RESUMO
The HTLV-I oncoprotein Tax interferes with DNA double strand break repair. Since non-homologous end joining (NHEJ) is a major pathway used to repair DNA double strand breaks we examined the effect of Tax on this pathway, with particular interest in the expression and function of Ku80, a critical component of the NHEJ pathway. Tax expression decreased Ku80 mRNA and protein levels, and repressed transcription from the Ku80 promoter. Conversely, Ku80 mRNA increased following siRNA knockdown of Tax in HTLV-I infected cells. Tax expression was associated with an elevated number of micronuclei and nucleoplasmic bridges, hallmarks of improper DNA double strand break repair. Our studies identified Tax as a transcriptional repressor of Ku80 that correlates with decreased DNA repair function. The reduction of Ku80 transcription by Tax may deplete the cell of an essential DNA break binding protein, resulting in reduced repair of DNA double strand breaks and accumulation genomic mutations.
Assuntos
Antígenos Nucleares/metabolismo , Proteínas de Ligação a DNA/metabolismo , Regulação Viral da Expressão Gênica/fisiologia , Produtos do Gene tax/metabolismo , Vírus Linfotrópico T Tipo 1 Humano/metabolismo , Antígenos Nucleares/genética , Linhagem Celular , Quebras de DNA de Cadeia Dupla , Reparo do DNA , DNA Viral/química , Proteínas de Ligação a DNA/genética , Produtos do Gene tax/genética , Vírus Linfotrópico T Tipo 1 Humano/genética , Humanos , Autoantígeno Ku , Regiões Promotoras Genéticas , RNA Mensageiro/genética , RNA Mensageiro/metabolismo , RNA Viral/genética , RNA Viral/metabolismoRESUMO
Genomic instability, a hallmark of leukemic cells, is associated with malfunctioning cellular responses to DNA damage caused by defective cell cycle checkpoints and/or DNA repair. Adult T-cell leukemia, which can result from infection with human T-cell leukemia virus type 1 (HTLV-1), is associated with extensive genomic instability that has been attributed to the viral oncoprotein Tax. How Tax influences cellular responses to DNA damage to mediate genomic instability, however, remains unclear. Therefore, we investigated the effect of Tax on cellular pathways involved in recognition and repair of DNA double-strand breaks. Premature attenuation of ATM kinase activity and reduced association of MDC1 with repair foci were observed in Tax-expressing cells. Following ionizing radiation-induced S-phase checkpoint activation, Tax-expressing cells progressed more rapidly than non-Tax-expressing cells toward DNA replication. These results demonstrate that Tax expression may allow premature DNA replication in the presence of genomic lesions. Attempts to replicate in the presence of these lesions would result in gradual accumulation of mutations, leading to genome instability and cellular transformation.
Assuntos
Proteínas de Ciclo Celular/antagonistas & inibidores , Reparo do DNA/fisiologia , Proteínas de Ligação a DNA/antagonistas & inibidores , Produtos do Gene tax/metabolismo , Vírus Linfotrópico T Tipo 1 Humano/fisiologia , Proteínas Serina-Treonina Quinases/antagonistas & inibidores , Proteínas Supressoras de Tumor/antagonistas & inibidores , Proteínas Adaptadoras de Transdução de Sinal , Proteínas Mutadas de Ataxia Telangiectasia , Linhagem Celular , Replicação do DNA , Humanos , Proteínas Nucleares/metabolismo , Linfócitos T/efeitos da radiação , Linfócitos T/virologia , Transativadores/metabolismoRESUMO
It is estimated that 15% of all cancers are etiologically linked to viral infection. Specific cancers including adult T-cell leukemia, hepatocellular carcinoma, and uterine cervical cancer are associated with infection by human T-cell leukemia virus type I, hepatitis B virus, and high-risk human papilloma virus, respectively. In these cancers, genomic instability, a hallmark of multistep cancers, has been explicitly linked to the expression of oncoproteins encoded by these viruses. This review discusses mechanisms utilized by these viral oncoproteins, Tax, HBx, and E6/E7, to mediate genomic instability and cellular transformation.