Apolipoprotein A-IV regulates chylomicron metabolism-mechanism and function

Dietary fat is an important mediator of atherosclerosis and obesity. Despite its importance in mediating metabolic disease, there is still much unknown about dietary fat absorption in the intestine and especially the detailed biological roles of intestinal apolipoproteins involved in that process. We were specifically interested in determining the physiological role of the intestinal apolipoprotein A-IV (A-IV) using A-IV knockout (KO) mice. A-IV is stimulated by fat absorption in the intestine and is secreted on nascent chylomicrons into intestinal lymph. We found that A-IV KO mice had reduced plasma triglyceride (TG) and cholesterol levels and that this hypolipidemia persisted on a high-fat diet. A-IV KO did not cause abnormal intestinal lipid absorption, food intake, or adiposity. Additionally, A-IV KO did not cause abnormal liver TG and cholesterol metabolism, as assessed by measuring hepatic lipid content, lipogenic and cholesterol synthetic gene expression, and in vivo VLDL secretion. Instead, A-IV KO resulted in the secretion of larger chylomicrons from the intestine into the lymph, and those chylomicrons were cleared from the plasma more slowly than wild-type chylomicrons. These data suggest that A-IV has a previously unknown role in mediating the metabolism of chylomicrons, and therefore may be important in regulating plasma lipid metabolism.     

Why does the gut choose apolipoprotein B48 but not B100 for chylomicron formation?

Chylomicrons produced by the human gut contain apolipoprotein (apo) B48, whereas very-low-density lipoproteins made by the liver contain apo B100. To study how these molecules function during lipid absorption, we examined the process as it occurs in apobec-1 knockout mice (able to produce only apo B100; KO) and in wild-type mice (of which the normally functioning intestine makes apo B48, WT). Using the lymph fistula model, we studied the process of lipid absorption when animals were intraduodenally infused with a lipid emulsion (4 or 6 micromol/h of triolein). KO mice transported triacylglycerol (TG) as efficiently as WT mice when infused with the lower lipid dose; when infused with 6 micromol/h of triolein, however, KO mice transported significantly less TG to lymph than WT mice, leading to the accumulation of mucosal TG. Interestingly, the size of lipoprotein particles from both KO and WT mice were enlarged to chylomicron-size particles during absorption of the higher dose. These increased-size particles produced by KO mice were not associated with increased apo AIV secretion. However, we found that the gut of the KO mice secreted fewer apo B molecules to lymph (compared with WT), during both fasting and lipid infusion, leading us to conclude that the KO gut produced fewer numbers of TG-rich lipoproteins (including chylomicron) than the wild-type animals. The reduced apo B secretion in KO mice was not related to reduced microsomal triglyceride transfer protein lipid transfer activity. We propose that apo B48 is the preferred protein for the gut to coat chylomicrons to ensure efficient chylomicron formation and lipid absorption.     

Overexpression of apolipoprotein A-IV enhances lipid transport in newborn swine intestinal epithelial cells

Apolipoprotein A-IV (apoA-IV) has myriad functions, including roles as a post-prandial satiety factor and lipid antioxidant. ApoA-IV is expressed in mammalian small intestine and is up-regulated in response to lipid absorption. In newborn swine jejunum, a high fat diet acutely induces a 7-fold increase in apoA-IV expression. To determine whether apoA-IV plays a role in the transport of absorbed lipid, swine apoA-IV was overexpressed in a newborn swine enterocyte cell line, IPEC-1, followed by analysis of the expression of genes related to lipoprotein assembly and lipid transport, as well as quantitation of lipid synthesis and secretion. A full-length swine apoA-IV cDNA was cloned, sequenced, and inserted into a Vp and Rep gene-deficient adeno-associated viral vector, containing the cytomegalovirus immediate early promoter/enhancer and neomycin resistance gene, and was used to transfect IPEC-1 cells. Control cells were transfected with the same vector minus the apoA-IV insert. Using neomycin selection, apoA-IV-overexpressing (+AIV) and control (-AIV) clones were isolated for further study. Both undifferentiated (-D) and differentiated (+D) +AIV cells expressed 40- to 50-fold higher levels of apoA-IV mRNA and both intracellular and secreted apoA-IV protein compared with -AIV cells. Expression of other genes was not affected by apoA-IV overexpression in a manner that would contribute to enhanced lipid secretion. +D +AIV cells secreted 4.9-fold more labeled triacylglycerol (TG), 4.6-fold more labeled cholesteryl ester (CE), and 2-fold more labeled phospholipid (PL) as lipoproteins, mostly in the chylomicron/very low density lipoprotein (VLDL) density range. ApoA-IV overexpression in IPEC-1 cells enhances basolateral TG, CE, and PL secretion in chylomicron/VLDL particles. This enhancement is not associated with up-regulation of other genes involved in lipid transport. ApoA-IV may play a role in facilitating enterocyte lipid transport, particularly in the neonate receiving a diet of high fat breast milk.     

Effects of high-fat diet, angiotensinogen (agt) gene inactivation, and targeted expression to adipose tissue on lipid metabolism and renal gene expression.

To address the role of angiotensinogen (agt) in lipid metabolism and its potential endocrine effects in vivo, we studied the effects of high-fat diet (HFD) on adult, 28-week-old agt knockout (KO) mice compared to wild type (WT) mice. Recent studies (Massiera et al., 2001) have demonstrated that reexpression of agt in adipose tissue of KO mice normalized adiposity, blood pressure, and kidney abnormalities. We therefore used microarray analysis to investigate changes in gene expression profile in kidneys of KO vs. Tg-KO mice, where agt expression is restricted to adipose tissue. Body weight, adiposity and insulin levels were significantly decreased (p < 0.05) in KO mice on a chow diet (CD) compared to WT mice, while circulating leptin levels were similar. On a high-fat diet, KO mice exhibited significantly lower bodyweight (p < 0.05), adiposity (p < 0.05), leptin, and insulin levels (p < 0.05) compared to WT mice. In agreement with previously reported changes in kidney histology, agt KO mice displayed altered expressions of genes involved in blood pressure regulation and renal function, but these levels were corrected by reexpression of agt in adipose tissue. Collectively, these findings further document important endocrine roles of adipocyte agt, in part via regulation of lipid metabolism and kidney homeostasis.     

Chronic high-fat diet affects intestinal fat absorption and postprandial triglyceride levels in the mouse

The effects of chronic fat overconsumption on intestinal physiology and lipid metabolism remain elusive. It is unknown whether a fat-mediated adaptation to lipid absorption takes place. To address this issue, mice fed a high-fat diet (40%, w/w) were refed or not a control diet (3%, w/w) for 3 additive weeks. Despite daily lipid intake 7.7-fold higher than in controls, fecal lipid output remained unchanged in mice fed the triglyceride (TG)-rich diet. In situ isolated jejunal loops revealed greater [1-(14)C]linoleic acid uptake without TG accumulation in mucosa, suggesting an increase in lipid absorption capacity. Induction both in intestinal mitotic index and in the expression of genes involved in fatty acid uptake, trafficking, and lipoprotein synthesis was found in high-fat diet mice. These changes were lipid-mediated, in that they were fully abolished in mice refed the control diet. A lipid load test performed in the presence or absence of the LPL inhibitor tyloxapol showed a sustained blood TG clearance in fat-fed mice likely attributable to intestinal modulation of LPL regulators (apolipoproteins C-II and C-III). These data demonstrate that a chronic high-fat diet greatly affects intestinal physiology and body lipid use in the mouse.     

Dietary modulation of circulating leptin levels: site-specific changes in fat deposition and ob mRNA expression.

In order to study the effects of diet on fat distribution, circulating leptin levels and ob mRNA expression, diets of different macronutrient composition were fed to lean mice and gold thioglucose-obese mice. A high-fat diet and 2 high-carbohydrate diets, one containing mostly high-glycaemic-index starch and the other containing low-glycaemic-index starch were fed ad libitum for 10 weeks and were compared to standard laboratory chow. Weight gain was attenuated by feeding low-glycaemic-index starch in all mice and by feeding a high-fat diet in lean mice. Reduced adiposity was seen in lean mice fed low-glycaemic-index starch, whereas increased adiposity was seen in both lean and obese mice fed on the high-fat diet. Circulating leptin levels, when corrected for adiposity, were decreased in all mice fed either the high-fat diet or the low-GI diet. In epididymal fat pads, decreased ob mRNA expression was seen after both high-fat and high-glycaemic-index starch feeding. These results show that diet macronutrient composition contributes to the variability of circulating leptin levels by the combined effects of diet on fat distribution and on site-specific changes in ob mRNA expression.     

Effect of myostatin depletion on weight gain, hyperglycemia, and hepatic steatosis during five months of high-fat feeding in mice

The marked hypermuscularity in mice with constitutive myostatin deficiency reduces fat accumulation and hyperglycemia induced by high-fat feeding, but it is unclear whether the smaller increase in muscle mass caused by postdevelopmental loss of myostatin activity has beneficial metabolic effects during high-fat feeding. We therefore examined how postdevelopmental myostatin knockout influenced effects of high-fat feeding. Male mice with ubiquitous expression of tamoxifen-inducible Cre recombinase were fed tamoxifen for 2 weeks at 4 months of age. This depleted myostatin in mice with floxed myostatin genes, but not in control mice with normal myostatin genes. Some mice were fed a high-fat diet (60% of energy) for 22 weeks, starting 2 weeks after cessation of tamoxifen feeding. Myostatin depletion increased skeletal muscle mass ～30%. Hypermuscular mice had ～50% less weight gain than control mice over the first 8 weeks of high-fat feeding. During the subsequent 3 months of high-fat feeding, additional weight gain was similar in control and myostatin-deficient mice. After 5 months of high-fat feeding, the mass of epididymal and retroperitoneal fat pads was similar in control and myostatin-deficient mice even though myostatin depletion reduced the weight gain attributable to the high-fat diet (mean weight with high-fat diet minus mean weight with low-fat diet: 19.9 g in control mice, 14.1 g in myostatin-deficient mice). Myostatin depletion did not alter fasting blood glucose levels after 3 or 5 months of high-fat feeding, but reduced glucose levels measured 90 min after intraperitoneal glucose injection. Myostatin depletion also attenuated hepatic steatosis and accumulation of fat in muscle tissue. We conclude that blocking myostatin signaling after maturity can attenuate some of the adverse effects of a high-fat diet.     

Enterocyte-specific ATGL overexpression affects intestinal and systemic cholesterol homeostasis

Enterocytes of the small intestine (SI) play an important role in maintaining systemic lipid levels by regulating dietary lipid absorption and postprandial lipoprotein secretion. An excessive amount of dietary-derived triglycerides (TGs) taken up by the apical side of enterocytes or basolaterally internalized lipoprotein remnants can be transiently stored in cytosolic lipid droplets (cLDs). As mice lacking adipose TG lipase (ATGL) in the SI display massive accumulation of cLDs but also delayed cholesterol absorption, we hypothesized that SI-specific overexpression of ATGL (Atgl iTg) might have beneficial effects on lipid homeostasis in the gut and possibly throughout the body. Here, we demonstrate that Atgl iTg mice had only modestly increased enzymatic activity despite drastically elevated Atgl mRNA levels (up to 120-fold) on chow diet, and was highly induced upon high-fat/high-cholesterol diet (HF/HCD) feeding. Atgl iTg mice showed markedly reduced intestinal TG concentrations after acute and chronic lipid challenge without affecting chylomicron TG secretion. Circulating plasma cholesterol levels were significantly lower in Atgl iTg mice under different feeding conditions, contrasting the accelerated uptake of dietary cholesterol into the circulation after HF/HCD feeding. In the fasted state, gene expression analysis revealed modulation of PPARα and liver X receptor (LXR) target genes by an increased fatty acid release, whereas the decreased plasma cholesterol concentrations in refed mice were more likely due to changes in HDL synthesis and secretion. We conclude that ATGL, in addition to its role in TG catabolism, plays a critical role in whole-body cholesterol homeostasis by modulating PPARα and LXR signaling in intestinal enterocytes.     

Decreased Enterobacteriaceae translocation due to gut microbiota remodeling mediates the alleviation of premature aging by a high-fat diet

Aging-associated microbial dysbiosis exacerbates various disorders and dysfunctions, and is a major contributor to morbidity and mortality in the elderly, but the underlying cause of this aging-related syndrome is confusing. SIRT6 knockout (SIRT6 KO) mice undergo premature aging and succumb to death by 4 weeks, and are therefore useful as a premature aging research model. Here, fecal microbiota transplantation from SIRT6 KO mice into wild-type (WT) mice phenocopies the gut dysbiosis and premature aging observed in SIRT6 KO mice. Conversely, an expanded lifespan was observed in SIRT6 KO mice when transplanted with microbiota from WT mice. Antibiotic cocktail treatment attenuated inflammation and cell senescence in KO mice, directly suggesting that gut dysbiosis contributes to the premature aging of SIRT6 KO mice. Increased Enterobacteriaceae translocation, driven by the overgrowth of Escherichia coli, is the likely mechanism for the premature aging effects of microbiome dysregulation, which could be reversed by a high-fat diet. Our results provide a mechanism for the causal link between gut dysbiosis and aging, and support a beneficial effect of a high-fat diet for correcting gut dysbiosis and alleviating premature aging. This study provides a rationale for the integration of microbiome-based high-fat diets into therapeutic interventions against aging-associated diseases.     

Apoprotein composition and turnover in rat intestinal lymph during steady-state triglyceride absorption.

Apoproteins of chylomicrons, very low density lipoprotein (VLDL), and a low density + high density fraction secreted by proximal and distal rat small intestine into mesenteric lymph were examined during triglyceride (TG) absorption. Apoprotein output and composition were determined and the turnover rates of labeled non-apoB (soluble) apoproteins in lipoprotein fractions were measured after an intraluminal [(3)H]leucine pulse during stable TG transport into lymph. The output of VLDL apoproteins exceeded that of chylomicrons during the absorption of 45 micro mol of TG per hour. More [(3)H]leucine was incorporated into VLDL than into chylomicrons and the decay of newly synthesized VLDL apoproteins was more rapid than that of chylomicrons, in part due to higher concentrations of apoA-I and apoA-IV with a rapid turnover rate. Chylomicrons from proximal intestine contained more apoA-I and less C peptides than chylomicrons from distal intestine. Ninety percent of [(3)H]leucine incorporated into soluble apoproteins was in apoA-I and apoA-IV, but little apoARP was labeled. The turnover rate of apoA-I and apoA-IV differed significantly in the lymph lipoproteins examined. Although total C peptide labeling was small, evidence for intestinal apoC-II formation and differing patterns of apoC-III subunit labeling was obtained. [(3)H]Leucine incorporation and apoprotein turnover rates in lipoprotein secreted by proximal and distal intestine were similar. The different turnover rates of apoA-I and apoA-IV in individual lipoproteins suggest that these A apoproteins are synthesized independently in the intestine.-Holt, P. R., A-L. Wu, and S. Bennett Clark. Apoprotein composition and turnover in rat intestinal lymph during steady-state triglyceride absorption.     

Protection against diet-induced obesity and obesity- related insulin resistance in Group 1B PLA2-deficient mice

Group 1B phospholipase A2 (PLA2) is an abundant lipolytic enzyme that is well characterized biochemically and structurally. Because of its high level of expression in the pancreas, it has been presumed that PLA2 plays a role in the digestion of dietary lipids, but in vivo data have been lacking to support this theory. Our initial study on mice lacking PLA2 demonstrated no abnormalities in dietary lipid absorption in mice consuming a chow diet. However, the effects of PLA2 deficiency on animals consuming a high-fat diet have not been studied. To investigate this, PLA2(+/+) and PLA2(-/-) mice were fed a western diet for 16 wk. The results showed that PLA2(-/-) mice were resistant to high-fat diet-induced obesity. This observed weight difference was due to decreased adiposity present in the PLA2(-/-) mice. Compared with PLA2(+/+) mice, the PLA2(-/-) mice had 60% lower plasma insulin and 72% lower plasma leptin levels after high-fat diet feeding. The PLA2(-/-) mice also did not exhibit impaired glucose tolerance associated with the development of obesity-related insulin resistance as observed in the PLA2(+/+) mice. To investigate the mechanism by which PLA(2)(-/-) mice exhibit decreased weight gain while on a high-fat diet, fat absorption studies were performed. The PLA(2)(-/-) mice displayed 50 and 35% decreased plasma [(3)H]triglyceride concentrations 4 and 6 h, respectively, after feeding on a lipid-rich meal containing [(3)H]triolein. The PLA(2)(-/-) mice also displayed increased lipid content in the stool, thus indicating decreased fat absorption in these animals. These results suggest a novel role for PLA(2) in the protection against diet-induced obesity and obesity-related insulin resistance, thereby offering a new target for treatment of obesity and diabetes.     

cis-acting determinants of basal and lipid-regulated apolipoprotein A-IV expression in mice

The levels of apolipoprotein A-IV (apoA-IV) mRNA are regulated by dietary lipid in the liver of both the mouse and rat. Thirteen different inbred mouse strains were fed a high lipid diet, and the effect on apoA-IV liver mRNA levels was examined. It was found that each strain responded in one of two ways. Mice of four strains had higher liver apoA-IV mRNA levels as compared with syngeneic mice fed a normal chow diet. Mice of the other nine strains had decreased liver apoA-IV mRNA levels as compared with syngeneic mice fed a normal chow diet. Using F1 hybrids between mice from BALB/c, C3H, and C57BL/6 and between 129 and C57BL/6, as well as recombinant inbred strains derived from a cross between BALB/c and C57BL/6, we have shown that both the normal level of liver apoA-IV mRNA in the chow-fed mice and the lipid-dependent regulation of apoA-IV mRNA levels are controlled by cis-acting genetic elements. The apoA-IV mRNA levels in mice fed a normal diet varied dramatically among strains, with the largest difference (90-fold) being between the 129/J inbred strain and the C57BL/6J strain. In addition, we have examined the expression of apoA-IV during mouse development. ApoA-IV mRNA is expressed early in mouse liver (16 days postcoitum), whereas others have shown previously that rat liver apoA-IV mRNA is undetectable until 14 days after birth. ApoA-IV mRNA levels in the intestine and apoA-I mRNA levels in the liver and intestine, by contrast, mirror the pattern seen in the rat.     

Effects of dietary Korean proso-millet protein on plasma adiponectin, HDL cholesterol, insulin levels, and gene expression in obese type 2 diabetic mice

We investigated the effect of dietary Korean proso-millet protein concentrate (PMP) on glycemic responses, plasma lipid levels, and the plasma level and gene expression of adiponectin in obese type 2 diabetic mice under normal and high-fat feeding conditions. The findings were that the feeding of PMP clearly elevated plasma high-density lipoprotein cholesterol (HDL cholesterol) and adiponectin levels and brought about effective reduction in the levels of glucose and insulin in mice under high-fat diet conditions as compared with a control diet. Gene expression study revealed that the diet up-regulated expression of adiponectin and down-regulated tumor necrosis factor-alpha (TNF-alpha). Considering the central role of adiponectin and HDL cholesterol in improving and ameliorating type 2 diabetes, obesity, and cardiovascular disease, our findings imply that PMP may have potential for therapeutic intervention in type 2 diabetes.     

Regulation of intestinal and hypothalamic apolipoprotein A-IV

This review discusses the regulation of the intestinal and hypothalamic apolipoprotein A-IV (apo A-IV) gene and protein expression. Apo A-IV is a glycoprotein secreted together with triglyceride-rich lipoproteins by the small intestine. Intestinal apo A-IV synthesis is stimulated by fat absorption, probably mediated by chylomicron formation. This stimulation of intestinal apo A-IV synthesis is attenuated by intravenous leptin infusion. Chronic ingestion of a high-fat diet blunts the intestinal apo A-IV in response to dietary lipid. Intestinal apo A-IV synthesis is also stimulated by members of the pancreatic polypeptide family, including peptide YY (PYY), neuropeptide Y (NPY), and pancreatic polypeptide (PP). Recently, apo A-IV was demonstrated to be present in the hypothalamus as well. Hypothalamic apo A-IV level was reduced by food deprivation and restored by lipid feeding. Intracerebroventricular administration of apo A-IV antiserum stimulated feeding and decreased the hypothalamic apo A-IV mRNA level, implying that feeding is intimately regulated by endogenous hypothalamic apo A-IV. Central administration of NPY significantly increased hypothalamic apo A-IV mRNA levels in a dose-dependent manner.     

Transport of lipid and apolipoproteins A-I and A-IV in intestinal lymph of the rat

Intestinal lipid absorption is associated with marked increases in the synthesis and secretion of apolipoprotein A-IV (apoA-IV) by the small intestine. Whether the increased intestinal apoA-IV synthesis and secretion results from increased fat uptake, increased cellular triglyceride (TG) content, or increased secretion of TG-rich lipoproteins by the enterocytes is unknown. Previous work from this laboratory has shown that a hydrophobic surfactant, Pluronic L-81 (L-81), is a potent inhibitor of intestinal formation of chylomicrons (CM), without reducing fat uptake or re-synthesis to TG. Furthermore, this inhibition can be reversed quickly by the cessation of L-81 infusion. Thus L-81 offers a unique opportunity to study the relationship between lymphatic TG, apoA-I and A-IV secretion. In this study, we studied the lymphatic transport of TG, apoA-I, and apoA-IV during both the inhibitory phase (L-81 infused together with lipid) and the subsequent unblocking phase (saline infusion). Two groups of lymph fistula rats were used, the control and the experimental rats. In the experimental rats, a phosphate-buffered taurocholate-stabilized emulsion containing 40 mumol [3H]triolein, 7.8 mumol of phosphatidylcholine, and 1 mg L-81 per 3 ml was infused at 3 ml/h for 8 h. This was then replaced by glucose-saline infusion for an additional 12 h. The control rats received the same lipid emulsion as the experimental rats, but without L-81 added, for 8 h. Lymph lipid was determined both by radioactivity and by glyceride-glycerol determination, and the apoA-I and apoA-IV concentrations were determined by rocket electroimmunophoresis assay. L-81 inhibited the rise in lymphatic lipid and apoA-IV output in the experimental rats after the beginning of lipid + L-81 infusion. Upon cessation of L-81 infusion, the mucosal lipid accumulated as a result of L-81 treatment was rapidly cleared into lymph as CM. This was associated with a marked increase in apoA-IV output; the maximal output was about 3 times that of the fasting level. There was a time lag of 4-5 h between the peak lymph lipid output and the peak lymph apoA-IV output during the unblocking phase in the experimental rats. There was also a comparable time lag between the maximal lipid and apoA-IV outputs in the control animals. Incorporation studies using [3H]leucine showed that apoA-IV synthesis was not stimulated during lipid + L-81 infusion, perhaps explaining the lack of increase in lymphatic A-IV secretion.(ABSTRACT TRUNCATED AT 400 WORDS)     

Fatty acid transport protein 4 is dispensable for intestinal lipid absorption in mice

FA transport protein 4 (FATP4), one member of a multigene family of FA transporters, was proposed as a major FA transporter in intestinal lipid absorption. Due to the fact that Fatp4(-/-) mice die because of a perinatal skin defect, we rescued the skin phenotype using an FATP4 transgene driven by a keratinocyte-specific promoter (Fatp4(-/-);Ivl-Fatp4(tg/+) mice) to elucidate the role of intestinal FATP4 in dietary lipid absorption. Fatp4(-/-);Ivl-Fatp4(tg/+) mice and wild-type littermates displayed indistinguishable food consumption, growth, and weight gain on either low or high fat (Western) diets, with no differences in intestinal triglyceride (TG) absorption or fecal fat losses. Cholesterol absorption and intestinal TG absorption kinetics were indistinguishable between the genotypes, although Western diet fed Fatp4(-/-);Ivl-Fatp4(tg/+) mice showed a significant increase in enterocyte TG and FA content. There was no compensatory upregulation of other FATP family members or any other FA or cholesterol transporters in Fatp4(-/-);Ivl-Fatp4(tg/+) mice. Furthermore, although serum cholesterol levels were lower in Fatp4(-/-);Ivl-Fatp4(tg/+) mice, there was no difference in hepatic VLDL secretion in-vivo or in hepatic lipid content on either a chow or Western diet. Taken together, our studies find no evidence for a physiological role of intestinal FATP4 in dietary lipid absorption in mice.     

Obesity and altered glucose metabolism impact HDL composition in CETP transgenic mice: a role for ovarian hormones

Mechanisms underlying changes in HDL composition caused by obesity are poorly defined, partly because mice lack expression of cholesteryl ester transfer protein (CETP), which shuttles triglyceride and cholesteryl ester between lipoproteins. Because menopause is associated with weight gain, altered glucose metabolism, and changes in HDL, we tested the effect of feeding a high-fat diet (HFD) and ovariectomy (OVX) on glucose metabolism and HDL composition in CETP transgenic mice. After OVX, female CETP-expressing mice had accelerated weight gain with HFD-feeding and impaired glucose tolerance by hyperglycemic clamp techniques, compared with OVX mice fed a low-fat diet (LFD). Sham-operated mice (SHAM) did not show HFD-induced weight gain and had less glucose intolerance than OVX mice. Using shotgun HDL proteomics, HFD-feeding in OVX mice had a large effect on HDL composition, including increased levels of apoA2, apoA4, apoC2, and apoC3, proteins involved in TG metabolism. These changes were associated with decreased hepatic expression of SR-B1, ABCA1, and LDL receptor, proteins involved in modulating the lipid content of HDL. In SHAM mice, there were minimal changes in HDL composition with HFD feeding. These studies suggest that the absence of ovarian hormones negatively influences the response to high-fat feeding in terms of glucose tolerance and HDL composition. CETP-expressing mice may represent a useful model to define how metabolic changes affect HDL composition and function.     

Lack of skeletal muscle liver kinase B1 alters gene expression,mitochondrial content,inflammation and oxidative stress without affecting high-fat diet-induced obesity or insulin resistance

Ad libitum high-fat diet (HFD) induces obesity and skeletal muscle metabolic dysfunction. Liver kinase B1 (LKB1) regulates skeletal muscle metabolism by controlling the AMP-activated protein kinase family, but its importance in regulating muscle gene expression and glucose tolerance in obese mice has not been established. The purpose of this study was to determine how the lack of LKB1 in skeletal muscle (KO) affects gene expression and glucose tolerance in HFD-fed, obese mice.KO and littermate control wild-type (WT) mice were fed a standard diet or HFD for 14 weeks. RNA sequencing, and subsequent analysis were performed to assess mitochondrial content and respiration, inflammatory status, glucose and insulin tolerance, and muscle anabolic signaling.KO did not affect body weight gain on HFD, but heavily impacted mitochondria-, oxidative stress-, and inflammation-related gene expression. Accordingly, mitochondrial protein content and respiration were suppressed while inflammatory signaling and markers of oxidative stress were elevated in obese KO muscles. KO did not affect glucose or insulin tolerance. However, fasting serum insulin and skeletal muscle insulin signaling were higher in the KO mice. Furthermore, decreased muscle fiber size in skmLKB1-KO mice was associated with increased general protein ubiquitination and increased expression of several ubiquitin ligases, but not muscle ring finger 1 or atrogin-1. Taken together, these data suggest that the lack of LKB1 in skeletal muscle does not exacerbate obesity or insulin resistance in mice on a HFD, despite impaired mitochondrial content and function and elevated inflammatory signaling and oxidative stress.     

Changes in gene expression foreshadow diet-induced obesity in genetically identical mice

High phenotypic variation in diet-induced obesity in male C57BL/6J inbred mice suggests a molecular model to investigate non-genetic mechanisms of obesity. Feeding mice a high-fat diet beginning at 8 wk of age resulted in a 4-fold difference in adiposity. The phenotypes of mice characteristic of high or low gainers were evident by 6 wk of age, when mice were still on a low-fat diet; they were amplified after being switched to the high-fat diet and persisted even after the obesogenic protocol was interrupted with a calorically restricted, low-fat chow diet. Accordingly, susceptibility to diet-induced obesity in genetically identical mice is a stable phenotype that can be detected in mice shortly after weaning. Chronologically, differences in adiposity preceded those of feeding efficiency and food intake, suggesting that observed difference in leptin secretion is a factor in determining phenotypes related to food intake. Gene expression analyses of adipose tissue and hypothalamus from mice with low and high weight gain, by microarray and qRT-PCR, showed major changes in the expression of genes of Wnt signaling and tissue re-modeling in adipose tissue. In particular, elevated expression of SFRP5, an inhibitor of Wnt signaling, the imprinted gene MEST and BMP3 may be causally linked to fat mass expansion, since differences in gene expression observed in biopsies of epididymal fat at 7 wk of age (before the high-fat diet) correlated with adiposity after 8 wk on a high-fat diet. We propose that C57BL/6J mice have the phenotypic characteristics suitable for a model to investigate epigenetic mechanisms within adipose tissue that underlie diet-induced obesity.