The regulation of hepatic tyrosine aminotransferase

Clinical relevance of thyroid cell models in redox research

Background

Thyroid-derived cell models are commonly used to investigate the characteristics of thyroid cancers. It is noteworthy that each in vitro single cell model system imitates only a few characteristics of thyroid cancer depending on e.g. source of cells or oncogene used to transform the cells.

Methods

In the current work we utilized rat thyroid cancer cell models to determine their clinical relevance in redox gene studies by comparing in vitro expression data to thyroid Oncomine microarray database. To survey the cell lines we analyzed mRNA expression of genes that produce superoxide anion (nox family), genes that catalyze destruction of superoxide anion to hydrogen peroxide (sod family), and genes that remove hydrogen peroxide from cellular environment (catalase, gpx family and prdx family).

Results

Based on the current results, rat thyroid PC Cl3, PC PTC1, PC E1A, or FRLT5 cell models can be used to study NOX2, NOX4, SOD2, SOD3, CATALASE, GPX1, GPX2, GPX5, PRDX2, and PRDX3 gene expression and function.

Conclusions

Redox gene expression in rat originated single cell model systems used to study human thyroid carcinogenesis corresponds only partly with human redox gene expression, which may be caused by differences in redox gene activation stimulus. The data suggest careful estimation of the data observed in rat thyroid in vitro models.

Electronic supplementary material

The online version of this article (doi:10.1186/s12935-015-0264-3) contains supplementary material, which is available to authorized users.

Maximising the use of freshly isolated human hepatocytes

INTRODUCTION

Freshly isolated human hepatocytes are the best model for predicting adverse drug reactions. However, their preparation and use present the investigator with many variables that are beyond their control. These include operation continuity and timing, size and number of cut surfaces on liver tissue and the prior history of the patient. To exploit the potential of freshly isolated human hepatocytes a method is required to preserve the cells in their initial in vivo like state. This experimental pausing allows experiments to be prioritised at convenient times of the day.

METHODS

A novel approach for selecting viable human hepatocytes by functional attachment to a gelatin gel is described rather than relying on their physical characteristics. The cells are preserved as a monolayer on the semi-solid support at 10°C as single spherical entities.

RESULTS

The hepatocytes can be released into suspension, when required, by a temperature transition to 37°C for 20min. The cells can be used in suspension or as a monolayer. The length of preservation depends upon the source tissue. Hepatocytes from normal liver can be maintained for at least 4days and demonstrated to have the same level of CYP3A4 and the enzymes involved in glucuronidation and sulphation as freshly isolated cells. Cells from fatty liver, attached to gelatin, vary in their preservation time but it is at least 24h and so confluent monolayers, that survive at 37°C can be generated the following day.

DISCUSSION

The technique enables freshly isolated human hepatocytes to be used more effectively. They can be preserved in times of plenty so more experimentation is possible. Alternatively, with poorer fatty cells the initial attachment on gelatin enables confluent monolayers of lipid rich cells to be studied.

Glucocorticoids stimulate hepatic and renal catecholamine inactivation by direct rapid induction of the dopamine sulfotransferase Sult1d1

During the stress response and metabolic fasting, glucocorticoids acting via the glucocorticoid receptor (GR) stimulate hepatic glucose production by activating specific gluconeogenic enzyme target genes. To characterize novel direct GR-regulated hepatic target genes under glucocorticoid control, we performed a whole genome gene expression microarray using dexamethasone-treated GR-null mice. Strongly induced previously characterized genes included phosphoenolpyruvate carboxykinase, serine dehydratase, tyrosine oxygenase, lipin 1, metallothionine, and cdkn1A. Novel induced genes included Ddit4, Fkbp5, Megf9, Sult1e1, and Sult1d1, and all were verified by real-time PCR. Sult1d1, a sulfotransferase, is a member of a large superfamily of detoxification enzymes and has an important role in the inactivation of endogenous dopamine-derived compounds, including the catecholamines. Treatment of primary mouse hepatocytes with dexamethasone for 6 h dramatically increased Sult1d1 mRNA levels, whereas cotreatment with RU-486, a GR antagonist, blocked induction by dexamethasone. Sult1d1 mRNA levels were also increased by dexamethasone in the kidney, a major site of Sult1d1 synthesis. Sult1d1 mRNA was localized by in situ hybridization to renal collecting ducts and was rapidly induced by glucocorticoids in renal inner medullary collecting duct (IMCD3) cells. Hepatic and renal Sult1d1 enzymatic activity was significantly induced in vivo in wild-type mice 6 h after dexamethasone treatment. Chromatin immunoprecipitation assay analysis upstream of the Sult1d1 gene promoter identified a glucocorticoid response element close to the neighboring glucocorticoid-responsive estrogen sulfotransferase Sult1e1 gene, indicating that both genes potentially share a common glucocorticoid response element. These results suggest that Sult1d1 in mice is directly induced by glucocorticoids and may attenuate elevated catecholamine activity during the stress response.

Hepatotoxin-induced hypertyrosinemia and its toxicological significance

A (1)H Nuclear Magnetic Resonance (NMR) spectroscopic investigation of the effects of single doses of four model hepatotoxins on male Sprague-Dawley rats showed that hypertyrosinemia was induced by three of the treatments (ethionine 300 mg/kg, galactosamine hydrochloride 800 mg/kg and isoniazid 400 mg/kg) but not by the fourth (thioacetamide 200 mg/kg). Concomitant histopathological and clinical chemistry analyses showed that hypertyrosinemia could occur with or without substantial hepatic damage and that substantial hepatic damage could occur without hypertyrosinemia. However, in the rats dosed with galactosamine hydrochloride, which showed highly variable amounts of liver damage at ca. 24 h after dosing, a clear relationship was found between the degree of hypertyrosinemia and the extent of the hepatic necrosis induced. In line with the cause of clinically observed Type II Tyrosinemia, we consider that the critical event in the onset of hepatotoxin-induced hypertyrosinemia is likely to be a reduction in hepatic tyrosine aminotransferase (TAT) activity. We discuss mechanisms by which TAT activity could be lost with special consideration given to pyridoxal 5'-phosphate (P5P) depletion and to the inhibition of protein synthesis. This analysis may have implications for the interpretation of clinical measures of liver status such as Fischer's ratio and the branched-chain tyrosine ratio (BTR).

Basal and glucocorticoid induced changes of hepatic glucocorticoid receptor during aging: relation to activities of tyrosine aminotransferase and tryptophan oxygenase

The characteristics of glucocorticoid receptors, their sensitivity to glucocorticoid as well as the basal and glucocorticoid induced thyrosine aminotranferase (TAT) and tryptophan oxygenase (TO) activities were studied in rat liver during aging. The concentration (N) and dissociation constant (K(d)) of glucocorticoid receptor (GR) significantly change during the aging both in untreated and dexamethasone treated animals. The level of receptors was lower in dexamethasone treated rats of all analyzed aged groups compared to untreated animals. In comparison to untreated groups, there was no correlation between the changes of N and K(d) during the lifespan. According to immunochemical analysis, the decline of receptor protein content occurs during lifespan. Dexamethasone treatment reduced the level of receptor protein compare to respective age group of untreated rats. The glucocorticoid-receptor (G-R) complexes from both untreated and treated animals underwent thermal activation, although the extent of activation was more pronounced in the case of untreated groups compared to treated animals. The magnitude of heat activation of receptor complexes was more pronounced in the liver of the youngest untreated rats compared to elderly ones, while the receptor activation between treated groups of studied ages has shown less significant differences. Besides, basal as well as induced TAT and TO activities after dexamethasone injection also showed age-related alterations. The observed alterations in GR might play a role in the changes of the cell responses to glucocorticoid during the age. This presumption is supported by detected changes in basal and dexamethasone induced TAT and TO activities during aging.

Membrane dips over nuclei correlate with DNA synthesis in spreading hepatocytes

Spreading of hepatocytes on different supports was examined using scanning electron microscopy. Positively charged Primaria plates gave a uniform morphology in 2 h. The spreading was rapid and the surface of the cells showed early prominent dips. The hepatocytes had one or two of these structures corresponding with nuclearity of the cells. The nuclear origin of the dips was confirmed after 6 h. The indentations contained solid structures the number, size, and shape of which were identical to the nucleoli seen by light microscopy. The spreading on the other supports was less uniform. Nuclear dips appeared more slowly and were less marked initially in their depths. The nuclear dipping was independent of cell density and took place under conditions under which the cells undergo phenotypic changes during culture. Individual phenotypic changes occur at different times and rates so that the initial signal for their onset cannot be determined with any certainty. However, the appearance of the dips was accompanied by DNA synthesis in the normally quiescent cells. The process stopped when the dipping was completed. The unavoidable change in nuclear morphology in spread cells may explain why maintenance of a spherical shape circumvents inappropriate DNA synthesis and maintains hepatocyte differentiation in vitro.

Ultrastructural changes in hypothermically preserved hepatocytes

Hypothermic preservation of hepatocytes on gelatin gels (10 degrees C) provided a stable system for investigating cold-induced changes culminating in cell death. Hepatocyte morphology remained unchanged during 9 days of preservation. Thereafter there was a progressive movement of organelles toward the center of the cell. During this process the mitochondria appeared to have a normal morphology suggesting that they are not the primary cause of the transition. When the movement was completed the mitochondria appeared aggregated and microvilli were no longer apparent on the cell surface.

Glucocorticoid receptors in ageing rats

The role of the glucocorticoid receptor (GR) in senescence was studied in rats of increasing age. Statistically significant changes in the number of GRs from rat liver were detected, whereas the affinity for the ligand triamcinolone acetonide (TA) did not change with increasing age, and was in the range of 1-2 nM. In all cases the number of receptors was lower in rats treated with hormone in vivo relative to untreated animals. In addition, we have found changes in GR activation, as measured by the binding to DNA cellulose in the mentioned age groups. Furthermore, expression of the glucocorticoid hormone (GH)-inducible gene, tyrosine amino transferase (TAT) also showed age-related alterations. We conclude that receptor function shows oscillatory changes during ageing. In addition, response to GH generally declines towards the older age. This specific periodicity in functional characteristics of the GR may reconcile conflicting results about the receptor number and properties during the ageing process, and marks particular age at which individual organism shows the highest or the lowest sensitivity to the actions of GH.

Fourth-generation model for corticosteroid pharmacodynamics: a model for methylprednisolone effects on receptor/gene-mediated glucocorticoid receptor down-regulation and tyrosine aminotransferase induction in rat liver

A fourth-generation pharmacokinetic/pharmacodynamic (PK/PD) model for receptor/genemediated effects of corticosteroids was developed. Male adrenalectomized Wistar rats received a 50 mg/kg i.v. bolus dose of methylprednisolone (MPL). Plasma concentrations of MPL, hepatic glucocorticoid receptor (GR) messenger RNA (mRNA) and GR density, tyrosine amino-transferase (TAT) mRNA, and TAT activity in liver were determined at various time points up to 72 hr after MPL dosing. Down-regulation of GR mRNA and GR density were observed: GR mRNA level declined to 45-50% of the baseline in 8-10 hr, and slowly returned to predose level in about 3 days; GR density fell to 0 soon after dosing and returned to the baseline in two phases. The first phase, occurring in the first 10 hr, entailed recovery from 0 to 30%. The second phase was parallel to the GR mRNA recovery phase. Two indirect response models were applied for GR mRNA dynamics regulated by activated steroid-receptor complex. A full PK/PD model for GR mRNA/GR down-regulation was proposed, including GR recycling theory. TAT mRNA began to increase at about 1.5 hr, reached the maximum at about 5.5 hr, and declined to the baseline at about 14 hr after MPL dosing. TAT induction followed a similar pattern with a delay of about 1-2 hr. A transcription compartment was applied as one of the cascade events leading to TAT mRNA and TAT induction. Pharmacodynamic parameters were obtained by fitting seven differential equations piecewise using the maximum likelihood method in the ADAPT II program. This model can describe GR down-regulation and the precursor/product relationship between TAT mRNA and TAT in receptor/gene-mediated corticosteroid effects.

Protein degradation in kidney proximal tubule cell monolayers

Isolated proximal tubule cells have been labelled with L-[4,5-3H]leucine prior to cell division. Histochemical staining demonstrated the purity of the cultures. The bicarbonate ion or a collagen support was required for cell growth. Different culture growth rates were established by varying these parameters. The proximal tubule marker enzyme, gamma-glutamyl transpeptidase, was expressed throughout the culture period (7-10 days) and the cells undergo a glycolytic shift, shown by an increase in the levels of lactate dehydrogenase. The specific activities of these enzymes were related to the growth conditions. Exponential rates of protein degradation were observed. The uptake of labelled exogenous hepatocyte proteins in proximal tubule cell cultures was completely suppressed in the presence of serum (10%, v/v) showing that endocytosis did not contribute to the observed measurements of intracellular protein degradation. The increased growth rates seen in cultures were accompanied by decreased rates of protein degradation. Use of the inhibitors of proteolysis, leupeptin and ammonium chloride, showed that the decrease was at the lysosomal level. The results suggest that targeting of inhibitors of lysosomal proteolysis, via low-molecular-weight proteins, may be useful in stimulating tubular regeneration in kidney disease.

Selective inhibition of the synthesis of phosphoenolpyruvate carboxykinase in freshly isolated proximal tubule cells

The behaviour of renal phosphoenolpyruvate carboxykinase (PEPCK) in non-growing kidney cell suspension culture was investigated. In heterogeneous kidney cells, glomeruli-proximal tubule mixtures and purified proximal tubule fragments, the enzyme activity fell with a t1/2 of 3.3 h. Hormonal additions did not modify the process. The fall was also observed in cells prepared from animals preinduced with dexamethasone. The in vitro response of PEPCK is tissue specific. The decline was not the result of a decrease in cell viability, enzyme leakage nor due to the mitochondrial translocation of cytosolic PEPCK. The response observed is due to the selective inhibition of the synthesis of the enzyme and its destruction by non-lysosomal mechanisms. Acute and chronic acidification do not affect PEPCK activity. However, the degradation of PEPCK is temperature dependent and is greatly accelerated by glutathione.

Hypophysectomy does not alter the acinar zonation of gluconeogenesis or the mitochondrial redox state in rat liver

The biochemical and functional heterogeneity of hepatocytes in different zones of the liver acinus may be related to the concentrations of hormones within the liver acinus. We examined the effects of hypophysectomy, which causes marked changes in plasma hormone levels and in activities of hepatic enzymes that are normally heterogeneously distributed, on the degree of metabolic zonation within the liver acinus. In hypophysectomized rats the activity of alanine aminotransferase was increased, but its normal zonation (predominance in the periportal zone) was preserved. The activity in cultured periportal and perivenous hepatocytes was increased by dexamethasone, but not by glucagon. Periportal hepatocytes from hypophysectomized rats expressed higher rates of gluconeogenesis in culture than did perivenous hepatocytes, irrespective of the absence or presence of dexamethasone, glucagon or insulin. Similar differences in rates of ketogenesis and in the mitochondrial redox state in response to glucagon were observed between periportal and perivenous hepatocytes from hypophysectomized rats as between cell populations from normal rats. Although hypophysectomy causes marked changes in hepatic enzyme activities, it does not alter the degree of zonation of alanine aminotransferase, gluconeogenesis or the mitochondrial redox state within the liver acinus.

Effect of antibiotics on the binding of pargyline to monoamine oxidase in cultured hepatocytes

The laser dye rhodamine 123 has been used to establish that the binding of [3H]pargyline to monoamine oxidase is a more sensitive indicator of mitochondrial perturbation than measurements of protein synthesis, secretion, or degradation. The amount of monoamine oxidase labelled depends on the antibiotic used. The labelling was considerably lower in the presence of gentamycin than in the presence of either chloramphenicol or of penicillin and streptomycin. The accumulation of gentamycin within the cells was the cause of the reduced labelling of monoamine oxidase which was not accompanied by an alteration in the metabolism of pargyline. The gentamycin effect can be prevented by incubating the cells in medium supplemented with methylamine prior to adding the monoamine oxidase inhibitor. Long term culture of cells with gentamycin can result in the cell culture medium becoming dark brown. Under these conditions the monoamine oxidase labelling is increased due to an inhibition of pargyline metabolism. The results indicate that the choice of antibiotic is important in patients being treated with both antibiotics and monoamine oxidase inhibitors.

Fate of the monoamine oxidase inhibitor pargyline in cultured hepatocytes

The interaction of the monoamine oxidase inhibitor pargyline with cultured hepatocytes has been studied. [Phenyl-3, benzyl-3H] pargyline (38 nM) rapidly enters the cells and a plateau of incorporation into a trichloroacetic acid insoluble form (monoamine oxidase) is reached after 2 hr. The level of labelling is lower in freshly isolated cells than in those in later culture. The maximum incorporation accounts for only 6% of the added radioactivity and produces a 9% inhibition of monoamine oxidase activity. The remaining [3H] pargyline is metabolized and quickly accumulates in the cell culture medium in a form which cannot label exogenous mitochondria. The metabolism of pargyline varies both qualitatively and quantitatively with culture age. In 0 hr and 20 hr-cultured cells one metabolite preferentially appears whilst in 140 hr cultured hepatocytes at least three metabolites are formed. The metabolism of [3H] pargyline in early culture is consistent with a cytochrome P-450 involvement. The use of [3H] pargyline to label monoamine oxidase in cultured hepatocytes offers several attractive features for studying the turnover of this enzyme. These include speed of interaction, non-reutilization, application to normal cells, controlled inhibition of monoamine oxidase and metabolism of non-specific label.

Effect of tyrosine administration on dopa accumulation in light- and dark-adapted retinas from normal and diabetic rats

The interaction of tyrosine concentration and lighting on in vivo dihydroxyphenylalanine (dopa) accumulation rate was studied in retinas of normal and diabetic rats. In both groups of rats, dopa accumulation and in vitro hydroxylase activity were higher in retinas exposed to light than in those adapted to darkness. In light-adapted diabetic rats, though, retinal tyrosine level, dopa accumulation, and in vitro tyrosine hydroxylase activity were all below normal. In both normal and diabetic rats exposed to light, tyrosine injection raised retinal tyrosine concentrations and stimulated dopa accumulation. Injection of tyrosine into dark-adapted rats raised retinal tyrosine level but did not enhance dopa accumulation. Together, these results suggest that in vivo retinal amacrine cells will vary their dopa accumulation rate as a function of substrate supply, but only in the light, when tyrosine hydroxylase is activated. They further indicate that dopa accumulation rate remains sensitive to tyrosine supply in the light-activated diabetic retina.

The synergistic interaction of hydrocortisone and dibutyryl cyclic AMP during enzyme induction in hybrids between rat C6 glioma cells and FU5AH hepatoma cells

The hormone-responsive enzymes tyrosine aminotransferase and glycerol-3-phosphate dehydrogenase were studied with respect to current models of the mechanism of glucocorticoid/cAMP interaction during the induction of enzyme activity in responsive cell hybrids between rat C6 glioma cells and rat FU5AH hepatoma cells. The results of experiments involving protein and mRNA synthesis inhibitors, sequential addition of inducers, and the assay of cyclic-AMP-dependent protein kinase could not be adequately explained by any one model of inducer interaction. Comparison of the hybrid clones revealed the presence of factors that may modify induction but that are not essential for synergistic induction.

Effects of cyclic AMP, glucocorticoids and insulin on the activities of phosphatidate phosphohydrolase, tyrosine aminotransferase and glycerol kinase in isolated rat hepatocytes in relation to the control of triacylglycerol synthesis and gluconeogenesis

Rat hepatocytes were incubated in monolayer culture in modified Leibovitz L-15 medium containing either 10% (v/v) newborn-calf serum or 0.2% (w/v) fatty-acid-poor bovine serum albumin. The addition of 100 nM-dexamethasone increased the activities of both phosphatidate phosphohydrolase and tyrosine aminotransferase by about 3.5-fold after 8h, and these activities continued to rise until at least 24h. Incubating the hepatocytes in the albumin-containing medium with 10 microM- or 100 microM-8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate increased the activities of the phosphohydrolase and aminotransferase by 2.6- and 3.4-fold respectively after 8h. These increases were blocked by actinomycin D. The increases in the activities that were produced by the cyclic AMP analogue and dexamethasone were independent and approximately additive. Insulin when added alone did not alter the phosphohydrolase activity, but it increased the aminotransferase activity by 34%. The dexamethasone-induced increase in the phosphohydrolase activity was completely blocked by 7-144 microM-insulin, whereas that of the aminotransferase was only partly suppressed. Insulin had no significant Effects on the increases in the activities of phosphatidate phosphohydrolase and tyrosine aminotransferase that were produced by the cyclic AMP analogue, but this may be because the analogue is fairly resistant to degradation by the phosphodiesterase. The activity of glycerol kinase was not significantly changed by incubating the hepatocytes with insulin, dexamethasone and the cyclic AMP analogue alone or in combinations. It is proposed that high concentrations of cyclic AMP and glucocorticoids increase the total activity of phosphatidate phosphohydrolase in the liver and provide it with an increased capacity for synthesizing triacylglycerols and very-low-density lipoproteins, which is expressed when the availability of fatty acids is high. There appears to be a co-ordinated hormonal control of triacyglycerol synthesis and gluconeogenesis in diabetes and in metabolic stress to enable the liver to supply other organs with energy.

Oleic acid promotes the activation and translocation of phosphatidate phosphohydrolase from the cytosol to particulate fractions of isolated rat hepatocytes

The incubation of hepatocytes with 1-4mM-oleate increased the total activity of phosphatidate phosphohydrolase that was measured in the presence of Mg2+ to about 2-fold. This was accompanied by an increase in the proportion of the enzyme that was isolated with the particulate fractions. Conversely, the addition of up to 4mM-oleate decreased the recovery of phosphatidate phosphohydrolase in the cytosolic fraction from about 70% to 3% when hepatocytes were lysed with digitonin. Most of the increase in the membrane-associated phosphohydrolase activity was isolated after cell fractionation in the microsomal fraction that was enriched with the endoplasmic-reticulum marker arylesterase. It is proposed that the translocation of phosphatidate phosphohydrolase facilitates the increased synthesis of triacylglycerols in the liver when it is presented with an increased supply of fatty acids.

Comparison of the degradative fate of monoamine oxidase in endogenous and transplanted mitochondrial outer membrane in rat hepatocytes. Implications for the cytomorphological basis of protein catabolism

The degradative fate of monoamine oxidase in endogenous and transplanted mitochondrial outer membrane has been compared in rat hepatocyte monolayers. Monoamine oxidase was specifically irreversibly radiolabelled by the suicide inhibitor [3H]pargyline. Hepatocyte monolayers were cultured in conditions in which rates of protein catabolism like those in vivo are maintained [Evans & Mayer (1983) Biochem. J. 216, 151-161]. Incubation of hepatocyte monolayers for 17 h with [3H]pargyline specifically radiolabels mitochondrial monoamine oxidase, as shown by Percoll-gradient fractionation of broken hepatocytes. Monoamine oxidase is degraded at a similar rate to that observed in liver in vivo (t1/2 approx. 63 h). The effects of leupeptin, methylamine and colchicine on the degradation of endogenous radiolabelled enzyme has been studied over prolonged culture periods. Culture of hepatocytes for periods of up to 80 h with inhibitors was not cytotoxic, as demonstrated by measurements of several intrinsic biochemical parameters. Leupeptin, methylamine and colchicine inhibit the degradation of endogenous monoamine oxidase by 60, 38 and 18% respectively. Monoamine oxidase in mitochondrial-outer-membrane vesicles introduced into hepatocytes by poly(ethylene glycol)-mediated vesicle-cell transplantation is degraded at a similar rate (t1/2 55 h) to the endogenous mitochondrial enzyme. Whereas leupeptin inhibits the degradation of endogenous and transplanted enzyme to a similar extent, methylamine and colchicine inhibit the degradation of transplanted enzyme to a much greater extent (85 and 56% respectively). Fluorescence microscopy (with fluorescein isothiocyanate-conjugated mitochondrial outer membrane) shows that transplanted mitochondrial outer membrane undergoes internalization and translocation to a sided perinuclear site, as observed previously with whole mitochondria [Evans & Mayer (1983) Biochem. J. 216, 151-161]. The effects of the inhibitors on the distribution of transplanted membrane material in the cell and inhibition of proteolysis show the importance of cytomorphology for intracellular protein catabolism.

Synergistic enzyme induction by glucocorticoids and cyclic AMP observed in glioma x hepatoma cell hybrids but not in their parents

Enzyme induction by hydrocortisone (HC) and dibutyryl cyclic AMP (dbcAMP) was studied in C6 rat glioma cells, FU5AH rat hepatoma cells, and five C6 x FU5AH hybrids. Hormone responsive enzymes from both parental lines were studied, including: tyrosine aminotransferase (TAT), alanine aminotransferase (AAT), glycerol phosphate dehydrogenase (GPDH), lactate dehydrogenase (LDH), and 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP). There was no overall dominance of one parental phenotype over the other in expression of uninduced or induced enzyme activity after fusion, and the hybrids possessed some enzymatic properties characteristic of both parents. GPDH was induced by dbcAMP in all five hybrids, and TAT was induced by dbcAMP in four of the hybrids, although neither of these enzymes were induced by dbcAMP in the parents. Furthermore, synergistic induction of these enzymes by HC and dbcAMP was observed in the hybrids but not in the parents. These hybrids provide a model system to study hormone interaction in enzyme induction.

Degradation of transplanted mitochondrial proteins by hepatocyte monolayers

Reductively [3H]methylated rat mitochondria and mitochondrial-outer-membrane vesicles and mitochondrial-outer-membrane vesicles where monoamine oxidase is irreversibly labelled by [3H]pargyline have been transplanted into hepatocytes by poly(ethylene glycol)-mediated organelle or organelle-vesicle cell fusion. During subsequent culture of hepatocyte monolayers for 4-5 days, under conditions which mimic endogenous catabolic rates in vivo the transplanted organelle proteins retain their degradation characteristics observed in vivo (e.g. mitochondria: average t 1/2 72.5 h; monoamine oxidase: t1/2 55 h). In all cases protein degradation with first-order kinetics is only observed after an initial lag period (i.e. 24-30 h after fusion). Transplantation of fluorescein-conjugated organelles showed that the fluorescent material is rapidly internalized (average t1/2 1-6 h) and uniformly distributed in the cytoplasm. During a subsequent 18-24 h period (which corresponds to the lag period for intracellular destruction of transplanted mitochondrial material) the transplanted material is translocated to assume a perinuclear distribution. The destruction of transplanted mitochondrial proteins is compared with endogenous mitoribosomally synthesized proteins (average t1/2 52.5 h). Percoll fractionation of cell homogenates containing transplanted mitochondrial outer membranes where the enzyme monoamine oxidase is irreversibly labelled with [3H]pargyline shows a distribution of enzyme similar to lysosomal acid phosphatase. After transplantation of reductively methylated 3H-labelled mitochondrial-outer-membrane vesicles the cells were treated with leupeptin to alter lysosomal density. This treatment leads to the predominant association of acid phosphatase with dense structures, whereas the 3H-labelled transplanted material predominantly does not change density. Therefore transplanted mitochondrial-outer-membrane proteins are found in intracellular vesicular structures from which the proteins are donated for destruction, at least in part, by a lysosomal mechanism.