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.