AP-1 transrepressing retinoic acid does not deplete coactivators or AP-1 monomers but may target specific Jun or Fos containing dimers

Mineralocorticoid Receptor (MR) trans-Activation of Inflammatory AP-1 Signaling: DEPENDENCE ON DNA SEQUENCE, MR CONFORMATION, AND AP-1 FAMILY MEMBER EXPRESSION

Glucocorticoids are commonly used to treat inflammatory disorders. The glucocorticoid receptor (GR) can tether to inflammatory transcription factor complexes, such as NFκB and AP-1, and trans-repress the transcription of cytokines, chemokines, and adhesion molecules. In contrast, aldosterone and the mineralocorticoid receptor (MR) primarily promote cardiovascular inflammation by incompletely understood mechanisms. Although MR has been shown to weakly repress NFκB, its role in modulating AP-1 has not been established. Here, the effects of GR and MR on NFκB and AP-1 signaling were directly compared using a variety of ligands, two different AP-1 consensus sequences, GR and MR DNA-binding domain mutants, and siRNA knockdown or overexpression of core AP-1 family members. Both GR and MR repressed an NFκB reporter without influencing p65 or p50 binding to DNA. Likewise, neither GR nor MR affected AP-1 binding, but repression or activation of AP-1 reporters occurred in a ligand-, AP-1 consensus sequence-, and AP-1 family member-specific manner. Notably, aldosterone interactions with both GR and MR demonstrated a potential to activate AP-1. DNA-binding domain mutations that eliminated the ability of GR and MR to cis-activate a hormone response element-driven reporter variably affected the strength and polarity of these responses. Importantly, MR modulation of NFκB and AP-1 signaling was consistent with a trans-mechanism, and AP-1 effects were confirmed for specific gene targets in primary human cells. Steroid nuclear receptor trans-effects on inflammatory signaling are context-dependent and influenced by nuclear receptor conformation, DNA sequence, and the expression of heterologous binding partners. Aldosterone activation of AP-1 may contribute to its proinflammatory effects in the vasculature.

Complexity of the RAR-mediated transcriptional regulatory programs

In the past several decades, intensive research in this field has uncovered a surprising number of regulatory factors and their associated enzymatic properties to reveal the network of complexes that function in activation and repression of the transcriptional programs mediated by nuclear receptors (NR). These factors and their associated complexes have been extensively characterized both biochemically and functionally [34, 87, 94]. Several principles have emerged: (1) It is widely recognized that ligand-dependent cofactor complexes mediating repression and activation exhibit ligand-dependent exchange. (2) These complexes mediate modifications of chromatin structure consequent to their binding at regulatory elements, particularly at promoter and enhancer Enhancer sites. (3) The concept about the rapid exchange of coregulatory complexes at regulatory sites has been suggested [88]. Key questions in the NR field have included: (a) What are the cofactors and exchange complexes used to mediate the ligand and signaling network-dependent switches in gene regulation programs; (b) Do long non-coding RNAs (lncRNAs) serve as regulatory "factors" for ligand-dependent gene programs, and do enhancers actually regulate transcription units encoding enhancer Enhancer non-coding RNAs (eRNAs) Enhancer RNA that might have functional significance; (c) What is the relationship between DNA damage repair machinery and transcriptional machinery? (d) Do Retinoic Acid Receptors (RAR) also regulate Pol III-dependent, non-coding repeat transcriptional units in stem cells? and (e) How have new technologies such as deep sequencing altered our ability to investigate transcriptional regulatory mechanisms utilized by NRs?

In situ proximity ligation detection of c-Jun/AP-1 dimers reveals increased levels of c-Jun/Fra1 complexes in aggressive breast cancer cell lines in vitro and in vivo

Genetic and biochemical studies have shown that selective interactions between the Jun, Fos, and activating transcription factor (ATF) components of transcription factor activating protein 1 (AP-1) exhibit specific and critical functions in the regulation of cell proliferation, differentiation, and survival. For instance, the ratio between c-Jun/c-Fos and c-Jun/ATF2 dimers in the cell can be a determining factor in the cellular response to oncogenic or apoptotic stimuli. Until recently, no methods were available to detect endogenous AP-1 complexes in cells and tissues in situ. Here, we validated the proximity ligation assay (PLA) for its ability to specifically visualize and quantify changes in endogenous c-Jun/c-Fos, c-Jun/ATF2, and c-Jun/Fra1 complexes by using, among others, partner-selective c-Jun mutants. Furthermore, we examined the levels of c-Jun/AP-1 dimers in cell lines representing different types of human breast cancer and found that aggressive basal-like breast cancer cells can be discriminated from much less invasive luminal-like cells by PLA detection of c-Jun/Fra1 rather than of c-Jun/ATF2 and c-Jun/c-Fos. Also in tumor tissue derived from highly metastatic basal-like MDA-MB231 cells, high levels of c-Jun/Fra1 complexes were detected. Together, these results demonstrate that in situ PLA is a powerful diagnostic tool to analyze and quantify the amounts of biologically critical AP-1 dimers in fixed cells and tissue material.

The role of retinoid deficiency and estrogens as cofactors in cervical cancer

Persistent infection with high-risk human papillomaviruses (HR-HPVs) is involved in cervical cancer (CC), a major cause of cancer mortality worldwide. Infection occurs primarily at the transformation zone (TZ), the most estrogen- and retinoid-sensitive region of the cervix. Development of CC affects a small percentage of HR-HPV-infected women and often takes decades after infection, suggesting that HR-HPV is a necessary but not sufficient cause of CC. Thus, other cofactors are necessary for progression from cervical HR-HPV infection to cancer such as long-term use of hormonal contraceptives, multiparity, smoking, as well as micronutrient depletion and in particular retinoid deficiency, which alters epithelial differentiation, cellular growth and apoptosis of malignant cells. Therefore, early detection of HR-HPV and management of precancerous lesions together with a profound understanding of additional risk factors could be a strategy to avoid this disease. In this review we focus on the synergic effect of estrogens, retinoid deficiency and HR-HPVs in the development of CC. These risk factors may act in concert to induce neoplastic transformation in squamous epithelium of the cervix, setting the stage for secondary genetic or epigenetic events leading to cervical cancer.

Basic fibroblast growth factor regulates persistent ERK oscillations in premalignant but not malignant JB6 cells

The regulation of extracellular signal-regulated kinase (ERK) oscillations in the context of wound healing and carcinogenesis have been investigated in premalignant and malignant JB6 mouse epidermal cells stimulated with basic fibroblast growth factor (bFGF) and 12-O-tetradecanoyl phorbol-13-acetate (TPA). In premalignant JB6 cells, bFGF stimulation (1) increases cellular phospho-ERK and phospho-c-Jun levels, (2) increases serum-dependent cell proliferation, (3) induces an apparent epithelial-to-mesenchymal transition, and (4) induces the persistent nuclear-cytosolic oscillation of an ERK1-green fluorescent protein (ERK1-GFP) chimera. In contrast, TPA induces persistent activation of ERK in the absence of oscillations and does not induce efficient migration. Treatment of malignant or transformed JB6 cells with bFGF is associated with a transient nuclear translocation of ERK1-GFP but not oscillations or efficient cell migration. Our data suggest that bFGF regulates ERK oscillations in premalignant but not malignant JB6 cells.

Dynamic and combinatorial control of gene expression by nuclear retinoic acid receptors (RARs)

Nuclear retinoic acid receptors (RARs) are transcriptional regulators controlling the expression of specific subsets of genes in a ligand-dependent manner. The basic mechanism for switching on transcription of cognate target genes involves RAR binding at specific response elements and a network of interactions with coregulatory protein complexes, the assembly of which is directed by the C-terminal ligand-binding domain of RARs. In addition to this scenario, new roles for the N-terminal domain and the ubiquitin-proteasome system recently emerged. Moreover, the functions of RARs are not limited to the regulation of cognate target genes, as they can transrepress other gene pathways. Finally, RARs are also involved in nongenomic biological activities such as the activation of translation and of kinase cascades. Here we will review these mechanisms, focusing on how kinase signaling and the proteasome pathway cooperate to influence the dynamics of RAR transcriptional activity.

Antagonistic regulation of cell-matrix adhesion by FosB and DeltaFosB/Delta2DeltaFosB encoded by alternatively spliced forms of fosB transcripts

Among fos family genes encoding components of activator protein-1 complex, only the fosB gene produces two forms of mature transcripts, namely fosB and DeltafosB mRNAs, by alternative splicing of an exonic intron. The former encodes full-length FosB. The latter encodes DeltaFosB and Delta2DeltaFosB by alternative translation initiation, and both of these lack the C-terminal transactivation domain of FosB. We established two mutant mouse embryonic stem (ES) cell lines carrying homozygous fosB-null alleles and fosB(d) alleles, the latter exclusively encoding DeltaFosB/Delta2DeltaFosB. Comparison of their gene expression profiles with that of the wild type revealed that more than 200 genes were up-regulated, whereas 19 genes were down-regulated in a DeltaFosB/Delta2DeltaFosB-dependent manner. We furthermore found that mRNAs for basement membrane proteins were significantly up-regulated in fosB(d/d) but not fosB-null mutant cells, whereas genes involved in the TGF-beta1 signaling pathway were up-regulated in both mutants. Cell-matrix adhesion was remarkably augmented in fosB(d/d) ES cells and to some extent in fosB-null cells. By analyzing ES cell lines carrying homozygous fosB(FN) alleles, which exclusively encode FosB, we confirmed that FosB negatively regulates cell-matrix adhesion and the TGF-beta1 signaling pathway. We thus concluded that FosB and DeltaFosB/Delta2DeltaFosB use this pathway to antagonistically regulate cell matrix adhesion.

Bovine dialyzable leukocyte extract modulates AP-1 DNA-binding activity and nuclear transcription factor expression in MCF-7 breast cancer cells

BACKGROUND

We have previously demonstrated that bovine dialyzable leukocyte extract (bDLE) induces death through an apoptosis mechanism in MCF-7 breast cancer cells. Depending on the cell type and stimulus, activating protein-1 (AP-1) has been shown to regulate cell proliferation and differentiation, the stress response, apoptosis and survival. It remains unknown whether AP-1 and other transcription factors are mechanisms by which bDLE induces cell death.

METHODS

To determine whether bDLE modulates the AP-1 DNA binding and gene expression, MCF-7 breast cancer cells were treated with bDLE (0, 1, 5, 10 U) for 72 h and evaluated by electrophoretic mobility shift assay, reverse transcriptase-polymerase chain reaction and Western blot assays.

RESULTS

bDLE induced inhibition of cell growth, suppressed the AP-1 DNA-binding activity, decreased c-Jun protein expression and modulated NFATx, NFATc, NFkappaB, c-Jun and c-Fos transcription factor gene expression in MCF-7 breast cancer cells.

DISCUSSION

The present data indicate that bDLE can block the AP-1 DNA-binding activity and expression of several transcriptions factors in breast cancer cells, which will have great potential in improving cancer therapy.

Retinoic acid receptor beta silences human papillomavirus-18 oncogene expression by induction of de novo methylation and heterochromatinization of the viral control region

Retinoic acid receptor beta2 (RAR beta2) is often down-regulated during the multistep process to cervical cancer. In that way, its inhibitory function on the transcription factor AP-1, indispensable to maintain human papillomavirus (HPV) gene expression is relieved. Using HPV-18 positive HeLa cells as a model system, we show that ectopic expression of RAR beta2 is able to down-regulate HPV-18 transcription by selectively abrogating the binding of AP-1 to the viral regulatory region in a ligand-independent manner. This resulted in down-regulation of the viral mRNAs at the level of initiation of transcription. Decreased oncogene expression was accompanied by a re-induction of cell cycle inhibitory proteins such as p53, p21(CIP1), and p27(KIP) as well as by a cessation of cellular growth. Reduced transcriptional activity as a consequence of AP-1 reduction by selective c-Jun degradation apparently targets the HPV-18 regulatory region for epigenetic modification such as de novo methylation and nucleosomal condensation. This mechanism is otherwise counterbalanced by active and abundant viral transcription in malignant cells, because RAR beta2 itself becomes inactivated during cervical carcinogenesis. Hence, our study shows that the temporal co-existence of a potential repressor and viral oncoproteins is mutually exclusive and provides evidence of a cross-talk between a nuclear receptor, AP-1, and the epigenetic machinery.

Transcriptional activities of retinoic acid receptors

Vitamin A derivatives plays a crucial role in embryonic development, as demonstrated by the teratogenic effect of either an excess or a deficiency in vitamin A. Retinoid effects extend however beyond embryonic development, and tissue homeostasis, lipid metabolism, cellular differentiation and proliferation are in part controlled through the retinoid signaling pathway. Retinoids are also therapeutically effective in the treatment of skin diseases (acne, psoriasis and photoaging) and of some cancers. Most of these effects are the consequences of retinoic acid receptors activation, which triggers transcriptional events leading either to transcriptional activation or repression of retinoid-controlled genes. Synthetic molecules are able to mimic part of the biological effects of the natural retinoic acid receptors, all-trans retinoic acid. Therefore, retinoic acid receptors are considered as highly valuable therapeutic targets and limiting unwanted secondary effects due to retinoid treatment requires a molecular knowledge of retinoic acid receptors biology. In this review, we will examine experimental evidence which provide a molecular basis for the pleiotropic effects of retinoids, and emphasize the crucial roles of coregulators of retinoic acid receptors, providing a conceptual framework to identify novel therapeutic targets.

Retinoic acid induced repression of AP-1 activity is mediated by protein phosphatase 2A in ovarian carcinoma cells

In previous studies we have shown that all-trans retinoic acid (atRA)-treatment of the atRA-sensitive ovarian carcinoma cell line CA-OV3 repressed AP-1 activity by about 50%, while a similar effect was not observed in the atRA-resistant ovarian carcinoma cell line, SK-OV3. These results suggested that the repression of AP-1 activity may be one of the mechanisms by which atRA inhibits the growth of atRA-sensitive CA-OV3 cells. In the present studies, we investigated further the molecular mechanism by which AP-1 activity is repressed by atRA. We show that the repression of AP-1 activity correlates with an increase in JunB protein expression and a decrease in N-terminal phosphorylation of c-Jun. The decrease in N-terminal phosphorylation of c-Jun does not appear to be modulated by JNK or ERK, since their protein expression patterns and kinase activity do not correlate with the repression of AP-1 activity following treatment with atRA. However, the activity of the protein phosphatase PP2A was found to increase 24 h following atRA treatment in CA-OV3 cells. Moreover, the catalytic subunit of PP2A was found to associate with c-Jun in vivo following atRA treatment. Since the inhibition of AP-1 activity following atRA treatment of CA-OV3 cells was abolished in the presence of specific PP2A inhibitors, it is likely that PP2A plays an important role in the atRA-induced repression of AP-1.

The GRIMs: a new interface between cell death regulation and interferon/retinoid induced growth suppression

Cytokines and vitamins play a central role in controlling neoplastic cell growth. The interferon (IFN) family of cytokines regulates antiviral, anti-tumor, antimicrobial, differentiation, and immune responses in mammals. Significant advances have been made with respect to IFN-induced signal transduction pathways and antiviral responses. However, the IFN-induced anti-tumor actions are poorly defined. Although IFNs themselves inhibit tumor growth, combination of IFNs with retinoids (a class of Vitamin A related compounds) strongly potentiates the IFN-regulated anti-tumor action in a number of cell types. To define the molecular mechanisms involved in IFN/retinoid (RA)-induced apoptosis we have employed a genetic approach and identified several critical genes. In this review, I provide the current picture of IFN- RA- and IFN/RA-regulated growth suppressive pathways. In particular, I focus on a novel set of genes, the genes-associated with retinoid-interferon induced mortality (GRIM). GRIMs may be novel types of tumor suppressors, useful as biological response markers and potentially novel targets for drug development.

Retinoids in nephrology: Promises and pitfalls

BACKGROUND

Retinoids, a family of vitamin A metabolites or analogs, play an important role in regulating cell proliferation, differentiation, and apoptosis.

METHODS

The biological importance of retinoids in the kidney and the potential of retinoids in the treatment of renal diseases are reviewed.

RESULTS

Vitamin A deficiency and mutations of retinoid nuclear receptors cause abnormalities in fetal kidneys, which might predispose to adult diseases such as hypertension. Further, the therapeutic value of retinoids in animal models of kidney diseases, such as lupus nephritis, puromycin aminonucleoside nephrosis, anti-glomerular basement membrane nephritis, mesangioproliferative nephritis, and acute renal allograft rejection has been unveiled recently. Retinoids target mesangial cells, podocytes, tubular epithelial cells, interstitial fibroblasts, as well as lymphocytes and macrophages. The anti-inflammation, anti-coagulation effects, and the proliferation- and immunity-modulating actions of retinoids, have been widely appreciated. Our recent in vitro data revealed a direct antifibrotic effect and a cytoprotective effect of retinoids in various renal cell types. In animal studies, the adverse effects of retinoids are generally minimal; however, the clinical use of retinoids in other diseases points to some major side effects. In addition, in vitro, retinoids can induce lipid accumulation in smooth muscle cells and macrophages and increase expression of some proinflammatory molecules, indicating that their clinical toxicity profile in the setting of renal diseases needs to be better understood.

CONCLUSION

Retinoids not only are important in renal development, but also show promise as a new generation of renal medication and deserve to be tested in clinical trials to clarify their full potential.

TGF-beta1 disrupts endotoxin signaling in microglial cells through Smad3 and MAPK pathways

Human formyl peptide receptor-like 1 and its mouse homologue formyl peptide receptor 2 (FPR2) are G protein-coupled receptors used by a number of exogenous and host-derived chemotactic peptides, including the 42 aa form of beta amyloid peptide, a causative factor of Alzheimer's disease. Functional FPR2 was induced by bacterial LPS in murine microglial cells, the resident phagocytic cells that play a pivotal role in inflammatory and immunological diseases in the CNS. To identify agents that may suppress microglial cell activation under proinflammatory conditions, we investigated the effect of TGF-beta1 on the expression of functional FPR2 by microglial cells activated by LPS. TGF-beta1 dose-dependently inhibited the mRNA expression and function of FPR2 in LPS-activated microglial cells. The inhibitory effect of TGF-beta1 was mediated by Smad3, a key signaling molecule coupled to the TGF-beta receptor, and the transcription coactivator, p300. Also, TGF-beta1 activates MAPKs in microglial cells that became refractory to further stimulation by LPS. These effects of TGF-beta1 culminate in the inhibition of LPS-induced activation of NF-kappaB and the up-regulation of FPR2 in microglial cells. Thus, TGF-beta1 may exert a protective role in CNS diseases characterized by microglial cell activation by proinflammatory stimulants.

Ectopic expression of nonliganded retinoic acid receptor beta abrogates AP-1 activity by selective degradation of c-Jun in cervical carcinoma cells

Expression of the nuclear retinoic acid receptor beta2 (RARbeta2) gene is often disturbed in cervical carcinoma cells. One important mechanism by which RARbeta2 can exert growth inhibitory function is based on its ability to repress the AP-1 transcription factor in a ligand-dependent manner. Because less is known about the biological effects of RARbeta in the absence of ligand, the corresponding cDNA was stably introduced into HPV18-positive HeLa cervical carcinoma cells. In the present study we describe a novel mechanism by which AP-1 becomes inactivated. Constitutive expression of nonliganded RARbeta abrogated both AP-1 binding affinity and activity by a selective degradation of the c-Jun protein as major dimerization partner, without substitution by other members of the Jun family. Blockage of the proteasomal pathway completely rescued c-Jun and reconstituted the AP-1 function. Moreover, HeLa RARbeta2 clones treated either with tumor necrosis factor-alpha or transfected with a constitutive active upstream mitogen-activated protein kinase (MEKK1Delta) also resulted in c-Jun phosphorylation and restoration of AP-1 affinity and functionality similar to that found in nontransfected parental HeLa cells. These data revealed an important cross-talk between trans-repression of AP-1 and nonliganded RARbeta in human papillomavirus-positive cells. Because AP-1 activity was not irreversibly disturbed, but could be switched on through activation of the Jun N-terminal kinase pathway, a model for the transient activation of AP-1 even in the presence of RARbeta as repressor is suggested.

Differential expression of retinoic acid receptor beta (RARβ) and the AP-1 transcription factor in normal, premalignant and malignant human laryngeal tissues

The anticancer effects of retinoids are mainly mediated by their nuclear receptors. Recent studies have demonstrated that retinoic acid receptor beta (RARbeta) plays a pivotal role from the early stages of laryngeal carcinogenesis; however, the exact mechanism of this detrimental effect has not yet been elucidated. One of the best-documented actions of retinoid receptors is the transrepression of activator protein-1 (AP-1) transcription factor activity, although this complex interplay has not been clarified. The present report is the first systematic morphological evaluation of the cross-talk of RARbeta and AP-1 transcription factor in a large series of human laryngeal tissues containing normal epithelium, premalignant lesions (hyperplasia and/or dysplasia) and squamous cell carcinoma. Immunohistochemical methodology was performed on formalin-fixed, paraffin-embedded sections by using a panel of monoclonal and polyclonal antibodies against RARbeta and the AP-1 components c-Jun, p-c-Jun (phosphorylated, active c-Jun) and c-Fos proteins. Their expression was screened and compared in 154 patients with various laryngeal histological entities. Nuclear expression of RARbeta, c-Jun, p-c-Jun and c-Fos was detected in 81 (89.2%), 48 (52.8%), 66 (72.6%) and 73 (80.3%), respectively, out of 91 specimens with normal-appearing laryngeal epithelium; in 86 (87.8%), 94 (95.9%), 94 (95.9%) and 94 (95.9%), respectively, out of 98 specimens with hyperplastic laryngeal epithelium; in 58 (56.8%), 92 (90.2%), 96 (94.1%) and 96 (94.1%), respectively, out of 102 specimens with dysplastic laryngeal epithelium; in 10 (22.3%), 41 (91.2%), 44 (97.8%) and 41 (91.2%), respectively, out of 45 specimens with well-differentiated squamous cell carcinoma; in 13 (30.3%), 37 (86%), 39 (90.7%) and 41 (95.3%), respectively, out of 43 specimens with moderately-differentiated squamous cell carcinoma; and in 8 (66.7%), 10 (83.3%), 12 (100%) and 12 (100%), respectively, out of 12 specimens with poorly-differentiated squamous cell laryngeal carcinoma. Statistical analysis and correlation of the intensity of nuclear immunostaining of the studied proteins among the various histological entities revealed statistically significant results. The progressive upregulation of the AP-1 transcription factor constituents and downregulation of the RARbeta protein detected from the onset of laryngeal tumorigenesis suggests an important role for the immediate-early AP-1/RARbeta on/off "switch" in the process of laryngeal carcinogenesis.

Assessment of the role of activator protein-1 on transcription of the mouse steroidogenic acute regulatory protein gene

cAMP-dependent mechanisms regulate the steroidogenic acute regulatory (StAR) protein even though its promoter lacks a consensus cAMP response-element (CRE, TGACGTCA). Transcriptional regulation of the StAR gene has been demonstrated to involve combinations of DNA sequences that provide recognition motifs for sequence-specific transcription factors. We recently identified and characterized three canonical 5'-CRE half-sites within the cAMP-responsive region (-151/-1 bp) of the mouse StAR gene. Among these CRE elements, the CRE2 half-site is analogous (TGACTGA) to an activator protein-1 (AP-1) sequence [TGA(C/G)TCA]; therefore, the role of the AP-1 transcription factor was explored in StAR gene transcription. Mutation in the AP-1 element demonstrated an approximately 50% decrease in StAR reporter activity. Using EMSA, oligonucleotide probes containing an AP-1 binding site were found to specifically bind to nuclear proteins obtained from mouse MA-10 Leydig and Y-1 adrenocortical tumor cells. The integrity of the sequence-specific AP-1 element in StAR gene transcription was assessed using the AP-1 family members, Fos (c-Fos, Fra-1, Fra-2, and Fos B) and Jun (c-Jun, Jun B, and Jun D), which demonstrated the involvement of Fos and Jun in StAR gene transcription to varying degrees. Disruption of the AP-1 binding site reversed the transcriptional responses seen with Fos and Jun. EMSA studies utilizing antibodies specific to Fos and Jun demonstrated the involvement of several AP-1 family proteins. Functional assessment of Fos and Jun was further demonstrated by transfecting antisense c-Fos, Fra-1, and dominant negative forms of Fos (A-Fos) and c-Jun (TAM-67) into MA-10 cells, which significantly (P < 0.01) repressed transcription of the StAR gene. Mutation of the AP-1 site in combination with mutations in other cis-elements resulted in a further decrease of StAR promoter activity, demonstrating a functional cooperation between these factors. Mammalian two-hybrid assays revealed high-affinity protein-protein interactions between c-Fos and c-Jun with steroidogenic factor 1, GATA-4, and CCAAT/enhancer binding protein-beta. These findings demonstrate that Fos and Jun can bind to the TGACTGA element in the StAR promoter and provide novel insights into the mechanisms regulating StAR gene transcription.

Differential regulation of protein expression, growth and apoptosis by natural and synthetic retinoids

All-trans retinoic acid (ATRA) can down regulate the anti-apoptotic protein Bcl-2 and the cell cycle proteins cyclin D1 and cdk2 in estrogen receptor-positive breast cancer cells. We show here that retinoids can also reduce expression of the inhibitor of apoptosis protein, survivin. Here we have compared the regulation of these proteins in MCF-7 and ZR-75 breast cancer cells by natural and synthetic retinoids selective for the RA receptors (RARs) alpha, beta, and gamma then correlated these with growth inhibition, induction of apoptosis and chemosensitization to Taxol. In both cell lines ATRA and 9-cis RA induced the most profound decreases in cyclin D1 and cdk2 expression and also mediated the largest growth inhibition. The RARalpha agonist, Ro 40-6055 also strongly downregulated these proteins although did not produce an equivalent decrease in S-phase cells. Only ATRA induced RARbeta expression. ATRA, 9-cis RA and 4-HPR initiated the highest level of apoptosis as determined by mitochondrial Bax translocation, while only ATRA and 9-cis RA strongly reduced Bcl-2 and survivin protein expression. Enumeration of dead cells over 96 h correlated well with downregulation of both survivin and Bcl-2. Simultaneous retinoid-mediated reduction of both these proteins also predicted optimal Taxol sensitization. 4-HPR was much weaker than the natural retinoids with respect to Taxol sensitization, consistent with the proposed requirement for reduced Bcl-2 in this synergy. Neither the extent of cell cycle protein regulation nor AP-1 inhibition fully predicted the antiproliferative effect of the synthetic retinoids suggesting that growth inhibition requires regulation of a spectrum of RAR-regulated gene products in addition even to pivotal cell cycle proteins.

Regulation of pancreatic stellate cell function in vitro: biological and molecular effects of all-trans retinoic acid

Pancreatic stellate cells (PSCs) are essentially involved in the development of pancreatic fibrosis, a constant feature of chronic pancreatitis and pancreatic cancer. Profibrogenic mediators, such as ethanol metabolites and cytokines, induce a PSC activation process that involves proliferation, enhanced production of extracellular matrix proteins and a phenotypic transition towards myofibroblasts which includes a loss of the characteristic retinoid-containing fat droplets. Here, we have analysed how exogenous all-trans retinoic acid (ATRA) affects activation of rat PSCs induced by sustained culture. Bromodeoxyuridine-incorporation assays indicated an ATRA-dependent inhibition of DNA synthesis. In contrast, ATRA did not affect expression of alpha-smooth muscle actin, a protein typical for myofibroblasts. Quantification of [3H]proline incorporation revealed a diminished collagen production in ATRA-treated PSCs. Furthermore, zymography experiments showed that supernatants of ATRA-exposed PSC cultures contained higher levels of matrix metalloproteinase-9 but not of matrix metalloproteinase-2 than untreated controls. At the level of intracellular signalling, ATRA had no effect on extracellular signal-regulated kinase activation after incubation of PSCs with the mitogen platelet-derived growth factor (PDGF). In addition, PDGF-induced DNA binding of activator protein-1 (AP-1) transcription factors was not inhibited by ATRA treatment. Luciferase reporter gene assays, however, revealed an ATRA-dependent transrepression of AP-1 in PDGF-stimulated PSCs. Together, the results indicate that exogenous ATRA displays inhibitory effects on PSC proliferation and collagen synthesis but does not block phenotypic transition towards myofibroblasts. We hypothesise that inhibition of AP-1 signalling may be involved in the mediation of biological effects of ATRA on PSCs.

Pdcd4 suppresses tumor phenotype in JB6 cells by inhibiting AP-1 transactivation

Transformation suppressor Pdcd4 is downregulated in transformed (Tx) mouse epidermal JB6 RT101 cells relative to transformation-resistant (P-) and susceptible (P+) variants. Whether Pdcd4 downregulation is necessary not only to induce transformation but also to maintain tumor phenotypes has not been determined previously. In the present study, overexpression of Pdcd4 cDNA in stably transfected RT101 cells resulted in 40% fewer anchorage-independent colonies that were smaller in size than the vector control colonies, indicating that elevated Pdcd4 expression is sufficient to suppress tumor phenotype. Transient transfection of Pdcd4 expression plasmid and 4 x AP-1 reporter gene showed that activation of AP-1-dependent transcription was inhibited by Pdcd4 expression in a concentration-dependent manner. In contrast, Pdcd4 did not inhibit serum response element-dependent transcription, indicating specificity. In a Gal4 fusion assay, Pdcd4 specifically inhibited activation of c-Jun and c-Fos activation domains, but did not inhibit activation of JunB, JunD, Fra-1, or Fra-2. Gel mobility shift assay demonstrated that c-Jun is the major component detected in the AP-1 complex in RT101 cells. Previous studies suggested that AP-1 activity is required for maintaining the transformed phenotype in RT101 cells. Thus, Pdcd4 suppresses tumor phenotype by inhibiting AP-1-dependent transcription, possibly through inhibiting c-Jun and c-Fos activation.

Retinoic acid repression of bone morphogenetic protein 4 in inner ear development

Bone morphogenetic protein 4 (BMP4) and retinoic acid are important for normal development of the inner ear, but whether they are linked mechanistically is not known. BMP4 antagonists disrupt semicircular canal formation, as does exposure to retinoic acid. We demonstrate that retinoic acid directly down-regulates BMP4 transcription in a mouse inner ear-derived cell line, and we identify a novel promoter in the second intron of the BMP4 gene that is a target of this regulation both in the cell line and in the mouse embryonic inner ear in vivo. The importance of this down-regulation is demonstrated in chicken embryos by showing that the retinoic acid effect on semicircular canal development can be overcome by exogenous BMP4.