Volume 397, Issue 7

Across a wide spectrum of cellular redox status, there emerges a dichotomy of responses in terms of cell survival/proliferation and cell death. Of note, there is emerging evidence that the anti-apoptotic protein, Bcl-2, in addition to its conventional activity of titrating the pro-apoptotic effects of proteins such as Bax and Bak at the mitochondria, also impacts cell fate decisions via modulating cellular redox metabolism. In this regard, both pro- and anti-oxidant effects of Bcl-2 overexpression have been described under different conditions and cellular contexts. In this short review, we attempt to analyze existing observations and present a probable explanation for the seemingly conflicting redox regulating activity of Bcl-2 from the standpoint of its pro-survival function. The consequential effect(s) of the dual redox functions of Bcl-2 are also discussed, particularly from the viewpoint of developing novel therapeutic strategies against cancers rendered refractory due to the aberrant expression of Bcl-2.

Cell division is a period of danger for cells, as inaccurate segregation of chromosomes can lead to loss of cell viability or aneuploidy. In order to protect against these dangers, cells ultimately initiate mitochondrial apoptosis if they are unable to correctly exit mitosis. A number of important chemotherapeutics exploit this response to delayed mitotic exit, but despite this, the molecular mechanism of the apoptotic timer in mitosis has proved elusive. Some recent studies have now shed light on this, showing how passage through the cell cycle fine-tunes a cell’s apoptotic sensitivity such that it can respond appropriately when errors arise.

Altered metabolism and resistance to cell death are typical hallmarks of cancer phenotype. Mitochondria are organelles central to cellular metabolism as well as to cell death induction. Hyperactivation of pro-survival and pro-proliferative pathways such as PI3K/AKT leads to cancer initiation, which affects mitochondria. Growing body of evidence indicates that oncogenes such as HER2 , EGFR and RAS , as well as the downstream members of the PI3K/AKT signaling pathway, directly regulate mitochondria by translocating to the organelle. Here we discuss evidence of this scenario and consider mechanisms for direct regulation of mitochondrial function. Being in close proximity to mitochondrial bioenergetics machinery as well as to the regulators/executors of programed cell death, oncogenes in mitochondria may be ideally placed to perform this task. This represents a thus far under-explored area, which may be relevant to better understanding of cancer initiation, progression and treatment.

Since entering our cells in an endosymbiotic event one billion years ago, mitochondria have shaped roles for themselves in metabolism, inflammation, calcium storage, migration, and cell death. Given this critical role in cellular homeostasis it is essential that they function correctly. Equally critical is the ability of a cell to remove damaged or superfluous mitochondria to avoid potential deleterious effects. In this review we will discuss the various mechanisms of mitochondrial clearance, with a particular focus on Parkin/PINK1-mediated mitophagy, discuss the impact of altered mitophagy in ageing and disease, and finally consider potential therapeutic benefits of targeting mitophagy.

Mitophagy is a selective autophagy pathway conserved in eukaryotes and plays an essential role in mitochondrial quality and quantity control. Mitochondrial fission and fusion cycles maintain a certain amount of healthy mitochondria and allow the isolation of damaged mitochondria for their elimination by mitophagy. Mitophagy can be classified into receptor-dependent and ubiquitin-dependent pathways. The mitochondrial outer membrane protein Atg32 is identified as the only known receptor for mitophagy in baker’s yeast, whereas mitochondrial proteins FUNDC1, NIX/BNIP3L, BNIP3 and Bcl2L13 are recognized as mitophagy receptors in mammalian cells. Earlier studies showed that ubiquitination and deubiquitination occurs in yeast, yet there is no direct evidence for an ubiquitin-dependent mitophagy pathway in this organism. In contrast, a ubiquitin-/PINK1-/Parkin-dependent mitophagy pathway was unraveled and was extensively characterized in mammals in recent years. Recently, a quantitative method termed synthetic quantitative array (SQA) technology was developed to identify modulators of mitophagy in baker’s yeast on a genome-wide level. The Ubp3-Bre5 deubiquitination complex was found as a negative regulator of mitophagy while promoting other autophagic pathways. Here we discuss how ubiquitination and deubiquitination regulates mitophagy and other selective forms of autophagy and what argues for using baker’s yeast as a model to study the ubiquitin-dependent mitophagy pathway.

The endoplasmic reticulum (ER) is the main coordinator of intracellular Ca 2+ signaling, protein synthesis, and folding. The ER is also implicated in the formation of contact sites with other organelles and structures, including mitochondria, plasma membrane (PM), and endosomes, thereby orchestrating through interorganelle signaling pathways, a variety of cellular responses including Ca 2+ homeostasis, metabolism, and cell death signaling. Upon loss of its folding capacity, incited by a number of stress signals including those elicited by various anticancer therapies, the unfolded protein response (UPR) is launched to restore ER homeostasis. The ER stress sensor protein kinase RNA-like ER kinase (PERK) is a key mediator of the UPR and its role during ER stress has been largely recognized. However, growing evidence suggests that PERK may govern signaling pathways through UPR-independent functions. Here, we discuss emerging noncanonical roles of PERK with particular relevance for the induction of danger or immunogenic signaling and interorganelle communication.

Necroptosis has recently been identified as an alternative form of programmed cell death that is characterized by defined molecular mechanisms. Reactive oxygen species (ROS) are involved in the regulation of numerous signaling pathways, as they are highly reactive and can cause (ir)reversible posttranslational modifications. While the role of ROS in other modes of cell death has been extensively studied, its impact on necroptotic signaling and cell death is far less clear. The current minireview discusses the evidence for and against a role of ROS in necroptosis.

Cardiac glycosides (CGs) or cardiotonic steroids, which constitute a group of naturally occurring compounds with a steroid-like structure, can act on Na + /K + -ATPase as a receptor and activate intracellular signaling messengers leading to a variety of cellular responses. Epidemiological studies have revealed that CGs, used for the treatment of cardiac disorders, may also be beneficial as anti-cancer agents. CGs, acting in combination with other chemotherapeutic agents, may significantly alter their efficiency in relation to cancer cell elimination, causing both sensitization and an increase in cancer cell death, and in some cases resistance to chemotherapy. Here we show the ability of CGs to modulate apoptotic response to conventionally used anti-cancer drugs. In combination with etoposide, CGs digoxin may enhance cytotoxic potential, thereby allowing the chemotherapeutic dose to be decreased and minimizing toxicity and adverse reactions. Mechanisms behind this event are discussed.

Dysregulation of the mitochondrial pathway of apoptosis, controlled by the BCL-2 family of proteins, leads to disease states including cancer. Rapid analysis of a cell’s dependency on the BCL-2 family of proteins is hindered by the complex interactions of more than a dozen proteins. Transcript or even protein levels are therefore generally insufficient to predict a cell’s response to perturbations like chemotherapy. Previously, we developed the JC-1 BH3 method to provide a same day functional assay to assess a cell’s propensity to undergo apoptosis and demonstrated its utility in predicting response to chemotherapy. We have now improved upon these methods to create a robust assay amenable to high throughput platforms using cytochrome c retention in formaldehyde fixed cells to remove the time sensitivity of JC-1 potential measurements. BH3 profiling by intracellular staining (iBH3) is suitable for 96- and 384-well formats, and can be used to directly screen candidate BH3-mimetic compounds for activity. When used as the final component of dynamic BH3 profiling (DBP), which uses a drug pretreatment prior to iBH3 to assess the change in profile due to treatment, it can predict the response of cells to chemotherapy days before they show signs of death.

Long QT syndrome refers to a characteristic abnormality of the electrocardiogram and it is associated with a form of ventricular tachycardia known as torsade-de-pointes and sudden arrhythmic death. It can occur as part of a hereditary syndrome or can be acquired usually because of drug administration. Here we review recent genetic, molecular and cellular discoveries and outline how they have furthered our understanding of this disease. Specifically we focus on compound mutations, genome wide association studies of QT interval, modifier genes and the therapeutic implications of this recent work.