Difference Between Autophagy and Apoptosis

Main Difference – Autophagy vs Apoptosis

Autophagy and apoptosis are self-degradative processes occurring naturally inside the cell, balancing the functioning of multicellular organisms during their lifetime. Autophagy helps the cell to survive under stressful conditions such as deficiency of nutrients. Apoptosis causes cell death due to either a physiological or pathological process. The main difference between  autophagy and apoptosis is that apoptosis is a predefined cell suicide, where the cell actively destroys itself, maintaining a smooth functioning in the body whereas autophagy is a self-degradative process of its own components, balancing the sources of energy during development.

This article explains,

1. What is Autophagy
      – Definition, Characteristics, Process
2. What is Apoptosis
      – Definition, Characteristics, Process
3. What is the difference between Autophagy and Apoptosis

What is Autophagy

Autophagy is a fundamental catabolic mechanism where the cell degrades either dysfunctional or unnecessary cellular components. This mechanism is driven by lysosomes. Autophagy promotes the survival of the cell during stressful conditions by maintaining cellular energy levels. Autophagosomes are formed by the fusion of dysfunctional organelles with the lysosomes. The formation of autophagosome is induced by a class 3 phosphoinositide-3-kinase, Atg 6 along with the ubiquitin or ubiquitin-like proteins. Autophagosome is trafficked through the cytosol in order to fuse with lysosomes, forming an autolysosome. An autolysosome is a double-membrane structure, where the degradation of the unnecessary organelle occurs by the hydrolytic enzymes contained by the lysosomes.

Autophagy recycles damaged proteins, aggregates, and organelles in the cell, clearing up the cell and providing building blocks in order to replace the depleted cellular components. It provides protection against the stress which is created by nutrient deprivation as well. Autophagy is pro-survival, allowing the cell to undergo stress. In contrast, apoptosis let the cell die. Sometimes, autophagy promotes the cell death by destroying active organelles in the cell like mitochondria, where the cell can no longer survive. Both autophagy and apoptosis are directly linked with each other. Autophagy can control apoptosis and apoptosis can control autophagy, the excessive autophagy let the cell dies. The mechanism of autophagy is shown in figure 1.

Figure 1: Autophagy

What is Apoptosis

Apoptosis is a programmed cell death (PCD), which leads to characteristic changes in the cell and finally death. It is a regular and controlled mechanism of the growth and development of an organism. It is also called cellular suicide since the cell itself takes part in the death. Apoptosis allows the maintaining of the balance of cell multiplication. The specific proteases called caspases are involved in the regulation of the whole process of apoptosis.

Apoptosis occurs through well-defined, consequent morphological changes. The cell membrane blebs, the cell shrinks by drying, the nucleus becomes fragmented, chromatin condenses, and finally chromosomal DNA becomes fragmented. Condensation of chromatin in the nucleus is a hallmark of apoptosis. Hence, apoptosis affects the nucleus, cell membrane, cytoplasm as well as mitochondria. Small membrane-bound vesicles called apoptotic bodies are formed, containing the cell contents. During apoptosis, the cellular contents are not released into the extracellular environment. The two mechanisms of autophagy and apoptosis are shown in figure 2.

Figure 2: Autophagy and Apoptosis

Difference Between Autophagy and Apoptosis

Names

Autophagy: Autophagy is called self-eating.

Apoptosis: Apoptosis is called cell suicide.

Definition

Autophagy:  Autophagy is a self-degradative process of its own components, balancing the sources of energy during development.

Apoptosis: Apoptosis is a pre-defined cell suicide, where the cell actively destroys itself, maintaining a smooth functioning in the body.

Role

Autophagy: Autophagy balances the energy sources in the cell depending on the cellular requirements.

Apoptosis: Apoptosis balances the number of cells in a multicellular organism.

Causes

Autophagy: Autophagy is caused by cellular stress like starvation.

Apoptosis: Apoptosis is caused by intracellular programs.

Lysosomes

Autophagy: Lysosomes are fused with autophagosomes, forming autolysosomes. Hydrolytic enzymes in lysosomes are involved in the degradation.

Apoptosis: The contents in lysosomes are not involved in the process of apoptosis.

Mitochondria

Autophagy: Mitochondria do not become leaky during autophagy.

Apoptosis: Mitochondria become leaky during apoptosis.

Survival

Autophagy: Autophagy allows the cell to survive stress.

Apoptosis: Apoptosis does not allow the cell to survive.

End Product

Autophagy: The end products can be used as building blocks during the regeneration of the loosed organelles. The generated waste is eliminated by exocytosis.

Apoptosis: The end products of the apoptosis are destroyed by the phagocytes.

Excessive Occurrence

Autophagy: Excessive autophagy leads to cell death.

Apoptosis: Excessive apoptosis leads to atrophy.

Inhibitors

Autophagy: The 3-Methyladenine is an inhibitor of autophagy.

Apoptosis: Z-VAD-FMK is one of the well-defined apoptosis modulator, which inhibits the process.

Conclusion

Autophagy and apoptosis are two self-degradative mechanisms found in cells of the multicellular organisms. Autophagy is involved in the destruction of dysfunctional organelles inside the cell, responding to environmental stress like starvation. Hence, autophagy maintains the balance of cellular energy sources. However, autophagy lets the cell survive in harsh conditions. In contrast, apoptosis lets the cell dies, maintaining the cell number in the organism during its development. It is required for the smooth functioning of the organism. Infections like pathological stimuli induce apoptosis as well. Thus, the main difference between autophagy and apoptosis is their influence on the cell.

Reference:
1. Eskelinen, Eeva-Liisa. “Macroautophagy in Mammalian Cells.” Madame Curie Bioscience Database [Internet]. U.S. National Library of Medicine, 01 Jan. 1970. Web. 02 Apr. 2017.
2. Gump, Jacob M., and Andrew Thorburn. “Autophagy and apoptosis- what’s the connection?” Trends in cell biology. U.S. National Library of Medicine, July 2011. Web. 02 Apr. 2017.
3. Alberts, Bruce. “Programmed Cell Death (Apoptosis).” Molecular Biology of the Cell. 4th edition. U.S. National Library of Medicine, 01 Jan. 1970. Web. 02 Apr. 2017.

Image Courtesy:
1. “Autophagy diagram PLoS Biology” By Juhasz G, Neufeld TP. Original images by Ryan Scott (B) and Dr. Noboru Mizushima (C). – Autophagy: A Forty-Year Search for a Missing Membrane Source. PLoS Biol 4(2): e36. doi:10.1371/journal.pbio.0040036 (CC BY 2.5) via Commons Wikimedia
2.”Paraptosis” By Jhayes21 – Own work (CC BY-SA 3.0) via Commons Wikimedia