The Australian Aboriginal people are known to have occupied the country more than 50,000 years ago, and currently, they constitute 3.3% of the total Australian population
[5][6]. Aboriginal people have developed a profound connection with their native flora and fauna. Their longstanding survival could have resulted from the prolonged use of medicinal plants in their diet and home remedies
[7], and they still use medicinal plants in their day-to-day life. However, as they have already merged with mainstream modern society, it has become crucial to properly document their vast indigenous knowledge for their future generation
[8]. The Northern Territory government, in collaboration with the Commonwealth in the 1980s, compiled “Traditional Bush Medicines,” an Aboriginal pharmacopeia of the Northern Territory
[9], a first-ever initiative to record dying Aboriginal medicinal lore. Since then, more collaborations have occurred between Aboriginal communities and scientists from various universities across Australia to explore Aboriginal medicinal plants
[10][11]. Exploring indigenous food and medicinal plants may give a wealth of potential candidates for novel therapeutics, and Australian native plants could be an intriguing source. The geographic isolation of Australia from Gondwana and other parts of the world for over 65 million years
[12][13] has become home to unique and complex flora, where approximately 85% of its vascular plants are endemic species
[10]. Plants growing in the tropics produce more phenolics, flavonoids, and terpenoids during adaptation to its extreme vegetative and climatic conditions
[14]. Phenolics and flavonoids are antioxidative and anti-inflammatory
[15][16][17], and thus tropical plants may yield novel drug leads for treating infectious and non-infectious diseases, including chronic inflammatory conditions
[18]. More than 900 medicinal plant species have been recorded in the Tropical region of Australia (shaded green in
Figure 1)
[19].
2. Ethnomedical Uses of Selected Medicinal Plants
Out of 78 tropical medicinal plants used by the Aboriginal people of Australia for treating inflammation and inflammatory-related diseases, 45 species were selected (Table 1).
Table 1. Ethnomedical uses and the compounds isolated from Aboriginal tropical medicinal plants of Australia.
Abbreviation: NA, not available.
3. Overview of the Anti-Inflammatory Mechanism of Action/Pathways
Bioactive crude extracts or isolated compounds from medicinal plants used by Aboriginal people attenuate inflammatory conditions through three major mechanisms, namely, (i) inhibition of nuclear factor kappa B (NF-κB) pathway, (ii) inhibition of iNOS (inducible nitric oxide synthase), and cyclooxygenase (COX-1 and COX-2) protein expression/activity, and (iii) inhibiting or downregulating NO production and various pro-inflammatory cytokines (such as interleukins, IL-1β, IL-6, interferon, IFN-γ, and tumour necrosis factor, TNF-α) or upregulating anti-inflammatory cytokines (such as IL-10 and transforming growth factor, TGF-β).
Briefly, NF-κB is a key to inducing gene expression of many pro-inflammatory cytokines (including IL1b, IL-6, IL-12p40, TNF-α, and COX-2) in innate and adaptive immune cells
[230]. NF-κB proteins consist of five members, including NF-κB1 (or p50), NF-κB2 (or p52), ReIA (or p65), ReIB, and c-ReI, and NF-κB activation occurs in two signalling pathways, canonical and alternative non-canonical pathways
[231], where both pathways are involved in regulating immune and inflammatory responses. Innate immune cells (e.g., macrophages, dendritic cells, and neutrophils) are central to innate immunity and inflammation. Innate immune cells have PRRs (pattern recognition receptors) that can detect microbial products, including pathogen-associated molecular patterns (PAMPS) and also damage-associated molecular patterns (DAMPS)—released by damaged cells and tissues
[232]. Five types of PRRs expressed by mammalian cells are toll-like receptors (TLRs), RIG-I-like receptors, NOD-like receptors (NLRs), C-type lectin-like receptors, and cytosolic DNA sensors
[231], each with distinct structures to bind with different PAMPs and DAMPs. All PRRs share a similar signalling pathway by activating the canonical NF-κB pathway, and this pathway either directly induces inflammation by expressing pro-inflammatory cytokines and chemokines or indirectly promoting inflammatory T cells differentiation
[233]. In response to different PAMPs and DAMPs, macrophages are activated and subsequently differentiate into M1 or M2, followed by the secretion of an array of cytokines and chemokines
[230]. Pro-inflammatory cytokines such as IL-1, IL-6, IL-12, and TNF-α are characteristic of M1 macrophages. TLR signals play a vital role in regulating macrophage polarization, and for instance, TLR4 ligand lipopolysaccharide (LPS) promotes the formation of M1 phenotypes
[230][234]. LPS is a bacterial-derived lipopolysaccharide that can induce inflammation in macrophage cells in in vitro assays. Activated M1 macrophages can also promote other inflammatory T cells, including Th1 and Th17 cells, and these cells, in turn, also mediate inflammation.
Inhibiting NO production is another anti-inflammatory mechanism shown by many compounds isolated from selected medicinal plants. In mammalian cells, NO is mainly produced from the
l-arginine:NO metabolic pathway by the enzyme called nitric oxide synthase (NOS), which has three isoforms of NOS—eNOS (endothelial NOS), nNOS (neuronal NOS), and iNOS
[235]. eNOS and nNOS produce a controlled amount of NO in endothelial cells and neurons, respectively, under the Ca
2+/calmodulin system
[236], while iNOS produces NO only upon activation by specific cytokines (e.g., IFN-γ) or microbial products (e.g., LPS). Sustained NO production enhances the formation of reactive nitrogen oxide species (RNOs), and prolonged exposure to such free radicals is harmful to healthy body cells
[237], whereby iNOS must be regulated.
4. Phytochemistry and Pharmacology of Medicinal Plants
4.1. Anti-Inflammatory Crude Extracts
Out of 45 anti-inflammatory medicinal plants included here, crude extracts from 30 species were already tested for anti-inflammatory activities in both in vitro and in vivo assays (Table 1). Pure compounds from 15 species have also been isolated and tested for their anti-inflammatory activities to validate their ethnopharmacological uses. Among 30 species, crude extracts of Acalypha wilkesiana, Brucea javanica, Centipeda minima, Euphorbia hirta, Melaleuca leucadendra, and Terminalia catappa are most widely studied against different inflammatory conditions. Most of the studies on crude extracts have shown that they inhibit NO, PGE2, iNOS productions, and COX-2 expression in murine macrophage cells (e.g., RAW 264.7 cells) stimulated with bacterial LPS. Moreover, they also inhibit the production of pro-inflammatory cytokines, mainly TNF-α, IL-1β, and IL-6.
For example, when LPS-induced RAW 264.7 cells were treated with an aqueous crude extract of
C. minima, there was a significant decrease in NO production at a 100 μg/mL concentration and also reduced inflammatory cytokines levels (TNF-α and IL-1β) significantly
[238]. Moreover, the aqueous extract also inhibited the expression of iNOS and COX-2 proteins by 80.2% and 71.2%, respectively, when incubated with LPS-activated RAW 264.7 cells for 24 h. The extract also significantly inhibits the expression of iNOS and COX-2 proteins in carrageenin-induced mice paw oedema
[238]. Chan et al.
[239] also observed a significant decrease in the expression of monocyte chemokine attractants, particularly CCL8, in LPS-stimulated RAW 264.7 cells by the crude extract of
C. minima, which could have contributed to the inhibition of monocyte chemotaxis and macrophage infiltration in DSS (dextran sodium sulphate)-induced acute colitis in C57BL/6J mice. The crude extract also inhibits the LPS-induced production of TNF-α and IL-1β, protecting HT22 (immortalized mouse hippocampal cell line) neuronal cells from inflammatory damage
[240]. Crude extracts from
A. wilkesiana,
E. hirta, and
M. leucadendra also showed a similar anti-inflammatory activity.
When Huang et al. (2017) studied the effect of oil emulsion from
B. javanica in DSS (Dextran Sodium Sulphate)-induced acute colitis mouse model (0.5, 1, and 2 g/kg), oil emulsion improved disease activity index, including colon length, and body weight
[241]. Additionally, when they analysed cytokines production,
B. javanica oil emulsion at higher concentrations (1 and 2 g/kg) has significantly (
p < 0.01) lowered the levels of six inflammatory cytokines (IL-1β, IL-6, IL-8, IL-17, IFN-γ, and TNF-α) in the colon tissues when compared to positive controls (sulfasalazine and azathioprine)
[241]. Crude extracts from
T. catappa bark, which Aboriginal people use to treat a sore throat, were anti-inflammatory. When Daram et al. (2021) compared ethanol and water extracts from
T. catappa bark, ethanol extract was better in inhibiting 50% of protein denaturation in the in vitro egg-albumin denaturation assay, and diclofenac was used as a positive control
[242]. Both extracts reduced carrageenan-induced paw oedema at a 500 mg/kg concentration in the rat. Leaves ethanol extract and chloroform fraction from
T. catappa were tested in chronic and acute models of 12-
O-tetradecanoylphorbol-13-acetate (TPA)-induced ear oedema. When crude extracts were applied topically at a 1 mg/kg concentration twice daily for four days, the chloroform fraction reduced oedema by 60% compared to ethanol extract (32%)
[222]. In the same study, chloroform fraction yielded two pure compounds showing anti-inflammatory activities, discussed in the subsequent sections.
4.2. Anti-Inflammatory Compounds
When 45 selected Aboriginal medicinal plants were reviewed for their phytochemical compositions and pharmacological properties, 40 species were studied for their phytochemistry. For rest of the five species, only crude extracts were studied. When compounds isolated from 40 Aboriginal medicinal plants (Table 1) were further reviewed, 83 compounds have shown various anti-inflammatory activities in in vitro cellular and in vivo animal models. Out of 83 anti-inflammatory compounds, majority were terpenes and terpenoids (30 compounds), followed by flavonoids (16 compounds), coumarins (10 compounds), alkaloids (6 compounds), glycosides, sterols, lignans, and carboxylic acids (3 compounds each). The rest of the compounds were phenolics, aldehydes, tannins (2 compounds each), pyrans, phenylpropanoids, and fatty acid esters (one compound each).