Hedgehog/GLI: Hidden Driver of HSC Activation in Fibrosis
The Hedgehog pathway drives hepatic stellate cell activation in MASH and liver fibrosis. We review SHH/GLI mechanisms and SMO inhibitors like Vismodegib.
Hedgehog/GLI Signaling Pathway: A "Hidden Driver" of Fibrosis
A developmental pathway reawakening in adult tissue—following Wnt/β-catenin, this is another "developmental program reactivation" pattern.
The Hedgehog (Hh) pathway is essential for fetal organ morphogenesis and stem cell maintenance, but is nearly silent in healthy adult liver. However, in MASH (Metabolic dysfunction-Associated Steatohepatitis) and chronic liver injury, it is aberrantly reactivated and strongly drives the activation of hepatic stellate cells (HSCs) and fibrosis progression. This article explains these mechanisms and the pathway's potential as a drug target.
1. Molecular Mechanism of Hedgehog Signaling
Ligands: The Three Hedgehog Family Members
Three Hh ligands exist in mammals:
- Sonic Hedgehog (Shh): The most widely studied; plays a major role in liver fibrosis.
- Indian Hedgehog (Ihh): Important in bone and cartilage formation; also expressed in the gastrointestinal tract.
- Desert Hedgehog (Dhh): Expressed in peripheral nerves and testis.
After synthesis, these ligands undergo lipid modifications (palmitoylation and cholesterol modification) before extracellular secretion.
Receptors and Signal Transduction: The PTCH1-SMO Derepression Mechanism
The defining feature of Hh signaling is its unique activation mode of "derepression."
- Signal OFF: In the absence of Hh ligand, the 12-pass transmembrane receptor Patched1 (PTCH1) continuously suppresses the 7-pass transmembrane protein Smoothened (SMO).
- Signal ON: When Shh or other ligands bind to PTCH1, PTCH1's suppression of SMO is released. SMO accumulates on the primary cilium and initiates the downstream signaling cascade.
- GLI Transcription Factor Activation: Activated SMO releases GLI transcription factors from the suppressor SUFU (Suppressor of Fused).
GLI Family: Balance of Activating and Repressive Forms
The GLI transcription factor family has three members with distinct functions:
| GLI Factor | Primary Function | Role in Fibrosis |
|---|---|---|
| GLI1 | Transcriptional activator (activating form only) | Amplification of Hh signaling, HSC activation marker |
| GLI2 | Both activating (GLI2A) and repressive (GLI2R) forms | Primary Hh signaling effector |
| GLI3 | Primarily functions as repressive form (GLI3R) | Negative regulator of Hh signaling |
When Hh signaling is activated, full-length GLI2 (activating form) is stabilized, and proteasomal degradation of GLI3 (conversion to repressive form) is inhibited. As a result, transcription of target genes by GLI1/2 is promoted.
2. Role of Hedgehog in Liver Fibrosis
HSC Activation and the Positive Feedback Loop
The involvement of Hh signaling in HSC activation is characterized by the following positive feedback loop:
- Shh secretion from injured hepatocytes: Hepatocytes undergoing ballooning degeneration secrete Shh.
- HSC response: Quiescent HSCs express PTCH1 and activate the SMO-GLI pathway in response to Shh.
- Differentiation into myofibroblasts: Activation of GLI2 induces fibrosis-related genes including α-SMA, type I collagen, and TIMPs.
- Activated HSCs themselves secrete Shh: Further activating surrounding HSCs in an autocrine/paracrine manner.
MASH and Ballooned Hepatocytes
Ballooned hepatocytes—a pathological hallmark of MASH—are a major source of Shh. Clinical studies have reported positive correlations between ballooning score, Shh expression, and fibrosis stage (J Hepatol). This suggests that Hh signaling is not merely an incidental phenomenon, but actively drives progression from MASH to fibrosis.
Ductular Reaction
Chronic liver injury frequently features abnormal proliferation of hepatic progenitor cells (oval cells) and cholangiocytes, known as ductular reaction. Hh signaling is a crucial regulator of this ductular reaction, promoting progenitor cell proliferation and biliary differentiation in a GLI2-dependent manner. The extent of ductular reaction correlates with fibrosis severity, so Hh signaling contributes to fibrosis through both parenchymal and non-parenchymal cells.
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3. Involvement in Fibrosis of Other Organs
Pulmonary Fibrosis (IPF)
High expression of Shh and GLI1 is confirmed in fibroblast foci of Idiopathic Pulmonary Fibrosis (IPF). Hh signaling promotes proliferation and collagen production in lung fibroblasts, contributing to IPF pathogenesis.
Renal Fibrosis
In renal fibrosis models such as unilateral ureteral obstruction (UUO), Shh secretion from tubular epithelium has been reported to activate GLI1 in interstitial fibroblasts, promoting fibrosis.
4. Crosstalk with Other Pathways
Coordination with Wnt/β-catenin
The Hh pathway coordinates with the Wnt/β-catenin pathway during development, and mutual enhancement is also observed in fibrosis:
- GLI1 promotes transcription of Wnt ligands.
- β-catenin and GLI1 cooperate on common target gene promoters.
- Simultaneous activation of both pathways induces stronger HSC activation and collagen production than activation of either alone.
Mutual Enhancement with TGF-β
The TGF-β/Smad pathway and Hh pathway have bidirectional crosstalk:
- TGF-β1 stimulation elevates Shh expression in HSCs.
- GLI2 directly binds the TGF-β1 promoter, enhancing TGF-β expression.
- This positive feedback establishes a "self-sustaining program" of fibrosis.
5. Therapeutic Strategies
SMO Inhibitors: Repurposing from Oncology
| Compound | Target | Approval Status | Preclinical Fibrosis Data |
|---|---|---|---|
| Vismodegib | SMO | Approved for basal cell carcinoma | Fibrosis reduction in CCl4 liver fibrosis model |
| Sonidegib | SMO | Approved for basal cell carcinoma | Efficacy in cholestatic liver fibrosis models |
| Cyclopamine | SMO | Research use | Antifibrotic effects in multi-organ fibrosis models |
SMO inhibitors have already been approved in oncology, which provides the advantage of an established safety profile. In preclinical MASH models and the CCl4 model, SMO inhibition has shown reduction in activated HSCs, suppression of collagen deposition, and decreased GLI1 expression.
Direct GLI Inhibitors
GLI inhibitors acting downstream of SMO can address SMO-independent GLI activation:
- GANT61: A small molecule that directly inhibits GLI1/GLI2 DNA binding. Suppresses HSC activation in liver fibrosis models.
- Arsenic trioxide (ATO): Promotes GLI2 degradation. A candidate for existing drug repositioning.
Challenges: Effects on Gut Epithelium and Stem Cells
Hh signaling is also critical for gut epithelial homeostasis, and systemic SMO inhibition carries the following adverse event risks:
- Taste disturbance, alopecia, muscle cramps (reported in Vismodegib clinical use)
- Gut epithelial damage: Dysfunction of the stem cell niche
- Skeletal effects: Risk of epiphyseal fusion in growing patients
To overcome these challenges, liver-targeted delivery systems (nanoparticles and HSC-directed liposomes) are under development.
Conclusion
The Hedgehog/GLI pathway is, alongside TGF-β and Wnt/β-catenin, a key driver pathway of fibrosis, playing a central role in HSC activation and fibrosis progression particularly in MASH. SMO inhibitors and GLI inhibitors show promising results in preclinical studies, but overcoming adverse effects from systemic administration is key to clinical application.
Advances in liver-specific targeting strategies and combination inhibition approaches with other pathways (TGF-β, Wnt) will be important themes in future drug discovery research.
Related Articles
- TGF-β/Smad Pathway: The Master Switch of Fibrosis
- Wnt/β-catenin: Why a Developmental Pathway Drives Fibrosis
- MASH Animal Models: Selection and Evaluation Guide
- CCl4 Liver Fibrosis Model
- MASH Treatment Drug Development Trends
- Myofibroblast Origins and Activation
References
- Omenetti A, et al. Hedgehog signaling in the liver. J Hepatol. 2011;54(2):366-373.
- Guy CD, et al. Hedgehog pathway activation parallels histologic severity of injury and contributes to liver fibrosis. Hepatology. 2012;55(6):1711-1721.
- Choi SS, et al. Hedgehog pathway activation and epithelial-to-mesenchymal transitions during myofibroblastic transformation of rat hepatic cells in culture and cirrhosis. Am J Physiol Gastrointest Liver Physiol. 2009;297(6):G1093-G1106.
- Syn WK, et al. Hedgehog-mediated epithelial-to-mesenchymal transition and fibrogenic repair in nonalcoholic fatty liver disease. Gastroenterology. 2009;137(4):1478-1488.
- Philips GM, et al. Hedgehog signaling antagonist promotes regression of both liver fibrosis and hepatocellular carcinoma in a murine model of primary liver cancer. PLoS One. 2011;6(9):e23943.