Date Published: September 27, 2018
Publisher: Public Library of Science
Author(s): Tatsuya Suzuki, Toru Okamoto, Hiroshi Katoh, Yukari Sugiyama, Shinji Kusakabe, Makoto Tokunaga, Junki Hirano, Yuka Miyata, Takasuke Fukuhara, Masahito Ikawa, Takashi Satoh, Sachiyo Yoshio, Ryosuke Suzuki, Masayuki Saijo, David C. S. Huang, Tatsuya Kanto, Shizuo Akira, Yoshiharu Matsuura, Glenn Randall.
BCL2 family proteins including pro-survival proteins, BH3-only proteins and BAX/BAK proteins control mitochondria-mediated apoptosis to maintain cell homeostasis via the removal of damaged cells and pathogen-infected cells. In this study, we examined the roles of BCL2 proteins in the induction of apoptosis in cells upon infection with flaviviruses, such as Japanese encephalitis virus, Dengue virus and Zika virus. We showed that survival of the infected cells depends on BCLXL, a pro-survival BCL2 protein due to suppression of the expression of another pro-survival protein, MCL1. Treatment with BCLXL inhibitors, as well as deficient BCLXL gene expression, induced BAX/BAK-dependent apoptosis upon infection with flaviviruses. Flavivirus infection attenuates cellular protein synthesis, which confers reduction of short-half-life proteins like MCL1. Inhibition of BCLXL increased phagocytosis of virus-infected cells by macrophages, thereby suppressing viral dissemination and chemokine production. Furthermore, we examined the roles of BCLXL in the death of JEV-infected cells during in vivo infection. Haploinsufficiency of the BCLXL gene, as well as administration of BH3 mimetic compounds, increased survival rate after challenge of JEV infection and suppressed inflammation. These results suggest that BCLXL plays a crucial role in the survival of cells infected with flaviviruses, and that BCLXL may provide a novel antiviral target to suppress propagation of the family of Flaviviridae viruses.
The genus Flavivirus belongs to the family Flaviviridae and includes the Japanese encephalitis virus (JEV), Dengue virus (DENV), West Nile virus (WNV) and Zika virus (ZIKV), all of which are mosquito-borne human pathogens [1,2]. The flaviviruses are internalized into dendritic cells, such as Langerhans cells , and keratinocytes in skin  through mosquito bites. Dendritic cells that are thus infected with flaviviruses are activated and migrate into the lymph nodes, facilitating viral spread into peripheral tissues  and the induction of visceral and/or central nervous system diseases. DENV infection may generate dengue haemorrhagic fever and dengue shock syndrome , while infection with either WNV or JEV can cause severe encephalitis [7,8]. Recently, an outbreak of ZIKV infection in Brazil was linked to birth defects, including microcephaly, and to Guillain-Barre syndrome in adults [9,10]. Therefore, the development of effective vaccines and antiviral drugs against the family of flaviviruses is an urgent need.
In this study, we showed that flavivirus infection suppresses expression of labile proteins such as MCL1 without observable cytotoxicity. Clear cell death, through activation of caspase 3/7, occurred only when BCLXL was inhibited. Moreover, we showed that treatment with BCLXL inhibitor (A-1331852) or deficiency of the BCLXL gene induces cell death upon infection with flaviviruses in several cancer cell lines and primary MEFs. Previous report showed that BCLXL and MCL1 predominantly inhibit activation of BAK , suggesting that a mechanism to inhibit BCLXL may induce BAK-dependent apoptosis in flavivirus-infected cells, therefore inhibition of BCLX alone could enhanced cell death in virus-infected cells. In addition, viral infection decreased MCL1 expression in a variety of human and murine cell lines. Although the magnitude of suppression of MCL1 expression varied depending on cell line, our data suggest that suppression of MCL1 expression upon propagation of flaviviruses is a common feature. Legionella infection also induces suppression of MCL1 expression . Although the molecular mechanisms of MCL1 protein expression in cells infected with Legionella remain unclear, the suppression of MCL1 through shut off of host translation might be part of the cellular innate response against infection with intracellular pathogens.