CELLULAR UPTAKE AND CATABOLISM OF HIGH-DENSITY-LIPOPROTEIN TRIACYLGLYCEROLS IN HUMAN CULTURED FIBROBLASTS - DEGRADATION BLOCK IN NEUTRAL LIPID STORAGE DISEASE

High-density lipoprotein (HDL)-[H-3]triolein (i.e. [H-3]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 I-125-HDL and HDL-[H-3]triolein, binding experiments (at 0 degrees C) followed by a short-time 'chase' at 37 degrees C showed that I-125 radioactivity was rapidly released in the culture medium (as trichloroacetic acid-precipitable material), whereas H-3 radioactivity remained associated with the cell. The cell-associated HDL-[H-3]triolein was rapidly degraded in normal fibroblasts, and the liberated [H-3]oleic acid was incorporated into newly biosynthesized phospholipids. In Wolman-disease fibroblasts HDL-[H-3]triolein was degraded at a normal rate, and thus independently of the lysosomal compartment. In contrast, the degradation of HDL-[H-3]triolein was blocked in fibroblasts from Neutral Lipid Storage Disease (NLSD), similarly to that of endogenously biosynthesized triacylglycerols [Radom, Salvayre, Negre, Maret and Douste-Blazy (1987) Eur. J. Biochem. 164, 703-708]. Trypsin-treated HDL-[H-3]triolein was also taken up by cells and degraded quite similarly to HDL-[H-3]triolein. In conclusion, all these data taken together suggest that HDL-[H-3]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 I-125-apoAs are released in the culture medium; (iii) directed to the same non-lysosomal catabolic pool (blocked in NLSD) as for endogenously biosynthesized triacylglycerols.