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Anti-Cancer Agents in Medicinal Chemistry, 2013, 13, 000-000
Sugar-Borate Esters – Potential Chemical Agents in Prostate Cancer Chemoprevention Romulus Ion Scorei1 and Radu Popa2,* 1
Department of Biochemistry, A.I. Cuza street, No. 13, University of Craiova, Craiova, Dolj, 200585, Romania; 2University Park Campus, ZHS 117, Mail Code 0740, University of Southern California, Los Angeles, 90089-0740 CA USA Abstract: The potential value of sugar-borate esters (SBEs) in the chemo-preventive therapy of prostate cancer has been reviewed. We propose that SBEs act as boron (B) vehicles, increasing the concentration of borate inside cancer cells relative to normal cells has been proposed. Increased intracellular concentration of borate activates borate transporters, but also leads to growth inhibition and apoptosis. The effects of SBEs on normal cells are less dramatic because SBEs are naturally-occurring biochemicals, common and abundant in some fruits and vegetables, and also because borate dissociated from SBEs in natural diet doses is easily exported from normal cells. Cancer cell lines that over-express sugar transporters or under-express borate export are potential targets for SBE-based therapy. With regard to efficiency against cancer cells and drug preparation requirements, trigonal cis-diol boric monoesters will be one of the most effective class of SBEs. Because negative correlation exists between borate intake and the incidence of prostate cancer, and because most cancer cells over-express sugar transporters, SBEs are proposed as a potential chemopreventive avenue in the fight against primary and recurrent prostate cancer.
Keywords: Boron, sugar-borate esters, prostate cancer, chemoprevention, diet, ribose, fructose, sugar transport. I. INTRODUCTION Chemoprevention is the use of chemical agents to reverse, suppress or prevent carcinogenic progression [1, 2]. Qualities sought in chemopreventing agents include efficiency against cancer progression, limited side effects, easiness of administration, known mechanisms of action and cost effectiveness in prolonged treatments . The high incidence and extended latency of prostate cancers [4, 5], makes them better targets for chemopreventive therapy. The main strategies for prostate cancer chemoprevention include: anti-inflammatory agents such as aspirin , non-steroidal anti-inflammatory agents  and statins ; targeting sex steroid hormones ; lowering oxidation levels ; and changes in diet [11-15]. Diet-based chemoprevention is complex and includes a combination of changes in nutritional intake balance and antioxidants [16-19]. The most important markers for evaluating the efficiency of prostate cancer chemoprevention are: prostate inflammation, benign prostatic hypertrophy, prostatic intraepithelial neoplasia (PIN) and the prostate-specific antigen (PSA) [20, 21]. In cancerous prostates, oxidation levels are high and inflammation creates additional freeradicals, which eventually escalate the evolution toward the precancerous condition PIN [22, 23]. PIN is a useful marker for studying chemoprevention because it does not influence the PSA level, may appear long before prostate cancer has been diagnosed (e.g. sometimes more than ten years later), and because about 85% of all men with prostate cancer also have PIN [24, 25]. Elevated PSA levels may be an indication of prostate cancer, but may also be associated with a non-cancerous prostatitis (i.e. prostate inflammation). Men with prostate cancer often have >4 ng/mL PSA level, but prostate cancer is also possible at lower PSA levels as well . The incidence of prostate cancer relative to PSA is 15% at 10 ng/mL PSA. This makes PSA a valuable tool for selecting human subjects for data mining and for studying chemoprevention .
*Address correspondence to this author University Park Campus, ZHS 117, Mail Code 0740, University of Southern California, Los Angeles, 900890740 CA USA; Tel: 1 503 725 9503; Fax: 1 503 725 3888; E-mail: [email protected]
Many connections were reported to exist between boron (B) and cancer. Boron deficiency was shown to be positively correlated with increased incidence of prostate cancer , lung cancer  and cervix cancer . Increased abundance of B in the environment and diet was negatively correlated with prostate hyperplasia , inhibited the activity of prostate and breast cancer cells [31-35], and lowered the incidence of prostate cancer and the mortality in prostate cancer  and lung cancer patients . Boron-containing compounds with anti-cancer effects include boric acid, borates, borate esters, boranes, borinic esters, boronic esters and oxazaborolidines [35, 36]. The drug Bortezomib (Velcade, Millennium Pharmaceuticals) is a boronate ester. Bortezomib is a proteasome inhibitor; it slows down the growth of prostate and breast cancer cells [37-39] and induces apoptosis in prostate cancer cells [37, 40], myeloma cells [41-43] mantle cell lymphoma , cell lung cancer , ovarian cancer , pancreatic cancer  and head and neck squamous cell carcinoma . In this study we review the potential of sugar-borate esters (another class of B compounds) as chemopreventive agents against prostate cancer. II. NATURAL B COMPOUNDS OF POTENTIAL VALUE IN PROSTATE CANCER CHEMOPREVENTION The microelement B is important to many types of organisms but its primary cellular function is unknown [34, 48, 49]. In many cases, the role of B seems to be related to its ability to form complexes with compounds having cis-diol configurations . Presently, B-based anticancer therapies include the use of boronate esters , and 10-B neutron capture therapy [52-54]. In this section we review effects of three classes of natural B compounds on living cells and in prostate cancer chemoprevention: (a) boric acid/borate; (b) polyketide-borate esters; and (c) sugar-borate esters. (a) Boric Acid (BA) Boric acid (BA) is an inhibitor of peptidases, proteases, proteasomes, arginase, nitric oxide synthase and transpeptidases [55, 56]. Its capacity to bind OH groups from NAD and serine may explain its capacity to inhibit dehydrogenases and serine proteases
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. BA has been proposed in the therapy of prostate carcinoma  partly because of its capacity to inhibit the activity of the PSA serine protease [57, 58]. The effect of BA on androgen-sensitive cancer cells may involve inhibition of PSA formation, but also effects on DNA polymerization, thymidilate synthesis, Sadenosylmethyltransferase activity, non-histone chromatin methylation, DNAse and RNAse [31, 59].” BA also inhibited androgen-independent cell lines (e.g. DU-145 and PC-3l) indicating that other (serine protease-independent mechanisms) may also exist . BA can control and inhibit the cell cycle (as seen in the proliferation of DU-145 cells) by inhibiting an agonist-stimulated release of Ca2+ from ryanodine receptor sensitive cell stores . In high doses (12.5 mM to 50 mM) BA slowed down cell replication or induced apoptosis in melanoma cells and some breast cancer cells [34, 60, 61]. (b) Polyketide-borate Esters Boromycin is a polyether-macrolide produced by Streptomyces antibioticus with antibiotic activity against Gram-positive bacteria . It acts at the cell membrane level and results in loss in intracellular potassium . It also selectively disrupts the cell cycle in some cancer cell types, making them sensitive to specific anticancer agents . Borophycin, a polyketide extracted from species of Nostoc , was shown to have inhibitory effects on
Scorei and Popa
several cancer cell lines . Tartrolons are macrolides with a chemical structure related to boromycin and aplasmomycin  and have antiviral and antineoplastic chemotherapeutic properties . Aplasmomycin, secreted by Streptomyces griseus, with antibiotic effects against Gram positive bacteria and Plasmodium berghei, was not yet verified against cancer cells, but it is similar in structure with tetralons which have anticancer properties . (c) Sugar-borate Esters BA, and its ionic form borate, produces esters with sugars called sugar-borate esters (SBEs). SBEs can be trigonal or tetragonal, and monoesters or diesters (Fig. 1) . Equilibrium between various SBEs in solution and the rates of inter-conversion depend on factors such as concentration, the type of sugar, pH and temperature. Sugars with high abundance of furanosic isomers and cis-diol groups (such as ribose and fructose) have increased reactivity and form numerous SBEs. At circumneutral or alkaline pH, electrically neutral trigonal mono-esters convert into electrically charged tetragonal esters. This transformation is important because the transport and effect of various SBEs on cells is controlled by both their chemical structure and electrical state. Relative to pyranosic esters, furanosic esters have higher association constant , which increases their shelf-stability and their value for SBEbased therapy . Calcium fructoborate (CF), one example of
Fig. (1). Chemical structures of main B-containing compounds, boric acid, borate and examples of common types of furanosic borate sugar esters (BSE) with ribose and fructose.
Sugar-Borate Esters – Potential Chemical Agents in Prostate Cancer
SBE used in the prevention and treatment of osteoporosis and osteoarthritis [70-72], is a natural plant-mineral complex found in plants, commercially available as nature-identical mineral complex produced by chemical synthesis . CF has been shown to inhibit MDA-MB-231 breast cancer cells, a metastatic cancer cell line that is estrogen-insensitive. Inside cells, CF acts as an antioxidant, induces over-expression of apoptosis-related proteins and eventually leads to apoptosis in a dosage-dependent manner . The ideal B-containing chemicals for chemotherapy should show low toxicity, high specificity in targeting cancer cells, and (once inside the cytoplasm) they should release toxic borate in solution for a long time. Healthy cells curb the toxicity of B by exporting borate. This, combined with the limited capacity of BA and borate to target cancer cells, significantly lowers the chemotherapeutic usefulness of BA and borate. Boronate esters show higher specificity for cancer cells than BA or borate , but they are also more toxic to normal cells . SBEs are less toxic than BA, are similar in structure with natural B compounds commonly found in fruits and vegetables [67, 74] and are better for targeting cancer cells than BA and borate . It was proposed that the capacity of SBEs to target cancer cells is also better than that of boronate esters . This is because SBEs (in monoester forms) are transported into cells via sugar transporters [74, 76] but also because many cancer cells show increased expression of sugar transporters [77-79]. Due to their dissociative properties, SBEs release less borate in solution than BA (Eqs. 1-3). Thus, once in the cytoplasm of cells with diminished capacity to export borate, SBEs release borate in solution for a longer time. Bo + OH- = B- (Ko = 8.5·104) Eq. 1 2 4 B L = B + L (K1 = 6.6·10 for L = fructose; and K1 = 1.82·10 for L = ribose) Eq. 2 B-L2 = B-L + L (K2 = 9.1·104 for L = fructose; and K2 = 6.3·104 for L = ribose) Eq. 3 where: Bo = boric acid; B- = borate; L = ligand; B-L = tetragonal borate monoesters and B-L2 = tetragonal borate diesters. Ko values represent the concentration of borate (in Eq.1 and Eq.2) and of monoester (in Eq.3) at equilibrium at a concentration of the reagent of 1M. Association constants for trigonal B esters are difficult to determine with precision because they evolve rapidly into tetragonal forms [68, 79-81]. For these reasons SBEs are thought to be appropriate for cancer chemoprevention and chemotherapy. We propose that: • during SBE-based therapy B is introduced into cells masked as a derivatized sugar; • trigonal monoesters (which are not electrically charged) are better than tetragonal SBEs or sugar borate diesters; • ribose SBEs are better than fructose SBEs (because cancer cells show particularly active RNA synthesis which requires large amounts of ribose); and • furanose SBEs drugs are better than pyranose SBEs drugs due to differences in shelf stability. III. THE ACTIVITY OF SBES IN NORMAL CELLS AND EFFECTS ON PROSTATE CANCER CELLS (a) SBEs as Anti-inflammatory Agents The link between inflammation and cancer occurs through production of cytokines and growth factors favoring cancer cell growth, the induction of COX-2 (a protein controlling the synthesis of prostaglandines linked with tumor proliferation), and the generation of mutagenic reactive chemical species of oxygen and nitrogen [82-84]. Cytokines produced during inflammation increase the expression of 5-lipoxygenase (5-LOX) , which leads to the formation of metabolites favoring cancer development. Inhibiting
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5-LOX-related metabolites triggers apoptosis in prostate cancer cells . The anti-inflammatory activity of B may be an outcome of suppressing serine proteases released by inflammation-activated white blood cells, by inhibition of leukotriene synthesis, by reduction of reactive oxygen species generated during neutrophils’ respiratory burst, or by suppression of T-cell activity and antibody concentrations . SBEs inhibit the synthesis of arachidonic acid (AA)-derived eicosanoids, a class of pro-inflammatory prostaglandins . High B intake leads to its incorporation in membrane phospholipids, partly substituting AA-derived eicosanoids, and increases the abundance of the omega-3-derived eicosapentaenoic acid . Treatments of LPS-stimulated RAW264.7 macrophages with CF inhibited the production of IL-1β and IL-6 cytokines and that of nitric oxide . Because these chemicals are inflammation mediators, it was proposed that CF can be used as an antiinflammatory agent [90, 91]. Adding CF to the diet of farm animals led to higher abundance of omega-3 polyunsaturated fatty acids (omega-3 PUFA) in the meat of pigs and in the meat and eggs of chicken [91-93]. Increased abundance of omega-3 PUFA relative to omega-6 PUFA was shown to increase the cellular resistance to inflammatory stimuli associated with cancer progression . (b) The Transport of B Into Cells In small amounts, B is needed by the cells, but excess BA and borate are toxic [95-97]. Cells regulate the internal concentration of borate via specialized transporters, though the mechanism of regulation is little understood [98, 99]. Electrically neutral B compounds cross the membrane passively [100-102]. The borate anion (Fig. 2) can be transported by aquaporins such as AQP9  or specialized transporters such as NABC1 in humans , ATR1 in yeast [104, 105] and BOR1 in plants . SBEs were shown to enter the blood stream at the intestine level , but how they cross the cell membrane remains unclear. Because borate and phosphate share many similarities in structure and chemical reactivity, they substitute for each other in many compounds and processes [106, 107]. They may also substitute each other during cross-membrane transport. SBE transport may occur via sugar transporters (most likely this applies to the SBEs that contain non-charged trigonal borate; Fig. 1), by facilitated diffusion, and by translocation in the case of SBEs containing electrically charged tetragonal borate (Fig. 1), . Sugar transporters are one of the likeliest avenues for introducing CF mono and diesters into cells . Putative candidates for this activity are the fructose transporter Glut5 and glucose-6-phospahte translocase. Differences in the expression of sugar transporters may explain why some cancer cells are more sensitive to SBEs than normal cells . B transporters influence the practicality of using BA in cancer treatments . Some cancer cells have impaired ability to eliminate excess borate and are about five times more prone to borate induced apoptosis than normal cells . Partly because of their diffusive properties, boronates are more efficient anticancer agents than BA. It is hypothesized that SBEs is more difficult to eliminate from cells than borate, because no specialized SBE transporters exist, because SBEs are most likely transported via sugar transporters (which in cancer cells are more active inward), and because the intracellular concentration and need for sugars and phosphate is always higher than that for BA. In support of this hypothesis, CF has been shown to trigger apoptosis in MDA231 breast cancer cells at concentrations 4-5 times lower than BA . (c) The Relationship Between B and Vitamin D3 The inverse correlation between exposure to solar radiation and the incidence and mortality of breast cancer was proposed to be associated with the production of vitamin D2 [109-110]. Vitamin D3 (calcitriol) is a derivative of D2, has important roles in regulating the prostatic cell growth and has demonstrated effects on the prostate cancer cell line LNCaP . Vitamin D3 arrests the cell cycle, induces apoptosis and inhibits metastases and the
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Fig. (2). Known and putative mechanisms for the transport of common boron compounds (boric acid, borate, boronates, borate sugar esters and boric acid sugar monoesters) into cells.
proliferation of prostate cancer cells [112-115]. Its tumor inhibition activity may be due to the induction of the cyclin dependent kinase inhibitor p21 and G1-G0 cell cycle arrest. This activity may explain the regression of cancer cell growth in rats treated with vitamin D3 . Vitamin D3 also initiates its own inactivation because it also induces the expression of CYP24, a protein that initiates the Vitamin D3 catabolism, a mechanism present in prostate cells . B in general and CF in particular, increase the intracellular concentration of vitamin D3 [70, 118]. This effect may be due to the fact that B up-regulates the 25-hydroxylation step or suppresses the vitamin D3 catabolic pathway . Because B readily forms covalent complexes with cis-vicinal dihydroxy compounds, it is reasonable to assume that B also forms complexes with 24,25dihydroxyvitamin D, the final product of the 25-OH-D reaction with 24-OH-hydroxylase (a vitamin D3 regulator). This complex acts as a competitive inhibitor for the 24-OH- hydroxylase reaction, or, as a down regulator of this enzyme. B may also be an inhibitor of microsomal enzymes (24-hydroxylase and estradiol hydroxylases) which catalyze the insertion of the hydroxyl group vicinal to the existing hydroxyl groups of steroids . For these reasons, combinations of vitamin D3 with chemical B compounds that are easy to assimilate and tend to accumulate into cells, are a potential strategy for hormonal cancer chemoprevention and chemotherapy . (d) B and Proteasome Inhibition Proteasomes are large protein complexes recycling ubiquitinated proteins. Several anti-apoptotic and proliferative signaling pathways require proteasomal activity. In the prooncogenic NF-κB pathway, which is activated and prevalent in the regulation of many types of tumors, NF-κB proteins are kept in inactive state by the IκB proteins [51, 118]. In order to remove this inhibition, IκBs have to be phosphorylated, poly-ubiquitylated and then recycled. Correct functioning of proteasomes appears to require that B exists in the cells within a specific concentration range. Both, too little or too much B inhibit proteasome activity [120, 121]. In high concentration, BA and boronic acid slow down the carcinogenic progression, because they block the degradation of IκBs, which in turn down-regulates the NF-κB signalling. The drug Bortezomib inhibits proteasomes, possibly through the formation of complexes with their active site(s) . The ensuing inhibition of the NF-κB signaling pathway reduces the expression of pro-
inflammatory response genes and up-regulates the cyclin-dependent kinase inhibitors p21Cip1 and p27Kip1. In turn, this increases the frequency of apoptosis in tumor cells. This mechanism may be part of the Bortezomib-based treatment of multiple myeloma, relapsed mantle-cell lymphoma  and prostate cancer [124,125]. The relationship between Bortezomib and apoptosis is complex and may be influenced by numerous factors. For example, over-expression of the anti-apoptotic protein Bcl-2 in H460 cells did not affect proteasomal activity, but decreased the effect of Bortezomib on apoptosis . BA, which dissociates in borate, inhibits the effect of Bortezomib on cancer cells. This may be due to competition between borate and boronate-esters for proteasome active site(s) . Understanding this interference is important in future studies of the efficiency of Bortezomib on proteasomes, in the presence of natural and artificial SBEs. This is particularly important in the case of neutral monoester SBEs, which release less borate by dissociation than BA. How will for example, a diet of fruits rich in SBEs (such as plums) or treatments with CF will influence the therapeutic efficiency of Bortezomib? (e) Effects on B Compounds on Enzymatic Activity Borates, borate esters and boronic esters interfere with the activity of many enzymes, including NADH-cytochrome b5 (cyt. b5) reductase [EC 22.214.171.124] , phosphorylases , alcohol dehydrogenas, xanthine oxidase, glutamyl transpeptidase and cytochromes . The mechanisms involved, albeit not fully clear, often include interactions with OH and cis-diol groups. Borate forms complexes with amino and hydroxy groups of proteins, targeting residues of lysine, glutamine, serine, hystidine and proline . It also binds polyhydroxy compounds such as sugars (mannitol, xylitol, sorbitol, glucose and fructose)  and ribose from NAD and FAD, phenols (catechol and pyrogallol) and α-hydroxy acids (2-hydroxyisobutyric acid, salicylic acid, and cis2-hydroxycyclopentanecarboxyric acid) . Borate stabilizes the tertiary structure and activity of alkaline phosphatase, protecting it from oxidative stress . This was attributed to borate creating a chemical bridge between carbohydrate residues and protecting 1,2diols and 1,3-diols. B was also shown to increases the resistance of heme-containing proteins (such as cyt c and metmyoglobin) to thermal stress . At biochemical and therapeutic levels, the main usefulness of B may be to protect heme proteins (Cyt c and metMb) from oxidative stress . Similar to proteasome-level
Sugar-Borate Esters – Potential Chemical Agents in Prostate Cancer
effects the activity of cytochrome c is regulated by the concentration of B and inhibited when B is either above or below some optimal thresholds [136, 137]. Inorganic additives enhancing the effect of B compounds on enzymes include calcium, magnesium and phosphorus . (f) Effects of Borate/phosphate Similarities Negative correlation has been reported to exist between the concentration of phosphate and borate in plants cells  and in normal and osteoporotic bones . Borate was shown to enhance phosphorylation and, in humans, was proposed to affect living cells via a mediator (putatively TNF-alpha) whose transduction signal involves a phosphorylation cascade . Though many similarities exist in structure and activity between borate and phosphate the borate:phosphate substitution is little studied. Phosphate esters are important in cellular energetics, biochemical activation, signal transduction and conformational switching. The borate:phosphate similarity, combined with borate’s ability to spontaneously esterify hydroxyl groups, suggests that phosphate ester recognition sites on proteins might exhibit significant affinity for non-enzymatically formed borate esters . For example, the complex between RNase A and 3′-deoxycytidine-2′-borate was shown to mimic the structure of the complex between RNase A and 2′-cytidine monophosphate inhibitor. The RNase A capacity to bind cyclic cytidine-2′,3′-borate ester, a structural homolog of the cytidine2′,3′-cyclic phosphate substrate, was also seen. In normal cells the borate:phosphate competition is little important, probably because in cells phosphate is thousands of times more abundant than borate, but also because in complexes such cytidine-2′,3′ - RNAse the affinity for phosphate is higher than the affinity for borate . Based on earlier evidence we expect that the effect of borate:phosphate similarity should increase in phosphate-starved cells treated with SBEs.
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IV. SBE-BASED TREATMENTS AND B TRANSPORT DEFICIENCY AS AN POTENTIAL AVENUE IN THE FIGHT AGAINST CANCER The recommended daily intake of B in humans is between 0.14 and 0.28 mg / day kg body weight (bw) [142, 143]. Increased exposure to about 2.5 and 24.8 mg borate / kg bw (used in the past to treat epilepsy), has has been shown to have non-lethal secondary effects such as alopecia, and some reversible effects such as dermatitis, anorexia and indigestion . The lethal daily dose of B is in the range 400–900 mg / kg bw . The genotoxicity of B was studied by Moore . The 50% lethal dose of B, as boric acid, for one time administration is very high (2.6 g per kg bw), close to that of table salt (3 g per kg bw) . In borax treatments of cultured lympohocites no effects were seen at 0.1 mg/ml, inhibition of proliferation occurred between 0.15 and 0.6 mg/ml, sister chromatin exchange increased at 0.6 mg/ml and the 50% inhibitory concentration was 0.9 mg/ml . We found no references linking B exposure with carcinogenesis. B sensitivity varies between normal and cancer cells, not all cancer cells are sensitive to B and the response to B vary among Bsensitive cancer cells. In cell cultures, cancer cells died at concentrations between 1 and 50 mM BA, and non-cancer cells were about 5 times more resistant to B . SBEs are appealing for therapy because they are less toxic to healthy cells than BA . In one study (Perry Scientific Inc, 2001, unpublished) the highesst dose of CF and B citrate administered to rats (by oral gavage) was 37.5 mg/kg at single dose administration. No toxicity was noted at this dosage equivalent to 2,250 mg CF (60 mg elemental B) for a 60-kg human . The median lethal dose (LD50) for CF is 18.75 g/kg (=0.525 g B), higher than that of BA (2.6 g/kg (=0.462 g B) (Perry scientific Inc. 2001). Thus, CF intake levels equivalent to single digits milligrams B per day are safe, and are close to BSE
Fig. (3). Diagram of proposed differences in cell transport between various types of SBEs. It is proposed that the most actively transported form of SBEs are trigonal boric monoesters (Fig. 1). Their transport is proposed to occur mainly via sugar transporters. As a result of this biased transport and subsequent hydrolysis in the cytoplasm the cell’s interrior is acidified and the tetragonal borate esters accumulate inside the cells. Cells with higher expression of sugar transporters (such as most cancer cells) and cells with impaired borate export are more vulnerable to SBE-based treatmetns using trigonal boric monoesters.
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intake via food. Based on apoptosis measurements, some cancer cells were shown to be more sensitive to B than normal cells [31, 61]. Mecham (2007) has shown that the sensitivity of cultured cancer cells was negatively correlated with the expression of the NaBC1 borate transporter, indicating that apart from its borate import function NaBC1 may also be used to regulate the level of intracellular borate through export. . Based on the fact that during treatments in the range 1-50 mM BA about 90 % of B was eliminated from the human body within 22-24 hours, the exposure treatments shown above, leading to the apoptosis of cancer cells, correspond with an approximate daily intake between 4.4 mg and 220 mg BA equivalent per kg bw. This is considered relatively safe to normal cells because it is below the toxic levels of B (see above), and close to the B exposure levels used in past for epilepsy treatments. For these reasons, B is a promising avenue for inducing apoptosis in some forms of cancer, including prostate. We propose that such treatments will be most efficient against cancers cell lines with under-expressed NaBC1 and when B is administered as SBE. The trigonal SBE would be particularly more efficient against cancer cells which unlike normal cells also show over-expression of sugar transporters (glucose and fructose channels), (Fig. 3) and against cancer forms with very slow progression (such as prostate cancer). V. CONCLUSIONS B is more toxic, and induces apoptosis, in some types of cancer cells (including prostate cancer) compared to non-cancer cells. SBEs are common in fruits and vegetables and are naturally absorbed by animal cells and are less toxic to normal cells than other B compounds such as inorganic borates, boric acid, boronates and boranes. The mechanism of transport for SBEs in animal cells is little understood. One putative avenue is via sugar transporters, in which case B enters the cells masked by a sugar. We propose that trigonal cis-diol isomers are some of the most effective SBEs against cancer cells. The effect of B on various forms of cancer cells varies, influenced by factors such as the level of expression of transport systems and the availability of phosphorus (relative to cellular needs for phosphorus). The involvement of B transporters in controlling cell proliferation is a novel research avenue in the fight against cancer. Some cancer cell lines will be more SBEs sensitive than others, because of higher expression of sugar transporters or diminished borate export capacity (Fig. 3). Because SBEs dissociate less than boric acid, they can be delivered discontinuously (e.g. through pelleted drugs), yet once inside target cells they will act as chronic releasers of toxic borate. This puts those cells having more active sugar import and diminished borate export at greater apoptosis risk. If SBEs prove to be beneficial in the fight against prostate cancer, the dietary strategy of persons belonging to age groups with increased prostate cancer risk and persons using anti-recurrence chemopreventive treatments should also include increase preponderance of foods and juices naturally rich in SBEs such as plums, avocado, nettle, walnuts, peaches, peanuts, honey and milk. CONFLICT OF INTEREST The author(s) confirm that this article content has no conflict of interest. ACKNOWLEDGEMENTS We thank the BioBoron Research Institute (Romania) for supporting this work. The authors contributed equally to this work. ABBREVIATIONS B = Boron BA = Boric Acid CF = Calcium Fructoborate
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omega-3 PUFA PIN PSA SBEs 5-LOX
omega-3 PolyUnsaturated Fatty Acids
= = = =
Prostatic Intraepithelial Neoplasia Prostate-Specific Antigen Sugar-Borate Esters 5-LipOXygenase
REFERENCES     
  
     
Sporn, M.B. Approaches to prevention of epithelial cancer during the preneoplastic period. Cancer Res., 1976, 36, 2699–2702. Sporn, M.B.; Suh, N. Chemoprevention of cancer. Carcinogenesis, 2000, 21(3), 525-530. Gupta, S. Prostate cancer chemoprevention: Current status and future prospects. Toxicol. Appl. Pharmacol., 2007, 224(3), 369-376. Nelson, W.G.; De Marzo, A.M.; Isaacs, W.B. Prostate Cancer. N. Engl. J. Med., 2003, 349, 366-381. Djulbegovic, M.; Beyth, R.J.; Neuberger, M.M.; Stoffs, T.L.; Vieweg, J.; Djulbegovic, B.; Dahm, P. Screening for prostate cancer: systematic review and metaanalysis of randomised controlled trials. Brit. Med. J., 2010, 341, c4543, doi:10.1136/ bmj.c4543. Leitzmann, M.F.; Stampfer, M.J.; Ma, J.; Chan, J.M.; Colditz, G.A.; Willett, W.C.; Giovannucci, E. Aspirin use in relation to risk of prostate cancer. Cancer Epidemiol. Biomarkers Prev., 2002, 11, 1108-1111. Ulrich, C.M.; Bigler, J.; Potter, J.D. Non-steroidal antiinflammatory drugs for cancer prevention: promise, perils and pharmacogenetics. Nature, 2006, 6, 130-140. Hamilton, R.J.; Freedland, S.J. Review of recent evidence in support of a role for statins in the prevention of prostate cancer. Curr. Opin. Urol., 2008, 18(3), 333-9. Jordan, V.C. A Century of Deciphering the Control Mechanisms of Sex Steroid Action in Breast and Prostate Cancer: The Origins of Targeted Therapy and Chemoprevention. Cancer Res., 2009, 69, 1243-1254. Hoque, A.; Ambrosone, C.B.; Till, C.; Goodman, P.J.; Tangen, C.; Kristal, A. Serum Oxidized Protein and Prostate Cancer Risk within the Prostate Cancer Prevention Trial. Cancer Prev. Res. (Phila), 2010, 3(4), 478–483, doi:10.1158/1940-6207.CAPR-090201. Bosetti, C.; Tzonou, A.; Lagiou, P.; Negri, E.; Trichopoulos, D.; Hsieh, C.C. Fraction of prostate cancer attributed to diet in Athens, Greece. Eur. J. Cancer Prev., 2000, 9(2), 119-23. Tseng, M.; Breslow, R.A.; Graubard, B.I.; Ziegler, R.G. Dairy, calcium and vitamin D intakes and prostate cancer risk in the National Health and Nutrition Examination Epidemiologic Followup Study cohort. Am. J. Clin. Nutr., 2005, 81, 1147-54. Park, S.Y.; Murphy, S.P.; Wilkens, L.; Stram, D.O.; Henderson, B.E.; Kolonel, L.N. Calcium, Vitamin D, and Dairy Product Intake and Prostate Cancer Risk: The Multiethnic Cohort Study. Am. J. Epid., 2007, 166(11), 1259-1269. Ma, W.L.; Chapman, K. A systematic review of the effect of diet in prostate cancer prevention and treatment. J. Hum. Nutr. Diet, 2009, 22, 187–199. Hori, S.; Butler, E.; McLoughlin, J. Prostate cancer and diet: Food for thought? Brit. J. Urol. Int., 2011, 107(9), 1348-1359. Block, G.; Patterson, B.; Subar, A. Fruit, vegetables and cancer prevention: a review of the epidemiological evidence. Nutr. Cancer, 1992, 18, 1–29. Donaldson, M.S. Nutrition and cancer: A review of the evidence for an anti-cancer diet. Nutr. J., 2004, 3, 19, doi:10.1186/14752891-3-19. Key, T.J.; Schatzkin, A.; Willett, W.C.; Allen, N.E.; Spencer, E.A.; Travis, R.C. Diet, nutrition and the prevention of cancer. Public Health Nutr., 2004, 7(1A), 187-200. Klein, E.A.; Platz, E.A.; Thompson. I.M. Epidemiology, etiology and prevention of prostate cancer. In: Campbell-Walsh Urology, Wein, A.J.; Kavoussi, L.R.; Novick, A.C.; Partin, A.V.; Peters, C.A. Eds.; 9th ed. Philadelphia: Saunders Elsevier, 2007; pp. 28542873. http://www.mdconsult.com/das/book/body/208746819-6/0/ 1445/0.html. Accessed Aug. 4, 2011.
Sugar-Borate Esters – Potential Chemical Agents in Prostate Cancer 
           
De Marzo, A.M.; DeWeese, T.L.; Platz, E.A.; Meeker, A.K.; Nakayama, M.; Epstein, J.I. Pathological and Molecular Mechanisms of Prostate Carcinogenesis: Implications for Diagnosis, Detection, Prevention, and Treatment. J. Cell Biochem., 2004, 91, 459–477. Cheng, I.; Witte, J.S.; Jacobsen, S.J.; Haque, R.; Quinn, V.P.; Quesenberry, C.P. Prostatitis, Sexually Transmitted Diseases, and Prostate Cancer: The California Men's Health Study. PLoS ONE, 2010, 5(1), e8736. doi:10.1371/journal.pone.0008736. McNeal, J.E.; Bostwick, D.G. Intraductal dysplasia: a premalignant lesion of the prostate. Hum. Pathol., 1986, 17(1), 64-71. Kelloff, G.J.; Boone, C.W.; Steele, V.E.; Crowell, J.A.; Lubet, R.; Sigman. C.C. Progress in Cancer Chemoprevention: Perspectives on Agent Selection and Short-Term Clinical Intervention. Cancer Res., 1994, 54, 2015s-2024s. Nelson, W.G.; De Marzo, A.M.; Deweese, T.L.; Lin, X.; Brooks, J.D.; Putzi, M.J. Preneoplastic prostate lesions: an opportunity for prostate cancer prevention. Ann. N.Y. Acad. Sci., 2001, 952, 135-44. Epstein, J.I.; Herawi, M. Prostate needle biopsies containing prostatic intraepithelial neoplasia or atypical foci suspicious for carcinoma: implications for patient care. J. Urol., 2006, 175(3), 820-34. Thompson, I.M.; Pauler, D.K.; Goodman, P.J.; Tangen, C.M.; Lucia, M.S.; Parnes, H.L. Prevalence of prostate cancer among men with a prostate-specific antigen level ≤ 4.0 ng per milliliter. N .Engl. J. Med., 2004, 350, 2239-2246. Barranco, W.T.; Hudak, P.F.; Eckhert, C.D. Evaluation of ecological and in vitro effects of boron on prostate cancer risk (United States). Cancer Cause Control, 2007, 18, 71-77. Mahabir, S.; Spitz, M.R.; Barrera, S.L.; Dong, Y.Q.; Eastham, C.; Forman, M.R. Dietary Boron and Hormone Replacement Therapy as Risk Factors for Lung Cancer in Women. Am. J. Epidemiol., 2008, 167, 1070-108. Korkmaz, M.; Uzgo, E.; Bakirdere, S.; Aydin, F.; Ataman Y. Effects of dietary boron on cervical cytopathology and on micronucleus frequency in exfoliated buccal cells. Environ. Toxicol., 2007, 22, 17-25. Müezzinoğlu, T.; Korkmaz, M.; Neşe, N.; Bakırdere, S.; Arslan, Y.; Ataman, O.Y.; Lekili, M. Prevalence of Prostate Cancer in High Boron-Exposed Population: A Community-Based Study. Biol. Trace Elem. Res., 2011, 144(1-3), 49-57. Barranco, W.T.; Eckhert, C.D. Boric acid inhibits human prostate cancer cell proliferation. Cancer Lett., 2004, 216(1), 21-29. Barranco, W.T.; Eckhert, C.D. Cellular changes in boric acid treated DU-145 prostate cancer cells. Brit. J. Cancer, 2006, 94, 884-890. Elegbede, A.F. Mechanism of boric acid analog cytotoxicity in breast cancer cells, M.S. thesis, University of Nevada Las Vegas, United States, 2007. Meacham, S.; Karakas, S.; Walace, A.; Altun, F. Boron in human health evidence for dietary recomandations and public policies. The Open Mineral Processing Journal, 2010, 3, 36-53. Scorei, R.; Popa, R. Boron Containing Compounds as Preventive and Chemotherapeutic Agents for Cancer. Anti-Cancer Agents in Medicinal Chemistry, 2010, 10, 346-351. Nielsen, F.H.; Meacham, S.L. Growing Evidence for Human Health Benefits of Boron. JEBCAM, 2011, doi: 10.1177/ 2156587211407638. Adams, J. Proteasome inhibition in cancer: development of PS-341. Semin. Oncol., 2001, 28, 613-619. Albanell, J.; Adams, J. Bortezomib, a proteasome inhibitor, in cancer therapy: From concept to clinic. Drugs Fut., 2002, 27(11), 1079-1092. Ludwig, H.; Khayat, D.; Giaccone, G.; Facon, T. Proteasome inhibition and its clinical prospects in the treatment of hematologic and solid malignancies. Cancer, 2005, 104, 1794–807. Paramore, A.; Frantz, S. Bortezomib. NRD, 2003, 2, 611-612. Curran, M.P.; McKeage, K. Bortezomib: a review of its use in patients with multiple myeloma. Drugs, 2009, 69(7), 859-88. Piro, E.; Molica, S. A Systematic Review on the Use of Bortezomib in Multiple Myeloma Patients with Renal Impairment: What Is the Published Evidence? Acta Haematol.-Basel, 2011, 126, 163-168. Frankel, A.; Man, S.; Elliott, P.; Adams, J.; Kerbel, R.S. Lack of Multicellular Drug Resistance Observed in Human Ovarian and
Anti-Cancer Agents in Medicinal Chemistry, 2013, Vol. 13, No. 0
  
       
   
      
Prostate Carcinoma Treated with the Proteasome Inhibitor PS3411. Clin. Cancer Res., 2000, 6, 3719-3728. Harel, S.; Delarue, R.; Ribrag, V.; Dreyling, M.; Hermine, O. Treatment of younger patients with mantle cell lymphoma. Semin. Hematol., 2011, 48(3), 194-207. Kennedy, B.; Gargoum, F.; Bystricky, B.; Curran, D.R.; O'Connor, T.M. Novel agents in the management of lung cancer. Curr. Med. Chem., 2010, 17(35), 4291-325. Nawrocki, S.T.; Carew, J.S.; Pino, M.S.; Highshaw, R.A.; Andtbacka, R.H.; Dunner, K. Jr. Aggresome Disruption: A Novel Strategy to Enhance Bortezomib-Induced Apoptosis in Pancreatic Cancer Cells. Cancer Res., 2006, 66, 3773-3781. Wang, F.; Arun, P.; Friedman, J.; Chen, Z.; Waes. C.V. Current and Potential Inflammation Targeted Therapies in Head and Neck Cancer. Curr. Opin. Pharmacol., 2009, 9(4), 389–395. Garcıa-Gonzalez, M.; Mateo, P.; Bonilla, I. Boron requirement for envelope structure and function in Anabaena PCC 7119 heterocysts. J. Exp. Bot., 1991, 42, 925–929. Devirian, T.; Volpe, S. The physiological effects of dietary boron. Crit. Rev. Food Sci. Nutr., 2003, 43, 219-231. Cakmak, I.; Romheld, V. Boron deficiency-induced impairments of cellular functions in plant. Plant Soil, 1997, 193, 71–83. Curran, M.P.; McKeage, K. Bortezomib: a review of its use in patients with multiple myeloma. Drugs, 2009, 69(7), 859-88. Beddoe, A.H. Boron neutron capture therapy. Brit. J. Radiol., 1997, 70, 665-667. Endo, Y.; Yoshimi, T.; Miyaura, C. Boron clusters for medicinal drug design: Selective estrogen receptor modulators bearing carborane. Pure Appl. Chem., 2003, 75, 1197-1205. Schinazi, R.F.; Hurwitz, S.J.; Liberman, I.; Glazkova, Y.; Mourier, N.S.; Olson, J. Tissue disposition of 5-o-carboranyluracil--a novel agent for the boron neutron capture therapy of prostate cancer. Nucleos. Nucleot. Nucl., 2004, 23(1-2), 291-306. Hunt, C.D. Regulation of enzymatic activity: one possible role of dietary boron in higher animals and humans. Biol. Trace Elem. Res., 1998, 66, 205-225. Bradke, T.; Hall, C.; Stephen, W.; Carper, S.W.; Plopper, G.E. Phenylboronic acid selectively inhibits human prostate and breast cancer cell migration and decreases viability. Cell Adhes. Migr., 2008, 2(3), 153–160. Gallardo-Williams, M.T.; Maronpot, R.R.; Wine, R.N.; Brunssen, S.H.; Chapin, R.E. Inhibition of the enzymatic activity of prostatespecific antigen by boric acid and 3- nitrophenyl boronic acid. The Prostate, 2003, 54, 44-49. Henderson, K.; Stella, Jr. S.L.; Kobylewski, S.; Eckhert, C.D. Receptor activated Ca2+ release is inhibited by boric acid in prostate cancer cells. PLoS One, 2009, 4(6), 1-10. Lilja, H. Structure, function, and regulation of the enzyme activity of prostate-specific antigen. World J. Urol., 1993, 11(4), 188-191. Acerbo, A.S.; Miller, L. Assessment of the chemical changes induced in human melanoma cells by boric acid treatment using infrared imaging. Analyst, 2009, 134, 1669-1674. Scorei, R.; Ciubar, R.; Ciofrangeanu, C.M.; Mitran, V.; Cimpean, A.; Iordachescu, D. Comparative effects of boric acid and calcium fructoborate on breast cancer cells. Biol. Trace Elem. Res., 2008, 122, 197-205. Pache, W.; Zähner, H. Metabolic products of microorganisms. Arch. Microbiol., 1969, 67, 156-165. Arai, M.; Koizumi, Y.; Sato, H.; Kawabe, T.; Suganuma, M.; Kobayashi, K. Boromycin abrogates bleomycin- induced G2 checkpoint. J. Antibiot., 2004, 57, 662-668. T. Rezanka, T.; K. Sigler, K. Biologically active compounds of semi-metals Phytochemistry,2008, 69, 585–606. Gademann, K.; Portmann, C. Secondary metabolites from Cyanobacteria: Complex structures and powerful bioactivities. Curr. Org. Chem., 2008, 12, 326-341. Irschik, H.; Schummer, D.; Gerth, K.; Hofle, G.; Reichenbach, H. The tartrolons, new boron-containing antibiotics from a myxobacterium, Sorangium cellulosum. J. Antibiot., 1995, 48, 26-30. Scorei, R.; Rotaru, P. Calcium fructoborate-potential antiinflammatory agent. Biol. Trace Elem. Res., 2011, 143(3), 1223-38. Chapelle, S.; Vercere, J.F. 11B and 13C NMR determination of the structures of borate complexes of pentoses and related sugars, Tetrahedron, 1988, 44, 4469-4482.
8 Anti-Cancer Agents in Medicinal Chemistry, 2013, Vol. 13, No. 0  
  
   
  
Scorei, I.R. Calcium fructoborate: plant-based dietary boron as potential medicine for cancer therapy. Front. Biosci., 2011, S3, 205-215. Reyes-Izquierdo, T.; Nemzer, B.; Gonzalez, A.E.; Zhou, Q.; Argumedo, R.; Shu C et al. Short-term Intake of Calcium Fructoborate Improves WOMAC and McGill Scores and Beneficially Modulates Biomarkers Associated with Knee Osteoarthritis: A Pilot Clinical Double-blinded Placebo-controlled Study. Am. J. Biomed. Sci. 2012; DOI: 10.5099/aj120200111 Militaru, C.; Donoiu, I.; Craciun, A.; Scorei,I.D.; Bulearca, A.M.; Scorei, R.I. Oral resveratrol and calcium fructoborate supplementation in subjects with stable angina pectoris: Effects on lipid profiles, inflammation markers, and quality of life. Nutrition, 2012, http://dx.doi.org/10.1016/j.nut.2012.07.006 Scorei, R.; Mitrut, P.; Petrisor, I.; Scorei, ID. A double-blind, placebo-controlled pilot study to evaluate the effect of calcium fructoborate on systemic inflammation and dyslipidemia markers for middle-aged people with primary osteoarthritis. Biol. Trace. Elem. Res. 2011;144(1-3):253-263. McAuley, E.M.; Bradke, T.A.; Plopper, G.E. Phenylboronic acid is a more potent inhibitor than boric acid of key signaling networks involved in cancer cell migration. Cell Adhes. Migr., 2011, 5(5), 382-6. Miljkovic, D.; Scorei, I.R.; Cimpoiasu, V.M.; Scorei, I.D. Calcium fructoborate: plant based dietary boron for human nutrition. J. Diet. Suppl., 2009, 6, 211-226. Bachelier, N.; Verchere, J.F. Formation of neutral complexes of boric acid with 1,3-diols in organic solvents and in aqueous solution. Polyhedron., 1995, 14(13-14), 2009-2017. Bachelier, N.; Chappey, C.; Langevin, D.; Métayer, M.; Verchère, J.F. Facilitated transport of boric acid by 1,3-diols through supported liquid membranes. J. Membrane Sci., 1996, 119(2), 285294. Ouahid, S.; Metayer, M.; Langevin, D.; Labbe, M. Sorption of glucose by an anion-exchange membrane in the borate form. Stability of the complexes and the facilitated transport of glucose. J. Membrane Sci., 1996, 114, 13-25. Macheda, M.L.; Rogers, S.; Best, J.D. Molecular and cellular regulation of glucose transporter (GLUT) proteins in cancer. J. Cell Physiol., 2005, 202, 654-662. Barrett, M.P.; Walmsley, A.R.; Gould, G.W. Structure and function of facilitative sugar transporters. Curr. Opin. Cell Biol., 1999, 11(4), 496-502. Davis, H.B.; Mott, C.J.B. Interaction of boric acid and borates with carbohydrates and related substances. J. Chem. Soc., Faraday Trans., 1980, 76, 1991-2002. Sponer, J.E.; Sumpter, B.G.; Leszczynski, J.; Sponer, J.; FuentesCabrera, M. Theoretical study on the factors controlling the stability of the borate complexes of ribose, arabinose, lyxose, and xylose. Chemistry, 2008, 14, 9990-9998. Benoit, V.; Relic, B.; de Leval, X.; Chariot, A.; Merville, M.P.; Bours. V. Regulation of HER-2 oncogene expression by cyclooxygenase-2 and prostaglandin E2. Oncogene, 2004, 23, 1631–1635. Lin, W.W.; Karin, M. A cytokine-mediated link between innate immunity, inflammation, and cancer. J. Clin. Invest., 2007, 117(5), 1175–1183. Grivennikov, S.I.; Greten, F.L.; Karin, M. Immunity, Inflammation and Cancer. Cell, 2010, 140(6), 883–899. Murakami, M.; Austen, K.F.; Bingham III, C.O.; Friend, D.S.; Penrose, J.F.; Arm, J.P. Interleukin-3 regulates development of the 5-lipoxygenase/leukotriene C-4 synthase pathway in mouse mast cells. J. Biol. Chem., 1996, 270, 2653–22656. Sarveswaran, S.; Thamilselvan, V.; Brodie, C.; Ghosh, J. Inhibition of 5-lipoxygenase triggers apoptosis in prostate cancer cells via down-regulation of protein kinase C-epsilon. Biochim. Biophys. Acta, 2011, 1813(12), 2108-2117. Hunt, C.D. Regulation of enzymatic activity: one possible role of dietary boron in higher animals and humans. Biol. Trace Elem. Res., 1998, 66, 205-225. Armstrong, T.A.; Spears, J.W. Effect of boron supplementation of pig diets on the production of tumor necrosis factor-a and interferon-g. J. Anim. Sci., 2003, 81, 2552-2561.
Scorei and Popa 
  
    
Scorei, I.R.; Ciofrangeanu, C.; Ion, R.; Cimpean, A.; Galateanu, B.; Mitran, V. In vitro effects of calcium fructoborate upon production of inflammatory mediators by LPS-stimulated RAW 264.7 macrophages. Biol. Trace Elem. Res., 2010, 135, 334-344. Scorei, R.; Cimpoiasu, V.M.; Iordachescu, D. In vitro evaluation of the antioxidant activity of calcium fructoborate. Biol. Trace Elem. Res., 2005, 107, 127-134. Scorei, R.; Ciubar, R.; Iancu, C.; Mitran, V.; Cimpean, A.; Iordachescu, D. In vitro effects of calcium fructoborate on fMLPstimulated human neutrophil granulocytes. Biol. Trace Elem. Res., 2007, 118, 27-37. Nielsen, F.H. Dietary fat composition modifies the effect of boron on bone characteristics and plasma lipids in rats. Biofactors, 2004, 20(3), 1-71. Nielsen, F.H.; Penland, J.G. Boron deprivation alters rat behavior and brain mineral composition differently when fish oil instead of safflower oil is the diet fat source. Nutr. Neurosci., 2006, 9(1-2), 105-112. Calder, P.C. N:3 Polyunsaturated fatty acids, inflammation, and inflammatory diseases. Am. J. Clin. Nutr., 2006, 83(suppl), 1505S– 1519S. Park, M.; Li, Q.; Shcheynikov, N.; Muallen, S.; Zeng, W. NaBC1 is a ubiquitous electrogenic Na+-coupled borate transporter essential for cellular boron homeostasis and cell growth and proliferation. Mol. Cell, 2004, 16(3), 331-341. EPA. Health effects support document for boron, United States Environmental Protection Agency Office of Water, 4304T, EPA Document number EPA-822-R-08-002, 2008. SSCS. Scientific Committee on Consumer Safety, SSCS/1249/09, Directorate General for Health and Consumers, European Commision, 2010. Liao, S.F.; Monegue, J.S.; Lindemann, M.D.; Cromwell, G.L.; Matthews, J.C. Dietary supplementation of boron differentially alters expression of borate transporter (NaBC1) mRNA by jejun and kidney of growing pigs. Biol. Trace Elem. Res., 2011, 143(2), 901-912. Sutton, T.; Baumann, U.; Hayes, J.; Collins, N.C.; Shi, B.J.; Schnurbusch, T. Boron-toxicity tolerance in barley arising from efflux transporter amplification. Science, 2007, 318(5855), 14461449. Raven, J.A. Short- and long-distance transport of boric acid in plants. New Phytol., 1980, 84, 231–249. Dordas, C.; Brown, P.H. Permeability of boric acid across lipid bilayers and factors affecting it. J. Membrane Biol., 2000, 175, 95– 105. Tanaka, M.; Fujiwara, T...Physiological roles and transport mechanisms of boron: perspectives from plants. Pflugers Arch – Eur. J. Physiol., 2008, 456, 671–677. Miwa, K.; Takano, J.; Omori, H.; Seki, M.; Shinozaki, K.; Fujiwara, T. Plants tolerant of high boron levels. Science, 2007, 318, 1417. Kaya, A.; Karakaya, H.C.; Fomenko, D.E.; Gladyshev, V.N.; Koc, A. Identification of a Novel System for Boron Transport: Atr1 Is a Main Boron Exporter in Yeast. Mol. Cell.Biol., 2009, 29(13), 3665–3674. Uluisik I, Kaya A, Fomenko DE, Karakaya HC, Carlson BA, et al. Boron Stress Activates the General Amino Acid Control Mechanism and Inhibits Protein Synthesis. PLoS ONE 2011, 6(11): e27772. doi:10.1371/journal.pone.0027772 Gabel, S.A.; London, R.E. Ternary borate-nucleoside complex stabilization by ribonuclease A demonstrates phosphate mimicry. J. Biol. Inorg. Chem., 2008, 13(2), 207-211. Sugiyama, M.; Hong, Z.; Greenberg, W.A.; Wong, C.H. In vivo selection for the directed evolution of L-rhamnulose aldolase from L-rhamnulose-1-phosphate aldolase (RhaD). Bioorg. Med. Chem., 2007, 15(17), 5905–5911. Calvo, M.B.; Figueroa, A.; Pulido, E.G.; Campelo, R.G.; Aparicio, L.A. Potential Role of Sugar Transporters in Cancer and Their Relationship with Anticancer Therapy. Int. J. Endocrinol., 2010, 2010, 205-357. Garland, C.F.; Garland, F.C.; Gorham, E.D.; Lipkin, M.; Newmark, H.; Mohr, S.B. The role of vitamin D in cancer prevention. Am. J. Public Health, 2006, 96(2), 252-256.
Sugar-Borate Esters – Potential Chemical Agents in Prostate Cancer       
   
   
  
Schwartz, G.G. Vitamin D and intervention trials in prostate cancer: from theory to therapy. Ann. Epidemiol., 2009, 19(2), 96102. Moffatt, K.A.; Johannes, W.U.; Miller, G.J. 1a,25-Dihydroxyvitamin D3 and Platinum Drugs Act Synergistically to Inhibit the Growth of Prostate Cancer Cell Lines. Clin. Cancer Res., 1999, 5, 695–703. Stewart, L.V.; Weigel, N.L. Vitamin D and Prostate Cancer. Exp. Biol. Med., 2004, 229, 4277-4284. Chen, T.C.; Holick, M.F. Vitamin D and prostate cancer prevention and treatment. Trends Endocrin. Met., 2003, 14(9), 423-30. Ingraham, B.A.; Bragdon, B.; Nohe, A. Molecular basis of the potential of vitamin D to prevent cancer. Curr. Med. Res. Opin., 2008, 24, 139–149. Donkena, K.V.; Karnes, R.J.; Young, C.Y.F. Vitamins and Prostate Cancer Risk. Molecules, 2010, 15, 1762-1783. Nickerson, T.; Huynh, H. Vitamin D analogue EB1089-induced prostate regression is associated with increased gene expression of insulin-like growth factor binding proteins. J. Endocrinol., 1999, 160, 223–229. Moreno, J.; Krishnan, A.V.; Feldman, D. Molecular mechanisms mediating the anti-proliferative effects of Vitamin D in prostate cancer. J. Steroid Biochem., 2005, 97, 31–36. Scorei, I.R. Boron Compounds in the Breast Cancer Cells Chemoprevention and Chemotherapy. In: Breast Cancer - Current and Alternative Therapeutic Modalities, Esra Gunduz and Mehmet Gunduz, Eds.; ISBN 978-953-307-776-5, Hard Cover, InTech Publication, November 2011; pp. 540. Miljkovic, D.; Miljkovic, N.; McCarty, M.F. Up-regulatory impact of boron on vitamin D function does it reflect inhibition of 24hydroxylase? Med. Hypotheses, 2004, 63(6), 1054-1056. Goldbach, H.E.; Wimmer, M.A. Boron in plants and animals: Is there a role beyond cell-wall structure? J. Plant Nutr. Soil Sci., 2007, 170, 39–48. Colak, S.; Geyikoglu, F.; Keles, O.N.; Türkez, H.; Topal, A.; Unal, B. The neuroprotective role of boric acid on aluminum chlorideinduced neurotoxicity. Toxicol. Ind. Health, 2011, 27(8), 700-710. Groll, M.; Berkers, C.R.; Ploegh, H.L.; Ovaa, H. Crystal structure of the boronic acid-based proteasome inhibitor bortezomib in complex with the yeast 20S proteasome. Structure, 2006, 14(3) 451-6. Driscoll, J.J.; Burris, J.; Annunziata, C.M. Targeting the Proteasome With Bortezomib in Multiple Myeloma: Update on Therapeutic Benefit as an Upfront Single Agent, Induction Regimen for Stem-Cell Transplantation and as Maintenance Therapy. Am. J. Ther., 2012, 19(2), 133–144. Papandreou, C.N.; Logothetis, C.J. Bortezomib as a potential treatment for prostate cancer. Cancer Res., 2004, 64(15), 5036-43. Whang, P.G.; Gamradt, S.C.; Gates, J.J.; Lieberman, J.R. Effects of the proteasome inhibitor bortezomib on osteolytic human prostate cancer cell metastases. Prostate Cancer P.D., 2005, 8, 327–334. Voortman, J.; Chęcińska, A.; Giaccone, G. The proteasomal and apoptotic phenotype determine bortezomib sensitivity of non-small cell lung cancer cells. Molecular Cancer, 2007, 6, 73. Scorei R. Regulation of therapeutic potential of boron-containing compounds. In: Encyclopedia of Metalloproteins, Kretsinger, Robert H.; Uversky, Vladimir N.; Permyakov, Eugene A. (Eds.) 2013, 1400 p. 100 illus. Strittmatter, P. Reversible direct hydrogen transfer from reduced pyridine nucleotides to cytocrome b5 reductase. J. Biol. Chem., 1964, 239, 3043-3050. Nzietchueng, R.M.; Dousset, B.; Franck, P.; Benderdour, M.; Nabet, P.; Hess, K. Mechanisms implicated in the effects of boron on wound healing. J. Trace Elem. Med. Biol., 2002, 16(4), 239-244. Hunt, C.D. Boron-binding biomolecules: a key to understanding the beneficial physiologic effects of dietary boron from prokaryotes to humans. In: Boron in Plant and Animal Nutrition, Goldbach,
Received: August 03, 2012
Revised: December 05, 2012
Accepted: December 06, 2012
Anti-Cancer Agents in Medicinal Chemistry, 2013, Vol. 13, No. 0
   
  
   
  
H.E., Rerkasem, B., Wimmer, M.A., Brown, P.H. Thellier, M., Bell, R.W., Eds.; New York, NY:Kluwer Academic Publishers, 2002, pp.21-36. Tate, S.S.; Meister, A. Serine-borate complex as a transition-state inhibitor of g-glutamyl transpeptidase. P. Natl. Acad. Sci. USA, 1978, 75, 4806–4809. Hunt, C.D. Regulation of enzymatic activity: one possible role of dietary boron in higher animals and humans. Biol. Trace Elem. Res., 1998, 66(1-3), 205-25. Kaup, Y.; Schmid, M.; Middleton, A.; Weser, U. Borate in mummification salts and bones from Pharaonic Egypt. J. Inorg. Biochem., 2003, 94(3), 214-20. Farooqui, H.; Mahmood, A.; Jairajpuri, D.S.; Ahmad, F.; Ali, S. Boron increases the transition temperature and enhances thermal stability of heme proteins. J. Therm. Anal. Calorim., 2011, 104(1), 339-342. Ali, S.; Farooqi, H.; Prasad, R.; Naime, M.; Routray, I.; Yadav, S.; Ahmad, F. Boron stabilizes peroxide mediated changes in the structure of heme proteins. Int. J. Biol. Macromol., 2010, 47(2), 109-115. Taler, G.; Navon, G.; Becker, O.M. The interaction of borate ions with cytochrome c surface sites: a molecular dynamics study. Biophys. J., 1998, 75(5), 2461–2468. Taler, G.; Eliav, U.; Navon, G. Detection and characterization of boric acid and borate ion binding to cytochrome c using multiple quantum filtered NMR. J. Magn. Reson., 1999, 141(2), 228-238. Meacham, S.L.; Taper, L.J.; Volpe, S.L. Effect of boron supplementation on blood and urinary calcium, magnesium, and phosphorus, and urinary boron in athletic and sedentary women. Am. J. Clin. Nutr., 1995, 61(2), 341-345. Camacho-Cristobal, J.J.; Rexach, J.; Gonzalez-Fontes, A. Boron in Plants: Deficiency and Toxicity. J. Integr. Plant Biol., 2008, 50(10), 1247-55. Noor, Z.; Sumitro, S.B.; Hidayat, M.; Rahim, A.H.; Sabarudin, A.; Umemura, T. Atomic Mineral Characteristics of Indonesian Osteoporosis by High-Resolution Inductively Coupled Plasma Mass Spectrometry. Scientific World Journal, 2012, doi:10.1100/2012/372972. Nzietchueng, R.M.; Dousset, B.; Franck, P.; Benderdour, M.; Nabet, P.; Hess, K. Mechanisms implicated in the effects of boron on wound healing. J. Trace Elem. Med. Biol., 2002, 16(4), 239-44. EPA. Health effects support document for boron, United States Environmental Protection Agency Office of Water, 4304T, EPA Document number EPA-822-R-08-002, 2008. SSCS. Scientific Committee on Consumer Safety, SSCS/1249/09, Directorate General for Health and Consumers, European Commission, 2010. EFSA. Opinion of the scientific panel on dietetic products, nutrition and allergies on the request from the commision related to the tolerable upper intake level of boron (Sodium Borate and Boric Acid). EFSA Journal, 2004, 80, 1-22. Weir, R.J.; Fisher, F.S. Toxicologic studies on borax and boric acid. Toxicol. Appl. Pharm., 1972, 23, 351-364. Moore, J.A. An assessment of boric acid and borax using the IEHR evaluative process for assessing human developmental and reproductive toxicity of agents. Reprod Toxicol, 1997, 11(1), 123160. Baker, S.J.; Tomsho, J.W.; Benkovic, S.J. Boron-containing inhibitors of synthetases. Chem. Soc. Rev., 2011, 40(8), 4279-4285. Pongsavee, M. Effect of borax on immune cell proliferation and sister chromatid exchange in human chromosomes. J. Occup. Med. Toxicol., 2009, 4, 27. Meacham, S.; Elwell, K.; Ziegler, S.; Carper, S. Boric Acid Inhibits Cell Growth in Breast and Prostate Cancer Cell Lines. Advances in Plant and Animal Boron Nutrition, 2007, Part II, 299-306, doi: 10.1007/978-1-4020-5382-5_29.