Two upstream elements activate transcription of a major histocompatibility complex class I gene in vitro

The Child with Recurrent Mycobacterial Disease

PURPOSE OF REVIEW

Many genetic conditions predispose affected individuals to opportunistic infections. A number of immunodeficiency diseases, including genetic defects termed Mendelian susceptibility to mycobacterial disease (MSMD), permit infection from many different strains of mycobacteria that would otherwise not cause disease. These include tuberculous and nontuberculous mycobacteria, and bacille Calmette-Guérin vaccine (BCG). Patients may present with infections from other organisms that depend on macrophage function for containment. Defects in multiple genes in the IL-12 and NFKB signaling pathways can cause the MSMD phenotype, some of which include IL12RB1, IL12B, IKBKG, ISG15, IFNGR1, IFNGR2, CYBB, TYK2, IRF8, and STAT1.

RECENT FINDINGS

Multiple autosomal recessive and dominant, and 2 X-linked recessive gene defects resulting in the MSMD phenotype have been reported, and others await discovery. This review presents the known gene defects and describes clinical findings that result from the mutations. If MSMD is suspected, a careful clinical history and examination and basic immunodeficiency screening tests will narrow the differential diagnosis. A specific diagnosis requires more sophisticated laboratory investigation. Genetic testing permits a definitive diagnosis, permitting genetic counseling. Mild cases respond well to appropriate antibiotic therapy, whereas severe disease may require hematopoietic stem cell transplantation.

The Icsbp locus is a common proviral insertion site in mature B-cell lymphomas/plasmacytomas induced by exogenous murine leukemia virus

ICSBP (interferon consensus sequence binding protein)/IRF8 (interferon regulatory factor 8) is an interferon gamma-inducible transcription factor expressed predominantly in hematopoietic cells, and down-regulation of this factor has been observed in chronic myelogenous leukemia and acute myeloid leukemia in man. By screening about 1200 murine leukemia virus (MLV)-induced lymphomas, we found proviral insertions at the Icsbp locus in 14 tumors, 13 of which were mature B-cell lymphomas or plasmacytomas. Only one was a T-cell lymphoma, although such tumors constituted about half of the samples screened. This indicates that the Icsbp locus can play a specific role in the development of mature B-lineage malignancies. Two proviral insertions in the last Icsbp exon were found to act by a poly(A)-insertion mechanism. The remaining insertions were found within or outside Icsbp. Since our results showed expression of Icsbp RNA and protein in all end-stage tumor samples, a simple tumor suppressor function of ICSBP is not likely. Interestingly, proviral insertions at Icsbp have not been reported from previous extensive screenings of mature B-cell lymphomas induced by endogenous MLVs. We propose that ICSBP might be involved in an early modulation of an immune response to exogenous MLVs that might also play a role in proliferation of the mature B-cell lymphomas.

Expression of nonclassical MHC class Ib genes: comparison of regulatory elements

Peptide binding proteins of the major histocompatibility complex consist of the "classical" class Ia and "nonclassical" class Ib genes. The gene organization and structure/function relationship of the various exons comprising class I proteins are very similar among the class Ia and class Ib genes. Although the tissue-specific patterns of expression of these two gene families are overlapping, many class Ib genes are distinguished by relative low abundance and/or limited tissue distribution. Further, many of the class Ib genes serve specialized roles in immune responses. Given that the coding sequences of the class Ia and class Ib genes are highly homologous we sought to examine the promoter regions of the various class Ib genes by comparison to the well characterized promoter elements regulating expression of the class Ia genes. This analysis revealed a surprising complexity of promoter structures among all class I genes and few instances of conservation of class Ia promoter regulatory elements among the class Ib genes.

The inverted CCAAT motif is an indispensable element of the enhancer B of the mouse major histocompatibility I H2-Kb gene

We have identified a strong binding of nuclear proteins derived from Ltk(-) fibroblasts to the enhancer B of the mouse MHC class I H2-K(b) gene. The inverted CCAAT motif and its adjacent upstream sequences have been revealed as protein-binding sites by electrophoretic mobility-shift, methylation interference, and DNase I footprint assays. Specific mutations in the inverted CCAAT motif as well as in the 5'-flanking cytosine pentanucleotide abrogated the formation of the major DNA-protein complex. Transcription of the chloramphenicol acetyltransferase (CAT) reporter gene driven by the H2-K(b) promoter in the Ltk(-) cell line was reduced substantially when a two-nucleotide mutation was introduced into the CCAAT element (CCAATCgcAT). The indicated two-nucleotide mutation decreased transcription initiated from both the homologous and a heterologous promoter. Furthermore, cotransfected MHC class II transactivator (CIITA) elevated the transcription of the reporter gene under the control of the H2-K(b) upstream sequences in the NIH 3T3 cell line. The intact enhancer B involving both the inverted CCAAT motif and the site alpha was found to play an indispensable role in the CIITA-mediated gene transactivation. The band-shift assay with the enhancer B probe revealed forming of a protein complex in a cooperative manner, which was again prevented by mutations in either element. Our results suggest an essential role of the inverted CCAAT element in the constitutive as well as inducible transcription of the mouse MHC class I genes.

Identification of CCAAT displacement protein (CDP/cut) as a locus-specific repressor of major histocompatibility complex gene expression in human tumor cells

Human major histocompatibility (MHC) class I antigen expression is important in controlling the metastatic growth of malignant tumors. Locus-specific down-regulation of MHC class I gene expression is frequently observed in human tumors, leading to decreased susceptibility to cytotoxic T-cell-mediated lysis. The mechanism of this down-regulation is incompletely understood. Here, we describe the identification of human CCAAT displacement protein (CDP/cut) as a locus-specific repressor of HLA-B and C gene expression. Transient and stable transfections in HeLa and K562 cells demonstrated the presence of a repressor element 650 base pairs upstream of the first exon of HLA-B7. A specific binding complex with the HLA-B7 and Cw2 repressor elements was demonstrated by EMSA. Formation of the EMSA complex was inhibited specifically with polyclonal antiserum to human CDP/cut, demonstrating that CDP/cut binds the HLA-B7 repressor element. The corresponding region of the HLA-A2 promoter neither repressed HLA-A2 gene expression nor bound CDP/cut. Overexpression of CDP/cut in cell lines deficient in CDP/cut resulted in a nearly 4-fold repression of reporter constructs containing the HLA-B7 repressor element but not the corresponding region of the HLA-A2 promoter. Repression of HLA-B and C gene expression by CDP/cut does not involve displacement of NF-Y, nor is CDP/cut associated with the histone deacetylase HDAC1 when bound to the HLA-B7 repressor element. To our knowledge, these results identify CDP/cut as the first example of a locus-specific repressor of MHC class I gene transcription in human tumor cells.

Differential Requirement of IFN Consensus Sequence Binding Protein for the Production of IL-12 and Induction of Th1-Type Cells in Response to IFN-γ

IFN-γ exerts multiple biological activities in the modulation of immune responses by the induction of transcription factors. One transcriptional factor of the IFN regulatory factor family found to be critical in regulating IL-12-dependent IFN-γ production in vivo following infectious challenge has been designated IFN consensus sequence-binding protein (ICSBP). In this study, the role of ICSBP in regulating type 1 responses to T cell-specific stimulation in vitro was assessed. Total splenocytes from ICSBP−/− mice stimulated with soluble anti-CD3 were markedly impaired in the production of IFN-γ compared with similarly stimulated cells from ICSBP+/+ mice. Consistent with the decrease in IFN-γ production, splenocytes from ICSBP−/− mice stimulated with anti-CD3 in the presence or absence of IFN-γ or a soluble CD40 ligand agonist failed to produce IL-12 p40 and IL-12 p70 protein; however, the deficient production of IFN-γ from ICSBP−/− mice could be restored by the addition of anti-CD28 Ab in an IL-12-independent manner. In contrast to the previous data, production of IFN-γ from naive CD4+/LECAM-1high cells of ICSBP−/− mice that had been primed in vitro with anti-CD3 was similar to or greater than that of ICSBP+/+ controls. In addition, the presence of IFN-γ in priming cultures enhanced both priming for IFN-γ and IL-12 responsiveness from ICSBP−/− CD4+ T cells. Overall, these results provide evidence that ICSBP is differentially required for the ability of IFN-γ to regulate type 1 cytokine responses from APCs and CD4+ T cells.

Human papillomavirus 16 E6 oncoprotein binds to interferon regulatory factor-3 and inhibits its transcriptional activity

Interferon regulatory factor-3 (IRF-3) was found to specifically interact with HPV16 E6 in a yeast two-hybrid screen. IRF-3 is activated by the presence of double-stranded RNA or by virus infection to form a stable complex with other transcriptional regulators that bind to the regulatory elements of the IFNbeta promoter. We show that IRF-3 is a potent transcriptional activator and demonstrate that HPV16 E6 can inhibit its transactivation function. The expression of HPV16 E6 in primary human keratinocytes inhibits the induction of IFNbeta mRNA following Sendai virus infection. The binding of HPV16 E6 to IRF-3 does not result in its ubiquitination or degradation. We propose that the interaction of E6 with IRF-3 and the inhibition of IRF-3's transcriptional activity may provide the virus a means to circumvent the normal antiviral response of an HPV16-infected cell.

Regulation of IRF and STAT gene expression by retinoic acid

Retinoic acid has antiproliferative and differentiative effects on many cell types. However, the molecular mechanisms involved in ATRA (all-trans retinoic acid) -dependent growth inhibition and cell differentiation are poorly understood. On the other hand, several different cytokine specific transcription factors such as signal transducers and activators of transcription (STAT) and interferon regulatory factors (IRF) are known to be instrumental in mediating differentiative, growth regulatory and antiproliferative effects in cells. The IRF family consists of six different proteins, of which IRF-1 has been demonstrated to have antiproliferative and tumor suppressive functions. We have shown that ATRA activates IRF-1 gene expression in several myeloid leukemia cell lines (HL-60, NB4, THP-1, U937), all of which respond to ATRA by growth inhibition. In addition, during ATRA-induced myeloid differentiation, gene expression of STAT1, STAT2, and p48 was upregulated. These proteins are involved in IFN-alpha specific signaling. ATRA-induced expression of IRF and/or STAT transcription factors may be one of the molecular mechanisms mediating growth inhibition by ATRA.

Induction of MHC class I expression by the MHC class II transactivator CIITA

Major histocompatibility complex (MHC) class I-deficient cell lines were used to demonstrate that the MHC class II transactivator (CIITA) can induce surface expression of MHC class I molecules. CIITA induces the promoter of MHC class I heavy chain genes. The site alpha DNA element is the target for CIITA-induced transactivation of class I. In addition, interferon-gamma (IFNgamma)-induced MHC class I expression also requires an intact site alpha. The G3A cell line, which is defective in CIITA induction, does not induce MHC class I antigen and promoter in response to IFNgamma. Trans-dominant-negative forms of CIITA reduce class I MHC promoter function and surface antigen expression. Collectively, these data argue that CIITA has a role in class I MHC gene induction.

Interferon regulatory factors and TFIIB cooperatively regulate interferon-responsive promoter activity in vivo and in vitro

Interferon regulatory factors (IRFs) bind to the interferon-stimulated response element (ISRE) and regulate interferon- and virus-mediated gene expression. IRF-1 acts as a transcriptional activator, while IRF-2 acts as a repressor. Here we show that IRF-1 and IRF-2 bind to both cellular TFIIB, a component of the basal transcription machinery, and recombinant TFIIB (rTFIIB) and that this protein-protein interaction facilitates binding of IRFs to the ISRE. A functional interaction between TFIIB and IRF was assessed by a newly established in vitro transcription assay in which recombinant IRF-1 (rIRF-1) stimulated transcription specifically from an ISRE-containing template. With this assay we show that rIRF-1 and rTFIIB cooperatively enhance the ISRE promoter in vitro. We found that the activity of an ISRE-containing promoter was cooperatively enhanced upon cotransfection of TFIIB and IRF-1 cDNAs into P19 embryonal carcinoma cells, further demonstrating functional interactions in vivo. The cooperative enhancement by TFIIB and IRF-1 was independent of the TATA sequence in the ISRE promoter but dependent on the initiator sequence (Inr) and was abolished when P19 cells were induced to differentiate by retinoic acid treatment. In contrast, cotransfection of TFIIB and IRF-1 into NIH 3T3 cells resulted in a dose-dependent repression of promoter activation which occurred in a TATA-dependent manner. Our results indicate the presence of a cell type-specific factor that mediates the functional interaction between IRFs and TFIIB and that acts in conjunction with the requirement of TATA and Inr for promoter activation.

Immunodeficiency and chronic myelogenous leukemia-like syndrome in mice with a targeted mutation of the ICSBP gene

Interferon consensus sequence binding protein (ICSBP) is a transcription factor of the interferon (IFN) regulatory factor (IRF) family. Mice with a null mutation of ICSBP exhibit two prominent phenotypes related to previously described activities of the IRF family. The first is enhanced susceptibility to virus infections associated with impaired production of IFN(gamma). The second is deregulated hematopoiesis in both ICSBP-/- and ICSBP+/- mice that manifests as a syndrome similar to human chronic myelogenous leukemia. The chronic period of the disease progresses to a fatal blast crisis characterized by a clonal expansion of undifferentiated cells. Normal mice injected with cells from mice in blast crisis developed acute leukemia within 6 weeks of transfer. These results suggest a novel role for ICSBP in regulating the proliferation and differentiation of hematopoietic progenitor cells.

Injury induces rapid changes in hepatocyte nuclear factor-1: DNA binding

BACKGROUND

Transcriptional regulation in the liver plays a critical role in mediating the acute phase response to injury. The molecular mechanisms driving these transcriptional events, however, are poorly defined in vivo. The liver-specific transcription factor hepatocyte nuclear factor (HNF)-1 binds to the 5' upstream region of many acute phase genes. To explore the connection between injury and transcriptional regulatory mechanisms, we investigated the effect of injury on HNF-1 binding activity.

METHODS

Liver nuclear extracts were prepared from animals after burn or anesthetized sham burn injury. HNF-1 binding activity, affinity, and off rate were assessed by electrophoretic mobility shift analysis.

RESULTS

HNF-1 binding activity decreased by 28% 1 1/2 hours after injury. The dissociation constant for HNF-1 increased from 0.6 nm to 11.8 nm at 1 1/2 hours after burn injury partly because of an increase in off rate for the HNF-1: DNA complex.

CONCLUSIONS

Burn injury leads to a significant decrease in HNF-1 binding activity as a result of decreased affinity of HNF-1 for DNA. These injury-induced alterations in binding of a liver-specific transcription factor for its DNA binding site represent a mechanism for rapidly modulating acute phase gene transcription in vivo.

Loss of MHC I transcription trans-activator in the bovine B-LCL, BL3.1

BL3.1, a variant derived from the BLV infected bovine B cell line, BL3, is distinguished by a loss of expression of MHC class I. All surface MHC I products were down-regulated in BL3.1 compared with BL3 correlating with a diminution in MHC I heavy chain transcription. Class I genes, including regulatory elements, showed no aberrations. The variant, BL3.1, did not differ from the parent cell line in expression of Bovine Leukemia Virus (BLV) or oncogene, c-myc. Transient transfection experiments determined the defect was trans rather than cis, and was due to loss of a trans-activator rather than gain of a trans-suppressor as evidenced by transient heterokaryon fusions. Southwestern blot analysis indicated that two DNA binding proteins associated with the MHC class I promoter were missing in BL3.1 cells. The specific response elements for these proteins in BL3 did not appear to be within the enhancerA region, the major enhancer region of the MHC I promoter.

Quantitative increases in DNA binding affinity and positional effects determine 9-cis retinoic acid induced activation of the retinoid X receptor beta homodimer

Retinoid X receptors (RXRs) exert transcriptional activities through heterodimerization with members of the nuclear hormone receptor superfamily. RXRs also act as homodimers and stimulate transcription from an RXR responsive element (RXRE) when bound to 9-cis-retinoic acid (9cRA). Here direct effects of 9cRA have been examined on biochemical and mechanistic parameters of RXR beta. It is shown that 9cRA significantly increases RXR beta homodimer binding affinity to an RXRE (Kd without ligand = 18 nM, Kd with ligand = 6 nM), while decreasing significantly the affinity of RXR beta/thyroid hormone receptor (T3R alpha) heterodimer binding to the same element. Effects on other response elements are also examined. The RXR beta homodimer was found to contact both halves of the RXRE direct repeat, irrespective of the effect of added ligand, while the RXR beta/T3R alpha heterodimer contacted the element only through a specific half-site. Binding of the homodimer to the element functionally activates RXR beta, since RXR beta enhanced transcription in vitro from a specific template in a ligand-dependent fashion. In agreement, transfection of RXR beta alone (but not RXR beta/T3R alpha) led to ligand-dependent activation of a reporter containing the RXRE. Taken together, 9cRA facilitates functional activation of the RXR beta homodimer in an element-dependent manner.

HLA class I allele (HLA-A2) expression defect associated with a mutation in its enhancer B inverted CAT box in two families

The present study shows a very highly diminished HLA-A2 cell surface expression with mendelian segregation in two nonrelated Spanish families. The associated haplotype included Cblank-B38-DRB1*1301-DQ6 in both families. cDNA sequence analysis in two members, one of each pedigree, revealed the presence of the commonest HLA-A2 allele (A*0201), without repetitive mutations that could indicate inappropriate or inefficient translation. Further, the coamplified 3'-untranslated region sequence was also the same described for HLA-A2. HLA-A transcription frequency by means of cDNA PCR-based cloning experiments and by Northern blotting pointed out a relatively low number of HLA-A2 mRNA molecules compared with the complementary HLA-A allele. 5'-Regulatory region sequences from two low-expressing HLA-A2 nonrelated individuals showed a unique and identical single point mutation at position -101 (T to C), when compared with all MHC class I alleles sequenced so far. Position -101 is located in the inverted CAT box associated with the MHC class I enhancer B. The fact that this is an extremely well-conserved position leads us to postulate that this change may be the only responsible for the defective expression of HLA-A2.

Recombinant thyroid hormone receptor and retinoid X receptor stimulate ligand-dependent transcription in vitro

The thyroid hormone and retinoid X receptors form a heterodimer with each other and mediate thyroid hormone (T3)-dependent transcription. Retinoid X receptor, in addition, forms a homodimer and mediates 9-cis-retinoic acid-dependent transcription. Here, recombinant thyroid hormone receptor and recombinant retinoid X receptor beta expressed from baculovirus vectors have been studied for ligand-mediated activation of transcription in vitro. We show that the two recombinant receptors, most likely as a heterodimer, cooperatively enhance transcription in vitro from a template containing functional T3 responsive elements. The enhancement was specific for the T3 responsive element and was greatest when T3 was added to the reaction (approximately 14-fold increase). Albeit to a lesser degree, the two receptors also directed transcription in the absence of T3. Template competition experiments suggest that the two receptors enhance formation of the preinitiation complex and that activation by T3 occurs when the ligand binds the receptor prior to (or during), but not after, the formation of the preinitiation complex. Although 9-cis-retinoic acid had no effect on the T3-dependent transcription, this ligand activated transcription in vitro directed by recombinant retinoic X receptor beta, most likely as a homodimer. This activation was observed when using nuclear extracts from embryonal carcinoma cells as a source of basal transcription factors, but not those from B lymphocytes. These results demonstrate that transcriptional activation mediated by T3 and 9-cis-retinoic acid can be reconstituted in vitro with the respective recombinant receptors.

HLA class I heavy-chain gene promoter elements mediating synergy between tumor necrosis factor and interferons

The cytokines tumor necrosis factor (TNF), beta interferon (IFN-beta), and IFN-gamma increase major histocompatibility complex class I molecule expression. A greater than additive (i.e., synergistic) induction of class I heavy-chain mRNA is observed in HeLa cells treated with TNF in combination with either type of IFN. To define the cis-acting elements mediating cytokine synergy, the promoter of a human major histocompatibility complex class I heavy-chain gene (HLA-B7) was placed in front of a reporter gene and transfected into HeLa cells. Deletion analysis mapped the elements required for synergy to a 40-bp region containing a kappa B-like element, which is necessary for the response to TNF, and an interferon consensus sequence (ICS), which is necessary for the responses to IFNs. When the orientation of these elements was reversed or their normal 20-bp spacing was reduced by 5 or 10 bp, i.e., one half or one full turn of the DNA helix, essentially equivalent responses were obtained, suggesting that these parameters are not critical. In electromobility shift assays, a p50-containing NF-kappa B nuclear factor from TNF-treated cells binds kappa B-containing probes, and ISGF-2 from IFN-gamma-treated cells binds ICS-containing probes. A probe containing both the kappa B and ICS elements (kappa B-ICS) forms a novel complex with nuclear factors isolated from cells treated with both TNF and IFN-gamma; this complex also forms when nuclear factors from individually cytokine-treated cells are mixed in vitro. The natural variant ICS found in HLA-A responds to IFN-gamma and can mediate synergy with TNF. However, the variant kappa B found in HLA-C does not respond to TNF, nor can it mediate synergy between TNF and IFN-gamma. These observations suggest that synergy between TNF and IFNs in the induction of HLA class I gene expression results from the sum of individual interactions of cytokine-activated enhancer-binding factors with the transcription initiation complex.

HLA class I heavy-chain gene promoter elements mediating synergy between tumor necrosis factor and interferons

The cytokines tumor necrosis factor (TNF), beta interferon (IFN-beta), and IFN-gamma increase major histocompatibility complex class I molecule expression. A greater than additive (i.e., synergistic) induction of class I heavy-chain mRNA is observed in HeLa cells treated with TNF in combination with either type of IFN. To define the cis-acting elements mediating cytokine synergy, the promoter of a human major histocompatibility complex class I heavy-chain gene (HLA-B7) was placed in front of a reporter gene and transfected into HeLa cells. Deletion analysis mapped the elements required for synergy to a 40-bp region containing a kappa B-like element, which is necessary for the response to TNF, and an interferon consensus sequence (ICS), which is necessary for the responses to IFNs. When the orientation of these elements was reversed or their normal 20-bp spacing was reduced by 5 or 10 bp, i.e., one half or one full turn of the DNA helix, essentially equivalent responses were obtained, suggesting that these parameters are not critical. In electromobility shift assays, a p50-containing NF-kappa B nuclear factor from TNF-treated cells binds kappa B-containing probes, and ISGF-2 from IFN-gamma-treated cells binds ICS-containing probes. A probe containing both the kappa B and ICS elements (kappa B-ICS) forms a novel complex with nuclear factors isolated from cells treated with both TNF and IFN-gamma; this complex also forms when nuclear factors from individually cytokine-treated cells are mixed in vitro. The natural variant ICS found in HLA-A responds to IFN-gamma and can mediate synergy with TNF. However, the variant kappa B found in HLA-C does not respond to TNF, nor can it mediate synergy between TNF and IFN-gamma. These observations suggest that synergy between TNF and IFNs in the induction of HLA class I gene expression results from the sum of individual interactions of cytokine-activated enhancer-binding factors with the transcription initiation complex.

Retinoic acid induction of major histocompatibility complex class I genes in NTera-2 embryonal carcinoma cells involves induction of NF-kappa B (p50-p65) and retinoic acid receptor beta-retinoid X receptor beta heterodimers

Retinoic acid (RA) treatment of human embryonal carcinoma (EC) NTera-2 (NT2) cells induces expression of major histocompatibility complex (MHC) class I and beta-2 microglobulin surface molecules. We found that this induction was accompanied by increased levels of MHC class I mRNA, which was attributable to the activation of the two conserved upstream enhancers, region I (NF-kappa B like) and region II. This activation coincided with the induction of nuclear factor binding activities specific for the two enhancers. Region I binding activity was not present in undifferentiated NT2 cells, but binding of an NF-kappa B heterodimer, p50-p65, was induced following RA treatment. The p50-p65 heterodimer was produced as a result of de novo induction of p50 and p65 mRNAs. Region II binding activity was present in undifferentiated cells at low levels but was greatly augmented by RA treatment because of activation of a nuclear hormone receptor heterodimer composed of the retinoid X receptor (RXR beta) and the RA receptor (RAR beta). The RXR beta-RAR beta heterodimer also bound RA responsive elements present in other genes which are likely to be involved in RA triggering of EC cell differentiation. Furthermore, transfection of p50 and p65 into undifferentiated NT2 cells synergistically activated region I-dependent MHC class I reporter activity. A similar increase in MHC class I reporter activity was demonstrated by cotransfection of RXR beta and RAR beta. These data show that following RA treatment, heterodimers of two transcription factor families are induced to bind to the MHC enhancers, which at least partly accounts for RA induction of MHC class I expression in NT2 EC cells.

Retinoic acid induction of major histocompatibility complex class I genes in NTera-2 embryonal carcinoma cells involves induction of NF-kappa B (p50-p65) and retinoic acid receptor beta-retinoid X receptor beta heterodimers

Retinoic acid (RA) treatment of human embryonal carcinoma (EC) NTera-2 (NT2) cells induces expression of major histocompatibility complex (MHC) class I and beta-2 microglobulin surface molecules. We found that this induction was accompanied by increased levels of MHC class I mRNA, which was attributable to the activation of the two conserved upstream enhancers, region I (NF-kappa B like) and region II. This activation coincided with the induction of nuclear factor binding activities specific for the two enhancers. Region I binding activity was not present in undifferentiated NT2 cells, but binding of an NF-kappa B heterodimer, p50-p65, was induced following RA treatment. The p50-p65 heterodimer was produced as a result of de novo induction of p50 and p65 mRNAs. Region II binding activity was present in undifferentiated cells at low levels but was greatly augmented by RA treatment because of activation of a nuclear hormone receptor heterodimer composed of the retinoid X receptor (RXR beta) and the RA receptor (RAR beta). The RXR beta-RAR beta heterodimer also bound RA responsive elements present in other genes which are likely to be involved in RA triggering of EC cell differentiation. Furthermore, transfection of p50 and p65 into undifferentiated NT2 cells synergistically activated region I-dependent MHC class I reporter activity. A similar increase in MHC class I reporter activity was demonstrated by cotransfection of RXR beta and RAR beta. These data show that following RA treatment, heterodimers of two transcription factor families are induced to bind to the MHC enhancers, which at least partly accounts for RA induction of MHC class I expression in NT2 EC cells.

Inhibition of estrogen-responsive gene activation by the retinoid X receptor beta: evidence for multiple inhibitory pathways

The retinoid X receptor beta (RXR beta; H-2RIIBP) forms heterodimers with various nuclear hormone receptors and binds multiple hormone response elements, including the estrogen response element (ERE). In this report, we show that endogenous RXR beta contributes to ERE binding activity in nuclear extracts of the human breast cancer cell line MCF-7. To define a possible regulatory role of RXR beta regarding estrogen-responsive transcription in breast cancer cells, RXR beta and a reporter gene driven by the vitellogenin A2 ERE were transfected into estrogen-treated MCF-7 cells. RXR beta inhibited ERE-driven reporter activity in a dose-dependent and element-specific fashion. This inhibition occurred in the absence of the RXR ligand 9-cis retinoic acid. The RXR beta-induced inhibition was specific for estrogen receptor (ER)-mediated ERE activation because inhibition was observed in ER-negative MDA-MB-231 cells only following transfection of the estrogen-activated ER. No inhibition of the basal reporter activity was observed. The inhibition was not caused by simple competition of RXR beta with the ER for ERE binding, since deletion mutants retaining DNA binding activity but lacking the N-terminal or C-terminal domain failed to inhibit reporter activity. In addition, cross-linking studies indicated the presence of an auxiliary nuclear factor present in MCF-7 cells that contributed to RXR beta binding of the ERE. Studies using known heterodimerization partners of RXR beta confirmed that RXR beta/triiodothyronine receptor alpha heterodimers avidly bind the ERE but revealed the existence of another triiodothyronine-independent pathway of ERE inhibition. These results indicate that estrogen-responsive genes may be negatively regulated by RXR beta through two distinct pathways.

Inhibition of estrogen-responsive gene activation by the retinoid X receptor beta: evidence for multiple inhibitory pathways

The retinoid X receptor beta (RXR beta; H-2RIIBP) forms heterodimers with various nuclear hormone receptors and binds multiple hormone response elements, including the estrogen response element (ERE). In this report, we show that endogenous RXR beta contributes to ERE binding activity in nuclear extracts of the human breast cancer cell line MCF-7. To define a possible regulatory role of RXR beta regarding estrogen-responsive transcription in breast cancer cells, RXR beta and a reporter gene driven by the vitellogenin A2 ERE were transfected into estrogen-treated MCF-7 cells. RXR beta inhibited ERE-driven reporter activity in a dose-dependent and element-specific fashion. This inhibition occurred in the absence of the RXR ligand 9-cis retinoic acid. The RXR beta-induced inhibition was specific for estrogen receptor (ER)-mediated ERE activation because inhibition was observed in ER-negative MDA-MB-231 cells only following transfection of the estrogen-activated ER. No inhibition of the basal reporter activity was observed. The inhibition was not caused by simple competition of RXR beta with the ER for ERE binding, since deletion mutants retaining DNA binding activity but lacking the N-terminal or C-terminal domain failed to inhibit reporter activity. In addition, cross-linking studies indicated the presence of an auxiliary nuclear factor present in MCF-7 cells that contributed to RXR beta binding of the ERE. Studies using known heterodimerization partners of RXR beta confirmed that RXR beta/triiodothyronine receptor alpha heterodimers avidly bind the ERE but revealed the existence of another triiodothyronine-independent pathway of ERE inhibition. These results indicate that estrogen-responsive genes may be negatively regulated by RXR beta through two distinct pathways.

Regulation of MHC gene expression

This review focuses on recent progress made in MHC regulation. The better characterization of proteins that interact with MHC class I and II promoters and the isolation of genes encoding several of these transcription factors, such as H-2RIIBP/RXR beta, NK kappa B, I-kappa B, hXBP-1 and NF-Y, allow the functional analysis of these molecules in MHC gene regulation. The application of new techniques, such as genomic in vivo footprinting analysis, to the study of these promoters provides insights into the status of in vivo protein-DNA interaction over these promoters. New insights have also been gained in the understanding of MHC-associated genes.

Interferon consensus sequence-binding protein, a member of the interferon regulatory factor family, suppresses interferon-induced gene transcription

We previously isolated a cDNA clone encoding interferon consensus sequence-binding protein (ICSBP), a member of the interferon regulatory factor (IRF) family, that binds to the interferon (IFN)-stimulated response element (ISRE) of many IFN-regulated genes. In this investigation, we studied the functional role of ICSBP by transient cotransfection of ICSBP cDNA with IFN-responsive reporter genes into the human embryonal carcinoma cell line N-Tera2. These cells were shown not to express ICSBP or IRF-2, thus allowing functional analysis of transfected cDNAs. Cotransfection of ICSBP into cells treated with retinoic acid or any of the IFNs (alpha, beta, or gamma) repressed expression of a chloramphenicol acetyltransferase reporter driven by the major histocompatibility complex class I gene promoter. Similarly, ICSBP repressed expression of chloramphenicol acetyltransferase reporters driven by the ISREs of the 2'-5' oligoadenylate synthetase, guanylate-binding protein, and ISG-15 genes in IFN-treated cells. The repression was dependent on the presence of the ISRE in the reporter. Deletion analysis showed that the putative N-terminal DNA binding domain of ICSBP by itself is capable of mediating the repression. Using the same cotransfection conditions as for ICSBP, a similar repression of these reporters was observed with IRF-2. Finally, ICSBP repressed the IRF-1-mediated induction of major histocompatibility complex class I and IFN-beta reporters in the absence of IFN or retinoic acid. Taken together, these results suggest that ICSBP is a negative regulatory factor capable of repressing transcription of target genes induced by IFN, retinoic acid, or IRF-1.

Interferon consensus sequence-binding protein, a member of the interferon regulatory factor family, suppresses interferon-induced gene transcription

We previously isolated a cDNA clone encoding interferon consensus sequence-binding protein (ICSBP), a member of the interferon regulatory factor (IRF) family, that binds to the interferon (IFN)-stimulated response element (ISRE) of many IFN-regulated genes. In this investigation, we studied the functional role of ICSBP by transient cotransfection of ICSBP cDNA with IFN-responsive reporter genes into the human embryonal carcinoma cell line N-Tera2. These cells were shown not to express ICSBP or IRF-2, thus allowing functional analysis of transfected cDNAs. Cotransfection of ICSBP into cells treated with retinoic acid or any of the IFNs (alpha, beta, or gamma) repressed expression of a chloramphenicol acetyltransferase reporter driven by the major histocompatibility complex class I gene promoter. Similarly, ICSBP repressed expression of chloramphenicol acetyltransferase reporters driven by the ISREs of the 2'-5' oligoadenylate synthetase, guanylate-binding protein, and ISG-15 genes in IFN-treated cells. The repression was dependent on the presence of the ISRE in the reporter. Deletion analysis showed that the putative N-terminal DNA binding domain of ICSBP by itself is capable of mediating the repression. Using the same cotransfection conditions as for ICSBP, a similar repression of these reporters was observed with IRF-2. Finally, ICSBP repressed the IRF-1-mediated induction of major histocompatibility complex class I and IFN-beta reporters in the absence of IFN or retinoic acid. Taken together, these results suggest that ICSBP is a negative regulatory factor capable of repressing transcription of target genes induced by IFN, retinoic acid, or IRF-1.