Transcriptional control of c-jun by retinoic acid

Expression and localization of the transcription factor JunD in the duct system of mouse submandibular gland

We studied the expression and localization of JunD, a component of the transcription factor activator protein-1 (AP-1), in the mouse submandibular gland with immunoblotting and immunohistochemistry. In adult mice, all seven Jun and Fos family members constituting the AP-1 complex were expressed more abundantly in the female gland than in the male gland, and JunD was the most abundant of the members. Immunoreactivity for JunD was localized exclusively in the duct system of the gland, in which it was localized to the nuclei of intercalated duct (ID) cells and a subpopulation of striated duct (SD) cells located adjacent to ID. In contrast, granular convoluted tubule (GCT) cells, which are much more abundant in the male gland, were devoid of JunD. During postnatal development of the male gland, JunD was lost from the duct cells as they differentiated to GCT cells at 3-5 weeks postpartum. When GCT differentiation was induced in adult female gland by testosterone administration, many JunD-negative SD cells were temporarily induced to express JunD after 6-24 hr, but those cells lost JunD as they completely converted to GCT cells by 48 hr. These results suggested that JunD is involved in the differentiation of the duct system of mouse submandibular gland, in which there is crosstalk between the androgen/androgen receptor system and the AP-1 complex.

[Induction of cell differentiation and development of new anticancer drugs]

Cell differentiation is essential for normal growth and homeostasis, and drug-induced differentiation of tumor cells into benign or normal cells is an important approach for anticancer chemotherapy. Studies of induction mechanisms for cell differentiation and discovery of differentiation-inducing factors are thus critical components of drug development. The Screening of differentiation-inducing factors, such as purified aldehyde reductase, a xenobiotic metabolite enzyme, that induces differentiation of human acute myeloid leukemia HL60 cells into monocyte/macrophage cells is described. Mechanisms of all-trans-retinoic acid (RA)-induced differentiation are also covered. RA is a potent inducer of HL60 cell differentiation and when used as a sole agent it can induce complete remission in patients with acute promyelocytic leukemia (APL). While one mechanism of the effect of RA involves RA nuclear receptors, retinoylation (a posttranslational modification of proteins by RA) may be a new nongenomic mechanism by which RA acts on cells. An early event in RA-induced differentiation may be retinoylation of RII alpha (regulatory subunits of cAMP-dependent protein kinase), in which RII alpha units are retinoylated and the retinoylated RII alpha is then translocated to the nucleus. Drugs can also be combined with RA in RA-differentiation therapy. Cytodifferation therapy by RA in APL patients exhibits limitations due to the resistance of relapsed patients to further RA treatment. This may occur through the induction of expression of various genes that reduce RA blood concentrations. Treatment with combinations of RA and other agents may be one way to reduce induction of those genes. Good candidates for such agents include cAMP-elevating agents, retinoids, steroids, and fatty acids that synergistically induce differentiation of HL60 cells. Two derivatives of falconensone A, falconensone A p-bromophenylhydrazone, which has a bromophenyl residue, and falconensone A dioxime, which possesses a hydroxy residue, were synthesized to incorporate features of RA and N-[4-hydroxyphenyl] retinamide. Both derivatives have exhibited more potent biological activity than the parent falconensone A in vitro and in vivo.

Protein kinase Calpha expression confers retinoic acid sensitivity on MDA-MB-231 human breast cancer cells

Retinoic acid activation of retinoic acid receptor alpha (RARalpha) induces protein kinase Calpha (PKCalpha) expression and inhibits proliferation of the hormone-dependent T-47D breast cancer cell line. Retinoic acid has no effect on proliferation or PKCalpha expression in a hormone-independent, breast cancer cell line (MDA-MB-231). To test the role of PKCalpha in retinoic acid-induced growth arrest of human breast cancer cells we established MDA-MB-231 cell lines stably expressing PKCalpha. Constitutive expression of PKCalpha did not affect proliferation of MDA-MB-231 cells but did result in partial retinoic acid sensitivity. Retinoic acid treatment of PKCalpha-MDA-MB-231 cells decreased proliferation (by approximately 40%) and inhibited serum activation of MAP kinases and induction of c-fos. Similar results were seen in MDA-MB-231 cells in which transcription of the transfected PKCalpha cDNA was reversibly induced by isopropyl beta-d-thiogalactoside. Expression of RARalpha in PKCalpha expressing MDA-MB-231 cells resulted in even greater retinoic acid responses, as measured by effects on cell proliferation, inhibition of serum signaling, and transactivation of an RARE-CAT reporter plasmid. In summary, PKCalpha synergizes with activated RARalpha to disrupt serum growth factor signaling, ultimately arresting proliferation of MDA-MB-231 cells.

Suppression of constitutive but not Il-1beta-inducible expression of monocyte chemoattractant protein-1 in mesangial cells by retinoic acids: intervention in the activator protein-1 pathway

Retinoic acid regulates a wide range of biologic processes, including inflammation. This study investigated the effect of all-trans-retinoic acid (t-RA) on the constitutive and cytokine-inducible expression of monocyte chemoattractant protein 1 (MCP-1) in rat mesangial cells. Serum-deprived mesangial cells exhibited substantial levels of MCP-1 mRNA, and the expression was markedly upregulated by interleukin-1beta (IL-1beta). Pretreatment with t-RA abrogated the constitutive mRNA expression but did not inhibit the IL-1beta-inducible expression. The similar effects were observed by 9-cis-RA. The suppressive effect of t-RA required retinoic acid receptors. t-RA did not affect the stability of MCP-1 mRNA, indicating that its suppressive effect was at the transcriptional level. Experiments that used pharmacologic and genetic inhibitors showed that the IL-1beta-inducible MCP-1 expression was dependent on nuclear factor-kappaB (NF-kappaB) and independent of activator protein 1 (AP-1). In contrast, the constitutive expression of MCP-1 was dependent on both NF-kappaB and AP-1. t-RA substantially inhibited the constitutive activity of AP-1 but did not inhibit NF-kappaB activity in mesangial cells. These data suggested that (1) constitutive and IL-1beta-inducible expression of MCP-1 was differently regulated by AP-1 and NF-kappaB and (2) t-RA inhibited selectively the constitutive expression of MCP-1 via intervention in the AP-1 pathway.

Suppression of apoptosis by all-trans-retinoic acid. Dual intervention in the c-Jun n-terminal kinase-AP-1 pathway

Retinoic acid induces apoptosis of various cells, whereas little is known about its anti-apoptotic potential. In this report, we describe an anti-apoptotic property of all-trans-retinoic acid (t-RA) in mammalian cells. Mesangial cells exposed to hydrogen peroxide (H2O2) exhibited shrinkage of the cytoplasm, membrane blebbing, condensation of nuclei, and DNA fragmentation. Pretreatment with t-RA attenuated the morphologic and biochemical hallmarks of apoptosis. t-RA also inhibited apoptosis of mesangial cells triggered by pyrrolidine dithiocarbamate, whereas it did not prevent tumor necrosis factor-alpha-induced apoptosis. The anti-apoptotic effect against H2O2 was similarly observed in NRK49F fibroblasts, but not in Madin-Darby canine kidney epithelial cells and ECV304 endothelial cells. Mesangial cells exposed to H2O2 undergo apoptosis via the activator protein 1 (AP-1)-dependent pathway. We found that t-RA abrogated the H2O2-induced expression of c-fos/c-jun and activation of AP-1. Furthermore, t-RA inhibited H2O2-triggered activation of c-Jun N-terminal kinase (JNK), and dominant-negative inhibition of JNK attenuated the H2O2-induced apoptosis. These data disclosed the novel potential of retinoic acid as an inhibitor of apoptosis. The anti-apoptotic action of t-RA was ascribed, at least in part, to dual suppression of the cell death pathway mediated by JNK and AP-1.

A network of mitogen-activated protein kinases links G protein-coupled receptors to the c-jun promoter: a role for c-Jun NH2-terminal kinase, p38s, and extracellular signal-regulated kinase 5

The expression of the c-jun proto-oncogene is rapidly induced in response to mitogens acting on a large variety of cell surface receptors. The resulting functional activity of c-Jun proteins appears to be critical for cell proliferation. Recently, we have shown that a large family of G protein-coupled receptors (GPCRs), represented by the m1 muscarinic receptor, can initiate intracellular signaling cascades that result in the activation of mitogen-activated protein kinases (MAPK) and c-Jun NH2-terminal kinases (JNK) and that the activation of JNK but not of MAPK correlated with a remarkable increase in the expression of c-jun mRNA. Subsequently, however, we obtained evidence that GPCRs can potently stimulate the activity of the c-jun promoter through MEF2 transcription factors, which do not act downstream from JNK. In view of these observations, we set out to investigate further the nature of the signaling pathway linking GPCRs to the c-jun promoter. Utilizing NIH 3T3 cells, we found that GPCRs can activate the c-jun promoter in a JNK-independent manner. Additionally, we demonstrated that these GPCRs can elevate the activity of novel members of the MAPK family, including ERK5, p38alpha, p38gamma, and p38delta, and that the activation of certain kinases acting downstream from MEK5 (ERK5) and MKK6 (p38alpha and p38gamma) is necessary to fully activate the c-jun promoter. Moreover, in addition to JNK, ERK5, p38alpha, and p38gamma were found to stimulate the c-jun promoter by acting on distinct responsive elements. Taken together, these results suggest that the pathway linking GPCRs to the c-jun promoter involves the integration of numerous signals transduced by a highly complex network of MAPK, rather than resulting from the stimulation of a single linear protein kinase cascade. Furthermore, our findings suggest that each signaling pathway affects one or more regulatory elements on the c-jun promoter and that the transcriptional response most likely results from the temporal integration of each of these biochemical routes.

Inducible and constitutive transcription factors in the mammalian nervous system: control of gene expression by Jun, Fos and Krox, and CREB/ATF proteins

This article reviews findings up to the end of 1997 about the inducible transcription factors (ITFs) c-Jun, JunB, JunD, c-Fos, FosB, Fra-1, Fra-2, Krox-20 (Egr-2) and Krox-24 (NGFI-A, Egr-1, Zif268); and the constitutive transcription factors (CTFs) CREB, CREM, ATF-2 and SRF as they pertain to gene expression in the mammalian nervous system. In the first part we consider basic facts about the expression and activity of these transcription factors: the organization of the encoding genes and their promoters, the second messenger cascades converging on their regulatory promoter sites, the control of their transcription, the binding to dimeric partners and to specific DNA sequences, their trans-activation potential, and their posttranslational modifications. In the second part we describe the expression and possible roles of these transcription factors in neural tissue: in the quiescent brain, during pre- and postnatal development, following sensory stimulation, nerve transection (axotomy), neurodegeneration and apoptosis, hypoxia-ischemia, generalized and limbic seizures, long-term potentiation and learning, drug dependence and withdrawal, and following stimulation by neurotransmitters, hormones and neurotrophins. We also describe their expression and possible roles in glial cells. Finally, we discuss the relevance of their expression for nervous system functioning under normal and patho-physiological conditions.

The Role of Transcription Factor PU.I in the Activity of the Intronic Enhancer of the Eosinophil-Derived Neurotoxin (RNS2) Gene

Eosinophil-derived neurotoxin (EDN) found in the granules of human eosinophils is a cationic ribonuclease toxin. Expression of the EDN gene (RNS2) in eosinophils is dependent on proximal promoter sequences in combination with an enhancer located in the first intron. We further define here the active region of the intron using transfections in differentiated eosinophilic HL60 cells. We show that a region containing a tandem PU.I binding site is important for intronic enhancer activity. This region binds multiple forms of transcription factor PU.I as judged by gel-shift analysis and DNA affinity precipitation. Importantly, introducing point mutations in the PU.I site drastically reduces the intronic enhancer activity, showing the importance of PU.I for expression of EDN in cells of the eosinophilic lineage.

The Role of Transcription Factor PU.I in the Activity of the Intronic Enhancer of the Eosinophil-Derived Neurotoxin (RNS2) Gene

Eosinophil-derived neurotoxin (EDN) found in the granules of human eosinophils is a cationic ribonuclease toxin. Expression of the EDN gene (RNS2) in eosinophils is dependent on proximal promoter sequences in combination with an enhancer located in the first intron. We further define here the active region of the intron using transfections in differentiated eosinophilic HL60 cells. We show that a region containing a tandem PU.I binding site is important for intronic enhancer activity. This region binds multiple forms of transcription factor PU.I as judged by gel-shift analysis and DNA affinity precipitation. Importantly, introducing point mutations in the PU.I site drastically reduces the intronic enhancer activity, showing the importance of PU.I for expression of EDN in cells of the eosinophilic lineage.

Epidermal growth factor induction of the c-jun promoter by a Rac pathway

The c-jun proto-oncogene encodes a transcription factor which is activated by mitogens both transcriptionally and by phosphorylation by Jun N-terminal kinase (JNK). We have investigated the cellular signalling pathways involved in epidermal growth factor (EGF) induction of the c-jun promoter. We find that two sequence elements, which bind ATF1 and MEF2D transcription factors, are required in HeLa cells, although they are not sufficient for maximal induction. Activated forms of Ras, RacI, Cdc42Hs, and MEKK increased expression of the c-jun promoter, while dominant negative forms of Ras, RacI, and MEK kinase (MEKK) inhibited EGF induction. These and previously published results suggest that EGF activates the c-jun promoter by a Ras-to-Rac-to-MEKK pathway. This pathway is similar to that used for posttranslational activation of c-jun by JNK.

Expressions of c-jun and jun-B proto-oncogenes in odontoblasts during development of bovine tooth germs

c-jun and jun-B genes are among the nuclear proto-oncogenes induced by growth factors such as the TGF-beta superfamily and play important roles in cell differentiation. These gene products enhance expressions of proteins including osteocalcin, alkaline phosphatase, and collagens. On the other hand, it is well-known that the TGF-beta superfamily affects odontoblast differentiation, and that differentiated odontoblasts express extracellular and membrane proteins as described above. However, there are few reports of factors that participate in the transcriptional regulation of odontoblasts. Especially, little is known about the expression of c-jun and jun-B genes. In this study, we focused on the examination of expressions of c-jun and jun-B genes in dental papillae of bovine tooth germs. Using in situ hybridization, we found that these genes were expressed only in the odontoblastic lineage, but not in other dental papilla cells. Levels of c-jun and jun-B mRNAs increased along the gradient of differentiation of odontoblasts. These levels of c-jun mRNAs were maintained in both young and mature odontoblasts. However, unlike the c-jun gene, expression of the jun-B gene became sparse in mature odontoblasts compared with young odontoblasts. For further analysis, Northern hybridization of total RNA extracted from differentiated odontoblasts was performed for the examination of levels of jun-B mRNAs, indicating that levels of jun-B mRNAs of mature odontoblasts were clearly less than those of young odontoblasts. These results suggest that c-jun and jun-B genes may participate in the transcriptional regulation of odontoblasts of bovine tooth germs, and may control the odontoblast phenotype. Furthermore, our results suggest that these genes can be markers of odontoblasts during dentinogenesis; especially, high expression of jun-B gene can be a marker of young odontoblasts that start to form the new dentin matrix.

Regulation of keratinocyte proliferation and differentiation by all-trans-retinoic acid, 9-cis-retinoic acid and 1,25-dihydroxy vitamin D3

We studied the effect of all-trans retinoic acid (all-trans-RA), 9-cis-retinoic acid (9-cis-RA) and 1,25-dihydroxy vitamin D3 (1,25(OH)2D3) on proliferation and differentiation of human keratinocytes cultured in a submerged culture system for up to 5 weeks and evaluated changes in cell morphology and in the expression of proliferation- and terminal differentiation-related genes on both the mRNA and the protein levels. Under control culture conditions, the expression of small proline-rich proteins (SPRR1 and SPRR2), involucrin, Ki67 and c-jun reached a maximum after 2 weeks in culture (1 week postconfluence) and then decreased as the tissue architecture of the cultures deteriorated. Upon simultaneous treatment with both retinoids and 1,25(OH)2D3 a culture was generated that remained stable for 4 weeks with at least eight living cell layers. Furthermore, this culture showed a pattern of SPRR2 and involucrin expression which closely resembled that of native epidermis, a maintained Ki67 expression and a strongly induced c-jun expression. Treatment with 1,25(OH)2D3 alone inhibited cell proliferation and stimulated cell differentiation resulting in acceleration of the differentiated phenotype and was accompanied by inhibition of c-jun and Ki67 expression and also, surprisingly by inhibition of SPRR1, SPRR2 and involucrin expression. In contrast, treatment with all-trans-RA and/or 9-cis-RA induced a more proliferative phenotype with a prolonged lifespan as compared to control cultures. SPRR1 was weakly repressed, SPRR2 was strongly repressed, a delayed induction of involucrin occurred, and c-jun and Ki67 expression were maintained. These results show that modulation of the composition of the medium by the addition of various vitamins results in changes in the balance between keratinocyte proliferation and differentiation which correspond to changes in the expression of proliferation and differentiation markers and prolongation of the culture lifespan.

AP-1, ETS, and transcriptional silencers regulate retinoic acid-dependent induction of keratin 18 in embryonic cells

The differentiation of both embryonal carcinoma (EC) and embryonic stem (ES) cells can be triggered in culture by exposure to retinoic acid and results in the transcriptional induction of both the endogenous mouse keratin 18 (mK18) intermediate filament gene and an experimentally introduced human keratin 18 (K18) gene as well as a variety of other markers characteristic of extraembryonic endoderm. The induction of K18 in EC cells is limited, in part, by low levels of ETS and AP-1 transcription factor activities which bind to sites within a complex enhancer element located within the first intron of K18. RNA levels of ETS-2, c-Jun, and JunB increase upon the differentiation of ES cells and correlate with increased expression of K18. Occupancy of the ETS site, detected by in vivo footprinting methods, correlates with K18 induction in ES cells. In somatic cells, the ETS and AP-1 elements mediate induction by a variety of oncogenes associated with the ras signal transduction pathway. In EC cells, in addition to the induction by these limiting transcription factors, relief from negative regulation is mediated by three silencer elements located within the first intron of the K18 gene. These silencer elements function in F9 EC cells but not their differentiated derivatives, and their activity is correlated with proteins in F9 EC nuclei which bind to the silencers and are reduced in the nuclei of differentiated F9 cells. The induction of K18, associated with the differentiation of EC cells to extraembryonic endoderm, is due to a combination of relief from negative regulation and activation by members of the ETS and AP-1 transcription factor families.

The reversion of highly tumorigenic cell lines to non-tumorigenic phenotype is associated with c-jun down-expression

Using model spontaneously reverting cell lines, c-jun, junB, junD and c-fos oncogene expression was investigated. c-jun, but not junB, junD or c-fos, was overexpressed in highly tumorigenic clones. The reversion of cells to the non-tumorigenic phenotype resulted in a dramatic decrease in c-jun expression. CAT assays revealed that c-jun overexpression in tumorigenic cells was associated with higher transcription activity. No correlation between c-jun oncogene expression and AP-1 transcription factor activity in tumorigenic and non-tumorigenic clones was found.

AP-1, ETS, and transcriptional silencers regulate retinoic acid-dependent induction of keratin 18 in embryonic cells

The differentiation of both embryonal carcinoma (EC) and embryonic stem (ES) cells can be triggered in culture by exposure to retinoic acid and results in the transcriptional induction of both the endogenous mouse keratin 18 (mK18) intermediate filament gene and an experimentally introduced human keratin 18 (K18) gene as well as a variety of other markers characteristic of extraembryonic endoderm. The induction of K18 in EC cells is limited, in part, by low levels of ETS and AP-1 transcription factor activities which bind to sites within a complex enhancer element located within the first intron of K18. RNA levels of ETS-2, c-Jun, and JunB increase upon the differentiation of ES cells and correlate with increased expression of K18. Occupancy of the ETS site, detected by in vivo footprinting methods, correlates with K18 induction in ES cells. In somatic cells, the ETS and AP-1 elements mediate induction by a variety of oncogenes associated with the ras signal transduction pathway. In EC cells, in addition to the induction by these limiting transcription factors, relief from negative regulation is mediated by three silencer elements located within the first intron of the K18 gene. These silencer elements function in F9 EC cells but not their differentiated derivatives, and their activity is correlated with proteins in F9 EC nuclei which bind to the silencers and are reduced in the nuclei of differentiated F9 cells. The induction of K18, associated with the differentiation of EC cells to extraembryonic endoderm, is due to a combination of relief from negative regulation and activation by members of the ETS and AP-1 transcription factor families.

Positive and negative regulation of the composite octamer motif of the interleukin 2 enhancer by AP-1, Oct-2, and retinoic acid receptor

The differentiating agent retinoic acid (RA) has been previously reported to interfere with 12-O-tetradecanoyl-phorbol-13-acetate (TPA)/Ca2+-induced signals for the regulation of the -96 to -66-bp octamer motif found in the enhancer for the interleukin (IL)-2 gene, which encodes a major T lymphocyte growth factor. The IL-2 octamer motif is a composite cis-element which binds Oct-1 and Oct-2 as well as a TPA/Ca2+-inducible nuclear factor, previously termed octamer-associated protein (OAP40). We show here that Oct-2, despite the presence of an active transcriptional activation domain, requires TPA/Ca2+-induced signals to strongly transactivate the IL-2 octamer motif in Jurkat T cells. This Oct-2-dependent transactivation is inhibited by RA. The presence of an intact COOH-terminal domain of Oct-2 contributes to both TPA/Ca2+-induced transactivation and the RA-mediated repression. We also show that both Fos and Jun components of the AP-1 factors participate in the OAP40 complex. Furthermore, transfected c-jun, jun-B, jun-D, c-fos, or Fos-B expression vectors partially substitute for TPA and Ca2+ and cooperate with Oct-2 for the transactivation of the combined OAP/octamer cis-element. Mutations of the genuine octamer-binding site abrogate both the binding of Oct-1 and Oct-2 and the TPA/Ca2+-induced transactivation of the OAP/octamer motif. OAP confers to Oct-2 responsivity to both TPA/Ca2+ and RA, since specific mutations of the AP-1/OAP-binding site significantly reduce the transactivation by Oct-2 in response to TPA and Ca2+ and abolish the inhibition by RA. Furthermore, retinoic acid receptor (RAR) alpha is able to inhibit in vitro the formation of the complex between the nuclear AP-1/OAP and its specific binding site, resulting in the interference with Oct-2-dependent cis-regulatory function of this AP-1 element. Therefore, we propose that the TPA/calcium-activated AP-1/OAP element is the main target of positive or negative regulatory signals influencing the IL-2 octamer motif, through synergism with Oct-2 and antagonism by RAR.

Regulation of intercellular adhesion molecule-1 expression by retinoic acid: analysis of the 5' regulatory region of the gene

Human intercellular adhesion molecule-1 (ICAM-1), a specific ligand for the lymphocyte-function-associated antigen-1, plays an important role in immune responses. ICAM-1 expression is regulated by various proinflammatory cytokines, by PMA, and by retinoic acid. In this study, we have investigated the mechanisms of transcriptional control involved in the stimulation of ICAM-1 gene expression by retinoic acid in SK-N-SH cells. Northern-blot analysis demonstrated that ICAM-1 mRNA is maximally induced at 24 hr, suggesting that it is not an early-response gene with respect to retinoic-acid responsiveness, whereas the retinoic acid receptor-beta mRNA level was maximal 12 hr following retinoic acid treatment. To analyze the 5'-regulatory region of the ICAM-1 gene, an EcoRI/SaII fragment spanning the first 1.3 kb upstream of the translational start site was used to direct the expression of a linked luciferase reporter gene in transient transfection assays in SK-N-SH cells. A 24-hr treatment of transfected cells with 10 microM retinoic acid resulted in a 10- to 13-fold increase in luciferase activity compared with untreated cells. Deletion mutant analysis revealed that a region located between -393 and -176 bp from the translational start site is critical for retinoic acid stimulation of luciferase activity. This region harbors a consensus sequence for a retinoic-acid-responsive element (RARE) homologous to the element found upstream of the alcohol dehydrogenase-3 gene. Co-transfection of expression vectors encoding the retinoic acid receptor-alpha, -beta, or -gamma, with reporter plasmids harboring the putative RARE, confirmed that the ICAM-1 gene is regulated by retinoic acid in a retinoic acid receptor-dependent fashion.

Altered gene expression in human leukemia K562 cells selected for resistance to etoposide

Sublines of K562 human leukemia cells were selected for resistance (30- to 80-fold) to etoposide by continuous exposure to 0.5 microM VP-16. Two etoposide-resistant cell lines, K/VP.5 and K/VP.5-1, showed a 5-fold reduction in levels of topoisomerase II alpha protein compared with K562 cells. Northern analysis indicated a 2.5-fold reduction in topoisomerase II alpha mRNA in etoposide-resistant cell lines, due in part to a 1.7-fold decrease in topoisomerase II mRNA stability with no change in transcription rate. Immunoblotting assays of electrophoresed cell lysates from VP-16-treated cells revealed less drug-induced covalent topoisomerase II/DNA adducts in resistant than in sensitive cells, suggesting a functional alteration in resistant cell topoisomerase II. Recent reports of specific topoisomerase II DNA binding sites near the promoter sites of growth response genes and alterations of gene expression in cells treated with topoisomerase II inhibitory drugs led to experiments to determine if the apparent functional alterations of topoisomerase II were accompanied by changes in the regulation of these genes. Therefore, the expression of several growth response genes was compared by northern analysis in parental K562 and both VP-16-resistant cell lines. Basal levels of c-myc were comparable for all three cell lines, but levels of c-jun and c-fos were elevated 2- to 4-fold in VP-16-resistant cell lines. Increased levels of c-fos and c-jun were not a result of altered rates of transcription, as determined by nuclear run-off assays. Exposure of both sensitive and resistant cells to 200 microM VP-16 for 5 hr resulted in no further changes in topoisomerase II mRNA levels but caused an additional 2- to 3-fold elevation in the level of c-jun mRNA, indicating that altered basal levels of this gene were not due to deregulation of this gene. Acquired VP-16 resistance in K/VP.5 and K/VP.5-1 cells was accompanied by reduced levels and altered activities of DNA topoisomerase II as well as changes affecting the expression of genes important for growth and differentiation.

Down-regulation of retinoic acid receptor activity associated with decreased alpha and gamma isoforms expression in F9 embryonal carcinoma cells differentiated by retinoic acid

F9 embryonal carcinoma cells differentiate in response to retinoic acid (RA). To investigate the regulation of RA receptors (RARs) expression during this process, cDNA probes specific for the major RAR isoforms were used. In contrast to the level of RAR beta 2 mRNA which was high in cells treated 5 days with RA and below detection in untreated cells, as previously described, the steady state levels of RAR alpha 1, alpha 2, gamma 1, and gamma 2 mRNAs were markedly decreased in the RA-differentiated cells as compared to untreated cells. The down-regulation of the RA-responsive system in differentiated cells was also evident in gel shift assays as a marked decrease in binding capacity to a retinoid acid response element (beta 2RARE), as well as in chloramphenicol acetyltransferase (CAT) assays as a sixfold decrease in RA-mediated transacting activity via this element. The down-regulation of RAR DNA-binding and transacting activity coincided with the burst in tissue plasminogen activator secretion and thus, occurred at the hinge between early and late differentiation. The down-regulation of RA responsiveness may constitute an important event in the transition between early and late differentiation stage in F9 cells.

Cooperation between retinoic acid and phorbol esters enhances human teratocarcinoma differentiation

This study explored cooperation between the retinoic acid (RA) and protein kinase C (PKC) pathways during differentiation of the multipotential human teratocarcinoma (TC) cell line NTERA-2 clone D1 (abbreviated NT2/D1). We report here that, compared to RA treatment alone, RA combined with the PKC stimulator 12-O-tetradecanoylphorbol-13-acetate (TPA) enhanced the regulated expression of the immunophenotypic differentiation markers SSEA-3, a globo-series carbohydrate, and the ganglio-series carbohydrate antigens GD2 and GD3. Northern analysis and transient transfection assays revealed that TPA co-treatment augmented the RA-induced expression and activation of the RA nuclear receptor-beta (RAR-beta), one early marker of RA response in NT2/D1 cells. This finding was extended with transient co-transfection experiments using a PKC-alpha expression vector which revealed that the PKC pathway can augment the activation of RAR-beta by RA. These experiments establish PKC as a modulator of RAR-beta expression in NT2/D1 cells. Similarly, experiments showed that RA can modulate activation of the PKC-responsive AP-1 complex, a transcription factor rapidly activated by TPA. Northern analysis and transient transfection assays revealed that, compared to TPA treatment alone, RA and TPA augmented the expression and transcriptional activity of AP-1 in NT2/D1 cells. In contrast, transient transfection assays revealed no cooperative effect between RA and TPA in HeLa cells, indicating that this effect in NT2/D1 cells is cell type-specific. In summary, these studies show that stimulation of the PKC second messenger pathway can modulate tumor differentiation and transcriptional activation of a retinoid receptor associated with RA response.

Transactivation of mouse insulin-like growth factor II (IGF-II) gene promoters by the AP-1 complex

The mouse insulin-like growth factor II gene (Igf2) is transcribed from three promoters (P1, P2 and P3), and is expressed in a tissue-specific and developmentally regulated fashion; however, little information is available on the transcription factors controlling Igf2 expression. The AP-1 complex is a transcription factor involved in the regulation of a variety of genes, including those encoding certain growth factors. We show that Igf2 P3 is transactivated by AP-1 in a transient expression assay, and that this effect is mediated through two non-consensus AP-1 binding sites characterised by DNA-protein interaction studies. Mutational analysis indicates these sites are required for AP-1 responsiveness and full promoter activity.

Mineralocorticoid and glucocorticoid receptor activities distinguished by nonreceptor factors at a composite response element

Mineralocorticoid and glucocorticoid hormones elicit distinct physiologic responses, yet the mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) bind to and activate transcription similarly from a consensus simple hormone response element (HRE). The activities of GR and MR at plfG, a 25-base pair composite response element to which both the steroid receptors and transcription factor AP1 can bind, are analyzed here. Under conditions in which GR represses AP1-stimulated transcription from plfG, MR was inactive. With the use of MR-GR chimeras, a segment of the NH2-terminal region of GR (amino acids 105 to 440) was shown to be required for this repression. Thus, the distinct physiologic effects mediated by MR and GR may be determined by differential interactions of nonreceptor factors with specific receptor domains at composite response elements.

Terminal differentiation in keratinocytes involves positive as well as negative regulation by retinoic acid receptors and retinoid X receptors at retinoid response elements

Terminal differentiation of epidermal keratinocytes is inhibited by 1 microM retinoic acid, a concentration which induces differentiation in a number of cell types, including F9 teratocarcinoma cells. The molecular basis for these opposing retinoid responses is unknown, although retinoic acid receptors (RARs) and retinoid X receptors (RXRs) have been detected in both cell types. When F9 cells are stably transfected with a truncated RAR alpha lacking the E/F domain necessary for ligand binding and RAR/RXR dimerization, action at retinoid response elements is suppressed and cells produce a retinoic acid-resistant phenotype; i.e., they are blocked in differentiation (A. S. Espeseth, S. P. Murphy, and E. Linney, Genes Dev. 3:1647-1656, 1989). If retinoid receptors influence epidermal differentiation only in a negative fashion, then suppression of transactivation at retinoid response elements would be expected to enhance, rather than block, keratinocyte differentiation. In this study, we show that surprisingly, even though constitutive expression of an analogous truncated RAR gamma in keratinocytes specifically suppressed transactivation at retinoid response elements, keratinocytes were blocked, rather than enhanced, in their ability to undergo morphological and biochemical features of differentiation. These findings demonstrate a direct and hitherto unrecognized role for RARs and RXRs in positively as well as negatively regulating epidermal differentiation. Additionally, our studies extend those of Espeseth et al. (Genes Dev. 3:1647-1656, 1989), indicating a novel RAR function independent of the E/F domain.

Differentiation and proliferation in mouse embryonal carcinoma cells

How cell commitment and differentiation are controlled in the early stages of embryogenesis is a problem that has long fascinated developmental biologists. Retinoic acid-induced differentiation of embryonal carcinoma cells in culture provides a model in which these questions can be explored. Recent work has yielded exciting insights into the central series of molecular changes which drives the commitment of these cells to formation of a new phenotype. Interacting with the key molecules in this central pathway is a variety of transcription factors, many of which show changes in availability and/or activity during differentiation. In various combinations, these modulate the activities of genes involved in both cell proliferation and in the production of extracellular matrix and other proteins characteristics of differentiated cells.

Terminal differentiation in keratinocytes involves positive as well as negative regulation by retinoic acid receptors and retinoid X receptors at retinoid response elements

Terminal differentiation of epidermal keratinocytes is inhibited by 1 microM retinoic acid, a concentration which induces differentiation in a number of cell types, including F9 teratocarcinoma cells. The molecular basis for these opposing retinoid responses is unknown, although retinoic acid receptors (RARs) and retinoid X receptors (RXRs) have been detected in both cell types. When F9 cells are stably transfected with a truncated RAR alpha lacking the E/F domain necessary for ligand binding and RAR/RXR dimerization, action at retinoid response elements is suppressed and cells produce a retinoic acid-resistant phenotype; i.e., they are blocked in differentiation (A. S. Espeseth, S. P. Murphy, and E. Linney, Genes Dev. 3:1647-1656, 1989). If retinoid receptors influence epidermal differentiation only in a negative fashion, then suppression of transactivation at retinoid response elements would be expected to enhance, rather than block, keratinocyte differentiation. In this study, we show that surprisingly, even though constitutive expression of an analogous truncated RAR gamma in keratinocytes specifically suppressed transactivation at retinoid response elements, keratinocytes were blocked, rather than enhanced, in their ability to undergo morphological and biochemical features of differentiation. These findings demonstrate a direct and hitherto unrecognized role for RARs and RXRs in positively as well as negatively regulating epidermal differentiation. Additionally, our studies extend those of Espeseth et al. (Genes Dev. 3:1647-1656, 1989), indicating a novel RAR function independent of the E/F domain.

Mapping of epidermal growth factor-, serum-, and phorbol ester-responsive sequence elements in the c-jun promoter

Expression of the nuclear proto-oncogene c-jun is rapidly and transiently induced by many growth factors, serum, and tumor promoters. The sequence elements in the c-jun promoter involved in serum or growth factor induction have not been identified. The c-jun promoter region between -117 and -72 contains binding sites for the transcription factors Sp1, CTF, and AP-1. An additional sequence element has been noted at position -59. This A+T-rich sequence, formerly proposed as a TFIID-binding site, conforms to the consensus binding sequence of a recently identified factor, RSRF (related to serum response factor). In this study, we mapped the sequences in the c-jun promoter responsible for epidermal growth factor (EGF), serum, and 12-O-tetradecanoylphorbol-13-acetate (TPA) induction by deletion and point mutational analysis. We found that the c-jun RSRF site is an important element for EGF and serum induction of the promoter and that there are several factors in HeLa nuclear extracts which specifically bind to this site. The RSRF site was also sufficient for EGF, serum, and TPA induction when assayed on a heterologous promoter. The c-jun AP-1 site was not required for EGF, serum, or TPA induction but was sufficient to mediate a weak response to these agents when assayed on a heterologous promoter. Double mutation of the RSRF and AP-1 sites suggests that there is an additional TPA-responsive element between -80 and +150 in the c-jun promoter.

Mapping of epidermal growth factor-, serum-, and phorbol ester-responsive sequence elements in the c-jun promoter

Expression of the nuclear proto-oncogene c-jun is rapidly and transiently induced by many growth factors, serum, and tumor promoters. The sequence elements in the c-jun promoter involved in serum or growth factor induction have not been identified. The c-jun promoter region between -117 and -72 contains binding sites for the transcription factors Sp1, CTF, and AP-1. An additional sequence element has been noted at position -59. This A+T-rich sequence, formerly proposed as a TFIID-binding site, conforms to the consensus binding sequence of a recently identified factor, RSRF (related to serum response factor). In this study, we mapped the sequences in the c-jun promoter responsible for epidermal growth factor (EGF), serum, and 12-O-tetradecanoylphorbol-13-acetate (TPA) induction by deletion and point mutational analysis. We found that the c-jun RSRF site is an important element for EGF and serum induction of the promoter and that there are several factors in HeLa nuclear extracts which specifically bind to this site. The RSRF site was also sufficient for EGF, serum, and TPA induction when assayed on a heterologous promoter. The c-jun AP-1 site was not required for EGF, serum, or TPA induction but was sufficient to mediate a weak response to these agents when assayed on a heterologous promoter. Double mutation of the RSRF and AP-1 sites suggests that there is an additional TPA-responsive element between -80 and +150 in the c-jun promoter.

Expression of the B cell-associated tyrosine kinase gene Lyn in primary neuroblastoma tumours and its modulation during the differentiation of neuroblastoma cell lines

The src-related intracellular protein tyrosine kinase Lyn is a signal transducing molecule for surface immunoglobulin M and is expressed predominantly in hemopoietic cells. We report here the expression of the lyn gene in human neuroblastoma. In surgical tumour samples lyn transcripts were found preferentially at early stages whereas they were barely detectable in highly malignant tumours. In a cloned human neuroblastoma cell line, Be(2)C, lyn mRNA levels increased during neuronal differentiation induced by retinoic acid. Lyn mRNA levels were undetectable and did not respond to retinoic acid in a glial-type neuroblastoma clone, SH-EP. Retinoic acid-induced glial differentiation was associated with a reduction of lyn transcripts in a clonal I-type neuroblastoma cell line, SH-IN, which shares properties of both neuronal- and glial-type clones. Like pp60c-src Lyn may be involved in a signalling pathway of neuroblasts committed to neuronal differentiation.

Activin synergistically increased c-jun mRNA in P19 embryonal carcinoma cells in the presence of retinoic acid

Activin and retinoids, which are involved in the induction and regulation of the early differentiation process in vertebrate embryogenesis, synergistically increased the amount of c-jun mRNA in P19 embryonal carcinoma cells, but activin alone had no effect. Among the retinoids, all-trans-retinoic acid most effectively increased c-jun mRNA. Activin (lng/ml) was sufficient to induce the synergistic increase of c-jun mRNA in P19 EC cells with all-trans-retinoic acid. The synergistic increase of the amount of c-jun mRNA by their cooperative action may be important in vertebrate development.

Upstream regions of the c-jun promoter regulate phorbol ester-induced transcription in U937 leukemic cells

To understand the mechanism by which phorbol esters (PMA) stimulate c-jun transcription in human leukemic cell line U937, we have mutated specific enhancer sequences within the c-jun promoter. We find in the region of DNA from -132 to +170 containing Sp1, C-TF and AP-1 sequences that mutation of the AP-1 sequence alone is not sufficient to abrogate transcription, and mutation of the Sp1 sequence increases transcription 4-fold. Although mutation of the CTF site had no effect, CTF and AP-1 mutations together totally abrogate PMA-induced transcription. In comparison mutations of either of these sites alone or together in a construct containing -1639/+740 of the c-jun promoter had no effect on transcription. Because this data suggested the possibility of other upstream control regions, we sequenced the promoter from -142 to -1639. This sequence demonstrates a greater than 70% homology between human, and mouse c-jun promoters for the region from -142 to -441, and a second AP-1-like site in the -183 to -192 region. Mutation of this site did not influence transcription by PMA. By making constructs containing varying portions of the promoter, we have identified the region between -142 and -711 to be responsible for mediating PMA-induced c-jun transcription.

JunB does not inhibit the induction of c-Jun during the retinoic acid induced differentiation of F9 cells

JunB, a member of the jun gene family of transcription factors, is distinguished from c-Jun by its differential activity on certain arrangements of promoter regulatory elements and the ability of JunB to inhibit the action of cJun in both transforming and trans-activating assays. We have tested the potential negative regulatory role of JunB during the retinoic acid induced differentiation of F9 murine embryonal carcinoma cells. Constitutive expression of high levels of JunB in F9 cells failed to inhibit the differentiation dependent induction of c-Jun or the coincident expression of differentiation markers keratin 8 and 18, tissue plasminogen activator, and laminin B1. Among these marker genes, keratin 18, has been shown to contain an AP-1 binding site, TGA(C/G)TCA, which is essential for high level, differentiation dependent expression and which is transactivated by Jun and Fos proteins. These results suggest that JunB does not play a major negative or positive regulatory role during the retinoic acid induced differentiation of F9 cells.

EGF-induced jun B-expression in transfected P19 embryonal carcinoma cells expressing EGF-receptors is dependent on Jun D

The TPA-inducible transcription factor AP-1, consisting of homo- or hetero-dimers of members of the Jun- and Fos-families, regulates transcription of a wide variety of genes containing the TPA response element (TRE). In P19 embryonal carcinoma (EC) cells, Jun D is the only component of AP-1 expressed, while in these cells until now none of the members of the jun- and fos-families have been found to be inducable by external stimuli. Here we demonstrate that Jun B is the only member of the Jun- and Fos-families that is induced by Epidermal Growth Factor (EGF) in transfected murine P19 EC cells, expressing functional human EGF receptors (hEGF-Rs). Induction of jun B can be mimicked in wild type P19 EC cells by the synergistic action of the phorbol ester TPA and the calcium ionophore A23187, through activation of signal transduction pathways, that are activated simultaneously by EGF. The EGF induced jun B expression in the hEGF-R expressing P19 EC cells is mediated by an inverted repeat (IR) sequence in the jun B promoter, previously shown to be responsive to both PKC and PKA signal transduction. Transactivation of the IR sequence by EGF can be blocked completely by prior expression of antisense Jun D, but not by antisense c-Jun. These studies therefore implicate Jun D in the regulation of immediate early gene expression by external stimuli.