We found that the risk of invasive breast cancer was significantly lower with estrogen–progestagen HRTs containing progesterone or dydrogesterone than with HRTs containing other progestagens. The latter group involved a variety of progestins whose associations with breast cancer risk did not differ significantly from one another. We also observed a significantly increased risk of breast cancer with the use of estrogen alone.
The effect of progestagens on breast tissue is complex and not completely understood. The mechanisms by which they act on cell proliferation include interaction with steroid receptors, growth factors and oncogenes, and with the cell-cycle and estrogen metabolizing enzymes [3
]. Because progestagens differ widely in their chemical structure, metabolism, pharmacokinetics and potency, it is reasonable to expect them to induce different responses in the breast [2
]. However, the effects of progestagens generally differ according to the experimental conditions, the duration of treatment and the dose concentration [3
]. As a result it is impossible to establish, on the basis of the available and often conflicting in vitro data, whether the predominant effect of a given progestagen is to stimulate or inhibit breast cell proliferation. This complicated and unresolved situation makes the results of real life studies like ours particularly interesting.
Our study is the first epidemiological study conducted on women that we know of, that evaluated the association of the estrogen–progesterone and estrogen–dydrogesterone combinations with breast cancer risk. A major finding is that these combinations may be safer than others. Studies of the effect of progesterone on breast cells have demonstrated that the hormone can exert either growth-promoting, neutral, or antiproliferative effects on the breast tissue [20
]. Recently, Wood et al. [22
] compared the effects of estradiol given with either medroxyprogesterone acetate or micronized progesterone on risk biomarkers for breast cancer in a postmenopausal primate model. In this randomized crossover trial, they found that, compared to placebo, estradiol + medroxyprogesterone acetate resulted in significantly greater proliferation (as measured by Ki67 expression) in lobular and ductal breast epithelium, while estradiol + micronized progesterone did not. This result supports our findings suggesting that, when combined with an estrogen, progesterone may have a safer risk profile in the breast compared with some other progestagens. The association of estrogen–dydrogesterone combinations with a nonsignificantly elevated relative risk in our study reinforces the plausibility of our finding since the retroprogesterone dydrogesterone is the progestin with the chemical structure and pharmacological effects closest to those of progesterone.
The high degree of androgenicity of progestins used in certain HRTs has been hypothesized to play a role in the increased risk of breast cancer [5
]. Our results do not support this hypothesis, as, when combined with an estrogen, neither promegestone, nomegestrol acetate, chlormadinone acetate or medrogestone (all nonandrogenic progestagens) nor cyproterone acetate (an antiandrogenic progestagen) had effects that differed significantly from that of norethisterone acetate (the most androgenic progestagen cited). These results are in line with those of two other European studies [10
], which found no difference between the effect of 19-nortestosterone derivatives and medroxyprogesterone acetate (a 17-hydroxyprogesterone derivative with lower androgenic potential than 19-nortestosterone derivatives), implying that other parameters must be involved. However, possible preferential prescribing of the nonandrogenic or antiandrogenic HRTs to women with signs of insulin resistance or hyperandrogenism, who are at higher risk of breast cancer [23
], could partly explain our findings.
In our study, estrogen alone was associated with a significantly lower increase in breast cancer risk than estrogen opposed with a progestagen (with the exception of progesterone or dydrogesterone), in line with the growing evidence that adding certain progestins to estrogen has an adverse impact on breast cancer risk [24
]. However, our finding of a 1.3-fold increased breast cancer risk associated with the use of estrogen alone (almost exclusively estradiol compounds, and mostly administered through the skin) differs with that of the WHI estrogen-alone trial which found a decreased risk when oral conjugated equine estrogens were used in a population of older and often overweight women [6
We had limited power to examine the effect of HRTs among past users as most women were still using HRT at the end of follow-up. However, our results are compatible with those of previous studies suggesting that the excess in risk associated with HRT use diminishes after treatment stop [4
The major strengths of our study are the range of HRTs evaluated and the fact that exposure was regularly updated during follow-up. This allowed us to (i) isolate the effects of each type of HRT, taking into account changes from one treatment to another by creating a separate “mixed” use category, and (ii) avoid the misclassification of users and nonusers, duration, or recency of use that can occur in prospective studies with a single baseline assessment of exposure.
Our results would have been only slightly changed if we had restricted our analyses to women who had not been using HRT before the baseline questionnaire (“incident users”). (We restricted analysis in this way in our previous study [16
], to avoid potential biases described by Ray [25
Analyses were adjusted for various potential confounders, and participants in the E3N cohort belong to a homogeneous occupational group (the great majority being teachers or teacher’s wives). This decreased the probability that the differences we found on risk between different estrogen–progestagen combinations are explained by confounding; in addition, there was no marked difference between users of the different types of estrogen–progestagen combinations regarding classical breast cancer risk factors, and stratified analyses yielded relative risks that were quite stable whatever the characteristics of the women (data not shown).
We were aware of the possibility of differential recall by HRT users and nonusers. We therefore ran a sensitivity analysis where exposure was included in the models in a prospective manner (i.e., using only the information on exposure reported in questionnaire i for the follow-up period between questionnaire i and questionnaire i + 1). Relative risks obtained with this sensitivity analysis were not below those obtained with our main analysis, showing that differential recall bias was unlikely to have occurred.
Analyses were controlled for previous mammograms, but a detection bias remains possible as women who use HRT have mammograms more frequently than nonusers. However, there is no reason why this bias should have been less marked for estrogen–progesterone or estrogen–dydrogesterone than for estrogen–other progestagens combinations. Of concern is the possibility that different estrogen–progestagen HRTs may influence breast density and hence alter mammographic sensitivity in a different way. However, in the PEPI trial, Greendale et al. found that, over 12 months, the adjusted absolute mean changes in mammographic percent density did not differ significantly between conjugated equine estrogens plus cyclic medroxyprogesterone acetate and with conjugated equine estrogens plus cyclic micronized progesterone [26
Nondifferential misclassification of HRT exposure, which was based on self-reported information, may have affected our results, most likely by diluting the magnitude of the relationship between HRTs and breast cancer risk, and reducing any real differences in the effects of different HRTs.
E3N is the first epidemiological study that we know of to be providing results indicating that estrogen–progesterone and estrogen–dydrogesterone combinations may be the least harmful estrogen–progestagen HRTs regarding breast cancer risk. However, more evidence is required before these results can be translated into firm clinical recommendations for the management of menopausal symptoms. In addition, the effect of these combinations in other diseases (e.g., coronary heart disease, venous thromboembolism and colorectal cancer) has also to be evaluated. We therefore encourage further studies and reflection on the links between estrogen–progesterone and estrogen–dydrogesterone HRTs and breast cancer.