Jianghuai Liu

CRD-BP mediates stabilization of βTrCP1 and c- myc mRNA in response to β-catenin signalling

Although constitutive activation of β-catenin/Tcf signalling is implicated in the development of human cancers, the mechanisms by which the β-catenin/Tcf pathway promotes tumorigenesis are incompletely understood. Messenger RNA turnover has a major function in regulating gene expression and is responsive to developmental and environmental signals. mRNA decay rates are dictated by cis-acting elements within the mRNA and by trans-acting factors, such as RNA-binding proteins (reviewed in refs 2, 3). Here we show that β-catenin stabilizes the mRNA encoding the F-box protein βTrCP1, and identify the RNA-binding protein CRD-BP (coding region determinant-binding protein) as a previously unknown target of β-catenin/Tcf transcription factor. CRD-BP binds to the coding region of βTrCP1 mRNA. Overexpression of CRD-BP stabilizes βTrCP1 mRNA and elevates βTrCP1 levels (both in cells and in vivo), resulting in the activation of the Skp1-Cullin1-F-box protein (SCF)βTrCP E3 ubiquitin ligase and in accelerated turnover of its substrates including IκB and β-catenin. CRD-BP is essential for the induction of both βTrCP1 and c-Myc by β-catenin signalling in colorectal cancer cells. High levels of CRD-BP that are found in primary human colorectal tumours exhibiting active β-catenin/Tcf signalling implicates CRD-BP induction in the upregulation of βTrCP1, in the activation of dimeric transcription factor NF-κB and in the suppression of apoptosis in these cancers.

Site-specific ubiquitination exposes a linear motif to promote interferon-α receptor endocytosis

Ligand-induced endocytosis and lysosomal degradation of cognate receptors regulate the extent of cell signaling. Along with linear endocytic motifs that recruit the adaptin protein complex 2 (AP2)–clathrin molecules, monoubiquitination of receptors has emerged as a major endocytic signal. By investigating ubiquitin-dependent lysosomal degradation of the interferon (IFN)-α/β receptor 1 (IFNAR1) subunit of the type I IFN receptor, we reveal that IFNAR1 is polyubiquitinated via both Lys48- and Lys63-linked chains. The SCFβTrcp (Skp1–Cullin1–F-box complex) E3 ubiquitin ligase that mediates IFNAR1 ubiquitination and degradation in cells can conjugate both types of chains in vitro. Although either polyubiquitin linkage suffices for postinternalization sorting, both types of chains are necessary but not sufficient for robust IFNAR1 turnover and internalization. These processes also depend on the proximity of ubiquitin-acceptor lysines to a linear endocytic motif and on its integrity. Furthermore, ubiquitination of IFNAR1 promotes its interaction with the AP2 adaptin complex that is required for the robust internalization of IFNAR1, implicating cooperation between site-specific ubiquitination and the linear endocytic motif in regulating this process.

Virus-induced unfolded protein response attenuates antiviral defenses via phosphorylation-dependent degradation of the type I interferon receptor.

Phosphorylation-dependent ubiquitination and degradation of the IFNAR1 chain of the type I interferon (IFN) receptor is regulated by two different pathways, one of which is ligand independent. We report that this ligand-independent pathway is activated by inducers of unfolded protein responses (UPR), including viral infection, and that such activation requires the endoplasmic reticulum-resident protein kinase PERK. Upon viral infection, activation of this pathway promotes phosphorylation-dependent ubiquitination and degradation of IFNAR1, specifically inhibiting type I IFN signaling and antiviral defenses. Knockin of an IFNAR1 mutant insensitive to virus-induced turnover or conditional knockout of PERK prevented IFNAR1 degradation, whether UPR-induced or virus-induced, and restored cellular responses to type I IFN and resistance to viruses. These data suggest that specific activation of the PERK component of UPR can favor viral replication. Interfering with PERK-dependent IFNAR1 degradation could therefore contribute to therapeutic strategies against viral infections.

Mammalian casein kinase 1α and its leishmanial ortholog regulate stability of IFNAR1 and type I interferon signaling

ABSTRACT Phosphorylation of the degron of the IFNAR1 chain of the type I interferon (IFN) receptor triggers ubiquitination and degradation of this receptor and, therefore, plays a crucial role in negative regulation of IFN-α/β signaling. Besides the IFN-stimulated and Jak activity-dependent pathways, a basal ligand-independent phosphorylation of IFNAR1 has been described and implicated in downregulating IFNAR1 in response to virus-induced endoplasmic reticulum (ER) stress. Here we report purification and characterization of casein kinase 1α (CK1α) as a bona fide major IFNAR1 kinase that confers basal turnover of IFNAR1 and cooperates with ER stress stimuli to mediate phosphorylation-dependent degradation of IFNAR1. Activity of CK1α was required for phosphorylation and downregulation of IFNAR1 in response to ER stress and viral infection. While many forms of CK1 were capable of phosphorylating IFNAR1 in vitro, human CK1α and L-CK1 produced by the protozoan Leishmania major were also capable of increasing IFNAR1 degron phosphorylation in cells. Expression of leishmania CK1 in mammalian cells stimulated the phosphorylation-dependent downregulation of IFNAR1 and attenuated its signaling. Infection of mammalian cells with L. major modestly decreased IFNAR1 levels and attenuated cellular responses to IFN-α in vitro. We propose a role for mammalian and parasite CK1 enzymes in regulating IFNAR1 stability and type I IFN signaling.

Ligand-independent pathway that controls stability of interferon alpha receptor.

Ligand-specific negative regulation of cytokine-induced signaling relies on down regulation of the cytokine receptors. Down regulation of the IFNAR1 sub-unit of the Type I interferon (IFN) receptor proceeds via lysosomal receptor proteolysis, which is triggered by ubiquitination that depends on IFNAR1 serine phosphorylation. While IFN-inducible phosphorylation, ubiquitination, and degradation requires the catalytic activity of the Tyk2 Janus kinase, here we found the ligand- and Tyk2-independent pathway that promotes IFNAR1 phosphorylation, ubiquitination, and degradation when IFNAR1 is expressed at high levels. A major cellular kinase activity that is responsible for IFNAR1 phosphorylation in vitro does not depend on either ligand or Tyk2 activity. Inhibition of ligand-independent IFNAR1 degradation suppresses cell proliferation. We discuss the signaling events that might lead to ubiquitination and degradation of IFNAR1 via ligand-dependent and independent pathways and their potential physiologic significance.

Inducible Priming Phosphorylation Promotes Ligand-independent Degradation of the IFNAR1 Chain of Type I Interferon Receptor

Phosphorylation-dependent ubiquitination and ensuing down-regulation and lysosomal degradation of the interferon α/β receptor chain 1 (IFNAR1) of the receptor for Type I interferons play important roles in limiting the cellular responses to these cytokines. These events could be stimulated either by the ligands (in a Janus kinase-dependent manner) or by unfolded protein response (UPR) inducers including viral infection (in a manner dependent on the activity of pancreatic endoplasmic reticulum kinase). Both ligand-dependent and -independent pathways converge on phosphorylation of Ser535 within the IFNAR1 degron leading to recruitment of β-Trcp E3 ubiquitin ligase and concomitant ubiquitination and degradation. Casein kinase 1α (CK1α) was shown to directly phosphorylate Ser535 within the ligand-independent pathway. Yet given the constitutive activity of CK1α, it remained unclear how this pathway is stimulated by UPR. Here we report that induction of UPR promotes the phosphorylation of a proximal residue, Ser532, in a pancreatic endoplasmic reticulum kinase-dependent manner. This serine serves as a priming site that promotes subsequent phosphorylation of IFNAR1 within its degron by CK1α. These events play an important role in regulating ubiquitination and degradation of IFNAR1 as well as the extent of Type I interferon signaling.

Inflammatory signaling compromises cell responses to interferon alpha

Interferon alpha (IFNα) is widely used for treatment of melanoma and certain other malignancies. This cytokine as well as the related IFNβ exerts potent anti-tumorigenic effects; however, their efficacy in patients is often suboptimal. Here, we report that inflammatory signaling impedes the effects of IFNα/β. Melanoma cells can secrete pro-inflammatory cytokines that inhibit cellular responses to IFNα/β via activating the ligand-independent pathway for the phosphorylation and subsequent ubiquitination and accelerated degradation of the IFNAR1 chain of type I IFN receptor. Catalytic activity of the p38 protein kinase was required for IFNAR1 downregulation and inhibition of IFNα/β signaling induced by proinflammatory cytokines such as interleukin 1 (IL-1). Activation of p38 kinase inversely correlated with protein levels of IFNAR1 in clinical melanoma specimens. Inhibition of p38 kinase augmented the inhibitory effects of IFNα/β on cell viability and growth in vitro and in vivo. The roles of inflammation and p38 protein kinase in regulating cellular responses to IFNα/β in normal and tumor cells are discussed.

Pathogen recognition receptor signaling accelerates phosphorylation-dependent degradation of IFNAR1.

An ability to sense pathogens by a number of specialized cell types including the dendritic cells plays a central role in hosts defenses. Activation of these cells through the stimulation of the pathogen-recognition receptors induces the production of a number of cytokines including Type I interferons (IFNs) that mediate the diverse mechanisms of innate immunity. Type I IFNs interact with the Type I IFN receptor, composed of IFNAR1 and IFNAR2 chains, to mount the host defense responses. However, at the same time, Type I IFNs elicit potent anti-proliferative and pro-apoptotic effects that could be detrimental for IFN-producing cells. Here, we report that the activation of p38 kinase in response to pathogen-recognition receptors stimulation results in a series of phosphorylation events within the IFNAR1 chain of the Type I IFN receptor. This phosphorylation promotes IFNAR1 ubiquitination and accelerates the proteolytic turnover of this receptor leading to an attenuation of Type I IFN signaling and the protection of activated dendritic cells from the cytotoxic effects of autocrine or paracrine Type I IFN. In this paper we discuss a potential role of this mechanism in regulating the processes of innate immunity.

Cigarette smoking products suppress anti-viral effects of Type I interferon via phosphorylation-dependent downregulation of its receptor

While negative effect of smoking on the resistance to viral infections was known, the underlying mechanisms remained unclear. Here we report that products of cigarette smoking compromise the cellular anti‐viral defenses by inhibiting the signaling induced by Type I interferon (IFN). Cigarette smoking condensate (but not pure nicotine) stimulated specific serine phosphorylation‐dependent ubiquitination and degradation of the IFNAR1 subunit of the Type I IFN receptor leading to attenuation of IFN signaling and decreased resistance to viral infection. This resistance was restored in cells where phosphorylation‐dependent degradation of IFNAR1 is abolished. We conclude that smoking compromises cellular anti‐viral defenses via degradation of Type I IFN receptor and discuss the significance of this mechanism for efficacy of IFN‐based therapies.

Raf inhibitor stabilizes receptor for the type I interferon but inhibits its anti-proliferative effects in human malignant melanoma cells.

Interferon alpha (IFNα) is widely used in treatment of malignant melanoma patients. This cytokine acts on cells by engaging Type I IFN receptor consisting of two subunits, (IFNAR1 and IFNAR2) followed by activation of Janus kinases (Jak). Levels of IFNAR1 (regulated via degradation mediated by the βTrcp E3 ubiquitin ligase) and IFNα signaling were reduced in 1205Lu melanoma cell line that harbors activated BRAF and exhibits high levels of βTrcp ubiquitin ligase. Expression of stabilized IFNAR1 in melanoma cells decreased their tumorigenicity. Furthermore, RNAi-mediated BRAF knockdown and pharmacologic inhibition of either Raf or MEK1 decreased levels of βTrcp and stabilized IFNAR1. However, despite causing stabilization of IFNAR1, Raf inhibitor BAY 43-9006 interfered with cellular responses to IFNα most likely due to its ability to directly inhibit Jak activity. We discuss the implications of this result for combination therapy with BAY 43-9006 and IFNα in melanoma patients.