Antiviral interferons (IFNs) provide one of the first lines of defense against virus infections. Whether produced endogenously in response to virus or used clinically in antiviral therapies, IFNs establish an “antiviral state” by transcriptionally inducing interferon-stimulated genes (ISGs) through the Janus kinase/signal transducer and activator of transcription (JAK/STAT) pathway. The signaling events upstream of IFN induction and downstream of IFN receptor engagement are complex and subject to various levels of positive and negative regulation. For example, STAT molecules are themselves induced by IFN, enforcing a positive feedback loop. By contrast, IFN also triggers production of suppressor of cytokine signaling (SOCS)1 and SOCS3, which dampen IFN signals in a classic negative feedback loop. Thus, multiple levels of regulation exist solely within the IFN pathway. Outside of the IFN cascade, other signaling pathways have been shown to cross-talk with IFN, including interleukin-6 and IFN-γ. In addition to these, Lupberger et al. now describe the influence of an additional well-known signaling molecule, the epidermal growth factor receptor (EGFR), on antiviral activity of IFN. Using a well-characterized, clinically approved EGFR inhibitor, the researchers provide data indicating that the EGFR pathway negatively regulates IFN antiviral signaling.
EGFR is a receptor tyrosine kinase that is activated by its extracellular ligand, epidermal growth factor (EGF). Engagement of EGFR by its ligand results in receptor dimerization and autophosphorylation, which triggers downstream signaling. The EGFR-signaling pathway results in cell proliferation, and defects in the pathway have been directly linked to cancer. Indeed, inhibition of EGFR by monoclonal antibodies or targeted inhibitors, such as erlotinib, have shown efficacy in certain human cancers, such as non-small-cell lung carcinoma.
EGFR has also been implicated in participating in cellular uptake pathways of several viruses, including hepatitis C virus (HCV), influenza A virus, adeno-associated virus serotype 6, and human cytomegalovirus. The current lines of evidence from several studies suggest that EGFR is itself a receptor for certain viruses, or that EGFR signaling promotes virus internalization. Less is known about the potential for EGFR signaling to intersect with cellular antiviral pathways. Lumberger et al. present data to dissect cross-talk between these two pathways in the context of HCV infection.
The same group had previously demonstrated a role for EGFR in HCV entry. When hepatoma cells were treated with the EGFR inhibitor, erlotinib, before HCV infection, they were less permissive to the virus. This prophylactic effect of erlotinib was attributed to impaired cellular uptake of HCV as a result of diminished EGFR activity. In the current study, the researchers set out determine the effects of erlotinib on IFN potency in cells that were already infected with HCV. In this therapeutic model, they show that low doses of erlotinib synergize with IFN-α-2a and IFN-α-2b to strongly impair HCV replication.
The researchers present two mechanisms that explain the increase in IFN potency in the presence of erlotinib (Fig. 1). The first mechanism targets a SOCS3/STAT3 axis. Similar to its ability to activate STAT1 and STAT2 by phosphorylation, IFN also activates STAT3. STAT3 is a known negative regulator of IFN antiviral responses,[8, 9] and SOCS3 has been shown to down-regulate STAT3 activity. The researchers show that reducing STAT3 levels by small interfering RNA or specific drug treatment resulted in increased HCV replication, suggesting that STAT3 is important for HCV infection in cell culture. Treatment of cells with both erlotinib and IFN reduced STAT3 phosphorylation, which correlated with increased SOCS3 expression. Thus, EGFR potentially influences the IFN pathway by suppressing IFN-induced SOCS3. Erlotinib relieves the repressive effect of EGFR on SOCS3, thereby promoting STAT3 antagonism and shifting the pathway in a manner that disfavors the virus.
The second mechanism involves direct effects on STAT1. Cells treated with EGF and IFN have lower levels of STAT1 homodimerization than cells treated with IFN alone, suggesting that EGFR signaling impairs STAT1 function. Erlotinib rescued the repressive effect of EGF on IFN-induced STAT1 homodimerization, which correlated with a concomitant rescue in ISG expression levels. Thus, in this mechanism, erlotinib relieves the repressive effect of EGFR on STAT1 function, thereby promoting antiviral gene expression and shifting the pathway in a direction that favors the host. This two-pronged mechanism of erlotinib action suggests that EGFR inhibition may be a useful way to boost the antiviral efficacy of IFN.
The intersection of IFN and EGFR pathways described in this study provides novel insight into cellular signaling cross-talk. These studies also provide a starting point to go deeper into the mechanisms of EGFR antagonism of IFN pathways. For example, EGFR impairs IFN-mediated STAT3, but not STAT1, phosphorylation. However, STAT1 homodimerization is affected. Thus, EGFR signaling may affect JAK1/tyrosine kinase 2 kinase activities and/or impair release of phosphorylated STAT1 from the cytoplasmic tail of the IFN-α receptor. Interestingly, the negative regulation of IFN signaling by EGFR described here will need to be examined in the context of a recent study showing cross-talk between Toll-like receptor 3 (TLR3) and EGFR. In fibroblasts, EGFR was shown to be required for TLR3 signaling and downstream antiviral responses. Thus, EGFR may play positive and negative regulatory roles with respect to antiviral immune signaling. Additional studies will be needed to delineate the context and specificity of these newly described roles for EGFR.
This study also points to potential implications of EGFR inhibition in the clinic. The researchers suggest that erlotinib may have value in the treatment of patients chronically infected with HCV, particularly in “hard-to-treat” patient populations. If so, the putative benefits would, of course, have to be weighed in comparison to the potential adverse effects of augmented IFN responses, as well as known side effects of erlotinib in cancer patients. Moreover, given the rapidly changing landscape of therapies for HCV, it is unclear how much longer IFN will be a mainstay of HCV treatment. Indeed, a major goal in the field is to eliminate the use of IFN in lieu of more targeted drug cocktails that have fewer side effects. IFN-free regimens are already showing remarkable potential in early clinical trials, with sustained virological response rates equal to or above those observed with IFN-based therapies. As the drugs and treatments improve, particularly in hard-to-treat patient populations, IFN's future as an HCV therapy remains uncertain. Nonetheless, the broad nature of IFN action against numerous viruses suggests that increasing its potency, with readily available drugs such as erlotinib, may be a viable option in certain viral contexts.
John W. Schoggins, Ph.D.
Department of Microbiology
University of Texas Southwestern Medical Center