image: LTr1 and DIM form as antitumor agents in mouse gastrointestinal tract from cruciferous vegetable-derived indole-3-carbinol (I3C). In the upper gastrointestinal tract, LTr1 generates through the Michael addition of 3-methyleneindolium (3MI, derived in situ from I3C) to DIM that is produced from I3C via the formaldehyde-releasing (major) and CO2-liberating (minor) pathways. The indole-recruiting pathway occurs primarily in large intestine where I3C and DIM formed from couplings of formaldehyde with one and two indole molecules, respectively. I3C, indole-3-carbinol; I3A, indole-3-carbaldehyde; I3CA, indole-3-carboxylic acid; DIM, 3,3'-diindolylmethane; LTr1, 2-(indol-3-ylmethyl)-3,3'-diindolylmethane; 3MI, 3-methyleneindolium; IS-1 and IS-2, intermediate salts 1 and 2, respectively; WMR, Wagner-Meerwein rearrangement. view more
Credit: ©Science China Press
This study was conceived and guided by Prof. Ren Xiang Tan (Nanjing University).
The cruciferous vegetables (e.g., broccoli, cabbage, and cauliflower) are globally consumed with the diverse health benefits believed to arise from their liberation of indole-3-carbinol (I3C). But I3C is chemically labile and can be rapidly metabolized in vivo, thereby raising questions regarding the I3C metabolic pathway and the real health-improving substances derived from the I3C ingestion. The authors identified the in vivo I3C metabolites including 2-(indol-3-ylmethyl)-3,3'-diindolylmethane (LTr1) and 3,3'-diindolylmethane (DIM, a commercialized nutraceutical and tumor therapeutic candidate entered into clinical trials). Unexpectedly, the authors found that LTr1 displays superior antitumor efficacy over DIM in diverse cancer cell lines and mouse models. This observation, along with its safety, motivated the team to decipher: 1) how I3C is metabolized into LTr1 in vivo, and 2) whether there are endogenous pathways toward these antitumor compounds.
To tackle the challenging issues, the reaction flux derailing (RFD) approach was conceptualized, designed, and deployed by using labeled substrate analogs, whose identical or similar reaction activity allows the metabolic intermediates to derail the I3C-to-LTr1 reaction flux according to rules of chemistry. The prowess and generalizability of the RFD methodology are validly showcased by its facilitation of clarifying: 1) the “I3CgDIMgLTr1” conversion pathways; 2) the stepwise formation of I3C and DIM from indole (a tryptophan catabolite) and formaldehyde (a common metabolite of methanol, creatine, and folate); and 3) the dependence of these processes on pHs of different gastrointestinal compartments in mice. Notably, Tan’s group has ascertained that in the upper (or acidic) gastrointestinal tract, LTr1 generates through the Michael addition of 3-methyleneindolium (3MI) to DIM that is produced from I3C via the formaldehyde-releasing (major) and CO2-liberating (minor) pathways. In the large intestine, ‘endogenous’ I3C and DIM form respectively from couplings of formaldehyde with one and two molecules of indole.
Overall, the work pinpoints the LTr1 druggability, establishes the RFD methodology that clarifies the post-ingestion conversion and endogenous formation of this important family of indoles, and thus symbolizes a movement in understanding the significance of I3C-releasing cruciferous vegetables.
See the article：Post-ingestion conversion of dietary indoles into anticancer agents
Journal
National Science Review