c-Jun represses transcription of the human chorionic gonadotropin alpha and beta genes through distinct types of CREs

Aberrant trophoblastic differentiation in human cancer: An emerging novel therapeutic target (Review)

Various types of human cancer may develop aberrant trophoblastic differentiation, including histological changes and altered expression of β‑human chorionic gonadotropin (β‑hCG). Aberrant trophoblastic differentiation in epithelial cancer is usually associated with poor differentiation, tumor metastasis, unfavorable prognosis and treatment resistance. Since β‑hCG‑targeting vaccines have failed in an early phase II trial, it is crucial to obtain a better understanding of the molecular pathogenesis of trophoblastic differentiation in human cancer. The present review summarizes the clinical and translational research on this topic with the aim of accelerating the development of an effective targeted therapy. Ectopic expression of β‑hCG promotes proliferation, migration, invasion, vasculogenesis and epithelial‑mesenchymal transition (EMT) in vitro, and enhances metastatic and tumorigenic capabilities in vivo. Signaling cascades modulated by β‑hCG include the TGF‑β receptor pathway, EMT‑related pathways, the c‑MET receptor tyrosine kinase and mitogen‑activated protein kinase/ERK pathways, and the SMAD2/4 pathway. Taken together, these findings indicated that TGF‑β receptors, c‑MET and ERK1/2 are potential therapeutic targets. Nevertheless, further investigation on the molecular basis of aberrant trophoblastic differentiation is mandatory to improve the design of precision therapy for this aggressive type of human cancer.

The glycoprotein mucin-1 negatively regulates GalNAc transferase 5 expression in pancreatic cancer

Aberrant expression of the glycoprotein mucin-1 (MUC1) has been associated with pancreatic cancer progression and metastasis as a result of mediating the oncogenic transcriptional regulation of target genes. In the present study, we demonstrate that MUC1 downregulates the expression of the tumor suppressor polypeptide N-acetylgalactosaminyltransferase 5 in pancreatic cancer. ChIP-on-chip analysis revealed that the MUC1 cytoplasmic tail binds to regulatory elements in the GALNT5 gene. Additionally, MUC1 increases binding of p53 and c-Jun and decreases the binding of Sp1 to the proximal promoter and exonic regions of GALNT5. We also observed that expression of N-acetylgalactosaminyltransferase 5 is inversionally proportional to MUC1 expression in human pancreatic cancer. These results demonstrate that MUC1 downregulates the expression of N-acetylgalactosaminyltransferase 5 in pancreatic cancer by modifying the promoter occupancy of transcription factors through its cytoplasmic domain.

Methylation status of human chorionic gonadotropin beta subunit promoter and TFAP2A expression as factors regulating CGB gene expression in placenta

OBJECTIVE

To evaluate mechanisms regulating the expression of CGB genes in placental tissues from uncomplicated pregnancies and chorionic samples from spontaneous miscarriages.

DESIGN

Molecular analyses in human samples.

SETTING

Laboratory of molecular biology.

PATIENT(S)

Nine placental samples from term deliveries and 21 chorionic samples from miscarriages at 7-13 weeks of gestation.

INTERVENTION(S)

None.

MAIN OUTCOME MEASURE(S)

The expression level of CGBs and genes encoding SP1, SP3, and AP2 transcription factors was analyzed using quantitative polymerase chain reaction (qPCR). The methylation status of the CGBs' promoter regions was determined using methylation-specific PCR.

RESULT(S)

The experiments showed significant differences in CGBs' expression and their regulation between placental and chorionic tissues. In placental tissues and chorionic tissues from 7 to 9 weeks of gestation, the expression level of CGBs was shown to be associated with the amount of TFAP2A transcripts. It was also demonstrated that variation in the expression level of CGB genes relies on changes in methylation of CGB3-9 and CGB1-2 promoter sequences.

CONCLUSION(S)

During pregnancy, regulation of hCG beta subunit genes expression correlates with both methylation of their promoters and TFAP2A expression level. The results suggest that these factors may be very influential in the early stages of pregnancy and may be associated with pregnancy outcome.

Opposite regulation of the human apolipoprotein M gene by hepatocyte nuclear factor 1 and Jun transcription factors

HDL is a negative risk factor for atherosclerosis because of its multiple atheroprotective functions. Inflammation converts HDL particles from anti-atherogenic to pro-atherogenic, and this transformation is associated with changes in HDL structure and composition. Apolipoprotein M (apoM) has been recently shown to play a role in the maturation of HDL in plasma and to protect from atherosclerosis. ApoM gene is expressed primarily in the liver and kidney and is down-regulated by pro-inflammatory signals. We now show that the human apoM promoter harbors a dual specificity regulatory element in the proximal region that binds hepatocyte nuclear factor 1 (HNF-1) and members of the AP-1 family of pro-inflammatory transcription factors (c-Jun and JunB). Overexpression of c-Jun or JunB repressed both the basal and the HNF-1-mediated transactivation of the human apoM promoter. Treatment of HepG2 cells with potent inflammation-inducing phorbol esters or overexpression of PKCα was associated with a marked inhibition of apoM gene expression in a c-Jun/JunB-dependent manner. We provide evidence for a novel mechanism of inflammation-induced transcriptional repression that is based on the competition between HNF-1 and Jun proteins for binding to the same regulatory region. A similar mechanism accounts for the down-regulation of the liver-specific apolipoprotein A-II gene by Jun factors. Our studies provide novel insights on the mechanisms that control the expression of liver-specific apolipoprotein genes during inflammation and could affect the maturation and the functionality of HDL particles.

Down-expression of PGC-1alpha partially mediated by JNK/c-Jun through binding to CRE site during apoptotic procedure in cerebellar granule neurons

In eukaryotes, mitochondria are critical for cellular bioenergetics and mediating apoptosis. The transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha) is an important regulator of mitochondrial biogenesis and function. However, the role of PGC-1alpha in neuronal apoptosis and its regulation by apoptotic pathway are still unknown. We demonstrated that PGC-1alpha expression was down-regulated in cerebellar granule neurons(CGNs) after activation of the JNK/c-Jun pathway by potassium deprivation. Overexpression of PGC-1alpha partially protected CGNs from potassium deprivation-induced apoptosis. JNK-specific inhibitors, SP600125 and CEP11004, partially blocked the inhibitory effects of JNK on PGC-1alpha expression and its promoter activity. Furthermore, ChIP assays revealed that c-Jun was able to bind to the CRE site (-188 to -180) in the PGC-1alpha promoter. In conclusion, these results suggest that down-expression of PGC-1alpha partially mediated by activation of JNK/c-Jun may be through the binding of c-Jun to the CRE site in the PGC-1alpha promoter, and it might be involved in potassium deprivation-induced apoptosis in CGNs.

New insights into the evolution of chorionic gonadotrophin

The glycoprotein hormones luteinizing hormone (LH) and chorionic gonadotrophin (CG) are crucial for reproduction, as LH induces sex hormone production and ovulation, and CG is essential for the establishment of pregnancy and fetal male sexual differentiation. Both consist of two heterodimeric peptides of which the alpha-subunit is common to both hormones whereas the beta-subunit is hormone-specific. The CGB gene was derived from LHB by gene duplication and frame shift mutation that led to a read-through into the formerly 3'-untranslated region, giving rise to the carboxyl-terminal peptide. Owing to nucleotide changes within the 5'-region of CGB, a new transcriptional start site and regulatory region was gained. These changes led to the specific expression of CGB in the placenta and its decrease in the pituitary. Recent findings on gonadotrophins led to an extended model for the sequence of events in the evolution of the CGB gene in primates and its tissue-specific expression.

Nerve Growth factor regulation of cyclin D1 in PC12 cells through a p21RAS extracellular signal-regulated kinase pathway requires cooperative interactions between Sp1 and nuclear factor-kappaB

The PC12 pheochromocytoma cell line responds to nerve growth factor (NGF) by exiting from the cell cycle and differentiating to induce extending neurites. Cyclin D1 is an important regulator of G1/S phase cell cycle progression, and it is known to play a role in myocyte differentiation in cultured cells. Herein, NGF induced cyclin D1 promoter, mRNA, and protein expression via the p21(RAS) pathway. Antisense- or small interfering RNA to cyclin D1 abolished NGF-mediated neurite outgrowth, demonstrating the essential role of cyclin D1 in NGF-mediated differentiation. Expression vectors encoding mutants of the Ras/mitogen-activated protein kinase pathway, and chemical inhibitors, demonstrated NGF induction of cyclin D1 involved cooperative interactions of extracellular signal-regulated kinase, p38, and phosphatidylinositol 3-kinase pathways downstream of p21(RAS). NGF induced the cyclin D1 promoter via Sp1, nuclear factor-kappaB, and cAMP-response element/activated transcription factor sites. NGF induction via Sp1 involved the formation of a Sp1/p50/p107 complex. Cyclin D1 induction by NGF governs differentiation and neurite outgrowth in PC12 cells.

Caffeine induction of Cyp6a2 and Cyp6a8 genes of Drosophila melanogaster is modulated by cAMP and D-JUN protein levels

Cytochrome P450 monooxygenases or CYPs, a family of endobiotics and xenobiotics metabolizing enzymes, are found in all organisms. We reported earlier that the promoters of Drosophila Cyp6a2 and Cyp6a8 genes are induced by caffeine. Since caffeine antagonizes adenosine receptor (AdoR) and inhibits cAMP phosphodiesterase (PDE), we used luciferase reporter gene to examine whether in SL-2 cells and adult Drosophila, induction of the two Cyp6 genes is mediated via AdoR and/or PDE pathway. Results showed that AdoR is not involved because AdoR agonists or antagonists do not affect the Cyp6 promoter activities. However, inhibition of PDE by specific inhibitors including caffeine causes induction of both Cyp6 gene promoters. We also found that flies mutant for dunce gene coding for cAMP-PDE, have higher Cyp6a8 promoter activity than the wild-type flies. We demonstrate that caffeine treatment increases intracellular cAMP levels, and cAMP treatment induces the Cyp6 gene promoters. Since both Cyp6 genes have multiple sites for JUN transcription factors, which generally play a positive role in cAMP pathway, effect of Drosophila jun (D-jun) on the Cyp6a8 promoter activity was examined. Results showed that the expression of D-jun sense plasmid causes downregulation rather than activation of the Cyp6a8 promoter. Conversely, expression of antisense plasmid increased the promoter activity. Interestingly, caffeine treatment decreased the D-JUN protein level in SL-2 cells as well as in adult flies. These results suggest that D-jun acts as a negative regulator, and caffeine induction of Cyp6a8 and Cyp6a2 genes is mediated by the upregulation of cAMP pathway and downregulation of the D-JUN protein level.

The evolution and genomic landscape of CGB1 and CGB2 genes

The origin of completely novel proteins is a significant question in evolution. The luteinizing hormone (LHB)/chorionic gonadotropin (CGB) gene cluster in humans contains a candidate example of this process. Two genes in this cluster (CGB1 and CGB2) exhibit nucleotide sequence similarity with the other LHB/CGB genes, but as a result of frameshifting are predicted to encode a completely novel protein. Our analysis of these genes from humans and related primates indicates a recent origin in the lineage specific to humans and African great apes. While the function of these genes is not yet known, they are strongly conserved between human and chimpanzee and exhibit three-fold lower diversity than LHB across human populations with no mutations that would disrupt the coding sequence. The 5'-upstream region of CGB1/2 contains most of the promoter sequence of hCGbeta plus a novel region proximal to the putative transcription start site. In silico prediction of putative transcription factor binding sites supports the hypothesis that CGB1 and CGB2 gene products are expressed in, and may contribute to, implantation and placental development.

Tumour necrosis factor-alpha impairs chorionic gonadotrophin beta-subunit expression and cell fusion of human villous cytotrophoblast

Growth factors expressed at the fetal-maternal interface modulate hormone expression of placental trophoblasts. The aim of this study was to investigate the effects of different cytokines on hCG subunit mRNA expression in differentiating villous cytotrophoblasts. Quantitative real-time PCR revealed a 1.8- and 6.9-fold increase of hCG-alpha and hCG-beta mRNA levels, respectively, between 36 and 60 h of term trophoblast syncytialization. Compared with controls, neither interleukin (IL)-1beta, IL-2, IL-4, IL-6, IL-10, IL-13 and IL-15 nor tumour necrosis factor (TNF)-alpha significantly altered hCG-alpha mRNA expression. Similarly, the ILs did not affect hCG-beta transcript levels. In contrast, TNF-alpha suppressed hCG-beta mRNA 3.8- and 1.8-fold at 36 and 60 h of term trophoblast differentiation. Accordingly, hCG secretion was impaired by TNF-alpha but not by the different ILs. Moreover, TNF-alpha reduced luciferase expression of reporter plasmids harbouring the proximal hCG-beta5 promoter to 35 and 77%, respectively, in primary term trophoblasts and trophoblastic SHGPL-5 cells. In addition, counting of nuclei in syncytialized, desmoplakin-negative areas revealed a 1.9-fold reduction of term trophoblast fusion in the presence of TNF-alpha. Similarly, floating explant cultures prepared from first trimester-denuded villi recovered the syncytium 2.8-fold less efficiently during 72 h of cytokine treatment. Concomitantly, TNF-alpha impaired induction of endogenous and secreted hCG-beta protein levels in these cultures. The data suggest that TNF-alpha decreases hCG-beta mRNA and protein expression by reducing gene transcription and trophoblast cell fusion. Suppression of these processes by TNF-alpha could partly explain the adverse effects of the cytokine on placental function and pregnancy outcome.

Characterization of a c-Jun-responsive module in the 5'-flank of the human CYP2J2 gene that regulates transactivation

The human cytochrome P450 2J2 (CYP2J2) generates cytoprotective epoxyeicosatrienoic acids from arachidonic acid. Expression of CYP2J2 is decreased in hypoxia, and the resultant decrease in CYP2J2-derived epoxyeicosanoids may contribute to the pathogenesis of cardiac ischaemia. Recent studies have indicated that AP-1 (activator protein-1) regulates CYP2J2 expression in normoxia and hypoxia. Down-regulation of CYP2J2 in hypoxic HepG2 cells was closely associated with the up-regulation of c-fos and transient transfection analysis demonstrated that c-Fos abolishes the activation of CYP2J2 by the AP-1 protein c-Jun. Deletion of the region between nt -122 and -50 upstream of the start codon in CYP2J2 prevented c-Jun transactivation. In this study we demonstrate that the sequence at -105/-95 is a major regulatory element that binds c-Jun and has a prominent role in CYP2J2 gene transactivation. Mutagenesis of both the -105/-95 region and the previously identified element at -56/-63 was required for complete loss of transactivation by c-Jun; separate mutagenesis of the -105/-95 element or, to a lesser extent, the -56/-63 element resulted in a partial loss of gene activation. In contrast to the behaviour of the -56/-63 element, c-Jun homodimers and c-Fos/c-Jun heterodimers bound to the -105/-95 element. These findings demonstrate that the c-Jun-responsive module between -122 and -50 in the CYP2J2 proximal promoter contains an atypical AP-1 element at -105/-95 that has a major role in c-Jun transactivation and acts in conjunction with the -56/-63 element to regulate expression.

Stimulation of hCG protein and mRNA in first trimester villous cytotrophoblast cells in vitro by glycodelin A

AIM

Human chorionic gonadotropin (hCG) is produced by fetal trophoblast cells and secreted into maternal circulation mainly in the first trimester of pregnancy. Another glycoprotein, glycodelin A, is one of the main products of the maternal decidua during this period. The purpose of this study was to investigate the effect of glycodelin A on hCG release by isolated cytotrophoblast cells in vitro.

METHODS

Cytotrophoblast cells were prepared from human first trimester placenta and incubated with varying concentrations of glycodelin A. Supernatants were assayed for hCG protein concentrations, and quantification of beta hCG mRNA was carried out by RT-PCR. Expression of hCG was analysed in stimulated trophoblast cells and in unstimulated controls by immunocytochemistry.

RESULTS

Glycodelin A induces a dose-dependent increase of hCG production. An increase of hCG expression was measured at 100 and 200 microg/mL glycodelin-A treatment in trophoblast cell culture by TaqMan assay on mRNA level. We found a moderate staining of hCG in control trophoblast cells, whereas a strong hCG staining was seen in glycodelin A-treated trophoblast cells.

CONCLUSIONS

HCG is a marker for the differentiation process of trophoblast cells. Our results suggest that glycodelin A secreted by the decidualized endometrium is involved in the regulation of hormones produced by the trophoblast.

Jun proteins modulate the ovary-specific promoter of aromatase gene in ovarian granulosa cells via a cAMP-responsive element

Estrogen is critical to both normal mammary gland and breast cancer development. Circulating levels of estrogen in premenopausal women are primarily determined by the action of aromatase in ovarian granulosa cells that converts testosterone to estradiol. In the current study, we unraveled an important role of Jun proteins in modulating ovary-specific aromatase expression. Ectopic expression of the Jun proteins in a human granulosa cell line significantly inhibited an ovary-specific promoter (PII) of the aromatase gene, whereas expression of dominant-negative mutants of Jun led to increased promoter activity. The Jun-mediated repression was specific to the aromatase promoter, as Jun proteins stimulated known AP1-responsive promoters in the same cellular context. Both the activation and basic leucine zipper domains of Jun were required for the transcriptional repression. Electrophoretic gel mobility assay showed that endogenous Jun proteins bound to a functionally important cAMP-responsive element (CRE) in the PII promoter-proximal region. Alteration of the CRE-like site impaired both the cAMP-responsive transcriptional activation and Jun-mediated repression. Furthermore, chromatin immunoprecipitation indicated the presence of cJun at the proximal region of the native PII promoter. Taken together, our work suggests that Jun proteins may attenuate estrogen biosynthesis by directly downregulating transcription of the aromatase gene in ovarian granulosa cells.

Estrogen-induced proliferation of normal endometrial glandular cells is initiated by transcriptional activation of cyclin D1 via binding of c-Jun to an AP-1 sequence

To explore the mechanism of estrogen-induced growth of normal endometrium, the transactivation system of the cyclin D1 gene was analysed using cultured normal endometrial glandular cells. Estradiol (E2) treatment of cultured normal endometrial glandular cells induced upregulation of c-Jun, and then cyclin D1 proteins, followed by serial expressions of cyclins E, A and B1 proteins. Increase in the mRNA expression of cyclin D1 preceded the protein expression of cyclin D1 under E2 treatment. A luciferase assay using deletion constructs of the cyclin D1 promoter indicated that E2-induced increase in transcriptional activity was observed in reporters containing AP-1-binding site sequence, and that in the absence of E2, cotransfection of c-Jun also showed increase of transcriptional activity in the same reporters with AP-1 sequence. A gel shift assay using nuclear extract from E2-treated endometrial glandular cells and AP-1 sequences of the cyclin D1 promoter indicated specific binding between c-Jun protein and the promoter. Transfection of c-jun antisense oligonucleotides to the glandular cells resulted in the suppression of the E2-induced upregulation of cyclin D1 mRNA and protein. These findings suggest that E2-induced proliferation of normal endometrial glandular cells is initiated by transcriptional activation of cyclin D1 via binding of c-Jun to the AP-1 sequences.

Adipocyte differentiation induces dynamic changes in NF-kappaB expression and activity

The adipocyte exerts an important role in energy homeostasis, both as depot for energy-rich triglycerides and as a source for metabolic hormones. Adipocytes also contribute to inflammation and the innate immune response. Although it can be physiologically beneficial to combine these two functions in a single cell type under some circumstances, the proinflammatory signals emanating from adipocytes in the obese state can have local and systemic effects that promote atherosclerosis and insulin resistance. The transcriptional machinery in the adipocyte that mediates these pro-inflammatory responses has remained poorly characterized to date. In particular, no information is currently available on the NF-kappaB family of transcription factors. Here, we show that adipogenesis is associated with changes in amount and subunit composition of the NF-kappaB complexes. NF-kappaB subunits p65 (RelA), p68 (RelB), and IkappaB are upregulated during fat cell differentiation. Correspondingly, basal NF-kappaB nuclear gel shift and luciferase reporter assays are induced in parallel during differentiation. Surprisingly, endotoxin sensitivity of the classical NF-kappaB pathway is substantially delayed and attenuated despite increased overall inflammatory response in the mature adipocyte, as judged by induction of IL-6 and TNF-alpha. As a reflection of the constitutively elevated NF-kappaB activity in the mature adipocyte, adipocytes (but not preadipocytes) exert a strong inflammatory stimulus on macrophages in vitro, suggesting a cross talk between adipocytes and interstitial macrophages in adipose tissue in vivo. These effects are mediated by a secretory product of adipocytes that is unlikely to be IL-6 or TNF-alpha.

Glycodelin A and differentiation of first trimester trophoblast cells in vitro

AIM

The glycoprotein, glycodelin A (GdA) is a main product of the maternal decidua in the first trimester of pregnancy and is secreted into the amniotic fluid. The purpose of this study was to investigate the effect of GdA on secretion and surface markers of isolated first trimester trophoblasts in vitro.

METHODS

Cytotrophoblasts were prepared from human first trimester placentae and incubated with varying concentrations of GdA or transfected separately with the expression plasmid of GdA. Supernatants were assayed for human chorionic gonadotropin (hCG) protein concentrations. Expression of human placental lactogen (hPL), mucin 1 (MUC1) and the Thomsen-Friedenreich (TF) epitope was analysed in stimulated trophoblast cells and in unstimulated controls by immunocytochemistry.

RESULTS

Glycodelin A induced a reduced expression of hPL compared with unstimulated controls. Expression of MUC1 was not affected by GdA. Freshly isolated trophoblast cells showed no TF expression but became positive for this antigen after 96 h of cultivation. GdA-stimulated trophoblast cells inhibited TF expression after 96 h of cultivation. GdA plasmids induced a significantly higher hCG production in transfected cells than in cells transfected with the empty plasmid.

CONCLUSIONS

The results obtained in this study suggest that GdA is involved in the differentiation of trophoblast cells. The treatment of GdA plasmid transfected trophoblast cells stimulated hCG production in isolated trophoblast cells and inhibited hPL and TF expression, suggesting a functional link between hCG and GdA.

Multiple and overlapping combinatorial codes orchestrate hormonal responsiveness and dictate cell-specific expression of the genes encoding luteinizing hormone

Normal reproductive function in mammals requires precise control of LH synthesis and secretion by gonadotropes of the anterior pituitary. Synthesis of LH requires expression of two genes [alpha-glycoprotein subunit (alphaGSU) and LHbeta] located on different chromosomes. Hormones from the hypothalamus and gonads modulate transcription of both genes as well as secretion of the biologically active LH heterodimer. In males and females, the transcriptional tone of the genes encoding alphaGSU and LHbeta reflects dynamic integration of a positive signal provided by GnRH from hypothalamic neurons and negative signals emanating from gonadal steroids. Although alphaGSU and LHbeta genes respond transcriptionally in the same manner to changes in hormonal input, different combinations of regulatory elements orchestrate their response. These hormone-responsive regulatory elements are also integral members of much larger combinatorial codes responsible for targeting expression of alphaGSU and LHbeta genes to gonadotropes. In this review, we will profile the genomic landscape of the promoter-regulatory region of both genes, depicting elements and factors that contribute to gonadotrope-specific expression and hormonal regulation. Within this context, we will highlight the different combinatorial codes that control transcriptional responses, particularly those that mediate the opposing effects of GnRH and one of the sex steroids, androgens. We will use this framework to suggest that GnRH and androgens attain the same transcriptional endpoint through combinatorial codes unique to alphaGSU and LHbeta. This parallelism permits the dynamic and coordinate regulation of two genes that encode a single hormone.

Combined Loss of INK4a and Caveolin-1 Synergistically Enhances Cell Proliferation and Oncogene-induced Tumorigenesis

Tumorigenesis is a multistep process that involves a series of genetic changes or "multiple hits," leading to alterations in signaling, proliferation, immortalization, and transformation. Many of the molecular factors that govern tumor initiation and progression remain unknown. Here, we evaluate the transformation suppressor potential of caveolin-1 (Cav-1) and its ability to cooperate with a well established tumor suppressor, the INK4a locus. To study the effects of loss of caveolin-1 on cellular transformation, we established immortalized primary mouse embryonic fibroblasts (MEFs) expressing and lacking caveolin-1 by interbreeding Cav-1 (+/+) and Cav-1 (-/-) mice with INK4a (-/-) mice. Analysis of these cells reveals that loss of caveolin-1 confers a significant growth advantage, as measured via cellular proliferation and cell cycle analysis. Loss of caveolin-1 in the INK4a (-/-) genetic background results in constitutive hyperactivation of the p42/44 MAP kinase cascade, decreased expression of p21(Cip1), as well as cyclin D1 and PCNA overexpression, consistent with their hyperproliferative phenotype. Importantly, in cells lacking Cav-1 expression, transformation by activated oncogenes (H-Ras(G12V) or v-Src) results in increased tumor growth in vivo (up to >40-fold). Finally, INK4a (-/-)/Cav-1 (-/-) mice demonstrate disturbed mammary epithelial ductal morphology, with hyperplasia, increased side-branching, and fibrosis. Our results provide important new evidence for the transformation suppressor properties of Cav-1 and the first molecular genetic evidence that Cav-1 cooperates with a tumor suppressor, namely the INK4a genetic locus.

Cytokines increase CRE binding but decrease CRE-mediated reporter activity in rat hepatocytes by increasing c-Jun

The cyclic AMP response element (CRE) has been implicated in the regulation of the expression of many genes and cellular processes important in hepatocyte function. CRE sites exist in the promoter regions of several genes expressed during inflammation. Numerous studies on the role of CRE in hepatocyte gene expression have been performed in resting hepatocytes, but the role of CRE during inflammation is unknown. To evaluate the regulation of CRE-mediated transcription during sepsis, cultured hepatocytes were exposed to proinflammatory cytokines and lipopolysaccharide (LPS) was injected into rats. Nuclear proteins were collected and CRE binding activity measured by electromobility shift assay (EMSA) using a consensus CRE oligonucleotide. CRE binding activity was increased in vitro by cytokines and in vivo by LPS administration but CRE-dependent reporter activity was decreased by cytokine stimulation. A c-jun N-terminal kinase (JNK) inhibitor reversed the cytokine-induced increase in CRE binding and increased CRE-dependent reporter activity. Supershift assays indicated that cyclic AMP response element binding protein (CREB) and c-Jun proteins were included in the CRE binding complex. CREB induced and c-Jun suppressed reporter activity using a CRE-dependent construct transfected into cultured primary hepatocytes. In conclusion, these data demonstrate that proinflammatory cytokines regulate CRE binding and activity in cultured hepatocytes and suggest that sepsis-induced changes in CRE binding may participate in the cellular response to inflammation.

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.

A central role for Ets-2 in the transcriptional regulation and cyclic adenosine 5'-monophosphate responsiveness of the human chorionic gonadotropin-beta subunit gene

Ets-2 has an important role in controlling the differentiation of the placenta. Here we show by truncation and mutational analysis that two closely spaced Ets-2 binding sites in the proximal promoter of the human chorionic gonadotropin beta5 (hCGbeta5) gene constitute a major enhancer for hCGbeta gene expression in JAr and JEG-3 human choriocarcinoma cells and in mouse NIH3T3 cells. Contrary to a previous report, we also demonstrate that the ability of Ets-2 to enhance transcription is subject to control by the Ras/MAPK pathway, although this relationship is less easily demonstrable in JAr and JEG-3 choriocarcinoma cells than in the 3T3 cells because the former already possess a fully activated MAPK pathway and contain Ets-2 phosphorylated at threonine residue at T72. Coexpression of Ets-2 and activated Ras in 3T3 cells led to activation of MAPK/ERK kinase 1/2, phosphorylation of Ets-2 at T72, and an approximately 120-fold up-regulation of reporter gene expression from a short (-175) hCGbeta promoter. Fold activation in JAr and JEG-3 cells was rather less (20- to 30-fold), but basal activity was much higher. These effects on promoter activity were largely reversed in presence of the MAPK inhibitor PD98059, which prevents ERK1/2 activation, and partially reversed by mutating T72 on Ets-2. We finally show that the ability of 8-bromoadenosine-cAMP to stimulate hCGbeta promoter activity in JAr and JEG-3 cells occurs with a short promoter lacking the upstream elements previously considered to be essential for cAMP activation of the gene and, through mutational analysis, confirm that the major cAMP effects on the hCGbeta promoter are mediated through the proximal Ets-2 enhancer. The data are consistent with the hypothesis that Ets-2 has a general and possibly essential role in controlling the activity of genes associated with trophectoderm differentiation.

Caveolin-1-deficient mice show accelerated mammary gland development during pregnancy, premature lactation, and hyperactivation of the Jak-2/STAT5a signaling cascade

It is well established that mammary gland development and lactation are tightly controlled by prolactin signaling. Binding of prolactin to its cognate receptor (Prl-R) leads to activation of the Jak-2 tyrosine kinase and the recruitment/tyrosine phosphorylation of STAT5a. However, the mechanisms for attenuating the Prl-R/Jak-2/STAT5a signaling cascade are just now being elucidated. Here, we present evidence that caveolin-1 functions as a novel suppressor of cytokine signaling in the mammary gland, akin to the SOCS family of proteins. Specifically, we show that caveolin-1 expression blocks prolactin-induced activation of a STAT5a-responsive luciferase reporter in mammary epithelial cells. Furthermore, caveolin-1 expression inhibited prolactin-induced STAT5a tyrosine phosphorylation and DNA binding activity, suggesting that caveolin-1 may negatively regulate the Jak-2 tyrosine kinase. Because the caveolin-scaffolding domain bears a striking resemblance to the SOCS pseudosubstrate domain, we examined whether Jak-2 associates with caveolin-1. In accordance with this homology, we demonstrate that Jak-2 cofractionates and coimmunoprecipitates with caveolin-1. We next tested the in vivo relevance of these findings using female Cav-1 (-/-) null mice. If caveolin-1 normally functions as a suppressor of cytokine signaling in the mammary gland, then Cav-1 null mice should show premature development of the lobuloalveolar compartment because of hyperactivation of the prolactin signaling cascade via disinhibition of Jak-2. In accordance with this prediction, Cav-1 null mice show accelerated development of the lobuloalveolar compartment, premature milk production, and hyperphosphorylation of STAT5a (pY694) at its Jak-2 phosphorylation site. In addition, the Ras-p42/44 MAPK cascade is hyper-activated. Because a similar premature lactation phenotype is observed in SOCS1 (-/-) null mice, we conclude that caveolin-1 is a novel suppressor of cytokine signaling.

Prolactin negatively regulates caveolin-1 gene expression in the mammary gland during lactation, via a Ras-dependent mechanism

Caveolin-1 is a 22-kDa integral membrane protein that has been suggested to function as a negative regulator of mitogen-stimulated proliferation in a variety of cell types, including mammary epithelial cells. Because much of our insight into caveolin-1 function has come from the study of human breast tumor-derived cell lines in culture, the normal physiological regulators of caveolin-1 expression in the mammary gland remain unknown. Here, we examine caveolin-1 expression in mice at different stages of mammary gland development. We show that caveolin-1 expression is significantly down-regulated during late pregnancy and lactation. Upon weaning, mammary gland expression of caveolin-1 rapidly returns to non-pregnant "steady-state" levels. Injection of virgin mice with a battery of hormones normally up-regulated during lactation demonstrates that prolactin is the main mediator of caveolin-1 down-regulation. Virtually identical results were obtained with human mammary epithelial cells (hTERT-HME1) in culture. In addition, we demonstrate that prolactin-mediated down-regulation of caveolin-1 expression occurs at the level of transcriptional control and via a Ras-dependent mechanism. Interestingly, in the mammary gland, both mammary epithelial cells and the surrounding mammary adipocytes show prolactin-mediated down-regulation of caveolin-1. This hormone-dependent regulation of caveolin-1 expression is specific to the mammary fat pad. Finally, we employed HC11 cells, a well-established model of mammary epithelial cell differentiation, to study the possible functional effects of caveolin-1 expression. In the presence of lactogenic hormones, recombinant expression of caveolin-1 in HC11 cells dramatically suppresses the induction of the promoter activity and the synthesis of beta-casein, an established reporter of lactogenic differentiation and milk production. These findings may explain why caveolin-1 levels are normally down-regulated during lactation. This report is the first demonstration that caveolin-1 levels are down-regulated during a normal physiological event in vivo, i.e. lactation, because previous reports have only documented that down-regulation of caveolin-1 occurs during cell transformation and tumorigenesis.

The regulation of human corticotrophin-releasing hormone gene expression in the placenta

Corticotrophin-releasing hormone (CRH) is a 41 amino acid neuropeptide that is expressed in the hypothalamus and the human placenta. Placental CRH production has been linked to the determination of gestational length in the human. Although encoded by a single copy gene, CRH expression in the placenta is regulated differently to the hypothalamus. Glucocorticoids stimulate CRH promoter activity in the placenta but inhibit it's activity in the hypothalamus, via mechanisms involving different regions of the CRH promoter. We discuss how various stimuli alter CRH promoter activity and why these responses are unique to the placenta.

Two distinct caveolin-1 domains mediate the functional interaction of caveolin-1 with protein kinase A

Numerous components of the cAMP-based signaling cascade, namely G-proteins and G- protein coupled receptors, adenylyl cyclase, and protein kinase A (PKA) have been localized to caveolae and shown to be regulated by the caveolar marker proteins, the caveolins. In order to gain mechanistic insights into these processes in vivo, we have assessed the functional interaction of caveolin-1 (Cav-1) with PKA using mutational analysis. As two regions of Cav-1 had previously been implicated in PKA signaling in vitro, we constructed Cav-1 molecules with mutations/deletions in one or both of these domains. Examination of these mutants shows that Cav-1 requires the presence of either the scaffolding domain or the COOH-terminal domain (but not both) to functionally interact with and inhibit PKA. Interestingly, in contrast to the wild-type protein, these Cav-1 mutants are not localized to caveolae microdomains. However, upon coexpression with wild-type Cav-1, a substantial amount of the mutants was recruited to the caveolae membrane fraction. Using the Cav-1 double mutant with both disrupted scaffolding and COOH-terminal domains, we show that wild-type Cav-1's inhibition of PKA signaling can be partially abrogated in a dose-responsive manner; i.e., the mutant acts in a dominant-negative fashion. Thus, this dominant-negative caveolin-1 mutant will be extremely valuable for assessing the functional role of endogenous caveolin-1 in regulating a variety of other signaling cascades.

AR suppresses transcription of the alpha glycoprotein hormone subunit gene through protein-protein interactions with cJun and activation transcription factor 2

Previously, we reported that the AR directly suppressed transcription of the alpha glycoprotein hormone subunit (alphaGSU) gene in a ligand-dependent fashion while ER had no effect. Mutagenesis studies of the alphaGSU promoter indicated that two elements were required for AR-mediated suppression: the alpha basal element and tandem cAMP response elements (CREs). Because several members of the bZip family of transcriptional proteins can bind the CREs, we used several functional assays to determine whether AR interacts selectively with cJun, activation transcription factor 2 (ATF2), or CRE binding protein (CREB). When tested by cotransfection with AR, cJun and ATF2 specifically rescued androgen-mediated suppression of the alphaGSU-reporter construct in a gonadotrope-derived cell line. In contrast, cotransfected CREB displayed no activity in this rescue assay. In fact, overexpression of CREB alone diminished activity of the alphaGSU promoter, suggesting that the transcriptional activity normally conferred by the tandem CREs in gonadotropes requires their occupancy by cJun/ATF2 heterodimers. Binding assays carried out with a glutathione-S-transferase-AR fusion protein indicated that the receptor itself also displayed a clear preference for binding cJun and ATF2. Furthermore, we ruled out the possibility that AR suppressed activity of the alphaGSU promoter by reducing synthesis of these bZip proteins. Additional experiments suggested that phosphorylation of AR or histone acetylation are unlikely requirements for AR suppression of alphaGSU promoter activity. Thus, our data suggest that AR suppresses activity of the alphaGSU promoter through direct protein-protein interactions with cJun and ATF2.

The regulation of human corticotrophin-releasing hormone gene expression in the placenta

Corticotrophin-releasing hormone (CRH) is a 41 amino acid neuropeptide that is expressed in the hypothalamus and the human placenta. Placental CRH production has been linked to the determination of gestational length in the human. Although encoded by a single copy gene, CRH expression in the placenta is regulated differently to the hypothalamus. Glucocorticoids stimulate CRH promoter activity in the placenta but inhibit it's activity in the hypothalamus, via mechanisms involving different regions of the CRH promoter. We discuss how various stimuli alter CRH promoter activity and why these responses are unique to the placenta.

Influence of caveolin-1 on cellular cholesterol efflux mediated by high-density lipoproteins

Caveolin-1 is a principal structural component of caveolae membranes. These membrane microdomains participate in the regulation of signaling, transcytosis, and cholesterol homeostasis at the plasma membrane. In the present study, we determined the effect of caveolin-1 expression on cellular cholesterol efflux mediated by high-density lipoprotein (HDL). We evaluated this effect in parental NIH/3T3 cells as well as in two transformed NIH/3T3 cell lines in which caveolin-1 protein levels are dramatically downregulated. Compared with parental NIH/3T3 cells, these two transformed cell lines effluxed cholesterol more rapidly to HDL. In addition, NIH/3T3 cells harboring caveolin-1 antisense also effluxed cholesterol more rapidly to HDL. However, this effect was not due to changes in total cellular cholesterol content. We further showed that chronic HDL exposure reduced caveolin-1 protein expression in NIH/3T3 cells. HDL exposure also inhibited caveolin-1 promoter activity, suggesting a direct negative effect of HDL on caveolin-1 gene transcription. Moreover, we showed that HDL-induced downregulation of caveolin-1 prevents the uptake of oxidized low-density lipoprotein in human endothelial cells. These data suggest a novel proatherogenic role for caveolin-1, i.e., regarding the uptake and/or transcytosis of modified lipoproteins.

Evidence that Myc isoforms transcriptionally repress caveolin-1 gene expression via an INR-dependent mechanism

The c-Myc oncoprotein contributes to oncogenesis by activating and repressing a repertoire of genes involved in cellular proliferation, metabolism, and apoptosis. Increasing evidence suggests that the repressor function of c-Myc is critical for transformation. Therefore, identifying and characterizing Myc-repressed genes is imperative to understanding the mechanisms of Myc-induced tumorigenesis. Here, we employ NIH 3T3 cell lines harboring c-Myc-ER or N-Myc-ER to dissect the relationship between Myc activation and caveolin-1 expression. In this well-established inducible system, treatment with estrogen like molecules, such as tamoxifen, leads to activation of Myc, but in a tightly controlled fashion. Using this approach, we show that Myc activation induces the repression of caveolin-1 expression at the transcriptional level. We also provide two independent lines of evidence suggesting that caveolin-1 is a direct target of Myc: (i) the effect of Myc activation on caveolin-1 expression is independent of new protein synthesis, as revealed through the use of cycloheximide; and (ii) Myc-mediated repression of the caveolin-1 promoter is dependent on an intact INR sequence. Moreover, we show that expression of caveolin-1, via an adenoviral vector approach, can suppress cell transformation that is mediated by Myc activation. In support of these observations, treatment with an adenoviral vector harboring anti-sense caveolin-1 specifically potentiates transformation induced by Myc activation. Taken together, our results indicate that caveolin-1 is a direct target of Myc repression, and they also provide evidence for an additional mechanism by which Myc repression can elicit a malignant phenotype.

Caveolin-1 regulates transforming growth factor (TGF)-beta/SMAD signaling through an interaction with the TGF-beta type I receptor

Transforming growth factor-beta (TGF-beta) signaling proceeds from the cell membrane to the nucleus through the cooperation of the type I and II serine/threonine kinase receptors and their downstream SMAD effectors. Although various regulatory proteins affecting TGF-beta-mediated events have been described, relatively little is known about receptor interactions at the level of the plasma membrane. Caveolae are cholesterol-rich membrane microdomains that, along with their marker protein caveolin-1 (Cav-1), have been implicated in the compartmentalization and regulation of certain signaling events. Here, we demonstrate that specific components of the TGF-beta cascade are associated with caveolin-1 in caveolae and that Cav-1 interacts with the Type I TGF-beta receptor. Additionally, Cav-1 is able to suppress TGF-beta-mediated phosphorylation of Smad-2 and subsequent downstream events. We localize the Type I TGF-beta receptor interaction to the scaffolding domain of Cav-1 and show that it occurs in a physiologically relevant time frame, acting to rapidly dampen signaling initiated by the TGF-beta receptor complex.

Caveolin-1 expression is down-regulated in cells transformed by the human papilloma virus in a p53-dependent manner. Replacement of caveolin-1 expression suppresses HPV-mediated cell transformation

Squamous cell carcinomas of the lung and cervix arise by neoplastic transformation of their respective tissue epithelia. In the case of cervical carcinomas, an increasing body of evidence implicates the human papillomavirus, HPV (types 16 and 18), as playing a pivotal role in this malignant transformation process. The HPV early genes E6 and E7 are known to inactivate the tumor suppressors p53 and Rb, respectively; this leads to disruption of cell cycle regulation, predisposing cells to a cancerous phenotype. However, the role of caveolin-1 (a putative tumor suppressor) in this process remains unknown. Here, we show that caveolin-1 protein expression is consistently reduced in a panel of lung and cervical cancer derived cell lines and that this reduction is not due to hyperactivation of p42/44 MAP kinase (a known negative regulator of caveolin-1 transcription). Instead, we provide evidence that this down-regulation event is due to expression of the HPV E6 viral oncoprotein, as stable expression of E6 in NIH 3T3 cells is sufficient to dramatically reduce caveolin-1 protein levels. Furthermore, we demonstrate that p53-a tumor suppressor inactivated by E6-is a positive regulator of caveolin-1 gene transcription and protein expression. SiHa cells are derived from a human cervical squamous carcinoma, harbor a fully integrated copy of the HPV 16 genome (including E6), and show dramatically reduced levels of caveolin-1 expression. We show here that adenoviral-mediated gene transfer of the caveolin-1 cDNA to SiHa cells restores caveolin-1 protein expression and abrogates their anchorage-independent growth in soft agar. Taken together, our results suggest that the HPV oncoprotein E6 down-regulates caveolin-1 via inactivation of p53 and that replacement of caveolin-1 expression can partially revert HPV-mediated cell transformation.

Caveolin-1 expression inhibits Wnt/beta-catenin/Lef-1 signaling by recruiting beta-catenin to caveolae membrane domains

Caveolin-1 is a principal component of caveolae membranes. In NIH 3T3 cells, caveolin-1 expression is dramatically up-regulated in confluent cells and localizes at areas of cell-cell contact. However, it remains unknown whether caveolin-1 is involved in cell-cell signaling. Here, we examine the potential role of caveolin-1 in regulating beta-catenin signaling. beta-Catenin plays a dual role in the cell, linking E-cadherin to the actin cytoskeleton and in Wnt signaling by forming a complex with members of the lymphoid enhancing factor (Lef-1) family of transcription factors. We show that E-cadherin, beta-catenin, and gamma-catenin (plakoglobin) are all concentrated in caveolae membranes. Moreover, we demonstrate that activation of beta-catenin/Lef-1 signaling by Wnt-1 or by overexpression of beta-catenin itself is inhibited by caveolin-1 expression. We also show that recombinant expression of caveolin-1 in caveolin-1 negative cells is sufficient to recruit beta-catenin to caveolae membranes, thereby blocking beta-catenin-mediated transactivation. These results suggest that caveolin-1 expression can modulate Wnt/beta-catenin/Lef-1 signaling by regulating the intracellular localization of beta-catenin.

Role of the cyclic AMP response element binding complex and activation of mitogen-activated protein kinases in synergistic activation of the glycoprotein hormone alpha subunit gene by epidermal growth factor and forskolin

The aim of these studies was to elucidate a role for epidermal growth factor (EGF) signaling in the transcriptional regulation of the glycoprotein hormone alpha subunit gene, a subunit of chorionic gonadotropin. Studies examined the effects of EGF and the adenylate cyclase activator forskolin on the expression of a transfected alpha subunit reporter gene in a human choriocarcinoma cell line (JEG3). At maximal doses, administration of EGF resulted in a 50% increase in a subunit reporter activity; forskolin administration induced a fivefold activation; the combined actions of EGF and forskolin resulted in synergistic activation (greater than eightfold) of the alpha subunit reporter. Mutagenesis studies revealed that the cyclic AMP response elements (CRE) were required and sufficient to mediate EGF-forskolin-induced synergistic activation. The combined actions of EGF and forskolin resulted in potentiated activation of extracellular signal-regulated kinase (ERK) enzyme activity compared with EGF alone. Specific blockade of ERK activation was sufficient to block EGF-forskolin-induced synergistic activation of the alpha subunit reporter. Pretreatment of JEG3 cells with a p38 mitogen-activated protein kinase inhibitor did not influence activation of the alpha reporter. However, overexpression of c-Jun N-terminal kinase (JNK)-interacting protein 1 as a dominant interfering molecule abolished the synergistic effects of EGF and forskolin on the alpha subunit reporter. CRE binding studies suggested that the CRE complex consisted of CRE binding protein and EGF-ERK-dependent recruitment of c-Jun-c-Fos (AP-1) to the CRE. A dominant negative form of c-Fos (A-Fos) that specifically disrupts c-Jun-c-Fos DNA binding inhibited synergistic activation of the alpha subunit. Thus, synergistic activation of the alpha subunit gene induced by EGF-forskolin requires the ERK and JNK cascades and the recruitment of AP-1 to the CRE binding complex.

Corticotropin-releasing hormone gene expression in primary placental cells is modulated by cyclic adenosine 3',5'-monophosphate

CRH, the principal neuropeptide regulator of pituitary ACTH secretion, is also expressed in placenta. Placental CRH has been linked to the process of human parturition. However, the mechanisms regulating transcription of the CRH gene in placenta remain unclear. cAMP signaling pathways play important roles in regulating the expression of a diverse range of endocrine genes in the placenta. Therefore, we have explored the effect of cAMP on CRH promoter activity in primary cultures of human placental cells. Both forskolin and 8-bromo-cAMP, activators of protein kinase A, can increase CRH promoter activity 5-fold in transiently transfected human primary placental cells, in a manner that parallels the increase in endogenous CRH peptide. Maximal stimulation of CRH promoter activity occurs at 500 micromol/L 8-bromo-cAMP and 10 micromol/L forskolin. Electrophoretic mobility shift assay and mutation analysis combined with transient transfection demonstrate that in placental cells cAMP stimulates CRH gene expression through a cAMP regulatory element in the proximal CRH promoter region and involves a placental nuclear protein interacting specifically with the cAMP regulatory element.

Constitutively active mitogen-activated protein kinase kinase 6 (MKK6) or salicylate induces spontaneous 3T3-L1 adipogenesis

Although much has been learned regarding the importance of p38 mitogen-activated protein kinase in inflammatory and stress responses, relatively little is known concerning its role in differentiation processes. Recently, we demonstrated that p38 mitogen-activated protein kinase activity is necessary for the differentiation of 3T3-L1 fibroblasts into adipocytes (Engelman, J. A., Lisanti, M. P., and Scherer, P. E. (1998) J. Biol. Chem. 273, 32111-32120). p38 activity is high during the initial stages of differentiation but decreases drastically as the fibroblasts undergo terminal differentiation into adipocytes. However, it remains unknown whether activation of p38 is sufficient to stimulate adipogenesis and whether the down-regulation of p38 activity in mature adipocytes is critical for maintaining adipocyte homeostasis. In this report, we have directly addressed these questions by analyzing 3T3-L1 cell lines harboring a specific upstream activator of p38 (a constitutively active mitogen-activated protein kinase kinase 6 (MKK6) mutant, MKK6(Glu)) under the control of an inducible promoter. Induction of MKK6(Glu) in 3T3-L1 fibroblasts spurs adipocyte conversion in the absence of the hormonal mixture normally required for efficient differentiation of wild-type cells. However, activation of p38 in adipocytes leads to cell death. Furthermore, treatment of 3T3-L1 fibroblasts with salicylate, a potent stimulator of p38, produces adipocyte-specific changes consistent with those observed with induction of MKK6(Glu). Expression of MKK6(Glu) in NIH-3T3 fibroblasts (cells that do not differentiate into adipocytes under normal conditions) is capable of converting these fibroblasts into lipid-laden fat cells following hormonal stimulation. Thus, p38 activation has pro-adipogenic effects in multiple fibroblast cell lines.

p42/44 MAP kinase-dependent and -independent signaling pathways regulate caveolin-1 gene expression. Activation of Ras-MAP kinase and protein kinase a signaling cascades transcriptionally down-regulates caveolin-1 promoter activity

Caveolin-1 is a principal component of caveolae membranes in vivo. Caveolin-1 mRNA and protein expression are down-regulated in NIH 3T3 cells in response to transformation by activated oncogenes, such as H-Ras(G12V) and v-Abl. The mechanisms governing this down-regulation event remain unknown. Here, we show that caveolin-1 gene expression is directly regulated by activation of the Ras-p42/44 MAP kinase cascade. Down regulation of caveolin-1 protein expression by Ras is independent of (i) the type of activating mutation (G12V versus Q61L) and (ii) the form of activated Ras transfected (H-Ras versus K-Ras versus N-Ras). Treatment of Ras or Raf-transformed NIH 3T3 cells with a well characterized MEK inhibitor (PD 98059) restores caveolin-1 protein expression. In contrast, treatment of v-Src and v-Abl transformed NIH 3T3 cells with PD 98059 does not restore caveolin-1 expression. Thus, there must be at least two pathways for down-regulating caveolin-1 expression: one that is p42/44 MAP kinase-dependent and another that is p42/44 MAP kinase-independent. We focused our efforts on the p42/44 MAP kinase-dependent pathway. The activity of a panel of caveolin-1 promoter constructs was evaluated using transient expression in H-Ras(G12V) transformed NIH 3T3 cells. We show that caveolin-1 promoter activity is up-regulated approximately 5-fold by inhibition of the p42/44 MAP kinase cascade. Using electrophoretic mobility shift assays we provide evidence that the caveolin-1 promoter (from -156 to -561) is differentially bound by transcription factors in normal and H-Ras(G12V)-transformed cells. We also show that activation of protein kinase A (PKA) signaling is sufficient to down-regulate caveolin-1 protein expression and promoter activity. Thus, we have identified two signaling pathways (Ras-p42/44 MAP kinase and PKA) that transcriptionally down-regulate caveolin-1 gene expression.

Regulation of cAMP-mediated signal transduction via interaction of caveolins with the catalytic subunit of protein kinase A

cAMP-dependent processes are essential for cell growth, differentiation, and homeostasis. The classic components of this system include the serpentine receptors, heterotrimeric G-proteins, adenylyl cyclase, protein kinase A (PKA), and numerous downstream target substrates. Evidence is accumulating that some members of this cascade are concentrated within membrane microdomains, termed caveolae and caveolae-related domains. In addition, the caveolin-1 protein has been shown to interact with some of these components, and this interaction inhibits their enzymatic activity. However, the functional effects of caveolins on cAMP-mediated signaling at the most pivotal step, PKA activation, remain unknown. Here, we show that caveolin-1 can dramatically inhibit cAMP-dependent signaling in vivo. We provide evidence for a direct interaction between caveolin-1 and the catalytic subunit of PKA both in vitro and in vivo. Caveolin-1 binding appears to be mediated both by the caveolin scaffolding domain (residues 82-101) and a portion of the C-terminal domain (residues 135-156). Further functional analysis indicates that caveolin-based peptides derived from these binding regions can inhibit the catalytic activity of purified PKA in vitro. Mutational analysis of the caveolin scaffolding domain reveals that a series of aromatic residues within the caveolin scaffolding domain are critical for mediating inhibition of PKA. In addition, co-expression of caveolin-1 and PKA in cultured cells results in their co-localization as seen by immunofluorescence microscopy. In cells co-expressing caveolin-1 and PKA, PKA assumed a punctate distribution that coincided with the distribution of caveolin-1. In contrast, in cells expressing PKA alone, PKA was localized throughout the cytoplasm and yielded a diffuse staining pattern. Taken together, our results suggest that the direct inhibition of PKA by caveolin-1 is an important and previously unrecognized mechanism for modulating cAMP-mediated signaling.

Activator protein-2 mediates transcriptional activation of the CYP11A1 gene by interaction with Sp1 rather than binding to DNA

The ovine P45 side chain cleavage (CYP11A1) enzyme gene, which catalyzes the initial enzymatic step in steroid hormone biosynthesis is transcriptionally regulated in cultured steroidogenic human trophoblastic JEG-3 cells. The ovine CYP11A1 promoter contains two GC-rich footprinted regions referred to as ovine footprints 5 (OF5) and OF3, which are well conserved among the CYP11A1 promoters of different species. These GC-rich sequences resemble activator protein-2 (AP-2)/Sp1 binding sites and were previously implicated in basal and cAMP-regulated activity of the bovine and ovine CYP11A1 promoters. In the current studies, AP-2 induced the ovine CYP11A1 promoter 4.5-fold in JEG-3 cells with full induction requiring the previously defined cAMP-responsive elements. Point mutation of OF3 abolished induction by AP-2, and OF3 was sufficient for induction by AP-2 when linked to a heterologous promoter. AP-2 induction of the CYP11A1 promoter required the basic region (N165-N278) and the carboxy terminus of AP-2 (N413-N437). In the course of investigating the mechanisms by which OF5 and OF3 regulated CYP11A1 transcription, we found that OF5 and OF3 bound Sp1 and Sp3 in JEG-3 cells. AP-2 did not bind OF5 or OF3 directly but rather formed a multiprotein complex with Sp1 in JEG-3 cells. AP-2 associated directly with Sp1 in vitro requiring the AP-2 basic region and the Sp1 carboxy terminus. AP-2 induced Sp1/Sp3 activity independently of AP-2 binding to DNA using a GAL4 paradigm. The Sp1 and Sp3 transactivation domains were linked to the DNA-binding domain of GAL4, and their activity was assessed using a luciferase reporter gene containing only the GAL4 DNA-binding sites linked to the minimal TATA site. AP-2 induced Sp1/ Sp3-GAL4 activity 3- to 4-fold, requiring both the amino and extreme carboxy terminus of AP-2. We conclude that AP-2 can bind to and stimulate Sp1 activity and induces the ovine CYP11A1 promoter through conserved Sp1/Sp3-binding sites in JEG-3 cells. The induction of Sp1 activity by AP-2 may contribute to the induction of other genes that bind Sp1.

Regulation of mouse SP-B gene promoter by AP-1 family members

The regulatory role of activator protein-1 (AP-1) family members in mouse surfactant protein (SP) B (mSP-B) promoter function was assessed in the mouse lung epithelial cell line MLE-15. Expression of recombinant Jun B and c-Jun inhibited mSP-B promoter activity by 50-75%. Although c-Fos expression did not alter mSP-B transcription, Jun D enhanced mSP-B promoter activity and reversed inhibition of mSP-B by c-Jun or Jun B. A proximal AP-1 binding site (-18 to -10 bp) was identified that overlaps a thyroid transcription factor-1 binding site. Mutation of this proximal AP-1 site blocked both Jun B inhibition and Jun D enhancement and partially blocked c-Jun inhibition of promoter activity. Promoter deletion mutants were used to identify additional sequences mediating the inhibitory effects of c-Jun in the distal region from -397 to -253 bp. The AP-1 element in this distal site (-370 to -364 bp) is part of a composite binding site wherein AP-1, cAMP response element binding protein, thyroid transcription factor-1, and nuclear factor I interact. Point mutation of the distal AP-1 binding site partially blocked c-Jun-mediated inhibition of the SP-B promoter. Both stimulatory (Jun D) and inhibitory (c-Jun/Jun B) effects of AP-1 family members on mSP-B promoter activity are mediated by distinct cis-acting elements in the mSP-B 5'-flanking region.

pp60(v-src) induction of cyclin D1 requires collaborative interactions between the extracellular signal-regulated kinase, p38, and Jun kinase pathways. A role for cAMP response element-binding protein and activating transcription factor-2 in pp60(v-src) signaling in breast cancer cells

The cyclin D1 gene is overexpressed in breast tumors and encodes a regulatory subunit of cyclin-dependent kinases that phosphorylate the retinoblastoma protein. pp60(c-src) activity is frequently increased in breast tumors; however, the mechanisms governing pp60(c-src) regulation of the cell cycle in breast epithelium are poorly understood. In these studies, pp60(v-src) induced cyclin D1 protein levels and promoter activity (48-fold) in MCF7 cells. Cyclin D1-associated kinase activity and protein levels were increased in mammary tumors from murine mammary tumor virus-pp60(c-src527F) transgenic mice. Optimal induction of cyclin D1 by pp60(v-src) involved the extracellular signal-regulated kinase, p38, and c-Jun N-terminal kinase members of the mitogen-activated protein kinase family. Cyclin D1 promoter activation by pp60(v-src) involved a cAMP response element-binding protein (CREB)/activating transcription factor 2 (ATF-2) binding site. Dominant negative mutants of CREB and ATF-2 but not c-Jun inhibited pp60(v-src) induction of cyclin D1. pp60(v-src) induction of CREB was blocked by the p38 inhibitor SB203580 or by mutation of CREB at Ser133. pp60(v-src) induction of ATF-2 was abolished by the c-Jun N-terminal kinase inhibitor JNK-interacting protein-1 or by mutation of ATF-2 at Thr69 and Thr71. CREB and ATF-2, which bind to a common pp60(v-src) response element, are transcriptionally activated by distinct mitogen-activated protein kinases. Induction of cyclin D1 activity by pp60(v-src) may contribute to breast tumorigenesis through phosphorylation and inactivation of the retinoblastoma protein.

CRE DNA binding proteins bind to the AP-1 target sequence and suppress AP-1 transcriptional activity in mouse keratinocytes

Previously, we have shown that nuclear extracts from cultured mouse keratinocytes induced to differentiate by increasing the levels of extra-cellular calcium contain Fra-1, Fra-2, Jun B, Jun D and c-Jun proteins that bind to the AP-1 DNA binding sequence. Despite this DNA binding activity, AP-1 reporter activity was suppressed in these cells. Here, we have detected the CREB family proteins CREB and CREMalpha as additional participants in the AP-1 DNA binding complex in differentiating keratinocytes. AP-1 and CRE DNA binding activity correlated with the induction of CREB, CREMalpha and ATF-1 and CREB phosphorylation at ser133 (ser133 phospho-CREB) in the transition from basal to differentiating keratinocytes, but the activity of a CRE reporter remained unchanged. In contrast, the CRE reporter was activated in the presence of the dominant-negative (DN) CREB mutants, KCREB and A-CREB, proteins that dimerize with CREB family members and block their ability to bind to DNA. The increase in CRE reporter activity in the presence of these mutants suggests that CRE-mediated transcriptional activity is suppressed in keratinocytes through protein-protein interactions involving a factor that dimerizes with the CREB leucine zipper. In experiments where the A-CREB mutant was co-transfected with an AP-1 reporter construct, transcriptional activity was also increased indicating that a CREB family member binds AP-1 sites and represses AP-1 transcriptional activity as well. Exogenous expression of the transcriptional repressor CREMalpha down-regulated both CRE and AP-1 reporters in keratinocytes suggesting that this factor may contribute to the suppression of AP-1 transcriptional activity observed in differentiating keratinocytes.

Identification of the cyclin D1 gene as a target of activating transcription factor 2 in chondrocytes

Endochondral bone growth is regulated by the rates of chondrocyte proliferation and differentiation. However, the intracellular mechanisms regulating these processes are poorly understood. Recently, interruption of the gene encoding the transcription factor activating transcription factor 2 (ATF-2) was shown to inhibit proliferation of chondrocytes in mice [Reimold, A. M., et al. (1996) Nature (London) 379, 262-265]. The target genes of ATF-2 that are responsible for this phenotype remain unknown. Here we report that the cyclin D1 gene is a direct target of ATF-2 in chondrocytes. ATF-2 is present in nuclear extracts from chondrogenic cell lines and binds, as a complex with a CRE-binding protein (CREB)/CRE modulator protein, to the cAMP response element (CRE) in the cyclin D1 promoter. Mutation of the cyclin D1 CRE caused a 78% reduction in the activity of the promoter in chondrocytes. Overexpression of ATF-2 in chondrocytes enhanced activity of the cyclin D1 promoter 3. 5-fold. In contrast, inhibition of endogenous ATF-2 or CREB by expression of dominant-negative inhibitors of CREB and ATF-2 significantly reduced the activity of the promoter in chondrocytes through the CRE. In addition, levels of cyclin D1 protein are greatly reduced in the chondrocytes of ATF-2-deficient mice. These data identify the cyclin D1 gene as a direct target of ATF-2 in chondrocytes and suggest that reduced expression of cyclin D1 contributes to the defective cartilage development of these mice.

An upstream negative regulatory element in human granulocyte-macrophage colony-stimulating factor promoter is recognised by AP1 family members

Granulocyte-macrophage colony-stimulating factor (GM-CSF) is a cytokine involved in haematopoiesis and host defence. Production of GM-CSF has been detected in tumour cells including the U87MG astrocytoma cell line. Previous studies have been focused on the regulatory role of the proximal region of the GM-CSF promoter. Our studies on the distal region of the promoter in U87MG cells identify a negative cis element (-1377/-1298) which contains a AP1-like site able to bind c-jun and c-fos transcription factors, according to the results of DNA/protein binding assays. Mutagenesis of the AP1-like site eliminates AP1 binding and the negative effect on promoter activity.

Fos family members induce cell cycle entry by activating cyclin D1

Expression of the fos family of transcription factors is stimulated by growth factors that induce quiescent cells to reenter the cell cycle, but the cellular targets of the Fos family that regulate cell cycle reentry have not been identified. To address this issue, mice that lack two members of the fos family, c-fos and fosB, were derived. The fosB-/- c-fos-/- mice are similar in phenotype to c-fos-/- mice but are 30% smaller. This decrease in size is consistent with an abnormality in cell proliferation. Fibroblasts derived from fosB-/- c-fos-/- mice were found to have a defect in proliferation that results at least in part from a failure to induce cyclin D1 following serum-stimulated cell cycle reentry. Although definitive evidence that c-Fos and FosB directly induce cyclin D1 transcription will require further analysis, these findings raise the possibility that c-Fos and FosB are either direct or indirect transcriptional regulators of the cyclin D1 gene and may function as a critical link between serum stimulation and cell cycle progression.

Reciprocal regulation of neu tyrosine kinase activity and caveolin-1 protein expression in vitro and in vivo. Implications for human breast cancer

Neu (c-erbB2) is a proto-oncogene product that encodes an epidermal growth factor-like receptor tyrosine kinase. Amplification of wild-type c-Neu and mutational activation of Neu (Neu T) have been implicated in oncogenic transformation of cultured fibroblasts and mammary tumorigenesis in vivo. Here, we examine the relationship between Neu tyrosine kinase activity and caveolin-1 protein expression in vitro and in vivo. Recent studies have suggested that caveolins may function as negative regulators of signal transduction. Our current results show that mutational activation of c-Neu down-regulates caveolin-1 protein expression, but not caveolin-2, in cultured NIH 3T3 and Rat 1 cells. Conversely, recombinant overexpression of caveolin-1 blocks Neu-mediated signal transduction in vivo. These results suggest a reciprocal relationship between c-Neu tyrosine kinase activity and caveolin-1 protein expression. We next analyzed a variety of caveolin-1 deletion mutants to map this caveolin-1-dependent inhibitory activity to a given region of the caveolin-1 molecule. Results from this mutational analysis show that this functional in vivo inhibitory activity is contained within caveolin-1 residues 32-95. In accordance with these in vivo studies, a 20-amino acid peptide derived from this region (the caveolin-1 scaffolding domain) was sufficient to inhibit Neu autophosphorylation in an in vitro kinase assay. To further confirm or refute the relevance of our findings in vivo, we next examined the expression levels of caveolin-1 in mammary tumors derived from c-Neu transgenic mice. Our results indicate that dramatic reduction of caveolin-1 expression occurs in mammary tumors derived from c-Neu-expressing transgenic mice and other transgenic mice expressing downstream effectors of Neu-mediated signal transduction, such as Src and Ras. Taken together, our data suggest that a novel form of reciprocal negative regulation exists between c-Neu and caveolin-1.

Dominant negative regulation by c-Jun of transcription of the uncoupling protein-1 gene through a proximal cAMP-regulatory element: a mechanism for repressing basal and norepinephrine-induced expression of the gene before brown adipocyte differentiation

The brown fat uncoupling protein-1 (ucp-1) gene is regulated by the sympathetic nervous system, and its transcription is stimulated by norepinephrine, mainly through cAMP-mediated pathways. Overexpression of the catalytic subunit of protein kinase A stimulated a chloramphenicol acetyltransferase expression vector driven by the 4.5-kb 5'-region of the rat ucp-1 gene. Mutant deletion analysis indicated the presence of the main cAMP-regulatory element (CRE) in the proximal region between -141 and -54. This region contains an element at -139/-122 able to confer enhancer and protein kinase A (PKA)-dependent activity to the basal thymidine kinase promoter. The potency of this element was much higher in differentiated than in nondifferentiated brown adipocytes. Gel shift analyses indicated that a complex array of proteins from brown fat nuclei bind to the -139/-122 element, among which CRE-binding protein (CREB) and Jun proteins were identified. In transfected brown adipocytes, c-Jun was a negative regulator of basal and PKA-induced transcription from the ucp-1 promoter acting through this proximal CRE region. A double-point mutation in the -139/-122 element abolished both PKA- and c-Jun-dependent regulation through this site, and overexpression of CREB blocked c-Jun repression. Thus, an opposite action of these two transcription factors on the -139/-122 CRE is proposed. c-Jun content in brown adipocytes differentiating in culture correlated negatively with both ucp-1 gene expression and the acquisition of the brown adipocyte morphology. These findings indicate that c-Jun provides a molecular mechanism to repress the basal and cAMP-mediated expression of the ucp-1 gene before the differentiation of the brown adipocyte.

Inhibition of cyclin D1 kinase activity is associated with E2F-mediated inhibition of cyclin D1 promoter activity through E2F and Sp1

Coordinated interactions between cyclin-dependent kinases (Cdks), their target "pocket proteins" (the retinoblastoma protein [pRB], p107, and p130), the pocket protein binding E2F-DP complexes, and the Cdk inhibitors regulate orderly cell cycle progression. The cyclin D1 gene encodes a regulatory subunit of the Cdk holoenzymes, which phosphorylate the tumor suppressor pRB, leading to the release of free E2F-1. Overexpression of E2F-1 can induce apoptosis and may either promote or inhibit cellular proliferation, depending upon the cell type. In these studies overexpression of E2F-1 inhibited cyclin D1-dependent kinase activity, cyclin D1 protein levels, and promoter activity. The DNA binding domain, the pRB pocket binding region, and the amino-terminal Sp1 binding domain of E2F-1 were required for full repression of cyclin D1. Overexpression of pRB activated the cyclin D1 promoter, and a dominant interfering pRB mutant was defective in cyclin D1 promoter activation. Two regions of the cyclin D1 promoter were required for full E2F-1-dependent repression. The region proximal to the transcription initiation site at -127 bound Sp1, Sp3, and Sp4, and the distal region at -143 bound E2F-4-DP-1-p107. In contrast with E2F-1, E2F-4 induced cyclin D1 promoter activity. Differential regulation of the cyclin D1 promoter by E2F-1 and E2F-4 suggests that E2Fs may serve distinguishable functions during cell cycle progression. Inhibition of cyclin D1 abundance by E2F-1 may contribute to an autoregulatory feedback loop to reduce pRB phosphorylation and E2F-1 levels in the cell.

Caveolin-mediated regulation of signaling along the p42/44 MAP kinase cascade in vivo. A role for the caveolin-scaffolding domain

The p42/44 mitogen-activated protein (MAP)-kinase cascade is a well-established signal transduction pathway that is initiated at the cell surface and terminates within the nucleus. More specifically, receptor tyrosine kinases can indirectly activate Raf, which in turn leads to activation of MEK and ERK and ultimately phosphorylation of Elk, a nuclear transcription factor. Recent reports have suggested that some members of p42/44 MAP kinase cascade can be sequestered within plasmalemmal caveolae in vivo. For example, morphological studies have directly shown that ERK-1/2 is concentrated in plasma membrane caveolae in vivo using immunoelectron microscopy. In addition, constitutive activation of the p42/44 MAP kinase cascade is sufficient to reversibly down-regulate caveolin-1 mRNA and protein expression. However, the functional relationship between the p42/44 MAP kinase cascade and caveolins remains unknown. Here, we examine the in vivo role of caveolins in regulating signaling along the MAP kinase cascade. We find that co-expression with caveolin 1 dramatically inhibits signaling from EGF-R, Raf, MEK-1 and ERK-2 to the nucleus. Using a variety of caveolin-1 deletion mutants, we mapped this in vivo inhibitory activity to caveolin-1 residues 32-95. Peptides derived from this region of caveolin 1 also inhibit the in vitro kinase activity of purified MEK-1 and ERK-2. Thus, we show here that caveolin-1 expression can inhibit signal transduction from the p42/44 MAP kinase cascade both in vitro and in vivo. Taken together with previous data, our results also suggest that a novel form of reciprocal negative regulation exists between p42/44 MAP kinase activation and caveolin-1 protein expression, i.e. up-regulation of caveolin-1 protein expression down-modulates p42/44 MAP kinase activity (this report) and up-regulation of p42/44 MAP kinase activity down-regulates caveolin-1 mRNA and protein expression.