Biomedical subjects

Karen Reue

Publications and source records attributed to Karen Reue.

18 recordsLinked to original sources

PubMed2006

Three mammalian lipins act as phosphatidate phosphatases with distinct tissue expression patterns.

We previously identified mutations in the Lpin1 gene, encoding lipin-1, as the underlying cause of lipodystrophy in the fatty liver dystrophy (fld) mutant mouse. Lipin-1 is normally expressed at high levels in adipose tissue and skeletal muscle, and deficiency in the fld mouse causes impaired adipose tissue development, insulin resistance, and altered energy expenditure. We also identified two additional lipin protein family members of unknown function, lipin-2 and lipin-3. Han et al. (Han, G. S., Wu, W. I., and Carman, G. M. (2006) J. Biol. Chem. 281, 9210-9218) recently demonstrated that the single lipin homolog in yeast, Smp2, exhibits phosphatidate phosphatase type-1 (PAP1) activity, which has a key role in glycerolipid synthesis. Here we demonstrate that lipin-1 accounts for all of the PAP1 activity in white and brown adipose tissue and skeletal muscle. However, livers of lipin-1-deficient mice exhibited normal PAP1 activity, indicating that other members of the lipin protein family could have PAP1 activity. Consistent with this possibility, recombinant lipin-2 and lipin-3 possess PAP1 activity. Each of the three lipin family members showed Mg2+-dependent activity that was specific for phosphatidate under the conditions employed. The different lipins showed distinct tissue expression patterns. Our results establish the three mammalian lipin proteins as PAP1 enzymes and explain the biochemical basis for lipodystrophy in the lipin-1-deficient fld mouse.

Adipose Tissue, Brown↗

PubMed2006

Approaches to lipid metabolism gene identification and characterization in the postgenomic era.

The availability of genomic resources has already had a tremendous impact on biomedical research. In this review, we describe how whole genome sequence and high-throughput functional genomics projects have facilitated the identification and characterization of important genes in lipid metabolism and disease. We review key approaches and lipid genes identified in the first years of this century and discuss how genomic resources are likely to streamline gene identification and functional characterization in the future.

Animals↗

PubMed2006

Lamins A and C but not lamin B1 regulate nuclear mechanics.

Mutations in the nuclear envelope proteins lamins A and C cause a broad variety of human diseases, including Emery-Dreifuss muscular dystrophy, dilated cardiomyopathy, and Hutchinson-Gilford progeria syndrome. Cells lacking lamins A and C have reduced nuclear stiffness and increased nuclear fragility, leading to increased cell death under mechanical strain and suggesting a potential mechanism for disease. Here, we investigated the contribution of major lamin subtypes (lamins A, C, and B1) to nuclear mechanics by analyzing nuclear shape, nuclear dynamics over time, nuclear deformations under strain, and cell viability under prolonged mechanical stimulation in cells lacking both lamins A and C, cells lacking only lamin A (i.e. "lamin C-only" cells), cells lacking wild-type lamin B1, and wild-type cells. Lamin A/C-deficient cells exhibited increased numbers of misshapen nuclei and had severely reduced nuclear stiffness and decreased cell viability under strain. Lamin C-only cells had slightly abnormal nuclear shape and mildly reduced nuclear stiffness but no decrease in cell viability under strain. Interestingly, lamin B1-deficient cells exhibited normal nuclear mechanics despite having a significantly increased frequency of nuclear blebs. Our study indicates that lamins A and C are important contributors to the mechanical stiffness of nuclei, whereas lamin B1 contributes to nuclear integrity but not stiffness.

Animals↗

PubMed2006

Agpat6--a novel lipid biosynthetic gene required for triacylglycerol production in mammary epithelium.

In analyzing the sequence tags for mutant mouse embryonic stem (ES) cell lines in BayGenomics (a mouse gene-trapping resource), we identified a novel gene, 1-acylglycerol-3-phosphate O-acyltransferase (Agpat6), with sequence similarities to previously characterized glycerolipid acyltransferases. Agpat6's closest family member is another novel gene that we have provisionally designated Agpat8. Both Agpat6 and Agpat8 are conserved from plants, nematodes, and flies to mammals. AGPAT6, which is predicted to contain multiple membrane-spanning helices, is found exclusively within the endoplasmic reticulum (ER) in mammalian cells. To gain insights into the in vivo importance of Agpat6, we used the Agpat6 ES cell line from BayGenomics to create Agpat6-deficient (Agpat6-/-) mice. Agpat6-/- mice lacked full-length Agpat6 transcripts, as judged by northern blots. One of the most striking phenotypes of Agpat6-/- mice was a defect in lactation. Pups nursed by Agpat6-/- mothers die perinatally. Normally, Agpat6 is expressed at high levels in the mammary epithelium of breast tissue, but not in the surrounding adipose tissue. Histological studies revealed that the aveoli and ducts of Agpat6-/- lactating mammary glands were underdeveloped, and there was a dramatic decrease in the size and number of lipid droplets within mammary epithelial cells and ducts. Also, the milk from Agpat6-/- mice was markedly depleted in diacylglycerols and triacylglycerols. Thus, we identified a novel glycerolipid acyltransferase of the ER, AGPAT6, which is crucial for the production of milk fat by the mammary gland.

1-Acylglycerol-3-Phosphate O-Acyltransferase↗

PubMed2006

Agpat6 deficiency causes subdermal lipodystrophy and resistance to obesity.

Triglyceride synthesis in most mammalian tissues involves the sequential addition of fatty acids to a glycerol backbone, with unique enzymes required to catalyze each acylation step. Acylation at the sn-2 position requires 1-acylglycerol-3-phosphate O-acyltransferase (AGPAT) activity. To date, seven Agpat genes have been identified based on activity and/or sequence similarity, but their physiological functions have not been well established. We have generated a mouse model deficient in AGPAT6, which is normally expressed at high levels in brown adipose tissue (BAT), white adipose tissue (WAT), and liver. Agpat6-deficient mice exhibit a 25% reduction in body weight and resistance to both diet-induced and genetically induced obesity. The reduced body weight is associated with increased energy expenditure, reduced triglyceride accumulation in BAT and WAT, reduced white adipocyte size, and lack of adipose tissue in the subdermal region. In addition, the fatty acid composition of triacylglycerol, diacylglycerol, and phospholipid is altered, with proportionally greater polyunsaturated fatty acids at the expense of monounsaturated fatty acids. Thus, Agpat6 plays a unique role in determining triglyceride content and composition in adipose tissue and liver that cannot be compensated by other members of the Agpat family.

1-Acylglycerol-3-Phosphate O-Acyltransferase↗

PubMed2006

Metabolic consequences of lipodystrophy in mouse models.

PURPOSE OF REVIEW: Adipose tissue is a key player in metabolic homeostasis through its role as an energy depot and endocrine organ. The characterization of mouse models of lipodystrophy, in which adipose tissue development and function are impaired, has shed new light on the mechanisms by which adipose tissue dysregulation may contribute to conditions such as insulin resistance, fatty liver, and beta-cell toxicity. RECENT FINDINGS: Here we review recent findings from mouse models with genetic alterations leading to reduced adipose tissue mass. The metabolic consequences depend on the basis for the adipose tissue reduction, and we classify mouse models into three categories based on whether they confer (1) lipoatrophy accompanied by insulin resistance, (2) reduced adipose tissue storage associated with enhanced energy expenditure, or (3) both lipoatrophic and energetic effects. SUMMARY: Reductions in the capacity of adipose tissue to store triglycerides or to secrete adipokine hormones lead to altered lipid metabolism and insulin resistance. In contrast, depletion of fat stores by virtue of enhanced energy metabolism does not produce undesirable metabolic effects. However, enhanced energy metabolism cannot overcome the deleterious effects of impaired adipokine production, as revealed by studies of models with both lipoatrophic and energetic effects.

Adipose Tissue↗

PubMed2006

Lipin expression is attenuated in adipose tissue of insulin-resistant human subjects and increases with peroxisome proliferator-activated receptor gamma activation.

Lipin-alpha and -beta are the alternatively spliced gene products of the Lpin1 gene, whose product lipin is required for adipocyte differentiation. Lipin deficiency causes lipodystrophy, fatty liver, and insulin resistance in mice, whereas adipose tissue lipin overexpression results in increased adiposity but improved insulin sensitivity. To assess lipin expression and its relation to insulin resistance in humans, we examined lipin-alpha and -beta mRNA levels in subjects with normal or impaired glucose tolerance. We found higher expression levels of both lipin isoforms in lean, insulin-sensitive subjects. When compared with normal glucose-tolerant subjects, individuals with impaired glucose tolerance were more insulin resistant, demonstrated higher levels of intramyocellular lipids (IMCLs), and expressed approximately 50% lower levels of lipin-alpha and -beta. In addition, there was a strong inverse correlation between adipose tissue lipin expression and muscle IMCLs but no evidence for an increase in muscle lipid oxidation. After treatment of the impaired glucose-tolerant subjects with insulin sensitizers for 10 weeks, pioglitazone (but not metformin) resulted in a 60% increase in the insulin sensitivity index (Si) and a 32% decrease in IMCLs (both P < 0.01), along with an increase in lipin-beta (but not lipin-alpha) expression by 200% (P < 0.005). Lipin expression in skeletal muscle, however, was not related to obesity or insulin resistance. Hence, high adipose tissue lipin expression is found in insulin-sensitive subjects, and lipin-beta expression increases following treatment with pioglitazone. These results suggest that increased adipogenesis and/or lipogenesis in subcutaneous fat, mediated by the LPIN1 gene, may prevent lipotoxicity in muscle, leading to improved insulin sensitivity.

Adipose Tissue↗

PubMed2006

Lipin deficiency impairs diurnal metabolic fuel switching.

Fatty liver is a common feature of both obesity and lipodystrophy, reflecting compromised adipose tissue function. The lipin-deficient fatty liver dystrophy (fld) mouse is an exception, as there is lipodystrophy without a fatty liver. Using a combination of indirect calorimetry and stable-isotope flux phenotyping, we determined that fld mice exhibit abnormal fuel utilization throughout the diurnal cycle, with increased glucose oxidation near the end of the fasting period and increased fatty acid oxidation during the feeding period. The mechanisms underlying these alterations include a twofold increase compared with wild-type mice in tissue glycogen storage during the fed state, a 40% reduction in hepatic glucose production in the fasted state, and a 27-fold increase in de novo fatty acid synthesis in liver during the fed state. Thus, the inability to store energy in adipose tissue in the fld mouse leads to a compensatory increase in glycogen storage for use during the fasting period and reliance upon hepatic fatty acid synthesis to provide fuel for peripheral tissues during the fed state. The increase in hepatic fatty acid synthesis and peripheral utilization provides a potential mechanism to ameliorate fatty liver in the fld that would otherwise occur as a consequence of adipose tissue dysfunction.

Animals↗

PubMed2005

Cross-species analyses implicate Lipin 1 involvement in human glucose metabolism.

Recent studies in the mouse have demonstrated that variations in lipin expression levels in adipose tissue have marked effects on adipose tissue mass and insulin sensitivity. In the mouse, lipin deficiency prevents normal adipose tissue development, resulting in lipodystrophy and insulin resistance, whereas excess lipin levels promote fat accumulation and insulin sensitivity. Here, we investigated the effects of genetic variation in lipin levels on glucose homeostasis across species by analyzing lipin transcript levels in human and mouse adipose tissues. A strong negative correlation was observed between lipin mRNA levels and fasting glucose and insulin levels, as well as an indicator of insulin resistance (HOMA-IR), in both mice and humans. We subsequently analyzed the allelic diversity of the LPIN1 gene in dyslipidemic Finnish families, as well as in a case-control sample of obese (n = 477) and lean (n = 821) individuals. Alleles were defined by genotyping seven single nucleotide polymorphisms (SNPs) of the critical DNA region over the LPIN1 gene. Intragenic SNPs and corresponding allelic haplotypes exhibited associations with serum insulin levels and body mass index (P = 0.002-0.04). Both the expression levels in adipose tissue across species and genetic data in human study samples highlight the importance of lipin in glucose homeostasis and imply that allelic variants of this gene have significance in human metabolic traits.

Adipose Tissue↗

PubMed2005

Alternatively spliced lipin isoforms exhibit distinct expression pattern, subcellular localization, and role in adipogenesis.

We recently identified mutations in the Lpin1 (lipin) gene to be responsible for lipodystrophy in the fatty liver dystrophy (fld) mouse strain. Previous studies revealed that lipin plays a critical role in adipogenesis, explaining the adipose-deficient phenotype of the fld mouse. In the current study, we demonstrate that alternative mRNA splicing generates two lipin isoforms, lipin-alpha and lipin-beta, which are differentially expressed during adipocyte differentiation. Lipin-alpha expression peaks at day 2 of 3T3-L1 cell differentiation, after which its levels gradually decrease. In contrast, lipin-beta expression is transiently elevated at 10 h, followed by a drop to background levels at 20 h and a gradual increase between days 2 and 6 of differentiation. The two lipin isoforms also exhibit differences in subcellular localization. Lipin-alpha is predominantly nuclear, whereas lipin-beta is primarily located in the cytoplasm of 3T3-L1 adipocytes, suggesting distinct cellular functions. Using primary mouse embryonic fibroblasts expressing either lipin-alpha or lipin-beta, we demonstrate functional differences between the two isoforms. Whereas lipin-alpha is required for adipocyte differentiation, the predominant effect of lipin-beta expression is the induction of lipogenic genes. In vivo, overexpression of lipin-beta specifically in mature adipocytes leads to elevated expression of lipogenic genes and adipocyte hypertrophy, confirming a role of lipin-beta in the regulation of lipogenesis. In conclusion, our data suggest that the two lipin isoforms have distinct, but complementary, functions in adipogenesis, with lipin-alpha playing a primary role in differentiation and lipin-beta being predominantly involved in lipogenesis.

3T3-L1 Cells↗

PubMed2005

Lipin, a lipodystrophy and obesity gene.

Lipodystrophy and obesity represent extreme and opposite ends of the adiposity spectrum and have typically been attributed to alterations in the expression or function of distinct sets of genes. We previously demonstrated that lipin deficiency impairs adipocyte differentiation and causes lipodystrophy in the mouse. Using two different tissue-specific lipin transgenic mouse strains, we now demonstrate that enhanced lipin expression in either adipose tissue or skeletal muscle promotes obesity. This occurs through diverse mechanisms in the two tissues, with lipin levels in adipose tissue influencing the fat storage capacity of the adipocyte, and lipin levels in skeletal muscle acting as a determinant of whole-body energy expenditure and fat utilization. Thus, variations in lipin levels alone are sufficient to induce extreme states of adiposity and may represent a mechanism by which adipose tissue and skeletal muscle modulate fat mass and energy balance.

Adipocytes↗

PubMed2004

Heterozygosity for Lmna deficiency eliminates the progeria-like phenotypes in Zmpste24-deficient mice.

Zmpste24 is a metalloproteinase required for the processing of prelamin A to lamin A, a structural component of the nuclear lamina. Zmpste24 deficiency results in the accumulation of prelamin A within cells, a complete loss of mature lamin A, and misshapen nuclear envelopes. Zmpste24-deficient (Zmpste24(-/-)) mice exhibit retarded growth, alopecia, micrognathia, dental abnormalities, osteolytic lesions in bones, and osteoporosis, which are phenotypes shared with Hutchinson-Gilford progeria syndrome, a human disease caused by the synthesis of a mutant prelamin A that cannot undergo processing to lamin A. Zmpste24(-/-) mice also develop muscle weakness. We hypothesized that prelamin A might be toxic and that its accumulation in Zmpste24(-/-) mice is responsible for all of the disease phenotypes. We further hypothesized that Zmpste24(-/-) mice with half-normal levels of prelamin A (Zmpste24(-/-) mice with one Lmna knockout allele) would be subjected to less toxicity and be protected from disease. Thus, we bred and analyzed Zmpste24(-/-)Lmna(+/-) mice. As expected, prelamin A levels in Zmpste24(-/-)Lmna(+/-) cells were significantly reduced. Zmpste24(-/-)Lmna(+/-) mice were entirely normal, lacking all disease phenotypes, and misshapen nuclei were less frequent in Zmpste24(-/-)Lmna(+/-) cells than in Zmpste24(-/-) cells. These data suggest that prelamin A is toxic and that reducing its levels by as little as 50% provides striking protection from disease.

Alleles↗

PubMed2004

Lamin B1 is required for mouse development and nuclear integrity.

Lamins are key structural components of the nuclear lamina, an intermediate filament meshwork that lies beneath the inner nuclear membrane. Lamins play a role in nuclear architecture, DNA replication, and gene expression. Mutations affecting A-type lamins have been associated with a variety of human diseases, including muscular dystrophy, cardiomyopathy, lipodystrophy, and progeria, but mutations in B-type lamins have never been identified in humans or in experimental animals. To investigate the in vivo function of lamin B1, the major B-type lamin, we generated mice with an insertional mutation in Lmnb1. The mutation resulted in the synthesis of a mutant lamin B1 protein lacking several key functional domains, including a portion of the rod domain, the nuclear localization signal, and the CAAX motif (the carboxyl-terminal signal for farnesylation). Homozygous Lmnb1 mutant mice survived embryonic development but died at birth with defects in lung and bone. Fibroblasts from mutant embryos grew under standard cell-culture conditions but displayed grossly misshapen nuclei, impaired differentiation, increased polyploidy, and premature senescence. Thus, the lamin B1 mutant mice provide evidence for a broad and nonredundant function of lamin B1 in mammalian development. These mutant mice and cell lines derived from them will be useful models for studying the role of the nuclear lamina in various cellular processes.

Animals↗

PubMed2004

Lipin expression preceding peroxisome proliferator-activated receptor-gamma is critical for adipogenesis in vivo and in vitro.

We recently identified mutations in the lipin gene, Lpin1, as the cause of lipodystrophy in the fatty liver dystrophy (fld) mouse. Here we identify impaired adipocyte differentiation as the basis for lipodystrophy in lipin-deficient mice and demonstrate that lipin is required for normal induction of the adipogenic gene transcription program. We found that the reduced adiposity in chow fed fld mice and resistance to obesity in fld mice fed a high-fat diet is associated with reduced adipogenic gene expression. Using primary mouse embryonic fibroblasts isolated from fld mice, we confirmed that lipin deficiency prevents normal lipid accumulation and induction of key adipogenic genes, including peroxisome proliferator-activated receptor (PPAR)gamma and CCAAT enhancer-binding protein (C/EBP)alpha. However, our previous studies of daily gene expression in differentiating 3T3-L1 preadipocytes indicated that lipin expression is undetectable until about day 3 of differentiation, at a point after PPARgamma and C/EBPalpha gene expression is established. This paradox was resolved by examining gene expression at 10-h intervals during 3T3-L1 cell differentiation, leading to detection of transient lipin expression at 10 h into the differentiation program, prior to the induction of PPARgamma and C/EBPalpha. Consistent with a requirement for lipin expression upstream of PPARgamma, differentiation of lipin-deficient mouse embryonic fibroblasts could be rescued by ectopic expression of PPARgamma. Thus, we conclude that lipin expression is required prior to PPARgamma during adipocyte differentiation.

3T3 Cells↗

PubMed2003

Cholesterol and cholate components of an atherogenic diet induce distinct stages of hepatic inflammatory gene expression.

Atherosclerosis in inbred mouse strains has been widely studied by using an atherogenic (Ath) diet containing cholesterol, cholic acid, and fat, but the effect of these components on gene expression has not been systematically examined. We employed DNA microarrays to interrogate gene expression levels in liver of C57BL/6J mice fed the following five diets: mouse chow, the Ath diet, or modified versions of the Ath diet in which either cholesterol, cholate, or fat were omitted. Dietary cholesterol and cholate produced discrete gene expression patterns. Cholesterol was required for induction of genes involved in acute inflammation, including three genes of the serum amyloid A family, three major histocompatibility class II antigen genes, and various cytokine-related genes. In contrast, cholate induced expression of genes involved in extracellular matrix deposition in hepatic fibrosis, including five collagen family members, collagen-interacting proteins, and connective tissue growth factor. The gene expression findings were confirmed by biochemical measurements showing that cholesterol was required for elevation of circulating serum amyloid A, and cholate was required for accumulation of collagen in the liver. The possibility that these gene expression changes are relevant to atherogenesis in C57BL/6J mice was supported by the observation that the closely related, yet atherosclerosis-resistant, C57BL/6ByJ strain was largely resistant to dietary induction of the inflammatory and fibrotic response genes. These results establish that cholesterol and cholate components of the Ath diet have distinct proatherogenic effects on gene expression and suggest a strategy to study the contribution of acute inflammatory response and fibrogenesis independently through dietary manipulation.

Animal Nutritional Physiological Phenomena↗

PubMed2002

A cluster of eight hydroxysteroid dehydrogenase genes belonging to the aldo-keto reductase supergene family on mouse chromosome 13.

A subclass of hydroxysteroid dehydrogenases (HSD) are NADP(H)-dependent oxidoreductases that belong to the aldo-keto reductase (AKR) superfamily. They are involved in prereceptor or intracrine steroid modulation, and also act as bile acid-binding proteins. The HSD family members characterized thus far in human and rat have a high degree of protein sequence similarity but exhibit distinct substrate specificity. Here we report the identification of nine murine AKR genes in a cluster on chromosome 13 by a combination of molecular cloning and in silico analysis of this region. These include four previously isolated mouse HSD genes (Akr1c18, Akr1c6, Akr1c12, Akr1c13), the more distantly related Akr1e1, and four novel HSD genes. These genes exhibit highly conserved exon/intron organization and protein sequence predictions indicate 75% amino acid similarity. The previously identified AKR protein active site residues are invariant among all nine proteins, but differences are observed in regions that have been implicated in determining substrate specificity. Differences also occur in tissue expression patterns, with expression of some genes restricted to specific tissues and others expressed at high levels in multiple tissues. Our findings dramatically expand the repertoire of AKR genes and identify unrecognized family members with potential roles in the regulation of steroid metabolism.

Alcohol Oxidoreductases↗

PubMed2001

The Diet1 locus confers protection against hypercholesterolemia through enhanced bile acid metabolism.

The C57BL/6ByJ (B6By) mouse strain is resistant to diet-induced hypercholesterolemia and atherosclerosis, despite its near genetic identity with the atherosclerosis-susceptible C57BL/6J (B6J) strain. We previously identified a genetic locus, Diet1, which is responsible for the resistant phenotype in B6By mice. To investigate the function of Diet1, we compared mRNA expression profiles in the liver of B6By and B6J mice fed an atherogenic diet using a DNA microarray. These studies revealed elevated expression levels in B6By liver for key bile acid synthesis proteins, including cholesterol 7alpha-hydroxylase and sterol-27-hydroxylase, and the oxysterol nuclear receptor liver X receptor alpha. Expression levels for several other genes involved in bile acid metabolism were subsequently found to differ between B6By and B6J mice, including the bile acid receptor farnesoid X receptor, oxysterol 7alpha-hydroxylase, sterol-12alpha-hydroxylase, and hepatic bile acid transporters on both sinusoidal and canalicular membranes. The overall expression profile of the B6By strain suggests a higher rate of bile acid synthesis and transport in these mice. Consistent with this interpretation, fecal bile acid excretion is increased 2-fold in B6By mice, and bile acid levels in blood and urine are elevated 3- and 18-fold, respectively. Genetic analysis of serum bile acid levels revealed co-segregation with Diet1, indicating that this locus is likely responsible for both increased bile acid excretion and resistance to hypercholesterolemia in B6By mice.

ATP Binding Cassette Transporter, Subfamily B↗

PubMedDate not supplied

Biphasic expression of lipin suggests dual roles in adipocyte development.

The identification of gene mutations that cause lipodystrophies, conditions characterized by a lack of normal adipose tissue, has revealed new proteins that play a role in adipocyte biology. Lipin is one such protein identified in a lipodystrophic mouse strain and found to be critical for normal adipocyte differentiation. Interestingly, lipin displays a biphasic expression pattern in adipocytes, with peaks of expression at two points during adipogenesis--a transient induction in preadipocytes prior to expression of peroxisome proliferator-activated receptor gamma, and a second wave of expression in mature adipocytes. Thus, lipin appears to have critical roles in both adipocyte differentiation and in the function of mature adipocytes.

Adipocytes↗