The interplay between oxLDL and caveolae/caveolin and their impact on endothelial dysfunction
Uptake and transcytosis of circulating oxLDL, together with oxidation of LDL in the subendothelium, play an important role in the development of atherosclerosis. As for LDL, these processes seem to involve caveolae-mediated mechanisms. Transcytosis of oxLDL via caveolae was suggested by Sun et al . (2010) who showed that two caveolae specific inhibitors, filipin and nocodazole, decrease the uptake of oxLDL by human umbilical vein endothelial cells (HUVEC). The lectin-like oxidized LDL receptor 1 (LOX-1) is the major receptor for binding, internalization and degradation of oxLDL in EC, and LOX-1 expression is up-regulated by oxLDL (Sawamura et al ., 1997; Li and Mehta, 2000) (Figure 2). Interestingly, also Cav-1 expression is up-regulated by oxLDL, while Cav-1 silencing results in decreased LOX-1 expression upon oxLDL administration, suggesting that caveolin participates in LOX-1 regulation. It is known that the binding of oxLDL to LOX-1 stimulates the development of atherosclerosis through different mechanisms involving: i) activation of MAPK proteins, which causes increased expression of adhesion molecules and chemoattractants; ii) stimulation of NADPH oxidase activity, leading to ROS production, oxidative stress, and consequent reduction of NO levels; and iii) activation of NF-kB signaling pathway, resulting in cytokine and adhesion molecule production as well as increased expression of LOX-1 itself, thus creating a vicious cycle of proinflammatory signaling (Kattoor et al ., 2019) (Figure 2).
More recently, the possible role of LOX-1 in oxLDL transcytosis has been questioned by Huang et al. (2019) who demonstrated that knockdown of LOX-1 did not alter the uptake of oxLDL in human aortic endothelial cells (HAEC). By contrast, a decrease in oxLDL transfer was observed when the expression of SR-B1 was downregulated, thus suggesting that transcytosis of oxLDL in EC occurs via the SR-B1 receptor rather than LOX-1 (Huang et al ., 2019) (Figure 2).
Further investigations are necessary to clarify the molecular pathways responsible for oxLDL uptake and transcytosis and the role of caveolin in these processes.
Caveolae are enriched in free cholesterol and changes in the content of this lipid can affect the morphology and signaling mediated by caveolae. To this regard, Smart et al . (1994) described for the first time that cholesterol oxidation results in the translocation of caveolin from plasma membrane to Golgi with a modest reduction in the number of caveolae. Later, Blair et al. (1999) were able to show that oxLDL can indeed deplete caveolar cholesterol and induce the transfer of Cav-1 and eNOS to intracellular compartments, thus enabling NO production (Figure 2).
The mechanism through which oxLDL may deplete cholesterol is unknown. It has been hypothesized that oxLDL may act as a cholesterol acceptor to remove cholesterol from cellular membranes rather than loading cells with cholesterol.  A higher efflux of cholesterol induced by oxLDL has also been proposed, in a mechanism that involves the binding of oxLDL to CD36 receptors. Finally, a redistribution of cholesterol between membrane-rich and cholesterol-poor domains has been proposed (Shentuet al ., 2010). Even though the effects of oxLDL on membrane cholesterol remain elusive and controversial, the effects of oxLDL on EC function impairment are very similar to those observed after experimental-induced cholesterol depletion, suggesting a common mechanism of action (Levitan and Shentu, 2011).
Zhu and coworkers (2005) demonstrated that oxLDL can inhibit the transcription of ATP-binding cassette transporter-1 (ABCA1) in HUVEC cells. This transporter mediates the active efflux of cholesterol and/or phospholipids. The regulation of ABAC1 by oxLDL occurs at the transcriptional level through the inhibition of endogenous LXR ligand production. The role of caveolin in cholesterol homeostasis is less understood. Overexpression of Cav-1 up-regulated ABCA1 expression and enhanced cholesterol efflux to extracellular effectors (Lin et al ., 2007). Conversely, Cav-1 knock down was associated with reduced free cholesterol and increased esterified cholesterol, but it had minimal effects on cellular cholesterol efflux (Frank et al ., 2006). Whether oxLDL might affect cholesterol homeostasis by directly interfering with Cav-1 levels is not known. It is well established that Cav-1 is regulated by cellular cholesterol levels (Bist et al ., 1997). In line with these observations, caveolin mRNA levels were found up-regulated by free cholesterol, but down-regulated by oxysterols in fibroblast monolayers (Fielding et al ., 1997).
Finally, an exchange in free cholesterol between plasma LDL particles and the luminal surface of EC is supposed to occur (Stender, 1982). In this context, oxLDL have been shown to induce an increase in endothelial stiffness by direct incorporation of oxysterols into the endothelial plasma membrane (Figure 2) (Shentu et al ., 2012). It has been hypothesized that this event could result in the disruption of the structure of lipid-ordered domains, including caveolae. Moreover, there is evidence that oxysterols interact with Cav-1 (Sleer et al ., 2001). These direct oxysterol effects might produce relevant consequences on caveolae-mediated signaling.
Thus, although the molecular mechanisms through which oxLDL lead to endothelial dysfunction must be still clarified, accumulating evidence point to a relevant role of direct/indirect disruption of cholesterol homeostasis/distribution which may in turn affect caveolae function and modulate signaling pathways relevant for the development of atherosclerosis.