Thrombopoietin (TPO) induces tyrosine phosphorylation and activation of STAT5 and STAT3

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Abstract
The growth and differentiation of megakaryocytes are regulated by thrombopoietin (TPO), a recently characterized cytokine which exerts its effects via a member of the hematopoietin receptor superfamily, c‐Mpl. Since many cytokines which bind hematopoietin receptors activate the STAT family of transcription factors, we investigated whether STAT proteins were activated by TPO. TPO induced the formation of a DNA‐binding complex recognizing a known STAT binding sequence. STAT5 was a major component of this DNA‐binding complex, and STAT5 was tyrosine phosphorylated in response to TPO. Additionally, TPO‐induced the tyrosine phosphorylation and DNA‐binding activity of STAT3. Together with the recent demonstration of JAK2 activation in response to TPO, the data presented here define a rapid signaling pathway likely to be important in TPO‐induced gene regulation.