Regulation of the cellular expression of secretory and cytosolic phospholipases A2, and cyclooxygenase-2 by peptide growth factors

Secretory group II (sPLA(2)) and cytosolic (cPLA(2)) phospholipases A(2) and cyclooxygenase-2 (Cox-2) play a pivotal role in release of proinflammatory eicosanoids. Excessive activity of sPLA(2) per se can also propagate inflammation. Endogenous control of the above enzymes has not been completely elucidated. We investigated the combined impact of promoting cytokines and inhibitory peptide growth factors on the expression of mRNA of the above enzymes, on protein content and extracellular release of sPLA(2) and on PGE(2) production in osteoblasts (FRCO). The synthesis and release of sPLA(2) were enhanced by about 20-fold by 0.5 ng/ml IL-1 beta or by 50 ng/ml of TNF alpha. Coaddition of both cytokines resulted in synergistic 150-fold increase in the release of sPLA(2) implying the existence of two paths of induction, IL-1 beta and TNF alpha markedly enhanced the transcription of sPLA(2) mRNA. Kinetic study showed that IL-1/TNF initiated sPLA(2) release after 12 h, reaching maximum at 48 h. IL-1 alpha was a weak stimulator of sPLA(2) release, whereas IL-6, IL-8, IGF, IFN-gamma, growth hormone, insulin and GM-CSF were not stimulatory. Peptide growth hormones TGF beta, PDGF-BB, EGF and bFGF markedly inhibited the extracellular release of sPLA(2). TGF beta and PDGF-BB significantly reduced the level of sPLA(2) mRNA, thus acting upon transcription whereas EGF and bFGF were not inhibitory, acting rather upon the translational or posttranslational steps. IL-1/TNF and growth factors had no significant effect on cPLA(2) mRNA expression. Cox-2 mRNA expression was markedly enhanced by IL-1/TNF and suppressed by all growth factors tested. Cytokines enhanced the extracellular release of PGE(2) and further enhancement was induced by growth factors with the exception of TGF beta. Cycloheximide abolished completely the release of sPLA(2) and markedly reduced the release of PGE(2) from cytokine-stimulated FRCO, regardless of whether growth factors were present or not. NS-398, a specific inhibitor of Cox-2 abolished almost completely the release of PGE(2) from cytokine-stimulated cells, regardless of the presence of growth factors. Thus, different signalling mechanisms are involved in the impact of growth factors on mRNA expression of sPLA(2), cPLA(2) and Cox-2. The differences between the impact on FRCO sPLA(2) and that reported in other cells, imply that endogenous control of arachidonic acid cascade is cell-specific. (C) 1998 Elsevier Science B.V. All rights reserved.