ATP-binding cassette transporter-2 (ABCA2) as a therapeutic target

Abstract

The ATP binding cassette transporter ABCA2 is primarily an endolysosomal membrane protein that demonstrates pleiotropic functionalities, coalescing around the maintenance of homeostasis of sterols, sphingolipids and cholesterol. It is most highly expressed in brain tissue and ABCA2 knockout mice express neurological defects consistent with aberrant myelination. Increased expression of the transporter has been linked with resistance to cancer drugs, particularly those possessing a steroid backbone and gene expression (in concert with other genes involved in cholesterol metabolism) was found to be regulated by sterols. Moreover, in macrophages ABCA2 is influenced by sterols and has a role in regulating cholesterol sequestration, potentially important in cardiovascular disease. Accumulating data indicate the critical importance of ABCA2 in mediating movement of sphingolipids within cellular compartments and these have been implicated in various aspects of cholesterol trafficking. Perhaps because the functions of ABCA2 are linked with membrane building blocks, there are reports linking it with human pathologies, including, cholesterolemias and cardiovascular disease, Alzheimer's and cancer. The present review addresses whether there is now sufficient information to consider ABCA2 as a plausible therapeutic target.

Keywords: ABCA2; Alzheimer; Cancer; Cholesterol; Sphingolipid; Transporter.

Figure 1. ABCA2 Structural Organization in Membrane Bilayer

ABCA2 is synthesized as a single polypeptide consisting of twelve transmembrane (TM) alpha helices that cross the lipid bilayer, spanning the aqueous (luminal) and cytosolic environments. Within the cytosolic compartment, are two nucleotide binding domains (NBD1 and NBD2) that bind and hydrolyze ATP to energize lipid translocation from the between the inner leaflet (IL) and outer leaflet (OL). The NBDs also contain the “ABC signature motif” common to all ATP transporters as well as Walker A and Walker B domains that function in binding hydrolysis of ATP.

Figure 2. Endosomal Maturation and Sphingolipid Salvage

Uptake of plasma membrane sphingolipids and glycolipids from the plasma membrane at coated pits and caveolae fuse with the membranes of early endosomes. Here, the membrane topology reverses as they pinch off intraluminal vesicles, where the lumen is derived from the cytosol. Progressive acidification of endosomes by H⁺/ATP pumps as they mature results in a dramatic decrease in pH to ~ 4.5 to 5.5 in late-endosomes/lysosomes. This pH is optimal for the activity of sphingolipid salvage pathway enzymes, e.g., Acid sphingomyelinase and Glucocerebrosidase. Catabolism of the complex sphingolipids and glycolipids generate ceramide that displaces free cholesterol from intraluminal membranes onto the soluble cholesterol carrier NPC2, which then transfers the cholesterol to NPC1 on the limiting membrane of the LE/LY. From the limiting membrane cholesterol sensors distribute cholesterol from the LE/LY to intracellular sites including the Golgi apparatus and the endoplasmic reticulum.

Figure 3. Hypothetical Model of ABCA2 Function in the LE/LY

Under reduced ABCA2 expression levels or ABCA2 loss of function (LOF) conditions e.g., ABCA2 RNAi or ABCA2 knockout, the ceramide generated from sphingolipid salvage displaces cholesterol from LE/LY intraluminal vesicles onto soluble NPC2, which then transfers the cholesterol to NPC1 on the limiting LE/LY membrane, where it is distributed to other intracellular sites. Genetic mutations (stable ABCA2 expression under constitutive promoter in cell lines) or pharmacological agents that increase ABCA2 expression (GOF) increase acid ceramidase activity and sphingosine production. Because of its lipid structure, sphingosine is less effective than ceramide in displacing cholesterol onto soluble NPC2 and ultimately transfers to NPC1 on the LE/LY limiting membrane, resulting in cholesterol sequestration in the LE/LY.

Figure 4. Negatively charged phospholipids promote sphingolipid salvage enzymatic activity

On the membranes of LE/LY intraluminal vesicles, the presence of negatively charged phospholipids, e.g., bis-(monoacylglycero) phosphate (BMP), phosphatidylserine (PS), phosphatidylinositol (PI), phosphatidylglycerol interact with positively charged (at acidic pH) Saposin (C and D) proteins and positively charged (at acidic pH) acid ceramidase and glucocerebrosidase to promote salvage pathway enzymatic activity. The mechanism for the establishment and maintenance of negatively charged phospholipid asymmetry has not been determined, but we theorize that it may be due to ABCA2 translocation of these substrates across the intraluminal membrane bilayer.