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Activin increases phosphorylation and decreases stability of the transcription factor Pit-1 in MtTW15 somatotrope
1Clayton Foundation Laboratories for Peptide Biology, Salk Institute, La Jolla, California 92037-1099, USA.
Abstract:
Activin is a polypeptide growth factor which exerts endocrine, paracrine, and autocrine effects in a variety of tissues. In the pituitary somatotrope, activin represses proliferation and growth hormone (GH) biosynthesis and secretion. We previously demonstrated that decreases in GH biosynthesis in MtTW15 somatotrope cells are due at least in part to decreased binding of the tissue-specific transcription factor, Pit-1, to the GH promoter, resulting in decreased transcription of the GH gene. The objective of the current study was to determine the extent to which activin-mediated decreases in GH transcription were the result of decreased Pit-1 activity and/or decreased Pit-1 protein content in MtTW15 cells. Activin caused rapid increases in Pit-1 phosphorylation, which were temporally correlated with decreases in GH DNA binding. Pit-1 phosphorylation preceded marked decreases in steady-state levels of Pit-1 protein. The rate of Pit-1 synthesis was only moderately decreased by activin, with a time-course similar to that observed for decreases in GH biosynthesis. However, Pit-1 stability was markedly decreased after more than 4 h of activin treatment. These data demonstrate that activin decreases GH expression in MtTW15 cells through multilevel regulation of Pit-1, which may represent a more general mechanism whereby activin and other transforming growth factor beta family members modulate gene expression through regulation of transcription factor activity as well as content.
Insights
Activin reduces growth hormone (GH) production by regulating the transcription factor Pit-1. This involves decreasing Pit-1 protein levels and stability, ultimately impacting GH gene expression in pituitary cells.
Area of Science:
- Endocrinology
- Molecular Biology
- Cell Signaling
Background:
- Activin, a polypeptide growth factor, influences various tissues.
- In pituitary somatotropes, activin inhibits growth hormone (GH) production.
- Previous work linked decreased GH biosynthesis to reduced Pit-1 transcription factor binding.
Purpose of the Study:
- To investigate how activin decreases GH transcription.
- To determine if decreased Pit-1 activity or protein content mediates this effect.
Main Methods:
- Utilized MtTW15 somatotrope cells.
- Assessed Pit-1 phosphorylation, DNA binding, protein levels, synthesis, and stability.
- Measured GH gene transcription and biosynthesis.
Main Results:
- Activin rapidly increased Pit-1 phosphorylation, correlating with decreased GH DNA binding.
- Pit-1 phosphorylation preceded significant reductions in steady-state Pit-1 protein.
- Activin moderately decreased Pit-1 synthesis but markedly reduced its stability after 4 hours.
Conclusions:
- Activin decreases GH expression in pituitary cells via multi-level regulation of Pit-1.
- This involves modulating both Pit-1 activity (phosphorylation) and content (stability).
- This mechanism may be shared by other TGF-beta family members for gene expression control.