Rapamycin Treatment of Tendon Stem/Progenitor Cells Reduces Cellular Senescence by Upregulating Autophagy.

The elderly population is prone to tendinopathy due to aging-related tendon changes such as cellular senescence and a decreased ability to modulate inflammation. Aging can render tendon stem/progenitor cells (TSCs) into premature senescence. We investigated the effects of rapamycin, a specific mTOR inhibitor, on the senescence of TSCs. We first showed that after treatment with bleomycin in vitro, rat patellar TSCs (PTSCs) underwent senescence, characterized by morphological alterations, induction of senescence-associated β-galactosidase (SA-β-gal) activity, and an increase in p53, p21, and p62 protein expression. Senescence of PTSCs was also characterized by the elevated expression of MMP-13 and TNF-α genes, both of which are molecular hallmarks of chronic tendinopathy. We then showed that rapamycin treatment was able to reverse the above senescent phenotypes and increase autophagy in the senescent PTSCs. The activation of autophagy and senescence rescue was, at least partly, due to the translocation of HMGB1 from the nucleus to the cytosol that functions as an autophagy promoter. By reducing TSC senescence, rapamycin may be used as a therapeutic to inhibit tendinopathy development in the aging population by promoting autophagy.