Inhibition of p90 ribosomal S6 kinase-mediated CCAAT/enhancer-binding protein β activation and cyclooxygenase-2 expression by salicylate

We have previously shown that salicylate at a pharmacological concentration suppresses CCAAT/enhancerbinding protein beta (C/EBP beta) binding, thereby reducing cyclooxygenase-2 (COX-2) and inducible nitric-oxide synthase expression (Saunders, M. A., Sansores-Garcia, L., Gilroy, D. W., and Wu, K. K. ( 2001) J. Biol. Chem. 276, 18897 - 18904; Cieslik, K., Zhu, Y., and Wu, K. K. ( 2002) J. Biol. Chem. 277, 49304 - 49310). We postulated that salicylate targets a kinase that phosphorylates and activates C/EBP beta. Here we report the identification of p90 ribosomal S6 kinase (RSK) as a target of salicylate. Salicylate inhibited RSK in vivo and blocked the activity of RSK2 purified from cells stimulated by phorbol 12-myristate 13-acetate (PMA). Mutation of the RSK-phosphorylation site (T266A) of C/EBP beta abrogated PMA-stimulated C/EBP beta binding activity. RSK activation was required for PMA-induced COX-2 transcriptional activation. Salicylate also inhibited Ras and extracellular signal-regulated kinase (ERK) activation induced by PMA. We conclude that salicylate inhibits C/EBP beta-mediated COX-2 transcriptional activation by blocking RSK activity and Ras signaling pathway.