Aryl Hydrocarbon Receptor Expression in Serum, Peripheral Blood Mononuclear Cells, and Skin Lesions of Patients With Atopic Dermatitis and Its Correlation With Disease Severity
Atopic dermatitis (AD)—a chronic, itchy skin condition affecting over 200 million people worldwide—often feels like a puzzle with missing pieces. Why do some people’s skin become inflamed and broken? Why does it flare up seemingly out of nowhere? A 2020 study from researchers at Peking University People’s Hospital in Beijing offers a new clue: a protein called the aryl hydrocarbon receptor (AhR), which acts as a “switch” for genes involved in skin health, detoxification, and immunity.
What Is AhR—And Why Does It Matter for AD?
AhR is a ligand-activated transcription factor—a protein that turns genes on or off when it binds to specific molecules (called ligands). These ligands can be natural (like flavonoids from fruits or tryptophan from food) or environmental (like pollutants such as dioxins). In healthy skin, AhR uses natural ligands to protect the skin barrier and calm inflammation. But when the pathway goes out of balance—say, from exposure to harmful chemicals or immune system changes—it can worsen conditions like AD.
Prior research linked AhR to psoriasis and vitiligo, but its role in AD was unclear. The Peking University team set out to change that by measuring AhR and its “partner” proteins (like CYP1A1, which breaks down toxins, and AhRR, which turns AhR off) in three key places: blood serum, immune cells from blood (peripheral blood mononuclear cells, or PBMCs), and skin lesions from people with AD.
How the Study Worked
The researchers recruited 29 adults with AD (diagnosed using standard criteria from both global and Chinese guidelines) and healthy controls (people without allergies or chronic diseases). They collected:
- Blood samples: To measure AhR levels in serum (via ELISA, a common lab test) and genetic activity of AhR and its partners in PBMCs (via real-time PCR, which tracks gene expression).
- Skin biopsies: From AD lesions and healthy skin (taken from people undergoing mole removal) to see where AhR was located.
All participants were untreated with steroids or immunosuppressants for at least 6 weeks to ensure results weren’t skewed by medication.
Key Findings: AhR Is Overactive in AD
The results painted a clear picture of an overactive AhR pathway in AD:
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Higher AhR in Blood and Immune Cells:
- People with AD had 22% higher AhR in their blood serum (41.26 pmol/L vs. 33.73 pmol/L in controls).
- In PBMCs (immune cells that drive inflammation), AhR levels were 57% higher than in controls. Two of its partners—CYP1A1 (toxin breakdown) and AhRR (AhR’s “off switch”)—were more than double in AD patients.
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AhR Correlates With AD Severity:
- The more AhR was present in PBMCs, the worse the AD was (measured by the EASI score, a standard tool for rating AD severity).
- Higher AhR in PBMCs also linked to higher levels of IL-6, a cytokine (immune protein) that fuels inflammation in AD.
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AhR Runs Amok in Skin Lesions:
- In healthy skin, AhR stays mostly in the deepest layer of the epidermis (the outer skin layer). In AD lesions? AhR was everywhere—especially in the upper, protective layers of the skin. It also showed up in blood vessels and immune cells (like lymphocytes) in the skin, which are key players in AD inflammation.
What Does This Mean for AD?
The study adds critical evidence that AhR isn’t just a “bystander” in AD—it’s an active participant in the disease’s biology. Here’s why that’s a big deal:
- AhR as a Treatment Target: Drugs that target AhR (like coal tar, a long-used AD therapy, or tapinarof, a newer FDA-approved cream) work by calibrating the pathway—using natural ligands to repair the skin barrier and reduce inflammation. This study supports why these treatments might help: they’re fixing an AhR system that’s out of balance.
- Inflammation Link: The correlation between AhR and IL-6 (a pro-inflammatory cytokine) suggests AhR is driving the “fire” in AD skin. Targeting AhR could help put that fire out.
- Skin Barrier Connection: AhR’s shift to the upper layers of AD skin hints it’s involved in the barrier damage that makes AD patients so prone to dryness and infection.
Limitations and Next Steps
The study had a small sample size (29 AD patients), so larger trials are needed to confirm the findings. It also didn’t explore why AhR is overactive in AD—whether it’s due to genetics, environmental exposure, or immune system changes. But as the researchers note, “Our results add new insight into the pathophysiology of AD.”
For people living with AD, this research is a reminder that even “hidden” proteins like AhR can hold the key to better treatments. For scientists, it’s a roadmap to developing therapies that target the root cause—not just the symptoms—of this frustrating condition.
Original study by Yu-Qing Hu, Ping Liu, Zhang-Lei Mu, and Jian-Zhong Zhang, published in Chinese Medical Journal (2020). doi.org/10.1097/CM9.0000000000000591
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