ARNT2 acts as the dimerization partner of SIM1 for the development of the hypothalamus.
(17/421)
One major function of the hypothalamus is to maintain homeostasis by modulating the secretion of pituitary hormones. The paraventricular (PVN) and supraoptic (SON) nuclei are major integration centers for the output of the hypothalamus to the pituitary. The bHLH-PAS transcription factor SIM1 is crucial for the development of several neuroendocrine lineages within the PVN and SON. bHLH-PAS proteins require heterodimerization for their function. ARNT, ARNT2, and BMAL1 are the three known general heterodimerization partners for bHLH-PAS proteins. Here, we provide evidence that Sim1 and Arnt2 form dimers in vitro, that they are co-expressed in the PVN and SON, and that their loss of function affects the development of the same sets of neuroendocrine cell types within the PVN and SON. Together, these results implicate ARNT2 as the in vivo dimerization partner of SIM1 in controlling the development of these neuroendocrine lineages. (+info)
Different transcriptional properties of mSim-1 and mSim-2.
(18/421)
The mSim-1 and mSim-2 gene products are mammalian homologues of the Drosophila Sim gene. The dSim gene product transactivates through a DNA binding site known as the CNS midline enhancer (CME) element. We have investigated the transcriptional properties of mSIM-1 and mSIM-2 mediated through the CME element in concert with their dimerization partners, ARNT and ARNT-2. The mSIM-1/ARNT heterodimer transactivates reporter constructs via the ARNT carboxy-terminus. However, mSIM-2 quenches ARNT transactivation. We find that mSIM-2 competes with mSIM-1 for binding to ARNT, suggesting a possible antagonism between these transcription factors. (+info)
Cardiovascular basic helix loop helix factor 1, a novel transcriptional repressor expressed preferentially in the developing and adult cardiovascular system.
(19/421)
We have cloned a cardiovascular-restricted basic helix-loop-helix factor that interacts with arylhydrocarbon receptor nuclear translocator (ARNT) in a yeast two-hybrid screen. Cardiovascular helix-loop-helix factor 1 (CHF1) is distantly related to the hairy family of transcriptional repressors. We analyzed its expression pattern during mouse embryo development. At day 8.5, the expression of CHF1 is first detected in the primitive ventricle of the primordial heart tube and persists throughout gestation. In rat hearts, this expression is down-regulated after birth, concurrent with terminal differentiation of cardiomyocytes. In the developing vasculature, CHF1 first appears in the dorsal aorta at day 9.0, which precedes the reported expression of smooth muscle cell markers, and persists into adulthood. In an in vitro system of smooth muscle cell differentiation, CHF1 mRNA was barely detectable in undifferentiated cells but was induced highly in differentiated smooth muscle cells. To determine whether CHF1 might affect the function of ARNT, we performed transfection studies. Co-transfection of CHF1 inhibited ARNT/EPAS1-dependent transcription by 85%, and this inhibition is dose-dependent. In electrophoretic mobility studies, CHF1 inhibited the binding of the ARNT/EPAS1 heterodimer to its target site. Our data suggest that CHF1 functions as a transcriptional repressor and may play an important role in cardiovascular development. (+info)
Expression of Ah receptor and dioxin-related genes in human uterine endometrium in women with or without endometriosis.
(20/421)
2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) has been suggested as a possible etiologic factor for endometriosis, a condition in which endometrium-like tissues are present outside the uterus. The prevailing view pertaining to the origin of endometriotic cells is that they are from eutopic endometrial cells which regurgitate through fallopian tubes. In order to get insight into the possible involvement of TCDD in the pathogenesis of endometriosis, we suspected that TCDD may act differently on the endometrium with or without endometriosis. To address this, we examined the presence of messenger RNAs of arylhydrocarbon receptor (AhR), AhR nuclear translocator (Arnt) and two dioxin-responsive genes, cytochrome P-450 1B1 (CYP1B1) and downstream of tyrosine kinases (p62(dok)), in the endometrium of women with or without endometriosis using semi-quantitative reverse transcription-polymerase chain reaction. All the genes were expressed throughout the menstrual cycle. The expression level of p62(dok) was higher in the proliferative phase than in the secretory phase. In contrast, the expression levels of AhR, Arnt and CYP1B1 seemed to be constant during the cycle. In terms of the comparison between non-endometriosis and endometriosis group, the mRNA levels of AhR, Arnt, CYP1B1 and p62(dok) were essentially similar. Interestingly, AhR mRNA level was significantly lower in smokers than in non-smokers. Based on the regression analysis, significant linear and positive correlations were observed between AhR and Arnt mRNA levels, and between Arnt and p62(dok) mRNA levels. In summary, expression of AhR and dioxin-related genes in the endometrium did not differ in women with or without endometriosis. (+info)
The t(1;12)(q21;p13) translocation of human acute myeloblastic leukemia results in a TEL-ARNT fusion.
(21/421)
The TEL/ETV6 gene is located at 12p13 and encodes a member of the ETS family of transcription factors. Translocated ETS leukemia (TEL) is frequently involved in chromosomal translocations in human malignancies, usually resulting in the expression of fusion proteins between the amino-terminal part of TEL and either unrelated transcription factors or protein tyrosine kinases. We have characterized a t(1;12)(q21;p13) translocation in an acute myeloblastic leukemia (AML-M2). At the protein level, the untranslocated TEL copy and, as a result of the t(1;12) translocation, a fusion protein between TEL and essentially all of aryl hydrocarbon receptor nuclear translocator (ARNT) are expressed. The involvement of ARNT in human leukemogenesis has not been previously described. The ARNT protein belongs to a subfamily of the "basic region helix-loop-helix" (bHLH) protein that shares an additional region of similarity called the PAS (Per, ARNT, SIM) domain. ARNT is the central partner of several heterodimeric transcription factors, including those containing the aryl hydrocarbon (dioxin) receptor (AhR) and the hypoxia-inducible factor 1alpha (HIF1alpha). Our results show that the TEL-ARNT fusion protein is the crucial product of the translocation and suggest that interference with the activity of AhR or HIF1alpha can contribute to leukemogenesis. (+info)
An aryl hydrocarbon receptor conformation acts as the functional core of nuclear dioxin signaling.
(22/421)
DNA-complexed heterodimers of the aryl hydrocarbon receptor (AhR) with the Ah receptor nuclear translocator (Arnt) are the molecular switches for nuclear signaling of 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD). AhR-Arnt heterodimers regulate genes involved in the metabolism of xenobiotics or fatty acids and various genes important for growth and differentiation. In this report several potent methods, such as the limited protease digestion, gel shift and gel shift clipping assays, allowed the investigation of ligand-stabilized conformations of AhR monomers in comparison to that of AhR-Arnt heterodimers. Interestingly, the ligand sensitivity of monomeric AhR was found to be very low at 25 nM, whereas DNA-dependent methods consistently provided EC(50) values between 0.12 and 0.6 nM for AhR in a heterodimeric complex, i. e. an approximate 100-fold higher ligand sensitivity. This indicates that complex formation of AhR with Arnt on DNA is an important and critical step in transforming AhR into a high affinity receptor for TCDD. A comparison of wild-type AhR with different C-terminal receptor truncations suggests that the PAS-B subregion of its PAS domain is of central importance for stabilization of a functional, i. e. ligand-sensitive, AhR-Arnt conformation, whereas the PAS-A subregion appears to be critical for dimerization of AhR and Arnt. In conclusion, the results of this study provide important information on the ligand sensitivity of AhR and AhR-Arnt heterodimer conformations. (+info)
The bioflavonoid galangin blocks aryl hydrocarbon receptor activation and polycyclic aromatic hydrocarbon-induced pre-B cell apoptosis.
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Bioflavonoids are plant compounds touted for their potential to treat or prevent several diseases including cancers induced by common environmental chemicals. Much of the biologic activity of one such class of pollutants, polycyclic aromatic hydrocarbons (PAH), is mediated by the aryl hydrocarbon receptor/transcription factor (AhR). For example, the AhR regulates PAH immunotoxicity that manifests as pre-B cell apoptosis in models of B cell development. Because bioflavonoids block PAH-induced cell transformation and are structurally similar to AhR ligands, it was postulated that some of them would suppress PAH-induced, AhR-dependent immunotoxicity, possibly through a direct AhR blockade. This hypothesis was tested using a model of B cell development in which pre-B cells are cultured with and are dependent on bone marrow stromal or hepatic parenchymal cell monolayers. Of seven bioflavonoids screened, galangin (3,5,7-trihydroxyflavone) blocked PAH-induced but not C(2)-ceramide- or H(2)O(2)-induced pre-B cell apoptosis. Because galangin blocked AhR-dependent reporter gene expression, AhR complex-DNA binding, and AhR nuclear translocation, inhibition of a relatively early step in AhR signaling was implicated. This hypothesis was supported by the ability of galangin to bind the AhR and stabilize AhR-90-kDa heat shock protein complexes in the presence of AhR agonists. These studies demonstrate the utility of pre-B cell culture systems in identifying compounds capable of blocking PAH immunotoxicity, define at least one mechanism of galangin activity (i.e., repression of AhR activation), and motivate the use of this and similar dietary bioflavonoids as relatively nontoxic inhibitors of AhR agonist activity and as pharmacologic agents with which to dissect AhR signaling pathways. (+info)
Efficient translation of mouse hypoxia-inducible factor-1alpha under normoxic and hypoxic conditions.
(24/421)
The heterodimeric hypoxia-inducible factor-1 (HIF-1), consisting of the subunits HIF-1alpha and HIF-1beta/ARNT, is a master transcriptional regulator of oxygen homeostasis. Under hypoxic conditions, HIF-1alpha levels very rapidly increase, mostly due to protein stabilization. However, translational regulation of HIF-1alpha has not been directly analyzed so far. Mouse HIF-1alpha exists as two mRNA isoforms (termed mHIF-1alphaI.1 and mHIF-1alphaI. 2) containing structurally different 5'-termini which might modulate translation initiation. Whereas the in vitro translation efficiency of these two mRNA isoforms was about equal, the mHIF-1alphaI.2 5'-untranslated region (5'-UTR) conferred significantly higher in vivo luciferase reporter gene activity than the mHIF-1alphaI.1 5'-UTR. Similar corresponding luciferase mRNA levels indicate translational rather than transcriptional alterations. Reporter gene expression was not affected upon exposure of transiently transfected cells to hypoxia (1% oxygen). Direct assessment of translational regulation by polysomal profile analysis of HeLaS3 cells showed that HIF-1alpha (and to a lower extent ARNT) mRNA was found mainly in the translationally active polyribosomal fractions under both normoxic and hypoxic conditions. In contrast, the association of mRNAs for beta-actin and ribosomal protein L28 with the polyribosomal fractions was substantially reduced under hypoxic conditions, suggesting decreased overall protein synthesis. Thus, efficient translation of mouse HIF-1alpha in a situation where the general translation efficiency is reduced represents a prerequisite for the very rapid accumulation of HIF-1alpha protein upon exposure to hypoxia. (+info)