Welcome to the Hepatitis B Research Review! This monthly blog shares recent scientific findings with members of Baruch S. Blumberg Institute (BSBI) labs and the hepatitis B (HBV) community. Technical articles concerning HBV, Hepatocellular Carcinoma, and STING protein will be highlighted as well as scientific breakthroughs in cancer, immunology, and virology. For each article, a brief synopsis reporting key points is provided as the BSBI does not enjoy the luxury of a library subscription. The hope is to disseminate relevant articles across our labs and the hep B community.
Summary: This month, researchers at Fudan University in Shanghai, China have identified activation of the cGAS/STING pathway by extracellular DNA as a mediator of radiation-induced liver disease. At the Pennsylvania State University College of Medicine in Hershey, PA, HBV researchers have elucidated the role of the host kinase protein CDK2 in phosphorylating the HBV core protein, leading to new cccDNA formation. Researchers from the University of Charlottesville in Virginia have characterized the “apoptotic metabolite secretome”, a select group of molecules released from cells undergoing apoptosis.
DNA sensing and associated type 1 interferon signaling contributes to progression of radiation-induced liver injury – Cellular & Molecular Immunology
This paper from Fudan University in Shanghai, China reveals the role of the cGAS/STING pathway in radiation-induced liver disease (RILD). Either radiation therapy (RT) or accidental exposure to ionizing radiation may cause RILD. RT is used to treat various cancers, including hepatocellular carcinoma (HCC). The dose of radiation used when treating HCC and gastrointestinal malignancies is limited by the risk of RILD as the liver is a highly radiosensitive organ. RILD is associated with a high mortality in patients with HCC and typically occurs within four months of receiving RT. RILD is characterized by hepatic injury due to the deposition of fibrin into the central veins and sinusoids of the liver. While the exact mechanism of RILD development is not well understood, it has been shown that hepatic nonparenchymal cells (NPCs) such as Kupffer cells, sinusoidal endothelial cells, and hepatic stellate cells play an important role. NPCs are cells in the liver that are not hepatocytes; they consist of immune cells, endothelial cells, pericytes, and other cell types. The cGAS/STING pathway is a component of the innate immune system in cells responsible for sensing double-stranded DNA (dsDNA) in the cytoplasm and subsequently initiating the expression and secretion of type 1 interferons (IFN-I). This publication identifies the cGAS/STING-mediated production of IFN-I by NPCs as a key mediator of RILD. The authors propose that RT induces massive hepatocyte apoptosis, resulting in a large amount of ectopic dsDNA which is then taken up by liver NPCs, resulting in the activation of cGAS and subsequently STING. In order to determine this, the group exposed wild-type (WT), cGAS knockout, and STING knockout mice to 30Gy of radiation. While livers of WT mice subjected to radiation showed increased steatosis (retention of lipids), mice lacking either cGAS or STING showed less at 48 hours as measured by histological staining. The knockout mice also showed reduced apoptosis in liver tissue at 48 hours as measured by terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) assay of histological sections. Additionally, histological staining of mouse liver tissues six weeks after radiation showed that the knockout mice had less veno-occlusive inflammation, an indicator of RILD. Next, the group showed that hepatocytes extracted from mice 24 hours following irradiation secrete much more dsDNA in vitro than NPCs extracted from the same liver. Furthermore, levels of cGAS, STING, IFN-α, IFN-β, and TLR9 mRNA transcripts were found to increase dramatically in liver NPCs but not in hepatocytes following radiation as measured by RT-qPCR. Additionally, expression levels of cGAS/STING-related genes TBK1, IRF3, ISG15, JAK1, TYK2, AKT1, AGBL5, TRIM32, RSAD2, and TTL4 were all increased in liver NPCs but not in hepatocytes following radiation. The group then showed that DNase treatment of mice during and after RT prevented increased expression levels of cGAS, STING, IFN-α, and IFN-β mRNAs. This result indicates that extracellular DNA is a trigger for RT-induced IFN-I secretion. Finally, the group showed that knockout of the IFNα and IFNβ receptors in mice reduced the amount of liver steatosis and apoptosis caused by RT. Additionally, blockade of IFN-I signaling with an interferon alpha and beta receptor subunit 1 (IFNAR1)-specific antibody did not negatively affect the tumor-reducing properties of RT in a mouse HCC model. This paper indicates that cGAS/STING-signaling in liver NPCs is a major cause of RILD. Extracellular DNA from hepatocytes killed during RT is taken up by NPCs where it activates cGAS/STING signaling to produce IFN-I. This finding could help scientists and clinicians devise ways to prevent RILD in patients undergoing RT for HCC or other cancers. Perhaps short-term immune modulators may be used in tandem with RT to prevent an excessive response of the innate immune system.
Role of Hepatitis B Virus Capsid Phosphorylation in Nucleocapsid Disassembly and Covalently Closed Circular DNA Formation – PLOS Pathogens
This paper from Dr. Jianming Hu’s laboratory at the Pennsylvania State University College of Medicine in Hershey, PA outlines the role of phosphorylation of the HBV core protein (HBc) in the HBV life cycle. HBV has a relaxed circular (RC) DNA genome which it delivers to the nucleus of hepatocytes. In the nucleus, the RC DNA is converted into covalently closed circular (CCC) DNA which is the viral transcriptional template for all HBV mRNA species including pregenomic RNA (pgRNA). Along with the viral reverse transcriptase (RT), pgRNA is packaged by HBc into newly formed nucleocapsids (NC) where it is reverse-transcribed to form RC DNA resulting in mature NCs. Mature NCs may either be enveloped and secreted as infectious virions or uncoat within the cell and further contribute to CCC DNA formation. Because CCC DNA is the reservoir of HBV in infected hepatocytes, its eradication is highly sought after and is required to achieve a true cure for the virus. This publication reports a model wherein HBc phosphorylation by the host protein cyclin-dependent kinase 2 (CDK2) facilitates the uncoating of newly formed NCs and their subsequent formation of CCC DNA. Previously, this group has found that CDK2 is a host kinase which is incorporated into HBV NCs. CDK2 is a highly conserved kinase (phosphorylating protein) which is essential during the G1, S, and G2 phases of the cell cycle. First, the group identified two S-P (serine-proline) motifs on the globular N-terminal domain (NTD) of HBc, S44 and S49 which are potential CDK2 substrates that are on the interior surface of assembled NCs. In order to mimic constitutive phosphorylation or to block phosphorylation of the serine residues, they were mutated to glutamic acid residues (N2E) or alanine residues (N2A) respectively. The phospho-mimetic mutant N2E showed decreased levels of pgRNA packaging into NCs as measured by native agarose gel electrophoresis (NAGE) and Southern blot following transfection of the constructs into HepG2 cells. After release from NCs into the nucleus, the RC DNA HBV genome takes the form of protein free (PF) RC DNA lacking the RT protein, prior to forming CCC DNA. The phospho-mimetic N2E mutant yielded more PF-RC DNA and CCC DNA than wild type (WT) HBV and conversely, the phospho-null N2A mutant yielded less of both species than WT HBV. These results show that while NCs phosphorylated at both S44 and S49 are less efficient at packaging pgRNA, they are more likely to uncoat and release their genomes into the nucleus. Next, PhoenixBio (PXB) primary human hepatocytes harvested from human-liver chimeric mice were infected with HBV and treated with two CDK2 small molecule inhibitors. PF DNA was then extracted from the cells and analyzed via Southern blot. Both CDK2 inhibitors dramatically reduced the level of CCC DNA formation as compared to the mock control. This result indicates that CDK2 activity within NCs modulates their stability causing them to uncoat and deliver their genomes to the nucleus as opposed to being exported as virions. This publication sheds light on the exact stages of HBc phosphorylation and how they affect CCC DNA formation. This work is important because understanding the molecular mechanisms of CCC DNA formation will help in the development HBV antivirals. Small molecules which interfere with specific stages of HBc phosphorylation and dephosphorylation may prove efficacious in preventing CCC DNA formation in individuals chronically infected with HBV.
Metabolites released from apoptotic cells act as tissue messengers – Nature
This paper from the University of Charlottesville in Virginia investigates the “apoptotic metabolite secretome” and its effect on neighboring cells. Apoptosis is a highly regulated form of programmed cell death (PCD) which accounts for approximately 90% of homeostatic cell turnover. Metabolites are small molecules that are the intermediates or end products of metabolism. Here, a panel of conserved apoptotic metabolites was identified in the supernatants of apoptotic cells using advanced spectroscopy techniques (spectroscopy-based metabolomics). Six metabolites were found to be secreted across a variety of cell types in response to various apoptosis inducers. These six metabolites are: adenosine monophosphate (AMP), guanosine 5′-monophosphate (GMP), creatine, spermidine, glycerol-3-phosphate (G3P), and adenosine triphosphate (ATP). These metabolites were all found in the supernatants of Jurkat cells (acute T cell leukemia) following exposure to UV irradiation as well as following treatment with anti-Fas antibody. These metabolites were also released from primary mouse bone-marrow-derived macrophages (BMDMs) treated with anthrax and primary mouse thymocytes treated with anti-Fas antibody. Additionally, lung and colon cancer cell lines A549 and HCT116 released four of these metabolites (ATP, spermadine, G3P, and creatine) when subjected to the BH3-mimetic ABT-737 (induces mitochondrial outer membrane permeabilization) as measured using commercial kits. Secretion of these metabolites was prevented by pretreatment of cells with the pan-caspase inhibitor zVAD, indicating apoptosis as the mechanism of release. The metabolites alanine, pyruvate, and creatinine were retained within apoptotic cells, showing that metabolite release was organized and not due to nonspecific rupture of apoptotic bodies. Because only specific metabolites were released during apoptosis, the group hypothesized that the opening of plasma membrane channels may determine the apoptotic secretome. Pannexin 1 (PANX1) is a membrane channel activated by caspase 3 and 7 cleavage during apoptosis. Previously, this group has demonstrated that PANX1 activation is responsible for the secretion of ATP and UTP from apoptotic cells, which function as “find me” signals to recruit phagocytes to perform efferocytosis. In order to determine the role of PANX1 activation in the apoptotic secretome, prior to UV irradiation, PANX1 was inhibited in Jurkat cells using two methods: pharmacological inhibition with the drugs trovafloxacin (Trovan) or spironolactone and generation of a cell line bearing a dominant-negative PANX1 mutation at the caspase cleavage site. Jurkat cells with inhibited or nonfunctional PANX1 showed less secretion of 25 metabolites released from UV-treated Jurkat cells as measured by spectroscopy-based metabolomics. Spermidine, GMP, AMP, and G3P were all secreted dependent upon PANX1 activation. Next, to test whether metabolic activity within the dying cell affects its secretome, the group chose to focus on the release of spermidine. Spermidine released from apoptotic cells naturally reduces local inflammation and counteracts autoimmunity. Interestingly, while spermidine was heavily secreted from apoptotic cells, its precursor molecule putrescine was not released at all. As the starting product of spermidine synthesis is arginine, the isotope carbon-13 (13C)-containing argenine was administered to Jurkat cells one minute prior to UV irradiation. Apoptotic cells showed 40% and 25% more incorporation of 13C label into putrescine and spermidine respectively than live cells at one hour post-UV. This indicates that in addition to the caspase-dependent opening of membrane channels, apoptotic cells also maintain or even upregulate certain metabolic pathways to contribute to the apoptotic secretome. Next, in order to test the effect of the apoptotic secretome on neighboring cells, supernatant from apoptotic Jurkat cells was administered to LR73 cells (phagocytic, Chinese hamster ovary). RNA-sequencing analysis of the LR73 cells after four hours in the apoptotic supernatant revealed altered transcription of programs linked to cytoskeletal rearrangements, inflammation, wound healing or tissue repair, antiapoptotic functions, metabolism and the regulation of cell size within the phagocyte. Finally, the group used two concoctions of PANX1-dependent metabolites to treat mouse models of inflammatory arthritis and lung-transplant rejection. Treatment with the metabolite mixtures resulted in significantly reduced inflammation and better clinical outcomes in both inflammatory disease models. This publication shows that apoptotic cells affect their microenvironment by secreting anti-inflammatory metabolites. It also demonstrates that apoptosis may be harnessed to ameliorate inflammatory diseases. Once fully elucidated, other forms of PCD may also prove useful in treating other diseases such as cancer and viral infections.
Meet our guest blogger, David Schad, B.Sc., Junior Research Fellow at the Baruch S. Blumberg Institute studying programmed cell death such as 