TRIB1 regulates LDL metabolism through CEBPα-mediated effects on the LDL receptor in hepatocytes
Katherine Quiroz-Figueroa, Cecilia Vitali, Donna M. Conlon, John S. Millar, John W. Tobias, Robert C. Bauer, Nicholas J. Hand, Daniel J. Rader
Katherine Quiroz-Figueroa, Cecilia Vitali, Donna M. Conlon, John S. Millar, John W. Tobias, Robert C. Bauer, Nicholas J. Hand, Daniel J. Rader
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Research Article Genetics Metabolism

TRIB1 regulates LDL metabolism through CEBPα-mediated effects on the LDL receptor in hepatocytes

Abstract

Genetic variants near the TRIB1 gene are highly significantly associated with plasma lipid traits and coronary artery disease. While TRIB1 is likely causal of these associations, the molecular mechanisms are not well understood. Here we sought to investigate how TRIB1 influences low density lipoprotein cholesterol (LDL-C) levels in mice. Hepatocyte-specific deletion of Trib1 (Trib1Δhep) in mice increased plasma cholesterol and apoB and slowed the catabolism of LDL-apoB due to decreased levels of LDL receptor (LDLR) mRNA and protein. Simultaneous deletion of the transcription factor CCAAT/enhancer-binding protein alpha (CEBPα) with TRIB1 eliminated the effects of TRIB1 on hepatic LDLR regulation and LDL catabolism. Using RNA-seq, we found that activating transcription factor 3 (Atf3) was highly upregulated in the livers of Trib1Δhep but not Trib1Δhep CebpaΔhep mice. ATF3 has been shown to directly bind to the CEBPα protein, and to repress the expression of LDLR by binding its promoter. Blunting the increase of ATF3 in Trib1Δhep mice reduced the levels of plasma cholesterol and partially attenuated the effects on LDLR. Based on these data, we conclude that deletion of Trib1 leads to a posttranslational increase in CEBPα, which increases ATF3 levels, thereby contributing to the downregulation of LDLR and increased plasma LDL-C.

Authors

Katherine Quiroz-Figueroa, Cecilia Vitali, Donna M. Conlon, John S. Millar, John W. Tobias, Robert C. Bauer, Nicholas J. Hand, Daniel J. Rader

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Figure 2

Hepatic deletion of Trib1 impairs LDL clearance due to decreased LDLR mRNA and protein.

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Hepatic deletion of Trib1 impairs LDL clearance due to decreased LDLR mR...
(A and B) LDL clearance in chow-fed mice 8 weeks after AAV injection (n = 5) and (C and D) in mice fed WTD for 19 weeks (24 weeks after AAV injection) (A–D) Mice were injected with 125I-radiolabeled LDL isolated from human plasma. Clearance of 125I-LDL was determined by measuring residual 125I activity at different time points after injection (from 2 minutes to 24 hours). Residual 125I activity is expressed as fraction of the total 125I activity, 2 minutes after injection. (B and D) Fractional catabolic rate (FCR) of LDL from A and C, calculated from the reciprocal of the area under a fitted biexponential curve, and it represents fraction of LDL cleared per hour. Results were confirmed in an independent cohort in chow-fed mice and 2 independent cohorts in WTD-fed mice. (E) VLDL clearance in chow-fed male mice 8 weeks after AAV injection. Mice were injected with 125I-radiolabeled human VLDL, and the total remaining counts were normalized to the injected dose at 1 minute after injection of 125IVLDL. (F) FCR of 125I-VLDL. (G) Hepatic transcript levels of LDLR in chow-fed male Trib1Δhep mice relative to control mice 8 weeks after AAV injection (n = 5). The relative quantity of mRNA normalized to the combined mean Ct of Mrlp19, Ywhaz, and Ipo8, and expressed relative to the mean of the control group. (H) Hepatic protein levels of LDLR and β-actin in control and Trib1Δhep mice (n = 5). Both LDLR mRNA and protein levels changes were confirmed in 3 independent cohorts. (A, C, and D) Data are expressed as mean ± SEM for the experimental group. (B, D, F, and G) Box plots indicate median and 25th and 75th percentiles, with whiskers extending to minimum and maximum values. Symbols indicate individual values. Significance was determined by 2-tailed, unpaired Student’s t test (**P ≤ 0.01, ***P ≤ 0.001, ****P ≤ 0.0001).