Abstract

Background: In Samoan traditional medicine, leaf homogenates of Psychotria insularum are used as an anti-inflammatory remedy for fever, body pain, swellings, wounds and skin and respiratory infections, but the bioactive constituents and underlying mechanism have remained unknown.

Results: Using integrated multi-omics analyses, the study showed that the P. insularum homogenate mediates its bioactivity through extracellular iron chelation, and used this mechanism to guide bioactivity-directed fractionation, isolating the iron-chelating constituents rutin and nicotiflorin. Immunological assays showed that the homogenate and its active constituent rutin are as effective as the anti-inflammatory drug ibuprofen, supporting its use in Samoan traditional medicine.

Conclusion: The study identified a mechanism supporting the anti-inflammatory use of the medicine as well as other potential applications of P. insularum homogenate not reported in traditional use. The synergy of combined approaches provides a molecular perspective on under-studied Samoan traditional medicine and develops methods that advance ethnobotany.

About the authors

Seeseei Molimau-Samasoni is a senior researcher at the Scientific Research Organisation of Samoa (SROS), working on medicinal uses of native and endemic Samoan plants. The study began in 2013 as part of her PhD at Victoria University of Wellington under the supervision of Dr Andrew Munkacsi.

Background

Natural compounds are reliable lead templates for new drugs with great development potential, and traditional medicines are an attractive yet challenging source for future research. Samoan traditional medicine frequently uses leaf homogenates of Psychotria insularum A.Gray (Rubiaceae), known locally as “matalafi”, to treat inflammation associated with fever, body pain, swellings, wounds, skin and respiratory infections and abdominal discomfort. However, the bioactive constituents and mechanism of action of the homogenate had not been determined.

Results

Figure 1. Chemical-genomic analysis of P. insularum homogenate

Figure 2. Expression levels of iron transporters are highly sensitive to P. insularum homogenate

To understand how the homogenate mediates its bioactivity, the authors prepared leaf homogenates together with Samoan traditional healers and analysed them by competitive haploinsufficiency profiling, GO enrichment, metal-rescue assays, iron-transporter expression and intracellular iron measurements. The results showed that the homogenate lowers intracellular iron through extracellular iron chelation, thereby mediating its bioactivity (Figures 1–3).

Figure 3. Treatment with P. insularum homogenate affects iron homeostasis

To identify the active constituents, the authors guided fractionation by testing the effects of sub-fractions and iron supplementation on yeast growth, and analysed the purified compounds by NMR. Two flavonol glycosides, rutin and nicotiflorin, were isolated; metabolomics showed that both chelate iron and contribute to the iron-chelation mechanism of the homogenate (Figures 4–5).

Figure 4. Bioactivity-guided fractionation of P. insularum homogenate

Figure 5. Native-spray metal metabolomics networks identify the iron-binding activity of rutin and nicotiflorin

Iron chelation is involved in many bioactive processes, including antioxidant, antibacterial and anti-inflammatory activity. To test whether the homogenate reduces inflammation, the authors compared the effects of different concentrations of the homogenate, its active constituent rutin, the extracellular iron chelator BPS and the anti-inflammatory drug ibuprofen on mediators of innate and adaptive immunity. The homogenate and rutin decreased pro-inflammatory and fever-inducing pathways while increasing anti-inflammatory pathways, supporting the traditional use (Figures 6–7).

Figure 6. Anti-inflammatory activity of P. insularum homogenate on activated mouse immune cells

Figure 7. Anti-inflammatory activity of P. insularum homogenate on activated mouse macrophages

Conclusion

By combining chemical genomics, analytical chemistry, metabolomics and immunology, the study revealed the extracellular iron-chelation mechanism of P. insularum homogenate, linked it to the traditional use of the medicine and identified its bioactive principles. As inflammation and iron chelation underlie many diseases, including cancer, neurodegenerative and cardiovascular diseases and diabetes, the results raise the possibility of applications beyond Samoan traditional medicine. Combining indigenous researcher–community engagement with interdisciplinary approaches, the study provides a molecular lens on under-studied Samoan medicine and develops methods that advance ethnobotany.

Original article: https://www.pnas.org/doi/10.1073/pnas.2100880118

Discussion

1. Spatial, temporal and varietal differences all affect the content and composition of flavonoids in plants. The plant material in this study came from cultivated plants in Samoa; to develop the homogenate further, wild and cultivated P. insularum from Samoa and wild P. insularum from other South Pacific islands should be collected, and natural variation in the plant's chemistry analysed.

2. Further development of the homogenate also requires an understanding of its pharmacodynamics.

3. Iron chelators can alter the microbiota, to which immune cells respond, so oral P. insularum homogenate may plausibly alter the gut microbiota, which could mediate additional effects.

4. Pathological cell death associated with many diseases — inflammation, neurodegeneration, periventricular leukomalacia and carcinogenesis — shows features of ferroptosis, and modulating ferroptosis by iron chelation may offer potential therapies. Since flavonoids are major constituents of many traditional medicinal extracts, it would be interesting to determine whether antioxidant and iron-chelating mechanisms mediating anti-ferroptotic activity also apply to other traditional medicines.