title: Mechanisms of unphosphorylated STAT3 transcription factor binding to DNA.
abstract: Phosphorylation of signal transducer and activator of transcription 3 (STAT3) on a single tyrosine residue in response to growth factors, cytokines, interferons, and oncogenes activates its dimerization, translocation to the nucleus, binding to the interferon γ (gamma)-activated sequence (GAS) DNA-binding site and activation of transcription of target genes. STAT3 is constitutively phosphorylated in various cancers and drives gene expression from GAS-containing promoters to promote tumorigenesis. Recently, roles for unphosphorylated STAT3 (U-STAT3) have been described in response to cytokine stimulation, in cancers, and in maintenance of heterochromatin stability. However, the mechanisms underlying U-STAT3 binding to DNA has not been fully investigated. Here, we explore STAT3-DNA interactions by atomic force microscopy (AFM) imaging. We observed that U-STAT3 molecules bind to the GAS DNA-binding site as dimers and monomers. In addition, we observed that U-STAT3 binds to AT-rich DNA sequence sites and recognizes specific DNA structures, such as 4-way junctions and DNA nodes, within negatively supercoiled plasmid DNA. These structures are important for chromatin organization and our data suggest a role for U-STAT3 as a chromatin/genome organizer. Unexpectedly, we found that a C-terminal truncated 67.5-kDa STAT3 isoform recognizes single-stranded spacers within cruciform structures that also have a role in chromatin organization and gene expression. This isoform appears to be abundant in the nuclei of cancer cells and, therefore, may have a role in regulation of gene expression. Taken together, our data highlight novel mechanisms by which U-STAT3 binds to DNA and supports U-STAT3 function as a transcriptional activator and a chromatin/genomic organizer.
authors.affiliation: Department of Oncology, Lombardi Comprehensive Cancer Center, Georgetown University Medical Center, Washington DC 20057, USA.
authors.forename: Olga A
authors.initials: OA
authors.lastname: Timofeeva
authors.forename: Sergey
authors.initials: S
authors.lastname: Chasovskikh
authors.forename: Irina
authors.initials: I
authors.lastname: Lonskaya
authors.forename: Nadya I
authors.initials: NI
authors.lastname: Tarasova
authors.forename: Lyuba
authors.initials: L
authors.lastname: Khavrutskii
authors.forename: Sergey G
authors.initials: SG
authors.lastname: Tarasov
authors.forename: Xueping
authors.initials: X
authors.lastname: Zhang
authors.forename: Valeriy R
authors.initials: VR
authors.lastname: Korostyshevskiy
authors.forename: Amrita
authors.initials: A
authors.lastname: Cheema
authors.forename: Lihua
authors.initials: L
authors.lastname: Zhang
authors.forename: Sivanesan
authors.initials: S
authors.lastname: Dakshanamurthy
authors.forename: Milton L
authors.initials: ML
authors.lastname: Brown
authors.forename: Anatoly
authors.initials: A
authors.lastname: Dritschilo
mesh_terms: D001665:Binding Sites; D045744:Cell Line, Tumor; D002843:Chromatin / Q000737:chemistry*; D004247:DNA / Q000737:chemistry*; D005260:Female; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D007700:Kinetics; D008297:Male; D018625:Microscopy, Atomic Force / Q000379:methods; D010766:Phosphorylation; D010957:Plasmids / Q000378:metabolism; D011485:Protein Binding; D020033:Protein Isoforms; D017434:Protein Structure, Tertiary; D050796:STAT3 Transcription Factor / Q000378:metabolism*; D013347:Subcellular Fractions
chemical_list: D002843:Chromatin; D020033:Protein Isoforms; D050796:STAT3 Transcription Factor; D004247:DNA