Use of an antagonist of HMGB1 in mice affected by malignant mesothelioma: a preliminary ultrasound and optical imaging study

Background

Malignant mesothelioma (MM) is an aggressive tumor, with a poor prognosis, usually unresectable due to late diagnosis, mainly treated with chemotherapy. BoxA, a truncated form of “high mobility group box 1” (HMGB1), acting as an HMGB1 antagonist, might exert a defensive action against MM. We investigated the potential of BoxA for MM treatment using experimental 40-MHz ultrasound and optical imaging (OI) in a murine model.

Methods

Murine MM cells infected with a lentiviral vector expressing the luciferase gene were injected into the peritoneum of 14 BALB/c mice (7 × 10⁴ AB1-B/c-LUC cells). These mice were randomized to treatment with BoxA (n = 7) or phosphate-buffered saline (controls, n = 7). The experiment was repeated with 40 mice divided into two groups (n = 20 + 20) and treated as above to confirm the result and achieve greater statistical power. Tumor presence was investigated by experimental ultrasound and OI; suspected peritoneal masses underwent histopathology and immunohistochemistry examination.

Results

In the first experiment, none of the 7 controls survived beyond day 27, whereas 4/7 BoxA-treated mice (57.1%) survived up to day 70. In the second experiment, 6/20 controls (30.0%) and 16/20 BoxA-treated mice (80.0%) were still alive at day 34 (p = 0.004). In both experiments, histology confirmed the malignant nature of masses detected using experimental ultrasound and OI.

Conclusion

In our preclinical experience on a murine model, BoxA seems to exert a protective role toward MM. Both experimental ultrasound and OI proved to be reliable techniques for detecting MM peritoneal masses.

CXCR4 engagement triggers CD47 internalization and antitumor immunization in a mouse model of mesothelioma

Boosting antitumor immunity has emerged as a powerful strategy in cancer treatment. While releasing T‐cell brakes has received most attention, tumor recognition by T cells is a pre‐requisite. Radiotherapy and certain cytotoxic drugs induce the release of damage‐associated molecular patterns, which promote tumor antigen cross‐presentation and T‐cell priming. Antibodies against the “do not eat me” signal CD47 cause macrophage phagocytosis of live tumor cells and drive the emergence of antitumor T cells. Here we show that CXCR4 activation, so far associated only with tumor progression and metastasis, also flags tumor cells to immune recognition. Both CXCL12, the natural CXCR4 ligand, and BoxA, a fragment of HMGB1, promote the release of DAMPs and the internalization of CD47, leading to protective antitumor immunity. We designate as Immunogenic Surrender the process by which CXCR4 turns in tumor cells to macrophages, thereby subjecting a rapidly growing tissue to immunological scrutiny. Importantly, while CXCL12 promotes tumor cell proliferation, BoxA reduces it, and might be exploited for the treatment of malignant mesothelioma and a variety of other tumors.

High-mobility group box 1 protein orchestrates responses to tissue damage via inflammation, innate and adaptive immunity, and tissue repair

A single protein, HMGB1, directs the triggering of inflammation, innate and adaptive immune responses, and tissue healing after damage. HMGB1 is the best characterized damage-associated molecular pattern (DAMP), proteins that are normally inside the cell but are released after cell death, and allow the immune system to distinguish between antigens that are dangerous or not. Notably, cells undergoing severe stress actively secrete HMGB1 via a dedicated secretion pathway: HMGB1 is relocated from the nucleus to the cytoplasm and then to secretory lysosomes or directly to the extracellular space. Extracellular HMGB1 (either released or secreted) triggers inflammation and adaptive immunological responses by switching among multiple oxidation states, which direct the mutually exclusive choices of different binding partners and receptors. Immune cells are first recruited to the damaged tissue and then activated; thereafter, HMGB1 supports tissue repair and healing, by coordinating the switch of macrophages to a tissue-healing phenotype, activation and proliferation of stem cells, and neoangiogenesis. Inevitably, HMGB1 also orchestrates the support of stressed but illegitimate tissues: tumors. Concomitantly, HMGB1 enhances the immunogenicity of mutated proteins in the tumor (neoantigens), promoting anti-tumor responses and immunological memory. Tweaking the activities of HMGB1 in inflammation, immune responses and tissue repair could bring large rewards in the therapy of multiple medical conditions, including cancer.

Human malignant mesothelioma is recapitulated in immunocompetent BALB/c mice injected with murine AB cells

Malignant Mesothelioma is a highly aggressive cancer, which is difficult to diagnose and treat. Here we describe the molecular, cellular and morphological characterization of a syngeneic system consisting of murine AB1, AB12 and AB22 mesothelioma cells injected in immunocompetent BALB/c mice, which allows the study of the interplay of tumor cells with the immune system. Murine mesothelioma cells, like human ones, respond to exogenous High Mobility Group Box 1 protein, a Damage-Associated Molecular Pattern that acts as a chemoattractant for leukocytes and as a proinflammatory mediator. The tumors derived from AB cells are morphologically and histologically similar to human MM tumors, and respond to treatments used for MM patients. Our system largely recapitulates human mesothelioma, and we advocate its use for the study of MM development and treatment.

Down syndrome fibroblasts and mouse Prep1-overexpressing cells display increased sensitivity to genotoxic stress

PREP1 (PKNOX1) maps in the Down syndrome (DS) critical region of chromosome 21, is overexpressed in some DS tissues and might be involved in the DS phenotype. By using fibroblasts from DS patients and by overexpressing Prep1 in F9 teratocarcinoma and Prep1^i/i MEF to single out the role of the protein, we report that excess Prep1 increases the sensitivity of cells to genotoxic stress and the extent of the apoptosis directly correlates with the level of Prep1. The apoptotic response of Prep1-overexpressing cells is mediated by the pro-apoptotic p53 protein that we show is a direct target of Prep1, as its depletion reverts the apoptotic phenotype. The induction of p53 overcomes the anti-apoptotic role of Bcl-X_L, previously shown to be also a Prep1 target, the levels of which are increased in Prep1-overexpressing cells as well. Our results provide a rationale for the involvement of PREP1 in the apoptotic phenotype of DS tissues and indicate that differences in Prep1 level can have drastic effects.

Induction of HoxB transcription by retinoic acid requires actin polymerization

We have analyzed the role of actin polymerization in retinoic acid (RA)-induced HoxB transcription, which is mediated by the HoxB regulator Prep1. RA induction of the HoxB genes can be prevented by the inhibition of actin polymerization. Importantly, inhibition of actin polymerization specifically affects the transcription of inducible Hox genes, but not that of their transcriptional regulators, the RARs, nor of constitutively expressed, nor of actively transcribed Hox genes. RA treatment induces the recruitment to the HoxB2 gene enhancer of a complex composed of "elongating" RNAPII, Prep1, beta-actin, and N-WASP as well as the accessory splicing components p54Nrb and PSF. We show that inhibition of actin polymerization prevents such recruitment. We conclude that inducible Hox genes are selectively sensitive to the inhibition of actin polymerization and that actin polymerization is required for the assembly of a transcription complex on the regulatory region of the Hox genes.

A Transcription-dependent Micrococcal Nuclease-resistant Fragment of the Urokinase-type Plasminogen Activator Promoter Interacts with the Enhancer

We show the interaction between the enhancer and the minimal promoter of urokinase-type plasminogen activator gene during active transcription by coupling micrococcal nuclease digestion of cross-linked, sonicated chromatin, and chromatin immunoprecipitation. This approach allowed the precise identification of the interacting genomic fragments, one of which is resistant to micrococcal nuclease cleavage. The interacting fragments form a single transcriptional control unit, as indicated by their common protein content. Furthermore, we show that the enhancer-MP interaction persists during the early stages of transcription and is lost upon alpha-amanitin treatment, indicating the requirement for active transcription. Our results support a looping model of interaction between the enhancer and the MP of the urokinase-type plasminogen activator gene.

Urokinase-type plasminogen activator

Urokinase-type plasminogen activator (uPA) is a serine protease involved in tissue remodeling and cell migration. At the gene level, the interplay between a complex enhancer, required for induced and basal transcription, and the minimal promoter finely tunes uPA expression. The active form of uPA is bound to its high affinity receptor on the cell surface, where specific inhibitors modulate its enzymatic activity. Such inhibitors also regulate the cell surface levels of uPA by triggering the internalization of the uPA-receptor-inhibitor complex. The role of uPA is not only linked to its action as an enzyme. In fact, the mere binding of uPA on the cell surface also brings about two events that broaden the spectrum of its biological functions: (1) a conformational change of the receptor, which, in turn, affects its interaction with other proteins; (2) a signal transduction which modulates the expression of apoptosis-related genes. Besides its applications as a thrombolytic agent and as a prognostic marker for tumors, uPA may provide the basis for other therapies, as the structure of the receptor-binding domain of uPA has become a model for the design of anti-cancer molecules.

Nuclear myosin VI enhances RNA polymerase II-dependent transcription

Myosin VI is the only myosin that moves toward the minus end of actin filaments, suggesting a unique biological function. Here, we show that myosin VI is present in the nucleus of mammalian cells where it colocalizes with newly transcribed mRNA and with RNA polymerase II (RNAPII) and is detected in the RNAPII complex. The colocalization and interaction of myosin VI with RNAPII require transcriptional activity. Chromatin immunoprecipitation (ChIP) demonstrates that myosin VI is recruited to the promoter and intragenic regions of active genes, encoding urokinase plasminogen activator (uPA), eukaryotic initiation factor 6 (p27/eIF6), and low-density lipoprotein receptor (LDLR), but not to noncoding, nonregulatory intergenic regions. Downregulation of myosin VI reduces steady-state mRNA levels of these genes in vivo, and antibodies to myosin VI reduce transcription in vitro. We suggest that myosin VI modulates RNAPII-dependent transcription of active genes, implicating the possibility of an actin-myosin based mechanism of transcription.

Pertussis toxin B-oligomer suppresses IL-6 induced HIV-1 and chemokine expression in chronically infected U1 cells via inhibition of activator protein 1

Pertussis toxin B-oligomer (PTX-B) inhibits HIV replication in T lymphocytes and monocyte-derived macrophages by interfering with multiple steps of the HIV life cycle. PTX-B prevents CCR5-dependent (R5) virus entry in a noncompetitive manner, and it also exerts suppressive effects on both R5- and CXCR4-dependent HIV expression at a less-characterized postentry level. We demonstrate in this study that PTX-B profoundly inhibits HIV expression in chronically infected promonocytic U1 cells stimulated with several cytokines and, particularly, the IL-6-mediated effect, a cytokine that triggers viral production in these cells independently of NF-kappaB activation. From U1 cells we have subcloned a cell line, named U1-CR1, with increased responsiveness to IL-6. In these cells, PTX-B neither down-regulated the IL-6R nor prevented IL-6 induced signaling in terms of STAT3 phosphorylation and DNA binding. In contrast, PTX-B inhibited AP-1 binding to target DNA and modified its composition with a proportional increases in FosB, Fra2, and ATF2. PTX-B inhibited IL-6-induced HIV-1 long-terminal repeat-driven transcription from A, C, E, and F viral subtypes, which contain functional AP-1 binding sites, but failed to inhibit transcription from subtypes B and D LTR devoid of these sites. In addition, PTX-B inhibited the secretion of IL-6-induced, AP-1-dependent genes, including urokinase-type plasminogen activator, CXCL8/IL-8, and CCL2/monocyte chemotactic protein-1. Thus, PTX-B suppression of IL-6 induced expression of HIV and cellular genes in chronically infected promonocytic cells is strongly correlated to inhibition of AP-1.

MAPK and JNK transduction pathways can phosphorylate Sp1 to activate the uPA minimal promoter element and endogenous gene transcription

Two upstream regions of the human urokinase (uPA) gene regulate its transcription: the minimal promoter (MP) and the enhancer element. The activity of the minimal promoter is essential for basal uPA transcription in prostate adenocarcinoma PC3 cells. Binding of a phosphorylated Sp1 transcription factor is, in turn, essential for the activity of the MP. Here we report that the Jun kinase (JNK) pathway is required for the basal activity of the MP and for the expression of the endogenous uPA gene in PC3 cells and for activated transcription in LNCaP cells. On the other hand, the p42/p44 mitogen-activated protein kinase (MAPK) pathway activates uPA gene expression through Sp1 phosphorylation in HeLa, LNCaP, and CCL39-derivative cells that do not typically express uPA in basal conditions. In HeLa cells the dominant-negative form of JNK interferes with the p42/p44 MAPK activation of the uPA-MP. The results suggest that the stress-activated protein kinase (SAPK)/JNK pathway plays an important role in the phosphorylation of Sp1, which, in turn, leads to basal or activated transcription from the uPA-MP element.

Binding of Sp1 to the proximal promoter links constitutive expression of the human uPA gene and invasive potential of PC3 cells

Activated transcription of the urokinase-type plasminogen activator (uPA) gene depends on the enhancer, located approximately 2 kb from the start of transcription. The proximal promoter, driving basal transcription, contains a GC-/GA-rich sequence immediately upstream of the TATA box. We have investigated the role played by this element in the transcription of the uPA gene in HeLa and PC3 cells, which do not express or constitutively express the gene, respectively. This region binds either Sp1 or Sp3, as monomers or multimers, but not a combination of the 2 proteins. The more efficient binding of Sp1 to the proximal promoter in PC3 cells is correlated to its phosphorylation state. Polymerase chain reaction (PCR)-coupled, chromatin immunoprecipitation experiments with anti-Sp1 antibodies indeed show an enrichment of proximal promoter sequences in PC3 cells and support the observed difference in transcription levels from proximal promoter constructs in HeLa versus PC3 cells. Furthermore, overexpression of Sp1 increases transcription from the reporter construct in HeLa cells, whereas in PC3 cells, overexpression of Sp3 does not reduce transcription from the same construct, indicating that the Sp1/Sp3 balance cannot be shifted. We conclude that the GC-/GA-rich element of the uPA regulatory region is an independent functional element, regulated by Sp family proteins. Phosphorylation of Sp1 determines the presence in vivo and the functionality of this element in PC3 cells. Thus, the cellular context determines the relevance of the GC-/GA-rich region in uPA gene transcription, which contributes to constitutive gene expression, related, in turn, to the invasive phenotype.

Transcriptional activation of urokinase by the Krüppel-like factor Zf9/COPEB activates latent TGF-beta1 in vascular endothelial cells

Understanding the regulation of genes controlling fibrinolysis and matrix homeostasis is essential for elucidating the basis of tissue repair. A recently described novel Krüppel-like factor, Zf9, is up-regulated in acute liver injury in activated hepatic stellate cells. Because Zf9 can be induced widely, its activity was examined in vascular endothelium, a key cell in vascular injury. Zf9 is induced as an immediate-early response gene in bovine aortic endothelial cells (BAECs) following treatment with serum or phorbol ester. Zf9 transcriptionally activates urokinase plasminogen activator (uPA). Recombinant Zf9-GST binds to wild-type but not mutated 'GC-box' motifs within the human uPA promoter (-63 to -32), with greatest affinity to the middle of 3 contiguous GC boxes. Transient transfection of Zf9 drives transactivation of a full-length uPA promoter- and GC box-construct, but not a uPA promoter-construct devoid of GC boxes. Transactivation of uPA by Zf9 is also supported in Drosophila S2 cells. Most importantly, transiently transfected Zf9 up-regulates endogenous uPA messenger RNA and activity in BAECs, resulting in increased bioactive transforming growth factor-beta (TGF-beta) via enhancement of proteolytic activation of the latent molecule. Furthermore, concomitant expression of Zf9 and uPA proteins was observed in arterial endothelial cells after balloon injury in rats, suggesting a potential role of Zf9 in uPA expression not only in vitro but also in vivo. These findings suggest a role of Zf9 in the injury response by enhancing uPA synthesis and subsequent activation of latent TGF-beta. (Blood. 2000;95:1309-1316)

Physical interaction between retinoic acid receptor and Sp1: mechanism for induction of urokinase by retinoic acid

Induction of urokinase plasminogen activator (uPA) by retinoic acid (RA) is the initial event preceding certain subsequent biological changes in vascular endothelial cells. We investigated the molecular mechanism by which RA stimulates the expression of uPA, which lacks a canonical RA receptor (RAR)-responsive element, in bovine and human aortic endothelial cells. Upon stimulation with RA, mRNA levels of RARalpha and beta transiently increased in parallel with the induction of uPA, and this increase was inhibited by cycloheximide. Results of transient transfection of RAR/RXR cDNAs and experiments using specific agonists and antagonists suggested that uPA induction is dependent upon RAR (initially, RARalpha) with the help of RXRalpha. Deletion analysis of the uPA promoter suggested that RAR/RXR acts on GC box region within the uPA promoter. This was further supported by inhibition of Sp1 binding to this region. Coimmunoprecipitation studies, glutathione S-transferase pull-down experiment, and mammalian two-hybrid assays suggested a physical interaction between RAR/RXR and Sp1. Furthermore, gel shift studies showed that the binding of Sp1 to the uPA GC box is significantly potentiated in the presence of RARs/RXRs. Finally, Sp1 and RAR/RXR synergistically enhanced the transactivation activity of the uPA promoter. These results suggest that (1) RA induces RARs mainly via RARalpha and that (2) RAR/RXR physically and functionally interact with Sp1, resulting in a potentiation of uPA transcription.

In vivo analysis of the state of the human uPA enhancer following stimulation by TPA

We have analysed in vivo the -2.0 kb enhancer of the human urokinase-type plasminogen activator (uPA) gene in HepG2 cells, in which gene expression can be induced by phorbol esters. The results reveal that, within the regulatory region, the enhancer, the silencer and the minimal promoter become hypersensitive to deoxyribonuclease I (DNase I) upon induction of transcription. The hypersensitivity of the enhancer can be reversed after removal of the inducer. In vivo footprinting analysis indicates that all the cis-acting elements of the enhancer, previously identified in vitro, are occupied in vivo upon 12-O-tetradecanoyl-phorbol-13-acetate (TPA) stimulation of HepG2 cells. Micrococcal nuclease (MNase) cleavage of this region fails to reveal discrete nucleosomal boundaries in vivo in close proximity of the enhancer, either before or after stimulation. Furthermore, this region does not lose its nucleosomal configuration after TPA induction of transcription. An approximately 600 bp long region around the enhancer becomes more, but not fully, accessible to restriction endonucleases upon stimulation. A time-course experiment shows that this accessibility reaches a plateau after a 1 h TPA treatment suggesting the persistent presence of nucleosomes. These results indicate that TPA induces the binding of transcription factors to the uPA enhancer without chromatin remodelling of this region.

Removal of domain D2 or D3 of the human urokinase receptor does not affect ligand affinity

The main ligand-binding determinant of the human urokinase receptor (uPAR) is located in the amino terminal domain D1, but when isolated this domain presents a 1500 fold lower affinity than the intact three-domain uPAR (D1D2D3). uPAR mutants missing either domain 2 (D1HD3) or domain 3 (D1D2) were expressed in murine LB6 cells and showed to be properly GPI-anchored to the cell surface. Binding assays with [125I]ATF demonstrated that these mutants possessed a normal (D1D2) or slightly reduced (D1HD3) affinity, indicating that a high ligand-affinity may be achieved by a combination of D1 with domain D2 or D3.

Incorporation of chromosomal proteins HMG-14/HMG-17 into nascent nucleosomes induces an extended chromatin conformation and enhances the utilization of active transcription complexes

The role of chromosomal proteins HMG-14 and HMG-17 in the generation of transcriptionally active chromatin was studied in a Xenopus laevis egg extract which supports complementary DNA strand synthesis and chromatin assembly. Chromosomal proteins HMG-14/HMG-17 enhanced transcription from a chromatin template carrying a 5S rRNA gene, but not from a DNA template. The transcriptional potential of chromatin was enhanced only when these proteins were incorporated into the template during, but not after, chromatin assembly. HMG-14 and HMG-17 stimulate transcription by increasing the activity, and not the number, of transcribed templates. They unfold the chromatin template without affecting the nucleosomal repeat or decreasing the content of histone B4. We suggest that HMG-14/HMG-17 enhance transcription by inducing an extended conformation in the chromatin fiber, perhaps due to interactions with histone tails in nucleosomes. By disrupting the higher order chromatin structure HMG-14/HMG-17 increase the accessibility of target sequences to components of the transcriptional apparatus.

The footprint of chromosomal proteins HMG-14 and HMG-17 on chromatin subunits

The position of chromosomal proteins HMG-14 and HMG-17 in nucleosome cores and in chromatosomes lacking linker histones has been mapped by hydroxyl radical footprinting. Both the nucleosome core and the H1/H5 depleted chromatosome can specifically bind two molecules of HMG-14/-17. The path of HMG-14 on the surface of chromatin subunits is indistinguishable from that of HMG-17. The bound HMGs protect the DNA from hydroxyl radical cleavage 25 base-pairs from the end of the DNA in nucleosome cores and in each of the two major grooves of the DNA flanking the nucleosomal dyad axis. Thus, in both cores and H1/H5-depleted chromatosomes the proteins bridge two adjacent DNA strands on the surface of the particles. The sites occupied by HMG near the end of the chromatosome-length particles are distinct from those occupied by the H1/H5 linker histones. In the region of the dyad axis the binding sites of HMGs overlap those of the linker histones. The placement of HMG-14/-17 near the nucleosomal dyad axis raises the possibility that interactions between histone H1 and HMGs may affect the transcriptional potential of chromatin.

Deposition of chromosomal protein HMG-17 during replication affects the nucleosomal ladder and transcriptional potential of nascent chromatin

A cell-free system from Xenopus eggs was used to study the role of chromosomal protein HMG-17 in the generation of the chromatin structure of transcriptionally active genes. Addition of HMG-17 protein to the extracts, which do not contain structural homologs of the HMG-14/-17 protein family, indicates the protein is incorporated into the nascent template during replication, prior to completion of chromatin assembly. The protein binds to and stabilizes the structure of the nucleosomal core thereby improving the apparent periodicity of the nucleosomal spacing of nascent chromatin. Assembly of HMG-17 into the nascent chromatin structure significantly increased the transcription potential of the 5S RNA gene and satellite I chromatin. Kinetic studies indicate that the increase in transcriptional potential is observed only when HMG-17 is incorporated into nucleosomes during chromatin assembly.

Nucleosome core binding region of chromosomal protein HMG-17 acts as an independent functional domain

Chromosomal proteins HMG-14 and HMG-17 have a modular structure. Here we examine whether the putative nucleosome-binding domain in these proteins can function as an independent module. Mobility shift assays with recombinant HMG-17 indicate that synthetic molecules can be used to analyze the interaction of this protein with the nucleosome core. Peptides corresponding to various regions of the protein have been synthesized and their interaction with nucleosome cores analyzed by mobility shift, thermal denaturation and DNase I digestion. A 30 amino acid long peptide, corresponding to the putative nucleosome-binding domain of HMG-17, specifically shifts the mobility of cores as compared to free DNA, elevates the tm of both the premelt and main melt of the cores and protects from DNase I digestion the same nucleosomal DNA sites as the intact protein. The binding of both the peptide and the intact protein is lost upon digestion of the histone tails by trypsin. The nucleosomal binding sites of the peptide appear identical to those of the intact protein. Thus, a region of the protein can acts as an independent functional domain. This supports the notion that HMG-14 and HMG-17 are modular proteins. This finding is relevant to the understanding of the function and evolution of HMG-14/-17, the only nucleosome core particle binding proteins known to date.

Recombinant human chromosomal proteins HMG-14 and HMG-17

Vectors for expressing human chromosomal proteins HMG-14 and HMG-17 in bacterial cultures under the control of the temperature-inducible lambda PL promoter have been constructed. The open reading frames of the cDNAs have been amplified by the polymerase chain reaction (PCR), utilizing amplimers containing desired restriction sites, thereby facilitating precise location of the initiation codon downstream from a ribosomal binding site. Expression of the recombinant proteins does not significantly affect bacterial growth. The rate of synthesis of the recombinant proteins is maximal during the initial stages of induction and slows down appreciably with time. After an initial burst of protein synthesis, the level of the recombinant protein in the bacterial extracts remains constant at different times following induction. Methods for rapid extraction and purification of the recombinant proteins are described. The recombinant proteins are compared to the proteins isolated from eucaryotic cells by electrophoretic mobility, Western analysis and nucleosome core mobility-shift assays. The ability of the proteins to shift the mobility of the nucleosome cores, but not that of DNA, can be used as a functional assay for these HMG proteins. A source for large quantities of human chromosomal proteins HMG-14 and HMG-17 will facilitate studies on their structure, cellular function and mechanism of interaction with nucleosomes.

Differentiation-dependent alteration in the chromatin structure of chromosomal protein HMG-17 gene during erythropoiesis

The expression of the gene coding for chromosomal protein HMG-17 is down regulated during chicken erythrocyte maturation. The transcriptional down regulation is associated with major alterations in the chromatin structure of this gene. The 5' region of the gene contains both constitutive and developmental stage-specific deoxyribonuclease I (DNase I) hypersensitive sites. The constitutive sites bracket the "CpG island" present in the gene, which remains hypomethylated throughout the various developmental stages. During erythropoiesis, the gene acquires a distinct structure that, upon digestion with micrococcal nuclease (MNase) yields an unusual repeat. Two nucleosomes, with a 200 base-pair repeat, are positioned immediately downstream from the start of transcription. Immediately downstream and upstream from these nucleosomes, the boundaries between MNase sites change to a 75 base-pair repeat, which indicates an unusual chromatin structure. The differentiation related changes in the DNase I and MNase digestion pattern in the 5' region of the gene suggest that sequences present in the first intron may be involved in gene regulation. The results may be relevant to the regulation of the entire HMG-14/-17 gene family.

Immunochemical analysis of the exposure of high mobility group protein 14 and 17 surfaces in chromatin

Antisera were elicited against synthetic peptides corresponding either to regions common to all members of the high mobility group 14 and 17 protein family protein or to distinct domains of the HMG-14 or HMG-17 subgroup. The antisera were used to probe the accessibility of various HMG domains in chromatin. Competitive enzyme-linked immunosorbent assays indicate that the central region of the proteins, which contains their DNA binding domain and is positively charged, is exposed to a smaller degree than the C-terminal region of the proteins, which has a net negative charge. The C-terminal regions of the HMG-14 and HMG-17 proteins are exposed and available to interact with other proteins.

Expression of chromosomal proteins HMG-14 and HMG-17 in transformed human cells

The relation between cellular phenotype and expression of chromosomal high mobility group proteins 14 and 17 (HMG-14 and HMG-17) has been examined in human cell lineages. Quantitation of HMG-14 and HMG-17 mRNA in several human cell lines revealed differences in both the steady state mRNA level and in the ratio of HMG-14 to HMG-17 mRNA. Analysis of phenotypically distinct derivatives of human bronchial epithelial cells revealed small differences between both the steady state mRNA levels and the relative amount of these proteins among the clonal variants. The effect of myeloid differentiation on the mRNA level of HMG-14 and HMG-17 was examined in the human promyelocytic leukemia cell line HL-60 following treatment with several granulocytic and monocytic differentiating agents. The ratio of HMG-17 mRNA to either HMG-14 or histone H4 mRNA varied among the cell phenotypes suggesting that phenotype switching may result in detectable alterations in the expression of the HMG-14 and HMG-17 genes. The data suggest that, although the ratio of HMG-14 to HMG-17 mRNA varies among human cell lines, these variations are relatively small.

Chromosomal protein HMG-14. Identification, characterization, and chromosome localization of a functional gene from the large human multigene family

The human HMG-14 multigene family is one of the largest retropseudogene families known. To identify and isolate a functional human HMG-14 gene, genomic clones, selected with the cDNA, were screened with a set of 6 oligonucleotides. A single genomic clone was isolated suggesting that the human genome contains few, and perhaps only one, functional genes. An 8882-base pair (bp) genomic clone containing the complete, 6804-bp-long human gene together with 850 bp 5' to the start of transcription and 1228 bp 3' to the end of transcription was sequenced. The gene is comprised of 6 exons ranging in size from 30 to 839 bp, two of which code for the entire DNA binding site of the protein, and has several features typical of "housekeeping" genes. Using human-rodent somatic cell hybrids, the HMG-14 gene was localized to human chromosome 21. A restriction fragment length polymorphism, useful for further analysis and mapping, has been detected. The present article, which describes the first isolation and characterization of a gene coding for chromosomal protein HMG-14, indicates that genes coding for HMG-14 and HMG-17 may share several distinctive characteristics. Comparison with the human and chicken HMG-17 genes reveals that all contain 6 exons, that all have exons of similar size, that all have 5' regions highly enriched in GC residues and that all have features typical of housekeeping genes.