Epigenetic modifications refer to changes in the expression and function of genes that regulate without any modification of the DNA sequence, mainly including DNA methylation, histone modifications, and noncoding RNA expression [33]. Compared to stable genetic events, epigenetic changes are highly malleable. Studies have found that epigenetic modifications may contribute to the development of multiple cancer types, which play a crucial role in the occurrence and progression of lung cancer [34, 35]. Moreover, there is growing evidence of a clear link between epigenetic modifications and cisplatin resistance in lung cancer [36] (Fig. 3), for example abnormal DNA methylation status, overexpression or downregulation of microRNAs, etc.

Epigenetic regulation is an important mechanism of cisplatin resistance in lung cancer

The figure systematically summarizes several key epigenetic regulatory mechanisms involved in cisplatin resistance in lung cancer. It illustrates the functional roles of DNA methylation, histone modifications, and noncoding RNAs (including miRNAs, lncRNAs, and circRNAs) in modulating gene expression: (1) DNA methylation silences tumor suppressor genes (e.g., NNT, HOXA11); (2) histone modifications (e.g., EZH2-mediated H3K27me3) alter chromatin structure; and (3) noncoding RNAs (miRNAs, lncRNAs, circRNAs) regulate drug efflux and apoptosis pathways. These epigenetic alterations can affect the transcriptional activity of critical genes, thereby influencing cellular processes such as apoptosis, DNA repair, drug uptake, and efflux, ultimately leading to cisplatin resistance.

##EZH2, enhancer of zeste homolog 2; ERCC1, excision repair cross-complementation group 1; H3K27me3, trimethylation of histone H3 lysine 27; PD-L1 programmed death-ligand 1; NNT, nicotinamide nucleotide transhydrogenase; HOXA11, homeobox A11; FOXF1, forkhead box F1; MALAT1, metastasis-associated lung adenocarcinoma transcript 1; PUMA, p53 upregulated modulator of apoptosis; JMJD1B, jumonji domain-containing protein 1B; PCNA, proliferating cell nuclear antigen.

DNA methylation is an important epigenetic modification that affects gene expression, transcription, and activity [37]. This process involves the transfer of methyl groups to the C5 position of cytosine, forming 5-methylcytosine, and is primarily catalyzed by DNA methyltransferases (DNMTs). Hypermethylation of gene promoters often leads to transcriptional repression, resulting in reduced gene expression [38, 39]. Aberrant methylation patterns, such as genome-wide hypomethylation and hypermethylation in specific gene promoter regions, are commonly observed in tumors, this indicates that abnormal methylation patterns are involved in the development and progression of tumors [40, 41]. In normal cells, however, CpG islands are generally unmethylated [42].

Several studies have shown that hypermethylation of DNA is strongly associated with cisplatin resistance [43,44,45]. Through comprehensive analysis of DNA methylation and mRNA expression profiles, researchers found that DNA methylation levels were significantly higher in cisplatin-resistant tumor samples for NSCLC [46]. Nicotinamide ribotide transhydrogenase (NNT) is a mitochondrial matrixlocalized enzyme [47]. Hypermethylation of tumor suppressor genes such as NNT and HOXA11 leads to their downregulation, promoting cisplatin resistance. For instance, NNT is silenced by DNA hypermethylation in cisplatin-resistant lung cancer cells, which promotes autophagy and cisplatin resistance [48]. Similarly, HOXA11 is hypermethylated and downregulated in cisplatin-resistant cells, while the expression level of HOXA11-AS is increased. HOXA11 is a member of the homeobox (HOX) gene family, HOXA11-AS, an oncogenic long-chain noncoding RNA, which is a natural antisense transcript of HOXA11 [49]. When the HOXA11 gene is knocked down, it induces cisplatin resistance in lung cancer cells; conversely, knocking down the HOXA11-AS gene increases the sensitivity of lung cancer cells to cisplatin. This inverse interaction between HOXA11 and HOXA11-AS promotes the occurrence of cisplatin resistance [50]. Table 1 summarizes the key genes associated with cisplatin resistance through DNA methylation, including NNT, HOXA11, and FOXF1.

It enumerates multiple DNA methylation-regulated target genes associated with cisplatin resistance in lung cancer, along with their regulatory mechanisms. Notably, genes including NNT, HOXA11, and CRTAC1exhibit significant hypermethylation in cisplatin-resistant cells, resulting in their transcriptional silencing. This downregulation consequently impairs critical cellular processes such as apoptosis, and transporter protein expression, ultimately enhancing tumor cell chemoresistance. Conversely, other genes (e.g., FOXF1 and MDR1) demonstrate hypomethylation-mediated overexpression, which similarly contributes to drug resistance.

Cisplatin resistance is mainly attributed to the hypermethylation of gene promoters. Methyltransferase inhibitors can reverse this hypermethylation, restoring cisplatin sensitivity. For instance, decitabine (DAC) and 5-aza-2’-deoxycytidine (5-AzadC) are methyltransferase inhibitors capable of reversing drug resistance. Low doses of 5-aza-2’-deoxycytidine and trichostatin A (TSA) can reduce DNA methylation levels and restore gene expression, thereby reversing resistance in A549/DDP cells [46]. Another methyltransferase inhibitor, decitabine (DAC), has been proved to reverse cisplatin resistance through demethylation in bladder cancer cells [45]. Moreover, the combination of methyltransferase inhibitors and cisplatin has shown a synergistic effect. For example, in cisplatin-resistant lung cancer cells, the DNA methyltransferase inhibitors azacytidine (AZA) and cisplatin together increased apoptosis by regulating levels of the metastasis inhibitor KiSS-1 [51]. Similarly, CRISPR/dCas9-based DNA methylation editing methods have demonstrated high efficiency and specificity in altering DNA methylation levels [52].

DNA methylation abnormalities in certain genes can serve as clinical biomarkers for predicting cisplatin efficacy in lung cancer and may even represent potential therapeutic targets after drug resistance develops. Analysis of the Gene Expression Omnibus (GEO) database in NSCLC patients revealed that Cartilage Acidic Protein 1 (CRTAC1), which is highly expressed in normal lung tissues, undergoes promoter hypermethylation leading to its downregulation in tumors. Functional studies demonstrated that CRTAC1 overexpression enhances cisplatin sensitivity by inducing Ca2⁺-dependent Akt1 degradation and promoting apoptosis [53]. Clinical data further supported that patients with high CRTAC1 expression exhibited significantly prolonged overall survival (OS) following cisplatin treatment, suggesting CRTAC1 as a promising predictive biomarker for cisplatin response. Additionally, hypermethylation of tumor suppressor genes plays a crucial role in cisplatin resistance. For instance, in NSCLC specimens, unmethylated IGFBP-3 promoter status was associated with prolonged disease-free survival (DFS) in stage I patients, indicating that IGFBP-3 methylation status may serve as a clinical biomarker for cisplatin efficacy [54]. Similarly, CDH13 promoter hypermethylation is frequently observed in NSCLC tissues, and demethylation-induced restoration of CDH13 expression reverses cisplatin resistance, highlighting its potential as both a predictive marker and a therapeutic target [55].

In summary, DNA hypermethylation is closely associated with cisplatin resistance in lung cancer. Inhibiting DNA methylation may reverse this resistance, potentially offering a promising treatment strategy for cisplatin-resistant lung cancer.

Noncoding RNA refers to RNA molecules that do not have the ability to encode proteins, including microRNA, long noncoding RNA, and circular RNA. These noncoding RNAs play crucial roles in various biological processes such as gene expression regulation, cell differentiation and development, and tumor occurrence and progression.

MiRNAs are single-stranded noncoding RNA molecules about 19–25 nucleotides long that regulate target genes by binding to their 3'untranslated region, inhibiting translation or promoting transcript degradation [56]. The role of miRNAs in the response of tumor cells to cisplatin and the development of cancer is becoming increasingly clear. Research has found that the expression of miRNAs in drug-resistant cancer cells differs significantly from that in drug-sensitive cells [57]. Aberrant expression of miRNAs has been associated with cisplatin sensitivity in lung cancer (Table 2).

This table summarizes the noncoding RNAs that are currently strongly associated with cisplatin sensitivity in lung cancer, detailing their expression alterations, target molecules, and functional mechanisms.

Regulatory effects of miRNAs: The expression of MiR-155 and MiR-488 is upregulated, acting on TP53 and eIF3a, respectively, inhibiting apoptosis and activating the NER pathway to enhance drug resistance. Expression of MiR-182-5p, MiR-27b, and MiR-204 is downregulated and is involved in drug resistance regulation by activating survival signaling, affecting EMT, or restoring apoptosis signaling.

Mediation mechanisms of LncRNAs: UCA1, HOXA-AS3, MALAT1, Linc00665, and HOXA11-AS are all upregulated, and they target different molecules or pathways to promote drug resistance by enhancing DNA repair, mediating EMT, activating signaling pathways, leading to cell cycle dysregulation, and upregulating anti-apoptotic genes.

CircRNA involvement pathways: Upregulation of the expression of CircPVT1, Circ_PIP5K1A, and Circ_CPA4, which enhance drug resistance by activating transporter efflux, modulating resistance pump function, and mediating immune escape, respectively. Hsa_circ_0001946, CircsSMARCA5 expression is downregulated and participates in the drug resistance process by affecting DNA damage repair and inhibiting tumor progression. In summary, the above epigenetic molecules are involved in drug resistance through various pathways such as regulating apoptosis, DNA repair, EMT, drug efflux, and immune escape.

For example, the oncogene miR-155 is upregulated in multiple human cancers and further increased in treatment-resistant tumors [58]. And, miR-155 has been shown to induce resistance to various chemotherapeutic agents, and its downregulation can resensitize tumors to chemotherapy. This process is mainly mediated through the miR-155/TP53 feedback loop. A combination of high miR-155 expression and low TP53 expression is significantly associated with shorter survival in lung cancer patients [59]. TP53, also known as p53, is a tumor suppressor gene located on human chromosome 17p13 [60]. In addition, miRNA-488 was found to be more expressed in A549/DDP-resistant cells and is associated with cisplatin resistance in two other NSCLC cells, H1299 and SK-MES-1 [61]. Conversely, some miRNAs are downregulated in cisplatin-resistant lung cancer cells. For instance, miR-182-5p is downregulated in cisplatin-resistant lung adenocarcinoma cells and modulates cisplatin resistance by directly targeting GLI2 in the hedgehog signaling pathway (HHSP) [62]. GLI2 is a transcriptional regulator of the hedgehog pathway, which is commonly reactivated in cancer [63]. Human bone marrow mesenchymal stem cell-derived exosome (BMSC-Exo) miR-193a can inhibit the progression of non-small cell lung cancer and promote the expression of miR-193a by downregulating its target gene LRRC1, thereby reducing cisplatin resistance in NSCLC cells [64].

MiRNAs contribute to cisplatin resistance in lung cancer by targeting specific genes and proteins. Compared to NSCLC patient samples, miR-27b is expressed at lower levels in adjacent normal tissue controls. Snail, as a target protein of miR-27b, is inhibited when miR-27b is highly expressed, and miR-27b increases the sensitivity of NSCLC cells to cisplatin and promotes epithelial–mesenchymal transition in lung cancer by targeting Snail [65]. miRNAs are also involved in multiple signaling pathways related to cisplatin resistance in lung cancer. For example, miR-29c negatively regulates the PI3K/Akt pathway to inhibit cisplatin resistance in NSCLC cells, while miR-204 reduces cisplatin resistance in NSCLC by inhibiting the caveolin-1/AKT/Bad pathway [66, 67]. Moreover, miRNAs can interact with each other and synergize with lncRNAs and circRNAs to influence chemosensitivity. For instance, Linc00173 acts as a competitive endogenous RNA (ceRNA) by “sponging” miRNA-218, thereby regulating the expression of Etk (Epithelial tyrosine kinase) and contributing to chemotherapy resistance in small cell lung cancer [68].

Long noncoding RNAs (lncRNAs) are a class of non-protein-coding transcripts longer than 200 nucleotides [69]. LncRNAs can influence the sensitivity of lung cancer cells to cisplatin through various mechanisms, including modulating the expression of key genes and interacting with miRNAs [70].

Aberrant expression of lncRNAs plays a significant role in tumorigenesis, development, and metastasis [71]. For instance, overexpression of lncRNA UCA1 has been shown to increase the proliferation and migration abilities of lung adenocarcinoma cells, enhance cisplatin resistance, and is more highly expressed in A549/DDP cells and lung adenocarcinoma tissues. This overexpression may be linked to increased levels of proliferating cell nuclear antigen (PCNA) and excision repair cross-complementation group 1 (ERCC1) [72]. Similarly, lncRNA UCA1 is upregulated in the tissues and cell lines of cisplatin-resistant ovarian cancer patients, including serum exosomes, and is involved in cisplatin resistance through the miR-143/FOSL2 pathway regulation [73]. Given its role in cancer progression and cisplatin resistance, UCA1 may serve as a potential diagnostic marker and therapeutic target. HOXA-AS3 is another lncRNA that is overexpressed in various human cancers, including lung cancer [74]. It is also significantly upregulated in lung cancer tissues and cells, and promotes cancer cell proliferation [75]. Recent findings suggest a potential link between HOXA-AS3 overexpression and cisplatin resistance. This overexpression enhances cisplatin resistance and induces EMT by downregulating HOXA3 expression [76]. LncRNAs can indirectly regulate ABC transporter gene expression through “sponge” miRNAs, or directly affect their chromatin status and promoter activity, thereby enhancing multidrug resistance as a whole [77]. LINC00707 has previously been reported to be an oncogene able to promote lung adenocarcinoma cell proliferation and metastasis. However, silencing LINC00707 can enhance cisplatin sensitivity in cisplatin-resistant NSCLC cells by inhibiting MRP1 and P-gp through spongy miR-145 [78].

MALAT1, also known as NEAT2, is an extensively studied lncRNA in cancer [79]. First identified in NSCLC patients, MALAT1 is upregulated in relation to cell proliferation, invasion, metastasis, evasion of apoptosis, and DNA repair defects, indicating its role as an oncogene [80, 83]. MALAT1 is closely linked to the development of drug resistance and has been shown to be upregulated in A549/DDP-resistant cell lines, reducing cisplatin sensitivity both in vitro and in vivo. This is likely due to the activation of STAT3, which upregulates MRP1 and MDR1 [81]. Additionally, MALAT1 forms a feedback loop with miR-101 and SOX9 to modulate cisplatin resistance through the Wnt signaling pathway [82]. Beyond cisplatin, MALAT1 has also been implicated in resistance to other chemotherapy drugs, such as gemcitabine, further enhancing resistance in NSCLC cells [84, 85]. MALAT1 could serve as a comprehensive target for cisplatin-resistant therapy in lung cancer. lncRNAs are key factors regulating cisplatin sensitivity in lung cancer (Table 2).

Circular RNAs (circRNAs) are covalently closed molecules that interact with mRNA and act as molecular sponges to regulate gene expression [86]. These RNAs have been found to be differentially expressed in tumors and normal tissues, including lung cancer, suggesting they may contribute to tumor development [87, 88]. Recent studies have shown that circRNAs are closely related to cisplatin resistance and may serve as novel therapeutic targets [89,90,91] (Table 2).

Some circRNAs are aberrantly expressed in resistant tissues or cell lines in lung cancer. For example, circPVT1 is highly expressed in lung adenocarcinoma and upregulated in cisplatin- and pemetrexed-resistant tissues and cell lines. Knocking down circPVT1 expression can resensitize resistant cells to these drugs. CircPVT1 acts as a competitive endogenous RNA (ceRNA) for miR-145-5p, which targets ABCC1, thus contributing to cisplatin resistance by regulating the miR-145-5p/ABCC1 axis [92]. CircPVT1 is also clinically significant in NSCLC patients treated with cisplatin and gemcitabine. The therapeutic effect can be assessed by detecting circRNA expression in serum, as chemoresistant patients have reduced circPVT1 expression post-treatment compared to chemosensitive patients [93]. MiR-545-3p can inhibit the proliferation and migration of NSCLC cells and promote cisplatin-induced apoptosis. Circ_0072083 enhances the cisplatin inhibitory effect in NSCLC cells through the miR-545-3p/CBLL1 axis [94]. Some downregulated circRNAs are also involved in cisplatin resistance. For instance, has-circ-0001946 downregulation promotes NSCLC cell proliferation, migration, and invasion, affecting cisplatin sensitivity by modulating the NER signaling pathway [95]. CircsSMARCA5, a tumor suppressor gene, is expressed at low levels in NSCLC tissues and cells. Overexpression of circsSMARCA5 inhibits cell proliferation and enhances sensitivity to cisplatin and gemcitabine [96] [97].

Additionally, dysregulated circRNAs contribute to cisplatin resistance through multiple pathways. ABC transporters, such as P-GP (MDR1) and MRP1, are associated with cisplatin resistance. Circ-0076305 is upregulated in drug-resistant NSCLC, leading to increased expression of these transporters [98]. Exosomes circ_PIP5K1A are overexpressed in NSCLC tissues and cells. Their knockout inhibits lung cancer progression and increases cisplatin sensitivity by modulating the miR-101/ABCC1 axis [99]. ABCC1, a member of the ATP-binding cassette subfamily C, reduces drug accumulation in cancer cells, contributing to resistance [100]. Exosomes are involved in cancer development and can promote tumor drug resistance [101]. Circ_CPA4 regulates Programmed death-ligand 1 (PD-L1) via the let-7 miRNA, promoting proliferation, invasion, EMT, and cisplatin resistance in lung cancer cells. This is due to PD-L1-containing exosomes increasing mRNA levels in stem cells and inactivating CD8 + T cells, leading to immune escape [102].

Although numerous studies have elucidated the regulatory roles of noncoding RNAs in cisplatin resistance in lung cancer, conflicting findings regarding their specific functions persist, reflecting the complexity and inconsistency in current mechanistic interpretations. For instance, miR-488 has been reported by some studies to promote cisplatin resistance in lung cancer by targeting eIF3a and upregulating the NER pathway, thereby enhancing tumor cell DNA damage repair capacity and diminishing cisplatin efficacy. However, in other cancer types such as gastric cancer (GC), miR-488 may function as a tumor suppressor, suggesting its role is highly context-dependent and potentially influenced by tissue-specific regulatory networks [103]. Similarly, the lncRNA HOXA11-AS was found to drive cisplatin resistance by modulating the miR-454-3p/STAT3 signaling axis. Yet, other studies indicate that HOXA11-AS forms a bidirectional regulatory loop with the HOXA11 gene, where its overexpression suppresses HOXA11 and promotes resistance, while its inhibition restores cisplatin sensitivity [104]. Such intricate interactions highlight that lncRNA-mediated regulation is not solely dependent on expression levels but also involves competitive endogenous RNA (ceRNA) mechanisms.

Furthermore, circPVT1 exhibits a dual role in cisplatin resistance. On one hand, it acts as a miR-145-5p sponge, upregulating ABCC1 expression and conferring resistance to cisplatin and pemetrexed in lung adenocarcinoma. On the other hand, while circPVT1 expression decreases post-treatment, it remains elevated compared to chemotherapy-sensitive patients, suggesting temporal or patient-specific heterogeneity in its functional impact [93]. These discrepancies may stem from variations across tumor types, different subtypes within the same cancer, or temporal dynamics during the development of cisplatin resistance. To address these inconsistencies, we can: (1) conduct large-scale clinical sample analyses to delineate expression patterns of specific ncRNAs across various lung cancer subtypes and treatment stages; (2) identify functionally stable ncRNAs; and (3) employ epigenetic editing tools like CRISPR/dCas9 to validate their causal regulatory relationships.

As we all know, noncoding RNAs play pivotal roles in mediating cisplatin resistance in lung cancer. These findings highlight the therapeutic potential of targeting specific noncoding RNAs to enhance cisplatin efficacy and overcome drug resistance. Furthermore, the dysregulated expression patterns of these noncoding RNAs in lung cancer tissues and cells may serve as promising biomarkers for predicting chemotherapy response and patient prognosis.

Histone modification, a crucial mechanism of epigenetic regulation, controls gene expression by altering the chemical modifications of histone tails (e.g., acetylation, methylation, phosphorylation) and affecting the structure and function of chromatin [33]. Recent studies have highlighted the role of histone modifications in cisplatin resistance in NSCLC.

Histone methylation can be regulated by histone methyltransferases (HMTs) and demethylases, primarily by the addition or removal of methyl groups from amino acids [105, 106]. These enzymes are involved in the occurrence and development of cancer and may even affect the efficacy of chemotherapy drugs [105, 107, 108]. MJD1 (also known as lysine demethylase 3 A [KDM3A]), a subfamily of JmjC histone demethylases, has been found to be upregulated in various cancers, promoting tumor growth and metastasis [108]. JMJD1B is overexpressed in lung cancer tissues and cell lines, increasing cell proliferation and invasion by inhibiting the expression of p53. Overexpressed JARID1B can also promote tumor aggressiveness by activating the c-MET signaling pathway and is associated with resistance to cisplatin and doxorubicin, which can be reversed by inhibiting their expression. This suggests that JARID1B may promote chemoresistance in NSCLC cells [109].

The zeste enhancer homolog 2 (EZH2) is a catalytic subunit of the multicomb inhibitory complex 2 (PRC2) that mediates the methylation of H3K27, resulting in transcriptional silencing of genes involved in cell differentiation [110]. Dysregulation or overexpression of EZH2 promotes oncogenic transformation by altering chromatin structure and increasing H3K27me3 levels [111]. EZH2, a histone methyltransferase with catalytic activity, is overexpressed in lung cancer cells, where it activates STAT3 signaling upon myristoylation, promoting accelerated proliferation of lung cancer cells [112]. However, knocking down EZH2 using specific inhibitors can inhibit lung cancer cell proliferation while enhancing chemosensitivity to cisplatin [