STAT1 Promotes Megakaryocytic Differentiation in a GATA-1-Independent Manner.

GATA-1 is a critical transcription factor that governs megakaryocyte development and differentiation. GATA-1-deficient megakaryocytes proliferate excessively and fail to undergo terminal differentiation, likely as a consequence of an aberrant gene expression program. In our studies to identify key signal transduction pathways that are dysregulated in the absence of GATA-1, we observed that STAT1, a gene of unknown function in megakaryocytes, was significantly downregulated in GATA-1-deficient megakaryocytes. Surprisingly, ectopic expression of STAT1, or IRF-1, a downstream effector of STAT1 signaling, promoted megakaryocytic differentiation of G1ME cells, a GATA-1-null and TPO-dependent erythromegakaryocytic cell line. Both STAT1- and IRF-1-transduced G1ME cells, but not GFP-expressing control cells, displayed multiple markers of terminal differentiation, including expression of CD42, as well as a significant fraction of polyploid cells. BrdU labeling assays confirmed that these transduced cells continued to effectively synthesize DNA, suggesting that STAT1 signaling switches these cells from mitosis to endomitosis. STAT1 and IRF-1 also modulated TPO signaling evidenced by enhanced STAT3 and STAT5 phosphorylation. Since STAT1 signaling is activated by IFN-g, we further investigated whether IFN-g-treatment also led to polyploidization of G1ME cells. IFN-g effectively promoted polyploidization of both G1ME cells and primary bone marrow-derived CD41⁺ cells in the absence of TPO. Finally, overexpression of STAT1 in primary bone marrow cells isolated from GATA-1 knockdown mice also promoted polyploidization in the absence of TPO. Taken together, these data reveal a GATA-1-independent mechanism by which STAT1 signaling promotes megakaryocyte differentiation.