Fig. 2

The roles of lipoproteins in the exogenous, endogenous, and reverse cholesterol pathways. Exogenous lipoprotein pathway (orange background): Lipid metabolism starts after food intake when chylomicrons are formed in the small intestine through the exogenous lipoprotein pathway [6]. Endogenous lipoprotein pathway (green background): The liver takes up chylomicron remnants, and the endogenous lipoprotein pathway then begins with the formation of VLDLs. VLDLs mainly transport TG from the liver to other tissues, in this process converting to IDLs and LDLs through the emission of fatty acids, and an increase in their cholesterol content [6]. Reverse cholesterol transport (RCT) pathway (blue background): The RCT delivers cholesterol from peripheral tissues back to the liver in both a direct and indirect manner. While in the direct RCT pathway effluxed cellular cholesterol is loaded onto HDLs for transportation, in the indirect RCT pathway cholesterol from HDLs is exchanged for TGs and transported by chylomicrons, VLDLs, and IDLs [6, 9]. In lipoprotein particles, yellow circles represent lipids and blue lines represent proteins. Solid arrows represent paths through which changes in lipoproteins occur. Segmented arrows represent paths that fatty acids follow. Dotted arrows represent paths that CE and TG follow. CE: cholesterol esters; TG: triglycerides; FFA: free fatty acids; CETP: cholesterol ester transfer protein; HTGL: hepatic triglyceride lipase; LCAT: lecithin:cholesterol acyltransferase; LPL: lipoprotein lipase