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Mohamed A1-Rubeai
the most important molecules required to maintain both high viability and
growth.
Serum is essential for the growth and survival of mammalian cells. It
provides growth factors, protein, lipids and other nutrients to cells in culture.
The removal of serum has been shown to induce apoptosis in many cell lines
such as hybridomas and plasmocytomas [21]. However, when serum or plasma
is present in the medium at a high concentration, apoptotic cell death is induced,
although such death can be prevented by the addition of thiol compounds such
as L-cyst(e)ine and L-tryptphan [65].
The mechanism by which the removal of growth factors induces apoptosis
has been shown to involve the expression of the protooncogene c-myc gene
product. Fibroblasts that express c-myc grow well in serum supplemented
medium but undergo apoptosis if the serum is removed [66]. It was also
observed that, in the absense of growth factors and down regulation of c-myc,
cell arrest in Go occurs. This suggests that c-myc may activate a regulatory
programme that leads either to proliferation or to apoptosis.
Apoptosis also occurs when cells are deprived of energy suppliers such as
glutamine and glucose. An important characteristic of apoptosis in continuous
cell lines of haemopoeitic origin is that it is induced by removal of glutamine or
glucose [21, 67]. The role of these molecules in the apoptotic pathways is not yet
clear but since both glutamine and glucose levels regulate both the cell growth
and catobolic pathway operation, their deprivation may prevent DNA synthesis
and block cells into G1, thereby triggering apoptosis. This suggestion, however,
does not explain why cells in S, G2 and M phases also undergo apoptosis. It is
more likely that apoptosis is initiated when maintenance energy required
to produce ATP to drive the biosynthesis of cellular material is reduced to a
critical level, resulting in the activation of a regulatory protein that promotes
apoptosis.
4.2 Genetic Regulation of Apoptosis
Although there have been suggestions that apoptosis may not be controlled at
the gene expression level, there is today a large body of data indicating that the
process of cell death is under genetic control and that the basic machinery to
induce apoptosis is present in all mammalian cells. However, the regulatory
pathway of programmed cell death is influenced by the type and origin of the
signal which in turn either inhibits or activates certain genes. Some of the earliest
evidence of the presence of the so called 'death genes' came from the studies on
the nematoed Caenorhabdities elegans. In this organism, 3 genes have been
identified: ced-3, ced-4 and ced-9 genes, the first two of which are required for all
somatic cell death to occur while the third gene is required to protect cells that
should survive from undergoing cell death (for review see [68]). The ced3 gene
product is similar to a family of mammalian cysteine proteases which includes
interleukin-1 ]3-converting enzyme (ICE). Many observations suggest that these
Mohamed A1-Rubeai
the most important molecules required to maintain both high viability and
growth.
Serum is essential for the growth and survival of mammalian cells. It
provides growth factors, protein, lipids and other nutrients to cells in culture.
The removal of serum has been shown to induce apoptosis in many cell lines
such as hybridomas and plasmocytomas [21]. However, when serum or plasma
is present in the medium at a high concentration, apoptotic cell death is induced,
although such death can be prevented by the addition of thiol compounds such
as L-cyst(e)ine and L-tryptphan [65].
The mechanism by which the removal of growth factors induces apoptosis
has been shown to involve the expression of the protooncogene c-myc gene
product. Fibroblasts that express c-myc grow well in serum supplemented
medium but undergo apoptosis if the serum is removed [66]. It was also
observed that, in the absense of growth factors and down regulation of c-myc,
cell arrest in Go occurs. This suggests that c-myc may activate a regulatory
programme that leads either to proliferation or to apoptosis.
Apoptosis also occurs when cells are deprived of energy suppliers such as
glutamine and glucose. An important characteristic of apoptosis in continuous
cell lines of haemopoeitic origin is that it is induced by removal of glutamine or
glucose [21, 67]. The role of these molecules in the apoptotic pathways is not yet
clear but since both glutamine and glucose levels regulate both the cell growth
and catobolic pathway operation, their deprivation may prevent DNA synthesis
and block cells into G1, thereby triggering apoptosis. This suggestion, however,
does not explain why cells in S, G2 and M phases also undergo apoptosis. It is
more likely that apoptosis is initiated when maintenance energy required
to produce ATP to drive the biosynthesis of cellular material is reduced to a
critical level, resulting in the activation of a regulatory protein that promotes
apoptosis.
4.2 Genetic Regulation of Apoptosis
Although there have been suggestions that apoptosis may not be controlled at
the gene expression level, there is today a large body of data indicating that the
process of cell death is under genetic control and that the basic machinery to
induce apoptosis is present in all mammalian cells. However, the regulatory
pathway of programmed cell death is influenced by the type and origin of the
signal which in turn either inhibits or activates certain genes. Some of the earliest
evidence of the presence of the so called 'death genes' came from the studies on
the nematoed Caenorhabdities elegans. In this organism, 3 genes have been
identified: ced-3, ced-4 and ced-9 genes, the first two of which are required for all
somatic cell death to occur while the third gene is required to protect cells that
should survive from undergoing cell death (for review see [68]). The ced3 gene
product is similar to a family of mammalian cysteine proteases which includes
interleukin-1 ]3-converting enzyme (ICE). Many observations suggest that these