Bioinformatics and systems biology have transformed the discovery of genes and pathways driving cancer progression and drug resistance. Integrating multi-omics data has revealed key dysregulated pathways and hub genes linked to GC resistance. Network and enrichment analyses provide insights into biomarkers and therapeutic targets, supporting precision oncology strategies to overcome resistance and enhance outcomes (
9-
15).
The main mechanisms of drug resistance in GC fall into eight categories (
Figure 1): Reduced drug uptake, increased drug efflux, impaired activation or enhanced inactivation of drugs, molecular target changes, enhanced DNA repair, dysregulated apoptosis, tumor microenvironment alterations, and phenotypic shifts such as EMT and stem-like traits (
16).
2.1. Reduced Drug Uptake
Reduced drug uptake is a key mechanism of chemoresistance in GC, driven by altered expression or genetic variants of transport proteins. The down-regulation of CTR1 (SLC31A1 gene) reduces cisplatin sensitivity, while higher expression of SLCO1B3 and other organic anion-transporting polypeptides (OATPs) isoforms (2B1, 3A1, 4A1, 5A1) facilitates the uptake of irinotecan, docetaxel, and methotrexate. The cancer-specific variant of OATP1B3 is highly expressed in GC, though its functional role remains unclear. Additionally, ENT1 contributes to pyrimidine analog uptake, but 5-fluorouracil (5-FU) resistance in GC suggests alternative mechanisms beyond ENT-mediated uptake. These transporters collectively influence drug sensitivity in GC (
16-
18).
2.2. Enhanced Drug Efflux
Genetic alterations in ATP-binding cassette (ABC) transporters contribute significantly to multidrug resistance (MDR) in GC. MDR1 (ABCB1) expression is regulated by deoxycytidine triphosphate pyrophosphatase 1 (DCTPP1), which influences promoter methylation; low DCTPP1 levels increase promoter methylation and reduce MDR1 expression, affecting resistance to cisplatin, oxaliplatin, and epirubicin. MRP1 (ABCC1) overexpression, often observed in cisplatin-resistant GC, is linked to acquired resistance. MRP2 (ABCC2) expression is influenced by single nucleotide polymorphisms (SNPs), such as the c.-24C > T variant, which affects drug response to oxaliplatin. The MRP4 (ABCC4) is highly expressed in cisplatin-resistant GC, with genetic inhibition improving sensitivity. BCRP (ABCG2) expression is associated with poor survival and resistance to cisplatin and 5-fluorouracil (5-FU), while genetic manipulation strategies like ribozymes have shown potential to overcome resistance. Similarly, copper transporters ATP7A and ATP7B exhibit increased expression in oxaliplatin- and cisplatin-resistant GC, though their precise genetic regulation requires further investigation. These genetic changes underscore potential targets for combating GC chemoresistance (
16,
17,
19,
20).
2.3. Decreased Levels of Active Therapeutic Agents
Chemoresistance in GC is driven by changes in metabolic enzymes that inactivate antitumor drugs or reduce pro-drug activation, resulting in lower intracellular levels of active agents. CYP2A6, responsible for converting tegafur to 5-fluorouracil (5-FU), is affected by single nucleotide polymorphisms (SNPs) that reduce its activity, impacting tumor sensitivity to 5-FU. Thymidine phosphorylase (TP), crucial for 5-FU activation, shows variable correlations with 5-FU sensitivity and patient prognosis. Overexpression of carboxylesterase 2 (CES2) enhances the activation of capecitabine and irinotecan, while dihydropyrimidine dehydrogenase (DPD) overexpression reduces sensitivity to 5-FU. Metallothioneins (MTs) are linked to resistance to irinotecan and cisplatin, with conflicting data on their role in prognosis. Glutathione-S-transferase (GST) enzymes, particularly GSTP1, contribute to resistance by forming inactive drug conjugates with glutathione, although specific variants like GSTP1*B are associated with better responses to 5-FU and oxaliplatin. UDP-glucuronosyltransferases (UGTs), including UGT1A1, influence irinotecan metabolism, with polymorphisms linked to variable outcomes in GC treatment. These genetic and expression changes highlight key mechanisms driving chemoresistance in GC (
16,
21-
23).
2.4. Alterations in Molecular Targets
Chemoresistance in GC is influenced by genetic changes in molecular targets critical for drug efficacy. Thymidylate synthase (TS), inhibited by 5-fluorouracil (5-FU), exhibits polymorphisms such as 2R/2R and 2R/3R genotypes in the TS enhancer region, which correlate with shorter overall survival (OS) in patients treated with 5-FU-based chemotherapy. Variations in DNA topoisomerase II (TOPO II) expression are linked to resistance against doxorubicin, while high β-tubulin-III (TUBB3) expression, a target of taxanes, is associated with resistance to docetaxel and paclitaxel. Additionally, microtubule-associated protein tau (MAPT) inversely correlates with paclitaxel sensitivity.
Tyrosine kinase receptors also play a role, with epidermal growth factor receptor (EGFR) overexpression common in GC but showing limited clinical efficacy with anti-EGFR therapies like cetuximab. HER2, targeted by trastuzumab, shows improved OS in HER2-positive patients, but resistance correlates with lower HER2 copy numbers. Vascular endothelial growth factor receptor-2 (VEGFR-2), targeted by ramucirumab and apatinib, influences survival outcomes, with apatinib demonstrating effectiveness by reversing MDR1 and BCRP-mediated resistance. For vascular endothelial growth factor (VEGF), low expression correlates with poor outcomes in bevacizumab-treated patients, while variants in the VEGF pathway (e.g., VEGF-A and VEGF-C) may predict treatment response. These genetic and molecular changes underscore the complexity of chemoresistance mechanisms in GC (
16,
24-
27).
2.5. Aberrant Over-Activation of DNA Repair Mechanisms
Chemoresistance in GC is heavily influenced by genetic changes in DNA repair mechanisms, which enhance tumor cell survival by counteracting drug-induced damage. Overexpression of ERCC1, a key player in the nucleotide-excision repair (NER) system, is linked to poor outcomes in GC patients treated with platinum-based therapies, with polymorphisms such as rs11615 and rs3212986 further affecting chemotherapy sensitivity. MicroRNAs such as miR-122 and miR-139-5p inversely regulate ERCC1 expression, with their induction restoring cisplatin sensitivity. Similarly, ERCC2 and ERCC4 overexpression contribute to resistance, though limited clinical evidence exists for ERCC4.
In base-excision repair (BER), increased XRCC1 expression is associated with cisplatin resistance, while its rs25487 polymorphism correlates with poor outcomes in oxaliplatin-treated patients. Mismatch repair (MMR) deficiencies, including loss of MLH1, result in microsatellite instability (MSI), present in up to 30% of GC cases, and contribute to reduced sensitivity to 5-fluorouracil (5-FU)-based treatments. Low MSI status, however, is linked to improved disease-free survival with adjuvant chemotherapy.
Finally, in homologous recombination (HR), reduced BRCA1 and BRCA2 expression is observed in GC, though the BRCA1 rs799917 polymorphism positively impacts survival in patients treated with taxane and cisplatin therapies. These genetic alterations underscore the role of DNA repair systems in mediating chemoresistance in GC (
16,
28-
33).
2.6. Imbalance in Apoptotic Regulation
Chemoresistance in GC is strongly influenced by genetic alterations in pro-apoptotic factors, impairing drug-induced apoptosis. TP53 mutations, including loss-of-function and gain-of-function variants such as those at Arg175, Gly245, and Arg248, result in diminished or oncogenic p53 activity, leading to enhanced HER2 expression and poorer outcomes with drugs like 5-fluorouracil (5-FU), paclitaxel, and cisplatin. The rs1042522 variant of TP53 is linked to worse response rates to chemotherapy. Loss of CDKN2A expression, due to promoter hypermethylation, is common in Epstein-Barr virus-associated GC and correlates with reduced efficacy of 5-FU therapy. Low expression of intrinsic pathway proteins, including BAX, BAK, BAD, BIM, and BLID, further reduces sensitivity to treatments like docetaxel, cisplatin, and 5-FU. Dysregulation of BIM and BAD by microRNAs, such as miR-BART20-5p and miR-501, promotes chemoresistance, while down-regulation of BLID inactivates caspases 3 and 9, fostering doxorubicin resistance. Impairments in the extrinsic apoptosis pathway, such as FADD loss and miR-633 overexpression targeting FADD, also contribute to resistance. These genetic changes highlight key apoptotic mechanisms driving GC chemoresistance (
16,
34-
37).
Chemoresistance in GC is also driven by genetic and molecular changes in survival pathways, which inhibit apoptosis and promote tumor cell resistance. Hyperactivation of the NF-κB pathway, commonly observed in GC, up-regulates anti-apoptotic factors like survivin, BCL-XL, and XIAP, contributing to cisplatin resistance and acquired chemoresistance. Elevated survivin serum levels correlate with responses to chemotherapy in advanced GC. The Wnt/β-catenin pathway, frequently hyperactive in GC, is exacerbated by Helicobacter pylori infection, which induces SOX9, NANOG, and OCT4 expression. Mutations in CTNNB1, APC, and FBXW7 are linked to poor overall survival (OS) and progression-free survival (PFS) in chemotherapy-treated patients. Loss of E-cadherin, which increases β-catenin levels, is more prevalent in chemoresistant GC. Hyperactivation of the Hedgehog pathway, influenced by
H. pylori and chemotherapy exposure, up-regulates GLI1, GLI2, and SHH, contributing to resistance to 5-FU and doxorubicin. The Notch pathway, up-regulated in cisplatin-resistant GC, promotes MDR1 and MRP1 expression via lncRNA AK022798, reducing caspase-3 and caspase-8 activity. High Notch1 expression correlates with non-responsiveness to 5-FU and cisplatin. Aberrations in the Hippo pathway lead to nuclear accumulation of YAP1 and TAZ, linked to poor responses to adjuvant chemotherapy. Increased activity of PI3K/AKT and JAK/STAT3 pathways is associated with reduced sensitivity to cisplatin and trastuzumab. These molecular changes collectively enhance GC survival and chemoresistance (
16,
38-
44).
2.7. Changes in Tumor Cell Microenvironment
Chemoresistance in GC is influenced by genetic and molecular changes within the tumor microenvironment, including interactions with stromal cells, blood vessels, and inflammatory cells. Hypoxia-inducible factor 1-alpha (HIF-1α), up-regulated in hypoxic conditions, promotes resistance to platinum derivatives by altering the expression of miR-27a, miR-421, and long noncoding RNA (lncRNA) PVT1, while also increasing MDR1, MRP1, and BCL-2 levels. Stanniocalcin 1 (STC1), enhanced under hypoxia, contributes to cisplatin resistance by up-regulating BCL-2 and reducing caspase activity.
Inflammation-driven factors also play a role. NR4A2, induced by prostaglandin E2, inhibits apoptosis and correlates with poor survival in 5-fluorouracil (5-FU)-treated patients. Cytokines like interleukin (IL)-6, IL-8, and IL-11 promote resistance through pathways involving NF-κB, ABCB1, and BCL-2, while IL-33 activates the JNK pathway to prevent apoptosis. Autocrine signaling via CCL2 maintains cisplatin resistance by inactivating autophagy through PI3K-AKT-mTOR signaling.
In exploring the molecular mechanisms underlying chemoresistance, the role of autophagy-related genes, such as FOXO3 and GAPDH, has been highlighted in various cancers, underscoring the potential for targeting autophagic pathways to enhance treatment efficacy. Overexpression of miR-23b-3p reverses resistance mediated by ATG-12 and HMGB2, while ATG-5 up-regulation is linked to shorter overall survival. Exosomes further contribute to chemoresistance. Mesenchymal stem cell-derived exosomes up-regulate ABC pumps, and tumor-associated macrophage-derived exosomes transfer miR-21a-5p, enhancing cisplatin resistance via the PI3K/AKT pathway.
Additionally, metabolic adaptations, such as glycolysis up-regulation and fatty acid oxidation driven by lncRNA HCP5, support resistance to 5-FU and oxaliplatin. These molecular and genetic changes highlight the complex role of the tumor microenvironment in GC chemoresistance (
15,
45-
52).
2.8. Changes in Cellular Phenotypes
Chemoresistance in GC arises from genetic changes and phenotypic shifts associated with EMT and cancer stem cells (CSCs). EMT, driven by factors such as transforming growth factor-beta (TGF-β), hepatocyte growth factor (HGF), and hypoxia-inducible factor 1-alpha (HIF-1α), promotes resistance through the up-regulation of CD168 (HMMR), vimentin, and N-cadherin. MicroRNA regulation further contributes, with miR-577 enhancing TGF-β signaling and increasing oxaliplatin resistance, and miR-187 up-regulating ERCC1/4, reducing cisplatin sensitivity.
Cancer stem cells, marked by surface proteins like CD44, CD24, CD133, CXCR4, and EpCAM, display intrinsic chemoresistance. CD44 splicing variants activate pathways such as Hedgehog, VEGF, and c-Met, driving resistance to 5-fluorouracil (5-FU), cisplatin, and anthracyclines. High CD133 expression up-regulates MDR1 and BCL-2 via the PI3K/AKT pathway, correlating with shorter overall survival (OS) in cisplatin/5-FU-treated patients. Additionally, CXCR4 expression in diffuse-type GC is linked to docetaxel resistance, while aldehyde dehydrogenase 1 (ALDH1) isoenzymes protect CSCs from oxidative damage, reducing 5-FU efficacy.
Key EMT regulators, such as LGR5 and DCLK1, promote resistance by activating pathways like Wnt/β-catenin and Notch, which enhance stemness and EMT markers, including SOX2, OCT4, and SNAIL. Prolonged exposure to oxaliplatin, doxorubicin, or trastuzumab induces EMT through β-catenin, Fas, and TGF-β-miR-200c-ZEB2 signaling. Resistance is further supported by YAP1-PI3K activation in trastuzumab-treated cells.
Lastly, CSCs characterized by low CD71 expression or high side-population cell activity, enriched during 5-FU treatment, exhibit elevated BCRP and MDR1 levels. These changes, along with SOX2-mediated ABCG2 expression, confer resistance to 5-FU, cisplatin, and doxorubicin, underscoring the role of EMT and CSCs in GC chemoresistance (
16,
42,
53-
63).