HuR-hnRNP interactions and the effect of cellular stress

Role of stress granules in tumorigenesis and cancer therapy

Stress granules (SGs) are membrane-less organelles that cell forms via liquid-liquid phase separation (LLPS) under stress conditions such as oxidative stress, ER stress, heat shock and hypoxia. SG assembly is a stress-responsive mechanism by regulating gene expression and cellular signaling pathways. Cancer cells face various stress conditions in tumor microenvironment during tumorigenesis, while SGs contribute to hallmarks of cancer including proliferation, invasion, migration, avoiding apoptosis, metabolism reprogramming and immune evasion. Here, we review the connection between SGs and cancer development, the limitation of SGs on current cancer therapy and promising cancer therapeutic strategies targeting SGs in the future.

The regulation of N6-methyladenosine modification in PD-L1-induced anti-tumor immunity

There is growing evidence that programmed death ligand-1 (PD-L1) has exciting therapeutic efficacy in hematological malignancy and partial solid tumors. However, many patients still face failure with the treatment of immune checkpoint blockade because of PD-L1 expression regulation during transcription and post-transcription processes, including N6-methyladenosine (m6A). Similar to the epigenetic regulation in DNA and histones, recent research has revealed the essential regulation of m6A modification in RNA nuclear export, metabolism and translation. Recent studies have shown that m6A-induced PD-L1 expression emerges as one of the main reasons for the immunological alteration in this process and contributes to the failure of T cell-induced anti-tumor immunity. The results of preclinical studies demonstrate the potential of m6A-targeted therapy in combination with immune checkpoint blockade. The comprehensive expression of m6A-related genes also provided the possibility to indicate the prognosis and to optimize the treatment for patients of various cancer types. In this review, we focus on the m6A modification in PD-L1 mRNA as well as the regulation of PD-L1 expression in cancer cells and summarize its clinical value in anti-PD-L1 cancer immune therapy.

Human Antigen R (HuR): A Regulator of Heme Oxygenase-1 Cytoprotection in Mouse and Human Liver Transplant Injury

BACKGROUND AND AIMS

Ischemia-reperfusion injury (IRI) represents a risk factor in liver transplantation (LT). We have shown that overexpression of heme oxygenase-1 (HO-1) mitigates hepatic IRI in LT recipients. Here, we hypothesized that human antigen R (HuR), the stabilizer of adenylate-uridylate (AU)-rich mRNAs, is required for hepatoprotection in LT.

APPROACH AND RESULTS

In an experimental arm, HuR/HO-1 protein expression was correlated with hepatic IRI phenotype. In an in vitro inflammation mimic model of hepatic warm IRI, induction of HuR/HO-1 and cytoplasmic localization following cytokine preconditioning were detected in primary hepatocyte cultures, whereas HuR silencing caused negative regulation of HO-1, followed by enhanced cytotoxicity. Using the HuR-inhibitor, we showed that HuR likely regulates HO-1 through its 3' untranslated region and causes neutrophil activation (CD69+/lymphocyte antigen 6 complex locus G [Ly6-G]). HuR silencing in bone marrow-derived macrophages decreased HO-1 expression, leading to the induction of proinflammatory cytokines/chemokines. RNA sequencing of HuR silenced transcripts under in vitro warm IRI revealed regulation of genes thymus cell antigen 1 (THY1), aconitate decarboxylase 1 (ACOD1), and Prostaglandin E Synthase (PTGES). HuR, but not hypoxia-inducible protein alpha, positively regulated HO-1 in warm, but not cold, hypoxia/reoxygenation conditions. HuR modulated HO-1 in primary hepatocytes, neutrophils, and macrophages under reperfusion. Adjunctive inhibition of HuR diminished microtubule-associated proteins 1A/1B light chain 3B (LC3B), a marker for autophagosome, under HO-1 regulation, suggesting a cytoprotective mechanism in hepatic IR. In a clinical arm, hepatic biopsies from 51 patients with LT were analyzed at 2 hours after reperfusion. Graft HuR expression was negatively correlated with macrophage (CD80/CD86) and neutrophil (Cathepsin G) markers. Hepatic IRI increased HuR/HO-1 expression and inflammatory genes. High HuR-expressing liver grafts showed lower serum alanine aminotransferase/serum aspartate aminotransferase levels and improved LT survival.

CONCLUSIONS

This translational study identifies HuR as a regulator of HO-1-mediated cytoprotection in sterile liver inflammation and a biomarker of ischemic stress resistance in LT.

Quantitative Proteomic Analysis of Small and Large Extracellular Vesicles (EVs) Reveals Enrichment of Adhesion Proteins in Small EVs

Extracellular vesicles (EVs) are important mediators of cell-cell communication due to their cargo content of proteins, lipids, and RNAs. We previously reported that small EVs (SEVs) called exosomes promote directed and random cell motility, invasion, and serum-independent growth. In contrast, larger EVs (LEVs) were not active in those assays, but might have unique functional properties. In order to identify protein cargos that may contribute to different functions of SEVs and LEVs, we used isobaric tags for relative and absolute quantitation (iTRAQ)-liquid chromatography (LC) tandem mass spectrometry (MS) on EVs isolated from a colon cancer cell line. Bioinformatics analyses revealed that SEVs are enriched in proteins associated with cell-cell junctions, cell-matrix adhesion, exosome biogenesis machinery, and various signaling pathways. In contrast, LEVs are enriched in proteins associated with ribosome and RNA biogenesis, processing, and metabolism. Western blot analysis of EVs purified from two different cancer cell types confirmed the enrichment of cell-matrix and cell-cell adhesion proteins in SEVs. Consistent with those data, we found that cells exhibit enhanced adhesion to surfaces coated with SEVs compared to an equal protein concentration of LEVs. These data suggest that a major function of SEVs is to promote cellular adhesion.

Embryonic lethal abnormal vision proteins and adenine and uridine-rich element mRNAs after global cerebral ischemia and reperfusion in the rat

Prolonged translation arrest correlates with delayed neuronal death of hippocampal CA1 neurons following global cerebral ischemia and reperfusion. Many previous studies investigated ribosome molecular biology, but mRNA regulatory mechanisms after brain ischemia have been less studied. Here we investigated the embryonic lethal abnormal vision/Hu isoforms HuR, HuB, HuC, and HuD, as well as expression of mRNAs containing adenine and rich uridine elements following global ischemia in rat brain. Proteomics of embryonic lethal abnormal vision immunoprecipitations or polysomes isolated from rat hippocampal CA1 and CA3 from controls or following 10 min ischemia plus 8 h of reperfusion showed distinct sets of mRNA-binding proteins, suggesting differential mRNA regulation in each condition. Notably, HuB, HuC, and HuD were undetectable in NIC CA1. At 8 h reperfusion, polysome-associated mRNAs contained 46.1% of ischemia-upregulated mRNAs containing adenine and rich uridine elements in CA3, but only 18.7% in CA1. Since mRNAs containing adenine and rich uridine elements regulation are important to several cellular stress responses, our results suggest CA1 is disadvantaged compared to CA3 in coping with ischemic stress, and this is expected to be an important contributing factor to CA1 selective vulnerability. (Data are available via ProteomeXchange identifier PXD004078 and GEO Series accession number GSE82146).

Epithelial Splicing Regulatory Protein 1 (ESRP1) is a new regulator of stomach smooth muscle development and plasticity

In vertebrates, stomach smooth muscle development is a complex process that involves the tight transcriptional or post-transcriptional regulation of different signalling pathways. Here, we identified the RNA-binding protein Epithelial Splicing Regulatory Protein 1 (ESRP1) as an early marker of developing and undifferentiated stomach mesenchyme. Using a gain-of-function approach, we found that in chicken embryos, sustained expression of ESRP1 impairs stomach smooth muscle cell (SMC) differentiation and FGFR2 splicing profile. ESRP1 overexpression in primary differentiated stomach SMCs induced their dedifferentiation, promoted specific-FGFR2b splicing and decreased FGFR2c-dependent activity. Moreover, co-expression of ESRP1 and RBPMS2, another RNA-binding protein that regulates SMC plasticity and Bone Morphogenetic Protein (BMP) pathway inhibition, synergistically promoted SMC dedifferentiation. Finally, we also demonstrated that ESRP1 interacts with RBPMS2 and that RBPMS2-mediated SMC dedifferentiation requires ESRP1. Altogether, these results show that ESRP1 is expressed also in undifferentiated stomach mesenchyme and demonstrate its role in SMC development and plasticity.

Poly(A)(+) mRNA-binding protein Tudor-SN regulates stress granules aggregation dynamics

Stress granules (SGs) and processing bodies (PBs) comprise the main types of cytoplasmic RNA foci during stress. Our previous data indicate that knockdown of human Tudor staphylococcal nuclease (Tudor-SN) affects the aggregation of SGs. However, the precise molecular mechanism has not been determined fully. In the present study, we demonstrate that Tudor-SN binds and colocalizes with many core components of SGs, such as poly(A)(+) mRNA binding protein 1, T-cell internal antigen-1-related protein and poly(A)(+) mRNA, and SG/PB sharing proteins Argonaute 1/2, but not PB core proteins, such as decapping enzyme 1 a/b, confirming that Tudor-SN is an SG-specific protein. We also demonstrate that the Tudor-SN granule actively communicates with the nuclear and cytosolic pool under stress conditions. Tudor-SN can regulate the aggregation dynamics of poly(A)(+) mRNA-containing SGs and selectively stabilize the SG-associated mRNA during cellular stress.

Lipoteichoic acid isolated from Lactobacillus plantarum inhibits melanogenesis in B16F10 mouse melanoma cells

Lipoteichoic acid (LTA) is a major component of the cell wall of Gram-positive bacteria. Its effects on living organisms are different from those of lipopolysaccharide (LPS) found in Gram-negative bacteria. LTA contributes to immune regulatory effects including anti-aging. In this study, we showed that LTA isolated from Lactobacillus plantarum (pLTA) inhibited melanogenesis in B16F10 mouse melanoma cells. pLTA reduced the cellular activity of tyrosinase and the expression of tyrosinase family members in a dose-dependent manner. The expression of microphthalmia-associated transcription factor (MITF), a key factor in the synthesis of melanin, was also decreased by pLTA. Further, we showed that pLTA activated melanogenesis signaling, such as extracellular signal-regulated kinase (ERK) and phosphatidylinositol 3-kinse (PI3K)/AKT. In addition, the expression of heterogeneous nuclear ribonucleoprotein A1 (hnRNP A1) and HuR, which are important RNA-binding proteins (RBPs), was reduced. pLTA likely degrades MITF via regulation of melanogenic signaling and RNA stability of melanogenic proteins, resulting in the reduction of melanin. Thus, our data suggest that pLTA has therapeutic potential for treating hyperpigmentation disorders and can also be used as a cosmetic whitening agent.

Involvement of ELAV RNA-binding proteins in the post-transcriptional regulation of HO-1

Heme oxygenase-1 (HO-1) is an inducible rate-controlling enzyme of heme catabolism. The cytoprotective function of HO-1 activity has been verified in multiple studies, and together with its by-products is considered a key component of the cellular stress response. The transcriptional induction of HO-1 has been largely studied in response to multiple forms of stressful stimuli but our understanding of HO-1 post-transcriptional control mechanisms in neuronal cells is currently lacking. In the present report we show the involvement of the RNA-binding proteins (RBPs) embryonic lethal abnormal vision (ELAV) in the regulation of HO-1 gene expression. Our study demonstrates a specific binding between HO-1 messenger RNA (mRNA) and ELAV proteins, accompanied by an increased expression of HO-1 at protein level, in a human neuroblastoma cell line treated with hemin. Clarifying the induction of HO-1 expression at post-transcriptional level may open therapeutic perspectives for treatments associated with the modulation of HO-1 expression.

Chemopreventive apigenin controls UVB-induced cutaneous proliferation and angiogenesis through HuR and thrombospondin-1

Plant flavonoid apigenin prevents and inhibits UVB-induced carcinogenesis in the skin and has strong anti-proliferative and anti-angiogenic properties. Here we identify mechanisms, by which apigenin controls these oncogenic events. We show that apigenin acts, at least in part, via endogenous angiogenesis inhibitor, thrombospondin-1 (TSP1). TSP1 expression by the epidermal keratinocytes is potently inhibited by UVB. It inhibits cutaneous angiogenesis and UVB-induced carcinogenesis. We show that apigenin restores TSP1 in epidermal keratinocytes subjected to UVB and normalizes proliferation and angiogenesis in UVB-exposed skin. Importantly, reconstituting TSP1 anti-angiogenic function in UVB-irradiated skin with a short bioactive peptide mimetic representing exclusively its anti-angiogenic domain reproduced the anti-proliferative and anti-angiogenic effects of apigenin. Cox-2 and HIF-1α are important mediators of angiogenesis. Both apigenin and TSP1 peptide mimetic attenuated their induction by UVB. Finally we identified the molecular mechanism, whereby apigenin did not affect TSP1 mRNA, but increased de novo protein synthesis. Knockdown studies implicated the RNA-binding protein HuR, which controls mRNA stability and translation. Apigenin increased HuR cytoplasmic localization and physical association with TSP1 mRNA causing de novo TSP1 synthesis. HuR cytoplasmic localization was, in turn, dependent on CHK2 kinase. Together, our data provide a new mechanism, by which apigenin controls UVB-induced carcinogenesis.

HuR-regulated mRNAs associated with nuclear hnRNP A1-RNP complexes

Post-transcriptional regulatory networks are dependent on the interplay of many RNA-binding proteins having a major role in mRNA processing events in mammals. We have been interested in the concerted action of the two RNA-binding proteins hnRNP A1 and HuR, both stable components of immunoselected hnRNP complexes and having a major nuclear localization. Specifically, we present here the application of the RNA-immunoprecipitation (RIP)-Chip technology to identify a population of nuclear transcripts associated with hnRNP A1-RNPs as isolated from the nuclear extract of either HuR WT or HuR-depleted (KO) mouse embryonic fibroblast (MEF) cells. The outcome of this analysis was a list of target genes regulated via HuR for their association (either increased or reduced) with the nuclear hnRNP A1-RNP complexes. Real time PCR analysis was applied to validate a selected number of nuclear mRNA transcripts, as well as to identify pre-spliced transcripts (in addition to their mature mRNA counterpart) within the isolated nuclear hnRNP A1-RNPs. The differentially enriched mRNAs were found to belong to GO categories relevant to biological processes anticipated for hnRNP A1 and HuR (such as transport, transcription, translation, apoptosis and cell cycle) indicating their concerted function in mRNA metabolism.

Multiple functions of the RNA-binding protein HuR in cancer progression, treatment responses and prognosis

The human embryonic lethal abnormal vision-like protein, HuR, is a member of the Hu family of RNA-binding proteins. Over the past decade, this ubiquitously expressed protein has been extensively investigated in cancer research because it is involved in the regulation of mRNA stability and translation in many cell types. HuR activity and function is associated with its subcellular distribution, transcriptional regulation, translational and post-translational modifications. HuR regulation of target mRNAs is based on the interaction between the three specific domains of HuR protein and one or several U- or AU-rich elements (AREs) in the untranslated region of target mRNAs. A number of cancer-related transcripts containing AREs, including mRNAs for proto-oncogenes, cytokines, growth factors, and invasion factors, have been characterized as HuR targets. It has been proposed that HuR has a central tumorigenic activity by enabling multiple cancer phenotypes. In this review, we comprehensively survey the existing evidence with regard to the diverse functions of HuR in caner development and progression. The current data also suggest that HuR might be a novel and promising therapeutic target and a marker for treatment response and prognostic evaluation.

Expression and functional studies on the noncoding RNA, PRINS

PRINS, a noncoding RNA identified earlier by our research group, contributes to psoriasis susceptibility and cellular stress response. We have now studied the cellular and histological distribution of PRINS by using in situ hybridization and demonstrated variable expressions in different human tissues and a consistent staining pattern in epidermal keratinocytes and in vitro cultured keratinocytes. To identify the cellular function(s) of PRINS, we searched for a direct interacting partner(s) of this stress-induced molecule. In HaCaT and NHEK cell lysates, the protein proved to be nucleophosmin (NPM) protein as a potential physical interactor with PRINS. Immunohistochemical experiments revealed an elevated expression of NPM in the dividing cells of the basal layers of psoriatic involved skin samples as compared with healthy and psoriatic uninvolved samples. Others have previously shown that NPM is a ubiquitously expressed nucleolar phosphoprotein which shuttles to the nucleoplasm after UV-B irradiation in fibroblasts and cancer cells. We detected a similar translocation of NPM in UV-B-irradiated cultured keratinocytes. The gene-specific silencing of PRINS resulted in the retention of NPM in the nucleolus of UV-B-irradiated keratinocytes; suggesting that PRINS may play a role in the NPM-mediated cellular stress response in the skin.