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Parathyroid hormone down-regulates the epidermal growth factor receptors in clonal osteoblastic mouse calvarial
S Ohta1, C Shigeno, I Yamamoto
1Department of Orthopedic Surgery, Kyoto University School of Medicine, Japan.
Abstract:
The effects of PTH on the binding and mitogenic activity of epidermal growth factor (EGF) were studied in clonal, PTH- and EGF-responsive mouse osteoblastic cell line, MC3T3-E1. Treatment of cells with synthetic rat PTH-(1-34) at 10(-10)-10(-8) M resulted in a dose-related decrease (maximally 30% relative to the control value) in the specific binding of a biologically active, 125I-iodolabeled derivative of recombinant human EGF ([125I]iodo-EGF). The PTH-induced decrease in EGF binding was time dependent, requiring at least 4 h of PTH treatment at 37 C for a maximal effect, completely reversible after cessation of PTH exposure and specific only to biologically active PTH. Scatchard analysis of saturation binding data revealed that the PTH-induced reduction of EGF binding was accounted for by a proportional decrease in the available EGF-binding sites without an alteration in binding affinity (Kd = 0.7-0.8 nM). PTH treatment resulted ina concomitant decrease in mitotic responsiveness to EGF (maximally 40-50% of control). Both of these down-regulatory effects of PTH were closely mimicked by forskolin, cholera toxin, or (Bu)2cAMP. Addition of either colchicine or cytochalasin-B during PTH treatment completely abolished the PTH-induced reduction of EGF binding. These data indicate that PTH down-regulates EGF receptors and reduces the mitotic responsiveness to EGF, probably via a cAMP-dependent cytoskeleton-mediated mechanism(s) in osteoblastic MC3T3-E1 cells.
Insights
Parathyroid hormone (PTH) reduces epidermal growth factor (EGF) binding and cell division in osteoblastic cells. This effect is mediated by a cAMP-dependent pathway involving the cytoskeleton.
Area of Science:
- Cell Biology
- Endocrinology
- Biochemistry
Background:
- Osteoblastic cells are crucial for bone remodeling.
- Epidermal growth factor (EGF) stimulates osteoblast proliferation.
- Parathyroid hormone (PTH) plays a key role in bone metabolism.
Purpose of the Study:
- To investigate the effects of PTH on EGF binding and mitogenic activity in osteoblastic cells.
- To elucidate the mechanism underlying PTH's regulation of EGF receptors.
Main Methods:
- Using the MC3T3-E1 mouse osteoblastic cell line.
- Treating cells with synthetic rat PTH-(1-34) and measuring EGF binding using [125I]iodo-EGF.
- Analyzing binding data with Scatchard analysis.
- Investigating the role of cAMP and cytoskeleton using forskolin, cholera toxin, (Bu)2cAMP, colchicine, and cytochalasin-B.
Main Results:
- PTH significantly decreased EGF binding in a dose- and time-dependent manner.
- PTH reduced the number of available EGF-binding sites without altering binding affinity.
- PTH treatment led to a decrease in mitotic responsiveness to EGF.
- cAMP-elevating agents mimicked these effects, while cytoskeleton inhibitors abolished them.
Conclusions:
- PTH down-regulates EGF receptors in osteoblastic cells.
- PTH reduces the mitotic response to EGF.
- These effects are likely mediated by a cAMP-dependent, cytoskeleton-dependent mechanism.