Inhibition of LONP1 Suppresses Pancreatic Cancer Progression Via c-Jun N-Terminal Kinase Pathway-Meditated Epithelial-Mesenchymal Transition

Mitochondrial quality control proteases and their modulation for cancer therapy

Mitochondria, the main provider of energy in eukaryotic cells, contains more than 1000 different proteins and is closely related to the development of cells. However, damaged proteins impair mitochondrial function, further contributing to several human diseases. Evidence shows mitochondrial proteases are critically important for protein maintenance. Most importantly, quality control enzymes exert a crucial role in the modulation of mitochondrial functions by degrading misfolded, aged, or superfluous proteins. Interestingly, cancer cells thrive under stress conditions that damage proteins, so targeting mitochondrial quality control proteases serves as a novel regulator for cancer cells. Not only that, mitochondrial quality control proteases have been shown to affect mitochondrial dynamics by regulating the morphology of optic atrophy 1 (OPA1), which is closely related to the occurrence and progression of cancer. In this review, we introduce mitochondrial quality control proteases as promising targets and related modulators in cancer therapy with a focus on caseinolytic protease P (ClpP), Lon protease (LonP1), high-temperature requirement protein A2 (HrtA2), and OMA-1. Further, we summarize our current knowledge of the advances in clinical trials for modulators of mitochondrial quality control proteases. Overall, the content proposed above serves to suggest directions for the development of novel antitumor drugs.

The Journey of Mitochondrial Protein Import and the Roadmap to Follow

Mitochondria are double membrane-bound organelles that play critical functions in cells including metabolism, energy production, regulation of intrinsic apoptosis, and maintenance of calcium homeostasis. Mitochondria are fascinatingly equipped with their own genome and machinery for transcribing and translating 13 essential proteins of the oxidative phosphorylation system (OXPHOS). The rest of the proteins (99%) that function in mitochondria in the various pathways described above are nuclear-transcribed and synthesized as precursors in the cytosol. These proteins are imported into the mitochondria by the unique mitochondrial protein import system that consists of seven machineries. Proper functioning of the mitochondrial protein import system is crucial for optimal mitochondrial deliverables, as well as mitochondrial and cellular homeostasis. Impaired mitochondrial protein import leads to proteotoxic stress in both mitochondria and cytosol, inducing mitochondrial unfolded protein response (UPR^mt). Altered UPR^mt is associated with the development of various disease conditions including neurodegenerative and cardiovascular diseases, as well as cancer. This review sheds light on the molecular mechanisms underlying the import of nuclear-encoded mitochondrial proteins, the consequences of defective mitochondrial protein import, and the pathological conditions that arise due to altered UPR^mt.

The mitochondrial unfolded protein response (UPRmt): shielding against toxicity to mitochondria in cancer

Mitochondria are essential for tumor growth and progression. However, the heavy demand for mitochondrial activity in cancer leads to increased production of mitochondrial reactive oxygen species (mtROS), accumulation of mutations in mitochondrial DNA, and development of mitochondrial dysfunction. If left unchecked, excessive mtROS can damage and unfold proteins in the mitochondria to an extent that becomes lethal to the tumor. Cellular systems have evolved to combat mtROS and alleviate mitochondrial stress through a quality control mechanism called the mitochondrial unfolded protein response (UPR^mt). The UPR^mt system is composed of chaperones and proteases, which promote protein folding or eliminate mitochondrial proteins damaged by mtROS, respectively. UPR^mt is conserved and activated in cancer in response to mitochondrial stress to maintain mitochondrial integrity and support tumor growth. In this review, we discuss how mitochondria become dysfunctional in cancer and highlight the tumor-promoting functions of key components of the UPR^mt.

Insight into the mitochondrial unfolded protein response and cancer: opportunities and challenges

The mitochondrial unfolded protein response (UPR^mt) is an evolutionarily conserved protective transcriptional response that maintains mitochondrial proteostasis by inducing the expression of mitochondrial chaperones and proteases in response to various stresses. The UPR^mt-mediated transcriptional program requires the participation of various upstream signaling pathways and molecules. The factors regulating the UPR^mt in Caenorhabditis elegans (C. elegans) and mammals are both similar and different. Cancer cells, as malignant cells with uncontrolled proliferation, are exposed to various challenges from endogenous and exogenous stresses. Therefore, in cancer cells, the UPR^mt is hijacked and exploited for the repair of mitochondria and the promotion of tumor growth, invasion and metastasis. In this review, we systematically introduce the inducers of UPR^mt, the biological processes in which UPR^mt participates, the mechanisms regulating the UPR^mt in C. elegans and mammals, cross-tissue signal transduction of the UPR^mt and the roles of the UPR^mt in promoting cancer initiation and progression. Disrupting proteostasis in cancer cells by targeting UPR^mt constitutes a novel anticancer therapeutic strategy.

AFF4 facilitates melanoma cell progression by regulating c-Jun activity

Melanoma is characterized by high mortality and poor prognosis due to metastasis. AFF4 (AF4/FMR2 family member 4), as a scaffold protein, is a component of the super elongation complex (SEC), and is involved in the progression of tumors, e.g., leukemia, head and neck squamous cell carcinoma (HNSCC). However, few studies on AFF4 have focused on melanoma. Here, AFF4 expression levels and clinicopathological features were evaluated in melanoma tissue samples. Then, we performed cell proliferation, migration and invasion assays in A375 and A2058 cells lines in vitro to evaluate the role of AFF4 in melanoma. The effects of AFF4 knockdown in vivo were characterized via a xenograft mouse model. Finally, the correlation between c-Jun and AFF4 protein levels in melanoma was analyzed by rescue assay and immunohistochemistry (IHC). We found that AFF4 expression was upregulated in melanoma tumor tissues and that AFF4 protein expression was also closely related to the prognosis of patients with cutaneous melanoma. Moreover, AFF4 could promote the invasion and migration of melanoma cells by mediating epithelial to mesenchymal transition (EMT). AFF4 might regulate c-Jun activity to promote the invasion and migration of melanoma cells. Importantly, c-Jun was regulated by the AFF4 promoted melanoma tumorigenesis in vivo. Taken together, AFF4 may be a novel oncogene that promotes melanoma progression through regulation of c-Jun activity.

Inhibition of LONP1 protects against erastin-induced ferroptosis in Pancreatic ductal adenocarcinoma PANC1 cells

Ferroptosis is identified as a regulated cell death mediated by iron accumulation and lipid peroxidation. The disturbances of mitochondrial morphology and function have been shown in this process. Mitochondrial Lon peptidase 1 (LONP1) is one of the main multi-function enzymes in regulating the mitochondrial function and cytological stability. To evaluate whether LONP1 take a role in ferroptosis, we applied erastin to initiate the ferroptosis in human pancreatic ductal adenocarcinoma (PDAC) cells. Here we show that erastin triggers cell death in both of oncogenic RAS mutant PANC1 cells and wild KRAS BxPC3 cells and the expression of LONP1 was up-regulated in this process. Gene inhibition of LONP1 only negatively regulates erastin-induced cell death and the alterations of molecular indicators in PANC1 cells. Furthermore, we show that inhibition of LONP1 activates the Nrf2/Keap1 signal pathway and up-regulates the expression of GPX4, a key peroxidase in regulating ferroptosis. Together, our results uncover a previously unappreciated mechanism coupling LONP1 to ferroptosis.