The pro-apoptotic BH3-only protein Bim regulates cell cycle progression of hematopoietic progenitors during megakaryopoiesis.

Summary Background: The pro-apoptotic BH3-only protein Bim is recognized as a pivotal regulator of apoptosis induced by the depletion of cytokines. In the present study, we examined the role of Bim in megakaryopoiesis.

Methods: Megakaryocyte (MK) progenitors obtained from bim knockout (KO) mice were analyzed in vitro for liability to apoptosis after the depletion of cytokines, ability to differentiate into MKs and proliferation/cell cycle progression in response to thrombopoietin (TPO). The production of platelets in vitro was evaluated by assaying the formation of proplatelets in MKs. Megakaryopoiesis in vivo was observed in a mouse model of thrombocytopenia induced by injecting fluorouracil (5-FU).

Results: Bim-deficient CD34-/c-kit+/Sca-1+/Lineage- stem cells and MKs were highly resistant to apoptosis induced by cytokine depletion, suggesting that Bim is involved in the apoptotic process in both stem cells and MKs. As bim KO mice exhibited splenomegaly and thrombocytopenia, splenectomized mice were used for experiments in vivo. Platelet recovery after 5-FU-induced thrombocytopenia was significantly delayed in bim KO mice. Corresponding with this, numbers of MKs in the recovery phase bone marrow were significantly reduced in bim KO mice. Culture of c-kit+/Lineage- progenitors with TPO revealed that Bim-deficient cells poorly proliferate and differentiate into CD41+ cells in comparison with wild-type (WT) cells. However, once differentiated into MKs, these cells matured normally. Furthermore, cell cycle analyses demonstrated that transition from the G1 to the S phase was delayed in Bim-deficient stem cells.

Conclusions: In the present study, we demonstrated that Bim plays a pivotal role in the regulation of cell cycle progression in hepatopoietic progenitors during megakaryopiesis.