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Effect of secretagogues on components of the secretory system in AtT-20 cells
1Department of Neuroscience, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205.
Abstract:
The mouse corticotrope tumor cell line AtT-20/D16v was used to investigate the effects of chronic treatment with various secretagogues on individual components of the secretory pathway. Secretagogues acting in part through receptors linked to guanine nucleotide-binding regulatory proteins [CRF and somatostatin (SS)] and agents by-passing membrane receptors (phorbol myristate acetate and dexamethasone) were examined. Effects on the secretory product were evaluated by measuring levels of pro-ACTH/endorphin mRNA and hormone secretion. Effects on posttranslational processing enzymes were evaluated by measuring levels of the mRNAs encoding carboxypeptidase-E and peptidyl-glycine-alpha-amidating monooxygenase (PAM); cellular levels of PAM activity were also measured. The mRNAs encoding the G-proteins in AtT-20 cells were identified, and secretagogue effects on the G-protein signal transduction system were evaluated by measuring levels of the mRNAs encoding (alpha s, alpha)i2, and beta 2. No single parameter adequately characterizes the regulatory state of the complex secretory apparatus. Although levels of pro-ACTH/endorphin (PAE) mRNA accurately reflected hormone secretion after chronic CRF or dexamethasone treatment, chronic SS treatment elevated PAE mRNA levels in the face of reduced hormone secretion. Levels of PAM mRNA generally changed in parallel with levels of PAE mRNA; in contrast, levels of carboxypeptidase-E mRNA were unaffected by any of the secretagogues tested. Secretagogues acting through distinct G-proteins (CRF and SS) as well as dexamethasone brought about a coordinate increase in the level of the mRNAs encoding the three G-protein subunits examined. Treatment with phorbol myristate acetate caused a slight decrease in the levels of the G-protein subunit mRNAs.
Insights
Chronic secretagogue treatment impacts the pituitary secretory pathway differently. While some agents alter pro-ACTH/endorphin mRNA and hormone secretion in parallel, others show dissociation, revealing complex regulatory mechanisms.
Area of Science:
- Endocrinology
- Molecular Biology
- Cell Biology
Background:
- The pituitary gland's secretory pathway is crucial for hormone regulation.
- Understanding how different stimuli affect this pathway is key to deciphering endocrine control.
- The AtT-20/D16v mouse corticotrope cell line serves as a model for studying these mechanisms.
Purpose of the Study:
- To investigate the chronic effects of various secretagogues on the pituitary secretory pathway.
- To analyze the impact on hormone production, processing enzymes, and G-protein signaling.
- To determine if different secretagogue pathways lead to distinct cellular responses.
Main Methods:
- Utilized the AtT-20/D16v mouse corticotrope tumor cell line.
- Administered chronic treatments with corticotropin-releasing factor (CRF), somatostatin (SS), phorbol myristate acetate (PMA), and dexamethasone.
- Measured pro-ACTH/endorphin (PAE) mRNA and hormone secretion.
- Assessed mRNA levels for carboxypeptidase-E and peptidyl-glycine-alpha-amidating monooxygenase (PAM).
- Quantified cellular PAM activity and G-protein subunit mRNA levels (alpha s, alpha i2, beta 2).
Main Results:
- Chronic CRF and dexamethasone treatments showed parallel increases in PAE mRNA and hormone secretion.
- Chronic SS treatment increased PAE mRNA but decreased hormone secretion, indicating a dissociation.
- PAM mRNA levels generally mirrored PAE mRNA levels, while carboxypeptidase-E mRNA remained unchanged.
- CRF, SS, and dexamethasone coordinately increased G-protein subunit mRNAs.
- PMA treatment slightly decreased G-protein subunit mRNAs.
Conclusions:
- No single parameter fully characterizes the regulatory state of the pituitary secretory apparatus.
- Different secretagogues, acting through distinct signaling pathways, elicit varied responses in hormone production and processing.
- G-protein signaling is modulated by chronic secretagogue exposure, suggesting its role in integrating cellular responses.