Type I interferons (IFNs) are critical for the antiviral immune response, and fine-tuning type I IFN production is critical to effectively clearing viruses without causing harmful immunopathology. We showed that the transcription factor Miz1 epigenetically repressed the expression of genes encoding type I IFNs in mouse lung epithelial cells by recruiting histone deacetylase 1 (HDAC1) to the promoters of Ifna and Ifnb. Loss of function of Miz1 resulted in augmented production of these type I IFNs during influenza A virus (IAV) infection, leading to improved viral clearance in vitro and in vivo. IAV infection induced Miz1 accumulation by promoting the cullin-4B (CUL4B)-mediated ubiquitylation and degradation of the E3 ubiquitin ligase Mule (Mcl-1 ubiquitin ligase E3; also known as Huwe1 or Arf-BP1), which targets Miz1 for degradation. As a result, Miz1 accumulation limited type I IFN production and favored viral replication. This study reveals a previously unrecognized function of Miz1 in regulating antiviral defense and a potential mechanism for influenza viruses to evade host immune defense.

The transcription factor Miz1 negatively regulates TNF-induced JNK activation and cell death by suppressing TRAF2 K63-polyubiquitination; upon TNF stimulation, the suppression is relieved by Mule/ARF-BP1-mediated Miz1 ubiquitination and subsequent degradation. It is not known how Mule is activated by TNF. Here we report that TNF activates Mule by inducing the dissociation of Mule from its inhibitor ARF. ARF binds to and thereby inhibits the E3 ligase activity of Mule in the steady state. TNF induces tyrosine phosphorylation of Mule, which subsequently dissociates from ARF and becomes activated. Inhibition of Mule phosphorylation by silencing of the Spleen Tyrosine Kinase (Syk) prevents its dissociation from ARF, thereby inhibiting Mule E3 ligase activity and TNF-induced JNK activation and cell death. Our data provides a missing link in TNF signaling pathway that leads to JNK activation and cell death.

The ubiquitin E3 ligase MDM2 is best known for its ability to suppress the tumor suppressor p53. However, MDM2 also targets other proteins for proteasomal degradation and accumulating evidence strongly suggests p53-independent roles of MDM2 in cancer. We previously reported that MDM2 promotes degradation of another ubiquitin E3 ligase HUWE1 by ubiquitination, particularly, which confers HER2+ breast cancer cells resistance to the HER2 inhibitor lapatinib. However, it remains unclear whether such a mechanism can operate in other cell types, independently of HER2 inhibitors. Moreover, in vivo evidence that supports HUWE1 degradation by MDM2 is missing. In the current study, we performed immunohistochemistry (IHC) to analyze expression levels of MDM2 and HUWE1 in normal organs, two breast cancer cohorts (A, n = 137 and B, n = 27), and a liposarcoma cohort (n = 45). Our results show that HUWE1 is ubiquitously expressed in healthy organs, where the oncoprotein MDM2 is undetectable. Likewise, in the majority of breast cancers regardless of their subtypes, MDM2 is below detectable levels, while HUWE1 is highly expressed. In contrast, in a subset of liposarcoma that is characterized by MDM2 overexpression, only 40% of these showed detectable HUWE1 protein. Importantly, despite the inverse association between MDM2 and HUWE1 protein levels, gene expression analysis in independent datasets revealed no such correlation at the mRNA level. Our results demonstrate the first in vivo evidence to support the hypothesis of MDM2-mediated HUWE1 degradation, which may help to understand the regulation of HUWE1 as well as p53-independent roles of MDM2.

The E3 ubiquitin ligase Mule/ARF-BP1 plays an important role in the cellular DNA damage response by controlling base excision repair and p53 protein levels. However, how the activity of Mule is regulated in response to DNA damage is currently unknown. Here, we report that the Ser18-containing isoform of the USP7 deubiquitylation enzyme (USP7S) controls Mule stability by preventing its self-ubiquitylation and subsequent proteasomal degradation. We find that in response to DNA damage, downregulation of USP7S leads to self-ubiquitylation and proteasomal degradation of Mule, which eventually leads to p53 accumulation. Cells that are unable to downregulate Mule show reduced ability to upregulate p53 levels in response to DNA damage. We also find that, as Mule inactivation is required for stabilization of base excision repair enzymes, the failure of cells to downregulate Mule after DNA damage results in deficient DNA repair. Our data describe a novel mechanism by which Mule is regulated in response to DNA damage and coordinates cellular DNA damage responses and DNA repair.