MEK/ERK pathway sustains p21 WAF1 expression. (A) RD cells were left untreated (-) or treated with TPA in the absence or in the presence of U0126 for the times indicated. (B) RD cells were transfected with the constitutively active form of HA-tagged-MEK1 (ca MEK1), -MEK2 (ca MEK2) or with the empty vector (CMV). (C) RD cells were transfected with control siRNA (siRNA-C) or ERK1 and ERK2 siRNAs (siRNAERK1-2) and then left untreated (-) or treated with TPA (+) for 4 days. Immunoblots of total lysates were performed using specific antibodies capable of recognising p21WAF1, total and phosphorylated ERK1/2, hemagglutinin (HA) and α-tubulin. The data shown are representative of three independent experiments for A and two for B and C.