Reciprocal effects of energy utilization on palmitate oxidation and esterification in hepatocytes of fed rats

Regulation of lipid metabolism and gene expression by fenofibrate in hamsters

Fenofibrate is a potent hypolipidemic agent that lowers plasma lipid levels and may thus decrease the incidence of atherosclerosis. Here we investigated the molecular mechanism of fenofibrate's hypolipidemic action by characterizing its in vivo effects on the expression of mRNAs and the activities of pivotal enzymes in cholesterol and triglyceride metabolism in the hamster. Treatment of hamsters with fenofibrate led to a dose-dependent reduction in serum cholesterol concentrations. Studies on the incorporation of [(14)C]acetate and [(14)C]mevalonate into cholesterol suggested that this effect occurs primarily through inhibition of cholesterol biosynthesis at steps prior to mevalonate. Fenofibrate decreased levels of hepatic enzyme activities and mRNAs for 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) synthase and HMG CoA reductase. A potential mechanism for transcriptional regulation of these enzymes is via SREBP-2 that we found to be suppressed 2-fold by fenofibrate. Fenofibrate also lowered circulatory triglyceride levels. In keeping with the effect, we observed strong suppression of fatty acid synthase, acetyl-CoA carboxylase and apolipoprotein C-III mRNA and stimulation of lipoprotein lipase and acyl-CoA oxidase mRNA in the liver of fenofibrate-treated hamsters. These observations suggest that the effect of fenofibrate on triglyceride metabolism is likely to be a result of both decreased fatty acid synthesis and increased lipoprotein lipase and acyl-CoA oxidase gene expression in the liver. Surprisingly, alterations in lipoprotein lipase, acyl-CoA oxidase, acetyl-CoA carboxylase, and apolipoprotein C-III could not be observed in hamster hepatocytes incubated with fenofibric acid in vitro. These observations raise the possibility that changes in these genes may be secondary to the metabolic alterations occurring in animals but not in cultured cells and thus that the effect of fenofibrate on these genes may be indirect.

Control of hepatocyte function on collagen foams: sizing matrix pores toward selective induction of 2-D and 3-D cellular morphogenesis

While microporous biopolymer matrices are being widely tested as cell culture substrates in hepatic tissue engineering, the microstructural basis for their control of cell differentiation is not well understood. In this paper, we studied the role of void size of collagen foams in directing the induction of liver-specific differentiated morphology and secretory activities of cultured rat hepatocytes. Hepatocytes cultured on collagen foams with subcellular sized pore diameters of 10 microm assumed a compact, cuboidal cell morphology, rapidly achieving monolayer coverage, and secreted albumin at the rate of 40 +/- 8 pg/cell/d. Increasing the pore size to 18 microm elicited a distinctly spread cellular phenotype, with discontinuous surface coverage, and albumin secretion rates declined precipitiously to 3.6 +/- 0.8 pg/cell/d. However, when collagen foams with an even higher average void size of 82 microm were used, hepatocytes exhibited high degree of spreading within an extensive three-dimensional cellular network, and exhibited high albumin secretory activity (26 +/- 0.6 pg/cell/d). The effect of void geometry on cellular ultrastructral polarity was further analyzed for the three void size configurations employed. The distribution of the cell-cell adhesion protein, E-cadherin, was primarily restricted to cell-cell contacts on the 10 microm foams, but was found to be depolarized to all membrane regions in cells cultured on the 18 and 82 microm foams. Vinculin-enriched focal adhesions were found to be peripherally clustered on cells cultured on 10 microm foams, but were found to redistribute to the entire ventral surface of cells cultured on the 18 and 82 microm foams. Overall, we demonstrate the significance of designing pore sizes of highly adhesive substrates like collagen toward selective cell morphogenesis in 2-D or 3-D. Subcellular and supercellular ranges of pore size promote hepatocellular differentiation by limiting 2-D cell spreading or effecting 3-D intercellular contacts, while intermediate range of pore sizes repress differentiation by promoting 2-D cell spreading.

Polymer substrate topography actively regulates the multicellular organization and liver-specific functions of cultured hepatocytes

This study examines the role of topography of porous synthetic polymer substrates in regulating the tissue-specific morphogenesis of cultured hepatocytes. Porous foams of amorphous 50/50 poly(D,L glycolic-co-lactic acid) (PGLA) with a wide range of controlled pore-size distributions ( approximately 1 to 100 microm) were used as culture model surfaces. We found that the induction of microporosity in PGLA substrates significantly improved cell attachment and viability in comparison to those observed on non-porous PGLA films. A detailed evaluation of cellular morphogenesis on the microporous matrices showed that hepatocellular organization was sensitively dependent on the topographical feature size of the foam surfaces. Foams with subcellular size voids ( approximately 3 microm) induced kinetics of two-dimensional hepatocyte reorganization, yet limited the extent of three-dimensional aggregation. In contrast, foams with supercellular size voids ( approximately 67-microm) restricted hepatocyte motility, thereby promoting the kinetics of 3D aggregation. At intermediate void sizes ( approximately 17 microm), both 2D and 3D reorganization kinetics were promoted. Albumin secretory kinetics progressively increased on all void size configurations, the most rapid and sustained kinetics observed in supercellular sized voids, which may serve to minimize cell-polymer contacts and maximize cell-cell contacts in 3D. Overall, these studies demonstrate that void topography of porous polymer substrates is a critical textural feature to induce short-term cell adhesion and viability, and to also selectively regulate the kinetics and extent of multicellular spreading versus 3D aggregation. By virtue of its effects on cell adhesion and morphogenesis, the void topography of nonphysiological polymer scaffolds also is a powerful variable to microengineer hepatospecific activity of tissue analogs.

Lipids up-regulate uncoupling protein 2 expression in rat hepatocytes

BACKGROUND & AIMS

Hepatic steatosis reflects the accumulation of triglycerides and free fatty acids in hepatocytes. Although lipids and their metabolites are potentially hepatotoxic, the absence of overt injury in fatty livers suggests that adaptive responses to lipid accumulation occur. Fatty acids induce mitochondrial uncoupling proteins (UCP) 2 and 3 in muscle and fat, providing a mechanism to dispose of excessive fatty acids. Although hepatocytes do not normally express uncoupling proteins, UCP-2 is expressed in hepatocytes of genetically obese mice with fatty livers, suggesting that lipids also induce UCP-2 in hepatocytes.

METHODS

To test whether lipids up-regulate hepatocyte UCP-2, cultures of rat hepatocytes were treated with lipid emulsions, linoleic or oleic acid, and UCP-2 expression was evaluated by Northern blotting and immunocytochemistry. Because increased reactive oxygen species (ROS) production may contribute to lipid-related UCP-2 induction, the DNA-binding activity of the ROS-activated transcription factor, NF-kappaB, was measured, and the effects of tert-butyl hydroperoxide (TBHP) and glutathione (GSH) on UCP-2 induction were also assessed.

RESULTS

Lipid emulsions increased the DNA-binding activity of NF-kappaB and resulted in a dose- and time-dependent induction of UCP-2 transcripts in cultured hepatocytes; after 24 hours, UCP-2 messenger RNA levels were increased 4.5-fold, and increased UCP-2 protein was shown by immunocytochemistry. Consistent with the possibility that ROS generated intracellularly during lipid metabolism participates in UCP-2 induction, addition of the cell-impermeable antioxidant GSH did not alter lipid-related induction of UCP-2. Furthermore, TBHP, which is known to increase hepatocyte mitochondrial ROS production, also increased UCP-2 messenger RNA levels.

CONCLUSIONS

Lipids increase ROS and induce UCP-2 in hepatocytes. Thus, the liver may adapt to an excessive supply of lipid substrates by inducing UCP-2 to facilitate substrate disposal while constraining ROS production.

Stearic acid unlike shorter-chain saturated fatty acids is poorly utilized for triacylglycerol synthesis and beta-oxidation in cultured rat hepatocytes

Utilization of stearate as compared to various saturated fatty acids for cholesterol and lipid synthesis and beta-oxidation was determined in primary culture of rat hepatocytes. At 0.5 mmol/L in the medium, stearate (18:0) adequately solubilized by albumin was less inhibitory to cholesterol synthesis from [2-14C] acetate than myristate (14:0) and palmitate (16:0) (68% vs. 91 and 88% inhibition, respectively). The rate of incorporation into cholesterol from [1-14C] stearate (3.0 +/- 0.6 nmol/mg protein/4 h) was 37-, 1.8-, and 7.8-fold of that from myristate, palmitate, and oleate, respectively. Conversely, the rate of [1-14C] stearate incorporation into total glycerolipids was 88-90% lower than that of labeled palmitate, myristate, and oleate. The rate of [1-14C] stearate incorporation into triacylglycerol (3.6 +/- 0.4 nmol/mg protein/4 h) was 6-8% of that from myristate, palmitate, oleate, and linoleate. The rate of stearate incorporation into phospholipids was the lowest among tested fatty acids, whereas the rate of mono- and diacylglycerol synthesis was the highest with stearate treatment. The rate of beta-oxidation as measured by CO2 and acid soluble metabolite production was also the lowest with [1-14C] stearate treatment at 22.7 nmol/mg protein/4 h, which was 35-40% of those from other [1-14C] labeled fatty acids. A greater proportion of stearate than other fatty acids taken up by the hepatocytes remained free and was not metabolized. Clearly, stearate as compared to shorter-chain saturated fatty acids was less efficiently oxidized and esterified to triacylglycerol in cultured rat hepatocytes.

Predominance of glucuronidation over sulfation in metabolism of 1-hydroxybenzo[a]pyrene by isolated rat hepatocytes

This study shows that 1-hydroxybenzo[a]pyrene glucuronide and 1-hydroxybenzo[a]pyrene sulfate are formed in isolated rat hepatocytes. Formation of these conjugates by hepatocytes incubated with 1-acetoxy-[G-3H]benzo[a]pyrene (100 microM) as a source of intracellular 1-hydroxy-[G-3H]benzo[a]pyrene was documented by comparison of the spectra of metabolites separated by HPLC with the spectra of 1-hydroxybenzo[a]pyrene glucuronide and 1-hydroxybenzo[a]pyrene sulfate standards. The rates of 1-hydroxybenzo[a]pyrene glucuronidation and sulfation were 7.72 +/- 1.03 and 0.68 +/- 0.02 nmol x mg dry wt.-1 x 30 min-1, respectively. The rate of 1-hydroxybenzo[a]pyrene glucuronide production by intact cells corresponded well with the total activity of UDP-glucuronosyltransferase(s) determined in permeabilized hepatocytes. Cryopreserved hepatocytes fully retained a high capacity to glucuronidate the benzo[a]pyrene phenol.

Preservation of the rate and profile of xenobiotic metabolism in rat hepatocytes stored in liquid nitrogen

A simple procedure for cryopreservation of rat hepatocytes that allows recovery of viable cells retaining activities of phase I and phase II drug metabolism equivalent to freshly isolated cells is described. The cooling process was initiated 30 min after incubation of freshly isolated hepatocytes at 37 degrees in Krebs-Ringer bicarbonate buffer containing 15 mM glucose to allow for metabolic equilibration. At the end of this period, hepatocyte suspensions were supplemented with 1.7% albumin, 13.3% dimethyl sulfoxide, and the synthetic buffers, 3-[N-morpholino]propanesulfonic acid (MOPS) and N-[2-hydroxyethyl]piperazine-N'-[2-ethanesulfonic acid] (HEPES). Hepatocytes were cooled in a stepwise manner to -196 degrees by holding the cells for 1 hr at -20 degrees and then for 1 hr at -70 degrees before transfer into liquid nitrogen. After thawing and removal of damaged cells by centrifugation in Percoll, the total recovery of viable hepatocytes subjected to freezing was about 42%. The contents of ATP, ADP, and AMP were not altered significantly in cells stored in liquid nitrogen. The metabolic competence of cryopreserved hepatocytes was further confirmed by their ability to synthesize urea from NH4Cl and ornithine at the same high rate that was observed in freshly isolated cells (693 +/- 68 and 740 +/- 68 nmol.mg dry wt-1 x hr-1, respectively). Similarly, cryopreservation did not affect drug-metabolizing systems as indicated by the metabolism of benzo[a]pyrene and 7-ethoxycoumarin, two model substrates. In both freshly isolated and cryopreserved hepatocytes, 7-ethoxycoumarin was O-deethylated to 7-hydroxycoumarin at essentially the same rates (8.66 +/- 0.75 and 8.25 +/- 0.53 nmol.mg dry wt-1.hr-1, respectively) and 7-hydroxycoumarin accumulated in hepatocyte suspensions almost exclusively in the conjugated form. The storage of hepatocytes in liquid nitrogen also did not affect the complex metabolism of benzo[a]pyrene to total oxygenated metabolites and, more importantly, to metabolites conjugated with glutathione, glucuronic acid, and sulfuric acid. Thus, cryopreserved hepatocytes represent a valid and convenient model to study drug biotransformation in intact cells.

Effect of medium-chain triglycerides (MCT) as an energy substrate after hepatectomy in rats with streptozotocin-induced diabetes

The suitability of energy substrates used by the remnant liver after a 70% partial hepatectomy was studied in relation to the hepatic energy status in diabetic rats. Rats with streptozotocin-induced diabetes mellitus underwent 70% hepatectomy, and were divided into three groups receiving total parenteral nutrition (TPN) for 24h. One group received standard TPN without fat, while two other groups received standard TPN with medium-chain triglycerides (MCTs) or long-chain triglycerides (LCTs) as a 10% lipid emulsion. All rats of the TPN group without fat died within 24h. Blood ketone body ratios (aceto-acetate/beta-hydroxybutyrate), energy charge levels of the remnant liver, and cumulative amounts of 14CO2 in the expired breath during the 6h following 14C-labeled fat emulsion administration (MCT or LCT) were all significantly higher in the MCT group than in the LCT group 24h after hepatectomy. These findings suggest that medium-chain fatty acids (MCFAs) as an energy substrate are superior to long-chain fatty acids (LCFAs) during the immediate posthepatectomy phase in diabetics.

Insulin increases the synthesis of phospholipid and diacylglycerol and protein kinase C activity in rat hepatocytes

The effects of insulin on phospholipid metabolism and generation of diacylglycerol (DAG) and on activation of protein kinase C in rat hepatocytes were compared to those of vasopressin and angiotension II. Insulin provoked increases in [3H]glycerol labeling of phosphatidic acid (PA), diacylglycerol (DAG), and other glycerolipids within 30 s of stimulation. Similar increases were also noted for vasopressin and angiotensin II. Corresponding rapid increases in DAG mass also occurred with all three hormones. As increases in [3H]DAG (and DAG mass) occurred within 30-60 s of the simultaneous addition of [3H]glycerol and hormone, it appeared that DAG was increased, at least partly, through the de novo synthesis of PA. That de novo synthesis of PA was increased is supported by the fact that [3H]glycerol labeling of total glycerolipids was increased by all three agents. Increases in [3H]glycerol labeling of lipids by insulin were not due to increased labeling of glycerol 3-phosphate, and were therefore probably due to activation of glycerol-3-phosphate acyltransferase. Unlike vasopressin, insulin did not increase the hydrolysis of inositol phospholipids. Insulin- and vasopressin-induced increases in DAG were accompanied by increases in cytosolic and membrane-associated protein kinase C activity. These findings suggest that insulin-induced increases in DAG may lead to increases in protein kinase C activity, and may explain some of the insulin-like effects of phorbol esters and vasopressin on hepatocyte metabolism.

Metabolism of benzo[a]pyrene and (-)-trans-benzo[a]pyrene-7,8-dihydrodiol by freshly isolated hepatocytes of brown bullheads

The metabolism of [3H]benzo[a]pyrene (BP) and (-)-trans-[14C]7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene (BP-7,8-diol) was studied in freshly isolated hepatocytes of the wild benthic fish, brown bullhead (Ictalurus nebulosus). Bullhead hepatocytes incubated with 40 microM [3H]BP for 1 h metabolized BP to water soluble metabolites which were separated on silica gel t.l.c. plates to reveal conjugates with glucuronic acid, glutathione, and sulfate (51%, 14% and 4% of total metabolites, respectively). Additional metabolites that were extractable with ethyl acetate were separated by reversed phase HPLC to reveal only two major metabolites: BP-9,10-dihydrodiol and BP-7,8-diol (13% and 2.6% of total metabolites, respectively). Hepatocytes isolated from individual fish displayed an 11-fold variability in the rates at which they metabolized BP (756 +/- 167 pmol x mg dry wt-1 x h-1), which correlated negatively (r = -0.7, P less than 0.01) with an 18-fold variability in the glycogen content of the cells. Hepatocytes isolated from the same fish, in parallel incubations under the same optimum conditions, metabolized BP-7,8-diol 4.5-fold faster than they metabolized BP. The variability in the rate of BP-7,8-diol metabolism was about 7-fold. Major metabolites included glutathione conjugates, glucuronides and sulfates (35%, 25% and 30% of total metabolites, respectively). These conjugates, like those formed from BP, were degradable with gamma-glutamyltransferase, beta-glucuronidase and arylsulfatase, respectively. Ethyl acetate extractable metabolites were predominantly isomeric benzo-ring tetrahydrotetrols (9% of total metabolites). In summary, this study indicates that during short-term incubations bull-head hepatocytes metabolize BP and BP-7,8-diol primarily to conjugated derivatives. The usefulness of thin-layer chromatography for the convenient determination of the rate of BP-7,8-diol metabolism is demonstrated.

Alpha-adrenergic suppression of very-low-density-lipoprotein triacylglycerol secretion by isolated rat hepatocytes

The effect of adrenaline on triacylglycerol synthesis and secretion was examined in isolated rat hepatocytes. Cells were incubated with 0.5 mM-[1-14C]oleate, and the accumulation of triacylglycerol and [14C]triacylglycerol was measured in the incubation medium. Triacylglycerol appearing in the medium was present in a form with properties similar to very-low-density lipoproteins. Triacylglycerol, [14C]triacylglycerol and [14C]phospholipid contents of hepatocytes were also determined. Addition of 10 microM-(-)adrenaline decreased accumulation of glycerolipid in the incubation medium and also decreased cellular [14C]phospholipid content. Prazosin abolished these effects, whereas propranolol did not. The hormone did not affect cellular triacylglycerol content or rates of incorporation of [1-14C]oleate into cell triacylglycerol. The effect of adrenaline on the removal of newly secreted triacylglycerol and the secretion of synthesized glycerolipid was also examined. The catecholamine did not affect rates of removal of newly secreted triacylglycerol. Adrenaline did inhibit the secretion of pre-synthesized lipid by the cells, as assessed by the appearance of radiolabelled triacylglycerol from hepatocytes that had been preincubated with [1,2,3-3H]-glycerol. Adrenaline did not affect rates of fatty acid uptake by hepatocytes, but did stimulate oxidation of [1-14C]oleate, principally to 14CO2.

Suppression of triglyceride secretion by epinephrine in isolated rat hepatocytes

The effect of epinephrine on triglyceride synthesis and secretion was examined in isolated rat hepatocytes. Epinephrine potently inhibited triglyceride secretion but did not affect cellular triglyceride content or the rate of incorporation of radiolabelled glycerol into cell triglyceride. The inhibitory effect of epinephrine was abolished by inclusion of the alpha-adrenergic antagonist prazosin but not the beta-antagonist propranolol.