Cancer prevention by traditional Chinese medicine and plant phytochemicals column
Prostate cancer and chemoprevention by natural dietary phytochemicals                          
Asia Abed Al-Mahmood1,2, Limin Shu3, Hyuck Kim3, Christina Ramirez2,4, Douglas Pung2, Yue Guo1,2,Wenji Li3, Ah-Ng Tony Kong2,3* 
1. Graduate Program in Pharmaceutical Sciences, Ernest Mario School of Pharmacy, Rutgers, The State University of New Jersey, Piscataway, NJ 08854, USA
2. Department of Pharmaceutics, Ernest Mario School of Pharmacy, Rutgers, The State University of New Jersey, Piscataway, NJ 08854, USA
3. Center for Cancer Prevention Research, Ernest Mario School of Pharmacy, Rutgers, The State University of New Jersey, Piscataway, NJ 08854, USA
4. Graduate Program in Cellular and Molecular Pharmacology, Rutgers, The State University of New Jersey, Piscataway, NJ 08854, USA    

Abstract: Prostate cancer is the second leading cancer among men in the United States. Several studies have correlated the developmentof prostate cancer with diet and life-style. Therefore, a balanced diet and improved life style might inhibit prostate cancer progression. Cancer chemoprevention has emerged as an important factor in controlling cancer development through natural or synthetic compounds. Oxidative stress is among the factors contributing to prostate cancer development. The transcription factor nuclear factor (erythroid-derived 2)-like 2 (Nrf2) controls detoxifying antioxidant enzymes expression by binding to the antioxidant response element (ARE) in the promoter of these genes to activate their expression. Many natural products can fight oxidative stress and protects cells from DNA damage by activating the Nrf2/ARE pathway. High consumption of fruits and vegetables can reduce disease incidence and invasive tumors. In this review, the roles of important fruit and vegetable phytochemicals in regulating prostate cancer progression and tumor growth are discussed.          
Keywords: Prostate cancer, Phytochemicals, Chemoprevention, Nuclear factor (erythroid-derived 2)-like 2, Oxidative stress, Antioxidant response element   
CLC number: R962                Document code: A                 Article ID: 10031057(2016)963318
1. Introduction
Prostate cancer is the most frequent type of cancer among males[1,2]. Approximately, one million cases are diagnosed each year across the world[1]. Several internaland external factors play a role in prostate cancer initiation. Prostate cancer can begin as a symptomatic or a latent period and progress to a more destructive stage. Externalfactors ranging from diet and exercise to lifestyle triggerthis transition. The most effective way to prevent prostate cancer is screening for early disease markers[2]. There is a direct link between the Western diet and obesity due to the high caloric and unsaturated fat content. Furthermore, the Western diet is poor in naturalcomponents, including fruits and vegetables. These factorscan support prostate cancer tumorigenesis[1]. It has been observed that Asians have a low incidence of prostate cancer. This has been attributed to the nature of their diet which is rich in natural plant products and low in fat and calories. Environment and life style thus play important roles in the initiation of aggressive tumors in the prostate[3].
Prostate cancer usually develops after the age of fifty years, with no clinical signs of disease. Because patients in the latent period of prostate cancer are not treated for the disease, it may progress to other diseases of the heart or lung or to other cancers[4] Several options are available to treat or manage prostate cancer, including surgery, radiation and chemotherapy[4,5]. Hormone therapy, proton beam therapy, cryosurgery and high intensity focused ultrasound (HIFU) are also used[4]. New products to manage and prevent prostate cancer progression have been developed after the existing methods were unsuccessful into treating progressive prostate cancer. Suppression of the precancerous stage and inhibition of cancer development could be achieved by using inexpensive natural components. As a result, a significant decline in prostate cancer-associated mortality has been observed[5].
In this review, we summarize the preventive and therapeutic role of some natural compounds in patients diagnosed with prostate cancer. These compounds include various types of natural plant products such as lycopene from tomato[6], curcumin from turmeric[7,8], sulforaphane, erucin[9], 3.3′- diindolymethane (DIM)[10]and phenethyl isothiocyanate (PEITC) from cruciferousvegetables[11], tocopherols from walnuts[12], anthocyaninsfrom purple corn[13], epigallocatechin-3-gallate (EGCG) from green tea[14], chrysin from honey[15], and tocopherols from honey[16] and vitamin E[17].
2. Risk factors and epidemiology of prostate cancer
Prostate cancer is considered the second-leading disease causing death among men in the US. The precisecause of prostate cancer has not been established, although many studies have aimed to determine its pathogenesis. Prostate cancer prevalence and fatality rates increase in men over fifty years old. With increasing age, the frequency of both clinical and histologic disease dramatically increases. With advanced age, the severity of prostate cancer also increases. It is more wide spread than other cancers[18]. The initiation of prostate cancer can be detected at an early stage. Studies have shown that presence of prostate cancer related foci in specimens from the prostate of men between twenty and forty years of age is not sufficient for diagnosis. However, these data suggest that cancer can develop at an early age, even though its incidence increases with age[19]. Family history and environment are important factors in developing prostate cancer. Approximately 10% of early prostate cancers are linked to a heritable cause. Prostate cancer is more frequently diagnosed in the US than in Asian countries[20]. There is a higher fatality rate (approximately 2.4) in African American men than in Caucasian men[21]. A family history of prostate cancer increases the risk of developing the disease and there is a nine fold higher risk of prostate cancer in men with a father and a brother affected with prostate cancer. In men with only a father or a brother diagnosed with prostate cancer, the risk is two-fold and three-fold higher, respectively[21]. The precise role of dietary fat in carcinogenesis is not clearly understood, but in vivo and ex vivo studies have identified a direct relationship between the growth of prostate cell lines and a high-fat diet. In addition, development of prostate cancer has been associated with activated signaling pathways, including cc chemokine receptor 2 (CCRC2) and serum monocyte chemoattractant protein-1 (MCP-1)[22]. The relationship between prostate cancer and physical activity, which is considered a variable factor, has been studied by the International Agency for Research on Cancer (IARC). A group of phytochemicals known as (Phytoestrogen) have a significant protective role against prostate cancer in animals (Table 1). These phytochemicals, which have estrogen-like properties, include lignans from flax seed and isoflavones, which are abundantly found in soybean. Phytoestrogens are considered some of the most important phytochemicals in plant food[23]

Table 1. Chemical structures of the phytochemicals.

3. Prostate cancer chemoprevention
Cancer chemoprevention is an efficient approach that involves using natural and synthetic compounds to suppress, hamper or reverse early cancer development[24].Chemoprevention can reduce mortality rates and unwanted effects of therapeutic agents, while decreasing disease prevalence by targeting characteristic pathways. Disease prevention can take one of three approaches: primary, secondary or tertiary. Primary prevention targets disease prevalence in healthy subjects. Secondary prevention aims to suppress disease development in persons in the nonmalignant stages. Tertiary prevention focuses on disease reoccurrence in patients whose disease is being managed after tumor identification[25] 
Reactive oxygen species (ROS)-induced inflammation is a main primary mechanism for cancer development including prostate cancer. Oxidative stress leads to ROS production, leading to DNA damage and cancer initiation including the prostate[26]. Reduced ROS production could be an effective therapeutic approach forprostate cancer and other cancers. ROS can be produced endogenously or resulted from exogenous sources. Inflammation, metabolism and mitochondrial activity are the primary endogenous ROS sources. Mitochondrial mutations can damage oxidative phosphorylation and electron transport, the latter of which result in ROS generation. Oxidative stress accumulates with age, leading to increased ROS production. The prevalence of prostate cancer is detected by the presence of a mutation or deletion in mitochondria DNA (mtDNA) that is also a sign of aging[27]. Prostate cancer is initiated by high-grade prostatic intraepithelial neoplasia (HGPIN). At this early stage, primary prevention can be useful for patients in the pemalignant stage. The prolonged multistage pathogenesis of prostate cancer makes specific pathway targeting a more suitable approach for cancer chemoprevention[25].
Antioxidant response element (ARE) is a cis-acting element responsible for controlling the protective response of certain enzymes to oxidative stress at a transcriptional level. AREs are often located in the promoter of certain enzymes, such as NADPH: quinoneoxidoreductase 1 (NQO1) and glutathione S-transferaseA2 (GSTA2), which are essential detoxifying enzymes[28]. The Nrf2/ARE signaling pathway controls several protective mechanisms and it activates more than 250 phase II genes termed “prolife genes” leading to protection of cells from death[29]. Phase II detoxifying enzymes, the main regulatory elements of cellular detoxification, include UDP-glucuronyl transferase, glutathione S-transferase (GST) and microsomal epoxide hydrolase[30].
Nuclear factor (erythroid-derived 2)–like 2 (Nrf2 or NFE2L2) is the primary transcriptional factor in the cap n’ collar family of basic leucine zipper (bZIP) transcription factors that have a major role in stimulating antioxidant genes by controlling the ARE. Stimulation of Nrf2/ARE-related genes is the first chemopreventive pathway for most phytochemicals that inhibit cancer progression[31]. In the absence of oxidative stress, Nrf2 is sequestered in the cytoplasm by binding to kelch-like-ECH-associated protein 1 (Keap1). Nuclear translocation of Nrf2 occurs when cysteine residues in Keap1 are irritated by oxidants or electrophiles. Nrf2 promotes transcription of antioxidant genes by heterodimerizing with small Maf proteins and binding to the ARE in the antioxidant gene promoters[32]. Several chemopreventive compounds can promote Nrf2 signaling, leading to stimulated production of phase II enzymes such as heme oxygenase (HO-1). The cellular protectiverole of HO-1 has been established in cancers, inflammation, atherosclerosis and neurodegenerative disorders. It mediates the breakdown of heme into iron, carbon monoxide and biliverdin. The current mechanistic model suggests that it induces the nuclear translocation of Nrf2[33], as shown in Figure 1. Several studies in rodents have reviewed the activation of the protective genes controlled by the ARE signaling pathway by natural and synthetic compounds found in their diets[34]. Cellular responses to oxidative stress are accomplished by Nrf2 translocation to the nucleus and escapes degradation. Under basal conditions, degradation maintains Nrf2 levels in the cell. The mechanism by which the stress response is controlled, is directly related to understandingthe molecular pathway of Nrf2 signaling activation by reactive oxidative species[35]. Many studies have revealed the effects of natural products on cancer chemoprevention, including the prostate. We review several natural phytochemicals and illustrate their effects and molecular mechanisms. 

Figure 1.
Nrf2-ARE Transcription Pathway. Under basal conditions, Nrf2 is sequestered in the cytoplasm by binding with keap1 and forming a complex. Nuclear translocation of Nrf2 occurs when cysteine residues in keap1 is irritated by oxidants or electrophiles. Nrf2 starts transcription of antioxidant genes by heterodimerizing with small Maf proteins and attaching to ARE in the promotor area of the antioxidant genes such as HO-1, NQO1, Gstm2 and UGT1A1. Exposure to certain phytochemicals can also induce expression of phase II detoxifying/antioxidant enzymes such as SFN, PEITC and DIM. 
4. Natural phytochemicals and prostate cancer
4.1. Lycopene from tomatoes
Lycopene, a member of the carotenoid family, is abundantly in tomato[36,37]. Other sources of lycopene are papaya, pink grape-fruit, pink guava, water melon, gac and red carrot[38]. Lycopene lacks vitamin A activity[39], but has been shown to decrease prostate-specific antigenand minimize DNA oxidation in several clinical trials. It has also been shown that high consumption of tomato-based products can increase lycopene levels and minimize aggressive prostate cancer[40]. Lycopene can also inhibit breast cancer, lung cancer and pancreaticcancer progression[41]. Lycopene inhibits cancer progression through its antioxidant activity, suggestingthat it protects proteins and lipids from oxidative damage. Several studies have shown that lycopene can inhibit DNA production and cell growth in several prostate cancer cell lines, including LNCaP, DU145 and PC3[36].
Lycopene inhibits prostate cancer progression by lowering insulin growth factor (IGF) levels. IGF has two cell surface receptors (IGF-1) and (IGF-2) with insulin-like sequences. IGF-1 primarily activates Akt signaling pathway, which is responsible for inhibiting apoptosis or programmed cell death and inducing cell reproduction. Lycopene exerts its antioxidant functions by scavenging free radicals and removing them, resulting in phase II detoxifying gene transcription. Lycopene’s antioxidant activity is due to its eleven bound bonds and the absence of Beta-ionone ring[42].
Lycopene and other carotenoids activate phase II enzymes, including NQO1 and GCS. The transcription factor Nrf2 is the main regulator of antioxidant enzyme expression, and it has been shown to induce the expressionof these enzymes in liver cancer cells and mammary cells. Lycopene also affects gap junction communication. Retenoids and carotenoids, including lycopene, can stimulate connexin 43 (CX43) expressionand increase gap junction communication. Ions, nutrients and low molecular weight molecules enter between cells through protein channels known as gap junctions in the cell membrane[43]. Among the connexin family, which includes Cx43, Cx50, Cx40 and Cx33, Cx43 is primarily responsible for cell growth. Studies have shown that activated Cx43 expression can lead to decreased fibroblast proliferation, but decreased Cx43 expression increased cell proliferation[42].
Mariani et al. estimated lycopene concentration in HGPIN patients. They determined the lycopene level in the prostate and plasma and studied its effects on prostate cancer development by administering a high-lycopene diet[44]. The study was conducted for six months, and the patients were divided into three groups (prostatitis, prostate cancer and HGPIN) depending on biopsy results. The cut-off value of lycopene level in the prostate was 1 ng/mL. The authors concluded that lycopene accumulation in the prostate can reduce apoptosis, oxidative injury and prostate-specific antigen. These specific markers are found in many cancers, including prostate cancer. This study suggested the important role of lycopene in the prostate. In this study, patients with prostate cancer had low lycopene concentrations in their prostates but high lycopene levels in the plasma. Compared with the HGPIN group, the six month follow-up study revealed low lycopene levels in the prostate[44] 
Uncontrolled cell growth is highly characteristic of cancer development, which can be caused by an imbalance in the cell cycle, specifically the down regulation of the first G1 phase. Studies using DU145 cells investigated the effects of lycopene and its metabolite apo-12-lycopeneon the cell cycle. These studies showed that cell cycle distribution was reversed. The number of cells in S phase decreased, whereas the number of cells in G1 and G2/M increased[45]. Lycopene can affect cyclin D1, which is related to the G0/G1 phase. Lycopene treatment (24 h) was sufficient to reduce cyclin D1. By contrast, the cyclin kinase inhibitors: p53, p27 and p21 were increased. Lycopene’s effect on the cell cycle has been attributed to its effects on the NF-kB signaling pathway[45].
The effects of lycopene on the cell cycle and apoptosis were also investigated by Soares et al. Benign prostate hyperplasia (BPH) and prostate cancer cell lines (PC3 and DU-145) were used. BPH cells, which are hormone-independent, did not respond to lycopene. By contrast, prostate cancer cells (PC3 and DU-145) are hormone-dependent and showed a significant response to low lycopene levels (25 µM). At this concentration, lycopene suppressed a high percentage of cell growth, and its inhibitory levels become saturated after 96 h oflycopene treatment, even though extracellular lycopene levels still increased. Lycopene treatment induced more apoptosis in DU-145 cells than PC-3 cells. After 96 h, DU-145 showed a five-fold increase in apoptosis, and PC3 had a 2.2-fold higher apoptosis rate compared with a 48-h treatment with 10 µM lycopene[46]. Genes responsible for prostate tumor growth include Bax, Bcl-2 and CK18. Both PC3 and DU-145 induced Bax and CK18 expression, but decreased Bcl-2 gene expression. The authors also reported a remarkable long G0/G1 phase and slow G2/M phase. G2/M inhibition results in serious effects on cell growth because cells cannot complete their division process, while blockade in G0/G1 phase can be reversible. Greater blockade in G2/M phase is responsible for the higher level of apoptosis in DU-145 cells than that in PC3 cells. The regulated expression of Bcl2 and Bax (high Bax and CK18 and low Bcl2) caused the apoptotic effect in BPH cells[46].
Lycopene can influence the prostate cancer cell migration and adhesion at low concentrations (0.01 µM). A previous study showed that lycopene can selectively inhibit the highly activated integrins in prostate cancer (αvβ3 and αvβ5)[45]. Integrins are closely associated with adhesion and migration of cancer cells[45,47] and are highly expressed in developed prostate cancer. The 22RV-1, PC3 and LNCaP cell lines have reduced integrin expression[45]. Elgass et al. investigated lycopene’s effect on prostate cancer cell adhesion and migration. The authors used normal prostate epithelial cells (PNT2) and the PC3 and DU-145 cells. Lycopene inhibited migration in both PC3 and DU-145 cells. Lycopene had a greater effect on DU-145 cells than PC3 cells. Lycopene had no effect on PNT2 cell migration, except at high concentrations[47]. A lycopene concentration of 1.15 µM/L was effective to reduce motility rate by 40% in PC3. Lycopene had a greater inhibitory effect on PC3 cell adhesion and a greater inhibitory effect on DU-145 cell migration. This was the first study that demonstrated lycopene’s effects on adhesion and migration of prostate cancer cell lines at physiologically relevant levels (12) µM[47].
4.2. Anthocyanins from purple corn color
Anthocyanins are natural compounds of the flavonoid family and exist as glycosides. These glycosides includegalactose, glucose, rhamnose, xylose or arabinose connectedto aglycone unit[48,49]. Several sources of anthocyanins are available, including purple corn, grape, berries, apples and purple cabbage[49]. Anthocyanins have multiple health benefits, including antioxidant and anti-inflammatory activities[50,51]. They can also improve mouse neuronal and heart function and suppress cancer progression. Cyanidin-3-glucoside, an anthocyanin present in purple corn color, can regulate hyperglycemia and obesity[51]. Anthocyanins are usually red, purple or blue due to the presence of metal ions and pH of the medium. Anthocyanins are soluble in water and in acidic solvents, as they have a positive charge compared with other flavonoids[49]. Anthocyanins are highly enriched in purple corn Zea mays L. seed extracts. The extract is called purple corn color maize morado color, and it is commonly used in the drinking solution chichi Morado in South America, particularly in Peru. Anthocyanins are responsible for the purple color of Zea mays L. corn seeds[48,52]
Purple corn colors (PCC) contain three anthocyanins: cyanidin-3-glucoside (C3G), pelargonidin-3-glucoside (Pg3G) and peonidin-3-glucoside (P3G)[13]. Anthocyaninscan scavenge free radicals and fight oxidative stress. A study using HepG2 cells and Caco-2 adenocarcinoma cells showed that cyanidin-3-glucoside significantly inhibits ROS synthesis. C3G can also decrease DNA and protein production in malignant cells. C3G also has a protective role against DNA damage in human colon cells[53].
Anthocyanin’s powerful antioxidant properties are related to the phenolic structure of these compounds. Hydroxyl radicals, hydrogen peroxide and singlet oxygen are the main reactive oxygen species taken by anthocyanins. They can also bind to proteins and exhibit defensive action by chelating metals. Anthocyanins have been reported to induce apoptosis in vitro. Anthocyanins and anthocyanidins have both been shown to induce a proapoptotic response by the intrinsic and extrinsic pathways. Regulation of FAS and FAS ligand occurs during the extrinsic pathway, whereas caspase-dependent pathway upregulation occurs through the intrinsic pathway. Release of cytochrome c is regulated through “potential? of mitochondrial membrane. Anthocyanins can suppress cancer progression by inhibiting angiogenesis through the decrease in vascular endothelial growth factor (VEGF) and its receptor. Suppression of VEGF-stimulating factors such as tumor necrosis factor alpha (TNF-α) can also inhibit VEGF expression. Suppression of H2O2 can also inhibit VEGF expression[49]. Anthocyanins, particularly PCC, can inhibit colon tumorigenesis. A previous study, induced colon tumorigenesis by 2-amino-1-methyl-6-phenylimidazo [4, 4-b] pyridine (PhIP) treatment is used. Studies on rats have shown that PhIP can induce colon cancer and breast cancer. Anthocyanins significantly inhibited PhIP-induced colon carcinogenesis,even when the percentage of purple corn color in the rat diet was as low as 5%. The presence of neoplasm was considered the end result of tumor formation. A PhIP concentration of 0.02% was sufficient to induce carcinogenesis. In this study, researchers used 1,2-dimethyl-hydrazine (DMH) as an indicator to identify anthocyanin’seffects on colon tumorigenesis. Azoxymethane is anotherinhibitor that can be used for the same purpose.The authors concluded that anthocyanins are an effective route of colon cancer chemoprevention, but further research is required to determine whether smaller amounts of anthocyanins have similar effects on colon cancer[52].
Anthocyanins also have significant suppression effects onin vitro and in vivo prostate carcinogenesis. Long et al.used anthocyanins on prostate cancer cell lines (LNCaP) and the transgenic rat for adenocarcinoma of prostate (TRAP) model. Anthocyanins successfully suppressed LNCaP cell growth after 72 h. qPCR and western blotting techniques revealed a significant reduction in cyclin D1, whereas there was a significant increase in the number of cells in the G0/G1 cell cycle phase. In the TRAP model, the probasin gene promoter controls the expression of simian virus 40T antigen. Androgen receptor regulates expression of this antigen. This study reported the clinical findings of anthocyanin treatment on prostate carcinogenesis. Development of prostate cancer in the TRAP model was indicated by the development of adenocarcinoma and high-grade prostatic intraepithelial neoplasia (HGPIN)[13].
The P3G, C3G and Pg3G anthocyanins can critically slow prostate carcinogenesis. These anthocyanins have been individually tested on LNCaP cells to determine the most effective component in purple corn color. P3G did not inhibit LNCaP cell growth, whereas C3Gand Pg3G significantly suppressed their growth. These results suggest that C3G and Pg3G are the most potent PCC compounds. The latter two compounds reduced cyclin D1 protein levels and increased the number of cells in G0/G1. These results may also be due to the presence of hydroxyl radicals in the anthocyanin structure, which could have a specific response on prostate cancer suppression. The authors concluded thatPCC successfully inhibited prostate cancer progression in vitro and in vivo. Mixture of PCC with its active components resulted in significant inhibition of LNCaPcell growth and the suppression of prostate tumorigenesisin vivo[13].
Anthocyanins exert their effects through various mechanisms. Hou explained the mechanism of anthocyanin action in a recent review. He illustrated the ability of anthocyanins to inhibit nitric oxide (NO) production. Bacterial or viral infection induces NO production, which can trigger oxidative stress and cancer. Cardiovascular disease and inflammation could be prevented with a food rich in anthocyanins, such as berries. Anthocyanins work by obstructing NO-induced oxidativestress. Anthocyanin can also work by affecting epidermalgrowth factor receptor (EGFR). Cyanidin and delphinidin can inhibit EGFR activity by suppressing tyrosine kinase[54].
Anthocyanins can also affect metastasis or progressionof cancer. Metastasis involves the controlled movementof chemotactic compounds by autocrine motility factors, including NDP-kinase, cell adhesion and secretion of specific enzymes, such as metalloproteinase (MMP). Delphinidin treatment can suppress MMP activity, but has no effects on cell adhesion or hepatotactic movement[54].
The phenolic structure of anthocyanins is responsible for their antioxidant properties, as they have significant free radical scavenging activity. This is effectively due to the presence of hydroxyl chains and a double-bondedstructure. This antioxidant activity is partially responsible for their anticancer activity. Anthocyanins can stimulate cancer cell apoptosis by accumulating intracellular ROS. They can also hamper cancer progression by actively transcribing detoxifying phase II enzymes. Notably,  their phenolic structure is responsible for their ability to chelate metals and bind to proteins[55]. The protective role of anthocyanins has been investigated in breast cancer cell lines. Fukamachi et al. examined the therapeutic effect of anthocyanins on breast cancer carcinogenesis in rats. Breast cancer was induced by 7,12-dimethylbenz[a]anthracene (DMBA). Both transgenic and non-transgenic rats were used in this study. Transgenic rats expressed a copy of the human c-Ha-Rasproto-oncogene, and PCC was administered as the anthocyanin source. C3G is the most potent anthocyanin. Protocatechuic acid (PC) is a metabolite that forms after C3G administration, and it has powerful antioxidantactivity. The authors concluded that PCC administration reduced tumor volume. Although the number of large tumors was effectively reduced by PCC, it could not inhibit cancer progression. The concentration of PCC used was 1%, which was sufficient to suppress tumor progression in the transgenic rats. In non-transgenic rats, 1% PCC could significantly inhibit massive tumor production and decrease the number of mammary tumors[48].  
In this study, anthocyanin acted by inhibiting the Ras signaling pathway and inducing apoptosis through caspase activation. Activated Ras signaling is associated with development of many tumors, including colon, breast, thyroid and lung adenocarcinoma. Abnormal Ras signaling has been found in 80% of tumors. Therefore, direct Ras inhibition can protect cells from oxidative stress and DNA damage, leading to apoptosis. PCC activates apoptosis through caspase-3 activation. The authors concluded that PCC affects mammary tumors by inactivating ERK and reducing Ras protein without altering Ras gene expression[48].
4.3.Nutrients from walnuts
Walnuts (Juglans regia L.) are found primarily in tropical areas around the world. They are mainly used as food, but their leaves, shells and husks (epicarps) may have medical and cosmetic uses. The walnut plant has long been associated with folk medicine[56], and several beneficial effects have been demonstrated. It has been reported that walnuts can improve many diseases including cancer and lifestyle-associated diseases such as hypertriglyceridemia, diabetes, arteriosclerosis, hypercholesterolemia and cardiovascular disease[57]. Consumption of walnuts can significantly inhibit systemic inflammation. Walnuts can also decrease omega-6 levels, as shown by studies on humans and animals. These studies indicated that walnuts improve endothelial-dependent vasodilatation[58].
Walnuts contain several nutrients, such as α-tocopherol[56] and gamma tocopherol[56,59]. They also contain phytosterols (primarily β-sitosterol), melatonin, fibers and polyphenols (primarily ellagitannin). Carotenoidsare also found in walnuts, as is α-linolenic acid (ALA, 18 C)[59]. β-Sitosterol, a sitosterol compound, is commonly in the Western diet. Other plant sterols in the Western diet include campesterol and stigmasterol. β-Sitosterol belongs to the phytosterol family and has a structure similar to that of cholesterol. Phytosterols are commonly found in the fatty part of nuts and can suppress inflammation, leading to decreased cholesterollevels. Phytosterols can also decrease cholesterol retention in the body[60].
Phytosterols possess several health benefits includingcancer prevention. They can activate “ceramide” cascades and induce apoptosis in cancer cell lines. β-Sitosterol can also induce cancer cell apoptosis. β-Sitosterol-inducedapoptosis has been shown in hormone and non-hormone-dependent prostate cancer cells (LNCaP and PC3 respectively), and in colon adenocarcinoma (HT-29) and breast cancer cell lines (MDA-MB-23), which are non-hormone-dependent[61]. Walnuts have antioxidant properties resulting from the presence of melatonin, a potent antioxidant compound, and other ingredients. Studies on rats have demonstrated melatonin’s protective role against oxidative stress. In a controlled trial, total glutathione and oxidized glutathione (GSSG) and catalase action were significantly improved in people who consumed walnut meal (WM)[62].
The fiber content of walnuts has a significant preventiverole in cancer, as it can suppress cancer progression. The European Prospective Investigation into Cancer and Nutrition (EPIC) study demonstrated the effect of dietary fiber content on cancer in women who consumednuts and seeds. The EPIC study demonstrated that consumption of fiber-rich foods decreased the risk of colorectal cancer by 21% whereas high fiber content from fruits led to increased risk of prostate cancer. By contrast, a French study showed that the risk of prostate cancer was not affected by fiber content from fruits and vegetables (insoluble), but the risk decreased with fiber content from legumes (soluble). Walnuts have been reported to inhibit prostate cancer using PC3 and LNCaP cell lines and in the TRAMP model[12,56,58].
Alshatwi et al. investigated walnut′s effects of on prostate cancer in PC3 cells. The walnut components examined included n-hexane, chloroform and methanol extracts of the green bark of walnut. Walnuts were used as air-dried green husk (WNGH). PC3 cell growth was significantly inhibited by these three organic walnut extracts, as they include apoptosis. n-Hexane was the most effective at inhibiting cell[56].
The same authors then examined induction of apoptosis by examining proapoptotic and antiapoptotic gene expression. They found a significant increase in p53, caspase 3 and Bax mRNA levels and a remarkable decrease in Bcl-2 mRNA. n-Hexane showed the highestinhibition of PC3 cell growth, possibly due to the presence of a diverse quantity of polyphenols in WNGH organic extract, which could be attributed to the type of solvent used. Bcl-2 and p53 family control apoptosis. P53 inhibits the transcription of antiapoptotic genes and activates proapoptotic gene transcription through a mitochondrial-regulated pathway. P53’ interaction with Bcl-2 induces Bax’s mitochondrial permeability. P53 can activate Bax by activating the caspase pathway. P53 enhances the Bax:Bcl-2 ratio to induce apoptosis. Caspase and cytochrome c activation induce apoptosis. In this study, P53 induced apoptosis by inhibiting Bcl-2and activating the Bax and caspase signaling pathway[56].
Davis et al. investigated the effects of a diet containing whole walnuts on the TRAMP prostate cancer model. The effect was compared with using high- and low-fat diet, and the α-tocopherol and γ-tocopherols levels were the same in all three food types. The authors concluded that high fat content did not affect prostate tumorigenesis, whereas whole-walnut intake resulted in a significant reduction in genitourinary intact tract (GUI) weight and the rate of tumor growth[12]. High IGF-1 is associatedwith increased prostate cancer risk, and high insulin and IGF-1 can stimulate cell growth and induce carcinogenesis. It is important to target IGF-1 as an initial strategy in controlling tumor growth as reported by several studies. A significant increase in tumor rate was obtained when IGF-1 was not targeted. Following fifteen weeks of a whole-walnut diet, the rate of tumor growth was decreased and a tumor volume significantly decreased. High resistin levels were associated with increased tumor growth. Suppression of resistin level was observed in individuals who consumed whole walnuts. LDL levels decreased with the whole-walnut diet compared with a high-fat diet. In this study, tumor growth-related factors were suppressed following the whole-walnut diet, and no specific fatty acid or individual tocopherol was responsible for the effects[12].
Walnut consumption effectively suppressed breast cancer tumorigenesis in mice[63,64]. The diverse content of walnuts helped fight cancer progression. As mentioned previously, walnuts have multiple components roles in carcinogenesis inhibition. The mechanism of walnuts needs to be carefully studied to determine the main component responsible for its cancer prevention role. An early study by Hardman et al. investigated the role ofα-Linolenic acid on breast cancer progression. α-Linolenic, a walnut component, has been reported to decrease the cancer growth rate. Transgenic mice (C3) Tag were used as a cancer model. These mice carried the SV40 T antigen, which induced cancer in the mammary glands. 10% corn oil was used as a negative control. High ALA levels are present in corn oil in amounts similar to those in the Western diet[64].
Higher levels of ALA are associated with walnut consumption than other nuts[63,64]. In one study, transgenic mice and their progeny were given a diet of walnuts, canola oil or corn oil. ALA is also in canola oil. The riskof breast cancer was effectively decreased by canola oil consumption during gestation and lactation. α-Linolenic also reduced cancer progression, but α-linoleic acid may induce tumorigenesis. The study found that walnut consumption resulted in the suppression of tumor progression. Decreased tumor growth was also found after walnut consumption by mice. Both walnuts and canola oil contain α-linolenic acid, and both have been shown to suppress cancer progression compared with corn oil. There are equal quantities of omega-3-fatty acid in walnuts and canola oil, but walnuts have a greater inhibitory effect[64].
The effect of walnut consumption on breast cancer was examined by Hardman et al. The authors used nude mice and induced breast cancer by injectingMDA-MB-231 human breast cancer cells. Corn oil was used as a negative control. Walnut consumption reducedbreast carcinogenesis. ω-3-Fatty-acids underwent elongation into saturated fatty acids with 20 or 22 C. These elongated ω-3-fatty acids included eicosapentanoic acid (EPA) and docoshexanoic acid (DHA), which are associated with inhibiting carcinogenesis or reducing tumor volume. These metabolic fatty acids are responsible for the protective and preventive roles of α-linolenic acid. Walnut consumption effectively provided high α-linolenic acid levels. Therefore, it was not necessary to add EPA or DHA to the diet, as the walnut diet couldprovide these important metabolic fatty acids[63]. Walnut consumption effectively slowed breast cancer growth, as shown by several mouse studies[63,64]. Walnut’s significant inhibitory role may not be due to a specific component, as they have many ingredients, each of which exerts a protective role against cancer. The α-linolenic acid in walnut could suppress breast cancer growth[64]. A walnut-rich diet suppressed breast carcinogenesis and malignant cell production decreased metastatic cell proliferation, leading to decreased cancer-related mortality[63].  
4.4. Sulforaphane from cruciferous vegetables
Sulforaphane is an organosulfur phytochemical foundin most cruciferous vegetables including cabbage, Brussels sprouts and broccoli. Many studies have demonstrated its anticancer and antioxidant roles. SFN exhibits antioxidant activity by stimulating expression of the Keap1/Nrf2 pathway. SFN also has beneficial roles in diabetes in that it can protect kidneys from diabetes-induced damage through its antioxidant action. This effect has been shown after four months treatment with SFN in mice with diabetes[65]. SFN is a member of the isothiocyanate family, which has been shown to have a role in inhibiting prostate cancer. Cruciferous vegetables contain isothiocyanate precursors known as glucosinolates[24]. SFN is released following glucoraphanin hydrolysis, a compound highly enriched in cruciferous vegetables, particularly in broccoli[66,67]. Glucoraphanin hydrolysis is achieved by thioglucosidase in the colon or thioglucosidase myrosinase in the plant[66].
SFN can stimulate nuclear translocation of Nrf2 and ARE gene expression by affecting the Keap1-Nrf2 complex. SFN interacts with the cysteine thiols on keap1 C151, C489, C583, inducing Nrf2 escape from the keap1 complex[68]. Nuclear translocation of Nrf2 enables Nrf2 binding to promoters of phase II enzymes and stimulates their release. SFN stimulation of phase II enzymes helps inhibit cancer initiation. Expression of phase II enzymes helps the cell remove reactive moleculesby combining “moities” and making them more soluble to increase their elimination. SFN upregulates phase II enzymes such as NQO1 and HO-1 to prevent cell death. SFN can also decrease the activity of electrophiles by neutralizing them. It can bind glutathione to electrophilesand reduce their activity. SFN is a potent glutathione S-transferase stimulator and neutralize electrophiles[69]. SFN can also induce apoptosis or programmed cell death and inhibit cell cycle in different cell lines, including in human colon cancer cells and prostate cancer cells (DU145). SFN can also induce cytochrome c release from the mitochondria and/or caspase 7 and 9 activation to activate the apotosis pathway. SFN can inhibit cell cycle in some cancer cell lines, including prostate cancer cell lines (PC3 and LNCaP) and human colon cancer cells (HCT116). Treatment of LNCaP cells with 10 µM SFN significantly inhibited the cell cycle at the G1/S phase. SFN was also shown to inhibit the cell cycle in HT-29 colon cancer cells at the G1 phase and reduce cyclin D1 levels and activate p21[67]. SFN is a promising chemopreventive agent that induces phase II enzyme expression, which converts toxic substances into more easily excreted products. Induction of phase II enzymes by SFN is proposed to occur by two mechanisms. First, it induces Nrf2 transcription; second, it modulates mitogen-activated protein kinase (MAPK) and induces ARE-gene transcription. The important signaling proteins c-jun N-terminal kinase JNK and p38 kinase are MAPK regulatory proteins and they can modulate the transduction of extracellular signals-induced intracellular functions[70]. SFN is highly studied for its role in cancer, as it can inhibit cancer growth and induce apotosis. SFN also inhibits cancer progression at the post-initiation stage. In prostate cancer, SFN induces apoptosis by inhibiting cell growth and the cell cycle. SFN can inhibit protein synthesis in PC3 cells, which is crucial for cell growth and viability. Translation is usually controlled by mTOR kinase. S6 kinase1 is one mTOR kinase target, and it regulates several processes in protein synthesis, such as starting and extending translation in addition to ribosome biogenesis. SFN treatment results in phosphorylation of S6K1 and leads to blocked transduction between mTOR and S6 kinase1. As a result, early translation inhibition occurs[71]. SFN can also induce apoptosis by inhibiting STAT3 activation. STAT3 is a signal transducer and activator of transcription 3 and is an important transcription factor in cancer development.SFN inhibits interleukin 6 (IL-6) associated with STAT3 stimulation. This study revealed that SFN can inhibit several genes associated with STAT3 activation such as cyclin D1, and inhibit prostate cancer cell survival[72].
SFN has been found to inhibit NFκB transcription. This transcription factor has two subunits p50 and p56. NFκB stimulation is found in several cancers, including colon and prostate cancer, and it is correlated with stimulated expression of genes associated with inflammation, cell growth and antiapoptotic events. SFN can down-regulate NFκB activity and inhibit p65-NFκB nuclear translocation. This effect was shown after treating PC3 cells with 20 µM SFN for one hour. Another cancer prevention target is the induction of apoptosis via induced ROS production. ROS production in the mitochondria requires high SFN doses. A study showed that ROS temporarily increased in DU145 cells after 10 µM SFN. Several events are associated with stimulated ROS production in the mitochondria, including “Disruption” in membrane potential and cytochrome c release, resulting in apoptosis[73]. A study using PC3 and DU154 cells investigated the role of SFN in apoptosis. The study revealed that ROS induced cell death through both the intrinsic and extrinsic pathways. Cytochrome c release and mitochondrial membrane potential disruption also occurred. SFN-induced apoptosis was reduced by BcL-xL. BcL-x is a member of the BcL-2 family of cell death regulators and it inhibits apotosis[74]. SFN protects skin cells from UV-induced inflammation. This effect was shown in C57BL/6J and C57BL/6J/Nrf2 (-/-) mice. This study concluded that SFN’s protective role throughNrf2 activation, and the damage was removed within 8 days. However, SFN had no effect on knockout mice[32].In a study using C57BL/6 TRAMP mice, SFN inhibitedtumor growth by activating apoptosis in mitochondria and activating the Nrf2/ARE signaling pathway. This resulted in induced expression of apoptosis-related genes such as Bax and a decrease in BcL-xL. Additionally, Nrf2/ARE signaling activation resulted in induced Nrf2 expression and its related genes HO-1 and decreased Keap1 expression[75]. SFN is a promising chemopreventive agent in prostate cancer and other cancers for its high efficacy. SFN has several molecular targets, and each is efficient in protecting cells.
4.5. Curcumin from turmeric
Curcumin, or diferuloylmethane, is the most importantingredient of the turmeric spice, Curcumalonga L.[32,76]. Curcumin is usually extracted from the rhizomes of the Curcuma longa Linn plant[8,76], which belongs to the Zingiberaceae family[76]. Curcumin has anti-inflammatoryand antioxidant characteristics[8,77] and anticancer effects. Several clinical trials have used curcumin as an anticancer agent because of its safety and tolerability. Curcumin has been found to be safe in amounts upto 12 g/d when administered orally. Its effects are likely due to curcumin’s powerful antioxidant properties, as it can protect normal cells and kill cancer cells. Therefore, curcumin is not toxic to normal healthy cells[78]
Curcumin has been used to treat and prevent several cancers, including prostate, lung, colorectal, breast, pancreatic, head and neck squamous cancers. Its effects in other cancers, such as multiple myeloma, have also been tested. Curcumin induces autophagy in metastatic cells. In a study using curcumin on CML K562 human chronic myeloid leukemia cells, curcumin induced autophagy and stimulated apoptosis, resulting in cell death. The mitochondrial membrane potential decreased and the caspase 3 signaling pathway was activated. There was a dose- and time-dependent decrease in cell viability due to increased apoptosis. Curcumin inhibits the cell cycle at G2/M phase, as shown in U373-MG and U87 MG glioma cells[79]. It is known that the oxidativestress is an important cause of inflammation leadingto cancer. In oxidative stress, activated macrophages or neutrophils produce reactive oxygen and nitrogen species. These ROS and RNS lead to cancer initiation. Curcumin scavenges free radicals, including nitric oxide. In a study using the mouse macrophages RAW 264.7 cell line, curcuminreduced inducible nitric oxide synthase (iNOS) levels caused by gamma interferon and lipopolysaccharide (LPS) treatments. Curcumin can significantly affect Nrf2 and NF-κB signaling to enhance HO-1 transcription, resulting in the prevention of oxidative stress[80].
In a study using renal epithelial cells, curcumin upregulated HO-1 expression and the expression of phase II genes by enhancing Nrf2 escape from the keap1 complex. This induced nuclear translocation of Nrf2 and activation and enhanced expression of detoxifying genes[80,81]. Curcumin has a significant effect on PCa cells in vivo. In a study using nude mice, an in situ cell death assay was used to measure apoptosis in tumors. The test showed that curcumin induced a significant increase in apoptosis and inhibited cell growth. In another study, curcumin induced TNFα related apoptosis ligand (TRAIL) to activate apoptosis in LNCap xenografts of nude mice. Curcumin also inhibited tumor growth. Many studies have investigated the combination of curcumin and phenethyl isothiocyanate (PEITC) on PCa. In a study using PC3 cells, 25 µM curcumin and 10 µM PEITC strongly inhibited Akt and epidermalgrowth factor phosphorylation. The combination significantly induced apoptosis by stimulating cleavageof caspase 3 and poly (ADP-ribose) polymerase (PARP)protein. Treatment also induced activation of phosphatidylinositol 3-kinase (PI3K)[82]. Prostate cancer metastasis can be prevented by inhibiting cell motility. A study using nude mice showed that inhibition of metalloproteinase MMP-2 and MMP-9 reduced the growth of prostate nodules. Metastasis results when cancer cell growth is highly expanded and starts to spread to other tissues. Metastasis significantly depends on the role of MMPs in enhancing cell motility. Curcumin has an important role in inhibiting inflammation in prostate cancer. COX-2 is highly expressed in prostate cancer, but curcumin can inhibit its expression by inhibiting several signaling pathways. These include dephosphorylation of c-jun N-terminal kinase (JNK) and protein kinase p38 resulting from mitogen-activated protein kinase-5 (MKP5) activation. As a result, the expression of proinflammatory cytokines such as IL-6 was inhibited. Inactivation of the NF-κB pathway also occurred. IL-6 is a proinflammatory cytokine and an autocrine growth factor in prostate cancer. Curcumin inhibits angiogenesis in prostate cancer cells, in addition to its effect on arresting cell cycle and inhibiting growth factors. As a result, curcumin can significantly inhibit prostate cancer metastasis. These effects have been shown in several studies using LNCapxenografts and androgen-sensitive and -insensitive prostate cancer cells[8]
Curcumin can inhibit cell proliferation by inhibiting the expression of multiple growth factors, such as VEGF and HER2. VEGF, such as Avastin, has a crucial role in angiogenesis. Curcumin can significantly inhibit VEGFin vivo, resulting in decreased angiogenesis. Activation of HER2 has been observed in prostate cancer, leading to tumor cell over-growth. Curcumin inhibits HER2 activity and leads to its degradation. Curcumin can also prevent its tyrosine kinase activity[83]. Curcumin is an important regulator of androgen receptor in prostate cancer, which is abnormally expressed in prostate cancer. β-Catenin can directly interact with the AR pathway, and it is an important target gene of Wnt signaling pathway, which is activated in many cancers including prostate. In a study using LNCap cells, Choi et al. foundthat curcumin significantly and dose-dependently inhibited androgen receptor activity. Degradation of β-catenin also resulted from increased phosphorylation. GSK-3β and PI3K/Akt are also upregulated in prostate cancer, and they are related to increased β-catenin activity. Curcumin significantly inhibited these signaling pathways and activated β-catenin degradation[84]. Many clinical trials have investigated the use of curcumin in controlling cancer due to its many targets, including inhibition of angiogenesis, cancer progression and tumorgrowth. These studies have been performed in vitro, in vivo and ex vivo and many attempts have been made to identify the relevant curcumin formula[76].
4.6. Triterepenoids from various plants and fruit surface wax
Triterepenoids are metabolic products of isopentenyl pyrophosphate oligomers found in many types of natural compounds[85,86]. Triterepenoids are present in plants such as oregano, thyme, rosemary, mistletoe, olives, cranberries, apples, blueberries, lavender and figs. Theyhave also been found in the wax of fruit peels or in other plants such as sea-weed. More than 20,000 types of triterepenoids have been identified[85]. Triterepenoids are a group of compounds that are synthesized by the cyclization of squalene[32,87,88], which is a triterpene compound and acts as a precursor to many steroids[32]. In Asian countries, triterpenoids are widely used as therapeutic agents. Natural triterpenoids, such as ursolic acid (UA) and oleanolic acid, have certain beneficial effects against cancer and inflammation, but these effects are relatively insignificant[87,88]. Synthetic oleananetriterpenoids (SO) result from alterations in the triterpenoid structure. Several studies have shown the significant effects of SO in controlling cancer and inflammation. SO induce apoptosis in cancer cells that are chemotherapy-resistant. They are cytoprotective and enhance cancer cell differentiation. Several SO are known, including 2-cyano-3,12-dioxooleana-1,9[11]-dien-28-oic acid (CDDO) and its methyl ester (CDDO-Me). Additional CDDO derivatives have been synthesized to improve their efficacy, such as amides including ethyl amide (CDDO-EA) and methyl amide (CDDO-Me), dinitrile (Di-CDDO) and the imidazolides (CDDO-IM). SO are multitarget agents, suggesting that they have several molecular mechanisms. SO can target STAT signaling, keap1 (the Nrf2 suppressor), transforming growth factor-β and IκB kinase (IKK)[89]. Synthesis of SO is important in controlling inflammation and oxidative stress, which are responsible for the initiation of many diseases including cancer. Many studies have reported the ability of SO to suppress nitric oxide production, which can be induced by LPS, interferon γ or TNFα and IL-1β. SO can activate the Nrf2 signaling pathway and induce the transcription of many cytoprotective genes, such as HO-1, superoxide dismutase, catalase, glutahthione-1 and UDP-glucuronosyltransferases. In the resting state, Nrf2 is tolerable for proteosomal degradation. SO can directly interact with Keap1 to allow Nrf2 to escape degradation. This induces the transcription of protective genes via ARE. Many in vitro and in vivo studies have demonstrated the ability of SO to induce Nrf2 signaling pathway. So are directly linkedto reduced oxidative stress and induced Nrf2 transcription and its related protective genes. Several studies have reported a role for SO in inhibiting carcinogens, which was related to its ability to stimulate the Nrf2/ARE signaling pathway[90]. CDDO and CDDO-IM treatment significantly induced HO-1 expression by stimulating Nrf2/ARE signaling in vitro and in vivo. In this study, breast cancer cell lines (T-47D an MCF10), lung carcinomacells (A549) and leukemia cells (THP-1 and U937) were used. HO-1 expression significantly increased after was CDDO-IM treatment. This compound is stronger than CDDO in inducing HO-1 expression. CDDO-IM successfully reduced oxidative stress in U937 cells at different concentrations. This assay included the addition of 2′,7′-dichlorofluorescin diacetate (H2DCFDA), and subsequent treatment cells with tert-butyl hydroperoxide (tBHP). The resultant compound is 2′,7′-dichlorofluorescein which can be measured by flow cytometry. tBHP-induced oxidation decreased after CDDO-IM treatment.A concentration of 100 nm/L CDDO-IM strongly inhibited oxidation (approximately 53%). This study also showed that activation of the Nrf2/ARE signaling pathway is necessary for CDDO-IM inhibition of oxidative stress. In Nrf2-deficient mice, CDDO-IM did not have an antioxidant activity. Increased HO-1 expression also requires Nrf2/ARE pathway activation[91].CDDO-IM significantly reduced LPS-induced inflammation by activating the Nrf2/ARE pathway. CDDO-IM decreased inflammation in macrophages and neutrophils[92,93]. LPS is an endotoxin that activates the TLR4-NF-κB pathway in macrophages and neutrophils, which induces the expression of proinflammatory proteins that result in cell death[93]. In these studies, CDDO-IM treatment increased Nrf2 transcription and its related cytoprotective genes, such as HO-1 and NQO-1. ROS and inflammatory mediator production were decreased upon CDDO-IM treatment. CDDO-IM significantly decreased LPS-induced inflammation[92,93]. The anti-inflammatory effect of CDDO-IM towards LPS was investigated using Nrf2-deficient mice nrf2 –/–and wild type animals nrf2 +/+. Nrf2 –/– neutrophils released higher levels of inflammatory cytokines and chemokines, suggesting a remarkable inflammatory effect on these cells. Expression of many antioxidant genes is controlled by Nrf2. CDDO-IM treatment significantly induced expression of the antioxidant genes in nrf2 +/+neutrophils, but not in nrf2 –/– neutrophils[93]. Syntheticoleanane triterpenoids (SO) activate apoptosis in different cancer cells. In leukemia and myeloma cancers, SO result in cancer cell apoptosis caused by increased ROS levels but not in normal lymphocytes. This suggests that SO are “selective” inducers of apoptosis. SO induce apoptosis in the (1–5) µM concentration range through various mechanisms. The primary mechanism of apoptosis induction is through extrinsic and intrinsic pathway activation. The type of apoptosis depends on the cancer cell type. The intrinsic mechanism is a mitochondrial pathway, while the extrinsic pathway is a death receptor pathway. These compounds can induce apoptosis by activating caspases 3, 8 and 9 and activating death receptors 4 and 5. They can also stimulate apoptosis-induced factor (AIF), resulting in cytochrome c release and BAX transfer to the mitochondria[89]. Many studies have shown that CDDO and its derivatives can induce apoptosis in different cancer cells[94–99]. CDDO induced apoptosis in osteosarcoma cells (Saos-2 and U20S) cells. CDDO treatment leads to caspase activation, apoptosis and cytochrome c release. Apoptosis is initiated by caspase 8 activation resulting in caspase 3 activation. Cytochrome c release and Bid cleavage occurred downstream of caspase 8 activation[97]. CDDO-IM induced leukemia cells apoptosis, primarily through caspase 3 activation. CDDO-IM also induced Bax expression[94]. SO can also induce apoptosis by disrupting the redox state in the cells. JNK activation results from antioxidant activity inhibition due to reduced glutathione levels in the mitochondria. Severalevents can occur that trigger apoptosis, including STAT signaling or NFκB inhibition[89]. At nanomolar doses, CDDO-IM induced apoptosis in pancreatic cancer cells (COLO357, PANC1). CDDO-IM also reduced the total glutathione in a dose- and time-dependent manner. GSH is an important protein that has a role in blocking apoptosis in addition to scavenging ROS produced from the mitochondria during metabolic processes[98]. CDDO also induced apoptosis myeloid leukemia cells (U-937, HL-60) by inducing caspase 3 cleavage and caspase 8 pathway activation. Cytochrome c was also released[95]. In prostate cancer, CDDO blocked cancer development in the TRAMP model. Additionally, CDDOinhibited prostate cancer cell growth in vitro andin vivo in both hormone-sensitive and -insensitive prostate cancer cells. In a study using the TRAMP C1 model, CDDO-Me induced apoptosis in TRAMP-C1 cells. CDDO-Me also inhibited signaling pathway activation in prostate cancer (Akt, mTOR and NF-κB). CDDO-Me suppressed prostate cancer development and significantly blocked cancer spread and metastasis in the TRAMP C1 model[100].
In another study using the CDDO compound in the TRAMP model, CDDO effectively inhibited cancer metastasis and induced apoptosis. It also inhibited angiogenesis and cell growth, thus limiting carcinogenesis in the prostate. CDDO also blocked cell growth and apoptosis regulatory pathways (Akt and NF-κB). CDDO inhibited antiapoptotic proteins BCL-2 and BcL-xL and survivin expression[101].
In a study using multiple prostate cancer cell lines, CDDO, CDDO-IM and CDDO-Me were used to investigatetheir effects on these cells. This study involved using both hormone-sensitive and -insensitive prostate cancer cells. The compounds induced apotosis through a caspase 8-mediated mechanism, but CDDO-IM was the most effective. CDDO-IM and CDDO induced apoptosis depending on the death receptors (DR4 and DR5). Very small CDDO, CDDO-IM and CDDO-Me concentrations were toxic to prostate cancer cells, suggested the high sensitivity of these cells to triterpenoid compounds[102]. In a study using LNCaP and PC3 cells, CDDO-Me induced apoptosis by activating both the mitochondrial and death receptor pathways. CDDO-IM blocked cell survival pathways, and CDDO-Me significantly inhibited NF-κB, p-AKT and mTOR. CDDO-Me also activated ROS production, which has a role in inducing apoptosis[103].
5. Summary
Prostate cancer is the second leading cause of death in the world. Several factors contribute to its initiation and development. The emergence of this disease is highly associated with diet and life style. Diets rich in fat and a sedentary life-style can induce prostate tumorigenesis.Many studies have found that healthy natural diets can reduce prostate cancer development and inhibit metastasis. An improved life-style can also help reduce the incidence of cancer. In general, eating a balanced diet can efficiently minimize tumor growth and inhibit its development to advanced prostate cancer. Triterpenoids,particularly synthetic oleanane (SO), have antiproliferative characteristics and can induce apoptosis in prostate cancer cell lines. Further research is needed to examine the effects of natural compounds on prostate cancer in vivo.
We thank all the members in Dr. Ah-Ng Tony Kong’s lab for their helpful discussion and preparation of this manuscript. This work was supported in part by Institutional Funds and by R01-CA118947, R01- CA152826, from the National Cancer Institute (NCI), R01AT007065from the National Center for Complementary and Alternative Medicines (NCCAM) and the Office of Dietary Supplements (ODS).
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Received: 2016-03-23, Revised: 2016-04-10, Accepted: 2016-04-30.
Foundation items: This work was supported in part by Institutional Funds and from the National Cancer Institute (Grant No. R01-CA118947, R01-CA152826), the National Center for Complementary and Alternative Medicines and the Office of Dietary Supplements (Grant No. R01AT007065).
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