Toxicology of the Lung: Environmental and Occupational Hazards

I was really distracted during this lecture for some reason. Hopefully I can write about it though :P

Be able to identify the major anatomical zones of the human lung and describe the prevailing cell types within each region.

The major anatomical zones for our purposes here are the upper respiratory tract (also known as the extrathoracic region), the lower respiratory tract (bronchial region) and alveolar region.

There are over 40 cell types in the lung, but thankfully we don't need to know about all of them. I'll just provide a quick overview of some of the cells that you will need to know.

The epithelium of the trachea and bronchi have mainly ciliated cells (which push mucus around), as well as "Clara-like cells" (a.k.a. bronchiolar secretoglobin cells (BSCs)). The latter cells express CYP450, which, as you should know from last year, metabolise a lot of drugs.

Small bronchi and bronchioles express the same cells, though Clara cells are more prevalent than ciliated cells in this region.

Alveoli have two main cell types: AEC I (alveolar epithelial type I cells) and AEC II. AEC I are thin, squamous looking cells that allow for gas exchange. AEC II are secretory cells that produce surfactants to lubricate the lung.

As mentioned above, Clara cells have CYP450. AEC II cells also have high levels of this enzyme. Some isoforms, such as CYP1A1, may be higher in smokers. CYP levels in the lung are, as you have probably guessed, nowhere near as high as in the liver (lung CYP is only around 10-30% of that in the liver). There are also plenty of other enzymes that can metabolise drugs.

Identify several factors that influence chemically‐induced pulmonary injury, including the role of metabolism, timing and duration of exposure, and the route of entry.

• Metabolism: As mentioned in an earlier post, some toxic compounds require bioactivation before producing their toxic effects, whereas others are dangerous right from the start.
• Route of entry: Some toxic compounds enter directly into the lung via inhalation. Others enter the body in different areas but end up affecting the lung anyway.
• Timing: Toxicity can be acute or chronic.
• Duration: Self-explanatory really...

One of the difficulties with toxicology is trying to figure out how much a person has been exposed to. In fact, there's a whole field called dosimetry which attempts to figure out ways to determine how much exposure an individual has had.

Identify chlorine and acrolein as major direct‐acting pulmonary toxicants, describing the main features of the oedematous acute lung injury accompanying intoxication with each agent, as well as basic aspects underlying their toxic actions.

Direct-acting pulmonary toxicants may cause damage by being electrophilic or by possessing extreme physicochemical properties. The two direct-acting toxicants covered in this lecture are chlorine (known for its role in WWI) and acrolein (mentioned earlier as a constituent of tobacco smoke).

Chlorine

Chlorine is a yellow-green gas that was used as a chemical weapon in World War I. Once inhaled, it can dissolve to form HCl and HOCl, which you should recognise as being bleaches. Neutrophils and macrophages are also activated, and neutrophil myeloperoxidase can produce even more HOCl. Furthermore, iNOS (inducible nitric oxide synthase) can produce NO, which can go on to form peroxynitrite (ONOO^-, which looks like "oh nooooo"). All of this can cause oedema, inflammation and so forth. The amount of gunk that gets generated in this process also clogs up the airways.

Throughout this process, reactive nitrogen species can react with tyrosine to form 3NT (3-nitrotyrosine). 3-NT can also serve as a marker of damage by reactive nitrogen species.

Acrolein

Acrolein, formed during the incomplete combustion of organic matter (including but not limited to tobacco, wood, plastics, polymers etc.), is highly electrophilic. This allows it to attack DNA, proteins and so on. Acrolein, unlike chlorine, has not been effectively used as a biological weapon due to its tendency to form polyacrolein chains which do not have these properties.

Acrolein is involved in a condition called Smoke Inhalation Injury (SII), which is, well, injury due to inhaling smoke. SII plays a major role in the morbidity and mortality of fire victims as it can cause pulmonary oedema. Interestingly enough, smoke produced from substances with low yields of acrolein tends to be less oedematogenic than smoke with higher yields of acrolein.

The events that take place in SII may be similar to those that take place in Acute Lung Injury (ALI). These events include activation of neutrophils and macrophages, which may increase production of free radicals and proteases. Additionally, blood vessels may become leakier due to damage, resulting in more fluid leaking out of the vessels, causing oedema.

Why is acrolein so bad? Acrolein is very reactive with many amino acids, allowing it to form a range of protein adducts (it's estimated that around 769 proteins can be adducted by acrolein). Many of these adducts contain a carbonyl group, allowing them to be trapped by certain reagents, such as biotin hydrazide. Acrolein adducts bound to biotin hydrazide can then be purified by using a column containing avidin, which binds to and traps biotin hydrazide. All of these trapped proteins can then be examined by using other techniques like agarose gel electrophoresis and mass spectometry.

Identify paraquat as an example of a major metabolism‐dependent pulmonary toxicant, showing appreciation for the factors underlying the vulnerability of the lung to this toxicant, and the role of free radicals in the onset of tissue injury.

Paraquat (PQ) is a herbicide that, if ingested, can injure the lung over 1-2 weeks by causing oedema, lesions and other fun things. It is metabolism-dependent, meaning that it needs to be metabolised before it can wreak havoc. PQ is metabolised in the lung (pulmonary bioactivation). (There is also extrapulmonary bioactivation, where a substance is bioactivated somewhere else. Cyclophosphamide is an example of one of these.)

If you manage to survive PQ poisoning, you're still not all clear: survivors of PQ poisoning are prone to Parkinson's disease. That's because paraquat is somewhat similar to MPTP which, as explained here, is toxic to dopaminergic neurons.

PQ readily builds up in the lungs because they can be taken up by polyamine transporters. Polyamines, as the name suggests, have multiple Ns in them. They play a role in a wide variety of events in the cell, from cell migration, mRNA stabilisation, chromatin function and so on. The distance between the Ns of paraquat is similar to the distance between the Ns of naturally-occurring polyamines, such as putrescine, spermidine and spermine, which makes it easy for the transporter to take up PQ.

So how does PQ do damage? When it is metabolised, it can form PQ free radicals. These PQ radicals can donate electrons to oxygen in order to form superoxide, which may combine with nitric oxide radicals to form ONOO^-, or peroxynitrite (yes, it's the oh no one again).

There are two main stages of PQ injury. In the first acute phase, or "destructive phase," AECs show swelling and disruption of organelles. In the second phase, or "proliferative phase," the alveolar space is filled with mononuclear profibroblasts, which become fibroblasts, leading to lung fibrosis.