apoB Secretion Research Articles

Suppressor of Cytokine Signaling-3 (SOCS-3) Induces Proprotein Convertase Subtilisin Kexin Type 9 (PCSK9) Expression in Hepatic HepG2 Cell Line

The suppressor of cytokine signaling (SOCS) proteins are negative regulators of the JAK/STAT pathway activated by proinflammatory cytokines, including the tumor necrosis factor-α (TNF-α). SOCS3 is also implicated in hypertriglyceridemia associated to insulin resistance. Proprotein convertase subtilisin kexin type 9 (PCSK9) levels are frequently found to be positively correlated to insulin resistance and plasma very low density lipoprotein (VLDL) triglycerides concentrations. The present study aimed to investigate the possible role of TNF-α and JAK/STAT pathway on de novo lipogenesis and PCSK9 expression in HepG2 cells. TNF-α induced both SOCS3 and PCSK9 in a concentration-dependent manner. This effect was inhibited by transfection with siRNA anti-STAT3, suggesting the involvement of the JAK/STAT pathway. Retroviral overexpression of SOCS3 in HepG2 cells (HepG2(SOCS3)) strongly inhibited STAT3 phosphorylation and induced PCSK9 mRNA and protein, with no effect on its promoter activity and mRNA stability. Consistently, siRNA anti-SOCS3 reduced PCSK9 mRNA levels, whereas an opposite effect was observed with siRNA anti-STAT3. In addition, HepG2(SOCS3) express higher mRNA levels of key enzymes involved in the de novo lipogenesis, such as fatty-acid synthase, stearoyl-CoA desaturase (SCD)-1, and apoB. These responses were associated with a significant increase of SCD-1 protein, activation of sterol regulatory element-binding protein-1c (SREBP-1), accumulation of cellular triglycerides, and secretion of apoB. HepG2(SOCS3) show lower phosphorylation levels of insulin receptor substrate 1 (IRS-1) Tyr(896) and Akt Ser(473) in response to insulin. Finally, insulin stimulation produced an additive effect with SOCS3 overexpression, further inducing PCSK9, SREBP-1, fatty acid synthase, and apoB mRNA. In conclusion, our data candidate PCSK9 as a gene involved in lipid metabolism regulated by proinflammatory cytokine TNF-α in a SOCS3-dependent manner.

Role of growth hormone-releasing hormone in dyslipidemia associated with experimental type 1 diabetes

Dyslipidemia associated with triglyceride-rich lipoproteins (TRLs) represents an important residual risk factor for cardiovascular and chronic kidney disease in patients with type 1 diabetes (T1D). Levels of growth hormone (GH) are elevated in T1D, which aggravates both hyperglycemia and dyslipidemia. The hypothalamic growth hormone-releasing hormone (GHRH) regulates the release of GH by the pituitary but also exerts separate actions on peripheral GHRH receptors, the functional role of which remains elusive in T1D. In a rat model of streptozotocin (STZ)-induced T1D, GHRH receptor expression was found to be up-regulated in the distal small intestine, a tissue involved in chylomicron synthesis. Treatment of T1D rats with a GHRH antagonist, MIA-602, at a dose that did not affect plasma GH levels, significantly reduced TRL, as well as markers of renal injury, and improved endothelial-dependent vasorelaxation. Glucagon-like peptide 1 (GLP-1) reduces hyperglucagonemia and postprandial TRL, the latter in part through a decreased synthesis of apolipoprotein B-48 (ApoB-48) by intestinal cells. Although plasma GLP-1 levels were elevated in diabetic animals, this was accompanied by increased rather than reduced glucagon levels, suggesting impaired GLP-1 signaling. Treatment with MIA-602 normalized GLP-1 and glucagon to control levels in T1D rats. MIA-602 also decreased secretion of ApoB-48 from rat intestinal epithelial cells in response to oleic acid stimulation in vitro, in part through a GLP-1-dependent mechanism. Our findings support the hypothesis that antagonizing the signaling of GHRH in T1D may improve GLP-1 function in the small intestine, which, in turn, diminishes TRL and reduces renal and vascular complications.

Characterization of a mutant form of human apolipoprotein B (Thr26_Tyr27del) associated with familial hypobetalipoproteinemia

We have previously identified a deletion mutant of human apoB [apoB (Thr26_Tyr27del)] in a subject with primary hypobetalipoproteinemia. The present study determined the effect of Thr26_Tyr27del mutation on apoB secretion using transfected McA-RH7777 cells. Transient or stable transfection of apoB-48 containing the Thr26_Tyr27del mutation showed drastically reduced secretion of the mutant as compared to wild-type apoB-48. No lipoproteins containing the mutant apoB-48 were secreted into the medium. Incubation of transfected cells in a lipid-rich medium in the presence of cycloheximide showed rapid turnover of cell-associated mutant apoB-48 as compared to that of wild-type apoB-48. Immunofluorescence experiments showed that the mutant apoB-48 was mostly localized in the endoplasmic reticulum. Treatment with the proteasomal inhibitor MG132 markedly attenuated the turnover of cell-associated mutant apoB-48, whereas treatment with inhibitors of autophagosomal/lysosomal function (e.g. 3-MA or ammonium chloride) had no effect. Taken together, these results indicated that the defective secretion of the Thr26_Tyr27del mutant was associated with increased intracellular degradation of apoB through the proteasome-dependent pathway.

Abstract 14589: Adiponectin Inhibits Hepatic Production of Triglyceride-Rich Apolipoprotein B Lipoproteins Through Both LDL-Receptor Dependent and Independent Mechanisms

Background and Rationale: Increasing plasma concentrations of the anti-inflammatory protein, adiponectin, are significantly and independently associated with a lower risk of coronary heart disease (CHD), while hypoadiponectinemia, conversely, is a strong risk factor for CHD. The mechanisms by which adiponectin mediates its cardioprotective role are not clearly understood. Here, we have elucidated a key cause of the cardioprotective effect of adiponectin, involving a direct inhibitory action on hepatic production of pro-atherogenic triglyceride-rich apolipoprotein B (apoB) lipoproteins, mediated through both LDL-receptor dependent mechanisms, involving PCSK9, and LDL-receptor independent mechanisms. Methods and Results: Cultured human hepatocytes (HepG2 cells) and primary human hepatocytes were treated with physiological concentrations of purified recombinant human adiponectin (10 μg/mL) for 24 hours. Adiponectin treatment reduced endogenous apoB cellular expression and secretion markedly by 60% and 30%, respectively, (P<0.01). Adiponectin also reduced apoB production stimulated by prior treatment of cells with oleate or with pro-inflammatory obese human serum (25% reduction for both, P<0.01). Adiponectin-mediated inhibition of cellular apoB production occurred through a substantial increase in levels of hepatocyte LDL-receptors (70%, P<0.01), which promote proteasome-mediated apoB degradation, and which resulted from a 50% reduction in cellular PCSK9 levels (P<0.01 for both). Multiple LDL-receptor independent effects of adiponectin involved mechanisms which reduce cellular apoB stability: (1) a decrease in cellular microsomal triglyceride transfer protein (MTP) (by 25%, (P<0.01); (2) an increase in AMP kinase (by 40%, P<0.01); and (3) marked stimulation of the Akt/Erk insulin signaling pathways. Conclusions: Adiponectin has a direct mitigating effect on hepatic apoB production through mechanisms mediated in part by the LDL-receptor and PCSK9, MTP and the insulin signaling pathway. These findings indicate that that inhibition of apoB may explain in large part adiponectin’s cardioprotective role. Furthermore, targeted adiponectin-based therapies may be a novel approach to inhibit hepatic apoB production.

Abstract 18667: Lipoprotein(a) Catabolism and Apolipoprotein(a) Secretion is Regulated by Sortilin

Elevated levels of lipoprotein(a) (Lp(a)) in plasma have been identified as an independent, causal risk factor for coronary heart disease. Lp(a) consists of a low-density lipoprotein (LDL)-like particle whose apolipoproteinB-100 (apoB-100) moiety is covalently linked to the unique glycoprotein apolipoprotein(a) (apo(a)). Recently, Lp(a) internalization by the LDL-receptor (LDLr) in hepatic cells and primary human fibroblasts has been shown to be regulated by proprotein convertase subtilisin/kexin type 9 (PCSK9). However, Lp(a)/apo(a) internalization still occurs even with a defective LDLr or in the presence of PCSK9 (in fibroblasts and hepatic cells, respectively), indicating a role for receptors other than the LDLr in Lp(a) catabolism. Hepatic sortilin has been identified as a potential receptor mediating LDL catabolism as well as the regulation of apoB-100 secretion. Sortilin localizes to the Golgi apparatus where it mediates trafficking of specific bound ligands to the lysosome. Sortilin also localizes to clathrin-coated pits in the plasma membrane where it can act as an internalization receptor. We demonstrate in the current study that Lp(a), but not apo(a), internalization in hepatic cells is influenced by sortilin overexpression. In the presence of PCSK9, Lp(a) internalization greatly increases with sortilin overexpression compared to control. These results suggest that hepatic sortilin has the ability to act as a receptor for Lp(a) catabolism, through the LDL-like moiety, in a manner that is not dependent on the LDLr. Furthermore, Lp(a) internalization increases with sortilin overexpression in primary human fibroblasts with a defective LDLr, again emphasizing an LDLr-independent role for sortilin as a receptor for Lp(a). Interestingly, an increase in apo(a) secretion is observed with sortilin overexpression in hepatic cells. Removal of the carboxyl-terminal tail of sortilin results in an inability to promote the secretion of apo(a), indicating a direct role for the sorting motifs present in this region of sortilin for the regulation of apo(a) secretion. Taken together, these results indicate novel roles for sortilin in both Lp(a) catabolism and apo(a) secretion.

Protective Effects of Alisma orientale Extract against Hepatic Steatosis via Inhibition of Endoplasmic Reticulum Stress.

Endoplasmic reticulum (ER) stress is associated with the pathogenesis of hepatic steatosis. Alisma orientale Juzepzuk is a traditional medicinal herb for diuretics, diabetes, hepatitis, and inflammation. In this study, we investigated the protective effects of methanol extract of the tuber of Alisma orientale (MEAO) against ER stress-induced hepatic steatosis in vitro and in vivo. MEAO inhibited the tunicamycin-induced increase in luciferase activity of ER stress-reporter constructs containing ER stress response element and ATF6 response element. MEAO significantly inhibited tunicamycin-induced ER stress marker expression including GRP78, CHOP, and XBP-1 in tunicamycin-treated Human hepatocellular carcinoma (HepG2) cells and the livers of tunicamycin-injected mice. It also inhibited tunicamycin-induced accumulation of cellular triglyceride. Similar observations were made under physiological ER stress conditions such as in palmitate (PA)-treated HepG2 cells and the livers of high-fat diet (HFD)-induced obese mice. MEAO repressed hepatic lipogenic gene expression in PA-treated HepG2 cells and the livers of HFD obese mice. Furthermore, MEAO repressed very low-density lipoprotein receptor (VLDLR) expression and improved ApoB secretion in the livers of tunicamycin-injected mice or HFD obese mice as well as in tunicamycin or PA-treated HepG2 cells. Alismol, a guaiane-type sesquiterpenes in Alisma orientale, inhibited GRP78 expression in tunicamycin-treated HepG2 cells. In conclusion, MEAO attenuates ER stress and prevents hepatic steatosis pathogenesis via inhibition of expression of the hepatic lipogenic genes and VLDLR, and enhancement of ApoB secretion.

SH2 domain-containing inositol 5-phosphatase (SHIP2) inhibition ameliorates high glucose-induced de-novo lipogenesis and VLDL production through regulating AMPK/mTOR/SREBP1 pathway and ROS production in HepG2 cells

Hepatic de-novo lipogenesis and production of triglyceride rich very low density lipoprotein (VLDL) is increased in the state of insulin resistance, however, the role of a negative regulator of the insulin signaling pathway, the SH2 domain-containing inositol 5-phosphatase (SHIP2) in this process, remains unknown. In the present study, we studied the molecular mechanisms linking SHIP2 expression to metabolic dyslipidemia using overexpression or suppression of SHIP2 gene in HepG2 cells exposed to high glucose (33mM). The results showed that high glucose induced SHIP2 mRNA and protein levels in HepG2 cells. Overexpression of the dominant negative mutant SHIP2 (SHIP2-DN) ameliorated high glucose-induced de-novo lipogenesis and secretion of apoB containing lipoprotein in HepG2 cells, as demonstrated by a reduction in both secreted apoB and MTP expression, and decreased triglyceride levels and the expression of lipogenic genes such as SREBP1c, FAS and ACC. Overexpression of the SHIP2-DN decreased high glucose-induced apoB containing lipoproteins secretion via reduction in ROS generation, JNK phosphorylation and Akt activation. Furthermore, using the specific inhibitor and activator, it was found that the AMPK/mTOR/SREBP1 is the signaling pathway that mediates the effects of SHIP2 modulation on hepatic de-novo lipogenesis. Taken together, these findings suggest that SHIP2 is an important regulator of hepatic lipogenesis and lipoprotein secretion in insulin resistance state.

SH2 domain-containing inositol 5-phosphatase (SHIP2) regulates de-novo lipogenesis and secretion of apoB100 containing lipoproteins in HepG2 cells

Hepatic de-novo lipogenesis and production of triglyceride rich VLDL are regulated via the phosphoinositide 3-kinase cascade, however, the role of a negative regulator of this pathway, the SH2 domain-containing inositol 5-phosphatase (SHIP2) in this process, remains unknown. In the present study, we investigated the molecular link between SHIP2 expression and metabolic dyslipidemia using overexpression or suppression of SHIP2 gene in HepG2 cells. The results showed that overexpression of the wild type SHIP2 gene (SHIP2-WT) led to a higher total lipid content (28%) compared to control, whereas overexpression of the dominant negative SHIP2 gene (SHIP2-DN) reduced total lipid content in oleate treated cells by 40%. Overexpression of SHIP2-WT also led to a significant increase in both secretion of apoB100 containing lipoproteins and de-novo lipogenesis, as demonstrated by an enhancement in secreted apoB100 and MTP expression, increased intra and extracellular triglyceride levels and enhanced expression of lipogenic genes such as SREBP1c, FAS and ACC. On the other hand, overexpression of the SHIP2-DN gene prevented oleate-induced de-novo lipogenesis and secretion of apoB100 containing lipoproteins in HepG2 cells. Collectively, these findings suggest that SHIP2 expression level is a key determinant of hepatic lipogenesis and lipoprotein secretion, and its inhibition could be considered as a potential target for treatment of dyslipidemia.

Glucagon-Like Peptide 2 (GLP-2) Stimulates Postprandial Chylomicron Production and Postabsorptive Release of Intestinal Triglyceride Storage Pools via Induction of Nitric Oxide Signaling in Male Hamsters and Mice.

The intestinal overproduction of apolipoprotein B48 (apoB48)-containing chylomicron particles is a common feature of diabetic dyslipidemia and contributes to cardiovascular risk in insulin resistant states. We previously reported that glucagon-like peptide-2 (GLP-2) is a key endocrine stimulator of enterocyte fat absorption and chylomicron output in the postprandial state. GLP-2's stimulatory effect on chylomicron production in the postabsorptive state has been confirmed in human studies. The mechanism by which GLP-2 regulates chylomicron production is unclear, because its receptor is not expressed on enterocytes. We provide evidence for a key role of nitric oxide (NO) in mediating the stimulatory effects of GLP-2 during the postprandial and postabsorptive periods. Intestinal chylomicron production was assessed in GLP-2-treated hamsters administered the pan-specific NO synthase (NOS) inhibitor L-N(G)-nitroarginine methyl ester (L-NAME), and in GLP-2-treated endothelial NOS knockout mice. L-NAME blocked GLP-2-stimulated apoB48 secretion and reduced triglycerides (TGs) in the TG-rich lipoprotein (TRL) fraction of the plasma in the postprandial state. Endothelial NOS-deficient mice were resistant to GLP-2 stimulation and secreted fewer large apoB48-particles. When TG storage pools were allowed to accumulate, L-NAME mitigated the GLP-2-mediated increase in TRL-TG, suggesting that NO is required for early mobilization and secretion of stored TG and preformed chylomicrons. Importantly, the NO donor S-nitroso-L-glutathione was able to elicit an increase in TRL-TG in vivo and stimulate chylomicron release in vitro in primary enterocytes. We describe a novel role for GLP-2-mediated NO-signaling as a critical regulator of intestinal lipid handling and a potential contributor to postprandial dyslipidemia.

Hepatocyte-Specific Depletion of UBXD8 Induces Periportal Steatosis in Mice Fed a High-Fat Diet.

We showed previously that UBXD8 plays a key role in proteasomal degradation of lipidated ApoB in hepatocarcinoma cell lines. In the present study, we aimed to investigate the functions of UBXD8 in liver in vivo. For this purpose, hepatocyte-specific UBXD8 knockout (UBXD8-LKO) mice were generated. They were fed with a normal or high-fat diet, and the phenotypes were compared with those of littermate control mice. Hepatocytes obtained from UBXD8-LKO and control mice were analyzed in culture. After 26 wk of a high-fat diet, UBXD8-LKO mice exhibited macrovesicular steatosis in the periportal area and microvesicular steatosis in the perivenular area, whereas control mice exhibited steatosis only in the perivenular area. Furthermore, UBXD8-LKO mice on a high-fat diet had significantly lower concentrations of serum triglyceride and VLDL than control mice. A Triton WR-1339 injection study revealed that VLDL secretion from hepatocytes was reduced in UBXD8-LKO mice. The decrease of ApoB secretion upon UBXD8 depletion was recapitulated in cultured primary hepatocytes. Accumulation of lipidated ApoB in lipid droplets was observed only in UBXD8-null hepatocytes. The results showed that depletion of UBXD8 in hepatocytes suppresses VLDL secretion, and could lead to periportal steatosis when mice are fed a high-fat diet. This is the first demonstration that an abnormality in the intracellular ApoB degradation mechanism can cause steatosis, and provides a useful model for periportal steatosis, which occurs in several human diseases.

Abstract 547: A Novel Abetalipoproteinemia Missense Mutation Highlights the Importance of N-terminal β-barrel in Microsomal Triglyceride Transfer Protein Function

Background: The use of microsomal triglyceride transfer protein (MTP) inhibitors is limited to severe hyperlipidemias due to associated hepatosteatosis. Comprehensive knowledge about the structure-function of MTP might help design new molecules that avoid steatosis. Characterization of mutations in MTP causing abetalipoproteinemia (ABL) have revealed that the central α-helical and C-terminal β-sheet domains are important for protein disulfide isomerase (PDI) binding and lipid transfer activity. Our aim was to identify and characterize ABL mutations in the N-terminal region to understand the function of this domain. Methods and Results: We identified a novel missense mutation (D169V) in a 4-month old Turkish male with severe signs of ABL. To study the effect of this mutation on MTP function, we created mutants via site directed mutagenesis. Although D169V was expressed in the endoplasmic reticulum and interacted with apoB17, it was unable to bind PDI, transfer lipids, and support apoB secretion. Computational modeling suggested that D169 could form an internal salt bridge with K187 and K189. Mutagenesis of these lysines to leucines abolished PDI heterodimerization, lipid transfer, and apoB secretion, without affecting apoB binding. Further, mutants with preserved charges (D169E, K187R, K189R) rescued these activities. Conclusions: D169V is detrimental because it disrupts an internal salt bridge leading to loss of PDI binding and lipid transfer activities; however, it does not affect apoB-binding. We propose that future inhibitors should avoid this region of the N-terminal domain.

Abstract 527: Decay of Hepatic and Serum Apolipoprotein(a) Levels is Slower than ApolipoproteinB Following Inhibition of Lipid Synthesis

The association of Lipoprotein(a) (Lp(a)) levels and increased cardiovascular risk is substantiated by human epidemiology, genetics and interventional apheresis studies. Lp(a) is a plasma lipoprotein consisting of a Low Density Lipoprotein (LDL) particle with one molecule of apolipoprotein B100 (ApoB100) covalently linked to one molecule of apolipoprotein(a). Published studies show that apo(a) synthesis and secretion from hepatoma cells is coupled to triglyceride synthesis and secretion, illustrating a point of control for Lp(a) production. The objective of this study is to compare Lp(a), apo(a) and apoB production, secretion and half-life in vivo to understand mechanisms controlling circulating levels of Lp(a). Since rodent species do not express endogenous apo(a), human apo(a) transgenic mice were created and bred with human apoB100 transgenic mice to generate Lp(a)-producing mice. The circulating level of Lp(a) in the transgenic mice is ~3.7 mg/dL that is comparable to 5.4 mg/dL measured in a healthy human subject. To inhibit triglyceride and VLDL secretion in liver, mice were treated with a microsomal triglyceride transfer protein inhibitor (MTPi), and plasma was collected in a time-course. This treatment allowed measurement of plasma Lp(a), apo(a) and apoB half-lives since secretion of nascent VLDL and LDL was blocked. MTPi caused an 81% reduction in cholesterol and 68% reduction in triglycerides after 5 days of treatment. ApoB decreased significantly within 6 hours of treatment and remained low for 5 days. The calculated half-life was 4.6 hours. By contrast, apo(a) and Lp(a) decreased significantly after 3 days of treatment with half-lives of 2.4 and 2.5 days, respectively, illustrating delayed catabolism of apo(a)/Lp(a). Hepatic Lp(a), apo(a) and apoB were decreased significantly after 5 days dosing, indicating that the apolipoproteins were likely degraded and not accumulated in the liver. Treatment of stable HepG2 cells expressing apo(a) with MTPi caused similar difference in Lp(a) and apoB half-lives. These models illustrate rapid decay of apoB, but not apo(a), when lipoprotein particle assembly is blocked, and they provide methods for future mechanistic studies of apo(a) assembly into Lp(a) and its catabolism.

Modulators of hepatic lipoprotein metabolism identified in a search for small-molecule inducers of tribbles pseudokinase 1 expression.

Recent genome wide association studies have linked tribbles pseudokinase 1 (TRIB1) to the risk of coronary artery disease (CAD). Based on the observations that increased expression of TRIB1 reduces secretion of VLDL and is associated with lower plasma levels of LDL cholesterol and triglycerides, higher plasma levels of HDL cholesterol and reduced risk for myocardial infarction, we carried out a high throughput phenotypic screen based on quantitative RT-PCR assay to identify compounds that induce TRIB1 expression in human HepG2 hepatoma cells. In a screen of a collection of diversity-oriented synthesis (DOS)-derived compounds, we identified a series of benzofuran-based compounds that upregulate TRIB1 expression and phenocopy the effects of TRIB1 cDNA overexpression, as they inhibit triglyceride synthesis and apoB secretion in cells. In addition, the compounds downregulate expression of MTTP and APOC3, key components of the lipoprotein assembly pathway. However, CRISPR-Cas9 induced chromosomal disruption of the TRIB1 locus in HepG2 cells, while confirming its regulatory role in lipoprotein metabolism, demonstrated that the effects of benzofurans persist in TRIB1-null cells indicating that TRIB1 is sufficient but not necessary to transmit the effects of the drug. Remarkably, active benzofurans, as well as natural products capable of TRIB1 upregulation, also modulate hepatic cell cholesterol metabolism by elevating the expression of LDLR transcript and LDL receptor protein, while reducing the levels of PCSK9 transcript and secreted PCSK9 protein and stimulating LDL uptake. The effects of benzofurans are not masked by cholesterol depletion and are independent of the SREBP-2 regulatory circuit, indicating that these compounds represent a novel class of chemically tractable small-molecule modulators that shift cellular lipoprotein metabolism in HepG2 cells from lipogenesis to scavenging.

Elevated retinol binding protein 4 induces apolipoprotein B production and associates with hypertriglyceridemia.

A high level of retinol binding protein 4 (RBP4) is reported to be associated with insulin resistance in humans. However, evidence from large-scale populations about the relationship between serum RBP4 and metabolic phenotypes is scarce. In the present study, we aimed to evaluate serum RBP4 distribution and its association with metabolic phenotypes among middle-aged and elderly Chinese. Serum concentrations of RBP4 in a cross-sectional sample of 2780 Chinese population aged 50-70 years old in Guangzhou were measured by ELISA. The mean of serum RBP4 concentration was 28.04 μg/mL for male and 37.76 μg/mL for female (P < .01), respectively. Circulating RBP4 was positively correlated with serum triglyceride and apolipoprotein B (apoB) concentrations. The odds ratio (OR) was substantially higher for hypertriglyceridemia (OR, 3.26; 95% confidence interval, 2.36-4.51) in the highest RBP4 quartile compared with those in the lowest quartile after multiple adjustment for confounders. Furthermore, serum RBP4 was significantly associated with fasting glucose, insulin levels, and homeostasis model assessment index-insulin resistance (HOMA-IR). Moreover, we showed that RBP4 enhanced microsomal triglyceride transfer protein (MTP) expression and activity via up-regulation of protein disulfide isomerase (PDI), suppressed low-density lipoprotein receptor (LDLR) expression, and impaired insulin-signaling pathway, leading to inductions in apoB secretion both in vitro and in vivo. Elevated circulating RBP4 concentrations were associated with higher risk of hypertriglyceridemia by inducing the secretion of triglyceride-rich apoB-containing lipoproteins.

Phospholipid Transfer Protein Plays a Major Role in the Initiation of Apolipoprotein B-containing Lipoprotein Assembly in Mouse Primary Hepatocytes

In this study, we tested the hypothesis that phospholipid transfer protein (PLTP) is a plausible mediator of phospholipid (PL) transfer to the N-terminal 1000 residues of apoB (apoB:1000) leading to the initiation of apoB-containing lipoprotein assembly. To this end, primary hepatocytes from wild type (WT) and PLTP knock-out (KO) mice were transduced with adenovirus-apoB:1000 with or without co-transduction with adenovirus-PLTP, and the assembly and secretion of apoB:1000-containing lipoproteins were assessed. PLTP deficiency resulted in a 65 and 72% reduction in the protein and lipid content, respectively, of secreted apoB:1000-containing lipoproteins. Particles secreted by WT hepatocytes contained 69% PL, 9% diacylglycerol (DAG), and 23% triacylglycerol (TAG) with a stoichiometry of 46 PL, 6 DAG, and 15 TAG molecules per apoB:1000. PLTP absence drastically altered the lipid composition of apoB:1000 lipoproteins; these particles contained 46% PL, 13% DAG, and 41% TAG with a stoichiometry of 27 PL, 10 DAG, and 23 TAG molecules per apoB:1000. Reintroduction of Pltp gene into PLTP-KO hepatocytes stimulated the lipidation and secretion of apoB:1000-containing lipoproteins by ∼3-fold; the lipid composition and stoichiometry of these particles were identical to those secreted by WT hepatocytes. In contrast to the WT, apoB:1000 in PLTP-KO hepatocytes was susceptible to intracellular degradation predominantly in the post-endoplasmic reticulum, presecretory compartment. Reintroduction of Pltp gene into PLTP-KO hepatocytes restored the stability of apoB:1000. These results provide compelling evidence that in hepatocytes initial recruitment of PL by apoB:1000 leading to the formation of the PL-rich apoB-containing initiation complex is mediated to a large extent by PLTP.

Health promoting properties of protein hydrolysates produced from oil palm (Elaeis guineensis) kernel

This study aimed to determine the lipid-lowering properties, antioxidant capacity (AC) and angiotensin-I converting enzyme (ACE)-inhibitory activity of oil palm kernel protein hydrolysates (OPKHs) that were produced using protease and pepsin-pancreatin hydrolysis. The effects of the OPKHs on apolipoprotein B (apoB) secretion was assessed using HepG2 cells as a model and the AC of the OPKHs was determined based on ABTS radical scavenging activity and ferric reducing antioxidant power (FRAP). Both protease and pepsin-pancreatin hydrolysates reduced apoB secretion significantly (p<0.05). The OPKHs scavenged ABTS radicals effectively and demonstrated a high reducing power even at a low concentration (1 mg/mL). The AC of the OPKHs was significantly correlated with the OPKHs protein content. However, the OPKHs demonstrated very low ACE-inhibitory activity. The pepsinpancreatin hydrolysate demonstrated significant lipidlowering properties, favourable AC and ACE inhibitory activity in compared to protease hydrolysate. Therefore, OPKH demonstrate the potential as a nutraceutical for functional foods.

Pharmacologic inhibition of sphingomyelin synthase (SMS) activity reduces apolipoprotein-B secretion from hepatocytes and attenuates endotoxin-mediated macrophage inflammation.

Sphingomyelin synthase (SMS) plays an important role in plasma atherogenic lipoprotein metabolism, inflammation, and the development of atherosclerosis. To understand whether the impaired apoB secretion and inflammation response is a direct result from lack of SMS activity, in this study, we prepared a series of compounds that inhibit SMS activity. Further, we characterized Dy105, the most potent inhibitor. We found that Dy105 treatment significantly reduces SM levels in SM-rich microdomain on cell membranes. Moreover, we found that SMS inhibition reduces apoB secretion in a human hepatoma cell line and reduces the activation of NFκB and p38, a MAP kinase, in bone marrow derived macrophages. These studies provided further evidence that SMS activity regulates atherogenic lipoprotein metabolism and inflammatory responses. Pharmacologic inhibition of SMS may be a new therapy for atherosclerosis by reducing apoB secretion, and reducing inflammation.

Hepatic overexpression of the prodomain of furin lessens progression of atherosclerosis and reduces vascular remodeling in response to injury

ObjectiveAtherosclerosis is a complex disease, involving elevated LDL-c, lipid accumulation in the blood vessel wall, foam cell formation and vascular dysfunction. Lowering plasma LDL-c is the cornerstone of current management of cardiovascular disease. However, new approaches which reduce plasma LDL-c and lessen the pathological vascular remodeling occurring in the disease should also have therapeutic value. Previously, we found that overexpression of profurin, the 83-amino acid prodomain of the proprotein convertase furin, lowered plasma HDL levels in wild-type mice. The question that remained was whether it had effects on apolipoprotein B (ApoB)-containing lipoproteins. MethodsAdenovirus mediated overexpression of hepatic profurin in Ldlr−/−mice and wild-type mice were used to evaluate effects of profurin on ApoB-containing lipoproteins, atherosclerosis and vascular remodeling. ResultsHepatic profurin overexpression resulted in a significant reduction in atherosclerotic lesion development in Ldlr−/−mice and a robust reduction in plasma LDL-c. Metabolic studies revealed lower secretion of ApoB and triglycerides in VLDL particles. Mechanistic studies showed that in the presence of profurin, hepatic ApoB, mainly ApoB100, was degraded by proteasomes. There was no effect on ApoB mRNA expression. Importantly, short-term hepatic profurin overexpression did not result in hepatic lipid accumulation. Blood vessel wall thickening caused by either wire-induced femoral artery injury or common carotid artery ligation was reduced. Profurin expression inhibited proliferation and migration in vascular smooth muscle cells in vitro. ConclusionThese results indicate that a profurin-based therapy has the potential to treat atherosclerosis by improving metabolic lipid profiles and reducing both atherosclerotic lesion development and pathological vascular remodeling.

Human FABP1 T94A variant impacts fatty acid metabolism and PPAR-α activation in cultured human female hepatocytes.

Although human liver fatty acid-binding protein (FABP1) T94A variant has been associated with nonalcoholic fatty liver disease and reduced ability of fenofibrate to lower serum triglycerides (TG) to target levels, molecular events leading to this phenotype are poorly understood. Cultured primary hepatocytes from female human subjects expressing the FABP1 T94A variant exhibited increased neutral lipid (TG, cholesteryl ester) accumulation associated with (1) upregulation of total FABP1, a key protein stimulating mitochondrial glycerol-3-phosphate acyltransferase (GPAM), the rate-limiting enzyme in lipogenesis; (2) increased mRNA expression of key enzymes in lipogenesis (GPAM, LPIN2) in heterozygotes; (3) decreased mRNA expression of microsomal triglyceride transfer protein; (4) increased secretion of ApoB100 but not TG; (5) decreased long-chain fatty acid (LCFA) β-oxidation. TG accumulation was not due to any increase in LCFA uptake, de novo lipogenesis, or the alternate monoacylglycerol O-acyltransferase pathway in lipogenesis. Despite increased expression of total FABP1 mRNA and protein, fenofibrate-mediated FABP1 redistribution to nuclei and ligand-induced peroxisome proliferator-activated receptor (PPAR-α) transcription of LCFA β-oxidative enzymes (carnitine palmitoyltransferase 1A, carnitine palmitoyltransferase 2, and acyl-coenzyme A oxidase 1, palmitoyl) were attenuated in FABP1 T94A hepatocytes. Although the phenotype of FABP1 T94A variant human hepatocytes exhibits some similarities to that of FABP1-null or PPAR-α-null hepatocytes and mice, expression of FABP1 T94A variant did not abolish or reduce ligand binding. Thus the FABP1 T94A variant represents an altered/reduced function mutation resulting in TG accumulation.

Abstract 597: Nobiletin Reduces Intestinal Lipoprotein Production in LDL Receptor-Null Mice With Diet-Induced Insulin Resistance

Insulin resistant subjects display excessive postprandial lipemia (PPL), defined by overproduction of intestinal triglyceride (TG)-rich, apoB48-containing chylomicrons (CM), which contribute to atherogenesis. However, the regulation of intestinal lipoprotein secretion by insulin and the mechanisms underlying elevated CM production in insulin resistant states remain poorly understood. In mice, the citrus flavonoid nobiletin prevents diet-induced obesity, dyslipidemia, insulin resistance, hepatic steatosis and VLDL overproduction. In the present study, we sought to characterize the aberrant PPL response in mice with diet-induced insulin resistance and hypothesized that nobiletin treatment would correct this abnormal response by normalizing intestinal insulin signaling and TG metabolism. Ldlr-/- mice were fed a high-fat, cholesterol-containing (HFHC) diet (42% fat, 0.2% cholesterol), a HFHC diet plus 0.3% w/w nobiletin or chow for 8 weeks (n=8 per group). Compared to chow, HFHC-fed mice displayed elevated TG (3-fold) within intestinal tissue following a 6 hour fast, which was completely normalized by nobiletin. Fasting-refeeding studies revealed impaired FoxO1 phosphorylation, increased mTOR phosphorylation and elevated Srebp1c mRNA in the jejunum of HFHC-fed mice, indicating intestinal insulin resistance. These parameters were normalized by nobiletin. Measurement of plasma TG following an oral gavage of 150ul olive oil revealed a 3-fold elevation in the total TG area under the curve, which was prevented by nobiletin. Intestinal CM production was determined by injection of mice with poloxamer-407 to inhibit plasma TG-rich lipoprotein lipolysis followed by a gavage containing olive oil and 3H-triolein. Although plasma TG was higher at baseline in HFHC-fed mice, no difference in secretion rates of TG mass or TG radioactivity was observed compared to nobiletin-treated or chow-fed mice. However, secretion of apoB48 was elevated by 20% in HFHC-fed mice, whereas nobiletin decreased apoB48 secretion to levels observed in chow-fed mice. These results indicate that nobiletin prevents intestinal TG accumulation and apoB48 overproduction induced by a HFHC diet, in part, through correction of intestinal insulin resistance.