Receptor-mediated nuclear translocation of growth hormone

We have previously shown that the growth hormone (GH) receptor-binding protein is associated with the nucleus. We show here both by electron microscopy and nuclear isolation that GH is subject to rapid nuclear translocation. The intracellular fate of intravenously injected 125I-bovine growth hormone (bGH) was examined in the rat hepatocyte by electron microscopic autoradiography. The hormone appeared rapidly at the plasma membrane, then sequentially in lysosomal and multivesicular bodies and/or the nuclear membrane before final translocation to the nuclear matrix. Maximal translocation to the nuclear matrix occurred within 30 min of injection. Nuclear translocation of 125I-hGH was also studied by isolation of nuclei from cells stably transfected with cDNAs encoding the GH receptor, GH-binding protein, and a membrane bound but cytoplasmic domain-deficient receptor. Specific internalization and nuclear translocation of hormone only occurred in cells transfected with the full-length receptor. The translocation was rapid and became saturated within 1 h after addition of hormone to the culture media. SDS-polyacrylamide gel electrophoresis of isolated nuclei showed that GH is transported to the nucleus as the intact molecule. Pretreatment of cells with lysosomotropic agents (chloroquine, ammonium chloride, and bacitracin) decreased hormone degradation and increased nuclear translocation of GH. The nuclear translocation of GH was achieved independent of the cytoskeletal system (microtubular, microfilament, and intermediate filament networks). Thus, GH is subject to rapid receptor-dependent nuclear translocation via the endosomal pathway.