Inducibility of carbamoylphosphate synthetase (ammonia) in cultures of embryonic hepatocytes: ontogenesis of the responsiveness to hormones

Purification of fetal mouse hepatoblasts by magnetic beads coated with monoclonal anti-e-cadherin antibodies and their in vitro culture

A simple, rapid, and reproducible method of fetal hepatoblast purification was established to investigate mechanisms controlling interactions between hepatoblasts and nonparenchymal cells during liver development. Because E-cadherin is exclusively expressed on the cell membrane of hepatoblasts, magnetic beads coated with monoclonal antibodies to an extracellular epitope of its molecule were used to purify hepatoblasts from a cell suspension prepared from 12.5-day fetal mouse livers. The purity and yield in the hepatoblast fraction prepared in our protocol were more than 90% and approximately 30%, respectively. The nonparenchymal fraction rarely contained hepatoblasts; the rate of hepatoblast contamination in this fraction was less than 1%. Separate cultures of these two fractions were compared with cocultures of both fractions. In culture of the hepatoblast fraction, hepatoblasts formed aggregates similar to a bunch of grapes via their loose adhesion, floating in the medium after 24 h, and dissociated into single cells from the aggregates after 120 h of culture. By contrast, in the mixed culture, the majority of hepatoblasts formed multicellular spheroids after 24 h, and these spheroids changed into monolayer cell sheets after 120 h of culture. The cells comprising these monolayer sheets abundantly expressed albumin and carbamoylphosphate synthase I. In the mixed culture, fibroblastic cells also proliferated extensively with spreading on glass slides and surrounded the hepatoblast or hepatocyte colonies. On the other hand, fibroblastic cells spreading on glass slides decreased gradually in cultures of the nonparenchymal cell fraction alone. These findings indicated that the coexistence of hepatoblasts and nonparenchymal cells may be essential for their mutual survival, proliferation, differentiation, and morphogenesis. The conditioned medium of fetal liver cell cultures could partially replace the effects of the nonparenchymal cells on hepatoblasts in vitro. Our isolation protocol for fetal mouse hepatoblasts using immunobeads can greatly facilitate studies on mechanisms of cell-cell interactions during liver development.

Foetal rise in hepatic enzymes follows decline in c-met and hepatocyte growth factor expression

BACKGROUND/AIMS

In the embryo, rapidly proliferating hepatocytes migrate from the liver primordium into the surrounding mesenchyme, whereas foetal hepatocytes are mitotically quiescent and accumulate hepatocyte-specific enzymes. We investigated the timing and topography of this behavioural switch.

METHODS

The expression of the c-met receptor and its ligand, hepatocyte growth factor (HGF), was investigated in prenatal rat liver by in situ hybridization, immunohistochemistry and western-blot analysis.

RESULTS

c-Met was expressed by hepatocytes and HGF by non-parenchymal liver cells. Their mRNA levels peaked during embryonic day (ED) 11-13. c-Met protein was weakly expressed in the entire liver during ED 11 and 12, but more abundantly at ED 13, when its expression withdrew to the hepatic periphery. Simultaneously, the periportal hepatocellular marker carbamoylphosphate synthetase began to accumulate in the centre of the liver. Although the definitive vascular architecture develops simultaneously, the downstream, pericentral hepatocytes began to express glutamine synthetase only 4 days later, suggesting a requirement for prior periportal hepatocyte maturation. Additionally, c-met protein appeared in the connective tissue surrounding the large veins. The c-met protein/mRNA ratio was substantially higher in non-epithelial cells (hepatic connective tissue, heart) than in endoderm-derived epithelia, including hepatocytes, indicating important post-transcriptional regulation.

CONCLUSIONS

The decline in c-met expression reflects the end of the embryonic phase and heralds the onset of the fetal, maturational phase of liver development.

A single regulatory module of the carbamoylphosphate synthetase I gene executes its hepatic program of expression

A 469-base pair (bp) upstream regulatory fragment (URF) and the proximal promoter of the carbamoylphosphate synthetase I (CPS) gene were analyzed for their role in the regulation of spatial, developmental, and hormone-induced expression in vivo. The URF is essential and sufficient for hepatocyte-specific expression, periportal localization, perinatal activation and induction by glucocorticoids, and cAMP in transgenic mice. Before birth, the transgene is silent but can be induced by cAMP and glucocorticoids, indicating that these compounds are responsible for the activation of expression at birth. A 102-bp glucocorticoid response unit within the URF, containing binding sites for HNF3, C/EBP, and the glucocorticoid receptor, is the main determinant of the hepatocyte-specific and hormone-controlled activity. Additional sequences are required for a productive interaction between this minimal response unit and the core CPS promoter. These results show that the 469-bp URF, and probably only the 102-bp glucocorticoid response unit, functions as a regulatory module, in that it autonomously executes a correct spatial, developmental and hormonal program of CPS expression in the liver.

Immunohistochemical analysis of development of desmin-positive hepatic stellate cells in mouse liver

Development of desmin-positive hepatic stellate cells was studied in mice using double immunofluorescent techniques and in vitro cultures with special attention given to their cell lineages. Several studies recently reported on the presence of cells that are immunologically reactive with both antidesmin and anticytokeratin antibodies in young fetal rat livers, and suggested the possibility that these cells give rise to hepatocytes and hepatic stellate cells. At early stages of mouse liver development, stellate cells with desmin-positive filaments were scattered in the liver parenchyma. However, the stellate cells definitely differed from hepatoblasts and hepatocytes in terms of their morphology and expression of desmin and hepatoblast and hepatocyte-specific E-cadherin in the liver. Fetal hepatoblasts and hepatocytes did not react with antidesmin antibodies, nor did desmin-positive stellate cells express E-cadherin in vivo and in vitro. Thus it is likely that desmin-positive stellate cells and hepatoblasts belong to different cell lineages. In the fetal liver, the desmin-positive stellate cells surrounded blood vessels, and extended their processes to haematopoietic cells and megakaryocytes. Many, but not all, hepatoblasts and hepatocytes were observed to be associated with the stellate cells. At fetal stages, cellular processes positive for desmin in the stellate cells were also thick compared with those in the adult liver, in which desmin-positive stellate cells lay in Disse's space and were closely associated with all hepatocytes. These developmental changes in the geography of desmin-positive cells in the liver parenchyma and their morphology may be associated with their maturation and interactions with other cell types.

Clonogenic colony-forming ability of flow cytometrically isolated hepatic progenitor cells in the murine fetal liver

Stem cells are defined as cells having multilineage differentiation potential and self-renewal capability. Hepatic stem cells have aroused considerable interest not only because of their developmental importance but also for their therapeutic potential. However, their presence in the liver has not yet been demonstrated. With the use of a fluorescence-activated cell sorter (FACS) and monoclonal antibodies, we attempted to ascertain whether hepatic stem cells are present in the murine fetal liver. For this purpose, we optimized a cell isolation technique for FACS sorting of fetal liver cells. When isolated CD45 TER119 cells (the non-blood cell fraction in the fetal liver) were tested for their clonogenic colony-forming ability, mechanical dissociation (pipetting) was the most suitable cell isolation technique for FACS sorting. We confirmed that these colonies contained not only cells expressing hepatocyte markers but also cells expressing cholangiocyte markers. To identify hepatic stem cells, studies must focus on CD45TER119- cells in the murine fetal liver.

Involvement of a cis-acting element in the suppression of carbamoyl phosphate synthetase I gene expression in the liver of carnitine-deficient mice

The expression of carbamoyl phosphate synthetase I (CPS) gene is suppressed in the liver of carnitine-deficient juvenile visceral steatosis (JVS) mice at weaning and under starvation at adult age. To clarify the suppression mechanism, we produced CPSL transgenic JVS mice carrying a transgene composed of the chloramphenicol acetyltransferase (CAT) gene with the upstream region (-12 kb to +138) of the rat CPS gene and CPSE transgenic JVS mice carrying a transgene composed of the luciferase gene with minimal promoter (299 bp from -161 to +138) and enhancer (469 bp around -6.3 kb) fragments of the rat gene. The expression of the CAT gene as well as the endogenous CPS was suppressed in CPSL transgenic JVS mice, but luciferase gene expression was not suppressed in CPSE transgenic JVS mice. We isolated the 5'-upstream region of the mouse CPS gene and identified an activator protein-1 (AP-1) site downstream of the minimum enhancer region of both rat and mouse CPS genes. In conjunction with the 313-bp mouse promoter region, the 714-bp mouse enhancer fragment conferred a cell-type-dependent hormone responsiveness. In rat primary cultured hepatocytes, the addition of oleic acid suppressed reporter gene expression induced by dexamethasone in the construct containing the enhancer fragment of 714 bp with the AP-1 site, but not in its AP-1 site mutants or in 519 bp without the AP-1 site. These results strongly suggest that direct protein-protein interaction between AP-1 and glucocorticoid receptor is not involved in the suppression of the CPS gene in JVS mice and that the AP-1 element is the cis-element which is responsible for the suppression.

Participation of different cell types in the restitutive response of the rat liver to periportal injury induced by allyl alcohol

BACKGROUND/AIM

Restitution of periportal liver necrosis induced by allyl alcohol involves proliferation and differentiation of putative liver stem cells. The participation of different non-epithelial cell types required to restore the liver cord structure in this process has not been well documented. The aim of the study was to determine the anatomic relationships among cells of liver lineage, extracellular matrix, and non-parenchymal cells during repair of periportal liver injury.

METHODS

Periportal liver injury in rats was induced by intraperitoneal injection of allyl alcohol. Cells of the liver lineage, as well as Kupffer cells, hepatic stellate cells, macrophages, and the extracellular matrix components fibronectin and laminin were localized using immunohistologic methods for 7 days after injury.

RESULTS

During the first day there was loss of periportal hepatocytes, as well as sinusoidal nonparenchymal cells, including macrophages, Kupffer cells and hepatic stellate cells. After day 1 macrophages appeared within the necrotic zone, increased until days 3-4, and then decreased to a few cells within reappearing sinusoids. At days 2-5 there was first proliferation of small "null" intraportal cells, which later acquired markers of ductular (OV-6, CKPan ) and liver cell differentiation (alphafetoprotein, carbamoylphosphate synthetase-I), eventually assuming mature hepatocyte morphology. There was also moderate bile duct hyperplasia with extension of small newly-formed ducts from the intraportal zone into the immediate periportal zone. Kupffer cells and hepatic stellate cells became enlarged at the borders of the necrotic and non-necrotic central zone and then appeared to migrate into the oval cell population expanding across the periportal zone. During the restitution phase, hepatic stellate cells were closely associated with the proliferating oval cells, surrounding small aggregates of oval cells which appeared to be forming liver cords. Kupffer cells also stained for fibronectin, and fibronectin was seen at the intersection of the injured portal and uninjured central zones and around the expanding oval cells. In some intraportal zones, the laminin surrounding the bile ducts was lost. It was speculated that this may permit proliferating ductular cells to migrate out of the bile ducts into the periportal zone. By days 6 and 7 most of the injured liver was restored to normal, with a few foci of chronic inflammation remaining.

CONCLUSIONS

There is a close anatomic relationship between immature liver lineage cells (oval/duct cells) and non-parenchymal cells during the restitutive repair of periportal injury. The nature of this relationship to the possible production of growth factors and expression of growth factor receptors by the cells involved during the restitution process is discussed.

Glucocorticoid receptor, C/EBP, HNF3, and protein kinase A coordinately activate the glucocorticoid response unit of the carbamoylphosphate synthetase I gene

A single far-upstream enhancer is sufficient to confer hepatocyte-specific, glucocorticoid- and cyclic AMP-inducible periportal expression to the carbamoylphosphate synthetase I (CPS) gene. To identify the mechanism of hormone-dependent activation, the composition and function of the enhancer have been analyzed. DNase I protection and gel mobility shift assays revealed the presence of a cyclic AMP response element, a glucocorticoid response element (GRE), and several sites for the liver-enriched transcription factor families HNF3 and C/EBP. The in vivo relevance of the transcription factors interacting with the enhancer in the regulation of CPS expression in the liver was assessed by the analysis of knockout mice. A strong reduction of CPS mRNA levels was observed in glucocorticoid receptor- and C/EBPalpha-deficient mice, whereas the CPS mRNA was normally expressed in C/EBPbeta knockout mice and in HNF3alpha and -gamma double-knockout mice. (The role of HNFbeta could not be assessed, because the corresponding knockout mice die at embryonic day 10). In hepatoma cells, most of the activity of the enhancer is contained within a 103-bp fragment, which depends for its activity on the simultaneous occupation of the GRE, HNF3, and C/EBP sites, thus meeting the requirement of a glucocorticoid response unit. In fibroblast-like CHO cells, on the other hand, the GRE in the CPS enhancer does not cooperate with the C/EBP and HNF3 elements in transactivation of the CPS promoter. In both hepatoma and CHO cells, stimulation of expression by cyclic AMP depends mainly on the integrity of the glucocorticoid pathway, demonstrating cross talk between this pathway and the cyclic AMP (protein kinase A) pathway.

The upstream regulatory region of the carbamoyl-phosphate synthetase I gene controls its tissue-specific, developmental, and hormonal regulation in vivo

The carbamoyl-phosphate synthetase I gene is expressed in the periportal region of the liver, where it is activated by glucocorticosteroids and glucagon (via cyclic AMP), and in the crypts of the intestinal mucosa. The enhancer of the gene is located 6.3 kilobase pairs upstream of the transcription start site and has been shown to direct the hormone-dependent hepatocyte-specific expression in vitro. To analyze the function of the upstream region in vivo, three groups of transgenic mice were generated. In the first group the promoter drives expression of the reporter gene, whereas the promoter and upstream region including the far upstream enhancer drive expression of the reporter gene in the second group. In the third group the far upstream enhancer was directly coupled to a minimized promoter fragment. Reporter-gene expression was virtually undetectable in the first group. In the second group spatial, temporal, and hormonal regulation of expression of the reporter gene and the endogenous carbamoyl-phosphate synthetase gene were identical. The third group showed liver-specific periportal reporter gene expression, but failed to activate expression in the intestine. These results show that the upstream region of the carbamoyl-phosphate synthetase gene controls four characteristics of its expression: tissue specificity, spatial pattern of expression within the liver and intestine, hormone sensitivity, and developmental regulation. Within the upstream region, the far upstream enhancer at -6.3 kilobase pairs is the determinant of the characteristic hepatocyte-specific periportal expression pattern of carbamoyl-phosphate synthetase.

Differentiation of the mouse hepatic primordium cultured in vitro

The differentiation of hepatic endodermal cells is affected by endodermal-mesodermal interactions. To examine the control mechanisms of this differentiation, we cultured mouse liver primordium and tissue recombinants of the hepatic endoderm with homo- or heterologous mesenchyme in vitro. When the hepatic primordia at somite stages 15-23 were cultured in vitro for 5-10 days, the endodermal cells differentiated into large hepatocytes expressing alpha-fetoprotein (AFP), albumin and carbamoylphosphate synthetase I (CPSI) and storing glycogen. AFP continued to be expressed in hepatocytes through culture for 10 days. Albumin and CPSI expression started in hepatocytes at 1 and 2 days after culture, respectively. Dexamethasone stimulated hepatocyte differentiation (expression of CPSI and glycogen accumulation) and large lumen formation of hepatocytes, but it did not change the commencement of differentiation. When the hepatic endoderm was recombined with hepatic mesenchyme or 4-day embryonic chick lung mesenchyme, clotted in Matrigel, which is a basement-membrane-like substratum, and cultured for 5 days in vitro, it differentiated into large hepatocytes expressing albumin and CPSI and accumulating glycogen. Lung mesenchyme promoted duct formation more efficiently than the hepatic mesenchyme did. However, the hepatic endodermal cells failed to differentiate into large hepatocytes when cultured with 6-day embryonic chick metanephric mesenchyme or with 2.5-day chick somitic mesenchyme, or cultured alone in Matrigel, suggesting that the endodermal cells require the presence of splanchnic mesoderm for their differentiation in vitro. Addition of HGF (hepatocyte growth factor), aFGF (acidic fibroblast growth factor), or bFGF (basic fibroblast growth factor) also did not support the survival of hepatic endodermal cells or hepatocyte differentiation in culture without mesenchyme. Matrigel and those growth factors might not be a suitable substitute for the mesenchyme.

The establishment of the hepatic architecture is a prerequisite for the development of a lobular pattern of gene expression

We have studied the expression patterns of ammonia-metabolising enzymes and serum proteins in intrasplenically transplanted embryonic rat hepatocytes by in situ hybridisation and immunohistochemical analysis. The enzymic phenotype of individually settled hepatocytes was compared with that of hepatocytes being organised into a three-dimensional hepatic structure. Our results demonstrate that development towards the terminally differentiated state with zonal differences in enzyme content requires the incorporation of hepatocytes into lobular structures. Outside such an architectural context, phenotypic maturation becomes arrested and hepatocytes linger in the protodifferentiated state. These features identify the foetal period as a crucial time for normal liver development and show that the establishment of the terminally differentiated hepatocellular phenotype, beginning with the differentiation of hepatocytes from the embryonic foregut, is realised via a multistep process.

The far-upstream enhancer of the carbamoyl-phosphate synthetase I gene is responsible for the tissue specificity and hormone inducibility of its expression

The role of the proximal promoter and the far-upstream enhancer in the hepatocyte-specific and hormonal regulation of the carbamoyl-phosphate synthetase I (CPS) gene was investigated in transient transfection assays using primary rat hepatocytes, hepatoma cells, and fibroblasts. These experiments revealed that the activity of the promoter is comparable in all cells tested and is, therefore, not responsible for tissue-specific expression. The 5'-untranslated region of the mRNA is a major, non-tissue specific stimulator of expression in FTO-2B hepatoma cells, acting at the post-transcriptional level. A 469-base pair DNA fragment, 6 kilobase pairs upstream of the transcription start-site in the CPS gene, confers strong hormone-dependent tissue specific expression, both in combination with the CPS promoter and a minimized viral thymidine kinase promoter. Sequences similar to a cyclic AMP-responsive element and a glucocorticosteroid-responsive element were found in the isolated enhancer. Substitutional mutations in these sites strongly affected hormone-induced expression. Analysis of the interaction between the enhancer and parts of the CPS promoter revealed that, in addition to the TATA box, the GAG box, a motif similar to the GC box near the TATA motif, is instrumental in conferring the enhancer activity.

The expression of liver-specific genes within rat embryonic hepatocytes is a discontinuous process

The onset of transcription and mRNA accumulation of two liver-specific genes, carbamoylphosphate synthase (CPS) and phosphoenolpyruvate carboxykinase (PEPCK) in individual embryonic rat hepatocytes was investigated with in situ hybridization. In vitro CPS and PEPCK mRNAs can be induced prematurely in monolayer cultures of embryonic rat hepatocytes by glucocorticosteroids and cyclic AMP, i.e. the hormones that also regulate the expression of these genes in vivo. Upon exposure to hormones the cultures showed an interhepatocyte heterogeneity in CPS and PEPCK mRNA content. The pattern of accumulation of nuclear CPS mRNA-precursors indicates that this heterogeneity is generated by intercellular differences in the timing of the onset of transcription. However, under induced steady-state conditions the heterogeneity in the hepatocyte population persisted. The degree of heterogeneity is inversely related to the half life of the gene product (i.e. higher for PEPCK than for CPS and higher for mRNAs than for the respective proteins) and to the concentrations of inducing hormones. Accordingly, the interhepatocyte heterogeneity was most pronounced for the nuclear CPS mRNA-precursor. In contrast, no intercellular differences in the rate of degradation of the mRNAs were seen. These observations reveal that although all hepatocytes can and do express the genes, transcription of a gene in a particular cell is a discontinuous process.

Expression patterns of ammonia-metabolizing enzymes in the liver, mesonephros, and gut of human embryos and their possible implications

Human and ungulate embryos can catabolize amino acids for energy production, whereas rodent embryos cannot, raising the question whether studies of rodent model systems are suitable for extrapolation to the human situation. Therefore, we investigated the expression of the amino acid- and ammonia-metabolizing enzymes glutaminase, glutamate dehydrogenase, glutamine synthase, carbamoylphosphate synthase, and arginase immunohistochemically in a graded series of human embryos and fetuses. During human development the expression of these enzymes is first seen in the liver, then in the mesonephric kidney, and finally in the small intestine. Such a simultaneous expression of nitrogen-metabolizing enzymes was not seen in any other organ. The early appearance of the enzymes involved in amino acid and ammonia metabolism in the human liver, compared to, for example, the rat liver, suggests that catabolism of amino acids may provide an important supply of metabolic energy for the human embryo. The coexpression of glutaminase, glutamate dehydrogenase, and carbamoylphosphate synthase, but not of arginase, in the mesonephros and the small intestine suggests that these organs are involved in the biosynthesis of intermediates of the ornithine cycle, e.g., arginine or citrulline. From a comparison of the developmental appearance of ornithine cycle enzymes in different mammalian species we postulate that an early appearance of these enzymes is generally associated with a relatively slow prenatal growth rate and the use of amino acids as metabolic fuel.

Metabolic zonation of the liver: regulation and implications for liver function

Liver parenchyma shows a remarkable heterogeneity of the hepatocytes along the porto-central axis with respect to ultrastructure and enzyme activities resulting in different cellular functions within different zones of the liver lobuli. According to the concept of metabolic zonation, the spatial organization of the various metabolic pathways and functions forms the basis for the efficient adaptation of liver metabolism to the different nutritional requirements of the whole organism in different metabolic states. The present review summarizes current knowledge about this heterogeneity, its development and determination, as well as about its significance for the understanding of all aspects of liver function and pathology, especially of intermediary metabolism, biotransformation of drugs and zonal toxicity of hepatotoxins.

Hepatocytes explanted in the spleen preferentially express carbamoylphosphate synthetase rather than glutamine synthetase

Urea cycle enzymes and glutamine synthetase are essential for NH3 detoxification and systemic pH homeostasis in mammals. Carbamoylphosphate synthetase, the first and flux-determining enzyme of the cycle, is found only in a large periportal compartment, and glutamine synthetase is found only in a small, complementary pericentral compartment. Because it is not possible to manipulate experimentally the intrahepatic distribution of carbamoylphosphate synthetase and glutamine synthetase, we looked for conditions in which explanted hepatocytes would exhibit either the carbamoylphosphate synthetase phenotype or glutamine synthetase phenotype. In the spleen hepatocytes either settle as individual cells or in small agglomerates. The dispersed cells only express the carbamoylphosphate synthetase phenotype. Within the agglomerates, sinusoids that drain on venules develop. Hepatocytes surrounding the venules stain only weakly for carbamoylphosphate synthetase but are strongly positive for glutamine synthetase. These observations were made for explanted embryonic hepatocytes (no prior expression of either carbamoylphosphate synthetase or glutamine synthetase), neonatal hepatocytes (compartments of gene expression not yet established) and adult periportal and pericentral hepatocytes.

The initiation of hepatocyte-specific gene expression within embryonic hepatocytes is a stochastic event

To gain insight into the mechanisms that govern the first steps of liver-specific enzyme accumulation upon hormone exposure, the initial accumulation of carbamoylphosphate synthetase, phosphoenolpyruvate carboxykinase, and arginase in monolayer cultures of Embryonic Day 14 rat hepatocytes was studied. By using different fluorescent labels the initial accumulation of two enzymes could be studied simultaneously in individual cells. Both microscopic and flow cytometric analyses showed that the initial expression of genes that are under the same hormonal control appears to lack the coordinated regulation of expression that is seen later in development. The coordination is gradually established during exposure to hormones. Once gene expression becomes coordinated, the enzyme content appears to increase continuously with time. Therefore, we postulate that within individual embryonic hepatocytes the initial intercellular heterogeneity in rate of accumulation of a particular protein may be the result of competition of different genes for an initially limiting supply of common regulatory factors, leading to random differences in the rate of accumulation of the respective gene products. This makes the initiation of liver-specific gene expression within the hepatocytes a stochastic event.

The initial accumulation of carbamoylphosphate synthetase in embryonic rat hepatocytes, and the cell cycle

The hormone-induced expression of the hepatocyte-specific enzyme carbamoylphosphate synthetase can take place in each phase of the cell cycle and is not restricted to the G1 or the G0 phase. To arrive at this conclusion, the cell cycle parameters of embryonic day 14 rat hepatocytes in vitro were determined by autoradiography after labeling with (3H)-TdR or with (3H)- and (14C)-TdR. An S-phase of approximately 14 h, a G2 + M-phase of 8 h, a G1-phase of 8-13 h and a total cell cycle of 30-35 h were measured. Freshly isolated embryonic hepatocytes have exponential growth parameter values, but shift to a steady state growth under culture conditions in the presence of hormones (glucocorticosteroids, thyroid hormones and cyclic AMP). The length of the S-phase and of the total cell cycle remain constant during the culture time. The time course of accumulation of carbamoylphosphate synthetase protein in embryonic hepatocytes is identical in all phases of the cell cycle. It is suggested that hormones, in particular glucocorticosteroids, simultaneously and independently regulate growth mode and gene expression in developing hepatocytes. The nucleotide-analogue 5-bromodeoxyuridine inhibits the hormone-induced expression of carbamoylphosphate synthetase only in cells that are exposed to the drug during early S-phase, indicating replication of the carbamoylphosphate synthetase gene in that part of the cell cycle.

Accumulation of carbamoylphosphate-synthetase and phosphoenolpyruvate-carboxykinase mRNA in embryonic rat hepatocytes. Evidence for translational control during the initial phases of hepatocyte-specific gene expression in vitro

The aim of this study was to establish whether the initial accumulation of hepatocyte-specific proteins after hormone induction is regulated at the pretranslational and/or the translational level. To this end, mRNA molar concentrations were determined and compared with rates of protein synthesis from previous studies [van Roon, M.A., Charles, R. & Lamers, W.H. (1987) Eur. J. Biochem. 165, 229-234]. In vivo, carbamoylphosphate-synthetase mRNA starts to accumulate at day 17 of pregnancy. Phosphoenolpyruvate-carboxykinase mRNA starts to accumulate only just prior to birth. Embryonic day 14 (i.e. 8 days before the expected day of birth), livers were chosen to study the regulation of the initiation of hepatocyte-specific mRNA accumulation in vitro. Accumulation of carbamoylphosphate-synthetase and phosphoenolpyruvate-carboxykinase mRNA is regulated by the same hormones as accumulation of the respective proteins. The rate at which carbamoylphosphate-synthetase and phosphoenolpyruvate-carboxykinase mRNA molecules accumulate in cultured embryonic hepatocytes is relatively low, compared to that of postnatal hepatocytes. However, the increase of the rate of synthesis of carbamoylphosphate-synthetase and phosphoenolpyruvate-carboxykinase protein is even 3-6-fold slower than that of mRNA. This shows that initially mRNAs accumulate intracellularly to a relatively high concentration without being efficiently translated or translatable. Only after the mRNA concentration reaches a plateau of 72 h and 48 h respectively, the cellular capacity to synthesize the respective proteins increases. Therefore, the translational efficiency is certainly one of the major rate-limiting factors of the initial phases of expression of the hepatocyte-specific genes for carbamoylphosphate synthetase and phosphoenolpyruvate carboxykinase.

Induction of the five urea-cycle enzymes by glucagon in cultured foetal rat hepatocytes

Foetal hepatocytes obtained from rats at different stages were cultured in order to investigate the inducibility of the five urea-cycle enzymes by glucagon and dibutyryl cyclic AMP (Bt2cAMP). When 18.5-day-old hepatocytes were cultured for 3 days with 10(-7) M glucagon, the activities of carbamoyl phosphate synthetase (CPS), argininosuccinase (ASL) and arginase were increased by 1.4-, 1.8- and 1.9-fold, respectively, as compared to controls. These effects were mimicked by 10(-4) M Bt2cAMP, but the activities of ornithine transcarbamylase (OTC) and argininosuccinate synthetase (ASS) were never changed by the addition of these compounds. Hepatocytes cultured at earlier stages were not responsive to glucagon unless dexamethasone was added simultaneously, suggesting that this steroid might induce some steps necessary for glucagon action. Bt2cAMP was effective as early as day 16.5 without requiring the presence of steroids. In addition, the effect of the cyclic nucleotide appeared additive or synergistic with that of dexamethasone. The simultaneous addition of actinomycin D did not affect the glucagon-induced increase in enzyme levels, thus suggesting a post-transcriptional effect of the hormone on the foetal enzyme activities. Insulin itself did not have any effect on the basal level of the enzyme activities and had only a moderate inhibitory effect on glucagon-induced ASL activity. This slight effect of insulin is in contrast with the marked inhibitory effect of dexamethasone on this enzyme activity that we described previously.

Hormonal regulation of carbamoyl-phosphate synthetase I synthesis in primary cultured hepatocytes and Reuber hepatoma H-35. Defective regulation in hepatoma cells

Regulation of carbamoyl-phosphate synthetase I (CPS) synthesis by various hormones was compared in primary cultured hepatocytes from adult rat and in Reuber hepatoma H-35 by pulse labeling of the cells with [35S]methionine. CPS synthesis in hepatocytes was stimulated 8-fold and 5-fold by dexamethasone and glucagon respectively. CPS synthesis in hepatocytes was synergically (about 50-fold) stimulated by a combination of dexamethasone and glucagon. Less synergic stimulation was observed by combining dexamethasone with N6, O2'-dibutyryladenosine 3',5'-monophosphate (dibutyryl-cAMP) or with isoproterenol. The basal level of CPS synthesis in hepatoma cells was higher than that in hepatocytes. CPS synthesis in hepatoma cells was stimulated by dexamethasone and dibutyryl-cAMP but the extent was only 3-fold and 1.8-fold respectively. The synergic effect of combination of dexamethasone and dibutyryl-cAMP was not observed in hepatoma cells. Neither glucagon nor isoproterenol exhibited an appreciable effect on CPS synthesis in hepatoma cells. Insulin and epinephrine suppressed CPS synthesis both in hepatocytes and hepatoma cells. The effect of epinephrine was indicated to be through alpha-adrenergic receptors. The effects of insulin and epinephrine were additive on CPS synthesis both in hepatocytes and hepatoma cells.

Regulation of mRNA levels for five urea cycle enzymes in rat liver by diet, cyclic AMP, and glucocorticoids

Adaptive changes in levels of urea cycle enzymes are largely coordinate in both direction and magnitude. In order to determine the extent to which these adaptive responses reflect coordinate regulatory events at the pretranslational level, measurements of hybridizable mRNA levels for all five urea cycle enzymes were carried out for rats subjected to various dietary regimens and hormone treatments. Changes in relative abundance of the mRNAs in rats with varying dietary protein intakes are comparable to reported changes in enzyme activities, indicating that the major response to diet occurs at the pretranslational level for all five enzymes and that this response is largely coordinate. In contrast to the dietary changes, variable responses of mRNA levels were observed following intraperitoneal injections of dibutyryl cAMP and dexamethasone. mRNAs for only three urea cycle enzymes increased in response to dexamethasone. Levels of all five mRNAs increased severalfold in response to dibutyryl cAMP at both 1 and 5 h after injection, except for ornithine transcarbamylase mRNA which showed a response at 1 h but no response at 5 h. Combined effects of dexamethasone and dibutyryl cAMP were additive for only two urea cycle enzyme mRNAs, suggesting independent regulatory pathways for these two hormones. Transcription run-on assays revealed that transcription of at least two of the urea cycle enzyme genes--carbamylphosphate synthetase I and argininosuccinate synthetase--is stimulated approximately four- to fivefold by dibutyryl cAMP within 30 min. The varied hormonal responses indicate that regulatory mechanisms for modulating enzyme concentration are not identical for each of the enzymes in the pathway.

The lining of the gut in the developing rat embryo. Its relation to the hypoblast (primary endoderm) and the notochord

A light microscopical study of the morphogenesis of the gut in the rat embryo was undertaken to provide a careful map of temporal changes in the topographical relations of the (definitive) endoderm, the notochord and the hypoblast (primary endoderm). The borderline between the (definitive) endoderm and the hypoblast that appears upon gastrulation defines the lateral extension of the future gut epithelium. Within this initially semiglobular disk, the foregut and hindgut originate sequentially as blind, rapidly growing pouches. Upon the turning of the embryo, the hardly growing peripheral part of the disk becomes located in the vitelline duct. Within the head process, endodermal and notochordal cells could not be separately identified. However, slightly more posteriorly notochordal cells are seen to become embedded into the endoderm of the foregut during gastrulation. This process is not seen over the hindgut and may explain why the detachment of the notochord from the (fore)gut begins caudally.

Synthesis, accumulation and turnover of carbamoylphosphate synthetase and phosphoenolpyruvate carboxykinase in cultures of embryonic rat hepatocytes

Glucocorticosteroid, thyroid hormones and cyclic AMP can induce the synthesis of carbamoylphosphate synthetase and phosphoenolpyruvate carboxykinase in cultures of hepatocytes as soon as these cells differentiate from the embryonic foregut. The low levels of both enzymes that can accumulate in such still protodifferentiated hepatocytes are due to low levels of enzyme synthesis. In cultures, the rate of synthesis of both enzymes increases continually in the presence of hormones, showing that maturation of the capacity for synthesis towards the postnatal, fully differentiated situation is occurring in these cells. The turnover rate of both enzymes in embryonic hepatocytes is lower in the presence of hormones than in the absence, but does not change during the culture period. In the presence of hormones the turnover rate is comparable to that found in adult rat liver in vivo. The development of the capacity to accumulate organ-specific enzymes in vitro (and hence the rate of enzyme synthesis) is found to be comparable to that in utero.

Developmental and hormonal regulation of carbamoyl-phosphate synthase gene expression in rat liver: evidence for control mechanisms at different levels in the perinatal period

Carbamoyl-phosphate synthase gene expression is found to be primarily regulated by conditions that enhance hepatic glucocorticosteroid levels (hormone injections) and cyclic AMP levels (induction of diabetes). After birth, changes in the level of carbamoyl-phosphate synthase protein follow changes in the level of carbamoylphosphate synthase mRNA, suggesting a pretranslational control mechanism. In fetal rats, carbamoyl-phosphate synthase gene expression is regulated by the same factors as in adults. However, both the level to which carbamoyl-phosphate synthase mRNA can accumulate and the extent to which mRNA can be translated appear to be limited, indicating control mechanisms at the pretranslational and translational level. Finally, in the immediate postnatal period, a transient but pronounced decrease in the rate of degradation of carbamoyl-phosphate synthase protein may play a role in the accumulation of the enzyme.

Amino acid environment determines expression of carbamoylphosphate synthetase and phosphoenolpyruvate carboxykinase in embryonic rat hepatocytes

A completely defined medium (EHM-1), which reflects the amino acid composition of fetal rat serum and contains albumin as the sole proteinaceous compound, allows the accumulation of carbamoylphosphate synthetase and phosphoenolpyruvate carboxykinase in the presence of dexamethasone, dibutyryl cyclic AMP, and triiodothyronine to approximately twice the level attained in a standard culture medium (RPMI 1640) supplemented with 10% fetal bovine serum (and hormones). Using the EHM-1 medium we could show that the capacity of hepatocytes to synthesize phosphoenolpyruvate carboxykinase in the presence of hormones is manifest as soon as the cells differentiate from the embryonic foregut (embryonic Day 11). Furthermore we could show that embryonic hepatocytes can become binuclear or polyploid when cultured in the presence of thyroid hormone.