Human Progesterone Receptor Binding to Mouse Mammary Tumor Virus Deoxyribonucleic Acid: Dependence on Hormone and Nonreceptor Nuclear Factor(s)

Abstract

Using crude progesterone receptor preparations from T47D human breast cancer cells, we show by immunoprecipitation assay that receptor specifically and with high affinity recognizes the hormone response element (HRE) of the mouse mammary tumor virus (MMTV). The use of crude preparations minimizes alterations of receptors or loss of associated factors that may occur during purification. Specific binding was obtained at 1:1 molar ratios of receptor to DNA, and HRE sequences are recognized with an affinity at least 3 orders of magnitude greater than nonspecific DNA. We have compared the DNA-binding activities of different forms of progesterone receptors. The unliganded 8S cytosol receptor had low but detectable binding activity for MMTV DNA. Addition of hormone to cytosol produced a small but consistent 2.5-fold increase. In vitro methods of transforming cytosol receptors from an 8S to a 4S species failed to increase DNA-binding further. By contrast, 4S receptors bound by R5020 in whole cells and extracted from nuclei by salt, displayed a substantially higher (average, 11-fold) binding activity than an equal number of unliganded cytosol receptors. The dissociation constants for cytosol and nuclear receptor binding to MMTV DNA were similar (approximately 2 x 10(-9) M). Thus, nuclear receptors possess a higher capacity for binding to specific recognition sequences. These results suggest that hormone or a hormone-dependent mechanism increases the intrinsic DNA-binding activity of receptors independent of receptor transformation from 8S to 4S. Further experiments indicate that a nonreceptor activity in nuclear extracts can increase the sequence-specific DNA-binding activity of cytosol receptors. This activity is present in both T47D cells and receptor-negative MDA-231 cells. We conclude that the higher DNA-binding activity of the nuclear receptor-hormone complex is due in part to receptor interaction with other nuclear proteins or factors. Such interactions may function to maintain receptors in a disaggregated active complex or to stabilize their binding to specific DNA sites.