Internalization and excretion of epidermal growth factor‐dextran‐associated radioactivity in cultured human squamous‐carcinoma cells

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Abstract
Certain tumour cells, such as squamous carcinomas and gliomas, can have an increased number of epidermal‐growth‐factor (EGF) receptors. The EGF receptors can in these cases be targets for toxic conjugates with specific binding. EGF‐based toxic conjugates are potential targeting agents. We have analyzed the internalization and excretion of 125I administered in the form of 125 I‐EGF‐dextran in squamous‐carcinoma A431 cells. 125I‐EGF without dextran was used for comparison. A431 cells have large numbers of EGF receptors and are capable both of recycling and of degradation of internalized receptor‐ligand complexes. The binding of 125I‐EGF‐dextran and 125I‐EGF was receptor‐specific, since both ligands competed with non‐radioactive EGF for binding. The amount of internalized 125I as a function of time increased continuously within 24 hr following administration of radioactivity as 125I‐EGF‐dextran. The time pattern was quite different when 125I‐EGF without dextran was applied. In the latter case, the amount of internalized radioactivity decreased already after a few hours, probably depending on degradation of EGF. Pre‐incubation of the cells with 125I‐EGF‐dextran or 125I‐EGF and analysis of retained and released 125I activity at different times after washing showed that the 125I activity was retained for longer periods of time when EGF‐dextran was used instead of EGF. About 30% of the internalized 125I activity was retained after 24 hr when EGF‐dextran was used, compared with excretion of nearly all the radioactivity within 5 hr when EGF was used. In some experiments a high concentration of non‐radioactive EGF, 5 μg/ml, was given to the cells after pre‐incubation with 125I‐EGF‐dextran. This changed the retention and excretion patterns, so that a larger amount of 125I was excreted in the macromolecular fraction and a smaller amount of 125I activity was retained in the cells. Gel chromatography of the 125I activity released into the culture medium showed that the variations in molecular weight were larger after administration of a high concentration of non‐radioactive EGF, most likely due to partial degradation of EGF‐dextran. The results regarding excretion are in conformity with a model of competition between recycling of EGF‐dextran‐EGF‐receptor complexes and “trapping” of EGF‐dextran in the lysosomes followed by slow degradation. For targeting purposes, it is worth noting that the radioactivity administered in the form of 125I‐EGF‐dextran had a longer retention time than when 125I‐EGF without dextran was used, and that the retention and excretion rates could be modified by post‐treatment with the receptor ligand itself.