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

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.