Catabolism of HDL

Catabolism of HDL

How is mature alpha-HDL removed from the circulation?

The lipids and proteins of mature alpha-HDL are removed from the circulation by direct pathways. These involve:
- selective lipid uptake by scavenger receptor B1 (SR-B1); and
- holoparticle (whole particle) uptake by apo E receptors, cubilin and probably also by other (as yet unknown) HDL receptors.
Indirect pathways involving the action of CETP and hepatic lipase also play a part:
- degradation of HDL phospholipids by endothelial lipase; and
- degradation of HDL phospholipids by sPLA₂.

Selective cholesterol uptake by SR-B1

What is SR-B1?

SR-B1 is a fatty-acylated and heavily glycosylated protein displaying a characteristic horseshoe-shaped structure with one extracellular, two transmembrane and two intracellular domains, where several intracellular binding partners dock.

What is the role of SR-B1?

SR-B1 mediates the selective uptake of cholesteryl esters from HDL (and also LDL) into hepatocytes and steroid-hormone-producing cells, without internalising HDL proteins [103,104].

Where is SR-B1 found?

SR-B1 is found in tissues with critical roles in cholesterol metabolism. These include the liver, adrenal glands and ovaries. Lower SR-B1 expression levels are found in cells relevant to the pathogenesis of atherosclerosis, such as macrophages, endothelial cells and smooth muscle cells [105–108].
SR-B1 is mainly (but not exclusively) recovered from membrane fractions containing caveolae – the 50–100 nm flask-shaped invaginations implicated in potocytosis, transcytosis and cellular signalling. In hepatocytes, SR-B1 is found both in canalicular and sinusoidal membranes, which is consistent with a postulated role of SR-B1 in lipid flux aimed at secretion of bile cholesterol. In addition, SR-B1 is found intracellularly in juxtanuclear endosomal recycling compartments indicating that SR-B1 is part of intracellular transport machinery. In cortical cells and ovarian granulosa cells SR-B1 is present in microvillar channels, where cholesterol-donating lipoproteins (HDL, LDL) are trapped [109–111].

What are SR-B1 ligands?

HDL is the primary ligand for SR-B1. Spherical alpha-HDL with a large diameter binds more effectively than small, discoidal pre-beta-HDL or reconstituted HDL particles containing apo A-I and phospholipids, which are in turn better ligands than free apo A-I. Other HDL apoproteins (apo A-II, C-III, E) either as free apoproteins or as phospholipid-containing liposomes also bind to SR-B1. Other ligands include oxidised HDL, native and oxidised LDL, advanced glycosylation end products (AGE) and negatively charged liposomes [112–117].

What is the mechanism of SR-B1?

The mechanism by which SR-B1 achieves the dissociation of lipids and proteins and the cellular uptake of cholesteryl esters is not yet understood.

It has been suggested that SR-B1 exerts a lipid transfer activity, for example, by forming a hydrophobic channel through which lipid particles diffuse from HDL particles into the plasma membrane. The preferred substrates for SR-B1-mediated lipid transfer are free cholesterol and cholesteryl esters followed by triglycerides. These lipids are then incorporated into the membranes. Following selective transfer of cholesteryl esters, which takes place mainly in caveolae, cytosolic complexes containing caveolin (caveola structural protein), annexin II and cyclophilins are formed and delivered to intracellular membranes. Free cholesterol reaches the endoplasmic reticulum, where it is re-esterified by acylcoenzyme A:cholesterol acetyltransferase (ACAT). Liver-type fatty acid binding protein (L-FABP) may mediate this process [118–120].
Another hypothesis assumes that apo E-free HDL particles are taken up with SR-B1 to endosomal recycling compartments, where cholesterol esters and triglycerides are hydrolysed. SR-B1 is then recycled back to the plasma membrane together with the lipid-depleted HDL particle, which is resecreted. This process, called retroendocytosis, has been observed in hepatocytes, macrophages and intestinal cells [110,111].

How is SR-B1-mediated HDL catabolism regulated?

Hepatic SR-B1 expression is regulated by a variety of factors including polyunsaturated fatty acids (stimulation) and alpha-tocopherol, insulin-like growth factor-1 and oestrogens (suppression) [118,121–123].
Peroxisome proliferator activated receptor (PPAR)-alpha agonists, such as fibrates, suppress SR-B1 on a post-transcriptional level [124].
Farnesoid X receptor (FXR), liver X receptor (LXR) and PPAR-gamma agonists, such as cholic acids, oxysterols and glitazones, respectively, elevate SR-B1 levels [125,126].
PDZ-domain containing protein (PDZK1, also called CLAMP) binds to the cytoplasmic C terminus of SR-B1 and stabilises the protein. Mutations in the C-terminus that abolish interaction between SR-B1 and PDZK1 prevent cell surface receptor expression in the liver. A PDZK1-deficient mouse is characterised by increased plasma HDL-C. Furthermore, a small PDZK1-associated protein (SPAP, also called DD96) regulates PDZK1 levels and thereby modulates plasma HDL-C [127–129].
Caveolin stabilises cholesterol ester-rich domains in the plasma membrane and diminishes SR-B1-mediated uptake of cholesterol ester [130,131].
Hepatic lipase hydrolyses the phospholipids of the surface layer of both HDL and plasma membrane and thereby enables the flux of cholesterol ester from the lipoprotein core into the plasma membrane. In addition, hepatic lipase may cooperate with SR-B1 in cholesterol ester selective uptake independently from its lipolytic activity [132,133].

SR-B1 also mediates cholesterol efflux from cells – to what extent does this occur?

In contrast to selective uptake of lipoprotein lipids, such as cholesterol ester, phospholipids and triglycerides, which is unidirectional, the movement of free cholesterol via SR-B1 is bidirectional occurring either from the lipoprotein towards the cell or the other way around. The direction of cholesterol flux is determined solely by the cholesterol gradient. HDL, LDL, other lipoproteins or small unilamellar phospholipid vesicles may function as acceptors/donors in SR-B1-mediated cholesterol flux:
- there is an absolute requirement for phospholipids as acceptors in SR-B1-mediated cholesterol efflux. No efflux to lipid-free apoproteins occurs via SR-B1. This is in contrast to ABCA1-mediated efflux. The efficiency of SR-B1-mediated cholesterol efflux specifically depends on the content of phosphatidylcholine in the acceptor particle [134,135];
- the binding of the acceptor to SR-B1 is critically important for SR-B1-mediated cholesterol efflux. In addition, SR-B1 is believed to facilitate the desorption rate of free cholesterol from the membrane by reorganising membrane lipid packing [136]; and
- the importance of SR-B1-mediated cholesterol efflux for HDL generation and for reverse cholesterol transport (RCT) remains unclear.

Endothelial lipase-dependent catabolism of HDL

What is endothelial lipase?

Endothelial lipase shows strong sequence similarities to lipoprotein and hepatic lipase and is a member of the same gene family [137].
In contrast to both lipoprotein lipase and hepatic lipase, endothelial lipase has little triglyceride hydrolase activity, but instead cleaves fatty acids from the sn-1 position of phosphatidylcholine. The enzyme is most active on lipids present in HDL [138].
Endothelial lipase is expressed mainly in endothelial cells and macrophages.

How does endothelial lipase contribute to HDL catabolism?

Endothelial lipase overexpression in the livers of mice leads to markedly reduced plasma concentration of HDL-C and apo A-I. Conversely, HDL-C levels are increased following inhibition of endothelial lipase with anti-endothelial lipase antibodies or in endothelial lipase 'knock-out' mice. In addition, some endothelial lipase polymorphisms are more common in persons with elevated HDL-C levels [139–141].
The position of endothelial lipase in the temporal sequence of events in HDL metabolism has not been determined. In particular, it is not clear whether endothelial lipase acts upstream of SR-B1 priming HDL for the interaction with this receptor, or downstream of SR-B1 removing phospholipids from HDL depleted of cholesteryl esters.

sPLA₂-dependent catabolism of HDL

sPLA₂ hydrolyses the phospholipid monolayer of HDL [142].
Transgenic overexpression of sPLA₂ in mice accelerates catabolism of both HDL apolipoproteins and cholesteryl esters, resulting in decreased plasma HDL-C levels [143].
sPLA₂ at least partially accounts for decreased HDL-C levels seen during acute-phase response [144].

Catabolism of HDL apolipoprotein

The removal of the protein constituents of HDL from the circulation is less well understood than the removal of HDL-associated lipids.

What are the potential mechanisms for catabolism of HDL apolipoproteins?

Potential mechanisms for the catabolism of HDL holoparticles and lipid-free apolipoproteins are endocytosis into liver and kidney cells, and into the placenta and yolk sac during pregnancy.

Cubilin and apo A-I uptake in the catabolism of HDL

Cubilin is a 460 kDa peripheral membrane glycoprotein expressed primarily on polarised epithelial cells and localised in apical membranes. Together with megalin, cubilin acts as a receptor for vitamin-binding proteins (eg, retinol-binding protein, intrinsic factor, transcobalamin, vitamin-D-binding protein), iron-complexed transferrin, metal-complexed metallothionein, hormones (eg, insulin, prolactin, epidermal growth factor, parathyroid hormone) and drugs (eg, polymyxin B, gentamicin). The cubilin–megalin complex mediates endocytosis and/or transcytosis of these ligands [145].

Cubilin has been identified as an HDL/apo A-I binding site in the epithelial cells of the proximal tubules of the kidney and the yolk sac [146].

Small HDL particles (diameter <8 nm) and lipid-free apo A-I are filtered by renal glomeruli into the primary urine. As cubilin-deficient men or dogs are known to have increased excretion of apo A-I in their urine, it has been suggested that cubilin may mediate the uptake of apo A-I into proximal tubule cells [147].

Cubilin has no transmembrane domain and, therefore, cannot mediate the internalisation of its ligands without co-receptors – it is possible that megalin, a member of the LDL receptor gene family, may carry out this role in the kidney and the yolk sac [148,149].

After internalisation, the ligands of cubilin (HDL, apo A-I, vitamin B12-intrinsic factor complex) are transported to lysosomes for degradation, therefore, it is:

unlikely that apo A-I of the primary urine taken up by proximal tubule cells re-enters the plasma compartment; and
unlikely that cubilin is an important determinant of HDL-C plasma concentration [146].

Catabolism of HDL

Apo E-containing HDL (which constitutes only a small proportion of HDL) is internalised by hepatic apo E receptors (LDL receptor, LDL receptor-related protein [LRP]) [150,151].

There is considerable evidence for the presence on liver cells of additional HDL receptors that mediate the catabolism of apo E-free HDL. Ligand blotting studies have identified HDL binding sites of different sizes in liver cells – these are candidates for receptors that mediate hepatic HDL-holoparticle uptake [152].

Morphological studies have also provided evidence for the binding and endocytosis of HDL:

some authors have demonstrated re-secretion of HDL that was internalised into liver cells [111,153].

Potential role of leptin in the catabolism of HDL

Hepatic binding, uptake, degradation and re-secretion of HDL are disturbed in ob/ob mice, which are deficient in leptin [154].

As ob/ob mice have elevated HDL-C levels caused by retarded HDL catabolism, it has been suggested that a hepatic leptin-regulated HDL receptor that regulates HDL-C levels by mediating holoparticle uptake into liver cells may be present [155].

Origin, remodelling and catabolism of HDL: issues

The origin of HDL particles containing both apo A-I and apo A-II is poorly understood - what is the current opinion? What research is being conducted to improve understanding?

What evidence is there for the role of cubilin in apo A-I uptake?

What evidence is there for the role of megalin in apo A-I uptake?

How likely is the presence of a hepatic leptin-regulated HDL receptor?

Morphological studies have also provided evidence for the binding and endocytosis of HDL - what evidence? How convincing is it?