The basal expression of RARα1, the only RARα isoform that was expressed in breast cancer cell lines and in most breast tumors, was supported by apo-ER but was unaffected by OH-Tam; RAR-β and -γ were not regulated by apo-ER.
We hypothesized that specific activation of a single retinoic acid receptor-alpha (RARalpha), without direct and concurrent activation of RARbeta and gamma, will inhibit mammary tumor oncogenesis in murine models relevant to human cancer.
In summary, the demonstration that estradiol increased RAR alpha mRNA levels in breast cancer cells supports the hypothesis that the correlation between RAR alpha and ER gene expression in breast tumors and breast cancer cell lines is due to estradiol augmentation of RAR alpha gene expression.