2Department of Research and Postgraduate in Food,
© The Author(s) 2020. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, sharing, adaptation, distribution and reproduction in any medium or format, for any purpose, even commercially, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
Although prostate epithelium concentrates zinc for the purpose of promoting citrate secretion, it loses its capacity to import zinc while undergoing malignant transformation. This exclusion of zinc may be necessary for the viability of prostate cancer, as measures which increase the intracellular zinc content of prostate cancers lead to cell death, oxidative stress, and a marked reduction in ATP, suggestive of mitochondrial damage. The anti-fungal drug clioquinol, which can act as a zinc ionophore, can markedly slow the growth of human prostate cancer in nude mice, and has been proposed as a clinical therapy for prostate cancer. However, clioquinol is currently only available as a topical agent, as it was linked to subacute myelo-optic neuropathy with oral use. A more practical option for promoting zinc transport may be offered by the nutraceutical zinc dipicolinate, a stable chelate in which four coordination positions of zinc are occupied by two molecules of the tryptophan metabolite picolinic acid. Zinc dipicolinate is a highly effective supplemental source of zinc that has been shown to be more potent than soluble zinc salts for alleviating the symptoms of acrodermatitis enteropathica, a genetic zinc deficiency disorder reflecting homozygous loss of functional ZIP4 zinc importers in enterocytes. This suggests that the zinc dipicolinate complex is sufficiently stable and lipophilic to transfer zinc across cellular membranes. If so, it may have potential for “smuggling” zinc into prostate cancer cells. Hence, cell culture and rodent studies to evaluate the impact of zinc dipicolinate on human prostate cancer are warranted.
Prostate cancer, zinc, clioquinol, picolinic acid, ZIP4
Prostate epithelium is characterized by high intracellular levels of zinc, particularly within the mitochondria. This intra-mitochondrial zinc is believed to promote the proper function of prostate epithelium by inhibiting aconitase activity, thereby causing an accumulation of citrate in the Krebs cycle[2,3]. Much of this citrate is exported into the seminal fluid, where it serves as an energy substrate for spermatozoa.
However, malignantly transformed prostate epithelium is far lower in intracellular zinc, reflected greatly diminished expression or activity of transporter proteins - ZIP1, ZIP2, and ZIP3 - that import zinc[4-8]. This loss of intracellular zinc appears to be essential to the viability of the transformed cells, as measures which restore high intracellular zinc levels - exposure to high extracellular zinc, or treatment with zinc ionophores such as pyrithione or clioquinol - slows their proliferation and up-regulates cell death[8-11]. In vivo, continual intravenous infusion of zinc, injection of zinc acetate directly into tumors, or parenteral administration of the zinc ionophore clioquinol has notably slowed the growth of human prostate cancers in nude mice[12-14]. In particular, administration of clioquinol was associated with an 85% growth retardation of a ZIP-1- deficient human prostate cancer.
In a range of human prostate cancer cells lines, increasing intracellular zinc with zinc pyrithione led to necrotic cell death associated with plummeting ATP levels, oxidative stress, and activation of ERK and PKC. The antioxidants N-acetylcysteine (NAC) and trolox protected against cell death in this system; NAC, but not trolox, likewise blunted the decline in ATP. Since prostate epithelium tends to concentrate zinc in mitochondria, it would be of interest to know whether excessive zinc uptake by mitochondria mediates the oxidative stress and reduction in ATP seen after prostate cancer cells are exposed to zinc pyrithione. In addition to inhibiting aconitase activity, zinc is also capable of inhibiting complex III of the respiratory chain, with a Ki of about 100 nmol/L[15-18].
Could malignant transformation of prostate epithelium somehow sensitize their mitochondria to the toxic impact of excessive zinc? The mitochondria of cancer cells are prone to structural abnormalities - possibly reflecting mutations in mitochondrial or nuclear DNA - which increase their propensity to produce superoxide[19,20]. Defects of the mitochondrial respiratory chain or of ATP synthase activity that moderately boost mitochondrial superoxide generation can be expected to promote cellular proliferation, angiogenesis, and mutagenesis; hence, they may act as tumor promoters, in which case these defects would be selected for[20-23]. The exceptionally high mitochondrial zinc levels of prostate epithelium presumably reflect increased expression or activity not only of ZIP1, but also of one or more zinc transporters -possibly ZnT2 - which import zinc into the mitochondrial inner matrix. In mammary epithelial cells, ZnT2 transports zinc into mitochondria, and over-expression of this protein lowers cellular ATP levels and oxygen consumption, and promotes apoptosis; oxidant production was not measured in this study.
If this increased intramitochondrial transport of zinc is maintained in transformed prostatic epithelial cells, then high mitochondrial zinc levels might interact with the mitochondrial abnormalities typical of cancer to induce severe dysfunction: excessive production of superoxide, decreased production of ATP, and further mitochondrial structural damage. This sequence of events could evidently be prevented by down-regulation of ZIP1 - which is what in fact is observed in transformed prostate epithelium.
In light of the utility of parenteral clioquinol for controlling growth of a prostate cancer in nude mice, it has been suggested that oral clioquinol could have potential as a therapeutic alternative for prostate cancer control. While it might indeed be the case that some sufficiently modest dose of clioquinol could prove useful in this regard, past clinical experience with oral administration of clioquinol as a fungicide or as a treatment for acrodermatitis enteropathica has been complicated by its association with subacute myelo-optic neuropathy, characterized by peripheral neuropathy and blindness[25,26]. Ten thousand patients in Japan were afflicted with this syndrome until oral use of clioquinol was discontinued in Japan. Hence, clioquinol is now available solely for topical use. The zinc-clioquinol chelate has been shown to lead to rapid mitochondrial damage and loss of mitochondrial membrane potential in a melanoma-derived cell line, possibly explaining the clinical toxicity of clioquinol.
However, an alternative strategy for boosting the intracellular zinc levels of clinical prostate cancer may be at hand. Zinc dipicolinate is a readily-available nutraceutical, originally patented by the U.S. Department of Agriculture, in which zinc is chelated by two molecules of the natural tryptophan metabolite picolinic acid; 4 coordination positions of zinc are occupied by picolinic acid in this complex. There is reason to suspect that, at least at neutral pH, zinc dipicolinate is sufficiently stable to carry zinc across bilipid layers. When children with acrodermatitis enteropathica (AE) were treated with either zinc dipicolinate or zinc sulfate, the dose of zinc required to prevent exacerbations of this disorder was found to be one-third as high with zinc dipicolinate, as opposed to zinc sulfate. AE is a hereditary zinc deficiency syndrome in which those afflicted are heterozygous for loss of function of ZIP4, the chief zinc importer expressed by the apical membranes of enterocytes[29,30]. The superior utility of zinc dipicolinate in this syndrome, as opposed to forms of zinc that ionize readily (such as zinc sulfate), seems likely to reflect the ability of the zinc dipicolinate chelate to carry zinc across enterocyte membranes in the absence of zinc transporter proteins. Furthermore, in healthy human subjects, when zinc was administered at 50 mg daily as either zinc dipicolinate, zinc citrate, or zinc gluconate, zinc dipicolinate was shown to have a significantly greater impact on zinc levels in erythrocytes, hair, and urine. When nursing rat mothers were fed zinc as either dipicolinate or acetate, the zinc content of the kidney or liver of nursing pups was higher after the dipicolinate supplement.
If zinc dipicolinate is sufficiently stable and lipophilic to “smuggle” zinc into enterocytes lacking ZIP4, might it not also be able transport zinc into prostate cancer cells lacking ZIP1 activity? This possibility could be readily tested in prostate cancer cell cultures and, if preliminary results are promising, in nude mice xenografted with human prostate cancer. The possibility that zinc dipicolinate supplementation might also have potential for prevention of prostate cancer might also be envisioned, as reduction in intracellular zinc is believed to arise at an early stage of prostate cancer evolution.
While therapies which boost intracellular zinc in prostate cancer might at best be expected to slow prostate cancer progression, the fact that such therapy might boost oxidative stress and lower ATP levels in prostate cancer cells raises the possibility that preceding zinc therapy might render prostate cancer more sensitive to hyperthermia and/or high-dose intravenous ascorbate. The selective susceptibility of cancer cells to high extracellular levels of ascorbate - which generate a high flux of hydrogen peroxide into the these cells - may reflect increased cancer production of superoxide, which can interact with hydrogen peroxide in a transition metal-catalyzed reaction to generate deadly hydroxyl radicals[34,35]. And the lethality of whole body-tolerable hyperthermia (42 °C) to cancer cells may be potentiated by hydrogen peroxide; conversely, overexpression of mitochondrial superoxide dismutase protects a prostate cancer cell line from 43 °C hyperthermia[36-39]. Mitochondrial superoxide production by zinc-treated cancer cells might be potentiated by concurrent treatment with dichloroacetate, which can increase the availability of pyruvate to mitochondria by inhibiting pyruvate dehydrogenase kinase; the latter is highly active in many cancers owing to up-regulated hypoxia-inducible factor-1 activity[34,40,41].
Conceived and wrote the first draft: McCarty MF
Provided suggestions that were incorporated into the final manuscript: Iloki-Assanga S, Lujan LLAvailability of data and materials
Not applicable.Financial support and sponsorship
None.Conflicts of interest
All authors declared that there is no conflict of interest.Ethical approval and consent to participate
Not applicable.Consent for publication
© The Author(s) 2020.
1. Liu Y, Franklin RB, Costello LC. Prolactin and testosterone regulation of mitochondrial zinc in prostate epithelial cells. Prostate 1997;30:26-32.DOIPubMed
2. Costello LC, Liu Y, Franklin RB, Kennedy MC. Zinc inhibition of mitochondrial aconitase and its importance in citrate metabolism of prostate epithelial cells. J Biol Chem 1997;272:28875-81.DOIPubMed
3. Costello LC, Franklin RB, Liu Y, Kennedy MC. Zinc causes a shift toward citrate at equilibrium of the m-aconitase reaction of prostate mitochondria. J Inorg Biochem 2000;78:161-5.DOIPubMed
4. Franklin RB, Milon B, Feng P, Costello LC. Zinc and zinc transporters in normal prostate and the pathogenesis of prostate cancer. Front Biosci 2005;10:2230-9.DOIPubMed PMC
5. Franklin RB, Feng P, Milon B, Desouki MM, Singh KK, et al. hZIP1 zinc uptake transporter down regulation and zinc depletion in prostate cancer. Mol Cancer 2005;4:32.DOIPubMed PMC
6. Desouki MM, Geradts J, Milon B, Franklin RB, Costello LC. hZip2 and hZip3 zinc transporters are down regulated in human prostate adenocarcinomatous glands. Mol Cancer 2007;6:37.DOIPubMed PMC
7. Zou J, Milon BC, Desouki MM, Costello LC, Franklin RB. hZIP1 zinc transporter down-regulation in prostate cancer involves the overexpression of ras responsive element binding protein-1 (RREB-1). Prostate 2011;71:1518-24.DOIPubMed PMC
8. Costello LC, Franklin RB, Zou J, Feng P, Bok R, et al. Human prostate cancer ZIP1/zinc/citrate genetic/metabolic relationship in the TRAMP prostate cancer animal model. Cancer Biol Ther 2011;12:1078-84.DOIPubMed PMC
9. Liang JY, Liu YY, Zou J, Franklin RB, Costello LC, et al. Inhibitory effect of zinc on human prostatic carcinoma cell growth. Prostate 1999;40:200-7.DOIPubMed PMC
10. Carraway RE, Dobner PR. Zinc pyrithione induces ERK- and PKC-dependent necrosis distinct from TPEN-induced apoptosis in prostate cancer cells. Biochim Biophys Acta 2012;1823:544-57.DOIPubMed
11. Hong SH, Choi YS, Cho HJ, Lee JY, Kim JC, et al. Antiproliferative effects of zinc-citrate compound on hormone refractory prostate cancer. Chin J Cancer Res 2012;24:124-9.DOIPubMed PMC
12. Feng P, Li TL, Guan ZX, Franklin RB, Costello LC. Effect of zinc on prostatic tumorigenicity in nude mice. Ann N Y Acad Sci 2003;1010:316-20.DOIPubMed
13. Shah MR, Kriedt CL, Lents NH, Hoyer MK, Jamaluddin N, et al. Direct intra-tumoral injection of zinc-acetate halts tumor growth in a xenograft model of prostate cancer. J Exp Clin Cancer Res 2009;28:84.DOIPubMed PMC
14. Franklin RB, Zou J, Zheng Y, Naslund MJ, Costello LC. Zinc ionophore (clioquinol) inhibition of human ZIP1-deficient prostate tumor growth in the mouse ectopic xenograft model: a zinc approach for the efficacious treatment of prostate cancer. Int J Cancer Clin Res 2016;3:37.DOIPubMed PMC
15. Link TA, von Jagow G. Zinc ions inhibit the QP center of bovine heart mitochondrial bc1 complex by blocking a protonatable group. J Biol Chem 1995;270:25001-6.DOIPubMed
16. Sensi SL, Yin HZ, Carriedo SG, Rao SS, Weiss JH. Preferential Zn2+ influx through Ca2+-permeable AMPA/kainate channels triggers prolonged mitochondrial superoxide production. Proc Natl Acad Sci U S A 1999;96:2414-9.DOIPubMed PMC
17. Park YH, Bae HC, Kim J, Jeong SH, Yang SI, et al. Zinc oxide nanoparticles induce HIF-1alpha protein stabilization through increased reactive oxygen species generation from electron transfer chain complex III of mitochondria. J Dermatol Sci 2018;91:104-7.DOIPubMed
18. Lorusso M, Cocco T, Sardanelli AM, Minuto M, Bonomi F, et al. Interaction of Zn2+ with the bovine-heart mitochondrial bc1 complex. Eur J Biochem 1991;197:555-61.DOIPubMed
19. Aykin-Burns N, Ahmad IM, Zhu Y, Oberley LW, Spitz DR. Increased levels of superoxide and H2O2 mediate the differential susceptibility of cancer cells versus normal cells to glucose deprivation. Biochem J 2009;418:29-37.DOIPubMed PMC
20. Wallace DC. Mitochondria and cancer. Nat Rev Cancer 2012;12:685-98.DOIPubMed PMC
21. Woo DK, Green PD, Santos JH, D’Souza AD, Walther Z, et al. Mitochondrial genome instability and ROS enhance intestinal tumorigenesis in APC(Min/+) mice. Am J Pathol 2012;180:24-31.DOIPubMed PMC
22. Copeland WC, Wachsman JT, Johnson FM, Penta JS. Mitochondrial DNA alterations in cancer. Cancer Invest 2002;20:557-69.DOIPubMed
23. Ježek J, Cooper KF, Strich R. Reactive oxygen species and mitochondrial dynamics: the Yin and Yang of mitochondrial dysfunction and cancer progression. Antioxidants (Basel) 2018;7:13.DOIPubMed PMC
24. Seo YA, Lopez V, Kelleher SL. A histidine-rich motif mediates mitochondrial localization of ZnT2 to modulate mitochondrial function. Am J Physiol Cell Physiol 2011;300:C1479-89.DOIPubMed PMC
25. Perez DR, Sklar LA, Chigaev A. Clioquinol: to harm or heal. Pharmacol Ther 2019;199:155-63.DOIPubMed PMC
26. Katsuyama M, Iwata K, Ibi M, Matsuno K, Matsumoto M, et al. Clioquinol induces DNA double-strand breaks, activation of ATM, and subsequent activation of p53 signaling. Toxicology 2012;299:55-9.DOIPubMed
27. Arbiser JL, Kraeft SK, van Leeuwen R, Hurwitz SJ, Selig M, et al. Clioquinol-zinc chelate: a candidate causative agent of subacute myelo-optic neuropathy. Mol Med 1998;4:665-70.PubMed PMC
28. Krieger I, Cash R, Evans GW. Picolinic acid in acrodermatitis enteropathica: evidence for a disorder of tryptophan metabolism. J Pediatr Gastroenterol Nutr 1984;3:62-8.DOIPubMed
29. Andrews GK. Regulation and function of Zip4, the acrodermatitis enteropathica gene. Biochem Soc Trans 2008;36:1242-6.DOIPubMed PMC
30. Wang X, Zhou B. Dietary zinc absorption: a play of Zips and ZnTs in the gut. IUBMB Life 2010;62:176-82.DOIPubMed
31. Barrie SA, Wright JV, Pizzorno JE, Kutter E, Barron PC. Comparative absorption of zinc picolinate, zinc citrate and zinc gluconate in humans. Agents Actions 1987;21:223-8.DOIPubMed
32. Evans GW, Johnson EC. Zinc concentration of liver and kidneys from rat pups nursing dams fed supplemented zinc dipicolinate or zinc acetate. J Nutr 1980;110:2121-4.DOIPubMed
33. Costello LC, Franklin RB. Decreased zinc in the development and progression of malignancy: an important common relationship and potential for prevention and treatment of carcinomas. Expert Opin Ther Targets 2017;21:51-66.DOIPubMed PMC
34. McCarty MF, Contreras F. Increasing superoxide production and the labile iron pool in tumor cells may sensitize them to extracellular ascorbate. Front Oncol 2014;4:249.DOIPubMed PMC
35. Ranzato E, Biffo S, Burlando B. Selective ascorbate toxicity in malignant mesothelioma: a redox Trojan mechanism. Am J Respir Cell Mol Biol 2011;44:108-17.DOIPubMed
36. Lord-Fontaine S, Averill DA. Enhancement of cytotoxicity of hydrogen peroxide by hyperthermia in chinese hamster ovary cells: role of antioxidant defenses. Arch Biochem Biophys 1999;363:283-95.DOIPubMed
37. Lord-Fontaine S, Averill-Bates DA. Heat shock inactivates cellular antioxidant defenses against hydrogen peroxide: protection by glucose. Free Radic Biol Med 2002;32:752-65.DOIPubMed
38. Razavi R, Harrison LE. Thermal sensitization using induced oxidative stress decreases tumor growth in an in vivo model of hyperthermic intraperitoneal perfusion. Ann Surg Oncol 2010;17:304-11.DOIPubMed
39. Venkataraman S, Wagner BA, Jiang X, Wang HP, Schafer FQ, et al. Overexpression of manganese superoxide dismutase promotes the survival of prostate cancer cells exposed to hyperthermia. Free Radic Res 2004;38:1119-32.DOIPubMed
40. McFate T, Mohyeldin A, Lu H, Thakar J, Henriques J, et al. Pyruvate dehydrogenase complex activity controls metabolic and malignant phenotype in cancer cells. J Biol Chem 2008;283:22700-8.DOIPubMed PMC
41. Hur H, Xuan Y, Kim YB, Lee G, Shim W, et al. Expression of pyruvate dehydrogenase kinase-1 in gastric cancer as a potential therapeutic target. Int J Oncol 2013;42:44-54.DOIPubMed PMC
McCarty MF, Assanga SI, Lujan LL. Could zinc dipicolinate be used to “smuggle” zinc into prostate cancer cells?. J Cancer Metastasis Treat 2020;6:20. http://dx.doi.org/10.20517/2394-4722.2020.47