Synthesis of phosphatidylcholine and phosphatidylglycerol by alveolar type II cells in primary culture

Codonopsis pilosula-derived glycopeptide dCP1 promotes the polarization of tumor-associated macrophage from M2-like to M1 phenotype

The majority of the immune cell population in the tumor microenvironment (TME) consists of tumor-associated macrophages (TAM), which are the main players in coordinating tumor-associated inflammation. TAM has a high plasticity and is divided into two main phenotypes, pro-inflammatory M1 type and anti-inflammatory M2 type, with tumor-suppressive and tumor-promoting functions, respectively. Considering the beneficial effects of M1 macrophages for anti-tumor and the high plasticity of macrophages, the conversion of M2 TAM to M1 TAM is feasible and positive for tumor treatment. This study sought to evaluate whether the glycopeptide derived from simulated digested Codonopsis pilosula extracts could regulate the polarization of M2-like TAM toward the M1 phenotype and the potential regulatory mechanisms. The results showed that after glycopeptide dCP1 treatment, the mRNA relative expression levels of some M2 phenotype marker genes in M2-like TAM in simulated TME were reduced, and the relative expression levels of M1 phenotype marker genes and inflammatory factor genes were increased. Analysis of RNA-Seq of M2-like TAM after glycopeptide dCP1 intervention showed that the gene sets such as glycolysis, which is associated with macrophage polarization in the M1 phenotype, were significantly up-regulated, whereas those of gene sets such as IL-6-JAK-STAT3 pathway, which is associated with polarization in the M2 phenotype, were significantly down-regulated. Moreover, PCA analysis and Pearson's correlation also indicated that M2-like TAM polarized toward the M1 phenotype at the transcriptional level after treatment with the glycopeptide dCP1. Lipid metabolomics was used to further explore the efficacy of the glycopeptide dCP1 in regulating the polarization of M2-like TAM to the M1 phenotype. It was found that the lipid metabolite profiles in dCP1-treated M2-like TAM showed M1 phenotype macrophage lipid metabolism profiles compared with blank M2-like TAM. Analysis of the key differential lipid metabolites revealed that the interconversion between phosphatidylcholine (PC) and diacylglycerol (DG) metabolites may be the central reaction of the glycopeptide dCP1 in regulating the conversion of M2-like TAM to the M1 phenotype. The above results suggest that the glycopeptide dCP1 has the efficacy to regulate the polarization of M2-like TAM to M1 phenotype in simulated TME.

Human cell receptors: potential drug targets to combat COVID-19

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causes the coronavirus disease 2019 (COVID-19). The World Health Organization (WHO) has announced that COVID-19 is a pandemic having a higher spread rate rather than the mortality. Identification of a potential approach or therapy against COVID-19 is still under consideration. Therefore, it is essential to have an insight into SARS-CoV-2, its interacting partner, and domains for an effective treatment. The present study is divided into three main categories, including SARS-CoV-2 prominent receptor and its expression levels, other interacting partners, and their binding domains. The first section focuses primarily on coronaviruses' general aspects (SARS-CoV-2, SARS-CoV, and the Middle East Respiratory Syndrome Coronaviruses (MERS-CoV)) their structures, similarities, and mode of infections. The second section discusses the host receptors which includes the human targets of coronaviruses like dipeptidyl peptidase 4 (DPP4), CD147, CD209L, Angiotensin-Converting Enzyme 2 (ACE2), and other miscellaneous targets (type-II transmembrane serine proteases (TTSPs), furin, trypsin, cathepsins, thermolysin, elastase, phosphatidylinositol 3-phosphate 5-kinase, two-pore segment channel, and epithelium sodium channel C-α subunit). The human cell receptor, ACE2 plays an essential role in the Renin-Angiotensin system (RAS) pathway and COVID-19. Thus, this section also discusses the ACE2 expression and risk of COVID-19 infectivity in various organs and tissues such as the liver, lungs, intestine, heart, and reproductive system in the human body. Absence of ACE2 protein expression in immune cells could be used for limiting the SARS-CoV-2 infection. The third section covers the current available approaches for COVID-19 treatment. Overall, this review focuses on the critical role of human cell receptors involved in coronavirus pathogenesis, which would likely be used in designing target-specific drugs to combat COVID-19.

Bleomycin induced apical-basal polarity loss in alveolar epithelial cell contributes to experimental pulmonary fibrosis

Idiopathic pulmonary fibrosis (IPF) is a fatal fibrosing interstitial lung disease with limited therapeutic options and a median survival of 3 years after diagnosis. Dysregulated epithelial regeneration is key event involved in initiating and sustaining IPF. The type II alveolar epithelial cells (AECIIs) play a crucial role for epithelial regeneration and stabilisation of alveoli. Loss of cell apical-basal polarity contributes to fibrosis. AECII has apical-basal polarity, but it is poorly understood whether AECII apical-basal polarity loss is involved in fibrosis. Bleomycin is a traditional inducer of pulmonary fibrosis. Here firstly we observed that bleomycin induced apical-basal polarity loss in cultured AECIIs. Next, cell polarity proteins lethal (2) giant larvae 1 (Lgl1), PAR-3A, aPKC and PAR-6B were investigated. We found bleomycin induced increases of Lgl1 protein and decreases of PAR-3A protein, and bleomycin-induced PAR-3A depression was mediated by increased-Lgl1. Then Lgl1 siRNA was transfected into AECIIs. Lgl1 siRNA prevented apical-basal polarity loss in bleomycin-treated AECIIs. At last, Lgl1-conditional knockout mice were applied in making animal models. Bleomycin induced pulmonary fibrosis, but this was attenuated in Lgl1-conditional knockout mice. Together, these data indicated that bleomycin mediated AECII apical-basal polarity loss which contributed to experimental pulmonary fibrosis. Inhibition of Lgl1 should be a potential therapeutic strategy for the disease.

Vitamin D attenuates lung injury via stimulating epithelial repair, reducing epithelial cell apoptosis and inhibits TGF-β induced epithelial to mesenchymal transition

Vitamin D regulates cell proliferation, inhibits cytokines release at sites of inflammation and reduces inflammatory responses. In this study, the aim was to investigate whether exogenous vitamin D attenuates LPS-induced lung injury via modulating epithelial cell proliferation, migration, apoptosis and epithelial mesenchymal transition (EMT). Murine and in vitro primary type II alveolar epithelial cell work were included in this study. In vivo, mice were mildly vitamin D deficient, 0.1, 1.5, 10 mg/kg 1,25(OH)₂-vitamin D₃ or 25(OH)-vitamin D₃ was administrated by means of an intra-gastric injection for 14 days pre-intra-tracheal (IT) LPS, which remarkedly promoted alveolar epithelial type II cells proliferation, inhibited ATII cells apoptosis and inhibited EMT, with the outcome of attenuated LPS-induced lung injury. In vitro, vitamin D stimulated epithelial cell scratch wound repair, reduced primary ATII cells apoptosis as well. Vitamin D promoted primary human ATII cells proliferation through the PI3K/AKT signaling pathway and activation of vitamin D receptor (VDR). Moreover, vitamin D inhibited EMT in response to TGF-β, which was vitamin D receptor dependent. In conclusion, vitamin D attenuates lung injury via stimulating ATII cells proliferation and migration, reducing epithelial cell apoptosis and inhibits TGF-β induced EMT. Together, these results suggest that vitamin D has therapeutic potential for the resolution of ARDS.

Synthesis of Phospholipids by Human Peritoneal Mesothelial Cells

Objective

To assess the capacity of cultured human peritoneal mesothelial cells to synthesize choline-containing phospholipids. The study compares the phospholipids secreted from cultured cells with those which we, and others, have identified in the dialysate of patients treated by continuous ambulatory peritoneal dialysis (CAPD).

Patients

CAPD effluent was collected from 8 patients who had been receiving CAPD treatment for at least 11 months and who had normal ultrafiltration.

Cell Cultures

Using human omental tissue, homogeneous cultures of mesothelial cells were established.

Methods

Synthesis of phospholipids by mesothelial cells was assessed following incubation with [methyl14C] choline chloride-a precursor capable of being in corporated into phosphatidylcholine (PtdCho) and sphingomyelin. Lipids from CAPD effluent, cultured cells, and cell medium were extracted in chloroform/methanol. Phospholipids were separated and identified by thin layer chromatography. Synthesis and secretion of PtdCho and other choline-containing lipids by the mesothelial cells were determined by β scintillation counting of the appropriate bands, while the fatty acid composition of the phospholipids was ascertained by gas liquid chromatography.

Results

Synthesis and secretion of PtdCho by mesothelial cells were observed during a 96-hour period. When maintained in medium replete with essential fatty acids, the fatty acid composition of the PtdCho synthesized by cultured mesothelial cells closely resembled that isolated from the peritoneal cavity.

Conclusion

The demonstration of phospholipid secretion from mesothelial cells, with a fatty acid composition similar to the phospholipids isolated from peritoneal dialysate, lends added support to the hypothesis that the mesothelial cells are the source of the peritoneal phospholipids. As such they offer a useful experimental system in which to study peritoneal phospholipid synthesis.

Wnt signaling regulates trans-differentiation of stem cell like type 2 alveolar epithelial cells to type 1 epithelial cells

Background

Type 2 alveolar epithelial cells (AT2s) behave as stem cells and show clonal proliferation upon alveolar injury followed by trans-differentiation (TD) into Type 1 alveolar epithelial cells (AT1s). In the present study we identified signaling pathways involved in the physiological AT2-to-AT1 TD process.

Methods

AT2 cells can be isolated from human lungs and cultured in vitro where they undergo TD into AT1s. In the present study we identified signaling pathways involved in the physiological AT2-to-AT1 TD process using Affymetrix microarray, qRT-PCR, fluorescence microscopy, and an in vitro lung aggregate culture.

Results

Affymetrix microarray revealed Wnt signaling to play a crucial role in the TD process. Wnt7a was identified as a ligand regulating the AT1 marker, Aquaporin 5 (AQP5). Artificial Neural Network (ANN) analysis of the Affymetrix data exposed ITGAV: Integrin alpha V (ITGAV), thrombospondin 1 (THBS1) and epithelial membrane protein 2 (EMP2) as Wnt signaling targets.

Conclusions

Wnt signaling targets that can serve as potential alveolar epithelial repair targets in future therapies of the gas exchange surface after injury. As ITGAV is significantly increases during TD and is regulated by Wnt signaling, ITGAV might be a potential target to speed up the alveolar healing process.

When Is an Alveolar Type 2 Cell an Alveolar Type 2 Cell? A Conundrum for Lung Stem Cell Biology and Regenerative Medicine

Generating mature, differentiated, adult lung cells from pluripotent cells, such as induced pluripotent stem cells and embryonic stem cells, offers the hope of both generating disease-specific in vitro models and creating definitive and personalized therapies for a host of debilitating lung parenchymal and airway diseases. With the goal of advancing lung-regenerative medicine, several groups have developed and reported on protocols using defined media, coculture with mesenchymal components, or sequential treatments mimicking lung development, to obtain distal lung epithelial cells from stem cell precursors. However, there remains significant controversy about the degree of differentiation of these cells compared with their primary counterparts, coupled with a lack of consistency or uniformity in assessing the resultant phenotypes. Given the inevitable, exponential expansion of these approaches and the probable, but yet-to-emerge second and higher generation techniques to create such assets, we were prompted to pose the question, what makes a lung epithelial cell a lung epithelial cell? More specifically for this Perspective, we also posed the question, what are the minimum features that constitute an alveolar type (AT) 2 epithelial cell? In addressing this, we summarize a body of work spanning nearly five decades, amassed by a series of "lung epithelial cell biology pioneers," which carefully describes well characterized molecular, functional, and morphological features critical for discriminately assessing an AT2 phenotype. Armed with this, we propose a series of core criteria to assist the field in confirming that cells obtained following a differentiation protocol are indeed mature and functional AT2 epithelial cells.

The role of myosin 1c and myosin 1b in surfactant exocytosis

Actin and actin-associated proteins have a pivotal effect on regulated exocytosis in secretory cells and influence pre-fusion as well as post-fusion stages of exocytosis. Actin polymerization on secretory granules during the post-fusion phase (formation of an actin coat) is especially important in cells with large secretory vesicles or poorly soluble secretions. Alveolar type II (ATII) cells secrete hydrophobic lipo-protein surfactant, which does not easily diffuse from fused vesicles. Previous work showed that compression of actin coat is necessary for surfactant extrusion. Here, we investigate the role of class 1 myosins as possible linkers between actin and membranes during exocytosis. Live-cell microscopy showed translocation of fluorescently labeled myosin 1b and myosin 1c to the secretory vesicle membrane after fusion. Myosin 1c translocation was dependent on its pleckstrin homology domain. Expression of myosin 1b and myosin 1c constructs influenced vesicle compression rate, whereas only the inhibition of myosin 1c reduced exocytosis. These findings suggest that class 1 myosins participate in several stages of ATII cell exocytosis and link actin coats to the secretory vesicle membrane to influence vesicle compression.

NMR-based metabonomics survey in rats envenomed by Hemiscorpius lepturus venom

About 1% of scorpion stings in Iran have been reported in cities and more than 5% in rural areas. The genus Hemiscorpous lepturus belongs to the family Hemiscorpiidae that is endemic in the south-western province of Khoozestan and other parts of western Iran. Although this species is responsible for only 10% of the reported stings, it is also responsible for 95% of mortalities. The heavy cytotoxic and neurotoxic venomous effects of scorpion sting may cause serious disorders such as erythema, purpuric changes, bulla, necrosis and ulcer, either alone or in combination. The toxic effects of H. lepturus venom cause vast changes in both primary and secondary metabolites of the victim, which finally lead to death if not treated early enough. Metabolomics is the systematic study of chemical fingerprints resulting from cytotoxic and pathogenic reactions of cells. We studied the toxic mechanism of H. lepturus venom on metabolome profiling of the victims along with biochemical pathways, organs and physiological detectable effects of this venom by the help of (1)H NMR. Our results showed that pyrimidine, histidine and tyrosine metabolisms, and steroid hormone biosynthesis were the most affected pathways. The major action of the crude venom is on the pancreas, and also on the nerve cells, spleen and mitochondria, causing acute seizures, which resemble the early markers of myocardial injury and seizure disorders.

Efficient gene delivery to primary alveolar epithelial cells by nucleofection

Primary alveolar epithelial cells play a pivotal role in lung research, particularly when focusing on gas exchange, barrier function, and transepithelial transport processes. However, efficient transfection of primary alveolar epithelial cells continues to be a major challenge. In the present study, we applied nucleofection, a novel method of gene and oligonucleotide delivery to the nucleus of cells by electroporation, to achieve highly efficient transfection of primary alveolar epithelial type II (ATII) cells. To quantify the amount of ATII cells effectively transfected, we applied a plasmid expressing GFP and assessed the amount of GFP-expressing cells by flow cytometry. Analysis of the nucleofected ATII cells revealed a concentration-dependent transfection efficiency of up to 50% when using 3-8 μg plasmid DNA without affecting cell viability. Nucleofection of cultured A549 and H441 cells yielded similar transfection rates. Importantly, nucleofection of ATII cells did not interfere with the integrity of ATII monolayers even with use of relatively high concentrations of plasmid DNA. In subsequent studies, we also efficiently delivered small interfering RNAs to ATII cells by nucleofection, thereby silencing Akt and the multiligand receptor megalin, which has been recently shown to play a key role in removal of excess protein from the alveolar space, and effectively inhibited megalin-driven uptake and transcellular transport of albumin in ATII cells. Thus we report successful transfection of primary rat alveolar epithelial cells with both plasmids and oligonucleotides via nucleofection with high viability and consistently good transfection rates without impairing key physiological properties of the cells.

Surfactant protein-C chromatin-bound green fluorescence protein reporter mice reveal heterogeneity of surfactant protein C-expressing lung cells

The regeneration of alveolar epithelial cells is a critical aspect of alveolar reorganization after lung injury. Although alveolar Type II (AT2) cells have been described as progenitor cells for alveolar epithelia, more remains to be understood about how their progenitor cell properties are regulated. A nuclear, chromatin-bound green fluorescence protein reporter (H2B-GFP) was driven from the murine surfactant protein-C (SPC) promoter to generate SPC H2B-GFP transgenic mice. The SPC H2B-GFP allele allowed the FACS-based enrichment and gene expression profiling of AT2 cells. Approximately 97% of AT2 cells were GFP-labeled on Postnatal Day 1, and the percentage of GFP-labeled AT2 cells decreased to approximately 63% at Postnatal Week 8. Isolated young adult SPC H2B-GFP(+) cells displayed proliferation, differentiation, and self-renewal capacity in the presence of lung fibroblasts in a Matrigel-based three-dimensional culture system. Heterogeneity within the GFP(+) population was revealed, because cells with distinct alveolar and bronchiolar gene expression arose in three-dimensional cultures. CD74, a surface marker highly enriched on GFP(+) cells, was identified as a positive selection marker, providing 3-fold enrichment for AT2 cells. In vivo, GFP expression was induced within other epithelial cell types during maturation of the distal lung. The utility of the SPC H2B-GFP murine model for the identification of AT2 cells was greatest in early postnatal lungs and more limited with age, when some discordance between SPC and GFP expression was observed. In adult mice, this allele may allow for the enrichment and future characterization of other SPC-expressing alveolar and bronchiolar cells, including putative stem/progenitor cell populations.

Surfactant phospholipid metabolism

Pulmonary surfactant is essential for life and is composed of a complex lipoprotein-like mixture that lines the inner surface of the lung to prevent alveolar collapse at the end of expiration. The molecular composition of surfactant depends on highly integrated and regulated processes involving its biosynthesis, remodeling, degradation, and intracellular trafficking. Despite its multicomponent composition, the study of surfactant phospholipid metabolism has focused on two predominant components, disaturated phosphatidylcholine that confers surface-tension lowering activities, and phosphatidylglycerol, recently implicated in innate immune defense. Future studies providing a better understanding of the molecular control and physiological relevance of minor surfactant lipid components are needed. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

LPS impairs phospholipid synthesis by triggering beta-transducin repeat-containing protein (beta-TrCP)-mediated polyubiquitination and degradation of the surfactant enzyme acyl-CoA:lysophosphatidylcholine acyltransferase I (LPCAT1)

Acyl-CoA:lysophosphatidylcholine acyltransferase 1 (LPCAT1) is a relatively newly described and yet indispensable enzyme needed for generation of the bioactive surfactant phospholipid, dipalmitoylphosphatidylcholine (DPPtdCho). Here, we show that lipopolysaccharide (LPS) causes LPCAT1 degradation using the Skp1-Cullin-F-box ubiquitin E3 ligase component, β-transducin repeat-containing protein (β-TrCP), that polyubiquitinates LPCAT1, thereby targeting the enzyme for proteasomal degradation. LPCAT1 was identified as a phosphoenzyme as Ser(178) within a phosphodegron was identified as a putative molecular recognition site for glycogen synthase kinase-3β (GSK-3β) phosphorylation that recruits β-TrCP docking within the enzyme. β-TrCP ubiquitinates LPCAT1 at an acceptor site (Lys(221)), as substitution of Lys(221) with Arg abrogated LPCAT1 polyubiquitination. LPS profoundly reduced immunoreactive LPCAT1 levels and impaired lung surfactant mechanics, effects that were overcome by siRNA to β-TrCP and GSK-3β or LPCAT1 gene transfer, respectively. Thus, LPS appears to destabilize the LPCAT1 protein by GSK-3β-mediated phosphorylation within a canonical phosphodegron for β-TrCP docking and site-specific ubiquitination. LPCAT1 is the first lipogenic substrate for β-TrCP, and the results suggest that modulation of the GSK-3β-SCFβ(TrCP) E3 ligase effector pathway might be a unique strategy to optimize dipalmitoylphosphatidylcholine levels in sepsis.

Alveolar epithelial transport in the adult lung

The alveolar surface comprises >99% of the internal surface area of the lungs. At birth, the fetal lung rapidly converts from a state of net fluid secretion, which is necessary for normal fetal lung development, to a state in which there is a minimal amount of alveolar liquid. The alveolar surface epithelium facing the air compartment is composed of TI and TII cells. The morphometric characteristics of both cell types are fairly constant over a range of mammalian species varying in body weight by a factor of approximately 50,000. From the conservation of size and shape across species, one may infer that both TI and TII cells also have important conserved functions. The regulation of alveolar ion and liquid transport has been extensively investigated using a variety of experimental models, including whole animal, isolated lung, isolated cell, and cultured cell model systems, each with their inherent strengths and weaknesses. The results obtained with different model systems and a variety of different species point to both interesting parallels and some surprising differences. Sometimes it has been difficult to reconcile results obtained with different model systems. In this section, the primary focus will be on aspects of alveolar ion and liquid transport under normal physiologic conditions, emphasizing newer data and describing evolving paradigms of lung ion and fluid transport. We will highlight some of the unanswered questions, outline the similarities and differences in results obtained with different model systems, and describe some of the complex and interweaving regulatory networks.

In vitro transdifferentiation of human fetal type II cells toward a type I-like cell

For alveolar type I cells, phenotype plasticity and physiology other than gas exchange await further clarification due to in vitro study difficulties in isolating and maintaining type I cells in primary culture. Using an established in vitro model of human fetal type II cells, in which the type II phenotype is induced and maintained by adding hormones, we assessed for transdifferentiation in culture toward a type I-like cell with hormone removal for up to 144 h, followed by electron microscopy, permeability studies, and RNA and protein analysis. Hormone withdrawal resulted in diminished type II cell characteristics, including decreased microvilli, lamellar bodies, and type II cell marker RNA and protein. There was a simultaneous increase in type I characteristics, including increased epithelial cell barrier function indicative of a tight monolayer and increased type I cell marker RNA and protein. Our results indicate that hormone removal from cultured human fetal type II cells results in transdifferentiation toward a type I-like cell. This model will be useful for continued in vitro studies of human fetal alveolar epithelial cell differentiation and phenotype plasticity.

Identification and characterization of a lysophosphatidylcholine acyltransferase in alveolar type II cells

Pulmonary surfactant is a complex of lipids and proteins produced and secreted by alveolar type II cells that provides the low surface tension at the air-liquid interface. The phospholipid most responsible for providing the low surface tension in the lung is dipalmitoylphosphatidylcholine. Dipalmitoylphosphatidylcholine is synthesized in large part by phosphatidylcholine (PC) remodeling, and a lysophosphatidylcholine (lysoPC) acyltransferase is thought to play a critical role in its synthesis. However, this acyltransferase has not yet been identified. We have cloned full-length rat and mouse cDNAs coding for a lysoPC acyltransferase (LPCAT). LPCAT encodes a 535-aa protein of approximately 59 kDa that contains a transmembrane domain and a putative acyltransferase domain. When transfected into COS-7 cells and HEK293 cells, LPCAT significantly increased lysoPC acyltransferase activity. LPCAT preferred lysoPC as a substrate over lysoPA, lysoPI, lysoPS, lysoPE, or lysoPG and prefers palmitoyl-CoA to oleoyl-CoA as the acyl donor. This LPCAT was preferentially expressed in the lung, specifically within alveolar type II cells. Expression in the fetal lung and in rat type II cells correlated with the expression of the surfactant proteins. LPCAT expression in fetal lung explants was sensitive to dexamethasone and FGFs. KGF was a potent stimulator of LPCAT expression in cultured adult type II cells. We hypothesize that LPCAT plays a critical role in regulating surfactant phospholipid biosynthesis and suggest that understanding the regulation of LPCAT will offer important insight into surfactant phospholipid biosynthesis.

Interaction between primary alveolar macrophages and primary alveolar type II cells under basal conditions and after lipopolysaccharide or quartz exposure

Intercellular communications between alveolar macrophages (AM) and alveolar epithelial type II (TII) cells have been suggested to be important in cellular responses. The main objective of this study was to improve our understanding of the interactions between AM and TII cells that might occur in the lung. In the present investigation, this interaction was studied under different interaction conditions (transwell or mixed coculture) and different exposure conditions (basal, lipopolysaccharide [LPS] exposure, or silica exposure). Studies also attempted different approaches to identify specific mediator(s) involved in this interaction. The following findings were made: (1) Surfactant released from TII cells appears to exert an inhibitory effect on AM activity. (2) Basal transwell coculture conditions are better than mixed coculture conditions to study AM/TII cell interactions, since the inhibitory effect of the surfactant in the transwell coculture is minimized. (3) AM/TII cell interaction is dependent on cell culture (transwell vs mixed) and exposure conditions. (4) Oxidants, tumor necrosis factor (TNF)-alpha, interleukin (IL)-1beta, prostaglandins, and leukotrienes probably do not independently affect the AM/TII intercellular interaction; instead, they appear to indirectly modulate the complex pathways of AM/TII communication.

Cloning and characterization of mouse lung-type acyl-CoA:lysophosphatidylcholine acyltransferase 1 (LPCAT1). Expression in alveolar type II cells and possible involvement in surfactant production

Phosphatidylcholine (1,2-diacyl-sn-glycero-3-phosphocholine, PC), is an important constituent of biological membranes. It is also the major component of serum lipoproteins and pulmonary surfactant. In the remodeling pathway of PC biosynthesis, 1-acyl-sn-glycero-3-phosphocholine (LPC) is converted to PC by acyl-CoA:lysophosphatidylcholine acyltransferase (LPCAT, EC 2.3.1.23). Whereas LPCAT activity has been detected in several tissues, the structure and detailed biochemical information on the enzyme have not yet been reported. Here, we present the cloning and characterization of a cDNA for mouse lung-type LPCAT (LPCAT1). The cDNA encodes an enzyme of 60 kDa, with three putative transmembrane domains. When expressed in Chinese hamster ovary cells, mouse LPCAT1 exhibited Ca(2+)-independent activity with a pH optimum between 7.4 and 10. LPCAT1 demonstrated a clear preference for saturated fatty acyl-CoAs, and 1-myristoyl- or 1-palmitoyl-LPC as acyl donors and acceptors, respectively. Furthermore, the enzyme was predominantly expressed in the lung, in particular in alveolar type II cells. Thus, the enzyme might synthesize phosphatidylcholine in pulmonary surfactant and play a pivotal role in respiratory physiology.

Identification and Characterization of a Gene Encoding Human LPGAT1, an Endoplasmic Reticulum-associated Lysophosphatidylglycerol Acyltransferase

Phosphatidylglycerol (PG) is an important membrane polyglycerolphospholipid required for the activity of a variety of enzymes and is a precursor for synthesis of cardiolipin and bis(monoacylglycerol) phosphate. PG is subjected to remodeling subsequent to its de novo biosynthesis to incorporate appropriate acyl content for its biological functions and to prevent the harmful effect of lysophosphatidylglycerol (LPG) accumulation. The enzymes involved in the remodeling process have not yet been identified. We report here the identification and characterization of a human gene encoding an acyl-CoA: lysophosphatidylglycerol acyltransferase (LPGAT1). Expression of the LPGAT1 cDNA in Sf9 insect and COS-7 cells led to a significant increase in LPG acyltransferase activity. In contrast, no significant acyltransferase activities were detected against glycerol 3-phosphate or a variety of lysophospholipids, including lysophosphatidylcholine, lysophosphatidylethanolamine, lysophosphatidylinositol, and lysophosphatidylserine. The recombinant human LPGAT1 enzyme recognized various acyl-CoAs and LPGs as substrates but demonstrated clear preference to long chain saturated fatty acyl-CoAs and oleoyl-CoA as acyl donors, which is consistent with the lipid composition of endogenous PGs identified from different tissues. Kinetic analyses of LPGAT1 expressed in COS-7 cells showed that oleoyl-LPG was preferred over palmitoyl-LPG as an acyl receptor, whereas oleoyl-CoA was preferred over lauroyl-CoA as an acyl donor. Consistent with its proposed microsomal origin, LPGAT1 was localized to the endoplasmic reticulum by subcellular fractionation and immunohistochemical analyses. Northern blot analysis indicated that the human LPGAT1 was widely distributed, suggesting a dynamic functional role of the enzyme in different tissues.

Freshly isolated rat alveolar type I cells, type II cells, and cultured type II cells have distinct molecular phenotypes

We used microarray analysis with Affymetrix rat chips to determine gene expression profiles of freshly isolated rat type I (TI) and TII cells and cultured TII cells. Our goals were 1) to describe molecular phenotypic "fingerprints" of TI and TII cells, 2) to gain insight into possible functional differences between the two cell types through differentially expressed genes, 3) to identify genes that might indicate potential functions of TI cells, since so little is known about this cell type, and 4) to ascertain the similarities and differences in gene expression between cultured TII cells and freshly isolated TI cells. For these experiments, we used preparations of isolated TI and TII cells that contained <2% cross-contamination. With a false discovery rate of 1%, 601 genes demonstrated over twofold different expression between TI and TII cells. Those genes with very high levels of differential expression may be useful as markers of cell phenotype and in generating novel hypotheses about functions of TI and TII cells. We found similar numbers of differentially expressed genes between freshly isolated TI or TII cells and cultured TII cells (698, 637 genes) and freshly isolated TI and TII cells (601 genes). Tests of sameness/difference including cluster dendrograms and log/log identity plots indicated major differences between the phenotypes of freshly isolated TI cell and cultured type II cell populations. The latter results suggest that experiments with TII cells cultured under these conditions should be interpreted with caution with respect to biological relevance to TI or TII cells.

Keratinocyte growth factor and the transcription factors C/EBP alpha, C/EBP delta, and SREBP-1c regulate fatty acid synthesis in alveolar type II cells

Strategies to stimulate endogenous surfactant production require a detailed understanding of the regulation of lipogenesis in alveolar type II cells. We developed culture conditions in which keratinocyte growth factor (KGF) stimulates fatty acid and phospholipid synthesis. KGF stimulated acetate incorporation into phosphatidylcholine, disaturated phosphatidylcholine, and phosphatidylglycerol more than 5% rat serum alone. To determine the mRNA levels of lipogenic enzymes and transport proteins, we analyzed gene expression by oligonucleotide microarrays. KGF increased the mRNA levels for fatty acid synthase, stearoyl-CoA desaturase-1 (SCD-1), and epidermal fatty acid-binding protein more than rat serum alone. In addition, KGF increased the mRNA levels of the transcription factors CCAAT/enhancer-binding protein alpha (C/EBPalpha) and C/EBPdelta as well as SREBP-1c (ADD-1), but not PPARgamma. These changes in C/EBPalpha and C/EBPdelta were confirmed by in situ hybridization. SCD-1 was also found to be highly expressed in alveolar type II cells in vivo. Furthermore, KGF increased protein levels of fatty acid synthase, C/EBPalpha, C/EBPdelta, SREBP-1, epidermal fatty acid-binding protein, and SCD. Finally, the liver X receptor agonist T0901317 increased acetate incorporation and SREBP-1 but not SREBP-2 protein levels. In summary, KGF stimulates lipogenesis in type II cells by a coordinated expression of lipogenic enzymes and transport proteins regulated by C/EBP isoforms and SREBP-1c.

Maintenance of the mouse type II cell phenotype in vitro

The purpose of this study was to identify culture conditions for maintenance of isolated mouse type II cells with intact surfactant protein (SP) and phospholipid production. Type II cells were isolated from 6-wk-old mice and cultured on Matrigel matrix-rat tail collagen (70:30 vol/vol) in bronchial epithelial cell growth medium minus hydrocortisone plus 5% charcoal-stripped FBS and 10 ng/ml keratinocyte growth factor. Under these conditions, type II cells actively produced surfactant phospholipids and proteins for at least 7 days. Synthesis and secretion of surfactant phospholipids and SP-A, -B, -C, and -D declined on day 1 of culture but recovered by day 3, reaching levels comparable to or exceeding freshly isolated cells by day 5. Abundant lamellar bodies were readily apparent in cells examined on days 5 and 7, and a surfactant pellet was recovered by centrifugation of media harvested on each day of culture. Secretion of SP-B, SP-C, and phosphatidylcholine was stimulated by phorbol 12-myristate 13-acetate and was inhibited by compound 48/80. When tested with a bubble surfactometer, surfactant secreted by type II cells on day 5 of culture lowered surface tension to 5.2 +/- 2.3 mN/m. This is the first description of the synthesis and secretion of a functional surfactant complex by mouse type II cells after 7 days in primary culture.

Glucose transporter gene expression in freshly isolated and cultured rat pneumocytes

Alveolar epithelium in situ takes up luminal glucose by cotransport with sodium. Cultured alveolar type II pneumocytes have only sodium-independent glucose uptake. It is unclear which isoforms are responsible for glucose transport in these cells and why sodium-glucose cotransport activity disappears during culture. GLUT1, GLUT4, GLUT5 and SGLT1 mRNA were detected in freshly isolated rat alveolar type II cells by reverse transcriptase-polymerase chain reaction. We show that SGLT1 mRNA was 90% lower in cells cultured in plastic wells for 2 or 4 days than in freshly isolated cells. mRNAs coding for the facilitated transporters were reduced from 40% (GLUT1) and 75% (GLUT4 and GLUT5) in cultured cells. Cells cultured at the air-liquid interface better preserved their phenotype as attested by significantly higher surfactant-associated protein mRNA levels. However, these cells had no higher GLUT1 and SGLT1 gene expression. Thus, alveolar type II cells lose sodium-glucose cotransport activity in part because of a decrease in mRNA levels. These changes in gene expression and/or mRNA stability may be an additional consequence of the shift towards the type I cell phenotype observed in cultured type II pneumocytes.

Effects of oligohydramnios on lung growth and maturation in the fetal rat

Oligohydramnios (OH) retards fetal lung growth by producing less lung distension than normal. To examine effects of decreased distension on fetal lung development, we produced OH in rats by puncture of uterus and fetal membranes at 16 days of gestation; fetuses were delivered at 21 or 22 days of gestation. Controls were position-matched littermates in the opposite uterine horn. OH lungs had lower weights and less DNA, protein, and water, but no differences in saturated phosphatidylcholine, surfactant proteins (SP)-A and -B, and mRNA for SP-A, -B, -C, and -D. To evaluate effects on epithelial differentiation, we used RTI(40) and RTII(70), proteins specific in lung to luminal surfaces of alveolar type I and II cells, respectively. At 22 days of gestation, OH lungs had less RTI(40) mRNA (P < 0.05) and protein (P < 0.001), but RTII(70) did not differ from controls. With OH, type I cells (in proportion to type II cells) covered less distal air space perimeter (P < 0.01). We conclude that OH, which retards lung growth, has little effect on surfactant and impedes formation of type I cells relative to type II cells.

Glucocorticoids regulate expression of the fatty acid synthase gene in fetal rat type II cells

Fatty acids are integral components of pulmonary surfactant, a mixture of phospholipids and specific proteins that lines the alveolar surface and is essential for normal lung function. There are developmental increases in fatty acid biosynthesis and surfactant production in late-gestation fetal lung, and both processes are accelerated by glucocorticoids. Fatty acid synthase (FAS) is a key enzyme in de novo fatty acid biosynthesis, and increased FAS activity is responsible for the developmental and hormone-induced increases in fatty acid biosynthesis in fetal lung. Using cultured fetal lung explants, it has been reported that dexamethasone (Dex) increases FAS activity, protein content, mRNA content and rate of transcription. However, FAS expression has not been measured in isolated type II cells, the cellular source of surfactant within the lung. In the present study we measured parameters of FAS expression in type II cells isolated from the lungs of Dex-treated rats. Pregnant rats were injected with Dex or saline on days 18 and 19 of gestation and the fetuses delivered on day 20. Type II cells and fibroblasts were then isolated from the fetal lungs. Dex increased FAS activity, protein content, mRNA content and rate of transcription in the type II cells but not in the fibroblasts. Increased FAS expression in fetal type II cells in response to Dex is consistent with a critical role for FAS in the biosynthesis of lung surfactant.

Parathyroid hormone-related protein and lung injury after pulmonary thromboendarterectomy

Parathyroid hormone-related protein (PTHrP) is an autocrine growth and differentiation factor for alveolar type II epithelial cells. Type II cells secrete pulmonary surfactant and are pluripotent cells with a role in alveolar epithelial repair after lung injury. The goals of this study were to investigate whether the levels of PTHrP in bronchoalveolar lavage liquid (BAL) varied between patients who did and did not develop lung injury after pulmonary thromboendarterectomy (PTE). BAL was performed in 48 patients undergoing PTE for unresolved pulmonary emboli. Samples were obtained following induction of anesthesia, following separation from cardiopulmonary bypass, and 48 h postoperatively. PTHrP was measured by radioimmunoassay. Lung injury was diagnosed in 23 patients on the basis of hypoxemia (PaO(2)/FIO(2) < 300) and the presence of lung infiltrates in the absence of infection or atelectasis. Patients with lung injury had significantly lower preoperative BAL levels of PTHrP, 21 (21-30) pg/ml (median, interquartile gap), compared to patients without lung injury, 34 (21-41) pg/ml (P < 0.05). Preoperative BAL PTHrP levels < 32 pg/ml predicted lung injury with a positive predictive value of 60% and negative predictive value of 82%. The odds of developing lung injury for patients with preoperative PTHrP levels below this cutpoint were 6 times the odds for patients with higher levels.

Release of hydrogen peroxide by rat type II pneumocytes in the prolonged culture

Type II pneumocytes (T II pneumocytes) produce hydrogen peroxide (H(2)O(2)), which may be potentially dangerous for the lung. These cells in culture differentiate to type I-like pneumocytes and it may reflect the differentiation which follows the injury of alveolar epithelium. This work was undertaken to estimate the H(2)O(2) release by T II pneumocytes, freshly isolated and cultured up to 8 days. The light and electron microscopy evaluation confirmed the differentiation of T II pneumocytes to type I-like cells. The release of H(2)O(2), estimated spectrofluorimetrically as homovanillic acid oxidation product obtained in the presence of horseradish peroxidase, was significantly higher at day 4 (0.63+/-0. 68nmol/mg protein/min, P</=0.02) and 6 (0.46+/-0.31, P</=0.001) compared to fresh cells (0.15+/-0.08). Phorbol esters increased H(2)O(2) release at day 2 (0.39+/-0.22 vs 0.16+/-0.08, P</=0.02) and the inhibition of protein kinase C resulted in the decrease at day 2 (0.14+/-0.06 vs 0.07+/-0.02, P</=0.025), day 6, (0.49+/-0.25 vs 0. 15+/-0.08, P</=0.005) and 8 (0.76+/-0.63 vs 0.23+/-0.29, P</=0.02). Inhibition of intracellular catalase resulted in a significant increase only at day 2 (0.23+/-0.1 vs 0.15+/-0.09, P</=0.05). Inhibition of mitochondrial respiratory chain decreased H(2)O(2) release at day 2 (0.13+/-0.11 vs 0.07+/-0.07, P</=0.002) and 4 (0. 75+/-0.88 vs 0.61+/-0.85, P</=0.002). These results indicate that alveolar epithelium may be a source of potentially dangerous ROS and that the cell differentiation is accompanied by the increase of H(2)O(2) production. Both mitochondrial respiratory chain and membrane-bound NADPH-oxidase may be responsible for the production of H(2)O(2) by T II pneumocytes.

Generation of an immortal differentiated lung type-II epithelial cell line from the adult H-2K(b)tsA58 transgenic mouse

This paper describes a new fully differentiated Type-II alveolar epithelial cell line designated T7, derived from transgenic H-2K(b)-tsA58 mice, capable of being passaged as an immortalized cloned cell line in culture. H-2K(b)-tsA58 mice harbor a temperature-sensitive (ts) mutant of the simian virus 40 (SV40) large tumor antigen (T antigen) under the control of the gamma-interferon (INF)-inducible mouse major histocompatibility complex H-2Kb promoter. When cultured under permissive conditions (33 degrees C and in the presence of gamma-INF) cells isolated from H-2Kb-tsA58 mice express the large T antigen, which drives the cells to proliferate. However, upon withdrawal of the gamma-INF and transfer of the cells to a higher temperature (39 degrees C), T antigen expression is turned off, the cells stop proliferating and differentiate. The T7 cell line is a clonal cell line originally derived from a Type-II cell-rich fraction isolated from lungs of H-2Kb-tsA58 mice. The T7 cells form confluent monolayers, and have a polarized epithelial cell morphology with tight junctions and apical microvilli. In addition, the T7 cells have distinct cytoplasmic lamellar bodies, which become more numerous and pronounced when the cells are grown under nonpermissive conditions. The T7 cells synthesize and secrete phosphatidylcholine and the three surfactant proteins, SP-A, SP-B, and SP-C. The T7 cell line is unique in that it is the first non-tumor-derived Type-II cell line capable of synthesizing and secreting the major components of surfactant. Based on the criteria studied, the T7 cell line is phenotypically very similar to normal Type-II cells. The T7 cell line, therefore, should prove a valuable experimental system to advance the study of the cell biology/physiology of surfactant metabolism and secretion as well as serve as a model for other studies of Type-II cell physiology.

Cyclosporin A protects lung function from hyperoxic damage

Cyclosporin A (CsA), an inhibitor of protein phosphatase 2B (calcineurin), has been shown to play a role in exocytosis and neutrophil mobility. Hyperoxia (>95% oxygen for 72 h) causes lung injury and reduces lung compliance. This model is indicative of deficiencies in surfactant and elicits a vigorous immune response leading to further damage. We examined the effects of CsA on surfactant-secreting lung alveolar type II cells. CsA enhances ATP-stimulated increases in whole cell capacitance in the presence of 2 mM extracellular Ca2+. This measurement corresponds with increases in exocytosis. Because of its effect on the immune system and exocytosis from type II cells, CsA was examined for its protective effects against hyperoxia-induced lung damage in mice. We found that CsA (50 mg. kg-1. day-1) attenuated hyperoxia-induced reductions in lung compliance when administered before or during 72 h of >95% oxygen (P < 0.05). CsA (10 mg. kg-1. day-1) also had a protective effect against hyperoxia-induced changes in neutrophil infiltration, capillary congestion, edema, and hyaline membrane formation. Wet lung weight-to-dry lung weight ratios did not show any significant changes after hyperoxia or hyperoxia plus CsA (P < 0. 05). CsA may be useful to treat patients undergoing prolonged high-oxygen therapy and possibly other lung injuries.

Mechanical distension modulates pulmonary alveolar epithelial phenotypic expression in vitro

The pulmonary alveolar epithelium is composed of two distinct types of cells, type I and type II cells, both of which are critical for normal lung function. On the basis of experiments of both nature and in vivo studies, it has been hypothesized that expression of the type I or type II phenotype is influenced by mechanical factors. We have investigated the effects of mechanical distension on the expression of specific markers for the type I and type II cell phenotypes in cultured alveolar type II cells. Rat alveolar type II cells were tonically mechanically distended in culture. Cells were analyzed for a marker for the type I phenotype (rTI40, an integral membrane protein specific for type I cells) and for markers for the type II phenotype [surfactant protein (SP) A, SP-B, and SP-C] as well as for glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Mechanical distension caused a 68 +/- 25% (n = 3) increase in mRNA content of rTI40 relative to undistended controls. In contrast, mechanical distension resulted in a decrease in mRNA content of SP-B to 35 +/- 19% (n = 3) and of SP-C to 20 +/- 6.7% (n = 3) of undistended controls. There was no effect on mRNA content of SP-A or GAPDH. The differences in mRNA content of SP-B and SP-C were found to be primarily due to changes at the transcriptional level by nuclear run-on assays. The effects on rTI40 appear to be due to posttranscriptional events. These data show that mechanical distension influences alveolar epithelial phenotypic expression in vitro, at least in part, at the transcriptional level.

E1A-induced immortalization of rat type II alveolar epithelial cells

Using a retroviral vector expressing the adenoviral 12S E1A gene product the authors have immortalized rat type II alveolar epithelial cells. For a period of time, the immortalized cells retain many of the ultrastructural characteristics of type II cells in situ, including the presence of lamellar bodies. By 250 days in culture, however, neither lamellar bodies, SP-A, nor a phospholipid profile characteristic of surfactant were present. The cell bind the lectin Maclura pomifera and stably express cytokeratins and the E1A gene product. The cell line also has a diploid karyotype, exhibits contact inhibition of growth, and does not grow in soft agar. E1A-immortalized cell lines should prove useful as models for study of certain aspects of type II alveolar epithelial cell function.

Direct toxic effect of bleomycin on alveolar type 2 cells

We studied the in vitro toxicity of bleomycin (BLM) on primary cultures of rat alveolar type 2 cells (T2 cells). It was shown that BLM was directly toxic for T2 cells in a dose- and time-dependent manner. Lung fibroblasts (LF) appear to be more resistant than T2 cells. Modulation of intracellular glutathione concentration was associated with changes in cytotoxicity. Furthermore, the addition of O-phenanthroline to the cellular medium reduced significantly BLM toxicity, suggesting the involvement of intra-cellular ferric ion. We also found that BLM toxicity was associated with a decreased release of phosphatidylcholine by T2 cells, the main component of surfactant. Protective effect of O-phenanthroline and the involvement of glutathione may be an alternative approach to the protection of BLM-induced damage.

Pulmonary epithelial cell proliferation in primary culture of alveolar type II cells

A small subpopulation of pulmonary epithelial cells (PE) proliferates in low-density primary culture of alveolar type II cells and forms colonies of cells that could be passaged for several generations and that in some respects maintain a differentiated phenotype of the alveolar type II cells. At this time it is not known if these cells are some form of progenitor epithelial cells or type II cells that are not fully differentiated in vitro. The proliferation of the PE cells was dependent on serum, alveolar macrophage-conditioned medium, and insulin being included in the culture medium. Under these conditions, approximately 0.5-1.0% of the seeded cells that adhered to the culture dishes were capable of forming colonies. Efficiency of colony formation increased to 5-10% in subsequent passages. PE cells maintained a high level (> 40%) of saturated phosphatidylcholine (PC) as a percentage of total PC throughout the culture period (> 28 days). However, the saturated PC content was not constant throughout the long-term culture period and the subsequent passages (41.3% at 29 days and 37.3% in the 3rd passage). These cells also contained numerous lamellar bodies and were able to bind the Maclura pomifera lectin. PE cells also expressed cytokeratin No. 19, as well as alkaline phosphatase activity, both possible markers for differentiated type II cells. However, PE cell synthesized low levels of Pg (approximately 2%), were squamous, and tended to form multiple strata, unlike the cuboidal type II cells in vivo. The cells did not exhibit immunocytochemically demonstrable surfactant-associated protein A (SP-A). Additional factors and culture requirements may be necessary for complete maturation of cultured PE cells. This was demonstrated by culturing PE cells on EHS matrix. Aggregates of cells surrounding a central lumen were formed after a few hours in culture and were maintained for 20 days. The cells contained lamellar bodies and some intercellular junctions. PE cells can be regarded as a highly selected subpopulation of pulmonary epithelial cells that concomitantly maintain proliferation and aspects of differentiated alveolar type II cells in long-term culture.

A rat monoclonal antibody specific for murine type 1 pneumocytes

A rat monoclonal antibody (MAb), 411-52, that binds specifically to murine pulmonary alveolar type 1 cells was developed. The cell-binding specificity of MAb 411-52 was assessed by light microscopy on immunoperoxidase-labeled tissue sections, electron microscopy on immunogold-labeled tissue blocks, and by flow cytometric analysis and fluorescence-activated cell sorting of immunofluorescently labeled cells enzymatically dissociated from murine lungs. The epitope recognized by MAb 411-52 was first detected in immunoperoxidase-stained sections of neonatal lungs of mice approximately 3 weeks after birth. In adult mice, the MAb 411-52-directed, immunoperoxidase-staining pattern was uniform throughout the lung parenchyma, was restricted to the luminal surfaces of alveoli, and was absent from type 2, endothelial, and interstitial cells, as well as from the epithelial cells of conducting airways. Electron microscopic analysis of immunogold-labeled lung tissue confirmed the type 1 cell binding specificity of MAb 411-52. Analysis by multiparameter, laser flow cytometry indicated that MAb 411-52 binds to 4.6 +/- 0.5% (mean +/- SD) of enzymatically dissociated cells from the lungs of normal adult mice. The absence of immunogold-labeling of type 2 cells suggested that the epitope recognized by MAb 411-52 might be a differentiation marker for the type 1 cell phenotype. With this MAb and standard immunohistochemical techniques, it is possible to visualize directly type 1 cells in paraffin sections.

Protein kinase C activation modulates arachidonic acid metabolism in cultured alveolar epithelial cells

Cultured alveolar type II cells can liberate esterified arachidonic acid (AA) and metabolize it predominantly via the cyclooxygenase pathway, and their capacity to do so increases as they alter their phenotype over time in culture. Little is known, however, about the regulation of AA metabolism in alveolar pneumocytes. We have examined the effects of protein kinase C (PKC) activation on arachidonate metabolism in primary cultures of rat alveolar epithelial cells studied at 2 and 7 days following isolation. The potent PKC activator phorbol myristate acetate (PMA) stimulated dose-dependent increases in free AA levels in both day 2 and day 7 cultures, with optimal stimulation at 50 nM. Greater stimulation was demonstrated for day 7 cells, and this was associated with greater prostanoid synthesis in response to PMA by day 7 than by day 2 cells. The capacity of PMA to "prime" epithelial cells for augmented AA liberation and metabolism in response to calcium ionophore A23187 (5 microM) was examined also. Significant priming by PMA was observed in both day 2 and day 7 cells; once again, augmentation of both free AA levels as well as prostaglandin E2 levels was greater for day 7 cells than for day 2 cells. That the capacity of PMA to modulate AA metabolism was mediated by activation of PKC was confirmed by demonstrating that (1) phorbol didecanoate, which lacks the ability to activate PKC, failed to activate AA metabolism; (2) pretreatment for 18 h with 1 microM PMA, which depletes cellular PKC, abolished subsequent AA metabolism activated by 50 nM PMA; and (3) the PKC inhibitor staurosporine abrogated increases in the quantities of both free AA and prostaglandin E2 in response to PMA. We conclude that activation of PKC increases the availability of AA for prostanoid synthesis in alveolar pneumocytes, and that this effect is more evident as type II cell differentiation is modeled during prolonged cultivation.

Vitamin E alters alveolar type II cell phospholipid synthesis in oxygen and air

Newborn rats were injected with vitamin E or placebo daily until 6 days after birth. The effect of vitamin E pretreatment on in vitro surfactant phospholipid synthesis was examined in isolated type II cells exposed to oxygen or air form 24 h in vitro. Type II cells were also isolated from untreated 6-day-old rats and cultured for 24 h in oxygen or air with control medium or vitamin E supplemented medium. These cells were used to examine the effect of vitamin E exposure in vitro on type II cell phospholipid synthesis and ultrastructure. Phosphatidylcholine (PC) synthesis was reduced in cells cultured in oxygen as compared with air. This decrease was not prevented by in vivo pretreatment or in vitro supplementation with vitamin E. Vitamin E pretreatment increased the ratio of disaturated PC to total PC and increased phosphatidylglycerol synthesis. The volume density of lamellar bodies in type II cells was increased in cells maintained in oxygen. Vitamin E did not affect the volume density of lamellar bodies. We conclude that in vitro hyperoxia inhibits alveolar type II cell phosphatidylcholine synthesis without decreasing lamellar body volume density and that supplemental vitamin E does not prevent hyperoxia-induced decrease in phosphatidylcholine synthesis.

Ontogeny of pulmonary alveolar epithelial markers of differentiation

We studied differentiation of the pulmonary epithelium in the periphery of fetal rat lung in vivo and in vitro by comparing the ontogeny of cell-surface glycoconjugates with that of surfactant phospholipids. Apical surface binding of the lectin Maclura pomifera agglutinin (MPA) and expression of a 200-kDa MPA-binding glycoprotein (MPA-gp200) was evident at 20 days gestation in type 2 cells, but did not correlate with ultrastructural features of type 2 cell differentiation. Epithelial cells isolated from peripheral lung of 18-day gestation fetal rats displayed hormone-sensitive surfactant synthesis prior to the hormone-insensitive expression of MPA-gp200. Expression of MPA-gp200 occurred in association with the appearance of many new apical surface proteins suggesting a hormone-independent process of polar membrane differentiation. Thus membrane and secretory differentiation are discordant and can be dissociated. In vivo binding of Ricinus communis 1 agglutinin (RCA1), an apical marker of the differentiated alveolar type 1 cell occurred in undifferentiated peripheral lung epithelial cells as early as 18 days gestation, disappeared from differentiating type 2 cells and appeared in differentiated type 1 cells. Both undifferentiated fetal epithelial cells at 18 days gestation and fully differentiated type 1 cells express multiple glycoproteins with terminal beta-linked galactose residues which bind RCA1. Some of these RCA1-binding glycoproteins appear to be similar. These observations suggest that alveolar epithelial type 1 cells may derive directly from undifferentiated peripheral lung epithelial cells as well as from fully differentiated type 2 cells. In addition, terminal differentiation of fetal lung peripheral epithelium into type 1 and type 2 cells may involve repression as well as induction of differentiation-related genes.

New concepts in molecular biology and ultrastructural pathology of the peritoneum: their significance for peritoneal dialysis

Continuous ambulatory peritoneal dialysis (CAPD) was developed into a life-maintaining therapy using a membrane whose fundamental biological characteristics were largely unknown. Recognition of this deficiency in our knowledge spurred a belated explosion of research that began with an exploration of the fine structure of the mesothelium. The monolayer of lining cells was found to be more sophisticated than previously imagined, being profusely carpeted with microvilli and bearing motile cilia, and in contrast to endothelium, was shown to possess a cytoplasm replete with organelles in which rough endoplasmic reticulum and lipid inclusions are prominent. Because these findings indicated possible secretory function, a link was sought between these observations and the discovery in effluent dialysate of phosphatidylcholine, a lubricant surfactant. Subsequently, comparison of mesothelial ultrastructure with that of type 2 pneumocytes revealed close concordance, while specialized fixation techniques developed for the preservation of lamellar bodies (the known storage vesicles of alveolar surfactant), when applied to mesothelium, for the first time revealed similar cytoplasmic inclusions. In vitro studies have shown that mesothelium, when incubated with radiolabeled precursor, is capable of synthesizing phosphatidylcholine, the principal constituent of pulmonary surfactant, in amounts similar to those produced by lung. The demonstration that the intensively studied type 2 pneumocyte and mesothelium both secrete lamellar bodies has opened up new possibilities in exploring the physiology, pharmacology, and pathology of the peritoneum. Recent work on mesothelial cell culture has shed new light on the factors involved in healing and regeneration. Recognition of the existence of subserosal multipotential cells and their importance in maintaining the integrity of the mesothelial cell layer is dawning. From the study of peritoneal biopsies in CAPD patients, evidence is accumulating that a process of nonenzymatic glycosylation of protein, similar to that which occurs in diabetes, is responsible for changes in stromal texture and the reduplication of basement membranes. Appreciation of stromal vulnerability to dialysate-induced accelerated aging following mesothelial loss may therefore require a new approach to peritoneal dialysis during peritonitis. Now that CAPD approaches clinical maturity there is increasing recognition of the need for strategies to ensure long-term preservation of the peritoneum as a dialyzing organ. Concomitantly there is a realization that these goals can only be attained through a much deeper appreciation of the molecular biology and pathology of the peritoneum itself.

Effect of lung collapse on alveolar surfactant in rabbits subjected to unilateral pneumothorax

To determine whether atelectasis might modify lung surfactant, we injected N2 into the right pleural space of adult rabbits. Daily, under sedation, pleural gas volume and pressure were measured and adjusted to 20 ml/kg and 0 to +2 cm H2O with N2. On the sixth day, pHa, PaCO2, PaO2, and FRC were measured. Pressure-volume diagrams or bronchoalveolar lavages (BAL) were performed separately on right and left lungs. Surfactant subfractions were obtained from BAL fluid, and total protein, LDH, and cell counts were determined. Phospholipid (PL) was assayed in lung homogenate, BAL fluid, and subfractions, and PL composition was determined on the largest BAL subfraction (P4). On the sixth day the pleural gas volume was 19.7 +/- 2.7 (SD) ml/kg, and PaO2 and FRC were significantly decreased. Air volume in excised right lungs at 30 cm H2O was 13.1 +/- 2.8 (SE) ml/kg with pneumothorax (PN) and 22.8 +/- 1.9 (SE) ml/kg in controls. Total PL was decreased 43% in BAL and 59% in P4 of collapsed lungs. Phosphatidylglycerol to phosphatidylinositol (PI) plus phosphatidylserine (PS) ratio of P4 was substantially decreased in both lungs of PN animals. Cell counts, LDH, and protein in BAL did not suggest inflammation or epithelial damage. We conclude that pneumothorax decreases the quantity of alveolar surfactant in the collapsed lung and alters its phospholipid composition toward the fetal pattern in both lungs, possibly due in part to the proliferative response of the lungs to pneumothorax.

Type I cell-like morphology in tight alveolar epithelial monolayers

The pulmonary alveolar epithelium separates air spaces from a fluid-filled interstitium and might be expected to exhibit high resistance to fluid and solute movement. Previous studies of alveolar epithelial barrier properties have been limited due to the complex anatomy of adult mammalian lung. In this study, we characterized a model of isolated alveolar epithelium with respect to barrier transport properties and cell morphology. Alveolar epithelial cells were isolated from rat lungs and grown as monolayers on tissue culture-treated Nuclepore filters. On Days 2-6 in primary culture, monolayers were analyzed for transepithelial resistance (Rt) and processed for electron microscopy. Mean cell surface area and arithmetic mean thickness (AMT) were determined using morphometric techniques. By Day 5, alveolar epithelial cells in vitro exhibited morphologic characteristics of type I alveolar pneumocytes, with thin cytoplasmic extensions and protruding nuclei. Morphometric data demonstrated that alveolar pneumocytes in vitro develop increased surface area and decreased cytoplasmic AMT similar to young type I cells in vivo. Concurrent with the appearance of type I cell-like morphology, monolayers exhibited high Rt (greater than 1000 omega.cm2), consistent with the development of tight barrier properties. These monolayers of isolated alveolar epithelial cells may reflect the physiological and morphological properties of the alveolar epithelium in vivo.

Carbon tetrachloride inhibits synthesis of pulmonary surfactant disaturated phosphatidylcholines and ATP production in alveolar type II cells

Other studies have shown that inhalation of carbon tetrachloride (CCl4) decreases the amount of pulmonary surfactant lining the alveolar surface. Therefore, we studied the effects of CCl4 on the synthesis of surfactant phosphatidylcholines (PCs) in rat alveolar type II cells in vitro. The rate of incorporation of choline, palmitate or glycerol into disaturated PC (DSPC) is decreased in a concentration-dependent manner. The CCl4 concentrations which cause maximal inhibition and 50% inhibition are similar for each substrate. The rate of incorporation of choline or glycerol into total PC is diminished to the same extent as their incorporation into DSPC. In addition, the rate of incorporation of glycerol into phosphatidylglycerol is decreased by the same extent as its incorporation into PC. All of these data suggest that there is a common site(s) at which CCl4 inhibits PC synthesis and that the inhibition occurs early in the biosynthetic pathway. However, individual enzymes involved in phospholipid synthesis do not seem to be affected by the solvent. Exposure of alveolar type II cells to CCl4 does cause a rapid and dramatic loss in cellular ATP, a cofactor required by some enzymes involved in PC synthesis. Studies with isolated lung mitochondria suggest that CCl4 inhibits the enzyme complex which catalyzes the synthesis of ATP from ADP. In addition, CCl4 causes a decrease in the amount of 3-O-methylglucose associated with type II cells, suggesting that glucose influx is impaired. This may also contribute to lower cellular ATP levels. The results of this study suggest that inhalation of CCl4 may impair surfactant phospholipid synthesis by decreasing ATP levels in alveolar type II cells.

The pulmonary toxicity of nitrosoureas

Drug-induced pulmonary fibrosis in cancer chemotherapy has become a more serious problem as treatment regimens are refined, larger total doses of cytotoxic drugs are administered and patient prognosis improves. The nitrosoureas are a class of chemotherapeutic agents whose clinical use is severely limited by drug-induced toxicities. The myelosuppression caused by nitrosourea administration can be managed clinically; however, the development of irreversible pulmonary fibrosis is a more serious clinical problem. Studies are needed to identify biochemical markers for early lung injury so that the amount of pulmonary tissue damage can be assessed and monitored. Additionally, considerable research is needed to understand the mechanisms by which these agents produce lung injury, so that therapeutic regimens can be developed to minimize or prevent lung toxicity. In understanding the mechanisms of toxicity, potentially the pulmonary injury caused by nitrosourea administration can be altered without affecting the clinical antitumor activity of this class of compounds. In the future, knowledge on mechanisms of drug-induced pulmonary injury can be used in the development of antineoplastic agents which are more disease specific and less pulmonary toxic.

Improved maintenance of adult rat alveolar type II cell differentiation in vitro: effect of hydrocortisone and cyclic AMP

We have examined the effect of hydrocortisone and cyclic AMP on the maintenance of lipid synthesis in primary cultures of adult rat alveolar type II cells. These hormones were tested in the presence of either 1% or 5% charcoal-stripped rat serum (CS-rat serum). The effect of substratum on responsiveness to these hormones was evaluated by comparing cells cultured for 4 days on tissue culture plastic, on floating type I collagen gels, on rat lung fibroblast feeder layers on floating collagen gels (floating feeder layers), and on Engelbreth-Holm-Swarm (EHS) tumor basement membrane gels. Type II cells cultured on floating feeder layers in medium containing 1% CS-rat serum and 10(-5) M hydrocortisone plus 0.5 mM dibutyryl cyclic AMP exhibited significantly increased incorporation of [14C]acetate into total lipids (238% of control). The hormone combination also increased the relative percentage of acetate incorporated into phosphatidylglycerol (PG; 7.3% versus 1.9%) and saturated phosphatidylcholine (PC; 43.6% versus 37.6%). The percentage of acetate incorporated into neutral lipids was significantly decreased by the addition of hormones (28.6% versus 70.0%). The addition of hydrocortisone and cyclic AMP to medium containing 5% CS-rat serum resulted in an increase in the relative incorporation of acetate into saturated PC (51.2% versus 46.4%), but had no effect on the relative incorporation of acetate into PG or on the incorporation of acetate into total lipids. Type II cells cultured on EHS gels in medium containing 1% CS-rat serum plus hydrocortisone and cyclic AMP showed increased acetate incorporation into total lipids (204% of control) and a relative decrease in the percentage of acetate incorporated into neutral lipids (16.9% versus 47.0%). The hormone combination also increased the relative incorporation of acetate into PG (4.4% versus 2.5%) and saturated PC (49.9% versus 42.1%). Hydrocortisone and cyclic AMP added to medium containing 5% CS-rat serum concentration increased the relative incorporation of acetate into saturated PC by type II cells on EHS gels, but these additions had no effect on acetate incorporation into PG. No responses to these soluble factors were seen when type II cells were cultured on floating type I collagen gels without feeder layers or on tissue culture plastic. These data indicate that there are positive interactions between substratum, soluble factors and serum in the maintenance of differentiated function of adult rat alveolar type II cells in vitro.

The pattern of cytokeratin synthesis is a marker of type 2 cell differentiation in adult and maturing fetal lung alveolar cells

During the last stages of fetal life, the immature epithelial cells of the rat lung alveolus develop the properties of mature type 2 cells. Adult type 2 cells rapidly lose these same properties when isolated and maintained in cell culture. We have examined the synthesis of cytokeratin proteins by adult type 2 cells as they lose their differentiated characteristics during 1 week in culture, and of immature fetal alveolar epithelial cells as they differentiate either in utero or when cultured on an extracellular matrix. Freshly isolated adult type 2 cells synthesize four cytokeratins which by electrophoretic mobilities and Western blot analysis correspond to human cytokeratins Nos. 7, 8, 18, and 19. During 7 days in culture synthesis of cytokeratin No. 19 is dramatically decreased and cytokeratin No. 18 becomes the predominant acidic cytokeratin produced. Fetal lung epithelial cells at 18 days gestation lack most characteristics of mature type 2 cells. When freshly isolated, these cells synthesize cytokeratins Nos. 7, 8, and 18 but make only minimal amounts of cytokeratin No. 19. When these cells are allowed to mature either in utero or in culture on a whole basement membrane extract, they develop both the morphological characteristics and the pattern of cytokeratin synthesis of fully developed type 2 cells, with cytokeratins No. 19 being the major acidic cytokeratin produced.

Identification and characterization of the pulmonary alveolar type II cell Maclura pomifera agglutinin-binding membrane glycoprotein

The lectin Maclura pomifera agglutinin (MPA) binds to the apical surface of pulmonary alveolar type II but not type I cells. We show that MPA binds to a single membrane glycoprotein in type II cells with a molecular mass of 230 kDa in the rabbit and 200 kDa in the rat. The glycoprotein has an abundance of terminal N-acetylgalactosamine residues. It is a hydrophilic integral membrane protein suggesting that it has an extensive extramembrane domain or is an ion channel. The glycoprotein is similar in rat and rabbit, with the exception that the rat glycoprotein is partially sialylated and is trypsin sensitive. The MPA-binding glycoprotein represents a new integral membrane marker of the apical domain of the pulmonary alveolar type II cell.