ALK1 opposes ALK5/Smad3 signaling and expression of extracellular matrix components in human chondrocytes.

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.

• 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)

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

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)