BINDING AND INTRACELLULAR FATE OF BETA-VERY LOW-DENSITY-LIPOPROTEIN IN ISOLATED RAT-LIVER PARENCHYMAL-CELLS

Binding of rabbit I-125-tyramine-cellobiose-beta-very low density lipoprotein ((ITC)-I-125-beta-VLDL) was saturable both in suspended and cultured rat liver parenchymal cells, and in isolated rat liver membrane fractions. The specific binding had K-D values ranging from 8 to 10 mu g/ml. At 37 degrees C beta-VLDL was internalized and degraded in parenchymal cells in culture, but only negligible degradation was measured in suspended cells. In cultured cells the degradation was inhibited by lysosomal and microtubular inhibitors, these agents had no effects in suspended cells. Studies of release suggested internalization in suspended cells and subcellular fractionation experiments confirmed that internalized (ITC)-I-125-beta-VLDL reached the lysosomal compartment in cultured cells, but not in suspended cells. Since lactosylated (lac-) beta-VLDL was taken up and degraded efficiently by suspended cells, these cells are not in general unable to transport large lipoprotein particles to the lysosomes. We believe that beta-VLDL does not dissociate from the receptor in the endosomes and therefore is retroendocytosed to the plasma membrane. In isolated parenchymal cells the beta-VLDL binding site is supposedly different from the methylamine-activated-alpha(2)-macroglobulin (ma-alpha(2)M) binding site, since; 1) Excess beta-VLDL did not reduce ma-alpha(2)M binding; 2) In suspended parenchymal cells ma-alpha(2)M was readily degraded, whereas beta-VLDL was not degraded and 3) The binding of beta-VLDL was not Ca++-dependent. We therefore conclude that beta-VLDL is taken up by an apolipoprotein E specific receptor on rat parenchymal liver cells that is a Ca++-independent receptor different from the alpha(2)M receptor and the low density lipoprotein receptor.