1 Introduction

A unique subset of cancer cells known as cancer stem cells (CSCs) is distinguished by their capacity to differentiate, self-renew, and promote tumor development, spread, and recurrence [1]. Initially discovered in hematological malignancies, CSCs have since been found in a variety of solid tumors, such as cancers of the breast, brain, colon, and pancreas. CSCs exhibit several defining features, including tumorigenicity, which is the capability to initiate and sustain tumor formation. Their adaptability allows them to survive in diverse microenvironments, an essential trait for tumor growth and metastasis. Moreover, they are marked by the expression of specific stem cell markers such as CD44, CD133, ALDH1, and Sox2, which play critical roles in their stem-like properties [2, 3]. A key aspect of CSC biology is their ability to maintain tumor heterogeneity through asymmetric cell division. This process produces differentiated cancer cells, which comprise the bulk of the tumor, and progeny that retain stem-like characteristics, ensuring a reservoir of cancer stem cells persists within the tumor [1]. CSCs also exhibit increased resistance to standard cancer therapies, such as chemotherapy and radiation. This resistance can be attributed to several factors, including their enhanced DNA repair mechanism. This quiescent state allows them to evade the effects of cytotoxic treatments and overexpression of drug efflux transporters, which pump out therapeutic agents before they can exert their effects [4]. Additionally, they influence the tumor microenvironment, which encourages metastasis and aids in immune evasion, allowing tumors to escape detection and destruction by the immune system [5, 6].

Most cancer drugs possess toxicity profiles that severely degrade their therapeutic benefit and restrict their ability to extend life. Two emerging technologies, drug delivery systems, and chemosensors, designed to increase the efficacy of cancer therapy (CT) drugs, have taken an age to deliver on their clinical potential in most cancers [7]. There were more vital “upstream” targets during the process of carcinogenesis than those responding to CT drugs in differentiated tumor cells, with consideration of the inability to dramatically elevate cancer survival rates [2]. Chemopreventive natural chemicals, which are frequently found in dietary sources, are a simple way to target CSCs’ self-renewal and differentiation programs [8]. Strong epidemiological evidence supports that dietary natural products (NPs) can prevent cells from transforming malignantly. In particular, studies indicate that plant-based diets are strongly associated with lower cancer risk [7]. The key features of CSCs potentially amenable to treatment with NPs are discussed in this review, such as self-renewal programs, differentiation programs, survival pathways, detoxification processes, and other postulated mechanisms involved in cancer recidivism [1]. To find potential significant targets for treatment, the CSC survival and growth pathways, which involve Wnt/β-catenin, Hedgehog, Notch, and PI3K/AKT/mTOR, are discussed [9]. Further discussed is how NPs modify detoxification mechanisms in CSCs, such as multidrug resistance (MDR). The source of NP activity derived from plants is of concern, with special attention given to evolutionary pressure’s role in creating pharmacologically active compounds. We conclude by presenting an overview of several NP compounds, organized by chemical class, that have shown direct or indirect activity against CSC-related pathways [3].

2 Mechanisms underlying Cancer stem cells

2.1 Key signaling pathways in CSCs

The continuous conjecture on the origins of CSCs complicates our knowledge of how these cells develop and sustain their signaling pathway. According to one theory, normal somatic stem cells’ self-renewal programs become dysregulated due to genetic instability, giving birth to their carcinogenic features [8]. On the other hand, CSCs may be derived from differentiated tumor cells that undergo phenotypic plasticity. This process gives them stem-like characteristics through the acquisition of several oncogenic mutations. In healthy stem cells, self-renewal is carefully regulated by transcription factor-mediated pathways that respond to external growth factor signals as part of the signal transduction process [10]. Conversely, dysregulation of transcription factor expression or activity in CSCs can lead to an abnormal self-renewal response, contributing to tumor progression as these neoplastic cells differentiate into proliferative tumor cells [10]. The major mechanistic pathways utilized by CSCs for pro-survival signaling and self-renewal include the WNT/β-catenin, Hedgehog, Notch, and PI3K/AKT/mTOR pathways, which are discussed in this context [11].

2.1.1 WNT/β-catenin pathway

The WNT/β-catenin signaling is essential for regulating the migration, apoptosis, and proliferation of differentiated cancer cells [12]. It also helps cancer stem cells of different cancer types continue to self-renew. When WNT attaches to the Frizzled receptor, it triggers this signaling cascade, which causes β-catenin to build up in the cytoplasm [13].

Normal cell adhesion is maintained by the epithelial cell adhesion protein E-cadherin, which holds β-catenin at the cell membrane. Adenomatous polyposis coli, casein kinase 1α, axin, and glycogen synthase kinase 3β (GSK3β) are among the proteins that β-catenin forms a multi-protein complex with when WNT signaling is not activated [8]. By regulating the stability and degradation of β-catenin, GSK3β plays a crucial role in regulating the WNT/β-catenin pathway. By encouraging its breakdown by the ubiquitin-proteasome system, GSK3β-dependent phosphorylation of β-catenin at the Ser33, Ser37, and Thr41 residues stops its nuclear translocation. Transcriptional co-activators like p300 and CREB binding protein (CBP) interact with activated β-catenin to promote the expression of downstream WNT target genes [14].

T-cell factor/lymphoid enhancer factor (TCF/LEF) is one of the transcription factor complexes linked to β-catenin’s migration into the nucleus and its activation of target genes. GSK3β suppresses tumors by blocking the classical WNT/β-catenin signaling pathway [15]. Cancer and normal stem cells have been shown to self-renew via the WNT/GSK3β/β-catenin signaling axis. It has been demonstrated that maintaining murine pluripotent stem cells requires inhibiting GSK3β activity [13]. Defective CSC formation is Breakpoint Cluster Region-Abelson (BCR-ABL) chronic myeloid leukemia (CML), which has also been linked to inactivating mutations of GSK3β. These results corroborate previous studies that connected the development of the disease to the nuclear accumulation of β-catenin in BCL-ABL CSCs [16].

WNT /GSK/β-catenin signaling in pre-leukemic and leukemic stem cells differs, according to mouse xenograft studies of mixed lineage myeloid leukemia (MML). Compared to pre-leukemic CSCs, tumorigenic CSCs exhibited higher levels of β-catenin, which resulted in increased self-renewal, higher tumor relapse rates, and worse survival outcomes. Radiation and chemotherapy resistance in xenografts has been linked to overexpression of β-catenin [17]. For example, Yang et al. discovered that hepatocellular carcinoma cells with constitutively active β-catenin developed cisplatin resistance. In contrast, chemoresistant progenitor-like cells were almost eliminated when β-catenin was removed [18].

2.1.2 Notch pathway

A highly conserved element throughout evolution, the Notch signaling system is essential for preserving cell variety and stem cell self-renewal. Several stem and progenitor cells use the four Notch proteins– Notch1 and Notch4– as transmembrane receptors [8]. Notch signaling is triggered when Delta-like and Jagged ligands attach to these receptors. Disintegrin and Metalloproteinase (ADAM) proteases and secretase proteolytic enzymes are involved in the cleavage process initiated by this binding. The Notch intracellular domain, which functions as a transcription factor for genes that support cell proliferation, such as c-Myc, cyclic D1, p21, and NF-κB, is released upon cleavage [19].

The Notch and NF-κB pathways have been shown to interact functionally, especially in hyperproliferative colon cancer. Furthermore, a recent study found that highly regenerative prostate luminal epithelial progenitor cells have increased proliferation through Notch signaling, which inhibits anoikis and promotes metastatic consequences [20].

Using a Drosophila model, Markstein et al. created a methodical way to screen small compounds against populations of cancer stem cells. Like mammalian CSCs, Drosophila intestinal stem cells are multipotent and can differentiate into various cell types. Crucially, evolutionarily conserved pathways like EGFR, HIPPO, AKT, and JAK-STAT are used by both Drosophila stem cells and mammalian CSCs. Because of these similarities, scientists can use Drosophila as a model to screen for cancer treatment molecules [21].

2.1.3 Hedgehog

The Hedgehog family of signaling molecules, which includes members such as Sonic Hedgehog (Shh), Indian Hedgehog (Ihh), and Desert Hedgehog (Dhh), plays a critical role in tissue development by regulating cellular differentiation, proliferation, and patterning [22]. The Hedgehog signaling pathway has emerged as a significant target for cancer therapy, given that dysregulation of this pathway has been implicated in a wide array of cancer types, including basal cell carcinoma, medulloblastoma, and pancreatic cancer [23].

In the context of pancreatic cancers, the transcription factor NF-ĸB serves as a crucial downstream mediator activated by the Shh pathway. Activation of NF-ĸB can facilitate tumor cell survival and proliferation, contributing to the aggressive nature of these tumors. In human leukemia, particularly Chronic Myeloid Leukemia (CML), research has shown that the CD44 + subpopulation of cells exhibits a predominant activation of Hedgehog signaling compared to their CD44- counterparts [24].

In a study conducted by Su et al., the researchers examined the role of Shh in the survival and growth of CML progenitor cells. They found that CML bone marrow stromal cells exhibited low levels of Shh protein, which was linked to the observation that CD44 + progenitor cells were less sensitive to exogenous Shh peptide but demonstrated increased sensitivity to cyclopamine, a known Hedgehog pathway inhibitor [25]. This differential response indicates that activation of Shh signaling can occur autonomously in these progenitor cells, potentially contributing to the pathogenesis and treatment resistance observed in CML. By understanding these mechanisms, researchers may identify more effective therapeutic strategies targeting the Hedgehog pathway in cancer treatment [8].

2.1.4 PI3K/AKT/mTOR

The PI3K/AKT/mTOR (PAM) signaling pathway is a highly conserved network that controls the cellular apoptotic process of cells. This pathway is frequently dysregulated in malignancies, aiding tumor growth and treatment resistance [26]. The tumor suppressor gene phosphatase and tensin homolog (PTEN), which controls PI3K signaling, frequently mutates, making the PI3K/AKT/mTOR pathway crucial in many malignancies. Cellular pro-survival modifications are fuelled by prolonged activation of downstream components, such as the AKT and mTOR kinases, caused by stimulated PI3K signaling [27].

It has been shown that PTEN loss mediates AKT activation in prostate cancer and improves the stemness traits of the CSC population. Furthermore, it has been demonstrated that PI3K/AKT’s interaction with other mitogenic and pro-survival pathways accelerates cancer development [28]. AKT’s inactivation of GSK3β may downregulate the WNT, Hedgehog, and Notch signaling pathways. Additionally, crosstalk has been shown to inhibit GSK3β and, through β-catenin, controlling mammary stem/progenitor cell activity [29]. Furthermore, tyrosine kinase receptors, GSK3β, and Bone Morphogenetic Protein 2 (BMP2) signaling have been shown to interact during the osteoblastic growth of human mesenchymal stem cells [30]. According to specific theories, PI3K signaling and nuclear β-catenin accumulation are required for colon cancer patients to activate canonical WNT signaling fully, and they may also be associated with a higher risk of distant metastasis [31].

2.2 Role of the tumor microenvironment in CSC maintenance

The maintenance, survival, and plasticity of CSCs are greatly influenced by the complex and dynamic ecology known as the tumor microenvironment (TME). Numerous biochemical cues and cellular interactions provided by this milieu play a crucial role in controlling CSC self-renewal, enhancing the potential for metastasis, and contributing to therapeutic resistance. The extracellular matrix (ECM), endothelial cells, immune cells, cancer-associated fibroblasts (CAFs), and different signaling molecules are some of the essential components of TME, and all are essential for maintaining CSCs [5, 32].

One of the most notable characteristics of the TME is hypoxia, a condition of low oxygen availability that is prevalent in many solid tumors. Hypoxia leads to the stabilization of hypoxia-inducible factors (HIFs), particularly HIF-1α, which are transcription factors that activate the expression of genes involved in angiogenesis, metabolism, and cell survival. These factors upregulate critical stemness-related pathways, including the WNT, Notch, and Hedgehog signaling pathways, which are essential for maintaining the undifferentiated and self-renewing phenotypes of CSCs [33]. Moreover, hypoxic conditions further contribute to therapy resistance by promoting adaptive stress responses in these cells [34, 35]. Because they release a variety of cytokines, including interleukin-6 (IL-6) and transforming growth factor-beta (TGF-β), CAFs are essential to the TME. It is well known that these cytokines promote the epithelial-to-mesenchymal transition (EMT), a biological mechanism that increases CSC adaptability and permits metastasis by letting epithelial cells acquire mesenchymal characteristics. Proteins that allow CSCs to infiltrate surrounding tissues and metastasize are essential for tumor growth [36, 37].

Tumor-associated macrophages (TAMs), myeloid-derived suppressor cells (MDSCs), and regulatory T cells (Tregs) are the components of the immunosuppressive milieu that is also present in the TME. Together, these immune cells suppress antigen presentation and increase the expression of immunological checkpoint proteins like PD-L1 to shield CSCs from immune surveillance. This fosters the formation of tumors by allowing CSCs to avoid immune system responses and proliferate [38, 39].

The TME’s ECM, abundant in laminin and fibronectin, offers vital biochemical and mechanical support that promotes CSC movement, adhesion, and survival. The behavior of CSCs can be strongly influenced by the structural characteristics of the extracellular matrix, which can strengthen their stem-like traits and increase their resistance to therapeutic interventions [40]. A feedback loop reinforcing the CSC population produces exosomes, tiny extracellular vehicles from CSCs that act as a channel for intercellular communication and can spread stemness features across tumor cells [41, 42]. The complex relationships between CSCs and TME create a protective niche that actively promotes tumor growth, metastasis, and resistance to traditional treatments and maintains CSC populations. This emphasizes the importance of developing therapeutic approaches that simultaneously target CSCs and the microenvironment around them to fight cancer and enhance treatment results effectively [35].

3 Potential of natural compounds in targeting CSCs

3.1 Challenges in targeting CSCs

3.1.1 Heterogeneity and plasticity

CSCs exhibit significant heterogeneity, not only within the individual tumor but also across various cancer types. This inherent variability poses a substantial challenge to formulating universal targeting strategies, as distinct populations of CSCs may exhibit divergent responses to treatment [43]. Moreover, CSCs are characterized by a remarkable degree of plasticity, enabling them to undergo dynamic transitions between stem-like and differentiated states in reaction to various environmental factors and therapeutic pressures. This capacity for adaptation complicates treatment efforts, as eliminating one specific CSC phenotype may inadvertently lead to the emergence of another equally or more resilient [44]. Consequently, a successful therapeutic strategy must account for this variability and plasticity, recognizing that targeting one CSC form may not suffice to eradicate the entire CSC population within the tumor [45].

3.1.2 Resistance to conventional therapies

In the CSC model, drug resistance occurs when progenitor cells survive drug exposure and subsequently differentiate into various lineages that carry different mutations, leading to a multidrug resistance (MDR) phenotype. This MDR in CSCs can arise from several mechanisms related to metabolism and drug transport, which include drug efflux, alteration of the cellular targets, reduced drug intake and transport, enhanced drug metabolism, and inactivation of drugs by enzymes [46,47,48]. CSCs often overexpress ATP-binding cassette (ABC) transporters, like P-glycoprotein (P-gp), which act as efflux pumps to expel chemotherapeutic drugs. This results in lower intracellular drug concentrations and decreases treatment efficacy, allowing CSCs to survive even in high chemotherapy doses [49, 50]. Additionally, CSCs have advanced DNA repair mechanisms, such as homologous recombination and non-homologous end joining, enabling them to quickly fix DNA damage from radiation or genotoxic agents. This enhanced repair capability improves their survival against standard therapies and fosters drug resistance, contributing to more aggressive tumor phenotypes and complicating cancer management [51,52,53]

3.1.3 Niche protection and immune evasion

CSCs reside in specialized microenvironments, or niches, which include hypoxic areas, perivascular regions, and areas rich in ECM components [54]. These niches protect CSCs, allowing them to evade treatments and immune system attacks. To avoid immune detection, CSCs downregulate surface antigens recognized by T-cells and secrete immunosuppressive factors like transforming growth factor-beta (TGF-β) [55]. This creates an immunosuppressive environment and hampers antitumor immune responses [32]. Additionally, Tregs and MDSCs support CSC maintenance. Tregs suppress immune responses, promoting tolerance, while MDSCs inhibit effector T cells and natural killer cells, making it difficult for the immune system to target tumors. These mechanisms allow CSCs to persist despite therapeutic efforts [56,57,58,59].

3.1.4 Metastatic potential

CSCs are pivotal in metastasis due to their ability to undergo epithelial-to-mesenchymal transition (EMT), enhancing their migratory and invasive properties. A significant challenge in targeting metastatic CSCs is their existence in various dynamic states that may differ from the primary tumor, affecting their behavior and response to therapies. Furthermore, the microenvironment plays a critical role in influencing these CSC states, complicating the development of effective treatments to eliminate these resilient cells and prevent metastasis [6, 60].

3.2 The potential of natural compounds to overcome the challenges in targeting CSCs

Numerous cancer treatment and prevention advancements have been made in the last few decades. Our increased knowledge of CSCs and chemoresistance mechanisms has aided researchers in investigating novel cancer treatment approaches. Numerous natural compounds have demonstrated biological activity against CSCs through their interactions with cell cycles, survival genes, and apoptotic genes [61]. These compounds exhibit multi-targeted actions that disrupt critical signaling pathways, including WNT/β-catenin, Notch, and Hedgehog, essential for the self-renewal and survival of cancer stem cells. For instance, curcumin and epigallocatechin gallate have been shown to inhibit WNT signaling activity, thereby leading to a decrease in CSC proliferation and self-renewal capabilities. Similarly, resveratrol and genistein effectively suppress Notch activation, further reducing the viability of CSCs [62].

In addition to targeting these signaling pathways, natural compounds play a significant role in overcoming drug resistance, which remains a considerable hurdle in CSC-targeted therapies. For example, berberine and capsaicin inhibit ATP-binding cassette (ABC) transporters. This inhibition increases the retention of chemotherapeutic agents within CSCs, enhancing their effectiveness [63]. Moreover, compounds such as sulforaphane, derived from cruciferous vegetables, and piperine induce apoptotic pathways in CSCs, thereby sensitizing these resilient cells to conventional chemotherapy [64].

TME is pivotal in the maintenance and functioning of CSCs, and several compounds have the potential to modulate this microenvironment [36]. Curcumin and resveratrol have been shown to decrease inflammatory cytokine levels, contributing to pro-tumorigenic milieu. Furthermore, these natural compounds can enhance the immune response against cancer by modulating immune checkpoints such as PD-1/PD-L1 and reprogramming TAMs toward an antitumor phenotype, promoting a more robust immune attack on cancer cells [65].

Despite their promising effects, challenges such as poor bioavailability and rapid metabolism hinder the clinical applicability of these compounds. Nevertheless, advances in nanotechnology and structural modifications that enhance their stability and absorption are being explored to improve their therapeutic efficacy. Continued research is essential to validate the role of these natural compounds in CSC-targeted therapy and to optimize their use in combination with existing treatment modalities for more effective cancer management [66].

4 Natural compounds in targeting CSCs

Possible targets for natural compounds to destroy cancer stem cells have been presented in Fig. 1.

Fig. 1
figure 1

Possible targets for natural substances to destroy cancer stem cells. Here, we’ve highlighted several methods: With the specific goal of impeding CSC regeneration and cancer relapse, the treatment targets surface biomarkers, signaling pathways that control CSC self-renewal and differentiation, drug-efflux pumps involved in apoptosis resistance, tumor microenvironmental signals that sustain CSC growth, manipulation of miRNA expression, and induction of CSC apoptosis and differentiation

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4.1 Induction of apoptosis in CSCs

Salinomycin is a monocarboxylic polyether antibiotic derived from Streptomyces albus. It has shown potent inhibitory effects on breast cancer stem cells (BCSCs) through multiple mechanisms. This compound selectively targets BCSCs, inducing apoptosis, inhibiting their proliferation, and reducing tumor invasiveness [67]. Research indicates that salinomycin can reduce the proportion of BCSCs by more than 100-fold compared to paclitaxel in mouse models. The mechanism by which salinomycin kills CSCs involves lysosomal iron sequestration, producing reactive oxygen species, lysosomal membrane permeabilization, and, ultimately, ferroptosis [68]. Studies conducted in 2011 demonstrated that salinomycin could induce apoptosis in human cancer cells at higher concentrations [69].

Piperine, an alkaloid in black pepper, inhibits glioblastoma cancer cells (GCSs) by targeting survivin, a protein linked to apoptosis inhibition and therapy resistance. This study shows that survivin is overexpressed in GSCs compared to normal brain cells. Treatment with piperine reduces survivin levels, leading to decreased stemness, proliferation, and invasion of GSCs. Additionally, piperine enhances the effectiveness of temozolomide, the standard glioblastoma chemotherapy, by increasing GSC sensitivity to the drug. These findings suggest piperine may be a promising adjuvant therapy for glioblastoma [70].

Quercetin, a natural flavonoid, exhibits anticancer effects by targeting breast cancer stem cells, specifically the CD44+/CD24- subpopulation, which is crucial for tumor initiation, progression, and drug resistance. The study demonstrates that quercetin alone and combined with doxorubicin significantly inhibits CSC proliferation, induces apoptosis, and causes cell cycle arrest at the G2/M phase. CSCs are known for their slow cell cycle progression and resistance to conventional chemotherapy, making them difficult to eradicate [71]. However, quercetin enhances doxorubicin’s cytotoxic effects by promoting apoptosis through modulating pro-apoptotic and anti-apoptotic gene expression and inhibiting drug efflux mechanisms [72].

Gingerols are primarily found in ginger and vary in chemical structure based on the length of their unbranched alkyl chains. Specifically, 6-gingerol has been reported to have potential cancer chemopreventive effects by influencing various stages of the metastatic process [73]. 6-Gingerol induces apoptosis in CSCs through multiple interconnected mechanisms. It downregulates metastasis-related proteins like MMP2 and MMP9, inhibiting cell adhesion, migration, and invasion. Disrupting cell cycle progression promotes growth arrest, often through upregulating inhibitors such as p21 and p27. Apoptosis is triggered via intrinsic and extrinsic pathways involving mitochondrial membrane disruption, cytochrome c release, caspase activation, and modulation of Bcl-2 family proteins [74]. Furthermore, studies have shown that treatment with 6-gingerol can induce growth arrest and apoptosis in human colorectal cancer stem cells. This effect appears to result from multiple mechanisms, including protein degradation and the involvement of several pathways, such as β-catenin, PKC, and GSK3β [75].

Resveratrol, a natural polyphenol, exhibits anticancer properties by targeting CSCs through various mechanisms. It suppresses fatty synthase (FAS) expression, leading to reduced lipid synthesis and induction of apoptosis in CSCs. This effect was observed in both in vitro and animal models, where resveratrol inhibited CSC growth without notable toxicity [76]. Additionally, resveratrol inhibits the WNT/β-catenin signaling pathway, which is crucial for maintaining CSC characteristics, thereby reducing proliferation and inducing autophagy in breast cancer stem-like cells [77].

4.2 Inhibition of CSC signaling pathways

Curcumin, a bioactive compound from turmeric, has shown promise for anticancer properties by influencing various signaling pathways. It inhibits the pro-inflammatory transcription factor NF-κB, which is crucial in reducing cancer-promoting effects in HPV-associated cervical cancer stem cells. Oesophageal squamous carcinoma poses significant challenges due to CSCs that drive treatment resistance [65]. Research has demonstrated that curcumin inhibits CSC by targeting the STAT-NF-κB signaling pathway, which is crucial for CSC survival and self-renewal. It effectively reduces STAT3 phosphorylation, thereby preventing its activation and nuclear translocation. Although STAT3 still interacts with NF-κB after curcumin treatment, the interaction is weakened, suggesting a disruption in their oncogenic signaling [78]. Additionally, curcumin decreases the CD44 + CSC population, a key marker of tumor-initiating cells, thereby reducing their ability to form spheres and migrate. This inhibition of tumorigenicity and invasiveness highlights curcumin’s potential as an anti-CSC agent in breast cancer therapy [79]. Furthermore, curcumin may potentially inhibit the Wnt/β-catenin and Sonic Hedgehog pathways associated with breast CSC self-renewal [80].

Sulforaphane, a compound found in cruciferous vegetables, downregulates Notch-1 expression in pancreatic cancer stem cells. Notch-1 is crucial for maintaining CSCs, and its down-regulation leads to reduced c-Rel expression, a key subunit of the NF-κB transcription factor. This reduction in Notch-1 disrupts the NF-κB signaling pathway, which is often upregulated in cancer and promotes cell survival and proliferation [81]. Thus, sulforaphane may inhibit NF-κB signaling in pancreatic CSCs, suggesting its potential as a therapeutic agent against pancreatic cancer by targeting pathways crucial for cancer cell growth and resistance to treatment [82].

Ginsenoside Rb1 is a natural saponin that is derived from the rhizome of Panax quinquefolius, and its metabolites can target Wnt/β-catenin signaling, which can inhibit the growth of cancer stem cells and reverse therapy resistance in ovarian cancer cells, both in vivo and in vitro [7]. These metabolites suppress ABCG2 and P-gp expression by disrupting the interaction between β-catenin and T-cell factor (TCF/LEF). The association of β-catenin with TCF/LEF is crucial for regulating its downstream functions, including the transcription of genes that regulate cell growth, division, and survival. Consequently, treating CSCs with ginsenoside Rb1 inhibits Wnt/β-catenin signaling, diminishing cell growth and proliferation [83].

Genistein, an isoflavonoid found in soybeans (Glycine max), inhibits breast cancer stem cells derived from MCF-7 cells by targeting the Hedgehog-Gil signaling pathway [84]. Studies show it reduces Smo and Gil1 expression at both mRNA and protein levels in these CSCs. This suggests natural compounds like genistein can exert anti-CSC effects by down-regulating key signaling molecules. This inhibition may reduce cancer cell growth and proliferation, highlighting the potential for natural compounds as effective anticancer agents. By targeting essential pathways in CSCs, we can develop novel treatments to combat breast cancer and improve outcomes [85, 86].

Epigallocatechin gallate (EGCG), a major catechin in green tea, inhibits the CSC pathway primarily by targeting the STAT3-NFkB signaling axis. In breast cancer cells, EGCG reduces the CSC phenotype by decreasing the expression of CD44, a key marker of tumor-initiating cells. Mechanistically, EGCG inhibits STAT3 phosphorylation, preventing its activation and nuclear translocation. This blocks the transcription of genes associated with self-renewal and survival, thereby reducing the CSC population [78]. Additionally, while the interaction between STAT3 and NFkB is retained, the overall activity of NFkB is suppressed, leading to decreased tumorigenicity and invasiveness [87].

The key mechanisms of the natural compounds targeting various cancer stem cell signaling pathways have been summarized in Table 1. Figure 2 represents the CSC signaling molecules targeted by the natural compounds.

Fig. 2
figure 2

A diagram illustrating the molecular signaling of CSCs and the impact of natural compounds on these molecules. WNT binding to the cell surface Frizzled receptors aid the movement and deactivation of the β-catenin disruption complex (APC/Axin/CK1/GSK3β complex). As a result, β-catenin accumulates and translocates to the nucleus to interact with transcription activators and initiate the transcription of target genes. Notch receptor-ligand binding exposes the cleavage site of the ADAM metalloprotease, allowing it to be cleaved by ADAM metalloprotease TACE and release of Notch extracellular truncation fragment, a γ-secretase substrate. This releases the intracellular domain NICD, which translocates to the nucleus and promotes target gene transcription. NPs inhibit Notch receptor expression and target gene expression. After the Hh binds to the Patched receptor, Smo is activated since Patched suppresses Smo when in its unbound state. Smo subsequently facilitates the Gli protein’s transport to the nucleus, resulting in the transcription of the downstream genes. In the PI3K/AKT/mTOR pathway, cytokines activate JAK, which stimulates PI3K. Phosphorylation of AKT by PI3K activation inhibits GSK3β and activates cell survival and metabolism. AKT also activates NF-κB to translocate to the nucleus and activate transcription of survival and inflammatory genes. The pathway can crosstalk with the Hedgehog signal pathway through Gli and NF-κB to regulate gene expression and cellular response

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Table 1 Natural compounds targeting various cancer stem cell signaling pathways
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4.3 Modulation of the tumor microenvironment

Research indicates that resveratrol impacts the CSCs directly and modifies the tumor microenvironment. By disrupting the interactions that typically support the survival and proliferation of stem-like breast cancer cells, resveratrol alters the dynamics in the tumor, creating a less favorable environment for cancer progression [99]. Moreover, it has been shown that resveratrol significantly impedes breast cancer stem cell proliferation, migration, and invasion capabilities. This compound also affects the expression levels of multiple oncogenic and stemness markers, including c-Myc, cyclin D1, matrix metalloproteinases 2 and 9, CSC marker CD44, and stemness factors SOX2 and Bmi-1 [100].

Pterostilbene (PTE), a natural compound found in blueberries, is suggested to have anticancer effects [101]. A study by Yang et al. found that PTE affects the tumor microenvironment in breast cancer by inducing pyroptosis through the BAX-caspase-3-GSDME pathway, selectively targeting CSC, targeting inflammatory responses that enhance the immune-mediated clearance of these resilient cells. By stimulating the secretion of IL-1β and IL-18, PTE recruits immune cells that can target and eliminate CSCs, which are known for their resistance to conventional therapies. Additionally, PTE boosts antitumor immune cells like CD44 + and CD8 + T cells, NK cells, and B cells, which can directly attack CSCs while reducing tumor-promoting cells such as Tregs, MDSCs, and M2-TAMs. These molecules facilitate CSC-driven tumor aggression [102]. By shifting macrophage polarization from M2 (pro-tumor) to M1 (antitumor) and promoting Th1-type immune responses, PTE further weakens the protective niche that CSCs rely on [103].

Wogonin, a flavonoid from the ancient Chinese medicinal plant Scutellaria baicalensis, and Scutellaria ocmulgee leaf extract have been reported to inhibit the secretion of TGFβ1 effectively [104, 105]. The cytokine is reported to play an essential role in enhancing Tregs activity, especially in the case of malignant gliomas. By disrupting TGFb1 signaling, leaf extract, and Wogonin decrease the activity of Tregs induced by this cytokine, thus opposing the immunosuppressive milieu generally established by tumors. This interference with Treg activity leads to the reversal of immunosuppression mediated by tumors, possibly improving the immune system’s capacity to target and destroy CSCs in malignant gliomas [106].

Baicalin and Baicalein are flavonoids derived from Scutellaria baicalensis that target CSC by modulating the TME to reduce CSC survival, immune invasion, and metastasis [107, 108]. They promote antitumor immune response by shifting TAMs toward an M1-like phenotype while suppressing M2-TAMs, enhancing T-cell responses, and regulating the Treg/Th17 balance, thereby counteracting CSC-mediated immunosuppression [109]. Additionally, baicalin reduces vascular endothelial growth factor (VEGF) expression, thereby blocking the PI3K/AKT/mTOR pathway disrupting the vascular niche that supports CSC proliferation and self-renewal [108]. By downregulating matrix metalloproteinases (MMPs), baicalin stabilizes the ECM, preventing CSC invasion and metastasis [110]. Furthermore, they disrupt tumor-associated platelet aggregation, limiting CSC evasion from immune surveillance. These multifaceted effects make Baicalin and Baicalein promising candidates for cancer therapy by reshaping the TME into a more antitumor state [111].

4.4 Targeting angiogenesis and metastasis

Angiogenesis and metastasis are crucial for tumorigenesis. Natural compounds modulate several molecular proteins involved in these processes. The detailed mechanisms of tumor angiogenesis and metastasis have been illustrated in Fig. 3.

Both in vivo and in vitro, curcumin exerted robust anti-metastatic activity against prostate DU145 cells. In a xenograft tumor model, it reduced DU145’s ability to metastasize. Tumor volume and levels of MMP-2 and MMP-9 fell sharply after curcumin administration. Curcumin treatment produced a striking reduction of lung metastases in a human breast cancer xenograft model. It suppressed the expression of intercellular adhesion molecule-1, vascular endothelial growth factor (VEGF), MMP-9, COX-2, and nuclear factor-kappa B (NF-κB) [112].

A flavone molecule named acacetin (5,7-dihydroxy-4’-methoxyflavone) commonly occurs in plant pigments, invariably occurs in vascular plants, and provides many of the colors of nature with their hues. It is anti-inflammatory, anti-peroxidative, and anticancer. Researchers examined the influence of acacetin on human prostate cancer DU-145 cells’ anti-metastasis. The cell adhesion ability of DU145 cells treated with acacetin (10 µM for 24 h) significantly decreased in a dose-dependent manner in the cell-matrix adhesion assay. Additionally, time course studies with a low dose of 1 µM of acacetin showed that it may significantly and time-dependently inhibit DU145 cell adhesion [113].

Anti-metastatic activity was studied of xanthorrhizol, a sesquiterpenoid molecule isolated from the rhizome of Curcuma xanthorrhiza. Tumor nodule formation was significantly lowered following xanthorrhizol treatment in an experimental mouse lung metastasis model (0.1, 0.2, 0.5, and 1.0 mg/kg, bw/day xanthorrhizol-treated group; 36%, 63%, 61%, and 52% reduction, respectively). The metastasis-related multiplex signal pathway of ERK, COX-2, and MMP-9 was significantly linked with the mode of xanthorrhizol’s anti-metastatic effect. This pathway was also augmented considerably by tumor cell injection into the tail vein. However, xanthorrhizol treatment decreased their expression [114].

Gou et al. investigated that the natural compound Eriocalyxin B (Eri B), an ent-kaurane diterpenoid purified from Isodon eriocalyx var. laxiflora, suppresses cancer metastasis through the inhibition of several mechanisms involved in the development of triple-negative breast cancer (TNBC). Eri B severely inhibited cell migration and adhesion to extracellular matrix proteins in TNBC cells, specifically MDA-MB-231. It repressed the expression of metastasis-associated proteins like EGFR, MEK1/2, ERK1/2, ZEB-1, FAK, MMP-2, MMP-9, vimentin, and slug—all of which are major players in the epithelial-mesenchymal transition (EMT) process and metastatic signaling. Eri B also suppressed the stemness characteristics of cancer stem-like cells by repressing ALDH1A1 expression and disrupting colony and sphere formation. In vivo, it suppressed lung and liver metastases in several breast cancer mouse models and altered the composition of the gut microbiome to potentially contribute to its anti-metastatic activity. Together, these findings validate Eriocalyxin B as an effective anti-metastatic natural compound for TNBC [115].

An ergostane-type triterpenoid, methyl antcinate A [17], was obtained from Antrodia camphorate fruiting bodies. Due to its anti-inflammatory, anticancer, and liver-protective properties, this medicinal fungus has been extensively used as an herbal medicine in Taiwan. Aside from being a potent anti-metastasis compound, methyl antcinate A can also enhance the expression of p53, a tumor suppressor known to hinder breast CSCs’ self-renewal capability. Compound 17 suppressed the self-renewal capability of breast CSC-like cells and repressed the expression of Hsp27 [116].

Fig. 3
figure 3

The mechanism of tumor angiogenesis and metastasis. MMPs, VEGF-A, HIF-1 A, cytokines, chemokines, and growth factors are some of the pro-angiogenic and pro-tumorigenic factors secreted by cancer cells. Endothelial cells (ECs) located in tip cells, which orient the forming vessels, and stalk cells, which contribute to vascular stability, are stimulated by VEGF-A. Besides, cytokines produced by cancer cells activate tumor-associated macrophages (TAMs) and cancer-associated fibroblasts (CAFs), which induce the invasion of cancer stem cells (CSCs). Circulating cancer cells (CTCs) extravasate, start to grow and disseminate after entering the target organ via the circulatory system

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4.5 Reversal of drug resistance

Berberine is a bioactive alkaloid of plant origin, mainly obtained from the rhizomes of Coptis chinensis Franch, one of the essential ingredients among the 50 crucial herbs in Traditional Chinese Medicine [117]. A landmark study identified that berberine significantly reduced the proportion of side-population cells in breast cancer, a subpopulation involved in chemotherapy resistance, by suppressing the expression of the ATP-binding cassette transporter ABCG2 [118]. To discuss berberine’s promise in drug resistance, especially as a modulator of breast cancer stem cells, another study emphasized modified liposomal formulations of berberine [63]. These sophisticated delivery systems had better permeation through cell membranes for efficient delivery into the mitochondria of breast CSCs. This mitochondrial sequestration not only disturbed mitochondrial function but also resulted in a significant downregulation of the drug efflux pumps ABCC1, ABCC2, and ABCC3 proteins that play a role in multidrug resistance in cancer cells [119, 120].

CSCs contribute to drug resistance by overexpressing ATP-binding cassette (ABC) transporters like P-gp, which actively pump out chemotherapeutic drugs, reducing their efficacy and leading to tumor recurrence. Derivatives of Sesquiterpenes from the Celastraceae family, such as celafolin A-1, sesquiterpene ester1, and celorbicol ester, function as MDR reversers by binding to the transmembrane domain of P-gp and regulating its ATPase activity. This interference prevents drug efflux, allowing increased intracellular drug accumulation in drug-resistant CSCs [61]. The study by Munoz-Martinez et al. demonstrated that dihydro-β-agarofuran sesquiterpenes effectively reverse MDR in human MDR1-transfected NIH-3T3 cells, with low IC50 values in resistant cancer models [121]. Parthenolide, a sesquiterpene lactone, was studied against CSC-derived chronic and acute myeloid leukemia by Guzman et al. This indicates the therapeutic potential of sesquiterpenes in fighting MDR in cancer [122].

Oxymatrine, another interesting compound, belongs to the quinolizidine alkaloid class from the root of the Chinese traditional medicinal herb Sophora flavescens Aiton [123]. Its complex pharmacological activities, including anti-apoptotic, anti-fibrotic, anti-inflammatory, and anticancer, have been previously reported [124]. Oxymatrine has demonstrated the potential to inhibit drug resistance in CSCs through multiple mechanisms. One notable approach involves the reversal of EMT, a process linked to chemoresistance. In colon cancer stem cells resistant to 5-fluorouracil, oxymatrine was found to reverse EMT by inactivating the NF-κB signaling pathway, thereby restoring chemosensitivity [125]. Additionally, oxymatrine has been observed to target side population cells, a subset enriched with drug-resistant CSCs. In studies on MCF-7 breast cancer cells, oxymatrine treatment resulted in a dose-dependent reduction in sub-population cells, indicating its efficacy against drug-resistant CSCs [126].

Arglabin is a naturally occurring lactone that functions as an inhibitor of farnesyl transferase. Arglabin is derived from Artemisia glabella, commonly known as a species of wormwood [127, 128]. Research has demonstrated that arglabin exhibits cytotoxic activity against acute myelogenous leukemia (AML) cells, highlighting its therapeutic potential in hematologic malignancies. Notably, it displays effectiveness against doxorubicin-resistant AML cell lines, indicating its ability to circumvent standard drug resistance mechanisms found in CSCs. This characteristic is fundamental, as CSCs often contribute to tumor recurrence and resistance to conventional chemotherapy [129].

The natural compounds that potentially target cancer stem cells have been summarized in Table 2.

Table 2 Natural compounds exert their effects by influencing aspects of CSCs
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4.6 Inhibition of invasion and migration

The inhibition of the invasive growth of malignant cells, for example, the highly invasive SK-Hep-1 human HCC cell line in vitro, by curcumin accounts for its anti-metastatic effect. Curcumin inhibited cancer cell migration and invasion by downregulating MMP-9 secretion in a dose-dependent manner [138]. Curcumin was observed to downregulate human breast cancer cells. In ER-negative MDA-MB-231 cells, the overexpression of MMP-2 (matrix metalloproteinase) and TIMP-1 (tissue inhibitor of metalloproteinase) also inhibited the levels of two angiogenic factors, i.e., VEGF (vascular endothelial growth factor) and b-FGF [139]. Curcumin also reduced the invasiveness seen in the transwell assay. The anti-invasive effect was concentration-dependent, and the lung adenocarcinoma CL1-5 cells’ concentration capacity varied greatly between its cytotoxicity (20 µM). Using microarray. Based on the analysis, including Western blotting and immunohistochemistry, curcumin (1–10 µM) caused downregulation of several invasion-related genes, including MMP14, neural adhesion molecule, and integrins [140].

Hepatocyte growth factor (HGF) plays a vital role in cancer-stromal interaction and invasion of cancer. HGF triggers c-Met autophosphorylation, causing enhanced proliferation, migration, and invasion of hypopharyngeal carcinoma cells. HGF also increases matrix metalloproteinase (MMP-9) and urokinase-type plasminogen activator (uPA) activity and activates Akt and Erk pathways. However, EGCG at 1 µM inhibits HGF-induced tumor motility, MMP-9 and uPA activities, and Akt and Erk pathway activation. These results indicate that EGCG may be a potent therapeutic drug to counteract HGF-induced invasion in patients with hypopharyngeal carcinoma [141].

Weng et al. studied the effect and mechanism of action of resveratrol and its methoxy counterparts on the invasion of highly metastatic human hepatocarcinoma cells. Compared with PMA treatment alone, the invasive activity of HepG2 cells was significantly reduced by 80% and 60%, respectively, after a 24-h treatment with 50 µM resveratrol and 3,5,4’-trimethoxy-trans-stilbene (MR-3). Upon treatment in Hep3B cells, resveratrol and MR-3 exhibited dose-dependent inhibitory effects against invasion and migration. 50 µM resveratrol and 1 µM MR-3, respectively, reduced the invasive activity of Hep3B cells drastically to 27% and 42%. These results suggest that MR-3 and resveratrol could be useful hepatoma cell invasion inhibitors [142].

The anti-invasive activity of the anthocyanins, present in Vitis coignetiae Pulliat fruits, called meoru in Korea, against human hepatoma Hep3B cells was investigated by Shin et al. Based on Matrix invasion experiments, the anthocyanins inhibited cell invasion in a dose-dependent manner at 400 µg/mL. They inhibited the invasion of human hepatoma Hep3B cells by nearly 75% compared to the controls. At this level, anthocyanins also inhibited the activation of NF-κB induced by tumor necrosis factor α and the synthesis of matrix metalloproteinase MMP-2/9 [143].

The anti-invasive activity of gambogic acid (GA) on MDA-MB-231 human breast cancer cells was explored by Qi et al. GA significantly inhibited cell adhesion, migration, and invasion in vitro, based on the outcomes of the heterotypic adhesion assay, wound migration assay, and chamber invasion experiment. GA’s adhesion, migration, and invasion ratios on MDA-MB-231 human breast cancer cells were 43.0%, 39.3%, and 49.7% at a dose of 1.2 µM. Western blotting and immunocytochemistry research findings indicated that GA could suppress MMP-2 and MMP-9 expression in MDA-MB-231 cells [144].

5 Limitations of targeting CSCs as a potential therapeutic avenue

Although an exciting option in cancer treatment, targeting CSCs has some significant limitations that neutralize their therapeutic efficacy and safety. CSCs are naturally resistant to traditional therapies because they have several unique biological features. They have highly effective DNA damage repair machinery that renders them immune to the cytotoxic actions of chemotherapy and radiotherapy. In addition, CSCs can be kept quiescent or slow-cycling and are, therefore, less accessible to drugs specifically targeting proliferating cells, including taxanes and vinca alkaloids. One of the leading causes of their drug resistance is the overexpression of ATP-binding cassette (ABC) transporters ABCB1, ABCG2, and ABCB5, which actively pump chemotherapeutic drugs out of the cells, decreasing intracellular drug levels and inducing multidrug resistance (MDR) [33, 145]. In addition, CSCs have high expression of detoxification enzymes, including aldehyde dehydrogenase 1 (ALDH1), which detoxifies reactive oxygen species (ROS) and chemotherapeutic drugs, hence supporting survival under chemical and oxidative stress [145]. These enzymes also occur in regular stem cells, making selective targeting difficult and increasing the threat of harming healthy regenerative tissue. Phenotypic plasticity is a further key challenge. CSCs can reversibly change from stem-like to non-stem-like states through epithelial-to-mesenchymal transition (EMT) and metabolic reprogramming (e.g., enhanced oxidative phosphorylation or fatty acid oxidation) [146]. Plasticity enables CSCs to escape therapies and restore tumors following initial treatment. The tumor microenvironment, or the CSC niche, also influences therapy resistance. The niche, comprised of stromal cells, immune cells, extracellular matrix, and soluble factors, delivers survival signals protecting the CSCs from therapy. It also establishes hypoxic and acidic conditions that make most drugs less effective and sustain CSC features. Notably, merely targeting CSCs without the breach of niche interactions will also be inadequate because the niche itself can facilitate CSC renewal and drug resistance [147, 148]. The molecular and genetic heterogeneity of CSCs is another challenge. Various tumors and even different areas within the same tumor can contain distinct populations of CSCs with other markers and drug sensitivities. This intra- and inter-tumoral heterogeneity complicates the design of a universal CSC-targeting therapy [33]. Additionally, existing CSC markers (e.g., CD133, CD44, ALDH1) are not specific since they are also present in normal stem cells or non-CSC cancer cells. This overlap poses a risk of off-target toxicity, such as hematopoietic or intestinal stem cells, essential for tissue homeostasis [147]. Additionally, there are limitations in preclinical models for investigating CSCs. Most in vitro and xenograft systems cannot mimic the multifaceted CSC-natural microenvironment interactions. Consequently, several promising CSC-targeting therapies in model experiments also falter in clinical trials because of a failure of these models to predict [147, 148].

6 Conclusion and future perspectives

This article explores the potential of natural compounds in targeting CSCs by disrupting key signaling pathways like WNT/β-catenin, Hedgehog, Notch, and PI3K/AKT/mTOR. These compounds can induce apoptosis and alter the tumor microenvironment, making them a promising alternative to conventional therapies, especially in inhibiting CSC proliferation and boosting chemosensitivity. Notable compounds such as curcumin, resveratrol, and epigallocatechin gallate exhibit anti-CSC activity. However, challenges like poor bioavailability, rapid metabolism, and the heterogeneity of CSC populations limit their effectiveness. Moreover, most research lacks robust clinical validation and often relies on in vitro or animal studies. The supportive tumor microenvironment also complies with treatment, necessitating strategies to optimize drug delivery and explore combination therapies with conventional treatment. Future research must focus on optimizing drug delivery systems, such as nanoparticle-based or liposomal formulations, to enhance natural compounds’ stability, bioavailability, and targeted delivery. Additionally, exploring combination therapies integrating natural compounds, understanding CSC characteristics, identifying specific biomarkers, and targeting the tumor microenvironment to enhance therapeutic outcomes. Comprehensive clinical and preclinical trials are essential to confirm the safety and efficacy of these natural compounds in cancer treatment. Overcoming these challenges may lead to more effective CSC-targeted therapies.