Lecithin:cholesterol acyltransferase: old friend or foe in atherosclerosis?

Chronic pharmacological activation of P2Y13 receptor in mice decreases HDL-cholesterol level by increasing hepatic HDL uptake and bile acid secretion

High level of high-density lipoprotein cholesterol (HDL-cholesterol) is inversely correlated to the risk of atherosclerotic cardiovascular disease. The protective effect of HDL is mostly attributed to their metabolic functions in reverse cholesterol transport (RCT), a process whereby excess cell cholesterol is taken up from peripheral cells and processed in HDL particles, and is later delivered to the liver for further metabolism and bile excretion. We have previously demonstrated that P2Y13 receptor is critical for RCT and that intravenous bolus injection of cangrelor (AR-C69931MX), a partial agonist of P2Y13 receptor, can stimulate hepatic HDL uptake and subsequent lipid biliary secretion without any change in plasma lipid levels. In the present study, we investigated the effect of longer-term treatment with cangrelor on lipoprotein metabolism in mice. We observed that continuous delivery of cangrelor at a rate of 35μg/day/kg body weight for 3days markedly decreased plasma HDL-cholesterol level, by increasing the clearance of HDL particles by the liver. These effects were correlated to an increase in the rate of biliary bile acid secretion. An increased expression of SREBP-regulated genes of cholesterol metabolism was also observed without any change of hepatic lipid levels as compared to non-treated mice. Thus, 3-day cangrelor treatment markedly increases the flux of HDL-cholesterol from the plasma to the liver for bile acid secretion. Taken together our results suggest that P2Y13 appears a promising target for therapeutic intervention aimed at preventing or reducing cardiovascular risk.

Modified lipoproteins provide lipids that modulate dendritic cell immune function

Both physiological and pathological situations can result in biochemical changes of low-density lipoproteins (LDL). Because they can deliver signals to dendritic cells (DC), these modified lipoproteins now appear as regulators of the immune response. Among these modified lipoproteins, oxidized LDL (oxLDL) that accumulate during inflammatory conditions have been extensively studied. Numerous studies have shown that oxLDL induce the maturation of DC, enhancing their ability to activate IFNγ secretion by T cells. LDL treated by secreted phospholipase A(2) also promote DC maturation. Among the bioactive lipids generated by oxidation or phospholipase treatment of LDL, lysophosphatidylcholine (LPC) and some saturated fatty acids induce DC maturation whereas some unsaturated fatty acids or oxidized derivatives have opposite effects. Among other factors, the nuclear receptor peroxisome-proliferator activated receptor γ (PPARγ) plays a crucial role in this regulation. Non-modified lipoproteins also contribute to the regulation of DC function, suggesting that the balance between native and modified lipoproteins, as well as the biochemical nature of the LDL modifications, can regulate the activation threshold of DC. Here we discuss two pathological situations in which the impact of LDL modifications on inflammation and immunity could play an important role. During atherosclerosis, modified LDL accumulating in the arterial intima may interfere with DC maturation and function, promoting a Th1 immune response and a local inflammation favoring the development of the pathology. In patients chronically infected, the hepatitis C virus (HCV) interferes with lipoprotein metabolism resulting in the production of infectious modified lipoproteins. These lipo-viral-particles (LVP) are modified low-density lipoproteins containing viral material that can alter DC maturation and affect specific toll-like receptor signaling. In conclusion, lipoprotein modifications play an important role in the regulation of immunity by delivering signals of danger to DC and modulating their function.