Effect of osmolarity on LDL binding and internalization in hepatocytes.
The present study has been performed to elucidate a possible role of cell volume in low-density lipoprotein (LDL) binding and internalization (LDL(b+i)). As shown previously, increase of extracellular osmolarity (OSMe) and K+ depletion, both known to shrink cells, interfere with the formation of clathrin-coated pits and thus with LDL(b+i). On the other hand, alterations of cell volume have been shown to modify lysosomal pH, which is a determinant of LDL(b+i). LDL(b+i) have been estimated from heparin-releasable (binding) or heparin-insensitive (internalization) uptake of 125I-labeled LDL. OSMe was modified by alterations of extracellular concentrations of ions, glucose, urea, or raffinose. When OSMe was altered by varying NaCl concentrations, LDL(b+i) decreased (by 0.5 +/- 0.1%/mM) with increasing OSMe and LDL(b+i) increased (by 1.2 +/- 0.1%/mM) with decreasing OSMe, an effect mainly due to altered affinity; the estimated dissociation constant amounted to 20.6, 48.6, and 131.6 micro/ml at 219, 293, and 435 mosM, respectively. A 25% increase of OSMe increased cytosolic (by 0.46 +/- 0.03) and decreased lysosomal (by 0.14 +/- 0.02) pH. Conversely, a 25% decrease of OSMe decreased cytosolic (by 0.28 +/- 0.02) and increased lysosomal (by 0.17 +/- 0.02) pH. Partial replacement of extracellular Na+ with K+ had little effect on LDL(b+i), although it swelled hepatocytes and increased lysosomal and cytosolic pH. Hypertonic glucose, urea, or raffinose did not exert similar effects despite a shrinking effect of hypertonic raffinose. Monensin, which completely dissipates lysosomal acidity, virtually abolished LDL(b+i). In conclusion, the observations reveal a significant effect of ionic strength on LDL(b+i). The effect is, however, not likely to be mediated by alterations of cell volume or alterations of lysosomal pH.
Krämer-Guth
A
A
Department of Internal Medicine, University of Freiburg, Germany.
Busch
G L
GL
Kaba
N K
NK
Schwedler
S
S
Wanner
C
C
Lang
F
F
D006528:Carcinoma, Hepatocellular; D005110:Extracellular Space / Q000502:physiology; D006493:Heparin / Q000494:pharmacology; D006801:Humans; D006863:Hydrogen-Ion Concentration; D006982:Hypertonic Solutions; D007038:Hypotonic Solutions; D007700:Kinetics; D008077:Lipoproteins, LDL / Q000378:metabolism*; D008099:Liver / Q000378:metabolism*; D008113:Liver Neoplasms; D008247:Lysosomes / Q000502:physiology; D009994:Osmolar Concentration; D011188:Potassium / Q000494:pharmacology; D011973:Receptors, LDL / Q000378:metabolism*; D014407:Tumor Cells, Cultured
D006982:Hypertonic Solutions; D007038:Hypotonic Solutions; D008077:Lipoproteins, LDL; D011973:Receptors, LDL; D006493:Heparin; D011188:Potassium