Differential Regulations and Roles of Mcl-1 and Bcl-Xl during Megakaryopoiesis

Backgrounds: Platelet plays an essential role in thrombosis and hemostasis and is produced from hematopoietic stem cells through a serious of differentiation and maturation processes called megakaryopoiesis. The major factor known to control platelet formation is thrombopoietin (TPO), but recently more proteins including apoptosis regulators have been reported to involve in megakaryopoiesis. Evolutionally conserved Bcl-2 family proteins are central regulators of apoptosis. Antiapoptotic Bcl-2 subfamily comprised of Bcl-xL, Mcl-1, Bcl-2, Bcl-w, and Bfl-1 plays a pivotal role in controlling cell death and survival under various conditions. According a recent study, Bcl-xL is a key molecular clock that determines the life span of platelets, but the role and regulation of Bcl-2 members in megakaryopoiesis are largely unknown.

Methods and Results: We have established an in vitro system of megakaryopoiesis and performed the profiling of Bcl-2 family genes during megakaryocytosis. We found that Bcl-xL and Mcl-1 were predominant molecules among other members of the pro-survival proteins as determined by quantitative RT-PCR. TPO differentially regulates Bcl-xL and Mcl-1 in Meg-01 cells, in which Bcl-xL protein was significantly up-regulated while the level of Mcl-1 was attenuated. Furthermore, the roles of Bcl-xL and Mcl-1 during megakaryopoiesis were determined by modulating the expression levels of two proteins. Overexpression of Mcl-1 prominently enhanced the viability of the cells, whereas the knockdown of Mcl-1 promoted apoptosis of the cells. In contrast, forced expression of Bcl-xL did not affect the cell survival but rather significantly stimulated differentiation of megakarycytes.

Conclusion: Bcl-xL and Mcl-1 are likely essential molecules during megakaryopoiesis. Moreover, the present study implies that although both Bcl-xL and Mcl-1 are members of antiapoptotic Bcl-2 family that promote the survival of different cells, these two proteins have distinctive and non-redundant functions in megakaryocytes.