Binding , Internalization , and Lysosomal Association of ' 25 1 - Human Growth Hormone in Cultured Human Lymphocytes : A Quantitative Morphological and Biochemical Study 360

'25 1-human growth hormone (' 25 1-hGH) binds specifically to receptors on cultured human lymphocytes (IM-9). When this process is studied by use of quantitative EM radioau-tography, under conditions of incubation at 15 °C for 5 min, the ligand is localized to the plasma membrane of the cell. At 30 ° and 37 ° C, however, ' 25 1-hGH is progressively internalized by the cell as a function of time. The internalized ligand is found predominantly in the Golgi region of the cells, with a five-fold preferential localization to membrane-bounded structures with the morphological and cytochemical characteristics of lysosomes. Up to 59% of these lysosome-like structures are positive for the acid phosphatase reaction under the conditions of incubation at 37 ° C for 120 min. When the cell-associated radioactivity after 15-120 min of incubation at 37 ° C is extracted in 1 M acetic acid and filtered on a Sephadex G-100 column, 58-73% of the material elutes as intact hGH. When cells are incubated with ' 25 1-hGH at 37 °C for 15-120 min, separated from the incubation medium, and washed and diluted 100-fold, the percent ' 25 1-hGH dissociable decreases as a function of increasing time of incubation. When cells are incubated with 1251-hGH for 15 min at 37°C and the radioactivity that dissociates from the cells during 15-90 min is studied, the labeled material appearing in the incubation medium is progressively degraded as a function of time of incubation. When the dissociation process is studied radioautograph-ically, grains are found both in plasma membrane and intracellular compartments after 30 min of association, but after 30 and 120 min of dissociation a higher proportion of grains are in the intracellular compartment. After 120 min of association, there is less dissociation from either compartment and a preferential increase of grains in the intracellular compartment. These data suggest that receptor-linked internalization of a polypeptide hormone provides a mechanism that couples degradation of the ligand with loss of the cell surface receptor. When a polypeptide hormone or growth factor binds to specific receptors on cell surfaces, several diverse events occur. In target cells á biological response is initiated, in both target and nontarget cells the ligand is inactivated or degraded (38), and the hormone-receptor interaction regulates the ambient receptor concentration (16, 26). By use of morphological probes, it has been shown directly that a variety of polypeptide hormones and growth factors bind to …