Lipoprotein lipase binds to low density lipoprotein receptors and induces receptor-mediated catabolism of very low density lipoproteins in vitro.
Lipoprotein lipase (LPL), the major enzyme responsible for the hydrolysis of plasma triglycerides, promotes binding and catabolism of triglyceride-rich lipoproteins by various cultured cells. Recent studies demonstrate that LPL binds to three members of the low density lipoprotein (LDL) receptor family, including the LDL receptor-related protein (LRP), GP330/LRP-2, and very low density lipoprotein (VLDL) receptors and induces receptor-mediated lipoprotein catabolism. We show here that LDL receptors also bind LPL and mediate LPL-dependent catabolism of large VLDL with Sf 100-400. Up-regulation of LDL receptors by lovastatin treatment of normal human foreskin fibroblasts (FSF cells) resulted in an increase in LPL-induced VLDL binding and catabolism to a level that was 10-15-fold greater than in LDL receptor-negative fibroblasts, despite similar LRP activity in both cell lines. This indicates that the contribution of LRP to LPL-dependent degradation of VLDL is small when LDL receptors are maximally up-regulated. Furthermore studies in LRP-deficient murine embryonic fibroblasts showed that the level of LPL-dependent degradation of VLDL was similar to that in normal murine embryonic fibroblasts. LPL also promoted the internalization of protein-free triglyceride emulsions; lovastatin-treatment resulted in 2-fold higher uptake in FSF cells, indicating that LPL itself could bind to LDL receptors. However, the lower induction of emulsion catabolism as compared with native VLDL suggests that LPL-induced catabolism via LDL receptors is only partially dependent on receptor binding by LPL and instead is primarily due to activation of apolipoproteins such as apoE. A fusion protein between glutathione S-transferase and the catalytically inactive carboxyl-terminal domain of LPL (GST-LPLC) also induced binding and catabolism of VLDL. However GST-LPLC was not as active as native LPL, indicating that lipolysis is required for a maximal LPL effect. Mutations of critical tryptophan residues in GST-LPLC that abolished binding to VLDL converted the protein to an inhibitor of lipoprotein binding to LDL receptors. In solid-phase assays using immobilized receptors, LDL receptors bound to LPL in a dose-dependent manner. Both LPL and GST-LPLC promoted binding of VLDL to LDL receptor-coated wells. These results indicate that LPL binds to LDL receptors and suggest that the carboxyl-terminal domain of LPL contributes to this interaction.
Medh
J D
JD
Department of Internal Medicine, University of Iowa College of Medicine, Iowa City, Iowa 52242, USA.
Bowen
S L
SL
Fry
G L
GL
Ruben
S
S
Andracki
M
M
Inoue
I
I
Lalouel
J M
JM
Strickland
D K
DK
Chappell
D A
DA
D000409:Alanine; D001665:Binding Sites; D002384:Catalysis; D002478:Cells, Cultured; D004655:Emulsions; D005217:Fat Emulsions, Intravenous / Q000378:metabolism; D005347:Fibroblasts; D005982:Glutathione Transferase; D006801:Humans; D007700:Kinetics; D008071:Lipoprotein Lipase / Q000378:metabolism*; D008079:Lipoproteins, VLDL / Q000302:isolation & purification / Q000378:metabolism*; D008297:Male; D016297:Mutagenesis, Site-Directed; D017354:Point Mutation; D011973:Receptors, LDL / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D012867:Skin / Q000378:metabolism; D014364:Tryptophan
D004655:Emulsions; D005217:Fat Emulsions, Intravenous; D008079:Lipoproteins, VLDL; D011973:Receptors, LDL; D011993:Recombinant Fusion Proteins; D014364:Tryptophan; D005982:Glutathione Transferase; D008071:Lipoprotein Lipase; D000409:Alanine