Positive feedback regulation of COX-2 expression by prostaglandin metabolites

Abstract.

Objectives: Previous studies in cell lines and tissues derived from mice lacking genes encoding cyclooxygenase (COX)-1 or −2 have demonstrated compensatory regulation between the two isoenzymes. To determine whether this compensation was driven by a mechanism that controls prostaglandin (PG) levels, we investigated the effects of PG availability on the regulation of COX and cytosolic phospholipase A₂ (cPLA₂), an upstream enzyme in the PG pathway.

Materials and methods: Mouse lung fibroblast cells were treated with various concentrations of PG metabolites including prostaglandin E₂ (PGE₂), 15-deoxy-Δ^12,14-prostaglandin J₂ (15d-PGJ₂), 6-keto PGF_1α and PGF_2α Cells were harvested for protein and mRNA analyses; culture supernatant was collected for prostaglandin assays.

Results: We observed 8- and 20-fold increase in basal COX-2 protein expression levels when cells were exposed to PGE₂ and 15d-PGJ₂, respectively. In the presence of IL-1β, PGE₂, 15d-PGJ₂, 6-keto PGF_1α and PGF_2α each enhanced COX-2 protein expression between 5- to 20-fold. Corresponding with the induction of COX-2 protein expression, the latter three PGs induced PGE₂ synthesis in a dose-dependent manner. Only PGE₂ and 15d-PGJ₂ induced COX-2 mRNA expression, although to a lower extent than protein induction. None of the PG metabolites tested showed significant effects on the level of COX-1 or cPLA₂ protein expression, except for PGF_2α, which increased IL-1β-induced cPLA₂ protein expression slightly.

Conclusion: Our results demonstrate that there is positive feedback regulation of COX-2 expression by PG metabolites, but not COX-1, indicating that PG levels per se do not play an important role in the compensatory regulation between the two COX isoenzymes, but may play an important role in mediating increased COX-2 expression and activity.