Caveolin-1 Is a Negative Regulator of Caveolae-mediated Endocytosis to the Endoplasmic Reticulum

Abstract

Caveolae are flask-shaped invaginations at the plasma membrane that constitute a subclass of detergent-resistant membrane domains enriched in cholesterol and sphingolipids and that express caveolin, a caveolar coat protein. Autocrine motility factor receptor (AMF-R) is stably localized to caveolae, and the cholesterol extracting reagent, methyl-␤-cyclodextrin, inhibits its internalization to the endoplasmic reticulum implicating caveolae in this distinct receptor-mediated endocytic pathway. Curiously, the rate of methyl-␤-cyclodextrinsensitive endocytosis of AMF-R to the endoplasmic reticulum is increased in ras-and abl-transformed NIH-3T3 cells that express significantly reduced levels of caveolin and few caveolae. Overexpression of the dynamin K44A dominant negative mutant via an adenovirus expression system induces caveolar invaginations sensitive to methyl-␤-cyclodextrin extraction in the transformed cells without increasing caveolin expression. Dynamin K44A expression further inhibits AMF-Rmediated endocytosis to the endoplasmic reticulum in untransformed and transformed NIH-3T3 cells. Adenoviral expression of caveolin-1 also induces caveolae in the transformed NIH-3T3 cells and reduces AMF-R-mediated endocytosis to the endoplasmic reticulum to levels observed in untransformed NIH-3T3 cells. Cholesterol-rich detergent-resistant membrane domains or glycolipid rafts therefore invaginate independently of caveolin-1 expression to form endocytosis-competent caveolar vesicles via rapid dynamin-dependent detachment from the plasma membrane. Caveolin-1 stabilizes the plasma membrane association of caveolae and thereby acts as a negative regulator of the caveolaemediated endocytosis of AMF-R to the endoplasmic reticulum.

Selective Caveolin-1–dependent Endocytosis of Glycosphingolipids

“…Our results are in apparent contrast to those of Nabi and coworkers who found that the overexpression of cav-1 negatively regulates the caveolar-mediated endocytosis of the autocrine motility factor receptor in NIH-3T3 cells (Le et al, 2002). One possible explanation is that there are distinct caveolar-mediated endocytic pathways with different characteristics (Le and Nabi, 2003).…”

Distinct Mechanisms of Agonist-induced Endocytosis for Human Chemokine Receptors CCR5 and CXCR4

“…Endocytosis of agonistoccupied CCR5 probably follows a similar pathway. However, as suggested (Le et al, 2002;Thomsen et al, 2002), caveolae may exist in lymphocytes too, but may be rapidly converted into endocytic vesicles. It is possible that the enhanced CCR5 trafficking during immune cell activation is due in part to up regulation of caveolin (reviewed in Harris et al, 2002), which may disrupt rafts.…”

“…Recent reports have shown that both DRMs and glycolipid rafts are able to undergo endocytosis in the absence of caveolin. One caveat in interpreting this is that caveolae could still represent a brief vesicular intermediate in such cells, and evidence from using budding inhibitors and caveolin overexpression indicates that this may be the case (Le et al, 2002;Thomsen et al, 2002;Le and Nabi, 2003).…”

“…Furthermore, caveolin-1 overexpression was shown to inhibit the clathrin-independent trafficking of autocrine motility factor receptor (AMF-R) and CTx-B to the ER and the Golgi, respectively (Le et al, 2002;Le and Nabi, 2003). Unlike AMF-R that follows a direct route from the plasma membrane to the ER, CTx-B is delivered to the Golgi after a transit via the early endosomes.…”

Endocytose sans clathrine

“…En cas de déplétion, l'endocytose est inhibée en raison de la perturbation du contenu en cholestérol des cellules et de la désorganisation des domaines lipidiques à la membrane plasmique. Le même effet est observé lors d'une surexpression de CAV1 [22,23]. Plusieurs protéines « cargo » empruntant une voie cavéolaire, sans que celle-ci soit exclusive, ont été identifiées.…”