Selenium and Bladder Cancer Risk: a Meta-analysis

Introduction

Worldwide, bladder cancer is one of the most common types of cancer, especially among men. The highest incidence is observed in Spain and Italy. The established risk factors for bladder cancer are smoking, which accounts for 60% of cases in men and 35% in women, occupational exposure to aromatic amines, high levels of arsenic intake, and schistosomiasis infection (1). Bladder cancer is a complex disease, and polymorphisms in low-penetrance genes are also involved in the development of this neoplasm. There is consistent evidence that NAT2 slow acetylator and GSTM1 null genotypes increase the risk of bladder cancer. Furthermore, NAT2 slow acetylator has been found to modulate the effect of smoking on bladder cancer, which represents one of the few known gene environment interactions involved in carcinogenesis (2). Although these factors explain more than half of the etiologic scenario of bladder cancer (3), a substantial fraction of the disease remains unexplained. The risk factors that remain may include a complex pattern of environmental exposures, difficult to measure with questionnaires. The increasing evidence of a role for trace metals and other environmental exposures in cancer acting through oxidative stress mechanisms (4, 5) suggests their further exploration in relation to bladder and other cancers.

Although the mechanism(s) by which selenium may act as an anticarcinogen are not fully known, several studies have observed an inverse association between selenium status and cancer, such as gastrointestinal, lung, and prostate cancers (6-9). However, for prostate cancer, there have been some conflicting results reported by clinical trials (10-12).

The present meta-analysis was conducted to summarize the association between high levels of selenium as measured in biological sample specimens and bladder cancer risk, and examine the possible causes of heterogeneity among published studies on this topic.

Results

The initial literature search identified 172 publications. After excluding duplicates and other publications according to selection criteria (Supplementary Table S2), three case-control, three nested case-control, and one case-cohort studies that examined the association between selenium status and bladder cancer risk were identified and used in the present analysis (Table 1). A nested case-control study of a Finnish population met almost all the inclusion criteria but could not be included because it lacked an adequate definition of reference and exposed groups (15). The total number of cases and controls/cohort members in the identified studies was 1,910 and 17,339, respectively. Four studies were done in the United States and three in Northern Europe. In four studies, selenium status was based on analysis of toenails, whereas in the remaining three, serum was the sample specimen used. The two oldest and smallest studies of the seven reported nonsignificant inverse associations (Table 1; refs. 16, 17). Three other studies reported the same tendency. Only two showed a significant decrease in the risk of bladder cancer among individuals with higher selenium levels (Table 1; refs. 18-22). In two case-control studies, toenail selenium concentrations were inversely associated with bladder cancer risk only among women, moderate smokers, and p53-positive cancers (Table 1; refs. 19, 22). Using a random effects model, the overall OR of bladder cancer risk for the highest compared with the lowest selenium status was 0.61 (95% CI, 0.42-0.87). A comparable result was found when using a fixed effects model (OR = 0.70; 95% CI, 0.58-0.84; Fig. 2).

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Figure 2.

Meta-analysis of the association of selenium with bladder cancer risk. ORs regard to comparisons of extreme categories of exposure in each study. The area of each square is proportional to the percentage weight of each individual study in the meta-analysis. Horizontal lines, 95% CIs. Diamond, the meta-OR from a random effects model.

Among the pooled studies, a moderate level of heterogeneity was detected (χ² = 15.32; degrees of freedom = 6; P = 0.018; I² = 60.8%; Fig. 2).

In the analysis stratified by gender, only women yielded significant decreased risk associated with selenium (OR = 0.55; 95% CI, 0.32-0.95) with a nonsignificant I² of 23.7% (Table 2). The sample specimen in which selenium was determined (serum or toenails) was found to be a source of heterogeneity (Table 2). Although selenium in toenails and in serum provided significant results, serum levels of selenium showed a stronger protective effect (OR = 0.33; 95% CI, 0.21-0.51). Stratifying the analysis according to smoking status decreased the heterogeneity found in the overall analysis, and results became similar between never and ever smokers (Table 2). As for the type of study, the stratified results were consistent with the pooled overall estimate. Moreover, heterogeneity increased in the stratum of case-control studies (χ² = 13.13; degrees of freedom = 2; P = 0.001; I² = 84.8%; Table 2).

Publication bias was not detected in the meta-analysis (coefficient = −1.84, P = 0.357), even when stratifying by the several study determinants (Table 2). As expected, the most influential studies in the analysis were found to be the largest ones (18, 19, 22).

Discussion

In the present meta-analysis, we observed a significant 39% decreased risk of bladder cancer associated with high levels of selenium by combining results from seven epidemiologic studies, conducted in different populations, which applied individual levels of selenium measured in serum or toenails. Although little is known on the role of selenium in bladder cancer, several systematic reviews and meta-analysis, both on observational (6-9) and interventional studies (10-12), have reported on the protective effect of selenium on cancer. The few clinical trials on selenium supplementation have reported conflicting and not yet conclusive results, with the Nutritional Prevention of Cancer (NPC) and the Supplémentation en Vitamines et Minéraux Antioxydants showing an inverse association between selenium and prostate cancer, and the Selenium and Vitamin E Cancer Prevention trial reporting nonsignificant increased risk of prostate cancer among those selenium-supplemented individuals (8, 10-12). The NPC also reported results on bladder cancer and other types of cancer, as secondary outcomes of the trial, but no clear association was found, probably due to insufficient number of incident cases (23).

The above-mentioned high selenium status refers to concentrations of selenium in toenails of ∼0.9 μg/g and in serum of ∼100 μg/L, or higher. Toenail and serum selenium levels are correlated⁴ (24, 25). Furthermore, a level of about 80 to 95 μg/L of selenium in serum is considered optimal for the maximization of glutathione peroxidases and selenoprotein P activities (26).

Selenium may exert anticarcinogenic effects mainly through selenoproteins, although the specific mechanisms are not yet fully known. Aberrant expression patterns of glutathione peroxidases and selenoprotein P, found in colorectal cancer, show that the antioxidant properties of selenoenzymes are relevant in carcinogenesis and tumor progression (27), particularly by scavenging reactive oxygen species and diminishing further oxidative damage. The protection of selenium against cancer is also linked to the activities of hydrogen selenide and selenomethionine present in cells, which may be responsible for modifying protein thiols and mimicking methionine, leading to higher methylating efficiency of RNA and thiols (28). Some studies suggest that this essential dietary trace element has antioxidant properties and that it produces effects on apoptosis, DNA repair, and carcinogen metabolism. To decrease the oxidative stress caused by exposure to arsenic, cadmium, or lead, the selenium requirement increases (29) as these metals act as selenium antagonists (30). Moreover, both organic and inorganic forms of selenium may enhance p53 activity toward either DNA repair or apoptosis (31). If selenium is in the form of seleno-l-methionine, the DNA repair branch of the p53 pathway is preferentially induced, which also involves Ref1 and Brca1 in a protein complex (32).

Stratified analyses by type of sample specimen (serum and toenails), smoking status, and study design also found that higher levels of selenium lead to a lower risk of bladder cancer, although each was identified as a potential source of heterogeneity. Serum and toenail measurements are both accepted as biomarkers of exposure to assess the exposure to selenium in the organism (33, 34), but they provide information on different time frames with toenail selenium reflecting longer term exposure (35, 36). Part of the heterogeneity found in the pooled overall analysis was also explained by the smoking status, which may be due to different ways of assessing that status and different definitions of smoking groups among studies. Although published evidence between selenium and smoking is not clear, several studies on healthy individuals show that smokers have lower levels of selenium (37, 38). Although study design explained some of the heterogeneity, more diversity was found among the case-control studies, which is probably the result of different criteria for selection of controls.

The summary effects for the two sexes were not similar in that a significant inverse association was observed for women but not men. An opposite gender pattern, with protective effects in men and not in women, was reported in a meta-analysis on selenium supplementation and primary cancer incidence and mortality (6). However, the trials in that meta-analysis were not specific for the effects of selenium on bladder cancer risk; two of the four trials that provided sex-specific data used a mix of compounds rather than selenium alone; and the effects of selenium on cancer incidence and mortality were based on levels of supplementation instead of internal levels of selenium. Moreover, only one of them assessed the internal levels of selenium of the individuals, and this showed that men had higher levels of selenium at baseline, which could perhaps explain the higher protection from cancer in those. Although no accepted explanation for these sex-specific differences exists, there are some hypotheses indicating different excretion rates, half-lives, and also sensitivity to potential toxic effects of selenium between genders (39, 40).

Publication bias was not evident in this meta-analysis. However, a note of caution is warranted because only seven epidemiologic studies were included in this meta-analysis and because they presented some heterogeneity. Nevertheless, the results of this meta-analysis and the heterogeneity found among studies are informative in the sense they emphasize the input of each study to the existing literature and the topics that entail further research.

In conclusion, this meta-analysis supports an inverse association between selenium concentration and bladder cancer risk. To further elucidate this relationship, efforts to quantify selenium and other trace metals in biological sample specimens at the individual level in large observational studies or randomized trials are needed. These are fundamental steps before suggesting selenium supplementation to bladder cancer patients.

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.