Variability of the intestinal uptake of lipids is genetically determined in mice

The Fabp4-Cre-Model is Insufficient to Study Hoxc9 Function in Adipose Tissue

Developmental genes are important regulators of fat distribution and adipose tissue (AT) function. In humans, the expression of homeobox c9 (HOXC9) is significantly higher in subcutaneous compared to omental AT and correlates with body fat mass. To gain more mechanistic insights into the role of Hoxc9 in AT, we generated Fabp4-Cre-mediated Hoxc9 knockout mice (ATHoxc9^-/-). Male and female ATHoxc9^-/- mice were studied together with littermate controls both under chow diet (CD) and high-fat diet (HFD) conditions. Under HFD, only male ATHoxc9^-/- mice gained less body weight and exhibited improved glucose tolerance. In both male and female mice, body weight, as well as the parameters of glucose metabolism and AT function were not significantly different between ATHoxc9^-/- and littermate control CD fed mice. We found that crossing Hoxc9 floxed mice with Fabp4-Cre mice did not produce a biologically relevant ablation of Hoxc9 in AT. However, we hypothesized that even subtle reductions of the generally low AT Hoxc9 expression may cause the leaner and metabolically healthier phenotype of male HFD-challenged ATHoxc9^-/- mice. Different models of in vitro adipogenesis revealed that Hoxc9 expression precedes the expression of Fabp4, suggesting that ablation of Hoxc9 expression in AT needs to be achieved by targeting earlier stages of AT development.

Role of MCP-1 on inflammatory processes and metabolic dysfunction following high-fat feedings in the FVB/N strain

Background

MCP-1 is known to be an important chemokine for macrophage recruitment. Thus, targeting MCP-1 may prevent the perturbations associated with macrophage-induced inflammation in adipose tissue. However, inconsistencies in the available animal literature have questioned the role of this chemokine in this process. The purpose of this study was to examine the role of MCP-1 on obesity-related pathologies.

Methods

Wild-type (WT) and MCP-1 deficient mice on an FVB/N background were assigned to either low-fat-diet (LFD) or high-fat-diet (HFD) treatment for a period of 16 weeks. Body weight and body composition were measured weekly and monthly, respectively. Fasting blood glucose and insulin, and glucose tolerance were measured at 16 weeks. Macrophages, T cell markers, inflammatory mediators, and markers of fibrosis were examined in the adipose tissue at sacrifice.

Results

As expected, HFD increased adiposity (body weight, fat mass, fat percent, and adipocyte size), metabolic dysfunction (impaired glucose metabolism and insulin resistance) macrophage number (CD11b⁺F480⁺ cells, and gene expression of EMR1 and CD11c), T cell markers (gene expression of CD4 and CD8), inflammatory mediators (pNFκB and pJNK, and mRNA expression of MCP-1, CCL5, CXCL14, TNF-α, and IL-6), and fibrosis (expression of IL-10, IL-13, TGF-β, and MMP2) (P<0.05). However, contrary to our hypothesis, MCP-1 deficiency exacerbated many of these responses resulting in a further increase in adiposity (body weight, fat mass, fat percent and adipocyte size), metabolic dysregulation, macrophage markers (EMR1), inflammatory cell infiltration, and fibrosis (formation of type I and III collagens, mRNA expression of IL-10 and MMP2) (P<0.05).

Conclusions

These data suggest that MCP-1 may be a necessary component of the inflammatory response required for adipose tissue protection, remodeling, and healthy expansion in the FVB/N strain in response to HFD feedings.

Mouse strain-dependent variation in obesity and glucose homeostasis in response to high-fat feeding

AIMS/HYPOTHESIS

Metabolic disorders are commonly investigated using knockout and transgenic mouse models. A variety of mouse strains have been used for this purpose. However, mouse strains can differ in their inherent propensities to develop metabolic disease, which may affect the experimental outcomes of metabolic studies. We have investigated strain-dependent differences in the susceptibility to diet-induced obesity and insulin resistance in five commonly used inbred mouse strains (C57BL/6J, 129X1/SvJ, BALB/c, DBA/2 and FVB/N).

METHODS

Mice were fed either a low-fat or a high-fat diet (HFD) for 8 weeks. Whole-body energy expenditure and body composition were then determined. Tissues were used to measure markers of mitochondrial metabolism, inflammation, oxidative stress and lipid accumulation.

RESULTS

BL6, 129X1, DBA/2 and FVB/N mice were all susceptible to varying degrees to HFD-induced obesity, glucose intolerance and insulin resistance, but BALB/c mice exhibited some protection from these detrimental effects. This protection could not be explained by differences in mitochondrial metabolism or oxidative stress in liver or muscle, or inflammation in adipose tissue. Interestingly, in contrast with the other strains, BALB/c mice did not accumulate excess lipid (triacylglycerols and diacylglycerols) in the liver; this is potentially related to lower fatty acid uptake rather than differences in lipogenesis or lipid oxidation.

CONCLUSIONS/INTERPRETATION

Collectively, our findings indicate that most mouse strains develop metabolic defects on an HFD. However, there are inherent differences between strains, and thus the genetic background needs to be considered carefully in metabolic studies.

Role of the gut in lipid homeostasis

Intestinal lipid transport plays a central role in fat homeostasis. Here we review the pathways regulating intestinal absorption and delivery of dietary and biliary lipid substrates, principally long-chain fatty acid, cholesterol, and other sterols. We discuss the regulation and functions of CD36 in fatty acid absorption, NPC1L1 in cholesterol absorption, as well as other lipid transporters including FATP4 and SRB1. We discuss the pathways of intestinal sterol efflux via ABCG5/G8 and ABCA1 as well as the role of the small intestine in high-density lipoprotein (HDL) biogenesis and reverse cholesterol transport. We review the pathways and genetic regulation of chylomicron assembly, the role of dominant restriction points such as microsomal triglyceride transfer protein and apolipoprotein B, and the role of CD36, l-FABP, and other proteins in formation of the prechylomicron complex. We will summarize current concepts of regulated lipoprotein secretion (including HDL and chylomicron pathways) and include lessons learned from families with genetic mutations in dominant pathways (i.e., abetalipoproteinemia, chylomicron retention disease, and familial hypobetalipoproteinemia). Finally, we will provide an integrative view of intestinal lipid homeostasis through recent findings on the role of lipid flux and fatty acid signaling via diverse receptor pathways in regulating absorption and production of satiety factors.

Essential fatty acids and psychiatric disorders

Psychiatric disorders are a significant source of disability worldwide. Increasing evidence indicates that disturbances of fatty acids and phospholipid metabolism can play a part in a wide range of psychiatric, neurological, and developmental disorders in adults. Essential fatty acids, ω-3 and ω-6 polyunsaturated fatty acids, play a central role in the normal development and functioning of the brain and central nervous system. The aim of this article is to discuss the overall insight into roles of essential fatty acids in the development of mental disorders (depression, schizophrenia, bipolar disorder) and, in light of the fact that disturbances of fatty acid metabolism can play a part in the above-mentioned disorders, to investigate the current knowledge of lipid abnormalities in posttraumatic stress disorder. The information in this review was obtained after extensive MEDLINE searching of each topic area through relevant published studies from the past 20 years. References from the obtained studies were also used. This review summarizes the knowledge in terms of essential fatty acids intake and metabolism, as well as evidence pointing to potential mechanisms of essential fatty acids in normal brain functioning and development of neuropsychiatric disorders. The literature shows that ω-3 fatty acids provide numerous health benefits and that changes in their concentration in organisms are connected to a variety of psychiatric symptoms and disorders, including stress, anxiety, cognitive impairment, mood disorders, and schizophrenia. Further studies are necessary to confirm ω-3 fatty acids' supplementation as a potential rational treatment in psychiatric disorders.

Diet-induced alterations in intestinal and extrahepatic lipid metabolism in liver fatty acid binding protein knockout mice

Liver fatty acid binding protein (L-FABP) is highly expressed in both enterocytes and hepatocytes and binds multiple ligands, including saturated (SFA), unsaturated fatty acids (PUFA), and cholesterol. L-fabp (-/-) mice were protected against obesity and hepatic steatosis on a high saturated fat (SF), high cholesterol "Western" diet and manifested a similar phenotype when fed with a high SF, low cholesterol diet. There were no significant differences in fecal fat content or food consumption between the genotypes, and fatty acid (FA) oxidation was reduced, rather than increased, in SF-fed L-fabp (-/-) mice as evidenced by decreased heat production and serum ketones. In contrast to mice fed with a SF diet, L-fabp (-/-) mice fed with a high PUFA diet were not protected against obesity and hepatic steatosis. These observations together suggest that L-fabp (-/-) mice exhibit a specific defect in the metabolism of SFA, possibly reflecting altered kinetics of FA utilization. In support of this possibility, microarray analysis of muscle from Western diet-fed mice revealed alterations in genes regulating glucose uptake and FA synthesis. In addition, intestinal cholesterol absorption was decreased in L-fabp (-/-) mice. On the other hand, and in striking contrast to other reports, female L-fabp (-/-) mice fed with low fat, high cholesterol diets gained slightly less weight than control mice, with minor reductions in hepatic triglyceride content. Together these data indicate a role for L-FABP in intestinal trafficking of both SFA and cholesterol.

Omega-3 fatty acid deficiency in major depressive disorder is caused by the interaction between diet and a genetically determined abnormality in phospholipid metabolism

Omega-3 fatty acids are a type of polyunsaturated fatty acid (PUFA). A growing body of evidence suggests that this form PUFA is a useful and well tolerated treatment for major depressive disorder, a common and serious mental illness. The efficacy of omega-3 PUFA is routinely explained as being due to a deficiency caused by inadequate dietary intake of this class of fatty acid. The hypothesis considered states that low omega-3 PUFA abundance in patients with major depressive and related disorders is due to an underlying genetically determined abnormality. The hypothesis can explain why although a specific and consistent deficit in omega-3, but not omega-6, PUFA occurs in major depressive and related disorders, the literature does not consistently support the notion that this is due to deficient dietary intake. Specifically it is hypothesized that having genetically determined low activity of fatty acid CoA ligase 4 and/or Type IV phospholipase A(2) combined with the low dietary availability of omega-3 PUFA results in reduced cellular uptake of omega-3 PUFA and constitutes a risk factor for depression. The hypothesis also has important consequences for the pharmacological treatment of depression in that it predicts that administering agents which enhance phospholipid synthesis, particularly those containing ethanolamine such as CDP-ethanolamine, should be effective antidepressants especially when co-administered with omega-3 PUFA.

Metabolic adaptations of three inbred strains of mice (C57BL/6, DBA/2, and 129T2) in response to a high-fat diet

Although it is now becoming more evident that the strain of mouse used to generate genetically modified models for the study of endocrine disorders contributes to the ensuing phenotype, metabolic characterization of these common strains used to produce genetically altered mice has been limited. The aim of this study therefore was to measure various metabolic parameters in C57BL/6, DBA/2, and 129T2 mice fed a control or a high-fat diet. Mice were fed either a control (7 g/100 g) or a high-fat (60 g/100 g) diet for 6 wk. During wk 6, spontaneous and voluntary physical activity and resting energy expenditure were determined. DBA/2 mice that consumed the control diet gained more weight and had larger regional fat pad depots than either C57BL/6 or 129T2 mice (P < 0.05). Spontaneous and voluntary activity was lower in 129T2 mice compared with DBA/2 or C57BL/6 mice (P < 0.05). Resting energy expenditure (corrected for body weight) was greater in C57BL/6 mice than in DBA/2 or 129T2 mice (P < 0.05), whereas glucose and fat oxidation did not differ among the 3 strains of mice. Plasma glucose concentrations in food-deprived mice were higher and insulin concentrations lower in 129T2 compared with C57BL/6 mice (P < 0.05), but were not affected by the high-fat diet in any of the 3 strains tested. This study shows that these 3 commonly used inbred strains of mice have different inherent metabolic characteristics. It further highlights that the background strain used to produce genetically modified mice is critical to the resultant phenotype.

Molecular cloning, genomic organization, genetic variations, and characterization of murine sterolin genes Abcg5 and Abcg8

Mammalian physiological processes can distinguish between dietary cholesterol and non-cholesterol, retaining very little of the non-cholesterol in their bodies. We have recently identified two genes, ABCG5 and ABCG8, encoding sterolin-1 and -2 respectively, mutations of which cause the human disease sitosterolemia. We report here the mouse cDNAs and genomic organization of Abcg5 and Abcg8. Both genes are arranged in an unusual head-to-head configuration, and only 140 bases separate their two respective start-transcription sites. A single TATA motif was identified, with no canonical CCAT box present between the two genes. The genes are located on mouse chromosome 17 and this complex spans no more than 40 kb. Expression of both genes is confined to the liver and intestine. For both genes, two different sizes of transcripts were identified which differ in the lengths of their 3' UTRs. Additionally, alternatively spliced forms for Abcg8 were identified, resulting from a CAG repeat at the intron 1 splice-acceptor site, causing a deletion of a glutamine. We screened 20 different mouse strains for polymorphic variants. Although a large number of polymorphic variants were identified, strains reported to show significant differences in cholesterol absorption rates did not show significant genomic variations in Abcg5 or Abcg8.

Dietary cholesterol absorption; more than just bile

Absorption of dietary cholesterol from the intestine is an important part of cholesterol homeostasis and represents the first step that allows dietary cholesterol to exert its metabolic effects. Although the role of bile salts in the initial absorption of dietary cholesterol, by the formation of emulsions, is readily appreciated, the recognition that other molecular mechanisms might govern this process is only recently gaining momentum. Not only does the intestine regulate the amount of dietary cholesterol that enters the body; it is very selective with regard to the sterols that are allowed in. The human intestine is responsible for absorbing a significant amount of cholesterol each day. In addition to approximately 0.5 g d(-1) of dietary cholesterol, many other sterols are also present in almost equal abundance in the normal diet. Approximately 0.4 g of plant sterols, such as sitosterol, brassicasterol and avanesterol, are also present. However, the human body seems to allow only cholesterol to enter and remain in the body, with almost negligible amounts of plant sterols being retained. That specific molecular mechanisms are responsible for this behavior is supported by the identification of the genetic defect(s) in a rare disorder, beta-sitosterolemia (MIM 210250), where this process is disrupted. Such studies are now beginning to throw light on sterol absorption and excretion and elucidate the molecular mechanisms that govern these processes.