Abstract. – OBJECTIVE: To identify themechanisms involved in paraquat (PQ)-inducedpulmonary injury.

MATERIALS AND METHODS: The mecha-nism of PQ-induced pulmonary injury was elu-cidated through both literature review and ex-perimentation.

RESULTS: As an effective herbicide, PQ hasbeen widely used in farmland and pasture, it hasa characteristic potent weeding effect. However,PQ is highly toxic to humans and there is nospecific medical treatment available. Paraquathas been used frequently by suicidal people toend their life; its mortality is > 90% in oral inges-tion cases. It has recently been recognized thatPQ causes respiratory failure and even deaththrough multiple organ failures, particularlythrough pulmonary fibrosis. However, the mech-anisms of PQ-induced pulmonary injury had notbeen clarified.

CONCLUSIONS: In this review, we systemati-cally elucidated the mechanism of PQ-inducedpulmonary injury, and concluded that PQ caus-es pulmonary injury through oxidative, alveolar,mitochondrial, lipid and metabolic enzyme dam-age, all of which can lead to death due to pul-monary fibrosis. With this in mind, we proposerecommendations for the treatment of PQ-in-duced pulmonary injury and provide theoreticalbases for subsequent treatment strategies.

Key Words:Paraquat, Pesticide, Mechanism of injury, Lung.

Introduction

Paraquat (PQ), also called Gramoxone andKewuzong, is the trade name for N, N’-dimethyl-4, 4’-bipyridinium dichloride. It is a water-solu-ble organic heterocyclic compound, and is a po-tent herbicide. It has been widely used in agricul-ture, as weed control agent for farmlands andpastures1. Even though no residual PQ is presentin crops and soil after spray and, therefore, PQ isnot considered toxic to humans via this route,

European Review for Medical and Pharmacological Sciences

Advances in the mechanism of paraquat-induced pulmonary injury

B. SUN1, Y.-G. CHEN2

1Department of Emergency, Binzhou Medical University Hospital, Binzhou, China2Department of Emergency, Qilu Hospital, Shandong University, Jinan, China

Corresponding Author: Yuguo Chen, MD; e-mail: chen_yuguo1@163.com 1597

there have been increasing incidences of PQ in-toxication. This is predominantly due to acciden-tal or intended ingestion by people. PQ is highlytoxic when ingested orally and there are no spe-cific medical treatments available; thus, the mor-tality in such cases can be as high as > 90%. Thelethal dose of PQ after ingestion is 1-3 g, accord-ing to a literature report2. PQ can cause multipleorgan injuries, especially pulmonary injury, sincePQ causes injury or even death to alveolar ep-ithelial cells. With alveolar cell destruction, fur-ther rupture of pulmonary capillaries leads to in-tra-alveolar hemorrhage and pulmonary infec-tion. Finally, fibrosis may ensue around pul-monary cells in the alveolar extra-cellular matrix,a condition known as “Paraquat Lung”3. Al-though the mechanisms of PQ-associated mortal-ity have been preliminarily elucidated, the mech-anisms of PQ-induced pulmonary injury have notbeen clarified. In this study, the mechanisms ofPQ-induced pulmonary injury are explained, acorresponding treatment is proposed, and theo-retical bases for subsequent treatment improve-ments are proposed.

Clinical and Pathological Manifestationsof Pulmonary Injury

Clinical Manifestations of PQ-induced Pulmonary Injury

Studies in PQ-poisoned patients found that inmild PQ intoxication cases, no significant pul-monary infiltration or pleural effusion is seen,but the pulmonary oxygen binding capacity isdecreased compared to the capacity in the normalpopulation4. However, in another study, it wasshown that in patients with moderate PQ intoxi-cation, chest stuffiness and shortness of breathoccur approximately 2 weeks after intoxication,and pulmonary dissection demonstrated signifi-cant lung swelling and pleural effusion5. In the

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same study, those patients with severe PQ intoxi-cation usually died within 1 week after intoxica-tion, and pulmonary dissection demonstrated se-vere emphysema and pleural effusion. Patientswith extremely severe PQ intoxication generallyhad dyspnea, shortness of breath and pulmonaryinfiltration within 24 h after intoxication, andpulmonary fibrosis would fast worsen and finallylead to death (Table I).

Pathological Manifestation of PQ-induced Pulmonary Injury

According to a report in the literature, PQ-caused damage to the body is predominantly af-fecting pulmonary structure and functions6. Earlypathological manifestations include pulmonaryedema of varying degrees in alveolar epithelialcells and increased levels of inflammatory fac-tors. Immediate pathological manifestation in-cluded emphysema of certain degree and medi-astinal emphysema in severe cases; while latepathological manifestations included pulmonaryfibrosis and pulmonary cyst formation. It was al-so reported that after PQ intoxication the alveolarsurface was found to be uneven as opposed to thenormally smooth one7. Pulmonary dissection ofmice administrated with different doses of PQdemonstrated many cytoplasmic vesicles in typeI alveolar epithelial cells earlier after intoxica-tion. These vesicles progressively burst and re-leased liquid content, which caused morphologi-cal changes of the alveolar cells. Numerousmacrophages usually accumulated around theseaberrant alveolar cells, the accumulation ofmacrophages, surrounding fibrin and protein par-ticles released from burst vesicles would evolveinto a lattice of fibroblasts proliferation8.

Mechanism of Pulmonary Injury

Oxidative DamagePQ can produce a large amount of reactive

oxygen species (ROS) through its interactionwith lung and other organs. These ROS can oxi-dize surrounding lipids and induce lipid peroxi-dation. Excessive ROS then consume reducingmolecules (e.g. Glutathione) and further causedamage to the lung and other organs9. Also, PQcan reach the lung through the circulation afteringestion, and accumulates in alveoli. Theamount of oxygen present in alveoli can inducethe production of PQ+ by reductases (e.g.NADPH), and lead to the generation of superox-ide anions (O2-) and PQ2+, O2- may be finallytransformed into hydrogen peroxide and hydrox-yl radicals with other pulmonary reductases andion (Fe2+)10. These strongly oxidative species canreadily obtain hydrogen atoms from alveolarlipids so as to result in alveolar cell injury.

Alveolar DamageAccording to the literature, PQ causes alveolar

damage through the following 3 pathways: pro-moting the demise of alveolar epithelial cells, de-stroying active molecules on alveolar surfaces,and destroying the alveolar structure11,12.

Promoting Death of AlveolarEpithelial Cells

Alveolar epithelial cells, especially type IIalveolar epithelial cells, play important roles inthe pulmonary injury induced by PQ: PQ cancause pulmonary fibrosis through aberration anddeath of type II alveolar epithelial cells13. Obser-vation of changes in alveolar epithelial cells at

B. Sun, Y.-G. Chen

Severity of Ingestion volume inintoxication ml of 25 mg/ml solution Main symptoms Main signs

Mild < 10 No significant symptoms No significant signs

Moderate 10-40 Dyspnea TachycardiaDizzinessGlossalgia Shortness of breathAnxiety Serum creatinine ↑Abdominal discomfort Oral mucosal necrosis

Severe > 40 Glossalgia Shortness of breathAnxiety Oral mucosal necrosisDyspnea TachycardiaComa JaundiceHiccup Serum creatinine ↑

Table I. The symptoms of acute PQ intoxication.

different times after exposure to PQ shows thatcell death occurs in type II alveolar epithelialcells 2 h after exposure to PQ, once the level ofROS in alveoli has significantly increased. Thissuggests that ROS may play 2 roles in the injuryof type II alveolar epithelial cells: (1) cause di-rect oxidative damage to alveoli, and (2) promoteapoptosis of alveolar cells through immune re-sponses induced by extracellular signaling mole-cules. The death of numerous alveolar epithelialcells leads to the accumulation of relevant mole-cules in alveoli and thus causes early pulmonaryfibrosis14.

Destroying Active Molecules onAlveolar Surfaces

Lipids on the alveolar surface decrease thesurface tension during alveolar contraction andexpansion, thus protecting the alveoli. Activemolecules (e.g. lipids) on the alveolar surfacecan act as the medium of inter-cellular exchangeof gasses15. The level of lipids (e.g. lecithin) onthe alveolar surface is altered during PQ intoxi-cation (the level is initially increased and thendecreases after removal of PQ). While the levelof lipids in the tissue fluid around alveoli is grad-ually increased, the decreased levels of lipids onalveolar surface lead to a decreased ability ofalveolar contraction and expansion. Persistentalveolar contraction or expansion readily de-stroys the alveolar structure, induces furtheralveolar swelling, and finally progresses into pul-monary fibrosis.

Destroying Alveolar StructureExamination of pulmonary tissue slices of PQ-

poisoned patients showed a continuum of alveo-lar damage and fibrosis, ranging from mild alve-olar damage and fibrosis in patients with mild in-toxication, to numerous cell deaths in alveoli andsevere pulmonary inflammation in severely in-toxicated patients16.

Mitochondrial DamageMitochondria are the major organelles provid-

ing energy; they are essential for normal cellulargrowth and development. It has been shown thatexposure to PQ may cause mitochondrial damageof varying degrees. PQ gets oxidized into super-oxide by reducing coenzymes after enteringcells. Superoxides may significantly damage mi-tochondrial structures, such as lipids, and causeirreversible mitochondrial damage. This nega-tively impacts the normal intracellular produc-

tion of energy, and further causes a series ofmetabolic disorders that lead to death of alveolarcells17.

Chromosomal DamageAlveolar cell death induced by PQ can occur

exclusively through irreversible chromosomaldamage18. Experiments demonstrate that DNAdamage of varying degrees or even DNA frag-mentation in alveolar cells and pulmonarymacrophages occurs after administration of PQ,leading to alveolar cell death10. Studies have re-ported that PQ-poisoned mice have severe DNAdamage in alveolar cells in comparison with nor-mal mice. Membrane transport proteins, immuneresponses and relevant signal pathways are af-fected to varying degrees, and the rate of DNAtranscription and translation can be significantlydecreased19. In contrast, the expression of apop-tosis-associated genes in alveolar cells, such asTRX, HO-1 and TGF-b1, is significantly up-reg-ulated with prolonged intoxication (Figure 1),and this indicates that PQ could induce pro-grammed cell death not only through damagingnormal intracellular DNA but also activating cel-lular apoptosis pathways.

Lipid DamageStudies suggest that PQ is present in the

body in the form of a strong oxidizing agent, itcan not only be reduced into a superoxide byreducing coenzymes in alveoli, but it can alsoact as an electron recipient to obtain electronsfrom superficial and intracellular lipids, andthus cause oxidation in alveolar cells20. Thelevel of lipid peroxide is significantly increasedin mice administered with PQ, and the level oflipids (lecithin) in alveoli rapidly decreases. Inexperiments, mice administered with vitaminE, or selenium-deficient mice, were easier topoison in comparison with normal mice, whilethose mice administered with reducing food(e.g. Vitamin C) were more difficult to poison.Therefore, lipid damage is associated with pul-monary injury induced by PQ.

Fibroblast ProliferationStudies have shown fibroblasts can be found

around alveolar cells in the lung, but are pre-dominantly localized around muscle cells. Theproliferation of fibroblasts in vivo is strictly reg-ulated; thus, the amount of fibroblasts is main-tained at a certain level20. However, dissectionof PQ-poisoned patients demonstrates massive

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proliferation of fibroblasts in the lung, which isnot seen in the lung of normal human subjects.Others found that those fibroblasts which prolif-erated massively in alveoli, are derived from a-smooth muscle cells21. As shown in Figure 2,pulmonary dissection and staining of PQ-poi-soned mice at different times demonstrates in-creasing degrees of pulmonary fibrosis overtime. This indicates that PA can damage pul-monary structures and functions by inducingpulmonary fibrosis. However, the specificmechanism of pulmonary fibrosis induced byPQ has not been elucidated and needs furtherinvestigation.

Treatment

Treatment With AntioxidantsAs previously stated, the main mechanism of

pulmonary injury induced by PQ is by its reduc-tion into a superoxide anion and hydroxyl radicalof strong oxidative ability by relevant reductasesafter entering into the body. These strong oxidiz-ing agents damage internal and superficial lipids,as well as signal transduction molecules in alve-oli. Therefore, oral administration of vitamin C,vitamin E, other strong reducing agents and an-tioxidants should be able to treat the oxidativedamage in the lungs induced by PQ.

B. Sun, Y.-G. Chen

Figure 1. Time course of the levels of TRX, HO-1 and TGF-b1.

Control Day 7 Day 14 Day 21

Rel

ativ

e le

vel o

f p

rote

ins

Control Day 7 Day 14 Day 21

Figure 1. Pulmonary fibrosis in mouse administrated with PQ.

Treatment With BlockersIn this review of the mechanisms of PQ-in-

duced pulmonary injury, we found that intracel-lular cytokines and intercellular signal transduc-tion molecules may play important roles in thepathogenesis of pulmonary fibrosis. As pul-monary fibrosis is the main reason for mortalitycaused by PQ, certain doses of these cytokinesand signaling blockers could by administrated totreat PQ intoxication. Pirfenidone has beenproved to be a highly effective cytokine blocker.Pharmacological experiments have confirmedpirfenidone as a good blocker for tumor necrosisfactor and connective tissue growth factor21.

Conclusions

This review explored the mechanisms of PQ-induced pulmonary injury and validated themechanisms through a series of experiments. Themechanisms of PQ-induced pulmonary injuryhad not been clarified until now, and there wasno specific treatment proposed. Therefore, an in-depth investigation of this mechanism was veryimportant to define the theoretical basis and de-velop a relevant treatment approach.

––––––––––––––––––––AcknowledgementsThis study was supported by the National Natural ScienceFoundation of China (81170136, 81100147, 81300103,81300219), the Taishan Scholar Program of ShandongProvince (ts20130911), the Specialized Research Fund for theDoctoral Program of Higher Education (20130131110048),and the Key Technology Research and Development Programof Science and Technology of Shandong Province(2014kjhm0102).

–––––––––––––––––-––––Conflict of InterestThe Authors declare that there are no conflicts of interest.

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