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Regulation of type I interferon: It’s HIP to be K2

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Science Signaling  19 Mar 2019:
Vol. 12, Issue 573, eaaw8549
DOI: 10.1126/scisignal.aaw8549


Uncontrolled expression of type I interferon (IFN-I) drives autoimmunity, necessitating the need for tight regulation. In this issue, Cao et al. reveal a role for the kinase HIPK2 in the transcriptional control of IFN-I during antiviral immune responses.

Type I interferons (IFN-α/β or IFN-I) are key cytokines in early antipathogen immune responses, where they can have both positive and negative roles in disease. IFN-I is generally expressed when pathogen-associated molecular patterns (PAMPs) are detected by one of several classes of cellular pattern recognition receptors (PRRs). In particular, pathogen nucleic acids are detected by intracellular signaling molecules, including the RIG-I–like receptors (RLRs), Toll-like receptors (TLRs), and dsDNA receptors such as the cGAS-STING pathway (1). Ligation of these molecules results in downstream activation of multiple transcription factors, including IFN regulatory factor-3 (IRF3), IRF7, and NF-κB, that cooperate to drive IFN-I expression (2). Secreted IFN-I then drives its own transcriptional response by binding to the heteromeric receptor complex of IFNAR1 and IFNAR2 to activate the Janus kinase–signal transducer and activator of transcription (JAK-STAT) pathway, which culminates in the expression of hundreds of genes. These so-called IFN-stimulated genes are responsible for the myriad of biological functions of IFN-I in pathogen defense, including direct intracellular restriction of pathogen replication, initiation of inflammatory responses, and activation of innate and adaptive cellular immune responses (3). While induced IFN-I expression is essential for recovery from virus infection in humans, constitutive expression of IFN-I is associated with severe forms of autoimmunity (4). Thus, understanding the regulation of IFN-I expression is important for determining how IFN-I functions in health and disease. In this issue of Science Signaling, Cao et al. (5) identify the kinase HIPK2 as functioning in the nucleus to regulate expression of IFN-I through the phosphorylation of transcription factor ELF4.

It has long been recognized that IFN-I expression is driven by coordinated activation of the transcription factors IRF3, IRF7, NF-κB, and AP-1 (2). The activity of each transcription factor must be induced from a latent state in the cytosol by the activity of upstream kinases. For example, the kinases TBK1 and IKKε phosphorylate IRF3 and IRF7 to promote IRF homodimerization, nuclear translocation, and DNA binding functions. Similarly, the IKKα/β kinases phosphorylate inhibitor of κB (IκB) to induce its degradation and relieve inhibition of NF-κB, thus allowing NF-κB to translocate to the nucleus and bind specific promoter sequences of target genes. In 2013, You et al. (6) identified an additional transcription factor, ETS-related transcription factor 4 (ELF4), that is critical for IFN-I gene expression after stimulation of various PRRs, including RLRs, TLRs, and cGAS-STING. In the cytosol, ELF4 is phosphorylated by TBK1 to induce its nuclear translocation and binding of specific DNA sequences, and this association increases the binding affinity of IRF3, IRF7, and NF-κB to enhancer elements within IFN-I promoter sequences (6). Thus, ELF4 may be an interesting therapeutic target in the modulation of IFN-I expression.

Cao et al. (5) have now identified an additional layer of ELF4 regulation after kinase activation in the nucleus. Vesicular stomatitis virus (VSV) is often used as a classic model of IFN-I induction by RNA viruses after ligation of RLR signaling. Examination of differential gene expression in wild-type and ELF4-deficient macrophages infected with VSV revealed ELF4-dependent expression of HIPK2, prompting the authors to examine the role of HIPK2 in VSV-induced IFN-I responses. Loss of HIPK2 in primary macrophages (HIPK2−/−) or in mice heterozygous for HIPK2 (HIPK2+/−) resulted in significantly lower IFN-I expression in response to infection with RNA viruses that stimulate RLR signaling, such as VSV and West Nile virus, while chemokine responses were unaffected. This lower IFN-I expression was associated with higher virus burden, suggesting that HIPK2 contributes directly to the antiviral IFN-I response. HIPK2 bound to ELF4. HIPK2-augmented IFN-I gene expression was dependent on both ELF4 interaction with its specific enhancer sequence in the IFN-I promoter and on IRF3/IRF7, suggesting that the role of HIPK2 was to promote ELF4 activity. Furthermore, the function of HIPK2 was through its kinase activity resulting in phosphorylation of ELF4 on residue Ser369, a site distinct from that targeted by TBK1. In strong support of this finding, mutation of Ser369 within ELF4 abrogated its ability to induce gene expression from IFN-I promoter sequences. Interestingly, HIPK2 is present in the cytosol in an autoinhibited state and is activated through cleavage by caspase-6 to remove the autoinhibitory domain (7). Cao et al. found that HIPK2 was cleaved by caspases after VSV infection and that a caspase-insensitive mutant of HIPK2 was unable to promote IFN-I expression. Moreover, deletion of the nuclear localization sequence (NLS) within HIPK2 not only prevented HIPK2 from moving to the nucleus after VSV infection but also prevented its cleavage, suggesting that caspase-mediated cleavage of HIPK2 occurred in the nucleus and not in the cytosol. This set of observations suggests a two-step model of HIPK2 activation after PRR activation, wherein HIPK2 is first induced to translocate to the nucleus, where it is activated by caspase-dependent cleavage to enable it to phosphorylate ELF4, also in the nucleus (Fig. 1).

Fig. 1 HIPK2 regulation of ELF4-dependent type I IFN.

RNA virus infection is detected through the RLRs, RIG-I and MDA5, which then interact with MAVS on mitochondria. This results in amplification of the signaling cascade, including the activation of the kinase TBK1 and downstream transcription factors IRF7, IRF3, and ELF4, among others. Cao et al. (5) demonstrate that the kinase HIPK2 is also induced after MAVS signaling to translocate to the nucleus, where it is cleaved by caspase-6, enabling HIPK2 to phosphorylate ELF4 and promote gene expression of multiple IFN-Is.


This work has revealed new levels of control of IFN-I gene expression by implicating both HIPK2 and caspase-6 as regulators of the antiviral response. The importance of specific antiviral responses is underscored by viral strategies to counter them (8). Cao et al. (5) demonstrated that virally encoded proteins known to interact with HIPK2 can suppress its role in driving IFN-I expression, including the US11 protein from herpes simplex virus 1 (HSV-1) or the E6 protein from human papillomavirus (HPV), both DNA viruses known to be sensed through the cGAS-STING pathway. Indeed, Cao et al. (5) also found that HIPK2 functions after cGAS-STING signaling, in addition to the RLRs. These observations support the conclusion that HIPK2 is likely broadly involved in antiviral IFN-I responses to both RNA and DNA viruses and further suggest that HIPK2 may be antagonized as part of specific virus IFN evasion strategies to promote virus replication or persistence. This work also raises multiple new questions. Specifically, how is HIPK2 activated to translocate from the cytosol to the nucleus in response to PRR signaling—does this occur via TBK1 phosphorylation simultaneously with ELF4 or is it achieved through a separate mechanism? In addition, what is the mechanism for caspase activation in this pathway? Although active caspase-6 is known to accumulate in the nucleus, how is this new putative function in IFN-I gene expression balanced with its known proapoptotic roles? Last, and perhaps most importantly, this work identifies two new targets, HIPK2 and caspase-6, that may be therapeutically amenable to manipulation, as they are both enzymes. A major challenge will be to test whether HIPK2 or caspase-6 can be inhibited to dampen IFN-I expression in the context of autoimmunity.


Acknowledgments: This work was supported by the Division of Intramural Research (DIR), National Institutes of Allergy and Infectious Diseases (NIAID), NIH.
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