| Prostaglandins are important mediators of a wide range of physiological processes. Cyclooxygenase-2 converts membrane-derived arachidonic acid to PGH2, the common precursor of prostaglandins. Induction of Cyclooxygenase-2 (COX-2) transcription is therefore a critical event in the regulation of physiological processes by prostaglandins. The tightly regulated balance between bone formation and resorption is regulated, at least in part, by prostaglandins secreted by osteoblasts. Macrophages, which play an important role in inflammation and host immune responses, direct local cytokine production and promote migration of additional immune cells to the site of inflammation in part through production and secretion of prostaglandins. Mast cells, also important cells in the immune system, secrete prostaglandins upon activation. The cis-acting promoter elements, transcription factors, and signaling kinases involved the induction of COX-2 transcription in these cells is not clear.; I generated wild-type and mutant COX-2 promoter-driven luciferase reporter plasmids, and transiently transfected these constructs into MC3T3-E1 osteoblasts, RAW 264.7 macrophages, and MMC-34 mast cells. In all of these cells, the cyclic-AMP response element (CRE) is required for COX-2 promoter activity. To varying degrees, presence of NF-IL6 (C/EBP) sites is also required for optimal promoter activity in these cells. A putative NF-kappa B site is not required for COX-2 promoter activity in osteoblasts, macrophages, or mast cells. Signaling to the CRE site in all these cells types requires the activity of the MEKK/JNK kinase pathway. In mast cells, this pathway requires activity of Ras. In contrast, activated receptor linkage to this pathway in macrophages requires the adapter protein ECSIT, and not Ras. COX-2 reporter induction in mast cells also requires a Ras dependent MAPKK/ERK signaling pathway. In contrast, this pathway is not required for COX-2 promoter activity in either osteoblasts or macrophages. COX-2 induction through the CRE in osteoblasts, macrophages, and mast cells involves the c-Jun transcription factor, and not the “classic” CRE-element binding protein CREB. Induction of the COX-2 promoter through the NF-IL6 sites in osteoblasts and macrophages requires the activity of members of the C/EBP transcription factor family, although in mast cells C/EBP proteins may also interact with c-Jun and the CRE site. |
