Modulation of nuclear receptor function by interacting proteins
Year: 2007
Type: dissertation
Abstract: Nuclear receptors are a family of transcription factors involved in many\nbiological processes. They represent key potentials as therapeutic\ntargets for several clinical conditions such as diabetes, obesity,\ncardiovascular disease and cancer, because their activity is modulated by\nsmall molecules. Collectively these diseases represent a large market for\nprescription drugs. The nuclear receptor family consists of 48 members.\nThe nuclear receptors have three conserved domains, the N-terminal\ntransactivation domain, the central DNA binding domain and the C-terminal\nligand binding domain. Nuclear receptors are ligand activated\ntranscription factors that are not only regulated by small lipophilic\nligands, but also by interaction with coactivators and corepressors as\nwell as other binding proteins. Protein-protein interactions are\nessential in many biological pathways and are also attractive for drug\ndiscovery. In this thesis I elucidate the functional significance of some\nnuclear receptor interacting proteins. I have investigated the mechanism\nof the corepressor receptor interacting protein 140 (RIP140) for\nrepression of glucocorticoid receptor (GR) and liver X receptor (LXR) and\nthe functional significance of the novel interacting proteins\npolyamine-modulated factor 1 (PMF-1) and germinal center associated\nnuclear protein (GANP) in glucocorticoid signalling.\n\nRIP140 represses ligand activated nuclear receptors, such as GR and LXR.\nIt is localized in small nuclear foci targeted by a 40 amino acid long\nsequence. Even though the foci targeting sequence in RIP140 overlaps with\nthe binding site of the corepressor C-terminal binding protein (CtBP),\ninteraction with CtBP is not essential for foci targeting. Upon\ncoexpression of ligand activated GR or LXR with RIP140 is redistributed\nto larger foci distinct from RIP140 foci. The redistribution of RIP140/GR\ninvolves RIP140 repression domains and the DNA binding domain of GR.\nRIP140 repression domains include the C-terminal receptor interacting\nLXXLL motifs binding to GR and the interaction domain of the corepressor\nCtBP. The repression of RIP140/LXR involves the integrity of the\nC-terminal domain of RIP140 including the LXXML motif. Jointly, these\nresults suggested that RIP140 represses in multiple ways including direct\nbinding to ligand activated NR and the formation and redistribution of\nintranuclear repressive protein complexes.\n\nPMF-1 is a transcription factor induced by polyamines, which are\nimportant regulators of cell growth and cell death and are implicated in\nglucocorticoid induced apoptosis. We identified PMF-1 to functionally\ninteract with RIP140 and GR. PMF-1 represses glucocorticoid induced GR\nactivity and has an intrinsic repression activity, which could contribute\nto the repressive action. Although a physical interaction was observed\nbetween PMF-1 and RIP140, we found that PMF-1 does not further enhance\nRIP140 repressive effect.\n\nWe identified GANP and the GANP splice variant MCM3 associated protein\n(MCM3AP) to bind to GR ligand binding domain. We found GANP to be a\nshuttling protein that shuttles between the nucleus and the cytoplasm,\nand contains nuclear localization and nuclear export signals. GANP and\nMCM3AP were initially reported as proteins binding to MCM3, which is a\nmember of the MCM protein complex that is involved in initiating of DNA\nreplication. We show that glucocorticoids regulate the chromatin loading\nof MCM3, inhibit DNA replication and arrest cells in the G1 phase of the\ncell cycle. We also show that MCM3AP counteracts the repressive activity\nof glucocorticoid on DNA replication. In conclusion we suggest that\ninteraction between GR, GANP/MCM3AP and MCM3 could imply a new way for\nglucocorticoid regulation of cell proliferation.
Source: Figshare
Author: Waffa Osman
Language: English
Subfield: Endocrinology
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W2164183100