4. Discussion
In the current study, we identified β-damascone from approximately 150
compounds as a novel immunomodulator through 2-step screening monitoring
of DC-mediated immunoresponses. By conducting in vivo experiments
using a mouse model, we demonstrated that oral intake of β-damascone,
which activates the NRF2 pathway, ameliorated CHS inflammation. NRF2
plays a role as a master regulator of antioxidant and anti-electrophilic
responses by transactivating the genes encoding antioxidant enzymes and
drug metabolizing enzymes. In a previous study using a
2,4-dinitrochlorobenzene (DNCB)-induced CHS model, NRF2-deficient mice
exhibited more severe inflammation accompanied by increased chemokine
production and neutrophil recruitment in the skin than control mice12. This observation that NRF2 deficiency deteriorates
CHS may be coincident with our results, indicating the important roles
of NRF2 in DC-mediated immune responses. In addition to CHS, various
immunorelated inflammatory diseases and autoimmune diseases, including
colitis and psoriasis, are reported to be exacerbated in NRF2 knockout
mice 13-16. Although the effects of NRF2 deficiency
are not restricted to DC function, because NRF2 is ubiquitously
expressed, these studies of NRF2-deficient mice support the possibility
that β-damascone could be useful for the prevention and/or treatment of
inflammatory diseases and autoimmune diseases. We found that β-damascone
significantly suppressed TLR7-mediated gene expression and protein
production of IL-6, IL-12p40, and TNF-α in BMDCs (Figure 3C and3F ). Because a topical application of imiquimod17,18, a ligand of TLR7 and TLR8, causes
psoriasis-like pathology in mouse skin through activation of dermal DCs
and Langerhans cells 19, we expect that β-damascone
ameliorates psoriasis, even from the point of view of modification of DC
function. We will evaluate the effects of β-damascone on various
immunorelated diseases, such as psoriasis, inflammatory bowel disease,
and multiple sclerosis using a mouse model in the near future.
Our results using Nrf2 -/- DCs indicated that
the suppressive effects of β-damascone on IL-12 production in DCs and
Th1 development activity of DCs are dependent on NRF2. In contrast, the
suppressive effects of β-damascone on the production of TNF-α and IL-6
were still observed in Nrf2 -/- DCs, suggesting
that β-damascone modulates DC function through additional target(s)
other than NRF2. It has been reported that the induction of the
proinflammatory cytokine genes, Il1a , Il1b , and Il6is inhibited by NRF2, which binds to these genes and subsequently
inhibits the recruitment of RNA polymerase II to target genes in
macrophages 20. However, under our experimental
conditions, we could not find a significant contribution of NRF2 in the
suppression of the production of IL-6 and TNF-α in DCs. Further detailed
analysis to clarify other target(s) of β-damascone could lead to the
development of novel anti-inflammatory immunomodulators.
In the current study, we identified β-damascone as the most effective
compound from an aroma library and confirmed that oral administration of
β-damascone ameliorated CHS. Although we obtained β-damascone as a
natural compound candidate of immunomodulator in the current study,
there is room for improvement; briefly, β-damascone could be developed
to a drag carrying stronger activity and lesser toxicity by chemical
structural modification. Further research using an organic chemical
approach considering β-damascone as a lead compound should be conducted
to develop a novel immunomodulator.