Binding, internalization, and lysosomal association of 125I-human growth hormone in cultured human lymphocytes: a quantitative morphological and biochemical study.

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Abstract
125I-human growth hormone (125I-hGH) binds specifically to receptors on cultured human lymphocytes (IM-9). When this process is studied by use of quantitative EM radioautography, under conditions of incubation at 15.degree. C for 5 min, the ligand is localized to the plasma membrane of the cell. At 30.degree. and 37.degree. C, 125I-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 5-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.degree. C for 120 min. When the cell-associated radioactivity after 15-120 min of incubation at 37.degree. 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 125I-hGH at 37.degree. C for 15-120 min, separated from the incubation medium, and washed and diluted 100-fold, the percent 125I-hGH dissociable decreases as a function of increasing time of incubation. When cells are incubated with 125I-hGH for 15 min at 37.degree. C and the radioactivity that dissociated 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 radioautographically, 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. Receptor-linked internalization of a polypeptide hormone may provide a mechanism that couples degradation of the ligand with loss of the cell surface receptor.

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