Endogenous Soluble TRAIL Contributes to the Survival and Growth of Human Mesenchymal Stem Cells

Background: TNF-related apoptosis-inducing ligand (TRAIL) is known to induce caspase-dependent apoptosis in tumor cells, while sparing normal cells. However, TRAIL is also able to induce proliferation in cells that are resistant to cell death induction, since it can also activate NF-κB-dependent signaling pathways. Human mesenchymal stem cells (hMSC) transfected with TRAIL have been used to treat cancer, with promising results in pre-clinical models, especially in gliomas. Regarding endogenous TRAIL expression in hMSC or its possible role in cell death, proliferation or differentiation, not much has been explored as yet. Methods: Cell death induction by soluble TRAIL (sTRAIL) or by TRAIL associated with liposomes (LUV-TRAIL) on hMSC was studied by annexin-V-FITC and 7-AAD staining and flow cytometry. The effect of sTRAIL, LUV-TRAIL or a blocking anti-TRAIL mAb on cell growth was studied by uptake of 5-bromo-2’-deoxyuridine (BrdU). Cell morphology after the treatments was studied by phase microscopy. Exosomes were separated from cell supernatant by sequential centrifugations and ultracentrifugation. Endogenous TRAIL expression in hMSC or in their derived exosomes was studied by immunoblot and flow cytometry. Results: We demonstrated that exogenous TRAIL alone, either in soluble form or associated with liposomes (LUVTRAIL), does not affect growth of hMSC. In addition, it is also unable to induce cell death in these cells. Nevertheless, endogenous TRAIL from hMSC did affect positively their proliferation in combination with growth factors present in the serum. We demonstrated the expression of endogenous TRAIL in hMSC, mainly with an intracellular distribution, and showed that it is secreted to the culture supernatant in soluble form, but not in association with exosomes, contributing in this way to hMSC viability and growth. Conclusion: These observations could be important if hMSC transfected with TRAIL are used as tumor treatment or in regenerative medicine, since secreted TRAIL could exert its anti-tumoral effect at the same time as hMSC would persist.