Retinoids are a class of polyisoprenoid lipids structurally related to vitamin A, that can have dramatic affects on the growth and differentiation of both normal and transformed cells. The differentiation of epithelial cells in general, and epidermal cells in particular, is extremely sensitive to retinoic acid. Direct antiproliferative affects of retinoic acid, often associated with a commitment to differentiation, have been demonstrated in myeloid leukemia, breast cancer, embryonal carcinoma, neuroblastoma, and melanoma cells. These generalized effects on cellular phenotype suggest that the likely mechanism of retinoid action is through selective alterations in gene expression of responsive cells, in a manner analogous to steroid hormones. However, progress into the mechanism of action of retinoids has been hampered by a lack of identifiable indices of eh direct effects of retinoids on gene expression. We have shown that retinoic acid directly enhances the transcription rate of the tissue transglutaminase gene in macrophages and HL-60 cells; thus the regulation of tissue transglutaminase gene expression will serve as a useful endpoint with which to investigate the molecular mechanism of retinoid action. We have cloned the mouse tissue transglutaminase gene, and propose to characterized the retinoid response element that mediates retinoic acid regulation. We will identify, through deletion mapping and transfection experiments, regions of he gene required for retinoid induced expression. Gel retardation and footprinting studies with purified receptor preparations will permit the identification of specific domains that contain retinoid regulatory elements. The precise boundaries of the response element(s) will then be determined using mutagenesis studies. These experiments will allow us to explore in detail the specific regulatory effect of retinoic acid on gene expression. The information we derive form these experiments should provide the initial progress toward an understanding of how retinoids affect cellular growth and differentiation.
