In the present study, the IRS-1 G972R variant was associated with circulating sex hormone levels in African-American patients. African-American carriers of the variant (A) allele had higher serum levels of E1, T, and free T, and decreased levels of SHBG than carriers of the wildtype allele. No statistically significant differences in serum hormone levels were observed for non-Hispanic white patients by genotype.
In general, African-American women have higher breast cancer mortality rates than whites [41
]. The reasons for this increased mortality are likely to be multifaceted including factors related to socioeconomic status, access to health care, course of treatment, or characteristics of the tumor such as tumor size, grade, or stage [41
]. Given that circulating estrogen levels are associated both with breast cancer onset [47
] and prognosis [2
], the results of the present study are consistent with a potential role of IRS-1
in racial/ethnic differences in breast cancer prognosis. However, there is no apparent association of the G972R variant with stage of disease in this data and the women have not been observed long enough to examine survival. The presence of an association between G972R and sex hormones and binding proteins in African-American, but not white patients, may indicate a chance association; however, it may also indicate that the variant is in linkage disequilibrium with a true causal variant. If, however, G972R is a functional variant, the observed differences by race/ethnicity may reflect the influence of genetic background or individual characteristics (e.g. weight) on penetrance.
As a functional variant or a marker of a causal variant in African-American breast cancer patients, G972R may be altering sex hormone and binding protein levels by decreasing insulin-stimulated signaling [25
]. It has also been suggested that the IRS-1
variant may contribute to insulin resistance [49
] and elevated insulin levels [50
] by impairing the ability of insulin to stimulate glycogen synthesis [51
] and glucose transport [53
]. This could potentially result in hyper-insulinemia [25
], a reduction in SHBG [54
], and a subsequent increase in circulating levels of E1
and T. However, these finding have not been supported by all studies [26
]. In our data, C-peptide (a surrogate measure of insulin) was not significantly different by genotype within African Americans and whites. Alternatively, it has been suggested by Ando et al [12
] that the IRS-1
variant might reduce ER binding capacity, which would in turn increase circulating levels of estrogen. Experimental studies are necessary to explore the possible mechanisms through which the IRS-1
variant may influence sex hormone levels, ER expression and function.
Previous epidemiological studies found that the IRS-1
G972R variant is associated with insulin resistance [25
], body fat distribution [27
], type II diabetes [28
], hyperlipidemia, and coronary artery disease [29
]. Other studies, however, have not confirmed these findings [50
]. The IRS-1 protein is expressed in a variety of solid tumors, including breast cancer, Wilms' tumors, rhabdomyosarcoma, liposarcoma, leiomyoma, leiomyosarcoma, and adrenal cortical carcinoma [64
]. A role for IRS-1 in the cross-talk between the estrogen, insulin, and IGF-1 pathways has been shown in experimental studies. In mice, IRS-1 plays a role in mammary gland development and this function is regulated by steroid hormones, especially the combination of estrogen and progesterone [65
]. In vitro
] have found that estrogen, especially E2
, can stimulate and increase the expression of IRS-1 protein levels in breast tumor cells resulting in enhanced insulin or IGF-1 signaling. The mechanisms through which the insulin or IGF-1 pathways influence the estrogen pathway are not as well characterized; however, experimental studies have shown that the IRS-1 protein can alter ER expression and function [12
]. For example, breast tumor cells with IRS-1 deficiency display up-regulation of ER protein expression and binding capacity, loss of insulin-estradiol synergism and loss of insulin-induced regulation of ER tyrosine phosphorylation [12
]. In total, the laboratory evidence suggests that IRS-1 may be an important mediator of the cross-talk or synergistic relationship between insulin/IGF-1 pathways and sex hormones in breast cancer. To our knowledge, human subject data characterizing the relationship between the IRS-1 variant and sex hormone levels have not previously been reported.
While we were able to detect an association between IRS-1, sex hormone levels and binding proteins among African-American patients, the generalizability of the findings across racial/ethnic groups is limited by our sample size. The coefficients of variation for some hormones, such as E2, were large and reflect the difficulty in measuring relatively low sex hormone levels in post-menopausal women. While we designed the analysis to include blood samples taken at 30+ months post-diagnosis, when most women would have finished chemotherapy or radiation therapy, the hormone levels of some breast cancer survivors may have been permanently reduced by past chemotherapy or radiotherapy; we do not expect this to differ by IRS-1 genotype. Tamoxifen, which is still used by many women at 30+ months post-diagnosis, could potentially influence circulating sex hormone levels. However, the association between hormone levels and genotype in African-American patients persisted, even after controlling for current tamoxifen use and in strata of current tamoxifen users versus non-users.
In summary, our study found a statistically significant association between the IRS-1 G972R variant and sex hormones in African-American post-menopausal breast cancer survivors that was not observed for white patients; these findings will need replication in additional studies. African-American women diagnosed with breast cancer are considered to have a poorer prognosis than white women diagnosed with a similar stage of disease; variation in the IRS-1 pathway may represent one factor that contributes to these differences. Follow-up of the HEAL Study cohort for disease-free survival and mortality in the next several years will provide an opportunity to assess the role of IRS-1 gene variation on breast cancer prognosis.