The connection between viral infections and metabolic dysfunction is an important clinical problem, yet the mechanisms linking these events are not understood. In this paper we provide in vivo evidence for a novel pathway linking viral infection to metabolic disease. We have shown that activation of IRF3 during the viral immune response leads to a profound suppression of RXRα mRNA and protein expression. Because RXRα serves as an obligatory heterodimeric partner for several nuclear receptors involved in metabolic control, these observations provide a molecular explanation for how viral infections can alter a range of metabolic pathways. As a consequence of RXRα suppression during viral infection, the expression of multiple downstream nuclear receptor target genes is compromised, including those required for liver detoxification of endogenous and exogenous compounds and those required for lipid metabolism. Moreover, the ability of viral infections to repress nuclear receptor function leads to hepatotoxicity in the context of endogenous toxins such as LCA and exogenous compounds such as ASA. These data provide a molecular mechanism to explain how viral infections may interfere with liver homeostasis and contribute to the pathogenesis of metabolic disease ().
Figure 8. Model of IRF3 nuclear receptor cross talk and biological consequence. Activation of IRF3 through pattern recognition receptors (PRRs) results in the induction of antiviral genes through type I IFNs or the repression of RXRα target genes through (more ...)
The clinical relevance of IRF3-mediated inhibition of liver metabolism is illustrated by its potential role in the pathogenesis of hepatic metabolic disorders that involve xenobiotics (drugs and chemicals) ingested during viral infections. One such disorder, Reye's syndrome, has yet to be explained mechanistically. It is known that ASA therapy during a viral infection in children can lead to fatty degeneration of the liver and encephalopathy (2
). Not specific to any virus in particular, Reye's syndrome is associated with chickenpox, influenza A or B, adenoviruses, hepatitis A viruses, paramyxovirus, picornaviruses, reoviruses, herpesviruses, measles, and varicella-zoster viruses (49
). Previous experiments have suggested that hepatotoxicity in Reye's syndrome results from a toxic combination of ASA metabolites and inflammatory cytokines generated in response to a viral infection (63
). It has also been shown that polyI:C can inhibit the metabolism of aspirin, and this has been suggested to occur through type I IFNs (52
). Our experimental model of polyI:C/VSV and ASA treatment, however, clearly demonstrates that hepatotoxicity and fatty degeneration occurs in an IRF3-dependent, type I IFN–independent manner, consistent with those seen during Reye's syndrome. Furthermore, it appears that this pathogenesis arises from IRF3 repression of RXRα and its hepatic target genes involved in ASA metabolism. We showed that this repression of RXRα blocks ASA and PCN induction of UGT1A6
, RXR heterodimer target genes involved in ASA metabolism, and results in increased mitochondrial damage by ASA, a known contributing factor to the pathogenesis of Reye's syndrome (56
). Our results therefore provide compelling evidence for the involvement of IRF3–nuclear receptor cross talk in the development of Reye's syndrome and suggest new therapeutic strategies for the prevention of hepatotoxicity associated with viral infections.
Our results also demonstrate that viral infections can alter the clearance of endogenous toxins that accumulate during normal metabolism. LCA, a secondary bile acid produced by intestinal bacteria, is metabolized by RXR heterodimers through the induction of cytochrome P450 family members such as CYP3A11, which catalyze the initial hydroxylation of LCA (67
). Mice deficient in hepatocyte PXR or RXRα exhibit functional defects in the expression of LCA metabolic genes (18
). Excess amounts of LCA disturb liver homeostasis and result in cholestasis, which can be alleviated by the activation of PXR/RXR with less toxic but more potent nuclear receptor agonists such as PCN (18
). In this work, we have shown that activation of IRF3 during viral infection inhibits PXR/RXR-dependent activation of CYP3A11
. Consequently, viral infections render mice highly susceptible to LCA-mediated cholestasis and hepatotoxicity. Interestingly, this mechanism may be relevant to viral-induced cholestasis in humans, as Epstein-Barr virus infections have been linked to cholestasis (68
). The molecular pathways elucidated in our study will likely provide a useful framework for further investigation into this connection.
IRF3 is a transcription factor best known for its function in type I IFN production during the innate immune response against viral infections. Our experiments have identified a new function for virally activated IRF3, repression of RXRα
, that is independent of the type I IFN pathway. We have shown that activation of IRF3 induces expression of the transcriptional repressor Hes1, which binds directly to the proximal promoter of RXRα
and recruits HDAC1 to repress transcription. Nevertheless, RXRα protein levels remain relatively stable in the absence of a nuclear receptor–activating signal. However, in combination with 26S proteosome complex activation by nuclear agonists (ASA, PCN, LG268, and GW3965), this pathway results in a biologically important loss of RXRα protein that would not be seen in the absence of IRF3 activation, where RXRα protein levels are replenished as new transcript is continually made. Although the repression of other nuclear receptors may contribute to our observed phenomenoms, mutation of RXRα
in hepatocytes results in similar in vivo defects in PXR/RXR target gene induction and increased LCA sensitivity, as seen in our experiments with polyI:C and VSV, providing further evidence that IRF3-mediated down-regulation of RXRα could contribute substantially to the pathogenesis of hepatic metabolic diseases (41
). Previous work has shown that nuclear receptor activation can inhibit IRF3 target genes (39
). It is possible that the down- regulation of RXRα may relieve this inhibitory effect and allow for optimal induction of IRF3 target genes involved in antiviral response. However, it is not clear whether this RXRα down-regulation will be beneficial overall or harmful to the host during a microbial infection.
The central role of RXRα in nuclear receptor signaling raises the possibility that IRF3–nuclear receptor cross talk may have implications for a variety of pathways and metabolic functions. The particular importance of the RXRα isoform is clear in that RXRα-deficient mice are embryonic lethal (19
). Furthermore, several tissue-specific RXRα-deficient mice have been described that point to diverse functions for this receptor (26
). Loss of RXRα has been demonstrated in our work and by others to inhibit some, but not all, RXR heterodimer target genes, suggesting that other factors may play overlapping roles in determining activation and maintenance of certain nuclear receptor target genes (31
). However, it is clear from our work and these genetic studies of RXRα that loss of RXRα would affect several nuclear receptor pathways. Thus, in addition to contributing to the pathogenesis of Reye's syndrome, IRF3 repression of RXRα may contribute to other diseases associated with viral infections. One such disease is atherosclerosis, where IRF3 activation contributes to negative regulation of LXR-related genes and cholesterol efflux (31
). It will be interesting to explore whether IRF3-dependent down-regulation of RXRα influences disorders such as Gianotti-Crosti syndrome in the skin (6
) and viral-linked diabetes (5
). IRF3–nuclear receptor cross talk provides a new understanding of the link between microbial infection and metabolic dysfunction and suggests novel targets for therapeutic intervention in these syndromes.