Administration of phosphatidylinositol (PI) toNew Zealand white rabbits increases HDL negative charge and stimulates reverse cholesterol transport. Intravenously administered PI (10 mg/kg) associated almost exclusively with the HDL fraction in rabbits. PI promoted an increase in the hepatic uptake of plasma free cholesterol (FC) and a 21-fold increase in the biliary secretion of plasma-derived cholesterol. PI also increased cholesterol excretion into the feces by 2.5-fold. PI directly affects cellular cholesterol metabolism. In cholesterol-loaded macrophages, PI stimulated cholesterol mass efflux to lipid-poor reconstituted HDL. PI was about half as effective as cAMP at stimulating efflux, and the effects of cAMP and PI were additive. In cultured HepG2 cells, PI-enriched HDL also enhanced FC uptake from HDL by 3-fold and decreased cellular cholesterol synthesis and esterification. PI enrichment had no effect on the selective uptake of cholesterol esters or on the internalization of HDL particles. PI-dependent metabolic events were efficiently blocked by inhibitors of protein kinase C and the inositol signaling cascade. The data suggest that HDL-PI acts via cell surface ATP binding cassette transporters and signaling pathways to regulate both cellular and intravascular cholesterol homeostasis. It has been known for over 30 years that the altered lipoprotein metabolism in many dyslipidemic states is associated with abnormally charged lipoprotein particles (1). There is now accumulating evidence that this abnormal charge may directly contribute to aberrant lipoprotein metabolism (2-5). Lipoproteins all exhibit a net negative charge, and this charge is determined by both the apolipoprotein and lipid constituents of the lipoprotein particle (6-8). The primary anionic lipid in lipoprotein particles is phosphatidylinositol (PI). While PI is a minor constituent (3-7%) of lipoprotein phospholipids (7, 9), studies suggest that it may be a critical component of chyle and an important regulator of lipoprotein secretion (10, 11). Our work has shown that HDL charge directly affects lipid metabolism by controlling interactions with interfacial enzymes (12-15) and cell surface molecules (16,17). It is now clear that PI affects lipoprotein metabolism both by controlling interfacial interactions and uniquely regulating intracellular signaling pathways.We have previously reported that a single intravenous injection of PI liposomes into fasted rabbits increases the net negative surface charge of HDL and almost completely inhibits lecithin:cholesterol acyltransferase (LCAT) (18). PI therefore directly acts to block the synthesis and storage of cholesteryl ester in the blood stream. In addition, PI appeared to stimulate reverse cholesterol transport (RCT) by promoting a 30-fold increase in the rate of clearance of free cholesterol (FC) from the circulation (18). Previous work has shown that infusion of lecithin liposomes can also promote cholesterol transport; however, the doses utilized to obtain the effect were about 30-fo...

A unique property of the extracellular matrix of J774 and THP-1 cells has been identified, which contributes to the ability of these cells to promote cholesterol efflux. We demonstrate high level apolipoprotein (apo) A-I binding to macrophage cells (THP-1 and J774) and to their extracellular matrix (ECM). However, high level apoA-I binding is not observed on fibroblasts, HepG2 cells, or U937 cells (a macrophage cell line that does not efflux cholesterol to apoA-I or bind apoA-I on their respective ECM). Binding to the ECM of THP-1 or J774 macrophages depends on the presence of apoA-I C-terminal helices and is markedly reduced with a mutant lacking residues 187-243 (apoA-I⌬(187-243)), suggesting that the hydrophobic C terminus forms a hydrophobic interaction with the ECM. ApoA-I binding is lost upon trypsin treatment or with Triton X-100, a preparation method that de-lipidates the ECM. However, binding is recovered with re-lipidation, and is preserved with ECM prepared using cytochalasin B, which conserves the endogenous phospholipid levels of the ECM. We also demonstrate that specific cholesterol efflux to apoA-I is much reduced in cells released from their native ECM, but fully restored when ECM-depleted cells are added back to ECM in the presence of apoA-I. The apoA-Imediated efflux is deficient in plated or suspension U937 macrophages, but is restored to high levels when the suspension U937 cells are reconstituted with the ECM of J774 cells. The ECM-dependent activity was much reduced in the presence of glyburide, indicating participation of ABCA1 (ATP-binding cassette transporter 1) in the efflux mechanism. These studies establish a novel binding site for apoA-I on the macrophage ECM that may function together with ABCA1 in promoting cholesterol efflux.