Prevention of DNA Replication Stress by CHK1 Leads to Chemoresistance Despite a DNA Repair Defect in Homologous Recombination in Breast Cancer.

Chromosomal instability not only has a negative effect on survival in triple-negative breast cancer, but also on the well treatable subgroup of luminal A tumors. This suggests a general mechanism independent of subtypes. Increased chromosomal instability (CIN) in triple-negative breast cancer (TNBC) is attributed to a defect in the DNA repair pathway homologous recombination. Homologous recombination (HR) prevents genomic instability by repair and protection of replication. It is unclear whether genetic alterations actually lead to a repair defect or whether superior signaling pathways are of greater importance. Previous studies focused exclusively on the repair function of HR. Here, we show that the regulation of HR by the intra-S-phase damage response at the replication is of overriding importance. A damage response activated by Ataxia telangiectasia and Rad3 related-checkpoint kinase 1 (ATR-CHK1) can prevent replication stress and leads to resistance formation. CHK1 thus has a preferred role over HR in preventing replication stress in TNBC. The signaling cascade ATR-CHK1 can compensate for a double-strand break repair error and lead to resistance of HR-deficient tumors. Established methods for the identification of HR-deficient tumors for Poly(ADP-Ribose)-Polymerase 1 (PARP1) inhibitor therapies should be extended to include analysis of candidates for intra-S phase damage response.

TGF-betal transgenic mouse model of thoracic irradiation: modulation of MMP-2 and MMP-9 in the lung tissue.

To investigate the effects of TGF-beta1 on the two gelatinases (MMP-2 and MMP-9), and their roles in lung remodeling after irradiation-induced lung injury. Expressions of TGF-beta1 were measured with western blot, and expressions of MMP-2 and MMP-9 were analyzed with zymography in a -TGF-beta1 transgenic mouse model after thoracic irradiation with 12 Gy. We found expressions of TGF-beta1 in the lung from the transgenic mice were three folds as compared to those from control mice. With densitometrical analysis, we found a significant decrease in MMP-9 activity in lung homogenates from the transgenic mice as compared with those from non-transgenic control mice 8 weeks after sham-irradiation (relative MMP-9 activity: C: 1.000 0.1091; TG: 0.4772 +/- 0.470 (n = 8, P < 0.05). But MMP-2 was constitutively expressed in the lung homogenates from the transgenic mice as compared to those from control mice 8 weeks after sham-irradiation (relative MMP-2 activity 8 weeks after sham-irradiation: C: 1.000 +/- 0.1556, TG: 1.0075 +/- 0.1472). Eight weeks after thoracic irradiation with 12 Gy, we observed a significant increase of MMP-2 and MMP-9 activity in lung homogenates from both transgenic and normal mice. In TGF-beta1 transgenic mice relative MMP-9 activity was increased to 1.5321 +/- 0.2217 folds 8 weeks after thoracic irradiation with 12 Gy as compared to those after sham-irradiation (1.000 +/- 0.2153), and relative MMP-2 activity was increased to 1.7142 +/- 0.4231 folds. Our results show that TGF-beta1 itself down-regulates activity of MMP-9, thereby decreases ECM degradation in lungs of TGF-beta1 transgenic mice. Also we find that ionizing irradiation upregulates both MMP-2 and MMP-9 activity. Over-expressions of MMP-9 and MMP-2 after lung irradiation are involved in the inflammatory response associated with radiation-induced lung injury, and maybe further in radiation-induced lung fibrosis.