Original Article

MG132, a proteasome inhibitor, enhances LDL uptake in HepG2 cellsin vitro by regulating LDLR and PCSK9 expression

Authors: Hong YAN, Yan-ling MA, Yu-zhou GUI, Shu-mei WANG, Xin-bo WANG, Fei GAO, Yi-ping WANG
DOI: 10.1038/aps.2014.52

Abstract

Hong YAN, Yan-ling MA, Yu-zhou GUI, Shu-mei WANG, Xin-bo WANG, Fei GAO, Yi-ping WANG*
State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, Shanghai 201203, China

Aim: Expression of liver low-density lipoprotein receptor (LDLR), a determinant regulator in cholesterol homeostasis, is tightly controlled at multiple levels. The aim of this study was to examine whether proteasome inhibition could affect LDLR expression and LDL uptake in liver cells in vitro.
Methods: HepG2 cells were examined. Real-time PCR and Western blot analysis were used to determine the mRNA and protein levels, respectively. DiI-LDL uptake assay was used to quantify the LDLR function. Luciferase assay system was used to detect the activity of proprotein convertase subtilisin/kexin type 9 (PCSK9, a major protein mediating LDLR degradation) promoter. Specific siRNAs were used to verify the involvement of PCSK9.

Results: Treatment of HepG2 cells with the specific proteasome inhibitor MG132 (0.03–3 μmol/L) dose-dependently increased LDLR mRNA and protein levels, as well as LDL uptake. Short-term treatment with MG132 (0.3 μmol/L, up to 8 h) significantly increased both LDLR mRNA and protein levels in HepG2 cells, which was blocked by the specific PKC inhibitors GF 109203X, Gö 6983 or staurosporine. In contrast, a longer treatment with MG132 (0.3 μmol/L, 24 h) did not change LDLR mRNA, but markedly increased LDLR protein by reducing PCSK9-mediated lysosome LDLR degradation. Furthermore, MG132 time-dependently suppressed PCSK9 expression in the HepG2 cells through a SREBP-1c related pathway. Combined treatment with MG132 (0.3 μmol/L) and pravastatin (5 μmol/L) strongly promoted LDLR expression and LDL uptake in HepG2 cells, and blocked the upregulation of PCSK9 caused by pravastatin alone.

Conclusion: Inhibition of proteasome by MG132 in HepG2 cells plays dual roles in LDLR and PCSK9 expression, and exerts a beneficial effect on cholesterol homeostasis.

Keywords: cholesterol homeostasis; LDLR; MG132; proteasome inhibition; PKC; PCSK9; pravastatin; HepG2 cells

The project was supported by the National Natural Science Foundation of China (No 81220108012, 61335007, 81371684, 81000666, 81171395, and 81328012).
* To whom correspondence should be addressed.
E-mail ypwang@mail.shcnc.ac.cn
Received 2014-03-18 Accepted 2014-05-16
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