title: Mxi1 isoforms are expressed in hematological cell lines and normal bone marrow.
abstract: Mxi1 protein is a basic helix-loop-helix, leucine zipper (bHLHZIP) transcriptional factor, which dimerizes with the Max protein. This heterodimer binds a specific DNA sequence (E-box) and suppresses transcription of target genes. On the other hand, c-Myc protein is also a bHLHZIP protein that dimerizes with Max, binds the identical E-box sequence but activates transcription of the target genes. We report that hematopoietic cells have three novel Mxi1 transcripts: Mxi-D lacks exon 3, which encodes the basic region; Mxi-ND lacks the N-terminal mSin3 binding region and the DNA binding region; and Mxi-NF lacks the N-terminal mSin3 binding region. In normal bone marrow and hematological cell lines, the dominantly expressed isoforms are Mxi-D and full-length Mxi1 (Mxi-F). In GST-pull down assays, the Mxi-D protein binds the Max protein and the PHA2 regions of mSin3 proteins. Whereas the Mxi-F/Max heterodimer binds the E-box sequence, Mxi-D/Max heterodimer can not bind this sequence in electrophoretic mobility shift assays. In reporter gene assay, Mxi-D suppresses transcription as strongly as Mxi-F. In colony formation assay using Rat1 fibroblast cells, Mxi-D cannot suppress clonal growth stimulated by c-Myc as strongly as Mxi-F. In summary, Mxi-D may play an important role in the c-Myc family protein network acting as a dominant negative isoform of Mxi-F since: i) Mxi-D can dimerize with Max in vitro; ii) Mxi-D/Max heterodimers cannot bind E-box in vitro, iii) Mxi-D cannot suppress clonal growth stimulated by c-Myc.
authors.affiliation: Division of Hematology/Oncology, Cedars-Sinai Medical Center, UCLA School of Medicine, Los Angeles, CA 90048, USA. kawamatan@cshs.org
authors.forename: Norihiko
authors.initials: N
authors.lastname: Kawamata
authors.forename: Kei-Ji
authors.initials: KJ
authors.lastname: Sugimoto
authors.forename: Sakura
authors.initials: S
authors.lastname: Sakajiri
authors.forename: Kazuo
authors.initials: K
authors.lastname: Oshimi
authors.forename: H Phillip
authors.initials: HP
authors.lastname: Koeffler
mesh_terms: D051792:Basic Helix-Loop-Helix Proteins; D001854:Bone Marrow Cells; D049109:Cell Proliferation; D004268:DNA-Binding Proteins / Q000096:biosynthesis* / Q000737:chemistry; D017930:Genes, Reporter; D018257:Helix-Loop-Helix Motifs; D019337:Hematologic Neoplasms / Q000235:genetics* / Q000473:pathology*; D006801:Humans; D020033:Protein Isoforms; D014157:Transcription Factors / Q000096:biosynthesis* / Q000737:chemistry; D014407:Tumor Cells, Cultured; D025521:Tumor Suppressor Proteins
chemical_list: D051792:Basic Helix-Loop-Helix Proteins; D004268:DNA-Binding Proteins; D020033:Protein Isoforms; D014157:Transcription Factors; D025521:Tumor Suppressor Proteins; C079440:MXI1 protein, human