Platelet activating factor induces expression of early response genes c-fos and TIS-1 in human epidermoid carcinoma A-431 cells

Platelet-Activating Factor Receptor Ligands Protect Tumor Cells from Radiation-Induced Cell Death

Irradiation generates oxidized phospholipids that activate platelet-activating factor receptor (PAFR) associated with pro-tumorigenic effects. Here, we investigated the involvement of PAFR in tumor cell survival after irradiation. Cervical cancer samples presented higher levels of PAF-receptor gene (PTAFR) when compared with normal cervical tissue. In cervical cancer patients submitted to radiotherapy (RT), the expression of PTAFR was significantly increased. Cervical cancer-derived cell lines (C33, SiHa, and HeLa) and squamous carcinoma cell lines (SCC90 and SCC78) express higher levels of PAFR mRNA and protein than immortalized keratinocytes. Gamma radiation increased PAFR expression and induced PAFR ligands and prostaglandin E2 (PGE2) in these tumor cells. The blocking of PAFR with the antagonist CV3938 before irradiation inhibited PGE2 and increased tumor cells death. Similarly, human carcinoma cells transfected with PAFR (KBP) were more resistant to radiation compared to those lacking the receptor (KBM). PGE2 production by irradiated KBP cells was also inhibited by CV3988. These results show that irradiation of carcinoma cells generates PAFR ligands that protect tumor cells from death and suggests that the combination of RT with a PAFR antagonist could be a promising strategy for cancer treatment.

Cigarette smoke induces cell motility via platelet-activating factor accumulation in breast cancer cells: a potential mechanism for metastatic disease

Most cancer deaths are a result of metastasis rather than the primary tumor. Although cigarette smoking has been determined as a risk factor for several cancers, its role in metastasis has not been studied in detail. We propose that cigarette smoking contributes to metastatic disease via inhibition of breast cancer cell platelet-activating factor acetylhydrolase (PAF-AH), resulting in PAF accumulation and a subsequent increase in cell motility. We studied several breast cell lines, including immortalized mammary epithelial cells (MCF-10A), luminal A hormone positive MCF-7, basal-like triple negative MDA-MB-468, and claudin-low triple-negative highly metastatic MDA-MB-231 breast tumor cells. We exposed cells to cigarette smoke extract (CSE) for up to 48 h. CSE inhibited PAF-AH activity, increased PAF accumulation, and increased cell motility in MDA-MB-231 metastatic triple negative breast cancer cells. The calcium-independent phospholipase A₂ (iPLA₂) inhibitor, (S) bromoenol lactone ((S)-BEL) was used to prevent the accumulation of PAF and further prevented the increase in cell motility seen previously when cells were exposed to CSE. Thus, iPLA₂ or PAF may represent a therapeutic target to manage metastatic disease, particularly in triple-negative breast cancer patients who smoke.

STAT5 mediates PAF-induced NADPH oxidase NOX5-S expression in Barrett's esophageal adenocarcinoma cells

We have shown that NADPH oxidase NOX5-S is overexpressed in Barrett's esophageal adenocarcinoma (EA) cells and may contribute to the progression from Barrett's esophagus (BE) to EA presumably by increasing cell proliferation and decreasing apoptosis (Fu X, Beer DG, Behar J, Wands J, Lambeth D, Cao W. J Biol Chem 281: 20368-20382, 2006). The mechanism(s) of NOX5-S overexpression in EA, however, is not fully understood. In SEG1 EA cells we found that acid treatment significantly increased platelet-activating factor (PAF) production, which in turn markedly increased NOX5-S expression and hydrogen peroxide (H(2)O(2)) production. Knockdown of NOX5-S by NOX5-S small interfering RNA (siRNA) blocked PAF-dependent H(2)O(2) production. PAF-dependent induction of NOX5-S expression and H(2)O(2) production were significantly decreased by the MAPK kinase 1 inhibitor PD-98059, by the cytosolic phospholipase A(2) (cPLA(2)) inhibitor AACOCF3, and by STAT5 downregulation with STAT5 siRNA. PAF significantly increased the phosphorylation of ERK1/2 MAPK, cPLA(2), and STAT5. Using inhibitors, we demonstrated that PAF-induced STAT5 phosphorylation depends on activation of ERK1/2 MAPK and cPLA(2), whereas PAF-induced cPLA(2) phosphorylation was associated with activation of ERK1/2 MAPK. Given that STAT5 bound to the c-sis-inducible element (TTCTGGTAA) of the NOX5-S promoter, overexpression of STAT5 significantly increased NOX5-S promoter activity. We conclude that acid-induced NOX5-S expression and H(2)O(2) production is mediated in part by production of PAF in SEG1 EA cells, and that PAF-induced increase in NOX5-S expression depends on sequential activation of ERK MAP kinases, cPLA(2), and STAT5 in these cells.

Critical role for matrix metalloproteinase-9 in platelet-activating factor-induced experimental tumor metastasis

In this study, the roles of matrix metalloproteinase (MMP)-2 and MMP-9 in platelet-activating factor (PAF)-induced experimental pulmonary metastasis of the murine melanoma cell, B16F10, were investigated. An injection of PAF resulted in increases in mRNA expression, protein levels and the activities of both MMP-2 and MMP-9 in the lungs. The overall expression of MMP-9 was stronger than that of MMP-2. The increased MMP-9 expression was inhibited by both NF-kappaB and AP-1 inhibitors, whereas the increased MMP-2 expression was inhibited by only AP-1 inhibitors. Immunohistochemical analysis revealed that MMP-9 was expressed in bronchial epithelial cells as well as in the walls of blood vessels, whereas MMP-2 expression was observed only in bronchial epithelial cells. PAF significantly enhanced the pulmonary metastasis of B16F10, which was inhibited by both NF-kappaB and c-jun inhibitors. MMP-9 inhibitor, but not that of MMP-2, completely inhibited PAF-induced B16F10 metastasis. These data indicate that MMP-9, the expression of which was regulated by NF-kappaB and AP-1, plays a critical role in PAF-induced enhancement of pulmonary melanoma metastasis.

Platelet-activating factor and human meningiomas

Meningiomas are common primary intracranial tumours. Platelet-activating factor (PAF) is an inflammatory and angiogenic lipid mediator involved in several types of cancer. The presence of PAF receptor (PAF-R) transcripts, the levels of PAF, the phospholipase A2 activity (PLA2, the enzymatic activity implicated in PAF formation) and the PAF acetylhydrolase activity (AHA, the PAF degrading enzyme) were investigated in 49 human meningiomas. PAF-R transcripts, PAF, PLA2 and AHA were detected in meningiomas. However, their levels did not correlate with biological parameters such as the tumour grade, the presence of associated oedema, necrosis, mitotic index as well as intensity of the neovascularization and chronic inflammatory response. In conclusion, PAF is present in meningiomas where it might act on tumour growth by altering the local angiogenic and/or cytokine networks as previously suggested for human breast and colorectal cancer.

Platelet-activating factor in cirrhotic liver and hepatocellular carcinoma

AIM: Platelet-activating factor (PAF) is a pro-inflammatory and angiogenic lipid mediator. Here we aimed to investigate levels of PAF, lyso-PAF (the PAF precursor), phospholipase A₂ (PLA₂, the enzymatic activity generating lyso-PAF), acetylhydrolase activity (AHA, the PAF degrading enzyme) and PAF receptor (PAF-R) transcripts in cirrhotic liver and hepatocellular carcinoma (HCC).

METHODS: Twenty-nine patients with HCC were enrolled in this study. Cirrhosis was present in fourteen patients and seven had no liver disease. Tissue PAF levels were investigated by a platelet-aggregation assay. Lyso-PAF was assessed after its chemical acetylation into PAF. AHA was determined by degradation of [³H]-PAF. PLA₂ levels were assessed by EIA. PAF-R transcripts were investigated using RT-PCR.

RESULTS: Elevated amounts of PAF and PAF-R transcripts 1 (leukocyte-type) were found in cirrhotic tissues as compared with non-cirrhotic ones. Higher amounts of PAF and PAF-R transcripts 1 and 2 (tissue-type) were found in HCC tissues as compared with non-tumor tissues. PLA₂, lyso-PAF and AHA levels were not changed in cirrhotic tissues and HCC.

CONCLUSION: While the role of PAF is currently unknown in liver physiology, this study suggests its potential involvement in the inflammatory network found in the cirrhotic liver and in the angiogenic response during HCC.

Platelet-activating factor and human thyroid cancer

OBJECTIVE

Platelet-activating factor (PAF) is a pro-inflammatory and angiogenic lipid mediator involved in several types of cancer in humans. The levels of PAF, lyso-PAF (the PAF precursor), phospholipase A2 activity (PLA2, the enzymatic activity implicated in lyso-PAF formation) and acetylhydrolase activity (AHA, the PAF-degrading enzyme) were investigated in various diseased thyroid tissues.

SUBJECTS

Control and diseased tissue of patients with a hyperplastic goitre (n = 14), a benign adenoma (n = 12) and a papillary thyroid carcinoma (n = 15) were investigated.

RESULTS

PAF receptor transcripts were found in the human thyroid tissue. PAF, lyso-PAF, PLA2 and AHA were present in control thyroid tissues, their levels being significantly correlated with each other, suggesting tiny regulations of the PAF metabolic pathways inside the thyroid gland. PAF, lyso-PAF, PLA2 and AHA levels remained unchanged in diseased tissues of patients with a hyperplastic goitre, a benign adenoma and a papillary thyroid carcinoma. No difference was found between PAF, lyso-PAF, PLA2 and AHA levels with respect to the TNM tumour status and the histological sub-type of papillary thyroid carcinoma. No correlation was found between tissue PAF levels and those of vascular endothelial growth factor and basic fibroblast growth factor, two angiogenic growth factors involved in thyroid cancer and that mediate their effect through PAF release in breast and colorectal cancer.

CONCLUSION

PAF, PAF receptor transcripts and the enzymatic activities implicated in PAF production and degradation are present in the thyroid gland. While the physiological role of PAF is presently unknown in thyroid physiology, this study highlights no evidence for a potentially important role of PAF during human thyroid cancer, a result that markedly differs from breast and colorectal ones.

Tissue concentrations of platelet-activating factor in colorectal carcinoma: inverse relationships with Dukes' stage of patients

The lipid mediator platelet-activating factor (PAF) plays a role in cancer. We investigated its presence in human colon carcinoma by assessing the levels of tissue phospholipase A(2) (PLA(2), the key enzyme in the generation of the lyso-PAF precursor), lyso-PAF, PAF and acetylhydrolase activity (AHA, the key enzyme in PAF degradation) in colorectal cancer patients and by correlating them with Dukes' classification. The results highlighted that the tumour tissues of Dukes' A and B patients had significantly higher PLA(2), lyso-PAF, PAF and AHA levels as compared with nontumour tissues. Dukes' C patients had higher PLA(2), lyso-PAF and AHA levels but unchanged PAF. Dukes' D patients had higher AHA levels but unchanged PLA(2), lyso-PAF and PAF. A pathophysiological role for PAF is suggested in human colon carcinoma.

Effects of PAF on histamine H1 receptor mRNA expression in rat trigeminal ganglia

The application of platelet-activating factor (PAF) to the nasal mucosa of humans has been shown to increase histamine-induced hyper-reactivity. To test the hypothesis that PAF acts by increasing the reactivity of sensory nerve endings in the nasal mucosa to histamine, we examined PAF-stimulated rat trigeminal nerve ganglion cells. We found that relatively low concentrations of PAF (10(-12)-10(-9) M) induced increased histamine H1 receptor mRNA expression. This increase appeared as early as 1 h after PAF stimulation, peaked at 4 h, and disappeared after 24 h. The PAF receptor antagonist WEB2086 inhibited the increased expression of histamine H1 receptor mRNA induced by PAF, suggesting that the effects of PAF are mediated by specific receptors. This PAF effect was abolished by actinomycin D, suggesting that PAF induces de novo transcription of histamine H1 and/or PAF receptor mRNA. PAF may be important in the hyper-responsiveness of nasal mucosa exposed to histamine.

Effects of posttraining intrahippocampal injections of platelet-activating factor and PAF antagonists on memory

The present experiments examined the effects of posttraining intrahippocampal injections of the degradative enzyme-resistant methylcarbamyl analog of the bioactive phospholipid platelet-activating factor (mc-PAF) and the platelet-activating factor (PAF) receptor antagonists BN52021 and BN 50730 on memory in male Long-Evans rats trained in a hidden platform version of the Morris water maze. Following an eight-trial training session, rats received a unilateral intrahippocampal injection of mc-PAF (0.5, 1.0, or 2.0 microgram/0.5 microliter), lyso-PAF (1.0 microgram/0.5 microliter), the cell surface PAF receptor antagonist BN 52021 (0.25, 0.5, or 1.0 micrigram/0.5 microliter/, the intracellular PAF receptor antagonist BN 50730 (2.0, 5.0, or 10.0 microgram/0.5 microliter), or vehicle (50% DMSO in 0.9% saline; 0.5 microliter). On a retention test conducted 24 h after training, the escape latencies of rats administered mc-PAF (1.0 or 2.0 microgram) were significantly lower than those of the vehicle-injected controls, demonstrating a memory-enhancing effect of mc-PAF. Injections of lyso-PAF, a structurally similar metabolite of PAF, had no influence on memory, indicating that the memory-enhancing effect of mc-PAF is not caused by membrane perturbation by the phospholipid. The retention test escape latencies of rats administered BN 52021 (0.5 microgram) and BN 50730 (5.0 or 10 microgram) were significantly higher than those of the controls, indicating a memory impairing effect of both PAF antagonists. When mc-PAF, BN 52021, or BN 50730 was administered 2 h posttraining, no effect on retention was observed, indicating a time-dependent effect of the neuroactive substances on memory storage. The findings suggest a role for endogenous PAF in hippocampal-dependent memory processes.

Platelet-activating factor in the modulation of excitatory amino acid neurotransmitter release and of gene expression

Selective phospholipids of synaptic membranes are reservoirs for lipid second messengers. 1-Alkyl-2 arachidonoyl glycero-3-phosphocholine is hydrolyzed by phospholipase A2 (PLA2) into two products: lyso-PAF, which is transacetylated to yield platelet-activating factor (PAF), and free arachidonic acid (20:4), which can undergo oxidative metabolism to eicosanoids. Alternative pathways of PAF synthesis, such as CoA-independent transacylase and the de novo route of synthesis, remain to be explored and compared to the PLA2-dependent route. At low concentrations, PAF is a retrograde messenger of LTP in CA1 hippocampal neurons, and is also a memory enhancer in inhibitor avoidance tasks. PAF enhances excitatory amino acid release in synaptic pairs from primary hippocampal cultures by a presynaptic mechanism. Ischemia and convulsions activate synaptic PLA2. Thus, increased concentrations of PAF promote massive glutamate exocytosis, glutamate receptor activation, and elevated intracellular calcium levels in target cells. As a result, calcium-sensitive cascades are affected. PAF thus had dual roles as a lipid mediator: under physiological conditions it modulates neurotransmitter release, but at high concentrations it becomes neurotoxic. Through an intracellular high affinity binding site, PAF activates the expression of immediate-early genes. Some of these genes encode transcription factors (e.g. zif-268, c-fos), and others encode enzymes (COX-2 or inducible prostaglandin synthase). PAF also activates the expression of metalloproteinases which participate in the remodeling of the extracellular matrix. These effects have been studied in cells in culture as well as in the brain. A PAF antagonist specific for the intracellular binding site inhibits COX-2 expression elicited by a single electroconvulsive shock or vasogenic edema. COX-1, the constitutive prostaglandin synthase, is not induced and is unaffected by the antagonist. Most of the cerebral induction occurs in the hippocampus and results from transcriptional activation. PAF mediated gene expression may be involved in neural plasticity as well as in pathophysiological conditions in which the neural tissue activates repair-injury pathways.

Identification of functional platelet-activating factor receptors on human keratinocytes

Platelet-activating factor (PAF) is a potent inflammatory mediator that has been shown to be produced by human keratinocytes and is thought to play a role in cutaneous inflammation. Immunofluorescence and radioligand binding studies were used to characterize PAF receptors (PAF-R) on human keratinocytes and the human epidermoid cell lines A-431 and HaCaT. Indirect immunofluorescence studies demonstrated anti-PAF-R staining of primary cultures of human keratinocytes, A-431 cells, and HaCaT cells. Primary cultures of human fibroblasts and the melanoma cell line SK-30 failed to show immunostaining above that seen with control antiserum. With indirect immunofluorescence studies of sections of normal human skin, a granular anti-PAF-R staining pattern was noted on the keratinocyte cell membranes. A-431 cells readily metabolized PAF by deacetylation-reacylation at 37 degrees C, but not at 4 degrees C. Binding studies on crude membrane preparations of A-431 cells conducted at 4 degrees C demonstrated specific binding that reached saturation by 120 min. Scatchard analysis of PAF binding data revealed a single class of high-affinity (KD = 6.3 +/- 0.3 nM) PAF binding sites. The immunofluorescence and radioligand binding sites were shown to be functional PAF-Rs, as 10 pM to 1 microM PAF increased intracellular calcium in primary cultures of human keratinocytes, A-431 cells, and HaCaT cells, whereas PAF treatment of primary cultures of human fibroblasts or the melanoma cell line SK-30 did not result in changes in the intracellular calcium concentration. The structurally dissimilar PAF-R antagonists CV-6209, Ro19-3704, and alprazolam all inhibited the PAF-induced calcium changes in A-431 cells. The CV-6209 inhibition was seen at doses that competed with the PAF binding to these cells. These studies provide the first evidence for the presence of a functional PAF-R expressed on human keratinocytes, suggesting that this lipid mediator may play an important role in normal keratinocytes or in inflammatory dermatology.

Mediators of injury in neurotrauma: intracellular signal transduction and gene expression

Membrane lipid-derived second messengers are generated by phospholipase A2 (PLA2) during synaptic activity. Overstimulation of this enzyme during neurotrauma results in the accumulation of bioactive metabolites such as arachidonic acid, oxygenated derivatives of arachidonic acid, and platelet-activating factor (PAF). Several of these bioactive lipids participate in cell damage, cell death, or repair-regenerative neural plasticity. Neurotransmitters may activate PLA2 directly when linked to receptors coupled to G proteins and/or indirectly as calcium influx or mobilization from intracellular stores is stimulated. The release of arachidonic acid and its subsequent metabolism to prostaglandins are early responses linked to neuronal signal transduction. Free arachidonic acid may interact with membrane proteins, i.e., receptors, ion channels, and enzymes, modifying their activity. It can also be acted upon by prostaglandin synthase isoenzymes (the constitutive prostaglandin synthase PGS-1 or the inducible PGS-2) and by lipoxygenases, with the resulting formation of different prostaglandins and leukotrienes. Glutamatergic synaptic activity and activation of postsynaptic NMDA receptors are examples of neuronal activity, linked to memory and learning processes, which activate PLA2 with the consequent release of arachidonic acid and platelet-activating factor (PAF), another lipid mediator. Both mediators may exert presynaptic and postsynaptic effects contributing to long-lasting changes in glutamate synaptic efficacy or long-term potentiation (LTP), PAF, a potential retrograde messenger in LTP, stimulates glutamate release. The PAF antagonist BN 52021 competes for receptors in presynaptic membranes and blocks this effect. PAF may also be involved in plasticity responses because PAF leads to the expression of early response genes and subsequent gene cascades. The PAF antagonist BN 50730, selective for PAF intracellular binding, blocks PAF-mediated induction of gene expression. A consequence of neural injury induced by ischemia, trauma, or seizures is an increased release of neurotransmitters, that in turn generates an overproduction of second messengers. Glutamate, a key player in excitotoxic neuronal damage, triggers increased permeation of calcium mediated by NMDA receptors and activation of PLA2 in postsynaptic neurons. NMDA receptor antagonists reduce the accumulation of free fatty acids and elicit neuroprotection in ischemic damage. Increased production of free arachidonic acid and PAF converges to exacerbate glutamate-mediated neurotransmission. These neurotoxic actions may be brought about by arachidonic acid-induced potentiation of NMDA receptor activity and decreased glutamate reuptake. On the other hand, PAF stimulates the further release of glutamate at presynaptic endings. The neuroprotective effects of the PAF antagonist BN 52021 in ischemia-reperfusion are due, at least in part, to an inhibition of presynaptic glutamate release. PAF also induces expression of the inducible prostaglandin synthase gene, and PAF antagonists selective for the intracellular sites inhibit this effect. The PAF antagonist also inhibits the enhanced abundance, due to vasogenic cerebral edema and ischemia-reperfusion damage, of inducible prostaglandin synthase mRNA in vivo. Therefore, PAF, an injury-generated mediator, may favor the formation of other cell injury and inflammation mediators by turning on the expression of the gene that encodes prostaglandin synthase.

Activation of mitogen-activated protein kinase and arachidonate release via two G protein-coupled receptors expressed in the rat hippocampus

Platelet-activating factor and somatostatin receptors, two G protein-coupled receptors expressed in the rat hippocampus, were analyzed for the downstream signaling pathways in Chinese hamster ovary cells stably expressing each receptor. Ligand stimulation to each CHO cell line induced (1) inhibition of forskolin-induced accumulation of cAMP, (2) arachidonate release, and (3) activation of mitogen-activated protein kinase and MAP kinase kinase. In contrast, inositol phosphate breakdown was seen only in the PAF-stimulated CHO cells. The induction of these signals accompanied no detectable Ras activation. Suppression of the signals by pertussis toxin was almost complete for the somatostatin receptor but partial for the PAF receptor, suggesting that the somatostatin receptor couples only with PTX-sensitive G protein, while the PAF receptor couples with both PTX-sensitive and -insensitive G proteins. A model of G protein-mediated signaling pathways was proposed in which the signals from Gi and those from Gq converge at MAP kinase kinase and lead to arachidonate release. The present system using CHO cells is useful for analyzing signaling pathways from G proteins to MAP kinase kinase and will thereby provide clues for understanding the mechanisms underlying the physiological and pathological events mediated by PAF, somatostatin, and other G protein-coupled receptors in the central nervous system and other tissues.

Platelet-activating factor and retinoic acid synergistically activate the inducible prostaglandin synthase gene

Platelet-activating factor (PAF), a potent lipid mediator generated in cell injury and in the inflammatory and immune responses, promotes transcriptional activation of several primary response genes. TIS10/PGS-2 is a primary response gene encoding the inducible form of prostaglandin synthase. The inductive effects of PAF and retinoic acid (RA), alone and in combination, were studied with the regulatory region of TIS10/PGS-2 transfected into an exponentially growing glioblastoma-neuroblastoma NG108-15 hybrid in the human SH-SY5Y neuroblastoma or in the NIH 3T3 cell. RA alone exhibited only a small inductive effect. However, in the presence of RA (100 nM), a PAF-dependent (1-50 nM) synergistic activation of luciferase reporter constructs driven by regulatory regions of the TIS10/PGS-2 gene was found. The hetrazepine BN-50730, an antagonist selective for intracellular PAF binding sites, inhibited PAF and RA induction of luciferase from the TIS10/PGS-2 promoter. Thus, the intracellular PAF binding site is involved in TIS10/PGS-2 expression. Induction is rapid, suggesting that the combination of PAF and RA activates a preexisting latent transcription factor(s). Deletion studies restrict the major PAF and RA cis-acting response element of the TIS10/PGS-2 gene to a 70-nucleotide sequence as an intracellular inducer of TIS10/PGS-2 expression. The synergistic effect of RA and PAF represents an unusual convergence of nuclear signaling pathways by which, through the modulation of preexisting transcription factors, specific gene expression can be upregulated. PAF-dependent induction of TIS10/PGS-2 expression may play a role in cell injury, differentiation, inflammation, and immune responses.

Platelet-activating factor in human endometrium

Platelet-activating factor (PAF) is a phospholipid actively produced by human endometrium and deeply involved in the processes of ovoimplantation and labor. We recently found that PAF represents a new autocrine growth factor for a human adenocarcinoma cell line, HEC-1A. Indeed, biologically active PAF is synthesized by HEC-1A cells, under progesterone control. In HEC-1A cells, PAF regulates intracellular calcium concentration ([Ca2+]), DNA synthesis and expression of early oncogenes. All these effects are blocked by the receptor antagonist L659,989. However, while nanomolar concentrations of PAF mobilize [Ca2+], only micromolar concentrations affect cell growth, suggesting heterogeneity of PAF receptors or signaling. Two distinct populations of PAF receptors are present in HEC-1A cells, which bind PAF in nanomolar and micromolar concentrations, respectively. Since HEC-1A cells are producing elevated concentrations of PAF and micromolar concentrations of the PAF antagonist L659,989 inhibit cell proliferation, an autocrine role for PAF is suggested in HEC-1A cells.

Platelet-activating factor is a messenger in the electroconvulsive shock-induced transcriptional activation of c-fos and zif-268 in hippocampus

Platelet-activating factor (PAF, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine), undetectable in resting neural tissue, accumulates in brain during seizures. A hetrazepine, BN-50730, is shown here to displace [3H]PAF-specific binding from microsomal, but not from synaptosomal membranes, indicating selectivity for a high affinity intracellular binding site. Rats pretreated with BN-50730 by intraperitoneal or intracerebroventricular injection exhibited an inhibition of the electroconvulsive shock (ECS)-induced expression of c-fos and zif-268 in hippocampus. A much more pronounced, dose-dependent inhibition of ECS-induced zif-268 mRNA in hippocampus by intracerebroventricular injection of BN-50730 was observed. It is concluded that, in the hippocampus, PAF is a mediator of the expression of zif-268 and, to a lesser extent, c-fos through an intracellular specific binding site. Thus, PAF may be a messenger in signal regulated zinc-finger transcription factors, and in other immediate-early genes involved in long-term synaptic plasticity changes.

Platelet-activating factor induces collagenase expression in corneal epithelial cells

Platelet-activating factor (PAF), a potent lipid mediator involved in inflammatory and immune responses, accumulates rapidly in response to injury in a variety of tissues, including the corneal epithelium. However, the precise role of this compound in the cascade of events following insult has not been defined. Here we examined the effect of PAF on gene expression in the epithelial cells of rabbit corneas in organ culture. We found that incubation with 100 nM methylcarbamoyl PAF, a nonhydrolyzable analog of PAF, produced rapid transient 2.8- and 3.5-fold increases in the expression of c-fos and c-jun, respectively, at 1 hr, followed by increased expression of the collagenase type I gene beginning at 3 hr and peaking at 14-fold by 8 hr. Addition of the protein-synthesis-inhibitor cycloheximide superinduced c-fos and c-jun, strongly potentiating the PAF effect, but inhibited the induction of collagenase type I expression, suggesting the existence of a transcriptional factor linking the two events. BN-50730, a selective antagonist of intracellular PAF-binding sites, blocked the expression of the immediate-early genes as well as the increase in collagenase type I mRNA. Our results suggest that one of the functions of PAF may be to enhance the breakdown of the extracellular matrix as a part of the remodeling process during corneal wound healing after injury. Pathologically, a PAF-induced overproduction of collagenase may be a factor in the development of corneal ulcers, as well as other pathophysiological conditions such as cartilage destruction in arthritis. If so, inhibitors of this signal-transduction pathway may be useful as tools for further investigation and, eventually, as therapeutic agents to treat such disorders.

Excitable membranes, lipid messengers, and immediate-early genes. Alteration of signal transduction in neuromodulation and neurotrauma

The physical nature of neuronal cells, particularly in the functional and morphological segregation of synapse, soma, and dendrites, imparts special importance on the integrity of their cell membranes for the localization of function, generation of intrinsic second messengers, and plasticity required for adaptation and repair. The component phospholipids of neural membranes are important sources of bioactive mediators that participate in such diverse phenomena as memory formation and cellular damage following trauma. A common role for PAF in these processes is established through the suppressive effects of its antagonists. Furthermore, being both an extracellular and intracellular agonist of phospholipase activation, in addition to being a product of phospholipase activity, PAF assumes a centralized role in the cellular metabolism following neural stimulation. The linkage of PAF to neural immediate-early gene expression, both in vitro and in vivo, suggests that its effects are initiating to long-term formative and reparative processes. Such a common link between destructive and plastic responses provides an important view of cellular and tissue maintenance in the nervous system.

Involvement of tyrosine kinase and protein kinase C in platelet-activating-factor-induced c-fos gene expression in A-431 cells

In A-431 cells, platelet-activating factor (PAF) induces the expression of c-fos and TIS-1 genes in both the absence and the presence of cycloheximide in a structurally specific and receptor-coupled manner. We have now investigated the molecular mechanisms of this response, particularly in relation to the role of protein kinases. Pretreatment of cells with genistein or methyl-2,5-dihydroxycinnamate (tyrosine kinase inhibitors) or staurosporine (a protein kinase C inhibitor) for 20 min abolished the c-fos expression induced by PAF. Interestingly, when genistein was added 90 s after addition of PAF, no inhibition was observed. Similarly, staurosporine did not inhibit c-fos expression when added 8 min after PAF addition to the cells. These inhibitions were dose-dependent (IC50 for staurosporine was 180 nM, and for genistein 50 microM). Simultaneous addition of PAF and phorbol 12-myristate 13-acetate (PMA) did not give a synergistic effect on c-fos expression. Pretreatment of cells with PMA had no effect on [3H]PAF binding, but abolished the PAF-induced gene expression. PAF-stimulated gene expression was desensitized if cells were pretreated with PAF. Interestingly, epidermal growth factor was able to stimulate c-fos expression in PAF-desensitized cells, and thus indicated involvement of distinct mechanisms for the two stimuli. Forskolin, an activator of adenylate cyclase, did not induce c-fos expression and had no effect on the PAF response. Exposure of cells to PAF for as little as 1 min, followed by its removal, was sufficient to activate the gene expression and demonstrated the rapidity and the exquisite nature of the signalling involved in this process. It is concluded that activation of PAF receptor (a proposed G-protein-coupled receptor) causes rapid production of signals which induce the expression of c-fos gene and that this is mediated via tyrosine kinase and protein kinase C.