LDL oxidation, oxysterol formation and endothelial cell dysfunction
Cholesterol is the most abundant lipid in eukaryotic cells where it is an important component of membranes. It is synthesized within the cells in the endoplasmic reticulum, although this organelle contains only 0.5-5% of the total cell cholesterol (Iuliano, 2011; Lange et al ., 1999). LDL are the main carriers of cholesterol and, as such, are important contributors to atherosclerotic lesion progression. Indeed, under physiological conditions, LDL penetrate the intima via transcytosis across EC, but this process is further stimulated by EC dysfunction. Subendothelial accumulation and retention of LDL is an early step in atherogenesis. Skalen and coworkers (2002) demonstrated that the extracellular matrix, mainly consisting of proteoglycans, plays a key role in the retention of LDL. On the other hand, oxidative modification of LDL prevents their interaction with proteoglycans and favors their uptake by macrophages through scavenger receptors, leading to cholesterol accumulation and foam cell formation (Öörni et al., 1997).
The oxidation of LDL is thought to mainly occur in the extracellular matrix underlying the arterial wall because in circulation they are protected by plasma antioxidants (Carmena et al ., 1996). Multiple mechanisms mediated by transition metals and enzymes are involved in LDL oxidation. However, at now, the physiological in vivo relevant mechanism is still not clear (Yoshida and Kisugi, 2010). Most of the cells present in the arterial intima can promote LDL oxidation in vitro by its enzymes, and ROS seem to play an active role. For example, high LDL levels and shear stress can enhance EC-mediated hydrogen peroxide (H2O2) production (Zouaoui Boudjeltia et al ., 2004). A relevant mechanism leading to oxidation of LDL occurs via myeloperoxidase (MPO) secreted by activated phagocytes. This enzyme generates oxLDL by producing hypochlorous acid (HOCl) from H2O2 and chloride. Zhang and coworker (2013) speculated that MPO could use the NADPH-derived H2O2 in order to produce HOCl, thus promoting the oxidation of LDL.
Oxidized LDL are particularly rich in oxysterols which are 27-atom carbon compounds formed after enzymatic or non-enzymatic cholesterol oxidation in vivo . However, oxysterols can also be derived from the diet. The methods of processing, preparation and storage expose the food to air, light or heat leading to the formation of oxysterols (Lordan et al ., 2009). Oxysterols circulate in the blood stream in both free and esterified forms, carried by lipoproteins. Interestingly, it has been demonstrated that the incorporation of oxysterols into LDL particles makes LDL more susceptible to oxidation (Vine et al ., 1998; Staprans et al ., 2003).
Oxysterols are involved in many physiological processes such as cholesterol metabolism, hormone and vitamin D synthesis, and transmembrane signaling as components of membrane microdomains enriched in cholesterol (lipid rafts and caveolae). On the other hand, accumulation of these compounds in tissues and organs has been associated with the progression of several diseases, such as atherosclerosis, neurodegenerative diseases, and cancer (Poli et al ., 2013; Voisin et al ., 2017). A growing body of evidence suggests that oxysterols and oxLDL play a key role in endothelial dysfunction impairing the formation/production of NO, increasing the formation of ROS and promoting the release of pro-inflammatory cytokines (Lubrano and Balzan, 2014; Maiolino et al ., 2013). Furthermore, it has been observed that oxysterols are able to remodel the endothelial layer by inducing endothelial dysfunction followed by cell death (Luchetti et al ., 2019; Luchetti et al ., 2015).