Caveolae pathway

As described above, mpCD is a commonly used inhibitor for both the clathrin and the caveolae pathway. See the section on Inhibiting Clathrin-Mediated Endocytosis for details. 

To highlight the caveolae pathway, a couple of substances are discussed and applied. The most commonly used markers besides the SV40 (17,20) are probably cholera toxin B subunit (CtxB) and caveolin-1-GFP. Folate and... 

Caveolae can mediate the delivery of CtxB that binds to GM1 ganglioside at the plasma membrane and is delivered to intracellular compartments. Cholera toxin, produced by Vibrio cholerae, consists of five identical subunits B and one A chain. In addition to labeled SV40 and caveolin-1-GFP, CtxB is one of the most commonly used caveolae markers. However, two groups reported that the toxin is internalized by either a clathrin-independent caveolae pathway or a clathrin-dependent uptake, bringing its selectivity/specificity into question (31,81,118). We controlled the suitability of this marker for COS-7 cells pretreated with CPZ, mpCD, and filipin and as expected, the uptake was not influenced by CPZ treatment but was strongly decreased by the latter two (data not shown). 

Rapacciuolo et al. (27) demonstrated that, following agonist activation, the PrAR can undergo both caveolar-mediated as well as clathrin-mediated internalization. Indeed, these authors showed that both internalization pathways make an approximately equal contribution to PrAR internalization. These authors further showed that the nature of the protein kinase that phosphorylated the receptor determined its internalization fate. For example, PKA-mediated phosphorylation of the PrAR resulted in internalization via the caveolae pathway. In contrast, GRK phosphorylation led to the expected arrestin/clathrin internalization pathway. 

Besides clathrin-mediated cell uptake, caveolae-mediated endocytosis is another well-investigated uptake mechanism. The caveolae pathway is particularly attractive for the delivery of proteins and nucleic acids because these vesicles have neutral pH and the lysosomes, where degradation takes place, are bypassed [8,13]. Nanoparticles... 

Serum albumin has also been tested as NPs for gene delivery. Mo et al. [88] encapsulated the DNA into human serum albumin (HSA) by a desolvationcrosslinking method to produce DNA-HSA NPs having a mean size of 120 nm and zeta potential of —44 mV. The DNA-HSA NPs were easily taken up by the cells via receptor-mediated endocytosis that involved primarily caveolae pathways. Within the cells, DNA-HSA NPs protected the DNA against nuclease attack and showed sustained release of DNA over 6 days without significant cytotoxicity. The overall transfection rate was found to be fivefold higher than obtained with Lipofectamine. 

Rejman J, Oberle V, Zuhom IS, Hoekstra D (2004) Size-dependent internalization of particles via the pathways of clathrin- and caveolae-mediated endocytosis. Biochem J 377 159-169... 

Other pinocytotic pathways also exist that do not depend on either caveolae or clathrin, although these are not as well defined [55]. Specific receptors continue to be internalized in the absence of clathrin or caveolin and these pathways can be monitored by following glycosyl phos-phatidylinositol (GPI (-anchored proteins. Nonclathrin, noncaveolin pathways may also be responsible for the reuptake of membrane in neuroendocrine cells after stimulated secretion. Some, but not all, of these pathways appear to require dynamin. 

Fig. 2.2 Cellular internalisation pathways proposed for carbon nanotubes (CNTs) (A) phagocytosis (B) membrane piercing by passive diffusion (C) caveolae-mediated endocytosis and (D) clathrin-mediated endocytosis... 

P. U. Le and I. Nabi. Distinct caveolae-mediated endocytic pathways target the Golgi apparatus and the endoplasmic reticulum. J. Cell Sci. 116 1059-1071 (2003). 

D. P. McIntosh, X.-Y. Tan, P. Oh, and J. E. Schnitzer. Targeting endothelium and its dynamic caveolae for tissue-specific transcytosis in vivo A pathway to overcome cell barriers to drug and gene delivery. Proc. Natl. Acad. Sci. USA 99 1996-2001 (2002). 

Figure 2 Proposed pathways for liposomal entry into the cell enhanced by peptides. These include direct cell entry suggested as the mechanism of entry by cell-penetrating peptides and receptor-mediated endocytosis by caveolae- and clathrin-dependent endocytosis.

Previous work has shown that the majority of cells internalize liposomes through an endocytic pathway (4,5). There are multiple pathways for internalization involving vesicles of 50 300 nm in diameter. These include clathrin-mediated endocytosis, caveolae-mediated endocytosis, phagocytosis, macropinocytosis, and nonclathrin- noncaveolae-dependent endocytosis (6). 

Figure 1 Endocytosis of liposomes five different routes into the cell. Multiple pathways can be used by the cell to internalize liposomes. Besides the well-characterized clathrin-mediated endocytosis, other pathways can be applied by the cell. Possible alternative pathways include phagocytosis or macropinocytosis—two pathways that internalize by an actin-driven protuberance of the plasma membrane. Other routes include the involvement of caveolae where substances are taken up into the cell bypass the traditional endosome/lysosome system (particles might escape from being degraded in lysosomes). Finally there exists an ill-defined mechanism that is neither mediated by caveolae nor by clathrin. In a single cell type, two or more of these mechanisms can coexist. Source Adapted from Ref 8.

It should be noted that the protein caveolin-1 is not necessarily an integral part of caveolae. The so-called lipid-raft mediated pathways have been studied mainly in cells that do not express caveolin-1. Apparently, these mechanisms are similar to caveolae uptake and are therefore discussed in the caveolae section. For detailed reviews on caveolae uptake, see Refs. (25-30). 

Cholera Toxin Subunit B. a marker for caveolae-dependent endocytosis. partly co-localizes with liposomal FITC>dextran indicating that pH-sensitive liposomes are taken up-at least to a certain extend— via caveotae pathway... 

The tyrosine kinase inhibitors genistein [4—50pM dissolved in 0.05% dimethyl sulfoxide (DMSO), 30 minutes] and herbimycin (500 pM dissolved in 0.05%i DMSO, 30 minutes) are described as being useful inhibitors of caveolae uptake (61,62), which can be applied to discriminate this pathway from clathrin-mediated uptake (63). It should be questioned if a block of the enzyme affects the uptake via caveolae uptake selectively. However, genistein is thought to inhibit the receptor-induced formation of caveolae (18). As for herbimycin, no comment can be given on its selectivity. 

Phorbol esters are described as being protein kinase C activators and should be selective inhibitors for caveolae uptake [1 pM Phorbol 12-myristate 13-acetate (PMA) for 30 minutes] (48,68). The inhibition occurs as quickly as five minutes after adding PMA to cell culture medium and is not reversible within five hours (MA104 cells). However, PMA has several other effects on several other pathways. An example of this is that it has been described to stimulate macropinocytosis and transcytosis (see the following). 

Probably the most appropriate marker for this pathway is caveolin-1 tagged with GFP, which after gene expression, is located at sites of caveolae as well as in the Golgi complex (20,26). 

Uptake of SV40 occurs specifically by the caveolar pathway. Less than 5% of internalized particles are found to pass through clathrin-coated pits (26). Even though SV40 was known to be a highly specific marker for caveolae uptake, the virus is now described as entering the cells via a different mechanisms (121). 

The use of the term marker is often misleading because it can be interpreted to imply an absolute specificity that does not reflect the dynamic nature of intracellular compartments. It is important to remember that the predominant location of these markers reflects a balance of trafficking pathways into and out of each compartment. Marker can be internalized not only by one but by different mechanisms. Even SV40, which is known to be a specific marker for caveolae uptake, is now also described as entering the cells via a different mechanism (121). Therefore, not just one but several inhibitors or markers for each mechanism should be applied and all results should be taken into account before reaching a conclusion. 

Cholesterol is an important structural component of cellular membranes, where it plays a role in modulating membrane fluidity and phase transitions, and, together with sphingomyelin, forms lipid rafts or caveolae, which are sites where proteins involved in diverse signaling pathways become concentrated. Furthermore, cholesterol is a precursor of oxysterols, steroid hormones, and bile acids. 

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