Interferon-stimulated Gene ISG12b2 is Localized to the Inner Mitochondrial Membrane and Mediates Virus-induced Cell Death

Interferons (IFNs) are crucial for host defence against viruses. Many IFN-stimulated genes (ISGs) induced by viral infection exert antiviral effects. Microarray analysis of gene expression induced in liver tissues of mice on dengue virus (DENV) infection has led to identification of the ISG gene ISG12b2. ISG12b2 is also dramatically induced on DENV infection of Hepa 1-6 cells (mouse hepatoma cell line). Here, we performed biochemical and functional analyses of ISG12b2. We demonstrate that ISG12b2 is an inner mitochondrial membrane (IMM) protein containing a cleavable mitochondrial targeting sequence and multiple transmembrane segments. Overexpression of ISG12b2 in Hepa 1-6 induced release of cytochrome c from mitochondria, disruption of the mitochondrial membrane potential, and activation of caspase-9, caspase-3, and caspase-8. Treatment of ISG12b2-overexpressing Hepa 1-6 with inhibitors of pan-caspase, caspase-9, or caspase-3, but not caspase-8, reduced apoptotic cell death, suggesting that ISG12b2 activates the intrinsic apoptotic pathway. Of particular interest, we further demonstrated that ISG12b2 formed oligomers, and that ISG12b2 was able to mediate apoptosis through both Bax/Bak-dependent and Bax/Bak-independent pathways. Our study demonstrates that the ISG12b2 is a novel IMM protein induced by IFNs and regulates mitochondria-mediated apoptosis during viral infection.