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Inhibitors of cellular proteolysis cause increased secretion from parathyroid cells
1Department of Anatomy and Cell Biology, University of Kansas Medical Center, Kansas City 66103.
Abstract:
The secretion but not the biosynthesis of parathyroid hormone and secretory protein 1 is strongly, negatively regulated by extracellular [Ca2+]. In order to examine the hypothesis that decreased degradation might directly cause increased secretion, we tested the effects of agents that suppress cellular degradation via different mechanisms: 3-methyladenine, chloroquine, and 1-deoxynojirimycin. When secretion was inhibited by high [Ca2+], all agents caused increased secretion, but at low [Ca2+] only deoxynojirimycin was effective. Results derived from the use of two radioimmunoassays for parathyroid hormone suggested that the increased secretion was due to reduced destruction of secretory vesicles by lysosomes, and to reduced proteolysis of their contents by secretory vesicle proteases.
Insights
Extracellular calcium regulates parathyroid hormone secretion by controlling its degradation. Inhibiting cellular degradation increases hormone secretion, suggesting reduced lysosomal and protease activity is key.
Area of Science:
- Endocrinology
- Cell Biology
- Molecular Biology
Background:
- Extracellular calcium (Ca2+) strongly regulates parathyroid hormone (PTH) and secretory protein 1 (SP1) secretion, but not their biosynthesis.
- The precise mechanism by which calcium influences PTH secretion remains incompletely understood.
Purpose of the Study:
- To investigate the hypothesis that decreased degradation of PTH and SP1 directly contributes to increased secretion.
- To explore the role of cellular degradation pathways in PTH secretion regulation.
Main Methods:
- Utilized agents that suppress cellular degradation via distinct mechanisms: 3-methyladenine, chloroquine, and 1-deoxynojirimycin.
- Administered these agents under conditions of both high and low extracellular calcium concentrations.
- Employed two radioimmunoassays (RIAs) to quantify parathyroid hormone levels.
Main Results:
- High extracellular calcium, which inhibits secretion, showed increased secretion upon treatment with all tested degradation inhibitors.
- At low extracellular calcium, only 1-deoxynojirimycin effectively increased secretion.
- Radioimmunoassays indicated that enhanced secretion resulted from reduced lysosomal destruction of secretory vesicles and decreased proteolysis of vesicle contents.
Conclusions:
- Cellular degradation pathways, particularly lysosomal activity and protease function, play a significant role in regulating PTH secretion.
- Inhibition of these degradation processes can lead to increased PTH secretion, especially under conditions where secretion is already suppressed by high calcium.
- These findings offer new insights into the post-biosynthetic regulation of parathyroid hormone secretion.