Direct transformation of rodent fibroblasts by
jaagsiekte sheep retrovirus DNA
Naoyoshi Maeda, Massimo Palmarini*, Claudio Murgia, and Hung Fan†
Cancer Research Institute and Department of Molecular Biology and Biochemistry, University of California, Irvine, CA 92697
Edited by John M. Coffin, Tufts University School of Medicine, Boston, MA, and approved January 29, 2001 (received for review November 17, 2000)
A
nimal retrovirus-induced cancers have played fundamental
roles in understanding the molecular basis of cancer (1).
Jaagsiekte sheep retrovirus (JSRV) is the causative agent of
ovine pulmonary carcinoma (OPC), a contagious lung cancer of
sheep (also known as sheep pulmonary adenomatosis or SPA)
(2–5). JSRV-induced OPC consists of transformed secretory
epithelial cells of the lungs: type II pneumocytes and Clara cells.
A characteristic feature of OPC tumors is the production of large
amounts of fluid secreted from the tumor cells (containing
infectious virus). OPC closely resembles human bronchiolo
alveolar carcinoma (BAC), an adenocarcinoma not associated
with cigarette smoking and whose etiology is currently unknown
(6). Thus OPC is an important model for understanding human
BAC pathogenesis.
Oncogenic retroviruses induce tumors by two mechanisms.
Acutely transforming retroviruses (typically replicationdefective) have captured a normal cell gene (a protooncogene)
and converted it into a viral oncogene. Acutely transforming
retroviruses typically induce rapid neoplasms in vivo, and they
frequently can transform cells in culture (7, 8). Retroviruses that
lack oncogenes also can induce tumors (nonacute retroviruses),
although they typically require longer incubation periods and
multiple rounds of infection in vivo (9). An important molecular
mechanism for these viruses is insertional activation of protooncogenes: insertion of a provirus in the vicinity of a cellular
protooncogene. This results in overexpression of the protooncogenes due to highly active transcriptional regulatory elements
in the retroviral long terminal repeat (LTR).
We recently obtained an infectious molecular clone of JSRV
(JSRV 21) and used virus recovered from it to demonstrate that
www.pnas.orgycgiydoiy10.1073ypnas.071547598
JSRV is necessary and sufficient to induce OPC in sheep (2).
However the mechanism of oncogenesis by JSRV has not been
clear. Examination of the nucleotide sequence of the JSRV
genome indicated the presence of typical structural genes for a
retrovirus (gag, pro, pol, and env), but there were no additional
genes with obvious characteristics of a viral oncogene (e.g.,
homology to a cellular protooncogene). However, JSRV does
contain an additional alternate ORF (orf-x) overlapping pol
whose significance is unknown (2, 5, 10, 11). Thus JSRV has the
genome organization more typical of a nonacute retrovirus. On
the other hand, when JSRV is used to experimentally induce
OPC in newborn lambs, multifocal tumor lesions appear quite
rapidly (4–6 weeks) (12, 13). This disease pattern is more typical
of acute transforming retroviruses.
In this study, we tested the hypothesis that JSRV contains a
gene with oncogenic potential. We report that JSRV DNA can
induce the foci of transformation when transfected into murine
NIH 3T3 cells. Additional experiments localized the transforming activity to the viral env gene.
Materials and Methods
Plasmids. The schematic organization of the plasmids used in this
study is shown in Fig. 1. pJSRV21 and pCMV2JS21 have been
described (2). Briefly, pJSRV21 is an infectious and oncogenic
molecular clone of JSRV; in pCMV2JS21 the U3 region in the
upstream LTR has been replaced by the cytomegalovirus (CMV)
immediate early promoter. pCMV2JS21Dorfx was obtained by
the introduction of two stop codons in the orf-x reading frame
of pCMV2JS21 (TTA to TAA at position 4821 and TTA to TAA
at position 4952 with respect to the JSRV21 sequence; ref. 2).
These mutations leave the amino acids in the overlapping pol
reading frame unaltered. Plasmid pCMV3JS21DGP was obtained by digesting pCMV3JS21 with PacI and BamHI and
religating the plasmid after a standard filling-in reaction (14).
PacI is immediately downstream from the splice donor for env
whereas BamHI is immediately upstream from the env splice
acceptor; thus pCMV3JS21DGP expresses the env gene.
pCMV3JS21 is identical to pCMV2JS21 with the exception of
the removal of some restriction sites in the multiple cloning site
of the backbone to facilitate cloning. pCMV3JS21DGPDStuI was
obtained from pCMV3JS21DGP by excision of the StuI fragment
encompassing the transmembrane region of the JSRV envelope
and the downstream LTR. pcDNA3.1(2) (Invitrogen) was used
as a negative control in the transformation assays (see below).
The plasmid pHR-IR-MuLV contained a Moloney murine leukemia virus (M-MuLV)-based vector expressing an activated
This paper was submitted directly (Track II) to the PNAS office.
Abbreviations: JSRV, jaagsiekte sheep retrovirus; OPC, ovine pulmonary carcinoma; CMV,
cytomegalovirus; LTR, long terminal repeat; RT, reverse transcription.
See commentary on page 4285.
*Present address: Department of Medical Microbiology and Parasitology, College of Veterinary Medicine, University of Georgia, Athens, GA 30602.
†To
whom reprint requests should be addressed. E-mail: hyfan@uci.edu.
The publication costs of this article were defrayed in part by page charge payment. This
article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C.
§1734 solely to indicate this fact.
PNAS u April 10, 2001 u vol. 98 u no. 8 u 4449 – 4454
CELL BIOLOGY
Jaagsiekte sheep retrovirus (JSRV) is the causative agent of ovine
pulmonary carcinoma, a unique animal model for human bronchioalveolar carcinoma. We previously isolated a JSRV proviral
clone and showed that it was both infectious and oncogenic. Thus
JSRV is necessary and sufficient for the development of ovine
pulmonary carcinoma, but no data are available on the mechanisms of transformation. Inspection of the JSRV genome reveals
standard retroviral genes, but no evidence for a viral oncogene.
However, an alternate ORF in pol (orf-x) might be a candidate for
a transforming gene. We tested whether the JSRV genome might
encode a transforming gene by transfecting an expression plasmid
for JSRV [pCMVJS21, driven by the cytomegalovirus (CMV) immediate early promoter] into mouse NIH 3T3 cells. Foci of transformed
cells appeared in the transfected cultures 2–3 weeks posttransfection; cloned transformants showed anchorage independence for
growth, and they expressed JSRV RNA. These results indicate that
the JRSV genome contains information with direct transforming
potential for NIH 3T3 cells. Transfection of a mutated version of
pCMVJS21 in which the orf-x protein was terminated by two stop
codons also gave transformed foci. Thus, orf-x was eliminated as
the candidate transforming gene. In addition, another derivative of
pCMVJS21 (pCMVJS21DGP) in which the gag, pol (and orf-x) coding
sequences were deleted also gave transformed foci. These results
indicate that the envelope gene carries the transforming potential.
This is an unusual example of a native retroviral structural protein
with transformation potential.
and fresh medium was added. Cells were maintained in culture
for 4–5 weeks with the media replaced every 3 days and
monitored microscopically. Foci of transformed cells were
counted 1 month after transfection. In parallel experiments,
transfected or mock-transfected cells were passaged every 3 days
and checked for changes in their morphology.
Colony Formation Assays. Cultures of JSRV-transformed NIH 3T3
cells that had been serially passaged were seeded in agar
suspension (104 cells per 6cm dish) as described (16). NIH 3T3
nontransformed cells were seeded in parallel. The cultures were
incubated for 4 weeks, and colonies were counted in a microscope at low magnification. The diameters of 100 colonies with
a diameter greater than 100 mm were determined with a
measuring reticle in the microscope.
Establishment of NIH 3T3 Transformed Cell Lines. Single cell clones
Fig. 1. Plasmids used in this study. They are described in Materials and
Methods. The restriction sites used for cloning are indicated. The two stop
codons in the orf-x reading frame of pCMV2JS21Dorfx are shown as vertical
bars underlined by asterisks. The broken lines in pCMV3JS21DGP and
pCMV3JS21DGPDStuI indicate deletions.
of transformed NIH 3T3 cells were obtained by dilution of
pCMV2JS21 transfected NIH 3T3 cells and growth into colonies
on monolayers followed by selective trypsinization of transformed colonies. Alternatively, single colonies were picked from
agar colony formation assays. DNA from 11 clones was extracted
by using a Dneasy Tissue kit (Qiagen). The presence of JSRV
DNA and RNA was confirmed by PCR for the JSRV LTR (17),
Southern or Northern blotting analysis using JSRV-specific
probes.
Infection of OHH1 with JSRV21 and Reverse Transcription (RT)–PCR. A
human H-ras gene (from the T24 bladder carcinoma). The H-ras
gene was downstream from an encephalomyocarditis virus internal ribosome entry site, and expression was under control of
the M-MuLV LTR (W. Hsiao and H.F., unpublished work).
Cell Culture. Human 293T cells (15) were grown in DMEM-10%
FBS. Mouse NIH 3T3 cells (obtained by R. Weinberg, Massachusetts Institute of Technology, Cambridge) were grown in
DMEM-10% calf serum. Both cell lines were grown in an
incubator at 37°C with 5% CO2. The OHH1 cell line (derived
from deer lung) was obtained from the ATCC (CRL-6195) and
was grown in DMEM-10% FBS.
RNA Extraction and Northern Blotting. To analyze the pattern of
expression of the plasmid constructs used in this study, 293T cells
('1 3 106 cells per 10-cm plate) were transfected with 28 mg of
plasmid DNA and the CalPhos mammalian transfection kit
(CLONTECH) as recommended by the manufacturer. Fortyeight hours after transfection, total RNA was extracted by using
a RNAqueous-Midi kit (Ambion, Austin, TX). RNA was extracted in the same way from JSRV-infected OHH1 cells, 3T3
transformed clones (see below), or lung tumors of OPC-affected
animals (provided by J. M. Sharp, Moredun Research Institute,
Penicuik, Scotland). RNA preparations were treated with
RNase-free DNase (Qiagen, Chatsworth, CA). Six to 10 micrograms of total RNA was denatured with glyoxalyDMSO and run
in a 1% agarose gel in 10 mM sodium phosphate and blotted to
nylon membranes (Hybond-N Plus, Amersham Pharmacia) by
using established methods (14). Membranes were hybridized to
32P-labeled JSRV env probes corresponding to the U3 region or
to nucleotides 5347–5530 of the JSRV21 sequence (env2) or to
nucleotides 6329–6641 (env-up) and subjected to autoradiography by exposure to x-ray film.
Transformation Assays. NIH 3T3 cells (3 3 105 per 10-cm tissue
culture plate) were transfected with 28 mg of plasmid DNA as
above. Independent experiments were performed with at least
two different plasmid preps for each construct. Approximately
12 h after transfection, cells were washed three times with PBS,
4450 u www.pnas.orgycgiydoiy10.1073ypnas.071547598
total of 2 3 105 OHH1 cells were plated in 5-cm tissue culture
dishes and infected with JSRV21 virus as described (18). Infectious JSRV was obtained by transfecting 293T cells with
pCMV2JS21 as described (2). Two micrograms of total RNA
from OHH1 cells at the 11th passage postinfection, control
OHH1 cells infected with heat-inactivated virus, or 293T cells
transfected with pCMV2JS21 were subjected to a RT reaction
primed with oligo(dT) using Omniscript (Quiagen) as recommended by the manufacturer. Three microliters of this reaction
was amplified by using an oligo(dT) primer as the reverse primer
and a forward primer designed in the JSRV untranslated gag
region before the splice donor site (TTCTCTAGAGGGCTCGAGCTCGACAGTTTTC). The resulting PCR products were
run in a 1% agarose gel and visualized under a UV transilluminator. The bands of interest were excised from the agarose gel,
and the DNA was purified by using QIAquick Gel Extraction kit
(Qiagen) and cloned into pBlueScript (Stratagene) as recommended by the manufacturer. Plasmids corresponding to PCR
bands from the OHH1-infected cells and 293T-transfected cells
were sequenced on an Applied Biosystems Prism 310 Genetic
Analyzer (Perkin–Elmer), using a BigDye Terminator DNA
cycle sequencing kit (Perkin–Elmer Applied Biosystems) as
recommended by the manufacturer.
Results
JSRV DNA Transforms NIH 3T3 Cells. To test the hypothesis that
JSRV might carry genetic information with direct oncogenic
potential, we performed a series of in vitro transformation assays
on mouse NIH 3T3 cells using plasmid DNA corresponding to
the full-length JSRV proviral genome. Initially, we used two
constructs: pJSRV21 and pCMV2JS21 (2) (Fig. 1). They both
contain the entire JSRV21 coding sequences and differ in the
promoteryenhancer regions. In pJSRV21, expression is driven by
the JSRV LTR whereas in pCMV2JS21 expression is driven by
the CMV immediate-early promoter (2). NIH 3T3 cells were
transfected with pJSRV21 and pCMV2JS21 and maintained in
culture for 4 weeks. Negative controls included NIH 3T3 cells
transfected with pcDNA3.1(2) or mock-transfected cells.
pcDNA3.1(2) was chosen as negative control because it contains the same CMV immediate early promoter as in
Maeda et al.
Table 1. Transformation assays performed in this study
Experiment
1
2
3
4
5
6
7
8
9
10
DNA(2)
pcDNA3.1(2)
pJSRV21
pCMV2JS21
pCMV2JS21Dorfx
pCMV3JS21DGP
PCMV3JS21DGPDStuI
pHR-IRMuLV (H-ras)
0
0
0
0
0
0
0
0
0
0
0
0
0
0
N.T.
0
0
0
0
0
0
4
1
0
1
N.T.
N.T.
N.T.
N.T.
N.T.
15
11
16
21
18
22
11
N.T.
N.T.
N.T.
14
12
12
22
14
N.T.
N.T.
N.T.
N.T.
N.T.
N.T.
N.T.
N.T.
N.T.
N.T.
31
23
15
13
18
N.T.
N.T.
N.T.
N.T.
N.T.
0
0
N.T.
N.T.
N.T.
N.T.
N.T.
N.T.
N.T.
N.T.
N.T.
N.T.
25
28
44
NIH-3T3 were transfected with the above indicated plasmids as described in Materials and Methods. Numbers indicate the number of foci counted at 28 days
posttransfection. N.T. 5 not tested.
after transfection, foci assumed a characteristic morphology with
the transformed cells radiating from the center of the focus (Fig.
2B); foci were generally visible to the naked eye at this stage.
Transfection of 3 3 105 NIH 3T3 cells with 28 mg pCMV2JS21
DNA resulted in the formation of an average of 16.2 foci per
experiment (SD 5 63.7) counted at 28 days posttransfection. No
foci were observed in the mock-transfected 3T3 cells nor in the
3T3 cells transfected with pcDNA3.1(2). pJSRV21 induced 0–4
foci in five experiments, indicative of significantly lower transformation efficiency. This finding was not surprising, because we
previously showed that the JSRV LTR is a weak promotery
enhancer in NIH 3T3 cells (19).
In parallel experiments, pCMV2JS21-transfected cells were
passaged every 3–4 days after transfection. In these cases, the
transformed cells were gradually selected and large foci of
transformed cells eventually occupied most of the plate by '6
weeks posttransfection (Fig. 2 C and D). These cells showed
anchorage-independent properties as witnessed by their capacity
to grow in soft agar, forming colonies with an average diameter
of 440 mm at 4 weeks. The parental untransformed NIH 3T3 cells
did not show any large colonies over 150 mm (Fig. 2 E and F).
JSRV DNA also transformed the Rat 6 cell line (16) at low
efficiency; in agar colony assays, a cloned Rat 6 transformant
formed large colonies (.200 mm) with an efficiency of 23%,
whereas parental Rat 6 cells remained as single cells (not shown).
We established 11 transformed cell lines from pCMV2JS21transfected cultures by single-cell cloning from colonies grown in
soft agar or by limiting dilution and picking individual transformed colonies. All 11 clones contained JSRV plasmid DNA as
established by JSRV LTR PCR (17) or Southern blotting (data
not shown), and they contained JSRV RNA as assessed by
Northern blotting (see below). The results obtained indicated
that one of the JSRV genes is able to transform NIH 3T3 cells
in vitro.
The JSRV orf-x Is Not Responsible for Transformation. We next
Fig. 2. Transformation of NIH 3T3 cells. NIH 3T3 cells were transfected with
28 mg of pCDNA3.1(2) (A) or pCMV2JS21 (B) and maintained in culture for 28
days. A typical focus of transformed cells is shown in B. In parallel experiments,
transfected cells were passaged every 3– 4 days. After 6 weeks, the cells
transfected with pCDNA3.1(2) (C) had the typical appearance of NIH 3T3 cells,
whereas the cells transfected with pCMV2JS21 (D) were morphologically
altered, with loss of contact inhibition. pCDNA3.1(2)-transfected NIH 3T3 cells
did not form large colonies in soft agar (E) whereas NIH 3T3 cells transformed
by pCMV2JS21 showed anchorage independent colonies (F). (Magnification:
340.)
Maeda et al.
investigated JSRV orf-x as the possible transforming gene. Orf-x
is an ORF overlapping pol that is conserved among different
JSRV isolates (10, 11) but has no obvious homologues in other
related type D and B retroviruses. To test the role of this gene
in transformation, we mutated pCMV2JS21 by the insertion of
two stop codons in the orf-x ORF; these stop codons do not
mutate the overlapping pol amino acid sequences. The resulting
plasmid, called pCMV2JS21Dorf-x (Fig. 1), was used in transformation assays as above. pCMV2JS21Dorf-x was able to
transform NIH 3T3 cells with the same efficiency as
pCMV2JS21; the average number of foci obtained in five
experiments was 14.4 (64.1) (Table 1). Thus we concluded that
the JSRV orf-x is not directly involved in cell transformation.
PNAS u April 10, 2001 u vol. 98 u no. 8 u 4451
CELL BIOLOGY
pCMV2JS21, but it does not contain JSRV sequences. Experiments were done independently five times with at least two
different plasmid preps.
In seven of seven experiments (see experiments 1–7 in Table
1) pCMV2JS21 was able to induce foci of cell transformation
(Fig. 2 A and B). The transformed foci became visible 2 weeks
after transfection. Transformed cells were recognizable by their
fusiform shape and the loss of contact inhibition, resulting in the
formation of foci of cells piling on top of each other. By 4 weeks
foci, respectively. Thus, the expression of the JSRV envelope
gene is sufficient to induce transformation of NIH 3T3 cells.
When compared with a plasmid expressing the activated c-ras
protooncogene from human T24 bladder carcinoma cells driven
by a murine leukemia virus LTR (pHR-IR-MuLV),
pCMV3JS21DGP induced foci with similar (ca. 2-fold less)
efficiency (Table 1, experiments 8–10).
Analysis of env Transcripts. To characterize the JSRV env gene,
Fig. 3. JSRV env transcripts. (A) Northern blotting of total RNA collected
from 293T cells transfected with pCMV2JS21, pCMV3JS21DGP, and mocktransfected 293T cells. Filters were hybridized with the env-2 probe as described in Materials and Methods. 293T cells transfected with pCMV3JS21DGP
lack the 7.5-kb band (corresponding to the full-length JSRV genome) because
the gag and pol sequences have been deleted. (B) Northern blotting of total
RNA from NIH 3T3 cells transformed by pCMV2JS21, parental NIH 3T3 cells,
lung from a healthy sheep, and lung from tumor lesions of two sheep affected
by OPC. Both the 7.5- and 2.4-kb transcripts are readily visible whereas the
1.2-kb transcript is present at a low level.
Expression of the JSRV Envelope Is Sufficient to Induce Cell Transformation. Because the JSRV orf-x was not responsible for trans-
formation, we next tested whether the env gene was responsible.
We constructed a plasmid derived from pCMV2JS21 where gag,
pro, pol (and consequently orf-x) were deleted. The only intact
gene of pCMV2JS21 was env; both the env splice donor and
splice acceptor sites were left intact. The resulting plasmid,
pCMV3JS21DGP, was able to induce transformation of NIH 3T3
cells. In two independent experiments (Table 1, experiments 6
and 7), pCMV3JS21DGP induced the formation of 31 and 23
we performed Northern blotting of RNA from human 293T
cells transiently transfected with either pCMV2JS21 or
pCMV3JS21Dorf-x, NIH 3T3 cell lines transformed by JSRV,
and tumors of OPC-affected sheep. We previously used transfected 293T cells to produce large amounts of infectious JSRV
(2) because they very efficiently express transiently transfected
plasmid DNA. Filters were hybridized with a probe corresponding to the first 183 bp of the envelope gene (env2). Three major
bands were detected in 293T cells transfected with pCMV2JS21,
in 3T3 transformed clones, and in OPC tumors (Fig. 3). As
expected, we detected JSRV-specific RNA of about 7.5 kb
corresponding to the full-length JSRV viral genome and a
transcript of about 2.5 kb corresponding to the spliced env
mRNA. A third band of '1.2 kb also was observed. In 293T cells
transfected with pCMV3JS21DGP, the 2.5- and the 1.2-kb bands
were present. The 1.2-kb transcript was very abundant (approximately the same intensity of the 2.4-kb env transcript) in
transfected 293T cells, while it was present at lower but detectable levels in 3T3 transformed clones and in OPC tumors. To
identify the structure of the 1.2-kb band (tr-env for truncated
env) Northern blots were hybridized in parallel with the env2
probe, a JSRV U3 probe, and another env probe corresponding
to nucleotides 6329–6641 (env-upstream) (Fig 4A). The 7.5-kb
and 2.5-kb bands were observed with all three probes. However
the tr-env band was observed only with the env2 probe. It was
particularly surprising that the JSRV U3 probe did not hybridize
with the tr-env RNA, because U3 is typically present in all of the
retroviral transcripts. These data suggested that tr-env RNA
might be prematurely polyadenylated.
To fully characterize the tr-env mRNA we performed RT-PCR
on total RNA from 293T cells transfected with pCMV2JS21 and
Fig. 4. Identification of JSRV env transcripts. (A) Northern blotting on 293T cells transfected with pCMV2JS21 vs. mock-transfected cells was carried out to
identify the structure of the 1.2-kb env transcript. Blot hybridization was carried out in parallel with a U3 probe and with the env-2 and env-upstream probes.
The 1.2-kb transcript is apparent only with the env-2 probe. (B) RT-PCR cloning mapped the 1.2-kb transcript (tr-env) as a prematurely polyadenylated mRNA
terminating at position 6301. (C) Transfection of plasmid pCMV3JS21DGPDStuI that expresses only the 1.2-kb tr-env mRNA in 293T cells and Northern blotting.
4452 u www.pnas.orgycgiydoiy10.1073ypnas.071547598
Maeda et al.
tr-env Is Not Sufficient for Transformation. To test whether the
full-length env mRNA andyor the tr-env mRNA are responsible
for transformation we repeated the transformation assays with
pCMV3JS21DGPDStuI. This construct is derived from
pCMV3JS21DGP by excision of a StuI fragment, which results in
a construct that should express only the 1.2-kb tr-env transcript
(the StuI site in env is 192 nt downstream from the cryptic
polyadenylation site of tr-env, at the boundary between surface
and the transmembrane region).
By Northern blotting, we confirmed that this plasmid expressed the tr-env mRNA in large quantities (Fig. 4C). However,
NIH 3T3 transformed with pCMV3JS21DGPDStuI did not show
any sign of transformation (Table 1, experiments 6 and 7). Thus,
the expression of the JSRV tr-env is not sufficient for transformation of NIH 3T3 cells.
Discussion
In these study, we have shown that transfection of JSRV DNA
into NIH 3T3 cells resulted in consistent induction of foci of
transformed cells with an efficiency similar to that observed for
known oncogenes. Rodent cells do not express a functional
receptor for JSRV (22), which indicates that the transformation
did not result from cell-to-cell spread of infectious virus. Thus
some viral gene product could directly induce transformation.
This was further supported by the fact that it was necessary to
drive expression of the JSRV sequences from a promotery
enhancer that is highly active in NIH 3T3 cells (the CMV
immediate early promoter), and that transformed cells contained JSRV DNA and expressed viral RNA.
We investigated which JSRV gene was responsible for the
transformation capacity. Mutation of the orf-x reading frame did
not decrease the transformation activity, eliminating orf-x as the
transforming gene. Further studies indicated that a deleted version
of pCMVJS21 lacking all coding regions except env retained the
ability to transform NIH 3T3 cells. Thus, an env gene product(s) is
apparently responsible for the transformation. Analysis of envcontaining transcripts indicated that in addition to the expected
spliced env mRNA, JSRV also encodes a novel truncated form of
env mRNA resulting from a cryptic cleavageypolyadenylation site
in env. This mRNA would encode a truncated form of env
polyprotein essentially containing only the surface domain. It is
noteworthy that the cryptic cleavageypolyadenylation site appears
to be conserved among three exogenous JSRV isolates, but it is
missing from the env genes of several endogenous JSRV-related
viruses present in the sheep genome (5, 11, 21). Transfection with
a pCMVJS21 derivative that could express only the truncated form
of env did not yield transformed foci. Thus, it appears that a
full-length env gene is necessary for transformation, although the
results do not rule out the possibility that expression of truncated
env protein also is required.
Maeda et al.
The finding that JSRV Env protein is able to transform NIH
3T3 cells is striking, because this protein is a structural component of virions, and productive infection of cells by most
retroviruses (even oncogenic ones) does not generally result in
transformation. These results also suggest that JSRV might be
considered an acute transforming virus, with the capacity to
directly initiate oncogenesis. The results also raise questions
about the mechanism of transformation. Because the Env protein is normally expressed on the surface of the virion (and of
infected cells), it seems most likely that transformation results
from interaction of Env protein with some other cell surface
protein. Three possibilities are particularly interesting. First, it is
possible that JSRV Env protein interacts with the murine
homologue of the JSRV receptor, and in doing so interferes with
a negative growth regulatory activity of the JSRV receptor. In
this regard, the JSRV receptor has been cloned from human cells
and shown to be the hyaluronidase-2 (HYAL-2) gene (A. D.
Miller, personal communication). It is striking that chromosomal
deletions of the region containing the HYAL-2 gene have been
observed in cell lines derived from human lung cancer (23),
suggestive of a tumor suppressor function for this protein. A
second possibility is that JSRV Env protein interacts with the
murine JSRV receptor, which results in a positive growth signal
through the JSRV receptor. Murine cells do not have functional
JSRV receptors, because these cells cannot propagate viral
infection (18) and they cannot be efficiently transduced with
retroviral vectors containing JSRV Env protein (22). However,
low level transduction of murine cells was actually observed (22).
Thus, JSRV Env can apparently interact with the murine
HYAL-2; this interaction is inefficient for viral entry, but it is
possible that it is sufficient for cell transformation. A third
possibility is that the JSRV Env protein can interact with a
distinct cell surface protein when it is causing transformation.
Finally, it is possible that some form or domain of Env protein
interacts with an intracellular protein to cause transformation.
At least one other retroviral Env protein has been described
as a viral oncogene. The spleen focus-forming (SFFV) component of the Friend erythroleukemia virus complex can cause
rapid proliferation of erythroid cells. SFFV encodes a deleted
version of a recombinant envelope protein (gp55) that is responsible for proliferation of erythroid cells, and it has been further
shown that gp55 protein binds to the erythropoietin receptor on
erythroid cells (24, 25). This binding leads to constitutive
activation of signal transduction pathways downstream from the
erythropoietin receptor (26–29). Recently, it has been reported
that the transforming gene of an avian hemangiosarcomainducing retrovirus is also env (30).
These results suggest that the JSRV Env protein may have direct
oncogenic potential during OPC tumorigenesis in sheep. This
suggestion would be consistent with experimentally induced disease
(intratracheal inoculation of virus in newborn lambs), where there
is rapid onset of disease and multiple tumor foci in the lungs (12,
31). However in the field, JSRV-induced OPC occurs much more
slowly, with considerable delay between evidence of infection and
appearance of the tumors (32). This finding suggests that in that
setting (and perhaps infection of older animals) additional factors
or events besides the intrinsic transforming potential of JSRV env
are required for development of disease. In the future, it will be
interesting to evaluate the relative roles of the oncogenic effects of
Env protein and other factors in OPC pathogenesis.
We thank Dr. J. M. Sharp (Moredun Research Institute, Penicuik,
Scotland) for providing tissues of animals affected by OPC and Dr. A.
Dusty Miller for communicating results before publication. We thank L.
Laimins for initially suggesting the pCMV2JS21 transfection experiment.
M.P. was a recipient of an American Cancer Society Ray and Estelle
Spehar Fellowship. This work is supported by National Institutes of
Health Grant RO1CA82564. Support from the University of California
Irvine Cancer Research Institute and the DNA sequencing core of the
Chao Family Comprehensive Cancer Center is acknowledged.
PNAS u April 10, 2001 u vol. 98 u no. 8 u 4453
CELL BIOLOGY
from deer OHH1 cells productively infected by JSRV21 virions.
The cDNAs obtained from these samples were amplified by PCR
using a forward primer in the untranslated gag region before the
splice donor site and oligo(dT) as reverse primer. The use of
human 293T cells and deer OHH1 allowed us to use PCR
primers that would otherwise cross-react with sheep endogenous
JSRV (enJSRVs) transcripts in sheep tissues (20, 21). A prominent PCR product of a size comparable to the tr-env RNA was
obtained from both cells (not shown). The RT-PCR products
were cloned and sequenced. The sequences of both products
revealed that JSRV tr-env RNA uses the splice donor and splice
acceptor of the canonical JSRV env mRNA (M.P., C. Murgia,
N.M., and H.F., unpublished work) but it is prematurely terminated and polyadenylated at nucleotide 6301 (Fig. 4B). Thirteen
nucleotides before the start of the poly(A) tail there is a putative
polyadenylation site in the JSRV env sequence (ATTAAA).
These results were consistent with the results obtained by the
Northern blot analysis.
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