Jd. Medh et al., Hepatic triglyceride lipase promotes low density lipoprotein receptor-mediated catabolism of very low density lipoproteins in vitro, J LIPID RES, 40(7), 1999, pp. 1263-1275
We demonstrate here that hepatic triglyceride lipase (HTGL) enhances VLDL d
egradation in cultured cells by a LDL receptor-mediated mechanism, VLDL bin
ding at 4 degrees C and degradation at 37 degrees C by normal fibroblasts w
as stimulated by HTGL in a dose-dependent manner. A maximum increase of up
to 7-fold was seen at 10 mu g/ml HTGL, Both VLDL binding and degradation we
re significantly increased (4-fold) when LDL receptors were up-regulated by
treatment with lovastatin, HTGL also stimulated VLDL degradation by LDL re
ceptor-deficient FH fibroblasts but the level of maximal degradation was 40
-fold lower than in lovastatin-treated normal fibroblasts, A prominent role
for LDL receptors was confirmed by demonstration of similar HTGL-promoted
VLDL degradation by normal and LRP-deficient murine embryonic fibroblasts,
HTGL enhanced binding and internalization of apoprotein-free triglyceride e
mulsions, however, this was LDL receptor-independent. HTGL-stimulated bindi
ng and internalization of apoprotein-free emulsions was totally abolished b
y heparinase indicating that it was mediated by HSPG, In a cell-free assay
HTGL competitively inhibited the binding of VLDL to immobilized LDL recepto
rs at 4 degrees C suggesting that it may directly bind to LDL receptors but
may not bind VLDL particles at the same time. We conclude that the ability
of HTGL to enhance VLDL degradation is due to its ability to concentrate l
ipoprotein particles on HSPG sites on the cell surface leading to LDL recep
tor-mediated endocytosis and degradation.