Mcl-1 (myeloid cell leukaemia-1) is an anti-apoptotic member of the Bcl-2 family protein, originally identified through the analysis of differentiating myeloid cells. We have recently reported that disruption of murine MCL-1 in adult murine hematopoiesis resulted in a complete deficiency of hematopoietic cells in the bone marrow, and that MCL-1 expression was activated in response to activation of c-Kit tyrosine kinase receptor signaling (

Science
307
;
1101
,
2005
). Here we propose that human MCL-1 also plays a critical role in maintenance of normal and malignant human hematopoiesis. The analysis of FACS-sorted human HSCs and myeloid progenitors demonstrated that, as in case of murine hematopoiesis, HSCs expressed hMCL-1 at the highest level, and that its expression gradually downregulated in committed myeloid progenitors such as CMPs and GMPs. We also found that unlike murine long-term HSCs that express c-Kit but not FLT3 receptor tyrosine kinase, human long-term HSCs express both of these receptors. Ligation of FLT3 receptor in purified human HSCs immediately induced the expression of MCL-1, suggesting that FLT3 supports human HSC survival through stimulating MCL-1 expression. We then evaluated whether hMCL-1 is expressed in leukemic stem cells. Thirty patients with acute myeloid leukemia (AML) were enrolled in this study. Within the immature CD34+ blast population, the CD34+CD38− fraction expressed higher levels of MCL-1 than those in more differentiated CD34+CD38+ fraction. In most cases, CD34+CD38− AML cells expressed 2 to 10-fold higher levels of MCL-1 transcripts as compared to normal HSCs. Interestingly, the group of samples expressed highest levels of MCL-1 constituted mostly of FLT3/internal tandem duplications (ITDs) positive AML, suggesting that constitutive activation of FLT3 signaling by FLT3 mutations might induce overexpression of MCL-1. A high level expression of MCL-1 was also found in MOLM-13 and MV4-11, FLT3/ITD positive AML cell lines. Treatment of MOLM-13 and MV4-11 with the small molecule tyrosine kinase inhibitor PKC 412, resulted in induction of apoptosis, which was associated with decreased expression of MCL-1 transcripts and proteins. Based on these data, MCL-1 might play a critical role in maintenance of normal and malignant HSCs in human at least through FLT3 signaling. Our data also suggest that constitutive activation of FLT3 signaling by FLT3 mutation might contribute to leukemic transformation through enforcing survival of AML stem cells.

Topics:
flt3 gene, hematopoietic stem cells, ms-like tyrosine kinase 3, signal transduction, proto-oncogene protein c-kit, impedance threshold device, leukemia, leukemia, myelocytic, acute, leukemic hematopoietic stem cell, ligation

Disclosure: No relevant conflicts of interest to declare.

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2006, The American Society of Hematology
2006
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