Thrombopoietin and the hematopoietic stem cell.

The molecular mechanisms that underlie the favorable effects of thrombopoietin on stem cell survival, self-renewal, and expansion are unknown. On the basis of known effects of HoxB4, HoxA9, and vascular endothelial cell growth factor on stem cells, we explored whether TPO might affect these pathways. We found that TPO enhances the transcription of HoxB4, induces the nuclear localization of HoxA9, and upregulates VEGF expression by reducing the ubiquination of hypoxia inducible factor.