title: Spontaneous γH2AX Foci in Human Solid Tumor-Derived Cell Lines in Relation to p21WAF1 and WIP1 Expression.
abstract: Phosphorylation of H2AX on Ser139 (γH2AX) after exposure to ionizing radiation produces nuclear foci that are detectable by immunofluorescence microscopy. These so-called γH2AX foci have been adopted as quantitative markers for DNA double-strand breaks. High numbers of spontaneous γH2AX foci have also been reported for some human solid tumor-derived cell lines, but the molecular mechanism(s) for this response remains elusive. Here we show that cancer cells (e.g., HCT116; MCF7) that constitutively express detectable levels of p21WAF1 (p21) exhibit low numbers of γH2AX foci (<3/nucleus), whereas p21 knockout cells (HCT116p21-/-) and constitutively low p21-expressing cells (e.g., MDA-MB-231) exhibit high numbers of foci (e.g., >50/nucleus), and that these foci are not associated with apoptosis. The majority (>95%) of cells within HCT116p21-/- and MDA-MB-231 cultures contain high levels of phosphorylated p53, which is localized in the nucleus. We further show an inverse relationship between γH2AX foci and nuclear accumulation of WIP1, an oncogenic phosphatase. Our studies suggest that: (i) p21 deficiency might provide a selective pressure for the emergence of apoptosis-resistant progeny exhibiting genomic instability, manifested as spontaneous γH2AX foci coupled with phosphorylation and nuclear accumulation of p53; and (ii) p21 might contribute to positive regulation of WIP1, resulting in dephosphorylation of γH2AX.
authors.affiliation: Department of Oncology, University of Alberta, Cross Cancer Institute, Edmonton, AB T6G 1Z2, Canada. razmik.mirzayans@albertahealthservices.ca.
authors.forename: Razmik
authors.initials: R
authors.lastname: Mirzayans
authors.affiliation: Department of Oncology, University of Alberta, Cross Cancer Institute, Edmonton, AB T6G 1Z2, Canada. bonnie.andrais@albertahealthservices.ca.
authors.forename: Bonnie
authors.initials: B
authors.lastname: Andrais
authors.affiliation: Department of Oncology, University of Alberta, Cross Cancer Institute, Edmonton, AB T6G 1Z2, Canada. april.scott@albertahealthservices.ca.
authors.forename: April
authors.initials: A
authors.lastname: Scott
authors.affiliation: Department of Oncology, University of Alberta, Cross Cancer Institute, Edmonton, AB T6G 1Z2, Canada. ywwang@ualberta.ca.
authors.forename: Ying W
authors.initials: YW
authors.lastname: Wang
authors.affiliation: Division of Nephrology, Department of Internal Medicine, University of California, Davis, CA 95616, USA. rhweiss@ucdavis.edu.|Department of Medicine, Mather VA Medical Center, Sacramento, CA 95655, USA. rhweiss@ucdavis.edu.
authors.forename: Robert H
authors.initials: RH
authors.lastname: Weiss
authors.affiliation: Department of Oncology, University of Alberta, Cross Cancer Institute, Edmonton, AB T6G 1Z2, Canada. david.murray5@albertahealthservices.ca.
authors.forename: David
authors.initials: D
authors.lastname: Murray
mesh_terms: D017209:Apoptosis; D045744:Cell Line, Tumor; D050759:Cyclin-Dependent Kinase Inhibitor p21 / Q000235:genetics*; D053903:DNA Breaks, Double-Stranded; D004260:DNA Repair; D017353:Gene Deletion; D015972:Gene Expression Regulation, Neoplastic; D055785:Gene Knockdown Techniques; D042822:Genomic Instability; D006657:Histones / Q000235:genetics / Q000378:metabolism; D006801:Humans; D009369:Neoplasms / Q000235:genetics* / Q000378:metabolism; D010749:Phosphoprotein Phosphatases / Q000235:genetics*; D010766:Phosphorylation; D000071636:Protein Phosphatase 2C
chemical_list: C493956:CDKN1A protein, human; D050759:Cyclin-Dependent Kinase Inhibitor p21; D006657:Histones; C000607458:PPM1D protein, human; D010749:Phosphoprotein Phosphatases; D000071636:Protein Phosphatase 2C
keywords: WIP1; apoptosis; p21WAF1; p53; γH2AX foci