Received for publication November 6, 2007.
Revised May 8, 2008.
Accepted for publication May 8, 2008.

Compounds that increase or mimic cAMP enhance tristetraprolin degradation in LPS-treated murine J774 macrophages

Abstract

Tristetraprolin (TTP) is a trans-acting factor that can regulate mRNA stability by binding to the cis-acting AU-rich element (ARE) in the 3'-untranslated region (3'-UTR) in mRNAs of certain transiently expressed genes. The best-studied target of TTP is tumor necrosis factor- (TNF-). By binding to ARE, TTP increases the degradation of TNF- mRNA, thereby reducing the expression of TNF-. We examined the effects of cAMP analogs and cAMP-elevating agents forskolin and ₂-agonists on lipopolysaccharide (LPS)-induced TTP mRNA and protein expression by quantitative real-time reverse transcriptase PCR and Western blotting in activated macrophages. All these agents caused a slight increase in LPS-induced expression of TTP mRNA. However, TTP protein levels were significantly reduced when the cells were treated with the combination of LPS and cAMP-elevating agent compared to LPS alone. Proteasome inhibitors MG132 and lactacystin increased TTP protein levels and abolished the effects of cAMP-enhancing compounds on TTP protein levels. The results suggest that mediators and drugs which enhance intracellular cAMP reduce TTP expression in macrophages exposed to inflammatory stimuli by increasing TTP degradation through the proteasome pathway.

Key words: b2-agonist, cAMP, inflammation, mRNA stability, proteasome, tristetraprolin