SIRT3 Suppresses Cyst Growth via Enhancing Mitochondrial Function in ADPKD

Background: ADPKD is characterized by progressive cyst formation leading to kidney failure. Mitochondrial dysfunction has been recognized as a pathogenetic factor of ADPKD. Polycystin 1 (PC1) undergoes a cleavage to form a C-terminal tail fragment (PC1-CTT) that translocates to nuclei and mitochondria to regulate signaling pathways. SIRT3, the primary mitochondrial NAD+-dependent deacetylase, enhances the functions of mitochondrial proteins, mitochondrial biogenesis and dynamics, and metabolic stability. However, the role of SIRT3 in regulating mitochondrial dysfunction in ADPKD and the interplay of SIRT3 and PC1-CTT remain unknown. Methods: To understand the role of SIRT3 in vivo, Pkd1RC/RC and Pkd1fl/fl;Pkhd1-Cre mice were treated with SIRT3 activator honokiol. To explore the mechanisms mediated by SIRT3 in regulating cyst growth, Pkd1 mutant cells were overexpressed with SIRT3 and treated with honokiol. To determine the interplay of SIRT3 and PC1-CTT, immunoprecipitation was performed. Results: The expression of SIRT3 mRNA and protein were downregulated in Pkd1 mutant renal epithelial cells and kidney tissues. Treatment with honokiol attenuated cyst progression in the kidneys of Pkd1RC/RC and Pkd1fl/fl;Pkhd1-Cre mice as seen by a decrease in cystic index, kidney weight/body weight ratios, blood urea nitrogen levels, and cystic lining epithelial cell proliferation. We found that SIRT3 protected renal epithelial cells from oxidative stress by deacetylating SOD2, thereby inhibiting the activation of Akt-mTOR pathway. Treatment with honokiol also led to a decrease of fragmentation and an improvement of elongation and interconnection of mitochondria. SIRT3 regulated the balance of fission and fusion through deacetylating its novel substrate, mitochondrial fission factor (MFF). SIRT3 colocalized and interacted with PC1-CTT in mitochondria, and deacetylated PC1-CTT. In addition, nuclear PC1-CTT promotes the transcription of SIRT3 through its interaction with PGC1α. Conclusion: The downregulation of SIRT3 promotes mitochondrial dysfunction and cyst growth in ADPKD by hyperacetylation of mitochondrial enzymes and PC1-CTT, which is transcriptionally regulated by nuclear PC1-CTT, forming a nuclear-mitochondrial regulatory axis of SIRT3-PC1-CTT. This study suggests that the activation of SIRT3 with activators, such as honokiol, may be a promising therapeutic strategy for ADPKD. Funding: Private Foundation Support