Activation of ER stress and mTORC1 suppresses hepatic sortilin-1 levels in obese mice
Ding Ai, … , Daniel J. Rader, Alan R. Tall
Ding Ai, … , Daniel J. Rader, Alan R. Tall
Published April 2, 2012
Citation Information: J Clin Invest. 2012;122(5):1677-1687. https://doi.org/10.1172/JCI61248.
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Activation of ER stress and mTORC1 suppresses hepatic sortilin-1 levels in obese mice

Abstract

Recent GWAS have identified SNPs at a human chromosom1 locus associated with coronary artery disease risk and LDL cholesterol levels. The SNPs are also associated with altered expression of hepatic sortilin-1 (SORT1), which encodes a protein thought to be involved in apoB trafficking and degradation. Here, we investigated the regulation of Sort1 expression in mouse models of obesity. Sort1 expression was markedly repressed in both genetic (ob/ob) and high-fat diet models of obesity; restoration of hepatic sortilin-1 levels resulted in reduced triglyceride and apoB secretion. Mouse models of obesity also exhibit increased hepatic activity of mammalian target of rapamycin complex 1 (mTORC1) and ER stress, and we found that administration of the mTOR inhibitor rapamycin to ob/ob mice reduced ER stress and increased hepatic sortilin-1 levels. Conversely, genetically increased hepatic mTORC1 activity was associated with repressed Sort1 and increased apoB secretion. Treating WT mice with the ER stressor tunicamycin led to marked repression of hepatic sortilin-1 expression, while administration of the chemical chaperone PBA to ob/ob mice led to amelioration of ER stress, increased sortilin-1 expression, and reduced apoB and triglyceride secretion. Moreover, the ER stress target Atf3 acted at the SORT1 promoter region as a transcriptional repressor, whereas knockdown of Atf3 mRNA in ob/ob mice led to increased hepatic sortilin-1 levels and decreased apoB and triglyceride secretion. Thus, in mouse models of obesity, induction of mTORC1 and ER stress led to repression of hepatic Sort1 and increased VLDL secretion via Atf3. This pathway may contribute to dyslipidemia in metabolic disease.

Authors

Ding Ai, Juan M. Baez, Hongfeng Jiang, Donna M. Conlon, Antonio Hernandez-Ono, Maria Frank-Kamenetsky, Stuart Milstein, Kevin Fitzgerald, Andrew J. Murphy, Connie W. Woo, Alanna Strong, Henry N. Ginsberg, Ira Tabas, Daniel J. Rader, Alan R. Tall

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Figure 7

Mechanism of sortilin-1 regulation by ATF3.

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Mechanism of sortilin-1 regulation by ATF3. (A) 2 putative ATF3 binding ...
(A) 2 putative ATF3 binding sites were predicted in the SORT1 promoter region. SNPs (asterisk) in CELSR2-PSRC1-SORT1 locis, which can regulate the expression of SORT1, identified a high association with CVD by GWAs studies. Two putative ATF3 binding sites were predicted in SORT1 promoter region. (B) Human SORT1 promoter fragments SORT1–2.66 kb, SORT1–674 bp, and SORT1–458 bp (see Methods) were inserted into the reporter system and cotransfected with or without ATF3 plasmid in the 293 cell line. (C) SORT1–674 bp was used as template to make a mutation construct (674M), and both were cotransfected with or without ATF3 in 293 cells. (D) Sortilin-1 expression was reduced by overexpressing adeno-Atf3 in mouse primary hepatocytes. Quantification is also shown. Empty adenoviral vector served as control. (E) EMSA was performed in the nuclear extract (NE) protein (5 μg) from 293 cells with biotin-labeled probes containing the ATF3 or mutant binding site in the SORT1 promoter at –538 bp. Cells were treated directly with DMSO control or thapsigargin and preincubated with cold probe or ATF3 antibody. (F and G) ChIP analysis in Li-Tsc1KO mice and Li-Tsc1fl/fl controls (F) and in ob/ob and control lean mice (G) using IgG or ATF3 antibody. The approximate –310 to –440 region in the mouse promoter is homologous to a region in the human promoter containing a proximal ATF3 binding site. ANOVA revealed significant differences for treatment with –310 to –440 primers (F, F3,12 = 58.40, P < 0.0001; G, F3,15 = 523.1, P < 0.0001) and IL-6 primers (F, F3,12 = 127.0, P < 0.0001; G, F3,15 = 269.9, P < 0.0001). *P < 0.05, unpaired t test. #P < 0.05 as shown by brackets, Bonferroni post-test.