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Examination of the Effects of Epidermal Growth Factor on the Production of Urokinase and the Expression of the Plasminogen Activator Receptor in a Human Colon Cancer Cell Line¹

Department of Pharmacology, Bristol-Baylor Laboratory, Baylor College of Medicine, Houston, Texas 77030

The expression of the plasminogen activator, urokinase, and the display of its receptor in response to growth factors were examined in a serum-free adapted colon cancer cell line, CBS_sf. Cells propagated in protein-free medium secreted 6.5 ± 1.0 ng/ml of urokinase/10⁶ cells in a 3-day period as determined by enzyme-linked immunosorbent assay. Inclusion of insulin or transferrin into the protein-free medium was without effect on this parameter. However, addition of epidermal growth factor (EGF) to the protein-free medium resulted in a 50% reduction in this parameter. This change was also reflected in the plasminogen-dependent solubilization of immobilized radioactive laminin. Plasminogen-supplemented conditioned medium derived from CBS_sf cells grown in protein-free medium solubilized 135,000 ± 25,000 dpm/10⁶ cells of radioactive substrate. This value was decreased to 59,000 ± 6,000 when conditioned medium was collected in the presence of EGF. Dose-response curves indicated that, while 0.5 ng/ml of EGF were suboptimal for the suppression of urokinase secretion, a concentration of 5.0 ng/ml had a maximum effect on this measurement. Northern hybridization studies indicated that the reduced plasminogen activator reflected, at least in part, translation of a less abundant transcript.

Examination of the colon carcinoma cell line for altered urokinase receptor display revealed that EGF caused a dose-dependent increase in the amount of radioactive urokinase bound. This did not reflect reduced occupation of binding sites with endogenous ligand. Scatchard manipulation of the binding data indicated that the increased amount of radioactive plasminogen activator bound to cells cultured with EGF reflected an increase in receptor number from 7,500 to 13,000 sites/cell.

Time course studies revealed that the decrease in urokinase secretion precedes changes in receptor display by 5 h. A 60% reduction in assayable urokinase was demonstrated in the conditioned medium from cells treated with the growth peptide for 10 h. However, a 24-h period was required to observe an increase (80%) in the amount of radioligand bound to EGF-treated cells.

These data suggest EGF to be a regulator of both urokinase production and urokinase receptor display in a colon cancer cell line.

¹ This work was supported by NIH Grant CA34432.

² To whom requests for reprints should be addressed.

Received 10/ 5/88. Revised 1/ 5/89. Accepted 2/ 6/89.

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