TPA induces translocation but not down-regulation of new PKC isoform eta in macrophages, MDCK cells and astrocytes

The interplay between estrogen receptor beta and protein kinase C, a crucial collaboration for medulloblastoma cell proliferation and invasion

Medulloblastoma (MB) is the most common and aggressive pediatric intracranial tumor. Estrogen receptor β (ERβ) expression correlates with MB development and its phosphorylation modifies its transcriptional activity in a ligand-dependent or independent manner. Using in silico tools, we have identified several residues in ERβ protein as potential targets of protein kinases C (PKCs) α and δ. Using Daoy cells, we observed that PKCα and PKCδ associate with ERβ and induce its phosphorylation. The activation of ERβ promotes MB cells proliferation and invasion, and PKCs downregulation dysregulates these steroid receptor mediated processes. Our data suggest that these kinases may play a crucial role in the regulation of the ERβ transcriptional activity. Overexpression of both PKCα and PKCδ in MB biopsies samples supports their relevance in MB progression.

Nuclear ERK: Mechanism of Translocation, Substrates, and Role in Cancer

The extracellular signal-regulated kinases 1/2 (ERK) are central signaling components that regulate stimulated cellular processes such as proliferation and differentiation. When dysregulated, these kinases participate in the induction and maintenance of various pathologies, primarily cancer. While ERK is localized in the cytoplasm of resting cells, many of its substrates are nuclear, and indeed, extracellular stimulation induces a rapid and robust nuclear translocation of ERK. Similarly to other signaling components that shuttle to the nucleus upon stimulation, ERK does not use the canonical importinα/β mechanism of nuclear translocation. Rather, it has its own unique nuclear translocation signal (NTS) that interacts with importin7 to allow stimulated shuttling via the nuclear pores. Prevention of the nuclear translocation inhibits proliferation of B-Raf- and N/K-Ras-transformed cancers. This effect is distinct from the one achieved by catalytic Raf and MEK inhibitors used clinically, as cells treated with the translocation inhibitors develop resistance much more slowly. In this review, we describe the mechanism of ERK translocation, present all its nuclear substrates, discuss its role in cancer and compare its translocation to the translocation of other signaling components. We also present proof of principle data for the use of nuclear ERK translocation as an anti-cancer target. It is likely that the prevention of nuclear ERK translocation will eventually serve as a way to combat Ras and Raf transformed cancers with less side-effects than the currently used drugs.

The Enigmatic Protein Kinase C-eta

Protein kinase C (PKC), a multi-gene family, plays critical roles in signal transduction and cell regulation. Protein kinase C-eta (PKCη) is a unique member of the PKC family since its regulation is distinct from other PKC isozymes. PKCη was shown to regulate cell proliferation, differentiation and cell death. It was also shown to contribute to chemoresistance in several cancers. PKCη has been associated with several cancers, including renal cell carcinoma, glioblastoma, breast cancer, non-small cell lung cancer, and acute myeloid leukemia. However, mice lacking PKCη were more susceptible to tumor formation in a two-stage carcinogenesis model, and it is downregulated in hepatocellular carcinoma. Thus, the role of PKCη in cancer remains controversial. The purpose of this review article is to discuss how PKCη regulates various cellular processes that may contribute to its contrasting roles in cancer.

PKCη deficiency improves lipid metabolism and atherosclerosis in apolipoprotein E-deficient mice

Genomewide association studies have shown that a nonsynonymous single nucleotide polymorphism in PRKCH is associated with cerebral infarction and atherosclerosis-related complications. We examined the role of PKCη in lipid metabolism and atherosclerosis using apolipoprotein E-deficient (Apoe^-/- ) mice. PKCη expression was augmented in the aortas of mice with atherosclerosis and exclusively detected in MOMA2-positive macrophages within atherosclerotic lesions. Prkch^+/+ Apoe^-/- and Prkch^-/- Apoe^-/- mice were fed a high-fat diet (HFD), and the dyslipidemia observed in Prkch^+/+ Apoe^-/- mice was improved in Prkch^-/- Apoe^-/- mice, with a particular reduction in serum LDL cholesterol and phospholipids. Liver steatosis, which developed in Prkch^+/+ Apoe^-/- mice, was improved in Prkch^-/- Apoe^-/- mice, but glucose tolerance, adipose tissue and body weight, and blood pressure were unchanged. Consistent with improvements in LDL cholesterol, atherosclerotic lesions were decreased in HFD-fed Prkch^-/- Apoe^-/- mice. Immunoreactivity against 3-nitrotyrosine in atherosclerotic lesions was dramatically decreased in Prkch^-/- Apoe^-/- mice, accompanied by decreased necrosis and apoptosis in the lesions. ARG2 mRNA and protein levels were significantly increased in Prkch^-/- Apoe^-/- macrophages. These data show that PKCη deficiency improves dyslipidemia and reduces susceptibility to atherosclerosis in Apoe^-/- mice, showing that PKCη plays a role in atherosclerosis development.

The unique protein kinase Cη: implications for breast cancer (review)

Deregulation of key signal transduction pathways that govern important cellular processes leads to cancer. The development of effective therapeutics for cancer warrants a comprehensive understanding of the signaling pathways that are deregulated in cancer. The protein kinase C (PKC) family has served as an attractive target for cancer therapy for decades owing to its crucial roles in several cellular processes. PKCη is a novel member of the PKC family that plays critical roles in various cellular processes such as growth, proliferation, differentiation and cell death. The regulation of PKCη appears to be unique compared to other PKC isozymes, and there are conflicting reports regarding its role in cancer. This review focuses on the unique aspects of PKCη in terms of its structure, regulation and subcellular distribution and speculates on how these features could account for its distinct functions. We have also discussed the functional implications of PKCη in cancer with particular emphasis on breast cancer.

Novel regulation of protein kinase C-η

Protein kinase C (PKC) is the receptor for tumor promoting phorbol esters, which are potent activators of conventional and novel PKCs, but persistent treatment with phorbol esters leads to downregulation of these PKCs. However, PKCη, a novel PKC isozyme, resists downregulation by tumor-promoting phorbol esters, but little is known about how PKCη level is regulated. Phosphorylation and dephosphorylation play an important role in regulating activity and stability of PKCs. In the present study, we have investigated the molecular mechanism of PKCη regulation. Several PKC activators, including phorbol 12,13-dibutyrate, 12-O-tetradecanoylphorbol-13-acetate and indolactam V caused upregulation of PKCη, whereas the general PKC inhibitor Gö 6983, but not the conventional PKC inhibitor Gö 6976 led to the downregulation of PKCη. Upregulation of PKCη was associated with an increase in phosphorylation of PKCη. Silencing of phosphoinositide-dependent kinase-1, which phosphorylates PKCη at the activation loop, failed to prevent PKC activator-induced upregulation of PKCη. Knockdown of PKCε but not PKCα inhibited PKC activator-induced upregulation of PKCη. Thus, our results suggest that the regulation of PKCη is unique and PKCε is required for the PKC activator-induced upregulation of PKCη.

Dieldrin-induced neurotoxicity: relevance to Parkinson's disease pathogenesis

Parkinson's disease (PD) is increasingly recognized as a neurodegenerative disorder strongly associated with environmental chemical exposures. Recent epidemiological data demonstrate that environmental risk factors may play a dominant role as compared to genetic factors in the etiopathogenesis of idiopathic Parkinson's disease. Identification of key genetic defects such as alpha-synuclein and parkin mutations in PD also underscores the important role of genetic factors in the disease. Thus, understanding the interplay between genes and environment in PD may be critical to unlocking the mysteries of this 200-year-old neurodegenerative disease. Pesticides and metals are the most common classes of environmental chemicals that promote dopaminergic degeneration. The organochlorine pesticide dieldrin has been found in human PD postmortem brain tissues, suggesting that this pesticide has potential to promote nigral cell death. Though dieldrin has been banned, humans continue to be exposed to the pesticide through contaminated dairy products and meats due to the persistent accumulation of the pesticide in the environment. This review summarizes various neurotoxic studies conducted in both cell culture and animals models following dieldrin exposure and discusses their relevance to key pathological mechanisms associated with nigral dopaminergic degeneration including oxidative stress, mitochondrial dysfunction, protein aggregation, and apoptosis.

Cyclosporin a promotes hair epithelial cell proliferation and modulates protein kinase C expression and translocation in hair epithelial cells

Cyclosporin A is an immunosuppressive agent known to cause hirsutism. The mechanisms of action that cause hirsutism have not been fully elucidated, however. We have previously reported that several selective protein kinase C inhibitors promote the growth of murine hair epithelial cells and stimulate anagen induction. In this paper, we report on an investigation of the mechanisms of action of hair-growing activity possessed by cyclosporin A from the viewpoint of whether it promotes hair epithelial cell growth or whether it modulates the expression or translocation of protein kinase C isozymes in hair epithelial cells. Our results indicate that cyclosporin A (over a wide dosage range of 1-1000 ng per ml) stimulates cultured murine hair epithelial cell growth to about 150%-160% relative to controls. We also observed growth-promoting effects on murine epidermal keratinocytes (about 140%) at the dose range of 1-100 ng per ml. At high dose ranges above 3 microg per ml, the growth of both cells was inhibited. On the other hand, we found that cyclosporin A reduces the overall expression of protein kinase C alpha, betaI, and betaII in cultured murine hair epithelial cells, and reduces the levels of protein kinase C alpha, betaI, betaII, and eta in the particulate fraction from cultured murine hair epithelial cells. From these results, we speculate that the hair-growing activity of cyclosporin A is at least partially attributable to its growth-promoting influence on hair epithelial cells sequential to its downregulation of some protein kinase C isozymes in hair epithelial cells or inhibition of translocation of some protein kinase C isozymes to the membrane or cytoskeleton of hair epithelial cells.

Nuclear factor kappa B is a molecular target for sulforaphane-mediated anti-inflammatory mechanisms

Sulforaphane (SFN), an aliphatic isothiocyanate, is a known cancer chemopreventive agent. Aiming to investigate anti-inflammatory mechanisms of SFN, we here report a potent decrease in lipopolysaccharide (LPS)-induced secretion of pro-inflammatory and pro-carcinogenic signaling factors in cultured Raw 264.7 macrophages after SFN treatment, i.e. NO, prostaglandin E(2), and tumor necrosis factor alpha. SFN did not directly interact with NO, nor did it inhibit inducible nitric-oxide synthase enzymatic activity. Western blot analyses revealed time- and dose-dependent reduction of LPS-induced inducible nitric-oxide synthase as well as Cox-2 protein expression, which was suppressed at the transcriptional level. To reveal the target of SFN beyond its anti-inflammatory action, we performed electrophoretic mobility shift assay analyses of transcription factor-DNA binding. Consequently, nuclear factor kappa B (NF-kappa B), a pivotal transcription factor in LPS-stimulated pro-inflammatory response, was identified as the key mediator. SFN selectively reduced DNA binding of NF-kappa B without interfering with LPS-induced degradation of the inhibitor of NF-kappa B nor with nuclear translocation of NF-kappa B. Because SFN can interact with thiol groups by dithiocarbamate formation, it may impair the redox-sensitive DNA binding and transactivation of NF-kappa B. Sulforaphane could either directly inactivate NF-kappa B subunits by binding to essential Cys residues or interact with glutathione or other redox regulators like thioredoxin and Ref-1 relevant for NF-kappa B function. Our data provide novel evidence that anti-inflammatory mechanisms contribute to sulforaphane-mediated cancer chemoprevention.

Activation-dependent degradation of protein kinase C eta

Prolonged activation of protein kinase Cs (PKCs) by long-term treatment of cells with phorbol ester tumor promoters down-regulates the expression of many PKCs. To investigate the molecular mechanisms involved in the down-regulation of PKC eta, we expressed the novel PKCs eta and θ and various mutant forms in baby hamster kidney cells. Upon overexpression, constitutively active PKC eta, but not wild type or kinase-dead PKC eta, underwent rapid degradation to generate several lower molecular weight polypeptides. When co-expressed with active kinases, kinase-dead PKC eta with a pseudosubstrate site mutation designed to give an active conformation was down-regulated while the wild type PKC eta was not. These results suggest requirements for kinase activity and an active conformation for down-regulation of PKC eta. Treatment with the proteasome inhibitors N-Ac-Leu-Leu-norleucinal and lactacystin led to accumulation of PKC eta proteolytic products and potentially ubiquitinated forms. While wild type PKC eta localizes mostly to the detergent-soluble fraction of the cell, a significant portion of full-length constitutively active PKC eta and of kinase-dead, active conformation PKC eta were found in the detergent-insoluble fraction. Several proteolytic fragments of constitutively active PKC eta also were found in the detergent insoluble fraction. These full-length and proteolytic fragments of PKC eta in the detergent-insoluble fraction accumulated further in the presence of proteasome inhibitors. These data suggest that active conformation PKC eta accumulates in the detergent-insoluble compartment, is degraded by proteolysis in the presence of kinase activity, and that the cleavage products undergo further degradation via ubiquitin-mediated degradation in the proteasome. Oncogene (2000) 19, 4263 - 4272

Antidepressant drugs inhibit glucocorticoid receptor-mediated gene transcription - a possible mechanism

1. Antidepressant drugs are known to inhibit some changes evoked by glucocorticoids, as well as a hyperactivity of hypothalamic-pituitary-adrenal (HPA) axis, often observed in depression. 2. The aim of present study was to investigate effects of various antidepressant drugs on the glucocorticoid-mediated gene transcription in fibroblast cells, stably transfected with an MMTV promoter (LMCAT cells). 3. The present study have shown that antidepressants (imipramine, amitriptyline, desipramine, fluoxetine, tianeptine, mianserin and moclobemide), but not cocaine, inhibit the corticosterone-induced gene transcription in a concentration- and a time-dependent manner. 4. Drugs which are known to augment clinical effects of medication in depressed patients (lithium chloride, amantadine, memantine), do not affect the inhibitory effects of imipramine on the glucocorticoid receptor (GR)-mediated gene transcription. 5. Inhibitors of phospholipase C (PLC), protein kinase C (PKC), Ca(2+)/calmodulin-dependent protein kinase (CaMK) and antagonists of the L-type Ca(2+) channel also inhibit the corticosterone-induced gene transcription. 6. Inhibitors of protein kinase A (PKA) and protein kinase G (PKG) are without effect on the GR-induced gene transcription. 7. Phorbol ester (an activator of PKC) attenuates the inhibitory effect of imipramine on the GR-induced gene transcription. 8. Imipramine decreases binding of corticosterone-receptor complex to DNA. 9. It is concluded that antidepressant drugs inhibit the corticosterone-induced gene transcription, and that the inhibitory effect of imipramine depends partly on the PLC/PKC pathway.

Post-transcriptional inhibition of lipopolysaccharide-induced expression of inducible nitric oxide synthase by Gö6976 in murine microglia

Glia in the brain respond to various toxins with an increased expression of inducible nitric oxide synthase (iNOS) and an increased production of nitric oxide (NO). Here, we report that lipopolysaccharide (LPS)-induced expression of iNOS was down-regulated post-transcriptionally through the destabilization of iNOS mRNA by the indolocarbazole compound, Gö6976, in murine microglia. This Gö6976 effect is specific for iNOS since tumor necrosis factor alpha was unaffected by the compound. Interestingly, the post-transcriptional effects ascribed to Gö6976 were not observed with other inhibitors of protein kinase A, C (PKC), G, or protein tyrosine kinases. Instead, these kinases appear to affect the iNOS/NO system at the transcriptional level. In the past, Gö6976 has been reported to be a rather specific inhibitor of PKC in vitro. Results from our experiments, through prolonged treatment with phorbol esters and with the various PKC inhibitors including phorbol ester-insensitive PKC isotype inhibitor, suggest that the Gö6976-mediated post-transcriptional regulation of iNOS gene expression and NO production in microglia is not mediated through its reputed effects on PKC activity. Since the effects of various neurotoxins and certain neurodegenerative diseases may be manifested through alterations in the iNOS/NO system, post-transcriptional control of this system may represent a novel strategy for therapeutic intervention.

The indolocarbazole Gö6976 protects neurons from lipopolysaccharide/interferon-gamma-induced cytotoxicity in murine neuron/glia co-cultures

The expression of inducible nitric oxide synthase (iNOS) and the production of nitric oxide (NO) after exposure to endotoxins has been implicated in immune-mediated neurotoxicity. The indolocarbazole compound Gö6976, which has been described as a selective protein kinase C (PKC) inhibitor in vitro, rescued neurons from lipopolysaccharide/interferon-gamma (LPS/IFNgamma)- or interleukin-1alpha/tumor necrosis alpha/IFNgamma (IL-1alpha/TNFalpha/IFNgamma)-induced cytotoxicity in murine primary neuron-glia co-cultures. Other compounds known to inhibit PKC, Ro31-8220, GF109203X, Gö7874, H7, staurosporine and H89, failed to rescue neurons from the LPS/IFNgamma-induced cytotoxicity. These results suggest that the neuroprotection by Gö6976 from the LPS/IFNgamma-induced neuronal cell death is not mediated through its reputed effects on PKC activity. The neuroprotection paralleled the inhibition of iNOS gene expression and NO production. However, further analyses correlating NO production with the extent of neurotoxicity suggested that additional mechanism(s) besides the inhibition of the iNOS/NO system may be responsible for the neuroprotective effects of Gö6976. An understanding of the mechanism underlying the neuroprotective effect of Gö6976 may provide key insights into potential interventions for immune-mediated neurodegenerative diseases.

Expression and translocation of protein kinase C isoforms in rat microglial and astroglial cultures

Cellular distribution and activation by phorbol myristate acetate (PMA) of classical (alpha, betaI, betaII,gamma), novel (delta, epsilon, theta, eta), and atypical (zeta, iota) protein kinase C (PKC) isoforms were studied in cultured rat neonatal microglial and astroglial cells by Western blot analysis. Among the classical isoforms, only betaII was expressed in microglia and astrocytes in the same abundance. The expression of betaI in microglia was less abundant, while PKCalpha was not detectable in this cell type. PKCgamma was absent in both cell populations. A different pattern of expression was also found for novel and atypical isoenzymes: Both cell types expressed delta, theta, eta, zeta, and iota isoforms, but PKCepsilon was absent in microglia and the expression of PKCzeta and PKCiota in these cells was low compared to astrocytes. The pattern of PKC distribution in cytosolic and particulate fractions as well as activation by short (10 min) and prolonged (4 hr) PMA treatment in both cell types were similar. On the whole, in comparison with astrocytes, PKC in microglial cells was less expressed, both in terms of number of isoforms and level of expression. The microglial profile of PKC isoforms differed from that of rat peritoneal macrophages, which did express PKCalpha. Preliminary evidence suggests that the ability of PMA to enhance cyclic AMP responses in astrocytes, but not in microglia, is related to the different pattern of expression of PKCalpha and PKCepsilon in the two cell types.

Antisense Oligonucleotides Targeting Protein Kinase C-α, -βI, or -δ But Not -η Inhibit Lipopolysaccharide-Induced Nitric Oxide Synthase Expression in RAW 264.7 Macrophages: Involvement of a Nuclear Factor κB-Dependent Mechanism

The signaling pathway for protein kinase C (PKC) activation and the role of PKC isoforms in LPS-induced nitric oxide (NO) release were studied in RAW 264.7 macrophages. The tyrosine kinase inhibitor genestein attenuated LPS-induced NO release and inducible nitric oxide synthase (iNOS) expression, as did the phosphoinositide-specific phospholipase C (PI-PLC) inhibitor U73122 and the phosphatidylcholine-specific phospholipase C (PC-PLC) inhibitor D609. LPS stimulated phosphatidylinositol (PI) hydrolysis and PKC activity in RAW cells; both were inhibited by genestein. The PKC inhibitors (staurosporine, calphostin C, Ro 31-8220, or Go 6976) or long-term 12-O-tetradecanoylphorbol 13-acetate (TPA) treatment also resulted in inhibition of LPS-induced NO release and iNOS expression. Western blot analysis showed expression of PKC-α, -βI, -δ, -η, and -ζ in RAW cells; down-regulation of PKC-α, -βI, and -δ, but not -η, was seen after long-term TPA treatment, indicating the possible involvement of one or all of PKC-α, -βI, and -δ, but not -η, in LPS-mediated effects. Treatment with antisense oligonucleotides for these isoforms further demonstrated the involvement of PKC-α, -βI, and δ, but not -η, in LPS responses. Stimulation of cells with LPS for 1 h caused activation of NF-κB in the nuclei by detection of NF-κB-specific DNA-protein binding; this was inhibited by genestein, U73122, D609, calphostin C, or antisense oligonucleotides for PKC-α, -βI, and -δ, but not -η. These data suggest that LPS activates PI-PLC and PC-PLC via an upstream tyrosine kinase to induce PKC activation, resulting in the stimulation of NF-κB DNA-protein binding, then initiated the expression of iNOS and NO release. PKC isoforms α, βI, and δ were shown to be involved in the regulation of these LPS-induced events.

Induction of differentiation in normal human keratinocytes by adenovirus-mediated introduction of the eta and delta isoforms of protein kinase C

Protein kinase C (PKC) plays a crucial role(s) in regulation of growth and differentiation of cells. In the present study, we examined possible roles of the alpha, delta, eta, and zeta isoforms of PKC in squamous differentiation by overexpressing these genes in normal human keratinocytes. Because of the difficulty of introducing foreign genes into keratinocytes, we used an adenovirus vector system, Ax, which allows expression of these genes at a high level in almost all the cells infected for at least 72 h. Increased kinase activity was demonstrated in the cells overexpressing the alpha, delta, and eta isoforms. Overexpression of the eta isoform inhibited the growth of keratinocytes of humans and mice in a dose (multiplicity of infection [MOI])-dependent manner, leading to G1 arrest. The eta-overexpressing cells became enlarged and flattened, showing squamous cell phenotypes. Expression and activity of transglutaminase 1, a key enzyme of squamous cell differentiation, were induced in the eta-overexpressing cells in dose (MOI)- and time-dependent manners. The inhibition of growth and the induction of transglutaminase 1 activity were found only in the cells that express the eta isoform endogenously, i.e., in human and mouse keratinocytes but not in human and mouse fibroblasts or COS1 cells. A dominant-negative eta isoform counteracted the induction of transglutaminase 1 by differentiation inducers such as a phorbol ester, 1alpha,25-dihydroxyvitamin D3, and a high concentration of Ca2+. Among the isoforms examined, the delta isoform also inhibited the growth of keratinocytes and induced transglutaminase 1, but the alpha and zeta isoforms did not. These findings indicate that the eta and delta isoforms of PKC are involved crucially in squamous cell differentiation.

Protein kinase C eta mediates lipopolysaccharide-induced nitric-oxide synthase expression in primary astrocytes

The signaling pathway involved in protein kinase C (PKC) activation and role of PKC isoforms in lipopolysaccharide (LPS)-induced nitric oxide (NO) release were studied in primary cerebellar astrocytes. LPS caused a dose- and time-dependent increase in NO release and inducible NO synthase (iNOS) expression. The tyrosine kinase inhibitor, genestein, the phosphatidylcholine-phospholipase C inhibitor, D609, and the phosphatidate phosphodrolase inhibitor, propranolol, attenuated the LPS effects, whereas the PI-PLC inhibitor, U73122, had no effect. The PKC inhibitors (staurosporine, Ro 31-8220, Go 6976, and calphostin C) also inhibited LPS-induced NO release and iNOS expression. However, long term (24 h) pretreatment of cells with 12-O-tetradecanoyl phorbol-13-acetate (TPA) did not affect the LPS response. Previous results have shown that TPA-induced translocation, but not down-regulation, of PKCeta occurs in astrocytes (Chen, C. C., and Chen, W. C. (1996) Glia 17, 63-71), suggesting possible involvement of PKCeta in LPS-mediated effects. Treatment with antisense oligonucleotides for PKCeta or delta, another isoform abundantly expressed in astrocytes, demonstrated the involvement of PKCeta, but not delta, in LPS-mediated effects. Stimulation of cells for 1 h with LPS caused activation of nuclear factor (NF)-kB in the nuclei as detected by the formation of a NF-kB-specific DNA-protein complex; this effect was inhibited by genestein, D609, propranolol, or Ro 31-8220 or by PKCeta antisense oligonucleotides, but not by long term TPA treatment. These data suggest that in astrocytes, LPS might activate phosphatidylcholine-phospholipase C and phosphatidylcholine-phospholipase D through an upstream protein tyrosine kinase to induce PKC activation. Of the PKC isoforms present in these cells, only activation of PKCeta by LPS resulted in the stimulation of NF-kB-specific DNA-protein binding and then initiated the iNOS expression and NO release. This is further evidence demonstrating that different members of the PKC family within a single cell are involved in specific physiological responses.

ATP-induced arachidonic acid release in cultured astrocytes is mediated by Gi protein coupled P2Y1 and P2Y2 receptors

ATP-induced arachidonic acid (AA) release was studied in [3H]AA-prelabeled cultured astrocytes. To characterize the P2 purinoceptor-mediated effect of ATP, the subtype-specific agonists 2-methylthio ATP (2-MeSATP) and UTP were compared. ATP, UTP, or 2-MeSATP induced a dose-dependent increase of [3H]AA release, with EC50 values of 22.7 microM, 29.4 microM, and 1.68 microM, respectively; alpha,beta-methyleneATP and adenosine had no effect. The order of potency was ATP = UTP > or = 2-MeSATP, indicating that ATP interacted with both P2Y1 and P2Y2 receptors to mediate AA release in astrocytes. The effect of ATP, UTP, or 2-MeSATP was markedly inhibited by pretreatment of cells with pertussis toxin. Ca2+ ionophore-A23187 and PKC activator-TPA mimicked the effects of these three agonists to stimulate AA release. ATP, UTP, and 2-MeSATP induced a rapidly initial rise of [Ca2+]i and a sustained [Ca2+]i increase. The AA release was blocked in the external Ca2+ free in condition the sustained [Ca2+]i increase was abolished. Both A23187- and TPA-induced AA release were also blocked in this condition. Furthermore, inorganic Ca2+ channel blocker Co2+ inhibited ATP, UTP, or 2-MeSATP induced AA release as well. Long-term (24 h) treatment of cells with TPA resulted in an attenuation of three agonists, TPA or A23187 response. Similarly, ATP or TPA promoted AA release was inhibited by the mitogen-activated protein kinase (MAPK) cascade inhibitor PD 98059. ATP, TPA, or A23187 induced an increase in the activity and tyrosine phosphorylation of p42 MAPK, as well as a molecular weight shift, consistent with phosphorylation, of cytosolic phospholipase A2 (cPLA2). ATP- and TPA-stimulated activation of p42 MAPK activity and tyrosine phosphorylation were inhibited by long-term TPA treatment, while A23187-stimulated effects were completely blocked. Furthermore, tyrosine phosphorylation and activation of p42 MAPK and mobility shift of cPLA2 induced by A23187 were reversed in the absence of external Ca2+, suggesting the involvement of PKCalpha in MAPK activation and mobility shift of cPLA2. Taken together, ATP-stimulated AA release was secondary to the activation of P2Y1 and P2Y2 receptors/PLC pathway. Ca2+ and PKC interact to regulate this response. Elevation of intracellular Ca2+, the mechanism involving extracellular Ca2+ influx, might act partly through PKCalpha activation and in turn MAPK might be activated, leading to cPLA2 phosphorylation and AA release.