In macrophages, the hydrolysis of cholesteryl esters (CHESTs) is the rate-limiting step in the removal of free cholesterol (CHOL) from these cells. CHOL is transported via transport vesicles and can be used for cellular functions or removed from the cell by ABCA1 to create new HDL particles. Accumulation of CHESTs in macrophage foam cells is key to atherosclerotic plaque formation (Dubland & Francis 2015).
Exit from lysosomes of CHOL derived from the hydrolysis of CHESTs in low-density lipoproteins (LDLs) requires the concerted effort of two proteins, membrane-bound Niemann-Pick C1 (NPC1) and soluble NPC2. In the second step, NPC2 transfers CHOL to the CHOL-binding pocket of the N-terminal domain of NPC1 (Infante et al. 2008). During the transfer of CHOL from NPC2 to NPC1, the orientation of CHOL is reversed, allowing insertion of its isooctyl side chain into the outer lysosomal membrane (Kwon et al. 2009).