Tripeptidyl peptidase II (TPPII or TPP2) degrades N-terminal tripeptides from proteins and peptides. Studies in both humans and mice have shown that TPPII deficiency is linked to cellular immune-senescence, lifespan regulation and the aging process. However, the mechanism of how TPPII participates in these processes is less clear. In this study, we established a chemical probe-based assay and found that although the mRNA and protein levels of TPPII were not altered during senescence, its enzymatic activity was reduced in senescent human fibroblasts. We also showed that elevation of the levels of the serine protease inhibitor serpinB2 reduced TPPII activity in senescent cells. Moreover, suppression of TPPII led to elevation in the amount of lysosomal contents as in well as TPPI (TPP1) and β-galactosidase activities, suggesting that lysosome biogenesis is induced to compensate for the reduction of TPPII activity in senescent cells. Together, this study discloses a critical role of the serpinB2-TPPII signaling pathway in proteostasis during senescence. Since serpinB2 levels can be increased by a variety of cellular stresses, reduction of TPPII activity through activation of serpinB2 might represent a common pathway for cells to respond to different stress conditions.

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