Constitutively active Raf-1 delays, and kinase-dead Raf-1 accelerates, erythroid differentiation and Fas expression in immortalized fetal liver erythroblasts. (A) The differentiation of I/11 cells transfected with empty vector was compared with that of I/11 cells expressing WTRaf-1, CARaf-1, and KDRaf-1, respectively. Hemoglobin content was determined between 0 and 96 hours after induction of differentiation. (B) Fas up-regulation during differentiation of I/11 erythroblasts expressing murine stem-cell virus-EGFP (empty vector) with full-length Raf-1 (WTRaf-1), a constitutively active Raf-1 (CARaf-1) or a kinase-dead mutant (KDRaf-1) was monitored by FACS analysis at the indicated time points. (C) CARaf-1 enforces, and KD-Raf-1 reduces, ERK phosphorylation in differentiating I/11 cells. The converse is observed for JNK/p38 phosphorylation. One representative experiment of 5 is shown in panels A-C.