TPA no longer augmented the FSH- or 8-bromo-cAMP-stimulated P450scc mRNA accumulation when IBMX was present. In cells treated with FSH for 24 h, IBMX augmented progesterone production, but paradoxically accentuated the inhibitory effect of TPA on steroidogenesis. These results indicate that IBMX converts TPA from a stimulatory into an inhibitory agent by an action unrelated to cAMP, and points to the need for caution in interpreting experiments with this drug.

Regulation of the renal Na-HCO3 cotransporter: V. mechanism of the inhibitory effect of parathyroid hormone

PTH administration decreases proximal HCO3 reabsorption and inhibits the brush border Na-H antiporter. We studied the effect of PTH on the renal Na-HCO3 cotransporter and examined whether this effect is mediated through the adenylate cyclase/cyclic AMP system or through the phospholipase A pathway. We studied the effect of PTH [1-34] on the Na-HCO3 cotransporter activity in rabbit renal basolateral membranes incubated with 50 microM ATP by measuring the 22Na uptake in the presence of HCO3 and gluconate. Na-HCO3 cotransporter activity (expressed in nmol/mg protein/3 seconds) was taken as the difference in 22Na uptake in the presence of HCO3 and gluconate. PTH (10(-10) M) completely inhibited Na-HCO3 cotransporter activity from 1.23 +/- 0.14 to -0.58 +/- 0.23, P < 0.001. This effect of PTH to inhibit the Na-HCO3 cotransporter was prevented by the polyclonal antibody against G alpha s indicating that PTH acts through G alpha s protein. Because G alpha s stimulates adenylate cyclase/cyclic AMP system, we examined the effect of PTH in the presence and in the absence of the adenylate cyclase inhibitor, dideoxyadenosine (DDA). DDA alone (10(-4) M) stimulated the Na-HCO3 cotransporter activity. In the presence of DDA, the net inhibitory effect of PTH was the same magnitude as that of control, suggesting the existence of other pathways for the effect of PTH on the cotransporter. Calmodulin inhibition also partially prevented the effect of PTH. To determine whether the inhibitory effect of PTH is mediated at least in part, through phospholipase A, we first examined the effect of PTH on arachidonic acid release and then measured the Na-HCO3 cotransporter activity in presence and in absence of arachidonic acid or eicosatetraynoic acid (ETA), an inhibitor of arachidonic acid metabolism. PTH significantly increased the release of arachidonic acid by isolated proximal tubule cells and arachidonic acid inhibited the Na-HCO3 cotransporter in basolateral membranes. ETA (3 microM) partially prevented the inhibitory effect of PTH. In cultured proximal tubule cells, PTH inhibited the HCO3-dependent 22Na uptake and ethoxyresorufin, an inhibitor of cytochrome P-450, blocked the inhibitory effect of PTH on the cotransporter. These results demonstrate that PTH inhibits the renal Na-HCO3 cotransporter through multiple mechanisms, that are mediated through G proteins, G alpha s and GP, and CaM-KII.

Adenine nucleotide and protein kinase C regulation of renal tubular epithelial cell wound healing

The present studies were done to determine the effect of selected adenine nucleotides on healing of wounds made by mechanically denuding areas in confluent monolayers of renal tubular epithelial cells. We found that hydrolyzable and nonhydrolyzable forms of ATP but not UTP stimulated healing of LLC-PK1 cell wounds, while both ATP and UTP promoted healing of MDCK cell wounds, suggesting that different subtypes of purinoceptors regulated wound healing in these cells. Stimulation of wound healing by ATP was equivalent in control cells and in cells in which irradiation suppressed proliferation, suggesting a prominent role for cell migration in the healing process. Since ATP receptors are often linked to activation of protein kinase C, the effect of a protein kinase C activator (4 beta-phorbol 12-myristate 13-acetate, PMA) on wound healing was studied. Long-term (24 hr) exposure to PMA inhibited while short-term (30-120 min) exposure to PMA enhanced cell wound healing. Two chemically and mechanistically dissimilar protein kinase C inhibitors (calphostin C and chelerythrine) inhibited LLC-PK1 and MDCK cell wound healing, and calphostin C prevented ATP enhancement of LLC-PK1 healing. These observations suggest a role for protein kinase C in regulation of basal and adenine nucleotide-stimulated wound healing. Adenosine triphosphate did not affect cell-cell adhesion of either LLC-PK1 or MDCK cells. Adenine nucleotides and PMA enhanced and calphostin C inhibited short-term adhesion of LLC-PK1 and MDCK cells to plastic and to other substrates such as fibronectin, laminin and collagen type IV.(ABSTRACT TRUNCATED AT 250 WORDS)

Growth-dependent accumulation of monoalkylglycerol in Madin-Darby canine kidney cells. Evidence for a role in the regulation of protein kinase C

1-O-Alkyl-sn-glycerol (alkylglycerol) forms the backbone of complex ether-linked glycerolipids, including biologically active lipids such as platelet-activating factor. Synthetic alkylglycerol itself possesses several potent pharmacological activities and has been shown to inhibit protein kinase C (PKC) in vitro. In spite of these properties, free alkylglycerol has been regarded only as a potential product of the inflammatory degradation of complex ether lipids rather than a natural cell constituent. To explore the possibility that endogenous alkylglycerol functions as a physiological regulator in normal cells, we measured its content, along with related monoglycerides and diglycerides, by high performance liquid chromatography and gas-liquid chromatography in Madin-Darby canine kidney (MDCK) cells. The content of free alkylglycerol increased up to 20-fold during the growth of MDCK cell cultures to a confluent density. The increase was greatest during the log phase of growth, in which the content of alkylglycerol rose from 6.0 +/- 1.3 nmol/10(8) cells in preconfluent cultures to 23.6 +/- 3.4 nmol/10(8) cells in confluent cultures. Analysis of the molecular species of alkylglycerol showed that the higher content in quiescent MDCK cells was due primarily to an increase in 1-O-octadecyl-sn-glycerol. In contrast, the levels of monoacylglycerol and the PKC activator diacylglycerol were lower in confluent, quiescent cultures than in preconfluent, proliferating cultures. A similar pattern of changes in the monoglyceride and diglyceride content was observed in interleukin-3-dependent CFTL-12 mast cells when cell proliferation was blocked by growth factor withdrawal. Growth of MDCK cells to a confluent density resulted in a decrease in particulate PKC enzyme activity to a level that was only 6% of that in proliferating cells. To explore whether the accumulation of cellular alkylglycerol contributes to growth-dependent changes in PKC activity, we examined the effects of adding alkylglycerol to the activity and subcellular distribution of the enzyme in MDCK cells. Treatment of cells with 1-O-dodecyl-sn-glycerol resulted in a decrease in the activity of membrane-associated PKC activity and inhibited 12-O-tetradecanoylphorbol-13-acetate-stimulated translocation of PKC from the cytosol to the membrane fraction. Alkylglycerol was also shown to inhibit the activity of purified PKC in vitro when present at levels similar to that of the diacylglycerol activator. We propose that the accumulation of alkylglycerol during the growth of MDCK cells to a confluent density contributes to the decrease in PKC activity. The control of cellular alkylglycerol levels may be a novel mechanism for the regulation of cellular physiology.

Signal transduction pathways leading to arachidonic acid release from neutrophilic HL-60 cells. The involvement of G protein, protein kinase C and phospholipase A2

Arachidonic acid release from undifferentiated and neutrophilic HL-60 cells was studied. In neutrophilic cells it was stimulated by N-formyl-Met-Leu-Phe and mastoparan by a mechanism involving Gi protein and phospholipase C and was largely dependent on diacyglycerol lipase. Maximum release from both cell types was achieved with fluoride and required cellular energy. Inhibitor studies suggest that arachidonic acid release by fluoride stimulation leads to phospholipase A2 activation with signal transduction involving phospholipase C and protein kinase C. Only neutrophilic cells responded to phorbol ester if Ca(2+)-ionophore was simultaneously present but this effect was abolished by extended treatment with phorbol ester. Thus, protein kinase C plays a major role in highly stimulated neutrophilic cells. These cells are differently equipped with protein kinase C isoenzymes compared with undifferentiated cells. In contrast, both cell types contain similar levels of type II and cytosolic phospholipases A2, the former being by far the more prevalent.

Evidence that ceramide selectively inhibits protein kinase C-alpha translocation and modulates bradykinin activation of phospholipase D

Sphingomyelinase (SMase) treatment (0.1 unit/ml for up to 30 min) of mouse epidermal (HEL-37) or human skin fibroblast (SF 3155) cells preincubated with [3H]serine to label the sphingomyelin pool caused the accumulation of labeled ceramide but not sphingosine or ceramide 1-phosphate. Incubation of HEL-37 cells with dioctanoylglycerol (diC8) or SF 3155 cells with bradykinin caused translocation of calcium/phosphatidylserine-dependent protein kinase C (PKC) activity to particulate material. In both cell lines the translocation was blocked by SMase treatment of the cells or by incubation with the cell-permeable ceramide analogue N-acetylsphingosine (C2-Cer). Western blot analysis indicated that treatment of HEL-37 cells with diC8 or SF 3155 cells with bradykinin resulted in the translocation of both PKC-alpha and PKC-espilon to particulate material. Treatment with SMase or C2-Cer specifically blocked the translocation of PKC-alpha but not that of PKC-epsilon. Pretreatment of cells with SMase or C2-Cer also inhibited the activation of phospholipase D activity induced by either diC8 (HEL-37 cells) or bradykinin (SF 3155 cells). The data provide strong evidence that ceramide can negatively regulate the translocation of PKC-alpha but not PKC-epsilon and further suggest that PKC-alpha may be involved in regulating phospholipase D activity.

Hydrogen peroxide activation of cytosolic phospholipase A2 in vascular smooth muscle cells

We have reported previously that hydrogen peroxide induces arachidonic acid release from prelabeled vascular smooth muscle cells. Here, we studied the effect of hydrogen peroxide on the phosphorylation of cytosolic phospholipase A2 in these cells. Hydrogen peroxide induced a rapid, time-dependent increase in the phosphorylation of cytosolic phospholipase A2. Hydrogen peroxide also increased arachidonic acid release from prelabeled cells in a time-dependent manner similar to that of phosphorylation of cytosolic phospholipase A2. Protein kinase C depletion significantly inhibited the hydrogen peroxide-stimulated cytosolic phospholipase A2 phosphorylation and arachidonic acid release. Hydrogen peroxide caused a time-dependent increase in mitogen activated protein kinase activity. Taken together, these findings suggest that cytosolic phospholipase A2 may, at least in part, contribute to arachidonic acid release induced by hydrogen peroxide and this effect appears to be mediated by protein kinase C and mitogen activated protein kinase.

Inhibition of phorbol ester-stimulated arachidonic acid release by alkylglycerols

Although synthetic analogs of alkylglycerol (AG), such as dodecylglycerol, possess potent biological activities, their mechanism of action has not been determined. We recently detected substantial amounts of AG in unstimulated MDCK cells (Warne, T.R. and Robinson, M. (1991) Anal. Biochem. 198, 302-307) raising the possibility that the endogenous compound may act as a biological mediator. In this study, we examined the effects of synthetic AG on the release of arachidonic acid and arachidonate metabolites (AA) from Madin Darby canine kidney (MDCK) cells in response to 12-O-tetradecanoylphorbol-13-acetate (TPA) in order to characterize its effects on this signalling pathway. Treatment of MDCK with AG potently inhibited the release of AA during subsequent stimulation with TPA. Dodecylglycerol, the most effective of a series of alkyglycerols tested, was active at concentrations as low as 3 microM. The sn-1 and sn-3 forms of AG were found to be equally potent inhibitors. The effects of AG on AA release were not the result of arachidonic acid redistribution among cellular lipids and were independent of the phospholipid source of the released AA. AG did not inhibit the release of AA from MDCK cells when bradykinin was used as a stimulus, indicating selectivity for the effects produced by phorbol esters. These results show that AG can function as a potent and specific inhibitor of TPA-mediated AA release. The ability of AG to regulate this signalling pathway in intact MDCK cells, together with its natural occurrence, suggests a potential bioregulatory role for the endogenous compound as an inhibitor of protein kinase C.

The role of protein kinase C in arachidonic acid release and prostaglandin E production from CHO cells transfected with EGF receptors

Arachidonic acid release and prostaglandin production are stimulated by both phorbol esters and growth factors in various cell types. Whereas phorbol esters activate and transmit a signal via protein kinase C, this pathway is not necessarily involved in growth factor signal transduction. We investigated the involvement of protein kinase C in the pathway of arachidonic acid metabolism by CHO cells transfected with full-length EGF receptor (CHOwt). Two isoforms of protein kinase C were identified in CHOwt cells, alpha and zeta. On downregulation, the parallel loss of phorbol ester-stimulated arachidonic acid release and the alpha-isoform suggests a possible involvement of this isoform in phospholipase A2 activation in these cells. In addition, we propose that the zeta-isoform may be separately involved in prostaglandin production as residual phorbol ester-stimulation of PGE production occurs in downregulated cells where PKC zeta is the sole remaining isoform. EGF stimulation of arachidonic acid release, as a measure of phospholipase A2 activation, and subsequent prostaglandin production are unaffected by inhibition of protein kinase C in CHOwt cells. Indeed one such inhibitor, staurosporine, augmented the EGF effect. These results suggest that PKC is not required for EGF activation of phospholipase A2 in these cells.

A phospholipase C inhibitor, U-73122, blocks TSH-induced inositol trisphosphate production, Ca2+ increase and arachidonic acid release in FRTL-5 thyroid cells

To characterize the role of phospholipase C (PLC)-mediated intrathyroid signal transduction by thyrotropin, we studied the effect of U-73122, an aminosteroid inhibitor of PLC-dependent activity, on TSH-activated PLC-Ca2+ and arachidonic acid (AA) signalling systems in cultured FRTL-5 rat thyroid cells. In the presence of extracellular Ca2+, TSH (0.1 microM) increased intracellular free calcium concentration ([Ca2+]i) by 63 +/- 6% with a sustained plateau phase, and AA release by 160 +/- 16%. By deletion of extracellular Ca2+, TSH induced a similar maximal [Ca2+]i increase, but the plateau phase and AA release were entirely suppressed. U-73122 (5 microM) inhibited TSH stimulation of 3H-labelled inositol trisphosphates (IP3) production by 73 +/- 3% (P < 0.01) in one study, and completely in another. U-73122 concentration-dependently blocked the TSH-induced Ca2+ increase in either the presence or absence of external Ca2+. U-73122 also showed a similar concentration-response inhibition of TSH-induced AA release. These results provide direct evidence of PLC mediation of TSH-stimulated signal transduction in FRTL-5 thyroid cells. TSH-induced external Ca2+ entry, as well as intracellular Ca2+ mobilization, is probably a PLC-mediated process. From an IP3-sensitive intracellular pool, TSH induces intracellular Ca2+ mobilization. External Ca2+ entry seems to be a prerequisite for TSH-induced AA release. U-73122 inhibition of both cytosolic Ca2+ increase and AA release further confirms [Ca2+]i dependence for TSH stimulation of AA release.

Translocation of protein kinase Cs in Aplysia neurons: evidence for complex regulation

We examined the activation of protein kinase C (PKC) produced by phorbol esters in Aplysia nervous tissue. Translocation of PKC in intact ganglia requires higher concentrations of phorbol esters than would be expected from: (1) their affinity for Aplysia PKCs measured in vitro; (2) their physiological effects on cultured Aplysia neurons; and (3) their actions on PKC in synaptosomes. Although phorbol esters enter intact ganglia slowly, delayed access to neurons is insufficient to account for the high concentrations needed for translocation. Increasing accessibility to the neural components of ganglia increases the rate at which translocation occurs, but does not affect the concentration of phorbol ester required. We suggest that this might best be explained by the presence of a competitive inhibitor at the binding site for phorbol esters in PKC. An indication for an inhibitor is that the concentration of phorbol esters needed for translocation in homogenates of nervous tissue is markedly decreased by diluting the extract. Preliminary characterization shows that the inhibitory activity is unusual: in addition to being competitive with lipid activators, it is soluble and tissue-specific. This type of inhibitor may be an important regulator of protein phosphorylation by PKC in neurons.

Protein kinase C isoform expression and regulation in the developing rat heart

To determine whether age-dependent differences in cardiac responses to autonomic agonists could result from developmental changes in protein kinase C (PKC) isoform expression, we probed extracts from the fetal, neonatal, and adult heart as well as cultured neonatal and isolated adult ventricular myocytes with specific antisera to calcium-dependent (alpha and beta) and calcium-independent (delta, epsilon and zeta) isoforms of the enzyme. Although PKC-beta immunoreactivity could not be detected in cultured neonatal or isolated adult ventricular myocytes, adult and neonatal myocytes expressed multiple other isoforms of PKC. Our studies revealed an age-dependent decline in the immunoreactivity for three PKC isoforms. PKC-alpha was detected in extracts from the fetal and 2-day-old neonatal heart as well as cultured neonatal rat ventricular myocytes. Only faint PKC-alpha immunoreactivity was detected in extracts from the adult heart, and PKC-alpha was not detected in extracts from isolated adult ventricular myocytes, suggesting that PKC-alpha resides in nonmyocyte elements in the adult heart. PKC-delta also was detected in greater abundance in fetal and neonatal than in adult myocardial extracts. The decline in PKC-alpha and PKC-delta expression occurred during the first 2 postnatal weeks. PKC-zeta was detected in greatest abundance in extracts from the fetal heart. PKC-zeta expression declined markedly by the second postnatal day, and only faint PKC-zeta immunoreactivity was detected in extracts from adult myocardium. Failure to detect PKC-zeta in extracts from isolated adult ventricular myocytes suggests that PKC-zeta resides primarily in nonmyocyte elements in the adult heart. PKC-epsilon was detected in all preparations, but it was detected in greatest abundance in extracts from neonatal hearts. In vitro sympathetic innervation of previously noninnervated neonatal ventricular myocytes or in vivo chemical sympathectomy of the neonatal heart did not modulate PKC isoform expression, suggesting that sympathetic innervation does not significantly regulate PKC isoform expression. PKC-alpha partitioned to the soluble fraction of unstimulated myocytes and was selectively translocated to the particulate fraction by Ca2+. In contrast, a major portion of the novel PKC isoforms partitioned to the particulate fraction of unstimulated myocytes. The subcellular distribution of novel PKC isoforms was not influenced by Ca2+. 12-O-Tetradecanoylphorbol 13-acetate (TPA, 300 nmol/L) induced translocation of soluble PKC-alpha, PKC-delta, and PKC-epsilon to the particulate fraction at 30 minutes and complete (PKC-alpha and PKC-delta) or 80% (PKC-epsilon) downregulation at 24 hours. PKC-zeta was not affected by TPA.(ABSTRACT TRUNCATED AT 400 WORDS)

Stimulation of arachidonic-acid release from Swiss 3T3 cells by recombinant basic fibroblast growth factor: independence from phosphoinositide turnover

In this study we have attempted to characterize the mechanism of recombinant bovine basic fibroblast growth factor (rbFGF)-induced release of arachidonic acid from prelabelled Swiss 3T3 fibroblasts. Recombinant bFGF caused the release of [3H]arachidonic acid from metabolically labelled cells in a dose- and time-dependent manner. This effect was maximal with 10 ng rbFGF/ml and became significant after a 30-min incubation. Although rbFGF was able to cause a modest increase in total inositol phosphate accumulation, an examination of the time-course of the latter effect revealed that enhanced [3H]arachidonic-acid release could not have been derived from phosphoinositide metabolism. Evidence suggesting that rbFGF-induced release of [3H]arachidonic acid was being mediated via a PLA2 pathway was obtained by pharmacological antagonism using mepacrine, a putative PLA2 inhibitor. Moreover, treatment of cells with neomycin failed to attenuate rbFGF-mediated release of [3H]arachidonic acid. Chelation of extracellular calcium by EGTA was found to abrogate rbFGF-induced liberation of [3H]arachidonic add. Down-regulation of protein kinase C (PKC) by prolonged treatment of cells with the phorbol ester, PMA, was observed to have no effect on the action of rbFGF on [3H]arachidonic add release from Swiss 3T3 fibroblasts. While rbFGF was found to cause the indomethacin-sensitive production of prostaglandin E2 (PGE2) in a dose-dependent manner, this effect was independent of rbFGF-induced reinitiation of DNA synthesis. Clearly, the effect of rbFGF on cellular DNA synthesis was being mediated independently of PGE2 biosynthesis. We discuss the potential importance of the PLA2-signalling pathway in the mechanism of action of fibroblast growth factors.

Formation of diacylglycerol, inositol phosphates, arachidonic acid and its metabolites in macrophages

Treatment of macrophages with zymosan, 4 beta-phorbol 12-myristate 13-acetate (PMA) and fluoride but not with A 23187 or arachidonic acid (delta Ach) leads to a generation of diacylglycerol (acyl2Gro). Formation of inositol phosphates is achieved with zymosan, only. An elevation of intracellular calcium is obtained with zymosan and A 23187 but not with PMA, fluoride or delta Ach. Prior treatment of the cells with phorbol ester for 3 h which has been shown recently to result in a down-regulation of protein kinase (PK) C-beta but not PKC-delta [Duyster, J., Schwende, H., Fitzke, E., Hidaka H. & Dieter P. (1993) Biochem. J. 292, 203-207] has no effect on the zymosan-induced formation of acyl2Gro or inositol phosphates but inhibits the PMA-induced generation of acyl2Gro. Down-regulation of PKC-delta by prior phorbol ester treatment for 24 h augments the zymosan-induced generation of acyl2Gro and inositol phosphates. The acyl2Gro lipase inhibitor RG 80267 inhibits the PMA-induced and fluoride-induced generation of prostaglandin (PG) E2, reduces the zymosan-induced release of PGE2 by 50% but has no effect on PGE2 formation of unstimulated, A 23187-treated or delta Ach-treated cells. Furthermore, RG 80267 enhances accumulation of delta Ach-labeled acyl2Gro in response to zymosan, PMA and fluoride. These data indicate that zymosan activates a phosphatidylinositol 4,5-bisphosphate-specific phospholipase (PL) C, that generation of acyl2Gro by PMA and fluoride occurs via hydrolysis of other phospholipids, that PKC-beta is involved in the PMA-induced generation of acyl2Gro and PKC-delta negatively modulates the zymosan-induced activation of PLC and PMA and fluoride induce a liberation of delta Ach from acyl2Gro, A 23187 activates the PLA2 pathway and zymosan stimulates both, the acyl2Gro- and PLA2-pathway.

1,25-Dihydroxyvitamin D-3 induces arachidonate mobilization in embryonic chick myoblasts

1,25-Dihydroxyvitamin D-3 (1,25(OH)2D3) which activates the phospholipase C (PLC)-protein kinase C (PKC) signalling pathway, induces within 1 min a dose-dependent (10(-11)-10(-7) M) increase in the release of [3H]arachidonic acid ([3H]AA) from prelabeled embryonic chick myoblasts. The response is dependent on extracellular calcium, since it is suppressed by EGTA and nifedipine, a Ca(2+)-channel blocker, and is mimicked by the calcium ionophore A23187. 1,25(OH)2D3-induced release of [3H]AA is not affected by neomycin (0.5 mM), an inhibitor of phosphoinositide hydrolysis. 12-o-tetradecanoylphorbol-13-acetate (TPA), a PKC activator, induces an extracellular Ca(2+)-independent release of [3H]AA and amplifies the release of AA stimulated by 1,25(OH)2D3. 1-(5-isoquinolinylsulfonyl)-2-methyl-piperazine (H7), a PKC inhibitor, markedly suppressed TPA as well as 1,25(OH)2D3-induced [3H]AA release. Down-regulation of cellular PKC abolishes the effect of the phorbol ester, and partially inhibits 1,25(OH)2D3-induced [3H]AA release. Temporally correlated with AA liberation, the hormone increases the formation of lysophosphatidylcholine (lysoPC) and lysophosphatidylethanolamine (lysoPE) and decreases the cellular content of PC and PE. These results indicate that part of AA release by 1,25(OH)2D3 derives from PLA2 activation and that the effects of the hormone are mediated by PKC in a mode independent of phosphoinositide hydrolysis by PLC.

A role for protein kinase C-alpha in zymosan-stimulated eicosanoid synthesis in mouse peritoneal macrophages

A possible regulatory function of protein kinase C (PKC) isoenzymes in zymosan-stimulated eicosanoid synthesis was studied in mouse peritoneal macrophages in culture. The addition of zymosan to intact cells labelled with [3H]arachidonic acid stimulated a time-dependent and concentration-dependent release of the fatty acid. There was a simultaneous marked increase in the synthesis of prostaglandin E2 and leukotriene C4. The protein-kinase inhibitor K-252a and the selective PKC inhibitor CGP41251 completely blocked zymosan-triggered arachidonic acid release as well as prostaglandin E2 and leukotriene C4 synthesis. In contrast, an inactive staurosporine derivative, CGP42700, failed to inhibit any of the zymosan-induced responses. The down-regulation of PKC by long-term treatment with phorbol 12-myristate 13-acetate eliminated zymosan-stimulated arachidonic acid release and eicosanoid synthesis (after 4-6 h treatment). By using specific antibodies it was observed that mouse macrophages express five PKC isoenzymes, PKC-alpha, -beta, -delta, -epsilon and -zeta. No PKC-gamma isoenzyme was detected. After exposure to phorbol 12-myristate 13-acetate, a complete depletion of PKC-beta was observed within 1 h and the complete depletion of PKC-alpha and PKC-delta isotypes was observed within 4 h. In contrast, PKC-epsilon was only partially down-regulated after a 24-h treatment with phorbol 12-myristate 13-acetate and PKC-zeta was not affected at all. These data indicate that PKC-alpha and PKC-delta isoenzymes are candidates for regulating prostaglandin and leukotriene production. From the potent inhibitory activities of K-252a and CGP41251, two compounds that reportedly display a higher selectivity for PKC-alpha compared to PKC-delta, it is suggested that PKC-alpha triggers arachidonic acid mobilization and eicosanoid synthesis in peritoneal macrophages.

Phorbol ester-induced neuritic alterations in the rat neocortex. Structural and immunocytochemical studies

In order to explore the effect of aberrant sprouting in the CNS, phorbol 12-myristate 13-acetate (PMA) was administered into the neocortex of adult rats. PMA is a growth-promoting agent that activates and eventually downregulates protein kinase C (PKC), and induces in the rat the expression of several genes, including amyloid precursor protein (APP). We found that multiple injections of 100 nM PMA into the rat neocortex promote, in the first week postinjection, a widespread vacuolization of the neuropil with a subsequent disruption of the synapses in the injection site, followed, at d 15, by the formation of abnormally distended clusters of neurites that resembled aberrant, sprouting axons. At d 30, fewer aberrant sprouts were observed, and many degenerating neurites were found. At the ultrastructural level, the PMA-induced abnormal neurites at d 7-15 resembled growth cones, whereas the dystrophic neurites at d 30 contained abundant dense and laminated bodies. Immunohistochemical analysis indicated that the abnormal neurites in the areas of denervation and PMA administration were positive with antisynaptophysin and antigrowth-associated protein 43 (GAP-43), with an increased APP immunoreactivity surrounding them. APP immunoreactivity around the injection site was mostly associated with pyramidal neurons and glial cells. Control experiments, where saline alone or 4 alpha-phorbol 12, 13-didecanoate (PDD, an inactive phorbol derivative) was injected, failed to show aberrant sprouting neurites. Further immunohistochemical analysis showed that the PMA-treated animals presented increased amyloid beta immunoreactivity in the pyramidal cells at the site of injection, when compared with control injections. These findings suggest that aberrant sprouting induced by overstimulation could be followed by neurodegeneration. Alternatively, PKC downregulation could directly induce the neurodegeneration, with a secondary sprouting response.

Interleukin-1 selectively potentiates bradykinin-stimulated arachidonic acid release from human synovial fibroblasts

Exposure of human synovial fibroblasts prelabelled with [3H]arachidonic acid to bradykinin causes a rapid and sustained increase in arachidonic acid release, a transient increase in cytosolic calcium and an increase in radiolabelled diacylglycerol. Activation of arachidonic acid release by bradykinin was potentiated by interleukin-1 added either simultaneously with bradykinin or to cultures 24 h before addition of bradykinin. In contrast, interleukin-1 did not modify bradykinin-induced increases in cytosolic calcium or diacylglycerol. The stimulation of arachidonic acid release in response to bradykinin, in the absence or presence of interleukin-1, was not affected by RHC-80267, an inhibitor of diacylglycerol kinase, suggesting that deacylation of diacylglycerol was not an important pathway of arachidonic acid production in cultures exposed to bradykinin. This conclusion is supported by the observation that increased release of arachidonic acid was not accompanied by increased release of [14C]stearic acid in cultures labelled with both isotopes. Bradykinin-stimulated release of arachidonic acid was prevented by down-regulating protein kinase C by pretreatment with phorbol 12-myristate 13-acetate and was unaffected by inhibitors of protein synthesis actinomycin D or cycloheximide. On the other hand, interleukin-1 amplification of bradykinin-stimulated release of arachidonic acid was blocked by actinomycin D and cycloheximide. The results from this study point to activation of phospholipase A2 as the source of arachidonic acid in response to bradykinin. Our data further indicate that interleukin-1 selectively potentiates bradykinin activation of a phospholipase A2 by a mechanism requiring protein synthesis, but has no effect on bradykinin activation of phospholipase C.

Over-expression of protein kinase C-alpha enhances platelet-derived growth factor- and phorbol ester- but not calcium ionophore-induced formation of prostaglandins in NIH 3T3 fibroblasts

Over-expression of human protein kinase C-alpha in murine NIH 3T3 fibroblasts is associated with an increased platelet-derived growth factor- and phorbol ester-mediated formation of prostaglandins, whereas the calcium ionophore-induced release of arachidonic acid metabolites is unaffected; however, the differences of arachidonic acid and prostaglandin formation are much more pronounced with platelet-derived growth factor than with phorbol ester. Platelet-derived growth factor induces an identical elevation of intracellular free calcium in control and protein kinase C-alpha over-expressing cells: the phorbol ester has no effect on intracellular free calcium in both cell lines. These results demonstrate that protein kinase C-alpha may couple to arachidonic acid cascade in NIH 3T3 fibroblasts.

Role of fatty acids in signal transduction: modulators and messengers

Many of the steps involved in signal transduction are regulated positively or negatively by fatty acids (FA) per se. FA have been shown to act both as modulators and messengers, particularly of signals triggered at the level of cell membranes. Enzymes and proteins of the cyclic AMP and the protein kinase C signalling pathways and those involving ion fluxes and mobilization are both activated and/or inhibited by FA. FA can also participate in a feedback control mechanism since phospholipases are themselves modulated by FA. FA, particularly arachidonic acid liberated from membrane phospholipids, are also second messengers in signal transduction, and a good example is the activation of protein kinase C by FA. FA play an important role in regulating the transmission of signals from the extracellular environment by acting as modulators and messengers within the complex intracellular network of relays.

Metabolic effects of short-chain ceramide and glucosylceramide on sphingolipids and protein kinase C

Recent studies have identified a potential role for glucosylceramide (GlcCer) in growth promotion and hormonal signalling. In an effort to demonstrate a growth-promoting activity of GlcCer, we prepared a GlcCer having a short-chain acid (octanoyl), in the belief that this glycolipid could be absorbed more readily and more uniformly by cultured cells. By using a mixture of two specific lecithins, dioleoylglycerophosphocholine and 1-stearoyl-2-palmitoylglycerophosphocholine, we were able to prepare dispersions containing a high molar proportion of the GlcCer and the related ceramide, octanoyl sphingosine. Unexpectedly, both sphingolipids inhibited protein and DNA synthesis in Madin-Darby canine kidney cells and produced large increases in the levels of the natural lipids, GlcCer, ceramide, free sphingosine, and an amine that may be glucosylsphingosine (GlcSph). Decreases were seen in the level of sphingomyelin and the proportion of protein kinase C in the cell membranes. The level of lactosylceramide was diminished by octanoyl GlcCer but elevated considerably by octanoyl sphingosine. Diacylglycerols were increased by the lecithins in the liposomes, but the exogenous sphingolipids had no effect. Octanoyl sphingosine labeled in the sphingoid base yielded labeled GlcCer and sphingomyelin labeled in both long-chain and very-long-chain fatty acid families, as well as the octanoyl version. The two families of ceramides, however, had relatively little radioactivity. Some of these changes are attributed to rapid hydrolysis of the added lipids with the formation, particularly from the ceramide, of sphingosine and its anabolic metabolite, GlcSph. Several observations support the idea that the octanoyl sphingosine inhibited the phosphocholinetransferase that synthesizes sphingomyelin while the octanoyl GlcCer inhibited GlcCer beta-glucosidase and GlcCer galactosyltransferase. The use of unnatural short-chain lipids in the study of cell growth and other phenomena may result in unexpected changes in related metabolites and the findings from such experiments should therefore be interpreted cautiously.

Extracellular ATP and ADP stimulate proliferation of porcine aortic smooth muscle cells

The mitogenic effect of extracellular ATP on porcine aortic smooth muscle cells (SMC) was examined. Stimulation of [3H]thymidine incorporation by ATP was dose-dependent; the maximal effect was obtained at 100 microM. ATP acted synergistically with insulin, IGF-1, EGF, PDGF, and various other mitogens. Incorporation of [3H]thymidine was correlated with the fraction of [3H]thymidine-labeled nuclei and changes in cell counts. The stimulation of proliferation was also determined by measurement of cellular DNA using bisbenzamide and by following the increase of mitochondrial dehydrogenase protein. The effect of ATP was not due to hydrolysis to adenosine, which shows synergism with ATP. ATP acted as a competence factor. The mitogenic effect of ATP, but not adenosine, was further increased by lysophosphatidate, phosphatidic acid, or norepinephrine. The inhibitor of adenosine deaminase, EHNA, stimulated the effect of adenosine but not ATP. The adenosine receptor antagonist theophylline depressed adenosine-induced mitogenesis. ADP and the non-hydrolyzable analogue adenosine 5'-[beta, gamma-imido]triphosphate (AMP-PNP) were equally mitogenic. Thus extracellular ATP stimulated mitogenesis of SMC via P2Y purinoceptors. The mechanism of ATP acting as a mitogen in SMC was further explored. Extracellular ATP stimulated the release of [3H]arachidonic acid (AA) and prostaglandin E2 (PGE2) into the medium, and enhanced cAMP accumulation in a dose-dependent fashion similar to ATP-induced [3H]thymidine incorporation. Inhibitors of the arachidonic acid metabolism pathway, quinacrine and indomethacin, partially inhibited the mitogenic effect of ATP but not of adenosine. Pertussis toxin inhibited ATP-stimulated DNA synthesis, AA release, PGE2 formation, and cAMP accumulation. Down-regulation of protein kinase C (PKC) by long-term exposure to phorbol dibutyrate (PDBu) partially prevented stimulation of DNA synthesis and activation of the AA pathway by ATP. The PKC inhibitor, staurosporine, antagonized mitogenesis stimulated by ATP. No synergistic effect was found when PDBu and ATP were added together. Therefore, a dual mechanism, including both arachidonic acid metabolism and PKC, is involved in ATP-mediated mitogenesis in SMC. In addition, ATP acted synergistically with angiotensin II, phospholipase C, serotonin, or carbachol to stimulate DNA synthesis. Finally, the possible physiological significance of ATP as a mitogen in SMC was further studied. The effect of endothelin and heparin, which are released from endothelial cells, on ATP-dependent mitogenesis was investigated. Extracellular ATP acted synergistically with endothelin to stimulate a greater extent of [3H]thymidine incorporation than was seen with PDGF plus endothelin. Heparin, believed to have a regulatory role, partially inhibited the stimulation of DNA synthesis caused both by ATP and PDGF.(ABSTRACT TRUNCATED AT 400 WORDS)

Allergic contact dermatitis releases soluble factors that stimulate melanogenesis through activation of protein kinase C-related signal-transduction pathway

Phenylazo-naphthol (PAN) allergy induces visibly well-defined and late-appearing hyperpigmentation of brownish yellow guinea pig skin in clear contrast to dinitrochlorobenzene (DNCB) allergy, which has very low incidence of hyperpigmentation. Skin extract from PAN allergy at 20-29 d post-challenge exhibited marked melanogenic stimulatory effects (3H2O release and 14C-thiouracil incorporation) when added to cultured guinea pig melanocytes. The time course in the appearance of melanogenic factor was definitely consistent with the induction pattern of visible pigmentation. By contrast, the addition of DNCB-challenged skin extract demonstrated no significant stimulating effect on melanogenesis in either assay system on any of the post-challenge days tested. Assay of intracellular inositol 1,4,5-trisphosphate formed through incubation with the melanocytes demonstrated that the PAN-allergy skin extract at day 28, which contains definite melanogenic factors, stimulated the formation of inositol 1,4,5-trisphosphate that occurs around 50 seconds in contrast to no or little increase with extracts obtained at days 0 and 1 post-challenge. Gel chromatographic analysis revealed that the PAN-allergy skin extract at day 28 contained a newly generated melanogenic fraction with a molecular weight of approximately 9000 Da which was also capable of stimulating DNA synthesis and activating the signal-transduction process (inositol trisphosphate formation) when added to guinea pig melanocytes. Both stimulations of melanogenesis and DNA synthesis by the 9000 Da fraction were completely abolished by the prior and simultaneous addition of protein kinase C (PKC) inhibitor (H-7) or its down-regulatory agent, phorbol 12,13-dibutyrate (PdBu). Taken together, these results suggest that PAN allergy provides a new mechanism of hypermelanization in which endogenous factors synthesized within skin induce the activation of signal-transduction pathways such as phosphoinositide turnover through ligands-receptor binding, resulting in the stimulation of melanocytes possibly through the activation of PKC.

Functional expression of human 5-HT1A receptors and differential coupling to second messengers in CHO cells

The signal transduction linkages of the cloned human 5-HT1A receptor as expressed stably in CHO cells were studied. A transfected clonal cell line which expresses 900 +/- 36 fmol 5-HT1A receptor/mg protein (designated CHO-5-HT1A/WT-27) responded to 5-HT and/or 8-OH-DPAT by coupling to several second messenger pathways. The 5-HT1A receptor inhibited, but did not stimulate, membrane adenylyl cyclase activity and whole cell cAMP accumulation in a dose-dependent manner (for 5-HT, IC50 = 146 +/- 27 and 55 +/- 12 nM, respectively). Activation of the receptor was associated with other signal transduction linkages: (i) a 40-50% increase in hydrolysis of inositol phosphates (for 5-HT, EC50 = 1.33 +/- 0.15 microM for 5-HT), (ii) a transient elevation of cytosolic Ca2+ levels (apparent at 1-100 microM 5-HT) which was not affected by chelation of extracellular Ca2+ by EGTA, and (iii) an augmentation of [3H]-arachidonic acid release pharmacologically with the calcium ionophore A23187 or by activation of endogenous thrombin or P2 purinergic receptors (for 5-HT, EC50 = 1.22 +/- 0.17 microM). This pathway may be an amplification mechanism for signaling in anatomic regions with high concentrations of several neuro-transmitters, hormones or autacoids, such as at neuronal junctions or near areas of platelet aggregation. All linkages were sensitive to pertussis toxin pre-treatment (IC50 approximately 0.5-0.6 ng/ml x 4.5 h for all pathways), suggesting the involvement of Gi protein(s) in these signal transduction pathways. Coupling to varied signal transduction pathways in a single cell system may be a common feature of receptors which classically inhibit adenylyl cyclase such as the 5-HT1A receptor.

Thrombin and histamine rapidly stimulate the phosphorylation of the myristoylated alanine-rich C-kinase substrate in human umbilical vein endothelial cells: evidence for distinct patterns of protein kinase activation

Human alpha-thrombin and histamine each stimulates protein phosphorylation in human umbilical vein endothelial cells (HUVEC). We have identified the most prominent of these phosphoproteins by immunoprecipitation as the human homolog of the widely distributed myristoylated alanine-rich C-kinase substrate (MARCKS). Stimulation by 0.1-10 U/ml of alpha-thrombin produces a time-dependent, sustained (plateau 3-5 min) level of MARCKS phosphorylation. MARCKS phosphorylation requires thrombin catalytic activity but not receptor binding and is also seen in response to stimulation by a peptide, TR (42-55), that duplicates a portion of the thrombin receptor tethered ligand created by thrombin proteolytic activity. One micromolar histamine, like alpha-thrombin, produces sustained phosphorylation of MARCKS (plateau 3-5 min). In contrast, 100 microM histamine results in rapid but transient MARCKS phosphorylation (peak 1-3 min). HUVEC treated with 100 microM histamine for 5 min can be restimulated by alpha-thrombin but not fresh histamine, suggesting that the histamine receptor was desensitized. MARCKS phosphorylation can also be induced by several exogenous protein kinase C (PKC) activators and both alpha-thrombin- and histamine-induced MARCKS phosphorylation are inhibited by the PKC antagonist staurosporine. However, while prolonged PMA pretreatment ablates histamine-induced MARCKS phosphorylation, the ability of thrombin to induce MARCKS phosphorylation is retained. These findings provide evidence for agonist-specific pathways of protein kinase activation in response to thrombin and histamine in HUVEC.

Different protein kinase C isozymes could modulate bradykinin-induced extracellular calcium-dependent and -independent increases in osteoblast-like MC3T3-E1 cells

Effects of protein kinase C (PKC) on bradykinin (BK)-induced intracellular calcium mobilization, consisting of rapid Ca2+ release from internal stores and a subsequent sustained Ca2+ inflow, were examined in Fura-2-loaded osteoblast-like MC3T3-E1 cells. The sustained Ca2+ inflow as inferred with Mn2+ quench method was blocked by Ni2+ and a receptor-operated Ca2+ channel blocker SK&F 96365, but not by nifedipine. The short-term pretreatment with phorbol 12-myristate 13-acetate (PMA), inhibited BK-stimulated Ca2+ inflow, and the prior treatment with PKC inhibitors, H-7 or staurosporine, enhanced the initial internal release and reversed the PMA effect. Moreover, 6 h pretreatment with PMA caused similar effect on the BK-induced inflow to that obtained with PKC inhibitors, whereas 24 h pretreatment was necessary to affect the internal release. On the other hand, the translocation and down-regulation of PKC isozymes were examined after PMA treatment of MC3T3-E1 cells by immunoblot analyses of PKCs with the isozyme-specific antibodies. 6 h treatment with PMA induced down-regulation of PKC beta, whereas longer treatment was needed for down-regulation of PKC alpha. Taken together, it was suggested that the BK-induced initial Ca2+ peak and the sustained Ca2+ inflow through the activation of a receptor-operated Ca2+ channel, are differentially regulated by PKC isozymes alpha and beta, respectively, in osteoblast-like MC3T3-E1 cells.

Proteinkinase C beta-isoform triggers the formation of prostanoids and superoxide in liver macrophages

The zymosan- and phorbolester-induced formation of prostanoids in cultured rat liver macrophages has been shown recently to be controlled by proteinkinase C (1). Using specific antibodies raised against the alpha-, beta-, gamma- and epsilon-isoforms of proteinkinase C, we show that proteinkinase-beta is the predominant isoform in rat liver macrophages. Northern blot analysis with a beta-isoform-specific c-DNA probe revealed the expression of m-RNA for proteinkinase-beta. In resting cells the beta-isoform of proteinkinase C is nearly equally distributed between the cytosolic and membrane fractions. Zymosan and phorbolester led to a translocation of proteinkinase-beta from the cytosol to the membranes, whereas exogenously added arachidonic acid and the calcium ionophore A23187 had no effect. These data indicate that the beta-isoform of proteinkinase C takes part in the prostaglandin and superoxide formation following PMA and zymosan treatment of rat liver macrophages.

Protein kinase C isoforms in multidrug resistant P388/ADR cells: a possible role in daunorubicin transport

To identify the role of protein kinase C (PKC) isoforms in multidrug resistance in tumor cells, we examined the PKC isoform pattern in the multidrug resistant P388/ADR cell line and studied the effect of down regulation of PKC isoforms on intracellular daunorubicin accumulation and P-glycoprotein expression. Using monoclonal antibodies to PKC alpha, beta and gamma and flow cytometry technique we showed that P388/ADR cells overexpressed PKC alpha and beta as compared to drug sensitive P388 cells. Prolonged treatment of P388/ADR cells with phorbol myristate acetate (PMA), a procedure that is known to down regulate PKC, resulted in the down regulation of total PKC activity and the PKC beta isoform (at the protein level) that was accompanied by the correction of daunorubicin accumulation in P388/ADR cells. The level of expression of P-glycoprotein in PMA treated cells was similar to that of untreated cells. These results suggest that PKC beta regulates the drug efflux function of P-glycoprotein.

Phospholipid metabolism in rat intestinal mucosa after oral administration of lysophospholipids

The present results indicate that PS, a phospholipid contained in small amounts in the human diet and not included in plasma lipoproteins, may be used to influence phospholipid metabolism in intestinal mucosal cells. Since PS influx into absorptive cells occurs after its hydrolysis to lysoPS, this metabolite may be used to increase the absorption of this phospholipid. These data show that lysoPS, after diffusion into intestinal cells, is sequentially converted into PS and PE (which make up a minor fraction of the lipids present in lipoproteins). As expected, lysoPS given together with radiolabeled unsaturated fatty acids was unable to promote their transfer into plasma lipoproteins. In this respect lysoPS differed from lysoPC, the latter increasing the appearance of dietary fatty acids in plasma. When given together, lysoPS decreased the lysoPC-induced transfer of unsaturated fatty acids into plasma. This effect required addition of triglycerides to the lipid mixture. In attempting to explain this triacylglycerol-dependent inhibition by lysoPS, we found that this phospholipid increased the incorporation of glycerol into mucosal cell PC. In contrast, lysoPC was inhibitory. Furthermore, in the presence of labeled inositol, lysoPC (but not lysoPS) promoted the appearance of labeled phosphatidylinositol. The data thus suggest that the two lysophospholipids differ in promoting the two main pathways of PC synthesis in the intestinal cells. While lysoPC favors PC synthesis by reacylation, lysoPS enhances the CDP-choline pathway of PC synthesis.

Inhibition of erythropoietin production by phorbol ester is associated with down-regulation of protein kinase C-alpha isoenzyme in hepatoma cells

The role of protein kinase C (PKC) in the control of erythropoietin (Epo) production was studied using the human hepatoma cell line HepG2. Inhibition of PKC by staurosporine and the selective PKC inhibitor CGP 41251 significantly reduced Epo formation. No inhibition occurred with the inactive staurosporine derivative CGP 42700. Treatment with phorbol 12-myristate 13-acetate (PMA) for 24 h dose-dependently inhibited Epo formation, thus suggesting that down-regulation of PKC might be responsible for this inhibition. Immunoblotting experiments showed that incubation of HepG2 cells with PMA for 24 h resulted in a selective and almost complete down-regulation of PKC-alpha. Thus, PKC-alpha may play a permissive role in Epo synthesis in HepG2 cells.