Protein Synthesis In Hepatocytes Research Articles

Importance of glycolysis for the energetics of anoxia-tolerant and anoxia-intolerant teleost hepatocytes.

The importance of glycolysis, as an ATP-producing and substrate-providing pathway, was studied in anoxia-tolerant (goldfish) and anoxia-intolerant (trout) hepatocytes. Inhibition of glycolysis with iodoacetic acid (IAA) left aerobic ATP production largely unaffected in hepatocytes from both species but caused a significant decrease of ATP contents in the goldfish cells. Ouabain-sensitive oxygen consumption (osVo2), an estimate of mitochondrial ATP production coupled to ATP consumption by the Na(+) pump, was significantly reduced in IAA-treated goldfish hepatocytes, whereas it was unaltered in trout hepatocytes. Partial reduction of mitochondrial respiration, achieved by titration with cyanide (CN), strongly stimulated glycolytic flux but did not affect ATP contents of hepatocytes from both species. Under these conditions, osVo2 became undetectable. Rb(+)-uptake rates, providing a direct estimate of Na(+)-pump activity, were in good agreement with estimates derived from osVo2 in IAA-treated cells, showing a decrease in goldfish and no change in trout. However, they indicated persistent Na(+)-pump activity despite the lack of osVo2 in CN-treated cells. Overall, these data indicate that in goldfish hepatocytes Na(+)-pump activity is more dependent on glycolytic ATP production as compared to trout hepatocytes. Protein synthesis of goldfish hepatocytes was inhibited in IAA- and CN-treated cells, possibly reflecting the hierarchical organization of energy metabolism. In trout hepatocytes, protein synthesis could be sustained at control levels, given that energetic substrate provision was not limited.

Role of apolipoprotein A-I in steroid-induced activation of DNA and protein synthesis in hepatocytes.

It is demonstrated that anabolic effect of steroid hormones is produced by steroid-apolipoprotein A-I complexes. These complexes accelerate not only protein, but also DNA synthesis and produce both cell hypertrophy and hyperplasia. For realization of their anabolic effect steroid hormones are modified by alpha- and beta-reductases in resident macrophages.

Short-Term Inhibition of Protein Synthesis in Mouse Hepatocytes Stimulate Their Proliferative Activity in vitro

Short-Term Inhibition of Protein Synthesis in Mouse Hepatocytes Stimulate Their Proliferative Activity in vitro

Action of adenosine on energetics, protein synthesis and K(+) homeostasis in teleost hepatocytes.

In a comparative study, we analysed the effects of adenosine on the energetics, protein synthesis and K(+ )homeostasis of hepatocytes from the anoxia-tolerant goldfish Carassius auratus and the anoxia-intolerant trout Oncorhynchus mykiss. The rate of oxygen consumption did not respond immediately to the addition of adenosine to the cells from either species, but showed a significant decrease in trout hepatocytes after 30 min. The anaerobic rate of lactate formation was not significantly affected by adenosine in goldfish hepatocytes, but was increased in trout cells. We also studied the effects of adenosine on the two most prominent ATP consumers in these cells, protein synthesis and Na(+)/K(+)-ATPase activity. Under aerobic conditions, adenosine inhibited protein synthesis of hepatocytes from goldfish by 51% and of hepatocytes from trout by 32%. During anoxia, the rate of protein synthesis decreased by approximately 50% in goldfish hepatocytes and by 90% in trout hepatocytes, and this decrease was not altered by the presence of adenosine. Adenosine inhibited normoxic Na(+)/K(+)-ATPase activity and K(+ )efflux by 20-35% in the cells of both species. An investigation into the mechanism underlying the inhibition of protein synthesis by adenosine indicated that, in the goldfish cells, adenosine acts via a membrane receptor-mediated pathway, i.e. the effect of adenosine was abolished by applying the A1 receptor antagonist 8-phenyltheophylline. In the trout, however, the uptake of adenosine into hepatocytes seems to be required for an effect on protein synthesis. [Ca(2+)](i) does not seem to be involved in the inhibition of protein synthesis by adenosine.

The effects of galactosamine on UTP levels in the livers of young, adult and old rats.

Galactosamine (GalN), a well-known hepatotoxin that depletes the cellular pool of uracil nucleotides, was previously shown to have greater impact on the inhibition of protein synthesis in hepatocytes of old rats as compared with young animals (Kmieć 1994, Ann. N.Y. Ac. Sci. 717, 216-225). In the present study we compared the effects of GalN on the nucleotide content (measured by ion-exchange HPLC) in the livers of young (4 months), adult (12 months), and old (24-26 months old) rats two hours after its intraperitoneal administration. UTP content of the livers of old control rats was significantly lower (by 28%) than that of young animals. GalN administration decreased the UTP content in the livers of young, adult and old rats by, respectively, 55%, 65% and 89%, and increased the content of UDP-sugars by 189%, 175% and 305%. The hepatic content of ATP, ADP, AMP, NAD, GTP except CTP did not differ significantly among the age groups of rats studied, and was not changed by GalN treatment. The content of CTP was significantly higher in old rats (P < 0.03) upon GalN treatment. The lower hepatic content of UTP may partially explain the increased sensitivity of hepatocytes and livers of old rats to the action of galactosamine, and possibly to other hepatotoxic compounds that decrease transcription in the liver.

The effect of conjugated linoleic acid on the antioxidant enzyme defense system in rat hepatocytes.

Short-term effects of physiological concentrations of conjugated linoleic acid (CLA) on membrane integrity, metabolic function, cellular lipid composition, lipid peroxidation, and antioxidant enzymes were examined using rat hepatocyte suspension cultures. Incubation with CLA (5-20 ppm) for 3 h decreased the ability of hepatocyte plasma membranes to exclude trypan blue by approximately 25%, and caused leakage of cytosolic lactate dehydrogenase (LDH) into the medium. The significant decrease (P< 0.02) in hepatocyte viability as measured by LDH leakage during cell incubation with 10 and 20 ppm CLA was not associated with significant changes in cellular ATP content. Protein synthesis in hepatocytes was elevated (P < 0.05) in the presence of 5 and 10 ppm CLA, but at a higher concentration (20 ppm), protein synthesis was similar to that of control cells. Gluconeogenesis was maintained in cells incubated with lower concentrations of CLA (5 and 10 ppm) but was decreased (P < 0.02) at the higher concentration. Incubation with 20 ppm CLA for 3 h did not affect the specific activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase, the rate-limiting enzyme of cholesterol synthesis. Both cis-9,trans-11/trans-9,cis-11, and cis-10,trans-12/trans-10,cis-12 isomers of CLA were incorporated to a similar level into hepatocytes. Levels ranged from 3.9 to 4.1%, respectively, of total fatty acids in neutral lipids, and from 0.7 to 0.8%, respectively, of total fatty acids in phospholipids. Cellular lipid peroxidation remained unchanged in the presence of CLA (5-20 ppm), despite significant inhibition (P < 0.05) of superoxide dismutase. Catalase activity was maintained near control levels in the presence of 5 and 10 ppm CLA but was significantly decreased in the presence of 20 ppm CLA. Glutathione peroxidase activity was significantly decreased in the presence of 10 ppm CLA. The apparent sensitivity of the antioxidant enzyme defense system of liver cells to CLA, coupled with the lack of effect of CLA on lipid peroxidation in cells, suggests that cytotoxic effects of CLA as described by LDH leakage and decreased gluconeogenesis were not mediated by a prooxidant action in hepatocytes.

The effects of amino acid supply on total and individual cellular protein synthesis in ovine hepatocytes

Protein nutrition, through altered amino-acid supply, regulates the quality and quantity of animal products, and deficiencies in supply of / response to amino-acids may be a limiting factor in ruminant efficiency. The effects of amino-acid supply may be indirect, via modulation of anabolic hormonal signalling. In particular, amino-acid limitation reduces insulin-like growth factor-1 (IGF-1) - and increases IGF-1 binding protein 1 (IGFBP-1) - production by the liver, and hence modulates the sensitivity of tissues to pituitary GH as an anabolic signal (Gluckman et al, 1987). The mechanisms through which amino-acids exert these selective effects on liver protein production are not understood.The present study sought to determine whether the effects of amino-acids on total cellular protein synthesis are sufficient to account for observed reductions in IGF-1 production in cultured ovine hepatocytes. In addition the use of two dimensional polyacrylamide gel electrophoresis (2D-PAGE) to visualise changes in translation of individual proteins caused by amino-acids in cultured ovine hepatocytes is described.

Malaria circumsporozoite protein inhibits protein synthesis in mammalian cells.

Native Plasmodium circumsporozoite (CS) protein, translocated by sporozoites into the cytosol of host cells, as well as recombinant CS constructs introduced into the cytoplasm by liposome fusion or transient transfection, all lead to inhibition of protein synthesis in mammalian cells. The following findings suggest that this inhibition of translation is caused by a binding of the CS protein to ribosomes. (i) The distribution of native CS protein translocated by sporozoites into the cytoplasm as well as microinjected recombinant CS protein suggests association with ribosomes. (ii) Recombinant CS protein binds to RNase-sensitive sites on rough microsomes. (iii) Synthetic peptides representing the conserved regions I and II-plus of the P.falciparum CS protein displace recombinant CS protein from rough microsomes with dissociation constants in the nanomolar range. (iv) Synthetic peptides representing region I from the P.falciparum CS protein and region II-plus from the P.falciparum, P.berghei or P.vivax CS protein inhibit in vitro translation. We propose that Plasmodium manipulates hepatocyte protein synthesis to meet the requirements of a rapidly developing schizont. Since macrophages appear to be particularly sensitive to the presence of CS protein in the cytosol, inhibition of translation may represent a novel immune evasion mechanism of Plasmodium.

Protective effect of linoleic acid on IFN gamma-induced cellular injury in primary culture hepatocytes.

We have previously demonstrated that treatment of hepatocytes with IFN gamma results a series of cellular injury processes, including DNA synthesis arrest, membrane breakage and apoptosis. In the present work, we show that IFN gamma suppresses cellular respiration and protein synthesis in hepatocytes, and that cellular respiration suppression is an early event in the IFN gamma-induced cellular injuries. Polyunsaturated fatty acids (PUFAs) increased cellular respiration of hepatocytes, but only linoleic acid showed some protective effect against IFN gamma-induced cellular respiration suppression. Linoleic acid also reduced other IFN gamma-mediated cellular injuries, including membrane breakage and protein synthesis inhibition. Like linoleic acid, fetal bovine serum also inhibited IFN gamma-induced cellular damage. Increased NAD levels were found in both IFN gamma-treated and non-treated hepatocytes following the addition of PUFAs, but clofibrate, a peroxisome proliferator, bromophenacyl bromide (BPB), an inhibitor of phospholipase, nordihydroguaiaretic acid (NDGA), an inhibitor of lipoxygenase, and arachidonic acid, a metabolite of linoleic acid, did not inhibit IFN gamma-induced cellular injury. In addition, the combination of linoleic acid and IFN gamma induced nitric oxide (NO) synthesis in hepatocytes. These results suggest that fatty acid may play an important role in liver homeostasis during chronic inflammatory states and sepsis.

Milk thistle (Silybum marianum) for the therapy of liver disease.

Silymarin, derived from the milk thistle plant, Silybum marianum, has been used for centuries as a natural remedy for diseases of the liver and biliary tract. As interest in alternative therapy has emerged in the United States, gastroenterologists have encountered increasing numbers of patients taking silymarin with little understanding of its purported properties. Silymarin and its active constituent, silybin, have been reported to work as antioxidants scavenging free radicals and inhibiting lipid peroxidation. Studies also suggest that they protect against genomic injury, increase hepatocyte protein synthesis, decrease the activity of tumor promoters, stabilize mast cells, chelate iron, and slow calcium metabolism. In this article we review silymarin's history, pharmacology, and properties, and the clinical trials pertaining to patients with acute and chronic liver disease.

Tetrahydroaminoacridine-induced apoptosis in rat hepatocytes

Tacrine (tetrahydroaminoacridine, THA) is currently administered to thousands of patients for the treatment of Alzheimer's disease. Unfortunately, THA therapy is often limited by this drugs' propensity to induce reversible hepatotoxicity. In the present study we investigated the mechanism of THA cytotoxicity by measuring the effect of THA on cell viability, protein synthesis activity and the induction of apoptosis in suspensions of freshly isolated rat hepatocytes. Our experimental findings indicate that THA-mediated apoptosis is responsible for the acute in vitro hepatotoxicity observed with this aminoacridine derivative. We found that THA-treated hepatocytes (0.1, 0.25 and 0.5 m m) demonstrated a significant and dose-dependent reduction in cellular protein synthesis activity (84, 55 and 5% of control activity, respectively) after 1 hr of incubation. However, in hepatocytes exposed to 0.1 and 0.25 m m THA, the inhibition of protein synthesis was short-lived. In these treated cells, protein synthesis activity returned to control levels (100%) by the fifth hr of incubation without a significant increase in cellular lactate dehydrogenase (LDH) leakage or the induction of apoptosis. In hepatocytes exposed to 0.5 m m THA, the near complete inhibition of protein synthesis was not reversible and a dramatic increase in LDH leakage (necrosis) was observed after 6 hr of treatment. In 0.5 m m THA-treated hepatocytes the appearance of apoptotic nuclei and cells were observed with electron microscopy following 2 hr of treatment (12% of total hepatocytes analysed) and steadily increased to 42% by the fifth hr (compared with 4% for control cells). We speculate that THA's ability to inhibit hepatocyte protein synthesis (>50%) and induce apoptosis may have an important role in the hepatotoxic episodes experienced by Alzheimer's patients taking this drug. However, the role of apoptosis in clinical THA-induced hepatotoxicity and the relevance of using rat hepatocyte suspensions as an in vitro model for THA hepatotoxicty in vivo require additional investigation.

Increase in cytosolic and mitochondrial protein synthesis in rat hepatocytes irradiated in vitro by HeNe laser

In order to gain an insight into the mechanism of cell photostimulation by laser light, protein synthesis was measured in hepatocytes irradiated with a low-power, continuous-wave HeNe laser (fluence, 0.24 J cm −2; fluence rate, 7 and 12 mW cm −2). As a result of irradiation the rate and amount of 35S-methionine incorporated into newly synthesized proteins increased, as demonstrated by gel electrophoresis and quantitative analysis of labelled protein bands. The stimulation of protein synthesis was fluence dependent, with a maximum stimulation at 0.24 J cm −2 for both fluence rates (12 and 7 mW cm −2). Both cytosolic and mitochondrial protein synthesis increased as a result of irradiation, as demonstrated by the measurement of hepatocytes previously treated with chloramphenicol and cycloheximide respectively. An initial investigation showed that stimulation of protein synthesis also occurred in hepatocytes irradiated with a non-coherent radiation source (fluence, 0.24 J cm −2).

The hepatocyte growth factor regulates the synthesis of acute-phase proteins in human hepatocytes: Divergent effect on interleukin-6-stimulated genes

Our study addressed the role of the human hepatocyte growth factor (HGF), a potent mitogen for mature rat and human hepatocytes, in the regulation of specific hepatic genes. The experimental evidence obtained in primary cultured human hepatocytes indicates that HGF regulates the synthesis of plasma proteins in a dose-response fashion. It stimulates the synthesis of the negative acute-phase proteins albumin, transferrin, and fibronectin, decreases that of alpha1-antichymotrypsin (ACT) and haptoglobin, and stimulates that of alpha2-macroglobulin (AMG), which in man is insensitive to inflammatory mediators. HGF had no effect on C-reactive protein (CRP) synthesis. These effects differ from those elicited by interleukin-6 (IL-6). The effects of HGF on fibrinogen and alpha1-antitrypsin were, however, similar to those induced by IL-6. The effects of HGF were also observed at the messenger RNA (mRNA) level. Time-course induction experiments showed that the effects of HGF on protein synthesis were delayed by about 48 to 72 hours, in contrast with the 12-hour lag found after IL-6 stimulation. Although the presence of glucocorticoids was not absolutely necessary for HGF to affect plasma protein synthesis, it moderately extended the effects. In pulse-chase experiments, it was found that the action of HGF was not due to an alteration of the rate of secretion of the proteins. The effects of HGF on the synthesis of albumin, transferrin, fibronectin, alpha1-antichymotrypsin, and haptoglobin could be counteracted by the simultaneous presence of IL-6 in the incubation media. A clear additive effect was observed only in the case of fibrinogen. No interaction was observed in the cases of CRP and AMG. The results of this study indicate that the effects of HGF on human hepatocytes may not simply be limited to its mitogenic activity, but that it also regulates hepatic-specific genes and antagonizes, in part, the action of IL-6.

The hepatocyte growth factor regulates the synthesis of acute-phase proteins in human hepatocytes: Divergent effect on interleukin-6-stimulated genes

Our study addressed the role of the human hepatocyte growth factor (HGF), a potent mitogen for mature rat and human hepatocytes, in the regulation of specific hepatic genes. The experimental evidence obtained in primary cultured human hepatocytes indicates that HGF regulates the synthesis of plasma proteins in a dose-response fashion. It stimulates the synthesis of the negative acute-phase proteins albumin, transferrin, and fibronectin, decreases that of alpha1-antichymotrypsin (ACT) and haptoglobin, and stimulates that of alpha2-macroglobulin (AMG), which in man is insensitive to inflammatory mediators. HGF had no effect on C-reactive protein (CRP) synthesis. These effects differ from those elicited by interleukin-6 (IL-6). The effects of HGF on fibrinogen and alpha1-antitrypsin were, however, similar to those induced by IL-6. The effects of HGF were also observed at the messenger RNA (mRNA) level. Time-course induction experiments showed that the effects of HGF on protein synthesis were delayed by about 48 to 72 hours, in contrast with the 12-hour lag found after IL-6 stimulation. Although the presence of glucocorticoids was not absolutely necessary for HGF to affect plasma protein synthesis, it moderately extended the effects. In pulse-chase experiments, it was found that the action of HGF was not due to an alteration of the rate of secretion of the proteins. The effects of HGF on the synthesis of albumin, transferrin, fibronectin, alpha1-antichymotrypsin, and haptoglobin could be counteracted by the simultaneous presence of IL-6 in the incubation media. A clear additive effect was observed only in the case of fibrinogen. No interaction was observed in the cases of CRP and AMG. The results of this study indicate that the effects of HGF on human hepatocytes may not simply be limited to its mitogenic activity, but that it also regulates hepatic-specific genes and antagonizes, in part, the action of IL-6. (Hepatology 1996 Jun;23(6):1345-52)

Effect of fumonisin B 1 on protein and lipid synthesis in primary rat hepatocytes

The effect of fumonisin B 1, (FB 1) on protein and lipid synthesis was evaluated in primary rat hepatocytes. FB 1 did not affect incorporation of [ 3H]leucine into hepatocytes at either non-toxic (150 μ m) or cytotoxic (500 μ m) concentrations indicating that protein synthesis was not affected. However, FB 1 significantly ( P < 0.01 to P < 0.0001) inhibited incorporation of [ 14C]palmitic acid into hepatocyte cultures implying that lipid synthesis was altered. Incorporation of the radiolabel was significantly ( P < 0.05 to P < 0.0001) lowered in triacylglycerol (TAG) and sphingomyelin fractions and increased in phosphatidylcholine (PC) and phosphatidylethanolamine (PEA) in both FB 1 concentrations. The incorporation pattern of [ 14C]palmitic acid closely resembles the changes in phospholipid levels in the treated cells. The sphingolipid, sphinganine (Sa), was significantly ( P < 0.0001) increased in treated cells but there was no significant difference between the toxic and non-toxic dose levels implying that the increased Sa level alone is not responsible for the in vitro toxicity. FB I significantly ( P < 0.01 to P < 0.001) decreased the level of free cholesterol within the cell, resulting in an increased PC:cholesterol ratio suggesting a more rigid membrane structure. Subsequent studies on the fatty acid (FA) profiles in PC and the neutral lipid, TAG, indicated that FB I significantly ( P < 0.05 to P < 0.0001) increased the levels of the polyunsaturated FAs C18:2n-6 and C20:4n-6 at both concentrations. The FB 1-induced changes to cellular membranes, specifically those related to FA changes in the major membrane phospholipids, and the altered FA content of the hepatocytes are likely to be key events in explaining the cytotoxic effects and altered growth responses induced by fumonisins in primary hepatocytes.

Protective effects of Osbeckia octandra against galactosamine and tert-butyl hydroperoxide induced hepatocyte damage

Ayurvedic and other ‘traditional’ medical practitioners in Sri Lanka use the mature leaves of the plant Osbeckia octandra for its hepatoprotective properties. In this study the effects of an aqueous extract of Osbeckia octandra against injury induced by d -galactosamine and tert -butyl hydroperoxide (TBH) were investigated in freshly isolated rat hepatocytes. The plant extract (500 μg/ml) significantly reduced the inhibition of protein synthesis (as assessed by the incorporation of 14 C-leucine into protein) in hepatocytes incubated for 1 h with 10 mM galactosamine by a mean of 25.6 ± 3.6% and decreased the release of cellular lactate dehydrogenase (LDH) and aspartate aminotransferase (AST) enzyme activities into the medium by 55.3% and 32.8%, respectively. With TBH, the plant extract decreased lipid peroxidation (estimated from malondialdehyde formation) by a mean of 29.9 ± 1.1% together with a 46.8% and 54.7% decrease in the release of LDH and AST, respectively into the incubation medium. Significant protection was also obtained when the Osbeckia extract was added to the incubation medium up to 30 min after pre-exposure of the hepatocytes to either galactosamine or, to a lesser extent, TBH. The results support the use of Osbeckia as a hepatoprotective agent.

Protective effects of Osbeckia octandra against galactosamine and tert-butyl hydroperoxide induced hepatocyte damage

Ayurvedic and other ‘traditional’ medical practitioners in Sri Lanka use the mature leaves of the plant Osbeckia octandra for its hepatoprotective properties. In this study the effects of an aqueous extract of Osbeckia octandra against injury induced by d-galactosamine and tert-butyl hydroperoxide (TBH) were investigated in freshly isolated rat hepatocytes. The plant extract (500 μg/ml) significantly reduced the inhibition of protein synthesis (as assessed by the incorporation of 14C-leucine into protein) in hepatocytes incubated for 1 h with 10 mM galactosamine by a mean of 25.6 ± 3.6% and decreased the release of cellular lactate dehydrogenase (LDH) and aspartate aminotransferase (AST) enzyme activities into the medium by 55.3% and 32.8%, respectively. With TBH, the plant extract decreased lipid peroxidation (estimated from malondialdehyde formation) by a mean of 29.9 ± 1.1% together with a 46.8% and 54.7% decrease in the release of LDH and AST, respectively into the incubation medium. Significant protection was also obtained when the Osbeckia extract was added to the incubation medium up to 30 min after pre-exposure of the hepatocytes to either galactosamine or, to a lesser extent, TBH. The results support the use of Osbeckia as a hepatoprotective agent.

Influence of monosaccharide derivatives on liver cell glycosaminoglycan synthesis: [formula omitted] (3-deoxy- d-galactose) and methyl (methyl 4-chloro-4-deoxy-,β-D-galactopyranosid) uronate

An improved, convenient synthesis of 3- deoxy- d -xylo- hexose (3-deoxy- d-galactose) has been developed, and the chemical synthesis of a novel monosaccharide derivative, methyl (methyl 4-chloro-4-deoxy-β- d-galactopyranosid)uronate (compound 10), is described. Using primary hepatocytes in culture, each was used to explore its effect on glycosaminoglycan (GAG) synthesis. In the absence of analogues hepatocytes synthesize primarily (92–95%) heparan sulphate. At 1 mM, 3-deoxy- d-galactose had little observable effect on either liver cell GAG or protein synthesis. At 10 mM and 20 mM, 3-deoxy- d-galactose reduced [ 3H]glucosamine and 35SO 4 incorporation into hepatocyte cellular GAGs to, respectively, 75% and 60% of the control cells. This inhibition of GAG synthesis occurred without any effect on hepatocyte protein synthesis, indicating that 3-deoxy- d-galactose's effect on GAG synthesis is not mediated through an inhibition of proteoglycan core protein synthesis. Furthermore, GAGS in the presence of 20 mM of the analogue were significantly reduced in size, 17 kDa vs. 66 kDa in untreated cells. These results reflect either impaired cellular GAG chain elongation, and/or altered GAG chain degradation. Compound 10 exhibited a concentration-dependent inhibition of both hepatocyte cellular GAG and protein synthesis. At concentrations of 5, 10 and 20 mM, compound 10 inhibited GAG and protein synthesis by 20, 65 and 90%, respectively. Exogenous uridine was able to restore partially the inhibition of protein synthesis, but was unable to reverse the effect of compound 10 on GAG synthesis. These results show that part of the effect of compound 10 on GAG synthesis is not mediated by an inhibition of proteoglycan core protein synthesis. GAGs in the presence of compound 10 are half as large as those in the absence of this compound (33 and 66 kDa, respectively). These results again may reflect either impaired cellular GAG chain elongation and/or altered GAG chain degradation. Potential metabolic routes for each analogue's effect are presented.

Interleukin-6 down-regulates expressions of the aldolase B and albumin genes through a pathway involving the activation of tyrosine kinase

Interleukin-6 plays a key role in mediating acute-phase protein synthesis in hepatocytes. However, the mechanism of how interleukin-6 regulates aldolase B and albumin syntheses in hepatocytes is not completely understood. In this study, using primary cultured rat hepatocytes, we have shown that interleukin-6 down-regulates expressions of the aldolase B and albumin genes in a dose- and time-dependent manner. We examined whether the decrease in aldolase B and albumin mRNA expressions by interleukin-6 reflected transcriptional down-regulation or stability of the mRNA. Actinomycin D and cycloheximide did not affect the interleukin-6-mediated decrease in the expressions of both genes. These results suggest that the decreased expressions of both genes induced by interleukin-6 is controlled at the transcriptional level, and that it is due neither to increased degredation of mRNA nor to synthesis of new proteins. Protein kinases play a fundamental role in the intracellular signal transduction. To examine the interleukin-6 signal pathway(s) leading to the decrease of aldolase B and albumin mRNA expressions, we tested various kinds of protein kinase inhibitors in this system. Herbimycin A, an inhibitor of tyrosine kinase(s), prevented the decrease in the expressions of aldolase B and albumin mRNAs by interleukin-6 H-7, an inhibitor of protein kinase C, prevented the decrease in the expression of albumin mRNA by interleukin-6, but did not induce recovery of that of aldolase B mRNA. These results suggest that a tyrosine kinase(s) or a herbimycin A-sensitive kinase(s) constitutes a common pathway for interleukin-6-mediated reduction of aldolase B and albumin mRNA expressions and that distinct pathways exist for the modes of expression of the two mRNAs.

Interleukin-6 and acute-phase proteins in head and neck cancer.

The acute-phase response is the answer of the organism to a disturbance of its homeostasis and is characterized by dramatic changes in the concentration of some plasma proteins defined as acute-phase proteins. In recent years several data have shown that interleukin-6 (IL-6) is the major inducer of acute-phase protein synthesis in human hepatocytes. Recently, we demonstrated higher IL-6 serum levels in head and neck cancer (HNC) patients than in healthy subjects. In the present study we examined the relationship between levels of IL-6 and of several acute-phase proteins, including C-reactive protein (CRP), alpha 1-antitrypsin (ATT), alpha 1-acid glycoprotein (AAG), haptoglobin (HPT) and fibrinogen. Eighteen patients were studied and had squamous cell carcinoma of the larynx (n = 9), oral cavity (n = 4), oropharynx (n = 3) and hypopharynx (n = 2). Proteins were measured at three time points before and three time points after surgery. Significant (P < 0.0001) relationships were found between IL-6 and CRP (r = 0.69), and fibrinogen (r = 0.51), whereas no correlation was found with AAT (r = 0.13, P = 0.56), AAG (r = 0.38; P = 0.07) and HPT (r = 0.16; P = 0.46). These data strongly suggest that IL-6 may play a key role in acute-phase protein synthesis in HNC and in regulation of the complex host response to malignancies.