To Die or not to Die

        Everything and everyone that is living has a limit to life, down to the single unit of living things our cells go through the same fate, death. During development, many regulatory mechanisms help direct and promote either cell differentiation and growth or the opposite cell death. Imagine trillions and trillions of cells accumulating.  If they  happened to live forever, it would cause many mechanisms that regulate physiological homeostasis to fail. We would all have high blood pressure and massive edema and eventually death if certain cells did not have a regulatory control mechanism. Cell death is necessary and specifically controlled. Three  main ways a cell can undergo cell death are apoptosis, necrosis, or autophagy. All three of these play vital roles in controlling many processes in development, the development and progression of major diseases and basic, daily functions. These cell death processes are also essential therapeutic targets to treat disease a variety of disease states.  Promoting death of cancer cells or preventing death of neurons  in neurodegenerative diseases like  Parkinson’s disease or  other tauopathies may be beneficial.

        During the development of cancer, defects in apoptosis pathway results in an accumulation of cells leads formation of a tumor. A crucial step in apoptosis involves activation of caspases (i.e., caspases 3, 7 and 9) by signaling and activating MEK through dimerization of the receptor which activates the ERK and MAPK which can phosphorylate caspase 9. Another way to phosphorylate caspase 9 is through mitosis and buildup of CDK1 and Cyclin B. Apaf-1 is also needed to form a complex with caspase 9 in order to activate caspase 3 and 7. This occurs by the BH3 pro cytochrome c activator and Bcl-2 a anti cytochrome c activator to bind to Apaf-1. After Caspases 3 and 7 are activated, apoptosis can be initiated (Fig. 1).  Overall apoptosis does not cause toxic build up at the site of cell death and has a very rapid turnover.

Figure 1: Caspase-9 is activates by formation of the apoptosome and by forming a multimeric complex with Apaf-1. When cytochrome c is released. This can be enhanced by proapoptotic signal BH3 or inhibited Bcl-2 an antiapoptotic protein class². 

        Like apoptosis, necrosis, is also a clinically important process of cell death.  Necrosis is involved in tissue damage that occurs during traumatic brain injury, myocardial infarctions, strokes and some liver diseases.  While it is also initiated following DNA fragmentation and dysregulate mitochondrial function, it fails to be subject to processed by lysosomal degradation.  Instead, necrotic cells spill their  toxic contents out into the extracellular space aiding in an increase in tissue damage and inflammation brought on by cytokines. Assays to distinguish different forms of cell death are used, including use of  a deoxynucleotidyl transferase-mediated biotin-dUTP nick-end labeling assay to detect DNA fragmentation³. Ischemia and hypoxia are important triggers of cell necrosis. While necrosis also has tightly regulated features, targeting these has limited  therapeutic value, since the damage is already done such as many agents in Spinal cord injuries have been related to inhibiting further necrosis and reducing inflammation through the process of apoptosis. Energy starvation or oxygen starvation, ROS production can generate pro-necrotic signals which consist of inflammatory mediators and their ligands. Cellular calcium overload accompanies depletion of intracellular adenosine triphosphate (ATP), since calcium pumps that sequester cytosolic calcium are energy dependent. Calcium overload is a potent inducer of cell death³. An indirect therapy suggests that activating protein kinase (MAP) turns necrosis signaling into stress activates induced apoptosis. Inhibition of p38 is another way to also reduce cellular necrosis. A common pathology associated with necrosis is pyknotic cells which degrade and increase toxicities to surrounding cells (Fig 1). This displays the importance of a balancing act of pro and antiapoptotic signals.

Figure 2: Distinctive features of the CSC (cancer stem cell) niche, including hypoxia, reduced nutrient availability and acidity (H⁺), promote high levels of basal autophagy in CSCs⁴. This figure explains the Basal autophagy pathway in different cancer cells that are self-renewing rapidly. These cells can undergo basil autophagy if high acidity, low glucose or oxygen is present acutely, this initiates different signaling mechanisms such as inhibition of ROS build up, migration leading to metastasis. And even further cell differentiation.

        Autophagy is the third natural cell death process and occurs when the body undergoes extreme change in pH, glucose deficiency, and hypoxic conditions. This pathway has the most potential for being targeted for the development of new therapies to treat  cancer. Autophagy can occur during fasting and inhibit production of reactive oxygen species and cell death.  Mechanistically it is controlled/initiated by the mTOR pathway and activates through the ATG/ULK1 and Class III PI3K complex. Next the autophagosome formation is meditated by ATG7,10,5,12. It them fuses with a lysosome degrades the waste into amino acids, lipids and nucleotides to be recycled to a new cell system. In a way autophagy cleanses the body to a degree and is a reset for cellular processes and resulting in a less toxic and efficient/stable cellular enviorment⁵.

        Taking all cell death pathways into account, we can better understand the pros and cons of cell death mechanisms therapeutically  for each specified disease whether its cancer and for example Hotchkiss et al classified that decreased apoptosis is associated with diseases in over half of neoplasms. Enhancing apoptosis TP53 guardian can reduce the risk of cancer by 8-fold. Also stated in a table Leukemia and multiple myeloma and colorectal cancers have been treated with obatoclax which induces apoptosis by inhibiting antiapoptotic Bcl-2 family members. Or activates death receptors for treating Hodgkin’s lymphoma⁶. Another example is giving hydroxychloroquine to inhibit autophagy in breast cancer and prostate cancer, which can ultimately inhibit the progression and metastasis of early stages of these cancers. It is important to look at death as good at times and appreciate the amount of interplay of genes and signaling cascades there are for normal biochemical processes within our complex systems. This all concludes the importance apoptosis is for normal function and health to our biological systems.

References:

(1)    Reed, J. C. (2000). Mechanisms of Apoptosis. The American Journal of Pathology, 157(5), 1415–1430. doi: 10.1016/s0002-9440(10)64779-7

(2)    Allan, Lindsey & Clarke, Paul. (2008). Allan LA, Clarke PRA mechanism coupling cell division and the control of apoptosis. SEB Exp Biol Ser 59: 257-265. SEB experimental biology series. 59. 257-65.

(3)    Szabó, C. (2005). Mechanisms of cell necrosis. Critical Care Medicine, 33(Suppl). doi: 10.1097/01.ccm.0000187002.88999.cf

(4)    Boya, P., Codogno, P., & Rodriguez-Muela, N. (2018). Autophagy in stem cells: repair, remodelling and metabolic reprogramming. Development, 145(4). doi: 10.1242/dev.146506

(5)    Yang, Z., & Klionsky, D. J. (2009). An Overview of the Molecular Mechanism of Autophagy. Current Topics in Microbiology and Immunology Autophagy in Infection and Immunity, 1–32. doi: 10.1007/978-3-642-00302-8_1

(6)    Hotchkiss, R. S., Strasser, A., McDunn, J. E., & Swanson, P. E. (2009). Cell death. The New England journal of medicine, 361(16), 1570–1583. doi:10.1056/NEJMra0901217

Amal Agarwal University of Kentucky MSMS student Class of 2019