Abstract | INTRODUCTION:
TGF-beta is a multifunctional regulator of chondrocyte proliferation, differentiation, and extracellular matrix production. Dysregulation of TGF-beta action has been implicated in cartilage diseases such as osteoarthritis. TGF-beta signaling is transduced through a pair of transmembrane serine/threonine kinases, known as the type I (ALK5) and type II receptors. However, recent studies on endothelial cells have identified ALK1 as a second type I TGF-beta receptor and have shown that ALK1 and ALK5 have opposing functions in these cells. Here we examined ALK1 expression and its regulation of TGF-beta signaling and responses in human chondrocytes. MATERIALS AND METHODS: ALK1 expression in human chondrocytes was examined by RT-PCR and Western blot. The ability of ALK1 to form complexes with other TGF-beta receptors was determined by affinity labeling/immunoprecipitation and by immunoprecipitation followed by Western blot. The effect of ALK1 on TGF-beta1-induced signaling and responses was determined by varying ALK1 expression levels and measuring transcriptional activity using promoter/ luciferase assays, Smad1/5 and Smad3 phosphorylation, and expression of type II collagen, PAI-1, and fibronectin. RESULTS: Our results indicate that ALK1 is expressed in human chondrocytes and that it is a component of the TGF-beta receptor system, associating with ALK5, type II TGF-beta receptor, endoglin, and betaglycan. Furthermore, we show that both ALK1 and ALK5 are needed for TGF-beta-induced phosphorylation of intracellular mediators Smad1/5, whereas only ALK5 is essential for TGF-beta1-induced phosphorylation of Smad3. In addition, our results show that ALK1 inhibits, whereas ALK5 potentiates, TGF-beta-induced Smad3-driven transcriptional activity and the expression of PAI-1, fibronectin, and type II collagen in chondrocytes. CONCLUSIONS: Our results suggest that ALK1 and ALK5 display opposing functions in human chondrocytes, implicating an essential role for ALK1 in the regulation of TGF-beta signaling and function in these cells.
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Authors | Kenneth W Finnson, Wendy L Parker, Peter ten Dijke, Midory Thorikay, Anie Philip |
Journal | Journal of bone and mineral research : the official journal of the American Society for Bone and Mineral Research
(J Bone Miner Res)
Vol. 23
Issue 6
Pg. 896-906
(Jun 2008)
ISSN: 1523-4681 [Electronic] United States |
PMID | 18333754
(Publication Type: Journal Article, Research Support, Non-U.S. Gov't)
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Chemical References |
- Receptors, Transforming Growth Factor beta
- SMAD1 protein, human
- SMAD3 protein, human
- SMAD5 protein, human
- Smad1 Protein
- Smad3 Protein
- Smad5 Protein
- Transforming Growth Factor beta1
- Protein Serine-Threonine Kinases
- ACVRL1 protein, human
- Activin Receptors, Type II
- Receptor, Transforming Growth Factor-beta Type I
- TGFBR1 protein, human
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Topics |
- Activin Receptors, Type II
(genetics, metabolism)
- Aged
- Cells, Cultured
- Chondrocytes
(drug effects, metabolism)
- Extracellular Matrix
(metabolism)
- Gene Expression Regulation
(drug effects)
- Humans
- Male
- Middle Aged
- Phosphorylation
(drug effects)
- Protein Serine-Threonine Kinases
(genetics, metabolism)
- Receptor, Transforming Growth Factor-beta Type I
- Receptors, Transforming Growth Factor beta
(genetics, metabolism)
- Signal Transduction
- Smad1 Protein
(metabolism)
- Smad3 Protein
(metabolism)
- Smad5 Protein
(metabolism)
- Transcription, Genetic
(drug effects, genetics)
- Transforming Growth Factor beta1
(pharmacology)
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