Pharmacological suppression of MEK1/2 attenuates ERK1/2 signaling, promotes apoptosis, and impairs IBV replication. Vero (A), H1299 (B), and DF-1 (C) cells were pre-incubated with U0126 (10 μM) or DMSO for 1 h, followed by IBV infection. 10 μM U0126 or DMSO was present throughout the infection. Mock infection was included as another control group. Cells were collected at 20 and 24 hpi and subjected to Western blot analysis. p-ERK1/2, ERK1/2, PARP, Bcl-2, Mcl-1, IBV N, and β-actin were detected. β-actin was included as a loading control. (PARP-FL: full-length PARP; PARP-C: cleaved PARP). The experiments were repeated in triplicate and the representative figures are shown. The signal of protein bands was determined by Image J software. The intensities of p-ERK1/2 or p-ERK2 were normalized to total ERK1/2 or total ERK2, the intensities of Bcl-2, Mcl-1, IBV N were normalized to β-actin, and the intensities of PARP-C were normalized to PARP-FL. The ratio of p-ERK1/2, p-ERK2, Bcl-2, and Mcl-1 of IBV infected cells to mock infected cells are shown as p-ERK1/2 (IBV + :−), p-ERK2 (IBV + :−), Bcl-2 (IBV + :−), and Mcl-1 (IBV + :−). The ratio of PARP-C and IBV N in U0126 treated cells to DMSO treated cells are shown as PARP-C (U0126:DMSO) and IBV N (U0126:DMSO).