PANC-1 human pancreatic carcinoma cells readily bound and internalized 12I-labeled epidermal growth factor (EGF). Bound 125I-labeled EGF was then partially processed to a number of high molecular weight acidic species. (5), and the gastrointestinal hormone cholecystokinin (6). Further, high concentrations of EGF decrease the number of EGF receptors on the cell surface through a process termed down-regulation (7). We now report an additional mechanism by which some cells may modulate EGF receptor function. In PANC-1 human pancreatic carcinoma cells, internalized EGF is processed mainly to a number of high molecular weight products. Two major products of processed EGF, along with intact EGF, are released into the culture medium. One of these products as well as intact EGF can rebind to the cell surface. These data suggest that in certain cells, recirculation of bindable EGF products may be important in the intracellular disposition of cell-surface EGF receptors.
MATERIALS AND METHODSCell Culture. PANC-1 human pancreatic carcinoma cells (CRL 1469) Internalization, Processing, and Degradation Studies. To monitor 125I-labeled EGF internalization, cultured cells were washed as for binding studies and then were incubated for 4 min at 40C with 500 mM NaCl, titrated to pH 2.5 with acetic acid (12). Radioactivity removed by acid treatment (surfacebound) and acid-resistant (internalized) radioactivity were then determined separately. Internalized ligand was localized with the aid of Percoll gradient fractionation of cell homogenates (11). Cell disruption was carried out as described (11), except that homogenization buffer contained 1 mM EDTA, 10 mM acetic acid, and 10 mM triethanolamine at pH 7.4, and was supplemented with 1 mM benzamidine and 0.25 mM phenylmethylsulfonyl fluoride (PMSF) to prevent in vitro proteolysis. Acid phosphatase, UDP-galactosyl transferase, and NADPH-cytochrome c reductase were assayed as reported (11) and served as markers for lysosomes, Golgi apparatus, and endoplasmic reticulum, respectively. To monitor 251I-labeled EGF processing, cell-bound and dissociated radioactivity were analyzed by IEF on horizontal agarose gels using internal standards generated with the aid of pI marker proteins (13). The locations of 125I-labeled proteins were determined by autoradiography of analytical gels after drying (13). Degradation of 125I-labeled EGF in incubation medium was monitored by analyzing supernatant radioactivity by Sephadex G-25 column chromatography.
RESULTSStudies with a highly purified species of EGF (pI 4.55) indicate that, after binding to target cells, the peptide is shortened by the sequential removal of carboxyl-terminal Abbreviations: EGF, epidermal growth factor; IEF, isoelectric focusing.
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