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 very-low-density lipoproteins (VLDL) or stored in lipid droplets (LDs). However, the regulatory mechanisms governing these two 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 single-nucleotide polymorphisms in SEC16B to be highly associated with serum lipid levels in humans. Hepatic Sec16b deficiency decreases serum lipid levels by impairing VLDL secretion through mechanisms that are at least partially independent of microsomal triglyceride transfer protein (MTP)-mediated ApoB lipidation and COPII-mediated intracellular trafficking. 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.