Constitutively expressed Oct-2 prevents immunoglobulin gene silencing in myeloma x T cell hybrids

POU2F2+ B cells enhance antitumor immunity and predict better survival in non small cell lung cancer

Immune checkpoint inhibitors are an effective adjuvant therapy for non-small cell lung cancer (NSCLC). Recent studies have highlighted the critical role of tumor-infiltrating B cells in tumor immunity. However, research specifically focusing on B cells in NSCLC is limited. This study aims to elucidate the role of POU2F2+ B cells in patient survival and immune cell infiltration in NSCLC. Pseudotime analysis was performed to identify B cell pseudotime-related gene sets from two single-cell RNA sequencing (scRNA-seq) datasets of NSCLC. Differentially expressed genes (DEGs) were identified from two NSCLC immunotherapy-related bulk RNA sequencing datasets. A Venn diagram was used to determine core genes shared between these datasets. Kaplan-Meier survival curves were utilized to analyze overall survival (OS). Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses were performed based on the differential genes between POU2F2+ and POU2F2- B cells. CIBERSORT analysis was conducted to compare the proportions of immune cell subpopulations between groups. Multiplex immunohistochemistry (mIHC) was used to localize POU2F2+ cells and measure distances between different immune cells. Three hallmark genes, POU2F2, CD2, and CST7, were identified as being associated with B cell maturation and immunotherapy efficacy in NSCLC. High expression of POU2F2 was associated with poorer OS in both LUAD and LUSC. However, the POU2F2+ B cell score specifically correlated with the OS of LUAD but not with LUSC. Further analysis using scRNA-seq and mIHC methods revealed that POU2F2 is predominantly expressed in B cells. In LUAD tumor tissues, POU2F2+ CD20+ B cells were spatially further from PD-1+ CD8+ T cells and CD206+ CD68+ macrophages compared to POU2F2- CD20+ B cells. In LUSC tumor tissues, POU2F2+ CD20+ B cells were spatially further from CD206+ CD68+ macrophages but showed no significant spatial difference from PD-1+ CD8+ T cells compared to POU2F2- CD20+ B cells. In patients with high POU2F2+ B cell scores, LUAD tissues showed an increased proportion of CD8+ T cells and M1 macrophages, and a decreased proportion of M2 macrophages. In contrast, in LUSC tissues, a high POU2F2+ B cell score was associated only with an increased proportion of M1 macrophages, with no significant differences in the proportions of CD8+ T cells or M2 macrophages between groups. This study elucidates the significant role of POU2F2+ B cells in influencing survival and immune cell infiltration in NSCLC. Our findings highlight POU2F2 as a novel target for NSCLC immunotherapy. Targeting POU2F2 may modulate the tumor immune microenvironment, enhance the infiltration and activity of critical immune cells, and ultimately improve patient survival.

Exploring sex-specific genetic architecture of coronary artery disease in Tehran: a cardiometabolic genetic study

BACKGROUND

The development of coronary artery disease (CAD) is influenced by sex and genetic factors. Genome-wide association studies (GWAS) have linked genetic loci to CAD, mostly in European populations. The study aims to find sex-related genetic differences in the Iranian population.

RESEARCH DESIGN AND METHODS

We conducted a sex-stratified GWAS with 4519 subjects (1832 males and 2687 females) in the discovery group and 922 subjects (495 males and 427 females) in the confirmation group of an Iranian cohort. We analyzed 9,141,124 variants using a genome-wide complex trait analysis (GCTA) tool.

RESULTS

We detected distinct genetic variants associated with CAD in males: rs34952209 [OR = 1.79; p = 5.216E-8], rs1432687863 [OR = 1.95; p = 8.477E-8], and in females, rs7314741 [OR = 1.67; p = 7.142-8E] positively influenced CAD risk. The CAD-associated SNPs that were obtained have been confirmed using independent samples. Rs3495229 May impact histone mark and Pou2f2 motifs, while rs7314741 in the LEM Domain Containing 3 (LEMD3) promoter may affect a regulatory motif for the STAT transcription factor. According to Roadmap and ENCODE data, Rs1432687863 is a new variant affecting CAD in males, potentially through H3K9me3 in the heart.

CONCLUSIONS

Our findings highlight the role of sex-specific genetic differences in CAD development, providing novel insights into disease pathways which is not appropriate using a sex-combined strategy. [Figure: see text].

POU2F2 promotes the proliferation and motility of lung cancer cells by activating AGO1

Background

To detect and investigate the expression of POU domain class 2 transcription factor 2 (POU2F2) in human lung cancer tissues, its role in lung cancer progression, and the potential mechanisms.

Methods

Immunohistochemical (IHC) assays were conducted to assess the expression of POU2F2 in human lung cancer tissues. Immunoblot assays were performed to assess the expression levels of POU2F2 in human lung cancer tissues and cell lines. CCK-8, colony formation, and transwell-migration/invasion assays were conducted to detect the effects of POU2F2 and AGO1 on the proliferaion and motility of A549 and H1299 cells in vitro. CHIP and luciferase assays were performed for the mechanism study. A tumor xenotransplantation model was used to detect the effects of POU2F2 on tumor growth in vivo.

Results

We found POU2F2 was highly expressed in human lung cancer tissues and cell lines, and associated with the lung cancer patients’ prognosis and clinical features. POU2F2 promoted the proliferation, and motility of lung cancer cells via targeting AGO1 in vitro. Additionally, POU2F2 promoted tumor growth of lung cancer cells via AGO1 in vivo.

Conclusion

We found POU2F2 was highly expressed in lung cancer cells and confirmed the involvement of POU2F2 in lung cancer progression, and thought POU2F2 could act as a potential therapeutic target for lung cancer.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12890-021-01476-9.

Genome-wide analysis of hepatic gene silencing in mammalian cell hybrids

Silencing of tissue-specific gene expression in mammalian somatic cell hybrids is a well-documented epigenetic phenomenon which is both profound (involving a large number of genes) and enigmatic. Our aim was to utilize whole-genome microarray analyses to determine the true extent of gene silencing on a genomic level. By comparing gene expression profiles of hepatoma×fibroblast cell hybrids with those of parental cells, we have identified over 300 liver-enriched genes that are repressed at least 5-fold in the cell hybrids, the majority of which are repressed at least 10-fold. Also, we identify nearly 200 fibroblast-enriched genes that are repressed at least 5-fold. Silenced hepatic genes include several that encode transcription factors and proteins involved in signal transduction pathways. These data suggest that extensive reprogramming occurs in cell hybrids, leading to a nearly global (although not complete) loss of tissue-specific gene expression.

Changes in replication, nuclear location, and expression of the Igh locus after fusion of a pre-B cell line with a T cell line

We have previously observed that replication and nuclear location of the murine Igh locus are developmentally regulated during B cell differentiation. In non-B, B, and plasma cells, sequences near the 3' end of the Igh locus replicate early in S while upstream Vh sequences replicate late in S, and the Igh locus is located near the nuclear periphery. In fact, in MEL non-B cells, replication of a 500-kb segment containing Igh-C and flanking sequences occurs progressively later throughout S by 3' to 5' unidirectional fork movement. In contrast, in pro- and pre-B cells, the entire 3-Mb Igh locus is located away from the nuclear periphery and replicates early in S by forks progressing in both directions. In this study, using an 18-81 (pre-B) x BW5147 (T) cell fusion system in which Igh expression is extinguished, we found that in all Igh alleles, Vh sequences replicated later in S than 3' Igh sequences (similar to that detected in BW5147), but the Igh locus was situated away from the nuclear periphery (similar to that observed in 18-81). Thus, pre-B cell-derived Igh genes had changes in replication timing, but not in nuclear location, whereas T cell-derived Igh genes changed their nuclear location but not their replication timing. These data are consistent with the silencing of a pre-B cell-specific replication program in the fusion hybrid cells and independent regulation of the nuclear location of Igh loci.

Phosphorylation of C/EBPalpha inhibits granulopoiesis

CCAAT/enhancer-binding protein alpha (C/EBPalpha) is one of the key transcription factors that mediate lineage specification and differentiation of multipotent myeloid progenitors into mature granulocytes. Although C/EBPalpha is known to induce granulopoiesis while suppressing monocyte differentiation, it is unclear how C/EBPalpha regulates this cell fate choice at the mechanistic level. Here we report that inducers of monocyte differentiation inhibit the alternate cell fate choice, that of granulopoiesis, through inhibition of C/EBPalpha. This inhibition is mediated by extracellular signal-regulated kinases 1 and/or 2 (ERK1/2), which interact with C/EBPalpha through an FXFP docking site and phosphorylate serine 21. As a consequence of C/EBPalpha phosphorylation, induction of granulocyte differentiation by C/EBPalpha or retinoic acid is inhibited. Our analysis of C/EBPalpha by fluorescent resonance energy transfer revealed that phosphorylation induces conformational changes in C/EBPalpha, increasing the distance between the amino termini of C/EBPalpha dimers. Thus, myeloid development is partly regulated by an ERK1/2-mediated change in the conformation of C/EBPalpha that favors monocyte differentiation by blocking granulopoiesis.

Critical Role for the Oct-2/OCA-B Partnership in Ig-Secreting Cells

B and T lymphocytes arise from a common precursor in the bone marrow, but ultimately acquire very different functions. The difference in function is largely attributable to the expression of tissue-specific transcription factors that activate discrete sets of genes. In previous studies we and others have shown that the specialized genes expressed by Ig-secreting cells cease transcription when these cells are fused to a T lymphoma. The extinguished genes include those encoding Ig, J chain, and the transcription factors Oct-2, PU.1, and the coactivator OCA-B. Remarkably, if we sustain Oct-2 expression during cell fusion, all the other tissue-specific genes of the Ig-secreting cell simultaneously escape silencing. This suggests that Oct-2 plays a central role in maintaining the gene expression program of these cells. In the present studies we have investigated the roles of the transcription factor PU.1 and the coactivator OCA-B within the hierarchy of regulatory factors that sustain Ig-secreting cell function. Our results show that OCA-B and Oct-2 are regulatory partners in this process and that PU.1 plays a subordinate role at this cell stage.

The amino terminal and E2F interaction domains are critical for C/EBP alpha-mediated induction of granulopoietic development of hematopoietic cells

The transcription factor C/EBP alpha (CCAAT/enhancer binding protein alpha) is critical for granulopoiesis. Gene disruption in mice blocks early granulocyte differentiation and disruption of C/EBP alpha function has been implicated in human acute myeloid leukemia (AML), but no systematic structure-function analysis has been undertaken to identify the mechanisms involved in C/EBP alpha-mediated granulocyte differentiation. Here we demonstrate that loss of either of 2 key regions results in disruption of C/EBP alpha granulocytic development: the amino terminus and specific residues residing on the non-DNA binding face of the basic region. Mutation of either results in loss of C/EBP alpha inhibition of E2F and down-regulation of c-Myc, but only mutation of the basic region results in loss of physical interaction with E2F. In contrast, while the amino terminal mutant retains the ability to interact with E2F, this mutant fails to bind a C/EBP alpha site efficiently, fails to activate C/EBP alpha target genes, and is also defective in inhibition of E2F activity. These results further emphasize the importance of inhibition of proliferative pathways in granulopoiesis and demonstrate that several regions of the C/EBP alpha protein are involved in this mechanism.

Disruption of the B-cell specific transcriptional program in HHV-8 associated primary effusion lymphoma cell lines

Primary effusion lymphoma (PEL) is a lymphoproliferative disease of B-cell origin that is associated with HHV-8 infection. PEL cells harbor a non-B, non-T phenotype and lack significant surface immunoglobulin (Ig) expression, a characteristic that has not been fully explained. In the present study, we demonstrate that PEL cells constitutively express interferon regulatory factor (IRF)-4, a transcription factor that regulates the activity of the immunoglobulin light-chain enhancer elements lambdaB and kappaE3' through binding to a composite Ets-IRF site. IRF-4 activity requires its physical interaction with PU.1, an Ets family member involved in the activation of genes essential for B-cell development. However, in PEL-derived B-cell lines, PU.1 expression was completely abrogated; expression of the B cell specific transcription factor Oct-2, which is known to regulate PU.1 expression, was also abolished. Moreover, the B-cell-specific coactivator of octamer factors, BOB-1/OcaB, was expressed at very decreased levels in PEL cells. Ectopic expression of Oct-2 was able to fully restore PU.1 promoter activity in the PEL cell line BCBL-1, while PU.1 expression also reconstituted the activity of the lambdaB Ets-IRF site. In addition, protein levels of BSAP/Pax-5 and IRF-8/ICSBP were undetectable in PEL cells. The pattern of transcription factor ablation observed in PEL was found to be comparable to that observed in classical Hodgkin's disease-derived cell lines, which also lack B-cell-specific surface markers. These observations indicate that disruption of the B-cell-specific transcriptional program is likely to contribute to the incomplete B-cell phenotype characteristic of PEL cells.

Transgenic targeting with regulatory elements of the human CD34 gene

The human CD34 gene is expressed on early progenitor and stem cells in the bone marrow. Here we report the isolation of the human CD34 locus from a human P1 artificial chromosome (PAC) library and the characterization and evaluation of this genomic fragment for expression of reporter genes in stable cell lines and transgenic mice. We show that a 160-kb fragment spanning 110 kb of the 5' flanking region and 26 kb of the 3' flanking region of the CD34 gene directs expression of the human CD34 gene in the bone marrow of transgenic mice. The expression of human CD34 transgenic RNA in tissues was found to be similar to that of the endogenous murine CD34 gene. Colony-forming cell assays showed that bone marrow cells staining positive for human CD34 consist of early progenitor cells in which expression of CD34 decreased with cell maturation. In order to test the construct for its ability to express heterologous genes in vivo, we used homologous recombination in bacteria to insert the tetracycline-responsive transactivator protein tTA. Analysis of transgenic human CD34-tTA mice by cross breeding with a strain carrying Cre recombinase under control of a tetracycline-responsive element demonstrated induction of Cre expression in mice in a pattern consistent with the expression of the human CD34 transgene.

Unique function for carboxyl-terminal domain of Oct-2 in Ig-secreting cells

The activity of Ig gene promoters and enhancers is regulated by two related transcription factors, Oct-1 (ubiquitous) and Oct-2 (B lineage specific), which bind the octamer motif (ATTTGCAT) present in these elements. As Ig promoter-binding factors, Oct-1 and Oct-2 each work together with a B lymphocyte-specific cofactor OCA-B/OBF-1/Bob-1 that interacts with them through their POU (DNA-binding) domains. Because both can mediate Ig promoter activity in B cells, there has been some question as to whether these two octamer-binding factors serve distinct functions in lymphocytes. We have shown previously that the silencing of B lymphocyte-specific genes in plasmacytoma x T lymphoma hybrids can be prevented by preserving Oct-2 expression. The pronounced effect of this transcription factor on the phenotype of plasmacytoma x T lymphoma hybrids established a critical role for Oct-2 not only in maintaining Ig gene expression, but in maintaining the overall genetic program of Ig-secreting cells. In the present study, we have explored the functional differences between Oct-1 and Oct-2 using chimeric Oct-1/Oct-2 proteins in cell fusion assays. Our results provide further evidence for an essential role for Oct-2 in Ig-secreting cells and identify the C-terminal domain of Oct-2 as responsible for its unique function in these cells.

Extinction of expression of the genes encoding haematopoietic cell-restricted transcription factors in T-lymphoma x fibroblast cell hybrids

We previously reported that expression of the T-cell receptor (TCR) alpha and lck genes is extinguished in hybrids between mouse T-lymphoma EL4 cells and mouse fibroblast B82 cells. In the present study, we found that the activities of the TCRalpha minimum enhancer and the lck promoter monitored by the luciferase or chloramphenicol acetyltransferase (CAT) assays were markedly inhibited in the hybrids. Expression of the TCF-1, LEF-1, GATA-3, Ikaros, c-myb and Fli-1 genes, which encode the haematopoietic cell-restricted transcription factors that appear to be responsible for the activities of the enhancer and the promoter, was fully extinguished or markedly suppressed in the hybrids. On the other hand, expression of the transcription factor genes observed in both parental cells, such as the AML1 and c-ets-1 genes, and that of the genes encoding ubiquitously expressed transcription factors, such as the E2A, CREB and c-ets-2 genes, was not significantly suppressed in the hybrids. These results suggest that the genes encoding haematopoietic cell-restricted transcription factors are targets for negative regulation in fibroblastic background and that the repression of these genes may consequently lead to suppression of the promoter and/or enhancer activities of several T-cell-specific structural genes in T-lymphoma x fibroblast cell hybrids.

Deletional analyses reveal an essential role for the hs3b/hs4 IgH 3' enhancer pair in an Ig-secreting but not an earlier-stage B cell line

The Ig heavy chain (IgH) locus is controlled by multiple regulatory sequences mapping both within the IgH transcription unit (E mu) and downstream (3') of IgH coding sequences (hs3a, hs1,2, hs3b and hs4). Enhancer knockout studies in mice have implicated E mu in the control of IgH variable region gene assembly, but single-enhancer knockouts involving the 3' IgH enhancers have yet to shed light on their function. Transfection studies in mice and cell lines have suggested that the 3' enhancers behave similarly to a locus control region as first identified in the beta-globin locus. We have exploited this property to form mini-loci in a surface Ig(+) and an Ig-secreting cell line as a means for studying the functions of the 3' IgH enhancers. Importantly, this experimental system allows for the analysis of enhancer function within the context of chromatin. The mini-loci consisted of an Ig gamma 2b transcription unit linked to the four murine 3' IgH enhancers. Using targeted deletions of enhancer pairs within these mini-loci, we have discovered a critical and apparently developmentally regulated role for the hs3b/hs4 enhancer pair in IgH transgene expression.

A Nuclear Factor Y (NFY) Site Positively Regulates the Human CD34 Stem Cell Gene

Proper regulation of the human CD34 gene requires a combinatorial action of multiple proximal and long-range, ciselements. This report shows that, like the murine CD34 5′ untranslated region (UTR), the corresponding region of the human CD34 gene is necessary for optimal promoter activity. We localized the most critical element of this region to base pairs +48/+75. Through oligonucleotide competition and antibody supershift experiments in electrophoretic mobility shift assays, we found that this sequence contains a binding site (CCAAT box) for the transcription factor NFY (nuclear factor Y), a factor mediating cell type-specific and cell-cycle regulated expression of genes. Mutating this site led to a 5-fold decrease in CD34 promoter activity in transient transfection experiments. Interestingly, NFY binds adjacently to the earlier identified c-myb binding site. Here we show that both binding sites are important for CD34 promoter function: mutating either site alone decreased CD34 promoter-driven reporter gene activity 4-fold. We also show that the integrity of the c-myb binding site is necessary for stabilization of NFY binding to its site. Such cooperation between c-myb, which is expressed in early hematopoietic cells, and NFY, which is expressed in many cell types, might contribute to specific activation of CD34 in stem cells. The CCAAT box motif was also noted in the 5′ UTR of the murine CD34 gene, however, NFY did not bind to this region. Thus, our results indicate that the functional similarities between the human and murine CD34 5′ UTRs are achieved through different molecular mechanism(s).

A Nuclear Factor Y (NFY) Site Positively Regulates the Human CD34 Stem Cell Gene

Proper regulation of the human CD34 gene requires a combinatorial action of multiple proximal and long-range, ciselements. This report shows that, like the murine CD34 5′ untranslated region (UTR), the corresponding region of the human CD34 gene is necessary for optimal promoter activity. We localized the most critical element of this region to base pairs +48/+75. Through oligonucleotide competition and antibody supershift experiments in electrophoretic mobility shift assays, we found that this sequence contains a binding site (CCAAT box) for the transcription factor NFY (nuclear factor Y), a factor mediating cell type-specific and cell-cycle regulated expression of genes. Mutating this site led to a 5-fold decrease in CD34 promoter activity in transient transfection experiments. Interestingly, NFY binds adjacently to the earlier identified c-myb binding site. Here we show that both binding sites are important for CD34 promoter function: mutating either site alone decreased CD34 promoter-driven reporter gene activity 4-fold. We also show that the integrity of the c-myb binding site is necessary for stabilization of NFY binding to its site. Such cooperation between c-myb, which is expressed in early hematopoietic cells, and NFY, which is expressed in many cell types, might contribute to specific activation of CD34 in stem cells. The CCAAT box motif was also noted in the 5′ UTR of the murine CD34 gene, however, NFY did not bind to this region. Thus, our results indicate that the functional similarities between the human and murine CD34 5′ UTRs are achieved through different molecular mechanism(s).

Cux/CDP homeoprotein is a component of NF-muNR and represses the immunoglobulin heavy chain intronic enhancer by antagonizing the bright transcription activator

Nuclear matrix attachment regions (MARs) flanking the immunoglobulin heavy chain intronic enhancer (Emu) are the targets of the negative regulator, NF-muNR, found in non-B and early pre-B cells. Expression library screening with NF-muNR binding sites yielded a cDNA clone encoding an alternatively spliced form of the Cux/CDP homeodomain protein. Cux/CDP fulfills criteria required for NF-muNR identity. It is expressed in non-B and early pre-B cells but not mature B cells. It binds to NF-muNR binding sites within Emu with appropriate differential affinities. Antiserum specific for Cux/CDP recognizes a polypeptide of the predicted size in affinity-purified NF-muNR preparations and binds NF-muNR complexed with DNA. Cotransfection with Cux/CDP represses the activity of Emu via the MAR sequences in both B and non-B cells. Cux/CDP antagonizes the effects of the Bright transcription activator at both the DNA binding and functional levels. We propose that Cux/CDP regulates cell-type-restricted, differentiation stage-specific Emu enhancer activity by interfering with the function of nuclear matrix-bound transcription activators.

Multiple control elements are required for expression of the human CD34 gene

Two cis regulatory elements of the human CD34 gene, the promoter and a 3' enhancer, have previously been described. In transient transfection assays, the promoter was not sufficient to direct cell type specific expression. In contrast, the 3' enhancer was active only in CD34+ cell lines, suggesting that this element might be responsible for stem cell-restricted expression of the CD34 gene. In the current work, through deletion and transient transfection experiments, we delineated the core enhancer sequence. We examined the role of this element upon stable integration. Our data suggested the presence of additional control elements. In order to identify them, using DNaseI hypersensitivity and methylation studies, we determined the chromatin structure of the entire CD34 locus. Amongst a number of DNaseI hypersensitive sites, we detected a strong CD34+ cell type-specific site in intron 4. This region, however, did not work as an enhancer by itself. By analyzing stable transfectants and transgenic animals, we demonstrated that the 3' enhancer and intron 4 hypersensitive regions, either alone or together, did not function as a locus control region upon chromosomal integration. In contrast, a 160kb genomic fragment encompassing the entire CD34 gene contained regulatory elements sufficient for high-level CD34 mRNA expression in murine stable lines. Our data indicate that combinatorial action of multiple, proximal and long-range, cis elements is necessary for proper regulation of CD34 expression.

CCAAT/enhancer binding protein alpha is a regulatory switch sufficient for induction of granulocytic development from bipotential myeloid progenitors

The transcription factor CCAAT/enhancer binding protein alpha (C/EBPalpha) regulates a number of myeloid cell-specific genes. To delineate the role of C/EBPalpha in human granulopoiesis, we studied its expression and function in human primary cells and bipotential (granulocytic/monocytic) myeloid cell lines. We show that the expression of C/EBPalpha initiates with the commitment of multipotential precursors to the myeloid lineage, is specifically upregulated during granulocytic differentiation, and is rapidly downregulated during the alternative monocytic pathway. Conditional expression of C/EBPalpha alone in stably transfected bipotential cells triggers neutrophilic differentiation, concomitant with upregulation of the granulocyte-specific granulocyte colony-stimulating factor receptor and secondary granule protein genes. Moreover, induced expression of C/EBPalpha in bipotential precursors blocks their monocytic differentiation program. These results indicate that C/EBPalpha serves as a myeloid differentiation switch acting on bipotential precursors and directing them to mature to granulocytes.

3′ IgH Enhancer Elements Shift Synergistic Interactions During B Cell Development

IgH gene expression is tightly controlled over the course of B cell development, B cell activation, and the subsequent differentiation of these cells into Ig-secreting plasmacytes. There are several transcriptional enhancers that map within and downstream of the IgH locus, and some of these have been clearly implicated in the developmental regulation of IgH gene assembly and expression. While some of the individual enhancers from this locus have been studied extensively, the functional interactions possible among this group of enhancers have been largely unexplored. In the present study, we have measured the transcriptional activities of combinations of enhancers introduced into B-lineage cell lines at several different developmental stages. We detected a developmental progression in which the 3′ enhancers are initially inactive, then become strongly active through synergistic interactions, and finally achieve a strong level of activity with little interdependency. The relative contributions of Eμ (the intron enhancer) and of the 3′ enhancers also change as a function of developmental stage. We discuss these results in light of parallel studies of developmental changes in transcription factor requirements.

Hepatocyte nuclear factor-4 prevents silencing of hepatocyte nuclear factor-1 expression in hepatoma x fibroblast cell hybrids

Hepatocyte nuclear factors-1alpha (HNF1alpha) and -4 (HNF4) are components of a liver-enriched transcription activation pathway which is thought to play a critical role in hepatocyte-specific gene expression, including activation of alpha1-antitrypsin gene expression. HNF1alpha, HNF4 and alpha1-antitrypsin (alpha1AT) genes are extinguished in hepatoma/fibroblast somatic cell hybrids, suggesting that fibroblasts contain a repressor-like activity. To determine the molecular basis for silencing of these genes in cell hybrids, ectopic expression of HNF1alpha and HNF4 was used. Results show that constitutive expression of HNF4 prevents extinction of HNF1alpha gene expression in hepatoma/fibroblast hybrids. In contrast, forced HNF1alpha expression failed to prevent extinction of the HNF4 locus in cell hybrids. Likewise, the alpha1AT gene remained silent in the presence of both HNF1alpha and HNF4. These results suggest that extinction of HNF1alpha is a simple lack-of-activation phenotype, whereas extinction of HNF4 andalpha1AT loci is more complex, perhaps involving negative regulation.

Octamer independent activation of transcription from the kappa immunoglobulin germline promoter

Previous analyses of immunoglobulin V region promoters has led to the discovery of a common octamer motif which is functionally important in the tissue-specific and developmentally regulated transcriptional activation of immunoglobulin genes. The germline promoters (Ko) located upstream of the J region gene segments of the kappa locus also contain an octamer motif (containing a single base pair mutation and referred to as the variant octamer) which has been shown previously to bind Oct-1 and Oct-2 transcription factors in vitro. To further elucidate the role of this variant octamer motif in the regulation of germline transcription from the unrearranged kappa locus, we have quantitated the relative binding affinity of Oct-1 and Oct-2 for the variant octamer motif and determined the functional role of this octamer motif in transcriptional activation. We find that, although the variant octamer motif binds Oct-1 and Oct-2 in vitro with 5-fold lower affinity than the consensus octamer motif, mutation of the variant octamer motif to either a consensus octamer or non-octamer motif has no effect on transcriptional activation from the germline promoter. We also find significant differences in activation of germline and V region promoters by kappa enhancers. Our results suggest that the germline promoters and V region promoters differ in their dependence on octamer for activation and respond differently to enhancer activation. These findings have important implications in regulation of germline transcription as well as concomitant activation of the V-J recombination of the kappa light chain locus.

Octamer binding factors and their coactivator can activate the murine PU.1 (spi-1) promoter

PU.1 (spi-1), a member of the Ets transcription factor family, is predominantly expressed in myeloid and B cells, activates many B cell and myeloid genes, and is critical for development of both of these lineages. Our previous studies (Chen, H. M., Ray-Gallet, D., Zhang, P., Hetherington, C. J., Gonzalez, D. A., Zhang, D.-E., Moreau-Gachelin, F., and Tenen, D. G. (1995) Oncogene 11, 1549-1560) demonstrate that the PU.1 promoter directs cell type-specific reporter gene expression in myeloid cell lines, and that PU.1 activates its own promoter in an autoregulatory loop. Here we show that the murine PU.1 promoter is also specifically and highly functional in B cell lines as well. Oct-1 and Oct-2 can bind specifically to a site at base pair -55 in vitro, and this site is specifically protected in B cells in vivo. We also demonstrate that two other sites contribute to promoter activity in B cells; an Sp1 binding site adjacent to the octamer site, and the PU.1 autoregulatory site. Finally, we show that the B cell coactivator OBF-1/Bob1/OCA-B is only expressed in B cells and not in myeloid cells, and that OBF-1/Bob1/OCA-B can transactivate the PU.1 promoter in HeLa and myeloid cells. This B cell restricted coactivator may be responsible for the B cell specific expression of PU.1 mediated by the octamer site.

Coordinate suppression of myeloma-specific genes and expression of fibroblast-specific genes in myeloma X fibroblast somatic cell hybrids

In most instances, fusion of differentiated cell types with fibroblasts has resulted in the extinction of the differentiation-specific traits of the non-fibroblast parental cell. To explore the genetic basis of this phenomenon, we have studied a series of somatic cell hybrids between mouse myeloma and fibroblasts. All the hybrids were adherent having a fibroblast-like phenotype. Molecular analysis revealed that plasma cell specific genes like the productively rearranged Ig genes, the J chain gene and genes for the cell surface markers CD20 and PC1, were extinguished in the hybrids. In contrast, fibroblast specific genes like fibronectin, alpha 2(I) and III collagens, as well as the receptor for fibroblast growth factor (flg), were expressed. Extinction was not due to chromosomal loss or lack of the relevant genes. To learn about the mechanism(s) of this phenomenon we have looked for the presence of positive and negative transcription factors in our hybrids. Expression of the PU.1 transcription factor, a member of the Ets transcription factor family normally expressed in B cells and macrophages, was lost in the cell hybrids. Interestingly, we found that the B-cell-specific Oct-2 transcription factor was still expressed at somewhat variable levels in several of the hybrid cell lines. In contrast, expression of the recently identified octamer coactivator BOB.1/OBF.1 was extinguished in all cell hybrids. This supports a critical role of this transcriptional coactivator for B-cell-specific gene expression. In addition, the Id and HLH462 genes coding for proteins known to repress bHLH transcription factors by formation of heterodimers, were found to be expressed at increased levels in fibroblasts and in the hybrids, indicating that their increased levels might also contribute to the suppression of myeloma-specific genes. Our results show that in myeloma x fibroblast hybrids, the phenotype of the fibroblast is dominant. It is suggested that fibroblasts contain regulatory "master" genes that are responsible for activation of the fibroblast differentiation pathway and suppress differentiation programs of other cell types.

Mammalian cell fusion in an electroporation device

We and others have been interested in the phenomenon of gene 'extinction' in somatic cell hybrids, reasoning that the study of this process is likely to reveal underlying mechanisms responsible for limiting the expression of specialized genes only to appropriate cell types. In the course of our studies in this area, we have developed a simple and economical method of fusing mammalian cells, using an electroporation device. In fusions between murine myeloma and T lymphoma lines, hybrid cell recoveries were typically one per 10(5) [corrected] input myeloma cells. Because of our interest in the regulation of immunoglobulin heavy chain (IgH) gene expression, we analyzed the hybrids for both IgH gene composition and expression. The hybrid lines were phenotypically indistinguishable from those generated by the more conventional, polyethylene glycol (PEG)-induced fusion protocol. There was a notable increase, however, in the number of hybrids that retained IgH-encoding chromosomes from both parental lines.

Cloning, functional characterization, and mechanism of action of the B-cell-specific transcriptional coactivator OCA-B

Biochemical purification and cognate cDNA cloning studies have revealed that the previously described transcriptional coactivator OCA-B consists of a 34- or 35-kDa polypeptide with sequence relationships to known coactivators that function by protein-protein interactions. Studies with a recombinant protein have proved that a single OCA-B polypeptide is the main determinant for B-cell-specific activation of immunoglobulin (Ig) promoters and provided additional insights into its mechanism of action. Recombinant OCA-B can function equally well with Oct-1 or Oct-2 on an Ig promoter, but while corresponding POU domains are sufficient for OCA-B interaction, and for octamer-mediated transcription of a histone H2B promoter, an additional Oct-1 or Oct-2 activation domain(s) is necessary for functional synergy with OCA-B. Further studies additional Oct-1 or Oct-2 activation domain(s) is necessary for functional synergy with OCA-B. Further studies show that Ig promoter activation by Oct-1 and OCA-B requires still other general (USA-derived) cofactors and also provide indirect evidence that distinct Oct-interacting cofactors regulate H2B transcription.