Apoptosis is important in controlling hematopoietic stem cell (HSC) numbers. However, the specific BCL-2 family member(s) that regulate HSC homeostasis are not precisely defined. We tested myeloid leukemia-1 (MCL-1) as an attractive candidate that is highly expressed in HSCs and regulated by growth factor signals. Inducible deletion of Mcl-1 in mice resulted in ablation of bone marrow. This resulted in the loss of early bone marrow progenitor populations, including HSCs. Moreover, growth factors including stem cell factor increased transcription of the Mcl-1 gene and required MCL-1 to augment survival of purified bone marrow progenitors. Deletion of Mcl-1 in other tissues, including liver, did not impair survival. Thus, MCL-1 is a critical and specific regulator essential for ensuring the homeostasis of early hematopoietic progenitors.
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