Receptor-mediated endocytosis and degradation of bovine growth hormone in rat liver.

Receptor-mediated endocytosis of radiolabelled bovine growth hormone (125I-bGH) via somatogenic receptors in the liver was studied following in vivo intraportal injection. At different times after injection, subcellular membrane fractions involved in binding (plasma membranes), endocytosis (endocytic vesicles) and degradation (lysosomes) of peptide hormones were isolated by sucrose density gradient centrifugation. These fractions were evaluated for the time-course accumulation of radiolabelled bGH and for the presence of internalized 125I-bGH-receptor complexes. These uptake studies indicate that after initial plasma membrane association of 125I-bGH, the ligand is transported in two successive endocytic compartments prior to arrival in lysosomes. The molecular weight of the somatogenic binders of male and female rat livers involved in internalization of 125I-bGH was determined to 95,000, 64,000, 55,000, 43,000 and 35,000, assuming a 1:1 binding of the hormone to the binder. These binders were seen in both endosomes and lysosomes, which suggests that growth hormone is transported to the lysosomes in a complex with its receptor. Binding and uptake of 125I-bGH was also compared in male and female rat livers, and endocytosis of 125I-bGH was compared to that of radiolabelled ovine prolactin (125I-oPrl). The specific uptake of 125I-bGH appeared not to be sexually differentiated in contrast to that of 125I-oPrl which showed a 35-fold higher uptake in female rat liver. Degradation of 125I-bGH was studied under in vitro binding assay conditions. A distinct 15,000 Da fragment was generated by plasma membrane, endosomal and lysosomal fractions. Based on protease inhibitor studies, a non-trypsin-like serine protease is suggested to be involved in the degradation of bGH. The 15,000 Da proteolytic fragment of GH can be affinity cross-linked to somatogenic binders of similar molecular weights as those involved in the binding of intact GH.