Serum lipoprotein functionality and cellular lipid transporters: focus on their possible role on cardiovascular risk modulation

Cardiovascular disease (CVD) is a leading cause of death with over 17 million deaths annually according to the World Health Organization. Among the major risk factors we find LDL that, accumulating in macrophages, lay the foundations of atherogenesis. This pro-atherogenic mechanism is opposed by the reverse transport of cholesterol (RCT), thanks in particular to its first step, the efflux of cellular cholesterol, mediated by the interaction of serum HDL with the different cell transporters. Studies show that HDL quality, beyond than quantity, is a better predictor of CVD risk. In this context, evaluating HDL functionality through cholesterol efflux capacity (CEC), would represent a better approach to stratify CVD risk, independently of classical risk factors. CEC is therefore an anti-atherogenic parameter, that has been demonstrated to be inverselly correlated with CVD risk. The amount of cholesterol inside cells is determined not only by the quantity that comes out of cells, but also by the quantity that enters. For this reason, a pro-atherogenic parameter has been identified to evaluate the ability of a serum to transfer cholesterol to the cell, called cholesterol loading capacity (CLC). Therefore, in the first project of this thesis we wanted to analyze CLC in patients with high levels of Lp(a) and at high risk of coronary heart disease (CAD) undergoing apheresis. Lp (a) is a lipoprotein more available to oxidation than LDL, thus representing a CVD risk factor. Among the gene determinants of Lp(a), to remember is a structure called KIV2, which determines by varying in number of copies, not only the quantity but also the size of the lipoprotein. Our study demostrated that particles of smaller size are associated with a higher number of Lp(a), while larger particles are associated with a smaller number. In addition larger Lp(a) particles determine a lower CLC and consequently a lower CVD risk. Not only do gene alterations represent a factor capable of influencing CVD risk, but metabolism would also represent a possible cause capable of influencing this risk. For this reason the aim the second project was to define a possible cause of the different CVD risk in patients with metabolic NAFLD compared to patients with NAFLD on a genetic basis. This disorder, caused by an excessive accumulation of TG in the liver, is actually a multifactorial pathology. In this study we observed how patients with the metabolic form were showing a decrease of serum CEC and an increase of serum CLC, which was not found in patients with genetic form. The levels of fat are not decisive, in fact, despite the same amount of fat, patients affected by the metabolic form have a lower CEC and a higher CLC than the genetic group, which had no different parameters compared to control subjects. Metabolic alterations would therefore appear to be the necessary cause of the increased CVD risk that afflicts patients with metabolic-based NAFLD, and the presence of metabolic syndrome would further aggravate the clinical picture. If on the one hand genetic and metabolic alterations are capable of influencing CVD risk, on the other hand an approach able to modify this risk would be represented by the diet. In the third project of this thesis we wanted to evaluate if two different dietary approaches, namely the Mediterranean diet (MD) and the vegetarian diet (VD), were able to affect CEC of the HDLs in healthy patients with low or medium CVD risk. VD was observed to reduce both Total-CEC and ABCA1-mediated CEC, while passive diffusion-mediated CEC was unchanged compared to the MD diet. The benefits of MD may be due to the lipid-lowering effects as well as the impact on some HDL functions, such as the antioxidant, vasodilatatory capacity and the ability to promote cholesterol efflux . In the last project, we evaluated a possible mechanism of action of a plasma CVD risk biomarker, namely anti-APO-AI antibodies. Anti-Apo-AI IgG are now known to be associated with various pathologies not only on inflammatory basis but also on immune ones. In this study we observed how these autoantibodies are able to maintain unchanged the expression of an important lipid metabolism regulator gene, namely SREBP-2, while increasing the expression of another TG metabolism gene, namely SREBP-1. Conversely, the expression of other genes, such as GPAT1 and FASN, which are also involved in metabolism, would be reduced. An induction of the inflammatory stimulus via TLR-2 would also occur in the presence of these antibodies. Therefore, Anti-Apo-AI IgG would appear to be capable of inducing lipid accumulation at the cellular level through this gene regulation, potentially responsible for the onset of diseases such as NAFLD.