Specific inhibition of p38 activation by ras rescues ras-induced premature senescence. (A) PD of BJ cells transduced at PD 18 with a vector control (BP) or Ha-RasV12 (Ras) in the presence of a vehicle control, SB203580, U0126, or U0126 and SB203580. Values are means ± standard deviations (SD) for triplicates. (B) Percentage of SA-β-gal-positive cells within cell populations transduced at PD 18 with a vector control or Ha-RasV12 in the presence of a vehicle control (Con), SB203580 (SB), U0126 (U), or U0126 and SB203580 (U+SB). Values are means ± SD for triplicates. (C) Western blot analysis of BJ cells transduced at PD 18 with a vector control (BP) or Ha-RasV12 (HaRasV12) in the presence of a vehicle control (Con), U0126 (U), or SB203580 (SB), showing the levels of Ras, phospho-ERK (P-ERK), ERK, and p16INK4A. The bottom panel (P-Elk1 fusion) shows the kinase activity of ERK immunoprecipitated from the same cells, as determined in an in vitro kinase assay using Elk1 as substrate. The phosphorylated substrate was visualized by Western blot analysis with an antibody against phospho-Elk1. (D) Dominant-negative alleles of p38β, p38γ, and p38δ rescued ras-induced senescence. BJ cells expressing a dominant-negative allele of p38β [p38β(AF)], p38γ [p38γ(AF)], p38δ [p38δ(AF)], or a vector control (BH) were transduced with Ha-RasV12 (Ras; open symbols) or a vector control (BP; filled symbols) at PD 34. The PD of these cells were followed starting from day 5 postinfection (designated as day 0). Values are means ± SD for triplicates.