Question

Which of the following lipoproteins is responsible for reverse cholesterol transport?

• A. Chylomicrons
• B. Very Low Density Lipoproteins
• C. Low Density Lipoproteins
• D. High Density Lipoproteins

Hint:

HDL is "good" because it performs "reverse" transport (tissues $\rightarrow$ liver), whereas LDL is "bad" because it transports cholesterol from the liver to peripheral tissues, which can promote atherosclerosis.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Lipoproteins are lipid-protein complexes that transport hydrophobic lipids (such as cholesterol, cholesterol esters, and triglycerides) through the aqueous environment of the bloodstream.
Different classes of lipoproteins are distinguished by their lipid-to-protein ratios, densities, and the specific apolipoproteins they carry.

Step 2: Detailed Explanation:
Reverse Cholesterol Transport (RCT) is a pathway that clears excess cholesterol from peripheral tissues and returns it to the liver for excretion in bile.
High-Density Lipoproteins (HDL) play the primary role in mediating RCT.
The process occurs in several key steps:
Efflux of free cholesterol: Nascent, lipid-poor HDL particles secreted by the liver and intestine pick up free cholesterol from peripheral cells (such as macrophages in arterial walls). This transport is mediated by cell-membrane proteins like ATP-binding cassette transporter A1 (ABCA1).
Esterification: The enzyme lecithin-cholesterol acyltransferase (LCAT), activated by ApoA-I on HDL, esterifies the free cholesterol. This converts it into highly hydrophobic cholesteryl esters, which move into the core of the HDL particle, transforming it into a mature, spherical HDL.
Delivery to the liver: The mature HDL delivers the cholesteryl esters back to the liver, either directly via binding to scavenger receptor class B type I (SR-BI) or indirectly via cholesteryl ester transfer protein (CETP), which transfers the esters to LDL in exchange for triglycerides.
The liver then converts this excess cholesterol into bile acids for excretion.
Because of this protective mechanism against plaque formation in arteries, HDL is referred to as "good cholesterol."

Step 3: Final Answer:
High Density Lipoproteins are the lipoprotein class responsible for reverse cholesterol transport.