Ligand-activated thyroid hormone and retinoic acid receptors inhibit growth factor receptor promoter expression

Decabromodiphenyl ether exposure reduces dabrafenib sensitivity of papillary thyroid carcinoma harboring BRAFV600E mutation through the EGFR-CRAF-MAPK pathway: An in vitro study

Decabromodiphenyl ether (BDE209) has been demonstrated to be associated with thyroid dysfunction and thyroid carcinoma risk as a widely used brominated flame retardants. Although dabrafenib has been confirmed to be a promising therapeutic agent for papillary thyroid carcinoma (PTC) harboring BRAFV600E mutation, the rapid acquired dabrafenib resistance has brought a great challenge to clinical improvement and the underpinning mechanisms remain poorly defined. By treating PTC-derived and normal follicular epithelial cell lines with BDE209, we assessed its impact on the MAPK pathway's activation and evaluated the resultant effects on cell viability and signaling pathways, utilizing methods such as Western blot, IF staining, and RNA-seq bioinformatic analysis. Our findings reveal that BDE209 exacerbates MAPK activation, undermining dabrafenib's inhibitory effects by triggering the EGFR pathway, thereby highlighting BDE209's potential to diminish the pharmacological efficacy of dabrafenib in treating BRAF-mutated PTC. This research underscores the importance of considering environmental factors like BDE209 exposure in the effective management of thyroid carcinoma treatment strategies.

Hyperthyroidism with concurrent FMS-like tyrosine kinase 3-internal tandem duplication-positive acute promyelocytic leukemia: A case report and review of the literature

Neutropenia is a common side-effect in hyperthyroid patients with long-term use of antithyroid drugs. This may be caused by drug-induced immune dysfunction or increased thyroxine hematologic toxicity, which usually returns to normal after medication is discontinued or the hyperthyroidism becomes well controlled. However, hyperthyroidism with pancytopenia is extremely rare. The current case report presents a hyperthyroid patient complicated with pancytopenia who had taken antithyroid drugs for 14 years. Bone marrow analysis revealed primary leuokocytes, indicating M3 acute leukemia. Genetic analysis revealed promyelocytic leukemia-retinoic acid receptor α fusion and FMS-like tyrosine kinase 3-internal tandem duplication. The genetic abnormality was also associated with thyroid hormonal functions. After a standard anti-M3 regimen was administed, the patient achieved complete remission and maintained stable thyroid functions. To the best of our knowledge, this is the first reported case of a patient with hyperthyroidism acquiring M3 leukemia harboring the FMS-like tyrosine kinase 3-internal tandem duplication.

Expression profiling and regulation of genes related to silkworm posterior silk gland development and fibroin synthesis

The posterior silk gland (PSG) is the most important suborgan responsible for the synthesis and secretion of silk core fibroin proteins in silkworm. Here, we performed genome-scale expression profiling analysis of silkworm PSG at the fourth molting (M4) and at day 1 (V1), day 3 (V3), day 5 (V5), and wandering stage (W) of the fifth instar by microarray analysis with 22 987 probes. We found that the five genes of silk proteins secreted from PSG including fibroin heavy (H) and light (L) chains, P25, seroin 1, and seroin 2 basically showed obvious up-regulation at V3 which lasted to V5, while slight down-regulation at W. The expression of translation-related genes including ribosomal proteins and translation initiation factors generally remained stable from M4 to V5, whereas it showed clear down-regulation at W. Clustering analysis of the 643 significantly differentially expressed transcripts revealed that 43 of the important genes including seroin 1 and sugar transporter protein had co-expression patterns which were consistent with the rate changes of fibroin synthesis and PSG growth. Pathway analysis disclosed that the genes in different clusters might have co-regulations and direct interactions. These genes were supposed to be involved in the fibroin synthesis and secretion. The differential expression of several hormone-related genes also suggested their functions on the regulation of PSG development and fibroin synthesis. 2D gel-based proteomics and phosphoproteomics profiling revealed that the phosphorylated proteins accounted for no more than one-sixth of the total proteins at each stage, which was much lower than the level in normal eukaryotic cells. Changes in the phosphorylation status and levels of several proteins such as actin-depolymerizing factor 1 and enolase might be deeply involved in fibroin secretion and tissue development. Shotgun proteomic profiling combined with label-free quantification analysis on the PSG at V3, V5, and W revealed that many small heat shock proteins (sHSP) were specially expressed at W, which was substantially consistent with the results from 2-DE analysis, and implied the close correlations of sHSP with the physiological states of PSG at W. A majority of significantly up-regulated proteins at V5 were related to ribosome pathway, which was different from the microarray results, implying that the translation-level regulation of ribosomal proteins might be critical for fibroin synthesis. In contrast, the ubiquitin-proteasome pathway related proteins appeared obviously up-regulated at W, suggesting that the programmed cell death process of PSG cells might be started before cocooning.

Reduced expression of epidermal growth factor receptors in rat liver during aging

Proliferative responsiveness of hepatocytes to epidermal growth factor (EGF) declines during aging. The role of EGF receptors in mediating age-dependent changes of EGF-induced mitogenic signaling in liver remains incompletely understood. We assessed EGF receptor expression levels in whole liver specimens as well as in freshly isolated and cultured hepatocytes from young adult and senescent Fischer 344 male rats. Hepatic EGF receptor messenger RNA and protein levels, and the number of high- and low-affinity receptor binding sites, decreased with aging. Ligand-induced EGF receptor activation, determined by receptor dimerization and tyrosine phosphorylation, was reduced in old animals in parallel with the age-related decline in receptor expression. Stimulation of the extracellular signal-regulated kinase pathway by EGF was also attenuated in hepatocytes from old animals. Our results implicate decreased expression of EGF receptors as a key determinant of reduced mitogenic signaling responsive to EGF stimulation of liver during aging.

GCF2: expression and molecular analysis of repression

GC-binding factor 2 (GCF2) is a transcriptional repressor that decreases activity of the epidermal growth factor receptor (EGFR) and other genes. We have mapped the gene for GCF2 by fluorescence in situ hybridization (FISH) to chromosome 2q37. Sequence analysis of the GCF2 gene and cDNA showed that the gene consists of eight exons and introns and spans 73 kbp of DNA. Northern blot analysis showed that GCF2 mRNA was differentially expressed in many human tissues and cell lines. GCF2 mRNA was expressed as a 4.2 kb mRNA in most human tissues with the highest expression level in peripheral blood leukocytes and lowest expression in brain and testis. Additional transcripts of 6.6, 2.9 and 2.4 kb were found in some tissues but the only transcript detected in cancer cell lines was 4.2 kb with high levels found in seven Burkitts' lymphoma cell lines. Western blot analysis showed that GCF2 protein is present at high levels in Burkitts' lymphoma and several other cancer cell lines. GCF2 was found in both nuclear and cytoplasmic compartments in cells. Deletion mutants of GCF2 revealed that amino acids 429-528 are required for both DNA binding and repression of the EGFR promoter. Furthermore, GCF2 was able to substantially decrease activator protein 2 (AP2) enhancement of the EGFR promoter. Thus, GCF2 is a transcriptional repressor overexpressed in cancer cell lines with a role in regulating expression of the EGFR.

Oestrogens and selective oestrogen receptor (ER) modulators regulate EGF receptor gene expression through human ER alpha and beta subtypes via an Sp1 site

Through the analysis of the transient expression of the luciferase reporter gene in HeLa cells, an evaluation has been made of the transcriptional activity of oestrogens and of selective oestrogen receptor (ER) modulators (SERMs), mediated by the alpha and beta isoforms of the ER, on the epidermal growth factor receptor gene promoter. Oestrogen-activated ERbeta presents a lower transcriptional activity compared with ERalpha, probably due to structural differences in the AF-1 regions of the receptors. Also SERMs induce different responses depending on the receptor isoform bound. Indeed, the phyto-oestrogens, genistein and daidzein, act as weak agonists of the oestrogenic activity via ERalpha, but as full agonists when bound to ERbeta. The synthetic SERM 4OH-tamoxifen, on the other hand, displays an opposite behaviour since it exerts a full agonist action through ERalpha, but acts as a full antagonist via ERbeta. As we have previously shown for ERalpha, an ERbeta/Sp1 functional synergism has also been highlighted, by means of gel mobility shift assays. Moreover, our results show that the sensitivity of target tissues to oestrogens and SERMs can be affected by coexpression of ERs, depending on the formation of appropriate levels of homo- and heterodimers, thus providing a useful approach to predict the effects of hormonal treatment.

Retinoic acid modulates the cell-cycle in fetal rat hepatocytes and HepG2 cells by regulating cyclin-cdk activities

Retinoic acid (RA), the most biologically active metabolite of vitamin A, is known to modulate cell proliferation, apoptosis and differentiation, with different effects depending on the cellular context. Retinoic acid can exert its effects by directly or indirectly influencing the expression of genes involved in the control of cell proliferation. In the present report we investigate the possible correlation between the antiproliferative, differentiative and apoptotic effects previously observed on rat hepatocytes and HepG2 cells, with a possible modulation of cell-cycle regulators. We demonstrate that RA induces growth arrest and differentiation in HepG2 cells by influencing the activities of cyclin-cdk complexes involved in the regulation of G1/S transition and S-phase progression, in particular by modifying the binding of these complexes to p21 and p27 inhibitors. In fetal cells, however, the induction of apoptosis and differentiation by RA was obtained via inhibition of cyclin D1-cdk4 activity, as result of an increased binding to the p16 inhibitor. Retinoic acid also modulates c-myc and Bcl-2 expression. In conclusion, our data suggest that RA could be useful to regulate the reversion of transformed phenotype and could also be utilized as a chemiopreventive agent in cells of hepatic origin.

Paradoxical triiodothyronine suppression of S14 transcription in permanent hepatic cell lines

Both in vivo and in primary rat hepatocyte culture, carbohydrate and triiodothyronine (T(3)) rapidly induce transcription of the rat S14 gene. To determine if regulation of this gene by T(3) is similar in human liver cells, we transfected the S14 upstream region into HepG2 cells. We chose this cell line because many others have used this cell line to study the effect of thyroid hormone on hepatic gene expression. We found that changing media glucose concentration did not affect S14 transcription. Furthermore, addition of T(3) to HepG2 cells caused a marked reduction of rat S14 transcription. This paradoxical reduction was dependent on cotransfection of the T(3) receptor. We obtained similar results in the other human hepatoma cell lines, HuH-7 and Hep3B. The paradoxical response was not limited to human cells. We found a similar response in the nonmalignant permanent mouse liver cell line, AML-12. This paradoxical response was specific to the S14 gene because transfection of all the cell lines with a CAT or luciferase reporter driven by a mouse mammary tumor virus promoter containing 1 or 4 copies of a palindromic thyroid hormone response element (TRE) showed marked induction by T(3). Our results show that T(3) abnormally regulates the S14 gene in proliferating liver cell lines of diverse origins. This paradoxical regulation by T(3) is caused by an interaction between T(3) and the thyroid hormone receptor. The factors that lead to this paradoxical response are not active in primary hepatocytes and normal intact liver.

Downregulation of the epidermal growth factor receptor by human cytomegalovirus infection in human fetal lung fibroblasts

Epidermal growth factor plays a key role in late fetal lung development and differentiation as well as in regulating surfactant protein A synthesis, which is involved in innate immunity of the lung. Here we show that human cytomegalovirus (HCMV), a known lung pathogen in connatal and postnatal infection of neonates as well as transplant recipients, completely down-regulates EGF receptor (EGF-R) on the surface of human fetal lung fibroblasts. Inhibition of EGF-R synthesis occurs on the transcriptional rather than on the posttranscriptional level. The effect essentially depends on expression of viral immediate early and/or early genes, as binding of ultraviolet light-inactivated virus to the cells had no effect on EGF-R expression. Furthermore, the anti-HCMV drug ganciclovir, which blocks HCMV DNA replication and late gene expression, cannot overcome HCMV-mediated inhibition of EGF-R, suggesting that immediate early or early gene products may be responsible for down-regulation of EGF-R. Interestingly, the glucocorticoid dexamethasone, which is used for its antiinflammatory action to prevent chronic lung disease in preterm infants, promotes HCMV-associated downregulation of the EGF-R by stimulation of viral gene expression. From these data it can be hypothesized that the pathogenesis of HCMV lung infection involves down-regulation of EGF-R and that congenital HCMV infection may cause retardation in lung maturation and surfactant protein synthesis.

A phosphorylation defective retinoic acid receptor mutant mimics the effects of retinoic acid on EGFR mediated AP-1 expression and cancer cell proliferation

BACKGROUND: The effects of the vitamin A metabolite retinoic acid (RA) are mediated at the transcriptional level by retinoic acid receptors (RAR). These proteins are part of a superfamily of transcription factors which activate target gene expression when bound to their respective ligands. In addition to ligand binding, heterodimerization with transcriptional cofactors and posttranslational modification such as phosphorylation are also critical for transactivation function. Previous studies have shown that phosphorylation of a serine residue at amino acid 77 in the RARalpha amino terminus was required for basal activation function of the transcription factor. RESULTS: We have determined that RA inhibits cyclin H and cdk7 expression thereby decreasing levels of phosphorylated RARalpha in human cancer cell lines. To determine the effects of decreased RARalpha phosphorylation in human cancer cells, we stably transfected a phosphorylation defective mutant RARalpha expression construct into SCC25 cultures. Cells expressing the mutant RARalpha proliferated more slowly than control clones. This decreased proliferation was associated with increased cyclin dependent kinase inhibitor expression and decreased S phase entry. In the absence of ligand, the RARalpha mutant inhibited AP-1 activity to an extent similar to that of RA treated control clones. Levels of some AP-1 proteins were inhibited due to decreased EGFR expression upstream in the signaling pathway. CONCLUSIONS: These results indicate that hypophosphorylated RARalpha can mimic the anti-AP-1 effects of RA in the absence of ligand.

Expression of thyroid hormone receptor/erbA genes is altered in human breast cancer

The relation between thyroid status and diseases and cancer is unclear. No detailed analysis of thyroid hormone receptor (TR) expression in human breast cancer has been reported. We have analysed the expression and mutational status of the TRalpha1, encoded by the c-erbA proto-oncogene, TRbeta1 and TRbeta2 isoforms in 70 sporadic breast cancers. Alterations in the RNA level of TRbeta1, TRalpha1, or both were found in a number of patients. No expression of TRbeta2 RNA was detected. Western blotting analysis confirmed the differences in expression at the protein level in those cases where sufficient tumor sample was available. Additionally, tumor-specific truncated TRbeta1 RNA was found in six patients. Strikingly, three transcripts shared the same breakpoint. Only one tumor carried the corresponding deletion at the genomic DNA level, suggesting that the remaining abnormal TRbeta1 transcripts are aberrant splicing products. Though no significant correlation was found between TRbeta1 alteration and any clinical parameter, it showed a tendency to associate with early age of onset (<50 years). Our results reveal specific alterations in the expression of TRbeta and TRalpha genes in a subset of breast cancer patients, suggesting that deregulation of thyroid hormone target genes may be involved in the generation of this neoplasia.

Thyroid hormone affects embryonic mouse lung branching morphogenesis and cellular differentiation

Although thyroid hormone (T(3)) influences epithelial cell differentiation during late fetal lung development, its effects on early lung morphogenesis are unknown. We hypothesized that T(3) would alter embryonic lung airway branching and temporal-spatial differentiation of the lung epithelium and mesenchyme. Gestational day 11.5 embryonic mouse lungs were cultured for 72 h in BGJb serum-free medium without or with added T(3) (0.2, 2.0, 10.0, or 100 nM). Evaluation of terminal bud counts showed a dose- and time-dependent decrease in branching morphogenesis. Cell proliferation was also significantly decreased with higher doses of T(3). Morphometric analysis of lung histology showed that T(3) caused a dose-dependent decrease in mesenchyme and increase in cuboidal epithelia and airway space. Immunocytochemistry showed that with T(3) treatment, Nkx2.1 and surfactant protein SP-C proteins became progressively localized to cuboidal epithelial cells and mesenchymal expression of Hoxb5 was reduced, a pattern resembling late fetal lung development. We conclude that exogenous T(3) treatment during early lung development accelerated epithelial and mesenchymal cell differentiation at the expense of premature reduction in new branch formation and lung growth.

Decreased mitogenic response to epidermal growth factor in human squamous cell carcinoma lines overexpressing epidermal growth factor receptor owing to limiting amounts of the adaptor protein Grb2: rescue by retinoic acid treatment

Growth factor receptors of the tyrosine kinase family regulate proliferation of a variety of cell types. In some human cancers, the epidermal growth factor receptor (EGFR) and its ligands often are overexpressed, leading to both constitutive and autocrine activation. Intracellular signaling via this receptor takes place through several mechanisms of action, including activation of ras and the mitogen-activated protein kinase (MAPK) pathway. Our previous studies have shown that human squamous cell carcinoma (SCC) lines overexpress EGFR and do not increase proliferation in response to exogenous epidermal growth factor (EGF). The vitamin A metabolite retinoic acid (RA) has been used as a chemotherapeutic drug in the treatment of SCC. RA decreases proliferation of SCC lines, in part owing to inhibition of EGFR expression. However, we previously found that treatment of SCC lines with inhibitory doses of RA sensitized cells to the proliferative effects of EGF. We now present a mechanism of action for this effect. RA inhibited expression of EGFR and proteins in the MAPK signaling pathway. Expression of these molecules returned to basal levels within 24 h after RA withdrawal. RA also inhibited autocrine secretion of EGF, which returned to basal levels with slower kinetics. During this time, addition of exogenous EGF stimulated mitosis in SCC lines. These data suggested that signaling proteins downstream of overexpressed EGFR may have limited the mitotic response in SCC lines. In support of this hypothesis, overexpression of the EGFR adaptor protein Grb2 increased cell proliferation and restored EGF-induced mitosis.

p53 Homologue p63 represses epidermal growth factor receptor expression

Tumor suppressor p53 has been shown to transactivate epidermal growth factor receptor (EGFR) expression through binding to a putative p53 responsive element in the EGFR promoter between nucleotides -265 and -239 (EGFRp53RE). Isotypes of p63 gene products, recently identified as p53 relatives, have a similar function to transactivate several p53 target gene promoters. However, our results indicate that TAp63gamma has a very low ability to bind to the EGFRp53RE and surprisingly represses both basal EGFR promoter activity and endogenous EGFR expression. Transient transfection assays show that the EGFR promoter region between -348 and -293, containing two Sp1 sites, is crucial for the repression of the EGFR expression by TAp63gamma. Mutations in these Sp1 sites in the reporter constructs result in loss of the TAp63gamma repression effect. We further show that TAp63gamma directly interacts with Sp1 by immunoprecipitation analysis and that TAp63gamma impairs Sp1 binding to the target DNA site in electrophoretic mobility shift assays. These results suggest that TAp63gamma is involved in the regulation of the EGFR gene expression through interactions with basal transcription factors.

Regulation of mucin gene expression in human tracheobronchial epithelial cells by thyroid hormone

We reported previously that the expression of the gene encoding MUC5AC mucin in human airway epithelial cells is controlled by retinoic acid via the retinoic acid receptor (RAR)-alpha and that 3,3',5-tri-iodothyronine (T(3)) inhibits the expression of MUC5AC. The purpose of the present study was to identify mechanisms mediating the effect of T(3). T(3) has been shown to inhibit gene expression via several mechanisms, either by enhancing or repressing the transcription of target genes or by the regulation of post-transcriptional events. Results showed that T(3) strongly inhibited MUC5AC-driven luciferase activity in normal human tracheobronchial epithelial cells that had been transiently transfected with a MUC5AC-luciferase reporter construct; however, it did not affect MUC5AC mRNA stability. These results indicate that T(3) suppresses MUC5AC expression at the transcriptional level. An analysis of deletion constructs showed that deletion of the region downstream of 3 kb resulted in markedly decreased levels of MUC5AC transcription in the absence of T(3) (i.e. under control conditions) as well as a loss of responsiveness to the inhibitory effects of T(3). This suggests that this region might contain elements important for the activation as well as the repression of MUC5AC transcription. To determine whether T(3) modulates retinoic-acid-dependent MUC5AC transcription via an alteration in the abundance of retinoid receptor proteins, we examined the type and abundance of these receptors in nuclear extracts of airway epithelial cells grown in the presence or absence of T(3). Western blots showed that T(3) markedly decreased several types of retinoid receptor while not affecting T(3) receptor proteins. Consistent with this finding were gel-shift assays revealing a decrease in RAR-retinoic acid response element complexes obtained from T(3)-treated cells. We propose that T(3) might inhibit retinoid-dependent MUC5AC expression by decreasing retinoid receptor levels and thereby decreasing the transcriptional activation of this gene for mucins.

Identification of an estrogen-mediated deoxyribonucleic acid-binding independent transactivation pathway on the epidermal growth factor receptor gene promoter

To investigate the estrogenic effects on the transcriptional regulation of the epidermal growth factor (EGF) receptor (EGFR) gene, we assayed its promoter ability to direct transcription of the luciferase reporter gene after transfection into HeLa cells. Our studies demonstrated a dose-dependent activation of the EGFR gene transcription by ligand-bound estrogen receptor alpha (ERalpha). This action was retained by the 36-bp core promoter fragment and did not require the receptor DNA binding domain, as demonstrated by analyzing the role of ERalpha deletion mutants on EGFR gene promoter-derived constructs. The 36-bp promoter fragment does not contain an estrogen response element but an imperfect thyroid hormone response element half-site that overlaps the Sp1 binding site. ERalpha does not bind this imperfect thyroid hormone response element half-site but is able to enhance binding of Sp1 to its site, in gel mobility shift assays, suggesting that the mechanism by which the receptor stimulated the transcription involved protein-protein interactions that replaced DNA binding. To explain this action, we propose a model in which induction of the EGFR gene expression by estrogens in HeLa cells is dependent upon the formation of a transcriptionally active ERalpha-Sp1 complex that binds to the GC-rich (Sp1) region of the minimal promoter.

Cloning and characterization of the promoter region of the rat epidermal growth factor receptor gene and its transcriptional regulation by nerve growth factor in PC12 cells

Our previous studies have shown that treatment of PC12 cells with nerve growth factor (NGF) causes a profound down-regulation of the epidermal growth factor receptor (EGFR) mRNA and protein. Further, the NGF-induced down-regulation of the EGFR is under transcriptional control. To elucidate the molecular mechanism of this down-regulation we have cloned a 2.7-kilobase sequence from the promoter region of the rat EGFR from a rat P1 library. Six transcriptional start sites were identified by 5'-rapid amplification of cDNA ends and primer extension. Sequence analysis showed a 62% overall homology with the human EGFR promoter region. To investigate its transcription, 1.1 kilobases of the 5'-flanking sequence were fused to a luciferase reporter gene. This sequence exhibited functional promoter activity in transient transfection experiments with PC12, C6, and CV-1 cells. Treatment of PC12 cells with NGF inhibited promoter activity. By transfection of promoter deletion constructs, a silencer element was found between nucleotides -260 and -181, and TCC repeat sequences appeared to be at least partially responsible for the down-regulation of the EGFR by NGF. Supportive evidence for the relevance of this sequence was obtained from gel mobility shift assays and by transfection of TCC mutation constructs. Our results demonstrate that TCC repeat sequences are required for the down-regulation of rat EGFR by NGF in PC12 cells and may lead to the identification of the NGF-responsive transcription factors.

The mechanisms of retinoic acid-induced regulation on the follicle-stimulating hormone receptor in rat granulosa cells

The present study was undertaken to identify the mechanisms underlying the effect of retinoic acid (RA) on follicle-stimulating hormone receptor (FSH-R) in rat granulosa cells. Treatment with FSH produced a substantial increase in FSH-R mRNA level, as was expected, while concurrent treatment with increasing concentrations of RA brought about dose-dependent decreases in FSH-induced FSH-R mRNA, with a maximal inhibition one-third lower than that induced by FSH alone. RA, either alone or in combination with FSH, did not affect intracellular cAMP levels, while it inhibited the effect of 8-Br-cAMP on FSH-R mRNA production. These results suggested that RA diminished the action of FSH on FSH-R expression at sites distal to cAMP generation in the granulosa cells. Whether the effect of RA and FSH on FSH-R mRNA levels was the result of decreased transcription and/or altered mRNA stability was also investigated. The rate of FSH receptor mRNA gene transcription, assessed by nuclear run-on transcription assay, was found to decrease by the addition of RA. On the other hand, the decay curves for the 2.4 kb FSH-R mRNA transcript in primary granulosa cells did not alter the slope of the FSH-R mRNA decay curve in the presence of RA. Our data suggests for the first time that the effect of RA on FSH-R expression is possibly mediated by the reduction of the FSH-R mRNA level due to a negative regulation of the FSH-R gene in the presence of FSH. These findings assist in understanding the molecular mechanism underlying the effect of RA on reproductive function in rat granulosa cells.

Retinoic acid (RA) represses follicle stimulating hormone (FSH)-induced luteinizing hormone (LH) receptor in rat granulosa cells

The present study was undertaken to identify the mechanisms underlying the effect of retinoic acid (RA) on the luteinizing hormone receptor (LH-R) in rat granulosa cells. Treatment with FSH produced a substantial increase in LH-R mRNA level, as was expected, while concurrent treatment with increasing concentrations of RA brought about dose-dependent decreases in FSH-induced LH-R mRNA. RA, either alone or in combination with FSH, did not affect intracellular cAMP levels, while it inhibited the effect of 8-Br-cAMP on LH-R mRNA production. Whether the effect of RA and FSH on LH-R mRNA levels was the result of decreased transcription and/or altered mRNA stability was also investigated. The rate of LH receptor mRNA gene transcription, assessed by nuclear run-on transcription assay, was inhibited by the addition of RA. The effect of RA on LH-R mRNA stability was determined by measuring the decay of LH receptor mRNA under conditions known to inhibit transcription. The decay curves for the 5.4-kb LH-R mRNA transcript showed a significant decrease after the addition of RA. It may be possible that RA not only inhibits FSH-induced transcription but also stimulates the production of destabilizing factors for the LH-R mRNA. These findings assist in understanding the molecular mechanism underlying the effect of RA on reproductive function in rat granulosa cells.

Nuclear receptor coregulators: cellular and molecular biology

Nuclear receptor coregulators are coactivators or corepressors that are required by nuclear receptors for efficient transcripitonal regulation. In this context, we define coactivators, broadly, as molecules that interact with nuclear receptors and enhance their transactivation. Analogously, we refer to nuclear receptor corepressors as factors that interact with nuclear receptors and lower the transcription rate at their target genes. Most coregulators are, by definition, rate limiting for nuclear receptor activation and repression, but do not significantly alter basal transcription. Recent data have indicated multiple modes of action of coregulators, including direct interactions with basal transcription factors and covalent modification of histones and other proteins. Reflecting this functional diversity, many coregulators exist in distinct steady state precomplexes, which are thought to associate in promoter-specific configurations. In addition, these factors may function as molecular gates to enable integration of diverse signal transduction pathways at nuclear receptor-regulated promoters. This review will summarize selected aspects of our current knowledge of the cellular and molecular biology of nuclear receptor coregulators.

Retinoids control the expression of c-erbB receptors in breast cancer cells

Nuclear retinoid and membrane c-erbB receptors participate in signal transduction systems that control mammary epithelial cell proliferation and differentiation. Recently, we demonstrated that c-erbB receptor activation stimulates retinoic acid receptor-alpha expression. We now report that retinoids reduce SK-BR-3 breast cancer cell growth by inhibiting the cell cycle and by inducing apoptosis. This is accompanied with reduced c-erbB expression as determined by FACS, Western, Northern, RT-PCR, and reporter assays. All-trans (ATRA) and 9-cis retinoic acid (9cRA) reduce c-erbB-1 protein to 50-100%, c-erbB-2 to 20-30%, and c-erbB-3 to 10-50% of control, depending on the concentration, respectively, without influencing the tyrosine phosphorylation status. Down-regulation of c-erbB-2 and -3 was seen at all levels analyzed, whereas c-erbB-1 mRNA remained unchanged. Retinoic acid-mediated down-regulation of growth and c-erbB-2 and -3 expression was also seen in MCF-7 cells. We conclude that retinoic acids are efficient repressors of c-erbB-2 and -3 gene expression, whereas c-erbB-1 is not markedly affected.

Transcriptional regulation of the EGF receptor promoter by HPV16 and retinoic acid in human ectocervical epithelial cells

We have previously demonstrated that human papillomavirus 16 (HPV16)-immortalized human ectocervical epithelial cells and cells derived from tumors which express HPV16 oncogenes express high levels of epidermal growth factor receptor (EGFR) compared to normal cervical cells. We have also shown that proliferation of these cells is inhibited by retinoid treatment. We have hypothesized that the retinoid inhibition of cell proliferation may be due to the retinoid-dependent reduction in EGFR level. In this study we examine the regulation of EGFR expression in cervical cells with emphasis on two aspects: (1) the mechanism of retinoid-dependent suppression of EGFR levels in HPV16-positive cells and (2) the mechanism of EGFR upregulation by HPV16. EGFR levels were found to be elevated 5-, 3. 7-, and 1.25-fold in the HPV16-immortalized ECE16-1, ECE16-D1, and ECE16-D2 cells, respectively, compared to normal cervical cells. Treatment of ECE16-1 and ECE16-D1 cells with retinoic acid suppresses proliferation, EGFR level, EGFR mRNA level, and EGFR promoter activity. The reduction in EGFR promoter activity appears to account for the reduction in EGFR protein and mRNA levels. In contrast, retinoic acid does not affect cell growth or EGFR level in ECE16-D2 cells or normal cervical cells. To study the mechanisms regulating EGFR expression in HPV16-positive cells, normal ECE cells were cotransfected with an EGFR promoter reporter plasmid and an expression plasmid encoding the HPV16 E6/E7 open reading frames. In the presence of E6/E7, EGFR promoter activity was increased by 2- to 3-fold, suggesting that the E6/E7 proteins are directly or indirectly responsible for the increased EGFR level and that the EGFR promoter contains the DNA elements necessary to mediate this response. Nevertheless expression of E6/E7 proteins did not confer retinoic acid regulation, as EGFR promoter activity remained elevated in normal cells cotransfected with pHPVE6/E7 and treated with retinoic acid. These results suggest that human papillomavirus and retinoic acid regulate EGFR levels by independent effects on the EGFR promoter.

Molecular cloning and characterization of a transcription regulator with homology to GC-binding factor

GC-binding factor (GCF) represses transcription of certain genes and is encoded by a 3.0-kilobase mRNA (Kageyama, R., and Pastan, I. (1989) Cell 59, 815-825). The GCF cDNA hybridizes to two additional mRNA species, 4.2 and 1.2 kilobases. We have used differential hybridization to identify a cDNA clone (termed GCF2) for the 4. 2-kilobase mRNA and find that it is highly expressed in HUT-102 cells. The open reading frame consists of 2256 nucleotides and encodes a protein of 752 amino acids with a calculated molecular mass of 83 kilodaltons. GCF2 expressed in vitro using reticulocyte lysates and Escherichia coli migrates as a 160-kilodalton protein in SDS-polyacrylamide gel electrophoresis but has a molecular mass of 83 kilodaltons as determined by mass spectrum analysis. GCF2 binds to epidermal growth factor receptor promoter fragments, and the major binding site is located between nucleotides -249 and -233. Cotransfection assays show that GCF2 acts to repress transcription from the epidermal growth factor receptor promoter in constructs containing the major GCF2 binding site and not when the site had been mutated. Thus, GCF2 is a newly identified transcriptional repressor with aberrant electrophoretic mobility.

Interactions with single-stranded and double-stranded DNA-binding factors and alternative promoter conformation upon transcriptional activation of the Htf9-a/RanBP1 and Htf9-c genes

The murine Htf9-a/RanBP1 and Htf9-c genes are divergently transcribed from a shared TATA-less promoter. Transcription of both genes is initiated on complementary DNA strands and is controlled by cell cycle-dependent mechanisms. The bidirectional promoter harbors a genomic footprint flanking the major transcription start site of both genes. Transient promoter assays showed that the footprinted element is important for transcription of both genes. Protein-binding experiments and antibody assays indicated that members of the retinoid X receptor family interact with the double-stranded site. In addition, distinct factors interact with single DNA strands of the element. Double-stranded binding factors were highly expressed in liver cells, in which neither gene is transcribed, while single-stranded binding proteins were abundant in cycling cells, in which transcription of both genes is efficient. In vivo S1 analysis of the promoter depicted an S1-sensitive organization in cells in which transcription of both genes is active; S1 sensitivity was not detected in conditions of transcriptional repression. Thus, the same element is a target for either retinoid X receptor factors, or for single-stranded binding proteins, and form distinct complexes in different cellular conditions depending on the DNA conformation in the binding site.

The zebrafish thyroid hormone receptor alpha 1 is expressed during early embryogenesis and can function in transcriptional repression

Nuclear receptors are a large family of ligand dependent transcription factors which participate in many diverse processes during development. In this report, we describe the cloning of the zebrafish thyroid hormone receptor alpha 1 (TR alpha 1) gene, the cellular counterpart of the viral oncogene v-erbA. TR alpha 1 is expressed during oogenesis and maternally supplied to the embryo. TR alpha 1 is expressed again after the mid blastula transition. By examining the effects of increased expression of TR alpha 1 on expression of a reporter gene which responds to both TR alpha 1 and retinoic acid receptors (RARs), we show that the zebrafish TR alpha 1 can act as a repressor during early zebrafish development before thyroid hormone is present in the embryo. In addition, our data suggest that TR alpha 1 can repress retinoic acid (RA)-signaling during early development. We propose that TR alpha 1 functions during early development as a transcriptional repressor, similar to the constitutive repressor activity of its viral counterpart v-erbA, which regulates anterior-posterior (A/P) patterning by repressing RA-signaling.

Tyrosine kinase signaling pathways control the expression of retinoic acid receptor-alpha in SK-BR-3 breast cancer cells

Breast carcinomas are frequently characterized by hyperactivated c-erbB receptor tyrosine kinase signaling. Combination of anti-proliferative retinoids with growth-inhibitory c-erbB-specific agents might induce therapeutic benefit. We demonstrate close interactions between the c-erbB and the retinoic acid receptor system in SK-BR-3 breast cancer cells. Epidermal growth factor and heregulin-beta1 activate c-erbB receptors and dose- and time-dependently up-regulate retinoic acid receptor-alpha (RAR-alpha) mRNA. Similar effects have been found for the growth-inhibitory c-erbB-2 receptor tyrosine kinase-activating antibody 4D5 and the tyrosine phosphatase inhibitor orthovanadate. In contrast, the tyrosine kinase-inhibitor herbimycin A reduces tyrosine-specific protein phosphorylation and down-regulates RAR-alpha. Our data demonstrate that the expression of RAR-alpha, which represents a key mediator of the anti-proliferative effects of retinoids in breast cancer cells, is regulated by modulators of tyrosine kinase signaling. The levels of RAR-beta and -gamma mRNAs, however, are not affected by such agents.

Transcriptional regulation of type I receptor tyrosine kinases in the mammary gland

The transcriptional regulation of the human EGFR3 and ERBB2 genes has been extensively studied, particularly in the context of their overexpression in breast cancer. Here we summarize published work detailing the transcription factors which interact with the promoters of these and the rat ERBB2 homologue, neu, genes and discuss their possible relevance to gene activation in cancer. In addition we review the biologically significant molecules which modulate expression of these genes and discuss the nuclear factors involved in mediating these responses. We also describe novel therapies which may result from these studies and highlight directions for future research into the control of expression of the EGFR and ERBB2 genes in the normal mammary gland and in breast cancer.

Negative regulation of the gene for the preprothyrotropin-releasing hormone from the mouse by thyroid hormone requires additional factors in conjunction with thyroid hormone receptors

To gain additional insights into the negative gene regulatory action by triiodothyronine (T3), we isolated a 2-kilobase pair 5'-flanking region of the mouse preprothyrotropin-releasing hormone (ppTRH) gene and characterized the DNA elements mediating inhibitory regulation by T3 in the promoter region. In GH4C1 cells, the expression of the 2-kilobase pair mouse ppTRH 5'-flanking region fused to the luciferase reporter gene occurred by transfection and was significantly suppressed by T3. In contrast, T3 suppression was not observed in T3 receptor (T3R)-deficient CV-1 cells, suggesting that T3Rs were required for the negative regulation. Cotransfected mouse T3R alpha1, beta1, and beta2 possessed indistinguishable potency for the negative regulation. Deletion analysis localized the element mediating the negative regulation to the region between -83 and +46, and the sequence downstream of the transcription start site (TSS) between +12 and +46 was found to be essential for the inhibitory regulation. In mobility shift assays, only T3R monomers bound to the element containing a T3 response element half-site at -57. No apparent T3R binding was observed to the element downstream of TSS. Neither the T3 response element half-site nor the element downstream of the TSS confer T3 suppression individually in heterologous promoters. These results indicate that the negative regulation of murine ppTRH gene by T3 might be mediated by the cooperation of T3R monomers with unknown factor(s) interacting with the element downstream of the TSS.

Retinoic acid-induced transcriptional modulation of the human interferon-gamma promoter

Disregulation of vitamin A metabolism is able to generate different immunological effects, including altered response to infection, reduced IgG production, and differential regulation of cytokine gene expression (including interleukin-2 and -4 and interferon-gamma (IFN-gamma)). In particular, IFN-gamma gene expression is significantly affected by vitamin A and/or its derivatives (e.g. retinoic acid (RA)). Here, we analyze the effect of retinoic acid on IFN-gamma transcription. Transient transfection assays in the human T lymphoblastoid cell line Jurkat demonstrated that the activation of the IFN-gamma promoter was significantly down-regulated in the presence of RA. Surprisingly, two different AP-1/CREB-ATF-binding elements situated in the initial 108 base pairs of the IFN-gamma promoter and previously shown to be critical for transcriptional activity were unaffected by RA. Utilizing promoter deletions and electrophoretic mobility shift analysis, we identified a USF/EGR-1-binding element cooperating in the modulation of IFN-gamma promoter activity by RA. This element was found to be situated in a position of the IFN-gamma promoter close to a silencer element previously identified in our laboratory. These results suggest that direct modulation of IFN-gamma promoter activity is one of the possible mechanisms involved in the inhibitory effect of retinoids on IFN-gamma gene expression.

Reconstitution of ionic channels from inner and outer membranes of mammalian cardiac nuclei

Recent reports suggest that the nuclear envelope possesses specific ion transport mechanisms that regulate the electrolyte concentrations within the nucleoplasm and perinuclear space. In this work, intact nuclei were isolated from sheep cardiac cells. After chromatin digestion, the nuclear envelopes were sonicated and four nuclear vesicle populations were separated by sucrose step gradients (SF1-SF4). These fractions were compared by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and their protein content was analyzed by Western blot, using lamin and SEC 61 antibodies. The lamins, which are associated with the inner nuclear membrane, were present in three fractions, SF2, SF3, and SF4, with a lower amount in SF2. The SEC 61 protein, a marker of the rough endoplasmic reticulum, was detected in small amounts in SF1 and SF2. Upon fusion of vesicles into bilayers, the activities of nuclear ionic channels were recorded in 50 mM trans/250 mM cis KCl or CsCl, pH 7.2. Two types of Cl- selective channels were recorded: a large conducting 150-180-pS channel displaying substates, and a low conducting channel of 30 pS. They were both spontaneously active into bilayers, and their open probability was poorly voltage dependent at negative voltages. Retinoic acid (10(-8) M) increases the po of the large Cl- conducting channel, whereas ATP modifies the kinetics of the low conductance anion selective channel. Our data also suggest that this anionic channel is mainly present in the SF3 and SF4 population. The presence of a 181 +/- 10 pS cation-selective channel was consistently observed in the SF2 population. The behavior of this channel was voltage dependent in the voltage range -80 to +60 mV. Furthermore, we report for the first time the activity of a channel exclusively present in the SF3 and SF4 fractions, shown to contain mainly inner membrane vesicles. This cation selective channel displays a 75-pS conductance in 50 mM trans/250 mM cis K-gluconate. It is concluded that the bilayer reconstitution technique is an attractive approach to studying the electrophysiological properties of the inner and outer membranes of the nuclear envelope.

Negative regulation of expression of the pituitary-specific transcription factor GHF-1/Pit-1 by thyroid hormones through interference with promoter enhancer elements

Expression of the growth hormone gene is due to the presence of the pituitary-specific transcription factor GHF-1/Pit-1. The action of the thyroid hormone T3 is mediated by nuclear receptors that regulate transcription by interaction with DNA elements located near promoters of the regulated genes. In this study, we show that T3 inhibits expression of the GHF-1/Pit-1 gene in rat pituitary GH4C1 cells by a novel mechanism that involves transcriptional interference with other regulatory elements of the promoter. Sequences between bp -90 and -200 of the rat GHF-1/Pit-1 gene which do not contain a hormone response element but contain two cyclic AMP-responsive elements mediate most of the repressive effect of T3. The hormone reduces basal levels of GHF-1/Pit-1 promoter activity and antagonizes its response to cyclic AMP and the tumor promoter TPA (12-O-tetradecanoylphorbol-13-acetate). A similar repression is found with a heterologous promoter that contains four copies of the cyclic AMP-responsive element motif. This regulation provides a novel example of the cross-talk between the thyroid hormone receptor and the signal transduction pathways used by different hormones and growth factors. Additionally, T3 interferes with in vitro binding of GHF-1/Pit-1 to a positive autoregulatory element located at bp -45 to -63 and has a detectable inhibitory effect on the activity of a promoter construct which extends to bp -90 of 5'-flanking DNA. The regulation of the transcription factor provides a novel example of negative transcriptional regulation by thyroid hormones.

Transcriptional repression of the interleukin-2 gene by vitamin D3: direct inhibition of NFATp/AP-1 complex formation by a nuclear hormone receptor

T-lymphocyte proliferation is suppressed by 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], the active metabolite of vitamin D3, and is associated with a decrease in interleukin 2 (IL-2), gamma interferon, and granulocyte-macrophage colony-stimulating factor mRNA levels. We report here that 1,25(OH)2D3-mediated repression in Jurkat cells is cycloheximide resistant, suggesting that it is a direct transcriptional repressive effect on IL-2 expression by the vitamin D3 receptor (VDR). We therefore examined vitamin D3-mediated repression of activated IL-2 expression by cotransfecting Jurkat cells with IL-2 promoter/reporter constructs and a VDR overexpression vector and by DNA binding. We delineated an element conferring both DNA binding by the receptor in vitro and 1,25(OH)2D3-mediated repression in vivo to a short 40-bp region encompassing an important positive regulatory element, NF-AT-1, which is bound by a T-cell-specific transcription factor, NFATp, as well as by AP-1. VDR DNA-binding mutants were unable to either bind to this element in vitro or repress in vivo; the VDR DNA-binding domain alone, however, bound the element but also could not repress IL-2 expression. These results indicate that DNA binding by VDR is necessary but not sufficient to mediate IL-2 repression. By combining partially purified proteins in vitro, we observed the loss of the bound NFATp/AP-1-DNA complex upon inclusion of VDR or VDR-retinoid X receptor. Order of addition and off-rate experiments indicate that the VDR-retinoid X receptor heterodimer blocks NFATp/AP-1 complex formation and then stably associates with the NF-AT-1 element. This direct inhibition by a nuclear hormone receptor of transcriptional activators of the IL-2 gene may provide a mechanistic explanation of how vitamin derivatives can act as potent immunosuppressive agents.

Regulation of the induction of ornithine decarboxylase in keratinocytes by retinoids

Treatment of SV40-transformed keratinocytes (Z114) with epidermal growth factor (EGF) resulted in an increase in ornithine decarboxylase (ODC) activity and a dose-dependent increase in ODC mRNA levels. Pretreatment of keratinocytes with all-trans-retinoic retinoic acid inhibited the EGF induction of ODC activity. In both quiescent and EGF-stimulated cells, all-trans-retinoic acid inhibited ODC gene transcription and lowered ODC mRNA levels, whereas glyceraldehyde phosphate dehydrogenase expression remained unaffected. Treatment with all-trans-retinoic acid for 24 h resulted in a dose- and time-dependent decrease of up to 52% in EGF binding to EGF receptors and a 30-75% decrease in EGF-receptor quantity. In addition, when cells were treated with both UV radiation and all-trans-retinoic acid, their effects were additive in causing a decrease in EGF binding. Blocking of EGF receptors with a neutralizing antibody for EGF receptors inhibited the induction of ODC activity by EGF. The effects of several other retinoids, including Ro15-0778, etretinate, Ro13-7410, etarotene, Ro40-8757, 13-cis-retinoic acid and acitretin, were also studied to determine their effects on EGF binding and ODC activity. Two of these other retinoids, 13-cis-retinoic acid and Ro13-7410, inhibited EGF binding the most (35-46%, P < 0.001); several others (etarotene, Ro40-8757 and etretinate) were less effective (7-16%), but significantly decreased EGF binding (P < 0.05), and two retinoids (Ro15-0778 and acitretin) showed no significant effect on EGF binding. In contrast, all of the retinoids tested inhibited the induction of ODC activity by EGF, although etretinate and Ro15-0778 were less effective. EGF signal transduction is important in ODC gene regulation, and retinoids are significant modulators of this pathway.

Negative regulation of a special, double AP-1 consensus element in the vimentin promoter: interference by the retinoic acid receptor

The growth-regulated vimentin gene contains a functional double AP-1 binding site formed by two nearly perfect inverted repeats. We present evidence for down-regulation of vimentin expression by the retinoic acid receptor (RAR) in two mesodermally derived cell types. By mutation analysis we prove that the double consensus element is responsible for this negative regulation. From in vitro protein-DNA interaction studies we conclude that AP-1 binding is inhibited at RAR amounts required for occupation of the cognate RAR binding site in nuclear extracts from 3T3 cells and differentiated embryonal carcinoma cells. Furthermore, we show that, unlike in other cases, trans-activation of the vimentin AP-1 enhancer element can occur in undifferentiated embryonal carcinoma cells, despite the low amount of Jun and Fos proteins present in these cells. Here, however, down-regulation by retinoic acid cannot be detected.

Interactions between the transforming growth factor beta (TGF beta) and retinoic acid signal transduction pathways in murine embryonic palatal cells

Regulation of expression of transforming growth factor-beta 3 (TGF-beta 3) and the cellular retinoic acid-binding proteins-I and II (CRABP-I, -II) by retinoic acid (RA) and TGF-beta was examined in primary cultures of murine embryonic palate mesenchymal (MEPM) cells. Northern blot hybridization revealed that RA and TGF-beta 1, beta 2 and beta 3 stimulated the expression of TGF-beta 3 mRNA within 24 hours of treatment. RA down-regulated the expression of CRABP-I mRNA and up-regulated the expression of CRABP-II mRNA in a time- and dose-dependent fashion. TGF-beta 1, beta 2 and beta 3 also down-regulated the expression of CRABP-I mRNA, while epidermal growth factor (EGF) and transforming growth factor alpha (TGF-alpha) were without effect. TGF-beta 1 also stimulated a dose-dependent increase in the expression of CRABP-II mRNA. Again EGF and TGF-alpha were without effect. Basic fibroblast growth factor (bFGF) elicited a slight inhibitory effect on CRABP-II and a slight stimulatory effect on CRABP-I mRNA expression. Thus, cells derived from the mammalian developing palate express CRABP-I and CRABP-II mRNAs, both of which may be regulated by RA and TGF-beta. These data constitute the first demonstration of an effect of TGF-beta on the expression of CRABP-I and CRABP-II and provide further evidence for cross-talk between RA and TGF-beta signal transduction pathways.

Nuclear retinoic acid receptor characterization in cultured human trophoblast cells: effect of retinoic acid on epidermal growth factor receptor expression

Vitamin A is an important factor during gestation and its metabolite, retinoic acid (RA), is a potent teratogen. However, RA action on the placenta is still poorly understood. In this study we analysed the presence of RARs and RXRs in human trophoblastic cells. We determined that RAR alpha was the more expressed form in term placenta, and that RAR beta was induced by RA treatment. Then we analysed RA effects on endocrine activities and on epidermal growth factor (EGF) receptor expression. We found that RA decreased 125I-labeled EGF binding and EGF-dependent phosphorylation. Furthermore, RA treatment led to a concentration-dependent decrease in the amount of EGFR protein expression. This treatment also decreased EGF receptor mRNA levels, suggesting transcriptional regulation of the EGF receptor. Thus we demonstrated that RA could interact with feto-placental development by modulating trophoblast EGF receptors expression, probably via its nuclear receptors.

Cholinergic differentiation of cultured sympathetic neurons induced by retinoic acid. Induction of choline acetyltransferase-mRNA and suppression of tyrosine hydroxylase-mRNA levels

Here we show that retinoic acid (RA) has the ability to alter the transmitter phenotype of cultured sympathetic neurons from newborn rats superior cervical ganglia (SCG). In the presence of RA, the level of choline acetyltransferase (ChAT) mRNA was increased, while the level of tyrosine hydroxylase (TH) mRNA was reduced in the cultured SCG neurons. Selective PCR amplification of different upstream regions of the ChAT-mRNA indicates that RA promotes the transcription of ChAT gene from R and M exons. The RA-induced upregulation of ChAT-mRNA level was significantly diminished by the chronic treatment with phorbol ester, suggesting that PKC has an important role in the induction of ChAT-mRNA in this system.

Thyroid hormone receptor can modulate retinoic acid-mediated axis formation in frog embryogenesis

Thyroid hormone receptor acts as a hormone-dependent transcriptional transactivator and as a transcriptional repressor in the absence of thyroid hormone. Specifically, thyroid hormone receptor can repress retinoic acid-induced gene expression through interactions with retinoic acid receptor. (Retinoic acid is a potent teratogen in the frog Xenopus laevis, acting at early embryonic stages to interfere with the formation of anterior structures. Endogenous retinoic acid is thought to act in normal anterior-posterior axis formation.) We have previously shown that thyroid hormone receptor RNA (alpha isotype) is expressed and polysome-associated during Xenopus embryogenesis preceding thyroid gland maturation and endogenous thyroid hormone production (D. E. Banker, J. Bigler, and R. N. Eisenman, Mol. Cell. Biol. 11:5079-5089, 1991). To determine whether thyroid hormone receptor might influence the effects of retinoic acid in early frog development, we have examined the results of ectopic thyroid hormone receptor expression on retinoic acid teratogenesis. We demonstrate that microinjections of full-length thyroid hormone receptor RNA protect injected embryos from retinoic acid teratogenesis. DNA binding is apparently essential to this protective function, as truncated thyroid hormone receptors, lacking DNA-binding domains but including hormone-binding and dimerization domains, do not protect from retinoic acid. We have shown that microinjections of these dominant-interfering thyroid hormone receptors, as well as anti-thyroid hormone receptor antibodies, increase retinoic acid teratogenesis in injected embryos, presumably by inactivating endogenous thyroid hormone receptor. This finding suggests that endogenous thyroid hormone receptors may act to limit retinoic acid sensitivity. On the other hand, after thyroid hormone treatment, ectopic thyroid hormone receptor mediates teratogenesis that is indistinguishable from the dorsoanterior deficiencies produced in retinoic acid teratogenesis. The previously characterized retinoic acid-responsive gene, Xhox.lab2, can be induced by thyroid hormone in embryos ectopically expressing thyroid hormone receptor and is less responsive to retinoic acid in such embryos. The fact that both thyroid hormone and retinoic acid can affect overlapping gene expression pathways to produce abnormal embryonic axes and can regulate the same early-expressed gene suggests a model in which thyroid hormone receptor blocks retinoic acid receptor-mediated teratogenesis by directly repressing retinoic acid-responsive genes.

EGF receptor expression, regulation, and function in breast cancer

Epidermal growth factor receptor (EGFR) overexpression correlates with both loss of estrogen receptor (ER) and poor prognosis in breast cancer. Interestingly, in normal breast EGFR appears to be expressed more frequently than in malignant tissue, and there may be a different relationship between ER and EGFR. A variety of cellular regulators, such as EGF, TGF alpha, phorbol esters, and steroid hormones, are capable of altering the level of EGFR expression in breast cells. However, much work remains to be done on the mechanistic details of EGFR regulation in this disease. The significance of EGFR as an oncogene in breast cancer is compounded by its potential interactions with other oncogenes such as c-erbB-2 and c-myc. Additionally, several recent studies have placed EGFR prominently in the signal transduction pathway, demonstrating that the EGFR-ligand system may play important roles throughout the course of malignant progression in breast cancer.

Mechanisms of EGF receptor regulation in breast cancer cells

Overexpression of the EGF receptor in breast cancer correlates with poor prognosis and failure on endocrine therapy for both ER-/EGFR+ and ER+/EGFR+ tumors, suggesting a role for EGFR in the progression to hormone independence. The identification of specific DNAse I hypersensitive site patterns for the EGFR gene in ER+ vs. ER- cells implicates regions of the EGFR first intron in up-regulation of EGFR, while estrogen regulation studies indicate the involvement of a repressor(s) in the maintenance of low levels of EGFR. Based on these findings, a multi-step model is proposed for the progression of breast cancer from a hormone-dependent, ER+/EGFR-phenotype to an aggressive, hormone-independent, ER-/EGFR+ stage.

Thyroid hormone receptor can modulate retinoic acid-mediated axis formation in frog embryogenesis

Thyroid hormone receptor acts as a hormone-dependent transcriptional transactivator and as a transcriptional repressor in the absence of thyroid hormone. Specifically, thyroid hormone receptor can repress retinoic acid-induced gene expression through interactions with retinoic acid receptor. (Retinoic acid is a potent teratogen in the frog Xenopus laevis, acting at early embryonic stages to interfere with the formation of anterior structures. Endogenous retinoic acid is thought to act in normal anterior-posterior axis formation.) We have previously shown that thyroid hormone receptor RNA (alpha isotype) is expressed and polysome-associated during Xenopus embryogenesis preceding thyroid gland maturation and endogenous thyroid hormone production (D. E. Banker, J. Bigler, and R. N. Eisenman, Mol. Cell. Biol. 11:5079-5089, 1991). To determine whether thyroid hormone receptor might influence the effects of retinoic acid in early frog development, we have examined the results of ectopic thyroid hormone receptor expression on retinoic acid teratogenesis. We demonstrate that microinjections of full-length thyroid hormone receptor RNA protect injected embryos from retinoic acid teratogenesis. DNA binding is apparently essential to this protective function, as truncated thyroid hormone receptors, lacking DNA-binding domains but including hormone-binding and dimerization domains, do not protect from retinoic acid. We have shown that microinjections of these dominant-interfering thyroid hormone receptors, as well as anti-thyroid hormone receptor antibodies, increase retinoic acid teratogenesis in injected embryos, presumably by inactivating endogenous thyroid hormone receptor. This finding suggests that endogenous thyroid hormone receptors may act to limit retinoic acid sensitivity. On the other hand, after thyroid hormone treatment, ectopic thyroid hormone receptor mediates teratogenesis that is indistinguishable from the dorsoanterior deficiencies produced in retinoic acid teratogenesis. The previously characterized retinoic acid-responsive gene, Xhox.lab2, can be induced by thyroid hormone in embryos ectopically expressing thyroid hormone receptor and is less responsive to retinoic acid in such embryos. The fact that both thyroid hormone and retinoic acid can affect overlapping gene expression pathways to produce abnormal embryonic axes and can regulate the same early-expressed gene suggests a model in which thyroid hormone receptor blocks retinoic acid receptor-mediated teratogenesis by directly repressing retinoic acid-responsive genes.