The interaction of sex steroid hormone receptors with the nuclear matrix (NM) of target and non-target tissue was investigated using a simple in vitro binding assay. Steroid receptors can recognize acceptor sites on the NM of target cells; androgen receptor binds with the highest apparent affinity to rat prostate NM; similarly estrogen receptor binds with the highest apparent affinity to uterine NM. Furthermore, the steroid receptor-NM interaction depends upon the hormonal status of the animal. The binding of androgen receptor to rat prostate NM was drastically reduced upon hormone withdrawal (castration) and fully recovered upon hormonal stimulation. When NM were prepared by an alternate method (DNase I digestion prior to high salt extraction) known to digest "active" chromatin, no preferential receptor binding to target tissue NM was observed. Although the NM fraction contains less than 1% of the total nuclear DNA, the matrix-associated DNA sequences seem to be, at least in part, responsible for specific receptor recognition. DNA extracted from the prostate NM was shown to be a potent competitor for androgen receptor binding as measured by DNA-cellulose competition experiments. Moreover, this DNA recognition also depends upon the hormonal status of the animal. These studies are consistent with the notion that hormonal manipulation induces changes in the NM-associated DNA sequences of steroid hormone target tissue.
AB - The receptor-binding factor (RBF) for the avian oviduct progesterone (Pg) receptor (PR) has previously been shown to be a unique 10-kDa nuclear matrix protein that generates high affinity PR-binding sites on avian DNA. This paper describes the use of Southwestern blot and DNA gel shift analyses with RBF protein to identify a minimal 54-base pair RBF-binding clement in the matrix-associated region (MAR) of the Pg-regulated c-myc gene promoter. This element contains a 5'-GC-rich domain and a 3'-AT-rich domain, the latter of which has a homopurine/homopyrimidine structure. The gel shift assays required the generation of an RBF-maltose fusion protein (RBF-MBP), which specifically binds this element and is supershifted when the anti-RBF polyclonal antibody is added. Computer analysis of the full-length amino acid sequence for RBF predicts a DNA-binding motif involving a β-sheet structure at the N-terminal domain. Southern blot analyses using nuclear matrix DNA suggests that there are dual MAR sites in the c-myc promoter, which flank an intervening domain containing the RBF element. The co-transfection of this MAR sequence, containing the RBF element and cloned into a luciferase reporter vector, together with an RBF expression vector construct, into steroid treated human MCF-7 cells, results in a decrease of the c-myc promoter activity relative to control transfections containing only the parent vector of the RBF expression construct. These data suggest that a unique chromatin/nuclear matrix structure, composed of the RBF-DNA element complex which is flanked by nuclear matrix attachment sites, serves to bind the PR and repress the c-myc promoter.