In this study population of women living in Western New York, we previously found that recent use of aspirin but not ibuprofen was inversely associated with breast cancer risk (29
). We found here that, for aspirin, this association did not differ by molecular subtype. For ibuprofen, recent use was associated with an increased risk of breast cancer defined as ER+/PR+, HER2−, p53− or luminal A/B, and a decreased risk of HER2 expressing tumors. With few exceptions, we observed no statistically significant exposure-response gradients for increasing frequency or intensity of aspirin or ibuprofen use in association with any molecular subtype. Similar to our findings with recent aspirin use, aspirin use during a woman’s adult lifetime was associated with similar reductions in risks for most tumor subtypes.
Because COX-2 expression has been associated in vitro
with estrogen synthesis (7
), we hypothesized that NSAID use would be inversely associated with risk of hormone receptor positive tumors. Authors of several case-control (18
) and cohort studies (17
), and one randomized controlled trial (26
), have examined the association of NSAID use and breast cancer stratified by hormone receptor status; however findings have been inconsistent (17
). Because previous studies have reported associations using different combinations of ER/PR expression status (e.g., ER+/PR+ and ER−/PR−), or ER and PR status separately, comparisons between our findings and others are challenging. Comparisons are further limited in that not all studies report associations for individual NSAIDs, assuming equal effects of this broad class of medications. In this study, aspirin use was associated with reduced risks of breast characterized by joint ER/PR status. Several others have reported similar findings (18
). In a recent analysis of the Iowa Women’s Health Study, Bardia et al. (27
), reported that ever aspirin use was associated with similar reductions in risk for ER+ (RR 0.77, 95% CI: 0.67–0.89), ER− (RR 0.78, 95% CI: 0.56–1.08), PR+ (RR 0.79, 95% CI: 0.68–0.92), and PR− (RR 0.73, 95% CI: 0.56–0.95) tumors; similar reductions in risk were observed when ER and PR were combined. In contrast, others have observed reduced risks of hormone receptor positive (i.e., ER+, ≥1 positive hormone receptor, or ER+/PR+) breast cancer only (21
), increased risks of hormone receptor negative breast cancers (19
), or no association (17
). In a randomized controlled trial of low-dose (100mg) aspirin taken every other day for 10 years compared to a placebo, there was no association with breast cancer risk overall (26
) or with breast cancer defined by ER or PR (28
). It is widely speculated, however, that this dose may be too low for chemoprevention (28
In contrast to our hypothesis, we observed increased risks of ER+/PR+ breast cancer in association with recent ibuprofen use. Several others have observed similar results. In the Multiethnic Cohort Study, there was an increase in hormone receptor positive breast cancer among women who were recent, short-term (≤1 year) users of non-aspirin NSAIDs compared to non-users (HR 1.29, 95% CI: 1.02–1.63), and no association among women with hormone receptor negative tumors (HR 1.06, 95% CI: 0.73–1.53), although greater duration of use at baseline was inversely associated with risk of hormone receptor positive tumors (17
). In the California Teachers Study, daily ibuprofen use was associated with increased breast cancer risk that did not differ by ER and PR status (19
), and in the Nurse’s Health Study II cohort of premenopausal women, non-aspirin NSAID use 2–3 times per week was associated with increased breast cancer risk (RR 1.37, 95% CI: 1.09–1.67), with no difference by hormone receptor status (25
). Our observation of inverse associations of breast cancers defined by ER and PR with aspirin use and an increased risk of hormone receptor positive tumors with ibuprofen use are inconsistent with our hypothesis that inhibition of COX-2 through the use of NSAIDs would result in the strongest reduction in risk of hormone receptor positive tumors.
This study is the first, to our knowledge, to report on the association of NSAID use with breast cancer characterized by HER2 expression, p53 mutation status, or combinations of ER, PR, and HER2. We found no difference in the association of aspirin use with tumors characterized by HER2 or p53, and use of ibuprofen was associated with increased risks of HER2− and p53− breast cancers. We know of only two other reports of the association of NSAIDs with any cancer characterized by p53 status and no others of breast cancer (32
). Freedman et al. (33
), reported no differences in the association of aspirin use with colon cancer stratified by p53 expression. Figueroa et al. (32
), reported no differences in the association of aspirin or any NSAID with esophageal or gastric cancers by p53 status in a population-based case-control study. Nevertheless, our findings suggest further investigations are needed.
The primary strength of this report is that it is the first to comprehensively examine the association of NSAIDs, and use of aspirin during a woman’s adult lifetime, with breast cancer subtypes defined by ER, PR, HER2, and p53. An additional strength is our ability to measure of the frequency and intensity of use of aspirin and ibuprofen.
This study also has several important limitations. Foremost, we were limited in power to detect differences between rare breast cancer subgroups. Our findings nevertheless merit further investigation. In addition, our classification of subtypes based on ER, PR, and HER2 is a surrogate for a more comprehensive nomenclature determined by tumor marker expression (12
); therefore the subtypes defined in this study, particularly the differences between luminal A and B tumors, may be misclassified. Because fluorescence in situ hybridization was not performed to validate tumors with an equivocal (i.e., 2+) HER2 score, and the agreement between IHC and medical records was good but not excellent, misclassification of HER2 status is possible. In addition, data collected on recent NSAID use were limited to frequency and intensity of use and information on NSAID dose, duration, or indication for use was not collected. Our classification of infrequent and regular users may misclassify participants who were regular users at an earlier time. However it is unlikely that such misclassification would differ by disease status or case group and would likely result in a bias of point estimates towards the null and widening of confidence intervals (34
). Doses of aspirin and ibuprofen made available over the counter and by prescription vary substantially; the absence of exposure-response gradients may be because of measurement error related to the lack of dose data. It is also possible that our results may be confounded by other unmeasured factors, in particular by the indication for NSAID use or use of other NSAIDs such as naproxen or selective COX-2 inhibitors such as celecoxib. However, adjustment for history of arthritis, hypertension, diabetes, or cerebrovascular disease had no effect on point estimates. Additionally, for analyses of adult lifetime aspirin, we were unable to assess confounding by an overall healthy lifestyle. As with any case-control study, possible effects of recall bias need to be considered, although it is unlikely that participants would differ in their ability to recall their NSAID exposure depending upon the molecular classification of their cancer. It is possible that our findings may be due, in part, to self-selection into the study. Though unlikely, if selection into the study was based upon use of aspirin or ibuprofen or some correlate of their use, our findings could be biased. Again, it is unlikely that that this bias would differ by tumor characteristics. Comparing cases with tumors blocks vs. those without, use of aspirin and ibuprofen was similar.
NSAIDs are thought to exhibit their anti-inflammatory and chemopreventive effects by non-selectively binding to the cyclooxygenase enzymes (COX-1 & 2; EC 220.127.116.11), which catalyze the synthesis of pro-inflammatory prostaglandins from arachidonic acid (35
). Among the prostaglandins, prostaglandin E2
is considered a powerful mitogen and potential chemopreventive target (35
has been shown to induce aromatase expression and de novo
estrogen synthesis in breast epithelia and stromal cells in vitro
; introduction of NSAIDs has been shown to reduce estrogen levels in a dose-dependent manner (8
). In humans, the association of NSAIDs with circulating estrogen levels has been inconsistent (37
). We know of no mechanism, however, by which ibuprofen would increase risk of breast cancer subtypes.
There is limited evidence that COX-2 expression is correlated with ER, PR, HER2, and p53 expression in breast tumors (40
). Findings of in vitro
studies among human invasive breast cancer cells suggest that HER2 oncogene activation regulates COX-2 expression in breast cancer (43
), inducing a positive feedback loop in which PGE2
in turn further induces HER2 expression (47
). Introduction of an NSAID has been shown to reduce HER2 expression (47
). P53 may also be associated with COX-2 expression in vitro
) and animal models of breast cancer give limited evidence that p53 expression is associated with COX-2 expression (43
The findings of this large, population-based case-control study support existing evidence that aspirin is inversely associated with breast cancer risk. Our findings do not support, however, the hypothesis that aspirin’s effects are differential by tumor subtype. Use of ibuprofen may be associated with increased risk of certain breast cancer subtypes associated with less aggressive phenotype and inversely associated with the risk of the aggressive HER2 expressing phenotype. Our findings provide preliminary evidence to suggest that aspirin and ibuprofen are heterogeneous in their effects, and lend further support to the hypothesis that the etiology of breast tumors differs by subtype. Epidemiologic studies of the association of NSAIDs with breast tumors characterized by molecular subtype should include detailed information of the timing and dose of a wide range of individual prescription and non-prescription NSAIDs. A better understanding of inflammation in relation to risk of breast cancer could significantly advance strategies for chemoprevention.