Viruses and RNA interference in mammalian cells

RNA silencing suppressors
The fact that B2 of both the Nodamura virus (NoV) and the Flock House virus (FHV) can
inhibit RNAi triggered artificially in human cells was proved by scientists in 2005 (Sullivan
and Ganem, 2005). The NoV is not enveloped ss(+)RNA virus. It occurs in nature as an
unapparent infection of mosquitoes in Japan, also asymptomatically infects pigs (Bailey et
al., 1974). Many viral RSS identification in mammalian cells include the use of shRNAs.
Scientist developed a highly sensitive assay of RNAi in mammalian cells, which shows that
RNAi triggered by either shRNAs or siRNAs, can be inhibited by the NoV B2 protein. In
the cell, NoV B2 binds to pre-Dicer substrate RNA and RISC-processed RNAs, inhibiting
the Dicer cleavage reaction. Both endogenous genes and transgenes encoding EGFP or
luciferase have been used as the reporter target gene in RNAi suppression assays. The assay
based on RNAi of a destabilized EGFP (dEGFP) reporter gene was developed to identify
inhibitors of RNAi in mammalian cells (Sullivan and Ganem, 2005). Less EGFP was
detected, when cotransfected with the anti-EGFP shRNA than, when co-transfected with the
control anti-luciferase (anti-Luc) irrelevant shRNA. The facts, that transfected siRNAs could
suppress viral replication by reducing viral RNA accumulation and that viral RSSs function
in mammalian cells, could enhance the evidence, that mammalian RNAi machinery
possesses an antiviral response (Gitlin et al, 2002).
VP35 protein of Ebola virus has RSS activity, which is functionally equivalent to that of the
HIV-1 Tat protein. VP35 can replace HIV-1 Tat and thereby support the replication of a Tatminus
HIV-1 variant. It shows that viruses need to counteract RNAi in order to replicate.
Also VP35 protein is known as IFN antagonist. The VP35 dsRNA-binding domain is
required for this RSS activity. (Haasnoot et al., 2007)
Influenza A, B, C viruses NS1 protein and vaccinia virus E3L protein were demonstrated to
be essential proteins that suppress RNA silencing in Drosophila. The major fact is, that
these mammalian virus proteins, are also distinct dsRNA-binding proteins essential for
pathogenesis by inhibiting IFN antiviral response. It was also demonstrated that the dsRNAbinding
domain of NS1 is essential and sufficient for RNAi suppression (Li et al., 2004).
Influenza A virus NS1 protein and vaccinia virus E3L protein are also capable to replace the
HIV-1 Tat RSS function (Haasnoot et al., 2007).
The adenovirus virus-associated RNAs I and II (VA-RNAI and II) are also capable of
inhibiting RNAi antivaral response. VA RNA are highly structured RNAs, about 160 nt in
length, encoded by adenoviral genome and expressed by host RNA polymerase III.
(Mathews and Shenk, 1991). At late infection human adenovirus inhibits RNAi antiviral
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