Isozyme differentiation of aldolase and pyruvate kinase in fetal, regenerating, preneoplastic, and malignant rat hepatocytes during culture

Targeting of several glycolytic enzymes using RNA interference reveals aldolase affects cancer cell proliferation through a non-glycolytic mechanism

In cancer, glucose uptake and glycolysis are increased regardless of the oxygen concentration in the cell, a phenomenon known as the Warburg effect. Several (but not all) glycolytic enzymes have been investigated as potential therapeutic targets for cancer treatment using RNAi. Here, four previously untargeted glycolytic enzymes, aldolase A, glyceraldehyde 3-phosphate dehydrogenase, triose phosphate isomerase, and enolase 1, are targeted using RNAi in Ras-transformed NIH-3T3 cells. Of these enzymes, knockdown of aldolase causes the greatest effect, inhibiting cell proliferation by 90%. This defect is rescued by expression of exogenous aldolase. However, aldolase knockdown does not affect glycolytic flux or intracellular ATP concentration, indicating a non-metabolic cause for the cell proliferation defect. Furthermore, this defect could be rescued with an enzymatically dead aldolase variant that retains the known F-actin binding ability of aldolase. One possible model for how aldolase knockdown may inhibit transformed cell proliferation is through its disruption of actin-cytoskeleton dynamics in cell division. Consistent with this hypothesis, aldolase knockdown cells show increased multinucleation. These results are compared with other studies targeting glycolytic enzymes with RNAi in the context of cancer cell proliferation and suggest that aldolase may be a useful target in the treatment of cancer.

The complexity of ERK1 and ERK2 MAPKs in multiple hepatocyte fate responses

Recent reports suggest that extracellular signal-regulated kinase (ERK1) and ERK2 mitogen-activated protein kinases (MAPK) may direct specific biological functions under certain contexts. In this study, we investigated the role of early and sustained epidermal growth factor (EGF) stimulation on long-term hepatocyte differentiation and the possible role of ERK1 and ERK2 in this process. We demonstrate a long-term survival and an elevated level of differentiation up to 3 weeks. The differentiation state of hepatocytes is supported by sustained expression of aldolase B, albumin, and the detoxifying enzymes CYP1A2, 2B2, and 3A23. Similarly to freshly isolated cells, cultured hepatocytes also retain the ability to respond to 3-methylcholanthrene (3MC) and phenobarbital (PB), two known CYP inducers. In addition, we show evidence that continuous MAPK/ERK kinase (MEK) inhibition enhances the level of differentiation. Using RNA interference approaches against ERK1 and ERK2, we demonstrate that this effect requires both ERK1 and ERK2 activity, whereas the specific ERK1 knockdown promotes cell survival and the specific ERK2 knockdown regulates cell proliferation. In conclusion, we demonstrate that early and sustained EGF stimulation greatly extends long-term hepatocyte survival and differentiation, and that inhibition of the ERK1/2 MAPK pathway potentiates these pro-survival/pro-differentiation phenotypes. We clearly attest that specific ERK1 and ERK2 MAPKs determine hepatocyte survival and proliferation, respectively, whereas dual inhibition is required to stabilize a highly differentiated state.

Proteomic analysis of apoptosis initiation induced by all-trans retinoic acid in human acute promyelocytic leukemia cells

The irreversible destiny of apoptosis in its early stage might play a critical role in the apoptosis of human acute promyelocytic leukemia (APL) cell line induced by all-trans retinoic acid (ATRA). To characterize protein alterations during the apoptosis-initiation phase and to understand the metabolic status at that time, we investigated the protein profiles in the apoptosis-initiation phase of APL cell line HL-60 by proteomic analysis. ATRA-withdrawal was conducted to demonstrate that there was committed initiation phase of apoptosis triggered by 10(-6) M ATRA at day 3. Only after that time point, ATRA-treated cells irreversibly went to apoptosis. Also at that time point, the positive regulators of apoptosis such as STAT3 increased at protein level, whereas negative regulators (Bcl-2 and p-STAT3) decreased. In addition, caspase-3 also increased after that time. Furthermore, comparative proteomic analysis was utilized to examine the protein expression profiles during the initiation stage of apoptosis. Our results showed 12 upregulated and 7 downregulated proteins experiencing twofold alteration, including key regulators of signal transduction such as G-proteins and nucleic receptors, proteins related with metabolism, oxidation and reduction, proteins associated with the nucleus and cytoskeleton-related proteins. Some of them could be positive modulators to trigger apoptosis, whereas others could contribute to intracellular defense against apoptosis induced by exogenous triggers. The results above suggest that there is a subtle balance between apoptosis and the intracellular defense against apoptosis. Once the balance is disturbed, cells would irreversibly initiate to undergo the execution of apoptosis.

Oxidative stress and gene regulation

Reactive oxygen species are produced by all aerobic cells and are widely believed to play a pivotal role in aging as well as a number of degenerative diseases. The consequences of the generation of oxidants in cells does not appear to be limited to promotion of deleterious effects. Alterations in oxidative metabolism have long been known to occur during differentiation and development. Experimental perturbations in cellular redox state have been shown to exert a strong impact on these processes. The discovery of specific genes and pathways affected by oxidants led to the hypothesis that reactive oxygen species serve as subcellular messengers in gene regulatory and signal transduction pathways. Additionally, antioxidants can activate numerous genes and pathways. The burgeoning growth in the number of pathways shown to be dependent on oxidation or antioxidation has accelerated during the last decade. In the discussion presented here, we provide a tabular summary of many of the redox effects on gene expression and signaling pathways that are currently known to exist.

Progression through G1 and S phases of adult rat hepatocytes

Regenerating liver, hepatocyte primary cultures and differentiated hepatoma cell lines are widely used to study the proliferation/differentiation/apoptosis equilibrium in liver. In hepatocytes, priming factors (TNF alpha, IL6) target G0/G1 transition while growth factors (HGF, EGF, TGF alpha) control a mid-late G1 restriction point. A characteristic pattern of cdk/cyclin expression is observed in hepatocytes, presumably related to their ability to proliferate a limited number of times and to undergo a reversible differentiation. Interestingly, cell-cell interactions between hepatocytes and liver biliary cells in co-cultures, result in a cell cycle arrest in mid G1 of hepatocytes which are insensitive to mitogens. Apoptosis exists in hepatocytes but is still poorly documented. However, hepatoma cell lines stimulated by TGF beta undergo cell death in a p53-independent pathway. In conclusion, the interplay of growth and apoptosis regulators and cell-cell interactions control the proliferation/differentiation/apoptosis balance which is a specific feature of hepatocytes.

Biosynthesis, surface expression and function of the fibronectin receptor after rat liver cell transformation to tumorigenicity

Zajdela hepatoma cells are poorly-adherent cells derived from an undifferentiated tumour and transplanted into rat. We compared the biosynthesis, structure and function of the fibronectin receptor in normal rat hepatocytes with that in Zajdela hepatoma cells. The rat hepatocyte fibronectin receptor has been isolated. It is composed of two subunits: alpha 5 (molecular mass 155 kDa) and beta 1 (molecular mass 115 kDa). However, its biosynthesis has not yet been described. Using polyclonal antibodies raised against each of the subunits of the receptor, we observed that the alpha 5-subunit was synthesized as a 155-kDa polypeptide in normal rat hepatocytes and Zajdela hepatoma cells. In contrast, the molecular mass of the beta 1-subunit was 130 kDa in Zajdela hepatoma cells versus 115 kDa in normal rat hepatocytes. Pulse-chase experiments showed that the apparent transition time from the 100-kDa beta 1-precursor to the 130-kDa mature form was abnormally prolonged in Zajdela hepatoma cells since the latter was not detected until 24 h, while the transition from the 100-kDa precursor to the 115-kDa mature form began within 3 h in normal rat hepatocytes. Digestion of both the normal rat hepatocytes and Zajdela hepatoma cells 100-kDa beta 1-precursors with endo-beta-N-acetylglucosaminidase H and peptide N-glycosidase yielded products from 100 kDa to 84 kDa and 82 kDa, respectively, as judged by SDS/PAGE, suggesting that the same polypeptide chain is synthesized in normal rat hepatocytes and in Zajdela hepatoma cells. Incubation of the mature normal rat hepatocyte beta 1-subunit with peptide N-glycosidase reduced its molecular mass from 115 kDa to 82 kDa, as judged by SDS/PAGE, while the molecular mass of the abnormal mature Zajdela hepatoma cell beta 1-subunit decreased from 130 to 110 kDa. Thus, in addition to alterations in the Asn-linked oligosaccharide processing, 'ascitic growth' induced other post-translational modifications in the Zajdela hepatoma cell beta 1-subunit. Furthermore, both the abnormal mature 130-kDa and precursor 100-kDa beta 1-subunits were detected on the surface of Zajdela hepatoma cells, associated with the alpha 5-subunit. The relationship between these structural alterations in the fibronectin receptor and the impaired Zajdela hepatoma cell binding to soluble fibronectin or to a coated fibronectin matrix that was observed in this study is discussed.

Identification of peanut agglutinin receptors related to the state of tumoral liver cell differentiation

The relationship between cell differentiation/tumorisation and plasma membrane glycoproteins was approached using peanut agglutinin (PNA) a lectin specific for the Gal-beta(1,3)GalNAc sequence and a homologous cell system consisted of normal rat hepatocytes (HyC) and a poorly differentiated hepatoma (ZHC). This work is focused on the molecular nature of PNA receptors. PNA bound strongly to ZHC, but bound very weakly, if at all to hepatocytes. After sialidase treatment this binding was slightly enhanced in ZHC and HyC. The total number of binding sites on ZHC was 9.6 x 10(6)/cell and 1.2 x 10(7)/cell before and after sialidase treatment respectively. In contrast, this number could not be calculated on HyC, even after sialidase treatment. The PNA receptors were isolated and identified from ZHC using affinity chromatography on immobilized PNA and lectin overlay. Two bands were revealed after SDS-PAGE of PNA receptors: a major one with a relative molecular mass of 160 kDa and a minor one of 110 kDa. The latter disappeared after sialidase treatment of ZHC suggesting the possibility that these two bands could be less and more sialylated forms of the PNA receptors, respectively. In contrast no PNA receptors could be detected on HyC. These PNA receptors could be considered O-linked glycoproteins containing the Gal-beta(1,3)GalNAc disaccharide because: i) PNA carbohydrate specificity toward this disaccharide found in this glycoprotein type; ii) their carbohydrate composition with Gal and GalNAc but not man residues; iii) their sensitivity to alkaline treatment; and iv) strong inhibition of PNA binding to ZHC with the Gal-beta(1,3)GalNAc structure. The absence of PNA receptors on HyC appeared to be related to the absence of this glycoprotein containing the disaccharide but not to the change or failure of glycosylation of the polypeptide chain of PNA receptors. The relationship between the presence of PNA receptors and differentiation/tumorisation phenomena as well as the mechanism that induced the expression of these receptors are discussed.

Differential entry of ricin into malignant and normal rat hepatocytes

We have compared the mechanisms of ricin binding to and entry into Zajdela hepatoma cells (ZHC) and normal rat hepatocytes (HyC). Lactose but not mannan was found to inhibit ricin binding to and toxicity on ZHC and HyC. This finding suggests that ricin binding, entry, and toxicity are expressed only through the galactose binding sites on ZHC and HyC. Nevertheless, the characteristics of ricin binding and its entry pathway appeared to be different in several respects in ZHC and HyC. Scatchard analysis of equilibrium data determined over a wide range of 125I-labeled ricin concentrations yielded a curvilinear plot for ZHC, while a straight line was obtained for HyC. These results indicate that only ZHC possess high-affinity receptors for ricin. Analysis of ricin toxicity on ZHC and HyC, in the presence of ammonium chloride or after K+-depletion in both cell types, suggests that the ricin bound to galactose receptors entered through neutral vesicles in ZHC, and through both neutral and acidic vesicles in HyC. The qualitative and quantitative differences found between the process of receptor-mediated endocytosis of ricin in ZHC and HyC might explain the differential sensitivity of the two cell types toward the toxin.

Lack of evidence for a pyruvate kinase isozyme shift in hepatocytes of the regenerating rat liver

1. Pyruvate kinase isozyme shift in a regenerating rat liver was studied. Rats were subjected to a 70% hepatectomy and the liver homogenate or hepatocyte preparations were obtained from the regenerating liver. 2. Using thin layer polyacrylamide gel electrophoresis, liver homogenates from an intact normal rat appeared to contain the L-type isozyme in the greatest number and M2-type to a lesser extent. 3. The ratio of the M2- to L-type increased in the preparations obtained from the regenerating liver. 4. In the hepatocyte preparations prepared from a regenerating rat liver by the conventional method, a small amount of M2-type isozyme was detected. 5. However, the M2-type isozyme was hardly detected in the highly purified hepatocyte preparations prepared using Percoll. 6. Similar results were obtained by separation of the enzyme by DEAE cellulose column chromatography. 7. These results suggest that there is no pyruvate kinase isozyme shift from L- to M2-type in hepatocytes in the course of regeneration. 8. The increased M2-type isozyme in the regenerating rat liver is considered to originate from nonparenchymal cells.

Changes in surface glycopeptides after malignant transformation of rat liver cells and during the regression of hepatoma cells

Normal liver cells, Zajdela's hepatoma cells, and regressing hepatoma cells were metabolically labeled with either radioactive glucosamine or mannose. Glycopeptides obtained by exhaustive pronase digestion of these cells were compared after fractionation by gel filtration on Bio-Gel P-6. Chemical analysis, affinity chromatography on immobilized lectins, alkaline treatment, and susceptibility toward endo-beta-N-acetylglucosaminidase and tunicamycin revealed dramatic changes in the glycopeptide patterns of transformed cells during the recovery of normal phenotype. The most prominent feature was the presence on the surface of hepatoma cells of a large glycopeptide, which was absent from normal liver cells and disappeared almost completely during the regression of hepatoma cells. This large glycopeptide had a Mr of 70,000, contained essentially O-glycosidically linked glycan chains, and did not result from a hypersialylation. N-glycosidically linked glycopeptides, high-mannose, and complex-type oligosaccharides were present in distinct proportions according to the differentiation state. Transformation of liver cells led to a reduction of high-mannose type oligosaccharides and an increase in the degree of branching of complex-type oligosaccharides. In addition, "bisected" glycopeptides were present only on hepatoma cells. The pattern of N-linked glycopeptides of normal liver cells was recovered during the regression of hepatoma cells. The origin of glycopeptide differences between normal and transformed cells and the evidence of a relation between carbohydrate changes, in particular the appearance of a large glycopeptide, and tumorigenicity are discussed.

Expression of c-fos oncogene during hepatocarcinogenesis, liver regeneration and in synchronized HTC cells

We have studied the expression of c-fos gene in rat hepatoma induced by DENA. An increase of c-fos mRNA concentration was observed after 8 days, but the maximal 5- to 6-fold increase was observed after 70 weeks. This increase was found in perinodular hepatocytes as well as in cancer nodules. c-fos expression was also enhanced during liver regeneration at a period corresponding to cell proliferation. In HTC cells the arrest of the cell cycle at early G1 phase by addition of sodium butyrate was accompanied by a strong increase of c-fos gene expression. However the c-fos mRNA rapidly decreased after removal of sodium butyrate during the progression of the cells in the cell cycle and increased transiently when the cells entered again in G1 phase.

Modifications of the expression of liver-specific and non-specific messenger RNAs during azo-dye hepatocarcinogenesis

The expression of specific and non-specific rat liver messenger RNAs has been studied during 3'-methyl-4-(dimethylamino)azobenzene (3'-MeDAB) carcinogenesis, using cDNA probes complementary to mRNAs encoding aldolase A and B, L-type pyruvate kinase, albumin, alpha-fetoprotein, transferrin and an unidentified 2.7 X 10(3)-base mRNA. mRNAs specific for undifferentiated cells, such as those encoding aldolase A and the unidentified 2.7 X 10(3)-base species were re-expressed very early, being easily detectable at the 1st week of 3'-MeDAB treatment. They reached a maximum of expression at the 4th week. Simultaneously the levels of aldolase B and L-type pyruvate kinase mRNAs dramatically decreased as compared to controls, but remained responsive to induction by a high-carbohydrate diet. Albumin and transferrin mRNA levels were only slightly modified in the course of the carcinogenic diet. At the terminal stage of hepatocarcinogenesis, i.e. in malignant hepatoma cells, expression and inducibility of aldolase B and L-type pyruvate kinase mRNAs were similar to those in normal adult rats while mRNAs specific for undifferentiated or foetal stages were also synthesized. The very early changes in gene expression for aldolases A and B, L-type pyruvate kinase and the 2.7 X 10(3)-base mRNA species could indicate that carcinogenic diet modifies gene control mechanisms long before inducing hepatoma.

Correlated increase of the expression of the c-ras genes in chemically induced hepatocarcinomas

The expression of the c-Ha-ras, the c-Ki-ras and the N-ras genes was measured by the dot blot technique in rat liver tumors induced by a short diethylnitrosamine (DENA) treatment and in the surrounding liver cells. A 2 to 25 times higher level of transcript was found as well in the surrounding cells, as in the tumor cells, as compared to the level in hepatocytes. In addition the increase of expression was parallel for the three ras genes. We conclude that this enhanced expression can be attributed to an epigenetic mechanism and it can, in certain cases, be dissociated from cell proliferation.

Modulation of functional activities in cultured rat hepatocytes

Rat hepatocytes isolated by enzymatic dissociation of the liver must attach in order to survive for more than a few hours. In conventional culture conditions, they rapidly lose their highly differentiated functions, e.g. adult isozymic forms, enzyme response to specific hormones and cytochrome P-450-dependent monooxygenase activities. Incompletely differentiated cells such as perinatal and regenerating hepatocytes, can transiently exhibit a more differentiated state. Therefore, regulation of hepatic functions, particularly enzyme activities cannot be studied for more than a few days. Hepatocyte survival rate and maintenance of specific functions are dependent on nutrient composition of the medium as well as the substrate. Complex matrices, particularly that derived from the connective liver biomatrix, appear to have an important favorable effect. However, regardless of culture conditions specific functions cannot be quantitatively maintained for more than several days. Recent observations strongly suggest that such a problem may be overcome by mimicking in vivo specific cell-cell interactions. Thus when co-cultured with a liver epithelial cell line, probably derived from biliary ductular cells, adult hepatocytes remain able to synthesize high levels of albumin and to conjugate drugs. In these conditions, the cells secrete an abundant heterogeneous extracellular material. The co-cultures can be maintained in a serum-free medium and specific liver functions can be altered experimentally. Such a model could be appropriate for studying long-term induction and modulation of liver enzyme activities under defined experimental conditions.

Comparative analysis of proteins labelled with [35S]methionine in the liver in vivo and in freshly isolated and short-term-cultured hepatocytes in vitro

[35S]Methionine-labelled liver proteins, analysed by one- or two-dimensional gel electrophoresis showed a strikingly similar pattern whether synthesized in vivo or by freshly isolated hepatocytes. In contrast, major qualitative and quantitative differences were observed with the patterns of labelled proteins found in cultured hepatocytes. The changes detectable very early (within 1 h) in culture affected preferentially the synthesis of cytoskeleton proteins (cytokeratins, actin, myosin), which was dramatically increased. Physical factors like cell attachment appear to be responsible for these changes which, however, occurred more rapidly in the presence of serum. Freshly isolated hepatocytes and short-term-cultured cells responded similarly to insulin and glucagon, which respectively increased and decreased the labelling of the whole set of cellular and exported proteins. Glucocorticoids caused either an increase or a decrease in the labelling of several proteins, but the effects were detectable only under chronic exposure of cultured hepatocytes. Based on these results, freshly isolated hepatocytes appear more representative of the liver in vivo than cultured hepatocytes, and therefore seem more suitable for short-term studies. However, cultured hepatocytes can be used for long-term studies since they maintain many specific liver functions and remain hormonally sensitive.