Integrin α2β1 Is a Positive Regulator of Collagenase (MMP-1) and Collagen α1(I) Gene Expression

Abstract

A classical model for studying the effects of extracellular matrix is to culture cells inside a three-dimensional collagen gel. When surrounded by fibrillar collagen, many cell types decrease the production of type I collagen, and the expression of interstitial collagenase (matrix metalloproteinase-1; MMP-1) is simultaneously induced. To study the role of the collagen-binding integrins α1β1 and α2β1 in this process, we used three different osteogenic cell lines with distinct patterns of putative collagen receptors: HOS cells, which express only α1β1 integrin, MG-63 cells, which express only α2β1 integrin, and KHOS-240 cells, which express both. Inside collagen gels, α1(I) collagen mRNA levels were decreased in HOS and KHOS-240 cells but not in MG-63 cells. In contrast, MMP-1 expression was induced in KHOS-240 and MG-63 cells but not in HOS cells. Transfection of MG-63 cells with α2 integrin cDNA produced cell clones overexpressing α2β1 integrin. Transfection of MG-63 cells with α2 integrin cDNA in an antisense orientation reduced the expression level of α2 integrin. These cell clones showed induction and reduction of mRNA levels for MMP-1, respectively. HOS cells normally lacking α2β1 integrin were forced to express it, and this prevented the down-regulation in the levels of α1(I) collagen mRNA when cells were grown inside collagen gels. The data indicate that the level of MMP-1 expression is regulated by the collagen receptor α2β1 integrin. The down-regulation of collagen α1(I) is mediated by another receptor. Integrin α2β1 may compete with it and thus be a positive regulator of collagen synthesis. A classical model for studying the effects of extracellular matrix is to culture cells inside a three-dimensional collagen gel. When surrounded by fibrillar collagen, many cell types decrease the production of type I collagen, and the expression of interstitial collagenase (matrix metalloproteinase-1; MMP-1) is simultaneously induced. To study the role of the collagen-binding integrins α1β1 and α2β1 in this process, we used three different osteogenic cell lines with distinct patterns of putative collagen receptors: HOS cells, which express only α1β1 integrin, MG-63 cells, which express only α2β1 integrin, and KHOS-240 cells, which express both. Inside collagen gels, α1(I) collagen mRNA levels were decreased in HOS and KHOS-240 cells but not in MG-63 cells. In contrast, MMP-1 expression was induced in KHOS-240 and MG-63 cells but not in HOS cells. Transfection of MG-63 cells with α2 integrin cDNA produced cell clones overexpressing α2β1 integrin. Transfection of MG-63 cells with α2 integrin cDNA in an antisense orientation reduced the expression level of α2 integrin. These cell clones showed induction and reduction of mRNA levels for MMP-1, respectively. HOS cells normally lacking α2β1 integrin were forced to express it, and this prevented the down-regulation in the levels of α1(I) collagen mRNA when cells were grown inside collagen gels. The data indicate that the level of MMP-1 expression is regulated by the collagen receptor α2β1 integrin. The down-regulation of collagen α1(I) is mediated by another receptor. Integrin α2β1 may compete with it and thus be a positive regulator of collagen synthesis.