In this study, we found that two functionally relevant adiponectin polymorphisms, +45 T→G (rs2241766) and +276 G→T (rs 1501299), were significantly associated with breast cancer risk. Furthermore, a polymorphism of ADIPOR1
, +10225 C→G (rs7539542), which has been shown to alter mRNA levels of the receptor (with the GG and CG polymorphisms increasing mRNA levels; ref. 26
), was also significantly associated with breast cancer risk. This study shows that adiponectin signaling, as assessed by a combination of functionally relevant SNPs, may predict breast cancer risk.
High BMI has been associated with postmenopausal breast cancer (33
). The prevailing theory behind the association between obesity and breast cancer is based on the increased levels of estrogens and/or insulin resistance observed in obese women (35
). The established association between breast cancer and insulin resistance and obesity has drawn interest for adipokines, a group of proteins synthesized in the adipose tissue (4
Adiponectin is secreted by adipose tissue, and its levels are inversely correlated to BMI. Several studies have shown that low levels of adiponectin are directly associated with increased breast cancer risk (11
). Adiponectin can act as a growth inhibitor for breast cancer cell lines. More specifically, adiponectin has been shown to inhibit the growth of the breast cancer cell lines MDA-MB-231 (38
) and T47D (15
). Furthermore, it was shown that in MCF-7 cells, growth stimulation with estradiol was suppressed by the presence of adiponectin (20
). Several theories exist as to the link between adiponectin and breast carcinogenesis. Adiponectin is inversely correlated with insulin levels and is also associated with IGF-binding proteins, which have been associated with breast cancer (39
). Adiponectin has also been shown to inhibit the production of TNF-α in macrophages and its actions in endothelial cells (41
), and has been found to bind several growth factors, such as fibroblast growth factor and platelet-derived growth factor-beta polypeptide, which can induce cell proliferation. Adiponectin also inhibits the activation of nuclear factor-κB, which is involved in breast cancer development (42
). One of the downstream elements in the adiponectin pathway is 5′-AMP-activated protein kinase (AMPK; refs. 43
). Once activated, AMPK targets mammalian target of rapamycin (44
), protein kinase B (43
), and the c-Jun-NH2
- kinase, and signal transducers and activators of transcription 3 pathways that are involved in breast carcinogenesis (45
There are several factors that have been shown to regulate adiponectin levels. A Mediterranean diet (46
) or a diet rich in whole grain and fat (47
) have been shown to increase adiponectin levels. Also, better glycemic control can increase adiponectin levels (47
). Physical activity has also been shown to influence adiponectin levels. In Zucker Diabetic Fatty rats, a rat model of DM, exercise was associated with a 28% increase in adiponectin levels (48
Polymorphisms in the adiponectin gene have also been shown to correlate with adiponectin serum levels. Rs266729 (−11365 C→G) has been found, although not consistently, to correlate with adiponectin levels with the GG genotype being associated with decreased adiponectin levels (25
). Rs1501299 (+276 G→T) has also been shown to correlate with adiponectin levels with the TT genotype having increased levels of adiponectin compared with GG (24
). Finally, rs2241766 (+45 T→G) has been associated with adiponectin levels with the GG correlating with increased adiponectin levels compared with TT (23
In this study, we found that two functionally significant adiponectin polymorphisms, rs1501299 and rs2241766, were significantly associated with breast cancer risk. More specifically, rs2241766 TG, which increases circulating levels of adiponectin, was associated with a 39% decreased risk for breast cancer (OR, 0.61; 95% CI, 0.46–0.80), whereas the homozygous GG genotype also showed a trend to decreased breast cancer risk (OR, 0.63; 95% CI, 0.32–1.25). Furthermore, rs1501299 TG and GG, which have been shown to decrease levels of circulating adiponectin, were associated with a 59% (OR, 1.59; 95% CI, 1.03–2.48) and 80% (OR, 1.80; 95% CI, 1.14–2.85) increased risk for breast cancer, respectively.
Given the previously shown functional significance of two of the SNPs tested (rs1501299 and rs2241766), we elected to divide patients into high, intermediate, and low signalers. Our hypothesis was that high signalers, i.e., individuals with higher predicted levels of adiponectin and/or expression of its receptor, would have a lower risk of breast cancer compared with low signalers. The findings provide support for our hypothesis in that individuals with intermediate and high adiponectin signaling had significantly lower breast cancer risk (OR, 0.64 and 0.15, respectively).
Haplotype-tagging SNPs for ADIPOR1
have also been shown to alter mRNA levels. More specifically, rs7539542 GG is associated with 30% to 40% lower ADIPOR1
mRNA levels than heterozygotes or CC homozygotes (26
). This SNP has also been associated with risk for DM and CAD (26
). In our analysis, we found that rs7539542 CC and CG, which are associated with higher mRNA levels, i.e., more adiponectin receptor 1, are associated with a 43% lower risk for breast cancer.
A limitation in our study is the lack of exact age at the time of blood draw for a small portion of our controls as only age range was available. To address this, we did analyses controlling for age. We also conducted sensitivity analyses using hypothetical models to show that the effect of lack of detailed age information in a portion of our samples was negligible. It is possible that age differences in cases and controls affected the allele frequencies observed. Nonetheless, this would be expected to create a bias toward the null hypothesis because it would overestimate the deleterious allele frequency in controls given that a fraction of younger women who would have developed breast cancer were not removed from the control group. Thus, the younger mean age of controls could have resulted in a bias toward the null hypothesis, resulting in a weaker association. Furthermore, inaccurate information with respect to any variable classification would result in “random misclassification,” which would also be expected to result in suppression of effect estimates with a trend toward the null hypothesis. Other limitations of this study are the lack of information on confounding factors such as BMI and family history of breast cancer. However, it has been shown that the association of adiponectin with breast cancer is independent of BMI (15
), and the presence of a strong family history of the disease would only weaken the power of our study. Our study also has several strengths. It includes a relatively large number of cases and controls from the same geographic location. The selection of our SNPs was based on previous studies, and the SNPs significantly associated with breast cancer are functionally significant. Although exposure was assessed in the context of a case control study, it is impossible that breast cancer would have changed SNP classification, which is already determined at birth. Thus, this study does fulfill the “time sequence” criterion for causality. This in association with existing epidemiologic evidence and biological plausibility support a causal association between these SNPs and risk for breast cancer.
There is currently only limited information on the allelic frequency of some of the SNPs studied in various ethnic groups.6
For example, the allelic frequency of rs2241766 has only been studied in 752 anonymous unrelated Japanese volunteers. There is no additional information on the allelic frequency in other populations besides the previously cited reports. With respect to rs2232853, the C:T allelic frequency differs between Caucasians (0.675:0.325), Asians (0.956:0.044), and sub-Saharan Africans (0.950:0.050). Finally, for rs7539542, the C:G allelic frequency also differs between Caucasians (0.712:0.288) and Asians (0.310:0.690), but there is no information on its frequency in other ethnic groups.
To our knowledge, this is the first study reporting an association of polymorphisms of adiponectin and its type I receptor with breast cancer risk. If confirmed in subsequent studies, our findings suggest that genetic variants of the adiponectin and adiponectin receptor 1 alter breast cancer risk. Our findings confirm the important role of adiponectin in breast cancer. In the future, we may be able to identify a population of breast cancer patients with low adiponectin levels, either due to genetic predisposition and/or environmental factors, who may benefit from adiponectin therapy. If these exciting results can be confirmed in other studies, the adiponectin axis may emerge as an important modifier of breast cancer risk.