The first 22 base pairs of the proximal promoter of the rat class I alcohol dehydrogenase gene is bipartite and interacts with multiple DNA-binding proteins

Ethanol impairs differentiation of human adipocyte stromal cells in culture

BACKGROUND

Bioinformatic resources suggest that adipose tissue expresses mRNAs for alcohol dehydrogenases (ADHs) and ALDH2, and epidemiological studies indicate that heavy alcohol use reduces adipose tissue mass. We therefore characterized the expression of alcohol metabolizing enzymes in human, rat and mouse adipose tissue, preadipocytes, and adipocytes, the ability of adipocytes to metabolize ethanol, and the effects of ethanol on differentiation of human adipose stromal cells (hASCs).

METHODS

Adipose tissue, preadipocytes, and adipocytes were collected from rodents or from humans undergoing bariatric surgery. hASCs were differentiated in vitro using standard methods. Gene expression and cellular differentiation were analyzed by Western blotting, RT-PCR, and microscopy.

RESULTS

Class I ADH was expressed in human > mouse > rat adipose tissue, whereas ALDH2 was high in all samples. ADH, catalase, and ALDH2 were induced during differentiation of hASCs. The presence of 50 mM ethanol markedly reduced the differentiation of hASCs; this effect was associated with inhibition of expression of transcription factors required for differentiation, but did not depend on the ability of the cells to metabolize ethanol.

CONCLUSIONS

Human adipose tissue expresses alcohol oxidizing enzymes. The presence of ethanol at physiologically relevant concentrations inhibits differentiation of hASCs. Ethanol could alter adipose tissue biology, inducing a form of acquired lipodystrophy, which is consistent with epidemiological studies.

Inhibition of albumin synthesis in chronic diseases: molecular mechanisms

The albumin gene is expressed specifically in the liver after birth, and this expression is regulated predominantly at the transcriptional level. Regulatory proteins occupy specific DNA sequences within the promoter and enhancer of the albumin gene. The interaction between the CCAAT/enhancer binding protein (C/EBP)-beta and the albumin DNA is critical for albumin synthesis. Cachexia-induced hypoalbuminemia is mediated by tumor necrosis factor (TNF)-alpha. In turn, TNF-alpha stimulates oxidative stress, NO synthesis, and phosphorylation of C/EBP-beta within its nuclear localization signal (NLS). Consequently, C/EBP-beta is exported from the nucleus, preventing it to act as a transcriptional factor on the albumin gene. Antioxidants, NOS inhibitors. and dominant negative, nonphosphorylatable C/EBP-beta peptides block phosphorylation of C/EBP-beta within the NLS and its nuclear export as well as rescue the abnormal albumin gene expression, suggesting potential therapeutic interventions.

Activation of the human alpha1(I) collagen promoter by leptin is not mediated by transforming growth factor beta responsive elements

Leptin increases human alpha1 (I) collagen mRNA and type I collagen production and enhances hepatic fibrosis in animal models of hepatic fibrosis. These effects of leptin on fibrogenesis may be mediated by TGFbeta1, since leptin increases the TGFbeta type II receptor and augments the effect of TGFbeta1 on collagen production by stellate cells. In this study, leptin increased the activity of the human alpha1 (I) collagen promoter in transfected stellate cells. Leptin did not further enhance the activation of the promoter induced by TGFbeta1. Leptin had no effects on the transfected TGFbeta-responsive p3TP-LUX plasmid, which contains 3 CAGA elements that are essential and sufficient for the induction by TGFbeta. Leptin did not increase significantly the binding of proteins to two TGFbeta1 responsive elements in the human alpha1 (I) collagen promoter. In conclusion, this study shows that leptin activates the alpha1 (I) collagen gene and that this effect is not mediated by TGFbeta responsive elements.

Ethanol induction of class I alcohol dehydrogenase expression in the rat occurs through alterations in CCAAT/enhancer binding proteins beta and gamma

Alcohol dehydrogenase (ADH) is the principal ethanol-metabolizing enzyme. Ethanol induces rat Class I ADH mRNA and activity by an as yet unknown mechanism. In the current study, adult male rats were fed an ethanol-containing diet by continuous intragastric infusion for 42 days. Hepatic Class I ADH mRNA, protein, and activity levels in the ethanol-infused rats increased 3.9-, 3.3-, and 1.7-fold, respectively (p <0.05). Cis-acting elements within the proximal promoter region of the ADH gene were studied by electrophoretic mobility shift assay (EMSA). Hepatic nuclear extract (HNE) binding to either the consensus or ADH-specific CCAAT/enhancer binding protein (C/EBP) sites was >2.4-fold greater in ethanol-fed rats (p <0.05) than controls. Antibody-specific EMSA assays demonstrated binding of the transcription factor C/EBPbeta to the C/EBP site. Western blot immunoblot analysis of HNEs demonstrated 3.5- and 2.3-fold increases in C/EBPbeta (LAP) and C/EBPdelta (p <0.05), respectively, in ethanol-fed rats compared with controls, whereas levels of the truncated C/EBPbeta (LIP) and C/EBPgamma were lower in ethanol-fed rats (p <0.05). HNE from ethanol-fed rats increased (3-fold) the in vitro transcription of rat Class I ADH (p <0.05), and mutation of the C/EBP element in the proximal promoter region blocked this effect. Antisera against LIP or C/EBPgamma enhanced transcription efficiency (p <0.05). These data provide the first evidence for the mechanism by which ethanol regulates rat hepatic Class I ADH gene expression in vivo. This mechanism involves the C/EBP site and the enhancer binding proteins beta and gamma.

Regulation of the mammalian alcohol dehydrogenase genes

This review focuses on the regulation of the mammalian medium-chain alcohol dehydrogenase (ADH) genes. This family of genes encodes enzymes involved in the reversible oxidation of alcohols to aldehydes. Interest in these enzymes is increased because of their role in the metabolism of beverage alcohol as well as retinol, and their influence on the risk for alcoholism. There are six known classes ADH genes that evolved from a common ancestor. ADH genes differ in their patterns of expression: most are expressed in overlapping tissue-specific patterns, but class III ADH genes are expressed ubiquitously. All have proximal promoters with multiple cis-acting elements. These elements, and the transcription factors that can interact with them, are being defined. Subtle differences in sequence can affect affinity for these factors, and thereby influence the expression of the genes. This provides an interesting system in which to examine the evolution of tissue specificity. Among transcription factors that are important in multiple members of this gene family are the C/EBPs, Sp1,USF, and AP1, HNF-1, CTF/NF-1, glucocorticoid, and retinoic acid receptors, and several as-yet unidentified negative elements, are important in at least one of the genes. There is evidence that cis-acting elements located far from the proximal promoter are necessary for proper expression. Three of the genes have upstream AUGs in the 5' nontranslated regions of their mRNA, unusual for mammalian genes. The upstream AUGs have been shown to significantly affect expression of the human ADH5 gene.

Regulation of CCAAT/enhancer binding protein alpha (C/EBP alpha) gene expression by thiazolidinediones in 3T3-L1 adipocytes

Thiazolidinediones are a class of antidiabetic drugs that induce preadipocyte differentiation by binding and activating peroxisome proliferator-activated receptor gamma 2. Although thiazolidinediones are commonly thought of as insulin-sensitizing agents, these drugs have opposing and antagonistic effects to that of insulin on CCAAT/enhancer binding protein alpha (C/EBP alpha) gene expression in fully differentiated 3T3-L1 adipocytes. Thiazolidinediones induce expression of C/EBP alpha mRNA and protein, while insulin stimulates a rapid decline in C/EBP alpha mRNA and protein. When added in combination, thiazolidinediones block the suppression of C/EBP alpha mRNA by insulin; however, thiazolidinediones do not block the insulin-induced decline in GLUT4 mRNA, indicating that repression of C/EBP alpha mRNA is not required for insulin to suppress expression of a C/EBP alpha-responsive gene such as GLUT4. Instead, insulin may regulate GLUT4 mRNA by inactivating C/EBP alpha through dephosphorylation as well as by inducing the expression of the dominant-negative form of C/EBP beta (liver inhibitory protein), since both of these processes occur in the presence of thiazolidinediones.

Signaling pathways through which insulin regulates CCAAT/enhancer binding protein alpha (C/EBPalpha) phosphorylation and gene expression in 3T3-L1 adipocytes. Correlation with GLUT4 gene expression

Treatment of 3T3-L1 adipocytes with insulin (IC50 approximately 200 pM insulin) or insulin-like growth factor-1 (IC50 approximately 200 pM IGF-1) stimulates dephosphorylation of CCAAT/enhancer binding protein alpha (C/EBPalpha), a transcription factor involved in preadipocyte differentiation. As assessed by immunoblot analysis of one- and two-dimensional PAGE, insulin appears to dephosphorylate one site within p30C/EBPalpha and an additional site within p42C/EBPalpha. Consistent with insulin causing dephosphorylation of C/EBPalpha through activation of phosphatidylinositol 3-kinase, addition of phosphatidylinositol 3-kinase inhibitors (e.g. wortmannin) blocks insulin-stimulated dephosphorylation of C/EBPalpha. In the absence of insulin, wortmannin or LY294002 enhance C/EBPalpha phosphorylation. Similarly, blocking the activity of FKBP-rapamycin-associated protein with rapamycin increases phosphorylation of C/EBPalpha in the absence of insulin. Dephosphorylation of C/EBPalpha by insulin is partially blocked by rapamycin, consistent with a model in which activation of FKBP-rapamycin-associated protein by phosphatidylinositol 3-kinase results in dephosphorylation of C/EBPalpha. The dephosphorylation of C/EBPalpha by insulin, in conjunction with the insulin-dependent decline in C/EBPalpha mRNA and protein, has been hypothesized to play a role in repression of GLUT4 transcription by insulin. Consistent with this hypothesis, the decline of GLUT4 mRNA following exposure of adipocytes to insulin correlates with dephosphorylation of C/EBPalpha. However, the repression of C/EBPalpha mRNA and protein levels by insulin is blocked with an inhibitor of the mitogen-activated protein kinase pathway without blocking the repression of GLUT4 mRNA, thus dissociating the regulation of C/EBPalpha and GLUT4 mRNAs by insulin. A decline in C/EBPalpha mRNA and protein may not be required to suppress GLUT4 transcription because insulin also induces expression of the dominant-negative form of C/EBPbeta (liver inhibitory protein), which blocks transactivation by C/EBP transcription factors.

Promoter regulation of a differentially expressed gene in the human colonic epithelial cell lines HT29-18 and HT29-18-C1

Gene A4 is transcriptionally activated upon enterocyte differentiation of the human colonic epithelial cell line HT29-18 and its highly differentiated subclone HT29-18-C1 [Oliva et al., Arch. Biochem. Biophys. 302 (1993) 183-192]. To characterize the mechanisms regulating the differential transcription of A4, we analyzed its immediate 5'-flanking region for regulatory elements. Promoter-linked transfection experiments of progressively deleted A4 5'-flanking sequences fused to the bacterial cat reporter gene suggest the presence of one negative and two positive DNA elements within the first 371 bp of the A4 promoter (pA4). DNase I footprint and electrophoretic mobility shift assays demonstrate that one positive element which contains the core binding sequence for the transcription factor, Sp1, mediates an equal level of transcription in the two cell types. The second positive element, localized between nucleotide positions--169 and -152, contains a sequence previously unrecognized as a transcription factor-binding site. This element mediates a twofold increase in the activity of pA4 in HT29-18-C1, as compared to HT29-18. Furthermore, nuclear extracts obtained from HT29-18-C1 contain a higher binding activity for this element than those from HT29-18. Southwestern blot analysis suggests that the protein interacting with this element has an estimated molecular mass of 50 kDa. We conclude that this protein may be involved in the differential regulation of A4 in these intestinal cell lines.

Molecular biological aspects of alcohol-induced liver disease

Molecular biological investigations have become a predominant methodology applied to the study of alcohol-induced liver disease. The enzymatic pathways responsible for ethanol metabolism, and their genetic as well as environmental control, have become the focus of detailed investigation. More recently, the significance of cytokines in the pathogenesis of alcohol-induced liver disease has also become a major area of speculation. This review focuses on the advances made in studies of two important enzymes responsible for alcohol metabolism, alcohol dehydrogenase and aldehyde dehydrogenase, as well as the investigation of the proinflammatory and profibrogenic cytokines involved in the process of hepatic fibrogenesis. The quality and quantity of new discoveries made in the field of alcohol-induced liver disease is impressive, especially when one realizes that molecular biological approaches have been employed in this area for only 15 years. However, in most cases the studies have been predominantly descriptive, with little direct relevance to the therapeutics of alcoholism and alcohol-induced organ injury. Because the groundwork has been laid, one hopes that the next 15 years will rectify this failure.

Insulin regulates transcription of the CCAAT/enhancer binding protein (C/EBP) alpha, beta, and delta genes in fully-differentiated 3T3-L1 adipocytes

The effect of insulin on expression of CCAAT/enhancer binding protein (C/EBP) alpha, beta, and delta was investigated in fully-differentiated 3T3-L1 adipocytes. Treatment of adipocytes with insulin stimulated rapid dephosphorylation of C/EBP alpha, and repressed the expression of C/EBP alpha within 2-4 h, with > 90% suppression occurring at 24 h. While insulin induced expression of C/EBP beta and C/EBP delta within 1 h and caused a > 20-fold increase by 4 h, expression returned to nearly pretreatment levels by 24 h. The insulin concentration dependence of these effects was consistent with involvement of the insulin receptor. Gel shift analysis revealed that 6 h of insulin treatment decreased the binding of nuclear C/EBP alpha while increasing binding of nuclear C/EBP beta and C/EBP delta. The reciprocal effects of insulin on the steady-state levels of C/EBP transcription factors can be accounted for kinetically and quantitatively by changes in their mRNA levels, which can be accounted for by effects on gene transcription. The effects of insulin on adipocyte gene transcription (e.g. GLUT4) may be mediated, at least in part, by down-regulation of C/EBP alpha and/or its dephosphorylation.

The adult enhancer factor-1, a Drosophila melanogaster transcriptional repressor, modulates the promoter activity of the rat class-I alcohol dehydrogenase-encoding gene

Expression of the Drosophila melanogaster alcohol dehydrogenase-encoding gene (ADH) in the adult fat body is controlled by the ADH adult enhancer site (AAE). The D. melanogaster transcription repressor, adult enhancer factor-1 (AEF-1), binds to AAE at a site which overlaps with a sequence recognized by the mammalian transcription factor, CCAAT/enhancer-binding protein alpha [C/EBP alpha; Falb and Maniatis, Genes Dev. 6 (1992a) 454-465]. C/EBP alpha also activates the promoter of the rat class-I ADH gene in a sequence-specific manner [Potter et al., Arch. Biochem. Biophys. 285 (1991a) 246-251]. In this study, we explored the possibility that D. melanogaster AEF-1 influences transcription of the rat class-I ADH. By DNase I footprint analysis, bacterially produced AEF-1 protects a region of DNA between nucleotides (nt) -22 and -36 of the rat class-I ADH promoter (pADH), just 5' to the binding site of C/EBP alpha, a result confirmed by the electrophoretic mobility shift assay (EMSA). Co-transfection of a rat pADH-CAT reporter construct with expression vectors containing C/EBP alpha, AEF-1, or both, indicates that AEF-1 inhibits induction of the rat pADH by C/EBP alpha. Moreover, rat liver nuclear extracts appear to contain AEF-1-like-binding activities to AAE by EMSA. These experiments suggest an evolutionarily conserved mechanism by which AEF-1 modulates expression of the D. melanogaster and rat ADH genes.

Selective induction of CCAAT/enhancer binding protein isoforms occurs during rat liver development

BACKGROUND/AIMS

Recent evidence suggests that CCAAT/enhancer binding protein (C/EBP) transcription factors may regulate hepatocyte terminal differentiation.

METHODS

To explore this possibility, the present study looked for variations in the expression or DNA binding activity of different C/EBP isoforms during rat postnatal liver development and determined which of the C/EBPs were expressed by adult hepatocytes in primary culture.

RESULTS

In intact rats, hepatocyte proliferation is active for 2-3 weeks after birth. During this period of postnatal liver growth, several liver-specific functions emerge and C/EBP alpha, beta, and delta isoforms are induced. Nuclear expression of the 36-kilodalton C/EBP delta protein increases immediately after birth, followed first by increases in the 38-kilodalton C/EBP beta protein expression and then by increases in the 42-kilodalton C/EBP alpha protein expression. Changes in C/EBP DNA binding activity accompany developmental increases in C/EBP proteins. Messenger RNAs of all three C/EBP isoforms are expressed by mature hepatocytes in primary culture.

CONCLUSIONS

Specific C/EBP isoforms are induced differentially during the course of rat postnatal liver development. Young adult rats and cultured adult hepatocytes express all three C/EBP isoforms. These results are consistent with (but do not prove) the theory that variations in C/EBP expression and function help regulate hepatocyte terminal differentiation.

Enhancer-site downstream binding protein activity is enriched in rat tissues that express the class I alcohol dehydrogenase gene

The activity of the rat class I alcohol dehydrogenase (ADH) is enriched in certain tissues including the liver, intestine and testis. The tissue-specific expression of the gene encoding ADH in the rat was studied and found to closely correlate with tissue isozymic activity. A factor designated enhancer-site downstream binding protein (EDBP) was recently identified in the rat liver and found to interact with the proximal promoter of the class I ADH gene. The distribution of EDBP in nuclear extracts obtained from various tissues was examined based on its sequence-specific DNA binding property and found to correlate with tissue ADH expression. These findings suggest that EDBP is potentially a positive regulatory factor which is involved in controlling the tissue-specific expression of the ADH gene.