Phosphatidylethanolamine-N-methyltransferase activity and dietary choline regulate liver-plasma lipid flux and essential fatty acid metabolism in mice

Nutritional metabonomics: applications and perspectives

Nowadays, nutrition focuses on improving health of individuals through diet. Current nutritional research aims at health promotion, disease prevention, and performance improvement. Modern analytical platforms allow the simultaneous measurement of multiple metabolites providing new insights in the understanding of the functionalities of cells and whole organisms. Metabonomics, "the quantitative measurement of the dynamic multiparametric metabolic response of living systems to pathophysiological stimuli or genetic modifications", provides a systems approach to understanding global metabolic regulations of organisms. This concept has arisen from various applications of NMR and MS spectroscopies to study the multicomponent metabolic composition of biological fluids, cells, and tissues. The generated metabolic profiles are processed by multivariate statistics to maximize the recovery of information to be correlated with well-determined stimuli such as dietary intervention or with any phenotypic data or diet habits. Metabonomics is thus uniquely suited to assess metabolic responses to deficiencies or excesses of nutrients and bioactive components. Furthermore, metabonomics is used to characterize the metabolic phenotype of individuals integrating genetic polymorphism, metabolic interactions with commensal and symbiotic partners such as gut microflora, as well as environmental and behavioral factors including dietary preferences. This paper reports several experimental key aspects in nutritional metabonomics, reviews its applications employing targeted and holistic approach analysis for the study of the metabolic responses following dietary interventions. It also reports the assessment of intra- and inter-individual variability in animal and human populations. The potentialities of nutritional metabonomics for the discovery of new biomarkers and the characterization of metabolic phenotypes are discussed in a context of their possible utilizations for personalized nutrition to provide health maintenance at the individual level.

Biomarker discovery using high-dimensional lipid analysis

PURPOSE OF REVIEW

High-dimensional lipid analysis technologies (lipidomics) provide researchers with an opportunity to measure lipids on an unprecedented scale. They do not, however, guarantee a fast track to new knowledge. The vast amount of data produced by these platforms presents a major hurdle to assembling valid knowledge and to the discovery of mechanistic biomarkers. This review examines strategies for improving the quality of high-dimensional lipid data and streamlining data analysis to increase the value of lipidomics platforms to research and commercial applications.

RECENT FINDINGS

Recent articles focus on careful study design and data analysis protocols. Authors offer detailed descriptions of study populations, analytical methods and data analysis, and highlight the use of practical data preprocessing and the incorporation of biological knowledge into data analysis.

SUMMARY

The field is moving towards more methodical and structured approaches to biomarker identification. Experimental designs focusing on well-defined outcomes have a better chance of producing biologically relevant results. The high-dimensional lipid analysis techniques available are varied, have different strengths and weaknesses, and must be chosen carefully depending on the experimental design and application. Many techniques for data analysis are available, but the most successful are those incorporating existing biological knowledge into the statistical analysis.

S-adenosylhomocysteine and polyunsaturated fatty acid metabolism in predementia syndromes and Alzheimer's disease

Recent epidemiological evidence showed that dietary fatty acids, antioxidants, and micronutrients appear to have a role in cognitive decline, and may permit a beneficial effect on the risk of dementia and predementia syndromes. We discussed in the present paper the issue of the suggested protective role of polyunsaturated fatty acids (PUFA) on Alzheimer's disease, mild cognitive impairment, and other predementia syndromes, supported by the findings of the Italian Longitudinal Study on Aging (ILSA) and other population-based studies. In particular, we discussed the possible metabolic link of plasma S-adenosylhomocysteine concentrations with PUFA erythrocyte composition in explaining this suggested protective role of fatty acids against dementia and predementia syndromes.

A metabolic link between S-adenosylhomocysteine and polyunsaturated fatty acid metabolism in Alzheimer's disease

There is evidence that vascular risk factors contribute to the pathology of Alzheimer's disease. Increased concentrations of circulating homocysteine are associated with vascular risk factors and Alzheimer's disease but the mechanisms involved are unclear. Homocysteine inhibits the hydrolysis of S-adenosylhomocysteine (SAH) which is a product inhibitor of S-adenosylmethionine (SAM) dependent methyltransferase reactions. It has been shown previously that SAH inhibits phosphatidylethanolamine N-methyltransferase (PEMT) in the liver. The activity of PEMT in the liver plays an important role in the methylation of phosphatidylethanolamine (PE) to phosphatidylcholine (PC) and the delivery of essential polyunsaturated fatty acids (PUFAs) to peripheral tissues. In the present study, the plasma concentrations of SAH, SAM and homocysteine and the erythrocyte composition of phosphatidylcholine (PC), phosphatidylethanolamine (PE) and their respective polyunsaturated fatty acid concentrations were determined in 26 patients with Alzheimer's disease and compared to those in 29 healthy control subjects. There was a significant increase in the plasma concentrations of SAH (p<0.001) and homocysteine (p<0.001) and a significant increase in the plasma concentrations of SAM (p<0.001) in the Alzheimer's patients. A significant positive correlation was found between the plasma concentrations of SAH and homocysteine (r=0.738, p<0.001). There was a negative correlation between the plasma concentrations of homocysteine and the ratio of SAM/SAH (r=-0.637, p<0.01). There was a significant decrease in the erythrocyte content of PC (p<0.001) and an increase in the erythrocyte content of PE (p<0.001) in the Alzheimer's patients. Plasma SAH concentrations were negatively related to erythrocyte PC concentrations (r=-0.286, p<0.01) and positively related to erythrocyte PE concentrations (r=0.429, p<0.001). The erythrocyte PC from Alzheimer's patients had a significant depletion of docosahexaenoic acid (DHA) (p<0.001) while there was no significant difference in the DHA content of erythrocyte PE. There was a significant negative correlation between plasma SAH and the DHA composition of erythrocyte PC (r=-0.271, p<0.001). This data may reflect the inhibition of hepatic PEMT activity by SAH in Alzheimer's disease. The decreased mobilization of DHA from the liver into plasma and peripheral tissues may increases the risk of atherosclerosis and stroke leading to chronic cerebral hypoperfusion. The evidence suggests that a metabolic link between the increased production of SAH and phospholipid metabolism may contribute to cerebrovascular and neurodegenerative changes in Alzheimer's disease.

The relationship between folate and docosahexaenoic acid in men

OBJECTIVE

Docosahexaenoic acid (DHA, 22:6n-3), an essential omega 3 fatty acid, may protect against disorders of emotional regulation as well as cardiovascular disease. Animal studies demonstrate that dietary folate can increase tissue concentrations of DHA, although the literature, to date, includes no human studies examining the possibility that folate status may affect plasma DHA concentrations. The objective of this study is to determine if the blood concentrations of folate and DHA are correlated in humans.

DESIGN

Retrospective study.

SETTING

An American research hospital.

SUBJECTS

A total of 15 normal and 22 hostile and aggressive subjects, with a mean age of 38 years.

METHODS

Concentrations of plasma polyunsaturated essential fatty acids and red blood cell folate (RBC folate) were obtained prior to 1996, before American flour was enriched with folate.

RESULTS

RBC folate was significantly correlated with plasma DHA, r=0.57, P=0.005 in the aggressive group. Age, smoking and alcohol consumption did not alter the results. No other essential fatty acids were significantly associated with RBC folate in either group.

CONCLUSIONS

The positive relationship between plasma DHA and RBC folate concentrations suggests that these two nutrients should be examined together in order to make the most accurate inferences about their relative contributions to disease pathogenesis. Our findings present one explanation why some conditions associated with hostility and low DHA status, such as cardiovascular disease and emotional disorders, are also associated with low folate status.

SPONSORSHIP

National Institutes of Health, National Institute on Alcohol Abuse and Alcoholism.

Lipid metabolism in human endothelial cells

Although lipids are largely involved in cardiovascular physiopathology, the lipid metabolism in endothelial cells remains largely unknown. Human umbilical vein endothelial cells (HUVECs) were used to investigate the metabolism of complex lipids. The membrane phospholipid homeostasis results from both de novo synthesis and remodelling that ensures the fine tuning of the phospholipid fatty acid composition. Using [(3)H]-glycerol and phosphoderivatives we showed the efficiency of glycerolipid synthesis from glycerol (0.9 nmol h(-1) mg proteins(-1)), but not from its phosphorylated form suggesting the requirement of a functional glycerol kinase in HUVECs. Conversely, the synthesis of triacylglycerols was very low (less than 5% of phospholipid synthesis). The incorporation rate of fatty acids into phospholipids showed that there is a specific fate for each fatty acid in respect to its chain length and saturation level. Moreover in steady state condition, increasing the long chain omega3 polyunsaturated fatty acids in the medium resulted in an increased polyunsaturated/saturated ratio in phospholipids (from 0.42 to 0.63). [(14)C]O(2) was produced form either [(14)C]-glucose or [(14)C]-palmitate indicating the functionality of the oxidation pathways, although beta-oxidation was less efficient than glucose oxidation. The endothelial cell lipid metabolism involves conventional pathways, with functional rates largely slower than in hepatocytes or in cardiomyocytes.

Genomic organization, promoter activity, and expression of the human choline transporter-like protein 1

Choline transporter-like (CTL) proteins of the CTL1 family are novel transmembrane proteins implicated in choline transport for phospholipid synthesis. In this study, we characterized the 5'-flanking region of the human (h)CTL1 gene and examined some of the possible mechanisms of its regulation, including promoter activity, splicing, and expression. The transcription start site of the hCTL1 gene was mapped by 5'-rapid amplification of cDNA ends (RACE), and the presence of two splice variants, hCTL1a and hCTL1b, was investigated using isoform-specific PCR and 3'-RACE. The hCTL1 promoter region of approximately 900 bp was isolated from MCF-7 human breast cancer cells. The promoter was TATA-less and driven by a long stretch of GC-rich sequence in accordance with widespread expression of hCTL1 at both mRNA and protein levels. Deletion analyses demonstrated that a very strong promoter is contained within 500 bp of the transcription start site, and more upstream regions did not increase its activity. The core promoter that conferred the minimal transcription is within the -188/+27-bp region, and its activity varied in human breast cancer and mouse skeletal muscle cells. Multiple motifs within the promoter regulatory region bound nuclear factors from both cultured cells and normal human skeletal muscle. The motifs within the three regions [S1 (-92/-61 bp), S2 (-174/-145 bp), and S3 (-289/-260 bp)] contained overlapping binding sites for hematopoietic transcription factors and ubiquitous transcription factors, in line with the expected gene function. Genomic analyses demonstrated a high conservation of hCTL1 and mouse CTL1 proximal promoters. Accordingly, mRNA profiles demonstrated that human splice variants were expressed ubiquitously, as demonstrated for the mouse transcripts; however, they differed from the profiles of rat CTL1 transcripts, which were more restricted to neurons and intestinal tissues. The shorter hCTL1b variant contained the cytosolic COOH-terminal motif L651KKR654 for endoplasmic reticulum retrieval/retention. This retention signal was conserved in hCTL1b and rat and mouse CTL1b and is typical for transmembrane proteins of type 1 topology.

Mice heterozygous for the Mdr2 gene demonstrate decreased PEMT activity and diminished steatohepatitis on the MCD diet

BACKGROUND/AIMS

The administration of a methionine and choline deficient (MCD) diet to mice serves as an animal model of NASH. The multidrug resistant 2 (Mdr2) P-glycoprotein encodes for the canalicular phospholipid transporter, and Mdr2 (+/-) mice secrete 40% less phosphatidylcholine than wild-type mice. We have hypothesized that phosphatidylethanolamine-N-methyl transferase (PEMT) up-regulation is a consequence of MCD diet administration, and is important for the pathogenesis of steatohepatitis in this model. However, the effect of decreased phosphatidylcholine secretion and modulation of PEMT on the development of diet-induced steatohepatitis in Mdr2 (+/-) mice has not been explored. Thus, the purpose of the study is to examine the effects of the MCD diet on Mdr2 (+/-) mice.

METHODS

Mdr2 (+/-) and Mdr2 (+/+) mice were treated with an MCD or control diet for up to 30 days, and the severity of steatohepatitis, PEMT activity and hepatic S-adenosylmethionine (SAM), and S-adenosylhomocysteine (SAH) levels were measured.

RESULTS

Serum ALT levels, hepatic inflammation, and PEMT activity were significantly lower, and hepatic SAM:SAH ratios were significantly higher in Mdr2 (+/-) mice at 7 and 30 days on the MCD diet.

CONCLUSIONS

Mdr2 (+/-) mice have diminished susceptibility to MCD diet-induced NASH, which is associated with a relative decrease in PEMT activity and increased SAM:SAH ratios.

Ethanolamine modulates DNA synthesis through epidermal growth factor receptor in rat primary hepatocytes

Ethanolamine (Etn) stimulates hepatocyte proliferation in vivo and in vitro; however, the physiological function of Etn in hepatocytes has yet to be elucidated. In the present study, we examined the effect of Etn using a primary culture of rat hepatocytes. The level of membrane phosphatidylethanolamine (PE) significantly decreased when the hepatocytes were cultured without Etn but increased to the level found in the liver when the culture medium was supplemented with 20- 50 microM Etn. Moreover, Etn stimulated DNA synthesis in a dose-dependent manner and had a synergistic effect with epidermal growth factor (EGF). A binding assay and Western blotting showed that the number of EGF receptors was 22- 30% lower in cells grown in the absence of Etn compared to those grown in its presence, but the respective Kd values were almost the same. Furthermore, tyrosine phosphorylation of the EGF receptor was significantly lower in cells grown without Etn. Phosphatidylcholine (PC) synthesis in the liver is unique in that it occurs via stepwise methylation of PE. We found that without Etn supplementation, bezafibrate-induced inhibition of PE methylation increased the level of PE by decreasing its conversion to PC and stimulated DNA synthesis. Moreover, the function of EGF in stimulating DNA synthesis was significantly enhanced under Etn-sufficient conditions. These data suggest that Etn is a nutritional factor required for synthesis of adequate PE, levels of which are important for hepatocyte proliferation.

Putting the 'Ome' in lipid metabolism

The recognition that altered lipid metabolism underlies many metabolic disorders challenging Western society highlights the importance of this metabolomic subset, herein referred to as the lipidome. Although comprehensive lipid analyses are not a recent concept, the novelty of a lipidomic approach lies with the application of robust statistical algorithms to highlight subtle, yet significant, changes in a population of lipid molecules. First-generation lipidomic studies have demonstrated the sensitivity of interpreting quantitative datasets with computational software; however, the innate power of comprehensive lipid profiling is often not exploited, as robust statistical models are not routinely utilized. Therefore, the current review aims to briefly describe the current technologies suitable for comprehensive lipid analysis, outline innovative mathematical models that have the ability to reveal subtle changes in metabolism, which will ameliorate our understanding of lipid biochemistry, and demonstrate the biological revelations found through lipidomic approaches and their potential implications for health management.

An overview of evidence for a causal relationship between dietary availability of choline during development and cognitive function in offspring

This review is part of a series intended for non-specialists that will provide an overview of evidence for causal relationships between micronutrient deficiencies and brain function. Here, we review 34 studies in rodents linking the availability of choline during gestation and perinatal development to neurological function or performance of offspring in cognitive and behavioral tests. Experimental designs, major results, and statistical criteria are summarized in Tables 1-4. Based on our reading of the literature, the evidence suggests that choline supplementation during development results in improved performance of offspring in cognitive or behavioral tests, and in changes in a variety of neurological functional indicators: (1) enhanced performance was observed, particularly on more difficult tasks; (2) increases (choline supplementation) or decreases (choline deficiency) were observed in electrophysiological responsiveness and size of neurons in offspring; and (3) supplementation resulted in some protection against adverse effects of several neurotoxic agents (including alcohol) in offspring. Discussion topics include methodological issues, such as the importance of independent replication, causal criteria, and uncertainties in interpreting test results.

Focused metabolomic profiling in the drug development process: advances from lipid profiling

The highly parallel analytical technologies comprising 'omics promised to dramatically improve drug development efficiency by increasing knowledge and improving decision-making capabilities. On this point, the 'omics have largely been a disappointment. The major reason genomics, transcriptomics and proteomics fail to improve decision making capabilities is that they produce so many false positive results that it is difficult to be sure that findings are valid. Metabolomics is not immune to this problem but, when practiced effectively, the technology can reliably produce knowledge to aid in decision making. In particular, focused metabolomics platforms - those that restrict their target analytes to those measured well by the technology - can produce data with properties that maximize sensitivity and minimize the false discovery problem. The most developed focused metabolomics area is lipid profiling.

Choline and homocysteine interrelations in umbilical cord and maternal plasma at delivery

BACKGROUND

Little is known about the interactions between choline and folate and homocysteine metabolism during pregnancy despite the facts that pregnancy places considerable stress on maternal folate and choline stores and that choline is a critical nutrient for the fetus. Choline, via betaine, is an important folate-independent source of methyl groups for remethylating homocysteine in liver.

OBJECTIVES

Our aims were to examine the intermediates of choline oxidation in maternal and umbilical cord plasma and to determine the relations between this pathway and folate-dependent homocysteine remethylation.

DESIGN

Blood samples were taken from 201 pregnant women and, at delivery, from the umbilical cord veins of their healthy, full-term infants. The blood samples were analyzed for plasma free choline, betaine, dimethylglycine, folate, vitamin B-12, total homocysteine (tHcy), and creatinine concentrations.

RESULTS

Choline concentrations in umbilical cord plasma were approximately 3 times those in maternal plasma (geometric x: 36.6 and 12.3 micromol/L, respectively; P < 0.0001). Betaine and dimethylglycine concentrations were also significantly higher in umbilical cord than in maternal plasma. Choline was positively associated with tHcy (r = 0.34, P < 0.0001), betaine (r = 0.58, P < 0.0001), and dimethylglycine (r = 0.30, P < 0.0001) in maternal blood. Much weaker relations were seen in the fetal circulation. In a multiple regression model, choline was a positive predictor of maternal tHcy, whereas vitamin B-12 and betaine were negative predictors.

CONCLUSIONS

The positive association between maternal choline and tHcy during pregnancy suggests that the high fetal demand for choline stimulates de novo synthesis of choline in maternal liver, with a resultant increase in tHcy concentrations. If this is confirmed, it may be appropriate to provide choline supplements during pregnancy to prevent elevated tHcy concentrations.

Metabolomics in Practice: Emerging Knowledge to Guide Future Dietetic Advice toward Individualized Health

The profession of dietetics can take an increasingly prominent role in managing health and patient care as clinicians gain access to three new resources: detailed information about the metabolic status of healthy individual clients, metabolic knowledge about the relationships between metabolite abundances and health, and bioinformatics tools that link clients' metabolism to their present and future health status. The current use of single biomarkers as indicators of disease will be replaced by comprehensive profiling of individual metabolites linked to an understanding of health and human metabolism--the emerging science now known as metabolomics. Industrial and academic initiatives are currently developing the analytical and bioinformatic technologies needed to assemble the quantitative reference databases of metabolites as the metabolic analog of the human genome. With these in place, dietetics professionals will be able to assess both the current health status of individuals and predict their health trajectories. Another important role for dietetics professionals will be to assist in the development of the tools and their application in predicting how an individual's specific metabolic pattern can be changed by diet, drugs, and lifestyle, with the goal of improving health and preventing the development of chronic diseases.

The nutritional phenotype in the age of metabolomics

The concept of the nutritional phenotype is proposed as a defined and integrated set of genetic, proteomic, metabolomic, functional, and behavioral factors that, when measured, form the basis for assessment of human nutritional status. The nutritional phenotype integrates the effects of diet on disease/wellness and is the quantitative indication of the paths by which genes and environment exert their effects on health. Advances in technology and in fundamental biological knowledge make it possible to define and measure the nutritional phenotype accurately in a cross section of individuals with various states of health and disease. This growing base of data and knowledge could serve as a resource for all scientific disciplines involved in human health. Nutritional sciences should be a prime mover in making key decisions that include: what environmental inputs (in addition to diet) are needed; what genes/proteins/metabolites should be measured; what end-point phenotypes should be included; and what informatics tools are available to ask nutritionally relevant questions. Nutrition should be the major discipline establishing how the elements of the nutritional phenotype vary as a function of diet. Nutritional sciences should also be instrumental in linking the elements that are responsive to diet with the functional outcomes in organisms that derive from them. As the first step in this initiative, a prioritized list of genomic, proteomic, and metabolomic as well as functional and behavioral measures that defines a practically useful subset of the nutritional phenotype for use in clinical and epidemiological investigations must be developed. From this list, analytic platforms must then be identified that are capable of delivering highly quantitative data on these endpoints. This conceptualization of a nutritional phenotype provides a concrete form and substance to the recognized future of nutritional sciences as a field addressing diet, integrated metabolism, and health.

Metabolomics: building on a century of biochemistry to guide human health

Medical diagnosis and treatment efficacy will improve significantly when a more personalized system for health assessment is implemented. This system will require diagnostics that provide sufficiently detailed information about the metabolic status of individuals such that assay results will be able to guide food, drug and lifestyle choices to maintain or improve distinct aspects of health without compromising others. Achieving this goal will use the new science of metabolomics - comprehensive metabolic profiling of individuals linked to the biological understanding of human integrative metabolism. Candidate technologies to accomplish this goal are largely available, yet they have not been brought into practice for this purpose. Metabolomic technologies must be sufficiently rapid, accurate and affordable to be routinely accessible to both healthy and acutely ill individuals. The use of metabolomic data to predict the health trajectories of individuals will require bioinformatic tools and quantitative reference databases. These databases containing metabolite profiles from the population must be built, stored and indexed according to metabolic and health status. Building and annotating these databases with the knowledge to predict how a specific metabolic pattern from an individual can be adjusted with diet, drugs and lifestyle to improve health represents a logical application of the biochemistry knowledge that the life sciences have produced over the past 100 years.

Choline transporter 1 maintains cholinergic function in choline acetyltransferase haploinsufficiency

Choline acetyltransferase (ChAT), the enzyme that synthesizes the neurotransmitter acetylcholine (ACh), is thought to be present in kinetic excess in cholinergic neurons. The rate-limiting factor in ACh production is the provision of choline to ChAT. Cholinergic neurons are relatively unique in their expression of the choline transporter 1 (CHT1), which exhibits high-affinity for choline and catalyzes its uptake from the extracellular space to the neuron. Multiple lines of evidence indicate that the activity of CHT1 is a key determinant of choline supply for ACh synthesis. We examined the interaction of ChAT and ChT activity using mice heterozygous for a null mutation in the Chat gene (Chat+/-). In these mice, brain ChAT activity was reduced by 40-50% relative to the wild type, but brain ACh levels as well as ACh content and depolarization-evoked ACh release in hippocampal slices were normal. However, the amount of choline taken up by CHT1 and ACh synthesized de novo from choline transported by CHT1 in hippocampal slices, as well as levels of CHT1 mRNA in the septum and CHT1 protein in several regions of the CNS, were 50-100% higher in Chat+/- than in Chat+/+ mice. Thus, haploinsufficiency of ChAT leads to an increased expression of CHT1. Increased ChT activity may compensate for the reduced ChAT activity in Chat+/- mice, contributing to the maintenance of apparently normal cholinergic function as reflected by normal performance of these mice in several behavioral assays.

Metabolomics in the opening decade of the 21st century: building the roads to individualized health

It is rapidly becoming possible to measure hundreds or thousands of metabolites in small samples of biological fluids or tissues. This makes it possible to assess the metabolic component of nutritional phenotypes and will allow individualized dietary recommendations. ASNS has to take action to ensure that appropriate technologies are developed and that metabolic databases are constructed with the right inputs and organization. The relations between diet and metabolomic profiles and between those profiles and health and disease must be established. ASNS also should consider the social implications of these advances and plan for their appropriate utilization.

Targeted deletion of hepatic CTP:phosphocholine cytidylyltransferase alpha in mice decreases plasma high density and very low density lipoproteins

CTP:phosphocholine cytidylyltransferase (CT) is the key regulatory enzyme in the CDP-choline pathway for the biosynthesis of phosphatidylcholine. Hepatic cells express both an alpha and a beta2 isoform of CT and can also synthesize phosphatidylcholine via the sequential methylation of phosphatidylethanolamine catalyzed by phosphatidylethanolamine N-methyltransferase. To ascertain the functional importance of CTalpha, we created a mouse in which the hepatic CTalpha gene was specifically inactivated by the Cre/LoxP procedure. In CTalpha knockout mice, hepatic CT activity (due to residual CTbeta2 activity as well as activity in nonhepatic cells) was 15% of normal, whereas phosphatidylethanolamine N-methyltransferase activity was elevated 2-fold compared with controls. Lipid analyses of the liver indicated that female knockout mice had reduced phosphatidylcholine levels and accumulated triacylglycerols. The plasma phosphatidylcholine concentration was reduced in the CTalpha knockout (independent of gender), as were levels of high density lipoproteins (cholesterol and apoAI) and very low density lipoproteins (triacylglycerols and apoB100). Experiments in which mice were injected with Triton WR1339 indicated that apoB secretion was decreased in hepatic-specific CTalpha knockout mice compared with controls. These results suggest an important role for hepatic CTalpha in regulating both hepatic and systemic lipid and lipoprotein metabolism.