Cellular uptake and catabolism of high-density-lipoprotein triacylglycerols in human cultured fibroblasts: degradation block in neutral lipid storage disease

High-density lipoprotein (HDL)-[3H]triolein (i.e. [3H]triolein incorporated into reconstituted HDL) was taken up by cultured fibroblasts through an apparently saturable process, competitively inhibited by non-labelled HDL and independent of the LDL receptor. Using 125I-HDL and HDL-[3H]triolein, binding experiments (at 0 degrees C) followed by a short-time ‘chase’ at 37 degrees C showed that 125I radioactivity was rapidly released in the culture medium (as trichloroacetic acid-precipitable material), whereas 3H radioactivity remained associated with the cell. The cell-associated HDL-[3H]triolein was rapidly degraded in normal fibroblasts, and the liberated [3H]oleic acid was incorporated into newly biosynthesized phospholipids. In Wolman-disease fibroblasts HDL-[3H]triolein was degraded at a normal rate, and thus independently of the lysosomal compartment. In contrast, the degradation of HDL-[3H]triolein was blocked in fibroblasts from Neutral Lipid Storage Disease (NLSD), similarly to that of endogenously biosynthesized triacylglycerols [Radom, Salvayre, Nègre, Maret and Douste-Blazy (1987) Eur. J. Biochem. 164, 703-708]. Trypsin-treated HDL-[3H]triolein was also taken up by cells and degraded quite similarly to HDL-[3H]triolein. In conclusion, all these data taken together suggest that HDL-[3H]triolein is: (i) associated with the cell through a process independent of intact apolipoprotein (apo) As, thus probably independent of an apoA-receptor-mediated uptake; (ii) internalized by cells, whereas 125I-apoAs are released in the culture medium; (iii) directed to the same non-lysosomal catabolic pool (blocked in NLSD) as for endogenously biosynthesized triacylglycerols.