Loss of Peter Pan (PPAN) Affects Mitochondrial Homeostasis and Autophagic Flux

Abstract

Nucleolar stress is a cellular response to inhibition of ribosome biogenesis or nucleolar disruption leading to cell cycle arrest and/or apoptosis. Emerging evidence points to a tight connection between nucleolar stress and autophagy as a mechanism underlying various diseases such as neurodegeneration and treatment of cancer. Peter Pan (PPAN) functions as a key regulator of ribosome biogenesis. We previously showed that human PPAN localizes to nucleoli and mitochondria and that PPAN knockdown triggers a p53-independent nucleolar stress response culminating in mitochondrial apoptosis. Here, we demonstrate a novel role of PPAN in the regulation of mitochondrial homeostasis and autophagy. Our present study characterizes PPAN as a factor required for maintaining mitochondrial integrity and respiration-coupled ATP production. PPAN interacts with cardiolipin, a lipid of the inner mitochondrial membrane. Down-regulation of PPAN enhances autophagic flux in cancer cells. PPAN knockdown promotes recruitment of the E3-ubiquitin ligase Parkin to damaged mitochondria. Moreover, we provide evidence that PPAN knockdown decreases mitochondrial mass in Parkin-expressing cells. In summary, our study uncovers that PPAN knockdown is linked to mitochondrial damage and stimulates autophagy.

Mitophagy in Diabetic Cardiomyopathy: Roles and Mechanisms

“…When the Wnt pathway is inhibited, β-catenin is degraded and the expression of the autophagy genes increases, promoting autophagy, and ameliorating cellular metabolic disorders ( Ziegler et al, 2018 ). The knockdown of Peter Pan ( PPAN ), a downstream gene in the Wnt signaling pathway that is expressed in mice and Xenopus laevis , promotes recruitment of the E3-ubiquitin ligase parkin to damaged mitochondria and reduces mitochondrial mass in parkin-expressing cells, which implies that Wnt signaling may regulate mitophagy ( Dannheisig et al, 2019 ). Furthermore, vitamin D3 upregulates autophagy by inhibiting the β-catenin/TCF4/mTOR signaling pathway, thereby improving cardiac function in DCM ( Wei et al, 2017 ).…”

Nucleolar Stress Functions Upstream to Stimulate Expression of Autophagy Regulators

“…The following siRNAs were Flexitube siRNAs from Qiagen (Hilden, Germany): si PPAN-A (SI00125545) [ 25 , 34 , 38 , 39 ], si SBDS-C (SI00711144) and si SBDS-D (SI03246390) [ 25 ], si UBF-A (SI00754992) and si UBF-B (SI04290839). Following custom si RNA sequences were previously reported [ 25 , 34 , 38 , 39 ] and were obtained from Horizon Discovery (Lafayette, CO, USA). The respective si RNA sequences are listed below:…”

“…U2OS cells were purchased from ATCC and HeLa cells were kindly provided by M. Kühl and were purchased from Sigma (Steinheim, Germany). HEK293A GFP-LC3 cells (ECACC 14050801) were purchased from Sigma (Steinheim, Germany) and were cultured in medium supplemented with 0.4 mg/mL G418 (Sigma, Steinheim, Germany) [ 34 , 40 ]. Cell lines were regularly tested for absence of mycoplasma and were used in low passage numbers.…”

“…Besides nucleolar stress induction by CX-5461, the chemotherapeutic ActinomycinD also has been shown to stimulate autophagy in cancer cells [ 24 , 31 , 32 ]. Moreover, loss of key nucleolar ribosome biogenesis factors connected to nucleolar stress induction, such as PPAN, NPM or the ribosomopathy factor SBDS have been linked to autophagy [ 15 , 31 , 33 , 34 ]. Overall, the increase of autophagic flux in the context of nucleolar stress is believed to initially possess compensatory anti-apoptotic effects.…”

A Glimmer of Hope: Maintain Mitochondrial Homeostasis to Mitigate Alzheimer’s Disease

“…proposed that nonfunctional mitochondrial proteins modestly induced by failed mitochondrial import can mark by SUMO (small ubiquitin-like modifier). Therefore, SUMOylation can serve as intraorganellar protein quality control [ 82 ]. Since mitophagy is a constant, dynamic and sophisticated cellular process, a combination of different mitophagy detection approaches, including cross-species evaluation, will improve the accuracy of measuring mitophagy and lead to a better understanding of mitophagy in the physiology of neurons, potentially contributing valuable information to the effort of advancing the therapy of neurodegenerative diseases.…”