Oxidative Stress in Prostate Cancer

Changing Research Concepts Towards a Novel Paradigm for Prevention and Therapeutics

A Paschos; R Pandya; W C M Duivenvoorden; J H Pinthus

Disclosures

Prostate Cancer Prostatic Dis. 2013;16(3):217-225. 

In This Article

Oxidative Stress in the Development of PC

In healthy cells, ROS are continuously formed and moderate alterations to redox homeostasis are pivotal for the physiological regulation of cellular functions via the activation or inactivation of transcription factors, metabolic enzymes and membrane channels, as well as in the regulation of calcium- and phosphorylation-dependent signaling pathways.[1] PC cells, in comparison to healthy cells, are characterized by innate oxidative stress, a hallmark of the aggressive phenotype of this disease.[3,9,10] In PC, increased ROS production caused by various processes (summarized in Table 1 and Figures 1 and 2) lead to oxidative stress, including intrinsic conditions such as metabolic alterations, androgen receptor activation and mutation-induced mitochondrial dysfunctions, and also extrinsic environmental factors such as inflammation, xenobiotic metabolism and hypoxia.[11]

Figure 1.

Oxygen (O2) is essential for life owing to its role in ATP generation and its participation in biosynthesis and detoxification processes. Oxygen is a bi-radical molecule and as such it has two antibonding electrons moving in parallel spins in separate orbital. Accordingly, when O2 accepts a single electron, it becomes a reactive oxygen molecule as it needs to complement its outer orbital by picking an electron from other molecules. In the process, it can damage DNA, cellular proteins and lipids, leading to mutations and other structural changes favoring carcinogenesis. A plethora of intrinsic and extrinsic factors contribute to the formation of reactive oxygen species (ROS). CoQ, coenzyme Q10; ds, double stranded.

Figure 2.

Schematic representation of various oxidative stress activators and the enzymatic and non-enzymatic scavenging mechanisms of reactive oxygen species produced in human cells. SOD, superoxide dismutase.

Nuclear and Mitochondrial DNA Mutations: Inactivation of Antioxidative Genes

Increasing evidence indicates that impaired cellular metabolism is a defining characteristic of PC underlying the concept of genetic development of malignant cells. Accepted is the general hypothesis that genomic and mitochondrial DNA (mtDNA) instability is linked to impaired mitochondrial function and altered energy metabolism.[12] Mutations acquired over lifetime most likely contribute to the instability and ROS formation promotes random mutagenic events through oxidative DNA damage,[13] hence linking aging to cancer.[14] Whether the genomic or mtDNA instability or specialized energy metabolism is responsible for the origin of PC remains controversial. When mutations in oncogenes, tumor- or metastasis-suppressor genes and DNA repair genes occur, alterations in critical cellular processes can cause aberrant growth and contribute to neoplastic transformation of normal prostate cells.[15,16] Studying the molecular events in PC progression identified several epigenetic events that dramatically influence oxidative stress in PC. In PC, reduced expression of glutathione (GSH)-S-transferase P1 (GSTP1) and nuclear factor-erythroid 2 p45-related factor 2 (Nrf2) via promoter hypermethylation is a frequently occurring event leading to alterations in prostatic redox homeostasis.[17] GSTP1 catalyzes the conjugation of many hydrophobic and electrophilic compounds (for example, ROS and xenobiotics) with reduced GSH, and has an important role in the ROS detoxification process. Similarly, Nrf2 is suppressed in PC, possibly epigenetically, as indicated in a transgenic adenocarcinoma mouse prostate model.[18] Nrf2,[19] a transcription factor, is implicated in the expression of antioxidative defense mechanisms by binding to genomic cis-acting antioxidative response elements.[20] In Nrf2-deficient mice[21] and microarray-based assays,[22] Nrf2 has been shown to modulate transcription of several genes whose protein products function as antioxidants and has a critical role in cellular defense against oxidative stress. These epigenetic events, hypermethylation of GSTP1 and Nrf2, profoundly reduce the cells' antioxidative capacity. Excess production of ROS by the sources described in Table 1 is thus unopposed leading to oxidative stress.

Altered Mitochondrial Metabolism

The ZIP1/zinc/citrate transformation concept of prostate carcinogenesis may link metabolic alterations in PC with increased mitochondrial ROS formation. Human prostate epithelial cells are uniquely characterized by their ability to accumulate and secrete high levels of citrate.[23] This is biochemically accomplished through the accumulation of high intracellular levels of zinc that inhibit aconitase in the Krebs cycle causing discontinued citrate oxidation.[24] Consequently, this physiological peculiarity of prostate cells requires unique metabolic/bioenergetic mitochondrial relationships with regard to the Krebs cycle enzymes (m-aconitase deactivation) and the NADH/H+/FADH2 sources of the oxidative phosphorylation. Prostate cells undergo metabolic transformation from specialized zinc-accumulating citrate-producing cells to citrate-oxidizing malignant cells that lose the ability to accumulate zinc. It is conceivable that the inability of PC cells to accumulate high levels of zinc initially results in a nascent, later in stabilized citrate oxidation, and the coupled ATP production likely becomes essential for progression.[23,24] However, increased compensatory enzymatic activities of the Krebs cycle enzymes, succinate dehydrogenase (CoQH2 pool increase), aconitase (possibly Fenton reaction) and possibly α-ketoglutarate dehydrogenase and pyruvate dehydrogenase,[25] substantially contribute to ROS production per se. These associated mitochondrial bioenergetic alterations accelerate both mitochondrial oxidative phosphorylation and alter NADH-H+/FADH2 ratios. Figure 3 represents the concept of the role of zinc and the caused altered citrate metabolism in ROS formation and the development of prostate malignancy.

Figure 3.

Schematic representation of the zinc/truncated citrate metabolism concept and its possible relationship to increased reactive oxygen species (ROS) formation if altered during the development of prostate cancer. Briefly, with aging the physiologic zinc accumulation process in prostatic tissue decreases. This leads slowly to aconitase reactivation and concomitant citrate cycle activation. An active citrate cycle promotes ROS formation by re-activation of enzymes, such as succinate dehydrogenase and aconitase. Changes in concentration and ratio of reduced electron carriers (NADH/H+ and FADH2) affect the ROS formation in the respiration chain. H indicates high levels and L indicates low levels.

In this context, calcium, another key regulator of mitochondrial function, acts at several levels within the mitochondrion to stimulate ATP synthesis (for example, α-ketoglutarate dehydrogenase, isocitrate dehydrogenase, pyruvate dehydrogenase, ATPase) and modulates mitochondrial ROS production. Furthermore, calcium is implicated in regulating the activity of NADPH-oxidase.[26]

Age-related Changes and PC

On the basis of the free radical theory of aging, cells age because they accumulate oxidative damage over time.[27] Age-related changes in the prostate may be associated with an increase in oxidative stress. The notion emerged that essentially all men with circulating androgens, regardless of diet, occupation, lifestyle or other factors, will develop microscopic PC if they live long enough.[11] Interestingly, the accumulation of zinc in citrate-producing prostate epithelial cells is highly regulated by androgens and prolactin,[28] indicating an association between age-related hormonal changes in the male body and metabolic alterations in the prostatic tissue.

Androgens and Prostatic Oxidative Stress

Androgens, and possibly other steroid hormones, appear to be crucial in PC development.

It has been demonstrated experimentally that androgens, via the androgen receptor, induce oxidative stress in normal[29] and cancerous prostate cells.[30–32] Mechanistically, the pro-oxidative effects of androgens can be related to their metabolic effects or to the activation of pro-oxidative signaling pathways. Androgens modulate the production of ROS via both the induction of fatty acid oxidation in the mitochondria[33] and via the induction of NADPH-oxidase activity.[8] Cellular uptake of fatty acids in PC cells is regulated by androgens[34] via an increase in the mRNA levels of carnitine palmitoyltransferase, the rate-limiting enzyme in lipid oxidation.[33]

An important source of ROS in cancer cells in general and PC cells in particular is the NOX family of ROS-generating NADPH-oxidases.[3] NOX members consist of up to seven multiunit enzymes, which transport electrons from NADPH across membranes to oxygen to generate superoxide by the reduction of oxygen in this process. NOX1 and NOX2 transcripts are found in human prostate[35] and NOX5 has been described in PC cell lines and samples.[36] Androgens increase the expression of the p22phox catalytic subunit as well as the gp91phox subunit of NOX in PC cells both in vitro and in vivo.[8] In response to castration, upregulation of NOX1, NOX2 and NOX4 has been described in the rat prostate,[37] reaffirming the hormonal control of NOX expression.

Diet and Environmental Agents

A wide variety of environmental and nutritional factors associated with the production or elimination of ROS have been implicated in the development and prevention of PC in epidemiological studies.[11] Fat consumption appears to be crucial and it is conceivable that a subsiding β-oxidation in prostate cells fosters increased mitochondrial ROS production (CoQH2 pool). The association between obesity and cancer is significant for several types of cancers, including PC.[38] Adiponectin, a hormone inversely associated with obesity, inhibits oxidative stress in human PC cells in a dose-dependent manner. Lower levels of adiponectin in obese individuals may result in higher levels of prostatic oxidative stress, which may explain the clinical association between obesity, hypoadiponectinemia and PC.[39]

Several foods containing natural antioxidants, such as polyphenols and flavonoids, and other phytonutrients, such as vitamins and minerals, are proposed to facilitate ROS detoxification. Nevertheless, specific recommendations for nutritional supplements or a specific diet, which may protect against prostatic oxidative stress, cannot be made for the lack of clinical evidence and heterogeneity of populations and diets.[40]

Pesticides have been implicated as risk factors for aggressive PC.[41] They have been shown to generate ROS as part of their detoxification process and some are considered endocrine-disrupting factors. Other compounds in this group include bisphenol A, polychlorinated biphenol and cadmium. They interfere with the activities of estrogen, prolactin, thyroxine and androgen, thus potentially affecting the process of prostate carcinogenesis.[42] Endocrine disruptors can interfere with hormonal activities through direct effects on the signaling or indirectly through inhibition of enzymatic activities of, for example, cytochrome P450 or estrogen sulfotransferases, to alter steroid metabolism.[43] Estrogens, similar to androgens, induce,[44] whereas thyroid hormones modulate, mitochondrial ROS formation.[45] Phytoestrogens, natural plant substances with estrogenic properties, are discussed to be potentially antioxidative.[46,47] However, clinical trials examining the role of a number of phytoestrogens have shown unclear associations between intake and PC risk.[48]

Inflammation and PC

Chronic inflammation and infection have been implicated in the pathogenesis of colon, esophagus, stomach, bladder and liver cancer,[49] but thus far not of PC. A meta-analysis reported an association between a history of sexually transmitted disease or prostatitis and PC, although these studies were limited by potential recall and detection bias.[50] Mechanistically, inflammatory cells are drawn to the site of infection, and consequently myeloperoxidase and phagocytic NADPH-oxidase-derived ROS (H2O2, chloramines, bromamines, and so on) are released. Parallel secretion of inflammatory cytokines accelerates inflammation processes and favors cellular ROS formation.[51] These pro-oxidative changes in the prostatic microenvironment may not be solely sufficient to induce prostate carcinogenesis, but when combined with genetic susceptibility and in particular defects in antioxidative genes, such as those encoding for GSTP1, and DNA repair enzymes, can induce PC.[52]

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