Activation of the TLR4-JNK but not the TLR4-ERK pathway induced by indole-3-acetic acid exerts anti-proliferative effects on Caco-2 cells.

We previously found that indole-3-acetic acid (IAA) produced from tryptophan by gut microbiota decreases the expression of tumor necrosis factor α (TNFα), which is implicated in the pathogenesis of colorectal cancer (CRC). The present study aimed to determine IAA involvement in the proliferation of CRC-derived Caco-2 cells. Cell proliferation was suppressed by IAA, whereas IAA-induced aryl hydrocarbon receptor activation had no impact. IAA activated extracellular signal-related (ERK) and c-Jun N-terminal (JNK) kinases, but not p38. Toll-like receptor 4 (TLR4) may be required to activate ERK and JNK, but only the TLR4-JNK pathway might elicit the anti-proliferative effects of IAA. Thus, IAA may be a ligand for TLR4 that contributes to inhibiting CRC cell proliferation by activating TLR4-mediated JNK. Because IAA did not induce cytotoxicity, inhibiting cell cycle progression might affect the anti-proliferative capacity of IAA. Therefore, colonic IAA accumulation might help to prevent CRC development and progression.

TLR4 may be a novel indole-3-acetic acid receptor that is implicated in the regulation of CYP1A1 and TNFα expression depending on the culture stage of Caco-2 cells.

Most studies of indole derivatives such as IAA produced by intestinal microbiota have been based on the premise that binding to AhR leads to biological responses. We previously revealed that IAA binds to more than one receptor, and thus the present study aimed to identify a new receptor for IAA and analyze its mechanism of action. We found that the TLR4 antagonist TAK-242 did not affect the IAA-induced increase in CYP1A1 expression at 3 h and decreased TNFα expression at 8 days. However, TAK-242 alleviated decreased TNFα expression induced by IAA at 2 days and promoted IAA-induced increased CYP1A1 expression by inhibiting JNK activation at 8 days. Taken together, TLR4 may be a novel IAA receptor with signaling pathways that regulate CYP1A1 and TNFα expression depending on the culture stage of Caco-2 cells. Furthermore, our findings offer important clues for elucidating the action mechanisms of indole derivatives that affect hosts.