Expression and function of TβRII-B, a variant of the type II TGF-β receptor, in human chondrocytes 1

SummaryObjectiveTransforming growth factor-β (TGF-β) has profound effects on chondrocyte proliferation and matrix production, and dysregulation of TGF-β action has been implicated in osteoarthritis. The mechanisms by which the diverse actions of TGF-β are regulated in chondrocytes are unclear. Although it is well documented that TGF-β signaling is transduced by types I and II receptors, other TGF-β receptors may play critical roles by regulating signaling receptor activity. Our objective was to examine the expression of TβRII-B, a splice variant of the type II TGF-β receptor, and to analyze its role in regulating TGF-β signaling in human chondrocytes.MethodsTβRII-B expression was examined in human cartilage tissue specimens, human chondrocyte cell lines C28/I2 and tsT/AC62, and human primary chondrocytes by Western blot and reverse-transcriptase-polymerase chain reaction. Ligand binding and heteromerization of TβRII-B with other TGF-β receptors on the cell surface were analyzed by affinity labeling, immunoprecipitation, and two-dimensional SDS-PAGE. Regulation of TGF-β responses by TβRII-B was determined by examining Smad2 phosphorylation, Smad3-specific signaling, transcriptional activity, and type II collagen levels.ResultsTβRII-B is expressed in normal and osteoarthritic human cartilage. Furthermore, it is a dynamic component of the TGF-β receptor system in human chondrocytes, forming heteromeric complexes with the types I and II TGF-β receptors, betaglycan and endoglin. Importantly, overexpression of TβRII-B leads to enhanced TGF-β signaling and responses in chondrocytes.ConclusionsThese results suggest that TβRII-B may play a key role in the regulation of TGF-β action in human chondrocytes.