Exploring the Molecular Mechanisms of Action of Samoan Medicinal Plants via Chemical Genetic Analyses

Natural products are a robust source of drug leads, and medicinal plants have been the source of natural products that are important pharmaceuticals in modern medicine. Samoan medicinal plants have been investigated in the past, but their potential as a source of new drug leads has not been realized. I hypothesized that determining the mechanism of action of Samoan medicinal plant extracts would provide insight into their pharmaceutical potential. The work described herein was carried out on 11 Samoan medicinal plants, from which 22 extracts were prepared. A bioactivity rate of 68% was determined when 15 of the 22 extracts inhibited the growth of yeast (Saccharomyces cerevisiae). The medicinal plant Psychotria insularum was the most potent, thus genome-wide analyses were completed using the haploid deletion mutant library of S. cerevisiae. Yeast strains deficient in iron transport were hypersensitive to the P. insularum aqueous extract. Further investigations showed that exogenous iron supplementation rescued the growth defect induced by P. insularum extracts, suggesting that P. insularum reduced intracellular iron. Fittingly, yeast cells treated with P. insularum extracts contained less intracellular iron than control cells. Paraxodically, the expression levels of iron transporter proteins were upregulated upon extract treatment. When we investigated iron-requiring cellular processes, we found that yeast cells treated with P. insularum extracts exhibited a respiratory deficient phenotype and reduced heme synthesis, indicative of an impaired cellular iron status. These findings suggested that P. insularum reduced bioavailable iron leading to the induction of the low iron response, and indeed the extracts of P. insularum were shown to chelate iron via the iron-chelating CAS assay. To translate results from yeast to mammalian cells, we treated primary murine macrophages with P. insularum extracts and detected an anti-inflammatory response, which we found to correlate with reduced activity of the iron-requiring aconitase enzyme. We further determined using pooled diploid mutant genetic analyses that the extracts of P. insularum did not have a genetic target. To identify the compound mediating the iron chelation mechanism, bioassay-guided isolation was conducted. Fractionation of the crude aqueous extract of P. insularum produced a relatively pure fraction that NMR and the acid-butanol assay identified as a condensed tannin. Together, these results indicate a relationship between iron chelation, a condensed tannin and inflammation. Further, we established an iron chelation mechanism of action by which P. insularum extracts are used to treat inflammation-associated symptoms in traditional Samoan medicine. Lastly, the findings presented here substantiate the reliability of plants with ethnobotanical background as sources for bioactive natural products