The following antibodies and assay kits were purchased (all from Cell Signaling Technology, Danvers, MA): monoclonal anti–mouse phospho p38 (Thr188/Tyr182) antibody, monoclonal anti–mouse phospho Erk1/2 antibody, p38 MAP kinase assay kit, Erk1/2 MAP kinase assay kit, rabbit anti–Erk1 antibody, rabbit anti–p38 antibody, mouse monoclonal anti–phospho-ERK, MKP-1 rabbit polyclonal IgG, MAP kinase kinase (MEK)-1 rabbit polyclonal IgG (12B), phospho-MEK-1/2 (Ser 218/Ser 222), Actin (H-196) rabbit polyclonal IgG antibody and rabbit polyclonal IgG, PP2A goat polyclonal IgG, phospho PP2A (Tyr307)-R rabbit polyclonal IgG, goat anti–mouse IgG-HRP, goat anti–rabbit IgG-HRP antibody, and MKP-1 short-interfering RNA (siRNA) transfection kit. The following were also purchased: control siRNA (Santa Cruz Biotechnology, Santa Cruz, CA); inhibitors PD98059, U0126, SB203580, and okadaic acid (Biomol, Plymouth Meeting, PA); Ro 31–8220 (Calbiochem, La Jolla, CA); sodium orthovanadate (Na3VO4), EGF, and bovine insulin (Sigma-Aldrich, St. Louis, MO); DMEM/F12 medium, fetal bovine serum (FBS), and tetracycline-inducible (tet-on) transfection kit (TREX; Invitrogen Corp., Carlsbad, CA).