title: Nonclassic functions of human topoisomerase I: genome-wide and pharmacologic analyses.
abstract: The biological functions of nuclear topoisomerase I (Top1) have been difficult to study because knocking out TOP1 is lethal in metazoans. To reveal the functions of human Top1, we have generated stable Top1 small interfering RNA (siRNA) cell lines from colon and breast carcinomas (HCT116-siTop1 and MCF-7-siTop1, respectively). In those clones, Top1 is reduced approximately 5-fold and Top2alpha compensates for Top1 deficiency. A prominent feature of the siTop1 cells is genomic instability, with chromosomal aberrations and histone gamma-H2AX foci associated with replication defects. siTop1 cells also show rDNA and nucleolar alterations and increased nuclear volume. Genome-wide transcription profiling revealed 55 genes with consistent changes in siTop1 cells. Among them, asparagine synthetase (ASNS) expression was reduced in siTop1 cells and in cells with transient Top1 down-regulation. Conversely, Top1 complementation increased ASNS, indicating a causal link between Top1 and ASNS expression. Correspondingly, pharmacologic profiling showed L-asparaginase hypersensitivity in the siTop1 cells. Resistance to camptothecin, indenoisoquinoline, aphidicolin, hydroxyurea, and staurosporine and hypersensitivity to etoposide and actinomycin D show that Top1, in addition to being the target of camptothecins, also regulates DNA replication, rDNA stability, and apoptosis. Overall, our studies show the pleiotropic nature of human Top1 activities. In addition to its classic DNA nicking-closing functions, Top1 plays critical nonclassic roles in genomic stability, gene-specific transcription, and response to various anticancer agents. The reported cell lines and approaches described in this article provide new tools to perform detailed functional analyses related to Top1 function.
authors: Laboratories of Molecular Pharmacology, National Institute of Diabetes and Digestive and Kidney Diseases, NIH, Bethesda, Maryland, USA.
authors: Ze-Hong
authors: ZH
authors: Miao
authors: Audrey
authors: A
authors: Player
authors: Uma
authors: U
authors: Shankavaram
authors: Yong-Hong
authors: YH
authors: Wang
authors: Drazen B
authors: DB
authors: Zimonjic
authors: Philip L
authors: PL
authors: Lorenzi
authors: Zhi-Yong
authors: ZY
authors: Liao
authors: Hong
authors: H
authors: Liu
authors: Tsutomu
authors: T
authors: Shimura
authors: Hong-Liang
authors: HL
authors: Zhang
authors: Ling-Hua
authors: LH
authors: Meng
authors: Yong-Wei
authors: YW
authors: Zhang
authors: Ernest S
authors: ES
authors: Kawasaki
authors: Nicholas C
authors: NC
authors: Popescu
authors: Mirit I
authors: MI
authors: Aladjem
authors: David J
authors: DJ
authors: Goldstein
authors: John N
authors: JN
authors: Weinstein
authors: Yves
authors: Y
authors: Pommier
mesh_terms: D001217:Aspartate-Ammonia Ligase / Q000096:biosynthesis / Q000235:genetics; D001943:Breast Neoplasms / Q000201:enzymology* / Q000235:genetics; D045744:Cell Line, Tumor; D002869:Chromosome Aberrations; D003110:Colonic Neoplasms / Q000201:enzymology* / Q000235:genetics; D004264:DNA Topoisomerases, Type I / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D015536:Down-Regulation; D015971:Gene Expression Regulation, Enzymologic; D015972:Gene Expression Regulation, Neoplastic; D015894:Genome, Human; D042822:Genomic Instability; D045325:HCT116 Cells; D006657:Histones / Q000096:biosynthesis / Q000235:genetics; D006801:Humans; D012333:RNA, Messenger / Q000235:genetics; D034741:RNA, Small Interfering / Q000235:genetics; D014162:Transfection
chemical_list: C468783:H2AX protein, human; D006657:Histones; D012333:RNA, Messenger; D034741:RNA, Small Interfering; D004264:DNA Topoisomerases, Type I; D001217:Aspartate-Ammonia Ligase