An assay system for the modulators of plasminogen activation on the cell surface

Discovery and characterisation of an antibody that selectively modulates the inhibitory activity of plasminogen activator inhibitor-1

Plasminogen activator inhibitor-1 (PAI-1) is a serine protease inhibitor (serpin) that regulates fibrinolysis, cell adhesion and cell motility via its interactions with plasminogen activators and vitronectin. PAI-1 has been shown to play a role in a number of diverse pathologies including cardiovascular diseases, obesity and cancer and is therefore an attractive therapeutic target. However the multiple patho-physiological roles of PAI-1, and understanding the relative contributions of these in any one disease setting, make the development of therapeutically relevant molecules challenging. Here we describe the identification and characterisation of fully human antibody MEDI-579, which binds with high affinity and specificity to the active form of human PAI-1. MEDI-579 specifically inhibits serine protease interactions with PAI-1 while conserving vitronectin binding. Crystallographic analysis reveals that this specificity is achieved through direct binding of MEDI-579 Fab to the reactive centre loop (RCL) of PAI-1 and at the same exosite used by both tissue and urokinase plasminogen activators (tPA and uPA). We propose that MEDI-579 acts by directly competing with proteases for RCL binding and as such is able to modulate the interaction of PAI-1 with tPA and uPA in a way not previously described for a human PAI-1 inhibitor.

Lysophosphatidylcholine induces urokinase-type plasminogen activator and its receptor in human macrophages partly through redox-sensitive pathway

Urokinase-type plasminogen activator (uPA) and its cell surface receptor (uPAR) have been shown to be expressed in macrophages in atherosclerotic arterial walls, but the regulatory mechanisms of their expression remain unclear. The present study was performed to examine the effects of lysophosphatidylcholine (lysoPC), an important atherogenic lipid, on the expression of uPA and uPAR in human monocyte-derived macrophages. LysoPC upregulated the mRNA expression of uPA and uPAR, and it increased the protein expression of uPA in the culture medium and bound to the cell surface and of uPAR in the particulate fraction of the cells. LysoPC significantly increased the binding of the amino-terminal fragment of uPA to the treated cells and the cell-associated plasminogen activator activity. LysoPC stimulated superoxide anion production and increased intracellular oxidant levels in the cells. The combined incubation with reduced glutathione diethyl ester or N-acetylcysteine, antioxidants, suppressed the upregulation of uPA and uPAR mRNA and the increase in plasminogen activator activity by lysoPC. uPA and uPAR mRNA expression was also induced by the incubation with xanthine and xanthine oxidase, a superoxide anion-generating system. The results suggest that lysoPC increased the expression of uPA and uPAR and their functional activities in human monocyte-derived macrophages, at least in part through a redox-sensitive mechanism. This coordinate increase in the expression of uPA and uPAR in human macrophages by lysoPC could play an important role in plaque formation and disruption, arterial remodeling, and angiogenesis in atherosclerotic arterial walls.

Sphingosylphosphorylcholine stimulates proliferation and upregulates cell surface-associated plasminogen activator activity in cultured human keratinocytes

Of the various sphingolipid metabolites, including sphingosine, sphingosylphosphorylcholine (SPC), dimethylsphingosine, sphingosine-1-phosphate, N-acetylsphingosine, and skin-specific ceramides, only SPC accelerated cutaneous wound healing in full-thickness excision wounds in genetically healing-impaired diabetic (db/db) mice. A histologic examination revealed that SPC promoted not only granulation tissue formation, but also the re-epithelization of epidermal keratinocytes. As the direct effects of SPC on keratinocytes are completely unknown, we investigated the effects of SPC on normal cultured human keratinocytes. SPC concentration-dependently enhanced DNA synthesis in keratinocytes, with an increase in intracellular calcium concentrations due to the release of calcium ions from intracellular stores. SPC upregulated cell surface plasminogen activity, and at the same time increased the cell surface expression of urokinase-type plasminogen activator (uPA) and urokinase-type plasminogen activator-receptor (uPA-R) in keratinocytes. Furthermore, SPC promoted the in vitro wound repair of cultured keratinocytes, which was partially blocked by an anti-uPA monoclonal antibody. Our results suggest that one of the mechanisms responsible for the SPC-mediated promotion of cutaneous wound healing seems to be an enhancement of re-epithelization caused by the direct stimulation of the proliferation of keratinocytes, and an activation of the uPA/uPA-R system, which enhances the migration of keratinocytes.

Plasminogen activator inhibitor type 2 is expressed in keratinocytes during re-epithelialization of epidermal defects

Plasminogen activation is observed in the human epidermis during re-epithelialization of epidermal defects. The activation reaction depends on plasminogen activators (PAs) associated with re-epithelializing keratinocytes. PA inhibitor type 2 (PAI-2) is thought to be a major epidermal PA inhibitor in keratinocytes. However, no data are available on the expression of PAI-2 in keratinocytes during epidermal regeneration. We have therefore analysed PAI-2 at the mRNA and protein level in keratinocyte cultures as well as in epidermal lesions in which re-epithelializing keratinocytes were apparent. We found that PAI-2 expression at the mRNA and protein level was negatively correlated with the cell density in regular keratinocyte cultures. In organotypic cocultures, in which the transition from a re-epithelializing to a sedentary phenotype can be studied, PAI-2 was most strongly expressed in early cultures prior to formation of a differentiated epidermis-like structure. We found a strong expression of PAI-2 in keratinocytes that re-epithelialized dermal burn wounds or lesions caused by the autoimmune blistering disease pemphigus vulgaris. Our results suggest that not only PAs, but also a major PA inhibitor, PAI-2, are expressed in keratinocytes that are actively involved in re-epithelialization.

Plasminogen activator system in pemphigus vulgaris

Pemphigus vulgaris (PV) is caused by autoantibodies against desmosomes and is characterized by intra-epidermal blisters. The pathology of PV has been linked with plasminogen activation in lesional epidermis. The plasminogen activator system (PA system) consists of urokinase-type plasminogen activator (uPA), tissue-type PA (tPA), as well as the two types of plasminogen activator inhibitors (PAI-1 and PAI-2). In keratinocytes, uPA binds to a specific cell surface receptor for uPA (uPA-R = CD87) in an autocrine manner. Cell-bound uPA is regulated by PAIs. The central PA system component plasminogen, which is present in plasma and interstitial fluids, is bound to the keratinocyte surface via plasmin(ogen) binding sites, where it can be activated by uPA-R-bound uPA. Cell surface-associated plasmin then mediates pericellular proteolysis. As the topographical organization of the distinct PA system components in lesional epidermis of PV remained elusive, we have performed the present immunohistological analysis of lesional and non-lesional epidermis of PV. In keratinocytes directly involved in the epidermal split formation, plasmin(ogen) was stained in nine of 10 cases, uPA-R and uPA in four of 10 cases and PAI-2 in seven of 10 cases. Together, acantholytic plasmin(ogen)+ keratinocytes appeared in three different phenotypes: uPA-R+/uPA+ and PAI-2+, uPA-R-/uPA- and PAI-2+, as well as uPA-R-/uPA- and PAI-2-. Our findings demonstrate that, in acantholytic keratinocytes of PV, PAs and PAIs appear as differentially regulated components of the PA system.

Phorbol-ester-stimulated human lymphoid cell lines produce a plasminogen activator modulator inducing cell-bound urokinase-type plasminogen activator in malignant tumor cell lines

The importance of cell-associated plasminogen activation in tumor invasion and metastasis is becoming increasingly evident. To clarify the modulators of cell-associated plasminogen activation in malignant states, we have recently established an assay system utilizing endogenous plasminogen activators on the cell surface. In the present study using the assay system, we found that the conditioned medium from phorbol 12-myristate 13-acetate (PMA)-stimulated human lymphoid cell lines, HUT 78 and Raji, strongly enhanced plasminogen activator (PA) activity on the surface of human malignant tumor cell lines (WI-38 VAI3 2RA, A431, A549 and HT-1080). The enhancing effect was inhibited by the addition of actinomycin D. By gel filtration, the active substances in PMA-stimulated HUT 78- and Raji-conditioned media were eluted in similar fractions corresponding to molecular weights of 60 to 80 kDa. The active substance was heat-labile. The enhanced PA activities were completely inhibited by anti-urokinase-type plasminogen activator (uPA) IgG. Moreover, the active substance was found to increase in cell-bound uPA antigen. These findings suggest that a population of activated lymphocytes produces a plasminogen activator modulator that induces uPA on the surface of malignant tumor cells.

Plasminogen activation in bullous pemphigoid immunohistology reveals urokinase type plasminogen activator, its receptor and plasminogen activator inhibitor type-2 in lesional epidermis

Keratinocytes synthesize urokinase-type plasminogen activator (uPA) and a specific cell surface receptor for uPA (uPA-R, CD 87). Plasminogen is present in plasma and interstitial fluids from where it is bound to cell surfaces via plasmin(ogen) binding sites. uPA binds to the uPA-R in an autocrine manner and activates cell-bound plasminogen: a mechanism, which provides plasmin for pericellular proteolysis. Cell-bound uPA is regulated by plasminogen activator inhibitor type-1 (PAI-1) or type-2 (PAI-2). Bullous pemphigoid is an autoimmune inflammatory skin disease characterized by subepidermal blisters. Although circumstantial evidence suggested plasminogen activation in lesional epidermis of bullous pemphigoid, immunohistological data on the type of plasminogen activators, on the uPA-receptor or the type of plasminogen activator inhibitors in the lesions of bullous pemphigoid are lacking so far. To obtain this information we have performed the present immunohistological study. The presence of uPA and its receptor as well as PAI-2 was disclosed in epidermal keratinocytes in the roof of the subepidermal blisters. Moreover, keratinocytes at the bottom of the blister, which most likely represent keratinocytes during reepithelialization were stained. Co-localization was found for uPA and its receptor, uPA and plasmin(ogen) as well as for uPA and PAI-2. In non-lesional epidermis of bullous pemphigoid only PAI-2 was found. We propose that the expression of uPA and uPA-R, as well as the upregulation of PAI-2 in keratinocytes of lesional epidermis is part of the repair and reepithelialization process following lesion formation, i.e. epidermo-dermal dyshesion, in bullous pemphigoid.

Retinoic acid enhances plasminogen activation on the cell surface

The importance of cell-associated plasminogen activation in the extracellular matrix degradation processes is becoming increasingly evident. To elucidate the modulators of net plasminogen activation on the cell surface, we have recently established an assay system. Using this system, we examined the effects of several candidate modulators on cell surface plasminogen activator in the human fibrosarcoma cell line HT-1080 and the SV40-transformed human lung fibroblast cell line WI-38 VA 13 2RA. Although the majority of the candidates had no effect or a selective effect on either cell line, only retinoic acid markedly enhanced cell surface plasminogen activator activity in both HT-1080 and WI-38 VA13 2RA cells in a time-dependent manner. The effect of retinoic acid was neutralized by actinomycin D. The enhanced activity was inhibited by anti-uPA IgG and by pretreatment with phosphatidylinositol-specific phospholipase C. These findings suggest that retinoic acid increases the amount of receptor-bound uPA via de novo synthesis, and that it plays an important role in modulating cell-associated plasminogen activation.

Binding and activation of plasminogen on the surface of osteosarcoma cells

Plasmin (Pm) is a broad action serine protease implicated in numerous physiological functions. In bone, Pm may play a role in growth, resorption, metastasis, and the activation of growth factors. The various components of the Pm system are known to bind and function on the cell surface of various cell types, but no pertinent data are available describing membrane-bound Pm or its zymogen, plasminogen (Pg), in either normal or neoplastic bone cells. We report here that Pg binds to the surface of the human osteosarcoma cell line MG-63 and is activated to Pm by endogenous urokinase plasminogen activator (uPA). These conclusions are based on experiments utilizing radiolabeled compounds and a cell surface proteolytic assay measuring amidolytic activity of Pm. 125I-Pg binding to cells was time dependent, saturable, reversible, and specific. Binding was characterized by a relatively low affinity (Kd approximately 0.9 microM) and a high capacity (approximately 7.5 x 10(6) sites/cell). The binding of 125I-Pg was associated with lysine binding sites of the plasminogen molecule. Activation of 125I-Pg to 125I-Pm occurred on the cell surface and was dependent upon cell bound uPA, as determined by inhibitory antibodies. Binding of Pg to MG-63 monolayers represented approximately 80% bound specifically to the cell surface and the remainder to the surrounding extra-cellular matrix. Either co-incubation with uPA or pre-incubation with Pm resulted in increased 125I-Pg binding to osteosarcoma cells. Cell surface Pm proteolytic activity was confirmed by an amidolytic chromogenic assay. Both Pm and Pg bound to cells with Pg being activated by endogenous uPA. Plasmin activated on the cell surface was partially protected from inhibition by alpha 2-antiPm (requiring Pm lysine binding site interaction) but inhibited by aprotinin, (interacting directly with the Pm catalytic site). Resistance of cell bound Pm to alpha 2-antiPm inhibition suggests that cell surface proteolysis can occur in the presence of a soluble Pm inhibitor known to exist in the extracellular space. Based on these results, we speculate that the various bone physiological processes implicating Pm may occur at or near the bone cell surface.