Insulin promotes the biosynthesis and secretion of apolipoprotein B-48 by altering apolipoprotein B mRNA editing

27-Hydroxycholesterol Inhibits Sterol Regulatory Element-Binding Protein 1 Activation and Hepatic Lipid Accumulation in Mice

OBJECTIVE

Although 27-hydroxycholesterol (27-HC) has been reported as a potent regulator of lipid homeostasis, its role in hepatic lipogenesis remains obscure. The present study was designed to investigate the impact of 27-HC on sterol regulatory element-binding protein 1 (SREBP-1) and hepatic steatosis.

METHODS

In this study, the 27-HC level in mice was upregulated by overexpressing CYP27A1 or treating primary hepatocytes with 27-HC, and then the hepatic lipid accumulation was detected.

RESULTS

27-HC inhibited hepatic lipid accumulation and decreased the levels of the mature active form of SREBP-1. The expression of lipogenic genes, including acetyl coenzyme A carboxylase, fatty acid synthase, stearoyl-coenzyme A desaturase-1, and glycerol-3-phosphate acyltransferase, were also suppressed after 27-HC intervention. Furthermore, 27-HC induced expression of insulin-induced gene-2 (Insig-2), an endoplasmic reticulum protein that prevents SREBP activation, both in vivo and in vitro. The inhibitory effect of 27-HC on SREBP-1 activation was absent when Insig-2 was silenced. Finally, coimmunoprecipitation showed that 27-HC promoted the binding of Insig-2 to SREBP-1.

CONCLUSIONS

These studies demonstrated the suppressive effect of 27-HC on hepatic lipid accumulation and revealed a novel mechanism by which 27-HC regulates lipogenesis.

Exploring the potential of the platelet membrane proteome as a source of peripheral biomarkers for Alzheimer's disease

Introduction

Peripheral biomarkers to diagnose Alzheimer's disease (AD) have not been established. Given parallels between neuron and platelet biology, we hypothesized platelet membrane-associated protein changes may differentiate patients clinically defined with probable AD from noncognitive impaired controls.

Methods

Purified platelets, confirmed by flow cytometry were obtained from individuals before fractionation by ultracentrifugation. Following a comparison of individual membrane fractions by SDS-PAGE for general proteome uniformity, equal protein weight from the membrane fractions for five representative samples from AD and five samples from controls were pooled. AD and control protein pools were further divided into molecular weight regions by one-dimensional SDS-PAGE, prior to digestion in gel. Tryptic peptides were analyzed by reverse-phase liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS). Ionized peptide intensities were averaged for each identified protein in the two pools, thereby measuring relative protein abundance between the two membrane protein pools. Log₂-transformed ratio (AD/control) of protein abundances fit a normal distribution, thereby permitting determination of significantly changed protein abundances in the AD pool.

Results

We report a comparative analysis of the membrane-enriched platelet proteome between patients with mild to moderate AD and cognitively normal, healthy subjects. A total of 144 proteins were determined significantly altered in the platelet membrane proteome from patients with probable AD. In particular, secretory (alpha) granule proteins were dramatically reduced in AD. Of these, we confirmed significant reduction of thrombospondin-1 (THBS1) in the AD platelet membrane proteome by immunoblotting. There was a high protein-protein connectivity of proteins in other pathways implicated by proteomic changes to the proteins that define secretory granules.

Conclusions

Depletion of secretory granule proteins is consistent with a preponderance of post-activated platelets in circulation in AD. Significantly changed pathways implicate additional AD-related defects in platelet glycoprotein synthesis, lipid homeostasis, amyloidogenic proteins, and regulators of protease activity, many of which may be useful plasma membrane-expressed markers for AD. This study highlights the utility of LC-MS/MS to quantify human platelet membrane proteins and suggests that platelets may serve as a source of blood-based biomarkers in neurodegenerative disease.

CideB protein is required for the biogenesis of very low density lipoprotein (VLDL) transport vesicle

Nascent very low density lipoprotein (VLDL) exits the endoplasmic reticulum (ER) in a specialized ER-derived vesicle, the VLDL transport vesicle (VTV). Similar to protein transport vesicles (PTVs), VTVs require coat complex II (COPII) proteins for their biogenesis from the ER membranes. Because the size of the VTV is large, we hypothesized that protein(s) in addition to COPII components might be required for VTV biogenesis. Our proteomic analysis, supported by Western blotting data, shows that a 26-kDa protein, CideB, is present in the VTV but not in other ER-derived vesicles such as PTV and pre-chylomicron transport vesicle. Western blotting and immunoelectron microscopy analyses suggest that CideB is concentrated in the VTV. Our co-immunoprecipitation data revealed that CideB specifically interacts with VLDL structural protein, apolipoprotein B100 (apoB100), but not with albumin, a PTV cargo protein. Confocal microscopic data indicate that CideB co-localizes with apoB100 in the ER. Additionally, CideB interacts with COPII components, Sar1 and Sec24. To investigate the role of CideB in VTV biogenesis, we performed an in vitro ER budding assay. We show that the blocking of CideB inhibits VTV budding, indicating a direct requirement of CideB in VTV formation. To confirm our findings, we knocked down CideB in primary hepatocytes and isolated ER and cytosol to examine whether they support VTV budding. Our data suggest that CideB knockdown significantly reduces VTV biogenesis. These findings suggest that CideB forms an intricate COPII coat and regulates the VTV biogenesis.

Tolfenamic acid suppresses cytochrome P450 2E1 expression in mouse liver

Non-steroidal anti-inflammatory drugs (NSAIDs) play a significant role in the chemoprevention of cancer. We recently showed the chemopreventive response of a NSAID, 2-[(3-chloro-2-methylphenyl)amino]benzoic acid) known as tolfenamic acid (TA) in N-nitrosomethylbenzylamine (NMBA)-induced esophageal tumors in rats. Pre-clinical studies showed that TA inhibits Specificity protein (Sp) transcription factors and acts as an anti-cancer agent in several cancer models; however the pertinent mechanisms associated with its chemopreventive response in esophageal cancer are not known. Since the bioactivation of carcinogens through cytochrome P450 (CYP) is critical for the induction of cancer, we have studied the effect of TA on critical CYP isozymes in mouse liver samples. Athymic nude mice were treated with vehicle (corn oil) or TA (50 mg kg(-1), 3 times per week) for 4 weeks. Protein extracts (whole cell lysates and microsomal fractions) were prepared from liver tissue and the expression of various CYP isozymes was determined by Western blot analysis. Rat (Sprague-Dawley) livers were harvested and primary hepatocyte cultures were treated with vehicle (DMSO) or TA (50 μM) and cell viability was assessed at 2 and 5 days post-treatment. TA caused remarkable decrease in the expression of CYP2E1 in both liver lysates and sub-cellular fraction, while its response on other tested isozymes was marginal. TA did not affect the body weight of animals (mice) and viability of rat hepatocytes. These results demonstrate that TA modulates the expression of CYP2E1 which is associated with the bioactivation of carcinogens without causing apparent toxicity. These data suggest that TA-induced inhibition of CYP2E1 attenuates the bioactivation of carcinogens potentially leading to the chemoprevention of NMBA-induced esophageal tumorigenesis in rats.

Metabolic regulation of APOBEC-1 complementation factor trafficking in mouse models of obesity and its positive correlation with the expression of ApoB protein in hepatocytes

APOBEC-1 Complementation Factor (ACF) is an RNA-binding protein that interacts with apoB mRNA to support RNA editing. ACF traffics between the cytoplasm and nucleus. It is retained in the nucleus in response to elevated serum insulin levels where it supports enhanced apoB mRNA editing. In this report we tested whether ACF may have the ability to regulate nuclear export of apoB mRNA to the sites of translation in the cytoplasm. Using mouse models of obesity-induced insulin resistance and primary hepatocyte cultures we demonstrated that both nuclear retention of ACF and apoB mRNA editing were reduced in the livers of hyperinsulinemic obese mice relative to lean controls. Coincident with an increase in the recovery of ACF in the cytoplasm was an increase in the proportion of total cellular apoB mRNA recovered in cytoplasmic extracts. Cytoplasmic ACF from both lean controls and obese mouse livers was enriched in endosomal fractions associated with apoB mRNA translation and ApoB lipoprotein assembly. Inhibition of ACF export to the cytoplasm resulted in nuclear retention of apoB mRNA and reduced both intracellular and secreted ApoB protein in primary hepatocytes. The importance of ACF for modulating ApoB was supported by the finding that RNAi knockdown of ACF reduced ApoB secretion. An additional discovery from this study was the finding that leptin is a suppressor ACF expression. Dyslipidemia is a common pathology associated with insulin resistance that is in part due to the loss of insulin controlled secretion of lipid in ApoB-containing very low density lipoproteins. The data from animal models suggested that loss of insulin regulated ACF trafficking and leptin regulated ACF expression may make an early contribution to the overall pathology associated with very low density lipoprotein secretion from the liver in obese individuals.

Insulin enhances the biogenesis of nuclear sterol regulatory element-binding protein (SREBP)-1c by posttranscriptional down-regulation of Insig-2A and its dissociation from SREBP cleavage-activating protein (SCAP).SREBP-1c complex

The regulation of lipid homeostasis by insulin is mediated in part by the enhanced transcription of the gene encoding sterol regulatory element-binding protein-1c (SREBP-1c). The nascent SREBP-1c is embedded in the endoplasmic reticulum (ER) and must be transported to the Golgi where two sequential cleavages generate its NH(2)-terminal fragment, nSREBP-1c. We have shown recently that in primary cultures of rat hepatocytes, insulin rapidly and selectively stimulates proteolytic processing of the nascent SREBP-1c by enhancing the affinity of the SREBP cleavage-activating protein (SCAP).SREBP-1c complex for coatomer protein complex II (COPII) vesicles. The SCAP.SREBP complex is retained in the ER by Insig proteins. We report here that insulin persistently stimulates controlled proteolysis of the nascent SREBP-1c by selectively reducing the level of Insig-2a protein via accelerated degradation of its cognate mRNA. Insulin enhanced the rate of turnover of Insig-2a mRNA via its 3'-untranslated region. Insulin-induced depletion of Insig-2a promotes association of the SCAP.SREBP-1c complex with COPII vesicles and subsequent migration to the Golgi where site-1 and site-2 proteases process the nascent SREBP-1c. Consistent with this mechanism, experimental knockdown of Insig-2a expression with small interfering RNA mimicked insulin-induced proteolysis of the nascent SREBP-1c, whereas exogenous expression of Insig-2a in hepatocytes led to reduced intramembrane proteolysis of the newly synthesized SREBP-1c. The action of insulin on the processing of the nascent SREBP-1c via Insig-2a was highly selective, as proteolysis of the newly synthesized SREBP-2 remained unchanged under identical conditions. On the basis of these data, we propose that the stimulation of SREBP-1c processing by insulin is mediated by a selective depletion of Insig-2a protein by promoting decay of its cognate mRNA. Thus, insulin-induced reduction in Insig-2a protein leads to an enhanced export of the SCAP.SREBP-1c complex from ER to the Golgi.

N-3 polyunsaturated fatty acids suppress insulin-induced SREBP-1c transcription via reduced trans-activating capacity of LXRalpha

Insulin coordinately up-regulates lipogenic gene transcription via induction of sterol regulatory element binding protein-1c (SREBP-1c). Conversely, polyunsaturated fatty acids (PUFA) decrease lipogenic gene transcription via suppression of SREBP-1c. We therefore examined the ability of n-3 PUFA to mitigate induction of SREBP-1c and its downstream lipogenic targets by insulin in primary rat hepatocyte cultures. Insulin induced expression of SREBP-1c mRNA 5-6 fold as well as rat SREBP-1c promoter activity. These effects were prevented by the n-3 fatty acids eicosapentaenoic acid (20:5 n-3; EPA) and docosahexaenoic acid (22:6 n-3, DHA), but not by the monounsaturated fatty acid oleic acid (18:1 n-6, OLA). N-3 fatty acids also effectively prevented insulin induction of the downstream lipogenic enzyme targets fatty acid synthase (FAS) and acetyl carboxyl coenzyme acetyltransferase-1 (ACC-1), and reduced de novo lipogenesis. The SREBP-1c promoter contains an insulin response unit consisting of tandem LXRalpha response elements (LXREs) as well as sites for NF-Y, Sp1, and SREBP-1c itself. The LXREs were identified as a primary site mediating suppression of SREBP-1c transcription by n-3 PUFA. DHA effectively prevented LXRalpha-dependent activation of both the wild type SREBP-1c promoter and the synthetic LXRE-driven promoter, and significantly blunted LXRalpha-dependent activation of a Gal4-LXRalpha chimeric protein thus demonstrating that n-3 PUFA effectively mitigate induction of SREBP-1c by insulin via reduced trans-activation of LXRalpha.

Insulin enhances post-translational processing of nascent SREBP-1c by promoting its phosphorylation and association with COPII vesicles

The regulation of lipid homeostasis by insulin is mediated in part by the enhanced transcription of the gene encoding SREBP-1c (sterol regulatory element-binding protein-1c). Nascent SREBP-1c is synthesized and embedded in the endoplasmic reticulum (ER) and must be transported to the Golgi in coatomer protein II (COPII) vesicles where two sequential cleavages generate the transcriptionally active NH(2)-terminal fragment, nSREBP-1c. There is limited indirect evidence to suggest that insulin may also regulate the posttranslational processing of the nascent SREBP-1c protein. Therefore, we designed experiments to directly assess the action of insulin on the post-translational processing of epitope-tagged full-length SREBP-1c and SREBP-2 proteins expressed in cultured hepatocytes. We demonstrate that insulin treatment led to enhanced post-translational processing of SREBP-1c, which was associated with phosphorylation of ER-bound nascent SREBP-1c protein that increased affinity of the SREBP-1c cleavage-activating protein (SCAP)-SREBP-1c complex for the Sec23/24 proteins of the COPII vesicles. Furthermore, chemical and molecular inhibitors of the phosphoinositide 3-kinase pathway and its downstream kinase protein kinase B (PKB)/Akt prevented both insulin-mediated phosphorylation of nascent SREBP-1c protein and its posttranslational processing. Insulin had no effect on the proteolysis of nascent SREBP-2 under identical conditions. We also show that in vitro incubation of an active PKB/Akt enzyme with recombinant full-length SREBP-1c led to its phosphorylation. Thus, insulin selectively stimulates the processing of SREBP-1c in rat hepatocytes by enhancing the association between the SCAP-SREBP-1c complex and COPII proteins and subsequent ER to Golgi transport and proteolytic cleavage. This effect of insulin is tightly linked to phosphoinositide 3-kinase and PKB/Akt-dependent serine phosphorylation of the precursor SREBP-1c protein.

Expression of the rat sterol regulatory element-binding protein-1c gene in response to insulin is mediated by increased transactivating capacity of specificity protein 1 (Sp1)

The induction of genes involved in lipid biosynthesis by insulin is mediated in part by the sterol regulatory element-binding protein-1c (SREBP-1c). SREBP-1c is directly regulated by insulin by transcriptional and post-transcriptional mechanisms. Previously, we have demonstrated that the insulin-responsive cis-acting unit of the rat SREBP-1c promoter is composed of several elements that include a sterol regulatory element, two liver X receptor elements, and a number of conserved GC boxes. Here we systematically dissected the role of these GC boxes and report that five bona fide Sp1-binding elements of the SREBP-1c promoter determine its basal and insulin-induced activation. Luciferase expression driven by the rat SREBP-1c promoter was accelerated by ectopic expression of Sp1, and insulin further enhanced the transactivation potential of Sp1. Introduction of a small interfering RNA against Sp1 reduced both basal and insulin-induced activation of the SREBP-1c promoter. We also found that Sp1 interacted with both SREBP-1c and LXRalpha proteins and that insulin promoted these interactions. Chromatin immunoprecipitation studies revealed that insulin facilitated the recruitment of the steroid receptor coactivator-1 to the SREBP-1c promoter. These studies identify a novel mechanism by which maximal activation of the rat SREBP-1c gene expression by insulin is mediated by Sp1 and its enhanced ability to interact with other transcriptional regulatory proteins.

Mouse models as tools to explore cytidine-to-uridine RNA editing

RNA editing is a process through which the nucleotide sequence specified in the genomic template is modified to produce a different nucleotide sequence in the transcript. RNA editing is an important mechanism of genetic regulation that amplifies genetic plasticity by allowing the production of alternative protein products from a single gene. There are two generic classes of RNA editing in nuclei, involving enzymatic deamination of either C-to-U or A-to-I nucleotides. The best characterized example of C-to-U RNA editing is that of apolipoprotein B (apoB), which is mediated by a holoenzyme that contains a minimal core composed of an RNA-specific cytidine deaminase apobec-1, and its cofactor apobec-1 complementation factor (ACF). C-to-U editing of apoB RNA generates two different isoforms--apoB100 and apoB48--from a single transcript. Both are important regulators of lipid transport and metabolism, and are functionally distinct. C-to-U apoB RNA editing is regulated by a range of factors including developmental, nutritional, environmental, and metabolic stimuli. Rodent models have provided a tractable system in which to study the effects of such stimuli on lipid metabolism. In addition, both transgenic and gene knockout experiments have provided important insights into gain and loss of function approaches for studying C-to-U RNA editing in a murine background.

Characterization of scavenger receptor class B, type I in Atlantic salmon (Salmo salar L.)

The scavenger receptor class B, type I (SR-BI) is an important player in regulation of mammalian lipid homeostasis. We therefore wanted to study this receptor in Atlantic salmon (Salmo salar L.), which requires a diet with particular high lipid content. We have for the first time cloned and characterized SR-BI from a salmonid fish. The predicted 494 amino acid protein contained two transmembrane domains, several putative N-glycosylation sites, and showed 72% sequence identity with the predicted homolog from zebrafish. SR-BI expression was analyzed by reverse transcription Real-Time PCR in several tissues, and a high relative expression in salmon midgut was detected, which may suggest that SR-BI has a role in uptake of lipids from the diet. We also expressed a construct of salmon myc-tagged SR-BI in salmon TO cells and HeLa cells, which gave a protein of approximately 80 kDa on reducing SDS-PAGE using an antibody against the myc-epitope. Immunofluorescence microscopy analyses of the salmon SR-BI protein in transiently transfected HeLa cells revealed staining in the cell periphery and in some intracellular membranes, but not in the nucleus, which indicated that the salmon protein may be a functional membrane protein. We also observed a high degree of co-localization using an anti-peptide SR-BI antiserum. We found that 20 microg mL(-1) insulin up-regulated the SR-BI mRNA levels in primary cultures of salmon hepatocytes relative to untreated cells. Oleic acid, EPA, DHA, or dexamethasone did not affect the relative expression of SR-BI in this liver model system. In conclusion, the salmon SR-BI cDNA encoded a protein with several features common to those of mammalian species. SR-BI gene expression was high in the intestine, which leads us to propose that SR-BI may contribute to the uptake of lipids from the diet.

ACTH reduces the rise in ApoB-48 levels after fat intake

It has been repeatedly demonstrated that ACTH administration lowers plasma lipid concentrations in man. The present study was designed to test the hypothesis, based on observations of decreased apolipoprotein B (ApoB) synthesis and secretion in vitro, that ACTH administration inhibits the postprandial output of ApoB in man. Therefore, we studied the response to a fat-rich meal supplemented with Vitamin A in eight healthy volunteers, who underwent this test without premedication, after 4 days administration of ACTH, and after 4 days administration of a glucocorticoid (betamethasone). As expected, fasting plasma levels of low-density lipoproteins (LDL)-cholesterol (-25%) and ApoB (-17%) decreased after ACTH, but not after betamethasone administration. Also, the elevation of plasma ApoB-48 in response to fat intake (to twice the basal levels) was markedly reduced after ACTH administration. However, the postprandial rise in plasma triglycerides and retinyl palmitate was unimpaired, suggesting that ACTH administration induced the secretion of fewer but larger chylomicrons. The effect of betamethasone on the postprandial response was similar but less pronounced. This study confirms earlier reports on the lipid-lowering effects of ACTH and supports our theory, based on in vitro studies, that the lipid-lowering effects of ACTH administration in man involves an inhibition of ApoB production.

Posttranslational processing of SREBP-1 in rat hepatocytes is regulated by insulin and cAMP

Insulin and cAMP have opposing effects on de novo fatty acid synthesis in liver and in cultured hepatocytes mediated by sterol-regulatory element binding protein (SREBP). To determine whether these agents regulate the cleavage of full-length SREBP to generate the transcriptionally active N-terminal fragment (nSREBP) in primary rat hepatocytes, an adenoviral vector (Ad-SREBP-1a) was constructed to constitutively express full-length SREBP-1a. Insulin increased, and dibutyryl (db)-cAMP inhibited, generation of nSREBP-1a from its full-length precursor. Insulin stimulated processing of SREBP-1a within 1h, and the effect was sustained for at least 24h. The initial stimulation of SREBP processing by insulin preceded measurable reduction in Insig-2 mRNA levels. Rat hepatocytes were also infected with an adenovirus expressing the nuclear form of SREBP-1c (Ad-nSREBP-1c). Insulin increased the half-life of constitutively expressed nSREBP-1c, and this effect of insulin was also inhibited by db-cAMP.

Insulin activates the rat sterol-regulatory-element-binding protein 1c (SREBP-1c) promoter through the combinatorial actions of SREBP, LXR, Sp-1 and NF-Y cis-acting elements

The enhanced synthesis of fatty acids in the liver and adipose tissue in response to insulin is critically dependent on the transcription factor SREBP-1c (sterol-regulatory-element-binding protein 1c). Insulin increases the expression of the SREBP-1c gene in intact liver and in hepatocytes cultured in vitro. To learn the mechanism of this stimulation, we analysed the activation of the rat SREBP-1c promoter and its truncated or mutated congeners driving a luciferase reporter gene in transiently transfected rat hepatocytes. The rat SREBP-1c promoter contains binding sites for LXR (liver X receptor), Sp1, NF-Y (nuclear factor-Y) and SREBP itself. We have found that each of these sites is required for the full stimulatory response of the SREBP-1c promoter to insulin. Mutation of either the putative LXREs (LXR response elements) or the SRE (sterol response element) in the proximal SREBP-1c promoter reduced the stimulatory effect of insulin by about 50%. Insulin and the LXR agonist TO901317 increased the association of SREBP-1 with the SREBP-1c promoter. Ectopic expression of LXRalpha or SREBP-1c increased activity of the SREBP-1c promoter, and this effect is further enhanced by insulin. The Sp1 and NF-Y sites adjacent to the SRE are also required for full activation of the SREBP-1c promoter by insulin. We propose that the combined actions of the SRE, LXREs, Sp1 and NF-Y elements constitute an insulin-responsive cis-acting unit of the SREBP-1c gene in the liver.

Inhibition of the synthesis of apolipoprotein B-containing lipoproteins

Increased serum concentrations of low density lipoproteins represent a major cardiovascular risk factor. Low-density lipoproteins are derived from very low density lipoproteins secreted by the liver. Apolipoprotein (apo)B that constitutes the essential structural protein of these lipoproteins exists in two forms, the full length form apoB-100 and the carboxy-terminal truncated apoB-48. The generation of apoB-48 is due to editing of the apoB mRNA which generates a premature stop translation codon. The editing of apoB mRNA is an important regulatory event because apoB-48-containing lipoproteins cannot be converted into the atherogenic low density lipoproteins. The apoB gene is constitutively expressed in liver and intestine, and the rate of apoB secretion is regulated post-transcriptionally. The translocation of apoB into the endoplasmic reticulum is complicated by the hydrophobicity of the nascent polypeptide. The assembly and secretion of apoB-containing lipoproteins within the endoplasmic reticulum is strictly dependent on the microsomal tricylceride transfer protein which shuttles triglycerides onto the nascent lipoprotein particle. The overall synthesis of apoB lipoproteins is regulated by proteosomal and nonproteosomal degradation and is dependent on triglyceride availability. Noninsulin dependent diabetes mellitus, obesity and the metabolic syndrome are characterized by an increased hepatic synthesis of apoB-containing lipoproteins. Interventions aimed to reduce the hepatic secretion of apoB-containing lipoproteins are therefore of great clinical importance. Lead targets in these pathways are discussed.

Identification of novel alternative splice variants of APOBEC-1 complementation factor with different capacities to support apolipoprotein B mRNA editing

Two novel mRNA transcripts have been identified that result from species- and tissue-specific, alternative polyadenylation and splicing of the pre-mRNA encoding the apolipoprotein B (apoB) editing catalytic subunit 1 (APOBEC-1) complementation factor (ACF) family of related proteins. The alternatively processed mRNAs encode 43- and 45-kDa proteins that are components of the previously identified p44 cluster of apoB RNA binding, editosomal proteins. Recombinant ACF45 displaced ACF64 and ACF43 in mooring sequence RNA binding but did not demonstrate strong binding to APOBEC-1. In contrast, ACF43 bound strongly to APOBEC-1 but demonstrated weak binding to mooring sequence RNA. Consequently ACF45/43 complemented APOBEC-1 in apoB mRNA editing with less efficiency than full-length ACF64. These data, together with the finding that all ACF variants were co-expressed in rat liver nuclei (the site of apoB mRNA editing), suggested that ACF variants might compete with one another for APOBEC-1 and apoB mRNA binding and thereby contribute to the regulation of apoB mRNA editing. In support for this hypothesis, the ratio of nuclear ACF65/64 to ACF45/43 decreased when hepatic editing was inhibited by fasting and increased when editing was re-stimulated by refeeding. These findings suggested a new model for the regulation of apoB mRNA editing in which the catalytic potential of editosomes is modulated at the level of their assembly by alterations in the relative abundance of multiple related RNA-binding auxiliary proteins and the expression level of APOBEC-1.

Developmental regulation of apolipoprotein B mRNA editing is an autonomous function of small intestine involving homeobox gene Cdx1

Apolipoprotein B mRNA editing is developmentally regulated in the human and rodent small intestine, changing from <1% at day 14 to approximately 90% by day 20 in the rat fetus. This regulation is coincident with the developmental formation of the crypt-to-villus axis functional unit, a continuous and rapidly renewing system involving cell generation, migration, and differentiation. Utilizing small intestine isografts implanted into the subcutaneous tissue of adult recipients, apolipoprotein B mRNA editing was developmentally up-regulated, parallel to that seen with an intact control. In contrast, apoB mRNA expression remains nearly constant in the isograft, unlike the normal intact small intestine. Immunohistochemical analyses demonstrated that apoB-48 protein existed predominantly in well differentiated enterocytes along the villus surface whereas apoB-100 was in the lamina propria and crypts. ApoB mRNA editing levels were very low in the crypt-like rat intestinal cell line, IEC-6 ( approximately 0.3%), but very high in well differentiated enterocytes ( approximately 91.5%). The expression of homeobox gene Cdx1 increased 18-fold in small intestine in vivo during the same time course when apoB mRNA editing increased from approximately 2 to approximately 90%. The overexpression of Cdx1 in IEC-6 cells increased apoB mRNA editing over 10-fold compared with the vector control. This increase was associated with a significant increase of activating factor ACF, a component of the apoB mRNA editing complex. Taken together, these data suggest that the developmental regulation of apoB mRNA editing is an autonomous cytodifferentiation function of small intestine for which homeobox gene Cdx1 may play an important role.

Interaction between growth hormone and insulin in the regulation of lipoprotein metabolism in the rat

The importance of insulin for the in vivo effects of growth hormone (GH) on lipid and lipoprotein metabolism was investigated by examining the effects of GH treatment of hypophysectomized (Hx) female rats with and without concomitant insulin treatment. Hypophysectomy-induced changes of HDL, apolipoprotein (apo)E, LDL, and apoB levels were normalized by GH treatment but not affected by insulin treatment. The hepatic triglyceride secretion rate was lower in Hx rats than in normal rats and increased by GH treatment. This effect of GH was blunted by insulin treatment. The triglyceride content in the liver changed in parallel with the changes in triglyceride secretion rate, indicating that the effect of the hormones on triglyceride secretion was dependent on changed availability of triglycerides for VLDL assembly. GH and insulin independently increased editing of apoB mRNA, but the effects were not additive. The expression of fatty-acid synthase (FAS), stearoyl-CoA desaturase-1 (SCD-1), and sterol regulatory element-binding protein-1c (SREBP-1c) was increased by GH treatment. Insulin and GH had no additive effects on these genes; instead, insulin blunted the effect of GH on SREBP-1c mRNA. In contrast to the liver, adipose tissue expression of SREBP-1c, FAS, or SCD-1 mRNA was not influenced by GH. In conclusion, the increased hepatic expression of lipogenic enzymes after GH treatment may be explained by increased expression of SREBP-1c. Insulin does not mediate the effects of GH but inhibits the stimulatory effect of GH on hepatic SREBP-1c expression and triglyceride secretion rate.

The editosome for cytidine to uridine mRNA editing has a native complexity of 27S: identification of intracellular domains containing active and inactive editing factors

Apolipoprotein B mRNA cytidine to uridine editing requires the assembly of a multiprotein editosome comprised minimally of the catalytic subunit,apolipoprotein B mRNA editing catalytic subunit 1 (APOBEC-1), and an RNA-binding protein, APOBEC-1 complementation factor (ACF). A rat homolog has been cloned with 93.5% identity to human ACF (huACF). Peptide-specific antibodies prepared against huACF immunoprecipitated a rat protein of similar mass as huACF bound to apolipoprotein B (apoB) RNA in UV cross-linking reactions, thereby providing evidence that the p66, mooring sequence-selective, RNA-binding protein identified previously in rat liver by UV cross-linking and implicated in editosome assembly is a functional homolog of huACF. The rat protein (p66/ACF) was distributed in both the nucleus and cytoplasm of rat primary hepatocytes. Within a thin section, a significant amount of total cellular p66/ACF was cytoplasmic, with a concentration at the outer surface of the endoplasmic reticulum. Native APOBEC-1 co-fractionated with p66/ACF in the cytoplasm as 60S complexes. In the nucleus, the biological site of apoB mRNA editing, native p66/ACF, was localized to heterochromatin and fractionated with APOBEC-1 as 27S editosomes. When apoB mRNA editing was stimulated in rat primary hepatocytes with ethanol or insulin, the abundance of p66/ACF in the nucleus markedly increased. It is proposed that the heterogeneity in size of complexes containing editing factors is functionally significant and reflects functionally engaged editosomes in the nucleus and an inactive cytoplasmic pool of factors.

Regulation of the rat SREBP-1c promoter in primary rat hepatocytes

We have cloned 5 kb of genomic DNA encompassing 1.72 kb of 5'-regulatory sequence and exons 1-c and 2 of the rat SREBP-1c gene. A 1.5-kb segment upstream from the transcription start site was ligated ahead of the luciferase reporter gene and tested for promoter activity by transient transfection assays in primary rat hepatocytes. We discovered that insulin strongly activated the full-length promoter, regardless of whether 5 or 20 mM glucose was in the culture medium during treatment. Stimulation by insulin was blocked by dibutyryl-cAMP and by polyunsaturated fatty acids, such as alpha-linolenic acid, gamma-linolenic acid, or eicosapentaenoic acid; palmitic or oleic acids, however, had no inhibitory effect. A truncated promoter containing 149 bp of 5' flanking DNA, including proximal NF-Y, E-box, SRE, and Sp1 sites, retained most of the response. This is the first report that insulin, cAMP, and polyunsaturated fatty acids modulate the proximal SREBP-1c promoter in rat hepatocytes mirroring physiological regulation of SREBP-1c in vivo.

Hepatic very low density lipoprotein-ApoB overproduction is associated with attenuated hepatic insulin signaling and overexpression of protein-tyrosine phosphatase 1B in a fructose-fed hamster model of insulin resistance

A fructose-fed hamster model of insulin resistance was previously documented to exhibit marked hepatic very low density lipoprotein (VLDL) overproduction. Here, we investigated whether VLDL overproduction was associated with down-regulation of hepatic insulin signaling and insulin resistance. Hepatocytes isolated from fructose-fed hamsters exhibited significantly reduced tyrosine phosphorylation of the insulin receptor and insulin receptor substrates 1 and 2. Phosphatidylinositol 3-kinase activity as well as insulin-stimulated Akt-Ser473 and Akt-Thr308 phosphorylation were also significantly reduced with fructose feeding. Interestingly, the protein mass and activity of protein-tyrosine phosphatase-1B (PTP-1B) were significantly higher in fructose-fed hamster hepatocytes. Chronic ex vivo exposure of control hamster hepatocytes to high insulin also appeared to attenuate insulin signaling and increase PTP-1B. Elevation in PTP-1B coincided with marked suppression of ER-60, a cysteine protease postulated to play a role in intracellular apoB degradation, and an increase in the synthesis and secretion of apoB. Sodium orthovanadate, a general phosphatase inhibitor, partially restored insulin receptor phosphorylation and significantly reduced apoB secretion. In summary, we hypothesize that fructose feeding induces hepatic insulin resistance at least in part via an increase in expression of PTP-1B. Induction of hepatic insulin resistance may then contribute to reduced apoB degradation and enhanced VLDL particle assembly and secretion.

Ethanol stimulates apolipoprotein B mRNA editing in the absence of de novo RNA or protein synthesis

Apolipoprotein B (apoB) mRNA editing involves a site-specific modification of cytidine to form uridine. The reaction is catalyzed in the nucleus by a multi-protein editosome. Rat hepatic editing is regulated during development, metabolically and in response to ethanol. Ethanol stimulated editing in hepatocytes within minutes of exposure. In the present study, we show that ethanol stimulated apoB mRNA synthesis and apoB mRNA editing. Significantly, the proportion of edited apoB mRNA also increased following ethanol treatment of transcription or translation arrested cells. These data suggested that ethanol could regulate editing activity using pre-existing editosomal proteins. In addition, the presence of a suppressor of apoB mRNA editing activity was suggested by the finding that inhibition of mRNA or protein synthesis alone was sufficient to increase the proportion of edited RNA. It is proposed that the level of editing activity observed in hepatocytes may be the end result of positive and negative regulatory proteins.

Long-term growth hormone excess induces marked alterations in lipoprotein metabolism in mice

The effects of long-term chronic growth hormone (GH) excess on lipid and lipoprotein metabolism were investigated in 8-mo-old bovine GH (bGH)-transgenic mice. Total body weight, serum cholesterol, insulin-like growth factor-I, and insulin levels were higher, whereas serum levels of glucose, free fatty acids, and triglycerides were lower in transgenic mice. Very low-density lipoprotein (VLDL) cholesterol levels were lower, and low-density lipoprotein (LDL) cholesterol levels were higher, in transgenic mice irrespective of gender, whereas only transgenic male mice had higher high-density lipoprotein cholesterol levels. Total serum apolipoprotein B (apoB) levels were not affected, but the amount of apoB in the LDL fraction was higher in transgenic mice. Hepatic LDL receptor expression was unchanged, whereas apoB mRNA editing and hepatic triglyceride secretion rate were reduced in bGH-transgenic male mice. Both lipoprotein lipase activity in adipose and heart tissue and beta-adrenergic-stimulated lipolysis were increased in transgenic male mice. The relative weight of adipose tissue was lower in transgenic mice, whereas hepatic triglyceride content was unchanged. Fat feeding of the mice equalized serum triglycerides and free fatty acids in bGH-transgenic and control mice. In summary, long-term GH excess is associated with marked alterations in lipid and lipoprotein metabolism, indicating decreased production and increased degradation of VLDL and preferential flux of fatty acids to muscle tissues.

Phosphorylation is a regulatory mechanism in apolipoprotein B mRNA editing

The editing of apolipoprotein B (apoB) mRNA is under tissue-specific, developmental and metabolic regulation. We found that multiple protein kinase inhibitors or activators increased apoB mRNA editing up to 2.5-fold in Caco-2 cells and 3-8-fold in McA7777 and FAO rat cells respectively. The phosphorylation-agent-induced modulation is independent of the apolipoprotein B editing catalytic subunit 1 (APOBEC-1) and of apoB mRNA expression levels, indicating the involvement of a protein modification, such as phosphorylation, regulating the cellular editing of apoB mRNA. Transient expression of protein kinase C-&theta; more than doubled apoB mRNA editing in FAO cells. Chronic exposure to ethanol, a treatment known to increase the expression of protein kinases and to change protein phosphorylation status, increased apoB mRNA editing in FAO cells up to 2.5-fold without increasing the mRNA abundance of APOBEC-1. The elimination of potential phosphorylation sites 47 and 72 of human APOBEC-1 decreased its activity to approx. one-eighth of control levels by a Ser(47)-->Ala mutation, but more than doubled the activity by a Ser(72)-->Ala mutation. The activity modulation was reversed by a Ser-->Asp mutation at sites 47 and 72, which introduced a phosphorylation-like carbonic acid group. Both human APOBEC-1 dephosphorylated by alkaline phosphase and the Ser(47,72)-to-alanine double mutant protein demonstrated a shifted isoelectric focusing pattern compared with the wild type, indicating phosphorylation at these sites. Taken together, these results suggest that phosphorylation might be an important mechanism in the regulation of apoB mRNA editing.

Studies with apolipoprotein A-II transgenic mice indicate a role for HDLs in adiposity and insulin resistance

Apolipoprotein A-II (apoA-II) is the second most abundant protein in HDLs. Genetic studies in humans have provided evidence of linkage of the apoA-II gene locus to plasma free fatty acid (FFA) levels and to type 2 diabetes, and transgenic mice overexpressing mouse apoA-II have elevated levels of both FFA and triglycerides. We now show that apoA-II promotes insulin resistance and has diverse effects on fat homeostasis. ApoA-II transgenic mice have increased adipose mass and higher plasma leptin levels than C57BL/6J control mice. Fasting glucose levels were similar between apoA-II transgenic and control mice, but plasma insulin levels were elevated approximately twofold in the apoA-II transgenic mice. Compared with control mice, apoA-II transgenic mice exhibited a delay in plasma clearance of a glucose bolus. Adipose tissue isolated from fasted apoA-II transgenic mice exhibited a 50% decrease in triglyceride hydrolysis compared with adipose tissue from control mice. This is consistent with a normal response of adipose tissue to the increased insulin levels in the apoA-II transgenic mice and may partially explain the increased fat deposition. Skeletal muscle isolated from fasted apoA-II transgenic mice exhibited reduced uptake of 2-deoxyglucose compared with muscles isolated from control mice. Our observations indicate that a primary disturbance in lipoprotein metabolism can result in several traits associated with insulin resistance, consistent with the hypothesis that insulin resistance and type 2 diabetes can, under certain circumstances, be related primarily to altered lipid metabolism rather than glucose metabolism.

APOLIPOPROTEIN B: mRNA editing, lipoprotein assembly, and presecretory degradation

Apolipoprotein (apo)B circulates in two distinct forms, apoB100 and apoB48. Human liver secretes apoB100, the product of a large mRNA encoding 4536 residues. The small intestine of all mammals secretes apoB48, which arises following C-to-U deamination of a single cytidine base in the nuclear apoB transcript, introducing a translational stop codon. This process, referred to as apoB RNA editing, operates through a multicomponent enzyme complex that contains a single catalytic subunit, apobec-1, in addition to other protein factors that have yet to be cloned. ApoB RNA editing also exhibits stringent cis-acting requirements that include both structural and sequence-specific elements-specifically efficiency elements that flank the minimal cassette, an AU-rich RNA context, and an 11-nucleotide mooring sequence-located in proximity to a suitably positioned (usually upstream) cytidine. C-to-U RNA editing may become unconstrained under circumstances where apobec-1 is overexpressed, in which case multiple cytidines in apoB RNA, as well as in other transcripts, undergo C-to-U editing. ApoB RNA editing is eliminated following targeting of apobec-1, establishing that there is no genetic redundancy in this function. Under physiological circumstances, apoB RNA editing exhibits developmental, hormonal, and nutritional regulation, in some cases related to transcriptional regulation of apobec-1 mRNA. ApoB and the microsomal triglyceride transfer protein (MTP) are essential for the assembly and secretion of apoB-containing lipoproteins. MTP functions by transferring lipid to apoB during its translation and by transporting triglycerides into the endoplasmic reticulum to form apoB-free lipid droplets. These droplets fuse with nascent apoB-containing particles to form mature, very low-density lipoproteins or chylomicrons. In cultured hepatic cells, lipid availability dictates the rate of apoB production. Unlipidated or underlipidated forms of apoB are subjected to presecretory degradation, a process mediated by retrograde transport from the lumen of the endoplasmic reticulum to the cytosol, coupled with multiubquitination and proteasomal degradation. Although control of lipid secretion in vivo is primarily achieved at the level of lipoprotein particle size, regulation of apoB production by presecretory degradation may be relevant in some dyslipidemic states.

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.

Calcium increases apolipoprotein B mRNA editing

ApoB-100 and apoB-48 are major components of chylomicrons, very low-density lipoprotein (VLDL) and low-density lipoprotein (LDL). The two proteins are generated from a single apoB mRNA by apoB mRNA editing which induces an in-frame stop codon in apoB mRNA. Apolipoprotein B (apoB) mRNA editing is an important determinant of the proportion of full-length (apoB-100) and truncated (apoB-48) proteins in total apoB metabolism. Calcium is involved in the regulation of secretion and synthesis of VLDL and apoB. In this paper, we demonstrate for the first time that the amount of edited apoB mRNA in the cultured cells Caco-2 and McA7777 is markedly increased by calcium. Increasing extracellular calcium concentration, calcium ionophore (A23187 and ionomycin) treatment, and depleting calcium stores and raising cytoplasmic calcium concentration by thapsigargin increase apoB mRNA editing up to threefold in a dose dependent manner. Calcium has no direct stimulative effect on apoB mRNA editing in an in vitro editing system. The editing increase by extracellular calcium is not related to alterations of APOBEC-1 mRNA expression. These data suggest that calcium is not only involved in the regulation of apolipoprotein metabolism but also apoB mRNA editing.

Acute modulation of the extent of apoB mRNA editing and the relative rates of syntheses of apoB48 and apoB100 in cultured rat hepatocytes by osmotic and other stress stimuli

The mRNA for apolipoprotein B (apoB) is edited by the enzyme APOBEC-1, which acts as part of a multiprotein complex or editosome. In cultured rat hepatocytes obtained from fed animals this results in the presence of edited and unedited apoB mRNA in a ratio of approximately 3:2 in the basal state. In this study we show that hyper-osmotic media, which induce cell shrinkage, resulted in an acute increase in the degree of editing of apoB mRNA (hypo-osmotic conditions had no effect). This increase was accompanied by a parallel and highly positively correlated change in the ratio of the rate of synthesis of apoB48 relative to that of apoB100. These changes occurred in the absence of any changes in the overall APOBEC-1 mRNA levels, indicating that the activation of editing occurred at a post-transcriptional level. Levels of total apoB mRNA were also unaffected by hyper-osmotic exposure of the cells indicating that changes in the relative rates of synthesis of apoB48 and apoB100 were due to post/translational events. Exposure of cells to anisomycin at concentrations (50 micrograms/ml) that inhibit protein synthesis or to the transcriptional inhibitor actinomycin D produced changes in the degree of apoB mRNA editing that were similar to those given by hyper-osmotic shock indicating that editing is able to respond acutely to transcriptional or translational inhibition. Anisomycin, at concentrations (50 ng/ml) that activate SAPK/JNK but do not inhibit protein synthesis, gave only a fraction of the effect of hyper-osmotic shock. SB203580, an inhibitor of p38 kinase, did not attenuate the effects of hyper-osmotic conditions on APOBEC-1 editing. These observations suggest that these MAPkinase pathways play a relatively minor part in the transduction of the osmotic stimulus to the editing mechanism. The hyper-osmotically-induced increase in apoB mRNA editing was also insensitive to PD98059 and wortmannin (inhibitors of MEK and PI3 kinase, respectively). These data provide evidence that apoB mRNA editing is capable of acute modulation independently of transcriptional or translational mechanisms and suggest that one or more components of the editosome may undergo post-translational activation.

Apolipoprotein B mRNA editing and apolipoprotein gene expression in the liver of hyperinsulinemic fatty Zucker rats: relationship to very low density lipoprotein composition

We previously demonstrated increased apolipoprotein B (apoB) mRNA editing, elevated levels of mRNA for the catalytic component of the apoB mRNA editing complex, apobec-1, and increased secretion of the product of the edited mRNA, apoB48, in very low density lipoproteins (VLDL) in primary cultures of Sprague-Dawley rat hepatocytes following insulin treatment. In order to determine the effect of in vivo hyperinsulinemia on these processes, we determined apoB mRNA editing, apobec-1 expression, hepatic expression of mRNA for apoB and other VLDL apoproteins, and the quantity and composition of plasma VLDL in the hyperinsulinemic fatty Zucker rat. Total apoB mRNA content of the livers of the fatty rats and lean littermates did not differ; however, edited apoB message coding for hepatic apo B48, and abundance of mRNA for the catalytic subunit of the apoB mRNA editing complex, apobec-1, was increased by 1.7- and 3.3-fold, respectively, in fatty rats. ApoCIII mRNA abundance was increased in livers of fatty rats as well, but the abundance of hepatic apoE mRNA in the fatty animal was not different from that of the lean rat. Hepatic apoAI mRNA abundance was also increased in the fatty rats. Associated with increased apoB mRNA editing, was the 1.7-fold increase in the fraction of apoB in plasma as apoB48 in fatty rats. VLDL-triglyceride and -apoB in plasma were 15- and 3-fold higher, respectively, in fatty Zucker rats compared to lean littermates, indicating both enrichment of VLDL with triglycerides and increased accumulation of VLDL particles. Increased hepatic expression of mRNA for apoCIII and apoAI was associated with increased content of apoC (and relative depletion of apoE) in VLDL of fatty rats, and plasma apoAI was increased in fatty Zucker rats, primarily in the HDL fraction. The current study provides further evidence that chronic exposure to high levels of insulin influences both the quantity of and lipid/apoprotein composition of VLDL in plasma. The increased apoC and decreased apoE (as well as increased triglyceride) content of VLDL in the fatty Zucker rat observed in the current study may affect VLDL clearance and therefore may be a factor in the observed accumulation of VLDL in the plasma of the fatty hyperinsulinemic Zucker rats.

Distinct promoters induce APOBEC-1 expression in rat liver and intestine

The expression of apolipoprotein (apo) B can be modulated by mRNA editing, a unique posttranscriptional base change in the apo B mRNA. Apo B-48, the translation product of edited apo B mRNA, is not a precursor of the atherogenic low density lipoproteins and lipoprotein(a). In humans and various other mammals, the apo B mRNA is edited in the intestine but not in the liver, which exclusively secretes apo B-100-containing lipoproteins as precursors for low density lipoprotein formation. In species such as the rat, mouse, dog, and horse, apo B mRNA is also edited in the liver, resulting in low plasma levels of low density lipoprotein. Editing of the apo B mRNA is mediated by the apo B mRNA-editing enzyme complex, of which the catalytic subunit APOBEC-1 is not expressed in the liver of species without hepatic editing. To understand the molecular basis for liver-specific expression of APOBEC-1 and the editing of hepatic apo B mRNA, the expression pattern and genomic organization of the rat APOBEC-1 gene have been characterized. The rat APOBEC-1 gene contains 6 exons and 2 promoters with distinct activities. The expression of APOBEC-1 in the rat liver is the result of a promoter located upstream, with tissue-specific exon use and alternate splicing within the 5'-untranslated region of APOBEC-1 mRNA encoded by exon 2. In addition to the liver, this promoter also induces APOBEC-1 expression in the spleen, lung, kidney, heart, and skeletal muscle. The promoter located downstream belongs to a new class of TATA-less promoters and is responsible for the abundant expression of APOBEC-1 in the intestine. Mapping of the transcriptional start sites and deletion analysis of the promoter regions by using luciferase as the reporter gene have defined the regulatory elements of both promoters. The downstream, intestine-specific promoter contains a negative regulatory element between -1100 and -500, which appears to restrict its activity to the intestine. The upstream, liver-specific promoter of the rat APOBEC-1 gene induces APOBEC-1 expression and editing of apo B mRNA in human hepatoma HuH-7 and Hep G2 cells. Understanding the molecular basis for the liver-specific expression of APOBEC-1 in the rat promises new strategies to induce APOBEC-1 expression in the human liver for the reduction of atherogenic lipoprotein levels by hepatic apo B mRNA editing.

Two distinct TATA-less promoters direct tissue-specific expression of the rat apo-B editing catalytic polypeptide 1 gene

The species and tissue specificity of apolipoprotein (apo) B mRNA editing is determined by the expression of apoB editing catalytic polypeptide 1 (APOBEC-1), the cytidine deaminase that catalyzes apoB mRNA editing. To understand the molecular mechanisms that regulate the transcription of APOBEC-1, we characterized rat APOBEC-1 cDNA and genomic DNA. cDNA cloning and RNase protection analysis showed two alternative promoters for the tissue-specific expression of APOBEC-1 in the liver and intestine, Pliv and Pint. Both promoters lack a TATA box, and Pint belongs to the MED-1 class of promoters, which initiate transcription at multiple sites. We also identified two allelic forms of the APOBEC-1 gene from the characterization of two rat APOBEC-1 P1 genomic clones, RE4 and RE5. The RE4 allele is 18 kilobases long and contains six exons and five introns, whereas the RE5 allele contains an additional approximately 8 kilobases of intron sequences and an extra exon encoding a 5'-untranslated region; however, the APOBEC-1 transcripts from the two alleles appear to have similar, if not identical, functions. Transgenic mouse studies showed that Pliv was preferentially used in the liver, kidney, brain, and adipose tissues, whereas Pint was preferentially used in the small intestine, stomach, and lung. Our results suggest that the tissue-specific expression of APOBEC-1 is governed by multiple regulatory elements exerting control over a single coding sequence. The presence or absence of these regulatory elements may determine the tissue-specific expression of APOBEC-1 in other mammalian species.

Selective effects of hydrocortisone on intestinal lipoprotein and apolipoprotein synthesis in the human fetus

Studies employing human fetal intestine have yielded much interesting information on the role of polarized enterocytes in fat absorption and transport. Using the organ culture model, we examined the influence of hydrocortisone on the synthesis and secretion of lipids and lipoproteins. Human jejunal explants were cultured for 5 days at 37 degrees C in serum-free medium containing either [14C]-oleic acid or [14C]-acetate, alone or supplemented with hydrocortisone (25 or 50 ng/ml). The uptake of [14C]-oleic acid was associated with the production of triglycerides, phospholipids, and cholesteryl esters, which were all affected by hydrocortisone. This hormonal agent (50 micrograms) led to the marked reduction of secreted triglycerides (43%, P < 0.01), phospholipids (39%, P < 0.01), and cholesteryl esters (36%, P < 0.05) without altering the characteristic distribution of tissue and medium lipid classes. Similarly, hydrocortisone significantly (P < 0.01) decreased (approximately 60%) the incorporation of [14C]-acetate into secreted free and esterified cholesterol in the medium. With [14C]-oleic acid as a precursor, hydrocortisone significantly diminished the delivery of chylomicrons and very low density lipoproteins to the medium while consistently enhancing the secretion of high density lipoproteins. In parallel, [35S]-methionine pulse-labeling of jejunal explants revealed the concomitant inhibitory effect of hydrocortisone on apo B-100 synthesis and hydrocortisone's stimulatory effect on apo B-48 and apo A-1. These studies suggest that glucocorticoids play a critical role in lipoprotein processing during intestinal development.

Apobec-1 and apolipoprotein B mRNA editing

Apolipoprotein (apo)B mRNA editing is a novel mechanism for the post-transcriptional regulation of gene expression in mammals. It consists of a C-->U conversion of the first base of the codon CAA, encoding glutamine-2153, to UAA, an in-frame stop codon, in apoB mRNA. Since its initial description in 1987, substantial progress has been made in the last few years on the mechanism of editing. Apobec-1, the catalytic component of the apoB mRNA editing enzyme complex, has been cloned. This article begins with an overview of the general biology of apoB mRNA editing. It then provides an in-depth analysis of the structure, evolution and possible mechanism of action of apobec-1. ApoB mRNA editing is the prototype of RNA editing in mammals. What we learn from apoB mRNA editing will be useful in our understanding of other examples of RNA editing in vertebrates which are being described with increasing frequency.

Dexamethasone enhanced by insulin, but not by thyroid hormones stimulates apolipoprotein B mRNA editing in cultured rat hepatocytes depending on the developmental stage

The increase of hepatic apolipoprotein B (apoB) mRNA editing during rat development was not affected by hypothyroidism. Furthermore, the addition of 3,3',5'-triiodothyronine (T3) to cultured hepatocytes taken from fetal, neonatal and adult rats had no effect on apoB mRNA editing. In contrast, dexamethasone markedly stimulated apoB mRNA editing in hepatocytes taken from neonates. This effect was enhanced by the addition of insulin. For the first time our data provide evidence that glucocorticoids together with insulin are important for the regulation of apoB mRNA editing during postnatal development, whereas thyroid hormones are not critical for this process.

Copper deficiency increases hepatic apolipoprotein B secretion and mRNA editing in rats

Male weanling Sprague-Dawley rats were assigned to copper-deficient (9.0 mumol Cu/kg diet) or copper-adequate (102 mumol Cu/kg diet) dietary treatments for 6 wk. Pulse-chase studies using freshly isolated rat liver parenchymal cells demonstrated that apolipoprotein B (apoB)-48 and apoB-100 syntheses were not altered, but secretion was increased twofold in hepatocytes derived from copper-deficient rats. Both plasma apoB-48 and apoB-100 levels were increased by copper deficiency, but only the apoB-48 increase was significant. Hepatic apoB mRNA editing, expressed as a ratio of apoB-48 mRNA to apoB-48 plus apoB-100 mRNA, was significantly increased from 60.8% in copper-adequate to 70.2% in copper-deficient rats. Moreover, hepatic apoB mRNA abundance was not significantly altered by copper deficiency. Thus the increased amount of nascent apoB-48 secreted into the medium as well as the enhanced apoB mRNA editing may have contributed to the differential increase in plasma apoB-48 over apoB-100 level in copper-deficient rats.

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.

Effects of diabetes mellitus on hepatocyte nuclear factor 1 decrease albumin gene transcription

We have previously reported that albumin gene transcription is reduced in diabetes mellitus (DM). The present study explored the mechanism by which albumin gene transcription is down-regulated in DM. Deletional studies and displacement of factors binding to site B of the albumin promoter indicated that the repressive effects of DM are mediated by nuclear factors binding to this site. Since hepatocyte nuclear factor 1 (HNF1) activates albumin promoter activity and is the predominant factor binding to site B, we examined HNF1. The abundance and binding activity of HNF1 were reduced in hepatonuclear extracts from diabetic compared to control rats. However, HNF1 mRNA levels were unchanged, suggesting that the effect of DM on HNF1 is at the post-transcriptional level. Extracts from diabetic animals also contained another protein, distinct from HNF1 and vHNF1, which bound to site B in gel retardation studies. In summary, our studies demonstrate that the reduced abundance and binding activity of HNF1 correlates with decreased albumin gene transcription in DM.

Chronic exogenous hyperinsulinaemia does not modify the acute inhibitory effect of insulin on the secretion of very-low-density lipoprotein triacylglycerol and apolipoprotein B in primary cultures of rat hepatocytes

Male Wistar rats were fitted with subcutaneous osmotic mini-pumps that delivered insulin at a constant rate of 0.20 i.u./h for 7 days. This treatment raised the plasma insulin concentration from 31 +/- 4 to 201 +/- 64 micro-i.u./ml. Hepatocytes prepared from the hyperinsulinaemic animals secreted very-low-density lipoprotein (VLDL) triacylglycerol (TAG) at a higher rate (172 +/- 21 microgram per 24 h per mg cell protein) than did those from sham-operated controls (109 +/- 12 microgram per 24 h per mg) (P<0.05). However, chronic exogenous hyperinsulinaemia had no stimulatory effect on the secretion of VLDL apolipoprotein B (apoB) in derived hepatocytes compared with those from the sham-operated controls (2.32 +/- 0.38 compared with 3.09 +/- 0.40 microgram per 24 h per mg). Hepatocytes from the hyperinsulinaemic rats thus secreted larger VLDL particles as evidenced by the increased TAG:apoB ratio (78.4 +/- 13.1 compared with 38.4 +/- 7.6; P<0.05). In hepatocytes from the hyperinsulinaemic rats a larger proportion of the newly synthesized TAG was secreted as VLDL. Hepatocytes from the hyperinsulinaemic and the sham-operated control animals were equally sensitive to the inhibitory effect of insulin added in vitro on the secretion of VLDL TAG. Insulin added in vitro to the culture medium of hepatocytes from hyperinsulinaemic animals significantly decreased the TAG:apoB ratio of the secreted VLDL. This change did not occur in hepatocytes from sham-operated rats. These results suggest that, in vivo, chronic hyperinsulinaemia is not in itself sufficient to desensitize the liver to the acute inhibitory effect of insulin on the secretion of VLDL.

Gene transfer of cytidine deaminase apoBEC-1 lowers lipoprotein(a) in transgenic mice and induces apolipoprotein B editing in rabbits

Apolipoprotein (apo) B100 is an essential component of low-density lipoproteins (LDL) and lipoprotein(a) [Lp(a)]. In mammals, apoB can be edited post-transcriptionally to encode a truncated form of apoB (apoB48) that is unable to form either of these atherogenic lipoproteins. To study the effect of increasing hepatic apoB editing activity on formation of Lp(a), a recombinant adenovirus encoding rat apoBEC-1, the cytidine deaminase component of the apoB mRNA editing complex, was administered to human apoB/apo(a) transgenic mice. This resulted in expression of apoBEC-1 in hepatocytes of these mice, increased hepatic editing of human apoB mRNA, and decreased plasma levels of human apoB100 and Lp(a). The apoBEC-1 recombinant adenovirus was also administered to rabbits, an animal which, like humans, naturally lacks hepatic apoB editing. Expression of the exogenous apoBEC-1 in rabbit liver resulted in editing of up to 10% of apoB mRNA. Hepatic apoB editing was associated with lower LDL levels in these rabbits relative to those treated with a control adenovirus. However, LDL levels were elevated significantly in both animals as a result of adenovirus injection. These studies demonstrate that introduction of the cytidine deaminase apoBEC-1 is sufficient to induce hepatic apoB editing in an animal lacking this activity, and that induction of editing could serve as a novel approach for lowering plasma concentrations of the atherogenic lipoproteins Lp(a) and LDL.

VLDL output by hepatocytes from obese Zucker rats is resistant to the inhibitory effect of insulin

The effects of insulin (0-780 nM) on the secretion of very-low-density lipoprotein (VLDL) apolipoprotein B (apoB) and triacylglycerol (TAG) in hepatocytes from obese Zucker rats and from their lean littermates were studied over a total period of 48 h in chemically defined culture medium. Cells from the obese Zucker rats initially secreted more TAG than those from the lean animals. Cells from the former were resistant to the inhibitory effect of insulin on the secretion of TAG. These changes were accompanied by an increased rate of TAG synthesis. Although the hepatocytes from the obese animals initially secreted less apoB than those from the lean, apoB output from the former could not be suppressed by insulin. Prolonging the length of the culture period resulted in the acquisition of sensitivity to the inhibitory effect of insulin in the hepatocytes from the obese rats. This occurred more rapidly for the secretion of TAG than of apoB. Under these conditions, the initial difference in the rate of TAG synthesis in the hepatocytes from the obese and from the lean animals was also abolished.