The protective role of pregnancy in breast cancer

Abstract

Epidemiological, clinical, and experimental data indicate that the risk of developing breast cancer is strongly dependent on the ovary and on endocrine conditions modulated by ovarian function, such as early menarche, late menopause, and parity. Women who gave birth to a child when they were younger than 24 years of age exhibit a decrease in their lifetime risk of developing breast cancer, and additional pregnancies increase the protection. The breast tissue of normally cycling women contains three identifiable types of lobules, the undifferentiated Lobules type 1 (Lob 1) and the more developed Lobules type 2 and Lobules type 3. The breast attains its maximum development during pregnancy and lactation (Lobules type 4). After menopause the breast regresses in both nulliparous and parous women containing only Lob 1. Despite the similarity in the lobular composition of the breast at menopause, the fact that nulliparous women are at higher risk of developing breast cancer than parous women indicates that Lob 1 in these two groups of women might be biologically different, or might exhibit different susceptibility to carcinogenesis. Based on these observations it was postulated that Lob 1 found in the breast of nulliparous women and of parous women with breast cancer never went through the process of differentiation, retaining a high concentration of epithelial cells that are targets for carcinogens and are therefore susceptible to undergo neoplastic transformation. These epithelial cells are called Stem cells 1, whereas Lob 1 structures found in the breast of early parous postmenopausal women free of mammary pathology, on the contrary, are composed of an epithelial cell population that is refractory to transformation, called Stem cells 2. It was further postulated that the degree of differentiation acquired through early pregnancy has changed the 'genomic signature' that differentiates Lob 1 of the early parous women from that of the nulliparous women by shifting the Stem cells 1 to Stem cells 2 that are refractory to carcinogenesis, making this the postulated mechanism of protection conferred by early full-term pregnancy. The identification of a putative breast stem cell (Stem cells 1) has, in the past decade, reached a significant impulse, and several markers also reported for other tissues have been found in the mammary epithelial cells of both rodents and humans. Although further work needs to be carried out in order to better understand the role of the Stem cells 2 and their interaction with the genes that confer them a specific signature, collectively the data presently available provide evidence that pregnancy, through the process of cell differentiation, shifts Stem cells 1 to Stem cells 2 -cells that exhibit a specific genomic signature that could be responsible for the refractoriness of the mammary gland to carcinogenesis. The protective effect of parity in breast cancerThe incidence of breast cancer has gradually increased in the United States and in most Western countries over the past few decades...

Age at first childbirth and oral contraceptive use are associated with risk of androgen receptor-negative breast cancer: the Malmö Diet and Cancer Cohort

“…Further, the breast tissue matures during breastfeeding, and it may take multiple pregnancies before the breast tissue is fully matured (36). In line with this, advanced age at first childbirth has been shown to increase both the risk of breast cancer in general (37) and to affect histologic types and ER/PR-defined breast cancer differently (1,38).…”

Cancer risk in Norwegian world class athletes

“…As the physical requirements within elite sports have increased over time and a sedate lifestyle has been more common in the general population, the contrast in activity level between these athletes and the general females may be larger than for former athletes. The protective effect of a full-term pregnancy is well established within breast cancer epidemiology (13,14) and, thus, separate analyses for nulliparous and parous women were performed. The results indicate a preventive effect from childbirths, although the differences were not statistically significant (not shown).…”

“…This subjective reporting of frequency and intensity level, over a long time 14 span, increases the risk of misclassification of lifetime activity. The response rate for the survey was 58.3%, but it did not appear to introduce major selection on sex, discipline or career period (Table 1).…”

Higher parity and shorter breastfeeding duration

“…[7][8][9] However, both parity and breastfeeding duration might contribute in combination to risk by altering the population of progenitor cells that are retained within breast parenchyma, a cell type that known to have enhanced survival potential under physiological or pathologic stress. 8,[10][11][12][13][14][15] Indeed, recruitment and expansion of progenitor cells within breast terminal ducts sustains epithelial expansion during pregnancy and lactation, and this is followed by epithelial differentiation or apoptosis during the course of weaning. 16 However, it is not known what minimum duration of breastfeeding might induce differentiation of breast progenitor cells, or whether a combination of higher parity with repeated short or absent breastfeeding might augment the pathogenesis of BC with undifferentiated phenotype.…”