Editors' ChoiceInfection

Enhancing and Inhibiting TGF-β Signaling in Infection

Sci. Signal.  13 Jan 2015:
Vol. 8, Issue 359, pp. ec9
DOI: 10.1126/scisignal.aaa6549

A pair of studies evaluates how pathogen infection affects the transforming growth factor–β (TGF-β) pathway. TGF-β signaling through the Smad family of transcriptional regulators induces the production of proteins of the extracellular matrix to which bacteria can bind, which can promote infection. TGF-β is released from cells as a latent, inactive complex, and the neuraminidase (NA) of influenza A virus activates TGF-β, thereby stimulating TGF-β signaling. Li et al. found that influenza A infection or exposure of a lung epithelial cell line increased the amount of active TGF-β in the culture medium, increased the proportion of cells with high amounts of the integrin subunit α5 and fibronectin, and specifically enhanced the adherence of bacteria that bind to fibronectin, but not those that bind to mannose. Inhibitors of the TGF-β receptor or Smad3 signaling blocked the influenza virus infection–mediated induction of the α5 integrin subunit and fibronectin and prevented the increased adhesion of the Group A Streptococcus (fibronectin-binding) bacteria, as well as the increased adhesion of other bacteria that commonly exhibit coinfection with influenza A. Smad3-deficient mice that were infected with influenza A and then exposed to Group A Streptococcus exhibited reduced bacterial load, and the number of cells with high abundance of α5 integrin and fibronectin was unchanged by influenza infection. Thus, viral neuraminidase-mediated activation of TGF-β may promote bacterial coinfection in influenza patients.

Whereas Li et al. identified an extracellular mechanism by which a virus can release TGF-β to enhance bacterial coinfection, Xu et al. characterized a mechanism by which the innate immune response blocks Smad-dependent signaling in virally infected cells. Infected cells detect the nucleic acid of a virus with cytosolic receptors of the RIG-I–like family (RLRs), which trigger a phosphorylation cascade that culminates in the activation of the transcription factors IRF3 and NF-κB to mediate antiviral and inflammatory responses. IRF3 is structurally similar to the Smad transcriptional regulators. Using mostly transfected cell lines, Xu et al. found that stimuli that phosphorylated and activated IRF3 or transfection of cells with the constitutively active mutant IRF3 5SD inhibited TGF-β–mediated regulation of reporter genes or endogenous target genes. Studies with the human epithelial cell line HaCaT, which had constitutively high IRF3 activity, showed that knocking down IRF3 or inhibiting its activation pharmacologically enhanced Smad3-dependent gene regulation that was stimulated by autocrine TGF-β activity. Knockdown of IRF3 in HaCaT cells increased TGF-β–induced Smad3 phosphorylation, whereas transfection of cells with IRF3 5SD inhibited Smad3 phosphorylation in response to TGF-β. Coimmunoprecipitation of proteins from cells cotransfected with various combinations of tagged forms of IRF3 (wildtype or 5SD), tagged Smad3, and constitutively active TGF-β receptor indicated that activated IRF3 interacted with Smad3, but that the presence of the activated receptor reduced this interaction, consistent with activated IRF3 interacting with nonphosphorylated Smad3, which may reduce the amount of Smad3 that can reach the receptor and become activated. Exposure of HepG2 cells (a hepatoma cell line) to double-stranded poly (I:C) RNA induced the coimmunoprecipitation of endogenous IRF3 and Smad3, but application of TGF-β to poly(I:C) RNA-transfected cells did not induce the colocalization of the two proteins, as determined by immunofluorescence microscopy, consistent with TGF-β receptor activity phosphorylating Smad3 and preventing the IRF3-Smad3 interaction. Activated IRF3 and Smad3 both form transcriptional regulatory complexes with the histone acetyltransferase p300, and in cotransfected cells, increasing the abundance of IRF3 5SD, but not wild-type IRF3, reduced the coimmunoprecipitation of tagged Smad3 with tagged p300 while increasing the coimmunoprecipitation of IRF3 S5D with tagged p300. Application of TGF-β to poly(I:C) RNA-transfected cells resulted in a decrease in the interaction of endogenous Smad3 with p300 and an increase in the interaction of p300 with IRF3, suggesting that activated IRF3 may compete with active Smad3 in the nucleus for binding to p300. Experiments with epithelial cell lines indicated that active IRF3 inhibited the TGF-β–stimulated epithelial-to-mesenchymal transition. Experiments with T cells showed that Sendai virus infection before exposure to TGF-β and interleukin-2 inhibited the induction of markers indicating differentiation of the cells into inhibitory T regulatory cells (iTregs) in a manner dependent on IRF3. These data indicated that the innate immune response to viral infection inhibits TGF-β signaling through competition between active IRF3 and the TGF-β receptor for inactive Smad3 and through competition between active Smad3 and active IRF3 for coregulators, such as p300.

This pair of studies demonstrates the complex nature of the interaction between the host and the pathogen.

N. Li, A. Ren, X. Wang, X. Fan, Y. Zhao, G. F. Gao, P. Cleary, B. Wang. Influenza viral neuraminidase primes bacterial coinfection through TGF-β–mediated expression of host cell receptors. Proc. Natl. Acad. Sci. U. S. A. 112, 238–243 (2015). [Abstract] [Full Text]

P. Xu, S. Bailey-Bucktrout, Y. Xi, D. Xu, D. Du, Q. Zhang, W. Xiang, J. Liu, A. Melton, D. Sheppard, H. A. Chapman, J. A. Bluestone, R. Derynck, Innate antiviral host defense attenuates TGF-β function through IRF3-mediated suppression of Smad signaling. Mol. Cell 56, 723–737 (2015). [PubMed]