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Thrombopoietin induces tyrosine phosphorylation and activation of the Janus kinase, JAK2
P J Tortolani1, J A Johnston, C M Bacon
1Arthritis and Rheumatism Branch, National Institute of Arthritis and Musculoskeletal and Skin Diseases, National Institutes of Health, Bethesda, MD 20892-1820, USA.
Abstract:
Thrombopoietin (TPO) is a recently characterized growth and differentiation factor for megakaryocytes and platelets that exerts its effects via the receptor, c-MpI. This receptor is a member of the hematopoietin receptor superfamily and is essential for megakaryocyte maturation; however, the molecular mechanisms of TPO and c-MpI action have not been elucidated. Recently, the Janus kinases have emerged as important elements in signaling via this family of receptors. In this report, we show that, in the M07e megakaryocytic cell line, which expresses c-MpI and proliferates in response to TPO, TPO induces phosphorylation of a number of substrates between 80 and 140 kD. Specifically, we show that stimulation with TPO induces the rapid tyrosine phosphorylation of a 130-kD protein that we identify as the Janus kinase, JAK2. However, no detectable tyrosine phosphorylation of JAK1, JAK3, or TYK2 was observed. TPO also induced activation of JAK2 phosphotransferase activity in vitro. Taken together, these data indicate that JAK2 likely plays a key role in TPO-mediated signal transduction.
Insights
Thrombopoietin (TPO) signaling involves the c-Mpl receptor. This study reveals that TPO rapidly activates Janus kinase 2 (JAK2) in megakaryocytic cells, indicating JAK2
Area of Science:
- Hematology
- Molecular Biology
- Cell Signaling
Background:
- Thrombopoietin (TPO) regulates megakaryocyte and platelet production via the c-Mpl receptor.
- The molecular mechanisms of TPO/c-Mpl signaling are not fully understood.
- Janus kinases (JAKs) are implicated in signaling pathways of hematopoietin receptors.
Purpose of the Study:
- To investigate the role of Janus kinases in TPO-mediated signal transduction.
- To identify specific JAKs involved in TPO signaling in megakaryocytic cells.
Main Methods:
- Utilized the M07e megakaryocytic cell line expressing c-Mpl.
- Stimulated cells with TPO and analyzed protein phosphorylation.
- Assessed Janus kinase activation using in vitro assays.
Main Results:
- TPO induced tyrosine phosphorylation of substrates between 80-140 kD.
- Specifically, TPO rapidly induced tyrosine phosphorylation of a 130-kD protein identified as Janus kinase 2 (JAK2).
- No detectable tyrosine phosphorylation of JAK1, JAK3, or TYK2 was observed; TPO activated JAK2 phosphotransferase activity.
Conclusions:
- JAK2 is a key mediator of TPO-induced signal transduction.
- This finding elucidates a crucial step in the molecular mechanism of TPO/c-Mpl action.