HDL – From Lipid Transport to Signaling
The HDL metabolism is still less understood compared to LDL metabolism as it is more versatile with regard to cholesterol transport and the receptors involved. The transport of cholesterol by HDL from the periphery back to the liver for disposal is called reverse cholesterol transport. The atheroprotective effects of HDL are complex, besides its role in cholesterol removal from the periphery HDL binding can initiate signaling cascades thereby affecting a multitude of metabolic targets.
HDL mediates lipid transfer by several different mechanism:
1) HDL mediates the transfer of lipids to target cells like hepatocytes by selective uptake, in which the particle is not degraded.
2) Plasma HDL and nascent HDL can act as an acceptor of cellular cholesterol.
3) HDL endocytosis and resecretion can mediate lipid transfer.
4) HDL catabolism can mediate uptake of lipids and their clearance from the bloodstream by the kidney and possibly other tissues.
The lipid transfer seems to be an exchange down a concentration gradient.
Examination of lipid trafficking at high resolution in hepatic cells
We are characterizing the uptake of HDL and its lipid (free and esterified cholesterol) in detail by combined light and electron microscopy. Therefore, diaminobenzidine photooxidation is utilized to convert fluorescent protein labels and cholesterol probes into electron dense precipitates. This method allows the localization of proteins and lipids first by the light microscope and then at nanometer resolution by transmission electron microscopy and electron tomography.
Transport of HDL and HDL- derived lipids through the endothelial barrier
HDL is a central player in cholesterol clearance from macrophages. To achieve the removal of excess cholesterol deposited in the arterial intima, HDL has to cross the endothelial barrier first to get into close proximity to macrophage foam cells found in atherosclerotic plaques. The mechanisms necessary for this transport are not fully understood.
We aim to analyze how HDL crosses the endothelial barrier by using light and electron microscopical methods enabling the visualization of HDL particles itself and its derived lipids using fluorescent probes. Overall this process of HDL transfer must encompass:
1) binding of HDL to the apical side of the endothelial cells to receptors/proteins and its concomitant uptake,
2) transport of HDL particles and their cholesterol / cholesteryl esters to the basolateral side of the endothelial cells and
3) excretion of HDL at the basolateral side of the endothelial cells.
We aim to analyze these steps and the factors involved using polarized endothelial cells.
Role of HDL in miRNA signaling
In a recent study Vickers et al. (Nature Cell Biol., 2011 13: 423ff) presented HDL as plasma transporter of miRNA to target recipient cells. These authors showed altered miRNA profiles in HDL of patients suffering from familial hypercholesterolemia pointing out an important role of HDL in intracellular communication and transport and delivery of miRNAs to cellular targets. HDL seems to be the major transporter for two miRNAs involved in the regulation of HDL receptors. We aim to analyze the targets of these miRNA in tissues in detail.