Inhibition of EGF-dependent mitogenesis by prostaglandin E2 in Syrian hamster embryo fibroblasts

Lipid metabolism can play an important role in the development and progression of human cancers. We have used Syrian hamster embryo (SHE) fibroblasts as a model system to study how lipid metabolites can alter cell proliferation and apoptosis. For example, the linoleic acid metabolite 13(S)-HpODE enhances EGF-dependent growth by inhibiting de-phosphorylation of the EGFR which leads to activation of the MAP kinase pathway. In contrast, the arachidonic acid metabolite, PGE(2), inhibits EGF-dependent mitogenesis and the expression of the proto-oncogenes c-myc, c-jun, and jun-B. In this study, we have investigated the mechanism by which PGE(2) attenuates these responses by studying the EGF signaling cascade in SHE cells. PGE(2) pretreatment caused a concentration-dependent decrease in EGF-dependent phosphorylation of MAP kinase and a corresponding inhibition of EGF-stimulated MAP kinase activity. Pretreatment of the SHE cells with PGE(2) had little effect on the magnitude of EGF-dependent receptor auto-phosphorylation and the phosphorylation of GAP suggesting a down-stream target. Treatment of cells with forskolin and EGF causes similar inhibition of MAP kinase phosphorylation as observed with PGE(2) and EGF. Since PGE(2) elevates cAMP in these cells, it may act by altering cAMP accumulation. Raf-1 activity can be inhibited by a cAMP-dependent process. Raf-1 activity, measured by phosphorylation of Mek-1, was attenuated by the addition of PGE(2). To determine if inhibition of Raf-1 activity causes inhibition of the MAP kinase pathway, cells were concomitantly incubated with PGE(2) and EGF. Inhibition of MAP kinase phosphorylation was observed. From these data, we propose that in SHE cells PGE(2) increases cAMP levels, which in turn causes inhibition of Raf-1 activity. The MAP kinase pathway is thus downregulated which decreases mitogenesis and proto-oncogene expression. This study demonstrates that an arachidonic acid metabolite can modulate phosphorylation and activity of key signal transduction proteins in a growth factor mitogenic pathway.