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Unité de Recherche en Santé Publique, Centre de Recherche du Centre Hospitalier de lUniversité Laval, Centre Hospitalier Universitaire de Québec, Beauport, Québec, G1E 7G9 Canada [A. D., P. A., S. D., É. D.]; Groupe de Recherche en Épidémiologie et Centre des Maladies du Sein, Pavillon Saint-Sacrement, Centre Hospitalier Affilié Universitaire de Québec, Québec, G1S 4L8 Canada [J. B., J. R.]; Département de Médecine Sociale et Préventive, Faculté de Médecine, Université Laval, Ste-Foy, G1K 7P4 Canada [P. A., J. B., É. D.]; and Unité de Recherche en Endocrinologie de la Reproduction, Centre de Recherche du Pavillon Saint-François-dAssise, Québec, G1L 3L5 Canada [S. D.]
| Abstract |
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Plasma lipid concentrations of 11 chlorinated pesticides and 14 polychlorinated biphenyl congeners were measured in 315 women newly diagnosed with breast cancer, 219 hospital-based controls, and 307 population controls from the Quebec City area (Canada). Concentrations of hormonally active organochlorines or their surrogates were compared between cases and controls as well as between groups of cases defined according to tumor size and axillary-lymph-node involvement.
We found similar levels of organochlorines in cases and controls and no relationship between the relative risk of breast cancer and organochlorine exposure. However, the probability of lymph-node invasion among cases increased with exposure to 1,1-dichloro-2,2-bis(4- chlorophenyl)ethylene [p,p'-DDE; odds ratio, 2.54; 95% confidence interval (CI), 1.205.35; between the highest and the lowest tertiles]. Furthermore, p,p'-DDE exposure was associated with a dose-related increased relative risk of exhibiting both lymph-node involvement and a large tumor. Indeed odds ratio raised to 2.33 (95% CI, 0.945.77) for the second tertile relative to the first tertile and reached 3.51 (95% CI, 1.418.73) for the third tertile relative to the first tertile. Similar associations were noted with ß-hexachlorocyclohexane, oxychlordane, and trans-nonachlor.
We conclude that exposure to persistent, hormonally active organochlorines during adulthood is not associated with breast cancer risk. The possibility that some organochlorines and especially p,p'-DDE may increase breast cancer aggressiveness deserves further attention.
| Introduction |
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Results from early human studies generally supported the existence of a relationship (16, 17, 18, 19) or suggested a possible link (20) between breast cancer risk and organochlorine exposure, more specifically with p,p'-DDE, the main metabolite of DDT. In contrast, recent studies involving larger sample sizes yielded negative results (21, 22, 23, 24, 25) . In particular, Hunter et al. (21) and Høyer et al. (25) , using a nested case control study design, failed to observe a relationship between p,p'-DDE or PCB plasma concentrations and breast cancer risk. However, Høyer et al. (25) reported that high plasma concentrations of dieldrin were associated with breast cancer risk.
Previous studies have focused solely on the risk of developing a new breast cancer. However, hormonally active organochlorines might also modulate cancer growth. The present study tests the hypothesis that the risk of developing breast cancer is related to exposure to selected organochlorines that either possess estrogenic properties themselves or are surrogates of past exposure to less persistent estrogenic compounds. In addition, we also investigated the possible relationship between exposure to organochlorines and two indicators of breast cancer aggressiveness and prognosis: axillary-lymph-node involvement and tumor size.
| Materials and Methods |
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Blood samples were obtained from cases and hospital controls after surgery and for cases before the initiation of chemotherapy or radiotherapy. A research nurse visited population controls at their residence, and blood sampling was performed during this visit. Information on demographic, anthropometric characteristics, life style habits, and reproductive history were obtained from cases and controls by telephone interview.
Assay Methods.
Blood samples (20 ml) were collected in vials containing EDTA and were
centrifuged (10 min, 5000 rpm), and the plasma was transferred in glass
vials prewashed with hexane. Plasma samples were stored frozen at
-20°C until the time of analysis. A 2-ml aliquot was extracted with
hexane, and the lipid extract was cleaned up on Florisil columns.
Fourteen PCB congeners (International Union of Pure and Applied
Chemistry numbers 28, 52, 99, 101, 105, 118, 128, 138, 153, 156, 170,
180, 183, 187) and 11 chlorinated pesticides or their metabolites
(aldrin,
-chlordane,
-chlordane, p,p'-DDT,
p,p'-DDE, hexachlorobenzene, ß-HCH, mirex,
cis-nonachlor, trans-nonachlor, oxychlordane)
were quantified in the eluate using a HP-5890 series II gas
chromatograph equipped with dual capillary columns and dual Ni-63
electron-capture detectors. Peaks were identified by their relative
retention times obtained on the two columns using a computer program
developed in-house. Quantification was mainly performed on the Ultra-1
column. The limit of detection, based on three times the average SD of
noise, was 0.02 µg/liter for PCB congeners and chlorinated
pesticides, with the exception of p,p'-DDT and ß-HCH (0.03
µg/liter). The average percentage recoveries were >95% for PCB
congeners and ranged from 90 to 103% for chlorinated pesticides. The
between-day precision ranged from 3.3 to 7.0% for PCB congeners and
from 5.5 to 14.2% for chlorinated pesticides.
Because organochlorines distribute mainly in body fat, and blood was collected from nonfasting participants, results are reported in µg/kg of plasma lipids to take into account the postprandial rise in blood lipids (26) . Total and free cholesterol, phospholipids, and triglycerides were quantified by enzymatic assays.
Statistical Analysis.
Characteristics of cases were compared to those of control groups using
Students t tests for continuous variables or
2 tests for categorical variables. Variance
analysis was used to compare the mean concentrations of organochlorines
between cases and controls. Organochlorine concentrations in plasma
lipids displayed log-normal distributions and therefore, these
statistical analyses were performed using the natural logarithm of
organochlorine concentrations. A concentration equal to half the
detection limit was assumed for samples with organochlorine levels
below the detection limit.
Point and interval estimates of relative risk were based on
unconditional logistic regression analysis. Quintile and tertile limits
of plasma organochlorine concentrations were based on the distribution
observed among controls. Risks were calculated relative to the lowest
category. Age (3040<, 4050<, 5060<,
60 years) and region of
residence (rural/urban) were included in all multivariate models. The
other variables tested for confounding were BMI
(kg/m2), total energy consumed, alcohol
consumption, age at first cigarette, number of fertile years, age at
first child, total breast feeding duration, use of oral contraceptive,
use of hormone therapy, first-degree family history of breast cancer,
history of benign breast disease, and time separating blood sampling
from surgery. A variable was considered as a confounder when its
inclusion in the model modified OR (adjusted for age and region of
residence) by >10%. All statistical analyses were performed using the
SAS software (SAS Institute Inc., Cary, NC).
The following organochlorines were detected in <70% of plasma
samples: aldrin,
-chlordane,
-chlordane,
cis-nonachlor, mirex, and PCB congeners 28, 52, 101, 105,
and 128. Regardless of their biological activity, these compounds were
excluded from statistical analyses because the precision of measurement
decreases when values approach the limit of detection. Out of those
remaining, the following compounds were selected that either display
estrogenic properties or are indicators of past exposure to less
persistent xenoestrogens: p,p'-DDT, p,p'-DDE,
ß-HCH, trans-nonachlor, and oxychlordane.
2,2',4,4',5,5'-Hexachlorobiphenyl (PCB-153) was selected as a surrogate
for all PCB congeners because this persistent congener is the most
abundant in plasma samples and exhibits a strong correlation
(r
0.72; P < 0.0001; Pearson
correlation coefficient) with other highly prevalent congeners (99,
118, 138, 156, 170, 180, 183, 187).
| Results |
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2 cm). In the crude
analysis, mean organochlorine plasma levels in this group of patients
were not different from those measured in patients with smaller tumors,
and the OR did not increase with plasma organochlorine concentrations
(data not shown). After adjusting for age, region of residence, BMI,
breast feeding duration, number of fertile years, and time separating
blood sampling from surgery, the relative risk of having a large tumor
increased with plasma concentrations of most organochlorines, although
statistically-significant ORs were observed only for ß-HCH (OR, 2.25;
95% CI, 1.124.51) and trans-nonachlor (OR, 2.27; 95% CI,
1.114.65), comparing the highest to the lowest tertiles (Table 4)
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The relation of breast cancer aggressiveness to plasma concentration of
p,p'-DDE was further explored by considering both tumor size
and lymph-node involvement in the same model (Table 5)
. ORs of having involved nodes after adjustment for tumor size and
confounding factors increased with tertiles of p,p'-DDE,
reaching 2.54 (95% CI, 1.205.35) for the highest tertile as compared
to the lowest. In contrast, ORs of having a large tumor after
adjustment for lymph-node involvement and confounding factors did not
increase with p,p'-DDE plasma concentrations (OR, 1.18; 95%
CI, 0.562.21; for the highest tertile as compared to the lowest).
However, using cases with a small tumor and without lymph-node
involvement as the referent group, the multivariate relative risk of
breast cancer characterized by both a large tumor and lymph-node
involvement increased in a dose-related manner with tertiles of
p,p'-DDE. The OR comparing the second tertile to the first
was 2.33 (95% CI, 0.945.77) and 3.51 (95% CI, 1.418.73) for the
third tertile relative to the first. Similar associations were noted
with ß-HCH (OR, 3.91; 95% CI, 1.4710.35; third tertile
versus first), oxychlordane (OR, 3.22; 95% CI, 1.188.80;
third tertile versus first), and trans-nonachlor
(OR, 3.92; 95% CI, 1.4210.82; third tertile versus
first).
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| Discussion |
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Because half-lives of the most persistent organochlorines are in the order of 1020 years, our measure of exposure is probably a good estimate of the body burden that prevailed in women during the decade before diagnosis. Hence, our conclusion and those of other case-control studies conducted on this subject only apply to organochlorine exposure occurring relatively late during adulthood, not to prenatal exposure or exposure during puberty.
Women diagnosed with breast cancer who had higher plasma concentrations
of p,p'-DDE, ß-HCH, oxychlordane, or
trans-nonachlor were more likely to show both a large tumor
(diameter
2 cm) and axillary-lymph-node involvement. These
results suggest that organochlorine exposure may influence the growth
or aggressiveness of the disease rather than initiate breast cancer, at
least in the range of organochlorine concentrations found in women from
the Quebec City area. With regard to breast cancer aggressiveness, only
Hunter et al. (21)
stratified cases according
to axillary-lymph-node involvement at diagnosis and reported similar
levels of p,p'-DDE and PCBs in both groups of women. It is
unclear whether these authors made an adjustment for confounding
factors. In the present study, crude comparisons of organochlorine
concentrations and OR calculations did not reveal any association
either with axillary-lymph-node involvement or tumor size.
Statistically significant associations were revealed only after
adjustment for confounding factors. Age was the most important
confounding factor because it was strongly associated with both
organochlorine plasma levels and lymph-node involvement. Log values of
p,p'-DDE concentration increased with age (r = 0.36; P = 0.0001; Pearson correlation coefficient),
and women <50 years of age were more likely to exhibit
axillary-lymph-node involvement than older women (55%
versus 37%;
2 = 8.2;
P = 0.004).
Alternatively, the toxicokinetics of persistent lipophilic organochlorines may be different in women developing a more aggressive disease, leading to higher plasma concentrations and artifactual associations. In particular, it might be argued that because blood samples in the present study were collected near the time of diagnosis, the disease process might have modulated organochlorine concentrations in plasma lipids. Indeed, severe weight loss can lead to an augmentation in organochlorine plasma lipid levels (27 , 28) . However, this is unlikely to have affected our results because severe weight loss is not expected in early stages of breast cancer. Few cases in our study reported losing weight during the year before diagnosis (4.5% of cases with positive lymph-node status and 4.3% of those with negative lymph-node status).
The mechanism by which organochlorines might influence the growth or aggressiveness of breast cancer is not clear but may involve the capacity of some of them to mimic or antagonize the effects of endogenous sex hormones. There are no data on the endocrine disrupting potential of oxychlordane and trans-nonachlor. Both compounds are indicators of past exposure to chlordane, which displayed weak estrogenic activity in a yeast-based ER assay (11) . ß-HCH was shown to stimulate the proliferation of MCF-7 and T47D breast cancer cell lines (10) . p,p'-DDT, the main component of technical DDT, also elicited weak estrogenic effects in several in vitro systems (9 , 29 , 30) . In addition to being an indicator of past DDT exposure, p,p'-DDE has been identified as a potent antiandrogen agent (31) . Androgens have been shown to inhibit the proliferation of hormone-responsive breast cancer cell lines (32) and the growth of mammary carcinoma in the rat (33) . This inhibitory effect on tumor growth was alleviated by blocking androgen receptors with flutamide (33) . The antiandrogen p,p'-DDE could in a similar way counteract cancer growth inhibition normally exerted by androgens and possibly accelerate breast cancer progression.
In conclusion, we observed no association between exposure during adulthood to various persistent, hormonally active organochlorines and breast cancer risk. However, this exposure was linked to an increased risk of having a large tumor and axillary-lymph-node involvement among cases. Replication of these results and additional studies examining other prognostic factors as well as survival are needed to determine whether exposure to organochlorines, especially to p,p'-DDE, can lead to more aggressive breast tumors and a less favorable clinical course.
| Acknowledgments |
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| Footnotes |
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1 Supported by Grant 4811-82 from the National
Cancer Institute of Canada. ![]()
2 To whom requests for reprints should be
addressed, at Unité de Recherche en Santé Publique, 2400
dEstimauville, Beauport, Québec, G1E 7G9 Canada. E-mail: edewailly{at}cspq.qc.ca ![]()
3 The abbreviations used are: PCB, polychlorinated
biphenyl; DDT, dichlorodiphenyltrichloroethane; o,p'-DDT,
2-(2-chlorophenyl)-2-(4-chlorophenyl)-1,1,1-trichloroethane;
p,p'-DDT, 2,2-bis(4-chlorophenyl)-1,1,1-trichloroethane;
p,p'-DDE,
1,1-dichloro-2,2-bis(4-chlorophenyl)ethylene; ß-HCH,
ß-hexachlorocyclo-hexane; BMI, body mass index; OR, odds ratio;
CI, confidence interval; ER, estrogen receptor. ![]()
Received 6/ 4/99; revised 11/ 2/99; accepted 11/30/99.
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