Internalization and degradation of receptor-bound hCG in granulosa cell cultures.

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Abstract
Monolayers of [rat] granulosa cells from preovulatory follicles were incubated for 3 h with 125I-human chorionic gonadotropin (hCG) and subsequently for 5 h in the presence or absence of a saturating dose of unlabeled hormone at 37.degree. C. Distribution of the labeled hormone during the 1st and 2nd incubations was followed by EM autoradiography. After 2-3 h of exposure to 125I-hCG, most of the label (80% and 69%, respectively) was associated with the cell membrane, whereas the remaining radioactivity was located within the cytoplasm. At the end of the 2nd incubation most of the cell-bound label (73%) was internalized and found mainly in lysosome-like structures, whereas 30-40% of the total radioactivity was released into the medium. Further release of the bound radioactivity to the medium (up to 70-80%) was evident at the end of longer incubation periods (up to 24 h). Most of the released radioactivity was associated with mono- and diiodotyrosine as determined by ion exchange chromatography. During the 1st incubation, the cells were desensitized to hCG, as indicated by lack of response in cyclic AMP accumulation to challenge with fresh hormone. Desensitization evidently precedes extensive internalization of the receptor-hormone complex and the internalized hormone is transported to lysosomes in which degradation may take place.