Receptor-mediated uptake and degradation of human chorionic gonadotropin: fate of the hormone subunits

LH-RH analogue carrying a cytotoxic radical is internalized by rat pituitary cells in vitro

The binding and internalization of a cytotoxic analogue of luteinizing hormone-releasing hormone (LH-RH), T-98 (agonist [D-Lys6]LH-RH linked to glutaryl-2-(hydroxymethyl)anthraquinone), by rat anterior pituitary cells was investigated. Analogue T-98 was bound to pituitary membrane binding sites for LH-RH with a high affinity (Kd = 1.2 nM) and was 17 times more potent in releasing luteinizing hormone (LH) from superfused rat pituitary cells than LH-RH. The labeling of this cytotoxic LH-RH analogue was carried out both with radioactive (125I) and nonradioactive iodine. Monoiodination of the Tyr5 residue of T-98 did not significantly affect its binding affinity but greatly decreased its LH-releasing activity to about 3% of the original value. Di-iodination in the same position lowered binding affinity twenty-threefold and further diminished LH-releasing potency. [125I]T-98 was found to bind very strongly to polystyrene, which precluded the use of regular tissue culture plasticware in our experiments. In pituitary cells cultured in glass vials, binding and internalization of [125I]T-98 were observed, which were time and temperature dependent, and which could be inhibited by excess unlabeled analogue. No enzymatic degradation of labeled T-98 was detected in the culture medium during the incubation. Our results indicate that T-98 is internalized by pituitary gonadotropes through receptor-mediated endocytosis. Because this new class of compounds was designed as anticancer drugs, our findings also suggest that this cytotoxic LH-RH agonist may also be internalized by LH-RH receptors present in breast, prostate, ovarian, and other tumors.

Intracellular localization and degradation of diphtheria toxin

The internalization of surface-bound diphtheria toxin (DT) in BS-C-1 cells correlated with its appearance in intracellular endosomal vesicles; essentially no toxin appeared within secondary lysosomal vesicles. In contrast, internalized epidermal growth factor (EGF) was localized within both endosomal and lysosomal vesicles. Upon preincubation of cells with leupeptin, a lysosomal protease inhibitor, a threefold increase in the accumulation of EGF into lysosomes was observed. Under identical conditions, essentially all of the diphtheria toxin remained within endosomes (less than 2% of the intracellular diphtheria toxin accumulated in the lysosomal fraction), indicating that the inability to detect diphtheria toxin in lysosomes was not due to its rapid turnover within this vesicle. Following internalization of EGF or DT, up to 40% of the ligand appeared in the medium as TCA-soluble radioactivity. EGF degradation was partially leupeptin-sensitive and markedly NH4Cl-sensitive, indicating lysosomal degradation. In contrast, DT A-fragment degradation was resistant to these inhibitors, while B-fragment showed only partial sensitivity. These data suggest that the bulk of endocytosed diphtheria toxin is localized within endosomes and degraded by a pathway essentially independent of lysosomes.

Binding and internalization in vivo of [125I]hCG in Leydig cells of the rat

The present study was performed to demonstrate the binding, mode of uptake, pathway and fate of iodinated human chorionic gonadotropin ([125I]hCG) by Leydig cells in vivo using electron microscope radioautography. Following a single injection of [125I]hCG into the interstitial space of the testis, the animals were fixed by perfusion with glutaraldehyde at 20 minutes, 1, 3, 6 and 24 hours. The electron microscope radioautographs demonstrated a prominent and qualitatively similar binding of the labeled hCG on the microvillar processes of the Leydig cells at 20 minutes, 1, 3, and 6 hours. The specificity of the [125I]hCG binding was determined by injecting a 100-fold excess of unlabeled hormone concurrently with the labeled hormone. Under these conditions, the surface, including the microvillar processes of Leydig cells, was virtually unlabeled, indicating that the binding was specific and receptor-mediated. In animals injected with labeled hCG and sacrificed 20 minutes later, silver grains were also seen overlying the limiting membrane of large, uncoated surface invaginations and large subsurface vacuoles with an electron-lucent content referred to as endosomes. A radioautographic reaction was also seen within multivesicular bodies with a pale stained matrix. At 1 hour, silver grains appeared over dense multivesicular bodies and occasionally over secondary lysosomes, in addition to the structures mentioned above, while at 3 and 6 hours, an increasing number of secondary lysosomes became labeled. At 24 hours, binding of [125I]hCG to the microvillar processes of Leydig cells persisted but was diminished, although a few endosomes, multivesicular bodies and secondary lysosomes still showed a radioautographic reaction. No membranous tubules that were seen in close proximity to, or in continuity with, endosomes and multivesicular bodies were observed to be labeled at any time interval. Likewise, an attempt to correlate silver grains with small coated or uncoated pits, the stacks of saccules of the Golgi apparatus and other Golgi-related elements including GERL, proved unsuccessful, since these structures were mostly unlabeled. These in vivo experiments thus demonstrate the specific binding of [125I]hCG to the plasma membrane of Leydig cells predominantly on their microvillar processes, and the subsequent internalization of the labeled hCG to secondary lysosomes. In addition, binding and internalization of hCG persisted for long periods of time.

Divergent metabolism of human chorionic gonadotropin subunits within rat ovarian lysosomes

Pseudopregnant rats were injected with either native human chorionic gonadotropin or with (125I)-human chorionic gonadotropin and their ovarian homogenates fractionated on Percoll density gradients. The levels of alpha and beta subunits within subcellular fractions were measured using radioimmunoassays specific for each subunit. Radioactivity measurements of fractions obtained from rats injected with (125I)-human chorionic gonadotropin were used as a separate index of alpha subunit distribution. The alpha subunit was primarily restricted to a combined plasma membrane/prelysosomal vesicle fraction. Immunoreactive beta subunit was present at high concentrations within both this plasma membrane/prelysosomal vesicle fraction and within lysosomes. The striking difference in alpha and beta subcellular distribution may arise from differential sensitivities to lysosomal enzymes.

Intracellular pathways of endocytosed tracers in Leydig cells of the rat

The endocytic activity of Leydig cells was examined by electron microscopy following the injection, into the interstitial space, of tracers used to examine fluid-phase endocytosis, ie, native ferritin and horseradish peroxidase-colloidal gold (horseradish peroxidase-gold), and adsorptive endocytosis, ie, cationic ferritin. At 5 minutes after injection, native ferritin or horseradish peroxidase-gold was present in the interstitial space and free in the lumen of large endocytic vesicles forming at the cell surface. At 15 minutes, these tracers appeared in the matrix of pale multivesicular bodies, while at 30 minutes and 1 hour, the matrix of dense multivesicular bodies became labeled. Beginning at 1 hour, dense membrane-delimited bodies identified cytochemically as lysosomes were labeled. In the case of cationic ferritin, two distinct pathways were taken. In one pathway, cationic ferritin was observed 5 minutes after injection bound to the plasma membrane of Leydig cells and to the membrane of small and large endocytic vesicles. At subsequent time intervals, cationic ferritin appeared consecutively in pale, dense multivesicular bodies and lysosomes. In a second pathway, cationic ferritin was observed at 5, 15, and 30 minutes bound to the membrane of vesicles of intermediate size seen near the cell surface. At 1, 1 1/2 and 2 hours, cationic ferritin-containing intermediate vesicles appeared in increasing number in the Golgi region. However, cationic ferritin was never observed in the Golgi saccules themselves. At later time intervals (3-6 hours), intermediate vesicles labeled with cationic ferritin progressively disappeared from the Golgi region and the cell. Thus in Leydig cells, while fluid-phase tracers reached lysosomes exclusively, cationic ferritin, a tracer of adsorptive endocytosis, not only reached the lysosomes, but was also carried by the intermediate vesicles to the Golgi region of the cell.