Decreased lipid synthesis in livers of mice with disrupted Site-1 protease gene

ABCG5 and ABCG8 require MDR2 for secretion of cholesterol into bile

The major pathway for the removal of cholesterol from the body is via secretion into the bile. Three members of the ATP binding cassette (ABC) family, ABCG5 (G5), ABCG8 (G8), and ABCB4 (MDR2), are required for the efficient biliary export of sterols. Here, we examined the interdependence of these three ABC transporters for biliary sterol secretion. Biliary lipid levels in mice expressing no MDR2 (Mdr2-/- mice) were compared with those of Mdr2-/- mice expressing 14 copies of a human G5 (hG5) and hG8 transgene (Mdr2-/-;hG5G8Tg mice). Mdr2-/- mice had only trace amounts of biliary cholesterol and phospholipids. The Mdr2-/-;hG5G8Tg mice had biliary cholesterol levels as low as those of Mdr2-/- mice. Thus, MDR2 expression is required for G5G8-mediated biliary sterol secretion. To determine whether the reduction in fractional absorption of dietary sterols associated with G5G8 overexpression is secondary to the associated increase in biliary cholesterol, we compared the fractional absorption of sterols in Mdr2-/-;hG5G8Tg and hG5G8Tg animals. Inactivation of MDR2 markedly attenuated the reduction in fractional sterol absorption associated with G5G8 overexpression. These results are consistent with the notion that increased biliary cholesterol secretion contributes to the reduction in fractional sterol absorption associated with G5G8 overexpression.

Marginal maternal biotin deficiency in CD-1 mice reduces fetal mass of biotin-dependent carboxylases

Marginal maternal biotin deficiency reduces hepatic activity of biotin-dependent carboxylases and causes high rates of fetal birth defects in mice. We tested the hypothesis that the decreased carboxylase activity observed in deficient dams and their offspring is mediated by decreased abundance of biotinylated carboxylases, decreased expression of their mRNAs, or both. During gestation, CD-1 mice were fed a diet that induced biotin deficiency or a biotin-sufficient diet. On gestational d 17, gravid uteri were removed, and each live fetus was examined grossly for defects. The expected high incidence of cleft palate (83%) in offspring was observed. In maternal and fetal liver, acetyl-CoA carboxylase, pyruvate carboxylase, propionyl-CoA carboxylase, and beta-methylcrotonyl-CoA carboxylase abundances were determined by Western blotting; the content of mRNAs for most of these enzymes and holocarboxylase synthetase was determined by real-time RT-PCR. Biotin deficiency significantly reduced the abundance of the carboxylases in maternal and fetal liver; neither the content of mRNAs for the carboxylases nor holocarboxylase synthetase changed. This study provides evidence that the decrease in carboxylase activities is attributable to a decrease in the abundance of biotinylated carboxylases; further, this effect is more severe in fetuses than dams.

The liver X receptor-{beta} is essential for maintaining cholesterol homeostasis in the testis

The liver X receptor (LXR)alpha and -beta has been found to play a central role in maintaining cellular cholesterol homeostasis. In this study we comprehensively investigated the effect of deleting LXRalpha and -beta on testicular morphology and function. In the absence of LXRbeta, excessive cholesterol accumulated in the Sertoli cells from 2.5 months, resulting in severe cellular disruption and dysregulation of spermatogenesis by 10 months of age. This correlated with gene expression analyses that clearly indicated that LXRbeta was the dominant transcript in the testis Although the LXRalpha(-/-) testis was normal, the LXRalpha(-/-)beta(-/-) testis presented with a more severe phenotype than the LXRbeta(-/-) mice, and males were infertile by 4 months of age, indicating LXRalpha may partially rescue the testicular phenotype. Although Leydig cells did not accumulate excessive cholesterol, declining serum and intratesticular androgen levels with age suggested that these cells were in fact less functional. Treatment of a Sertoli cell line with the LXR agonist T0901317 led to increased expression of known LXR target genes like ATP binding cassette-G1 and sterol regulatory binding protein-1c; similar results were observed in wild-type testis after in vivo administration, suggesting the LXR is functioning in the same way as in other tissues. Ordinarily increased levels of cholesterol activate intracellular sensors to decrease these levels; however, the increasing amount of cholesterol in the Sertoli cells indicates improper control of cholesterol metabolism when LXRbeta is absent. Although the precise molecular mechanism at this time remains unclear, our study highlights the crucial role for LXRbeta in retaining cholesterol homeostasis in Sertoli cells.

Effects of diet and genetic background on sterol regulatory element-binding protein-1c, stearoyl-CoA desaturase 1, and the development of the metabolic syndrome

Both environmental and genetic factors play important roles in the development of the metabolic syndrome. To elucidate how these factors interact under normal conditions, C57Bl/6 (B6) and 129S6/SvEvTac (129) mice were placed on a low-fat or high-fat diet. Over 18 weeks, the 129 strain developed features of the metabolic syndrome, notably obesity, hyperinsulinemia, and glucose intolerance only on the high-fat diet; the B6 strain on the other hand developed these features on both diets. High-fat feeding of both strains led to decreased serum triglycerides, hepatic steatosis, and hypercholesterolemia; however, B6 mice developed worse steatosis and a larger increase in LDL cholesterol. Both B6 background and high-fat feeding increased sterol regulatory element-binding protein-1c (SREBP-1c), a key regulator of lipogenic gene transcription, and its downstream targets. Stearoyl-CoA desaturase 1 (SCD1), an enzyme that regulates monounsaturated fatty acid (MUFA) synthesis, was also increased at the mRNA and enzyme activity levels by both high-fat feeding and B6 background. Furthermore, lipid analysis revealed increased hepatic triglycerides and MUFAs in B6 and high-fat-fed mice. Thus, dietary fat and genetic background act through SREBP-1c and SCD1 to affect hepatic lipid metabolism contributing to the development of the metabolic syndrome.

Decreased plasma cholesterol and hypersensitivity to statins in mice lacking Pcsk9

PCSK9 encodes proprotein convertase subtilisin/kexin type 9a (PCSK9), a member of the proteinase K subfamily of subtilases. Missense mutations in PCSK9 cause an autosomal dominant form of hypercholesterolemia in humans, likely due to a gain-of-function mechanism because overexpression of either WT or mutant PCSK9 reduces hepatic LDL receptor protein (LDLR) in mice. Here, we show that livers of knockout mice lacking PCSK9 manifest increased LDLR protein but not mRNA. Increased LDLR protein led to increased clearance of circulating lipoproteins and decreased plasma cholesterol levels (46 mg/dl in Pcsk9(-/-) mice versus 96 mg/dl in WT mice). Statins, a class of drugs that inhibit cholesterol synthesis, increase expression of sterol regulatory element-binding protein-2 (SREBP-2), a transcription factor that activates both the Ldlr and Pcsk9 genes. Statin administration to Pcsk9(-/-) mice produced an exaggerated increase in LDLRs in liver and enhanced LDL clearance from plasma. These data demonstrate that PCSK9 regulates the amount of LDLR protein in liver and suggest that inhibitors of PCSK9 may act synergistically with statins to enhance LDLRs and reduce plasma cholesterol.

A biphasic response of hepatobiliary cholesterol metabolism to dietary fat at the onset of obesity in the mouse

Human obesity is associated with abnormal hepatic cholesterol homeostasis and resistance to leptin action. Because leptin administration to rodents promotes the biliary elimination of plasma cholesterol, this study was designed to elucidate a pathophysiological role for leptin during the development of obesity. We fed mice diets containing high or low saturated fat contents. Before and after the onset of obesity, we measured downstream targets of leptin action and evaluated plasma, hepatic, and biliary cholesterol metabolism. Although not obese at 28 days, mice fed a high fat diet became hyperleptinemic. Sensitivity to leptin was evidenced by downregulation of both hepatic stearoyl CoA desaturase-1 and fatty acid synthase. Due principally to upregulation of adenosine triphosphate-binding cassette proteins A1 and G5, plasma high density lipoprotein (HDL) cholesterol concentrations increased, as did relative secretion rates of biliary cholesterol. A smaller, more hydrophilic bile salt pool decreased intestinal cholesterol absorption. In this setting, hepatic cholesterol synthesis was downregulated, indicative of increased uptake of plasma cholesterol. After 56 days of high fat feeding, obesity was associated with leptin resistance, as evidenced by marked hyperleptinemia without downregulation of stearoyl CoA desaturase-1 or fatty acid synthase and by upregulation of hepatic cholesterol and bile salt synthesis. Hypercholesterolemia was attributable to overproduction and decreased clearance of large HDL(1) particles. In conclusion, before the onset of obesity, preserved leptin sensitivity promotes biliary elimination of endogenous cholesterol in response to dietary fat. Leptin resistance due to obesity leads to a maladaptive response whereby newly synthesized cholesterol in the liver is eliminated via bile.

The sterol response element binding protein regulates cyclooxygenase-2 gene expression in endothelial cells

We previously demonstrated that cholesterol deprivation increases endothelial cyclooxygenase-2 (COX-2)-dependent prostacyclin [prostaglandin I2 (PGI2)] production in vitro. Cholesterol directly regulates gene transcription through the sterol response element binding protein (SREBP). In this work, we demonstrate that SREBP directly regulates COX-2 expression. Cholesterol reduces human COX-2 promoter-luciferase reporter construct activity in transiently transfected endothelial cells. Conversely, cotransfection with a constitutively active mutant SREBP increases COX-2 promoter activity. SREBP-1a and -2 specifically bind a putative sterol response element (SRE) sequence in the COX-2 promoter. This sequence competes for SREBP binding to a low density lipoprotein receptor consensus sequence in an electromobility-shift assay. These data indicate that endothelial COX-2 is regulated by cholesterol via the SREBP pathway. The present study identifies COX-2 as the first vascular gene without a clear role in lipid metabolism transactivated by SREBP, and suggests that enhanced production of PGI2 through this pathway may be an additional benefit of cholesterol-lowering therapies.

ABCG1 has a critical role in mediating cholesterol efflux to HDL and preventing cellular lipid accumulation

Here we demonstrate that the ABC transporter ABCG1 plays a critical role in lipid homeostasis by controlling both tissue lipid levels and the efflux of cellular cholesterol to HDL. Targeted disruption of Abcg1 in mice has no effect on plasma lipids but results in massive accumulation of both neutral lipids and phospholipids in hepatocytes and in macrophages within multiple tissues following administration of a high-fat and -cholesterol diet. In contrast, overexpression of human ABCG1 protects murine tissues from dietary fat-induced lipid accumulation. Finally, we show that cholesterol efflux to HDL specifically requires ABCG1, whereas efflux to apoA1 requires ABCA1. These studies identify Abcg1 as a key gene involved in both cholesterol efflux to HDL and in tissue lipid homeostasis.

Delineation of molecular changes in intrahepatic cholesterol metabolism resulting from diminished cholesterol absorption

The absorption of cholesterol by the small intestine is a major route for the net entry of cholesterol into the body and can therefore affect the plasma low density lipoprotein-cholesterol (LDL-C) concentration. These studies used ezetimibe, a potent inhibitor of cholesterol absorption, to delineate the biochemical and molecular changes in intrahepatic metabolism and biliary lipid secretion when there is a major reduction in chylomicron cholesterol delivery to the liver. In female LDL receptor (LDLR)-deficient (LDLR-/-) mice fed a basal diet containing ezetimibe (0-10 mg/day/kg body weight), cholesterol absorption was reduced up to 91%, fecal neutral sterol excretion was increased up to 4.7-fold, and plasma total cholesterol concentrations decreased by up to 18%. Blocking cholesterol absorption prevented the accumulation of very low density lipoproteins and LDL in the circulation of LDLR-/- mice fed a lipid-rich diet. In female LDLR+/+ mice fed the lipid-rich diet with ezetimibe, the relative mRNA level for the LDLR in the liver was 2-fold greater than in matching mice given the lipid-rich diet alone. We conclude that in the mouse the reduction in plasma LDL-C levels induced by blocking cholesterol absorption reflects both a diminished rate of LDL-C production and a modest increase in hepatic LDLR expression.

Compensatory increase in fatty acid synthesis in adipose tissue of mice with conditional deficiency of SCAP in liver

The escort protein SCAP transports SREBPs from ER to Golgi where the active domains are released to activate genes for fatty acid (FA) and cholesterol synthesis. Mice with conditional SCAP deficiency in liver (L-Scap-) manifest marked reductions in hepatic lipid synthesis. Here, we show that the decreased FA synthesis in liver is balanced by an equal increase in nonhepatic tissues, primarily adipose tissue. Extrahepatic synthesis of FAs preserves adipose mass, even when L-Scap- mice consume a fat-free diet. This compensatory response disappears upon fasting, implicating a role for insulin, the major hormonal activator of FA synthesis. This response is mediated by an insulin-dependent increase in adipocyte SREBP-1c and its target mRNAs. In epididymal fat of L-Scap- mice, phosphorylated Akt, Glut-4 mRNA, and glucose uptake are also increased, indicating insulin hypersensitivity. Plasma VLDL triglycerides are dramatically reduced in L-Scap- mice, underscoring the benefits of synthesizing FAs in fat rather than liver.

Expression of ABCG5 and ABCG8 is required for regulation of biliary cholesterol secretion

The major pathway for elimination of cholesterol in mammals is via secretion into bile. Biliary cholesterol secretion is mediated by the ATP-binding cassette (ABC) transporters ABCG5 (G5) and ABCG8 (G8) and is stimulated by cholesterol and by the non-cholesterol steroids cholate and diosgenin. To define the relationship between G5G8 expression and biliary cholesterol secretion, we measured G5 and G8 mRNA levels and biliary cholesterol concentrations in genetically manipulated mice expressing 0, 1, 2, 5, 10, or 16 copies of the two genes. Biliary cholesterol levels varied directly with G5G8 copy number and hepatic mRNA levels over a >16-fold range. Thus neither delivery of cholesterol to the transporter nor levels of cholesterol acceptors in bile were limiting under these conditions. In wild-type mice, cholate and diosgenin both increased biliary cholesterol concentrations 2-3-fold. The increase in biliary cholesterol content was dependent on expression of G5 and G8; neither steroid increased biliary cholesterol levels in G5G8-/- mice. Cholate treatment was associated with a farnesoid X receptor (FXR)-dependent increase in hepatic mRNA and protein levels of G5 and G8. In contrast to cholate, diosgenin treatment did not affect G5G8 expression. Diosgenin increased the expression of several pregnane X receptor (PXR) target genes and the choleretic effect of diosgenin was reduced by approximately 70% in PXR knock-out mice. Thus G5 and G8 are required to modulate biliary cholesterol secretion in response to cholate and diosgenin, but the choleretic effects of these two steroids are mediated by different mechanisms requiring FXR and PXR, respectively.

Disruption of cholesterol homeostasis by plant sterols

The ABC transporters ABCG5 and ABCG8 limit absorption and promote excretion of dietary plant sterols. It is not known why plant sterols are so assiduously excluded from the body. Here we show that accumulation of plant sterols in mice lacking ABCG5 and ABCG8 (G5G8-/- mice) profoundly perturbs cholesterol homeostasis in the adrenal gland. The adrenal glands of the G5G8-/- mice were grossly abnormal in appearance (brown, not white) due to a 91% reduction in cholesterol content. Despite the very low cholesterol levels, there was no compensatory increase in cholesterol synthesis or in lipoprotein receptor expression. Moreover, levels of ABCA1, which mediates sterol efflux, were increased 10-fold in the G5G8-/- adrenals. Adrenal cholesterol levels returned to near-normal levels in mice treated with ezetimibe, which blocks phytosterol absorption. To determine which plant sterol(s) caused the metabolic changes, we examined the effects of individual plant sterols on cholesterol metabolism in cultured adrenal cells. Addition of stigmasterol, but not sitosterol, inhibited SREBP-2 processing and reduced cholesterol synthesis. Stigmasterol also activated the liver X receptor in a cell-based reporter assay. These data indicate that selected dietary plant sterols disrupt cholesterol homeostasis by affecting two critical regulatory pathways of lipid metabolism.

Maintaining cholesterol homeostasis: Sterol regulatory element-binding proteins

The molecular mechanism of how hepatocytes maintain cholesterol homeostasis has become much more transparent with the discovery of sterol regulatory element binding proteins (SREBPs) in recent years. These membrane proteins are members of the basic helix-loop-helix-leucine zipper (bHLH-Zip) family of transcription factors. They activate the expression of at least 30 genes involved in the synthesis of cholesterol and lipids. SREBPs are synthesized as precursor proteins in the endoplasmic reticulum (ER), where they form a complex with another protein, SREBP cleavage activating protein (SCAP). The SCAP molecule contains a sterol sensory domain. In the presence of high cellular sterol concentrations SCAP confines SREBP to the ER. With low cellular concentrations, SCAP escorts SREBP to activation in the Golgi. There, SREBP undergoes two proteolytic cleavage steps to release the mature, biologically active transcription factor, nuclear SREBP (nSREBP). nSREBP translocates to the nucleus and binds to sterol response elements (SRE) in the promoter/enhancer regions of target genes. Additional transcription factors are required to activate transcription of these genes. Three different SREBPs are known, SREBPs-1a, -1c and -2. SREBP-1a and -1c are isoforms produced from a single gene by alternate splicing. SREBP-2 is encoded by a different gene and does not display any isoforms. It appears that SREBPs alone, in the sequence described above, can exert complete control over cholesterol synthesis, whereas many additional factors (hormones, cytokines, etc.) are required for complete control of lipid metabolism. Medicinal manipulation of the SREBP/SCAP system is expected to prove highly beneficial in the management of cholesterol-related disease.

ACAT2 deficiency limits cholesterol absorption in the cholesterol-fed mouse: impact on hepatic cholesterol homeostasis

Acyl CoA:cholesterol acyltransferase (ACAT) 2 is the major cholesterol-esterifying enzyme in mouse enterocytes and hepatocytes. Male ACAT2(+/+) and ACAT2(-/ -) mice were fed chow containing added cholesterol (0%-0.500% w/w) for 24 days. Over this range, fractional cholesterol absorption in the ACAT2(+/+) mice fell from 41.4% +/- 6.6% to 21.0% +/- 5.2%, and in their ACAT2(-/-) counterparts it fell from 35.1% +/- 4.5% to 7.9% +/- 0.8%. The mass of dietary cholesterol absorbed (mg/d per 100 g body weight) increased from 1.2 +/- 0.2 to 14.7 +/- 4.4 in the ACAT2(+/+) mice and from 1.0 +/- 0.2 to 5.5 +/- 0.6 in those without ACAT2. In the ACAT2(+/+) mice, hepatic cholesterol concentrations increased as a function of intake despite compensatory changes in cholesterol and bile acid synthesis and in the expression of adenosine triphosphate-binding cassette transporter G5 (ABCG5) and ABC transporter G8 (ABCG8). In contrast, in ACAT2(-/-) mice in which the amount of cholesterol absorbed at the highest intake was only 37% of that in the ACAT2(+/+) mice, suppression of synthesis was a sufficient adaptive response; there was no change in bile acid synthesis, ABCG5/G8 expression, or hepatic cholesterol concentration. The expression of adenosine triphosphate-binding cassette transporter A1 (ABCA1) in the jejunum was markedly elevated in the ACAT2(-/-) mice, irrespective of dietary cholesterol level. In conclusion, although ACAT2 deficiency limits cholesterol absorption, the extent to which it impacts hepatic cholesterol homeostasis depends on cholesterol intake. Loss of ACAT2 activity may result in unesterified cholesterol being absorbed via an ABCA1-mediated basolateral efflux pathway.

Post-transcriptional Regulation of Low Density Lipoprotein Receptor Protein by Proprotein Convertase Subtilisin/Kexin Type 9a in Mouse Liver

Lipid homeostasis is transcriptionally regulated by three DNA-binding proteins, designated sterol regulatory element-binding protein (SREBP)-1a, -1c, and -2. Oligonucleotide arrays hybridized with RNA made from livers of transgenic SREBP-1a, transgenic SREBP-2, and SREBP cleavage-activating protein knockout mice recently identified 33 genes regulated by SREBPs in liver, four of which had no known connection to lipid metabolism. One of the four genes was PCSK9, which encodes proprotein convertase subtilisin/kexin type 9a, a protein that belongs to the proteinase K subfamily of subtilases. Mutations in PCSK9 are associated with an autosomal dominant form of hypercholesterolemia. Here, we demonstrate that hepatic overexpression of either wild-type or mutant PCSK9 in mice results in hypercholesterolemia. The hypercholesterolemia is due to a post-transcriptional event causing a reduction in low density lipoprotein (LDL) receptor protein prior to the internalization and recycling of the receptor. Overexpression of PCSK9 in primary hepatocytes and in mice lacking the LDL receptor does not alter apolipoprotein B secretion. These data are consistent with PCSK9 affecting plasma LDL cholesterol levels by altering LDL receptor protein levels via a post-transcriptional mechanism.

NARC-1/PCSK9 and Its Natural Mutants

The discovery of autosomal dominant hypercholesterolemic patients with mutations in the PCSK9 gene, encoding the proprotein convertase NARC-1, resulting in the missense mutations suggested a role in low density lipoprotein (LDL) metabolism. We show that the endoplasmic reticulum-localized proNARC-1 to NARC-1 zymogen conversion is Ca2+-independent and that within the zymogen autocatalytic processing site SSVFAQ [downward arrow]SIP Val at P4 and Pro at P3' are critical. The S127R and D374Y mutations result in approximately 50-60% and > or =98% decrease in zymogen processing, respectively. In contrast, the double [D374Y + N157K], F216L, and R218S natural mutants resulted in normal zymogen processing. The cell surface LDL receptor (LDLR) levels are reduced by 35% in lymphoblasts of S127R patients. The LDLR levels are also reduced in stable HepG2 cells overexpressing NARC-1 or its natural mutant S127R, and this reduction is abrogated in the presence of 5 mm ammonium chloride, suggesting that overexpression of NARC-1 increases the turnover rate of the LDLR. Adenoviral expression of wild type human NARC-1 in mice resulted in a maximal approximately 9-fold increase in circulating LDL cholesterol, while in LDLR-/- mice a delayed approximately 2-fold increase in LDL cholesterol was observed. In conclusion, NARC-1 seems to affect both the level of LDLR and that of circulating apoB-containing lipoproteins in an LDLR-dependent and -independent fashion.

Maternal dietary iron restriction modulates hepatic lipid metabolism in the fetuses

Maternal dietary Fe restriction reduced fasting plasma cholesterol and triglyceride (TG) concentrations in the fetuses, as well as decreased plasma TG levels in the adult offspring. To investigate how maternal Fe restriction was affecting fetal lipid metabolism, we investigated whether there were changes in liver lipid metabolism in the full-term fetuses. There was a approximately 27% (P < 0.05) increase in cholesterol but approximately 29% reduction (P = 0.01) in TG concentrations in the liver of the Fe-restricted fetuses. Hepatic mRNA levels of cholesterol 7alpha hydroxylase and liver X receptor-alpha (LXRalpha) were reduced by approximately 50% (P < 0.01) and approximately 34% (P < 0.01), respectively. As LXRalpha regulates expression of sterol response element binding protein-1c (SREBP-1c) expression, we measured SREBP-1c expression. There was an approximately 43% (P < 0.001) reduction in mRNA levels of SREBP-1c and its response genes, including acetyl-CoA carboxylase by approximately 35% (P = 0.01), fatty acid synthase by approximately 18% (P = 0.05), and diacylglycerol acyltransferase by approximately 19% (P = 0.03). Furthermore, protein levels of CD36 were reduced by approximately 27% (P = 0.02) in Fe-restricted fetuses. In conclusion, changes in liver cholesterol and TG concentrations in Fe-restricted fetuses may be coordinated through reduced expression of heme-containing cholesterol 7alpha hydroxylase and its regulator LXRalpha, mainly via downregulation of expression of genes in bile acid synthesis and fatty acid synthesis pathways.

Apolipoprotein E Receptors Are Required for Reelin-induced Proteasomal Degradation of the Neuronal Adaptor Protein Disabled-1

The cytoplasmic adaptor protein Disabled-1 (Dab1) is necessary for the regulation of neuronal positioning in the developing brain by the secreted molecule Reelin. Binding of Reelin to the neuronal apolipoprotein E receptors apoER2 and very low density lipoprotein receptor induces tyrosine phosphorylation of Dab1 and the subsequent activation or relocalization of downstream targets like phosphatidylinositol 3 (PI3)-kinase and Nckbeta. Disruption of Reelin signaling leads to the accumulation of Dab1 protein in the brains of genetically modified mice, suggesting that Reelin limits its own action in responsive neurons by down-regulating the levels of Dab1 expression. Here, we use cultured primary embryonic neurons as a model to demonstrate that Reelin treatment targets Dab1 for proteolytic degradation by the ubiquitin-proteasome pathway. We show that tyrosine phosphorylation of Dab1 but not PI3-kinase activation is required for its proteasomal targeting. Genetic deficiency in the Dab1 kinase Fyn prevents Dab1 degradation. The Reelin-induced Dab1 degradation also depends on apoER2 and very low density lipoprotein receptor in a gene-dose dependent manner. Moreover, pharmacological blockade of the proteasome prevents the formation of a proper cortical plate in an in vitro slice culture assay. Our results demonstrate that signaling through neuronal apoE receptors can activate the ubiquitin-proteasome machinery, which might have implications for the role of Reelin during neurodevelopment and in the regulation of synaptic transmission.

Insulin Regulation of Sterol Regulatory Element-binding Protein-1 Expression in L-6 Muscle Cells and 3T3 L1 Adipocytes

Sterol regulatory element-binding proteins (SREBPs) are transcription factors that regulate enzymes required for cholesterol and fatty acid synthesis. Expression of SREBP-1 is enhanced by insulin; however, the actual insulin-signaling cascades employed are yet unclear. We determined the roles of the phosphatidylinositol (PI) 3-kinase and mitogen-activated protein (MAP) kinase-dependent pathways in the effect of mediating insulin on SREBP-1 in L-6 skeletal muscle cells and 3T3 L1 adipocytes, using wortmannin or LY294002 to inhibit the PI 3-kinase pathway, and PD98059 to inhibit the MAP kinase-dependent pathway. In myocytes, insulin increased SREBP-1 protein in a dose-dependent manner. 1 and 10 nm insulin significantly increased expression of total cellular SREBP-1 protein at 24 and 48 h, nuclear SREBP-1 protein at 24 h, and SREBP-1a mRNA at 24 h. Although wortmannin and LY294002 had no effect on this aspect of insulin action, PD98059 completely blocked each of these responses. Transfection of a dominant negative mutant of Ras similarly blocked the insulin effect on SREBP-1. In contrast, in adipocytes, the insulin effect on SREBP-1 was mediated via the PI 3-kinase and not the MAP kinase pathway. In conclusion, although insulin increases skeletal muscle SREBP-1 expression in a dose-dependent fashion via the MAP kinase-dependent signaling pathway, insulin action on adipocyte SREBP-1 is mediated via the PI 3-kinase signaling pathway. In the state of insulin resistance, characterized by selective inhibition of the PI 3-kinase pathway, the usual stimulation of lipogenesis by insulin in adipocytes may be inhibited, whereas intramyocellular lipogenesis via the MAP kinase pathway of insulin may continue unabated.

Bile acids lower triglyceride levels via a pathway involving FXR, SHP, and SREBP-1c

We explored the effects of bile acids on triglyceride (TG) homeostasis using a combination of molecular, cellular, and animal models. Cholic acid (CA) prevents hepatic TG accumulation, VLDL secretion, and elevated serum TG in mouse models of hypertriglyceridemia. At the molecular level, CA decreases hepatic expression of SREBP-1c and its lipogenic target genes. Through the use of mouse mutants for the short heterodimer partner (SHP) and liver X receptor (LXR) alpha and beta, we demonstrate the critical dependence of the reduction of SREBP-1c expression by either natural or synthetic farnesoid X receptor (FXR) agonists on both SHP and LXR alpha and LXR beta. These results suggest that strategies aimed at increasing FXR activity and the repressive effects of SHP should be explored to correct hypertriglyceridemia.

Mice Lacking Thioredoxin-interacting Protein Provide Evidence Linking Cellular Redox State to Appropriate Response to Nutritional Signals

Thioredoxin-interacting protein (Txnip) is a ubiquitous protein that binds with high affinity to thioredoxin and inhibits its ability to reduce sulfhydryl groups via NADPH oxidation. HcB-19 mice contain a nonsense mutation in Txnip that eliminates its expression. Unlike normal animals, HcB-19 mice have approximately 3-fold increase in insulin levels when fasted. The C-peptide/insulin ratio is normal, suggesting that the hyperinsulinemia is due to increased insulin secretion. Fasted HcB-19 mice are hypoglycemic, hypertriglyceridemic, and have higher than normal levels of ketone bodies. Ablation of pancreatic beta-cells with streptozotocin completely blocks the fasting-induced hypoglycemia/hypertriglyceridemia, suggesting that these abnormalities are due to excess insulin secretion. This is supported by increased hepatic mRNA levels of the insulin-inducible, lipogenic transcription factor sterol-responsive element-binding protein-1c and two of its targets, acetyl-CoA carboxylase and fatty acid synthase. During a prolonged fast, the hyperinsulinemia up-regulates lipogenesis but fails to down-regulate hepatic phosphoenolpyruvate carboxykinase mRNA expression. Hepatic ratios of reduced:oxidized glutathione, established regulators of gluconeogenic/glycolytic/lipogenic enzymes, were elevated 30% in HcB-19 mice, suggesting a loss of Txnip-enhanced sulfhydryl reduction. The altered hepatic enzymatic profiles of HcB-19 mice divert phosphoenolpyruvate to glyceroneogenesis and lipogenesis rather than gluconeogenesis. Our findings implicate Txnip-modulated sulfhydryl redox as a central regulator of insulin secretion in beta-cells and regulation of many of the branch-points of gluconeogenesis/glycolysis/lipogenesis.

Overexpression of Insig-1 in the livers of transgenic mice inhibits SREBP processing and reduces insulin-stimulated lipogenesis

In the current studies we generated transgenic mice that overexpress human Insig-1 in the liver under a constitutive promoter. In cultured cells Insig-1 and Insig-2 have been shown to block lipid synthesis in a cholesterol-dependent fashion by inhibiting proteolytic processing of sterol regulatory element-binding proteins (SREBPs), membrane-bound transcription factors that activate lipid synthesis. Insig's exert this action in the ER by binding SREBP cleavage-activating protein (SCAP) and preventing it from escorting SREBPs to the Golgi apparatus where the SREBPs are processed to their active forms. In the livers of Insig-1 transgenic mice, the content of all nuclear SREBPs (nSREBPs) was reduced and declined further upon feeding of dietary cholesterol. The nuclear content of the insulin-induced SREBP isoform, SREBP-1c, failed to increase to a normal extent upon refeeding on a high-carbohydrate diet. The nSREBP deficiency produced a marked reduction in the levels of mRNAs encoding enzymes required for synthesis of cholesterol, fatty acids, and triglycerides. Plasma cholesterol levels were strongly reduced, and plasma triglycerides did not exhibit their normal rise after refeeding. These results provide in vivo support for the hypothesis that nSREBPs are essential for high levels of lipid synthesis in the liver and indicate that Insig's modulate nSREBP levels by binding and retaining SCAP in the ER.

Rosiglitazone induction of Insig-1 in white adipose tissue reveals a novel interplay of peroxisome proliferator-activated receptor gamma and sterol regulatory element-binding protein in the regulation of adipogenesis

Insulin-induced gene 1 (INSIG-1) is a key regulator in the processing of the sterol regulatory element-binding proteins (SREBPs). We demonstrated that Insig-1 is regulated by peroxisome proliferator-activated receptor gamma (PPARgamma) providing a link between insulin sensitization/glucose homeostasis and lipid homeostasis. Insig-1 was identified as a PPARgamma target gene using microarray analysis of mRNA from the white adipose tissue of diabetic (db/db) animals treated with PPARgamma agonists. Insig-1 was induced in subcutaneous (9-fold) and epididymal (4-fold) fat pads from db/db mice treated for 8 days with the PPARgamma agonist rosiglitazone (30 mg/kg/day). This in vivo response was confirmed in differentiated C3H10T1/2 adipocytes treated with rosiglitazone. To elucidate the molecular mechanisms regulating INSIG-1 expression, we cloned and characterized the human INSIG-1 promoter. Co-expression of PPARgamma and RXRalpha transactivated the INSIG-1 promoter in the presence of PPARgamma agonists. This induction was attenuated when a dominant negative PPARgamma construct was transfected into cells. Furthermore, a PPARgamma antagonist repressed the transactivation of the INSIG-1 promoter-reporter construct. Truncations of the promoter resulted in the identification of a PPAR response element that mediated the regulation of the promoter. We demonstrated with recombinant proteins that the PPARgamma/RXRalpha heterodimer binds directly to this PPAR response element. In addition to regulation by PPARgamma/RXRalpha, we demonstrated that the INSIG-1 promoter is regulated by transcriptionally active SREBP. The sterol response element was identified 380 base pairs upstream of the transcriptional start site. These findings suggest that the regulation of Insig-1 by PPARgamma agonists could in turn regulate SREBP processing and thus couple insulin sensitizers with the regulation of lipid homeostasis.

A mouse model of sitosterolemia: absence of Abcg8/sterolin-2 results in failure to secrete biliary cholesterol

BACKGROUND

Mutations in either of two genes comprising the STSL locus, ATP-binding cassette (ABC)-transporters ABCG5 (encoding sterolin-1) and ABCG8 (encoding sterolin-2), result in sitosterolemia, a rare autosomal recessive disorder of sterol trafficking characterized by increased plasma plant sterol levels. Based upon the genetics of sitosterolemia, ABCG5/sterolin-1 and ABCG8/sterolin-2 are hypothesized to function as obligate heterodimers. No phenotypic difference has yet been described in humans with complete defects in either ABCG5 or ABCG8. These proteins, based upon the defects in humans, are responsible for regulating dietary sterol entry and biliary sterol secretion.

METHODS

In order to mimic the human disease, we created, by a targeted disruption, a mouse model of sitosterolemia resulting in Abcg8/sterolin-2 deficiency alone. Homozygous knockout mice are viable and exhibit sitosterolemia.

RESULTS

Mice deficient in Abcg8 have significantly increased plasma and tissue plant sterol levels (sitosterol and campesterol) consistent with sitosterolemia. Interestingly, Abcg5/sterolin-1 was expressed in both liver and intestine in Abcg8/sterolin-2 deficient mice and continued to show an apical expression. Remarkably, Abcg8 deficient mice had an impaired ability to secrete cholesterol into bile, but still maintained the ability to secrete sitosterol. We also report an intermediate phenotype in the heterozygous Abcg8+/- mice that are not sitosterolemic, but have a decreased level of biliary sterol secretion relative to wild-type mice.

CONCLUSION

These data indicate that Abcg8/sterolin-2 is necessary for biliary sterol secretion and that loss of Abcg8/sterolin-2 has a more profound effect upon biliary cholesterol secretion than sitosterol. Since biliary sitosterol secretion is preserved, although not elevated in the sitosterolemic mice, this observation suggests that mechanisms other than by Abcg8/sterolin-2 may be responsible for its secretion into bile.

Development of protein-based inhibitors of the proprotein of convertase SKI-1/S1P: processing of SREBP-2, ATF6, and a viral glycoprotein

Processing of membrane-bound transcription factors such as sterol regulatory element-binding proteins (SREBPs) and the ER-stress response factor ATF6, and glycoproteins of some hemorrhagic fever viruses are initiated by the proprotein convertase SKI-1/S1P. So far, no cellular protein-based inhibitor of the hydrophobic-amino acid specific SKI-1 is known. The prosegment of the basic-amino acid specific convertases (e.g. furin and PC5) or alpha(1)-PDX, a variant of alpha(1)-antitrypsin (alpha(1)-AT) exhibiting an RIPR(358) sequence at the reactive site loop, were shown to potently inhibit these secretory proteinases. Accordingly, we tested the SKI-1-inhibitory potential of various point mutants of either the 198 amino acid preprosegment of SKI-1-(1-198) or alpha(1)-AT. Transient transfections data showed that, out of numerous mutants studied, the R134E prosegment mutant or the alpha(1)-AT reactive site loop variants RRVL(358), RRYL(358) and RRIL(358) are the best specific cellular inhibitors of SKI-1. The observed inhibition of the processing of endogenous SREBP-2, exogenous ATF6 and a PDGF-A (RRLL(86)) variant were >55% and reach approximately 80% in stable transfectants. We also show that SKI-1 forms SDS-stable complexes with these alpha(1)-AT variants, but not with wild-type alpha(1)-AT or alpha(1)-PDX. Finally, these inhibitors were also shown to affect the processing and stability of the Crimean-Congo hemorrhagic fever virus glycoprotein.

Site-1 protease is required for cartilage development in zebrafish

gonzo (goz) is a zebrafish mutant with defects in cartilage formation. The goz phenotype comprises cartilage matrix defects and irregular chondrocyte morphology. Expression of endoderm, mesoderm, and cartilage marker genes is, however, normal, indicating a defect in chondrocyte morphogenesis. The mutated gene responsible for the goz phenotype, identified by positional cloning and confirmed by phosphomorpholino knockdown, encodes zebrafish site-1 protease (s1p). S1P has been shown to process and activate sterol regulatory element-binding proteins (SREBPs), which regulate expression of key enzymes of lipid biosynthesis or transport. This finding is consistent with the abnormal distribution of lipids in goz embryos. Knockdown of site-2 protease, which is also involved in activation of SREBPs, results in similar lipid and cartilage phenotypes as S1P knockdown. However, knockdown of SREBP cleavage-activating protein, which forms a complex with SREBP and is essential for S1P cleavage, results only in lipid phenotypes, whereas cartilage appears normal. This indicates that the cartilage phenoptypes of goz are caused independently of the lipid defects.

The rat STSL locus: characterization, chromosomal assignment, and genetic variations in sitosterolemic hypertensive rats

BACKGROUND

Elevated plant sterol accumulation has been reported in the spontaneously hypertensive rat (SHR), the stroke-prone spontaneously hypertensive rat (SHRSP) and the Wistar-Kyoto (WKY) rat. Additionally, a blood pressure quantitative trait locus (QTL) has been mapped to rat chromosome 6 in a New Zealand genetically hypertensive rat strain (GH rat). ABCG5 and ABCG8 (encoding sterolin-1 and sterolin-2 respectively) have been shown to be responsible for causing sitosterolemia in humans. These genes are organized in a head-to-head configuration at the STSL locus on human chromosome 2p21.

METHODS

To investigate whether mutations in Abcg5 or Abcg8 exist in SHR, SHRSP, WKY and GH rats, we initiated a systematic search for the genetic variation in coding and non-coding region of Abcg5 and Abcg8 genes in these strains. We isolated the rat cDNAs for these genes and characterized the genomic structure and tissue expression patterns, using standard molecular biology techniques and FISH for chromosomal assignments.

RESULTS

Both rat Abcg5 and Abcg8 genes map to chromosome band 6q12. These genes span ~40 kb and contain 13 exons and 12 introns each, in a pattern identical to that of the STSL loci in mouse and man. Both Abcg5 and Abcg8 were expressed only in liver and intestine. Analyses of DNA from SHR, SHRSP, GH, WKY, Wistar, Wistar King A (WKA) and Brown Norway (BN) rat strains revealed a homozygous G to T substitution at nucleotide 1754, resulting in the coding change Gly583Cys in sterolin-1 only in rats that are both sitosterolemic and hypertensive (SHR, SHRSP and WKY).

CONCLUSIONS

The rat STSL locus maps to chromosome 6q12. A non-synonymous mutation in Abcg5, Gly583Cys, results in sitosterolemia in rat strains that are also hypertensive (WKY, SHR and SHRSP). Those rat strains that are hypertensive, but not sitosterolemic (e.g. GH rat) do not have mutations in Abcg5 or Abcg8. This mutation allows for expression and apparent apical targeting of Abcg5 protein in the intestine. These rat strains may therefore allow us to study the pathophysiological mechanisms involved in the human disease of sitosterolemia.

Stimulation of cholesterol excretion by the liver X receptor agonist requires ATP-binding cassette transporters G5 and G8

Liver X receptor (LXR) is a nuclear receptor that plays a crucial role in orchestrating the trafficking of sterols between tissues. Treatment of mice with a potent and specific LXR agonist, T0901317, is associated with increased biliary cholesterol secretion, decreased fractional cholesterol absorption, and increased fecal neutral sterol excretion. Here we show that expression of two target genes of LXRalpha, the ATP-binding cassette (ABC) transporters Abcg5 and Abcg8, is required for both the increase in sterol excretion and the decrease in fractional cholesterol absorption associated with LXR agonist treatment. Mice expressing no ABCG5 and ABCG8 (G5G8(-/-) mice) and their littermate controls were treated for 7 days with T0901317. In wild type animals, treatment with the LXR agonist resulted in a 3-fold increase in biliary cholesterol concentrations, a 25% reduction in fractional cholesterol absorption, and a 4-fold elevation in fecal neutral sterol excretion. In contrast, the LXR agonist did not significantly affect biliary cholesterol levels, fractional cholesterol absorption, or neutral fecal sterol excretion in the G5G8(-/-) mice. Thus Abcg5 and Abcg8 are required for LXR agonist-associated changes in dietary and biliary sterol trafficking. These results establish a central role for ABCG5 and ABCG8 in promoting cholesterol excretion in vivo.

Liver-specific mRNA for Insig-2 down-regulated by insulin: implications for fatty acid synthesis

Insig-1 and -2 are closely related proteins of the endoplasmic reticulum (ER) that block proteolytic activation of sterol regulatory element-binding proteins (SREBPs), transcription factors that activate the synthesis of cholesterol and fatty acids in liver and other organs. When cellular cholesterol levels are high, Insig proteins bind and trap SREBP cleavage-activating protein (SCAP), retaining it in the ER and preventing it from escorting SREBPs from ER to the site of proteolytic activation in the Golgi complex. Here, we report the discovery of a liver-specific transcript of Insig-2, designated Insig-2a. This transcript and the ubiquitous transcript, designated Insig-2b, differ through the use of different promoters that produce different noncoding first exons that splice into a common second exon. Although the Insig-2a and -2b mRNAs encode identical proteins, they differ in patterns of regulation. Insig-2a is the predominant transcript in livers of fed animals, and it is selectively down-regulated by insulin. Insig-2a mRNA increases when mice are fasted, and it declines when they are refed. The transcript also increases in livers of rats whose insulin-secreting pancreatic beta cells have been destroyed by streptozotocin, and it is reduced when insulin is injected. The insulin-mediated fall in Insig-2a may allow SREBP-1c to be processed, thereby allowing insulin to stimulate fatty acid synthesis, even under conditions in which hepatic cholesterol levels are elevated.

Endoproteolytic processing of the lymphocytic choriomeningitis virus glycoprotein by the subtilase SKI-1/S1P

The envelope glycoprotein (GP) of lymphocytic choriomeningitis virus (LCMV) is posttranslationally cleaved into two subunits. We show here that this endoproteolytic processing is not required for transport to the cell surface but is essential for LCMV GP to mediate infectivity of pseudotyped retroviral vectors. By systematic mutational analysis of the LCMV GP cleavage site, we determined that the consensus motif R-(R/K/H)-L-(A/L/S/T/F)(265) is essential for the endoproteolytic processing. In agreement with the identified consensus motif, we show that the cellular subtilase SKI-1/S1P cleaves LCMV GP.

Selective proteolytic processing of rat hepatic sterol regulatory element binding protein-1 (SREBP-1) and SREBP-2 during postnatal development

Sterol regulatory element-binding protein-1c (SREBP-1c) plays a major role in hepatic lipogenic gene expression. In adult animals, insulin and oxysterols induce SREBP-1c gene transcription, whereas polyunsaturated fatty acids suppress the nuclear content of SREBP-1c through pre-translational regulatory mechanisms. A decline in nuclear SREBP-1 is associated with suppression of hepatic lipogenesis. In contrast to adult rats, hepatic lipogenesis in preweaned neonatal rats is low. Ingestion of milk fat by the neonate may contribute to low hepatic lipogenesis. In this report, we tested the hypothesis that low lipogenic gene expression prior to weaning correlates with low mRNA(SREBP-1c), as well as low precursor and nuclear forms of SREBP-1. In contrast to expectations, levels of mRNA(SREBP-1c) and the 125-kDa SREBP-1 precursor in livers of preweaned rats was comparable with adult levels. Despite high levels of SREBP-1 precursor, mature (65 kDa) SREBP-1 was not detected in rat liver nuclei prior to 18 days postpartum. Weaning rats at 21 days postpartum was accompanied by a rise in nuclear SREBP-1 levels as well as increased lipogenic gene expression. In contrast, SREBP-2 was present in rat liver nuclei, and its target gene, HMG-CoA reductase, was expressed above adult levels prior to weaning. These studies indicate that, prior to weaning, SREBP-2 but not SREBP-1 is proteolytically processed to the mature form. As such, SREBP-2-regulated genes are active. Failure of SREBP-1 to be processed to the mature form <18 days postpartum correlates with low hepatic lipogenic gene expression. This mechanism differs from the hormonal and fatty acid-mediated pre-translational control of SREBP-1c in adult liver.

Identification of two mammalian reductases involved in the two-carbon fatty acyl elongation cascade

The de novo synthesis of fatty acids occurs in two distinct cellular compartments. Palmitate (16:0) is synthesized from acetyl-CoA and malonyl-CoA in the cytoplasm by the enzymes acetyl-CoA carboxylase 1 and fatty acid synthase. The synthesis of fatty acids longer than 16 carbons takes place in microsomes and utilizes malonyl-CoA as the carbon source. Each two-carbon addition requires four sequential reactions: condensation, reduction, dehydration, and a final reduction to form the elongated fatty acyl-CoA. The initial condensation reaction is the regulated and rate-controlling step in microsomal fatty acyl elongation. We previously reported the cDNA cloning and characterization of a murine long chain fatty acyl elongase (LCE) . Overexpression of LCE in cells resulted in the enhanced addition of two-carbon units to C12-C16 fatty acids, and evidence was provided that LCE catalyzed the initial condensation reaction of long chain fatty acid elongation. The remaining three enzymes in the elongation reaction have not been identified in mammals. Here, we report the identification and characterization of two mammalian enzymes that catalyze the 3-ketoacyl-CoA and trans-2,3-enoyl-CoA reduction reactions in long and very long chain fatty acid elongation, respectively.

Stearoyl-CoA desaturase, a short-lived protein of endoplasmic reticulum with multiple control mechanisms

Stearoyl-CoA desaturase (SCD) is a short-lived, polytopic membrane-bound non-heme iron enzyme localized primarily in the endoplasmic reticulum. SCD is required for the biosynthesis of monounsaturated fatty acids, and plays a key role in hepatic synthesis of triglycerides and very-low-density lipoproteins. The intracellular concentration of SCD fluctuates in a wide range in response to complex and often competing hormonal and dietary factors. A combination of transcriptional regulation and rapid protein degradation produces transient elevations of SCD enzyme activity in response to physiologic demands. Dysregulation of SCD has been implicated in non-alcoholic fatty liver disease, hyperlipidemia, and obesity.

A low fish oil inhibits SREBP-1 proteolytic cascade, while a high-fish-oil feeding decreases SREBP-1 mRNA in mice liver: relationship to anti-obesity

Rodents fed fish oil showed less obesity with a reduction of triglyceride synthesis in liver, relative to other dietary oils, along with a decrease of mature form of sterol regulatory element binding protein-1 (SREBP-1) and activation of peroxisome proliferator-activated receptor alpha (PPARalpha). Decrease of mature SREBP-1 protein by fish oil feeding was due to either inhibition of SREBP-1 proteolytic cascade or to decrease of its mRNA. To clarify its mechanism and relation to antiobesity effect, mice were fed fish oil in a range from 10 to 60 energy percent (en%). Fish oil feeding decreased body weight and fat mass in a dose-dependent manner, in parallel with PPARalpha activation and a decrease of SREBP-1 mRNA. However, compared with 0 en% fish oil feeding, 10 en% fish oil feeding decreased mature SREBP-1 protein by 50% with concomitant decreases of lipogenic genes, while precursor SREBP-1 protein rather increased by 1.3-fold. These data suggest that physiological doses of fish oil feeding effectively decrease expression of liver lipogenic enzymes by inhibiting SREBP-1 proteolytic cascade, while substantial decrease of SREBP-1 expression is observed in its pharmacological doses, and that activation of PPARalpha rather than SREBP-1 decrease might be related to the antiobesity effect of fish oil feeding.

Disruption of Abcg5 and Abcg8 in mice reveals their crucial role in biliary cholesterol secretion

Cholesterol and other sterols exit the body primarily by secretion into bile. In patients with sitosterolemia, mutations in either of two ATP-binding cassette (ABC) half-transporters, ABCG5 or ABCG8, lead to reduced secretion of sterols into bile, implicating these transporters in this process. To elucidate the roles of ABCG5 and ABCG8 in the trafficking of sterols, we disrupted Abcg5 and Abcg8 in mice (G5G8(-/-)). The G5G8(-/-) mice had a 2- to 3-fold increase in the fractional absorption of dietary plant sterols, which was associated with an approximately 30-fold increase in plasma sitosterol. Biliary cholesterol concentrations were extremely low in the G5G8(-/-) mice when compared with wild-type animals (mean = 0.4 vs. 5.5 micromol ml) and increased only modestly with cholesterol feeding. Plasma and liver cholesterol levels were reduced by 50% in the chow-fed G5G8(-/-) mice and increased 2.4- and 18-fold, respectively, after cholesterol feeding. These data indicate that ABCG5 and ABCG8 are required for efficient secretion of cholesterol into bile and that disruption of these genes increases dramatically the responsiveness of plasma and hepatic cholesterol levels to changes in dietary cholesterol content.

Disruption of Abcg5 and Abcg8 in mice reveals their crucial role in biliary cholesterol secretion

Cholesterol and other sterols exit the body primarily by secretion into bile. In patients with sitosterolemia, mutations in either of two ATP-binding cassette (ABC) half-transporters, ABCG5 or ABCG8, lead to reduced secretion of sterols into bile, implicating these transporters in this process. To elucidate the roles of ABCG5 and ABCG8 in the trafficking of sterols, we disrupted Abcg5 and Abcg8 in mice (G5G8(-/-)). The G5G8(-/-) mice had a 2- to 3-fold increase in the fractional absorption of dietary plant sterols, which was associated with an approximately 30-fold increase in plasma sitosterol. Biliary cholesterol concentrations were extremely low in the G5G8(-/-) mice when compared with wild-type animals (mean = 0.4 vs. 5.5 micromol ml) and increased only modestly with cholesterol feeding. Plasma and liver cholesterol levels were reduced by 50% in the chow-fed G5G8(-/-) mice and increased 2.4- and 18-fold, respectively, after cholesterol feeding. These data indicate that ABCG5 and ABCG8 are required for efficient secretion of cholesterol into bile and that disruption of these genes increases dramatically the responsiveness of plasma and hepatic cholesterol levels to changes in dietary cholesterol content.

Liver X receptors in the central nervous system: from lipid homeostasis to neuronal degeneration

Liver X receptors (LXRalpha and -beta) are nuclear receptors abundant in the liver where they are regulators of lipid homeostasis. Both LXRs are also expressed in the brain, but their roles in this tissue remain to be clarified. We examined the brains of mice in which the genes of both LXRalpha and -beta have been disrupted and found several severe abnormalities. One of the most striking features is that the lateral ventricles are closed and lined with lipid-laden cells. In addition, there are enlarged brain blood vessels, especially in the pars reticularis of the substantia nigra and in the globus pallidus. Other features of the brains are excessive lipid deposits, proliferation of astrocytes, loss of neurons, and disorganized myelin sheaths. Electron micrographs revealed that, as mice aged, lipid vacuoles accumulated in astrocytes surrounding blood vessels. Comparison of mRNA profiles in LXR knockout mice and wild-type littermates showed that expression of several LXR target genes involved in cholesterol efflux from astrocytes was reduced. These findings show that LXRs have an important function in lipid homeostasis in the brain, and that loss of these receptors results in neurodegenerative diseases. Further characterization of the role of LXRs in the brain could lead to new insights into the etiology and treatment of some neurodegenerative disorders.

Overexpression of ABCG5 and ABCG8 promotes biliary cholesterol secretion and reduces fractional absorption of dietary cholesterol

Two ATP-binding cassette (ABC) transporters, ABCG5 and ABCG8, have been proposed to limit sterol absorption and to promote biliary sterol excretion in humans. To test this hypothesis, a P1 clone containing the human ABCG5 and ABCG8 genes was used to generate transgenic mice. The transgenes were expressed primarily in the liver and small intestine, mirroring the expression pattern of the endogenous genes. Transgene expression only modestly affected plasma and liver cholesterol levels but profoundly altered cholesterol transport. The fractional absorption of dietary cholesterol was reduced by about 50%, and biliary cholesterol levels were increased more than fivefold. Fecal neutral sterol excretion was increased three- to sixfold and hepatic cholesterol synthesis increased two- to fourfold in the transgenic mice. No significant changes in the pool size, composition, and fecal excretion of bile acids were observed in the transgenic mice. Transgene expression attenuated the increase in hepatic cholesterol content induced by consumption of a high cholesterol diet. These results demonstrate that increased expression of ABCG5 and ABCG8 selectively drives biliary neutral sterol secretion and reduces intestinal cholesterol absorption, leading to a selective increase in neutral sterol excretion and a compensatory increase in cholesterol synthesis.

New perspectives in the regulation of hepatic glycolytic and lipogenic genes by insulin and glucose: a role for the transcription factor sterol regulatory element binding protein-1c

The regulation of hepatic glucose metabolism has a key role in whole-body energy metabolism, since the liver is able to store (glycogen synthesis, lipogenesis) and to produce (glycogenolysis, gluconeogenesis) glucose. These pathways are regulated at several levels, including a transcriptional level, since many of the metabolism-related genes are expressed according to the quantity and quality of nutrients. Recent advances have been made in the understanding of the regulation of hepatic glycolytic, lipogenic and gluconeogenic gene expression by pancreatic hormones, insulin and glucagon and glucose. Here we review the role of the transcription factors forkhead and sterol regulatory element binding protein-1c in the inductive and repressive effects of insulin on hepatic gene expression, and the pathway that leads from glucose to gene regulation with the recently discovered carbohydrate response element binding protein. We discuss how these transcription factors are integrated in a regulatory network that allows a fine tuning of hepatic glucose storage or production, and their potential importance in metabolic diseases.

Biosynthesis and cellular trafficking of the convertase SKI-1/S1P: ectodomain shedding requires SKI-1 activity

Subtilisin kexin isozyme-1 (SKI-1)/site 1 protease is a mammalian subtilase composed of distinct functional domains. Among the major substrates of SKI-1 are the sterol regulatory element-binding proteins, regulating cholesterol and fatty acid homeostasis. Other substrates include the stress response factor activating transcription factor-6, the brain-derived neurotrophic factor, and the surface glycoproteins of highly infectious viruses belonging to the family of Arenaviridae. Domain deletion and/or point mutants were used to gauge the role of the various domains of SKI-1. Biosynthesis, cellular trafficking, and sterol regulatory element-binding protein-2 cleavage activity were used as diagnostic tools. Results revealed that Arg(130) and Arg(134) are critical for the autocatalytic primary processing of the prosegment and for the subsequent efficient exit of SKI-1 from the endoplasmic reticulum. Functional mapping of the growth factor cytokine receptor motif suggested a folding role within the endoplasmic reticulum. Microsequencing of the remaining membrane-bound stub following ectodomain shedding of SKI-1 localized the shedding site to KHQKLL(953) downward arrow. Site-directed mutagenesis, in vitro cleavage of a synthetic peptide containing the shedding site, and inhibitor studies favor an autocatalytic event occurring at a non-canonical SKI-1 recognition sequence, with P2 and P1 Leu being very critical. In conclusion, multiple domains ensuring optimal functional characteristics control SKI-1 activity and cellular trafficking.

Diminished hepatic response to fasting/refeeding and liver X receptor agonists in mice with selective deficiency of sterol regulatory element-binding protein-1c

Two treatments, fasting/refeeding and administration of liver X receptor (LXR) agonists, elevate the mRNA for sterol regulatory element-binding protein-1c (SREBP-1c) and enhance lipid synthesis in liver. These treatments do not affect the mRNA for SREBP-1a, an alternative transcript from the same gene. Through homologous recombination, we eliminated the exon encoding SREBP-1c from the mouse genome, leaving the SREBP-1a transcript intact. On a normal diet, livers of SREBP-1c(-/-) mice manifested reductions in multiple mRNAs encoding enzymes of fatty acid and triglyceride synthesis, including acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS). In contrast, SREBP-1c(-/-) livers showed a compensatory increase in hepatic SREBP-2 mRNA, accompanied by increased mRNA levels for cholesterol biosynthetic enzymes. In fasted/refed animals, ACC and FAS mRNAs rose, but not to the same extent as in wild-type livers. The refeeding-induced increase in SREBP-1c(-/-) mice was greater than in mice lacking SREBP cleavage-activating protein (SCAP), in which all nuclear SREBPs are absent. Thus, SREBP-2 and/or SREBP-1a can substitute partially for SREBP-1c in permitting an insulin-mediated increase in ACC and FAS mRNAs. In contrast, mRNAs for several other lipogenic enzymes (glucose-6-phosphate dehydrogenase, malic enzyme, glycerol-3-phosphate acyltransferase, and stearoyl-CoA desaturase-1) showed a complete failure of the normal inductive response to refeeding, indicating specific reliance on SREBP-1c. Moreover, these mRNAs, as well as multiple other lipogenic mRNAs, showed a markedly blunted response to the LXR agonist T090137, indicating an essential role of SREBP-1c in the LXR response.

A rapid fluorometric assay for the proteolytic activity of SKI-1/S1P based on the surface glycoprotein of the hemorrhagic fever Lassa virus

The subtilase subtilisin kexin isozyme-1 (SKI-1)/site 1 protease (S1P), has been implicated in the processing of Lassa virus glycoprotein C (GP-C) precursor into GP1 and GP2 that are responsible for viral fusion with the host cell membrane. Here, we studied in vitro the kinetics of this cleavage by hSKI-1 using an intramolecularly quenched fluorogenic (IQF) peptide, Q-GPC(251-263) [Abz-(251)Asp-Ile-Tyr-Ile-Ser-Arg-Arg-Leu-Leu/Gly-Thr-Phe-Thr(263)-3-NitroTyr-Ala-CONH(2)], containing the identified site. The measured V(max (app))/K(m (app)) was compared to those for other IQF SKI-substrates. Q-GPC(251-263) is cleaved 10-fold more efficiently than the previously known best SKI-substrate, Q-hproSKI(134-142). This study confirmed the role of SKI-1 in GP-C processing and provides a novel, rapid and efficient enzymatic assay of SKI-1.