Disturbed alternative splicing of FIR (PUF60) directed cyclin E overexpression in esophageal cancers

// Yukiko Ogura 1 , Tyuji Hoshino 2 , Nobuko Tanaka 3 , Guzhanuer Ailiken 4 , Sohei Kobayashi 1, 3 , Kouichi Kitamura 3, 4 , Bahityar Rahmutulla 5 , Masayuki Kano 1 , Kentarou Murakami 1 , Yasunori Akutsu 1 , Fumio Nomura 3 , Sakae Itoga 3 , Hisahiro Matsubara 1 and Kazuyuki Matsushita 3 1 Department of Frontier Surgery, Graduate School of Medicine, Chiba University, Chiba, Japan 2 Department of Physical Chemistry, Graduate School of Pharmaceutical Sciences, Chiba University, Chiba, Japan 3 Department of Laboratory Medicine & Division of Clinical Genetics and Proteomics, Chiba University Hospital, Chiba, Japan 4 Department of Molecular Diagnosis, Graduate School of Medicine, Chiba University, Chiba, Japan 5 Department of Molecular Oncology, Graduate School of Medicine, Chiba University, Chiba, Japan Correspondence to: Kazuyuki Matsushita, email: kmatsu@faculty.chiba-u.jp Keywords: esophageal squamous cell carcinoma (ESCC); alternative splicing (AS); FBW7; FIR (PUF60); cyclin E Received: May 25, 2017     Accepted: March 22, 2018     Published: May 01, 2018 ABSTRACT Overexpression of alternative splicing of far upstream element binding protein 1 (FUBP1) interacting repressor (FIR; poly(U) binding splicing factor 60 [PUF60]) and cyclin E were detected in esophageal squamous cell carcinomas (ESCC). Accordingly, the expression of FBW7 was examined by which cyclin E is degraded as a substrate via the proteasome system. Expectedly, FBW7 expression was decreased significantly in ESCC. Conversely, c-myc gene transcriptional repressor FIR (alias PUF60; U2AF-related protein) and its alternative splicing variant form (FIRΔexon2) were overexpressed in ESCC. Further, anticancer drugs (cis-diaminedichloroplatinum/cisplatin [CDDP] or 5-fluorouracil [5-FU]) and knockdown of FIR by small interfering RNA (siRNA) increased cyclin E while knockdown of FIRΔexon2 by siRNA decreased cyclin E expression in ESCC cell lines (TE1, TE2, and T.Tn) or cervical SCC cells (HeLa cells). Especially, knockdown of SAP155 (SF3b1), a splicing factor required for proper alternative splicing of FIR pre-mRNA, decreased cyclin E. Therefore, disturbed alternative splicing of FIR generated FIR/FIRΔexon2 with cyclin E overexpression in esophageal cancers, indicating that SAP155 siRNA potentially rescued FBW7 function by reducing expression of FIR and/or FIRΔexon2. Remarkably, Three-dimensional structure analysis revealed the hypothetical inhibitory mechanism of FBW7 function by FIR/FIRΔexon2, a novel mechanism of cyclin E overexpression by FIR/FIRΔexon2-FBW7 interaction was discussed. Clinically, elevated FIR expression potentially is an indicator of the number of lymph metastases and anti-FIR/FIRΔexon2 antibodies in sera as cancer diagnosis, indicating chemical inhibitors of FIR/FIRΔexon2-FBW7 interaction could be potential candidate drugs for cancer therapy. In conclusion, elevated cyclin E expression was, in part, induced owing to potential FIR/FIRΔexon2–FBW7 interaction in ESCC.