Tristetraprolin (TTP) is a zinc-finger-containing AU-rich elements (ARE)-binding protein. AREs presented in the 3'untranslated region (UTR) of mRNAs from many proto-oncogenes, cytokines, and growth factors may be targets for regulation of messenger RNA stability. In this study, we observed that many immediate early genes (IEGs) were induced during the early differentiation of 3T3-L1 preadipocytes and their ARE-containing transcripts were degraded rapidly. Immunoprecipitation followed by RT-PCR analysis showed that two of IEG mRNAs, COX-2 (cyclooxygenase-2) and MKP-1 (mitogen-activated protein kinase phosphatase), were the target of TTP. Biotinylated MKP-1 AREs also could bring down TTP and the other ARE-binding protein HuR. RNA EMSA and competition assays showed that each of three AREs located in 3'UTR of MKP-1 mRNA has differential binding affinity to TTP. Sequence analysis of 3'UTR of IEG mRNAs suggested that TTP may prefer binding to UUAUUUAUU sequence. Taken together, our results implied that TTP may target specific ARE-containing IEGs' mRNAs such as COX-2 and MKP-1 mRNAs to modulate their expression post-transcriptionally. (c) 2007 Elsevier Inc. All rights reserved.