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We previously reported that combining a phosphoinositide 3-kinase (PI3K) inhibitor with a poly-ADP Rib polymerase (PARP)-inhibitor enhanced DNA damage and cell death in breast cancers that have genetic aberrations in BRCA1 and TP53. Here, we show that enhanced DNA damage induced by PI3K inhibitors in this mutational background is a consequence of impaired production of nucleotides needed for DNA synthesis and DNA repair. Inhibition of PI3K causes a reduction in all four nucleotide triphosphates, whereas inhibition of the protein kinase AKT is less effective than inhibition of PI3K in suppressing nucleotide synthesis and inducing DNA damage. Carbon flux studies reveal that PI3K inhibition disproportionately affects the nonoxidative pentose phosphate pathway that delivers Rib-5-phosphate required for base ribosylation. In vivo in a mouse model of BRCA1-linked triple-negative breast cancer (K14-Cre BRCA1(f/f)p53(f/f)), the PI3K inhibitor BKM120 led to a precipitous drop in DNA synthesis within 8 h of drug treatment, whereas DNA synthesis in normal tissues was less affected. In this mouse model, combined PI3K and PARP inhibition was superior to either agent alone to induce durable remissions of established tumors.

Selenium, an essential micronutrient known for its cancer prevention properties, is incorporated into a class of selenocysteine-containing proteins (selenoproteins). Selenoprotein H (SepH) is a recently identified nucleolar oxidoreductase whose function is not well understood. Here we report that seph is an essential gene regulating organ development in zebrafish. Metabolite profiling by targeted LC-MS/MS demonstrated that SepH deficiency impairs redox balance by reducing the levels of ascorbate and methionine, while increasing methionine sulfoxide. Transcriptome analysis revealed that SepH deficiency induces an inflammatory response and activates the p53 pathway. Consequently, loss of seph renders larvae susceptible to oxidative stress and DNA damage. Finally, we demonstrate that seph interacts with p53 deficiency in adulthood to accelerate gastrointestinal tumor development. Overall, our findings establish that seph regulates redox homeostasis and suppresses DNA damage. We hypothesize that SepH deficiency may contribute to the increased cancer risk observed in cohorts with low selenium levels.

REV1/POLζ-dependent mutagenic translesion synthesis (TLS) promotes cell survival after DNA damage but is responsible for most of the resulting mutations. A novel inhibitor of this pathway, JH-RE-06, promotes cisplatin efficacy in cancer cells and mouse xenograft models, but the mechanism underlying this combinatorial effect is not known. We report that, unexpectedly, in two different mouse xenograft models and four human and mouse cell lines we examined in vitro cisplatin/JH-RE-06 treatment does not increase apoptosis. Rather, it increases hallmarks of senescence such as senescence-associated β-galactosidase, increased p21 expression, micronuclei formation, reduced Lamin B1, and increased expression of the immune regulators IL6 and IL8 followed by cell death. Moreover, although p-γ-H2AX foci formation was elevated and ATR expression was low in single agent cisplatin-treated cells, the opposite was true in cells treated with cisplatin/JH-RE-06. These observations suggest that targeting REV1 with JH-RE-06 profoundly affects the nature of the persistent genomic damage after cisplatin treatment and also the resulting physiological responses. These data highlight the potential of REV1/POLζ inhibitors to alter the biological response to DNA-damaging chemotherapy and enhance the efficacy of chemotherapy. [ABSTRACT FROM AUTHOR]