Mutations in the IGF-II pathway that confer resistance to lytic reovirus infection
© Sheng et al; licensee BioMed Central Ltd. 2004
Received: 21 July 2004
Accepted: 27 August 2004
Published: 27 August 2004
Viruses are obligate intracellular parasites and rely upon the host cell for different steps in their life cycles. The characterization of cellular genes required for virus infection and/or cell killing will be essential for understanding viral life cycles, and may provide cellular targets for new antiviral therapies.
A gene entrapment approach was used to identify candidate cellular genes that affect reovirus infection or virus induced cell lysis. Four of the 111 genes disrupted in clones selected for resistance to infection by reovirus type 1 involved the insulin growth factor-2 (IGF-II) pathway, including: the mannose-6-phosphate/IGF2 receptor (Igf2r), a protease associated with insulin growth factor binding protein 5 (Prss11), and the CTCF transcriptional regulator (Ctcf). The disruption of Ctcf, which encodes a repressor of Igf2, was associated with enhanced Igf2 gene expression. Plasmids expressing either the IGF-II pro-hormone or IGF-II without the carboxy terminal extension (E)-peptide sequence independently conferred high levels of cellular resistance to reovirus infection. Forced IGF-II expression results in a block in virus disassembly. In addition, Ctcf disruption and forced Igf2 expression both enabled cells to proliferate in soft agar, a phenotype associated with malignant growth in vivo.
These results indicate that IGF-II, and by inference other components of the IGF-II signalling pathway, can confer resistance to lytic reovirus infection. This report represents the first use of gene entrapment to identify host factors affecting virus infection. Concomitant transformation observed in some virus resistant cells illustrates a potential mechanism of carcinogenesis associated with chronic virus infection.
Viruses as obligate intracellular parasites rely upon the host cell for different steps in their life cycle, including attachment, disassembly, transcription, translation, reassembly, and egress. Consequently, characterization of these cellular processes will be essential for any understanding of viral life cycles, and may provide cellular targets for new antiviral therapies.
The susceptibility to virus infection varies greatly among different cell types, and virus-resistant cells frequently emerge post-infection [1–4]. This suggests that host cell contributions to the virus life cycle, although complex, have genetic determinants. We therefore used a genetic approach to identify cellular genes required for infection by reovirus, a small cytolytic RNA virus that replicates in the cytoplasm. A gene trap retrovirus was use to create libraries of rat intestinal epithelial (RIE-1) cell clones in which each clone contained a single gene disrupted by an integrated retrovirus. The mutant libraries were then infected with reovirus, and resistant clones were selected. We hypothesized that genes mutated by gene entrapment may confer reovirus resistance as a result of either haploinsufficiency or loss of heterozygosity and could be identified by characterizing the genes disrupted by the entrapment vector. From these experiments we have isolated 152 clones and have characterized mutations in 111 different genes, providing potential candidates required for reovirus infection and/or cell killing. Many of the disrupted genes have known or imputed functions, and several are known to function in the same or related pathways. For example, four mutations affected genes in the insulin growth factor-2 (IGF-II) pathway, including genes encoding the IGF-ll/manose-6-phosphate receptor [5, 6] (Igf2r, locus ID 25151), the IGF binding protein 5 protease  (Prss11, locus ID 65164, 2 clones), and CTCF (Ctcf, locus ID 83726), a transcriptional repressor of the IGF-II gene (Igf2) involved in paternal imprinting.
The frequency of mutations involving the IGF-II pathway led us to investigate the role of IGF-II in reovirus infection. Clone 6B72, which contains a mutation in Ctcf , was found to over express Igf2 transcripts, consistent with the known role of CTCF as a transcriptional repressor of the Igf2 gene. Moreover, forced expression of IGF-II in RIE-1 cells was sufficient to confer cellular resistance to lytic reovirus infection. Enforced IGF-II expression also transformed RIE-1 cells to anchorage independent growth, a phenotype associated with malignant change. These results represent the first use of gene entrapment to identify components of host cell metabolism required for virus infection and illustrate a potential mechanism of carcinogenesis associated with chronic virus infection.
Disruption of the CTCF gene results in cells resistant to lytic infection by reovirus
Igf2 transcripts induced by CTCF disruption
Resistance to reovirus lytic infection results from increased Igf2 expression
To determine whether Igf2 confers resistance to reovirus infections, clones of RIE-1 cells over-expressing the full-length Igf2 transcript or the splice variant (Igf2 sv ) were generated and examined for their capacity to resist lytic infection. As shown in Figure 5 expression of wild type (Igf2), but not the splice variant (Igf2 sv ) increased the resistance of RIE-1 cells to reovirus infection by over 100 fold. However, when the Igf2 sv was transfected into 6B72 cells, the ability of 6B72 cells to survive infection was abolished. Expression of the Igf2 gene in an anti-sense orientation caused no significant difference in the capacity of 6B72 cells to resist infection (data not shown). These studies suggest that increased Igf2 expression in 6B72 cells is associated with their capacity to resist reovirus infection, and that the Igf2 sv encodes a trans-dominant isoform that blocks the activity of Igf2.
The IGF-II hormone confers resistance to lytic infection
Over expression of igf2 confers resistance to other reovirus sertotypes
IGF-II expressing RIE-1 cell has delayed disassembly of virus particle
CTCF deficient RIE cells grow in soft agar, a consequence of increased Igf2 expression
Insertional mutagenesis provides an approach to identify genes associated with selectable cellular phenotypes. We have isolated over 100 potential clones with mutations in genes that may play roles in the life cycle of reovirus. In the present study, one clone resistant to reovirus lytic infection contained a provirus inserted into the gene encoding the CTCF transcriptional regulator. CTCF binding motifs are present in many genes, including Igf2 , H19 , and Myc . However, since 3 other clones selected for reovirus resistance contained mutations in the IGF-II pathway, the role of IGF-II in virus-resistance was investigated further. Reduced expression of the Ctcf gene was associated with enhanced Igf2 expression in virus-resistant cells, while forced expression of the Igf2 gene in the parental RIE-1 line was sufficient to confer resistance to lytic reovirus infection.
By inference, the recovery of inserts affecting other genes in the IGF-II signalling pathway suggests that mutations in multiple genes may affect the same phenotype by acting on a common pathway. The insert in Igf2r was found to decrease the expression of the gene as assayed by northern blot analysis (data not shown). As IGF-II targets the igf2r to lysosomal degradation, mutations in the genes encoding either the receptor (Igf2r) or its ligand (Igf2) will affect the activity of the other, and result in a reduced endosomal trafficking of hydrolases necessary for reovirus disassembly . Our data indicates there is a decrease in virus disassembly in 6B72 cells, consistent with a block at this step in morphogenesis. Further studies will be required to assess if and how inserts in the Prss11 and Igf2r genes influence reovirus resistance.
As the entry, disassembly, transcription, translation, and repacking of viruses share common features; we anticipate that common cellular pathways will influence infection by other virus families. Indeed, Igf2r has been implicated in herpes simplex and zoster virus infection [30, 31]. However, the present study is the first to show a direct connection between Igf2 gene expression and resistance to lytic virus infection. By using constructs that encode the mature hormone without the E-peptide, we were able to show that forced IGF-II expression is sufficient to confer a reovirus resistant phenotype. These results differ from other studies in which reovirus replication was enhanced by treatment of RIE-1 cells with insulin. The latter effect is presumably caused by enhanced virus replication associated with cell proliferation .
Expression of the Igf2 gene is frequently elevated in common childhood and adult neoplasms [27, 32–38] and has been associated with tumour progression and metastasis [39, 40]. Igf2 also transformed RIE-1 cells to anchorage-independence, a phenotype that predicts the potential for malignant growth in vivo. Virus-resistant 6B72 cells also grew in soft agar, presumably as a result of enhanced IGF-II expression. Since plasmids expressing only IGF-II were less active in transforming RIE-1 cells to anchorage independence than vectors expressing the entire pro-hormone, further study is needed to determine whether the capacity of RIE-1 cells to proliferate in soft agar is enhanced by the E-peptide or other products derived from the carboxy terminus of the pro-hormone. In other studies, the E-peptide enhanced insulin secretion from β-cells , and may play a role in cellular transformation .
cDNA clones of an alternatively spliced Igf2 transcript (Igf2 sv ), blocked the ability of IGF-II to promote reovirus resistance and anchorage independent growth in a trans-dominant manner. The protein coding sequence of Igf2 sv contains a frame shift in the E-peptide region and lacks a site [20, 41] required for the proteolytic processing of the pro-hormone. The alternative splice site is used very infrequently [only one EST (AA259833) in dbEST was similarly spliced] and thus probably plays no physiological role. Further studies are planned to determine molecular basis for the dominant negative activity of Igf2 sv .
6B72 cells were highly resistant to reovirus infection as determined by virus yield and cell survival at different times post-infection. Virus resistance was a genetically selected trait manifested by clonally pure cell populations, could be conferred by enforced IGF-II expression, and involved decreases in virus disassembly. Although decreased virus disassembly is sufficient to explain virus resistance, we do not exclude the possibility that other mechanisms may contribute to the resistance of 6B72 cells, since any genetic selection may generate clones with multiple, independent mutations. The fact that an early step in infection (uptake/disassembly) is defective in 6B72 cells makes it very difficult to test whether downstream steps might also be affected.
While the original impetus of our studies was to understand the replication cycle of intracellular pathogens that cause acute and chronic infectious diseases, the finding of cell growth phenotypes associated with virus resistance is of some interest. It has been proposed that lytic viruses may used to treat certain malignancies [42–45]. However, based upon our observations, such therapy may carry a risk associated with selection of virus resistant cell clones with enhanced growth/survival potential. Additionally, chronic infections contribute to the development of a number of human cancers [46–49]. While the carcinogenic process is not well understood, cell proliferation associated with inflammation is also thought to contribute to tumour promotion [50, 51]. The present study illustrates how carcinogenesis could also be influenced by selection for virus-resistant cells with mutations in genes affecting cell proliferation or survival.
This is the first reported use of gene entrapment to identify host genes affecting the susceptibility of cells to virus infection. These results indicate that IGF-II, and by inference other components of the IGF-II signaling pathway, can confer high levels of resistance to lytic reovirus infection. IGF-II expression specifically blocked virus disassembly. Ctcf disruption and forced Igf2 expression both enabled cells to proliferate in soft agar, a phenotype associated with malignant growth in vivo. Therefore, these results illustrate a potential indirect mechanism of viral carcinogenesis by which cells selected to virus resistance may also have enhanced oncogenic potential.
Entrapment mutagenesis and selection of reovirus resistant clones
To identify genes required for reovirus lytic infection, a gene trap retrovirus shuttle vector, U3NeoSV1, was used to generate mutagnized rat intestinal (RIE-1) cells . RIE-1 cells were infected with the gene trap vector at a multiplicity of infection <0.1, and were selection in media containing G418 sulfate (0.7 mg/ml) (Clontech, Palo Alto, CA, USA). Twenty libraries of mutant RIE-1 cells, each consisting of 104 independent gene entrapment events, were generated and expanded until each mutant clone was represented by approximately 103 sibling cells. These cells were plated at low density and incubated in serum-free media for 3 days until they became quiescent, infected overnight with reovirus serotype 1 at a multiplicity of infection of 30 plaque forming units (pfu) per cell. The infected cells were detached with trypsin, DMEM medium containing 10% fetal bovine serum (FBS) (Hyclone Laboratories, Inc., Logan, UT, USA) was added and the cells were allowed to reattach. After 4–6 hours the medium was replaced with serum-free medium and the cells were incubated for several days until only a few cells remained attached to the culture flask. Cells that survived the selection were allowed to form colonies that were expanded for further analysis.
Reovirus stocks and infectivity assays
Reovirus type 1 (Lang) and reovirus type 3 (Dearing) were previously described . A stock of reovirus that was passaged twice in L cells was purified  and the purified virus band was fluorescein labelled as previously described . For some experiments the top component, consisting of virus particles that are devoid of genome, was used to study the entry pathway .
Survival of parental L- and RIE-1 cells and RIE-1 and L-cells transfected with Igf2 constructs was determined in 96-well plates seeded at 5 × 104 per well. On the following day, serial dilutions of reovirus type 1 or type 3 were added in 100 μl of media and cells were incubated at 37°C and 5%CO2 for 1 hour. Cells were washed three times in PBS, and fresh media was added containing 0.1% anti-reovirus antibodies to inhibit secondary infection. Cells were incubated for 4 to 5 days, and surviving cells visualized with gentian violet. Studies were repeated a minimum of three times.
To determine the titre of reovirus present in cells, cells were frozen and thawed three times and plaque assays were performed as previously described . Titres of virus were repeated twice.
For experiments involving fluorescein labeled reovirus, cells were grown on glass slides and fixed following appropriate times with 4% paraformaldehyde, dehydrated, and mounting in cytoseal acrylic resin (Stephens Scientific, Cornwall NJ, USA) to improve clarity and prevent bleaching. Fluorescence microscopy was performed using an Axiophot microscope (Carl Zeiss, Inc., Thornwood, NY, USA), with a 40×/1.3 plan Neofluar objective and fluorescein filter set. Images were captured with a low-light, cooled CCD camera (Micromax, Photometries, Inc., Tucson, AZ, USA).
Identification of genes disrupted by gene entrapment
To identify the gene disrupted by the vector in clones surviving reovirus infection, the shuttle-vector property of U3NeoSV1 was utilized. Regions of genomic DNA adjacent to the U3NeoSV1 provirus were cloned by plasmid rescue, and sequenced . Sequencing was done using an automated sequencer (ABI 3700 DNA Analyzer, Applied Biosystems, Foster City, CA, USA), and the results obtained were compared to databases available in the public domain (BLAST nr, est, and hgts). The probability of a match to sequences in the databases occurring by chance alone varies due to interspecies conservation and the length of the match. Matches to characterized genes were considered significant if the interspecies matches had a probability score p <10-5 and involved non-repetitive sequences. As indicated, virtually all of the genes identified had matches to murine or human gene sequences with p < 10-10 and rat with p < 10-20.
Igf2 expression was assessed by northern blot hybridization
Total RNA was isolated from cultured cells using Trizole reagent (Gibco BRL, Gaithersburg, MD, USA). 5 μg of RNA was separated on 1.2% agarose gel, and transferred to a nitrocellose membrane. Membranes were hybridized with random prime-labeled (Strategene, Cedar Creek, TX, USA) probes corresponding to a full length of Igf2 cDNA and either glyceraldehyde dehydrogenase (GAPDH) or β-actin cDNA.
Igf2 cDNA isolation and expression
Rat Igf2 cDNAs were obtained using reverse transcriptase PCR (RT-PCR). Total RNA was extracted from RIE and 6B72 cells using Trizole reagent (Life Technologies, Rockville, MD, USA). RT was performed on 1 μg of total RNA (PTC-100 programmable Thermal Controller, MJ Research. Inc, Watertown, MA, USA). A pair of primers was designed according to rat sequences: CTTCCAGGTACCAATGGGGATC (forward) and TTTGGTTCACTGATGGTTGCTG (reverse). A 500 bp DNA was amplified under following conditions: 95°C, 1 min; 40 cycles of 95°C 30 seconds, 60°C 30 seconds and 68°C 3 minutes; 68°C 10 min; 4°C.
Cells were washed with PBS and lysed in SDS Lamelli buffer. Protein concentration was determined using the bicinchoninc acid protein assay (Sigma-Aldrich Corp., St. Louis, MO, USA). 20 μg of protein extract was loaded in each lane of a 10% SDS-PAGE and run at 100 V. Protein was transferred to a nitrocellulose membrane at 22 V overnight at 4°C. The membrane was washed three times with TBST (50 mM Tris pH 7.5, 150 mM NaCI, 0.05 % Tween 20) and then incubated in blocking buffer (TBST and 5% non fat dry milk, pH 7.5) for 1 hour at room temperature. The membrane was then incubated with anti-mouse, CTCF (1:500, BD Transduction laboratories) and β-actin (1:3000, Sigma-Aldrich Corp., St. Louis, MO, USA) in blocking buffer overnight at 4°C. Following 3 washes, the membranes were incubated with goat anti-mouse secondary antibody (1:20,000, Jackson ImmunoResearch Laboratories, West Grove, PA, USA) for 1 hour at room temperature, followed by three 15-min washings. Immune complexes were visualized by addition of chemiluminescence reagent (Renaissance, DuPont NEN, Boston, MA, USA) and the membrane was exposed to Kodak XAR-5 film (Eastman Kodak Co., Rochester, NY, USA).
Cells were cultured to semi-confluence and plasmids expressing wild type and variant IGF-II transcripts were transfected into RIE-1 and 6B72 or L-cells using SuperFect Reagent (Qiagen, Inc. Valencia, CA, USA) according to the manufacturer's protocol. After 48 hours, transfected cells were passaged, 1:10, into medium containing hygromicin B (selective medium) at a concentration determined to kill 100% of non-transfected cells (150 mg/ml for RIE-1 and 6B72 cells, 650 mg/ml for L-cells). Cells were maintained in selective medium until clones appeared.
Soft agar colony forming assay
Dual layers of sea plaque agarose were made with the bottom layer consisting of a 50:50 mixture of 1.6% agarose solution 1:1 and 2X medium. The bottom layer was allowed to set for 4 hours, and then a 50:25:25 solution consisting of 2X medium, 1.6% stock agarose, and 1X medium containing cells, at a final concentration of 5000 cells/ml, was vortexed in a conical tube, and 2 ml was added to each well. Following 30 minutes at room temperature to allow the upper layer to set, plates were incubated at 37°C, 5% CO2 incubator for 7–10 days and checked for colony formation by microscopy.
Analysis of viral protein expression in infected cells
Cells were plated at 1.5 × 106 per well in 2 ml of medium in 6-well plates and allowed to sit over night. Cells were washed with phosphate-buffered saline (PBS), pH 7.4, and then infected with reovirus type 1 at the specified MOI. Virus was allowed to adsorb to cells for 1.5 hours at 4°C, washed twice with serum-free medium and incubated at 37°C and 5% CO2. At the indicated times, cells were scraped and lysed in Tris lysis buffer (10 mM Tris [pH 7.5], 2.5 mM MgCI2, 100 mM NaCI, 0.5% Triton x-100, 1 tablet Protease Inhibitor Cocktail Tablets [Roche Applied Science, Indianapolis, IN, USA] per 10 ml). After 30 min on ice, Laemmli sample buffer (Bio-Rad Laboratories, Hercules, CA, USA) were added to cell lysate samples (1:1). Protein samples were loaded in a 12% SDS-PAGE gel and run at 100 V. Protein was transferred to a nitrocellulose membrane at 100 V for 1 hour on ice. The membrane was washed three times with TBST (50 mM Tris pH 7.5, 150 mM NaCI, 0.05 % Tween 20) and then incubated in blocking buffer (TBST and 5% non fat dry milk, pH 7.5) for 1 hour at room temperature. The membranes were then incubated with rabbit anti-reovirus type 1 (1:50) and β-actin (1:3000, Sigma-Aldrich Corp., St. Louis, MO, USA) antibodies in blocking buffer overnight at 4°C. Following 3 washes in TBST, the membranes were incubated with goat anti-rabbit (for reovirus) or goat anti-mouse (β-actin) secondary antibodies (1:20,000, Jackson ImmunoResearch Laboratories, West Grove, PA, USA) for 1 hour at room temperature, followed by three 15-min washes. Immune complexes were visualized by addition of chemiluminescence reagent (Renaissance, DuPont NEN, Boston, MA, USA) and the membrane was exposed to Kodak XAR-5 film (Eastman Kodak Co., Rochester, NY, USA).
Cell Proliferation Assay
RIE-1, 6B72, or Igf2 transfected RIE-1 or 6B72 cells were seeded at 5 × 104 per well in 96-well plates, incubated at 37°C and 5% CO2. At 4, 6, 18, 48 hours post plating, 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium [MTS], and an electron coupling reagent (phenazine methosulfate, [PMS]) were added at 20 μl per well (CellTiter 96 Aqueous Non-Radioactiver Cell Proliferation Assay, Promega, Madison, Wl). Plates were incubated for 2 hours, and then the absorbance was determined at 490 nm. Each set of conditions was repeated in triplicate.
This work was supported by Public Health Service Grants (R01HG00684 to HER, RO1CA682383 to DHR), by a grant from the Kleberg Foundation, partially supported by Cancer Centre (Core) grant P30CA42014, and Avatar BioSci, Inc. Fluorescence microscopy was performed, in part, through the use of the VUMC Cell Imaging Shared Resource, supported by NIH grants CA68485, DK20593, DK58404 and HD15052. We would like to thank J. Hawiger, T. Hodge, and E. Eisenberg for review of the manuscript. B. Mooneyhan provided expert secretarial assistance.
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