Role of dexamethasone and insulin on the development of the five urea-cycle enzymes in cultured rat foetal hepatocytes.
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ABSTRACT:
SUBMITTER: Husson A
PROVIDER: S-EPMC1144581 | biostudies-other | 1985 Jan
REPOSITORIES: biostudies-other
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Project description:Foetal-rat hepatocytes were cultured in primary monolayer culture, and activity changes of argininosuccinate synthetase (ASS, EC 6.3.4.5) and argininosuccinase (ASL, EC 4.3.2.1) were followed under defined hormone conditions. In hormone-free medium, cultured cells maintained the enzyme activities at values equal to those of freshly isolated cells for at least 3 days. Continuous addition of dexamethasone produced the development of the two enzyme activities, but only after the first 20h of culture. Under these conditions, urea production by the foetal hepatocytes was concomitantly increased in the culture medium. Pretreatment with dexamethasone for 20h was sufficient to produce the development of ASL activity within the 2 following days. Introduced alone, glucagon induced an increase of ASL activity, but did not affect the ASS activity. The most powerful stimulation of ASS and ASL could be observed in cultured hepatocytes if glucagon and dexamethasone were added simultaneously or sequentially. These results indicated that the development of the receptor complex for the induction of urea-cycle enzymes appears early before birth and established that glucocorticoids amplify the glucagon stimulation of these enzyme activities during foetal life.
| S-EPMC1152502 | biostudies-other
Project description:Insulin-stimulated glycogenesis and insulin degradation were studied simultaneously at 37 degrees C in cultured foetal hepatocytes grown for 2-3 days in the presence of cortisol. Degradation of cell-associated insulin, as measured by trichloroacetic acid precipitation, was significant after 4 min in the presence of 1-3 nM-125I-labelled insulin. This process became maximal (30% of insulin degraded) after 20 min, a time when binding-state conditions were achieved. No insulin-degradative activity was detected in a medium that had been exposed to cells. At steady-state, the appearance of insulin degradation products in the medium was linearly dependent on time (1.5 fmol/min per 10(6) cells at 1nM-125I-labelled insulin). Chloroquine (3-50 microM), bacitracin (0.1-10 mM) and NH4Cl (1-10 mM) inhibited insulin degradation as soon as this became detectable and caused an increase in the association of insulin to hepatocytes after 20 min. Lidocaine and dansylcadaverine had similar effects, whereas N-ethylmaleimide, aprotinin, phenylmethanesulphonyl fluoride and leupeptin were found to be ineffective. Chloroquine, and also bacitracin, at concentrations that inhibited insulin degradation, decreased the insulin-stimulated incorporation of [14C]glucose into glycogen over 2 h. This effect of chloroquine was specific, since it did not modify the basal glycogenesis, or the glycogenic effect of a glucose load in the absence of insulin. It therefore appears that the receptor-mediated insulin degradation (or some associated pathway) is functionally related to the glycogenic effect of insulin in foetal hepatocytes.
| S-EPMC1158116 | biostudies-other
Project description:An improved non-perfusion method for the preparation of cultured foetal-rat hepatocytes is described. Digestion of the liver with collagenase and deoxyribonuclease I gave yields of 40 X 10(6) hepatocytes/g of liver. The plating efficiency of hepatocytes in medium with 10 microM-cortisol was 50%. Cell morphology and metabolism were maintained through 3 days of monolayer culture, with minimal contamination by haematopoietic cells or fibroblasts. The cultured cells bound and degraded 125I-insulin in a time- and dose-dependent manner. The estimated ED50 for competitive binding at 37 degrees C was 1.1 nM. Curvilinear Scatchard plots were observed, with estimates of 16 500 high-affinity sites (Kd = 813 pM) and 53 000 low-affinity sites (Kd = 23 nM) per cell. The cultured cells demonstrated a glycogenic response to insulin, with an estimated ED50 of 120 pM. The degree of glycogenic response to insulin varied with time in culture: 500% above basal on day 1, 200% on day 2, and only 150% on day 3. Cultured foetal cells also exhibited a time-dependent uptake of 2-aminoisobutyric acid, which, in contrast with previous reports with adult cells, was not stimulated by the presence of 10 nM-insulin. Cultured foetal hepatocytes may provide an interesting model with which to study the relationship between insulin-receptor binding and insulin action.
| S-EPMC1144261 | biostudies-other
Project description:Binding and degradation of 125I-labelled insulin were studied in cultured foetal hepatocytes after exposure to the protein-synthesis inhibitors tunicamycin and cycloheximide. Tunicamycin (1 microgram/ml) induced a steady decrease of insulin binding, which was decreased by 50% after 13 h. As the total number of binding sites per hepatocyte was 20000, the rate of the receptor degradation could not exceed 13 sites/min per hepatocyte. Cycloheximide (2.8 micrograms/ml) increased insulin binding by 30% within 6 h, an effect that persisted for up to 25 h. This drug had a specific inhibitory effect on the degradation of proteins prelabelled for 10 h with [14C]glucosamine, without affecting the degradation of total proteins. Chronic exposure to 10 nM-insulin neither decreased insulin binding nor modified the effect of the drugs. The absence of down-regulation of insulin receptors cannot be attributed to rapid receptor biosynthesis in foetal hepatocytes. Cellular insulin degradation, which is exclusively receptor-mediated, was determined by two different parameters. First, the rate of release of degraded insulin into the medium was 600 molecules/min per hepatocyte with 1 nM labelled hormone, and increased (preincubation with cycloheximide) or decreased (tunicamycin) as a function of the amount of cell-bound insulin. Secondly, the percentage of cell-bound insulin degraded was not changed by the presence of protein-synthesis inhibitors (25-30%). The stability of insulin degradation suggested that this process was dependent on long-life proteinase systems. Such differences in degradation rates and cycloheximide sensitivity imply that hormone- and receptor-degradation processes utilize distinct pathways.
| S-EPMC1153126 | biostudies-other
Project description:Sequential changes in the numbers of cell-surface receptors induced by a transitory exposure to insulin in cultured 18-day foetal-rat hepatocytes were investigated in the presence of drugs and at a temperature of 22 degrees C, which inhibit cellular insulin degradation. Chloroquine (70 microM) and monensin (3 microM) did not greatly change the initial rate of internalization of cell-surface receptor sites after exposure to 10 nM-insulin, but led to a steady state after 20 min, which represented 40% of the initial binding, compared with 5 min and 60% in the absence of the drug. Moreover, these drugs strongly decreased the proportion of receptor sites recovered at the cell surface after subsequent removal of the hormone. They were ineffective when insulin was not present. The removal of monensin together with the hormone allowed partial restoration of cell-surface receptor sites and degradation of cell-associated insulin to start again at the initial speed, indicating a reversible effect of the drug. During this phase, the drug concentration-dependence for the two effects showed that receptor recycling was restored with concentrations of monensin not as low as for insulin degradation. The effect of vinblastine (50-100 microM) was similar to that of chloroquine and monensin, whereas no modification in the internalization and recovery processes was observed in the presence of bacitracin concentrations (1-3 mM) that inhibit insulin degradation by 70%. A temperature of 22 degrees C did not prevent the receptor internalization, but had a slowing effect on the recycling process, which appeared to vary in experiments where insulin degradation remained inhibited. The present study shows that the process of insulin degradation mediated by receptor endocytosis is not a prerequisite for insulin-receptor recycling in cultured foetal hepatocytes.
| S-EPMC1148318 | biostudies-other
Project description:The insulin-receptor cycle was investigated in cultured foetal rat hepatocytes by determining the variations in insulin-binding sites at the cell surface after short exposure to the hormone. Binding of 125I-insulin was measured at 4 degrees C after dissociation of prebound native insulin. Two protocols were used: exchange binding assay and binding after acid treatment; both gave the same results. Cell-surface 125I-insulin-receptor binding decreased sharply (by 40%) during the first 5 min of 10 nM-insulin exposure (t1/2 = 2 min) and remained practically constant thereafter; subsequent removal of the hormone restored the initial binding within 10 min. This fall-rise sequence corresponded to variations in the number of insulin receptors at the cell surface, with no detectable change in receptor affinity. The reversible translocation of insulin receptors from the cell surface to a compartment not accessible to insulin at 4 degrees C was hormone-concentration- and temperature-dependent. SDS/polyacrylamide-gel electrophoresis after cross-linking of bound 125I-insulin to cell-surface proteins with disuccinimidyl suberate showed that these variations were not associated with changes in Mr of binding components, in particular for the major labelled band of Mr 130,000. The insulin-receptor cycle could be repeated after intermittent exposure to insulin. Continuous or intermittent exposure to the hormone gave a similar glycogenic response, contrary to the partial effect of a unique short (5-20 min) exposure. A relationship could be established between the repetitive character of the rapid insulin-receptor cycle and the maximal expression of the biological effect in cultured foetal hepatocytes.
| S-EPMC1147330 | biostudies-other
Project description:The antagonistic effects of insulin and glucagon on glycogen formation and mobilization were studied in cultured 18-day foetal rat hepatocytes with regard to different modes of exposure. Hormone combinations were achieved with a constant dose of 10 nM-insulin (maximal for the glycogenic effect of this hormone) and increasing doses of glucagon [from 0.03 to 10 nM: concn. causing half-maximal response (ED50) = 0.3 nM)]. When insulin and glucagon were added simultaneously, increasing glucagon concentrations progressively depressed the glycogenic effect of insulin and 0.3 nM-glucagon antagonized the insulin effect completely. The maximal glycogenolytic effect of glucagon was observed at concentrations greater than 1 nM. When the two hormones were introduced successively, with an interval of 4 h between additions, the effect of the second hormone was always fully expressed between 4 and 8 h. at which time the effect of the first hormone had ceased; the dominance of glucagon over insulin was also lost, due to cell desensitization to glucagon. Both continuous or intermittent (10 min on/10 min off periods) exposure to insulin and/or glucagon gave similar antagonistic effects, while in cells exposed to insulin plus glucagon alternating with exposure to insulin or glucagon alone, the glycogenic effect of insulin was less or more antagonized respectively by glucagon. Whatever the situation, the results obtained could not be related to antagonism by a glucagon-induced rise in either cyclic AMP levels (ED50 = 3 nM) or cell-surface hormone binding. Thus, depending on the hormonal state and the mode of hormone administration, regulation of glycogenesis in cultured foetal hepatocytes appears to be different from that predicted by the insulin/glucagon molar ratio, which is strikingly altered in the perinatal period.
| S-EPMC1151198 | biostudies-other
Project description:The pathways of glycogen synthesis from glucose were studied using double-isotope procedures in 18-day cultured foetal-rat hepatocytes in which glycogenesis is strongly stimulated by insulin. When the medium containing 4 mM-glucose was supplemented with [2-3H,U-14C]glucose or [3-3H,U-14C]glucose, the ratios of 3H/14C in glycogen relative to that in glucose were 0.23 +/- 0.04 (n = 6) and 0.63 +/- 0.09 (n = 8) respectively after 2 h. This indicates that more than 75% of glucose was first metabolized to fructose 6-phosphate, whereas 40% reached the step of the triose phosphates prior to incorporation into glycogen. The stimulatory effect of 10 nM-insulin on glycogenesis (4-fold) was accompanied by a significant increase in the (3H/14C in glycogen)/(3H/14C in glucose) ratio with 3H in the C-2 position (0.29 +/- 0.05, n = 6, P less than 0.001) or in the C-3 position (0.68 +/- 0.09, n = 8, P less than 0.01) of glucose, whereas the effect of a 12 mM-glucose load (3.5-fold) did not alter these ratios. Fructose (4 mM) displaced [U-14C]glucose during labelling of glycogen in the presence and absence of insulin by 50 and 20% respectively, and produced under both conditions a similar increase (45%) in the (3H/14C in glycogen)/(3H/14C in glucose) ratio when 3H was in the C-2 position. 3-Mercaptopicolinate (1 mM), an inhibitor of gluconeogenesis from lactate/pyruvate, further decreased the already poor labelling of glycogen from [U-14C]alanine, whereas it increased both glycogen content and incorporation of label from [U-14C]serine and [U-14C]glucose with no effect on the relative 3H/14C ratios in glycogen and glucose with 3H in the C-3 position of glucose. These results indicate that an alternative pathway in addition to direct glucose incorporation is involved in glycogen synthesis in cultured foetal hepatocytes, but that insulin preferentially favours the classical direct route. The alternative foetal pathway does not require gluconeogenesis from pyruvate-derived metabolites, contrary to the situation in the adult liver.
| S-EPMC1133514 | biostudies-other
Project description:Isolated hepatocytes from female rats were cultured in HI-WO/BA medium for 6 days. To the medium was added oleate, ethanol, dexamethasone and insulin. With oleate To alone, triacylglycerol accumulated; ethanol augmented the accumulation by 90%. To the best of our knowledge, this is the first demonstration that ethanol in vitro increases the content of triacylglycerol in liver cells. Further addition of dexamethasone or insulin did not alter the accumulation of triacylglycerol, indicating that these hormones did not play permissive roles for the effect of ethanol in the present system. Dexamethasone and insulin, in the absence of ethanol, increased the accumulation of triacylglycerol by 30% and 50% respectively. The concentration of glycerol 3-phosphate was increased in the presence of ethanol; however, with time the concentration of glycerol 3-phosphate declined almost to control values, while the accumulation of triacylglycerol continued linearly; this suggests that the effect of ethanol was not mediated via fluctuations in the concentration of glycerol 3-phosphate. These results are discussed in relation to earlier investigations in vivo and in vitro.
| S-EPMC1153135 | biostudies-other
Project description:Mitochondrial enzymes involved in energy transformation are organized into multiprotein complexes that channel the reaction intermediates for efficient ATP production. Three of the mammalian urea cycle enzymes: N-acetylglutamate synthase (NAGS), carbamylphosphate synthetase 1 (CPS1), and ornithine transcarbamylase (OTC) reside in the mitochondria. Urea cycle is required to convert ammonia into urea and protect the brain from ammonia toxicity. Urea cycle intermediates are tightly channeled in and out of mitochondria, indicating that efficient activity of these enzymes relies upon their coordinated interaction with each other, perhaps in a cluster. This view is supported by mutations in surface residues of the urea cycle proteins that impair ureagenesis in the patients, but do not affect protein stability or catalytic activity. We find the NAGS, CPS1, and OTC proteins in liver mitochondria can associate with the inner mitochondrial membrane (IMM) and can be co-immunoprecipitated. Our in-silico analysis of vertebrate NAGS proteins, the least abundant of the urea cycle enzymes, identified a protein-protein interaction region present only in the mammalian NAGS protein-"variable segment," which mediates the interaction of NAGS with CPS1. Use of super resolution microscopy showed that NAGS, CPS1 and OTC are organized into clusters in the hepatocyte mitochondria. These results indicate that mitochondrial urea cycle proteins cluster, instead of functioning either independently or in a rigid multienzyme complex.
| S-EPMC7860981 | biostudies-literature