Expression patterns of mRNAs for alpha-fetoprotein and albumin in the developing rat: the ontogenesis of hepatocyte heterogeneity

Renal type a intercalated cells contain albumin in organelles with aldosterone-regulated abundance

Albumin has been identified in preparations of renal distal tubules and collecting ducts by mass spectrometry. This study aimed to establish whether albumin was a contaminant in those studies or actually present in the tubular cells, and if so, identify the albumin containing cells and commence exploration of the origin of the intracellular albumin. In addition to the expected proximal tubular albumin immunoreactivity, albumin was localized to mouse renal type-A intercalated cells and cells in the interstitium by three anti-albumin antibodies. Albumin did not colocalize with markers for early endosomes (EEA1), late endosomes/lysosomes (cathepsin D) or recycling endosomes (Rab11). Immuno-gold electron microscopy confirmed the presence of albumin-containing large spherical membrane associated bodies in the basal parts of intercalated cells. Message for albumin was detected in mouse renal cortex as well as in a wide variety of other tissues by RT-PCR, but was absent from isolated connecting tubules and cortical collecting ducts. Wild type I MDCK cells showed robust uptake of fluorescein-albumin from the basolateral side but not from the apical side when grown on permeable support. Only a subset of cells with low peanut agglutinin binding took up albumin. Albumin-aldosterone conjugates were also internalized from the basolateral side by MDCK cells. Aldosterone administration for 24 and 48 hours decreased albumin abundance in connecting tubules and cortical collecting ducts from mouse kidneys. We suggest that albumin is produced within the renal interstitium and taken up from the basolateral side by type-A intercalated cells by clathrin and dynamin independent pathways and speculate that the protein might act as a carrier of less water-soluble substances across the renal interstitium from the capillaries to the tubular cells.

Cellular localization and biochemical analysis of mammalian CDC50A, a glycosylated β-subunit for P4 ATPases

CDC50 proteins are β-subunits for P4 ATPases, which upon heterodimerization form a functional phospholipid translocation complex. Emerging evidence in mouse models and men links mutations in P4 ATPase genes with human disease. This study analyzed the tissue distribution and cellular localization of CDC50A, the most abundant and ubiquitously expressed CDC50 homologue in the mouse. The authors have raised antibodies that detect mouse and human CDC50A and studied CDC50A localization and glycosylation status in mouse liver cells. CDC50A is a terminal-glycosylated glycoprotein and is expressed in hepatocytes and liver sinusoidal endothelial cells, where it resides in detergent-resistant membranes. In pancreas and stomach, CDC50A localized to secretory vesicles, whereas in the kidney, CDC50A localized to the apical region of proximal convoluted tubules of the cortex. In WIF-B9 cells, CDC50A partially costains with the trans-Golgi network. Data suggest that CDC50A is present as a fully glycosylated protein in vivo, which presumes interaction with distinct P4 ATPases.

Hepatic progenitor cells, stem cells, and AFP expression in models of liver injury

Adult hepatocytes and liver-cell progenitors play a role in restoring liver tissue after injury. For the study of progenitor cells in liver repair, experimental models included (a) surgical removal of liver tissue by partial hepatectomy; (b) acute injury by carbontetrachloride; (c) acute injury by d-galactosamine (GalN) and N-nitrosomorpholine (NNM); and (d) chemical hepatocarcinogenesis by feeding NNM in low and high doses. Serological and immunohistological detection of alpha-fetoprotein gene expression served to follow pathways of cellular differentiation. Stem cells were not required in models of surgical removal of parenchyma and in carbon tetrachloride intoxication of adult hepatocytes. In contrast, regeneration of liver occurred through biliary epithelial cells in injuries induced by GalN and NNM. These biliary epithelial cells, collectively called oval cells, are most probably derived from the canals of Hering. Proliferating bile duct cells reached a level of differentiation with reactivation of foetal genes and significant alpha-1-fetoprotein (AFP) synthesis signalling a certain degree of retrodifferentiation with potential stemness. Due to the same embryonic origin of bile ducts and hepatocytes, biliary epithelium and its proliferating progeny (oval cells) have a defined role in liver regeneration as a transit and amplification compartment. In their early proliferation stage, oval cells were heavily engaged in DNA synthesis ([3H]thymidine labelling). Pulse-chase experiments during experimental hepatocarcinogenesis exhibited their development into hepatocytes with high risk for transformation and leading to foci of altered hepatocytes. Hepatocellular carcinomas may arise either from proliferating/differentiating oval cells or from adult hepatocytes; both cell types have stem-like properties. AFP-positive and AFP-negative carcinomas occurred in the same liver. They may represent random clonal origin. The heterogeneity of phenotypic marker (AFP) correlated with a process of retrodifferentiation.

Small hepatocytes in culture develop polarized transporter expression and differentiation

Rat small hepatocytes have been shown to proliferate in culture and to form organoids with differentiated hepatocytes in vitro. To evaluate the degree of polarized transporter differentiation of rat small hepatocytes during 9 weeks of culturing, we studied the time-dependent expression and subcellular localization of the major bile salt and organic anion transport systems of hepatocytes [i.e. the basolateral sodium-taurocholate co-transporting protein (Ntcp), organic-anion-transporting polypeptide 1b2 (Oatp1b2), the canalicular bile-salt export pump (Bsep) and multidrug-resistance-associated protein 2 (Mrp2)]. Small hepatocytes proliferated and differentiated in culture and formed sharply demarcated colonies as assessed by morphology, α-fetoprotein, albumin and Mrp1 expression. Polarized surface transporter expression was evident after 5 weeks of culturing for Ntcp, Oatp1b2 and Mrp2, and after 7 weeks for Bsep. After 9 weeks in culture, the vast majority of matured hepatocytes expressed Ntcp/Oatp1b2 at the basolateral and Bsep/Mrp2 at the canalicular plasma-membrane domains. This polarized transporter expression was accompanied by canalicular secretion of fluorescein-diacetate and cholylglycyl-fluorescein. Furthermore, an anastomizing three-dimensional network of bile canaliculi developed within piling-up colonies. These data demonstrate that cultured rat small hepatocytes acquire a fully differentiated transporter expression phenotype during their development into hepatic `organoid-like' clusters of mature hepatocytes. Thereby, the time-dependent sequence of transporter expression mirrored the ontogenesis of transporter expression in developing rat liver, supporting the concept that small hepatocytes correspond to the hepatocyte lineage derived from embryonic hepatoblasts and/or from a different pool of `committed hepatocyte progenitor cells'.

The onecut transcription factor hepatocyte nuclear factor-6 controls B lymphopoiesis in fetal liver

Mouse genetic models have helped to identify transcription factors that are expressed by hemopoietic cells and control their differentiation into lymphoid cells. However, little is known on transcription factors that are involved in this process, but are expressed in nonhemopoietic cells of the microenvironment. We show in this study that inactivation of the gene coding for hepatocyte nuclear factor-6 (HNF-6) in mice led to B lymphopenia in the bone marrow and spleen. This phenotype disappeared shortly after birth when fetal B lymphopoiesis is no longer active, pointing to a defect in fetal liver. Indeed, the number of B cells was decreased in this organ as well. An analysis of B cell developmental markers in fetal liver cells showed that B lymphopoiesis was impaired just beyond the pre-pro B cell stage. Hemopoietic cells from hnf6(-/-) fetal liver could reconstitute the lymphoid system when injected into scid mice. Because parenchymal cells, but not hemopoietic cells, expressed hnf6 in normal liver, we concluded that HNF-6 controls B lymphopoiesis in fetal liver and that HNF-6 exerts this control indirectly by acting in parenchymal cells. The involvement, in the B cell defect of hnf6(-/-) fetuses, of genes known to exert such an indirect control was ruled out by expression analysis, including microarrays, and by in vivo rescue experiments. This work identifies HNF-6 as the first noncell-intrinsic transcription factor known to control B lymphopoiesis specifically in fetal liver.

Cell-autonomous and signal-dependent expression of liver and intestine marker genes in pluripotent precursor cells from Xenopus embryos

Early regulatory events in respect to the embryonic development of the vertebrate liver are only poorly defined. A better understanding of the gene network that mediates the formation of hepatocytes from pluripotent embryonic precursor cells may help to establish in vitro protocols for hepatocyte differentiation. Here, we describe our first attempts to make use of early embryonic explants from the amphibian Xenopus laevis in order to address these questions. We have identified several novel embryonic liver and intestine marker genes in a random expression pattern screen with cDNA libraries derived from the embryonic liver anlage and from the adult liver of Xenopus laevis. Based on their embryonic expression characteristics, these genes, together with the previously known ones, can be categorized into four different groups: the liver specific group (LS), the liver and intestine group A (LIA), the liver and intestine group B (LIB), and the intestine specific group (IS). Dissociation of endodermal explants isolated from early neurula stage embryos reveals that all genes in the LIB and IS groups are expressed in a cell-autonomous manner. In contrast, expression of genes in the LS and LIA groups requires cell-cell interactions. The regular temporal expression profile of genes in all four groups is mimicked in ectodermal explants from early embryos, reprogrammed by co-injection of VegT and beta-catenin mRNAs. FGF signaling is found to be required for the induction of liver specific marker (LS group) gene expression in the same system.

Position-dependent activity of alpha -fetoprotein enhancer element III in the adult liver is due to negative regulation

alpha-Fetoprotein (AFP) transcription is activated early in hepatogenesis, but is dramatically repressed within several weeks after birth. AFP regulation is governed by multiple elements including three enhancers termed EI, EII, and EIII. All three AFP enhancers continue to be active in the adult liver, where EI and EII exhibit high levels of activity in pericentral hepatocytes with a gradual reduction in activity in a pericentral-periportal direction. In contrast to these two enhancers, EIII activity is highly restricted to a layer of cells surrounding the central veins. To test models that could account for position-dependent EIII activity in the adult liver, we have analyzed transgenes in which AFP enhancers EII and EIII were linked together. Our results indicate that the activity of EIII is dominant over that of EII, indicating that EIII is a potent negative regulatory element in all hepatocytes except those encircling the central veins. We have localized this negative activity to a 340-bp fragment. This suggests that enhancer III may be involved in postnatal AFP repression.

Differential expression of apolipoprotein E messenger RNA within the rat liver lobule determined by in situ hybridization

Apolipoprotein (Apo) E plays a key role in the metabolism of lipoproteins. It also modulates immunoregulation, cell growth and differentiation and the response to nerve injury. The liver is a major site of ApoE synthesis. Most of the circulating ApoE is thought to be of hepatic origin with most synthesized in hepatocytes. We showed that total liver ApoE messenger RNA (mRNA) levels were greater in normal adult female rats than in male and that gender-specific patterns of liver ApoE mRNA expression were present by in situ hybridization. In the male liver, the signal was strongest in the portal area, decreasing toward the central vein with the weakest signal in pericentral hepatocytes, resulting in a hepatic lobular gradient of expression. In female liver, a strong periportal signal also was observed that decreased in Zone 2, similar to that in males, but which then increased in pericentral hepatocytes resulting in a bowl-like distribution in marked contrast with that of the male. The results suggest that ApoE mRNA level is regulated differentially in hepatocytes within the liver plate and that the regulation is gender-dependent. Further, the results suggest that in males, hepatocytes in the portal area are the major contributors of ApoE to the plasma and/or sinusoidal pool, whereas in females, both portal and central area hepatocytes play an equal role.

Cellular aspects of alpha-fetoprotein reexpression in tumors

The cellular basis of AFP synthesis in normal development, liver regeneration, hepatocarcinogenesis and in tumors is discussed in the review. The attempt is made to interpret the production of AFP by germ cell and liver tumors as a consequence of their origin from the cell types producing AFP in normal conditions. Thus, AFP in germ cell tumors is explained by the development of the yolk sac visceral endoderm (YSVE) in teratocarcinomas, since YSVE is the first site of AFP synthesis in the embryo. The next site of AFP production is embryonal hepatoblast and just hepatoblastomas are the maximal producers of AFP among liver cancers. The reason for AFP resumption in hepatocellular carcinomas (HCC) is not yet clear. This problem is discussed in the light of possible role of oval cells in the HCC origin and the concept of the two states of the mature hepatocyte, associated and non-associated with AFP production. The crucial role of extracellular matrix in the control of AFP-producing state of hepatocyte is emphasized.

Pancreatic expression of keratinocyte growth factor leads to differentiation of islet hepatocytes and proliferation of duct cells

Keratinocyte growth factor, (KGF), a member of the fibroblast growth factor (FGF) family, is involved in wound healing. It also promotes the differentiation of many epithelial tissues and proliferation of epithelial cells as well as pancreatic duct cells. Additionally, many members of the highly homologous FGF family (including KGF), influence both growth and cellular morphology in the developing embryo. We have previously observed elevated levels of KGF in our interferon-gamma transgenic mouse model of pancreatic regeneration. To understand the role of KGF in pancreatic differentiation, we generated insulin promoter-regulated KGF transgenic mice. Remarkably, we have found that ectopic KGF expression resulted in the emergence of hepatocytes within the islets of Langerhans in the pancreas. Additionally, significant intra-islet duct cell proliferation in the pancreata of transgenic KGF mice was observed. The unexpected appearance of hepatocytes and proliferation of intra-islet duct cells in the pancreata of these mice evidently stemmed directly from local exposure to KGF.

Cholesterol 7alpha-hydroxylase (CYP7A): patterns of messenger RNA expression during rat liver development

Cholesterol 7-hydroxylase is a rate-limiting enzyme in bile acid synthesis, a major pathway for cholesterol catabolism. It plays a crucial role in postnatal development and survival. In an adult liver, its activity and messenger RNA (mRNA) are heterogeneously distributed with concentration in the pericentral area. We defined how the pattern of cholesterol 7-hydroxylase mRNA evolves during rat liver development, correlated this with its total liver mRNA levels, and determined when its heterogeneous pattern of expression is established. Cholesterol 7-hydroxylase mRNA was undetectable in 18-day-old fetal livers by Northern blot. It was increased markedly in newborns with a homogeneous liver lobular distribution as determined by in situ hybridization. At postnatal day four, mRNA levels were markedly decreased with concomitant appearance of a lobular gradient: mRNA was detected only in a few hepatocytes located around efferent venules. At 22 days, the time of highest mRNA expression, a marked extension of the gradient towards the periportal area was observed, indicating that the increase in total liver cholesterol 7-hydroxylase mRNA level was a result of recruitment of hepatocytes upstream from the central vein area. By 28 days, the adult pattern was observed. Thus, expression of cholesterol 7-hydroxylase mRNA is tightly regulated during rat liver development, both temporally and spatially supporting its critical role in normal postnatal development.

Immunohistochemical localization of arginase II and other enzymes of arginine metabolism in rat kidney and liver

Arginine is a precursor for the synthesis of urea, polyamines, creatine phosphate, nitric oxide and proteins. It is synthesized from ornithine by argininosuccinate synthetase and argininosuccinate lyase and is degraded by arginase, which consists of a liver-type (arginase I) and a non-hepatic type (arginase II). Recently, cDNAs for human and rat arginase II have been isolated. In this study, immunocytochemical analysis showed that human arginase II expressed in COS-7 cells was localized in the mitochondria. Arginase II mRNA was abundant in the rat small intestine and kidney. In the kidney, argininosuccinate synthetase and lyase were immunostained in the cortex, intensely in proximal tubules and much less intensely in distal tubules. In contrast, arginase II was stained intensely in the outer stripes of the outer medulla, presumably in the proximal straight tubules, and in a subpopulation of the proximal tubules in the cortex. Immunostaining of serial sections of the kidney showed that argininosuccinate synthetase and arginase II were colocalized in a subpopulation of proximal tubules in the cortex, whereas only the synthetase, but not arginase II, was present in another subpopulation of proximal tubules. In the liver, all the enzymes of the urea cycle, i.e. carbamylphosphate synthetase I, ornithine transcarbamylase, argininosuccinate synthetase and lyase and arginase I, showed similar zonation patterns with staining more intense in periportal hepatocytes than in pericentral hepatocytes, although zonation of ornithine transcarbamylase was much less prominent. The implications of these results are discussed.

In situ hybridization demonstration of albumin mRNA in B6C3F1 murine liver and hepatocellular neoplasms

In situ hybridization was used to detect albumin mRNA in normal liver and hepatocellular neoplasms in 20 male B6C3F1 mice between 17 and 24 months of age. Positive signals for albumin were observed consistently in the cytoplasm of hepatocytes in normal liver, particularly in periportal areas. No signals were observed in other cells, such as Kupffer's cells, mesenchymal cells, or bile duct epithelium. Of hepatocellular adenomas, 11/11 (100%) stained positively for albumin mRNA, whereas 14/15 (93%) of primary hepatocellular carcinomas showed positive expression. Albumin mRNA was also detected in extrahepatic metastases of hepatocellular carcinoma, including 9/15 (60%) of pulmonary neoplasms and 5/12 (42%) of metastases at other sites. The pulmonary metastases of hepatocellular carcinoma frequently exhibited a glandular, papillary, or sarcomatous histologic appearance. The presence of albumin in these tumors, lacking characteristic hepatocellular phenotype, is a potential determinant of hepatic lineage. We conclude that in situ hybridization for albumin mRNA in mice is a useful tool in the differential diagnosis of hepatocellular carcinoma, particularly in the case of pulmonary metastasis. This technique may also enable recognition of hepatocyte differentiation in glandular structures with phenotypic features of biliary cells, as seen in mixed hepatocellular-cholangial neoplasms.

Flow cytometric characterization of isolated porcine hepatocyte suspensions for liver support

The high yield hepatocyte isolation necessary for hybrid liver assist devices (LAD) unavoidably increases contamination by nonparenchymal cells and depresses hepatocyte viability and functions. We have developed a flow cytometric procedure that improves quality control of the isolations. Cells present in these preparations were labeled by immunofluorescent antibody staining against cytokeratin 8, 18 as well as vimentin to identify hepatocytes, fibroblasts, and endothelial cells. Antibody staining against albumin and carbamoylphosphate synthetase allowed assessment of levels of albumin and carbamoylphosphate synthetase based on the hepatocyte relative fluorescence intensity. Hepatocyte P450 enzyme activity was measured by its ability to convert 5,6-methoxycarbonylfluorescein, a nonfluorescent substrate, to an intracellular fluorescent product. Flow cytometric methods of cell type identification and cell function assessment are fast and accurate and can be applied to commercial cell production. They may also provide an avenue for the enrichment of otherwise heterogeneous hepatocyte suspensions with cells presenting the specific functions desired for an hybrid liver assist devices.

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.

Distribution of albumin, alpha 1-inhibitor 3 and their respective mRNAs in periportal and perivenous rat hepatocytes isolated by the digitonin-collagenase technique

The expression of albumin and alpha 1-inhibitor 3 genes was investigated in rat cell suspensions enriched in periportal (n = 10) and perivenous (n = 10) hepatocytes obtained by the digitonin-collagenase technique. The degree of enrichment of the cell suspensions was assessed: (1) by enzymic assays for the periportal marker alanine aminotransferase and for the perivenous marker glutamine synthetase; and (2) by their content of mRNAs for the periportal marker hepatic glutaminase and for glutamine synthetase. The existence of an antegrade intra-lobular gradient for albumin and alpha 1-inhibitor 3 mRNAs was demonstrated, with periportal:perivenous ratios of 2.33 and 3.80, respectively. However, no gradient was demonstrated for the respective protein contents with corresponding ratios of 0.98 and 1.21. A certain degree of overlap existed between periportal and perivenous suspensions for their content in albumin and alpha 1-inhibitor 3 mRNAs. A morphometrical analysis of the surface of digitonin-permeabilized hepatic tissue revealed that this overlap could be explained by a variable extent of permeabilization of the mediolobular zone from one rat to another and from one lobule to another in a given animal. These results suggest that while the digitonin-collagenase technique is well suited for studies in vitro of proteins expressed in sharp intra-lobular gradients or restricted to an intra-lobular compartment, it is not completely reliable for proteins distributed in continuous moderate intra-lobular gradients, such as albumin and alpha 1-inhibitor 3.

Members of the CAAT/enhancer-binding protein, hepatocyte nuclear factor-1 and nuclear factor-1 families can differentially modulate the activities of the rat alpha-fetoprotein promoter and enhancer

The promoter of the rat alpha-fetoprotein (AFP) gene, which makes the expression of the developmentally regulated AFP gene specific to the liver, is a putative target for transcription factors of the CAAT/enhancer-binding protein (C/EBP), hepatocyte nuclear factor-1 (HNF-1) and nuclear factor-1 (NF-1) families. We have evaluated the influence of these factors on the activity of the AFP promoter by transfection of HepG2 hepatoma cells with the appropriate expression vector plus a CAT plasmid under the control of the AFP promoter. A similar plasmid bearing the rat albumin promoter was used as a control. C/EBP alpha, C/EBP beta and D-binding protein (DBP) acted as trans-activators on the AFP promoter, whereas liver inhibitory protein (LIP), a truncated form of C/EBP beta, was a potent negative regulator of the promoter. C/EBP alpha also bound to and stimulated the activity of the AFP enhancer at -2.5 kb. Interestingly, HNF-1 beta was found to be more potent than HNF-1 alpha in activating the AFP promoter. This effect was specific, as it did not occur with the rat albumin promoter. HNF-1 beta, which is produced earlier than HNF-1 alpha during liver development, would thus have the greater influence on the AFP promoter in early development. Both HNF-1s allowed expression of the AFP promoter in cells of nonhepatic origin. Overexpression of NF-1 induced a specific decrease in the activity of the AFP promoter. This strongly suggests that competition between NF-1 and HNF-1 for binding to their overlapping binding sites on the AFP promoter is critical for modulating its activity. Thus changing combinations of these trans-acting factors may tightly modulate the AFP promoter activity in the course of liver development and carcinogenesis.

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.

Distribution and activity of glutamine synthase and carbamoylphosphate synthase upon enlargement of the liver lobule by repeated partial hepatectomies

Glutamine synthase and carbamoylphosphate synthase show a strikingly heterogeneous and fully complementary distribution in the rat liver. In the human liver, however, there is a midlobular zone where both enzymes are absent. The diameter of the human liver lobule is approximately twice the size of the rat lobule. To investigate whether lobule size is a major determinant for the expression patterns of glutamine synthase and carbamoylphosphate synthase, Wistar strain rats were partially hepatectomized 3 times, at weekly or monthly intervals. Due to hepatic regeneration the cross-sectional area of the liver lobules increased twofold. However, a midlobular zone which lacked expression of both glutamine synthase and carbamoylphosphate synthase did not develop in these livers, thus showing that lobular size is not a major determinant for the distribution patterns of glutamine and carbamoylphosphate synthase. The twofold increase in the cross-sectional area of the liver lobule was associated with a similar reduction in the relative number of glutamine synthase-positive cells and in the enzyme content of the liver, indicating that the regeneration process does not affect the pericentral pattern of glutamine synthase expression. After regeneration was complete, the glutamine synthase content in the liver was restored to its original value, demonstrating a twofold increase in the cellular concentration of glutamine synthase-positive hepatocytes. An increase in the diameter of the liver lobule was only seen after the first partial hepatectomy. Liver growth following subsequent partial hepatectomies can be explained by an increase in the length of the liver lobule and/or by splitting of liver lobules. The zonal distribution of DNA replication, which is characteristic of the first partial hepatectomy, is lost after repeated partial hepatectomies. Furthermore, evidence was obtained that the signal for inducing DNA synthesis may originate at the level of single liver units.

Human fetoacinar pancreatic protein: an oncofetal glycoform of the normally secreted pancreatic bile-salt-dependent lipase

A fetoacinar pancreatic protein (FAP) associated with the ontogenesis, differentiation and oncogenic transformation of the human exocrine pancreas has been purified from pancreatic juices of patients suffering from pancreatitis or duodenal cancers invading the pancreas [Escribano and Imperial (1989) J. Biol. Chem. 264, 21865-21871]. This protein has striking similarities, i.e. M(r), amino acid composition and N-terminal sequence, to the bile-salt-dependent lipase (BSDL) of normal human pancreatic secretion. The aim of this study was to gain further insight into the nature of the two proteins. Reactivity with the mouse monoclonal antibody J28 (mAb J28), which characterizes FAP, and enzyme activity could not be dissociated during biochemical purification of BSDL. Furthermore, a polyclonal antiserum raised against purified human BSDL reacted completely with FAP in Western-blot analysis giving additional support to the idea of similar molecular structures for BSDL and FAP. However, by the same technique, mAb J28 reacted with a relatively restricted population of BSDL molecules. The classical BSDL preparation could be separated into molecules bearing the J28 epitope and those devoid of it by immunoaffinity on immobilized mAb J28. The two subpopulations had identical N-terminal sequences and some differences in their amino acid compositions. However, they had different carbohydrate compositions. J28-epitope-bearing molecules were active on BSDL substrates, although their specific activity was decreased. These results are consistent with the existence of two closely related polypeptide chains with different glycan counterparts. Therefore, if the name FAP is reserved for molecules bearing the J28 epitope, which is linked to a carbohydrate-dependent structure. FAP could represent an oncofetal-related variant of BSDL. Our result is the first demonstration of the existence of an oncofetal-type subpopulation of an otherwise normally secreted human pancreatic enzyme.

The zonal expression of alpha-fetoprotein transgenes in the livers of adult mice

The developmental regulation of the alpha-fetoprotein (AFP) gene in liver results in high-level expression in the fetus, followed by dramatic transcriptional repression after birth. We have examined the mouse AFP gene for transcriptional control sequences that may be involved in its postnatal repression in liver. We showed previously that removal of a DNA region between positions -250 base pairs (bp) and -838 bp of the AFP gene resulted in the persistence of expression of an AFP minigene in the postpartum liver of transgenic mice (Vacher and Tilghman, Science 250:1732-1735, 1990). This study examines the distribution of these transgene transcripts in liver using in situ hybridization. We show that there is a zonal distribution of minigene transcripts in the adult livers of these animals. Hepatocytes surrounding the central veins express high levels of minigene transcripts, while hepatocytes in the intermediate and portal areas contain few, if any, transcripts. Quantitative RNAse protection analysis shows a decrease in transgene RNA levels after birth, consistent with repression in all but a small subset of hepatocytes. These results indicate that repression in the pericentral hepatocytes is dependent upon the presence of a cis-acting, negative-regulatory domain, which is located between the enhancers and the proximal promoter of the AFP gene. In contrast, this domain is not essential for complete repression of AFP transgenes in the intermediate zone and periportal hepatocytes.

The dynamics of the expression of C/EBP mRNA in the adult rat liver lobulus qualifies it as a pericentral mRNA

A hybridocytochemical approach has been applied to establish whether the gene for the C/EBP mRNA might be involved in the topographical regulation of gene expression in adult rat liver. To that end the spatial distribution of the mRNA of C/EBP has been compared to that of the mRNAs of glutamine synthetase (GS), phosphoenolpyruvate carboxykinase (PEPCK) and glucokinase (GK) in normal adult livers, in livers from dexamethasone-treated animals and in livers from starved animals refed with glucose for 4 h. In normal rat liver, in situ hybridization with a probe for C/EBP mRNA revealed a low density of apparently homogeneously distributed grains, indicating low levels of C/EBP mRNA. In contrast, the livers of the experimentally-treated animals revealed a zonal distribution of the mRNA of C/EBP with the highest density of grains around the central venules. The dynamics of the pattern of expression of C/EBP mRNA are virtually identical to that of the GK mRNA. These data qualify C/EBP mRNA as a pericentral mRNA and suggest a role for the C/EBP protein in the topographical regulation of the expression of the GK mRNA.

Pericentral expression pattern of glucokinase mRNA in the rat liver lobulus

The spatial distribution of glucokinase mRNA (GK mRNA) in rat liver was studied by in situ hybridization under normal and inducing conditions. GK mRNA was first detectable in the liver parenchyma of neonatal rats of 1.5 days. The density of grains decreases in a central-portal direction. This pattern remains essentially unchanged up to 15 days, after which the adult type of distribution gradually starts to develop, i.e. low density of grains indicating low levels of GK mRNA, in which no gradient of expression could be visualized. Within 2 h after an oral glucose load to starved animals, the GK mRNA expression pattern changed from hardly detectable to a clear gradient with the highest grain density around the terminal central venules. Within 6 h relatively high levels of grains, almost homogeneously distributed across the liver lobule, were observed. Glucocorticosteroid treatment also induced GK mRNA in the pericentral area. It is concluded that the observed induction pattern qualifies GK mRNA as a pericentral mRNA suggesting that the pericentral expression pattern of the protein is primarily regulated at the pretranslational level.

Conformational effects of volatile anesthetics on the membrane-bound acetylcholine receptor protein: facilitation of the agonist-induced affinity conversion

The rate of the carbamylcholine-induced affinity conversion of the membrane-bound acetylcholine receptor protein from Torpedo californica is enhanced by pretreatment of the membranes under an atmosphere of 3% halothane or 1% chloroform. The enhancement is much more pronounced in the presence of low rather than high concentrations of carbamylcholine since the volatile anesthetics alter the apparent dissociation constant for carbamylcholine from 17 to 3 microM without affecting the first-order rate constant for the ligand-induced conversion (0.07 s-1). These results indicate that the acetylcholine receptor is assuming a conformational form with intermediate affinity for carbamylcholine in addition to the previously described low- and high-affinity forms. The dissociation constants for carbamylcholine obtained from kinetic studies of the carbamylcholine-induced transition are 3-15-fold lower than those obtained as inhibition constants from the rate of 125I-labeled alpha-bungarotoxin binding to the low-affinity conformer of the acetylcholine receptor protein. This pattern, observed in both the presence and absence of anesthetic, provides further evidence that the acetylcholine receptor has nonequivalent ligand binding sites for carbamylcholine.