Transforming Growth Factor β/Smad3 Signaling Regulates IRF-7 Function and Transcriptional Activation of the Beta Interferon Promoter

ABSTRACT The rapid induction of alpha interferon (IFN-α) and IFN-β expression plays a critical role in the innate immune response against viral infection. We studied the effects of transforming growth factor β (TGF-β) and its intracellular effectors, the Smads, on the function of IRF-7, an essential transcription factor for IFN-α and -β induction. IRF-7 interacted with Smads, and IRF-7, but not IRF-3, cooperated with Smad3 to activate IFN-β transcription. This transcriptional cooperation occurred at the IRF-binding sequences in the IFN-β promoter, and dominant-negative interference with TGF-β receptor signaling and Smad3 function decreased IRF-7-mediated transcription. Furthermore, elimination of Smad3 expression in Smad3−/− fibroblasts delayed and decreased double-stranded RNA-induced expression of endogenous IFN-β, whereas restoration of Smad3 expression enhanced IFN-β induction. The IRF-7-Smad3 cooperativity resulted from the regulation of the transactivation activity of IRF-7 by Smad3, and dominant-negative interference with Smad3 function decreased IRF-7 activity. Consistent with the regulation by Smad3, the transcriptional activity of IRF-7 depended on and was regulated by TGF-β signaling. Our studies underscore a role of TGF-β/Smad3 signaling in IRF-7-mediated induction of IFN-β expression.

Roles of Autocrine TGF-β Receptor and Smad Signaling in Adipocyte Differentiation

TGF-β inhibits adipocyte differentiation, yet is expressed by adipocytes. The function of TGF-β in adipogenesis, and its mechanism of action, is unknown. To address the role of TGF-β signaling in adipocyte differentiation, we characterized the expression of the TGF-β receptors, and the Smads which transmit or inhibit TGF-β signals, during adipogenesis in 3T3-F442A cells. We found that the cell-surface availability of TGF-β receptors strongly decreased as adipogenesis proceeds. Whereas mRNA levels for Smads 2, 3, and 4 were unchanged during differentiation, mRNA levels for Smads 6 and 7, which are known to inhibit TGF-β responses, decreased severely. Dominant negative interference with TGF-β receptor signaling, by stably expressing a truncated type II TGF-β receptor, enhanced differentiation and decreased growth. Stable overexpression of Smad2 or Smad3 inhibited differentiation and dominant negative inhibition of Smad3 function, but not Smad2 function, enhanced adipogenesis. Increased Smad6 and Smad7 levels blocked differentiation and enhanced TGF-β–induced responses. The inhibitory effect of Smad7 on adipocyte differentiation and its cooperation with TGF-β was associated with the C-domain of Smad7. Our results indicate that endogenous TGF-β signaling regulates the rate of adipogenesis, and that Smad2 and Smad3 have distinct functions in this endogenous control of differentiation. Smad6 and Smad7 act as negative regulators of adipogenesis and, even though known to inhibit TGF-β responses, enhance the effects of TGF-β on these cells.