Glioma Cell Invasion: Regulation of Metalloproteinase Activity by TGF-β

Matrix metalloproteinases (MMPs) are a family of extracellular endopeptidases that selectively degrade components of the extracellular matrix. MMPs are implicated in tumor cell invasion because they mediate the breakdown of the basal membrane. In addition, they seem to be important for the creation and maintenance of a microenvironment that facilitates tumor cell survival. Among the essential characteristics of human malignant gliomas are infiltrative growth, angiogenesis and suppression of antitumor immune surveillance. Transforming growth factor-beta (TGF-β) is intimately involved in the regulation of these processes. We have previously demonstrated that TGF-β promotes the migration of LN-18 and LN-229 glioma cells via a process that may involve the upregulation of αVβ3 integrin expression. Furthermore, we have defined a novel pathway for hepatocyte growth factor (HGF)-induced glioma cell migration and invasion which requires the induction of TGF-β2 expression. Here, we demonstrate that TGF-β2 induces MMP-2 expression and suppresses tissue inhibitor of metalloproteinases (TIMP)-2 expression and that concentration-dependently promotes the invasion of U87MG and LN-229 glioma cells in a matrigel invasion assay. Similarly, ectopic expression of the anti-apoptotic BCL-xL protein leads to enhanced matrigel invasion by LN-18 and LN-229 glioma cells. We outline the possible interrelations of TGF-β, proteins of the BCL-2 family, integrins and metalloprotease activity. By virtue of its promotion of glioma invasion and its growth regulatory and immunomodulatory properties, TGF-β continues to be one of the most promising targets for the experimental therapy of human malignant glioma.