Shedding of a soluble form of BMP receptor-IB controls bone cell responses to BMP.
Bone morphogenetic proteins (BMP) are members of the transforming growth factor beta (TGF-beta) superfamily and are involved in a wide variety of biological processes, including osteoblast differentiation and bone healing. The activities of the BMP are mediated by signal transduction via three BMP receptors (BMPR-IA, -IB and -II), which are thus essential for the biological actions of the BMP. Although the precise mechanisms which control the BMPR are not yet known, it is possible that post-translational regulation of these cell surface antigens by shedding could modulate their expression and thereby at least partly determine the response of the cells to the BMP. To test this possibility, the present study has examined whether soluble forms of the BMPR are produced by shedding from primary human bone cells in vitro. The results showed that human bone cells expressed both mRNA transcripts and antigens corresponding to BMPR-IA, -IB and -II. Incubation of the cells with phorbol 12-myristate 13-acetate (PMA), a potent inducer of proteolytic shedding, resulted in a pronounced decrease in cell surface expression of all three BMPR and, concurrently, the presence of "soluble" forms of these antigens in culture supernatants. Moreover, PMA treatment significantly reduced the level of BMP-2-induced Smad1/5 phosphorylation, a major early activation step in signal transduction initiated by BMP/BMPR interaction. It is notable that, while treatment of bone cells with interleukin-1beta (IL-1beta) also reduced the level of surface BMPR-IB, this inflammatory cytokine had no effect on BMPR-IA or -II levels, hence only the soluble form of BMPR-IB was detected. Furthermore, in addition to down-regulating BMP-2-induced Smad1/5 phosphorylation, IL-1beta also caused a reduction in the level of BMP-2-induced alkaline phosphatase activity and osteocalcin expression, both closely associated with bone cell differentiation. In conclusion, our study has provided evidence, for the first time, that BMPR can be modulated at the cell surface by the shedding of a soluble form of the antigen, resulting in a markedly diminished response to BMP-2 in vitro.
Singhatanadgit
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W
Division of Biomaterials and Tissue Engineering, Eastman Dental Institute, University College London, 256 Gray's Inn Road, London WC1X 8LD, United Kingdom.
Salih
V
V
Division of Biomaterials and Tissue Engineering, Eastman Dental Institute, University College London, 256 Gray's Inn Road, London WC1X 8LD, United Kingdom.
Olsen
I
I
Division of Biomaterials and Tissue Engineering, Eastman Dental Institute, University College London, 256 Gray's Inn Road, London WC1X 8LD, United Kingdom. Electronic address: i.olsen@eastman.ucl.ac.uk.
D015153:Blotting, Western; D055396:Bone Morphogenetic Protein 2; D052004:Bone Morphogenetic Protein Receptors / Q000235:genetics / Q000378:metabolism*; D019485:Bone Morphogenetic Proteins / Q000378:metabolism*; D001842:Bone and Bones / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D002454:Cell Differentiation / Q000187:drug effects; D002478:Cells, Cultured; D006801:Humans; D053583:Interleukin-1beta / Q000494:pharmacology; D010006:Osteoblasts / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D011485:Protein Binding / Q000187:drug effects; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D051898:Smad1 Protein / Q000378:metabolism; D051902:Smad5 Protein / Q000378:metabolism; D012995:Solubility; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology; D014158:Transcription, Genetic / Q000187:drug effects / Q000235:genetics; D016212:Transforming Growth Factor beta / Q000378:metabolism
C525718:BMP2 protein, human; D055396:Bone Morphogenetic Protein 2; D019485:Bone Morphogenetic Proteins; D053583:Interleukin-1beta; D020033:Protein Isoforms; D012333:RNA, Messenger; D051898:Smad1 Protein; D051902:Smad5 Protein; D016212:Transforming Growth Factor beta; D052004:Bone Morphogenetic Protein Receptors; D013755:Tetradecanoylphorbol Acetate