The molecular mechanisms predisposing to atherosclerotic aneurysm formation remain undefined. Nevertheless, rupture of aortic aneurysms is a major cause of death in Western societies, with few available treatments and poor long-term prognosis. Indirect evidence suggests that matrix metalloproteinases (MMPs) and plasminogen activators (PAs) are involved in its pathogenesis. MMPs are secreted as inactive zymogens (pro-MMPs), requiring activation in the extracellular compartment. Plasmin, generated from the zymogen plasminogen by tissue-type plasminogen activator (t-PA) or urokinase-type plasminogen activator (u-PA; refs 14,15), has been proposed as a possible activator in vitro, but evidence for such a role in vivo is lacking. Analysis of atherosclerotic aorta in mice with a deficiency of apoliprotein E (Apoe-/-; ref. 18), singly or combined with a deficiency of t-PA (Apoe-/-:Plat-/-) or of u-PA (Apoe-/-:Plau-/-; ref. 19), indicated that deficiency of u-PA protected against media destruction and aneurysm formation, probably by means of reduced plasmin-dependent activation of pro-MMPs. This genetic evidence suggests that plasmin is a pathophysiologically significant activator of pro-MMPs in vivo and may have implications for the design of therapeutic strategies to prevent aortic-wall destruction by controlling Plau gene function.

Explants of human endometrium were cultured to study the release of matrix metalloproteinases (MMPs). Analysis of conditioned media by zymography revealed latent and active forms of co ,genase (MMP-1, EC 3.4.24.7), 72-kDa gelatinase A (MMP-2, EC 3.4.24.24), and 92-kDa gelatinase B EC 3.4.24.35). These proteinases were identified by their Mr, their inhibition by tissue inhibitor of metalloproteinases, and the activation of their zymogens by trypsin or aminophenylmer acetate. In the absence of sex hormone, explants released large a ts of enzyme activities, as measured by densitometry of zymogra or in soluble assays. Physiological concentrations of progesterone (10-200 nM) almost totally abolished the release of coilagenase, of total gelatinase activity, and of the active form of gelatinase B and largely inhibited the release of the active form of gelatinase A. These effects, which were antgoz by mifepristone (RU 38486), suggest that progesterone restrains endometrial tissue breakdown by blocking the secretion and activation of MMPs.The stroma of the human endometrium shows striking structural changes, especially during the second half of the menstrual cycle (1). If no pregnancy develops, interstitial shrinkage due to extracellular-matrix breakdown occurs and leads to hemorrhagia and mucosal shedding. The mechanisms of endometrial-tissue breakdown initiated by the fall of plasma progesterone and estradiol are still obscure. A major role of lysosomal enzymes (2) is not supported by biochemical evidence (3), whereas matrix metalloproteinases (MMPs), a family of neutral nonlysosomal enzymes, appear to be plausible agents of the remodeling of the extracellular matrix of the human endometrium (4).MMPs share similar structural domains and properties (for review, see ref. 5), are secreted as zymogens in the extracellular space, are activated by proteolytic cleavage, and are inhibited by the tissue inhibitors of metalloproteinases (TIMPs). A key feature in the regulation of extracellularmatrix degradation seems to be a fine local balance between MMPs, their activators, and their inhibitors. Interstitial collagenase (MMP-1, EC 3.4.24.7) specifically cleaves the fibrillar collagens into one-fourth and three-fourth fragments, which are susceptible to further degradation by other proteases, among which are two other MMPs, gelatinase A (MMP-2, 72 and 65 kDa, EC 3.4.24.24) and gelatinase B (MMP-9, 92 and 84 kDa, EC 3.4.24.35). Both enzymes are also able to degrade other substrates present in human endometrium (1), such as collagen types V and IV. A third subclass of MMPs has a broad spectrum of substrates and comprises stromelysin 1 (MMP-3, EC 3.4.24.17) and matrilysin EC 3.4.24.23).We have studied the possible involvement of MMPs in human endometrial remodeling, using a culture system that maintains the tissue in a similar microenvironment as in situ and responds to sex hormones for several days (3). This system allowed us to demonstrate that the human endometrium secretes collagenase and gelatinases A and B. The secr...

1. Cathepsin B, a tissue (lysosomal) proteinase, and two humoral proteinases, plasmin and kallikrein, activate the latent collagenase ('procollagenase') which is released by mouse bone explants in culture. Other lysosomal proteinases (carboxypeptidase B, cathepsin C and D) and thrombin did not activate the procollagenase. Dialysis of the culture fluids against 3M-NaSCN at 4 degrees C and, for some culture fluids, prolonged preincubation at 25 degrees C also caused the activation of procollagenase. 2. In all these cases, activation of procollagenase involved at least two successive steps: the activation of an endogenous latent activator present in the culture fluids and the activation of procollagenase itself. 3. An assay method was developed for the endogenous activator. Human serum, bovine serum albumin, casein and cysteine inhibited the endogenous activator at concentrations that did not influence the collagenase activity. N-Ethylmaleimide and 4-hydroxy-mercuribenzoate stimulated the endogenous activator, but iodoacetate had no effect. 4. It is proposed that cathepsin B, kallikrein and plasmin may play a role in the physiological activation of latent collagenase and thus initiate degradation of collagen in vivo. This may occur whatever the molecular nature of procollagenase (zymogen or enzyme-inhibitor complex) might be.

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