Retinoic acid inhibition of a tumour protease immobilised on cell surfaces and in free solution

Retinoids are inhibitors of tumour cell proliferation in culture and have been shown to suppress carcinogenesis and decrease the levels of proteases. The present study has demonstrated that retinoic acid is a potential non-competitive inhibitor of a protease (GB) immobilised on the surfaces of tumour cells in thin sections and free GB in solution. The enzymic status of GB on the cell surfaces in sections has been determined by challenging the retinoic acid-treated cells with a second fluorescent inhibitor (9-AA), followed by fluorescence microscopic analysis. The inhibition of cell surface GB by retinoic acid was demonstrated to be reversible. The activity of soluble GB has been measured by the MUGB assay in the presence and absence of retinoic acid. It is suggested that retinoic acid acts on GB by interacting with a binding site, different from the active site, and causes major conformational changes, resulting in enzyme inhibition. It is possible that the modulation of GB activity by retinoic acid may play a role in the control of cell migration and metastasis.

Inhibition of a tumour protease with 3,4-dichloroisocoumarin, pentamidine-isethionate and guanidino derivatives

Guanidinobenzoatase (GB) is a cell surface proteolytic enzyme capable of degrading fibronectin, and is associated with tumour cells and cells capable of migration. The location of active GB in sections has been demonstrated with 9-aminoacridine (9-AA), a competitive inhibitor of GB. 3,4-Dichloroisocoumarin (3,4-DCI) and pentamidine isethionate (PI) are inhibitors of trypsin-like enzymes. It has now been demonstrated that 3,4-DCI, PI, and guanidino derivative compounds are significant inhibitors of GB, on the surfaces of lung squamous cell carcinoma cells in frozen sections and free GB in solution. Dexamethasone acetate (DMA) and medroxy-progesterone (MP) did not show any significant inhibition of GB activity. These molecules lack a reactive chloride or guanidino groups and are thought to react at the nuclear level, rather than directly on this cell surface protease. Kinetic studies have shown that 3,4-DCI, PI and guanidino derivatives are reversible competitive inhibitors of GB, as determined in vitro on the purified enzyme. The inhibition resulting with 3,4-DCI was a time-dependent process. It is suggested that these inhibitors interact with GB by binding to its active site, resulting in the formation of enzyme-inhibiter complexes (GB-I). The GB-I complexes can be dissociated with SDS treatment, resulting in the regain of GB activity.

The interactions of protein inhibitors with tumour proteases studied in solution and immobilised on cell surfaces in frozen sections

The cell surface protease guanidinobenzoatase (GB) has been purified from human colonic and lung carcinoma tissue by an affinity step involving the binding of the enzyme either onto fibrin fibrils or onto agmatine-sepharose. The inhibitor protein (I) was extracted from the cytoplasm of tumour cells and isolated by an affinity step involving the binding of I to GB on the surface of cultured carcinoma cells. The interaction of GB and I in solution was followed by kinetic studies employing the release of the fluorescent 4-methylumbelliferone (MU) from the synthetic substrate 4-methylumbelliferyl-p-guanidinobenzoate (MUGB). The interaction of soluble I with membrane bound GB was followed by using the yellow fluorescent probe 9-aminoacridine (9AA) which binds to active GB but not to GB-I. The results of these studies demonstrated the presence of isoenzymic froms of GB which were recognized specifically by their appropriate isoinhibitor, isolated from the appropriate cell type. This high degree of selectivity suggests a cell specific regulatory role for the inhibitors and the possibility that they might be used for the delivery of cytotoxic molecules to the surface of specific types of tumour cells.

Association and dissociation of a protease and its inhibitor on the surface of lung squamous cell carcinoma cells

Squamous cell carcinoma cells possess a cell surface protease, referred to as guanidinobenzoatase (GB). GB is a plasminogen-activator-like enzyme which can be located by the fluorescent probe 9-amino acridine in frozen sections. Fluorescence microscopy has been used to study the inhibition of this GB, the displacement of inhibitor from GB, the displacement of GB from the cell surface receptor and the preparation of both active GB and inhibitor, obtained from these frozen sections of tumour tissue.

Observations on the conformational changes in the structure of a cell surface protease, followed by its ability to be recognised by competitive and non-competitive inhibitors

Lung tumour cells possess a cell surface protease which can be inhibited by a cytoplasmic protein inhibitor extracted from these cells. The dissociation of this enzyme-inhibitor complex on the surface of tumour cells in sections treated with 10(-4) M sodium dodecyl sulphate has been studied. The dissociation of the inhibitor and regain of enzymic activity was followed by the use of a fluorescent probe which binds to the active centre of the cell surface enzyme in a competitive manner.

Influence of angiotensins (I, II, & III), bradykinin and arachidonic acid on renomedullary PGE production in vitro

Renomedullary tissue from rabbit or rat was incubated with angiotensin I, II, III, arachidonic acid, bradykinin, indomethacin and meclofenamate to study their effect on PGE2 production. Arachidonic acid and bradykinin enhanced PGE2 production significantly. Indomethacin and meclofenamate inhibited PGE2 production by more than 70%. Angiotensin I, II and III did not influence PGE2 production. These results suggest that bradykinin and arachidonic acid stimulate PGE2 production by a direct cellular action whereas the angiotensins do not.