title: Garcinol, a histone acetyltransferase inhibitor, radiosensitizes cancer cells by inhibiting non-homologous end joining.
abstract: OBJECTIVE
  Non-homologous end joining (NHEJ), a major pathway used to repair DNA double-strand breaks (DSBs) generated by ionizing radiation (IR), requires chromatin remodeling at DSB sites through the acetylation of histones by histone acetyltransferases (HATs). However, the effect of compounds with HAT inhibitory activities on the DNA damage response (DDR), including the NHEJ and cell cycle checkpoint, as well as on the radiosensitivity of cancer cells, remains largely unclear. Here, we investigated whether garcinol, a HAT inhibitor found in the rinds of Garcinia indica fruit (called mangosteens), has effects on DDR, and whether it can be used for radiosensitization.
  
  
  METHODS
  The following assays were used to examine the effect of garcinol on the inhibition of DSB repair, including the following: a conventional neutral comet assay; a cell-based assay recently developed by us, in which NHEJ repair of DSBs on chromosomal DNA was evaluated; the micrococcal nuclease sensitivity assay; and immunoblotting for autophosphorylation of DNA-dependent protein kinase catalytic subunit (DNA-PKcs). We assessed the effect of garcinol on the cell cycle checkpoint after IR treatment by analyzing the phosphorylation levels of checkpoint kinases CHK1 and CHK2 and histone H3, and by cell cycle profile analysis using flow cytometry. The radiosensitizing effect of garcinol was assessed by a clonogenic survival assay, whereas its effects on apoptosis and senescence were examined by annexin V and senescence-associated β-galactosidase (SA-β-Gal) staining, respectively.
  
  
  RESULTS
  We found that garcinol inhibits DSB repair, including NHEJ, without affecting cell cycle checkpoint. Garcinol radiosensitized A549 lung and HeLa cervical carcinoma cells with dose enhancement ratios (at 10% surviving fraction) of 1.6 and 1.5, respectively. Cellular senescence induced by IR was enhanced by garcinol.
  
  
  CONCLUSIONS
  These results suggest that garcinol is a radiosensitizer that inhibits NHEJ and facilitates senescence without impairing activation of the cell cycle checkpoint.
authors.affiliation: Division of Multistep Carcinogenesis, National Cancer Center Research Institute, Chuo-ku, Tokyo, Japan.
authors.forename: Takahiro
authors.initials: T
authors.lastname: Oike
authors.forename: Hideaki
authors.initials: H
authors.lastname: Ogiwara
authors.forename: Kohta
authors.initials: K
authors.lastname: Torikai
authors.forename: Takashi
authors.initials: T
authors.lastname: Nakano
authors.forename: Jun
authors.initials: J
authors.lastname: Yokota
authors.forename: Takashi
authors.initials: T
authors.lastname: Kohno
mesh_terms: D059447:Cell Cycle Checkpoints / Q000187:drug effects*; D002460:Cell Line; D002470:Cell Survival / Q000187:drug effects; D016922:Cellular Senescence / Q000187:drug effects; D000071877:Checkpoint Kinase 1; D064447:Checkpoint Kinase 2; D059766:DNA End-Joining Repair / Q000187:drug effects*; D006367:HeLa Cells; D051548:Histone Acetyltransferases / Q000037:antagonists & inhibitors*; D006657:Histones / Q000378:metabolism; D006801:Humans; D010936:Plant Extracts / Q000494:pharmacology; D011494:Protein Kinases / Q000378:metabolism; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D011836:Radiation Tolerance / Q000187:drug effects* / Q000235:genetics; D011838:Radiation-Sensitizing Agents / Q000494:pharmacology*; D013729:Terpenes / Q000494:pharmacology*
chemical_list: D006657:Histones; D010936:Plant Extracts; D011838:Radiation-Sensitizing Agents; D013729:Terpenes; D051548:Histone Acetyltransferases; D011494:Protein Kinases; D064447:Checkpoint Kinase 2; C000605867:CHEK1 protein, human; C578634:CHEK2 protein, human; D000071877:Checkpoint Kinase 1; D017346:Protein Serine-Threonine Kinases; C054597:garcinol