Persistent Parity-Induced Changes in Growth Factors, TGF-β3, and Differentiation in the Rodent Mammary Gland

Abstract

Epidemiological studies have repeatedly demonstrated that women who undergo an early first full-term pregnancy have a significantly reduced lifetime risk of breast cancer. Similarly, rodents that have previously undergone a full-term pregnancy are highly resistant to carcinogen-induced breast cancer compared with age-matched nulliparous controls. Little progress has been made, however, toward understanding the biological basis of this phenomenon. We have used DNA microarrays to identify a panel of 38 differentially expressed genes that reproducibly distinguishes, in a blinded manner, between the nulliparous and parous states of the mammary gland in multiple strains of mice and rats. We find that parity results in the persistent down-regulation of multiple genes encoding growth factors, such as amphiregulin, pleiotrophin, and IGF-1, as well as the persistent up-regulation of the growth-inhibitory molecule, TGF-beta3, and several of its transcriptional targets. Our studies further indicate that parity results in a persistent increase in the differentiated state of the mammary gland as well as lifelong changes in the hematopoietic cell types resident within the gland. These findings define a developmental state of the mammary gland that is refractory to carcinogenesis and suggest novel hypotheses for the mechanisms by which parity may modulate breast cancer risk.

The ADAM17–amphiregulin–EGFR Axis in Mammary Development and Cancer

“…Although at least six EGFR ligands are expressed at one stage or another during mammary development, only AREG is strongly upregulated at puberty and then dramatically repressed during and after pregnancy; kinetics that are consistent with the importance of EGFR in ductal rather than alveolar development [24,29]. And indeed, instead of forming a competent ductal tree that fills the entire mammary fat pad, AREG-deficient mice form what could be called an inadequate bush-a small mammary shrub of insufficient size to nourish pups [25].…”

“…Otherwise, the only other ErbB ligand that was readily detected in the post-pubertal mammary gland was the ErbB3/ ErbB4 ligand neuregulin-4, which was more highly expressed in the stroma than in TEBs or ducts. Although these array data leave open the possibility that AREG is weakly expressed in the stroma or locally induced in the immediate peri-epithelial stroma, in situ hybridization data specifically confirm that AREG is substantially and exclusively expressed in the epithelium of the developing ducts and TEBs [25,29]. Thus although other EGFR ligands can support ductal growth in culture and in vivo if provided exogenously, only AREG is expressed at significant levels in epithelial ducts during their actual outgrowth.…”

“…Although this alveolar compaction could very well reflect alveolar crowding as a consequence of impaired ductal outgrowth, some EGFR ligands may still affect lactational differentiation. Indeed, EGF is strongly upregulated during late pregnancy and lactation [24,29]. Moreover, if EGF were involved in lactational differentiation, then its participation would help to explain why triple EGF/AREG/TGFα-deficient mothers had an even more severe lactational insufficiency than did dams that were deficient in AREG alone [25].…”

Stem Cells, Hormones, and Mammary Cancer

“…Microarray evidence suggests that pregnancy mediates persistent changes in the gene expression profile in parous females [11,12]. These pregnancy-induced changes can be imitated through a transient administration of hormones, in particular estrogen and progesterone or human chorionic gonadotropin.…”

“…Twenty-eight days after the last treatment, they identified approximately 100 differentially expressed loci. In a more comprehensive study, D'Cruz and colleagues [11] utilized oligonucleotide arrays to examine differences in the expression profile of approximately 5,500 genes between parous mice and their nulliparous controls. These initial results were verified by more laborious methods (northern blot analysis and in situ hybridization) and across several mouse strains as well as in two rat models.…”