Initiation and promotion are major stages in the multistage carcinogenesis process. Formation of initiating carcinogen-DNA base adducts leads to heritable genetic changes, but the tumor-promoting events induced by complete carcinogens have not, as yet, been elucidated. Oxidant production and oxidative DNA damage induced by phorbol esters (i.e., 12-O-tetradecanoyl-phorbol-13-acetate) are associated with tumor promotion, while antioxidants and inhibitors of oxidative DNA damage suppress promotion and carcinogenesis. Our goal was to establish whether a carcinogen that requires oxidative metabolism for its activity can also induce oxidant production and DNA base oxidation. We found that topical treatment of SENCAR mice with 7,12-dimethylbenz[a]anthracene, which induces tumors in 40-50% of the mice, also causes hydrogen peroxide production and formation of oxidized bases (i.e., 8-hydroxyl-2'-deoxyguanosine and 5-hydroxymethyl-2'-deoxyuridine) in epidermal DNA. The levels of oxidized bases were of comparable magnitude to those mediated by the potent tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate. The oxidized bases persisted over several weeks in epidermal DNA. These oxidative events appear to be temporally associated with inflammatory responses that include edema and polymorphonuclear leukocyte infiltration, which remained elevated over longer periods of time and at higher levels than those induced by phorbol ester. Because these processes are usually associated with tumor promotion, our results support the conjecture that oxidative events may be involved in what is operationally referred to as the tumor promotion process by 7,12-dimethylbenz[a]anthracene.
[Show abstract][Hide abstract] ABSTRACT: We evaluated the protective effects of EA, a promising dietary constituent against degenerative diseases, on the clastogenic action of the model carcinogen DMBA in vitro on human hepatoma cells (HepG2) and in vivo on bone marrow of mice, using the frequencies of induced micronuclei as the end point. Pre-, post- and simultaneous treatments with EA and the carcinogen were carried out in vitro. Simultaneous treatment with EA caused a statistically significant increase of DMBA induced MN, suggesting a direct interaction between the two agents. No significant reduction in DMBA induced MN was found by pre-or post treatment with EA .Similar effects were observed in the toxicity assay. In in vivo experiments, EA pre-treatment did not affect the frequencies of MN in PCEs of bone marrow induced by DMBA. A good correlation was found between in vitro and in vivo experiments. Our results did not reveal any clear indication on the efficacy of EA on the induction of micronuclei by DMBA. EA by itself did not show any harmful effects.
"Hence, we have just initiated a comprehensive repetition of DMBA/TPA treatment where we will have a representative sample of animals from different stages following the application of this carcinogen in order to answer this question through quantification of apoptosis and senescence of Spalax skin and muscle tissue at the area of the carcinogen application. Furthermore, considering the high tolerance of Spalax to oxidative stress and the fact that DMBA is metabolized among others into ROS that cause oxidative DNA damage in the skin , the above experiment will allow us to compare the ROS levels upon DMBA application in Spalax and mice. "
[Show abstract][Hide abstract] ABSTRACT: Background
Subterranean blind mole rats (Spalax) are hypoxia tolerant (down to 3% O2), long lived (>20 years) rodents showing no clear signs of aging or aging related disorders. In 50 years of Spalax research, spontaneous tumors have never been recorded among thousands of individuals. Here we addressed the questions of (1) whether Spalax is resistant to chemically-induced tumorigenesis, and (2) whether normal fibroblasts isolated from Spalax possess tumor-suppressive activity.
Treating animals with 3-Methylcholantrene (3MCA) and 7,12-Dimethylbenz(a) anthracene/12-O-tetradecanoylphorbol-13-acetate (DMBA/TPA), two potent carcinogens, confirmed Spalax high resistance to chemically induced cancers. While all mice and rats developed the expected tumors following treatment with both carcinogens, among Spalax no tumors were observed after DMBA/TPA treatment, while 3MCA induced benign fibroblastic proliferation in 2 Spalax individuals out of12, and only a single animal from the advanced age group developed malignancy 18 months post-treatment. The remaining animals are still healthy 30 months post-treatment. In vitro experiments showed an extraordinary ability of normal Spalax cultured fibroblasts to restrict malignant behavior in a broad spectrum of human-derived and in newly isolated Spalax 3MCA-induced cancer cell lines. Growth of cancer cells was inhibited by either direct interaction with Spalax fibroblasts or with soluble factors released into culture media and soft agar. This was accompanied by decreased cancer cell viability, reduced colony formation in soft agar, disturbed cell cycle progression, chromatin condensation and mitochondrial fragmentation. Cells from another cancer resistant subterranean mammal, the naked mole rat, were also tested for direct effect on cancer cells and, similar to Spalax, demonstrated anti-cancer activity. No effect on cancer cells was observed using fibroblasts from mouse, rat or Acomys. Spalax fibroblast conditioned media had no effect on proliferation of noncancerous cells.
This report provides pioneering evidence that Spalax is not only resistant to spontaneous cancer but also to experimentally induced cancer, and shows the unique ability of Spalax normal fibroblasts to inhibit growth and kill cancer cells, but not normal cells, either through direct fibroblast-cancer cell interaction or via soluble factors. Obviously, along with adaptation to hypoxia, Spalax has evolved efficient anti-cancer mechanisms yet to be elucidated. Exploring the molecular mechanisms allowing Spalax to survive in extreme environments and to escape cancer as well as to kill homologous and heterologous cancer cells may hold the key for understanding the molecular nature of host resistance to cancer and identify new anti-cancer strategies for treating humans.
"Arsenite can act as a co-carcinogen with UV light in a mouse skin model (Rossman et al., 2001a). Chemicals possessing tumor initiating and/or enhancing properties, such as 7,12-dimethyl benz[a]anthracene (DMBA), 12-O- tetradecanoylphorbol-13-acetate (TPA), and UV radiation commonly induce chronic inflammation and oxidative stress (Wei and Frenkel, 1993; Frenkel et al., 1995; Soriani et al., 1999). It is thought that inflammatory reactions, characterized by the modulation of various cytokines and other inflammatory factors, may play an important role during the process of carcinogenesis [reviewed in (Frenkel, 1992; Shacter and Weitzman, 2002)]. "
[Show abstract][Hide abstract] ABSTRACT: Recent evidence suggests that inflammatory cytokines and growth factors contribute to arsenite (As)-induced human carcinogenesis. We investigated the expression of inflammatory cytokine mRNAs during the transformation process induced by chronic As exposure in non-tumorigenic human osteogenic sarcoma (N-HOS) cells using gene arrays, and results were confirmed by RT-PCR and protein arrays. Caffeic acid phenethyl ester (CAPE), a naturally occurring immunomodulating agent, was used to evaluate the role of inflammatory factors in the process of As-mediated N-HOS cell transformation and in As-transformed HOS (AsT-HOS) cells. We found that an 8-week continuous exposure of N-HOS to 0.3 μM arsenite resulted in HOS cell transformation. That exposure also caused substantial decreases in inflammatory cytokine mRNAs, such as interleukin (IL) IL-1α, IL-2, IL-8, IL-18, MCP-1, TGF-β2, and TNF-α, while it increased c-jun mRNA in a time-dependent manner. Co-incubation of N-HOS with As and CAPE (0.5-2.5 μM) prevented As-mediated declines in cytokine mRNAs in the co-treated cells, as well as their transformation to anchorage independence, while it caused decreases in c-jun mRNA. CAPE (up to 10 μM) had no effect on growth of N-HOS cells. However, CAPE (1-10 μM) treatment of AsT-HOS cells inhibited cell growth, induced cell cycle G2/M arrest, and triggered apoptosis, accompanied by changes in cytokine gene expression, as well as decreases in cyclin B1 and cdc2 abundance. Resveratrol (RV) and (-)•epigallocatechin gallate (EGCG), preventive agents present in grapes and green tea, respectively, induced similar changes in AsT-HOS cell growth but required much higher doses than CAPE to cause 50% growth arrest (<2.5 μM CAPE versus 25 μM RV or 50 μM EGCG). Overall, our findings suggest that inflammatory cytokines play an important role in the suppressive effects of CAPE on As-induced cell transformation and in the selective cytotoxicity of CAPE to As-transformed HOS cells.
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