Noncoding RNAs: an emerging modulator of drug resistance in pancreatic cancer

Pancreatic cancer is the eighth leading cause of cancer-related deaths worldwide. Chemotherapy including gemcitabine, 5-fluorouracil, adriamycin and cisplatin, immunotherapy with immune checkpoint inhibitors and targeted therapy have been demonstrated to significantly improve prognosis of pancreatic cancer patients with advanced diseases. However, most patients developed drug resistance to these therapeutic agents, which leading to shortened patient survival. The detailed molecular mechanisms contributing to pancreatic cancer drug resistance remain largely unclear. The growing evidences have shown that noncoding RNAs (ncRNAs), including microRNAs (miRNAs), long noncoding RNAs (lncRNAs) and circular RNAs (circRNAs), are involved in pancreatic cancer pathogenesis and development of drug resistance. In the present review, we systematically summarized the new insight on of various miRNAs, lncRNAs and circRNAs on drug resistance of pancreatic cancer. These results demonstrated that targeting the tumor-specific ncRNA may provide novel options for pancreatic cancer treatments.


Introduction
Pancreatic cancer (PC), a highly lethal human malignancy, ranks the fourth leading cause of cancer-related death in the United States and the eighth worldwide. PC patients had only 5% overall survival rate at 5-years after diagnosis (Von Hoff et al., 2009). A total of 458,918PC cases were diagnosed all around the world and 432,242 deaths were reported in 2018 (Bray et al., 2018). Pancreatic ductal adenocarcinoma (PDAC) is the most common pathological type, accounting for 85%-90% of all PC patients. Due to lack of the effectively early diagnosis methods, the vast majority of PC patients are diagnosed at advanced disease stages and tumors are unresectable. Systematical therapy with chemotherapy, immunotherapy and targeted therapy drugs with or without radiation therapy is only viable alternative for advanced PC patients. Currently, chemotherapeutic agents applied in PC treatments include gemcitabine (GEM), 5-fluorouracil (5-FU), adriamycin (ADM), cisplatin (DDP), oxaliplatin, nab-paclitaxel, irinotecan and capecitabine (Springfeld et al., 2019). Targeted therapy drugs for PC include small molecule tyrosine kinase inhibitor (TKI) erlotinib targeting epidermal growth factor receptor (EGFR), oral mammalian target of rapamycin (mTOR) inhibitor everolimus and small molecule multi-target receptor TKI sunitinib which targets RTK. GEM combined with erlotinib is commonly used to treat locally advanced PC or metastatic PC (Irigoyen et al., 2017;Halfdanarson et al., 2019). Everolimus and sunitinib were applied in PC patients with progressive neuroendocrine tumors that cannot be resected or have metastatic spread (Angelousi et al., 2017;Nunez et al., 2019). In the aspect of immunotherapy, there are several monoclonal antibodies of immune checkpoint inhibitors (ICIs), such as ICIs of cytotoxic T lymphocyte antigen-4 (CTLA-4) (ipilimumab and trimetamumab), as well as ICIs of programmed cell death protein 1(PD-1) (pidilizumab, nivolumab and pembrolizumab) (Weiss et al., 2018;Kamath et al., 2020). Unfortunately, intrinsic drug resistance or acquired drug resistance severely limits the applications of these therapeutic agents. The deregulations of cell cycle control, apoptosis, DNA damage repair, autophagy, epithelial-mesenchymal transition (EMT), ABC transporters and cancer stem cells (CSCs), have been reported to be associated with drug resistance in PC (Modi et al., 2016;Kuwada et al., 2018;Liu Q G et al., 2018;Ma T et al., 2018;Knudsen et al., 2019;Qian et al., 2019;Yang Z et al., 2020). Up to now, the molecular mechanisms for development of drug resistance in PC still remain largely unclear.
Noncoding RNAs (ncRNAs) transcripts, a series of RNA molecules lacking of proteins coding potentials, constitute about 70% of human genome and modulate most signaling pathways, physiological and pathological processes (Jadeja et al., 2020;Song et al., 2020;Su et al., 2020;Wang et al., 2020;Wei et al., 2020). Multiple types of ncRNAs have been identified, such as microRNAs (miRNAs), long ncRNAs (lncRNAs), circular RNAs (circRNAs), small nucleolar RNAs (snoRNAs) and PIWI-interacting RNAs (piRNAs) Pammer et al., 2020;Qin et al., 2020;Xu W et al., 2020;Zhang Y et al., 2020;Zhao et al., 2020). A number of miRNAs, lncRNAs and circRNAs have been shown to be related to various cell behaviors, such as cell growth, apoptosis, cell cycle progression, EMT and autophagy (Luo et al., 2023;Xu, 2023). It has been demonstrated that many dysregulated ncRNAs in PC not only might serve as biomarkers of diagnosis and prognosis, but also contribute to resistance development to therapeutic agents and irradiation (Hao et al., 2019;Xu et al., 2019;Li et al., 2020b;Franses et al., 2020;Nguyen et al., 2020;Ye et al., 2020;Yu et al., 2020). Although a large number of ncRNAs have been found to be involved in PC chemoresistance, only a few ncRNAs were reported to confer resistance to targeted therapy in PC. miR-142-5p has been shown to modulate the expression of PD-L1 in PC cells and promote anti-tumor immunity , nevertheless, no studies have reported the impacts of ncRNAs on immunotherapy resistance of PC cells.
Considering the critical and complicated roles of ncRNAs in PC drug resistance development, herein, we systematically summarized the underlying molecular mechanisms how miRNAs, lncRNAs and circRNAs confer drug resistance in PC. miRNAs and drug resistance miRNAs, a subclass of sncRNAs, negatively regulate target gene expression through binding to the 3′-untranslated region (3′-UTR) of the target mRNA and are involved in multiple cellular process, such as cell difference, proliferation, apoptosis, cell cycle progression, angiogenesis, EMT and CSC formation. Several dysregulated miRNAs, functioning as oncogenes or tumor suppressors, have been identified to be involved in resistance development to therapeutic agents in PC (Li et al., 2020a;An and Zheng, 2020;Guo et al., 2020;Jiang et al., 2020;. Compared with naive cells, some abnormally expressed miRNAs have been reported in various drug-resistant PC cells (Dhayat et al., 2015;Shen et al., 2015;Tian et al., 2016). The involvement of miRNAs in PC resistance to GEM, 5-FU and other drugs are summarized below.

MiRNAs and GEM resistance
GEM, a kind of cytosine nucleoside derivative, is currently the first-line standard chemotherapeutic agent to treat PC patients. However, only a few PC patients can maintain sensitivity to GEM chemotherapy and subsequently, inevitable drug resistance often leads to low response rate and poor treatment efficacy. A variety of miRNAs have been found to be associated with GEM resistance in PC. Multiple mechanisms, such as diminished apoptosis, increased DNA repair, disordered cell cycle, decreased intracellular drug accumulation due to increased expression of drug efflux transporters, EMT, as well as CSCs, play crucial roles in miRNAs-mediated development of GEM resistance in PC. Oncogenic and tumor suppressor miRNAs involved in GEM resistance are summarized in details (Table 1; Table 2).
Recently, it has been found that exosomal miR-106b deriving from CAFs, could confer GEM resistance through targeting TP53INP1 in PC (Fang et al., 2019). By contrast, exosomes deriving from pancreatic CSCs with GEM resistance could deliver miR-210 and transform GEM-sensitive cells to drug-resistance cells via activating the mTOR signaling pathway . EMT and CSCs also play crucial roles in development of drug resistance. It has been shown that several miRNAs conferred GEM resistance of PC via modulating EMT and CSCs. miR-301, a highly expressed miRNA in GEM-resistant Capan-2 and PANC-1 cells, could trigger EMT and potentiate GEM resistance through inhibiting cadherin 1 (CDH1) expression (Funamizu et al., 2019). miR-301a-3p has also been shown to confer GEM resistance via suppression of PTEN expression (Xia et al., 2017). Hypoxia, a prevalent phenomenon during tumorigenesis, has been found to promote resistance of chemotherapy and radiotherapy in PC. miR-301a, a hypoxia-sensitive miRNA, has been shown to be involved in hypoxia-induced GEM resistance via targeting P63 family member TAp63 in PC (Luo et al., 2018). In addition, through suppressing a pro-apoptotic gene of Bcl2-related ovarian killer (BOK) and EMT marker vimentin and N-cadherin, exogenous expression of miR-296-5p has been found to weaken the apoptosis induced by GEM, indicating that targeting miR-296-5p may have therapeutic potential to overcome GEM resistance of PC patients (Okazaki et al., 2020). Ectopic expression of miR-1246 could promote CSC-like properties and GEM resistance via inhibiting expression of CCNG2. Moreover, high expression levels of miR-1246 in PC tissues predicted poor prognosis of patients (Hasegawa et al., 2014).
In addition, oncogenic miR-125a, miR-10a-5p and miR-342-3p have also been found to enhance GEM resistance of PC cells through targeting A20, transcription factor activating protein 2 gamma (TFAP2C) and Krüppel-like factor 6 (KLF6), respectively (Yao et al., 2016;Xiong et al., 2018;Ma et al., 2019). Plasma miR-744 might be a valuable marker to predict poor prognosis and GEM resistance through PC patients' plasma miRNA profiling analyses (Miyamae et al., 2015). Interestingly, a novel miR-135b-BMAL1-YY1 signaling, which could promote tumorigenesis and GEM resistance in pancreas, has been identified . In addition, miR-320c could confer GEM resistance via modulating expression of SMARCC1, a core subunit of chromatin remodeling complex of switch/sucrose nonfermentable (SWI/SNF) in PC cells . Also, miR-93-5p could promote GEM resistance in PC cells through silencing expression of its target gene PTEN and, thus activating the PI3K/Akt pathway. miR-331-3p could confer GEM resistance in PC cells through targeting ST7L and activating the Wnt/β-Catenin signaling (Zhan et al., 2020;Wu et al., 2021). miR-3178 has also been found to promote GEM resistance via activating the RhoB/PI3K/Akt signaling and upregulation ABC transporters (Gu et al., 2022).

Tumor suppressive miRNAs and GEM resistance
Multiple tumor suppressive miRNAs also participate in regulating GEM resistance of PC cells through different mechanisms, such as apoptosis, cell proliferation, cell cycle, EMT, CSCs, autophagy and glycolysis. miR-33a, for example, could suppress GEM resistance and cell proliferation in PC via targeting Pim-3 and inhibiting the Akt/GSK-3β/β-catenin signaling pathway (Liang et al., 2015b). Similarly, miR-33a could enhance the sensitivity to GEM in human PC cells through inhibiting β-catenin nuclear translocation, suppressing survivin, cyclin D1 (CCND1) and multi-drug resistance 1 (MDR-1) transcription, as well as reducing protein expression of N-cadherin, slug and vimentin (Liang et al., 2015a). miR-497, a downregulated miRNA in GEM-resistant PC cells, could reverse GEM resistance through silencing expression of its target genes FGF2 and FGFR1 (Xu et al., 2014). It has been shown that overexpression of miR-210 could also potentiate GEM sensitivity via suppressing expression of its target gene ABCC5 (Amponsah et al., 2017). In addition, tumor suppressors miR-1285, miR-608, miR-506, miR-30a-5p and miR-146a-5p have been demonstrated to impair GEM resistance of PC by silencing expression of YAP1/ EGFR/β-catenin, ribonucleotide reductase M1 (RRM1)/CDA, sphingosine kinase 1 (SPHK1)/Akt/NF-κB, FOXD1/ERK and the tumor necrosis factor receptor-associated factor 6 (TRAF6)/NF-kB p65/P-gp signaling, respectively Huang et al., 2017;Rajabpour et al., 2017;Zhou et al., 2019;Meng et al., 2020). miR-34 has been found to induce apoptosis, cell cycle arrest in G1 and G2/M phase and sensitize PC cells to GEM via suppressing expression of its target genes Bcl-2 and Notch1/2. Moreover, miR-34 could inhibit growth of CSCs and tumor spheres in vitro and tumorigenesis in vivo (Ji et al., 2009). Additionally, miR-34 has also been shown to potentiate GEM-induced apoptosis of PC cells through inhibiting expression of Slug and elevating expression of p53 upregulated modulator of apoptosis (PUMA) (Zhang Q A et al., 2018). miR-494, a miRNA with decreased levels in PC tissues and cells, could  (2021) a miRNAs, downregulated in gemcitabine resistant pancreatic cancer cells. This table shows 39 miRNAs, whose expression levels and potential targets in gemcitabine resistance of pancreatic cancer.
Tumor suppressive miRNAs are also involved in regulating the EMT process and/or CSC formation which have been associated with development of GEM resistance in PC. For instance, tumor suppressors miR-509-5p and miR-1243 have been found to enhance GEM sensitivity through modulating expression of EMT markers E-cadherin, Z O -1, Zinc finger E-box binding homeobox transcription factor 1 (ZEB1) and Snail in PC (Hiramoto et al., 2017). In addition, miR-200c, a miRNA with significantly reduced levels in human CSCs of PC (PCSCs), could effectively overcome GEM resistance and diminish colony formation of PCSCs (Ma et al., 2015). Similarly, in Capan-1, Capan-2, PANC-1, MIAPaCa-2, BxPC-3 and PL45 PC cell lines, miR-200b levels have been found to be negatively correlated with GEM resistance. Moreover, miR-200b overexpression could enhance GEM sensitivity by modulating expression of EMT markers ZEB and CDH1 (Funamizu et al., 2019). In addition, miR-125a-3p and miR-3656 could also potentiate GEM sensitivity of PC cells through silencing Fyn and RHOF and interfering EMT process, respectively (Yang S Z et al., 2017;Liu B et al., 2018). The miR-17-92 cluster miRNAs, which were downregulated in chemoresistant PCSCs, could reverses GEM resistance and quiescence through the NODAL/ACTIVIN/TGF-β1 signaling (Cioffi et al., 2015). Moreover, miR-205 has been shown to suppress the PCSCs proliferation and reduce GEM resistance via silencing expression of tubulin beta 3 class III (TUBB3), RRM1 and ZEB1 (Chaudhary et al., 2017). Similarly, tumor suppressor miR-497, downregulated in CSCs from BxPC-3 and ASPC-1 PC cells and PC tissues, has also been found to inhibit GEM resistance and metastasis via directly targeting NF-κB1. On the contrary, suppression of miR-497 could dramatically contribute to GEM resistance, migration and invasion of PC CSCs .

miRNAs and 5-FU resistance
As a thymidylate synthase inhibitor, 5-FU is commonly used for PC treatments in clinic. 5-FU leads to apoptosis and cell cycle arrest through interfering DNA replication, RNA function and protein synthesis. It has been demonstrated that several oncogenic or tumor suppressor miRNAs contribute to the resistance to 5-FU in PC (Table 3).
Multiple oncogenic miRNAs, such as miR-21, miR-221, miR-296-5p, miR-320a and miR-499a-5p, have been found to be involved in 5-FU resistance in PC. For example, miR-21 could confer 5-FU resistance in human PATU8988 and PANC-1 PC cells via inhibiting the expression of tumor suppressor genes PTEN and PDCD4 . In tumor-initiating stem-like PC cells (L3.6 pL), suppression of miR-21 and miR-221 have been shown to reduce side population (SP) cell fraction and reverse 5-FU resistance . Moreover, low expression of miR-21 not only was associated with good prognosis of PDAC cases treated with 5-FUbased adjuvant regimens in two independent cohorts, but also could potentiate the sensitivity to 5-FU in PL45 and HPAF-II PC cells (Hwang et al., 2010). miR-221-3p could promote cell proliferation, EMT and 5-FU resistance via targeting the RB1 3′-UTR region in PC . In addition, oncogenic miR-296-5p and miR-320a contribute to 5-FU resistance of PC cells by modulating BOK, vimentin, N-cadherin and PDCD4 expression levels, respectively Okazaki et al., 2020). miR-499a-5p could promote cell proliferation, migration and 5-FU resistance in PC cells through targeting PTEN and activating the PI3K/Akt pathway. Moreover, miR-499a-5p has been shown to influence the expression of MDR-related genes, including adenosine triphosphate (ATP) binding cassette subfamily B member 1 (P-gp), ATP binding cassette subfamily C member 1 (MRP1), and ATP binding cassette subfamily G member 2 (BCRP) (Ouyang et al., 2021).
By contrast, several tumor suppressor miRNAs can reverse 5-FU resistance of PC. MiR-137, for instance, has been found to be markedly downregulated in PC cell lines and tissues. Overexpression of miR-137 could sensitize cells to 5-FU through inhibiting pleiotropic growth factor (PTN) expression . In addition, miR-138-5p and miR-494, which were both downregulated in PC tissues and cell lines, have been shown to increase 5-FU sensitivity through targeting vimentin, SIRT1 and c-myc expression, respectively Yu et al., 2015).
miRNAs and resistance to other drugs ADM, DDP, oxaliplatin and FOLFIRINOX (a combination regimen of folinicacid, 5-FU, irinotecan and oxaliplatin), targeted Frontiers in Cell and Developmental Biology frontiersin.org therapy drugs and immunotherapy agents are also used during PC clinical treatments. It has been found that several tumor suppressor miRNAs, including miR-137, miR-142 and miR-212, could weaken ADM resistance of PC. For example, through targeting ATG5 and improving autophagy, exogenous expression of miR-137 could enhance ADM sensitivity and promote apoptosis in PC cells (Wang Z C et al., 2019). Interestingly, the plectin-1(PL-1)/miR-212 nanoparticles could significantly promote ADM-induced apoptosis and autophagy by silencing the expression of ubiquitin specific peptidase 9 X-linked (USP9X) in PC cells . Oncogenic miR-223 has been shown to promote proliferation and DDP resistance via targeting forkhead transcription factor O subfamily 3a (FoxO3a) in PC cells . In human MiaPaCa2 and BxPC3 PC cells harboring P53 mutations, exogenous expression of tumor suppressor miR-34 could not only result in cell cycle arrest, apoptosis, the reduced tumor-initiating cell population and tumor sphere growth, but also sensitize the cells to DDP through down-regulating Notch1/2 and Bcl-2 expression (Ji et al., 2009). miR-100, a downregulated miRNA in PC tissues and cell lines, could increase DDP sensitivity and suppress tumor growth in vivo via targeting fibroblast growth factor receptor 3 (FGFR3) . In addition, it has been found that miR-374b, a downregulated miRNA in DDP-resistant PC cell line BxPC3-R, contributed to the acquired DDP resistance, at least partly by targeting ATP7A (ATPase, Cu 2+ Transporting, Alpha Polypeptide) and clusterin (CLU) (Schreiber et al., 2016). Tumor suppressor miR-1291-5p could act as a metabolism regulator and potentiate the sensitivity to DDP via diminishing glucose transporter protein type 1 (GLUT1) expression and GLUT1mediated glycolysis in ASPC-1 and PANC-1 PC cells . Laura and colleagues showed that inhibition of miR-181a-5p could potentiate oxaliplatin sensitivity of PC cells via suppressing expression of ATM. Moreover, PC patients with better response to FOLFIRINOX displayed lower levels of miR-181a-5p both in cancerous tissues and plasma specimens (Meijer et al., 2020). Interestingly, through regulation of DNA damage, miR-1307 has been shown to modulate FOLFIRINOX sensitivity in PDAC cells (Carotenuto et al., 2021). In addition to be involved in chemoresistance, a few miRNAs have been shown to confer resistance to targeted therapy and immunotherapy in PC. For instance, silencing oncogeneic miR-21 could potentiate the sensitivity to sunitinib in PDAC (Passadouro et al., 2014). Izumchenko and colleagues showed that silencing of tumor suppressor miR-200 could upregulate the expression of negative EGFR regulator of mitogen-inducible gene 6 (MIG6) in the process of transforming growth factor β (TGFβ)-mediated EMT. Moreover, the ratio of MIG6 mRNA to miR-200 (MIG6 mRNA/miR-200) was negatively correlated with erlotinib response not only in cancer cell lines with diverse tissue origins in vitro, but also in xenografts derived from PC patients carrying wild-type EGFR in vivo (Izumchenko et al., 2014). Similarly, tumor suppressor miR-497 could impact erlotinib resistance via modulating expression levels of fibroblast growth factor 2 (FGF2) and fibroblast growth factor receptor 1 (FGFR1) (Xu et al., 2014). Through inhibiting the expression of erythropoietin-producing hepatocellular receptor 2 (EphA2), tumor suppressor miR-124 has also been found to improve erlotinib sensitivity in Capan-1 PC cells with K-RAS mutations . Table 4.

lncRNAs and drug resistance
LncRNAs could be divided into four types according to their location in the genome: intronic lncRNAs, intergenic lncRNAs, divergent lncRNAs and antisense lncRNAs (Lee, 2012). Accumulating evidences demonstrated that multiple lncRNAs, functioning as oncogenes or tumor suppressors, contribute to tumorigenesis, disease progression and therapy response by regulating specific target genes or signaling pathways (Johnsson et al., 1991;Feng et al., 2020;Gong et al., 2020;He et al., 2020; Qu et al., 2021). In PC, lncRNAs have been found to be involved in development of drug resistance (Table 5; Table 6).

lncRNAs and GEM resistance
Similar with miRNAs, several lncRNAs have been shown to participate in GEM resistance in PC, including oncogenic lncRNAs and tumor suppressive lncRNAs molecules (Table 5).
HOTTIP, an overexpressed lncRNA in PDAC tissues and cells, has been found to promote GEM resistance by modulating levels of HOXA13 in PDAC cells. In contrast, silencing HOTTIP could lead to enhanced sensitivity of PC cells to GEM . LncRNA GSTM3TV2, which was significantly upregulated in GEM-resistant PC cells, could confer GEM resistance by competitively sponging let-7 and subsequently up-regulating expression of L-type amino acid transporter 2 (LAT2) and oxidized low-density lipoprotein receptor 1 (OLR1). Moreover, the increased levels of GSTM3TV2 in PC tissues have been significantly associated with worse prognosis, indicating that GSTM3TV2 may be a novel prognostic marker and therapeutic target in PC . LncRNA MALAT-1, initially identified as a prognostic marker for lung cancer patients, was involved in PC chemoresistance. It has been demonstrated that MALAT-1 could not only reduce GEM sensitivity, but also potentiate the proportion and self-renewal ability of PCSCs, by up-regulating expression of self-renewal related factor Sox2 (Jiao et al., 2015). Interestingly, over-expression of lncRNA PVT1 could promote GEM resistance in naïve ASPC-1 PC cells (You et al., 2011). Moreover, PVT1 could confer the resistance to GEM in PC via regulating the miR-619-5p/Pygo2 and miR-619-5p/ATG14 axes, which lead to dysregulated autophagic activities and the Wnt/βcatenin signaling . Similarly, silencing of PVT1 could suppress autophagy and promote GEM sensitivity through modulating the miR-143/HIF-1α/VMP1 axis in PC (Liu S L et al., 2021). In addition, oncogenic lncRNA TUG1 has also been shown to contribute to GEM resistance of PDAC cells via inducing expression of SCH772984 which is an ERK pathway suppressor . LncRNA DiGeorge syndrome critical region gene 5 (DGCR5) could enhance GEM resistance via functioning as a ceRNA through sponging miR-3163 to inhibit the Wnt/β-catenin signaling and modulate expression of DNA topoisomerase 2-alpha (TOP2A) . LncRNA HIF1A-AS1 (antisense RNA1 of HIF-1α) has been found to prevent GEM sensitivity of PC cells through activating the AKT/YB1/HIF-1α signaling and promoting glycolysis (Xu F et al., 2021a). Interestingly, lncRNA urothelial carcinoma-associated 1 (UCA1), which was delivered by hypoxic pancreatic stellate cells (PSCs)-derived exosomes (HPSC-EXO), has also been shown to enhance GEM resistance in PC (Chi et al., 2021). Through enhancer of zeste homolog 2 (EZH2) -mediated epigenetic modification, lncRNA small nucleolar RNA host gene 16 (SNHG16) has been found to confer GEM resistance via diminishing SMAD family member (Smad4) expression in PC . Via sponging miR-139-5p and activating Wnt pathway, and subsequently increasing the expression of ezh2, lncRNA SH3BP5-AS1 has been demonstrated to contribute to GEM resistance of PC cells . Accumulating evidences demonstrated that CAFs are critically involved in chemoresistance (Qin et al., 2019;Yang et al., 2022). LINC00460, a lncRNA molecule mainly located in the cytoplasm, has been shown not only to correlate with GEM response in PDAC patients, but also to regulate GEM resistance of CAFs through mediating the cellular communication of PDAC cancer cells and CAFs by platelet derived growth factor subunit A (PDGFA) associated protein 1 (PDAP1)/PDGFA/PDGFR signaling pathway (Zhu et al., 2022).
Additionally, EMT and CSCs have also been shown to promote lncRNAs-mediated chemotherapy resistance. Linc-DYNC2H1-4, an upregulated intergenic lncRNA in GEM-resistant BxPC-3-GEM PC cells, could promote EMT and stemness of the parental sensitive cells. In cells, Linc-DYNC2H1-4 could sponge miR-145 and elevate levels of several EMT key players including ZEB1 and CSC markers including Lin28, Nanog, Sox2 and Oct4 (Gao et al., 2017). LncRNA SLC7A11-AS1, which was over-expressed in PDAC tissues and GEM-resistant cell lines, has also been found to potentiate PDAC stemness and GEM resistance through stabilizing nuclear factor erythroid-2-related factor 2 (NRF2) and stimulating intracellular reactive oxygen species (ROS) (Yang Z et al., 2020). In addition, oncogenic lncRNA SBF2-AS1 could promote EMT and GEM resistance in PC via sponging miR-142-3p and up-regulating expression of twinfilin 1 (TWF1) . Autophagy also contributes to drug resistance and cancer progression (Kim and Lee, 2014;Levy et al., 2017;Wu and Zhang, 2020). It has been found that lncRNAs HLA complex P5 (HCP5) and SNHG14 could confer GEM resistance of PC through sponging miR-214-3p and miR-101 and activating autophagy Zhang et al., 2019). LncRNA LINC01559 has also been found to confer GEM resistance by promoting autophagy and inhibiting apoptosis . In addition, oncogenic lncRNAs LINC00346, small nucleolar RNA host gene 8 (SNHG8) and HOST2, have also been shown to confer GEM resistance of PC via regulating the miR-188-3p/BRD4 axis, the caspase-3/PARP axis and apoptosis (Song et al., 2018;Shi et al., 2019;An and Cheng, 2020). Additionally, lncRNA ANRIL could enhance GEM resistance of PC cells through inhibiting miR-181a expression and modulating autophagy induced by HMGB1 .
Mesenchymal stem cells (MSCs), a crucial cell type in tumor micro-environment, may contribute to drug resistance in multiple neoplasms via producing protective cytokines or influencing gene expression (Houthuijzen et al., 2012;. It has been found that extracellular vesicle -loaded oncogenic lncRNA nuclear paraspeckle assembly transcript 1 (NEAT1) from adipose-derived MSCs could promote GEM resistance in PC by regulating the miR-491-5p/snail/suppressor of cytokine signaling 3 (SOCS3) signaling pathway (Wu et al., 2023).

Tumor suppressor lncRNAs and GEM resistance
Several tumor suppressor lncRNAs have also been found to be associated with the resistance to therapeutic agents in PC. For instance, lncRNA growth arrest-specific 5 (GAS5) has been demonstrated to suppress GEM resistance of PC cells through inhibiting miR-221 expression and increasing SOCS3 expression . Interestingly, GAS5 has also been found to antagonize GEM resistance in PC cells via negatively regulating miR-181c-5p expression and subsequently activating the Hippo signaling (Gao et al., 2018). AB209630, an evidently downregulated lncRNA in PDAC tissues, could improve GEM sensitivity of PDAC cells by modulating the PI3K/Akt pathway. Moreover, high levels of lncRNA AB209630 were associated with good prognosis of PDAC patients . MEG3, which is a downregulated lncRNA in PC tissues and cells, has been found to sensitize GEM in PC cells. On the contrary, silencing MEG3 led to inhibited GEM sensitivities. Moreover, low expression of MEG3 in PC patients were associated with GEM resistance and poor outcomes . LncRNA DLEU2L (deleted in lymphocytic leukemia 2-like), which was downregulated in PC tissues, has been shown to lessen GEM resistance of PC cells via modulating expression of BRCA2 (Xu F et al., 2021b). Through modulating the miR-3163/USP44 axis in PC cells, tumor suppressive lncRNA DBH-AS1 has been found to reverse GEM resistance and inhibit cell growth . Most recently, down syndrome critical region 9 (DSCR9), a downregulated lncRNA in PC cells and tissues, has also been shown to suppress the proliferation, invasion and GEM resistance by miR-21-5p/BTG anti-proliferation factor 2 (BTG2) axis .

lncRNAs and resistance to other drugs
In addition to participate in GEM resistance, lncRNAs have also been shown to promote other drugs resistance, such as 5-FU, DDP, oxaliplatin, FOLFIRINOX and targeted therapy agents (Table 6).

Oncogenic lncRNAs and resistance to other drugs
LncRNA TUG1 could promote 5-FU resistance in PC cells via inhibiting expression of miR-376b-3p and elevating dihydropyrimidine dehydrogenase (DPD) expression (Tasaki et al., 2021). HOTTIP has been found to promote DDP resistance of PC cells through silencing miR-137 (Yin et al., 2020). Similarly, oncogenic lncRNA UPK1A-AS1 could confer oxaliplatin resistance induced by paracrine IL8 derived from CAFs via activating Ku70 and Ku80 interaction and, thus, promoting  . Interestingly, oncogenic lncRNA HOX transcript antisense gene RNA (HOTAIR) has been demonstrated to confer resistance to TNF-related apoptosis-inducing ligand (TRAIL) via interacting with the epigenetic regulator EZH2 and inhibiting TRAIL receptor death receptor 5 (DR5) expression. Silencing HOTAIR could enhance TRAIL-induced apoptosis in TRAIL-resistant PC cells. On the contrary, HOTAIR over-expression could inhibit apoptosis induced by TRAIL in TRAIL-sensitive cells, indicating that HOTAIR may act as a potential therapeutic target to conquer TRAIL-resistance in PC (Yang R M et al., 2017). In addition, it has been shown that lncRNA small nucleolar RNA host gene 7 (SNHG7) could modulate activities of MSCs and confer FOLFIRINOX resistance in PC through the Notch1/Jagged1/Hes-1 signaling pathway . Aerobic glycolysis, a hallmark of PC, has been shown to promote tumorigenesis and progression via multiple mechanisms. Glycolysis-related lncRNA LINC02432 could inhibit ferroptosis and predict drug sensitivity to EGFR inhibitors (afatinib and sapitinib), MEK inhibitors (trametinib, PD0325901 and selumetinib) and ERK inhibitors (VX-11e, Ulicocitinib, SCH772984, and ERK_6604) in PC by regulating miR-98-5p/hexokinase 2 (HK2) axis (Tan et al., 2022).

Tumor suppressive lncRNAs and resistance to other drugs
Unlike tumor suppressive miRNA, few tumor suppressor lncRNA has been found to be associated with other drugs resistance. Until now, only tumor suppressive lncRNA GAS5 could prevent 5-FU resistance in PC cells by sponging miR-181c-5p and subsequently activating the Hippo signaling (Gao et al., 2018).

circRNAs and drug resistance
CircRNAs, a special class of ncRNAs with covalently closed-loop structure, have been shown to function as important regulator in various tumors, including PC (Kristensen et al., 2019;Wong et al., 2020). Amounting data indicated that circRNAs function in carcinogenesis and progression of PC (Shao et al., 2018;Chen W et al., 2019). Importantly, several circRNAs have been associated with development of drug resistance in PC (Table 7).
CircHIPK3, an upregulated circRNA in PC tissues and GEMresistant PC cells, has also been found to confer GEM resistance through regulating RASSF1 expression . Through depriving the suppression of miR-23b-3p on expression level of deacetylase SIRT1 and leading to increased glycolysis, circZNF91 has been shown to confer GEM resistance in PC cells (Zeng et al., 2021). Oncogenic circ-MTHFD1L could induce DNA damage repair and confer GEM resistance in PDAC through the miR-615-3p/RPN6 axis. On the contrary, inhibition of circ-MTHFD1L combined with olaparib could reverse GEM resistance . Similarly, circ_ Frontiers in Cell and Developmental Biology frontiersin.org 0074298 has also been found to promote GEM resistance and PC progression through sponging miR-519 and regulating SMOC expression . Interestingly, circFARP1 could enable CAFs to promote GEM resistance in PC via the leukemia inhibitory factor (LIF)/STAT3 axis . CircLMTK2, an overexpressed circRNA in GEM-resistant PC cells and PC tissues, could contribute to GEM resistance via modulating p21 (RAC1) activated kinase 1 (PAK1) by sponging miR-485-5p (Lu et al., 2022). Similarly, tumor suppressive circRNA has also been demonstrated to participate in the resistance to GEM. For example, tumor suppressor circ_0092367 could inhibit EMT and enhance GEM sensitivity in PC cells through regulating the miR-1206/epithelial splicing regulatory protein 1 (ESRP1) axis .

circRNAs and resistance to other drugs
To date, no report has shown that oncogenic circRNAs could involve in the resistance to other drugs. In the aspect of tumor suppressor circRNAs, only circ_0013587 has been found to reverse erlotinib resistance through modulating the miR-1227/E-cadherin signaling in PC cells (Xu H et al., 2021).

Conclusion
Many ncRNAs have been shown to be involved in the pathogenesis and progression of PC, indicating the potential roles of ncRNAs as biomarkers for early diagnosis, as promising prognostic and predictive markers for the identification of candidates amenable to adjuvant treatment, enabling a personalized clinical approach. Interestingly, a number of studies attempted to investigate the diagnostic value of circulating miRNAs in PC (Previdi et al., 2017). Recently, a pilot study has proposed that an exosomal four miRNA biomarker panel, consisting of miR-93-5p, miR-339-3p, miR-425-5p, and miR-425-3p, may provide a promising avenue for PC screening (Makler and Asghar, 2023). Although the diagnostic and prognostic potential of ncRNAs have shown the promising results, large and An outlined diagram of miRNAs, lncRNAs and circRNAs participated in the resistance to therapeutic agents in pancreatic cancer. Several miRNAs, lncRNAs, and circRNAs have been shown to be associated with pancreatic cancer drug resistance via regulating apoptosis, cell cycle arrest, DNA damage repair, cell proliferation, autophagy, epithelial-mesenchymal transition (EMT), drug efflux transporter, reactive oxygen species, glycolysis and cancer stem cells (CSC) through regulating specific signaling pathways and target genes.
Frontiers in Cell and Developmental Biology frontiersin.org prospective validation studies should be implemented before they can enter clinical practice. Also, ncRNAs can be used as targets for novel therapeutics of PC patients. Accumulating evidences indicated that different kinds of ncRNAs have been reported to be involved in drug resistance of PC. As shown in Figure 1, we summarized how miRNAs, lncRNAs and circRNAs contribute to resistance development to therapeutic agents and the underlying molecular mechanisms in PC. Interestingly, drug resistance may be reversed through targeting specific endogenous miRNAs, lncRNAs and/or circRNAs. Several approaches could be employed to inhibit expression of dysregulated ncRNAs, such as small interfering RNAs (siRNAs), short hairpin RNAs (shRNAs), antagomirs, anti-oligonucleotides (ASOs), clustered regulatory interspaced short palindromic repeatsassociated endonuclease 9 (CRISPR/Cas9)-based genome editing, small molecule inhibitors of ncRNAs, and artificial miRNA sponges (Lin et al., 2020;Pandya et al., 2020). The strategies which could restore the normal levels of tumor suppressor ncRNAs involved in drug resistance of PC include to replace or substitute these ncRNAs through synthetic ncRNA-like molecules, for example, miRNA mimics. However, multiple challenges for therapeutic targeting ncRNAs remain to be faced, such as off-target effects, lack of efficient carrier systems, immune related toxicities, tolerability and other side effects. Progresses in ncRNAs delivery vector systems help to increase the potential for ncRNA-based treatment. Due to the improved circulation time and diminished recognition by the immune system of host, nanoparticles have been shown to function as efficient vectors for various gene therapies in PC patients, including siRNAs and miRNAs (Kurtanich et al., 2019). Through searching the database of http://clinicaltrials.gov, no clinical trials based on ncRNAs therapeutics in PC were found currently. The combined therapeutic modalities based on the manipulation of ncRNAs' levels and traditional treatments, i.e. chemotherapy, molecular targeted therapy or immunotherapy, may be a promising approach to overcome drug resistance and help to improve prognosis of advanced PC patients. Nevertheless, a serious issue is how to select the appropriate target molecules from a huge number of ncRNA candidates. More importantly, clinical studies and translational studies are also needed for conquering drug resistance in PC.

Author contributions
MY conceived and designed this study. LW and MY collected data, conceived this review, provided project funding, and wrote the manuscript. JS, XW, YiH, LHan, LHuang, YZ, YX, and NZ performed the initial work. NZ critically revised the manuscript for important intellectual content. All authors contributed to the article and approved the submitted version.

Funding
This work was financially supported by the Natural Science

Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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