Poxvirus Growth Factors Related to Epidermal Growth Factor

6 Pages·0.579 MB·English

by  Mcfadden G., Moyer R.

Poxvirus Growth Factors
Related to Epidermal
Growth Factor
Grant McFadden1,* and Richard Moyer2
1The John P. Robarts Research Institute and Department of Microbiology and Immunology,
The University of Western Ontario, 1400 Western Road, London, Ontario, N6G 2V4, Canada
2Department of Molecular Genetics and Microbiology, University of Florida College of Medicine,
PO Box 100266, Gainesville, FL 32610-0266, USA
*corresponding author tel: [phone], fax: [phone], e-mail: [email protected]
DOI: 10.1006/rwcy.2000.03014.
SUMMARY been associated with proliferative diseases that range
from benign tumors to disseminating lesions asso-
ciated with excessive levels of cellular replication. An
Many poxviruses encode homologs of the cellular
example of this phenomenon is provided by Shope
epidermal growth factor (EGF)/transforming growth
fibroma virus (SFV), which causes benign fibromas in
factor (cid:11) (TGF(cid:11)) family of ligands. Like their cellular
adult rabbits and atypical invasive fibrosarcomas in
counterparts, the poxviral growth factors are
newborn rabbits or immunosuppressed adult rabbits.
expressed as secreted proteins that bind to members
Other examples include Yaba tumor virus, which
of the erbB family of cellular receptors and induce a
induces subcutaneous histiocytomas in humans and
mitogenic response in the target cells. The first dis-
primates, and molluscum contagiosum virus, which
coveredviralexample,andprototypicmemberofthis
induces wart-like lesions in humans.
family, is the vaccinia growth factor (VGF), which is
Many of the features of these tumor-like lesions
synthesized as a membrane precursor and then later
induced by poxviruses suggest that some members of
processed by proteolytic cleavage to the mature sec-
thisfamilycaneitherexpressorinducesecretedgrowth
retedligand.Itisbelievedthatpoxvirusesutilizethese
factors that stimulate uninfected cells within lesions
growth factors as paracrine stimulators of neighbor-
into unscheduled cell division. Indeed, as early as
ing uninfected cells in viral lesions to facilitate subse-
1963,Katoetal.speculatedthata‘growthpromoting
quentroundsofvirusreplicationandcell–cellspread.
factor’mightbesecretedfromcellsinfectedwithSFV
that induce neighboring fibroblasts to proliferate in
an uncontrolled paracrine-like fashion (Kato et al.,
BACKGROUND
1963). This prediction was verified in the mid-1980s
when sequencing studies revealed the presence of a
Discovery
gene (designated 19K) in vaccinia virus (strain WR)
that encoded a 140 amino acid protein which bore
Although poxviruses do not replicate in the nuclei of striking similarity to a family of host growth factors
infectedcells,nordotheyimmortalizeorpermanently related to epidermal growth factor (EGF) and trans-
transform cells, infections by some members have forming growth factor (cid:11) (TGF(cid:11)) (Blomquist et al.,
842 Grant McFadden and Richard Moyer
1984;Brownetal.,1985;Reisner,1985).Subsequently, Alternative names
it was shown that the poxvirus proteins were indeed
bona fide functional ligands for the cellular EGF
See Table 1.
receptors.
Structure
Table 1 Poxvirus growth factors related to EGF/TGF(cid:11)
As shown in Figure 1, vaccinia growth factor (VGF)
Virus (strain) Name Amino % homology contains a characteristic spacing of six key cysteine
acids residueswhichdeterminesthefoldingdomainsofmost
membersoftheEGF/TGF(cid:11)superfamily.Thismotifis
Vaccinia (Copenhagen) VGF 142 100
mirroredintheotherpoxvirushomologsaswell.
Vaccinia (WR) VGF 140 99
Cowpox (GRI-90) CGF 138 99 Main activities and
Variola (Bangladesh) VaGF 140 97 pathophysiological roles
Myxoma (Lausanne) MGF 85 42
Shope fibroma (Kasza) SFGF 80 44 Since the discovery of VGF, related growth factors
have been detected in other poxviruses (see Table 1).
Figure 1 Comparison of vaccinia growth factor (VGF) with other members of the EGF-like
familyofgrowthfactors.FeaturesoftheVGFprimarysequenceincludeasignalsequencecleavage
site (arrow) at amino acid 20, a proteolytic cleavage site (arrow) at amino acid 96 to release the
mature76aminoacidsecretedligand,andthetransmembranespanningdomainfromaminoacids
105to124.Thesequenceoftheconservedcysteine-richregion,aminoacid45–80,isshownaligned
toVGFhomologs.Aminoacidsconservedamongthepoxvirussequencesareboxedin;conserved
cysteine residues are indicated by the black circles. Adapted from McFadden et al. (1995).
Poxvirus Growth Factors Related to Epidermal Growth Factor 843
All are believed to be secreted from infected cells and secreted from vaccinia-infected cells, competes with
are bona fide ligands of the EGF receptor (ErbB) EGF for binding to the EGF receptor and induces
superfamily. The poxvirus growth factors are receptor phosphorylation (Stroobant et al., 1985;
reviewed extensively elsewhere (McFadden et al., Twardzik et al., 1985; King et al., 1986). The native
1995), but all are believed to confer a selective protein is antigenic, but antibodies to VGF show
advantage to the virus by stimulating quiescent cells only limited crossreactivity with EGF and no reac-
into S phase within virus-infected cells. tion with TGF(cid:11). The VGF coding sequence
includes an N-terminal signal sequence and C-ter-
minal hydrophobic domain, similar to that of
GENE AND GENE REGULATION
TGF(cid:11) (see Figure 1). VGF is first synthesized as a
glycosylated membrane-spanning surface protein,
Accession numbers which is then cleaved by unknown protease(s) to
generate a 77 amino acid (22kDa) mature ligand
which is released into the extracellular environment
GenBank:
(Stroobant et al., 1985; Chang et al., 1988). The
Vaccinia virus (Copenhagen): M35027
secreted VGF is mitogenic, can substitute for
Vaccinia virus (WR): J02421
Cowpox virus (GRI-90): Y11842/Y15035 TGF(cid:11) in TGF(cid:11)/(cid:12) transformation assays and
Variola virus (Bangladesh): L22579 scores positively on standard wound-healing assays
Myxoma virus (Lausanne): M15806 (Stroobant et al., 1985; Twardzik et al., 1985;
King et al., 1986; Schultz et al., 1987). Furthermore,
Shope fibroma virus (Kasza): M15921
VGF induces tyrosine phosphorylation of phos-
pholipase C(cid:13)1 in a fashion similar to EGF (Kim
Regulatory sites and corresponding et al., 1995).
The Shope fibroma growth factor (SFGF) and
transcription factors
myxoma growth factor (MGF) genes have been
detected in members of the leporipoxvirus genus
All poxvirus growth factor genes are expressed from
(Upton and McFadden, 1986; Chang et al., 1987;
early viral promoters under the strict control of the
Upton et al., 1987). SFGF is an 80 amino acid
viral transcriptional apparatus, and thus expression
protein that demonstrates 37% identity to VGF and
levels are relatively independent of the infected cell
TGF(cid:11) within the central Cys-rich domain (Figure 1).
type, provided that the infection is productive and
MGF has 85 amino acids, and has 80% overall
allows the full range of virus gene expression.
identity to SFGF. Like VGF, both MGF and SFGF
have N-terminal signal sequences, but unlike VGF
neither possesses a C-terminal hydrophobic domain.
PROTEIN
SFGF and MGF are expressed from early viral
promoters as secreted glycoproteins (Chang et al.,
Accession numbers
1990) and a synthetic nonglycosylated peptide
spanning the central residues of MGF and SFGF
Vaccinia virus (Copenhagen): 121611 have been shown to function as active EGF-like
Vaccinia virus (WR): 121612 ligands upon members of the ErbB receptor family
Cowpox virus (GRI-90): 1808608 (Lin et al., 1988; Ye et al., 1988; Lin et al., 1991;
Variola virus (Bangladesh): 438919 Tzahar et al., 1998).
Myxoma virus (Lausanne): 332300
Shope fibroma virus (Kasza): 333601
Important homologies
Sequence
Generally, VGF-like genes derived from poxviruses
See Figure 1. that are from the same taxonomic genus are closely
related to each other, but less to members from
another genus (McFadden et al., 1995). Thus,
Description of protein
members of the orthopoxvirus genus (vaccinia, cow-
pox, variola) encode growth factors that share over
The two growth factors most extensively studied are 97% sequence identity to each other in the conserved
VGF and MGF/SFGF (see Table 1). VGF is Cys-rich central domain, while the two members of
844 Grant McFadden and Richard Moyer
theleporipoxvirusgenus(SFVandmyxoma)are85% IN VIVO BIOLOGICAL
identical to each other in this same region, but both
ACTIVITIES OF LIGANDS IN
are<50%identicaltomembersintheothergenus,or
ANIMAL MODELS
to the cellular members of the EGF/TGF(cid:11) super-
family (see Figure 1).
Normal physiological roles
Posttranslational modifications It has been proposed that the biological role(s) of the
poxvirus growth factors is to reduce the dependence
of the virus on the S phase of the cell cycle, when
These growth factors are all secreted as highly
infecting normally quiescent cell populations within
glycosylated proteins from cells infected with a
infected tissues (McFadden et al., 1995). Thus,
variety of poxviruses. Their crystal structures have
mitogenic stimulation of neighboring cells before
notbeendeterminedtodate,buttheirprimaryamino
virus infection would be predicted to upregulate
acid sequences reveal the consensus six-cysteine
the cellular macromolecular synthesis and related
spacing motif illustrated in Figure 1.
metabolic pathways needed for optimal virus replica-
tion. Deletions of the VGF gene in vaccinia strain
WR were used to show that VGF expression is
RECEPTOR UTILIZATION dispensable for replication in cultured cells, but
causes significant attenuation of virus virulence in
mice (Buller et al., 1988a,b). Infection of chicken egg
The EGF/TGF(cid:11) superfamily of ligands recognize
chorioallantoic membranes revealed that pocks
members of the cellular ErbB family, which now
caused by VGF-minus vaccinia contain fewer pro-
includesfourdistinctmembers(PrigentandLemoine,
liferating cells and lower levels of virus replication
1992; Pinkas-Kramarski et al., 1998). When synthetic
(Buller et al., 1988a).
peptide versions of VGF, MGF, and SFGF were
When the SFGF and MGF genes were analyzed
tested for receptor specificities, it was discovered
within knockout recombinant myxoma virus con-
that SFGF exhibits broad specificity, VGF is specific
structs, the resultant viruses grew normally in
for ErbB-1 homodimers, while MGF uniquely
cultured cells, but exhibited attenuated disease
binds the ErbB-2/3 heterodimer (Tzahar et al.,
characteristics in infected rabbits (Opgenorth et al.,
1998). Unexpectedly, the poxviral growth factors
1992a,b).Cassette replacementexperiments have also
interacted with these receptors with 1–3 orders of
indicated that VGF, SFGF or TGF(cid:11) can all func-
magnitude less affinity than the cellular ligands,
tionally replace MGF in recombinant myxoma
but were just as effective mitogens (or even more
viruses (Opgenorth et al., 1993).
potent in some cases) because the virus ligand/
receptor complexes were poorly downregulated and
hence caused prolonged signal transduction (Tzahar
et al., 1998). Thus, poxviruses have ‘engineered’ Transgenic overexpression
members of the EGF/TGF(cid:11) family to prevent the
natural ErbB desensitization pathway and thus
TransgenicmicewhichconstitutivelyexpressMGFor
increase the efficiency of ligand-induced mitogenic
SFGF exhibit hyperproliferation of epithelial and
signaling.
stromal cells (Strayer et al., 1993).
IN VITRO ACTIVITIES Pharmacological effects
In vitro findings
The available data indicate that the various poxvirus
growthfactorsrelatedtoEFG/TGF(cid:11)arebiologically
With the singular exception of preventing ErbB activeligandsoftheErbB-familyofcellularreceptors
receptor downregulation following ligand stimula- andthattheyprovideabiologicalstrategytostimulate
tion, all the poxvirus growth factors induce the mitogenesis within infected lesions so as to provide a
normal spectrum of downstream signaling events more favorable environment for virus replication. In
associated with the EFG/TGF(cid:11)-induced pathways an epithelial wound-healing model VGF was some-
(Kim et al., 1995; Tzahar et al., 1998). what more potent than TGF(cid:11) (Schultz et al., 1987).
Poxvirus Growth Factors Related to Epidermal Growth Factor 845
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