Increased generation of reactive oxygen species (ROS) and an altered redox status have long been observed in cancer cells, and recent studies suggest that this biochemical property of cancer cells can be exploited for therapeutic benefits. Cancer cells in advanced stage tumours frequently exhibit multiple genetic alterations and high oxidative stress, suggesting that it might be possible to preferentially eliminate these cells by pharmacological ROS insults. However, the upregulation of antioxidant capacity in adaptation to intrinsic oxidative stress in cancer cells can confer drug resistance. Abrogation of such drug-resistant mechanisms by redox modulation could have significant therapeutic implications. We argue that modulating the unique redox regulatory mechanisms of cancer cells might be an effective strategy to eliminate these cells.

Targeting cancer cells by ROS-mediated mechanisms: a radical therapeutic approach?

ROS Function in Redox Signaling and Oxidative Stress

Reactive oxygen species (ROS) homeostasis and redox regulation in cellular signaling

Otto Warburg pioneered quantitative investigations of cancer cell metabolism, as well as photosynthesis and respiration. Warburg and co-workers showed in the 1920s that, under aerobic conditions, tumour tissues metabolize approximately tenfold more glucose to lactate in a given time than normal tissues, a phenomenon known as the Warburg effect. However, this increase in aerobic glycolysis in cancer cells is often erroneously thought to occur instead of mitochondrial respiration and has been misinterpreted as evidence for damage to respiration instead of damage to the regulation of glycolysis. In fact, many cancers exhibit the Warburg effect while retaining mitochondrial respiration. We re-examine Warburg's observations in relation to the current concepts of cancer metabolism as being intimately linked to alterations of mitochondrial DNA, oncogenes and tumour suppressors, and thus readily exploitable for cancer therapy.

Otto Warburg's contributions to current concepts of cancer metabolism

Tumor Cell Metabolism: Cancer's Achilles' Heel

Mutations in mitochondrial DNA (mtDNA) occur at high frequency in human tumors, but whether these mutations alter tumor cell behavior has been unclear. We used cytoplasmic hybrid (cybrid) technology to replace the endogenous mtDNA in a mouse tumor cell line that was poorly metastatic with mtDNA from a cell line that was highly metastatic, and vice versa. Using assays of metastasis in mice, we found that the recipient tumor cells acquired the metastatic potential of the transferred mtDNA. The mtDNA conferring high metastatic potential contained G13997A and 13885insC mutations in the gene encoding NADH (reduced form of nicotinamide adenine dinucleotide) dehydrogenase subunit 6 (ND6). These mutations produced a deficiency in respiratory complex I activity and were associated with overproduction of reactive oxygen species (ROS). Pretreatment of the highly metastatic tumor cells with ROS scavengers suppressed their metastatic potential in mice. These results indicate that mtDNA mutations can contribute to tumor progression by enhancing the metastatic potential of tumor cells.

/pdf/ros-generating-mitochondrial-dna-mutations-can-regulate-4vseb8w1d7.pdf

ROS-generating mitochondrial DNA mutations can regulate tumor cell metastasis.

The extract of ginger (Zingiber officinale Roscoe) and its major pungent components, [6]-shogaol and [6]-gingerol, have been shown to have an anti-proliferative effect on several tumor cell lines. However, the anticancer activity of the ginger extract in pancreatic cancer is poorly understood. Here, we demonstrate that the ethanol-extracted materials of ginger suppressed cell cycle progression and consequently induced the death of human pancreatic cancer cell lines, including Panc-1 cells. The underlying mechanism entailed autosis, a recently characterized form of cell death, but not apoptosis or necroptosis. The extract markedly increased the LC3-II/LC3-I ratio, decreased SQSTM1/p62 protein, and enhanced vacuolization of the cytoplasm in Panc-1 cells. It activated AMPK, a positive regulator of autophagy, and inhibited mTOR, a negative autophagic regulator. The autophagy inhibitors 3-methyladenine and chloroquine partially prevented cell death. Morphologically, however, focal membrane rupture, nuclear shrinkage, focal swelling of the perinuclear space and electron dense mitochondria, which are unique morphological features of autosis, were observed. The extract enhanced reactive oxygen species (ROS) generation, and the antioxidant N-acetylcystein attenuated cell death. Our study revealed that daily intraperitoneal administration of the extract significantly prolonged survival (P = 0.0069) in a peritoneal dissemination model and suppressed tumor growth in an orthotopic model of pancreatic cancer (P < 0.01) without serious adverse effects. Although [6]-shogaol but not [6]-gingerol showed similar effects, chromatographic analyses suggested the presence of other constituent(s) as active substances. Together, these results show that ginger extract has potent anticancer activity against pancreatic cancer cells by inducing ROS-mediated autosis and warrants further investigation in order to develop an efficacious candidate drug.

/pdf/anticancer-effect-of-ginger-extract-against-pancreatic-6alfxahngc.pdf

Anticancer Effect of Ginger Extract against Pancreatic Cancer Cells Mainly through Reactive Oxygen Species-Mediated Autotic Cell Death.

Interleukin-33 (IL-33) was recently shown to be involved in the inflammatory tumour microenvironment and the progression of colorectal cancer (CRC). We report here that the expression level of sST2, a soluble form of the IL-33 receptor (ST2L), is inversely associated with the malignant growth of CRC. sST2 is downregulated in high-metastatic cells compared with low-metastatic human and mouse CRC cells. Knockdown of sST2 in low-metastatic cells enhances tumour growth, metastasis and tumour angiogenesis, whereas its overexpression in high-metastatic cells suppresses these processes. Circulating and intratumourally administered sST2-Fc fusion protein reduce tumour growth, metastatic spread and tumour angiogenesis in mice bearing high-metastatic CRC. Mechanistically, sST2 suppresses IL-33-induced angiogenesis, Th1- and Th2-responses, macrophage infiltration and macrophage M2a polarization. In conclusion, we show that sST2 negatively regulates tumour growth and the metastatic spread of CRC through modification of the tumour microenvironment. Thus, the IL-33/ST2L axis may be a potential therapeutic target in CRC.

Soluble IL-33 receptor sST2 inhibits colorectal cancer malignant growth by modifying the tumour microenvironment.

Generation of various kinds of trans-mitochondrial mice, mito-mice, each carrying mtDNAs with a different pathogenic mutation, is required for precise investigation of the pathogenesis of mitochondrial diseases. This study used two respiration-deficient mouse cell lines as donors of mtDNAs with possible pathogenic mutations. One cell line expressed 45-50% respiratory activity due to mouse mtDNAs with a T6589C missense mutation in the COI gene (T6589C mtDNA) and the other expressed 40% respiratory activity due to rat (Rattus norvegicus) mtDNAs in mouse cells. By cytoplasmic transfer of these mtDNAs to mouse ES cells, we isolated respiration-deficient ES cells. We obtained chimeric mice and generated their F(6) progeny carrying mouse T6589C mtDNAs by its female germ line transmission. They were respiration-deficient and thus could be used as models of mitochondrial diseases caused by point mutations in mtDNA structural genes. However, chimeric mice and mito-mice carrying rat mtDNAs were not obtained, suggesting that significant respiration defects or some deficits induced by rat mtDNAs in mouse ES cells prevented their differentiation to generate mice carrying rat mtDNAs.

/pdf/generation-of-trans-mitochondrial-mice-carrying-homoplasmic-4ejijb8p8q.pdf

Generation of trans-mitochondrial mice carrying homoplasmic mtDNAs with a missense mutation in a structural gene using ES cells

Hypoxia has been shown to promote metastasis of cancer cells through induction of epithelial-mesenchymal transition (EMT). It is also known to cause generation of reactive oxygen species (ROS). We investigated here the role of ROS in hypoxia-induced EMT and whether attenuation of ROS by antioxidants suppresses hypoxia-induced EMT and metastasis of human pancreatic cancer cells in a xenograft nude mouse model. PANC-1 and MiaPaCa-2 cells exposed to hypoxia (1 % O2) showed increased ROS generation and characteristic changes of EMT such as morphological changes, enhanced invasiveness, and upregulation of EMT regulators, SLUG, SNAI1 and TWIST. The antioxidants N-acetylcysteine (NAC) and ebselen significantly suppressed EMT and the expression of EMT regulators during hypoxia. NAC abrogated activation of HIF-1α and NF-κB, both of which were found to play an active role in hypoxia-induced EMT. Administration of NAC to nude mice with orthotopic tumors suppressed the expression of EMT regulators in hypoxic areas and significantly inhibited hepatic metastasis. Together, the present findings demonstrate that attenuation of ROS by antioxidants suppresses hypoxia-induced EMT and metastatic phenotype, suggesting that antioxidants may be of therapeutic value in treating pancreatic cancers.

Attenuation of reactive oxygen species by antioxidants suppresses hypoxia-induced epithelial-mesenchymal transition and metastasis of pancreatic cancer cells.

Miho Akimoto

Papers

ROS-generating mitochondrial DNA mutations can regulate tumor cell metastasis.

Anticancer Effect of Ginger Extract against Pancreatic Cancer Cells Mainly through Reactive Oxygen Species-Mediated Autotic Cell Death.

Soluble IL-33 receptor sST2 inhibits colorectal cancer malignant growth by modifying the tumour microenvironment.

Generation of trans-mitochondrial mice carrying homoplasmic mtDNAs with a missense mutation in a structural gene using ES cells

Attenuation of reactive oxygen species by antioxidants suppresses hypoxia-induced epithelial-mesenchymal transition and metastasis of pancreatic cancer cells.