Many cancer drugs exert their therapeutic effect by inducing oxidative stress in the cancer cells. Oxidative stress compromises cell survival by inflicting lesions in macromolecules like DNA. Cancer cells rely on enhanced antioxidant metabolism and increased DNA repair function to survive oxidative assault. PARP1, a protein that senses DNA-strand breaks and orchestrates their repair, has an important role in the repair of oxidative DNA damage. Berberine, an alkaloid compound present in many herbal plants, is capable of inducing oxidative DNA damage and downregulating homologous recombination repair (HRR) in cancer cells. In this study, we demonstrated that berberine and PARP inhibitor niraparib have a synthetic lethal effect on ovarian cancer cells. Oxidative DNA damage was greatly induced by berberine in ovarian cancer cells. In addition, the level of RAD51 and the capacity of HRR were also reduced by berberine. Correspondingly, PARP became hyperactivated in response to berberine treatment. Cancer cells treated with berberine and niraparib in combination exhibited greatly increased apoptosis and remarkably reduced tumor growth in vivo. Together, the results indicate that by inducing oxidative DNA damage and downregulating HRR in cancer cells berberine is able to further sensitize cancer cells to PARP inhibition. Our findings demonstrate a potential therapeutic value of combined application of berberine and PARP inhibitors in ovarian cancer treatment.

No abstract

Reactive oxygen species (ROS) serve as critical signals in various cellular processes. Excessive ROS cause cell death or senescence and mediates the therapeutic effect of many cancer drugs. Recent studies showed that ROS increasingly accumulate during G2/M arrest, the underlying mechanism, however, has not been fully elucidated. Here, we show that in cancer cells treated with anticancer agent TH287 or paclitaxel that causes M arrest, mitochondria accumulate robustly and produce excessive mitochondrial superoxide, which causes oxidative DNA damage and undermines cell survival and proliferation. While mitochondrial mass is greatly increased in cells arrested at M phase, the mitochondrial function is compromised, as reflected by reduced mitochondrial membrane potential, increased SUMOylation and acetylation of mitochondrial proteins, as well as an increased metabolic reliance on glycolysis. CHK1 functional disruption decelerates cell cycle, spares the M arrest and attenuates mitochondrial oxidative stress. Induction of mitophagy and blockade of mitochondrial biogenesis, measures that reduce mitochondrial accumulation, also decelerate cell cycle and abrogate M arrest-coupled mitochondrial oxidative stress. These results suggest that cell cycle progression and mitochondrial homeostasis are interdependent and coordinated, and that impairment of mitochondrial homeostasis and the associated redox signaling may mediate the antineoplastic effect of the M arrest-inducing chemotherapeutics. Our findings provide insights into the fate of cells arrested at M phase and have implications in cancer therapy.

No abstract

Autophagy has been well documented to play an important role in maintaining genomic stability. However, in addition to directly engulfing and digesting the damaged organelles and chromatin fragments, autophagy can affect many cellular processes including DNA damage response, regulation of redox homeostasis, and cell division; it remains to be determined to what extent each of those processes contributes to the maintenance of genomic stability. We here examined the role of autophagy-dependent redox regulation in the maintenance of genomic stability in two cancer cell lines (HT1080 and U2OS) and mesenchymal stem cells (MSCs) using micronuclei MN, also referred to as cytoplasmic chromatin fragments, as a marker. Our results showed that the spontaneous and genotoxic stress-induced frequencies of MN in cancer cells were significantly reduced by autophagy activators rapamycin and Torin1, and the reduction in MN was accompanied by a reduction in reactive oxygen species (ROS). Increased micronucleation in senescent MSCs, in which autophagic flux is blocked, was also attenuated by rapamycin, together with a reduction in ROS. Inhibition of autophagy by chloroquine (CQ) or ATG5 depletion, on the other hand, resulted in an increased frequency of MN, though a ROS elevation in response to autophagy inhibition was only observed in MSCs. Importantly, the induction of MN by autophagy inhibition in MSCs could be abrogated by antioxidant N-acetylcysteine (NAC). In contrast to the reported impairment of CHK1 activation in Atg7-deficient mouse embryonic fibroblasts, we found that the level of phosphorylated CHK1 was increased by CQ or ATG5 depletion but decreased by rapamycin or Torin1, suggesting that the increased genomic instability by defective autophagy is not caused by insufficient activation of CHK1-homologous recombination cascade. Together, our findings suggest that redox homeostasis regulated by autophagy contributes substantially to the maintenance of genomic stability in certain contexts.