Autoregulation of inducible prostaglandin G/H synthase in osteoblastic cells by prostaglandins

Abstract

Prostaglandins (PGs) have been postulated to amplify their own production by stimulating cyclic adenosine monophosphate activity, which in turn stimulates PG production. We examined regulation of messenger RNA levels for the inducible and constitutive prostaglandin G/H synthases, PGHS-2 and PGHS-1, in murine osteoblastic MC3T3-E1 cells, which express both PGHS-1 and PGHS-2, and in rat osteoblastic Py1a cells, which express only PGHS-2. Prostaglandins E₂, F_2α, and D₂ induced PGHS-2 mRNA in both cell lines under serum-free conditions and stimulated small increases in PGHS-1 mRNA levels in MC3T3-E1 cells. PGE₂ (1 μM) increased the transcription rate of PGHS-2 mRNA 9-fold at 2 h in serum-free cells and also induced PGHS-2 protein. In the presence of arachidonic acid or serum, PGs also increased medium PGE₂. Both forskolin, a protein kinase A activator, and phorbol 12-myristate 13-acetate (PMA), a protein kinase C (PKC) activator, have previously been shown to induce PGHS-2 mRNA in MC3T3-E1 cells, but in the present study only PMA induced PGHS-2 expression in Pyla cells. The induction of PGHS-2 mRNA in Py1a cells by PGs was inhibited by chelerythrine, a PKC inhibitor, and blocked by 24 h of pretreatment with PMA. The 2 h serum stimulation of PGHS-2 mRNA in MC3T3-E1 cells was inhibited 40–50% by three structurally unrelated nonsteroidal anti-inflammatory drugs (NSAIDs), suggesting that endogenous PGs also amplify PG production through induction of PGHS-2. We conclude that the mechanism for autoamplification is in part transcriptional and may involve multiple pathways. We speculate that the induction of PGHS-2 by PGs may be the means for expanding PG responses to small and intermittent signals, such as mechanical strains.