INTERNALIZATION; INTRACELLULAR BINDING OF GONADOTROPIN

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Nuclei, lysosomes, rough ER and Golgi as well as plasma membranes of bovine corpora lutea contain specific binding sites for 125I-human chorionic gonadotropin (125I-hCG). The internalized 125I-hCG associated with the receptors in all of the above intracellular organelles in an intact form. 125I-hCG degradation is not a primary consequence of 125I-hCG internalization during the first 2 hrs of incubation. These findings may represent, at least in part, the intracellular pathway of gonadotropin receptor synthesis and recycling. However, this pathway is neither documented unequivocally for gonadotropin receptors, nor does the data exist to show that the intracellular binding sites have no functional significance. Functional significance for internalized gonadotropin binding to the intracellular organelles is suggested by the following, a) diversity of intracellular organelles to which internalized 125I-hCG binds b) very high binding to some of the intracellular organelles (i.e., Golgi) c) nuclear association and d) the lack of high levels of hormone degradation during the first 2 hrs of incubation. To evaluate the functional significance, we will study the direct hCG effects on nuclear RNA synthesis, activity of RNA polymerases, chromatin density and solubility changes, exit of mRNA, nucleoside triphosphatase activity, phosphorylation and dephosphorylation, 125I-hCG binding and internalization in luteal cytoplasts and their responsiveness to hCG and cyclic AMP. The direct hCG effects on lysosomal, RER and Golgi intrinsic enzyme activities, phosphorylation and dephosphorylation will be studied along with determination of sidedness (cisternal or cytoplasmic) of receptors and capability of isolated lysosomes to degrade hormone and/or receptors. Corresponding organelle controls (no hormone added) will be run simultaneously. Plasma membranes will also be included in some of the experiments for comparison. Any effects seen will be extended to include hormonal and tissue specificity and possibility of cyclic AMP mimicking hCG. In vivo confirmation of observed effects in vitro will be sought using superovulated rats. Whether the concentration of hormone at which the effects are seen will correspond to physiological levels and apparent dissociation constant for hormone binding will be closely scruntinized. The results obtained may open up new and exciting possibilities in gonadotropin action which were never thought to exist before.