Endocytosis of hepatic lipase and lipoprotein lipase into rat liver hepatocytes in vivo is mediated by the low density lipoprotein receptor-related protein.
In isolated cell studies, the internalization and degradation of hepatic lipase (HL) has been linked to its binding to the low density lipoprotein receptor-related protein (LRP). We have utilized the receptor-associated protein (RAP), a universal inhibitor of high affinity ligand binding to LRP, to evaluate the participation of LRP in the endocytosis of HL and lipoprotein lipase (LPL). We isolated a total endosome fraction from rat livers after a 30-min infusion of recombinant RAP, administered as a glutathione S-transferase conjugate (GST-RAP). GST-RAP infusion had no effect on the concentration of HL in liver homogenates, but its concentration in blood plasma increased progressively by 20%, and enrichment over homogenate of HL in endosomes was reduced by 50% as compared with infusion of GST alone. The concentrations of LPL in liver and plasma were 1.4 and 0.5%, respectively, those of HL, but endosomal enrichment of the two enzymes was similar ( approximately 10-fold). GST-RAP infusion had no effect on the concentration of LPL in liver but increased its concentration in blood plasma by 250% and reduced its endosomal enrichment by 95% or greater. GST-RAP infusion also reduced endosomal enrichment of LRP by 40%, but enrichment of several other endocytic receptors was unaffected. Endosomal enrichment of several membrane trafficking proteins associated with the endocytic pathway in hepatocytes was unaffected by GST-RAP with the exception of early endosome endosome antigen 1, which was reduced by 85%. We conclude that HL is partially and LPL almost exclusively taken up into rat hepatocytes after binding to the endocytic receptor LRP.
Vergés
Marcel
M
Cardiovascular Research Institute, University of California, San Francisco, California 94143, USA.
Bensadoun
Andre
A
Herz
Joachim
J
Belcher
John D
JD
Havel
Richard J
RJ
D000818:Animals; D004705:Endocytosis; D022781:Hepatocytes / Q000378:metabolism; D008049:Lipase / Q000378:metabolism*; D008071:Lipoprotein Lipase / Q000378:metabolism*; D008099:Liver / Q000378:metabolism*; D026503:Low Density Lipoprotein Receptor-Related Protein-1 / Q000378:metabolism*; D008297:Male; D051381:Rats; D017207:Rats, Sprague-Dawley; D015398:Signal Transduction
D026503:Low Density Lipoprotein Receptor-Related Protein-1; D008049:Lipase; D008071:Lipoprotein Lipase