Binding of lipoproteins and regulation of cholesterol synthesis in cultured mouse adipose cells

Abstract

Binding of human lipoproteins to cultured mouse Ob17 preadipose and adipose cells was studied, using labeled VLDL, LDL and apoprotein E-free HDL. In each case, saturation curves were obtained, yielding linear Scatchard plots. The K d values were found to be respectively 6.4, 31 and 24 μg/ml for VLDL, LDL and apoprotein E-free HDL, whereas the maximal numbers of binding sites per cell were 4.2 · 10 4, 1.5 · 10 4 and 2.5 · 10 5. The binding of 125I-LDL was competitively inhibited by LDL > VLDL > total HDL ; human LDL and mouse LDL were equipotent in competition assays. Methylated LDL and apoprotein E-free HDL were not competitors. In contrast, the binding of 125I-apoprotein E-free HDL was competitively inhibited by apoprotein E-free HDL > total HDL and the binding of 125I-HDL 3 by mouse HDL. Thus, mouse adipose cells possess distinct apoprotein B, E and apoprotein E-free HDL binding sites which can recognize heterologous or homologous lipoproteins. The cell surface receptor of LDL in mouse preadipose cells shows similarities with that described for human fibroblasts, since: (1) the LDL binding initiated the process of internalization and degradation of the apoprotein B and apoprotein E-containing lipoproteins; (2) receptor-mediated uptake of cholesterol LDL led to a parallel but incomplete decrease in the [ 14C]acetate incorporation into cholesterol and in the activity of HMG-CoA reductase. Growing (undifferentiated) or growth-arrested cells (differentiated or not) showed no significant changes in the K d values for lipoprotein binding. In contrast, the maximal number of binding sites correlated with the proliferative state of the cells and was independent of cell differentiation. The results are discussed with respect to cholesterol accumulation in adipose cells.