Plasminogen activator-dependent pericellular proteolysis

Hydrocortisone regulates the dynamics of plasminogen activator and plasminogen activator inhibitor expression in cultured murine keratinocytes

The plasminogen activators tPA and uPA, and their inhibitors, PAI-1 and PAI-2, have been associated with epithelial homeostasis and wound healing. In these studies, we investigate the effect of the steroid hormone hydrocortisone, a commonly used therapeutic modality for skin, on PAs/PAIs in serum- and plasminogen-free primary cultures of murine keratinocytes. SDS-PAGE fibrin zymography showed that addition of 1 microM hydrocortisone to cultures significantly reduced tPA fibrinolytic activity in both cell extracts and conditioned medium. uPA activity in conditioned medium was similarly inhibited. Cells were also cultured in the presence of dibutyryl cyclic AMP (dbcAMP). dbcAMP (5 mM) alone enhanced uPA and tPA fibrinolytic activity in conditioned medium, but this increase was diminished in the presence of 1 microM hydrocortisone. Immunoblots revealed a three- to fivefold induction of free PAI-1 by hydrocortisone which was partially blocked by dbcAMP. Northern blots showed that PAI-1 mRNA increased threefold 2 h after addition of hydrocortisone and remained elevated at least 8 h. In contrast, uPA and tPA mRNA were unchanged over the same time course. uPA, tPA, and PAI-1 mRNA increased in the presence of dbcAMP; levels remained elevated at least 8 h. HC suppressed the induction of uPA and tPA by dbcAMP. Studies directed at identifying plasminogen mRNA showed that in this culture system, keratinocytes produce no plasminogen mRNA either in the presence or in the absence of hydrocortisone or dbcAMP. Collectively, these results show that keratinocyte plasminogen activator activity is suppressed by hydrocortisone as a function of increased PAI-1 combined with an attenuation of PA induction by agents that increase intracellular cAMP. These results provide additional information to further define the mechanism by which glucocorticoids inhibit wound healing.

Cholesterol feeding accentuates the cyclosporine-induced elevation of renal plasminogen activator inhibitor type 1

Long-term cyclosporine (CsA) therapy is accompanied by the occurrence of hypercholesterolemia and renal interstitial fibrosis. The present study investigates the effect of dietary cholesterol on CsA-induced lipid disturbances in the rat and on CsA nephrotoxicity. Since plasminogen activator inhibitor type 1 (PAI-1) is a major inhibitor of matrix degradation and elevated plasma PAI-1 levels are reported to be associated with increased low-density lipoprotein (LDL) cholesterol, PAI-1 was examined in the kidneys of rats fed a sodium-deficient diet, with or without cholesterol. After nine weeks, both diet groups were subdivided into a CsA-treated group and a vehicle-treated group. Although cholesterol feeding significantly aggravated CsA-induced renal function impairment, CsA-induced histological lesions were comparable in both diet groups. Cholesterol feeding significantly decreased high-density lipoprotein (HDL) cholesterol irrespective of the treatment, while CsA treatment significantly elevated serum triglycerides irrespective of the diet. Cholesterol feeding alone did not increase the number of infiltrating cells in the renal interstitium. In contrast, in both diet groups CsA treatment caused a significant influx of macrophages, while combined treatment with CsA and cholesterol additionally elevated the number of T-helper cells in the cortex. In all rats, PAI-1 immunostaining was found mainly in intracellular vesicles (lysosomes) in proximal tubules, which stained most intensely in fibrotic areas of kidneys from CsA-treated rats. Cholesterol feeding enhanced the CsA-induced elevation of renal PAI-1 immunostaining to a significant level. These results show that, although serum creatinine, PAI-1 staining and T cell influx were significantly increased in the cholesterol-fed CsA-treated group compared to the other groups, renal CsA-induced histological lesions were not influenced by cholesterol feeding after short-term (3 weeks) CsA administration. To what extent the more pronounced proximal tubular PAI-1 (inhibitor of matrix degradation) immunostaining in fibrotic areas in the cortex of cholesterol-fed CsA-treated rats contributes to the progression of CsA-induced renal fibrosis remains to be determined.

Regulation of vascular smooth muscle cell migration and proliferation in vitro and in injured rat arteries by a synthetic matrix metalloproteinase inhibitor

Smooth muscle cell (SMC) migration and proliferation and extracellular matrix remodeling are essential aspects of the arterial response to injury, vessel development, and atherogenesis. Matrix metalloproteinase (MMP) expression is associated with SMC proliferation and migration after arterial injury. To assess the role of MMPs in SMC proliferation and migration and intimal thickening, we measured the effect of the synthetic MMP inhibitor BB94 (Batimastat) on DNA synthesis and migration of SMCs in vitro as well as the formation of a neointima after balloon injury to the rat carotid artery. BB94 dose-dependently inhibited SMC migration induced by platelet-derived growth factor (PDGF)-BB through a filter coated with a thick basement membrane matrix (Matrigel) layer but did not show any inhibitory effect on SMC migration through a lightly coated filter. At concentrations up to 1 mumol/L, BB94 did not alter DNA synthesis induced by PDGF-AA or PDGF-BB. Treatment with 30 mg BB94.kg-1.d-1 IP for 7 or 14 days after balloon injury to the rat carotid artery decreased the total number of intimal SMC nuclei and suppressed intimal thickening. SMC proliferation (5-bromo-2'-deoxyuridine labeling) was decreased in the media at 2 days, whereas it was increased in the intima at 7 but not 14 days. These results suggest that BB94 inhibits intimal thickening after arterial injury by decreasing SMC migration and proliferation and support the conclusion that MMPs play a significant role in regulating intimal thickening in injured arteries.

Novel thrombolytic agents

The fibrinolytic system comprises an inactive pro-enzyme, plasminogen, that is converted by plasminogen activators to the active enzyme, plasmin, that degrades fibrin. Two immunologically distinct plasminogen activators have been identified: tissue-type plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA). Plasminogen activation is regulated by specific molecular interactions between its main components, as well as by controlled synthesis and release of plasminogen activator inhibitors, primarily from endothelial cells. The observed association between abnormal fibrinolysis and a tendency toward bleeding or thrombosis demonstrates the (patho)physiological importance of the fibrinolytic system. Transgenic animals are a suitable experimental model to examine the in vivo impact of fibrinolytic components in thrombosis and thrombolysis. Inactivation, by homologous recombination, of the tissue-type plasminogen activator genes in mice impairs thrombolysis in a significant manner whereas inactivation of the plasminogen activator-1 gene enhances the rate of spontaneous lysis. Despite their widespread use all currently available thrombolytic agents suffer from a number of significant limitations, including resistance to reperfusion, the occurrence of acute coronary reocclusion and bleeding complications. Therefore, the quest for thrombolytic agents with a higher thrombolytic potency, specific thrombolytic activity and/or a better fibrin-selectivity continues. Several lines of research toward improvement of thrombolytic agents are being explored, including the construction of mutants and variants of plasminogen activators, chimeric plasminogen activators, conjugates of plasminogen activators with monoclonal antibodies, or plasminogen activators from animal or bacterial origin.

Overview on fibrinolysis: plasminogen activation pathways on fibrin and cell surfaces

Plasminogen activation at the surface of fibrin or of cell membranes is a sophisticated specialized system for localized extracellular proteolysis implicated in a large variety of biological functions (fibrinolysis, cell migration and extracellular matrix degradation). Assembly of plasminogen and/or activators at specific binding sites induces conformational changes that make accessible the scissile peptide bond of plasminogen and exposes the active centre of the tissue-type plasminogen activator. The mechanism of activation by pro-urokinase, a second type of activator that binds to cell membrane but not to fibrin, is far from being understood. It may be able, however, in contrast to urokinase, to specifically activate plasminogen bound to partially degraded fibrin. An extremely low Km and high catalytic rate are characteristic of the process of activation at surfaces. In contrast, activation in liquid phase by tissue-type plasminogen activator proceeds at an extremely low catalytic rate. The initiation and amplification of plasminogen activation depend on specific interactions between the modular constitutive units of these proteins and binding sites present on cell or fibrin surfaces. Thus, the most important mechanism for the acceleration of fibrinolysis and pericellular proteolysis is the unveiling of carboxy-terminal lysine residues on these surfaces, to which plasminogen may bind. Since plasminogen bound to carboxy-terminal lysines of progressively degraded fibrin or membranes is readily transformed into plasmin by fibrin-bound t-PA, this mechanism represents the most important pathway for the acceleration and amplification of fibrinolysis. Alpha-2-antiplasmin, by inhibiting plasmin release from surfaces, regulates the extent and rate of this process but has no effect on fibrin-bound or membrane-bound plasmin. Lipoprotein(a), a particle possessing a plasminogen-like apolipoprotein, apo(a), may interfere with this mechanism by inhibiting the specific binding of plasminogen to lysine residues in membrane or fibrin surfaces.

Proteolysis and invasiveness of brain tumors: role of urokinase-type plasminogen activator receptor

The cellular receptor for urokinase-type plasminogen activator (uPAR) in glioblastoma cell lines has been identified and found to be similar to the uPAR expressed by other tumor cell lines. Increased levels of uPAR have been found in primary malignant brain tumor tissues, especially highly malignant glioblastoma, and, to a lesser degree, in malignant astrocytomas, suggesting that this receptor might be involved in efficient activation of pro-uPA and confinement of uPA activity on the cell surface of invading brain tumors. The cell surface uPARs in gliomas could constitute an optimum environment for the generation and activity of plasmin, which is known to play a crucial role in the dissolution of the extracellular matrix during tumor cell invasion. In situ hybridization studies have shown that uPAR mRNA is expressed abundantly in tumor cells and is consistently present at the invasive edges of malignant gliomas. These results imply that uPAR is involved in plasmin-catalyzed proteolysis during glioma invasion and that interference with the uPA:uPAR interactions could constitute a novel approach for developing therapeutic strategies to counteract invasion of brain tumors.

Membrane-associated procollagenase of leukemic cells is activated by urokinase-type plasminogen activator

Murine leukemic cells from the WEHI-3B line, present in the cell surface a latent collagenase activity which is activated proteolytically. In this paper we show that this enzyme is activated by plasmin generated by the activity of a urokinase-like plasminogen activator (u-PA) also present in the surface of these cells. Using a reverse fibrin autography method we found that u-PA is the major proteolytic activity present in the cell membranes. This fact suggests that u-PA could represent a normal activating system for this collagenase.

Reduced tissue type plasminogen activator activity of the gastroduodenal mucosa in peptic ulcer disease

The gastroduodenal mucosa has a rich blood supply. An active fibrinolytic system is presumably required to maintain vascular patency, and impairment may result in reduced blood flow, focal tissue necrosis, and peptic ulcerogenesis. Tissue type and urokinase type plasminogen activator activity (expressed as mIU/mg protein) and plasminogen activator inhibitor type-1 antigen were assayed in homogenates of gastric and duodenal biopsy specimens taken from patients with: normal endoscopy (controls) (n = 14); active duodenal ulcer (n = 21); healed duodenal ulcer (n = 12); and active benign gastric ulcer (n = 15). In controls mean duodenal tissue type plasminogen activator activity was 4110 and urokinase type plasminogen activator activity 150; gastric tissue type plasminogen activator was 2760 and urokinase type plasminogen activator 170; plasminogen activator inhibitor type-1 was generally undetectable. At the edge of active duodenal ulcers tissue type plasminogen activator was considerably reduced, 2220 (p < 0.001) whereas urokinase type plasminogen activator was raised, 290 (p < 0.01). At the edge of active benign gastric ulcers tissue type plasminogen activator was substantially reduced, 1160 (p < 0.001) but urokinase type plasminogen activator was unchanged. At the scar of healed duodenal ulcers tissue type plasminogen activator was slightly reduced, 3290, but urokinase type plasminogen activator was increased, 308 (p < 0.05). H2 receptor antagonist treatment had little effect on tissue type or urokinase type plasminogen activator activity. Plasminogen activator inhibitor type-1 was increased at the edge of active ulcers (p < 0.05) especially when tissue type plasminogen activity was low (r = -0.61, p < 0.05). These findings are consistent with the hypothesis that impaired fibrinolytic activity may be implicated in peptic ulcerogenesis.