5-methoxytryptophan protects MSCs from stress induced premature senescence by upregulating FoxO3a and mTOR

5-methoxytryptophan (5-MTP) is a newly discovered tryptophan metabolite which controls stressinduced inflammatory signals. To determine whether 5-MTP protects against stress-induced mesenchymal stem cell (MSC) senescence, we incubated bone marrow-derived MSC (BM-MSC) in highglucose medium or regular medium for 2 weeks followed by addiction of 5-MTP (10 mu M) or vehicle for 48 h. 5-MTP reduced p16 and p21 expression, senescence-associated beta-Gal (SA-beta-Gal) and IL-6 secretion and increased BrdU incorporation. 5-MTP exerted a similar effect on BM-MSC senescence induced by a sublethal concentration of H2O2. 5-MTP enhanced FoxO3a expression and increased superoxide dismutase and catalase activities in HG BM-MSCs. Silencing of FoxO3a with siRNA abrogated 5-MTPmediated reduction of SA-beta-Gal and IL-6 secretion but not p21 or p16. Since mechanistic target of rapamycin (mTOR) is involved in cellular senescence, we determined whether 5-MTP influences mTOR expression. Our data reveal that mTOR protein level was depressed in HG-MSC which was rescued by 5-MTP. Rapamycin abrogated 5-MTP-mediated suppression of p16, p21, SA-beta-Gal and IL-6 and rise of BrdU incorporation. Our findings suggest that 5-MTP protects MSCs against stress-induced senescence via FoxO3a and mTOR upregulation and has potential to improve cell expansion for cell therapy.