Positive feedback regulation of COX-2 expression by prostaglandin metabolites

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.

Mouse lung fibroblast cells were treated with various concentrations of PG metabolites including prostaglandin E2 (PGE2), 15-deoxy-delta(12,14)-prostaglandin J2 (15d-PGJ2), 6-keto PGF1alpha and PGF2alpha. Cells were harvested for protein and mRNA analyses; culture supernatant was collected for prostaglandin assays.

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-1beta, PGE2, 15d-PGJ2, 6-keto PGF1alpha and PGF2alpha 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 PGF2alpha, which increased IL-1beta-induced cPLA2 protein expression slightly.

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.