Targeted cholesterol efflux.

In the process of reverse cholesterol transport, high-density lipoprotein (HDL) transports excess cholesterol from peripheral cells back to the liver, where cholesterol is recycled or metabolized and excreted. On the cellular level, 2 membrane-associated ATP-binding cassette (ABC) cholesterol transporters, ABCA1 and ABCG1, play a major role in cholesterol efflux from the cell to extracellular acceptors. ABCA1 mediates cholesterol efflux to lipid-poor apoA-I, which then forms a nascent HDL, and ABCG1 is involved in cholesterol efflux to HDL. Maintaining efficient cholesterol efflux is important for normal cell function. Abca1 mice develop atherosclerosis and myeloproliferative disease. Abcg1 mice have excessive cholesterol accumulation in the lungs and impaired insulin secretion, and Abca1 mice demonstrate an accelerated aging phenotype. Administering or genetically raising levels of apoA-I inhibits atherogenesis and tumor growth in mice. This rather abbreviated summary and many other studies, not referred to due to limited space, show the importance of efficient cholesterol efflux for normal cellular function and suggest that modulation of cholesterol efflux is a promising therapeutic strategy for various human diseases.

Current studies pursue 2 therapeutic goals: (1) to ensure efficient functioning of the ABCA1 and ABCG1-powered cellular efflux machine and (2) to boost the levels and improve the functionality of apoA-I and HDL. These approaches aim to achieve a systemic improvement of cholesterol efflux and reverse cholesterol transport in general. The question we would like to raise in this commentary is, would it be possible to achieve targeted, organ- or even cell-specific regulation of cholesterol efflux? Are there physiological mechanisms that provide localized regulation of cholesterol efflux? And if such mechanisms exist, can they be used to design targeted cholesterol efflux therapeutic approaches?

Recent studies suggest that apoA-I binding protein (AIBP) is such a non-cell-autonomous, secreted regulator of cholesterol efflux, and that its highly regulated spatiotemporal expression in animal tissues modulates local processes via enhanced cholesterol efflux from targeted cells. So far, 2 examples have been reported. One is from John Herr’s group, which studied AIBP in the context of sperm capacitation. They found that AIBP is released from the sperm into the media during in vitro capacitation and hypothesized that this event represented an autocrine regulation of cholesterol efflux, which has been previously observed during sperm capacitation. However, this hypothesis was not tested experimentally.

The second example is the work from our laboratory showing that AIBP facilitates cholesterol efflux from endothelial cells (EC) and thereby regulates angiogenesis. AIBP binds apoA-I and HDL as well as EC (Fig. 1), and our data suggest the presence of an AIBP receptor, but its identity is yet unknown. Although the exact mechanism by which AIBP promotes cholesterol efflux from EC is not completely understood, its most immediate result—the disruption of lipid rafts—has important consequences. As is the case with many other cell surface receptors,, lipid rafts support VEGF-induced dimerization of VEGFR2, the major angiogenic receptor in EC. AIBP/HDL remove cholesterol from EC, leading to lipid raft reduction, reduced VEGFR2 dimerization and inhibited VEGFR2 endocytosis, signaling, and angiogenesis in vitro.

Figure 1. AIBP-targeted cholesterol efflux from EC disrupts lipid rafts and inhibits angiogenesis. VEGFR2 signaling in response to VEGF requires the receptor dimerization, which occurs in the cholesterol-rich membrane microdomains, often designated as lipid rafts. As the name implies, AIBP binds apoA-I and HDL; it also binds EC in a saturable manner, which suggests a receptor-mediated binding, but the hypothetical AIBP receptor (AIBP-R) has not yet been identified. AIBP increases the overall HDL binding to EC and at the same time reduces the affinity of HDL binding, thereby creating conditions for accelerated cholesterol efflux. Enhanced removal of cholesterol from the EC plasma membrane disrupts lipid rafts, which, in turn, inhibits VEGFR2 signaling and VEGF-induced angiogenesis. HDL transports cholesterol removed from the cell (in free and esterified forms) to the liver.

When we examined aibp mRNA expression in developing zebrafish embryos, we noticed its spatial expression in somites at 24–36 h post fertilization (hpf), the time when segmental arteries sprout from the dorsal aorta and grow dorsally in between somites. The aibp expression in somites ceased by 48 hpf. Knockdown of aibp resulted in elevated cholesterol, increased abundance of lipid rafts in the ECs of segmental arteries and their chaotic growth inside the somites, which is an off-limits area in wild-type embryos. We concluded that the spatiotemporal expression of Aibp in somites of developing embryos restricts angiogenesis to the dorsal-only direction by controlling lipid raft content and VEGFR2 signaling.

In our opinion, this newly discovered, AIBP-mediated mechanism of angiogenesis regulation exemplifies a new paradigm, according to which localized regulation of cholesterol efflux—via soluble proteins, such as AIBP, and possibly other mechanisms—is involved in regulation of developmental, physiologic, and pathologic processes. Human AIBP (APOA1BP) mRNA is ubiquitously expressed, but secretion of the AIBP protein seems to be highly regulated, as it has been shown in sperm during capacitation and in renal proximal tubular cells in response to apoA-I and HDL. Further, we speculate that if the AIBP cellular binding is indeed receptor-mediated, this would enable recruitment of HDL to a specific receptor cluster and result in targeted disruption of lipid rafts associated with the receptors forming the cluster. Our results with the VEGFR2 signaling inhibition support such a mechanism. The hypothesis of targeted cholesterol efflux, if further supported by future experimental work, may have important implications for basic understanding of biological processes and for forming new therapeutic approaches to human disease.