Dex maintains Raf-1 expression in proliferating and differentiating primary erythroblasts. (A) Effect of Dex on the amount of Raf-1, Fas, and phosphorylated ERK detected in proliferating (A) and differentiating (B) erythroblasts. Protein expression and phosphorylation were determined by immunoblotting. The experiment was repeated twice with comparable results. (C) Regulation of Raf-1 and Fas expression by activated ERK maintains the equilibrium between renewal and differentiation of erythroid precursor cells—a working model. In differentiating erythroblasts, Raf-1 is required for ERK activation. Activated ERK is responsible for maintaining Raf-1 expression and suppressing Fas expression. Glucocorticoids maintain Raf-1 expression and therefore renewal. Withdrawal of the renewal stimulus causes the down-regulation of the Raf-1/ERK axis and allows the expression of Fas, which in turn up-regulates the ASK1/JNK/p38 module, suppresses ERK activation and brings about differentiation. See text for details. → stimulation of expression; —-▸ enzymatic activation; —X suppression of expression; — suppression of enzymatic activity.