Reverse cholesterol transport is a multi-step process resulting in the net movement of cholesterol from peripheral tissues back to the liver first via entering the lymphatic system, then the bloodstream.

Cholesterol Cholesterol is any of a class of certain organic molecules called lipids. It is a sterol (or modified steroid), a type of lipid. Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell membr ...

from non-hepatic peripheral tissues is transferred to HDL by the ABCA1 ( ATP-binding cassette transporter).http://biochemistry.med.uoc.gr/photos/kardasis_research-07.gif in
/ref> Apolipoprotein A1 (ApoA-1), the major protein component of HDL, acts as an acceptor, and the

phospholipid Phospholipids, are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an alcohol residue (usually a glycerol molecule). Marine phospholipids ty ...

component of HDL acts as a sink for the mobilised cholesterol. The cholesterol is converted to cholesteryl esters by the

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...

LCAT ( lecithin-cholesterol acyltransferase). The cholesteryl esters can be transferred, with the help of CETP ( cholesterylester transfer protein) in exchange for triglycerides, to other lipoproteins (such as LDL and VLDL), and these lipoproteins can be taken up by secreting unesterified cholesterol into the

bile Bile (from Latin ''bilis''), or gall, is a dark-green-to-yellowish-brown fluid produced by the liver of most vertebrates that aids the digestion of lipids in the small intestine. In humans, bile is produced continuously by the liver (liver b ...

or by converting cholesterol to bile acids. Adiponectin induces ABCA1-mediated reverse cholesterol transport from macrophages by activation of

PPAR-γ In the field of molecular biology, the peroxisome proliferator-activated receptors (PPARs) are a group of nuclear receptor proteins that function as transcription factors regulating the expression of genes. PPARs play essential roles in the reg ...

and LXRα/β. Uptake of HDL₂ is mediated by hepatic lipase, a special form of

lipoprotein lipase Lipoprotein lipase (LPL) (EC 3.1.1.34, systematic name triacylglycerol acylhydrolase (lipoprotein-dependent)) is a member of the lipase gene family, which includes pancreatic lipase, hepatic lipase, and endothelial lipase. It is a water-solubl ...

found only in the liver. Hepatic lipase activity is increased by androgens and decreased by

estrogen Estrogen or oestrogen is a category of sex hormone responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three major endogenous estrogens that have estrogenic hormonal a ...

s, which may account for higher concentrations of HDL₂ in women. Discoidal (Nascent) HDL: Initially, HDL is discoidal in shape because it lacks esterified cholesterol but as it keeps accumulating free cholesterol in it, the enzyme LCAT keeps esterifying the free cholesterol. When the HDL molecule is cholesterol rich, its shape is changed into more spherical and it becomes less dense (HDL 2). This is carried to the liver to release all the esterified cholesterol into the liver.

References

{{DEFAULTSORT:Reverse Cholesterol Transport Biochemistry Lipids Lipoproteins Metabolism