Conditioned media from explants of human colorectal and gastric
tumors in short-term organ culture were analysed for
plasminogen activator activity, activity toward the synthetic
urokinase substrate,
Spectrozyme-UK, and for the presence of
urokinase antigen using monospecific goat antibody, by
enzyme-linked
immunosorbent assay. Comparisons were made between primary
tumors, adjacent normal mucosa and metastatic lesions. These analyses were carried out on unfractionated culture fluids and on fractions obtained by fast
protein liquid chromatography separation using Superose 6
gels.
Plasminogen activator activity, tested by azocaseinolysis in the presence of added
plasminogen, was restricted to peaks of 55 kD and 155 kD. These were of the
urokinase type as shown by specific immunoinhibition and by absorption by an antiurokinase antibody-Affigel 10 column.
Spectrozyme-UK, in addition to these peaks, detected a series of higher molecular weight activities, the largest of which appeared in the void volume, and were therefore of greater than 10(6) molecular weight. These activities were greatly increased by inclusion of trace
plasmin indicating that these components were mostly in their
proenzyme forms. The characteristics of these very large
enzymes were similar to those isolated earlier from a human
lung cancer cell line. Comparison of the primary and metastatic
tumors confirmed earlier observations showing that
urokinase secretion by the metastatic
tumors was greatly reduced in comparison with the primary
tumors: in the colon
carcinomas it was 10 per cent of the value for the primary, in the gastric
tumors 3 per cent, whether means or medians were compared (P less than 0.0001). This large difference was characteristic only of
plasminogen activator secretion assayable by azocaseinolysis; activities toward
Spectrozyme-UK, and
antigen reacting with anti-
urokinase antibody, were considerably less different in the two groups. In individual tissues, no correlation was found between the amount of extractable
plasminogen activator and amounts secreted, or between the latter and the amount of
lactic acid released. It is postulated that the greatly reduced
plasminogen activator secretion by explants of metastatic
tumors may be a phenotypic characteristic of distinct advantage for
cancer cells destined to initiate metastatic foci, and may contribute to the ability of circulating
cancer cells to lodge in the blood vessels of the target organ.