Decoding the Role of CD271 in Melanoma

The roles of cancer stem cells and therapeutic implications in melanoma

Melanoma is a highly malignant skin tumor characterized by high metastasis and poor prognosis. Recent studies have highlighted the pivotal role of melanoma stem cells (MSCs)-a subpopulation of cancer stem cells (CSCs)-in driving tumor growth, metastasis, therapeutic resistance, and recurrence. Similar to CSCs in other cancers, MSCs possess unique characteristics, including specific surface markers, dysregulated signaling pathways, and the ability to thrive within complex tumor microenvironment (TME). This review explored the current landscape of MSC research, discussing the identification of MSC-specific surface markers, the role of key signaling pathways such as Wnt/β-catenin, Notch, and Hedgehog (Hh), and how interactions within the TME, including hypoxia and immune cells, contribute to MSC-mediated drug resistance and metastatic behavior. Furthermore, we also investigated the latest therapeutic strategies targeting MSCs, such as small-molecule inhibitors, immune-based approaches, and novel vaccine developments, with an emphasis on their potential to overcome melanoma progression and improve clinical outcomes. This review aims to provide valuable insights into the complex roles of MSCs in melanoma biology and offers perspectives for future research and therapeutic advances against this challenging disease.

Leveraging Neural Crest-Derived Tumors to Identify NF1 Cancer Stem Cell Signatures

Neurofibromatosis type 1 (NF1) is a genetic disorder that predisposes individuals to develop benign and malignant tumors of the nerve sheath. Understanding the signatures of cancer stem cells (CSCs) for NF1-associated tumors may facilitate the early detection of tumor progression. Background: Neural crest cells, the cell of origin of NF1-associated tumors, can initiate multiple tumor types, including melanoma, neuroblastoma, and schwannoma. CSCs within these tumors have been reported; however, identifying and targeting CSC populations remains a challenge. Results: This study aims to leverage existing studies on neural crest-derived CSCs to explore markers pertinent to NF1 tumorigenesis. By focusing on the molecular and cellular dynamics within these tumors, we summarize CSC signatures in tumor maintenance, progression, and treatment resistance. Conclusion: A review of these signatures in the context of NF1 will provide insights into NF1 tumor biology and pave the way for developing targeted therapies and improving treatment outcomes for NF1 patients.

Unveiling the Dynamic Interplay between Cancer Stem Cells and the Tumor Microenvironment in Melanoma: Implications for Novel Therapeutic Strategies

Cutaneous melanoma still represents a significant health burden worldwide, being responsible for the majority of skin cancer deaths. Key advances in therapeutic strategies have significantly improved patient outcomes; however, most patients experience drug resistance and tumor relapse. Cancer stem cells (CSCs) are a small subpopulation of cells in different tumors, including melanoma, endowed with distinctive capacities of self-renewal and differentiation into bulk tumor cells. Melanoma CSCs are characterized by the expression of specific biomarkers and intracellular pathways; moreover, they play a pivotal role in tumor onset, progression and drug resistance. In recent years, great efforts have been made to dissect the molecular mechanisms underlying the protumor activities of melanoma CSCs to provide the basis for novel CSC-targeted therapies. Herein, we highlight the intricate crosstalk between melanoma CSCs and bystander cells in the tumor microenvironment (TME), including immune cells, endothelial cells and cancer-associated fibroblasts (CAFs), and its role in melanoma progression. Specifically, we discuss the peculiar capacities of melanoma CSCs to escape the host immune surveillance, to recruit immunosuppressive cells and to educate immune cells toward an immunosuppressive and protumor phenotype. We also address currently investigated CSC-targeted strategies that could pave the way for new promising therapeutic approaches for melanoma care.

Therapeutic advances of targeting receptor tyrosine kinases in cancer

Receptor tyrosine kinases (RTKs), a category of transmembrane receptors, have gained significant clinical attention in oncology due to their central role in cancer pathogenesis. Genetic alterations, including mutations, amplifications, and overexpression of certain RTKs, are critical in creating environments conducive to tumor development. Following their discovery, extensive research has revealed how RTK dysregulation contributes to oncogenesis, with many cancer subtypes showing dependency on aberrant RTK signaling for their proliferation, survival and progression. These findings paved the way for targeted therapies that aim to inhibit crucial biological pathways in cancer. As a result, RTKs have emerged as primary targets in anticancer therapeutic development. Over the past two decades, this has led to the synthesis and clinical validation of numerous small molecule tyrosine kinase inhibitors (TKIs), now effectively utilized in treating various cancer types. In this manuscript we aim to provide a comprehensive understanding of the RTKs in the context of cancer. We explored the various alterations and overexpression of specific receptors across different malignancies, with special attention dedicated to the examination of current RTK inhibitors, highlighting their role as potential targeted therapies. By integrating the latest research findings and clinical evidence, we seek to elucidate the pivotal role of RTKs in cancer biology and the therapeutic efficacy of RTK inhibition with promising treatment outcomes.

Pharmacological agents targeting drug-tolerant persister cells in cancer

Current cancer therapy can be effective, but the development of drug resistant disease is the usual outcome. These drugs can eliminate most of the tumor burden but often fail to eliminate the rare, "Drug Tolerant Persister" (DTP) cell subpopulations in residual tumors, which can be referred to as "Persister" cells. Therefore, novel therapeutic agents specifically targeting or preventing the development of drug-resistant tumors mediated by the remaining persister cells subpopulations are needed. Since approximately ninety percent of cancer-related deaths occur because of the eventual development of drug resistance, identifying, and dissecting the biology of the persister cells is essential for the creation of drugs to target them. While there remains uncertainty surrounding all the markers identifying DTP cells in the literature, this review summarizes the drugs and therapeutic approaches that are available to target the persister cell subpopulations expressing the cellular markers ATP-binding cassette sub-family B member 5 (ABCB5), CD133, CD271, Lysine-specific histone demethylase 5 (KDM5), and aldehyde dehydrogenase (ALDH). Persister cells expressing these markers were selected as the focus of this review because they have been found on cells surviving following drug treatments that promote recurrent drug resistant cancer and are associated with stem cell-like properties, including self-renewal, differentiation, and resistance to therapy. The limitations and obstacles facing the development of agents targeting these DTP cell subpopulations are detailed, with discussion of potential solutions and current research areas needing further exploration.

Mechanisms of Melanoma Progression and Treatment Resistance: Role of Cancer Stem-like Cells

Melanoma is the third most common type of skin cancer, characterized by its heterogeneity and propensity to metastasize to distant organs. Melanoma is a heterogeneous tumor, composed of genetically divergent subpopulations, including a small fraction of melanoma-initiating cancer stem-like cells (CSCs) and many non-cancer stem cells (non-CSCs). CSCs are characterized by their unique surface proteins associated with aberrant signaling pathways with a causal or consequential relationship with tumor progression, drug resistance, and recurrence. Melanomas also harbor significant alterations in functional genes (BRAF, CDKN2A, NRAS, TP53, and NF1). Of these, the most common are the BRAF and NRAS oncogenes, with 50% of melanomas demonstrating the BRAF mutation (BRAFV600E). While the successful targeting of BRAFV600E does improve overall survival, the long-term efficacy of available therapeutic options is limited due to adverse side effects and reduced clinical efficacy. Additionally, drug resistance develops rapidly via mechanisms involving fast feedback re-activation of MAPK signaling pathways. This article updates information relevant to the mechanisms of melanoma progression and resistance and particularly the mechanistic role of CSCs in melanoma progression, drug resistance, and recurrence.

Novel cellular systems unveil mucosal melanoma initiating cells and a role for PI3K/Akt/mTOR pathway in mucosal melanoma fitness

BACKGROUND

Mucosal Melanomas (MM) are highly aggressive neoplasms arising from mucosal melanocytes. Current treatments offer a limited survival benefit for patients with advanced MM; moreover, the lack of pre-clinical cellular systems has significantly limited the understanding of their immunobiology.

METHODS

Five novel cell lines were obtained from patient-derived biopsies of MM arising in the sino-nasal mucosa and designated as SN-MM1-5. The morphology, ultrastructure and melanocytic identity of SN-MM cell lines were validated by transmission electron microscopy and immunohistochemistry. Moreover, in vivo tumorigenicity of SN-MM1-5 was tested by subcutaneous injection in NOD/SCID mice. Molecular characterization of SN-MM cell lines was performed by a mass-spectrometry proteomic approach, and their sensitivity to PI3K chemical inhibitor LY294002 was validated by Akt activation, measured by pAkt(Ser473) and pAkt(Thr308) in immunoblots, and MTS assay.

RESULTS

This study reports the validation and functional characterization of five newly generated SN-MM cell lines. Compared to the normal counterpart, the proteomic profile of SN-MM is consistent with transformed melanocytes showing a heterogeneous degree of melanocytic differentiation and activation of cancer-related pathways. All SN-MM cell lines resulted tumorigenic in vivo and display recurrent structural variants according to aCGH analysis. Of relevance, the microscopic analysis of the corresponding xenotransplants allowed the identification of clusters of MITF-/CDH1-/CDH2 + /ZEB1 + /CD271 + cells, supporting the existence of melanoma-initiating cells also in MM, as confirmed in clinical samples. In vitro, SN-MM cell lines were sensitive to cisplatin, but not to temozolomide. Moreover, the proteomic analysis of SN-MM cell lines revealed that RICTOR, a subunit of mTORC2 complex, is the most significantly activated upstream regulator, suggesting a relevant role for the PI3K-Akt-mTOR pathway in these neoplasms. Consistently, phosphorylation of NDRG1 and Akt activation was observed in SN-MM, the latter being constitutive and sustained by PTEN loss in SN-MM2 and SN-MM3. The cell viability impairment induced by LY294002 confirmed a functional role for the PI3K-Akt-mTOR pathway in SN-MM cell lines.

CONCLUSIONS

Overall, these novel and unique cellular systems represent relevant experimental tools for a better understanding of the biology of these neoplasms and, as an extension, to MM from other sites.

Feline SCCs of the Head and Neck Display Partial Epithelial-Mesenchymal Transition and Harbor Stem Cell-like Cancer Cells

Squamous cell carcinoma of the head and neck (HNSCC) is a malignant cancer disease in humans and animals. There is ample evidence that the high plasticity of cancer cells, i.e., their ability to switch from an epithelial to a mesenchymal, endothelial, and stem cell-like phenotype, chiefly contributes to progression, metastasis, and multidrug resistance of human HNSCCs. In feline HNSCC, the field of cancer cell plasticity is still unexplored. In this study, fourteen feline HNSCCs with a known feline papillomavirus (FPV) infection status were subjected to histopathological grading and subsequent screening for expression of epithelial, mesenchymal, and stem cell markers by immunohistochemistry (IHC) and immunofluorescence staining (IF). Irrespective of the FPV infection status, all tumors except one corresponded to high-grade, invasive lesions and concurrently expressed epithelial (keratins, E-cadherin, β-catenin) and mesenchymal (vimentin, N-cadherin, CD146) proteins. This finding is indicative for partial epithelial-mesenchymal transition (pEMT) events in the lesions, as similarly described for human HNSCCs. IF double staining revealed the presence of CD44/CD271 double-positive cells notably within the tumors' invasive fronts that likely correspond to cancer stem cells. Taken together, the obtained findings suggest that feline HNSCCs closely resemble their human counterparts with respect to tumor cell plasticity.

Promising Markers in the Context of Mesenchymal Stem/Stromal Cells Subpopulations with Unique Properties

The heterogeneity of the mesenchymal stem/stromal cells (MSCs) population poses a challenge to researchers and clinicians, especially those observed at the population level. What is more, the lack of precise evidences regarding MSCs developmental origin even further complicate this issue. As the available evidences indicate several possible pathways of MSCs formation, this diverse origin may be reflected in the unique subsets of cells found within the MSCs population. Such populations differ in specialization degree, proliferation, and immunomodulatory properties or exhibit other additional properties such as increased angiogenesis capacity. In this review article, we attempted to identify such outstanding populations according to the specific surface antigens or intracellular markers. Described groups were characterized depending on their specialization and potential therapeutic application. The reports presented here cover a wide variety of properties found in the recent literature, which is quite scarce for many candidates mentioned in this article. Even though the collected information would allow for better targeting of specific subpopulations in regenerative medicine to increase the effectiveness of MSC-based therapies.

Spatial tissue proteomics reveals distinct landscapes of heterogeneity in cutaneous papillomavirus-induced keratinocyte carcinomas

Infection with certain cutaneous human papillomaviruses (HPV), in conjunction with chronic ultraviolet (UV) exposure, are the major cofactors of non-melanoma skin cancer (NMSC), the most frequent cancer type worldwide. Cutaneous squamous cell carcinomas (SCCs) as well as tumors in general represent three-dimensional entities determined by both temporal and spatial constraints. Whole tissue proteomics is a straightforward approach to understand tumorigenesis in better detail, but studies focusing on different progression states toward a dedifferentiated SCC phenotype on a spatial level are rare. Here, we applied an innovative proteomic workflow on formalin-fixed, paraffin-embedded (FFPE) epithelial tumors derived from the preclinical animal model Mastomys coucha. This rodent is naturally infected with its genuine cutaneous papillomavirus and closely mimics skin carcinogenesis in the context of cutaneous HPV infections in humans. We deciphered cellular networks by comparing diverse epithelial tissues with respect to their differentiation level and infection status. Our study reveals novel regulatory proteins and pathways associated with virus-induced tumor initiation and progression of SCCs. This approach provides the basis to better comprehend the multistep process of skin carcinogenesis.

Reactive Oxygen Species Regulation of Chemoresistance and Metastatic Capacity of Melanoma: Role of the Cancer Stem Cell Marker CD271

BRAF mutations are present in 30-50% of cases of cutaneous melanoma, and treatment with selective BRAF and MEK inhibitors has been introduced. However, the development of resistance to these drugs often occurs. Chemo-resistant melanoma cells show increased expression of CD271, a stem cell marker that features increased migration. Concordantly, resistance to the selective inhibitor of oncogenic BRAFV600E/K, vemurafenib, is mediated by the increased expression of CD271. It has recently been shown that the BRAF pathway leads to an overexpression of the NADPH oxidase Nox4, which produces reactive oxygen species (ROS). Here, we examined in vitro how Nox-derived ROS in BRAF-mutated melanoma cells regulates their drug sensitivity and metastatic potential. We demonstrated that DPI, a Nox inhibitor, reduced the resistance of a melanoma cell line (SK-MEL-28) and a primary culture derived from a BRAFV600E-mutated biopsy to vemurafenib. DPI treatment affected the expression of CD271 and the ERK and Akt signaling pathways, leading to a drop in epithelial-mesenchymal transition (EMT), which undoubtedly promotes an invasive phenotype in melanoma. More importantly, the scratch test demonstrated the efficacy of the Nox inhibitor (DPI) in blocking migration, supporting its use to counteract drug resistance and thus cell invasion and metastasis in BRAF-mutated melanoma.

[CUP syndrome-diagnostics from the perspective of pathology]

PROBLEM

Histologic and immunohistologic workup of tumor material from metastases of a previously unknown primary tumor is important for identifying their origin, but is often insufficient for this purpose without clinical oncologic and radiologic evaluation.

PROCEDURE

In the initial cancer of unknown primary (CUP) situation, histologic and immunohistochemical workup together with clinicoradiologic correlations contribute significantly to the identification of the primary tumor. There are now accepted guidelines to follow when there is an initial CUP situation. Molecular diagnostic tools can be used to investigate changes at the nucleic acid level, which can provide clues about the primary tumor, including potential targets for therapy. If, despite broad and interdisciplinary diagnostics, it is not possible to identify the primary tumor, the diagnosis is CUP syndrome. If a true CUP situation is present, it is important to assign the tumor to a tumor class or a specific therapy-sensitive subgroup as best as possible so that the best possible treatment can be given. However, for a final assignment to a primary tumor or a final classification as CUP, a comparison with medical oncological and imaging findings is indispensable.

CONCLUSION

When CUP is suspected, close interdisciplinary collaboration between pathology, medical oncology, and imaging is essential to achieve a viable classification as CUP or identification of a presumptive primary tumor, in the interest of providing the most specific and effective therapy for affected individuals.

A Comprehensive Analysis of Cutaneous Melanoma Patients in Greece Based on Multi-Omic Data

Cutaneous melanoma (CM) is the most aggressive type of skin cancer, and it is characterised by high mutational load and heterogeneity. In this study, we aimed to analyse the genomic and transcriptomic profile of primary melanomas from forty-six Formalin-Fixed, Paraffin-Embedded (FFPE) tissues from Greek patients. Molecular analysis for both germline and somatic variations was performed in genomic DNA from peripheral blood and melanoma samples, respectively, exploiting whole exome and targeted sequencing, and transcriptomic analysis. Detailed clinicopathological data were also included in our analyses and previously reported associations with specific mutations were recognised. Most analysed samples (43/46) were found to harbour at least one clinically actionable somatic variant. A subset of samples was profiled at the transcriptomic level, and it was shown that specific melanoma phenotypic states could be inferred from bulk RNA isolated from FFPE primary melanoma tissue. Integrative bioinformatics analyses, including variant prioritisation, differential gene expression analysis, and functional and gene set enrichment analysis by group and per sample, were conducted and molecular circuits that are implicated in melanoma cell programmes were highlighted. Integration of mutational and transcriptomic data in CM characterisation could shed light on genes and pathways that support the maintenance of phenotypic states encrypted into heterogeneous primary tumours.

Decoding molecular programs in melanoma brain metastases

Melanoma brain metastases (MBM) variably respond to therapeutic interventions; thus determining patient's prognosis. However, the mechanisms that govern therapy response are poorly understood. Here, we use a multi-OMICS approach and targeted sequencing (TargetSeq) to unravel the programs that potentially control the development of progressive intracranial disease. Molecularly, the expression of E-cadherin (Ecad) or NGFR, the BRAF mutation state and level of immune cell infiltration subdivides tumors into proliferative/pigmented and invasive/stem-like/therapy-resistant irrespective of the intracranial location. The analysis of MAPK inhibitor-naive and refractory MBM reveals switching from Ecad-associated into NGFR-associated programs during progression. NGFR-associated programs control cell migration and proliferation via downstream transcription factors such as SOX4. Moreover, global methylome profiling uncovers 46 differentially methylated regions that discriminate BRAF^mut and wildtype MBM. In summary, we propose that the expression of Ecad and NGFR sub- classifies MBM and suggest that the Ecad-to-NGFR phenotype switch is a rate-limiting process which potentially indicates drug-response and intracranial progression states in melanoma patients.

p75NTR prevents the onset of cerebellar granule cell migration via RhoA activation

Neuronal migration is one of the fundamental processes during brain development. Several neurodevelopmental disorders can be traced back to dysregulated migration. Although substantial efforts have been placed in identifying molecular signals that stimulate migration, little is known about potential mechanisms that restrict migration. These restrictive mechanisms are essential for proper development since it helps coordinate the timing for each neuronal population to arrive and establish proper connections. Moreover, preventing migration away from a proliferative niche is necessary in maintaining a pool of proliferating cells until the proper number of neuronal progenitors is attained. Here, using mice and rats, we identify an anti-migratory role for the p75 neurotrophin receptor (p75NTR) in cerebellar development. Our results show that granule cell precursors (GCPs) robustly express p75NTR in the external granule layer (EGL) when they are proliferating during postnatal development, however, they do not express p75NTR when they migrate either from the rhombic lip during embryonic development or from the EGL during postnatal development. We show that p75NTR prevented GCP migration by maintaining elevated levels of active RhoA. The expression of p75NTR was sufficient to prevent the migration of the granule cells even in the presence of BDNF (brain-derived neurotrophic factor), a well-established chemotactic signal for this cell population. Our findings suggest that the expression of p75NTR might be a critical signal that stops and maintains the GCPs in the proliferative niche of the EGL, by promoting the clonal expansion of cerebellar granule neurons.

Drug-Tolerant Persister Cells in Cancer Therapy Resistance

One of the current stumbling blocks in our fight against cancer is the development of acquired resistance to therapy, which is attributable to approximately 90% of cancer-related deaths. Undercutting this process during treatment could significantly improve cancer management. In many cases, drug resistance is mediated by a drug-tolerant persister (DTP) cell subpopulation present in tumors, often referred to as persister cells. This review provides a summary of currently known persister cell subpopulations and approaches to target them. A specific DTP cell subpopulation with elevated levels of aldehyde dehydrogenase (ALDH) activity has stem cell-like characteristics and a high level of plasticity, enabling them to switch rapidly between high and low ALDH activity. Further studies are required to fully elucidate the functions of ALDH-high DTP cells, how they withstand drug concentrations that kill other cells, and how they rapidly adapt under levels of high cellular stress and eventually lead to more aggressive, recurrent, and drug-resistant cancer. Furthermore, this review addresses the processes used by the ALDH-high persister cell subpopulation to enable cancer progression, the ALDH isoforms important in these processes, interactions of ALDH-high DTPs with the tumor microenvironment, and approaches to therapeutically modulate this subpopulation in order to more effectively manage cancer.

Tumor Cell Plasticity in Equine Papillomavirus-Positive Versus-Negative Squamous Cell Carcinoma of the Head and Neck

Squamous cell carcinoma of the head and neck (HNSCC) is a common malignant tumor in humans and animals. In humans, papillomavirus (PV)-induced HNSCCs have a better prognosis than papillomavirus-unrelated HNSCCs. The ability of tumor cells to switch from epithelial to mesenchymal, endothelial, or therapy-resistant stem-cell-like phenotypes promotes disease progression and metastasis. In equine HNSCC, PV-association and tumor cell phenotype switching are poorly understood. We screened 49 equine HNSCCs for equine PV (EcPV) type 2, 3 and 5 infection. Subsequently, PV-positive versus -negative lesions were analyzed for expression of selected epithelial (keratins, β-catenin), mesenchymal (vimentin), endothelial (COX-2), and stem-cell markers (CD271, CD44) by immunohistochemistry (IHC) and immunofluorescence (IF; keratins/vimentin, CD44/CD271 double-staining) to address tumor cell plasticity in relation to PV infection. Only EcPV2 PCR scored positive for 11/49 equine HNSCCs. IHC and IF from 11 EcPV2-positive and 11 EcPV2-negative tumors revealed epithelial-to-mesenchymal transition events, with vimentin-positive cells ranging between <10 and >50%. CD44- and CD271-staining disclosed the intralesional presence of infiltrative tumor cell fronts and double-positive tumor cell subsets independently of the PV infection status. Our findings are indicative of (partial) epithelial–mesenchymal transition events giving rise to hybrid epithelial/mesenchymal and stem-cell-like tumor cell phenotypes in equine HNSCCs and suggest CD44 and CD271 as potential malignancy markers that merit to be further explored in the horse.

Tracking of Melanoma Cell Plasticity by Transcriptional Reporters

Clonal evolution and cellular plasticity are the genetic and non-genetic driving forces of tumor heterogeneity, which in turn determine tumor cell responses towards therapeutic drugs. Several lines of evidence suggest that therapeutic interventions foster the selection of drug-resistant neural crest stem-like cells (NCSCs) that establish minimal residual disease (MRD) in melanoma. Here, we establish a dual-reporter system, enabling the tracking of NGFR expression and mRNA stability and providing insights into the maintenance of NCSC states. We observed that a transcriptional reporter that contained a 1-kilobase fragment of the human NGFR promoter was activated only in a minor subset (0.72 ± 0.49%, range 0.3-1.5), and ~2-4% of A375 melanoma cells revealed stable NGFR mRNA. The combination of both reporters provides insights into phenotype switching and reveals that both cellular subsets gave rise to cellular heterogeneity. Moreover, whole transcriptome profiling and gene-set enrichment analysis (GSEA) of the minor cellular subset revealed hypoxia-associated programs that might serve as potential drivers of an in vitro switching of NGFR-associated phenotypes and relapse of post-BRAF inhibitor-treated tumors. Concordantly, we observed that the minor cellular subset increased in response to dabrafenib over time. In summary, our reporter-based approach provides insights into plasticity and identified a cellular subset that might be responsible for the establishment of MRD in melanoma.

Oral Cancer Stem Cells: Therapeutic Implications and Challenges

Head and neck squamous cell carcinoma (HNSCC) is currently one of the 10 most common malignancies worldwide, characterized by a biologically highly diverse group of tumors with non-specific biomarkers and poor prognosis. The incidence rate of HNSCC varies widely throughout the world, with an evident prevalence in developing countries such as those in Southeast Asia and Southern Africa. Tumor relapse and metastasis following traditional treatment remain major clinical problems in oral cancer management. Current evidence suggests that therapeutic resistance and metastasis of cancer are mainly driven by a unique subpopulation of tumor cells, termed cancer stem cells (CSCs), or cancer-initiating cells (CICs), which are characterized by their capacity for self-renewal, maintenance of stemness and increased tumorigenicity. Thus, more understanding of the molecular mechanisms of CSCs and their behavior may help in developing effective therapeutic interventions that inhibit tumor growth and progression. This review provides an overview of the main signaling cascades in CSCs that drive tumor repropagation and metastasis in oral cancer, with a focus on squamous cell carcinoma. Other oral non-SCC tumors, including melanoma and malignant salivary gland tumors, will also be considered. In addition, this review discusses some of the CSC-targeted therapeutic strategies that have been employed to combat disease progression, and the challenges of targeting CSCs, with the aim of improving the clinical outcomes for patients with oral malignancies. Targeting of CSCs in head and neck cancer (HNC) represents a promising approach to improve disease outcome. Some CSC-targeted therapies have already been proven to be successful in pre-clinical studies and they are now being tested in clinical trials, mainly in combination with conventional treatment regimens. However, some studies revealed that CSCs may not be the only players that control disease relapse and progression of HNC. Further, clinical research studying a combination of therapies targeted against head and neck CSCs may provide significant advances.