Tissue specificity of L-pyruvate kinase transgenes results from the combinatorial effect of proximal promoter and distal activator regions

The L-type pyruvate kinase (L-PK) gene is regulated by diet and hormones and expressed at high levels in the hepatocytes, enterocytes, and proximal tubular cells of the kidney and at low levels in the endocrine pancreatic cells. Two regulatory regions have been shown to be important in transgenic mice to confer on a reporter gene a similar tissue-specific and diet-responsive expression: a proximal promoter fragment, with binding sites for the tissue-specific hepatocyte nuclear factors 1 and 4, and presence of the glucose-response element (GIRE) and a distal activator corresponding to a liver-specific hypersensitive site at -3000 bp with respect to the cap site. Although the proximal promoter is able to confer by itself tissue-specific expression on a reporter gene, its activity in vivo is strongly stimulated by the distal activator. To determine the possible role of the distal region on diet responsiveness and tissue specificity of the L-PK gene expression, we have created lines of transgenic mice in which the gene for SV40 T antigen (Tag) was directed by composite regulatory sequences consisting of the L-PK promoter and different enhancers: either the SV40 early enhancer (SV) or the H enhancer of the aldolase A gene (H). The induction of the composite H-PK/Tag and SV-PK/Tag transgenes by a carbohydrate-rich diet in the liver was similar to that of the endogenous L-PK gene. This suggests that in fasted mice the L-PK promoter, and especially the GIRE, is able to silence the activating influence of a strong viral enhancer such as the SV40 enhancer. The H-PK/Tag mice expressed the transgene similarly to the endogenous gene, except in the pancreas, where expression was practically undetectable. Consistently, whereas L-PK/Tag mice develop insulinomas, H-PK/Tag mice develop only hepatomas. In contrast, the transgene expression was partly aberrant in SV-PK/Tag mice. In addition to a normal activation of the transgene in the liver, a strong expression was also detected in the kidney medulla, whereas the transgene was practically silent in enterocytes. Finally, the effect of the distal region (-2070 to -3200) on an ubiquitous promoter was tested by ligating the distal L-PK gene fragment in front of a thymidine kinase/CAT transgene. Such a transgene was constantly expressed in the pancreas and, strikingly, in the brain. It appears, therefore, that the L-PK distal activator exhibits, by itself, a certain neuropancreatic specificity required in combination with the proximal promoter for L-PK gene expression in pancreas endocrine cells.

Endothelial-specific deletion of connexin40 promotes atherosclerosis by increasing CD73-dependent leukocyte adhesion

BACKGROUND

Endothelial dysfunction is the initiating event of atherosclerosis. The expression of connexin40 (Cx40), an endothelial gap junction protein, is decreased during atherogenesis. In the present report, we sought to determine whether Cx40 contributes to the development of the disease.

METHODS AND RESULTS

Mice with ubiquitous deletion of Cx40 are hypertensive, a risk factor for atherosclerosis. Consequently, we generated atherosclerosis-susceptible mice with endothelial-specific deletion of Cx40 (Cx40del mice). Cx40del mice were indeed not hypertensive. The progression of atherosclerosis was increased in Cx40del mice after 5 and 10 weeks of a high-cholesterol diet, and spontaneous lesions were observed in the aortic sinuses of young mice without such a diet. These lesions showed monocyte infiltration into the intima, increased expression of vascular cell adhesion molecule-1, and decreased expression of the ecto-enzyme CD73 in the endothelium. The proinflammatory phenotype of Cx40del mice was confirmed in another model of induced leukocyte recruitment from the lung microcirculation. Endothelial CD73 is known to induce antiadhesion signaling via the production of adenosine. We found that reducing Cx40 expression in vitro with small interfering RNA or antisense decreased CD73 expression and activity and increased leukocyte adhesion to mouse endothelial cells. These effects were reversed by an adenosine receptor agonist.

CONCLUSIONS

Cx40-mediated gap junctional communication contributes to a quiescent nonactivated endothelium by propagating adenosine-evoked antiinflammatory signals between endothelial cells. Alteration in this mechanism by targeting Cx40 promotes leukocyte adhesion to the endothelium, thus accelerating atherosclerosis.

[Connexins and junctional channels. Roles in the spreading of cardiac electrical excitation and heart development]

The electrical activity in heart is generated in the sinoatrial node and then propagates to the atrial and ventricular tissues. The junctional channels that couple the cardiomyocytes are responsible for this propagation process. These channels are dodecamers of transmembrane proteins of the connexin (Cx) family. Four Cxs - Cx30.2, -40, -43 and -45--have been demonstrated to be synthesized in the cardiomyocytes. In addition, each of these Cxs has a unique expression pattern in the myocardium. A fruitful approach of the role of these Cxs in the cardiac functions came with the development of transgenic mouse models. It has been shown that Cx43 was mainly involved in influx propagation in the ventricles and that inactivation in the cardiomyocytes of the gene of this Cx predisposed to development of cardiac abnormalities. Cx40 very significantly contributes to the propagation of electrical activity in the atria and the conduction system. Cx45 is essential to coordinate the synchronization of contractile activities of embryonic cardiomyocytes and for the normal progress of cardiogenesis. Finally, Cx30.2 contributes to the slowing of propagation of excitation in the atrioventricular node. These observations enable to better understand the relationships between alteration in Cx expression or gap junction remodelling and arrhythmias in the human heart.

Gap junctional connexins in the developing mouse cardiac conduction system

Gap junctional channels which couple myocytes mediate conduction phenomena in the heart. These channels are dodecamers of transmembrane proteins belonging to the connexin family (Cx). Three Cxs, Cx43, -40 and -45, have been found to be expressed in cardiomyocytes. Each of them has a distinct spatiotemporal pattern of expression, which is regulated during development. In the adult mouse heart, Cx43 is expressed in all the working myocytes and most of the conductive myocytes; Cx45 is weakly expressed in all conductive myocytes, including those of the nodal tissues; Cx40 expression is restricted to the atria and ventricular conduction system. Analysis of mice with deletions of Cx genes has provided evidence that Cx43, -40 and -45, and consequently the gap junctional channels they form, are involved in both heart function and development. For example, Cx40 deficiency results in sinoatrial conduction impairments, a significant decrease of the conduction velocity in the atria, and a delay of the propagation of impulse in the His bundle. Transgenic mouse lines with modified Cx40 genes are now being used to draw up a detailed map of the conduction system in the adult and developing heart, and to identify the regulatory elements involved in the transcriptional regulation of the Cx40 gene. Some preliminary results of these studies are described.

Embryonic development is disrupted by modest increases in vascular endothelial growth factor gene expression

Previous work has shown that heterozygocity for a null mutation of the VEGF-A gene, resulting in a 50% reduction in VEGF-A expression, is embryonic lethal at embroyonic day (E) 9.5 in mice. We now show that two- to threefold overexpression of VEGF-A from its endogenous locus results in severe abnormalities in heart development and embryonic lethality at E12.5-E14. The mutant embryos displayed an attenuated compact layer of myocardium, overproduction of trabeculae, defective ventricular septation and abnormalities in remodeling of the outflow track of the heart. In addition, aberrant coronary development was characterized by formation of oversized epicardial vessels, apparently through vasculogenesis. We infer that embryonic survival requires a narrow window of VEGF-A expression.

Multiple developmental roles of VEGF suggested by a LacZ-tagged allele

Vascular endothelial growth factor (VEGF) is an angiogenic factor and a potent stimulator of microvascular permeability. It is a mitogen specific for endothelial cells. The expression of VEGF and its two receptors, Flk-1 and Flt-1, is pivotal for the proper formation of blood vessels in embryogenesis as shown by gene-targeting experiments. Interestingly, the loss of even a single allele of VEGF led to embryonic lethality between day E9.5 and day E10.5 in the mouse. To assess the role of VEGF during embryonic development we decided to tag VEGF expression with LacZ, by inserting an IRES (internal ribosome entry site)-LacZ reporter cassette into the 3' untranslated region of the gene. This alteration enabled us to monitor VEGF expression throughout embryonic development at single-cell resolution. beta-Galactosidase expression from the altered VEGF locus was first observed prior to gastrulation and was detectable at all stages of vascular development in the embryo. Later, the specific cellular distribution and the level of VEGF expression indicated its pleiotropic role in development. High expression levels seemed to be associated with vasculogenesis and permeability, whereas lower levels were associated with angiogenesis and cell migration. In addition, we found VEGF expression in a subtype of endothelial cells present in the endocardium. We believe that the LacZ-tagged allele we have generated offers a precise means of detecting VEGF expression under a variety of physiological and pathological conditions.

In vivo effects of activated H-ras oncogene expressed in the liver and in urogenital tissues

Transgenic mouse technology provides a direct genetic approach to in vivo carcinogenesis. In order to determine the oncogenic potential of an activated ras gene in liver, kidney and intestine, we created transgenic mice expressing the human H-ras oncogene under control of the L-type pyruvate-kinase gene. This gene is expressed in hepatocytes, enterocytes, proximal tubular cells of the kidney and endocrine pancreatic cells. Depending on lines, we observed hepatocarcinoma, polycystic kidney disease and an unexpected epididymis hyperplasia. These transgenic mice are an interesting model of polycystic kidney disease, and complete our study of the tissue-specificity of oncogene action.

Transimmortalized mouse intestinal cells (m-ICc12) that maintain a crypt phenotype

This study describes the properties of a clone of immortalized cells (m-ICc12 cells) derived from the bases of small intestinal villi from 20-day-old fetuses of L-type pyruvate kinase (L-PK)/ TAg1 transgenic mice. The mice harbor the simian virus 40 large T antigen under the control of the 5' regulatory sequence from the L-PK gene. m-ICc12 cells expressed nuclear large T antigen, had a prolonged life span, and were nontumorigenic when injected into nude mice. They formed confluent monolayers of cuboid cells separated by tight junctions, developed dense, short apical microvilli, and formed domes. They also possessed cytokeratins, villin, aminopeptidase N, dipeptidyl-peptidase IV, and glucoamylase and retained crypt cell features, including intracellular sucrase isomaltase and alpha-L-fucose glycoconjugates accumulation and expression of the polymeric immunoglobulin receptor and the cystic fibrosis transmembrane conductance regulator gene. Thus the m-ICc12 cell line obtained by targeted oncogenesis in transgenic mice maintained in culture several important properties and differentiated functions of intestinal crypt cells.

Identification and functional characterization of an erythroid-specific enhancer in the L-type pyruvate kinase gene

The rat L-type pyruvate kinase gene is transcribed either from promoter L in the liver or promoter L' in erythroid cells. We have now cloned and functionally characterized an erythroid-specific enhancer, mapped in the fetal liver as hypersensitive site B (HSSB) at 3.7 kilobases upstream from the promoter L'. Protein-DNA interactions were examined in the 200-base pair core of the site by in vivo footprinting experiments. In the fetal liver, footprints were revealed at multiple GATA and CACC/GT motifs, whose association is the hallmark of erythroid-specific regulatory sequences. Functional analysis of the HSSB element in transgenic mice revealed properties of a cell-restricted enhancer. Indeed, this element was able to activate the linked ubiquitous herpes simplex virus thymidine kinase promoter in erythroid tissues. The activation was also observed in a variety of nonerythroid tissues known to synthesize GATA-binding factors. In the context of L'-PK transgenes, HSSB was not needed for an erythroid-specific activation of the L' promoter, while it was required to stimulate the L' promoter activity to a proper level. Finally, HSSB cannot be replaced by strong ubiquitous viral or cellular enhancers, suggesting a preferential interaction of the HSSB region with the L' promoter.

Transcription factors and aldolase B gene expression in microdissected renal proximal tubules and derived cell lines

Renal expression of the aldolase B isoenzyme and transcription factors previously shown to regulate the aldolase B gene promoter in the liver were analyzed in whole kidney, microdissected tubules, and the two PKSV-PCT and PKSV-PR proximal tubule cell lines derived from transgenic mice. Aldolase B gene expression appeared restricted to the proximal tubule, the site where HNF1 alpha, HNF1 beta, C/EBP alpha, and DBP transcripts were also abundant. Compared to the liver, another organ synthesizing aldolase B, proximal tubules from the kidney were characterized by the absence of HNF3 and the presence of higher ratio of HNF1 beta/HNF1 alpha transcripts. The same features were conserved in both PKSV-PCT and PKSV-PR proximal tubule cell lines. Transactivation experiments in PKSV-PCT cultured cells showed that HNF1 alpha, C/EBP alpha, and DBP behave as transactivators of the 190-bp aldolase B gene promoter, and that HNF1 beta had a low transactivating efficiency. HNF1 beta, as well as HNF3, antagonized the HNF1 alpha-dependent transactivation of the aldolase B promoter. The fact that both HNF1 beta and HNF3 factors play similar negative roles by competitively binding close to or on the HNF1 site could suggest that, in proximal tubule renal cells, HNF1 beta has the same attenuator effect on the aldolase B gene promoter as HNF3 in hepatocytes. Thus, these results indicate that such models of established renal tubule cell lines, which have conserved the same features of parental cells, represent valuable tools for studies of the regulation of genes expressed in proximal tubules of the kidney.

Expression of the L-type pyruvate kinase gene and the hepatocyte nuclear factor 4 transcription factor in exocrine and endocrine pancreas

The L-pyruvate kinase (L-PK) gene is slightly active in normal and tumoral endocrine pancreatic tissues while, in vivo, this gene is not transcribed in the exocrine pancreas. Nevertheless, the L-PK gene is re-expressed at a very low level in cultured 266.6 cells derived from an exocrine pancreas carcinoma. The L-PK gene is early activated in endodermal tissues, e.g. yolk sac and primitive intestine; it remains transcribed in fetal pancreas. In adult, L-PK gene expression is restricted to some endocrine cells. Hepatocyte nuclear factor (HNF) 1 and HNF4 are the main tissue-restricted transcription factors involved in tissue-specific expression of the L-PK gene. HNF1 concentration is similar in liver and all pancreatic cells. HNF4 concentration is high in liver, much lower in islets of Langerhans, endocrine pancreatic tumors, and cultured insulinoma cells, and is scarcely detectable in adult exocrine pancreas. This distribution of HNF4 parallels the expression of the L-PK gene. In vivo footprinting experiments show that the HNF1 binding site is similarly occupied in both adult liver and adult pancreas, in which this gene is practically inactive. In this latter tissue, however, the HNF4 binding site is differently occupied with respect to the liver. Since the chromatin structure remains open around the L-PK promoter in pancreas, the L-PK gene can probably be re-expressed under certain circumstances, for instance in cancerous pancreatic cells.

Influence of the content in transcription factors on the phenotype of mouse hepatocyte-like cell lines (mhAT)

We have described new well-differentiated mouse hepatocyte-like cell lines (mhAT) derived from transgenic mice expressing simian virus 40 large T antigen under the control of antithrombin III gene promoter (Exp. Cell Res. (1992) 200, 175-185). In an attempt to understand the phenotypic variations of the different cell lines, we analyzed their content in liver-specific transcription factors at the level of both the proteins, by gel shift analysis, and the mRNA, by quantitative reverse-transcription PCR. Moreover, the transactivating ability of endogenous HNF1 alpha and C/EBP alpha was also evaluated by measuring the activity of transfected synthetic promoters consisting of DNA element homopolymers upstream of a TATA box. High levels of HNF1, HNF3, and HNF4 transcription factors were maintained in mhAT cells. In contrast, C/EBP alpha was much more variable between the different cell lines and was less abundant than it was in vivo, in the liver. We investigated the influence of HNF1 alpha and C/EBP alpha on the activity of transfected liver-specific promoters. HNF1 alpha was not limiting for the activity of transfected liver-type pyruvate kinase and albumin promoters. In contrast, the activity of the albumin promoter in the different lines was clearly dependent on the C/EBP alpha content, which seems, therefore, to be an essential factor modulating the expression of this gene in HNF1 alpha-containing cells. This work shows that the correlations between promoter activities and transacting factor contents in a panel of well-differentiated cultured cells can be used to determine the respective role of transcription factors on the strength of some promoters.

The pyruvate kinase gene as a model for studies of glucose-dependent regulation of gene expression in the endocrine pancreatic beta-cell type

The insulinoma beta-cell line INS-1 expresses the L-type pyruvate kinase gene at high level and responds to a rise in extracellular glucose by strong induction of gene expression. Following the addition of glucose to the culture medium in the 3.5-33 mM concentration range, the cellular level of L-type pyruvate kinase mRNA increases within 2 h and reaches a maximum 15-fold above basal in 8-12 h. By run-on nuclear assay, the relative transcription rate of the pyruvate kinase gene is shown to increase 4-fold at maximal stimulation, suggesting that both transcriptional and post-transcriptional effects contribute to mRNA accumulation. The glucose effect is totally suppressed by the hexokinase inhibitor mannoheptulose, indicating a requirement for glucose phosphorylation. The mRNA induction is not inhibited in glutamine-free culture medium or by azaserine, suggesting that the hexosamine biosynthetic pathway is not involved. Moreover, metabolism along the glycolytic pathway does not appear to be an absolute requisite, since 2-deoxyglucose partly mimics the inductive effect of glucose. The glucose effect on the pyruvate kinase gene is reversibly antagonized by agents increasing intracellular cAMP. In addition, the effect is highly specific to the pyruvate kinase gene. Neither proinsulin I mRNA nor glucokinase mRNA are increased in glucose-stimulated INS-1 cells. Short term transfection with CAT plasmids driven by the pyruvate kinase L promoter reveals specific glucose-inducible reporter activity with the 183-base pair promoter region upstream of the cap site. Within this region, the previously described L4 cis-acting element is crucial for glucose responsiveness, as demonstrated by the fact that a plasmid with a mutation in this element does not elicit glucose-inducible CAT activity. Induction of L-type pyruvate kinase mRNA occurs in the islets of rats subjected to fasting and carbohydrate refeeding. In conclusion, the L-type pyruvate kinase gene provides an interesting model of glucose-regulated gene in the endocrine beta-cell type.

Diet-dependent carcinogenesis of pancreatic islets and liver in transgenic mice expressing oncogenes under the control of the L-type pyruvate kinase gene promoter

The liver-type pyruvate kinase (L-PK) gene is controlled positively by insulin and carbohydrates, negatively by glucagon and fasting. Diet-inducible models of carcinogenesis were obtained using the L-PK gene promoter and regulatory sequences to control the expression of c-myc and SV40 T oncogenes in transgenic mice. L-PK/c-myc and L-PK/Tag animals fed a carbohydrate-rich diet developed hepatocarcinomas. In addition, L-PK/Tag animals developed diet-dependent, aggressive endocrine pancreatic tumors, preceded by islet hyperplasia involving the different analysed cell populations (alpha, beta and delta). Expression of the L-PK gene was demonstrated in pancreatic tumors, in rat isolated islets and in rat insulinoma-derived cells (RIN line), revealing a new tissue specificity of the L-PK gene. Our results suggest that this gene may be expressed in islet progenitor cells from which the different mature endocrine cells derive.

Molecular cloning and expression of a Tetrahymena pyriformis ubiquitin fusion gene coding for a 53-amino-acid extension protein

The genome of Tetrahymena pyriformis has been shown to contain a ubiquitin multigene family consisting of several polyubiquitin genes and at least one ubiquitin fusion gene. We report here the isolation and characterization of one genomic clone (pTU11), that encodes a ubiquitin extension protein. A comparison of the predicted amino acid sequence of the ubiquitin extension protein gene of T. pyriformis with those from other organisms indicated a high degree of homology. However, the Tetrahymena ubiquitin extension protein contains 53 and not 52 amino acids. This feature is different from all ubiquitin 52-amino-acid extension protein genes thus far sequenced. Furthermore, we found an array of four cysteine residues similar to those found in nucleic acid binding proteins. Also, the C-terminal sequence possesses a conserved motif which may represent a nuclear translocation signal. The ubiquitin 53-amino-acid extension protein gene encodes the smallest class of ubiquitin mRNAs in T. pyriformis.