The interplay between non-coding RNAs and alternative splicing: from regulatory mechanism to therapeutic implications in cancer

Alternative splicing (AS) is a common and conserved process in eukaryotic gene regulation. It occurs in approximately 95% of multi-exon genes, greatly enriching the complexity and diversity of mRNAs and proteins. Recent studies have found that in addition to coding RNAs, non-coding RNAs (ncRNAs) are also inextricably linked with AS. Multiple different types of ncRNAs are generated by AS of precursor long non-coding (pre-lncRNAs) or precursor messenger RNAs (pre-mRNAs). Furthermore, ncRNAs, as a novel class of regulators, can participate in AS regulation by interacting with the cis-acting elements or trans-acting factors. Several studies have implicated abnormal expression of ncRNAs and ncRNA-related AS events in the initiation, progression, and therapy resistance in various types of cancers. Therefore, owing to their roles in mediating drug resistance, ncRNAs, AS-related factors and AS-related novel antigens may serve as promising therapeutic targets in cancer treatment. In this review, we summarize the interaction between ncRNAs and AS processes, emphasizing their great influences on cancer, especially on chemoresistance, and highlighting their potential values in clinical treatment.


ene express
on, involves the removal of introns from precursor messenger RNAs (pre-mRNAs) and the merging of the exons to form mature mRNAs [2][3][4].Alternative selection of spliced exons, also known as alternative splicing (AS), can result in different protein products from a single primary transcript, and thus AS acts as a predominant post-transcriptional regulatory mechanism of gene expression [5,6].

In addition to coding RNAs, non-coding RNAs (ncRNAs) are also inextricably linked with AS.Non-coding RNAs (ncRNAs) are functional RNAs that do not encode proteins.The advancement in high-throughput sequencing in the ast decades has allowed us to identify a large number of ncRNAs, including long non-coding RNAs (lncRNAs),


Ivyspring

International Publisher microRNAs (miRNAs), circular RNAs (circRNAs), small nucleolar RNAs (snoRNAs) and small nuclear RNA (snRNAs), which are generated by AS of pre-lncRNAs or pre-mRNAs [7][8][9][10][11].R

ent studie
have shown that ncRNAs, originally considered as transcriptional "junks", play vital roles in gene expression regulation, including AS [12][13][14][15][16][17].Extensive interactions between ncRNAs and AS have been reported, which contribute to the complexity of gene regulation in cancer.Therefore, deciphering their interplay would help to determine cancer pathogenesis of cancer and provide new insights into cancer therapy.In this review, we summarize the molecular mechanisms and potential roles of the interaction between ncRNAs and AS in the development, progression and multi-drug resistance (MDR) of various cancers and discuss the latest development in therapeutic strategies targeting AS or AS-related ncRNAs.


The regulation mechanism of RNA splicing

Pre-mRNA splicing in the eukaryotic cell was first discovered in 1977 using an electron microscope [18][19][20].To date, our understanding of RNA splicing has been greatly improve

As seen in Figure 1, RNA splicing is a hi
hly regulated process, which requires coordination between spliceosomes, cis-acting elements, and trans-acting proteins to remove introns from pre-mRNAs and merge the protein-coding exons to generate mature mRNAs [21].

The introns contain three important sites, the 5' splice site (5'SS), branch point site (BPS), and 3' splice site (3'SS), which are short conserved sequences (Figure 1A).In general, the RNA splicing process is a two-step tra sesterification reaction that begins with a nucleophilic attack at the 5' SS (also known as the splice donor) by a 2'-hydroxyl group of the BP adenosine.This reaction creates a cleaved 5' exon and a lariat structure containing the intron and the 3' exon.Subsequently, the 3'-hydroxyl group on the detached 5' exon attacks the 3' SS (also referred as the splice acceptor), which removes the intron and ligates the exons to produce a mature mRNA [22,23].

In the splicing process, the generation of spliceosome plays an important function (Figure 1B).The spliceosome is a large multimeric ribonucleoprotein (RNP) complex which consists of five small nuclear ribonucleoproteins (snRNPs , including U1, U2, U4/U6, and U5.The U1 and U2 snRNPs recognize the 5'SS and the BP sequence, respectively, and functionally generate the pre-spliceosome.The pre-spliceosome then associates with the preassembled U4/U6/U5 tri-snRNP to form the fully assembled spliceosome to execute the splicing function [24][25][26][27].

In addition, some other important cis-acting elements on pre-mRNA such as intronic splice enhancers and silencers, exonic splice enhancers and silencers (ISEs, ISSs, ESEs, and ESSs, respectively) are important for the splicing regul tion by recruitment of splicing factors such as serine/arginine-rich (SR) proteins and heterogenous nuclear ribonucleoproteins (hnRNPs) (Figure 1A).The SR proteins contain one or more RNA recognition motifs that bind to ESEs and ISEs on the pre-mRNA to recruit other SFs.The hnRNPs are generally more diverse in their RNA-binding domain and preferentially interact with the splicing silencers [28][29][30].

More than 95% of human genes undergo AS, which contributes to the diversity of RNA transcripts and protein products [31][32][33].Figure 1C illustrates different types of AS, including exon skipping (ES), intron retention (IR), alternat ve 5' or 3' splice site selection (A5SS or A3SS) and mutually exclusive exons (MXE) [15,34].AS is regulated by cis-acting elements in pre-mRNAs and trans-acting factors, whose mutation or deregulation could lead to aberrant AS events and is usually associated with tumorigenesis.


NcRNAs produced by RNA splicing

Similar to mRNAs, the majority of ncRNAs are produced by the splicing of primary transcripts (also known as host genes).In this section, we discuss the splicing mechanisms underlying the biogenesis of va

ous ncRNAs, including lncRNAs, c
rcRNA, snoRNAs and sno-derived RNAs (sdRNAs), and clarify their biological significance in tumorigenesis and progression (Figure 2, Table 1 and 2).


AS of lncRNAs from pre-lncRNAs

LncRNAs are >200 nucleotide-long RNA transcripts that do not encode proteins.They are generated from the splicing of pre-lncRNAs (Figure 2A).AS of lncRNAs produces different isoforms of lncRNAs that might e

rt different functions in cance
[35][36][37][38][39].For example, a multi-exon lncRNA PXN-AS1, which is regulated by SFs MBML3 and DDX17, could be spliced into multiple isoforms in hepatocellular carcinoma (HCC) [35].MBNL3 promotes the inclusion of the exon 4 of PXN-AS1 to produce PXN-AS1-L, which inhibits myeloid cell leukemia (Mcl)-mediated cell apoptosis in a PXN-dependent manner [35].Whereas DDX17 induces the retention of the intron 3 of PXN-AS1 to produce a novel aberrant isoform, PXN-AS1-IR3, which promotes HCC metastasis by inducing MYC transcription activation [36].The cis-acting elements on the primary transcription product include a 5' splice site (5'SS), 3' splice site (3'SS), branch point site (BPS), polypyrimidine tract (PPT), and splicing regulatory elements (SREs) in the proximity of splice site.The SREs are subdivided into intronic and exonic splice enhancers and silencers (ISEs, ISSs, ESEs, and ESSs, respectively).SR proteins as splicing activators enhance the utilization of splice sites by preferentially combining with ESEs and ISEs; and conversely, hnRNPs as splicing repressors inhibit the binding to the splice sites by interacting with ESSs and ISSs.(B) U1 snRNP recognizes 5'SS and binds it via base pairing, while SF1, U2AF2 and U2AF1 combines separately the BPS, PPT and 3'SS, forming early complex E. Then U2 snRNP replaces SF1 and binds to the BPS to form complex A. The U4/U6-U5 tri-snRNP complex is subsequently recruited to form a pre-catalytic spliceosome.The complex B is rearranged to form catalytically activated complex B*, which catalyzes the first transesterification reaction to produce Complex C, followed by the second transesterification reaction.Lastly, exons are interlinked to form mature mRNA, and the introns are degraded rapidly as the lariat and snRNPs are recovered.(C) RNA splicing consists of constitutive splicing (CS) and alternative splicing (AS), and various AS modes are generated based on the multiple splice sites and ways of exon linking, including exon skipping (ES), alternative 5' or 3' splice site selection (A5SS or A3SS), mutually exclusive exons (MXE) and intron retention (IR).For instance, CDR1as, circSMARCA5 and circPPP1R12A affect cancer progression through sponging miR-7, forming an R-loop with parental DNA, and producing peptides, respectively.(D) SnoRNAs (SNORA23 and SNORA42) are produced by splicing of the intronic region of the host genes, while sno-derived RNAs (sdRNAs) sdRNA68 and sdRNA104 are produced by FUS-mediated self-splicing of some snoRNAs.Both snoRNAs and sdRNAs are reported to alter the process of tumorigenesis in some cancers.

Moreover, AS of lncRNA PCAT19 also generates two isoforms, namely PCAT19-short and PCAT19-long, which exhibit reciprocal expression in pancreatic cancer (PCa) [37].The PCAT19-long isoform interacts with HNRNPAB to activate a subset of cell-cycle genes associated with PCa progression, such as CHEK1 and AURKB, while the PCAT19-short isoform possesses potential tumor suppressive function [37].In addition, NEAT1, a well-known oncogenic lncRNA, produces two isoforms, NEAT1_1 and NEAT1_2 [40,41].Among these, NEAT1_2 is significantly upregulated in papillary thyroid carcinoma (PTC) and non-small cell lung cancer (NSCLC) compared with that in noncancerous tissues [38,39,42].In PTC, NEAT1_2 significantly promotes cell growth and metastasis by acting as a sponge of miR-106b-5p to derepress ATAD2 expression [3

onal activity, consequently activating its downstre
m genes and promoting LUAD cell proliferation and invasion [44].Altogether, these findings uncovered the

vel lncRNA-mediated functions of pr
-mRNAs in cancer.


ES (exon 4) promotes tumorigenesis and progression

Ovarian cancer [133] * (-) refers to down-regulated AS products of target genes or suppressed AS events.


CircRNAs produced by back-splicing

CircRNAs are closed-loop RNA molecules produced by back splicing of the parental genes, in which the downstream splice site is reversed and joined to the upstream splice site [10].Previously, circRNAs were regarded as accidental "splicing noise" or by-products with few biological functions.However, increasing evidence suggests that circRNAs exert vital roles, such as miRNAs sponging, transcription regulation, and peptides encoding [45][46][47] (Figure 2C).For instance, circRNA CDR1as sponges miR-7 to enhance the stability of miR-7 targets including E2F3, CKAP4, and TGFBR2, thus promoting tumor growth and progression [45,48].Additionally, CircSMARCA5, derived from the back-splicing of exon 15 and exon 16 of SMARCA5, binds to the genomic location of SMARCA5 to form an R-loop, which pauses transcription at exon 15 of SMARCA5 and produces truncated nonfunctional proteins, thus increasing sensitivity to cisplatin chemotherapy of breast cancer [46].Moreover, CircPPP1R12A is generated by reverse splicing exon 24 and 25 of PPP1R12A pre-mRNA, and encodes a 73-amino acid peptide, called circPPP1R12A-73aa [47].CircPPP 1R12A-73aa pro

tes proliferation and metastasis of colon cancer thro
gh activating the Hippo-YAP signaling pathway [47].Therefore, circRNAs produced by splicing exhibit significant effects associated with carcinogenesis.


Intronic snoRNAs and sdRNAs produced by RNA splicing

The majority of snoRNAs are processed from the introns of snoRNA host genes (SNHGs) [49] (Figure 2D).For example, SNORA23 is generated by the splicing of the intronic region of the IPO-7 gene, and its elevated levels significantly promote cancer cell survival and invasion in pancreatic ductal adenocarcinoma [50].Furthermore, SNORA42 is spliced from the intron 10 of the KIAA0 07 gene, and its elevated levels play oncogenic roles via driving the malignant phenotype in NSCLC cells [51].Therefore, snoRNAs spliced from SNHGs perform essential functions during tumor development.

A preprint by Plewka P et al. argued a novel molecular mechanism of snoRNA self-splicing (Figure 2D), in which snoRNAs SNORD104 and SNORD68 interacted with RNA-binding protein (RBP) FUS and were further spliced into smaller sdRNAs (sdRNA104 and sdRNA68, respectively) [52].FUS-dependent sdRNA68 and sdRNA104 regulated the expression of two colorectal cancer (CRC)-promoting genes including KCNQ10T1-001 antisense transcript and BRE mRNA [52,53].Therefore, sdRNAs, as spliced products of snoRNAs, play critical pathological functions during tumor proliferation and progressi

.

Collectively, a numerous of ncRNAs generat
d by RNA splicing serve as potential prognostic and therapeutic biomarkers, indicating potential candidate targets for tumor prevention and treatment.


Regulation of alternative splicing by ncRNAs

Recent studies have identified that ncRNAs exert their functions as regulators of AS events.As illustrated in Fig

e 3, ncRNAs regulate AS of pre-mRNAs by influencing the tran
-acting factors or cis-acting elements, thus resulting in abnormal splicing of certain oncogenes or tumor suppressors (Table 1 and 2).


NcRNAs regulate AS through influencing trans-acting factors

NcRNAs alter the expression and function of splicing factors (SFs) via interacting with SFs as "Interactors" or "Hijackers", participating in competing endogenous RNA (ceRNA) regulatory network, encoding functional peptides and chromatin remodeling, thereby triggering tumor-associated alternative pre-mRNA splicing events.

Figure 3. Regulation of alternative splicing by ncRNAs.Noncoding RNAs including lncRNAs, circRNAs, miRNAs, and snoRNAs regulate the occurrence of carcinogenesis-related AS events through two main splicing regulatory mechanisms.One way is to interact with the trans-acting factors, comprising acting as "Interactors" or "Hijackers" of SFs, participating in ceRNA regulatory network, encoding functional peptides and modulating chromatin signatures, all of which influence the expression and function of SFs.The second

y is ncRNAs form dsRNA duplexes with t
e cis-acting elements, thus affecting tumorigenesis.The symbols (+) and (-) denote that ncRNAs act as oncogenes and tumor suppressor genes, respectively.


NcRNAs acting as "Interactors" of SFs

NcRNAs recruit and stabilize SFs, and positively regulate the process of pre-mRNA splicing as "Interactors" of SFs, consequently influencing the pathogenesis process of tumorigenesis and progression [54][55][56] (Figure 3).Exemplified by linc01232, which significantly upregulates HNRNPA2B1 protein expression by suppressing its ubiquitin-mediated degradation in PCa cells [54].Subsequently, the stabilized HNRNPA2B1 participates in the AS of A-Raf pre-mRNA and facilitates the formation of full-length A-Raf (A-Raf FL) isoform, thus promoting PCa cells metastasis [54].Besides, lncRNA DGCR5 indirectly regulates AS of Mcl-1 pre-mRNA by interacting with serine-and arginine-rich splicing factor 1 (SRSF1) to increase its stability, and contributes to the generation of the full length of Mcl-1 (Mcl-1L, anti-apoptotic isoform), thus facilitating the carcinogenesis of esophageal squamous cell carcinoma (ESCC) [55].Additionally, lncRNA SNH

interacts with hnRNPA1, which trigg
rs hnRNPA1mediated splicing of PKM pre-mRNA and promotes the expression of PKM2 over PKM1, consequently enhancing aerobic glycolysis in CRC cells [56].


NcRNAs acting as "Hijackers" of SFs

Accumulating studies have supported that ncRNAs, including lncRNA, circRNAs and snoRNAs, function as "Hijackers", which competitively bind to SFs and thereby inhibit the interaction between SFs and target pre-mRNAs in the context of tumorigenesis and development [57][58][59] (Figure 3).For example, lncRNA PNCTR recruits many RBPs PTBP1 to the peri-nucleolar compartment and blocks its binding with CHEK2 pre-mRNA [57].The interaction between PTBP1 and CHEK2 pre-mRNA results in the upregulation of a CHEK2 isoform containing exon 8 and enhances the cell survival of cancer cells [57].Furthermore, it has been convincingly found that circSMARCA5 hijacks SRSF1 and impedes the binding of SRSF1 protein to vascular endothelial growth factor A (VEGFA) pre-mRNA, thus reducing the ratio of pro-angiogenic (Iso8a) to anti-angiogenic (Iso8b) isoforms and inhibiting angiogenesis in glioblastoma multiforme (GBM) cells [58].In addition, SNORD27 is a C/D box snoRNA that competitively binds to U1 snRNAs and several SFs to form RNP complexes in HeLa cells [59].The RBPs facilitate the

ipping of exon 12 in E2F7 pre-mRNA and inhibit the
inclusion of silent exon in MAP4K3, ZBTB37, FER, and ABCA8 pre-mRNAs, thereby influencing E2F7-dependent cell cycle regulation [59].


NcRNAs alter SF expression by the ceRNA mechanism

The expression of SFs is regulated by various types of ncRNAs, which can functionally influence the outcomes of AS in cancer [60][61][62] (Figure 3).MiRNAs suppress SF expression by directly attaching to the 3'-untranslated region (3'-UTR) of the SF transcripts [60,63].For instance, miR-92a reduces the expression of RNA-biding motif 4 (RBM4) by targeting the RBM4 mRNA, which leads to elevated levels of exon 10-included nPTB transcript via an AS-coupled nonsense-mediated decay (NMD) mechanism [60].Subsequently, nPTB affects the splicing of FGFR2 and PKM2 and promotes the isoform FGFR2 and PKM2 of these two genes, respectively, thereby altering metabolic signature of CRC cells [60].More importantly, the ceRNA machinery can alter SF expression involving ncRNAs such as lncRNA, circRNAs, and pseudogenes, and these transcripts regulate each other through competing for shared miRNA regulators at the post-transcriptional level.Since microRNA response elements (MREs) exist on mRNAs, lncRNAs, circRNAs etc., and these RNAs can competitively sponge miRNAs by recognizing the same MREs [64][65][66].Thus, lncRNAs and circRNAs can functionally act as ceRNAs to sponge miRNAs and therefor modulate the expression of miRNA-targeted SF mRNAs [61,62].Taking lncRNA CCAT1 as an example, it sponges miR-490 and indirectly upregulates the expression of hnRNPA1 and subsequently facilitates hnRNPA1-mediated AS events, leading to the migration and metastasis of gastric cancer [61].Similarly, circRNA100146 directly spon

s miR-361-3p and miR-615-5p and leads to promoting SF3B3 expressio
, consequently accelerating NSCLC cell proliferation and invasion through SF3B3-mediated AS regulation [62].


NcRNAs affect the function of SFs by encoding functional peptides

NcRNAs can encode "hidden" peptides that regulate RNA splicing [67,68].As shown in Figure 3, LOC90024 encodes a splicing regulatory small protein (SRSP) that enhances the binding of SRSF3 to the exon 3 of Sp4 pre-mRNA, which induces the generation of long Sp4 isoform (encoding L-Sp4 protein), whereas suppresses short Sp4 isoform (encoding S-Sp4 peptide), ultimately promoting CRC tumorigenesis and progression [67].LncRNA HOXB-AS3 encodes a conserved 53-aa peptide named HOXB-AS3, which competitively recognizes the arginine residues in the RGG motif of hnRNPA1 and antagonizes hnRNPA1dependent PKM splicing, leading to the inhibition of PKM2 isoform and glucose metabolism in CRC cells [68].Taken together, these stud

s suggest that the ncRNAs-encoded functional peptides play important ro
es in AS regulation and may serve as novel targets for peptide-based anti-tumor drugs in the future.


NcRNAs alter the function of SFs by modulation of chromatin signatures

LncRNAs participate in the establishment of cell-specific splicing events by regulation of chromatin conformation signatures [69,70].An antisense lncRNA, asFGFR2, recruits histone demethylase KDM2a and polycomb repressive complex 2 (PRC2) to the FGFR2 locus creating an adverse chromatin environment that antagonizes the recruitment of splicing regulatory factor complex MRG15-PTB [69] (Figure 3).This regulatory mechanism eventually promotes the generation of exon IIIb-containing isoform FGFR2 IIIb, which inhibits tumorigenicity of HCC cells [69,71].Besides, ENST00000501665.2, a splicing form of lncRNA OIP5-AS1, facilitates the interaction of chromatin-remodeling complexes SWI/ SNF with the promoter of OIP5 by directly binding to multi le nuclear RBPs including a of the SWI/SNF multi-subunit molecular complex, which leads to activated transcription and splicing of OIP5 oncogene [70].

Altogether, these findings demonstrate that ncRNAs influence the e

ression and function of SFs to regulate mul
iple cancer-related AS events through different mechanisms, adding complexity into the ncRNAs-AS network.


NcRNAs regulate AS via cis-acting elements

Cis-natural antisense transcripts (cis-NATs) are a new class of RNAs transcribed from the opposite strand of a coding gene and regulate gene expression by forming the double-stranded RNA (dsRNA) with the complementary region [72,73].Cis-NATs can regulate AS of their antisense pre-mRNA, involved in diverse cellular functions during carcinogenesis [74][75][76] (Figure 3).Villamizar O et al. discovered that FAS-AS1 (SAF), a cis-NAT transcribed from the antisense strand of FAS, interacts with FAS pre-mRNA to form RNA duplexes.These duplexes recruit SPF45 to mediate exon skipping of FAS and upregulate soluble Fas (sFas) protein to protect cells against FasL-induced apoptosis [74].Similarly, lncRNA ZEB2 (Sip1) interacts with the 5'-untranslated region (5'-UTR) of ZEB2 to form a dsRNA [75].This interaction blocks the splicing of a large intron located in the 5'UTR of ZEB2 which contains an internal ribosome entry site (IRES) critical for Zeb2 expression, thus promoting EMT of cancer cells through upregulating Zeb2 protein levels [75].Moreover, lncRNA UXT-AS1, transcribed from the antisense strand of UXT, binds to the cis-acting element within UXT pre-mRNA [76].The binding reduces the pro-apoptotic UXT1 transcripts, meanwhile increases the pro-proliferative UXT2 transcripts, thereby accelerating CRC progression [76].Altogether, thes

publications reveal a novel mechanism of ncRNAs-mediated regul
tion of AS in cancer cells by RNA duplex formation with the parental pre-mRNA.


NcRNA-regulated AS events mediate drug resistance in oncology

Chemotherapy and targeted therapy are frequently used in cancer treatment.However, multi-drug resistance (MDR) continues to hinder the clinical effects of chemotherapy.AS provides an opportunity for the pro-oncogenes to gain a new function that facilitates cancer cells evade from chemotherapy [77].For instance, Androgen receptor splice variant 7 (ARV7) is associated with abiraterone resistance in castration-resistant prostate cancer [78].Besides, Δ16HER2 splice variant is associated with lapatinib resistance in breast cancer [79].Therefore, owing to the important roles and promising clinical value of AS in drug resistance, as well as above mentioned extensive interplay between ncRNAs and AS, we summarized the interactive network of ncRNA-AS in drug resistance.This may provide new insights into unde

tanding the MDR mechanism and identifying novel targets for preventing or
reversing drug resistance in cancers (Figure 4 and Table 3).


NcRNAs trigger AS of cell death-related genes to mediate chemoresistance

Apoptosis-evading cancer cells have a critical role in chemoresistance.NcRNAs regulate the AS events of apoptosis-related genes such as the BCL-2 family, pro-apoptotic caspases, and FAS, thus altering the process of apoptosis and affecting the efficacy of chemotherapeutic drugs [80][81][82] (Figure 4).LncRNA UCA1 is abnormally upregulated in cisplatin-resistant oral squamous cell carcinoma (OSCC) cells and sponges miR-184 to enhance SF1 expression.This augments SF1-mediated splicing of BCL-2 family genes containing Mcl-1 and increases BCL2 protein expression, thereby preventing apoptosis [80,83,84].Furthermore, miR-193a-3p interacts with SRSF2 to increase the anti-apoptotic variant of BCL-X (BCL-XL) and decrease the pro-apoptotic variant of caspase 9 (caspase 9a), leading to cisplatin resistance in CD44 + gastric cancer cells [81].Interestingly, EZH2, the catalytic subunit of the PRC2 involved in H3K27 methylation, hyper-methylates the lncRNA FAS-AS1 promoter and represses the FAS-AS1 expression in chemo-resistant B-cell lymphoma [82].The reduced FAS-AS1 expression causes increasing of soluble Fas r ceptor (sFAS) in a RBM5-dependent manner, which further suppresses apoptosis and leads to acquirement of chemoresistance [82,85].

Autophagy is another fundamental mechanism that affects the sensitivity of cancer cells to anticancer agents, inhibition of which can also trigger apoptosis in cancer cells [86].Increasing studies have indicated that the interplay of ncRNAs-AS mediates drug resistance in cells by regulating autophagy [87,88].For example, lncRNA CRN E interacts with SRSF6 protein and reduces its stability, thus reducing AS of PICALM and inhibiting S-to-L isoform switch [87].

Finally, CRNDE suppresses 5-FU/oxaliplatin resistance via attenuating autophagy flux in gastric cancer cells [87].Another study suggested that estrogen inhibits lncRNA EGOT in a dose-dependent manner in breast cancer [88].Low expression of EGOT interferes with pre-ITPR1/EGOT dsRNA formation and hnRNPH1 recruitment, consequently reducing autophagosome accumulation by downregulating I

R1 protein, and ultimately enhancing paclitaxel resistance [88].NcRNA-regulated AS events
ediate drug resistance in oncology.Some lncRNAs, miRNAs and circRNAs impact drug resistance of cancer cells by triggering AS of cell death-related genes and metabolism-related genes.AS of cell death-related genes decreases the ratio of pro-/anti-apoptotic splicing isoforms to promote apoptotic tolerance in tumor cells, while AS of metabolism-related genes causes the upregulation of PKM2 expression to promote glycolysis in cancer cells.*(-) refers to down-regulated AS products of target genes.


NcRNAs trigger AS of metabolism-related genes to facilitate chemo-and targeted resistance

Cancer cells depend on aerobic glycolysis for ATP production and promoting rapid growth, which are also critical for the development of chemo-and targeted resistance [89].PKM is a critical enzyme in glycolysis metabolism, its splicing is crucial for metabolic regulation [90].AS of PKM pre-mRNA generates two isoforms, PKM1 and PKM2, containing exon 9 and exon 10, respectively.Interestingly, PKM2 encodes a protein that can catalyze glycolysis, and is correlated with poor prognosis in cancer patients, whereas PKM1 encodes a protein that can promote oxidative phosphorylation Hence, aberrant AS of PKM pre-mRNA triggered by ncRNAs can alter the sensitivity of cancer cells to chemotherapeutic agents [94,95] (Figure 4).A study found that hnRNPA1 can bind to the flanking sequence of exon 9, resulting in exon 10 inclusion and thus sustaining a high PKM2/PKM1 ratio [96].However, miR-374b represses hnRNPA1 expression through targeting its 3'UTR, subsequently inhibiting PKM2 and glycolysis to re-sensitize sorafenib-resistant HCC cells [94].It seems beyond dispute that AS of PKM regulated by miR-374b overexpression plays a significant role in overcoming sorafeni

resistance in HCC.Another study indicated that CIRS122 (hsa_cir
_0005963) produced by back splicing upregulates the expression of PKM2 by sponging miR-122, which stimulates glycolysis and leads to oxaliplatin resistance in CRC cells [95].This intercellular signal delivery could thus be used as a potential strategy for treating oxaliplatin-resistant CRC [95].Overall, these findings support that ncRNAs promote drug resistance in tumor cells by enhancing PKM2-mediated glycolysis, highlighting their potential as targets to reverse drug resistance in cancer cells.


Targeting AS of ncRNAs or AS-related ncRNAs for cancer therapy

Aberrant AS events are emerging as additional hallmarks of various cancers [33,[97][98][99][100][101].Currently, strategies targeting AS, such as small molecule targeting SFs or spliceosome, splice-switching antisense oligonucleotides (SSOs) and RNA or protein isoforms have exhibited potential values in clinical treatment (Figure 5).H3B-8800 is an orally available small molecule targeting SF3B1, which has entered phase I of clinical trials for the treatment of hematological malignancies [102,103].Besides, SSOs are typically synthetic short-stranded RNAs that are designed to base-pair with cis-acting elements of target pre-mRNA, thus facilitating the conversion of splicing isoforms via blocking the binding of SFs to the pre-mRNA [104].Li et al. found that BCL-X SSO targeting the exon 2 of BCL-X pre-mRNA significantly elevates the BCL-XS/BCL-XL ratio and promotes glioma cell apoptosis [105].In addition, antisense oligonucleotides (ASOs) targeting RNA isoforms are also emerging as pharmacological agents [106].Li et al. identified an oncogenic lncRNA AC104041.1,and designed LNA-modified ASO targeting two splice variants of AC104041.1 which exhibited potent anti-tumor activity for head and neck squamous carcinoma (HNSCC) [107].Moreover, alternative tumor-specific antigens (TSAs) have recently been evaluated and considered to be bona fide targets of anti-cancer immunity [108,109].Volpe G et al. reported that in Philadelphia chromosome-positive hematological malignancies, novel BCR-ABL transcripts are generated by AS, whose translational products contain C-terminal amino acid sequence derived from the out of reading frame (OOF) of the ABL gene [110].The presence of OOF-peptide can stimulate specific cytotoxic T lymphocyte reaction, suggesting that the BCR-ABL-OOF isoforms may be novel neoantigens for chronic myeloid leukemia therapy [110].Some recent studies showed that small molecule inhibitors can also be used as a potential source of tumor antigens and can be used in immunotherapy [111,112].Elizabeth A et al. found that triple-negative breast cancer (TNBC) cells produce many intron-retained double-strand RNAs when treated with H3B-8800 [111].These new antigens in turn activate the antiviral immune response and further induce exogenous apoptosis [111].Moreover, Lu X et al. revealed that sulfonamide derivative, Indisulam (E7070), degrades RBM39 in a dose-dependent manner and induces new antig ns in cancer cells, thereby stimulating anti-tumor immune response and enhancing the efficacy of immune checkpoint inhibitors [112].Additionally, agents that mediate splicing isoform-specific degradation have also been developed [113].For instance, the AR-V7 degrades both AR-V7 isoform and full-length AR for prostate cancer, while DT2216 degrades the anti-apoptotic splicing isoform BCL-XL for liquid and solid cancers [114].Therefore, these small molecules have shown promising effects of reducing the tumor burden in a variety of cancers [114].

In addition

these AS-targeting strategie
, ncRNAs as novel splicing regulators also hold new promise in cancer therapeutic (Figure 5).In B-cell lymphoma, 3-Deazaneplanocin A (DZNep) and ibrutinib could inhibit EZH-mediated methylation of lncRNA FAS-AS1 promoter and upregulate FAS-AS1, thus enhancing FAS-mediated apoptosis of cancer cells [82,115,116].These findings indicate that FAS-AS1 might be a promising target for lymphoma treatment and provide a rationale for the synergistic combination of EZH inhibitors and chemotherapy for lymphoma treatment.


Conclusions and perspectives

Previously studies focused on AS related-coding genes, yet ncRNA-associated AS events have gained increasing interest, especially in the field of tumor epigenetics.This review highlights the interaction between cRNAs and AS in cancer and summarizes distinct types of ncRNAs-mediated mechanisms involved in the aberrant regulation of AS events in various cancers.Owing to the extensive interactions between ncRNAs and AS and their mutual influence on cancer progression, AS-related ncRNAs have emerged as predictive biomarkers of chemotherapy and as potential targets for combination therapy.Hence, we also discussed the interplay of ncRNAs and AS in drug-resistant cells and the recent developments in cancer therapies targeting AS or AS-related ncRNAs.

Currently, researches on ncRNAs-AS network are conducted at a single gene level, since original papers usually performed rescue experiments to prove the concept.However, this simple model of single gene cannot fully reflect the complexity of the ncRNAs-AS interactive network or explain their important roles in cancers.Therefore, the mechanisms and functions of ncRNAs-AS network need to be further investigated, especially using bioinformatics to identify AS isoforms or AS-related ncRNAs that serve as prognostic biomarkers or therapeutic targets [117,118].Recently, Deng et al. developed a comprehensive database LncAS2Cancer, which provides information regarding AS of lncRNAs across human cancers, as well as predicts the potential interaction between lncRNA and AS in ca

ers [118].Howe
er, to systematically explore the interplay between ncRNAs and AS, the development of high through-put sequencing methods detecting ncRNApre-mRNA or ncRNA-SF interaction is demanded.Nevertheless, the evidence to date is sufficient to demonstrate the importance of ncRNA-AS interplay in cancer.The development of effective drugs or strategies to target AS events of ncRNAs or AS-related ncRNAs, and their combination with current chemo-, targeted-, or immuno-therapies hold the promise for combating cancer in the future.


Abbreviations

AS, Alternative splicing; NcRNAs, Non-coding RNAs; SR, Serine/arginine-rich; pre-mRNA, precursor messenger RNA; RNP, Ribonucleoprotein; Strategies targeting AS of ncRNAs or AS-related ncRNAs for cancer therapy.Several antitumor strategies have been de