STAT5 Is a Negative Regulator of Megakaryopoiesis.

The transcription factor STAT5 plays a critical role in self-renewal and lineage commitment of hematopoietic stem cels (HSCs). We have recently shown in CB CD34⁺ cells that persistent activation of STAT5 results in enhanced self-renewal and induces erythroid differentiation, while myelopoiesis was severely impaired. In this study we analyzed the function of STAT5 during megakaryocyte (MK) differentiation. Using RNA interference we downregulated STAT5 expression in peripheral blood CD34⁺ cells. Cells were transduced with a lentiviral construct encoding eGFP and a short-hairpin RNA for STAT5 or with a control vector expressing YFP. Transduction efficiencies were determined by flow cytometric detection of eGFP/YFP and ranged from 40%–80%. Decreased STAT5 expression was verified by Western blot and quantitative RT-PCR (65% lower expression in STAT5 RNAi cells versus YFP⁺ control cells, p=<0.01). To assess the effects of decreased STAT5 expression on the progenitor pool, the transduced populations were sorted and enumerated in colony assays. Downregulation of STAT5 significantly increased the number of MK progenitors (1.9-fold, p=0.02) and resulted in a decrease of erythroid progenitors (0.6-fold, p<0.01), but did not affect the number of granulocyte/monocyte progenitors. Prospective isolation of immature and commited progenitors are currently being performed to distinguish if certain progenitor subsets are re-directed to the MK lineage by low STAT5 expression. Next, we analyzed the role of STAT5 during MK differentiation in unilineage suspension cultures. Transduced cells were cultured in serum-free medium containing TPO and SCF and scored for cell counts and expression of MK markers weekly. The percentage of transduced cells and their number did not differ when STAT5 RNAi cells were compared to controls, indicating that downregulation of STAT5 provides no proliferative advantage or disadvantage. However, the expression of glycoproteins IIb/IIIa (CD41) and Ib (CD42) was upregulated, and an increase in large, polyploid cells was observed in STAT5 RNAi cultures (15.2% polyploid cells versus 4.9% in control cells at day 7, p<0.01). RT-PCR analysis of transcription factors predominantly expressed in MK and erythroid lineages revealed that Runx1 and Erg were increased in STAT5 RNAi cells, and the level of Scl was decreased compared to control cells. These observations correlate with the increased MK differentiation observed in STAT5 downregulated cells. Together, these data demonstrate that the expression level of STAT5 has important effects regarding the type of hematopoietic cell development, with high levels favouring erythroid development and low levels enhancing MK differentiation and maturation.