Plasma folate level, urinary arsenic methylation profiles, and
urothelial carcinoma susceptibility
Yung-Kai Huang
a, Yeong-Shiau Pu
b, Chi-Jung Chung
c, Horng-Sheng Shiue
a,d,
Mo-Hsiung Yang
e, Chien-Jen Chen
f,g, Yu-Mei Hsueh
h,*aGraduate Institute of Medical Sciences, College of Medicine, Taipei Medical University, Taipei, Taiwan bDepartment of Urology, National Taiwan University College of Medicine, Taipei, Taiwan
cGraduate Institute of Public Health, Taipei Medical University, Taipei, Taiwan dDepartment of Chinese Medicine, Chang Gung Memorial Hospital, Taipei, Taiwan
eDepartment of Nuclear Science, National Tsing-Hua University, Hsinchu, Taiwan fGenomic Research Center, Academia Sinica, Taipei, Taiwan
gGraduate Institute of Epidemiology, National Taiwan University, Taipei, Taiwan
hDepartment of Public Health, School of Medicine, Taipei Medical University, No. 250 Wu-Hsing Street, Taipei 110, Taiwan Received 27 June 2007; accepted 18 October 2007
Abstract
To elucidate the influence of folate concentration on the association between urinary arsenic profiles and urothelial carcinoma (UC)
risks in subjects without evident arsenic exposure, 177 UC cases and 488 controls were recruited between September 2002 and May 2004.
Urinary arsenic species including inorganic arsenic, monomethylarsonic acid (MMA
V) and dimethylarsinic acid (DMA
V) were determined
by employing a high performance liquid chromatography-linked hydride generator and atomic absorption spectrometry procedure. After
adjustment for suspected risk factors of UC, the higher indicators of urinary total arsenic levels, percentage of inorganic arsenic,
percent-age of MMA
V, and primary methylation index were associated with increased risk of UC. On the other hand, the higher plasma folate
levels, urinary percentage of DMA
Vand secondary methylation index were associated with decreased risk of UC. A dose–response
rela-tionship was shown between plasma folate levels or methylation indices of arsenic species and UC risk in the respective quartile strata. The
plasma folate was found to interact with urinary arsenic profiles in affecting the UC risk. The results of this study may identify the
sus-ceptible subpopulations and provide insight into the carcinogenic mechanisms of arsenic even at low arsenic exposure.
2007 Elsevier Ltd. All rights reserved.
Keywords: Plasma folate level; Urinary arsenic species; Interaction; Urothelial carcinoma
1. Introduction
A urinary bladder cancer in Asia is considered a minor
incidence cancer compared to the US and other Western
countries. Urothelial carcinoma (UC) is a heterogeneous
disease influenced by both environmental exposure and
genetic factors. Folate is a water soluble B vitamin, and
present in cells as a family of structurally related derivatives
comprised of 2-amino-4-hydroxypteridine linked through a
methylene carbon to p-amino-benzoylpolyglutamate and it
is the donor of one-carbon groups in both DNA
methyla-tion and DNA synthesis (
Suh et al., 2001; Stanger, 2002
).
0278-6915/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.fct.2007.10.017
Abbreviations: SAM, S-adenosylmethionine; UC, urothelial carci-noma; InAs, inorganic arsenic (AsIII+ AsV); MMAV, monomethylarsonic acid; DMAV, dimethylarsinic acid; %InAs, inorganic arsenic percentage; %MMAV, monomethylarsonic acid percentage; %DMAV, dimethylarsinic acid percentage; PMI, primary methylation index; SMI, secondary methylation index; FFQ, food-frequency questionnaire; OR, odds ratio; CI, confidence interval.
*
Corresponding author. Tel.: +886 2 27361661x6513; fax: +886 2 27384831.
E-mail address:ymhsueh@tmu.edu.tw(Y.-M. Hsueh).
www.elsevier.com/locate/foodchemtox Food and Chemical Toxicology 46 (2008) 929–938
The epidemiologic evidence relating folate intake and the
risk of bladder cancer is contradictory and limited (
Bruem-mer et al., 1996; Michaud et al., 2000; Zeegers et al., 2001;
Schabath et al., 2005
). These studies used the
food-fre-quency questionnaire (FFQ) to estimate the folate content
from food intake and to assess the relationship between
folate intake and risk of bladder cancer. The estimation of
folate from FFQ may influence by the recall and
informa-tion bias; therefore, plasma folate of subjects used as an
exposure marker is the one of methods to prevent recall bias
(
Szklo and Nieto, 2007
). Because plasma folate reflects the
dietary folate intake (
Stanger, 2002
), quantification of
folate in biological samples may be a more reliable index
for cancer risk than estimated folate from the FFQ.
Arsenic is widely distributed in nature and is spread in
the environment mainly by water. Ingestion of inorganic
arsenic from arterial well water increases the worldwide
bladder cancer risk (
Chen et al., 1985, 1992; Bates et al.,
1992; Abernathy et al., 2003
). The metabolism of inorganic
arsenic involves reduction and oxidative methylation (
Kit-chin, 2001; Thomas et al., 2001, 2004; Vahter, 2002; Styblo
et al., 2002
). After ingestion of inorganic arsenic, the
pen-tavalent inorganic arsenic (arsenate, As
V) is readily reduced
to trivalent inorganic arsenic (arsenite, As
III) in red blood
cells (
Vahter, 1981
) and subsequently methylated to
monomethylarsonic acid (MMA
V), and to dimethylarsinic
acid (DMA
V) in the liver (
Buchet et al., 1981a,b
).
Evalua-tion of arsenic methylaEvalua-tion efficiency is mainly based on the
relative amounts of the different metabolites present in
urine. Previous epidemiological studies from Taiwan were
reported that higher cumulative arsenic exposure and less
efficient methylation activities were detected in skin and
bladder cancer patients than in healthy controls (
Hsueh
et al., 1995, 1997; Yu et al., 2000; Chen et al., 2003b, 2005
).
The evidence for nutritional regulation of arsenic
meth-ylation and excretion in humans is limited and rarely
con-sidered as a disease risk. A case–control study in West
Bengal showed a modestly increased risk of arsenic related
skin lesions for individuals with the lowest quintiles of
die-tary folate intake than those with higher quintiles (
Mitra
et al., 2004
). A recent study found that high plasma folate
levels were associated with efficient arsenic methylation
pattern (
Gamble et al., 2005
). These studies all focused
on subjects who had high arsenic exposure. The arsenic
concentration allowance in public water supplies in Taiwan
was 50 lg/L and a new standard of 10 lg/L was announced
in 2000. We designed a case–control study to assess the
association between individual plasma folate levels and
arsenic methylation capability on UC risk among a
popu-lation having no evident arsenic exposure in Taiwan.
2. Materials and methods
2.1. Study subjects
One hundred and seventy-one patients, age range 24–93 years, with pathologically proven UC were recruited from the Department of Urol-ogy, National Taiwan University Hospital, between September 2002 and
May 2004. Pathological verification of UC was done by routine urological methods including endoscopic biopsy or surgical resection of urinary tract tumors followed by histopathological examination by board-certified pathologists. A total of 488 control subjects with no evidence of UC or any other malignancy were recruited from a hospital-based pool, including those receiving senior citizen health examinations at Taipei Medical University Hospital and those receiving health examinations at Taipei Municipal Wan Fang Hospital. These three hospitals are medical center and their clinical clients’ bases are similar and located in Taipei approxi-mately 200–300 km away from the arsenic-contaminated areas in Taiwan. In this study, no case subjects or controls have lived in the arsenic-con-taminated areas in southwestern (Chen et al., 2003b) or northeastern Taiwan (Chiou et al., 2001). Although we only collected tap water from 37 UC cases and determined the total arsenic levels, the mean ± standard error was 17.14 ± 0.55 lg/L. However, urinary total arsenic levels in cases and controls were 24.47 ± 2.56 lg/L and 24.85 ± 1.06 lg/L, respectively (p-value is 0.89 for Student’s t-test). These results may indicate no differ-ence in arsenic exposure between cases and controls.
2.2. Questionnaire interview and specimens collection
Well-trained personnel carried out standardized personal interviews based on a structured questionnaire. Information collected included demographic and socioeconomic characteristics, general potential risk factors for malignancies such as lifestyle, quantified details of alcohol consumption, cigarette smoking, exposure to potential occupational and environmental carcinogens such as hair dyes and pesticides, chronic medication history, consumption of conventional and alternative medi-cines, and personal and family history of urological diseases. Regular alcohol drinkers referred to those who consumed alcohol three or more days per week, continuing for at least six months. The Research Ethics Committee of National Taiwan University Hospital, Taipei Medical University Hospital and Taipei Municipal Wan Fang Hospital approved the study. All subjects provided informed consent forms before specimen’s collection and questionnaire interview. The study was consistent with the World Medical Association Declaration of Helsinki.
After the questionnaire interview, a 10-mL blood sample was drawn into an EDTA-treated tube and centrifuged at 3000 rpm for 15 min at room temperature after collection. Plasma was separated and stored at 80 C until analysis. Urine samples were collected simultaneously and drawn into a 1% nitric acid rinsed PE bottle, and stored at20 C until used for urinary arsenic speciation. Because questionnaire and biospeci-mens were obtained before UC cases’ acceptance with surgery, radio-therapy, or chemoradio-therapy, any influence of treatment is unlikely.
2.3. Plasma folate assays
Plasma folate levels were determined using a competitive immunoassay kit (Diagnostic Products Corporation, Los Angeles, CA) according to the manufacturer’s instructions. All plasma samples were processed under dim yellow light. Laboratory personnel were unaware of the case–control status. The coefficient of variation was used to test the reliability and the mean coefficient of variation for 23 pairs of replicate plasma samples was 8.8%.
2.4. Determination of urinary arsenic species
It has been shown that urinary arsenic species are stable for at least six months when preserved at20 C (Chen et al., 2002); thus, the urine assay was performed within six months post-collection. Frozen urine samples were thawed at room temperature, dispersed by ultrasonication, filtered through a Sep-Pak C18 column (Mallinckrodt Baker Inc., NJ) and the levels of AsIII, AsV, MMAVand DMAVwere determined. A 200 lL ali-quot of urine was used for the determination of arsenic species by high performance liquid chromatography (Hitachi 7110, Naka, Japan) using columns obtained from Phenomenex (Nucleosil, Torrance, CA). The contents of inorganic arsenic and their metabolites were quantified by
hydride generator-atomic absorption spectrometry (Hsueh et al., 1998). The concentrations of the four arsenic species in a standard solution, a sample, and a sample spiked standard solution were determined by using
on-line HPLC-HG-AAS, respectively. Recovery rates of the four arsenic species were estimated according to the following calculation: [(sample spiked standard solution concentration) sample concentration]/ Table 1
Demographic characteristics, urothelial carcinoma risk variables, plasma folate level, and odds ratio of urothelial carcinoma in UC patients and controls Cases (N = 171) Controls (N = 488) Odds ratio (95% CI)
Gender, n (%) Male 116 (67.84) 286 (58.61) 1.00 Female 55 (32.16) 202 (41.39) 0.67 (0.46–0.97)* Education, n (%) Elementary school 78 (45.61) 136 (27.87) 1.00 High school 60 (35.09) 171 (35.04) 0.61 (0.41–0.92)* University 33 (19.30) 181 (37.09) 0.32 (0.20–0.51)** Smoking, n (%) Non-smokers 93 (54.39) 335 (68.65) 1.00
Light smokers (<22 pack-years) 21 (12.28) 77 (15.78) 0.98 (0.58–1.68)
Heavy smokers (P22 pack-years) 57 (33.33) 76 (15.57) 2.70 (1.79–4.08)**
Alcohol consumption, n (%) Never 111 (64.91) 288 (59.02) 1.00 1.00 Occasional 22 (12.87) 136 (27.87) 0.42 (0.25–0.69)** Regular 38 (22.22) 64 (13.11) 1.54 (0.97–2.43)*** 1.89 (1.21–2.96)** Age Mean ± SE 64.60 ± 0.99 63.51 ± 0.69 p = 0.37a Folate level (ng/mL) Mean ± SE 7.31 ± 0.41 12.29 ± 0.25 p < 0.01a
SE: standard error. * p < 0.05. ** p < 0.01. ***0.1 > p > 0.05.
a p-Value for student’s t-test.
Table 2
Distribution of the plasma folate level and urinary arsenic profile among subgroups of demographic characteristics N Folate level (ng/mL) Urinary arsenic level
(lg/g creatinine)
InAs (%) MMA (%) DMA (%)
Mean ± SE Mean ± SE Mean ± SE Mean ± SE Mean ± SE
Gender Male 402 10.75 ± 0.30 27.25 ± 1.34 6.21 ± 0.36 9.96 ± 0.49a 83.83 ± 0.63a Female 257 11.41 ± 0.38 31.01 ± 1.57 5.60 ± 0.51 6.48 ± 0.48 87.92 ± 0.72 Education Elementary school 214 10.91 ± 0.43 34.94 ± 1.57 5.1 ± 0.38 8.31 ± 0.55 86.59 ± 0.71 High school 231 11.23 ± 0.39 27.63 ± 1.84b 6.58 ± 0.61 9.45 ± 0.59 83.97 ± 0.91 University 214 10.86 ± 0.40 23.65 ± 1.78b 6.18 ± 0.51 7.99 ± 0.72 85.83 ± 0.86 Smoking status Non-smokers 428 11.21 ± 0.28c 28.96 ± 1.29 5.46 ± 0.35 7.69 ± 0.42c 86.86 ± 0.56 Light smokers (<22 pack-years) 98 12.13 ± 0.67c 24.63 ± 1.52 6.58 ± 0.83 9.21 ± 1.11 84.21 ± 1.30 Heavy smokers (P22 pack-years) 133 9.53 ± 0.49 30.94 ± 2.68 7.17 ± 0.72 11.12 ± 0.78 81.71 ± 1.19 Alcohol consumption Never 399 11.07 ± 0.29 29.51 ± 1.50 5.72 ± 0.38 8.26 ± 0.47 86.02 ± 0.63 Occasional 158 11.65 ± 0.51d 26.90 ± 1.43 5.56 ± 0.57 8.86 ± 0.76 85.58 ± 1.04 Regular 102 9.75 ± 0.63 28.41 ± 2.00 7.58 ± 0.81 9.57 ± 0.80 82.86 ± 1.06
SE: standard error.
a p-Value for student’s t-test, p < 0.0001.
b Significant different (p < 0.05) from elementary school group by ANOVA and Scheffe’s test. c Significant different (p < 0.05) from heavy smokers group by ANOVA and Scheffe’s test.
standard solution concentration· 100. Recovery rates for AsIII, DMAV, MMAVand AsVranged between 93.8% and 102.2% with detection limits of 0.02, 0.06, 0.07 and 0.10 lg/L, respectively. Urinary concentration of the sum of inorganic arsenic, MMAVand DMAVwas normalized against urinary creatinine levels (lg/g creatinine). The tap water was digested by 65% nitric acid and determined the total arsenic by HG-AAS. The stan-dard reference material, SRM 2670, containing 480 ± 100 lg/L inorganic arsenic was obtained from the National Institute of Standards and Technology (NIST, Gaithersburg, MD), and was used as a quality stan-dard and analyzed along with urine samples. The mean value of arsenic of SRM 2670 determined by our system was 507 ± 17 (SD) lg/L (n = 4). Arsenic methylation indices were assessed as the percentages of various urinary arsenic species in the sum of inorganic arsenic, MMAV and DMAV. The primary methylation index (PMI) was defined as the ratio between MMAV and inorganic arsenic (AsIII+ AsV) levels and the sec-ondary methylation index (SMI) was defined as the ratio between DMAV and MMAV(Hsueh et al., 1998; Vahter, 2002).
3. Statistical methods
Student’s t-test was used to compare the differences in
continuous variables between UC cases and controls.
Logistic regression models were used to estimate the
uni-variate and multiuni-variate-adjusted odds ratio (OR) and
the 95% confidence interval (CI). Cutoff points for
contin-uous variables were the respective quartiles of the controls.
For the joint effect analysis, the cutoff points for the plasma
folate levels and arsenic methylation indices were the
medi-ans of the controls, respectively. The joint effects of
ciga-rette smoking and plasma folate and urinary arsenic
methylation indices, or plasma folate levels and urinary
arsenic methylation indices on the UC risk were evaluated
by estimating the synergy index. An observed synergy
index value that departs substantially from the expected
additive null, i.e., synergy index greater than 1, suggests
an additive interaction effect. The OR values and their
var-iance covarvar-iance matrix were then used to calculate synergy
index and 95% CIs (
Hosmer and Lemeshow, 1992
). SAS
version 8.2 was used for all statistical analyses.
4. Results
The UC risks were significantly influenced by gender,
education level, cigarette smoking status, alcohol
consump-tion and plasma folate levels strata (
Table 1
). Males or
Table 3
Multivariate-adjusted ORs and 95% CI for associations of plasma folate levels and arsenic methylation capability with the risk of urothelial carcinoma
Quartiles p-Value for trenda
Q1 Q2 Q3 Q4
Folate (ng/mL)
Range <7.89 7.90–11.49 11.50–15.99 P16.00
Case/control 104/121 33/120 24/123 10/124
OR (95% CI)b 1.00 0.33 (0.20–0.54)*** 0.22 (0.13–0.38)*** 0.09 (0.04–0.19)*** <0.0001
Total arsenic (lg/g creatinine)
Range <13.09 13.10–20.29 20.30–30.59 P30.60
Case/control 13/121 21/121 47/122 90/123
OR (95% CI)b 1.00 1.48 (0.69–3.12) 3.22 (1.62–6.27)*** 6.26 (3.21–12.22)*** <0.0001 Percentage of inorganic arsenic (%)
Range <1.49 1.50–3.69 3.70–6.29 P6.30 Case/control 24/121 40/122 41/122 66/123 OR (95% CI)b 1.00 1.67 (0.93–3.00) 1.67 (0.93–3.01) 2.52 (1.44–4.41)** 0.002 Percentage of MMA (%) Range <0.89 0.9–5.89 5.90–10.89 P10.90 Case/control 25/121 27/122 39/121 80/124 OR (95% CI)b 1.00 0.98 (0.53–1.82) 1.41 (0.79–2.51) 2.75 (1.61–4.71)** <0.0001 Percentage of DMA (%) Range <81.89 81.90–89.19 89.20–94.39 P94.40 Case/control 73/121 49/122 33/121 16/124 OR (95% CI)b 1.00 0.66 (0.42–1.05) 0.46 (0.27–0.76)** 0.22 (0.12–0.42)*** <0.0001
Primary methylation index
Range <0.29 0.30–1.39 1.40–2.79 P2.80
Case/control 73/121 49/122 33/121 16/124
OR (95% CI)b 1.00 1.05 (0.57–1.93) 1.44 (0.87–2.57) 1.99 (1.13–2.48)* 0.0063
Secondary methylation index
Range <6.59 6.60–10.59 10.60–19.29 P19.30 Case/control 26/105 32/106 44/106 57/106 OR (95% CI)b 1.00 0.51 (0.30–0.85)** 0.32 (0.18–0.57)** 0.28 (0.15–0.51)** <0.0001 * p < 0.05. **p < 0.01. *** p < 0.001.
a p-Value for trend for category variables.
lower education or regular alcohol drinkers or lower
plasma folate levels subjects had significantly higher UC
risk than females or higher education or alcohol
non-drink-ers or higher plasma folate levels subjects. There was no
significant difference of the mean age of cases at 64.60 years
and controls at 63.51 years. Folate (7 lmol/L) in plasma
was recommended as normal standard by
Institute of
Med-icine (1998)
. In this study, the proportion under 7 lmol/L
plasma folate levels was 57% (98/171) and 18% (88/488)
in UC cases and controls, respectively.
The distribution of the plasma folate levels and urinary
arsenic profiles among subgroups of gender, education,
cig-arette smoking, and alcohol consumption was shown in
Table 2
. Male had a higher urinary %MMA
Vand a lower
%DMA
Vthan female. The higher total urinary arsenic
lev-els were observed for subjects who had education level of
elementary school than those had high school and
univer-sity. Compared to heavy smokers, non-smokers had a
higher folate levels and a lower %MMA
V. Light smokers
had a higher folate levels than heavy smokers. Occasional
alcohol drinkers had a higher folate levels than regular
alcohol drinkers. The results of
Table 2
suggested that
male, lower education level, heavy smokers, and regular
alcohol drinkers may have an inefficient methylation
pro-cess of metabolizing arsenic to DMA
V.
Table 3
presents the multivariate-adjusted ORs for the
associations between plasma folate levels or arsenic
methyl-ation indices and UC risk. In general, there was a dose–
response association between the quartile of plasma folate
levels or arsenic methylation indices and UC risk after
adjustment for suspected UC risk factors. The plasma
folate levels appeared to have an inverse association with
the risk of UC having OR of quartiles strata of 1.0, 0.33,
0.22, and 0.09, respectively (p < 0.0001 for the trend test).
The creatinine-adjusted total arsenic levels appeared to
have an increased UC risk, the OR of quartiles strata were
1.0, 1.48, 3.22, and 6.26, respectively (p < 0.0001 for the
trend test). Subjects with either lower %MMA
V, or lower
%InAs, or lower PMI, or higher %DMA
Vor higher SMI
were suggested a more efficient capacity to methylate
inor-ganic arsenic to DMA
V, and the more efficient capacity
was the less risk of UC.
Table 4
examined the joint effects of the plasma folate
levels in combination with various arsenic methylation
profiles. The folate concentrations were divided into two
categories based on the median values of controls. Subjects
with a higher plasma folate levels and possessing efficient
arsenic methylation profiles were the reference group.
The OR was 5.24 (95% CI, 1.93–14.20) for individuals with
a higher plasma folate levels and a higher total arsenic
lev-els. The OR was 5.93 (95% CI, 2.19–16.01) for individuals
with a lower folate levels and lower total arsenic levels as
compared to those with a higher folate and a lower total
arsenic levels. The highest risk group occurred in those
with a lower folate levels and a higher total arsenic levels
having an adjusted OR 19.58 and 95% CI, 7.63–50.23.
Table 4
Joint effects of plasma folate level and arsenic methylation capability index on urothelial carcinoma risk
Arsenic methylation profiles Folate (ng/mL) S index (95% CI)
P11.5 <11.5
Case/control OR (95% CI) Case/control OR (95% CI)
Total arsenic (lg/g creatinine)
<20.30 5/118 1.00 29/125 5.93 (2.19–16.01)** 2.02 (1.24–3.28)**
P20.30 29/129 5.24 (1.93–14.20)** 108/116 19.58 (7.63–50.23)***
Percentage of inorganic arsenic (%)
<3.70 10/124 1.00 54/119 5.78 (2.77–12.03)*** 1.18 (0.71–1.98) P3.70 24/123 2.37 (1.07–5.23)* 83/122 8.30 (4.02–17.12)*** Percentage of MMA (%) <5.90 17/122 1.00 35/121 2.12 (1.10–4.08)* 4.43 (1.18–16.57)* P5.90 12/125 0.92 (0.44–1.91) 102/120 5.61 (3.10–10.16)** Percentage of DMA (%) P89.20 19/105 1.00 39/106 3.10 (1.54–6.24)** 2.51 (1.25–5.03)** <89.20 12/106 1.68 (0.97–3.57) 89/106 7.98 (4.15–15.35)***
Primary methylation index
<1.40 19/105 1.00 39/106 2.10 (1.12–3.96)* 4.64 (0.74–29.07)
P1.40 12/106 0.60 (0.27–1.33) 89/106 4.25 (3.38–7.58)***
Secondary methylation index
P10.60 13/98 1.00 29/89 2.40 (1.16–4.98)* 3.77 (1.29–10.96)*
<10.60 14/90 1.19 (0.52–2.71) 91/96 7.00 (3.58–13.67)**
Adjusted for age, sex, educational attainment, smoking status (pack-year), and alcohol consumption. * p < 0.05.
** p < 0.01. ***p < 0.001.
Furthermore, the interaction between plasma folate levels
and urinary total arsenic levels were statistically significant
on the additive scale (S index 2.02, p < 0.01). The
phenom-ena were identical to the %InAs, %MMA
V, %DMA
V, PMI
and SMI. Except for %InAs and PMI, interactions between
arsenic methylation profiles and folate on additive scale
were statistically significant.
Table 5
examined the joint effects of cigarette smoking
and plasma folate levels or cigarette smoking and various
arsenic methylation indices for UC risk. The reference
group was the non-smokers or light smokers with a high
plasma folate levels or an efficient arsenic methylation
pro-files. Heavy smokers with a higher plasma folate levels had
2.52-fold (95% CI, 1.08–5.85) risk of UC. A 4.51-fold UC
risk (95% CI, 2.71–7.51) for non-smokers or light smokers
with a lower plasma folate levels, and the risk increased to
8.25 (95% CI, 1.23–16.09) for heavy smokers with a lower
plasma folate levels. Similar results were obtained for the
various arsenic methylation profiles. For example,
non-smokers or light non-smokers with the higher total arsenic levels
had an OR of 4.27 (95% CI, 2.39–7.31), heavy smokers with
the lower total arsenic levels had an OR of 2.95 (95% CI,
1.32–6.61). The highest risk was found in heavy smokers
with the higher total arsenic levels (adjusted OR 8.89;
95% CI, 4.38–18.03). The joint effect was shown statistically
insignificant between cigarette smoking and plasma folate
levels or between cigarette smoking and urinary arsenic
indices. Comparing
Tables 4 and 5
, the interaction was
sta-tistically significant between the plasma folate levels and the
arsenic methylation indices on UC risk but not between the
plasma folate levels and cigarette smoking on UC risk.
5. Discussion
High folate levels and efficient arsenic methylation
profiles are associated with a decreased risk of UC. Both
factors exhibited a strong interaction on UC risk. More
interestingly, the marked interaction between folate and
arsenic methylation indices was greater than that between
cigarette smoking and folate or between cigarette smoking
and arsenic methylation indices. This study demonstrated
that the UC risk is enhanced by a synergistic interaction
between low plasma folate levels and inefficient arsenic
methylation capability among a population having no
evident arsenic exposure in Taiwan.
Numerous
epidemiological
studies
had
conflicting
results between folate intake and bladder cancer risk. In
a folate supplement case–control study, total folate intake
was inversely related with bladder cancer, OR 0.54, 95%
CI, 0.31–0.93 for the highest quartile compared to the
Table 5
Joint effects of smoking status, and plasma folate level or arsenic methylation capability index on urothelial carcinoma risk Folate and arsenic methylation
profiles
Smoking status S index (95% CI)
Non-smokers or light smokers (<22 pack years) Heavy smokers (>22 pack years)
Case/control OR (95% CI) Case/control OR (95% CI)
Plasma folate level (ng/mL)
P11.50 23/123 1.00 11/34 2.52 (1.08–5.85)* 1.44 (0.72–2.87)
<11.50 91/199 4.51 (2.71–7.51)*** 46/42 8.25 (1.23–16.09)***
Total arsenic (lg/g creatinine)
<20.30 20/104 1.00 14/39 2.95 (1.32–6.61)** 1.51 (0.76–2.99)
P20.30 94/208 4.27 (2.49–7.31)*** 43/37 8.89 (4.38–18.03)***
Percentage of inorganic arsenic (%)
<3.70 46/211 1.00 18/32 2.16 (1.05–4.45)* 1.32 (0.46–3.74) P3.70 68/201 1.55 (1.00–2.93)* 39/44 3.26 (1.76–6.03)*** Percentage of MMA (%) <5.90 37/217 1.00 15/26 2.79 (1.25–6.24)* 1.03 (0.44–2.42) P5.90 77/195 2.25 (1.43–3.54)*** 42/50 4.16 (2.24–7.73)*** Percentage of DMA (%) P89.20 26/216 1.00 13/29 2.33 (1.02–5.37)* 1.34 (0.58–3.08) <89.20 78/196 2.44 (1.52–3.89)*** 44/47 4.75 (2.52–8.90)***
Primary methylation index
<1.40 37/183 1.00 21/28 3.21 (1.53–6.72)** 0.70 (0.28–1.75)
P1.40 67/167 1.94 (1.21–3.11)** 34/45 3.22 (1.69–6.14)***
Secondary methylation index
P10.60 34/163 1.00 8/24 1.38 (0.53–3.63) 1.97 (0.68–6.71)
<10.60 65/146 2.24 (1.37–3.65)** 40/40 4.22 (2.19–8.11)***
Adjusted for age, sex, educational attainment, and alcohol consumption. * p < 0.05. ** p < 0.01. *** p < 0.001.
lowest quartile (
Sharp and Little, 2004; Schabath et al.,
2005
). Other studies did not observe any association
between folate and bladder cancer risk (
Michaud et al.,
2002; Holick et al., 2005
). The evidence for a relationship
between folate and bladder cancer from epidemiological
studies is limited and mainly focused on the folate ingested
from dietary food rather than the concentration in the
body. Plasma folate is a precise marker to reflect dietary
folate intake (
Stanger, 2002
) and the result of this study
found that the UC risk was raised with the plasma folate
levels decreasing. Other nutrition such as methionine,
vita-mins B-6 and B-12, which interacted metabolically with
folate in the one-carbon metabolism processes, may also
influence cancer risk (
Bailey, 2003
). Dark-green vegetables
and certain other fruits and vegetables are rich sources of
folate and B vitamins (
Gebhardt et al., 2007
), and
epidemi-ological study suggested that high fruits and vegetables
intake reduced the risk of bladder cancer (
Steinmaus
et al., 2000
).
Besides nutrition, life styles such as cigarette smoking or
regular alcohol drinking also influenced the bioavailability
of folate. In accordance with previous studies (
Piyathilake
et al., 1994; Schabath et al., 2005
), reduced plasma folate
level was observed with either heavy cigarette smoking or
regular alcohol drinking in our study. Recently a study
showed that cigarette smoking might result in localized
deficiencies of folate coenzymes, tetrahydrofolate and
5,10-methylenetetrahydrofolate (
Gabriel et al., 2006
).
On the other hand, alcohol drinking also affected the
folate-dependent
metabolism
including
inhibition
of
enzymes central to one-carbon metabolism (methionine
synthase, methylenetetrahydrofolate reductase, methionine
adenosyltransferase 1A, glycine N-methyltransferase, and
S-AdoHcyst hydrolase), and stimulation of serine synthesis
and inhibition of thymidine synthesis (
Mason and Choi,
2005
). These evidences suggested that the folate
bioavail-ability was influenced by cigarette smoking and alcohol
drinking.
A recent study showed that bladder cancer mortality
declined gradually after eliminating arsenic exposure from
artesian well water by improving the drinking water supply
system in southwest Taiwan (
Yang et al., 2005
). This
find-ing substantiates the association between arsenic exposure
and bladder cancer risk. A previous study evaluated the
relationship between UC risk and arsenic exposure by
focusing on the total arsenic levels in drinking water
(
Chiou et al., 2001
). It would be more relevant if urinary
arsenic species were used as indicators of arsenic
metabo-lism (
Francesconi and Kuehnelt, 2004; Steinmaus et al.,
2005
). The order of toxicity in arsenic species by oral
expo-sure in mice was As
III> DMA
V> MMA
V(
Shiomi, 1994
).
Humans excreted appreciable amounts of MMA
Vin the
urine compared with other mammals. The methylated
metabolites were virtually undetectable in the urine of
mar-moset monkeys that were administered with inorganic
arsenic (
Vahter et al., 1995; Vahter, 2002
). Studies
indi-cated that marmoset, tamarin monkeys and guinea-pigs
were deficient in methyltransferase activity (
Zakharyan
et al., 1996; Healy et al., 1997
). Rat revealed significantly
high methylating activity and it is the only species that
excreted significant amounts of TMAO (
Aposhian, 1997;
Cohen et al., 2006
). The variation in the metabolism of
inorganic arsenic between human and other animals
may result in variant susceptibility to inorganic arsenic
carcinogenesis.
This study demonstrated that the profile of urinary
arsenic metabolites was significantly associated with the
risk of UC. A high PMI and low SMI indicated an
accumu-lation of MMA
Vby an increased upstream input and a
reduced downstream output of the arsenic methylation
pathway metabolites. In addition to bladder cancer, our
previous study reported that skin cancer patients had
higher indicators of %InAs and %MMA
V, and had lower
indicators of %DMA
V, and PMI than healthy controls
(
Hsueh et al., 1997
). These results are compatible with
the study of
Chen et al. (2003a,b)
that showed skin and
bladder cancer patients had a lower SMI in high chronic
arsenic exposure area. Arsenic methylated metabolites in
urine have been reported to be biomarkers for disease
states and disease susceptibility in other ethnicities (
Valen-zuela et al., 2005
). The key metabolic intermediates,
MMA
IIIand DMA
III, have been identified in human urine
(
Mandal et al., 2001
), and these trivalent methylated
arsen-icals are more toxic than inorganic arsenic compounds
(
Styblo et al., 2000; Petrick et al., 2001
). Levels of trivalent
methylated metabolites in the urine are expected to be
sig-nificantly low, since these metabolites have short half-lives
and, therefore, were considered not to be suitable markers
for arsenic methylation at the present time (
Mass et al.,
2001; Gong et al., 2001; Nesnow et al., 2002; Francesconi
and Kuehnelt, 2004
). The arsenic methylation pattern
may remain stable over time and be influenced by factors
such as methylation related enzymes, genes, environmental
exposure, smoking habits and diet (
Francesconi and
Kueh-nelt, 2004; Steinmaus et al., 2005
).
This study discovered the synergy indices of the plasma
folate and urinary methylation profiles ranged from 1.18 to
4.64, revealing significant synergistic interactions between
folate and total arsenic levels, %MMA
V, %DMA
V, or
SMI. Another study also reported that %DMA
Vwas
sig-nificantly positive and %MMA
Vwas negatively related to
the plasma folate levels (
Gamble et al., 2005
). Recently, a
clinical
study
showed
that
folate
supplementation
improved the arsenic methylation efficiency, such as a
low urinary %MMA
Vand a high %DMA
Vin the
supple-mented group compared to the placebo group (
Gamble
et al., 2006
). These observations suggested that subjects
with low folate levels were at a high disease risk especially
when they had low arsenic methylation capability. Indeed,
the arsenic methylation profiles may be altered by the
folate status in the body.
Previous animal studies provided evidence that folate
can influence arsenic methylation, excretion, and toxicity.
(
Kim, 2000; Moyers and Bailey, 2001; Townsend et al.,
2004
). Folate is essential for methylation in the human
body and its metabolism is important for the biosynthesis
of S-adenosylmethionine (SAM), a substrate for
methyla-tion including DNA methylamethyla-tion (
Choi and Mason, 2000;
Gregory and Quinlivan, 2002; Choi and Friso, 2005
) and
arsenic methylation (
Vahter, 1981, 2002
). Inorganic arsenic
is enzymatically methylated and consumes SAM in the
bio-transformation process. The molecular mechanism of
arsenic carcinogenesis may cause DNA damage or alter
the methylation status of DNA (
Zhao et al., 1997; Okoji
et al., 2002; Huang et al., 2004; Reichard et al., 2007
); this
process is similar to folate deficiency. In human colon
epithelial cells, a folate deficiency increased uracil
mis-incorporation 2–3-fold and lowered the cells’ capacity for
DNA repair in response to oxidation or alkylation (
Choi
et al., 1998; Duthie et al., 2000b
). Uracil mis-incorporation
or DNA strand breakage was significantly increased in rat
lymphocytes after 4–8 weeks or 10 weeks of folate deficient
diet intake (
Duthie et al., 2000a,b,c
). The individual
sus-ceptibility may result from the differences in the genes
con-trolling the metabolism of xenobiotics, DNA repair, cell
transport, immune responses, antioxidant defenses and cell
cycle control (
Huang et al., 2004
). Several animal studies
reported that folate binding or transport gene knock-out
mice or rabbits decreased biotransformation and excretion
of arsenic, and these animals were more susceptible to
arsenic and induced the defects (
Vahter and Marafante,
1987; Spiegelstein et al., 2003, 2005a,b; Spuches et al.,
2005
). Based on these observations, the interaction between
folate and the arsenic methylation pathways may cause UC
risk through the DNA methylation or DNA repair
systems.
One limitation of this study is that the UC cases are
pre-valent cases and we cannot rule out the possibility that
folate and/or arsenic methylation patterns changed after
the participants became UC cases. Dietary habits lacked
from the questionnaires are another limitation of this
study; however, the plasma folate levels are a good
bio-marker to reflect the dietary folate intake (
Piyathilake
et al., 1994; Stanger, 2002
).
Inorganic arsenic is a human carcinogen; however, a
good animal model has not yet been found. The arsenic
methylation process may be less efficient and leads to more
severe toxicity in humans than in several other species
(
Cohen et al., 2006
). In this study, we established the
dose–response relationships among two credible markers,
plasma folate levels and arsenic methylation indices, and
UC risk. It was indicated that folate interacted with urinary
arsenic profiles in affecting the UC risk. These results may
identify the susceptible subpopulations and provide insight
into the carcinogenic mechanisms of arsenic even at low
arsenic exposure.
Acknowledgement
This study was supported by Grants
NSC91-3112-B-038-0019,
NSC92-3112-B-038-001,
NSC93-3112-B-038-001,
NSC94-2314-B-038-023,
NSC95-2314-B-038-007,
and
NSC-96-2314-B-002-311 from the National Science
Coun-cil, Executive Yuan, ROC. We thank Dr. Ying-Chin Lin
of the Health Management Center, Taipei Medical
Univer-sity Municipal Wan Fang Hospital for recruitment of the
healthy controls.
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