Cholesterol Trafficking: An Emerging Therapeutic Target for Angiogenesis and Cancer

Abstract

Cholesterol is an essential structural component of cellular membranes. In addition to the structural role, it also serves as a precursor to a variety of steroid hormones and has diverse functions in intracellular signal transduction. As one of its functions in cell signaling, recent evidence suggests that cholesterol plays a key role in regulating angiogenesis. This review discusses the role of cholesterol in angiogenesis, with a particular emphasis on cholesterol trafficking in endothelial cell signaling. Small molecule inhibitors of cholesterol trafficking and their preclinical and clinical development targeting angiogenesis and cancer are also discussed.

Keywords: NPC1; angiogenesis; cholesterol trafficking; lysosome; mTOR.

Figure 1

Intracellular cholesterol trafficking. Biosynthesized cholesterol from endoplasmic reticulum (ER) is delivered to the cell membrane directly or via the Golgi. Circulating cholesterol (low-density lipoprotein (LDL)-cholesterol) is delivered through the endosomal trafficking system, where LDL binds to the LDL receptor in the clathrin-coated pits on the cell membrane and internalizes via endocytosis. The endosomes are then mature and are fused with lysosomes, where cholesterol is released from LDL and transported out from the endosomal system. Through anterograde (ER to Golgi) and retrograde (Golgi to ER) trafficking, cholesterol is delivered to cellular organelles and plasma membrane. Inter-compartmental cholesterol delivery can be done via sterol transfer proteins, such as oxysterol binding protein (OSBP) and OSBP-related protein (ORP), or by membrane (vesicular) trafficking along cytoskeletal proteins using kinesin or dynein motors.

Figure 2

Cholesterol transport in the lysosome. Upon being delivered to the lysosome, LDL and cholesteryl esters are hydrolyzed by lysosomal acid lipase (LAL). Unesterified cholesterol in the lysosome lumen binds to Niemann-Pick disease type C (NPC) NPC2 and is then transferred to the N-terminal domain (NTD) of NPC1 in the inner-membrane side of the lysosome. Cholesterol is transferred from the NTD to the sterol-sensing domain (SSD) of NPC1 and transported out of the lysosome. Several cholesterol trafficking inhibitors including itraconazole, cepharanthine, and astemizole, as well as U18666A, are known to bind to the SSD and interfere with the cholesterol transport by NPC1. This effect leads to an accumulation of free cholesterol inside the lysosome. The immunofluorescence images are from endothelial cells stained with filipin (cholesterol-specific fluorescent dye). The left immunofluorescence image is control HUVEC with a high level of cholesterol in the cell membrane and other intracellular compartments. The right one is an example of HUVEC treated with cepharanthine, showing the strong accumulation of cholesterol in the lysosomes and the depletion of cholesterol in other cellular compartments.

Figure 3

Endothelial cell cholesterol level and angiogenic signaling molecules. The proper level of cholesterol in the membrane is critical for membrane structure and the function of membrane binding angiogenic signaling proteins, such as mTOR and VEGFR2. Depletion of cholesterol by the inhibition of cholesterol trafficking leads to the dissociation of mTOR from the lysosomal membrane and the inhibition of VEGFR2 dimerization on the cell membrane, causing the inhibition of their signaling pathways.