Long non-coding RNAs as the critical regulators of doxorubicin resistance in tumor cells

Resistance against conventional chemotherapeutic agents is one of the main reasons for tumor relapse and poor clinical outcomes in cancer patients. Various mechanisms are associated with drug resistance, including drug efflux, cell cycle, DNA repair and apoptosis. Doxorubicin (DOX) is a widely used first-line anti-cancer drug that functions as a DNA topoisomerase II inhibitor. However, DOX resistance has emerged as a large hurdle in efficient tumor therapy. Furthermore, despite its wide clinical application, DOX is a double-edged sword: it can damage normal tissues and affect the quality of patients’ lives during and after treatment. It is essential to clarify the molecular basis of DOX resistance to support the development of novel therapeutic modalities with fewer and/or lower-impact side effects in cancer patients. Long non-coding RNAs (lncRNAs) have critical roles in the drug resistance of various tumors. In this review, we summarize the state of knowledge on all the lncRNAs associated with DOX resistance. The majority are involved in promoting DOX resistance. This review paves the way to introducing an lncRNA panel marker for the prediction of the DOX response and clinical outcomes for cancer patients.

by causing DNA damage that prevents mitosis in tumor cells. It also promotes apoptosis by stimulating topoisomerase II to cut DNA strands. However, despite its wide clinical application, DOX is a double-edged sword: it damages normal tissues, thus negatively affecting the quality of patients' lives during and after treatment. It has toxic effects on normal heart, brain, kidney and liver tissues [7].
Clarifying the molecular basis of DOX resistance could enable the development and introduction of novel therapeutic modalities with fewer and/or lower-impact side effects in cancer patients. Various genetic mutation and epigenetic mechanisms can be related with DOX resistance. Mutations in ABC transporter family members such as ABCB1 [8], ABCBG2 [9] and MRP1 [10], as well as DNA repair factors such as p53 [11][12][13][14] are considered to be the major causes of DOX resistance. There is also evidence for a role of epigenetic aberration in chemoresistance [15][16][17].
Long non-coding RNAs (lncRNAs) are involved in various cellular processes via transcriptional regulation of their target genes. They can also function as oncogenes or tumor suppressors [18,19]. Based on their biogenesis, lncRNAs are categorized as intergenic, antisense, intronic, overlapping or full lapping [18]. Antisense (AS) lncRNAs are the largest category, making up about 70% of the long non-coding transcriptome [20].
LncRNAs have important effects on tumorigenesis through their modulation of various pathophysiological processes, including the stability of mRNA, RNA splicing, chromatin remodeling and miRNA sponging ( Fig. 1) [21][22][23][24][25][26]. Their deregulation is one of the main obstacles for the effectiveness of chemotherapy [27][28][29]. They are involved in chemotherapeutic responses through their regulation of histone modification and DNA methylation. Since epigenetic signatures are inheritable and reversible, they have been suggested as effective biomarkers for the prediction of chemotherapeutic outcomes [29]. This review summarizes the molecular mechanisms whereby lncRNAs affect DOX responses in tumor cells (Fig. 2, Table 1).

Breast cancer
Various screening and therapeutic methods have been used against breast cancer [30], but in advanced stages, many patients still develop invasive carcinoma and have poor prognosis [31]. Although Doxorubicin is one of the most efficient drugs for breast cancer treatment, drug resistance can be observed after several treatments [32]. About 30% of breast cancer patients who receive chemotherapy experienced the poor prognosis that is associated with the expression of multidrug resistance proteins [33]. It has been reported that XIST increases tumor cells proliferation and suppresses apoptosis in DOX-treated MDA-MB-231 cells through upregulation of the anillin actinbinding protein (ANLN). XIST was suggested to be a competitive endogenous RNA that increases the levels of ANLN expression via miR-200c-3p targeting [34].
Linc00152 is a critical factor during the progression of various cancers, including lung, liver, and colorectal cancer (CRC) [35][36][37]. Mitosis and the cell cycle could be modulated by Linc00152 in Hela cells [38]. During EMT, epithelial tumor cells gain mesenchymal properties through reduced adhesion and increased motility. This process is involved in early stages of tumor metastasis [39,40]. Linc00152 upregulation has been reported in breast cancer tissues and cell lines, where it increased the levels of cell growth, migration, EMT and DOX-resistance [41].
Multidrug resistance protein 1 (MRP1) is a member of the ATP-binding cassette (ABC) C superfamily, which is involved in MDR of different tumors [42,43]. Increased linc00518 and MRP1 expression levels have been reported in breast cancer tissue and cell lines. Higher expressions of linc00518 and MRP1 were also observed in MDR breast tumor cells (MCF-7/DOX) in comparison with parental cells (MCF-7). Drug resistance could be improved through regulation of the miR-199a/MRP1 axis in breast cancer tissue. Linc00518 upregulated the MRP1 via MiR-199a sponging. The resistance of the        MCF-7/DOX cell line was also increased toward DOX, VCR, and PTX treatments via the miR-199a/MRP1 axis [44]. AKT is a Ser/Thr kinase involved in cell proliferation, apoptosis, and migration. It inhibits BAD pro-apoptotic factor via phosphorylation, which results in disassembly from BCL-2/BCL-X. AKT also upregulates the pro-survival genes via NF-κB activation. There are significant correlations between increased levels of AKT1 expression and resistance toward paclitaxel [45]. The PI3K/AKT/mTOR pathway has a pivotal regulatory role in the cell cycle, cell proliferation, metabolism, and protein synthesis [46,47]. It has been reported that HOTAIR promoted DOX sensitivity via repression of the PI3K/AKT/mTOR axis. Inhibition of HOTAIR markedly decreased the expression of MDR proteins, which resulted in reduced cell survival and the promotion of apoptosis in DOXR-MCF-7 cells. Moreover, the CASP3, BCL-2, and BAX expression levels were significantly altered following HOTAIR inhibition, which increased apoptosis in DOXR-MCF-7 cells [48].
Breast cancer stem cells are a sub-population of tumor cells that have the ability of self-renewal, EMT, and chemoresistance [49]. Since SND1 is able to bind with other proteins and nucleic acids, it can regulate various proteins, including transcription factors and co-regulatory factors [50]. STAT6, STAT5, and c-MYB are SND1-associated cofactor proteins [51]. SND1 is also involved in splicing through its Tudor-SN domain [52], and mRNA stabilization through staphylococcal nuclease-like domains [53]. It has been reported that there is a correlation between linc00668 upregulation and lymph node metastasis in BC patients. Linc00668 induces cell invasion, self-renewal properties, and DOX resistance in BC cells through SND1 binding to upregulate SMAD2/3/4 [54].
The most significant signs of the EMT process are vimentin upregulation and E-cadherin downregulation [55]. There is a negative correlation between the E-cadherin expression and tumor progression in breast cancer patients [56]. EMT progression is regulated by SNAI1, which is an EMT-specific transcription factor that represses E-cadherin expression and promotes tumor invasion [57]. Vimentin is a type III intermediate filament produced by fibroblasts and endothelial cells. Tumor invasion can be decreased through vimentin downregulation as a consequence of re-epithelialized cells [58]. Annexin A1 (ANXA1) is in the calcium-dependent phospholipid-binding protein family involved in anti-inflammation [59]. It has also pivotal roles in the regulation of cell proliferation, adhesion, and metastasis [60]. The canonical TGF-β signaling pathway modulates EMT. Moreover, TGF-β can induce EMT via non-canonical pathways, including the ERK1/2, GTPase, and p38 MAPK pathways [55]. It has been reported that DCST1-AS1 increases TGF-β-induced EMT and DOX resistance via ANXA1 targeting in breast cancer cells. DCST1-AS1 inhibition also regulates TGF-β-induced production of MMP2 and MMP9 [61].
LncRNAs have a critical role in the chemoresistance of breast tumor cells through interactions with transcription factors. C/EBPβ is a transcription factor regulated by LINC00160, which targets TFF3. C/EBPβ is associated with a poor prognosis in estrogen receptor-negative and metastatic mammary tumors [62]. TFF3 is more highly expressed in metastatic breast cancer than in the non-metastatic type [63]. LINC00160 is associated with paclitaxel resistance and DOX resistance in MCF-7 and BT474 cells, respectively. Overexpression of LINC00160 is correlated with poor overall survival in BC tissues. LINC00160 upregulated TFF3 via C/EBPβ, which resulted in DOX-resistance in BT474 cells [64]. LncRNA in non-homologous end-joining pathway 1 (LINP1) is an oncogene that suppresses tumor growth and metastasis. LINP1 upregulation has a positive association with drug-resistance and unfavorable prognosis, and is seen in breast cancer cells resistant to 5-FU and doxorubicin. It has been reported that LINP1 regulates the cell cycle via CDK4, CCND1 and CCND3 modulations. LINP1 suppresses apoptosis and induces EMT process. There is a negative correlation between P53 and LINP1. The 5-FU and DOX resistance of breast cancer cells are increased by LINP1. LINP1 represses CASP9/ BAX and CASP8/9 expressions induced by 5-FU and DOX, respectively. There is also a correlation between the levels of LINP1 expression and tumor metastasis and stage [65].
H19 is an imprinted lncRNA that is only active when inherited maternally. H19 imprinting is regulated by a cis-acting upstream sequence that is involved in the regulation of DNA methylation and replication of parental chromosomes [66]. H19 has a pivotal role during tumorigenesis: its upregulation is observed in about 70% of breast cancer patients [67,68]. It has been reported that there is a significant H19 upregulation in DOX-resistant BC cells. H19 regulates DOX-resistance through upregulation of CUL4A and ABCB1/MDR1 [69]. Poly (ADP-ribose) polymerase (PARP) is involved in the detection of DNA damage. It employs DNA repair proteins through ADP-ribose binding. It is also involved in cell cycle and transcriptional regulations [70]. It has been reported that there is a significant H19 upregulation in BC tissues compared with their normal margins. There is also significant H19 upregulation in DOX-resistant tissues and cell lines. H19 increases DOX-resistance via PARP-1 targeting in breast tumor cells [71].

Osteosarcoma
Osteosarcoma (OS) is the most frequent bone tumor among adolescents and children, accounting for up to 20% of bone malignancies. Cisplatin, doxorubicin, or methotrexate is considered to be the standard treatment methods for advanced osteosarcoma. However, 40-45% of osteosarcoma patients are resistant toward doxorubicin treatment [72]. Taurine upregulated gene 1 (TUG1) is an oncogenic lncRNA that is associated with chemoresistance in various cancers [73,74]. TUG1 functions in post-transcriptional regulation through miRNA sponging and interacting with PRC2 complex [75]. TUG1 recruits EZH2 to downregulate CDK inhibitors such as p16 and p21 in gastric carcinoma (GC) [76]. It is also involved in tumor cell proliferation and migration through regulation of the Hedgehog, PI3K/AKT, and WNT signaling pathways in HCC and OS cells [77,78]. Polydatin is a stilbenoid glucoside isolated from some plants that is involved in cell proliferation inhibition and apoptosis induction [79,80]. AKT phosphorylation is critical for cell survival. TUG1 promotes osteosarcoma proliferation and invasion via AKT activation. In a positive feedback, AKT also upregulates TUG1 [81]. Polydatin inhibits tumor cells through suppression of the PI3K/AKT and PDGF/AKT pathways [81,82]. It has been reported that polydatin inhibits osteosarcoma cell proliferation and reduces DOX-resistance via TUG1 downregulation. Since polydatin treatment in TUG1-silenced cells decreases AKT phosphorylation, inhibition of TUG1/AKT axis is required for its regulation of DOX-resistance in osteosarcoma cells [83]. Forkhead box C2 (FOXC2) is a critical transcription factor in tumor angiogenesis and MDR, functioning through EMT promotion [84]. ABCB1 plays a significant role in pumping external molecules through ATP hydrolysis that reduces the chemosensitivity of tumor cells [85]. FOXC2-AS1 is an lncRNA that regulates FOXC2 to promote DOX resistance via ABCB1 upregulation [86]. It is involved in the regulation of intracellular Ca 2+ levels and the activation of the Ca 2+ -FAK signaling pathway [87]. It downregulates p15 and inhibits apoptosis via recruitment of EZH2 and SU212 [88]. FOXC2-AS1 and FOXC2 upregulations were observed in DOX-resistant osteosarcoma tissues and cell lines. FOXC2-AS1 is involved in FOXC2 upregulation through the formation of a stable RNA duplex, which upregulates ABCB1 in DOX-resistant osteosarcoma cells [86]. Simultaneous high expression levels of FOXC2-AS1 and ABCB1 are the main reason for DOX-resistance in OS cells. Silencing FOXC2-AS1 and ABCB1 reduces tumor growth during doxorubicin treatment. FOXC2-AS1 regulates the methylation of ABCB1 via PRC2, which results in ABCB1 downregulation [89].
SNHG12 is an lncRNA involved in the tumorigenesis of various cancers, including papillary thyroid carcinoma (PTC), GC, OS, and glioma [98][99][100][101]. It can affect the Wnt/ β-catenin pathway in PTC proliferation and metastasis [99]. It can also modulate the NOTCH2 pathway, which promotes OS metastasis and growth [101]. SNHG12 upregulates CRKL through miR-320 targeting that results in AKT/ERK activation in GC [102]. As a member of the BCL2 protein family, MCL1 plays a pivotal role in chemoresistance and apoptosis. It has been reported that SNHG12 decreases DOX sensitivity through miR-320a downregulation and MCL1 upregulation [103].
In vivo and in vitro experiments confirmed that doxorubicin-resistant OS cell lines and patients have higher expression levels of LINC00426 than their parental counterparts. Therefore, an unfavorable prognosis and no effective response to DOX are the consequences of LINC00426 overexpression. LINC00426 increases DOX resistance by targeting miR-4319 in OS cells [104].
CTA downregulation has been reported in DOX-resistant OS cells. CTA promotes apoptosis and suppresses autophagy by targeting miR-210 in OS cells. Its downregulation correlates with poor prognosis in OS patients. CTA significantly upregulates Cas-p8ap2 and AIFM3 [105].
ABCB1 is one of the MDR-associated genes involved in drug efflux from tumor cells [106]. FENDRR is an lncRNA involved in heart development through its binding to PRC2 and TrxG/MLL complexes [107]. A significant association has been reported between FENDRR downregulation and DOX-resistance in OS cells. FENDRR downregulates ABCB1 and ABCC1. It suppresses DOX resistance and induces OS cells apoptosis [108].

Gastric cancer
Gastric cancer (GC) remains one of the most frequent malignancies and the third leading cause of neoplasm-related death globally [109,110]. Approximately two-thirds of patients are detected in advanced tumor stages [111,112]. Although, there is an effective response to chemotherapy in GC patients with advanced tumors, drug resistance is also a major cause of tumor growth [113].
HOTAIR is a lncRNA that binds to PRC2 and the LSD1/CoREST/REST complex [114]. It also increases HOXA1 hypermethylation via DNMT1 and DNMT3b upregulations [115]. An association between HOTAIR upregulation and advanced stage GC tumors has been reported. HOTAIR increases DOX resistance, cell proliferation and migration by targeting miR-217, resulting in GPC5 and PTPN14 upregulations in GC cells [116].
Urothelial carcinoma associated 1 (UCA1) is a non-coding RNA that has been detected in bladder cancer for the first time [117]. It is in human endogenous retrovirus H gene family, which is highly expressed in malignant bladder cancer [117]. UCA1 upregulation promotes cell survival in bladder cancer during treatment with cisplatin [118]. It also induces DOX resistance in breast cancer tissue [119]. Its upregulation also positively correlates with poor differentiation, high grade, and poor overall survival. Knockdown of UCA1 inhibits tumor cell proliferation. DOX can promote apoptosis in SGC7901/DOX cells by silencing UCA1, and also lead to cleavage of PARP protein and BCL-2 downregulation. UCA1 had an oncogenic role in GC via regulation of cell proliferation and DOX resistance [120].
MiR-27b is known as a tumor suppressor that is downregulated in GC [121,122]. It acts as an anti-angiogenic factor through its targeting of VEGF-C in GC [122]. Significant UCA1 upregulation has been observed in GC tissues, which was negatively correlated with miR-27b. Downregulation of UCA1 induces expression of miR-27b, resulting in a reduction in the level of anti-apoptotic proteins such as BCL2 and promotion of apoptotic proteins such as CASP3 in gastric tumor cells [123].
Myocyte enhancer factor 2D (MEF2D) is a transcription factor that is upregulated in various cancers, such as osteosarcoma [124], leukemia [125] and GC [126]. MEF2D has a key role in tumorigenesis, promoting proliferation, invasion and metastasis via repression of cell cycle arrest proteins, apoptosis, and the induction of the VEGF and TGF-b1 signaling pathways [126,127]. LncR-D63785 upregulation has been reported in gastric tumor cells. Reduced lncR-D63785 expression represses cell proliferation, invasion and metastasis. LncR-D63785 downregulation promotes the DOX-sensitivity of GC cells to apoptosis via the miR-422a/MEF2D axis. The expression levels of KLK4, FOXG1, FOXQ1 and FOXE1 are also reduced by miR-422a. Positive correlations exist between the lncR-D63785, miR422a and MEF2D expressions in DOX-resistant GC cells [128].
NEAT1 is a component of the paraspeckle nuclear bodies involved in the transcriptional regulation of various genes. It has an oncogenic role in various tumors, including GC [129,130]. NEAT1 upregulation that inhibited cell proliferation and invasion has been reported in GC. Its upregulation has also been observed in DOX-resistant GC cells [131].
MRUL is an lncRNA that upregulates P-gp in MDR gastric tumor cells. MRUL silencing significantly downregulate the Bcl-2/Bax ratio, RPS13, and RPL23 while significantly upregulating JNK1 and CPP32 in the presence of DOX. Drug-induced apoptosis increases following MRUL depletion in GC cells [132].

Leukemia and lymphoma
Acute myeloid leukemia (AML) is a heterogeneous bone marrow malignancy [133]. DOX is the most commonly prescribed chemotherapeutic agent for AML treatment, but chemoresistance is a big challenge [134].
KCNQ1OT1 is reported in vaious tumors [135,136]. It has interactions with G9a methyltransferase and the PRC2 complex [137]. Tetraspanin3 (Tspan3) is a cell-surface protein that regulates signal transductions in cell development, growth, the immune response and tumorigenesis [138]. Significant KCNQ1OT1 upregulation has been observed in DOX-resistant AML tissues. Its knockdown increases the DOX sensitivity and suppresses the cell proliferation and invasion of AML cells. It regulates the DOX response through miR-193a-3p targeting that inhibits Tspan3 [139].
Enhancer of zeste homolog 2 (EZH2) is a histone methyltransferase component of the PRC2 complex that can epigenetically methylate H3K27 to inhibit gene expression [140]. TUG1 overexpression has been reported in DOX-resistant AML tissues and cells. Interestingly, EZH2 is recruited through TUG1 to methylate and downregulate miR-34a, resulting in DOX resistance in AML cells [141].
HOXA-AS2 is located between the HOXA3 and HOXA4 genes. It acts as an oncogenic factor in promoting cell survival, proliferation and invasion [142,143]. It is upregulated in various types of tumors and this state significantly correlates with poor prognosis. Its overexpression has been seen in patients who received DOX. HOXA-AS2 functions as a ceRNA of miR-520c-3p to upregulate S100A4, resulting in DOX-resistance of AML cells [144].
The PI3K/AKT/mTOR signaling pathway plays a pivotal role in the proliferation, differentiation and viability of hematopoietic cells [145,146]. A correlation between linc00239 expression and tumor cell proliferation and migration in AML cells has been observed. Linc00239 significantly increases the DOX-resistance of KG-1 and HL-60 cells through phosphorylation of AKT and mTOR, resulting in PI3K/ATK/mTOR pathway activation [147].
HuR is a member of RBP family. It stabilizes mRNA via binding to AU-rich elements, located in the 3′-UTRs of RNA [153,154]. An association between FENDRR downregulation and MDR1 expression in DOX resistant CML cells has beenreported. FENDRR decreases the DOX-resistance of tumor cells by downregulating MDR1 through HuR and targeting miR-184 in CML cells [155]. DOX is one of the common treatments for Burkitt lymphoma (BL) [156], although the majority of patients have no DOX response [157]. PI3K/AKT/mTOR is a nominated pathway in lymphoma chemoresistance. Eukaryotic translation initiation factor 4E (EIF4E) is a target of the mTOR pathway, which can affect numerous cancer phenotypes [158,159]. MCM3AP-AS1 reportedly increases the DOX resistance of BL cells through miR-15a sponging and EIF4E upregulation [160].

Liver cancer
Hepatocellular carcinoma (HCC) is one of the leading causes of cancer-related death in the world [161,162]. MALAT1 is an oncogenic lncRNA that promotes tumor progression and chemoresistance through various mechanisms, such as miRNA sponging and autophagy induction [163]. It is involved in alternative splicing via regulation of SR proteins [164]. It has also critical roles in various signaling pathways, such as Hippo, PI3K-AKT, MAPK, WNT and NF-κB [165][166][167][168]. MALAT1 upregulation has been shown in MDR-HCC cells. HIF-2a upregulates MALAT1, which subsequently targets miR-216b during MDR regulation in HCC cells [169].
Neuro-oncological ventral antigen 1 (Nova1) is a neuron-specific RNA-binding protein that functions as an oncogene involved in the aberrant immune response [170], the resistance of cancer cells to hypoxia-related apoptosis induction [171], and tumor progression [172]. Nova1 upregulation has been observed in Huh-7 cells, and is associated with cell proliferation, migration, invasion and poor prognosis in HCC [173].
MALAT1 and Nova1 upregulations have been reported for DOX-resistant hepatic tumor cells in comparison with DOX-sensitive cells. MALAT1 upregulation correlates with tumor cell proliferation, invasion and chemoresistance through Nova1 regulation. It sponges miR-3129-5p in DOX-resistant cells. MALAT1 depletion triggered DOXresistance in HCC cells by repressing the proliferation, migration, invasion and promotion of apoptosis through the MALAT1/miR-3129-5p/Nova1 axis [174].
NEAT1 has an important role in the integrity of paraspeckles. Its upregulation has been observed in sorafenib-and DOX-resistant HCC cells. Paraspeckles have been observed in DOX-resistant HCC cells [175].
LncARSR is activated by AKT to target miR-34 and miR-449, which results in sunitinib resistance of renal cancer cells through AXL and c-MET upregulations [176]. Correlations have been shown between lncARSR upregulation and the large tumor size, advanced BCLC stage, poor prognosis, and DOX resistance of HCC cells. LncARSR induces DOX resistance in both in vitro and in vivo studies through PTEN targeting that activates the PI3K-AKT signaling pathway [177].
Growth arrest-specific 5 (GAS5) is an lncRNA associated with a variety of biological mechanisms, such as cell proliferation, survival and DOX resistance, via regulation of The miR-21/PTEN axis. GAS5 upregulation in HCC cells is associated with metastasis to lymph nodes and shorter overall survival time in HCC patients. It also has a key role in DOX-resistance in both in vitro and in vivo studies. GAS5 inhibits the expression of miR-21, which results in PTEN upregulation [178]. H19 is a maternally expressed gene product that functions as a tumor suppressor or oncogene. H19 reportedly inhibits HCC cell proliferation following sorafenib or doxorubicin treatments [179].

Colorectal cancer
LncRNA X-inactive specific transcript (XIST) is considered the most significant regulator of X chromosome inactivation in mammals via the PRC complex [180]. It also promotes NOTCH signaling by targeting miR-137, which results in NOTCH-1 upregulation [181]. It has been suggested that the deregulation of XIST plays an important role in tumor progression and prognosis [182].
Overexpression of serum and glucocorticoid-regulated kinase 1 (SGK1; one of the AGC serine/threonine protein kinases) has been associated with proliferative activity, apoptosis, adhesion and drug-resistance in numerous types of epithelial cancer [183,184]. There is a correlation between SGK1 and DOX-mediated apoptosis in renal cancer [185]. Downregulation of SGK1 reduces cell proliferation and migration and promotes 5-FU-mediated apoptosis induction [186]. XIST upregulation has been reported in DOX-resistant CRC cells. XIST increases DOX resistance through miR-124 sponging that results in SGK1 upregulation in CRC cells [187].
BRAF-activated noncoding RNA (BANCR) is an lncRNA involved in tumorigenesis in various cancer types, such as lung cancer, GC, thyroid cancer and osteosarcoma [188]. Chromosomal segregation 1-like (CSE1L) plays a critical role in apoptosis, survival, chromosome assembly, nuclear transportation, microvesicle formation and metastasis [189,190]. BANCR and CSE1L overexpressions have been observed in CRC cells. Direct correlations have been found between CSE1L and BANCR expressions and the clinicopathological features of CRC. BANCR increases CSE1L expression through miR-203 sponging in CRC tissue. There is significant miR-203 downregulation in CRC cells in comparison with controls. BANCR downregulation inhibits tumor progression and promotes the sensitivity of CRC cells to DOX by modulating the miR-203/CSE1L axis [191].
The NODAL signaling pathway has a key role in the regulation of chemoresistance in cancer stem cells (CSCs) [192,193]. NODAL signaling can be protected by GAS5, contributing to the preservation and chemoresistance of CSCs. GAS5 is a pivotal factor in the proliferation of CSCs, and thus to tumor promotion and metastasis. It also plays a key role in drug-resistance. Knockdown of GAS5 improves chemo-sensitivity and apoptosis in the tumor cells treated with 5-FU and DOX [194].

Thyroid and gall bladder cancers
Thyroid cancer remains the most frequent endocrine malignancy worldwide. It has a high mortality rate [195]. Anaplastic thyroid carcinoma (ATC) is the most aggressive and recurrent type of thyroid tumor that is commonly treated with DOX [196]. However, overexpression of multidrug resistance proteins causes drug resistance in such patients [197].
Signal transducer and activator of transcription 3 (STAT3) is a transcription factor activated by cytokines and growth factors involved in inflammation, tumor cell proliferation and invasion [198][199][200]. INO80 is involved in DNA repair and transcription [201]. Lipoprotein receptor-related protein 6 (LRP6) is targeted by PTCSC3, resulting in repression of glioma cell proliferation via suppression of WNT signaling pathway [202]. It has been reported that PTCSC3 downregulates INO80 by targeting STAT3, which reduces the DOX-resistance of ATC [203]. Gallbladder cancer is the most aggressive cancer type observed in the biliary tract. It ranks as the fifth most frequent malignancy in digestive tracts worldwide. Many patients have poor prognosis because of diagnosis in the advanced stage due to the unclear and non-specific symptoms. Autophagy has a paradoxical role in oncogenesis. The cytoprotective role of autophagy leads to stress tolerance which enables tumor resistance toward chemotherapy [204].
Gallbladder cancer drug resistance-associated lncRNA1 (GBCDRlnc1) is a unique lncRNA mediating resistance to chemotherapy. GBCDRlnc1 upregulation has been reported in gallbladder tumor cells. GBCDRlnc1 maintained PGK1 stability by inhibiting its ubiquitination leading to ATG5 and ATG12 downregulations in DOX-resistant tumor cells. GBCDRlnc1 upregulation correlates with poorer histological grade and advanced tumor stage [205].

Prostate and urothelial cancers
PRC2 is in the methyltransferase protein family, which methylates lysine of histone H3 to suppress gene expression. The PRC2 complex is comprised of several components, including EZH1, EZH2, SUZ12 and EED [206,207]. Significant LINC-PINT upregulation has been observed in clear cell renal cell carcinoma (ccRCC) cells, correlating with sex, pT and tumor stage. The LINC-PINT levels also negatively correlate with DFS and OS in patients. LINC-PINT induces cell proliferation, but represses apoptosis via EZH2 targeting in ccRCC cells. DOX upregulates P53 and LINC-PINT in ccRCC tissues [208].
GAS5 is a tumor suppressor that is downregulated in HCC, GC and ovarian cancer [209][210][211]. Its downregulation has also been reported in bladder transitional cell carcinoma (BTCC) tissues and cells, where it is associated with higher grades of cancer. It inhibits cell proliferation and DOX resistance in BTCC cells through downregulation of BCL-2 [212]. There are also HOTAIR upregulations in transitional cell carcinoma (TCC) tissues and cells and these correlate with higher histological grades, shorter overall survival, and reduced DOX sensitivity [213].
Lysyl oxidase-like 1 (LOXL1) is an extracellular matrix (ECM) protein in the the copper-dependent monoamine lysyl oxidase family, which is involved in oxidation of collagens and elastin [214]. LOXL1-AS1 is located in the opposite strand of LOXL1 [215]. The epidermal growth factor receptor (EGFR) is a member of the receptor tyrosine kinases (RTK) family, participating in cell proliferation, differentiation and tumor progression [216,217]. Overexpression of EGFR has been reported in a variety of tumor types [218]. It has been reported that EGFR regulates LOXL1-AS1 expression via miR-let-7a-5p in prostate cancer (PCa) cells. LOXL1-AS1 is downregulated in DOX-resistant PCa cells compared with DOX-sensitive cells. There is a significant miR-let-7a-5p upregulation in DOX-resistant PCa cells. MiR-let-7a-5p reduces the promoting role of LOXL1-AS1 on DOX-resistant cell proliferation [219].

Conclusions
Despite its wide clinical applications, DOX can affect the quality of life of cancer patients due to side effects during and after treatment. Clarifying the molecular basis of DOX resistance is essential for the development of novel therapeutic strategies with fewer and less impactful side effects in cancer patients. LncRNAs have critical