Lytic HSV-1 infection induces the multifunctional transcription factor Early Growth Response-1 (EGR-1) in rabbit corneal cells
© Bedadala et al; licensee BioMed Central Ltd. 2011
Received: 24 February 2011
Accepted: 27 May 2011
Published: 27 May 2011
Herpes simplex virus type-1 (HSV-1) infections can cause a number of diseases ranging from simple cold sores to dangerous keratitis and lethal encephalitis. The interaction between virus and host cells, critical for viral replication, is being extensively investigated by many laboratories. In this study, we tested the hypothesis that HSV-1 lytic infection triggers the expression of important multi-functional transcription factor Egr1. The mechanisms of induction are mediated, at least in part, by signaling pathways such as NFκB and CREB.
SIRC, VERO, and 293HEK cell lines were infected with HSV-1, and the Egr-1 transcript and protein were detected by RT-PCR and Western blot, respectively. The localization and expression profile of Egr-1 were investigated further by immunofluorescence microscopy analyses. The recruitment of transcription factors to the Egr-1 promoter during infection was studied by chromatin immunoprecipitation (ChIP). Various inhibitors and dominant-negative mutant were used to assess the mechanisms of Egr-1 induction and their effects were addressed by immunofluorescence microscopy.
Western blot analyses showed that Egr-1 was absent in uninfected cells; however, the protein was detected 24-72 hours post treatment, and the response was directly proportional to the titer of the virus used for infection. Using recombinant HSV-1 expressing EGFP, Egr-1 was detected only in the infected cells. ChIP assays demonstrated that NFкB and cAMP response element binding protein (CREB) were recruited to the Egr-1 promoter upon infection. Additional studies showed that inhibitors of NFкB and dominant-negative CREB repressed the Egr-1 induction by HSV-1 infection.
Collectively, these results demonstrate that Egr-1 is expressed rapidly upon HSV-1 infection and that this novel induction could be due to the NFкB/CREB-mediated transactivation. Egr-1 induction might play a key role in the viral gene expression, replication, inflammation, and the disease progression.
List of Abbreviations
cAMP response element binding protein
Dulbecco's modified Eagles medium
Egr1 binding element
Early growth response
fetal bovine serum
green fluorescent protein
herpes virus entry mediator
nerve growth factor
phosphate buffered saline
Statens Seruminstitut Rabbit Cornea
tumor necrosis factor
derived from kidney epithelial cells of the African green monkey.
Herpes simplex virus type-1 (HSV-1) is a common pathogen with worldwide seroprevalence rates ranging from 50% to 90% [1–3]. It is a neurotropic virus that is in the subfamily of alpha herpesvirinae. Initial or primary infection with HSV-1 mostly occurs during childhood in mucoepithelial surfaces and is generally mild or asymptomatic. Upon infection of epithelial cells, HSV-1 initiates lytic replication, and at the end of this cycle, the virus infects sensory neurons proximal to the site of primary infection. Virions can travel via retrograde axonal transport to the cell bodies of neurons in the trigeminal ganglia where lifelong latency is established . The latent virions can reactivate due to unknown causes and prohibit subsequent lytic infections. The most common form of lytic infection is the cold sore or fever blister where viral replication takes place in the orofacial mucosa . Infection of the brain leads to herpes encephalitis, a dangerous condition that can cause permanent neurological damage with high mortality . Orofacial infection, although less threatening, represents the major route for transmission to a naive host.
In addition to encephalopathy and orofacial infection, HSV-1 also infects the eyes leading to 8 cases of herpetic keratitis per 100,000 population every year . Keratitis refers to inflammation of corneal epithelium due to a number of irritants such as infections by viruses, bacteria, and fungi. Herpetic keratitis is the major cause of corneal blindness, with unilateral visual impairment occurring in at least one-third of patients with recurrent disease .
Lytic infections of HSV-1 are typically characterized by the entry of virus into the cell and translocation of the viral genome into the nucleus followed by replication and evasion of host defenses in order to disperse and persist in the environment. Gene expression of HSV-1 is tightly regulated in a cascade fashion. The three temporal classes of genes are designated immediate-early (α), early (β) and late (γ) genes . There are five α genes, designated ICP0, ICP4, ICP22, ICP27, and ICP47. The α genes were defined by the presence of the cis element for induction by VP16, which interacts with cellular factors, including the protein Oct-1, a homeobox protein, to activate viral immediate early (IE) gene transcription in trans [9, 10]. The expression of β genes requires the expression of α genes, especially functional ICP4. ICP0 enhances the ability of ICP4 to trans-activate γ genes . The expression of γ gene has been shown to be blocked by effective concentrations of DNA synthesis inhibitors . All of these steps require interaction between viral and host components. The communication between virus and host factors is being investigated by many laboratories but is not understood completely, and identification of host factors that interact with the virus remains a challenge. Additional information will identify new drug targets for treatment and will aid in understanding the complex processes of initiation of HSV-1 latency and reactivation.
Our laboratory examined the hypothesis that HSV-1 infection of epithelial cells including a cell line from rabbit cornea rapidly induced the expression of the cellular transcription factor early growth response-1 (Egr-1), also known as NGFI-a, Zif268, Krox24, TIS8, and ZENK. The Early Growth Response (EGR) family belongs to the C2H2-type zinc-finger proteins [13–15]. Egr-1 functions as a convergence point for many signaling cascades and is known to play an important role in regulating inflammation, cell proliferation, and apoptosis . Our previous transient cotransfection studies with promoter plasmids and Egr-1 expression vector showed that Egr-1 regulated HSV-1 ICP22 and ICP4 promoter activity . In the present study, we demonstrate for the first time that Egr-1 protein is induced rapidly in cells such as SIRC and VERO upon infection. Using inhibitors of the transcription factors CREB and NFκB, we further investigate the mechanism by which HSV-1 induces Egr-1 activation
Cells, viruses and culture conditions
VERO (derived from kidney epithelial cells of the African Green Monkey) cells and HEK293 cells were grown in DMEM supplemented with 10% FBS. SIRC (Rabbit cornea) cells obtained from ATCC (Cat#: CCL-60) were grown in MEM supplemented with 10% FBS. The RE-EGFP strain of HSV-1 was used throughout the study . Unless otherwise indicated, all viral infections were carried with a MOI of 5 for 1 hour.
Western Blot analysis
Protein extract was subjected to 10% sodium dodecyl sulfate polyacrylamide gel electrophoresis and transferred onto nitrocellulose membranes. The blots were blocked using PBS with 5% (wt/vol) non-fat dry milk and washed in PBS. Rabbit anti-Egr-1 polyclonal antibody (Santa Cruz SC-110x) or cAMP response element binding (CREB) Ser-133 phosphorylation (Abcam#ab32096) was used at a dilution of 1:1,000. Anti-a-Tubulin mouse antibody (Calbiochem, Cat#: CP06, San Diego, CA) was added at a dilution of 1:10,000. The chemiluminiscent signal from the membranes was detected by Syngene GeneGnome HR Bioimaging system (Frederick, MD). The protocol was performed essentially as described by the manufacturer.
Approximately 20,000 cells were placed in a multi-chamber slide (Cat# 354104 BD Falcon) with respective media one day before infection. The cells were pretreated for 3 hours with inhibitors Bay 11-7082 (Calbiochem#196871) and NBD binding peptide (Calbiochem Cat#: 480025) for the NFкB inhibition study. After infection cells were rinsed once with 2 ml PBS for 5 min and fixed with 100% methanol at -20°C. Slides were incubated with 2% normal blocking serum followed by incubation with primary antibody for Egr-1 (Millipore# MAB10073) overnight at 4°C. The slides were then incubated with fluorescent conjugated secondary antibody (Invitrogen cat# A21424), at RT for 1 hr. Finally, the slides were mounted with fluorescent mounting medium containing DAPI. The expression of enhanced green fluorescent protein (EGFP), red fluorescence, and DAPI staining were accessed by an Olympus fluorescence microscope (IX71) coupled with an Olympus digital camera photo apparatus (DP71). Imaging analysis was performed by using Olympus DP controller software.
Reverse transcriptase PCR (RT-PCR)
Total RNA was extracted from the cells using trizol reagent (Invitrogen). RT-PCR was performed using Superscript One-Step RT-PCR (Invitrogen) with 0.5 μg of total RNA and primer set for reaction. Their sequences are as follows: Egr-1: 5'-AGA CCA GTT ACC CCA GCC AAA C-3'and 5'-AAA ATG TCA GTG TTC GGC GTG-3'; EGFP: 5'-GCA GAA GAA CGG CAT CAA GGT G-3' and 5'-TGG GTG CTC AGG TAG TGG TTG TC-3'. The reverse transcription/PCR reaction was carried out at 45°C for 20 min followed by 25 cycles of 94°C for 30 s, 55°C for 30 s, and 68°C for 30 s. The RT-PCR products were analyzed by 2% agarose gel electrophoresis.
Chromatin Immunoprecipitation (ChIP) assays
Cell monolayers were treated with 1% formaldehyde solution for 10 min at room temperature. Cells were then harvested and subjected to sonication. The lysed samples were centrifuged for 10 min at 13,000 rpm at 4°C and the supernatant was diluted 10-fold with RIPA buffer containing protease inhibitor. Immunoprecipitation was then performed with Dynabeads Protein A (Invitrogen, Cat#: 100.01D) with antibodies against CREB (Abcam#ab32096) and NFкB (Abcam#ab7970). To analyze immunoprecipitated DNA, PCR amplification was performed with primers against Egr-1 promoter: 5'-TGG GGG GCT TCA CGT CAC TC-3' and 5'-AAG TTC TGC GGC TGG ATC TCT C-3''. The products were analyzed by 2% agarose gel electrophoresis.
Transfection studies involving the dominant negative mutants of CREB
Approximately 20,000 cells were placed in a multichamber slide (Cat# 354104 BD Falcon) with respective media one day before transfection. The cells were cotransfected with phMGFP (Promega Cat #E6421) and one of the vectors (pCMV-CREB133 or pCMV-KCREB) from the CREB dominant negative vector set (Clonetech Cat# 631925) using lipofectamine (Invitrogen). fter 2 days, the cells were infected with virus (MOI = 1) for 1 hour. The cells were then incubated in fresh medium for 24 hours. Immunofluorescence was carried out using anti-Egr-1 antibody as described above.
Egr-1 protein is induced by HSV-1 infection
Egr-1 mRNA was detected at 1 hour post infection
NFкB and pCREB were recruited to the Egr1 promoter upon infection
Dominant-negative CREB decreased the Egr-1 induction observed upon infection
NFкB inhibition reduced Egr-1 expression by infection
Egr-1 is involved in the gene expression and the replication of several viruses such as Epstein Barr virus [31, 32], HSV-1 [17, 33, 34], JC virus , orthopox virus , murine corona virus , rabies virus [38, 39], borna disease virus , human foamy virus , Japanese encephalitis virus , HTLV-I [41, 42] and HTLV-II . In most cases, Egr-1 was induced after the virus targeted neural cells and lymphocytes. Our results have demonstrated an induction of Egr-1 protein upon HSV-1 infection in epithelial cells such as VERO and SIRC (a corneal cell line). This observation suggests that Egr-1 could play a role in host cellular responses to HSV-1 lytic infection.
Several pathways are known to regulate the induction of Egr-1 including p38/MAPK , JNK , MEK/ERK , CREB pathway , and NFкB activation [27, 28]. By inhibiting the pathways known to be involved in Egr1 induction, we determined that NFкB and CREB play roles in Egr1 expression induced by HSV-1. CREB is activated and stabilized by HSV-1 ICP10 and can modulate viral-induced apoptosis . Studies have reported the presence of NFкB regulatory sequence  and CRE  in the human Egr-1 promoter. Transient activation of NFкB during the first few minutes of HSV-1 infection has been reported [48, 49]. In addition, most of the up-regulated mRNA upon infection was NFкB-dependent . Furthermore, NFкB-dependent gene expression is directly related to a number of stress-induced activities in eukaryotic cells . Since Egr-1 is induced by stress and is known to control a variety of divergent cellular responses, it is possible that this induction is NFкB-dependent. Therefore from the host cell standpoint, viral infection can be considered as a stress and Egr1 is rapidly induced to turn on various cellular responses.
Our previous study showed that Egr1 can bind to the Egr1 binding element (EBE) located in the intron of ICP22 and thus regulate the activity of both ICP22 and ICP4 . Since Egr1 is expressed in neural tissues, it could play a role in the maintenance of latency and subsequent reactivation. In addition, our preliminary studies revealed that Egr1 induced by viral infection was sufficient to interact with the ICP22 EBE during the lytic infection of SIRC and the over-expression of Egr-1 enhanced HSV-1 gene expression, replication, and release of infectious viruses (unpublished data). Together, these results suggest the involvement of Egr-1 in HSV-1 gene expression/replication via regulation of α-genes. Recombinant virus over-expressing Egr-1 and stable cell lines containing siRNA repressing Egr-1 are being constructed to further investigate the roles of Egr-1 during HSV-1 lytic infections.
It is not known which viral proteins are required for Egr-1 induction. It is likely that the binding of the viruses to the cell surface is sufficient to trigger the expression of Egr-1. Viral binding to the target cell is mediated by envelope glycoprotein gB, gC, or gD and the entry is mediated by gD to one of the cell surface receptors, such as the Herpes Virus Entry Mediator (HVEM)), heparan sulfate, nectin-1/2, and cell adhesion molecules from the immunoglobulin superfamily . HVEM belongs to the superfamily of TNF (tumor necrosis factor)/NGF (nerve growth factor) receptors , and has been identified independently as TR2 (TNF receptor like-2) . The Egr-1 gene can be regulated by TNF  and NGF  in cultured cells, and NFкB can be activated by the gD/HVEM . Therefore, Egr-1 induction quite possibly can be correlated to the binding of viral glycoprotein to HVEM or other cellular surface receptors. Additional experiments are required to determine the induction mechanisms of Egr-1 by viral infection.
In summary, we have shown for the first time that Egr1 can be induced rapidly by HSV-1 lytic infection in epithelial cells such as VERO and SIRC. This induction may be due to transactivation by the NFкB and/or CREB-mediated signaling cascades. The Egr1 induced by HSV-1 in corneal cells could have implications in viral pathogenesis, inflammation, and disease progression. More experiments to determine the role of Egr-1 are underway, especially in other models of infection.
This publication was made possible by NIH Grant R15EY020803 to SVH. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of NIH. We thank Ms. Trisha Chiasson for manuscript editing. Support from University of Maryland Eastern Shore is acknowledged. LG is supported by R15EY020803. JRP is supported by NCRR Grant P20RR016456. JMH is supported in part by NEI EY006311, Research to Prevent Blindness Senior Scientific Investigator Award, and LSU Eye Center Core Grant NEI EY02377.
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