Our preclinical findings that human chorionic gonadotropin (hCG) inhibits mammary cancer initiation and progression in rodents led us to postulate that this hormone might be useful for breast cancer prevention in women. Towards this goal we have designed experiments for determining whether the protection conferred by hCG is the result of the induction of a genomic signature that is specific for this hormone, or whether pregnancy or ovarian steroid hormones would induce a similar signature. We utilized virgin Sprague-Dawley rats that at the age of 45 days were randomized into four groups: hCG group, in which the rats received a daily intraperitoneal injection of 100 IU/hCG for 21 days; Pregnancy group, in which the rats were mated; Estrogen+Progesterone group, in which each rat had implanted subcutaneously one pellet of 0.72 mg 17β-estradiol (E2) and one of 200 mg progesterone, and Virgin Control group, which consisted of age-matched virgin rats injected with saline. RNA was extracted from mammary gland tissues from the four groups of animals at 42 days post-treatment or postpartum, respectively, times when the mammary glands exhibit their lowest susceptibility to carcinogenesis. Gene expression profiles were studied employing an oligonucleotide microarray containing 1,300 known genes. The microarrays were hybridized using 30 pmol of fluorescent probe; the green channel or the Cy3 fluorescent dye was used with the control group and the Cy5 or the red channel corresponded with the treatments or the pregnancy groups. The microarrays were washed and scanned in the Affymetrix 428 fluorescent scanner. After normalization of the data with Lowess method we combined the replicates of 5 animals in each group in order to obtain the mean (log scale) of the genes expressed with the standard deviation. We selected the up and down modulated genes by designing a cut off of 2.5 log scale respect to the control group, with p2.5 log2 folds) (p<0.01) and commonly expressed, whereas these genes were not expressed in the E+P group. The genomic signature induced by hCG and pregnancy encompassed the following genes: activators or repressors of transcription, i.e., CREB1, p21, and GADD153, apoptosis; i.e., caspase 1; growth factors, i.e., TGF-beta3; cell division control, i.e. p21 Cdk 1 inhibitor; DNA repair genes, i.e., GADD153 and Casein kinase delta; tumor suppressor genes, i.e., p53, Cell-surface antigens, i.e., B7.1, CD-28, and CD44 antigen. We validated the levels of gene expression of p53, p21, GADD153, CD28 and Casein Kinase delta 1, by RT-PCR in the mammary glands of hCG-treated virgin rats and in parous rats. These data indicate that hCG, like pregnancy, induces permanent genomic changes that are not reproduced by steroid hormones. (This work was supported by NIH grant RO1-CA94098).

[Proc Amer Assoc Cancer Res, Volume 46, 2005]