Stathmin expression and megakaryocyte differentiation: a potential role in polyploidy

Megakaryopoiesis is characterized by two major processes, acquisition of lineage-specific markers and polyploidization. Polyploidy is a result of endomitosis, a process that is characterized by continued DNA replication in the presence of abortive mitosis. Stathmin is a major microtubule-regulatory protein that plays an important role in the regulation of the mitotic spindle. Our previous studies had shown that inhibition of stathmin expression in human leukemia cells results in the assembly of atypical mitotic spindles and abnormal exit from mitosis. We hypothesized that the absence of stathmin expression in megakaryocytes might be important for their abortive mitosis.

The experimental models that we used were human K562 and HEL cell lines that can be induced to undergo megakaryocytic differentiation and primary murine megakaryocytes generated by in vitro culture of bone marrow cells. The megakaryocytic phenotype was evaluated by flow cytometry and light microscopy. The DNA content (ploidy) was analyzed by flow cytometry. Stathmin expression was analyzed by Western and Northern blotting and by RT-PCR.

Our studies showed an inverse correlation between the level of ploidy and the level of stathmin expression in megakaryocytic cell lines and in primary cells. More importantly, inhibition of stathmin expression in K562 cells enhanced the propensity of these cells to undergo endomitosis and to become polyploid upon induction of megakaryocytic differentiation. In contrast, inhibition of stathmin expression interfered with the ability of the cells to acquire megakaryocyte-specific markers of differentiation.

Based on these observations, we propose a model of megakaryopoiesis in which stathmin expression is necessary for the proliferation and differentiation of early megakaryoblasts and its suppression in the later stages of megakaryocytic maturation is necessary for polyploidization.