For every cell, there is a time to live and a time to die.
There are two ways in which cells die:
Cells that are damaged by injury, such as by
- they are killed by injurious agents
- they are induced to commit suicide
undergo a characteristic series of changes:
- mechanical damage
- exposure to toxic chemicals
Cells that are induced to commit suicide:
- they (and their organelles like mitochondria) swell (because the ability of the plasma membrane to control the passage of ions and water is disrupted)
- the cell contents leak out, leading to
- inflammation of surrounding tissues
The pattern of events in death by suicide is so orderly that the process is often called programmed cell death or PCD. The cellular machinery of programmed cell death turns out to be as intrinsic to the cell as, say, mitosis.
Programmed cell death is also called apoptosis. (There is no consensus yet on how to pronounce it; some say APE oh TOE sis; some say uh POP tuh sis.)
There are two different reasons.
- have their mitochondria break down with the release of cytochrome c
- develop bubble-like blebs on their surface
- have the chromatin (DNA and protein) in their nucleus degraded
- break into small, membrane-wrapped, fragments
- The phospholipid phosphatidylserine, which is normally hidden within the plasma membrane is exposed on the surface.
- This is bound by receptors on phagocytic cells like macrophages and dendritic cells which then engulf the cell fragments.
- The phagocytic cells secrete cytokines that inhibit inflammation.
- The resorption of the tadpole tail at the time of its metamorphosis into a frog occurs by apoptosis.
- The formation of the fingers and toes of the fetus requires the removal, by apoptosis, of the tissue between them.
- The sloughing off of the inner lining of the uterus (the endometrium) at the start of menstruation occurs by apoptosis.
- The formation of the proper connections (synapses) between neurons in the brain requires that surplus cells be eliminated by apoptosis
- Cells infected with viruses
- One of the methods by which cytotoxic T lymphocytes (CTLs) kill virus-infected cells is by inducing apoptosis [diagram of the mechanism]. (And some viruses mount countermeasures to thwart it.)
- Cells of the immune system
- As cell-mediated immune responses wane, the effector cells must be removed to prevent them from attacking body constituents. CTLs induce apoptosis in each other and even in themselves. Defects in the apoptotic machinery is associated with autoimmune diseases such as lupus erythematosus and rheumatoid arthritis.
- Cells with DNA damage
- Damage to its genome can cause a cell
Cells respond to DNA damage by increasing their production of p53. p53 is a potent inducer of apoptosis. Is it any wonder that mutations in the p53 gene, producing a defective protein, are so often found in cancer cells (that represent a lethal threat to the organism if permitted to live)?
- to disrupt proper embryonic development leading to birth defects
- to become cancerous.
The balance between:
- Cancer cells
- Radiation and chemicals used in cancer therapy induce apoptosis in some types of cancer cells.
The continued survival of most cells requires that they receive continuous stimulation from other cells and, for many, continued adhesion to the surface on which they are growing.
Some examples of positive signals:
- the withdrawal of positive signals; that is, signals needed for continued survival
- the receipt of negative signals
- growth factors for neurons
- Interleukin-2 (IL-2), an essential factor for the mitosis of lymphocytes
Receipt of negative signals
There are 2 different mechanisms by which a cell commits suicide by apoptosis.
- increased levels of oxidants within the cell
- damage to DNA by these oxidants or other agents like
- molecules that bind to specific receptors on the cell surface and signal the cell to begin the apoptosis program.
These death activators include:
- Tumor necrosis factor - alpha ("TNF") that binds to the TNF receptor
- Lymphotoxin (also known as tumor necrosis factor - beta) that also binds to the TNF receptor
- Fas ligand (FasL), a molecule that binds to a cell-surface receptor named Fas (also called CD95)
- one generated by signals arising within the cell
- the other triggered by death activators binding to receptors at the cell surface.
- Fas ligand (FasL)
Apoptosis triggered by internal signals
- In a healthy cell, the outer membranes of its mitochondria express the protein Bcl-2 on their surface.
- Bcl-2 is bound to a molecule of the protein Apaf-1.
- Internal damage in the cell causes Bcl-2
- to release Apaf-1
- to no longer keep cytochrome c from leaking out of the mitochondria
- The released cytochrome c and Apaf-1 bind to molecules of caspase 9.
- The resulting complex of
is called the apoptosome.
- cytochrome c
- caspase 9
- (and ATP)
- These aggregate in the cytosol.
- Caspase 9 is one of a family of over a dozen caspases. They are all proteases. They get their name because they cleave proteins - mostly each other - at aspartic acid (Asp) residues).
- Caspase 9 cleaves and, in so doing, activates other caspases.
- The sequential activation of one caspase by another creates an expanding cascade of proteolytic activity (rather like that in blood clotting and complement activation) which leads to
- digestion of structural proteins in the cytoplasm
- degradation of chromosomal DNA and
- phagocytosis of the cell
Example (right): When cytotoxic T cells recognize (bind to) their target,
- Fas and the TNF receptor are integral membrane proteins with their receptor domains exposed at the surface of the cell
- binding of the complementary death activator (FasL and TNF respectively) transmits a signal to the cytoplasm that leads to
- activation of caspase 8
- caspase 8 (like caspase 9) initiates a cascade of caspase activation leading to
- phagocytosis of the cell.
- they produce more FasL at their surface.
- This binds with the Fas on the surface of the target cell leading to its death by apoptosis.
The early steps in apoptosis are reversible - at least in C. elegans. In some cases, final destruction of the cell is guaranteed only with its engulfment by a phagocyte.
Apoptosis and Cancer
Some cancer-causing viruses use tricks to prevent apoptosis of the cells they have transformed.
Even cancer cells produced without the participation of viruses may have tricks to avoid apoptosis.
- Two human papilloma viruses (HPV) have been implicated in causing cervical cancer. One of them produces a protein (E6) that binds and inactivates the apoptosis promoter p53.
- Epstein-Barr Virus (EBV), the cause of mononucleosis and a cause of Burkitt's lymphoma
- produces a protein similar to Bcl-2
- produces another protein that causes the cell to increase its own production of Bcl-2. Both these actions make the cell more resistant to apoptosis (thus enabling the cancer cell to continue to proliferate).
- Some B-cell leukemias and lymphomas express high levels of Bcl-2, thus blocking apoptotic signals they may receive. The high levels result from a translocation of the BCL-2 gene into an enhancer region for antibody production. [Discussion].
- Melanoma (the most dangerous type of skin cancer) cells avoid apoptosis by inhibiting the expression of the gene encoding Apaf-1.
- Some cancer cells, especially lung and colon cancer cells, secrete elevated levels of a soluble "decoy" molecule that binds to FasL, plugging it up so it cannot bind Fas. Thus, cytotoxic T cells (CTL) cannot kill the cancer cells by the mechanism shown above.
- Other cancer cells express high levels of FasL, and can kill any cytotoxic T cells (CTL) that try to kill them because CTL also express Fas (but are protected from their own FasL).
Apoptosis and AIDS
The hallmark of AIDS (acquired immunodeficiency syndrome) is the decline in the number of the patient's CD4+ T cells (normally about 1000 per microliter (µl) of blood). CD4+ T cells are responsible, directly or indirectly (as helper cells), for all immune responses. When their number declines below about 200 per µl, the patient is no longer able to mount effective immune responses and begins to suffer a series of dangerous infections.
What causes the disappearance of CD4+ T cells?
HIV (human immunodeficiency virus) invades CD4+ and one might assume that it this infection by HIV that causes the great dying-off of CD4+ T cells. However, that appears not to the main culprit. Fewer than 1 in 100,000 CD4+ T cells in the blood of AIDS patients are actually infected with the virus.
So what kills so many uninfected CD4+ cells?
The answer is clear: apoptosis.
The mechanism is not clear. There are several possibilities.
One of them:
For many years it has been known that certain parts of the body
- All T cells, both infected and uninfected, express Fas.
- Expression of a HIV gene (called Nef) in a HIV-infected cell causes
- the cell to express high levels of FasL at its surface
- while preventing an interaction with its own Fas from causing it to self-destruct.
- However, when the infected T cell encounters an uninfected one (e.g. in a lymph node), the interaction of FasL with Fas on the uninfected cell kills it by apoptosis.
are "immunologically privileged sites". Antigens within these sites fail to elicit an immune response.
- the anterior chamber of the eye
- the testes
It turns out that cells in these sites differ from the other cells of the body in that they express high levels of FasL at all times.
Thus antigen-reactive T cells, which express Fas, would be killed when they enter these sites.
This finding raises the possibility of a new way of preventing graft rejection.
If at least some of the cells on a transplanted kidney, liver, heart, etc. could be made to express high levels of FasL, that might protect the graft from attack by the T cells of the host's cell-mediated immune system. If so, then the present need for treatment with immunosuppressive drugs for the rest of the transplant recipient's life would be reduced or eliminated.
So far, the results in animal experiments have been mixed. Allografts engineered to express FasL have shown increased survival for kidneys but not for hearts or islets of Langerhans.
23 July 2001