Estrogens and breast cancer.

Estrogens have been associated with an increase in breast cancer risk. Yet emerging clinical and experimental evidence points to progestogens [endogenous progesterone or synthetic progesterone (progestin)] as the primary hormonal driver underlying seemingly estrogen-associated breast cancer risk. Estrogens may contribute to breast cancer risk indirectly by induction of the progesterone receptor and thus amplifying progesterone signaling. Large studies of hormonal contraceptives suggest that the small increase in breast cancer risk from hormonal contraceptives is mainly attributable to progestins, not estrogens. Estrogen-plus-progestin hormone replacement therapy (HRT) has consistently shown an increase in breast cancer risk among postmenopausal women, whereas estrogen-alone HRT has little impact on breast cancer risk in naturally or surgically menopausal women. In particular, the long-term follow-up of the Women's Health Initiative (WHI) randomized trials suggests a benefit of estrogen alone. Recent data further indicate that endogenously elevated estrogen during assisted reproductive technology (ART) exhibits little adverse effect on or potentially a reduction in breast cancer risk and recurrence. Also, accumulating evidence suggests that inhibition of progesterone signaling is a critical mechanism underlying the risk-reducing and therapeutic effects of antiestrogens. Estrogen HRT has shown an array of proven benefits, including ameliorating menopausal symptoms and improving bone health. Collective evidence thus suggests that estrogen HRT is likely to offer health benefits to perimenopausal or postmenopausal women, including breast cancer survivors, as well as young BRCA 1/2 carriers with prophylactic oophorectomy for ovarian cancer prevention. • Without a progestogen, estrogen alone appears to have little or no impact on breast cancer risk. • Estrogen may increase breast cancer risk by inducing the progesterone receptor and augmenting progesterone signaling. • Progestins—not estrogens—from hormonal contraceptives appear to be responsible for a small increase in breast cancer risk. • In postmenopausal women using HRT, progestins are likely the primary hormonal agent that elevates breast cancer risk. • Estrogen HRT, coupled with minimal progestogen, might be acceptable for women in need of HRT, including BRCA1/2 carriers. [ABSTRACT FROM AUTHOR]