'Nuclear' oncogenes convert extracellular stimuli into changes in the genetic program

Genetic Control of Flower Development by Homeotic Genes in Antirrhinum majus

Homeotic mutants have been useful for the study of animal development. Such mutants are also known in plants. The isolation and molecular analysis of several homeotic genes in Antirrhinum majus provide insights into the underlying molecular regulatory mechanisms of flower development. A model is presented of how the characteristic sequential pattern of developing organs, comprising the flower, is established in the process of morphogenesis.

Involvement of Ras and AP-1 in Helicobacter pylori-induced expression of COX-2 and iNOS in gastric epithelial AGS cells

Helicobacter pylori (H. pylori) is an important risk factor for chronic gastritis, peptic ulcer, and gastric cancer. The genetic differences of H. pylori isolates play a role in the clinical outcome of the infection. Inflammatory genes including cyclooxygenase-2 (COX-2) and inducible nitric oxide synthase (iNOS) are involved in H. pylori gastritis. Transcription factor AP-1 is composed of c-Fos and c-Jun and mediates inflammation and carcinogenesis. Ras acts as a regulator for AP-1 activation in various cells. We investigated whether H. pylori in a Korean isolate (HP99), a cagA ( + ), vacA ( + ) strain, induces the expression of c-Fos and c-Jun for AP-1 activation to induce COX-2 and iNOS and whether HP99-induced expressions of COX-2 and iNOS are mediated by Ras and AP-1, determined by the expressions of c-Fos and c-Jun, in gastric epithelial AGS cells, using transfection with mutant genes for Ras (ras N-17) and c-Jun (TAM-67). As a result, HP99 induced the expression of c-Fos and c-Jun and the expressions of COX-2 and iNOS in AGS cells. Transfection with mutant genes for Ras or c-Jun suppressed HP99-induced expressions of COX-2 and iNOS in AGS cells. In conclusion, H. pylori in a Korean isolate induces the expression of COX-2 and iNOS via AP-1 activation, which may be mediated by Ras and the expression of c-Fos and c-Jun in gastric epithelial cells.

Natural biology of polyomavirus middle T antigen

"It has been commented by someone that 'polyoma' is an adjective composed of a prefix and suffix, with no root between--a meatless linguistic sandwich" (C. J. Dawe). The very name "polyomavirus" is a vague mantel: a name given before our understanding of these viral agents was clear but implying a clear tumor life-style, as noted by the late C. J. Dawe. However, polyomavirus are not by nature tumor-inducing agents. Since it is the purpose of this review to consider the natural function of middle T antigen (MT), encoded by one of the seemingly crucial transforming genes of polyomavirus, we will reconsider and redefine the virus and its MT gene in the context of its natural biology and function. This review was motivated by our recent in vivo analysis of MT function. Using intranasal inoculation of adult SCID mice, we have shown that polyomavirus can replicate with an MT lacking all functions associated with transformation to similar levels to wild-type virus. These observations, along with an almost indistinguishable replication of all MT mutants with respect to wild-type viruses in adult competent mice, illustrate that MT can have a play subtle role in acute replication and persistence. The most notable effect of MT mutants was in infections of newborns, indicating that polyomavirus may be highly adapted to replication in newborn lungs. It is from this context that our current understanding of this well-studied virus and gene is presented.

Heat shock factor 1 represses Ras-induced transcriptional activation of the c-fos gene

Heat shock factor 1, the critical molecular regulator of the stress response is conserved throughout eukaryotic organisms and activates the transcription of heat shock genes. We now show that heat shock factor 1 inhibits the expression of c-fos, an immediate early gene that controls responses to extracellular stimuli for growth and differentiation. Heat shock factor 1 inhibits the transcription of the c-fos gene and antagonizes the activating effects of the signal transducing protein Ras on the c-fos promoter and on the promoter of another Ras responsive gene uPA. This property was specific for heat shock factor 1; c-fos repression was not seen with the structurally related protein heat shock factor 2. Repression involved different molecular mechanisms compared with those involved in transcriptional activation by heat shock factor 1 and specifically did not require binding to the c-fos promoter. Thus, in addition to its known role as a transcriptional activator of the cellular heat shock response, heat shock factor 1 also antagonizes the expression of Fos, a key component of the ubiquitous AP-1 transcription factor complex and as such could influence multiple aspects of cell regulation.

Systemic administration of an anabolic dose of prostaglandin E2 induces early-response genes in rat bones

Systemic administration of prostaglandins of the E series (PGEs) has an anabolic effect in bone. A large part of this osteogenic effect is due to recruitment of osteoblasts from their precursors. However, the immediate events initiated by the administration of an anabolic dose of PGEs or their target cells within bone tissue are not known. In this study we used Northern analysis to explore the induction of early-response genes in bone tissue following a single injection of an anabolic dose of PGE2 (6 mg/kg) and in situ hybridization to localize the responding cells. The mRNA levels of c-fos, c-jun, junB and early growth response gene-1 were markedly elevated in the tibial metaphysis as early as 15 min postinjection and returned to basal level by 180-300 min. The induction of c-fos was the earliest (significant at 15 min) and the greatest (sixfold at 60 min) and that of the other genes was smaller. Early-response gene expression was induced in the calvaria as well. Numerous cells in bone marrow (both in the tibia and calvaria) expressed high levels of c-fos in response to PGE2. In the tibia, these cells were localized in the secondary spongiosa and diaphysis and were absent from the primary spongiosa. Many, but not all, expressing cells were in relative proximity to cancellous or endosteal surfaces. In the calvaria, these cells were found in the marrow "windows" within the bony plate. Mature osteoblasts and osteoclasts were negative. Based on many reports of the stimulation of cancellous bone formation in tibiae of similar animals by PGE2 and the increased bone formation we found in the calvarial marrow spaces, the best candidate for these cells is a bone marrow-resident osteoblast precursor. The induction of early-response genes may thus be the first step in a chain of events which leads to the anabolic effect of PGE2 in vivo.

Expression of c-fos and jun protooncogenes in ovine trophoblasts in relation to interferon-tau expression and early implantation process

Expression of c-fos and jun protooncogenes was analyzed in the ovine extraembryonic trophoblast from days 14-18 of gestation, using Northern and Western blotting and immunohistochemistry. This study was carried out in relation to the early implantation process and the expression of interferon-tau, which is secreted in large amounts for a few days before implantation. Our results demonstrated that c-fos, c-jun, and junB were differently expressed in the ovine trophoblast around the time of implantation. The c-fos mRNA and protein were detected at high levels prior to attachment and decreased thereafter, following the pattern of expression of interferon-tau, whereas c-jun expression was maintained at relatively high levels during the implantation process. By contrast, the levels of junB mRNA and protein decreased prior to attachment. Immunohistochemical studies indicated that JunB, like C-Fos and interferon-tau, was no longer expressed in the trophoblastic cells which had established cellular contacts with the uterine epithelium. A striking finding in this study is the temporal correlation between the accumulation of c-Fos and c-Jun proteins and the expression of the interferon-tau (days 14 and 15 of gestation). We also showed by gel-retardation assays that an AP-1-like site present in the promoter of one interferon-tau gene was functional in vitro, as judged by its ability to bind day-15 trophoblast nuclear protein extracts. Nuclear proteins binding to this site had the characteristics of AP-1, as judged by the ability to be competed efficiently by a consensus TRE (12.0-tetradecanoyl phorbol 13-acetate-responsive element)-site oligonucleotide and by antibodies to c-Fos and Jun proteins. These results suggest that Fos and Jun could form regulatory complexes of interferon-tau expression and/or are regulated by common mechanisms which are still unknown.

Regulation of bradykinin B2 receptors by the ras oncogene: evidence for multiple mechanisms

The objective of this study was to probe the molecular mechanisms underlying the increase in sensitivity of cells to bradykinin (BK) following expression of a transforming Ha-ras oncogene. We used native NIH3T3 fibroblast (3T3) cells and 3T3 cells transfected with a glucocorticoid sensitive oncogenic Ha-ras construct (DT cells). DT cells incubated in the presence of 1 microM dexamethasone (DEX) for 24 hr expressed a relatively high level of membrane-bound Ha-Ras protein, BK B2 receptor mRNA, and B2 receptor binding as determined by Western blotting with anti-Ha-Ras antibodies, reverse transcriptase-polymerase chain reaction using B2 receptor-specific primers, and specific [3H]BK binding, respectively. BK also stimulated a significant B2 receptor-mediated increase in [3H]thymidine incorporation in the cells both alone and in synergy with epidermal growth factor. In the absence of DEX, the DT cells expressed a considerably lower but yet clearly significant level of Ha-Ras. Under this condition, receptor mRNA and receptor binding remained maximally expressed. On the other hand, BK was unable to stimulate any increase in [3H]thymidine incorporation. In contrast to DT cells, no Ha-Ras, receptor mRNA, receptor binding, or BK-stimulated, B2 receptor-mediated [3H]thymidine incorporation was detected in 3T3 cells (+/- DEX). However, BK stimulated a transient increase in the level of intracellular free Ca2+ in the 3T3 cells indicating that these cells express a small number of functional B2 receptors. In all, these results show that oncogenic Ha-Ras regulates the sensitivity of 3T3 cells to BK through at least two different mechanisms. One mechanism occurs at a relatively low level of Ha-Ras and involves an increase in B2 receptor mRNA and expressed B2 receptor levels, and another mechanism occurs at a relatively high level of Ha-Ras and involves an increase in B2 receptor-mediated mitogenic signaling.

H-89 inhibits collagenase induction by phorbol ester through a mechanism that does not involve protein kinase A

Cyclic adenosine monophosphate (cAMP)-dependent protein kinase A (PKA) regulates the activity of growth-factor-induced pathways at the level of cytoplasmic kinases and nuclear transcription factors. We observed that H-89, an inhibitor of PKA, induced mitogen-activated protein (MAP) kinase activity in a 12V-ras-transformed fibroblast cell line. In contrast, H-89 inhibited phorbol-ester-mediated induction of MAP kinase, junB messenger ribonucleic acid (mRNA), and collagenase mRNA in these cells. Phorbol-ester stimulation of a collagenase-promoter reporter construct was also inhibited by H-89. However, stimulation of the collagenase promoter was not inhibited by overexpression of the PKA-inhibitory protein PKI. These data suggest that H-89 inhibits the activity of an enzyme required for phorbol-ester induction of collagenase mRNA, but that this inhibition does not occur at the level of PKA.

Cellular factors binding to a novel cis-acting element mediate steroid hormone responsiveness of mouse mammary tumor virus promoter

Steroid hormone receptors regulate mouse mammary tumor virus (MMTV) gene expression by binding to hormone response DNA elements present in the long terminal repeat. Tissue-specific expression of MMTV is unlikely to be regulated by steroid hormone-receptor complex alone, and mammary cell-specific factors might play a role in the hormone-induced transcriptional activation. In this report we have investigated the function of a novel cis-acting element designated Kil (-204 to -188) which is located adjacent to the distal glucocorticoid response element, in steroid hormone-induced transcription of MMTV. Electrophoretic mobility shift assays indicate that cellular factors bind to the Kil element, and dexamethasone stimulation results in alterations in the binding pattern of proteins in this region. By transient transfection assays using wild type and deletion mutants of the Kil element, we show that this novel cis-acting element is necessary for hormone-induced transcription of MMTV and functions in mammary tumor cells but not in NIH/3T3 cells. Mutagenesis of the Kil sequence suggests that the entire Kil element functioning as one unit is necessary for hormone-induced transcription of MMTV. When placed in the context of heterologous promoters, neither Kil element nor glucocorticoid response element is able to induce significant hormone-induced transcription of MMTV. The presence of both the DNA elements in tandem results in optimal induction of transcription in the presence of steroid hormones. Our results also indicate that the Kil element functions in human breast carcinoma cell lines such as T47D and MCF-7. These results suggest that Kil element in combination with distal glucocorticoid response element functions as a mammary cell-specific enhancer to regulate MMTV transcription.

Differential regulation of AP1 activity by retinoic acid in hormone-dependent and -independent breast cancer cells

We have studied the role of the AP1 transcription factor in the progression of human breast carcinomas. This progression is characterized by a loss of dependence for proliferation on mitogenic hormones, and is also linked to loss of responsiveness to the growth inhibitor retinoic acid (RA). In the hormone-dependent breast tumor cell line MCF7 mitogenic stimulation was found to be linked to an enhancement of AP1 transcriptional activity, while growth inhibition by RA was parallelled by decreased AP1 activity. AP1 binding activity to its consensus DNA sequence was rapidly reduced in RA treated cells, in the absence of any noticeable change in expression of AP1 constituents. AP1 overexpression abrogated RA repression in MCF7 cells. In hormone-independent cell lines (BT20, Hs578T, MDA-MB231, MDA-MB468) autonomous proliferation was associated with an increased background AP1 activity. Interestingly, these cells are refractory to growth inhibition by RA, which can only be partly explained by underexpression of RA receptors. In these cells RA did not repress AP1 transactivation unless RA receptors were overexpressed by means of cotransfection with an expression vector. This suggests that the high background levels of AP1 activity in the autonomously growing cells are associated with prevention of RA inhibition of AP1 activity to occur. Therefore, increased AP1 activity may not only play a role in progression of breast tumors towards hormone-insensitivity but may also contribute to the RA resistance of such cells.

Molecular characterization of the in vivo alkylating agent resistant murine EMT-6 mammary carcinoma tumors

The expression of several early-response genes and genes associated with malignant disease was assessed in the EMT-6/parent tumor and the EMT-6/CTX and EMT-6/CDDP in vivo resistant tumor lines growing as tumors or as monolayers in culture. In the absence of treatment the levels of mRNA for the genes c-jun, c-fos, c-myc, Ha-ras and p53 were increased in the EMT-6/CTX and EMT-6/CDDP as compared with the EMT-6/parent tumor, whereas the expression of erb-2 was similar in all three tumors. Although the cells from each of the three tumors show increased expression of early response genes after exposure to cisplatin (CDDP; 100 microM, 2 h) or 4-Hydroxyperoxycyclophosphamide (4-HC; 100 microM, 2 h) in culture, in mRNA extracted from tumor tissue these changes are absent or very small. Both C-jun and erb-2 were detectable in liver. There was increased expression of both of these genes in the livers of tumor-bearing animals as compared with non-tumor-bearing animals. The highest expression of both c-jun and erb-2 occurred in the livers of animals bearing the EMT-6/CDDP tumor. Treatment of the animals with CDDP or cyclophosphamide, in general, resulted in increased expression of both genes at 6 h post treatment. The increased expression of these genes may impart metabolic changes in the tumors and/or hosts that contribute to the resistance of these tumors to specific antitumor alkylating agents.

c-Fos transcriptional activity stimulated by H-Ras-activated protein kinase distinct from JNK and ERK

Ras proteins exert their mitogenic and oncogenic effects through activation of downstream protein kinases. An important question is how Ras-generated signals reach the nucleus to activate downstream target genes. AP-1, a heterodimeric complex of Jun and Fos proteins, which activates mitogen-inducible genes, is a major nuclear target of Ras. Ras can stimulate AP-1 activity by inducing c-fos transcription, a process which is probably mediated by the ERK1 and -2 mitogen-activated protein (MAP) kinases, which phosphorylate the transcription factor Elk-1/TCF. Besides inducing transcription from fos and jun genes, mitogens and Ras proteins enhance AP-1 activity through phosphorylation of c-Jun. Phosphorylation of the c-Jun activation domain leads to c-jun induction through an autoregulatory loop. Ras- and ultra-violet-responsive protein kinases that phosphorylate c-Jun on serine residues at positions 63 and 73 and stimulate its transcriptional activity have been identified. These proline-directed kinases, termed JNKs, are novel MAP kinases. It is not clear, however, whether c-Jun is the only recipient and JNK the only transducer of the Ras signal to AP-1 proteins. A short sequence surrounding the major JNK phosphorylation site of c-Jun is conserved in c-Fos and is part of its activation domain, suggesting that c-Fos may be similarly regulated. Here we show that Ras does indeed augment the transcriptional activity of c-Fos through phosphorylation at Thr 232, the homologue of Ser 73 of c-Jun. However, this is mediated by a novel Ras- and mitogen-responsive proline-directed protein kinase that is different from JNKs and ERKs. Therefore, at least three types of proline-directed kinases transmit Ras- and mitogen-generated signals to the transcriptional machinery.

Regulation of uterine collagenase gene expression: interactions between serotonin and progesterone

This report seeks to further define the requirements for the previously established induction of collagenase gene expression by serotonin and inhibition by progesterone in primary cultures of rat uterine smooth muscle cells. Detectable increases in collagenase production were observed after as little as 3 h exposure of cells to 5 microM serotonin, with maximal induction occurring after approximately 8 h of exposure. The apparent half-life of collagenase mRNA upon removal of serotonin was estimated to be approximately 12 h, and was not dependent on the duration of induction. Inhibition by either cycloheximide or progesterone showed similar half lives for collagenase mRNA, however a much shorter half-life (6 h) was obtained in the presence of actinomycin D. These experiments suggest that neither serotonin induction nor progesterone inhibition of collagenase synthesis represents a primary effect on collagenase gene transcription. Rather they appear to be secondary to changes that occur at one or more primary intermediate genes whose induction or decay must occur prior to changes in collagenase transcription. The progesterone receptor antagonist, RU-486, abrogates the ability of progesterone to inhibit serotonin-induced collagenase gene expression, indicating that the effects of progestins in this system likely are receptor-mediated. Finally, the present studies demonstrate that pretreatment of cells for times as long as 5 days with medroxyprogesterone in the absence of serotonin is unable to prevent subsequent serotonin-induced collagenase mRNA increases. These data suggest the possibility of a unique interaction between the molecular pathways of inducer and inhibitor, one in which serotonin may help mediate the progesterone-dependent repression of the levels of collagenase mRNA.

Retinoic acid resistance of estradiol-independent breast cancer cells coincides with diminished retinoic acid receptor function

Retinoic acid (RA) strongly inhibits proliferation of the estrogen (E2)-dependent human breast cancer cell lines MCF7, T47D, and ZR75-1, but not the E2-independent and E2 receptor (ER)-negative lines MDA-MB231, MDA-MB468, BT20 and Hs578T. The specific sensitivity of the E2-dependent cell lines seems not to be caused by an inhibitory effect of RA on ER functioning since RA inhibited the proliferative response not only to E2 but also to insulin. Furthermore, endogenous RA receptors (RARs) hardly impaired transcriptional activation of an E2 responsive element-tk-CAT reporter construct. RAR alpha mRNA was highly expressed in the RA-responsive lines, but not in the unresponsive lines, except BT20. With the exception of Hs578T, also RAR beta mRNA expression was low in the unresponsive lines. While in the dependent lines and Hs578T RA activated RA responsive element-dependent transcriptional activity, this response was very low in MDA-MB231, MDA-MB468, and BT20, suggesting that the RA resistance of these latter three ER-negative lines is due to underexpression of functional RARs. Our results suggest that the loss of functional RARs may be a frequent event, leading to RA unresponsiveness of ER-negative breast cancer cells. This implies that both the steroid and retinoid receptor status of breast tumors may be used to predict a successful treatment with retinoids.

Gene structure and expression analysis of the epidermal growth factor receptor, transforming growth factor-alpha, myc, jun, and metallothionein in human ovarian carcinomas. Classification of malignant phenotypes

This study reports the structure and expression rates of genes of the transforming growth factor-alpha (TGF-alpha) signal transduction pathway (TGF-alpha, epidermal growth factor receptor [EGF-R], jun, myc, and metallothionein [MT]) in 47 specimens of ovarian cancer and 21 nonmalignant tissues. The objective was to establish a direct correlation between the genetic activities and the malignant phenotype of the ovarian cancer. The Southern blot technique identified four samples with myc amplification and two with rearranged EGF-R genes. By using the S1 nuclease assay, the analysis of myc transcription showed a similar use of both promotors. Although the size of the investigated transcripts was unaltered, significant differences in the transcription rates were noticed in malignant tissue probes (using northern blot analysis and RNAase protection assay). The following results of messenger RNA analysis in ovarian cancer were observed: EGF-R, negative in 25%, low in 65%, and strongly positive in 10%; TGF-alpha, negative in 34%, low in 36%, and strongly positive in 30%; myc, negative in 8%, low in 64%, and strongly positive in 28%; jun, negative in 4%, low in 58%, and strong in 38%; and MT, low in 80% and strongly positive in 20%. In most nonmalignant tissues studied, no or only a low expression of TGF-alpha, EGF-R, and myc. was found. A comparison of these messenger RNA results with the clinical data from tumors showed four different subgroups of ovarian carcinomas. The results of chemotherapy were known in 32 cases. Tumors with negative or low expression rates of all investigated genes did not respond to chemotherapy; 13 of 18 tumors with high expression rates did respond. Additional signal transduction chains distinct from the TGF-alpha pathway, however, are likely to influence both the expression and activity of transcription factors and MT.

Glutathione S-transferases: gene structure and regulation of expression

The current knowledge about the structure of GST genes and the molecular mechanisms involved in regulation of their expression are reviewed. Information derived from the study of rat and mouse GST Alpha-class, Ya genes, and a rat GST Pi-class gene seems to indicate that a single cis-regulatory element, composed of two adjacent AP-1-like binding sites in the 5'-flanking region of these GST genes, is responsible for their basal and xenobiotic-inducible activity. The identification of Fos/Jun (AP-1) complex as the trans-acting factor that binds to this element and mediates the basal and inducible expression of GST genes offers a basis for an understanding of the molecular processes involved in GST regulation. The induction of expression of Fos and Jun transcriptional regulatory proteins by a variety of extracellular stimuli is known to mediate the activation of target genes via the AP-1 binding sites. The modulation of the AP-1 activity may account for the changes induced by growth factors, hormones, chemical carcinogens, transforming oncogenes, and cellular stress-inducing agents in the pattern of GST expression. Recent observations implying reactive oxygen as the transduction signal that mediates activation of c-fos and c-jun genes are presently considered to provide an explanation for the induction of GST gene expression by chemical agents of diverse structure. The possibility that these agents may all induce conditions of oxidative stress by various pathways to activate expression of GST genes that are regulated by the AP-1 complex is discussed.

Targeted degradation of c-Fos, but not v-Fos, by a phosphorylation-dependent signal on c-Jun

The proto-oncogene products c-Fos and c-Jun heterodimerize through their leucine zippers to form the AP-1 transcription factor. The transcriptional activity of the heterodimer is regulated by signal-dependent phosphorylation and dephosphorylation events. The stability of c-Fos was found to also be controlled by intracellular signal transduction. In transient expression and in vitro degradation experiments, the stability of c-Fos was decreased when the protein was dimerized with phosphorylated c-Jun. c-Jun protein isolated from phorbol ester-induced cells did not target c-Fos for degradation, which suggests that c-Fos is transiently stabilized after stimulation of cell growth. v-Fos protein, the retroviral counterpart of c-Fos, was not susceptible to degradation targeted by c-Jun.

Regulation of collagen I gene expression by ras

Although transformation of rodent fibroblasts can lead to dramatic changes in expression of extracellular matrix genes, the molecular basis and physiological significance of these changes remain poorly understood. In this study, we have investigated the mechanism(s) by which ras affects expression of the genes encoding type I collagen. Levels of both alpha 1(I) and alpha 2(I) collagen mRNAs were markedly reduced in Rat 1 fibroblasts overexpressing either the N-rasLys-61 or the Ha-rasVal-12 oncogene. In fibroblasts conditionally transformed with N-rasLys-61, alpha 1(I) transcript levels began to decline within 8 h of ras induction and reached 1 to 5% of control levels after 96 h. In contrast, overexpression of normal ras p21 had no effect on alpha 1(I) or alpha 2(I) mRNA levels. Nuclear run-on experiments demonstrated that the transcription rates of both the alpha 1(I) and alpha 2(I) genes were significantly reduced in ras-transformed cells compared with those in parental cells. In addition, the alpha 1(I) transcript was less stable in transformed cells. Chimeric plasmids containing up to 3.6 kb of alpha 1(I) 5'-flanking DNA and up to 2.3 kb of the 3'-flanking region were expressed at equivalent levels in both normal and ras-transformed fibroblasts. However, a cosmid clone containing the entire mouse alpha 1(I) gene, including 3.7 kb of 5'- and 4 kb of 3'-flanking DNA, was expressed at reduced levels in fibroblasts overexpressing oncogenic ras. We conclude that oncogenic ras regulates the type I collagen genes at both transcriptional and posttranscriptional levels and that this effect, at least for the alpha 1(I) gene, may be mediated by sequences located either within the body of the gene itself or in the distal 3'-flanking region.

Downregulation of JE and KC genes by glucocorticoids does not prevent the G0----G1 transition in BALB/3T3 cells

The effects of glucocorticoid hormones on the expression of the growth factor-inducible genes JE, KC, and c-myc were analyzed in parental BALB/3T3 and polyomavirus middle-T antigen-transfected cell lines. Northern (RNA) blot hybridization and run-on transcription analysis showed that (i) glucocorticoid hormones selectively inhibit JE and KC expression at the transcriptional level and (ii) the downregulatory effect of glucocorticoids on JE and KC expression is partial for serum-stimulated and middle T antigen-transformed cells and total for quiescent and exponentially growing cells. Gel mobility assays using AP-1 oligonucleotides showed a positive correlation between glucocorticoid downregulating effect and presence of the AP-1 complex. JE and KC downregulation by means of the AP-1 complex may play a role in the actions of glucocorticoids as anti-inflammatory and antitumor agents. The ability of glucocorticoids to downregulate JE and KC was used to investigate the relevance of these genes to the mitogenic response to serum growth factors. Hydrocortisone did not alter the basal DNA synthesis level displayed by quiescent 3T3 cells, but it potentiated both the mitogenic effect of platelet-derived growth factor and c-myc induction by serum growth factors. Upon serum restimulation, untreated and dexamethasone-treated quiescent 3T3 cultures entered the S phase after an identical time lag (G1). These results suggest that (i) JE and KC are not necessary for the G0----G1----S transition and (ii) c-myc overexpression is likely to be the basis for the potentiating effect of glucocorticoids on serum growth factors.

Downregulation of JE and KC genes by glucocorticoids does not prevent the G0----G1 transition in BALB/3T3 cells

The effects of glucocorticoid hormones on the expression of the growth factor-inducible genes JE, KC, and c-myc were analyzed in parental BALB/3T3 and polyomavirus middle-T antigen-transfected cell lines. Northern (RNA) blot hybridization and run-on transcription analysis showed that (i) glucocorticoid hormones selectively inhibit JE and KC expression at the transcriptional level and (ii) the downregulatory effect of glucocorticoids on JE and KC expression is partial for serum-stimulated and middle T antigen-transformed cells and total for quiescent and exponentially growing cells. Gel mobility assays using AP-1 oligonucleotides showed a positive correlation between glucocorticoid downregulating effect and presence of the AP-1 complex. JE and KC downregulation by means of the AP-1 complex may play a role in the actions of glucocorticoids as anti-inflammatory and antitumor agents. The ability of glucocorticoids to downregulate JE and KC was used to investigate the relevance of these genes to the mitogenic response to serum growth factors. Hydrocortisone did not alter the basal DNA synthesis level displayed by quiescent 3T3 cells, but it potentiated both the mitogenic effect of platelet-derived growth factor and c-myc induction by serum growth factors. Upon serum restimulation, untreated and dexamethasone-treated quiescent 3T3 cultures entered the S phase after an identical time lag (G1). These results suggest that (i) JE and KC are not necessary for the G0----G1----S transition and (ii) c-myc overexpression is likely to be the basis for the potentiating effect of glucocorticoids on serum growth factors.

Regulation of collagen I gene expression by ras

Although transformation of rodent fibroblasts can lead to dramatic changes in expression of extracellular matrix genes, the molecular basis and physiological significance of these changes remain poorly understood. In this study, we have investigated the mechanism(s) by which ras affects expression of the genes encoding type I collagen. Levels of both alpha 1(I) and alpha 2(I) collagen mRNAs were markedly reduced in Rat 1 fibroblasts overexpressing either the N-rasLys-61 or the Ha-rasVal-12 oncogene. In fibroblasts conditionally transformed with N-rasLys-61, alpha 1(I) transcript levels began to decline within 8 h of ras induction and reached 1 to 5% of control levels after 96 h. In contrast, overexpression of normal ras p21 had no effect on alpha 1(I) or alpha 2(I) mRNA levels. Nuclear run-on experiments demonstrated that the transcription rates of both the alpha 1(I) and alpha 2(I) genes were significantly reduced in ras-transformed cells compared with those in parental cells. In addition, the alpha 1(I) transcript was less stable in transformed cells. Chimeric plasmids containing up to 3.6 kb of alpha 1(I) 5'-flanking DNA and up to 2.3 kb of the 3'-flanking region were expressed at equivalent levels in both normal and ras-transformed fibroblasts. However, a cosmid clone containing the entire mouse alpha 1(I) gene, including 3.7 kb of 5'- and 4 kb of 3'-flanking DNA, was expressed at reduced levels in fibroblasts overexpressing oncogenic ras. We conclude that oncogenic ras regulates the type I collagen genes at both transcriptional and posttranscriptional levels and that this effect, at least for the alpha 1(I) gene, may be mediated by sequences located either within the body of the gene itself or in the distal 3'-flanking region.

Control of transcription factors by signal transduction pathways: the beginning of the end

Signal transduction pathways regulate gene expression by modulating the activity of nuclear transcription factors. The mechanisms that control the activity of two groups of sequence-specific transcription factors, the AP-1 and CREB/ATF proteins, are described. These factors serve as a paradigm explaining the transfer of regulatory information from the cell surface to the nucleus.

An ATF/CREB binding motif is required for aberrant constitutive expression of the MHC class II DR alpha promoter and activation by SV40 T-antigen

Constitutive expression of major histocompatibility complex class II (MHC II) antigens normally occurs in B-lymphocytes and antigen presenting cells of the monocyte/macrophage lineage. However, many malignant tumours and transformed cells express these proteins aberrantly. We demonstrate here that the MHC II DR alpha promoter is constitutively active both in the SV40 large T antigen transformed cell line, COS, and in CV1 cells from which they are derived. As an approach to understanding the molecular mechanisms underlying aberrant DR alpha expression we have examined the cis- and trans-acting requirements for DR alpha transcription in these cell types. Electrophoretic mobility shift assays showed that the region immediately 3' to the X-box was bound by a member of the ATF/CREB family of transcription factors. Using deletions and point mutations in the DR alpha promoter we demonstrate that, in contrast to B-cells, the octamer motif and conserved X- and Y-boxes make only a minor contribution to promoter function while single point mutations in the ATF/CREB motif reduced transcription up to 20-fold. In addition, we show that the DR alpha promoter is activated by SV40 large T-antigen and that activation requires an intact ATF/CREB motif. Similar data were obtained using B16 melanoma cells. These results suggest that the ATF/CREB motif may be a target for transcription deregulation in several transformed cell types.

Overexpression of Mos, Ras, Src, and Fos inhibits mouse mammary epithelial cell differentiation

Mammary epithelial cells terminally differentiate in response to lactogenic hormones. We present evidence that oncoprotein overexpression is incompatible with this hormone-inducible differentiation and results in striking cellular morphological changes. In mammary epithelial cells in culture, lactogenic hormones (glucocorticoid and prolactin) activated a transfected beta-casein promoter and endogenous beta-casein gene expression. This response to lactogenic hormone treatment was paralleled by a decrease in cellular AP-1 DNA-binding activity. Expression of the mos, ras, or src (but not myc) oncogene blocked the activation of the beta-casein promoter induced by the lactogenic hormones and was associated with the maintenance of high levels of AP-1. Mos expression also increased c-fos and c-jun mRNA levels. Overexpression of Fos and Jun from transiently transfected constructs resulted in a functional inhibition of the glucocorticoid receptor in these mouse mammary epithelial cells. This finding clearly suggests that glucocorticoid receptor inhibition arising from oncogene expression will contribute to the block in hormonally induced mammary epithelial cell differentiation. Expression of Src resulted in the loss of the normal organization and morphological phenotype of mammary epithelial cells in the epithelial/fibroblastic line IM-2. Activation of a conditional c-fos/estrogen receptor gene encoding an estrogen-dependent Fos/estrogen receptor fusion protein also morphologically transformed mammary epithelial cells and inhibited initiation of mammary epithelial differentiation-associated expression of the beta-casein and WDNM 1 genes. In response to estrogen treatment, the cells displayed a high level of AP-1 DNA-binding activity. Our results demonstrate that high cellular AP-1 levels contribute to blocking the ability of mammary epithelial cells in culture to respond to lactogenic hormones. This and other studies indicate that the oncogene products Mos, Ras, and Src exert their effects, at least in part, by stimulating cellular Fos and probably cellular Jun activity.

Casein kinase II is a negative regulator of c-Jun DNA binding and AP-1 activity

c-Jun, a major component of the inducible transcription factor AP-1, is a phosphoprotein. In nonstimulated fibroblasts and epithelial cells, c-Jun is phosphorylated on a cluster of two to three sites abutting its DNA-binding domain. Phosphorylation of these sites inhibits DNA binding, and their dephosphorylation correlates with increased AP-1 activity. We show that two of these sites, Thr-231 and Ser-249, are phosphorylated by casein kinase II (CKII). Substitution of the third site, Ser-243, by Phe interferes with phosphorylation of the inhibitory sites in vivo and by purified CKII in vitro. Microinjection into living cells of synthetic peptides that are specific competitive substrates or inhibitors of CKII results in induction of AP-1 activity and c-Jun expression. Microinjection of CKII suppresses induction of AP-1 by either phorbol ester or an inhibitory peptide. These results suggest that one of the roles of CKII, a major nuclear protein kinase with no known functions, is to attenuate AP-1 activity through phosphorylation of c-Jun.

Overexpression of Mos, Ras, Src, and Fos inhibits mouse mammary epithelial cell differentiation

Mammary epithelial cells terminally differentiate in response to lactogenic hormones. We present evidence that oncoprotein overexpression is incompatible with this hormone-inducible differentiation and results in striking cellular morphological changes. In mammary epithelial cells in culture, lactogenic hormones (glucocorticoid and prolactin) activated a transfected beta-casein promoter and endogenous beta-casein gene expression. This response to lactogenic hormone treatment was paralleled by a decrease in cellular AP-1 DNA-binding activity. Expression of the mos, ras, or src (but not myc) oncogene blocked the activation of the beta-casein promoter induced by the lactogenic hormones and was associated with the maintenance of high levels of AP-1. Mos expression also increased c-fos and c-jun mRNA levels. Overexpression of Fos and Jun from transiently transfected constructs resulted in a functional inhibition of the glucocorticoid receptor in these mouse mammary epithelial cells. This finding clearly suggests that glucocorticoid receptor inhibition arising from oncogene expression will contribute to the block in hormonally induced mammary epithelial cell differentiation. Expression of Src resulted in the loss of the normal organization and morphological phenotype of mammary epithelial cells in the epithelial/fibroblastic line IM-2. Activation of a conditional c-fos/estrogen receptor gene encoding an estrogen-dependent Fos/estrogen receptor fusion protein also morphologically transformed mammary epithelial cells and inhibited initiation of mammary epithelial differentiation-associated expression of the beta-casein and WDNM 1 genes. In response to estrogen treatment, the cells displayed a high level of AP-1 DNA-binding activity. Our results demonstrate that high cellular AP-1 levels contribute to blocking the ability of mammary epithelial cells in culture to respond to lactogenic hormones. This and other studies indicate that the oncogene products Mos, Ras, and Src exert their effects, at least in part, by stimulating cellular Fos and probably cellular Jun activity.

Direct stimulation by estrogen of growth factor signal transduction pathways in human breast cancer cells

Estrogen is thought to stimulate the proliferation of human breast tumors indirectly, through induced production of autocrine polypeptide growth factors. Constitutive production of such growth factors would lead to the loss of 17 beta-estradiol (E2)-dependence that is associated with progression of the disease. Our data, however, do not support this hypothesis and suggest that hormone-dependent breast tumor cell lines like MCF7 do not react to the growth factors which they produce. Moreover, we provide evidence that E2 directly stimulates proliferation by inducing, like many growth factors, the c-fos proto-oncogene. E2 by itself, however, is poorly mitogenic. This may be caused by the lack of induction of genes from the jun family, whose gene products are necessary for dimerization with the c-fos encoded protein, leading to an important step in growth factor signalling pathways; stimulation of TPA responsive element (TRE)-dependent transcriptional activity. In combination with insulin-like growth factors, efficient inducers of c-jun in these cells, E2 synergistically stimulates proliferation and TRE-activity. Constitutive TRE-activation may lead to loss of E2-dependence.

Down-regulation of cellular proto-oncogenes during inhibition of rat parotid acinar cell proliferation

The role of cell surface galactosyltransferase in mediating isoproterenol-induced parotid gland hypertrophy and hyperplasia was examined in rat parotid gland acinar cells. Introduction of the transferase modifier, alpha-lactalbumin, or galactosyltransferase-associated kinase inhibitor trifluoperazine, into beta-agonist-treated rats prevented acinar cell proliferation as determined by [3H]thymidine incorporation after 96 h of treatment. However, [3H]thymidine incorporation into DNA after 24 h of treatment, with injection of a combination of isoproterenol/alpha-lactalbumin or isoproterenol/trifluoperazine, was similar to injections of isoproterenol alone; suggesting that acinar cells could be stimulated to undergo a single round of DNA synthesis. Northern blot analysis of myc and fos expression followed a similar pattern of down-regulation to control levels after 96 h but not after 24 h. Hybridization with erb B showed little change with proliferation, confirming previous observations on protein levels of the EGF-receptor in acinar cells. Western blot analysis of nuclear protein expression of myc revealed that isoproterenol caused an increase in a 62-kDa protein which was again down-regulated with inhibition of cell proliferation. Analysis of protein levels of Rb110 protein showed no change in protein level in the nucleus with cell proliferation, but did show an associated increase in protein phosphorylation in response to growth stimulation.

Post-transcriptional effect of ultraviolet light on gene expression in human cells. Stabilization of cytokine-induced and poly(I).poly(C)-induced messenger RNA

It is well established that ultraviolet light modulates gene expression in mammalian cells, particularly at transcriptional and post-translational levels. The present study was undertaken to investigate whether the fate of mRNA is also altered in ultraviolet-light-irradiated human cells. In order to facilitate distinction between transcriptional and post-transcriptional effects, this analysis has focused on six genes whose transcription is conditional on the supply of exogenous inducers, interferon-alpha, interleukin-1 alpha or the double-stranded RNA, poly(I).poly(C). Human cells induced to express these genes were found to retain a significantly higher concentration of corresponding transcripts when irradiated with ultraviolet light at the end of the inducing treatment. This stimulation was due to dose-dependent ultraviolet-light stabilization of preformed mRNA, as shown by run-on and pulse/chase experiments. This work uncovers a new facet of the cellular response to genotoxic stresses, i.e. extension of the life-span of transcription products. Whether this stabilizing effect contributes to cell recovery by promoting gene expression remains to be determined.

Presence of an estradiol response region in the mouse c-fos oncogene

We have previously shown that the intracellular content of c-fos mRNA is rapidly induced (within 1 to 3 hours) in ovariectomized rat or mouse uteri following administration of estradiol. This induction is sensitive to actinomycin D but not to protein synthesis inhibitor puromycin, indicating an effect of estradiol at the transcriptional level, possibly mediated by the estrogen receptor. We have used transient transfection assays with defined regions of the mouse c-fos gene ligated to a reporter plasmid expressing chloramphenicol acetyl transferase to study regulation of this gene by estrogens. These recombinants were transfected in two different estrogen-responsive cell lines, GH4 and MCF-7, and stimulated with estradiol. A two- to five-fold induction of chloramphenicol acetyl transferase activity was observed with a construct containing the intact c-fos promoter and 351 bases of 5'-flanking sequence (-351/+44). A similar induction by estrogen is observed with the endogenous c-fos gene in the two cell lines as determined by RNA blot analysis. Estrogen induction is lost when a construct containing -135/+44 region of the c-fos gene is transfected. Plasmid containing the consensus estrogen response element GGTCAnnnTGACC derived from vitellogenin gene is induced 10- to 50-fold in both estrogen-responsive cell lines. Under identical conditions, the oligonucleotide containing the perfect palindrome GGTCTnnnAGACC, present around the -209 region of the c-fos gene, is completely silent when transfected under the control of thymidine kinase promoter. Additional transfection analysis with a number of c-fos promoter constructs has narrowed the estrogen response region to within the -278 to -135 region upstream of the c-fos promoter.

In vivo activation of human immunodeficiency virus type 1 long terminal repeat by UV type A (UV-A) light plus psoralen and UV-B light in the skin of transgenic mice

UV irradiation has been shown to activate the human immunodeficiency virus type 1 (HIV-1) long terminal repeat (LTR) in cell culture; however, only limited studies have been described in vivo. UV light has been categorized as UV-A (400 to 315 nm), -B (315 to 280 nm), or -C (less than 280 nm); the longer wavelengths are less harmful but more penetrative. Highly penetrative UV-A radiation constitutes the vast majority of UV sunlight reaching the earth's surface but is normally harmless. UV-B irradiation is more harmful but less prevalent than UV-A. In this report, the HIV-1 LTR-luciferase gene in the skin of transgenic mice was markedly activated when exposed to UV-B irradiation. The LTR in the skin of transgenic mice pretreated topically with a photosensitizing agent (psoralen) was also activated to similar levels when exposed to UV-A light. A 2-h exposure to sunlight activated the LTR in skin treated with psoralen, whereas the LTR in skin not treated with psoralen was activated after 7 h of sunlight exposure. The HIV-1 LTR-beta-galactosidase reporter gene was preferentially activated by UV-B irradiation in a small population of epidermal cells. The transgenic mouse models carrying HIV-1 LTR-luciferase and LTR-beta-galactosidase reporter genes have been used to demonstrate the in vivo UV-induced activation of the LTR and might be used to evaluate other environmental factors or pharmacologic substances that might potentially activate the HIV-1 LTR in vivo.

Stringent integrity requirements for both trans-activation and DNA-binding in a trans-activator, Oct3

POU-specific and POU-homeo domains of Oct3 were produced in Echerichia coli for characterization of DNA binding to the octamer sequence. POU domain protein including A, B and H domains could bind to the octamer sequence efficiently and specifically, and DNase I footprint analysis gave an indistinguishable protection pattern between recombinant POU protein of Oct3 and native Oct3 from undifferentiated P19 cells. Truncated mutants, which contained B-specific and H domains or the H domain only, showed no binding activity, indicating that both of POU-specific and POU-homeo domains are essential for binding activity to octamer sequence. Furthermore, a 6 amino acid deletion from the N-terminal region of the A-specific domain is enough to destroy the binding activity. As for trans-activation, the N-terminal region is essential and sufficient. Deletion of the N-terminal proline-rich region rapidly eliminated trans-activating activity. These data strongly indicate the stringent integrity requirements for both trans-activation and DNA-binding domains in Oct3.

A new cAMP response element in the transcribed region of the human c-fos gene

In NIH 3T3 cells the c-fos gene is induced rapidly and transiently by cAMP. As shown by the analysis of 3T3 cells stably transfected with promoter mutants of the human c-fos gene this induction does not depend on the dyad symmetry element (position -320 to -300), but involves at least two other non-related sites: an element located around position -60 resembling the cAMP response element of the fibronectin and somatostatin genes (which has been described before), and an element located between positions +18 and +38. Destruction of one or the other element in the c-fos gene reduces cAMP inducibility. The cAMP response of c-fos promoter CAT gene constructs also depends on these elements in transient transfection assays. When cloned in front of the albumin TATA box, both elements independently mediate cAMP inducibility. These elements do not bind the same protein as shown in gel retardation analyses, suggesting that two different cAMP inducible factors mediate the activation of the c-fos gene by cAMP.

Depletion of c-myc with specific antisense sequences reverses the transformed phenotype in ras oncogene-transformed NIH 3T3 cells

ras oncogene-transformed NIH 3T3 cells expressing glucocorticoid-inducible antisense c-myc cDNA transcripts at levels sufficient to deplete c-myc protein lost their transformed morphology and the ability to grow in soft agar; their ability to form tumors in nude mice was also impaired. These changes were dependent on the continuous expression of the antisense sequences. No major effects on plating efficiencies, growth rates in monolayer culture, or immortalization were observed in the revertant cells, indicating that the observed effects were not a toxic consequence of c-myc protein depletion. Transfection with the same vector expressing c-myc in the sense orientation or other control vectors had no effect on transformation. These results suggest that a certain minimum level of expression of c-myc is required for the maintenance of ras transformation in NIH 3T3 cells.

Alternative splicing of fosB transcripts results in differentially expressed mRNAs encoding functionally antagonistic proteins

We show that serum-stimulated fibroblasts transiently express two different forms of fosB mRNA, which are generated by alternative splicing of the transcript from a single gene. In addition to the known long form (fosB-L), encoding a protein of 338 amino acids (FosB-L), a second shorter form (fosB-S) with a deletion of 140 bp was detected. This deletion creates a stop codon 3' to the leucine repeat, giving rise to a protein of 237 amino acids (FosB-S) lacking the carboxyl terminus of FosB-L. Only the long FosB form efficiently induces transformation in mouse and rat fibroblast cell lines and trans-represses the c-fos promoter. Both of these functions are suppressed by coexpressed FosB-S. Upon serum stimulation, maximum expression of the oncogenic fosB-L form precedes the expression of the antagonistic fosB-S form, indicating a new mechanisms regulating the action of members of the Fos family. However, FosB-L and FosB-S do not differ in all trans-regulatory properties: Trans-activation of a 5x TRE-CAT reporter construct in HeLa and NIH-3T3 cells was found with both FosB forms. These observations suggest a correlation between fosB-induced transformation and trans-repression, thus pointing to different mechanisms involved in transformation by fosB and c-fos/v-fos.

Depletion of c-myc with specific antisense sequences reverses the transformed phenotype in ras oncogene-transformed NIH 3T3 cells

ras oncogene-transformed NIH 3T3 cells expressing glucocorticoid-inducible antisense c-myc cDNA transcripts at levels sufficient to deplete c-myc protein lost their transformed morphology and the ability to grow in soft agar; their ability to form tumors in nude mice was also impaired. These changes were dependent on the continuous expression of the antisense sequences. No major effects on plating efficiencies, growth rates in monolayer culture, or immortalization were observed in the revertant cells, indicating that the observed effects were not a toxic consequence of c-myc protein depletion. Transfection with the same vector expressing c-myc in the sense orientation or other control vectors had no effect on transformation. These results suggest that a certain minimum level of expression of c-myc is required for the maintenance of ras transformation in NIH 3T3 cells.

Signal transduction and gene control

Extracellular signals regulate gene expression by triggering signal transduction cascades that result in the modulation of transcription factor activity. This is most commonly achieved by changes in the phosphorylation state of these nuclear proteins. Phosphorylation affects transcription factor activity at several distinct levels. It can modulate their intracellular localization by controlling the association with other proteins, have both negative and positive effects on their DNA-binding activity, and modulate the activity of their transcriptional activation domains. In addition to phosphorylation, protein-protein interactions also have an important role in mediating a crosstalk at the nuclear level between different signalling pathways.

Ha-Ras augments c-Jun activity and stimulates phosphorylation of its activation domain

Ha-Ras augments c-Jun-mediated transactivation by potentiating the activity of the c-Jun activation domain. Ha-Ras also causes a corresponding increase in phosphorylation of specific sites in that part of the c-Jun protein. A Ha-Ras-induced protein kinase cascade resulting in hyperphosphorylation of the c-Jun activation domain could explain how these oncoproteins cooperate to transform rat embryo fibroblasts.

How does p21ras transform cells?

Oncogenic forms of p21ras are found in a wide range of human tumors. However, the mechanism by which p21ras transforms remains obscure. Genetic evidence has identified a domain of p21ras that is involved with interaction with an effector molecule required for transformation. Two proteins, GAP and the tumor suppressor NF1, interact with p21ras in this region but it is an unresolved puzzle whether either of these is the an unresolved puzzle whether either of these is the effector. After interaction with an effector, two downstream events--activation of protein kinase C and another pathway--are necessary for induction of DNA synthesis by oncogenic p21ras; however, morphological transformation does not require activation of protein kinase C.

Oestrogen directly stimulates growth factor signal transduction pathways in human breast cancer cells

We have studied the mechanism by which 17 beta-oestradiol (E2) stimulates breast cancer proliferation using the MCF7 cell line as a model system. We provide evidence that E2 directly stimulates cellular proliferation by inducing, like many growth factors, the c-fos proto-oncogene. E2 by itself, however, is poorly mitogenic and it does not induce genes from the jun family, whose gene products are necessary for heterodimerization with the c-fos encoded protein (Fos), leading to an important step in growth factor signalling pathways, stimulation of the 12-O-tetradecanoyl-phorbol-13-acetate responsive element (TRE)-dependent transcriptional activity. In combination with insulin-like growth factors (IGFs), efficient inducers of c-jun in breast cancer cells, E2 synergistically stimulates TRE-activity and proliferation. This direct stimulation by E2 of growth factor signalling pathways suggest that E2 can directly induce proliferation, independent from autocrine growth factors.