Inhibition of the TPO-induced mitogenic response by GF109203X. (A) Inhibition of the TPO-induced mitogenic response as function of GF109203X concentration. UT-7/mpl 5.1 cells were treated for 48 hours with 5 ng/mL TPO and the indicated concentrations of GF109203X. [³H]Thymidine incorporation into DNA was measured as described in Materials and Methods. Results are expressed as a percentage of the response observed in the cells treated with TPO alone. (B) Effect of GF109203X on the dose response for the induction of mitogenesis stimulated by TPO. Cells were treated for 48 hours with various concentrations of TPO, in the absence (○) or presence (•) of 3 μmol/L GF109203X. [³H]Thymidine incorporation into DNA was measured as described in Materials and Methods. Results are expressed as a percentage of the maximal response. Mean values from one representative experiment performed in triplicate are shown in each case. Error bars (SDs), which were smaller or equal to the size of the symbol, are not visible.