PI3-kinase activity modulates apo B available for hepatic VLDL production in apobec-1-/- mice

Hepatic selective insulin resistance at the intersection of insulin signaling and metabolic dysfunction-associated steatotic liver disease

Insulin resistance (IR) is a major pathogenic factor in the progression of MASLD. In the liver, insulin suppresses gluconeogenesis and enhances de novo lipogenesis (DNL). During IR, there is a defect in insulin-mediated suppression of gluconeogenesis, but an unrestrained increase in hepatic lipogenesis persists. The mechanism of increased hepatic steatosis in IR is unclear and remains controversial. The key discrepancy is whether insulin retains its ability to directly regulate hepatic lipogenesis. Blocking insulin/IRS/AKT signaling reduces liver lipid deposition in IR, suggesting insulin can still regulate lipid metabolism; hepatic glucose metabolism that bypasses insulin's action may contribute to lipogenesis; and due to peripheral IR, other tissues are likely to impact liver lipid deposition. We here review the current understanding of insulin's action in governing different aspects of hepatic lipid metabolism under normal and IR states, with the purpose of highlighting the essential issues that remain unsettled.

Can modulators of apolipoproteinB biogenesis serve as an alternate target for cholesterol-lowering drugs?

Understanding the molecular defects underlying cardiovascular disease is necessary for the development of therapeutics. The most common method to lower circulating lipids, which reduces the incidence of cardiovascular disease, is statins, but other drugs are now entering the clinic, some of which have been approved. Nevertheless, patients cannot tolerate some of these therapeutics, the drugs are costly, and/or the treatments are approved for only rare forms of disease. Efforts to find alternative treatments have focused on other factors, such as apolipoproteinB (apoB), which transports cholesterol in the blood stream. The levels of apoB are regulated by endoplasmic reticulum (ER) associated degradation as well as by a post ER degradation pathway in model systems, and we suggest that these events provide novel therapeutic targets. We discuss first how cardiovascular disease arises and how cholesterol is regulated, and then summarize the mechanisms of action of existing treatments for cardiovascular disease. We then review the apoB biosynthetic pathway, focusing on steps that might be amenable to therapeutic interventions.

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.

Perilipin-2 Deletion Impairs Hepatic Lipid Accumulation by Interfering with Sterol Regulatory Element-binding Protein (SREBP) Activation and Altering the Hepatic Lipidome

Perilipin-2 (PLIN2) is a constitutively associated cytoplasmic lipid droplet coat protein that has been implicated in fatty liver formation in non-alcoholic fatty liver disease. Mice with or without whole-body deletion of perilipin-2 (Plin2-null) were fed either Western or control diets for 30 weeks. Perilipin-2 deletion prevents obesity and insulin resistance in Western diet-fed mice and dramatically reduces hepatic triglyceride and cholesterol levels in mice fed Western or control diets. Gene and protein expression studies reveal that PLIN2 deletion suppressed SREBP-1 and SREBP-2 target genes involved in de novo lipogenesis and cholesterol biosynthetic pathways in livers of mice on either diet. GC-MS lipidomics demonstrate that this reduction correlated with profound alterations in the hepatic lipidome with significant reductions in both desaturation and elongation of hepatic neutral lipid species. To examine the possibility that lipidomic actions of PLIN2 deletion contribute to suppression of SREBP activation, we isolated endoplasmic reticulum membrane fractions from long-term Western diet-fed wild type (WT) and Plin2-null mice. Lipidomic analyses reveal that endoplasmic reticulum membranes from Plin2-null mice are markedly enriched in ω-3 and ω-6 long-chain polyunsaturated fatty acids, which others have shown inhibit SREBP activation and de novo lipogenesis. Our results identify PLIN2 as a determinant of global changes in the hepatic lipidome and suggest the hypothesis that these actions contribute to SREBP-regulated de novo lipogenesis involved in non-alcoholic fatty liver disease.

Sortilin facilitates VLDL-B100 secretion by insulin sensitive McArdle RH7777 cells

Studies examining the relationship between cellular sortilin and VLDL-B100 secretion demonstrate inconsistent results. Current studies explore the possibility that discrepancies may be related to insulin sensitivity. McArdle RH7777 cells (McA cells) cultured under serum enriched conditions lose sensitivity to insulin. Following incubation in serum-free DMEM containing 1% BSA, McA cells become insulin responsive and demonstrate reduced apo B secretion. Current studies indicate that insulin sensitive McA cells express lower cellular sortilin that corresponds with reduction in VLDL-B100 secretion without changes in mRNA of either sortilin or apo B. When sortilin expression is further reduced by siRNA knockdown (KD), there are additional decreases in VLDL-B100 secretion. A crystal structure of human sortilin (hsortilin) identifies two binding sites on the luminal domain for the N- and C-termini of neurotensin (NT). A small organic compound (cpd984) was identified that has strong theoretical binding to the N-terminal site. Both cpd984 and NT bind hsortilin by surface plasmon resonance. In incubations with insulin sensitive McA cells, cpd984 was shown to enhance VLDL-B100 secretion at each level of sortilin KD suggesting cpd984 acted through sortilin in mediating its effect. Current results support a role for sortilin to facilitate VLDL-B100 secretion which is limited to insulin sensitive McA cells. Inconsistent reports of the relationship between VLDL-B100 secretion and sortilin in previous studies may relate to differing functions of sortilin in VLDL-B100 secretion depending upon insulin sensitivity.

Insulin dependent apolipoprotein B degradation and phosphatidylinositide 3-kinase activation with microsomal translocation are restored in McArdle RH7777 cells following serum deprivation

Previous studies in rat hepatocytes demonstrated that insulin-dependent apolipoprotein (apo) B degradation (IDAD) is lost when cells are maintained for 3 d under enriched culture conditions. Loss of IDAD correlates with increased expression of protein tyrosine phosphatase 1B (PTP1B) known to be associated with resistance to insulin signaling in the liver. McArdle RH7777 hepatoma (McA) cells cultured in serum containing medium are resistant to IDAD; demonstrate a 30% increase in apo B secretion, and express increased levels of PTP1B protein and mRNA. In addition, insulin-stimulated Class I phosphatidylinositide 3-kinase (PI3K) activity of anti-pY immunoprecipitates is severely blunted. IDAD resistance in McA cells correlates with diminished translocation of insulin-stimulated pY-IRS1 to intracellular membranes. Incubation of McA cells with RK682, a protein tyrosine phosphatase inhibitor, is sufficient to restore IDAD in resistant McA cells. Overall, results further support the importance of Class I PI3K activity in IDAD, and suggest that loss of this activity is sufficient to cause resistance. Although other factors are involved in downstream events including sortilin binding to apo B, autophagy, and lysosomal degradation, loss of signal generation and reduced localization of Class I PI3K to intracellular membranes plays a significant role in IDAD resistance.

The Regulation of Lipid Deposition by Insulin in Goose Liver Cells Is Mediated by the PI3K-AKT-mTOR Signaling Pathway

Background

We previously showed that the fatty liver formations observed in overfed geese are accompanied by the activation of the PI3K-Akt-mTOR pathway and an increase in plasma insulin concentrations. Recent studies have suggested a crucial role for the PI3K-Akt-mTOR pathway in regulating lipid metabolism; therefore, we hypothesized that insulin affects goose hepatocellular lipid metabolism through the PI3K-Akt-mTOR signaling pathway.

Methods

Goose primary hepatocytes were isolated and treated with serum-free media supplemented with PI3K-Akt-mTOR pathway inhibitors (LY294002, rapamycin, and NVP-BEZ235, respectively) and 50 or 150 nmol/L insulin.

Results

Insulin induced strong effects on lipid accumulation as well as the mRNA and protein levels of genes involved in lipogenesis, fatty acid oxidation, and VLDL-TG assembly and secretion in primary goose hepatocytes. The stimulatory effect of insulin on lipogenesis was significantly decreased by treatment with PI3K-Akt-mTOR inhibitors. These inhibitors also rescued the insulin-induced down-regulation of fatty acid oxidation and VLDL-TG assembly and secretion.

Conclusion

These findings suggest that the stimulatory effect of insulin on lipid deposition is mediated by PI3K-Akt-mTOR regulation of lipogenesis, fatty acid oxidation, and VLDL-TG assembly and secretion in goose hepatocytes.

Insulin resistance induces posttranslational hepatic sortilin 1 degradation in mice

Insulin promotes hepatic apolipoprotein B100 (apoB100) degradation, whereas insulin resistance is a major cause of hepatic apoB100/triglyceride overproduction in type 2 diabetes. The cellular trafficking receptor sortilin 1 (Sort1) was recently identified to transport apoB100 to the lysosome for degradation in the liver and thus regulate plasma cholesterol and triglyceride levels. Genetic variation of SORT1 was strongly associated with cardiovascular disease risk in humans. The major goal of this study is to investigate the effect and molecular mechanism of insulin regulation of Sort1. Results showed that insulin induced Sort1 protein, but not mRNA, in AML12 cells. Treatment of PI3K or AKT inhibitors decreased Sort1 protein, whereas expression of constitutively active AKT induced Sort1 protein in AML12 cells. Consistently, hepatic Sort1 was down-regulated in diabetic mice, which was partially restored after the administration of the insulin sensitizer metformin. LC-MS/MS analysis further revealed that serine phosphorylation of Sort1 protein was required for insulin induction of Sort1 in a casein kinase 2-dependent manner and that inhibition of PI3K signaling or prevention of Sort1 phosphorylation accelerated proteasome-dependent Sort1 degradation. Administration of a PI3K inhibitor to mice decreased hepatic Sort1 protein and increased plasma cholesterol and triglyceride levels. Adenovirus-mediated overexpression of Sort1 in the liver prevented PI3K inhibitor-induced Sort1 down-regulation and decreased plasma triglyceride but had no effect on plasma cholesterol in mice. This study identified Sort1 as a novel target of insulin signaling and suggests that Sort1 may play a role in altered hepatic apoB100 metabolism in insulin-resistant conditions.

Silencing of ANGPTL 3 (angiopoietin-like protein 3) in human hepatocytes results in decreased expression of gluconeogenic genes and reduced triacylglycerol-rich VLDL secretion upon insulin stimulation

Homozygosity of loss-of-function mutations in ANGPTL3 (angiopoietin-like protein 3)-gene results in FHBL2 (familial combined hypolipidaemia, OMIM #605019) characterized by the reduction of all major plasma lipoprotein classes, which includes VLDL (very-low-density lipoprotein), LDL (low-density lipoprotein), HDL (high-density lipoprotein) and low circulating NEFAs (non-esterified fatty acids), glucose and insulin levels. Thus complete lack of ANGPTL3 in humans not only affects lipid metabolism, but also affects whole-body insulin and glucose balance. We used wild-type and ANGPTL3-silenced IHHs (human immortalized hepatocytes) to investigate the effect of ANGPTL3 silencing on hepatocyte-specific VLDL secretion and glucose uptake. We demonstrate that both insulin and PPARγ (peroxisome-proliferator-activated receptor γ) agonist rosiglitazone down-regulate the secretion of ANGPTL3 and TAG (triacylglycerol)-enriched VLDL1-type particles in a dose-dependent manner. Silencing of ANGPTL3 improved glucose uptake in hepatocytes by 20–50% and influenced down-regulation of gluconeogenic genes, suggesting that silencing of ANGPTL3 improves insulin sensitivity. We further show that ANGPTL3-silenced cells display a more pronounced shift from the secretion of TAG-enriched VLDL1-type particles to secretion of lipid poor VLDL2-type particles during insulin stimulation. These data suggest liver-specific mechanisms involved in the reported insulin-sensitive phenotype of ANGPTL3-deficient humans, featuring lower plasma insulin and glucose levels.

We show that silencing of ANGPTL3 in human hepatocytes in addition to reducing secretion of TAG-enriched VLDL upon insulin stimulation enhances glucose uptake and improves insulin response. Thus, our data provide insight into the lower insulin and glucose levels observed in humans with ANGPTL3 loss-of-function mutation.

Transmembrane protein 55B is a novel regulator of cellular cholesterol metabolism

OBJECTIVE

Interindividual variation in pathways affecting cellular cholesterol metabolism can influence levels of plasma cholesterol, a well-established risk factor for cardiovascular disease. Inherent variation among immortalized lymphoblastoid cell lines from different donors can be leveraged to discover novel genes that modulate cellular cholesterol metabolism. The objective of this study was to identify novel genes that regulate cholesterol metabolism by testing for evidence of correlated gene expression with cellular levels of 3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR) mRNA, a marker for cellular cholesterol homeostasis, in a large panel of lymphoblastoid cell lines.

APPROACH AND RESULTS

Expression array profiling was performed on 480 lymphoblastoid cell lines established from participants of the Cholesterol and Pharmacogenetics (CAP) statin clinical trial, and transcripts were tested for evidence of correlated expression with HMGCR as a marker of intracellular cholesterol homeostasis. Of these, transmembrane protein 55b (TMEM55B) showed the strongest correlation (r=0.29; P=4.0E-08) of all genes not previously implicated in cholesterol metabolism and was found to be sterol regulated. TMEM55B knockdown in human hepatoma cell lines promoted the decay rate of the low-density lipoprotein receptor, reduced cell surface low-density lipoprotein receptor protein, impaired low-density lipoprotein uptake, and reduced intracellular cholesterol.

CONCLUSIONS

Here, we report identification of TMEM55B as a novel regulator of cellular cholesterol metabolism through the combination of gene expression profiling and functional studies. The findings highlight the value of an integrated genomic approach for identifying genes that influence cholesterol homeostasis.

Insulin-dependent apolipoprotein B degradation is mediated by autophagy and involves class I and class III phosphatidylinositide 3-kinases

Insulin acutely stimulates the degradation of apolipoprotein B (apo B) which decreases very low density lipoprotein (VLDL) secretion by liver. Insulin-dependent apo B degradation (IDAD) occurs following phosphatidylinositide 3-kinase (PI3K) activation and involves lysosomal degradation. Insulin suppression of apo B secretion is blocked by over-expression of phosphatase and tensin homologue (PTEN) in McArdle RH7777 (McA) cells suggesting the importance of Class I PI3K generated PI (3,4,5) triphosphate (PIP3) in IDAD. Classical autophagy inhibitors including 3-methyladenine, L-asparagine and bafilomycin A1 also blocked the ability of insulin to suppress apo B secretion by rat hepatocytes (RH) suggesting that IDAD occurs through an autophagy-related mechanism. IDAD is also blocked following over-expression in McA cells of a dominant negative kinase-defective Vps34, a class III PI3K that generates PI 3-monophosphate required for autophagy. Vps34 inhibition of IDAD occurs without altering insulin-dependent S473 phosphorylation of Akt indicating PI3K/PIP3/Akt signaling is intact. Cellular p62/SQSTM1, an inverse indicator of autophagy, is increased with insulin treatment consistent with the known ability of insulin to inhibit autophagy, and therefore the role of insulin in utilizing components of autophagy for apo B degradation is unexpected. Thapsigargan, an inducer of endoplasmic reticulum (ER) stress, and a recently demonstrated autophagy inhibitor, blocked apo B secretion which contrasted with other autophagy inhibitors and mutant Vps34 results which were permissive with respect to apo B secretion. Pulse chase studies indicated that intact B100 and B48 proteins were retained in cells treated with thapsigargan consistent with their accumulation in autophagosomal vacuoles. Differences between IDAD and ER stress-coupled autophagy mediated by thapsgargin suggest that IDAD involves an unique form of autophagy. Insulin action resulting in hepatic apo B degradation is novel and important in understanding regulation of hepatic VLDL metabolism.

Insulin-stimulated degradation of apolipoprotein B100: roles of class II phosphatidylinositol-3-kinase and autophagy

Both in humans and animal models, an acute increase in plasma insulin levels, typically following meals, leads to transient depression of hepatic secretion of very low density lipoproteins (VLDL). One contributing mechanism for the decrease in VLDL secretion is enhanced degradation of apolipoprotein B100 (apoB100), which is required for VLDL formation. Unlike the degradation of nascent apoB100, which occurs in the endoplasmic reticulum (ER), insulin-stimulated apoB100 degradation occurs post-ER and is inhibited by pan-phosphatidylinositol (PI)3-kinase inhibitors. It is unclear, however, which of the three classes of PI3-kinases is required for insulin-stimulated apoB100 degradation, as well as the proteolytic machinery underlying this response. Class III PI3-kinase is not activated by insulin, but the other two classes are. By using a class I-specific inhibitor and siRNA to the major class II isoform in liver, we now show that it is class II PI3-kinase that is required for insulin-stimulated apoB100 degradation in primary mouse hepatocytes. Because the insulin-stimulated process resembles other examples of apoB100 post-ER proteolysis mediated by autophagy, we hypothesized that the effects of insulin in autophagy-deficient mouse primary hepatocytes would be attenuated. Indeed, apoB100 degradation in response to insulin was significantly impaired in two types of autophagy-deficient hepatocytes. Together, our data demonstrate that insulin-stimulated apoB100 degradation in the liver requires both class II PI3-kinase activity and autophagy.

Insulin suppression of apolipoprotein B in McArdle RH7777 cells involves increased sortilin 1 interaction and lysosomal targeting

Insulin suppresses secretion of very low density lipoprotein (VLDL) apolipoprotein (apo) B in primary rodent hepatocytes (RH) by favoring the degradation of B100, the larger form of apo B, through post-endoplasmic reticulum proteolysis. Sortilin 1 (sort1), a multi-ligand sorting receptor, has been proposed as a mediator of lysosomal B100 degradation by directing B100 in pre-VLDL to lysosomes rather than allowing maturation to VLDL and secretion. The purpose of our studies was to investigate the role of sort1 in insulin-dependent degradation of apo B. Using liver derived McArdle RH7777 (McA) cells, we demonstrate that insulin suppresses VLDL B100 secretion via a phosphatidylinositide 3-kinase (PI3K) dependent process that is inhibitable by wortmannin in a fashion similar to RH. Using McA cells and in situ cross-linking, we demonstrate that insulin acutely (30min) stimulates the interaction of B100 with sort1. The insulin-induced interaction of sort1-B100 is markedly enhanced when lysosomal degradation is inhibited by Bafilomycin A1 (BafA1), an inhibitor of lysosomal acidification. As BafA1 also prevents insulin suppressive effects on apo B secretion, our results suggest that sort1-B100 interaction stimulated by insulin transiently accumulates with BafA1 and favors B100 secretion by default.

Selective hepatic insulin resistance, VLDL overproduction, and hypertriglyceridemia

Insulin plays a central role in regulating energy metabolism, including hepatic transport of very low-density lipoprotein (VLDL)-associated triglyceride. Hepatic hypersecretion of VLDL and consequent hypertriglyceridemia leads to lower circulating high-density lipoprotein levels and generation of small dense low-density lipoproteins characteristic of the dyslipidemia commonly observed in metabolic syndrome and type 2 diabetes mellitus. Physiological fluctuations of insulin modulate VLDL secretion, and insulin inhibition of VLDL secretion upon feeding may be the first pathway to become resistant in obesity that leads to VLDL hypersecretion. This review summarizes the role of insulin-related signaling pathways that determine hepatic VLDL production. Disruption in signaling pathways that reduce generation of the second messenger phosphatidylinositide (3,4,5) triphosphate downstream of activated phosphatidylinositide 3-kinase underlies the development of VLDL hypersecretion. As insulin resistance progresses, a number of pathways are altered that further augment VLDL hypersecretion, including hepatic inflammatory pathways. Insulin plays a complex role in regulating glucose metabolism, and it is not surprising that the role of insulin in VLDL and lipid metabolism will prove equally complex.

Translational control mechanisms in metabolic regulation: critical role of RNA binding proteins, microRNAs, and cytoplasmic RNA granules

Regulated cell metabolism involves acute and chronic regulation of gene expression by various nutritional and endocrine stimuli. To respond effectively to endogenous and exogenous signals, cells require rapid response mechanisms to modulate transcript expression and protein synthesis and cannot, in most cases, rely on control of transcriptional initiation that requires hours to take effect. Thus, co- and posttranslational mechanisms have been increasingly recognized as key modulators of metabolic function. This review highlights the critical role of mRNA translational control in modulation of global protein synthesis as well as specific protein factors that regulate metabolic function. First, the complex lifecycle of eukaryotic mRNAs will be reviewed, including our current understanding of translational control mechanisms, regulation by RNA binding proteins and microRNAs, and the role of RNA granules, including processing bodies and stress granules. Second, the current evidence linking regulation of mRNA translation with normal physiological and metabolic pathways and the associated disease states are reviewed. A growing body of evidence supports a key role of translational control in metabolic regulation and implicates translational mechanisms in the pathogenesis of metabolic disorders such as type 2 diabetes. The review also highlights translational control of apolipoprotein B (apoB) mRNA by insulin as a clear example of endocrine modulation of mRNA translation to bring about changes in specific metabolic pathways. Recent findings made on the role of 5'-untranslated regions (5'-UTR), 3'-UTR, RNA binding proteins, and RNA granules in mediating insulin regulation of apoB mRNA translation, apoB protein synthesis, and hepatic lipoprotein production are discussed.

Insulin resistance in obesity as the underlying cause for the metabolic syndrome

The metabolic syndrome affects more than a third of the US population, predisposing to the development of type 2 diabetes and cardiovascular disease. The 2009 consensus statement from the International Diabetes Federation, American Heart Association, World Heart Federation, International Atherosclerosis Society, International Association for the Study of Obesity, and the National Heart, Lung, and Blood Institute defines the metabolic syndrome as 3 of the following elements: abdominal obesity, elevated blood pressure, elevated triglycerides, low high-density lipoprotein cholesterol, and hyperglycemia. Many factors contribute to this syndrome, including decreased physical activity, genetic predisposition, chronic inflammation, free fatty acids, and mitochondrial dysfunction. Insulin resistance appears to be the common link between these elements, obesity and the metabolic syndrome. In normal circumstances, insulin stimulates glucose uptake into skeletal muscle, inhibits hepatic gluconeogenesis, and decreases adipose-tissue lipolysis and hepatic production of very-low-density lipoproteins. Insulin signaling in the brain decreases appetite and prevents glucose production by the liver through neuronal signals from the hypothalamus. Insulin resistance, in contrast, leads to the release of free fatty acids from adipose tissue, increased hepatic production of very-low-density lipoproteins and decreased high-density lipoproteins. Increased production of free fatty acids, inflammatory cytokines, and adipokines and mitochondrial dysfunction contribute to impaired insulin signaling, decreased skeletal muscle glucose uptake, increased hepatic gluconeogenesis, and β cell dysfunction, leading to hyperglycemia. In addition, insulin resistance leads to the development of hypertension by impairing vasodilation induced by nitric oxide. In this review, we discuss normal insulin signaling and the mechanisms by which insulin resistance contributes to the development of the metabolic syndrome.

Insulin signaling to hepatic lipid metabolism in health and disease

The increasing prevalence of overnutrition and reduced activity has led to a worldwide epidemic of obesity. In many cases, this is associated with insulin resistance, an inability of the hormone to direct its physiological actions appropriately. A number of disease states accompany insulin resistance such as type 2 diabetes mellitus, the metabolic syndrome, and non-alcoholic fatty liver disease. Though the pathways by which insulin controls hepatic glucose output have been of intense study in recent years, considerably less attention has been devoted to how lipid metabolism is regulated. Thus, both the proximal signaling pathways as well as the more distal targets of insulin remain uncertain. In this review, we consider the signaling pathways by which insulin controls the synthesis and accumulation of lipids in the mammalian liver and, in particular, how this might lead to abnormal triglyceride deposition in liver during insulin-resistant states.

Acute suppression of apo B secretion by insulin occurs independently of MTP

Secretion of apolipoprotein (apo) B-containing lipoproteins by the liver depends mainly upon apo B availability and microsomal triglyceride transfer protein (MTP) activity and is subject to insulin regulation. Hepatic MTP mRNA expression is negatively regulated by insulin which correlates with inhibition of apo B secretion suggesting that insulin might suppress apo B secretion through an MTP-dependent mechanism. To investigate this possibility, we examined the acute effect of insulin on hepatic MTP expression and activity levels in vivo utilizing apobec-1(-/-) mice. Insulin did not significantly alter hepatic MTP mRNA levels or lipid transfer activity 2h following injection, but suppressed expression of genes important in gluconeogenesis. To study the specific role of MTP, we expressed human MTP (hMTP) in primary rat hepatocytes using adenoviral gene transfer. Increased expression of hMTP resulted in a 47.6±17.9% increase in total apo B secreted. Incubation of hepatocytes with insulin suppressed apo B secretion by 50.1±10.8% in cells over-expressing hMTP and by 53.0±12.4% in control transfected hepatocytes. Results indicate that even under conditions of increased hepatic apo B secretion mediated by MTP, responsiveness of hepatocytes to insulin to suppress apo B secretion is maintained.

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.

Interleukin-6 mediates hepatic hypersecretion of apolipoprotein B

Obesity and type 2 diabetes are associated with insulin resistance (IR), increased circulating proinflammatory cytokines, and hypertriglyceridemia, the latter being caused by overproduction of hepatic very low density lipoprotein (VLDL). One cytokine strongly linked with development of hepatic IR is interleukin-6 (IL-6). Our objective was to evaluate IL-6 effects on hepatic apolipoprotein B (apoB) and VLDL secretion and to examine possible linkages between cytokine signaling and insulin-suppressive effects on lipoprotein secretion. Of the cytokines examined, only IL-6 stimulated secretion of apoB-containing lipoproteins in a dose-dependent manner. Both B100 and B48 secretion were significantly increased in VLDL and in lipoproteins with a density >1.019 g/ml. The ability of insulin to suppress hepatic apoB secretion was maintained in hepatocytes treated with IL-6. Pulse-chase studies indicated that enhanced apoB synthesis was the primary mechanism for increased lipoprotein secretion, which corresponded with higher abundance of apoB mRNA. Because IL-6 did not alter the decay rate of apoB mRNA transcripts, results support that increased apoB mRNA levels are the result of enhanced apob gene transcription. Increased apoB-lipoprotein secretion was also detected with oncostatin M (OSM), supporting involvement of the signal-transducing protein, gp130. Increased suppressor of cytokine signaling (SOCS) 3 expression negated IL-6 and OSM effects and significantly reduced cellular apoB mRNA abundance. We conclude that IL-6 favors secretion of apoB-containing lipoproteins by increasing availability of apoB through changes in apob gene transcription. These changes may contribute to hypersecretion of VLDL associated with obesity, particularly under conditions where SOCS3 is not overexpressed to an extent capable of overcoming IL-6-stimulated apob gene transcription.

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.

FoxO1 and hepatic lipid metabolism

PURPOSE OF REVIEW: This review summarizes recent research implicating Forkhead box (Fox)O1, a key transcription factor in glucose metabolism, in the regulation of hepatic lipid metabolism. Insulin dysregulation leading to hypertriglyceridemia is associated with increased hepatic VLDL secretion. FoxO1 is integrated in action with other regulatory factors in VLDL metabolism. The role of FoxO1 is defined in context of recent controversies. RECENT FINDINGS: FoxO1 regulates transcription of microsomal triglyceride transfer protein and apolipoprotein (apo)CIII involved in hepatic assembly and postsecretory catabolism of VLDL. Insulin activation of Akt leads to the phosphorylation of FoxO1 with nuclear exclusion and loss of transcriptional activity. Reduced insulin action increases FoxO1 activity and induces microsomal triglyceride transfer protein favoring VLDL assembly and induces apoCIII reducing peripheral triglyceride catabolism. This new mechanistic link between insulin resistance and VLDL overproduction and hypertriglyceridemia compounds effects of other known VLDL regulatory factors. SUMMARY: This review highlights recent advances in research of insulin regulation of hepatic VLDL metabolism. Formation of VLDL requires lipid, apoB structural protein, and microsomal triglyceride transfer protein. FoxO1 is a major factor in hepatic microsomal triglyceride ransfer protein regulation. A unifying hypothesis is presented linking regulation of the three necessary hepatic components for VLDL assembly with insulin action and insulin resistance.

Betaine administration corrects ethanol-induced defective VLDL secretion

Our previous studies, demonstrating ethanol-induced alterations in phosphatidylcholine (PC) synthesis via the phosphatidylethanolamine methyltransferase (PEMT) pathway, implicated a defect in very low-density lipoprotein (VLDL) secretion in the pathogenesis of hepatic steatosis. The objective of this study was to determine whether VLDL secretion was reduced by chronic ethanol consumption and whether betaine supplementation, that restores PEMT activity and prevents the development of alcoholic steatosis, could normalize VLDL secretion. The VLDL secretion in rats fed with control, ethanol and the betaine supplemented diets was determined using Triton WR-1339 to inhibit plasma VLDL metabolism. We observed reduced VLDL production rates in chronic alcohol-fed rats compared to control animals. Supplementation of betaine in the ethanol diet increased VLDL production rate to values significantly higher than those observed in the control diet-fed rats. To conclude, chronic ethanol consumption impairs PC generation via the PEMT pathway resulting in diminished VLDL secretion which contributes to the development of hepatic steatosis. By increasing PEMT-mediated PC generation, betaine results in increased fat export from the liver and attenuates the development of alcoholic fatty liver.

The many intersecting pathways underlying apolipoprotein B secretion and degradation

Because the levels of secreted apolipoprotein B (apoB) directly correlate with circulating serum cholesterol levels, there is a pressing need to define how the biosynthesis of this protein is regulated. Most commonly, the concentration of a secreted, circulating protein corresponds to transcriptionally and/or translationally regulated events. By contrast, circulating apoB levels are controlled by degradative pathways in the cell that select the protein for disposal. This article summarizes recent findings on two apoB disposal pathways, endoplasmic reticulum (ER)-associated degradation and autophagy, and describes a role for post-ER degradation in the increased circulating lipid levels in insulin-resistant diabetics.

Overindulgence and metabolic syndrome: is FoxO1 a missing link?

Excessive production of triglyceride-rich VLDL, which can result from dietary overindulgence, underlies metabolic syndrome--a combination of disorders including high blood pressure, obesity, high triglyceride, and insulin resistance--and places individuals at increased risk of developing cardiovascular disease and type 2 diabetes. However, the link between VLDL overproduction and insulin resistance has remained unclear. VLDL assembly in the liver is catalyzed by microsomal triglyceride transfer protein (MTP). In this issue of the JCI, Kamagate et al. investigate the events controlling hepatic MTP expression and VLDL production and secretion (see the related article beginning on page 2347). They demonstrate that MTP is a target of the transcription factor FoxO1 and that excessive VLDL production associated with insulin resistance is caused by the inability of insulin to regulate FoxO1 transcriptional activation of MTP.

Inhibition of apoB secretion from HepG2 cells by insulin is amplified by naringenin, independent of the insulin receptor

Hepatic overproduction of apolipoprotein B (apoB)-containing lipoproteins is characteristic of the dyslipidemia associated with insulin resistance. Recently, we demonstrated that the flavonoid naringenin, like insulin, decreased apoB secretion from HepG2 cells by activation of both the phosphoinositide-3-kinase (PI3-K) pathway and the mitogen-activated protein kinase/extracellular-regulated kinase (MAPK(erk)) pathway. In the present study, we determined whether naringenin-induced signaling required the insulin receptor (IR) and sensitized the cell to the effects of insulin, and whether the kinetics of apoB assembly and secretion in cells exposed to naringenin were similar to those of insulin. Immunoblot analysis revealed that insulin stimulated maximal phosphorylation of IR and IR substrate-1 after 10 min, whereas naringenin did not affect either at any time point up to 60 min. The combination of naringenin and submaximal concentrations of insulin potentiated extracellular-regulated kinase 1/2 activation and enhanced upregulation of the LDL receptor, downregulation of microsomal triglyceride transfer protein expression, and inhibition of apoB-100 secretion. Multicompartmental modeling of apoB pulse-chase studies revealed that attenuation of secreted radiolabeled apoB in naringenin- or insulin-treated cells was similar under lipoprotein-deficient or oleate-stimulated conditions. Naringenin and insulin both stimulated intracellular apoB degradation via a kinetically defined rapid pathway. Therefore, naringenin, like insulin, inhibits apoB secretion through activation of both PI3-K and MAPK(erk) signaling, resulting in similar kinetics of apoB secretion. However, the mechanism for naringenin-induced signaling is independent of the IR. Naringenin represents a possible strategy for reduction of hepatic apoB secretion, particularly in the setting of insulin resistance.

Increased hepatic steatosis and insulin resistance in mice lacking hepatic androgen receptor

Early studies demonstrated that whole-body androgen receptor (AR)-knockout mice with hypogonadism exhibit insulin resistance. However, details about the mechanisms underlying how androgen/AR signaling regulates insulin sensitivity in individual organs remain unclear. We therefore generated hepatic AR-knockout (H-AR(-/y)) mice and found that male H-AR(-/y) mice, but not female H-AR(-/-) mice, fed a high-fat diet developed hepatic steatosis and insulin resistance, and aging male H-AR(-/y) mice fed chow exhibited moderate hepatic steatosis. We hypothesized that increased hepatic steatosis in obese male H-AR(-/y) mice resulted from decreased fatty acid beta-oxidation, increased de novo lipid synthesis arising from decreased PPARalpha, increased sterol regulatory element binding protein 1c, and associated changes in target gene expression. Reduced insulin sensitivity in fat-fed H-AR(-/y) mice was associated with decreased phosphoinositide-3 kinase activity and increased phosphenolpyruvate carboxykinase expression and correlated with increased protein-tyrosine phosphatase 1B expression.

CONCLUSION

Together, our results suggest that hepatic AR may play a vital role in preventing the development of insulin resistance and hepatic steatosis. AR agonists that specifically target hepatic AR might be developed to provide a better strategy for treatment of metabolic syndrome in men.

Presecretory oxidation, aggregation, and autophagic destruction of apoprotein-B: a pathway for late-stage quality control

Hepatic secretion of apolipoprotein-B (apoB), the major protein of atherogenic lipoproteins, is regulated through posttranslational degradation. We reported a degradation pathway, post-ER pre secretory proteolysis (PERPP), that is increased by reactive oxygen species (ROS) generated within hepatocytes from dietary polyunsaturated fatty acids (PUFA). We now report the molecular processes by which PUFA-derived ROS regulate PERPP of apoB. ApoB exits the ER; undergoes limited oxidant-dependent aggregation; and then, upon exit from the Golgi, becomes extensively oxidized and converted into large aggregates. The aggregates slowly degrade by an autophagic process. None of the oxidized, aggregated material leaves cells, thereby preventing export of apoB-lipoproteins containing potentially toxic lipid peroxides. In summary, apoB secretory control via PERPP/autophagosomes is likely a key component of normal and pathologic regulation of plasma apoB levels, as well as a means for remarkably late-stage quality control of a secreted protein.

The physiological and molecular regulation of lipoprotein assembly and secretion

Triglycerides are insoluble in water and yet are transported at milligram per millilitre concentrations in the bloodstream. This is made possible by the ability of the liver and intestine to assemble lipid-protein emulsions (i.e. lipoproteins), which transport hydrophobic molecules. The assembly of triglyceride-rich lipoproteins requires the coordination of protein and lipid synthesis, which occurs on the cytoplasmic surface of the endoplasmic reticulum (ER), and their concerted assembly and translocation into the luminal ER secretory pathway as nascent lipoprotein particles. The availability of lipid substrate for triglyceride production and the machinery for lipoprotein assembly are highly sensitive to nutritional, hormonal, and genetic modulation. Disorders in lipid metabolism or an imbalance between lipogenesis and lipoprotein assembly can lead to hyperlipidemia and/or hepatic steatosis. We selectively review recently-identified machinery, such as transcription factors and nuclear hormone receptors, which provide new clues to the regulation of lipoprotein secretion.