Assembly and Secretion of Apo B Containing Lipoproteins: The dyslipidemia of insulin resistance is very common and is associated with increased assembly and secretion of atherogenic apolipoprotein B (apoB) containing lipoproteins, particularly the triglyceride (TG)-enriched very low density lipoproteins (VLDL). Recently, hepatic steatosis, or non-alcoholic fatty liver disease (NAFLD), has emerged as an additional component of the phenotype. Pharmacologic approaches that would benefit both plasma lipid levels and hepatic fat are limited. A direct path to reducing plasma levels of atherogenic apoB-containing lipoproteins would be inhibition of apoB or TG synthesis and/or assembly into a VLDL. However, efforts to take this path have been associated with severe steatosis e.g., inhibition of microsomal triglyceride transfer protein (MTP) or modest steatosis e.g., inhibition of apoB synthesis using antisense oligonucleotides (ASO). Our preliminary data suggests that autophagy can play an important role in protecting the liver from steatosis while effectively reducing VLDL secretion. Thus, when we treated mice with MTP ASO, significant reductions in VLDL secretion were associated with severe steatosis. However, treatment with apoB ASO led to similar reductions in VLDL secretion but no increased steatosis. Further investigations revealed that apoB ASO treatment stimulated endoplasmic reticulum (ER) autophagy that was associated with increased fatty acid (FA) oxidation. Using this as a foundation, we will test the following hypothesis that reduced availability of apoB in the ER, without concomitant reductions in ER TG, stimulates autophagy, which protects the liver from steatosis with the following aims:
Aim 1 a. To characterize the effects of inhibiting apoB synthesis, without altering TG delivery to the ER lumen, on hepatic lipid homeostasis and autophagy using mouse models.
Aim 1 b. To characterize the molecular basis for ER autophagy in cells with inhibited apoB synthesis. We will then extend our studies beyond the specific model of inhibition of apoB synthesis to address the more common states where hepatic lipid metabolism is perturbed: increased FA delivery to the liver, insulin resistance with increased apoB secretion, and perturbations of ER function associated with ER stress. Here we will test the hypothesis that ER autophagy is a mechanism for maintaining homeostasis in the secretory pathway whenever abnormal VLDL assembly and/or secretion lead to ER stress and the UPR with two aims:
Aim 2 a. To characterize, in greater detail the mechanism whereby FA-induced ER stress and the UPR stimulate autophagy.
Aim 2 b. To characterize the effects of overexpression of apoB on ER stress, the UPR, and autophagy. The detailed and broad-based studies of autophagy and apoB proposed herein could provide new approaches to reducing VLDL secretion while avoiding hepatic steatosis.
Assembly and Secretion of Apo B Containing Lipoproteins: High levels of triglyceride (fat) and the bad LDL cholesterol are commonly present in people with metabolic syndrome or type 2 diabetes mellitus, and add to their risk for developing cardiovascular disease. We are studying how triglyceride and cholesterol are put together in the liver, with a protein called apoB, to make a package (VLDL) that enters the bloodstream and can itself get into the artier wall or, after delivery the triglyceride to fat tissue for storage, ecome LDL. Recently, more and more people with high levels of blood triglyceride and bad cholesterol have been identified to also have fatty liver. The link between fatty liver and too much fat in the blood is under intensive examination and our studies hope to shed new light on this problem. If we are successful at increasing our knowledge about these issues, we may find ways to reduce both fatty liver and heart disease.
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