Abstract
Intermittent administration of parathyroid hormone (PTH) dramatically increases bone mass and currently is one of the most effective treatments for osteoporosis. However, the detailed mechanisms are still largely unknown. Here we demonstrate that conditioned media from PTH-treated osteoblastic and osteocytic cells contain soluble chemotactic factors for bone marrow mesenchymal progenitors, which express a low amount of PTH receptor (PTH1R) and do not respond to PTH stimulation by increasing cAMP production or migrating toward PTH alone. Conditioned media from PTH-treated osteoblasts elevated phosphorylated Akt and p38MAPK amounts in mesenchymal progenitors and inhibition of these pathways blocked the migration of these progenitors toward conditioned media. Our previous and current studies revealed that PTH stimulates the expression of amphiregulin, an epidermal growth factor (EGF)-like ligand that signals through the EGF receptor (EGFR), in both osteoblasts and osteocytes. Interestingly, conditioned media from PTH-treated osteoblasts increased EGFR phosphorylation in mesenchymal progenitors. Using several different approaches, including inhibitor, neutralizing antibody, and siRNA, we demonstrate that PTH increases the release of amphiregulin from osteoblastic cells, which acts on the EGFRs expressed on mesenchymal progenitors to stimulate the Akt and p38MAPK pathways and subsequently promote their migration in vitro. Furthermore, inactivation of EGFR signaling specifically in osteoprogenitors/osteoblasts attenuated the anabolic actions of PTH on bone formation. Taken together, these results suggest a novel mechanism for the therapeutic effect of PTH on osteoporosis and an important role of EGFR signaling in mediating PTH's anabolic actions on bone.
Publication types
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Research Support, N.I.H., Extramural
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Research Support, Non-U.S. Gov't
MeSH terms
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Amphiregulin
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Animals
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Blotting, Western
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Bone Density
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Bone Marrow / drug effects*
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Bone Marrow / metabolism
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Cell Movement / drug effects
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Cell Proliferation / drug effects
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Chemotaxis / drug effects
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Culture Media, Conditioned / pharmacology
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EGF Family of Proteins
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ErbB Receptors / antagonists & inhibitors
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ErbB Receptors / genetics
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ErbB Receptors / metabolism*
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Female
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Flow Cytometry
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Glycoproteins / antagonists & inhibitors
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Glycoproteins / genetics
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Glycoproteins / metabolism*
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Humans
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Immunoenzyme Techniques
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Intercellular Signaling Peptides and Proteins / genetics
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Intercellular Signaling Peptides and Proteins / metabolism*
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Male
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Mesenchymal Stem Cells / cytology
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Mesenchymal Stem Cells / drug effects*
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Mesenchymal Stem Cells / metabolism
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Mice
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Mice, Knockout
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Osteoblasts / cytology
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Osteoblasts / drug effects*
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Osteoblasts / metabolism
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Osteocytes / cytology
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Osteocytes / drug effects*
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Osteocytes / metabolism
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Osteogenesis
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Parathyroid Hormone / pharmacology*
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Phosphorylation
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Proto-Oncogene Proteins c-akt / antagonists & inhibitors
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Proto-Oncogene Proteins c-akt / metabolism
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RNA, Messenger / genetics
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RNA, Small Interfering / genetics
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Rats
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Real-Time Polymerase Chain Reaction
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Receptor, Parathyroid Hormone, Type 1 / genetics
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Receptor, Parathyroid Hormone, Type 1 / metabolism
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Reverse Transcriptase Polymerase Chain Reaction
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Signal Transduction
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p38 Mitogen-Activated Protein Kinases / antagonists & inhibitors
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p38 Mitogen-Activated Protein Kinases / metabolism
Substances
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AREG protein, human
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Amphiregulin
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Areg protein, mouse
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Areg protein, rat
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Culture Media, Conditioned
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EGF Family of Proteins
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Glycoproteins
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Intercellular Signaling Peptides and Proteins
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Parathyroid Hormone
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RNA, Messenger
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RNA, Small Interfering
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Receptor, Parathyroid Hormone, Type 1
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ErbB Receptors
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Proto-Oncogene Proteins c-akt
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p38 Mitogen-Activated Protein Kinases