Cardiovascular diseases cause 17 million deaths annually, which is estimated to increase to 23 million deaths by the year 2030. One of the major risk factors for the pathogenesis of cardiovascular diseases is increased secretion of very-low density lipoproteins (VLDL) by the liver; however, reduced VLDL-secretion causes fatty liver disease. Synthesis and secretion of VLDL by the liver plays an important role in maintaining overall lipoprotein homeostasis. Assembly of VLDL occurs along with the expression of apolipoproteinB-100 (apoB100) and its lipidation at the endoplasmic reticulum (ER) level. Once formed in the ER lumen, the nascent VLDL is transported to the Golgi for its maturation. In the Golgi compartment, the nascent VLDL acquires apolipoproteinAI (apoAI), more triglycerides, and its apoB100 undergoes phosphorylation and glycosylation. These modifications are necessary for VLDL-exit from the trans-Golgi network (TGN) and this step is mediated by post-Golgi VLDL transport vesicle (PG-VTV). The transport of mature VLDL from the TGN to the plasma membrane (PM) is required for its secretion by the liver but remains to be studied. Our group has shown that the nascent VLDL particles do not contain apoAI, however, VLDL acquires apoAI in the cis-Golgi compartment. Interestingly, apoAI comes off the VLDL as soon as VLDL is secreted into the blood. We hypothesised that apoAI plays an important role in post-TGN VLDL trafficking and thus controls VLDL secretion by the liver. To determine the role of apoAI in the formation of PG-VTV and VLDL secretion, we knocked down apoAI in the hepatocytes using apoAI specific siRNA. The deficiency of apoAI did not have any effect on the expression of apoB100 and other apolipoprotein synthesis that are involved in VLDL synthesis; however, VLDL secretion was significantly reduced. Next, we overexpressed apoAI using plasmid with apoAI gene sequence and checked for the effects in VLDL secretion from the hepatocytes. We observed a significant increase in VLDL secretion from apoAI-overexpressing hepatocytes which is consistent with knockdown results. To determine the role of apoAI in post-TGN trafficking of the mature VLDLs, we isolated sub-cellular organelles from apoAI knockout (apoAI KO) and control mice. Subsequently, we performed in vitro PG-VTV budding assays to assess the effect of apoAI silencing on PG-VTV formation from the TGN. Our results strongly suggest that the deficiency of apoAI increases PG-VTV formation (i.e. TGN-exit of mature VLDL) but significantly reduces VLDL-triglyceride secretion from the hepatocytes. We conclude that apoAI controls VLDL secretion by the liver by regulating post-TGN trafficking of mature VLDL.
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Master of Science (M.S.)
College of Medicine
Molecular Biology and Microbiology
Biotechnology; Professional Science Master's
Length of Campus-only Access
Masters Thesis (Campus-only Access)
Gurwani, Bhavesh, "Apolipoprotein-AI Regulates Hepatic VLDL Secretion by Controlling Intracellular VLDL-Trafficking" (2016). Electronic Theses and Dissertations. 5458.