ARTICLES

Cholesterol efflux is a key process that prevents cholesterol accumulation within cells, maintaining lipid homeostasis and protecting against diseases such as atherosclerosis. The primary pathway for cholesterol efflux is mediated by the ATP-binding cassette transporter A1 (ABCA1), which facilitates the transport of cholesterol to apolipoprotein A-I (ApoA-I). This process results in the formation of high-density lipoproteins (HDL), which carry cholesterol to the liver for excretion. The activity of ABCA1 is regulated by Liver X Receptor (LXR), a nuclear receptor activated by oxysterols, which are oxidized derivatives of cholesterol produced when cellular cholesterol levels rise. Once activated, LXR upregulates ABCA1 expression, promoting cholesterol export from cells and preventing cholesterol buildup. Additionally, ABCG1 plays a complementary role by exporting cholesterol to mature HDL particles, further contributing to cholesterol efflux, particularly in cells like macrophages.

When cholesterol homeostasis is disrupted, efflux pathways become impaired, leading to cholesterol accumulation within cells. This can result from reduced LXR activity or decreased expression of efflux transporters like ABCA1, often caused by inflammatory conditions. Chronic inflammation, triggered by excess cholesterol, further disrupts these pathways. One of the most important mechanisms linking cholesterol accumulation to inflammation is NLRP3 inflammasome activation, which is triggered by cholesterol crystal formation. Cholesterol crystals physically disrupt lysosomes, leading to the activation of the NLRP3 inflammasome and the release of pro-inflammatory cytokines like IL-1β. IL-1β amplifies the inflammatory response and activates the NF-κB pathway, which in turn suppresses LXR activity and reduces ABCA1 expression, creating a vicious cycle of cholesterol accumulation and inflammation.

A key factor contributing to the disruption of cholesterol efflux is the overactivation of SREBP-2 (Sterol Regulatory Element-Binding Protein 2). SREBP-2 is a transcription factor that controls cholesterol synthesis and uptake by upregulating the expression of HMG-CoA reductase (HMGCR), the rate-limiting enzyme in cholesterol biosynthesis. Importantly, HMGCR is the target of statins, a class of drugs that lower cholesterol by inhibiting this enzyme. This process predominantly occurs in the liver, the main organ responsible for cholesterol synthesis. SREBP-2 also promotes the expression of the LDL receptor (LDLR), which facilitates cholesterol uptake from low-density lipoproteins (LDL). Excessive SREBP-2 activation, which can be driven by mTORC1 activation, leads to overproduction of cholesterol, overwhelming the cell's capacity for efflux and resulting in intracellular cholesterol accumulation.

mTORC1 activation not only increases cholesterol synthesis but also plays a role in autophagy dysfunction, which contributes to cholesterol accumulation. Autophagy is a critical process for degrading and recycling cellular components, including lipids and damaged organelles. When mTORC1 is overactivated, it inhibits autophagy, impairing cellular processes like mitophagy (the degradation of damaged mitochondria) and lipophagy (the degradation of lipid droplets). This disruption in autophagy results in the accumulation of damaged mitochondria and lipid droplets, further promoting cholesterol buildup and exacerbating cellular dysfunction.

A key consequence of autophagy dysfunction is mitochondrial dysfunction, which significantly contributes to ROS production. When mitophagy is inhibited due to autophagy dysfunction, damaged mitochondria accumulate, leading to inefficient electron transport and excess ROS production. ROS causes oxidative damage to lipids, proteins, and DNA, exacerbating cellular dysfunction. This oxidative stress activates NF-κB signaling, which not only drives a sustained pro-inflammatory response but also suppresses LXR activity, further impairing cholesterol efflux through reduced ABCA1 expression. Additionally, the failure to clear lipid droplets via lipophagy worsens intracellular lipid accumulation, reinforcing the cycle of mitochondrial damage, ROS production, and inflammation. In this pro-inflammatory environment, oxidized LDL (oxLDL) can activate TLR4, further promoting inflammation and perpetuating cholesterol accumulation.

To avoid toxicity from free cholesterol accumulation in cellular membranes, excess cholesterol is stored in its esterified form as cholesteryl esters. The enzyme responsible for this process is acyl-coenzyme A: cholesterol acyltransferase (ACAT), which is found in the endoplasmic reticulum (ER). ACAT catalyzes the esterification of cholesterol by attaching a fatty acid from fatty acyl-CoA to cholesterol, producing cholesteryl esters. These esters are stored in lipid droplets within the cell, particularly in the liver and adipose tissue. Cholesterol esterification helps regulate free cholesterol levels in the plasma membrane and other cellular compartments. Under certain conditions, such as in foam cell formation in atherosclerosis, cholesteryl esters can accumulate excessively, contributing to disease progression. When needed, cholesteryl esters are hydrolyzed by cholesteryl ester hydrolases (CEH) to release free cholesterol for membrane synthesis or steroid hormone production.

Intracellular trafficking also plays a critical role in maintaining cholesterol homeostasis. Cholesterol is synthesized in the endoplasmic reticulum (ER) and then transported to various cellular compartments such as the plasma membrane, mitochondria, and lysosomes. Specialized lipid transport proteins, such as Niemann-Pick C1 (NPC1) and Niemann-Pick C2 (NPC2), facilitate the movement of cholesterol out of lysosomes after the degradation of low-density lipoprotein (LDL). When these transport mechanisms are impaired, such as in Niemann-Pick disease, cholesterol accumulates in lysosomes, leading to cellular dysfunction. Additionally, cholesterol is transferred between the ER and plasma membrane through vesicular transport and non-vesicular routes, involving proteins like StAR-related lipid transfer (START) domains and oxysterol-binding proteins (OSBPs). This trafficking network ensures that cholesterol is distributed where it is needed, particularly in the plasma membrane where it maintains membrane fluidity, or in mitochondria where it is involved in steroidogenesis. Disruptions in this trafficking system can contribute to cholesterol accumulation in specific organelles, exacerbating metabolic dysfunction and disease.

In conclusion, maintaining cholesterol homeostasis requires a careful balance between efflux, synthesis, uptake, storage, and intracellular trafficking. Efflux pathways, mediated by transporters like ABCA1 and ABCG1 under the control of LXR, prevent excess cholesterol buildup, while SREBP-2 regulates cholesterol synthesis and uptake, ensuring cells acquire sufficient cholesterol when needed. To prevent toxicity from free cholesterol, the enzyme ACAT esterifies excess cholesterol, storing it as cholesteryl esters in lipid droplets, which can be mobilized when required for membrane synthesis or hormone production. Disruptions in these processes, such as impaired efflux, overactivation of cholesterol synthesis, or defective cholesterol storage and trafficking, can lead to cholesterol accumulation in specific organelles, including lysosomes and the plasma membrane. This imbalance is further exacerbated by inflammation, mitochondrial dysfunction, and autophagy impairment, all of which contribute to the accumulation of cholesterol and cholesteryl esters. The formation of cholesterol crystals activates the NLRP3 inflammasome, while oxidized cholesterol and ROS promote inflammation through pathways like TLR4, creating a cycle of inflammation and lipid overload. Understanding the complex interplay between these mechanisms—efflux, synthesis, storage, trafficking, and inflammatory responses—is essential for developing targeted therapies to prevent or reverse cholesterol-related diseases.