Cytokine gene regulation by PGE2, LTB4 and PAF

The initial response of the host to noxious stimuli produces a nonspecific inflammatory response. A more specific immune response is believed to be modulated by two classes of molecules: lipid mediators (PG, LT and PAF) and cytokines, synthesized by phagocytes and parenchyreal cells. In this review we discuss the increasing evidence of the interrelationship between eicosanoids, PAF and cytokines: IL-1 and TNF induce PG synthesis in various cells and PG, in turn, modulate cytokine production. We focused on the regulatory effects of LTB 4, PGE 2 and PAF on cytokine gene expression.

degrees of inflammation. Among the soluble mediators which participate in this inflammatory response and which may also modulate the subsequent more specific immune response, two classes of molecules have emerged as the principal protagonists" lipid mediators, derived from cell membrane phospholipids, and cytokines, synthesized by phagocytes and parenchymal cells.
Within minutes of stimulation, eicosanoids such as prostaglandins (PG) and leukotrienes (LT), as well as platelet activating factor (PAF) are produced by the action of phospholipase a 2 (PLA2) on the membrane phospholipid 1-0-alkyl-2-arachidonoyl-sn-glycero-3-phosphocholine and subsequent oxidative metabolism of the freed arachidonic acid or acetylation of the remaining 1-alkyl-2-1ysoglycerophosphocholine (lyso-PAF) molecule, respectively. These lipid mediators in turn can act on a variety of cell populations, often, including the cells which produced them. Their bioactions are thought to be mediated by specific cell membrane receptors. Recently, a PAF receptor from guineapig lung has been cloned 2 and its proposed structure suggests that it is associated with a G protein.
Functional studies had previously suggested that this may also be the case with LTB 4 receptors.
Postreceptor events and signal transduction pathways in cellular responses triggered by LT, PG and PAF are objects of intense research using metabolic inhibitors and receptor antagonists. In the present review, we will focus on the regulatory effects of LTB4, PGE2 and PAF on cytokine gene expression (Table 1). TNFz is a cytokine produced preferentially by activated macrophages, but also by NK cells and neutrophils. TNFz may play a role in immune modulation 4 and anti-tumour defences, in addition to being a potent mediator for numerous inflammatory responses, such as endotoxic shock, adult respiratory distress syndrome and bowel necrosis. -8 In these conditions, however, synergy with other mediators, including PAF, may be needed for expression of disease.
When human monocytes are exposed to graded concentration of LTB4, their cell-free supernatants contain increased amounts of TNFcz which peaks at 8-16 h. The maximal effect of LTB 4 is observed at concentrations of 10-1 M. Endogenous lipoxygenase metabolites may also be involved in enhanced TNFz production following some stimuli, such as silica, asbestos, 1 PAF 11 or lipopolysaccharide (LPS). 2 Evidence for such an involvement is derived from use of 5-1ipoxygenase (5-LO) inhibitors which can partially or totally block TNFz production. Addition of exogenous LTB4 can restore TNFz production under certain Mediators of Inflammation. Vol circumstances. 11 In contrast, inhibition of 5-LO activation using MK-886, which binds the 5lipoxygenase-activating protein (FLAP), does not affect TNF production 13 in response to phorbol ester, Concanavalin A, LPS or zymosan. TNFo gene expression can be stimulated by phorbol esters, LPS or TNFo itself. Under these conditions, inhibition of PLA 2 by bromophenacyl bromide or quinacrine, or of lipoxygenases by ketoconazole or nordihydroguaiaretic acid (NDGA) results in inhibition of TNFo mRNA accumulation and TNFo gene transcription. 14<6 On the other hand, exogenous LTB 4 increases TNFz mRNA. 14 Interestingly, the dual cyclooxygenose (CO)/5-LO inhibitor, tebufelone, at 20-25/M, inhibits TNFo mRNA accumulation while enhancing TNFo production. 7 In comparison to 5-LO metabolites, CO metabolites such as PGE2 have been shown to inhibit TNFo production at high concentrations, 18'19 presumably by augmenting cAMP levels in the cells. Low concentrations of PGE2, however, appear to stimulate guanylate cyclase and result in augmented TNFo production. 2'21 TNF mRNA accumulation is also inhibited by PGE2, 22 an effect associated with decreased TNF transcription 14 (Fig. 1). PAF can enhance TNFo production by rat alveolar macrophages 11 and human monocytes 23-2s and myeloid cells. 26 In the macrophages, the activation pathway may involve 5-LO, since inhibitors such as NDGA or AA-861 can abrogate the effect of PAF. 1 In human monocytes and myeloid cell lines, the involvement of the 5-LO pathway in PAF-mediated TNFo production is still unclear. PAF induces, however, a bimodal dose-response pattern in these cells, with both a nanomolar and a femtomolar concentration peak of activation. 2  Interleukin-1 (IL-1)

IL-1 is a key protagonist of immune and inflammatory events, being involved, among other effects, in T-cell activation by accessory cells and in
induction of fever and other features of inflammation. iv It is produced principally by monocytes/macrophages, 28  Addition of exogenous LTB 4 to monocytes stimulates IL-1/ transcription and mRNA accumulation (Rola-Pleszczynski and Stankova, in preparaproduction in PAF-treated monocytes follows a bimodal pattern, with peak activities at 10 -13 and 10 -8 M P_AF. 24 '39 In rats treated with a continuous infusion of PAF via an osmotic mini-pump, IL-1 production by splenic macrophages was enhanced by lower doses of PAF, while higher doses had an inhibitory effect. 4 At the present time, it is unclear whether PAF regulates IL-lo and IL-lfi production via transcriptional or post-transcriptional mechanisms. IL-lfl mRNA expression in THP-1 cells, however, can be enhanced by 10-1 M PAF. 41 Interleukin-6 IL-6 is a multifunctional cytokine produced by monocytes, macrophages, endothelial cells, fibroblasts, keratinocytes, T-cells and some tumour cells. Its numerous synonyms reflect its various biological activities, as B-cell stimulatory factor 2, interferon #2, hybridoma-plasmacytoma growth factor, hepatocyte-stimulating factor. 42-46 IL-6 is an important regulator of Tand B-cell functions, haematopoiesis and acute phase responses. 47'48 Infectious agents, endotoxin and the inflammatory cytokines TNF0 and IL-1 can induce IL-6 production, while dysregulation of IL-6 expression is associated with certain chronic inflammatory, autoimmune and haematopoietic disorders. The findings that IL-6 production is associated with inflammatory states suggests that its production may also be modulated by inflammatory lipid mediators. When human monocytes are cultured in the presence of graded concentrations of LTB4, a significant stimulation of production of bioactive and immunoreactive IL-6 is observed. 29'31 Nanomolar concentrations of LTB 4 are optimal, while the c0-oxidation products 20-OH-LTB4 and 20-COOH-LTB 4 are only 22% and 2% effective, respectively. LTB 4 induces an accumulation of IL-6 mRNA in treated monocytes with a superposable doseresponse, and maximal accumulation at 1 h. While IL-6 mRNA half-life in untreated ceils is approximately 1 h, it is extended to 3h in LTB4-treated monocytes. Moreover, nuclear transcription of IL-6 mRNA is augmented at 30 min by a factor of five in LTB4-treated cells. Furthermore, LTB4-treated monocytes contain a nuclear protein factor (NF) which binds to a promoter region of the IL-6 gene, called NF-IL-6-binding domain, and which may be involved, among other factors, in LTB4-mediated induction of IL-6 gene transcription. 31 PAF can also stimulate IL-6 production by human monocytes in the picomolar range and this stimulation is enhanced by prior treatment with TNF0 or IFN/. 49 In contrast, rat alveolar macrophages require a second stimulus, such as LPS or MDP, to respond to PAF with augmented IL-6 production. 5 This augmented production is brought down to baseline by pretreatment of the cells with the FLAP inhibitor MK-886 or the 5-LO inhibitor AA-861, suggesting that under these conditions, the action of PAF is mediated by endogenous 5-LO metabolites. Human PMN and endothelial cells can also respond to PAF with enhanced IL-6 production and IL-6 mRNA accumulation (unpublished).

Conclusions
There is increasing evidence to suggest that the production of eicosanoids, PAF and cytokines may be interrelated: IL-1 and TNF induce PG synthesis in various cells [51][52][53] and PG, in turn, modulate cytokine production, as discussed in this review. In contrast, LT can augment IL-1, IL-6 and TNF production and endogenous LT production may play a role in TNF and IL-6 synthesis. IL-1, TNF and IFN can also induce the synthesis of PAF in several cell types, including endothelial cells, neutrophils and macrophages, 54-56 while PAF can, in turn, augment IL-1, IL-6 and TNF production by rat and human cells. Such a positive feedback loop at this level, with potential for amplification of immune or inflammatory responses, may be counterbalanced by the negative feedback action of IL-6 on both IL-1 and TNF. 57'58 This negative feedback may account for the limited production of IL-1 by LTB4-stimulated monocytes which readily produce large amounts of IL-6. It may also explain the augmented production of IL-1 and TNF observed after treatment of monocytes with a dual CO/5-LO inhibitor, a treatment which may preferentially inhibit IL-6 production (Fig. 2).