ncRNA-Encoded Peptides or Proteins and Cancer

Non-coding RNAs (ncRNAs) are unique RNA transcripts that have been widely identified in the eukaryotic genome and have been shown to play key roles in the development of many cancers. However, the rapid development of genome-wide translation profiling and ribosome profiling has revealed that a small number of small open reading frames (sORFs) within ncRNAs actually have peptide- or protein-coding potential. The peptides or proteins encoded by ncRNA (HOXB-AS3, encoded by long ncRNA [lncRNA]; FBXW7-185aa, PINT-87aa, and SHPRH-146aa, encoded by circular RNA [circRNA]; and miPEP-200a and miPEP-200b, encoded by primary miRNAs) have been shown to be critical players in cancer development and progression, through effects upon the regulation of glucose metabolism, the epithelial-to-mesenchymal transition, and the ubiquitination pathway. In this review, we summarize the reported peptides or proteins encoded by ncRNAs in cancer and explore the application of these peptides or proteins in the development of anti-tumor drugs and the identification of relevant therapeutic targets and tumor biomarkers. Non-coding RNAs (ncRNAs) are unique RNA transcripts that have been widely identified in the eukaryotic genome and have been shown to play key roles in the development of many cancers. However, the rapid development of genome-wide translation profiling and ribosome profiling has revealed that a small number of small open reading frames (sORFs) within ncRNAs actually have peptide- or protein-coding potential. The peptides or proteins encoded by ncRNA (HOXB-AS3, encoded by long ncRNA [lncRNA]; FBXW7-185aa, PINT-87aa, and SHPRH-146aa, encoded by circular RNA [circRNA]; and miPEP-200a and miPEP-200b, encoded by primary miRNAs) have been shown to be critical players in cancer development and progression, through effects upon the regulation of glucose metabolism, the epithelial-to-mesenchymal transition, and the ubiquitination pathway. In this review, we summarize the reported peptides or proteins encoded by ncRNAs in cancer and explore the application of these peptides or proteins in the development of anti-tumor drugs and the identification of relevant therapeutic targets and tumor biomarkers. An increasing number of studies have found that dysregulation of the central dogma of DNA > RNA > protein can lead to the development of diseases such as cancer. Although proteins represent a fundamental function and end-product of genetic information, less than 2% of the genome codes for proteins.1Anastasiadou E. Jacob L.S. Slack F.J. Non-coding RNA networks in cancer.Nat. Rev. Cancer. 2018; 18: 5-18Crossref PubMed Scopus (1023) Google Scholar, 2Djebali S. Davis C.A. Merkel A. Dobin A. Lassmann T. Mortazavi A. Tanzer A. Lagarde J. Lin W. Schlesinger F. et al.Landscape of transcription in human cells.Nature. 2012; 489: 101-108Crossref PubMed Scopus (3618) Google Scholar The remaining transcripts are called non-coding RNAs (ncRNAs). ncRNAs are classified into small nucleolar RNAs (snoRNAs), microRNAs (miRNAs), circular RNAs (circRNAs), and long ncRNA (lncRNA).3Adams B.D. Parsons C. Walker L. Zhang W.C. Slack F.J. Targeting noncoding RNAs in disease.J. Clin. Invest. 2017; 127: 761-771Crossref PubMed Scopus (465) Google Scholar, 4Zhu S. Wang J. He Y. Meng N. Yan G.R. Peptides/Proteins Encoded by Non-coding RNA: A Novel Resource Bank for Drug Targets and Biomarkers.Front. Pharmacol. 2018; 9: 1295Crossref PubMed Scopus (46) Google Scholar Recent studies have reversed the perception that ncRNAs represent "junk" transcriptional products. Subsequent studies have shown that ncRNAs are key regulators that mediate many fundamental cellular processes, such as development, differentiation, proliferation, transcription, post-transcriptional modifications, apoptosis, and cell metabolism. In general, ncRNAs naturally link related genetic networks to influence a variety of fundamental protein effectors that drive specific cellular biological responses and determine cell fate. As a consequence, the dysregulation of ncRNAs plays an important role in the development of many diseases. Indeed, ncRNAs, as oncogenic drivers or tumor suppressors, are dysregulated in a variety of cancers.5Jiang H. Huang G. Zhao N. Zhang T. Jiang M. He Y. Zhou X. Jiang X. Long non-coding RNA TPT1-AS1 promotes cell growth and metastasis in cervical cancer via acting AS a sponge for miR-324-5p.J. Exp. Clin. Cancer Res. 2018; 37: 169Crossref PubMed Scopus (65) Google Scholar, 6Xiong H. Chen R. Liu S. Lin Q. Chen H. Jiang Q. MicroRNA-183 induces epithelial-mesenchymal transition and promotes endometrial cancer cell migration and invasion in by targeting CPEB1.J. Cell. Biochem. 2018; 119: 8123-8137Crossref PubMed Scopus (28) Google Scholar, 7Huang J.Z. Chen M. Zeng M. Xu S.H. Zou F.Y. Chen D. Yan G.R. Down-regulation of TRPS1 stimulates epithelial-mesenchymal transition and metastasis through repression of FOXA1.J. Pathol. 2016; 239: 186-196Crossref PubMed Scopus (39) Google Scholar In general, canonical open reading frames (ORFs; defined as comprising >101 codons) are translated and produce annotated proteins with predictable functions.8Couso J.P. Patraquim P. Classification and function of small open reading frames.Nat. Rev. Mol. Cell Biol. 2017; 18: 575-589Crossref PubMed Scopus (142) Google Scholar However, the absolute requirement for a starting codon (methionine) and the arbitrary size restriction of ORFs have limited the identification of transcripts with non-canonical protein coding capacity.9Jackson R. Kroehling L. Khitun A. Bailis W. Jarret A. York A.G. Khan O.M. Brewer J.R. Skadow M.H. Duizer C. et al.The translation of non-canonical open reading frames controls mucosal immunity.Nature. 2018; 564: 434-438Crossref PubMed Scopus (104) Google Scholar pri-miRNA and lncRNAs contain a 5′ end modification called capping and 3′ end polyadenylated tails, just like mRNAs produced by RNA polymerase II.10Waterhouse P.M. Hellens R.P. Plant biology: Coding in non-coding RNAs.Nature. 2015; 520: 41-42Crossref PubMed Scopus (33) Google Scholar, 11van Heesch S. van Iterson M. Jacobi J. Boymans S. Essers P.B. de Bruijn E. Hao W. MacInnes A.W. Cuppen E. Simonis M. Extensive localization of long noncoding RNAs to the cytosol and mono- and polyribosomal complexes.Genome Biol. 2014; 15: R6Crossref PubMed Scopus (270) Google Scholar These features led to the assumption that the former might also encode proteins. Although lacking terminal 5′ caps and 3′ polyadenylated tails, circRNAs can initiate translation through internal ribosome entry sites (IRESs) or m6A modification.12Legnini I. Di Timoteo G. Rossi F. Morlando M. Briganti F. Sthandier O. Fatica A. Santini T. Andronache A. Wade M. et al.Circ-ZNF609 Is a Circular RNA that Can Be Translated and Functions in Myogenesis..Mol. Cell. 2017; 66: 22-37.e9Abstract Full Text Full Text PDF PubMed Scopus (1313) Google Scholar, 13Yang Y. Fan X. Mao M. Song X. Wu P. Zhang Y. Jin Y. Yang Y. Chen L.L. Wang Y. et al.Extensive translation of circular RNAs driven by N6-methyladenosine.Cell Res. 2017; 27: 626-641Crossref PubMed Scopus (1045) Google Scholar ncRNAs (lncRNAs, circRNAs, and miRNAs) have been deemed non-coding because of the arbitrary size restriction and the presumed insignificance of their small open reading frames (sORFs). On the basis of their short length, sORFs have been considered to defy standard computational detection of protein-coding potential.8Couso J.P. Patraquim P. Classification and function of small open reading frames.Nat. Rev. Mol. Cell Biol. 2017; 18: 575-589Crossref PubMed Scopus (142) Google Scholar Interestingly, recent advances in next-generation sequencing (genome-wide translation profiling, Ribo-Seq) and developments in bioinformatics have led to the identification of several sORFs from RNAs annotated as non-coding at a genome-wide scale.14Plaza S. Menschaert G. Payre F. In Search of Lost Small Peptides.Annu. Rev. Cell Dev. Biol. 2017; 33: 391-416Crossref PubMed Scopus (77) Google Scholar Deep RNA sequencing profiling reveals the prevalence of sORFs scattered throughout all types of transcripts. Ribosome profiling enables the detection of protein translation at a genome-wide scale, obtaining comprehensive, high-quality protein translation rates and quantifying protein expression and abundance by sequencing ribosome-protected fragments (RPFs).15Olexiouk V. Crappé J. Verbruggen S. Verhegen K. Martens L. Menschaert G. sORFs.org: a repository of small ORFs identified by ribosome profiling.Nucleic Acids Res. 2016; 44: D324-D329Crossref PubMed Scopus (82) Google Scholar Proteomic methods, such as mass spectrometry (MS), further confirm the existence of an unexpected variety of peptides or proteins, which represent an overlooked reservoir of bioactive molecules encoded by ncRNAs. In addition, some integrated web-based resources, such as the sORFs.org repository (http://sORFs.org) and SmProt (http://bioinfo.ibp.ac.cn/SmProt/), have supported the growing number of studies focusing on peptides or proteins encoded by ncRNAs.15Olexiouk V. Crappé J. Verbruggen S. Verhegen K. Martens L. Menschaert G. sORFs.org: a repository of small ORFs identified by ribosome profiling.Nucleic Acids Res. 2016; 44: D324-D329Crossref PubMed Scopus (82) Google Scholar, 16Hao Y. Zhang L. Niu Y. Cai T. Luo J. He S. Zhang B. Zhang D. Qin Y. Yang F. Chen R. SmProt: a database of small proteins encoded by annotated coding and non-coding RNA loci.Brief. Bioinform. 2018; 19: 636-643PubMed Google Scholar Recently, a limited number of peptides or proteins encoded by ncRNAs have been validated, and they displayed important biological and pathological functions in the occurrence and development of tumors. These include a peptide encoded by lncRNA, some proteins or peptides by circRNAs, and a protein and a peptide by pri-miRNAs. A conserved 53-aa peptide encoded by the putative lncRNA HOXB-AS3 suppresses colon cancer (CRC) growth by regulating the alternative splicing of pyruvate kinase M (PKM) and tumor metabolic reprogramming.17Huang J.Z. Chen M. Chen Gao X.C. Zhu S. Huang H. Hu M. Zhu H. Yan G.R. A Peptide Encoded by a Putative lncRNA HOXB-AS3 Suppresses Colon Cancer Growth.Mol. Cell. 2017; 68: 171-184.e6Abstract Full Text Full Text PDF PubMed Scopus (358) Google Scholar FBXW7-185aa, PINT-87aa, and SHPRH-146aa are encoded by the circRNAs FBXW7, PINTexon2, and SHPRH, respectively, and inhibit glioma growth.18Yang Y. Gao X. Zhang M. Yan S. Sun C. Xiao F. Huang N. Yang X. Zhao K. Zhou H. et al.Novel Role of FBXW7 Circular RNA in Repressing Glioma Tumorigenesis.J. Natl. Cancer Inst. 2018; 110: 304-315Crossref PubMed Scopus (681) Google Scholar, 19Zhang M. Huang N. Yang X. Luo J. Yan S. Xiao F. Chen W. Gao X. Zhao K. Zhou H. et al.A novel protein encoded by the circular form of the SHPRH gene suppresses glioma tumorigenesis.Oncogene. 2018; 37: 1805-1814Crossref PubMed Scopus (418) Google Scholar, 20Zhang M. Zhao K. Xu X. Yang Y. Yan S. Wei P. Liu H. Xu J. Xiao F. Zhou H. et al.A peptide encoded by circular form of LINC-PINT suppresses oncogenic transcriptional elongation in glioblastoma.Nat. Commun. 2018; 9: 4475Crossref PubMed Scopus (379) Google Scholar A protein and a peptide (miPEP-200a and miPEP-200b) encoded by pri-miRNA (miR-200a and miR-200b, respectively) inhibit the migration of prostate cancer cells via suppressing the process of epithelial-to-mesenchymal transition (EMT).21Fang J. Morsalin S. Rao V.N. Reddy E.S.P. Decoding of Non-Coding DNA and Non-Coding RNA: Pri-Micro RNA-Encoded Novel Peptides Regulate Migration of Cancer Cells.J. Pharm. Sci. Pharmacol. 2017; 3: 23-27Crossref Google Scholar In this review, we summarize the limited examples of the reported peptides or proteins encoded by ncRNAs (lncRNA, circRNA, and pri-miRNA) in cancers. The new mode of action of ncRNAs provides new perspectives and new horizons for cancer treatment and diagnosis. Moreover, we also discuss the great potential applications of peptides or proteins encoded by ncRNAs in developing anti-tumor drugs, therapeutic targets, and biomarkers. lncRNA is a ncRNA with a transcript length of more than 200 nt that does not encode proteins.22Wapinski O. Chang H.Y. Long noncoding RNAs and human disease.Trends Cell Biol. 2011; 21: 354-361Abstract Full Text Full Text PDF PubMed Scopus (1635) Google Scholar A large number of cancer-related lncRNAs have been discovered based on cancer cell transcriptome profiling.23Gibb E.A. Brown C.J. Lam W.L. The functional role of long non-coding RNA in human carcinomas.Mol. Cancer. 2011; 10: 38Crossref PubMed Scopus (1393) Google Scholar lncRNAs play critical roles in cancer development by regulating chromatin remodeling,24Gupta R.A. Shah N. Wang K.C. Kim J. Horlings H.M. Wong D.J. Tsai M.C. Hung T. Argani P. Rinn J.L. et al.Long non-coding RNA HOTAIR reprograms chromatin state to promote cancer metastasis.Nature. 2010; 464: 1071-1076Crossref PubMed Scopus (4166) Google Scholar transcription,25Kino T. Hurt D.E. Ichijo T. Nader N. Chrousos G.P. Noncoding RNA gas5 is a growth arrest- and starvation-associated repressor of the glucocorticoid receptor.Sci. Signal. 2010; 3: ra8Crossref PubMed Scopus (990) Google Scholar, 26Hung T. Wang Y. Lin M.F. Koegel A.K. Kotake Y. Grant G.D. Horlings H.M. Shah N. Umbricht C. Wang P. et al.Extensive and coordinated transcription of noncoding RNAs within cell-cycle promoters.Nat. Genet. 2011; 43: 621-629Crossref PubMed Scopus (937) Google Scholar RNA editing,27Salameh A. Lee A.K. Cardó-Vila M. Nunes D.N. Efstathiou E. Staquicini F.I. Dobroff A.S. Marchiò S. Navone N.M. Hosoya H. et al.PRUNE2 is a human prostate cancer suppressor regulated by the intronic long noncoding RNA PCA3.Proc. Natl. Acad. Sci. USA. 2015; 112: 8403-8408Crossref PubMed Scopus (182) Google Scholar RNA degradation,28Gong C. Maquat L.E. lncRNAs transactivate STAU1-mediated mRNA decay by duplexing with 3′ UTRs via Alu elements.Nature. 2011; 470: 284-288Crossref PubMed Scopus (949) Google Scholar translational regulation,29Yang F. Zhang H. Mei Y. Wu M. Reciprocal regulation of HIF-1α and lincRNA-p21 modulates the Warburg effect.Mol. Cell. 2014; 53: 88-100Abstract Full Text Full Text PDF PubMed Scopus (405) Google Scholar, 30Wu D. Yang B. Chen J. Xiong H. Li Y. Pan Z. Cao Y. Chen J. Li T. Zhou S. et al.Upregulation of long non-coding RNA RAB1A-2 induces FGF1 expression worsening lung cancer prognosis.Cancer Lett. 2018; 438: 116-125Crossref PubMed Scopus (31) Google Scholar RNA splicing,31Bernard D. Prasanth K.V. Tripathi V. Colasse S. Nakamura T. Xuan Z. Zhang M.Q. Sedel F. Jourdren L. Coulpier F. et al.A long nuclear-retained non-coding RNA regulates synaptogenesis by modulating gene expression.EMBO J. 2010; 29: 3082-3093Crossref PubMed Scopus (559) Google Scholar and miRNA sponges32Nie W. Ge H.J. Yang X.Q. Sun X. Huang H. Tao X. Chen W.S. Li B. LncRNA-UCA1 exerts oncogenic functions in non-small cell lung cancer by targeting miR-193a-3p.Cancer Lett. 2016; 371: 99-106Crossref PubMed Scopus (324) Google Scholar (Figures 1A–1H). Interestingly, in addition to the RNA-based activity of lncRNA molecules, our group identified a peptide encoded by a lncRNA in cancer cells (Figure 1I).17Huang J.Z. Chen M. Chen Gao X.C. Zhu S. Huang H. Hu M. Zhu H. Yan G.R. A Peptide Encoded by a Putative lncRNA HOXB-AS3 Suppresses Colon Cancer Growth.Mol. Cell. 2017; 68: 171-184.e6Abstract Full Text Full Text PDF PubMed Scopus (358) Google Scholar lncRNA HOXB-AS3 was annotated as a lncRNA in the NONCODE and other lncRNA databases. We found that lncRNA HOXB-AS3 encoded a conserved 53-aa peptide and demonstrated that the HOXB-AS3 peptide, encoded by lncRNA HOXB-AS3, but not the lncRNA HOXB-AS3 itself, suppressed CRC cell growth, colony formation, migration, invasion, and tumorigenesis by regulating tumor energy metabolism17Huang J.Z. Chen M. Chen Gao X.C. Zhu S. Huang H. Hu M. Zhu H. Yan G.R. A Peptide Encoded by a Putative lncRNA HOXB-AS3 Suppresses Colon Cancer Growth.Mol. Cell. 2017; 68: 171-184.e6Abstract Full Text Full Text PDF PubMed Scopus (358) Google Scholar (Figure 1I). The conclusion that lncRNA HOXB-AS3 encodes a peptide was supported by multiple lines of evidence. First, the start codon of the HOXB-AS3 ORF had the ability to reactivate the GFPmut ORF, in which the start codon 5′-ATGGTG-3′ was mutated to 5′-ATTGTT-3′, in exogenous expression assays. Second, the existence of the HOXB-AS3 peptide was further confirmed by MS analysis. Third, the HOXB-AS3 peptide was further validated to be naturally and endogenously produced in many cells and tissues. Fourth, the expression of the HOXB-AS3 peptide could be blocked by using anti-HOXB-AS3 translation blocking antisense oligonucleotides. Finally, endogenously produced HOXB-AS3 peptides were shown not to represent the processed products of a longer pro-protein. Moreover, the HOXB-AS3 peptide and lncRNA levels were downregulated in highly metastatic colon, breast, nasopharyngeal, and ovarian cancer cells and in primary CRC tissues compared with their matched parental cell lines and adjacent normal colon tissues. Decreased HOXB-AS3 peptide levels were significantly associated with more advanced clinical stages. Low HOXB-AS3 peptide levels indicated a poor prognosis for CRC patients. Specifically, the mean overall survival time of CRC patients with higher HOXB-AS3 peptide expression was 1.6 times that of patients with lower HOXB-AS3 peptide expression. CRC patients with higher levels of the HOXB-AS3 peptide had a lower risk for death compared with patients with lower levels of the HOXB-AS3 peptide. The HOXB-AS3 peptide, but not lncRNA HOXB-AS3, suppressed CRC cancer cell growth, colony formation, migration, invasion, and tumor growth in vitro and in vivo. The HOXB-AS3 peptide lacks homology to other proteins or peptides. To further investigate the mechanism of action of the HOXB-AS3 peptide in inhibiting cancer progression, the proteins interacting with the HOXB-AS3 peptide were identified by interactomics. A total of 485 proteins that bound to the HOXB-AS3 peptide were identified. Surprisingly and interestingly, most of the proteins that bind to the HOXB-AS3 peptide are functionally linked to RNA splicing. Furthermore, the HOXB-AS3 peptide inhibited the hnRNP A1-dependent splicing of PKM and the formation of the PKM2 isoform, and subsequently suppressed glucose metabolic reprogramming and tumorigenesis by competitively binding to the arginine residues in the RGG motif of hnRNP A1 (Figure 1I).17Huang J.Z. Chen M. Chen Gao X.C. Zhu S. Huang H. Hu M. Zhu H. Yan G.R. A Peptide Encoded by a Putative lncRNA HOXB-AS3 Suppresses Colon Cancer Growth.Mol. Cell. 2017; 68: 171-184.e6Abstract Full Text Full Text PDF PubMed Scopus (358) Google Scholar Since circRNAs were identified in RNA viruses in 1976,33Sanger H.L. Klotz G. Riesner D. Gross H.J. Kleinschmidt A.K. Viroids are single-stranded covalently closed circular RNA molecules existing as highly base-paired rod-like structures.Proc. Natl. Acad. Sci. USA. 1976; 73: 3852-3856Crossref PubMed Scopus (1407) Google Scholar they have been considered a class of ncRNAs comprising closed continuous-loop structures lacking terminal 5′ caps and 3′ polyadenylated tails.34Chen L.L. Yang L. Regulation of circRNA biogenesis.RNA Biol. 2015; 12: 381-388Crossref PubMed Scopus (1244) Google Scholar Recently, taking advantage of RNA sequence technology, thousands of novel circRNAs have been discovered.35Zheng L.L. Li J.H. Wu J. Sun W.J. Liu S. Wang Z.L. Zhou H. Yang J.H. Qu L.H. deepBase v2.0: identification, expression, evolution and function of small RNAs, LncRNAs and circular RNAs from deep-sequencing data.Nucleic Acids Res. 2016; 44: D196-D202Crossref PubMed Scopus (172) Google Scholar An increasing number of studies have demonstrated that circRNAs have important physiological functions. For instance, circRNAs can regulate transcription and mRNA splicing by interacting with RNA polymerase II and snRNPs (Figures 2A and 2E ).36Zhang Y. Zhang X.O. Chen T. Xiang J.F. Yin Q.F. Xing Y.H. Zhu S. Yang L. Chen L.L. Circular intronic long noncoding RNAs.Mol. Cell. 2013; 51: 792-806Abstract Full Text Full Text PDF PubMed Scopus (1529) Google Scholar, 37Li Z. Huang C. Bao C. Chen L. Lin M. Wang X. Zhong G. Yu B. Hu W. Dai L. et al.Exon-intron circular RNAs regulate transcription in the nucleus.Nat. Struct. Mol. Biol. 2015; 22: 256-264Crossref PubMed Scopus (1852) Google Scholar circRNAs can also regulate protein localization and activity (Figures 2B and 2D).38Ashwal-Fluss R. Meyer M. Pamudurti N.R. Ivanov A. Bartok O. Hanan M. Evantal N. Memczak S. Rajewsky N. Kadener S. circRNA biogenesis competes with pre-mRNA splicing.Mol. Cell. 2014; 56: 55-66Abstract Full Text Full Text PDF PubMed Scopus (1951) Google Scholar, 39Abdelmohsen K. Panda A.C. Munk R. Grammatikakis I. Dudekula D.B. De S. Kim J. Noh J.H. Kim K.M. Martindale J.L. Gorospe M. Identification of HuR target circular RNAs uncovers suppression of PABPN1 translation by CircPABPN1.RNA Biol. 2017; 14: 361-369Crossref PubMed Scopus (508) Google Scholar Additionally, circRNAs can act as miRNA sponges to combine with miRNA and influence tumorigenesis and metastasis in hepatocellular carcinoma (Figure 2C),40Yu J. Xu Q.G. Wang Z.G. Yang Y. Zhang L. Ma J.Z. Sun S.H. Yang F. Zhou W.P. Circular RNA cSMARCA5 inhibits growth and metastasis in hepatocellular carcinoma.J. Hepatol. 2018; 68: 1214-1227Abstract Full Text Full Text PDF PubMed Scopus (463) Google Scholar CRC,41Hsiao K.Y. Lin Y.C. Gupta S.K. Chang N. Yen L. Sun H.S. Tsai S.J. Noncoding Effects of Circular RNA CCDC66 Promote Colon Cancer Growth and Metastasis.Cancer Res. 2017; 77: 2339-2350Crossref PubMed Scopus (475) Google Scholar and other cancers. In addition to their RNA-based regulatory functions, circRNAs have been reported to encode proteins or peptides.12Legnini I. Di Timoteo G. Rossi F. Morlando M. Briganti F. Sthandier O. Fatica A. Santini T. Andronache A. Wade M. et al.Circ-ZNF609 Is a Circular RNA that Can Be Translated and Functions in Myogenesis..Mol. Cell. 2017; 66: 22-37.e9Abstract Full Text Full Text PDF PubMed Scopus (1313) Google Scholar, 18Yang Y. Gao X. Zhang M. Yan S. Sun C. Xiao F. Huang N. Yang X. Zhao K. Zhou H. et al.Novel Role of FBXW7 Circular RNA in Repressing Glioma Tumorigenesis.J. Natl. Cancer Inst. 2018; 110: 304-315Crossref PubMed Scopus (681) Google Scholar, 19Zhang M. Huang N. Yang X. Luo J. Yan S. Xiao F. Chen W. Gao X. Zhao K. Zhou H. et al.A novel protein encoded by the circular form of the SHPRH gene suppresses glioma tumorigenesis.Oncogene. 2018; 37: 1805-1814Crossref PubMed Scopus (418) Google Scholar, 20Zhang M. Zhao K. Xu X. Yang Y. Yan S. Wei P. Liu H. Xu J. Xiao F. Zhou H. et al.A peptide encoded by circular form of LINC-PINT suppresses oncogenic transcriptional elongation in glioblastoma.Nat. Commun. 2018; 9: 4475Crossref PubMed Scopus (379) Google Scholar, 42Pamudurti N.R. Bartok O. Jens M. Ashwal-Fluss R. Stottmeister C. Ruhe L. Hanan M. Wyler E. Perez-Hernandez D. Ramberger E. et al.Translation of CircRNAs.Mol. Cell. 2017; 66: 9-21.e7Abstract Full Text Full Text PDF PubMed Scopus (1089) Google Scholar In certain tissues, many endogenous circRNAs, including circ-Mbl(42), circ-ZNF609(12), circ-FBXW7(18), circ-SHPRH(19), and circPINTexon2(20), have been shown to be translated from IRESs and N6-methyladenosine (m6A) residues under certain conditions.13Yang Y. Fan X. Mao M. Song X. Wu P. Zhang Y. Jin Y. Yang Y. Chen L.L. Wang Y. et al.Extensive translation of circular RNAs driven by N6-methyladenosine.Cell Res. 2017; 27: 626-641Crossref PubMed Scopus (1045) Google Scholar, 43Chen C.Y. Sarnow P. Initiation of protein synthesis by the eukaryotic translational apparatus on circular RNAs.Science. 1995; 268: 415-417Crossref PubMed Scopus (501) Google Scholar Furthermore, it has been shown that some proteins or peptides translated from circRNA play important regulatory roles in tumorigenesis and tumor progression (Figure 2F).18Yang Y. Gao X. Zhang M. Yan S. Sun C. Xiao F. Huang N. Yang X. Zhao K. Zhou H. et al.Novel Role of FBXW7 Circular RNA in Repressing Glioma Tumorigenesis.J. Natl. Cancer Inst. 2018; 110: 304-315Crossref PubMed Scopus (681) Google Scholar, 19Zhang M. Huang N. Yang X. Luo J. Yan S. Xiao F. Chen W. Gao X. Zhao K. Zhou H. et al.A novel protein encoded by the circular form of the SHPRH gene suppresses glioma tumorigenesis.Oncogene. 2018; 37: 1805-1814Crossref PubMed Scopus (418) Google Scholar, 20Zhang M. Zhao K. Xu X. Yang Y. Yan S. Wei P. Liu H. Xu J. Xiao F. Zhou H. et al.A peptide encoded by circular form of LINC-PINT suppresses oncogenic transcriptional elongation in glioblastoma.Nat. Commun. 2018; 9: 4475Crossref PubMed Scopus (379) Google Scholar Recently, Yang et al.18Yang Y. Gao X. Zhang M. Yan S. Sun C. Xiao F. Huang N. Yang X. Zhao K. Zhou H. et al.Novel Role of FBXW7 Circular RNA in Repressing Glioma Tumorigenesis.J. Natl. Cancer Inst. 2018; 110: 304-315Crossref PubMed Scopus (681) Google Scholar reported that FBXW7-185aa, a protein encoded by circRNA FBXW7 (circ-FBXW7), plays a crucial role in glioma carcinogenesis and in patient clinical prognosis. The authors utilized RNA sequencing (RNA-seq) to analyze 31,145 differentially expressed circRNAs between 10 human glioblastomas and their paired adjacent normal tissues, and explored the potential capabilities of circRNA to encode peptides or proteins by matching with them in the circRNADb database. Then, they focused their research on circ-FBXW7, which had the greatest differential expression between the cancerous and normal groups. In accordance with the positive match in the circRNADb database, Yang et al.18Yang Y. Gao X. Zhang M. Yan S. Sun C. Xiao F. Huang N. Yang X. Zhao K. Zhou H. et al.Novel Role of FBXW7 Circular RNA in Repressing Glioma Tumorigenesis.J. Natl. Cancer Inst. 2018; 110: 304-315Crossref PubMed Scopus (681) Google Scholar revealed that circ-FBXW7 possessed a spanning junction open reading frame. Further, the authors showed that circ-FBXW7 encoded a 185-aa (approximately 22-kDa) protein, FBXW7-185aa, expressed from the IRES in a 5′-cap-independent manner. Cells stably overexpressing circ-FBXW7 and FBXW7-Flag exhibited cell-cycle G0/G1 phase arrest and cell growth inhibition, in contrast with the negative control cells. In addition, knocking down circ-FBXW7 in anaplastic astrocytoma cells to reduce the expression of FBXW7-185aa accelerated cell-cycle progression and increased cell proliferation. Consistent with the RNA-seq analysis, it was confirmed that the expression of circ-FBXW7 and FBXW7-185aa in glioma samples was downregulated. In 38 glioblastoma clinical samples, both circ-FBXW7 and FBXW7-185aa levels were significantly reduced, and the overall survival time of glioblastoma patients with higher circ-FBXW7 expression was longer than that of patients with lower circ-FBXW7 expression. These results indicate that circ-FBXW7 and FBXW7-185aa might serve as prognostic markers for glioblastoma. High expression of circ-FBXW7 might positively correlate with glioblastoma patient overall survival time. In addition, Yang et al.18Yang Y. Gao X. Zhang M. Yan S. Sun C. Xiao F. Huang N. Yang X. Zhao K. Zhou H. et al.Novel Role of FBXW7 Circular RNA in Repressing Glioma Tumorigenesis.J. Natl. Cancer Inst. 2018; 110: 304-315Crossref PubMed Scopus (681) Google Scholar found that the expression of c-Myc was lower in both circ-FBXW7- and FBXW7-185aa-overexpressing cells. The authors further strengthened the evidence that FBXW7-185aa overexpression in cancer cells inhibited the proliferation of tumor cells and cell-cycle acceleration by antagonizing the deubiquitination of c-Myc induced by USP28. FBXW7α, a well-defined E3 ligase encoded by the FBXW7 gene, inhibited tumorigenesis by targeting c-Myc for ubiquitination degradation. USP28 is a deubiquitination enzyme that regulates and controls c-Myc stability by binding with FBXW7α. Unlike FBXW7α, FBXW7-185aa cannot directly target c-Myc. However, there is a higher binding affinity between FBXW7-185aa and USP28. Specifically, increased expression of FBXW7-185aa in cancer cells could competitively bind USP28 so as to free FBXW7α. Therefore, FBXW7-185aa indirectly increased the ubiquitination of c-Myc and induced its degradation by antagonizing USP28.18Yang Y. Gao X. Zhang M. Yan S. Sun C. Xiao F. Huang N. Yang X. Zhao K. Zhou H. et al.Novel Role of FBXW7 Circular RNA in Repressing Glioma Tumorigenesis.J. Natl. Cancer Inst. 2018; 110: 304-315Crossref PubMed Scopus (681) Google Scholar Without a doubt, the discovery of circ-FBXW7 and its derived protein FBXW7-185aa, and their role in glioma provided a new direction for cancer research. Recently, it was reported that circular SHPRH (circ-SHPRH) encodes a functional protein, SHPRH-146aa, as a tumor suppressor in human glioblastoma.19Zhang M. Huang N. Yang X. Luo J. Yan S. Xiao F. Chen W. Gao X. Zhao K. Zhou H. et al.A novel protein encoded by the circular form of the SHPRH gene suppresses glioma tumorigenesis.Oncogene. 2018; 37: 1805-1814Crossref PubMed Scopus (418) Google Scholar SHPRH-146aa encoded by circ-SHPRH, which contains an ORF driven by an IRES, was verified by liquid chromatography-tandem MS (LC-MS/MS). circ-SHPRH was significantly decreased in glioblastoma tissues compared with normal brain tissues. The levels of SHPRH-146aa in 81% of glioblastoma patients (49/60 cases) was significantly reduced. Patients with higher expression of SHPRH-146aa or SHPRH had longer survival times than the control group. Cell proliferation rates and malignant phenotype of U251 and U373 stably overexpressing SHPRH-146aa cells were significantly reduced compared with those of control cells. The ability of in vivo tumor proliferation is reduced through subcutaneous injections of U251 and U373 cells, which stably overexpress circ-SHPRH. The proliferating cell nuclear antigen (PCNA) protein levels were reduced