Alterations in albumin secretion and total protein synthesis in livers of thyroidectomized rats.

Perfused rat livers and isolated rat hepatocytes exhibited a 50% decrease in the secretion of both albumin and total secretory proteins after thyroidectomy. In contrast, synthesis of non-secretory proteins was decreased by only 20% from the rates observed in liver preparations from euthyroid rats. These observations suggested a disproportionate effect of thyroidectomy on the synthesis of secretory proteins compared with non-secretory proteins. Disproportionate decreases in the synthesis of albumin in other endocrine-deficient states such as hypophysectomy and diabetes had previously been shown to be associated with decreases of similar magnitude in the relative abundance of albumin-mRNA sequences. In contrast, thyroidectomy did not affect the activity or amount of albumin mRNA in total liver poly(A)-containing RNA when assayed by cell-free translation and by hybridization with complementary DNA, respectively. Furthermore, labelling experiments in vivo demonstrated that albumin synthesis represented 12.9 +/- 0.5% and 12.4 +/- 0.4% of total protein synthesis in livers of thyroidectomized and euthyroid rats respectively. Therefore the fall in secretion of albumin and total secretory protein after thyroidectomy did not appear to be a reflection of disproportionate decreases in the synthesis of these proteins. Instead, defects in steps involved in the post-synthetic processing and secretion of albumin are suggested. A number of comparisons, including ribosome half-transit times, the size distributions of total and albumin-synthesizing polyribosomes, and the fraction of RNA present as inactive ribosomes, provided evidence that the overall decrease in protein synthesis after thyroidectomy was not due to generalized alterations in translational processes. Instead, the decrease in total protein synthesis appeared to reflect the RNA content of the liver, which fell in proportion to th decrease in protein synthesis.     

Synthesis and secretion of ceruloplasmin by isolated rat hepatocytes

The synthesis and secretion of ceruloplasmin (Cp) by isolated rat hepatocytes were investigated. Cp released by liver cells appeared to have properties similar to those of the blood-circulating protein, i. e. Mr, oxidase activity, immunological specificity and the peptide set of tryptic fingerprints. The polypeptides with Mr of 130,000, 65,000, 48,000 and 18,000 were revealed in Cp isolated from the incubation medium. These results suggest the susceptibility of the single-chain protein molecule (Mr 130,000) to limited proteolysis which is accomplished by the proteases released from the cells. When fresh serum was added to the incubation medium, the proteolytic degradation of Cp proceeded at a much slower rate, which led to an increase in the content of excreted polypeptides with Mr 130,000. The secretion was strongly diminished by the addition of colchicine to the medium. The time of Cp molecule synthesis on membrane-bound polyribosomes (3.5 min) was determined.     

Studies on the differential response to insulin on the stimulation of amino acid incorporation into protein in isolated hepatocytes containing different levels of glycogen.

Effect of insulin on amino acid incorporation into protein by isolated rat liver hepatocytes was studied. A two to three-fold increase in the incorporation of U-¹⁴C-Leucine and U-¹⁴C-Phenylalanine into protein by insulin (100 μUnits) was observed in isolated hepatocytes containing high glycogen. This effect was abolished by the addition of glucagon (3 × 10^?6M). No stimulation in amino acid incorporation by insulin was observed when isolated hepatocytes contained low or no glycogen. Electron micrographs of incubated cells show that in the presence of insulin more normal parallel strands of polyribosomes are maintained as compared to control cell preparation.     

Effect of post-ischaemic recovery on albumin synthesis and relative amount of translatable albumin messenger RNA in rat liver.

In liver cells recovering from reversible ischaemia, total protein synthesis by postmitochondrial supernatant and membrane-bound and free polyribosomes is not different from that in sham-operated controls. However, the relative proportion of specific proteins is changed, since the incorporation of [3H]leucine in vivo into liver albumin, relative to incorporation into total protein, as determined by precipitation of labelled albumin with the specific antibody, decreases by 40-50% in post-ischaemic livers. Cell-free synthesis by membrane-bound polyribosomes and poly(A)-enriched RNA isolated from unfractionated liver homogenate shows that the decrease in albumin synthesis in liver of rats recovering from ischaemia is due to the relative decrease in translatable albumin mRNA.     

Functional modification of rat liver ribosomes by the in vitro action of N-methyl-N'-nitro-N-nitrosoguanidine

The mechanism by which N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) inhibits protein synthesis has been studied in a rat liver cell free system. Using preformed aminoacyl-tRNA it was observed that incorporation of amino acid into polyribosomal protein was inhibited in the presence of low concentration of MNNG. This inhibition was not reversed by increasing the concentration of soluble factors. Transfer RNAs modified previously by treatment with MNNG and subsequently esterified with amino acids were transferred to polyribosomes with the same efficiency as those species which were not modified. Polyribosomes, on the other hand, lost activity to incorporate amino acids after pretreatment with MNNG. This inactivation was dependent on the concentration of MNNG with which polyribosomes were treated. When poly(U) was used with MNNG-treated polyribosomes, its translation, after correction for endogenous translation, was also found to be significantly low as compared to the case with untreated polyribosomes. Purified ribosomes stripped of endogenous mRNA when treated with increasing concentrations of MNNG progressively lost ability to support polyphenylalanine synthesis programmed by poly(U). The treated ribosomes, however, neither inhibited the activity of control ribosomes nor induced any loss of fidelity of translation by poly(U). It is concluded that MNNG inhibits protein synthesis through functional inactivation of ribosomes resulting from direct modification of ribosomal proteins possibly involving nitroguanidination of lysine residues.     

Isolation and characterization of a novel liver-specific gene, hepassocin, upregulated during liver regeneration

By differential cDNA cloning coupled with Xenopus oocyte expression screening, we isolated a cDNA encoding a novel protein, termed 'hepassocin', the expression of which is upregulated in the regenerating rat liver. The cDNA contained a single open reading frame encoding a protein of 314 amino acids (ca. 34 kDa), including 24 amino acids of signal sequence. The protein expressed from the cDNA in Verots cells had activity to stimulate DNA synthesis in primary rat hepatocytes and was of 66 kDa or 34 kDa, under non-reducing or reducing conditions, respectively. Using an affinity column conjugated with the antibody raised against a peptide in a hydrophilic region, we purified hepassocin from the rat liver: it had a DNA synthesis-stimulating activity in hepatocytes. The hepassocin obtained here was 66 kDa, and the 34 kDa protein obtained under reducing conditions contained five cysteine residues, indicating that hepassocin is active as a homodimer. Northern blot analysis revealed that hepassocin mRNA (1.4 kb in length) occurred only in the liver, and in situ hybridization studies revealed its presence in parenchymal hepatocytes but not in endothelial cells. Furthermore, the expression of hepassocin mRNA was upregulated during compensatory hyperplasia after partial hepatectomy and regeneration after galactosamine treatment in the rat liver. These results suggest that hepassocin plays an important role in stimulating liver cell growth, through an autocrine mechanism.     

Mouse liver cell culture. I. Hepatocyte isolation

A method for isolation of mouse liver cells by a two-step perfusion with calcium and magnesium-free Hanks' salt solution followed by a medium containing collagenase is described. Several variations of the commonly used procedure for rat liver cell isolation were quantitatively compared with respect to cell yield and viability. The optimal isolation technique involved perfusion through the hepatic portal vein and routinely produced an average of 2.3 x 10(6) viable liver cells/g body weight. Optimal perfusate collagenase concentration was found to be 100 U of enzyme activity per milliliter of perfusate. Light and electron microscopic evaluation of liver morphology after several steps of the isolation showed distinct morphologic changes in hepatocytes and other liver cells during perfusion. After perfusion with Hanks' calcium- and magnesium-free solution, many hepatocytes exhibited early reversible cell injury. These changes included vesiculation and slight swelling of the endoplasmic reticulum as well as mitochondrial matrix condensation. Subsequent to perfusion with collagenase, the majority of hepatocytes appeared connected to one another only by tight junctional complexes at the bile canaliculi. Multiple evaginations were seen on the outer membrane resembling microville and probably represented the remains of cell-to-cell interdigitations between hepatocytes and sinusoidal lining cells from the space of Disse. The cytoplasmic injury seen after Hanks' perfusion was reversed after collagenase perfusion. After mechanical dispersion, isolated mouse hepatocytes were spherical in shape and existed as individual cells; many (80 to 85%) were binucleated under hase contrast light microscopy. By electron microscopy, cells appeared morphologically similar in cytoplasmic constitution to that seen in intact nonaltered liver cells.     

IL-6 functions as an exocrine hormone in inflammation. Hepatocytes undergoing acute phase responses require exogenous IL-6

We have previously shown that IL-6 is the major monocyte- and fibroblast-derived regulator of acute phase protein gene expression and synthesis in hepatocytes in inflammation. Recently, we and others have shown that rat and human hepatoma cells express IL-6 mRNA, and the question arose as to whether normal hepatocytes express IL-6 and whether any such expression occurs under normal physiologic conditions or is seen in inflammation. Poly A+ mRNA of liver from normal rats and from rats undergoing an acute phase response was not positive when probed with cDNA for rat IL-6 under conditions in which macrophage mRNA was strongly positive. We then compared poly A+ mRNA from purified hepatocytes freshly isolated from normal rats--from rats that were undergoing an acute inflammatory response and from freshly isolated normal hepatocytes that had been cultured for 24 h in the presence or absence of dexamethasone (microM). Only the mRNA from normal hepatocytes cultured for 24 h in the absence of any glucocorticoid was obviously positive for IL-6. The increased expression of gamma-fibrinogen mRNA indicated the presence of inflammation. These results confirm the identification of IL-6 as an exogenous hormone for regulating normal hepatic acute phase protein synthesis in inflammation and rules out an autocrine mechanism being active in the liver in normal homeostasis.     

Biosynthesis of rat liver transhydrogenase in vivo and in vitro

The biosynthesis of pyridine dinucleotide transhydrogenase, a homodimeric inner mitochondrial membrane redox-linked proton pump, has been studied in isolated rat hepatocytes. Newly synthesized transhydrogenase, having an apparent molecular weight identical to the enzyme of isolated liver mitochondria, was selectively immunoprecipitated from detergent extracts of isolated hepatocytes which were labeled with [35S]methionine. That the enzyme is a nuclear gene product is indicated since 1) synthesis was inhibited by cycloheximide, but not by chloramphenicol and 2) no synthesis could be demonstrated in hepatocyte ghosts which are competent only in mitochondrial translation. In addition to the mature form of the enzyme, a species about 2000 daltons larger was also immunoprecipitated from pulse-labeled cells. The half-life of the larger form during a subsequent chase at 37 degrees C was about 2 min, whereas the mature form was not degraded. The relationship between the two forms of the enzyme was established by in vitro studies. A protein approximately 2000 daltons larger than mature transhydrogenase was immunoisolated from a rabbit reticulocyte lysate system programmed with sucrose gradient fractionated rat liver mRNA. This protein was converted to a species having the same size as mature enzyme after incubation with either intact rat liver mitochondria or a soluble matrix fraction derived from mitoplasts. These studies indicate that transhydrogenase is synthesized in the cytoplasm as a higher molecular weight precursor which is post-translationally processed to the mature protein by a soluble matrix protease during or after membrane insertion.     

Cholesterol 7 alpha-hydroxylase in isolated rat liver cells.

1. The activity of cholesterol 7 alpha-hydroxylase found in the 10000 x g supernatant prepared from isolated rat liver cells was comparable to that found with microsomal fractions from whole liver. 2. The activity of cholesterol 7 alpha-hydroxylase from cells prepared from livers of rats fed the bile salt sequestering agent cholestyramine was 2--3 fold higher than the activity of this enzyme found in cells isolated from animals on a control diet. 3. On incubation of hepatocytes in a suitable medium at 37 degrees C, cholesterol 7 alpha-hydroxylase activity declined to about 50% of its original value after three hours despite the fact that the cells retained a high level of viability over 5--6 h as measured by various sensitive criteria. 4. The decrease in cholesterol 7 alpha-hydroxylase activity was observed whether cholestyramine was included in the diet or excluded from the diet of the animals used as sources of the liver cells. 5. The change in cholesterol 7 alpha-hydroxylase activity seen on incubation of the cells was not affected by including in the incubation medium additional nutrients such as amino acids, the glucocorticoid cortisol, phospholipid dispersions, or sodium taurocholate. 6. Changing the incubation medium in which the cells were suspended at regular intervals during the three-hour experiments failed to prevent this decline in the cholesterol 7 alpha-hydroxylase activity during the incubation of these cells. 7. Although isolated liver cells have been shown to lose glutathione on incubation, addition of physiological levels of this compound did not prevent the decline in cholesterol 7 alpha-hydroxylase activity. 8. Cycloheximide addition to the incubation medium accelerated the decrease in cholesterol 7 alpha-hydroxylase activity. This suggests that some protein synthesis associated with cholesterol 7 alpha-hydroxylase activity occurs during the incubation and inhibition of such protein synthesis accelerated the decrease in this enzyme activity. 9. The cytochrome P-450 content of the 10000 x g supernatant prepared from hepatocytes declined slowly to about 65% of its original value after four hours of incubation at 37 degrees C. This decline in the 10000 x g supernatant cytochrome P-450 content may partly explain the observed loss of cholesterol 7 alpha-hydroxylase activity during incubations in vitro. 10. Isolated hepatocytes rapidly take up radioactively labelled sodium cholate. Subsequent excretion of the radioactivity was also very rapid even in the presence of large amounts of this bile salt in the medium.     

Isolation and subfractionation on ficoll gradients of adult rat hepatocytes. Size, morphology, and biochemical characteristics of cell fractions

The recirculating perfusion of adult rat liver with a Ca-++-free Hanks' solution produces a release of the adhesiveness of cells and a cleaving of the desmosomes. The addition of collagenase and hyaluronidase to the perfusion medium leads to complete dissociation of the liver tissue into a mixture of isolated cells and cell cords in which the hepatocytes remain connected with specific junctional differentiations, namely the gap and tight junctions. Individual cells are released by submitting the suspension of cell trabeculae to a gentle rolling. The gap junctions are ruptured at least in one of the two adjacent cells and remain generally attached to the other cell taking with them a small portion of cytoplasm. This technique of isolation of hepatocytes yields about 60-65% of the parenchymal cells contained in a liver; endothelial cells and other cells of the connective tissue are not recovered. The ultrastructural preservation of the isolated hepatocytes is excellent and the glucose-6-phosphatase activity, confined to the endoplasmic reticulum, appears unaltered in most cells. Protein, DNA and RNA recovery in the preparations of isolated hepatocytes is satisfactory, amounting to 70% of that found in liver homogenate; glycogen, the most labile component examined, is partly lost or degraded during the manipulations. Cell diameters measured by different methods confirm the preservation of the original volume of the in situ hepatocytes and the presence of more than one type of parenchymal cell. By submitting this heterogeneous cell population to an isopycnic density gradient centrifugation, two types of hepatocytes can be distinguished: the light hepatocytes, with a mean diameter of 20.5 mum and a mean density of 1.10, are characterized by an extended smooth- walled endoplasmic reticulum entrapping dispersed alpha-glycogen particles; the heavy hepatocytes, with a mean diameter of 19.0 mum and a mean density of 1.14, present a relatively reduced compartment of smooth endoplasmic reticulum, but large accumulations of glycogen. It is suggested that the cell fraction of low density is enriched in centrolobular cells and the high density fraction in perilobular hepatocytes.     

Identification, purification, and immunochemical characterization of a tocopherol-binding protein in rat liver cytosol.

Tocopherol binding activity accompanying a rat liver cytosolic protein with molecular weight of 30-36 kDa has been demonstrated previously, although the isolation of the protein has not been reported. We now report the purification of an alpha-tocopherol-binding protein (TBP) from rat liver cytosol utilizing three chromatographic procedures: gel filtration, Affi-Gel Blue affinity chromatography, and chromatofocusing. Three peaks of specific alpha-tocopherol-binding activity were resolved on Affi-Gel Blue, referred to as AFB-1A, 1B, and 2. A 32-kDa homogeneous form was obtained after chromatofocusing of AFB-1B. D-alpha-[3H]tocopherol was displaced from homogeneous TBP in the presence of 500-fold excess of nonlabeled alpha-tocopherol, indicating the specificity of the binding. Anti-TBP rabbit antisera identified only one protein in rat hepatic cytosol on Western blotting. TBP immunoreactivity was found in the cytosol of rat liver and the lysate of fractionated hepatocytes, but not in the cytosol of other organs (including the heart, spleen, testes, and lung) nor in the lysate of fractioned Ito cells, endothelial cells, or Kupffer cells isolated from rat liver. Semi-quantitative ELISA demonstrated that rat liver cytosol contained approximately 2 mg TBP/g of cytosol protein. This immunoreactivity was associated with only the 30-36 kDa gel filtration fractions of rat liver cytosol and with both AFB-1A and -1B but not with AFB-2.     

Indirect immunoprecipitation by rat liver polyribosomes using antibodies to tyrosine aminotransferase]

A fraction of rat liver polyribosomes is isolated, which in its immunochemical characteristics considerably enriched with polyribosomes capable to synthesize hydrocortisone-induced liver tyrosine aminotransferase isoenzyme. This specific polyribosome fraction was purified by immunochemical fractionation of total liver polyribosomes using indirect precipitation. The content of polyribosomes in immunoprecipitates comprise 0.4-0.8% of its initial amount (before immunochemical fractionation). The ratio of specific polyribosomes in immunoprecipitates varies from 20 to 45%, which corresponds to 25-100-fold purification. The data obtained suggest that the method of indirect precipitation can be an efficient step in the isolation procedure of individual mRNA.     

Behavior of RNA and protein synthesis during the acclimatization of the carp. Studies with isolated hepatocytes

1. Carp hepatocytes were isolated by dissociating the liver tissue with collagenase. The procedure yields viable cells with highly preserved ultrastructural and metabolic features. The isolated cells were able to self-aggregated and form tissue. 2. RNA and protein synthesis activity was significantly higher in the carp hepatocytes from summer acclimatized fish compared to the activity present in the cold adapted animals. 3. RNA synthesis assayed in carp hepatocytes suspensions obtained from summer and winter acclimatized fish exhibited a behaviour consistent with an inverse compensation to the cold acclimatization state, being apparently repressed, whereas protein synthesis did not show a compensatory activity.     

Cultured Ito cells of rat liver express the alpha 2-macroglobulin gene

Ito cells were isolated from rat liver and kept in culture for up to 13 days. The capability of the Ito cells to synthesize alpha 2-macroglobulin was analyzed at different times after isolation and by pulse-chase experiments. Newly synthesized alpha 2-macroglobulin was determined by immunoprecipitation followed by sodium dodecyl sulfate/polyacrylamide gel electrophoresis and fluorography. alpha 2-Macroglobulin synthesis was hardly detectable in Ito cells and their media 3 days after plating. However, 5-11 days after the isolation of the cells, increasing amounts of alpha 2-macroglobulin were synthesized. The results of pulse-chase experiments performed on day 7 showed that radioactively labeled alpha 2-macroglobulin decreased in the intracellular compartment and increased in the culture medium. alpha 2-Macroglobulin was identified by immunoprecipitation and sodium dodecyl sulfate/polyacrylamide gel electrophoresis under reducing and non-reducing conditions. Furthermore, when unlabeled alpha 2-macroglobulin was added during the immunoprecipitation, a competition was observed. Incubation of pancreatic elastase with culture medium of rat Ito cells or rat hepatocytes led to the same cleavage products as found with alpha 2-macroglobulin. alpha 2-Macroglobulin-specific mRNA could be demonstrated by Northern blot analysis of total RNA extracted from rat Ito cells. Under the conditions where alpha 2-macroglobulin was synthesized in Ito cells, no synthesis of alpha 1-macroglobulin, alpha 1-inhibitor 3, alpha 1-proteinase inhibitor, alpha 1-acid glycoprotein, alpha 1-acute-phase globulin (T-kininogen) and albumin could be demonstrated. It is concluded that alpha 2-macroglobulin is a true secretory protein of rat Ito cells in culture. This could be of importance for collagen metabolism in liver diseases.     

Insulin effects on protein synthesis and secretion in primary cultures of amphibian hepatocytes.

The objective of the present study was to determine the effects of insulin on amphibian hepatocytes in primary culture. Hepatocytes were isolated from adult bullfrogs by collagenase perfusion and maintained as monolayers in serum-free medium. Cells cultured in the continuous presence of insulin exhibited a relatively constant rate of protein secretion over the first four to five days, whereas controls showed an almost three-fold decrease over the same time period. The decline in secreted proteins was equally represented in most exported proteins, except that serum albumin secretion showed twice as much of a decrease relative to the other proteins. The maintenance of protein secretion by insulin was the result of its effect on protein synthesis. The rate of protein synthesis was measured by the incorporation of (3H)-leucine into protein using culture medium containing 0.5 mM leucine, a condition where the specific radioactivity of leucyl-tRNA was shown to be equal to that of (3H)-leucine in the medium. Cultures maintained with insulin for 60 hours synthesized protein at two to three times the rate found in non-insulin treated controls whose rate of protein synthesis was first detectably decreased after nine hours of culture in the insulin-free medium. Sedimentation profiles of polyribosomes from hepatocytes maintained for 60 hours without insulin showed proportionately fewer ribosomes in large polysomes and more in monosomes and free ribosomal subunits than ribosomes from cells cultured with insulin. This result suggests that the decrease in protein synthesis found in the absence of insulin is due to a defect in initiation. Insulin does not exert its effect by regulating cellular levels of ATP; no change in ATP content was found in cells maintained with or without insulin. The results show that insulin maintains high levels of protein synthesis and secretion in amphibian hepatocytes. The hepatocytes in monlayer culture provide a system to study the molecular mechanisms involved in the translational control of protein synthesis by insulin.     

Isolation of ribonuclease-free intact polyribosomes from rat kidney.

High levels of RNAase present in rat kidney have prevented isolation of intact polyribosomes from this tissue. This problem has been circumvented by a thorough in situ arterial perfusion of rat kidney, coupled with homogenization of the perfused rat kidney in heparin and detergents-fortified high-speed supernatant prepared from rat liver. This procedure reduced RNAase activity in the homogenate by as much as 70%. Sedimentation of the polyribosomes from this homogenate through a layer of 2.0 M sucrose resulted in a 78--80% yield of polyribosomes from the rat kidney. The resulting polyribosomal pellet contained less than 8% of the RNAase activity present in polyribosomes from non-perfused rat kidney. The remaining RNAase activity was separated from the larger polyribomes by sucrose density gradient centrifugation. The majority of the polyribosomes were larger than tetramers. This procedure also incrased both the yield and size of polyribosomes from rat and mouse liver.     

A general method for isolation of individual polyribosomes and pure mRNAs by immunosorption technique

A highly specific procedure has been developed for the isolation of individual polyribosomes by immunosorbents containing immobilized antigen-antibody complexes. Polyribosomes synthesizing immunoglobulin IgGl and serum albumin were quantificated and isolated in the native state from MOPC 21A plasmacytoma and rat liver cells. For albumin polyribosomes, the presumptive messenger RNA was demonstrated; it contained three components: 20S, 16S and 9S.Abbreviations IgG murine immunoglobulin - RSA rat serum albumin - RGG rat gamma-globulin - EDTA ethylene diamine tetraacetate - SDS sodium dodecyl sulphate