Abstract
The hormone thrombopoietin (TPO) induces proliferation of megakaryocytic progenitors and augments agonist-induced mobilization of Ca<sup>2+</sup> in platelets. Because the action of TPO is not restricted to the megakaryocytic lineage, we studied the occurrence of TPO receptor mRNA and protein, and effects of TPO on cytosolic Ca<sup>2+</sup> levels and DNA synthesis in human umbilical vein endothelial cell cultures (HUVECs). Polymerase chain reaction following reverse transcription (RT-PCR) of total mRNA revealed that TPO receptor (MPL) mRNA was expressed only at low level in our samples. TPO receptor protein was not detectable in HUVEC lysates investigated by immunoprecipitation and immunoblotting. In contrast to vascular endothelial growth factor (VEGF), TPO did neither alter fura2 fluorescence as a measure of cytosolic Ca<sup>2+</sup> levels nor increase 5-bromo-2''-deoxyuridine incorporation into DNA of HUVECs. In conclusion, our data demonstrate that HUVECs are neither structurally nor functionally responsive to TPO..
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