Rad4TopBP1 associates with Srr2, an Spc1 MAPK-regulated protein, in response to environmental stress

Topoisomerase IIβ binding protein 1 serves as a novel prognostic biomarker for stage II-III colorectal cancer patients

BACKGROUND

Colorectal cancer (CRC) is a commonly diagnosed human malignancy worldwide. Accumulating evidence has suggested DNA repair related proteins widely participate in CRC initiation and development. TOPBP1 is recently identified as a novel regulator for DNA repair, however, its biological role in CRC remains unknown.

METHODS

Firstly, the bioinformatics analysis was utilized to investigate the expression and clinical significance of TOPBP1 in CRC patients. Then, a retrospective study enrolling 129 stage II/III CRC patients was performed for validation. The CCK-8, colony formation, transwell assay and xenograft model were used to clarify the biological impact of TOPBP1 on CRC cells. Finally, transcriptome sequencing was performed to investigate the potential oncogenic mechanisms regulated by TOPBP1 in CRC development.

RESULTS

The expression of TOPBP1 was significantly higher in CRC tissues than that in normal tissues. High TOPBP1 expression was an independent unfavorable prognostic factor for overall and disease-free survival in II/III CRC patients. Knockdown of TOPBP1 not only significantly inhibited the proliferation, colony formation, invasion, migration and epithelial-mesenchymal transition (EMT) molecular phenotype of CRC cells, while the opposite was for TOPBP1 expression. Moreover, knockdown of TOPBP1 slowed down the growth speed of xenografts. The transcriptome sequencing identified MAP3K3 as a downstream gene of TOPBP1 and MAP3K3 knockdown inhibited the EMT molecular phenotype in CRC cells. Finally, the rescue assay indicated MAP3K3 overexpression counteracted the inhibitory effect of TOPBP1 knockdown on the proliferation, colony formation, invasion, migration and EMT phenotype of CRC cells.

CONCLUSION

TOPBP1 promotes the malignant progression of CRC through MAP3K3 induced EMT. TOPBP1 is a promising clinical biomarker or therapeutical target for CRC patients.

Stress activated protein kinase pathway modulates homologous recombination in fission yeast

Rad52 is a key player in homologous recombination (HR), a DNA repair pathway that is dedicated to double strand breaks repair and recovery of perturbed replication forks. Here we show that fission yeast Rad52 homologue is phosphorylated when S phase cells are exposed to ROS inducers such as ultraviolet A radiation or hydrogen peroxide, but not to ultraviolet C or camptothecin. Phosphorylation does not depend on kinases Chk1, Rad3, Tel1 or Cdc2, but depends on a functional stress activated protein kinase (SAPK) pathway and can be partially prevented by anti-oxidant treatment. Indeed, cells lacking Sty1, the major fission yeast MAP kinase of the SAPK pathway, do not display Rad52 phosphorylation and have UVA induced Rad52 foci that persist longer if compared to wild type cells. In addition, spontaneous intrachromosomal HR is diminished in cells lacking Sty1 and, more precisely, gene conversion is affected. Moreover, HR induced by site-specific arrest of replication forks is twice less efficient in cells that do not express Sty1. Importantly, impairing HR by deletion of the gene encoding the recombinase Rhp51 leads to Sty1 dependent Rad52 phosphorylation. Thus, SAPK pathway impinges on early step of HR through phosphorylation of Rad52 in cells challenged by oxidative stress or lacking Rhp51 and is required to promote spontaneous gene conversion and recovery from blocked replication forks.

Involvement of fission yeast Clr6-HDAC in regulation of the checkpoint kinase Cds1

Modification of the N-terminal tail of histones is required for various nuclear processes. Here, we show that fission yeast Clr6-HDAC (histone deacetylase) regulates the checkpoint kinase Cds1 when DNA replication encounters a stressful condition. We found that the global level of acetylation of histone H4 was constant throughout the normal cell cycle, but was reduced significantly when the cell recovered from the HU-induced cell cycle arrest (or slow DNA replication). We identified the Clr6-HDAC as a component responsible for the reduction in the level of the H4 acetylation. Although DNA replication was completed, the HU-induced cell cycle arrest could not be released even after removal of HU in the clr6-1 mutant. Under this experimental condition, Cds1 kinase was maintained active and remained bound tightly to chromatin. We also demonstrated that Cds1 was active even after treatment with caffeine, an inhibitor for ATM/ATR that is an activator of Cds1. These results indicate that inactivation of Cds1 requires functional Clr6-HDAC independently of the conventional DNA replication checkpoint. When DNA replication is impeded, Clr6-HDAC activity may monitor damage on chromatin structure/environment, which is required for inactivation of Cds1.