Selenium in Oncology

12 Selenium in Oncology

G. N. Schrauzer

Introduction

Selenium is an essential trace element recognized as a cancer-protective agent and is increasingly employed as an adjuvant in cancer therapy. Whereas for cancer prevention organic nutritional forms of selenium (Se) are used, sodium selenite is the preferred form of selenium for therapeutic applications. Sodium selenite is administered primarily to reduce side effects of chemotherapy and radiation therapy. Patients are typically receiving 300–1000 μg Se/day as sodium selenite orally or by infusion for one to five days prior to and during chemotherapy or radiation, and subsequently oral doses of 100–300 μg Se/day for maintenance. Sodium selenite is also used in conjunction with biological therapies and in the management of secondary or postoperative lymphedema.

Selenium in Cancer Prevention

Epidemiological and Case–Control Studies

Evidence for cancer protective properties of selenium was first obtained through comparisons of U.S. cancer mortalities in low and high selenium regions and by correlating cancer mortalities in different countries with dietary selenium intake parameters (1–5). The cancer-protective properties of selenium were subsequently demonstrated in animal studies, as well as in vitro with various tumor cell lines. Further supporting evidence for cancer-protecting effects of selenium was obtained through case-control studies (6–11) which, for the most part, demonstrated that low serum or plasma selenium levels, or, in later studies, levels of selenoprotein P (12, 13), were indicative of increased cancer risk. Other workers using toenail selenium levels as indices of selenium status reached similar conclusions for cancers of lung (14), stomach (15), and invasive prostate cancer (16).

Effects of Selenium-Antagonistic Elements

The cancer-protecting effects of selenium are counteracted by selenium-antagonistic elements that may be found in foods, the drinking water, or the environment (17–23). Some of these elements, arsenic (As), lead, mercury, and cadmium, for example, are known to inhibit selenium-dependent enzymes, impede selenium uptake, or to form unreactive selenides accumulating in organs and tissues, while others counteract the cancer-protective effects of selenium indirectly by stimulating oxygen radical production. Accordingly, the selenium requirement increases in the presence of such elements. These findings are relevant to occupational medicine: A Swedish study (24) revealed that lung tissues from foundry workers who had died from lung and other types of cancer exhibit much higher arsenic/selenium ratios than those from foundry workers who had died from heart disease or accidents. Higher arsenic/selenium ratios were observed in the scalp hair of miners of a tin mine in China who developed lung cancer than those who did not (25). This prompted studies to reduce the cancer risk of exposed workers by means of selenium. In one such study (26), supplemental selenium at 300 ug Se/day increased blood and hair selenium as well as plasma glutathione peroxidase (GSH-Px) activity and reduced the concentration of lipid hydroperoxides compared with placebo. In addition, measurements of unscheduled DNA synthesis revealed the DNA of lymphocytes of the selenium-supplemented miners to be less damaged than that of miners receiving placebo, a result consistent with a reduction of cancer risk in the supplemented group. For accurate assessments of cancer risk multielement analyses are necessary and are producing promising results (for additional discussion see refs. 27–29). Zinc, while itself essential for normal cellular functions, also abolishes the cancer-protecting effects of selenium if supplied in excessive amounts (29). Zinc in addition is specifically required by the tumor for growth and progression and accumulates in its actively growing surface layer (30), and there is evidence for its direct interaction with selenium in vivo (31, 32).

Cancer Prevention Trials with Selenium

From the 1980s onward, several cancer prevention trials with selenium have been conducted. The first trial which showed a protective effect of selenium A was performed from 1985–1989 in Qidong, a region north of Shanghai with a high incidence of primary liver cancer (PLC) and hepatitis B (HB) (33–35). Subjects in one commune in the center of the endemic area receiving table salt fortified with sodium selenite experienced a drop of PLC and HB incidence to approximately one-half of the incidences observed in control populations maintained on ordinary salt. Another intervention trial was conducted from 1984 to 1991 in Linxian, Henan Province, China, a region with high incidence of esophageal cancer (36). In this trial the cancer protective effects of several vitamins and minerals including selenium were tested against a placebo. While supplemental retinol, zinc, riboflavin, niacin, molybdenum, and vitamin C were ineffective, supplemental selenium combined with p-carotene and vitamin E lowered the total mortality by 9% and the cancer mortality by 13%. However, best known is the trial directed from 1983–1996 by L.C. Clark et al. (37), which involved 1332 conservatively treated, mostly male, former nonmelanoma skin cancer patients. The consecutively enrolled subjects either received selenium (200μg/day) in the form of selenomethionine incorporated in yeast or a placebo for up to seven years. In the selenium-supplemented group, lung cancer mortality was reduced by 53%, the incidence of cancer of prostate by 63%, of colon and rectum by 58%. Selenium supplementation had no effect on skin cancer recurrence, which, however, is not unexpected as selenium at the dosage chosen does not act on premalignant or transformed cell populations that the study subjects may have harbored. In view of the significant protective effects observed, a new trial designated “SELECT” was started in May 2001. In this largest cancer prevention trial ever to be conducted, 32 000 men will be enrolled in 400 centers in the United States, Canada, and Puerto Rico. Study subjects will receive 200 μg of selenium as L-selenomethionine (in this case not in yeast), with or without vitamin E, or a placebo (38). Its primary aim is to show if a supplement of selenium in the form of selenomethionine will lower the incidence of prostate cancer, and whether the protective effect of selenium can be further increased by vitamin E. Secondary end points, i. e., cancers developing at other sites, will also be monitored, the study is scheduled to be completed in 2012.

Mechanisms of Anticarcinogenic Action

The mechanisms by which selenium protects against the development of cancer are complex. They depend on the dosage and chemical form of selenium and the nature of the initiating agent and hence are probably multifactorial. In the following only some of the many mechanisms will be delineated.

Effects of Selenium Deficiency on Gene Expression

Cells grown or maintained in selenium deficient media do not become spontaneously malignant, they still require a carcinogenic stimulus, which may be chemical, physical, viral or genetic. However, an insufficient supply of selenium evidently results in adaptive changes facilitating the malignant transformation of cells. The extent of these changes has recently been elucidated through a study (39) with cells from the intestine of mice. When these cells were grown under conditions of selenium deficiency, as many as 44 of their genes were up-regulated, among them genes associated with DNA repair and the protection against oxidative stress and genes controlling cell cycle. In addition, at least 24 genes were down-regulated, those involved with selenoprotein synthesis, the synthesis of enzymes involved in detoxification (cytochrome P450, GSH S-transferase, epoxide hydrolase) as well as the synthesis of enzymes regulating lipid transport, angiogenesis, cell adhesion, cell cycle, and cell growth. All these changes point to diminished resistance to carcinogenic stress factors. The loss of cell cycle control alone, for example, may increase the error rate during DNA replication and prevent DNA repair. The downregulation of the detoxifying enzymes may prevent carcinogens to be metabolized.

Antimutagenic and Antiviral Effects

The down-regulation of selenoprotein synthesis will result in low activities of key selenoenzymes protecting cellular DNA against free radical-induced mutations. The glutathione peroxidases, as is well known, prevent the accumulation of H₂O₂ and of lipid hydroperoxides in cells and tissues. Cells deficient in GSH-Px thus will more likely suffer mutations that will push them further toward becoming malignant. The protective action extends to the genomes of viruses that might be present in a dormant state but which may be activated and become pathogenic through an oxygen radical-induced mutation, as was demonstrated with a nonpathogenic coxsackie B3 virus strain (40). It has been suggested (41, 22), by analogy, that selenium could prevent mutations of other viruses, preventing them from becoming oncogenic.

The Putative Role of the Thioredoxin Reductases

In addition to GSH-Px, thioredoxin reductases (TRxRs) (42) deserve particular attention, whose primary function is to catalyze the reduction of the oxidized forms of thioredoxins back into their reduced forms. Thioredoxins are ubiquitous low molecular weight proteins containing two Cys–SH groups in their active reduced forms and a disulfide (Cys–S–S–Cys)-linkage in their oxidized forms. Thioredoxins serve as electron donors in a multitude of biologically important reactions, such as DNA synthesis, DNA repair, gene transcription, cell growth, apoptosis, detoxification reactions, including those involving carcinogens, and they in addition help to maintain the functioning of the immune system. The TRxRs have surprisingly low substrate specificity. In addition to the thioredoxins they also reduce protein disulfide isomerase, selenite, selenodiglutathione, nitrosoglutathione, glutathione peroxidase, H₂O₂, and lipid hydroperoxide reductase, alloxan, and vitamin K, NK lysine disulfide, lipoic acid, dehydroascorbic acid, and the ascorbyl free radical (43). In the physiological hierarchy of the selenoenzymes, the TRxRs fall behind the GSH-Px. Thus, for optimal TRxR activity, higher amounts of selenium are required than for the saturation of GSH-Px activity. The cancer-protective effects of selenium undoubtedly also involves other selenoproteins, of which 25 are encoded in the human genome (44) and whose functions have not been elucidated.

Effect on DNA Methylation

However, some protective functions of selenium are not associated with any selenoenzyme. The interaction of selenium with toxic or carcinogenic metals as discussed above belongs to this category; the methylation of selenium is another. Selenium is methylated under physiological conditions. As an acceptor of biogenic methyl groups selenium was shown to prevent the hypermethylation of DNA (45), an early activating step in benzo(α) pyrene carcinogenesis (46).

Oncogene Inactivation

Among the effects on the genomic level of tumor cells, selenium has been shown to inactivate the oncogene c-myc and to activate c-fos, causing the partial retransformation of human hepatoma cells (47). Related to these effects of selenium is the inactivation of MAZ, the c-myc activating zinc finger protein, which regulates the activation of c-myc (48). These effects of selenium are counteracted by zinc, providing a deeper insight into the mechanism of the zinc-selenium antagonism mentioned above. Selenium has also been shown to inhibit the activation of the nuclear transcription factor NFKB, to activate p53, and to induce apoptosis (49). Selenium may be viewed, in these processes, to act as a catalyst of cellular respiration; the interdependence of its anticarcinogenic effects on oxygen availability has been stressed (50).

Selenium in Cancer Chemotherapy

Reduction of Side Effects of Chemotherapeutic Agents

Cancer patients as a rule present with subnormal blood selenium levels and signs of increased lipid peroxidation (51). They thus are in a state of weakened resistance to oxidative stress to begin with and will suffer more damage during therapy as oxygen radical production is increased by many cytotoxic agents as well as during radiation therapy. The administration of selenium thus is indicated as a general supportive measure, as well as a means of diminishing additional oxygen radical damage occurring during chemotherapy and radiation. For example, the nephrotoxicity of cisplatin, believed to be caused by the stimulation of oxygen radical production in the kidney, was prevented by selenium in rats if administered one hour prior to the cytostatic agent (52–54). In humans, selenium protected even when given after cisplatin: In patients with ovarian cancer treated with cisplatin, sodium selenate and vitamin E prevented the rise of Creatine Kinase serum values, an indicator of kidney damage (55). In addition, the selenium treatment prevented the drop of the leukocyte counts in these patients so that blood transfusions were not required. In other studies sodium selenite was shown to reduce the cardiotoxicity of adriamycin (56, 57) caused by oxidative damage of the cardiac muscle. The reduction of toxic side effects by sodium selenite also extends to cytotoxic agents not necessarily caused by oxidative damage, such as the highly cytotoxic polyamine synthase inhibitors MGBG, EHNA, ARA-A, and DFMO, administered to rats with transplanted human prostate cancer cells (58). The question whether selenium interferes with the therapeutic efficacy of cytotoxic agents has since also been addressed in in-vitro studies with different human cancer cell lines (59). The therapeutic efficacy of doxorubicin, docetaxel, 5-FU, MTX, and mafosfamide against MDA-MB-231 breast cancer cells in vitro was not affected by increasing amounts of sodium selenite in the culture medium. In addition, the antiproliferative activities of cisplatin, etoposide, gemcitabine, or mitomycin C against human A549 lung cancer cells remained unaltered, while the inhibitory activity of docetaxel against these cells was actually increased. An enhancement of the antiproliferative or inhibitory activities of 5-FU, oxaliplatin, and irinotecan by selenite was also observed with HCT116 and SW620 colon cancer cells.

Current Therapeutic Selenite Dosage Recommendations

Patients receiving chemotherapy are given, orally or by infusion, 1000 μg of selenium as sodium selenite per day for 1–5 days prior to and during chemotherapy. The dosage is subsequently lowered to 500, 300, 200, and 100 μg selenium/day. Sodium selenite administered according to this scheme is generally well tolerated as even at the maximum of 1000 μg selenium/day, which is given only for a few days, it is still below the “individual toxic level” of 1600 μg/day for long-term selenium intake (60). Additional sodium selenite dosage schemes for specific applications are also given in Chapter 21 (p. 247), as well as in a recently published monograph (61).

Anti-Inflammatory Effects

Inflammatory reactions are known to occur after treatment with certain chemotherapeutic agents. Vinorelbine is one such agent. Given by infusion, this agent causes phlebitis of the arm in sensitive patients, which often forces the interruption or cessation of the treatment. A pretreatment of such patients with sodium selenite prior to Vinorelbine infusion was found to significantly reduce or prevent this adverse reaction (62).

Diminution of Drug Resistance

The enhanced production of oxygen radicals and increased lipid peroxidation during cytotoxic drug therapy activates antioxidative defense mechanisms of tumor cells (63) resulting in an increase of their intracellular glutathione (GSH) concentrations and ultimately the development of drug resistance (64–66). Selenite reacts with GSH to yield selenodiglutathione, GSSeSG, and related species, which in the presence of oxygen catalyze the oxidation of GSH to GSSG. Depletion of the tumor cells of GSH resensibilizes them against the cytostatic agent and may simultaneously induce p53 activity and apoptosis resulting in cell destruction (67).