Ploidy class-dependent variations during 24 h of glucose-6-phosphate and succinate dehydrogenase activity and single-stranded RNA content in isolated rat hepatocytes

Transcripts of ceruloplasmin but not hepcidin, both major iron metabolism genes, exhibit a decreasing pattern along the portocentral axis of mouse liver

BACKGROUND/AIMS

During iron overload of dietary origin, iron accumulates predominantly in periportal hepatocytes. A gradient in the basal and normal transcriptional control of genes involved in iron metabolism along the portocentral axis of liver lobules could explain this feature. Therefore, we aimed at characterizing, by quantitative RT-PCR, the expression of iron metabolism genes in adult C57BL/6 mouse hepatocytes regarding lobular localisation, with special emphasis to cell ploidy, considering its possible relationship with lobular zonation.

METHODS

We used two methods to analyse separately periportal and perivenous liver cells: 1) a selective liver zonal destruction by digitonin prior to a classical collagenase dissociation, and 2) laser capture microdissection. We also developed a method to separate viable 4N and 8N polyploid hepatocytes by flow cytometer.

RESULTS

Transcripts of ceruloplasmin, involved in iron efflux, were overexpressed in periportal areas and the result was confirmed by in situ hybridization study. By contrast, hepcidin 1, hemojuvelin, ferroportin, transferrin receptor 2, hfe and L-ferritin mRNAs were not differentially expressed according to either lobular zonation or polyploidisation level.

CONCLUSIONS

At variance with glutamine or urea metabolism, iron metabolism is not featured by a metabolic zonation lying only on a basal transcriptional control. The preferential periportal expression of ceruloplasmin raises the issue of its special role in iron overload disorders involving a defect in cellular iron export.

Regulation of the spatiotemporal pattern of expression of the glutamine synthetase gene

Glutamine synthetase, the enzyme that catalyzes the ATP-dependent conversion of glutamate and ammonia into glutamine, is expressed in a tissue-specific and developmentally controlled manner. The first part of this review focuses on its spatiotemporal pattern of expression, the factors that regulate its levels under (patho)physiological conditions, and its role in glutamine, glutamate, and ammonia metabolism in mammals. Glutamine synthetase protein stability is more than 10-fold reduced by its product glutamine and by covalent modifications. During late fetal development, translational efficiency increases more than 10-fold. Glutamine synthetase mRNA stability is negatively affected by cAMP, whereas glucocorticoids, growth hormone, insulin (all positive), and cAMP (negative) regulate its rate of transcription. The signal transduction pathways by which these factors may regulate the expression of glutamine synthetase are briefly discussed. The second part of the review focuses on the evolution, structure, and transcriptional regulation of the glutamine synthetase gene in rat and chicken. Two enhancers (at -6.5 and -2.5 kb) were identified in the upstream region and two enhancers (between +156 and +857 bp) in the first intron of the rat glutamine synthetase gene. In addition, sequence analysis suggests a regulatory role for regions in the 3' untranslated region of the gene. The immediate-upstream region of the chicken glutamine synthetase gene is responsible for its cell-specific expression, whereas the glucocorticoid-induced developmental appearance in the neural retina is governed by its far-upstream region.

Effects of partial hepatectomy, phenobarbital and 3-methylcholanthrene on kinetic parameters of glucose-6-phosphate and phosphogluconate dehydrogenase in situ in periportal, intermediate and pericentral zones of rat liver lobules

Glucose-6-phosphate dehydrogenase (G6PDH) and phosphogluconate dehydrogenase (PGDH) are heterogeneously distributed in liver lobules of female rats. The maximum activity of both enzymes is approximately twice higher in intermediate and pericentral zones than in periportal zones. Enzyme activities and their distribution patterns were manipulated by partial hepatectomy and treatment with phenobarbital (PB) or 3-methylcholanthrene (3-MC). Vmax values of G6PDH for glucose-6-phosphate decreased mainly in intermediate and pericentral zones after partial hepatectomy, whereas they increased after PB treatment. Vmax values of PGDH for phosphogluconate decreased after partial hepatectomy in both zones, whereas other treatments did not have any effect. The affinity of G6PDH for glucose-6-phosphate was similar in all zones and it was decreased 2-3 fold by PB and 3-MC treatment. The affinity of PGDH for phosphogluconate was 1.4-2.3 times lower in intermediate and pericentral zones than in periportal zones of all livers tested and was not affected by treatment. From these data it can be concluded that not only the maximum activity of enzymes may differ in periportal, intermediate and pericentral zones of the liver lobule but also the affinity of enzymes for their substrates. The implication of these findings is that metabolic flux rates as they occur in vivo in these different metabolic compartments may be significantly different from predictions on the basis of maximum enzyme activities as detected immunohistochemically, microchemically or cytophotometrically.

The stem cells of the liver ? a selective review

The current status of the much-debated question of the still-hypothetical stem cells of the liver is reviewed, with an emphasis on their role in hepatocarcinogenesis. The widely held view of the primacy of the hepatocyte, notably of the mononuclear diploid type, in this process--the "hepatocytic theory"--has been compared with variants of the "stem cell hypothesis" based on the "non-parenchymal epithelial cells" of the liver--the "oval" or biliary ductular cells, the "nondescript periductular" cells and the "primitive" bipotential epithelial cells. An attempt has been made to concentrate mainly on the more recent publications, in an effort to balance the conflicting opinions expressed by comparing results obtained by the newer procedures currently in use. Despite some interesting and relevant findings it appears that the evidence in favour of the stem-cell hypothesis is still circumstantial and that the hepatocytic theory has not been invalidated. Presumably the question of the hepatic stem cells will be answered when the riddle of hepatocarcinogenesis has been solved.

Cytophotometric analysis of alkaline phosphatase and 5'-nucleotidase activity in regenerating rat liver after partial hepatectomy

Alkaline phosphatase and 5'-nucleotidase activities were analysed cytophotometrically in cryostat sections of female rat liver after partial hepatectomy. Alkaline phosphatase activity increased rapidly after operation up to a maximum seven-fold rise at 24 h in comparison with sham operated or control rats. There was no indication of preferential localization of alkaline phosphatase activity in either periportal or pericentral areas at any time point in control rats, sham operated rats or hepatectomized rats. Microscopical observation revealed that (a) all alkaline phosphatase activity was present at the bile canalicular surface of hepatocytes and (b) hepatocytes in mitosis did not show any increase in activity. These findings indicate that the high alkaline phosphatase activity after partial hepatectomy is not involved primarily in proliferation processes because cell division mainly takes place periportally. It may be needed for enhanced bile secretion by conversion of intracellular phosphorylcholine into choline which can be transported into the bile. The intracellular phosphorylcholine level is high after operation due to changes in phospholipid metabolism. 5'-Nucleotidase appeared to be three times higher pericentrally than periportally under normal conditions. Partial hepatectomy caused a 40 per cent decrease in activity in pericentral areas and only a small decrease periportally. It has been suggested that 5'-nucleotidase plays a role in breakdown of messenger RNA and its activity in control liver could be considerably lower periportally because plasma protein synthesis mainly takes place in this area.(ABSTRACT TRUNCATED AT 250 WORDS)

Age-related changes of glucose-6-phosphate dehydrogenase activity in mouse oocytes

Glucose-6-phosphate dehydrogenase (G6PDH) activity was measured in follicular oocytes and in ovulated eggs of prepubertal, adult and aged mice. G6PDH activity in ovulated eggs was 60% of the activity in follicular oocytes in all age groups. The mean G6PDH activity was significantly higher in follicular oocytes of adult mice than in oocytes of both prepubertal and aged mice. In aged mice, the decreased mean activity in follicular oocytes as well as in ovulated eggs was mainly due to a high percentage of cells with extremely low activity (25 and 18%, respectively). The percentage of preovulatory oocytes with low activity in prepubertal mice was 9% and in adult mice 0.3%. For ovulated eggs these percentages were 0% for both prepubertal and adult mice. In every age group, all ovulated eggs showed a normal morphology. When ovulated eggs with extremely low G6PDH activity can still be fertilized, it can be questioned whether this loss of activity could cause disturbances in development of (preimplantation) embryos. Our findings emphasize the potentialities of investigating intact singly oocytes for changes in enzyme activities, which could be applied as parameters for 'quality control' of these cells.

Diurnal variation in 5'-nucleotidase activity in rat liver. A quantitative histochemical study

The diurnal variation of 5'-nucleotidase activity in periportal and pericentral areas of rat liver parenchyma has been determined with quantitative histochemical means. 5'-Nucleotidase activity was estimated using microdensitometry in cryostat sections after being incubated with a medium according to Wachstein and Meisel (1957). It appeared that 5'-nucleotidase activity was significantly higher in pericentral areas than in periportal areas throughout the daily cycle and showed a maximum at the end of the light period. It was concluded that 5'-nucleotidase activity may be related with the capacity to diminish messenger RNA resulting in protein breakdown.

Detection of metabolic changes in hepatocytes by quantitative cytochemistry

Studies by means of quantitative histochemistry and cytochemistry have greatly contributed to the knowledge of metabolic changes in liver parenchymal cells. In the present paper recent work along this line is reviewed with emphasis on three topics, polyploidy as a source of metabolic heterogeneity, proteolysis in the regulation of hepatocyte cell mass and ischemic injury of hepatocytes. In all three fields, accuracy and precision of information obtained by quantitative histochemical means has been greatly enhanced by a thorough knowledge of the mechanisms of histochemical reactions obtained by fundamental work on matrix chemistry, and well-considered application of optical measuring tools and conditions of measurement. These are the principles put forward by van Duijn since the pioneer period of histochemistry and to whom this review is dedicated.

Ploidy class-dependent metabolic changes in rat hepatocytes after partial hepatectomy

Glucose-6-phosphate dehydrogenase (G6PDH), succinate dehydrogenase (SDH) activity and the single-stranded RNA (ssRNA) content of isolated hepatocytes of different ploidy classes from adult male rats have been studied after partial hepatectomy using quantitative cytochemical means. The SDH activity and ssRNA content in all classes of hepatocytes are decreased during the first hours after operation followed by an increase above control values. The increase of both SDH activity and ssRNA content is significant only in the mononuclear diploid (MD) cells but not in the hepatocytes of higher ploidy classes and is related with the mitotic wave at 32 h after hepatectomy. After the mitotic wave, the values quickly return to normal levels. The G6PDH activity does not show any significant change in hepatocytes other than MD cells. In MD cells the G6PDH activity is elevated on a highly significant level up to a maximum value of 3.5 times the control value at 48 h after operation. The G6PDH activity in MD cells is returned to normal values within 14 days after operation. It is concluded that: 1. The MD cells show a distinct metabolic behaviour due to their function as stem cells of liver parenchyma and retain at least some of their fetal characteristics. 2. G6PDH activity is not a transformation-linked discriminant for neoplastic metabolism.