REGULATION OF THE CELLULAR EXPRESSION OF SECRETORY AND CYTOSOLIC PHOSPHOLIPASES A(2), 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 proi nflammatory eicosanoids. Excessive activity of sPLA(2) per se can also propagate inflammation. Endogenous control of the above enzymes has n ot been completely elucidated. We investigated the combined impact of promoting cytokines and inhibitory peptide growth factors on the expre ssion of mRNA of the above enzymes, on protein content and extracellul ar 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 c ytokines resulted in synergistic 150-fold increase in the release of s PLA(2) implying the existence of two paths of induction, IL-1 beta and TNF alpha markedly enhanced the transcription of sPLA(2) mRNA. Kineti c study showed that IL-1/TNF initiated sPLA(2) release after 12 h, rea ching maximum at 48 h. IL-1 alpha was a weak stimulator of sPLA(2) rel ease, 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 inhibit ory, 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 an d suppressed by all growth factors tested. Cytokines enhanced the extr acellular release of PGE(2) and further enhancement was induced by gro wth factors with the exception of TGF beta. Cycloheximide abolished co mpletely the release of sPLA(2) and markedly reduced the release of PG E(2) from cytokine-stimulated FRCO, regardless of whether growth facto rs were present or not. NS-398, a specific inhibitor of Cox-2 abolishe d almost completely the release of PGE(2) from cytokine-stimulated cel ls, regardless of the presence of growth factors. Thus, different sign alling 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 e ndogenous control of arachidonic acid cascade is cell-specific. (C) 19 98 Elsevier Science B.V. All rights reserved.