3.2.2 Regulation of WNV proteins on apoptosis
Yang et al.(Joo-Sung et al., 2002) demonstrated for the first time that transfection of WNV-C in neuronal and non-neuronal cells can cause inflammation and apoptosis. Further studies have shown that WNV-C induces apoptosis by destroying mitochondrial membranes and activating caspase-9 and caspase-3, and the 3’-terminal region of the capsid protein is related to its apoptosis-inducing function. In addition, the WNV-C protein phosphorylated by protein kinase C (PKC) can enhance its binding and nucleus co-localization with HDM2 protein, a p53 negative regulator, thereby stabilizing p53 and then inducing its apoptosis target protein Bax(Bhuvanakantham, Cheong, & Ng, 2010; M. Yang et al., 2010). Despite the substantial evidence supporting the pro-apoptotic effect of capsid protein, Urbanowski et al.(Urbanowski & Hobman, 2013) found that WNV-C inhibits cell apoptosis by activating Phosphatidylinositol-3-kinase (PI3K)-Akt pro-survival pathway in four mammalian cells (A549, HEK293T, Vero-76, BHK-21). The discrepancy between these reports may be due to the capsid protein containing an 18-amino-acid-residue signal peptide of prM in the earlier reports.
In 2006, Liu et al.(W. J. Liu et al., 2006) mutated the alanine at position 30 of the WNV-NS2A protein to proline (A30P) and found that the virulence of the virusviral was significantly weakened. Further, Melian et al. (Melian et al., 2013) used this mutant and wild strain to infect IFN⁃β-deficient cells, and found that the DNA degradation of the mutant group was reduced and the number of TUNEL-labeled positive cells decreased significantly, indicating that the WNV⁃NS2A protein plays a role in IFN-independent apoptosis. Moreover, the expression of NS2B-NS3 or NS3 protein, but not NS2B, can induce apoptosis. Further studies have showed that WNV-NS3 can trigger apoptosis mediated either individually or together by the caspases-8 and -3 pathways, and the protease and helicase domains of NS3 protein are both essential for inducing apoptosis(Ramanathan et al., 2006). The thorough knowledge of the apoptotic pathways driven by NS2B-NS3 and NS3 will undoubtedly provide valuable insights for development of novel therapeutics for this viral infection.