Transforming Growth Factor- Regulates Collagen Gel Contraction by Increasing 21 Integrin Expression in Osteogenic Cells (*)

Abstract

The contraction of floating collagen gels is suggested to mimic the reorganization of collagenous matrix during development and tissue healing. Here, we have studied two osteogenic cell lines, namely MG-63 and HOS, and a chemically transformed subclone of HOS cells, HOS-MNNG. Transforming growth factor-β (TGF-β), a putative regulator of bone fracture healing, increased collagen gel contraction by MG-63 and HOS-MNNG, but not by HOS cells. Our data show that TGF-β-induced fibronectin synthesis is not sufficient for the process. Instead, anti-β1 integrin antibodies could prevent the contraction. There are three different integrin heterodimers that are known to mediate the cell-collagen interaction, namely α1β1, α2β1, and α3β1. In MG-63 cells TGF-β increased the expression of α2β1 integrin and decreased the expression of α3β1 integrin, whereas α1β1 integrin is not expressed. HOS cells had no α2β1 integrin, neither did TGF-β induce its expression. However, HOS-MNNG cells expressed more α2β1 integrin when treated with TGF-β. Thus, we suggest that the mechanism of the enhanced collagen gel contraction by TGF-β is the increased expression of α2β1 integrin heterodimer. To further test this hypothesis, we expressed a full-length α2 integrin cDNA in HOS cells and in MG-63 cells. We obtained HOS cell clones that expressed α2β1 heterodimer, and the ability of these cells to contract collagen gels was greatly enhanced. Furthermore, the contraction by MG-63 cells transfected with α2 integrin cDNA was enhanced, and the contraction by cells transfected with antisense oriented α2 integrin cDNA was decreased. Thus, both in MG-63 and HOS cells the increased α2 integrin expression alone was sufficient for the enhanced contraction of collagen gels. Furthermore, the amount of α2 integrin is critical for the process, and its decrease leads to diminished ability to contract gels.