Association of ABCA1 with Syntaxin 13 and Flotillin-1 and Enhanced Phagocytosis in Tangier Cells

Abstract

The ATP-binding cassette transporter A1 (ABCA1) facilitates the cellular release of cholesterol and choline-phospholipids to apolipoprotein A-I (apoA-I) and several studies indicate that vesicular transport is associated with ABCA1 function. Syntaxins play a major role in vesicular fusion and have also been demonstrated to interact with members of the ABC-transporter family. Therefore, we focused on the identification of syntaxins that directly interact with ABCA1. The expression of syntaxins and ABCA1 in cultured human monocytes during M-CSF differentiation and cholesterol loading was investigated and syntaxins 3, 6, and 13 were found induced in foam cells together with ABCA1. Immunoprecipitation experiments revealed a direct association of syntaxin 13 and full-length ABCA1, whereas syntaxin 3 and 6 failed to interact with ABCA1. The colocalization of ABCA1 and syntaxin 13 was also shown by immunofluorescence microscopy. Silencing of syntaxin 13 by small interfering RNA (siRNA) led to reduced ABCA1 protein levels and hence to a significant decrease in apoA-I-dependent choline-phospholipid efflux. ABCA1 is localized in Lubrol WX-insoluble raft microdomains in macrophages and syntaxin 13 and flotillin-1 were also detected in these detergent resistant microdomains along with ABCA1. Syntaxin 13, flotillin-1, and ABCA1 were identified as phagosomal proteins, indicating the involvement of the phagosomal compartment in ABCA1-mediated lipid efflux. In addition, the uptake of latex phagobeads by fibroblasts with mutated ABCA1 was enhanced when compared with control cells and the recombinant expression of functional ABCA1 normalized the phagocytosis rate in Tangier fibroblasts. It is concluded that ABCA1 forms a complex with syntaxin 13 and flotillin-1, residing at the plasma membrane and in phagosomes that are partially located in raft microdomains.

ABCA7 expression is regulated by cellular cholesterol through the SREBP2 pathway and associated with phagocytosis

“…In contrast, phagocytic activity increased significantly in the ABCA1-deficient fibroblasts (Fig. 12C), consistent with the findings in cells from Tangier disease patients (34). Thus, endogenous ABCA7 functions not to mediate HDL biogenesis but rather to regulate phagocytic function.…”

The roles of flotillin microdomains – endocytosis and beyond

“…Proteins related to flotillins are found in bacteria (Lopez and Kolter, 2010), plants (Haney and Long, 2010), fungi and most metazoans, yet are curiously absent from C. elegans and yeast. They localise to both the plasma membrane and to late endosomes or lysosomes, and have also been detected in maturing phagosomes, early endosomes and exosomes (Babuke et al, 2009;Bared et al, 2004;Glebov et al, 2006;Okabayashi and Kimura, 2010;Staubach et al, 2009).…”

Scaffolding microdomains and beyond: the function of reggie/flotillin proteins

“…Accordingly, reggies seem to be quite closely associated with the Src-family kinases lck and fyn [29,41,42], as shown by both co-immunoprecipitation and co-localization at the light microscopic (LM) and electron microscopic (EM) level. Several large transmembrane proteins have also been co-immunoprecipitated with reggies, including ABCA1 [55], an ABC transporter implicated in cholesterol transport to highdensity lipoprotein particles. Co-immunoprecipitation of the thrombin receptor PAR-1 with reggie-1 from melanoma cell lines [56] and identification of an interaction between neuroglobin and reggie-2 in a yeast two-hybrid screen [57] suggest a function of reggies in G-protein-coupled receptor signaling.…”