The distal enhancer implicated in the developmental regulation of the tyrosine aminotransferase gene is bound by liver-specific and ubiquitous factors

Hepatic SRC-1 activity orchestrates transcriptional circuitries of amino acid pathways with potential relevance for human metabolic pathogenesis

Disturbances in amino acid metabolism are increasingly recognized as being associated with, and serving as prognostic markers for chronic human diseases, such as cancer or type 2 diabetes. In the current study, a quantitative metabolomics profiling strategy revealed global impairment in amino acid metabolism in mice deleted for the transcriptional coactivator steroid receptor coactivator (SRC)-1. Aberrations were hepatic in origin, because selective reexpression of SRC-1 in the liver of SRC-1 null mice largely restored amino acids concentrations to normal levels. Cistromic analysis of SRC-1 binding sites in hepatic tissues confirmed a prominent influence of this coregulator on transcriptional programs regulating amino acid metabolism. More specifically, SRC-1 markedly impacted tyrosine levels and was found to regulate the transcriptional activity of the tyrosine aminotransferase (TAT) gene, which encodes the rate-limiting enzyme of tyrosine catabolism. Consequently, SRC-1 null mice displayed low TAT expression and presented with hypertyrosinemia and corneal alterations, 2 clinical features observed in the human syndrome of TAT deficiency. A heterozygous missense variant of SRC-1 (p.P1272S) that is known to alter its coactivation potential, was found in patients harboring idiopathic tyrosinemia-like disorders and may therefore represent one risk factor for their clinical symptoms. Hence, we reinforce the concept that SRC-1 is a central factor in the fine orchestration of multiple pathways of intermediary metabolism, suggesting it as a potential therapeutic target that may be exploitable in human metabolic diseases and cancer.

Organ-specific gene expression: the bHLH protein Sage provides tissue specificity to Drosophila FoxA

FoxA transcription factors play major roles in organ-specific gene expression, regulating, for example, glucagon expression in the pancreas, GLUT2 expression in the liver, and tyrosine hydroxylase expression in dopaminergic neurons. Organ-specific gene regulation by FoxA proteins is achieved through cooperative regulation with a broad array of transcription factors with more limited expression domains. Fork head (Fkh), the sole Drosophila FoxA family member, is required for the development of multiple distinct organs, yet little is known regarding how Fkh regulates tissue-specific gene expression. Here, we characterize Sage, a bHLH transcription factor expressed exclusively in the Drosophila salivary gland (SG). We show that Sage is required for late SG survival and normal tube morphology. We find that many Sage targets, identified by microarray analysis, encode SG-specific secreted cargo, transmembrane proteins, and the enzymes that modify these proteins. We show that both Sage and Fkh are required for the expression of Sage target genes, and that co-expression of Sage and Fkh is sufficient to drive target gene expression in multiple cell types. Sage and Fkh drive expression of the bZip transcription factor Senseless (Sens), which boosts expression of Sage-Fkh targets, and Sage, Fkh and Sens colocalize on SG chromosomes. Importantly, expression of Sage-Fkh target genes appears to simply add to the tissue-specific gene expression programs already established in other cell types, and Sage and Fkh cannot alter the fate of most embryonic cell types even when expressed early and continuously.

Foxa2 integrates the transcriptional response of the hepatocyte to fasting

Survival during prolonged food deprivation depends on the activation of hepatic gluconeogenesis. Inappropriate regulation of this process is a hallmark of diabetes and other metabolic diseases. Activation of the genes encoding gluconeogenic enzymes is mediated by hormone-responsive transcription factors such as the cyclic AMP response element binding protein (CREB) and the glucocorticoid receptor (GR). Here we show using cell-type-specific gene ablation that the winged helix transcription factor Foxa2 is required for activation of the hepatic gluconeogenic program during fasting. Specifically, Foxa2 promotes gene activation both by cyclic AMP, the second messenger for glucagon, and glucocorticoids. Foxa2 mediates these effects by enabling recruitment of CREB and GR to their respective target sites in chromatin. We conclude that Foxa2 is required for execution of the hepatic gluconeogenic program by integrating the transcriptional response of the hepatocyte to hormonal stimulation.

Comparison of the tyrosine aminotransferase cDNA and genomic DNA sequences of normal mink and mink affected with tyrosinemia type II

Type II tyrosinemia, designated Richner-Hanhart syndrome in humans, is a hereditary metabolic disorder with autosomal recessive inheritance characterized by a deficiency of tyrosine aminotransferase activity. Mutations occur in the human tyrosine aminotransferase gene, resulting in high levels of tyrosine and disease. Type II tyrosinemia occurs in mink, and our hypothesis was that it would also be associated with mutation(s) in the tyrosine aminotransferase gene. Therefore, the transcribed cDNA and the genomic tyrosine aminotransferase gene were sequenced from normal and affected mink. The gene extended over 11.9 kb and had 12 exons coding for a predicted 454-amino-acid protein with 93% homology with human tyrosine aminotransferase. FISH analysis mapped the gene to chromosome 8 using the Mandahl and Fredga (1975) nomenclature and chromosome 5 using the Christensen et al. (1996) nomenclature. The hypothesis was rejected because sequence analysis disclosed no mutations in either cDNA or introns that were associated with affected mink. This suggests that an unlinked gene regulatory mutation may be the cause of tyrosinemia in mink.

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.

Hepatic nuclear factor-3 (HNF-3 or Foxa2) regulates glucagon gene transcription by binding to the G1 and G2 promoter elements

Glucagon gene expression in the endocrine pancreas is controlled by three islet-specific elements (G3, G2, and G4) and the alpha-cell-specific element G1. Two proteins interacting with G1 have previously been identified as Pax6 and Cdx2/3. We identify here the third yet uncharacterized complex on G1 as hepatocyte nuclear factor 3 (HNF-3)beta, a member of the HNF-3/forkhead transcription family, which plays an important role in the development of endoderm-related organs. HNF-3 has been previously demonstrated to interact with the G2 element and to be crucial for glucagon gene expression; we thus define a second binding site for this transcription on the glucagon gene promoter. We demonstrate that both HNF-3alpha and -beta produced in heterologous cells can interact with similar affinities to either the G1 or G2 element. Pax6, which binds to an overlapping site on G1, exhibited a greater affinity as compared with HNF-3alpha or -beta. We show that both HNF-3beta and -alpha can transactivate glucagon gene transcription through the G2 and G1 elements. However, HNF-3 via its transactivating domains specifically impaired Pax6-mediated transactivation of the glucagon promoter but had no effect on transactivation by Cdx2/3. We suggest that HNF-3 may play a dual role on glucagon gene transcription by 1) inhibiting the transactivation potential of Pax6 on the G1 and G3 elements and 2) direct activation through G1 and G2.

Identification of new regulatory sequences far upstream of the mouse monocyte chemoattractant protein-1 gene

We systematically searched for sequences influencing the expression of the mouse monocyte chemoattractant protein-1 (MCP-1) gene (Scya2) by mapping DNase I hypersensitive sites (HS) in the chromatin of mesangial cells in a 40-kb interval around the gene. We found nine HS located between -24 kb and +12.7 kb. Three HS coincided with previously known regulatory sequences (HS-2.4, HS-1.0, and HS-0.2). We tested two of the previously unknown HS located far upstream of Scya2 (HS-19.4 and HS-16.3) in transfection experiments using luciferase reporter constructs and mouse mesangial cells as recipients. In transient transfections, both HS had a moderate effect on basal promoter activity as well as promoter activity stimulated by tumor necrosis factor-alpha. In stable transfection experiments, we found much higher activity. A DNA fragment containing HS-19.4 and HS-16.3 caused a considerable increase in the number of stably integrated luciferase copies. We determined the nucleotide sequence of the 5' flanking region to -28.6 kb. Computer-assisted sequence analysis did not yield evidence of an additional gene. These HS are located within the 5' flanking region of a gene cluster consisting of Scya2 (MCP-1), Scya7 (MCP-3), Scya11 (eotaxin), Scya12 (MCP-5), and Scya8 (MCP-2). This report represents the first comprehensive chromatin analysis of the mouse MCP-1 locus leading to the identification of a complex regulatory region located far upstream of Scya2.

Transducin-like enhancer of split proteins, the human homologs of Drosophila groucho, interact with hepatic nuclear factor 3beta

Members of the hepatic nuclear factor 3 (HNF3) family, including HNF3alpha, HNF3beta, and HNF3gamma, play important roles in embryonic development, the establishment of tissue-specific gene expression, and the regulation of gene expression in differentiated tissues. We found, using the glutathione S-transferase pull-down method, that the transducin-like Enhancer of split (TLE) proteins, which are the human homologs of Drosophila Groucho, directly associate with HNF3beta. Conserved region II of HNF3beta (amino acids 361-388) is responsible for the interaction with TLE1. A mammalian two-hybrid assay was used to confirm that this interaction occurs in vivo. Overexpression of TLE1 in HepG2 and HeLa cells decreases transactivation mediated through the C-terminal domain of HNF3beta, and Grg5, a naturally occurring dominant negative form of Groucho/TLE, also increases the transcriptional activity of this region of HNF3. These results lead us to suggest that TLE proteins could influence the expression of mammalian genes regulated by HNF3.

Identification of transacting factors responsible for the tissue-specific expression of human glucose transporter type 2 isoform gene. Cooperative role of hepatocyte nuclear factors 1alpha and 3beta

We investigated transacting factors binding to the cis-element important in tissue-specific expression of the human glucose transporter type 2 isoform (GLUT2) gene. By transient transfection assay, we determined that the 227-base pair fragment upstream of the ATG start site contained promoter activity and that the region from +87 to +132 (site C) was responsible for tissue-specific expression. DNase I footprinting and electrophoretic mobility shift assay indicated that site C contained one binding site for hepatocyte nuclear factor 1 (HNF1) and two binding sites for HNF3. The mutations at positions +101 and +103, which are considered to be critical in binding HNF1 and HNF3, resulted in a 53% decrease in promoter activity, whereas the mutation of the proximal HNF3 binding site (+115 and +117) reduced promoter activity by 28%. The mutations of these four sites resulted in marked decrease (70%) in promoter activity as well as diminished bindings of HNF1 and HNF3. A to G mutation, which causes conversion of the HNF1 and HNF3 binding sequence to the NF-Y binding site, resulted in a 22% decrease in promoter activity. We identified that both HNF1 and HNF3 function as transcriptional activators in GLUT2 gene expression. Coexpression of the pGL+74 (+74 to +301) construct with the HNF1alpha and HNF3beta expression vectors in NIH 3T3 cells showed the synergistic effect on GLUT2 promoter activity compared with the expression of HNF1alpha, HNF3beta, or a combination of HNF1beta and HNF3beta. These data suggest that HNF1alpha and HNF3beta may be the most important players in the tissue-specific expression of the human GLUT2 gene.

Cooperative regulation of CYP2C12 gene expression by STAT5 and liver-specific factors in female rats

The purpose of this study was to clarify the mechanism(s) responsible for the growth hormone (GH)-induced expression of the CYP2C12 gene. To identify a functional GH-responsive element (GHRE) in vivo, we performed the direct injection of promoter-luciferase chimeric genes into female rat livers. The results showed that the luciferase activity was decreased to approximately 20% by the deletion of the sequence between nucleotides -4213 and -4161. Within this region, two copies of a possible GHRE were present. The sequence of the GHRE was overlapped with that of an interferon-gamma-activated sequence, known to be recognized by the signal transducer and activator of transcription (STAT) proteins. In fact, a supershift assay showed that STAT5 was capable of binding to the core sequence of the GHRE. Furthermore, a luciferase assay with reporter plasmids, Delta-4161/-3781, mutated hepatocyte nuclear factor-4 (HNF-4), and mutated HNF-6, revealed that the GH-stimulated expression of the CYP2C12 gene was regulated cooperatively by STAT5, HNF-4, HNF-6, and the factor(s) that binds to the elements, 2C12-I (-4095 to -4074) and 2C12-II (-4072 to -4045). The cooperative regulation by STAT5 and the liver-enriched transcription factors account for the GH-dependent and the liver-specific expression of the CYP2C12 gene in female rats.

The transcription factor CCAAT/enhancer-binding protein beta regulates gluconeogenesis and phosphoenolpyruvate carboxykinase (GTP) gene transcription during diabetes

CCAAT/enhancer-binding protein (C/EBP) beta and C/EBPalpha are members of the c/ebp gene family and are highly expressed in mammalian liver and adipose tissue. C/EBPalpha is essential for adipogenesis and neonatal gluconeogenesis, as shown by the C/EBPalpha knockout mouse. C/EBPbeta binds to several sequences of the phosphoenolpyruvate carboxykinase (PEPCK) gene promoter with high affinity, and C/EBPbeta protein is increased 200% in the livers of streptozotocin-diabetic mice, concurrent with increased PEPCK mRNA. To elucidate the role of C/EBPbeta in the control of gluconeogenesis during diabetes, we studied the levels of plasma metabolites and hormones related to energy metabolism during diabetes in adult mice heterozygous and homozygous for a null mutation of the gene for C/EBPbeta. We also examined the expression of PEPCK and glucose 6-phosphatase mRNAs and regulation of blood glucose, including the contribution of gluconeogenesis to blood glucose in c/ebpbeta-/- mice. C/EBPbeta was not essential to basal PEPCK mRNA levels. However, C/EBPbeta deletion affected streptozotocin-diabetic response by: (a) delaying hyperglycemia, (b) preventing the increase of plasma free fatty acids, (c) limiting the full induction of PEPCK and glucose 6-phosphatase genes, and (d) preventing the increase in gluconeogenesis rate. Gel supershifts of transcription factor C/EBPalpha, bound to CRE, P3I, and AF-2 sites of the PEPCK promoter, was not increased in diabetic c/ebpbeta-/- mouse liver nuclei, suggesting that C/EBPalpha does not substitute for C/EBPbeta in the diabetic response of liver gene transcription. These results link C/EBPbeta to the metabolic and gene regulatory responses to diabetes and implicate C/EBPbeta as an essential factor underlying glucocorticoid-dependent activation of PEPCK gene transcription in the intact animal.

Upstream region of rat serum albumin gene promoter contributes to promoter activity: presence of functional binding site for hepatocyte nuclear factor-3

Transcription of the serum albumin gene occurs almost exclusively in the liver and is controlled in part by a strong liver-specific promoter. The upstream region of the serum albumin gene promoter is highly conserved among species and is footprinted in vitro by a number of nuclear proteins. However, the role of the upstream promoter region in regulating transcription and the identity of the transcription factors that bind to this region have not been established. In the present study, deletion analysis of the rat serum albumin promoter in transiently transfected HepG2 cells demonstrated that elimination of the region between -207 and -153 bp caused a two-fold decrease in promoter activity (P<0.05). Additional analysis of the -207 to -124 bp promoter interval led to the identification of two potential binding sites for hepatocyte nuclear factor-3 (HNF-3) located at -168 to -157 bp (site X) and -145 to -134 bp (site Y). Electrophoretic mobility-shift assays performed with the HNF-3 X and Y sites demonstrated that both sites are capable of binding HNF-3alpha and HNF-3beta. Placement of a single copy of the HNF-3 X site upstream from a minimal promoter increased promoter activity by about four-fold in HepG2 cells, and the reporter construct containing this site could be transactivated if co-transfected with an HNF-3 expression construct. Furthermore, inactivation of the HNF-3 X site by site-directed mutagenesis within the context of the -261 bp albumin promoter construct resulted in a 40% decrease in transcription (P<0.05). These results indicate that the positive effect of the -207 to -153 bp promoter interval is attributable to the presence of the HNF-3 X site within this interval. Additional results obtained with transfected HepG2 cells suggest that the HNF-3 Y site plays a lesser role in activation of transcription than the X site.

Further characterization of the glucocorticoid response unit in the phosphoenolpyruvate carboxykinase gene. The role of the glucocorticoid receptor-binding sites

Phosphoenolpyruvate carboxykinase (PEPCK) catalyzes the rate-limiting step of gluconeogenesis. The activity of this enzyme is controlled by several hormones, including glucocorticoids, glucagon, retinoic acid, and insulin, that principally affect the rate of transcription of the PEPCK gene. Glucocorticoids induce PEPCK gene transcription through a complex glucocorticoid response unit that consists of, from 5' to 3', accessory factor elements AF1 and AF2; two noncanonical glucocorticoid receptor-binding sites, GR1 and GR2; a third accessory factor element, AF3; and a cAMP-response element, CRE. A complete glucocorticoid response is dependent on the presence of both GR-binding sites, all three accessory elements, and the CRE. In this study we assess the relative roles of GR1 and GR2 in the context of the glucocorticoid response unit and use a combination of binding and function assays to compare GR1 and GR2 to glucocorticoid response elements (GREs) that conform closely to the consensus sequence. The relative binding affinity of GR follows the order: consensus GRE >> GR1 > GR2. Mutations that disrupt the binding of GR to GR1 result in a major reduction of the glucocorticoid response, whereas similar mutations of GR2 have a much smaller effect. Unlike the simple consensus GRE, neither GR1 nor GR2 mediate a glucocorticoid response through a heterologous promoter. The accessory elements appear to have different functional roles. AF2 is still needed for a maximal glucocorticoid response when GR1 is converted to a high-affinity GR-binding element, but AF1 and AF3 are not required.

5' sequences direct developmental expression and hormone responsiveness of tyrosine aminotransferase in primary cultures of fetal rat hepatocytes

Tyrosine aminotransferase (TyrAT) is one of several gluconeogenic enzymes which appear postnatally in humans and rodents in response to increased glucocorticoid and glucagon levels and decreased insulin. Primary cultured fetal rat hepatocytes older than day 15 of gestation (>E15) transcribe the TyrAT gene in response to the synergistic effect of dexamethasone and N6,2'-O-dibutyryl-adenosine 3',5'-monophosphate (Bt2cAMP), whereas less mature hepatocytes (<E15) do not [Shelly, L. L. & Yeoh, G. C. T. (1991) Eur. J. Biochem. 199, 475-481]. Therefore, we consider >E15 hepatocytes, and not <E15 hepatocytes, to be determined. This study reports that 11.1 kb of sequences upstream of the TyrAT transcription start site, which include a cAMP-responsive element (CRE) and a glucocorticoid-responsive element (GRE), are required for correct developmental regulation of gene expression in determined fetal hepatocytes. In contrast, the TyrAT CRE alone does not have this capability. Dexamethasone augments basal and Bt2cAMP-stimulated activity of the TyrAT CRE alone, suggesting that synergism may be due to interaction between the glucocorticoid and cAMP-signaling pathways. However, Bt2cAMP does not further increase dexamethasone-induced activity of the 11.1 kb 5' sequences when the TyrAT CRE is removed, thus excluding interaction of Bt2cAMP with the glucocorticoid pathway. Finally, insulin inhibition of dexamethasone-induced gene transcription is shown to be conferred by TyrAT 5' sequences. This study shows that cellular components, other than those which mediate hormonal regulation of genes, are required for determination of hepatocytes with respect to TyrAT. Since this phenomenon is observed with transient transfections, it is unlikely to involve higher-order chromatin structure.

Five years on the wings of fork head

Since its discovery five years ago the conserved family of fork head/HNF-3-related transcription factors has gained increasing importance for the analysis of gene regulatory mechanisms during embryonic development and in differentiated cells. Different members of this family, which is defined by a conserved 110 amino acid residues encompassing DNA binding domain of winged helix structure, serve as regulatory keys in embryogenesis, in tumorigenesis or in the maintenance of differentiated cell states. The purpose of this review is to summarize the accumulating amount of data on structure, expression and function of fork head/HNF-3-related transcription factors.

In vitro transcription of the rat insulin-like growth factor-I gene

Although the liver is the major source of circulating insulin-like growth factor-I (IGF-I), relatively little is known about the regulation of IGF-I gene transcription in this tissue. Since transcripts are initiated largely in exon 1, we established an in vitro transcription system to evaluate activation of transcription via the major exon 1 initiation site. Transcription of a G-free cassette reporter was directed by rat IGF-I genomic fragments, and the adenovirus major late promoter was used as an internal control. Tissue specificity was demonstrated by a 60-90% decrease in transcripts with spleen extracts as compared with liver. 54 base pairs (bp) of upstream sequence were sufficient to direct IGF-I gene transcription, and activity increased 5-fold with 300 bp of upstream sequence. DNase I footprinting revealed four protected regions between -300 and -60 bp; binding was confirmed by gel shift analysis, and tissue specificity was demonstrated by reduced shifts with spleen extracts. The necessity of transcription factor binding to such sites was established by competition analysis, which revealed a specific decrease in IGF-I transcription in the presence of a competing fragment. Use of this in vitro transcription system should permit analysis of the function of individual transcription factors involved in regulation of IGF-I gene expression.

Tissue-specific enhancer-promoter interactions regulate high level constitutive expression of matrix metalloproteinase 2 by glomerular mesangial cells

The 72-kDa gelatinase A (MMP-2) is a central mediator of the response of the intrinsic glomerular mesangial cell to inflammatory stimuli and is regulated in a unique, cell-specific manner. We isolated a 6-kilobase pair genomic fragment of the rat MMP-2 gene and sequenced and characterized 1686-base pair of the 5'-flanking region. Using a series of 5' deletion constructs of the proximal 5'-flanking region, a strong MMP-2 enhancer element was identified. Gel shift and mutational analyses suggest tha the enhancer region represents the binding site for complex transcription factor demonstrating separable DNA-binding and transcriptional activating domains. The presence and activity of the enhancer element was evaluated in several cell types with varying capabilities to synthesize MMP-2 including mesangial cells, glomerular epithelial cells, and the monocytic U937 cell. Although binding activity was present in all cell types studied, enhancer activity was demonstrated only in mesangial and glomerular epithelial cells. Additional transcriptional control resided in a tissue-specific promoter, which supported transcription only in mesangial cells. These results indicate that the final control of mesangial cell-specific synthesis of MMP-2 derives from an interaction between the strong enhancer element and the tissue-specific MMP-2 promoter.

The hepatocyte nuclear factor-3/forkhead transcription regulatory family in development, inflammation, and neoplasia

HNF-3/FKH genes are a large family of transcriptional activators. They are expressed in specific developmental and tissue patterns. Indeed, several of them are known to be essential for normal development (e.g. Dfkh and slp-1,2). Mutation within one of these genes produces mutant fruitfly embryos that are unable to survive. This family shares conserved DNA binding and transcriptional activation domains. The DNA binding domain has been crystallized, and its structure determined. Although it has resemblance to helices of homeodomains and H5 histones, it represents a new DNA binding motif, which has been called the 'winged helix,' because it contains additional interactive peptide regions called termed wings. Subtle amino acid variations in a region adjacent to the DNA recognition helix influence the recognition specificity of each HNF-3/FKH protein and therefore confer selectivity in promoter regulation. Members of this family are important in regulating the inflammatory response of the liver (the three HNF-3 genes). In addition, several members may be important in blood cell development (H3 and 5-3). Finally, two of these genes have been found to produce neoplasia (qin and FKHR). As investigation progresses, the mechanism by which these genes regulate development, inflammation and neoplasia will become more clear.

The efficiency of nuclear processing of the tyrosine aminotransferase mRNA transcript increases after partial hepatectomy

Following a two-thirds partial hepatectomy, an approximately fivefold increase in the levels of nuclear and total mRNA for tyrosine aminotransferase was observed at 1 h and 1.5 h, respectively, and a return to the levels of the quiescent state, i.e. the levels found in non-operated livers from adrenalectomized rats, was established 16 h post-hepatectomy. The increase in mRNA levels was not accounted for by a comparable change in the rate of transcription of the gene which, at 0.5 h post-hepatectomy, reached a maximum value that amounted to only 1.4-fold the value for quiescent liver. Subsequent changes in the transcription rate largely accounted for the changes in mRNA levels observed later on. Although tyrosine aminotransferase mRNA levels were equal in quiescent and 16-h-regenerating liver, the rate of transcription of the gene in quiescent liver was threefold higher than the rate in 16-h-regenerating liver. The maintenance of a higher rate of gene transcription in quiescent liver, as compared to regenerating liver, was shown to depend on ongoing protein synthesis. The possibility that the high rate of gene transcription was due to blockage or pausing during transcript elongation in quiescent liver was excluded. The inference is that the pronounced increase in tyrosine aminotransferase mRNA levels within 1 h of partial hepatectomy is largely due to a rapid increase in the efficiency of nuclear processing of the primary transcript.

Morphology, growth, and gene expression in five newly isolated murine hepatocellular tumor cell lines

Five murine hepatocellular tumor cell lines (HepM-1-5) were isolated and grown in a synthetic medium added with hormones, growth factors and/or serum. The morphology of these lines ranged from a nearly homogeneous epithelial-like shape (HepM-2) to a stromal appearance (HepM-1). The remaining lines displayed a mixed morphology. For their proliferation all of the cell lines retained a clear dependence on the extracellular calcium level and hormonal and/or serum growth factors and, rather homogeneously, they did not express the albumin, alpha-fetoprotein (with the exception of HepM-2 cells), tyrosine aminotransferase, and ornithine transcarbamylase genes, whereas they all exhibited discrete levels of the ornithine aminotransferase mRNA. Only HepM-3 and HepM-5 lines expressed the procollagen type I gene.

Isoproterenol/tannin-dependent R15 expression in transgenic mice is mediated by an upstream parotid control region

Transgenic mice were used to locate the cis-acting DNA elements that are essential for tissue-specific and inducible expression of the rat proline-rich protein gene, R15. Chimeric genes with up to 10 kb of R15 5'-flanking region fused to chloramphenicol acetyltransferase (CAT) or polyomaviral large T-antigen (PyLT) reporter genes were tested. Our results demonstrate that (1) the isoproterenol/tannin-inducible, parotid-specific transgene expression requires an upstream cis-regulatory domain, namely the parotid control region, which extends from -6 to -1.7 kb of the R15 gene; (2) this parotid control region functions with a heterologous promoter and is indispensable for achieving a reproducible chromosomal position-independent transgene expression; (3) deletion of the R15 5'-flanking region up to -1.7 kb results in a pleiotropic effect on the transgene expression, which includes ectopic (nonsalivary) reporter expression and lack of inducibility by either the beta-agonist isoproterenol or dietary tannin stimulation; (4) when the -10 to -6 kb region from the R15 gene is deleted in the construct, the inducible expression in the parotid glands of the transgenic mice decreases by over 30-fold, but position-independent and tissue-specific transgene expression is retained. Moreover, the mechanism of induction by either catecholamine isoproterenol or dietary tannin appears to be through a beta 1-adrenergic receptor-mediated pathway for both normal (non-transgenic) and transgenic animals.