ERK activation regulates Raf-1 and Fas expression during erythroid differentiation. (A-B) Chemical inhibition of MEK accelerates erythroid differentiation and increases Fas surface expression. I/11 cells were treated with the MEK inhibitor PD98059 (40 μg, 72 hours) during differentiation. Hemoglobin content (A) and Fas expression (B) were monitored between 0 and 96 hours of differentiation. (C) Chemical inhibition of MEK decreases Raf-1 protein expression and ERK phosphorylation in differentiating I/11 cells. Protein expression and phosphorylation were determined by immunoblotting. (A-C) One representative experiment of 5 is shown. (D) Chemical inhibition of MEK decreases Raf-1 mRNA levels and increases Fas mRNA levels in differentiating I/11 cells. mRNA levels were determined by RT-PCR, which was repeated twice with comparable results.