Reverse cholesterol transport (RCT) is a multistep process comprising removal of excess cholesterol from cells in the body and delivery to the liver for excretion into the small intestine. [1]
Enhancing reverse cholesterol transport is considered a potential strategy for preventing and treating atherosclerosis and associated diseases such as cardiovascular disease and stroke. [2] Atherosclerosis is caused by the build-up in arterial blood vessels of atherosclerotic plaques. These consist mostly of foam cells, which are macrophages overloaded with cholesterol and other lipids. Foam cells and other cells in peripheral tissues can hand over their excess cholesterol to high-density lipoprotein (HDL) particles. These will transport the cholesterol via the lymph and then the blood stream to the liver, from where it will be excreted with bile into the small intestine. Reverse cholesterol transport thereby works against the build-up of atherosclerotic plaques from dying foam cells.
In more detail, reverse cholesterol transport proceeds in the following steps:
Through these steps, RCT plays a vital role in maintaining cholesterol homeostasis and preventing the accumulation of cholesterol in peripheral tissues, thereby reducing the risk of cardiovascular diseases.
While excess fat (lipids) can simply be catabolized (burned) by cells as energy source, cholesterol's complex molecular structure cannot be efficiently catabolized. Therefore, excess peripheral cholesterol is recycled to the liver via RCT.
Adiponectin induces ABCA1-mediated reverse cholesterol transport from macrophages by activation of PPAR-γ and LXRα/β. [5]
High-density lipoprotein cholesterol (HDL-C) refers to the total cholesterol content carried by all HDL particles in the bloodstream. Traditionally the amount of HDL-C is used as a proxy to measure the amount of HDL particles, and from there a proxy for the reverse cholesterol transport capacity. However, a number of conditions that increase reverse cholesterol transport (e.g. being male) will reduce HDL-C due to the greater clearance of HDL, making such a test unreliable. In fact, when many known correlates of CVD risks are controlled for, HDL-C does not have any correlation with cardiovascular event risks. In this way, HDL-C only seems to serve as an imperfect, but easy-to-measure, proxy for a healthy lifestyle. [6]
The actual cholesterol efflux capacity (CEC) is measured directly: one takes a blood sample from the patient, isolates the serum, and removes any ApoB-containg particles from it. Mouse macrophages are incubated in an ACAT inhibitor and radioisotope-labelled cholesterol, then have their efflux ability "woken up" with an ABCA1 agonist before use. They are then mixed with the prepared serum. The macrophages are then recovered to quantify their change in radioactivity compared to a control batch. Any extra loss in radioactivity is interpreted to have been taken up by the HDL particles in the patient's serum. [7] (This test does not account for the liver-bile-feces part of the transport.)
The cholesterol efflux capacity (CEC) has much better correlation with CVD risks and CVD event frequencies, even when controlling for known correlates. [6] Many drugs affect enzymes and receptors involved in the transport process: