The extracellular matrix protein tenascin is secreted by osteoblasts but absent from mineralized bone matrix. The current study was undertaken to test the hypothesis that tenascin regulates osteoblast behaviour. Three osteoblast-like cell lines UMR-106, ROS-17/2.8 (rat) and SAOS-2 (human) were used to investigate the role of tenascin in osteoblast morphology, differentiation and proliferation. Two of three cell lines adhered specifically to tenascin, remaining round and failing to spread. Tenascin as a substratum stimulated alkaline phosphatase activity (a marker of osteoblast differentiation) in two of three cell lines. Moreover, anti-tenascin in the medium caused a reduction in alkaline phosphatase levels in all three cell lines. Anti-tenascin also inhibited collagen synthesis, an important osteoblast function. Since it seemed possible that tenascin may exert its effects on cell function through its ability to cause cell rounding, the ability of cell shape change alone to influence alkaline phosphatase levels was investigated. Cells were incubated in the presence of cytochalasin D and alkaline phosphatase levels assayed. Alkaline phosphatase activity was not elevated by cytochalasin D treatment, indicating that cell rounding alone is insufficient to mimic the effect of tenascin. Anti-tenascin caused a slight increase in proliferation of SAOS-2 cells, indicating that tenascin is itself inhibitory. In ROS 17/2.8 and UMR-106 cells, in contrast, proliferation was inhibited by anti-tenascin. The results presented here indicate that tenascin is able to stimulate osteoblastic differentiation and that endogenous tenascin helps to maintain the functional state of cultured osteoblast-like cells.

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