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Transforming growth factor-beta: a down-regulator of the parathyroid hormone-related protein receptor in renal
F Law1, J P Bonjour, R Rizzoli
1Department of Medicine, Geneva University Hospital, Switzerland.
Abstract:
We have recently provided evidence for the ability of transforming growth factor-beta 1 (TGF beta) to modulate PTH-related protein (PTHrP)-mediated responses in opossum kidney (OK) cells through reducing the number of PTHrP receptor-binding sites. In the present studies, we investigated the possible mechanisms by which TGF beta might regulate PTHrP receptor density in OK cells, an area that has remained largely unexplored. The steady state level of PTHrP receptor mRNA was time dependently reduced by TGF beta treatment, with the nadir (approximately 3-fold decrease) between 6-10 h, preceding the maximal inhibition on PTHrP receptor binding at 18 h. We then assessed whether the 41% reduction in binding consequent to 18-h TGF beta exposure was reversible. PTHrP-binding activity recovered considerably after 24 h (23% decrease compared with controls) and almost completely by 48 h. However, the addition of monensin or cycloheximide, but not actinomycin (at a dose effective in preventing TGF beta action in this system) during the 24-h recovery period prevented restoration of PTHrP binding. Upon removal of TGF beta, the PTHrP receptor message showed a trend toward recovery in the ensuing 24 h. Therefore, TGF beta provides an example of heterologous desensitization of the PTHrP receptor in OK epithelial cells by decreasing the expression of the receptor message. The desensitization was reversible, and the first 24-h recovery phase was dependent on synthesis and processing of new receptor proteins.
Insights
Transforming growth factor-beta 1 (TGF-β) reduces parathyroid hormone-related protein (PTHrP) receptor expression in kidney cells. This desensitization is reversible and requires new protein synthesis for recovery.
Area of Science:
- Cell biology
- Endocrinology
- Molecular biology
Background:
- Transforming growth factor-beta 1 (TGF-β) previously shown to modulate parathyroid hormone-related protein (PTHrP) responses.
- TGF-β reduces PTHrP receptor-binding sites in opossum kidney (OK) cells.
- Mechanisms of TGF-β regulation of PTHrP receptor density in OK cells were unexplored.
Purpose of the Study:
- Investigate mechanisms by which TGF-β regulates PTHrP receptor density in OK cells.
- Determine the role of mRNA expression and protein synthesis in TGF-β-induced PTHrP receptor desensitization.
- Assess the reversibility of TGF-β-mediated PTHrP receptor downregulation.
Main Methods:
- Treatment of OK cells with TGF-β.
- Measurement of PTHrP receptor mRNA levels using quantitative methods.
- Assessment of PTHrP receptor binding activity.
- Evaluation of receptor recovery in the presence of monensin, cycloheximide, or actinomycin.
- Analysis of receptor mRNA recovery after TGF-β removal.
Main Results:
- TGF-β treatment time-dependently reduced PTHrP receptor mRNA levels, with a 3-fold decrease between 6-10 hours.
- Maximal inhibition of PTHrP receptor binding (41%) occurred at 18 hours.
- PTHrP binding recovered significantly within 24-48 hours after TGF-β removal.
- Recovery of PTHrP binding was inhibited by monensin or cycloheximide, but not actinomycin.
- PTHrP receptor mRNA showed a trend toward recovery within 24 hours post-TGF-β removal.
Conclusions:
- TGF-β induces heterologous desensitization of the PTHrP receptor in OK cells by decreasing receptor mRNA expression.
- The observed desensitization is reversible.
- The initial phase of receptor recovery depends on the synthesis and processing of new receptor proteins.