High Density Lipoproteins and Arteriosclerosis

It has been suggested that the signal transduction pathway initiated by apoA-I activates key proteins involved in cellular lipid efflux. We investigated apoA-Imediated cAMP signaling in cultured human fibroblasts induced with (22R)-hydroxycholesterol and 9-cis-retinoic acid (stimulated cells). Treatment of stimulated fibroblasts with apoA-I for short periods of time (<45 min) increased ATP binding cassette A1 (ABCA1) phosphorylation in a concentration-dependent manner. Concomitantly, apoA-I increased the intracellular level of cAMP in a concentration-and time-dependent manner. The maximal cAMP level was reached within 10 min at 10 g/ml apoA-I representing a 1-fold increase. The ability of apoA-I to mediate cAMP production was only observed in stimulated fibroblasts. Furthermore, overexpression of ABCA1 in Chinese hamster ovary cells resulted in a 1.5-fold increase in apoA-I-mediated cAMP accumulation as compared with untransfected cells. In contrast, forskolin increased cAMP production significantly in unstimulated fibroblasts as well as in untransfected Chinese hamster ovary cells. Pharmacological inhibition of protein kinase A (H89) completely blocked apoA-I-mediated ABCA1 phosphorylation. Naturally occurring mutations of ABCA1 associated with Tangier disease (C1477R, 2203X, and 2145X) severely reduced apoA-I-mediated cAMP production, ABCA1 phosphorylation, 125 I-apoA-I binding, and lipid efflux, without affecting forskolin-mediated cAMP elevation. In contrast, the protein kinase A catalytic subunit was able to phosphorylate ABCA1 similarly from mutant and normal cell lines in vitro. Together, our results indicate that apoA-I activates ABCA1 phosphorylation through the cAMP/ protein kinase A-dependent pathway, apoA-I-mediated cAMP production required high level expression of functional ABCA1, and Tangier disease mutants have defective apoA-I-mediated cAMP signaling. These findings suggest that apoA-I may activate cAMP signaling through G protein-coupled ABCA1 transporter.

Section: Discussionsupporting

confidence: 56%

Apolipoprotein A-I Activates Cellular cAMP Signaling through the ABCA1 Transporter

Haidar

Denis

Marcil

et al. 2004

“…It is postulated that this is due to the role of HDL in reverse cholesterol transport from the cells of the arterial wall (1,2). Reverse cholesterol transport involves the HDL-mediated transport of cholesterol from extrahepatic tissues to the liver, allowing for cells to rid themselves of excess cholesterol and maintain cholesterol balance.…”

Section: Hdlmentioning

confidence: 99%

ABCA1 Is Essential for Efficient Basolateral Cholesterol Efflux during the Absorption of Dietary Cholesterol in Chickens

Mulligan

Flowers

Tebon

et al. 2003

113

The ATP-binding cassette transporter A1 (ABCA1) participates in the efflux of cholesterol from cells. It remains unclear whether ABCA1 functions to efflux cholesterol across the basolateral or apical membrane of the intestine. We used a chicken model of ABCA1 dysfunction, the Wisconsin hypoalpha mutant (WHAM) chicken, to address this issue. After an oral gavage of radioactive cholesterol, the percentage appearing in the bloodstream was reduced by 79% in the WHAM chicken along with a 97% reduction in the amount of tracer in high density lipoprotein. In contrast, the percentage of radioactive cholesterol absorbed from the lumen into the intestine was not affected by the ABCA1 mutation. Liver X receptor (LXR) agonists have been inferred to decrease cholesterol absorption through activation of ABCA1 expression. However, the LXR agonist T0901317 decreased cholesterol absorption equally in both wild type and WHAM chickens, indicating that the effect of LXR activation on cholesterol absorption is independent of ABCA1. The ABCA1 mutation resulted in accumulation of radioactive cholesterol ester in the intestine and the liver of the WHAM chicken (5.0-and 4.4-fold, respectively), whereas biliary lipid concentrations were unaltered by the WHAM mutation. In summary, ABCA1 regulates the efflux of cholesterol from the basolateral but not apical membrane in the intestine and the liver.

“…4B), the most important acceptors of cell-derived phospholipids and cholesterol (reviewed in Ref. 48). Furthermore, another study has shown that proteolysis leads to dissociation of apoA-I from HDL, especially on the less buoyant HDL 3 , and produces an unstable HDL population (49).…”

Section: Table II Physical Properties and Lcat Kinetic Data For Reconmentioning

confidence: 99%

Proteolytic Degradation and Impaired Secretion of an Apolipoprotein A-I Mutant Associated with Dominantly Inherited Hypoalphalipoproteinemia

McManus

Scott

Franklin

et al. 2001

We have devised a combined in vivo, ex vivo, and in vitro approach to elucidate the mechanism(s) responsible for the hypoalphalipoproteinemia in heterozygous carriers of a naturally occurring apolipoprotein A-I (apoA-I) variant (Leu 159 to Arg) known as apoA-I Finland (apoA-I FIN ). Adenovirus-mediated expression of apoA-I FIN decreased apoA-I and high density lipoprotein cholesterol concentrations in both wild-type C57BL/6J mice and in apoA-I-deficient mice expressing native human apoA-I (hapoA-I). Interestingly, apoA-I FIN was degraded in the plasma, and the extent of proteolysis correlated with the most significant reductions in murine apoA-I concentrations. ApoA-I FIN had impaired activation of lecithin:cholesterol acyltransferase in vitro compared with hapoA-I, but in a mixed lipoprotein preparation consisting of both hapoA-I and apoA-I FIN there was only a moderate reduction in the activation of this enzyme. Importantly, secretion of apoA-I was also decreased from primary apoA-I-deficient hepatocytes when hapoA-I was co-expressed with apoA-I FIN following infection with recombinant adenoviruses, a condition that mimics secretion in heterozygotes. Thus, this is the first demonstration of an apoA-I point mutation that decreases LCAT activation, impairs hepatocyte secretion of apoA-I, and makes apoA-I susceptible to proteolysis leading to dominantly inherited hypoalphalipoproteinemia. Plasma concentrations of high density lipoprotein (HDL)1 cholesterol (HDL-C) are inversely correlated with the risk of developing coronary heart disease (1). However, the complex and often poorly understood etiology for variations in HDL-C concentrations within the general population has made the therapeutic control of HDL levels an elusive target to date. This is attributed to the intricate nature of HDL metabolism that involves many components including the major HDL structural protein apolipoprotein A-I (apoA-I) and multiple factors required for cholesteryl ester (CE) formation, lipolysis, lipid transfer, cellular lipid efflux, and cell surface interactions (reviewed in Refs. 2-4). Nascent HDLs that are derived from the liver and intestine are poorly lipidated (2, 5) and must acquire additional lipids for their maturation into the more stable ␣-migrating HDLs in plasma. Defective clearance of triglyceride-rich lipoproteins (TRL) is recognized as a major determinant of HDL-C concentrations. Recessive mutations in lipoprotein lipase and its major activator protein apolipoprotein C-II, which result in impaired hydrolysis of TRL, also contribute to low HDL-C concentrations. Also the efficient conversion of free cholesterol (FC) to CE on HDL by lecithin:cholesterol acyltransferase (LCAT) is necessary for HDL maturation and depends on apoA-I as its physiological activator (reviewed in Refs. 2-4). Recent work has also highlighted the importance of both the ATP-binding cassette transporter A1 (ABCA1) protein and phospholipid transfer protein (PLTP) in maintaining normal HDL-C concentrations. PLTP-deficient mice have HDL-C levels that...