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Implication of Aster-mediated transport and endocytosis in intestinal absorption and human bioavailability of fat-soluble vitamins

This study demonstrates that Aster-mediated non-vesicular transport and endocytic pathways are critical for the intestinal absorption and human bioavailability of fat-soluble vitamins D and E, while vitamin A utilizes an Aster-independent route.

Original authors: Emmanuelle Reboul, Angélique Berthomé, Donato Vairo, Marine Chotard, Charles Desmarchelier, Mark Zumaraga, Patrick Borel, Ángela Bravo-Núñez, Charlotte Sabran, Charlène Sirvins

Published 2026-06-30

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Emmanuelle Reboul, Angélique Berthomé, Donato Vairo, Marine Chotard, Charles Desmarchelier, Mark Zumaraga, Patrick Borel, Ángela Bravo-Núñez, Charlotte Sabran, Charlène Sirvins

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 ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed.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: How Your Body Absorbs Fat-Soluble Vitamins

Imagine your intestine as a busy customs checkpoint at an airport. When you eat food containing fat-soluble vitamins (Vitamins A, D, and E), these vitamins arrive in a "luggage cart" called a micelle (a tiny bubble of fat and water).

For a long time, scientists knew how these vitamins got into the customs building (the intestinal cell), but they didn't know exactly how they got moved from the front door to the back exit, where they could be loaded onto a truck (chylomicron) to travel through your body.

This paper investigates two specific "moving crews" inside the cell that might be responsible for this transport:

The "Aster" Crew: A team of proteins that act like a non-stop conveyor belt, moving vitamins directly across the cell floor.
The "Endocytosis" Crew: A team that uses tiny "bubbles" or scoops to pick up vitamins and carry them through the cell.

The Experiment: Blocking the Moving Crews

The researchers used two main tools to figure out which crew does what:

Cell Cultures: They grew human intestinal cells in a lab dish (like a mini-intestine).
Mice: They fed mice special diets with vitamins and a drug that blocks the "Aster" crew.
Human Data: They looked at the DNA of healthy men to see if natural genetic differences affected how well their bodies absorbed Vitamin D.

What They Found (The Results)

1. The "Aster" Crew is Vital for Vitamins D and E, but not A

The Analogy: Imagine the Aster crew is a specialized moving truck that only carries heavy furniture (Vitamins D and E).
The Finding: When the researchers blocked the Aster crew (using a drug called AI-3d), the cells stopped sending Vitamin D and Vitamin E out the back door. The vitamins got stuck inside the cell.
The Twist: However, Vitamin A (Retinol) didn't care about the Aster crew. Even when the Aster crew was blocked, Vitamin A still got to the back door. This is because Vitamin A has its own dedicated "mover" (a protein called CRBPII) that doesn't need the Aster crew.

2. The Aster Crew Works with Other Transporters

The Analogy: The Aster crew doesn't work alone; they need a "loading dock" manager.
The Finding: The Aster crew seems to work hand-in-hand with another protein called NPC1L1 (which helps bring cholesterol in). If you block NPC1L1, the Aster crew can't do its job either.
Another Partner: They also found that the Aster crew works alongside SR-BI, another protein that senses fats. If you block SR-BI and the Aster crew, the transport of Vitamins D and E drops even further, suggesting they have overlapping jobs.

3. The "Bubble" Method (Endocytosis) Also Helps

The Analogy: Besides the conveyor belt, there's also a "scooping" method where the cell grabs a vitamin and puts it in a tiny bubble to carry it across.
The Finding: When the researchers blocked this "scooping" method, the secretion of Vitamins D and E dropped significantly. It seems the cell uses both the conveyor belt (Aster) and the scooping bubbles to get these vitamins out.

4. What Happens in Real Life (Mice and Humans)

In Mice: When they gave mice the drug to block the Aster crew, the mice had lower levels of Vitamin E in their blood and liver after eating. This confirmed that the Aster crew is important for getting these vitamins into the bloodstream in a living animal.
In Humans: The researchers looked at the DNA of men who had taken Vitamin D supplements. They found that men with specific genetic variations (SNPs) in the GRAMD1B gene (the gene that builds the Aster crew) had different levels of Vitamin D in their blood.
- Simple translation: Some people's "Aster crew" is built slightly differently due to their genes, which might explain why some people absorb Vitamin D better than others.

Summary of the "Story"

Vitamin A has its own special ticket and doesn't need the Aster crew to get through the cell.
Vitamins D and E rely heavily on the Aster crew to move from the front of the cell to the back.
The Aster crew works with other proteins (NPC1L1 and SR-BI) and also uses "bubble scoops" (endocytosis) to do the job.
If you block the Aster crew, Vitamins D and E get stuck, and less of them reach your bloodstream.
Your genes (specifically the GRAMD1B gene) might determine how efficient your personal Aster crew is, affecting how well you absorb Vitamin D.

What This Paper Does Not Say

It does not say that taking a specific supplement will fix a genetic issue.
It does not recommend using the drug (AI-3d) to treat humans.
It does not claim that blocking these pathways is a cure for any disease.
It simply identifies how the vitamins move inside the cell and notes that genetics play a role in this process.

Technical Summary: Implication of Aster-mediated transport and endocytosis in intestinal absorption and human bioavailability of fat-soluble vitamins

Problem Statement
The intracellular trafficking mechanisms of fat-soluble vitamins (A, D, and E) within enterocytes remain incompletely understood. While their uptake at the brush border membrane involves specific transporters like SR-BI and NPC1L1, the subsequent non-vesicular transport from the plasma membrane to the endoplasmic reticulum (ER) and the role of vesicular endocytosis in this process are unclear. Previous research established that Aster proteins (encoded by GRAMD1A/B/C) facilitate non-vesicular cholesterol transport from the plasma membrane to the ER, and that NPC1L1 enriches the membrane with cholesterol to recruit Aster proteins. However, it was unknown whether Aster proteins and endocytic pathways similarly govern the intracellular routing of vitamins D and E, or if vitamin A utilizes distinct machinery (e.g., CRBPII). Furthermore, the genetic determinants of interindividual variability in the bioavailability of these vitamins, specifically regarding GRAMD1 genes, had not been explored in humans.

Methodology
The study employed a multi-faceted approach combining in vitro cell models, in vivo animal experiments, and a secondary analysis of a human clinical trial:

In Vitro Models (Caco-2 TC7): Differentiated human intestinal cells were used to assess vitamin uptake and basolateral secretion. The study utilized pharmacological inhibitors:
- AI-3d: A specific inhibitor of Aster-mediated cholesterol transfer.
- Ezetimibe: An inhibitor of NPC1L1.
- BLT1: An inhibitor of SR-BI.
- Chlorpromazine: An inhibitor of clathrin-coated pit assembly (endocytosis).
- MβCD: A cholesterol-depleting agent affecting lipid raft/caveolae endocytosis.
- CRISPR-Cas9: SCARB1-knockout (KO) Caco-2 clones were generated to validate SR-BI's role in vitamin E transport.
  Experiments measured vitamin uptake, cellular esterification (for Vitamin A), and basolateral secretion under varying micellar cholesterol concentrations and time points.
In Vivo Models (Mice): C57BL/6 male mice were subjected to postprandial experiments involving oral gavage of emulsions containing $\gamma$ -tocopherol or cholecalciferol. Mice were pre-treated with AI-3d (10 mg/kg) to inhibit Aster function. Plasma, intestinal segments, and liver samples were analyzed for vitamin concentrations and gene expression.
Human Genetic Analysis: A secondary analysis was conducted on data from a randomized crossover clinical trial involving healthy adult males. The study investigated the association between Single Nucleotide Polymorphisms (SNPs) in GRAMD1B and GRAMD1C and the bioavailability of vitamins D and E (measured as chylomicron AUC). Statistical models adjusted for metabolic covariates (age, BMI, fasting lipids, baseline vitamin levels).

Key Results

Aster Proteins and Vitamin E/D Transport: In Caco-2 cells, inhibition of Aster proteins (AI-3d) significantly reduced the basolateral secretion of $\alpha$ -tocopherol (Vitamin E) and cholecalciferol (Vitamin D) by up to 58% and 50%, respectively. Crucially, Aster inhibition did not affect the uptake of these vitamins, nor did it impact retinol (Vitamin A) uptake or esterification. This suggests Aster proteins act downstream of membrane uptake, specifically in intracellular trafficking to the ER.
Cooperation with Membrane Transporters:
- NPC1L1: Combined inhibition of NPC1L1 (ezetimibe) and Aster proteins did not further reduce secretion compared to NPC1L1 inhibition alone, suggesting Aster-mediated transport occurs downstream of NPC1L1 uptake.
- SR-BI: Inhibition of SR-BI (BLT1) alone had a modest effect, but combined inhibition with Aster proteins (AI-3d) resulted in an additional reduction in Vitamin E and D secretion. This indicates that SR-BI and Aster proteins cooperate, and SR-BI may also function independently in this pathway.
Role of Endocytosis: Inhibition of clathrin-mediated endocytosis (chlorpromazine) and lipid raft disruption (MβCD) significantly decreased the secretion of Vitamins D and E, and the esterification of Vitamin A. This supports the hypothesis that vesicular endocytosis contributes to the intracellular routing of these vitamins, potentially alongside or distinct from Aster-mediated non-vesicular transport.
In Vivo Validation: In mice, AI-3d treatment tended to reduce postprandial plasma responses of dietary $\gamma$ -tocopherol and significantly lowered hepatic $\gamma$ -tocopherol accumulation. While dietary cholecalciferol responses were less affected, the data aligns with the in vitro findings of impaired secretion/trafficking.
Human Genetic Associations: In the human cohort, no significant associations were found between GRAMD1 variants and Vitamin E bioavailability. However, five SNPs in GRAMD1B (rs12291891, rs2078158, rs1274214, rs732211, rs1148093) were significantly associated with Vitamin D bioavailability, but only after adjusting for metabolic covariates (age, BMI, fasting lipids, and baseline Vitamin D). These SNPs are intronic and show strong regulatory potential, suggesting they influence gene expression rather than protein structure.

Significance and Claims
The authors claim this work provides the first evidence of the involvement of Aster proteins and endocytic pathways in the intestinal absorption and bioavailability of fat-soluble vitamins D and E.

Mechanistic Insight: The study distinguishes the trafficking mechanisms of fat-soluble vitamins: Vitamin A relies on specific proteins like CRBPII and is Aster-independent, whereas Vitamins D and E utilize Aster-mediated non-vesicular transport from the plasma membrane to the ER.
Pathway Integration: The findings suggest a cooperative model where membrane transporters (NPC1L1, SR-BI) facilitate uptake, followed by Aster-mediated intracellular transfer and potentially endocytic routing for secretion.
Genetic Basis: The identification of GRAMD1B variants associated with Vitamin D bioavailability in humans supports the physiological relevance of Aster-mediated pathways and offers a potential explanation for interindividual variability in vitamin status.

The paper maintains a modest tone regarding its limitations, acknowledging the use of pharmacological inhibitors which may lack absolute specificity, the limitations of the Caco-2 model (low secretion capacity), and the exploratory nature of the genetic associations due to sample size. The authors conclude that further studies in larger cohorts are required to confirm the genetic associations and clarify the underlying mechanisms.