Posttranscriptional regulation of collagen alpha1(I) mRNA in hepatic stellate cells

EPRS1 Controls the TGF- Signaling Pathway via Interaction with TβRI in Hepatic Stellate Cell

Glutamyl-prolyl-tRNA synthetase 1 (EPRS1) is known to associated with fibrosis through its catalytic activity to produce prolyl-tRNA. Although its catalytic inhibitor halofuginone (HF) has been known to inhibit the TGF-β pathway as well as to reduce prolyl-tRNA production for the control of fibrosis, the underlying mechanism how EPRS1 regulates the TGF-β pathway was not fully understood. Here, we show a noncatalytic function of EPRS1 in controlling the TGF-β pathway and hepatic stellate cell activation via its interaction with TGF-β receptor I (TβRI). Upon stimulation with TGF-β, EPRS1 is phosphorylated by TGF-β-activated kinase 1 (TAK1), leading to its dissociation from the multi-tRNA synthetase complex and subsequent binding with TβRI. This interaction increases the association of TβRI with SMAD2/3 while decreases that of TβRI with SMAD7. Accordingly, EPRS1 stabilizes TβRI by preventing the ubiquitin-mediated degradation of TβRI. HF disrupts the interaction between EPRS1 and TβRI, and reduces TβRI protein levels, leading to inhibition of the TGF-β pathway. In conclusion, this work suggests the novel function of EPRS1 involved in the development of fibrosis by regulating the TGF-β pathway and the antifibrotic effects of HF by controlling both of EPRS1 functions.

The Beneficial Effects of Triterpenic Acid and Acteoside in an In Vitro Model of Nonalcoholic Steatohepatitis (NASH)

Triterpenic acid (TA) and acteoside (ACT), the major components of APPLIVER and ACTEOS, respectively, have been reported to exert hepatoprotective effects, but the molecular mechanisms remain elusive, particularly in the NAFLD/NASH context. We assessed their effects in our well-established in vitro model resembling the pathophysiological mechanisms involved in NASH. Human hepatocytes and hepatic stellate cells were exposed to free fatty acids (FFA) alone or in combination with APPLIVER and ACTEOS as a mono- or co-culture. Steatosis, inflammation, generation of reactive oxygen species (ROS), and collagen deposition were determined. ACTEOS reduced both the TNF-α and ROS production, and, most importantly, attenuated collagen deposition elicited by the excess of FFA in the co-culture model. APPLIVER also showed inhibition of both TNF-α production and collagen deposition caused by FFA accumulation. The compounds alone did not induce any cellular effects. The present study showed the efficacy of APPLIVER and ACTEOS on pathophysiological mechanisms related to NASH. These in vitro data suggest that these compounds deserve further investigation for possible use in NASH treatment.

Targeting Type I Collagen for Cancer Treatment

Collagen is the most abundant protein in animals. Interactions between tumor cells and collagen influence every step of tumor development. Type I collagen is the main fibrillar collagen in the extracellular matrix and is frequently up-regulated during tumorigenesis. The binding of type I collagen to its receptors on tumor cells promotes tumor cell proliferation, epithelial-mesenchymal transition, and metastasis. Type I collagen also regulates the efficacy of tumor therapies, such as chemotherapy, radiotherapy, and immunotherapy. Furthermore, type I collagen fragments are diagnostic markers of metastatic tumors and have prognostic value. Inhibition of type I collagen synthesis has been reported to have anti-tumor effects in animal models. However, collagen has also been shown to possess anti-tumor activity. Therefore, the roles that type I collagen plays in tumor biology are complex and tumor type-dependent. In this review, we discuss the expression and regulation of synthesis of type I collagen, as well as the role up-regulated type I collagen plays in various stages of cancer progression. We also discuss the role of collagen in tumor therapy. Finally, we highlight several recent approaches targeting type I collagen for cancer treatment. This article is protected by copyright. All rights reserved.

Metabolic Reprogramming of Liver Fibrosis

Liver fibrosis is an excessive and imbalanced deposition of fibrous extracellular matrix (ECM) that is associated with the hepatic wound-healing response. It is also the common mechanism that contributes to the impairment of the liver function that is observed in many chronic liver diseases (CLD). Despite the efforts, no effective therapy against fibrosis exists yet. Worryingly, due to the growing obesity pandemic, fibrosis incidence is on the rise. Here, we aim to summarize the main components and mechanisms involved in the progression of liver fibrosis, with special focus on the metabolic regulation of key effectors of fibrogenesis, hepatic stellate cells (HSCs), and their role in the disease progression. Hepatic cells that undergo metabolic reprogramming require a tightly controlled, fine-tuned cellular response, allowing them to meet their energetic demands without affecting cellular integrity. Here, we aim to discuss the role of ribonucleic acid (RNA)-binding proteins (RBPs), whose dynamic nature being context- and stimuli-dependent make them very suitable for the fibrotic situation. Thus, we will not only summarize the up-to-date literature on the metabolic regulation of HSCs in liver fibrosis, but also on the RBP-dependent post-transcriptional regulation of this metabolic switch that results in such important consequences for the progression of fibrosis and CLD.

A Type I Collagen-Targeted MR Imaging Probe for Staging Fibrosis in Crohn's Disease

Fibrostenosis is a serious complication of Crohn's disease (CD), affecting approximately one-half of all patients. Surgical resection is the typical clinical end due to ineffective antifibrotic therapy mainly through anti-inflammatory treatment and fibrosis can be reverted only at early stages. Mover, human fibrotic disorders is known to be associated with aging process. Thus, accurate monitoring of the progression of fibrosis is crucial for CD management as well as can be benefit to aging related fibrosis. The excessive deposition of type I collagen (ColI) is the core point in major complications of fibrosis, including that in patients with CD and aging related fibrosis. Therefore, a MR imaging probe (EP-3533) targeted ColI was employed to stage bowel fibrosis in CD using a rat model and to compare its efficiency with the common MR imaging contrast medium gadopentetatedimeglumine (Gd-DTPA). The bowel fibrotic rat model was established with different degrees of bowel fibrosis, were scanned using a 3.0-T MRI scanner with a specialized animal coil. MRI sequence including T ₁ mapping and T1-weighed imaging were performed before and after injecting the MRI probe (EP-3533 or Gd-DTPA). The T ₁ relaxation time (T ₁ value) and change in the contrast-to-noise ratio (ΔCNR) were measured to evaluate bowel fibrosis. Masson's trichrome staining was performed to determine the severity of fibrosis. EP-3533 offered a better longitudinal relaxivity (r₁) with 67.537 L/mmol·s, which was approximately 13 times that of Gd-DTPA. The T ₁ value on bowel segments was reduced in the images from EP-3533 compared to that from Gd-DTPA (F = 16.478; p < 0.001). Additionally, a better correlation between ΔCNR calculated from EP-3533 imaging and bowel fibrosis (AUC = 0.846) was determined 10 min after enhanced media administration than with Gd-DTPA (AUC = 0.532). The 10th-minute ΔCNR performed using the ColI probe showed the best correlation with the severity of bowel fibrosis (r = 0.538; p = 0.021). Our results demonstrates that targeted MRI probe (EP-3533) supplies a better enhanced effect compared to Gd-DTPA and could be a promising method to evaluate the progression and monitor the therapeutic response of bowel fibrosis.

Proteomic Changes of Activated Hepatic Stellate Cells

Hepatic stellate cells (HSC) are the major cellular drivers of liver fibrosis. Upon liver inflammation caused by a broad range of insults including non-alcoholic fatty liver, HSC transform from a quiescent into a proliferating, fibrotic phenotype. Although much is known about the pathophysiology of this process, exact cellular processes which occur in HSC and enable this transformation remain yet to be elucidated. In order to investigate this HSC transformation, we employed a simple, yet reliable model of HSC activation via an increase in growth medium serum concentration (serum activation). For that purpose, immortalized human LX-2 HSC were exposed to either 1% or 10% fetal bovine serum (FBS). Resulting quiescent (1% FBS) and activated (10% FBS) LX-2 cells were then subjected to in-depth mass spectrometry-based proteomics analysis as well as comprehensive phenotyping. Protein network analysis of activated LX-2 cells revealed an increase in the production of ribosomal proteins and proteins related to cell cycle control and migration, resulting in higher proliferation and faster migration phenotypes. Interestingly, we also observed a decrease in the expression of cholesterol and fatty acid biosynthesis proteins in accordance with a concomitant loss of cytosolic lipid droplets during activation. Overall, this work provides an update on HSC activation characteristics using contemporary proteomic and bioinformatic analyses and presents an accessible model for HSC activation. Data are available via ProteomeXchange with identifier PXD029121.

Study on the effect of Mongolian medicine Qiwei Qinggan Powder on hepatic fibrosis through JAK2/STAT3 pathway

This research aimed to evaluate the antihepatic fibrosis effect and explore the mechanism of Qiwei Qinggan Powder (QGS-7) in vivo and in vitro. Carbon tetrachloride (CCl4)-treated rats and hepatic stellate cells (HSCs) were used. QGS-7 treatment significantly improved the liver function of rats as indicated by decreased serum enzymatic activities of alanine aminotransferase, aspartate transaminase, and alkaline phosphatase. Meanwhile, the hydroxyproline of liver was significantly decreased. Histopathological results indicated that QGS-7 alleviated liver damage and reduced the formation of fibrosis septa. Moreover, QGS-7 significantly attenuated expressions of Alpha smooth muscle actin, Collagen I, Janus kinase 2 (JAK2), phosphorylation-JAK2, signal transducer and activator of transcription 3 (STAT3), phosphorylation-STAT3 in the rat hepatic fibrosis model. QGS-7 inhibited HSC proliferation and promoted it apoptosis. QGS-7 may affect hepatic fibrosis through JAK2/STAT3 signaling pathway so as to play an antihepatic fibrosis role.

Advances in poly(rC)-binding protein 2: Structure, molecular function, and roles in cancer

Poly(rC)-binding protein 2 (PCBP2) is an RNA-binding protein that is characterized by its ability to interact with poly(C) with high affinity in a sequence-specific manner. PCBP2 contains three K homology domains, which are consensus RNA-binding domains that play a role in recognizing and combining with RNA and DNA. The specific structure and localization of PCBP2 lay the foundation for its multiple roles in transcriptional, posttranscriptional, and translational processes, even in iron metabolism. Numerous studies have indicated that PCBP2 expression is increased in many cancer types. PCBP2 is considered as an oncogene that promotes tumorigenesis, development of cancer cells, and metastasis. Here, we summarized the current evidence regarding PCBP2 in the proliferation, migration, invasion of cancer cells, and drug resistance, aiming to clarify the molecular mechanisms of PCBP2 in cancer. Results from this review suggest that an in-depth study of PCBP2 in cancer may provide novel biomarkers for prognostic or therapeutic purposes.

Discovery of a Lead Compound for Specific Inhibition of Type I Collagen Production in Fibrosis

Fibrosis is a major medical problem caused by excessive synthesis of the extracellular matrix, composed predominantly of type I collagen, in various tissues. There are no approved antifibrotic drugs, and the major obstacle in finding clinically relevant compounds is the lack of specificity of current experimental drugs for type I collagen. Here we describe the discovery of a lead compound that specifically inhibited secretion of type I collagen by fibroblasts in culture at IC₅₀ = 4.5 μM. The inhibition was specific for type I collagen, because secretion of fibronectin was not affected. In vitro, the compound inhibited binding of LARP6, the master regulator of translation of type I collagen mRNAs, to the 5' stem-loop sequence element which regulates their translation. Because binding of LARP6 to collagen mRNAs is crucial for the development of fibrosis, this inhibitor represents a promising lead for optimization into specific antifibrotic drugs.

Silencing PCBP2 normalizes desmoplastic stroma and improves the antitumor activity of chemotherapy in pancreatic cancer

Rationale: Dense desmoplastic stroma is a fundamental characteristic of pancreatic ductal adenocarcinoma (PDAC) and comprises up to 80% of the tumor mass. Type I collagen is the major component of the extracellular matrix (ECM), which acts as a barrier to impede the delivery of drugs into the tumor microenvironment. While the strategy to deplete PDAC stroma has failed in clinical trials, normalization of the stroma to allow chemotherapy to kill the tumor cells in the “nest” could be a promising strategy for PDAC therapy. We hypothesize that silencing the poly(rC)-binding protein 2 (αCP2, encoded by the PCBP2 gene) leads to the destabilization and normalization of type I collagen in the PDAC stroma.

Methods: We develop a micro-flow mixing method to fabricate a peptide-based core-stabilized PCBP2 siRNA nanocomplex to reverse the accumulation of type I collagen in PDAC tumor stroma. Various in vitro studies were performed to evaluate the silencing activity, cellular uptake, serum stability, and tumor penetration of the PCBP2 siRNA nanocomplex. We also investigated the penetration of small molecules in stroma-rich pancreatic cancer spheroids after the treatment with the PCBP2 siRNA nanocomplex. The anti-tumor activity of the PCBP2 siRNA nanocomplex and its combination with gemcitabine was evaluated in an orthotopic stroma-rich pancreatic cancer mouse model.

Results: Silencing the PCBP2 gene using siRNA reverses the accumulation of type I collagen in human pancreatic stellate cells (PSCs) and mouse NIH 3T3 fibroblast cells. The siRNA nanocomplex significantly reduces ECM production and enhances drug penetration through desmoplastic tumor stroma. The combination of gemcitabine with the PCBP2 siRNA nanocomplex markedly suppresses the tumor progression in a desmoplastic PDAC orthotopic mouse model.

Conclusion: This approach provides a new therapeutic avenue to improve the antitumor efficacy of PDAC therapies by normalizing tumor stroma using the PCBP2 siRNA nanocomplex.

Advances in the clinical use of collagen as biomarker of liver fibrosis

INTRODUCTION

Hepatic fibrosis is the excessive synthesis and deposition of extracellular matrix including collagen in the tissue. Chronic liver insult leads to progressive parenchymal damage, portal hypertension, and cirrhosis. Determination of hepatic collagen by invasive liver biopsy is the gold standard to estimate severity and stage of fibrosis. However, this procedure is associated with pain, carries the risk of infection and bleeding, and is afflicted with a high degree of sampling error. Therefore, there is urgent need for serological collagen-derived markers to assess collagen synthesis/turnover.

AREAS COVERED

Biochemical properties of collagens, cellular sources of hepatic collagen synthesis, and regulatory aspects in collagen expression. Markers are discussed suitable to estimate hepatic collagen synthesis and/or turnover. Discussed studies were identified through a PubMed search done in May 2020 and the authors' topic knowledge.

EXPERT OPINION

Hepatic fibrosis is mainly characterized by accumulation of collagen-rich scar tissue. Although traditionally performed liver biopsy is still standard in estimating hepatic fibrosis, there is evidence that noninvasive diagnostic scores and collagen-derived neo-epitopes provide clinical useful information. These noninvasive tests are less expensive than liver biopsy, better tolerated, safer, and more acceptable to patients. Therefore, these tests will lead to dramatic changes in diagnosis.

Structure of the PCBP2/stem-loop IV complex underlying translation initiation mediated by the poliovirus type I IRES

The poliovirus type I IRES is able to recruit ribosomal machinery only in the presence of host factor PCBP2 that binds to stem-loop IV of the IRES. When PCBP2 is cleaved in its linker region by viral proteinase 3CD, translation initiation ceases allowing the next stage of replication to commence. Here, we investigate the interaction of PCBP2 with the apical region of stem-loop IV (SLIVm) of poliovirus RNA in its full-length and truncated form. CryoEM structure reconstruction of the full-length PCBP2 in complex with SLIVm solved to 6.1 Å resolution reveals a compact globular complex of PCBP2 interacting with the cruciform RNA via KH domains and featuring a prominent GNRA tetraloop. SEC-SAXS, SHAPE and hydroxyl-radical cleavage establish that PCBP2 stabilizes the SLIVm structure, but upon cleavage in the linker domain the complex becomes more flexible and base accessible. Limited proteolysis and REMSA demonstrate the accessibility of the linker region in the PCBP2/SLIVm complex and consequent loss of affinity of PCBP2 for the SLIVm upon cleavage. Together this study sheds light on the structural features of the PCBP2/SLIV complex vital for ribosomal docking, and the way in which this key functional interaction is regulated following translation of the poliovirus genome.

Selective deletion of MyD88 signaling in α-SMA positive cells ameliorates experimental intestinal fibrosis via post-transcriptional regulation

Intestinal fibrosis leading to strictures remains a significant clinical problem in inflammatory bowel diseases (IBD). The role of bacterial components in activating intestinal mesenchymal cells and driving fibrogenesis is largely unexplored. Tamoxifen-inducible α-SMA promoter Cre mice crossed with floxed MyD88 mice were subjected to chronic dextran sodium sulfate colitis. MyD88 was deleted prior to or after induction of colitis. Human intestinal myofibroblasts (HIMF) were exposed to various bacterial components and assessed for fibronectin (FN) and collagen I (Col1) production. RNA sequencing was performed. Post-transcriptional regulation was assessed by polysome profiling assay. Selective deletion of MyD88 in α-SMA-positive cells prior to, but not after induction of, experimental colitis decreased the degree of intestinal fibrosis. HIMF selectively responded to flagellin with enhanced FN or Col1 protein production in a MyD88-dependent manner. RNA sequencing suggested minimal transcriptional changes induced by flagellin in HIMF. Polysome profiling revealed higher proportions of FN and Col1 mRNA in the actively translated fractions of flagellin exposed HIMF, which was mediated by eIF2 alpha and 4EBP1. In conclusion, selectivity of flagellin-induced ECM secretion in HIMF is post-transcriptionally regulated. The results may represent a novel and targetable link between the gut microbiota and intestinal fibrogenesis.

Technology for Discovery of Antifibrotic Drugs: Phenotypic Screening for LARP6 Inhibitors Using Inverted Yeast Three Hybrid System

Fibrosis is defined by excessive production of type I collagen in various organs. Excessive type I collagen production in fibrosis is stimulated by binding of RNA protein LARP6 to the structural element of collagen mRNAs, the 5' stem loop (5'SL). The LARP6-dependent regulation is specific for type I collagen and critical for fibrosis development. Inhibitors of LARP6 binding have potential to be specific antifibrotic drugs, as evidenced by the discovery of one such inhibitor. To create technology for phenotypic screening of additional compounds we developed an inverted yeast three hybrid system. The system is based on expression of human LARP6 and a short RNA containing the 5'SL of human collagen α1(I) mRNA in Saccharomyces cerevisiae cells. The cells were engineered in such a way that when LARP6 is bound to 5'SL RNA they fail to grow in a specific synthetic medium. Dissociation of LARP6 from 5'SL RNA permits the cell growth, allowing identification of the inhibitors of LARP6 binding. The assay simply involves measuring optical density of cells growing in multiwall plates and is pertinent for high throughput applications. We describe the specificity of the system and its characteristics for high throughput screening. As a proof of principle, the result of one screen using collection of FDA approved drugs is also presented. This screen demonstrates that using this technology discovery of novel LARP6 inhibitors is possible.

Discovery and evaluation of inhibitor of LARP6 as specific antifibrotic compound

Fibrosis is characterized by excessive production of type I collagen. Biosynthesis of type I collagen in fibrosis is augmented by binding of protein LARP6 to the 5' stem-loop structure (5'SL), which is found exclusively in type I collagen mRNAs. A high throughput screen was performed to discover inhibitors of LARP6 binding to 5'SL, as potential antifibrotic drugs. The screen yielded one compound (C9) which was able to dissociate LARP6 from 5' SL RNA in vitro and to inactivate the binding of endogenous LARP6 in cells. Treatment of hepatic stellate cells (liver cells responsible for fibrosis) with nM concentrations of C9 reduced secretion of type I collagen. In precision cut liver slices, as an ex vivo model of hepatic fibrosis, C9 attenuated the profibrotic response at 1 μM. In prophylactic and therapeutic animal models of hepatic fibrosis C9 prevented development of fibrosis or hindered the progression of ongoing fibrosis when administered at 1 mg/kg. Toxicogenetics analysis revealed that only 42 liver genes changed expression after administration of C9 for 4 weeks, suggesting minimal off target effects. Based on these results, C9 represents the first LARP6 inhibitor with significant antifibrotic activity.

Small RNA Sequencing Reveals Dlk1-Dio3 Locus-Embedded MicroRNAs as Major Drivers of Ground-State Pluripotency

Ground-state pluripotency is a cell state in which pluripotency is established and maintained through efficient repression of endogenous differentiation pathways. Self-renewal and pluripotency of embryonic stem cells (ESCs) are influenced by ESC-associated microRNAs (miRNAs). Here, we provide a comprehensive assessment of the "miRNome" of ESCs cultured under conditions favoring ground-state pluripotency. We found that ground-state ESCs express a distinct set of miRNAs compared with ESCs grown in serum. Interestingly, most "ground-state miRNAs" are encoded by an imprinted region on chromosome 12 within the Dlk1-Dio3 locus. Functional analysis revealed that ground-state miRNAs embedded in the Dlk1-Dio3 locus (miR-541-5p, miR-410-3p, and miR-381-3p) promoted pluripotency via inhibition of multi-lineage differentiation and stimulation of self-renewal. Overall, our results demonstrate that ground-state pluripotency is associated with a unique miRNA signature, which supports ground-state self-renewal by suppressing differentiation.

mTORC1 phosphorylates LARP6 to stimulate type I collagen expression

Excessive deposition of type I collagen causes fibrotic diseases. Binding of La ribonucleoprotein domain family, member 6 (LARP6) to collagen mRNAs regulates their translation and is necessary for high type I collagen expression. Here we show that mTORC1 phosphorylates LARP6 on S348 and S409. The S348A/S409A mutant of LARP6 acts as a dominant negative protein in collagen biosynthesis, which retards secretion of type I collagen and causes excessive posttranslational modifications. Similar effects are seen using mTORC1 inhibitor rapamycin or by knocking down raptor. The S348A/S409A mutant weakly interacts with the accessory protein STRAP, needed for coordinated translation of collagen mRNAs. The interaction of wt LARP6 and STRAP is also attenuated by rapamycin and by raptor knockdown. Additionally, in the absence of S348/S409 phosphorylation LARP6 is sequestered in increasing amounts at the ER membrane. We postulate that phosphorylation of S348/S409 by mTORC1 stimulates the interaction of LARP6 and STRAP to coordinate translation of collagen mRNAs and to release LARP6 from the ER for new round of translation. These mechanisms contribute to high level of collagen expression in fibrosis.

Activated hepatic stellate cells play pivotal roles in hepatocellular carcinoma cell chemoresistance and migration in multicellular tumor spheroids

Most Hepatocellular carcinoma (HCC) are resistant to conventional chemotherapeutic agents and remain an unmet medical need. Recently, multiple studies on the crosstalk between HCC and their tumor microenvironment have been conducted to overcome chemoresistance in HCC. In this study, we formed multicellular tumor spheroids (MCTS) to elucidate the mechanisms of environment-mediated chemoresistance in HCC. We observed that hepatic stellate cells (HSCs) in MCTS significantly increased the compactness of spheroids and exhibited strong resistance to sorafenib and cisplatin relative to other types of stromal cells. Increased collagen 1A1 (COL1A1) expression was apparent in activated HSCs but not in fibroblasts or vascular endothelial cells in MCTS. Additionally, COL1A1 deficiency, which was increased by co-culture with HSCs, decreased the cell-cell interactions and thereby increased the therapeutic efficacy of anticancer therapies in MCTS. Furthermore, losartan, which can inhibit collagen I synthesis, attenuated the compactness of spheroids and increased the therapeutic efficacy of anticancer therapies in MCTS. Meanwhile, activated HSCs facilitated HCC migration by upregulating matrix metallopeptidase 9 (MMP9) in MCTS. Collectively, crosstalk between HCC cells and HSCs promoted HCC chemoresistance and migration by increasing the expression of COL1A1 and MMP9 in MCTS. Hence, targeting HSCs might represent a promising therapeutic strategy for liver cancer therapy.

Identification of poly(rC) binding protein 2 (PCBP2) as a target protein of immunosuppressive agent 15-deoxyspergualin

15-Deoxyspergualin (DSG) is an immunosuppressive agent being clinically used. Unlike tacrolimus and cyclosporine A, it does not inhibit the calcineurin pathway, and its mechanism of action and target molecule have not been elucidated. Therefore, we previously prepared biotinylated derivative of DSG (BDSG) to fish up the target protein. In the present research, we identified poly(rC) binding protein 2 (PCBP2) as a DSG-binding protein using this probe. DSG was confirmed to bind to PCBP2 by pull-down assay. Intracellular localization of PCBP2 was changed from the nucleus to the cytoplasm by DSG treatment. DSG inhibited the cell growth, and over-expression of PCBP2 reduced the anti-proliferative activity of DSG. PCBP2 is known to regulate various proteins including STAT1/2. Thus, we found PCBP2 as the first target protein of DSG that can explain the immunosuppressive activity.

Unfolded protein response induced by Brefeldin A increases collagen type I levels in hepatic stellate cells through an IRE1α, p38 MAPK and Smad-dependent pathway

Unfolded protein response (UPR) triggered as a consequence of ER stress has been shown to be involved in the development of different pathologies, including fibrotic disorders. In the present paper we explore the role played by UPR on a key fibrogenic parameter in the liver: collagen type I levels in activated hepatic stellate cells (HSC). Using Brefeldin A (BFA) as an ER stress inducer we found that UPR correlated with enhanced mRNA and protein levels of collagen type I in a cell line of immortalized non-tumoral rat HSC. Analysis of the three branches of UPR revealed the activation of IRE1α, PERK and ATF6 in response to BFA, although PERK activation was shown not to be involved in the fibrogenic action of BFA. BFA also activated p38 MAPK in an IRE1α-dependent way and the p38 MAPK inhibitor SB203580 prevented the increase in collagen type I mRNA and protein levels caused by BFA, suggesting the involvement of this kinase on this effect. Analysis of Smad activation showed that phosphorylated nuclear levels of Smad2 and 3 were increased in response to BFA treatment. Inhibition of Smad3 phosphorylation by SIS3 prevented the enhancement of collagen type I levels caused by BFA. Pretreatment with IRE1α and p38 MAPK inhibitors also prevented the increased p-Smad3 accumulation in the nucleus, suggesting an IRE1α-p38 MAPK-Smad pathway to be responsible for the fibrogenic action of BFA on HSC.

LARP6 Meets Collagen mRNA: Specific Regulation of Type I Collagen Expression

Type I collagen is the most abundant structural protein in all vertebrates, but its constitutive rate of synthesis is low due to long half-life of the protein (60-70 days). However, several hundred fold increased production of type I collagen is often seen in reparative or reactive fibrosis. The mechanism which is responsible for this dramatic upregulation is complex, including multiple levels of regulation. However, posttranscriptional regulation evidently plays a predominant role. Posttranscriptional regulation comprises processing, transport, stabilization and translation of mRNAs and is executed by RNA binding proteins. There are about 800 RNA binding proteins, but only one, La ribonucleoprotein domain family member 6 (LARP6), is specifically involved in type I collagen regulation. In the 5'untranslated region (5'UTR) of mRNAs encoding for type I and type III collagens there is an evolutionally conserved stem-loop (SL) structure; this structure is not found in any other mRNA, including any other collagen mRNA. LARP6 binds to the 5'SL in sequence specific manner to regulate stability of collagen mRNAs and their translatability. Here, we will review current understanding of how is LARP6 involved in posttranscriptional regulation of collagen mRNAs. We will also discuss how other proteins recruited by LARP6, including nonmuscle myosin, vimentin, serine threonine kinase receptor associated protein (STRAP), 25 kD FK506 binding protein (FKBP25) and RNA helicase A (RHA), contribute to this process.

Mice deficient in poly(C)-binding protein 4 are susceptible to spontaneous tumors through increased expression of ZFP871 that targets p53 for degradation

Poly(C)-binding protein 4 (PCBP4), also called MCG10 and a target of p53, plays a role in the cell cycle and is implicated in lung tumor suppression. Here, we found that PCBP4-deficient mice are prone to lung adenocarcinoma, lymphoma, and kidney tumor and that PCBP4-deficient mouse embryo fibroblasts (MEFs) exhibit enhanced cell proliferation but decreased cellular senescence. We also found that p53 expression is markedly reduced in PCBP4-deficient MEFs and mouse tissues, suggesting that PCBP4 in turn regulates p53 expression. To determine how PCBP4 regulates p53 expression, PCBP4 targets were identified by RNA immunoprecipitation followed by RNA sequencing (RNA-seq). We found that the transcript encoding ZFP871 (zinc finger protein 871; also called ZNF709 in humans) interacts with and is regulated by PCBP4 via mRNA stability. Additionally, we found that ZFP871 physically interacts with p53 and MDM2 proteins. Consistently, ectopic expression of ZFP871 decreases-whereas knockdown of ZFP871 increases-p53 protein stability through a proteasome-dependent degradation pathway. Moreover, loss of ZFP871 reverses the reduction of p53 expression by lack of PCBP4, and thus increased expression of ZFP871 is responsible for decreased expression of p53 in the PCBP4-deficient MEFs and mouse tissues. Interestingly, we found that, like PCBP4, ZFP871 is also regulated by DNA damage and p53. Finally, we showed that knockdown of ZFP871 markedly enhances p53 expression, leading to growth suppression and apoptosis in a p53-dependent manner. Thus, the p53-PCBP4-ZFP871 axis represents a novel feedback loop in the p53 pathway. Together, we hypothesize that PCBP4 is a potential tissue-specific tumor suppressor and that ZFP871 is part of MDM2 and possibly other ubiquitin E3 ligases that target p53 for degradation.

The RNA-binding protein PCBP2 inhibits Ang II-induced hypertrophy of cardiomyocytes though promoting GPR56 mRNA degeneration

Poly(C)-binding proteins (PCBPs) are known as RNA-binding proteins that interact in a sequence-specific fashion with single-stranded poly(C). This family can be divided into two groups: hnRNP K and PCBP1-4. PCBPs are expressed broadly in human and mouse tissues and all members of the PCBP family are related evolutionarily. However, their physiological or pathological functions in the hearts remain unknown. Here we reported that PCBP2 is an anti-hypertrophic factor by inhibiting GPR56 mRNA stability. We found the downregulation of PCBP2 in human failing hearts and mouse hypertrophic hearts. PCBP2 knockdown promoted angiotensin II (Ang II)-induced hypertrophy (increase in cell size, protein synthesis and activation of fetal genes) of neonatal cardiomyocytes and H9C2 cells, while PCBP2 overexpression obtained oppose effects. Furthermore, PCBP2 was shown to inhibit GPR56 expression by promoting its mRNA degeneration in cardiomyocytes. Finally, we knocked down GPR56 in cardiomyocytes and found that GPR56 promoted Ang II-induced cardiomyocyte hypertrophy and it contributed to PCBP2 effects on cardiomyocyte hypertrophy.

Hepatic stellate cells and liver fibrosis

Hepatic stellate cells are resident perisinusoidal cells distributed throughout the liver, with a remarkable range of functions in normal and injured liver. Derived embryologically from septum transversum mesenchyme, their precursors include submesothelial cells that invade the liver parenchyma from the hepatic capsule. In normal adult liver, their most characteristic feature is the presence of cytoplasmic perinuclear droplets that are laden with retinyl (vitamin A) esters. Normal stellate cells display several patterns of intermediate filaments expression (e.g., desmin, vimentin, and/or glial fibrillary acidic protein) suggesting that there are subpopulations within this parental cell type. In the normal liver, stellate cells participate in retinoid storage, vasoregulation through endothelial cell interactions, extracellular matrix homeostasis, drug detoxification, immunotolerance, and possibly the preservation of hepatocyte mass through secretion of mitogens including hepatocyte growth factor. During liver injury, stellate cells activate into alpha smooth muscle actin-expressing contractile myofibroblasts, which contribute to vascular distortion and increased vascular resistance, thereby promoting portal hypertension. Other features of stellate cell activation include mitogen-mediated proliferation, increased fibrogenesis driven by connective tissue growth factor, and transforming growth factor beta 1, amplified inflammation and immunoregulation, and altered matrix degradation. Evolving areas of interest in stellate cell biology seek to understand mechanisms of their clearance during fibrosis resolution by either apoptosis, senescence, or reversion, and their contribution to hepatic stem cell amplification, regeneration, and hepatocellular cancer.

Role of LARP6 and nonmuscle myosin in partitioning of collagen mRNAs to the ER membrane

Type I collagen is extracellular matrix protein composed of two α1(I) and one α2(I) polypeptides that fold into triple helix. Collagen polypeptides are translated in coordination to synchronize the rate of triple helix folding to the rate of posttranslational modifications of individual polypeptides. This is especially important in conditions of high collagen production, like fibrosis. It has been assumed that collagen mRNAs are targeted to the membrane of the endoplasmic reticulum (ER) after translation of the signal peptide and by signal peptide recognition particle (SRP). Here we show that collagen mRNAs associate with the ER membrane even when translation is inhibited. Knock down of LARP6, an RNA binding protein which binds 5' stem-loop of collagen mRNAs, releases a small amount of collagen mRNAs from the membrane. Depolimerization of nonmuscle myosin filaments has a similar, but stronger effect. In the absence of LARP6 or nonmuscle myosin filaments collagen polypeptides become hypermodified, are poorly secreted and accumulate in the cytosol. This indicates lack of coordination of their synthesis and retro-translocation due to hypermodifications and misfolding. Depolimerization of nonmuscle myosin does not alter the secretory pathway through ER and Golgi, suggesting that the role of nonmuscle myosin is primarily to partition collagen mRNAs to the ER membrane. We postulate that collagen mRNAs directly partition to the ER membrane prior to synthesis of the signal peptide and that LARP6 and nonmuscle myosin filaments mediate this process. This allows coordinated initiation of translation on the membrane bound collagen α1(I) and α2(I) mRNAs, a necessary step for proper synthesis of type I collagen.

Origin of myofibroblasts in the fibrotic liver in mice

Hepatic myofibroblasts are activated in response to chronic liver injury of any etiology to produce a fibrous scar. Despite extensive studies, the origin of myofibroblasts in different types of fibrotic liver diseases is unresolved. To identify distinct populations of myofibroblasts and quantify their contribution to hepatic fibrosis of two different etiologies, collagen-α1(I)-GFP mice were subjected to hepatotoxic (carbon tetrachloride; CCl4) or cholestatic (bile duct ligation; BDL) liver injury. All myofibroblasts were purified by flow cytometry of GFP(+) cells and then different subsets identified by phenotyping. Liver resident activated hepatic stellate cells (aHSCs) and activated portal fibroblasts (aPFs) are the major source (>95%) of fibrogenic myofibroblasts in these models of liver fibrosis in mice. As previously reported using other methodologies, hepatic stellate cells (HSCs) are the major source of myofibroblasts (>87%) in CCl4 liver injury. However, aPFs are a major source of myofibroblasts in cholestatic liver injury, contributing >70% of myofibroblasts at the onset of injury (5 d BDL). The relative contribution of aPFs decreases with progressive injury, as HSCs become activated and contribute to the myofibroblast population (14 and 20 d BDL). Unlike aHSCs, aPFs respond to stimulation with taurocholic acid and IL-25 by induction of collagen-α1(I) and IL-13, respectively. Furthermore, BDL-activated PFs express high levels of collagen type I and provide stimulatory signals to HSCs. Gene expression analysis identified several novel markers of aPFs, including a mesothelial-specific marker mesothelin. PFs may play a critical role in the pathogenesis of cholestatic liver fibrosis and, therefore, serve as an attractive target for antifibrotic therapy.

The types of hepatic myofibroblasts contributing to liver fibrosis of different etiologies

Liver fibrosis results from dysregulation of normal wound healing, inflammation, activation of myofibroblasts, and deposition of extracellular matrix (ECM). Chronic liver injury causes death of hepatocytes and formation of apoptotic bodies, which in turn, release factors that recruit inflammatory cells (neutrophils, monocytes, macrophages, and lymphocytes) to the injured liver. Hepatic macrophages (Kupffer cells) produce TGFβ1 and other inflammatory cytokines that activate Collagen Type I producing myofibroblasts, which are not present in the normal liver. Secretion of TGFβ1 and activation of myofibroblasts play a critical role in the pathogenesis of liver fibrosis of different etiologies. Although the composition of fibrogenic myofibroblasts varies dependent on etiology of liver injury, liver resident hepatic stellate cells and portal fibroblasts are the major source of myofibroblasts in fibrotic liver in both experimental models of liver fibrosis and in patients with liver disease. Several studies have demonstrated that hepatic fibrosis can reverse upon cessation of liver injury. Regression of liver fibrosis is accompanied by the disappearance of fibrogenic myofibroblasts followed by resorption of the fibrous scar. Myofibroblasts either apoptose or inactivate into a quiescent-like state (e.g., stop collagen production and partially restore expression of lipogenic genes). Resolution of liver fibrosis is associated with recruitment of macrophages that secrete matrix-degrading enzymes (matrix metalloproteinase, collagenases) and are responsible for fibrosis resolution. However, prolonged/repeated liver injury may cause irreversible crosslinking of ECM and formation of uncleavable collagen fibers. Advanced fibrosis progresses to cirrhosis and hepatocellular carcinoma. The current review will summarize the role and contribution of different cell types to populations of fibrogenic myofibroblasts in fibrotic liver.

Structural determinants of human ζ-globin mRNA stability

Background

The normal accumulation of adult α and β globins in definitive erythrocytes is critically dependent upon processes that ensure that the cognate mRNAs are maintained at high levels in transcriptionally silent, but translationally active progenitor cells. The impact of these post-transcriptional regulatory events on the expression of embryonic ζ globin is not known, as its encoding mRNA is not normally transcribed during adult erythropoiesis. Recently, though, ζ globin has been recognized as a potential therapeutic for α thalassemia and sickle-cell disease, raising practical questions about constitutive post-transcriptional processes that may enhance, or possibly prohibit, the expression of exogenous or derepresssed endogenous ζ-globin genes in definitive erythroid progenitors.

Methods

The present study assesses mRNA half-life in intact cells using a pulse-chase approach; identifies cis-acting determinants of ζ-globin mRNA stability using a saturation mutagenesis strategy; establishes critical 3′UTR secondary structures using an in vitro enzymatic mapping method; and identifies trans-acting effector factors using an affinity chromatographical procedure.

Results

We specify a tetranucleotide 3′UTR motif that is required for the high-level accumulation of ζ-globin transcripts in cultured cells, and formally demonstrate that it prolongs their cytoplasmic half-lives. Surprisingly, the ζ-globin mRNA stability motif does not function autonomously, predicting an activity that is subject to structural constraints that we subsequently specify. Additional studies demonstrate that the ζ-globin mRNA stability motif is targeted by AUF1, a ubiquitous RNA-binding protein that enhances the half-life of adult β-globin mRNA, suggesting commonalities in post-transcriptional processes that regulate globin transcripts at all stages of mammalian development.

Conclusions

These data demonstrate a mechanism for ζ-globin mRNA stability that exists in definitive erythropoiesis and is available for therapeutic manipulation in α thalassemia and sickle-cell disease.

Absence of feedback regulation in the synthesis of COL1A1

AIM

Recent studies have emphasized the importance of the extracellular microenvironment in modulating cell growth, motility, and signalling. In this study we have evaluated the ability of a fibroblast derived-extracellular matrix (fd-ECM) to regulate type I collagen synthesis and degradation in fibroblasts.

MAIN METHODS

Fibroblasts were plated on plastic (control) or on fd-ECM and type I collagen synthesis and degradation was evaluated. MTT, western blotting, real time PCR, zymographic analysis and inhibitor assays were utilised to investigate the molecular mechanism of type I collagen regulation by the fd-ECM.

KEY FINDINGS

Fibroblasts plated on fd-ECM showed significant downregulation in the production of type I collagen and COL1A2 messenger ribonucleic acid (mRNA) whilst COL1A1 mRNA remained unchanged. Cells grown on fd-ECM exhibited increased matrix metalloproteases (MMPs) and their corresponding mRNAs. The use of transforming growth factor β (TGF-β) and MMP inhibitors showed that the excess COL1A1 polypeptide chains were degraded by the combined action of MMP-1, MMP-2, MMP-9 and cathepsins.

SIGNIFICANCE

These results show the crucial role played by proteases in regulating extracellular matrix protein levels in the feedback regulation of connective tissue gene expression.

Specific enrichment of the RNA-binding proteins PCBP1 and PCBP2 in chief cells of the murine gastric mucosa

RNA-binding proteins and corresponding post-transcriptional controls play critical roles in gene expression. The poly-(C) binding proteins, PCBPs (αCPs, hnRNPEs), comprise a well-characterized family of abundant RNA-binding proteins that impact on RNA processing in the nucleus as well as mRNA stability and translation in the cytoplasm. Here we demonstrate that PCBP1 and PCBP2 are abundantly expressed in the gastric epithelium with prominent enrichment in specific cell types within the gastric glandular mucosa. The spatial and intracellular patterns of PCBP1 and PCBP2 expression in these regions are highly correlated. Remarkably, we observe that these proteins are present in the nuclear and cytoplasmic compartments of zymogenic chief cells while they are restricted to the nuclear compartment in acid-secreting parietal cells and poorly expressed in pit cells that line the gland exit. This specificity of expression patterns and subcellular localization of PCBP1 and PCBP2, along with their appearance in the precursor tissues of the gastric epithelium during early postnatal development, suggests these RNA-binding proteins play specific roles in cell differentiation and organismal development within the gastric glandular epithelium.

Serine-threonine kinase receptor-associated protein (STRAP) regulates translation of type I collagen mRNAs

Type I collagen is the most abundant protein in the human body and is composed of two α1(I) and one α2(I) polypeptides which assemble into a triple helix. For the proper assembly of the collagen triple helix, the individual polypeptides must be translated in coordination. Here, we show that serine-threonine kinase receptor-associated protein (STRAP) is tethered to collagen mRNAs by interaction with LARP6. LARP6 is a protein which directly binds the 5' stem-loop (5'SL) present in collagen α1(I) and α2(I) mRNAs, but it interacts with STRAP with its C-terminal domain, which is not involved in binding 5'SL. Being tethered to collagen mRNAs, STRAP prevents unrestricted translation, primarily that of collagen α2(I) mRNAs, by interacting with eukaryotic translation initiation factor 4A (eIF4A). In the absence of STRAP, more collagen α2(I) mRNA can be pulled down with eIF4A, and collagen α2(I) mRNA is unrestrictedly loaded onto the polysomes. This results in an imbalance of synthesis of α1(I) and α2(I) polypeptides, in hypermodifications of α1(I) polypeptide, and in inefficient assembly of the polypeptides into a collagen trimer and their secretion as monomers. These defects can be partially restored by supplementing STRAP. Thus, we discovered STRAP as a novel regulator of the coordinated translation of collagen mRNAs.

RNA protein interactions governing expression of the most abundant protein in human body, type I collagen

Type I collagen is the most abundant protein in human body. The protein turns over slowly and its replacement synthesis is low. However, in wound healing or in pathological fibrosis the cells can increase production of type I collagen several hundred fold. This increase is predominantly due to posttranscriptional regulation, including increased half-life of collagen messenger RNAs (mRNAs) and their increased translatability. Type I collagen is composed of two α1 and one α2 polypeptides that fold into a triple helix. This stoichiometry is strictly regulated to prevent detrimental synthesis of α1 homotrimers. Collagen polypeptides are co-translationally modified and the rate of modifications is in dynamic equilibrium with the rate of folding, suggesting coordinated translation of collagen α1(I) and α2(I) polypeptides. Collagen α1(I) mRNA has in the 3' untranslated region (UTR) a C-rich sequence that binds protein αCP, this binding stabilizes the mRNA in collagen producing cells. In the 5' UTR both collagen mRNAs have a conserved stem-loop (5' SL) structure. The 5' SL is critical for high collagen expression, knock in mice with disruption of the 5' SL are resistant to liver fibrosis. the 5' SL binds protein LARP6 with strict sequence specificity and high affinity. LARP6 recruits RNA helicase A to facilitate translation initiation and associates collagen mRNAs with vimentin and nonmuscle myosin filaments. Binding to vimentin stabilizes collagen mRNAs, while nonmuscle myosin regulates coordinated translation of α1(I) and α2(I) mRNAs. When nonmuscle myosin filaments are disrupted the cells secrete only α1 homotrimers. Thus, the mechanism governing high collagen expression involves two RNA binding proteins and development of cytoskeletal filaments.

Norovirus genome circularization and efficient replication are facilitated by binding of PCBP2 and hnRNP A1

Sequences and structures within the terminal genomic regions of plus-strand RNA viruses are targets for the binding of host proteins that modulate functions such as translation, RNA replication, and encapsidation. Using murine norovirus 1 (MNV-1), we describe the presence of long-range RNA-RNA interactions that were stabilized by cellular proteins. The proteins potentially responsible for the stabilization were selected based on their ability to bind the MNV-1 genome and/or having been reported to be involved in the stabilization of RNA-RNA interactions. Cell extracts were preincubated with antibodies against the selected proteins and used for coprecipitation reactions. Extracts treated with antibodies to poly(C) binding protein 2 (PCBP2) and heterogeneous nuclear ribonucleoprotein (hnRNP) A1 significantly reduced the 5'-3' interaction. Both PCBP2 and hnRNP A1 recombinant proteins stabilized the 5'-3' interactions and formed ribonucleoprotein complexes with the 5' and 3' ends of the MNV-1 genomic RNA. Mutations within the 3' complementary sequences (CS) that disrupt the 5'-3'-end interactions resulted in a significant reduction of the viral titer, suggesting that the integrity of the 3'-end sequence and/or the lack of complementarity with the 5' end is important for efficient virus replication. Small interfering RNA-mediated knockdown of PCBP2 or hnRNP A1 resulted in a reduction in virus yield, confirming a role for the observed interactions in efficient viral replication. PCBP2 and hnRNP A1 induced the circularization of MNV-1 RNA, as revealed by electron microscopy. This study provides evidence that PCBP2 and hnRNP A1 bind to the 5' and 3' ends of the MNV-1 viral RNA and contribute to RNA circularization, playing a role in the virus life cycle.

Tacrolimus (FK506) prevents early stages of ethanol induced hepatic fibrosis by targeting LARP6 dependent mechanism of collagen synthesis

Tacrolimus (FK506) is a widely used immunosuppressive drug. Its effects on hepatic fibrosis have been controversial and attributed to immunosuppression. We show that in vitro FK506, inhibited synthesis of type I collagen polypeptides, without affecting expression of collagen mRNAs. In vivo, administration of FK506 at a dose of 4 mg/kg completely prevented development of alcohol/carbon tetrachloride induced liver fibrosis in rats. Activation of hepatic stellate cells (HSCs) was absent in the FK506 treated livers and expression of collagen α2(I) mRNA was at normal levels. Collagen α1(I) mRNA was increased in the FK506 treated livers, but this mRNA was not translated into α1(I) polypeptide. No significant inflammation was associated with the fibrosis model used. FK506 binding protein 3 (FKBP3) is one of cellular proteins which binds FK506 with high affinity. We discovered that FKBP3 interacts with LARP6 and LARP6 is the major regulator of translation and stability of collagen mRNAs. In the presence of FK506 the interaction between FKBP3 and LARP6 is weakened and so is the pull down of collagen mRNAs with FKBP3. We postulate that FK506 inactivates FKBP3 and that lack of interaction of LARP6 and FKBP3 results in aberrant translation of collagen mRNAs and prevention of fibrosis. This is the first report of such activity of FK506 and may renew the interest in using this drug to alleviate hepatic fibrosis.

RNA-binding protein PCBP2 modulates glioma growth by regulating FHL3

PCBP2 is a member of the poly(C)-binding protein (PCBP) family, which plays an important role in posttranscriptional and translational regulation by interacting with single-stranded poly(C) motifs in target mRNAs. Several PCBP family members have been reported to be involved in human malignancies. Here, we show that PCBP2 is upregulated in human glioma tissues and cell lines. Knockdown of PCBP2 inhibited glioma growth in vitro and in vivo through inhibition of cell-cycle progression and induction of caspase-3-mediated apoptosis. Thirty-five mRNAs were identified as putative PCBP2 targets/interactors using RIP-ChIP protein-RNA interaction arrays in a human glioma cell line, T98G. Four-and-a-half LIM domain 3 (FHL3) mRNA was downregulated in human gliomas and was identified as a PCBP2 target. Knockdown of PCBP2 enhanced the expression of FHL3 by stabilizing its mRNA. Overexpression of FHL3 attenuated cell growth and induced apoptosis. This study establishes a link between PCBP2 and FHL3 proteins and identifies a new pathway for regulating glioma progression.

Active stabilization of human endothelial nitric oxide synthase mRNA by hnRNP E1 protects against antisense RNA and microRNAs

Human endothelial nitric oxide synthase (eNOS) mRNA is highly stable in endothelial cells (ECs). Posttranscriptional regulation of eNOS mRNA stability is an important component of eNOS regulation, especially under hypoxic conditions. Here, we show that the human eNOS 3' untranslated region (3' UTR) contains multiple, evolutionarily conserved pyrimidine (C and CU)-rich sequence elements that are both necessary and sufficient for mRNA stabilization. Importantly, RNA immunoprecipitations and RNA electrophoretic mobility shift assays (EMSAs) revealed the formation of heterogeneous nuclear ribonucleoprotein E1 (hnRNP E1)-containing RNP complexes at these 3'-UTR elements. Knockdown of hnRNP E1 decreased eNOS mRNA half-life, mRNA levels, and protein expression. Significantly, these stabilizing RNP complexes protect eNOS mRNA from the inhibitory effects of its antisense transcript sONE and 3'-UTR-targeting small interfering RNAs (siRNAs), as well as microRNAs, specifically, hsa-miR-765, which targets eNOS mRNA stability determinants. Hypoxia disrupts hnRNP E1/eNOS 3'-UTR interactions via increased Akt-mediated serine phosphorylation (including serine 43) and increased nuclear localization of hnRNP E1. These mechanisms account, at least in part, for the decrease in eNOS mRNA stability under hypoxic conditions. Thus, the stabilization of human eNOS mRNA by hnRNP E1-containing RNP complexes serves as a key protective mechanism against the posttranscriptional inhibitory effects of antisense RNA and microRNAs under basal conditions but is disrupted under hypoxic conditions.

Markers in nonalcoholic steatohepatitis

Nonalcoholic fatty liver disease (NAFLD), the most common liver disorder worldwide, encompasses a spectrum of abnormal liver histology ranging from simple steatosis to nonalcoholic steatohepatitis (NASH) and cirrhosis. Population studies show that NAFLD is strongly associated with insulin resistance, obesity, type 2 diabetes mellitus, and lipid abnormalities. In the context of hepatic steatosis, factors that promote cell injury, inflammation, and fibrosis include oxidative stress, early mitochondrial dysfunction, endoplasmic reticulum stress, iron accumulation, apoptosis, adipocytokines, and stellate cell activation. The exact NASH prevalence is unknown because of the absence of simple noninvasive diagnostic tests. Although liver biopsy is the "gold standard" for the diagnosis of NASH, other tests are needed to facilitate the diagnosis and greatly reduce the requirement for invasive liver biopsy. In addition, the development of new fibrosis markers in NASH is needed to facilitate the assessment of its progression and the effectiveness of new therapies. The aim of this chapter, which is overview of biomarkers in NASH, is to establish a systematic approach to laboratory findings of the disease.

Role of cytokine receptor-like factor 1 in hepatic stellate cells and fibrosis

AIM

To elucidate the role of cytokine receptor-like factor 1 (CRLF1) in hepatic stellate cells and liver fibrosis.

METHODS

Rat hepatic stellate cells (HSCs) were isolated by Nykodenz gradient centrifugation and activated by culturing in vitro. Differentially expressed genes in quiescent and culture activated HSCs were identified using microarrays. Injections of carbon tetrachloride (CCl(4)) for 4 wk were employed to induce liver fibrosis. The degree of fibrosis was assessed by Sirius red staining. Adenovirus expressing CRLF1 was injected through tail vein into mice to achieve overexpression of CRLF1 in the liver. The same adenovirus was used to overexpress CRLF1 in quiescent HSCs cultured in vitro. Expression of CRLF1, CLCF1 and ciliary neurotrophic factor receptor (CNTFR) in hepatic stellate cells and fibrotic livers was analyzed by semi-quantitative reverse transcription-polymerase chain reaction and Western blotting. Expression of profibrotic cytokines and collagens was analyzed by the same method.

RESULTS

CRLF1 is a secreted cytokine with unknown function. Human mutations suggested a role in development of autonomous nervous system and a role of CRLF1 in immune response was implied by its similarity to interleukin (IL)-6. Here we show that expression of CRLF1 was undetectable in quiescent HSCs and was highly upregulated in activated HSCs. Likewise, expression of CRLF1 was very low in normal livers, but was highly upregulated in fibrotic livers, where its expression correlated with the degree of fibrosis. A cofactor of CLRF1, cardiotrophin-like cytokine factor 1 (CLCF1), and the receptor which binds CRLF1/CLCF1 dimer, the CNTFR, were expressed to similar levels in quiescent and activated HSCs and in normal and fibrotic livers, indicating a constitutive expression. Overexpression of CLRF1 alone in the normal liver did not stimulate expression of profibrotic cytokines, suggesting that the factor itself is not pro-inflammatory. Ectopic expression in quiescent HSCs, however, retarded their activation into myofibroblasts and specifically decreased expression of type III collagen. Inhibition of type III collagen expression by CRLF1 was also seen in the whole liver. Our results suggest that CLRF1 is the only component of the CRLF1/CLCF1/CNTFR signaling system that is inducible by a profibrotic stimulus and that activation of this system by CLRF1 may regulate expression of type III collagen in fibrosis.

CONCLUSION

By regulating activation of HSCs and expression of type III collagen, CRLF1 may have an ability to change the composition of extracellular matrix in fibrosis.

Withaferin-A reduces type I collagen expression in vitro and inhibits development of myocardial fibrosis in vivo

Type I collagen is the most abundant protein in the human body. Its excessive synthesis results in fibrosis of various organs. Fibrosis is a major medical problem without an existing cure. Excessive synthesis of type I collagen in fibrosis is primarily due to stabilization of collagen mRNAs. We recently reported that intermediate filaments composed of vimentin regulate collagen synthesis by stabilizing collagen mRNAs. Vimentin is a primary target of Withaferin-A (WF-A). Therefore, we hypothesized that WF-A may reduce type I collagen production by disrupting vimentin filaments and decreasing the stability of collagen mRNAs. This study is to determine if WF-A exhibits anti-fibrotic properties in vitro and in vivo and to elucidate the molecular mechanisms of its action. In lung, skin and heart fibroblasts WF-A disrupted vimentin filaments at concentrations of 0.5-1.5 µM and reduced 3 fold the half-lives of collagen α1(I) and α2(I) mRNAs and protein expression. In addition, WF-A inhibited TGF-β1 induced phosphorylation of TGF-β1 receptor I, Smad3 phosphorylation and transcription of collagen genes. WF-A also inhibited in vitro activation of primary hepatic stellate cells and decreased their type I collagen expression. In mice, administration of 4 mg/kg WF-A daily for 2 weeks reduced isoproterenol-induced myocardial fibrosis by 50%. Our findings provide strong evidence that Withaferin-A could act as an anti-fibrotic compound against fibroproliferative diseases, including, but not limited to, cardiac interstitial fibrosis.

Influence of physiologic folate deficiency on human papillomavirus type 16 (HPV16)-harboring human keratinocytes in vitro and in vivo

Although HPV16 transforms infected epithelial tissues to cancer in the presence of several co-factors, there is insufficient molecular evidence that poor nutrition has any such role. Because physiological folate deficiency led to the intracellular homocysteinylation of heterogeneous nuclear ribonucleoprotein E1 (hnRNP-E1) and activated a nutrition-sensitive (homocysteine-responsive) posttranscriptional RNA operon that included interaction with HPV16 L2 mRNA, we investigated the functional consequences of folate deficiency on HPV16 in immortalized HPV16-harboring human (BC-1-Ep/SL) keratinocytes and HPV16-organotypic rafts. Although homocysteinylated hnRNP-E1 interacted with HPV16 L2 mRNA cis-element, it also specifically bound another HPV16 57-nucleotide poly(U)-rich cis-element in the early polyadenylation element (upstream of L2L1 genes) with greater affinity. Together, these interactions led to a profound reduction of both L1 and L2 mRNA and proteins without effects on HPV16 E6 and E7 in vitro, and in cultured keratinocyte monolayers and HPV16-low folate-organotypic rafts developed in physiological low folate medium. In addition, HPV16-low folate-organotypic rafts contained fewer HPV16 viral particles, a similar HPV16 DNA viral load, and a much greater extent of integration of HPV16 DNA into genomic DNA when compared with HPV16-high folate-organotypic rafts. Subcutaneous implantation of 18-day old HPV16-low folate-organotypic rafts into folate-replete immunodeficient mice transformed this benign keratinocyte-derived raft tissue into an aggressive HPV16-induced cancer within 12 weeks. Collectively, these studies establish a likely molecular linkage between poor folate nutrition and HPV16 and predict that nutritional folate and/or vitamin-B(12) deficiency, which are both common worldwide, will alter the natural history of HPV16 infections and also warrant serious consideration as reversible co-factors in oncogenic transformation of HPV16-infected tissues to cancer.

Contribution of the first K-homology domain of poly(C)-binding protein 1 to its affinity and specificity for C-rich oligonucleotides

Poly-C-binding proteins are triple KH (hnRNP K homology) domain proteins with specificity for single stranded C-rich RNA and DNA. They play diverse roles in the regulation of protein expression at both transcriptional and translational levels. Here, we analyse the contributions of individual αCP1 KH domains to binding C-rich oligonucleotides using biophysical and structural methods. Using surface plasmon resonance (SPR), we demonstrate that KH1 makes the most stable interactions with both RNA and DNA, KH3 binds with intermediate affinity and KH2 only interacts detectibly with DNA. The crystal structure of KH1 bound to a 5′-CCCTCCCT-3′ DNA sequence shows a 2:1 protein:DNA stoichiometry and demonstrates a molecular arrangement of KH domains bound to immediately adjacent oligonucleotide target sites. SPR experiments, with a series of poly-C-sequences reveals that cytosine is preferred at all four positions in the oligonucleotide binding cleft and that a C-tetrad binds KH1 with 10 times higher affinity than a C-triplet. The basis for this high affinity interaction is finally detailed with the structure determination of a KH1.W.C54S mutant bound to 5′-ACCCCA-3′ DNA sequence. Together, these data establish the lead role of KH1 in oligonucleotide binding by αCP1 and reveal the molecular basis of its specificity for a C-rich tetrad.

A novel role of RNA helicase A in regulation of translation of type I collagen mRNAs

Type I collagen is composed of two α1(I) polypeptides and one α2(I) polypeptide and is the most abundant protein in the human body. Expression of type I collagen is primarily controlled at the level of mRNA stability and translation. Coordinated translation of α(I) and α2(I) mRNAs is necessary for efficient folding of the corresponding peptides into the collagen heterotrimer. In the 5' untranslated region (5' UTR), collagen mRNAs have a unique 5' stem-loop structure (5' SL). La ribonucleoprotein domain family member 6 (LARP6) is the protein that binds 5' SL with high affinity and specificity and coordinates their translation. Here we show that RNA helicase A (RHA) is tethered to the 5' SL of collagen mRNAs by interaction with the C-terminal domain of LARP6. In vivo, collagen mRNAs immunoprecipitate with RHA in an LARP6-dependent manner. Knockdown of RHA prevents formation of polysomes on collagen mRNAs and dramatically reduces synthesis of collagen protein, without affecting the level of the mRNAs. A reporter mRNA with collagen 5' SL is translated three times more efficiently in the presence of RHA than the same reporter without the 5' SL, indicating that the 5' SL is the cis-acting element conferring the regulation. During activation of quiescent cells into collagen-producing cells, expression of RHA is highly up-regulated. We postulate that RHA is recruited to the 5' UTR of collagen mRNAs by LARP6 to facilitate their translation. Thus, RHA has been discovered as a critical factor for synthesis of the most abundant protein in the human body.

Liver fibrosis protects mice from acute hepatocellular injury

BACKGROUND & AIMS

Development of fibrosis is part of the pathophysiologic process of chronic liver disease. Although it is considered deleterious, it also represents a form of tissue repair. Deposition of extracellular matrix changes the cellular environment of the liver; we investigated whether it increases resistance to noxious stimuli and the role of changes in intracellular signaling to hepatocytes in mediating this effect.

METHODS

Primary cultures of mouse hepatocytes were exposed to type I collagen (COL1); cell injury was assessed by morphologic and biochemical criteria. The expression of Bcl-2 family members was evaluated by immunoblot analyses. Activation of extracellular signal-regulated kinase (ERK) was assessed using phospho-specific antibodies. Liver fibrosis was induced by repeated administration of thioacetamide or carbon tetrachloride to mice; mice were then exposed to Fas antibodies.

RESULTS

Hepatocytes exposed to COL1 were more resistant to a variety of hepatotoxins, in a dose-dependent manner, and had lower levels of Bad, Bid, and Bax proapoptotic proteins compared with control hepatocytes. Activation of ERK1/2 was stronger and quicker in hepatocytes exposed to COL1. The MEK1/2 inhibitors U0126 and PD98059 reversed the protective effects of COL1 and the decrease in proapoptotic proteins. Hepatocytes isolated from ERK1(-/-) mice were insensitive to the protective effect of COL1. Fibrotic livers from wild-type mice had high levels of phospho-ERK1 and were resistant to Fas-induced cell death. ERK1(-/-) mice lost this effect.

CONCLUSIONS

Production of COL1 during liver fibrosis induces a hepatoprotective response that is mediated by activation of ERK1 signaling.

PCBP2 siRNA reverses the alcohol-induced pro-fibrogenic effects in hepatic stellate cells

PURPOSE

Type I collagen accumulates during liver fibrosis primarily because α-complex protein-2 (αCP(2)), encoded by the poly(rC) binding protein 2 (PCBP2) gene, binds to the 3' end of the collagen mRNA and increases its half-life. This study aimed to reverse the pro-fibrogenic effect of alcohol on hepatic stellate cells (HSCs) by silencing the PCBP2 gene with siRNA.

METHODS

The silencing effects of a series of predesigned PCBP2 siRNAs were evaluated in the rat hepatic stellate cell line, HSC-T6. The pro-fibrogenic effects of alcohol on the expression levels of PCBP2 and type-I collagen were examined by several methods. The effect of PCBP2 siRNA on the stability of type I collagen α1(I) mRNA was investigated by an in vitro mRNA decay assay.

RESULTS

We identified one potent PCBP2 siRNA that reversed the alcohol-induced expression of PCBP2 in HSCs. The decay rate of the collagen α1(I) mRNA increased significantly in HSCs treated with the PCBP2 siRNA.

CONCLUSION

This study provides the first evidence that alcohol up-regulates the expression of PCBP2, which subsequently increases the half-life of collagen α1(I) mRNA. Silencing of PCBP2 using siRNA may provide a promising strategy to reverse the alcohol-induced pro-fibrogenic effects in HSCs.

Myocardin-related transcription factor-A complexes activate type I collagen expression in lung fibroblasts

Pulmonary fibrosis is characterized by the excessive deposition of a collagen-rich extracellular matrix. The accumulation of collagen within the lung interstitium leads to impaired respiratory function. Furthermore, smooth muscle actin-positive myofibroblasts within the fibrotic lung contribute to disease progression. Because collagen and smooth muscle cell α-actin are coordinately expressed in the setting of fibrosis, the hypothesis was tested that specific transcriptional regulators of the myocardin family might also regulate collagen gene expression in myofibroblasts. Myocardin-related transcription factors (MRTFs), through their interaction with the serum-response factor (SRF) on CArG box regulatory elements (CC(A/T)6GG), are important regulators of myofibroblast differentiation. MRTF-A transactivated type I collagen gene reporters as much as 100-fold in lung myofibroblasts. Loss of functional MRTF-A using either a dominant negative MRTF-A isoform, shRNA targeting MRTF-A, or genetic deletion of MRTF-A in lung fibroblasts significantly disrupted type I collagen synthesis relative to controls. Analysis of the COL1A2 proximal promoter revealed a noncanonical CArG box (CCAAACTTGG), flanked by several Sp1 sites important for MRTF-A activation. Chromatin immunoprecipitation experiments confirmed the co-localization of MRTF-A, SRF, and Sp1 bound to the same region of the COL1A2 promoter. Mutagenesis of either the noncanonical CArG box or the Sp1 sites significantly disrupted MRTF-A activation of COL1A2. Together, our findings show that MRTF-A is an important regulator of collagen synthesis in lung fibroblasts and exhibits a dependence on both SRF and Sp1 function to enhance collagen expression.

Incrimination of heterogeneous nuclear ribonucleoprotein E1 (hnRNP-E1) as a candidate sensor of physiological folate deficiency

The mechanism underlying the sensing of varying degrees of physiological folate deficiency, prior to adaptive optimization of cellular folate uptake through the translational up-regulation of folate receptors (FR) is unclear. Because homocysteine, which accumulates intracellularly during folate deficiency, stimulated interactions between heterogeneous nuclear ribonucleoprotein E1 (hnRNP-E1) and an 18-base FR-α mRNA cis-element that led to increased FR biosynthesis and net up-regulation of FR at cell surfaces, hnRNP-E1 was a plausible candidate sensor of folate deficiency. Accordingly, using purified components, we evaluated the physiological basis whereby L-homocysteine triggered these RNA-protein interactions to stimulate FR biosynthesis. L-homocysteine induced a concentration-dependent increase in RNA-protein binding affinity throughout the range of physiological folate deficiency, which correlated with a proportionate increase in translation of FR in vitro and in cultured human cells. Targeted reduction of newly synthesized hnRNP-E1 proteins by siRNA to hnRNP-E1 mRNA reduced both constitutive and L-homocysteine-induced rates of FR biosynthesis. Furthermore, L-homocysteine covalently bound hnRNP-E1 via multiple protein-cysteine-S-S-homocysteine mixed disulfide bonds within K-homology domains known to interact with mRNA. These data suggest that a concentration-dependent, sequential disruption of critical cysteine-S-S-cysteine bonds by covalently bound L-homocysteine progressively unmasks an underlying RNA-binding pocket in hnRNP-E1 to optimize interaction with FR-α mRNA cis-element preparatory to FR up-regulation. Collectively, such data incriminate hnRNP-E1 as a physiologically relevant, sensitive, cellular sensor of folate deficiency. Because diverse mammalian and viral mRNAs also interact with this RNA-binding domain with functional consequences to their protein expression, homocysteinylated hnRNP-E1 also appears well positioned to orchestrate a novel, nutrition-sensitive (homocysteine-responsive), posttranscriptional RNA operon in folate-deficient cells.

miRNA studies in in vitro and in vivo activated hepatic stellate cells

AIM

To understand which and how different miRNAs are implicated in the process of hepatic stellate cell (HSC) activation.

METHODS

We used microarrays to examine the differential expression of miRNAs during in vitro activation of primary HSCs (pHSCs). The transcriptome changes upon stable transfection of rno-miR-146a into an HSC cell line were studied using cDNA microarrays. Selected differentially regulated miRNAs were investigated by quantitative real-time polymerase chain reaction during in vivo HSC activation. The effect of miRNA mimics and inhibitor on the in vitro activation of pHSCs was also evaluated.

RESULTS

We found that 16 miRNAs were upregulated and 26 were downregulated significantly in 10-d in vitro activated pHSCs in comparison to quiescent pHSCs. Overexpression of rno-miR-146a was characterized by marked upregulation of tissue inhibitor of metalloproteinase-3, which is implicated in the regulation of tumor necrosis factor-α activity. Differences in the regulation of selected miRNAs were observed comparing in vitro and in vivo HSC activation. Treatment with miR-26a and 29a mimics, and miR-214 inhibitor during in vitro activation of pHSCs induced significant downregulation of collagen type I transcription.

CONCLUSION

Our results emphasize the different regulation of miRNAs in in vitro and in vivo activated pHSCs. We also showed that miR-26a, 29a and 214 are involved in the regulation of collagen type I mRNA.

miRNA studies in in vitro and in vivo activated hepatic stellate cells

AIM

To understand which and how different miRNAs are implicated in the process of hepatic stellate cell (HSC) activation.

METHODS

We used microarrays to examine the differential expression of miRNAs during in vitro activation of primary HSCs (pHSCs). The transcriptome changes upon stable transfection of rno-miR-146a into an HSC cell line were studied using cDNA microarrays. Selected differentially regulated miRNAs were investigated by quantitative real-time polymerase chain reaction during in vivo HSC activation. The effect of miRNA mimics and inhibitor on the in vitro activation of pHSCs was also evaluated.

RESULTS

We found that 16 miRNAs were upregulated and 26 were downregulated significantly in 10-d in vitro activated pHSCs in comparison to quiescent pHSCs. Overexpression of rno-miR-146a was characterized by marked upregulation of tissue inhibitor of metalloproteinase-3, which is implicated in the regulation of tumor necrosis factor-α activity. Differences in the regulation of selected miRNAs were observed comparing in vitro and in vivo HSC activation. Treatment with miR-26a and 29a mimics, and miR-214 inhibitor during in vitro activation of pHSCs induced significant downregulation of collagen type I transcription.

CONCLUSION

Our results emphasize the different regulation of miRNAs in in vitro and in vivo activated pHSCs. We also showed that miR-26a, 29a and 214 are involved in the regulation of collagen type I mRNA.

A novel role of vimentin filaments: binding and stabilization of collagen mRNAs

The stem-loop in the 5' untranslated region (UTR) of collagen α1(I) and α2(I) mRNAs (5'SL) is the key element regulating their stability and translation. Stabilization of collagen mRNAs is the predominant mechanism for high collagen expression in fibrosis. LARP6 binds the 5'SL of α1(I) and α2(I) mRNAs with high affinity. Here, we report that vimentin filaments associate with collagen mRNAs in a 5'SL- and LARP6-dependent manner and stabilize collagen mRNAs. LARP6 interacts with vimentin filaments through its La domain and colocalizes with the filaments in vivo. Knockdown of LARP6 by small interfering RNA (siRNA) or mutation of the 5'SL abrogates the interaction of collagen mRNAs with vimentin filaments. Vimentin knockout fibroblasts produce reduced amounts of type I collagen due to decreased stability of collagen α1(I) and α2(I) mRNAs. Disruption of vimentin filaments using a drug or by expression of dominant-negative desmin reduces type I collagen expression, primarily due to decreased stability of collagen mRNAs. RNA fluorescence in situ hybridization (FISH) experiments show that collagen α1(I) and α2(I) mRNAs are associated with vimentin filaments in vivo. Thus, vimentin filaments may play a role in the development of tissue fibrosis by stabilizing collagen mRNAs. This finding will serve as a rationale for targeting vimentin in the development of novel antifibrotic therapies.