Retraction Note: Selective killing of cancer cells by a small molecule targeting the stress response to ROS

Intratumor Heterogeneity and Treatment Resistance of Solid Tumors with a Focus on Polyploid/Senescent Giant Cancer Cells (PGCCs)

Single cell biology has revealed that solid tumors and tumor-derived cell lines typically contain subpopulations of cancer cells that are readily distinguishable from the bulk of cancer cells by virtue of their enormous size. Such cells with a highly enlarged nucleus, multiple nuclei, and/or multiple micronuclei are often referred to as polyploid giant cancer cells (PGCCs), and may exhibit features of senescence. PGCCs may enter a dormant phase (active sleep) after they are formed, but a subset remain viable, secrete growth promoting factors, and can give rise to therapy resistant and tumor repopulating progeny. Here we will briefly discuss the prevalence and prognostic value of PGCCs across different cancer types, the current understanding of the mechanisms of their formation and fate, and possible reasons why these tumor repopulating "monsters" continue to be ignored in most cancer therapy-related preclinical studies. In addition to PGCCs, other subpopulations of cancer cells within a solid tumor (such as oncogenic caspase 3-activated cancer cells and drug-tolerant persister cancer cells) can also contribute to therapy resistance and pose major challenges to the delivery of cancer therapy.

Exploiting Mitochondrial Vulnerabilities to Trigger Apoptosis Selectively in Cancer Cells

The transformation of normal cells to the cancerous stage involves multiple genetic changes or mutations leading to hyperproliferation, resistance to apoptosis, and evasion of the host immune system. However, to accomplish hyperproliferation, cancer cells undergo profound metabolic reprogramming including oxidative glycolysis and acidification of the cytoplasm, leading to hyperpolarization of the mitochondrial membrane. The majority of drug development research in the past has focused on targeting DNA replication, repair, and tubulin polymerization to induce apoptosis in cancer cells. Unfortunately, these are not cancer-selective targets. Recently, researchers have started focusing on metabolic, mitochondrial, and oxidative stress vulnerabilities of cancer cells that can be exploited as selective targets for inducing cancer cell death. Indeed, the hyperpolarization of mitochondrial membranes in cancer cells can lead to selective importing of mitocans that can induce apoptotic effects. Herein, we will discuss recent mitochondrial-selective anticancer compounds (mitocans) that have shown selective toxicity against cancer cells. Increased oxidative stress has also been shown to be very effective in selectively inducing cell death in cancer cells. This oxidative stress could lead to mitochondrial dysfunction, which in turn will produce more reactive oxygen species (ROS). This creates a vicious cycle of mitochondrial dysfunction and ROS production, irreversibly leading to cell suicide. We will also explore the possibility of combining these compounds to sensitize cancer cells to the conventional anticancer agents. Mitocans in combination with selective oxidative-stress producing agents could be very effective anticancer treatments with minimal effect on healthy cells.

The influence of alkaloids on oxidative stress and related signaling pathways

Alkaloids have always attracted scientific interest due to either their positive or negative effects on human beings. This review aims to summarize their antioxidant effects by both classical in vitro scavenging assay and at the cellular level. Since most in vitro studies used the DPPH (2,2-diphenyl-1-picrylhydrazyl) radical scavenging assay, the results from those studies are summed up in the first part of the article. In the second part, available data on the effect of alkaloids on NADPH-oxidase, the key enzyme for reactive oxygen species production, at the cellular level, are summarized. More than 130 alkaloids were tested by DPPH assay. However, due to methodological differences, a direct comparison is hardly possible. It can be at least concluded that some of them were either similar to or even more active than standard antioxidants and the number of aromatic hydroxyl groups seems to be the major determinant for the activity. The data on inhibition of NADPH-oxidase activity by alkaloids demonstrated that there is little relationship to the DPPH assay. The mechanism seems to be based on inhibition of synthesis, activation or translocation of NADPH-oxidase subunits. In some alkaloids, activation of the nuclear factor Nrf2 pathway was documented to be the grounds for inhibition of NADPH-oxidase. Interestingly, many alkaloids can behave both as anti-oxidants and pro-oxidants depending on conditions and pro-oxidation might be the reason for activation of Nrf2. Available data on other "antioxidant" transcription factors FOXOs and PPARs are also mentioned.

Impact of DNA repair pathways on the cytotoxicity of piperlongumine in chicken DT40 cell-lines

Piperlongumine is a naturally-occurring small molecule with various biological activities. Recent studies demonstrate that piperlongumine selectively kills various types of transformed cells with minimal toxicity to non-transformed cells by inducing a high level of reactive oxygen species (ROS). ROS generates various types of DNA lesions, including base modifications and single strand breaks. In order to examine the contribution of ROS-induced DNA damage to the cytotoxicity by piperlongumine, various DNA repair-deficient chicken DT40 cell-lines with a single DNA repair gene deletion were tested for cellular sensitivity to piperlongumine. The results showed that cell lines defective in homologous recombination (HR) display hyper-sensitivity to piperlongumine, while other cell lines with a deficiency in non-homologous end joining (NHEJ), base excision repair (BER), nucleotide excision repair (NER), Fanconi anemia (FA) pathway, or translesion DNA synthesis (TLS) polymerases, show no sensitivity to piperlongumine. The results strongly implicate that double strand breaks (DSBs) generated by piperlongumine are major cytotoxic DNA lesions. Furthermore, a deletion of 53BP1 or Ku70 in the BRCA1-deficient cell line restored cellular resistance to piperlongumine. This strongly supports the idea that piperlongumine induces DSB- mediated cell death. Interestingly, piperlongumine makes the wild type DT40 cell line hypersensitive to a PARP-inhibitor, Olaparib. The results implicate that piperlongumine inhibits HR. Further analysis with cell-based HR assay and the kinetic study of Rad51 foci formation confirmed that piperlongumine suppresses HR activity. Altogether, we revealed novel mechanisms of piperlongumine-induced cytotoxicity.