Developmental changes in the expression of genes involved in cholesterol biosynthesis and lipid transport in human and rat fetal and neonatal livers

Where does fetal and embryonic cholesterol originate and what does it do?

The development of a single-celled fertilized egg, through the blastocyst stage of a ball of cells and the embryonic stage when almost all organ systems begin to develop, and finally to the fetal stage where growth and physiological maturation occurs, is a complex and multifaceted process. A change in metabolism during gestation, especially when organogenesis occurs, can lead to abnormal development and congenital defects. Although many studies have described the roles of specific proteins in development, the roles of specific lipids, such as sterols, have not been studied as intensely. Sterol's functions in development range from being a structural component of membranes to regulating the patterning of the forebrain through sonic hedgehog to regulating expression of key proteins involved in metabolic processes. This review focuses on the roles of sterols in embryonic and fetal development and metabolism. Potential sources of cholesterol for the fetus and embryo are also discussed.

Differential expression of peroxisomal matrix and membrane proteins during postnatal development of mouse brain

In peroxisomal biogenesis disorders, serious neurological abnormalities can be observed in the patients and the respective knockout mouse models. As a prerequisite for a better understanding of the relationship between the absence of peroxisomes and the observed neuropathology, knowledge of the regional and cell-type specific distribution of peroxisomal proteins in mouse brain is necessary. Therefore, we investigated the expression of distinct peroxins, peroxisomal membrane and matrix proteins (e.g. Pex5p, Pex14p, Pex13p, PMP70, catalase, peroxisomal thiolase, Acox1, "SKL"-PTS1 proteins) by indirect immunofluorescence 1) in primary cultures of the medial neocortex, hippocampus, and cerebellum of newborn mice and 2) in paraffin sections of mouse brain of different ages (newborn-adult). Quantitative analysis revealed a comparable abundance (number/microm(2)) of peroxisomes in cultured neurons and astrocytes of all three brain regions. In contrast, catalase immunoreactivity was higher in cultured astrocytes than in neurons. In mouse brain tissue, the abundance of peroxisomes decreased by half during postnatal development, also exhibiting prominent differences between distinct brain regions and cell types. Catalase protein levels in neuronal peroxisomes, however, decreased much more strongly in the neocortex, CA1-3 areas of the hippocampus, dentate gyrus, cerebellar nuclei, and cerebellar cortex but remained high in Bergmann glia and other astrocytes, epithelial cells of the choroid plexus, and ependyma. Similar age-dependent changes were found for thiolase and Acox1 protein levels. Developmental changes were confirmed by Western blot analysis using enriched peroxisomal and cytosolic fractions of the brain tissue as well as by measurement of catalase activity.

Inability to fully suppress sterol synthesis rates with exogenous sterol in embryonic and extraembyronic fetal tissues

The requirement for cholesterol is greater in developing tissues (fetus, placenta, and yolk sac) as compared to adult tissues. Here, we compared cholesterol-induced suppression of sterol synthesis rates in the adult liver to the fetal liver, fetal body, placenta, and yolk sac of the Golden Syrian hamster. Sterol synthesis rates were suppressed maximally in non-pregnant adult livers when cholesterol concentrations were increased. In contrast, sterol synthesis rates were suppressed only marginally in fetal livers, fetal bodies, placentas, and yolk sacs when cholesterol concentrations were increased. To begin to elucidate the mechanism responsible for the blunted response of sterol synthesis rates in fetal tissues to exogenous cholesterol, the ratio of sterol regulatory element-binding protein (SREBP) cleavage-activating protein (SCAP) to Insig-1 was measured in these same tissues since the ratio of SCAP to the Insigs can impact SREBP processing. The fetal tissues had anywhere from a 2- to 6-fold greater ratio of SCAP to Insig-1 than did the adult liver, suggesting constitutive processing of the SREBPs. As expected, the level of mature, nuclear SREBP-2 was not different in the fetal tissues with different levels of cholesterol whereas it was different in adult livers. These findings indicate that the suppression of sterol synthesis to exogenous sterol is blunted in developing tissues and the lack of response appears to be mediated at least partly through relative levels of Insigs and SCAP.

Lipid metabolism in pregnancy and its consequences in the fetus and newborn

During early pregnancy there is an increase in body fat accumulation, associated with both hyperphagia and increased lipogenesis. During late pregnancy there is an accelerated breakdown of fat depots, which plays a key role in fetal development. Besides using placental transferred fatty acids, the fetus benefits from two other products: glycerol and ketone bodies. Although glycerol crosses the placenta in small proportions, it is a preferential substrate for maternal gluconeogenesis, and maternal glucose is quantitatively the main substrate crossing the placenta. Enhanced ketogenesis under fasting conditions and the easy transfer of ketones to the fetus allow maternal ketone bodies to reach the fetus, where they can be used as fuels for oxidative metabolism as well as lipogenic substrates. Although maternal cholesterol is an important source of cholesterol for the fetus during early gestation, its importance becomes minimal during late pregnancy, owing to the high capacity of fetal tissues to synthesize cholesterol. Maternal hypertriglyceridemia is a characteristic feature during pregnancy and corresponds to an accumulation of triglycerides not only in very low-density lipoprotein but also in low- and high-density lipoprotein. Although triglycerides do not cross the placental barrier, the presence of lipoprotein receptors in the placenta, together with lipoprotein lipase, phospholipase A2, and intracellular lipase activities, allows the release to the fetus of polyunsaturated fatty acids transported as triglycerides in maternal plasma lipoproteins. Normal fetal development needs the availability of both essential fatty acids and long chain polyunsaturated fatty acids, and the nutritional status of the mother during gestation has been related to fetal growth. However, excessive intake of certain long chain fatty acids may cause both declines in arachidonic acid and enhanced lipid peroxidation, reducing antioxidant capacity.

Immunological quantitation and localization of ACAT-1 and ACAT-2 in human liver and small intestine

By using specific anti-ACAT-1 antibodies in immunodepletion studies, we previously found that ACAT-1, a 50-kDa protein, plays a major catalytic role in the adult human liver, adrenal glands, macrophages, and kidneys but not in the intestine. Acyl-coenzyme A:cholesterol acyltransferase (ACAT) activity in the intestine may be largely derived from a different ACAT protein. To test this hypothesis, we produced specific polyclonal anti-ACAT-2 antibodies that quantitatively immunodepleted human ACAT-2, a 46-kDa protein expressed in Chinese hamster ovary cells. In hepatocyte-like HepG2 cells, ACAT-1 comprises 85-90% of the total ACAT activity, with the remainder attributed to ACAT-2. In adult intestines, most of the ACAT activity can be immunodepleted by anti-ACAT-2. ACAT-1 and ACAT-2 do not form hetero-oligomeric complexes. In differentiating intestinal enterocyte-like Caco-2 cells, ACAT-2 protein content increases by 5-10-fold in 6 days, whereas ACAT-1 protein content remains relatively constant. In the small intestine, ACAT-2 is concentrated at the apices of the villi, whereas ACAT-1 is uniformly distributed along the villus-crypt axis. In the human liver, ACAT-1 is present in both fetal and adult hepatocytes. In contrast, ACAT-2 is evident in fetal but not adult hepatocytes. Our results collectively suggest that in humans, ACAT-2 performs significant catalytic roles in the fetal liver and in intestinal enterocytes.

Gene expression related to cholesterol metabolism in mouse brain during development

Although a large amount of cholesterol is known to be needed for brain maturation and differentiation, cholesterol metabolism during these periods remains unclear. To elucidate the developmental regulation of cholesterol metabolism in the brain, we investigated the expression of 3-hydroxy-3-methyglutaryl-coenzyme A (HMG-CoA) reductase (EC 1.1.1.34), low-density-lipoprotein (LDL) receptor and very-low-density-lipoprotein (VLDL)/apolipoprotein E (apo E) receptor (VLDL receptor) using RNase protection assay (RPA) to quantitate mRNA levels in mouse brain, liver and kidney during development. Messenger RNA levels of HMG-CoA reductase in the brain decreased with age, and those levels at -5 (5 days before birth) and 5 days after birth were significantly higher than the control level of adult mice. The period from -5 to 5 days might correspond to stages of active biogenesis of the membranes of brain cells. The mRNA level of HMG-CoA reductase in the liver was also high at -5 days; a finding that correlated with cell proliferation. On the other hand, mRNA levels of the LDL and VLDL receptors in the brain did not change markedly during development. These results suggest that de novo cholesterol biosynthesis in brain cells plays a major role in the supply of cholesterol to the developing brain, rather than the uptake of cholesterol from serum lipoproteins through lipoprotein receptors.

Modulation of neuronal voltage-gated calcium channels by farnesol

The modulation of presynaptic voltage-dependent calcium channels by classical second messenger molecules such as protein kinase C and G protein betagamma subunits is well established and considered a key factor for the regulation of neurotransmitter release. However, little is known of other endogenous mechanisms that control the activity of these channels. Here, we demonstrate a unique modulation of N-type calcium channels by farnesol, a dephosphorylated intermediate of the mammalian mevalonate pathway. At micromolar concentrations, farnesol acts as a relatively non-discriminatory rapid open channel blocker of all types of high voltage-activated calcium channels, with a mild specificity for L-type channels. However, at 250 nM, farnesol induces an N-type channel-specific hyperpolarizing shift in channel availability that results in approximately 50% inhibition at a typical neuronal resting potential. Additional experiments demonstrated the presence of farnesol in the brain (rodents and humans) at physiologically relevant concentrations (100-800 pmol/g (wet weight)). Altogether, our results indicate that farnesol is a selective, high affinity inhibitor of N-type Ca(2+) channels and raise the possibility that endogenous farnesol and the mevalonate pathway are implicated in neurotransmitter release through regulation of presynaptic voltage-gated Ca(2+) channels.

HMG-CoA reductase and ACAT inhibitors act synergistically to lower plasma cholesterol and limit atherosclerotic lesion development in the cholesterol-fed rabbit

Given the beneficial effects of HMG-CoA reductase and ACAT inhibitors on hypercholesterolemia and atherosclerosis, we hypothesized that coadministration would improve the hypolipidemic response and not only limit lesion development but also alter the cellular composition of atherosclerotic lesions so as to induce a stable atherosclerotic lesion morphology. Plasma total cholesterol exposure was reduced 29 and 39% with atorvastatin (2.5 mg/kg) and CI-976 (5 mg/kg), respectively, and 60% upon coadministration due primarily to reductions in VLDL-cholesterol. Modest changes in liver cholesterol ester (CE) content were observed with atorvastatin or CI-976; however, a striking 48% reduction was noted upon coadministration. Liver HMG-CoA reductase mRNA levels were reduced 73% by cholesterol feeding and drug treatment did not prevent the reduction; however, atorvastatin alone and upon coadministration blunted the decrease in LDL receptor mRNA levels. The CE content of the iliac-femoral was unaffected by atorvastatin but was reduced 35% by CI-976 and 53% upon coadministration. Thoracic aortic CE content was reduced 38% by atorvastatin, 48% by CI-976 and 80% upon coadministration. Iliac-femoral lesion and macrophage area were reduced 48 and 67% by atorvastatin, respectively, and 68 and 81% by CI-976 but upon coadministration only an 85% reduction in macrophage area was noted. Aortic arch cross-sectional lesion and macrophage area were unaffected by atorvastatin, decreased 72-80% by CI-976 and reduced 87-92% upon coadministration. We conclude that inhibition of HMG-CoA reductase and ACAT acts synergistically to lower plasma total and lipoprotein cholesterol levels and to limit the development of atherosclerotic lesions in the cholesterol-fed rabbit by presumably regulating cholesterol trafficking pathways within liver and vascular cells.

Age-related changes in catalytic activity, enzyme mass, mRNA, and subcellular distribution of hepatic neutral cholesterol ester hydrolase in female rats

Activity and protein mass of hepatic neutral cholesteryl ester hydrolase (CEH) were measured in liver cytosol and washed microsomes of female Sprague-Dawley rats aged 3, 4, 7, 9, 13, and 16 wk. CEH mRNA was also measured. The microsomal component varied with age and contributed a greater fraction of total activity in females than previously reported in males. Nevertheless, the cytosolic component accounted for 62-80% of activity and 77-94% of immunoreactive protein in postmitochondrial fractions. Cytosolic and microsomal CEH specific activities, relative to total protein, decreased 94 and 83%, respectively, from 3 to 4 wk, prior to onset of puberty at 5 wk, and increased 360 and 137%, respectively, from 12 to 16 wk. These results contrast with an earlier study, in which cytosolic CEH activity of males increased with puberty and declined after 12 wk. Although cytosolic CEH was activated by protein kinase A and inhibited by alkaline phosphatase treatment at all ages, protein kinase activation peaked at 4 wk, coinciding with the initial decrease in specific activity. Specific activity in cytosol and microsomes correlated with CEH mass at all ages, suggesting that this CEH accounts for most variation in cellular activity. In contrast, CEH mRNA varied little from 3-16 wk, indicating that transcriptional regulation does not make a major contribution to the variation in CEH activity and mass in females, although it may make an important contribution to male-female differences in CEH expression. Specific activities of cytosolic and microsomal CEH, relative to immunoreactive CEH protein mass, exhibited changes consistent with posttranslational regulation. These results indicate gender-specific multivalent regulation of hepatic CEH by posttranslational mechanisms during development of female rats.

Age-related changes in mRNA, protein and catalytic activity of hepatic neutral cholesterol ester hydrolase in male rats: evidence for transcriptional regulation

Messenger RNA, protein mass and catalytic activity of hepatic neutral cholesteryl ester hydrolase (CEH) were measured in male Sprague-Dawley rats, aged 6, 8, 9.5, 12 and 24 weeks (wks). CEH mRNA increased 101% from 6 to 9.5 wks, corresponding to onset of puberty, and declined by 52% from 12 to 24 wks. CEH mass was highly correlated with mRNA levels at all ages, increasing 170% from 6 to 9.5 wks and declining 61% from 12 to 24 wks. CEH activity was highly correlated with mass and mRNA from 8-24 wks, but was greater at 6 wks than the activity predicted by the measured mass. In all age groups, activity was consistently increased by activation of endogenous protein kinase A and consistently inhibited by alkaline phosphatase, suggesting that age-related differences in catalytic activity were not due to differences in the level of enzyme phosphorylation. These data suggest transcriptional regulation and indicate an important role for CEH in cholesterol homeostasis in the developing rat.

Phospholipid-transfer proteins and their mRNAs in developing rat lung and in alveolar type-II cells

Gene expression of non-specific lipid-transfer protein (nsL-TP; identical with sterol carrier protein 2) and phosphatidylinositol-transfer protein (PI-TP) was investigated in developing rat lung. During the late prenatal period (between days 17 and 22) there is a 7-fold increase in the level of nsL-TP and a 2-fold rise in that of PI-TP. The prenatal increases in the levels of nsL-TP and PI-TP are accompanied by parallel increases in the levels of their mRNAs, indicating pretranslational regulation. Compared with whole lung, isolated alveolar type-II cells are enriched in nsL-TP and its mRNA, but not in PI-TP and its mRNA. The observation that the levels of nsL-TP and its mRNA in rat lung show a pronounced increase in the period of accelerated surfactant formation, together with the observation that the surfactant-producing type-II cells are enriched in nsL-TP and its mRNA, suggest that nsL-TP plays a role in the metabolism of pulmonary surfactant.

HMG-CoA reductase mRNA in the post-implantation rat embryo studied by in situ hybridization

3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase (EC 1.1.1.34) is the rate limiting step in the mevalonate pathway that produces isoprenoids and cholesterol. Inhibitors of HMG-CoA reductase are teratogenic in vivo and induce neural tube defects in rat embryo culture, effects which appear unrelated to cholesterol deficiency. This study is the first to localize HMG-CoA reductase mRNA by in situ hybridization (ISH). Expression of reductase mRNA was examined in post-implantation rat embryos, and for control purposes in rat liver and UT-1 cells, using a digoxigenin-11 (dig-11) labelled cRNA probe. Eighteen-day fetal liver showed heavy but patchy hybridization, and adult rat liver showed strong hybridization only on some periportal hepatocytes, which was absent in livers of fasted animals. UT-1 cells stimulated to overexpress HMG-CoA reductase mRNA were strongly positive with the same probe. Control hybridizations with sense strand RNA probe, or with cRNA probe on pre-RNased tissue were negative. Strong hybridization signal for HMG-CoA reductase mRNA was observed in all tissues of the post-implantation rat embryo, from egg cylinder to 30 somite stages (7 to 12 days). Heavy signal was noted in primitive ectoderm and neural tube. The wide embryonic and extraembryonic distribution and abundance of HMG-CoA reductase mRNA may reflect developmental requirements for products of the mevalonate pathway, e.g., isoprenoids for post-translational farnesylation of p21ras.

Quaking phenotype influences brain lipid-related mRNA levels

Although lipids compose almost 80% of myelin, the influence of quaking on mRNAs encoding lipid biosynthetic enzymes and transport proteins has not been previously reported. Understanding the influence of quaking on myelin-specific and lipid-related mRNAs will be useful in determining the mechanism of the quaking defect. Stearoyl CoA desaturase (SCD) catalyzes a key step in the biosynthesis of oleic acid (C18:1, n-9), a major fatty acid in myelin. SCD, LDL receptor (LDLR) and apolipoprotein E (Apo E) mRNA levels are all reduced in neonatal quaking brains. In contrast to brain, quaking hepatic LDLR and Apo E mRNA levels are normal. These results indicate that lipid-related mRNAs are reduced in neonatal quaking brain, but the quaking liver is unaffected. The quaking defect influences gene expression in multiple cell types of glial lineage in the developing CNS.

Functions of fatty acid binding proteins

Cytosolic fatty acid binding proteins (FABP) belong to a gene family of which eight members have been conclusively identified. These 14-15 kDa proteins are abundantly expressed in a highly tissue-specific manner. Although the functions of the cytosolic FABP are not clearly established, they appear to enhance the transfer of long-chain fatty acids between artificial and native lipid membranes, and also to have a stimulatory effect on a number of enzymes of fatty acid metabolism in vitro. These findings, as well as the tissue expression, ligand binding properties, ontogeny and regulation of these proteins provide a considerable body of indirect evidence supporting a broad role for the FABP in the intracellular transport and metabolism of long-chain fatty acids. The available data also support the existence of structure- and tissue-specific specialization of function among different members of the FABP gene family. Moreover, FABP may also have a possible role in the modulation of cell growth and proliferation, possibly by virtue of their affinity for ligands such as prostaglandins, leukotrienes and fatty acids, which are known to influence cell growth activity. FABP structurally unrelated to the cytosolic gene family have also been identified in the plasma membranes of several tissues (FABPpm). These proteins have not been fully characterized to date, but strong evidence suggest that they function in the transport of long-chain fatty acids across the plasma membrane.