Dasatinib (BMS-354825) is a novel, oral, potent, multi-targeted
kinase inhibitor of Bcr-Abl and
Src family kinases (SFK) and is a promising
cancer therapeutic agent. Preclinical data indicate that
dasatinib is 325-fold more potent than
imatinib against cells expressing wild-type Bcr-Abl, and that
dasatinib is active against 18 of 19 Bcr-Abl mutations known to cause
imatinib resistance. Phase I clinical data show that
dasatinib is well tolerated and highly effective for the treatment of
imatinib-resistant/
imatinib-intolerant
chronic myelogenous leukemia (CML) and
Philadelphia chromosome-positive
acute lymphoblastic leukemia. However, the molecular mechanism of action of
dasatinib is not fully understood. In this study, we confirm that
dasatinib inhibits
tyrosine phosphorylation of SFKs, including Src, Hck, and Lyn, in K562 human CML cells. Significantly, downstream
signal transducer and activator of transcription 5 (Stat5) signaling is also blocked by
dasatinib as shown by decreases in levels of phosphorylated Stat5 and Stat5
DNA-binding activities. In addition,
dasatinib down-regulates expression of Stat5 target genes, including Bcl-x, Mcl-1, and
cyclin D1. Consistent with these results, blockade of Stat5 signaling by
dasatinib is accompanied by inhibition of cell proliferation and induction of apoptosis. Surprisingly, Stat5
DNA-binding activities are enhanced with increasing cell density, which is associated with resistance to apoptosis by
dasatinib. Our findings indicate that inhibition of Stat5 signaling downstream of Bcr-Abl/SFKs contributes to the action of
dasatinib, and, conversely, that increasing cell density up-regulates Stat5 activation and confers resistance to
dasatinib. Moreover, the level of phosphorylated Stat5 in CML cells represents a mechanistically relevant
biomarker for monitoring inhibition of Bcr-Abl signaling by
dasatinib in CML patients using convenient immunocytochemical assays.