Particulates and the proteome: How airborne particles affect the lungs

Dusty lungs

If you live in an urban environment and leave your windows open, you will undoubtedly see a fine layer of dark dust begin to settle inside your house. This black material is known as airborne particulate matter and it comes in a variety of sizes but it is the smaller stuff that we should be worried about. Particulates with a diameter of less than 2.5 µm (PM2.5) are known to be associated with a number of health problems.

These relatively small particles have been positively associated with lung disease due to the ease with which they can enter the respiratory tract. They are also thought to initiate oxidative damage in the lungs which can lead to lung cancer, as well as contributing to cell death, known as apoptosis, and other diseases.

The composition of the particulates varies geographically, depending upon the location of nearby roads, industrial sites, construction sites and the like. However, they are generally composed of trace metals, carbon, sulphate, nitrate and ammonium and a number of organic pollutants such as the polycyclic aromatic hydrocarbons (PAHs).

Despite the acknowledged link between PM2.5 and disease, little is known about how the particles affect the body. This has just been remedied to a certain extent by a team of Chinese scientists who have used toxicoproteomics to examine how the particles affect the levels of proteins. Heqing Shen, Jie Zhang and colleagues from the Institute of Urban Environment, Chinese Academy of Sciences, Xiamen, chose to study the effects of PM2.5 on human lung epithelial cells (A549), which line the lungs and protect them. Similar work has been carried out on larger particles, but proteomics studies on PM2.5 have been largely ignored apart from a few isolated studies.

Proteins affected by particulates

The PM2.5 were collected from the rooftop of their institute, about 15 m from the ground. The site was located in a suburban area containing schools, highways, buildings under construction and a sea bay. The filters in the air samplers were removed and mixed with water to create the water-soluble fraction for further investigation.

Before incubation, the particles were characterised by inductively coupled plasma mass spectrometry and ion chromatography for the inorganic components and by gas chromatography for the principal organic components, expected to be PAHs. This confirmed the absence of PAHs and the presence of 28 elements and 5 ions, suggesting that inorganic species are the key toxic components of the particulates.

Metals like calcium, sodium, aluminium, potassium and iron were most abundant but there were also significant concentrations of heavy metals like zinc, manganese, arsenic, chromium and copper. Ammonium, fluoride, chloride, nitrate and sulphate ions were also detected.

The PM2.5 extract was mixed with cultures of A549 cells for up to 72 hours before the proteins were extracted from these and control cells and subjected to conventional 2D differential gel electrophoresis. After imaging the gels, the protein spots with significant abundance variations between control and PM2.5 cells were extracted for mass spectrometric identification. A total of 27 proteins spots had altered abundances, 12 being up-regulated and 15 down-regulated. After identification, they were subjected to network analysis to see which biochemical processes were implicated.

Oxidative stress and the rest

The protein functions affected by exposure to PM2.5 were oxidative stress, energy metabolism, signal transduction, protein synthesis and degradation and transcriptional regulation. Proteins involved with the mitochondrial structure and cytoskeletal system were also prominent. The involvement of oxidative stress was not unexpected. It was implicated by the modified regulation of several proteins, including glutathione-S-transferase P1, endoplasmic reticulum oxidoreductin 1-like protein and prohibitin, all of which are associated with cellular oxidation.

The metabolism in A549 cells was also affected by the presence of PM2.5, exemplified by the up-regulation of an aldehyde dehydrogenase and a decrease in another protein which is generally involved in the regulation of energy metabolism and balance. Three proteins involved in signal transduction and others involved in protein synthesis and degradation, as well as the other cell processes, were also found to have altered abundances, demonstrating their involvement in the response to invasion by PM2.5. One of the principal mechanisms of PM2.5 toxicity was confirmed to be apoptosis, illustrated by a flow cytometric analysis and a study of gene expression levels, which found that three genes involved with apoptosis were increased under particulate stress.

Clearly, the response of lung epithelial cells to small particulates is complex, involving a range of different cellular processes as well as cell death. The researchers think that it is likely that the co-actions of the metal components are responsible, given their dominance in the particulates.

As well as shedding light on the way that cells react when contaminated with PM2.5, the results could be used to find a set of biomarkers which signal that someone has been subjected to exposure to PM2.5 particles.