Repression of cyclooxygenase-2 and prostaglandin E2 release by dexamethasone occurs by transcriptional and post-transcriptional mechanisms involving loss of polyadenylated mRNA.
The two cyclooxygenase (COX) isoforms convert arachidonic acid to precursor prostaglandins (PGs). Up-regulation of COX-2 is responsible for increased PG production in inflammation and is antagonized by corticosteriods such as dexamethasone. In human pulmonary A549 cells, interleukin-1beta (IL-1beta) increases prostaglandin E2 (PGE2) synthesis via dexamethasone-sensitive induction of COX-2. Nuclear run-off assays showed that COX-2 transcription rate was repressed 25-40% by dexamethasone, while PGE2 release, COX activity, and COX-2 protein were totally repressed. At the mRNA level, complete repression of COX-2 was only observed at later (6 h) time points. Preinduced COX-2 mRNA was also potently repressed by dexamethasone, yet suppression of transcription by actinomycin D showed little effect. This dexamethasone-dependent repression involved a reduced COX-2 mRNA half-life, was blocked by actinomycin D or cycloheximide, and was antagonized by the steroid antagonist RU38486. Repression of IL-1beta-induced PGE2 release, COX activity, and COX-2 protein by actinomycin D was only effective within the first hour following IL-1beta treatment, while dexamethasone was effective when added up to 10 h later, suggesting a functional role for post-transcriptional mechanisms of repression. Following dexamethasone treatment, shortening of the average length of COX-2 mRNA poly(A) tails was observed. Finally, ligation of the COX-2 3'-UTR to a heterologous reporter failed to confer dexamethasone sensitivity. In conclusion, these data indicate a major role for post-transcriptional mechanisms in the dexamethasone-dependent repression of COX-2 that require de novo glucocorticoid receptor-dependent transcription and translation. This mechanism involves shortening of the COX-2 poly(A) tail and requires determinants other than just the 3'-UTR for specificity.
Newton
R
R
Department of Thoracic Medicine, National Heart and Lung Institute, Imperial College School of Medicine, Dovehouse Street, London, SW3 6LY, United Kingdom. robert.newton@ic.ac.uk
Seybold
J
J
Kuitert
L M
LM
Bergmann
M
M
Barnes
P J
PJ
D003513:Cycloheximide / Q000494:pharmacology; D051546:Cyclooxygenase 2; D003609:Dactinomycin / Q000494:pharmacology; D003907:Dexamethasone / Q000494:pharmacology*; D015232:Dinoprostone / Q000378:metabolism*; D004790:Enzyme Induction; D006727:Hormone Antagonists / Q000494:pharmacology; D006801:Humans; D007375:Interleukin-1 / Q000494:pharmacology / Q000502:physiology; D007527:Isoenzymes / Q000096:biosynthesis / Q000235:genetics*; D007700:Kinetics; D008168:Lung; D008175:Lung Neoplasms; D008565:Membrane Proteins; D015735:Mifepristone / Q000494:pharmacology; D011451:Prostaglandin-Endoperoxide Synthases / Q000096:biosynthesis / Q000235:genetics*; D012323:RNA Processing, Post-Transcriptional / Q000187:drug effects*; D012333:RNA, Messenger / Q000187:drug effects / Q000235:genetics* / Q000378:metabolism; D012091:Repetitive Sequences, Nucleic Acid; D014158:Transcription, Genetic / Q000187:drug effects*; D014407:Tumor Cells, Cultured
D006727:Hormone Antagonists; D007375:Interleukin-1; D007527:Isoenzymes; D008565:Membrane Proteins; D012333:RNA, Messenger; D003609:Dactinomycin; D015735:Mifepristone; D003907:Dexamethasone; D003513:Cycloheximide; D051546:Cyclooxygenase 2; C496540:PTGS2 protein, human; D011451:Prostaglandin-Endoperoxide Synthases; D015232:Dinoprostone