Secretion of triacylglycerol-enriched very-low-density lipoproteins (VLDLs) from the liver is vital for maintaining plasma lipid homeostasis. However, the process of VLDL assembly and lipidation is not well characterized. Here, we observed that liver of Cideb null mice had higher levels of triacylglycerols accompanied by low level of VLDL secretion. Furthermore, VLDL particles secreted from hepatocytes of Cideb null mice have low levels of triacylglycerols but normal levels of apoB. We also observed that Cideb is localized to endoplasmic reticulum and lipid droplets. Importantly, we have identified apoB as a Cideb-interacting protein. By infecting adenoviruses expressing various Cideb truncations into hepatocytes of Cideb null mice, we found that Cideb requires both its apoB-binding and lipid droplet association domains to restore the secretion of triacylglycerol-enriched VLDL particles. Our data suggest that Cideb promotes the formation of triacylglycerol-enriched VLDL particles and provides a molecular insight into VLDL lipidation and maturation in hepatocytes.

High levels of dietary saturated fat have been closely associated with the development of hepatic steatosis, but the factors that mediate this process remain elusive. Here, we observed that the level of cell death-inducing DNA fragmentation factor-alpha-like effector a (Cidea) expression was highly correlated with the severity of hepatic steatosis in humans. Overexpression of Cidea in mouse liver resulted in increased hepatic lipid accumulation and the formation of large lipid droplets (LDs). In contrast, mice with a Cidea deficiency had decreased lipid accumulation and alleviated hepatic steatosis when they received a high-fat-diet feeding or in ob/ob mice. Furthermore, the knockdown of Cidea in livers of ob/ob mice resulted in significantly reduced hepatic lipid accumulation and smaller LDs. Importantly, we observed that Cidea expression in hepatocytes was specifically induced by saturated fatty acids (FAs), and such induction was reduced when sterol response element-binding protein (SREBP)1c was knocked down. In contrast, the overexpression of SREBP1c restored the saturated FA-induced expression of Cidea. In addition, we observed that the stability of Cidea protein in hepatocytes increased significantly in response to treatment with FAs. Conclusion: Cidea plays critical roles in promoting hepatic lipid accumulation and in the development of hepatic steatosis by acting as a sensor that responds to diets that contain FAs. (Hepatology 2012;56:95–107)

triglycerides (TAG) and cholesterol and for the development of metabolic disorders, including obesity, diabetes, and hepatic steatosis. It is believed that VLDL assembly and maturation involves two steps ( 1, 2 ). The fi rst step is the formation of lipid-poor pre-VLDL particles involving the cotranslational lipidation of apolipoprotein B (apoB) with a few lipids, aided by microsomal triglyceride transfer protein (MTP) ( 3, 4 ). The second step is the transfer of large quantities of lipids, likely from TAG-rich lipid droplets (LD), to the pre-VLDL particles, converting the pre-VLDL particles into lipid-rich, mature VLDL particles ( 2-5 ). Thus far, the subcellular site of the addition of bulk lipids to pre-VLDL particles, the source of the lipids, and the factors promoting VLDL lipidation remain unclear. The endoplasmic reticulum (ER) has been suggested as the VLDL lipidation site because the majority of TAG is synthesized in the ER, and apoB-100 has been shown to be localized to the ER membrane ( 2, 6, 7 ). Others believe that the pre-VLDL particles are assembled in the ER and then exit, with the fi nal VLDL lipidation and maturation occurring primarily within the Golgi apparatus or post-ER compartments ( 8-11 ). For example, the biochemical separation of the lipoprotein particles from McA-RH7777 cells indicated that mature VLDL particles accumulate in the Golgi but not in the ER fractions ( 11,12 ). In addition, the late addition of bulk lipids to pre-VLDL particles is independent of both MTP activity and new triglyceride synthesis ( 12, 13 ).The availability of neutral lipids and the capacity to transfer TAG to pre-VLDL particles are crucial for VLDL lipidation and maturation. The LD, a subcellular organelle, Abstract Regulation of hepatic very low density lipoprotein (VLDL) assembly and maturation is crucial in controlling lipid homeostasis and in the development of metabolic disorders, including obesity, hepatic steatosis, and insulin resistance. Cideb, a member of cell death-inducing DFF45-like effector (CIDE) protein family, has been previously shown to promote VLDL lipidation and maturation. However, the precise subcellular location of Cideb-mediated VLDL lipidation and the factors modulating its activity remain elusive. In addition to its localization to endoplasmic reticulum (ER) and lipid droplets (LD), we observed that Cideb was also localized to the Golgi apparatus. Mature and lipid-rich VLDL particles did not accumulate in the Golgi apparatus in Cideb ؊ / ؊ livers. Interestingly, we observed that hepatic perilipin 2/adipose differentiation-related protein (ADRP) levels were markedly increased in Cideb ؊ / ؊ mice. Liver-specifi c knockdown of perilipin 2 in Cideb ؊ / ؊ mice resulted in the reduced accumulation of hepatic triglycerides (TAG), increased VLDL-TAG secretion, and the accumulation of mature TAG-rich VLDL in the Golgi apparatus. These data reveal that Cideb and perilipin 2 play opposing roles in controlling VLDL lipidation and hepatic lipid homeostasis. China,