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Journal of Virology, November 1998, p. 9303-9306, Vol. 72, No. 11
0022-538X/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Use of a Prenylation Inhibitor as a Novel
Antiviral Agent
Jeffrey S.
Glenn,1,*
James C.
Marsters Jr.,2 and
Harry B.
Greenberg1,3
Division of
Gastroenterology,1 and
Department of
Microbiology and Immunology,3 Stanford
University School of Medicine and Veterans Administration Medical
Center, Palo Alto, California 94305-5487, and
Bioorganic
Chemistry, Genentech Inc., South San Francisco, California
940802
Received 3 March 1998/Accepted 24 July 1998
 |
ABSTRACT |
No specific therapy exists for hepatitis delta virus (HDV), which
can cause severe liver disease. Molecular genetic studies have
implicated the prenylation site of large delta antigen as a critical
determinant of HDV particle assembly. We have established a cell
culture model which produces HDV-like particles, and we show that delta
antigen prenylation can be pharmacologically inhibited by the
prenylation inhibitor BZA-5B. Furthermore, BZA-5B specifically abolishes particle production in a dose-dependent manner. These results
demonstrate that the use of such a prenylation inhibitor-based antiviral therapy may be feasible and identify a novel class of potential antiviral agents.
 |
TEXT |
Hepatitis delta virus (HDV) is a
novel viral pathogen that is an important cause of acute and chronic
liver disease in various parts of the world (4, 14, 16, 18, 24,
29). There is currently no effective treatment for HDV
infections. Recent advances in our understanding of the viral life
cycle have revealed new targets for antiviral therapy.
The HDV particle's core contains the 1.7-kb single-stranded circular
genomic RNA (32) and the virally encoded small and large
delta antigens. The particle core is encapsulated by a lipid envelope
embedded with hepatitis B virus (HBV) surface antigen (HBsAg) proteins
(2). HBsAg is provided by HBV, which accounts for the
occurrence of HDV infections only in the presence of an HBV infection.
Infectious HDV particles can be produced in vitro by cells transfected
with cloned DNAs containing portions of the HBV and HDV genomes
(28, 33). Virus-like particles can also be produced in the
absence of genome replication. Indeed, cotransfection into cultured
cells of plasmids encoding only large delta antigen and HBsAg is
sufficient for the production and release of particles (5,
31). Essential to large delta antigen's requisite role in HDV
assembly are the last four amino acids at the carboxyl terminus
(Cys-Arg-Pro-Gln-COOH), which together form a "CXXX box," a motif
recognized by prenyltransferase enzymes as a substrate for covalent
addition of a prenyl lipid to the CXXX box cysteine (13, 21, 26,
34). Prenyl lipid addition to delta antigen may help target the
protein to cellular membranes containing HBsAg and may help trigger
virus assembly (7, 10).
When the large delta antigen's CXXX box is destroyed by genetic
mutation, prenylation is prevented and particle production is abolished
(11, 19). While these studies demonstrated the critical role
of prenylation in HDV virion morphogenesis, a strategy that uses
mutagenesis to disrupt delta antigen prenylation in natural infections
would be impractical. Therefore, we investigated whether virion
assembly is similarly inhibited when delta antigen prenylation is
prevented by pharmacologic means
by using a drug that specifically
inhibits the enzyme responsible for the transfer of the prenyl lipid to
delta antigen.
Construction and characterization of a particle-producing cell
line.
We constructed a permanent cell line capable of continuously
producing HDV-like particles. Briefly, a clone of NIH 3T3 cells stably
transfected with pSVL-large, which expresses large delta antigen
(11), and pHygro, which encodes hygromycin resistance, was
further cotransfected with SV24H, which expresses HBsAg (3), and pRCCMV (Invitrogen), which encodes G418 resistance. Cells were
transfected with Lipofectamine (Gibco BRL) according to the manufacturer's directions and selected with hygromycin B and G418. One
of the resulting clones, termed LH, was selected for further characterization.
Confluent LH cells were washed twice with phosphate-buffered saline,
harvested in cell lysis buffer (50 mM Tris [pH 8.8], 2% sodium
dodecyl sulfate [SDS]), and analyzed for the presence of large delta
antigen. Briefly, samples were subjected to SDS-polyacrylamide gel
electrophoresis (PAGE) with 12% resolving gels, followed by transfer
to Immobilon polyvinylidene difluoride membranes (Millipore), essentially as described previously (11). The blots were
treated with a human antibody against delta antigen (11) and
with alkaline phosphatase-conjugated rabbit antibody to human
immunoglobulin G (Promega), followed by chemifluorescence development
(with a kit from Amersham) and detection (STORM 840; Molecular
Dynamics). The results (Fig. 1, lanes 2)
confirm the presence of large delta antigen in LH cells. In addition,
similar to other cell lines stably transfected with cDNA encoding HBsAg
(17, 20, 27), LH cells abundantly express and constitutively
secrete HBsAg into the media, as measured by a commercial assay
(Auszyme; Abbott Laboratories). Finally, LH cells also produce and
release HDV-like particles that contain both HBsAg and delta antigen.
The particles can be isolated from clarified medium supernatants by
either immunoprecipitation with a monoclonal antibody to HBsAg (Abbott
Laboratories) (Fig. 1A, lane 1) or ultracentrifugation through a 20%
sucrose cushion and collection of the pellet (25) (Fig. 1B,
lane 1). The LH cell line is thus well suited to pharmacologic studies
dependent on precise reproducibility and aimed at measuring the effect
of various inhibitors on particle production.

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FIG. 1.
LH cells produce HDV-like particles. (A) HDV-like
particles (lane 1) were immunoprecipitated from clarified medium
supernatants (Sup.) of LH cells with a monoclonal antibody to HBsAg and
subjected to immunoblot analysis for the presence of large delta
antigen. LH cells remaining on the dish were harvested in cell lysis
buffer and an aliquot (lane 2) was included in the immunoblot analysis.
(B) Particles (lane 1) were isolated by ultracentrifugation of
clarified medium supernatants of LH cells and an aliquot was subjected
to immunoblot analysis, along with a sample of LH cell lysate (lane 2).
L, large delta antigen. The locations of prestained molecular mass
markers are shown at the left (in kilodaltons).
|
|
Effect of BZA-5B on large delta antigen prenylation.
We next
wished to identify a compound capable of inhibiting large delta antigen
prenylation. For this purpose, we chose to test BZA-5B, a drug
originally synthesized as a specific prenyltransferase inhibitor and
known to inhibit prenylation of the oncoprotein H-RasV12
(22). BZA-5B can abrogate the prenylation-dependent,
H-RasV12-mediated transformation of Rat-1 cells without
observed gross cellular toxicity (15).
To determine the effect of BZA-5B on large delta antigen prenylation,
we performed in vitro translation-prenylation reactions,
essentially as described previously (
11), with the addition
of BZA-5B. Briefly, combined in vitro transcription-translation
reactions with rabbit reticulocyte lysates (Promega) were
programmed
with a plasmid encoding large delta antigen in
the presence of
[5-
3H]mevalonate (60 Ci/mmol
[
R,
S]; American Radiolabeled Chemicals),
a metabolic precursor of prenyl lipids. A carrier (final
concentrations
of 0.5 mM dithiothreitol [DTT] and 0.05% dimethyl
sulfoxide [DMSO])
or various concentrations of BZA-5B dissolved in
the carrier were
included in the reactions. Aliquots of each
reaction mixture were
then subjected to SDS-PAGE and either
fluorography (
11) (Fig.
2A) or
immunoblot analysis for delta antigen as described above
(Fig.
2B).

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FIG. 2.
BZA-5B inhibits prenylation of large delta antigen.
Combined in vitro transcription-translation reactions were performed
with rabbit reticulocyte lysates programmed with water (lanes 1) or a
plasmid encoding large delta antigen (lanes 2 to 7) in the presence of
[5-3H]mevalonate and either water (lanes 2), a carrier
(0.5 mM DTT and 0.05% DMSO) (lanes 3), or a carrier with 5, 10, 25, or
50 µM BZA-5B, as indicated. Aliquots (1 µl) were subjected to
SDS-PAGE and either fluorography (A) or immunoblot analysis (B). L,
large delta antigen.
|
|
In the absence of BZA-5B, [
3H]mevalonate was incorporated
into large delta antigen (Fig.
2A, lane 3), indicating that it
undergoes
prenylation, as previously shown (
11). BZA-5B had
no apparent
effect on the translation of large delta antigen (Fig.
2B),
whereas
profound inhibition of posttranslational prenyl lipid
modification
was observed at 5 µM BZA-5B. No prenylation was
detectable at
concentrations of 25 µM and above (Fig.
2A, lanes 4 to
7). Thus,
BZA-5B appears to be a potent inhibitor of large delta
antigen
prenylation. These results are in good agreement with recent
data
showing that large delta antigen is prenylated by
farnesyltransferase
(
23), the prenyltransferase most
sensitive to BZA-5B (
15).
Our results also suggested that
BZA-5B would be a good candidate
for inhibiting HDV particle
production.
Effect of BZA-5B on particle production.
To test the
hypothesis that an inhibitor of delta antigen prenylation can prevent
virus-like-particle production, the effect of BZA-5B on LH-cell
particle production was studied (Fig. 3). LH cells were seeded at low confluency in 100-mm-diameter dishes (15). Duplicate dishes were grown in media containing a
carrier (0.5 mM DTT and 0.05% DMSO
to minimize oxidation and enhance
cellular penetration of the compound) alone or a carrier plus various
concentrations of BZA-5B. After four medium changes, made every other
day, portions (2.5 ml) of the respective final clarified medium
supernatants were quantitatively analyzed for the presence of HDV-like
particles with the centrifugation-over-sucrose-cushion and immunoblot
procedures described above (Fig. 3A). The large delta antigen bands
were quantitated with the ImageQuant (Molecular Dynamics) software package. All quantitations were performed within the linear range of
the chemifluorescence detection, as determined by serial dilutions of
large delta antigen standards. As a control for nonspecific inhibition
of protein synthesis and secretion, HBsAg was quantitated in duplicate
aliquots (10 µl) of each medium supernatant sample with a commercial
assay (Auszyme; Abbott Laboratories). After the medium supernatants
were collected, the underlying LH cells were harvested and counted, and
a fraction of them were subjected to immunoblot analysis (Fig. 3B). The
percentage of control particles per cell and of HBsAg per cell was then
calculated for each concentration of BZA-5B and plotted as a function
of drug concentration (Fig. 3C).

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FIG. 3.
BZA-5B inhibits HDV-like-particle production. Duplicate
dishes of LH cells were grown in media containing a carrier (0.5 mM DTT
and 0.05% DMSO) alone (lanes 1) or a carrier with either 10, 25, or 50 µM BZA-5B. Clarified medium supernatants were analyzed for the
presence of HDV-like particles by quantitative immunoblot analysis (A).
The underlying cells were harvested and counted, and the presence of
large delta antigen was analyzed by immunoblotting (B). Only one of the
duplicate set of blots is shown. (C) HBsAg was quantitated in duplicate
aliquots of each medium supernatant sample, and the percentage of
control HBsAg per cell (light bars) and of control particles per cell
(dark bars), determined from the experiment whose results are shown in
panel A, at each concentration of BZA-5B is plotted. Error bars
represent the average deviations.
|
|
As shown in Fig.
3A, a significant inhibition of particle production
was observed with 10 µM BZA-5B compared to that in the
control with
the carrier alone (lane 2 versus lane 1). At 50 µM
BZA-5B, particle
production is reduced to below the level of detection
(Fig.
3A, lane
4). The inhibition of particle formation was not
due to a decrease in
the cellular pool of delta antigen (Fig.
3B). To assess whether
BZA-5B's inhibition of particle production
was secondary to a more
general effect on secretion or cell metabolism,
rather than direct
prenylation inhibition, we measured the HBsAg
contained in the
collected medium supernatants. HBsAg does not
harbor a CXXX box
and it is therefore not subject to prenylation,
although the
known constitutive secretion of HBsAg alone into
the media could
be affected by nonspecific toxicity. As indicated
in Fig.
3C, however,
BZA-5B selectively abolished prenylation-dependent
HDV-like particle
release while exerting no such effect on the
constitutive secretion of
HBsAg.
As with most pharmacologic inhibitor experiments, it is possible that
the inhibition of particle formation is unrelated to
BZA-5B's effect
on delta antigen prenylation. Since particle formation,
however, can be
similarly abolished when delta antigen prenylation
is specifically
prevented by a completely different and nonpharmacologic
means

namely,
genetic mutation of the prenylation site on delta
antigen
(
11)

our data suggests that it is indeed the specific
ability of BZA-5B to inhibit delta antigen prenylation (Fig.
2)
that is
responsible for preventing particle formation (Fig.
3).
These results have obvious implications for a new type of antiviral
therapy based on prenylation inhibition. Such an antiviral
strategy may
be able to be applied to other viruses found to have
similarly
prenylated proteins. While delta antigen was the first
viral protein
shown to undergo prenylation, analysis of sequence
data banks reveals
the presence of CXXX box-containing proteins
in numerous other viruses,
including herpes simplex virus, cytomegalovirus,
and hepatitis A virus
(
10).
As an initial viral target, HDV is particularly attractive because
preventing prenylation of large delta antigen might have
two antiviral
consequences. First, as we show here, particle assembly
could be
blocked. Second, as a result of not being released in
the form of viral
particles, the large delta antigen concentration
within infected cells
may increase. Because large delta antigen
is a potent
trans-dominant inhibitor of HDV genome replication
(
6,
12), the antiviral effect of prenylation inhibition on
assembly
could thus be amplified by an additional suppression
of viral genome
replication.
Strategies designed to inhibit viral prenylation could affect host cell
prenylation as well. Several factors, however, may
help limit the
potential for intolerable side effects. Normal
cellular prenylation is
accomplished by a family of prenyltransferases
(
34). Thus,
selective inhibition of the prenyltransferase that
modifies delta
antigen may not affect host cell functions which
depend on other
prenyltransferases. Some substrates can be prenylated
by more than one
prenyltransferase (
1,
30). Such potential
cross-specificity
may help mitigate unwanted prenylation inhibition
of critical cellular
proteins by BZA-5B. Indeed, BZA-5B is surprisingly
well tolerated in a
variety of experimental systems (9, 22); in
our experiments, we
observed no gross cellular toxicity and a
mild (30 to 50%) inhibition
of growth rate at the highest BZA-5B
concentrations. In addition, viral
assembly may be more sensitive
than key host cell functions to the
effects of prenylation inhibitors.
It is possible that inhibiting the
prenylation of only a fraction
of the large delta antigen in a nascent
virus particle may be
sufficient for abrogating normal assembly of the
entire particle.
Ultimately, the true benefit-to-risk ratio will need
to be determined
for each clinical application. In the case of HDV,
there are established
animal models (
8) which may now be
suitable for further evaluating
the proposed antiviral strategy.
 |
ACKNOWLEDGMENTS |
We thank Bruno Bordier, Anson Lowe, Thomas Merigan, and Edward
Mocarski for helpful discussions and critical reading of the manuscript.
This work was supported by grants from the Stanford Digestive Disease
Center and the Veterans Administration. J.S.G. is also the recipient of
a Howard Hughes Medical Institute Physician Postdoctoral Fellowship.
 |
FOOTNOTES |
*
Corresponding author. Mailing address: Department of
Medicine, Division of Gastroenterology, MC 5487, Stanford University School of Medicine, MSLS, P-304, 1201 Welch Rd., Palo Alto, CA 94305-5487. Phone: (650) 723-6661. Fax: (650) 723-5488. E-mail: Jeffrey.glenn{at}stanford.edu.
 |
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Journal of Virology, November 1998, p. 9303-9306, Vol. 72, No. 11
0022-538X/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
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