Very low density lipoproteins stimulate surfactant lipid synthesis in vitro

Mosa K, Almehdi AM, El‐Keblawy AA, Soliman SSM. Lipid-based therapies against SARS-CoV-2 infection

Viruses have evolved to manipulate host lipid metabolism to benefit their replication cycle. Enveloped viruses, including coronaviruses, use host lipids in various stages of the viral life cycle, particularly in the formation of replication compartments and envelopes. Host lipids are utilised by the virus in receptor binding, viral fusion and entry, as well as viral replication. Association of dyslipidaemia with the pathological development of Covid-19 raises the possibility that exploitation of host lipid metabolism might have therapeutic benefit against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). In this review, promising host lipid targets are discussed along with potential inhibitors. In addition, specific host lipids are involved in the inflammatory responses due to viral infection, so lipid supplementation represents another potential strategy to counteract the severity of viral infection. Furthermore, switching the lipid metabolism through a ketogenic diet is another potential way of limiting the effects of viral infection. Taken together, restricting the access of host lipids to the virus, either by using lipid inhibitors or supplementation with exogenous lipids, might significantly limit SARS-CoV-2 infection and/or severity.

Serum cholesterol as a predictor of mortality among the elderly patients with pneumonia in the emergency department

BACKGROUND

Reduced cholesterol levels are associated with poor outcomes in critically ill patients. However, the effect of reduced cholesterol levels on the prognosis of patients with community-acquired pneumonia (CAP) is unclear. This study aimed to investigate the association between serum total cholesterol levels and the clinical outcomes of elderly patients with CAP.

METHODS

This was a retrospective observational study that included elderly (≥65 years) CAP patients hospitalized through emergency department between January 2016 and December 2019. We collected their baseline characteristics and laboratory data, including total cholesterol levels at the time of admission. Univariate and multivariate analyses were performed to determine the association between total cholesterol levels and 14-day in-hospital mortality.

RESULTS

A total of 380 patients were included. The overall 14-day in-hospital mortality rate was 12.37%. Survivors had higher total cholesterol levels than non-survivors (median, 125 mg/dL; interquartile range [IQR], 102-151 mg/dL versus median, 100 mg/dL; IQR, 83-126 mg/dL; p < 0.001). Multivariate analysis using a logistic regression model showed that a total cholesterol level of <97 mg/dL was independently associated with 14-day in-hospital mortality in patients with CAP (odds ratio, 2.93; 95% confidence interval, 1.13-7.599; p = 0.027).

CONCLUSIONS

A decreased level of total cholesterol was associated with increased short-term mortality in elderly patients with CAP. Initial total cholesterol levels may be a useful biomarker to predict the outcome of patients with CAP.

Alveolar lipids in pulmonary disease. A review

Lung lipid metabolism participates both in infant and adult pulmonary disease. The lung is composed by multiple cell types with specialized functions and coordinately acting to meet specific physiologic requirements. The alveoli are the niche of the most active lipid metabolic cell in the lung, the type 2 cell (T2C). T2C synthesize surfactant lipids that are an absolute requirement for respiration, including dipalmitoylphosphatidylcholine. After its synthesis and secretion into the alveoli, surfactant is recycled by the T2C or degraded by the alveolar macrophages (AM). Surfactant biosynthesis and recycling is tightly regulated, and dysregulation of this pathway occurs in many pulmonary disease processes. Alveolar lipids can participate in the development of pulmonary disease from their extracellular location in the lumen of the alveoli, and from their intracellular location in T2C or AM. External insults like smoke and pollution can disturb surfactant homeostasis and result in either surfactant insufficiency or accumulation. But disruption of surfactant homeostasis is also observed in many chronic adult diseases, including chronic obstructive pulmonary disease (COPD), and others. Sustained damage to the T2C is one of the postulated causes of idiopathic pulmonary fibrosis (IPF), and surfactant homeostasis is disrupted during fibrotic conditions. Similarly, surfactant homeostasis is impacted during acute respiratory distress syndrome (ARDS) and infections. Bioactive lipids like eicosanoids and sphingolipids also participate in chronic lung disease and in respiratory infections. We review the most recent knowledge on alveolar lipids and their essential metabolic and signaling functions during homeostasis and during some of the most commonly observed pulmonary diseases.

A New Frontier in Immunometabolism. Cholesterol in Lung Health and Disease

The lung has a unique relationship to cholesterol that is shaped by its singular physiology. On the one hand, the lungs receive the full cardiac output and have a predominant dependence on plasma lipoprotein uptake for their cholesterol supply. On the other hand, surfactant lipids, including cholesterol, are continually susceptible to oxidation owing to direct environmental exposure and must be cleared or recycled because of the very narrow biophysical mandates placed upon surfactant lipid composition. Interestingly, increased lipid-laden macrophage "foam cells" have been noted in a wide range of human lung pathologies. This suggests that lipid dysregulation may be a unifying and perhaps contributory event in chronic lung disease pathogenesis. Recent studies have shown that perturbations in intracellular cholesterol trafficking critically modify the immune response of macrophages and other cells. This minireview discusses literature that has begun to demonstrate the importance of regulated cholesterol traffic through the lung to pulmonary immunity, inflammation, and fibrosis. This emerging recognition of coupling between immunity and lipid homeostasis in the lung presents potentially transformative concepts for understanding lung disease and may also offer novel and exciting avenues for therapeutic development.

Surfactant Lipids at the Host-Environment Interface. Metabolic Sensors, Suppressors, and Effectors of Inflammatory Lung Disease

The lipid composition of pulmonary surfactant is unlike that of any other body fluid. This extracellular lipid reservoir is also uniquely susceptible by virtue of its direct and continuous exposure to environmental oxidants, inflammatory agents, and pathogens. Historically, the greatest attention has been focused on those biophysical features of surfactant that serve to reduce surface tension at the air-liquid interface. More recently, surfactant lipids have also been recognized as bioactive molecules that maintain immune quiescence in the lung but can also be remodeled by the inhaled environment into neolipids that mediate key roles in inflammation, immunity, and fibrosis. This review focuses on the roles in inflammatory and infectious lung disease of two classes of native surfactant lipids, glycerophospholipids and sterols, and their corresponding oxidized species, oxidized glycerophospholipids and oxysterols. We highlight evidence that surfactant composition is sensitive to circulating lipoproteins and that the lipid milieu of the alveolus should thus be recognized as susceptible to diet and common systemic metabolic disorders. We also discuss intriguing evidence suggesting that oxidized surfactant lipids may represent an evolutionary link between immunity and tissue homeostasis that arose in the primordial lung. Taken together, the emerging picture is one in which the unique environmental susceptibility of the lung, together with its unique extracellular lipid requirements, may have made this organ both an evolutionary hub and an engine for lipid-immune cross-talk.

Effect of chronic intermittent hypoxia on triglyceride uptake in different tissues

Chronic intermittent hypoxia (CIH) inhibits plasma lipoprotein clearance and adipose lipoprotein lipase (LPL) activity in association with upregulation of an LPL inhibitor angiopoietin-like protein 4 (Angptl4). We hypothesize that CIH inhibits triglyceride (TG) uptake via Angptl4 and that an anti-Angptl4-neutralizing antibody would abolish the effects of CIH. Male C57BL/6J mice were exposed to four weeks of CIH or intermittent air (IA) while treated with Ab (30 mg/kg ip once a week). TG clearance was assessed by [H(3)]triolein administration retroorbitally. CIH delayed TG clearance and suppressed TG uptake and LPL activity in all white adipose tissue depots, brown adipose tissue, and lungs, whereas heart, liver, and spleen were not affected. CD146+ CD11b- pulmonary microvascular endothelial cells were responsible for TG uptake in the lungs and its inhibition by CIH. Antibody to Angptl4 decreased plasma TG levels and increased TG clearance and uptake into adipose tissue and lungs in both control and CIH mice to a similar extent, but did not reverse the effects of CIH. The antibody reversed the effects of CIH on LPL in adipose tissue and lungs. In conclusion, CIH inactivates LPL by upregulating Angptl4, but inhibition of TG uptake occurs predominantly via an Angptl4/LPL-independent mechanism.

Dyslipidemia induces opposing effects on intrapulmonary and extrapulmonary host defense through divergent TLR response phenotypes

Dyslipidemia influences innate immune responses in the bloodstream, but whether and how pulmonary innate immunity is sensitive to circulating lipoproteins is largely unknown. To define whether dyslipidemia impacts responses to bacteria in the airspace and, if so, whether differently from its effects in other tissues, airspace, bloodstream, and i.p. responses to LPS and Klebsiella pneumoniae were investigated using murine models of dyslipidemia. Dyslipidemia reduced neutrophil (PMN) recruitment to the airspace in response to LPS and K. pneumoniae by impairing both chemokine induction in the airspace and PMN chemotaxis, thereby compromising pulmonary bacterial clearance. Paradoxically, bacteria were cleared more effectively from the bloodstream during dyslipidemia. This enhanced systemic response was due, at least in part, to basal circulating neutrophilia and basal TLR4/MyD88-dependent serum cytokine induction and enhanced serum cytokine responses to systemically administered TLR ligands. Dyslipidemia did not globally impair PMN transvascular trafficking to, and host defense within all loci, because neutrophilia, cytokine induction, and bacterial clearance were enhanced within the infected peritoneum. Peritoneal macrophages from dyslipidemic animals were primed for more robust TLR responses, reflecting increased lipid rafts and increased TLR4 expression, whereas macrophages from the airspace, in which cholesterol was maintained constant during dyslipidemia, had normal responses and rafts. Dyslipidemia thus imparts opposing effects upon intra- and extrapulmonary host defense by inducing tissue-divergent TLR response phenotypes and dysregulating airspace/blood compartmental levels of PMNs and cytokines. We propose that the airspace is a "privileged" site, thereby uniquely sensitive to dyslipidemia.

c-Jun N-terminal kinase regulates CTP:phosphocholine cytidylyltransferase

CTP:phosphocholine cytidylyltransferase (CCTalpha) is a rate-regulatory enzyme required for phosphatidylcholine (PtdCho) synthesis. CCTalpha is also a phosphoenzyme, but the physiologic role of kinases on enzyme function remains unclear. We report high-level expression of two major isoforms of the c-Jun N-terminal kinase family (JNK1 and JNK2) in murine lung epithelia. Further, JNK1 and JNK2 phosphorylated purified CCTalpha in vitro, and this was associated with a dose-dependent decrease (approximately 40%) in CCT activity. To evaluate JNK in vivo, lung epithelial cells were infected with a replication defective adenoviral vector encoding murine JNK2 (Adv-JNK2) or an empty vector. Adv-JNK2 infection, unlike the empty vector, markedly increased JNK2 expression concomitant with increased incorporation of [32P]orthophosphate into endogenous CCTalpha. Although Adv-JNK2 infection only modestly reduced CCT activity, it reduced PtdCho synthesis by approximately 30% in cells. These observations suggest a role for JNK kinases as negative regulators of phospholipid synthesis in murine lung epithelia.

Distribution of the lipolysis stimulated receptor in adult and embryonic murine tissues and lethality of LSR-/- embryos at 12.5 to 14.5 days of gestation

The lipolysis stimulated receptor (LSR) recognizes apolipoprotein B/E-containing lipoproteins in the presence of free fatty acids, and is thought to be involved in the clearance of triglyceride-rich lipoproteins (TRL). The distribution of LSR in mice was studied by Northern blots, quantitative PCR and immunofluorescence. In the adult, LSR mRNA was detectable in all tissues tested except muscle and heart, and was abundant in liver, lung, intestine, kidney, ovaries and testes. During embryogenesis, LSR mRNA was detectable at 7.5 days post-coitum (E7) and increased up to E17 in parallel to prothrombin, a liver marker. In adult liver, immunofluorescence experiments showed a staining at the periphery of hepatocytes as well as in fetal liver at E12 and E15. These results are in agreement with the assumption that LSR is a plasma membrane receptor involved in the clearance of lipoproteins by liver, and suggest a possible role in steroidogenic organs, lung, intestine and kidney). To explore the role of LSR in vivo, the LSR gene was inactivated in 129/Ola ES cells by removing a gene segment containing exons 2-5, and 129/Ola-C57BL/6 mice bearing the deletion were produced. Although heterozygotes appeared normal, LSR homozygotes were not viable, with the exception of three males, while the total progeny of genotyped wild-type and heterozygote pups was 345. Mortality of the homozygote embryos was observed between days 12.5 and 15.5 of gestation, a time at which their liver was much smaller than that of their littermates, indicating that the expression of LSR is critical for liver and embryonic development.

Oxidized lipoproteins inhibit surfactant phosphatidylcholine synthesis via calpain-mediated cleavage of CTP:phosphocholine cytidylyltransferase

We investigated effects of pro-atherogenic oxidized lipoproteins on phosphatidylcholine (PtdCho) biosynthesis in murine lung epithelial cells (MLE-12). Cells surface-bound, internalized, and degraded oxidized low density lipoproteins (Ox-LDL). Ox-LDL significantly reduced [3H]choline incorporation into PtdCho in cells by selectively inhibiting the activity of the rate-regulatory enzyme, CTP:phosphocholine cytdylyltransferase (CCT). Ox-LDL coordinately increased the cellular turnover of CCTalpha protein as determined by [35S]methionine pulse-chase studies by inducing the calcium-activated proteinase, calpain. Forced expression of calpain or exposure of cells to the calcium ionophore, A23187, increased CCTalpha degradation, whereas overexpression of the endogenous calpain inhibitor, calpastatin, attenuated Ox-LDL-induced CCTalpha degradation. The effects of Ox-LDL on CCTalpha breakdown were attenuated in calpain-deficient cells. In vitro calpain digestion of CCTalpha isolated from cells transfected with truncated or internal deletion mutants indicated multiple cleavage sites within the CCTalpha primary structure, leading to the generation of a 26-kDa (p26) fragment. Calpain hydrolysis of purified CCTalpha generated p26, which upon NH2-terminal sequencing localized a calpain attack site within the CCTalpha amino terminus. Expression of a CCTalpha mutant where the amino-terminal cleavage site and a putative carboxyl-terminal hydrolysis region were modified resulted in an enzyme that was significantly less sensitive to proteolytic cleavage and restored the ability of cells to synthesize surfactant PtdCho after Ox-LDL treatment. Thus, these results provide a critical link between proatherogenic lipoproteins and their metabolic target, CCTalpha, resulting in impaired surfactant metabolism.

Maternal loading with very low-density lipoproteins stimulates fetal surfactant synthesis

We examined whether administration of very low-density lipoproteins (VLDL) to pregnant rats increases surfactant phosphatidylcholine (PtdCho) content in fetal pre-type II alveolar epithelial cells. VLDL-triglycerides are hydrolyzed to fatty acids by lipoprotein lipase (LPL), an enzyme activated by heparin. Fatty acids released by LPL can incorporate into the PtdCho molecule or activate the key biosynthetic enzyme cytidylyltransferase (CCT). Dams were given BSA, heparin, VLDL, or VLDL with heparin intravenously. Radiolabeled VLDL given to the pregnant rat crossed the placenta and was distributed systemically in the fetus and incorporated into disaturated PtdCho (DSPtdCho) in pre-type II cells. Maternal administration of VLDL with heparin increased DSPtdCho content in cells by 45% compared with control (P < 0.05). VLDL produced a dose-dependent, saturable, and selective increase in CCT activity. VLDL did not significantly alter immunoreactive CCT content but increased palmitic, stearic, and oleic acids in pre-type II cells. Furthermore, hypertriglyceridemic apolipoprotein E knockout mice contained significantly greater levels of DSPtdCho content in alveolar lavage and CCT activity compared with either LDL receptor knockout mice or wild-type controls that have normal serum triglycerides. Thus the nutritional or genetic modulation of serum VLDL-triglycerides provides specific fatty acids that stimulate PtdCho synthesis and CCT activity thereby increasing surfactant content.

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.

Lung function, permeability, and surfactant composition in oleic acid-induced acute lung injury in rats

Although acute lung injury (ALI) is associated with inflammation and surfactant dysfunction, the precise sequence of these changes remains poorly described. We used oleic acid to study the pathogenesis of ALI in spontaneously breathing anesthetized rats. We found that lung pathology can occur far more rapidly than previously appreciated. Lung neutrophils were increased approximately threefold within 5 min, and surfactant composition was dramatically altered within 15 min. Alveolar cholesterol increased by approximately 200%, and even though disaturated phospholipids increased by approximately 30% over 4 h, the disaturated phospholipid-to-total phospholipid ratio fell. Although the alveolocapillary barrier was profoundly disrupted after just 15 min, with marked elevations in lung fluid ((99m)Tc-labeled diethylenetriamine pentaacetic acid) and (125)I-labeled albumin flux, the lung rapidly began to regain its sieving properties. Despite the restoration in lung permeability, the animals remained hypoxic even though minute ventilation was increased approximately twofold and static compliance progressively deteriorated. This study highlights that ALI can set in motion a sequence of events continuing the respiratory failure irrespective of the alveolar surfactant pool size and the status of the alveolocapillary barrier.

Tumor necrosis factor-alpha inhibits expression of CTP:phosphocholine cytidylyltransferase

We investigated the effects of tumor necrosis factor alpha (TNFalpha), a key cytokine involved in inflammatory lung disease, on phosphatidylcholine (PtdCho) biosynthesis in a murine alveolar type II epithelial cell line (MLE-12). TNFalpha significantly inhibited [(3)H]choline incorporation into PtdCho after 24 h of exposure. TNFalpha reduced the activity of CTP:phosphocholine cytidylyltransferase (CCT), the rate-regulatory enzyme within the CDP-choline pathway, by 40% compared with control, but it did not alter activities of choline kinase or cholinephosphotransferase. Immunoblotting revealed that TNFalpha inhibition of CCT activity was associated with a uniform decrease in the mass of CCTalpha in total cell lysates, cytosolic, microsomal, and nuclear subfractions of MLE cells. Northern blotting revealed no effects of the cytokine on steady-state levels of CCTalpha mRNA, and CCTbeta mRNA was not detected. Incorporation of [(35)S]methionine into immunoprecipitable CCTalpha protein in pulse and pulse-chase studies revealed that TNFalpha did not alter de novo synthesis of enzyme, but it substantially accelerated turnover of CCTalpha. Addition of N-acetyl-Leu-Leu-Nle-CHO (ALLN), the calpain I inhibitor, or lactacystin, the 20 S proteasome inhibitor, blocked the inhibition of PtdCho biosynthesis mediated by TNFalpha. TNFalpha-induced degradation of CCTalpha protein was partially blocked by ALLN or lactacystin. CCT was ubiquitinated, and ubiquitination increased after TNFalpha exposure. m-Calpain degraded both purified CCT and CCT in cellular extracts. Thus, TNFalpha inhibits PtdCho synthesis by modulating CCT protein stability via the ubiquitin-proteasome and calpain-mediated proteolytic pathways.

Effect of phosphocholine cytidylyltransferase overexpression on phosphatidylcholine synthesis in alveolar type II cells and related cell lines

Disaturated phosphatidylcholine (DSPC) is the predominate phospholipid component of lung surfactant. In the alveolar type II cell, the cytidine diphosphocholine (CDP-choline) pathway is the major biosynthetic pathway for DSPC. To investigate the hypothesis that phosphocholine cytidylyltransferase (CT) is the rate-limiting enzyme in the CDP-choline pathway, rat alveolar type II cells or lung tumor-derived cell lines (A549 or H441) with type II cell features were transfected with CT complementary DNA (cDNA). Cell fractions were subsequently assayed for CT protein and activity, and cell rates of DSPC synthesis were determined. In all cases, cell CT protein and activity were increased after transfection with CT cDNA but not after control transfection. Rat type II cells, but not A549 or H441 cells, increased the rate of DSPC synthesis after transfection with CT cDNA. Exposure of type II cells transfected with CT cDNA to palmitic acid resulted in a further increase in CT protein and activity. Exposure to dexamethasone resulted in increased CT protein and activity and increased synthesis of DSPC. The results confirm that CT has a rate-limiting and regulatory role in the synthesis of type II cell DSPC, and raise possibilities for novel therapeutic interventions.

Identification of sex-specific differences in surfactant synthesis in rat lung

Delayed lung maturation and lower levels of surfactant phosphatidylcholine have been previously identified in male fetuses compared with female fetuses in several species. We investigated the mechanisms for sex differences in surfactant content by examining parameters of phosphatidylcholine turnover and biosynthesis; the latter was evaluated by measuring metabolic steps within the biosynthetic pathway. Compared with male lung cells, freshly isolated lung cells from female fetuses contained higher levels of disaturated phosphatidylcholine, a marker of surfactant lipid. Female mixed monolayer cultures exhibited a 71% increase in choline incorporation into disaturated phosphatidylcholine compared with male cultures. Male cultures exhibited significantly greater release of [3H]-arachidonic acid into the medium compared with females, suggesting sex differences in phospholipase activity. However, pulse-chase studies showed no sex differences in degradation of disaturated phosphatidylcholine, which was confirmed by assays of phospholipase A2, phosphatidylcholine-specific phospholipase C, and phospholipase D. Female mixed lung cells, however, had greater rates of cellular choline transport and activity of cytidylyltransferase, the rate-regulatory enzyme for phosphatidylcholine synthesis. Separate studies showed that exposure of sex-specific pretype II cell cultures to cortisol-stimulated fibroblast-conditioned medium plus transforming growth factor-beta-neutralizing antibody stimulated cytidylyltransferase activity to a greater extent in male cells compared with female cells. These studies indicate that sex differences in surfactant phospholipid content are not due to differences in phospholipid turnover, but rather differential regulation of specific metabolic steps within the surfactant biosynthetic pathway. The data also support a role for transforming growth factor-beta as a negative regulator of a key surfactant biosynthetic enzyme within male lungs.

Cloning of a unique lipase from endothelial cells extends the lipase gene family

A new lipoprotein lipase-like gene has been cloned from endothelial cells through a subtraction methodology aimed at characterizing genes that are expressed with in vitro differentiation of this cell type. The conceptual endothelial cell-derived lipase protein contains 500 amino acids, including an 18-amino acid hydrophobic signal sequence, and is 44% identical to lipoprotein lipase and 41% identical to hepatic lipase. Comparison of primary sequence to that of lipoprotein and hepatic lipase reveals conservation of the serine, aspartic acid, and histidine catalytic residues as well as the 10 cysteine residues involved in disulfide bond formation. Expression was identified in cultured human umbilical vein endothelial cells, human coronary artery endothelial cells, and murine endothelial-like yolk sac cells by Northern blot. In addition, Northern blot and in situ hybridization analysis revealed expression of the endothelial-derived lipase in placenta, liver, lung, ovary, thyroid gland, and testis. A c-Myc-tagged protein secreted from transfected COS7 cells had phospholipase A1 activity but no triglyceride lipase activity. Its tissue-restricted pattern of expression and its ability to be expressed by endothelial cells, suggests that endothelial cell-derived lipase may have unique functions in lipoprotein metabolism and in vascular disease.

Endogenous surfactant metabolism in critically ill infants measured with stable isotope labeled fatty acids

Little is known about endogenous surfactant metabolism in infants, because radioactive isotopes used for this purpose in animals cannot be used in humans. We developed a novel and safe method to measure the endogenous surfactant kinetics in vivo in humans by using stable isotope labeled fatty acids. We infused albumin-bound [U-13C]palmitic acid (PA) and [U-13C]linoleic acid (LLA) for 24 h in eight critically ill infants (mean+/-SD: weight: 3.7+/-1.3 kg: age: 51.3+/-61.6 d) who required mechanical ventilation. The 13C enrichment of PA and LLA in surfactant phosphatidylcholine (PC), obtained from tracheal aspirates, was measured by gas chromatography combustion interface-isotope ratio mass spectrometry. We measured a significant incorporation of both 13C-PA and 13C-LLA into surfactant PC. PC-PA and PC-LLA became enriched after 8.7+/-4.9 h (range: 3.4-17.3) and 10.0+/-7.2 h (range: 3.0-22.4), respectively; the times at maximum enrichment were 49.2+/-8.9 and 45.6+/-19.3 h, respectively. The fractional synthesis rate of surfactant PC-PA ranged from 0.4 to 3.4% per h, whereas the fractional synthesis rate of PC-LLA ranged from 0.5 to 3.8% per h. The surfactant PC-PA and PC-LLA half-lives ranged from 16.8 to 177.7 and 23.8 to 144.4 h, respectively. This method provides new data on surfactant metabolism in infants requiring mechanical ventilation. We found that synthesis of surfactant from plasma PA and LLA is a slow process and that there were marked differences in PC kinetics among infants. This variability could be related to differences in lung disease and could affect the clinical course of the respiratory failure.

Expression of plasma phospholipid transfer protein mRNA in normal and emphysematous lungs and regulation by hypoxia

The lung is the major site expressing plasma phospholipid transfer protein (PLTP) mRNA in humans and mice, suggesting that this protein might have an important role in maintaining normal function of this organ. In the lung of human collagenase transgenic mice, an emphysematous animal model, PLTP mRNA was 3-fold higher than in control mice. However, the mRNA in other tissues was not changed. To further assess the expression and function of PLTP, we measured PLTP mRNA level in lung tissue of two emphysematous patients and found that the mRNA was 4-fold higher than in control subjects. In situ hybridization on mouse lung suggested positive staining in alveolar type II epithelial cells. In addition, immortalized rat alveolar pre-type II epithelial cells and freshly isolated mature rat alveolar type II epithelial cells both highly expressed PLTP mRNA, and the former cells actively secreted PLTP activity into the medium. To examine the possible mechanisms leading to high levels of PLTP expression in vivo, we exposed the pre-type II cells to hypoxia and demonstrated induction of PLTP mRNA and a coordinate increase in secreted PLTP activity. Thus, the PLTP gene is highly expressed in alveolar type II epithelial cells and is induced during hypoxia and in emphysema. These observations suggest that a hypoxic stimulus occurring in emphysema may be a novel mechanism that contributes to enhanced expression of PLTP.