Developmental coordinate expression of triacylglycerol and small molecular weight apoB synthesis and secretion by rat hepatocytes

Fatty Acid Accumulations and Transcriptome Analyses Under Different Treatments in a Model Microalga Euglena gracilis

With the continuous growth of the world’s population and the increasing development of industrialization, the demand for energy by human beings has been expanding, resulting in an increasingly severe energy crisis. Microalgae are considered the most potential alternatives to traditional fossil fuels due to their many advantages, like fast growth rate, strong carbon sequestration capacity, and low growth environment requirements. Euglena can use carbon sources such as glucose, ethanol, and others for heterotrophic growth. Moreover, Euglena is highly adaptable to the environment and has a high tolerance to various environmental stresses, such as salinity, heavy metals, antibiotics, etc. Different treatments of Euglena cells could affect their growth and the accumulation of bioactive substances, especially fatty acids. To expand the industrial application of Euglena as a potential biodiesel candidate, we determine the physiological responses of Euglena against environmental stresses (antibiotics, heavy metals, salinity) or carbon resources (glucose and ethanol), and evaluate the potential for higher quality and yield of fatty acid with a high growth rate. Adding glucose into the culture media increases cell biomass and fatty acid production with high-quality biodiesel characters. The transcriptome analysis helped explore the possible regulation and biosynthesis of fatty acids under different treatments and exploited in the improvement of biodiesel production. This study provides insights for further improvement and various culture treatments for Euglena-based biodiesel and jet fuels.

Linking the low-density lipoprotein receptor-binding segment enables the therapeutic 5-YHEDA peptide to cross the blood-brain barrier and scavenge excess iron and radicals in the brain of senescent mice

Introduction

Iron accumulates in the brain during aging, which catalyzes radical formation, causing neuronal impairment, and is thus considered a pathogenic factor in Alzheimer's disease (AD). To scavenge excess iron-catalyzed radicals and thereby protect the brain and decrease the incidence of AD, we synthesized a soluble pro-iron 5-YHEDA peptide. However, the blood-brain barrier (BBB) blocks large drug molecules from entering the brain and thus strongly reduces their therapeutic effects. However, alternative receptor- or transporter-mediated approaches are possible.

Methods

A low-density lipoprotein receptor (LDLR)-binding segment of Apolipoprotein B-100 was linked to the 5-YHEDA peptide (bs-5-YHEDA) and intracardially injected into senescent (SN) mice that displayed symptoms of cognitive impairment similar to those of people with AD.

Results

We successfully delivered 5-YHEDA across the BBB into the brains of the SN mice via vascular epithelium LDLR-mediated endocytosis. The data showed that excess brain iron and radical-induced neuronal necrosis were reduced after the bs-5-YHEDA treatment, together with cognitive amelioration in the SN mouse, and that the senescence-associated ferritin and transferrin increase, anemia and inflammation reversed without kidney or liver injury.

Discussion

bs-5-YHEDA may be a mild and safe iron remover that can cross the BBB and enter the brain to relieve excessive iron- and radical-induced cognitive disorders.

Arylacetamide deacetylase attenuates fatty-acid-induced triacylglycerol accumulation in rat hepatoma cells

Mobilization of hepatic triacylglycerol stores provides substrates for mitochondrial beta-oxidation and assembly of VLDLs; however, the identity of lipolytic enzymes involved in the regulation of this process remains largely unknown. Arylacetamide deacetylase (AADA) shares homology with hormone-sensitive lipase and therefore could potentially participate in hepatic lipid metabolism, including the regulation of hepatic triacylglycerol levels. We have established McArdle-RH7777 (rat hepatoma) cell lines stably expressing mouse AADA cDNA and performed metabolic labeling as well as lipid mass analyses. Expression of AADA cDNA in McArdle-RH7777 cells significantly reduced intracellular triacylglycerol levels and apolipoprotein B secretion and increased fatty acid oxidation. These results suggest that fatty acids released by AADA-mediated hydrolysis of lipids are channeled for -oxidation rather than for the assembly of lipoproteins.

Distinct ontogenic patterns of overt and latent DGAT activities of rat liver microsomes

We have studied the ontogeny of the two functional diacylglycerol acyltransferase (DGAT) activities (overt and latent) during postnatal development in rat liver. We find that the ontogenic patterns of the two are highly distinct. Overt DGAT shows a transient rise in activity up to day 4 postnatally, after which it declines until weaning; thereafter, it increases steadily to reach high adult values that may contribute to the high rates of turnover of cytosolic triacylglycerol (TAG). By contrast, latent DGAT activity increases continuously during the suckling period but falls sharply upon weaning onto chow but not onto a high-fat diet. Rates of TAG secretion by hepatocytes are higher than in the adult during the first 7 days after birth, and are largely dependent on the mobilization of the abundant intrahepatocyte TAG as a source of acyl moieties. When the hepatic steatosis is cleared (after day 7) the TAG secretion rate declines by 80% to reach adult values. Quantification of the content of mRNA for the DGAT1 and DGAT2 genes does not show correlation with either of the DGAT activities. We conclude that post-translational modification may play an important role in the overt and latent distribution of DGAT activity in the liver microsomal membrane.

Characterization of the rodent genes for arylacetamide deacetylase, a putative microsomal lipase, and evidence for transcriptional regulation

In the current study, we have determined the cDNA and the genomic sequences of the arylacetamide deacetylase (AADA) gene in mice and rats. The AADA genes in the rat and mouse consist of five exons and have 2.4 kilobases of homologous promoter sequence upstream of the initiating ATG codon. AADA mRNA is expressed in hepatocytes, intestinal mucosal cells (probably enterocytes), the pancreas and also the adrenal gland. In mice, there is a diurnal rhythm in hepatic AADA mRNA concentration, with a maximum 10 h into the light (post-absorptive) phase. This diurnal regulation is attenuated in peroxisome proliferator-activated receptor alpha knockout mice. Intestinal but not hepatic AADA mRNA was increased following oral administration of the fibrate, Wy-14,643. The homology of AADA with hormone-sensitive lipase and the tissue distribution of AADA are consistent with the view that AADA plays a role in promoting the mobilization of lipids from intracellular stores and in the liver for assembling VLDL. This hypothesis is supported by parallel changes in AADA gene expression in animals with insulin-deficient diabetes and following treatment with orotic acid.

A role for Sp1 in the transcriptional regulation of hepatic triacylglycerol hydrolase in the mouse

Microsomal triacylglycerol hydrolase (TGH) hydrolyzes stored triacylglycerol in cultured hepatoma cells (Lehner, R., and Vance, D. E. (1999) Biochem. J. 343, 1-10). We studied expression of TGH in murine liver and found both protein and mRNA increased dramatically at 27 days after birth. Nuclear run-on assays demonstrated that this was due to increased transcription. We cloned 542 base pairs upstream of the transcriptional start site of the murine TGH gene. Electrophoretic mobility shift assays demonstrated enhanced binding of hepatic nuclear proteins from 27-day-old mice to the murine TGH promoter, yielding three differentially migrating complexes. DNase I footprint analysis localized these complexes to two distinct regions: site A contains a putative Sp binding site, and site B contains a degenerate E box. We transfected primary murine hepatocytes with a series of 5'-deletion constructs upstream of the reporter luciferase cDNA. Positive control elements were identified in a segment containing site A. Competitive electrophoretic mobility shift assays and supershift assays demonstrated that site A binds Sp1 and Sp3. Transcriptional activation assays in Schneider SL-2 insect cells demonstrated that Sp1 is a potent activator of the TGH promoter. These experiments directly link increased TGH expression at the time of weaning to transcriptional regulation by Sp1.

Direct effects of growth hormone on production and secretion of apolipoprotein B from rat hepatocytes

The aim of this study was to investigate the direct effects of growth hormone (GH) on production and secretion of apolipoprotein B (apoB)-containing lipoproteins from hepatocytes. Bovine GH (5-500 ng/ml) was given for 1 or 3 days to rat hepatocytes cultured on laminin-rich matrigel in serum-free medium. The effects of GH were compared with those of 3 nM insulin and 500 microM oleic acid. GH increased the editing of apoB mRNA, and the proportion of newly synthesized apoB-48 (of total apoB) in the cells and secreted into the medium changed in parallel. GH increased total secretion of apoB-48 (+30%) and apoB-48 in very low density lipoproteins (VLDL) more than twofold. Total apoB-100 secretion decreased 63%, but apoB-100-VLDL secretion was unaffected by GH. Pulse-chase studies indicated that GH increased intracellular early degradation of apoB-100 but not apoB-48. GH had no effect on apoB mRNA or LDL receptor mRNA levels. The triglyceride synthesis, the mass of triglycerides in the cells, and the VLDL fraction of the medium increased after GH incubation. Three days of insulin incubation had effects similar to those of GH. Combined incubation with oleic acid and GH had additive effects on apoB mRNA editing and apoB-48-VLDL secretion. In summary, GH has direct effects on production and secretion of apoB-containing lipoproteins, which may add to the effects of hyperinsulinemia and increased flux of fatty acids to the liver during GH treatment in vivo.

Update on solid-phase extraction for the analysis of lipid classes and related compounds

This article provides information on the different procedures and methodologies developed when solid-phase extraction (SPE) is used for lipid component separation. The analytical systematics, established by different authors and designed to separate groups of compounds and also specific components by using a combination of chromatographic supports and solvents are presented. The review has been divided into three parts, which we consider well defined: edible fats and oils, fatty foods and biological samples. Separations of non-polar and polar lipids is the most extensive systematic, although many other published methods have been established to isolate specific components or a reduced number of components from edible fats and oils, fatty foods or biological samples susceptible to further analysis by other quantitative techniques.

Coordinate regulation of lipogenesis, the assembly and secretion of apolipoprotein B-containing lipoproteins by sterol response element binding protein 1

Stable plasmid-driven expression of the liver-specific gene product cholesterol 7alpha-hydroxylase (7alpha-hydroxylase) was used to alter the cellular content of transcriptionally active sterol response element binding protein 1 (SREBP1). As a result of stable expression of 7alpha-hydroxylase, individual single cell clones expressed varying amounts of mature SREBP1 protein. These single cell clones provided an opportunity to identify SREBP1-regulated genes that may influence the assembly and secretion of apoB-containing lipoproteins. Our results show that in McArdle rat hepatoma cells, which normally do not express 7alpha-hydroxylase, plasmid-driven expression of 7alpha-hydroxylase results in the following: 1) a linear relationship between (i) the cellular content of mature SREBP1 and 7alpha-hydroxylase protein, (ii) the relative expression of 7alpha-hydroxylase mRNA and the mRNA's encoding the enzymes regulating fatty acid, i.e. acetyl-CoA carboxylase and sterol synthesis, i.e. HMG-CoA reductase, (iii) the relative expression of 7alpha-hydroxylase mRNA and microsomal triglyceride transfer protein mRNA, a gene product that is essential for the assembly and secretion of apoB-containing lipoproteins; 2) increased synthesis of all lipoprotein lipids (cholesterol, cholesterol esters, triglycerides, and phospholipids); and 3) increased secretion of apoB100 without any change in apoB mRNA. Cells expressing 7alpha-hydroxylase contained significantly less cholesterol (both free and esterified). The increased cellular content of mature SREBP1 and increased secretion of apoB100 were concomitantly reversed by 25-hydroxycholesterol, suggesting that the content of mature SREBP1, known to be decreased by 25-hydroxycholesterol, mediates the changes in the lipoprotein assembly and secretion pathway that are caused by 7alpha-hydroxylase. These data suggest that several steps in the assembly and secretion of apoB-containing lipoproteins by McArdle hepatoma cells may be coordinately linked through the cellular content of mature SREBP1.

Selective recruitment of ApoB-48 for the assembly of VLDL in rat triacylglycerol-enriched hepatocytes

Primary rat hepatocyte cultures were enriched in cellular triacylglycerol (TAG) by exposure to extracellular oleate for 3 days. Control cells were cultured for the same time without oleate. The large increase in TAG secretion into the medium of TAG-enriched cells during the final 24 hours (225 +/- 30 versus 40 +/- 10 micrograms/mg cell protein [control cells], P < .01) was not accompanied by a similar change in apolipoprotein B (apoB) secretion (4.22 +/- 0.94 versus 3.72 +/- 0.75 micrograms/mg per 24 hours, respectively). Instead, TAG-enriched cells recruited a larger proportion of apoB for the synthesis of very low density lipoprotein (VLDL), the secretion of which was substantially higher under these circumstances (1.46 +/- 0.39 versus 0.34 +/- 0.06 microgram apoB per milligram cell protein per 24 hours, P < .05). The increase in VLDL assembly was accompanied by a selective 2.5-fold increase (P < .05) in the specific recruitment of apoB-48. There was no significant increase in the amount of apoB-100, which appeared in the VLDL fraction when cells were enriched with TAG. Under these circumstances there was an increase in net cellular synthesis of apoB-48 (5524 +/- 667 versus 2505 +/- 598 disintegrations per minute per milligram protein per hour, P < .05). The net cellular synthesis of apoB-100 was unchanged compared with that observed in control cell cultures (1548 +/- 237 versus 2000 +/- 897 dpm/ mg per hour, respectively). A large proportion of the total secreted apoB was associated with small particles of density higher than VLDL, even when VLDL output was maximally stimulated, suggesting that apoB was oversecreted and in excess of the cells' requirement to transport TAG.

Upregulation of low density lipoprotein receptor activity by tumor necrosis factor, a process independent of tumor necrosis factor-induced lipid synthesis and secretion

It has been shown that tumor necrosis factor (TNF) rapidly upregulates expression of the low density lipoprotein (LDL) receptors on Hep G2 cells and acutely stimulates hepatic lipid synthesis and secretion in vivo. It may thus be possible that TNF-induced expression of LDL receptors is secondary to a decrease in cellular cholesterol content caused by TNF-stimulated lipid secretion. In order to know whether TNF upregulates LDL receptors by depletion of the cellular cholesterol content, the present experiments were designed to study the temporal relationship between TNF-stimulated expression of LDL receptor activity and TNF-induced changes in lipid synthesis and secretion in an in vitro setting by using Hep G2 cells (a highly differentiated human hepatoma cell line) as a hepatocyte model. Hep G2 cells were incubated with TNF (usually 2.5 nmol/L) for certain periods, and LDL receptor activity was evaluated by measuring [125I]LDL binding at 4 degrees C; lipid synthesis and secretion were assayed by measuring [3H]glycerol incorporation into triglycerides and phospholipids as well as [14C]acetate incorporation into cholesterol. We found that a 30-h exposure of the cells to TNF was needed for the effect of TNF to be seen on lipid synthesis and secretion as measured by incorporation of [3H]glycerol into triglycerides and phospholipids, whereas TNF rapidly (in several hours) upregulated LDL receptor activity. TNF stimulated triglyceride synthesis, but did not stimulate phospholipid synthesis. On the other hand, TNF stimulated phospholipid secretion, but did not stimulate triglyceride secretion. Exposure of the cells to TNF for 16 or 24 h neither decreased cholesterol synthesis nor stimulated cholesterol secretion as measured by [14C]acetate incorporation into cholesterol.(ABSTRACT TRUNCATED AT 250 WORDS)

Liver regrowth and apolipoprotein B secretion by rat hepatocytes following partial hepatectomy

Apolipoprotein (apo) B is an essential component for the assembly and secretion of lipoproteins. The current report examines apo B production using primary cultures of hepatocytes derived from rats 3 to 21 days after partial hepatectomy (PH) to determine the effects of liver regrowth on apo B. Studies indicate that hepatocytes stimulated by PH have a two-thirds reduction in net apo B production 3 to 7 days after surgery, which coincides with the period of maximum rate of liver regrowth. Both higher (apo BH)- and lower-molecular-weight (apo BL) apo B are synthesized and secreted after PH, indicating the presence of edited apo B mRNA in hepatocytes. Hepatocytes derived from PH rats are more sensitive to insulin inhibition of apo B secretion compared with controls, suggesting an enhanced effect of insulin on newly replicated hepatocytes. Epidermal growth factor (EGF), a key regulator of liver regrowth following PH, potentiates the inhibitory action of insulin on apo B secretion in control hepatocytes and those derived from rats 2 to 3 weeks after PH. However, the potentiating effect of EGF on insulin inhibition of apo B is not discernible in hepatocytes 3 to 7 days after PH. The short-term in vitro hormonal effects occurring even with decreased apo B production suggest that this pathway remains available following PH to balance lipoprotein secretion with lipid and energy requirements necessary for liver regeneration.

Plasma lipids, lipoproteins and post-heparin lipases in ponies with hyperlipaemia

The metabolic origins of equine hyperlipaemia were investigated by analysing the concentration and composition of plasma lipoproteins in 18 ponies with the condition. The mean concentrations of cholesterol, triglyceride and very low density lipoproteins (VLDL) were increased by 4-, 52- and 19-fold, respectively, compared with a control group of 18 healthy ponies. These increases were due to the appearance of a buoyant VLDL fraction (VLDL1) not present in healthy ponies. The mean diameter of VLDL1 particles was 44% greater than control VLDL, and the particles were enriched in triglyceride and free cholesterol and depleted of cholesteryl esters, phospholipid and protein. The apolipoprotein (apo) B-100 content of VLDL1 was reduced and the ratio of apoB-100 to apoB-48 particles was 1:1, compared with 2:1 in control VLDL. The VLDL1 was also enriched in apoE, but had normal complements of apoC-II and apoC-III. The conventional VLDL (called VLDL2), LDL and HDL fractions were moderately enriched with triglyceride, and HDL contained increased amounts of apoE, apoC-II and apoC-III. The activities of lipoprotein lipase and hepatic lipase, the enzymes responsible for the catabolism of VLDL and their remnants, were increased by 2- and 3-fold, respectively, in response to the increased concentrations of their substrates. The composition of VLDL1 suggested that the liver was maximising the secretion of triglyceride by producing larger number of VLDL particles that accommodated a greater mass of triglyceride by having apoB-48 rather than apoB-100 as their structural protein. Plasma free fatty acid (FFA) concentrations were elevated in 17 of the 18 ponies, suggesting that increased FFA flux might be the stimulus for hepatic triglyceride synthesis and VLDL secretion. We conclude that overproduction, rather than defective catabolism, of VLDL was the cause of the hyperlipidaemia and that lipid lowering agents which reduce VLDL synthesis, by decreasing adipose lipolysis and FFA flux, are candidates for the management of hyperlipaemia.

The ontogeny and site of intestinal lipid and lipoprotein synthesis

The developmental aspects of characteristic intestinal lipoprotein synthesis, chlomicrons (CM), very low density lipoproteins (VLDL) and high density lipoproteins (HDL), are unknown. Our objective was to determine the ontogeny of intestinal lipid and lipoprotein synthesis in both the jejunum and the ileum. Explants of the jejunum and the ileum from fetal (F) (18-19 days of gestation), suckling (S) (5 days old) and weaning (W) (23 days old) rats were cultured in the presence of [14C]-oleic acid to examine lipid synthesis. The results indicate the following. (1) The incorporation of oleic acid is higher for the fetal explants. However, the efficiency of esterification of free fatty acids (FFA) into triglycerides (TG) in the jejunum increases with age (33% F, 37% S, 48% W) (P less than 0.05, by ANOVA). (2) The same profile is found at the ideal site for the incorporation of oleic acid. However, the capacity for the synthesis of TG is more intense at the suckling period (34% F, 54% S, 42% W) (P less than 0.05, by ANOVA). (3) The relative content of TG in CM changes with age: F, 90-93%; S, 80-84%; W, 33-40%. (4) A low percentage of TG content is found in CM at the weaning period while high levels are detectable in VLDL (40-42%). (5) A most significant difference is noted between the jejunum and the ileum in TG synthesis at the suckling period. The ileum synthesizes 53% more TG than the jejunum (P less than 0.025). (6) HDL particles contain substantial amounts of FFA. Nevertheless, they were also found to be able to transport TG mainly in the suckling rat. Thus, this study demonstrates that with growth the rat is able to synthesize CM, VLDL and HDL. Our findings indicate changes in the synthesis of intestinal lipids and lipoproteins, depending on both the development and the site, which suggests an ontogeny. These modifications can be attributed to dietary and hormonal influences present during the period of development.

Tumor necrosis factor-alpha, interleukin-1, and interferon alpha stimulate triglyceride synthesis in HepG2 cells

Tumor necrosis factor-alpha (TNF-alpha), interleukin-1 (IL-1), and interferon alpha (IFN-alpha) stimulate hepatic lipogenesis in vivo. We now show that TNF-alpha, IL-1, and IFN-alpha stimulate lipogenesis as measured by the incorporation of 3H-glycerol into triglyceride in cultured HepG2 cells. Incubation of HepG2 cells for approximately 24 hours with TNF or IL-1 was required to see stimulation of lipogenesis, with this effect increasing over the next four days. TNF stimulated lipogenesis by 2.4-fold after 72 hours of incubation, while a 3.5-fold stimulation was seen with IL-1. The half maximal concentration for TNF stimulation of hepatic lipogenesis was 4 ng/mL, while that for IL-1 was 0.3 ng/mL. Cells treated with TNF or IL-1 also showed increased secretion of labeled triglyceride into the media. IFN-alpha stimulated the incorporation of 3H-glycerol into triglyceride by 39% after 72 hour's incubation. In contrast, IFN-gamma had no effect on lipogenesis in HepG2 cells. These data suggest that cytokines can directly stimulate the synthesis of triglycerides in cultured Hep G2 liver cells in vitro.

Quantitation of endogenous liver apolipoprotein B mRNA editing

The mRNA for apolipoprotein B is translated into either a high molecular weight (apo BH) or low molecular weight (apo BL) form of the protein depending on a novel form of RNA processing known as RNA editing. Apo BH mRNA editing is both tissue-specific and hormonally regulated and involves transition of cytidine to uridine at codon 2153 thereby converting a glutamine codon (CAA) to a translational stop codon (UAA). Three methods for quantitating the endogenous levels of liver apo B mRNA editing were compared: (1) Southern blot hybridization with discriminative thermal washes, (2) competimer-hybridization with discriminative thermal washes and (3) competimer-polymerase chain reaction (competimer-PCR). The data suggest that hybridization and PCR can yield similar quantitation when competing oligonucleotides are used. Based on competimer-PCR it is proposed that 40% and 85% of normal rat liver and small intestine apo B mRNA (respectively) are edited.