An FGF7-FGFR2-KLF4 feedback loop sustains anti-inflammatory signaling in epithelial cells

The study identifies a self-sustaining FGF7-FGFR2-KLF4 feedback loop in epithelial cells that suppresses pro-inflammatory gene expression, offering a potential therapeutic strategy for treating inflammatory skin and epithelial diseases.

The Gist

The Big Picture: A "Peacekeeper" for Your Skin

Imagine your skin is a bustling city. Sometimes, due to injury, infection, or irritation, the city goes into a state of emergency. Sirens are blaring (inflammation), the police are swarming, and the buildings are under stress. This is good for a short time to fix a problem, but if the emergency never ends, the city gets destroyed. This is what happens in chronic skin diseases like eczema or psoriasis.

Scientists have long known that a molecule called FGF7 acts like a "bodyguard" for skin cells, helping them survive attacks from radiation or chemicals. But this new study discovered something even cooler: FGF7 is also a master peacekeeper. It doesn't just protect the buildings; it actively tells the police to stand down and stops the emergency alarms from ringing.

The Main Characters

1. FGF7 (The Peacekeeper): A signal molecule that tells skin cells to calm down and stay safe.
2. FGFR2 (The Doorbell): A receptor on the skin cell's surface. FGF7 rings this doorbell to get inside.
3. KLF4 (The City Manager): A protein inside the cell that controls the "switches" for thousands of genes. Think of it as the mayor who decides which laws get passed.
4. IL-6 (The Siren): A protein that screams "FIRE!" to the immune system. Too much IL-6 means too much inflammation.

The Story of the Discovery

1. The "Silence the Siren" Effect

The researchers treated human skin cells with FGF7 and then tried to set them on fire (using inflammatory triggers). They found that FGF7 didn't just help the cells survive the fire; it actually stopped the fire alarms (IL-6) from going off in the first place.

It was like having a fire extinguisher that also had a remote control to turn off the smoke detectors. This happened even when the cells were under heavy attack.

2. How Does the Peacekeeper Work? (The Mechanism)

The scientists wanted to know how FGF7 told the cell to stop screaming. They followed the signal like a detective:

• Step 1: FGF7 rings the FGFR2 doorbell.
• Step 2: This triggers a chain reaction inside the cell (the MAPK pathway), which is like sending a messenger to the City Manager's office.
• Step 3: The messenger finds KLF4 (the City Manager).
• Step 4: Here is the twist: FGF7 doesn't fire KLF4 or lock him in a closet. Instead, it changes KLF4's job description.

Normally, KLF4 might help turn on the "Fire Alarm" (IL-6) genes. But when FGF7 arrives, it shakes KLF4's hand and changes who KLF4 is talking to. Suddenly, KLF4 stops turning on the alarms and starts turning them off. It's like the City Manager suddenly deciding to issue a "Calm Down" decree instead of a "Lockdown" decree.

3. The Secret Loop (The Self-Sustaining Cycle)

The most fascinating part of the study is a "feedback loop" they discovered. It's a bit like a self-reinforcing friendship.

• FGF7 helps KLF4 calm things down.
• But KLF4 also helps build more FGFR2 (the doorbell) and its helpers.
• Why does this matter? If you have more doorbells, the Peacekeeper (FGF7) can ring them more easily, which keeps KLF4 happy, which keeps building more doorbells.

It creates a positive feedback loop that keeps the skin in a "calm and protected" state for a long time. It's like a self-sustaining peace treaty.

Why This Matters for Real Life

The Problem: In diseases like eczema or psoriasis, the skin is stuck in a cycle of inflammation. The "Fire Alarms" are stuck on, and the immune system keeps attacking the skin, damaging the barrier.

The Solution: This study suggests that we could treat these diseases by boosting the FGF7-FGFR2-KLF4 peacekeeping team.

• Currently, there is a drug called Palifermin (a version of FGF7) used to help cancer patients whose mouths get sore from chemo.
• This research suggests we could use this same drug (or similar ones) to treat skin inflammation.

The Catch: The study also found that in inflamed skin, the "doorbells" (FGFR2) sometimes get broken or disappear. If the doorbell is broken, the Peacekeeper can't get in to do his job. So, the best treatment might be a combination: Give the Peacekeeper (FGF7) AND fix the Doorbells (FGFR2) at the same time.

The Takeaway

This paper reveals a hidden superpower of our skin cells. They have an internal "off switch" for inflammation that is controlled by a team of three: FGF7, FGFR2, and KLF4. By understanding how this team works together, scientists hope to develop better ways to stop chronic skin inflammation, not just by patching the damage, but by teaching the skin how to keep the peace on its own.

Technical Summary

1. Problem Statement

Chronic and excessive inflammation are hallmarks of many human diseases, including atopic dermatitis (AD) and psoriasis. While Fibroblast Growth Factor 7 (FGF7) is known for its cytoprotective and barrier-repair functions in epithelial tissues, its direct role as a regulator of inflammatory gene expression was not fully characterized. Previous studies suggested FGF7 might indirectly reduce inflammation by improving epithelial integrity, but a direct mechanism suppressing pro-inflammatory signaling within epithelial cells remained elusive. Furthermore, the specific transcriptional machinery mediating this effect and the existence of regulatory feedback loops sustaining this anti-inflammatory state were unknown.

2. Methodology

The study employed a multi-disciplinary approach combining transcriptomics, proteomics, molecular biology, and in vivo models:

• Cell Models: Human immortalized keratinocytes (HaCaT), primary human keratinocytes (HPKs), and Caco-2 intestinal epithelial cells.
• Stimuli: Cells were treated with FGF7 (and FGF10) alone or in combination with pro-inflammatory stimuli: poly(I:C) (dsRNA mimic), TNFα, 2'3'-cGAMP (STING activator), and 3p-hpRNA (RIG-I agonist).
• Genomic & Transcriptomic Analysis:
  • Bulk RNA-seq: Performed on HaCaT cells under homeostatic and inflammatory conditions to identify differentially expressed genes (DEGs).
  • ISMARA Analysis: Used to predict transcription factor (TF) motif activities based on gene expression changes.
  • ChIP-seq Data Mining: Utilized public databases (GTRD) to map TF binding sites.
• Proteomics:
  • Co-Immunoprecipitation (Co-IP) & Mass Spectrometry: KLF4 was pulled down from FGF7-treated vs. control cells to identify changes in the KLF4 interactome.
• Functional Assays:
  • Luciferase Reporter Assays: Used constructs with the IL6 promoter (long and short variants, including KLF4 binding site deletions) and KLF4 response elements to measure transcriptional activity.
  • Pharmacological Inhibition: Used inhibitors for FGFR (BGJ398, AZD4547, erdafitinib), MEK (U0126), and PI3K (LY294002).
  • Genetic Manipulation: CRISPR/Cas9 knockout of FGFR2 in HPKs and siRNA-mediated knockdown of KLF4 in HaCaT/HPKs.
  • Protein Delivery: Membrane-permeable KLF4-TAT fusion protein to induce KLF4 activity.
• In Vivo Model: A psoriasiform inflammation model in C57BL/6 mice induced by topical Imiquimod (Aldara) cream, with intradermal co-injection of FGF7.
• Histology & Immunofluorescence: Analysis of ear thickness, transepidermal water loss (TEWL), and immune cell infiltration (CD3+, Ly6G+, CD68+).

3. Key Contributions

• Discovery of Direct Anti-Inflammatory Function: Demonstrated that FGF7 acts as a global negative regulator of inflammatory gene expression in epithelial cells, independent of its cytoprotective effects.
• Identification of a Novel Signaling Axis: Defined the FGF7-FGFR2-MAPK-KLF4 axis as the primary mechanism for this suppression.
• Elucidation of a Feedback Loop: Uncovered a reciprocal regulatory circuit where KLF4 not only mediates FGF7's anti-inflammatory effects but also positively regulates the expression of FGFR2 and its downstream effector FRS2A, thereby sustaining the signaling pathway.
• Mechanistic Insight into KLF4: Showed that FGF7 does not alter KLF4 protein levels or nuclear localization but modulates its transcriptional activity via changes in its protein interactome (specifically reducing interactions with co-activators like RUVBL2).

4. Key Results

A. FGF7 Suppresses Pro-Inflammatory Genes

• Transcriptomics: FGF7 treatment suppressed ~1,000 genes, including a cluster of Interferon-Stimulated Genes (ISGs) and pro-inflammatory cytokines (e.g., IL6, IL1R1, TRPV3).
• IL-6 Regulation: FGF7 rapidly and significantly reduced IL6 mRNA and protein levels in both basal and inflammatory states. This suppression was dependent on FGFR2, MEK/ERK signaling, and transcription (blocked by Actinomycin D).
• NF-κB Independence: Unlike other pathways, FGF7 did not inhibit NF-κB phosphorylation (p65) or nuclear translocation, indicating a distinct mechanism of action.

B. KLF4 is the Central Mediator

• Motif Analysis: ISMARA identified KLF4 (and PATZ1) motifs as the most strongly suppressed transcription factors upon FGF7 treatment.
• Interactome Changes: FGF7 treatment significantly altered the KLF4 interactome, reducing the abundance of specific binding partners involved in transcriptional regulation and chromatin remodeling (e.g., RUVBL2, NKAP, TP63).
• Functional Validation:
  • Loss of Function: KLF4 knockdown increased IL6 expression and abolished the suppressive effect of FGF7 on the IL6 promoter.
  • Binding Sites: Deletion of KLF4 binding sites (5'-CACCC-3') in the IL6 promoter abolished FGF7-mediated repression.
  • Mechanism: FGF7 reduces KLF4's transcriptional activity at pro-inflammatory targets without changing KLF4 protein abundance or nuclear/cytoplasmic distribution.

C. The Positive Feedback Loop

• KLF4 Regulates FGFR2: KLF4 knockdown reduced mRNA and protein levels of FGFR2 and its critical signaling adaptor FRS2A.
• Direct Transcriptional Control: KLF4 binds directly to the FRS2A and FGFR2 promoters. Overexpression of KLF4 (via TAT-fusion) increased FGFR2 and FRS2A expression.
• Sustained Signaling: This creates a KLF4-dependent feedback loop: FGF7 activates KLF4 (via MAPK), which then upregulates FGFR2/FRS2A, ensuring sustained sensitivity to FGF7 and continuous anti-inflammatory signaling.

D. In Vivo Relevance

• In the Imiquimod-induced psoriasis model, FGF7 co-treatment reduced epidermal thickness and significantly lowered the expression of Il6, Rsad2, and Ifnl3 in the skin.
• Notably, FGFR2 expression was downregulated in inflamed skin, suggesting that maintaining FGFR2 levels (potentially via KLF4) is crucial for therapeutic efficacy.

5. Significance and Implications

• Therapeutic Potential: The study proposes that activating the FGF7-FGFR2-KLF4 axis is a promising strategy for treating chronic inflammatory skin diseases (e.g., atopic dermatitis, psoriasis) and other epithelial disorders.
• Clinical Translation: Since Palifermin (recombinant FGF7) is already FDA-approved for mucositis, these findings support expanding its use to inflammatory conditions.
• Novel Mechanism: The discovery of a feedback loop where an anti-inflammatory transcription factor (KLF4) sustains the receptor expression (FGFR2) required for its own activation offers a new paradigm for understanding epithelial homeostasis.
• Targeting the Loop: The authors suggest that combining FGF7 therapy with strategies to maintain or restore FGFR2 expression (which is often lost in inflamed tissues) could enhance clinical outcomes.

In summary, this paper redefines FGF7 not just as a tissue repair factor, but as a potent, direct anti-inflammatory agent in epithelial cells, operating through a sophisticated KLF4-mediated transcriptional program and a self-sustaining feedback loop.