Study of the Photoattachment of Estrogen Receptor to the Nuclear Acceptor Sites in Human Breast Cancer Cells *

A model for estrogen hormone action has been in vogue for over a decade.’.’ For example, the steroid binds to a specific cytoplasmic receptor and the hormone receptor-complex then migrates to the nucleus, where it presumably binds to chromatin to trigger off a specific alteration in gene expression.’-5 Precisely what happens in the nucleus is still unknown. Although the nature of specific interactions between estrogen receptors and the genetic material has been extensively ~tudied,’.~ there is, as yet, no evidence for direct contact between the receptor and the DNA in the target cell nucleus. I f there is an intimate contact between the receptor protein and DNA, and if we introduce a short cross-link between the protein and DNA, then the position of this cross-link will identify the point of contact. Our approach to this problem was to study the photochemical attachment of estrogen-receptor complex to bromouracil-substituted DNA (photochemically reactive DNA) in the intact nucleus of human breast cancer cells, MCF-7. We selected MCF-7 cells because they are estrogen responsive and contain estrogen receptor, which can translocate from the cytosol to the nucIeus.’ Our experiment was designed to follow the effect of irradiation on the extractability of the nuclear receptor as follows: I . Cells were grown in the presence of bromouracil, so that the bromouracil was incorporated into the nuclear DNA 2. The bromouracil-substituted cells were labeled with ’H-estradiol, so that the cytoplasmic receptor complex translocated to the nucleus 3. The cells were irradiated with near-uv light (-312 nm) to form a covalent cross-link between the receptor and the DNA