A novel megakaryocyte differentiation factor from mouse placenta.

Megakaryocyte differentiation and subsequent platelet production are regulated by a network of growth factors and cytokines. We hypothesized that pregnancy-specific regulatory factors also may participate in the modulation of megakaryocytopoiesis and thrombopoiesis. We identified a mouse placental prolactin-like protein hormone with an activity similar to interleukin 6 in targeting megakaryocytes and inducing cell differentiation. The receptor for this placental hormone is present on megakaryocytes from pregnant and nonpregnant female mice and from male mice, and from humans, suggesting that this signaling pathway (if not necessarily this particular ligand) is broadly functional both in terms of physiologic state and evolution. Thus, studying the biologic activities of the large family of placental prolactin-like proteins represents a potentially valuable approach to the discovery of novel hematopoietic signaling pathways.