ABCG1 regulates mouse adipose tissue macrophage cholesterol levels and ratio of M1 to M2 cells in obesity and caloric restriction

In addition to triacylglycerols, adipocytes contain a large reserve of unesterified cholesterol. During adipocyte lipolysis and cell death seen during severe obesity and weight loss, free fatty acids and cholesterol become available for uptake and processing by adipose tissue macrophages (ATMs). We hypothesize that ATMs become cholesterol enriched and participate in cholesterol clearance from adipose tissue. We previously showed that ABCG1 is robustly upregulated in ATMs taken from obese mice and further enhanced by caloric restriction. Here, we found that ATMs taken from obese and calorie-restricted mice derived from transplantation of WT or Abcg1-deficient bone marrow are cholesterol enriched. ABCG1 levels regulate the ratio of classically activated (M1) to alternatively activated (M2) ATMs and their cellular cholesterol content. Using WT and Abcg1(-/-) cultured macrophages, we found that Abcg1 is most highly expressed by M2 macrophages and that ABCG1 deficiency is sufficient to retard macrophage chemotaxis. However, changes in myeloid expression of Abcg1 did not protect mice from obesity or impaired glucose homeostasis. Overall, ABCG1 modulates ATM cholesterol content in obesity and weight loss regimes leading to an alteration in M1 to M2 ratio that we suggest is due to the extent of macrophage egress from adipose tissue.

Genetic modulation of diabetic nephropathy among mouse strains with Ins2 Akita mutation

Diabetic nephropathy (DN) is a major complication of diabetes and the leading cause of end‐stage renal disease. DN is characterized by changes in kidney structure and function but the underlying genetic and molecular factors are poorly understood. We used a mouse diversity panel to explore the genetic basis of DN traits in mice carrying the Ins2 Akita mutation. Twenty‐eight Akita strains were generated by breeding this panel to DBA/2.Akita mice. Male F1 diabetic and nondiabetic littermates were evaluated for DN‐related traits. Urine albumin‐to‐creatinine ratios (ACRs), volume and cystatin C as well as blood urea nitrogen and lipoprotein levels varied significantly among the diabetic strains. For most Akita strains, ACR values increased 2‐ to 6‐fold over euglycemic control values. However, six strains exhibited changes in ACR exceeding 10‐fold with two strains (NOD/ShiLt and CBA) showing 50‐ to 83‐ fold increases. These increases are larger than previously reported among available DN mouse models establishing these strains as useful for additional studies of renal function. ACRs correlated with cystatin C (P = 0.0286), a measure of hyperfiltration and an interstitial tubular marker associated with DN onset in humans suggesting that tubule damage as well as podocyte‐stress contributed to reduced kidney function assessed by ACR. Although large changes were seen for ACRs, severe nephropathology was absent. However, glomerular hypertrophy and collagen IV content were found to vary significantly among strains suggesting a genetic basis for early onset features of DN. Our results define the range of DN phenotypes that occur among common inbred strains of mice.

Diabetic nephropathy (DN) is characterized by changes in kidney structure and function but the underlying genetic and molecular factors are poorly understood. We used a mouse diversity panel to explore the genetic basis of DN traits in mice carrying the Ins2 Akita mutation. Twenty‐eight Akita strains on different genetic backgrounds were evaluated for DN‐related traits and the results define the range of DN phenotypes that occur among common inbred strains of mice.

Genetic determinants of atherosclerosis, obesity, and energy balance in consomic mice

Metabolic diseases such as obesity and atherosclerosis result from complex interactions between environmental factors and genetic variants. A panel of chromosome substitution strains (CSSs) was developed to characterize genetic and dietary factors contributing to metabolic diseases and other biological traits and biomedical conditions. Our goal here was to identify quantitative trait loci (QTLs) contributing to obesity, energy expenditure, and atherosclerosis. Parental strains C57BL/6 and A/J together with a panel of 21 CSSs derived from these progenitors were subjected to chronic feeding of rodent chow and atherosclerotic (females) or diabetogenic (males) test diets, and evaluated for a variety of metabolic phenotypes including several traits unique to this report, namely fat pad weights, energy balance, and atherosclerosis. A total of 297 QTLs across 35 traits were discovered, two of which provided significant protection from atherosclerosis, and several dozen QTLs modulated body weight, body composition, and circulating lipid levels in females and males. While several QTLs confirmed previous reports, most QTLs were novel. Finally, we applied the CSS quantitative genetic approach to energy balance, and identified three novel QTLs controlling energy expenditure and one QTL modulating food intake. Overall, we identified many new QTLs and phenotyped several novel traits in this mouse model of diet-induced metabolic diseases.

Hybrid mouse diversity panel: a panel of inbred mouse strains suitable for analysis of complex genetic traits

We have developed an association-based approach using classical inbred strains of mice in which we correct for population structure, which is very extensive in mice, using an efficient mixed-model algorithm. Our approach includes inbred parental strains as well as recombinant inbred strains in order to capture loci with effect sizes typical of complex traits in mice (in the range of 5% of total trait variance). Over the last few years, we have typed the hybrid mouse diversity panel (HMDP) strains for a variety of clinical traits as well as intermediate phenotypes and have shown that the HMDP has sufficient power to map genes for highly complex traits with resolution that is in most cases less than a megabase. In this essay, we review our experience with the HMDP, describe various ongoing projects, and discuss how the HMDP may fit into the larger picture of common diseases and different approaches.

RNA-binding protein IGF2BP2/IMP2 is required for laminin-β2 mRNA translation and is modulated by glucose concentration

Laminin-β2 (LAMB2) is a critical component of the glomerular basement membrane as content of LAMB2 in part determines glomerular barrier permeability. Previously, we reported that high concentrations of glucose reduce expression of this laminin subunit at the translational level. The present studies were undertaken to further define systems that control Lamb2 translation and the effect of high glucose on those systems. Complementary studies were performed using in vitro differentiation of cultured podocytes and mesangial cells exposed to normal and elevated concentrations of glucose, and tissues from control and diabetic rats. Together, these studies provide evidence for regulation of Lamb2 translation by IMP2, an RNA binding protein that targets Lamb2 mRNA to the actin cytoskeleton. Expression of Imp2 itself is regulated by the transcription factor HMGA2, which in turn is regulated by the microRNA let-7b. Elevated concentrations of glucose increase let-7b, which reduces HMGA2 expression, in turn reducing IMP2 and LAMB2. Correlative changes in kidney tissues from control and streptozotocin-induced diabetic rats suggest these control mechanisms are operative in vivo and may contribute to proteinuria in diabetic nephropathy. To our knowledge, this is the first time that translation of Lamb2 mRNA has been linked to the actin cytoskeleton, as well as to specific RNA-binding proteins. These translational control points may provide new targets for therapy in proteinuric disorders such as diabetic nephropathy where LAMB2 levels are reduced.

Deficiency of lymphotoxin-α does not exacerbate high-fat diet-induced obesity but does enhance inflammation in mice

Lymphotoxin-α (LTα) is secreted by lymphocytes and acts through tumor necrosis factor-α receptors and the LTβ receptor. Our goals were to determine whether LT has a role in obesity and investigate whether LT contributes to the link between obesity and adipose tissue lymphocyte accumulation. LT deficient (LT(-/-)) and wild-type (WT) mice were fed standard pelleted rodent chow or a high-fat/high-sucrose diet (HFHS) for 13 wk. Body weight, body composition, and food intake were measured. Glucose tolerance was assessed. Systemic and adipose tissue inflammatory statuses were evaluated by quantifying plasma adipokine levels and tissue macrophage and T cell-specific gene expression in abdominal fat. LT(-/-) mice were smaller (20%) and leaner (25%) than WT controls after 13 wk of HFHS diet feeding. LT(-/-) mice showed improved glucose tolerance, suggesting that, in WT mice, LT may impair glucose metabolism. Surprisingly, adipose tissue from rodent chow- and HFHS-fed LT(-/-) mice exhibited increased T lymphocyte and macrophage infiltration compared with WT mice. Despite the fact that LT(-/-) mice exhibited an enhanced inflammatory status at the systemic and tissue level even when fed rodent chow, they were protected from enhanced diet-induced obesity and insulin resistance. Thus, LT contributes to body weight and adiposity and is required to modulate the accumulation of immune cells in adipose tissue.

Direct recruitment of insulin receptor and ERK signaling cascade to insulin-inducible gene loci

OBJECTIVE

Insulin receptor (IR) translocates to the nucleus, but its recruitment to gene loci has not been demonstrated. Here, we tested the hypothesis that IR and its downstream mitogenic transducers are corecruited to two prototypic insulin-inducible genes: early growth response 1 (egr-1), involved in mitogenic response, and glucokinase (Gck), encoding a key metabolic enzyme.

RESEARCH DESIGN AND METHODS

We used RNA and chromatin from insulin-treated rat hepatic tumor cell line expressing human insulin receptor (HTC-IR) and livers from lean and insulin-resistant ob/ob glucose-fed mice in quantitative RT-PCR and chromatin immunoprecipitation studies to determine gene expression levels and associated recruitment of RNA polymerase II (Pol II), insulin receptor, and cognate signaling proteins to gene loci, respectively.

RESULTS

Insulin-induced egr-1 mRNA in HTC-IR cells was associated with corecruitment of IR signaling cascade (IR, SOS, Grb2, B-Raf, MEK, and ERK) to this gene. Recruitment profiles of phosphorylated IR, B-Raf, MEK, and Erk along egr-1 transcribed region were similar to those of elongating Pol II. Glucose-feeding increased Gck mRNA expression in livers of lean but not ob/ob mice. In lean mice, there was glucose feeding-induced recruitment of IR and its transducers to Gck gene synchronized with elongating Pol II. In sharp contrast, in glucose-fed ob/ob mice, the Gck recruitment patterns of active MEK/Erk, IR, and Pol II were asynchronous.

CONCLUSIONS

IR and its signal transducers recruited to genes coupled to elongating Pol II may play a role in maintaining productive mRNA synthesis of target genes. These studies suggest a possibility that impaired Pol II processivity along genes bearing aberrant levels of IR/signal transducers is a previously unrecognized facet of insulin resistance.

Receptors for tumor necrosis factor-alpha play a protective role against obesity and alter adipose tissue macrophage status

TNF-alpha signals through two receptors, TNFR1 and TNFR2. Our goals were: 1) determine the role of TNFRs in obesity and metabolic disease and 2) investigate whether TNFRs contribute to the link between obesity and adipose tissue macrophage infiltration and polarization. R1(-/-)R2(-/-) (RKO) and wild-type (WT) mice were fed standard chow or a high-fat/high-sucrose diet (HFHS) over 14 wk. Body composition, food intake, and energy expenditure were measured. Oral glucose tolerance and insulin sensitivity tests assessed glucose homeostasis. Adipose tissue and systemic inflammatory status were evaluated by quantifying plasma adipokine levels and macrophage-specific gene expression in fat. RKO mice were heavier (10%) and fatter (18%) than WT controls at 4 wk of age and were 26% heavier and 50% fatter than WT after 14 wk of HFHS diet feeding. Age- and diet-adjusted 24-h oxygen consumption, activity, and respiratory exchange ratio were significantly reduced in RKO mice. Obese RKO mice were markedly insulin resistant, suggesting that intact TNFR signaling is not required for the effect of obesity to impair glucose metabolism. Adipose tissue from HFHS-fed RKO mice exhibited increased macrophage infiltration, but compared with WT mice, macrophage phenotypic markers featured a predominance of antiinflammatory M2 over proinflammatory M1 cells. TNFRs play a physiological role to limit body weight and adiposity by modestly increasing metabolic rate and fatty acid oxidation, and they are required for obesity-induced activation of adipose tissue macrophages. Despite these effects, TNFRs are not required for obesity-induced insulin resistance.

Overexpression of apolipoprotein A5 in mice is not protective against body weight gain and aberrant glucose homeostasis

Apolipoprotein A5 (APOA5) is expressed primarily in the liver and modulates plasma triglyceride levels in mice and humans. Mice overexpressing APOA5 exhibit reduced plasma triglyceride levels. Because there is a tight association between plasma triglyceride concentration and traits of the metabolic syndrome, we used transgenic mice overexpressing human APOA5 to test the concept that these mice would be protected from diet-induced obesity and insulin resistance. Male and female transgenic and wild-type mice on the FVB/N genetic background were fed standard rodent chow or a diet rich in fat and sucrose for 18 weeks, during which time clinical phenotypes associated with obesity and glucose homeostasis were measured. We found that APOA5 transgenic (A5tg) mice were resistant to diet-induced changes in plasma triglyceride but not total cholesterol levels. Body weights were similar between the genotypes for females and males, although male A5tg mice showed a modest but significant increase in the relative size of inguinal fat pads. Although male A5tg mice showed a significantly increased ratio of plasma glucose to insulin, profiles of glucose clearance as evaluated after injections of glucose or insulin failed to reveal any differences between genotypes. Overall, our data showed that there was no advantage to responses to diet-induced obesity with chronic reduction of plasma triglyceride levels as mediated by overexpression of APOA5.

Mice deficient in apolipoprotein E but not LDL receptors are resistant to accelerated atherosclerosis associated with obesity

The aims of this study were to determine whether mice induced to become obese also exhibited accelerated atherosclerosis, and to determine whether obesity itself or dyslipidemia associated with obesity enhanced atherosclerosis. Wild-type (C57BL/6) mice and mice deficient for the low density lipoprotein receptor (LDLR-/-) or apolipoprotein E (apoE-/-) were fed a low fat, rodent chow diet or a high fat, high sucrose (diabetogenic) diet to induce obesity. As compared with wild-type mice, diabetogenic diet-fed LDLR-/- mice became more obese and developed severe dyslipidemia. Consequently, atherosclerotic lesions were increased in the LDLR-/- mice 3.7-fold over chow fed values. ApoE-/- mice showed weight gain profiles similar to those observed for wild-type mice. However, no differences in plasma lipid levels, lipoprotein profiles or atherosclerotic lesion areas were observed between chow-fed and diabetogenic diet-fed apoE-/- mice. These data demonstrate that lipid storage and partitioning as mediated by the low density lipoproteins (LDL) receptor or apoE-/- have profound and opposing consequences for dyslipidemia and atherosclerosis susceptibility associated with obesity.

Hyperglycemia and hyperlipidemia act synergistically to induce renal disease in LDL receptor-deficient BALB mice

Diabetic nephropathy is the leading cause of end-stage renal disease in Western countries, but only a portion of diabetic patients develop diabetic nephropathy. Dyslipidemia represents an important aspect of the metabolic imbalance in diabetic patients. In this study, we addressed the impact of combined hyperlipidemia and hyperglycemia on renal pathology. Kidneys from wild-type (WT) or LDL receptor-deficient BALB/cBy mice (BALB.LDLR-/-) were examined at 22 weeks of age. Diabetes was induced by administration of streptozotocin and mice were randomly assigned to either standard chow or Western diet. Chow fed BALB.LDLR-/- mice did not demonstrate renal abnormalities, whereas BALB. LDLR-/- mice fed a Western diet showed occasional glomerular and tubulointerstitial foam cells. Diabetic WT mice had modestly increased glomerular cellularity and extracellular matrix. Hyperlipidemic and diabetic BALB.LDLR-/- mice exhibited an increase in glomerular cellularity and extracellular matrix, accumulation of glomerular and tubulointerstitial foam cells and mesangial lipid deposits. The tubular epithelium demonstrated pronounced lipid induced tubular degeneration with increased tubular epithelial cell turnover. Hyperlipidemia and hyperglycemia seem to act synergistically in inducing renal injury in the BALB.LDLR-/- mouse. This model of diabetic nephropathy is unique in its development of tubular lesions and may represent a good model for hyperlipidemia-exacerbated diabetic nephropathy.

Early intraneuronal Abeta deposition in the hippocampus of APP transgenic mice

Increasing evidence suggests that intraneuronal amyloid-beta (Abeta) accumulation may be an early event in Alzheimer's disease (AD) pathogenesis. However direct in vivo evidence regarding initial Abeta seeding is missing. Using an APP transgenic mouse model, our sensitive immunocytochemical procedures revealed a novel intraneuronal Abeta deposition in the somas of hippocampal CA1/subiculum neurons far in advance of the occurrence of extracellular Abetaplaques. These deposits increased exponentially with age and were elevated approximately 4-fold (p < 0.001) by high fat/high cholesterol diet. Abeta40 and Abeta42 were the major constituents of these deposits and were co-localized with lysosomal markers. Our results are consistent with the notion that the earliest Abeta deposition occurs intraneuronally, prior to extracellular amyloid plaque formation.

The effect of apolipoprotein E deficiency on islet amyloid deposition in human islet amyloid polypeptide transgenic mice

AIMS/HYPOTHESIS

Islet amyloid deposits are present in over 85% of Type 2 diabetic patients and have been suggested to be pathogenic. The mechanism that converts islet amyloid polypeptide (IAPP), the unique component of these deposits, into amyloid fibrils in vivo is not known. The amino acid sequence of IAPP is critical but insufficient for beta-pleated sheet formation. As apolipoprotein E (apoE), another component of islet amyloid deposits, plays a critical role in amyloid formation in Alzheimer's disease, we hypothesised that apoE could play an important role in islet amyloid formation.

METHODS

Transgenic mice expressing the human form of IAPP ( hIAPP (+/0)) were crossbred with apoE deficient ( apoE (-/-)) mice and followed for 12 months, at which time the prevalence and severity of islet amyloid, as well as plasma glucose, hIAPP, immunoreactive insulin (IRI) and lipid concentrations were measured.

RESULTS

The prevalence and severity of islet amyloid after one year of follow up were comparable among hIAPP (+/0) mice that were apoE (+/+), apoE (+/-) or apoE (-/-). Differences in glucose tolerance, lipid abnormalities or changes in pancreatic content or plasma concentrations of hIAPP and/or IRI did not account for these findings.

CONCLUSION/INTERPRETATION

Our data shows that, unlike in the localized amyloidosis in the brain characteristic of Alzheimer's disease, apoE is not critical for islet amyloid formation in a transgenic mouse model of Type 2 diabetes mellitus. These results indicate that the mechanisms of localised amyloid formation probably vary among different amyloid-associated disorders. Therefore, therapeutic strategies targeting apoE might not apply equally to patients with different amyloid associated diseases.

The murine macrophage apoB-48 receptor gene (Apob-48r): homology to the human receptor

Previously we cloned the human macrophage apolipoprotein B-48 receptor (ApoB-48R) and documented its expression in human atherosclerotic foam cells (1). Now we have identified and characterized the murine macrophage apob-48r cDNA gene sequence and its chromosomal location. The cDNA (3,615 bp) -deduced amino acid (aa) sequence (942 aa) is approximately 45% identical to the human macrophage APOB-48R, but not to other known gene families. The murine Apob-48r gene, like the human APOB-48R gene, consists of four exons interrupted by three small introns and is syntenically located on chromosome 7. Functionally significant conserved domains include an N-terminal hydrophobic domain, a glycosaminoglycan attachment site, an N-glycosylation site, and an ExxxLL internalization motif C-terminal to the putative internal transmembrane domain. Two conserved coiled-coil domains are likely involved in the spontaneous homodimerization that generates the active dimeric ligand binding species (mouse, approximately 190 kDa; human, approximately 200 kDa). Transfection of the murine apoB-48R into Chinese hamster ovary cells (CHOs) confers apoB-48R function: rapid, high-affinity, specific uptake of known triglyceride-rich lipoprotein ligands of the apoB-48R and, of note, uptake of the cholesteryl ester-rich apoB-48-containing very low density lipoproteins that accumulate in atherosclerosis-prone apoE-deficient mice. Uptake of these ligands by murine apoB-48R-transfected CHOs causes saturable, visible cellular triglyceride and cholesterol accumulation in vitro that resemble foam cells of atherosclerotic lesions. In aggregate, the data presented here and that previously published suggest that the apoE-independent murine apoB-48R pathway may contribute to the spontaneous development of atherosclerotic lesions rich in macrophage-derived foam cells observed in apoE-deficient mice, a murine model of human atherosclerosis.

Renal injury in apolipoprotein E-deficient mice

Hyperlipidemia is thought to accelerate the progression of renal diseases, but the mechanisms by which hyperlipidemia exerts its deleterious effect is still poorly understood. The aim of this study was to describe the renal pathology in a hyperlipidemic mouse strain, the apolipoprotein E-deficient mice (apoE-/-). Renal specimens from a total of 34 mice were studied, including 19 apoE-/- females at the age of 36 weeks, 9 apoE-/- females at the age of 24 weeks, and 6 wild-type females (C57BL/6) as controls. Kidneys were evaluated by histologic examination, immunohistochemistry, and electron microscopy. Immunohistochemistry was used to detect MAC-2-expressing monocyte/macrophages, and the proliferation marker PCNA. Glomerular cell number, glomerular matrix area, and glomerular area were quantified by morphometry. Glomerular lesions in apoE-/- mice were characterized by macrophage accumulation, commonly with foam cell appearance, deposition of extracellular matrix, glomerular hyperplasia, and at times prominent mesangiolysis associated with capillary microaneurysms. Some cases demonstrated lipid deposits filling glomerular capillaries. Arterioles of the vascular pole demonstrated a "foamy" degeneration of smooth muscle cells. These lesions related to hyperlipidemia in this well-established mouse strain have not been previously described. Because this mouse strain is among the most widely studied for interventions aimed at altering hyperlipidemia and the progression of atherosclerosis, we believe that our observations may be of major importance for the accurate interpretation of interventional studies in this strain and offer a new opportunity to study mechanisms of hyperlipidemic renal injury.

Loss of lymphotoxin-alpha but not tumor necrosis factor-alpha reduces atherosclerosis in mice

Inflammatory processes are involved with all phases of atherosclerotic lesion growth. Tumor necrosis factor-alpha (TNFalpha) is an inflammatory cytokine that is thought to contribute to lesion development. Lymphotoxin-alpha (LTalpha) is also a proinflammatory cytokine with homology to TNFalpha. However, its presence or function in lesion development has not been investigated. To study the role of these molecules in atherosclerosis, the expression of these cytokines in atherosclerotic lesions was examined. The presence of both cytokines was observed within aortic sinus fatty streak lesions. To determine the function of these molecules in regulating lesion growth, mice deficient for TNFalpha or LTalpha were examined for induction of atherosclerosis. Surprisingly, loss of TNFalpha did not alter lesion development compared with wild-type mice. This brings doubt to the generally held concept that TNFalpha is a "proatherogenic cytokine." However, LTalpha deficiency resulted in a 62% reduction in lesion size. This demonstrates an unexpected role for LTalpha in promoting lesion growth. The presence of LTalpha was observed in aortic sinus lesions suggesting a direct role of LTalpha in modulating lesion growth. To determine which receptor mediated these responses, diet-induced atherosclerosis in mice deficient for each of the TNF receptors, termed p55 and p75, was examined. Results demonstrated that loss of p55 resulted in increased lesion development, but loss of p75 did not alter lesion size. The disparity in results between ligand- and receptor-deficient mice suggests there are undefined members of the TNF ligand and receptor signaling pathway involved with regulating atherogenesis.

Diet-induced hypercholesterolemia enhances brain A beta accumulation in transgenic mice

Epidemiological data show correlations between hypercholesterolemia and Alzheimer's disease (AD). We test the hypothesis that hypercholesterolemia modulates Abeta deposition in mice overexpressing the human APP695 Swedish mutation (K670N and M671L) (TgAPPsw). Feeding mice a high fat/high cholesterol (HFHC) diet for 7-10 months increased total cholesterol levels by 4-fold. The extent of Abeta immunostained plaque-like deposits were significantly higher for mice fed the HFHC diet as compared with mice fed rodent chow. Extent of deposits correlated inversely with plasma levels of HDL and directly to apolipoprotein E. Overall, plasma lipoproteins may be an important factor in induction of AD-like plaques in mice. The lowering of plasma lipids may be therapeutic for AD patients.

Accumulation of biglycan and perlecan, but not versican, in lesions of murine models of atherosclerosis

Proteoglycan accumulation within the arterial intima has been implicated in lipoprotein retention and in atherosclerosis progression in humans. Two commonly studied murine models of atherosclerosis, the apolipoprotein E (apoE)-deficient (apoE-/-) mouse and the low density lipoprotein receptor-deficient (LDLR-/-) mouse, develop arterial lesions similar to those of human atherosclerosis. However, specific proteoglycan classes that accumulate in lesions of these mice and their relation to the retention of specific apolipoproteins have not been previously determined. In this report, we characterized the distribution of proteoglycans (versican, biglycan, and perlecan) and apolipoproteins (apoB, apoA-I, and apoE) in proximal aortic lesions of chow-fed apoE-/- and LDLR-/- mice at 10, 52, and 73 weeks of age. We observed that similar to the apoE-/- mice, the LDLR-/- mice develop intermediate and advanced plaques within 52 weeks of age. Perlecan and biglycan (both are proteoglycans) appeared early in lesion development with distinct expression patterns as the plaques advanced. Versican, a major proteoglycan detected in human plaques, was mostly absent in both strains. ApoA-I and apoB were detected in early through advanced lesions in regions of proteoglycan accumulation in both strains. Our results indicate that proteoglycans may contribute to the retention of lipoproteins at the earliest stage of atherosclerosis in murine models of atherosclerosis.

LDL receptor but not apolipoprotein E deficiency increases diet-induced obesity and diabetes in mice

The aim of this study was to determine whether phenotypes associated with type 2 diabetes are altered in dyslipidemic obese mice. C57BL/6 wild-type, low-density lipoprotein (LDL) receptor-deficient (LDLR-/-), and apolipoprotein E-deficient (apoE-/-) mice were fed a high-fat, high-carbohydrate diet (diabetogenic diet), and the development of obesity, diabetes, and hypertriglyceridemia was examined. Wild-type mice became obese and developed hyperglycemia, but not hypertriglyceridemia, in response to this diet. LDLR-/- mice fed the diabetogenic diet became more obese than wild-type mice and developed severe hypertriglyceridemia and hyperleptinemia. Surprisingly, glucose levels were only modestly higher and insulin levels and insulin-to-glucose ratios were not strikingly different from those of wild-type mice. In contrast, diabetogenic diet-fed apoE-/- mice were resistant to changes in glucose and lipid homeostasis despite becoming obese. These data suggest that modifications in lipoprotein profiles associated with loss of the LDL receptor or apoE function have profound and unique consequences on susceptibility to diet-induced obesity and type 2 diabetic phenotypes.

Mutation of the RIIbeta subunit of protein kinase A prevents diet-induced insulin resistance and dyslipidemia in mice

The mechanisms by which obesity contributes to diabetic phenotypes remain unclear. We evaluated the role of protein kinase A (PKA) signaling events in mediating diabetes associated with obesity. PKA comprises two regulatory subunits and two catalytic subunits and is activated by cAMP. The RIIbeta regulatory subunit is abundantly expressed in adipose tissue and brain. Knockout mice lacking this subunit are lean and display remarkable resistance to diet-induced obesity. We investigated whether these mice were also resistant to diet-induced diabetes and whether this effect was dependent on reduced adiposity. Mice were fed a high-fat, high-carbohydrate diet and weight gain and diabetes phenotypes were examined. RIIbeta(-/-) mice displayed decreased body weights, reduced insulin levels, improved insulin sensitivity, and improved total-body glucose disposal as compared with wild-type controls. Plasma levels of VLDL and LDL cholesterol were also reduced in high fat-fed RIIbeta(-/-) mice compared with wild-type mice. Taken together, these data demonstrate that loss of RIIbeta protects mice from diet-induced obesity, insulin resistance, and dyslipidemia.

Increased expression of GPI-specific phospholipase D in mouse models of type 1 diabetes

Glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) is a high-density lipoprotein-associated protein. However, the tissue source(s) for circulating GPI-PLD and whether serum levels are regulated are unknown. Because the diabetic state alters lipoprotein metabolism, and liver and pancreatic islets are possible sources of GPI-PLD, we hypothesized that GPI-PLD levels would be altered in diabetes. GPI-PLD serum activity and liver mRNA were examined in two mouse models of type 1 diabetes, a nonobese diabetic (NOD) mouse model and low-dose streptozotocin-induced diabetes in CD-1 mice. With the onset of hyperglycemia (2- to 5-fold increase over nondiabetic levels), GPI-PLD serum activity and liver mRNA increased 2- to 4-fold in both models. Conversely, islet expression of GPI-PLD was absent as determined by immunofluorescence. Insulin may regulate GPI-PLD expression, because insulin treatment of diabetic NOD mice corrected the hyperglycemia along with reducing serum GPI-PLD activity and liver mRNA. Our data demonstrate that serum GPI-PLD levels are altered in the diabetic state and are consistent with liver as a contributor to circulating GPI-PLD.

Iron overload diminishes atherosclerosis in apoE-deficient mice

It has been proposed that elevated levels of tissue iron increase the risk for atherosclerosis, perhaps by favoring the formation of pro-atherogenic oxidized LDL. Working with apoE-deficient (apoE(-/-)) mice, which do not require a high-fat diet to develop atherosclerosis, we compared the effects of standard diet (0.02% iron) or a 2% carbonyl iron diet. After 24 weeks, mice fed the 2% carbonyl iron diet had twice as much iron in their plasma, a ninefold increase in bleomycin-detectable free iron in their plasma, and ten times as much iron in their livers as control mice. Dietary iron overload caused a modest (30%) rise in plasma triglyceride and cholesterol. Nevertheless, this regimen did not exacerbate, but rather reduced the severity of atherosclerosis by 50%, and it failed to elevate hepatic levels of heme oxygenase mRNA, which is induced by many different oxidative insults in vitro. Moreover, hepatic levels of protein-bound dityrosine and ortho-tyrosine, two markers of metal-catalyzed oxidative damage in vitro, failed to rise in iron-overloaded animals. Our observations suggest that elevated serum and tissue levels of iron are not atherogenic in apoE(-/-) mice. Moreover, they call into question the hypothesis that elevated levels of tissue iron promote LDL oxidation and oxidative stress in vivo.

Validation of whole-body magnetic resonance spectroscopy as a tool to assess murine body composition

OBJECTIVE

To evaluate proton magnetic resonance spectroscopy (MRS) as a tool for the non-invasive assessment of murine body composition.

DESIGN

Twenty C57/BL6 male mice with a wide range of body adiposities underwent both pre- and post-mortem whole-body MRS to assess body composition. MRS measures were compared to the results obtained by chemical carcass analysis, the current 'gold standard' for determination of body composition.

MEASUREMENTS

Areas under the curve (AUC) for lipid and water peaks of whole body MRS spectra (AUClipid and AUCH2O, respectively) were used to determine percentages of body fat (%FATMRS) and fat free mass by MRS (%FFMMRS). Total body fat, total body water, fat free mass, and total lean mass were determined by chloroform/methanol extraction of lipid from dessicated whole carcass and compared to MRS measures (%FATMRS, %FFMMRS, AUClipid, and AUCH2O). The variability of the MRS technique was assessed by determining the coefficients of variation (COV) associated with %FATMRS, AUClipid, and AUCH2O for mice of three different adiposities.

RESULTS

%FATMRS in live mice was highly correlated with body fat percentage (r=0.994, P<0.001) and total body fat (r=0.980, P<0.001) derived from chemical carcass analysis over a broad range of adiposities (7-48% body fat content by carcass analysis). There was no difference in %FATMRS measured pre- vs post-mortem (r=1.00, P<0.001). AUClipid was highly correlated with chemically derived total fat mass (r=0.996, P<0.001) and body fat percentage (r=0.981, P<0.001), while %FFMMRS was strongly correlated to chemical determinations of percentage body water (r=0.994, P<0. 001), percentage fat free mass (r=0.993, P<0.001), and percentage lean mass (r=0.792, P<0.001). AUCH2O was strongly associated with carcass analysis determinations of total body water (r=0.964, P<0. 001), total fat free mass (r=0.953, P<0.001), and total lean mass (r=0.89, P<0.001). In mice of 6%, 12%, and 43% body fat, COVs determined for %FATMRS and AUClipid were less than 10%. The COVs for AUCH2O were less than 2%.

CONCLUSIONS

MRS provides precise, accurate, rapid, and non-invasive measures of body fat, body water, fat free mass, and lean mass in living mice with a broad range of adiposities.

Impaired superoxide production due to a deficiency in phagocyte NADPH oxidase fails to inhibit atherosclerosis in mice

Superoxide, the reduced form of molecular oxygen, has been implicated in the genesis of vascular disease. One potential mechanism involves oxidation of low density lipoprotein into an atherogenic particle. A second involves reaction with nitric oxide to generate peroxynitrite, a highly oxidizing intermediate. A third involves regulation of signal transduction in artery wall cells. One well-characterized pathway for superoxide production resides in macrophages, the cellular hallmark of the early atherosclerotic lesion. Macrophages contain a membrane-bound NADPH oxidase that reduces oxygen to superoxide. In the current studies, we used mice that are deficient in the gp91-phox subunit of the NADPH oxidase-a model of chronic granulomatous disease (CGD)-to explore the role of superoxide in atherosclerotic vascular disease. Wild-type and CGD mice on the C57BL/6 background received a high-fat diet for 20 weeks to induce hypercholesterolemia. At the end of this period, the 2 strains of mice had comparable plasma lipid levels, and their atherosclerotic lesions were similar in size. We also crossed CGD mice with apolipoprotein E-deficient (apoE-/-) mice to generate spontaneously hypercholesterolemic animals that lacked functional NADPH oxidase. After 24 weeks, the CGD-apoE-/- animals had lower plasma cholesterol and triglyceride levels than did the apoE-/- animals, but there was no difference in the extent of atherosclerotic plaque. Our findings suggest that superoxide generated by the NADPH oxidase of phagocytes does not promote atherosclerosis in mice with either diet-induced or genetic forms of hypercholesterolemia.

Plasma and vessel wall lipoprotein lipase have different roles in atherosclerosis

Lipoprotein lipase (LPL) is a key enzyme in lipoprotein metabolism, and has been hypothesized to exert either pro- or anti-atherogenic effects, depending on its localization. Decreased plasma LPL activity is associated with the high triglyceride (TG);-low HDL phenotype that is often observed in patients with premature vascular disease. In contrast, in the vessel wall, decreased LPL may be associated with less lipoprotein retention due to many potential mechanisms and, therefore, decreased foam cell formation. To directly assess this hypothesis, we have distinguished between the effects of variations in plasma and/or vessel wall LPL on atherosclerosis susceptibility in apoE-deficient mice. Reduced LPL in both plasma and vessel wall (LPL(+/-)E(-/-)) was associated with increased TG and increased total cholesterol (TC) compared with LPL(+/+)E(-/-) sibs. However despite their dyslipidemia, LPL(+/-)E(-/-) mice had significantly reduced lesion areas compared to the LPL(+/+)E(-/-) mice. Thus, decreased vessel wall LPL was associated with decreased lesion formation even in the presence of reduced plasma LPL activity. In contrast, transgenic mice with increased plasma LPL but with no increase in LPL expression in macrophages, and thus the vessel wall, had decreased TG and TC and significantly decreased lesion areas compared with LPL(+/+)E(-/-) mice. This demonstrates that increased plasma LPL activity alone, in the absence of an increase in vessel wall LPL, is associated with reduced susceptibility to atherosclerosis. Taken together, these results provide in vivo evidence that the contribution of LPL to atherogenesis is significantly influenced by the balance between vessel wall protein (pro-atherogenic) and plasma activity (anti-atherogenic).

Initiation of hyperinsulinemia and hyperleptinemia is diet dependent in C57BL/6 mice

C57BL/6 female mice were fed high fat diets containing different types of carbohydrate (sucrose or corn starch) and contents of cholesterol (0.03 % or 1 %) to identify early metabolic changes leading to increases in leptin levels and eventual insulin resistance. Under identical dietary fat conditions, type of carbohydrate and cholesterol content contributed to the timing of leptin increases. Mice fed a high-fat, high-sucrose diet showed early (4 weeks) and robust increases in circulating insulin and leptin levels (2-fold and 5-fold, respectively). In contrast, mice fed this diet with added cholesterol or with sucrose substituted by corn starch led to marked delays (8-10 weeks) in the elevations of insulin and leptin, although body weight gains were nearly identical among test diet groups. Thus, sucrose in combination with saturated fat played a specific role in initiating early metabolic changes associated with elevated leptin and insulin levels. Because leptin levels were most reflective of changes in insulin, our data support a role for insulin in determining plasma leptin levels in mice.

Increased glycogen synthase kinase-3 activity in diabetes- and obesity-prone C57BL/6J mice

Although the precise mechanisms contributing to insulin resistance and type 2 diabetes are unknown, it is believed that defects in downstream components of the insulin signaling pathway may be involved. In this work, we hypothesize that a serine/threonine kinase, glycogen synthase kinase-3 (GSK-3), may be pertinent in this regard. To test this hypothesis, we examined GSK-3 activity in two inbred mouse strains known to be susceptible (C57BL/6J) or resistant (A/J) to diet-induced obesity and diabetes. Examination of GSK-3 in fat, liver, and muscle tissues of C57BL/6J mice revealed that GSK-3 activity increased twofold in the epididymal fat tissue and remained unchanged in muscle and liver of mice fed a high-fat diet, compared with their low-fat diet-fed counterparts. In contrast, GSK-3 activity did not change in the epididymal fat tissue of A/J mice, regardless of the type of diet they were fed. In addition, both basal and diet-induced GSK-3 activity was higher (2.3- and 3.2-fold, respectively) in the adipose tissue of C57BL/6J mice compared with that in A/J mice. Taken together, our studies suggest an unsuspected link between increased GSK-3 activity and development of insulin resistance and type 2 diabetes in fat tissue of C57BL/6J mice, and implicate GSK-3 as a potential factor contributing to susceptibility of C57BL/6J mice to diet-induced diabetes.

Murine phospholipid hydroperoxide glutathione peroxidase: cDNA sequence, tissue expression, and mapping

Phospholipid hydroperoxide glutathione peroxidase (PHGPx), also known as glutathione peroxidase 4 (GPX4), is a 19-kDa, monomeric enzyme that protects cells from lipid peroxide-mediated damage by catalyzing the reduction of lipid peroxides. PHGPx is synthesized in two forms, as a 194-amino acid peptide that predominates in gonadal tissue and localizes to mitochondria, and as a 170-amino acid protein that predominates in most somatic tissues and localizes to the cytoplasm. With the rapid amplification of cDNA ends (RACE) procedure, an 876-bp PHGPx cDNA was amplified from mouse testis, and a 767-bp PHGPx cDNA was amplified from mouse heart. The cDNA sequences were identical except that the testis cDNA contained an additional 109 bp at its 5' end. With a partial cDNA with complete homology to both the testis and myocardial PHGPx cDNAs, the murine tissue distribution of PHGPx mRNA expression was determined by Northern blotting. Highest level of PHGPx expression was found in the testis, followed by the kidney, heart and skeletal muscle, liver, brain, lung, and spleen. Northern blotting performed with a cDNA specific for the longer PHGPx transcript demonstrated that this longer PHGPx transcript was present only in the testis. A 1.4-kb PHGPx genomic fragment was amplified from murine kidney DNA and used to map the PHGPx gene by linkage analysis of restriction fragment length variants (RFLVs). The murine PHGPx gene (Gpx4) was mapped to a region of murine Chromosome (Chr) 10, located 43 cM from the centromere, that is syntenic with the human locus, which is located at the terminus of the short arm of human Chr 19. This information may be valuable in characterizing the role of PHGPx in modulating susceptibility to lipid peroxide-mediated injury in inbred murine strains and for targeted disruption of the gene.

Dietary antioxidants inhibit development of fatty streak lesions in the LDL receptor-deficient mouse

Oxidized low density lipoprotein (LDL) promotes atherogenesis. Although pharmacological antioxidants such as probucol inhibit both LDL oxidation and atherosclerosis in hyperlipidemic animals, the effects of natural antioxidants such as vitamin E are inconclusive. To further determine the effects of supplemental dietary antioxidants in vivo, we evaluated whether combined dietary antioxidants (0.1% vitamin E, 0.5% beta-carotene, and 0.05% vitamin C) inhibit LDL oxidation and fatty streak lesion development in homozygous LDL receptor-null (LDLR-/-) mice fed a high-fat, high-cholesterol diet. An additional group of mice were fed black tea, which has been shown to inhibit LDL oxidation in vitro. After receiving a high-fat, high-cholesterol diet for 8 weeks, the combined antioxidant-supplemented (antioxidant) group (n=18), tea group (n=19), and control group (n=17) had equivalent plasma cholesterol levels. LDL oxidation, as measured by the lag phase of conjugated diene formation, was markedly inhibited in the antioxidant group compared with the tea or control groups [mean lag phases=143+/-7 (antioxidant), 100+/-5 (tea), and 84+/-4 (control) minutes; P<0.0001 antioxidant versus tea or control]. The cross-sectional surface area of fatty streak lesions in the aortic sinus was reduced by 60% in the antioxidant group compared with both the tea and control groups (P<0.0001 antioxidant versus tea or control). There was no difference in lesion area between tea and control groups. Although both LDL oxidation and atherosclerosis were significantly inhibited in the antioxidant group, no correlation between lag phase values and lesion size was observed among individual animals. Furthermore, black tea did not inhibit fatty streak development in LDLR-/- mice. These data suggest that combined natural dietary antioxidants inhibit both LDL oxidation and atherogenesis in animals with elevated LDL but that inhibition of LDL oxidation alone may not prevent the development of atherosclerosis.

Mouse glycosylphosphatidylinositol-specific phospholipase D (Gpld1) characterization

Glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) is an 110-kDa monomeric protein found in the circulation that is capable of degrading the GPI anchor utilized by dozens of cell-surface proteins in the presence of detergent. This protein is relatively abundant (5-10 microgram/ml in human serum), yet its sites of synthesis, gene structure, and overall function are unclear. It is our purpose to use the mouse system to determine its putative roles in lipid transport, pathogen control, and diabetes. We have isolated murine full-length cDNA for GPI-PLD from a pancreatic alpha cell library. The deduced amino acid sequence shows 74% homology to bovine and human GPI-PLD. There is a single structural gene (Gpld1) mapping to mouse Chromosome (Chr) 13, and among nine tissues, liver showed the greatest abundance of GPI-PLD mRNA. Genetic differences in serum GPI-PLD activity were seen among four mouse strains, and no correlation was seen between GPI-PLD activity and circulating levels of high density lipoproteins in these mice. This is the first report of map position and genetic regulation for Gpld1. This information will enable us to further study the expression and function of GPI-PLD in normal and pathological conditions.

Obesity and diabetes in TNF-alpha receptor- deficient mice

TNF-alpha may play a role in mediating insulin resistance associated with obesity. This concept is based on studies of obese rodents and humans, and cell culture models. TNF elicits cellular responses via two receptors called p55 and p75. Our purpose was to test the involvement of TNF in glucose homeostasis using mice lacking one or both TNF receptors. C57BL/6 mice lacking p55 (p55(-)/-), p75, (p75(-)/-), or both receptors (p55(-)/-p75(-)/-) were fed a high-fat diet to induce obesity. Marked fasting hyperinsulinemia was seen for p55(-)/-p75(-)/- males between 12 and 16 wk of feeding the high-fat diet. Insulin levels were four times greater than wild-type mice. In contrast, p55(-)/- and p75(-)/- mice exhibited insulin levels that were similar or reduced, respectively, as compared with wild-type mice. In addition, high-fat diet-fed p75(-)/- mice had the lowest body weights and leptin levels, and improved insulin sensitivity. Obese (db/db) mice, which are not responsive to leptin, were used to study the role of p55 in severe obesity. Male p55(-)/-db/db mice exhibited threefold higher insulin levels and twofold lower glucose levels at 20 wk of age than control db/db expressing p55. All db/db mice remained severely insulin resistant based on fasting plasma glucose and insulin levels, and glucose and insulin tolerance tests. Our data do not support the concept that TNF, acting via its receptors, is a major contributor to obesity-associated insulin resistance. In fact, data suggest that the two TNF receptors work in concert to protect against diabetes.

Dietary isoflavones reduce plasma cholesterol and atherosclerosis in C57BL/6 mice but not LDL receptor-deficient mice

Susceptibility to atherosclerosis is determined by a combination of genetic and environmental factors, including diet. Consumption of diets rich in soy protein has been claimed to protect against the development of atherosclerosis. Potential mechanisms include cholesterol lowering, inhibition of lipoprotein oxidation and inhibition of cell proliferation by soy proteins or isoflavones, such as genistein, that are present in soy. This study was designed to determine whether soy isoflavones confer protection against atherosclerosis in mice and whether they reduce serum cholesterol levels and lipoprotein oxidation. C57BL/6 and LDL receptor-deficient (LDLr-null) mice were fed soy protein-based, high fat diets with isoflavones present (IF+, 20.85 g/100 g protein, 0.027 g/100 g genistein, 0.009 g/100 g daidzein) or diets from which isoflavones, and possibly other components, had been extracted (IF-, 20.0 g/100 g protein, 0.002 g/100 g genistein, 0.001 g/100 g daidzein). Because LDLr-null mice develop extensive atherosclerosis and hypercholesterolemia after minimal time on a high fat diet, they were fed the diets for 6 wk, whereas C57BL/6 mice were fed the diets for 10 wk. Plasma cholesterol levels did not differ between LDLr-null mice fed IF- and those fed IF+, but were 30% lower in C57BL/6 mice fed the IF+ diet than in those fed the IF- diet. Susceptibility of LDL to oxidative modification, measured as the lag phase of conjugated diene formation in LDLr-null mice, was not altered by isoflavone consumption. All LDLr-null mice developed atherosclerosis, and the presence or deficiency of dietary isoflavones did not influence atherosclerotic lesion area. In contrast, atherosclerotic lesion area was significantly reduced in C57BL/6 mice fed IF+ compared with those fed IF-. Thus, this study demonstrates that although the isoflavone-containing diet resulted in a reduction in cholesterol levels in C57BL/6 mice, it had no effect on cholesterol levels or on susceptibility of LDL to oxidative modification in LDLr-null mice. Further, dietary isoflavones did not protect against the development of atherosclerosis in LDLr-null mice but did decrease atherosclerosis in C57BL/6 mice. These findings suggest that soy isoflavones might lower cholesterol levels by increasing LDL receptor activity, and the reduction in cholesterol may offer some protection against atherosclerosis.

C57BL/6 mice fed high fat diets as models for diabetes-accelerated atherosclerosis

Non-insulin-dependent diabetes mellitus (NIDDM) is a major risk factor for the development of atherosclerosis in humans. The development of an animal model that displays accelerated atherosclerosis associated with NIDDM will aid in elucidating the mechanisms that associate these disorders. C57BL/6 mice may provide such a model system. This strain becomes obese, hyperglycemic and insulin resistant when fed a high fat diet (diabetogenic diet) and is susceptible to atherosclerotic lesion development when fed a separate high fat diet containing cholesterol and bile acids (atherogenic diet). This report tests whether a diet commonly used to induce atherosclerosis also provokes a diabetic phenotype and whether a diet used to induce diabetes provokes the development of aortic fatty streak lesions. Mice of strains C57BL/6, C3H/He, BALB/c and seven recombinant inbred (RI) strains were fed an atherogenic diet for 14 weeks and glucose parameters were measured. No correlation was observed between atherosclerosis susceptibility and fasting insulin or glucose levels, or glucose clearance following short-term insulin or glucose treatment. Analysis of the RI strains suggested that multiple genes control these glucose metabolic parameters. Feeding the diabetogenic diet for 14 weeks to C57BL/6 mice induced obesity and diabetes and 2-fold increases in plasma lipoprotein concentrations. Also, small aortic sinus lipid deposits were observed in 40% of the mice. Thus, analysis of the diabetogenic diet fed C57BL/6 mouse may provide an important tool for further studies of diabetes accelerated vascular disease.

High-density lipoprotein-binding protein (HBP)/vigilin is expressed in human atherosclerotic lesions and colocalizes with apolipoprotein E

Accumulation of cholesteryl esters within cells of the arterial intima is a hallmark of atherosclerosis. A small number of proteins have been shown in vitro to be upregulated by cellular cholesterol loading, including apolipoprotein E (apoE) and the recently cloned HDL-binding protein (HBP), but only apoE has been shown to be upregulated in cholesterol-loaded cells in atherosclerosis. To determine whether HBP (also called vigilin) might be expressed in human atherosclerosis, immunohistochemistry and in situ hybridization were performed on coronary arteries of 18 patients. HBP/vigilin was detected on all endothelial cells. HBP/vigilin mRNA and protein also were detected on a subset of macrophages and occasionally on smooth muscle cells (SMC) in atherosclerotic plaques but were not detected on these cell types in nondiseased coronary intima. The majority of HBP/vigilin-expressing macrophages were foam cells, but HBP/vigilin expression also was detected rarely in nonfoam cell macrophages. Foam cell macrophage HBP/vigilin expression was present in 100% of atherosclerotic quadrants, and nonfoam cell macrophage HBP/vigilin expression was present in 6% of atherosclerotic quadrants. HBP/vigilin-expressing human plaque cells also expressed apoE. However, HBP/vigilin was detected in cardiac myocyte foam cells of an apoE-deficient mouse, demonstrating that HBP/vigilin expression can occur independently of apoE. These results suggest that in vivo HBP/vigilin expression is upregulated by intracellular cholesterol loading but also that other factors present in atherosclerotic plaques may upregulate HBP/vigilin. Although the exact function of HBP/vigilin is unknown, its expression in plaque macrophages suggests a role for this molecule in atherogenesis.

Relationship between lipoprotein lipase and high density lipoprotein cholesterol in mice: modulation by cholesteryl ester transfer protein and dietary status

Plasma lipoprotein lipase (LPL) activity correlates with high density lipoprotein (HDL) cholesterol levels in humans. However, in several mouse models created either through transgenesis or targeted inactivation of LPL, no significant changes in HDL cholesterol values have been evident. One possible explanation for this species difference could be the absence of plasma cholesteryl ester transfer protein (CETP) activity in mice. To explore this possibility and further investigate interactions between LPL and CETP modulating HDL cholesterol levels in vivo, we examined the relationship between LPL activity and HDL levels in mice expressing the simian CETP transgene, compared with littermates not carrying the CETP gene. On a chow diet, increasing LPL activity was associated with a trend towards increased HDL levels (51 +/- 29 vs. 31 +/- 4 mg/dL highest vs. lowest tertiles of LPL activity, P = 0.07) in mice expressing CETP, while no such effects were seen in the absence of CETP (65 +/- 12 vs. 61 +/- 15 mg/ dL). Furthermore, in the presence of CETP, a significant positive correlation between LPL activity and HDL cholesterol was evident (r = 0.15, P = 0.006), while in the absence of CETP no such correlation was detected (r = 0.15, P = 0.36), highlighting the interactions between LPL and CETP in vivo. When mice were challenged with a high fat, high carbohydrate diet, strong correlations between LPL activity and HDL cholesterol were seen in both the presence (r = 0.45, P = 0.03) and absence (r = 0.73, P < 0.001) of CETP. Therefore, under altered metabolic contexts, such as those induced by dietary challenge, the relation between LPL activity and HDL cholesterol may also become evident. Here we have shown that both genetic and environmental factors may modulate the association between LPL activity and HDL cholesterol, and provide explanations for the absence of any changes in HDL values in mice either transgenic or with targeted disruption of the LPL gene.

Accelerated atherosclerosis in mice lacking tumor necrosis factor receptor p55

TNF-alpha (TNF) is produced primarily from macrophages and promotes numerous inflammatory reactions associated with atherosclerosis including the induction of vascular adhesion molecules and the recruitment and proliferation of monocyte/macrophages. There are two receptors known to elicit TNF responses, termed p55 and p75. Since p55 is thought to play the primary role in inflammatory processes, we postulated that the absence of p55 in mice would protect against atherosclerosis. In contrast, C57BL/6 mice lacking p55 had aortic sinus lesion sizes 2.3-fold larger than C57BL/6 wild type mice when fed an atherogenic diet (37,123 +/- 3485 microm2 versus 16, 688 +/- 2887 microm2, respectively, p < 0.0004). Plasma lipid levels were not different between strains. A 3-fold increase in the uptake and degradation of acetylated low density lipoprotein for p55-null as compared with wild type mice was demonstrated in cultured peritoneal macrophages. Immunohistochemical staining for scavenger receptor protein in the aortic sinus was more intense in lesions from the p55-null mice as compared with wild type controls. Our results support the concept that increased scavenger receptor activity contributes to excessive fatty streak formation. We conclude that TNF p55 receptors protect against atherosclerotic lesion development in the mouse.

High levels of circulating beta-amyloid peptide do not cause cerebral beta-amyloidosis in transgenic mice

We have established transgenic mice that constitutively overproduce the signal sequence and the 99-amino-acid carboxyl-terminal region of the human beta-amyloid precursor protein. The transgenic mice strongly expressed the transgene in multiple tissues under the control of a cytomegalovirus enhancer/chick beta-actin promoter. There were exceptionally high levels of beta-amyloid peptides in the plasma (approximately 17 times or more compared with the human plasma level). Although some transgenic mice from one founder line developed amyloidosis in the intestine, no neuropathology was found in transgenic mice up to age 29 months. Given the absence of cerebral beta-amyloidosis despite extremely high levels of circulating beta-amyloid peptides in the transgenic mice, the results suggest that local cerebral metabolism of beta-amyloid precursor protein may play a predominant role in cerebral beta-amyloidosis in transgenic mice. Such transgenic mice may be useful for the investigation of the etiology of the disease and for the establishment of therapeutic strategies.

Expression of apolipoprotein E inhibits aggregation of the C-terminal fragments of beta-amyloid precursor protein

An important role of apolipoprotein E in the amyloidogenesis of Alzheimer's disease is suggested by an accumulation of apolipoprotein E in beta-amyloid plaques and a genetic association between Alzheimer's disease and one of the allelic variants (APOE4) of apolipoprotein E. Overexpression of a C-terminal region of beta-amyloid precursor protein brings about aggregation of the C-terminal fragments in COS cells. This COS cell culture system was used to study effects of apolipoprotein E on aggregation of the C-terminal fragments. When both apolipoprotein E and the C-terminal fragments were overexpressed in COS cells, Western blot analyses revealed significant inhibition of aggregation of the C-terminal fragments. No significant differences between apolipoprotein E3 and E4 in the inhibitory activities were found by this method. Apolipoprotein E may inhibit formation of amyloid fibrils.

Increased atherosclerosis in streptozotocin-induced diabetic mice

Premature and extensive atheroscleroses involving renal, peripheral, and cardiovascular sites remain major complications of diabetes mellitus. Controversy exists as to the contribution of hyperglycemia versus elevated local or systemic concentrations of insulin to atherosclerosis risk. In this report, we developed the first murine model susceptible to both atherosclerosis and diabetes to determine which diabetogenic factors contribute to vascular disease. C57BL/6 and BALB/c mice were treated with multiple low-dose streptozotocin (STZ) or control citrate buffer and fed rodent chow or an atherogenic-promoting (Ath) diet for 12-20 wk. STZ treatment resulted in sustained hyperglycemia (250-420 mg/dl) and a modest reduction in plasma insulin levels for both strains regardless of diet. Citrate-treated C57BL/6 mice fed the Ath diet showed extensive oil red O-staining fatty streak aortic sinus lesions (20,537+/-2,957 micron2), the size of which did not differ for Ath-fed mice treated with STZ (16,836+/-2,136 micron2). In contrast, hyperglycemic BALB/c mice fed the Ath diet showed a 17-fold increase in atherosclerotic lesion area (7,922+/-2,096 micron2) as compared with citrate-treated mice fed the Ath diet (467+/-318 micron2). Correlations between lesion size and plasma glucose levels were significant for BALB/c (r = 0.741, P < 0.009), but not C57BL/6 (r = 0.314, P<0.3) mice. Lesion size correlated significantly with plasma cholesterol for C57BL/6 (r = 0.612, P<0.03) but not BALB/c (r = 0.630, P<0.1) mice. Immunohistochemistry showed that aortic sinus lesions from both strains contained macrophages, but smooth muscle cells were clearly present in lesions of BALB/c mice. In summary, we present the first small animal model showing accelerated atherosclerosis in response to hyperglycemia. Fatty streaks resembled those of human type II lesions in that both macrophages and smooth muscle cells were evident. In addition, our results support the concept that hyperglycemia as opposed to hyperinsulinemia contributes heavily to risk of atherosclerosis.

Hyper- and hypo-responsiveness to dietary fat and cholesterol among inbred mice: searching for level and variability genes

A concept proposed by Berg (Berg, K. 1989. Arteriosclerosis. 9: I-50-I-58) is that a combination of level and variability genes determine an individual's overall plasma lipid levels and atherosclerotic risk. Our goal was to determine which inbred mouse strains could be used to identify candidate level and variability genes controlling lipid levels and atherosclerosis susceptibility. Nine common inbred mouse strains were examined for responsiveness with respect to plasma lipoprotein and tissue lipid levels upon feeding diets rich in cholesterol and fat. Marked quantitative variations were observed in plasma cholesterol and triglyceride levels among mice fed rodent chow and the high fat test diets. Mice of strains DBA/2 and AKR appeared to be hyporesponsive to diets containing high levels of fat and cholesterol as compared to rodent chow. In contrast, several strains were primarily hyperresponsive to either dietary fat or cholesterol, or both ingredients. Determination of cholesterol absorption for selected strains fed test diets suggested that decreased cholesterol absorption, in part, contributes to hyporesponsiveness as seen in DBA/2 mice. Levels of mRNA for cholesterol 7 alpha-hydroxylase were estimated and shown to vary markedly among strains. An inverse correlation was seen among strains between cholesterol 7 alpha-hydroxylase mRNA, and plasma and hepatic cholesterol levels for some diets. Thus, genes controlling cholesterol absorption and bile acid synthesis are candidates for further study as level and variability genes affecting plasma cholesterol levels. Overall, inbred mouse strains will prove useful for identifying genes controlling level and variability traits.

Alteration of lipid profiles in plasma of transgenic mice expressing human lipoprotein lipase

Lipoprotein lipase (LPL) is a key enzyme required for the hydrolysis of triglyceride-rich particles. To assess the effects of increased plasma LPL on lipoprotein levels, transgenic mice expressing human LPL (hLPL) were produced. Abundant hLPL transcripts were detected in RNA from different tissues of transgenic mice which resulted in an increase in post-heparin plasma LPL activity of approximately 154%. On rodent chow (p = 0.01) and after a 16-h fast (p = 0.001), plasma triglycerides in transgenic mice were decreased by approximately 50% as compared to littermate controls. Gel filtration chromatography showed a 2-3-fold decrease in very low density lipoprotein triglycerides and cholesterol enrichment of low density lipoprotein. Transgenic mice maintained on a high carbohydrate diet exhibited a 78% (p = 0.03) lowering of low density lipoprotein and very low density lipoprotein cholesterol levels, in addition to a 68% (p = 0.01) lowering of total to high density lipoprotein cholesterol (TC/HDL-C) ratios compared to controls. The distribution of apoA-I and A-II were similar in the transgenics and their non-transgenic littermates, while the apoE distribution was mildly altered in the plasma from the transgenic mice. These data demonstrate that moderate increases in total LPL activity are associated with significant changes in lipoprotein levels and altered composition of lipoprotein particles.

Regulation by nutritional status of lipids and apolipoproteins A-I, A-II, and A-IV in inbred mice

This study illustrates that genetic strain and feeding status can markedly influence tissue lipid concentrations and mRNA levels of apolipoprotein genes. C57BL/6 and BALB/c mice were maintained for 2 weeks on four test diets differing in amount of cholesterol and type of fat, and fasted for 4 h or 16 h prior to collection of tissues. For both strains, the primary effect of fasting from 4 h to 16 h was to paradoxically elevate triglyceride levels in plasma and liver, and to elevate hepatic apoA-IV mRNA levels. Triglyceride secretion rates, estimated after the injection of Triton WR-1339, suggested that elevations in plasma triglyceride levels were due to reduced clearance of very low density lipoproteins. Although plasma glucose levels decreased with fasting time for both strains, insulin levels decreased for BALB/c but not C57BL/6 mice regardless of diet. This suggests that factors thought to be mediated by insulin, (e.g., plasma free fatty acid concentrations; hepatic apoA-IV mRNA levels) may be influenced by local changes in insulin sensitivity, which are controlled genetically and are not reflected by plasma insulin levels. In summary, nutritional status influences a constellation of factors involved in lipid transport that also show strong genetic components and may influence subsequent analyses of gene expression in the mouse system.

Genetically determined body weight loss in mice fed diets containing salmon oil

Several reports describe adverse effects of dietary fish oil. We examined the influence of dietary salmon oil (138 g/kg diet) fed without or with 5 g supplemental cholesterol/kg diet on body weight and plasma lipid concentrations of inbred mice. Salmon oil contained 0.17 g naturally occurring cholesterol/kg diet. Mice used were BALB/c, C57BL/6 and seven recombinant inbred strains derived from BALB/c and C57BL/6 (CXB). Parental strains BALB/c and C57BL/6 maintained or gained body weight when fed both salmon oil diets. Mice of recombinant inbred strains showed weight gain except for CXB-E and -H mice. Although CXB-E mice lost approximately 12% of initial body weight after 10 d of consuming either salmon oil diet, no further reductions in body weight were seen. CXB-H mice maintained or gained weight when fed the salmon oil-high cholesterol diet but showed a steady decline in body weight (up to 30% of initial weight) while consuming the salmon oil-low cholesterol diet. The biochemical basis for weight loss in CXB-H mice was studied and results suggest effects of diet on satiety and/or lipid utilization. Because nonparental body weight phenotypes were observed among recombinant inbred strains, body weight response to salmon oil feeding is controlled by multiple genes.