Plasma Protein Synthesis Research Articles

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)

Chronic Protein Deficiency Differentially Affects the Kinetics of Plasma Proteins in Young Pigs

The use of plasma protein concentrations to assess protein-nutritional status has been questioned because concentrations and kinetics are affected by factors other than protein intake. To determine the effect of protein deficiency on plasma protein concentration and synthesis, two groups of four piglets consumed diets containing either 20 or 3% protein. After 8 wk, 2H3-leucine was infused intravenously to measure the fractional and absolute synthesis rates (FSR and ASR) of albumin, transferrin, retinol binding protein (RBP), transthyretin (TTR), a new peptide called TTR2, the high density apolipoprotein (HDL-apoA-1), fibrinogen, and haptoglobin. Compared with controls, protein-deficient pigs had significantly lower (P < 0.05) plasma albumin, RBP and TTR2 concentrations, significantly slower (P < 0.05) FSR of fibrinogen, HDL-apoA-1, transferrin, and TTR2, significantly lower (P < 0.05) ASR of albumin, fibrinogen, transferrin, and TTR2, and a significantly higher (P < 0.05) ASR of TTR. Fibrinogen and transferrin concentrations did not differ between groups, but transthyretin concentration was higher in protein-deficient pigs. These results suggest that protein-nutritional status cannot be predicted from the concentrations of all plasma proteins, that chronic protein deficiency affects the rate of synthesis of only some plasma proteins, and that the kinetic response of plasma proteins to protein restriction cannot be predicted from measurements of plasma concentrations.

Cell Sources for Bioartificial Liver Support

The present review discusses hepatocyte sources for a bioartificial liver. Intended requirements for cell sources are for example: synthesis of plasma proteins, detoxification and regulation. The need for highly differentiated hepatocytes is stressed. Furthermore, the gap between this objective on the one hand and the real possibilities as they appear today on the other is shown. Alternatives to primarily isolated hepatocytes are discussed, thereby elucidating the limits of established cell lines. In summary, it is postulated that the results expected from a bioartificial liver, are closely related to the source and type of cells used.

Isolation and characterization of cell-specific cDNA clones from a subtractive library of the ocular ciliary body of a single normal human donor: transcription and synthesis of plasma proteins.

A subtractive cDNA library was developed for the purpose of identifying cell-specific genes expressed within the human ocular ciliary body, a tissue responsible for regulating aqueous humor secretion and intraocular pressure. Partial DNA sequence of a large number of cDNA clones and homology searches of nucleic acid and protein databases revealed significant homologies to at least 90 independently known genes. A group of biologically significant genes that were previously not known to have transcriptional expression in the ciliary body, complement component C4; alpha 2 macroglobulin; selenoprotein-P; and apolipoprotein D, were further demonstrated by Northern hybridization. Antibodies to these and other proteins (i.e., tyrosinase-related protein and pigment epithelium-derived factor) confirmed their cell-restricted expression in ciliary epithelial cells (pigmented, nonpigmented), or vascular endothelial cells. We provide evidence that two human plasma proteins, complement component C4 and alpha 2-macroglobulin are metabolically labeled with [35S]methionine in ciliary processes explants, suggesting that the ciliary body is an organ of synthesis and secretion of plasma proteins present in aqueous humor. These results challenge the notion that plasma proteins in aqueous humor are imported from outside of the eye. The subtractive cDNA library reported in this work should be very useful for identifying potential candidate genes in ocular abnormalities affecting the ciliary body, or involved in the regulation of intraocular pressure.

Effects of hepatocyte growth factor on the growth and metabolism of human hepatocytes in primary culture

The effect of recombinant human hepatocyte growth factor (h-rHGF), a potent mitogen for hepatocytes, was investigated in primary cultures of human hepatocytes. Here, we describe a series of experiments to investigate the kinetics of its mitogenic action, as well as its metabolic effects on cultured human hepatocytes. The hrHGF is a potent signal for initiating DNA synthesis in human hepatocytes, with maximal stimulatory effects at 10 ng/mL (0.1 pmol/L). The kinetics of DNA synthesis showed a lag of about 48 to 72 hours, followed by a maximum at 96 hours. At least 48 hours of continuous exposure to h-rHGF are required to initiate DNA synthesis in quiescent human hepatocytes. Cell cycle analysis by flow cytometry showed that most of quiescent 2c cells have left GO/G1 and entered the cell cycle (S and G2/M phases) by 96 hours of continuous exposure to h-rHGF. When compared with other growth factors, h-rHGF was a much more potent mitogen. The effects of 10 ng/mL (0.1 pmol/L) h-rHGF on DNA synthesis were only achieved by 1.5 pmol/L epidermal growth factor (EGF), 0.1 μmol/L insulin, or 1 μmol/L glucagon. It is noteworthy that the effect of h-rHGF was potentiated by glucagon but not by insulin or EGF. The stimulatory effect of HGF on DNA synthesis was gradually inhibited by h-rHGF transforming growth factor beta (TGF-β) in the range 1 to 10 ng/ml. The HGF also influenced the expression of other hepatic genes. This mitogenic factor stimulated the synthesis of the negative acute-phase plasma protein, albumin, and inhibited the synthesis of the positive one, α 1-antichymotrypsin, whereas it moderately influenced the synthesis of fibrinogen. Although glucagon per se had no effects on cell growth, the synergism between glucagon and h-rHGF could be of relevance facilitating the mobilization of glycogen and the stimulation of human hepatocyte growth.

Actions of 3,5,3′-tri-iodothyronine on the synthesis and secretion of major plasma proteins by a human hepatoblastoma cell line (Hep G2)

We have elucidated the action of tri-iodothyronine (T3) on the synthesis and secretion of seven major plasma proteins in a human hepatoblastoma cell line, Hep G2, and established an in vitro experimental model of human liver cells for the study of the mechanism of the action of thyroid hormone. Hep G2 cells cultured in serum-free medium were treated with various concentrations of T3. During the first 24 h of T3 treatment, accumulation of alpha-fetoprotein in the medium was decreased in a dose-dependent manner (10(-11)-10(-8) M), and the inhibitory effect was enhanced during the second 24 h of T3 treatment. On the other hand, alpha 1-antitrypsin accumulation in the medium during the second 24 h of hormone treatment was decreased by T3 (10(-9)-10(-8) M), although no change in accumulation was observed during the first 24 h of T3 treatment. The newly synthesized [35S]Met-labelled alpha 1-acid glycoprotein was increased by T3 and reached 3.4-fold within 37 h of 10(-8) M T3 treatment. The stimulatory effect increased time-dependently (4.6-fold after 61 h). In contrast, the synthesis of alpha-fetoprotein was reduced to half of that of the control after T3 treatment for 37 h. Although the content of newly synthesized [35S]alpha 1-antitrypsin was not affected by 10(-8) M T3 treatment during 3 days of hormone treatment, the accumulation of alpha 1-antitrypsin in the medium decreased to 87%; in contrast, total cellular newly synthesized alpha 1-antitrypsin increased to 105-130% of that of the control.(ABSTRACT TRUNCATED AT 250 WORDS)

Behavior of the small GTP-binding protein rab6 in the liver of normal rats and rats presenting an acute inflammatory reaction

While it is known that the small GTP-binding protein rab6 is localized in vitro to the Golgi apparatus of several mammalian cells, its behaviour in vivo has not yet been investigated. The aim of this work was to compare by immunocytochemistry and immunoblotting the distribution of rab6 in hepatocytes from normal rats and from rats with an acute inflammatory reaction, a circumstance where the synthesis and secretion of plasma proteins by the hepatocytes is increased and which is accompanied by several changes in the Golgi apparatus. Our results show that in normal rats, rab6 was present in all hepatocytes irrespective of the location of the cell in the hepatic lobule. At the ultrastructural level, rab6 was mainly visible on the three Golgi saccules, but in some cells it appeared to be absent in saccules corresponding to the cis or the trans saccule. The inflammatory reaction was accompanied by an increase of the immunocytochemical labelling at the light and electron microscopy levels. However, by immunoblotting, no differences in the total amount of rab6, nor in its subcellular distribution were found in liver cells after acute inflammatory reaction. These results demonstrate that rab6 is restricted in vivo to the Golgi apparatus and that no significant redistribution occurs during an acute inflammatory reaction.

Influence of Transcriptional Regulation and mRNA Stability on Hemopexin Gene Expression in Regenerating Liver

The hepatic response to systemic injury is characterized by alterations in the synthesis of plasma proteins, while acute injury to the liver can lead to rapid proliferation of hepatocytes. The hemopexin gene was found to be markedly induced in rat liver following both sham surgery (SS) and 70% partial hepatectomy (PH), models of systemic injury and hepatic proliferation, respectively. Transcriptional and post-transcriptional regulation of this gene was evaluated to examine the mechanisms of hemopexin mRNA expression in these models. Significant transcriptional activation was observed within 6 h of either surgery, with a more pronounced effect after PH. In both processes, transcription rates returned to baseline values by 24 h after surgery, although marked elevations in mRNA steady-state levels were noted for at least 72 h. At each time point, levels of hemopexin mRNA were more abundant following PH than after SS, in part due to greater transcriptional induction. In addition, posttranscriptional mechanisms appeared to contribute to the increased expression of hemopexin post-PH. The in vivo half-life of the 1.6-kb hemopexin transcript was determined to be considerably greater than 12 h in control, sham-operated, and PH animals. The exceptionally long mRNA half-life appears to be an important but complex factor in the kinetics of hemopexin gene regulation.

Polydnavirus infection inhibits synthesis of an insect plasma protein, arylphorin.

The wasp Campoletis sonorensis injects a segmented, double-stranded DNA polydnavirus (CsPDV) along with its egg during parasitization of Heliothis virescens larvae. After parasitization, CsPDV protects the wasp egg and larva by selectively disabling the host's cellular immune response. Other host physiological systems including growth and development are affected to the apparent benefit of the parasite. To begin the characterization of the biochemical effects and mode of action of CsPDV on host growth, the titre of a developmentally regulated insect storage protein, arylphorin, was studied. Parasitized or virus-infected insects had substantially less circulating arylphorin than control insects. Fat bodies from parasitized larvae also synthesized less arylphorin in vitro. However, Northern blots of total RNA from parasitized and non-parasitized, control insects showed that the arylphorin transcript level was unaffected by parasitization suggesting a biochemical block at the translational level. In vitro translation followed by immunoprecipitation of arylphorin indicated that the mRNA was present and translatable at equal levels in both parasitized and control insects. Injection of purified virus elicited the response observed in naturally parasitized larvae, demonstrating that the effect on arylphorin synthesis is mediated, either directly or indirectly, by polydnavirus gene product(s).

The acute phase response: General aspects

The acute phase response in a given individual represents the integrated sum of multiple, separately regulated changes. Although many of these changes commonly occur together in affected individuals, clinical experience indicates that not all of them occur in all individuals, indicating that they must be individually regulated. For example, febrile patients may have normal blood levels of CRP and vice versa, leukocytosis does not always accompany other acute phase phenomena, and many instances of discordance between levels of the various acute phase proteins are seen. Cytokines function as part of a complex regulatory network, a signalling language in which information is conveyed to cells by combinations, and perhaps sequence, of intercellular messenger molecules. The effects of combinations of cytokines are complex. To use a somewhat crude simile, individual cytokines can be thought of as words which bear informational content and which may, on occasion, communicate a complete message. More commonly, however, the actual messages received by cells probably resemble sentences, in which combinations and sequences of words convey information. Currently available data suggest that hepatocytes receive a complex mixture of humoral or paracrine signals during the acute phase response. These are integrated by multiple interacting signal transducing mechanisms to cause finely regulated changes in plasma protein synthesis. Regulation largely occurs by transcriptional control, but post-transcriptional mechanisms, including translational regulation, may also participate. Both the extracellular and intracellular mechanisms that mediate the response of the hepatocyte to inflammatory stimuli appear to be highly complex and involve multiple overlapping, concurrent and parallel pathways. Enough is known at present to conclude that IL-6 is a major participant in these plasma protein changes. Regulation of non-hepatocyte acute phase phenomena has not been delineated as thoroughly, but clearly involves a number of inflammation-associated cytokines.

Measurement of plasma protein synthesis rate in infant pig: an investigation of alternative tracer approaches.

To devise a new method to measure plasma protein synthesis, we tested the hypothesis that, when [U-13C]glucose is used to produce [U-13C]alanine, plasma pyruvate and alanine will be in isotopic equilibrium with the alanine used to synthesize plasma proteins. The incorporation of labeled leucine, lysine, and alanine into very-low-density lipoprotein (VLDL) apolipoprotein B (apoB)-100, albumin, and fibrinogen was measured in seven infant pigs by infusing [U-13C]glucose, [2H3]leucine, and [2H4]lysine. The plateau enrichments of plasma alanine (2.29 +/- 0.29), pyruvate (2.5 +/- 0.33), and apoB-alanine (2.33 +/- 0.25) were not different. The fractional synthesis rates of fibrinogen and albumin calculated using the isotopic enrichments of apoB-bound lysine, leucine, and alanine as the precursor were similar to those based on plasma alanine. These results suggest that the intrahepatic precursor alanine pool and plasma alanine were in isotopic equilibrium. Thus plasma protein synthesis can be measured by infusing [U-13C]glucose and using plasma alanine as precursor.

Characterization of a chicken hepatoma cell line with a specific defect in fibrinogen secretion.

This study characterizes plasma protein synthesis and its hormonal regulation in a chicken hepatoma cell line, with particular emphasis on fibrinogen. Whereas virtually all aspects of hemopexin, transferrin and albumin production in these cells corresponded to those of cultured primary hepatocytes, fibrinogen was not secreted. Analysis of fibrinogen subunit synthesis revealed a specific defect in synthesis of one subunit, gamma, correlating with a lack of its mRNA. Pulse-chase and electron microscopic studies demonstrate that, despite the inability of these cells to secrete the A alpha and B beta subunits produced, there is no long-term accumulation of unsecreted fibrinogen. The B beta fibrinogen subunits are largely degraded 2 hr after synthesis. During this time, approximately half of the A alpha subunits are degraded; the rest are converted to the glycosylated form. The implications of this type of defect with respect to the pathogenesis of fibrinogen storage disease are discussed.

Plasma protein synthesis and serum amino acids in dry and lactating dairy cows.

Blood serum concentrations of protein, albumin, prealbumin, transferrin, amino acids and urea were measured in 31 healthy cows 0 to 6 weeks before and 3 to 8 weeks after parturition. In comparison to the precalving values the concentrations of albumin, prealbumin and transferrin were all lower after parturition. Alanine, glutamine, leucine, methionine, serine, and urea concentrations were also lower after calving. Multiple correlation analysis between plasma protein and amino acid concentrations reveals that the synthesis of export proteins in liver may be reduced due to limitation in amino acid availability.

Amino Acids Suppress Intracellular Protein Degradation in Rat Liver during Parenteral Nutrition

The effects of variations in the amino acid supply on the rates of synthesis and degradation of liver proteins and on the rate of synthesis and secretion of plasma proteins were investigated. Rats were nourished by infusion of total parenteral nutrition solutions containing four different levels (0, 1.65, 3.3 and 6.6%) of amino acids for 7 d. The fractional rate of total protein synthesis in the liver was determined by injecting a flooding dose of [3H]phenylalanine. The proportion of newly synthesized proteins retained and exported by the liver was estimated by injecting a tracer dose of [14C]leucine and then measuring the protein radioactivity remaining in the liver and present in the plasma after secretion was completed. The rate of plasma albumin synthesis was significantly lower in the 0 and 1.65% amino acid groups than in the other groups. The fractional synthesis rates of liver domestic proteins, however, were essentially the same in rats administered all levels of amino acids except for the 0% amino acid group, which showed a slightly higher value than the other groups. The fractional degradation rates of liver domestic proteins, calculated as the difference between the fractional synthesis rate of liver domestic proteins and the net gain of liver proteins, were found to be inversely related (r = -0.999, P < 0.05) to the level of amino acids in infusion solutions up to 3.3% amino acids. It was concluded that protein degradation plays the predominant role in the regulation of liver protein mass.

The Role of the Adrenals in the Acute Phase Response to Interleukin-1 and Tumor Necrosis Factor-α

The purpose of this study was to investigate the role of the adrenals, particularly the glucocorticoids, in the acute phase response following daily injections for 5 days of recombinant human interleukin-1α,β and tumor necrosis factor-α (TNFα). Adult weight-stable freely fed or pair-fed (to cytokine-injected mice) mice (C57B1/6J) with and without adrenals were used. Adrenalectomized animals showed a sensitivity to both IL-1α and -1β (40 ng IL-1/day) greater than 10- fold higher than that of normal mice (420 ng IL-1/day) in regard to mortality and anorexia. Microscopic examination of tissue specimens from adrenalectomized IL-1α,β-injected mice did not reveal any histologic alterations in lung, kidney, liver, brain, or gastrointestinal tract to explain the mortality. This mortality was not prevented by physiologic replacement doses of hydrocortisone (10-20 μg/day); however, a pharmacological dose of 2.5 mg hydrocortisone/day abolished completely the increased toxicity to IL-1α,β and the anorectic response to IL-1α,β and TNFα. Increased toxicity (mortality) was not observed in adrenalectomized animals with TNFα at the dose interval used (450 ng TNFα/day and lower). The hepatic acute phase response (liver weight and RNA and liver protein content) was increased by both IL-1α,β and TNFα in a glucocorticoid-independent way. The cytokine-induced alterations of plasma concentrations of acute phase proteins (serum amyoid P, transferrin, complement C3) were significantly dependent on glucocorticoids, while the decline in plasma albumin was not. Glucocorticoids acted as both a positive and a negative modulator of cytokine-regulated synthesis of individual plasma proteins. Plasma IL-6 was highest in mice with adrenals receiving IL-1α injections, significantly higher than that found in adrenalectomized animals. The results demonstrate that several acute phase responses to IL-1α,β and TNFα in the liver cells are independent of the presence of glucocorticoids, while several other reactions are either potentiated or attenuated by glucocorticoids. The increased toxicity to IL-1 in the absence of adrenals may contribute to the increased mortality at stress in patients with Addison's disease.

Prothrombin Synthesis in Rat Hepatoma H4IIEC3 Cells: Response to the Proposed “Coagulopoietin Factor”

Based on studies in intact animals, the presence of humoral factors, "coagulopoietins", which regulate the synthesis of vitamin K-dependent plasma proteins has been proposed. These proposed factors are produced in response to vitamin K deficiency, coumarin treatment, or specific antibody depletion of vitamin K-dependent clotting factors. The production of prothrombin by rat hepatoma H4IIEC3 cells has now been shown to be dependent on the source of bovine serum in the media. Cells grown in serum from cows treated with dicoumarol produce about 20% more prothrombin in 24 h than those cells grown in control serum. The humoral factor causing this response is present early in the course of dicoumarol treatment, and the increase in prothrombin production is dependent on the amount of serum from a dicoumarol-treated cow in the media. Based on membrane filtration studies, the factor appears to be associated with the protein fraction of serum.

Plasma composition in the nephrotic syndrome.

The nephrotic syndrome is a consequence of urinary loss of intermediate sized plasma proteins and the resulting homeostatic responses to those losses. Plasma protein composition is changed greatly. Intermediate sized proteins, including albumin, transferrin, IgG, hormone binding proteins, and low molecular weight inhibitors of the clotting cascade, are lost in the urine and their concentration in plasma reduced. Synthesis of many proteins secreted by the liver is increased either at the level of transcription or posttranscriptionally. Synthesis of several liver-derived proteins is increased in the absence of their urinary loss, suggesting that hypoalbuminemia or reduced plasma oncotic pressure (tau) stimulates the production or reduces the rate of catabolism of these proteins. Their plasma levels, including those of lipoproteins and elements of the coagulation cascade, are increased. Plasma tau falls and plasma viscosity increases because of the replacement of intermediate sized plasma proteins by larger ones. The plasma concentration of several proteins lost in the urine but not secreted by the liver, such as erythropoietin and IgG, are not defended by increased synthesis, suggesting that increased synthesis of plasma proteins is primarily confined to the liver. Loss of both liver-derived and non-liver-derived proteins may cause reduced immunity, anemia, and deficiency syndromes. Urinary loss of albumin alone is not responsible for decreased plasma pi. The relationship between plasma protein concentration and pi is greatly disturbed in nephrotic rats. In contrast, the relationship between pi and plasma protein concentration is nearly the same in rats with hereditary analbuminemia (NAR) and normal rats, despite the absence of albumin from the plasma of NAR. When proteinuria is induced in NAR the relationship between plasma protein concentration and pi becomes identical to that in nephrotic animals, although no albumin was lost in the urine of NAR.

Acute‐phase plasma protein response to cholera intoxication in healthy and diabetic rats

The aim of the present study is twofold: to establish the response of hepatic machinery of plasma protein biosynthesis to cholera intoxication, and to examine the same response of alloxan-diabetic hepatocytes with minimal capacity of synthesis of plasma proteins. Direct lesion of hepatic plasma membranes via ip administration of cholera toxin to male rats resulted in a typical acute-phase response (APR) of plasma proteins, which had regressed to levels similar to those of healthy controls approximately at 240 h postintoxication. The d 2 response to a single 0.16 mg/kg body weight dose was typified by a 23% reduction in the level of albumin, but a 6- and 24-fold increase in the levels of fibrinogen and alpha-1-acid glycoproteins, respectively. This response was similar (in direction but not in magnitude) to the acute-phase reaction to a simple subcutaneous administration of carrageenan. The intoxication was accompanied by a massive leakage, into the peritoneal cavity, of plasma fluid, which embraced the complete profile of acute-phase reactants. A three-step mechanism is proposed to account for the observations as follows: (1) There is a rapid formation of a stable complex between subunit B of the toxin and ganglioside GM1 of hepatic plasma membrane. An APR is induced in response to the alteration(s) of hepatic plasma membranes. (2) The release, from the choleragen-membrane complex, of polypeptide A1 and its subsequent penetration of the hepatic membrane result in both activation of adenylate cyclase and increased vascular permeability of hepatic membranes. This leads, in turn, to exudation of components of plasma fluid in the peritoneal cavity of intoxicated rats. An alternate rationale for this exudation is the slow leakage of plasma proteins out of the blood vascular system (possibly through microvesicles) into the peritoneal cavity of cholera intoxicated rats. The spectrum of acute-phase hepatic secretory components was mirrored in the corresponding peritoneal exudate. (3) The increased hepatic membrane flow provides the continued renewal of plasma membrane proteins required for its eventual repair by either endocytosis or sloughing off the toxin-bound membrane segments into the circulatory system, thus producing regression of APR. Livers of diabetic rats, an already established model in terms of APR, responded to ip administration of cholera toxin by increased biosynthesis of the identified plasma proteins and a marked reduction in total free-glucose in serum.(ABSTRACT TRUNCATED AT 400 WORDS)

Meal stimulation of albumin synthesis: a significant contributor to whole body protein synthesis in humans.

The present studies were performed to test the hypothesis that the liver, by increasing the synthesis of specific plasma proteins during the absorption of an amino acid meal, may play an important role in the temporary "storage" of ingested essential amino acids and to explore the effects of glucocorticosteroids and recombinant human growth hormone (rhGH) on these processes. The fractional synthetic rates of albumin and fibrinogen were determined using simultaneous infusions of intravenous [1-14C]leucine and intraduodenal [4,5-3H]leucine after 22 h fasting and during absorption of glucose and amino acids in four groups of normal subjects treated for 1 wk with placebo, prednisone (0.8 mg.kg-1.day-1), rhGH (0.1 mg.kg-1.day-1), or combined treatment. When compared with the fasted state and independent of the route of tracer delivery and hormonal treatment, albumin, but not fibrinogen, synthesis increased (P < 0.0001) during absorption of a mixed glucose amino acid meal in all groups. This increase in albumin synthesis accounted for 28% of the increase in whole body protein synthesis associated with feeding and for 24, 22, and 14% in the prednisone, rhGH, and combined treatment groups, respectively. These data suggest that the stimulation of albumin synthesis observed during feeding prevents irreversible oxidative losses of a significant fraction of ingested essential amino acids and may serve as a vehicle to capture excess dietary amino acids and transport them to peripheral tissues to sustain local protein synthesis.

Partially hepatectomized rats: a model for the study of the effect of toxins on the plasma protein profiles of nascent hepatocytes.

A useful framework is proposed for unifying the synthesis of plasma proteins and their degradation by, or release from, liver cells of intact and partially hepatectomized rats, in which synthesis and release of acute-phase plasma proteins occur in synchrony with the internalization and catabolism of plasma and extracellular proteins. The catabolism of proteins and other hepato-intracellular glycoproteins during sepsis or trauma is essential to provide constituent amino acids and carbohydrates for the synthesis of acute-phase plasma proteins. Increases in the plasma levels of acute-phase response proteins in sham-operated rats reached a maximum between 1 and 2 d after mock surgery, and had returned virtually to control levels within 6 d. By contrast, acute-phase proteins in the plasma of partially hepatectomized rats were decreased by 10-20% of their initial values after 24 h. A maximum acute-phase response on d 7 after the operation was characterized by an increase of 181, 445, and 19% for alpha-1-acid glycoprotein, hepatoglobin, and hemopexin, whereas other acute-phase proteins remained below control levels, for example, by 11, 25, and 38% for albumin, transferrin, and prealbumin, respectively. This delayed response suggests that the nascent liver cells had inherited the capacity of the parent cells to respond to inflammatory signal and had synthesized acute-phase plasma proteins. Accordingly, a time frame for the application of toxin to nascent hepatocytes is suggested. An increased activity (300 +/- 50%) for both bound and free neuraminidase in remnant liver tissue 19 h post partial hepatectomy suggested that hepatic regenerating factor(s) were produced in liver tissue via the hepatic bound and/or free neuraminidase-mediated desialylation of humoral substrates. By contrast, circulating levels of lysosomal enzymes alpha-fucosidase and beta-N-acetyl-D-glucosaminidase were increased marginally after 24 h but had returned nearly to control levels after 7 d, suggesting that lysosomal acid hydrolases do not play a major role in regenerative DNA synthesis, mitosis, or in the synthesis of acute-phase plasma proteins.