Redox-Mediated and Ionizing-Radiation-Induced Inflammatory Mediators in Prostate Cancer Development and Treatment

Abstract

Significance: Radiation therapy is widely used for treatment of prostate cancer. Radiation can directly damage biologically important molecules; however, most effects of radiation-mediated cell killing are derived from the generated free radicals that alter cellular redox status. Multiple proinflammatory mediators can also influence redox status in irradiated cells and the surrounding microenvironment, thereby affecting prostate cancer progression and radiotherapy efficiency. Recent Advances: Ionizing radiation (IR)–generated oxidative stress can regulate and be regulated by the production of proinflammatory mediators. Depending on the type and stage of the prostate cancer cells, these proinflammatory mediators may lead to different biological consequences ranging from cell death to development of radioresistance. Critical Issues: Tumors are heterogeneous and dynamic communication occurs between stromal and prostate cancer cells, and complicated redox-regulated mechanisms exist in the tumor microenvironment. Thus, antioxidant and anti-inflammatory strategies should be carefully evaluated for each patient at different stages of the disease to maximize therapeutic benefits while minimizing unintended side effects. Future Directions: Compared with normal cells, tumor cells are usually under higher oxidative stress and secrete more proinflammatory mediators. Thus, redox status is often less adaptive in tumor cells than in their normal counterparts. This difference can be exploited in a search for new cancer therapeutics and treatment regimes that selectively activate cell death pathways in tumor cells with minimal unintended consequences in terms of chemo- and radio-resistance in tumor cells and toxicity in normal tissues. Antioxid. Redox Signal. 20, 1481–1500.

Introduction

Cancer is a major health issue throughout the world and accounts for about 25% of all deaths in the United States. Prostate cancer has accounted for ∼29% of newly diagnosed cancer cases and 9% of cancer deaths in men in 2012 (167). The common forms of treatment for prostate cancer are surgery, radiation, chemotherapy, and hormone management (181). Radiation therapy can be used to treat localized disease or as part of a curative therapy to prevent cancer recurrence after surgical removal of the primary tumor. Unfortunately, the disease recurs and progresses to an advanced stage in as many as 30%–40% of prostate cancer patients treated with radiation (181). Contributing factors that influence radiation therapy outcomes are as follows: the presence of radiation-resistant prostate cancer cells and cancer stem cells; the complexity of the tumor microenvironment, such as hypoxia; increased inflammatory cytokine and growth factor secretion; and elevated relevant receptor expression.

Consideration of the effects of radiation therapy should not be limited to isolated cells since the entire tissue plays a role in determining the response of individual cells to any regulatory or damaging signals (13, 148). The localized release of radiation energy generates free radicals, mainly by ionization of water, which constitutes about 80% of cell mass, and produces various reactive oxygen species (ROS). The ROS can then rapidly diffuse and react with other molecules to damage DNA, protein, and lipid targets. This ROS-mediated effect of ionizing radiation (IR) is suspected to have caused a majority of radiation-induced damage (13, 66). Different types of cells in tumor tissues are subjected to complex regulatory mechanisms depending on their interactions with other cells and cellular products in the microenvironment, such as interleukin-1β (IL-1β), IL-6, IL-8, tumor necrosis factor-alpha (TNF-α), and transforming growth factor-beta (TGF-β). Altered cytokine expression can alter many signaling pathways that converge on a few important transcription factors, including nuclear factor kappa B (NF-κB), activator protein-1 (AP-1), and signal transducers and activators of transcription (STATs). These transcription factors also upregulate the expression of several cytokines, such as IL-1β and TNF-α (105). Such positive feedback loops amplify radiation- or oxidative-stress-induced inflammation, which may persist chronically (156). Because ROS play crucial dual roles in inducing cancer development (initiation, promotion, and progression) and maintaining metabolic homeostasis, both prooxidant- and antioxidant-based agents have been developed for cancer prevention and treatment (63, 183).

This article reviews commonly elevated cytokines and growth factors, such as IL-6, IL-8, TNF-α, and TGF-β, as major mediators of IR response found in prostate cancer after radiation therapy, and discusses different redox signaling pathways and redox-sensitive transcription factors controlled by these proteins. The biological significance of this information can be particularly useful in understanding the development of cancer radioresistance and improving radiation therapeutic effects in humans.

Radiation in Prostate Cancer Treatment

IR and radiation therapy

Cancer radiotherapy is the medical use of IR to control or kill malignant cells. For prostate cancer treatment, radiation is most commonly given by an external source (external beam radiotherapy), but it may also be administered by inserting small radioactive seeds directly into the tumor (brachytherapy), which is appropriate for some men with early prostate cancer (181). Delivery of a lethal dose of radiation to a tumor lesion while minimizing damage to normal surrounding tissues is one of the major challenges of radiotherapy. The accuracy and precision of radiotherapy has improved as imaging technology has improved, especially the use of 3-dimensional conformal radiation therapy and intensity-modulated radiation therapy (164).

External beam radiotherapy can deliver two types of radiation that damage DNA and other macromolecules of cancerous cells: photons, such as X-rays and γ-rays, and charged particles, such as protons and electrons. X-rays are generated extranuclearly from X-ray machines whereas γ-rays are produced intranuclearly from radioactive materials. While they differ in the source of generation, X-rays and γ-rays share the same radiophysical properties (66, 113, 148) (Fig. 1). Direct action of IR is the interaction of radiation beams or particles with critical target molecules in cells, such as DNA, to cause various types of damage in DNA structure, leading to lethal chromosome aberrations (113). Indirect action of radiation is a multicellular effect by water radiolysis that produces free radicals, such as hydrated electrons (e⁻_aq), ionized water (H₂O⁺), hydroperoxyl radical (HO₂^•), hydrogen radical (H^•), and hydroxyl radical (^•OH), which can diffuse far enough to reach and damage the DNA, protein, and lipid targets (13, 66, 113, 148). Direct action and indirect action of IR are closely linked; for example, direct damage to DNA by IR can induce ROS generation via histone H2AX-mediated mechanisms that involve NADPH oxidase 1 (NOX1) and Ras-related C3 botulinum toxin substrate 1 (Rac1) GTPase (83). To a large extent, it is these free radicals that break chemical bonds, produce chemical changes, and initiate the chain of events that results in the final expression of biological damage.

FIG. 1.

Electromagnetic spectrum of radiation and medical use of ionizing radiation (IR). Direct action of IR leads to damages in DNA structure. Indirect action of radiation results from water radiolysis, which produces free radicals that can diffuse far enough to reach and damage the DNA.

It has been reported that intracellular ROS levels are quickly increased after exposure to IR and that elevated levels of ROS are sustained for several hours after initial IR exposure (32, 179). NOX is responsible, in part, for a late increase in intracellular superoxide generation after exposure to IR (32, 179). IR-induced mitochondrial dysfunction, especially decreased electron transport chain complex I activity, produces a feed forward loop that contributes to persistent oxidative stress after irradiation (198). Since the mitochondrion is the most important energy-generating organelle, mitochondrial dysfunction due to direct effects of IR or indirect effects mediated by ROS may result in alteration or adaptive responses of metabolic pathways involved in cancer development. Free radicals may amplify and prolong the deleterious effects of radiation, leading to chronic oxidative stress, alteration of multiple metabolic pathways, normal tissue injury, cell death, and other bystander effects [reviewed in (142)].

Radioresistance: an important impediment to prostate cancer treatment

The development of resistance to radiation is one of the worst obstacles of prostate cancer radiotherapy. Some of the molecular entities associated with radioresistance have been identified in prostate cancer (71, 88, 95, 171); however, the underlying mechanisms are still not well understood. While it is possible to increase radiation doses to a level that ensures complete irradiation of cancer, the use of higher doses of radiation may cause unacceptable serious side effects to normal tissues. Thus, it is important to develop strategies that can sensitize tumor cells to radiation treatment and/or can protect normal tissue from radiation damage.

Radiotherapy mainly induces cell death by generating oxidative stress, and cellular antioxidant status also affects normal tissue injury and tumor sensitivity to radiation treatment [reviewed in (71)]. Inhibiting prooxidant enzymes, such as cyclooxygenase-2 (75), and overexpression or upregulation of antioxidant enzymes, such as extracellular superoxide dismutase (ECSOD) (84) and heme oxygenase-1 (200), have been shown to protect against radiation-induced thoracic, lung, and skin injuries (141). Radiation-resistant mice have been shown to have higher levels of superoxide dismutase (SOD) and catalase activities compared with radiation-sensitive mice (69). Manganese superoxide dismutase (MnSOD) upregulation has been implicated in adaptive response induced by low or fractionated doses of IR, leading to radioresistance (71, 73). MnSOD is one of the most important antioxidant enzymes located exclusively in mitochondria, the main source of ROS (115). The levels and activities of MnSOD modulate cellular redox status and influence the effects of chemotherapy and radiotherapy; therefore, MnSOD may confer radioresistance through its antioxidant enzyme activity. Our previous studies have demonstrated that selective inhibition of RelB-induced MnSOD after irradiation can sensitize prostate cancer cells to radiation treatment (71, 79), confirming the importance of MnSOD in radioresistance.

Depending on the level of tolerance to oxidative stress, different cancer cell types may exhibit an opposite response to ROS elevation. For example, the anticancer drug 2-methoxyestradiol (2-ME) is associated with upregulation of MnSOD as an adaptive response that protects pancreatic cancer cells from increased ROS (202). In contrast, 2-ME can sensitize radioresistant MCF-7/FIR breast cancer cells by activating apoptosis, arresting the cell cycle, and further enhancing radiation-induced ROS (152). Therefore, applying redox-modulating reagents, such as ascorbate (49), arsenic trioxide (34), selenite (76), or a metalloporphyrin antioxidant mimetic (MnTE-2-PyP) (112), in combination with IR can either increase the cell-killing effect of IR or protect against radiation-induced oxidative stress to surrounding normal tissues.

The neuroendocrine differentiation (NED) of prostate cancer cells is closely correlated with radioresistance (43, 185). In the prostate gland, neuroendocrine (NE) cells are <1% of total epithelial cells; however, the number of NE-like cells increases in advanced prostate cancer (129). Fractionated IR can induce NED in the LNCaP prostate cancer cell line by activation of the cAMP response element binding protein (CREB) and cytoplasmic sequestration of activating transcription factor 2 (ATF2) (43). NE-like cells can dedifferentiate to a proliferating state, which may contribute to tumor recurrence (43, 44). Interestingly, by utilizing LNCaP cell clones with stably overexpressed MnSOD with lower superoxide levels and higher H₂O₂ levels, Quiros-Gonzalez et al. showed that MnSOD upregulation was sufficient to drive NE differentiation, resulting in androgen independence and cell survival in prostate cancer cells (145). It has been proposed that the balance between O₂^•− and H₂O₂ can determine pathways that drive the NED process (145). Thus, it is conceivable that MnSOD might affect NED by modulating the rate of H₂O₂ production and the balance between O₂^•− and H₂O₂. Further investigation of the roles of MnSOD in regulating prostate cancer cell NED and the significance of NED in prostate cancer radioresistance and recurrence may lead to discoveries that can be explored to overcome radioresistance.

ROS and Prostate Cancer

Reactive species, which include ROS and reactive nitrogen species (RNS), are categorized into two groups: free radicals that contain one or more unpaired electrons, such as superoxide (O₂^•−), ^•OH, and nitric oxide (NO^•), and nonradicals, such as hydrogen peroxide (H₂O₂). Biological organisms are able to maintain a delicate redox homeostasis because they contain a complex intracellular “redox buffer” network, including both enzymatic and nonenzymatic antioxidants. The major enzyme defense system against ROS includes SOD, catalase, glutathione peroxidase, peroxiredoxin, and glutathione S-transferase (GST) (74). In addition to these antioxidant enzymes, small thiol-containing peptides, such as glutathione (GSH), glutaredoxin, and thioredoxin (Trx) systems, also help to scavenge ROS and maintain appropriate redox homeostasis (157).

The redox status (oxidizing/reducing conditions) of cells can regulate various transcription factors/activators, such as AP-1, NF-κB, and p53, thereby influencing target gene expression and modulating cellular signaling pathways. Requisite levels of ROS and RNS must be present for normal physiological function of living organisms (177). An increase in production of reactive species and/or a decrease in antioxidants can lead to oxidative stress, which can damage DNA, inhibit cellular enzyme activities, and induce cell death through activation of kinases and caspase cascades (154). Oxidative stress resulting from an imbalance between prooxidants and antioxidants that favors the former is believed to play a critical role in prostate carcinogenesis and prostate cancer progression [reviewed in (64)].

Sources of ROS

ROS derived from incomplete reduction of oxygen can be produced either endogenously (e.g., mitochondria respiration) or exogenously (e.g., IR) (80, 125) (Fig. 2). The most important endogenous source of ROS is the mitochondrial electron transport chain (ETC). The electrons that leak from some components of mitochondrial ETC lead to a one-electron reduction of O₂ and generation of superoxide radicals (O₂^•−). Superoxide can be dismutated by SOD to yield hydrogen peroxide and O₂. In the presence of transition metal ions, especially iron ions, hydrogen peroxide is subsequently converted through Fenton and Haber-Weiss reactions to a hydroxyl radical, which is the most toxic form of ROS, leading to various types of lipid peroxidation, protein modification, and particularly oxidative DNA damages, such as 8-hydroxy-deoxyguanosine (148). Somatic mutations in the mitochondrial genome are relatively frequent events in prostate cancer. Compared with nuclear DNA, mitochondrial DNA (mtDNA) is more susceptible to radiation-induced loss of integrity due, in part, to the lack of protective histones, an inefficient DNA repair system, and continuous exposure to the mutagenic effect of ROS (187), which is exacerbated by GSH depletion in mitochondria (120). ROS-induced mtDNA damage can alter polypeptides encoded by mtDNA for respiratory complexes, resulting in additional decreased electron transfer activity and increased ROS generation, thereby establishing a vicious cycle of oxidative stress (102) and decline in mitochondria energy production after initial oxidative damage of mtDNA (101). Mitochondrial dysfunction that causes persistent oxidative stress may contribute to radiation-induced genomic instability (198).

FIG. 2.

Scheme of cellular reactive oxygen species (ROS) generation and the antioxidant system. ROS generated from extracellular or intracellular sources can damage nuclear and mitochondrial DNA. Various transcription factors, such as nuclear factor kappa B (NF-κB), p53, activator protein-1 (AP-1), signal transducers and activators of transcription (STAT)3, Nrf2, HIF-1α, Sp1, PPARγ, and Ref1 are modulated by ROS. Examples of extracellular sources of ROS: radiation, carcinogens, inflammation, and hypoxia. Intracellular sources of ROS: ETC, electron transport chain; XO, xanthine oxidase; LPX, lipoxygenase; P450, cytochrome P450; COX, cyclooxygenase. Antioxidant system: MnSOD; Cu/ZnSOD; ECSOD, extracellular superoxide dismutase; GSH, glutathione; GPx, glutathione peroxidase; Prx, peroxiredoxin; GR, glutathione reductase; TrxR, thioredoxin reductase; TRXox, oxidized thioredoxin; TRXre, reduced thioredoxin; HIF-1α, hypoxia-inducible factor 1-alpha; PPARγ, peroxisome proliferator-activated receptor gamma.

ROS can also be generated by other enzymes, such as xanthine oxidase, membrane-associated NOX, and cytochrome P450 in endoplasmic reticulum, and oxidases in peroxisomes (97). The association of NOX enzymes with prostate cancer growth and malignant phenotypes has been extensively reviewed (86, 98).

ROS and prostate cancer progression

Increased oxidative stress plays a significant role in several physiological/pathological situations, such as aging and aging-associated diseases. Prostate cancer is closely associated with aging as it occurs frequently in older men. Prostate cancer cells generally have a higher level of oxidative stress compared with normal prostate cells, and the level of oxidative stress is associated with prostate cancer occurrence, recurrence, and progression (10, 21, 98). It has been demonstrated that, at an early stage of cancer development, tumor cells are exposed to high oxidative stress, in part due to inhibition of various antioxidant enzyme activities (Fig. 3). Lower levels of antioxidant enzymes, such as MnSOD, copper/zinc SOD, and catalase (10, 21) and defects in several classes of GSTs (22) have been observed repeatedly in prostate adenocarcinoma compared with benign prostate cells and tissues. However, after cancer has progressed, ROS partially render cancer cells more dependent on the function of antioxidant enzymes, such as SODs, to protect against damages caused by increased levels of superoxide radicals (115). Our laboratory has provided in vivo evidence and has proposed a model for the underlying molecular mechanism by which p53 differentially regulates MnSOD expression between early and advanced stages of cancer (46).

FIG. 3.

Role of oxidative stress in cancer development and radioresistance. IR-induced genomic instability and oxidative stress are closely related to each other. Many human cancer cells harbor low levels of antioxidants at early stages of a tumor, whereas cancer cells may eventually become resistant to radiation treatment and/or chemotherapy and possess high levels of antioxidants at advanced stages of a tumor. With cancer development, tumor cells are continuously facing increased oxidative stress and risk of genomic instability.

The protein levels of some antioxidant enzymes and signaling molecules are associated with cellular redox status. The proteins play a more dominant role when activation/inhibition of enzymatic activity and redox modification during redox signaling, or in response to cellular redox change in specific cellular compartments, occurs (Table 1). For example, it has been suggested that redox-sensitive molecule Trx1 functions as a protective cellular antioxidant and its upregulation protects cancer cells from oxidative stress (122). However, despite a significant increase in its protein level, oxidation of nuclear Trx1 resulted in a loss of antioxidant activity, which clearly demonstrates that when redox imbalance occurs, prostate cancer cells adapt to oxidative stress (161). Therefore, characterization of redox-sensitive protein structure and cellular localization, identification of potential redox modifications based on structure information and modeling strategies, and investigation of different functions before and after modification will provide insightful knowledge of cellular redox status at each stage of cancer development.

Table 1.

Roles of MnSOD Expression or Activity and MnSOD-Regulated Cellular Redox Status in Different Stages of Tumor Development

Tumor development	Normal cells	Early stage of cancer	Advanced stage of cancer
MnSOD level or activity	High	Low	Very high
	↓prooxidant	↑prooxidant	↑prooxidant
Cellular redox status	↑antioxidant	↓antioxidant	↑antioxidant
	Low oxidative stress	Modest oxidative stress	High oxidative stress
Sensitivity to ROS	++	+++	+
Implications in radiotherapy	Protect normal cells from oxidative insults such as radiation by inducing adaptive responses	Promote radiation-mediated cell killing due to low level of antioxidant capacity	Protect tumor cells from radiation and modulate neuroendocrine-like differentiation leading to radioresistance

Based on the biomedical property of increased ROS and altered redox status in cancer cells, many avenues of research have been proposed to modulate the unique redox regulatory mechanisms of cancer cells for therapeutic benefits (183). Mitochondrial ROS have been shown to promote proinflammatory cytokine expression (27) and NLRP3 inflammasome activity (174). Blocking androgen-induced ROS production by inhibiting polyamine oxidase could delay prostate cancer progression and prolong survival of animals when spontaneous prostate cancer develops (14). A highly oxidizing condition is strongly cytotoxic and is the primary mechanism for tumor cell killing by radiation therapy and some chemotherapeutics, such as Taxol and Adriamycin. Since tumor cells are under more oxidative stress and normal cells usually carry higher redox-buffering capacity, specific mild prooxidants, such as parthenolide, have been shown to be redox-modulating reagents capable of selectively pushing tumor cells beyond their tolerance to oxidative stress and sensitizing cancer cells to radiation-induced cell killing (178).

ROS are not only involved in radioresistance but also are implicated in prostate cancer progression and castration resistance. Growth and proliferation of castration-resistant prostate cancer are mediated by gain-of-function changes in the androgen receptor (AR) and AR reactivation. MnSOD downregulation is directly responsible for AR reactivation in prostate cancer and occurs through an ROS-mediated mechanism (163). Shiota et al. have extensively reviewed both the effects of AR signaling on oxidative stress and the effects of oxidative stress on AR signaling in the context of prostate cancer, especially castration-resistant prostate cancer (165). Castration-induced oxidative stress may promote AR overexpression through transcription factor Twist1 overexpression, which may result in a gain of castration resistance (166). Thus, modulating redox status to sensitize cells and overcome radioresistance may result in castration resistance, which diminishes the therapeutic benefits of redox modulation. Thus, the stage of prostate cancer development and AR signaling must be carefully determined before introducing redox intervention strategies.

Role of ROS in the interaction between stroma and prostate cancer cells

It is becoming increasingly clear that the microenvironment is crucial to prostate cancer cell survival, progression, metastasis [reviewed in (20)], and resistance to chemotherapy and/or radiotherapy. Redox status within such a microenvironment is complicated at all stages of prostate cancer development, due to the considerable heterogeneity of the cellular composition of stroma and tumors. IR generates highly reactive free radicals, and it has been well documented that stromal components, such as cancer-associated fibroblasts (CAFs), tumor-associated macrophages (TAMs), and endothelial cells, enhance oxidative stress, which promotes tumor progression (2, 50).

As the most abundant cell type in the microenvironment of solid tumors, fibroblasts are particularly prominent in prostatic carcinoma (Fig. 4). The origin of CAFs and their significance in determining cancer aggressiveness have been elegantly demonstrated previously (109, 134). Cunha and colleagues have shown that CAFs contribute to prostate tumor growth and metastatic potential. Human prostatic CAFs grown using initiated human prostatic epithelial cells dramatically stimulated the growth and altered histological characteristics of the epithelial cell population. However, this effect was not observed when normal prostatic fibroblasts were grown using initiated epithelial cells under the same experimental conditions (134).

FIG. 4.

Radiotherapy-induced cell killing and unintended effects on tumor stromal components leading to inflammatory mediator secretion. Inflammatory mediators induced by IR include epidermal growth factor (EGF), fibroblast growth factor (FGF), interferon-γ (IFN-γ), transforming growth factor-beta (TGF-β), interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-α), and IL-8. Both radiation treatment and IR-induced inflammatory mediators increase ROS levels within a tumor microenvironment, which contributes to radioresistance and recurrence of cancer.

It has been observed that ROS formation increased immediately after irradiation and continued for several hours, resulting in the production of 8-oxoguanine, which is a product of oxidative DNA damage (132). Oxidative damage in DNA is repaired mainly via the base excision repair (BER) pathway (42). The BER pathway is initiated by removal of the base by DNA glycosylases, leaving an intact abasic (AP) site. Subsequently, AP endonuclease 1 (APE1/Ref1) nicks the damaged DNA strand upstream of the AP site, creating a 3′-hydroxyl terminus and a 5′-deoxyribose phosphate group flanking the gap (172). APE1/Ref-1 (APE1) possesses not only DNA repair functions but also transcriptional regulatory activities, controlling cellular response to oxidative stress (180). APE1 has been identified as a protein with nuclear redox activity, inducing the DNA binding activity of several transcription factors, such as AP-1, NF-κB, hypoxia-inducible factor-1α, p53, Myb, and the ATF/CREB family [reviewed in (180)]. Thus, while IR-induced ROS lead to oxidative DNA damage, their repair can also contribute to cellular redox status, at least in part, through APE1/Ref-1 functions.

Due to diffusibility and abundance, multiple ROS and inflammatory mediators associated with aging, infection, or IR exposure may provide a permissive environment for cancer development. Compelling experimental and clinical evidence indicates that ROS-mediated stromal–epithelial interactions in both normal and malignant prostatic environments involve a number of soluble factors and their corresponding receptors (37). ECSOD plays predominant roles in scavenging superoxide in the extracellular space where redox state regulates intracellular signaling or tumor growth. ECSOD-derived H₂O₂ can promote vascular endothelial growth factor (VEGF) signaling in caveolin-enriched lipid rafts and stimulate endothelial cell migration and proliferation through oxidative inactivation of protein tyrosine phosphatases (PTPs), such as density-enhanced protein tyrosine phosphatase-1 (DEP-1) and PTP1B (135). VEGF is critical for not only angiogenesis but also prostate-cancer-mediated osteoblastic activity (93). IR modulates VEGF expression through multiple mitogen activated protein kinases (MAPK)–dependent pathways (137) and enhances glioma cell motility through vascular endothelial growth factor receptor 2 (VEGFR2) signaling pathways (87). Since prostate cancer cells lack expression of specific VEGF receptors, especially VEGFR2, IR-induced VEGFs are more likely to promote prostate cancer progression indirectly through their functions in stromal cells, in particular, endothelial cell survival, and as a chemotactic agent within the tumor microenvironment (93). In addition to endothelial cells, CAFs can also exert their cancer-promoting roles through release of growth factors, such as TGF-β and epidermal growth factor (EGF), as well as chemokines (2). Oxidative-stress-dependent monocarboxylate transporter 4 expression in CAFs is closely involved in stromal–epithelial lactate shuttling (190). According to a recently proposed model, an increase of ROS in CAFs drives tumor-stroma coevolution, DNA damage, and aneuploidy in cancer cells (109). Pavlides et al. have provided detailed information on how CAFs accelerate tumor growth and metastasis via oxidative stress, mitophagy, and aerobic glycolysis (138). The multiple roles of ROS in these new metabolic coupling interactions and models suggest that certain redox-modulation-based therapeutic methods can be helpful when used in combination with traditional radiotherapy in prostate cancer treatment.

The radiation-induced bystander effect, mediated through gap junctions and inflammatory responses, is defined as the response of cells to their irradiated neighbors (142). Many types of cancer-infiltrating immune cells, such as macrophages, dendritic cells, and T cells, are important stromal components of a prostate tumor as well as being prominent bystander targets of radiotherapy. More information on the mechanisms by which IR influences tumor-associated immune responses and various immune cells to secrete different inflammatory mediators appear in recent reviews (38, 123). Thus, activated immune cells are not limited to induction of antitumor immunity; they are also involved in creating an immunosuppressive and prooxidant network that promotes tumor progression and facilitates immune evasion. Since tumor cells are often under higher oxidative stress with disregulated and/or less adaptive redox buffering capacity than their normal counterparts, tumor cells are probably less able to cope with additional incremental increases in oxidative stress than normal cells are, which can be explored to enhance antitumor immunity while minimizing the possibility of unintended tumor progression and evasion.

Radiotherapy-Induced Inflammatory Mediator Secretion

When cells are exposed to IR, DNA damage generated from either direct or indirect effects of IR induces a multicellular program through a variety of signaling pathways to start DNA repair and prevent the proliferation of damaged cells. Such programs are usually mediated by soluble factors composed of cytokines, growth factors, and chemokines, which perform functions in both tumors and stroma to determine the fate of affected cells (13). IR exposure commonly induces stromal cells, especially CAFs, into a senescence-like phenotype in an altered tumor microenvironment. The so-called senescence-activated secretory pathways in senescent stromal fibroblasts generate an inflammatory environment through the secretion of proinflammatory cytokines and proteases (50). These soluble factors can exert paracrine or autocrine functions mediated by their respective receptors or interactive partners to promote prostate cancer progression and to create a continuous loop that pushes prostate cancer to a more aggressive state.

Chronic inflammatory mediator secretion associated with aging has been involved in the etiology and progression of prostate cancer. A chronic inflammatory microenvironment leads to an increased fraction of epithelial cells that proliferate in local atrophy lesions, an event called proliferative inflammatory atrophy (PIA) (149). PIA may progress to high-grade intraepithelial neoplasia and prostate cancer (96). That a relationship exists between a chronic inflammatory microenvironment and prostate cancer is gaining wide acceptance (39, 67). The evidence that the microenvironment is altered as a result of radiotherapy, especially that various types of cytokines are generated, has been elegantly reviewed (13, 142). There are many types of mediators induced by IR (13), including EGF (45), fibroblast growth factor (13), interferon-γ (54), TGF-β (78), proinflammatory cytokines IL-6 (35), TNF-α (201), the chemokine IL-8 (128), and others (111). A range of studies has shown that a clear difference exists in the level of circulating cytokines in prostate cancer patients compared with normal or benign controls and changes in levels of circulating cytokines after radiation exposure and/or androgen deprivation therapy (26). For the purpose of brevity, we will only highlight the signaling pathways mediated by IL-6, IL-8, TNF-α, and TGF-β induced by IR, as well as their implications in prostate cancer malignancy and their potential significance in radiotherapy of prostate cancer.

Interleukin-6

IL-6 is a multifunctional cytokine that signals through a cell-surface type 1 cytokine receptor complex composed of the ligand-binding protein of IL-6Rα (also called CD126) and the signal-transducing component gp130 (CD130) (62). Another type of receptor for IL-6 is a soluble IL-6 receptor (sIL-6R) that lacks a membrane-signaling domain but can bind with IL-6 and then with the membrane receptor β chain (gp130) to mediate the intracellular signaling pathways (153). IL-6 mainly activates the Janus kinase (JAK)/STAT3 signaling pathway (110), but it also participates in the MAPK and phosphatidyl inositol 3-kinase (PI3K)/Akt pathways to influence a wide range of biological activities in tumor cells (150). IL-6 also acts as an autocrine and/or paracrine proliferative factor in prostate cancer cell lines (133). IL-6 treatment not only stimulates the IL-6 autocrine loop but also activates insulin-like type I growth factor receptor (IGF-1R) signaling. This STAT3-mediated cooperation between IL-6 signaling and IGF-1R signaling in the prostate plays a critical role in facilitating prostate malignancy and epithelial–mesenchymal transition (EMT). STAT3 has been shown to promote oncogenesis in human cancer through Src oncogene transformation of STAT3 cells (199). In addition to its classical role in the nucleus, STAT3 modified by serine phosphorylation augments oxidative phosphorylation in mitochondria and supports cellular transformation by the oncogene Ras (58). Considering the different cellular localization of STAT3 implicated in intracellular energy metabolism and a variety of redox-sensitive genes, it will be interesting to investigate mitochondrial functions and cellular transformation under IR-induced IL-6 activation (Fig. 5).

FIG. 5.

IL-6-mediated Jak-STAT3 signaling pathway. IL-6 signals through a cell-surface type 1 cytokine receptor complex composed of the ligand-binding protein of IL-6Rα and the signal-transducing component gp130 (glycoprotein of 130 kDa). IL-6 can also bind to soluble IL-6 receptor (sIL-6R), which lacks a membrane-signaling domain, and then with gp130 to mediate Jak phosphorylation and activation. Activated Jak family tyrosine kinases further phosphorylate STAT3, which in turn translocate to the nucleus and regulate target gene transcription. Many types of growth factors, such as EGF, vascular endothelial growth factor (VEGF), FGF, and insulin-like type I growth factor (IGF), can aggregate with respective receptors and activate STAT3 signaling pathways.

Different cell types, such as B and T cells, macrophages, monocytes, fibroblasts, and certain tumor cells, can synthesize IL-6 (92), which regulates various cellular functions, including immune response, proliferation, apoptosis, angiogenesis, and differentiation (41). Several clinical studies have reported that elevated serum levels of IL-6 and sIL-6R are associated with metastasis and castration resistance, suggesting that IL-6 correlates with prostate cancer progression and patient morbidity (1, 25, 126). Most clinical data support the biological role of the IL-6 pathway in prostate cancer, especially in an advanced castration-resistant prostate cancer patient where the significance of the IL-6 pathway is mediated by crosstalk between IL-6 and the AR pathway (9). Under androgen deprivation conditions, IL-6 is able to promote intracellular synthesis of androgens in the prostate (36), resulting in AR activation and upregulation of AR-targeted prostate specific antigen (PSA) expression, via STAT3 and MAPK signaling pathways (9), as well as an androgen enhancer region within the human PSA promoter (184).

Even though increased IL-6 may indicate the presence of an advanced prostate cancer tumor in a patient or in in vivo experiments, some in vitro results that support the significance of the IL-6 pathway in the growth of prostate cancer cells are still controversial. IL-6 can act as either a growth inducer (144) or inhibitor (70) in androgen-dependent LNCaP cells (9). It is possible that IL-6-induced growth arrest may be associated with NED (175). The presence of NE-like cells has been correlated with a radioresistant phenotype and an unfavorable prognosis (43, 185).

IL-6 signaling is tightly regulated by several negative feedback inhibitors, including suppressors of cytokine signaling, Src-homology 2 containing protein tyrosine phosphatases, and protein inhibitors of activated STATs (40, 99). The mechanisms by which these inhibitors regulate IL-6 intracellular signaling pathways have been reviewed previously in detail (40). Since IL-6 also plays an inhibitory role in prostate cancer cells depending on signaling crosstalk and the difference between the cancer and normal cells in redox status and adaptive response to oxidative stress may influence the signaling crosstalk, blocking IL-6 with antibody or signaling inhibitors may instead promote prostate cancer progression. Thus, it is necessary to identify the role of IL-6 signaling in specific situations before applying anti-IL-6-related therapy.

Interleukin-8

IL-8, also known as CXCL8, is a chemoattractant chemokine. IL-8 is usually associated with inflammation that predisposes cells to produce different chemokines for malignant transformation or progression (170). IL-8 secretion is increased by oxidative stress from either intracellular or extracellular sources. IL-8 can stimulate the recruitment of inflammatory cells, which further elevates oxidant stress mediators, thereby making IL-8 a key parameter in localized inflammation (186). Two cell-surface G protein-coupled receptors, CXCR1 and CXCR2 (72), are responsible for the binding of IL-8 and regulating target gene expression through downstream signaling pathways, such as activation of serine/threonine kinases, protein tyrosine kinases, and Rho-GTPases (189). Depletion of CXCR1 leads to inhibition of IL-8-mediated androgen-independent tumor growth by increasing proapoptotic proteins and decreasing antiapoptotic proteins (160).

Increased IL-8 expression is associated with both a high Gleason score and a tumor pathologic stage (56). Elevation of IL-8 expression has been linked to various markers of the progression of prostate cancer up to an advanced stage, such as castration resistance, metastasis, and enhanced angiogenesis in vitro (7, 159), in vivo (7, 197), and in human patients (30). It has been shown that IL-8 signaling, which is endogenous and induced by a TNF-related apoptosis-inducing ligand, can modulate the extrinsic apoptosis pathway in prostate cancer cells through direct transcriptional regulation of c-FLIP, an endogenous caspase-8 inhibitor, and reduce the propensity of prostate cancer cells to undergo apoptosis (194). Therefore, inhibiting IL-8 signaling may be a promising strategy to sensitize advanced prostate cancer to chemotherapy. The reduction of intrinsic IL-8 potentiates ansamycin-based heat shock protein 90 cytotoxicity by several mechanisms, including inhibition of IL-8-induced NF-κB activity (158), cell cycle arrest at the G1/S boundary, and increased spontaneous apoptosis as well as enhancement of the efficacy of multiple chemotherapeutic drugs, such as Docetaxel, Staurosporine, and Rapamycin (168).

There are currently seven known CXC chemokine receptors in mammals, named CXCR1 through CXCR7. Various types of crosstalk exist between different chemokine-mediated signaling pathways due to remarkable redundancy within chemokines with multiple chemokine bindings to similar or the same receptor(s) and multiple receptors binding with similar or the same chemokine(s) (53). It has been shown that IL-8 can upregulate CXCR7 expression and the ligand-independent functions of CXCR7, which usually binds to CXCL11 and CXCL12 ligands to promote the growth, proliferation, and angiogenesis of prostate cancer cells by increasing epidermal growth factor receptor (EGFR) and ERK1/2 phosphorylation (169). Therefore, the effects of IL-8/IL-8 receptor signaling pathways in prostate cancer progression and radiation sensitivity may be orchestrated by communication and/or interaction with many chemokines and their receptors, such as CXCR1–7.

The correlation between IL-8 signaling and AR signaling pathways has been reviewed previously (170, 189). Proteomic data illustrate that the androgen-stimulated LNCaP cells have increased expression of IL-8 (55), which is dependent on AR. Additionally, IL-8 signaling also increases AR expression and alters the distribution and transcriptional activation of AR, leading to increased expression of AR-targeted genes (159). Since the transition of prostate cancer to an androgen-independent state is partially due to IL-8-signaling-induced AR activation, targeting IL-8 expression and signaling pathways may significantly enhance the efficacy of androgen ablation therapy.

In addition to establishing the importance of IL-8 in developing chemoresistance (193), our laboratory found that the RelB-mediated NF-κB alternative pathway plays a crucial role in IL-8 upregulation, which enhances the radioresistance of prostate cancer cells (196). This result is consistent with the observation that RelB promotes prostate cancer progression and radioresistance (197). The relationship of the NF-κB pathway, increased antioxidant capacity, and resistance to radiation treatment in many tumor cell types has been well documented (61, 147). It has been demonstrated, and reviewed, that RelB regulates MnSOD gene expression and the radioresistance of prostate cancer cells (71, 79). Selective inhibition of the RelB-mediated NF-κB alternative pathway can, to a remarkable degree, sensitize prostate cancer cells to IR-induced killing (71). Thus, it will be interesting to investigate the relationship of radiosensitizing effects of IL-8 signaling blockage with either inhibitors of IL-8 receptors or monoclonal antibodies against IL-8. This strategy may synergistically facilitate the killing of castration-resistant and/or radiation-resistant prostate cancer cells.

Tumor necrosis factor-alpha

TNF-α is synthesized as a 26 kDa (233 amino acids) membrane-bound pro-peptide (pro-TNF) and is released as a 17 kDa soluble polypeptide (157 amino acids) after cleavage by the TNF-converting enzyme (119). The action of TNF-α is mediated by two distinct receptors, TNF-receptor I (55 kDa, TNFRI) (106), which mediates the majority of TNF-α biological activities, and receptor II (75 kDa, TNFRII) (173), with both having an affinity for TNF-α in human tissues. An imbalance between prosurvival and apoptosis signals by TNF-α-initiated signaling pathways (Fig. 6) has been implicated in malignancies of the colon (52), ovary (151), breast (149), and prostate (117).

FIG. 6.

TNFα-regulated major cellular signaling pathways. Binding of TNFα to TNFR1/2 leads to the rapid phosphorylation of the NF-κB, ERK, p38, and JNK pathways, and activates a group of transcription factors, such as NF-κB, Elk1, and AP-1, in the nucleus. In addition to these pro-survival pathways, TNF-α can induce apoptosis through receptor-mediated caspase activation, and caspase-dependent and -independent components of the mitochondrial cell death pathway. A balance between these intracellular signaling pathways determines whether cells will die or survive after exposure to TNF-α. TNFα-mediated ROS generation is mainly derived from mitochondria and membrane-associated NADPH oxidase, which contributes to signaling pathways. JNK, c-Jun N-terminal Kinase.

TNF-α is one of the central factors involved in stress responses, including response to radiation exposure, because of its ability to induce rapid hemorrhagic necrosis via selective destruction of tumor blood vessels and generate specific T-cell antitumor immunity (100). Antagonists of TNF-α action have been developed for the treatment of rheumatoid arthritis and other inflammatory diseases (11). When present chronically in the tumor microenvironment, TNF-α is a major mediator of cancer-related inflammation. In addition to maintaining homeostasis of the immune system, inflammation, and host defense, TNF-α also plays paradoxical roles in cancer promotion and progression pathways leading to activation of NF-κB and AP-1 transcription factor complexes [reviewed by Balkwill (11)]. Circulating TNF-α is normally not detectable in healthy individuals but can be detected in some cancer patients (25, 117). While it remains to be determined whether TNF-α elevation in prostate cancer patients is the cause or consequence of cancer development and progression, a relatively consistent association between increased TNF-α and cachexia in patients with prostate carcinoma (127, 140) has been determined, and it is one of the most devastating conditions of late stages of cancer. TNF-α plays multiple roles in changes related to cancer cachexia, such as altered nitrogen metabolism associated with cachexia (3), blockage of muscle differentiation associated with muscle tissue regeneration (29), and activation of transcription factors NF-κB and AP-1 to increase proteolysis (182) [reviewed in (8)].

TNF-α is often produced in response to oxidative stress and it acts, at least in part, by causing oxidative stress in its target cells. Mitochondria are the primary generators of ROS, which contribute to the TNF-α-initiated signaling pathway (81). TNF-α-induced ROS, which can be inhibited by mitochondrial-specific MnSOD overexpression, may oxidize and inhibit c-Jun N-terminal Kinase (JNK)-inactivating phosphatases, and sustained JNK activation is required for cytochrome c release and caspase 3 cleavage as well as necrotic cell death (81). NOX activation is involved in TNF-α-induced ROS production, depending on cell type and the extent of TNF-α exposure (90, 104). It has been shown that acute TNF-α exposure induces rapid (within 5 min) p47^phox phosphorylation and increases p47^phox-TNF-α receptor-associated factor 4 association and membrane translocation, which further mediates p47^phox-p22^phox complex formation, leading to NADPH-dependent O₂^•− production (104). The binding of TNF-α to TNFR1 can activate NOX1 or NOX2 to generate ROS in early endosome (131). During TNF-α-induced necrotic cell death, Nox1 is activated by forming a complex with TRADD (TNF-receptor-associated protein with death domain), RIP1 (Receptor Interacting Protein 1), and Rac1 (90). This NOX-dependent redox-regulated mechanism plays a key role in TNF-α-induced necrotic cell death.

Most studies referenced before used a relatively high dose of TNF-α to induce an acute and pro-cell death response. However, chronic elevation of TNF-α at a relatively low level can result in cytoprotection, which is related to increased levels of antioxidant, antiapoptotic, and other defense proteins, such as thioredoxins and MnSOD. Increased mitochondrial ROS production induced by TNF-α leads to activation of nuclear genes, especially NF-κB. In human and mouse ovarian cancer, TNF-α maintains TNFR1-dependent IL-17 production by CD4+ cells, which leads to myeloid cell recruitment into the tumor microenvironment and enhances tumor growth (31). TNF-α can be produced when NF-κB is activated and TNF-α is also an important stimulus of NF-κB signaling and additional cytokine production. The NF-κB signaling pathway is critical in supporting cancer-related inflammation and malignant progression as well as maintaining the immunosuppressive phenotype of TAMs (65). TNF-α may play a role in the initiation of an androgen-independent state in prostate cancer through its ability to inhibit AR sensitivity (118). The interplay of NF-κB and B-myb contributes to negative regulation of AR expression by TNF-α (94). Immunohistochemistry results show that nuclear localization of NF-κB family member p65 is associated with PSA relapse, the first sign of prostate cancer recurrence, while cytoplasmic expression does not (47). Our laboratory demonstrated that the RelB-mediated alternative NF-κB pathway is involved in prostate cancer aggressiveness and radiation resistance (71, 79, 197). TNF-α functions as a potent inducer of the NF-κB signaling pathway and mediates the crosstalk between the classical and alternative NF-κB signaling pathways, as well as interactions with AR (according to our unpublished data). Thus, it is important to investigate in prostate cancer the effects of TNF-α production after chemo/radiotherapy and the potential influences of TNF-α on the activation of the RelB-mediated alternative NF-κB pathway.

The expression and activation of several genes and kinases, such as cyclooxygenase-2, Cyclin D1, the Bcl-2 family, survivin, Akt, and EGFR, are regulated by NF-κB in various tumor cells (103). The therapeutic potential and benefit of targeting NF-κB in cancer and the possible complications and pitfalls associated with NF-κB modulation have been reviewed and explored (15). Inhibition of NF-κB has been proposed as a means to treat cancer or to overcome chemoresistance and radioresistance in cancer therapy (103). Inhibition of IR-induced NF-κB activation sensitizes Ki-Ras transformed prostate epithelial cells (267b1/K-Ras) to IR (88). Selective inhibition of RelB nuclear activation and downregulation of RelB-targeted MnSOD gene expression improve IR-induced killing of PC3 cells (71).

Novel strategies have been proposed to target TNF-α-mediated signaling for treatment of human prostate cancer. For example, Gambogic acid can inhibit TNF-α-induced invasion of human prostate cancer PC3 cells in vitro by inhibiting the PI3K/Akt and NF-κB pathways (107). TNF-α induces MnSOD expression, which mediates delayed radioprotection (124) through an NF-κB binding site located within the second intron of the sod2 gene (115). The natural compound curcumin acts as a potent radiosensitizer in PC3 cells by inhibiting TNF-α-mediated NF-κB activity, resulting in bcl-2 protein downregulation (33). There is a caveat to targeting TNF-α in prostate cancer therapy. TNF-α synergizes with γ-irradiation to induce apoptosis in LNCaP cells through a mechanism that may involve increased production of ceramide at 48–72 h after exposure (91). Anti-TNF-α treatment may mitigate the effect of γ-irradiation. Depending on TNF-α dose and prostate cancer cell type, different isoforms of C/EBPβ may regulate cell growth and confer TNF-α resistance to prostate cancer cells (89). Although TNF-α is clearly linked with prostate cancer progression and radioresistance, it may also contribute to tumor immune surveillance and apoptosis-mediated antitumor pathways. Since TNF-α expression is subjected to redox regulation, the difference between tumor and normal cells in their redox regulation and adaptive redox buffering capacity can be exploited to shift the paradoxical effects TNF-α toward increased immune surveillance and tumor cell apoptosis.

Transforming growth factor-beta

TGF-β is a ubiquitous cytokine that plays a critical role in numerous pathways regulating homeostasis and injury response as well as in the progression of human cancer. Prior to tumor initiation and during early phases of tumor progression, TGF-β acts as a tumor suppressor. At later stages of cancer development, TGF-β promotes processes associated with tumor aggressiveness, such as cell invasion, dissemination, and immune invasion (48, 114). In mammals, there are three TGF-β isoforms: TGF-β1, TGF-β2, and TGF-β3. With the assistance of the coreceptors endoglin and betaglycan (known as type III receptors or TGFβRIII), active TGF-β binds to cell surface type I (TGFβRI) and type II (TGFβRII) serine/threonine kinase receptors, which phosphorylate and activate the Smad family of signal transducers (48) (Fig. 7).

FIG. 7.

TGF-β-mediated classical Smads signaling pathway. With the assistance of TGFβRIII, active TGF-β (three isoforms, i.e., TGF-β1, TGF-β2, and TGF-β3) binds to cell surface TGFβRI and TGFβRII, which phosphorylate and activate the Smad family of signal transducers.

Once activated by TGF-β binding to the receptors, Smad2 and Smad3 associate with Smad4 and translocate to the nucleus where they regulate the transcription of genes involved in cell cycle arrest and apoptosis, which are essential to the tumor suppressor role of the TGF-βs in normal epithelial cells and at early stages of carcinogenesis (18, 19). TGF-β-induced growth arrest is mediated by the inhibition of cyclin-dependent kinases and the downregulation of myelocytomatosis oncogene cellular homolog [reviewed in (48)]. Mutational inactivation of TGF-β signal-transduction components, such as the TGF-β type II receptor (TGFβRII) (60) or its mediators, Smad2 and Smad4, leads to defective TGF-β signaling in some cancers (17, 59). Pu et al. developed a transgenic adenocarcinoma of mouse prostate-based prostate cancer transgenic mouse model that harbors the dominant negative mutant TGF-β type II receptor in epithelial cells to characterize the in vivo consequences of inactivated TGF-β signaling on prostate tumor initiation and progression, and found that disruption of TGF-β signaling in vivo accelerated pathologic malignant changes in the prostate by altering the kinetics of prostate growth and inducing EMT (143). These findings indicate that TGF-β exerts its tumor suppressor functions through inhibition of cell proliferation, induction of apoptosis, and regulation of autophagy.

TGF-β is expressed at high levels in the later stages of tumor development (192), during which it is utilized as a potent promoter of cell motility, invasion, metastasis, and tumor stem cell maintenance, as demonstrated in experimental prostate cancer models (121). Local TGF-β1 elevation has been associated with tumor grade, pathologic stage, and lymph node metastasis in prostate cancer patients (162). Although some investigators were not able to find a discriminative difference in the serum concentration of TGF-β1 in benign prostate hyperplasia and prostate cancer (195), elevated levels of plasma TGF-β1 (77), TGF-β2 (139), and urinary TGF-β1 (139) were found in patients with prostate cancer.

TGF-β1 plays a critical role in tumor–stromal cell interactions and modulates the growth of prostate cancer, either positively or negatively, through the balance between the amounts of IGF-1 and IGF binding protein-3 (85). Resistance to TGF-β-mediated growth arrest results in highly malignant phenotypes with increased EMT, tumor invasion, metastatic dissemination, and evasion of immune surveillance (114). Interestingly, TGF-β1 activates IL-6, which has been implicated in the malignant progression of prostate cancers, as described earlier, via multiple signaling pathways, including Smad2, NF-κB, JNK, and Ras (136). Zhu and Kyprianou have provided a detailed description of the crosstalk between AR and growth factors, including TGF-β-mediated signaling pathways, in prostate cancer cells (203). Smad3, a downstream mediator of the TGF-β signaling pathway, can function as a coregulator to enhance AR-mediated transactivation and increase AR-targeted PSA gene expression (82). Considering the correlation between increased circulating levels of TGF-β1 with invasion (77), metastasis (77), and poor prognosis in patients with prostate cancer (162), TGF-β1 could be an additional serum marker for prostate cancer (16, 176).

TGF-β acts as an important mediator for response to IR, and its signaling is tightly regulated by redox status within tumor cells and the tumor microenvironment. IR has been shown to induce the release and activation of TGF-β in cells and tissues (13). A mechanistic study in a cell-free system demonstrated that oxidation of the TGF-β latent complex acts as a sensor of oxidative stress to mediate the release and activation of TGF-β1 and orchestrate cellular responses to damage (12). More aspects of TGF-β biology, particularly its involvement in the microenvironmental response to IR, have been described elegantly (13). Intracellular redox equilibrium is essential for constitutive AP-1-dependent TGF-β1 expression (57). Nitric oxide downregulates TGF-β1 expression in prostate cancer cells at the transcriptional level by suppressing the de novo synthesis of TGF-β1 mRNA (188). TGF-β1 induces a stromal oxidant/antioxidant imbalance as a result of elevated NOX4-dependent ROS production and inhibits the expression of the MnSOD and catalase (116) that may be critical in the acquisition of epithelial migratory properties (23). In addition, TGF-β1 decreases ETC complex IV activity by decreasing phosphorylation of the subunit 6b of glycogen synthase kinase 3, which contributes to senescence-associated mitochondrial ROS generation (28). The significant roles of TGF-β in modulating tumor intracellular and extracellular redox statuses suggest that TGF-β signaling is involved in mediating cell autonomous, local, and systemic responses, which together regulate the initiation, promotion, progression, and prognosis of prostate cancer.

Radiotherapy-induced TGF-β activation may have undesirable side effects that are implicated in late tissue damage, such as fibrosis (6, 108). Several studies support the use of TGF-β inhibitors to ameliorate IR toxicity to normal tissues (51, 146). Anticancer therapies, such as IR or doxorubicin, may accelerate the steps of tumor progression, such as EMT and metastasis, due to the promoting effect of TGF-β within the tumor microenvironment (4, 130). This effect can be abrogated by administration of a pan-TGF-β neutralizing antibody (19). Current strategies to target TGF-β in radiotherapy mainly focus on general inhibition of TGF-β signaling. It has been shown that blockade of TGF-β signaling prior to irradiation attenuates DNA damage responses, increases clonogenic cell death, and promotes tumor growth delay and, thus, enhances radiation response and prolonged survival in patients with breast cancer (24) and glioblastoma (68), but renders a lung cancer cell line more radioresistant (191). Genetic differences and tumor specificity can be important factors in determining the radiosensitizing effect of TGF-β inhibition in radiotherapy. For an example, a hypofunctional genetic haplotype of the TGFB1 gene encoding TGF-β1 is associated with lower TGF-β1 plasma concentrations and increased sensitivity to radiation-induced chromosomal aberrations and apoptosis in lymphoid cells (155). There are three major approaches to inhibit TGF-β signaling: targeting TGF-β synthesis using antisense molecules and using ligand traps that sequester TGF-β and small-molecule inhibitors that hinder the kinase activity of TGF-β receptors [reviewed in (5, 114)]. Since IR-induced TGF-β may not only provide a survival benefit to cancer cells that are radioresistant but also accelerate tumor progression, targeted disruption of the TGF-β signaling pathway for therapeutic intervention may be an effective adjuvant in cancer radiotherapy.

Conclusion and Future Perspectives

Radiation therapy is generally used to treat early stage and inoperable locally advanced prostate cancer. Radiation kills prostate cancer cells and extends long-term patient survival by direct and indirect actions leading to macromolecule damages and altered redox signaling. However, IR is also responsible for the induction of neoplastic transformation and tumor progression as well as normal tissue injuries. The development of radioresistance is a significant impediment to prostate cancer treatment. The side effects and late complications that result from IR exposure limit the full potential of radiotherapy efficacy. Considering the heterogeneity of tumors, dynamic communications between stromal and prostate cancer cells as well as the complicated redox-regulated mechanisms within the tumor microenvironment—simply applying generalized anti-inflammatory strategies—might result in unintended adverse effects. Thus, it is important to develop individualized treatment regimes that will be most effective and will not disrupt antitumor immunity in individual patients. Additionally, redox-dependent proinflammatory mediator production from the directly exposed cells and their neighboring nonirradiated cells, as the bystander effect of radiotherapy, may play a critical role in the response of cells and tissues to IR. The key roles of IR-induced cytokines and growth factors and their interference with prostate cancer radiotherapy have been extensively discussed in this review with an emphasis on IL-6, IL-8, TNF-α, and TGF-β. These major cytokines, which are induced by IR in prostate cancer treatment, are not only involved in modulating redox balance but are also subjected to regulation by various oxidative stresses. Compared and contrasted to normal cells, tumor cells are usually under a higher oxidative stress and secrete more proinflammatory mediators. An incremental increase in oxidative stress to the extent that is still within the adaptive redox buffering capability of normal cells may overwhelm the less adaptive redox buffering capability of cancer cells, thereby selectively disrupting the redox state in tumor cells and activating the apoptotic or necrotic pathway, which leads to selective killing of tumor cells. Thus, modulation of IR-induced oxidative stress and inflammatory cytokine signaling may provide a better basis for enhancing radiation-mediated killing in prostate cancer treatment with minimal normal tissue damage.

Abbreviations Used

2-ME

2-methoxyestradiol

3DCRT

3-dimensional conformal radiation therapy

AP-1

activator protein-1

APE1

AP endonuclease 1

AP site

abasic site

AR

androgen receptor

ATF

activating transcription factor

BER

base excision repair

CAF

cancer associated fibroblast

COX-2

cyclooxygenase-2

CREB

cAMP response element binding protein

Cu/ZnSOD

copper/zinc superoxide dismutase

DEP-1

density-enhanced protein tyrosine phosphatase-1

ECSOD

extracellular superoxide dismutase

EGF

epidermal growth factor

EGFR

epidermal growth factor receptor

EMT

epithelial mesenchymal transition

ETC

mitochondrial electron transport chain

FGF

fibroblast growth factor

GPx

glutathione peroxidase

GR

glutathione reductase

Grx

glutaredoxin

GSH

glutathione

GST

glutathione S-transferase

HIF-1α

hypoxia-inducible factor-1alpha

HO-1

heme oxygenase-1

IGF

insulin-like type I growth factor

IGF-1R

insulin-like type I growth factor receptor

IL-1β

interleukin-1beta

IL-6

interleukin-6

IL-8

interleukin-8

IR

ionizing radiation

Jak

Janus kinase

JNK

c-Jun N-terminal kinase

LPX

lipoxygenase

MAPK

mitogen-activated protein kinase

MnSOD

manganese superoxide dismutase

mtDNA

mitochondrial DNA

MYC

myelocytomatosis oncogene cellular homolog

NE

neuroendocrine

NED

neuroendocrine differentiation

NF-κB

nuclear factor kappa B

NOX

NADPH oxidase

PPARγ

peroxisome proliferator-activated receptor gamma

PI3K

phosphatidyl inositol 3-kinase

PIA

proliferative inflammatory atrophy

PIAS

protein inhibitors of activated STATs

Prx

peroxiredoxin

PSA

prostate-specific antigen

PTP

protein tyrosine phosphatase

Rac1

ras-related C3 botulinum toxin substrate 1

RIP1

receptor Interacting Protein 1

RNS

reactive nitrogen species

ROS

reactive oxygen species

SH2

Src-homology 2

sIL-6R

soluble IL-6 receptor

SOD

superoxide dismutase

STAT

signal transducers and activators of transcription

TACE

TNF-converting enzyme

TAM

tumor-associated macrophage

TGF-β

transforming growth factor-beta

TNAF4

TNF-α receptor-associated factor 4

TNF-α

tumor necrosis factor-alpha

TNFRI

TNF-receptor I

TRAIL

TNF-related apoptosis-inducing ligand

TRAMP

transgenic adenocarcinoma of mouse prostate

Trx

thioredoxin

TRXox

oxidized thioredoxin

TrxR

thioredoxin reductase

TRXre

reduced thioredoxin

VEGF

vascular endothelial growth factor

VEGFR2

vascular endothelial growth factor receptor 2

XO

xanthine oxidase

Acknowledgments

This work was supported by NIH grants CA 115801 and CA 143428 and the Edward P. Evans Foundation.