Sodium Butyrate Reduces Brain Amyloid-β Levels and Improves Cognitive Memory Performance in an Alzheimer's Disease Transgenic Mouse Model at an Early Disease Stage

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by progressive cognitive decline and neuropathological features, including abnormal deposition of amyloid-β (Aβ) peptides, intracellular neurofibrillary tangles, and neuronal death. Identifying therapeutics which can reduce memory deficits at an early stage of the disease has the advantage of slowing or even reversing disease progression before irreversible brain damage has occurred. Consequently, in this study, we investigated the ability of the histone deacetylase inhibitor sodium butyrate (NaB) to attenuate memory deficits in the 5xFAD mouse model of AD following a 12-week feeding regimen. 5xFAD mice demonstrate a unique time course of Aβ pathology, developing Aβ plaques as early as 2 months. Male mice were assigned to either a control diet or a NaB-supplemented diet which was administered at either 5 mg/kg/day, or 15 mg/kg/day for 12 weeks (each group, N = 15). Supplementation commenced at an early disease stage (8-10 weeks of age). Behavioral testing (contextual and cued fear conditioning) was undertaken, and brain Aβ levels measured, at the end of the 12-week intervention. NaB had profound effects on Aβ levels and on associative learning and cognitive functioning. A 40% reduction in brain Aβ levels and a 25% increase in fear response in both the cued and contextual testing was observed in the NaB-treated animals compared to the control group. These findings suggest that NaB warrants further investigation as a potential therapeutic agent in the treatment of cognitive deficits associated with early stages of AD.

Serum high-density lipoprotein is associated with better cognitive function in a cross-sectional study of aging women

Purpose/Aim of the study: Poor cardiovascular health, including obesity and altered lipid profiles at mid-life, are linked to increased risk of Alzheimer's disease (AD). The biological mechanisms linking cardiovascular health and cognitive function are unclear though are likely to be multifactorial. This study examined the association between various lipoproteins and cognitive functioning in ageing women.

MATERIALS AND METHODS

We investigated the relationship between readily available biomarkers (i.e. serum lipoprotein) and cognitive decline in domains associated with increased risk of AD (e.g. episodic verbal memory performance and subjective memory complaint). We report cross-sectional data investigating the relationship between serum total cholesterol, triglycerides, high-density lipoprotein (HDL-C) and low-density lipoprotein with verbal memory and learning ability in 130 women with and without memory complaints (n = 71 and 59, respectively) drawn from a study investigating cognitively healthy Western Australians (average age 62.5 years old).

RESULTS

After statistical modelling that controlled for the effects of age, depression and apolipoprotein E genotype, HDL-C was significantly associated with better verbal learning and memory performance, specifically short and long delay-free recalls (F = 3.062; p < .05 and F = 3.2670; p < .05, respectively).

CONCLUSION

Our cross-sectional findings suggest that the positive effect of HDL-C on verbal memory may be present much earlier than previously reported and provide further support for the role of HDL-C in healthy brain ageing. Further exploration of the protective effect of HDL-C on cognitive function in ageing is warranted through follow-up, longitudinal studies.

Zinc affects the proteolytic stability of Apolipoprotein E in an isoform-dependent way

The pathological role of zinc in Alzheimer's disease (AD) is not yet fully elucidated, but there is strong evidence that zinc homeostasis is impaired in the AD brain and that this contributes to disease pathogenesis. In this study we examined the effects of zinc on the proteolysis of synthetic Apolipoprotein E (ApoE), a protein whose allelic variants differentially contribute to the onset/progression of disease. We have demonstrated that zinc promotes the proteolysis (using plasma kallikrein, thrombin and chymotrypsin) of synthetic ApoE in an isoform-specific way (E4>E2 and E3), resulting in more ApoE fragments, particularly for ApoE4. In the absence of exogenous proteases there was no effect of metal modulation on either lipidated or non-lipidated ApoE isoforms. Thus, increased zinc in the complex milieu of the ageing and AD brain could reduce the level of normal full-length ApoE and increase other forms that are involved in neurodegeneration. We further examined human plasma samples from people with different ApoE genotypes. Consistent with previous studies, plasma ApoE levels varied according to different genotypes, with ApoE2 carriers showing the highest total ApoE levels and ApoE4 carriers the lowest. The levels of plasma ApoE were not affected by either the addition of exogenous metals (copper, zinc or iron) or by chelation. Taken together, our study reveals that zinc may contribute to the pathogenesis of AD by affecting the proteolysis of ApoE, which to some extent explains why APOE4 carriers are more susceptible to AD.

Bone mineral density, adiposity, and cognitive functions

Cognitive decline and dementia due to Alzheimer's disease (AD) have been associated with genetic, lifestyle, and environmental factors. A number of potentially modifiable risk factors should be taken into account when preventive or ameliorative interventions targeting dementia and its preclinical stages are investigated. Bone mineral density (BMD) and body composition are two such potentially modifiable risk factors, and their association with cognitive decline was investigated in this study. 164 participants, aged 34–87 years old (62.78 ± 9.27), were recruited for this longitudinal study and underwent cognitive and clinical examinations at baseline and after 3 years. Blood samples were collected for apolipoprotein E (APOE) genotyping and dual energy x-ray absorptiometry (DXA) was conducted at the same day as cognitive assessment. Using hierarchical regression analysis, we found that BMD and lean body mass, as measured using DXA were significant predictors of episodic memory. Age, gender, APOE status, and premorbid IQ were controlled for. Specifically, the List A learning from California Verbal Learning Test was significantly associated with BMD and lean mass both at baseline and at follow up assessment. Our findings indicate that there is a significant association between BMD and lean body mass and episodic verbal learning. While the involvement of modifiable lifestyle factors in human cognitive function has been examined in different studies, there is a need for further research to understand the potential underlying mechanisms.

Evaluation of Ion-exchange Microspheres as Carriers for the Anticancer Drug Doxorubicin: In-vitro Studies

A comparison study of doxorubicin loading, release characteristics and stability within sodium and hydrogen forms of ion-exchange resin microspheres has been performed. It was demonstrated that resins in the Na+ form, although having lower drug loading capacity, showed similar release profiles to resins in the H+ form but still maintain all the drug activity. Resins in the H+ form, despite having high drug loading capacity, caused drug degradation within microspheres due to their strong acidic nature. Therefore, in comparison with the H+ form, resins in the Na+ form can be considered as better carriers for doxorubicin in terms of sustaining the release of drug and maintaining drug activity. Other factors such as the degree of resin cross-linkage and drug/resin mixing time have also been examined in relation to drug loading and release characteristics. Overall, this study demonstrated the significance of the characteristics of matrix materials and their influence on the drug activity and microsphere performance in-vitro.

APOE genotype results in differential effects on the peripheral clearance of amyloid-beta42 in APOE knock-in and knock-out mice

The epsilon4 allele of apolipoprotein E (APOE) is currently the major genetic risk factor identified for Alzheimer's disease (AD). Previous in vivo data from our laboratory has demonstrated that amyloid-beta (Abeta) is rapidly removed from the plasma by the liver and kidney and that the rate of its clearance is affected by ApoE in C57BL/6J and APOE-/- mice. To expand upon these findings, we assessed the peripheral clearance of human synthetic Abeta42 in APOE epsilon2, epsilon3, and epsilon4 knock-in and APOE knock-out mice injected with lipidated recombinant apoE2, E3, and E4 protein. Our results show that APOE does influence the rate at which the mice are able to clear Abeta42 from their bloodstream. Both APOE epsilon4 mice and APOE knock-out mice treated with lipidated recombinant apoE4 demonstrated increased retention of plasma Abeta42 over time compared to APOE epsilon2/APOE knock-out rE2 and APOE epsilon3/APOE knock-out rE3 mice. These findings suggest that the peripheral clearance of Abeta42 is significantly altered by APOE genotype. Given that APOE epsilon4 is a risk factor for AD, then these novel findings provide some insight into the role of ApoE isoforms on the peripheral clearance of Abeta which may impact on clearance from the brain.

Functional effects of genetic polymorphism in inflammatory genes in subjective memory complainers

A number of genetic risk factors have been identified for Alzheimer's disease (AD) including genes involved in the inflammatory response (interleukin 1A, [IL-1α (-889)], interleukin 1B (IL-1β [+3953]), and tumor necrosis factor (TNF [-308 and -850]). We investigated the prevalence and functional consequences (baseline cognitive performance, plasma cytokine levels) of possession of these putative genetic risk factors within a group of subjective memory complainers (SMC, n = 226) and age and sex matched noncomplainers (NMC, n = 167). We observed no effect of any of the genetic factors investigated on cognitive performance. Further, there was no difference in the frequency of the disease-associated alleles, or cytokine levels between subjective memory complainers and noncomplainer participants. There was no relationship between TNF polymorphisms and TNF levels. There was a significant increase in plasma IL-1β levels in those homozygous for the disease-associated allele (i.e., IL-1β +3953 TT). Follow-up longitudinal assessments on this cohort will provide insight as to how these polymorphisms may affect the risk of cognitive decline over time.

Profiling brain and plasma lipids in human APOE epsilon2, epsilon3, and epsilon4 knock-in mice using electrospray ionization mass spectrometry

It is known that apolipoprotein E (ApoE) is essential for normal lipid metabolism. ApoE is the major apolipoprotein in the central nervous system and plays a key role in neurobiology by mediating the transport of cholesterol, phospholipids, and sulfatides. We therefore examined APOE epsilon2, epsilon3, and epsilon4 knock-in mice, using electrospray ionization mass spectrometry to determine if APOE genotype or age leads to altered levels in the brain of a number of glycerophospholipids (phosphatidylinositol, PI; phosphatidylethanolamine, PE; phosphatidic acid, PA, phosphatidylserine, PS; phosphatidylcholine, PC), sphingolipids (sphingomyelin, SM; ceramide, Cer), cholesterol, and triacylglycerols. We observed slight changes within individual PI, PE, PC, Cer, and SM lipid levels in APOE epsilon2 and epsilon4 mice compared to APOE epsilon3 mice. However, overall, we did not observe any major effects in APOE epsilon4 knock-in mice for the levels of the glycerophospholipids measured, as compared to APOE epsilon2 and epsilon3 mice. Our findings indicate that variations in ApoE isoforms do not per se affect bulk lipid homeostasis in the brain. These findings indicate that APOE epsilon4 is not associated with disturbances in brain sterol or sphingolipids in the absence of environmental factors.

Novel phage peptides attenuate beta amyloid-42 catalysed hydrogen peroxide production and associated neurotoxicity

Amyloid-beta (Abeta) peptides play a central role in the pathogenesis of Alzheimer's disease. There is accumulating evidence that supports the notion that the toxicity associated with human Abeta (both 40 and 42) is dependent on its superoxide dismutase (SOD)-like activity. We developed a novel screening method involving phage display technology to identify novel peptides capable of inhibiting Abeta's neurotoxicity. Two random peptide libraries containing 6-mer and 15-mer peptide inserts were used and resulted in the identification of 25 peptides that bound human Abeta (40 or 42). Here, we show that two of the three most enriched peptides obtained significantly reduced Abeta42's SOD-like activity. A 15-mer peptide reduced Abeta42 neurotoxicity in a dose-dependent manner as evidenced by a reduction in LDH release. These findings were confirmed in the independent MTT assay. Furthermore, comparative analysis of the 15-mer peptide with Clioquinol, a known inhibitor of Abeta's metal-mediated redox activity, showed the 15-mer peptide to be equipotent to this metal chelator, under the same experimental conditions. These agents represent novel peptides that selectively target and neutralise Abeta-induced neurotoxicity and thus provide promising leads for rational drug development.

Cholesterol metabolism and transport in the pathogenesis of Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disorder, affecting millions of people worldwide. Apart from age, the major risk factor identified so far for the sporadic form of AD is possession of the epsilon4 allele of apolipoprotein E (APOE), which is also a risk factor for coronary artery disease (CAD). Other apolipoproteins known to play an important role in CAD such as apolipoprotein B are now gaining attention for their role in AD as well. AD and CAD share other risk factors, such as altered cholesterol levels, particularly high levels of low density lipoproteins together with low levels of high density lipoproteins. Statins--drugs that have been used to lower cholesterol levels in CAD, have been shown to protect against AD, although the protective mechanism(s) involved are still under debate. Enzymatic production of the beta amyloid peptide, the peptide thought to play a major role in AD pathogenesis, is affected by membrane cholesterol levels. In addition, polymorphisms in several proteins and enzymes involved in cholesterol and lipoprotein transport and metabolism have been linked to risk of AD. Taken together, these findings provide strong evidence that changes in cholesterol metabolism are intimately involved in AD pathogenic processes. This paper reviews cholesterol metabolism and transport, as well as those aspects of cholesterol metabolism that have been linked with AD.

Association of cardiovascular factors and Alzheimer's disease plasma amyloid-beta protein in subjective memory complainers

A strong link is indicated between cardiovascular disease (CVD) and risk for developing Alzheimer's disease (AD), which may be exacerbated by the major AD genetic risk factor apolipoprotein Eepsilon4 (APOEepsilon4). Since subjective memory complaint (SMC) may potentially be an early indicator for cognitive decline, we examined CVD risk factors in a cohort of SMC. As amyloid-beta (Abeta) is considered to play a central role in AD, we hypothesized that the CVD risk profile (increased LDL, reduced HDL, and increased body fat) would be associated with plasma Abeta levels. We explored this in 198 individuals with and without SMC (average age = 63 years). Correlations between Abeta40 and HDL were observed, which were stronger in non-APOEepsilon4 carriers (rho = -0.315, p < 0.001) and in SMC (rho = -0.322, p = 0.01). There was no relationship between percentage body fat and Abeta40 in this cohort. Age and HDL remained predictive for plasma Abeta40 using multivariate regression analysis. We report a novel negative association between HDL and Abeta, which if demonstrated to be causal has implications for the development of lifestyle interventions and/or novel therapeutics. The relationship between HDL and Abeta and the potential significance of such an association needs to be validated in a larger longitudinal study.

Apolipoprotein E, cholesterol metabolism, diabetes, and the convergence of risk factors for Alzheimer's disease and cardiovascular disease

High fat diets and sedentary lifestyles are becoming major concerns for Western countries. They have led to a growing incidence of obesity, dyslipidemia, high blood pressure, and a condition known as the insulin-resistance syndrome or metabolic syndrome. These health conditions are well known to develop along with, or be precursors to atherosclerosis, cardiovascular disease, and diabetes. Recent studies have found that most of these disorders can also be linked to an increased risk of Alzheimer's disease (AD). To complicate matters, possession of one or more apolipoprotein E epsilon4 (APOE epsilon4) alleles further increases the risk or severity of many of these conditions, including AD. ApoE has roles in cholesterol metabolism and Abeta clearance, both of which are thought to be significant in AD pathogenesis. The apparent inadequacies of ApoE epsilon4 in these roles may explain the increased risk of AD in subjects carrying one or more APOE epsilon4 alleles. This review describes some of the physiological and biochemical changes that the above conditions cause, and how they are related to the risk of AD. A diversity of topics is covered, including cholesterol metabolism, glucose regulation, diabetes, insulin, ApoE function, amyloid precursor protein metabolism, and in particular their relevance to AD. It can be seen that abnormal lipid, cholesterol and glucose metabolism are consistently indicated as central in the pathophysiology, and possibly the pathogenesis of AD. As diagnosis of mild cognitive impairment and early AD are becoming more reliable, and as evidence is accumulating that health conditions such as diabetes, obesity, and coronary artery disease are risk factors for AD, appropriate changes to diets and lifestyles will likely reduce AD risk, and also improve the prognosis for people already suffering from such conditions.

Does pravastatin increase chylomicron remnant catabolism in postmenopausal women with type 2 diabetes mellitus?

OBJECTIVE

We investigated the effects of pravastatin on chylomicron remnant catabolism measured with a 13C stable isotope breath test and plasma apolipoprotein (apo) B-48 and remnant-like particle (RLP)-cholesterol in postmenopausal women with type 2 diabetes mellitus.

PATIENTS AND MEASUREMENTS

Nineteen postmenopausal women with type 2 diabetes were randomized to receive 40 mg/day pravastatin or no treatment for 6 weeks followed by a 2-week washout period, and crossed over for a further 6 weeks. Fractional catabolic rate (FCR) of a chylomicron remnant-like emulsion was determined from 13CO2 enrichment in the breath and plasma using isotope-ratio mass spectrometry and multicompartmental modelling. Plasma apo B-48 and RLP-cholesterol concentrations were also measured as static markers of chylomicron remnant metabolism.

RESULTS

Pravastatin significantly reduced plasma concentrations of cholesterol (5.9 +/- 0.3 vs. 4.8 +/- 0.2 mmol/l; P < 0.001), low density lipoprotein (LDL)-cholesterol (3.5 +/- 0.2 vs. 2.6 +/- 0.2 mmol/l; P < 0.001), triglyceride (2.1 +/- 0.3 vs. 1.7 +/- 0.2 mmol/l; P = 0.017), non-high density lipoprotein (HDL)-cholesterol (4.4 +/- 0.3 vs. 3.3 +/- 0.2 mmol/l; P < 0.001), lathosterol/total cholesterol ratio (2.6 +/- 0.2 vs. 2.0 +/- 0.3, P = 0.035), apo B-100 (1.1 +/- 0.1 vs. 0.8 +/- 0.1 g/l; P = 0.001), apo B-48 (4.8 +/- 0.9 vs. 3.3 +/- 0.6 mg/l; P = 0.016), and RLP-cholesterol (31.4 +/- 8.2 vs. 18.6 +/- 4.6 mg/dl; P = 0.024). Pravastatin was also associated with an increase in sitosterol/total cholesterol ratio (2.8 +/- 0.3 vs. 3.1 +/- 0.3, P = 0.029). Chylomicron remnant-like emulsion catabolism was not, however, significantly altered by pravastatin estimated by either breath or plasma clearance measurements.

CONCLUSIONS

In postmenopausal women, pravastatin decreases plasma concentrations of remnant lipoproteins by a mechanism that may relate chiefly to inhibition of remnant production, but this requires further evaluation.

Alzheimer's disease amyloid-beta peptide modulates apolipoprotein E isoform specific receptor binding

The major protein component of the extracellular deposits in Alzheimer's disease (AD) is a 4 kDa peptide termed amyloid-beta (Abeta). This peptide is known to bind apolipoprotein E (apoE), a key mediator of lipoprotein transport, in an isoform specific manner. Whilst these isoform specific effects on apoE are well recognized, the functional significance of this interaction is poorly understood. Here, we investigated the influence of Abeta on apoE-mediated lipoprotein binding to cells using fluorescently tagged lipoprotein-like emulsions. Using this approach, we demonstrate that Abeta enhanced the normally poor binding of apoE2 lipoprotein-like particles to fibroblasts in culture, whilst markedly reducing the binding of apoE3 and apoE4. This suggests that the action of apoE isoforms on cellular lipoprotein or cholesterol metabolism is differentially modulated by Abeta. This also suggests that Abeta may also compromise apoE function in the Alzheimer disease affected brain.

Obesity and post-prandial lipid metabolism. Feast or famine?

Both in Western countries and in third world countries there is an increasing incidence of obesity. Obesity per se or insulin resistance associated with obesity may increase cardiovascular risk factors including dyslipidemia, hypertension and Type 2 diabetes. Over the past decade the understanding has increased of specific mediators in the hypothalamus that are involved in regulating food intake and body weight. In obese humans fasting plasma lipids can be normal but postprandial lipid metabolism is abnormal with an accumulation of triglyceride-rich remnant lipoproteins. In viscerally obese men chylomicron remnant catabolism was markedly decreased when compared with lean individuals. The decreased clearance of chylomicron remnants in viscerally obese subjects may be explained by competition between chylomicron remnants and the increased hepatic production of VLDL for clearance by low density lipoprotein receptors. Increased food intake in rodent models of obesity was shown to be associated with a delay in the catabolism of remnant lipoprotein particles. Prevention of hyperphagia was found to correct the impairment in the metabolism of remnant lipoproteins. Under fasting and food restricted conditions the improvement of remnant metabolism was associated with an increased oxidation of remnant lipids as determined by a novel stable isotope breath test. Anti-obesity and lipid lowering drugs have been used for the treatment of obesity. Inhibitors of cholesterol synthesis inhibitors (statins) have been shown to be effective in treating dyslipidemia. Inhibition of cholesterol synthesis with Atorvastatin was shown to improve chylomicron metabolism by increasing chylomicron remnant catabolism in obese subjects as assessed by the newly developed stable isotope breath test.

Mutation screening of the N-myc downstream-regulated gene 1 (NDRG1) in patients with Charcot-Marie-Tooth Disease

In a previous study, we have shown that N-myc downstream-regulated gene 1 (NDRG1), classified in databases as a tumor suppressor and heavy metal-response protein, is mutated in hereditary motor and sensory neuropathy Lom (HMSNL), a severe autosomal recessive form of Charcot-Marie-Tooth (CMT) disease. The private founder mutation R148X, causing HMSNL in patients of Romani ethnicity, has so far remained the only molecular defect linking NDRG1 to a specific disease phenotype. Here we report the first study aiming to assess the overall contribution of this gene to the pathogenesis of peripheral neuropathies, in cases where the most common causes of CMT disease have been excluded. Sequence analysis of NDRG1 in 104 CMT patients of diverse ethnicity identified one novel disease-causing mutation, IVS8-1G>A (g.2290787G>A), which affects the splice-acceptor site of IVS8 and results in the skipping of exon 9. The phenotype of the IVS8-1G>A homozygote was very closely related to that of HMSNL patients. In addition, we have detected homozygosity for the known R148X mutation in two affected individuals. Mutations in NDRG1 thus accounted for 2.88% of our overall group of patients, and for 4.68% of cases with demyelinating neuropathies. No other variants were identified in the coding sequence, whereas 12 single nucleotide polymorphisms were observed in the introns. Hum Mutat 22:129-135, 2003.

Chylomicron remnant metabolism studied with a new breath test in postmenopausal women with and without type 2 diabetes mellitus

OBJECTIVES

The kinetic basis for the effect of type 2 diabetes mellitus (DM) on postprandial lipoproteins has not been fully established. We investigated chylomicron remnant metabolism using a stable isotope breath test and fasting measurements of plasma apolipoprotein (apo) B-48 and apoC-III concentrations in postmenopausal women with and without type 2 DM.

PATIENTS

Twenty-four postmenopausal women without DM and 14 postmenopausal women with diet-controlled DM of similar age and body mass index (BMI) were studied in the postabsorptive state.

METHODS

The fractional catabolic rate (FCR) of an intravenously injected chylomicron remnant-like emulsion was determined from the appearance of 13CO2 in the breath using isotope-ratio mass spectrometry and multicompartmental modelling. apoB-48, a marker of particle number of intestinal lipoproteins, was determined immunoelectrophoretically. apoC-III was measured by immunoturbidimetric assay.

RESULTS

Compared with the nondiabetic women, the women with DM had significantly higher plasma apoB-48 concentration (16.40 +/- 1.18 mg/l vs. 13.0 +/- 0.9 mg/l; mean +/- standard error mean; P = 0.021), higher plasma apoC-III concentration (204.24 +/- 15.18 mg/l vs. 170.74 +/- 10.75 mg/l; P = 0.042) and lower FCR of the chylomicron remnant-like emulsion (0.06 +/- 0.05 pools/h vs. 0.12 +/- 0.02 pools/h; P < 0.001). In the diabetic patients, the FCR of the emulsion was correlated significantly with plasma apoB-48 levels (r = -0.641, P = 0.007) but not with apoC-III levels.

CONCLUSIONS

In postmenopausal women, diabetes mellitus appears to decrease the catabolism of chylomicron remnants and result in an accumulation of these particles in plasma. This may chiefly be due to decreased clearance by hepatic receptors related to an effect of insulin resistance. Impairment in the catabolism of chylomicron remnants may contribute to increased risk of atherosclerosis in postmenopausal women with type 2 diabetes mellitus.

Apolipoprotein E influences amyloid-beta clearance from the murine periphery

The relationship between amyloid-beta protein (Abeta) metabolism and Alzheimer's disease is currently poorly understood. While it is well known that the generation of Abeta results from enzymatic cleavage of its parent molecule, the amyloid beta protein precursor (AbetaPP), there is little information available regarding its in vivo clearance. The E4 isoform of apolipoprotein E (apoE) has been associated with poor clearance of Abeta under in vitro conditions. This is thought to be due to its poor ability to bind Abeta compared with the other common isoforms, apoE2 and apoE3. Although cell culture studies support the notion that Abeta clearance depends upon apoE isoform, validation of these findings requires Abeta clearance studies in vivo. In this study, we examined the clearance of Abeta in vivo from the periphery in mice that expressed apoE (C57BL/6J) or lacked apoE (APOE knockout). We measured the clearance of peripherally injected Abeta over time and additionally, the quantities sequestered by peripheral organs. Western blot analysis of the murine plasma indicated that the half-life of Abeta in the periphery was approximately 15 minutes. The livers of the C57BL/6J mice were found to have sequestered approximately 40% of the total injected Abeta at 90 minutes post-injection, whilst their kidneys contained 5% of the total injected Abeta. In contrast, the livers and kidneys of the APOE knockout animals were found to contain no detectable Abeta. These findings indicate that Abeta is rapidly removed from the plasma by murine peripheral tissues and the rate of its clearance is affected by apoE.

Effect of Simvastatin on markers of triglyceride-rich lipoproteins in familial hypercholesterolaemia

BACKGROUND

We have previously shown elevated fasting plasma concentrations of intestinal remnants, as reflected by apolipoprotein (apo) B-48 and remnant-like particle-cholesterol (RLP-C) in patients with heterozygous familial hypercholesterolaemia (FH). We now investigate the effect of an HMG-CoA reductase inhibitor (simvastatin) on chylomicron remnant metabolism using the measurement of fasting apoB-48 and RLP-C in FH patients after long- and short-term simvastatin therapy and after a wash-out period. We also piloted the response of a breath test, involving the measurement of the fractional catabolic rate (FCR) of an intravenously injected chylomicron remnant-like emulsion labeled with cholesteryl (13)C-oleate.

METHODS

Fifteen FH patients were studied after > 6 months 40 mg day(-1) simvastatin treatment (long-term), a wash-out period (4 weeks), and 4 weeks of simvastatin treatment (short-term). Apolipoprotein B-48 was determined by SDS-PAGE and Western blotting/enhanced chemiluminescence and RLP-C by an immunoseparation assay. The FCR of the chylomicron remnant-like emulsion was determined from the appearance of (13)CO(2) in the breath and by multicompartmental mathematical modelling.

RESULTS

Both long- and short-term treatment with simvastatin were associated with decreases in the plasma concentration of apoB-48 (P < 0.05) and RLP-C (P < 0.001), but there was no significant change in the FCR of the emulsion.

CONCLUSIONS

We suggest that long- and short-term treatments with simvastatin have comparable effects in decreasing the plasma concentration of triglyceride-rich remnants in heterozygous FH, as measured by fasting apoB-48 and RLP-C. The mechanisms for this may involve decreased production of hepatic and possibly intestinal lipoproteins, and/or up-regulation of hepatic receptor clearance pathways, but these changes are apparently not associated with a change in remnant clearance as measured kinetically by the (13)CO(2) breath test.

Effect of atorvastatin on chylomicron remnant metabolism in visceral obesity: a study employing a new stable isotope breath test

Elevated plasma concentration of chylomicron remnants may be causally related to atherosclerosis in obesity. We examined the effect of atorvastatin on chylomicron remnant metabolism in 25 obese men with dyslipidaemia. A remnant-like emulsion labeled with cholesteryl [(13)C]oleate was injected intravenously into patients; the fractional catabolic rate (FCR) of the remnant-like emulsion was determined by measurement of (13)CO(2) in the breath and analyzed using compartmental modelling. Compared with placebo, atorvastatin significantly decreased the plasma concentrations of total cholesterol, triglycerides, LDL cholesterol, apolipoprotein B (apoB), and lathosterol (P < 0.001). ApoB-48 and remnant-like particle-cholesterol (RLP-C) both decreased significantly by 23% (P = 0.002) and 33% (P = 0.045), respectively. The FCR of the remnant-like emulsion increased significantly from 0.054 +/- 0.008 to 0.090 +/- 0.010 pools/h (P = 0.002). The decrease in RLP-C was associated with the decrease in plasma triglycerides (r = 0.750, P = 0.003). Furthermore, the change in FCR of remnant-like emulsions was inversely associated with the change in LDL-C (r = -0.575, P = 0.040), suggesting removal of LDL and chylomicron remnants by similar hepatic receptor pathways. We conclude that in obese subjects, inhibition of cholesterol synthesis with atorvastatin decreases the plasma concentrations of both LDL-C and triglyceride-rich remnants and that this may be partially due to an enhancement in hepatic clearance of these lipoproteins.

Food restriction normalizes chylomicron remnant metabolism in murine models of obesity as assessed by a novel stable isotope breath test

Evidence is increasing that defective metabolism of postprandial remnants of triglyceride-rich lipoproteins contributes to atherogenesis. In obesity, postprandial lipemia is increased by mechanisms that are not currently established. In the present study, a recently developed (13)CO(2) breath test was used to assess the metabolism of chylomicron remnants (CR) in obese mice. Six murine obese models ob/ob, fat/fat, New Zealand Obese (NZO), db/db, gold thioglucose (GTG)-treated and agouti (A(y)) were studied. All obese mice were hyperphagic and their breath test metabolism was markedly impaired (P < 0.01) compared with control, nonobese mice. The breath test was also impaired (P < 0.01) in all obese mice except A(y) mice after 24-h food deprivation. However, after restriction to the food intake of paired control mice for 6 wk, the breath test in all obese mice improved to values of control, nonobese mice. The obese NZO, fat/fat and ob/ob mice had significant (P < 0.02) weight loss when food restricted, whereas A(y), GTG, and db/db mice did not. In all obese mice, plasma cholesterol levels decreased (P < 0.02) after the 6-wk period of food restriction. Plasma triglyceride levels significantly decreased (P < 0.02) in NZO, GTG and db/db mice, but not in other obese mice. Plasma glucose levels were significantly decreased (P < 0.02) after the 6-wk period in the obese mice except for the A(y) and NZO mice; levels were greater in food-restricted db/db mice. Although some of the obese models such as db/db were diabetic, our data suggest that the defective breath test was independent of diabetes because all obese and diabetic models responded similarly to food restriction. Impaired hepatic catabolism of CR was excluded as a cause of the abnormal breath tests. In summary, the impairment (P < 0.05) in remnant metabolism as assessed by the breath test in obese mice was corrected by food restriction, associated with improvements in plasma glucose, triglyceride and cholesterol levels.

Preliminary experience with a new stable isotope breath test for chylomicron remnant metabolism: a study in central obesity

We aimed to investigate the metabolism of chylomicron remnants in the postabsorptive state employing a new stable isotope breath test in centrally obese men without overt hyperlipidaemia. Groups of 12 centrally obese and 12 non-obese men of similar age and with similar plasma cholesterol and triacylglycerol (triglyceride) levels were studied. The catabolism of chylomicron remnants was measured using an intravenous injection of a remnant-like emulsion containing cholesteryl [(13)C]oleate. Isotopic enrichment of (13)CO(2) in breath was determined using isotope-ratio mass spectrometry, and a multi-compartmental model (SAAM II program) was used to estimate the fractional catabolic rate (FCR) of the chylomicron remnant-like particles. The plasma concentrations of low-density lipoprotein (LDL)-cholesterol, non-high-density lipoprotein (HDL)-cholesterol and insulin were significantly higher (P<0.05) in the obese than the control subjects. The obese subjects had significantly lower HDL-cholesterol (P<0.05) and, in particular, a decreased FCR of the remnant-like particles compared with lean subjects (0.061+/-0.014 and 0.201+/-0.048 pools/h respectively; P=0.016). In the obese group, the FCR of remnant-like particles was inversely associated with the waist/hip ratio, and with plasma triacylglycerol, cholesterol, LDL-cholesterol and non-HDL-cholesterol levels. In multiple regression analysis, the waist/hip ratio was the best predictor of the FCR of the emulsion. In conclusion, this new test suggests that postabsorptive chylomicron remnant catabolism is impaired in centrally obese subjects without overt hyperlipidaemia. This defect may be due to the degree of adiposity.

Chylomicron remnant metabolism in familial hypercholesterolaemia studied with a stable isotope breath test

Chylomicron remnant metabolism was studied using a stable isotope breath test in 25 patients with familial hypercholesterolaemia (FH) (10 homozygotes, 15 heterozygotes), and in 15 normolipidaemic controls. A lipid emulsion mimicking the composition of chylomicron remnants and labelled with cholesteryl (13)C-oleate was injected intravenously; (13)CO(2) was measured subsequently in breath using isotope-ratio mass spectrometry. The fractional catabolic rate (pools/h) of the emulsion, derived from a compartmental model, did not differ significantly among the groups: homozygous FH mean 0.20 (S.E.M. 0.05), heterozygous FH 0.12 (0.02), controls 0.16 (0.03). We suggest that the catabolism of chylomicron remnants from plasma is not impaired in FH and that the hepatic uptake of these particles is not dependent on functional LDL receptors.

Chylomicron remnant metabolism in familial dyslipidemias studied with a remnant-like emulsion breath test

We have developed a stable isotope breath test for the assessment of chylomicron remnant metabolism and report the results from the breath test in human subjects selected for disorders of chylomicron or remnant metabolism. In type I hyperlipemia, the phenotype is extreme hypertriglyceridemia due to a lack of lipoprotein lipase activity, which causes the failure of remnant formation. The type III dyslipidemia phenotype is caused by the inefficient removal of chylomicron remnants from plasma, generally because of homozygosity for apolipoprotein E2 alleles. The breath test was predicted to be abnormal in type III hyperlipemia, whereas a priori in type I hyperlipemia defective remnant clearance was not anticipated. Subjects were injected with lipid emulsions prepared with a composition similar to normal chylomicron remnants. The emulsions contained cholesteryl ester incorporating the stable nonradioactive isotope (13)C in the fatty acid moiety. End exhalation breath was collected at intervals after intravenous injection of the remnant-like emulsions and analyzed for (13)C enrichment by isotope-ratio mass spectrometry. Compared with the group of normolipemic men, the fractional catabolic rate of remnants measured by the breath test was significantly decreased (P = 0.006) in subjects with type III dyslipidemia. In the group with type I hyperlipemia, the fractional catabolic rate was not different (P = 0.233) from the control group. Therefore, the underlying capacity for remnant catabolism was normal in this group of markedly hypertriglyceridemic subjects. By short-circuiting the step of lipolysis, the remnant-like emulsion breath test provides direct information about remnant clearance and metabolism, which should assist in investigations of postprandial lipid metabolism.

Relative roles of mitochondrial and peroxisomal fatty acid oxidation in the metabolism of chylomicron remnants in rats and mice as assessed by a stable-isotope breath test

We have developed a stable isotope breath test to trace physiological remnant metabolism. Validity of the test depends on the injected lipid emulsion mimicking chylomicron remnant (CR) clearance and on subsequent metabolism of the emulsion cholesteryl ester (CE). Oxidation of CE fatty acids could involve both mitochondrial and peroxisomal pathways. In the present studies various agents were used to inhibit the binding of remnants, CE hydrolysis or mitochondrial fatty acid oxidation. Treatment of mice with suramin or lactoferrin markedly delayed the clearance and metabolism of remnants as shown by the significantly lower enrichment of 13CO(2) in the breath when compared with untreated mice. In hepatectomized rats injected with remnant-like emulsions, enrichment with 13CO(2) was virtually abolished. Treatment of mice with chloroquine or rats with methyl palmoxirate (an inhibitor of mitochondrial fatty acid oxidation) markedly impaired the recovery of label in the breath. Compared with mice fasted overnight, Intralipid by gavage decreased the breath enrichment with 13CO(2) consistent with competition between endogenous CR and the injected emulsion particles. These findings show that the breath test reliably measures the metabolism of CR and that CE fatty acid is metabolised by mitochondrial pathways.

Relative roles of LDLr and LRP in the metabolism of chylomicron remnants in genetically manipulated mice

Remnant-like emulsions labeled with cholesteryl [(13)C]-oleate were prepared with lipid compositions similar to remnants derived from triacylglycerol-rich lipoproteins. When injected into the bloodstream of conscious mice, the remnant-like emulsions were metabolized in the liver leading to the appearance of (13)CO(2) in the breath. Previously, using this technique, we found that remnant metabolism was significantly impaired but not completely inhibited in mice lacking low density lipoprotein receptors (LDLr). We have now found in mice with non-functional low density lipoprotein receptor-related protein (LRP) that breath enrichment of (13)CO(2) was significantly decreased, indicating that the LRP also plays an important role in the metabolism of chylomicron remnants (CR). The enrichment of (13)CO(2) in the expired breath was negligible in mice lacking both LDLr and receptor-associated protein (-/-), essential for normal function of LRP. In mice pre-injected with gluthatione S-transferase-receptor-associated protein to block LRP binding, there was a marked inhibition of the appearance of (13)CO(2) in the expired breath of homozygous LDLr-deficient mice, supporting the role of LRP in vivo. Whether or not LDLr were present, in mouse and human fibroblast cells human apoE3 or E4 but not apoE2 were essential for binding of remnant-like emulsions, while lactoferrin and suramin completely inhibited binding. We conclude that in normal mice LDLr are important for the physiological metabolism of CR. When LDLr are absent the evidence supports a role for the LRP in the uptake of CR in liver cells and in fibroblasts, with binding characteristics for CR-associated apoE similar to LDLr.

Plasma clearance of chylomicrons from butterfat is not dependent on saturation: studies with butterfat fractions and other fats containing triacylglycerols with low or high melting points

BACKGROUND

Dietary fats influence plasma lipids, and changes in the clearance and metabolism of postprandial lipoproteins can affect atherosclerosis. Butterfat is considered hypercholesterolemic but contains a multitude of constituent fatty acids.

OBJECTIVES

We determined triacylglycerol and cholesteryl ester clearances of lymph chylomicrons derived from butterfat, fractions of butterfat, and other dietary fats.

METHODS

Radiolabeled lymph chylomicrons resulting from the intestinal absorption of different fats were reinjected into recipient rats to measure plasma clearance. Plasma clearance of [14C]triacylglycerol was used as an indicator of chylomicron lipolysis whereas clearance of [3H]cholesteryl ester was used as an indicator of chylomicron remnant removal.

RESULTS

[3H]Cholesteryl ester clearance was slower from chylomicrons derived from a solid, high-saturated-butterfat fraction than from whole butterfat, but clearance of chylomicrons from other fractions did not correlate with the fractions' saturated fatty acid contents. Clearance of cholesteryl esters in chylomicrons derived from cocoa butter, palm oil, and butterfat was slower than clearance of cholesteryl esters in chylomicrons derived from safflower oil. Hepatic uptakes of cholesteryl esters were generally lower for chylomicrons from all butterfat fractions, cocoa butter, and palm oil.

CONCLUSIONS

In contrast with minor effects on the lipolysis of chylomicron triacylglycerols, chylomicron remnant removal was strongly influenced by the type of dietary fat, with slower cholesteryl ester clearances for saturated fats with higher melting points. However, remnant removal and hepatic uptake of chylomicrons from whole butterfat and fractions of butterfat were not correlated with fat saturation. The mechanisms of this apparent paradox remain unknown but may be attributable to acyl arrangements in the lipid classes of chylomicrons that influence the association with apolipoproteins and receptors and hence remnant removal.

Chylomicron remnant uptake is regulated by the expression and function of heparan sulfate proteoglycan in hepatocytes

Chylomicron remnants transport cholesterol from the intestine, and are removed from the circulation principally by the liver. While hepatic receptors, including the low density lipoprotein (LDL) receptor account for endocytosis, heparan sulfate proteoglycans (HSPG) participate in the initial binding of remnants to liver cells. To explore the interactions between HSPG and endocytosis of remnants, in the present study the expression of HSPG was inhibited in HepG2 cells transfected by a synthetic antisense oligodeoxynucleotide SYN5. Immunofluorescent staining by a monoclonal anti-syndecan antibody showed significant reduction in the expression of syndecan in SYN5-treated cells compared with control cells. Remnant binding decreased by about 50-70% in SYN5-transfected cells. Monoclonal antibodies to either heparan sulphate or the LDL receptor decreased binding by about 60-65%. The glycosylation inhibitor beta-nitrophenylxylopyranoside inhibited remnant uptake by 25%, whereas 4-nitrophenyl-beta-D-galactopyranoside had no effect on remnant binding. Heparinase completely abolished binding at appropriate concentrations. Heparitinase was less effective than hep arinase in inhibiting remnant binding. Suramin completely abolished the remnant binding. Poly-arginine, poly-lysine, and protamine all reduced remnant uptake by the cells, as did polybrene, a synthetic polycation, suggesting a role of cation-anion interactions in remnant binding. Brefeldin A, colchicine, and monensin caused the fluorescence associated with remnants to persist within the cells, confirming that blockers of tubulovesicular processes and Golgi function inhibit the intracellular transport and degradation of the remnants. Our results show that remnant binding to liver cells depends on the LDL receptor, on the expression of HSPG core proteins, and on the functionality of heparan sulfate in HSPG.

A 13CO2 breath test to assess the metabolism of triglyceride-rich lipoprotein remnants in mice

Remnant-like emulsions were prepared with lipid compositions similar to remnants derived from triacylglycerol-rich lipoproteins. When injected into the bloodstream of conscious mice the remnant-like emulsions labeled with cholesteryl[13C]oleate were metabolized in the liver and the appearance of 13CO2 in the breath was measured. In control mice injected with remnant-like emulsions labeled with cholesteryl[13C]oleate, enrichment of 13CO2 in the breath peaked at 45 min and then decreased markedly by 3 h. In apoE-deficient (-/-) mice no enrichment was found and in low density lipoprotein receptor (LDLr)-deficient (-/-) mice the appearance of 13CO2 in the breath was markedly decreased. These findings were consistent with the ability of the breath test to detect defects in remnant metabolism. The breath test was useful in detecting a defect in remnant metabolism in LDLr heterozygote (+/-) mice, in which the appearance of 13CO2 in the breath was less by 45 min but remained elevated for the duration of the experiment when compared with control mice. In hepatic lipase-deficient (-/-) mice no defect in remnant metabolism was found. Under fasting conditions, the enrichment of 13CO2 in the breath after injection of emulsion was markedly increased when compared with fed mice, indicating that the metabolism of the injected remnant-like emulsion was probably competed for by post-prandial particles under fed conditions. Our findings show that a 13C breath test can be used to assess the metabolism of remnants. The test provides a useful and sensitive method for non-invasive testing of remnant metabolism in experimental animals.

Sterol side chain length and structure affect the clearance of chylomicron-like lipid emulsions in rats and mice

In previous work we found that sterols such as cholesterol were essential for physiological plasma clearance of lipid emulsions mimicking the structure of mammalian triglyceride-rich lipoproteins. In the present study we compared the clearances of emulsions prepared with sterols of varying alkyl chain length (straight chains, n-C3 to n-C7, or branched chains, i-C5 to i-C10) at the C-17 position. Our studies show that the length of the alkyl chain at the C-17 position of sterols markedly affects the removal of remnant particles from the plasma of rats traced by emulsion cholesteryl oleate label. An alkyl chain of 7 carbons or more was needed for normal remnant clearance. Straight and branched chains of similar length were cleared similarly, showing that the presence of a branch at the end of the alkyl chain had no effect on remnant clearance. For side chains of 7 carbons or less, substitution of sterols with an unsaturation in the alkyl chain close to the terminal carbon markedly decreased the clearance of remnants. Triolein label was used to estimate lipolysis of the injected emulsions. Lipolysis was little affected by the structure of the sterol side chain, except that lipolysis was markedly higher with emulsions containing sterols with an alkyl chain having 4 carbon atoms (n-C4) or with an unsaturation in the 4 carbon alkyl chain. We conclude that the length of the alkyl side chain is an important element in the essentiality of cholesterol as a regulator of metabolism of lipid emulsion models of triglyceride-rich lipoproteins.

Effect of the ACAT inhibitor CL 277,082 on apolipoprotein B48 transport in mesenteric lymph and on plasma clearance of chylomicrons and remnants

Inhibitors of acyl CoA:cholesterol acyltransferase (ACAT) activity previously have been found to decrease the absorption of cholesterol and to be effective antiatherosclerotic agents. Effects on chylomicron (CM) transport could contribute to these effects. No previous study has examined the effect of inhibition of ACAT activity on the intestinal lymph output of apolipoprotein (apo) B48 or on the clearance from plasma of lymph CM. In this study, we selected 2,4-difluoro-phenyl-N[[4-(2,2-dimethylpropyl)phenyl]methyl]-N-( hepthyl)urea (CL 277,082) to inhibit intestinal ACAT activity and measured its effects on the output of lipids and apo B48 in intestinal lymph. Compared with control untreated rats, treatment with CL 277,082 decreased the lymph outputs of apo B48 and triglyceride. Associated with the effects on transport, the lymph CM were smaller in diameter in rats treated with CL 277,082. The unesterified cholesterol content of lymph CM was markedly increased and the cholesteryl ester (CE) content was decreased. The contents of triglyceride were decreased and phospholipid was increased. Labeled CM were prepared by feeding donor rats with a test meal containing 3H-cholesterol and 14C-fatty acid. Traced by the CE label in lymph CM in both control rats and rats treated with CL 277,082, the remnants derived after intravenous injection of CM from rats treated with CL 277,082 were cleared significantly more slowly than CM from untreated rats. Moreover, less CE label was recovered in the livers of both groups of rats after injection of CM from rats treated with CL 277,082. Recovery in the spleen was significantly higher in recipient rats injected with CM from rats treated with CL 277,082 when compared with injections of CM obtained from untreated rats. We conclude that the metabolism of CM is affected by treatment with CL 277,082, partly due to the changes in lymph CM composition and partly due to other effects on the recipient rat.

Effects of particle size and number on the plasma clearance of chylomicrons and remnants

Lymph chylomicrons of different sizes are known to be cleared at different rates, but the underlying mechanism for this effect has not been resolved. To investigate the differences in clearance rates between small and large particles, chylomicron-like lipid emulsions labeled with radioactive triolein and cholesteryl oleate were injected into conscious rats. The clearance from plasma of small emulsion particles was significantly slower than large when equal lipid masses of small and large particles were injected. Similar results were obtained in clearance studies with lymph chylomicrons. When equal numbers of either small or large emulsion particles were injected into rats, the clearance of the triolein label from large particles was significantly slower than small particles but no significant difference was found in the clearance of the remnants (traced by the cholesteryl oleate label) derived from small and large particles. However, when increased numbers of either small or large particles were injected, the clearances of emulsion triolein and remnants were significantly decreased. Larger particles were found to be lipolyzed significantly less than small. Simultaneous injections showed competition for removal of large and small particles, suggesting competition for a common, saturable removal process. Our findings provide evidence that particle number and size are determinants of the rates of plasma clearance of the triglyceride-rich lipoproteins and the results are consistent with a saturable process. Our data also show that particle number is more important than size and higher numbers of particles markedly affect the clearance of triglyceride-rich lipoproteins. However particle uptake by the liver is not sensitive to remnant size.

Intracellular localization and metabolism of chylomicron remnants in the livers of low density lipoprotein receptor-deficient mice and apoE-deficient mice. Evidence for slow metabolism via an alternative apoE-dependent pathway

The metabolism of chylomicron remnants in mice deficient in low density lipoprotein receptor (LDLr) or apolipoprotein E (apoE) was compared with that of control C57BL/6J mice. Mice were injected intravenously with chylomicron-like emulsions labeled with radioactive lipids. Blood samples were taken at fixed time intervals from the retro-orbital sinus, and clearance rates of the lipoproteins were assessed from the decline in plasma radioactivities. To follow the intracellular pathway of remnants in the liver, emulsions labeled with a fluorescent cholesteryl ester (BODIPY) were injected, and liver sections were processed and assayed by laser confocal microscopy. Catabolism of remnant cholesteryl esters was assessed by injecting emulsions labeled with cholesteryl[1-14C]oleate and measuring the expired CO2 from each animal. In apoE-deficient mice, remnant removal from plasma was totally impeded, while the clearance of remnants in LDLr-deficient mice was similar to that in C57BL/6J control mice. The confocal micrographs of livers 20 min after injection of fluorescent chylomicron-like emulsions showed evenly distributed fluorescent particles in the hepatocytes from control mice. In contrast, the fluorescent particles were mainly located in sinusoidal spaces in LDLr-deficient mice. Three hours after injection the livers from control mice showed few fluorescent particles, indicating that remnants have been catabolized, while the sections from LDLr-deficient mice were still highly fluorescent. Micrographs from apoE-deficient mice showed no fluorescent particles in the liver at any time after injection. Measurement of expired radioactive CO2 after injection of emulsions labeled in the fatty acid moiety of cholesteryl oleate indicated that remnant metabolism was slower in the LDLr-deficient mice and essentially nil in the apoE-deficient mice. Control mice had expired 50% of the injected label by 3 h after injection. We conclude that under normal circumstances, chylomicron remnants are rapidly internalized by LDLr and catabolized in hepatocytes, with a critical requirement for apoE. When LDLr is absent, remnants are taken up by a second apoE-dependent pathway, first to the sinusoidal space of the liver, with subsequent slow endocytosis and slow catabolism. Hepatic clearance via this second pathway is increased by heparin, inhibited by lactoferrin, heparinase, and suramin, and down-regulated by feeding a high fat diet.

Measurement of expired carbon dioxide to assess the metabolism of remnant lipoproteins

The cholesteryl esters transported from the intestine in chylomicrons are delivered to the liver. Hepatocytes take up chylomicron remnants by receptor-mediated endocytosis and the cholesteryl esters are subsequently degraded. In this study we measured the appearance in breath of labeled carbon dioxide after injection of chylomicron-like emulsions labeled with radioactive cholesteryl [1-14C]oleate. Measurements by the breath test provide an integrated assessment of capacity for clearance and subsequent metabolism of the remnants of the triglyceride-rich lipoproteins. In normal rats, mice, and rabbits injected with the radioactive emulsions, label appeared in the breath after a delay of approximately 30 min, appreciably slower than the appearance of label after injection of emulsions labeled with [14C]triolein or of [14C]oleic acid complexed with albumin. To test for the ability of the procedure to detect defects in remnant clearance, labeled emulsions were injected into diabetic rats, apoE-deficient mice, low density lipoprotein receptor (LDLr)-deficient mice, and Watanabe heritable hyperlipidemic (WHHL) rabbits with defective low density lipoprotein receptors. In rats made diabetic by treatment with streptozotocin the appearance of 14CO2 in breath was slower than in normal control rats. This finding was consistent with previous evidence from our laboratory that remnant clearance is defective in diabetic rats. In LDLr-deficient mice the appearance of 14CO2 was slower when compared with control mice and in apoE-deficient mice the appearance of 14CO2 was extremely small. In homozygous WHHL rabbits, the appearance of 14CO2 in breath was much slower than in normal control rabbits, while in heterozygous WHHL rabbits an intermediate level of appearance was found, consistent with our previous findings of defective remnant clearance in WHHL rabbits. Emulsions with cholesterol omitted, previously found to be cleared from plasma much slower than chylomicron-like emulsions, had much slower appearance of label in breath. The breath test as described is consistent with predicted metabolism of chylomicron remnants and therefore provides a useful means of assessment of remnant catabolism in the intact animal.

Effects of triacylglycerol-saturated acyl chains on the clearance of chylomicron-like emulsions from the plasma of the rat

We previously found that a single saturated acyl chain at the glycerol 2-position affected the metabolism of chylomicrons. The explanation for the effect is not clear, but could be reproduced by saturated monoacylglycerols. In the present work we have extended our measurements to several different triacylglycerols containing one or two saturated chains in specific locations in an attempt to define structural features that affect chylomicron clearance. Lipid emulsions containing triacylglycerol, egg yolk phosphatidylcholine, free cholesterol, cholesteryl oleate (CO) and labelled with 3H-CO and [14C]triolein (OOO) were prepared as models of lymph chylomicrons. When injected intravenously into rats, the metabolism of the emulsions was influenced by the acyl chains of the constituent triacylglycerols. Compared with emulsions containing OOO as the only triacylglycerol, plasma clearances of emulsion [3H]CO were extremely slow in emulsions containing either 1,2-dioleoyl-3-stearoylglycerol (OOS) or 1-stearoyl-2,3-dioleoylglycerol (SOO). As little as 10% of SOO in mixture with OOO slowed the clearance, and increasing proportions of SOO in OOO emulsions progressively slowed the removal of OOO and CO labels from plasma. With 50% and 100% SOO in the emulsions clearance was negligible. In emulsions containing the triacyl-sn-glycerols, 1,3-dimyristoyl-2-oleoylglycerol (MOM), 1,3-dipalmitoyl-2-oleoylglycerol (POP), 1-oleoyl-2,3-distearoylglycerol (OSS) or 1-palmitoyl-2-oleoyl-3-stearoylglycerol (POS), clearance rates of CO and OOO labels from plasma were significantly decreased compared with control OOO emulsions. With emulsions prepared with the triacylglycerols, 1-oleoyl-2,3-dimyristoylglycerol (OMM) and 1-oleoyl-2,3-dipalmitoylglycerol (OPP), clearances of CO label were significantly slower than with control OOO emulsions, while the removal of OOO label was not significantly affected. The uptake of CO label in the liver was decreased in conjunction with the lower rates of clearance of emulsion CO from the plasma. The clearance from plasma of 1,3-distearoyl-2-oleoylglycerol (SOS) emulsions was similar to the control OOO emulsions, but significantly more emulsion OOO label was taken up by the liver. Emulsions made with the triacylglycerols extracted from natural cocoa butter, which contained a high proportion of saturated acyl chains, were cleared similarly to the control OOO emulsions. Our findings indicate that the plasma clearance of triacylglycerol-rich lipoprotein particles depends upon the specific arrangements of the acyl chains of the constituent triacylglycerols, and not necessarily on the overall saturation of the triacylglycerols.(ABSTRACT TRUNCATED AT 400 WORDS)

Lipid and apolipoprotein B48 transport in mesenteric lymph and the effect of hyperphagia on the clearance of chylomicron-like emulsions in insulin-deficient rats

In insulin-deficient streptozotocin-treated rats the intestine is hypertrophic and cholesterol synthesis and transport from the intestine are increased. The increased load of cholesterol is transported through the mesenteric lymph in chylomicrons. Clearance from plasma of injected chylomicrons is slowed in insulin-deficient rats, but the underlying mechanisms are currently unresolved. Hyperphagia may increase the size of chylomicrons which could contribute to defective chylomicron clearance in insulin-deficiency. In the present experiments we compared the size and number of chylomicrons in mesenteric lymph of control rats and diabetic rats infused with fat at two levels. In control and diabetic lymph-cannulated rats, as the infused dose of lipid increased the transport of triglyceride increased substantially compared with fasted rats. In contrast the transport of apoB48 increased by only a small amount during fat transport. Therefore, increased lipid transport was accomplished mostly by increased particle size, with only small increases in numbers of particles in intestinal lymph. Insulin-deficiency had no effect on triglyceride or apoB48 transport in lymph. Calculations suggested that each chylomicron particle contained a single molecule of apoB48. When hyperphagia in diabetic rats was prevented, the plasma triglycerides were decreased but the slow plasma clearance of injected chylomicron-like emulsions persisted. Hyperphagia, therefore, was unconnected to the impairment in chylomicron metabolism in insulin-deficient rats. Changes in the association with plasma apolipoproteins, in the expression of receptors for uptake of chylomicron remnants or in exposure to endothelial lipases may be responsible for the defective clearance of triacylglycerol-rich lipoproteins.

The effect of insulin deficiency on the plasma clearance and exchange of high-density-lipoprotein phosphatidylcholine in rats

Triolein/cholesteryl oleate/cholesterol/phosphatidylcholine emulsions designed to model the lipid composition of chylomicrons were injected intravenously into control and streptozotocin-treated insulin-deficient rats. As previously described for lymph chylomicrons, the emulsion triolein was hydrolysed and phosphatidylcholine was transferred to the plasma high-density lipoproteins (HDL). This mechanism was used to introduce a phospholipid label into HDL in vivo. The subsequent clearance of phospholipid radioactivity from the plasma of insulin-deficient rats was significantly slower than in controls (P less than 0.025). Plasma clearance was similarly slower in insulin-deficient rats after injection of HDL that was previously labelled with radioactive phospholipids. After injection, the phospholipid label redistributed rapidly between the large-particle fraction of plasma lipoproteins (very-low- and low-density lipoproteins), and the lighter and heavier fractions of HDL. Compared with control rats, in insulin-deficient rats less of the phospholipid label was distributed to the lighter HDL fraction and more to the heavier HDL fraction, and this difference was not due to changes in activity of lecithin: cholesterol acyltransferase or in the apparent activity of phospholipid transfer protein. In insulin-deficient rats the changes in HDL phospholipid clearance and exchange appeared to be secondary to the associated hypertriglyceridaemia and the related changes in distribution of phospholipids between classes of plasma lipoproteins.

Dose response and toxicity of doxorubicin microspheres in a rat tumor model

The therapeutic response and toxic effects of chemotherapy using several doses of doxorubicin in conventional solution form or bound to an ion-exchange resin were compared in a rat tumor model, to assess the relationship of drug dose to therapeutic efficacy and associated toxicity. Single bolus injections of 3.0, 4.5, 6.0, 7.5 and 9.0 mg/kg were administered via the abdominal aorta to rats bearing hindlimb tumors. Tumor size was measured serially and the growth rates of treated groups were compared with a control growth curve. In addition, the effect of empty microspheres on tumor growth rate was assessed. The levels of circulating white blood cells were measured and compared to control levels to provide an indication of the severity of bone marrow toxicity experienced by each form of treatment. Finally, any difference in the distribution of doxorubicin to tumor, hindlimb and cardiac tissue following administration of doxorubicin as free drug or on microspheres was ascertained. Empty ion-exchange resin exerted a small although significant detrimental effect on tumor growth which may be explained by the embolization of microspheres in the precapillary blood vessels of the tumor resulting in a transient delay in tumor growth rate. The lowest dose of doxorubicin produced a significantly better therapeutic response when administered in the free drug form, but higher doses elicited an equivalent delay in tumor growth for both drug microsphere and free drug groups in a dose-dependent manner, with the maximum anti-tumor response occurring at the highest dose. Treatment with free doxorubicin at high doses resulted in significant reductions of circulating white blood cells suggesting the occurrence of bone marrow toxicity.(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions between model triacylglycerol-rich lipoproteins and high-density lipoproteins in rat, rabbit and man

There are inverse relationships between HDL cholesterol and plasma triacylglycerol concentrations in normal and in hypertriglyceridemic individuals. To investigate the interactions between triacylglycerol-rich lipid particles and HDL, a lipid emulsion model of the triacylglycerol-rich lipoproteins was prepared. When emulsion particles were incubated with rat high-density lipoproteins (HDL) in the presence of lipid transfer activity (d greater than 1.21 g/ml fractions) from rabbit or human plasma there was a rapid bi-directional exchange of cholesteryl oleate (CO) and phospholipid (PL) labels between lighter and heavier fractions of HDL and emulsion particles. The transfers of CO and PL labels between both light and heavy fractions of HDL and the emulsion particles were increased with increasing amounts of emulsion added to the incubations. Incubation with the d greater than 1.21 g/ml fraction from rat plasma resulted in only a small exchange of CO whereas PL exchange was similar to rabbit and human plasma. Retinyl palmitate label was not transferred from emulsion particles to the HDL fractions even in the presence of lipid transfer activity from rabbit or human plasma. The present study shows that the transfer protein-mediated exchanges of surface and core lipids between HDL and the triacylglycerol-rich lipoproteins are affected by the quantity of triacylglycerol-rich particles in the system. This mechanism may contribute to the inverse relationships between plasma triacylglycerol concentrations and HDL concentrations in normal and hypertriglyceridemic individuals.