The primary superoxide scavenger, SOD, has become an important potential therapeutic modality for cancer treatment in view of the link between reactive oxygen species (ROS) and carcinogenesis. ...Ironically, these same ROS also play a critical role in anticancer therapies that depend on ROS generation for their action rendering the use of SOD, in these settings, as a potential interfering factor that could decrease therapeutic efficacy. This complicating situation arises since ROS are well recognized for playing a dual role as both beneficial signaling molecules as well as deleterious damaging species in biological systems. A growing body of evidence has shown that ROS act as secondary messengers in intracellular signaling cascades, which stimulate and support the oncogenic phenotype of cancer cells. On the other hand, ROS also function as antitumorigenic species since they can induce cellular senescence and death. Therefore, the appropriate application of these antioxidant enzymes is a critical consideration in designing proper strategies for both prevention and treatment of malignant disorders. This chapter reviews evidence that suggests SOD as an emerging therapeutic agent as well as a target for cancer treatment. A discussion is also devoted to the practical challenges that limit the use of SOD manipulation as an anticancer approach.
The effects of overexpression of human manganese superoxide dismutase (MnSOD) on cell proliferation and response to oxidative stress in rat glioma cells were studied. MnSOD-overexpressing cells had a ...2- to 14-fold increase in MnSOD activity, but did not have consistent changes in the activities of CuZnSOD, catalase, or glutathione peroxidase. Cells with more than a 5-fold increase in MnSOD activity became more sensitive to radiation, 1,3-bis(2-chloroethyl)-1-nitrosourea, and buthionine sulfoximine and had a lower growth rate than parental and vector control cells. The sensitivity to 1,3-bis(2-chloroethyl)-1-nitrosourea was partially reduced by pyruvate, a H2O2 scavenger. Our results suggest that overexpression of MnSOD can cause an imbalance of antioxidant enzymes, which we hypothesize results in an elevation of intracellular H2O2. Overexpression of MnSOD can either inhibit cell proliferation or increase cell death by oxidative agents, depending on the levels of peroxide-removing enzymes.
Quantitative transplantation techniques have been employed to study radiogenic cancer initiation frequency and cell interactions during promotion/progression in grafted clonogenic rat thyroid ...epithelial cells. The graft recipients were surgically thyroidectomized and maintained on a diet containing less than 50 ng iodine per g. The results confirm that radiogenic initiation is a common cellular event; one of approximately 32 surviving 5-Gy-irradiated thyroid clonogens gave rise to cancer in grafts initially containing approximately 11 clonogens per transplantation site. The data demonstrate that the efficiency of promotion/progression is inversely related to grafted irradiated cell number. As the number of transplanted surviving irradiated clonogens was increased progressively from approximately 11 to approximately 720 clonogens per graft site, the carcinoma frequency per grafted clonogen progressively decreased to one per approximately 920. Addition of unirradiated thyroid cells to the transplant inocula further suppressed promotion/progression of radiation-initiated thyroid clonogens. Furthermore, the probability of vascular invasion, a reflection of metastatic potential in carcinomas which arose from irradiated grafted thyroid clonogens, was reduced by addition of unirradiated thyroid cells to the transplant inocula. Assays of thyroid stimulating hormone (TSH) titers in the sera of thyroidectomized rats 44 weeks after transplantation of clonogenic thyroid cells indicate that the suppression of neoplastic promotion/progression observed with increased numbers of cells per graft site is due at least in part to feed-back inhibition of TSH production by thyroid hormone of graft origin. Whether local cellular interactions are also involved in this inhibitory process is currently under investigation.
Maspin is a tumor suppressor whose expression is lost in many advanced breast cancers. Maspin has been shown to inhibit cell motility, invasion and metastasis; however, its precise role in normal ...mammary epithelium remains to be elucidated. Although expression of maspin mRNA is low or absent in most human breast cancer cells, the maspin gene is rarely re-arranged or deleted. We hypothesized that aberrant cytosine methylation and chromatin condensation of the maspin promoter participates in the silencing of maspin expression during neoplastic progression. To test this hypothesis, we compared cultured normal human mammary epithelial cells (HMECs) to 9 cultured human breast cancer cell lines. HMECs expressed maspin mRNA and displayed a completely non-methylated maspin gene promoter with an open chromatin structure. In contrast, 7 of 9 breast cancer cell lines had no detectable maspin expression and 6 of these 7 maspin-negative breast cancer cell lines also displayed an aberrant pattern of cytosine methylation of the maspin promoter. Interestingly, the maspin promoter was completely methylated in maspin-negative normal peripheral blood lymphocytes. This indicates that the maspin promoter is not a functional CpG island and that cytosine methylation of this region may contribute to normal tissue- restricted gene expression. Chromatin accessibility studies with MCF-7 cells, which lack maspin expression and have a methylated maspin promoter, showed a closed chromatin structure compared with HMECs. Moreover, maspin gene expression could be re-activated in MCF-7 cells by treatment with 5- aza-2'-deoxycytidine, a DNA demethylating agent. Thus, aberrant cytosine methylation and heterochromatinization of the maspin promoter may silence maspin gene expression, thereby contributing to the progression of human mammary cancer.
This chapter focuses on superoxide dismutases (SOD), a group of enzymes important for removing biologically generated superoxide anion radical (O2.−). These enzymes function by catalyzing the ...dismutation of superoxide radicals to hydrogen peroxide and oxygen, and their action helps to protect cells from oxidation of lipids, proteins and DNA. These enzymes are crucial in maintaining a proper balance of superoxide within specific cellular compartments that is essential for normal cellular signaling and stress responses. Three forms of SOD exist in humans: copper and zinc containing SOD (CuZnSOD), manganese containing SOD (MnSOD) and extracellular SOD (ecSOD). Each of the three SOD proteins plays a unique physiological role based in part on its tissue distribution and sub cellular localization. For example CuZnSOD is generally found in the cytosol, but can also be localized in the nucleus under certain conditions. CuZnSOD protects proteins, lipids and nuclear DNA from oxidation. In contrast, MnSOD, an enzyme required for life in an oxygen atmosphere, is located specifically in the mitochondrial matrix and protects the respiratory machinery and mitochondrial DNA from oxidative damage. ecSOD is found both extracellularly and on the plasma membrane. Its expression is greatest in endothelial cells of blood vessels where its key role appears to be regulating superoxide interactions with nitric oxide. Superoxide and nitric oxide react to form peroxynitrite, a toxic species. This reaction decreases the bioavailability of nitric oxide so it can no longer function to maintain vascular tone and health. The balance of superoxide and nitric oxide is therefore vital for proper vasculature function. In disease processes, dysregulation of SODs results in a pathogenic imbalance of superoxide, leading to profound effects on cells and tissue. For example, aberrant structure or activity of CuZnSOD has been linked to the neurological disease amyotrophic lateral sclerosis (ALS) and developmental abnormalities seen in Down’s syndrome. Similarly, decreases in the function of MnSOD have been shown to play a role in carcinogenesis. Finally, cardiovascular disease is associated with poor vascular tone following lowered nitric oxide bioavailability. Decreased ecSOD function results in increased peroxynitrite formation that causes damage to the vasculature, thus accelerating cardiovascular disease. In summary, SODs are an essential group of enzymes that must be present at appropriate levels, structure, and locations to maintain a healthy balance of superoxide, necessary for development and disease prevention.
During cancer progression there is an accumulation of genetic and epigenetic changes that dictate the phenotypic diversity of a tumor population. Epigenetics is a heritable change in phenotype that ...is independent of a change in the genotype. The focus of this chapter is to review, in part, the literature indicating that an “epigenetic switch” occurs in cancer progression and to detail the evidence of one particular gene product, maspin, as a paradigm of epigenetic control. The regulation of maspin expression in normal epithelial tissue and the multiple mechanisms by which maspin is inappropriately silenced in cancer is discussed. Understanding the molecular features of the epigenetic switch is likely to provide a therapeutic opportunity to induce transcriptional reprogramming and prevent devastating disease processes such as epithelial cancer metastasis.
The goal of this chapter is to promote the value of studying maspin regulation as a paradigm for loss of transcriptional control during cancer progression and to highlight the importance of this ...endeavor in developing a comprehensive picture of the epigenetics of the malignant phenotype. We will attempt to do this through a discussion of the structure and functions of the serpin superfamily of proteins, with an emphasis on maspin, its discovery as a tumor suppressor, and its functional role in cancer. The control of maspin expression in normal tissue by epigenetic mechanisms will be described and how this underlying mechanism is compromised in cancer leading to the inappropriate silencing of maspin in cancers derived from maspin-positive cell types, as well as the activation of maspin in cancers derived from normally maspin-negative cell types. Finally, we will close with speculation that maspin may represent an inaugural member of a class of cell-type restricted genes involved in cancer cause and progression that are controlled by epigenetic mechanisms. During transformation, epigenetic instability and mischief results in a loss of control in the expression of these genes. We propose that these genes, through metastable epigenetic switching mechanisms, can be turned off and on in response to environmental stresses and cues in the cancer cell, thereby allowing tumor cells a phenotypic plasticity that appears necessary for the challenges a tumor cell and its progeny must undertake to migrate from primary tumor site to distant metastatic site. It is proposed that this epigenetic switch can be targeted by therapeutics designed to transcriptional reprogram tumor cells and flip the switch back to non-malignant behavior.
We have presented evidence that the functional thyroid follicles (follicular units, FU) which are formed in grafts of monodispersed rat thyroid cells, and hence the thyroid tumors which later develop ...in such grafts, are clonal in origin. Transplantation assays indicate that the clonogens comprise ?? 1% of the cells in monodispersed suspensions of normal thyroid tissue. Carcinogenesis studies show that neoplastic initiation of thyroid clonogens by radiation is a common event. Promotion-progression to cancer from radiation initiated clonogens has, however, been shown to be inversely related to the total grafted thyroid cell number; i.e. more tumors develop per irradiated clonogen in grafts of small cell numbers than of large cell numbers. Recent studies have been designed to investigate: a) whether the cell number-dependent inhibition of promotion-progression is mediated by remote hormonal feed-back, local cell-cell interactions, or both; b) the cell population kinetics of the clonogen subpopulation during goitrogenesis and goiter involution; and c) the effect of prolonged exposure to high levels of TSH (thyrotropin) on the capacity of the clonogens to give rise to functional FU. The results indicate that local cell-cell interactions play an important role in the cell numberdependent suppression of neoplastic promotion-progression. They also show that if sufficient thyroid cells are grafted, the thyroid-pituitary axis can be reestablished in thyroidectomized rats fed normal diets. In such animals given iodine deficient diets, the FU that develop in the thyroid grafts shift their secretory pattern to increase the ratio of T3 (triiodothyronine) to T4 (thyroxine), and thus conserve the available iodine. Finally, the clonogenic subpopulation is conserved during both goitrogenesis and goiter involution. When they are transplanted to thyroidectomized recipients, clonogens from two types of goiters form FU that are morphologically indistinguishable from those that develop in grafts of normal thyroid clonogens. Furthermore, the secretion of T3 and T4 by such grafts is dependent on the grafted clonogen number, and hence FU formation, and not on the total number of thyroid cells transplanted. We conclude that the thyroid clonogens, the presumptive cancer progenitor cells, have many of the characteristics of stem cells.
