Early full-term pregnancy affords lifetime protection against the development of breast cancer. Parity-induced protection can be reproduced in a carcinogen-induced rat mammary carcinoma model, but the molecular mechanisms of this protection against carcinogenic stimuli in rat mammary glands have not been fully characterized. To gain a better understanding of these molecular mechanisms, we used an oligonucleotide microarray to examine gene expression in parous and age-matched virgin (AMV) mammary glands of Lewis rats before and after carcinogen (N-methyl-N-nitrosourea; MNU) treatment. Parous mammary glands before MNU treatment showed up-regulation of multiple differentiation-related genes, such as whey acidic protein (Wap), casein beta (Csn2), casein gamma (Csng), lipopolysaccharide binding protein (Lbp), secreted phosphoprotein 1 (Spp1) and glycosylation-dependent cell adhesion molecule 1 (Glycam1). Also, parous mammary glands before MNU treatment exhibited down-regulation of growth-related genes such as regenerating islet-derived 3 alpha (Reg3a), mesothelin (Msln), insulin-like growth factor 2 (Igf2) and insulin-like growth factor binding protein 4 (Igfbp4). After MNU treatment, AMV mammary glands exhibited up-regulation of growth-related genes, such as Msln, cell division cycle 2 homolog A (Cdc2a), Igf2, Igfbp4, stathmin 1 (Stmn1) and homeobox, msh-like 1 (Msx1), whereas expression of these genes remained low in parous mammary glands. AMV mammary glands also exhibited marked up-regulation of Cdc2a and Stmn1 in response to MNU. After MNU treatment, the PCNA labeling index increased significantly in AMV mammary epithelial cells (13.7±1.1%), but remained low in parous mammary glands (3.6±0.4%). The response of AMV mammary glands to carcinogenic stimuli includes up-regulation of growth-related genes and increased cell proliferation. The lack of a similar response in parous mammary glands may explain parity-induced protection against mammary tumor development.

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