Smad Proteins Suppress CCAAT/Enhancer-binding Protein (C/EBP) β- and STAT3-mediated Transcriptional Activation of the Haptoglobin Promoter

Abstract

Activin A, a member of the transforming growth factor β (TGFβ) superfamily, blocks interleukin (IL)-6 biological functions. The molecular basis of the influence of this TGFβ signaling on the IL-6 receptor triggered cascade is unknown. We studied IL-6-induced secretion of the acute phase protein haptoglobin by hepatoma cells. Overexpression of the C/EBPβ gene, a downstream effector in the IL-6 pathway, activated transcription from the haptoglobin promoter. This was abolished by either a constitutively active form of activin A type IB receptor (CAactRIB) or by a combination of Smad3 and Smad4. Similarly, Smads abolished transcriptional activation by co-stimulation with IL-6 and STAT3. The transcription co-activator p300 partially overcame the suppressive effect of Smads. Electrophoretic mobility shift assays indicated that C/EBPβ binding to haptoglobin promoter DNA was reduced by over-expression of CAac-tRIB and Smad4. We thus show that Smad proteins operate as transcription inhibitors on target genes of the IL-6 induced pathway. The effect of Smads is exerted on components of the transcription activation complex and may also involve interference with DNA binding. This study thus depicts molecular sites of interaction between the TGFβ superfamily and the IL-6 signaling cascades.