PARP-2 depletion results in lower radiation cell survival but cell line-specific differences in poly(ADP-ribose) levels.

Poly(ADP-ribose) polymerase-2 (PARP-2) activity contributes to a cells' poly(ADP-ribosyl)ating potential and like PARP-1, has been implicated in several DNA repair pathways including base excision repair and DNA single strand break repair. Here the consequences of its stable depletion in HeLa, U20S, and AS3WT2 cells were examined. All three PARP-2 depleted models showed increased sensitivity to the cell killing effects on ionizing radiation as reported in PARP-2 depleted mouse embryonic fibroblasts providing further evidence for a role in DNA strand break repair. The PARP-2 depleted HeLa cells also showed both higher constitutive and DNA damage-induced levels of polymers of ADP-ribose (PAR) associated with unchanged PARP-1 protein levels, but higher PARP activity and a concomitant lower PARG protein levels and activity. These changes were accompanied by a reduced maximal recruitment of PARP-1, XRCC1, PCNA, and PARG to DNA damage sites. This PAR-associated phenotype could be reversed in HeLa cells on re-expression of PARP-2 and was not seen in U20S and AS3WT2 cells. These results highlight the complexity of the relationship between different members of the PARP family on PAR metabolism and suggest that cell model dependent phenotypes associated with the absence of PARP-2 exist within a common background of radiation sensitivity.

Cdk5 promotes DNA replication stress checkpoint activation through RPA-32 phosphorylation, and impacts on metastasis free survival in breast cancer patients.

Cyclin dependent kinase 5 (Cdk5) is a determinant of PARP inhibitor and ionizing radiation (IR) sensitivity. Here we show that Cdk5-depleted (Cdk5-shRNA) HeLa cells show higher sensitivity to S-phase irradiation, chronic hydroxyurea exposure, and 5-fluorouracil and 6-thioguanine treatment, with hydroxyurea and IR sensitivity also seen in Cdk5-depleted U2OS cells. As Cdk5 is not directly implicated in DNA strand break repair we investigated in detail its proposed role in the intra-S checkpoint activation. While Cdk5-shRNA HeLa cells showed altered basal S-phase dynamics with slower replication velocity and fewer active origins per DNA megabase, checkpoint activation was impaired after a hydroxyurea block. Cdk5 depletion was associated with reduced priming phosphorylations of RPA32 serines 29 and 33 and SMC1-Serine 966 phosphorylation, lower levels of RPA serine 4 and 8 phosphorylation and DNA damage measured using the alkaline Comet assay, gamma-H2AX signal intensity, RPA and Rad51 foci, and sister chromatid exchanges resulting in impaired intra-S checkpoint activation and subsequently higher numbers of chromatin bridges. In vitro kinase assays coupled with mass spectrometry demonstrated that Cdk5 can carry out the RPA32 priming phosphorylations on serines 23, 29, and 33 necessary for this checkpoint activation. In addition we found an association between lower Cdk5 levels and longer metastasis free survival in breast cancer patients and survival in Cdk5-depleted breast tumor cells after treatment with IR and a PARP inhibitor. Taken together, these results show that Cdk5 is necessary for basal replication and replication stress checkpoint activation and highlight clinical opportunities to enhance tumor cell killing.