Hepatic SEC16B regulates lipid homeostasis by coordinating VLDL secretion and lipid droplet expansion
Wei Lu, Zhiming Zhao, Donald Molina, Huaxun Fan, Ruicheng Shi, Ye Tian, Raja Gopoju, Tiantian Yang, Xinyuan Zhang, Yanqiao Zhang, Kai Zhang, Jaume Amengual, Bo Wang
Wei Lu, Zhiming Zhao, Donald Molina, Huaxun Fan, Ruicheng Shi, Ye Tian, Raja Gopoju, Tiantian Yang, Xinyuan Zhang, Yanqiao Zhang, Kai Zhang, Jaume Amengual, Bo Wang
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Research Article Hepatology Metabolism

Hepatic SEC16B regulates lipid homeostasis by coordinating VLDL secretion and lipid droplet expansion

Abstract

The liver plays a critical role in lipid homeostasis, where lipids are either secreted as VLDLs or stored in lipid droplets (LDs). However, the regulatory mechanisms governing these 2 interconnected processes remain poorly understood. Here, we demonstrate that SEC16B functions as a lipid-responsive regulator in the liver, promoting VLDL secretion and LD expansion to handle lipid flux and maintain lipid homeostasis. Genome-wide association studies have identified SNPs in SEC16B to be highly associated with serum lipid levels in humans. Hepatic Sec16b deficiency decreases serum lipid levels by impairing VLDL secretion via disruption of COPII-mediated intracellular trafficking and through mechanisms partially independent of microsomal triglyceride transfer protein–mediated ApoB lipidation. SEC16B partially localizes at ER-LD contact sites and promotes LD expansion by facilitating the targeting of ER proteins to LDs. More importantly, suppression of Sec16b dramatically lowers serum lipid levels and reduces atherosclerotic lesion size in Ldlr null mice. These data reveal a mechanism that coordinates VLDL and LD metabolism and suggest SEC16B as a potential therapeutic target for atherosclerosis treatment.

Authors

Wei Lu, Zhiming Zhao, Donald Molina, Huaxun Fan, Ruicheng Shi, Ye Tian, Raja Gopoju, Tiantian Yang, Xinyuan Zhang, Yanqiao Zhang, Kai Zhang, Jaume Amengual, Bo Wang

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Figure 4

SEC16B modulates the interaction between SEC13 and SEC31A, thereby promoting ApoB secretion.

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SEC16B modulates the interaction between SEC13 and SEC31A, thereby promo...
(A) Immunostaining and quantification of relative intensity of SAR1-GTP (red) in shCON and shSEC16B Huh7 cells cultured in DMEM+OA for 6 hours. Scale bars: 8 μm. (B) Immunostaining and quantification of relative intensity of SAR1-GTP (red) in F/F and LKO primary hepatocytes cultured in maintenance medium. Scale bars: 8 μm. (C) Representative Western blot images and quantification of ApoB levels in the cell lysate and medium of shCON and shSEC16B Huh7 cells cultured in DMEM+OA for 16 hours. Cells were transfected with vector control or SAR1A-HA (n = 4). (D–F) Colocalization (D) and Pearson’s coefficients (E and F) of endogenous SEC31A (purple), SEC24B (red), and exogenous SEC23A (green) in shCON and shSEC16B Huh7 cells transfected with eGFP-SEC23A and cultured in DMEM+OA for 6 hours (n = 3). Scale bars: 8 μm. (G and H) Colocalization (G) and Pearson’s coefficients (H) of endogenous SEC24B (green) and SEC31A (red) in shCON and shSEC16B Huh7 cells cultured in DMEM+OA for 6 hours (n = 3). Scale bars: 8 μm. (I and J) Colocalization (I) and Pearson’s coefficients (J) of endogenous SEC13 (green) and SEC31A (red) in shCON and shSEC16B Huh7 cells cultured in DMEM+OA for 6 hours (n = 3). Scale bars: 8 μm. (K and L) Co-IP (K) and quantification (L) of SEC31A interaction with SEC13 in shCON and shSEC16B Huh7 cells cultured in DMEM+OA for 6 hours. The relative interaction levels between SEC31A and SEC13 were determined by normalizing the SEC13 signal to the SEC31A signal within the immunocomplex (n = 3). (M) SEC16B-FLAG and HA-SEC13 co-IP assay in HEK293T cells (n = 3). (N) Colocalization and Pearson’s coefficients of exogenous SAR1A (green) and endogenous SEC31A (red) in shCON and shSEC16B Huh7 cells transfected with SAR1A-HA and cultured in DMEM+OA for 6 hours (n = 3). Scale bars: 8 μm. Values are presented as mean ± SEM or as violin plots. Statistical analysis was performed with 2-way ANOVA (C) or 2-tailed Student’s t test (A, B, E, F, H, J, L, and N). *P < 0.05, ***P < 0.001, ****P < 0.0001.