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Signaling by the Mpl receptor involves IKK and NF-kappaB
Ying Zhang1, Shishinn Sun, Zhengyu Wang
1Department of Biochemistry, Whitaker Cardiovascular Institute, Cancer Research Center, Boston University School of Medicine, Boston, Massachusetts 02118, USA.
Abstract:
Binding of tumor necrosis factor-alpha (TNF-alpha) to its receptor activates IKK complex, which leads to inducement of NF-kappaB activity. Here we report that activation of Mpl ligand is also linked to IKK and NF-kappaB activity. Mpl ligand, also known as thrombopoietin (TPO) or megakaryocyte growth and development factor (MGDF), induces megakaryocyte differentiation and inhibition of mitotic proliferation, followed by induction of polyploidization and fragmentation into platelets. The latter process is often observed in megakaryocytes undergoing apoptosis. Treatment of a Mpl ligand-responding megakaryocytic cell line with this cytokine led to an immediate, transient increase in IKK activity followed by a profound decrease in this kinase activity over time. This decrease was not due to an effect on the levels of the IKK regulatory components IKKalpha and IKKbeta. Proliferating megakaryocytes displayed a constitutive DNA-binding activity of NF-kappaB p50 homodimers and of NF-kappaB p50-p65 heterodimers. As expected, reduced IKK activity in Mpl ligand-treated cells was associated with a significant reduction in NF-kappaB DNA binding activity and in the activity of a NF-kappaB-dependent promoter. Our study is thus the first to identify a constitutive NF-kappaB activity in proliferating megakaryocytes as well as to describe a link between Mpl receptor signaling and IKK and NF-kappaB activities. Since a variety of proliferation-promoting genes and anti-apoptotic mechanisms are activated by NF-kappaB, retaining its low levels would be one potential mechanism by which inhibition of mitotic proliferation is maintained and apoptosis is promoted during late megakaryopoiesis.
Insights
Mpl ligand signaling, also known as thrombopoietin, influences NF-kappaB activity in megakaryocytes. This study reveals a link between Mpl ligand, IKK, and NF-kappaB, impacting megakaryocyte proliferation and apoptosis.
Area of Science:
- Molecular Biology
- Cell Signaling
- Hematopoiesis
Background:
- Tumor necrosis factor-alpha (TNF-alpha) receptor binding activates IKK complex, inducing NF-kappaB activity.
- Mpl ligand (thrombopoietin/TPO) signaling is crucial for megakaryocyte differentiation, proliferation inhibition, polyploidization, and platelet formation.
- Megakaryocyte apoptosis is associated with polyploidization and fragmentation into platelets.
Purpose of the Study:
- To investigate the relationship between Mpl ligand activation and IKK/NF-kappaB activity.
- To identify the role of NF-kappaB in megakaryocyte proliferation and apoptosis during late megakaryopoiesis.
Main Methods:
- Treatment of a Mpl ligand-responding megakaryocytic cell line with Mpl ligand.
- Assessing IKK activity over time.
- Analyzing DNA-binding activity of NF-kappaB p50 homodimers and p50-p65 heterodimers.
- Evaluating NF-kappaB-dependent promoter activity.
Main Results:
- Mpl ligand treatment caused a transient increase followed by a sustained decrease in IKK activity.
- This decrease in IKK activity was independent of IKKalpha and IKKbeta levels.
- Proliferating megakaryocytes exhibited constitutive NF-kappaB p50 homodimer and p50-p65 heterodimer DNA-binding activity.
- Reduced IKK activity correlated with decreased NF-kappaB DNA binding and NF-kappaB-dependent promoter activity.
Conclusions:
- This study establishes the first link between Mpl receptor signaling and IKK/NF-kappaB activities.
- Mpl ligand signaling modulates IKK and NF-kappaB activity in megakaryocytes.
- Downregulation of NF-kappaB activity, mediated by Mpl signaling, may contribute to the inhibition of mitotic proliferation and promotion of apoptosis in late megakaryopoiesis.