Phosphorylation of human La protein at Ser 366 by casein kinase II contributes to hepatitis B virus replication and...

Human La protein (hLa) is a multifunctional RNA-binding protein involved in the regulation of hepatitis B virus (HBV) expression. Casein kinase II (CK2), a protein kinase, is known to activate hLa by phosphorylating Ser(366). Tetrabromobenzimidazole (TBBz) has been shown to be a specific inhibitor of CK2 activity, which suggests that TBBz may be useful for reducing HBV gene expression. The aim of our study was to determine whether inhibition of CK2 by TBBz and decreased phosphorylation of hLa Ser(366) (pLa) would reduce HBV gene expression. pLa and total La expression levels were evaluated by immunohistochemistry in human liver tissues with or without HBV infection. HepG2.2.15 cells (an HBV-expressing cell line) were treated with TBBz, and cell viability and pLa levels were evaluated. Knockdown of hLa and CK2 levels by specific siRNA and mutant hLa Ala(366) were utilized to establish the roles of pLa and CK2 in HBV gene expression. HBV DNA replication and HBsAg and HBeAg levels were analysed in HepG2.2.15 cell supernatants by standard methods. pLa was significantly overexpressed in HBV-infected human liver samples. TBBz decreased the phosphorylation of hLa, which coincided with decreased HBV expression. Mutant hLa Ala(366) had reduced viral expression compared with hLa Ser(366) treatment in hLa siRNA knockdown cells. Knockdown of CK2 also decreased the HBV parameters. hLa plays a key role in the regulation of HBV gene expression in a CK2-dependent mechanism via phosphorylation of hLa at Ser(366).