Activation of TrkA-RON fails to support erythroid differentiation and delays differentiation in the presence of EPO. (A-B) TrkA-RON-expressing erythroblasts were cultured in medium containing iron-loaded transferrin without additional factor (NF; ○), supplemented with EPO (2 U/mL; •) or with NGF (10 ng/mL; □). Total cell numbers (A) and mean cell volume (B; fl) are depicted. (C) On day 4 of the experiment, hemoglobin content was measured (Hb/cell volume in arbitrary units) in empty vector-transduced cultures and TrkA-RON-expressing cultures seeded in the absence of factor (-) or in the presence of EPO or NGF. (D-F) TrkA-RON-expressing erythroblasts were seeded in differentiation medium lacking growth factor (NF; ○) or in medium supplemented with EPO (2 U/mL; •) or EPO (2 U/mL) plus NGF (10 ng/mL; □). Cumulative cell number (D), cell volume (E; fl), and hemoglobin accumulation (F; Hb/cell volume, in arbitrary units) were determined at daily intervals. (G) Schematic representation of the results. Empty vector-transduced cultures and TrkA-RON-expressing cultures could be expanded in the presence of EPO/SCF/Dex and differentiated in the presence of EPO. When NGF was substituted for EPO, TrkA-RON-expressing cultures were expanded, whereas control cells became pyknotic. However, NGF failed to support the differentiation of TrkA-RON-expressing erythroblasts. In combination with EPO, NGF delayed differentiation of the TrkA-RON-expressing erythroblasts.