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iInternational Journal of Cancer, Vol. 117, No. 4. (2005), pp. 561-568./ibr /br /Absence of
estrogen receptor (ER) and progesterone receptor (PR) is the hallmark of most hormone-independent
breast cancers. Previous studies demonstrated that reactivation of PR expression in
hormone-independent MDA-MB-231 breast cancer cells enabled progesterone to suppress cell growth
both in vitro and in vivo. We determined the whole genomic effect of progesterone in PR-transfected
MDA-MB-231 cells. We identified 151 progesterone-regulated genes with expression changes >
3-fold after 24 hr treatment. Most are novel progesterone target genes. Real-time RT-PCR analysis
of 55 genes showed a 100% confirmation rate. Twenty-six genes were regulated at both 3 and 24 hr.
Studies using translation inhibitor suggest that most of the 26 genes are primary progesterone
target genes. Progesterone consistently suppressed the expression of genes required for cell
proliferation and metastasis and increased the expression of many tumor-suppressor genes.
Progesterone also consistently decreased the expression of DNA repair and chromosome maintenance
genes, which may be part of the mechanism leading to cell cycle arrest. These data suggest
potential usefulness of progestin in combating ER-negative but PR-positive breast cancer and
indicate that progesterone can exert a strong anticancer effect in hormone-independent breast
cancer following PR reactivation. The identification of many novel progesterone target genes open
up new avenues for in-depth elucidation of progesterone-mediated molecular networks. © 2005
Wiley-Liss, Inc.br /iJoyce Leo, Suk Wang, Chun Guo, Swee Aw, Yi Zhao, Jin Li, Kam Hui, Valerie
Lin/i

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