Hepatic microsomal triglyceride transfer protein messenger RNA concentrations are increased by dietary cholesterol in hamsters

The lipid substrate preference of CETP controls the biochemical properties of HDL in fat/cholesterol-fed hamsters

Cholesteryl ester transfer protein (CETP) modulates lipoprotein metabolism by transferring cholesteryl ester (CE) and triglyceride (TG) between lipoproteins. However, differences in the way CETP functions exist across species. Unlike human CETP, hamster CETP prefers TG over CE as a substrate, raising questions regarding how substrate preference may impact lipoprotein metabolism. To understand how altering the CE versus TG substrate specificity of CETP might impact lipoprotein metabolism in humans, we modified CETP expression in fat/cholesterol-fed hamsters, which have a human-like lipoprotein profile. Hamsters received adenoviruses expressing no CETP, hamster CETP, or human CETP. Total plasma CETP mass increased up to 70% in the hamster and human CETP groups. Hamsters expressing human CETP exhibited decreased endogenous hamster CETP, resulting in an overall CE:TG preference of plasma CETP that was similar to that in humans. Hamster CETP overexpression had little impact on lipoproteins, whereas human CETP expression reduced HDL by 60% without affecting LDL. HDLs were TG enriched and CE depleted and much smaller, causing the HDL3:HDL2 ratio to increase threefold. HDL from hamsters expressing human CETP supported higher LCAT activity and greater cholesterol efflux. The fecal excretion of HDL-associated CE in human CETP animals was unchanged. However, much of this cholesterol accumulated in the liver and was associated with a 1.8-fold increase in hepatic cholesterol mass. Overall, these data show in a human-like lipoprotein model that modification of CETP's lipid substrate preference selectively alters HDL concentration and function. This provides a powerful tool for modulating HDL metabolism and impacting sterol balance in vivo.

Microsomal triglyceride transfer protein in the ectoparasitic crustacean salmon louse (Lepeophtheirus salmonis)

The salmon louse, Lepeophtheirus salmonis, is an endemic ectoparasite on salmonid fish that is challenging for the salmon farming industry and wild fish. Salmon lice produce high numbers of offspring, necessitating sequestration of large amounts of lipids into growing oocytes as a major energy source for larvae, most probably mediated by lipoproteins. The microsomal triglyceride transfer protein (MTP) is essential for the assembly of lipoproteins. Salmon lice have three L. salmonis MTP (LsMTP) transcript variants encoding two different protein isoforms, which are predicted to contain three β-sheets (N, C, and A) and a central helical domain, similar to MTPs from other species. In adult females, the LsMTPs are differently transcribed in the sub-cuticular tissues, the intestine, the ovary, and in the mature eggs. RNA interference-mediated knockdown of LsMTP in mature females gave offspring with significantly fewer neutral lipids in their yolk and only 10-30% survival. The present study suggests the importance of LsMTP in reproduction and lipid metabolism in adult female L. salmonis, a possible metabolic bottleneck that could be exploited for the development of new anti-parasitic treatment methods.

Cloning and characterization of microsomal triglyceride transfer protein gene and its potential connection with peroxisome proliferator-activated receptor (PPAR) in blunt snout bream (Megalobrama amblycephala)

Microsomal triglyceride transfer protein (MTTP), a major intracellular protein capable of transferring neutral lipids, plays a pivotal role in the assembly and secretion of apolipoprotein B-containing lipoproteins. In this study, MTTP cDNA was firstly cloned from the liver of blunt snout bream (Megalobrama amblycephala), the full-length cDNA covered 3457-bp with an open reading frame of 2661-bp, which encodes 886 amino acids, including a putative signal peptide of 24 amino acids long. After the feeding trial, a graded tissue-specific expression pattern of MTTP was observed and high expression abundance in the liver and intestine indicated its major function in lipid transport in this fish species. In addition, expression of genes encoding MTTP as well as peroxisome proliferator-activated receptor (PPAR), which are transcription factors and serve as key regulators in lipid homoeostasis, was all affected by dietary lipid and choline supplementations. Elevated dietary lipid levels significantly increased the liver, intestinal and muscle MTTP mRNA abundance. Additionally, the down-regulation of MTTP expression in the liver and muscle was observed when fish were fed with inadequate choline supplementation in high-fat diet, yet up-regulated as supplementing extra choline in diet. Expressions of PPARα and PPARβ in the liver and muscle showed similar trend of MTTP expression. The results suggested the potential connection of MTTP and PPAR in response to different dietary nutritional factors. Furthermore, extra choline supplementations could promote lipid transfer and enhance fatty acid oxidation, which indicated a molecular mechanism of choline on diminishing fat accumulation in blunt snout bream.

In vivo biochemical and gene expression analyses of the antioxidant activities and hypocholesterolaemic properties of Tamarindus indica fruit pulp extract

BACKGROUND

Tamarindus indica (T. indica) is a medicinal plant with many biological activities including anti-diabetic, hypolipidaemic and anti-bacterial activities. A recent study demonstrated the hypolipidaemic effect of T. indica fruit pulp in hamsters. However, the biochemical and molecular mechanisms responsible for these effects have not been fully elucidated. Hence, the aims of this study were to evaluate the antioxidant activities and potential hypocholesterolaemic properties of T. indica, using in vitro and in vivo approaches.

METHODOLOGY/PRINCIPAL FINDINGS

The in vitro study demonstrated that T. indica fruit pulp had significant amount of phenolic (244.9 ± 10.1 mg GAE/extract) and flavonoid (93.9 ± 2.6 mg RE/g extract) content and possessed antioxidant activities. In the in vivo study, hamsters fed with high-cholesterol diet for ten weeks showed elevated serum triglyceride, total cholesterol, HDL-C and LDL-C levels. Administration of T. indica fruit pulp to hypercholesterolaemic hamsters significantly lowered serum triglyceride, total cholesterol and LDL-C levels but had no effect on the HDL-C level. The lipid-lowering effect was accompanied with significant increase in the expression of Apo A1, Abcg5 and LDL receptor genes and significant decrease in the expression of HMG-CoA reductase and Mtp genes. Administration of T. indica fruit pulp to hypercholesterolaemic hamsters also protected against oxidative damage by increasing hepatic antioxidant enzymes, antioxidant activities and preventing hepatic lipid peroxidation.

CONCLUSION/SIGNIFICANCE

It is postulated that tamarind fruit pulp exerts its hypocholesterolaemic effect by increasing cholesterol efflux, enhancing LDL-C uptake and clearance, suppressing triglyceride accumulation and inhibiting cholesterol biosynthesis. T. indica fruit pulp has potential antioxidative effects and is potentially protective against diet-induced hypercholesterolaemia.

Microsomal triglyceride transfer protein and nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease is currently one of the most common forms of liver disease, covering cases from simple steatosis without inflammation, to cases of steatohepatitis and fibrosis, and may lead to liver cirrhosis and hepatocellular carcinoma. The pathophysiology of nonalcoholic fatty liver disease is based on multiple events; changes in the secretion of lipoproteins can lead to steatosis. Liver lipid secretion is mediated by apoB100 and microsomal triglyceride transfer protein (MTP). The pharmacological suppression of MTP is suggested as a possible treatment for hyperlipidemia, although the upregulation of this protein can be a treatment for nonalcoholic steatohepatitis.

The use of human tissues and cells in biomedical research: the unusual suspects

There are compelling reasons to search for alternatives to the use of animals in medical and pharmaceutical research. Aside from the obvious animal welfare issues, both the well-established differences between animal models and humans, and the inherent inter-individual variability in human biological responses, indicate that human-based alternatives are urgently required. However, any such alternative must out-perform the animal-based alternative, otherwise there will be little or no uptake and adoption by end-users. Data obtained from inbred animal models is often highly reproducible, and is therefore attractive to researchers in the fields of biomedical and pharmaceutical research. The inter-individual variability observed during human volunteer and human tissue-based studies is often considered to be problematic, and has been highlighted further with the advent of the 'omics' technologies, which generate large biological datasets. However, the variability in both baseline data and response to pharmacological or toxicological challenge observed in human tissues potentially contains a veritable gold mine of information, which may be critical for the advancement of drug discovery.

Mechanisms underlying different responses of plasma triglyceride to high-fat diets in hamsters and mice: roles of hepatic MTP and triglyceride secretion

Hypertriglyceridemia, closely associated with insulin resistance, is induced on high-fat diets (HFD) in humans but not in mouse models. Mechanisms underlying this species difference are still unclear. Hamsters resemble humans in lipoprotein metabolism. Here by comparing the responses to HFD in hamsters and mice, we found that hepatic TG secretion, MTP expression and plasma free fatty acid (FFA) level were increased in hamsters on HFD feeding but decreased in mice. Although hepatic steatosis and de novo lipogenesis were induced by HFD feeding in both models, cholesterol biosynthesis was inhibited in mice but not in hamsters. Moreover, in insulin deficient state, HFD increased plasma TG level, hepatic TG secretion, MTP expression and plasma FFA level in both models. In summary, distinct changes of MTP expression, in correlation with hepatic TG secretion, underlie the opposite responses of plasma TG levels to high-fat diets in hamsters and mice. Furthermore, hepatic TG secretion and MTP expression seems to be associated with plasma FFA level and cholesterol biosynthesis but not hepatic steatosis or de novo lipogenesis.

Microsomal triglyceride transfer protein in plasma and cellular lipid metabolism

PURPOSE OF REVIEW

This review summarizes recent advances about the role of microsomal triglyceride transfer protein in plasma and tissue lipid homeostasis.

RECENT FINDINGS

Microsomal triglyceride transfer protein emerged as a phospholipid transfer protein and acquired triacylglycerol transfer activity during evolution from invertebrates to vertebrates. These activities are proposed to participate in 'nucleation' and 'desorption' steps during the biosynthesis of primordial apoB-containing lipoproteins. Microsomal triglyceride transfer protein also transfers phospholipids to the glycolipid antigen presentation molecule CD1d. Under physiologic conditions, plasma apoB-containing lipoproteins and microsomal triglyceride transfer protein expression exhibit diurnal variations synchronized by food and light. Microsomal triglyceride transfer protein is regulated at the transcriptional level. HNF4alpha is critical for its transcription. Other transcription factors along with coactivators and corepressors modulate microsomal triglyceride transfer protein expression. Reductions in microsomal triglyceride transfer protein mRNA and activity are related to steatosis in HCV-3 infected patients. CCl4 induces steatosis by enhancing proteasomal degradation of microsomal triglyceride transfer protein and can be partially avoided by inhibiting this degradation. Chemical antagonists cause hepatosteatosis, but this was not seen in the absence of fatty acid binding protein.

SUMMARY

Microsomal triglyceride transfer protein is a target to lower plasma lipids and to reduce inflammation in certain immune disorders. More knowledge is required, however, regarding its regulation and its role in the biosynthesis of apoB-containing lipoproteins and CD1d.

Diurnal regulation of microsomal triglyceride transfer protein and plasma lipid levels

Plasma lipids are maintained within a narrow physiologic range and exhibit circadian rhythmicity. Plasma triglyceride and cholesterol levels were high in the night due to changes in apolipoprotein B-lipoproteins in ad libitum fed rats and mice maintained in a 12-h photoperiod. Absorption of [(3)H]triolein or [(3)H]cholesterol was higher at 2400 h than at 1200 h, indicating that intestinal lipoprotein production shows diurnal variation. Moreover, intestinal microsomal triglyceride transfer protein (MTP) activity, protein, mRNA, and gene transcription showed diurnal variations and were high at 2400 h. Similar to the small intestine, hepatic MTP activity, protein, and mRNA levels also changed significantly within a day. MTP was induced in fasted animals soon after refeeding. When mice were subjected to restricted feeding, MTP expression was high at the expected time of food availability. In contrast, extended exposures to light and dark completely abolished rhythmicity in MTP expression and plasma lipid levels. These studies show that MTP expression and plasma lipid undergo diurnal regulation and exhibit peaks and nadirs at similar times and suggest that diurnal modulation of MTP is a major determinant of daily changes in plasma lipids. Furthermore, environmental factors, such as food and light, play an important role in MTP regulation.

The assembly of triacylglycerol-rich lipoproteins: an essential role for the microsomal triacylglycerol transfer protein

Raised plasma triacylglycerol is an independent risk factor for cardiovascular disease, and an understanding of factors which regulate the synthesis and degradation of lipoproteins which carry triacylglycerol in the blood may lead to novel approaches to the treatment of hypertriacylglycerolaemia. An active microsomal triacylglycerol transfer protein (MTP) is essential for the assembly of particles which transport triacylglycerol through the circulation. After absorption in the intestine, dietary fat and fat-soluble vitamins are incorporated into chylomicrons in the intestinal epithelial cells, and these lipoproteins reach the bloodstream via the lymphatic system. Patients with the rare genetic disorder, abetalipoproteinaemia, in which MTP activity is absent, present clinically with fat-soluble vitamin and essential fatty acid deficiency, indicating a key role for MTP in the movement of fat into the body. The triacylglycerol-rich lipoprotein found in fasting blood, VLDL, is assembled in the liver by an MTP-dependent process similar to chylomicron assembly, and transports triacylglycerol to extra-hepatic tissues such as adipose tissue and heart. In the absence of MTP activity, VLDL are not synthesized and only extremely low levels of triacylglycerol are present in the blood. Dietary components, including fat, cholesterol and ethanol, can modify the expression of the MTP gene and, hence, MTP activity. The present review summarizes current knowledge of the role of MTP in the assembly and secretion of triacylglycerol-rich lipoproteins, and the regulation of its activity in both animal and cell systems.

Genes that affect cholesterol synthesis, cholesterol absorption, and chylomicron assembly: the relationship between the liver and intestine in control and streptozotosin diabetic rats

Chylomicrons and very low-density lipoproteins (VLDLs) are abnormal in diabetes. The aim of this study was to compare the expression of Niemann-Pick C1-like1 (NPC1L1), adenosine triphosphate-binding cassette (ABC) proteins G5 and G8, microsomal triglyceride transfer protein (MTP), and 3-hydroxy-3-methylglutaryl coenzyme A (HMGCoA) reductase in the fasting and fed states in nondiabetic Sprague-Dawley rats fed a high-fat/cholesterol diet and to examine the messenger RNA (mRNA) expression of these proteins in the liver and intestine of diabetic and control animals using streptozotosin diabetic cholesterol-fed rats. Chylomicron and VLDL concentrations were significantly lower after a 12-hour fast in fasted compared with fed rats (P < .02). There was no change with fasting in mRNA expression of any of the genes in the intestine, but MTP level was significantly lower in the liver after the 12-hour fast (P < .01). There was a positive correlation between intestinal NPC1L1 mRNA and chylomicron cholesterol (P < .01) and between hepatic NPC1L1 mRNA and VLDL cholesterol (P < .01). The diabetic rats had significantly higher chylomicron and VLDL cholesterol, triglyceride, and apolipoprotein B-48 and B-100 levels compared with control rats (P < .0001). They had significantly increased NPC1L1 and MTP mRNA in both liver and intestine (P < .05 and P < .0005, respectively), and ABCG5 and ABCG8 mRNA were significantly reduced (P < .05). HMGCoA reductase mRNA was increased in diabetic animals (P < .01). In conclusion, fasting intestinal gene expression reflects the fed state. In diabetes, intestinal and hepatic gene expression correlates with abnormalities in chylomicron and VLDL cholesterol.

Microsomal triglyceride transfer protein is required for yolk lipid utilization and absorption of dietary lipids in zebrafish larvae

Although the absorption, transport, and catabolism of dietary lipids have been studied extensively in great detail in mammals and other vertebrates, a tractable genetic system for identifying novel genes involved in these physiologic processes is not available. To establish such a model, we monitored neutral lipid by staining fixed zebrafish larvae with oil red o (ORO). The head structures, heart, vasculature, and swim bladder stained with ORO until the yolk was consumed 6 days after fertilization (6 dpf). Thereafter, the heart and vasculature no longer had stainable neutral lipids. Following a high-fat meal, ORO stained the intestine and vasculature of 6 dpf larvae, and whole-larval triacylglycerol (TAG) and apolipoprotein B levels increased. Levels of microsomal triglyceride transfer protein (Mtp), the protein responsible for packaging TAG and betalipoproteins into lipoprotein particles, were unchanged by feeding. Since the developing zebrafish embryo expresses mtp in the yolk cell layer, liver, and intestine, we determined the effect of targeted knockdown of Mtp expression using an antisense morpholino oligonucleotide approach (Mtp MO) on the transport of yolk and dietary lipids. Mtp MO injection led to loss of Mtp expression and of lipid staining in the vasculature, heart, and head structures. Mtp MO-injected larvae were smaller than age-matched, uninjected larvae, consumed very little yolk, and did not absorb dietary neutral lipids; however, they absorbed a short chain fatty acid that does not require Mtp for transport. Importantly, the vasculature appeared unaffected in Mtp MO-injected larvae. These studies indicate that zebrafish larvae are suitable for genetic studies of lipid transport and metabolism.

Developmental expression and nutritional regulation of a zebrafish gene homologous to mammalian microsomal triglyceride transfer protein large subunit

The microsomal triglyceride transfer protein (MTP) large subunit is required for the assembly and secretion of apolipoprotein B-containing lipoproteins. We have found a zebrafish mtp homologous gene coding a protein with 54% identity with human MTP large subunit with the most conserved regions distributed in the corresponding predicted alpha-helical and C- and A-sheet domains. In situ hybridizations showed that zebrafish mtp transcripts were distributed in the yolk syncytial layer during early embryogenesis and in anterior intestine and liver from 48 hr postfertilization onward. Real-time quantitative RT-PCR confirmed the developmental regulation and tissue-specificity of mtp expression. A significant pretranslational up-regulation of mtp expression was observed in the anterior intestine after feeding. The nutritional regulation of zebrafish mtp expression observed in the anterior intestine supports the notion that this protein, similar to mammalian MTP large subunit, could be a factor implicated directly or indirectly in large lipid droplets accumulation observed in the fish enterocyte after feeding.

Transcriptional regulation of human microsomal triglyceride transfer protein by hepatocyte nuclear factor-4alpha

Microsomal triglyceride transfer protein (MTP) catalyzes the assembly of triglyceride (TG)-rich apolipoprotein B-containing liver (e.g., VLDL) and intestinal (e.g., chylomicron) lipoproteins. The human MTP gene promoter is reported here to associate in vivo with endogenous hepatocyte nuclear factor-4alpha (HNF-4alpha) and to be transactivated or transsuppressed by overexpressed or by dominant negative HNF-4alpha, respectively. Human MTP (hMTP) transactivation by HNF-4alpha is accounted for by the concerted activity of distal (-83/-70) and proximal (-50/-38) direct repeat 1 elements of the hMTP promoter that bind HNF-4alpha. Transactivation by HNF-4alpha is specifically antagonized by chicken ovalbumin upstream promoter. Transcriptional activation of hMTP by HNF-4alpha is mediated by HNF-4alpha domains engaged in ligand binding and ligand-driven transactivation and is further complemented by HNF-4alpha/HNF-1alpha synergism that involves the HNF-4alpha activation function 1 (AF-1) domain. hMTP transactivation by HNF-4alpha is specifically inhibited by beta,beta-tetramethyl-hexadecanedioic acid acting as an HNF-4alpha antagonist ligand. hMTP transactivation by HNF-4alpha may account for the activation or inhibition of MTP expression and the production of TG-rich lipoproteins by agonist (e.g., saturated fatty acids) or antagonist [e.g., (n-3) PUFA, hypolipidemic fibrates, or Methyl-substituted dicarboxylic acid (Medica) compounds] HNF-4alpha ligands.

Small Intestine and Liver Microsomal Triacylglycerol Transfer Protein in the Bovine and Rat: Effects of Dietary Coconut Oil

The bovine liver is characterized by a chronic low capacity to secrete triacylglycerols (TAG). In situations favoring their hepatic synthesis, such as coconut oil feeding, TAG accumulate, leading to a lipid infiltration in the liver of preruminant calves. To assess the possible role of the microsomal TAG transfer protein (MTP) in this phenomenon and to put into evidence a tissue-specific regulation in the bovine species, we compared by Western blot the content in both MTP subunits in the liver and in different portions of the small intestine in preruminant calves and in growing rats receiving coconut oil or beef tallow as the sole source of fat in the diet. The pattern of MTP distribution was similar between calf and rat tissues, the jejunum being the major site for both MTP expression and intestinal absorption of dietary lipid endproducts. Concentrations of the MTP large and small subunits were 10- to 20-fold lower and 2- to 3-fold lower, respectively, in calf than in rat tissues, including the liver. Coconut oil in the diets of calves and rats did not significantly affect the expression of MTP large subunits even though TAG content was strongly increased 12-fold in the calf liver. These results clearly indicated that calf liver handled fat metabolically in a manner different from rat liver. However, present experimental conditions did not allow proof that MTP was directly related to the accumulation of fat in calf liver.

Dietary oxidized cholesterol decreases expression of hepatic microsomal triglyceride transfer protein in rats

The aim of this study was to compare the effects of dietary oxidized cholesterol and pure cholesterol on plasma and very low density lipoprotein (VLDL) lipids and on some parameters of VLDL assembly and secretion in rats fed two different dietary fats. Four groups of male growing Sprague-Dawley rats were fed diets containing pure or oxidized cholesterol (5 g/kg diet) with either coconut oil or salmon oil as dietary fat (100 g/kg diet) for 35 days. Rats fed oxidized cholesterol supplemented diets had significantly lower concentrations of triglycerides and cholesterol in plasma and VLDL than rats fed pure cholesterol supplemented diets irrespective of the type of fat. In addition, rats fed oxidized cholesterol supplemented diets had significantly lower relative concentrations of microsomal triglyceride transfer protein messenger ribonucleic acid (mRNA) than rats fed pure cholesterol supplemented diets. In contrast, hepatic lipid concentrations and the relative concentration of apolipoprotein B mRNA were not influenced by the dietary factors investigated. Parameters of hepatic lipogenesis (relative mRNA concentration of sterol regulatory element binding protein-1c and activity of glucose-6-phosphat dehydrogenase) were significantly reduced by feeding fish oil compared to coconut oil, but were not affected by the type of cholesterol. In conclusion, the data of this study suggest, that dietary oxidized cholesterol affects VLDL assembly and/or secretion by reducing the synthesis of MTP but not by impairing hepatic lipogenesis or synthesis of apolipoprotein B.

Sex difference in hepatic microsomal triglyceride transfer protein expression is determined by the growth hormone secretory pattern in the rat

Microsomal triglyceride transfer protein (MTP) is essential and rate limiting for the assembly and secretion of apoB-containing lipoproteins. The aim of this study was to investigate whether gender and GH influence hepatic MTP expression. We used intact, gonadectomized, or hypophysectomized (Hx) adult Sprague Dawley rats. Gonadal steroids and insulin were given as a daily sc injection for 7 d. GH was given for 7 d either as a continuous infusion or as two daily injections (2 x GH) to mimic the feminine and masculine GH secretory patterns, respectively. MTP mRNA and MTP and protein disulfide isomerase protein expression was measured. MTP mRNA, and protein expression was higher in females than in males. Gonadectomy abolished the sex difference, and treatment with gonadal steroids restored the sex difference in MTP mRNA levels. MTP mRNA expression was not influenced in either sex by 2 wk of cholesterol (1% wt/wt) feeding. Hx decreased MTP mRNA in females but not in males. A continuous GH infusion increased MTP mRNA and protein expression in intact males but not in females. A continuous GH infusion to Hx females normalized MTP mRNA and protein expression, but 2 x GH had no effect. Also, insulin treatment had no effect. In summary, MTP expression is sex differentiated and regulated by the sexually dimorphic secretory pattern of GH at the level of mRNA. These results are important for the understanding of the effects of gender and GH in the regulation of very low-density lipoprotein assembly and secretion.

Dietary diacylglycerol suppresses high fat diet-induced hepatic fat accumulation and microsomal triacylglycerol transfer protein activity in rats

We have recently shown that the long-term ingestion of dietary diacylglycerol (DAG) mainly containing 1,3-isoform reduces body fat accumulation in humans as compared to triacylglycerol (TAG) with the same fatty acid composition. The fat reduction in this human experiment was most pronounced in visceral fat and hepatic fat. Recent animal studies have also indicated that dietary DAG induces alteration of lipid metabolism in the rat liver. In the present study, the dietary effects of DAG on high fat diet-induced hepatic fat accumulation and hepatic microsomal triglyceride transfer protein (MTP) activity were examined in comparison with those of TAG diet in rats. When the TAG oil content was increased from 10 to 30 g/100 g diet, hepatic TAG concentration, hepatic MTP activity and MTP large subunit mRNA levels were significantly increased after 21 days. However, when the dietary TAG oil (30 g/100 g diet) was replaced with the same concentration of DAG oil with the same fatty acid composition, the increase of the TAG concentration and the MTP activity in the liver were significantly less and the mRNA levels remained unchanged. The MTP activity levels correlated significantly with hepatic TAG concentration.These results showed that dietary DAG may suppress high fat diet-induced MTP activity in the liver, and indicated the possibility that hepatic TAG concentration may regulate hepatic MTP activity.

Garlic inhibits microsomal triglyceride transfer protein gene expression in human liver and intestinal cell lines and in rat intestine

Epidemiologic studies have suggested that fresh garlic has lipid-lowering activity. Because the microsomal triglyceride transfer protein (MTP) plays a pivotal role in the assembly and secretion of apolipoprotein B (apoB)-containing lipoproteins, we evaluated the effect of garlic on the expression of the MTP gene in vitro in cell lines and in vivo in rats. Fresh garlic extract (FGE) reduced MTP mRNA levels in both the human hepatoma HepG2 and intestinal carcinoma Caco-2 cells in dose-dependent fashion; significant reductions were detected with 3 g/L FGE. Maximal 72 and 59% reductions, respectively, were observed with 6 g/L FGE. To evaluate the in vivo effect of garlic on MTP gene expression, rats were given a single oral dose of fresh garlic homogenate (FGH), with hepatic and intestinal MTP mRNA measured 3 h after dosing. Rats fed FGH had significantly (46% of the control) lower intestinal MTP mRNA levels compared with the control rats, whereas hepatic MTP mRNA levels were not affected. These results suggest a new mechanism for the hypolipidemic effect of fresh garlic. Long-term dietary supplementation of fresh garlic may exert a lipid-lowering effect partly through reducing intestinal MTP gene expression, thus suppressing the assembly and secretion of chylomicrons from intestine to the blood circulation.

Microsomal triglyceride transfer protein: does insulin resistance play a role in the regulation of chylomicron assembly?

We have previously demonstrated that diabetes is associated with an increase in intestinal microsomal triglyceride transfer protein (MTP) mRNA in both the rat and rabbit models. The present study was designed to investigate the relationship between MTP expression and chylomicron assembly in an insulin resistant non-diabetic animal model. Ten insulin resistant Zucker obese fa/fa rats and ten lean fa/minus sign rats were examined at 8-10 weeks of age. The lymph duct was cannulated and lymph collected for 4 h. Lymph chylomicrons were isolated by ultracentrifugation and their composition determined. RNA was extracted from intestinal mucosa and from the liver. MTP mRNA was measured using the RNase protection assay. Blood sugar in the fatty rats was significantly higher (6.3 +/-1.2 vs. 5.4 +/-0.4 P<0.05) and plasma insulin was almost six times that of the lean rats (P<0.001). Plasma cholesterol and phospholipid but not triglyceride were significantly increased in the obese animals (P<0.01). Obese animals secreted significantly more lymph chylomicron apo B48 (0.05 +/-0.02 vs. 0.02 +/-0.01 mg/h P<0.005), triglyceride (9.7 +/-5.3 vs. 3.8+/-1.9 mg/h P<0.005) and phospholipid (1.5 +/-0.7 vs. 0.4 +/-0.3 mg/h P<0.001). The only difference in the chylomicron particle composition between the two groups was a significant increase in phospholipid (P<0.01). Intestinal MTP mRNA expression was significantly higher in the fatty compared to the lean rats (22.1 +/-9.5 vs. 7.8+/-5.6 amol MTP mRNA/microg total RNA P<0.001) as was hepatic MTP mRNA expression (6.9 +/-3.5 vs. 3.4 +/-1.5 amol MTP mRNA/microg total RNA, P<0.01). Thus in this animal model of insulin resistance, increased MTP, which was associated with increased chylomicron particle number, may play a crucial role in the development of atherosclerosis.

Effects of fatty acids and growth hormone on liver fatty acid binding protein and PPARalpha in rat liver

The aim of this study was to investigate the interaction between long-chain fatty acids (LCFA) and growth hormone (GH) in the regulation of liver fatty acid binding protein (LFABP) and peroxisome proliferator-activated receptor-alpha (PPARalpha). Cultured rat hepatocytes were given oleic acid (OA; 500 microM) and GH (100 ng/ml) for 3 days. LFABP mRNA increased 3.6-fold by GH and 5.7-fold by OA, and combined incubation with GH and OA increased LFABP mRNA 17.6-fold. PPARalpha mRNA was decreased 50% by GH, but OA had no effect. Hypophysectomized (Hx) female rats were treated with L-thyroxine, cortisol, GH, and dietary fat for 7 days. PPARalpha mRNA levels were three- to fourfold higher in Hx than in normal female rats. GH decreased PPARalpha mRNA 50% in Hx rats. Dietary triglycerides (10% corn oil) increased LFABP mRNA and cytosolic LFABP about twofold but had no effect on PPARalpha mRNA in Hx rats. GH and dietary triglycerides had an additive effect on LFABP expression. Dietary triglycerides increased mitochondrial hydroxymethylglutaryl-CoA synthase mRNA only in the presence of GH. The diet increased serum triglycerides in Hx rats, and GH treatment prevented this increase. Addition of cholesterol to the diet did not influence LFABP levels but mitigated increased hepatic triglyceride content. In summary, these studies show that GH regulates LFABP expression independently of PPARalpha. Moreover, GH has different effects on PPARalpha-responsive genes and does not counteract the effect of LCFA on the expression of these gene products.

The role of the microsomal triglygeride transfer protein in abetalipoproteinemia

The microsomal triglyceride transfer protein (MTP) is a dimeric lipid transfer protein consisting of protein disulfide isomerase and a unique 97-kDa subunit. In vitro, MTP accelerates the transport of triglyceride, cholesteryl ester, and phospholipid between membranes. It was recently demonstrated that abetalipoproteinemia, a hereditary disease characterized as an inability to produce chylomicrons and very low-density lipoproteins in the intestine and liver, respectively, results from mutations in the gene encoding the 97-kDa subunit of the microsomal triglyceride transfer protein. Downstream effects resulting from this defect include malnutrition, very low plasma cholesterol and triglyceride levels, altered lipid and protein compositions of membranes and lipoprotein particles, and vitamin deficiencies. Unless treated, abetalipoproteinemic subjects develop gastrointestinal, neurological, ophthalmological, and hematological abnormalities.

Etiology of fatty liver in dairy cattle: effects of nutritional and hormonal status on hepatic microsomal triglyceride transfer protein

We conducted three experiments to determine the effects of nutritional and hormonal status on microsomal triglyceride transfer protein (MTP) activity and mass. In experiment 1, 18 nonlactating Holstein cows, 75 d before expected calving date, in their second gestation or greater were monitored from d 75 to 55 prepartum. Cows were fed a control diet from d 75 to 62 prepartum for covariable measurements. From d 61 to 55 prepartum, six cows continued to receive the control diet, six cows were restricted to 2.3 kg of grass hay/d, and six cows were fed the control diet plus 1.8 kg of concentrate/d and 500 ml of propylene glycol given 2 times/d as an oral drench. Plasma glucose and serum insulin concentrations were highest in cows that received propylene glycol and lowest in feed restricted cows. Plasma nonesterified fatty acids (NEFA) and liver triglyceride (TG) concentrations were highest in feed restricted cows and not different between cows that received the control diet and cows that received propylene glycol. Hepatic MTP activity and mass were not affected by treatment in experiment 1. In experiment 2, bovine hepatocytes isolated from the caudate process of five preruminating Holstein bull calves were incubated with either 0, 0.5, 1.0, or 2.0 mM NEFA for 48 h. Intracellular TG increased linearly as NEFA concentration in the media increased. Concentration of NEFA in the incubation media had no effect on MTP activity or mass. There was a quadratic effect of concentration of NEFA in the incubation media on MTP mRNA. In experiment 3, bovine hepatocytes isolated from the caudate process of five preruminating Holstein bull calves were incubated with 2 mM [1-14C]oleate for 24 h to accumulate TG, followed by a 36-h period of TG depletion, during which hepatocytes were incubated with no hormone, 10 nM insulin, or 10 nM glucagon. There was no effect of insulin or glucagon on intracellular TG, MTP activity or mass. Cells incubated with no hormone had higher levels of MTP mRNA compared to cells incubated with insulin or glucagon during the depletion period. Results suggest that hepatic MTP mRNA may be affected by TG accumulation, insulin, and glucagon in vitro. However, hepatic MTP activity and mass are not affected by nutritional status of nonlactating dairy cows, TG accumulation in vitro, or insulin and glucagon in vitro.

Differences in activity of hepatic microsomal triglyceride transfer protein among species

Five sows, five cows, five hens, six guinea pigs, six rabbits, and six rats were used in a study to determine if hepatic microsomal triglyceride transfer protein activity differed among species that varied in site of fatty acid synthesis and rate of hepatic triglyceride export. No differences in plasma nonesterified fatty acids were seen among species. Plasma concentrations of glucose were highest in the hen, intermediate in the rat, guinea pig, and rabbit and lowest in the sow and cow. Liver triglyceride was low in all species with the only significant difference being between the hen and the guinea pig (4.7 and 1.1%, DM basis, respectively). No microsomal triglyceride transfer protein activity was found in muscle. The cow, rat, and guinea pig had the lowest levels and the hen and rabbit the highest levels of duodenal microsomal triglyceride transfer protein activity. Hepatic microsomal triglyceride transfer protein activity was significantly higher in the sow than the other species. Hepatic microsomal triglyceride transfer protein activity was 1.51, 1.63, 2.36, 2.72, 2.95, and 6.70 nmole triolein transferred/h/mg microsomal protein for the guinea pig, rabbit, cow, rat, hen, and sow, respectively. Microsomal triglyceride transfer protein activity in duodenal tissue was 18.0, 18.6, 19.2, 33.4, 113, and 161% of hepatic microsomal triglyceride transfer protein activity for the sow, cow, rat, guinea pig, hen, and rabbit, respectively. Hepatic microsomal triglyceride transfer protein activity scaled to liver weight and metabolic body size was 2.69, 3.36, 4.58, 5.83, 7.49, and 22.3 nmole triolein transferred in the liver/min/kg body weight0.75 for the rabbit, guinea pig, rat, hen, cow, and sow, respectively. There was little relationship between previously published rates for triglyceride export and hepatic microsomal triglyceride transfer protein activity measured in this experiment.