Transcriptional and posttranscriptional regulation of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase during liver regeneration

Molecular characterization of fructose-1,6-bisphosphatase 1b in blunt snout bream Megalobrama amblycephala and the transcriptional response to glucose loading after the adaptation to high-carbohydrate diets

This study aimed to characterize the full-length complementary DNA (cDNA) of fructose-1,6-bisphosphatase 1b (FBP1b) from fish Megalobrama amblycephala, and investigate its transcriptional response to glucose administration after the adaptation to high-carbohydrate diets. The cDNA obtained covered 1435 bp with an open reading frame of 1014 bp. Sequence alignment and phylogenetic analysis revealed a high degree of conservation (76-96%) among most fish and other vertebrates, retaining one N-linked glycosylation site, one N-terminal acetylation site, 13 phosphorylation sites, one fructose-1,6-bisphosphatase (FBPase) active site, five metal-binding sites, four substrate-binding sites, and several AMP-binding sites. The highest messenger RNA (mRNA) level of FBP1b was observed in liver followed by intestine, whereas relatively low values were detected in heart, gill, and eye. Then, the mRNA levels of FBP1b and the FBPase activity were both determined in the liver of fish injected intraperitoneally with 1.67 g glucose per kilogram body weight after being fed two dietary carbohydrate levels (30 and 42%) for 11 weeks. After the glucose load, the mRNA levels of FBP1b in both treatments decreased significantly to the basal value at 8 h and showed a slight increase afterward. However, the enzymatic activity showed no statistical difference during the first 4 h, but increased remarkably with further increasing times. In addition, both the mRNA levels and activities decreased significantly with increasing dietary carbohydrate levels. The results indicated that the FBP1b of M. amblycephala shared a high similarity with that of the other vertebrates. Its mRNA expression in liver was downregulated remarkably by a glucose administration, as also held true after the long-term adaptation of a carbohydrate-rich diet.

Pfkfb3 is transcriptionally upregulated in diabetic mouse liver through proliferative signals

The ubiquitous isoform of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (uPFK-2), a product of the Pfkfb3 gene, plays a crucial role in the control of glycolytic flux. In this study, we demonstrate that Pfkfb3 gene expression is increased in streptozotocin-induced diabetic mouse liver. The Pfkfb3/-3566 promoter construct linked to the luciferase reporter gene was delivered to the liver via hydrodynamic gene transfer. This promoter was upregulated in streptozotocin-induced diabetic mouse liver compared with transfected healthy cohorts. In addition, increases were observed in Pfkfb3 mRNA and uPFK-2 protein levels, and intrahepatic fructose-2,6-bisphosphate concentration. During streptozotocin-induced diabetes, phosphorylation of both p38 mitogen-activated protein kinase and Akt was detected, together with the overexpression of the proliferative markers cyclin D and E2F. These findings indicate that uPFK-2 induction is coupled to enhanced hepatocyte proliferation in streptozotocin-induced diabetic mouse liver. Expression decreased when hepatocytes were treated with either rapamycin or LY 294002. This shows that uPFK-2 regulation is phosphoinositide 3-kinase-Akt-mammalian target of rapamycin dependent. These results indicate that fructose-2,6-bisphosphate is essential to the maintenance of the glycolytic flux necessary for providing energy and biosynthetic precursors to dividing cells.

Zonal differences in effects of HGF/SF and EGF on DNA synthesis in hepatocytes under fed or starved conditions

Zonal differences of DNA synthesis in hepatocytes induced by hepatocyte growth factor and/or scatter factor (HGF/SF) and epidermal growth factor (EGF) were investigated using male Wistar rats under fed or starved conditions. Overall, DNA synthesis was greater in fed rats than in starved rats. The predominance of EGF in periportal hepatocytes (PPH) on zonal DNA synthesis was reversed by starved conditions, but the predominance of HGF/SF on zonal DNA synthesis in perivenous hepatocytes (PVH) was not influenced by nutritional conditions. 125I-labeled EGF and 125I-labeled HGF/SF-receptor binding studies revealed no significant difference between PPH and PVH in starved or fed rats. To investigate the mechanism of the signal transduction pathway, we used genistein, an inhibitor of tyrosine kinase. Genistein had different effects on zonal difference in EGF and HGF/SF. In EGF, 1 microgram/ml genistein abolished zonal differences, but in HGF/SF 1 microgram/ml genistein did not abolish zonal differences. These data suggest that, in contrast to HGF/SF, zonal difference of DNA synthesis by EGF was dependent on nutritional conditions and DNA synthesis induced by HGF/SF and EGF might be related to tyrosine kinase, but the influence of tyrosine kinase on DNA synthesis was different between HGF/SF and EGF.

Expression of the F-type 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase mRNA during liver regeneration

The expression of the F-type 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase mRNA was studied during liver regeneration by three independent assays: Northern blot analysis, reverse transcription-polymerase chain reaction and ribonuclease protection. We demonstrate the presence of F-type mRNA in foetal and adult rat livers and a transient increase in its levels with a maximum at 12 h after partial liver resection. The time course of F-type mRNA induction differs from that reported for the L-type isoform, suggesting differences in the regulation of the expression of F- and L-type isoforms of the bifunctional enzyme during liver regeneration.

Gene expression of glucokinase regulatory protein in regenerating rat liver

The activity and messenger RNA (mRNA) levels of glucokinase, and the concentration and mRNA levels of its regulatory protein, were analyzed during liver regeneration. The activity of glucokinase and the concentration of its regulatory protein decreased to 30% and 50%, respectively, after liver resection, remaining low after 1 week. No significant variations in the level of these proteins were found in sham-operated animals. The regulatory protein/glucokinase molar ratio increased during the replicative phase, to a maximum at 48 hours. The mRNA levels of glucokinase and of its regulatory protein decreased rapidly after partial hepatectomy to minimum values at 6 hours (15%) and at 12 hours (4%), respectively, returning to normal values at 24 hours and 168 hours, respectively. Sham-operated animals showed a similar decrease in mRNA levels during the prereplicative phase of liver regeneration, suggesting that the initial effects observed in the gene expression of these proteins were due to surgical stress. During the replicative phase, a specific inhibition of the regulatory protein's gene expression was observed in the regenerating liver. A decrease in the content of regulatory protein and the glucokinase activity, and an increase in the molar ratio of these two proteins correlate with the observed decrease in glycolytic flux, providing further evidence that the phosphorylation of glucose is a control point in the glycolytic/gluconeogenic flux during liver regeneration.

Molecular sites of regulation of expression of the rat cationic amino acid transporter gene

Cat-1 is a protein with a dual function, a high affinity, low capacity cationic amino acid transporter of the y+ system and the receptor for the ecotropic retrovirus. We have suggested that Cat-1 is required in the regenerating liver for the transport of cationic amino acids and polyamines in the late G1 phase, a process that is essential for liver cells to enter mitosis. In our earlier studies we had shown that the cat-1 gene is silent in the quiescent liver but is induced in response to hormones, insulin, and glucocorticoids, and partial hepatectomy. Here we demonstrate that cat-1 is a classic delayed early growth response gene in the regenerating liver, since induction of its expression is sensitive to cycloheximide, indicating that protein synthesis is required. The peak of accumulation of the cat-1 mRNA (9-fold) by 3 h was not associated with increased transcriptional activity of the cat-1 gene in the regenerating liver, indicating post-transcriptional regulation of expression of this gene. Induction of the cat-1 gene results in the accumulation of two mRNA species (7.9 and 3.4 kilobase pairs (kb)). Both mRNAs hybridize with the previously described rat cat-1/2.9-kb cDNA clone. However, the 3' end of a longer rat cat-1 cDNA (rat cat-1/6.5-kb) hybridizes only to the 7.9-kb mRNA transcript. Sequence analysis of this clone indicated that the two mRNA species result from the use of alternative polyadenylation signals. The 6. 5-kb clone contains a number of AT-rich mRNA destabilizing sequences which is reflected in the half-life of the cat-1 mRNAs (90 min for 7. 9-kb mRNA and 250 min for 3.4-kb mRNA). Treatment of rats with cycloheximide superinduces the level of the 7.9-kb cat-1 mRNA in the kidney, spleen, and brain, but not in the liver, suggesting that cell type-specific labile factors are involved in its regulation. We conclude that the need for protein synthesis for induction of the cat-1 mRNA, the short lived nature of the mRNAs, and the multiple sites for regulation of gene expression indicate a tight control of expression of the cat-1 gene within the regenerating liver and suggest that y+ cationic amino acid transport in liver cells is regulated at the molecular level.

Hepatocyte growth factor and transforming growth factor beta regulate 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase gene expression in rat hepatocyte primary cultures

Hepatocyte growth factor (HGF) and transforming growth factor beta (TGF-beta) are believed to be of major importance for hepatic regeneration after liver damage. We have studied the effect of these growth factors on fructose 2,6-bisphosphate (Fru-2,6-P2) levels and the expression of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (6PF2K/Fru-2,6-BPase) in rat hepatocyte primary cultures. Our results demonstrate that HGF activates the expression of the 6PF2K/Fru-2,6-BPase gene by increasing the levels of its mRNA. As a consequence of this activation, the amount of 6PF2K/Fru-2,6-BPase protein and 6-phosphofructo-2-kinase activity increased, which was reflected by a rise in Fru-2,6-P2 levels. In contrast, TGF-beta decreased the levels of 6PF2K/Fru-2,6-BPase mRNA, which led to a decrease in the amount of 6PF2K/Fru-2,6-BPase protein and Fru-2,6-P2. The different actions of HGF and TGF-beta on 6PF2K/Fru-2,6-BPase gene expression are concomitant with their effect on cell proliferation. Here we show that, in the absence of hormones, primary cultures of hepatocytes express the F-type isoenzyme. In addition, HGF increases the expression of this isoenzyme, and dexamethasone activates the L-type isoform. HGF and TGF-beta were able to inhibit this activation.

Transcriptional repression of specific host genes by the mycovirus Cryphonectria hypovirus 1

The hypovirus CHV1, which infects the plant-pathogenic fungus Cryphonectria parasitica, causes a distinct range of symptoms in its host that include reduced virulence expression, reduced sporulation, and reduced pigmentation. The virus, however, has little or no effect on fungal growth in culture. The visual symptoms are associated with reduced accumulation of a small number of host mRNAs and proteins. Four of the host genes encoding these down-regulated mRNAs have been characterized; they include two genes encoding a fungal sex pheromone (Vir1 and Vir2), a gene encoding an extracellular laccase (Lac1), and a gene encoding a cell wall hydrophobin (Crp). Expression of most other host proteins appears to be unaffected by the virus. These four genes can serve as reporter genes in studies of the effect of the virus on host gene expression. It is hypothesized that the four genes are coordinately down-regulated by the virus and probably are associated in a regulatory cascade. This hypothesis was tested by measuring the relative transcription rate of each gene in virus-infected and uninfected isogenic strains of the fungus by using nuclear run-on assays. The effects of the virus on transcription of these genes generally mirrored the observed effects of the virus on relative accumulation of the mRNAs of each gene. Although repressed transcription cannot account for all of the effects of the virus on mRNA accumulation of these four reporter genes, it is the predominant effect.

Gene expression of urea cycle enzymes following two-thirds partial hepatectomy in the rat

The effect of reduction of functional liver mass on the expression of enzyme systems for hepatic urea synthesis was assessed in rats following two-thirds partial hepatectomy. Results were related to normal, fed rats and to sham-operated rats, with identical timing for surgery and feeding. Among the five urea cycle enzymes the mRNA steady-state level was higher in hepatectomized than in sham-operated rats for carbamoyl phosphate synthetase and arginino-succinate lyase. The level for albumin mRNA remained close to that of the controls. Relative transcription rates were found to be increased for carbamoyl phosphate synthetase, arginino-succinate synthase and arginase. For albumin the transcription rate was drastically reduced initially, but recovered gradually during the experimental period. The data indicate that the expression of urea cycle enzymes, in particular that of carbamoyl phosphate synthetase which is the rate-limiting step, is up-regulated by partial hepatectomy. This helps to maintain urea synthesis rate at a normal or near normal level during the period of reduced liver mass, confirming metabolic studies. In contrast, the transcription for albumin was reduced. The immediate increase in urea cycle enzyme expression during the period of acute hepatocyte loss is consistent with the view that it is vitally important that urea synthesis, in contrast to e.g. albumin synthesis, remains intact when the metabolic capacity of the liver is reduced.

High levels of glucose-6-phosphatase gene and protein expression reflect an adaptive response in proliferating liver and diabetes

The regenerating liver after partial hepatectomy is one of the few physiologic models of cellular proliferation in the adult animal. During hepatic regeneration, the animal is able to maintain metabolic homeostasis despite the acute loss of two thirds of hepatic tissue. In examining the molecular mechanisms regulating hepatic regeneration, we isolated novel immediate-early genes that are rapidly induced as the remnant liver undergoes the transition from its normal quiescent state into the G1 phase of the cell cycle. One of the most rapidly and highly induced genes which we initially termed RL-1, encodes rat glucose-6-phosphatase (rG6Pase). G6Pase mRNA peaks at 30 min and 36-48 h after hepatectomy correlating with the first and second rounds of cell division. This finding is compatible with studies that showed that G6Pase enzyme activity increases during liver regeneration. However, the increase in G6Pase mRNA is much more dramatic, indicating that it is a more sensitive indicator of this regulation. G6Pase gene expression peaks in the perinatal time period in the liver and remains elevated during the first month of life. The expression of the G6Pase gene is also dramatically elevated in BB diabetic rats, again higher than the enzyme elevation, and its relative induction after partial hepatectomy is blunted in these animals. Insulin treatment of partially hepatectomized diabetic animals downregulates the expression of G6Pase mRNA. Using specific antibodies against G6Pase, we detect a 36-kD G6Pase protein, and its level is elevated in regenerating and diabetic livers. The pattern of G6Pase mRNA expression appears to reflect similar changes in insulin and glucagon levels which accompany diabetes and hepatic proliferation. The elevation of G6Pase expression in these conditions is indicative of its importance as a regulator of glucose homeostasis in normal and abnormal physiologic states.

Calmodulin expression during rat liver regeneration

We have investigated the messenger RNAs expressed from the three calmodulin genes during rat liver regeneration. The results revealed that all the calmodulin transcripts increased from 8 hr after a partial hepatectomy, although differences in the timing and the level of expression from the three genes were observed. Calmodulin I transcripts peaked at 16 hr, whereas calmodulin II and calmodulin III progressively increased from 8 to 24 hr. At 24 hr after surgery, calmodulin I, calmodulin II and the 2.3 kb calmodulin III transcripts reached values of a 6-fold increase, whereas the 0.8 kb product of calmodulin III increased 25-fold. At 30 hr the levels of all the calmodulin transcripts were similar to those observed at 24 hr. The transcription rates of the three calmodulin genes augmented after hepatectomy (calmodulin I and calmodulin II twofold and calmodulin III fourfold), indicating that the elevation of the calmodulin transcripts could be, at least partially, the result of this increase in the transcription rates. The total calmodulin concentration also increased twofold at 24 hr after hepatectomy. We also report that the administration of the beta-adrenergic blocker, D,L-propranolol inhibited the accumulation of calmodulin protein without significantly affecting the increase of the messenger RNAs. These results indicate that the expression of calmodulin observed during liver regeneration could be regulated by cyclic AMP at the translational or posttranslational level.

Gene expression of enzymes regulating ketogenesis and fatty acid metabolism in regenerating rat liver

Levels of mRNA for mitochondrial 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase, carnitine palmitoyltransferase I (CPT I) and carnitine palmitoyltransferase II (CPT II), fatty acid synthase (FAS) and actin were analysed during liver regeneration. mRNA levels for mitochondrial HMG-CoA synthase decreased rapidly, reaching a minimum 12 h after partial hepatectomy and returning to normal at 24-36 h. In contrast, CPT I, CPT II and FAS mRNAs increased throughout the period examined. Expression of actin increased significantly during regeneration. Levels of mRNA for mitochondrial HMG-CoA synthase also decreased as a result of surgical stress, although the effect of hepatectomy was much greater. We determined the levels of mitochondrial HMG-CoA synthase using specific antibodies. The amount of protein rapidly decreased, although less markedly than the corresponding mRNA levels. These results show that the decrease described in ketogenesis in partially hepatectomized rats correlated with the decrease in the expression of mitochondrial HMG-CoA synthase, suggesting that this enzyme may also be a control point in ketogenesis in the regenerating liver, as it is in normal and diabetic rats.

Gene expression of regulatory enzymes of glycolysis/gluconeogenesis in regenerating rat liver

Levels of mRNA for glucokinase, L-pyruvate kinase, fructose-1,6-bisphosphatase and phosphoenolpyruvate carboxykinase were analysed during liver regeneration. Levels of mRNA for glycolytic enzymes (glucokinase and L-pyruvate kinase) decreased rapidly after partial hepatectomy. Glucokinase mRNA increased at 16-24 h, returning to normal values after this time. L-pyruvate kinase mRNA recovered control levels at 168 h. In contrast, phosphoenolpyruvate carboxykinase mRNA increased rapidly after liver resection and remained high during the regenerative process. However, the levels of fructose-1,6-bisphosphatase mRNA were not modified significantly. These results correlate with the reported increased rate of gluconeogenesis and changes in enzyme levels after partial hepatectomy. The effect of stress on the mRNA levels was also studied. All enzymes showed variations in their mRNA levels after the surgical stress. In general, the differences were more pronounced in regenerating liver than in sham-operated animals, being practically normalized at 24 h.