High density lipoproteins

Features of HDL include:

• High density lipoprotein (HDL) can receive excess cholesterol from the tissues. The cholesterol can then be transferred out of HDL to the liver and to other lipoproteins such as Very low-density lipoprotein (VLDL)
• much of the cholesterol received by HDL is unesterified cholesterol. The capacity to receive unesterified cholesterol is increased by the presence of an enzyme, lecithin:cholesterol acyl transferase (LCAT) which esterifies cholesterol. The cholesteryl ester is more hydrophobic than free cholesterol and thus is more tightly packed into the HDL particle. The ability to convert cholestroyl esters from free cholesterol thus allows the HDL to pick up relatively more free cholesterol
• HDL particles have a half-life of days
• the majority of the mass of HDL consists of protein. It accounts for approximately 20% of circulating cholesterol and carries relatively large amounts of phospholipid
• HDL is anti-athrogenic; its concentration is inversely related to the risk of coronary artery disease. This may be due to the capacity of HDL to transfer cholesterol from the tissues to the liver
• HDL levels are often low if there is hypertriglyceridaemia:
  • the presence of high levels of triglyceride promotes the high rate of transfer of cholesterol from HDL to VLDL (also there is a similar promotion of transfer of cholesterol from large, buoyant LDL to VLDL). These transfers of cholesterol are mediated by cholesteryl ester transfer protein (CETP) and there is also a transfer of triglyceride from VLDL to both HDL and LDL. The subsequent removal of triglyceride from cholesterol-depleted LDL by hepatic lipase results in the formation of small, dense LDL

Are high levels of HDL anti-atherogenic?

• .."According to HEART UK experts the protective effect of HDL appears to reach its maximum when levels are roughly 1.5mmol/L. Beyond this, higher levels of HDL may not provide additional protection and current research is suggesting that levels in excess of 2.3mmol/L may behave more like LDL cholesterol, raising the risk of developing cardiovascular disease and heart problems, particularly in women who are going through, or are post menopause...
• If you have a family history of early heart disease and your HDL is very high (above 3 mmol/L) and there is no obvious cause then ideally you should be referred to a lipid clinic for further investigations. If both total and HDL cholesterol are high (total cholesterol over 7.5 mmol/L with HDL cholesterol over 2.5 mmol/L in women) then treatment decisions are better guided by the levels of bad cholesterol (LDL and/or non-HDL-cholesterol) than by the Total/HDL-cholesterol ratio which may be misleading in these circumstances.."

Reference:

• High Density Lipoprotein (HDL). HEART UK Factsheet (Accessed 30/11/2020)