Blotting was performed as previously described (22 (link)). Antibodies used were: Actin #A2066 from Sigma (St. Louis, MO), ERK2 #sc-1647, MEK1 #sc-219 from Santa Cruz Biotechnology (Santa Cruz, CA), Sprouty2 #07-524 from Millipore (Billerica, MA), and pERK1/2 #9101, pMEK1/2 #9121, p21Cip1 #2946, p27Kip1 #2552 from Cell Signaling (Danvers, MA). Chemiluminence was visualized with VersaDoc MultiImager and quantified using Quantity-One software (Bio-Rad, Hercules, CA).