α-Tocopherol reduces VLDL secretion through modulation of intracellular ER-to-Golgi transport of VLDL

Avoiding hepatic steatosis is crucial for preventing liver dysfunction, and one mechanism by which this is accomplished is through synchronization of the rate of very low density lipoprotein (VLDL) synthesis with its secretion. Endoplasmic reticulum (ER)-to-Golgi transport of nascent VLDL is the rate-limiting step in its secretion and is mediated by the VLDL transport vesicle (VTV). Recent in vivo studies have indicated that α-tocopherol (α-T) supplementation can reverse steatosis in nonalcoholic fatty liver disease, but its effects on hepatic lipoprotein metabolism are poorly understood. Here, we investigated the impact of α-T on hepatic VLDL synthesis, secretion, and intracellular ER-to-Golgi VLDL trafficking using an in vitro model. Pulse-chase assays using [3H]-oleic acid and 100 µmol/L α-T demonstrated a disruption of early VLDL synthesis, resulting in enhanced apolipoprotein B-100 expression, decreased expression in markers for VTV budding, ER-to-Golgi VLDL transport, and reduced VLDL secretion. Additionally, an in vitro VTV budding assay indicated a significant decrease in VTV production and VTV-Golgi fusion. Confocal imaging of lipid droplet (LD) localization revealed a decrease in overall LD retention, diminished presence of ER-associated LDs, and an increase in Golgi-level LD retention. We conclude that α-T disrupts ER-to-Golgi VLDL transport by modulating the expression of specific proteins and thus reduces VLDL secretion.

Cathepsin B regulates hepatic lipid metabolism by cleaving liver fatty acid-binding protein

Synthesis and secretion of hepatic triglycerides (TAG) associated with very-low-density lipoprotein (VLDL) play a major role in maintaining overall lipid homeostasis. This study aims to identify factors affecting synthesis and secretion of VLDL-TAG using the growth hormone-deficient Ames dwarf mouse model, which has reduced serum TAG. Proteomic analysis coupled with a bioinformatics-driven approach revealed that these mice express greater amounts of hepatic cathepsin B and lower amounts of liver fatty acid-binding protein (LFABP) than their wildtype littermates. siRNA-mediated knockdown of cathepsin B in McA-RH7777 cells resulted in a 39% increase in [³H]TAG associated with VLDL secretion. Cathepsin B knockdown was accompanied by a 74% increase in cellular LFABP protein levels, but only when cells were exposed to 0.4 mm oleic acid (OA) complexed to BSA. The cathepsin B knockdown and 24-h treatment with OA resulted in increased CD36 expression alone and additively. Co-localization of LFABP and cathepsin B was observed in a distinct Golgi apparatus-like pattern, which required a 1-h OA treatment. Moreover, we observed co-localization of LFABP and apoB, independent of the OA treatment. Overexpression of cathepsin B resulted in decreased OA uptake and VLDL secretion. Co-expression of cathepsin B and cathepsin B-resistant mutant LFABP in McA-RH7777 cells resulted in an increased TAG secretion as compared with cells co-expressing cathepsin B and wildtype LFABP. Together, these data indicate that cathepsin B regulates VLDL secretion and free fatty acid uptake via cleavage of LFABP, which occurs in response to oleic acid exposure.

Silencing of Small Valosin-containing Protein-interacting Protein (SVIP) Reduces Very Low Density Lipoprotein (VLDL) Secretion from Rat Hepatocytes by Disrupting Its Endoplasmic Reticulum (ER)-to-Golgi Trafficking

The transport of nascent very low density lipoprotein (VLDL) particles from the endoplasmic reticulum (ER) to the Golgi determines their secretion by the liver and is mediated by a specialized ER-derived vesicle, the VLDL transport vesicle (VTV). Our previous studies have shown that the formation of ER-derived VTV requires proteins in addition to coat complex II proteins. The VTV proteome revealed that a 9-kDa protein, small valosin-containing protein-interacting protein (SVIP), is uniquely present in these specialized vesicles. Our biochemical and morphological data indicate that the VTV contains SVIP. Using confocal microscopy and co-immunoprecipitation assays, we show that SVIP co-localizes with apolipoprotein B-100 (apoB100) and specifically interacts with VLDL apoB100 and coat complex II proteins. Treatment of ER membranes with myristic acid in the presence of cytosol increases SVIP recruitment to the ER in a concentration-dependent manner. Furthermore, we show that myristic acid treatment of hepatocytes increases both VTV budding and VLDL secretion. To determine the role of SVIP in VTV formation, we either blocked the SVIP protein using specific antibodies or silenced SVIP by siRNA in hepatocytes. Our results show that both blocking and silencing of SVIP lead to significant reduction in VTV formation. Additionally, we show that silencing of SVIP reduces VLDL secretion, suggesting a physiological role of SVIP in intracellular VLDL trafficking and secretion. We conclude that SVIP acts as a novel regulator of VTV formation by interacting with its cargo and coat proteins and has significant implications in VLDL secretion by hepatocytes.