Evidence for Activation of the TGF-β1 Promoter by C/EBPβ and Its Modulation by Smads

Abstract

The transforming growth factor-β1 (TGF-β1) is a cytokine involved in many biological events inlcuding immunosuppression, angiogenesis, cell growth, and apoptosis. Expression of TGF-β1 at the transcriptional level is controlled by a series of ubiquitous and specialized factors whose activities can be modulated by a variety of signaling events. Here we demonstrate that activity of the TGF-β1 promoter is increased by C/EBPβ, a DNA-binding transcription factor whose activity can be influenced by several immunomodulators, in astrocytes and microglial cells. Interestingly, expression of Smad3 and Smad4, the downstream regulators of the TGF-β1-signaling pathway, impairs the activity of C/EBPβ on the TGF-β1 promoter. Further, we demonstrate that MH2, a common domain among Smads that has protein-binding activities, interacts with C/EBPβ and decreases its association with a region of the TGF-β1 promoter that is responsive to C/EBPβ activation. Interestingly, the p65 subunit of nuclear factor-κB (NF-κB), which also interacts with C/EBPβ, cooperates with MH2 and decreased DNA-binding and transcriptional activities of C/EBPβ on the TGF-β1 promoter. These observations indicate that an autoregulatory mechanism, involving the MH2 domain of Smads, modulates activation of the TGF-β1 promoter by C/EBPβ. Further, our results show that the interplay between NF-κB and C/EBPβ has an impact on the ability of C/EBPβ to stimulate TGF-β1 transcription, hence, suggesting that the cross-communication of signaling pathways that modulate NF-κB and C/EBPβ may dictate the level of TGF-β1 promoter activity.