Apoptosis – Concept, phases, function and relationship with cancer

We explain what is apoptosis, what it does and what are its phases. We also talk about neuronal apoptosis and the differences with necrosis.

cell apoptosis
Apoptosis is a controlled process of cell death.

What is apoptosis?

Apoptosis is a cellular self-destruction mechanism of cellular that allows the body to control the development and growth of cell, and rule out those with anomalies or defects. This process of programmed cell death operates through genetically controlled cellular signals and has an important preventive function in the body.

This process can occur in the body in two ways:

  • Negative induction. By isolating the cells destined to die, removing its growth factors, cutting suppressive activity or completely clipping it off from the the surrounding cells.
  • Positive induction. By activating proteins or other types of organic compounds that trigger cell death, or even sending conflicting signals to the cells marked to die.

In both cases, apoptosis occurs methodically, in an organized fashion, not chaotic but following strict guidelines for every “cell suicide”. The immune system will eventually take care of the rest, eliminating the residues.

Apoptosis is an integral part of the body’s protection and renewal mechanism. It should not affect the cell system or produce significant damage. If necessary, young cells of the same type as those eliminated are produced at the same rate, in the same time.

Apoptosis functions

Apoptosis plays a vital planning role in the organism, and takes care of the following functions:

  • Get rid of abnormal cells who were born undersized, who have abnormalities or who have been infected with viruses or have suffered DNA damage.
  • Eliminate some old and faulty cells and replace them with new cells that fulfill the same function, keeping the body healthy. This is particularly important in the case of the body’s defense cells, which can develop a tendency to attack healthy tissues by mistake.
  • Participate during the early formation of the organism during key stages of its development, such as embryonic stages when tissue must be lost or separated. For example, when the fingers are formed, they are united by a membrane. In order to separate the fingers, the cells of the membrane must be programmed to die. The same happens to the uterine endometrium during menstruation.

Apoptosis phases

Apoptosis has two recognizable phases, which are:

  • Decision phase. The apoptosis process begins with the reception by certain cells of a death signal, or the instruction to commit suicide. Then the cell must “decide” if it will survive or start the death processes. Mitochondria are fundamental organelles during the decision phase: they generate a multiproteic chemical complex able to release intramitochondrial content such as cytochrome C, certain hormones of the caspase family and other triggers of apoptosis.
  • Execution phase. Once the cell has “decided” to die, a process of degradation of chromatin proteins begins inside it, setting in motion everything secreted in the previous phase by the mitochondria. This involves a series of ordered biochemical reactions, culminating in cellular autolysis – or self-disintegration by releasing own enzymes. The execution phase ends up with a residue clean-up controlled by the immune system, and the removal of the unwanted material from the tissue.

Apoptosis and necrosis

cell apoptosis tissue necrosisNecrosis is a chaotic process that does not affect individual cells but entire tissues.

Apoptosis and necrosis should not be confused. The first is a natural, healthy and orderly process. On the contrary, necrosis it the unscheduled and unwanted cell death, known as tissue death. Necrosis can put the organism’s health at risk.

The fundamental difference is that necrosis is a chaotic, accidental and irreversible process, during which a massive number of cell begin to die massively, affecting the whole tissue.

Necrosis may occur from various causes: uncontrolled bacterial infections, the interruption of blood flow to certain tissues (vascular accidents) or the action of toxins such as poisons, lethal substances or high-level radiation.

It is also common when a person’s limbs have been exposed to extreme cold. Such cases often lead to amputation, as the necrosis spreads throughout the body and can cause a general septic reaction (generalized sepsis).

Neural apoptosis

The cells of the nervous system and the brain, called neurons, also go through the natural process of apoptosis, during which old neurons are supplanted by fresh ones. However, the (re)generation of neurons is much slower and more sporadic, in comparison  than the rest of the ordinary cells of the body.

Consequently, over time, our nervous system deteriorates, causing loss of brain efficiency, late reactions or even loss of certain functions, as it happens when we get old. In fact, many of the mental ailments afflicting aged people, such as senile dementia, depend on this process.

There are other pathologies, such as epilepsy or Alzheimer’s disease, in which neural apoptosis is combined with a malfunction of the glial cells. They are responsible for absorbing and discarding the remains of dead neurons and, when there is a malfunction, the waste wouldn’t be removed and could cause problems. The waste accumulates and interferes with the regular functioning of the brain, causing the loss of brain mass or leaving scars and injuries that contribute to the perpetuation of the problem.

Scientific experiments are currently performed on apoptosis, with the goal of finding a cure for the aforementioned pathologies and other related ailments, such as cancer.

Apoptosis and cancer

Various internal or external causes can lead to the appearance of flawed cells, carriers of damaged DNA. The cell attempts to repair the harm caused by the damaged DNA or, if that is impossible, will sentence itself to programmed death. This way, the body prevents damaged cells from reproducing, spreading the genetic failure.

Should the intended mechanisms fail, the same immune system can exert pressure to force the cell to undergo apoptosis. If the process is successful, the proliferation of possible cancer cells, for example, is prevented.

The problem is that many precancerous cells do not respond to internal or external apoptosis signals. They start to divide uncontrollably and create tumors, crazed masses of cells reproducing without stopping.

This is the reason why many of the current studies on cancer focus on understanding why cancer cells block the natural apoptosis functions. A possible cure could be an external intervention able to reignite the process, without applying invasive and highly destructive therapies, such as radiotherapy or chemotherapy.