Characterization of the human granulocyte-macrophage colony-stimulating factor gene promoter: an AP1 complex and an Sp1-related complex transactivate the promoter activity that is suppressed by a YY1 complex

Sp1 cooperates with the ets transcription factor, GABP, to activate the CD18 (beta2 leukocyte integrin) promoter

CD18, the beta chain of the leukocyte integrins, plays a crucial role in immune and inflammatory responses. CD18 is expressed exclusively by leukocytes, and it is transcriptionally regulated during the differentiation of myeloid cells. The ets factors, PU.1 and GABP, bind to three ets sites in the CD18 promoter, which are essential for high level myeloid expression of CD18. We now identify two binding sites for the transcription factor, Sp1, that flank these ets sites. Sp1 is the only factor from myeloid cells that binds to these sites in a sequence-specific manner. Mutagenesis of these sites abrogates Sp1 binding and significantly reduces the activity of the transfected CD18 promoter in myeloid cells. Transfection of Sp1 into Drosophila Schneider cells, which otherwise lack Sp1, activates the CD18 promoter dramatically. GABP also activates the CD18 promoter in Schneider cells. Co-transfection of Sp1 and GABP activates CD18 more than the sum of their individual effects, indicating that these factors cooperate to transcriptionally activate myeloid expression of CD18. These studies support a model of high level, lineage-restricted gene expression mediated by cooperative interactions between widely expressed transcription factors.

Activation of the ferritin H enhancer, FER-1, by the cooperative action of members of the AP1 and Sp1 transcription factor families

We have previously reported that the adenovirus E1A oncogene represses the transcription of the H subunit of the mouse ferritin gene. Subsequent analyses defined FER-1, a 37-nucleotide sequence located 4.1 kilobases proximal to the start site of transcription, as the target of E1A-mediated transcriptional repression and as an enhancer of the ferritin H gene. FER-1 is composed of an AP1-like sequence followed by an element with dyad symmetry. To achieve maximal enhancer activity and transcriptional repression by E1A, both elements were essential. Using gel retardation assays, we now demonstrate that the binding complex for the AP1-like sequence of FER-1 contains JunD, FosB, and ATF1. Furthermore, JunD and FosB were able to activate FER-1 enhancer activity by transient cotransfection with ferritin H-chloramphenicol acetyltransferase reporter constructs. This augmented enhancer activity was inhibited by E1A. In addition, we have defined the minimal sequence in the dyad element of FER-1 required for protein interaction. This was determined to be a C-rich sequence to which Sp1 and Sp3 bind. Experiments with recombinant proteins indicate that members of both transcription factor families simultaneously bind FER-1. Taken together, these results elucidate molecular mechanisms involved in the transcriptional regulation of a pivotal gene in iron metabolism and provide insights into the contribution of the Sp1 family to the activation of AP1-dependent enhancers.

Transcriptional regulation of the rat poly(ADP-ribose) polymerase gene by Sp1

Expression of the gene encoding poly(ADP-ribose) polymerase (PARP), although ubiquitous, nevertheless varies substantially between tissues. We have recently shown that Sp1 binds five distinct target sequences (US-1 and F1-F4) in the rat PARP (rPARP) gene promoter. Here we used deletion analyses and site-directed mutagenesis to address the regulatory function played by these Sp1 sites on the basal transcriptional activity directed by the rPARP promoter. Transfection experiments revealed that the most proximal Sp1 site is insufficient by itself to direct any promoter activity. In addition, a weak negative regulatory element was identified between positions -101 and -60. The rPARP promoter directed high levels of chloramphenicol acetyltransferase activity in Jurkat T-lymphoblastoid and Ltk- fibroblast cells but only moderate levels in pituitary GH4C1 and liver HTC cells, correlating with the amounts of PARP detected in these cells by western blot analysis. However, the reduced promoter efficiency in HTC and GH4C1 cells did not result from the lack of Sp1 activity in these cells but suggested that yet uncharacterized regulatory proteins might turn off PARP gene expression by binding negative regulatory elements from the rPARP promoter. Similarly, site-directed mutagenesis on the three most proximal Sp1 elements suggested the influence exerted by Sp1 on the rPARP promoter activity to vary substantially between cell types. It also provided evidence for a basal rPARP promoter activity driven through the recognition of unidentified cis-acting elements by transcription factors other than Sp1.

Detection and characterization of Sp1 binding activity in human chondrocytes and its alterations during chondrocyte dedifferentiation

We have detected DNA binding activity for a synthetic oligonucleotide containing an Sp1 consensus sequence in nuclear extracts from human chondrocytes. Changes in the levels of Sp1 oligonucleotide binding activity were examined in nuclear extracts from freshly isolated human chondrocytes, from chondrocytes that had been cultured under conditions that allowed the maintenance of a chondrocyte-specific phenotype on plastic dishes coated with the hydrogel poly(2-hydroxyethyl methacrylate), and from chondrocytes induced to dedifferentiate into fibroblast-like cells by passage in monolayer culture on plastic substrata. It was observed that Sp1 binding was 2-3-fold greater in nuclear extracts from dedifferentiated chondrocytes than in nuclear extracts from either freshly isolated chondrocytes or from cells cultured in suspension. The Sp1 binding activity was specific, since it was competed by unlabeled Sp1 but not by AP1 or AP2. The addition of a polyclonal antibody against Sp1 to nuclear extracts from freshly isolated chondrocytes or to extracts isolated from chondrocytes cultured in monolayer decreased the binding of Sp1 by approximately 85%. However, when the same experiment was carried out with nuclear extracts prepared from cells cultured on poly(2-hydroxyethyl methacrylate)-coated plates, only a very slight inhibition of Sp1 binding was observed. When fragments of the COL2A1 promoter containing putative Sp1 binding sites amplified by polymerase chain reaction were examined, it was found that the amounts of DNA-protein complex formed with nuclear extracts from dedifferentiated chondrocytes were 2-3-fold greater than the amounts formed with nuclear extracts from freshly isolated chondrocytes or from cells cultured in suspension. Quantitation of DNA binding activity by titration experiments demonstrated that nuclear extracts from fibroblast-like cells contained approximately 2-fold greater Sp-1 specific binding activity than nuclear extracts from chondrocytes. The direct role of Sp1 in type II collagen gene transcription was demonstrated by co-transfection experiments of COL2A1 promoter-CAT constructs in Drosophila Schneider line L2 cells that lack Sp1 homologs. This is the first demonstration of Sp1 binding activity in human chondrocytes and of differences in Sp1 DNA binding activity between differentiated and dedifferentiated chondrocytes.

Sp3 encodes multiple proteins that differ in their capacity to stimulate or repress transcription

The product of the retinoblastoma (Rb) susceptibility gene ( RB-1 ) regulates expression of a variety of growth control genes via discrete promoter elements termed retinoblastoma control elements (RCEs). We have previously shown that RCEs are bound and regulated by a common set of ubiquitously expressed nuclear proteins of 115, 95 and 80 kDa, termed retinoblastoma control proteins (RCPs). We have also previously determined that Sp3 and Sp1, two members of the Sp family of transcription factors, encode the 115 and 95 kDa RCPs respectively and that Rb stimulates Sp1/Sp3-mediated transcription in vivo. In this report we have extended these results by determining that the 80 kDa RCP arises from Sp3 mRNA via translational initiation at two internal sites located within the Sp3 trans -activation domain. Internally initiated Sp3 proteins readily bind to Sp1 binding sites in vitro yet have little or no capacity to stimulate transcription of Sp-regulated genes in vivo. Instead, these Sp3-derived proteins function as potent inhibitors of Sp1/Sp3- mediated transcription. Since cell cycle- or signal- induced expression of a variety of genes, including p21 waf1/cip1, p15 INK4B, CYP11A, mdr1 and acetyl-CoA carboxylase, have been mapped to GC-rich promoter elements that bind Sp family members, we speculate that alterations of the protein and/or DNA binding activities of internally initiated Sp3 isoforms may account in part for the regulation of such differentially expressed genes.

A competitive mechanism of CArG element regulation by YY1 and SRF: implications for assessment of Phox1/MHox transcription factor interactions at CArG elements

In the promoters of many immediate early genes, including c-fos, CArG DNA regulatory elements mediate basal constituitive expression and rapid and transient serum induction. CArG boxes also occur in the promoters of muscle-specific genes, including skeletal alpha-actin, where it confers muscle-specific expression. These elements are regulated, at least in part, by the ubiquitous transcription factors serum response factor (SRF) and YY1. The homeobox transcription factor Phox1/MHox has also been implicated in regulation of the c-fos CArG element and is thought to function by facilitating SRF binding to DNA. Here, we provide in vitro and in vivo evidence that the mechanism of YY1 repression of CArG elements results from competition with SRF for overlapping binding sites. We describe in detail the binding sites of YY1 and SRF through serial point mutations of the skeletal alpha-actin proximal CArG element and identify a mutation that dramatically reduces YY1 binding but retains normal SRF binding. YY1 competes with SRF for binding to wild-type CArG elements, but not to this point mutant in vitro. This mutant is sufficient for muscle-specific expression in vivo but is much less sensitive to repression by YY1 overexpression. We utilized the YY1/SRF competition to address the role of Phox1 at these elements. Phox1 overexpression did not diminish YY1-mediated repression, suggesting that transcriptional activation by Phox1 does not result from enhanced SRF binding to these elements. These methods may prove to be useful for assessing interactions between other CArG element regulatory factors.

Inhibition of activator protein 1 activity and neoplastic transformation by aspirin

Aspirin, along with its analgesic-antipyretic uses, is now also being considered for prevention of cardiovascular disease, cancer, and treatment of human immunodeficiency virus infection. Although many of aspirin's pharmacological actions are related to its ability to inhibit prostaglandin biosynthesis, some of its beneficial therapeutic effects are not completely understood. Transcription factor activator protein 1 (AP-1) is critical for the induction of neoplastic transformation and induction of multiple genes involved in inflammation and infection. We have used the JB6 mouse epidermal cell lines, a system that has been used extensively as an in vitro model for the study of tumor promotion and anti-tumor promotion, to study the anti-carcinogenesis effect of aspirin at the molecular level. Aspirin and aspirin-like salicylates inhibited the activation of AP-1 in the same dose range as seen for the inhibition of tumor promoter-induced transformation. The inhibition of AP-1 and tumor promoter-induced transformation in JB6 cells occurs through a prostaglandin independent- and an Erk1- or Erk2-independent pathway. The mechanism of AP-1 and transformation inhibition in this cell culture model may involve the elevation of H+ concentration. The inhibition effects on the activation of AP-1 activity by aspirin and aspirin-like salicylates may further explain the anti-carcinogenesis mechanism of action of these drugs.

Sp1 binding plays a critical role in Erb-B2- and v-ras-mediated downregulation of alpha2-integrin expression in human mammary epithelial cells

The human alpha2-integrin gene is transcriptionally downregulated in a nontumorigenic human mammary epithelial cell line, MTSV1-7, and its clonal variant HB2, overexpressing the Erb-B2 oncogene. In this study, we have used deletion mutations within the alpha2-integrin promoter inserted 5' of the chloramphenicol acetyltransferase or luciferase reporter genes to identify the element that is responsible for the Erb-B2-mediated downregulation. The results of the transient-transfection assay showed that the Sp1 binding element located in the core region (positions --64 to +1) of the alpha2-integrin promoter plays an essential role in the alpha2-integrin promoter activity and its downregulation by Erb-B2. By gel shift assay, we have demonstrated that this element binds with a high degree of affinity not only to Sp1, but also to Sp3. The downregulation of the alpha2-integrin promoter activity could also be achieved by overexpression of v-Hras (v-ras), suggesting that the signals generated by Erb-B2, which lead to downregulation of the alpha2-integrin gene expression, may proceed through the ras pathway. Both the Erb-B2- and the v-ras-overexpressing cells exhibited a Sp1 DNA binding activity lower than that of the parental line, while the relative levels of Sp1 protein in these cells were not altered. The Erb-B2- and v-ras-mediated downregulation could be reversed by the overexpression of Sp1 and by a dominant negative variant of ras (rasN17), confirming the importance of Sp1 and the ras pathway. The inhibitory effects of Erb-B2 on transcriptional activity of the alpha2-integrin promoter were observed in transient-cotransfection assays using alpha2-integrin reporter plasmids and plasmids expressing the Erb-B2 or v-ras oncogene. The same effects were seen when an alpha2-integrin reporter gene construct was transfected into MTSV1-7 or HB2 cells permanently overexpressing Erb-B2 or v-ras. The effects of Erb-B2 or v-ras on the transcriptional activity of the alpha2-integrin promoter were observed in nontumorigenic luminal epithelial cell lines (MTSV1-7 and HB2) as well as in the breast cancer cell line T47D. These data suggest that in luminal epithelial cells and the breast cancers which develop from them, the Erb-B2 proto-oncogene signaling leads to inhibition of (alpha)2(beta)1-integrin gene expression and could contribute to the disruption of tissue architecture seen in breast cancers.

YY1 represses human papillomavirus type 16 transcription by quenching AP-1 activity

YY1 is a multifunctional transcription factor that has been shown to regulate the expression of a number of cellular and viral genes, including the human papillomavirus (HPV) oncogenes E6 and E7. In this study, we have analyzed the YY1-mediated repression of the HPV type 16 (HPV-16) E6-E7 promoter. A systematic analysis to identify YY1 sites present in the HPV-16 long control region showed that of 30 potential YY1 binding motifs, 24 bound purified recombinant YY1 protein, but only 10 of these were able to bind YY1 when nuclear extracts of HeLa cells were used. Of these, only a cluster of five sites, located in the vicinity of an AP-1 motif, were found to be responsible for repressing the HPV-16 P97 promoter. All five sites were required for repression, the mutation of any one site giving rise to a four- to sixfold increase in transcriptional activity. The target for YY1-mediated repression was identified as being a highly conserved AP-1 site, and we propose that AP-1 may represent a common target for YY1 repression. We also provide data demonstrating that YY1 can bind the transcriptional coactivator CREB-binding protein and propose a potentially novel mechanism by which YY1 represses AP-1 activity as a result of this YY1-CREB-binding protein interaction.