🧬 biology

STING is a novel channel to facilitate cholesterol loading into Endoplasmic Reticulum membrane

This study reveals that STING functions as a novel transmembrane channel in the endoplasmic reticulum that recruits Aster-escorted cholesterol from the plasma membrane to maintain cholesterol homeostasis, a mechanism whose disruption leads to altered cholesterol distribution and reduced systemic corticosterone levels.

Original authors: Chen Wang, Munire Maimaiti, Jie Mei, Yuxin Xu, Ziwei Zhong, Xiaonan Ma, Chenhui Li, Chunmeng Sun, Haiyang Hu, Ze Hong

Published 2026-07-03

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CC BY 4.0

Original authors: Chen Wang, Munire Maimaiti, Jie Mei, Yuxin Xu, Ziwei Zhong, Xiaonan Ma, Chenhui Li, Chunmeng Sun, Haiyang Hu, Ze Hong

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

The Big Picture: A New Job for an Old Guard

For a long time, scientists thought STING was just a security guard inside our cells. Its main job was to sound the alarm when it detected dangerous DNA (like from a virus), triggering the immune system to fight back.

However, this paper reveals that STING has a second, ancient, and very different job: it acts as a specialized delivery truck for cholesterol. Specifically, it helps move cholesterol from the cell's outer wall (the Plasma Membrane) into the cell's internal warehouse (the Endoplasmic Reticulum, or ER).

The Problem: A Traffic Jam at the Door

Imagine a cell as a busy city.

The Plasma Membrane (PM) is the city's outer fence.
The Endoplasmic Reticulum (ER) is the central warehouse where the city manages its supplies.
Cholesterol is a vital building material needed to keep the city's walls strong and flexible.

Usually, a helper protein called Aster acts like a forklift. It picks up excess cholesterol sitting on the outer fence (PM) and drives it toward the warehouse (ER). But here was the mystery: The forklift could drop the cholesterol at the warehouse door, but no one knew how the cholesterol actually got inside the warehouse walls.

The Discovery: STING is the Loading Dock

The researchers found that STING is the missing piece of the puzzle. It isn't just a guard; it's a tunnel or a loading dock built right into the warehouse wall.

The Handoff: The forklift (Aster) brings the cholesterol to the warehouse door.
The Tunnel: STING has a special tunnel made of four connected pockets (like a series of rooms).
The Loading: STING grabs the cholesterol from the forklift and guides it through its tunnel, dropping it safely inside the ER membrane.

What Happens When the Truck Breaks?

The scientists tested this by removing STING from mice and cells (like taking the loading dock away).

The Traffic Jam: Without STING, the cholesterol delivered by the forklift couldn't get inside the warehouse. It got stuck piling up at the outer fence (the Plasma Membrane).
The False Alarm: The warehouse (ER) thought, "Oh no! We are out of cholesterol!" Even though there was plenty of cholesterol in the cell, it was all stuck outside.
The Overreaction: Because the warehouse was empty, the cell's manager (a protein called SREBP2) panicked. It started shouting, "Make more cholesterol!" This caused the cell to overproduce cholesterol unnecessarily, even though the total amount in the cell didn't change much—it was just in the wrong place.

The "Compound 53" (C53) Experiment

The researchers used a drug called Compound 53 (C53). Think of this drug as a plug that fits perfectly into STING's loading tunnel.

When they plugged the tunnel with C53, the cholesterol couldn't get inside the warehouse.
Just like removing STING, this caused the same traffic jam at the fence and the same panic from the manager (SREBP2).
This proved that STING's tunnel is the only way cholesterol gets loaded into the ER in this specific process.

Why This Matters for the Whole Body

The study looked at the intestines of mice, which are responsible for absorbing cholesterol from food.

Normal Mice: Their intestinal cells use the STING tunnel to pull dietary cholesterol from the gut into their cells and then into the bloodstream.
Mice Without STING (or with the tunnel plugged): They couldn't absorb dietary cholesterol well. The cholesterol stayed stuck in the gut or on the cell surface and never made it into the blood.
The Result: These mice had lower levels of cholesterol in their blood and liver. They also had lower levels of a stress hormone called corticosterone, suggesting their bodies were struggling to maintain balance.

The Bottom Line

This paper rewrites the rulebook on STING. It shows that before STING became a famous immune system alarm, it evolved to be a cholesterol transporter. It acts as a bridge, ensuring that cholesterol delivered by the "Aster forklifts" actually gets loaded into the cell's internal warehouse. Without this tunnel, the cell gets confused, thinks it's starving for cholesterol, and starts making too much of it, while the body fails to absorb cholesterol from food efficiently.

Technical Summary: STING as a Novel Channel for Cholesterol Loading into the Endoplasmic Reticulum

Problem Statement
The Stimulator of Interferon Genes (STING) is a well-characterized endoplasmic reticulum (ER)-resident transmembrane protein that acts as a central hub for innate immune responses, specifically sensing cytosolic DNA via cGAS to induce type I interferon responses. While STING's role in immunity is established, its function in the resting state remains poorly understood. Concurrently, the mechanism by which cholesterol, delivered from the plasma membrane (PM) to the ER by Aster proteins (GRAMD1 family), is inserted into the ER membrane has remained a molecular conundrum. Although ER cholesterol levels regulate STING activation, it is unknown whether STING reciprocally regulates cholesterol metabolism or facilitates its uptake into the ER.

Methodology
The study employed a multi-faceted approach combining in vivo mouse models, in vitro cell biology, structural biology, and pharmacological interventions:

Genetic Models: The authors utilized Sting1 knockout (KO) mice, Cgas KO mice (to distinguish STING-specific effects from cGAS), and intestinal epithelial-specific Sting1 conditional KO (Sting1 cKO) mice.
Metabolomics and Biochemistry: Steroid-targeted metabolomics, quantitative biochemical assays for cholesterol and corticosterone, and qPCR/Western blotting were used to assess systemic and tissue-specific cholesterol homeostasis and SREBP2 pathway activation.
Cellular Imaging and Fractionation: Subcellular fractionation (PM vs. ER), immunofluorescence using cholesterol sensors (ALOD4, GRAM-187L), and live-cell confocal imaging with Bodipy-cholesterol were used to track cholesterol distribution and transport kinetics.
Molecular Interaction Studies: Co-immunoprecipitation (Co-IP), Surface Plasmon Resonance (SPR), cholesterol pull-down assays, and 22-NBD-cholesterol binding assays were used to characterize the physical interaction between STING, Aster proteins, and cholesterol.
Structural Modeling and Mutagenesis: Molecular docking (Schrödinger Maestro) predicted cholesterol-binding pockets. Site-directed mutagenesis of these pockets (e.g., L51, L139, Y319) validated their functional role.
Reconstituted Systems: A liposome-based Förster Resonance Energy Transfer (FRET) assay was developed to measure cholesterol transfer from PM-like donor liposomes to ER-like acceptor liposomes in the presence of purified Aster and STING proteins.
Pharmacology: The study utilized Compound 53 (C53), a known STING blocker, to test its effect on cholesterol transport channels.

Key Contributions and Results

STING Deficiency Disrupts Cholesterol Homeostasis:
- In vivo, Sting1 KO mice exhibited significantly reduced serum, liver, and small intestine cholesterol levels compared to Wild Type (WT), despite unchanged body weight or triglyceride levels.
- This reduction triggered a compensatory upregulation of SREBP2-dependent cholesterol biosynthetic genes (e.g., Hmgcr, Hmgcs1, Ldlr) in the liver, intestine, and adrenal glands.
- Crucially, Cgas KO mice showed no such defects, confirming that the phenotype is specific to STING, not cGAS.
- Intestinal epithelial-specific Sting1 KO mice recapitulated the systemic cholesterol reduction, indicating that STING in enterocytes is critical for dietary cholesterol absorption.
STING Regulates Cholesterol Partitioning Between PM and ER:
- While total cellular cholesterol remained unchanged in STING-deficient cells, the distribution was altered: cholesterol accumulated at the plasma membrane (PM) while ER cholesterol levels dropped.
- Live-cell imaging revealed that in WT cells, exogenous cholesterol rapidly moved from the PM to the ER. In Sting1 KO cells, cholesterol remained trapped at the PM.
- Pharmacological inhibition of Aster proteins (AI-3d) blocked cholesterol transport in WT cells but had no effect in STING-deficient cells, placing STING downstream of Aster in the transport pathway.
STING Functions as a Cholesterol Channel:
- Interaction: STING physically interacts with the Aster-B protein in a cholesterol-dependent manner. The cytosolic ASTER domain of Aster-B binds to multiple regions of STING (TMD, LBD, CTT).
- Binding: STING directly binds cholesterol. The presence of the Aster ASTER domain significantly enhances STING's affinity for cholesterol.
- Channel Structure: Molecular docking and mutagenesis identified four tandem hydrophobic pockets (Pocket 1–4) spanning the cytosolic, linker, and transmembrane regions of STING. Pocket 1 (cytosolic) recognizes Aster-delivered cholesterol, while Pockets 2–4 facilitate its translocation into the ER membrane.
- Functional Validation: In the liposome FRET assay, reconstituting STING into ER-like liposomes significantly enhanced Aster-mediated cholesterol transfer. Mutations in the predicted pockets (e.g., L139F/K) abolished this transport.
Pharmacological Blockade:
- Compound 53 (C53), which binds to the lower part of the STING transmembrane domain (overlapping with Pocket 4), competitively inhibits STING-cholesterol binding.
- C53 treatment in WT mice reduced dietary cholesterol absorption and systemic cholesterol levels, mimicking the Sting1 KO phenotype. This effect was absent in Sting1 KO mice, confirming C53 acts specifically through the STING cholesterol channel.

Significance and Claims
The paper claims to identify a "primordial function" of STING in lipid metabolism, distinct from its canonical immune signaling role. The authors propose that STING acts as a newly defined cholesterol transfer channel (CTC) that loads Aster-escorted cholesterol from the plasma membrane into the ER membrane.

This discovery establishes a reciprocal regulatory loop:

Cholesterol $\to$ STING: ER cholesterol retention stabilizes STING in the ER, preventing its activation.
STING $\to$ Cholesterol: STING facilitates ER cholesterol loading; its absence leads to ER cholesterol depletion, triggering SREBP2 activation to restore homeostasis.

The study suggests that the integration of STING into the cholesterol regulatory network likely occurred during early metazoan evolution, coordinating membrane homeostasis in complex multicellular environments. Furthermore, the ability of C53 to block this channel highlights a potential mechanism for modulating dietary cholesterol absorption and systemic lipid levels, linking innate immune signaling proteins directly to metabolic regulation. The authors note that while this insight has broad pathophysiological relevance given the roles of cholesterol in atherosclerosis and neurodegeneration, the precise contributions to these diseases await future investigation.