Abstract
Eucalyptus oil (EO) used in traditional medicine continues to prove useful for aroma therapy in respiratory ailments; however, there is a paucity of information on its mechanism of action and active components. In this direction, we investigated EO and its dominant constituent 1,8–cineole (eucalyptol) using the murine lung alveolar macrophage (AM) cell line MH-S. In an LPS-induced AM inflammation model, pre-treatment with EO significantly reduced (P ≤0.01or 0.05) the pro-inflammatory mediators TNF-α, IL-1 (α and β), and NO, albeit at a variable rate and extent; 1,8-cineole diminished IL-1 and IL-6. In a mycobacterial-infection AM model, EO pre-treatment or post-treatment significantly enhanced (P ≤0.01) the phagocytic activity and pathogen clearance. 1,8-cineole also significantly enhanced the pathogen clearance though the phagocytic activity was not significantly altered. EO or 1,8-cineole pre-treatment attenuated LPS-induced inflammatory signaling pathways at various levels accompanied by diminished inflammatory response. Among the pattern recognition receptors (PRRs) involved in LPS signaling, the TREM pathway surface receptor (TREM-1) was significantly downregulated. Importantly, the pre-treatments significantly downregulated (P ≤0.01) the intracellular PRR receptor NLRP3 of the inflammasome, which is consistent with the decrease in IL-1β secretion. Of the shared downstream signaling cascade for these PRR pathways, there was significant attenuation of phosphorylation of the transcription factor NF-κB and p38 (but increased phosphorylation of the other two MAP kinases, ERK1/2 and JNK1/2). 1,8-cineole showed a similar general trend except for an opposite effect on NF-κB and JNK1/2. In this context, either pre-treatment caused a significant downregulation of MKP-1 phosphatase, a negative regulator of MAPKs. Collectively, our results demonstrate that the anti-inflammatory activity of EO and 1,8-cineole is modulated via selective downregulation of the PRR pathways, including PRR receptors (TREM-1 and NLRP3) and common downstream signaling cascade partners (NF-κB, MAPKs, MKP-1). To our knowledge, this is the first report on the modulatory role of TREM-1 and NLRP3 inflammasome pathways and the MAPK negative regulator MKP-1 in context of the anti-inflammatory potential of EO and its constituent 1,8-cineole.
Highlights
The growing prevalence of infection-associated and other inflammatory conditions and diseases in modern healthcare necessitates novel therapeutic interventions
The results showed that eucalyptus oil (EO) and 1,8 cineole do not adversely affect the viability of lung macrophages (Fig 1) up to a reasonably high concentration (0.05%); the 0.02% concentration appeared to be the safe level to use and was employed in all subsequent experiments
This pro-inflammatory response was significantly (P 0.01 or P 0.05) reduced when the cells were pre-treated with EO (0.02%) for 3 hours before the LPS challenge, the effect varied with the inflammatory mediator and the time-point of incubation (Fig 2)
Summary
The growing prevalence of infection-associated and other inflammatory conditions and diseases in modern healthcare necessitates novel therapeutic interventions. Synthetic steroidal and non-steroidal anti-inflammatories and antibiotics commonly used for treating inflammation and infection conditions often result in undesirable side effects and health consequences [1]. This has evoked renewed global interest for alternative safe therapeutics from natural sources. Natural products continue to inspire the design of novel therapeutics for various disease conditions; little is known regarding their mode of action and active components. One of the important medicinal plant products is eucalyptus oil (EO) derived from Eucalyptus globulus belonging to the family Myrtaceae, of which several species are found throughout the world [5]. Independent studies have reported analgesic and anti-inflammatory properties of EO and its major component 1, 8- cineole, which accounts for up to more than 70% of eucalyptus oil’s content depending on the source species of Eucalyptus [8,9,10,11]
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