Mitophagy, which is a conserved cellular process for selectively removing damaged or unwanted mitochondria, is critical for mitochondrial quality control and the maintenance of normal cellular physiology. However, the precise mechanisms underlying mitophagy remain largely unknown. Prior studies on mitophagy focused on the events in the mitochondrial outer membrane. PHB2 (prohibitin 2), which is a highly conserved membrane scaffold protein, was recently identified as a novel inner membrane mitophagy receptor that mediates mitophagy. Here, we report a new signaling pathway for PHB2-mediated mitophagy. Upon mitochondrial membrane depolarization or misfolded protein aggregation, PHB2 depletion destabilizes PINK1 in the mitochondria, which blocks the mitochondrial recruitment of PRKN/Parkin, ubiquitin and OPTN (optineurin), leading to an inhibition of mitophagy. In addition, PHB2 overexpression directly induces PRKN recruitment to the mitochondria. Moreover, PHB2mediated mitophagy is dependent on the mitochondrial inner membrane protease PARL, which interacts with PHB2 and is activated upon PHB2 depletion. Furthermore, PGAM5, which is processed by PARL, participates in PHB2-mediated PINK1 stabilization. Finally, a ligand of PHB proteins that we synthesized, called FL3, was found to strongly inhibit PHB2-mediated mitophagy and to effectively block cancer cell growth and energy production at nanomolar concentrations. Thus, our findings reveal that the PHB2-PARL-PGAM5-PINK1 axis is a novel pathway of PHB2-mediated mitophagy and that targeting PHB2 with the chemical compound FL3 is a promising strategy for cancer therapy. Abbreviations: AIFM1: apoptosis inducing factor mitochondria associated 1; ATP5F1A/ATP5A1: ATP synthase F1 subunit alpha; BAF: bafilomycin A 1 ; CALCOCO2/NDP52: calcium binding and coiled-coil domain 2; CCCP: chemical reagent carbonyl cyanide m-chlorophenyl hydrazine; FL3: flavaglines compound 3; HSPD1/HSP60: heat shock protein family D (Hsp60) member 1; LC3B/MAP1LC3B: microtubule associated protein 1 light chain 3 beta; MEF: mouse embryo fibroblasts; MPP: mitochondrial-processing peptidase; MT-CO2/COX2: mitochondrially encoded cytochrome c oxidase II

The MST/YAP (mammalian Ste20-like kinase/Yes-associated protein 2) pathway plays an important role in hepatocellular carcinoma (HCC). Although post-translational modification-especially MST/Lats (large tumor suppressor)-mediated phosphorylation and PP1 (protein phosphatase-1)-mediated dephosphorylation-has been found to regulate the activity of YAP2, very little is known about its acetylation. In our experiments, we observed that the expression of SIRT1 is significantly upregulated in the tumor samples of the hepatocarcinoma patients, and SIRT1 mRNA level positively correlates with connective tissue growth factor (CTGF) mRNA level. We then found that SIRT1 deacetylates YAP2 protein in HCC cells and SIRT1-mediated deacetylation increases the YAP2/TEAD4 association, leading to YAP2/TEAD4 transcriptional activation and upregulated cell growth in HCC cells. Moreover, knockdown of SIRT1 blocks the cisplatin (CDDP)-induced nuclear translocation of YAP2 and enhances the chemosensitivity of HCC cells to CDDP treatment. Together, our findings reveal a new regulatory mechanism of YAP2 by the SIRT1-mediated deacetylation that may be involved in HCC tumorigenesis and drug resistance.

Lung metastasis and relapse in osteosarcoma (OS) patients indicate poor prognosis. Here, we identified significantly decreased expression of miR-382 in highly metastatic OS cell lines and relapsed OS samples compared to their parental cell lines and primary OS samples, respectively. In addition, our clinical data showed that the miR-382 expression level was inversely associated with relapse and positively associated with metastasis-free survival in OS patients. The overexpression of miR-382 suppressed epithelial-mesenchymal transition (EMT) and metastasis. This overexpression also decreased the cancer stem cell (CSC) population and function in OS cells. In contrast, inhibition of miR-382 stimulated EMT and metastasis and increased CSC population in OS cells. In addition, our in vivo experiments showed that the overexpression of miR-382 inhibited CSC-induced tumor formation, and the combination of miR-382 with doxorubicin prevented disease relapse in OS patients. Furthermore, we demonstrated that miR-382 exerted its tumor-suppressing potential by directly targeting Y box-binding protein 1 (YB-1) in OS. Taken together, our findings suggest that miR-382 functions as a tumor suppressor function and that the overexpression of miR-382 is a novel strategy to inhibit tumor metastasis and prevent CSC-induced relapse in OS.

LSS results in higher 5-year survival rates and better functional outcomes as indicated by MSTS scores in patients with limb osteosarcomas.