Progesterone promotes cell proliferation and development of the normal mammary gland and is implicated in the etiology of breast cancer. The specific functions of progesterone receptor isoforms PRA and PRB in the normal mammary gland are critical to understanding the molecular mechanisms of progesterone action in these processes. PRA is most highly expressed during puberty, an important widow of exposure sensitivity that can increase or decrease breast cancer risk later in life. While PRB is essential for glandular development in pregnancy, the functions of PRA have not been elucidated. To identify PRA-regulated genes, mammary epithelial cells were obtained from pubertal or adult mouse mammary glands and cultured as “organoids” in a 3-D culture system. Progestin (P) treatment causes organoids to develop a cellular organization similar to ducts, mimicking in vivo ductal development. P-induced gene expression was analyzed with whole mouse genome microarrays. We identified 69 P-regulated genes in both pubertal and adult epithelium, 38 genes regulated uniquely in adult epithelium, and 96 genes regulated uniquely in pubertal epithelium. In both pubertal and adult epithelium, inflammation-related genes were prominently upregulated. Many of these genes are not previously known to be P-regulated. Additionally, genes involved in programmed cell death show pubertal upregulation, while those involved in cell adhesion show pubertal and adult upregulation, perhaps reflecting progesterone’s role in organ development. Among the inflammatory genes most dramatically upregulated were serum amyloid A1, A2, and A3. We confirmed P-induced serum amyloid A1 protein expression in organoids. Serum amyloid A proteins are implicated in the induction of inflammation, and are reported to increase expression of proinflammatory factors in white blood cells, and to play a role in recruitment and adhesion of these cells to sites of infection and injury. Supporting the notion that progestins can induce an inflammatory state, we found that mammary glands of ovariectomized mice show increased infiltration of white blood cells upon 5-day progesterone treatment. Inflammatory cells are implicated in normal mammary gland development and the inflammatory state is implicated in tumor progression in many studies. Progestin induction of inflammatory genes suggests a novel role for progestins in mammary gland development and function, and presents a possible link between progestins and inflammation in the etiology of breast cancer. An inflammatory mechanism may underlie the increased breast cancer risk associated with hormone replacement therapy that includes progestins.
This work is supported by the Breast Cancer and the Environment Research Center: NIH/NIEHS/NCI U01 ES12800.