SU5416 blocks the TPO-dependent growth of UT-7/TPO cells but not TPO signaling. (A) After 24 hours of growth factor deprivation, UT-7/TPO cells were pretreated with the indicated concentration of SU5416 for 1 hour and then stimulated with 100 ng/mL hTPO for 10 minutes. Whole-cell lysates were prepared and activation of ERK, Akt, and Jak2 was analyzed by Western blotting with antibodies specific for the phosphorylated (activated) form of each protein. As controls, the membranes were reprobed with antibodies for the total level of each protein. (B). UT-7/TPO cells were placed in culture with 10 ng/mL hTPO and the indicated concentrations of SU5416. After 72 hours in culture, cell proliferation was assessed by a methylthiotetrazole (MTT) assay; ^*P < .05, ^**P < .01 versus no SU5416.