The role of caveolae and LDL receptors in LDL transcytosis
EC control LDL shuttling across the vessel wall by a process named caveolae-mediated transcytosis. Indeed, the diameter of LDL particles is about 20-30 nm which is much larger than that of gap-junctions (3-6 nm) between adjacent cells in continuous endothelium (Iuliano et al ., 2001). Thus, the only way for LDL to cross the endothelium is through transcytosis. In detail, transcytosis may occur via fluid phase or receptor-mediated ligand uptake (Fung et al ., 2018). The endocytosis of LDL by the LDL receptor (LDLR) has been reported to mediate LDL uptake in the blood-brain barrier, but since this process leads to LDL degradation into the lysosomes, it does not explain the accumulation of LDL in the subendothelium of systemic circulation (Dehouck et al ., 1997). Moreover, it has been observed that the LDLR mediated pathway is downregulated at high concentrations of LDL, while an LDLR-independent pathway is enhanced in conditions of hypercholesterolemia (Vasile et al ., 1983). Thus, transcytosis in EC of systemic circulation appears to be LDLR-independent and, importantly, requires the presence of caveolae (Figure 1).
Caveolae are specialized plasma membrane subdomains consisting of 50-100 nm invaginations of the apical plasma membrane that detach as vesicles to shuttle their cargo to the basolateral membrane where they fuse and release their content (Figure 1).
Caveolae are present in most cell types but are particularly abundant in EC, adipocytes, fibroblasts, and smooth muscle cells (Chidlow and Sessa, 2010). Like lipid rafts, caveolae are rich in cholesterol, glycosphingolipids and lipid-anchored proteins. Differently from lipid rafts, caveolae are coated with the protein caveolin, a cholesterol binding protein (Sharma et al ., 2010). Three caveolin isoforms (Cav-1, -2, 3), which are expressed at different densities in different cell types, have been identified so far. Cav-1 and Cav-2 are the most expressed in EC where caveolae cover up to 40% of the luminal surface of vascular endothelium. In addition, caveolae are involved in signal transduction being equipped with a complete set of effector proteins, from extracellular receptors to intracellular transducers, including heterotrimeric GTP-binding proteins, protein kinase C, endothelin 1 and acetylcholine receptors, ATP-dependent Ca2+ pump, and the small GTP-binding protein Ras. Cav-1 acts as a molecular hub able to regulate the signalling of these specific molecules.
Recent studies by Ramirez et al . (2019) demonstrated that Cav-1 deletion suppresses atherosclerosis by attenuating LDL transcytosis. In particular, the authors showed that LDL accumulation in atherosclerosis-prone areas was significantly reduced in Cav-1 deficient mice. The number of caveolae and Cav-1 protein levels in the EC luminal plasma membrane may be locally affected by hemodynamic and mechanical stress, thus favouring LDL infiltration in atherosclerosis-prone regions (Boyd et al ., 2003; Frank and Lisanti, 2006).
Both scavenger receptor class B type1 (SR-B1) and activin receptor-like kinase 1 (ALK1) receptors, which are localized within caveolae, seem to be involved in LDL loading and subsequent trafficking across the EC. The role of SR-B1 in LDL transcytosis has been investigated by Amstronget al. (2015) who provided evidence that the infiltration of LDL in the subendothelial space is inhibited in SR-B1 deficient mice. More recently, Haung et al. (2019) demonstrated that SR-B1 directly binds LDL and recruits/activates Rac1 that, in turn, is required to sustain SR-B1-mediated LDL uptake. Interestingly, it was reported a higher SR-B1 expression level in atherosclerosis-prone regions of mouse aorta, before lesion formation, and in human atherosclerotic arteries compared with normal arteries. This observation supports the notion that atherosclerosis is favored by increased LDL transcytosis in altered areas of the endothelial barrier rather than by paracellular leak. A second receptor involved in LDL transcytosis has been identified by Kraehling and coworkers (2016) who provided evidence that ALK1 functions as a low affinity receptor for LDL in EC. ALK1 is an EC-restricted transforming growth factor β-type receptor that mediates LDL transcytosis independently of its kinase activity. ALK1 is localized in endothelial caveolae where functionally interacts with Cav-1, and co-localization of the proteins is drastically reduced under conditions of cholesterol depletion from the plasma membrane (Santibanez et al ., 2008).
Recently, Gerbod-Giannone et al . (2019) demonstrated that LDL endocytosis is affected in EC deficient in Cav-1 or in CD36, suggesting that CD36 may be involved in the transcytosis of native LDL across the endothelium as well. However, data reported by Huang et al . (2019) point to a role of CD36 receptor in LDL uptake but not in transcytosis, while ALK1 and SR-B1 are the only receptors involved in transcytosis, with a predominant role for the latter.
While there are multiple evidences converging on the role of Cav-1, contrasting results are reported in the literature concerning the type of receptors involved in LDL transcytosis. Since LDL uptake and transcytosis are important contributors to atherosclerotic lesion development and receptors represent important pharmacological targets, these discrepancies underly the need to further explore the mechanism(s) involved.