NADH-activated cell-free transfer between Golgi apparatus and plasma membranes of rat liver.

This report concerns development of a cell-free system from rat liver to study transport of membrane constituents from the Golgi apparatus to the plasma membrane. Highly purified Golgi apparatus as donor and a mixture of sheets and vesicles as plasma membrane acceptor fractions were combined to analyze requirements for lipid and protein transport. In the reconstituted system, the Golgi apparatus donor was in suspension. To measure transfer, membrane constituents of the donor membranes were radiolabeled with [3H]acetate (lipids) or [3H]leucine (proteins). The plasma membrane vesicles were used as the acceptor and were unlabeled and immobilized on nitrocellulose for ease of recovery and analysis. The reconstituted cell-free transfer was dependent on temperature, but even at 37 degrees C, the amount of transfer did not increase with added ATP, was not specific for any particular membrane fraction or subfraction nor was it facilitated by cytosol. ATP was without effect both in the presence or absence of a cytosolic fraction capable of the support of cell-free transfer in other systems. In contrast to results with ATP, NADH added to the reconstituted system resulted in an increased amount of transfer. A further increase in transfer was obtained with NADH plus a mixture of ascorbate and dehydroascorbate to generate ascorbate free radical. The transfer of labeled membrane constituents from the Golgi apparatus to the plasma membrane supported by NADH plus ascorbate radical was stimulated by a cytosol fraction enriched in less than 10 kDa components. This was without effect in the absence of NADH/ascorbate radical or with ATP as the energy source. Specific transfer was inhibited by both N-ethylmaleimide and GTP gamma S. The findings point to the possibility of redox activities associated with the trans region of the Golgi apparatus as potentially involved in the transport of membrane vesicles from the Golgi apparatus to the cytoplasmic surface of the plasma membrane.     

Requirement for guanosine triphosphate for cholera-toxin-catalysed incorporation of adenosine diphosphate ribose into rat liver plasma membranes and for activation of adenylate cyclase.

Sphingomyelin in mixed dispersion with bile salts was hydrolysed by the solubilized sphingomyelinase of rat brain lysosomes. In parallel studies, physical properties of these dispersions were determined. The kinetic curves that described the rate of hydrolysis as a function of increasing concentrations of bile salt were multiphasic. A region of very low activity was followed by an ascending portion, a peak, a descending portion, a trough and a second ascending portion. The positions of the initiation points, peaks and troughs were found to be a function of the respective ratios of the bile salt to sphingomyelin for the detergent sodium taurodeoxycholate, but of the absolute concentration of the detergent for sodium taurocholate. Turbidity studies suggested that hydrolysis of sphingomyelin begins at a bile salt concentration that solubilizes the lipid and incorporates it into a mixed micelle with the detergent. Ultracentrifugation studies suggested that the sizes of the mixed aggregates of detergent and lipid were a function of the ratio of taurodeoxycholate to sphingomyelin, but of the absolute concentration of the bile salt, for sodium taurocholate.     

Phosphatidylcholine breakdown in rat liver plasma membranes. Roles of guanine nucleotides and P2-purinergic agonists

Release of P-choline and choline from purified rat plasma membrane preparations was increased by GTP and its less hydrolyzable analogues, whereas other nucleotide triphosphates had little or no effect. Stimulation by guanosine 5'-(3-O-thiol)triphosphate (GTP gamma S) was dependent upon magnesium, inhibited by guanosine 5'-(2-O-thiol)diphosphate, and independent of calcium. ATP and ADP (1-100 microM) markedly enhanced the GTP gamma S stimulation of P-choline plus choline release but had no effect alone. ADP was as effective as ATP and nonhydrolyzable ATP analogues produced a similar or greater stimulation, whereas AMP and adenosine were much less effective. Vasopressin (0.1 microM) also produced a small stimulation. Under conditions in which protein kinase C was activated, PMA also stimulated the response to GTP gamma S but was ineffective in its absence. P-choline was the initial product which was hydrolyzed to choline. Guanine nucleotide and purinergic effects were also apparent on phosphatidylcholine degradation. EGTA, at 0.5 mM, completely removed purinergic stimulation but did not affect P-choline plus choline released in response to GTP gamma S alone. Prior treatment of plasma membranes with cholera toxin or prior injection of animals with islet-activating protein did not affect the stimulation of P-choline plus choline release either by GTP gamma S alone or by GTP gamma S plus ATP. These results indicate that a phosphatidylcholine phospholipase C is coupled to purinergic receptors in rat liver plasma membranes by a GTP-binding protein. Hydrolysis of phosphatidylcholine could contribute to hepatic diacylglycerol levels and thus influence protein kinase C activity.     

Effect of serum lipoproteins on the adenylate cyclase activity of rat liver plasma membranes.

Four rat lipoprotein classes [lymph chylomicrons, VLD (very-low-density), LD (low-density) and HD (high-density) lipoproteins] were tested for their ability to affect basal adenylate cyclase (EC 4.6.1.1) activity of rat liver plasma membranes. All the lipoproteins, with the exception of lymph chylomicrons, effectively increase the enzyme activity. VLD lipoproteins are the most active class (67% maximal increase), followed by HD lipoproteins (33%) and LD lipoproteins (23%). The effect of VLD lipoproteins is additive to that elicited by GTP or GTP plus glucagon (at least within a certain concentration range). VLD lipoproteins affect only the Vmax. of the enzyme, not the Km.     

Sphingomyelin and ceramide-phosphoethanolamine synthesis by microsomes and plasma membranes from rat liver and brain

Pulse-chase experiments showed that phosphatidylethanolamine (PE) was the direct precursor for ceramide-phosphoethanolamine, a sphingomyelin analogue, in the same way as phosphatidylcholine was for sphingomyelin. Ceramide-phosphoethanolamine could be identified by incorporation of radioactivity from labeled PE, as well as by its stability in alkaline methanolysis and its ability to be methylated by S-adenosyl-methionine. Ceramide-phosphoethanolamine synthesis from labeled exogenous PE seemed to be independent of exogenous ceramide; it was proportional to the amount of incubated membrane, when taking into account the isotopic dilution of labeled precursor by endogenous PE. Sphingomyelin synthesis, which was demonstrated using natural PC as a substrate, was not possible using dipalmitoyl-PC. The formation of sphingomyelin and ceramide-phosphoethanolamine was demonstrated in microsomes and plasma membranes from rat brain and liver.     

Analytical study of microsomes and isolated subcellular membranes from rat liver VIII. Subfractionation of preparations enriched with plasma membranes, outer mitochondrial membranes, or Golgi complex membranes

Preparations enriched with plasmalemmal, outer mitochondrial, or Golgi complex membranes from rat liver were subfractionated by isopycnic centrifugation, without or after treatment with digitonin, to establish the subcellular distribution of a variety of enzymes. The typical plasmalemmal enzymes 5'-nucleotidase, alkaline phosphodiesterase I, and alkaline phosphatase were markedly shifted by digitonin toward higher densities in all three preparations. Three glycosyltransferases, highly purified in the Golgi fraction, were moderately shifted by digitonin in both this Golgi complex preparation and the microsomal fraction. The outer mitochondrial membrane marker, monoamine oxidase, was not affected by digitonin in the outer mitochondrial membrane marker, monoamine oxidase, was not affected by digitonin in the out mitochondrial membrane preparation, in agreement wit its behavior in microsomes. With the exception of NADH cytochrome c reductase (which was concentrated in the outer mitochondrial membrane preparation), typical microsomal enzymes (glucose-6-phosphatase, esterase, and NADPH cytochrome c reductase) displayed low specific activities in the three preparations; except for part of the glucose-6-phosphatase activity in the plasma membrane preparation, their density distributions were insensitive to digitonin, as they were in microsomes. The influence of digitonin on equilibrium densities was correlated with its morphological effects. Digitonin induced pseudofenestrations in plasma membranes. In Golgi and outer mitochondrial membrane preparations, a few similarly altered membranes were detected in subfractions enriched with 5'-nucleotidase and alkaline phosphodiesterase I. The alterations of Golgi membranes were less obvious and seemingly restricted to some elements in the Golgi preparation. No morphological modification was detected in digitonin-treated outer mitochondrial membranes. These results indicate that each enzyme is associated with the same membrane entity in all membrane preparations and support the view that there is little overlap in the enzymatic equipment of the various types of cytomembranes.     

Effect of Cerium on liver lipids metabolism and plasma lipoproteins synthesis in the rat

After a single injection of Cerium chloride to female rats, liver triglycerides concentration increases sharply and plasma lipids decrease to about one half the initial level. In these conditions, the respiratory quotient of treated rats $\text{in}\text{vivo}$ decreases by 20% indicating a lowered fatty acid oxidation. Incorporation of [3H]-oleate into liver lipids shows an important accumulation of newly synthetized triglycerides without any significant effect on phospholipids. Lipoproteins production estimated by [3H]-oleate and [14C]-leucine incorporation into plasma very low-, low- and high-density lipoproteins shows a 50% inhibition synthesis of lipoprotein.     

The synthesis of apoproteins of very low density lipoproteins isolated from the Golgi apparatus of rat liver.

The incorporation of [3H]leucine in vivo into very low density lipoproteins (VLDL) from the rat hepatic Golgi apparatus and serum was studied. A Golgi-rich fraction isolated on a discontinuous sucrose gradient between 0.5 and 1.1 M was found to contain VLDL having common antigenic determinants with serum VLDL. The incorporation of the [3H]leucine into the Golgi VLDL and serum VLDL suggested a precursor-product relationship. Analysis of the apoproteins of the Golgi VLDL by polacrylamide gel electrophoresis revealed protein bands with similar mobility to those of serum VLDL, except that the former contained virtually no rapidly migrating peptides with the mobility of serum apo-C-II and apo-C-III. The pattern of incorporation of the [3H]leucine into the apoproteins was similar in VLDL from Golgi apparatus and serum, except for the absence of radioactivity in the area of the gel of Golgi apo-VLDL corresponding to apo-C-II and apo-C-III. The radioactive amino acid was incorporated predominantly into the Golgi apo-VLDL bands with similar mobility to apo-B and an apoprotein or group of apoproteins containing the arginine-rich peptide of serum VLDL. In vitro incubation of the Golgi VLDL with [3H]leucine-labeled HDL resulted in the acquisition of a number of proteins, including the rapidly migrating proteins. Administration of colchicine prior to the injection of [3H]leucine resulted in the appearance of gel bands and radioactivity in the apo-C-II and apo-C-III areas of Golgi apo-VLDL, suggesting that these can be acquired if secretion of VLDL is slowed or inhibited. The hepatic Golgi apparatus was then divided into fractions of predominantly forming face (GF3) or secretory granules (GF1). After polyacrylamide gel electrophoresis of the apo-VLDL from GF, no visible bands or incorporation of [3H]leucine was found in the region of apo-C-II or apo-C-III. However VLDL from GF1, showed visible and radioactive bands in the apo-C-II and apo-C-III area although they represented a much smaller proportion of the total apoprotein than was found in the corresponding serum apo-VLDL. In the isolated perfused liver the percentage incorporation of [3H]leucine into the rapidly migrating apoproteins of Golgi VLDL was considerably less than that found in the corresponding apoproteins of perfusate VLDL, where circulating C lipoproteins are virtually absent. The data indicate that nascent VLDL begins to acquire the C-II and C-III apoproteins during its passage through the Golgi apparatus but that the main acquisition occurs during or after secretion into the space of Disse.     

Identification of lipoprotein-binding proteins in rat liver Golgi apparatus membranes

The low density lipoprotein (LDL) receptor has been shown to be a plasma membrane glycoprotein responsible for the cellular binding and endocytosis of plasma lipoproteins. Inasmuch as the Golgi apparatus has been shown to participate in glycoprotein processing and in the assembly of plasma lipoproteins by hepatic and intestinal epithelial cells, the present studies were designed to test the hypothesis that lipoprotein receptors are present within Golgi membranes. Utilizing ligand blotting with a variety of iodinated lipoproteins, several lipoprotein-binding proteins were identified in rat liver Golgi membranes at apparent molecular weights (Mr) 200,000, 160,000, 130,000, 120,000, 100,000, 80,000, and 70,000. The 130,000 protein was the most prominent and was identified as the mature LDL receptor by its binding characteristics and an Mr characteristic of the plasma membrane receptor. Enzymatic deglycosylation studies suggested that the 120,000 and 100,000 proteins were LDL receptor precursors lacking sialic acid. Antibody to the LDL receptor recognized all the bands on immunoblots except the 70,000 protein, with the 130,000 protein being the most prominent. Isolation of the Golgi fractions in the presence of protease inhibitors did not eliminate any of the proteins recognized by the antibody but did result in sharper bands on the blots. Additionally, we investigated the hypothesis that conditions that regulate plasma membrane LDL receptors also cause detectable changes in receptors in Golgi membranes. All the binding proteins were increased in Golgi membranes from rats treated with 17-alpha-ethynylestradiol. Colchicine caused an accumulation of 120,000 Mr protein, suggesting blockage of final sialylation in the trans Golgi. When protein synthesis was inhibited by cycloheximide, there was no reduction of mature LDL receptors in Golgi membranes, consistent with recycling of receptors through this organelle.     

Synchrony between DNA synthesis and thymidine incorporation into DNA in regenerating rat liver

DNA synthesis in regenerating liver was studied to determine whether the onset of stimulated DNA synthesis preceded the onset of increased incorporation of thymidine into DNA. Thymidine incorporation into hepatic DNA was not stimulated 15 h after operation, but was stimulated after 18 h; peak stimulation occurred 30 h after operation. Thymidine kinase activity was stimulated 24 h after operation; highest kinase activity was observed at 36 h. The onset of stimulated DNA synthesis was estimated by following the incorporation of labeled aspartic acid, sodium formate, adenine or orotic acid into appropriate DNA bases, viz., thymine, adenine, adenine or cytosine, respectively. Incorporation of adenine and orotic acid was stimulated between 15 h and 18 h after operation; incorporation of aspartic acid and sodium formate was stimulated between 18 h and 21 h after operation.The incorporation of thymidine into DNA was accelerated by stress stimulus and was inhibited by hydrocortisone. Changes in thymidine kinase activity also were correspondingly accelerated or delayed. Incorporation of labeled thymidine, adenine, formate, orotic acid or thymine into appropriate DNA bases, viz., thymine, adenine, adenine, cytosine or thymine, respectively, was stimulated by stress stimulus or was inhibited by hydrocortisone.It was concluded from these data that stimulation of DNA synthesis and of thymidine incorporation into DNA was essentially synchronized in regenerating rat liver. Results from this study were compared with results from similar studies in 2 other tissues, and the limitations, attendant with using thymidine incorporation into DNA as an indicator of stimulated DNA synthesis, were discussed.