Comorbidity-associated glutamine deficiency is a predisposition to severe COVID-19

SARS-CoV-2 vaccinations have greatly reduced COVID-19 cases, but we must continue to develop our understanding of the nature of the disease and its effects on human immunity. Previously, we suggested that a dysregulated STAT3 pathway following SARS-Co-2 infection ultimately leads to PAI-1 activation and cascades of pathologies. The major COVID-19-associated metabolic risks (old age, hypertension, cardiovascular diseases, diabetes, and obesity) share high PAI-1 levels and could predispose certain groups to severe COVID-19 complications. In this review article, we describe the common metabolic profile that is shared between all of these high-risk groups and COVID-19. This profile not only involves high levels of PAI-1 and STAT3 as previously described, but also includes low levels of glutamine and NAD+, coupled with overproduction of hyaluronan (HA). SARS-CoV-2 infection exacerbates this metabolic imbalance and predisposes these patients to the severe pathophysiologies of COVID-19, including the involvement of NETs (neutrophil extracellular traps) and HA overproduction in the lung. While hyperinflammation due to proinflammatory cytokine overproduction has been frequently documented, it is recently recognized that the immune response is markedly suppressed in some cases by the expansion and activity of MDSCs (myeloid-derived suppressor cells) and FoxP3+ Tregs (regulatory T cells). The metabolomics profiles of severe COVID-19 patients and patients with advanced cancer are similar, and in high-risk patients, SARS-CoV-2 infection leads to aberrant STAT3 activation, which promotes a cancer-like metabolism. We propose that glutamine deficiency and overproduced HA is the central metabolic characteristic of COVID-19 and its high-risk groups. We suggest the usage of glutamine supplementation and the repurposing of cancer drugs to prevent the development of severe COVID-19 pneumonia.

An aberrant STAT pathway is central to COVID-19

COVID-19 is caused by SARS-CoV-2 infection and characterized by diverse clinical symptoms. Type I interferon (IFN-I) production is impaired and severe cases lead to ARDS and widespread coagulopathy. We propose that COVID-19 pathophysiology is initiated by SARS-CoV-2 gene products, the NSP1 and ORF6 proteins, leading to a catastrophic cascade of failures. These viral components induce signal transducer and activator of transcription 1 (STAT1) dysfunction and compensatory hyperactivation of STAT3. In SARS-CoV-2-infected cells, a positive feedback loop established between STAT3 and plasminogen activator inhibitor-1 (PAI-1) may lead to an escalating cycle of activation in common with the interdependent signaling networks affected in COVID-19. Specifically, PAI-1 upregulation leads to coagulopathy characterized by intravascular thrombi. Overproduced PAI-1 binds to TLR4 on macrophages, inducing the secretion of proinflammatory cytokines and chemokines. The recruitment and subsequent activation of innate immune cells within an infected lung drives the destruction of lung architecture, which leads to the infection of regional endothelial cells and produces a hypoxic environment that further stimulates PAI-1 production. Acute lung injury also activates EGFR and leads to the phosphorylation of STAT3. COVID-19 patients' autopsies frequently exhibit diffuse alveolar damage (DAD) and increased hyaluronan (HA) production which also leads to higher levels of PAI-1. COVID-19 risk factors are consistent with this scenario, as PAI-1 levels are increased in hypertension, obesity, diabetes, cardiovascular diseases, and old age. We discuss the possibility of using various approved drugs, or drugs currently in clinical development, to treat COVID-19. This perspective suggests to enhance STAT1 activity and/or inhibit STAT3 functions for COVID-19 treatment. This might derail the escalating STAT3/PAI-1 cycle central to COVID-19.

The immune regulatory protein B7-H3 promotes osteoblast differentiation and bone mineralization

B7-H3, a member of the B7 family of the Ig superfamily proteins, is expressed on the surface of the antigen-presenting cells and down-regulates T cell functions by engaging an unknown counterreceptor on T cells. Although B7-H3 is ubiquitously expressed, its potential nonimmune functions have not been addressed. We found that B7-H3 is highly expressed in developing bones during embryogenesis and that its expression increases as osteoblast precursor cells differentiate into mature osteoblasts. In vitro bone formation by osteoblastic cells was inhibited when B7-H3 function was interrupted by the soluble recombinant protein B7-H3-Fc. Analysis of calvarial cells derived from neonatal B7-H3 knockout (KO) mice revealed normal numbers of osteoblast precursor cells possessing a normal proliferative capacity. However, the B7-H3-deficient calvarial cells exhibited impaired osteogenic differentiation, resulting in decreased mineralized bone formation in vitro. These results suggest that B7-H3 is required for the later phase of osteoblast differentiation. Although B7-H3 KO mice had no gross skeletal abnormalities, they displayed a lower bone mineral density in cortical (but not trabecular) bones compared with WT controls. Consistent with the reduced bone mineral density, the femurs of B7-H3 KO mice were more susceptible to bone fracture compared with those of WT mice. Taken together, these results indicate that B7-H3 and its unknown counterreceptor play a positive regulatory role in bone formation. In addition, our findings identified B7-H3 as another molecule that has a dual role in the bone-immune interface.

Interaction of B7RP-1 with ICOS negatively regulates antigen presentation by B cells

Stimulation of T cells through the T cell receptor is insufficient for optimal T cell activation. A second activation signal is necessary, being usually provided by the costimulatory molecule CD28. Recently, additional costimulatory pathways have been identified, including inducible costimulator (ICOS) and its ligand B7RP-1. We have examined the role of the B7RP-1/ICOS costimulatory pathway on antigen presentation by B cells, using the I-Ak and I-Ek-positive CH27 B cell line and several different T cell lines. We found that CH27 expressed B7RP-1 and PD-L1 whereas the T cell lines expressed ICOS and PD-1. In the presence of HEL, the T cell hybridomas C10 and 3A9 released IL-2, which is indicative of antigen-specific T cell activation by the CH27 cells. Unexpectedly, blocking antibodies for B7RP-1 and ICOS enhanced the IL-2 response in both T cells. As expected, an increase in the production of IL-2 was seen when blocking antibodies for PD-1 were used. Blocking with antibodies for I-Ak, CD28, B7.1, and B7.2 lead to a decrease in IL-2 production. Additionally we tested a Th1 and a Th2 T cell clone. Blockade of B7RP-1/ICOS lead to an increased IFN-gamma response in Th1 cells (A.E7) and an increased IL-4 response in Th2 cells (D10.G4.1). Intracellular staining also showed an increase in cytokine production when the B7RP-1/ICOS pathway was blocked. In conclusion, the B7RP-1/ICOS pathway is negatively regulating T cell activation by B cells and may play a role similar to that of the PD-L1/PD-1 pathway.

The B7 family member B7-H3 preferentially down-regulates T helper type 1-mediated immune responses

We investigated the in vivo function of the B7 family member B7-H3 (also known as B7RP-2) by gene targeting. B7-H3 inhibited T cell proliferation mediated by antibody to T cell receptor or allogeneic antigen-presenting cells. B7-H3-deficient mice developed more severe airway inflammation than did wild-type mice in conditions in which T helper cells differentiated toward type 1 (T(H)1) rather than type 2 (T(H)2). B7-H3 expression was consistently enhanced by interferon-gamma but suppressed by interleukin 4 in dendritic cells. B7-H3-deficient mice developed experimental autoimmune encephalomyelitis several days earlier than their wild-type littermates, and accumulated higher concentrations of autoantibodies to DNA. Thus, B7-H3 is a negative regulator that preferentially affects T(H)1 responses.

Potent activity of soluble B7RP-1-Fc in therapy of murine tumors in syngeneic hosts

We have characterized a receptor:ligand pair, ICOS:B7RP-1, that is structurally and functionally related to CD28:B7.1/2. We reported previously that B7RP-1 costimulates T cell proliferation and immune responses (Yoshinaga et al., Nature 1999;402:827-32; Guo et al., J Immunol 2001;166:5578-84; Yoshinaga et al., Int Immunol 2000;12:1439-47). We report that B7RP-1-Fc causes rejection or growth inhibition of Meth A, SA-1 and EMT6 tumors in syngeneic mice. Established Meth A tumors were rejected effectively with a single dose of B7RP-1-Fc, however, the treatment was less effective on larger tumors. Mice that rejected Meth A tumors previously by Day 30, also rejected a subsequent Meth A challenge on Day 60, without additional B7RP-1-Fc treatment, indicating a long-lived memory response. Tumor cells believed to be less immunogenic, such as P815 and EL-4 cells, were less responsive to this treatment. The EL-4 responsiveness to the B7RP-1-Fc treatment was enhanced, however, by pre-treatment of the mice with cyclophosphamide. As expected, T cells appeared to be targeted by B7RP-1-Fc treatment. Thus, the administration of soluble B7RP-1-Fc may have therapeutic value in generating or enhancing anti-tumor activity in a clinical setting.

Inducible costimulator protein controls the protective T cell response against Listeria monocytogenes

The inducible costimulator protein (ICOS) was recently identified as a costimulatory molecule for T cells. Here we analyze the role of ICOS for the acquired immune response of mice against the intracellular bacterium Listeria monocytogenes. During oral L. monocytogenes infection, low levels of ICOS expression were detected by extracellular and intracellular Ab staining of Listeria-specific CD4(+) and CD8(+) T cells. Blocking of ICOS signaling with a soluble ICOS-Ig fusion protein markedly impaired the Listeria-specific T cell responses. Compared with control mice, the ICOS-Ig treated mice generated significantly reduced numbers of Listeria-specific CD8(+) T cells in spleen and liver, as determined by tetramer and intracellular cytokine staining. In contrast, the specific CD8(+) T cell response in the intestinal mucosa did not appear to be impaired by the ICOS-Ig treatment. Analysis of the CD4(+) T cell response revealed that ICOS-Ig treatment also affected the specific CD4(+) T cell response. When restimulated with listerial Ag in vitro, reduced numbers of CD4(+) T cells from infected and ICOS-Ig-treated mice responded with IFN-gamma production. The impaired acquired immune response in ICOS-Ig treated mice was accompanied by their increased susceptibility to L. monocytogenes infection. ICOS-Ig treatment drastically enhanced bacterial titers, and a large fraction of mice succumbed to the otherwise sublethal dose of infection. Thus, ICOS costimulation is crucial for protective immunity against the intracellular bacterium L. monocytogenes.

Inducible costimulator costimulates cytotoxic activity and IFN-gamma production in activated murine NK cells

The functions of NK cells are regulated by the balance of activating and inhibitory signals. The inhibitory NK cell receptors are well understood; however, less is known about the activating signaling pathways. To explore whether a costimulatory receptor, inducible costimulator (ICOS), is involved in NK cell function, we assessed the role of ICOS in NK cell-mediated cytotoxicity and cytokine production. In addition, to determine whether ICOS contributes to the elimination of tumors in vivo, we examined the tumor growth survival of mice injected with a tumor expressing the ICOS ligand, B7RP-1. We found that ICOS was up-regulated by cytokine stimulation in murine NK cells. Consistent with ICOS expression on activated NK cells, ICOS-dependent cytotoxicity and IFN-gamma production were observed, and appeared to require signaling through the phosphoinositide 3-kinase pathway. Interestingly, ICOS-mediated stimulation allowed activated NK cells to kill more efficiently tumor cells expressing MHC class I. Furthermore, fewer metastases appeared in the liver and spleen of mice injected with the ICOS ligand-expressing tumor compared with mice bearing the parental tumor. These results indicate that NK cell functions are regulated by ICOS.

Essential role of NF-kappa B-inducing kinase in T cell activation through the TCR/CD3 pathway

NF-kappa B-inducing kinase (NIK) is involved in lymphoid organogenesis in mice through lymphotoxin-beta receptor signaling. To clarify the roles of NIK in T cell activation through TCR/CD3 and costimulation pathways, we have studied the function of T cells from aly mice, a strain with mutant NIK. NIK mutant T cells showed impaired proliferation and IL-2 production in response to anti-CD3 stimulation, and these effects were caused by impaired NF-kappa B activity in both mature and immature T cells; the impaired NF-kappa B activity in mature T cells was also associated with the failure of maintenance of activated NF-kappa B. In contrast, responses to costimulatory signals were largely retained in aly mice, suggesting that NIK is not uniquely coupled to the costimulatory pathways. When NIK mutant T cells were stimulated in the presence of a protein kinase C (PKC) inhibitor, proliferative responses were abrogated more severely than in control mice, suggesting that both NIK and PKC control T cell activation in a cooperative manner. We also demonstrated that NIK and PKC are involved in distinct NF-kappa B activation pathways downstream of TCR/CD3. These results suggest critical roles for NIK in setting the threshold for T cell activation, and partly account for the immunodeficiency in aly mice.

Renal tubular epithelial expression of the costimulatory molecule B7RP-1 (inducible costimulator ligand)

MHC class II-expressing renal tubular epithelial cells (TEC) are able to present foreign peptide antigens to T cells. The costimulatory signals that are required for effective T cell activation upon antigen presentation by TEC have not been characterized. Various cultured TEC lines were examined for expression of the recently described costimulatory molecule B7RP-1 (B7h), a ligand of the T cell molecule inducible costimulator (ICOS), and expression was compared with that of B7.1, B7.2, and CD40. B7RP-1 and CD40 were abundantly expressed by cultured murine and human TEC, whereas B7.1 and B7.2 could not be detected. Stimulation with lipopolysaccharide or tumor necrosis factor-alpha did not induce B7.1 or B7.2 expression and did not alter B7RP-1 expression. Interestingly, interleukin-2 production by T cell hybridomas after antigen presentation by TEC was enhanced by blocking antibodies to B7RP-1 and ICOS. In contrast, blocking antibodies to B7RP-1 or ICOS exerted inhibitory effects on anti-CD3-activated murine splenocyte proliferation. Immunohistochemical staining of normal human kidneys demonstrated strong constitutive B7RP-1 expression in distal tubules, collecting ducts, and urothelium. In human kidneys with allograft rejection or interstitial nephritis, distinct B7RP-1 staining was also detected in proximal tubules, in areas of mononuclear infiltration. In conclusion, the B7RP-1/ICOS pathway negatively regulates T cell activation upon MHC class II-restricted antigen presentation by TEC. Because B7RP-1 is also expressed by tubules in vivo, it can be speculated that the B7RP-1/ICOS pathway could play an inhibitory role in TEC-mediated immune activation in the kidney.

Stimulatory effects of B7-related protein-1 on cellular and humoral immune responses in mice

Inducible costimulator (ICOS) and B7-related protein-1 (B7RP-1) constitute a receptor-ligand pair involved in T cell costimulation. In this study, the stimulatory effects of B7RP-1 on cellular and humoral immune responses were investigated giving mice a construct with the extracellular domain of murine B7RP-1 fused with human IgG1 Fc (B7RP-1-Fc). B7RP-1-Fc stimulated contact hypersensitivity (CH) given near either the time of sensitization or challenge with oxazolone. When given near challenge time, B7RP-1-Fc stimulated CH more than a construct containing the extracellular domain of murine B7.2 and Fc (B7.2-Fc). B7RP-1-Fc increased the number of cells in lymph nodes draining the skin sensitized with oxazolone, especially activated T cells. B7RP-1-Fc also increased the ability of the cells in these lymph nodes to induce CH when transfused into naive mice. B7RP-1-Fc stimulated the production of anti-keyhole limpet hemocyanin (KLH) Ab, increasing anti-KLH IgG, IgG2a, and IgE, whereas B7.2-Fc did not affect this production. B7RP-1-Fc also increased the number of cells in lymph nodes draining the skin immunized with KLH and their production of IFN-gamma, IL-4, and IL-10 in response to KLH. Finally, B7RP-1-Fc increased the presence of eosinophils in the bronchoalveolar lavage and lungs of mice sensitized and challenged with OVA so to mount an asthmatic reaction. B7RP-1-Fc stimulates both cellular and humoral immune responses in vivo by increasing number and function of T and B cells reacting to Ag exposure.

ICOS is essential for effective T-helper-cell responses

The outcome of T-cell responses after T-cell encounter with specific antigens is modulated by co-stimulatory signals, which are required for both lymphocyte activation and development of adaptive immunity. ICOS, an inducible co-stimulator with homology to CD28, is expressed on activated, but not resting T cells, and shows T-cell co-stimulatory function in vitro. ICOS binds specifically to its counter-receptor B7RP-1 (refs 5,6,7), but not to B7-1 or B7-2. Here we provide in vivo genetic evidence that ICOS delivers a co-stimulatory signal that is essential both for efficient interaction between T and B cells and for normal antibody responses to T-cell-dependent antigens. To determine the physiological function of ICOS, we generated and characterized gene-targeted ICOS-deficient mice. In vivo, a lack of ICOS results in severely deficient T-cell-dependent B-cell responses. Germinal centre formation is impaired and immunoglobulin class switching, including production of allergy-mediating IgE, is defective. ICOS-deficient T cells primed in in vivo and restimulated in vitro with specific antigen produce only low levels of interleukin-4, but remain fully competent to produce interferon-gamma.

Characterization of a new human B7-related protein: B7RP-1 is the ligand to the co-stimulatory protein ICOS

Optimal T cell activation requires the interactions of co-stimulatory molecules, such as those in the CD28 and B7 protein families. Recently, we described the co-stimulatory properties of the murine ligand to ICOS, which we designated as B7RP-1. Here, we report the co-stimulation of human T cells through the human B7RP-1 and ICOS interaction. This ligand-receptor pair interacts with a K:(D) approximately 33 nM and an off-rate with a t((1/2)) > 10 min. Interestingly, tumor necrosis factor (TNF)-alpha differentially regulates the expression of human B7RP-1 on B cells, monocytes and dendritic cells (DC). TNF-alpha enhances B7RP-1 expression on B cells and monocytes, while it inhibits it on DC. The human B7RP-1-Fc protein or cells that express membrane-bound B7RP-1 co-stimulate T cell proliferation in vitro. Specific cytokines, such as IFN-gamma and IL-10, are induced by B7RP-1 co-stimulation. Although IL-2 levels are not significantly increased, B7RP-1 co-stimulation is dependent on IL-2. These experiments define the human ortholog to murine B7RP-1 and characterize its interaction with human ICOS.

T-cell co-stimulation through B7RP-1 and ICOS

T-cell activation requires co-stimulation through receptors such as CD28 and antigen-specific signalling through the T-cell antigen receptor. Here we describe a new murine costimulatory receptor-ligand pair. The receptor, which is related to CD28 and is the homologue of the human protein ICOS, is expressed on activated T cells and resting memory T cells. The ligand, which has homology to B7 molecules and is called B7-related protein-1 (B7RP-1), is expressed on B cells and macrophages. ICOS and B7RP-I do not interact with proteins in the CD28-B7 pathway, and B7RP-1 co-stimulates T cells in vitro independently of CD28. Transgenic mice expressing a B7RP-1-Fc fusion protein show lymphoid hyperplasia in the spleen, lymph nodes and Peyer's patches. Presensitized mice treated with B7RP-1-Fc during antigen challenge show enhanced hypersensitivity. Therefore, B7RP-1 exhibits co-stimulatory activities in vitro and in vivo. ICOS and B7RP-1 define a new and distinct receptor-ligand pair that is structurally related to CD28-B7 and is involved in the adaptive immune response.

Folding and purification of a recombinantly expressed interferon regulatory factor, IRF-4

Interferon regulatory factor 4 (IRF-4), an intracellular, multidomain protein, is a member of the interferon regulatory factor family and a lymphoid-specific transcription factor that can form a ternary complex with DNA and the transcription factor PU.1. Recombinant human IRF-4 was expressed in Escherichia coli and purified from the soluble cell extract and the insoluble inclusion bodies. The inclusion bodies were solubilized with guanidinium-hydrochloride and sequentially buffer exchanged into urea- and then NaCl-containing solutions. This two-step process for the removal of the denaturants was the critical step to allow for the correct folding of IRF-4. Following purification through immobilized metal affinity, hydrophobic interaction, and gel permeation chromatographies, the renatured protein was shown to be structurally and physically equivalent to a sample of IRF-4 produced in the soluble fraction of E. coli cells. This was confirmed by near and far UV circular dichroism analysis, including thermal stability analysis. The purified IRF-4 was also shown to be capable of binding DNA in a PU.1-dependent manner by electrophoretic mobility shift analysis. The protein folding and purification methods are suitable for producing large quantities of full-length IRF-4.

A Neu differentiation factor (NDF) domain essential for proliferation and alterations in morphology of colonic epithelial cells in vitro

The Neu Differentiation Factors (NDFs, also termed "heregulins") are a family of proteins that were first isolated as ligands for the HER2 (ergB2, or p185neu) receptor protein tyrosine kinase. Here we show that NDF acts to stimulate the proliferation and alter the cellular morphology of colonic epithelial cells in culture. Dramatic NDF-induced changes in cellular morphology were noted in the colonic epithelial cell line, LIM 1215. In addition, the expression of specific cell proteins, such as carcinoembryonic antigen and integrin beta 4, was induced in LIM 1215 cells by NDF. These effects were more pronounced with the beta isoform than with the alpha isoform of NDF. The EGF-homology domain of NDF beta was sufficient to stimulate the proliferation and alteration in cell morphology. The use of chemically synthesized chimeric NDF alpha and NDF beta proteins enabled use to identify a region of seven amino acids in the EGF-homology domain of NDF beta that is required for both activities. These in vitro experiments suggest that NDF may act as a regulator of growth and differentiation of colonic epithelial cells in vivo.

Transformation of NIH 3T3 cells by HER3 or HER4 receptors requires the presence of HER1 or HER2

Members of the epidermal growth factor receptor (EGFR) subfamily of receptor protein tyrosine kinases have been implicated in the pathogenesis of various malignancies. The ability of one EGFR subfamily member to influence, or function synergistically with, another is likely to be a general feature of these receptors. To assess the role of receptor heterodimerization, we analyzed the ability of Neu differentiation factor (NDF) to induce cell growth and transformation of NIH 3T3 cells transfected with different combinations of the EGFR subfamily of receptors. NDF induced mitogenesis, but not transformation, of cells expressing either HER3 or HER4 alone. However, NDF-induced cell transformation was observed when either HER1 or HER2 was coexpressed with HER3 or HER4. In analogous receptor phosphorylation experiments, NDF-induced transphosphorylation appears to be correlated with synergistic transformation of NIH 3T3 cells. Interestingly, transphosphorylation between HER1 and HER4 can be stimulated by either EGF or NDF.

Molecular cloning of LSIRF, a lymphoid-specific member of the interferon regulatory factor family that binds the interferon-stimulated response element (ISRE)

Interferon regulatory factor (IRF) genes encode a family of DNA-binding proteins that are involved in the transcriptional regulation of type-I interferon and/or interferon-inducible genes. We report here the characterization of LSIRF, a new member of the IRF gene family cloned from mouse spleen by the polymerase chain reaction using degenerate primers. LSIRF was found to encode a 51 kDa protein that shares a high degree of amino acid sequence homology in the DNA-binding domain with other IRF family members. LSIRF expression was detectable only in lymphoid cells. In contrast to other IRF genes, LSIRF expression was not induced by interferons, but rather by antigen-receptor mediated stimuli such as plant lectins, CD3 or IgM crosslinking. In in vitro DNA binding studies, LSIRF was able to bind to the interferon-stimulated response element (ISRE) of the MHC class I promoter. The expression pattern and DNA binding activities suggest that LSIRF plays a role in ISRE-targeted signal transduction mechanisms specific to lymphoid cells.

Roles of SWI1, SWI2, and SWI3 proteins for transcriptional enhancement by steroid receptors

The SWI1, SWI2, and SWI3 proteins, which are required for regulated transcription of numerous yeast genes, were found also to be essential for rat glucocorticoid receptor function in yeast; the receptor failed to activate transcription in strains with mutations in the SWI1, SWI2, or SWI3 genes. Certain mutations in genes encoding components of chromatin, identified as suppressors of swi mutations, partially relieved the SWI- requirement for receptor function. Immunoprecipitation of glucocorticoid receptor derivatives from wild-type (SWI+) yeast extracts coprecipitated the SWI3 protein; such receptor-SWI3 complexes were not detected in swi1- or swi2- mutant strains, implying that a complex of multiple SWI proteins may associate with the receptor. Prior incubation of a Drosophila embryo transcription extract with the yeast SWI3-specific antibody inhibited receptor function in vitro whereas the antibody had no effect if added after initiation complex formation. Thus, positive regulation by the glucocorticoid receptor in vivo and in vitro appears to require its interaction, at an early step, with one or more SWI proteins.

Signaling and regulation by a mammalian glucocorticoid receptor in Drosophila cells

We demonstrate that the rat glucocorticoid receptor enhanced transcription in cultured Drosophila cells from Drosophila promoters linked to glucocorticoid response elements (GREs); promoters either containing or lacking a TATA box were rendered hormone inducible. Enhancement was dependent on the receptor, GREs, and the presence of an agonist ligand such as dexamethasone. The specific activities and relative efficacies of a series of potential ligands were generally similar in Drosophila and mammalian cells, except that dexamethasone mesylate, a potent antagonist in mammalian cells, was a strong agonist in Drosophila cells. A composite GRE, which mediates either positive or negative glucocorticoid regulation in animal cells depending on the presence and composition of the AP-1 transcription factor, conferred hormone-dependent enhancement, but not repression, in Drosophila cells. These results indicate that factors in addition to the receptor and GRE sequences participate as determinants of both signal transduction and transcriptional regulation by the glucocorticoid receptor, and that Drosophila cells carry functional homologs of many or all of those factors. Moreover, receptor activity can be exploited to obtain regulated gene expression in Drosophila.

Transcription factor interactions: selectors of positive or negative regulation from a single DNA element

The mechanism by which a single factor evokes opposite regulatory effects from a specific DNA sequence is not well understood. In this study, a 25-base pair element that resides upstream of the mouse proliferin gene was examined; it conferred on linked promoters either positive or negative glucocorticoid regulation, depending upon physiological context. This sequence, denoted a "composite" glucocorticoid response element (GRE), was bound selectively in vitro both by the glucocorticoid receptor and by c-Jun and c-Fos, components of the phorbol ester-activated AP-1 transcription factor. Indeed, c-Jun and c-Fos served as selectors of hormone responsiveness: the composite GRE was inactive in the absence of c-Jun, whereas it conferred a positive glucocorticoid effect in the presence of c-Jun, and a negative glucocorticoid effect in the presence of c-Jun and relatively high levels of c-Fos. The receptor also interacted selectively with c-Jun in vitro. A general model for composite GRE action is proposed that invokes both DNA binding and protein-protein interactions by receptor and nonreceptor factors.

Human transcription factor TFIIIC2 specifically interacts with a unique sequence in the Xenopus laevis 5S rRNA gene

Transcription factor TFIIIC2 derived from human cells is required for tRNA-type gene transcription and binds with high affinity to the essential B-box promoter element of tRNA-type genes. Although 5S rRNA genes contain no homology with the tRNA-type gene B box, we show that TFIIIC2 is also required for Xenopus laevis 5S rRNA gene transcription. TFIIIC2 protected an approximately 30-base-pair (-10 to +18) region of a Xenopus 5S rRNA gene from DNase I digestion. This region, which spanned the transcription start site, included sequences that are highly conserved among eucaryotic 5S rRNA genes and have no homology with the B-box sequence of tRNA genes. Mutation of the TFIIIC2-binding site reduced transcription of the 5S rRNA gene by a factor of 10 in HeLa cell extracts. Methylation of C residues within the TFIIIC2-binding site interfered with binding of TFIIIC2. These results suggest a role of the TFIIIC2-binding sequence in 5S rRNA gene transcription. In addition, the 5S rRNA gene binding site and the tRNA-type gene B-box sequence did not compete with each other for binding to TFIIIC2 any better than did an unrelated DNA sequence, indicating that TFIIIC2 interacts with 5S rRNA genes and tRNA-type genes through separate DNA-binding domains or polypeptides.

In vitro transcription enhancement by purified derivatives of the glucocorticoid receptor

Mammalian glucocorticoid receptors enhance transcription from linked promoters by binding to glucocorticoid response element (GRE) DNA sequences. Understanding the mechanism of receptor action will require biochemical studies with purified components. Enhancement was observed in vitro with derivatives of the receptor that were expressed in Escherichia coli, purified, and added to a cell-free extract from Drosophila embryo nuclei. Transcription from promoters linked to one or multiple GREs was selectively enhanced by as much as six times. The effect was weaker with only one GRE, and enhancement was abolished by a point mutation that inactivates the GRE in vivo.

Purification and characterization of transcription factor IIIC2

Transcription factor IIIC2 (TFIIIC2), together with other transcription factors (TFIIIB and TFIIIC1), is required for the in vitro transcription of tRNA and adenovirus VA genes by RNA polymerase III. Previous studies have shown that TFIIIC2 is a high molecular weight (approximately 500,000) protein which binds with high affinity to the B-box promoter element of tRNA-type genes. A polypeptide of Mr approximately 250,000 is in close association with DNA in the specific complex between TFIIIC2 and the B-box promoter element. Here we describe the purification of TFIIIC2 by a factor of approximately 25,000 from nuclear extracts of HeLa cells by ionic exchange, affinity, and hydrophobic chromatography and sedimentation velocity centrifugation. The most purified fractions contain polypeptides of approximately 230 kDa (corresponding to the polypeptide which can be cross-linked to VA1 DNA), 110, 100, 80, and 60 kDa which co-sediment with TFIIIC2 B-box specific binding and in vitro transcriptional activities.

DNA-binding properties and characterization of human transcription factor TFIIIC2

Interaction between the B-block region of adenovirus VA1 DNA and the human RNA polymerase III transcription factor (TFIII) C2 was analyzed using a gel DNA-binding assay. The retarded band corresponding to the specific complex between TFIIIC2 and the regulatory B-block region was identified by DNase I footprint analysis, competition experiments, and gel shift assays using mutated and truncated virus-associated (VA) 1 DNA probes. The equilibrium constants for the binding reaction with the complete VA1 gene were determined. TFIIIC2 was found to bind to non-specific DNA sequences with a relatively low affinity (equilibrium constant Kn = 6 x 10(4) M-1), and to the B-block sequence with a high affinity (specific constant Ks = 2 x 10(11) M-1). Assuming one site per molecule, the total concentration of binding sites [C0] in the TFIIIC2-containing fractions ranged between 0.6 and 1.6 x 10(-10) M. This corresponded to 1500 TFIIIC2 molecules extracted per 293 cell. Sedimentation analysis of TFIIIC2 on a sucrose gradient showed both VA1 DNA binding activity and in vitro transcription activity cosedimenting with an apparent coefficient of 17-18 S, consistent with a molecular weight of 400-500 kDa. UV cross-linking of a 5-bromo 2'-deoxyuridine-containing, 32P-labeled VA1 probe with the TFIIIC2 fraction, followed by DNase I digestion and analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, revealed a single 32P-labeled band migrating as a 250-kDa polypeptide. Compared with the sedimentation data, this result suggests that native TFIIIC2 may be a dimer of the approximately 250-kDa polypeptide.

Inhibition of host cell RNA polymerase III-mediated transcription by poliovirus: inactivation of specific transcription factors

The inhibition of transcription by RNA polymerase III in poliovirus-infected cells was studied. Experiments utilizing two different cell lines showed that the initiation step of transcription by RNA polymerase III was impaired by infection of these cells with the virus. The observed inhibition of transcription was not due to shut-off of host cell protein synthesis by poliovirus. Among four distinct components required for accurate transcription in vitro from cloned DNA templates, activities of RNA polymerase III and transcription factor TFIIIA were not significantly affected by virus infection. The activity of transcription factor TFIIIC, the limiting component required for transcription of RNA polymerase III genes, was severely inhibited in infected cells, whereas that of transcription factor TFIIIB was inhibited to a lesser extent. The sequence-specific DNA-binding of TFIIIC to the adenovirus VA1 gene internal promoter, however, was not altered by infection of cells with the virus. We conclude that (i) at least two transcription factors, TFIIIB and TFIIIC, are inhibited by infection of cells with poliovirus, (ii) inactivation of TFIIIC does not involve destruction of its DNA-binding domain, and (iii) sequence-specific DNA binding by TFIIIC may be necessary but is not sufficient for the formation of productive transcription complexes.

Resolution of human transcription factor TFIIIC into two functional components

tRNA genes and adenovirus viral-associated (VA) genes are transcribed by RNA polymerase III. Transcription of these genes in vitro requires two protein fractions containing transcription factors designated TFIIIB and TFIIIC, in addition to RNA polymerase III. We report that the TFIIIC fraction derived from human cells in culture can be separated into two functional components, which we call TFIIIC1 and TFIIIC2. Both TFIIIC1 and TFIIIC2 fractions are required for in vitro transcription of the VA1 gene. In DNase I "footprinting" experiments, the TFIIIC2 fraction protects the internal control region termed the B block. Addition of the TFIIIC1 fraction extends the footprint over the internal control region called the A block. TFIIIC1 activity is the limiting transcription factor activity required for VA1 transcription in the crude extract. TFIIIC2 activity sediments as a large component of approximately 18 S, while TFIIIC1 activity sediments at approximately 9 S. These data indicate that the two activities are unique components and when added together reconstitute TFIIIC activity.