FGF2-dependent resistance is mediated by activation of FGFR-RAS-RAF-MEK-ERK pathway, whereas FGF2-independent resistance is mediated by reactivation of BCR-ABL. (A) Untreated K562 cells, K562 cells treated for 24 hours with 1 μM IM, and K1 cells after 1 month of culture in 10 ng/mL FGF2 and 1 μM IM were lysed and used to probe a human phospho-kinase array (Proteome Profiler, RnD). The dots corresponding to pERK1/2 and pSTAT5 are indicated. (B) K562 cells were treated for 48 hours in the indicated conditions under short-term culture conditions. The long-term cultured cells were grown continuously in IM and FGF2 as described in Figure 1. Cell lysates were collected 48 hours after replacement of media, FGF2, and inhibitor (IM and PD173074). The cells were then lysed as described with western blot analysis as in “Methods.” RAS-GTP was evaluated at 24 hours using immunoprecipitation.