Aberrant catalytic cycle and impaired lipid transport into intracellular vesicles in ABCA3 mutants associated with nonfatal pediatric interstitial lung disease

The ATP-binding cassette transporter ABCA3 mediates uptake of choline-phospholipids into intracellular vesicles and is essential for surfactant metabolism in lung alveolar type II cells. We have shown previously that ABCA3 mutations in fatal surfactant deficiency impair intracellular localization or ATP hydrolysis of ABCA3 protein. However, the mechanisms underlying the less severe phenotype of patients with ABCA3 mutation are unclear. In this study, we characterized ABCA3 mutant proteins identified in pediatric interstitial lung disease (pILD). E292V (intracellular loop 1), E690K (adjacent to Walker B motif in nucleotide binding domain 1), and T1114M (8th putative transmembrane segment) mutant proteins are localized mainly in intracellular vesicle membranes as wild-type protein. Lipid analysis and sucrose gradient fractionation revealed that the transport function of E292V mutant protein is moderately preserved, whereas those of E690K and T1114M mutant proteins are severely impaired. Vanadate-induced nucleotide trapping and photoaffinity labeling of wild-type and mutant proteins using 8-azido-[(32)P]ATP revealed an aberrant catalytic cycle in these mutant proteins. These results demonstrate the importance of a functional catalytic cycle in lipid transport of ABCA3 and suggest a pathophysiological mechanism of pILD due to ABCA3 mutation.     

ABCA3-mediated choline-phospholipids uptake into intracellular vesicles in A549 cells

ABCA3 is proposed to function as a lung surfactant lipid transporter. Here we report ABCA3-dependent lipid uptake into intracellular vesicles in lung adenocarcinoma A549 cells. A549 cells stably expressing GFP-tagged wild-type ABCA3 (A549/ABCA3(WT)) had larger LAMP3-positive vesicles than their parental cells as well as A549 cells expressing a Walker A motif mutant (A549/ABCA3(N568D)). The choline-phospholipids level in A549/ABCA3(WT) was increased 1.25-fold compared to that in A549 and A549/ABCA3(N568D) cells, while the cholesterol levels were similar. Sucrose gradient fractionation analysis in A549/ABCA3(WT) cells revealed that choline-phospholipids were enriched in low-density and nile red-positive vesicles. Electronmicroscopic analysis showed multilamellar vesicles in A549/ABCA3(WT) cells. These results indicate that ABCA3 mediates ATP-dependent choline-phospholipids uptake into intracellular vesicles.     

Some ABCA3 mutations elevate ER stress and initiate apoptosis of lung epithelial cells

Background

ABCA3 transporter (ATP-binding cassette transporter of the A subfamily) is localized to the limiting membrane of lamellar bodies, organelles for assembly and storage of pulmonary surfactant in alveolar epithelial type II cells (AECII). It transports surfactant phospholipids into lamellar bodies and absence of ABCA3 function disrupts lamellar body biogenesis. Mutations of the ABCA3 gene lead to fatal neonatal surfactant deficiency and chronic interstitial lung disease (ILD) of children. ABCA3 mutations can result in either functional defects of the correctly localized ABCA3 or trafficking/folding defects where mutated ABCA3 remains in the endoplasmic reticulum (ER).

Methods

Human alveolar epithelial A549 cells were transfected with vectors expressing wild-type ABCA3 or one of the three ABCA3 mutant forms, R43L, R280C and L101P, C-terminally tagged with YFP or hemagglutinin-tag. Localization/trafficking properties were analyzed by immunofluorescence and ABCA3 deglycosylation. Uptake of fluorescent NBD-labeled lipids into lamellar bodies was used as a functional assay. ER stress and apoptotic signaling were examined through RT-PCR based analyses of XBP1 splicing, immunoblotting or FACS analyses of stress/apoptosis proteins, Annexin V surface staining and determination of the intracellular glutathion level.

Results

We demonstrate that two ABCA3 mutations, which affect ABCA3 protein trafficking/folding and lead to partial (R280C) or complete (L101P) retention of ABCA3 in the ER compartment, can elevate ER stress and susceptibility to it and induce apoptotic markers in the cultured lung epithelial A549 cells. R43L mutation, resulting in a functional defect of the properly localized ABCA3, had no effect on intracellular stress and apoptotic signaling.

Conclusion

Our data suggest that expression of partially or completely ER localized ABCA3 mutant proteins can increase the apoptotic cell death of the affected cells, which are factors that might contribute to the pathogenesis of genetic ILD.     

On the mechanism for efficient repression of the interleukin-6 promoter by glucocorticoids: enhancer, TATA box, and RNA start site (Inr motif) occlusion.

The feedback inhibition of interleukin-6 (IL-6) gene expression by glucocorticoids represents a regulatory link between the endocrine and immune systems. The mechanism of the efficient repression of the IL-6 promoter by dexamethasone (Dex) was investigated in HeLa cells transiently transfected with plasmid constructs containing different IL-6 promoter elements linked to the herpesvirus thymidine kinase gene (tk) promoter and the bacterial chloramphenicol acetyltransferase gene (cat) and cotransfected with cDNA vectors constitutively expressing either the active wild-type or inactive mutant human glucocorticoid receptor (GR). The induction by interleukin-1, tumor necrosis factor, phorbol ester, or forskolin of IL-6-tk-cat chimeric constructs containing a single copy of the IL-6 DNA segment from -173 to -151 (MRE I) or from -158 to -145 (MRE II), which derive from within the multiple cytokine- and second-messenger-responsive enhancer (MRE) region, was strongly repressed by Dex in a wild-type GR-dependent fashion irrespective of the inducer used. The induction by pseudorabies virus of an IL-6 construct containing the IL-6 TATA box and the RNA start site ("initiator" or Inr element) but not the MRE region was also repressed by Dex in the presence of wild-type GR. DNase I footprinting showed that the purified DNA-binding fragment of GR bound across the MRE, the TATA box, and the Inr site in the IL-6 promoter; this footprint overlapped that produced by proteins present in nuclear extracts from uninduced or induced HeLa cells. Imperfect palindromic nucleotide sequence motifs moderately related to the consensus GR-responsive element (GRE) motif were present at the Inr, the TATA box, and the MRE II site in the IL-6 promoter; although MRE I and a GR-binding site between -201 and -210 in IL-6 both lacked a discernible inverted repeat motif, their sequences showed considerable similarity with negative GRE sequences in other Dex-repressed genes. Surprisingly, chimeric genes containing MRE II, which lacks a recognizable GACGTCA cyclic AMP- and phorbol ester-responsive motif, were strongly induced by both phorbol ester and forskolin, suggesting that MRE II (ACATTGCACAATCT) may be the prototype of a novel cyclic AMP- and phorbol ester-responsive element. Taken together, these observations suggest that ligand-activated GR represses the IL-6 gene by occlusion not only of the inducible IL-6 MRE enhancer region but also of the basal IL-6 promoter elements.     

Functional and trafficking defects in ATP binding cassette A3 mutants associated with respiratory distress syndrome

Members of the ATP binding cassette (ABC) protein superfamily actively transport a wide range of substrates across cell and intracellular membranes. Mutations in ABCA3, a member of the ABCA subfamily with unknown function, lead to fatal respiratory distress syndrome (RDS) in the newborn. Using cultured human lung cells, we found that recombinant wild-type hABCA3 localized to membranes of both lysosomes and lamellar bodies, which are the intracellular storage organelles for surfactant. In contrast, hABCA3 with mutations linked to RDS failed to target to lysosomes and remained in the endoplasmic reticulum as unprocessed forms. Treatment of those cells with the chemical chaperone sodium 4-phenylbutyrate could partially restore trafficking of mutant ABCA3 to lamellar body-like structures. Expression of recombinant ABCA3 in non-lung human embryonic kidney 293 cells induced formation of lamellar body-like vesicles that contained lipids. Small interfering RNA knockdown of endogenous hABCA3 in differentiating human fetal lung alveolar type II cells resulted in abnormal, lamellar bodies comparable with those observed in vivo with mutant ABCA3. Silencing of ABCA3 expression also reduced vesicular uptake of surfactant lipids phosphatidylcholine, sphingomyelin, and cholesterol but not phosphatidylethanolamine. We conclude that ABCA3 is required for lysosomal loading of phosphatidylcholine and conversion of lysosomes to lamellar body-like structures.     

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.     

ABCA3 is a lamellar body membrane protein in human lung alveolar type II cells.

The ABCA3 gene, of the ABCA subclass of ATP-binding cassette (ABC) transporters, is expressed exclusively in lung. We report here the cloning, molecular characterization, and distribution of human ABCA3 in the lung. Immunoblot analysis using the specific antibody reveals a 150-kDa protein in the crude membrane fraction of human lung. Immunohistochemical analyses of alveoli show that ABCA3 is expressed only in the type II cells expressing surfactant protein A. At the ultrastructural level, ABCA3 immunoreactivity was detected mostly at the limiting membrane of the lamellar bodies. Since members of the ABCA transporter family are known to be involved in transmembrane transport of endogenous lipids, our findings suggest that ABCA3 plays an important role in the formation of pulmonary surfactant in type II cells.     

Human ABCA3, a product of a responsible gene for abca3 for fatal surfactant deficiency in newborns, exhibits unique ATP hydrolysis activity and generates intracellular multilamellar vesicles

ABCA3 is highly expressed at the membrane of lamellar bodies in alveolar type II cells, in which pulmonary surfactant is stored. ABCA3 gene mutations cause fatal surfactant deficiency in newborns. We established HEK293 cells stably expressing human ABCA3 and analyzed the function. Exogenously expressed ABCA3 is glycosylated and localized at the intracellular vesicle membrane. ABCA3 is efficiently photoaffinity labeled by 8-azido-[alpha(32)P]ATP, but not by 8-azido-[gamma(32)P]ATP, when the membrane fraction is incubated in the presence of orthovanadate. Photoaffinity labeling of ABCA3 shows unique metal ion-dependence and is largely reduced by membrane pretreatment with 5% methyl-beta-cyclodextrin, which depletes cholesterol. Electron micrographs show that HEK293/hABCA3 cells contain multivesicular, lamellar body-like structures, which do not exist in HEK293 host cells. Some fuzzy components such as lipids accumulate in the vesicles. These results suggest that ABCA3 shows ATPase activity, which is induced by lipids, and may be involved in the biogenesis of lamellar body-like structures.     

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.