Development of drug resistance due to BCR-ABL point mutations and the persistence of
leukemia initiating cells has become a major obstacle for
tyrosine kinase inhibitors (TKIs) in the treatment of
chronic myeloid leukemia (CML). The BCR-ABL
protein is an important client
protein of
heat shock protein 90 (Hsp90).
BIIB021, an orally available Hsp90 inhibitor, has activity against various
cancer cells. However, little is known about the inhibitory effect of
BIIB021 on CML cells. We evaluated the inhibitory effects of
BIIB021 on K562, K562/G (an
imatinib-resistant cell lines), as well as 32D mouse leukemic cells expressing wild-type BCR-ABL (b3a2, 32Dp210) and T315I mutant BCR-ABL (32Dp210-T315I) cells. Our data showed that
BIIB021 induced significant growth inhibition and apoptosis that was predominantly mediated by the mitochondrial pathway.
BIIB021 also resulted in proteasomal degradation of BCR-ABL
proteins. In addition to induction of apoptosis, we report for the first time that
BIIB021 induced autophagic response as evidenced by the formation of autophagosome, increased conversion of
microtubule-associated protein light chain 3 (LC3)-I to LC3-II, decreased p62 (SQSTM1)
protein levels. Further study suggested that Akt-mTOR-Ulk1 signaling pathway was involved in BIIB021-triggered autophagy. Moreover, blocking autophagy using pharmacological inhibitor
3-methyladenine and
bafilomycin A1 significantly enhanced cell death and apoptosis induced by
BIIB021, indicating the cytoprotective role of autophagy in BIIB021-treated CML cells. Collectively, these data provide possible molecular mechanisms for the antileukemic effect of
BIIB021 on
imatinib-sensitive and -resistant CML cells and provide new insights into the future application of
BIIB021 in the clinical treatment of CML.