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J Gerontol A Biol Sci Med Sci. 2015 Apr;70(4):495-502. doi: 10.1093/gerona/glu164. Epub 2014 Sep 18.

Exogenous estrogen as mediator of racial differences in bioactive insulin-like growth factor-I levels among postmenopausal women.

Author information

1
Department of Epidemiology, Division of Cancer Prevention and Population Sciences, The University of Texas MD Anderson Cancer Center, Houston. sjung@mdanderson.org.
2
Department of Epidemiology and Prevention, Division of Public Health Sciences, Wake Forest School of Medicine, Winston-Salem, North Carolina.
3
Division of Cancer Prevention and Control, College of Medicine, Ohio State University, Columbus.
4
Department of Epidemiology, Division of Cancer Prevention and Population Sciences, The University of Texas MD Anderson Cancer Center, Houston.

Abstract

BACKGROUND:

The role of exogenous estrogen use in racial differences in insulin-like growth factor-I (IGF-I) levels which affect cancer risk is unclear. We investigated whether the relationship between race and circulating bioactive IGF-I proteins was mediated by exogenous estrogen and the extent to which exogenous estrogen influenced the race-IGF-I relationship in postmenopausal women.

METHODS:

This cross-sectional study included 636 white and 133 African American postmenopausal women enrolled in an ancillary study of the Women's Health Initiative Observational Study. To assess exogenous estrogen use (nonusers [n = 262] vs users [n = 507]) as a mediator of the race-IGF-I relationship, we used the Baron-Kenny method and an estimation of the proportional change in the odd ratios for IGF-I levels on race plus a bootstrapping test for the significance of the mediation effect.

RESULTS:

Compared with white women, African American women were more likely to have high IGF-I levels and less likely to use exogenous estrogen. After accounting for race, estrogen nonusers had higher IGF-I levels than estrogen users did. Among oral contraceptive ever users, exogenous estrogen had a strong mediation effect (67%; p = .018) in the race-IGF-I relationship. In the women with a history of hypertension, exogenous estrogen explained racial differences in IGF-I levels to a modest degree (23%; p = .029).

CONCLUSIONS:

Exogenous estrogen use has a potentially important role in disparities in IGF-I bioactivity between postmenopausal African American and white women. A history of oral contraceptive use and hypertension may be part of the interconnected hormonal pathways related to racial differences in IGF-I levels.

KEYWORDS:

Exogenous estrogen; Insulin-like growth factor-I; Mediation; Postmenopausal women.

PMID:
25238773
PMCID:
PMC4447798
DOI:
10.1093/gerona/glu164
[Indexed for MEDLINE]
Free PMC Article

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