Recent advances on regulatory roles of HuD and promoter hypermethylation for miR-204 in lung cancer development

Abstract

Lung cancer, responsible for the highest number of cancer-related deaths globally, comprises small cell lung cancer (SCLC) and non-small cell lung cancer (NSCLC), two subtypes with fundamentally different molecular mechanisms and disease trajectories. The tumor-suppressive microRNA miR-204 is frequently inactivated in lung cancer and plays a central role in driving progression and metastasis. Recent work has uncovered striking differences in how miR-204 is downregulated in each subtype. In SCLC, the RNA-binding protein HuD stabilizes oncogenic long non-coding RNAs (lncRNAs) and PFN2 mRNA, leading to reduced miR-204-5p levels and increased cell migration. By contrast, in NSCLC, epigenetic silencing via miR-204 promoter hypermethylation decreases its expression, derepressing NUAK1, activating mTOR signaling, and promoting tumor progression. Despite operating through distinct pathways, both mechanisms converge on miR-204 loss to foster malignancy. This minireview summarizes these recent insights into the HuD-lncRNA axis and epigenetic regulation of miR-204, evaluates their clinical implications, and discusses emerging multimodal approaches that target the miR-204 network for more effective precision therapy in lung cancer.

Key Words: miR-204; HuD; Promoter methylation; Lung cancer; NUAK1; PFN2

Core Tip: miR-204 is a pivotal tumor suppressor in lung cancer, with its inactivation driven by distinct mechanisms across subtypes: In small cell lung cancer, the RNA-binding protein HuD stabilizes oncogenic long non-coding RNAs LYPLAL1-DT and PFN2 mRNA, sequestering miR-204-5p to derepress PFN2 and promote cytoskeletal remodeling and metastasis via a self-amplifying positive feedback loop. In non-small cell lung cancer, promoter hypermethylation mediated by DNA methyltransferase directly silences miR-204 transcription, derepressing NUAK1 to activate mTOR signaling and drive cancer progression. Though operating through discrete upstream pathways and targeting different downstream effectors, the both pathways converge on loss of miR-204 function to foster malignancy. This dual-track regulatory network may provide a basis for subtype-specific molecular typing, prognostic prediction, and targeted intervention strategies, offering new insights for precision therapy of lung cancer.

INTRODUCTION

According to the latest global cancer statistics, lung cancer remains the leading cause of cancer-related mortality, accounting for more than 2.3 million new cases and 1.8 million deaths annually. Lung cancer is broadly classified into small cell lung cancer (SCLC), which constitutes approximately 15% of cases and is characterized by rapid proliferation, early metastasis, and near-ubiquitous inactivation of TP53 and RB1. And non-small cell lung cancer (NSCLC), which accounts for approximately 85% of cases and typically progresses more slowly yet is frequently diagnosed at advanced stages, resulting in persistently low five-year survival rates[1]. In contrast, NSCLC constitutes approximately 85% of all lung cancer cases and encompasses various subtypes, including adenocarcinoma, squamous cell carcinoma, and large cell carcinoma. Despite the relatively slow progression of NSCLC, the absence of characteristic early symptoms and the atypical imaging presentations of certain subtypes often result in patients being diagnosed at a locally advanced stage or with distant metastases. Consequently, the five-year survival rate has persistently remained between 15% and 20%, underscoring the severe prognosis associated with this condition[2].

The onset and progression of lung cancer result from the interplay of multiple factors, including genetic variations, epigenetic modifications, and intricate disruptions in post-transcriptional regulatory networks[3]. Numerous studies have demonstrated that the inactivation of critical tumor suppressor genes, along with the activation of driver genes, forms the foundation of the molecular pathology of lung cancer[4]. For example, mutations in the TP53 and RB1 genes are nearly ubiquitous in SCLC, while mutations in KRAS, EGFR, ALK, and other genes are commonly identified in NSCLC. These genetic alterations serve as driving forces in the malignant progression of tumors by disrupting cell cycle regulation, facilitating evasion of apoptosis, and modifying signal transduction pathways[5]. Alongside genetic mutations, epigenetic modifications significantly contribute to the progression of lung cancer[6]. Mechanisms including DNA methylation, hydroxymethylation, histone modifications, and chromatin remodeling play a crucial role in regulating gene expression without altering the underlying DNA sequences, thus significantly influencing the determination of cell fate[7]. Elevated methylation levels in promoter regions are associated with the transcriptional silencing of tumor suppressor genes, whereas reduced methylation in specific enhancer regions can result in the activation of oncogenic pathways[8]. Recent studies have increasingly shown that epigenetic dysregulation is intricately linked to the aggressiveness, drug resistance, and prognosis of lung cancer[9].

The interplay between epigenetic and genetic alterations is increasingly recognized as a significant pathogenic factor in lung cancer, particularly through the disruption of non-coding RNA (ncRNA) networks[10]. ncRNA encompasses several categories, including long non-coding RNA (lncRNA), circular RNA, and various forms of microRNA (miRNA). Of these, miRNA has garnered significant attention in oncological research due to its capacity to regulate numerous target genes and finely modulate cellular functions[11]. miRNA generally consists of 19 to 25 nucleotides. By annealing to and interacting with the 3’ untranslated region (3’ UTR) of target messenger RNA (mRNA), miRNA facilitates translation inhibition or mRNA degradation, thereby modulating gene expression networks at the post-transcriptional level[12]. Beyond their well-documented intracellular functions, miRNA exert universal multi-target regulatory roles that extend across host-microenvironment interactions and systemic homeostasis, underscoring their broad biological significance[13]. miR-204 is a highly conserved endogenous miRNA, initially identified in retinal and neural tissues. It plays a critical role in regulating cellular processes, including the inhibition of cell differentiation, survival, migration, and invasion[14]. In a range of solid tumors, including breast cancer, gastric cancer, and liver cancer, miR-204 exhibits definitive tumor suppressor properties. Its reduced expression is frequently correlated with tumor progression and unfavorable prognosis[15]. In the realm of lung cancer research, a growing body of evidence indicates that miR-204 plays a critical role in inhibiting tumor growth and metastasis through the targeting of oncogenes and the regulation of signaling pathways. The inactivation of miR-204 is frequently attributed to distinct upstream mechanisms, offering a novel perspective for comprehending the heterogeneity of lung cancer[16].

In the investigation of the molecular mechanisms underlying lung cancer, RNA-binding proteins (RBPs) have increasingly been recognized as central mediators of post-transcriptional and translational regulation, elucidating their significant roles[17]. HuD (neuronal RBP HuD, ELAVL4) is a member of the ELAV family, characterized as an evolutionarily conserved RBP with a pivotal role in mRNA stability and localization during neuronal development. HuD extends the half-life of target mRNAs by binding to AU-rich elements via its RNA recognition motif (RRM), thereby preventing degradation by exonucleases. Beyond its functions in the nervous system, aberrant expression and functional alterations of HuD in various tumors have garnered significant research interest. In the context of lung cancer, HuD overexpression has been positively associated with enhanced migratory capacity and increased invasiveness of tumor cells. Notably, in small cell lung cancer, HuD not only stabilizes the oncogenic lncRNA LYPLAL1-DT but also directly binds to and prolongs the stability of PFN2 mRNA[18]. As a competitive endogenous RNA, LYPLAL1-DT functions by sequestering miR-204-5p, thereby preventing its interaction with target mRNAs and inhibiting downstream effects. This sequestration alleviates the suppressive impact of miR-204 on PFN2 expression. The subsequent upregulation of PFN2 contributes to cytoskeletal remodeling and enhances cellular motility, which in turn facilitates the invasion and metastasis of SCLC. This HuD-lncRNA-miR-204-PFN2 regulatory axis underscores the critical role of RBP in integrating post-transcriptional regulatory networks with epigenetic contexts[19].

Conversely, in non-small cell lung cancer, the inactivation of miR-204 is predominantly characterized by hypermethylation within the promoter region. This methylation of CpG islands, facilitated by DNMT, can directly obstruct the binding of transcription factors, thereby leading to a reduction in the transcriptional levels of miR-204[20]. The down-regulation of miR-204 mitigates its suppressive impact on NUAK1. NUAK1, an AMPK-associated serine/threonine kinase, has the capability to activate the mTOR signaling pathway, subsequently promoting the phosphorylation of p70S6K1 and 4E-BP1. This activation enhances protein synthesis and cellular growth, thereby conferring increased migratory and invasive capacities to NSCLC cells. Clinical investigations have further demonstrated a significant negative correlation between miR-204 expression levels in NSCLC tumor tissues and NUAK1 expression, indicating a potential pathological association within this axis[21]. It is important to highlight the substantial differences between SCLC and NSCLC regarding the mechanisms of miR-204 inactivation and the subsequent downstream effector molecules. In SCLC, miR-204 inactivation primarily occurs through the HuD-mediated lncRNA sponge mechanism, targeting downstream PFN2 to enhance cellular motility. Conversely, in NSCLC, miR-204 is inactivated via epigenetic silencing, which subsequently activates the mTOR pathway to facilitate cellular growth. Statements concerning promoter methylation in NSCLC refer to the mature miR-204 species, most commonly miR-204-5p, whereas miR-204-3p is noted separately where necessary. The phenomenon underscores the ability of a single miRNA to achieve convergence through multiple molecular networks across different tumor contexts.

This narrative mini-review is based on a structured literature search of PubMed, Web of Science, and Scopus databases performed up to May 2026. Search terms included combinations of “miR-204” OR “miR-204-5p” OR “miR-204-3p” AND (“lung cancer” OR “SCLC” OR “NSCLC” OR “small cell lung cancer” OR “non-small cell lung cancer”) AND (“HuD” OR “ELAVL4” OR “promoter methylation” OR “hypermethylation” OR “PFN2” OR “NUAK1”). Primary experimental studies (in vitro, in vivo), human tissue analyses, and relevant reviews were included. Studies not directly addressing miR-204 regulation in lung cancer subtypes were excluded. As a narrative synthesis, formal risk-of-bias assessment and meta-analysis were not performed. The evidence is therefore presented with explicit distinction between preclinical and clinical findings, and the limitations of this approach are acknowledged.

HuD-MEDIATED MIR-204 REGULATORY MECHANISM IN SCLC

SCLC represents one of the most aggressive subtypes of lung cancer, comprising approximately 15% of all diagnosed lung cancer cases[22]. The clinical characteristics of this condition include a notably short doubling time, early onset of lymph node involvement, and distant metastasis, with approximately 70% of patients presenting with extensive-stage disease at diagnosis. In contrast to NSCLC, SCLC exhibits a high frequency of tumor suppressor gene inactivation, such as the biallelic deletion of TP53 and RB1, along with extensive epigenetic remodeling at the genomic level, which underpins its high invasiveness[23]. Recent advancements in the study of ncRNA have highlighted miR-204, an evolutionarily conserved tumor suppressor miRNA, as a focal point of research due to its low expression in SCLC and its associated regulatory network. Substantial evidence suggests that reduced expression of miR-204 is closely linked to increased migratory and invasive capabilities of SCLC, as well as poor prognosis[16].

In SCLC, the RBP HuD has been identified as a critical upstream regulator of miR-204 inactivation, influencing the oncogenic signaling cascade via a distinctive dual stabilization mechanism. HuD, a member of the ELAV protein family, possesses three RRMs that specifically bind to AREs. This binding interaction delays the degradation of target RNA, consequently extending its half-life[24]. In SCLC, HuD is not only aberrantly overexpressed in tumor cells but is also detected in certain tumor-associated fibroblasts, indicating its potential role in remodeling the tumor microenvironment. Research indicates that HuD can directly bind to a specific sequence within the lncRNA LYPLAL1-DT, significantly enhancing its stability and facilitating its accumulation in the cytoplasm to levels sufficient for functioning as a competing endogenous RNA (ceRNA)[25]. The 3’ end of LYPLAL1-DT contains a site complementary to miR-204-5p, enabling it to sequester miR-204 through a “molecular sponge” mechanism. This sequestration prevents miR-204 from binding to its natural target mRNAs, thereby indirectly alleviating its inhibitory effects on downstream oncogenic factors[26].

A crucial component within the HuD-LYPLAL1-DT-miR-204 axis is PFN2[27]. PFN2 is a cytoskeletal regulatory protein involved in the dynamic equilibrium of actin polymerization and depolymerization. Overexpression of PFN2 has been shown to augment pseudopodia formation, adhesion plaque remodeling, and cellular motility, thereby facilitating migration and invasion in various malignant tumors. In the context of SCLC, the miRNA miR-204 targets the 3’ UTR of PFN2 mRNA, typically mediating translational repression or degradation under normal conditions[19]. When LYPLAL1-DT sequesters a substantial amount of miR-204-5p, the suppression of PFN2 is alleviated, leading to an increase in its expression levels. Notably, HuD not only stabilizes LYPLAL1-DT but also directly interacts with PFN2 mRNA, protecting it from exonuclease-mediated degradation. This interaction establishes a dual stabilizing effect of HuD on both LYPLAL1-DT and PFN2 mRNA. This mechanism forms a self-amplifying positive feedback loop: An increase in HuD results in elevated levels of LYPLAL1-DT and PFN2, which in turn sequesters more miR-204, further enhancing PFN2 expression and consequently promoting cell motility[28]. This feedback loop is notably prominent in SCLC and may represent a significant contributing factor to its high degree of invasiveness. The functional experiments conclusively demonstrated the carcinogenic potential of the aforementioned regulatory axis. In SCLC cell lines, exogenous overexpression of HuD/ELAVL4 significantly upregulated LYPLAL1-DT and PFN2, reduced miR-204-5p levels, and enhanced migration and invasion. These effects were confirmed in xenograft models where HuD-overexpressing cells formed larger tumors with elevated PFN2 expression[29]. Conversely, the suppression of HuD or the targeting of LYPLAL1-DT using antisense oligonucleotides can reestablish the expression of miR-204 and inhibit PFN2, thereby attenuating cellular migration and invasion capabilities. In an in vivo study utilizing a subcutaneous transplanted tumor model in nude mice, it was observed that the tumor volume and weight generated by HuD-overexpressing. SCLC cells were significantly greater compared to the control group. Immunohistochemical analysis revealed an upregulation of PFN2 protein expression, and the presence of more invasive pseudopod-like structures at the tumor margins[30]. These findings were corroborated by in vitro data, thereby establishing the pivotal role of the HuD-lncRNA-miR-204-PFN2 axis in the malignant progression of SCLC.

The analysis of clinical samples has further substantiated the pathological correlation of this axis. Multiple independent studies have examined surgical or biopsy specimens from patients with SCLC and have consistently observed that the mRNA or protein levels of HuD, LYPLAL1-DT, and PFN2 are elevated in tumor tissues compared to adjacent non-tumorous tissues[31]. Furthermore, the elevated expression of these markers is significantly associated with reduced progression-free survival and overall survival in patients[32]. Receiver operating characteristic curve analysis demonstrated that the simultaneous detection of the three markers could improve the discriminatory power for identifying high-risk SCLC patients, indicating their potential utility as prognostic biomarkers[33]. Multi-marker panels consistently outperform single biomarkers because they capture complementary molecular dimensions of tumor biology, thereby increasing both diagnostic sensitivity and specificity while reducing the impact of inter-patient heterogeneity. Similar combined-detection strategies have proven beneficial in other solid tumors[34]. Furthermore, a concurrent elevation in HuD and PFN2 levels was observed in the chemotherapy-resistant SCLC subpopulation, implying that this axis may be linked to treatment resistance. This association warrants further investigation in future studies.

PROMOTER HYPERMETHYLATION REGULATES MIR-204 IN NSCLC

In contrast to the highly invasive SCLC, NSCLC exhibits a distinct pattern of miR-204 inactivation at the molecular level[35]. Numerous studies have demonstrated that the reduced expression of miR-204 in NSCLC is predominantly attributed to the widespread epigenetic modification of promoter hypermethylation, rather than being primarily influenced by RBP or lncRNA sponge mechanisms[36]. DNA methylation involves the addition of a methyl group to the fifth carbon atom of cytosine within CpG dinucleotides, resulting in the formation of 5-methylcytosine, a process largely catalyzed by DNMT (Such as DNMT1, DNMT3A, DNMT3B)[37]. Promoter hypermethylation or its proximal CpG islands impedes transcription factor binding, attracts methylation-binding proteins, and induces the formation of a compact chromatin structure, ultimately resulting in transcriptional silencing. This epigenetic regulatory mechanism is notably prominent in adenocarcinoma and squamous cell carcinoma of NSCLC, with a higher methylation degree frequently observed in cases associated with smoking[38].

In NSCLC, the prevalence of hypermethylation of the miR-204 promoter ranges from 40% to 60%, demonstrating a positive correlation with tumor stage and lymph node metastasis. Techniques such as methylation-specific PCR and pyrosequencing, among others, have been extensively employed to detect this epigenetic marker. It has been established that reduced expression of miR-204 is strongly associated with a high level of methylation[39]. The transcriptional suppression of miR-204 functionally alleviates its inhibitory effect on various oncogenic target genes, among which NUAK1 is particularly significant. NUAK1 is a member of the AMPK-related serine/threonine kinase family and plays a critical role in cytoskeletal remodeling, cell migration, and stress adaptation[40]. Under physiological conditions, miR-204 modulates the protein expression of NUAK1 by interacting with the 3’ UTR of NUAK1 mRNA, thereby inhibiting its translation or promoting its degradation.

The downregulation of miR-204 expression, attributed to promoter methylation, results in the alleviation of NUAK1 inhibition, thereby leading to an upregulation of its protein expression. Subsequently, the activated NUAK1 is capable of phosphorylating and activating the mTOR signaling pathway[21]. The mTOR functions as a central intracellular hub for sensing nutritional and growth factor signals, integrating various upstream pathways including PI3K/AKT and MAPK. It facilitates ribosomal biosynthesis and cap-dependent translation initiation through the phosphorylation of downstream effectors such as p70S6K1 and 4E-BP1, thereby accelerating protein synthesis. This process enhances the cell cycle progression and metabolic activity. In NSCLC cells, aberrant activation of the mTOR pathway is closely associated with increased cell volume, enhanced migratory capacity, and an invasive phenotype[41]. Experimental studies have shown that artificially restoring miR-204 expression, either through miR-204 mimic introduction or demethylation treatment, in NSCLC cell lines such as A549 and H1299, significantly reduces NUAK1 and phosphorylated mTOR levels. This restoration inhibits the phosphorylation of p70S6K1 and 4E-BP1, ultimately diminishing the cells’ migratory and invasive capabilities. Conversely, the knockdown of miR-204 enhances the activity of the aforementioned signaling pathway[42].

In contrast to the oncogenic axis of HuD-lncRNA-miR-204-PFN2 in SCLC, the inactivation mechanism of miR-204 in NSCLC is predominantly characterized by epigenetic silencing. This silencing is primarily driven by DNMT-mediated promoter methylation, as opposed to the stabilizing influence of RBP. Furthermore, the downstream effector molecules differ between the two cancer types; SCLC primarily involves the cytoskeleton regulatory factor PFN2, whereas NSCLC facilitates growth and invasion by activating the mTOR signaling pathway through NUAK1[43]. Nevertheless, the convergence between the two phenomena is evident in the functional loss of miR-204, which ultimately results in the activation of pro-cancer signaling pathways and facilitates the migration and invasion of lung cancer cells. This intersection underscores the pivotal role of miR-204 as a cross-subtype tumor suppressor node. Furthermore, it implies that restoring miR-204 expression or reversing its epigenetic silencing could possess broad-spectrum therapeutic potential against lung cancer.

In lung squamous cell carcinoma (LUSC) and lung adenocarcinoma (LUAD), promoter hypermethylation of miR-204 can act synergistically with subtype-specific ceRNA mechanisms. In LUAD, the oncogenic lncRNA LINC00483 is markedly overexpressed, sponging miR-204-3p and thereby derepresses ETS1, promoting proliferation, migration and invasion. Concurrently, promoter methylation further lowers miR-204-3p levels and is associated with poorer survival[44,45].

In LUSC, LINC01980 similarly functions as a ceRNA that directly inhibits miR-204-3p, its knockdown suppresses malignant phenotypes in a miR-204-3p dependent manner. When both hypermethylation and elevated LINC01980 expression coexist, miR-204-3p function is more completely lost, resulting in greater tumor aggressiveness and reduced chemotherapy response. In addition to the NUAK1-mTOR axis common to NSCLC, miR-204-3p targets are dependent on the tissue context (Table 1). For example, ETS1 in LUAD and SULF2 in models of acute lung injury, illustrating the functional versatility of this miRNA across pulmonary pathologies[46].

Table 1 Dysregulation of miR-204 and downstream oncogenic axes in small cell lung cancer and non-small cell lung cancer subtypes.

Comparison item	Mechanism	Upstream driving factors	The influence on miR-204	Main downstream effector molecules	Clinical significance	Ref.
SCLC	HuD-regulated axis	HuD binds and stabilizes LYPLAL1-DT and PFN2 mRNA	Indirectly inhibit miR-204-5p	PFN2	Elevated levels of HuD, LYPLAL1-DT, and PFN2 correlate with reduced PFS/OS	[19,29]
NSCLC (LUAD/LUSC)	DNMT-mediate promoter hypermethylation	Promoter hypermethylation	Directly inhibit miR-204-5p	NUAK1 → Activate the mTOR pathway	Increased NUAK1 expression and poor prognosis	[21,40]
	LINC00483 axis (LUAD)	Significantly upregulate LINC00483	Sponging, miR-204-3p	ETS1	High LINC00483 expression correlates with shorter LUAD survival, advanced TNM stage, larger tumors, and positive lymph node metastasis	[44,45]
	LINC01980 axis (LUSC)	High expression of LINC01980	Directly inhibit miR-204-3p	The specific target gene has not been clearly identified	Elevated LINC01980 expression associated with increased malignancy in LUSC	[46]

SCLC: Small cell lung cancer; NSCLC: Non-small cell lung cancer; LUAD: Lung adenocarcinoma; LUSC: Lung squamous cell carcinoma.

DISCUSSION

miR-204 is identified as a definitive tumor suppressor miRNA in the two primary subtypes of lung cancer, SCLC and NSCLC. Its functional loss not only results in increased proliferation, migration, and invasion capabilities of tumor cells but is also significantly associated with reduced patient survival and elevated recurrence risk. This suggests that the inactivation of miR-204 is a critical factor in the progression of lung cancer[47]. It is important to highlight that, despite the distinct mechanisms of miR-204 inactivation in SCLC and NSCLC, both result in the same functional loss of miR-204, subsequently unleashing downstream pro-cancer signals and promoting malignant phenotypes. In SCLC, the inactivation mechanism involves the RBP HuD, which stabilizes a dual regulatory axis consisting of the oncogenic lncRNA LYPLAL1-DT and PFN2 mRNA. Conversely, in NSCLC, inactivation is directly initiated by promoter hypermethylation, leading to transcriptional silencing. This “multiple input-single output” convergence model underscores the dual characteristics of miRNA within the tumor network, highlighting their roles as both adaptable and central components (Figure 1).

Figure 1 Dual-track regulatory mechanisms of miR-204 inactivation in small cell lung cancer and non-small cell lung cancer. In small cell lung cancer (left panel), HuD binds to and stabilizes both the oncogenic LYPLAL1-DT (lncRNA) and PFN2 mRNA. LYPLAL1-DT functions as a competing endogenous RNA that indirectly inhibits miR-204-5p, thereby relieving miR-204-5p-mediated repression of PFN2. Elevated PFN2 promotes actin cytoskeletal remodeling, cancer cell proliferation, migration, and invasion. HuD further stabilizes PFN2 mRNA, forming a self-amplifying positive-feedback loop. In non-small cell lung cancer (right panel), DNMT-mediate promoter hypermethylation, directly suppressing miR-204 transcription, leading to NUAK1 upregulation. It activates the mTOR signaling cascade, leading to phosphorylation of 4E-BP1 and p70S6K1, enhanced protein synthesis and cancer progression. The both pathways converge on loss of miR-204 function despite distinct upstream mechanisms. SCLC: Small cell lung cancer; NSCLC: Non-small cell lung cancer.

At the level of commonality, the suppression of miR-204 in both SCLC and NSCLC markedly increased the motility and invasiveness of tumor cells, and demonstrated a consistently poor prognostic correlation with its reduced expression in clinical samples. For example, elevated expression levels of HuD, LYPLAL1-DT, and PFN2 in SCLC are linked to accelerated tumor growth and early metastasis[19]; In NSCLC, the methylation status of the miR-204 promoter was concurrently observed with elevated NUAK1 expression and activation of the mTOR signaling pathway, serving as a prognostic indicator of poorer survival outcomes[21]. These findings enhance the recognition of miR-204 as a cross-subtype tumor suppressor factor and propose that evaluating its expression or activity may serve as a potential biomarker for lung cancer classification and risk assessment.

SCLC and NSCLC exhibit distinct histological and molecular characteristics in the upstream mechanisms leading to miR-204 inactivation. In SCLC, there is a pronounced reliance on the HuD-mediated lncRNA sponge mechanism. Specifically, HuD interacts with and stabilizes LYPLAL1-DT via its RRM domain. This stabilization facilitates the sequestration of miR-204-5p, thereby alleviating its suppression of PFN2. Concurrently, HuD directly binds to PFN2 mRNA, creating a positive feedback loop that enhances the pro-invasive signaling pathway. This post-transcriptional regulatory model, orchestrated by RBP, is particularly significant in SCLC with neuroendocrine features. Conversely, in NSCLC, miR-204 silencing is predominantly attributed to promoter hypermethylation mediated by DNMT. This epigenetic mechanism is prevalent in both adenocarcinoma and squamous cell carcinoma subtypes and is partially correlated with smoking history and specific driver mutations, such as those in the EGFR gene[39]. By alleviating the inhibition of NUAK1, the mTOR-P70S6K1/4E-BP1 axis is subsequently activated downstream, thereby enhancing protein synthesis and promoting cell growth. This variation underscores the fundamental differences among various subtypes of lung cancer with respect to gene expression regulatory networks, epigenetic landscapes, and signaling preferences, and also offers a foundation for precise therapeutic interventions.

These two pathways elucidate the quintessential attributes of the “multi-input-multi-output” miRNA network within diverse tumor contexts: A single miRNA is capable of integrating multiple signals from RBP, epigenetic modifications, and transcription factors, thereby achieving convergent pathological outcomes through the targeting of various effector molecules. The distinctive function of HuD in SCLC warrants particular attention, as it not only contributes to the regulation of RNA stability but may also influence the overall structure of the ceRNA network by modulating the spatial distribution of lncRNAs[48]. Future research should investigate the differential expression of HuD across various molecular subtypes of SCLC, examine its synergistic or antagonistic interactions with other RBPs, and explore its potential involvement in signal transduction within the tumor microenvironment. Such studies would enhance our comprehension of the multifunctional roles of RBPs in the pathogenesis of lung cancer.

Clinically, the integrated assessment of miR-204 expression and methylation status, along with HuD, LYPLAL1-DT, and PFN2 levels, is anticipated to facilitate the development of a comprehensive molecular typing and prognostic prediction model, thereby enhancing the capacity for early detection and risk stratification of lung cancer. Regarding intervention strategies, for SCLC, the formulation of HuD functional inhibitors or LYPLAL1-DT targeted degradation approaches could be pursued to disrupt the HuD-lncRNA-miR-204-PFN2 axis[49]. In the context of NSCLC, the application of demethylating agents, such as 5-azacitidine, or miR-204 mimics may be investigated to restore their tumor suppressor functions[50]. Moreover, since both pathways ultimately converge on the functional restoration of miR-204, cross-subtype miR-204 replacement therapies could provide a unified platform for combination therapy, potentially enhancing therapeutic efficacy and overcoming the challenges posed by subtype heterogeneity. But any future subtype-specific sampling or intervention strategy would need to distinguish SCLC from NSCLC (within NSCLC, distinguish adenocarcinoma from squamous-cell carcinoma) at the time of biopsy or resection. Feasibility is currently limited by the requirement for adequate tissue for both histologic subtyping and molecular assays (LYPLAL1-DT quantification or promoter methylation analysis), and by the absence of prospectively validated thresholds.

In addition, current evidence is limited by preclinical-only data, heterogeneity, incomplete strand discrimination, unresolved delivery and toxicity barriers, along with the absence of prospective trials, rendering liquid biopsy and AI applications speculative.

CONCLUSION

While SCLC and NSCLC exhibit distinct mechanisms regarding the inactivation of miR-204, both underscore the pivotal role of miR-204 as a cross-subtype tumor suppressor hub. The HuD-lncRNA axis and promoter methylation constitute the two principal paradigms of miRNA regulation in lung cancer. Comprehensive research in these areas not only broadens the theoretical framework of miRNA network regulation but also paves the way for precise classification, prognostic evaluation, and personalized treatment of lung cancer. With advancements in multi-omics technologies and targeted interventions, the dual-track regulatory network of miR-204 in lung cancer has evolved from mechanistic understanding to clinical application, establishing a novel paradigm for addressing the issue of lung cancer heterogeneity. This advancement is poised to redefine the trajectory of precision treatment for lung cancer and is anticipated to substantially enhance patients’ quality of life, thereby inaugurating a new era in lung cancer diagnosis and treatment, transitioning from mere “suppression” to active “regulation”[51].

Future research should systematically delineate the complete target spectrum of HuD in SCLC and conduct a comprehensive analysis of the interaction network between HuD, lncRNAs, and mRNAs utilizing high-throughput methodologies such as CLIP-seq, RNA pull-down, and RIP-seq. It is imperative to elucidate HuD’s binding preferences and functional variations across different SCLC molecular subtypes. Such investigations will not only uncover novel mechanisms beyond HuD’s regulation of miR-204 but also potentially identify previously unrecognized targets associated with the tumor microenvironment and drug resistance. SCLC is characterized by high invasiveness, propensity for metastasis, and a current paucity of effective therapeutic targets. Therefore, a thorough understanding of HuD’s multifunctional role as an RBP, particularly its interactions with ncRNA, may offer new avenues for therapeutic intervention. Given HuD’s extensive regulatory effects, the development of functional inhibitors targeting HuD and strategies for the degradation of lncRNAs represent promising directions for intervention, potentially providing novel therapeutic approaches for SCLC[52].

Epigenetic interventions continue to be of substantial importance in addressing the hypermethylation of the miR-204 promoter in NSCLC. As a predominant subtype of lung cancer, NSCLC is characterized by genomic and epigenetic modifications that significantly contribute to tumorigenesis and progression, with promoter methylation being particularly prevalent. The application of low-dose, more controllable demethylating agents, such as 5-Aza-2’-deoxycytidine (5-Aza-CdR) and its derivatives, or novel DNMT inhibitors, is anticipated to reinstate miR-204 expression while minimizing severe toxicities and side effects[50]. These agents may also enhance the efficacy of targeted therapies or immunotherapies through synergistic interactions. Notably, 5-Aza-CdR, a commonly employed demethylating agent, has demonstrated potential in augmenting therapeutic responses across various tumor types. By reactivating miR-204, its tumor suppressor functions can be restored, potentially reversing the malignant phenotype and thereby enhancing the therapeutic outcomes in NSCLC[53].

Building upon the dual-track inactivation mechanism of miR-204 in lung cancer, future therapeutic strategies may be developed as comprehensive cross-subtype regimens. For example, in SCLC, targeting the HuD-lncRNA axis is proposed, whereas in NSCLC, the focus is on demethylation, followed by the administration of miR-204 mimics to uniformly restore its tumor suppressor function. This approach may address the heterogeneity among subtypes and enhance therapeutic efficacy. Currently, treatment strategies for lung cancer are challenged by tumor heterogeneity, particularly in achieving precise targeting across different subtypes. By concurrently addressing various miRNA inactivation mechanisms, a comprehensive intervention across subtypes can be realized, thereby improving therapeutic outcomes[54]. Furthermore, liquid biopsy, as a non-invasive monitoring technique, presents the potential for real-time dynamic assessment of miR-204 methylation or HuD expression levels, offering a novel non-invasive tool for evaluating treatment efficacy and providing early warnings of recurrence. The implementation of this strategy may lead to significant advancements in the early diagnosis, treatment evaluation, and recurrence prediction of lung cancer[55]. To further modernize dynamic monitoring, the integration of artificial intelligence (AI) with multimodal data, including genomics, imaging and clinical indicators, which holds considerable promise for optimizing risk stratification and guiding precision treatment strategies in lung cancer[56].

Despite the therapeutic promise of restoring miR-204 function through mimics or replacement strategies, the clinical translation of RNA-based therapeutics is hindered by inherent delivery barriers, including rapid nuclease-mediated degradation, poor cellular permeability, and potential systemic toxicity. Recent advances in targeted nanomedicine platforms provide effective solutions to these challenges. Lipid nanoparticles, extracellular vesicles (exosomes), and functionalized biomaterials have shown remarkable capability in encapsulating and protecting nucleic acids, facilitating tumor-specific accumulation, minimizing off-target effects, and enabling controlled release within the tumor microenvironment. These delivery systems, as comprehensively reviewed in emerging nanomedicine strategies for solid tumor therapy, can be readily adapted to improve the safety and efficacy of miR-204-based interventions across lung cancer subtypes[57]. Because SCLC and NSCLC display profound intratumoral and microenvironmental heterogeneity, bulk sequencing or cell-line data may mask subpopulation-specific dynamics. Integrating single-cell RNA sequencing with pathway-activity-based cell-discrimination algorithms offers a powerful means to delineate cell-type-specific miR-204 regulation within the lung-cancer microenvironment, thereby refining our understanding of how the HuD-lncRNA and promoter-methylation axes operate in distinct cellular niches[58].