Twenty-four h post-transfection or -infection, cells were lysed by incubation with Passive Lysis Buffer (Promega) for 15 min at room temperature. Cell lysates were assayed for luciferase activity using the Dual-Luciferase assay system (Promega) according to the manufacturer's protocols, and the relative light units (RLU) were determined using a Centro LB 960 Luminometer (Berthold Technologies). The ratio of firefly luciferase/Gaussia luciferase activity after co-transfection of both reporter constructs (norm-Fluc/Gluc) was normalized to the ratio of firefly luciferase/Gaussia luciferase activity after single transfection of reporter constructs, which was set at 1.