Traditional Chinese medicine has been used with beneficial effects on cancer for thousands of years and in more recent times has emerged in western medicine as a field in its own right. Herbal medicine-derived phytochemicals include curcumin, resveratrol and berberine.
Thus it would be no surprise if I revealed this month’s focus of work, from Joseph Tintelnot, Nicola Gagliani and colleagues from the University of Hamburg and other centres in Germany together with the New York University Grossman School of Medicine, to be about novel plant-derived chemicals that have anti-cancer effects.
But it isn’t.
Their study focuses on pancreatic ductal adenocarcinoma (PDAC), a disease predicted to be the second most deadly cancer by 2040. This is because when first detected it has a high incidence of metastatic disease, the result being that fewer than half of all patients respond to the primary treatment — chemotherapy — and the 5-year survival rate remains at about 6% (see Speed Dating Drugs for latest on development of drugs for PDAC).
What Tintelnot et alia did was to use shotgun metagenomic sequencing (i.e. sequenced all genes in all the organisms in the sample, including microbes) and metabolomic screening to show that a metabolite of the amino acid tryptophan, called indole-3-acetic acid (3-IAA), is present at higher levels in patients who respond to chemotherapy treatment. [By way of background, the standard treatment for metastatic PDAC is polychemotherapy, either with 5-fluorouracil, irinotecan and oxaliplatin in combination with folinic acid (known as FOLFIRINOX), or with gemcitabine and nab-paclitaxel (nab-paclitaxel is an albumin-bound nanoparticle formulation of paclitaxel)].
Gut bacteria boost chemotherapy. (a) Some gut bacteria use the amino acid tryptophan to produce 3-IAA. (b) 3-IAA is taken up by neutrophils (a type of white blood cell) and oxidized by the enzyme myeloperoxidase into toxic molecules. These enter cancer cells in which they increase the level of reactive oxygen species (ROS). This inhibits a cellular degradation process called autophagy, mediated by vesicles called autophagosomes, enhancing chemotherapy. From Li & McAllister 2023.
So far so useful — but here’s the remarkable bit. 3-IAA is a plant hormone widespread in bacteria that inhabit the rhizosphere of plants — the soil zone surrounding plant roots. Sometimes called the microbe storehouse, it’s where microbes interact with plant roots and rhizobacteria release plant growth-promoting chemicals. Their presence is affected by nitrogen deposition and by drought.
Once the Hamburg folk had found 3-IAA their metagenomic screen came up with the source: two bacterial strains, Bacteroides fragilis and Bacteroides thetaiotaomicron, strains known to make 3-IAA in plant roots. These days you can guess the next experiment: take your mouse model of metastatic pancreatic cancer, use mice that lack their own microbiota (germ-free) and give them faecal transplants from people whose PDAC had responded to chemotherapy. Sure enough, the mice had smaller tumours after chemotherapy than mice given microbiota from human non-responders.
Next question: given that production of 3-IAA involves the amino acid tryptophan and is mediated by bacterial species in the gut, could a tryptophan-rich diet also result in an anticancer effect? Indeed such a diet had a striking anti-tumour effect in mice given chemotherapy (reduced tumour size by about half). The effect seems to be specific to 3-IAA because other tryptophan-derived metabolites, such as indole-3-propionic acid and hippuric acid, had no effect.
Furthermore, screening tumour environment for immune cells showed that 3-IAA production was associated with the presence of neutrophils — cells known to be involved in the anti-tumour effects observed in people who respond to chemotherapy.
So neutrophils can sense signals from microbes in the gut that are more usually associated with plant-nourishing microbiota! Neutrophils are known to affect tumour spread and in this PDAC model they convert a bacterial metabolite into something that inhibits autophagy and makes cancer cells more susceptible to toxic drugs.
How extraordinary is all that?
References
Tintelnot J, Xu Y, Lesker TR, Schönlein M, Konczalla L, Giannou AD, Pelczar P, Kylies D, Puelles VG, Bielecka AA, Peschka M, Cortesi F, Riecken K, Jung M, Amend L, Bröring TS, Trajkovic-Arsic M, Siveke JT, Renné T, Zhang D, Boeck S, Strowig T, Uzunoglu FG, Güngör C, Stein A, Izbicki JR, Bokemeyer C, Sinn M, Kimmelman AC, Huber S, Gagliani N. Microbiota-derived 3-IAA influences chemotherapy efficacy in pancreatic cancer. Nature. 2023 Mar;615(7950):168-174. doi: 10.1038/s41586-023-05728-y. Epub 2023 Feb 22. PMID: 36813961; PMCID: PMC9977685.
Li L, McAllister F. A gut reaction can tune tumour fate during chemotherapy. Nature. 2023 Mar;615(7950):36-37. doi: 10.1038/d41586-023-00476-5. PMID: 36814015.