PDGFRβ and GSK3β: Fidaxomicin’s Role in Intestinal Fibroblasts

### PDGFRβ and GSK3β: Key Players in Intestinal Fibrosis

#### **PDGFRβ (Platelet-Derived Growth Factor Receptor Beta):**

• **Function:** PDGFRβ is a receptor tyrosine kinase that plays a critical role in cellular signaling, particularly in fibroblast activation, proliferation, and migration. It regulates tissue remodeling, wound healing, and extracellular matrix production.
• **Role in Fibrosis:** In Crohn’s disease (CD), PDGFRβ is strongly overexpressed in fibrotic intestinal regions, driving fibroblast activation and excessive collagen deposition, which contributes to intestinal fibrosis.
• **Mechanism:** Upon binding its ligand, PDGF-BB, PDGFRβ undergoes phosphorylation, triggering downstream signaling pathways that promote fibrogenesis, including activation of the GSK3β pathway.

#### **GSK3β (Glycogen Synthase Kinase 3 Beta):**

• **Function:** GSK3β is a serine/threonine kinase involved in various cellular processes, including inflammation, cell proliferation, and differentiation. It also regulates collagen synthesis and extracellular matrix remodeling.
• **Role in Fibrosis:** In CD-associated fibrosis, GSK3β is activated downstream of PDGFRβ signaling. Phosphorylation of GSK3β contributes to fibroblast activation and collagen production, exacerbating fibrotic progression.
• **Signaling Axis:** PDGFRβ–GSK3β signaling is central to fibrogenesis. Inhibiting this pathway can suppress fibroblast activation and collagen deposition.

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### Fidaxomicin’s Role in Intestinal Fibroblasts

#### **Overview of Fidaxomicin:**

• Fidaxomicin is an FDA-approved antibiotic primarily used to treat Clostridioides difficile infections. It has a gut-restricted bioactivity, meaning it acts locally in the gastrointestinal tract without systemic absorption.

#### **Antifibrotic Mechanism in Intestinal Fibroblasts:**

1. **Targeting PDGFRβ:**

• Fidaxomicin binds strongly to PDGFRβ, as confirmed by molecular docking studies (binding energy: −8.5 kcal/mol).
• It inhibits PDGFRβ phosphorylation, effectively downregulating PDGFRβ signaling.
• This results in reduced activation of intestinal fibroblasts and suppression of collagen gene expression (e.g., COL1A1 and COL1A2), thereby mitigating fibrosis.

2. **Modulation of GSK3β:**

• Fidaxomicin suppresses GSK3β phosphorylation induced by patient-derived exosomes (CDSE), which is dependent on PDGFRβ modulation.
• This action further inhibits fibroblast activation and collagen synthesis, confirming the PDGFRβ–GSK3β signaling axis as a therapeutic target.

3. **Gene Expression Suppression:**

• Fidaxomicin downregulates key fibrosis-related genes, including **COL1A1**, **COL1A2**, and **PDGFRB**, in patient-derived explants.
• Importantly, it does not affect fibroblast migration or epithelial-mesenchymal transition, suggesting its effects are specific to fibrogenesis.

#### **Combined Antifibrotic and Anti-Inflammatory Effects:**

• Fidaxomicin reduces inflammatory markers such as IL-8 and TNF-α in peripheral blood mononuclear cells, indicating dual antifibrotic and anti-inflammatory properties.
• This makes it particularly suited for treating fibrostenotic Crohn’s disease, which involves both fibrosis and inflammation.

#### **In Vivo Efficacy:**

• In the SAMP1/YitFc CD mouse model:
• Oral fidaxomicin significantly lowered fibrosis scores and collagen gene expression.
• It reduced overall disease activity without altering the gut microbiota, showcasing its safety and tolerability.

#### **Mechanistic Specificity:**

• Overexpression of **Pdgfrb** or **Gsk3b** nullified fidaxomicin’s antifibrotic effects, confirming the critical roles of these targets in its mechanism of action.

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### **Therapeutic Potential of Fidaxomicin:**

• Fidaxomicin’s gut-restricted bioactivity, strong antifibrogenic efficacy, and safety profile position it as a promising candidate for targeted therapy in fibrostenotic Crohn’s disease.
• By inhibiting the PDGFRβ–GSK3β signaling axis, fidaxomicin addresses the therapeutic gap in treating intestinal fibrosis, which current anti-inflammatory agents fail to achieve.