Retinoic Acid Receptor-ß:Role in the development and prevention of breast cancer

Breast cancer is one of the most often diagnosed tumors in the western world with rising incidence over the past 40 years. In contrast to other diagnosed diseases (for instance myocardial infarction and cerebrovascular diseases), in breast cancer no strategies for prevention have been identified till now. In the last few years the vitamin-A derivative retinoic acid (RA) alone or in combination with various cytokines has been shown to be a potent substance in prevention and treatment of cancer. the effects of retinoids are mediated by two classes of nuclear receptors: the retinoic acid receptors (RARs) and the retinoid X receptors (RXRs), members of the steroid-thyroid hormone receptor superfamily. Both types of receptors are coded for by three distinct genes, alpha, beta, and gamma. Autoregulation of the RAR-beta gene presumably plays a critical role in amplifying the RA response. Recently, it was found that RAR-beta is not expressed in a number of malignant tumors. Loss of heterozygosity at the chromosom 3p24 was found in a high percentage of breast cancer specimens and in normal terminal ductal- lobular units adjacent to the carcinoma, but not in normal breast tissue distant to the carcinomas of the same patients. The locus 3p24 encodes the gene for the RAR-beta. In breast cancer cell lines RAR-beta mediates apoptosis and loss of RAR-beta contributes to the tumorigenicity of human mammary epithelial cells. There is much evidence that RAR-beta mediates apoptosis by modulating mitochondrial function (e.g. downregulation of bcl-2 oncogene and release of cytochrome c) and subsequent activation of enzymes (e.g. CPP32) which are involved in late stages of apoptosis. Our working hypothesis is as follows: Patients with familiar breast cancer own one allel with a mutated gene for RAR-beta; during life-time the second allel will be mutated; no functioning RAR-beta is present in mammary epithelial cells, no apoptosis via modulation of mitochondrial function could be initiated; an imbalance between growth-promoting and apoptosis-inducing factors develops and breast cancer is initiated. To prove this hypothesis we plan to perform the following analyses: (1) Looking for mutations in the RAR-beta promoter and gene in patientsl ymphocytes and breast cancer specimens of these patients. (2) If a mutation of the promoter could be found it should be functional characterized. (3) Expression of RARs and bcl-2 should be studied in breast cancer specimens and correlated to the apoptotic index. (4) RAR-beta should be stable transfected in breast cancer cell lines; induction of apoptosis, bcl-2 expression, cytochrom-c and ATP concentration, mitochondrial function and expression of the CPP32 should be evaluated before and after treatment with RA. In this study we will evaluate the RAR-beta to be a gene which is involved in the development of familiar or sporadic breast cancer; if this will prove to be true, RA or selective binding RARs (via autoregulation of the RAR- beta on the non mutated allel) should be tested in patients which are at high risk for breast cancer.