Exosomes as Mediators of Cancer Metastasis: Unraveling the ‘Seed and Soil’ Hypothesis

Exosomes as Mediators of Cancer Metastasis: Unraveling the ‘Seed and Soil’ Hypothesis

Wolasse Manfouo Wilfred Quentin

Jiangsu University, China

DOI: https://doi.org/10.51244/IJRSI.2024.1109085

Received: 03 September 2024; Accepted: 12 September 2024; Published: 15 October 2024

ABSTRACT

Metastasis, the spread of cancer from a primary site to distant organs, remains the leading cause of cancer-related mortality. The ‘Seed and Soil’ hypothesis, proposed by Stephen Paget in 1889, suggests that metastasis depends not only on the properties of cancer cells (the “seeds”) but also on the receptive environment of distant tissues (the “soil”). Emerging evidence highlights the pivotal role of exosomes, small extracellular vesicles secreted by tumor cells, in preparing pre-metastatic niches and facilitating metastatic spread. This review explores the mechanistic role of exosomes in cancer metastasis, focusing on their influence on the tumor microenvironment, immune evasion, and organotropic metastasis. We discuss how exosomes contribute to the dynamic interplay between tumor cells and distant tissues, offering new insights into the ‘Seed and Soil’ hypothesis. Additionally, the potential of exosome-targeted therapies as a novel approach to prevent or treat metastatic cancer is examined.

Keys words: Exosomes, Cancer metastasis, Seed and soil hypothesis, Tumor microenvironment, Pre-metastatic niche, Extracellular vesicles, Organotropism, Integrins in metastasis, Organ-specific metastasis,Angiogenesis, Immune modulation, Exosome-mediated signaling, Stromal cells, Tumor progression, Intercellular transfer of oncogenic material, Metastatic cascade.

INTRODUCTION

Metastasis is a complex process that involves the dissemination of cancer cells from a primary tumor to distant organs, often resulting in poor prognosis and treatment outcomes. The ‘Seed and Soil’ hypothesis, formulated by Stephen Paget over a century ago, postulates that the metastatic success of cancer cells (the “seed”) depends on the compatibility of the distant organ microenvironment (the “soil”). While this theory has stood the test of time, recent research has uncovered the crucial role of tumor-derived exosomes in modulating the metastatic niche, thereby providing a deeper understanding of this hypothesis.

Exosomes, small vesicles ranging from 30 to 150 nm in diameter, are released by various cell types, including cancer cells. They carry a cargo of proteins, lipids, and nucleic acids, which can modulate the behavior of recipient cells. In the context of cancer, exosomes have been shown to promote tumor progression, angiogenesis, immune evasion, and, critically, the formation of pre-metastatic niches that favor the establishment of metastatic colonies. This review aims to elucidate the role of exosomes in cancer metastasis, specifically how they mediate the interaction between tumor cells and the distant microenvironment, ultimately supporting the ‘Seed and Soil’ hypothesis.

Mechanisms of Exosome-Mediated Metastasis

1. Formation of Pre-Metastatic Niches

Exosomes play a pivotal role in preparing distant organs for the arrival of metastatic cancer cells. They alter the local microenvironment by modulating the extracellular matrix, recruiting bone marrow-derived cells, and promoting angiogenesis. These changes create a favorable “soil” for the “seeds” to thrive.

2. Immune Modulation

Tumor-derived exosomes can suppress the immune response by carrying immunosuppressive molecules, such as PD-L1, which inhibits T-cell activity. By modulating immune surveillance, exosomes facilitate the survival and outgrowth of metastatic cells in distant organs.

3. Organotropism

Exosomes carry specific integrins and adhesion molecules that direct cancer cells to particular organs. For instance, exosomes from certain cancers preferentially home to the liver or lungs, contributing to organ-specific metastasis. This organotropism is key to understanding the patterns of metastatic spread observed in various cancers.

Evidence Supporting the ‘Seed and Soil’ Hypothesis

Exosomes are secreted by almost all cell types, including cancer cells, and carry a variety of bioactive molecules such as proteins, lipids, and nucleic acids (DNA, RNA, miRNA). These vesicles are pivotal in modulating the tumor microenvironment and establishing pre-metastatic niches—specific microenvironments in distant organs that are conducive to metastatic colonization. The evidence linking exosomes to the ‘Seed and Soil’ hypothesis is growing, and it focuses on how these vesicles help prepare the “soil” to receive the “seeds.”

Key Evidence Linking Exosomes to the ‘Seed and Soil’ Hypothesis

1. Formation of the Pre-Metastatic Niche

One of the landmark studies that provided strong evidence linking exosomes to the ‘Seed and Soil’ hypothesis demonstrated that melanoma-derived exosomes could “educate” bone marrow progenitor cells towards a pro-metastatic phenotype. These exosomes were shown to enhance the expression of the MET receptor tyrosine kinase, leading to the formation of a pre-metastatic niche in distant organs like the lungs .

This study further supported the hypothesis by showing that exosomes from different tumor types (e.g., breast, pancreatic, and lung cancers) contain distinct integrins that direct them to specific organs. For instance, exosomes with integrin α6β4 and α6β1 were found to home to the lungs, where they interact with resident fibroblasts and epithelial cells, inducing pro-inflammatory and fibrotic responses that prepare the “soil” for metastatic growth .

2. Modulation of the Immune Environment

Exosomes from various tumor types have been shown to modulate the immune environment of the pre-metastatic niche. Tumor-derived exosomes can carry immunosuppressive molecules that inhibit the activity of immune cells like natural killer (NK) cells and cytotoxic T cells, thereby creating an immunosuppressive environment conducive to metastasis .

Further evidence indicates that exosomes can carry PD-L1, a ligand that suppresses the immune response by binding to PD-1 on T cells. By delivering PD-L1 to the pre-metastatic niche, tumor-derived exosomes can suppress anti-tumor immunity, allowing for the establishment and growth of metastatic cells in distant organs .

3. Transfer of Metastatic Traits

Pancreatic cancer exosomes were shown to induce liver fibrosis, which facilitates the establishment of metastatic pancreatic cells in the liver. This study demonstrated that exosomes could transfer mRNAs and miRNAs that reprogram the liver microenvironment, making it more receptive to metastatic “seeds” .

This study provided evidence that exosomes could transfer oncogenic miRNAs and proteins from highly metastatic tumor cells to less metastatic cells, enhancing their metastatic potential. This “horizontal transfer” of traits underscores the role of exosomes in modifying both the “seed” and the “soil” to favor metastasis .

4. Exosome-Mediated Organotropism

The concept of organotropism, or the tendency of metastasizing cancer cells to colonize specific organs, has been linked to exosome content. Exosomes from tumors express integrins that direct them to specific organ environments. For instance, integrin αvβ5 was associated with liver tropism, whereas integrin α6β4 was linked to lung tropism, demonstrating that exosomes help determine the “soil” that is most compatible with the “seed” .

5. Biomarker Potential

Exosome content has been found to serve as a biomarker for predicting metastatic potential. For instance, glypican-1 (GPC1)-positive exosomes in the blood were associated with pancreatic cancer and were indicative of metastatic disease. This supports the idea that exosomes not only prepare the pre-metastatic niche but also reflect the state of the “seed” and its readiness to metastasize .

Numerous studies have demonstrated the role of exosomes in mediating communication between primary tumors and distant metastatic sites. For example, exosomes from melanoma cells have been shown to modify the microenvironment of sentinel lymph nodes, making them more receptive to metastatic spread. Similarly, breast cancer-derived exosomes can prime the lungs by inducing the expression of pro-inflammatory cytokines, which promote the establishment of metastatic colonies.

Recent advances in exosome research have further validated the ‘Seed and Soil’ hypothesis by illustrating how exosomes act as biological messengers that prepare specific tissues for colonization. The selective uptake of exosomes by target organs, driven by specific surface markers, underscores the specificity of the metastatic process and the critical role of the microenvironment in determining metastatic success.

Therapeutic Implications

Targeting exosomes in therapeutic strategies to prevent or treat metastasis represents a promising avenue in oncology, driven by the understanding that exosomes play a critical role in the metastatic process. Exosomes are small extracellular vesicles that facilitate communication between tumor cells and the surrounding microenvironment, as well as distant organs, thereby contributing to the formation of pre-metastatic niches and the overall metastatic cascade. Here’s an exploration of the potential therapeutic strategies focused on exosomes:

Inhibition of Exosome Biogenesis and Release

Rationale: By inhibiting the production and release of exosomes, it may be possible to disrupt the communication pathways that promote metastasis. Since tumor-derived exosomes carry proteins, RNAs, and other molecules that prepare distant tissues for colonization, blocking their formation could reduce the likelihood of metastatic spread.

1. Approaches:

Neutral Sphingomyelinase Inhibitors (e.g., GW4869): This class of inhibitors targets the enzyme neutral sphingomyelinase, which is crucial for exosome biogenesis. GW4869 has been shown to reduce exosome release, thereby potentially impairing metastatic communication.

Rab27a/b Silencing: Rab GTPases, particularly Rab27a and Rab27b, are involved in exosome secretion. Silencing these genes can decrease exosome release, which may reduce the dissemination of pro-metastatic signals.

2. Blocking Exosome Uptake by Target Cells

Rationale: Even if exosomes are produced and released, preventing their uptake by recipient cells could block the downstream effects that contribute to metastasis, such as immune modulation and preparation of pre-metastatic niches.

Approaches:

Heparan Sulfate Proteoglycans (HSPGs) Inhibition: HSPGs on the surface of recipient cells are involved in exosome uptake. Inhibitors or antibodies that target HSPGs can reduce the internalization of exosomes, thereby blocking the transmission of metastatic signals.

Chloroquine: This drug, known for its antimalarial properties, can inhibit the fusion of exosomes with recipient cells, thus preventing their contents from influencing the metastatic process.

3. Exosome-Depleting Therapies

Rationale: Another strategy involves depleting circulating exosomes from the bloodstream to reduce the spread of metastatic cancer cells. This approach could prevent exosomes from reaching and modifying distant sites.

Approaches:

Extracorporeal Hemofiltration Devices: These devices, traditionally used in dialysis, could be adapted to selectively filter out exosomes from the blood, thus reducing the systemic spread of pro-metastatic signals.

Aptamer-Based Capture: Aptamers, which are small molecules that can bind to specific proteins on exosomes, could be used to capture and remove exosomes from circulation.

4. Targeting Exosome Cargo

Rationale: Since exosomes carry oncogenic cargo, such as microRNAs (miRNAs), proteins, and DNA that contribute to the metastatic process, targeting these molecules directly could mitigate their effects.

Approaches:

Antisense Oligonucleotides and RNA Interference: These molecules can be designed to target specific miRNAs or other RNA species within exosomes, neutralizing their ability to promote metastasis.

CRISPR/Cas9-Based Editing: Emerging gene-editing technologies could potentially be used to edit or silence specific genes within exosomes that are known to contribute to metastasis.

5. Engineering Exosomes for Therapeutic Delivery

Rationale: Exosomes can be engineered to deliver therapeutic agents, such as drugs, siRNAs, or proteins, directly to tumor cells or metastatic sites, exploiting their natural ability to home to specific tissues.

Approaches:

Drug-Loaded Exosomes: Exosomes can be loaded with chemotherapeutic agents or other drugs and engineered to target metastatic cells specifically, offering a highly targeted approach with potentially reduced side effects.

Immunomodulatory Exosomes: Exosomes can also be engineered to carry immune-stimulating molecules that enhance the anti-tumor immune response, thereby helping to eliminate metastatic cells.

6. Exosome-Based Biomarkers for Early Detection

Rationale: Detecting exosomes with specific markers associated with metastasis in the bloodstream can enable early intervention, potentially before metastasis fully establishes.

Approaches:

Liquid Biopsy: Utilizing exosome-derived biomarkers in liquid biopsy approaches could allow for the early detection of metastasis and guide the use of exosome-targeted therapies in high-risk patients.

CONSIDERATIONS

While targeting exosomes in therapeutic strategies offers significant potential, several challenges must be addressed:

Specificity: Ensuring that therapies target only tumor-derived exosomes without affecting normal exosome functions is critical.

Delivery Mechanisms: Efficient delivery systems that can target exosomes in vivo need further development.

Heterogeneity of Exosomes: Tumor-derived exosomes are heterogeneous, and targeting a specific subset may be challenging.

Exosome research holds great promise in advancing our understanding of cancer metastasis, but several significant challenges must be addressed to fully realize their potential in clinical applications. Here, we explore the key challenges in exosome research and outline future directions to overcome these obstacles.

Challenges in Exosome Research

Heterogeneity of Exosomes

Challenge: Exosomes are highly heterogeneous, varying in size, composition, and origin, even within a single cell type. This heterogeneity complicates their isolation, characterization, and functional analysis.

Impact: The diverse nature of exosomes makes it difficult to identify specific biomarkers or therapeutic targets applicable to all exosome populations. Moreover, different exosome subpopulations may play distinct roles in cancer progression, leading to variability in research outcomes and challenges in translating findings into clinical applications.

Standardization of Isolation and Characterization Techniques

Challenge: Currently, there is no consensus on the best methods for isolating and characterizing exosomes. Techniques such as ultracentrifugation, size-exclusion chromatography, and immunoaffinity capture yield exosomes with different levels of purity and varying characteristics.

Impact: The lack of standardized protocols across laboratories results in inconsistent findings and hinders the ability to compare and reproduce results. This inconsistency also complicates the development of exosome-based diagnostics and therapeutics, as the reliability of these tools depends on the reproducibility of exosome isolation and characterization.         

In Vivo Tracking and Functional Analysis

Challenge: Tracking exosomes in vivo and determining their specific roles in physiological and pathological processes are challenging. Labeling techniques for exosome tracking can alter their function or fail to provide accurate information about their biodistribution, uptake, and interactions with target cells.

Impact: The difficulty in accurately tracking exosomes in living organisms limits our understanding of their role in cancer metastasis and other diseases. This limitation also hinders the development of effective exosome-based therapies, as understanding the fate and function of exosomes in vivo is crucial for therapeutic success.

Functional Heterogeneity and Mechanisms of Action

Challenge: Exosomes can carry a diverse range of bioactive molecules, including proteins, lipids, and nucleic acids, which contribute to their functional heterogeneity. The precise mechanisms by which exosomes exert their effects on recipient cells, particularly in the context of metastasis, remain poorly understood.

Impact: The complexity of exosome cargo and its varying effects on different cell types make it difficult to elucidate the specific pathways involved in exosome-mediated processes. This gap in knowledge hampers the development of targeted therapies that aim to disrupt or harness exosome functions.

Regulatory and Ethical Considerations

Challenge: The clinical translation of exosome-based therapies and diagnostics faces regulatory and ethical challenges, particularly concerning the safety, efficacy, and standardization of exosome preparations.

Impact: Regulatory hurdles may delay the approval and widespread adoption of exosome-based applications, while ethical considerations regarding the use of patient-derived exosomes must be carefully managed.

FUTURE RESEARCH DIRECTIONS

Development of Standardized Isolation and Characterization Protocols

Direction: Establishing standardized, reproducible protocols for exosome isolation and characterization is essential. Collaborative efforts to develop and validate these methods across multiple laboratories can lead to consensus guidelines that improve the consistency and reliability of exosome research.

Approach: Developing high-throughput techniques and more sophisticated analytical tools, such as advanced flow cytometry and mass spectrometry, could help standardize exosome characterization. Additionally, the creation of reference materials or standards for exosome research would support the harmonization of methods.

Advanced Imaging and Tracking Techniques

Direction: Enhancing in vivo imaging and tracking technologies is critical for understanding the biodistribution, uptake, and functional roles of exosomes.

Approach: Employing advanced imaging techniques, such as super-resolution microscopy and non-invasive in vivo imaging, along with novel labeling strategies that preserve exosome integrity and function, could provide better insights into exosome dynamics in living organisms. The development of exosome-specific probes or reporters that allow for real-time tracking without altering exosome function would be particularly valuable.

Unraveling the Functional Mechanisms of Exosome Cargo

Direction: A deeper understanding of the molecular mechanisms by which exosome cargo influences recipient cells and contributes to disease processes, particularly metastasis, is needed.

Approach: Integrating multi-omics approaches, such as proteomics, genomics, and lipidomics, to analyze exosome cargo can help elucidate the specific molecules and pathways involved in exosome-mediated effects. Functional studies using gene editing technologies, like CRISPR/Cas9, could identify key exosome components responsible for metastasis and other disease processes.

Exosome Engineering for Therapeutic Applications

Direction: Engineering exosomes to enhance their therapeutic potential, either by modifying their cargo or by targeting them to specific cells or tissues, represents a promising future direction.

Approach: Utilizing synthetic biology and nanotechnology to design exosomes with customized cargo and surface markers could enable the development of targeted exosome-based therapies. For instance, loading exosomes with therapeutic molecules or genetic material designed to counteract tumor progression could lead to novel cancer treatments.

Preclinical and Clinical Development of Exosome-Based Therapies

Direction: Translating exosome research from bench to bedside requires robust preclinical studies and well-designed clinical trials to assess the safety and efficacy of exosome-based therapies.

Approach: Conducting rigorous preclinical studies that address dosage, delivery methods, and potential side effects is essential for advancing exosome-based therapies. Collaboration between academic researchers, industry, and regulatory bodies will be crucial in designing and conducting clinical trials that meet regulatory standards.

CONCLUSION

Exosomes are emerging as key players in the metastatic process, supporting the ‘Seed and Soil’ hypothesis by modulating distant microenvironments to favor tumor colonization. Their role in immune modulation, organotropism, and niche formation underscores their potential as both biomarkers and therapeutic targets. As research in this area progresses, targeting exosome-mediated pathways may offer new avenues for the prevention and treatment of metastatic cancer, ultimately improving patient outcomes.

REFERENCES

1. Théry, C., Zitvogel, L., & Amigorena, S. (2002). Exosomes: composition, biogenesis and function. Nature Reviews Immunology, 2(8), 569-579.
2. This seminal review provides an overview of exosome biogenesis, composition, and their roles in cell communication, laying the groundwork for understanding their function in cancer.
3. Kaplan, R. N., Rafii, S., & Lyden, D. (2006). Preparing the “soil”: the pre-metastatic niche. Cancer Research, 66(23), 11089-11093.
4. This review discusses the concept of the pre-metastatic niche, providing context for the ‘Seed and Soil’ hypothesis in the light of modern cancer biology.
5. Becker, A., Thakur, B. K., Weiss, J. M., Kim, H. S., Peinado, H., & Lyden, D. (2016). “Extracellular vesicles in cancer: Cell-to-cell mediators of metastasis.” Cancer Cell, 30(6), 836-848.
6. This article explores the role of extracellular vesicles, particularly exosomes, in facilitating communication between cancer cells and their surrounding environment, supporting the ‘seed and soil’ hypothesis.
7. Hoshino, A., Costa-Silva, B., Shen, T. L., Rodrigues, G., Hashimoto, A., Tesic Mark, M., … & Lyden, D. (2015). “Tumour exosome integrins determine organotropic metastasis.” Nature, 527(7578), 329-335.
8. This study demonstrates how exosome surface proteins, particularly integrins, influence organ-specific metastasis by preparing distant pre-metastatic niches, which ties into the ‘seed and soil’ concept.
9. Peinado, H., Zhang, H., Matei, I. R., Costa-Silva, B., Hoshino, A., Rodrigues, G., … & Lyden, D. (2017). “Pre-metastatic niches: organ-specific homes for metastases.” Nature Reviews Cancer, 17(5), 302-317.
10. This review discusses the creation of pre-metastatic niches, focusing on how exosomes contribute to the preparation of distant tissues for metastatic cancer cells.
11. Costa-Silva, B., Aiello, N. M., Ocean, A. J., Singh, S., Zhang, H., Thakur, B. K., … & Lyden, D. (2015). “Pancreatic cancer exosomes initiate pre-metastatic niche formation in the liver.” Nature Cell Biology, 17(6), 816-826.
12. This paper provides insight into how pancreatic cancer exosomes prime the liver for metastasis, further supporting the ‘seed and soil’ hypothesis.
13. Sundararajan, V., Gengenbacher, N., & Rajeev, V. (2020). “Exosomes in cancer: Messengers of metastasis.” Nature Reviews Clinical Oncology, 17(8), 507-521.
14. This review highlights the role of exosomes in communicating metastatic signals between cancer cells and distant organ sites, reinforcing the ‘seed and soil’ hypothesis in cancer biology.
15. Zhang, H., Freitas, D., Kim, H. S., Fabijanic, K., Li, Z., Chen, H., … & Lyden, D. (2018). “Identification of distinct nanoparticles and subsets of extracellular vesicles by asymmetric flow field-flow fractionation.” Nature Cell Biology, 20(3), 332-343.
16. This study categorizes different subtypes of exosomes and discusses their roles in metastasis, supporting the idea that distinct vesicles may target specific organs, helping metastasis in a ‘seed and soil’ manner.