The androgen (AR) and glucocorticoid receptors (GR) are related ligand-activated transcriptional regulators which bind the same cis-acting element and are coexpressed in a variety of cell types. Despite a shared DNA binding site, these receptors mediate diverse cellular responses. To explain this paradox, the existence of cell-specific factors that interact with, and modulate the function of, distinct receptors has been proposed. Prostate epithelial cell growth is sensitive to androgens, but is not affected by glucocorticoids, even though both AR and GR are expressed in these cells. We have recently isolated a unique panel of prostate epithelial cell lines from normal rats and have used these cell lines to examine cell-specific steroid responses. In this study, we compared the abilities of AR and GR to enhance transcription of several different reporter genes regulated by simple (i.e., noncompsite) hormone response elements (HREs) in prostate and nonprostate cell lines. The cell-specific effect occurred independently of the AR hormone binding domain and could be observed with a GAL4 fusion protein containing only the AR N-terminal regulatory domain. Gel shift analyses showed that the relative DNA binding affinity of AR for a probe containing a simple HRE was similar in prostate and nonprostate cell extracts. Presently, the only factors known to mediate steroid receptor-specific gene regulation are cJun and cFos, but there were no cell-specific differences in the functional levels of these proteins which could account for a preferential effect on AR-dependent transcription. Taken together, these results suggest that cell-specific activities exist which can preferentially modulate transcriptional transactivation by AR.
The glucocorticoid receptor (GR) is a ligand-regulated transcription factor that controls genes necessary to initiate glucocorticoid-induced thymocyte apoptosis. We have performed a genetic analysis of thymocyte cell death by isolating and characterizing a panel of GR+ dexamethasone-resistant mutants of the murine WEHI7.2 thymocyte cell line. These apoptosis-defective (Apt-) mutants were used to identify previously unknown early steps in the apoptotic pathway. The Apt- mutants contain nonglucocorticoid receptor, recessive mutations in genes that represent multiple complementation groups. These mutations block apoptosis induced by dexamethasone, gamma irradiation, and c-AMP treatment before the point where Bcl-2 exerts its protective effect. We propose that different signals share a common apoptotic pathway, and that the induction of apoptosis involves multiple precommitment steps that can be blocked by recessive mutations.
Apoptosis induction by staurosporine, ceramide, and Fas stimulation was investigated in the mouse thymoma cell line W7.2 and a panel of dexamethasone (dex)-resistant W7.2 mutant cell lines, Apt3.8, Apt4.8 and Apt5.8, and a Bcl-2 transfected W7.2 cell line (Wbcl2). While W7. 2 cells were found to be sensitive to these apoptosis inducers, the Apt- mutants and Wbcl2 cells were shown to be resistant to some or all of the treatments. Specifically, all three Apt- mutants and Wbcl2 cells were found to be resistant to ceramide and Fas-mediated apoptosis, whereas, Apt4.8 and Apt5.8 were sensitive to staurosporine-induced apoptosis under conditions in which Apt3.8 and Wbcl2 cells were resistant. Measurements of caspase activity and cytochrome c release in cytosolic extracts of dex and staurosporine-treated cells indicated that the recessive Apt- mutations effect steps upstream of mitochondrial dysfunction. Steady-state RNA levels of apoptosis-associated gene transcripts showed that the observed differential resistance of the Apt- cell lines could not be explained by altered expression of numerous Bcl-2 or Fas related genes. Transient transfection of human Fas gene coding sequences into the Apt- mutants and Wbcl2 cells did not induce apoptosis, even though these same cell lines were sensitive to ectopic expression of the FADD and caspase 8 genes. Taken together, these data provide genetic evidence for the existence of shared components in the dex- and Fas-mediated apoptotic pathways in W7.2 cells.
In an effort to understand the molecular basis of androgen action in the prostate, we isolated androgen receptor (AR) cDNA from rat ventral prostate cells and analyzed the transcriptional regulatory activity of the encoded protein in a cotransfection assay. We found that AR is capable of inducing chloramphenicol acetyltransferase activity more than 20-fold using the mouse mammary tumor virus LTR as a source of androgen response elements. This induction was observed in both monkey CV1 cells and human HeLa cells, neither of which contains endogenous functional AR, and was entirely dependent on added androgens. Deletion mapping studies showed that carboxy-terminal deletions of approximately 250 amino acids convert AR into a constitutive activator of transcription. In addition, a chimeric receptor protein containing the amino-terminus and DNA-binding domains of AR fused to the previously defined ligand domain of the glucocorticoid receptor was found to be fully functional based on dexamethasone-induced chloramphenicol acetyltransferase activity. Our results support the prediction that androgens modulate rates of transcriptional initiation, suggesting that posttranscriptional effects of androgens are secondary responses. Moreover, these data reveal that, like other steroid receptors, AR contains a number of distinct regulatory regions important for normal activity. The isolation and characterization of fully functional AR sequences will facilitate the use of molecular genetics to study complex androgen responses in target tissues such as the prostate.
Based on the finding that some transcription factors contain multiple transcriptional regulatory activities, we constructed a panel of rat androgen receptor (AR) mutants containing small internal deletions and point mutations within the amino-terminal region of the receptor. Trans-activation assays in CV-1 cells using AR-responsive reporter genes were performed and led to the identification of two noncontiguous trans-activation regions in the AR amino terminus. One of these regions, termed activator function 1a (AF-1a) is a highly-conserved 14-amino acid segment that is predicted to form a beta-turn followed by an acidic amphipathic alpha-helix. Point mutagenesis within AF-1a revealed that two adjacent hydrophobic residues were required for full AR trans-activation function, as arginine substitutions resulted in a 60% reduction in transcriptional activity. A second amino-terminal region was also identified and has been designated AF-1b. Deletion of the 65-amino acid AF-1b segment, which contains numerous glutamate and aspartate residues, caused a 55% decrease in trans-activation function. An AF-1a/AF-1b double mutant retains less than 10% trans-activation function compared with wild-type AR, suggesting that AF-1a and AF-1b may each contribute separately to maximal AR activity. To determine whether AF-1a and AF-1b play a role in AR-mediated trans-repression of AP-1 function, we tested single and double AF-1a/AF-1b mutants in a transient trans-repression assay. Our results showed that neither AF-1a nor AF-1b was required for AP-1 trans-repression, demonstrating that AR-mediated trans-repression and trans-activation are discrete functions.