Prostaglandin D₂ generation from human lung mast cells is catalysed exclusively by cyclooxygenase-1

Mast cells are an exceptionally rich source of prostaglandin D₂ (PGD₂). PGD₂ is pro-inflammatory and can cause bronchoconstriction. The enzyme cyclooxygenase (COX) is central to the generation of prostanoids such as PGD₂. Non-steroidal anti-inflammatory drugs (NSAIDs) inhibit COX. COX exists as two isoforms, COX-1 and COX-2. The principal aim of this study was to establish whether COX-1 and/or COX-2 mediates PGD₂ generation from human lung mast cells. Mast cells were isolated from human lung tissue and purified by flotation over Percoll and immunomagnetic bead separations. The cells were activated with anti-IgE or Stem Cell Factor (SCF). The generation of PGD₂ was determined by ELISA. The effects of NSAIDs (aspirin, ibuprofen, diclofenac, naproxen, indomethacin), COX-1 selective (FR122047), and COX-2 selective (celecoxib) inhibitors on PGD₂ generation were determined. The expression of COX-1 and COX-2 in mast cells was determined by Western blotting. All the NSAIDs tested abrogated stimulated PGD₂ generation from mast cells except aspirin which was only weakly effective. FR122047 was an effective inhibitor of PGD₂ generation (EC₅₀ ~25nM) from mast cells whereas celecoxib was ineffective. Immunoblotting indicated that COX-1 was strongly expressed in all mast cell preparations while COX-2 expression was weak. No induction of COX-2 was observed following activation of mast cells. These findings indicate that COX-1 is the principal isoform involved in generating PGD₂ from human lung mast cells. These studies provide insight into the potential behaviour of NSAIDs in the context of respiratory diseases.