Abstract PR04: Exogenous estrogen as a mediator of racial differences in insulin-like growth factor-I levels among postmenopausal women

Abstract

Abstract Purpose: The role of exogenous estrogen use in explaining racial/ethnic differences in insulin-like growth factor-I (IGF-I) levels in relation to cancer risk is not clear. We investigated whether the relationship between race and circulating bioactive IGF-I proteins is mediated by exogenous estrogen and the extent to which exogenous estrogen explained the race–IGF-I relationship in postmenopausal women. Methods: This cross-sectional study included 636 white and 133 black postmenopausal women enrolled in an ancillary study of the Women's Health Initiative Observational Study between February 1995 and July 1998. The race–IGF-I relationship was analyzed using ordinal regression, and quartiles of molar ratios of IGF-I/IGF binding protein-3 were used as a proxy of bioactive IGF-I outcomes. To assess exogenous estrogen as a mediator of the race–IGF-I relationship, we used the Baron-Kenny method and an estimation of the proportional change in the odds ratios for race on IGF-I levels plus a bootstrapping test for the significance of the mediation effect. Results: Compared with white women, black 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. When women were stratified by oral contraceptive (OC) ever use, among OC ever users, exogenous estrogen showed a strong mediation effect (67%; P=0.018) in the race–IGF-I relationship. Moreover, when women were classified by a history of hypertension, the IGF-I levels of women with a history of hypertension were higher than those of women with no history of hypertension. Of the women with a history of hypertension, exogenous estrogen explained differences in IGF-I levels between white and black women to a modest degree (23%; P=0.029). Conclusions: Exogenous estrogen use has a potentially important role in racial/ethnic disparities in cancer risk among postmenopausal women. This abstract is also presented as Poster A38. Citation Format: Su Yon Jung, Jennifer Hays-Grudo, Electra Paskett, Stephen D. Hursting, Jenifer Fenton, Michael Pollak, Mara Vitolins, Shine Chang. Exogenous estrogen as a mediator of racial differences in insulin-like growth factor-I levels among postmenopausal women. [abstract]. In: Proceedings of the Twelfth Annual AACR International Conference on Frontiers in Cancer Prevention Research; 2013 Oct 27-30; National Harbor, MD. Philadelphia (PA): AACR; Can Prev Res 2013;6(11 Suppl): Abstract nr PR04.