Pregnancy can both reduce and increase lifetime
breast cancer risk, and it also induces a short-term, transient increase in risk. Several biological mechanisms have been proposed to explain the protective effect, including pregnancy-induced increase in circulating
estrogen levels leading to reduced
estrogen receptor (ER) expression and activity. Persistent changes in ER-regulated gene expression may then alter the response of the breast to postpregnancy hormonal exposures originating, for example, from food. Understanding how pregnancy increases
breast cancer risk has received less attention. Human studies indicate that those women who were exposed to an elevated pregnancy estrogenic environment, such as women who took the
synthetic estrogen diethylstilbestrol or who had the highest circulating
estrogen levels at the beginning or end of pregnancy, are at increased risk of developing
breast cancer. There is also evidence that elevated
leptin levels, for example, in pregnant women who gained excessive amount of weight, increase later
breast cancer risk. This may reflect a close interaction between
estradiol (E2), ER, and
leptin. Our preclinical study suggests that an exposure to excess pregnancy E2 and
leptin levels reverses the protective changes in genomic signaling pathways seen in the breast/mammary gland of parous women and rodents. Recent findings indicate that involution - the period after lactation when the breast regresses back to prepregnancy stage - may be related to some pregnancy-associated breast
cancers. Importantly, in a preclinical model, the increase can be reversed by anti-inflammatory treatment, offering hope that the increase in lifelong
breast cancer risk induced by late first pregnancy or by an exposure of pregnant women to an excessive hormonal environment may be reversible.