Autophagy: regulation and role in development

Tetrandrine is a potent cell autophagy agonist via activated intracellular reactive oxygen species

Background

Autophagy is an evolutionarily conserved cellular process that involves the lysosomal degradation of proteins and organelles and the recycling of cellular components to ensure cellular survival under external or internal stress. Numerous data has indicated that autophagy can be successfully targeted for the treatment of multiple cancers. We have previously demonstrated that tetrandrine, a bisbenzylisoquinoline alkaloid isolated from the broadly used Chinese medicinal herb Stephaniae tetrandrae, exhibits potent antitumor effects when used either alone or in combination with other drugs.

Results

In the present study, we showed that tetrandrine is a broad-spectrum potent autophagy agonist. Although low-dose tetrandrine treatment does not affect cell viability, it can potently induce autophagy in a variety of cell lines, including cancerous cells and nontumorigenic cells. The autophagy inhibitors 3-methyladenine (3-MA) and chloroquine (CQ), effectively blocked tetrandrine-induced autophagy. Moreover, tetrandrine significantly triggered the induction of mitophagy. The underlying mechanisms are associated with the tetrandrine-induced production of intracellular reactive oxygen species (ROS), which plays a critical role in tetrandrine-induced autophagy.

Conclusions

Here, we report that tetrandrine is a potent cell autophagy agonist and may have a wide range of applications in the fields of antitumor therapy and basic scientific research.

Autophagy in resin monomer-initiated toxicity of dental mesenchymal cells: a novel therapeutic target of N-acetyl cysteine

A proposed schematic model of autophagy involvement in resin monomer-initiated toxicity of dental mesenchymal cells and as a novel therapeutic target of NAC.

Beclin-1 deficiency in the murine ovary results in the reduction of progesterone production to promote preterm labor

Autophagy is an important cellular process that serves as a companion pathway to the ubiquitin-proteasome system to degrade long-lived proteins and organelles to maintain cell homeostasis. Although initially characterized in yeast, autophagy is being realized as an important regulator of development and disease in mammals. Beclin1 (Becn1) is a putative tumor suppressor gene that has been shown to undergo a loss of heterozygosity in 40-75% of human breast, ovarian, and prostate cancers. Because Becn1 is a key regulator of autophagy, we sought to investigate its role in female reproduction by using a conditional knockout approach in mice. We find that pregnant females lacking Becn1 in the ovarian granulosa cell population have a defect in progesterone production and a subsequent preterm labor phenotype. Luteal cells in this model exhibit defective autophagy and a failure to accumulate lipid droplets needed for steroidogenesis. Collectively, we show that Becn1 provides essential functions in the ovary that are essential for mammalian reproduction.

Inhibition of H3K9 methyltransferase G9a repressed cell proliferation and induced autophagy in neuroblastoma cells

Histone methylation plays an important role in gene transcription and chromatin organization and is linked to the silencing of a number of critical tumor suppressor genes in tumorigenesis. G9a is a histone methyltransferase (HMTase) for histone H3 lysine 9. In this study, we investigated the role of G9a in neuroblastoma tumor growth together with the G9a inhibitor BIX01294. The exposure of neuroblastoma cells to BIX01294 resulted in the inhibition of cell growth and proliferation, and BIX01294 treatment resulted in the inhibition of the tumorigenicity of neuroblastoma cells in NOD/SCID mice. Therefore, G9a may be a potential therapeutic target in neuroblastoma. Moreover, we found several specific characteristics of autophagy after BIX01294 treatment, including the appearance of membranous vacuoles and microtubule-associated protein light chain 3 (LC3B). Similar results were observed in G9a-knockdown cells. In conclusion, our results demonstrated that G9a is a prognostic marker in neuroblastoma, and revealed a potential role of G9a in regulating the autophagy signaling pathway in neuroblastoma.

Defective autophagy in Parkinson's disease: lessons from genetics

Parkinson's disease (PD) is the most prevalent neurodegenerative movement disorder. Genetic studies over the past two decades have greatly advanced our understanding of the etiological basis of PD and elucidated pathways leading to neuronal degeneration. Recent studies have suggested that abnormal autophagy, a well conserved homeostatic process for protein and organelle turnover, may contribute to neurodegeneration in PD. Moreover, many of the proteins related to both autosomal dominant and autosomal recessive PD, such as α-synuclein, PINK1, Parkin, LRRK2, DJ-1, GBA, and ATPA13A2, are also involved in the regulation of autophagy. We propose that reduced autophagy enhances the accumulation of α-synuclein, other pathogenic proteins, and dysfunctional mitochondria in PD, leading to oxidative stress and neuronal death.

Proteotoxicity: an underappreciated pathology in cardiac disease

In general, in most organ systems, intracellular protein homeostasis is the sum of many factors, including chromosomal state, protein synthesis, post-translational processing and transport, folding, assembly and disassembly into macromolecular complexes, protein stability and clearance. In the heart, there has been a focus on the gene programs that are activated during pathogenic processes, but the removal of damaged proteins and organelles has been underappreciated as playing an important role in the pathogenesis of heart disease. Proteotoxicity refers to the adverse effects of damaged or misfolded proteins and even organelles on the cell. At the cellular level, the ultimate outcome of uncontrolled or severe proteotoxicity is cell death; hence, the pathogenic impact of proteotoxicity is maximally manifested in organs with no or very poor regenerative capability such as the brain and the heart. Evidence for increased cardiac proteotoxicity is rapidly mounting for a large subset of congenital and acquired human heart disease. Studies carried out in animal models and in cell culture have begun to establish both sufficiency and, in some cases, the necessity of proteotoxicity as a major pathogenic factor in the heart. This dictates rigorous testing for the efficacy of proteotoxic attenuation as a new strategy to treat heart disease. This review article highlights some recent advances in our understanding of how misfolded proteins can injure and are handled in the cell, examining the emerging evidence for targeting proteotoxicity as a new therapeutic strategy for heart disease. This article is part of a Special Issue entitled "Protein Quality Control, the Ubiquitin Proteasome System, and Autophagy."

Distribution, cleavage and lipidation of Atg8 fusion proteins in Spodoptera litura Sl-HP cells

Atg8 proteins fused with tags are commonly used to detect autophagy. The expression patterns of Lepidopteran insect Atg8 are relatively well documented. However, the influence of protein tags on characterization of Atg8 is still not very clear. Our results showed that endogenous Spodoptera litura Atg8 and HA tagged Atg8 driven by the baculovirus ie2 promoter were enriched in cytoplasm. The recombinant plasmid pEGFP-Atg8(EGFP) in which Atg8 contained a stop codon was constructed and expressed. Green fluorescence was accumulated in cytoplasm. However, red fluorescence was located in both cytoplasm and nucleoplasm in most cells transfected with the recombinant plasmid pmCherry-Atg8(EGFP). In contrast to pEGFP-Atg8(EGFP), green fluorescence was also located in both cytoplasm and nucleoplasm in most cells transfected with the recombinant plasmid pie2/EGFP-Atg8 driven by the baculovirus ie2 promoter in which the CMV promoter and EGFP nucleotide sequences were removed, and the high level of the EGFP-Atg8 expression significantly increased its abundance in nucleoplasm. HA-Atg8 expressed at high level through baculovirus under the control of polyherin promoter was also localized in cytoplasm and nucleoplasm. The cleavage of mCherry-Atg8 was different from that of EGFP-Atg8. Both the mutant mCherry-Atg8F77/79A resulting in non-cleavage of the Atg8 and the mutant mCherry-Atg8G exposing its glycine residue at the end of C-terminus were also localized in cytoplasm and nucleoplasm. The increase of autophagosomes decreased the abundance of mCherry-Atg8 in nucleoplasm. In addition, the ratio of HA-Atg8-PE/HA-Atg8 was less than that of endogenous Atg8-PE/Atg8. These results demonstrated that the Atg8 is located in both nucleus and cytoplasm when expressed at high level and exported to the cytoplasm when autophagy is activated, and the fusion tags of Atg8 might have influence on the processing of Atg8 fusion proteins.

Proteostasis in endoplasmic reticulum--new mechanisms in kidney disease

Cells use an exquisite network of mechanisms to maintain the integrity and functionality of their protein components. In the endoplasmic reticulum (ER), these networks of protein homeostasis--referred to as proteostasis--regulate protein synthesis, folding and degradation via the unfolded protein response (UPR) pathway. The UPR pathway has two components: the adaptive UPR pathway, which predominantly maintains the ER function or ER proteostasis, and the apoptotic UPR pathway, which eliminates dysfunctional cells that have been subject to long-term or severe ER stress. Dysregulation of the UPR pathway often occurs in glomerular or tubulointerstitial cells under a pathogenic microenvironment, such as oxidative stress, glycative stress or hypoxia. A defective UPR is highly deleterious to renal cell function and viability and is thereby implicated in the pathophysiology of various kidney diseases. Accumulating evidence provides a link between the UPR pathway and mitochondrial structure and function, indicating the important role of ER proteostasis in the maintenance of mitochondrial homeostasis. Restoration of normal proteostasis, therefore, holds promise in protecting the kidney from pathogenic stresses as well as ageing. This Review is focused on the role of the ER stress and UPR pathway in the maintenance of ER proteostasis, and highlights the involvement of the derangement of ER proteostasis and ER stress in various pathogenic stress signals in the kidney.

LOX-1, oxidant stress, mtDNA damage, autophagy, and immune response in atherosclerosis

As a major receptor for oxidized low density lipoprotein (ox-LDL), lectin-like oxidized low-density lipoprotein receptor-1 (LOX-1) is upregulated in many pathophysiological events, including endothelial cell dysfunction and smooth muscle cell growth, as well as monocyte migration and transformation into foam cells, which are present in atherosclerosis and myocardial ischemia. Excessive production of reactive oxygen species (ROS) increases LOX-1 expression, induces mitochondrial DNA damage, and activates autophagy. Damaged mitochondrial DNA that escapes from autophagy induces an inflammatory response. This paper reviews the potential link between LOX-1, mitochondrial DNA damage, autophagy, and immune response in atherosclerosis.