ADAR1 Biology Can Hinder Effective Antiviral RNA Interference.

Viruses constantly evolve and adapt to the antiviral defenses of their hosts. The biology of viral circumvention of these selective pressures can often be attributed to the acquisition of novel antagonistic gene products or by rapid genome change that prevents host recognition. To study viral evasion of RNA interference (RNAi)-based defenses, we established a robust antiviral system in mammalian cells using recombinant Sendai virus designed to be targeted by endogenous host microRNAs (miRNAs) with perfect complementarity. Using this system, we previously demonstrated the intrinsic ability of positive-strand RNA viruses to escape this selective pressure via homologous recombination, which was not observed in negative-strand RNA viruses. Here, we show that given extensive time, escape of miRNA-targeted Sendai virus was enabled by host adenosine deaminase acting on RNA 1 (ADAR1). Independent of the viral transcript targeted, ADAR1 editing resulted in disruption of the miRNA-silencing motif, suggesting an intolerance for extensive RNA-RNA interactions necessary for antiviral RNAi. This was further supported in Nicotiana benthamiana, where exogenous expression of ADAR1 interfered with endogenous RNAi. Together, these results suggest that ADAR1 diminishes the effectiveness of RNAi and may explain why it is absent in species that utilize this antiviral defense system. IMPORTANCE All life at the cellular level has the capacity to induce an antiviral response. Here, we examine the result of imposing the antiviral response of one branch of life onto another and find evidence for conflict. To determine the consequences of eliciting an RNAi-like defense in mammals, we applied this pressure to a recombinant Sendai virus in cell culture. We find that ADAR1, a host gene involved in regulation of the mammalian response to virus, prevented RNAi-mediated silencing and subsequently allowed for viral replication. In addition, the expression of ADAR1 in Nicotiana benthamiana, which lacks ADARs and has an endogenous RNAi system, suppresses gene silencing. These data indicate that ADAR1 is disruptive to RNAi biology and provide insight into the evolutionary relationship between ADARs and antiviral defenses in eukaryotic life.

Anticancer effect of inactivated Sendai virus strain Tianjin on human osteosarcoma HOS cells.

Ultraviolet-inactivated Sendai virus strain Tianjin (UV-Tianjin) has been proved to have antitumor effects in many kinds of tumor cells. Here, we investigated the anticancer properties of UV-Tianjin on human osteosarcoma HOS cells and the underlying molecular mechanism. Apoptosis, intracellular reactive oxygen species (ROS) levels and mitochondrial membrane potential were determined by flow cytometry analysis. The expression levels of apoptosis-related proteins were tested by western blotting. The results showed that UV-Tianjin concentration-dependently induced apoptosis in HOS cells. UV-Tianjin-induced apoptosis was mediated by the mitochondrial pathway, which was confirmed by mitochondrial dysfunction, downregulation of B-cell lymphoma 2 (Bcl-2), B-cell lymphoma-xL (Bcl-xL) and myeloid cell leukemia-1 (Mcl-1), upregulation of B-cell lymphoma 2 associated X protein (Bax) and Bcl-2 Homologous Antagonist/Killer (Bak), as well as the cleavage of caspase-9 and -3. Further analysis showed that UV-Tianjin augmented the phosphorylation of c-Jun N-terminal kinase, the extracellular-regulated kinase and p38, the major components of mitogen-activated protein kinase (MAPK) pathways, as well as the generation of ROS. Moreover, UV-Tianjin-induced apoptosis was remarkably attenuated by MAPK inhibitors and ROS inhibitor. Taken together, our results indicated that UV-Tianjin exerts antitumor effects by inducing mitochondria-dependent apoptosis involving ROS generation and MAPK pathway in human osteosarcoma HOS cells.