In this paper we report a new myeloid differentiation effect of bortezomib (BTZ) in acute myeloid leukemia (AML) cell lines and primary patient-derived AML cells; this effect was assayed by investigating growth-inhibition, cell morphology, differentiation markers, and nitro-blue tetrazolium reduction. We show that BTZ induces the phosphorylation of several mitogen-activated protein (MAP) kinases, including MEK/ERK, c-Jun N-terminal kinase (JNK), and p38 MAPK. BTZ-induced cell differentiation is almost completely reversed by PD98059, an inhibitor of MEK, which also attenuates the increase in phospho-JNK p46. However, p38 activation does not appear to be required for the differentiation induced by BTZ. Furthermore, the differentiation effect of BTZ is associated with increased protein level of signal transducer and activator of transcription-1 (STAT1), a molecular determinant of myeloid differentiation, due to effects on both its synthesis and degradation. In short, this study reveals that BTZ activates the MEK/ERK cascade, which further up-regulates the expression and activity of the key myeloid transcription factor STAT1, thus promoting myeloid differentiation. These findings contribute to an unexpected potential mechanism for the antitumor activity of BTZ in AML.
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