Muscle-specific overexpression of human lipoprotein lipase in mice causes increased intracellular free fatty acids and induction of peroxisomal enzymes

PPAR-Mediated Toxicology and Applied Pharmacology

Peroxisome proliferator-activated receptors (PPARs), members of the nuclear hormone receptor family, attract wide attention as promising therapeutic targets for the treatment of multiple diseases, and their target selective ligands were also intensively developed for pharmacological agents such as the approved drugs fibrates and thiazolidinediones (TZDs). Despite their potent pharmacological activities, PPARs are reported to be involved in agent- and pollutant-induced multiple organ toxicity or protective effects against toxicity. A better understanding of the protective and the detrimental role of PPARs will help to preserve efficacy of the PPAR modulators but diminish adverse effects. The present review summarizes and critiques current findings related to PPAR-mediated types of toxicity and protective effects against toxicity for a systematic understanding of PPARs in toxicology and applied pharmacology.

The tissue distribution of lipoprotein lipase determines where chylomicrons bind

To determine the role of LPL for binding of lipoproteins to the vascular endothelium, and for the distribution of lipids from lipoproteins, four lines of induced mutant mice were used. Rat chylomicrons labeled in vivo with [(14)C]oleic acid (primarily in TGs, providing a tracer for lipolysis) and [(3)H]retinol (primarily in ester form, providing a tracer for the core lipids) were injected. TG label was cleared more rapidly than core label. There were no differences between the mouse lines in the rate at which core label was cleared. Two minutes after injection, about 5% of the core label, and hence chylomicron particles, were in the heart of WT mice. In mice that expressed LPL only in skeletal muscle, and had much reduced levels of LPL in the heart, binding of chylomicrons was reduced to 1%, whereas in mice that expressed LPL only in the heart, the binding was increased to over 10%. The same patterns of distribution were evident at 20 min when most of the label had been cleared. Thus, the amount of LPL expressed in muscle and heart governed both the binding of chylomicron particles and the assimilation of chylomicron lipids in the tissue.

Intrinsic and extrinsic regulation of cardiac lipoprotein lipase following diabetes

Cardiac lipoprotein lipase (LPL) is a pivotal enzyme controlling heart metabolism by providing the majority of fatty acids required by this organ. From activation in cardiomyocytes to secretion to the vascular lumen, cardiac LPL is regulated by multiple pathways, which are altered during diabetes. Hence, dimerization/activation of LPL is modified following diabetes, a process controlled by lipase maturation factor 1. The role of AMP-activated protein kinase, protein kinase D, and heparan sulfate proteoglycans, intrinsic factors that regulate the intracellular transport of LPL is also shifted, and is discussed. More recent studies have identified several exogenous factors released from endothelial cells (EC) and adipose tissue that are required for proper functioning of LPL. In response to hyperglycemia, both active and latent heparanase are released from EC to facilitate LPL secretion. Diabetes also increased the expression of glycosylphosphatidylinositol-anchored high density lipoprotein-binding protein 1 (GPIHBP1) in EC, which mediates the transport of LPL across EC. Angiopoietin-like protein 4 secreted from the adipose tissue has the potential to reduce coronary LPL activity. Knowledge of these intrinsic and extrinsic factors could be used develop therapeutic targets to normalize LPL function, and maintain cardiac energy homeostasis after diabetes.

Hypertriglyceridemic Pancreatitis Caused by the Oral Contraceptive Agent Estrostep

Norethindrone acetate/ethinyl estradiol (Estrostep; Warner Chilcott, Rockaway, New Jersey) is an "estrophasic" type of oral contraceptive, which combines a continuous low progestin dose with a gradually increasing estrogen dose. In clinical trials, this medication failed to produce clinically significant changes in serum lipid levels. We report a case of severe hypertriglyceridemia-induced acute pancreatitis in a 24-year-old woman caused by Estrostep, occurring nearly 10 years after she began using the drug. The patient was admitted to the medical intensive care unit (ICU) for aggressive volume resuscitation and management of severe electrolyte abnormalities. Laboratory studies obtained on admission indicated severe hypertriglyceridemia (2,200 mg/dL), hyponatremia (120 mEq/L), and hypocalcemia (0.78 mmol/L). Amylase and lipase levels were also elevated (193 and 200 U/L, respectively). Ranson score calculated after 48 hours of admission was 4, and her Acute Physiology and Chronic Health Evaluation (APACHE) IV score was 35. Treatment included an insulin infusion, ω-3 fatty acid esters, and gemfibrozil. The insulin infusion reduced serum triglyceride levels by 50% after 1 day of treatment and to 355 mg/dL by day 7 of her ICU course. We believe that this is the first reported case of severe, acute hypertriglyceridemia-induced pancreatitis caused by this medication.

Safety profile of recombinant adeno-associated viral vectors: focus on alipogene tiparvovec (Glybera®)

There has been great interest over the past two decades in developing gene therapies (GTs) to treat a variety of diseases; however, translating research findings into clinical treatments have proved to be a challenge. A major milestone in the development of GT has been achieved with the approval of alipogene tiparvovec (Glybera(®)) in Europe for the treatment of familial lipoprotein lipase deficiency. At this important stage with the evolution of GT into the clinic, this review will examine the safety aspects GT with adeno-associated virus (AAV) vectors. The topics that will be covered include acute reactions, immunological reactions to the AAV capsid and expressed transgene, viral biodistribution and shedding, DNA integration and carcinogenicity. These safety aspects of GT will be discussed with a focus on alipogene tiparvovec, in addition to other AAV vector GT products currently in clinical development.

Hypertriglyceridemia-induced pancreatitis created by oral estrogen and in vitro fertilization ovulation induction

Hypertriglyceridemia is one of the known causes of pancreatitis. Estrogen treatment can aggravate hypertriglyceridemia by increasing very low density lipoprotein secretion and reducing hepatic triglyceride lipase. In this paper, we present 3 patients who developed severe hypertriglyceridemia with conditions that increased estrogen. Two patients were found to have genetic lipoprotein lipase deficiency and were treated with birth control pills. The third was a patient with polycystic ovary disease who was receiving ovulation induction therapy for in vitro fertilization.

Skeletal muscle damage and impaired regeneration due to LPL-mediated lipotoxicity

According to the concept of lipotoxicity, ectopic accumulation of lipids in non-adipose tissue induces pathological changes. The most prominent effects are seen in fatty liver disease, lipid cardiomyopathy, non-insulin-dependent diabetes mellitus, insulin resistance and skeletal muscle myopathy. We used the MCK(m)-hLPL mouse distinguished by skeletal and cardiac muscle-specific human lipoprotein lipase (hLPL) overexpression to investigate effects of lipid overload in skeletal muscle. We were intrigued to find that ectopic lipid accumulation induced proteasomal activity, apoptosis and skeletal muscle damage. In line with these findings we observed reduced Musculus gastrocnemius and Musculus quadriceps mass in transgenic animals, accompanied by severely impaired physical endurance. We suggest that muscle loss was aggravated by impaired muscle regeneration as evidenced by reduced cross-sectional area of regenerating myofibers after cardiotoxin-induced injury in MCK(m)-hLPL mice. Similarly, an almost complete loss of myogenic potential was observed in C2C12 murine myoblasts upon overexpression of LPL. Our findings directly link lipid overload to muscle damage, impaired regeneration and loss of performance. These findings support the concept of lipotoxicity and are a further step to explain pathological effects seen in muscle of obese patients, patients with the metabolic syndrome and patients with cancer-associated cachexia.

Over-expression of human lipoprotein lipase in mouse mammary glands leads to reduction of milk triglyceride and delayed growth of suckling pups

Background

The mammary gland is a conserved site of lipoprotein lipase expression across species and lipoprotein lipase attachment to the luminal surface of mammary gland vascular endothelial cells has been implicated in the direction of circulating triglycerides into milk synthesis during lactation.

Principal Findings

Here we report generation of transgenic mice harboring a human lipoprotein lipase gene driven by a mammary gland-specific promoter. Lipoprotein lipase levels in transgenic milk was raised to 0.16 mg/ml, corresponding to an activity of 8772.95 mU/ml. High lipoprotein lipase activity led to a significant reduction of triglyceride concentration in milk, but other components were largely unchanged. Normal pups fed with transgenic milk showed inferior growth performances compared to those fed with normal milk.

Conclusion

Our study suggests a possibility to reduce the triglyceride content of cow milk using transgenic technology.

Apoptosis and fibrosis are early features of heart failure in an animal model of metabolic cardiomyopathy

In previous experiments, we observed signs of cardiac failure in mice overexpressing lipoprotein lipase (LPL) under the control of a muscle specific promotor and in peroxisome proliferators activated receptor alpha (PPARalpha) knockout mice overexpressing LPL under the control of the same promotor. In our current investigations, we focussed on morphological consequences and changes in mRNA and protein expression in hearts from these animals. mRNA expression was analysed by differential display analysis and Northern blot as well as by cDNA microarray analysis followed by pathway analysis. Protein expression was examined using immunoblot and immunohistochemistry. Fibrosis was determined by chromotrope aniline blue staining for collagen. A distinct increase in the expression of alpha-tubulin mRNA was noted in hearts of all mutant mouse strains compared with the control. This result was paralleled by increased alpha-tubulin protein expression. Using cDNA microarray analysis, we detected an activation of apoptosis, in particular an increase of caspase-3 expression in hearts of mice overexpressing LPL but not in PPARalpha knockout mice overexpressing LPL. This finding was confirmed immunohistochemically. In addition, we identified a distinct interstitial increase in collagen and an increase around blood vessels. In our mouse model, we detect mRNA and protein changes typical for cardiomyopathy even before overt clinical signs of heart failure. In addition, a small but distinct increase in the rate of apoptosis of cardiomyocytes and fibrotic changes contributes to cardiac failure in mice overexpressing LPL, whereas additional deficiency in PPARalpha seems to protect hearts from these effects.

Adeno-associated virus LPL(S447X) gene therapy in LDL receptor knockout mice

BACKGROUND

Overexpression of lipoprotein lipase (LPL) protects against atherosclerosis in genetically engineered mice. We tested whether a gene therapy vector that delivers human (h) LPL(S447X) cDNA to skeletal muscle could induce similar effects.

METHODS

LDL receptor knockout (LDLr-/-) mice were injected intramuscular (i.m.) with adeno-associated virus serotype 1 (AAV1) LPL(S447X) or PBS. Four weeks later they were started on an atherogenic diet for 12 weeks. After termination, atherosclerosis was assessed and homogenates of muscle and liver tissue were analyzed.

RESULTS

AAV1-treated mice showed hLPL concentrations of 768+/-293 ng/mL in post-heparin plasma associated with 48% reductions of fasting triglycerides (TG) levels (p<0.0001). In the absence of an effect on total cholesterol (TC) levels, no effects on atherosclerosis were found. An increase in lipid content of injected muscles was accompanied by a significant decrease of TG (-20%, p<0.0001) and free cholesterol (FC) content (-24%, p<0.0001) in liver homogenates.

CONCLUSIONS

The data show that transgenic hLPL(S447X) on top of endogenous murine LPL reduces fasting TG levels in plasma but has no effect on atherosclerosis in LDLr-/- mice. While lipid accumulation in the injected muscle was anticipated, this coincided with an interesting decrease of both TG and FC in liver homogenates.

Gene therapy for lipoprotein lipase deficiency: working toward clinical application

Lipoprotein lipase (LPL) deficiency causes hypertriglyceridemia and recurrent, potentially life-threatening pancreatitis. There currently is no adequate treatment for this disease. Previously, we showed that intramuscular administration of an adeno-associated virus serotype 1 (AAV1) vector encoding the human LPL(S447X) variant cDNA (AAV1-LPL(S447X)) normalized the dyslipidemia of LPL-/- mice for more than 1 year. In preparation for a clinical trial, we evaluated the safety and biodistribution of AAV1-LPL(S447X) in wild-type mice and fully characterized six LPL-deficient patients. Toxicological analysis in mice showed that intramuscular administration was well tolerated. Acute inflammatory response markers were transiently increased, and anti- AAV1 antibodies were generated. Histological analyses indicated a dose-dependent reversible spleen hyperplasia, and myositis at the injection sites. Biodistribution data showed short-term vector leakage from injection sites into the circulation, followed by liver-mediated clearance. Persistence of vector DNA was limited to the injected muscle and draining lymph nodes, and spread to reproductive organs was limited. Characterization of LPL-deficient patients showed that all patients presented with hypertriglyceridemia and recurrent pancreatitis. LPL catalytic activity was absent, but LPL protein levels were 20-100% of normal. Myoblasts derived from skeletal muscle biopsies of these patients were efficiently transduced by AAV1-LPL(S447X) and secreted active LPL. These data support the initiation of a clinical trial in LPL-deficient patients, for which regulatory approval has been granted.

Control of muscle bioenergetic gene expression: implications for allometric scaling relationships of glycolytic and oxidative enzymes

Muscle metabolic properties vary with body size, with larger animals relying relatively less on oxidative metabolism as a result of lower specific activities of mitochondrial enzymes and greater specific activities of glycolytic enzymes. While many have argued reasons why such relationships might be grounded in physical relationships, an explanation for the regulatory basis of the differences in enzyme levels remains unexplored. Focusing on skeletal muscle, we review potential cellular and genetic explanations for the relationship between bioenergetic enzymes and body mass. Differences in myonuclear domain (the ratio of fiber volume to nuclei number) in conjunction with constitutive expression may explain part of the variation in mitochondrial content among fiber types and species. Superimposed on such constitutive determinants are (1) extrinsic signalling pathways that control the muscle contractile and metabolic phenotype and (2) intrinsic signalling pathways that translate changes in cellular milieu (ions, metabolites, oxygen,redox) arising through the contractile phenotype into changes in enzyme synthesis. These signalling pathways work through transcriptional regulation,as well as post-transcriptional, translational and post-translational regulation, acting via synthesis and degradation.

Unusual metabolic characteristics in skeletal muscles of transgenic rabbits for human lipoprotein lipase

BACKGROUND

The lipoprotein lipase (LPL) hydrolyses circulating triacylglycerol-rich lipoproteins. Thereby, LPL acts as a metabolic gate-keeper for fatty acids partitioning between adipose tissue for storage and skeletal muscle primarily for energy use. Transgenic mice that markedly over-express LPL exclusively in muscle, show increases not only in LPL activity, but also in oxidative enzyme activities and in number of mitochondria, together with an impaired glucose tolerance. However, the role of LPL in intracellular nutrient pathways remains uncertain. To examine differences in muscle nutrient uptake and fatty acid oxidative pattern, transgenic rabbits harboring a DNA fragment of the human LPL gene (hLPL) and their wild-type littermates were compared for two muscles of different metabolic type, and for perirenal fat.

RESULTS

Analyses of skeletal muscles and adipose tissue showed the expression of the hLPL DNA fragment in tissues of the hLPL group only. Unexpectedly, the activity level of LPL in both tissues was similar in the two groups. Nevertheless, mitochondrial fatty acid oxidation rate, measured ex vivo using [1-(14C)]oleate as substrate, was lower in hLPL rabbits than in wild-type rabbits for the two muscles under study. Both insulin-sensitive glucose transporter GLUT4 and muscle fatty acid binding protein (H-FABP) contents were higher in hLPL rabbits than in wild-type littermates for the pure oxidative semimembranosus proprius muscle, but differences between groups did not reach significance when considering the fast-twitch glycolytic longissimus muscle. Variations in both glucose uptake potential, intra-cytoplasmic binding of fatty acids, and lipid oxidation rate observed in hLPL rabbits compared with their wild-type littermates, were not followed by any modifications in tissue lipid content, body fat, and plasma levels in energy-yielding metabolites.

CONCLUSIONS

Expression of intracellular binding proteins for both fatty acids and glucose, and their following oxidation rates in skeletal muscles of hLPL rabbits were not fully consistent with the physiology rules. The modifications observed in muscle metabolic properties might not be directly associated with any LPL-linked pathways, but resulted likely of transgene random insertion into rabbit organism close to any regulatory genes. Our findings enlighten the risks for undesirable phenotypic modifications in micro-injected animals and difficulties of biotechnology in mammals larger than mice.

Conserved and muscle-group-specific gene expression patterns shape postnatal development of the novel extraocular muscle phenotype

Current models in skeletal muscle biology do not fully account for the breadth, causes, and consequences of phenotypic variation among skeletal muscle groups. The muscle allotype concept arose to explain frank differences between limb, masticatory, and extraocular (EOM) muscles, but there is little understanding of the developmental regulation of the skeletal muscle phenotypic range. Here, we used morphological and DNA microarray analyses to generate a comprehensive temporal profile for rat EOM development. Based upon coordinate regulation of morphologic/gene expression traits with key events in visual, vestibular, and oculomotor system development, we propose a model that the EOM phenotype is a consequence of extrinsic factors that are unique to its local environment and sensory-motor control system, acting upon a novel myoblast lineage. We identified a broad spectrum of differences between the postnatal transcriptional patterns of EOM and limb muscle allotypes, including numerous transcripts not traditionally associated with muscle fiber/group differences. Several transcription factors were differentially regulated and may be responsible for signaling muscle allotype specificity. Significant differences in cellular energetic mechanisms defined the EOM and limb allotypes. The allotypes were divergent in many other functional transcript classes that remain to be further explored. Taken together, we suggest that the EOM allotype is the consequence of tissue-specific mechanisms that direct expression of a limited number of EOM-specific transcripts and broader, incremental differences in transcripts that are conserved by the two allotypes. This represents an important first step in dissecting allotype-specific regulatory mechanisms that may, in turn, explain differential muscle group sensitivity to a variety of metabolic and neuromuscular diseases.

Controlling muscle mitochondrial content

Mitochondrial content, a chief determinant of aerobic capacity, varies widely among muscle types and species. Mitochondrial enzyme levels in vertebrate skeletal muscles vary more than 100-fold, from fish white muscle to bird flight muscles. Recent studies have shed light on the transcriptional regulators that control mitochondrial gene expression in muscle fiber differentiation and development, and in the context of pathological conditions such as neuromuscular disease and obesity. While the transcriptional co-activator PGC-1alpha (peroxisome proliferator-activated receptor gamma co-activator 1) has emerged as a master controller of mitochondrial gene expression, it is important to consider other mechanisms by which coordinated changes in mitochondrial content could arise. These studies, largely using biomedical models, provide important information for comparative biologists interested in the mechanistic basis of inter-species variation in muscle aerobic capacity.

Transgenic mice expressing lipoprotein lipase in adipose tissue. Absence of the proximal 3'-untranslated region causes translational upregulation

Lipoprotein lipase (LPL) is a key enzyme in lipoprotein and adipocyte metabolism. Defects in LPL can lead to hypertriglyceridemia and the subsequent development of atherosclerosis. The mechanisms of regulation of this enzyme are complex and may occur at multiple levels of gene expression. Because the 3'-untranslated region (UTR) is involved in LPL translational regulation, transgenic mice were generated with adipose tissue expression of an LPL construct either with or without the proximal 3'-UTR and driven by the aP2 promoter. Both transgenic mouse colonies were viable and expressed the transgene, resulting in a 2-fold increase in LPL activity in white adipose tissue. Neither mouse colony exhibited any obvious phenotype in terms of body weight, plasma lipids, glucose, and non-esterified fatty acid levels. In the mice expressing hLPL with an intact 3'-UTR, hLPL mRNA expression approximately paralleled hLPL activity. However in the mice without the proximal 3'-UTR, hLPL mRNA was low in the setting of large amounts of hLPL protein and LPL activity. In previous studies, the 3'-UTR of LPL was critical for the inhibitory effects of constitutively expressed hormones, such as thyroid hormone and catecholamines. Therefore, these data suggest that the absence of the 3'-UTR results in a translationally unrepressed LPL, resulting in a moderate overexpression of adipose LPL activity.

Origins and consequences of mitochondrial variation in vertebrate muscle

This review addresses the mechanisms by which mitochondrial structure and function are regulated, with a focus on vertebrate muscle. We consider the adaptive remodeling that arises during physiological transitions such as differentiation, development, and contractile activity. Parallels are drawn between such phenotypic changes and the pattern of change arising over evolutionary time, as suggested by interspecies comparisons. We address the physiological and evolutionary relationships between ATP production, thermogenesis, and superoxide generation in the context of mitochondrial function. Our discussion of mitochondrial structure focuses on the regulation of membrane composition and maintenance of the three-dimensional reticulum. Current studies of mitochondrial biogenesis strive to integrate muscle functional parameters with signal transduction and molecular genetics, providing insight into the origins of variation arising between physiological states, fiber types, and species.

Lipolysis of triglyceride-rich lipoproteins generates PPAR ligands: evidence for an antiinflammatory role for lipoprotein lipase

Increased levels of triglyceride-rich lipoproteins provoke lipid accumulation in the artery wall, triggering early inflammatory responses central to atherosclerosis like endothelial adhesion molecule expression. The endogenous mechanisms limiting such reactions remain poorly defined. Lipoprotein lipase (LPL) plays a central role in lipid metabolism by hydrolyzing triglyceride rich lipoproteins and releasing fatty acids. We found that LPL treatment reversed tumor necrosis factor alpha and very low-density lipoprotein (VLDL)-stimulated endothelial vascular cell adhesion molecule 1 (VCAM1) induction and VCAM1 promoter responses, thus recapitulating effects reported with synthetic peroxisome proliferator-activated receptor (PPAR) agonists. In fact, these LPL effects on VCAM1 were absent in endothelial cells isolated from PPAR alpha-deficient mice. This finding suggests a novel antiinflammatory role for LPL. Further studies reveal specificity for PPAR activation through lipolysis in regards to lipoprotein substrate (VLDL >> LDL > HDL), PPAR isoform (PPAR alpha >> PPAR delta > PPAR gamma), and among fatty acid-releasing lipases. These PPAR responses required intact LPL catalytic activity. In vivo, transgenic mice overexpressing LPL had increased peroxisome proliferation, but not in the genetic absence of PPAR alpha. Although human plasma possesses minimal PPAR alpha activation despite containing abundant free fatty acids, marked PPAR alpha activation is seen with human plasma after LPL is added in vitro or systemically released in vivo. These data suggest a previously uncharacterized pathway in which the key lipolytic enzyme LPL can act on circulating lipoproteins to generate PPAR alpha ligands, providing a potentially important link between lipoprotein metabolism and distal PPAR alpha transcriptional effects.

Myocardial dysfunction and male mortality in peroxisome proliferator-activated receptor alpha knockout mice overexpressing lipoprotein lipase in muscle

Free fatty acids (FFA) are liberated from triglyceride-rich lipoproteins by lipoprotein lipase (LPL) and are considered to be a principal energy source for the heart. The peroxisome proliferator-activated receptor alpha (PPARalpha) is a key regulator of FFA catabolism. To investigate its role in cardiac muscle metabolism, transgenic mice overexpressing LPL in skeletal and cardiac muscle were bred on a PPARalpha knockout background. Fifty-five percent of male animals lacking PPARalpha and overexpressing LPL died within 4 months after birth. In contrast, females of this genotype stayed alive. Deceased animals exhibited cardiopulmonary congestion but had no increase of neutral lipids in the heart. Changes in plasma glucose, FFA, lactate, and triglycerides did not clearly account for gender-specific differences in mortality; however, they indicated a critical role for PPARalpha during fasting. Analysis of cardiac function revealed that in isolated perfused hearts, left ventricular developed pressure (a measure of contractility) was markedly lower in PPARalpha knockout mice overexpressing LPL compared with controls. Glucose uptake of isolated perfused hearts was significantly higher in PPARalpha knockout mice with both normal or increased LPL expression. However, uptake of FFA was not different among genotypes. In contrast, fasted FFA levels were significantly lower in cardiac muscle of PPARalpha knockout mice with normal LPL expression (-26%) and PPARalpha knockout mice overexpressing LPL (-38%) compared with controls. Our results indicate a critical role for PPARalpha in myocardial pump function and suggest that mouse models combining different genetic effects such as PPARalpha knockout mice overexpressing muscle LPL may be useful to study cardiomyopathies.

Animal models of myositis

Current animal models of human myositis include spontaneous, induced, and transgenic models. Although it is clear that none of these models possesses all the features of the human diseases, they may provide insight into the pathophysiologic mechanisms, and possibly the therapy, of inflammatory muscle disease. Because the human IIMs are phenotypically heterogeneous, but may be divided into more homogeneous subgroups based upon clinical or serologic features, it is possible that different pathogeneses are involved in different subgroups. It is unlikely that any single model would reproduce all features of the human disease. It may be possible, however, to gain insight into some subgroups of the human disease if certain animal models faithfully reproduce one or more subtypes or aspects of the IIMs. Because immunogenetic risk factors, and exposure to certain environmental agents important in triggering myositis in genetically susceptible persons, may be necessary components for human disease induction, transgenic approaches to humanizing murine immune systems and a better understanding of environmental risk factors will be productive avenues for future research. Additional investigations into the molecular basis of the human myositis syndromes and the pathogenesis of the spontaneous, induced, and transgenic animal models should ultimately allow for better understanding and therapy of these diseases.

Overexpression of muscle lipoprotein lipase and insulin sensitivity

PURPOSE OF REVIEW

The number of people affected with obesity and type 2 diabetes has reached epidemic proportions worldwide. Insulin resistance, a common feature of both conditions, has come under intense investigation. This review focuses on our current understanding of the insulin signaling cascade and potential mechanisms of regulation.

RECENT FINDINGS

Recent studies have concentrated on inhibition of insulin-stimulated glucose uptake by free fatty acids as the primary cause of insulin resistance, particularly in muscle, a major site of insulin-stimulated glucose disposal. Mouse models of muscle-specific lipoprotein lipase overexpression permit closer examination of the consequences of lipid oversupply to muscle. Such mice exhibit whole-body and muscle insulin resistance, accompanied by increased accumulation of intramyocellular triglyceride and other fatty acid metabolites (i.e. long-chain acyl coenzyme A, diacylglycerol, and ceramide). These molecules may impede glucose transport by interfering with insulin signal transduction. The mechanisms for the inhibitory effect of free fatty acids on insulin-stimulated glucose transport are complex, and multiple pathways may be involved. Although key molecules have been identified, no single, clearly defined pathway has been established.

SUMMARY

The mouse model of muscle-specific lipoprotein lipase overexpression allows closer examination of increased free fatty acid delivery to the muscle and of effects on insulin sensitivity. Further study of this model may provide additional insight into the role that lipids play in the development of insulin resistance, and may possibly help to identify novel approaches to prevention or treatment.

Altered levels of growth-related and novel gene transcripts in reproductive and other tissues of female mice overexpressing spermidine/spermine N1-acetyltransferase (SSAT)

Overexpression of SSAT (polyamine catabolic enzyme) in female mice results in impaired ovarian folliculogenesis and uterine hypoplasia. To identify the molecular basis for this, the gene expression profiles in uterus and ovary and for comparison, liver and kidney, from non-transgenic (NT) and SSAT transgenic (ST) mice were compared. The mRNA abundance for lipoprotein lipase and glyceraldehyde-3-phosphate dehydrogenase was elevated in all four ST (>NT) tissues. The translation initiation factor-3 subunit 5 mRNA, and transcripts related to endogenous murine leukemia provirus (MLV-related) and murine retrovirus-related sequences (MuRRS) were decreased in ST tissues. A novel calmodulin-related mRNA was strongly induced in ST liver and kidney. SSAT overexpression was associated with increased levels of IGF-binding protein-2 (IGFBP-2) in the uterus and ovary, and a reduction in IGFBP-3 mRNA levels in the uterus. Exogenous spermidine and spermine elevated endogenous IGFBP-2 and SSAT mRNA abundance, whereas, putrescine stimulated IGFBP-2 mRNA abundance and transfected IGFBP-2 gene promoter activity in human (Hec-1-A) uterine cells. Sp1 and BTEB1 mRNAs that encode transcription factors for the IGFBP-2 gene also were induced in some ST tissues. The data suggest that SSAT and polyamines are important for the control of molecular pathways underlying reproductive tract tissue growth, phenotype, and function.

cDNA cloning and analysis of tissue-specific expression of mouse peroxisomal straight-chain acyl-CoA oxidase

Straight-chain acyl-CoA oxidase is the first and rate limiting enzyme in the peroxisomal beta-oxidation pathway catalysing the desaturation of acyl-CoAs to 2-trans-enoyl-CoAs, thereby producing H2O2. To study peroxisomal beta-oxidation we cloned and characterized the cDNA of mouse peroxisomal acyl-CoA oxidase. It consists of 3778 bp, including a 1983-bp ORF encoding a polypeptide of 661 amino-acid residues. Like the rat and human homologue the C-terminus contains an SKL motif, an import signal present in several peroxisomal matrix proteins. Sequence analysis revealed high amino-acid homology with rat (96%) and human (87%) acyl-CoA oxidase in addition to minor homology ( approximately 40%) with other related proteins, such as rabbit trihydroxy-cholestanoyl-CoA oxidase, human branched chain acyl-CoA oxidase and rat trihydroxycoprostanoyl-CoA oxidase. Acyl-CoA oxidase mRNA and protein expression were most abundant in liver followed by kidney, brain and adipose tissue. During mouse brain development acyl-CoA oxidase mRNA expression was highest during the suckling period indicating that peroxisomal beta-oxidation is most critical during this developmental stage. Comparing tissue mRNA levels of peroxisome proliferator-activated receptor alpha and acyl-CoA oxidase, we noticed a constant relationship in all tissues investigated, except heart and adipose tissue in which much more, and respectively, much less, peroxisome proliferator-activated receptor alpha mRNA in proportion to acyl-CoA oxidase mRNA was found. Our data show that acyl-CoA oxidase is an evolutionary highly conserved enzyme with a distinct pattern of expression and indicate an important role in lipid metabolism.