Identification of the promoter sequences involved in the interleukin-6 dependent expression of the rat alpha 2-macroglobulin gene

[Hepatology. New research results in its significance for the understanding of liver diseases]

By two exemplary clinical situations--acute viral hepatitis, acute-phase reaction of the liver--the significance of basic research for the understanding of clinical phenomena and for the development of new diagnostic and therapeutic procedures is demonstrated. The very different phenomena following infection with the hepatitis-B-virus can be explained by the variation in the interactions of virus and liver cell, by the immune reaction of the host, and by mutants of the virus. The reaction of the liver to an extrahepatic infection is mediated by interleukin-6, and characterized by an alteration in protein metabolism. The synthesis of acute-phase proteins is increased. The proteins confine the local injury and establish the homeostasis of the organism.

Interleukin 6 (IL-6) and its receptor: their role in plasma cell neoplasias

Interleukin 6 (IL-6) is a multifunctional cytokine regulating immune response acute phase reaction and hematopoiesis. IL-6 plays a critical role in B cell differentiation to plasma cells and is a potent growth factor for plasmacytoma and myeloma. Abnormal production of IL-6 has been suggested to be involved in polyclonal plasma cell abnormalities and plasma cell neoplasias. The deregulated expression of the IL-6 gene in transgenic mice resulted in the generation of malignant plasmacytoma. Based on these findings, it could be considered that continuous IL-6 gene expression plays an essential role in a multistep oncogenesis of plasma cell neoplasias. The role of IL-6 and its receptor in the generation of plasma cell neoplasias and the mechanisms of the IL-6 gene expression and IL-6 receptor-mediated signal transduction are described.

Effects of cytokines on the liver

Cytokines are essential for the communication not only between the liver and extrahepatic sites but also within the liver itself. Cytokines regulate the intermediary metabolism of amino acids, proteins, carbohydrates, lipids and minerals. Cytokines partially interact with classical hormones such as glucocorticoids, resulting in a complex network of mutual control. Since many cytokines exert growth factor-like activities in addition to their specific proinflammatory effects, the distinction between cytokines and growth factors is somewhat artificial. The liver is an important site of synthesis and the major clearance organ for several cytokines. In liver disease, cytokines are involved in the onset of intrahepatic immune responses (e.g., during viral hepatitis), in liver regeneration (e.g., after partial hepatectomy) and in the fibrotic and cirrhotic transformation of the liver such as chronic chemical injury or viral infection. Further studies of cytokine actions may lead to a better understanding of liver diseases and to the development of new immunomodulating therapeutic options.

Genomic organization of the alpha chain of the human C4b-binding protein gene

C4b-binding protein (C4bp) is a serum glycoprotein that is one of the regulators of the complement activation (RCA) family. This protein is composed of structurally related 70-kDa (alpha chain) and 45-kDa (beta chain) polypeptides. The alpha chain of C4bp (C4bp alpha) consists of eight short consensus repeats (SCR), which constitute the amino-terminal 491 residues. Human C4bp is also one of the acute-phase reactants. In order to clarify the genetic basis of the SCR and to understand the regulatory mechanisms of C4bp synthesis, we isolated 6 genomic DNA clones covering all of the human C4bp alpha gene. This gene consists of 12 exons and spans about 40 kb. Each of the SCRs is encoded by a single exon, except for the second SCR (SCR II), which is encoded by two separate exons, demonstrating that human C4bp alpha has a split SCR at the genomic level. The 5' flanking region was sequenced up to 380 bases upstream from the putative transcription initiation site. Several possible binding sites for transcription factors were identified.

Distinct regulation of the interleukin-1 and interleukin-6 response elements of the rat haptoglobin gene in rat and human hepatoma cells

The transcription rate of the haptoglobin (Hp) gene is stimulated by interleukin-1 (IL-1), IL-6, and dexamethasone in rat hepatoma (H-35) cells. To identify the cis-acting regulatory elements responsive to these hormones, various lengths of 5' Hp gene-flanking regions, including the promoter, were inserted into chloramphenicol acetyltransferase gene expression vectors and transiently introduced into H-35 cells. The first 4 kb of 5' region mediated a severalfold increase in expression after treatment with IL-6 and dexamethasone. No response to IL-1 was detectable. When, however, upstream sequences were deleted to position -165 relative to the transcription start site, a significant stimulation by IL-1 was gained without appreciably affecting the IL-6 response. With the apparent removal of an inhibitory sequence, the promoter-proximal 165-bp region also displayed a severalfold enhanced response to the combination of dexamethasone, IL-1, and IL-6. The sequence from -165 to -147, termed the A-element, was found to be crucial for all hormone regulatory functions. Two copies of the A-element linked to a heterologous promoter responded to the three hormones, but to a lesser degree than in the Hp gene promoter context. The regulatory elements of the rat Hp gene were similarly active in human hepatoma cells. Optimal regulation by IL-6 in HepG2 cells was, however, independent of the A-element. The A-element functioned in these cells exclusively as an IL-1 response sequence. The results suggest that genomic sequences upstream of the rat Hp gene suppress the regulation by specific cytokines more prominently in transient expression assays than in the normal chromosomal context. Moreover, the functional comparison indicated that specific regulatory regions of the rat Hp gene do not function identically in different hepatic cell types.

IL-6DBP, a nuclear protein involved in interleukin-6 signal transduction, defines a new family of leucine zipper proteins related to C/EBP

We analyzed a family of proteins from hepatoma cell nuclei that bind to interleukin-6 responsive elements (IL-6REs) of several acute-phase genes. This family is characterized by leucine zipper domains compatible with that of the CCAAT/enhancer binding protein (C/EBP). A cDNA clone coding for a member of the family, IL-6DBP, was isolated; it is strongly homologous to C/EBP in the region of the basic domain and in the leucine zipper sequence. IL-6DBP and C/EBP can interact in vitro to form heterodimers that bind to DNA with the same specificity as the respective homodimers, and they can interact functionally in vivo. Both the DNA binding activity and the trans-activating capacity of IL-6DBP are induced in hepatoma cells by treatment with IL-6 through a posttranslational mechanism, implicating it as a nuclear target of IL-6 and as a mediator of the IL-6-dependent transcriptional activation of liver genes during the acute-phase response.

Distinct regulation of the interleukin-1 and interleukin-6 response elements of the rat haptoglobin gene in rat and human hepatoma cells

The transcription rate of the haptoglobin (Hp) gene is stimulated by interleukin-1 (IL-1), IL-6, and dexamethasone in rat hepatoma (H-35) cells. To identify the cis-acting regulatory elements responsive to these hormones, various lengths of 5' Hp gene-flanking regions, including the promoter, were inserted into chloramphenicol acetyltransferase gene expression vectors and transiently introduced into H-35 cells. The first 4 kb of 5' region mediated a severalfold increase in expression after treatment with IL-6 and dexamethasone. No response to IL-1 was detectable. When, however, upstream sequences were deleted to position -165 relative to the transcription start site, a significant stimulation by IL-1 was gained without appreciably affecting the IL-6 response. With the apparent removal of an inhibitory sequence, the promoter-proximal 165-bp region also displayed a severalfold enhanced response to the combination of dexamethasone, IL-1, and IL-6. The sequence from -165 to -147, termed the A-element, was found to be crucial for all hormone regulatory functions. Two copies of the A-element linked to a heterologous promoter responded to the three hormones, but to a lesser degree than in the Hp gene promoter context. The regulatory elements of the rat Hp gene were similarly active in human hepatoma cells. Optimal regulation by IL-6 in HepG2 cells was, however, independent of the A-element. The A-element functioned in these cells exclusively as an IL-1 response sequence. The results suggest that genomic sequences upstream of the rat Hp gene suppress the regulation by specific cytokines more prominently in transient expression assays than in the normal chromosomal context. Moreover, the functional comparison indicated that specific regulatory regions of the rat Hp gene do not function identically in different hepatic cell types.

Structure, hormonal regulation, and identification of the interleukin-6- and dexamethasone-responsive element of the rat haptoglobin gene

Hepatic expression of the haptoglobin (Hp) gene in mammalian species is stimulated severalfold during an acute-phase reaction. To identify the molecular mechanism responsible for this regulation, the single-copy rat Hp gene has been isolated. The genomic sequences showed a high degree of homology with the primate Hp gene. Activity of the rat Hp gene was increased in cultured liver cells by interleukin-1 (IL-1), IL-6, and glucocorticoids. The genomic Hp gene sequence spanning from -6500 to +6500, when transiently introduced into human hepatoma (HepG2) cells, directed IL-6- and dexamethasone-stimulated expression of rat Hp mRNA and protein. No response to IL-1 was detected, suggesting that the corresponding regulatory element(s) might lie outside of the tested gene sequences. An IL-6- and dexamethasone-responsive element has been localized to the promoter proximal region -146 to -55. Although the nucleotide sequences of this rat Hp gene region showed substantial divergence from that of the human gene, analysis of sequential 5' and 3' deletion constructs indicated an arrangement of functional IL-6 response elements in the rat Hp promoter sequence comparable to that of the human homolog. The magnitude of IL-6 regulation through the rat Hp gene promoter was severalfold lower than that of the human Hp gene. The reduced activity could be ascribed to a single-base difference in an otherwise conserved sequence corresponding to an active element in the human gene. The IL-6 response of the rat Hp element was improved severalfold by substituting that base with the human nucleotide.

Structure, hormonal regulation, and identification of the interleukin-6- and dexamethasone-responsive element of the rat haptoglobin gene

Hepatic expression of the haptoglobin (Hp) gene in mammalian species is stimulated severalfold during an acute-phase reaction. To identify the molecular mechanism responsible for this regulation, the single-copy rat Hp gene has been isolated. The genomic sequences showed a high degree of homology with the primate Hp gene. Activity of the rat Hp gene was increased in cultured liver cells by interleukin-1 (IL-1), IL-6, and glucocorticoids. The genomic Hp gene sequence spanning from -6500 to +6500, when transiently introduced into human hepatoma (HepG2) cells, directed IL-6- and dexamethasone-stimulated expression of rat Hp mRNA and protein. No response to IL-1 was detected, suggesting that the corresponding regulatory element(s) might lie outside of the tested gene sequences. An IL-6- and dexamethasone-responsive element has been localized to the promoter proximal region -146 to -55. Although the nucleotide sequences of this rat Hp gene region showed substantial divergence from that of the human gene, analysis of sequential 5' and 3' deletion constructs indicated an arrangement of functional IL-6 response elements in the rat Hp promoter sequence comparable to that of the human homolog. The magnitude of IL-6 regulation through the rat Hp gene promoter was severalfold lower than that of the human Hp gene. The reduced activity could be ascribed to a single-base difference in an otherwise conserved sequence corresponding to an active element in the human gene. The IL-6 response of the rat Hp element was improved severalfold by substituting that base with the human nucleotide.

Acute-phase reaction induces a specific complex between hepatic nuclear proteins and the interleukin 6 response element of the rat alpha 2-macroglobulin gene

Interleukin 6 (IL-6) was established as a transcriptional inducer of the rat alpha 2-macroglobulin gene, a prototype liver acute-phase gene. Maximum induction occurred when the 5' flanking sequences of this gene (position -209 to -43) directed expression from the gene's own TATA box and transcription start site. Removal of the hexanucleotide CTGGGA (position -164 to -159) abolished 60-70% of the hormonal induction in FAO1 rat hepatoma cells. This hexanucleotide was defined as the IL-6 response element (IL-6-RE). The IL-6-RE is well conserved in the cytokine-responsive regions of other acute-phase genes and serves as a binding site for nuclear proteins. A characteristic DNA-protein complex (complex I) was formed with nuclear proteins from normal rat livers. A different, hormone-inducible complex (complex II) was assembled specifically with nuclear proteins from acute-phase rat livers or from IL-6-treated human Hep 3B hepatoma cells. Complex II was competitively inhibited by oligonucleotides representing the conserved IL-6-RE sequence from other acute-phase genes. Thus, the proteins building complex II likely participate in a general signal transduction mechanism mediating the transcriptional activation by IL-6 of several acute-phase genes.

Interleukin-6 and the acute phase response

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Nuclear factors interacting with an interleukin-6 responsive element of rat alpha 2-macroglobulin gene

During acute inflammation, a group of liver-derived plasma proteins, acute phase proteins (APPs), increase in concentration. Interleukin-6 (IL-6) is responsible for this increase via the induction of APP gene expression. We have identified an IL-6 responsive cis-acting element (IL-6RE) of gene encoding a typical APP, rat alpha 2-macroglobulin (alpha 2M). The IL-6RE contains a sequence that is conserved among the 5'-flanking regions of various APP genes. Introduction of mutations into the conserved sequence revealed that the sequence, termed IL-6RE core, is a critical and essential component of IL6-RE. Nuclear factors binding to the IL-6RE core were identified in livers of normal and inflamed rats. Mobility shift pattern and DNase I footprinting profile indicated that the factors from normal and inflamed stages recognized the same sequence but were distinct from each other. These results suggested that the regulation of alpha 2M gene expression may involve mutually exclusive interaction of stage-specific trans-acting factors.