Sterol regulatory element binding proteins downregulate LDL receptor-related protein (LRP1) expression and LRP1-mediated aggregated LDL uptake by human macrophages.
OBJECTIVE
In the extracellular intima, extracellular matrix proteoglycans favor LDL retention and aggregation (agLDL). In contrast to native LDL (nLDL), agLDL induces high intracellular cholesteryl ester (CE) accumulation in macrophages. It has been suggested that LDL receptor-related protein (LRP1) is involved in agLDL binding and internalization by macrophages. The aim of this work was to analyze whether sterol regulatory element binding proteins (SREBPs) modulate LRP1 expression and LRP1-mediated agLDL uptake by human monocyte-derived macrophages (HMDM).


RESULTS
The treatment of HMDM with small anti-LRP1 interfering RNA (siRNA-LRP1) led to the specific inhibition of LRP1 mRNA expression and also to the inhibition of LRP1 protein expression in these cells. In siRNA-LRP1-treated HMDM, CE accumulation from agLDL uptake (84.66+/-5 microg CE/mg protein) was reduced by 95.76+/-5.22%. This suggests that LRP1 plays a pivotal role in agLDL uptake by HMDM. N-acetyl-leucyl-leucyl-norleucinal (ALLN), an inhibitor of SREBP catabolism, maintained high levels of active SREBP-2 and SREBP-1 even in the presence of nLDL and agLDL. Therefore, ALLN induced LDL receptor (LDLR) upregulation. Concomitantly, a strong downregulation of LRP1 mRNA and LRP1 protein was observed in ALLN-treated macrophages. By decreasing LRP1 expression levels, ALLN reduced CE accumulation from agLDL at all tested concentrations.


CONCLUSIONS
These results suggest that high levels of active SREBPs downregulate LRP1 expression and intracellular CE accumulation in HMDM.
Llorente-Cortés
Vicenta
V
Cardiovascular Research Center, CSIC-ICCC, Hospital de la Santa Creu i Sant Pau, and Departamento de Biologia Celular, School of Medicine, Universidad de Barcelona, Spain.
Royo
Teresa
T
Otero-Viñas
Marta
M
Berrozpe
Maria
M
Badimon
Lina
L
D000704:Analysis of Variance; D050197:Atherosclerosis / Q000378:metabolism; D015153:Blotting, Western / Q000379:methods; D002478:Cells, Cultured; D002788:Cholesterol Esters / Q000378:metabolism; D003331:Coronary Vessels / Q000378:metabolism; D015536:Down-Regulation; D005786:Gene Expression Regulation; D006801:Humans; D007976:Leupeptins / Q000494:pharmacology; D008077:Lipoproteins, LDL / Q000378:metabolism*; D026503:Low Density Lipoprotein Receptor-Related Protein-1 / Q000235:genetics* / Q000378:metabolism; D008264:Macrophages / Q000187:drug effects / Q000378:metabolism*; D034622:RNA Interference; D012333:RNA, Messenger / Q000032:analysis; D034741:RNA, Small Interfering / Q000494:pharmacology*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D018709:Statistics, Nonparametric; D051779:Sterol Regulatory Element Binding Proteins / Q000378:metabolism*
D002788:Cholesterol Esters; D007976:Leupeptins; D008077:Lipoproteins, LDL; D026503:Low Density Lipoprotein Receptor-Related Protein-1; D012333:RNA, Messenger; D034741:RNA, Small Interfering; D051779:Sterol Regulatory Element Binding Proteins; C063261:acetylleucyl-leucyl-norleucinal