Unveiling therapeutic avenues targeting xCT in head and neck cancer

Abstract

Head and neck cancer (HNC) remains a major global health burden, prompting the need for innovative therapeutic strategies. This review examines the role of the cystine/glutamate antiporter (xCT) in HNC, specifically focusing on how xCT contributes to cancer progression through mechanisms such as redox imbalance, ferroptosis, and treatment resistance. The central questions addressed include how xCT dysregulation affects tumor biology and the potential for targeting xCT to enhance treatment outcomes. We explore recent developments in xCT-targeted current and emerging therapies, including xCT inhibitors and novel treatment modalities, and their role in addressing therapeutic challenges. This review aims to provide a comprehensive analysis of xCT as a therapeutic target and to outline future directions for research and clinical application.

Introduction

Head and neck cancer (HNC) stands as the seventh most prevalent malignancy globally, with tumors manifesting across diverse anatomical sites, including the oral cavity, pharynx, larynx, nasal cavity, and paranasal sinus [1]. Mucosal linings of these regions harbor HNCs, which predominantly originate from epithelial cells, except for thyroid cancers, and progressively advance, often transitioning to a mesenchymal or poorly differentiated state, thereby promoting migration, invasion, metastasis, and treatment resistance [2]. HNC encompasses various subtypes with distinct clinical features. For example, HPV-associated oropharyngeal cancer is highly prevalent in Western countries, while nasopharyngeal cancer, closely linked to Epstein-Barr virus (EBV) infection, is more common in East Asian populations but rare in Western countries [3]. With the emergence of organ preservation strategies in current HNC treatments, systemic therapy has gained prominence, either in combination with radiation or for induction purposes before definitive treatment [4]. Immunotherapy, including immune checkpoint inhibitors like pembrolizumab and nivolumab, has emerged as an advanced therapeutic option, demonstrating promising results in improving survival outcomes in recurrent or metastatic HNC [5]. However, there remains an urgent imperative to devise novel treatment approaches to enhance the therapeutic ratio, mitigate adverse effects, and substantially improve survival outcomes for HNC patients [6].

Ferroptosis, a distinctive form of regulated cell demise marked by iron-dependent lipid peroxide accumulation, emerges as a critical phenomenon in cancer biology [7]. The seminal discovery of ferroptosis induction by erastin, a selective lethal agent against oncogenic RAS, in 2012 unveiled this novel cell death modality, characterized by the accrual of lipid peroxides and redox-active iron, ultimately culminating in cellular membrane oxidative injury and demise [7, 8]. The intracellular labile iron pool is central to ferroptosis orchestration, pivotal in generating highly reactive radicals through the Fenton reaction, driving lipid peroxidation and cellular membrane damage [9]. Enzymatic reactions catalyzed by iron-dependent enzymes such as 12/15-LOX, P450 oxidoreductase, and prostaglandin-endoperoxide synthase 2 fuel the reaction between ferrous iron and polyunsaturated fatty acid-phospholipids (PUFA-PLs), fostering lipid peroxidation [10]. Conversely, radical-trapping antioxidant systems, including glutathione peroxidase 4 (GPX4), counteract lipid peroxidation propagation, safeguarding cells from excessive oxidative damage [11].

The Hippo signaling pathway, mainly through its downstream effectors YAP (Yes-associated protein) and TAZ (Transcriptional co-activator with PDZ-binding motif), is increasingly recognized as a critical player in regulating cancer processes, including ferroptosis. In HNC, dysregulation of the Hippo pathway can lead to enhanced YAP/TAZ activity, contributing to tumor progression and treatment resistance. have shown that YAP activation can increase ferroptosis susceptibility by upregulating key modulators like ACSL4 and TFRC, which are involved in lipid peroxidation and iron uptake [12, 13]. Furthermore, TAZ has been implicated in ROS regulation through mechanisms such as the EMP1-NOX4 axis. These findings suggest that the Hippo pathway’s influence on ferroptosis could be a potential therapeutic target in HNC, especially in cases where this pathway is dysregulated.

Notably, the system xc^– cystine/glutamate exchange transporter integral in maintaining cellular redox homeostasis through cystine uptake and glutathione (GSH) biosynthesis emerges as a pivotal regulator in ferroptosis modulation (Fig. 1) [8]. This system, comprising SLC3A2 heavy chain and SLC7A11 (xCT) light chain solute carrier family, orchestrates cystine import coupled with glutamate export, critically influencing intracellular cysteine levels and GSH synthesis [14]. Inhibition of xCT disrupts cysteine availability and GSH biosynthesis, precipitating ferroptosis through lipid peroxidation overload [8]. Given the profound implications of ferroptosis and the xCT/GSH axis in cancer biology, this review endeavors to elucidate the intricate interplay of these molecular pathways and their potential as therapeutic targets for innovative HNC management strategies.

Fig. 1.

Mechanism of ferroptosis induction through the xCT/GPX4 axis. The cystine/glutamate antiporter (xCT) is the key facilitator of cystine import concomitant with glutamate export. Intracellular cysteine production, vital for glutathione (GSH) biosynthesis, hinges on cystine uptake, which is supplemented by partial de novo cysteine synthesis through the transsulfuration pathway. As a crucial cofactor, GSH enables glutathione peroxidase 4 (GPX4) to detoxify lipid peroxides into lipid alcohols. Consequently, depletion of GSH triggers iron-catalyzed lipid peroxidation, instigating ferroptosis—a non-apoptotic form of cell death. The Fenton reaction, catalyzed by ferrous iron from the labile iron pool and hydrogen peroxide, generates hydroxyl or peroxyl radicals. These radicals initiate lipid peroxidation cascades, disrupting cell membrane integrity and ultimately leading to cell demise

Targeting xCT for therapeutic intervention in head and neck cancer

Previous investigations have underscored the potential of xCT inhibition in impeding tumor progression and enhancing apoptosis in HNC (Fig. 1). Irinotecan, a well-established chemotherapeutic agent, operates as a topoisomerase I inhibitor, instigating DNA damage and cell demise [15]. Its active metabolite, SN-38, demonstrated efficacy in retarding tumor growth in FaDu tumor xenografts derived from a patient with hypopharyngeal cancer [16]. Elevated GSH synthesis, driven by augmented cystine uptake via xCT overexpression, contributes to drug resistance [17]. Irinotecan-mediated xCT downregulation in FaDu HNC models led to reduced GSH levels and a subsequent rise in ROS production, underscoring its potential to circumvent chemoresistance [16]. Another study elucidated the impact of xCT inhibition on apoptosis induction in CD44v-expressing HNSCC. The cell adhesion molecule CD44 variant (CD44v) interacts with xCT, promoting cystine uptake and GSH synthesis, thereby conferring stem-like cancer cells with reactive oxygen species (ROS) defense and chemoresistance [17, 18]. Selective apoptosis induction in CD44v-expressing head and neck squamous cell carcinoma (HNSCC), inherently resistant to EGFR-targeted therapy, was achieved through xCT inhibition, disrupting the GSH-dependent antioxidant system [18]. Notably, highly CD44v-expressing tumors exhibited resistance to EGFR-targeted chemotherapy, emphasizing the role of the CD44v-xCT axis in chemoresistance [19]. However, the dependency on CD44 and xCT diminished with HNSCC cell differentiation in vivo, rendering them less responsive to xCT inhibitory interventions. Notably, sulfasalazine, a specific xCT inhibitor, selectively suppressed tumor growth and chemoresistance in CD44v-dependent HNSCC cells, offering a potential avenue to deplete undifferentiated cancer cells and sensitize the remaining population to chemotherapeutic agents, including EGFR-targeted therapy.

Cisplatin, a time-honored antineoplastic agent, continues to hold prominence in the therapeutic armamentarium against a spectrum of solid tumors, encompassing bladder, ovarian, testicular, lung, gastric, colorectal, and HNC [20]. While a multimodal treatment approach integrating surgery, radiotherapy, and chemoradiation is standard in HNC management, cisplatin, as a first-line chemotherapeutic agent, is pivotal, especially in organ-preserving protocols [21]. However, cisplatin’s clinical utility is hampered by its propensity for moderate-to-severe organ toxicity and the development of resistance, posing challenges in effective cancer management [22]. Resistance mechanisms, including altered drug accumulation, enhanced DNA repair, and drug inactivation, underlie the limited efficacy of cisplatin in clinical practice [23]. Notably, cisplatin-induced upregulation of xCT expression and subsequent elevation of GSH levels contribute to resistance in tongue squamous cell carcinoma (SCC) cells [24]. Combination therapy involving sulfasalazine and cisplatin has demonstrated efficacy in reducing GSH levels, triggering ROS accumulation, and activating cell death pathways in tongue cancer cells. Moreover, pharmacological or genetic inhibition of xCT has shown promise in eradicating cisplatin-resistant HNC cells in vitro and in vivo by inducing ferroptosis, a form of regulated cell death characterized by lipid peroxide accumulation [25]. The synergistic effect of aspirin and sorafenib co-treatment in cisplatin-resistant HNC cells underscores the potential of combination therapies targeting xCT to overcome resistance. This regimen induces xCT inhibition, GSH depletion, and ROS accumulation, culminating in ferroptotic cell death, offering a promising strategy against resistant HNC [26]. The combination of acetylsalicylic acid (aspirin), a non-steroidal inflammation drug [27], and sorafenib, a multikinase inhibitor targeting the RAF-MEK-ERK protein pathway and receptor tyrosine kinase [28], synergized to induce ferroptosis in the cisplatin-resistant HNC cells, even at low concentrations [26]. Furthermore, sorafenib’s ability to induce ferroptosis in therapy-resistant, metastatic nasopharyngeal carcinoma (NPC) cells by inhibiting ERK and xCT highlights the pivotal role of xCT in NPC occurrence and progression, underscoring its potential as a novel therapeutic target [29]. Therefore, xCT plays an important role in the occurrence and progression of NPC, suggesting being a novel target for NPC treatment.

Human Papillomavirus (HPV) infection has emerged as a significant etiological factor in oropharyngeal SCC. It stands as a pivotal prognostic determinant in HNSCC, often correlating with improved responses to chemoradiation therapy [30]. Examining ferroptosis gene expression in HNSCC cohorts from The Cancer Genome Atlas revealed markedly lower expression levels of two system xc^– subunits, SLC7A11 and SLC3A2, in HPV-positive tumors compared to their HPV-negative counterparts [31]. Notably, the expression of HPV16 oncoproteins E6 and E7 heightened the susceptibility of HNSCC cells to erastin-induced ferroptosis by impeding cystine import via xCT and inhibiting GSH synthesis, a phenomenon not observed with RSL3, a GPX4 inhibitor [31]. These findings underscore the potential of targeting xCT to induce ferroptosis as a promising therapeutic avenue in HPV-positive HNSCC, offering new insights into managing this subtype of HNC.

Mechanistic insights into xCT regulation and its role in HNC progression

The burgeoning body of evidence underscores xCT as an intriguing biomarker pertinent to the progression and prognosis of HNC. Notably, in laryngeal SCC, xCT expression exhibited a significant upsurge compared to benign laryngeal pathologies like vocal polyps and atypical hyperplasias [32]. Immunohistochemical analyses revealed a robust correlation between xCT expression and the markers of cellular proliferation, Ki-67, and p53. Functionally, xCT was found to intricately regulate cell cycle progression in laryngeal SCC cell lines, implicating its pivotal role in tumorigenesis. In oral SCC, overexpression of xCT, particularly in conjunction with CD44, was associated with aggressive clinicopathological features such as advanced T classification, perineural invasion, lymphovascular invasion, and nodal metastasis in a sizable patient cohort [33]. Notably, the adhesion molecule CD44 variant (CD44v) formed a molecular complex with SLC7A11, reinforcing the latter’s stability. CD44, through its interaction with the ubiquitin hydrolase otubain-1 (OTUB1), bolstered the stability of xCT, thereby inhibiting ferroptosis, a form of regulated cell death (Figs. 1 and 2). Conversely, the depletion of CD44v disrupted this interaction, leading to xCT inactivation and promoting ferroptosis in cancer cells. Intriguingly, high expression levels of xCT, either alone or in tandem with CD44, but not CD44 alone, emerged as independent predictors of unfavorable recurrence-free, disease-specific, and overall survival outcomes in oral cancer patients, underscoring the potential utility of the CD44/xCT axis as a prognostic biomarker and therapeutic target in oral SCC [33]. Furthermore, the sensitivity to xCT-targeted therapy in HNSCC appears intricately linked with glutaminolysis-related genes such as glutamate dehydrogenase (GDH) and the glutamine transporter ASCT2 [34]. Inhibition of xCT via sulfasalazine instigated metabolic reprogramming, characterized by augmented levels of the tricarboxylic acid (TCA) cycle intermediate α-ketoglutarate (α-KG), concurrent with diminished intracellular cysteine and GSH content, culminating in oxidative stress-induced damage within HNSCC tumors (Fig. 1).

Fig. 2.

Modulators of xCT and their therapeutic implications in head and neck cancer. Cellular uptake of cysteine primarily relies on the cystine transporter xCT, facilitating cystine import from extracellular sources. The regulation of xCT expression in cancer cells is multifaceted, encompassing transcriptional, translational, and posttranslational processes, as elucidated in the main text. Given the heightened intracellular reactive oxygen species (ROS) levels in cancer cells, necessitating elevated cysteine and GSH levels, aberrant xCT expression is prevalent across various human cancers. Notably, this aberration renders cancer cells vulnerable to selective xCT inhibitors or the enzyme cyst(e)inase

The intricate orchestration of molecular players governing xCT expression and function unveils novel avenues for understanding and combating HNC. Notably, the tumor suppressor protein p53 emerges as a pivotal regulator, exerting direct transcriptional repression on xCT and thereby impeding ferroptotic cell demise induced by a spectrum of ferroptosis inducers [35]. Concurrently, ATF3, a ubiquitous stress sensor, exerts basal repression on the xCT promoter independent of p53 (Fig. 2). Under conditions of ferroptosis induction precipitated by erastin treatment or cystine deficiency, ATF3 assumes a pivotal role in downregulating xCT expression, depleting intracellular GSH, and ultimately promoting ferroptosis in malignant cells [36]. Furthermore, hotspot mutations of p53 at codons R248 and R175 frequently manifest in HNSCC specimens, correlating with dismal survival outcomes attributed to heightened metastatic propensities [37, 38]. Intriguingly, these high-risk TP53 mutations harbor gain-of-function (GOF) properties pertaining to xCT regulation, orchestrating its rapid degradation via the ubiquitin-proteasome system [39]. The mutant p53-mediated transcriptional modulation of xCT stands poised as a critical determinant shaping the therapeutic efficacy of low-dose paclitaxel in conjunction with RSL3, offering fresh insights into synergistic cell demise mediated by the interplay of ferroptosis and p53 [40]. Moreover, xCT expression emerges as a pivotal determinant dictating the response to APR-246, a mutant p53 reactivator currently undergoing clinical scrutiny in TP53-mutated human cancers [41]. The delineation of xCT expression as a broadly applicable biomarker for APR-246 sensitivity across diverse human malignancies holds promise for patient stratification in future clinical investigations, paving the way for its judicious application in clinical practice.

Within the intricate landscape of HNC, the Kelch-like ECH-associated protein 1 (Keap1)-nuclear erythroid 2-related factor (Nrf2)-activator protein-1 (AP-1)/antioxidant response element (ARE) pathway emerges as a central player orchestrating cellular responses to oxidative stress, notably modulating the transcriptional activation of genes crucial for antioxidant defense, including xCT [42]. Under basal conditions, the transcription factor Nrf2 is subject to rapid degradation mediated by the E3 ubiquitin ligase Keap1; however, oxidative insults disrupt Nrf2 degradation, allowing its stabilization and subsequent binding to the ARE, thereby fostering antioxidant responses crucial for redox homeostasis [43]. In the realm of cancer biology, aberrant activation of the Keap1-Nrf2 axis confers resistance to ferroptosis by upregulating xCT and bolstering cysteine and GSH-mediated antioxidant defenses [43]. Furthermore, Nrf2 exerts its cytoprotective effects by orchestrating the expression of genes involved in GSH biosynthesis, iron metabolism, and broader antioxidant responses, collectively fortifying malignant cells against ferroptotic stimuli [44]. Consequently, the activation of the Nrf2-ARE pathway emerges as a hallmark of ferroptosis resistance in refractory HNCs, underscoring its potential utility as a predictive marker for assessing cancer cell susceptibility to ferroptosis induction following xCT inhibition [45]. Moreover, in the quest for therapeutic strategies for overcoming ferroptosis resistance in HNC, the sesquiterpene lactone artesunate emerges as a promising candidate, selectively inducing iron-dependent, ROS-accumulated ferroptosis in HNC cells while sparing normal counterparts [46]. Nonetheless, the efficacy of artesunate may be compromised in cisplatin-resistant HNCs characterized by hyperactivation of the Nrf2-ARE pathway, which blunts ferroptotic responses [47]. Intriguingly, inhibition of the Nrf2-ARE axis restores artesunate sensitivity and potentiates ferroptosis induction in resistant HNC cells, offering a novel avenue for overcoming therapeutic resistance in HNC [47].

Beyond the realms of Nrf2 signaling, the intricate network of cellular pathways governing ferroptosis resistance in HNC extends to include the mammalian target of rapamycin complex 2 (mTORC2), which emerges as a pivotal regulator of amino acid metabolism within the cancer milieu [48]. Through the orchestrated activation of the AKT signaling cascade, mTORC2 exerts its influence by directly phosphorylating serine 26 of SLC7A11, thereby modulating cystine uptake and glutamate efflux crucial for cellular redox balance and stress adaptation [48]. Moreover, the regulatory prowess of mTORC1 emerges as a double-edged sword in the context of ferroptosis, as recent investigations unveil its paradoxical role as a negative modulator of ferroptosis via upregulation of xCT in laryngeal SCC [49]. Within this intricate regulatory framework, endoplasmic reticulum oxidoreductase 1 alpha (ERO1α) emerges as a pivotal mediator of ferroptosis resistance in mTORC1-hyperactivated cancer cells, orchestrating the transcriptional activation of xCT through activation of the interleukin-6 (IL-6)/signal transducer and activator of transcription 3 (STAT3) signaling axis [49]. Remarkably, inhibition of ERO1α sensitizes mTORC1-driven tumors to ferroptosis induction by erastin, offering a novel therapeutic avenue for overcoming ferroptosis resistance in mTORC1-related malignancies [49]. In light of these findings, the convergence of mTORC2 and mTORC1 signaling pathways emerges as a pivotal nexus in regulating ferroptosis resistance in HNC, offering promising targets for therapeutic intervention to overcome treatment resistance and enhance patient outcomes.

Within the intricate tapestry of HNC pathogenesis, interleukin-6 (IL-6) emerges as a central player, orchestrating a symphony of inflammatory responses contributing to tumor development and progression [50]. Through its intricate interplay with the tumor microenvironment and immune system, IL-6 fosters a milieu conducive to tumorigenesis, amplifying its effects through synergistic interactions with signal transducer and activator of transcription 3 (STAT3) and nuclear factor-kappa B (NF-κB) [51]. However, the influence of IL-6 extends beyond its canonical inflammatory roles, as recent investigations uncover its pivotal involvement in ferroptosis resistance during HNSCC carcinogenesis [52]. Immunopositivity of IL-6 surges in tandem with lipid peroxidation, marking the malignant transformation from premalignant lesions to HNSCC. Strikingly, IL-6 emerges as a transcriptional activator of xCT, underpinning the malignant progression accompanied by heightened expression of phospho-JAK2 and phospho-STAT3. Through its adept modulation of xCT expression, IL-6 profoundly influences erastin-induced ferroptosis resistance in HNSCC cells, culminating in tumor growth promotion in vivo [52]. In essence, IL-6 is a pivotal player in the complex interplay of pathways governing ferroptosis resistance in HNSCC, offering novel insights into the mechanisms driving carcinogenesis and potential therapeutic avenues for intervention.

Mitochondrial pyruvate carrier 1 (MPC1), a pivotal inner membrane protein facilitating the transfer of pyruvate into the mitochondria, holds sway over the delicate balance of cancer cell dynamics, orchestrating shifts in epithelial-mesenchymal transition (EMT) and glutaminolysis [53]. Intricately regulated by histone lysine demethylase 5 A (KDM5A), its expression acts as a linchpin in a signaling cascade that propels cancer cell progression [54]. Notably, erlotinib-tolerant persister cancer cells undergo a phenotypic metamorphosis marked by heightened KDM5A levels and diminished MPC1 expression, rendering them more susceptible to ferroptosis [55]. The clinical implications of MPC1 extend to its prognostic significance, as evidenced by its association with overall survival outcomes in an HNC cohort sourced from the TCGA datasets. MPC1 inhibition, under its ability to perpetuate mesenchymal traits and sustain glutaminolysis, heightens susceptibility to sulfasalazine-induced ferroptosis, hinting at the therapeutic promise of KDM5A-MPC1 axis modulation in potentiating ferroptosis in persister cancer cells.

Moreover, NPC undergoes a fateful rendezvous with integrin ß3 (ITGB3) ferried by platelet-derived extracellular vesicles (P-EVs), heralding a cascade of events culminating in distant metastasis [56]. The influx of ITGB3, facilitated by P-EVs, stabilizes xCT protein levels, setting ablaze the MAPK/ERK/ATF4/Nrf2 axis, an orchestrated dance that quells ferroptosis and paves the path for NPC metastasis. Thus, targeting P-EV-mediated tumor metastasis unveils a promising avenue in the diagnostic and therapeutic realms of NPC. Conversely, the expression levels of the MAP kinase, Erk1/2, emerge as a harbinger of HNSCC cell fate in the face of ferroptosis induction by erastin [57]. Perturbation of Erk1/2 signaling via ravoxertinib instigates a domino effect, bolstering the efficacy of erastin by upregulating superoxide dismutase 1 and 2, thereby reining in lipid peroxidation. In light of these findings, Erk1/2 expression levels serve as a litmus test for predicting HNSCC responses to xCT inhibitors, offering a beacon of hope in the quest for personalized therapeutic interventions.

MicroRNAs (miRNAs) emerge as pivotal orchestrators of gene expression, governing intricate post-transcriptional regulatory networks pivotal in tumorigenesis and therapeutic response [58]. These tiny yet potent molecules, derived from a basal RNA-silencing pathway, deftly manipulate mRNA stability and translation, shaping the cellular landscape in health and disease. Within the realm of cancer, miRNAs wield profound influence, regulating diverse processes from development to metastasis and therapy resistance [59]. Evidence mounts for the interplay between miRNAs and ferroptosis in human malignancies, with miR-375 emerging as a key player in modulating this iron-dependent cell death pathway in oral SCC [60]. Downregulated in oral cancer, miR-375 reigns as a tumor suppressor, dampening the expression of xCT, a crucial player in ferroptosis regulation. Complementing this, miR-125b-5p and miR-34c-3p also take center stage, orchestrating ferroptosis through their interactions with xCT in oral cancer cells [61, 62]. Enhancer of Zeste homolog 2 (EZH2), a member of the polycomb-repressive complex 2 (PRC2) superfamily, is highly expressed in almost all human cancers and is considered a potent tumor marker [63]. EZH2 transcription inhibits miR-125b-5p, which affects the role of miR-125b-5p in inhibiting ferroptosis by xCT in oral tongue SCC [64]. Overexpression of EZH2 and xCT inhibits ferroptosis induced by erastin, as targeting miR-125b-5p in tongue SCC. Also, miR-34c-3p influences the development of oral SCC by directly targeting xCT and inhibiting cell proliferation in oral cancer cells [62]. This study showed low expression of miR-34c-3p in oral SCC, negatively regulating xCT expression, promoting ferroptosis, and suppressing cell proliferation. Our observations provide new insights into oral SCC and present a potentially useful therapeutic strategy for OSCC. MiR-26a-5p expression was downregulated in oral SCC tissues and cell lines [65]. Mir-26a mimic inhibited cancer cell viability and promoted erastin-induced ferroptosis in oral SCC by targeting binding to the SLC7A11 3’UTR.

Circular noncoding RNAs (circRNAs) represent a unique class of regulatory molecules characterized by their closed-loop structure devoid of traditional mRNA features like 5′ end caps and 3′ end poly(A) tails [66]. Operating as versatile modulators of gene expression, circRNAs orchestrate intricate regulatory networks, acting as sponges for miRNAs RNA-binding proteins and even participating in protein/peptide translations [67]. CircRNAs emerge as promising candidates for predictive biomarkers and therapeutic targets in the landscape of human diseases and cancers, particularly in cancer treatment [68]. CircCDR1as (also known as has_circ_0001946) emerges as a key player in oral SCC, where its upregulation correlates with the downregulation of miR-876-5p [69]. Direct interaction between CircCDR1as and miR-876-5p unleashes a cascade of events driving OSCC progression and invasion, underscoring CircCDR1as’ pivotal role as an oncogenic driver by elevating xCT expression via miR-876-5p modulation. Similarly, CircPVT1 influences oral SCC progression by manipulating the miR-143-3p/xCT axis, a mechanism orchestrated through the MAPK signaling pathway [70]. Extending beyond oral SCC, the impact of circRNAs on xCT expression is also evident in laryngeal SCC, where Circ_0120175 emerges as a potent promoter of tumor proliferation, migration, and invasion through the miR-330-3p/xCT axis [71]. Linked to advanced tumor classifications, nodal metastasis, and reduced survival, Circ_0120175 underscores the broader significance of circRNAs in driving aggressive tumor behavior across different subtypes of HNC.

Understanding the intricate regulatory mechanisms of xCT provides a foundation for exploring targeted therapies in HNC. These insights pave the way for novel therapeutic strategies aimed at modulating xCT activity, which are discussed in the following section.

Recent advances in targeting xCT for therapeutic applications in cancer

Numerous substances have surfaced as inhibitors of xCT, including compounds like erastin, imidazole ketone erastin (IKE), sulfasalazine, and sorafenib [72, 73]. These compounds belong to the class of ferroptosis inducers known as class 1 FINs, capable of triggering ferroptosis by obstructing cystine uptake via xCT (Table 1). Erastin, renowned as a class 1 FIN, can remarkably target and eliminate cancer cells carrying the oncogenic RAS mutation [74]. Nevertheless, erastin’s clinical application is hindered by its poor metabolic stability and solubility [72]. IKE, a derivative of erastin that boasts improved stability and solubility, showcases potent activity at nanomolar levels, making it a promising candidate for preclinical ferroptosis studies [75]. However, IKE has yet to advance in clinical trials for cancer treatment. Sulfasalazine and sorafenib, approved by the U.S. Food and Drug Administration for arthritis and cancer treatment, respectively, exert their anti-tumor effects by inhibiting xCT transporter activity and ferroptosis in vivo [25, 26, 72]. Recent research has unveiled HG106 as a potent xCT inhibitor, demonstrating significant efficacy in suppressing tumor growth and prolonging survival in preclinical models of KRAS-mutated lung adenocarcinoma [73]. Additionally, an engineered and pharmacologically optimized human cyst(e)inase enzyme exhibited promising results in curtailing tumor growth in xenograft models of PDAC, prostate, and breast cancer [76, 77]. Administration of cyst(e)inase led to the depletion of serum L-cysteine and L-cystine pools, doubling the median survival time in TCL1-Tg: p53^−/− mice resembling chronic lymphocytic leukemia [76]. In summary, despite the efficacy of class 1 FIN agents in preclinical studies, their translation into successful clinical trials for cancer patients remains limited. Consequently, there is an urgent need to develop novel xCT inhibitors with enhanced efficacy and minimal side effects and to subject them to rigorous preclinical and clinical evaluations.

Table 1.

Key findings and limitations of therapeutic strategies targeting xCT in cancer research

Therapeutic strategy	Key findings	Effects	Drawbacks/limitations
Erastin	Inhibits xCT, inducing ferroptosis and targeting cancer cells with oncogenic RAS mutation	Reduces tumor size, effective against RAS-mutant tumors	Poor metabolic stability, limited clinical application
IKE (Imidazole Ketone Erastin)	Derivative of Erastin with improved stability and solubility	Effective at nanomolar levels, promising in preclinical studies	Not yet advanced to clinical trials
Sulfasalazine	Inhibits xCT transporter and induces ferroptosis, FDA-approved for arthritis	Exhibits anti-tumor effects through xCT inhibition	Specific side effects related to its approved uses
Sorafenib	Inhibits xCT transporter and induces ferroptosis, FDA-approved for cancer treatment	Anti-tumor effects	Potential liver toxicity and other side effects
HG106	Potent xCT inhibitor showing efficacy in suppressing tumor growth and extending survival in KRAS-mutated lung adenocarcinoma models	Significant tumor growth inhibition, prolongs survival	Limited clinical trial data
Irinotecan (SN-38)	Mediates xCT downregulation	Rise in ROS production in FaDu HNC xenografts	A well-established chemotherapeutic agent (topoisomerase I inhibitor)
Human Cyst(e)inase	Depletes serum L-cysteine and L-cystine, extends survival in TCL1-Tg: p53-/- mice	Tumor growth inhibition, extended survival in mouse models	Limited application to specific cancer models
Ce6-erastin	Combines photodynamic therapy (PDT) and ferroptosis, enhances tumor accumulation and ROS levels under light irradiation	Increases therapeutic efficacy in oral tongue SCC	PDT’s oxygen supply issues need addressing
Sonodynamic Therapy (SDT)	Utilizes sonosensitizers and ultrasound to generate ROS, provides deep tissue penetration	Effective ROS generation, penetrates up to 10 cm into tissues	Requires specialized ultrasound equipment
Chemodynamic Therapy (CDT)	Employs Cu-based nanocatalysts for Fenton-like reactions, generating toxic hydroxyl radicals	ROS generation within tumor microenvironment, disrupts autophagic flux	Non-specific radical effects and challenges in broader application
Triptolide (TPL)	Induces GSDME-mediated pyroptosis, suppresses Nrf2/xCT axis, and promotes lipid peroxidation	Strong anti-cancer effects, particularly in HNSCC	Potential for specific cancer efficacy, possible side effects
Astaxanthin (ATX)	Enhances ROS production when combined with ionizing radiation, decreases intracellular GSH levels	Synergistic cytotoxicity against oral SCC cells, reduces tumor growth	Potential for skin irritation and other side effects

Photodynamic therapy (PDT) revolutionizes the delivery of photosensitizers to tumors, followed by targeted irradiation, while fully preserving tissue integrity [78]. PDT’s efficacy relies on the synergy of light, a light-activatable photosensitizer, and oxygen, initiating photodynamic reactions that yield cytotoxic ROS, culminating in direct tumor cell demise [79]. Its minimal systemic impact and negligible normal tissue toxicity position PDT as a compelling therapeutic avenue, boasting remarkable cosmetic outcomes and organ preservation in oncological contexts. However, the heightened levels of ROS and oxygen induced by hypoxia within the tumor microenvironment often compromise PDT’s tumoricidal potential, an obstacle that could be substantially surmounted through strategic integration with ferroptosis [80, 81]. Enter the realm of carrier-free nanodrugs, exemplified by chlorine e6 and erastin (Ce6-erastin), meticulously engineered through supramolecular interactions to deliver a dual punch of erastin-induced ferroptosis alongside PDT in oral tongue SCC [82]. Chlorin e6 (Ce6), an FDA-approved photosensitizer, achieves heightened tumor accumulation via various nanosystems, thereby augmenting its therapeutic efficacy [83]. Under light irradiation, sustained oxygen generation ensures an optimal photochemical cascade, culminating in elevated ROS levels at the tumor site. The Ce6-erastin nanoparticles harness intracellular ROS accumulation, deplete GSH reservoirs, and induce lipid peroxidation, thereby triggering ferroptosis through xCT inhibition in CAL-27 oral SCC cells and xenografts in vivo. This innovative fusion of PDT and ferroptosis heralds a promising frontier in cancer therapeutics, potentially furnishing a blueprint for more potent PDT platforms and efficacious cancer treatments.

Traditional PDT confronts challenges due to its limited tissue penetration depth and the skin phototoxicity associated with photosensitizers [84]. Enter sonodynamic therapy (SDT), an alternative modality harnessing sonosensitizers to induce ROS under ultrasound (US) stimulation, boasting an impressive tissue penetration depth of up to 10 cm [85]. The efficacy of SDT hinges on selecting optimal sonosensitizers characterized by high sonosensitizing efficiency and robust ROS generation [86]. Meanwhile, the advent of copper (Cu)-based nanocatalysts has propelled the emergence of chemodynamic therapy (CDT), a modality catalyzing the generation of toxic hydroxyl radicals within the tumor microenvironment akin to Fenton-like reactions [87]. Enter copper nanodots (Cu NDs), seamlessly amalgamating chemodynamic and sonodynamic therapies, leveraging reversible valence conversion of Cu^II/CU^I upon US stimulation to catalyze the conversion of O₂ and H₂O into hydroxyl radicals [88]. Moreover, Cu NDs intricately disrupt autophagic flux by impeding the fusion of autophagosomes and lysosomes, amplifying lipid peroxidation-induced ferroptosis and incapacitating xCT and GPX4 within HNSCC cells. Noteworthy is their real-time monitoring potential under MR imaging and their safe urinary excretion owing to inherent hydrophilicity and ultrasmall size. This multifaceted sono-sensitizer/catalyst, poised at the intersection of US-responsive spatiotemporal chemodynamic and sonodynamic therapies, presents a compelling avenue for efficient cancer management.

Nature’s pharmacopeia harbors potent compounds capable of inducing cell death by impeding xCT function and fostering lipid peroxidation accumulation. Triptolide (TPL), a natural diterpenoid epoxide extracted from the Chinese traditional herb Tripterygium wilfordii Hook f (TWHf), wields formidable anti-cancer prowess across various human malignancies [89]. Several TPL derivatives have advanced into phase I/II clinical trials for cancer management [90]. TPL administration orchestrates the robust demise of HNSCC cells via gasdermin E (GSDME)-mediated pyroptosis [91]. This multifaceted treatment regimen suppresses c-myc and mitochondrial hexokinase II expression, culminating in GSDME cleavage. Moreover, TPL intervention effectively stifles the Nrf2/xCT axis while fostering lipid peroxidation accumulation. In tandem with erastin, TPL unleashes a potent synergistic effect, profoundly impeding HNSCC growth in vitro and xenograft mouse models. Meanwhile, astaxanthin (ATX), a ketocarotenoid naturally synthesized in freshwater microalgae Haematococcus pluvialis and yeast fungus Xanthophyllomyces dendrorhous, emerges as a stalwart contender [92]. Renowned for its antioxidant supremacy, surpassing vitamin E by a staggering 550-fold, ATX enjoys recognition as a safe color additive in animal and fish feed [93]. When introduced alongside ionizing radiation (IR) in oral SCC, ATX precipitates a decline in intracellular GSH levels, coupled with the accumulation of malondialdehyde and heightened intracellular iron levels within tumor cells [94]. This strategic amalgamation of ATX and IR elicits synergistic cytotoxicity against oral SCC cells, manifesting in pronounced tumor growth suppression via xCT and GPX4 inhibition while promoting ACSL4 expression. Thus, ATX holds promise as an adjuvant in IR treatment, bolstering ferroptosis induction in oral SCC cells.

Most xCT-targeting strategies are still in preclinical stages, with limited progress to clinical trials. To date, xCT-targeting agents such as erastin and imidazole ketone erastin (IKE) are primarily in preclinical stages due to challenges like poor metabolic stability and solubility. Notable exceptions include TPL, which has advanced to phase I/II trials, and FDA-approved drugs like sulfasalazine, sorafenib, and irinotecan, which, while not specifically targeting xCT in new trials, demonstrate related therapeutic effects. Challenges hindering further clinical development include issues with metabolic stability and solubility, ensuring target specificity, and navigating regulatory and funding hurdles. These factors collectively complicate the transition from preclinical successes to effective clinical therapies. Addressing the identified challenges will be crucial for advancing these therapies from research settings to clinical practice.

Conclusions and perspectives

In conclusion, the investigation into xCT targeting for therapeutic intervention in head and neck cancer (HNC) underscores its potential as a pivotal therapeutic strategy. The multifaceted role of xCT in redox homeostasis, ferroptosis modulation, and tumor progression highlights its significance as a promising therapeutic target in HNC. The elucidation of the intricate regulatory mechanisms governing xCT expression and activity provides a foundation for developing novel therapeutic modalities to disrupt xCT-mediated pathways in HNC. Moreover, the identification of various compounds, including natural products and nanodrugs, as potent xCT inhibitors underscores the diverse arsenal available for targeted therapy in HNC.

Moving forward, future research endeavors should prioritize unraveling the molecular intricacies underlying xCT dysregulation in HNC pathogenesis and treatment resistance. Furthermore, there is an urgent need to develop highly specific and potent xCT inhibitors with enhanced therapeutic efficacy and reduced off-target effects. Clinical translation of xCT-targeted therapies necessitates robust preclinical validation and well-designed clinical trials to assess their safety, efficacy, and long-term outcomes in HNC patients. Collaboration among researchers, clinicians, and pharmaceutical industries will be pivotal in accelerating the translation of xCT-targeted therapies into clinical practice.

Looking ahead, the integration of xCT-targeted therapies into multimodal treatment regimens holds promise for improving the overall survival and quality of life of HNC patients. By harnessing the synergistic effects of xCT inhibition with conventional therapies such as chemotherapy, radiotherapy, and immunotherapy, we can overcome treatment resistance and enhance therapeutic outcomes in HNC. Furthermore, the advent of precision medicine approaches, including biomarker-driven patient stratification and personalized treatment strategies, will optimize the therapeutic efficacy of xCT-targeted therapies. Ultimately, the continued exploration of xCT targeting in cancer therapy offers a beacon of hope in pursuing more effective and tailored treatments for HNC and beyond.

Abbreviations

ACSL4

Acyl-CoA synthetase long-chain family member 4

AMPK

AMP-activated protein kinase

ARE

Antioxidant response element

ATF4

Activating transcription factor 4

BAP1

BRCA1‑associated protein‑1

BRD4

Bromodomain-containing protein 4

CD44v

CD44 variant

CDT

Chemodynamic therapy

circRNAs

Circular noncoding RNAs

EGFR

Epidermal growth factor receptor

EMT

Epithelial-mesenchymal transition

FINs

Ferroptosis inducers

GPX4

Glutathione peroxidase 4

GSH

Glutathione

GSSG

Glutathione disulfide

HNSCC

Head and neck squamous cell carcinoma

HPV

Human Papillomavirus

IKE

Imidazole ketone erastin

IL-6

Interleukin-6

Keap1

Kelch-like ECH-associated protein 1

miRNAs

MicroRNAs

MPC1

Mitochondrial pyruvate carrier 1

mTORC2

Mammalian target of rapamycin complex 2

NADPH

Nicotinamide adenine dinucleotide phosphate

NPC

Nasopharyngeal carcinoma

Nrf2

Nuclear erythroid 2-related factor

OTUB1

Ubiquitin hydrolase otubain-1

PDT

Photodynamic therapy

PLs

Phospholipids

PUFA

Polyunsaturated fatty acid

ROS

Reactive oxygen species

SLC7A11 (xCT)

Solute carrier family 7 member 11

SCC

Squamous cell carcinoma

SDT

Sonodynamic therapy

STAT3

Signal transducer and activator of transcription 3

system xc^–

Cystine/glutamate exchange transporter

Author contributions

J.L.: Conceived the structure of the article and wrote the manuscript. J.L.R.: Contributed to planning and manuscript writing, and acquired funding.

Funding

This study was supported by the National Research Foundation of Korea (NRF) grant, funded by the Ministry of Science and ICT (MSIT), Republic of Korea (No. 2019R1A2C2002259).

Data availability

No datasets were generated or analysed during the current study.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.