by Manisha Kashyap, PhD
This article is based on an open-access study published February 27, 2026 in Cell Death Discovery titled: “Exosomal S100A9 Promotes Lung Metastasis of Adenoid Cystic Carcinoma Via Activating Cancer-Associated Fibroblasts.”
Introduction
Adenoid cystic carcinoma (ACC) is a malignant epithelial tumor that arises primarily in glandular tissues, particularly the salivary glands. Although local tumor control can often be achieved through surgery and radiotherapy, distant metastasis—especially to the lungs—remains the primary cause of mortality. Long-term survival drops significantly once metastasis occurs, with reported 10-year survival rates as low as approximately 29% in metastatic cases.
Despite advances in cancer treatment, effective systemic therapies for metastatic ACC remain limited, largely due to an incomplete understanding of the biological mechanisms driving metastasis. This has shifted research focus toward the tumor microenvironment (TME)—a complex network of stromal cells, signaling molecules, and extracellular components that actively contribute to cancer progression.
Among these components, cancer-associated fibroblasts (CAFs) play a central role in shaping tumor behavior. This study investigates how tumor cells communicate with fibroblasts via exosomes and identifies S100A9 as a key mediator of this interaction, ultimately promoting lung metastasis.
The Tumor Microenvironment and Fibroblast Dynamics
The tumor microenvironment is not just a passive structure; it actively supports tumor growth and spread. Within this environment, fibroblasts can be transformed into CAFs, which contribute to:
- Extracellular matrix remodeling
- Angiogenesis
- Secretion of cytokines and growth factors
- Suppression of anti-tumor immune responses
Using single-cell RNA sequencing, the study revealed significant cellular heterogeneity within ACC tumors, identifying multiple cell types including epithelial cells, fibroblasts, immune cells, and endothelial cells.
Importantly, strong communication networks were observed between malignant epithelial cells and fibroblasts, particularly a subset of fibroblasts expressing FAP and α-SMA, which are markers of activated CAFs. These CAFs were shown to interact closely with tumor cells undergoing epithelial–mesenchymal transition (EMT), a process critical for metastasis.
Exosomes: Messengers of Tumor Progression
Exosomes are small extracellular vesicles that can carry proteins, RNA, DNA, and other biomolecules between cells. Unlike free signaling molecules, exosomes provide a protected delivery system, enabling tumor cells to influence both nearby and distant tissues.
In ACC, tumor-derived exosomes were found to contain specific proteins that reflect the aggressiveness of the tumor. Proteomic analysis revealed distinct differences between exosomes from low-metastatic and highly metastatic cells, highlighting their role in disease progression.
Among these proteins, S100A9 emerged as a key factor, consistently elevated in both tumor cells and their derived exosomes.
S100A9: A Central Molecular Driver
S100A9 is a calcium-binding protein known for its role in inflammation and tumor progression. This study demonstrated that:
- S100A9 is significantly upregulated in metastatic ACC cells
- It is highly enriched in tumor-derived exosomes
- Its expression correlates with lung metastasis and advanced disease stage
- Higher levels are associated with shorter disease-free survival
Additionally, circulating exosomes from patients with metastatic ACC showed elevated S100A9 levels, suggesting its potential as a non-invasive biomarker for predicting metastasis.
Mechanism: Reprogramming Fibroblasts into CAFs
A major finding of the study is that exosomal S100A9 can reprogram normal fibroblasts into activated CAFs.
This transformation is characterized by:
- Increased expression of FAP and α-SMA
- Enhanced migration and contractility
- Elevated secretion of inflammatory cytokines (IL-6, IL-8, IL-1β)
- Increased production of matrix-degrading enzymes (MMP2, MMP9)
Experimental evidence showed that:
- Exosomes from highly metastatic cells strongly activated fibroblasts
- Overexpression of S100A9 enhanced this effect
- Knockdown or inhibition of S100A9 reduced fibroblast activation
This confirms that S100A9 is the key functional cargo driving fibroblast reprogramming.
Signaling Pathways: IL-17, TNF-α, and NF-κB
The study identified three major signaling pathways involved in fibroblast activation:
- IL-17 pathway
- TNF-α pathway
- NF-κB pathway
These pathways function in coordination:
- S100A9 stimulates IL-17 and TNF-α production
- These cytokines synergistically activate NF-κB
- NF-κB drives fibroblast activation and inflammatory responses
Blocking NF-κB signaling significantly reduced fibroblast activation, demonstrating its central regulatory role.
Formation of the Premetastatic Niche
Before tumor cells even reach the lungs, S100A9-containing exosomes prepare the environment for metastasis.
- In vivo experiments showed that exosomal S100A9:
- Increased fibroblast activation markers in lung tissue
- Elevated fibronectin, MMP9, and VEGFR1 levels
- Created a microenvironment favorable for tumor colonization
This process is known as premetastatic niche formation, which significantly enhances the likelihood of metastasis.
Tumor Progression Through EMT
Activated fibroblasts further promote cancer progression by inducing epithelial–mesenchymal transition (EMT) in ACC cells.
This includes:
- Downregulation of E-cadherin
- Upregulation of N-cadherin
- Increased tumor cell migration, invasion, and proliferation
These changes make cancer cells more aggressive and capable of spreading to distant organs.
The S100A9–CAF–IL-17 Feedback Loop
The study describes a powerful feedback mechanism:
- Tumor cells release S100A9-rich exosomes
- Fibroblasts become activated CAFs
- CAFs secrete IL-17 and inflammatory factors
- Tumor cells undergo EMT and increased invasion
- Metastasis is enhanced and the cycle continues
This loop represents a self-sustaining system that drives tumor progression.
Simplified Methods Overview
To investigate this mechanism, the researchers used a combination of advanced techniques:
- Gene expression analysis to identify key proteins
- Proteomics to analyze exosomal content
- Single-cell RNA sequencing to map tumor cell interactions
- Cell culture experiments to study fibroblast activation
- Animal models to observe metastasis in vivo
- Pathway analysis to identify signaling mechanisms
These complementary approaches allowed for a comprehensive understanding of how S100A9 influences tumor progression.
Study Limitations
While the findings are significant, the study acknowledges several limitations:
- The clinical sample size was relatively small, which may affect statistical strength
- Larger, multicenter studies are needed to validate findings
- The animal models primarily represent later stages of metastasis rather than the full metastatic process
Further research is needed to fully understand all molecular interactions within the tumor microenvironment
Future Directions
The study highlights several important areas for future research:
- Validation of S100A9 as a clinical biomarker in larger patient cohorts
- Development of therapies targeting the S100A9–IL-17 axis
- Exploration of exosome-based diagnostics and treatments
- Creation of more comprehensive models to study the full metastatic cascade
These directions could significantly improve the management of metastatic ACC.
Therapeutic Implications
Targeting the pathways identified in this study offers promising treatment strategies:
- Inhibiting S100A9 to block fibroblast activation
- Targeting IL-17 signaling to disrupt tumor progression
- Blocking NF-κB pathway to reduce inflammation and metastasis
By disrupting this signaling network, it may be possible to break the cycle driving metastasis.
Conclusion
This study provides critical insight into the mechanisms underlying ACC metastasis. It demonstrates that exosomal S100A9 acts as a central regulator of tumor–stromal interactions, transforming fibroblasts into tumor-promoting CAFs and creating a microenvironment that supports metastasis.
Through activation of inflammatory signaling pathways and induction of EMT, this process establishes a self-reinforcing loop that drives disease progression.
Targeting this pathway represents a promising strategy for improving outcomes in patients with metastatic ACC.
[Cell Death Discovery article]
About the BioQuick Writer
Dr. Manisha Kashyap (Dr. MK)
BDS (Hons), MSc, PhD
Manisha Kashyap earned a BS in Dentistry (Gold Medalist) and an MS in Clinical Research, and she is a driven PhD scholar in Health Policy and Management.
With a rich background as a Drug Safety Analyst for top-tier bio pharmaceutical companies, she blends clinical expertise with a passion for advancing healthcare accessibility and innovation through her writing career for the love of science and technology via MedKnow Research Writing Solutions linkedin.com/in/medknowresearchwritingsolutionsbydrmk
Dr. Kashyap’s PhD thesis is on “Artificial Intelligence (AI) and Public Health: Harnessing the Power of AI for Enhanced Public Health Policy and Management.”