Proteasome-mediated degradation of RNase L in response to phorbol-12-myristate-13-acetate (PMA) treatment of mouse L929 cells

Negative Regulation of the Innate Immune Response through Proteasomal Degradation and Deubiquitination

The rapid and dynamic activation of the innate immune system is achieved through complex signaling networks regulated by post-translational modifications modulating the subcellular localization, activity, and abundance of signaling molecules. Many constitutively expressed signaling molecules are present in the cell in inactive forms, and become functionally activated once they are modified with ubiquitin, and, in turn, inactivated by removal of the same post-translational mark. Moreover, upon infection resolution a rapid remodeling of the proteome needs to occur, ensuring the removal of induced response proteins to prevent hyperactivation. This review discusses the current knowledge on the negative regulation of innate immune signaling pathways by deubiquitinating enzymes, and through degradative ubiquitination. It focusses on spatiotemporal regulation of deubiquitinase and E3 ligase activities, mechanisms for re-establishing proteostasis, and degradation through immune-specific feedback mechanisms vs. general protein quality control pathways.

The Roles of RNase-L in Antimicrobial Immunity and the Cytoskeleton-Associated Innate Response

The interferon (IFN)-regulated endoribonuclease RNase-L is involved in multiple aspects of the antimicrobial innate immune response. It is the terminal component of an RNA cleavage pathway in which dsRNA induces the production of RNase-L-activating 2-5A by the 2′-5′-oligoadenylate synthetase. The active nuclease then cleaves ssRNAs, both cellular and viral, leading to downregulation of their expression and the generation of small RNAs capable of activating retinoic acid-inducible gene-I (RIG-I)-like receptors or the nucleotide-binding oligomerization domain-like receptor 3 (NLRP3) inflammasome. This leads to IFNβ expression and IL-1β activation respectively, in addition to broader effects on immune cell function. RNase-L is also one of a growing number of innate immune components that interact with the cell cytoskeleton. It can bind to several cytoskeletal proteins, including filamin A, an actin-binding protein that collaborates with RNase-L to maintain the cellular barrier to viral entry. This antiviral activity is independent of catalytic function, a unique mechanism for RNase-L. We also describe here the interaction of RNase-L with the E3 ubiquitin ligase and scaffolding protein, ligand of nump protein X (LNX), a regulator of tight junction proteins. In order to better understand the significance and context of these novel binding partners in the antimicrobial response, other innate immune protein interactions with the cytoskeleton are also discussed.

RNase-L control of cellular mRNAs: roles in biologic functions and mechanisms of substrate targeting

RNase-L is a mediator of type 1 interferon-induced antiviral activity that has diverse and critical cellular roles, including the regulation of cell proliferation, differentiation, senescence and apoptosis, tumorigenesis, and the control of the innate immune response. Although RNase-L was originally shown to mediate the endonucleolytic cleavage of both viral and ribosomal RNAs in response to infection, more recent evidence indicates that RNase-L also functions in the regulation of cellular mRNAs as an important mechanism by which it exerts its diverse biological functions. Despite this growing body of work, many questions remain regarding the roles of mRNAs as RNase-L substrates. This review will survey known and putative mRNA substrates of RNase-L, propose mechanisms by which it may selectively cleave these transcripts, and postulate future clinical applications.

Pathologic effects of RNase-L dysregulation in immunity and proliferative control

The endoribonuclease RNase-L is the terminal component of an RNA cleavage pathway that mediates antiviral, antiproliferative and immunomodulatory activities. Inactivation or dysregulation of RNase-L is associated with a compromised immune response and increased risk of cancer, accordingly its activity is tightly controlled and requires an allosteric activator, 2',5'-linked oligoadenylates, for enzymatic activity. The biological activities of RNase-L are a result of direct and indirect effects of RNA cleavage and microarray analyses have revealed that RNase-L impacts the gene expression program at multiple levels. The identification of RNase-L-regulated RNAs has provided insights into potential mechanisms by which it exerts antiproliferative, proapoptotic, senescence-inducing and innate immune activities. RNase-L protein interactors have been identified that serve regulatory functions and are implicated as alternate mechanisms of its biologic functions. Thus, while the molecular details are understood for only a subset of RNase-L activities, its regulation by small molecules and critical roles in host defense and as a candidate tumor suppressor make it a promising therapeutic target.

Ribosomal protein mRNAs are primary targets of regulation in RNase-L-induced senescence

The endoribonuclease RNase-L requires 2',5'-linked oligoadenylates for activation, and mediates antiviral and antiproliferative activities. We previously determined that RNase-L activation induces senescence; to determine potential mechanisms underlying this activity, we used microarrays to identify RNase-L-regulated mRNAs. RNase-L activation affected affected a finite number of transcripts, and thus does not lead to a global change in mRNA turnover. The largest classes of downregulated transcripts, that represent candidate RNase-L substrates, function in protein biosynthesis, metabolism and proliferation. Among these, mRNAs encoding ribosomal proteins (RPs) were particularly enriched. The reduced levels of four RP mRNAs corresponded with a decrease in their half lives and a physical association with an RNase-L-ribonucleoprotein (RNP) complex in cells, suggesting that they represent authentic RNase-L substrates. Sequence and structural analysis of the downregulated mRNAs identified a putative RNase-L target motif that was used for the in silico identification of a novel RNase-L-RNP-interacting transcript. The downregulation of RP mRNAs corresponded with a marked reduction in protein translation, consistent with the roles of RP proteins in ribosome function. Our data support a model in which the RNase-L-mediated degradation of RP mRNAs inhibits translation, and may contribute to its antiproliferative, senescence inducing and tumor suppressor activities.

RNase L: its biological roles and regulation

2'-5'oligoadenylate-dependent ribonuclease L (RNase L) is one of the key enzymes involved in the function of interferons (IFNs), a family of cytokines participating in innate immunity against viruses and other microbial pathogens. Upon binding with its activator, 5'-phosphorylated, 2'-5' linked oligoadenylates (2-5A), RNase L degrades single-stranded viral and cellular RNAs and thus plays an important role in the antiviral and antiproliferative functions of IFNs. In recent years, evidence has revealed that RNase L displays a broad range of biological roles which are summarized in this review.

Interplay between protein synthesis and degradation in the CNS: physiological and pathological implications

Compromise of the ubiquitin-proteasome system (UPS) is a potential basis for multiple physiological abnormalities and pathologies in the CNS. This could be because reduced protein turnover leads to bulk intracellular protein accumulation. However, conditions associated with compromised UPS function are also associated with impairments in protein synthesis, and impairment of UPS function is sufficient to inhibit protein synthesis. These data suggest that the toxicity of UPS inhibition need not depend on gross intracellular protein accumulation, and indicate the potential for crosstalk between the UPS and protein-synthesis pathways. In this review, we discuss evidence for interplay between the UPS and protein-synthesis machinery, and outline the implications of this crosstalk for physiological and pathological processes in the CNS.

Role of 2-5A-dependent RNase-L in senescence and longevity

Senescence is a permanent growth arrest that restricts the lifespan of primary cells in culture, and represents an in vitro model for aging. Senescence functions as a tumor suppressor mechanism that can be induced independent of replicative crisis by diverse stress stimuli. RNase-L mediates antiproliferative activities and functions as a tumor suppressor in prostate cancer, therefore, we examined a role for RNase-L in cellular senescence and aging. Ectopic expression of RNase-L induced a senescent morphology, a decrease in DNA synthesis, an increase in senescence-associated beta-galactosidase activity, and accelerated replicative senescence. In contrast, senescence was retarded in RNase-L-null fibroblasts compared with wild-type fibroblasts. Activation of endogenous RNase-L by 2-5A transfection induced distinct senescent and apoptotic responses in parental and Simian virus 40-transformed WI38 fibroblasts, respectively, demonstrating cell type specific differences in the antiproliferative response to RNase-L activation. Replicative senescence is a model for in vivo aging; therefore, genetic disruption of senescence effectors may impact lifespan. RNase-L-/- mice survived 31.7% (P<0.0001) longer than strain-matched RNase-L+/+ mice providing evidence for a physiological role for RNase-L in aging. These findings identify a novel role for RNase-L in senescence that may contribute to its tumor suppressive function and to the enhanced longevity of RNase-L-/- mice.

Double-stranded RNA-dependent protein kinase (PKR) is downregulated by phorbol ester

The double-stranded RNA-dependent protein kinase (PKR) is one of the key mediators of interferon (IFN) action against certain viruses. PKR also plays an important role in signal transduction and immunomodulation. Understanding the regulation of PKR activity is important for the use of PKR as a tool to discover and develop novel therapeutics for viral infections, cancer and immune dysfunction. We found that phorbol 12-myristate 13-acetate (PMA), a potent activator of protein kinase C (PKC), decreased the level of autophosphorylated PKR in a dose- and time-dependent manner in IFN-treated mouse fibroblast cells. Polyinosinic-polycytidylic acid (poly I:C) treatment enhanced the activity of PKR induced by IFN, but did not overcome the PMA-induced reduction of PKR autophosphorylation. Western blot analysis with a monoclonal antibody to mouse PKR revealed that the decrease of PKR autophosphorylation in cells by PMA was a result of PKR protein degradation. Selective PKC inhibitors blocked the degradation of PKR stimulated by PMA, indicating that PKC activity was required for the effect. Furthermore, we also found that proteasome inhibitors prevented PMA-induced down regulation of PKR, indicating that an active proteasome is required. Our results identify a novel mechanism for the post-translational regulation of PKR.