Pulmonary Surfactant Monolayers Research Articles

A Review on Investigating the Interactions between Nanoparticles and the Pulmonary Surfactant Monolayer with Coarse-Grained Molecular Dynamics Method.

Pulmonary drug delivery has garnered significant attention due to its targeted local lung action, minimal toxic side effects, and high drug utilization. However, the physicochemical properties of inhaled nanoparticles (NPs) used as drug carriers can influence their interactions with the pulmonary surfactant (PS) monolayer, potentially altering the fate of the NPs and impairing the biophysical function of the PS monolayer. Thus, the objective of this review is to summarize how the physicochemical properties of NPs affect their interactions with the PS monolayer. Initially, the definition and properties of NPs, as well as the composition and characteristics of the PS monolayer, are introduced. Subsequently, the coarse-grained molecular dynamics (CGMD) simulation method for studying the interactions between NPs and the PS monolayer is presented. Finally, the implications of the hydrophobicity, size, shape, surface charge, surface modification, and aggregation of NPs on their interactions with the PS monolayer and on the composition of biomolecular corona are discussed. In conclusion, gaining a deeper understanding of the effects of the physicochemical properties of NPs on their interactions with the PS monolayer will contribute to the development of safer and more effective nanomedicines for pulmonary drug delivery.

Numerical investigations of translocation characteristics of multiple silica nanoparticles across pulmonary surfactant monolayer

Understanding translocation characteristics of multiple silica nanoparticles (SiNPs) across pulmonary surfactant (PS) is crucial for their applications in inhaled nanoparticle drugs. Therefore, we employed coarse-grained molecular dynamics method to simulate the translocation process of paired SiNPs (P-SiNPs). It is found that crossing or embedding process of 6 and 8 nm spherical P-SiNPs takes significantly longer due to cooperative effects, with times increased by 4.8–14.6 and 8.8–24.8 times, respectively. The influence of size differences between P-SiNPs is complex. Notably, 8 nm SiNP completes its crossing in a pinocytosis-like manner. The presence of same-sized ellipsoidal SiNPs significantly disrupts PS monolayer, and 8 nm ellipsoidal SiNP induces the formation of a crescent-shaped groove and even a pore. In conclusion, the translocation process of multiple SiNPs is cooperative process influenced by size, concentration, size differences, and shape. To mitigate potential toxic side effects, it is recommended to use smaller-sized spherical SiNPs as drug carriers.

Numerical investigations of translocation characteristics of nano-silica lunar dust across pulmonary surfactant monolayer

The interactions between nano-silica lunar dust (NSLD) on the moon surface and pulmonary surfactant (PS) monolayer will pose risks to astronaut health in future manned lunar exploration missions, but the specifics of these interactions are unknown. This study investigates them using the coarse-grained molecular dynamics method considering different sizes (5, 10, and 15 nm) and shapes (sphere, ellipsoid, and cube), with special focus on the unique morphology of NSLDs with bugles. The key findings are as follows: (1) The 10 nm and 15 nm NSLDs embed in the PS monolayer through the major sphere of spherical-type, major ellipsoid of ellipsoidal-type, or one edge of cubic-type NSLDs upon contact the PS monolayer. (2) Adsorbed NSLDs cause a higher Sz value (ASz > 0.84), while embedded NSLDs cause a lower Sz value (0.47 < ASz < 0.83) that decreases with an increase in the number of bulges. (3) The embedding process absorbs 50–342 dipalmitoylphosphatidylcholine (DPPC) molecules, reducing the PS monolayer area by 0.21%–6.05%. NSLDs with bulges absorb approximately 9–126 additional DPPC molecules and cause a 0.05%–3.22% reduction in the PS monolayer area compared to NSLDs without bulges. (4) NSLDs move obliquely or vertically within the PS monolayer, displaying two distinct stages with varying velocities. Their movement direction and speed are influenced by the increasing complexity of NSLD with more bulges on them. In general, larger NSLDs with sharper shapes and increasing complex morphology of more bulges cause more significant damages to the PS monolayer. These findings have implications for safeguarding astronaut health in future manned lunar exploration missions.

Adsorption of CMIT/MIT on the Model Pulmonary Surfactant Monolayers.

Polyhexamethylene guanidine (PHMG) is a guanidine-based chemical that has long been used as an antimicrobial agent. However, recently raised concerns regarding the pulmonary toxicity of PHMG in humans and aquatic organisms have led to research in this area. Along with PHMG, there are concerns about the safety of non-guanidine 5-chloro-2-methylisothiazol-3(2H)-one/2-methylisothiazol-3(2H)-one (CMIT/MIT) in human lungs; however, the safety of such chemicals can be affected by many factors, and it is difficult to rationalize their toxicity. In this study, we investigated the adsorption characteristics of CMIT/ MIT on a model pulmonary surfactant (lung surfactant, LS) using a Langmuir trough attached to a fluorescence microscope. Analysis of the π-A isotherms and lipid raft morphology revealed that CMIT/MIT exhibited minimal adsorption onto the LS monolayer deposited at the air/water interface. Meanwhile, PHMG showed clear signs of adsorption to LS, as manifested by the acceleration of the L o phase growth with increasing surface pressure. Consequently, in the presence of CMIT/MIT, the interfacial properties of the model LS monolayer exhibited significantly fewer changes than PHMG.

Effect of lipid-polymer hybrid nanoparticles on the biophysical function and lateral structure of pulmonary surfactant: Mechanistic in vitro studies

The interaction between inhaled drug-loaded nanoparticles and pulmonary surfactant (PS) is critical for the efficacy and safety of inhaled nanomedicines. Here, we investigated the effect of small interfering RNA (siRNA)-loaded lipid-polymer hybrid nanoparticles (LPNs), which are designed for treatment of lung inflammation, on the physiological function of PS. By using biophysical in vitro methods we show that siRNA-loaded LPNs affect the biophysical function and lateral structure of PS. We used the Langmuir monolayer technique to demonstrate that LPNs display intrinsic surface activity by forming interfacial films that collapse at 49 mN/m, and they competitively inhibit the adsorption and spreading of PS components at the air–liquid interface. However, LPNs are excluded from the interface into the aqueous subphase at surface pressures above 49 mN/m, and hence they overcome the PS monolayer film barrier. Epifluorescence microscopy data revealed that LPNs influence the lateral structure of PS by: (i) affecting the nucleation, shape, and growth of compression-driven segregated condensed PS domains, and (ii) facilitating intermixing of liquid-expanded and tilted-condensed domains. However, the total surface area occupied by a highly condensed phase, presumably enriched in the highly surface tension-reducing dipalmitoylphosphatidylcholine, remained constant upon exposure to LPNs. These results suggest that surface-active LPNs influence the lateral structure of PS during translocation from the interface into the subphase, but LPNs do apparently not affect the biophysical function of PS under physiologically relevant conditions.

Numerical investigations of translocation characteristics of non-spherical silica nanoparticles across pulmonary surfactant monolayer under different respiratory states using coarse-grained molecular dynamics method

Understanding translocation characteristics of non-spherical silica nanoparticles (SiNPs) across pulmonary surfactant (PS) monolayer is of great significance for their applications in inhaled nanoparticles (NPs) drugs, as their shapes may impact drug delivery efficiency. Therefore, this study has performed coarse-grained molecular dynamics (CGMD) simulation to study the characteristics of SiNPs across the PS monolayer and adsorption of dipalmitoylphosphatidylcholine (DPPC) molecules. It is found that SiNPs smaller than 8 nm are more likely to cross the PS monolayer in expansion state than other states and crossing times required for ellipsoidal SiNPs are the shortest, and when SiNPs are in penetrating and embedding states, the number of DPPC molecules adsorbed by bulged-spherical SiNPs is the highest. In conclusion, ellipsoidal SiNPs have the strongest crossing ability and bulged-spherical SiNPs cause the greatest damage. Therefore, it is suggested to adopt ellipsoidal SiNPs and avoid bulged-spherical SiNPs when improving transportation efficiencies of spherical SiNP drug carriers.

Effect of Amphotericin B on the Thermodynamic Properties and Surface Morphology of the Pulmonary Surfactant Model Monolayer during Respiration.

During the COVID-19 pandemic, the treatment of pulmonary fungal infection faced noteworthy challenges. Amphotericin B has shown promising therapeutic effects as an inhalation treatment for pulmonary fungal infections, especially those associated with the COVID-19 virus, due to its rare resistance. However, because the drug frequently produces renal toxicity, its effective dose is limited in clinical use. In this work, the DPPC/DPPG mixed monolayer was used as the pulmonary surfactant monolayer to study the interaction between amphotericin B and the pulmonary surfactant monolayer during inhalation therapy using the Langmuir technique and atomic force microscopy. The effects of different molar ratios of AmB on the thermodynamic properties and surface morphology of the pulmonary surfactant monolayer at different surface pressures was evaluated. The results showed that when the molar ratio of AmB to lipids in the pulmonary surfactant was less than 1:1, the main intermolecular force was attractive at a surface pressure greater than 10 mN/m. This drug had little effect on the phase transition point of the DPPC/DPPG monolayer, but decreased the height of the monolayer at 15 mN/m and 25 mN/m. When the molar ratio of AmB to lipids was greater than 1:1, the intermolecular force was mainly repulsive at a surface pressure greater than 15 mN/m, and AmB increased the height of the DPPC/DPPG monolayer at both 15 mN/m and 25 mN/m. These results are helpful in understanding the interaction between the pulmonary surfactant model monolayer and different doses of drugs at various surface tensions during respiration.

Surfactant Proteins SP-B and SP-C in Pulmonary Surfactant Monolayers: Physical Properties Controlled by Specific Protein-Lipid Interactions.

The lining of the alveoli is covered by pulmonary surfactant, a complex mixture of surface-active lipids and proteins that enables efficient gas exchange between inhaled air and the circulation. Despite decades of advancements in the study of the pulmonary surfactant, the molecular scale behavior of the surfactant and the inherent role of the number of different lipids and proteins in surfactant behavior are not fully understood. The most important proteins in this complex system are the surfactant proteins SP-B and SP-C. Given this, in this work we performed nonequilibrium all-atom molecular dynamics simulations to study the interplay of SP-B and SP-C with multicomponent lipid monolayers mimicking the pulmonary surfactant in composition. The simulations were complemented by z-scan fluorescence correlation spectroscopy and atomic force microscopy measurements. Our state-of-the-art simulation model reproduces experimental pressure-area isotherms and lateral diffusion coefficients. In agreement with previous research, the inclusion of either SP-B and SP-C increases surface pressure, and our simulations provide a molecular scale explanation for this effect: The proteins display preferential lipid interactions with phosphatidylglycerol, they reside predominantly in the lipid acyl chain region, and they partition into the liquid expanded phase or even induce it in an otherwise packed monolayer. The latter effect is also visible in our atomic force microscopy images. The research done contributes to a better understanding of the roles of specific lipids and proteins in surfactant function, thus helping to develop better synthetic products for surfactant replacement therapy used in the treatment of many fatal lung-related injuries and diseases.

S2 Subunit of SARS-CoV-2 Spike Protein Induces Domain Fusion in Natural Pulmonary Surfactant Monolayers.

Pulmonary surfactant has been attempted as a supportive therapy to treat COVID-19. Although it is mechanistically accepted that the fusion peptide in the S2 subunit of the S protein plays a predominant role in mediating viral fusion with the host cell membrane, it is still unknown how the S2 subunit interacts with the natural surfactant film. Using combined bio-physicochemical assays and atomic force microscopy imaging, it was found that the S2 subunit inhibited the biophysical properties of the surfactant and induced microdomain fusion in the surfactant monolayer. The surfactant inhibition has been attributed to membrane fluidization caused by insertion of the S2 subunit mediated by its fusion peptide. These findings may provide novel insight into the understanding of bio-physicochemical mechanisms responsible for surfactant interactions with SARS-CoV-2 and may have translational implications in the further development of surfactant replacement therapy for COVID-19 patients.

The aggregation of carbon nanotubes deteriorates their adverse effects on pulmonary surfactant monolayer

The wide use of carbon nanotubes (CNTs) increases the concern of their potential risks upon human exposure, especially through respiratory tract. The small size of CNTs allows them to deposit into the deep lung where they inevitably interact with pulmonary surfactant (PS) monolayer and thus could profoundly influence the biophysical function of PS monolayer. Using coarse-grained molecular dynamics simulations, we studied how the aggregation state of CNTs regulated the interactions of different sized CNTs with the PS monolayer at the air-water interface. Compared with the dispersed CNTs, the aggregated CNTs induced more severe structure disturbance of the PS monolayer at both compression and expansion stages. The increased diameter, initial attack angle, and configuration irregularity of the aggregated CNTs enhanced the lipid disturbance around the CNTs and promoted the pore formation on the PS monolayer under a high surface tension. As a result, the aggregated CNTs tended to inhibit the biophysical function of the PS monolayer and were more difficult to be cleared from the monolayer than the dispersed CNTs. Our results suggest that the aggregated CNTs, rather than the dispersed ones, should be given more weight in assessing the pneumonic nanotoxicity caused by the inhaled CNTs.

Quantitative Evaluation on the Degradation Process of the Pulmonary Surfactant Monolayer When Exposed to Low-Level Ozone of Ambient Environment.

Ozone is a potent environmental oxidant with high chemical reactivity and is present in the ambient environment at a low level of a few tens of ppb. However, only limited information is known about low-level ozone's influence on the respiratory system. In the present study, we systematically investigated the degradation of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), which is one of the major components of the pulmonary surfactant (PS), enabling breath function of the lung exposed to low ambient-level ozone (40 ± 10 ppb). Using the liquid chromatography-mass spectrometry technique combined with the Langmuir-Blodgett approach, we first tracked the degradation process of POPC molecules by determining the degradation products during exposure to the ambient environment. As a result, we found that the POPC molecules can be readily degraded from the C═C moiety in 45 min, yielding an aldehyde-type product of 1-palmitoyl-2-(9'-oxo-nonanoyl)-sn-glycero-3-phosphocholine (POnPC) and a trace amount of an acid-type one of 1-palmitoyl-2-azelaoyl-sn-glycero-3-phosphocholine (PAzPC), as well as a pair of secondary ozonide (SOZ) isomers. Furthermore, with prolonged exposure, the SOZ stayed constant but the yield of PAzPC significantly increased with the decrease in POnPC. The low-level ozone-induced oxidation mechanisms for unsaturated lipids are discussed based on the quantitative analyses of these experimental observations. The present results demonstrate that the ground-level ozone is strong enough to induce dramatic oxidation damage to the unsaturated lipids of the PS. These oxidized species may trigger the lung's inflammatory response and be used as biomarkers for oxidative stress and inflammation.

Permeation of Nanoparticles into Pulmonary Surfactant Monolayer: In Situ X-ray Standing Wave Studies.

High-resolution X-ray techniques were applied to examine the effects of gold nanoparticles (size <5 nm) on natural pulmonary surfactant and pure DPPC monolayers preliminarily formed on water subphase in a Langmuir trough. Hydrophobic and hydrophilic nanoparticles were delivered from nanoaerosol using electrodeposition method. Grazing incidence diffraction, X-ray reflectivity, and X-ray standing wave measurements allow to monitor the changes in molecular organization of lipid monolayer and to locate the position of gold nanoparticles. X-ray experiments were performed over a period of 9-14 h. The obtained results evidenced that, on a long time scale, the deposition of nanoparticles, even at low doses, can induce pronounced alterations in lipid monolayer. The presented data can help to elucidate the mechanism of pulmonary translocation of inhaled nanoparticles that is of special interest for biomedical investigations of potential risk of nanoaerosols for human health.

Aerosolizable Lipid-Nanovesicles Encapsulating Voriconazole Effectively Permeate Pulmonary Barriers and Target Lung Cells.

The entire world has recently been witnessing an unprecedented upsurge in microbial lung infections. The major challenge encountered in treating the same is to ensure the optimum drug availability at the infected site. Aerosolization of antimicrobials, in this regard, has shown immense potential owing to their localized and targeted effect. Efforts, therefore, have been undertaken to systematically develop lung-phosphatidylcholine-based lipid nanovesicles of voriconazole for potential management of the superinfections like aspergillosis. LNVs, prepared by thin-film hydration method, exhibited a globule size of 145.4 ± 19.5 nm, polydispersity index of 0.154 ± 0.104 and entrapment efficiency of 71.4 ± 2.2% with improved in vitro antifungal activity. Aerodynamic studies revealed a microdroplet size of ≤5 μm, thereby unraveling its promise to target the physical barrier of lungs effectively. The surface-active potential of LNVs, demonstrated through Langmuir-Blodgett troughs, indicated their ability to overcome the biochemical pulmonary surfactant monolayer barrier, while the safety and uptake studies on airway-epithelial cells signified their immense potential to permeate the cellular barrier of lungs. The pharmacokinetic studies showed marked improvement in the retention profile of voriconazole in lungs following LNVs nebulization compared to pristine voriconazole. Overall, LNVs proved to be safe and effective delivery systems, delineating their distinct potential to efficiently target the respiratory fungal infections.

Molecular dynamics simulation insights into the cellular uptake of elastic nanoparticles through human pulmonary surfactant.

The interaction between inhaled nanoparticles (NPs) and the pulmonary surfactant (PS) monolayer has drawn significant attention due to its potential in drug delivery design and application for respiratory therapeutics in active and passive cellular uptake pathways. Even though much attention has been given to explore the interaction between NPs and the PS monolayer, the effects of the NP elasticity on the translocation across the PS monolayer have not been thoroughly studied. Here, we performed a series of coarse-grained (CG) molecular dynamics simulations to study active or passive cellular uptake pathways of three NPs with different elasticities through a PS monolayer. The differences between active and passive pathways underly the enhanced targeting ability by ligand–receptor interaction (L–R interaction). In the active or passive cellular uptake pathways, it is found that the increase in stiffness level leads to a higher penetrability of NPs at the same time range. The soft NP has always been withheld inside the PS monolayer due to the lowest level of elasticity, while the other two types of NPs penetrate through the PS monolayer as the simulation progresses toward the end. The NPs in the active cellular uptake pathways take a longer time to penetrate the PS monolayer, resulting in a longer average penetration distance of approximately 40.55% and a higher average number of contacts, approximately 36.11%, than passive cellular uptake pathways, due to the L–R interaction. Moreover, it demonstrates that NPs in active cellular uptake pathways have a significantly higher targeting ability with the PS monolayer. We conclude that the level of NP elasticities has a substantial link to the penetrability in active or passive cellular uptake pathways. These results provide valuable insights into drug delivery and nanoprobe design for inhaled NPs within the lungs.

Molecular basis of transport of surface functionalised gold nanoparticles to pulmonary surfactant.

Ligands like alkanethiol (e.g. dodecanethiol, hexadecanethiol, etc.) and polymers (e.g. poly(vinyl pyrrolidone), polyethylene glycol-thiol) capped to the gold nanoparticles (AuNPs) are widely used in biomedical field as drug carriers and as promising materials for probing and manipulating cellular processes. Ligand functionalised AuNPs are known to interact with the pulmonary surfactant (PS) monolayer once reaching the alveolar region. Therefore, it is crucial to understand the interaction between AuNPs and PS monolayers. Using coarse-grained molecular dynamics simulations, the effect of ligand density, and ligand length have been studied for two classes of ligands on a PS model monolayer consisting of DPPC, POPG, cholesterol and SP-B (mini-peptide). The ligands considered in this study are alkanethiol and polyethylene glycol (PEG) thiol as examples of hydrophobic and hydrophilic ligands, respectively. It was observed that the interaction between AuNPs and PS changes the biophysical properties of PS monolayer in compressed and expanded states. The AuNPs with hydrophilic ligand, can penetrate through the monolayer more easily, while the AuNPs with hydrophobic ligand are embedded in the monolayer and participated in deforming the monolayer structure particularly the monolayer in the compressed state. The bare AuNPs hinder to lower the monolayer surface tension value at the interface, however introducing ligand to the bare AuNPs or increasing the ligand length and density have an impact of lowering of monolayer surface tension to a minor extent. The simulation results guide the design of ligand protected NPs as drug carriers and can identify the nanoparticles' potential side effects on lung surfactant.

Simulated Breathing: Application of Molecular Dynamics Simulations to Pulmonary Lung Surfactant

In this review, we delve into the topic of the pulmonary surfactant (PS) system, which is present in the respiratory system. The total composition of the PS has been presented and explored, from the types of cells involved in its synthesis and secretion, down to the specific building blocks used, such as the various lipid and protein components. The lipid and protein composition varies across species and between individuals, but ultimately produces a PS monolayer with the same role. As such, the composition has been investigated for the ways in which it imposes function and confers peculiar biophysical characteristics to the system as a whole. Moreover, a couple of theories/models that are associated with the functions of PS have been addressed. Finally, molecular dynamic (MD) simulations of pulmonary surfactant have been emphasized to not only showcase various group’s findings, but also to demonstrate the validity and importance that MD simulations can have in future research exploring the PS monolayer system.

Understanding the Functional Properties of Lipid Heterogeneity in Pulmonary Surfactant Monolayers at the Atomistic Level.

Pulmonary surfactant is a complex mixture of lipids and proteins lining the interior of the alveoli, and constitutes the first barrier to both oxygen and pathogens as they progress toward blood circulation. Despite decades of study, the behavior of the pulmonary surfactant at the molecular scale is poorly understood, which hinders the development of effective surfactant replacement therapies, useful in the treatment of several lung-related diseases. In this work, we combined all-atom molecular dynamics simulations, Langmuir trough measurements, and AFM imaging to study synthetic four-component lipid monolayers designed to model protein-free pulmonary surfactant. We characterized the structural and dynamic properties of the monolayers with a special focus on lateral heterogeneity. Remarkably, simulations reproduce almost quantitatively the experimental data on pressure-area isotherms and the presence of lateral heterogeneities highlighted by AFM. Quite surprisingly, the pressure-area isotherms do not show a plateau region, despite the presence of liquid-condensed nanometer-sized domains at surface pressures larger than 20 mN/m. In the simulations, the liquid-condensed domains were small and transient, but they did not coalesce to yield a separate phase. They were only slightly enriched in DPPC and cholesterol, and their chemical composition remained very similar to the overall composition of the monolayer membrane. Instead, they differed from liquid-expanded regions in terms of membrane thickness (in agreement with AFM data), diffusion rates, as well as acyl chain packing and orientation. We hypothesize that such lateral heterogeneities are crucial for lung surfactant function, as they allow both efficient packing, to achieve low surface tension, and sufficient fluidity, critical for rapid adsorption to the air–liquid interface during the breathing cycle.

In Situ Monitoring of the Unsaturated Phospholipid Monolayer Oxidation in Ambient Air by HD-SFG Spectroscopy

The pulmonary surfactant monolayer is indispensable for the respiratory system. Recently, it was reported that some unsaturated lipids of the pulmonary surfactants are oxidized by low-level ozone in ambient air. However, the molecular-level understanding of the reaction mechanism is still limited due to technical difficulties. We applied heterodyne-detected sum frequency generation (HD-SFG) spectroscopy to probe the reaction process of an unsaturated phospholipid monolayer (1-palmitoyl-2-oleoyl-glycero-3-phosphocholine, POPC), which is one of the major lipids in the pulmonary surfactant, under low-level ozone (30 ± 5 ppb). The HD-SFG spectroscopy realized the accurate peak assignments of the spectra and the identification of molecular species with high sensitivity, which were impossible with previous measurements. The time-resolved spectra indicated that the C═C moiety in the unsaturated alkyl chain is selectively oxidized by ozone with a time constant of 22 ± 3 min by first-order reaction kinetics. Furthermore, it was revealed for the first time that the reaction product of the POPC monolayer under low-level ozone is not the carboxylic form but the aldehyde form based on the vibrational spectroscopy results. The present study has deepened our molecular-level understanding of the oxidation mechanism of unsaturated lipids that are widely found in many biological systems.

Real-time monitoring of the effect of carbon nanoparticles on the surface behavior of DPPC/DPPG Langmuir monolayer

Air pollution has become increasingly serious. Fine particulate matter (PM2.5) is the most well-known air pollutant, which leads to some common respiratory diseases when inhaled into the lungs to certain concentration. However, there is a lack of research on the process of dynamically monitoring the real-time effect of nanoparticles on the pulmonary surfactant monolayer. In this study, the DPPC/DPPG monolayer is prepared by the Langmuir method to simulate the lung surfactant monolayer during respiration and the carbon nanoparticles are introduced to the monolayer under different surface pressures to simulate the real dynamic process of inhaling nanoparticles during breathing. The effect of carbon nanoparticles on the surface behavior of DPPC/DPPG monolayer in real-time was examined in details by a combination of surface pressure (π)-area (A) isotherms, compressibility modulus (Cs-1)-surface pressure (π) isotherms and the Brewster angle microscopy (BAM). The results have shown that the introduction of carbon nanoparticles under different surface pressures affects the properties of lipid monolayers. The added carbon nanoparticles under lower surface pressure are easy to penetrate the lipid molecules to inhibit monolayer phase transition. When the carbon nanoparticles are introduced to the monolayer under higher surface pressure, they tend to self-aggregate to reduce the monolayer stability rather than interact with lipid tail chains. This work not only confirms the exotic hydrophobic carbon nanoparticles retain in the DPPC/DPPG monolayer irreversibly and affect the surface behavior of monolayer during respiration, but also opens a new idea for real-time monitoring of the effects of PM2.5 on lung health.

Benzalkonium chloride and cetylpyridinium chloride induce apoptosis in human lung epithelial cells and alter surface activity of pulmonary surfactant monolayers

Quaternary ammonium compounds (e.g., benzalkonium chloride (BAC) and cetylpyridinium chloride (CPC)) constitute a group of cationic surfactants are widely used for personal hygiene and medical care despite the potential pulmonary toxicity. To examine whether BAC and CPC aerosols deposited in the alveolar region alter pulmonary function, we studied the effects on pulmonary surfactant using two-step in vitro models; cytotoxicity using A549 alveolar epithelial cell and changes in surface activity of the pulmonary surfactant monolayer using both Surfacten® and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC). Cell viability was decreased with BAC and CPC dose-dependently. A comparison of cytotoxicity among BAC homologues with different length of alkyl chain showed that C16-BAC, which has the longest alkyl chain, was more cytotoxic than C12- or C14-BAC. Caspase-3/7 activity and cleaved form of caspase-3 and PARP were increased in BAC- and CPC-exposed cells. The elevated caspase-3/7 activity and their cleaved active forms were abolished by caspase-3-inhibitor. Furthermore, we examined the features of the surface pressure/trough area (π-A) isotherm by the Langmuir-Wilhelmy method and atomic force microscopy (AFM) images of lipid monolayers on a subphase containing BAC, CPC, or pyridinium chloride (PC, as a control). The π-A isotherms showed that addition of BAC or CPC yielded dose-dependent increases in surface pressure without compression, indicating that BAC and CPC expand the isotherm to larger areas at lower pressure. The collapse pressure diminished with increasing concentration of CPC. Topographic images indicated that BAC and CPC resulted in smaller condensed lipid domains compared to the control. Conversely, PC without hydrocarbon tail group, showed no cytotoxicity and did not change the isotherms and AFM images. These results indicate that BAC and CPC cause cell death via caspase-3-dependent apoptotic pathway in A549 cells and alter the alveolar surfactant activity. These effects can be attributed to the long alkyl chain of BAC and CPC.