Expression and role of cyclooxygenase isoforms in alveolar and peritoneal macrophages

Prostaglandin synthesis represents one means by which macrophages modulate inflammation. The initial enzyme in the metabolism of arachidonic acid to prostaglandins is cyclooxygenase (COX). Both constitutive (COX-1) and inducible (COX-2) isoforms are recognized. We previously showed that COX activity of rat peritoneal macrophages (PM) exceeds that of alveolar macrophages (AM). In this study, we correlated the steady-state levels of COX-1 and COX-2 proteins with COX activity in resident AM and PM. Freshly obtained AM contained lower levels of COX-1 than did fresh PM. Neither contained substantial amounts of COX-2 in the basal state, but both cell types demonstrated induction when cultured with lipopolysaccharide; once again, COX-2 levels in PM exceeded those in AM. Despite COX-2 induction under these circumstances, its contribution to prostaglandin production appeared to be modest. We conclude that, although both isoforms of COX are expressed in rat AM and PM, COX-1 is responsible for the majority of enzyme activity in both the basal and stimulated states. The lesser prostaglandin synthetic capacity of AM than of PM appears to be the consequence of lower steady-state levels of both COX proteins.