Removal of cellular cholesterol by pre-beta-HDL involves plasma membrane microsolubilization

Area of Science:

• Biochemistry
• Cell Biology
• Lipid Metabolism

Background:

• High density lipoprotein (HDL) plays a key role in reverse cholesterol transport.
• The mechanism of cholesterol removal by lipid-poor pre-beta-HDL mediated by apolipoprotein (apo) A-I is not fully understood.
• Aqueous diffusion is a known mechanism for cholesterol removal by HDL.

Purpose of the Study:

• To elucidate the molecular mechanism of cholesterol removal from peripheral cells mediated by lipid-poor apolipoprotein A-I.
• To investigate the initial rates and concentration-dependence of cholesterol and phospholipid efflux to apoA-I.
• To determine the characteristics of the cholesterol pool accessible for apoA-I-mediated removal.

Main Methods:

• Incubation of human fibroblasts with lipid-free human apoA-I.
• Measurement of initial rates of unesterified cholesterol and phospholipid efflux.
• Analysis of concentration-dependence of lipid efflux to determine kinetic parameters (Km).

Main Results:

• Both cholesterol and phospholipid are released from fibroblasts upon incubation with apoA-I.
• The concentration-dependence for cholesterol and phospholipid efflux suggests a membrane microsolubilization process.
• A finite pool (approx. 1%) of plasma membrane cholesterol is accessible, limited by phospholipid removal, not apoA-I availability.

Conclusions:

• Membrane microsolubilization by apoA-I is the primary mechanism for initial cholesterol removal from cells in reverse cholesterol transport.
• This process involves the release of a limited amount of phospholipid, restricting the extent of cholesterol solubilization.
• The relative contribution of microsolubilization versus aqueous diffusion depends on apoA-I lipidation state.