Altered gene expression profile in chemically induced rat mammary adenocarcinomas and its modulation by an aromatase inhibitor

In the present study, competitive cDNA library screening (CCLS) and cDNA mi croarray analyses were employed to identify differentially expressed genes in methylnitrosourea-induced rat mammary adenocarcinomas. The preliminary s creening of 100 000 plaques by CCLS identified 1217 clones with differentia l expression. Dot-blot analysis of the isolated clones verified differentia l expression in 471 distinct genes. Confirmation of these 471 genes was con ducted by performing reverse transcription-polymerase chain reactions, and a total of 160 genes were confirmed after comparing six rat mammary adenoca rcinomas and three normal rat mammary glands. Fifty-nine of these showed lo wer expression in the adenocarcinomas while the remaining 101 were overexpr essed in the tumors. Employing a cDNA microarray containing 588 known genes revealed an additional 33 differentially expressed genes in these tumors. Importantly, most of the identified genes demonstrated relatively reproduci ble overexpression or underexpression in individual tumors. Many of the alt ered genes determined by cDNA microarray analysis were oncogenes, tumor sup pressor genes, or genes involved in cell cycle control and apoptosis. CCLS identified many others not previously associated with mammary carcinogenesi s, including a novel gene named RMT-7. Preliminary studies to determine the applicability of this gene expression approach for detecting potential bio markers for cancer chemoprevention was evaluated in rat mammary tumors obta ined from animals treated with vorozole, a potent aromatase inhibitor. When genes exhibiting differential expression as determined by CCLS or cDNA mic roarray analysis were examined in control and vorozole-treated tumors, expr ession of 19 genes was found to be modulated significantly in tumors treate d with vorozole. Further investigations into these identified genes should contribute significantly to our understanding of the molecular mechanisms o f rat mammary tumorigenesis. In addition, the identified genes may become u seful targets for drug development and potential biomarkers for monitoring treatment and prevention of breast cancer in humans.