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.