SCF- and Epo-dependent mitogenesis and inhibition of apoptosis in FDC2 and FDC2–wt-ER cells. Parental FDCP1-WEHI2 cells (ie, FDC2 cells) were transfected stably with an Epo receptor expression vector (pXM-ER) and were selected in Epo to yield FDC2–wt-ER cells. In FDC2–wt-ER cells and parental FDC2 cells, the ability of IL-3, Epo, and SCF to promote mitogenesis and/or survival was then tested. (A) Mitogenic signaling was assayed based on cytokine-stimulated rates of [³H] thymidine incorporation. To account for any minor differences in plating, values were normalized versus maximal responsiveness to IL-3 in each cell line. (B) FDC2 and FDC2–wt-ER cells were cultured for 7 hours in Optimem medium, 1% FBS in the presence (or absence) of SCF (220 ng/mL or 6.6 nmol/L), Epo (20 U/mL or 4 nmol/L), or IL-3 (20% W3CM, ≥ maximal mitogenic dose). Apoptosis then was assayed based on observed levels of intranucleosomal DNA fragmentation. Levels of fragmentation were analyzed quantitatively by phosphor-imaging.