Transcriptional regulation of tristetraprolin by transforming growth factor-β in human T cells

Transforming growth factor-beta (TGF-beta) is a pleiotropic cytokine that plays a critical role in modulating immune response and inflammation. We employed the Affymetrix cDNA microarray system to detect genes whose expression is regulated by TGF-beta1 in a human T cell line HuT78. Tristetraprolin (TTP), a protein involved in the degradation of tumor necrosis factor-alpha (TNF-alpha) mRNA, was found to be up-regulated by TGF-beta. This up-regulation was confirmed by reverse transcriptase-PCR analysis that revealed a rapid and transient induction of TTP mRNA by TGF-beta1 in HuT78 cells, primary human T cells, and THP-1 macrophage-monocyte cells. In addition, de novo protein synthesis was not required for this induction, suggesting that TTP is regulated by TGF-beta at the transcriptional level. To delineate the transcriptional regulation of the TTP gene, a 2.7-kb human TTP promoter region (-2682 to +56 bp relative to the transcription initiation site) was isolated. We found that this promoter was stimulated by TGF-beta1 or a constitutively active TGF-beta type I receptor via TGF-beta-specific Smad proteins. Furthermore, a series of TTP promoter deletion constructs were used to localize the Smad-responsive region to the -583 to -263 bp portion of the promoter. In this region, the TTP promoter contained a stretch of putative Smad-binding elements that had a synergistic effect in mediating Smad activation of the promoter. These putative Smad-binding element-containing sequences were also able to bind Smad3 and Smad4 proteins purified in vitro. As TGF-beta- and TTP-deficient mice exhibit overlapping phenotypes manifested by multifocal inflammation and autoimmunity, our findings that TTP transcription is under the control of TGF-beta signaling would indicate a potential role of TTP in mediating the immune suppressive action of TGF-beta in vivo.