Levels of mRNAs coding for lipogenic enzymes in rat lung upon fasting and refeeding and during perinatal development

Harlequin ichthyosis model mouse reveals alveolar collapse and severe fetal skin barrier defects

Harlequin ichthyosis (HI), which is the most severe genodermatosis, is caused by loss-of-function mutations in ABCA12, a member of the ATP-binding cassette transporter family. To investigate the pathomechanism of HI and the function of the ABCA12 protein, we generated ABCA12-deficient mice (Abca12(-/-)) by targeting Abca12. Abca12(-/-) mice closely reproduce the human HI phenotype, showing marked hyperkeratosis with eclabium and skin fissure. Lamellar granule abnormalities and defective ceramide distribution were remarkable in the epidermis. Skin permeability assay of Abca12(-/-) fetuses revealed severe skin barrier dysfunction after the initiation of keratinization. Surprisingly, the Abca12(-/-) mice also demonstrated lung alveolar collapse immediately after birth. Lamellar bodies in alveolar type II cells of the Abca12(-/-) mice lacked normal lamellar structures. The level of surfactant protein B, an essential component of alveolar surfactant, was reduced in the Abca12(-/-) mice. Fetal therapeutic trials with systemic administration of retinoid or dexamethasone, which are effective for HI and respiratory distress, respectively, to the pregnant mother mice neither improved the skin phenotype nor extended the survival period. Our HI model mice reproduce the human HI skin phenotype soon after the initiation of fetal skin keratinization and provide evidence that ABCA12 plays pivotal roles in lung and skin barrier functions.

ABCA3 as a Lipid Transporter in Pulmonary Surfactant Biogenesis

ABCA3 protein is expressed predominantly at the limiting membrane of the lamellar bodies in alveolar type II cells, and mutations in the ABCA3 gene cause lethal respiratory distress in newborn infants. To investigate the function of ABCA3 protein, we generated Abca3-deficient mice by targeting Abca3. Full-term Abca3(-/-) newborn pups died within an hour after birth because of acute respiratory failure. Ultrastructural analysis revealed abnormally dense lamellar body-like organelles and no normal lamellar bodies in Abca3(-/-) alveolar type II cells. TLC and electrospray ionization mass spectrometry analyses of lipids in the pulmonary interstitium showed that phosphatidylcholine and phosphatidylglycerol, which contain palmitic acid and are abundant in normal surfactant lipids, were dramatically decreased in Abca3(-/-) lung. These findings indicate that ABCA3 plays an essential role in pulmonary surfactant lipid metabolism and lamellar body biogenesis, probably by transporting these lipids as substrates.

Characterization and classification of ATP-binding cassette transporter ABCA3 mutants in fatal surfactant deficiency

The ATP-binding cassette transporter ABCA3 is expressed predominantly at the limiting membrane of the lamellar bodies in lung alveolar type II cells. Recent study has shown that mutation of the ABCA3 gene causes fatal surfactant deficiency in newborns. In this study, we investigated in HEK293 cells the intracellular localization and N-glycosylation of the ABCA3 mutants so far identified in fatal surfactant deficiency patients. Green fluorescent protein-tagged L101P, L982P, L1553P, Q1591P, and Ins1518fs/ter1519 mutant proteins remained localized in the endoplasmic reticulum, and processing of oligosaccharide was impaired, whereas wild-type and N568D, G1221S, and L1580P mutant ABCA3 proteins trafficked to the LAMP3-positive intracellular vesicle, accompanied by processing of oligosaccharide from high mannose type to complex type. Vanadate-induced nucleotide trapping and ATP-binding analyses showed that ATP hydrolysis activity was dramatically decreased in the N568D, G1221S, and L1580P mutants, accompanied by a moderate decrease in ATP binding in N568D and L1580P mutants but not in the G1221S mutant, compared with the wild-type ABCA3 protein. In addition, mutational analyses of the Gly-1221 residue in the 11th transmembrane segment and the Leu-1580 residue in the cytoplasmic tail, and homology modeling of nucleotide binding domain 2 demonstrate the significance of these residues for ATP hydrolysis and suggest a mechanism for impaired ATP hydrolysis in G1221S and L1580P mutants. Thus, surfactant deficiency because of ABCA3 gene mutation may be classified into two categories as follows: abnormal intracellular localization (type I) and normal intracellular localization with decreased ATP binding and/or ATP hydrolysis of the ABCA3 protein (type II). These distinct pathophysiologies may reflect both the severity and effective therapy for surfactant deficiency.

Structure and regulation of acetyl-CoA carboxylase genes of metazoa

Acetyl-CoA carboxylase (ACC) plays a fundamental role in fatty acid metabolism. The reaction product, malonyl-CoA, is both an intermediate in the de novo synthesis of long-chain fatty acids and also a substrate for distinct fatty acyl-CoA elongation enzymes. In metazoans, which have evolved energy storage tissues to fuel locomotion and to survive periods of starvation, energy charge sensing at the level of the individual cell plays a role in fuel selection and metabolic orchestration between tissues. In mammals, and probably other metazoans, ACC forms a component of an energy sensor with malonyl-CoA, acting as a signal to reciprocally control the mitochondrial transport step of long-chain fatty acid oxidation through the inhibition of carnitine palmitoyltransferase I (CPT I). To reflect this pivotal role in cell function, ACC is subject to complex regulation. Higher metazoan evolution is associated with the duplication of an ancestral ACC gene, and with organismal complexity, there is an increasing diversity of transcripts from the ACC paraloges with the potential for the existence of several isozymes. This review focuses on the structure of ACC genes and the putative individual roles of their gene products in fatty acid metabolism, taking an evolutionary viewpoint provided by data in genome databases.

Lipogenesis in fetal rat lung: importance of C/EBPalpha, SREBP-1c, and stearoyl-CoA desaturase

Alveolar type II cells increase lipogenesis and convert glycogen into the phospholipids of surfactant in the late term fetal lung. Recent studies suggest that CCAAT/enhancing-binding protein (C/EBP) isoforms and sterol regulatory element binding protein (SREBP)-1c regulate fatty acid synthesis in adult type II cells in vitro. To define the temporal relationships and enzymes involved in lipogenesis in fetal rat lung, the mRNA levels of selected transcription factors and enzymes were determined. There was an increase in the mRNA levels of C/EBPalpha, C/EBPbeta, C/EBPdelta, peroxisomal proliferator-activated receptor gamma (PPARgamma), and SREBP-1c, but not SREBP-1a or SREBP-2 from fetal Days 19-21. There was also an increase in the mRNA levels of fatty acid synthase, stearoyl-CoA desaturase 1 (SCD-1), fatty acid translocase, glycerol-3-P acyl transferase, and phosphatidate cytidylyltransferase. By in situ hybridization, there was detectible expression of fatty acid synthase, SCD-1, and C/EBPalpha along the alveolar septae with the same distribution pattern as surfactant protein-C, whereas PPARgamma expression appeared to be restricted to macrophages. Regulation of lipogenesis at the mRNA level is predominately on enzymes of fatty acid synthesis and appears to be regulated by C/EBPalpha and SREBP-1c. SCD-1 and phosphatidate cytidylyltransferase are important components of the lipogenic response in the fetal lung that have not been recognized previously.

Lipid deprivation increases surfactant phosphatidylcholine synthesis via a sterol-sensitive regulatory element within the CTP:phosphocholine cytidylyltransferase promoter

Lipid-deprived mice increase alveolar surfactant disaturated phosphatidylcholine (DSPtdCho) synthesis compared with mice fed a standard diet by increasing expression of CTP:phosphocholine cytidylyltransferase (CCT), the rate-limiting enzyme for DSPtdCho synthesis. We previously observed that lipid deprivation increases mRNA synthesis for CCT [Ryan, McCoy, Mathur, Field and Mallampalli (2000) J. Lipid Res. 41, 1268-1277]. To evaluate regulatory mechanisms for this gene, we cloned the proximal approximately 1900 bp of the 5' flanking sequence of the murine CCT gene, coupled this to a luciferase reporter, and examined transcriptional regulation in a murine alveolar epithelial type II cell line (MLE-12). The core promoter was localized to a region between -169 and +71 bp, which exhibited strong basal activity comparable with the simian virus 40 promoter. The full-length construct, from -1867 to +71, was induced 2-3-fold when cells were cultured in lipoprotein-deficient serum (LPDS), similar to the level of induction of the endogenous CCT gene. By deletional analysis the sterol regulatory element (SRE) was localized within a 240 bp region. LPDS activation of the CCT promoter was abolished by mutation of this SRE, and gel mobility-shift assays demonstrated specific binding of recombinant SRE-binding protein to this element within the CCT promoter. These observations indicate that sterol-regulated expression of CCT is mediated by an SRE within its 5' flanking region.

Pre-translational regulation by glucocorticoid of fatty acid and phosphatidylcholine synthesis in type II cells from fetal rat lung

Exposure to fibroblast-conditioned cortisol-containing medium increased fatty acid synthase activity and fatty acid synthase, acetyl-CoA carboxylase and ATP citrate lyase mRNA abundance in fetal type II alveolar epithelial cells. Both fibroblast conditioning and cortisol in the medium were required for maximal effect on the mRNA levels, indicating involvement of mesenchymal-epithelial interaction in the cortisol effects. The observed effects provide evidence for an earlier hypothesis that increased activity of CTP:phosphocholine cytidylyltransferase in lung tissue caused by glucocorticoid is due to increased fatty acid synthesis. However, evidence suggesting pre-translational regulation of this enzyme by glucocorticoid was also found.

Genomic distribution of three promoters of the bovine gene encoding acetyl-CoA carboxylase alpha and evidence that the nutritionally regulated promoter I contains a repressive element different from that in rat

The enzyme acetyl-CoA carboxylase alpha (ACC-alpha) is rate-limiting for the synthesis of long-chain fatty acids de novo. As a first characterization of the bovine gene encoding this enzyme, we established the entire bovine ACC-alpha cDNA sequence (7041 bp) and used experiments with 5' rapid amplification of cDNA ends to determine the heterogeneous composition of 5' untranslated regions, as expressed from three different promoters (PI, PII and PIII). The individual locations of these promoters have been defined within an area comprising 35 kbp on Bos taurus chromosome 19 ('BTA19'), together with the segmentation of the first 14 exons. Primer extension analyses reveal that the nutritionally regulated PI initiates transcription from at least four sites. PI transcripts are much more abundant in adipose and mammary-gland tissues than in liver or lung. A 2.6 kb promoter fragment drives the expression of reporter genes only weakly in different model cells, irrespective of stimulation with insulin or dexamethasone. Thus bovine PI is basically repressed, like its analogue from rat. Finely graded deletions of PI map two separate elements, which have to be present together in cis to repress bovine PI. The distal component resides within a well-preserved Art2 retroposon element. Thus sequence, structure and evolutionary origin of the main repressor of PI in bovines are entirely different from its functional counterpart in rat, which had been identified as a (CA)(28) microsatellite. We show that, in different mammalian species, unrelated genome segments of different origins have been recruited to express as functionally homologous PI the ancient and otherwise highly conserved ACC-alpha-encoding gene.

Transcriptional regulation of the lung fatty acid synthase gene by glucocorticoid, thyroid hormone and transforming growth factor-beta 1

Fatty acid synthase (FAS) is a key enzyme in the biosynthesis of lung surfactant. FAS expression in fetal lungs is increased by glucocorticoids and this effect is largely due to increased transcription. The stimulatory effect of glucocorticoid on FAS expression is antagonized by thyroid hormone and transforming growth factor-beta 1 (TGF-beta 1). To determine the glucocorticoid responsive regions of the FAS gene we employed deletion analysis and reporter gene assays. A549 cells were transfected with various FAS gene constructs ligated to the firefly luciferase gene and cultured with dexamethasone (Dex) for 24 h after which luciferase activity was measured. Dex increased luciferase expression in response to a fragment in the promoter and 5'-flanking region of the FAS gene, from -1592 to +65 bp. This increase was antagonized by triiodothyronine (T(3)) and TGF-beta 1. Serial deletions showed that the full response to Dex and T(3) were retained in the 89 bp -33/+56 bp fragment whereas the response to TGF was mediated by the immediately upstream -104/-34 bp sequence. The Dex responsive region of the FAS gene could not be separated from the minimal promoter showing that they are intimately associated. The extents of Dex stimulation and antagonism by T(3) and TGF in A549 cells were similar to those noted on parameters of FAS expression in fetal lung explants. These data show that the effects of Dex, T(3) and TGF on FAS expression are mediated by DNA sequences in the promoter region of the gene.

Regulation of mammalian acetyl-coenzyme A carboxylase

Long-chain fatty acids are involved in all aspects of cellular structure and function. For controlling amounts of fatty acids, cells are endowed with two acetyl-coenzyme A carboxylase (ACC) systems. ACC-alpha is the rate-limiting enzyme in the biogenesis of long-chain fatty acids, and ACC-beta is believed to control mitochondrial fatty acid oxidation. These two isoforms of ACC control the amount of fatty acids in the cells. Phosphorylation and dephosphorylation of ACC-alpha cause enzyme inactivation and activation, respectively, and serve as the enzyme's short-term regulatory mechanism. Covalently modified enzymes become more sensitive toward cellular metabolites. In addition, many hormones and nutrients affect gene expression. The gene products formed are heterogeneous and tissue specific. The ACC-beta gene is located on human chromosome 12; the cDNA for this gene has just been cloned. The gene for the alpha-isoform is located on human chromosome 17. The catalytic core of the beta-isoform is homologous to that of the alpha-isoform, except for an additional peptide of about 150 amino acids at the N terminus. This extra peptide sequence makes the beta-form about 10,000 daltons larger, and it is thought to be involved in the unique role that has been assigned to this enzyme. The detailed control mechanisms for the beta-isoform are not known.

Glucocorticoid and cAMP increase fatty acid synthetase mRNA in human fetal lung explants

During late fetal development, synthesis of surfactant phospholipid requires a large supply of fatty acid precursor. Fatty acid synthetase is a regulatory enzyme for de novo fatty acid synthesis in lung as well as other lipogenic tissues. In this study, we report hormonal induction of FAS mRNA in human fetal lung explants (16-23 week gestation) cultured up to 7 days in Waymouth's medium (no serum) supplemented with dexamethasone (Dex, 10 nM) or agents that increase cAMP (8-Br-cAMP, 0.1 mM; isobutylmethylxanthine, 0.1 mM; forskolin, 0.01 mM; PGE1, 0.01 mM). Exposure of explants to Dex or cAMP agents increased FAS mRNA content by 6 h and maximal stimulation occurred at 72 h for Dex (approx. 3-fold increase) and 24 h for cAMP (approx. 2-fold increase). In the presence of both Dex and cAMP there was a synergistic increase in FAS mRNA content at all times (approx. 11-fold increase at 72 h). Induction of FAS mRNA was specific for steroids with glucocorticoid activity, reversible on removal of hormone, and was half-maximal at 2-3 nM Dex consistent with receptor mediation. Actinomycin D blocked induction by Dex but not by cAMP suggesting a transcriptional effect by glucocorticoid but not by cAMP. T3, which increases phosphatidylcholine synthesis, did not induce FAS mRNA. The findings indicate that both glucocorticoid and cAMP increase FAS gene expression consistent with an important role for FAS in regulating the supply of fatty acid for surfactant phospholipid synthesis.

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.

Pre-translational regulation of lipid synthesizing enzymes and surfactant proteins in fetal rat lung in explant culture

In hormone-free explant cultures of 18-day fetal rat lung the levels of the mRNAs for fatty acid synthase, ATP citrate lyase and surfactant proteins A, B, and C, increased as they do in vivo. Also CTP:phosphocholine cytidylyltransferase mRNA increased spontaneously. Unlike in vivo, malic enzyme mRNA increased drastically in cultured explants. Culture with dexamethasone increased the abundance of fatty acid synthase and surfactant protein mRNAs, but considerably depressed that of malic enzyme mRNA. Dexamethasone had little effect on CTP:phosphocholine cytidylyltransferase mRNA, supporting the concept that the increased activity of this enzyme caused by glucocorticoid is due to increased fatty acid synthesis.

Evidence that CTP:choline-phosphate cytidylyltransferase is regulated at a pretranslational level in rat liver after partial hepatectomy

Regulation of CTP:choline-phosphate cytidylyltransferase activity was studied in regenerating rat liver. The formation of phosphatidylcholine from [14C]choline in hepatocytes isolated from regenerating liver at 22 h after surgery was increased 1.9-fold when compared with hepatocytes from sham-operated animals. This effect was accompanied by a 1.4-fold increase in cytosolic cytidylyltransferase activity as well as by a 1.5-fold increase in the amount of immunoreactive cytidylyltransferase protein, and a 1.7-fold increase in [35S]methionine incorporation into cytidylyltransferase protein. Northern blot analysis of cytidylyltransferase mRNA showed two signals at 1.5 and 5.0 kb. Partial hepatectomy caused a significant 2-3-fold increase in the 1.5-kb and 5.0-kb messengers at 12 h after surgery. During the next 10 h after partial hepatectomy cytidylyltransferase mRNA levels slightly decreased, although they were still elevated in comparison with sham-operated rats 20-22 h after surgery. In contrast to the elevated cytidylyltransferase mRNA levels, the amount of acetyl-CoA carboxylase mRNA did not increase between 12 and 22 h after surgery, which is in line with the unchanged activity of this enzyme. In conclusion, our data demonstrate that in regenerating liver phosphatidylcholine biosynthesis and cytidylyltransferase activity are regulated at a pretranslational level.

Regulation of malic enzyme gene expression by nutrients, hormones, and growth factors in fetal hepatocyte primary cultures

The culture of fetal hepatocytes for 64 h in medium supplemented with 5 mM glucose, T3, insulin, and dexamethasone resulted in the coordinate precocious expression of malic enzyme mRNA, protein, and specific activity. T3 was the main inducer; meanwhile, insulin exerted a small synergistic effect when added with T3. Dexamethasone had a potentiation effect on the T3 response of malic enzyme mRNA expression regardless of the presence of insulin. This effect of dexamethasone on T3 response of malic enzyme mRNA expression was time (64 h) and glucose dependent. Glucagon, and to a greater degree dibutyryl-cAMP, repressed malic enzyme mRNA as well as protein expression by T3 and dexamethasone, in the absence of insulin. Glucose and other carbon sources such as lactate-pyruvate or dihydroxyacetone induced the abundance of malic enzyme mRNA in the absence of hormones. Insulin and T3 produced a high accumulation of malic enzyme mRNA in lactate-pyruvate medium, this effect being decreased by dexamethasone. EGF suppressed the induction produced by T3 and dexamethasone on malic enzyme mRNA, while the expression of beta-actin mRNA remained essentially unmodified.

Effects of fasting, refeeding, and fasting with T3 administration on Na-K,ATPase in rat skeletal muscle

It is known that Na-K,adenosine triphosphatase (ATPase) in cell membranes represents an important consumer of cellular energy, eg, adenosine triphosphate (ATP), and that the concentration and activity of this enzyme change in a dose-dependent manner with serum thyroid hormone levels. To examine the hypothesis that low triiodothyronine (T3) syndrome represents a cellular adaptation in generalized severe illnesses that saves tissue energy expenditure, we measured the muscle Na-K,ATPase concentration and its activity in rats that led to low T3 syndrome induced by fasting. The Na-K,ATPase concentration was measured by 3H-ouabain binding to soleus muscle, and its activity was measured by 42K uptake in the contralateral soleus muscle. The effects of refeeding or T3 administration on Na-K,ATPase in soleus muscle in fasted rats were also examined. Na-K,ATPase concentration and activity were both increased in hyperthyroid rats and decreased in hypothyroid rats. In the fasting state, they were decreased to as low as the levels seen in hypothyroidism. Furthermore, with fasting + refeeding or fasting + T3 administration, Na-K,ATPase in soleus muscle returned to the normal level. These results suggest that tissue energy expenditure, as assessed by Na-K,ATPase, in skeletal muscles of fasted rats with low T3 syndrome is actually decreased to levels seen in hypothyroidism, due at least partly to the decrease in serum T3 concentrations, and that there exist some adaptation mechanisms in the peripheral tissues for the accommodation of energy metabolism in the body through decreased thyroxine (T4) to T3 conversion.

Acetyl-coenzyme A carboxylase mRNA metabolism in the rat liver

The acetyl-coenzyme A carboxylase (ACC) gene contains two promoters (PI and PII), both of which are active in the liver. Various physiological stimuli affect one, or both of the promoters of the ACC gene, and result in the generation of two classes of ACC mRNAs which differ in the composition of their 5' untranslated regions (5' UTR). We have analyzed the amounts of the two major mRNAs species that are generated from each of these promoters in order to examine the regulation of ACC gene activity in the liver under different physiological conditions. Our findings can be summarized as follows: (1) In liver from normal animals, fed a complete laboratory chow ad libitum, the level of class 2 ACC mRNA species generated by PII is very low. These mRNA species disappear on starvation. Refeeding starved animals with a fat-free diet stimulates both PI and PII with different time courses of induction: PII responds quickly and PII gene products accumulate to maximum levels within 18 hours, while the PI response, as measured by the accumulation of class 1 mRNAs, shows a lag period of 6 hours before reaching maximal levels at the end of a 24-hour refeeding period. The half-lives estimated from the induction kinetics were 4.4 hours for class 2 mRNAs and 11.8 hours for class 1 mRNAs. Reinstatement of starvation causes an almost instantaneous disappearance of class 1 mRNA species, as compared with class 2 mRNA species. This rapid decay of PI transcripts suggests that factors stabilizing this class of ACC mRNAs contribute to the steady-state levels reached after the dietary induction.(ABSTRACT TRUNCATED AT 250 WORDS)