What is apoptosis, and why is it important? (2024)

Biological mechanisms

The term apoptosis is often used interchangeably with programmed cell death. In the strictest sense, programmed cell death may be applied to other forms of cell death that require gene expression without fulfilling some, or all, of the morphological criteria of apoptosis.2 Whatever the definition, studies clearly show that apoptosis is genetically regulated.

In its simplest model, the stages of apoptosis may be considered as initiation, genetic regulation, and effector mechanisms (figure).3 Initiators of apoptosis include anticancer drugs, gamma and ultraviolet irradiation, deprivation of survival factors such as interleukin-1, and various other cytokines that activate “death receptors” such as Fas and tumour necrosis factor receptors. Through a variety of pathways, these stimuli in turn generate a characteristic pattern of gene expression.

The bcl-2 family of genes is the best studied and includes at least 20 members; some are pro-apoptotic or “death genes” and some are anti-apoptotic or “survival genes,” including bcl-2 itself. The tumour suppressor gene p53 is also a well characterised apoptotic agent. The principal effectors are a family of proteases termed caspases. Studies in the nematode Caenorhabditis elegens, the fruitfly Drosophila, and the mouse indicate that the molecular machinery of apoptosis is evolutionarily conserved and intrinsic to all metazoan cells.

Assessment of apoptosis

Morphological assessment is the standard method for identifying and quantifying apoptosis. Other approaches include the use of fluorescence dyes to stain for condensed nuclei or exposed cell surface phosphatidylserine and the detection of fragmented DNA by terminal transferase mediated dUTP-biotin nick end labelling (TUNEL). Levels of cell death in a tissue are often expressed as an apoptotic index. This approach has serious limitations, mainly because of uncertainty about the duration of apoptosis (which may be less than six hours).4 Thus, for instance, a small number of apoptotic cells observed in a static analysis may reflect a considerable contribution to cell turnover.

Physiological role

The first role of apoptosis is during intrauterine development. It helps to sculpture organ shape and carve out the interdigital webs of the fingers and toes. Apoptotic mechanisms are important determinants of fetal abnormalities; experiments have shown that wild type p53 mouse embryos will readily abort after radiation induced teratogenesis, whereas p53 null embryos will not.5 Both the nervous system and the immune system arise through overproduction of cells followed by the apoptotic death of those that fail to establish functional synaptic connections or productive antigen specificities.

Such massacre or altruistic behaviour requires a tightly regulated system. In adulthood, about 10 billion cells die every day simply to keep balance with the numbers of new cells arising from the body's stem cell populations. This normal homoeostasis is not just a passive process but regulated through apoptosis. The same mechanisms serve to “mop up” damaged cells. With ageing, apoptotic responses to DNA damage may be less tightly controlled and exaggerated, contributing to degenerative disease. Alternatively, the apoptotic responses may show reduced sensitivity, contributing to susceptibility to cancer.6

Altered apoptosis and disease

There is now a long list of diseases associated with altered cell survival.7 Increased apoptosis is characteristic of AIDS; neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis; ischaemic injury after myocardial infarction, stroke, and reperfusion; and in autoimmune diseases such as hepatitis and graft versus host disease. Decreased or inhibited apoptosis is a feature of many malignancies, autoimmune disorders such as systemic lupus erythematosus, and some viral infections.

The role of apoptosis in cancer has probably received the greatest research effort.8 Observations that patterns of spontaneous and induced apoptosis differ between the small and large intestine has led to a plausible explanation for the differences in incidence of cancer between these two sites.9 Studies in p53 null mice show an increased preponderance of premature tumours and offer strong evidence that such apoptotic related genes are pivotal to development of tumours.

In addition, tumours develop methods to evade elimination by the immune system; one such mechanism involves tumours expressing Fas, which enables them to delete (by apoptosis) antitumour lymphocytes. This phenomenon is known as the “tumour counterattack.”10 There is also increasing evidence that systemic stimuli such as insulin-like growth factor I (anti-apoptotic) and insulin-like growth factor binding protein 3 (pro-apoptotic) may influence the development and progression of many common cancers.11

Potential treatments

This brief review has shown that many human diseases may result when cells die that shouldn't or others live that should die. Modulation of apoptotic processes may thus offer valuable methods of treatment. It is now known that many existing drugs (for example, non-steroidal anti-inflammatories) act by altering the levels of apoptosis. Virtually all cytotoxic drugs and radiotherapy programmes induce apoptosis in tumour cells, and resistance to apoptosis is associated with treatment failure. These therapies also induce apoptosis in normal cells, and side effects on bone marrow, gut, and oral mucosa limit the dose that can be used. Many more new treatment strategies are currently in preclinical trials and show promise (box).3,12 If future clinical studies are fruitful, this translation from basic science to clinical practice will be unique as it will affect not just one, but a broad range of disorders—and many patients will benefit.

Footnotes

References