Calpain-5 mediates STIP1 degradation via triggering N-degron pathway to suppress epithelial ovarian cancer progression and CAPN5low-STIP1high status indicates Pimitespib susceptibility

Rapid progression is considered the major contributor to the poor prognosis of epithelial ovarian cancer (EOC), while the underlying mechanism remains elusive. Herein, we identified that Calpain-5 (CAPN5), a well-known protease, was significantly downregulated across various types of EOC, and acted as a protective indicator for prognosis. Phenotypically, CAPN5 overexpression greatly restrained the proliferation and invasion of EOC, but induced apoptosis, while knocking down CAPN5 exerted opposite effects. Moreover, CAPN5 markedly inhibited epithelial-to-mesenchymal transition (EMT). Mechanistically, CAPN5 exerted its inhibitory effect mainly via downregulating STIP1, a well-recognized oncoprotein in EOC, evidenced by that silencing STIP1 could almost abolish the promotional effects of CAPN5 knockdown on EOC progression. Further experiments demonstrated that CAPN5 catalyzes STIP1 cleavage at R160 residue, exposing the novel Asn-initiated N-terminal fragment of STIP1, which triggered the Arg/N-degron pathway to mediate the K169 ubiquitination of STIP1 fragment by the ATE1-UBR4 cascade. Consequently, the ubiquitinated STIP1 fragment underwent proteasomal degradation. Notably, applying an approved HSP90i, Pimitespib, can effectively suppress CAPN5-low/STIP1-high EOC progression and exert a synergetic effect with Cisplatin. In summary, CAPN5 is a novel suppressor in EOC by acting as an initiator for the N-degron pathway to induce STIP1 degradation, and a CAPN5 low -STIP1 high status served as an indicator to guide Pimitespib application.