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Short Communication |
1 Channing Laboratory, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School; 2 Department of Epidemiology, Harvard School of Public Health; and 3 Division of Endrocrinology, Diabetes, and Metabolism, Department of Medicine, Beth Israel Deaconess Medical Center, Boston, Massachusetts; and 4 Departments of Medicine and Oncology, McGill University, Montreal, Quebec, Canada
Requests for reprints: Heather Eliassen, Channing Laboratory, Brigham and Women's Hospital, 181 Longwood Avenue, Boston, MA 02115. Phone: 617-525-2104; Fax: 617-525-2008. E-mail: heather.eliassen{at}channing.harvard.edu
| Abstract |
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| Introduction |
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During insulin secretion, proinsulin is cleaved into equimolar amounts of c-peptide and insulin. Although insulin levels fluctuate acutely with meals, c-peptide has a longer half-life and is therefore a useful marker of insulin secretion (8). In previous prospective studies of c-peptide or insulin levels, nonsignificant positive (9-11), null (12, 13), and inverse (14) associations have been observed, with conflicting findings by menopausal status (10, 11, 13).
We conducted a nested case-control study within the Nurses' Health Study II to examine the associations of insulin and c-peptide with breast cancer risk among 317 cases and 634 matched controls, the majority of whom were premenopausal at blood collection.
| Materials and Methods |
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Breast cancer cases were identified on biennial questionnaires; the National Death Index was searched for nonresponders. Cases had no previously reported cancer diagnosis and were diagnosed with breast cancer after blood collection but before June 1, 2003. Overall, 317 cases of breast cancer were reported and confirmed by medical record review (n = 298) or by the nurse herself (n = 19). Given the 99% confirmation rate upon medical record review, these latter cases were included. Mean time from blood draw to diagnosis was 31 months (range = 1-87). Each case was matched to two controls (total n = 634) on age (±2 years), menopausal status at blood collection and diagnosis (premenopausal, postmenopausal, unknown), month/year of blood draw (±2 months), race/ethnicity (African American, Asian, Hispanic, Caucasian, Other), luteal day (timed samples only, date of next period date of luteal draw, ±1 day), and for each blood collection, time of day (±2 hours), and fasting status (<2 h, 2-4, 5-7, 8-11,
12). For each matching variable, >90% of matches were exact.
Laboratory Assays
Insulin and c-peptide were assayed in luteal and untimed samples. Insulin was measured in fasting samples only (n = 211 cases and 414 controls) by RIA (Linco Research, Inc., St. Charles, MO) in the laboratory of Dr. Christos Mantzoros. c-Peptide was assayed using ELISA (Diagnostic Systems Laboratory, Webster, TX) in the laboratory of Dr. Michael Pollak. Estradiol and testosterone were assayed at Quest Diagnostics (San Juan Capistrano, CA) by RIA following extraction and celite column chromatography. Free estradiol and testosterone were calculated per Sodergard et al. (16). SHBG was measured at the Royal Marsden Hospital by chemiluminescent immunoassay with the Immulite auto-analyzer (Diagnostic Products, Gwynedd, United Kingdom).
Case-control sets were assayed together and were ordered randomly and labeled to mask case-control status. Samples were assayed in two batches; the inter-assay coefficients of variation from masked replicate samples were <6% for insulin and c-peptide and <14% for estradiol, SHBG, and testosterone.
Statistical Analysis
We identified and excluded statistical outliers [ref. 17; insulin >40 µU/mL (n = 8), luteal free estradiol > 10.6 pg/mL (n = 1)]. Several samples had missing hormone values related to technical difficulties or low sample volume; the final case/control sample sizes for insulin and c-peptide analyses were 208/409 and 316/629, respectively. Quartile cut points and Spearman correlation coefficients were based on control distributions. We used mixed effects regression, by case-control set to account for matching, to test the paired differences in log-transformed hormone levels between cases and controls.
We used conditional logistic regression to estimate relative risks (RR) and 95% confidence intervals (95% CI). Multivariate models adjusted for common breast cancer risk factors, including body mass index (BMI) at age 18, BMI at blood collection, ages at menarche and first birth, parity, family history of breast cancer, and history of benign breast disease. We used unconditional logistic regression, adjusting for matching factors, in stratified analyses; results from multivariate unconditional and conditional logistic regression models were essentially identical. Tests for trend were conducted by modeling continuous quartile median concentrations and calculating the Wald statistic. Tests for interaction compared the slope of the quartile medians between groups (Wald test). All Ps were based on two-sided tests and were considered statistically significant if <0.05.
| Results |
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Insulin and c-peptide levels were similar between cases and controls (Table 1 ). c-Peptide levels were lower among women who gave fasting blood samples and, again, similar between cases and controls. Insulin and c-peptide levels were both moderately correlated with BMI (r = 0.41 and 0.40, respectively) and fairly strongly correlated with one another (r = 0.68; Table 2 ). Both insulin and c-peptide were modestly, but significantly, inversely associated with follicular total estradiol and total testosterone (insulin: r = 0.24 and 0.12, respectively; c-peptide: r = 0.19 and 0.20, respectively). Insulin and c-peptide were significantly positively associated with free testosterone (r = 0.24 and 0.20, respectively) and inversely associated with SHBG (r = 0.36 and 0.39, respectively).
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2.29 ng/mL: RR, 1.1; 95% CI, 0.6-2.2; Ptrend = 0.87) or fasting-specific cut points (top quartile
1.82 ng/mL: RR, 1.2; 95% CI, 0.6-2.1; Ptrend = 0.62). When we adjusted for sex steroid hormones and SHBG, the results were unchanged with the exception of adjusting for luteal estradiol (RR, 1.8; 95% CI, 0.9-3.8; Ptrend = 0.24) and luteal free estradiol (RR, 1.7; 95% CI, 0.8-3.6; Ptrend = 0.31).
The associations among insulin, c-peptide, and breast cancer risk also were similar among cases with ductal tumors and by tumor size (
2 versus >2 cm; data not shown). No substantial differences were observed when stratifying by age, BMI, waist-to-hip ratio, or time since diagnosis. Additionally, exclusion of cases diagnosed in the first 2 years following blood draw did not change the results. When we included both insulin and c-peptide in the same model, insulin results were unchanged, and the RR for the top quartile of c-peptide increased to 1.4 (95% CI, 0.6-3.2), although the trend was still nonsignificant (P = 0.44).
| Discussion |
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Insulin and insulin resistance have been hypothesized to increase breast cancer risk and may factor into the association between weight and breast cancer risk. Insulin may affect risk independently because it increases proliferation in breast cancer cells, and because the insulin receptor is expressed in normal and malignant breast tissue (1, 2). Alternatively, insulin may affect risk indirectly by inhibiting SHBG production (3) and increasing circulating ovarian androgens (19). Finally, insulin increases the bioavailability of insulin-like growth factor I, which may be associated with breast cancer (20), by inhibiting the synthesis of the insulin-like growth factor binding protein 1 (21). Although these hypotheses indicate a role for insulin in breast carcinogenesis, the inverse association between adiposity and breast cancer among premenopausal women (22) suggests that the association between insulin and breast cancer may be complex in premenopausal women.
Results from observational studies have been mixed. Positive associations with diabetes have been reported in prospective studies among postmenopausal women or all women combined (23), but no associations were observed among premenopausal women in two studies (24, 25). In the Women's Health Study, higher levels of hemoglobin A1c were significantly inversely associated with risk among postmenopausal, but not premenopausal, women (26). Nonsignificant positive (9-11), null (12), and inverse (13, 14) associations have been observed with c-peptide (9, 10, 13) or insulin (11, 12, 14) in prospective studies. In the most recent study, within the EPIC cohort, c-peptide was inversely associated with risk among younger women (
50 at diagnosis; top versus bottom quintile: RR, 0.7; 95% CI, 0.4-1.2; Ptrend = 0.05; ref. 13), similar to the estimate for insulin in a Swedish study (top versus bottom quartile: RR, 0.6; 95% CI, 0.3-1.2; Ptrend = 0.31; ref. 14). Although we observed an inverse association between insulin levels and risk, the lack of trend and the inconsistency with the c-peptide results suggests this may be a chance finding. We would expect similar results for the two biomarkers given that proinsulin is cleaved into equimolar amounts of insulin and c-peptide, and we used fasting insulin levels to avoid the acute fluctuations with meals (8). An inverse association among premenopausal women is possible if insulin is merely a marker of adiposity, and adjustment for BMI in the EPIC analyses did attenuate the estimate to the null (13). However, adjustment for BMI in our analyses resulted in a slightly stronger association. Given the inverse correlations of insulin and c-peptide with estradiol, and the positive association we observed between estradiol and breast cancer risk (5), it is logical that adjustment for estradiol increased the RRs for both insulin and c-peptide, but neither trend was significant.
Our study has several strengths, including the prospective nature, with blood samples collected before diagnosis. Although c-peptide levels are less susceptible than insulin to meal-related fluctuations, we were able to analyze both hormones and perform secondary analyses of c-peptide among women with fasting blood samples. In addition, we were able to analyze estrogen/progesterone receptorpositive and invasive cases separately. However, we were somewhat limited in that we could not examine estrogen/progesterone receptornegative tumors or an interaction by menopausal status. Although we used a single blood sample, insulin levels have been shown to be fairly consistent over time (ICC = 0.70 for samples collected a year apart; ref. 11). Finally, our case numbers limited our ability to detect meaningful differences among subgroups.
Higher insulin levels were inversely associated with risk, but we did not observe a similar association with c-peptide. Thus, high levels of insulin and c-peptide do not seem to be substantial risk factors for breast cancer among predominantly premenopausal women.
| Acknowledgments |
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| Footnotes |
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 8/14/06; revised 10/30/06; accepted 11/ 3/06.
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