Endoglin differentially regulates TGF-β-induced Smad2/3 and Smad1/5 signalling and its expression correlates with extracellular matrix production and cellular differentiation state in human chondrocytes.
OBJECTIVE
Transforming growth factor-β (TGF-β) plays a critical role in cartilage homeostasis and deregulation of its signalling is implicated in osteoarthritis (OA). TGF-β isoforms signal through a pair of transmembrane serine/threonine kinases known as the type I and type II TGF-β receptors. Endoglin is a TGF-β co-receptor that binds TGF-β with high affinity in the presence of the type II TGF-β receptor. We have previously shown that endoglin is expressed in human chondrocytes and that it forms a complex with the TGF-β signalling receptors. However, the functional significance of endoglin expression in chondrocytes is unknown. Our objective was to determine whether endoglin regulates TGF-β/Smad signalling and extracellular matrix (ECM) production in human chondrocytes and whether its expression varies with chondrocyte differentiation state.


METHODS
Endoglin function was determined by overexpression or antisense morpholino/siRNA knockdown of endoglin in human chondrocytes and measuring TGF-β-induced Smad phosphorylation, transcriptional activity and ECM production. Alterations in endoglin expression levels were determined during subculture-induced dedifferentiation of human chondrocytes and in normal vs OA cartilage samples.


RESULTS
Endoglin enhances TGF-β1-induced Smad1/5 phosphorylation and inhibits TGF-β1-induced Smad2 phosphorylation, Smad3-driven transcriptional activity and ECM production in human chondrocytes. In addition, the enhancing effect of endoglin siRNA knockdown on TGF-β1-induced Smad3-driven transcription is reversed by ALK1 overexpression. Furthermore, endoglin levels are increased in chondrocytes following subculture-induced dedifferentiation and in OA cartilage as compared to normal cartilage.


CONCLUSIONS
Together, our results suggest that endoglin regulates the balance between TGF-β/ALK1/Smad1/5 and ALK5/Smad2/3 signalling and ECM production in human chondrocytes and that endoglin may represent a marker for chondrocyte phenotype.
Finnson
K W
KW
Division of Plastic Surgery, Department of Surgery, McGill University, Montreal, Quebec, Canada.
Parker
W L
WL
Chi
Y
Y
Hoemann
C D
CD
Goldring
M B
MB
Antoniou
J
J
Philip
A
A
D015703:Antigens, CD / Q000378:metabolism* / Q000494:pharmacology*; D015153:Blotting, Western; D002358:Cartilage, Articular / Q000378:metabolism; D002454:Cell Differentiation / Q000187:drug effects; D002478:Cells, Cultured; D019902:Chondrocytes / Q000166:cytology / Q000378:metabolism*; D000071063:Endoglin; D005109:Extracellular Matrix / Q000378:metabolism*; D005786:Gene Expression Regulation; D006801:Humans; D007150:Immunohistochemistry; D010003:Osteoarthritis / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D011956:Receptors, Cell Surface / Q000378:metabolism*; D051896:Smad Proteins, Receptor-Regulated / Q000378:metabolism*; D016212:Transforming Growth Factor beta / Q000494:pharmacology*
D015703:Antigens, CD; C064206:ENG protein, human; D000071063:Endoglin; D011956:Receptors, Cell Surface; D051896:Smad Proteins, Receptor-Regulated; D016212:Transforming Growth Factor beta