Biomedical subjects

Larbi Amazit

Publications and source records attributed to Larbi Amazit.

8 recordsLinked to original sources

PubMed2006

Specific amino acid residues in the basic helix-loop-helix domain of SRC-3 are essential for its nuclear localization and proteasome-dependent turnover.

SRC-3/AIB1/ACTR/pCIP/RAC3/TRAM-1 is a primary transcriptional coactivator for the estrogen receptor. Here we report that deletion of the SRC-3 basic helix-loop-helix (bHLH) domain blocks its proteasome-dependent turnover. We further identified two residues (K17 and R18) in the SRC-3 bHLH domain that are essential for its stability. Moreover, we found that the bHLH domain contains a bipartite nuclear localization signal (NLS). SRC-3 NLS mutants block its translocation into the nucleus, and this correlates with its insensitivity to proteasome-dependent turnover. SRC-3 shows a time-dependent decay in the presence of cycloheximide which is not apparent for the cytoplasmic mutant. Fusion of a simian virus 40 T antigen NLS to the cytoplasmic localized SRC-3 mutant drives it back into the nucleus and restores its proteasomal sensitivity. In addition, the cytoplasmic mutants are inactive for transcriptional coactivation and cancer cell growth. Taken together, our data indicate that proteasome-dependent turnover of SRC-3 occurs in the nucleus and that two amino acid residues in the bHLH domain provide a signal for its nuclear localization and proteasome-dependent degradation as well as for regulation of SRC-3 transcriptional coactivator capacity.

Active Transport, Cell Nucleus↗

PubMed2006

Oncogenic steroid receptor coactivator-3 is a key regulator of the white adipogenic program.

The white adipocyte is at the center of dysfunctional regulatory pathways in various pathophysiological processes, including obesity, diabetes, inflammation, and cancer. Here, we show that the oncogenic steroid receptor coactivator-3 (SRC-3) is a critical regulator of white adipocyte development. Indeed, in SRC-3(-/-) mouse embryonic fibroblasts, adipocyte differentiation was severely impaired, and reexpression of SRC-3 was able to restore it. The early stages of adipocyte differentiation are accompanied by an increase in nuclear levels of SRC-3, which accumulates to high levels specifically in the nucleus of differentiated fat cells. Moreover, SRC-3(-/-) animals showed reduced body weight and adipose tissue mass with a significant decrease of the expression of peroxisome proliferator-activated receptor gamma2 (PPARgamma2), a master gene required for adipogenesis. At the molecular level, SRC-3 acts synergistically with the transcription factor CAAT/enhancer-binding protein to control the gene expression of PPARgamma2. Collectively, these data suggest a crucial role for SRC-3 as an integrator of the complex transcriptional network controlling adipogenesis.

3T3 Cells↗

PubMed2006

Ligand-controlled interaction of histone acetyltransferase binding to ORC-1 (HBO1) with the N-terminal transactivating domain of progesterone receptor induces steroid receptor coactivator 1-dependent coactivation of transcription.

Modulators of cofactor recruitment by nuclear receptors are expected to play an important role in the coordination of hormone-induced transactivation processes. To identify such factors interacting with the N-terminal domain (NTD) of the progesterone receptor (PR), we used this domain as bait in the yeast Sos-Ras two-hybrid system. cDNAs encoding the C-terminal MYST (MOZ-Ybf2/Sas3-Sas2-Tip60 acetyltransferases) domain of HBO1 [histone acetyltransferase binding to the origin recognition complex (ORC) 1 subunit], a member of the MYST acetylase family, were thus selected from a human testis cDNA library. In transiently transfected CV1 cells, the wild-type HBO1 [611 amino acids (aa)] enhanced transcription mediated by steroid receptors, notably PR, mineralocorticoid receptor, and glucocorticoid receptor, and strongly induced PR and estrogen receptor coactivation by steroid receptor coactivator 1a (SRC-1a). As assessed by two-hybrid and glutathione-S-transferase pull-down assays, the HBO1 MYST acetylase domain (aa 340-611) interacts mainly with the NTD, and also contacts the DNA-binding domain and the hinge domains of hormone-bound PR. The HBO1 N-terminal region (aa 1-340) associates additionally with PR ligand-binding domain (LBD). HBO1 was found also to interact through its NTD with SRC-1a in the absence of steroid receptor. The latter coassociation enhanced specifically activation function 2 activation function encompassed in the LBD. Conversely, the MYST acetylase domain specifically enhanced SRC-1 coupling with PR NTD, through a hormone-dependent mechanism. In human embryonic kidney 293 cells expressing human PRA or PRB, HBO1 raised selectively an SRC-1-dependent response of PRB but failed to regulate PRA activity. We show that HBO1 acts through modification of an LBD-controlled structure present in the N terminus of PRB leading to the modulation of SRC-1 functional coupling with activation function 3-mediated transcription. Importantly, real-time RT-PCR analysis also revealed that HBO1 enhanced SRC-1 coactivation of PR-dependent transcription of human endogenous genes such as alpha-6 integrin and 11beta-hydroxydehydrogenase 2 but not that of amphiregulin. Immunofluorescence and confocal microscopy of human embryonic kidney-PRB cells demonstrated that the hormone induces the colocalization of HBO1 with PR-SRC-1 complex into nuclear speckles characteristic of PR-mediated chromatin remodeling. Our results suggest that HBO1 might play an important physiological role in human PR signaling.

Animals↗

PubMed2006

High-resolution, high-throughput microscopy analyses of nuclear receptor and coregulator function.

Steroid nuclear receptors are ligand-dependent transcription factors that have been studied since the early 1960s by principally biochemical and reporter assay approaches. From these studies an elegant and complex model of nuclear receptor transcription regulation has been developed. Inherent to both biochemical and reporter assay approaches is the generation of averaged responses and it is not generally considered that individual cells could exhibit quite varied responses. In some cases, recent microscopic single-cell analyses provide markedly different responses relative to traditional approaches based on population averaging and underscore the need to continue refinement of the current model of nuclear receptor-regulated transcription. While single-cell analyses of nuclear receptor action have been hindered by the predominantly qualitative nature of the approach, high-throughput microscopy is now available to resolve this issue. This chapter demonstrates the utility of high-throughput microscopic analyses of nuclear receptor and nuclear receptor coregulator function. The ability of high-throughput microscopy to generate physiologically appropriate test populations by filtering based on morphological and protein of interest expression criteria is demonstrated. High-resolution, high-throughput microscopy is illustrated that provides quantitative subcellular information for both androgen and estrogen receptors. Efforts are ongoing to develop model systems that provide additional multiplex data and with refined image analyses to achieve true high-content imaging screens.

Algorithms↗

PubMed2004

Selective recruitment of p160 coactivators on glucocorticoid-regulated promoters in Schwann cells.

In the nervous system, glucocorticoid hormones play a major role during development and throughout life. We studied the mechanisms of action of the glucocorticoid receptor (GR) and its interactions with p160 coactivator family members [steroid receptor coactivator (SRC)-1 (a and e), SRC-2 and SRC-3] in mouse Schwann cells (MSC80). We found that the three p160s were expressed in MSC80 cells. We have shown by functional overexpression and RNA interference experiments that the recruitment of these coactivators by the GR is promoter dependent. A minimal promoter containing two glucocorticoid response elements, (GRE)2-TATA, recruits SRC-1 (a and e) and SRC-3, whereas SRC-2 is excluded. Within the context of the more complex mouse mammary tumor virus promoter, GR recruits SRC-1e and SRC-2, whereas SRC-1a and SRC-3 are not implicated. Furthermore, we have identified cytosolic aspartate aminotransferase as a GR target gene in MSC80 cells by microarray experiments. The GR recruits exclusively SRC-1e in the context of the cytosolic aspartate aminotransferase promoter. Because SRC-1 is the omnipresent coactivator of GR, we further investigated the interactions between GR and this coactivator in Schwann cells by reporter assays and immunocytochemistry experiments with deleted forms of SRC-1. We have shown that SRC-1 unexpectedly interacts with GR via its two nuclear receptor binding domains, thus providing a novel mechanism of GR signaling within the nervous system.

Animals↗

PubMed2003

Subcellular localization and mechanisms of nucleocytoplasmic trafficking of steroid receptor coactivator-1.

Steroid hormone receptors are ligand-stimulated transcription factors that modulate gene transcription by recruiting coregulators to gene promoters. Subcellular localization and dynamic movements of transcription factors have been shown to be one of the major means of regulating their transcriptional activity. In the present report we describe the subcellular localization and the dynamics of intracellular trafficking of steroid receptor coactivator 1 (SRC-1). After its synthesis in the cytoplasm, SRC-1 is imported into the nucleus, where it activates transcription and is subsequently exported back to the cytoplasm. In both the nucleus and cytoplasm, SRC-1 is localized in speckles. The characterization of SRC-1 nuclear localization sequence reveals that it is a classic bipartite signal localized in the N-terminal region of the protein, between amino acids 18 and 36. This sequence is highly conserved within the other members of the p160 family. Additionally, SRC-1 nuclear export is inhibited by leptomycin B. The region involved in its nuclear export is localized between amino acids 990 and 1038. It is an unusually large domain differing from the classic leucine-rich NES sequences. Thus SRC-1 nuclear export involves either an alternate type of NES or is dependent on the interaction of SRC-1 with a protein, which is exported through the crm1/exportin pathway. Overall, the intracellular trafficking of SRC-1 might be a mechanism to regulate the termination of hormone action, the interaction with other signaling pathways in the cytoplasm and its degradation.

Amino Acid Sequence↗

PubMed2003

Sumoylation of the progesterone receptor and of the steroid receptor coactivator SRC-1.

SUMO-1 (small ubiquitin-like modifier) conjugation regulates the subcellular localization, stability, and activity of a variety of proteins. We show here that SUMO-1 overexpression markedly enhances progesterone receptor (PR)-mediated gene transcription. PR undergoes a sumoylation at lysine 388 located in its N-terminal domain. However, sumoylation of the receptor is not responsible for enhanced transcription because substitution of its target lysine did not abolish the effect of SUMO-1 and even converted the receptor into a slightly more active transactivator. Furthermore estrogen receptor alpha (ERalpha)-driven transcription is also enhanced by SUMO-1 overexpression contrasting with the absence of sumoylation of this receptor. We thus analyzed SUMO-1 conjugation to the steroid receptor coactivator SRC-1. We showed that this protein contains two major sites of conjugation at Lys-732 and Lys-774. Sumoylation was shown to increase PR-SRC-1 interaction and to prolong SRC-1 retention in the nucleus. It did not prevent SRC-1 ubiquitinylation and did not exert a clear effect on the stability of the protein. Overexpression of SUMO-1 enhanced PR-mediated gene transcription even in the presence of non-sumoylated mutants of SRC-1. This observation suggests that among the many protein partners involved in steroid hormone-mediated gene regulation several are probably targets of SUMO-1 modification.

Amino Acid Sequence↗

PubMed2002

RANK (receptor activator of nuclear factor kappa B) and RANK ligand are expressed in giant cell tumors of bone.

In giant cell tumors of bone (GCTBs), the mesenchymal stromal cells are the neoplastic cells and induce recruitment and formation of osteoclasts (OCs). Studies on recently discovered members of the tumor necrosis factor receptor-ligand family have demonstrated a crucial role of RANKL (receptor activator of nuclear factor kappa B [RANK] ligand) expressed by osteoblast/stromal cells and of its receptor RANK expressed by OCs during OC differentiation and activation. OCs typically are present in large numbers in GCTBs, suggesting that these tumors may contain cells expressing factors that stimulate OC precursor recruitment and differentiation. We used immunohistochemical analysis to study RANKL and RANK expression in 5 GCTBs. Multinucleated cells and some mononuclear cells showed strong positive staining with anti-RANK antibodies; RANKL was present in a subset of mononuclear cells that did not express the hematopoietic lineage cell marker CD45, a feature that identified them as mesenchymal tumor cells. Our results suggest that RANKL expression may have a role in the pathogenesis of GCTBs and in the formation of the large OC population present in these tumors.

Animals↗