In order to address the effect of trypsin on PRV replication,
PK15 cells were infected (3 h, m.o.i. of 20) and then treated
or not with trypsin (Becton Dickinson, ~ 410 BAEE units
ml21, 50 mM, 5 min). We performed this delayed treatment
to exclude the possibility of a direct effect of trypsin on virus
particles. The cells were then returned to medium to let the
infection proceed for 13 or 24 h. Viral growth was evaluated
by titrating the virus in the supernatants using a plaque
assay. When exposed to trypsin, PRV-infected cells subsequently
produced more infectious virus (Fig. 1a, left panel),
as indicated by the statistical Mann–Whitney test analysis
(P<0?05 at t=24 h). Similar results were observed by using
sucrose gradient-purified virus (Fig. 1a, right panel) and in
Madin–Darby bovine kidney (MDBK; Fig. 1b) or ST (data
not shown) cells. Thus, trypsin triggers an intracellular
process leading to increased virus production in PRVinfected
cells. To our knowledge, this is the first report
demonstrating that proteases can mediate the enhancement
of viral production through an effect on the infected cell.
Whether this occurs through protease-activated receptors
(Ossovskaya & Bunnett, 2004) and whether this is
accompanied by an increased synthesis of virus proteins
remain to be determined. Activation of the ERK1/2
intracellular signalling pathway is known to be associated
with increased viral growth in several models of viral
infection (Luo et al., 2002; Pleschka et al., 2001) and
activation of this pathway has been reported in proteasemediated
signalling (Brown et al., 2001; DeFea et al., 2000).
Thus, in order to investigate its possible involvement in the
increased virus production induced by trypsin in infected
cells, PK15 cells were either mock- or PRV-infected and
treated 3 h later with trypsin for 5, 30 or 120 min or left
unexposed for the same length of time. Activation of the
ERK1/2 pathway was then assessed by Western blot analysis
using an anti-phospho-ERK antibody (Cell Signaling
Technology). The results (Fig. 2a) showed that, in mockinfected
cells, ERK1/2 was phosphorylated after a 5 min
exposure to trypsin, whilst after exposure for a longer time
(between 30 and 120 min), the phosphorylation dropped
below the basal level. These results were consistent with
previous reports showing that trypsin treatment transiently
activates the ERK1/2 pathway in several cell lines
(Brown et al., 2001; DeFea et al., 2000). In contrast, virus and
trypsin triggered a synergistic activation of ERK1/2 that
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