Interaction of Progesterone Receptor with Chromatin

Basic Sciences

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Abstract

The goal of this project is to understand the mechanism by which the progesterone receptor (PR) interacts with and remodels chromatin at target genes in vivo using the promoters for the FKBP51 gene and the mouse mammary tumor virus (MMTV) as model systems. In the non-activated state, the MMTV promoter has a chromatin structure repressive to transcription when it exists in cells as replicating chromatin. Upon binding of the liganded glucocorticoid receptor (GR), the promoter undergoes a chromatin remodeling event which is mechanistically involved in the activation of transcription. Our previous work has shown that the GR and PR have different requirements for chromatin remodeling at the MMTV promoter even though they bind to the same DNA sequences in the promoter. Our observations may form the basis for a mechanism by which the GR and PR control expression of distinct sets of target genes in vivo. This is particularly relevant in the mammary gland where the GR and PR can coexist in the same cell types. We have established that the PR exists in two distinct functional states in cultured mammary adenocarcinoma cells. In one state, it can neither remodel chromatin nor activate transcription at the MMTV promoter; thus its action may be restricted to target genes which do not require remodeling prior to activation. In addition, this form of the PR can be activated by other signal transduction pathways in a progestin-independent fashion. In the second functional state, the PR is able to remodel and activate the MMTV promoter in chromatin, but is refractory to ligand-independent activation. Thus, this form of the PR responds only to its ligand but would be able to activate target genes even in a repressive chromatin environment. We have also shown that the PR can be converted from the first state to the second by some form of cellular processing. This may represent a mechanism by which cells can restrict or expand the activity of the PR in vivo. Because molecular chaperones play a critical role in the processing of steroid receptors, we have employed an immunoprecipitation method to isolate the PR in its native forms and examine its association with chaperones. This has been done in collaboration with Dr. David Smith (Mayo Clinic Scottsdale) who has provided us with reagents and expertise invaluble to the project. We find that the unliganded PR forms a variety of complexes which are differentially distributed in its two functional states. PR which cannot remodel chromatin but is activated by ligand-independent mechanisms is enriched in a tightly-bound nuclear form which appears to be either dimerized and chaperone-free or complexed with hsp90, p23, and the large immunophilin, FKBP51. In contrast, PR which can activate MMTV in chromatin and is refractory to ligand-independent acitvation exists mostly in a cytoplasmic or loosely-bound nuclear form complexed with hsp90 and p23. We speculate that the balance between the cytoplasmic and tightly-bound nuclear state indicates that PR/chaperone interactions can be regulated by means other than the presence of progestins. In addition, the balance of these interactions may have a significant impact on the function of the PR. Current efforts are concentrated on further characterization of the PR complexes and the basis of their equilibrium. The classic model of steroid receptor action is that the ligand-occupied receptor binds directly to recognition sequences in the promoter of a target gene. However, upon characterization of an increasing number of steroid-regulated genes, it is becoming clear that there are other mechanisms of steroid receptor-induced activation that may not involve a direct interaction with DNA. The promoter for the large immunophilin, FKBP51, is upregulated by progestins and glucocorticoids. However, the promoter has no classic steroid response elements, rather, the regulation appears to be mediated through SP1 sites by an unknown mechanism. A number of other steroid-regulated promoters are also rich in SP1 sites. We have initiated a project to characterize the mechanism by which PR activates this promoter in chromatin, comparing it directly with the mechanism by which PR activates the MMTV promoter in the same cells. Lastly, in collaboration with Dr. Anna Riegel at Georgetown School of Medicine, we are characterizing the mechanism by which a truncated steroid receptor activator, AIB1D3, synergizes with PR to activate target genes. This protein is a truncated version of AIB1 (amplified in breast cancer) and has been found to be overexpressed in human mammary tumors. It potently enhances PR-dependent transcription in some experimental systems. We are examining its function at the MMTV promoter in chromatin as well as its cellular distribution.