Integrin expression by primary and immortalized human chondrocytes: evidence of a differential role for alpha 1 beta 1 and alpha 2 beta 1 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 functio n is poorly understood. The objective of this study was to determine how th e relative expression of different integrins may be modulated in relation t o the differentiated state and proliferative capacity of the chondrocyte. Design: Integrin expression by four different cell lines of human chondrocy tes immortalized with Simian virus 40 large T-antigen (SV40-TAg) was studie d and compared to primary chondrocytes. Differences in alpha 1 and alpha 2 integrin subunit expression were utilized to further study the role of thes e integrins in mediating adhesion to types II and VI collagen. Results: The overall cell-surface levels of pi-containing integrins were hi gher on all four immortalized cell lines which expressed over 10-fold highe r levels of alpha 2 and alpha 3 integrin subunits compared to primary cells . However, primary cells expressed higher levels of the alpha 1 integrin su bunit which was not expressed by T/C28a4 cells and expressed at variable an d lower levels in the other lines. Levels of the a3 integrin subunit were s ignificantly greater on the highly proliferative juvenile costal chondrocyt e lines (T/C-28a4, C-2812, and C-20a4) compared to primary articular chondr ocytes and tsT/AC-62 cells which were derived from adult articular chondroc ytes. 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 significa nt 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 type s 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% r espectively. Conclusions: Immortalization with SV40-TAg results in altered integrin expr ession by chondrocytes. Changes in the relative levels of alpha 1, alpha 2, and alpha 3 subunits may significantly alter the manner in which chondrocy tes interact with types II and VI collagen in the extracellular matrix. (C) 2000 OsteoArthritis Research Society International.