Processing of growth hormone by rat adipocytes in primary culture: differentiation between release of intact hormone...

Rat adipocytes in primary culture have been used to study the intracellular processing of GH. These classic target cells for GH have been shown to process GH through two pathways: a nondegradative pathway which resulted in the rapid release of intact GH, and a slower, degradative pathway which involved the degradation of GH and release of degraded ligand. Differentiation between the two pathways was on the basis of differences in their kinetics and temperature dependence. The present study has investigated the relative characteristics of the two pathways further. Incubation of [125I]human GH ([125I]hGH)-preloaded adipocytes with extracellular unlabeled hGH (400 ng/ml) resulted in an increase in the absolute amount of [125I]hGH released. The increased amount of [125I]hGH released was all intact. Extracellular, unlabeled hGH had no effect on the rate or amount of degraded [125I]hGH released. This suggests that the nondegradative pathway is sensitive to the number of internalized hormone-receptor complexes and that GH which is not immediately degraded or stored in the degradative pathway, is redirected and processed via the faster non-degradative pathway. Ammonium chloride (known to inhibit the lysosomal degradation of many polypeptide hormones) markedly inhibited the absolute amount of [125I]hGH released from preloaded adipocytes. This inhibition was due to an effect on the release of degraded [125I]hGH. NH4Cl had no effect on the rate or amount of intact [125I]hGH released. Finally, it was found that dinitrophenol and sodium fluoride (agents known to deplete cellular energy) inhibited the release of degraded GH but not intact GH suggesting that the degradative pathway involves an energy-dependent step, most likely the fusion of hormone-containing vesicles with the lysosomal membrane. The mechanism of release of intact hormone by energy-independent means is not yet known. These data indicate that the processing of GH by cultured rat adipocytes is complex and involves at least two independently regulated pathways, a predominant degradative route and a nondegradative route. Further studies are required to assess the possible roles of these pathways in the metabolic actions of GH in adipocytes.