Arsenic Exposure and Cancer Mortality in a US-based Prospective Cohort: the Strong Heart Study

Baseline data collection

Study visits were performed by trained and certified examiners following a standard protocol (19), and included a questionnaire (sociodemographic factors, smoking status, alcohol use and medical history), a physical examination (height, weight, blood pressure) and biospecimen collection (blood, urine). Hypertension was defined as mean systolic blood pressure ≥140 mm Hg, mean diastolic blood pressure ≥90 mm Hg, or antihypertensive medication. Diabetes was defined as fasting plasma glucose ≥126 mg/dL, 2-h post-load plasma glucose ≥200 mg/dL, hemoglobin A1c ≥6.5%, or use of insulin or oral hypoglycemic agents. Plasma creatinine was measured by an alkaline-picrate rate method to estimate glomerular filtration rate (eGFR). Urine creatinine was measured by an automated alkaline picrate methodology (19).

Urine arsenic and creatinine determinations

The analytical methods to determine arsenic species for this study have been described in detail (20). Inorganic arsenic (arsenite, arsenate), monomethylarsonate (MMA), dimethylarsinate (DMA), and arsenobetaine plus other arsenic cations were measured using anion-exchange high-performance liquid chromatography (Agilent 1100 HPLC, Agilent Technologies, Waldbronn, Germany) coupled with inductively coupled plasma mass spectrometry (Agilent 7700x ICPMS). The limits of detection were 0.1μg/L for inorganic arsenic, MMA, DMA and arsenobetaine plus other cationic arsenic species. The percentages of participants with concentrations below the limit of detection were 5.3% for inorganic arsenic, 0.7 % for MMA, 0.03 % for DMA and 2.1% for arsenobetaine plus other cations. For undetectable concentrations, we divided the limit of detection by √2. The interassay coefficients of variation were 6.0% for inorganic arsenic, 6.5% for MMA, 5.9% for DMA and 6.5% for arsenobetaine plus other cations. Arsenobetaine concentrations were low (median 0.76 μg/L, interquartile range 0.47–1.70 μg/L) confirming that seafood intake is rare in the SHS population.

Cancer mortality follow-up

Follow-up for mortality was complete for 99.8% of the study population. Death certificates were obtained from the State Departments of Health. If the death certificate indicated that an autopsy had been performed, the medical examiner’s report was obtained. Death certificate codes were recorded according to the International Classification of Diseases, 9th Revision (ICD-9) by a single nosologist. In addition to overall cancer deaths, we evaluated the following cancers: esophagus and stomach (ICD-9 150–151), colon and rectum (ICD-9 153–154), liver and intrahepatic bile ducts (ICD-9 155) and gallbladder and extrahepatic bile ducts (ICD-9 156) (ICD-9 155 and 156 were combined due to small sample size and similar results in separate analyses), pancreas (ICD-9 157), trachea, bronchus and lung (ICD-9 162) (referred from now on as lung cancer), breast (ICD-9 174), prostate (ICD-9 185), kidney (ICD-9 189.0) and lymphatic and hematopoietic tissue (ICD-9 200–208). Skin and bladder cancers were not evaluated as the corresponding number of deaths was very small (2 and 1, respectively).

Time to event was calculated from the date of baseline examination to the date of death or to December 31, 2008 whichever occurred first. The mean follow-up time among participants who did not have a cancer death was 14.6 years.

Sources of arsenic exposure in the Strong Heart Study

Participants in the Strong Heart Study rely either on small public water systems or on private wells. At the time of the study, arsenic concentrations in public drinking water systems for the study communities ranged from <10 up to 61 μg/L in Arizona and from <10 up to 21 μg/L in North and South Dakota. Levels in private wells are not known but given arsenic concentrations in groundwater in those regions (29), it is likely that arsenic levels exceeded 10 and even 50 μg/L. In Oklahoma, arsenic levels in public water systems were <10 μg/L, and similar to other populations with low arsenic levels in drinking water (30), we expect diet to be the main source of arsenic exposure in Oklahoma.

Epidemiologic evidence at high levels of exposure

Several studies have evaluated the association between lung cancer and high levels of arsenic exposure. In 10,591 participants from Southwestern Taiwan (27), the hazard ratios for lung cancer incidence were 1.09, 2.28, 3.03 and 3.29 for arsenic concentrations in drinking water of 10–99, 100–299, 300–699 and ≥700 μg/L, respectively, compared to <10 μg/L. In a case-control study from Antofagasta (Chile) (31), the odds ratio for lung cancer comparing the highest (200–400 μg/L) to the lowest (<10 μg/L) categories of arsenic in drinking water was 8.9 (95%CI: 4.0–19.6). Lung cancer mortality was also increased among young adults in Northern Chile who were exposed to high doses of arsenic in utero (32).

Regarding prostate cancer, ecological evidence from Southwestern Taiwan showed increased prostate cancer mortality compared to the overall Taiwanese population (2). Based on the Taiwanese evidence, the IARC concluded there was evidence for a dose-response relationship (1). In a population from Utah exposed to moderate arsenic levels in drinking water, the SMR for prostate cancer compared to the overall US population was 1.48 (95%CI: 1.07–1.91) (33).

Epidemiologic evidence for an association between arsenic and pancreas cancer is scarce. Japanese individuals exposed during infancy to high arsenic levels through contaminated milk showed an SMR for pancreatic cancer of 1.79 (95%CI: 1.23–2.61) compared to unexposed (17). In a case-control study from Spain (18), at low-moderate arsenic in drinking water (34), the odds ratio for exocrine pancreatic cancer comparing the highest (≥0.11 μg/g) to the lowest (≤0.05 μg/g) toenail arsenic quartiles was 2.02 (1.08–3.78).

Few studies have evaluated the association between arsenic and overall cancer mortality. In a prospective cohort including 115,903 participants in Bangladesh, the hazard ratios (95%CI) for cancer mortality were 1.10 (0.77–1.59), 1.44 (1.06–1.95), 1.75 (1.28–2.40) and 1.56 (1.06–2.30) comparing arsenic concentrations in drinking water of 10–49, 50–149, 150–299 and ≥300 to <10 μg/L, respectively (35).

Epidemiologic evidence at low-moderate levels of exposure

Evidence at low-moderate arsenic levels remains limited for most cancers. Bladder cancer has been the most frequently studied cancer, with inconsistent findings (33, 36, 37). For lung cancer, a case-control study conducted in New Hampshire and Vermont counties found an odds ratio for small-cell and squamous-cell carcinoma of 2.75 (95% CI 1.39–5.91) comparing toenail arsenic concentrations ≥0.114 vs. <0.05 μg/g (38). In our study, histological information was not available and we cannot evaluate if the association found with lung cancer mortality was also related to those cancer types. At very low levels of exposure, a Danish cohort of 57,053 participants found no association between arsenic in drinking water (mean 1.2 μg/L) with lung, liver, bladder, kidney or prostate cancer incidence over 10 years of follow-up (36). Our study, conducted in a population exposed to low-moderate arsenic levels, extends previous evidence for mortality associated with several cancer types but included too few cases to evaluate some cancers of interest, including bladder and skin cancers.

Mechanistic evidence

Despite the extensive epidemiologic evidence, the mechanisms for arsenic carcinogenicity remain unclear. A major challenge has been to develop animal models, with transplacental models being essential to advance our understanding of arsenic carcinogenesis (39). Some of the accepted mechanisms involve genetic and epigenetic changes, oxidative stress, enhanced cell proliferation, mitochondrial damage and modulation of gene expression (40–42). For prostate cancer, arsenic accelerates prostate cancer cell progression by inducing androgen independence (10). Little is known about mechanisms for arsenic related pancreatic cancer, although arsenic induces oxidative stress and apoptosis of pancreatic β-cells (43) and acinar cells (44), as well as changes in the pancreatic vasculature (45).

Lymphatic and hematopoietic cancers

In occupational settings (46, 47), and in infants exposed through milk powder (17), arsenic exposure has been associated with increased blood cancer mortality. Arsenic trioxide is used in the treatment of some leukemias such as acute promyelocytic leukemia (48), and is under investigation for multiple myeloma (49). Arsenic trioxide pharmacotherapy has been related to cytotoxicity and apoptosis in cancer cells (50). Since we could not specifically evaluate the association between arsenic and specific lymphatic or hematopoietic cancers due to the small number of cases, this finding requires replication in other populations and needs to be interpreted with caution.

Strengths and limitations

The prospective design and long follow-up, the low rate of follow-up losses, the careful standardization and quality control of data collection and laboratory analyses, and the high sensitivity of our assay for urine arsenic species are major strengths of this study (19, 20). Urine arsenic, an integrated biomarker of ongoing exposure, has a relatively short half-life. Previous evidence in the SHS, however, showed relatively constant concentrations over a 10 year-period (51), indicating that a single measure reflects long-term arsenic exposure in this population. Moreover, the Strong Heart Study is a very stable population and most of the participants were born on the communities and have lived there all their lives. The estimated migration rates during the study period are 0.5% in Arizona, 1.5% in Oklahoma, and 2.7% in North and South Dakota. Finally, this study provides information on environmental cancer determinants in American Indian communities, a population understudied for cancer risk that is disproportionately exposed to arsenic in drinking water compared to the general US population (51).

Our study has some limitations. First, cancer mortality is an imperfect outcome to study tumors with relatively good prognosis, such as bladder and skin cancer. Second, we relied on death certificates, which may lack accuracy compared to clinical or pathology records (52). Cancer deaths, however, are considered to be better coded than other causes of death. Third, the number of deaths and statistical power was limited for some cancers, for subgroup analyses, and to detect a potential small increase in overall cancer mortality. For instance, we could not evaluate the association with some cancer deaths separately such as colon and rectal cancer, or esophagus and stomach cancer. Fourth, we did not account for multiple comparisons, although the number of statistically significant associations expected to happen by chance for a total of 9 cancer-specific associations is less than one. Fifth, we could not account for family history of cancer or for clustering of arsenic exposure. Finally, arsenic levels in drinking water were only available at the community level. Arsenic in urine, however, correlates well with levels in drinking water (53) and integrates multiple sources of exposure including food (54).

Financial support: This work was supported by the National Heart Lung and Blood Institute (grant HL090863 and by SHS grants HL41642, HL41652, HL41654 and HL65521). Dr. Esther Garcia-Esquinas was supported by a Río Hortega (CM10/00332) research training grant from the Spanish Ministry of Economy and Competitiveness (Carlos III Institute of Health). Dr. Navas-Acien had full access to all the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.