L-triiodothyronine rescues human mesenchymal stem cells from H2O2-induced cell death by eliciting mitochondrial turnover

Background: Ninety percent of stem cells dies following myocardial transplantation. The mitochondrial turnover promotes cell survival by removing damaged mitochondria and favours biogenesis of new functional organelles. Parkin2 (PARK2) is a novel enhancer of mitophagy, while mitochondrial transcription factor A (TFAM) is a well known mediator of mitochondrial biogenesis. We hypothesize that 3,5,3'-Levo-triiodothyronine (L-T3) prevents death of human mesenchymal stem cells (hMSCs) in ischemic tissue by enhancing PARK2- and TFAM-mediated mitochondrial turnover. Methods: hMSCs were exposed for 2h to H2O2 (100?M) and then treated with L-T3 at physiological concentration (3nM for 24h, T3-SCs) or vehicle (SCs). Caspase-3 activation and mitochondrial membrane depolarization were measured, trough confocal immunofluorescence and FACS analysis respectively, as markers of cell damage and apoptosis. The cell level of PARK2 and TFAM were measured through western blot. Regulation of mitochondrial dynamic was visualized trough confocal imaging by using the mitochondrial probe mitotrack-green. All the experiments were repeated after PARK2 or TFAM downregulation with selective short interfering-RNA(siRNA). Results: Significant increase of caspase 3-activation and mitochondrial depolarization were observed in stressed SCs, but not in T3-SCs. TFAM expression was significantly reduced by 71.4% in SCs compared to unstressed cells, but the parkin expression was unchanged. L-T3 treatment of stressed hMSCs increased the expression of TFAM (2.2±0.2 vs. 1±0.1 a.u., p<0.05) and parkin (2.7±0.1 vs. 1.6±0.1 a.u., p<0.05) compared to SCs, without affecting cell differentiation. Parkin or TFAM silencing completely abolished the prosurvival effects of L-T3 in hMSCs exposed to H2O2 in concomitance with the induction of altered mitochondrial network characterized by excessive mitochondrial fusion. Conclusions: L-T3 treatment at physiological dose increases survival of hMSCs in presence of oxidative stress by enhancing mitochondrial turnover trough a PARK2 and TFAM dependent mechanism.