Triglyceride rich lipoproteins and Apolipoprotein C-III in Atherosclerosis

Abstract

While the causal role of low-density lipoprotein cholesterol (LDL-C) in atherosclerosis and the clinical benefit of lipid lowering with statins are well established, the impact of triglyceride rich lipoproteins (TRL) in cardiovascular disease is less well understood. Given the important association between hypertriglyceridaemia and both obesity and type 2 diabetes, mechanistic studies are required to further understand the role of TRL as both a causal factor and potential target for therapeutic modification. This thesis aims to investigate the impact of both TRL and Apolipoprotein C-III (ApoC-III), an important factor that regulates TRL metabolism, in atherosclerosis. It demonstrated the adverse effect of oxidised TRL on endothelial cells following co-incubation studies in vitro. It also described the presence of ApoC-III within atherosclerotic lesions in an animal model of diabetes and dyslipidaemia, with evidence of a direct correlation between plaque levels of ApoC-III with both the burden and inflammatory composition of plaques. Additional studies demonstrated inverse correlations between hepatic levels of triglyceride and ApoC-III with expression of factors involved in the generation of high-density liporptoeins (HDL) and the promotion of reverse cholesterol transport. Modification of LDL by myeloperoxidase (MPO), a peroxidase enzyme secreted by leukocytes, has been established to promote vascular inflammation and cholesterol uptake by macrophages, a critical step in foam cell formation. In cell studies, we demonstrated that MPO modified TRL (MPO-TRL) exerted an adverse effect in human umbilical vein endothelial cells (HUVEC), as evidenced by an upregulation of mRNA expression of pro-inflammatory adhesion molecules. MPO-TRL co-incubation also resulted in a reduction in endothelial cell proliferation which can be restored by co-incubation with HDL. Cells treated with MPO-TRL also demonstrated an increase in expression of proteins involved in responses to hypoxia (HIF1ɑ) and in angiogenesis (VEGF) and a reduced expression of the cholesterol transporter ABCG1. We are further interested in whether triglyceride mediator ApoC-III exerts similar adverse effect in atherosclerosis in the settings of mouse models with dyslipidaemia and diabetes. We demonstrated that ApoC-III was present within plaque and the presence was positively associated with lesion size and inflammatory marker CD68. ApoC-III has been well characterised to induce endothelial cell inflammation, whether ApoC-III will induce inflammation in other vascular cells will be of additional interest in future studies. We have also found that using patients’ serum stratified with different levels of triglyceride, ApoC-III levels inversely correlated with HepG2 expression of both PPARɑ and cholesterol transporter ABCA1, important factors which involved in the synthesis of both ApoAI and HDL and subsequent effects on lipid transport. Further experiments are needed to demonstrate the correlation of ApoC-III in HDL metabolism in large, prospective cohorts. In summary, these observations described potential adverse effects of triglycerides and ApoC-III on vascular cells, atherosclerosis and lipid metabolism. The findings support potential causal effets and novel targets for therapeutic targeting to prevent atherosclerotic disease.