3.3. Activities Found by DNA Microarray Assays (Silent Estrogens)
Activities found by DMA are often detected as cell signals in specific pathways, such as angiogenesis, ErbB/HER, MAPK, nuclear receptor, and ubiquitin/proteasome signaling pathways, and/or in cell functions, such as apoptosis, autophagy, cell cycle/DNA damage/cytoskeletal formation, cellular metabolism, chromatin/epigenesis regulation, development/differentiation, immunology/inflammation response, neurological diseases, and translational control [183]. While most of these cell signaling pathways and cell functions can be detected by other technologies, there might be some activities that can be detected exclusively by DMA. One such activity is by a group of estrogens, silent estrogens, which show estrogenic gene expression profiles without showing positive effects on cell proliferation [13].
Estrogen is a female hormone that is responsible for various biological and physiological activities, including receptor-mediated stimulation of the proliferation of cells in tissues such as the breast and ovary. Several chemicals and mixtures of chemicals, such as brefeldin A [138], licorice extracts [150], and oil degradation products [184], were found to show gene expression profiles similar to that for estrogen, although they did not stimulate the proliferation of estrogen receptor-positive breast cancer MCF-7 cells. Although the signaling pathway for cell proliferation could theoretically be separated from those for other cell functions, this separation has not been possible because most of the cells examined for estrogenic activity contain estrogen receptors and the technologies used were not suited to such a purpose. Recent findings about more complicated signaling pathways/networks, such as autocrine/paracrine/homeostatic networks and crosstalk/bypassing of cell signals, include pathways not necessarily involving cell proliferation or the cells containing estrogen receptors [185,186]. Thus, estrogenic activity can be detected even for silent estrogens because DMA can separate various signaling pathways, and the similarity of chemicals can be analyzed at the levels of gene expression and cell signaling.