Apoptosis and Cell Culture Technology
239
and non-lymphoid cell types [51-53]. However, due to the inability of anti-Fas
antibody to induce apoptosis in all myeloma cells it has been suggested that
variability in Fas expression may determine sensitivity to apoptosis [54].
The apoptosis pathway may be regulated in a number of different ways,
depending on the stress signal, the expression of several genes and the microenvironment at the site of activation. Apoptosis has been shown to require the
active synthesis of RNA and protein. In a number of circumstances the suppression of mRNA or protein by inhibitors such as actinomycin D and cycloheximide has led to the activation of apoptosis [55-59]. By contrast, these
inhibitors in some other situations either have no blocking effect or can actually
induce apoptosis [60, 61]. To explain such an apparent contradiction, it has
been suggested that if protein synthesis is inhibited, for apoptosis to continue the
cell must have all the necessary machinery for invoking the process of cell death
[41]. It is also suggested that cells may contain regulatory proteins that can
either inhibit or promote cell death and that catabolism of such proteins
determines whether stopping RNA or protein synthesis suppresses or induces
apoptosis [62]. It is increasingly clear that the signalling molecules in apoptosis
are also involved in other phenotypic expression such as proliferation and
differentiation and therefore it is difficult to separate the action of these molecules into distinct functional mechanisms.
4.1 Apoptosis Induced by Removal of Growth Factors and Nutrient
Deprivation
Cells survive and proliferate in culture when they are provided with adequate
amounts of nutrients and a suitable physical environment. The extracellular
environment must meet the essential requirement of cells to survive and whenever these conditions cease to exist, the cells begin to die. Growth factors and
energy suppliers are considered the most significant factors that influence the
biological activities of cells. The depletion of these nutrients may not only block
proliferation but also can induce apoptosis. It is now widely accepted that both
proliferation and apoptosis are controlled by an integrated mechanism and that
the signals which trigger or prevent one process also influence other aspects of
cellular activity. For example, the presence of mitogens to induce cell proliferation and survival factors to inhibit apoptosis are both necessary for maintainenance of cell viability [63]. Various types of growth factors, such as
insulin-like growth factor, interleukin 2, platelet-derived growth factors and
macrophage colony stimulating factor, have been reported to inhibit apoptosis
in many cell types (see Table 4). It is suggested that all animal cells are dependent
on the presence of at least one growth factor [64]. However, the main effect of
these growth factors is not necessarily to enhance both survival and proliferation as some of them are considered poor mitogens but potent survival factors
and vice versa. In cell culture technology, where continuous cell lines are
used, survival factors which provide anti-apoptic properties are probably
239
and non-lymphoid cell types [51-53]. However, due to the inability of anti-Fas
antibody to induce apoptosis in all myeloma cells it has been suggested that
variability in Fas expression may determine sensitivity to apoptosis [54].
The apoptosis pathway may be regulated in a number of different ways,
depending on the stress signal, the expression of several genes and the microenvironment at the site of activation. Apoptosis has been shown to require the
active synthesis of RNA and protein. In a number of circumstances the suppression of mRNA or protein by inhibitors such as actinomycin D and cycloheximide has led to the activation of apoptosis [55-59]. By contrast, these
inhibitors in some other situations either have no blocking effect or can actually
induce apoptosis [60, 61]. To explain such an apparent contradiction, it has
been suggested that if protein synthesis is inhibited, for apoptosis to continue the
cell must have all the necessary machinery for invoking the process of cell death
[41]. It is also suggested that cells may contain regulatory proteins that can
either inhibit or promote cell death and that catabolism of such proteins
determines whether stopping RNA or protein synthesis suppresses or induces
apoptosis [62]. It is increasingly clear that the signalling molecules in apoptosis
are also involved in other phenotypic expression such as proliferation and
differentiation and therefore it is difficult to separate the action of these molecules into distinct functional mechanisms.
4.1 Apoptosis Induced by Removal of Growth Factors and Nutrient
Deprivation
Cells survive and proliferate in culture when they are provided with adequate
amounts of nutrients and a suitable physical environment. The extracellular
environment must meet the essential requirement of cells to survive and whenever these conditions cease to exist, the cells begin to die. Growth factors and
energy suppliers are considered the most significant factors that influence the
biological activities of cells. The depletion of these nutrients may not only block
proliferation but also can induce apoptosis. It is now widely accepted that both
proliferation and apoptosis are controlled by an integrated mechanism and that
the signals which trigger or prevent one process also influence other aspects of
cellular activity. For example, the presence of mitogens to induce cell proliferation and survival factors to inhibit apoptosis are both necessary for maintainenance of cell viability [63]. Various types of growth factors, such as
insulin-like growth factor, interleukin 2, platelet-derived growth factors and
macrophage colony stimulating factor, have been reported to inhibit apoptosis
in many cell types (see Table 4). It is suggested that all animal cells are dependent
on the presence of at least one growth factor [64]. However, the main effect of
these growth factors is not necessarily to enhance both survival and proliferation as some of them are considered poor mitogens but potent survival factors
and vice versa. In cell culture technology, where continuous cell lines are
used, survival factors which provide anti-apoptic properties are probably