Endothelial lipase facilitates low-density lipoprotein (LDL) uptake in LDL receptor deficiency by a heparan sulfate proteoglycan-dependent mechanism
Listed in
This article is not in any list yet, why not save it to one of your lists.Abstract
Background
Current lipid-lowering drugs reduce low-density lipoprotein (LDL) cholesterol by enhancing the LDL receptor (LDLR) pathway and are relatively ineffective in patients with Familial Hypercholesterolemia (FH) due to a dysfunctional LDLR. Angiopoietin-like 3 (ANGPTL3) inhibitors reduce LDL cholesterol in FH patients through an uncharacterized, LDLR-independent pathway that requires endothelial lipase (EL). Kinetic studies in FH patients showed that ANGPTL3 inhibitors directly enhanced LDL catabolism; however, EL’s role in this pathway remains unclear. Here, we aim to investigate the mechanisms by which EL mediates LDLR-independent uptake of LDL in hepatocytes.
Methods
CRISPR/Cas9-generated control and LDLR-KO HepG2 cells were transfected with an empty plasmid or a plasmid encoding the human LIPG gene, and the cellular uptake of fluorescent human LDL was measured by FACS. Additionally, to test the contribution of heparan sulfate proteoglycans (HSPG), cellular LDL uptake was assessed with and without the pre-incubation with heparin or a cocktail of heparinases. Finally, LDL uptake was measured after incubating cells with tetrahydrolipstatin (THL) to inhibit EL enzymatic activity.
Results
As expected, LDLR-KO HepG2 cells showed an 80% reduction in LDL uptake compared to controls (p<0.001). Remarkably, EL overexpression almost fully rescued LDL uptake in LDLR-KO cells (p<0.001), while no effect was observed in control cells. EL-mediated LDL uptake was completely blocked by heparinases and heparin in LDLR-KO cells, suggesting a crucial role of HSPG in the EL-mediated LDL uptake. Notably, treatment with THL reduced the cellular LDL uptake in LDLR-KO cells overexpressing EL (p=0.0015).
Conclusions
EL facilitates the uptake of LDL in hepatocytes through an LDLR-independent, HSPG-dependent pathway that involves EL activity. Our data provides an alternative mechanism to explain the reduction of LDL cholesterol induced by ANGPTL3 inhibitors. This pathway represents a potential druggable target to treat FH.
