See other articles in this series on pages 1227–1230 and 1231–1236 in this issue. in amnionic membranes, fibrin clots, type I collagen, or basement membrane matrices. In these assays, endothelial cells acquire a migratory or invasive phenotype, reorganize the extracellular matrix, and in some cases recapitulate the tubular morphology of microvessels complete with lumen formation. In many early studies, the extracellular matrix was viewed as a barrier to endothelial cell invasion. The principal role of MMP activity was to remove this barrier and allow endothelial cell migration. Recent studies challenge this notion and suggest that cell–extracellular matrix interactions profoundly influence cell behavior. These interactions not only influence MMP production but are subject to modulation and regulation by MMP activity. In this way, MMP activity can directly and indirectly mediate the angiogenic response.

Treatment of endothelial cells with exogenous pro-MMP-2 induces a dose-dependent morphologic change consistent with an angiogenic response (ie, tube formation)(8) However, this effect reaches a plateau, and addition of further MMP-2 begins to reverse tube formation. These effects are dependent on MMP-2 activity and are inhibited by TIMP-2. This suggests that exogenous pro-MMP-2 is activated by the endothelial cells. But how do endothelial cells activate pro-MMP-2? Capillary endothelial cells cultured on twodimensional type I collagen gels produce low, constitutive levels of pro-MMP-2 with little endogenous activation of this protease. However, when placed in threedimensional type I collagen gels, there is a marked increase in pro-MMP-2 steady-state transcript levels (9). After three days in culture, the increase in MMP-2 secretion and activation is pronounced. This effect on MMP-2 expression and activation is coordinate with enhancement of MT-1-MMP transcription and expression. The current model for MT-1-MMP–mediated activation of pro-MMP-2 involves TIMP-2 as a receptor (Figure 1a and ref. 7). Binding of pro-MMP-2 to the MT-1-MMP/TIMP-2 complex localizes this pro-MMP-2 on the cell surface, and activation is initiated by the proteolytic action of a second, TIMP-2–free MT-1-MMP molecule at the Asn37-Leu38 bond of the MMP-2 propeptide (10). Binding of pro-MMP-2 to TIMP-2 is mediated by the COOH-terminal hemopexin-like domain (often referred to as PEX), present in most soluble MMPs. The structure of the PEX domain of MMPs consists of a four-bladed propeller composed of antiparallel β sheets. The TIMP-2 binding domain is localized by mutational analysis to the junction of modules III and IV of the MMP-2 PEX domain (11). The study by Haas et al.(9) presents correlative data that supports a direct role for MT-1-MMP in the activation of pro-MMP-2 produced by endothelial cells cultured in three-dimensional collagen I gel. Culture of these cells on or in reconstituted basement membrane did not enhance MMP-2 production or MT-1-MMP–mediated activation. This is an interesting contrast, as many view basement membrane as the first barrier that endothelial cells must cross to initiate an angiogenic response. Are other mechanisms of MMP activation operative in endothelial cells in contact with basement membrane and/or provisional matrix? When rat endothelial cells are cultured for longer periods