Cholesterol Transport to the Endoplasmic Reticulum

Abstract

Most cholesterol in mammalian cells is stored in the plasma membrane (PM). Cholesterol transport from the PM to low-sterol regulatory regions of the endoplasmic reticulum (ER) controls cholesterol synthesis and uptake, and thereby influences the rates of cholesterol flux between tissues of complex organisms. Cholesterol transfer to the ER is also required for steroidogenesis, oxysterol and bile acid synthesis, and cholesterol esterification. The ER-resident Aster proteins (Aster-A, -B, and -C) form contacts with the PM to move cholesterol to the ER in mammals. Mice lacking Aster-B have low adrenal cholesteryl ester stores and impaired steroidogenesis because of a defect in cholesterol transport from high-density lipoprotein (HDL) to the ER. This work reviews the molecular characteristics of Asters, their role in HDL- and low-density lipoprotein (LDL)-cholesterol movement, and how cholesterol transferred to the ER is utilized by cells. The roles of other lipid transporters and of membrane lipid organization in maintaining aspects of cholesterol homeostasis are also highlighted.

Figure 1.

Three pools of cholesterol in the plasma membrane. The accessible pool (green) is available for Aster-mediated transport to the endoplasmic reticulum (ER). The sphingomyelin-sequestered pool (pink) is not available for transport but can be made accessible by hydrolysis of plasma membrane sphingomyelin. The essential pool (blue) is sequestered by phospholipids and proteins and is not available for transport. Depletion of the essential pool results in impaired plasma membrane (PM) integrity. The probes ALOD4 and PFO* bind accessible cholesterol, OlyA binds sphingomyelin (SM)-sequestered cholesterol, and the essential pool currently lacks probes. (Figure based on Das et al. 2014.)

Figure 2.

The domain structure of Aster-A, -B and -C. The GRAM domain interacts with accessible cholesterol and phosphatidylserine (PS) at the plasma membrane. The ASTER domain binds and transports cholesterol. The transmembrane (TM) domain anchors the proteins to the endoplasmic reticulum (ER).

Figure 3.

Crystal structure of the ASTER domain of Aster-A (mouse). The ribbons are colored from blue to red moving from the amino to the carboxyl terminus. The 25-OHC is displayed as atomic spheres (gray = carbon, red = oxygen). The structure is rotated 90° about the indicated axis in the right-hand display. The sterol-binding pocket is located between a curved β-sheet and an extended carboxy-terminal helix. Structure determined with Dr. John Schwabe. (Figure reprinted from Sandhu et al. 2018 with permission from Elsevier © 2018.)

Figure 4.

Liver X receptor (LXR) signaling in macrophages. Excess unesterified cholesterol is converted to oxysterols. Oxysterols activate LXR to promote cholesterol efflux by inducing ABCA1. LXRs also induce Aster-B to deliver unesterified cholesterol from the plasma membrane (PM) to endoplasmic reticulum (ER), where acyl-CoA:cholesterol acyltransferase (ACAT) resides. Fatty acid substrates for ACAT-mediated esterification of excess cholesterol are provided by LXR-dependent induction of SREBP-1c (and its targets fatty acid synthase, acetyl-CoA carboxylase, and stearoyl-CoA desaturase).

Figure 5.

Lipoprotein–cholesterol transport to the endoplasmic reticulum (ER). High-density lipoprotein (HDL) cholesterol is funneled into the plasma membrane (PM) by SR-B1. Low-density lipoprotein (LDL) cholesterol is endocytosed by the LDL receptor (LDLR) and released in lysosomes before transport to the PM. When PM sphingomyelin (SM)/cholesterol complexes are saturated, cholesterol becomes accessible for Aster-mediated transport to the ER. The Aster GRAM domain recognizes cholesterol in the presence of PS. At the ER, cholesterol is esterified by acyl-CoA:cholesterol acyltransferase (ACAT) for storage in lipid droplets or converted to 25-hydroxycholesterol by 25-hydroxylase. Cholesterol can also be transported to mitochondria to undergo further modifications including conversion to 27-hydroxcholesterol, bile acids (liver), or steroid hormones (adrenal, gonads). Cholesterol and oxysterols in the ER enhance the proteolytic degradation of 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGCR) and the binding of insulin-induced gene proteins (INSIGs) to SREBP cleavage-activating protein (SCAP) to prevent SREBP-2 movement to the nucleus.