Cancer is characterized by substantial cellular heterogeneity. Although tumor cells may originate from a common malignant clone, they can acquire different molecular and functional characteristics during tumor development.

Within this heterogeneous population, a subset of cells can demonstrate stem-like properties, including self-renewal and the ability to generate diverse tumor-cell populations. These cells are commonly described as cancer stem cells or cancer stem-like cells.

The cancer stem-cell concept proposes that a relatively small population of tumor cells can maintain tumor growth and contribute to disease progression. Importantly, cancer stem cells are not necessarily a fixed population. Their properties can change in response to genetic alterations, epigenetic regulation, environmental signals, and interactions with surrounding cells.

Cancer stem cells have attracted considerable interest because of their potential role in tumor recurrence and metastasis. Conventional treatment may eliminate a large proportion of rapidly dividing tumor cells while leaving behind resistant populations with stem-like characteristics.

Surviving cells can subsequently regenerate the tumor.

Similarly, cancer stem cells may possess properties that allow them to migrate from the primary tumor, survive in circulation, establish metastatic colonies, and adapt to distant tissue environments.

Understanding the molecular mechanisms responsible for these properties may therefore provide opportunities for developing treatments that prevent recurrence and metastatic progression.

Biological Characteristics of Cancer Stem Cells

Cancer stem cells are generally characterized by three major properties: self-renewal, differentiation, and tumor initiation.

Self-renewal allows these cells to maintain a stem-like population through repeated cell divisions.

Differentiation allows cancer stem cells to generate more specialized tumor-cell populations that constitute much of the tumor mass.

Tumor-initiation capacity refers to the ability of certain cells to generate tumors when introduced into appropriate experimental systems.

Cancer stem cells may also exhibit enhanced survival mechanisms.

These include increased DNA repair, resistance to oxidative stress, altered metabolism, increased drug efflux, and reduced susceptibility to apoptosis.

However, the exact characteristics of cancer stem cells vary considerably among tumor types.

Cancer Stem-Cell Markers

Several cell-surface and intracellular molecules have been associated with cancer stem-cell populations.

These markers should not be considered universal identifiers of cancer stem cells. Their expression can vary according to tumor type, disease stage, and microenvironmental conditions.

Molecular Signaling Pathways

Cancer stem-cell properties are regulated by several developmental and oncogenic signaling pathways.

The Wnt/β-catenin pathway plays an important role in maintaining stem-cell characteristics and regulating proliferation.

Notch signaling contributes to cell fate determination and has been associated with cancer stem-cell maintenance in several malignancies.

Hedgehog signaling can regulate self-renewal and differentiation.

The PI3K/AKT pathway promotes cell survival and metabolic adaptation.

JAK/STAT signaling can contribute to stemness and inflammatory responses.

Transforming growth factor-β can promote epithelial-to-mesenchymal transition and stem-like phenotypes.

These pathways do not operate independently. Their interactions can create highly adaptable cancer-cell populations.

Wnt/β-Catenin Signaling

Wnt signaling is one of the most extensively studied pathways associated with cancer stemness.

Activation of Wnt signaling can stabilize β-catenin and promote transcription of genes involved in proliferation and stem-cell maintenance.

Abnormal Wnt signaling is particularly important in colorectal cancer.

Persistent activation can contribute to maintenance of tumor-initiating populations and may facilitate tumor recurrence.

Notch and Hedgehog Signaling

Notch signaling regulates cell differentiation and tissue homeostasis.

Abnormal Notch activation has been associated with maintenance of cancer stem-cell populations in several tumors.

Hedgehog signaling similarly regulates developmental processes and tissue renewal.

Aberrant Hedgehog pathway activation can support self-renewal and survival of cancer stem-like cells.

These pathways are therefore being investigated as potential therapeutic targets.

Cancer Stem Cells and Epithelial-to-Mesenchymal Transition

Epithelial-to-mesenchymal transition is a cellular process in which epithelial cells acquire characteristics associated with mesenchymal cells.

During EMT, cells can lose epithelial adhesion properties and gain increased motility and invasive capacity.

Importantly, EMT can also increase stem-like characteristics.

Transcription factors such as SNAIL, SLUG, TWIST, and ZEB1 can regulate EMT-associated gene expression.

The connection between EMT and cancer stemness may help explain why highly invasive tumor cells can also demonstrate increased treatment resistance.

Cancer Stem Cells and Tumor Recurrence

Tumor recurrence occurs when cancer returns after apparently successful treatment.

One potential mechanism involves survival of resistant tumor cells that remain after treatment.

Cancer stem cells may contribute to this process because of their ability to enter slow-cycling or quiescent states.

Many chemotherapeutic drugs preferentially affect rapidly proliferating cells.

A relatively dormant cancer stem-cell population may therefore survive treatment.

Following removal of therapeutic pressure, surviving cells can re-enter the cell cycle and regenerate a tumor.

This model provides a possible explanation for recurrence occurring months or years after initial treatment.

Figure 1. Cancer Stem Cells in Tumor Recurrence and Metastasis

                     PRIMARY TUMOR

                          │

                          ▼

               ┌─────────────────────┐

               │  Cancer Stem Cells  │

               └─────────────────────┘

                    │          │

          ┌─────────┘          └─────────┐

          ▼                              ▼

   Treatment Survival               EMT Activation

          │                              │

          ▼                              ▼

      Dormancy                     Migration / Invasion

          │                              │

          ▼                              ▼

   Minimal Residual Disease       Circulation / Dissemination

          │                              │

          ▼                              ▼

   Tumor Regeneration              Distant Organ

          │                         Colonization

          ▼                              │

       RECURRENCE ◄──────────────────────┘

                          │

                          ▼

                      METASTASIS

Figure 1: Proposed relationship between cancer stem-cell survival, treatment resistance, dormancy, tumor recurrence, epithelial-to-mesenchymal transition, and metastatic dissemination.

Cancer Stem Cells and Therapeutic Resistance

Cancer stem cells can survive conventional treatment through multiple mechanisms.

Increased expression of ATP-binding cassette transporters can reduce intracellular accumulation of chemotherapy.

Enhanced DNA repair can facilitate recovery from treatment-induced DNA damage.

Antioxidant systems can protect cells from oxidative stress.

Anti-apoptotic signaling can reduce treatment-induced cell death.

Quiescence can further reduce sensitivity to treatments that primarily affect rapidly dividing cells.

These characteristics can collectively create a resistant population capable of surviving therapy.

Role of the Tumor Microenvironment

Cancer stem-cell behavior is strongly influenced by the surrounding microenvironment.

Cancer-associated fibroblasts can release growth factors and cytokines that support stemness.

Hypoxic niches can activate hypoxia-inducible factors and promote stem-like characteristics.

Interactions with extracellular matrix components can influence cell adhesion, migration, and signaling.

Immune cells can also modify cancer stem-cell activity through inflammatory and immunosuppressive signals.

Thus, cancer stemness is not exclusively a cell-intrinsic property.

Hypoxic Stem-Cell Niches

Hypoxia is a common characteristic of solid tumors.

Low oxygen levels activate hypoxia-inducible factors, which regulate metabolism, angiogenesis, survival, and cellular adaptation.

Hypoxic environments can promote stem-like phenotypes and increase resistance to treatment.

Hypoxia may therefore create specialized niches in which cancer stem cells can survive.

These niches may protect tumor cells from therapeutic stress and contribute to disease recurrence.

Cancer Stem Cells and Metastasis

Metastasis requires tumor cells to acquire migratory and invasive capabilities.

Cancer stem cells can contribute to this process by activating EMT-associated pathways and increasing cellular plasticity.

After entering the circulation, disseminated tumor cells must survive substantial physiological stress.

Stem-like properties may help these cells withstand unfavorable conditions.

After reaching a distant organ, disseminated cells must adapt to the new tissue environment.

Only a subset may successfully establish metastatic colonies.

Cancer stem-cell properties may facilitate this process through self-renewal and adaptation.

Pre-Metastatic Niches

Primary tumors can influence distant organs before metastatic cells arrive.

Tumor-derived cytokines, growth factors, and extracellular vesicles can modify distant tissues.

These changes can recruit stromal and immune cells and alter extracellular matrix composition.

The resulting environment may become more favorable for metastatic colonization.

Cancer stem cells may exploit these preconditioned environments during metastatic development.

Dormancy and Late Recurrence

Tumor-cell dormancy is an important biological phenomenon.

Disseminated tumor cells can remain inactive for prolonged periods before beginning to proliferate.

Dormant cells may be difficult to detect clinically and may be relatively resistant to conventional therapies.

Cancer stem-cell characteristics may support this dormant state.

Changes in the surrounding microenvironment can eventually provide signals that permit dormant cells to resume proliferation.

This may contribute to late recurrence in certain cancers.

Review Design

The present article was prepared as a narrative review focusing on the biological and molecular role of cancer stem cells in tumor recurrence and metastasis.

Literature Search

Relevant publications were considered from major biomedical databases and peer-reviewed journals.

Search terms included combinations of “cancer stem cells,” “tumor recurrence,” “metastasis,” “self-renewal,” “stemness,” “epithelial-to-mesenchymal transition,” “tumor microenvironment,” “cancer dormancy,” “Wnt,” “Notch,” “Hedgehog,” and “therapeutic resistance.”

Inclusion Criteria

Studies addressing cancer stem-cell biology, tumor initiation, recurrence, metastasis, treatment resistance, stemness-associated signaling, or tumor–microenvironment interactions were considered relevant.

Data Synthesis

The available evidence was organized around cancer stem-cell characteristics, molecular pathways, treatment resistance, tumor recurrence, metastatic dissemination, and therapeutic implications.

Results

The reviewed evidence indicates that cancer stem-like populations can contribute to tumor initiation, progression, recurrence, and metastasis.

These cells demonstrate self-renewal and differentiation capacity and can survive therapeutic stress through multiple mechanisms.

Major signaling pathways associated with cancer stemness include Wnt/β-catenin, Notch, Hedgehog, PI3K/AKT, JAK/STAT, and TGF-β.

EMT can promote both invasive behavior and stem-like characteristics.

Cancer stem cells can also interact with hypoxic niches, fibroblasts, immune cells, and extracellular matrix components.

These interactions may increase resistance to treatment and facilitate metastatic colonization.

The cancer stem-cell model provides an important framework for understanding tumor heterogeneity and treatment failure.

Traditional cancer treatment often focuses on eliminating the bulk tumor population.

Although this can produce substantial tumor reduction, surviving cells may retain the capacity to regenerate the disease.

Cancer stem cells are particularly important in this context because they possess biological characteristics that favor long-term survival.

Their ability to enter quiescent states is one possible mechanism.

A slowly dividing cell may be less vulnerable to therapies that depend on active cell division.

Enhanced DNA repair and antioxidant activity can provide additional protection.

Drug-efflux transporters can reduce intracellular drug exposure, while anti-apoptotic signaling can prevent cell death.

These mechanisms may operate simultaneously.

The relationship between cancer stem cells and EMT is also important.

EMT increases migration and invasion while potentially increasing stemness.

This creates a population capable of both surviving treatment and disseminating to distant organs.

However, cancer stemness should not be considered a permanently fixed cellular state.

Increasing evidence suggests that differentiated tumor cells may acquire stem-like properties in response to environmental stress.

Inflammatory signaling, hypoxia, chemotherapy, and changes in the extracellular matrix can all contribute to this plasticity.

This concept has important therapeutic implications because simply eliminating a pre-existing stem-cell population may not be sufficient if other tumor cells can acquire stem-like characteristics.

The tumor microenvironment is therefore an important component of cancer stem-cell biology.

Cancer-associated fibroblasts can provide growth and survival signals, while hypoxic regions can support stem-like phenotypes.

Interactions with immune cells may also influence stemness and treatment response.

Consequently, successful therapeutic approaches may need to target both cancer stem-cell pathways and the microenvironmental signals that maintain them.

Therapeutic Strategies Targeting Cancer Stem Cells

Several therapeutic strategies are being investigated to target cancer stem-cell populations.

Potential targets include Wnt/β-catenin, Notch, Hedgehog, PI3K/AKT, and JAK/STAT signaling.

Surface molecules such as CD44, CD133, and other tumor-associated markers have also been investigated.

Metabolic vulnerabilities provide another possible strategy.

Cancer stem cells may depend on particular metabolic pathways that differ from those used by bulk tumor cells.

Targeting these metabolic adaptations could potentially reduce the survival of resistant populations.

However, selective elimination of cancer stem cells remains challenging because some stem-cell markers are also expressed by normal tissue stem cells.

Future Perspectives

Future studies should focus on understanding cancer stem-cell plasticity rather than treating stemness as a completely fixed characteristic.

Single-cell sequencing can identify distinct cellular states within tumors.

Spatial technologies can reveal the location of stem-like cells and their interactions with surrounding cells.

Longitudinal sampling may help determine how cancer stem-cell populations change during treatment.

Another important research direction is the development of combination therapies that target stem-cell signaling together with conventional chemotherapy, immunotherapy, or targeted therapy.

Preventing the development of resistance may ultimately require simultaneous targeting of tumor-cell plasticity and the microenvironment

Cancer stem cells represent an important component of tumor heterogeneity and may contribute significantly to recurrence, metastasis, and therapeutic resistance.

Their self-renewal capacity allows them to maintain tumor populations, while quiescence, enhanced DNA repair, drug efflux, antioxidant defense, and resistance to apoptosis can facilitate survival during treatment.

The interaction between cancer stem cells and the tumor microenvironment further promotes their persistence.

Wnt/β-catenin, Notch, Hedgehog, PI3K/AKT, JAK/STAT, and TGF-β pathways contribute to the maintenance of stem-like properties.

EMT can connect stemness with invasion and metastatic dissemination.

A major challenge is that cancer stemness can be dynamic, allowing non-stem tumor cells to acquire stem-like properties under environmental pressure.

Future therapeutic strategies should therefore combine direct targeting of stem-cell-associated pathways with approaches that modify the tumor microenvironment and prevent cellular plasticity.

Improved molecular characterization of cancer stem-cell populations may ultimately contribute to more effective strategies for preventing tumor recurrence and metastatic disease.

1. Reya T, Morrison SJ, Clarke MF, Weissman IL. Stem cells, cancer, and cancer stem cells. Nature. 2001;414:105–111.
2. Al-Hajj M, Wicha MS, Benito-Hernandez A, Morrison SJ, Clarke MF. Prospective identification of tumorigenic breast cancer cells. Proceedings of the National Academy of Sciences. 2003;100:3983–3988.
3. Clevers H. The cancer stem cell: premises, promises and challenges. Nature Medicine. 2011;17:313–319.
4. Batlle E, Clevers H. Cancer stem cells revisited. Nature Medicine. 2017;23:1124–1134.
5. Kreso A, Dick JE. Evolution of the cancer stem cell model. Cell Stem Cell. 2014;14:275–291.
6. Magee JA, Piskounova E, Morrison SJ. Cancer stem cells: impact, heterogeneity, and uncertainty. Cancer Cell. 2012;21:283–296.
7. Visvader JE, Lindeman GJ. Cancer stem cells: current status and evolving complexities. Nature Reviews Cancer. 2012;12:764–775.
8. Brabletz T, Jung A, Spaderna S, Hlubek F, Kirchner T. Migrating cancer stem cells—an integrated concept of malignant tumour progression. Nature Reviews Cancer. 2005;5:744–749.
9. Shibue T, Weinberg RA. EMT, CSCs, and drug resistance: the mechanistic link and clinical implications. Nature Reviews Clinical Oncology. 2017;14:611–629.
10. Nusse R, Clevers H. Wnt/β-catenin signaling, disease, and emerging therapeutic modalities. Cell. 2017;169:985–999.
11. Takebe N, Miele L, Harris PJ, et al. Targeting Notch, Hedgehog, and Wnt pathways in cancer stem cells. Nature Reviews Clinical Oncology. 2015;12:445–464.
12. Hanahan D, Weinberg RA. Hallmarks of cancer: the next generation. Cell. 2011;144:646–674.
13. Hanahan D. Hallmarks of cancer: new dimensions. Cancer Discovery. 2022;12:31–46.
14. Quail DF, Joyce JA. Microenvironmental regulation of tumor progression and metastasis. Nature Medicine. 2013;19:1423–1437.
15. Joyce JA, Pollard JW. Microenvironmental regulation of metastasis. Nature Reviews Cancer. 2009;9:239–252.
16. Batlle E, Massagué J. Transforming growth factor-β signaling in immunity and cancer. Immunity. 2019;50:924–940.
17. Valent P, Bonnet D, De Maria R, et al. Cancer stem cell definitions and terminology: the devil is in the detail. Nature Reviews Cancer. 2012;12:767–775.
18. Phi LTH, Sari IN, Yang YG, et al. Cancer stem cells and resistance to cancer therapy. International Journal of Molecular Sciences. 2018;19:1553.
19. Lytle NK, Barber AG, Reya T. Stem cell fate in cancer growth, progression and therapy resistance. Nature Reviews Cancer. 2018;18:669–680.
20. Hanahan D, Coussens LM. Accessories to the crime: functions of cells recruited to the tumor microenvironment. Cancer Cell. 2012;21:309–322.