The tumor microenvironment: regulation by MMP-independent effects of tissue inhibitor of metalloproteinases-2

Abstract

Proteolytic remodeling of the extracellular matrix is an important component of disease progression in many chronic disease states and is the initiating event in the formation of the tumor microenvironment in cancer. It is the balance of extracellular matrix degrading enzymes, the matrix metalloproteinases (MMPs) and their endogenous inhibitors that determine the extent of tissue remodeling. Unchecked MMP activity can result in significant tissue damage, facilitate disease progression and is associated with host responses to pathologic injury such as angiogenesis and inflammation. The tissue inhibitors of metalloproteinases (TIMPs) have been shown to regulate MMP activity. However, recent findings demonstrate that the tissue inhibitor of metalloproteinases-2 (TIMP-2) inhibits the mitogenic response of human microvascular endothelial cells to growth factors, such as VEGF-A and FGF-2 in vitro and angiogenesis in vivo. The mechanism of this effect is independent of metalloproteinase inhibition. Our lab is the first to demonstrate a cell-surface signaling receptor for a member of the TIMP family and suggest that TIMP-2 functions to regulate cellular responses to growth factors. These new findings are discussed in terms of a model of TIMP-2 regulation of cellular functions in the tumor microenvironment.

Fig. 1

Multiple pathways of TIMP-2/α3β1 signaling. TIMP-2 binding to α3β1 initiates receptor tyrosine kinase inactivation via the action of the protein tyrosine phosphatase activity. Cell cycle arrest is mediated by de novo synthesis of p27^Kip1, that down-regulate the cyclin-dependent kinases 4 and 2, resulting in hypophosphorylation of pRb and cell cycle arrest in G1. TIMP-2 also mediates activation of the small G protein Rap1 via a mechanism involving altered association of paxillin scaffolding proteins, guanidine exchange factors and ultimately results in enhanced expression of RECK. This suggests that in addition to arresting cellular proliferation, TIMP-2 also seems to promote expression of cellular differentiation markers

Fig. 2

TIMP-2 controls cell behavior directly through α3β1 integrin receptors and indirectly by modulating the activity of MMPs. In physiological quiescent states high levels of free TIMP-2 promotes cellular quiescence and maintenance of the differentiated state. This occurs independently of MMP inhibitory action. As local concentrations of angiogenic factors increase, endothelial cells respond by increasing MMP production and limiting TIMP-2 expression. Increasing concentrations of activated MMPs acts as a sink to reduce free TIMP-2 concentrations, limiting interaction of TIMP-2 with α3β1, thus reducing the growth inhibitory effects. As active MMP concentrations continue to increase, TIMP-2 concentrations may be insufficient to completely inhibit MMP activity. At low concentrations TIMP-2 is insufficient to inhibit MMP activity, and actually enhances MMP-2 activation (via MT1-MMP-dependent mechanism) resulting in remodeling of extracellular matrix, facilitating angio-invasion