Factors Affecting Thrombopoietin Production from Bone Marrow Stromal Cells.

Thrombopoietin (TPO) is the primary regulator of megakaryopoiesis and therefore also the most important determinant of the number of platelets in circulation. The regulation of TPO blood concentration is complex, with at least a significant component mediated by removal and degradation of the hormone by the mature circulating platelet mass. In this way, if the production of TPO were fixed, when the number of platelets rises, an increased amount of TPO is removed from the plasma, resulting in an inverse relationship between platelet counts and TPO concentration. However, several studies hint at the existence of additional mechanisms in which TPO production is altered in response to physiological and pathologic conditions. We, and others, have previously reported that TPO mRNA is increased in the bone marrow of mice or humans after either immune- or radiation-mediated thrombocytopenia, although hepatic levels remain unchanged by this manipulation. To further explore the mechanism(s) of this effect, we utilized in vitro marrow stromal cell models to study the effects of blood proteins on TPO production. As an initial hypothesis we determined if platelet-derived proteins in serum might suppress TPO production from both a marrow stromal cell line, OP9, or primary murine marrow stromal cells derived from long-term hematopoietic cultures. As assessed by quantitative RT-PCR we found that TPO mRNA levels increase a mean of 2.8-fold±0.7 (n = 3) twelve hours following the removal of serum from the culture. Similar results were obtained from primary murine marrow stromal cells; TPO mRNA levels rose a mean of 2.9-fold±0.9 (n = 4) within sixteen hours of serum deprivation. In contrast, TPO transcript levels were unaffected by the same manipulation of serum in the hepatocyte cell line HEPA1c1c7. As the removal of serum might have induced a stress response in the cells, we tested whether other cell stressors might mimic this response; we found that neither UV irradiation nor treatment with toxic metals such as nickel, cobalt, or cadmium produced any rise in TPO mRNA in marrow stromal cells. Furthermore, to test whether a cell cycle arrest triggered by serum deprivation might mediate these effects, we treated stromal cells with cell cycle inhibitors, but failed to find any affect on TPO transcript levels in stromal cells. Further biochemical fractionation of serum suggested that one or more distinct proteins is responsible for this effect, demonstrated by the ability of both ammonium sulfate precipitation and ion exchange chromatography to partition the suppressive effects of serum. However, the active agent in serum is not TPO itself, as the addition of 150 ng/ml of the pure hormone did not suppress stromal cell TPO transcript levels. Knowledge of this novel regulatory mechanism should be useful in treating platelet disorders and perhaps also during stem cell transplantation, a setting in which TPO is known to play a vital role.