RNA-binding motifs of hnRNP K are critical for induction of antibody diversification by activation-induced cytidine deaminase

hnRNPs in antiviral innate immunity

During virus infection, many host proteins are redirected from their normal cellular roles to restrict and terminate infection. Heterogeneous nuclear ribonucleoproteins (hnRNPs) are cellular RNA-binding proteins critical to host nucleic acid homeostasis, but can also be involved in the viral infection process, affecting virus replication, assembly and propagation. It has become evident that hnRNPs play important roles in modulation of host innate immunity, which provides critical initial protection against infection. These novel findings can potentially lead to the leveraging of hnRNPs in antiviral therapies. We review hnRNP involvement in antiviral innate immunity, in humans, mice and other animals, and discuss hnRNP targeting as a potential novel antiviral therapeutic.

Hnrnpk protects against osteoarthritis through targeting WWC1 mRNA and inhibiting Hippo signaling pathway

Osteoarthritis (OA) is an age-related or post-traumatic degenerative whole joint disease characterized by the rupture of articular cartilage homeostasis, the regulatory mechanisms of which remain elusive. This study identifies the essential role of heterogeneous nuclear ribonucleoprotein K (hnRNPK) in maintaining articular cartilage homeostasis. Hnrnpk expression is markedly downregulated in human and mice OA cartilage. The deletion of Hnrnpk effectively accelerates the development of post-traumatic and age-dependent OA in mice. Mechanistically, the KH1 and KH2 domain of Hnrnpk bind and degrade the mRNA of WWC1. Hnrnpk deletion increases WWC1 expression, which in turn leads to the activation of Hippo signaling and ultimately aggravates OA. In particular, intra-articular injection of LPA and adeno-associated virus serotype 5 expressing WWC1 RNA interference ameliorates cartilage degeneration induced by Hnrnpk deletion, and intra-articular injection of adeno-associated virus serotype 5 expressing Hnrnpk protects against OA. Collectively, this study reveals the critical roles of Hnrnpk in inhibiting OA development through WWC1-dependent downregulation of Hippo signaling in chondrocytes and defines a potential target for the prevention and treatment of OA.

Undervalued and novel roles of heterogeneous nuclear ribonucleoproteins in autoimmune diseases: Resurgence as potential biomarkers and targets

Autoimmune diseases are mainly characterized by the abnormal autoreactivity due to the loss of tolerance to specific autoantigens, though multiple pathways associated with the homeostasis of immune responses are involved in initiating or aggravating the conditions. The heterogeneous nuclear ribonucleoproteins (hnRNPs) are a major category of RNA-binding proteins ubiquitously expressed in a multitude of cells and have attracted great attentions especially with their distinctive roles in nucleic acid metabolisms and the pathogenesis in diseases like neurodegenerative disorders and cancers. Nevertheless, the interplay between hnRNPs and autoimmune disorders has not been fully elucidated. Virtually various family members of hnRNPs are increasingly identified as immune players and are pertinent to all kinds of immune-related processes including immune system development and innate or adaptive immune responses. Specifically, hnRNPs have been extensively recognized as autoantigens within and even beyond a myriad of autoimmune diseases, yet their diagnostic and prognostic values are seemingly underestimated. Molecular mimicry, epitope spreading and bystander activation may represent major putative mechanisms underlying the presence of autoantibodies to hnRNPs. Besides, hnRNPs play critical parts in regulating linchpin genes expressions that control genetic susceptibility, disease-linked functional pathways, or immune responses by interacting with other components particularly like microRNAs and long non-coding RNAs, thereby contributing to inflammation and autoimmunity as well as specific disease phenotypes. Therefore, comprehensive unraveling of the roles of hnRNPs is conducive to establishing potential biomarkers and developing better intervention strategies by targeting these hnRNPs in the corresponding disorders. This article is categorized under: RNA in Disease and Development > RNA in Disease RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

Post-transcriptional checkpoints in autoimmunity

Post-transcriptional regulation is a fundamental process in gene expression that has a role in diverse cellular processes, including immune responses. A core concept underlying post-transcriptional regulation is that protein abundance is not solely determined by transcript abundance. Indeed, transcription and translation are not directly coupled, and intervening steps occur between these processes, including the regulation of mRNA stability, localization and alternative splicing, which can impact protein abundance. These steps are controlled by various post-transcription factors such as RNA-binding proteins and non-coding RNAs, including microRNAs, and aberrant post-transcriptional regulation has been implicated in various pathological conditions. Indeed, studies on the pathogenesis of autoimmune and inflammatory diseases have identified various post-transcription factors as important regulators of immune cell-mediated and target effector cell-mediated pathological conditions. This Review summarizes current knowledge regarding the roles of post-transcriptional checkpoints in autoimmunity, as evidenced by studies in both haematopoietic and non-haematopoietic cells, and discusses the relevance of these findings for developing new anti-inflammatory therapies.

Emerging roles for heterogeneous ribonuclear proteins in normal and malignant B cells

Heterogeneous nuclear ribonucleoproteins (hnRNPs) are among the most abundantly expressed RNA binding proteins in the cell and play major roles in all facets of RNA metabolism. hnRNPs are increasingly appreciated as essential for mammalian B cell development by regulating the carefully ordered expression of specific genes. Due to this tight regulation of the hnRNP-RNA network, it is no surprise that a growing number of genes encoding hnRNPs have been causally associated with the onset or progression of many cancers, including B cell neoplasms. Here we discuss our current understanding of hnRNP-driven regulation in normal, perturbed, and malignant B cells, and the most recent and emerging therapeutic innovations aimed at targeting the hnRNP-RNA network in lymphoma.

Multiple functions of heterogeneous nuclear ribonucleoproteins in the positive single-stranded RNA virus life cycle

The heterogeneous nuclear ribonucleoproteins (hnRNPs) are a diverse family of RNA binding proteins that are implicated in RNA metabolism, such as alternative splicing, mRNA stabilization and translational regulation. According to their different cellular localization, hnRNPs display multiple functions. Most hnRNPs were predominantly located in the nucleus, but some of them could redistribute to the cytoplasm during virus infection. HnRNPs consist of different domains and motifs that enable these proteins to recognize predetermined nucleotide sequences. In the virus-host interactions, hnRNPs specifically bind to viral RNA or proteins. And some of the viral protein-hnRNP interactions require the viral RNA or other host factors as the intermediate. Through various mechanisms, hnRNPs could regulate viral translation, viral genome replication, the switch of translation to replication and virion release. This review highlights the common features and the distinguish roles of hnRNPs in the life cycle of positive single-stranded RNA viruses.

LCDR regulates the integrity of lysosomal membrane by hnRNP K-stabilized LAPTM5 transcript and promotes cell survival

Here, we report that the long noncoding RNA lysosome cell death regulator (LCDR) mediates the survival of cancer cells, counteracting the effects of apoptosis triggered by lysosomal cell death pathways. Mechanistically, LCDR, as a cofactor for heterogenous nuclear ribonucleoprotein K (hnRNP K) to potentiate the stabilization of lysosomal membrane protein lysosomal-associated protein transmembrane 5 (LAPTM5), prevents lysosomal membrane permeabilization and promotes cancer cell survival. Clinically, LCDR, hnRNP K, and LAPTM5 are significantly up-regulated in lung adenocarcinoma (LUAD) patients. Targeting LCDR via nanoparticles-mediated RNA interference technology increases cell death in vitro and inhibits the growth of patient-derived xenografts of LUAD in vivo. Our study demonstrates that LCDR contributes to cancer pathology by regulating LCDR-mediated apoptosis.

Lysosome plays important roles in cellular homeostasis, and its dysregulation contributes to tumor growth and survival. However, the understanding of regulation and the underlying mechanism of lysosome in cancer survival is incomplete. Here, we reveal a role for a histone acetylation–regulated long noncoding RNA termed lysosome cell death regulator (LCDR) in lung cancer cell survival, in which its knockdown promotes apoptosis. Mechanistically, LCDR binds to heterogenous nuclear ribonucleoprotein K (hnRNP K) to regulate the stability of the lysosomal-associated protein transmembrane 5 (LAPTM5) transcript that maintains the integrity of the lysosomal membrane. Knockdown of LCDR, hnRNP K, or LAPTM5 promotes lysosomal membrane permeabilization and lysosomal cell death, thus consequently resulting in apoptosis. LAPTM5 overexpression or cathepsin B inhibitor partially restores the effects of this axis on lysosomal cell death in vitro and in vivo. Similarly, targeting LCDR significantly decreased tumor growth of patient-derived xenografts of lung adenocarcinoma (LUAD) and had significant cell death using nanoparticles (NPs)-mediated systematic short interfering RNA delivery. Moreover, LCDR/hnRNP K/LAPTM5 are up-regulated in LUAD tissues, and coexpression of this axis shows the increased diagnostic value for LUAD. Collectively, we identified a long noncoding RNA that regulates lysosome function at the posttranscriptional level. These findings shed light on LCDR/hnRNP K/LAPTM5 as potential therapeutic targets, and targeting lysosome is a promising strategy in cancer treatment.

hnRNPK-derived cell-penetrating peptide inhibits cancer cell survival

hnRNPK is a multifunctional protein that plays an important role in cancer cell proliferation and metastasis via its RNA- and DNA-binding properties. Previously we showed that cell-penetrating peptides derived from the RGG RNA-binding domain of SAFA (hnRNPU) disrupt cancer cell proliferation and survival. Here we explore the efficacy of a peptide derived from the RGG domain of hnRNPK. This peptide acts in a dominant-negative manner on several hnRNPK functions to induce death of multiple types of cancer cells. The peptide phenocopies the effect of hnRNPK knockdown on its mRNA-stability targets such as KLF4 and EGR1 and alters the levels and locations of long non-coding RNAs (lncRNAs) and proteins required for nuclear and paraspeckle formation and function. The RGG-derived peptide also decreases euchromatin as evidenced by loss of active marks and polymerase II occupancy. Our findings reveal the potential therapeutic utility of the hnRNPK RGG-derived peptide in a range of cancers.

The RNA regulatory programs that govern lymphocyte development and function

Lymphocytes require of constant and dynamic changes in their transcriptome for timely activation and production of effector molecules to combat external pathogens. Synthesis and translation of messenger (m)RNAs into these effector proteins is controlled both quantitatively and qualitatively by RNA binding proteins (RBPs). RBP-dependent regulation of RNA editing, subcellular location, stability, and translation shapes immune cell development and immunity. Extensive evidences have now been gathered from few model RBPs, HuR, PTBP1, ZFP36, and Roquin. However, recently developed methodologies for global characterization of protein:RNA interactions suggest the existence of complex RNA regulatory networks in which RBPs co-ordinately regulate the fate of sets of RNAs controlling cellular pathways and functions. In turn, RNA can also act as scaffolding of functionally related proteins modulating their activation and function. Here we review current knowledge about how RBP-dependent regulation of RNA shapes our immune system and discuss about the existence of a hidden immune cell epitranscriptome. This article is categorized under: RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

RNA-binding motifs of hnRNP K are critical for induction of antibody diversification by activation-induced cytidine deaminase

Activation-induced cytidine deaminase (AID) is the key enzyme for class switch recombination (CSR) and somatic hypermutation (SHM) to generate antibody memory. Previously, heterogeneous nuclear ribonucleoprotein K (hnRNP K) was shown to be required for AID-dependent DNA breaks. Here, we defined the function of major RNA-binding motifs of hnRNP K, GXXGs and RGGs in the K-homology (KH) and the K-protein-interaction (KI) domains, respectively. Mutation of GXXG, RGG, or both impaired CSR, SHM, and cMyc/IgH translocation equally, showing that these motifs were necessary for AID-dependent DNA breaks. AID-hnRNP K interaction is dependent on RNA; hence, mutation of these RNA-binding motifs abolished the interaction with AID, as expected. Some of the polypyrimidine sequence-carrying prototypical hnRNP K-binding RNAs, which participate in DNA breaks or repair bound to hnRNP K in a GXXG and RGG motif-dependent manner. Mutation of the GXXG and RGG motifs decreased nuclear retention of hnRNP K. Together with the previous finding that nuclear localization of AID is necessary for its function, lower nuclear retention of these mutants may worsen their functional deficiency, which is also caused by their decreased RNA-binding capacity. In summary, hnRNP K contributed to AID-dependent DNA breaks with all of its major RNA-binding motifs.