The concentric structure of the developing gut is regulated by Sonic hedgehog derived from endodermal epithelium.
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The embryonic gut of vertebrates consists of endodermal epithelium, surrounding mesenchyme derived from splanchnic mesoderm and enteric neuronal components derived from neural crest cells. During gut organogenesis, the mesenchyme differentiates into distinct concentric layers around the endodermal epithelium forming the lamina propria, muscularis mucosae, submucosa and lamina muscularis (the smooth muscle layer). The smooth muscle layer and enteric plexus are formed at the outermost part of the gut, always some distance away from the epithelium. How this topographical organization of gut mesenchyme is established is largely unknown. Here we show the following: (1) Endodermal epithelium inhibits differentiation of smooth muscle and enteric neurons in adjacent mesenchyme. (2) Endodermal epithelium activates expression of patched and BMP4 in adjacent non-smooth muscle mesenchyme, which later differentiates into the lamina propria and submucosa. (3) Sonic hedgehog (Shh) is expressed in endodermal epithelium and disruption of Shh-signaling by cyclopamine induces differentiation of smooth muscle and a large number of neurons even in the area adjacent to epithelium. (4) Shh can mimic the effect of endodermal epithelium on the concentric stratification of the gut. Taken together, these data suggest that endoderm-derived Shh is responsible for the patterning across the radial axis of the gut through induction of inner components and inhibition of outer components, such as smooth muscle and enteric neurons. (+info)
Post-transcriptional regulation of thyroid hormone receptor expression by cis-acting sequences and a naturally occurring antisense RNA.
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Thyroid hormone (T(3)) coordinates growth, differentiation, and metabolism by binding to nuclear thyroid hormone receptors (TRs). The TRalpha gene encodes T(3)-activated TRalpha1 (NR1A1a) as well as an antagonistic, non-T(3)-binding alternatively spliced product, TRalpha2 (NR1A1b). Thus, the TRalpha1/TRalpha2 ratio is a critical determinant of T(3) action. However, the mechanisms underlying this post-transcriptional regulation are unknown. We have identified a non-consensus, TRalpha2-specific 5' splice site and conserved intronic sequences as key determinants of TRalpha mRNA processing. In addition to these cis-acting elements, a novel regulatory feature is the orphan receptor RevErbAalpha (NR1D1) gene, which is transcribed from the opposite direction at the same locus and overlaps the TRalpha2 coding region. RevErbAalpha gene expression correlates with a high TRalpha1/TRalpha2 ratio in a number of tissues. Here we demonstrate that coexpression of RevErbAalpha and TRalpha regulates the TRalpha1/TRalpha2 ratio in intact cells. Thus, both cis- and trans-regulatory mechanisms contribute to cell-specific post-transcriptional regulation of TR gene expression and T(3) action. (+info)
Constitutive activation of retinoic acid receptor beta2 promoter by orphan nuclear receptor TR2.
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The orphan nuclear receptor TR2 functions as a constitutive activator for the endogenous retinoic acid receptor beta2 (RAR(beta2)) gene expression in P19 embryonal carcinoma cells and for reporters driven by the RAR(beta2) promoter in COS-1 cells. The activation of RAR(beta2) by TR2 is mediated by the direct repeat-5 (DR5) element located in the RAR(beta2) promoter. Furthermore, cAMP exerts an enhancing effect on the activation of RAR(beta2) by TR2, which is mediated by the cAMP response element located in the 5'-flanking region of the DR5. The constitutive activation function-1 (AF-1) of TR2 is mapped to amino acid residues 10-30 in its N-terminal A segment. A direct molecular interaction occurs between CREMtau and TR2, detected by co-immunoprecipitation, which is mediated by the N-terminal AB segment of TR2. In gel mobility shift assays, TR2 competes with P19 nuclear factor binding to the RAR(beta2) promoter, and TR2 and CREMtau bind simultaneously to this DNA fragment. The role of TR2 in the early events of RA signaling process is discussed. (+info)
Silencing mediator of retinoic acid and thyroid hormone receptors, as a novel transcriptional corepressor molecule of activating protein-1, nuclear factor-kappaB, and serum response factor.
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Silencing mediator of retinoic acid and thyroid hormone receptors (SMRT) is known to interact with Sin3 and recruit the histone deacetylases (HDACs) that lead to hypoacetylation of histones and transrepression of target transcription factors. Herein, we found that coexpression of SMRT significantly repressed transactivations by activating protein-1 (AP-1), nuclear factor-kappaB (NFkappaB), and serum response factor (SRF) in a dose-dependent manner, but not in the presence of trichostatin A, a specific inhibitor of HDAC. Similarly, coexpression of HDAC1 and mSin3A also showed repressive effects. Consistent with these results, the C-terminal region of SMRT directly interacted with SRF, the AP-1 components c-Jun and c-Fos, and the NFkappaB components p50 and p65, as demonstrated by the yeast and mammalian two hybrid tests as well as the glutathione S-transferase pull down assays. Thus, we concluded that SMRT serves to recruit Sin3/HDACs to SRF, NFkappaB, and AP-1 in vivo and modulate their transactivation. (+info)
Overexpression of thyroid hormone receptor beta1 is associated with thyrotropin receptor gene expression and proliferation in a human thyroid carcinoma cell line.
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To correlate the differentiation phenotype of two human thyroid cancer cell lines with their expression of various molecular markers, we analyzed the mRNA levels of four thyroid-specific genes, including thyrotropin receptor (TSHR), thyroglobulin (Tg), thyroid transcription factor-1 (TTF-1), and paired-box containing transcription factor-8 (PAX-8) genes. The results showed a differentiation-status-related pattern in which a well-differentiated cell line (WRO) expressed all the four genes, in contrast to an anaplastic cell line (ARO) that expressed TTF-1 and reduced levels of TSHR, but no Tg or PAX-8 genes. Furthermore, to verify the finding of concomitant loss of beta subtype thyroid hormone receptor (TRbeta) and TSHR gene expression in neoplastic thyroid tumors (Bronnegard et al. 1994), we examined the expression levels of TRbeta1 gene in these cell lines. Whereas the WRO cells produced an abundant amount of TRbeta1 protein detectable by immunoprecipitation, the ARO cells produced none. This new observation prompted us to investigate whether overexpression of TRbeta1 protein in ARO cells might produce changes in the differentiation phenotypes. We found that the level of expression of the TSHR gene and the proliferative index of ARO cells were significantly upregulated in the cells stably transfected with wild-type TRbeta1. These findings suggest that TRbeta1 protein overexpression can affect the differentiation phenotypes and induce more efficient cell proliferation of the anaplastic ARO cells. (+info)
Mechanism of liver-selective thyromimetic activity of SK&F L-94901: evidence for the presence of a cell-type-specific nuclear iodothyronine transport process.
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The thyromimetic compound SK&F L-94901 shows more potent thyromimetic activity in the liver than in the pituitary gland or heart when administered to rats. The mechanisms of liver-selectivity of SK&F L-94901 were examined using cultured rat hepatoma cells (dRLH-84) and rat pituitary tumor cells (GH3), both of which showed saturable cellular uptake of tri-iodothyronine (T(3)). When isolated nuclei with partial disruption of the outer nuclear membrane were used, SK&F L-94901 competed for [(125)I]T(3) binding to nuclear receptors almost equally in dRLH-84 and GH3 cells. SK&F L-94901 also did not discriminate thyroid hormone receptors (TR) alpha1 and beta1 in terms of binding affinity and activation of the thyroid hormone responsive element. In intact cells, however, SK&F L-94901 was a more potent inhibitor of nuclear [(125)I]T(3) binding in dRLH-84 cells than in GH3 cells at an early phase of the nuclear uptake process and after binding equilibrium. These data suggest that SK&F L-94901 is more effectively transported to nuclear TRs in hepatic cells than in pituitary cells and therefore shows liver-selective thyromimetic activity. In conclusion, SK&F L-94901 discriminates hepatic cells and pituitary cells at the nuclear transport process. The cellular transporters responsible for this discrimination were not evident. (+info)
The p53/retinoblastoma-mediated repression of testicular orphan receptor-2 in the rhesus monkey with cryptorchidism.
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Whereas the linkage of infertility to cryptorchidism, the failure of the testis to descend into the scrotum at birth, has been well documented, the detailed molecular mechanism remains unclear. Here we report that the testicular orphan receptor-2 (TR2) expression, which modulates many signal pathways, was completely repressed in the surgery-induced cryptorchidism of the rhesus monkey. Further studies link TR2 repression to the induction of p53 and results suggest that induced p53 could repress TR2 expression via the p53-->p21-->CDK-->Rb-->E2F signal pathway. In return, TR2 could also control the expression of p53 and Rb through the regulation of human papillomavirus 16 E6/E7 genes. Together, our data suggest a feedback control mechanism between TR2 and p53/Rb tumor suppressors, which might play important roles in male infertility associated with cryptorchidism. (+info)
Zebrafish ftz-f1 gene has two promoters, is alternatively spliced, and is expressed in digestive organs.
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Fushi-tarazu Factor-1 (FTZ-F1) is a family of nuclear receptors involved in various developmental processes. We have cloned a zebrafish FTZ-F1 gene, termed ff1, which belongs to the fetoprotein transcription factor/liver receptor homologue-1 (FTF/LRH-1) subgroup of the FTZ-F1 family. Four transcripts arise as a result of differential promoter usage and alternative splicing at the 3'-most exons. The longer transcript, form A, encodes a transcriptional activator. The shorter transcript, form B, lacks the activation domain, and hence could not activate transcription. The difference in promoter usage generates FF1 proteins with different N-terminal sequences. All four transcripts appear to be expressed in most of the adult tissues, whereas, during embryo development, the IIA form is the predominant transcript. Reverse transcriptase-PCR and in situ hybridization experiments showed that the ff1 transcript is expressed in the hypothalamus, spinal cord, mandibular arch and digestive organs, including pancreas, liver, and intestine. The expression of ff1 in the digestive organs implies its function in gut development. (+info)