Integrin expression by primary and immortalized human chondrocytes:: evidence of a differential role for α1β1 and α2β1 integrins in mediating chondrocyte adhesion to types II and VI collagen

Objective: Chondrocytes have been shown to express pi-containing integrins both in vitro and in situ, but their role in regulating chondrocyte function is poorly understood. The objective of this study was to determine how the relative expression of different integrins may be modulated in relation to the differentiated state and proliferative capacity of the chondrocyte. Design: Integrin expression by four different cell lines of human chondrocytes immortalized with Simian virus 40 large T-antigen (SV40-TAg) was studied and compared to primary chondrocytes. Differences in alpha 1 and alpha 2 integrin subunit expression were utilized to further study the role of these integrins in mediating adhesion to types II and VI collagen. Results: The overall cell-surface levels of pi-containing integrins were higher on all four immortalized cell lines which expressed over 10-fold higher levels of alpha 2 and alpha 3 integrin subunits compared to primary cells. However, primary cells expressed higher levels of the alpha 1 integrin subunit which was not expressed by T/C28a4 cells and expressed at variable and lower levels in the other lines. Levels of the a3 integrin subunit were significantly greater on the highly proliferative juvenile costal chondrocyte lines (T/C-28a4, C-2812, and C-20a4) compared to primary articular chondrocytes and tsT/AC-62 cells which were derived from adult articular chondrocytes. Expression of alpha 5 was similar among primary cells and cell lines except on C-20/A4 cells which had an average of over 4-fold higher levels. None of the primary or immortalized chondrocytes tested expressed significant levels of alpha 4. Cell adhesion assays revealed that both alpha 1 beta 1 and alpha 2 beta 1 could serve as chondrocyte adhesion receptors for types II and VI collagen. In cell lines expressing both integrins, alpha 1 beta 1 was the preferential receptor for type VI collagen while alpha 2 beta 1 was the preferential receptor for type II collagen. Rather than inhibiting adhesion, Incubation with the a3 blocking antibody P1B5 increased adhesion of C-28/12 cells to both fibronectin and type II collagen by 67% and 100% respectively. Conclusions: Immortalization with SV40-TAg results in altered integrin expression by chondrocytes. Changes in the relative levels of alpha 1, alpha 2, and alpha 3 subunits may significantly alter the manner in which chondrocytes interact with types II and VI collagen in the extracellular matrix. (C) 2000 OsteoArthritis Research Society International.