Hepatic ABCA1 deficiency is associated with delayed apolipoprotein B secretory trafficking and augmented VLDL triglyceride secretion

ATP binding cassette transporter A1 (ABCA1) is a membrane transporter that facilitates nascent HDL formation. Tangier disease subjects with complete ABCA1 deficiency have <5% of normal levels of plasma HDL, elevated triglycerides (TGs), and defective vesicular trafficking in fibroblasts and macrophages. Hepatocyte-specific ABCA1 knockout mice (HSKO) have a similar lipid phenotype with 20% of normal plasma HDL levels and a two-fold elevation of plasma TGs due to hepatic overproduction of large, triglyceride-enriched VLDL. We hypothesized that enhanced VLDL TG secretion in the absence of hepatocyte ABCA1 is due to altered intracellular trafficking of apolipoprotein B (apoB), resulting in augmented TG addition to nascent VLDL. We found that trafficking of newly synthesized apoB through the secretory pathway was delayed in ABCA1-silenced rat hepatoma cells and HSKO primary hepatocytes, relative to controls. Endoglycosidase H treatment of cellular apoB revealed a likely delay in apoB trafficking in post-ER compartments. The reduced rate of protein trafficking was also observed for an adenoviral-expressed GPI-linked fluorescent fusion protein, but not albumin, suggesting a selective delay of secretory cargoes in the absence of hepatocyte ABCA1. Our results suggest an important role for hepatic ABCA1 in regulating secretory trafficking and modulating VLDL expansion during the TG accretion phase of hepatic lipoprotein particle assembly.

The constitutive lipid droplet protein PLIN2 regulates autophagy in liver

Excess triglyceride (TG) accumulation in the liver underlies fatty liver disease, a highly prevalent ailment. TG occurs in the liver sequestered in lipid droplets, the major lipid storage organelle. Lipid droplets are home to the lipid droplet proteins, the most abundant of which are the perilipins (PLINs), encoded by 5 different genes, Plin1 to Plin5. Of the corresponding gene products, PLIN2 is the only constitutive and ubiquitously expressed lipid droplet protein that has been used as a protein marker for lipid droplets. We and others reported that plin2^-/- mice have an ∼60% reduction in TG content, and are protected against fatty liver disease. Here we show that PLIN2 overexpression protects lipid droplets against macroautophagy/autophagy, whereas PLIN2 deficiency enhances autophagy and depletes hepatic TG. The enhanced autophagy in plin2^-/- mice protects against severe ER stress-induced hepatosteatosis and hepatocyte apoptosis. In contrast, hepatic TG depletion resulting from other genetic and pharmacological manipulations has no effect on autophagy. Importantly, PLIN2 deficiency lowers cellular TG content in wild-type mouse embryonic fibroblasts (MEFs) via enhanced autophagy, but does not affect cellular TG content in atg7^-/- MEFs that are devoid of autophagic function. Conversely, adenovirus-shAtg7-mediated hepatic Atg7 knockdown per se does not alter the hepatic TG level, suggesting a more complex regulation in vivo. In sum, PLIN2 guards its own house, the lipid droplet. PLIN2 overexpression protects against autophagy, and its downregulation stimulates TG catabolism via autophagy.

Very Low Density Lipoprotein Assembly Is Required for cAMP-responsive Element-binding Protein H Processing and Hepatic Apolipoprotein A-IV Expression

Hepatic apolipoprotein A-IV (apoA-IV) expression is correlated with hepatic triglyceride (TG) content in mouse models of chronic hepatosteatosis, and steatosis-induced hepatic apoA-IV gene expression is regulated by nuclear transcription factor cAMP-responsive element-binding protein H (CREBH) processing. To define what aspects of TG homeostasis regulate hepatic CREBH processing and apoA-IV gene expression, several mouse models of attenuated VLDL particle assembly were subjected to acute hepatosteatosis induced by an overnight fast or short term ketogenic diet feeding. Compared with chow-fed C57BL/6 mice, fasted or ketogenic diet-fed mice displayed increased hepatic TG content, which was highly correlated (r² = 0.95) with apoA-IV gene expression, and secretion of larger, TG-enriched VLDL, despite a lower rate of TG secretion and a similar or reduced rate of apoB100 secretion. When VLDL particle assembly and secretion was inhibited by hepatic shRNA-induced apoB silencing or genetic or pharmacologic reduction in microsomal triglyceride transfer protein (MTP) activity, hepatic TG content increased dramatically; however, CREBH processing and apoA-IV gene expression were attenuated compared with controls. Adenovirus-mediated reconstitution of MTP expression proportionately restored CREBH processing and apoA-IV expression in liver-specific MTP knock-out mice. These results reveal that hepatic TG content, per se, does not regulate CREBH processing. Instead, TG mobilization into the endoplasmic reticulum for nascent VLDL particle assembly activates CREBH processing and enhances apoA-IV gene expression in the setting of acute steatosis. We conclude that VLDL assembly and CREBH activation play key roles in the response to hepatic steatosis by up-regulating apoA-IV and promoting assembly and secretion of larger, more TG-enriched VLDL particles.

Characterization of circulating APOL1 protein complexes in African Americans

APOL1 gene renal-risk variants are associated with nephropathy and CVD in African Americans; however, little is known about the circulating APOL1 variant proteins which reportedly bind to HDL. We examined whether APOL1 G1 and G2 renal-risk variant serum concentrations or lipoprotein distributions differed from nonrisk G0 APOL1 in African Americans without nephropathy. Serum APOL1 protein concentrations were similar regardless of APOL1 genotype. In addition, serum APOL1 protein was bound to protein complexes in two nonoverlapping peaks, herein referred to as APOL1 complex A (12.2 nm diameter) and complex B (20.0 nm diameter). Neither of these protein complexes associated with HDL or LDL. Proteomic analysis revealed that complex A was composed of APOA1, haptoglobin-related protein (HPR), and complement C3, whereas complex B contained APOA1, HPR, IgM, and fibronectin. Serum HPR was less abundant on complex B in individuals with G1 and G2 renal-risk variant genotypes, relative to G0 (P = 0.0002-0.037). These circulating complexes may play roles in HDL metabolism and susceptibility to CVD.

Biogenesis and cytotoxicity of APOL1 renal risk variant proteins in hepatocytes and hepatoma cells

Two APOL1 gene variants, which likely evolved to protect individuals from African sleeping sickness, are strongly associated with nondiabetic kidney disease in individuals with recent African ancestry. Consistent with its role in trypanosome killing, the pro-death APOL1 protein is toxic to most cells, but its mechanism of cell death is poorly understood and little is known regarding its intracellular trafficking and secretion. Because the liver appears to be the main source of circulating APOL1, we examined its secretory behavior and mechanism of toxicity in hepatoma cells and primary human hepatocytes. APOL1 is poorly secreted in vitro, even in the presence of chemical chaper-ones; however, it is efficiently secreted in wild-type transgenic mice, suggesting that APOL1 secretion has specialized requirements that cultured cells fail to support. In hepatoma cells, inducible expression of APOL1 and its risk variants promoted cell death, with the G1 variant displaying the highest degree of toxicity. To explore the basis for APOL1-mediated cell toxicity, endoplasmic reticulum stress, pyroptosis, autophagy, and apoptosis were examined. Our results suggest that autophagy represents the predominant mechanism of APOL1-mediated cell death. Overall, these results increase our understanding of the basic biology and trafficking behavior of circulating APOL1 from the liver.

Uncleaved ApoM signal peptide is required for formation of large ApoM/sphingosine 1-phosphate (S1P)-enriched HDL particles

Apolipoprotein M (apoM), a plasma sphingosine 1-phosphate (S1P) carrier, associates with plasma HDL via its uncleaved signal peptide. Hepatocyte-specific apoM overexpression in mice stimulates formation of both larger nascent HDL in hepatocytes and larger mature apoM/S1P-enriched HDL particles in plasma by enhancing hepatic S1P synthesis and secretion. Mutagenesis of apoM glutamine 22 to alanine (apoM(Q22A)) introduces a functional signal peptidase cleavage site. Expression of apoM(Q22A) in ABCA1-expressing HEK293 cells resulted in the formation of smaller nascent HDL particles compared with wild type apoM (apoM(WT)). When apoM(Q22A) was expressed in vivo, using recombinant adenoviruses, smaller plasma HDL particles and decreased plasma S1P and apoM were observed relative to expression of apoM(WT). Hepatocytes isolated from both apoM(WT)- and apoM(Q22A)-expressing mice displayed an equivalent increase in cellular levels of S1P, relative to LacZ controls; however, relative to apoM(WT), apoM(Q22A) hepatocytes displayed more rapid apoM and S1P secretion but minimal apoM(Q22A) bound to nascent lipoproteins. Pharmacologic inhibition of ceramide synthesis increased cellular sphingosine and S1P but not medium S1P in both apoM(WT) and apoM(Q22A) hepatocytes. We conclude that apoM secretion is rate-limiting for hepatocyte S1P secretion and that its uncleaved signal peptide delays apoM trafficking out of the cell, promoting formation of larger nascent apoM- and S1P-enriched HDL particles that are probably precursors of larger apoM/S1P-enriched plasma HDL.

Localization of APOL1 protein and mRNA in the human kidney: nondiseased tissue, primary cells, and immortalized cell lines

Although APOL1 gene variants are associated with nephropathy in African Americans, little is known about APOL1 protein synthesis, uptake, and localization in kidney cells. To address these questions, we examined APOL1 protein and mRNA localization in human kidney and human kidney-derived cell lines. Indirect immunofluorescence microscopy performed on nondiseased nephrectomy cryosections from persons with normal kidney function revealed that APOL1 protein was markedly enriched in podocytes (colocalized with synaptopodin and Wilms' tumor suppressor) and present in lower abundance in renal tubule cells. Fluorescence in situ hybridization detected APOL1 mRNA in glomeruli (podocytes and endothelial cells) and tubules, consistent with endogenous synthesis in these cell types. When these analyses were extended to renal-derived cell lines, quantitative RT-PCR did not detect APOL1 mRNA in human mesangial cells; however, abundant levels of APOL1 mRNA were observed in proximal tubule cells and glomerular endothelial cells, with lower expression in podocytes. Western blot analysis revealed corresponding levels of APOL1 protein in these cell lines. To explain the apparent discrepancy between the marked abundance of APOL1 protein in kidney podocytes observed in cryosections versus the lesser abundance in podocyte cell lines, we explored APOL1 cellular uptake. APOL1 protein was taken up readily by human podocytes in vitro but was not taken up efficiently by mesangial cells, glomerular endothelial cells, or proximal tubule cells. We hypothesize that the higher levels of APOL1 protein in human cryosectioned podocytes may reflect both endogenous protein synthesis and APOL1 uptake from the circulation or glomerular filtrate.

Hepatic apolipoprotein M (apoM) overexpression stimulates formation of larger apoM/sphingosine 1-phosphate-enriched plasma high density lipoprotein

Apolipoprotein M (apoM), a lipocalin family member, preferentially associates with plasma HDL and binds plasma sphingosine 1-phosphate (S1P), a signaling molecule active in immune homeostasis and endothelial barrier function. ApoM overexpression in ABCA1-expressing HEK293 cells stimulated larger nascent HDL formation, compared with cells that did not express apoM; however, the in vivo role of apoM in HDL metabolism remains poorly understood. To test whether hepatic apoM overexpression increases plasma HDL size, we generated hepatocyte-specific apoM transgenic (APOM Tg) mice, which had an ∼3-5-fold increase in plasma apoM levels compared with wild-type mice. Although HDL cholesterol concentrations were similar to wild-type mice, APOM Tg mice had larger plasma HDLs enriched in apoM, cholesteryl ester, lecithin:cholesterol acyltransferase, and S1P. Despite the presence of larger plasma HDLs in APOM Tg mice, in vivo macrophage reverse cholesterol transport capacity was similar to that in wild-type mice. APOM Tg mice had an ∼5-fold increase in plasma S1P, which was predominantly associated with larger plasma HDLs. Primary hepatocytes from APOM Tg mice generated larger nascent HDLs and displayed increased sphingolipid synthesis and S1P secretion. Inhibition of ceramide synthases in hepatocytes increased cellular S1P levels but not S1P secretion, suggesting that apoM is rate-limiting in the export of hepatocyte S1P. Our data indicate that hepatocyte-specific apoM overexpression generates larger nascent HDLs and larger plasma HDLs, which preferentially bind apoM and S1P, and stimulates S1P biosynthesis for secretion. The unique apoM/S1P-enriched plasma HDL may serve to deliver S1P to extrahepatic tissues for atheroprotection and may have other as yet unidentified functions.

ApoB-containing lipoproteins regulate angiogenesis by modulating expression of VEGF receptor 1

Despite the clear major contribution of hyperlipidemia to the prevalence of cardiovascular disease in the developed world, the direct effects of lipoproteins on endothelial cells have remained obscure and are under debate. Here we report a previously uncharacterized mechanism of vessel growth modulation by lipoprotein availability. Using a genetic screen for vascular defects in zebrafish, we initially identified a mutation, stalactite (stl), in the gene encoding microsomal triglyceride transfer protein (mtp), which is involved in the biosynthesis of apolipoprotein B (ApoB)-containing lipoproteins. By manipulating lipoprotein concentrations in zebrafish, we found that ApoB negatively regulates angiogenesis and that it is the ApoB protein particle, rather than lipid moieties within ApoB-containing lipoproteins, that is primarily responsible for this effect. Mechanistically, we identified downregulation of vascular endothelial growth factor receptor 1 (VEGFR1), which acts as a decoy receptor for VEGF, as a key mediator of the endothelial response to lipoproteins, and we observed VEGFR1 downregulation in hyperlipidemic mice. These findings may open new avenues for the treatment of lipoprotein-related vascular disorders.

Hepatic ABC transporters and triglyceride metabolism

PURPOSE OF REVIEW

Elevated plasma triglyceride and reduced HDL concentrations are prominent features of metabolic syndrome and type 2 diabetes. Individuals with Tangier disease also have elevated plasma triglyceride concentrations and very low HDL, resulting from mutations in ATP-binding cassette transporter A1 (ABCA1), an integral membrane protein that facilitates nascent HDL particle assembly. Past studies attributed the inverse relationship between plasma HDL and triglyceride to intravascular lipid exchange and catabolic events. However, recent studies also suggest that hepatic signaling and lipid mobilization and secretion may explain how HDL affects plasma triglyceride concentrations.

RECENT FINDINGS

Hepatocyte-specific ABCA1 knockout mice have markedly reduced plasma HDL and a two-fold increase in triglyceride due to failure to assemble nascent HDL particles by hepatocytes, causing increased catabolism of HDL apolipoprotein A-I and increased hepatic production of triglyceride-enriched VLDL. In-vitro studies suggest that nascent HDL particles may induce signaling to decrease triglyceride secretion. Inhibition of microRNA 33 expression in nonhuman primates augments hepatic ABCA1, genes involved in fatty acid oxidation, and decreases expression of lipogenic genes, causing increased plasma HDL and decreased triglyceride levels.

SUMMARY

New evidence suggests potential mechanisms by which hepatic ABCA1-mediated nascent HDL formation regulates VLDL-triglyceride production and contributes to the inverse relationship between plasma HDL and triglyceride.

Hepatic ABCA1 and VLDL triglyceride production

Elevated plasma triglyceride (TG) and reduced high density lipoprotein (HDL) concentrations are prominent features of metabolic syndrome (MS) and type 2 diabetes (T2D). Individuals with Tangier disease also have elevated plasma TG concentrations and a near absence of HDL, resulting from mutations in ATP binding cassette transporter A1 (ABCA1), which facilitates the efflux of cellular phospholipid and free cholesterol to assemble with apolipoprotein A-I (apoA-I), forming nascent HDL particles. In this review, we summarize studies focused on the regulation of hepatic very low density lipoprotein (VLDL) TG production, with particular attention on recent evidence connecting hepatic ABCA1 expression to VLDL, LDL, and HDL metabolism. Silencing ABCA1 in McArdle rat hepatoma cells results in diminished assembly of large (>10nm) nascent HDL particles, diminished PI3 kinase activation, and increased secretion of large, TG-enriched VLDL1 particles. Hepatocyte-specific ABCA1 knockout (HSKO) mice have a similar plasma lipid phenotype as Tangier disease subjects, with a two-fold elevation of plasma VLDL TG, 50% lower LDL, and 80% reduction in HDL concentrations. This lipid phenotype arises from increased hepatic secretion of VLDL1 particles, increased hepatic uptake of plasma LDL by the LDL receptor, elimination of nascent HDL particle assembly by the liver, and hypercatabolism of apoA-I by the kidney. These studies highlight a novel role for hepatic ABCA1 in the metabolism of all three major classes of plasma lipoproteins and provide a metabolic link between elevated TG and reduced HDL levels that are a common feature of Tangier disease, MS, and T2D. This article is part of a Special Issue entitled: Triglyceride Metabolism and Disease.

Abstract 431: Impact of Liver-Specific Transgenic Overexpression of Drosophila and Human MTP on Diet-Induced Hepatic Steatosis and Lipoprotein Secretion

Microsomal triglyceride transfer protein (MTP) is essential for the formation of apolipoprotein B (apoB)-containing lipoproteins. Previous results established that invertebrate forms of MTP (e.g., Drosophila ) are capable of phospholipid (PL) transfer, whereas vertebrate forms of MTP (e.g., human) engage in both PL and triglyceride (TG) transfer. To determine, in vivo, whether the PL and TG transfer activities of MTP play different roles in lipoprotein formation and protection from hepatic steatosis, we created B6D2F1 mice that express human MTP (hMTP) or Drosophila MTP (dMTP) under the control of the apoE 5’ proximal promoter and 3’ hepatic control region in the plasmid pLIV11. Immunoblot analysis confirmed that transgenic expression was limited to the liver and that expression of the transgenes did not impact the abundance of endogenous mouse MTP. To assess the ability of MTP to protect mice from hepatic steatosis, dMTP transgenic mice, hMTP transgenic, and wild type (WT) littermates were placed on a high fat (45% from lard) and cholesterol (0.2%) diet for eight weeks. Mice displayed no significant differences in plasma TG or total plasma cholesterol; however, hMTP mice displayed a trend toward reduced liver TG content, indicating that chronic MTP overexpression may protect the liver from hepatic lipid accumulation. Surprisingly, however, a 2-fold increase in liver TG was observed in two separate founder lines of dMTP transgenic mice. H&E staining confirmed an increase in neutral lipid accumulation in mice expressing the dMTP transgene, relative to both WT and hMTP mice. Quantitative PCR analysis did not reveal changes in expression of genes responsible for hepatic lipogenesis or lipid oxidation. These data complement earlier studies using adenovirus-mediated expression, indicating that acute hMTP and dMTP expression in liver-specific MTP knockout mice can reverse hepatic steatosis caused by MTP deficiency. In contrast, chronic hepatic overexpression of dMTP in a wild type background appears to interfere with endogenous lipoprotein formation and secretion. Understanding the basis for this phenotype may provide insights into how the specific lipid transfer and other activities of MTP contribute to apoB lipoprotein formation.

Abstract 109: Role of ApoAIV in ApoB Lipoprotein Assembly and Triglyceride Transport

Apolipoprotein A-IV (apoAIV) is a lipid binding protein expressed primarily in mammalian intestine and also in rodent liver. While in vitro studies have suggested a role of apoAIV in lipoprotein formation and triglyceride (TG) transport, limited evidence supports such a function in vivo. To explore if hepatic apoAIV is uniformly regulated by increased hepatic TG levels, as is the case in the intestine, we studied three distinct mouse models of hepatic steatosis. These included high fat diet (HFD)-fed wild type mice, mice overexpressing a constitutive active form of SREBP-1a (SREBP-1aTg), and mice in which hepatic TG hydrolysis is blunted (mice treated with an anti-sense oligonucleotide (ASO) targeting knockdown of CGI-58/ABHD5). High fat diet-fed mice displayed a 4.5-fold increase in liver TG content and a 10-fold increase in apoAIV mRNA expression and protein mass. SREBP-1aTg mouse liver demonstrated an ∼35-fold increase in hepatic TG, apoAIV mRNA and apoAIV protein. A linear correlation was observed between hepatic TG content and apoAIV mRNA expression in the HFD fed wild type and SREBP-1aTg mice (r2 = 0.6852; p < 0.01). Interestingly, while the TG content of CGI-58/ABHD5 knockdown animals increased by 4-fold, no corresponding increase in apoAIV expression was observed. To determine if the increase in apoAIV expression in the steatotic livers of SREPB-1aTg mice plays a role in the secretion of TG-rich apoB-containing lipoproteins, we crossed these mice with apoAIV knock out mice (A4-KO). The SREBP1a-Tg/A4-KO mice did not demonstrate a change in liver to body weight ratio or a change in hepatic TG content. Despite the large induction of apoAIV in the livers of SREBP1a-Tg mice, no difference was observed in hepatic TG production rates or VLDL particle size when apoAIV was deficient (SREBP1a-Tg/A4-KO). These results indicate that hepatic apoAIV expression is upregulated under conditions of enhanced lipogenesis, but is not altered under condition of defective TG hydrolysis. Furthermore, even under conditions of a 35-fold induction, apoAIV does not exert a detectable impact on TG transport or VLDL particle characteristics in mouse liver.

Phospholipid transfer activity of microsomal triglyceride transfer protein produces apolipoprotein B and reduces hepatosteatosis while maintaining low plasma lipids in mice

Microsomal triglyceride transfer protein (MTP), essential for apolipoprotein B (apoB) biosynthesis, evolved as a phospholipid transfer protein and acquired triglyceride transfer activity during a transition from invertebrates to vertebrates. But it is unknown whether MTP directly transfers lipids onto apoB in vivo and, if it does, whether both neutral and polar lipid transfer activities of MTP are critical for lipoprotein assembly. The molecular bases for differences in lipid transfer activities with respect to distinct domains in Drosophila MTP (dMTP) and human MTP (hMTP) are not obvious because both proteins have very similar primary, secondary, and tertiary structures. We used an in vivo approach to delineate physiological significance of these distinct lipid transfer activities by expressing dMTP (transfers phospholipids) and hMTP (transfers phospholipids and triglycerides) orthologs using adenoviruses in liver-specific MTP-deficient (L-MTP(-/-)) mice that have low plasma and high hepatic lipids. Both orthologs improved plasma lipids but plasma triglycerides were lower in dMTP mice due to lower hepatic triglyceride and apoB production. Hepatosteatosis in L-MTP(-/-) mice was ameliorated to similar levels by both. Attenuation of hepatosteatosis upon dMTP expression pertained to enhanced β-oxidation with no changes in lipogenesis. Phospholipid transfer activity of MTP promoted biogenesis of both apoB48 and apoB100-containing very low density lipoprotein in addition to a phospholipid-rich apoB48-containing high-density lipoprotein particle. Triglyceride transfer activity augmented the biosynthesis of triglyceride-rich lipoproteins by increasing the formation of these particles in the lumen of the endoplasmic reticulum.

CONCLUSION

Based on these findings, we posit that the selective inhibition of MTP triglyceride transfer activity might reduce hyperlipidemia while protecting liver from excess lipid accumulation.

ApoA-IV modulates the secretory trafficking of apoB and the size of triglyceride-rich lipoproteins

Although the evidence linking apoA-IV expression and triglyceride (TG)-rich lipoprotein assembly and secretion is compelling, the intracellular mechanisms by which apoA-IV could modulate these processes remain poorly understood. We therefore examined the functional impact of apoA-IV expression on endogenous apoB, TG, and VLDL secretion in stably transfected McA-RH7777 rat hepatoma cells. Expression of apoA-IV modified with the endoplasmic reticulum (ER) retention signal KDEL (apoA-IV-KDEL) dramatically decreased both the rate and efficiency of endogenous apoB secretion, suggesting a presecretory interaction between apoA-IV-KDEL and apoB or apoB-containing lipoproteins. Expression of native apoA-IV using either a constitutive or tetracycline-inducible promoter delayed the initial rate of apoB secretion and reduced the final secretion efficiency by ∼40%. However, whereas apoA-IV-KDEL reduced TG secretion by 75%, expression of native apoA-IV caused a 20-35% increase in TG secretion, accompanied by a ∼55% increase in VLDL-associated apoB, an increase in the TG:phospholipid ratio of secreted d < 1.006 lipoproteins, and a 10.1 nm increase in peak VLDL(1) particle diameter. Native apoA-IV expression had a negligible impact on expression of the MTP gene. These data suggest that by interacting with apoB in the secretory pathway, apoA-IV alters the trafficking kinetics of apoB-containing TG-rich lipoproteins through cellular lipidation compartments, which in turn, enhances particle expansion and increases TG secretion.

Biogenesis of apolipoprotein A-V and its impact on VLDL triglyceride secretion

Apolipoprotein A-V (apoA-V) is a potent regulator of intravascular triglyceride (TG) metabolism, yet its plasma concentration is very low compared with that of other apolipoproteins. To examine the basis for its low plasma concentration, the secretion efficiency of apoA-V was measured in stably transfected McA-RH7777 rat hepatoma cells. Pulse-chase experiments revealed that only ∼20% of newly synthesized apoA-V is secreted into culture medium within 3 h postsynthesis and that ∼65% undergoes presecretory turnover; similar results were obtained with transfected nonhepatic Chinese hamster ovary cells. ApoA-V secreted by McA-RH7777 cells was not associated with cell surface heparin-competable binding sites. When stably transfected McA-RH7777 cells were treated with oleic acid, the resulting increase in TG synthesis caused a reduction in apoA-V secretion, a reciprocal increase in cell-associated apoA-V, and movement of apoA-V onto cytosolic lipid droplets. In a stably transfected doxycycline-inducible McA-RH7777 cell line, apoA-V expression inhibited TG secretion by ∼50%, increased cellular TG, and reduced Z-average VLDL(1) particle diameter from 81 to 67 nm; however, no impact on apoB secretion was observed. These data demonstrate that apoA-V inefficiently traffics within the secretory pathway, that its intracellular itinerary can be regulated by changes in cellular TG accumulation, and that apoA-V synthesis can modulate VLDL TG mobilization and secretion.

A novel role for ABCA1-generated large pre-beta migrating nascent HDL in the regulation of hepatic VLDL triglyceride secretion

In Tangier disease, absence of ATP binding cassette transporter A1 (ABCA1) results in reduced plasma HDL and elevated triglyceride (TG) levels. We hypothesized that hepatocyte ABCA1 regulates VLDL TG secretion through nascent HDL production. Silencing of ABCA1 expression in oleate-stimulated rat hepatoma cells resulted in: 1) decreased large nascent HDL (>10 nm diameter) and increased small nascent HDL (<10 nm) formation, 2) increased large buoyant VLDL1 particle secretion, and 3) decreased phosphatidylinositol-3 (PI3) kinase activation. Nascent HDL-containing conditioned medium from rat hepatoma cells or HEK293 cells transfected with ABCA1 was effective in increasing PI3 kinase activation and reducing VLDL TG secretion in ABCA1-silenced hepatoma cells. Addition of isolated large nascent HDL particles to ABCA1-silenced hepatoma cells inhibited VLDL TG secretion to a greater extent than small nascent HDL. Similarly, addition of recombinant HDL, but not human plasma HDL, was effective in attenuating TG secretion and increasing PI3 kinase activation in ABCA1-silenced cells. Collectively, these data suggest that large nascent HDL particles, assembled by hepatic ABCA1, generate a PI3 kinase-mediated autocrine signal that attenuates VLDL maturation and TG secretion. This pathway may explain the elevated plasma TG concentration that occurs in most Tangier subjects and may also account, in part, for the inverse relationship between plasma HDL and TG concentrations in individuals with compromised ABCA1 function.

Targeted deletion of hepatocyte ABCA1 leads to very low density lipoprotein triglyceride overproduction and low density lipoprotein hypercatabolism

Loss of ABCA1 activity in Tangier disease (TD) is associated with abnormal apoB lipoprotein (Lp) metabolism in addition to the complete absence of high density lipoprotein (HDL). We used hepatocyte-specific ABCA1 knock-out (HSKO) mice to test the hypothesis that hepatic ABCA1 plays dual roles in regulating Lp metabolism and nascent HDL formation. HSKO mice recapitulated the TD lipid phenotype with postprandial hypertriglyceridemia, markedly decreased LDL, and near absence of HDL. Triglyceride (TG) secretion was 2-fold higher in HSKO compared with wild type mice, primarily due to secretion of larger TG-enriched VLDL secondary to reduced hepatic phosphatidylinositol 3-kinase signaling. HSKO mice also displayed delayed clearance of postprandial TG and reduced post-heparin plasma lipolytic activity. In addition, hepatic LDLr expression and plasma LDL catabolism were increased 2-fold in HSKO compared with wild type mice. Last, adenoviral repletion of hepatic ABCA1 in HSKO mice normalized plasma VLDL TG and hepatic phosphatidylinositol 3-kinase signaling, with a partial recovery of HDL cholesterol levels, providing evidence that hepatic ABCA1 is involved in the reciprocal regulation of apoB Lp production and HDL formation. These findings suggest that altered apoB Lp metabolism in TD subjects may result from hepatic VLDL TG overproduction and increased hepatic LDLr expression and highlight hepatic ABCA1 as an important regulatory factor for apoB-containing Lp metabolism.

Alternative splicing attenuates transgenic expression directed by the apolipoprotein E promoter-enhancer based expression vector pLIV11

The plasmid vector pLIV11 is used commonly to achieve liver-specific expression of genes of interest in transgenic mice and rabbits. Expression is driven by the human apolipoprotein (apo)E 5' proximal promoter, which includes 5 kb of upstream sequence, exon 1, intron 1, and 5 bp of exon 2. A 3.8 kb 3' hepatic control region, derived from a region approximately 18 kb downstream of the apoE gene, enhances liver-specific expression. Here, we report that cDNA sequences inserted into the multiple cloning site (MCS) of pLIV11, which is positioned just downstream of truncated exon 2, can cause exon 2 skipping. Hence, splicing is displaced to downstream cryptic 3' splice acceptor sites causing deletion of cloned 5' untranslated mRNA sequences and, in some cases, deletion of the 5' end of an open reading frame. To prevent use of cryptic splice sites, the pLIV11 vector was modified with an engineered 3' splice acceptor site inserted immediately downstream of truncated apoE exon 2. Presence of this sequence fully shifted splicing of exon 1 from the native intron 1-exon 2 splice acceptor site to the engineered site. This finding confirmed that sequences inserted into the MCS of the vector pLIV11 can affect exon 2 recognition and provides a strategy to protect cloned sequences from alternative splicing and possible attenuation of transgenic expression.

Apolipoprotein M expression increases the size of nascent pre beta HDL formed by ATP binding cassette transporter A1

Apolipoprotein M (apoM) is a novel apolipoprotein that is reportedly necessary for pre beta HDL formation; however, its detailed function remains unknown. We investigated the biogenesis and properties of apoM and its effects on the initial steps of nascent pre beta HDL assembly by ABCA1 in HEK293 cells. Transiently transfected apoM was localized primarily in the endomembrane compartment. Pulse-chase analyses demonstrated that apoM is inefficiently secreted, relative to human serum albumin, and that approximately 50% remains membrane-associated after extraction with sodium carbonate, pH 11.5. To investigate the role of apoM in nascent pre beta HDL formation, ABCA1-expressing or control cells, transfected with empty vector, apoM, or C-terminal epitope-tagged apoM (apoM-C-FLAG), were incubated with (125)I-apoA-I for 24 h. Conditioned media were harvested and fractionated by fast-protein liquid chromatography (FPLC) to monitor HDL particle size. Pre beta HDL particles were formed effectively in the absence of apoM expression; however, increased apoM expression stimulated the formation of larger-sized nascent pre beta HDLs. Immunoprecipitation with anti-apoA-I antibody followed by apoM Western blot analysis revealed that little secreted apoM was physically associated with pre beta HDL. Our results suggest that apoM is an atypical secretory protein that is not necessary for ABCA1-dependent pre beta HDL formation but does stimulate the formation of larger-sized pre beta HDL. We propose that apoM may function catalytically at an intracellular site to transfer lipid onto pre beta HDL during or after their formation by ABCA1.

LDL cholesteryl oleate as a predictor for atherosclerosis: evidence from human and animal studies on dietary fat

This review focuses on the relationships among dietary fat type, plasma and liver lipid, and lipoprotein metabolism and atherosclerosis. Dietary polyunsaturated fatty acids are beneficial for the prevention of coronary artery atherosclerosis. By contrast, dietary monounsaturated fatty acids appear to alter hepatic lipoprotein metabolism, promote cholesteryl oleate accumulation, and confer atherogenic properties to lipoproteins as shown in data from experimental animal studies. Polyunsaturated fat appears to provide atheroprotection, at least in part, because it limits the accumulation of cholesteryl oleate in favor of cholesteryl linoleate in plasma lipoproteins.

Structural and dynamic interfacial properties of the lipoprotein initiating domain of apolipoprotein B

To better understand the earliest steps in the assembly of triglyceride (TG)-rich lipoproteins, we compared the biophysical and interfacial properties of two closely related apolipoprotein B (apoB) truncation mutants, one of which contains the complete lipoprotein initiating domain (apoB20.1; residues 1-912), and one of which, by virtue of a 50 amino acid C-terminal truncation, is incapable of forming nascent lipoproteins (apoB19; residues 1-862). Spectroscopic studies detected no major differences in secondary structure, and only minor differences in conformation and thermodynamic stability, between the two truncation mutants. Monolayer studies revealed that both apoB19 and apoB20.1 bound to and penetrated egg phosphatidylcholine (EPC) monolayers; however, the interfacial exclusion pressure of apoB20.1 was higher than apoB19 (25.1 mN/m vs. 22.8 mN/m). Oil drop tensiometry revealed that both proteins bound rapidly to the hydrophobic triolein/water interface, reducing interfacial tension by approximately 20 mN/m. However, when triolein drops were first coated with phospholipids (PL), apoB20.1 bound with faster kinetics than apoB19 and also displayed greater interfacial elasticity (26.9 +/- 0.8 mN/m vs. 22.9 +/- 0.8 mN/m). These data establish that the transition of apoB to assembly competence is accompanied by increases in surface activity and elasticity, but not by significant changes in global structure.

Monounsaturated fatty acids and atherosclerosis: opposing views from epidemiology and experimental animal models

A substantial body of epidemiologic data has shed light on the potential protective effects of the Mediterranean diet against atherosclerosis in humans. Many believe the reason the Mediterranean diet is atheroprotective is the elevated consumption of olive oil, an oil poor in saturated fatty acids (SFA) and highly enriched in monounsaturated fatty acids (MUFA). Based on human feeding studies, the American Heart Association and the US Food and Drug Administration have advocated for the consumption of MUFA as a more healthy replacement for SFA. However, using experimental animal models in which extent of atherosclerosis can be directly measured following dietary intervention, it has been demonstrated that MUFA-enriched diets are not atheroprotective when compared with SFA-enriched diets. Hence, the current body of experimental evidence refutes the idea that MUFAs per se are atheroprotective; therefore much additional work is needed to determine which aspects of the Mediterranean diet are indeed heart healthy.

CGI-58 facilitates the mobilization of cytoplasmic triglyceride for lipoprotein secretion in hepatoma cells

Comparative Gene Identification-58 (CGI-58) is a member of the alpha/beta-hydrolase family of proteins. Mutations in the human CGI-58 gene are associated with Chanarin-Dorfman syndrome, a rare autosomal recessive genetic disease in which excessive triglyceride (TG) accumulation occurs in multiple tissues. In this study, we investigated the role of CGI-58 in cellular lipid metabolism in several cell models and discovered a role for CGI-58 in promoting the packaging of cytoplasmic TG into secreted lipoprotein particles in hepatoma cells. Using both gain-of-function and loss-of-function approaches, we demonstrate that CGI-58 facilitates the depletion of cellular TG stores without altering cellular cholesterol or phospholipid accumulation. This depletion of cellular TG is attributable solely to augmented hydrolysis, whereas TG synthesis was not affected by CGI-58. Furthermore, CGI-58-mediated TG hydrolysis can be completely inhibited by the known lipase inhibitors diethylumbelliferyl phosphate and diethyl-p-nitrophenyl phosphate, but not by p-chloro-mercuribenzoate. Intriguingly, CGI-58-driven TG hydrolysis was coupled to increases in both fatty acid oxidation and secretion of TG. Collectively, this study reveals a role for CGI-58 in coupling lipolytic degradation of cytoplasmic TG to oxidation and packaging into TG-rich lipoproteins for secretion in hepatoma cells.

Identification of a novel isoform of microsomal triglyceride transfer protein

Microsomal triglyceride transfer protein (MTP) has been studied extensively, primarily because of its role in the assembly of very low density lipoproteins by the liver and chylomicrons by the intestine. Recent studies have suggested that MTP may also play key roles in other cellular processes. In this paper we report the identification of a novel splice variant of MTP in mice. This isoform, MTP-B, has a unique first exon located approximately 2.7 kilobases upstream of canonical MTP (MTP-A) exon 1. The alternative exon encodes 35 amino acids compared with 20 amino acids encoded by exon 1 of MTP-A. MTP-B represents approximately 90% of total MTP mRNA in mouse adipocytes and 3T3-L1 cells and <5% in mouse liver and intestine. Expression of the alternate isoform in mouse liver was confirmed by mass spectrometry. Co-transfection of COS cells with truncated forms of apoB and either MTP-A or MTP-B demonstrated that both isoforms are effective in the assembly and secretion of nascent apoB-containing lipoproteins. Confocal microscopy of 3T3-L1 cells transfected with enhanced green fluorescent protein or DsRed fusions of the two proteins revealed that MTP-A is localized to the endoplasmic reticulum, whereas MTP-B localizes primarily to the Golgi complex in these cells. We conclude that MTP-B functions similarly to MTP-A in lipoprotein assembly. However, in nonlipoprotein-secreting cells, such as the adipocyte, MTP-B may have different localization properties, perhaps reflecting a distinct role in lipid storage and mobilization.

ACAT2 stimulates cholesteryl ester secretion in apoB-containing lipoproteins

Previous studies in nonhuman primates revealed a striking positive correlation between liver cholesteryl ester (CE) secretion rate and the development of coronary artery atherosclerosis. CE incorporated into hepatic VLDL is necessarily synthesized by ACAT2, the cholesterol-esterifying enzyme in hepatocytes. We tested the hypothesis that the level of ACAT2 expression, in concert with cellular cholesterol availability, affects the CE content of apolipoprotein B (apoB)-containing lipoproteins. In a model system of lipoprotein secretion using COS cells cotransfected with microsomal triglyceride transfer protein and truncated forms of apoB, ACAT2 expression resulted in a 3-fold increase in microsomal ACAT activity and a 4-fold increase in the radiolabeled CE content of apoB-lipoproteins. After cholesterol-cyclodextrin (Chol-CD) treatment, CE secretion was increased by 27-fold in ACAT2-transfected cells but by only 7-fold in control cells. Chol-CD treatment also caused the percentage of CE in the apoB-lipoproteins to increase from 3% to 33% in control cells and from 16% to 54% in ACAT2-transfected cells. In addition, ACAT2-transfected cells secreted 3-fold more apoB than control cells. These results indicate that under all conditions of cellular cholesterol availability tested, the relative level of ACAT2 expression affects the CE content and, hence, the potential atherogenicity, of nascent apoB-containing lipoproteins.

Heritable, diet-induced hyperlipidemia in California mice (Peromyscus californicus) is due to increased hepatic secretion of very low density lipoprotein triacylglycerol

California mice (Peromyscus californicus) develop type II diabetes mellitus when fed a high-fat diet. We undertook the current studies to determine whether hyperlipidemia precedes the development of insulin resistance and to establish breeding colonies of hyperlipidemic and normolipidemic mice. For 6 wk, mice (n = 24) received a diet containing 25.8% of energy from fat. Mice representing the upper and lower quartiles of serum triacylglycerol (TAG) response (mean, >1000 mg/dl versus <300 mg/dl, respectively; 6 mice per group) were designated as high (HR) and low (LR) responders, respectively, and were used for further study. After 12 wk of consuming the high-fat diet, HR mice remained hypertriglyceridemic and developed hyperinsulinemia (5.1 +/- 1.3 ng/ml), hypercholesterolemia (309.3 +/- 31.0 mg/dl), and hyperglycemia (205.9 +/- 30.3 mg/dl) compared with LR mice. HR mice were not hyperphagic or obese. Offspring of HR x HR mice had elevated serum TAG concentrations (mean, 1752.2 +/- 209.7 mg/dl), hypercholesterolemia, hyperinsulinemia, and mild hyperglycemia by 5.5 mo of age. Mating HR male and LR female mice produced HR, intermediate, and LR progeny. HR mice had elevated serum concentrations of cholesterol, and plasma concentrations of high density lipoprotein cholesterol, and the very low density lipoprotein TAG compared with LR mice. Lipoprotein lipase and hepatic lipase activities did not differ between HR and LR mice. Studies of in vivo hepatic TAG production indicated that the hyperlipidemia of HR mice is a consequence of TAG hypersecretion.

Phospholipid Transfer Activity of Microsomal Triacylglycerol Transfer Protein Is Sufficient for the Assembly and Secretion of Apolipoprotein B Lipoproteins

Human microsomal triacylglycerol transfer protein (hMTP) is essential for apolipoprotein B (apoB)-lipoprotein assembly and secretion and is known to transfer triacylglycerols, cholesterol esters, and phospholipids. To understand the relative importance of each lipid transfer activity, we compared the ability of hMTP and its Drosophila ortholog (dMTP) to assemble apoB lipoproteins and to transfer various lipids. apoB48 secretion was induced when co-expressed with either hMTP or dMTP in COS cells, and oleic acid supplementation further augmented secretion without altering particle density. C-terminal epitope-tagged dMTP (dMTP-FLAG) facilitated the secretion of apoB polypeptides in the range of apoB48 to apoB72 but was approximately 50% as efficient as hMTP-FLAG. Comparison of lipid transfer activities revealed that although phospholipid transfer was similar in both orthologs, dMTP was unable to transfer neutral lipids. We conclude that the phospholipid transfer activity of MTP is sufficient for the assembly and secretion of primordial apoB lipoproteins and may represent its earliest function evolved for the mobilization of lipid in invertebrates. Identification of MTP inhibitors, which selectively affect transfer of a specific lipid class, may have therapeutic potential.

Self-association and lipid binding properties of the lipoprotein initiating domain of apolipoprotein B

The amino-terminal 20.1% of apolipoprotein B (apoB20.1; residues 1-912) is sufficient to initiate and direct the formation of nascent apoB-containing lipoprotein particles. To investigate the mechanism of initial lipid acquisition by apoB, we examined the lipid binding and interfacial properties of a carboxyl-terminal His6-tagged form of apoB20.1 (apoB20.1H). ApoB20.1H was expressed in Sf9 cells and purified by nickel affinity chromatography. ApoB20.1H was produced in a folded state as characterized by formation of intramolecular disulfide bonds and resistance to chemical reduction. Dynamic light scattering in physiological buffer indicated that purified apoB20.1H formed multimers, which were readily dissociable upon the addition of nonionic detergent (0.1% Triton X-100). ApoB20.1H was incapable of binding dimyristoylphosphatidylcholine multilamellar vesicles, unless its multimeric structure was first disrupted by guanidine hydrochloride. However, apoB20.1H multimers spontaneously dissociated and bound to the interface of naked and phospholipid-coated triolein droplets. These data reveal that the initiating domain of apoB contains solvent-accessible hydrophobic sequences, which, in the absence of a hydrophobic lipid interface or detergent, engage in self-association. The high affinity of apoB20.1H for neutral lipid is consistent with the membrane binding and desorption model of apoB-containing lipoprotein assembly.

Overexpression of apolipoprotein A-IV enhances lipid secretion in IPEC-1 cells by increasing chylomicron size

Intestinal apolipoprotein A-IV expression is highly regulated by dietary lipid in newborn swine, suggesting a role in lipid absorption. Constitutive overexpression of apoA-IV in newborn swine enterocytes enhances basolateral secretion of triacylglycerol (TG) in TG-rich lipoproteins 4.9-fold (Lu, S., Yao, Y., Meng, S., Cheng, X., and Black, D. D. (2002) J. Biol. Chem. 277, 31929-31937). To investigate the mechanism of this enhancement, IPEC-1 cells were transfected with a tetracycline-regulatable expression system (Tet-On). In cells incubated with oleic acid, a dose response relationship was observed between medium doxycycline concentration and basolateral apoA-IV and TG secretion. Similarly regulated expression of apoA-I did not enhance lipid secretion. The mean diameter of TG-rich lipoproteins secreted from doxycycline-treated cells was larger than from untreated cells (87.0 nm versus 53.4 nm). Basolateral apoB secretion decreased. Using the same expression system, full-length human apoA-IV (376 amino acids); a "pig-like" human apoA-IV, lacking the C-terminal EQQQ repeats (361 amino acids); and a "chicken-like" apoA-IV, further truncated to 343 amino acids, were expressed in IPEC-1 cells. With increasing protein secretion, cells expressing the full-length human apoA-IV displayed a 2-fold increase in TG secretion; in sharp contrast, cells expressing the pig-like human apoA-IV displayed a 25-fold increase in TG secretion and a 27-fold increase in lipoprotein diameter. When human apoA-IV was further truncated to yield a chicken-like protein, TG secretion was inhibited. We conclude that overexpression of swine apoA-IV enhances basolateral TG secretion in a dose-dependent manner by increasing the size of secreted lipoproteins. These data suggest that the region in the human apoA-IV protein from residues 344 to 354 is critical to its ability to enhance lipid secretion, perhaps by enabling the packaging of additional core TG into chylomicron particles. The EQQQ-rich region may play an inhibitory or modulatory role in chylomicron packaging in humans.

Current biology of MTP: implications for selective inhibition

The microsomal triglyceride transfer protein (MTP), along with its partner, protein disulphide isomerase, performs a wide range of lipid transport functions necessary for maintenance of whole-body lipid homeostasis. In this review, we summarize the recent deluge of comparative and functional genomic data that have forced a radical re-appraisal of the evolutionary processes that established the major lipid transport pathway in man, and the different structural and lipid transfer roles MTP plays within it. This is followed by an overview of MTP structure-function relationships, highlighting two newly identified functional roles: first, the production of small, apolipoprotein (apo)B-containing lipoprotein particles in cardiac myocytes and, second, the lipidation of a major histocompatibility complex class-I related molecule (CD1d) that presents glycolipid antigens to distinct subsets of natural killer T cells. We also discuss the interactions of MTP with proteins such as apoB and CD1d, and the complex mechanisms regulating MTP transcription in different cell types and nutritional states. The past five years has witnessed remarkable progress in teasing out the different functionalities of MTP, and the properties of the different molecules that inhibit MTP activity, data that are likely to underpin the design of the next generation of MTP/apoB inhibitors for preventing cardiovascular disease attributable to the increased production of atherogenic lipoproteins.

Evolution and mechanism of apolipoprotein B-containing lipoprotein assembly

PURPOSE OF REVIEW

Apolipoprotein B-containing lipoprotein assembly and secretion is critical for lipid absorption and triglyceride homeostasis, and plays a role in atherogenesis and the pathobiology of type 2 diabetes and obesity. This review highlights recent insights into the evolutionary, structural, and cell biology of hepatic and intestinal pathways for lipid mobilization, and the mechanisms and regulation of lipoprotein assembly and secretion.

RECENT FINDINGS

Until recently it was assumed that microsomal triglyceride transfer protein-dependent apolipoprotein B-containing lipoprotein assembly was a unique adaptation associated with vertebrate lipid homeostasis. However, it is now clear that microsomal triglyceride transfer protein (MTP) exists in species whose last common ancestor diverged over 550 million years ago. In its long evolutionary history, the MTP gene has given rise to a series of paralogous lipid transport proteins, all of which require MTP for their biogenesis. During its evolution, MTP has acquired new functions, enabling it to participate in a disparate array of lipid mobilization and transport pathways, ranging from primitive lipoprotein assembly to antigenic lipid presentation. In addition to the complex and multifunctional role of MTP in apolipoprotein B assembly, other factors responsible for the generation of secretion-coupled lipids and the modulation of apolipoprotein B production are emerging.

SUMMARY

The phylogenic dissection of MTP and apolipoprotein B function, coupled with ongoing structural and biochemical analyses, provide significant insights into the mechanisms of lipid mobilization and secretion. Some of these factors and processes may be targeted therapeutically to modulate the quantitative and qualitative aspects of apolipoprotein B production.

Microsomal triglyceride transfer protein promotes the secretion of Xenopus laevis vitellogenin A1

Vitellogenins (Vtg) are ancient lipid transport and storage proteins and members of the large lipid transfer protein (LLTP) gene family, which includes insect apolipophorin II/I, apolipoprotein B (apoB), and the microsomal triglyceride transfer protein (MTP). Lipidation of Vtg occurs at its site of synthesis in vertebrate liver, insect fat body, and nematode intestine; however, the mechanism of Vtg lipid acquisition is unknown. To explore whether Vtg biogenesis requires the apoB cofactor and LLTP family member, MTP, Vtg was expressed in COS cells with and without coexpression of the 97-kDa subunit of human MTP. Expression of Vtg alone gave rise to a approximately 220-kDa apoprotein, which was predominantly confined to an intracellular location. Coexpression of Vtg with human MTP enhanced Vtg secretion by 5-fold, without dramatically affecting its intracellular stability. A comparison of wild type and a triglyceride transfer-defective form of MTP revealed that both were capable of promoting Vtg secretion, whereas only wild type MTP could promote the secretion of apoB41 (amino-terminal 41% of apoB). These studies demonstrate that the biogenesis of Vtg is MTP-dependent and that MTP is the likely ancestral member of the LLTP gene family.

Cloning and Characterization of Cyclooxygenase-1b (Putative Cyclooxygenase-3) in Rat

A splice variant of cyclooxygenase-1 (COX-1), COX-1b (previously termed as COX-3), has been identified in canine tissues as an acetaminophen-sensitive isoform, but the sequence of COX-1b mRNA and the encoded protein are not known in rats. We cloned and sequenced rat COX-1b mRNA from cerebral endothelial cells. Sequence analysis indicated that the 98-base pair intron-1 of COX-1 gene remains unprocessed in the COX-1b mRNA, causing a frameshift mutation and a 127-amino acid open reading frame with no sequence similarity with known cyclooxygenases. Transient and permanent transfection of COS-7 cells with a vector containing the rat COX-1b cDNA resulted in synthesis of a protein of the expected size. We generated an affinity-purified polyclonal antibody against the rat COX-1b protein. Western blot analysis of rat tissues using this antibody demonstrated the likely existence of rat COX-1b protein in vivo with the highest expression in heart, kidney, and neuronal tissues. Our results on both stable and on transiently transfected COS-7 cells suggest that rat COX-1b does not have cyclooxygenase activity and does not have any effect on the inhibition of prostaglandin production by acetaminophen. Because this protein has a completely different amino acid sequence than COX-1 and COX-2 and it does not have cyclooxygenase activity, we suggest a name cyclooxygenase variant protein to distinguish it from the known prostaglandin-synthesizing cyclooxygenase isoforms.

apoA-IV tagged with the ER retention signal KDEL perturbs the intracellular trafficking and secretion of apoB

To examine the role of apolipoprotein A-IV (apoA-IV) in the intracellular trafficking and secretion of apoB, COS cells were cotransfected with microsomal triglyceride transfer protein (MTP), apoB-41 (amino terminal 41% of apoB), and either native apoA-IV or apoA-IV modified with the carboxy-terminal endoplasmic reticulum (ER) retention signal, KDEL (apoA-IV-KDEL). As expected, apoA-IV-KDEL was inefficiently secreted relative to native apoA-IV. Coexpression of apoB-41 with apoA-IV-KDEL reduced the secretion of apoB-41 by approximately 80%. The apoA-IV-KDEL effect was specific, as neither KDEL-modified forms of human serum albumin or apoA-I affected apoB-41 secretion. Similar results were observed in McA-RH7777 rat hepatoma cells, which express endogenous MTP. The full inhibitory effect of apoA-IV-KDEL on apoB secretion was observed only for forms of apoB containing a minimum of the amino-terminal 25% of the protein (apoB-25). However, apoA-IV-KDEL inhibited the secretion of both lipid-associated and lipid-poor forms of apoB-25. Dual-label immunofluorescence microscopy of cells transfected with native apoA-IV and apoB-25 revealed that both apolipoproteins were localized to the ER and Golgi, as expected. However, when apoA-IV-KDEL was cotransfected with apoB-25, both proteins localized primarily to the ER. These data suggest that apoA-IV may physically interact with apoB in the secretory pathway, perhaps reflecting a role in modulating the process of triglyceride-rich lipoprotein assembly and secretion.

Quality control in the apoA-I secretory pathway

From a total of 47 known apolipoprotein A-I (apoA-I) mutations, only 18 are linked to low plasma HDL apoA-I concentrations, and 78% of these map to apoA-I helices 6 and 7 (residues 143-186). Gene transfer and transgenic mouse studies have shown that several helix 6 apoA-I mutations have reduced hepatic HDL production. Our objective was to examine the impact of helix 6 modifications on intracellular biosynthetic processing and secretion of apoA-I. Cells were transfected with wild-type or mutant apoA-I, radiolabeled with [(35)S]Met/Cys, and then placed in unlabeled medium for up to 4 h. Results show that >90% of newly synthesized wild-type apoA-I was secreted by 60 min. Over the same length of time, only 20% of helix 6 deletion mutant (Delta 6 apoA-I) was secreted, whereas 80% remained cell associated. Microscopic and biochemical studies revealed that cell-associated Delta 6 apoA-I was located predominantly within the cytoplasm as lipid-protein inclusions, whereas wild-type apoA-I was localized in the endoplasmic reticulum/Golgi. Results using other helix deletions or helix 6 substitution mutations indicated that only complete removal of helix 6 resulted in massive cytoplasmic accumulation. These data suggest that alterations in native apoA-I conformation can lead to aberrant trafficking and accumulation of apolipoprotein-phospholipid structures. Thus, conformation-dependent alterations in intracellular trafficking and turnover may underlie the reduced plasma HDL concentrations observed in individuals harboring deletion mutations within helix 6.

Identification of the lipoprotein initiating domain of apolipoprotein B

We have explored the minimum sequence requirement for the initiation of apolipoprotein B (apoB)-mediated triglyceride-rich lipoprotein assembly. A series of apoB COOH-terminal truncation mutants, spanning a range from apoB34 (amino acid residues 1-1544 of apoB100) to apoB19 (residues 1-862) were transfected into COS cells with and without coexpression of the microsomal triglyceride transfer protein (MTP). ApoB34, -25, -23, -21, -20.5, and -20.1 underwent efficient conversion to buoyant lipoproteins when coexpressed with MTP. ApoB19.5 (amino acids 1-884) also directed MTP-dependent particle assembly, although at reduced efficiency. When apoB19.5 was truncated by another 22 amino acids to form apoB19, MTP-dependent lipoprotein assembly was abolished. Analysis of the lipid stoichiometry of secreted lipoproteins revealed that all apoB truncation mutants formed spherical particles containing a hydrophobic core. Even highly truncated assembly-competent forms of apoB, such as apoB19.5 and 20.1, formed lipoproteins with surface:core lipid ratios of <1. We conclude that the translation of the first approximately 884 amino acids of apoB completes a domain capable of initiating nascent lipoprotein assembly. The composition of lipids recruited into lipoproteins by this initiating domain is consistent with formation of small emulsion particles, perhaps by simultaneous desorption of both polar and neutral lipids from a saturated bilayer.

Structure and interfacial properties of human apolipoprotein A-V

Apolipoprotein A-V (apoA-V), the newest member of the plasma apolipoprotein family, was recently discovered by comparison of the mouse and human genomes. Studies in rodents and population surveys of human apoA-V polymorphisms have noted a strong effect of apoA-V on plasma triglyceride levels. Toward the elucidation of the biologic function of apoA-V, we used spectroscopic and surface chemistry techniques to probe its structure and interfacial activity. Computer-assisted sequence analysis of apoA-V predicts that it is very hydrophobic, contains a significant amount of alpha-helical secondary structure, and probably is composed of discrete structural regions with varying degrees of lipid affinity. Fluorescence spectroscopy of recombinant human apoA-V provided evidence of tertiary folding, and light scattering studies indicated that apoA-V transforms dimyristoylphosphatidylcholine vesicles into discoidal complexes with an efficiency similar to that of apoA-I. Surface chemistry techniques revealed that apoA-V displays high affinity, low elasticity, and slow binding kinetics at hydrophobic interfaces, properties we propose may retard triglyceride-rich particle assembly. Metabolic labeling and immunofluorescence studies of COS-1 cells transfected with human apoA-V demonstrated that apoA-V is poorly secreted, remains associated with the endoplasmic reticulum, and does not traffic to the Golgi. Given that overexpression of the apoA-V gene lowers plasma triglycerides in mice, these data together suggest that apoA-V may function intracellularly to modulate hepatic VLDL synthesis and/or secretion.

A Drosophila microsomal triglyceride transfer protein homolog promotes the assembly and secretion of human apolipoprotein B. Implications for human and insect transport and metabolism

The assembly and secretion of triglyceride-rich lipoproteins in vertebrates requires apolipoprotein B (apoB) and the endoplasmic reticulum-localized cofactor, microsomal triglyceride transfer protein (MTP). Invertebrates, particularly insects, transport the majority of their neutral and polar lipids in lipophorins; however, the assembly of lipophorin precursor particles was presumed to be MTP-independent. A Drosophila melanogaster expressed gene sequence (CG9342), displaying 23% identity with human MTP, was recently identified. When coexpressed in COS cells, CG9342 promoted the assembly and secretion of apoB34 and apoB41 (N-terminal 34 and 41% of human apoB). The apoB34-containing particles assembled by human MTP and CG9342 displayed similar peak densities of approximately 1.169 g/ml and similar lipid compositions. However, CG9342 displayed differential sensitivities to two inhibitors of human MTP and low vesicle-based lipid transfer activity, in vitro. In addition, important predicted structural distinctions exist between the human and Drosophila proteins suggesting overlapping but not identical functional roles. We conclude that CG9342 and human MTP are orthologs that share only a subset of functions, consistent with known differences in intracellular and extracellular aspects of vertebrate and invertebrate lipid transport and metabolism.

ApoA-I secretion from HepG2 cells: evidence for the secretion of both lipid-poor apoA-I and intracellularly assembled nascent HDL

The goal of this study was to determine whether apolipoprotein A-I (apoA-I) is lipidated before secretion by HepG2 cells. ApoA-I was extracted from microsomes after radiolabeling with [(35)S]Met/Cys. After ultracentrifugal flotation, d < 1.25 g/ml and d > 1.25 g/ml fractions were immunoprecipitated and analyzed by SDS-PAGE. Under steady state radiolabeling conditions, 20% of extracted microsomal apoA-I floated at d < 1.25 g/ml. Pulse-chase experiments demonstrated that the percentage of microsomal apoA-I associated with lipid peaked between 2 and 8 min postsynthesis. Density gradient ultracentrifugation, and nondenaturing gradient gel electrophoresis of HepG2 cell medium, indicated that 50% of secretory apoA-I existed as small HDL particles with a diameter of approximately 7.5 nm. These and additional data suggested that approximately 20% of newly secreted apoA-I is lipidated intracellularly and another 30% is secreted in lipid-free or lipid-poor form but acquires sufficient lipid to become small HDL within 1 h of secretion, with little further maturation over the time course of the incubation (2 h).These results indicate that a process exists for the presecretory intracellular assembly of apoA-I with lipid in HepG2 cells and that apoA-I is secreted in both lipid-poor and lipidated forms.

Lipoprotein assembly capacity of the mammary tumor-derived cell line C127 is due to the expression of functional microsomal triglyceride transfer protein

C127, a murine mammary tumor-derived cell line, is capable of lipidating and secreting apolipoprotein B-41 (apoB-41) in the apparent absence of microsomal triglyceride transfer protein (MTP). Using a semiquantitative reverse transcriptase-coupled polymerase chain reaction, mouse MTP mRNA was detected in C127 cells at approximately 10-20% of the relative abundance of human MTP in HepG2 cells. Radiolabeling of C127 cells with [35S]methionine and [35S]- cysteine followed by immunoprecipitation with anti-MTP antibodies identified a band with an electrophoretic mobility identical to that of authentic mouse MTP. Cotransfection of apoB-41 and the MTP 97-kDa subunit in C127 cells enhanced apoB secretion by approximately 5-fold relative to apoB-41 transfection alone, suggesting that MTP is limiting in these cells. To establish that MTP expression is responsible for apoB-containing lipoprotein assembly in C127 cells, the effects of the MTP inhibitor BMS-200150 were examined. Secretion of apoB-41 by C127 cells was inhibited to the same extent observed in COS-1 cells cotransfected with apoB-41 and MTP. These results suggest that low MTP expression, and not the expression or overexpression of another known or novel factor(s), is responsible for apoB assembly and secretion in C127 cells and further supports the essential nature of MTP in the biogenesis of apoB-containing lipoproteins. .

Very-low-density lipoprotein assembly and secretion

The assembly of apolipoprotein B (apoB) into VLDL is broadly divided into two steps. The first involves transfer of lipid by the microsomal triglyceride transfer protein (MTP) to apoB during translation. The second involves fusion of apoB-containing precursor particles with triglyceride droplets to form mature VLDL. ApoB and MTP are homologs of the egg yolk storage protein, lipovitellin. Homodimerization surfaces in lipovitellin are reutilized in apoB and MTP to achieve apoB-MTP interactions necessary for first step assembly. Structural modeling predicts a small lipovitellin-like lipid binding cavity in MTP and a transient lipovitellin-like cavity in apoB important for nucleation of lipid sequestration. The formation of triglyceride droplets in the endoplasmic reticulum requires MTP however, their fusion with apoB may be MTP-independent. Second step assembly is modulated by phospholipase D and A2. Phospholipases may prime membrane transport steps required for second step fusion and/or channel phospholipids into a pathway for VLDL triglyceride production. The enzymology of VLDL triglyceride synthesis is still poorly understood; however, it appears that ACAT2 is the sole source of cholesterol esters for VLDL and chylomicron assembly. VLDL production is controlled primarily at the level of presecretory degradation. Recently, it was discovered that the LDL receptor modulates VLDL production through its interactions with nascent VLDL in the secretory pathway.