Neonatal extinction of liver lipoprotein lipase expression

CSF1R-dependent macrophages control postnatal somatic growth and organ maturation

Homozygous mutation of the Csf1r locus (Csf1rko) in mice, rats and humans leads to multiple postnatal developmental abnormalities. To enable analysis of the mechanisms underlying the phenotypic impacts of Csf1r mutation, we bred a rat Csf1rko allele to the inbred dark agouti (DA) genetic background and to a Csf1r-mApple reporter transgene. The Csf1rko led to almost complete loss of embryonic macrophages and ablation of most adult tissue macrophage populations. We extended previous analysis of the Csf1rko phenotype to early postnatal development to reveal impacts on musculoskeletal development and proliferation and morphogenesis in multiple organs. Expression profiling of 3-week old wild-type (WT) and Csf1rko livers identified 2760 differentially expressed genes associated with the loss of macrophages, severe hypoplasia, delayed hepatocyte maturation, disrupted lipid metabolism and the IGF1/IGF binding protein system. Older Csf1rko rats developed severe hepatic steatosis. Consistent with the developmental delay in the liver Csf1rko rats had greatly-reduced circulating IGF1. Transfer of WT bone marrow (BM) cells at weaning without conditioning repopulated resident macrophages in all organs, including microglia in the brain, and reversed the mutant phenotypes enabling long term survival and fertility. WT BM transfer restored osteoclasts, eliminated osteopetrosis, restored bone marrow cellularity and architecture and reversed granulocytosis and B cell deficiency. Csf1rko rats had an elevated circulating CSF1 concentration which was rapidly reduced to WT levels following BM transfer. However, CD43hi non-classical monocytes, absent in the Csf1rko, were not rescued and bone marrow progenitors remained unresponsive to CSF1. The results demonstrate that the Csf1rko phenotype is autonomous to BM-derived cells and indicate that BM contains a progenitor of tissue macrophages distinct from hematopoietic stem cells. The model provides a unique system in which to define the pathways of development of resident tissue macrophages and their local and systemic roles in growth and organ maturation.

Lipoprotein Lipase Up-regulation in Hepatic Stellate Cells Exacerbates Liver Fibrosis in Nonalcoholic Steatohepatitis in Mice

Lipoprotein lipase (LPL) plays a central role in incorporating plasma lipids into tissues and regulates lipid metabolism and energy balance in the human body. Conversely, LPL expression is almost absent in normal adult livers. Therefore, its physiological role in the liver remains unknown. We aimed to elucidate the role of LPL in the pathophysiology of nonalcoholic steatohepatitis (NASH), a hepatic manifestation of obesity. Hepatic stellate cell (HSC)–specific LPL‐knockout (Lpl^HSC‐KO) mice, LPL‐floxed (Lpl^fl/fl) mice, or double‐mutant toll‐like receptor 4–deficient (Tlr4^−/−) Lpl^HSC‐KO mice were fed a high‐fat/high‐cholesterol diet for 4 weeks to establish the nonalcoholic fatty liver model or an high‐fat/high‐cholesterol diet for 24 weeks to establish the NASH model. Human samples, derived from patients with nonalcoholic fatty liver disease, were also examined. In human and mouse NASH livers, serum obesity‐related factors, such as free fatty acid, leptin, and interleukin‐6, dramatically increased the expression of LPL, specifically in HSCs through signal transducer and activator of transcription 3 signaling, as opposed to that in hepatocytes or hepatic macrophages. In the NASH mouse model, liver fibrosis was significantly reduced in Lpl^HSC‐KO mice compared with that in Lpl^fl/fl mice. Nonenzymatic LPL‐mediated cholesterol uptake from serum lipoproteins enhanced the accumulation of free cholesterol in HSCs, which amplified TLR4 signaling, resulting in the activation of HSCs and progression of hepatic fibrosis in NASH. Conclusion: The present study reveals the pathophysiological role of LPL in the liver, and furthermore, clarifies the pathophysiology in which obesity, as a background factor, exacerbates NASH. The LPL‐mediated HSC activation pathway could be a promising therapeutic target for treating liver fibrosis in NASH.

A screen in mice uncovers repression of lipoprotein lipase by microRNA-29a as a mechanism for lipid distribution away from the liver

Identification of microRNAs (miRNAs) that regulate lipid metabolism is important to advance the understanding and treatment of some of the most common human diseases. In the liver, a few key miRNAs have been reported that regulate lipid metabolism, but since many genes contribute to hepatic lipid metabolism, we hypothesized that other such miRNAs exist. To identify genes repressed by miRNAs in mature hepatocytes in vivo, we injected adult mice carrying floxed Dicer1 alleles with an adenoassociated viral vector expressing Cre recombinase specifically in hepatocytes. By inactivating Dicer in adult quiescent hepatocytes we avoided the hepatocyte injury and regeneration observed in previous mouse models of global miRNA deficiency in hepatocytes. Next, we combined gene and miRNA expression profiling to identify candidate gene/miRNA interactions involved in hepatic lipid metabolism and validated their function in vivo using antisense oligonucleotides. A candidate gene that emerged from our screen was lipoprotein lipase (Lpl), which encodes an enzyme that facilitates cellular uptake of lipids from the circulation. Unlike in energy-dependent cells like myocytes, LPL is normally repressed in adult hepatocytes. We identified miR-29a as the miRNA responsible for repressing LPL in hepatocytes, and found that decreasing hepatic miR-29a levels causes lipids to accumulate in mouse livers.

CONCLUSION

Our screen suggests several new miRNAs are regulators of hepatic lipid metabolism. We show that one of these, miR-29a, contributes to physiological lipid distribution away from the liver and protects hepatocytes from steatosis. Our results, together with miR-29a's known antifibrotic effect, suggest miR-29a is a therapeutic target in fatty liver disease.

Effect of starvation on activities and mRNA expression of lipoprotein lipase and hormone-sensitive lipase in tilapia (Oreochromis niloticus x O. areus)

A 4-week study was conducted to determine the effect of starvation on activities and mRNA expression of lipoprotein lipase (LPL) and hormone-sensitive lipase (HSL) in hybrid tilapia (Oreochromis niloticus x O. areus). The tissue samples were sampled once a week. Results showed that body weight (BW) and hepatosomatic index (HSI) were decreased significantly (P < 0.05) during starvation. The percentages of crude fat and crude protein in the whole body and the fat content in muscle decreased significantly (P < 0.05), while the rate of moisture and crude ash increased significantly (P < 0.05). The response of LPL, HSL activities and mRNA expression in tissues was tissue dependent. The activities of LPL and HSL in muscle at day 7 were elevated by 2.5 times (P < 0.05) and 11.8 times (P < 0.05) of the value at day 0, respectively, and both then decreased to pre-starvation levels at day 14 and finally stabilized at a certain level afterward. LPL and HSL mRNA abundance in muscle remained relatively stable between 0 and 14 day; then, a significant increase was seen after 14 days. In the liver, LPL activity maintained a significantly increasing trend during starvation, while HSL activity rose dramatically at day 7 of starvation by 2.35 times (P < 0.05) and finally stabilized at a certain level. The mRNA abundance of liver LPL increased significantly during the whole process of starvation (P < 0.05), whereas the mRNA abundance of liver HSL decreased significantly at day 7 of starvation, elevating significantly afterward (P < 0.05).

An integrated functional genomic study of acute phenobarbital exposure in the rat

Background

Non-genotoxic carcinogens are notoriously difficult to identify as they do not damage DNA directly and have diverse modes of action, necessitating long term in vivo studies. The early effects of the classic rodent non-genotoxic hepatocarcinogen phenobarbital have been investigated in the Fisher rat using a combination of metabolomics and transcriptomics, to investige early stage mechanistic changes that are predictive of longer term pathology.

Results

Liver and blood plasma were profiled across 14 days, and multivariate statistics used to identify perturbed pathways. Both metabolomics and transcriptomics detected changes in the liver which were dose dependent, even after one day of exposure. Integration of the two datasets associated perturbations with specific pathways. Hepatic glycogen was decreased due to a decrease in synthesis, and plasma triglycerides were decreased due to an increase in fatty acid uptake by the liver. Hepatic succinate was increased and this was associated with increased heme biosynthesis. Glutathione synthesis was also increased, presumably in response to oxidative stress. Liquid Chromatography Mass Spectrometry demonstrated a remodeling of lipid species, possibly resulting from proliferation of the smooth endoplasmic reticulum.

Conclusions

The data fusion of metabolomic and transcriptomic changes proved to be a highly sensitive approach for monitoring early stage changes in altered hepatic metabolism, oxidative stress and cytochrome P450 induction simultaneously. This approach is particularly useful in interpreting changes in metabolites such as succinate which are hubs of metabolism.

Quantitative trait locus mapping and identification of Zhx2 as a novel regulator of plasma lipid metabolism

BACKGROUND

We previously mapped a quantitative trait locus on chromosome 15 in mice contributing to high-density lipoprotein cholesterol and triglyceride levels and now report the identification of the underlying gene.

METHODS AND RESULTS

We first fine-mapped the locus by studying a series of congenic strains derived from the parental strains BALB/cJ and MRL/MpJ. Analysis of gene expression and sequencing followed by transgenic complementation led to the identification of zinc fingers and homeoboxes 2 (Zhx2), a transcription factor previously implicated in the developmental regulation of alpha-fetoprotein. Reduced expression of the protein in BALB/cJ mice resulted in altered hepatic transcript levels for several genes involved in lipoprotein metabolism. Most notably, the Zhx2 mutation resulted in a failure to suppress expression of lipoprotein lipase, a gene normally silenced in the adult liver, and this was normalized in BALB/cJ mice carrying the Zhx2 transgene.

CONCLUSIONS

We identified the gene underlying the chromosome 15 quantitative trait locus, and our results show that Zhx2 functions as a novel developmental regulator of key genes influencing lipoprotein metabolism.

Lipoprotein lipase: from gene to obesity

Lipoprotein lipase (LPL) is a multifunctional enzyme produced by many tissues, including adipose tissue, cardiac and skeletal muscle, islets, and macrophages. LPL is the rate-limiting enzyme for the hydrolysis of the triglyceride (TG) core of circulating TG-rich lipoproteins, chylomicrons, and very low-density lipoproteins (VLDL). LPL-catalyzed reaction products, fatty acids, and monoacylglycerol are in part taken up by the tissues locally and processed differentially; e.g., they are stored as neutral lipids in adipose tissue, oxidized, or stored in skeletal and cardiac muscle or as cholesteryl ester and TG in macrophages. LPL is regulated at transcriptional, posttranscriptional, and posttranslational levels in a tissue-specific manner. Nutrient states and hormonal levels all have divergent effects on the regulation of LPL, and a variety of proteins that interact with LPL to regulate its tissue-specific activity have also been identified. To examine this divergent regulation further, transgenic and knockout murine models of tissue-specific LPL expression have been developed. Mice with overexpression of LPL in skeletal muscle accumulate TG in muscle, develop insulin resistance, are protected from excessive weight gain, and increase their metabolic rate in the cold. Mice with LPL deletion in skeletal muscle have reduced TG accumulation and increased insulin action on glucose transport in muscle. Ultimately, this leads to increased lipid partitioning to other tissues, insulin resistance, and obesity. Mice with LPL deletion in the heart develop hypertriglyceridemia and cardiac dysfunction. The fact that the heart depends increasingly on glucose implies that free fatty acids are not a sufficient fuel for optimal cardiac function. Overall, LPL is a fascinating enzyme that contributes in a pronounced way to normal lipoprotein metabolism, tissue-specific substrate delivery and utilization, and the many aspects of obesity and other metabolic disorders that relate to energy balance, insulin action, and body weight regulation.

Lipoprotein lipase expression in livers of morbidly obese patients could be responsible for liver steatosis

BACKGROUND

Most patients with morbid obesity develop non-alcoholic fatty liver disease (NAFLD). The origins of lipid deposition in the liver and the effects of bariatric surgery in the obese with NAFLD are controversial.

METHODS

We analyzed lipids and lipoprotein lipase (LPL) in both plasma and liver biopsies performed before and 12-18 months after Roux-en-Y gastric bypass surgery in 26 patients.

RESULTS

In the livers of morbidly obese patients, the levels of LPL messenger RNA (mRNA) were higher (4.5-fold) before surgery than afterwards than control livers. In these patients, LPL activity was also significantly higher (91 +/- 7 mU/g) than in controls (51 +/- 3 mU/g, p = 0.0026) and correlated with the severity of the liver damage. All hepatic lipids were significantly increased in obese patients; however, after bariatric surgery, these lipids, with the exception of NEFA, tended to recover to normal levels.

CONCLUSIONS

The liver of obese patients presented higher LPL activity than controls, and unlike the controls, this enzyme could be synthesized in the liver because it also present LPL mRNA. The presence of the LPL activity could enable the liver to capture circulating triacylglycerides, thus favoring the typical steatosis observed in these patients.

Gender-specific programmed hepatic lipid dysregulation in intrauterine growth-restricted offspring

OBJECTIVE

Intrauterine growth restriction demonstrates increased risk of adult metabolic syndrome. The associated hyperlipidemia results from obesity or programmed metabolic abnormalities. Because lipid homeostasis is regulated by the liver, we hypothesized that hepatic structure and lipid content in intrauterine growth restriction would reflect a primary lipid dysfunction.

STUDY DESIGN

From 10 days to term gestation, control pregnant rats received ad libitum diet; study rats were 25% food-restricted (FR). All dams received ad libitum diet throughout lactation. At 3 weeks of age, hepatic lobule size and lipid profile of the pups were determined.

RESULTS

At 3 weeks of age, body and liver weights of the pups were comparable with controls, although with reduced hepatic lobule size. FR males had increased hepatic triglyceride and cholesterol content with elevated sterol regulatory element-binding protein-1c, fatty acid synthase, and lipoprotein lipase expression; FR females exhibited decreased hepatic cholesterol levels. Plasma lipid levels were unchanged in FR males and females.

CONCLUSION

Developmental programming results in sex-dependent altered lipid metabolism with increased risk in males.

Relationship between lipoprotein lipase and peroxisome proliferator-activated receptor-α expression in rat liver during development

The present study was addressed to determine whether the high expression of peroxisome proliferator-activated receptor-alpha (PPAR-alpha) in rat liver during the perinatal stage plays a role in the induction of liver lipoprotein lipase (LPL) expression and activity. Parallel increases in liver mRNA PPAR-alpha and LPL activity were found in newborn rats, and after a slight decline, values remained elevated until weaning. Anticipated weaning for 3 days caused a decline in those two variables as well as in the mRNA LPL level, and a similar change was also found in liver triacylglycerol concentration. Force-feeding with Intralipid in 10-day-old rats or animals kept fasted for 5 h showed high mRNA-PPARalpha and -LPL levels as well as LPL activity with low plasma insulin and high FFA levels, whereas glucose and a combination of glucose and Intralipid produced low mRNA-PPARalpha and -LPL levels as well as LPL activity. Under these latter conditions, plasma insulin and FFA levels were high in those rats receiving the combination of glucose and Intralipid, whereas plasma FFA levels were low in those force-fed with glucose. It is proposed that the hormonal and nutritional induction of liver PPAR-alpha expression around birth and its maintained elevated level throughout suckling is responsible for the induction of liver LPL-expression and activity during suckling.

Effect of dietary fatty acids on lipoprotein lipase gene expression in the liver and visceral adipose tissue of fed and starved red sea bream Pagrus major

Juvenile red sea bream Pagrus major were fed either a commercial diet (diet 1) or diets supplemented with 10% oleate (diet 2), 5% oleate+5% linoleate (diet 3) or 5% oleate+5% n-3 polyunsaturated fatty acid mixture (diet 4) for 4 weeks. Following the conditioning period, the effects of dietary fatty acids on lipoprotein lipase (LPL) gene expression in the liver and visceral adipose tissue of fed (5 h post-feeding) and starved (48 h post-feeding) fish were investigated by competitive polymerase chain reaction. Fish liver showed substantial LPL mRNA expression that is not found in adult rat liver. When compared with diet 1, diets 2-4 tended to increase the LPL mRNA level in the liver, but tended to decrease it in the visceral adipose tissue under the fed condition. The reciprocal regulation of the liver and visceral adipose LPL mRNA abundance by dietary fatty acids was comparable to that of rat brown and white adipose tissue, respectively. The change in the LPL mRNA level by fatty acids was not completely consistent with the degree of fatty acid unsaturation. Our results indicate that the regulatory effect of dietary fatty acids on LPL gene expression was tissue-specific and related to feeding conditions, but was not solely dependent on the degree of unsaturation of fatty acids.

Down-regulation of adipose tissue lipoprotein lipase during fasting requires that a gene, separate from the lipase gene, is switched on

During short term fasting, lipoprotein lipase (LPL) activity in rat adipose tissue is rapidly down-regulated. This down-regulation occurs on a posttranslational level; it is not accompanied by changes in LPL mRNA or protein levels. The LPL activity can be restored within 4 h by refeeding. Previously, we showed that during fasting there is a shift in the distribution of lipase protein toward an inactive form with low heparin affinity. To study the nature of the regulatory mechanism, we determined the in vivo turnover of LPL activity, protein mass, and mRNA in rat adipose tissue. When protein synthesis was inhibited with cycloheximide, LPL activity and protein mass decreased rapidly and in parallel with half-lives of around 2 h, and the effect of refeeding was blocked. This indicates that maintaining high levels of LPL activity requires continuous synthesis of new enzyme protein. When transcription was inhibited by actinomycin, LPL mRNA decreased with half-lives of 13.3 and 16.8 h in the fed and fasted states, respectively, demonstrating slow turnover of the LPL transcript. Surprisingly, when actinomycin was given to fed rats, LPL activity was not down-regulated during fasting, indicating that actinomycin interferes with the transcription of a gene that blocks the activation of newly synthesized LPL protein. When actinomycin was given to fasted rats, LPL activity increased 4-fold within 6 h, even in the absence of refeeding. The same effect was seen with alpha-amanitin, another inhibitor of transcription. The response to actinomycin was much less pronounced in aging rats, which are obese and insulin-resistant. These data suggest a default state where LPL protein is synthesized on a relatively stable mRNA and is processed into its active form. During fasting, a gene is switched on whose product prevents the enzyme from becoming active even though synthesis of LPL protein continues unabated.

The effects of feeding condition and dietary lipid level on lipoprotein lipase gene expression in liver and visceral adipose tissue of red sea bream Pagrus major

The effects of feeding condition and dietary lipid level on lipoprotein lipase (LPL) gene expression in the liver and visceral adipose tissue of red sea bream Pagrus major were investigated by competitive polymerase chain reaction. Not only visceral adipose tissue but also liver of red sea bream showed substantial LPL gene expression. In the liver, starvation (at 48 h post-feeding) drastically stimulated LPL gene expression in the fish-fed low lipid diet, but had no effect in the fish fed high lipid diet. Dietary lipid level did not significantly affect the liver LPL mRNA level under fed condition (at 5 h post-feeding). In the visceral adipose tissue, LPL mRNA number per tissue weight was significantly higher in the fed condition than in the starved condition, irrespective of the dietary lipid levels. Dietary lipid levels did not affect the visceral adipose tissue LPL mRNA levels under fed or starved conditions. Our results demonstrate that both feeding conditions and dietary lipid levels alter the liver LPL mRNA levels, while only the feeding conditions but not dietary lipid levels cause changes in the visceral adipose LPL mRNA level. It was concluded that the liver and visceral adipose LPL gene expression of red sea bream seems to be regulated in a tissue-specific fashion by the nutritional state.

Developmental and pharmacological regulation of apolipoprotein C-II gene expression. Comparison with apo C-I and apo C-III gene regulation

Increased plasma triglyceride concentrations are often observed in metabolic disorders predisposing to coronary heart disease. Among the major determinants of plasma triglyceride metabolism are the apolipoproteins (apos) of the C class, C-I, C-II, and C-III. Whereas physiological concentrations of apo C-II are required for lipolysis of triglycerides by lipoprotein lipase (LPL), overexpression of all 3 C apolipoproteins leads to hypertriglyceridemia. In the present study, we investigated apo C-II gene regulation under conditions associated with profound changes in plasma triglyceride metabolism, ie, during postnatal development and after treatment with the triglyceride-lowering fibrate drugs, and compared its expression to that of apo C-I and apo C-III. Whereas the expression of both apo C-I and apo C-III is low in fetal liver, increases gradually after birth, and attains maximal levels after weaning, apo C-II gene expression is already detectable in the fetal liver, increases rapidly immediately after birth, and remains elevated throughout suckling. Thus, the increased ingestion of lipids during suckling is met by an earlier induction of apo C-II, the obligatory activator for LPL, compared with apo C-III and apo C-I, which antagonize triglyceride catabolism. Treatment of rats with fibrates decreased apo C-II gene expression in the liver, but not in the intestine, whereas apo C-I gene expression did not change. The decrease of liver apo C-II mRNA levels after fenofibrate occurred in a time- and dose-dependent manner and was reversible but appeared less pronounced than the decrease of apo C-III mRNA. Apo C-II mRNA levels were not affected after treatment with BRL49653, a peroxisome proliferator-activated receptor (PPAR)gamma-specific ligand, suggesting that fibrates act on apo C-II expression via PPARalpha. Addition of fenofibric acid to primary rat and human hepatocytes resulted in a decrease of apo C-II expression. In conclusion, fibrates decrease gene expression of apo C-II and apo C-III, but not apo C-I, in rat and human hepatocytes. This decrease of apo C-II and apo C-III gene expression, together with a lowered apo C-III to apo C-II ratio, should result in an improved clearance of triglyceride-rich remnant lipoproteins from plasma, without hampering triglyceride lipolysis by LPL.

Hepatic regeneration induces changes in lipoprotein lipase activity in several tissues and its re-expression in the liver

We examined the expression of lipoprotein lipase (LPL) gene and LPL activity following a two-thirds hepatectomy and during liver regeneration. In most of the tissues studied, LPL activity increased a few hours after partial hepatectomy, but soon returned to normal levels. The greatest increase was found in the adrenal glands, plasma and liver. This increase in LPL activity in the liver could be partially due to an increase in the influx of the enzyme from extrahepatic tissues. There is, however, also a re-expression of LPL mRNA in the liver after partial hepatectomy (during the first hours). It is well known that LPL is expressed in the liver of neonatal animals, but progressively decreases during post-natal development, to reach adult levels around the time of weaning. Our results show by the first time that the remaining liver re-expresses LPL gene during the regeneration process and that the hepatocytes de-differentiate and acquire some of the neonatal characteristics. The increase in LPL mRNA will contribute to the rise in LPL activity after hepatectomy. This presence of LPL could enable the liver to take up fatty acids from the circulating triacylglycerols, which are needed as energetic and plastic substrates during the process of hepatic regeneration.

Expression and regulation of the lipoprotein lipase gene in human adrenal cortex

Lipoprotein lipase (LPL), an enzyme which hydrolyzes triglycerides and participates in the catabolism of remnant lipoproteins, plays a crucial role in energy and lipid metabolism. The goal of this study was to analyze the expression and regulation of the LPL gene in human adrenals. Reverse transcriptase-polymerase chain reaction amplification and sequence analysis demonstrated the presence of LPL mRNA in fetal and adult human adrenal cortex. Furthermore, the human adrenocortical carcinoma cell line, NCI-H295, expresses LPL mRNA and protein, which is localized to the outer cellular membrane as demonstrated by immunofluorescence confocal microscopy and can be released in the medium by heparin addition. To asses whether the LPL gene is regulated by agents regulating adrenal steroidogenesis, NCI-H295 cells were treated with activators of second messenger systems. Whereas the calcium-ionophore A23187 did not affect LPL gene expression, treatment with phorbol 12-myristate 13-acetate decreased LPL mRNA levels in a time- and dose-dependent manner. This decrease after phorbol 12-myristate 13-acetate was associated with diminished heparin-releasable LPL mass and activity in the culture medium. Addition of the cAMP analog 8-Br-cAMP to NCI-H295 cells resulted in a rapid, but transient dose-dependent induction of LPL mRNA. Treatment with the protein synthesis inhibitor cycloheximide gradually induced, whereas simultaneous addition of cAMP and cycloheximide superinduced LPL mRNA levels. Nuclear run-on analysis indicated that the effects of cAMP and cycloheximide occurred at the transcriptional and post-transcriptional level, respectively. Transient co-transfection assays demonstrated that the first 230 base pairs of the proximal LPL promoter contain a cAMP-responsive element activated by protein kinase A and transcription factors belonging to the CREB/CREM family. These data indicate that LPL is expressed in human adrenal cortex and regulated in NCI-H295 adrenocortical carcinoma cells by activators of the protein kinase A and protein kinase C second messenger pathways in a manner comparable to P450scc, which catalyzes the first step in adrenal steroidogenesis. These observations suggest a role for LPL in adrenal energy and/or lipid metabolism and possibly in steroidogenesis.

Hormonal regulation of lipoprotein lipase activity from 5-day-old rat hepatocytes

Lipoprotein lipase (LPL) activity is known to be synthesized, active and functional in the 1-day-old rat liver: it peaks just at birth triggered by parturition. During suckling LPL mRNA, LPL synthesis and LPL activity are still high at 5 days and then fade reaching adult values at weaning. How LPL expression is gradually extinguished is not known. Therefore we studied the effect of different doses of several hormones on LPL activity released by incubated hepatocytes from 5-day-old rats. In the presence of heparin the release of LPL activity in the medium was linear until 3 h and was always significantly increased vs. without heparin. At 3 h in the presence of heparin the main hormonal effects were: dose-dependent increase (30-60%) with dexamethasone; dose-dependent increase (20-60%) with glucagon; dose-independent decrease (50-60%) with ethinylestradiol, testosterone, progesterone and prolactin; no effect with insulin; 20-40% increase with adrenaline < 1 mM but 40-50% decrease with noradrenaline < 10 microM. Increase of LPL release by glucagon and adrenaline agrees with the increased LPL expression we previously found in an undifferentiated hepatoma cell line when the adenylate cyclase/protein kinase A pathway was activated. The effect of glucagon is concordant with our previous observations that fasting increases liver LPL activity in neonatal rats. The high estradiol levels known to be present in male and female 9-19-day-old rats might contribute to liver LPL extinction during suckling.

Expression in mouse embryos and in adult mouse brain of three members of the amyloid precursor protein family, of the alpha-2-macroglobulin receptor/low density lipoprotein receptor-related protein and of its ligands apolipoprotein E, lipoprotein lipase, alpha-2-macroglobulin and the 40,000 molecular weight receptor-associated protein

We have analysed by northern blotting and by in situ hybridization the expression patterns of eight different genes during the second half of mouse embryonic development and in adult mouse brain: we compared the messenger RNA levels of amyloid precursor protein and of the two amyloid precursor protein-like proteins 1 and 2 and we have analysed expression of apolipoprotein E and of its main receptor in brain, the alpha-2-macroglobulin/low density lipoprotein receptor-related protein and three other ligands: the proteinase inhibitor alpha-2-macroglobulin, the modifying enzyme lipoprotein lipase and the 44,000 molecular weight heparin binding protein, a ligand of unknown function. During embryogenesis the temporal expression pattern differs considerably for the three members of the amyloid precursor proteins. Total embryo messenger RNA levels of amyloid precursor protein and amyloid precursor protein-like protein 2 increased progressively, while amyloid precursor protein-like protein 1 messenger RNA showed a burst of synthesis between days 10 and 13 post-coitum. Significantly, expression of the alpha-2-macroglobulin/low density lipoprotein receptor-related protein and of its associated protein, the 44,000 molecular weight heparin binding protein, exhibited their most important increase very similar to that of amyloid precursor protein-like protein 1, between days 10 and 13 post-coitum. Apolipoprotein E, lipoprotein lipase and alpha-2-macroglobulin messenger RNA levels in total embryos increased progressively, beginning most pronounced at days 13, 15 and 17, respectively. In mouse embryos, in situ hybridization established amyloid precursor protein, amyloid precursor protein-like protein 2 and alpha-2-macroglobulin/low density lipoprotein receptor-related protein messenger RNA to be expressed in most organs, with the notable exception of the liver, while expression of the other studied proteins was much more restricted. Among adult mouse tissues, the genes investigated were expressed very prominently in brain, except for lipoprotein lipase and for the complete absence of alpha-2-macroglobulin messenger RNA. In adult mouse brain, the cortex and hippocampus exhibited strong signals for most genes analysed. Exceptions are lipoprotein lipase and apolipoprotein E messenger RNAs, and the absent alpha-2-macroglobulin messenger RNA. Several interesting features, similarities as well as differences, between brain tissue sections hybridized with probes for amyloid precursor protein, amyloid precursor protein-like proteins 1 and 2 and between alpha-2-macroglobulin/low density lipoprotein receptor-related protein and heparin binding protein-44 were observed and are described. The results are further discussed in view of the known or anticipated physiological functions of the proteins examined and of their possible role in the etiology of Alzheimer's disease.

Developmental extinction of liver lipoprotein lipase mRNA expression might be regulated by an NF-1-like site

The molecular mechanism underlying the extinction of lipoprotein lipase (LPL) expression in rat liver during development was investigated. A mouse (BWTG3) and a rat (7777) hepatoma, both of which exhibit characteristics of fetal hepatocytes, were found to contain LPL mRNA, whereas the more differentiated human (Hep G2 and Hep 3B) or rat (Fa32) hepatoma cell lines did not. Somatic cell hybrids between LPL-producing hepatoma cells and non-LPL-producing cells, such as adult rat hepatocytes or fibroblasts, exhibited extinction of LPL gene expression. Assay of expression of nested deletions in the 5' regulatory sequences of the LPL gene in the Hep G2 cell line and in BWTG3 cells localized sequences involved in the suppression of LPL production to a region between -591 and -288 relative to the transcription initiation site. A site with sequence homology to a glucocorticoid responsive element (GRE) was shown not to play an important role in the extinction process. A novel transcription factor, termed RF-1-LPL, was shown to bind to an NF-1-like site in this region. In contrast to neonatal animals, in adult animals an additional protein complex (RF-2-LPL), was formed on the NF-1-like site, suggesting that this sequence might recruit a trans-acting factor involved in the extinction of LPL gene expression in adult rat liver.

Lipoprotein lipase: recent contributions from molecular biology

Lipoprotein lipase (LPL) is a glycoprotein enzyme that is produced in several cells and tissues. LPL belongs to a large lipase gene family that includes, among others, hepatic lipase and pancreatic lipase. After secretion, LPL becomes anchored on the luminal surface of the capillary endothelial cells. There it hydrolyzes triglycerides in triglyceride-rich lipoproteins, generating free fatty acids that can serve either as a direct energy source or can be stored. Through this action LPL plays a pivotal role both in energy and in lipoprotein metabolism. LPL production is regulated in a tissue-specific fashion by developmental, hormonal, and nutritional factors. The recent availability of the regulatory sequences of the LPL gene will greatly facilitate these regulatory studies in the future. In man, several mutations resulting in familial LPL deficiency have been delineated at a molecular level. The study of these mutations is not only very beneficial from a clinical point of view but also contributes in a major way to our understanding of the structure-function relationship of LPL and other lipases. In this review major attention is given to molecular studies relating to the regulation of LPL production, to the defects underlying LPL deficiency, and to structure-function relationship of the lipases.