Metabolic effects of bacitracin in isolated rat hepatocytes

Lack of metabolic effects of cholecystokinin on hepatocytes

We previously reported that the liver was the major organ that extracts small, biologically active, circulating forms of cholecystokinin. Although our work indicated extensive degradation of cholecystokinin extracted from plasma during its transit across the hepatocyte, it was unclear whether cholecystokinin might also have a physiological effect on this cell before its intracellular degradation. Therefore we tested the hypothesis that cholecystokinin has a direct biological effect on hepatocytes. Using freshly isolated or cultured hepatocytes, we studied whether cholecystokinin-octapeptide alters protein synthesis, affects amino acid transport or influences cytosolic free calcium concentrations. Using liver slices, we also determined the effect of cholecystokinin-octapeptide on cyclic nucleotide levels. Cholecystokinin-octapeptide, up to a concentration of 1 mumol/L, had no effect on the incorporation of radiolabeled amino acids into total hepatocyte protein; in contrast, comparable molar amounts of insulin stimulated protein synthesis by as much as 37% (ED50 = 1.5 x 10(-10) mol/L). Although insulin and glucagon stimulated the transport into hepatocytes of 14C-alpha-aminoisobutyric acid, a nonmetabolizable amino acid analog, cholecystokinin-octapeptide had no affect Cholecystokinin-octapeptide also did not affect either the concentration of calcium in individual hepatocytes, as measured by digitized video microscopy using Fura-2, or the levels of cyclic AMP or cyclic GMP in liver slices. Our results show that cholecystokinin has no effect on protein synthesis, on amino acid transport or on hepatocyte calcium and cyclic nucleotide levels. These and our previous data suggest that the primary outcome of hepatic extraction of cholecystokinin is hormone degradation.

Bacitracin and a bacitracin-zinc complex damage DNA and carbohydrate in the presence of iron and copper salts

The antibiotic bacitracin binds metal ions such as zinc, copper and iron, and is often prepared for clinical and veterinary use as a zinc complex to enhance stability. Here, we show that bacitracin and its zinc complex are able to reduce copper and iron salts with the formation of hydrogen peroxide. This results in damage to the bacitracin molecule and the release of some oxidising radicals into free solution that attack detector molecules such as deoxyribose and DNA. Damage to deoxyribose and DNA can be detected by the formation of thiobarbituric acid-reactive material and this damage can be protected against by the addition of scavengers of the hydroxyl radical implicating the OH radical in the degradation observed.

Regulation of flux through pyruvate dehydrogenase and pyruvate carboxylase in rat hepatocytes. Effects of fatty acids and glucagon

The regulation of flux through pyruvate dehydrogenase (PDH) and pyruvate carboxylase (PC) by fatty acids and glucagon was studied in situ, in intact hepatocyte suspensions. The rate of pyruvate metabolized by carboxylation plus decarboxylation was determined from the incorporation of [1-14C]pyruvate into 14CO2 plus [14C]glucose. The flux through PDH was determined from the rate of formation of 14CO2 from [1-14C]pyruvate corrected for other decarboxylation reactions (citrate cycle, phosphoenolpyruvate carboxykinase and malic enzyme), and the flux through PC was determined by subtracting the flux through PDH from the total pyruvate metabolized. With 0.5 mM pyruvate as substrate the ratio of flux through PDH/PC was 1.9 in hepatocytes from fed rats and 1.4 in hepatocytes from 24 h-starved rats. In hepatocytes from fed rats, octanoate (0.8 mM) and palmitate (0.5 mM) increased the flux through PDH (59-76%) and PC (80-83%) without altering the PDH/PC flux ratios. Glucagon did not affect the flux through PDH but it increased the flux through PC twofold, thereby decreasing the PDH/PC flux ratio to the value of hepatocytes from starved rats. In hepatocytes from starved rats, fatty acids had similar effects on pyruvate metabolism as in hepatocytes from fed rats, however glucagon did not increase the flux through PC. 2[5(4-Chlorophenyl)pentyl]oxirane-2-carboxylate (100 microM) an inhibitor of carnitine palmitoyl transferase I, reversed the palmitate-stimulated but not the octanoate-stimulated flux through PDH, in cells from fed rats, indicating that the effects of fatty acids on PDH are secondary to the beta-oxidation of fatty acids. This inhibitor also reversed the stimulatory effect of palmitate on PC and partially inhibited the flux through PC in the presence of octanoate suggesting an effect of POCA independent of fatty acid oxidation. It is concluded that the effects of fatty acids on pyruvate metabolism are probably secondary to increased pyruvate uptake by mitochondria in exchange for acetoacetate. Glucagon favours the partitioning of pyruvate towards carboxylation, by increasing the flux through pyruvate carboxylase, without directly inhibiting the flux through PDH.

Effects of 2[5(4-chlorphenyl)pentyl]oxirane-2-carboxylate on fatty acid synthesis and fatty acid oxidation in isolated rat hepatocytes

The effects of the hypoketonaemic and hypoglycaemic compound 2[5(4-chlorophenyl)pentyl]oxirane-2-carboxylate (POCA) on fatty acid synthesis and fatty acid oxidation in rat hepatocytes were examined. Two microM-POCA caused a small stimulation of fatty acid synthesis which might be due to an increased flux through pyruvate dehydrogenase. Ten to one hundred microM-POCA inhibited (40-70%) fatty acid synthesis. At low concentrations (less than or equal to 5 microM) POCA was a more powerful inhibitor of fatty acid oxidation than of synthesis, but at higher concentrations (10-100 microM) the inhibition of synthesis and oxidation was similar. One hundred microM POCA-CoA inhibited acetyl-CoA carboxylase by about 22% and 100 microM-palmitoyl-CoA by about 33%. Since POCA was a more potent inhibitor of fatty acid synthesis than palmitate, but POCA-CoA did not inhibit acetyl-CoA carboxylase more strongly than palmitoyl-CoA, it is suggested that POCA-CoA may inhibit fatty acid synthase directly.

Bacitracin enhances intracellular accumulation of insulin in rat adipocytes

Bacitracin (1 mg/ml) markedly increased (approx. 75%) the cell-associated specifically bound 125I-labelled insulin without altering the affinity of the binding sites. Bacitracin also exerted a modest inhibitory effect on the degradation of insulin in the incubation medium determined as radioactivity not precipitated by trichloroacetic acid (from 9.6 to 4.8%). The effect on insulin binding was about 5-times as sensitive as the effect on degradation. The increased binding was due to intracellular accumulation of radioactivity which could not be removed by treating the cells with trypsin. This increase was not seen when the internalization process was reduced by ATP-depletion or low temperature. Since the trypsin-sensitive fraction of cell-associated radioactivity was apparently not altered, it is suggested that bacitracin, in addition to its well-known inhibition of extracellular degradation, also inhibits the intracellular degradation of insulin.

Monolayer culture of parenchymal rat hepatocytes on collagen-coated microcarriers. A hepatocyte system for short- and long-term metabolic studies

A method is described for the attachment to and monolayer culture of adult rat hepatocytes on collagen-coated or fibronectin-coated microbeads or both in a chemically defined serum-free medium. Protein synthesis measured by the incorporation of [3H]leucine into protein was four-fold higher in the hepatocyte microcarrier cultures than in isolated hepatocyte suspensions. The hepatocyte microcarrier cultures showed acute responsiveness to insulin of fatty acid synthesis, glucose incorporation into glycogen, and decarboxylation of [1-14 C]pyruvate. Microcarrier-cultured hepatocytes have the combined advantages of monolayer culture and suspension systems. They are a potential tool for the study of long-term as well as acute effects of hormones.

The effect of inhibitors of insulin processing on generation of insulin intermediate products from human fibroblast as detected by high performance liquid chromatography (HPLC)

To assess the role of various modulators of insulin processing on cell-associated A14-125I-insulin intermediates in human fibroblasts, we have studied the effect of N-ethylmaleimide (NEM), chloroquine, bacitracin, dansylcadavarine, and phenylarsine oxide on generation of these intermediate products with the use of HPLC. NEM completely inhibited generation of intermediate peaks or iodotyrosine. Chloroquine inhibited conversion of A14-125I-insulin to iodotyrosine by about 75 percent and the remaining A14-125I-insulin was not susceptible to acid wash. Bacitracin, dansylcadavarine, and phenylarsine oxide, on the other hand, stimulated formation of intermediate products with concomitant inhibition of iodotyrosine formation. We conclude that there are at least three components of insulin degradation in human fibroblasts. These include the sulfhydryl group inhibitor-sensitive, the intracellular chloroquine-sensitive, and membrane site inhibitor-sensitive components.

Effect of insulin on ketogenesis and fatty acid synthesis in rat hepatocytes incubated with dichloroacetate

In parenchymal liver cells isolated from fed rats, insulin increased the formation of 14CO2 from [1-14C]pyruvate (and presumably the flux through pyruvate dehydrogenase) by 14%. Dichloroacetate, an activator of the pyruvate dehydrogenase complex, stimulated this process by 133%. As judged from the conversion of [2-14C]pyruvate to 14CO2, the tricarboxylic acid cycle activity was not affected by insulin, but it was depressed by dichloroacetate. In hepatocytes from fed rats, incubated with glucose as the only carbon source, dichloroacetate caused a stimulation (31%) of fatty acid synthesis, measured as 3H incorporation from 3H2O into fatty acid, and an increased (134%) accumulation of ketone bodies (acetoacetate + D-3-hydroxybutyrate). Dichloroacetate did not affect ketone body formation from [14C]palmitate, suggesting that the increased accumulation of ketone bodies resulted from acetyl-CoA derived from pyruvate. Insulin stimulated fatty acid synthesis in hepatocytes from fed rats. In the combined presence of insulin plus dichloroacetate, fatty acid synthesis was more rapid than in the presence of either insulin or dichloroacetate, whereas the accumulation of ketone bodies was smaller than in the presence of dichloroacetate alone. Although pyruvate dehydrogenase activity, which is rate-limiting for fatty acid synthesis in hepatocytes from fed rats, is stimulated both by insulin and by dichloroacetate, the reciprocal changes in fatty acid synthesis and ketone body accumulation brought about by insulin in the presence of dichloroacetate suggest that insulin is also involved in the regulation of fatty acid synthesis at a mitochondrial site after pyruvate dehydrogenase, possibly at the partitioning of acetyl-CoA between citrate and ketone body formation.

The binding of somatostatin to rat synaptosomal membranes: effects of detergent solubilisation and of bacitracin

Specific high and low affinity binding sites for somatostatin have been demonstrated in rat brain synaptosomal membranes using 125I [Tyr"] somatostatin tracer. When incubations were carried out in the presence of low concentrations of bacitracin (0.01% w/v), a 66% increase in association constant of the high affinity binding site to Ka = 1.71 X 10(10) M-1 and a 70% increase in the Ka of the low affinity site to 0.034 X 10(10) M-1 were observed; a similar (50%) increase in the binding capacity of the low affinity site but no increase in the binding capacity of the high affinity site was seen. These results may indicate a direct effect of bacitracin on the membrane that is distinct from its effect as an inhibitor of tracer degradation. The somatostatin-binding capacity of the membranes was solubilised by treatment with Triton X-100 and the apparent molecular weight of the receptor-ligand-detergent micelle complex was estimated by gel filtration to be approximately 250,000.

Role of hepatocyte plasma membrane in insulin degradation

An important role of the cell membrane in insulin degradation by cultured rat hepatocytes is supported by studies using the surface-active antibiotic bacitracin. Bacitracin inhibited degradation of cell-associated insulin (both randomly and A14 labeled) by 80-90% at 15 degrees C and by 60% at 37 degrees C. At 37 degrees C, inhibition of degradation was observed only with bacitracin present during dissociation and was accompanied by a compensatory increase in release of trichloroacetic acid (TCA)-precipitable insulin. This profile suggests inhibition of insulin degradation on the membrane after either primary binding or diacytosis (endocytosis-reverse endocytosis). In contrast, at 15 degrees C, bacitracin's inhibitory effect was greater with the antibiotic present during association and was not accompanied by a compensatory increase in TCA-precipitable insulin. This profile was compatible with inhibition of partial cleavage of insulin on the membrane. Internalization and degradation through chloroquine-sensitive pathways may be required to complete degradation at this temperature because chloroquine exhibited an inhibitory effect on insulin degradation equally potent to that of bacitracin at 15 degrees C (no effect at 37 degrees C).

Amino acid metabolism in hepatocytes isolated from lactating rats

Parenchymal hepatocytes isolated from lactating rats had similar rates of amino acid incorporation into protein, but increased rates of urea formation compared to hepatocytes from non-lactating rats. The increased urea formation may be due to increased amino acid transport and degradation. The liver contributes to the increased utilization of amino acids during lactation.