Kinase‐dependent, retinoic acid receptor‐independent up‐regulation of cyclooxygenase‐2 by all‐trans retinoic acid in human mesangial cells

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Abstract
Preliminary results in human mesangial cells (MC) suggested that all-trans retinoic acid (ATRA) increased the expression of COX-2 and the production of prostaglandin E2 (PGE2), a PG with anti-inflammatory effects in MC. The aim of this work is to confirm that ATRA increases the expression of COX-2 in MC and to examine the mechanisms involved. Cultured MC were treated with ATRA. COX expression and kinase activity were analyzed by Western blot. Transcriptional mechanisms were analyzed by Northern blot, RT-PCR and promoter assays. COX-2 and COX-1 expression and PGE2 production were increased by ATRA. COX-2 played a role in PGE2 production as production was only partially inhibited by COX-1 inhibitor SC-560. COX-2 up-regulation by ATRA was due to transcriptional mechanisms as pre-incubation with actinomycin D abolished it and ATRA increased the expression of COX-2 mRNA and the activity of a human COX-2 promoter construct, whereas post-transcriptional mechanisms were not found. Retinoic acid receptors (RAR) were not involved in the up-regulation of COX-2 by ATRA since it was not inhibited by RAR-pan-antagonists and the RAR-pan-agonist TTNPB did not up-regulate COX-2. Instead ATRA might act through a sustained activation of extracellular signal-regulated kinase 1/2 (ERK1/2) since up-regulation of COX-2 was prevented by inhibition of the activation of ERK1/2 with PD098059. Also ERK1/2, as well as downstream signalling proteins from ERK1/2, remained phosphorylated when COX-2 increased 24 h later. These results highlight the relevance of RAR-independent mechanisms to the biological effects of ATRA.

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