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Effects of hyperosmolarity on ligand processing and receptor recycling in the hepatic galactosyl receptor system
Author(s) -
Oka Janet A.,
Weigel Paul H.
Publication year - 1988
Publication title -
journal of cellular biochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.028
H-Index - 165
eISSN - 1097-4644
pISSN - 0730-2312
DOI - 10.1002/jcb.240360208
Subject(s) - endocytosis , internalization , mannitol , endocytic cycle , receptor , receptor mediated endocytosis , sucrose , osmotic concentration , biochemistry , ligand (biochemistry) , biology , chemistry , biophysics
Binding, endocytosis, and degradation of asialo‐orosomucoid (ASOR) mediated by the galactosyl (Gal) receptor were examined in isolated rat hepatocytes in complete media supplemented with an osmolite. The specific binding of 125 I‐ASOR to cells at 4°C was unaffected by up to 0.4 M sucrose or NaCl. Unlike sucrose or NaCl, mannitol stimulated 125 I‐ASOR binding at low concentrations but inhibited binding at higher concentrations. Continuous internalization at 37°C, which requires receptor recycling, was completely blocked at 0.2 M sucrose or 0.15 M NaCl, corresponding in each case to a total osmolality of about 550 mmol/ kg. This effect was reversed and endocytic function was restored by washing the cells, indicating that cell viability was unaffected. The rate of degradation of internalized 125 I‐ASOR was also inhibited by increasing sucrose concentrations. This inhibition is due to a block in the delivery of ligand to lysosomes and not an effect on degradation per se. In the presence of 0.2 M sucrose, the rate and extent of endocytosis of surface‐bound 125 I‐ASOR were, respectively, 33.0 ± 8.1% and 69.4 ± 10.5% (n = 8) of the control without sucrose. Under these conditions, the dissociation of internalized receptor‐ASOR complexes was completely inhibited. When sucrose was added, the effect on the endocytosis of surface‐bound 125 I‐ASOR was virtually immediate. Previous studies showed that about 40% of the surface‐bound 125 I‐ASOR which is internalized can return to the cell surface still bound to receptor (Weigel and Oka: J Biol Chem 259:1150, 1984). If 0.2 M sucrose was added after endocytosis occurred, 125 I‐ASOR still returned to the cell surface, although the rate and extent of return were inhibited by more than 50%. Interestingly, hyperosmolarity is the only treatment we have found which can reversibly inhibit, although only partially, the endocytosis of surface‐bound 125 I‐ASOR.