Plasminogen activator in cultured Lewis lung carcinoma cells measured by chromogenic substrate assay.

A chromogenic substrate assay for the plasminogen activator (PA) activity of Lewis lung carcinoma cells has been developed. The cells were incubated with plasminogen, the activation of which to plasmin was measured by the amidolysis of the chromogenic substrate S-2251. This was routinely performed as a 4h serum-free assay, but a variation lasting 24 h, in medium supplemented with plasminogen-free inhibitor-reduced serum, produced similar results. The assay also detected PA released into the medium. PA activity was proportional to cell density, and the assay was non-toxic to the cells. Assays were performed on cultures derived from primary and metastatic tumours. Host cells were effectively eliminated from such cultures but, because of an initial phase of tumour-cell death, PA assays were not carried out until cultures became established. No consistent difference was detected between PA levels in primary and metastatic cultures. However, these cultures were shown to be atypical of the parent tumour; they grew slowly when reinjected at the primary site, and their metastatic potential was impaired.

Plasminogen activation transforms the morphology of quiescent 3T3 cell monolayers and initiates growth.

Plasminogen activator of cell origin converts the plasma protein plasminogen to the proteolytic enzyme plasmin. Recently, high levels of activator have been observed to be particularly associated with tumours and transformed cells, and a functional relationship between plasminogen activation and malignancy has been proposed. In this paper we have attempted to induce transformation-like morphology and growth in a population of confluent quiescent cells in tissue culture, by inducing plasminogen activation. Untransformed 3T3 cells grown to confluence in plasminogen-free medium were subjected to plasminogen activation by the addition of urokinase and plasminogen or plasminogen-containing acid-treated serum, or plasmin. Under these conditions, the previously well ordered monolayers became disrupted, with multilayering, and discontinuities in the cell sheet, and the cells simultaneously grew to significantly higher densities. Removal of the plasmin-containing medium supplements effected some restoration of normal morphology. Thus, lhen plasmin was present 3T3 cells did not become transformed, but expresses transformation-like features. Well ordered monolayer morphology and quiescence in 3T3 cells at confluence are therefore dependent upon the absence of plasminogen activation.

Substrate retention of fractured retraction fibres during detachment of trypsinized BHK21 fibroblasts.

When BHK21 fibroblasts adhering to glass were incubated in trypsin they became spherical within a few minutes. They did not, however, respond simultaneously; the most trypsin-resistant cells in an unsynchronized population were of greater length, had more processes projecting from the cell body, and were more spread out than the most trypsin sensitive cells. In no instance did trypsin detach cells from their substrate, even when the incubation period was prolonged to 5 h in trypsin concentrations almost sufficient to cause cell lysis. Scanning electron-microscope observations showed that initially flat cells rounded up in trypsin to reveal persistent adhesion sites joined to the cell body by retraction fibres. Such cell could be dislodged only by agitation; when this occurred parts of the cell remained attached to the substrate in the form of small spheres and fibres; these were remnants of the retraction fibres and adhesion sites. We propose that the adhesion sites are not susceptible to proteolytic degradation, presumably because of steric hindrance, and that this causes detachment to be dependent upon mechanical dislocation. We suggest that some of the descriptions of substrate-associated 'cell exudates' in the literature may refer to these cell remnants on the substrate which consist of membrane-bound fragments of cytoplasm rather than secreted products.

Effect of a protease inhibitor on the adhesion of Ehrlich ascites cells to host cells in vivo.

Ehrlich ascites tumours (EAT) were grown in mice by injecting 1 × 10(6) cells intraperitoneally. In mice which received one or more injections of 30 mg soybean trypsin inhibitor (TI) i.p. during tumour growth, the number of recoverable tumour cells was significantly reduced by up to 92%. Also, the mean size of these cells was significantly smaller.When the rate of labelled thymidine incorporation in vitro was compared in TI-treated and control cells, no significant differences were detected. However, when the population doubling time of EAT cells in vivo was calculated, it was apparent that recoverable TI-treated cells were dividing more rapidly than controls. Consequently, the reduced number of cells recovered from TI-treated mice did not result from a reduced growth rate.Viability, assessed by trypan blue dye exclusion and rate of labelled chromium release, was the same in TI-treated and control cell populations. Thus TI was nontoxic to EAT cells and the reduced number of cells from treated tumours was not therefore due to cytotoxicity.Scanning electron microscopy revealed that normal EAT cells did not adhere to internal host surfaces but that after TI treatment they adhered in large numbers to produce an appearance which resembled a confluent monolayer. This binding to host tissue accounted for the reduction in the number of cells recovered from TI-treated animals. We propose that TI acts as a protease inhibitor to prevent intrinsic proteolytic enzyme activity at the tumour cell surface. This activity would normally destroy the binding sites required for adhesion to host tissue.