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Determination of the Effects of Epidermal Growth Factor on Urokinase Secretion and Urokinase Receptor Display in a Well-Differentiated Human Colon Carcinoma Cell Line¹

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

The regulation of urokinase secretion and receptor display in a well-differentiated colon carcinoma cell line, GEO, adapted to serum-free conditions was examined. In protein-free medium, the cell line secreted 0.8 ± 0.1 ng/ml/10⁶ cells of urokinase in a 3-day period as determined by an enzyme-linked immunosorbent assay. This value was elevated 4-fold when the cells were cultivated in the presence of epidermal growth factor (EGF) but not insulin or transferrin. Propagation of the cell line with any combination of these growth factors was not superior to EGF alone in inducing urokinase secretion. The presence of EGF raised the radioactive laminin-solubilizing activity of the conditioned medium. In the absence of the growth factor, spent medium supplemented with plasminogen solubilized 23,000 ± 7,000 dpm/10⁶ cells of the immobilized laminin. This value was increased to 95,000 ± 10,000 dpm/10⁶ cells when the cultures were grown with EGF. Northern analysis indicated that the elevated level of the plasminogen activator protein by EGF was a consequence of a more abundant urokinase transcript.

The stimulation of urokinase secretion by EGF was accompanied by a reduction of radioactive urokinase binding to the cell line. The reduction in plasminogen activator binding was not further enhanced by insulin or transferrin. In addition, these latter growth factors, by themselves, were ineffective in altering the amount of plasminogen activator bound. The attenuation in ¹²⁵I-labeled urokinase binding did not reflect occupation of the receptors with endogenous ligand as acid pretreatment was without effect on the binding profile. Scatchard analysis revealed that the altered urokinase binding by EGF reflected a decrease in receptor number from 14,000 ± 1,500 to 8,000 ± 1,500 sites per cell. The temporal relationship of urokinase secretion and receptor display was examined. Changes in either parameter required an EGF exposure period of 10 h or more. Further amplification of the EGF effects was seen with longer incubation times with the growth peptide.

These opposite effects of EGF on urokinase secretion and receptor display may suggest a homeostatic control mechanism for keeping the plasminogen activator system in check. The ability of the cell line to express biological characteristics associated with a well-differentiated colon cell type may reflect its capacity to suppress a system which is usually associated with the transformed state.

¹ This work was supported by NIH Grant CA34432.

² To whom requests for reprints should be addressed, at Department of Pharmacology, Bristol-Baylor Laboratory, One Baylor Place, Baylor College of Medicine, Houston, TX 77030.

Received 8/10/88. Revised 12/12/88. Accepted 1/19/89.