Castilho Leda R., Moraes Angela Maria (Ed.) Animal Cell Technology: From Biopharmaceuticals to Gene Therapy

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decrease in cell size, by a controlled and uniform DNA fragmentation,

and by speciﬁc changes in the chromatin, which becomes highly con-

densed and can appear concentric with the nuclear membrane. The only

alteration that may be observed in the organelles is a swelling of the

endoplasmic reticulum (ER) (McCarthy, 2002). However, during cultiva-

tion processes in vitro, apoptosis ends up in a stage called secondary

necrosis (Figure 7.3; see color section), where, due to the absence of

specialized phagocytic cells, the apoptotic cells and apoptotic bodies

release their contents into the extracellular medium, as occurs in necrosis

(Al-Rubeai, 1998).

7.4 Inﬂuence of environmental conditions on the induction of

cell death

During in vitro cultivation of animal cells, any change relative to the

optimal environmental conditions may result in a rapid decrease in cell

viability. This phenomenon is more frequently observed in cell lines that

exhibit high susceptibility to apoptosis (Singh and Al-Rubeai, 1998). Any

agent or condition that alters the cell metabolism may activate pro-

grammed cell death. Cells exposed to high stress levels due, for instance,

to the presence of a high concentration of toxins, to large changes in pH,

or to high agitation rates, generally die by necrosis, because there is no

time for the cell to generate a response, that is, cell death is instantaneous.

At intermediate stress levels, the cell can be damaged without instanta-

neously dying, having time to activate its programmed death mechanism.

Under low levels of environmental stress, the cells may produce heat-

shock proteins (HSPs), which allow their survival until the stimulus is

eliminated. However, once a threshold level of resistance is surpassed,

survival becomes impossible and the cells activate their apoptotic path-

ways (Cotter and Al-Rubeai, 1995).

Cells in culture are usually exposed to variations in environmental

conditions and, depending upon these conditions, cells can survive or die.

In biotechnological processes, the usual aim is to maintain cells alive and

productive. There are a large number of factors that inﬂuence the quality

of a cell culture, including an appropriate supply of nutrients, pH, shear

stress, accumulation of metabolites, and oxygen availability. Studies have

already shown that several alterations in culture conditions, such as the

lack of nutrients or serum, increase in osmotic pressure, and maintaining

cells in suspension culture, may induce apoptotic mechanisms. The critical

factors determining apoptosis vary between cell lines (Singh et al., 1994;

Zanghi et al., 1999; Kim and Lee, 2002).

7.4.1 Depletion of nutrients and growth factors

Cells survive and proliferate according to the addition of appropriate

amounts of nutrients and growth factors, which are provided by the

culture media. When these requirements are not met, cells die. The

availability of nutrients and growth factors is considered the most signiﬁ-

cant factor inﬂuencing the biological activity of cells. The lack or depletion

152 Animal Cell Technology

of these substances can not only lead to arrest of proliferation, but also to

cell death by apoptosis.

In the cells, control of proliferation and apoptosis is carried out by an

integrated signaling mechanism that activates or avoids one of these

processes and also inﬂuences other aspects of cell activity. For instance,

the presence of mitotic agents that induce proliferation, and survival

factors that inhibit apoptosis, are necessary for maintaining cell viability

(Al-Rubeai, 1998). It is important, however, to distinguish between mito-

gens and survival factors. Insulin-like growth factor 1 (IGF-1) is a potent

survival factor, but a poor mitogen. Many cytokines are good mitogens,

but poor survival factors, since they induce cell division but do not avoid

apoptosis. In this way, induction of cell growth and apoptosis inhibition

become independent events. Macrophage-colony stimulating factor

(M-CSF), for instance, promotes cell survival when present at low concen-

trations, but promotes proliferation at high concentrations. Thus, the

correct balance of growth and survival factors is an important considera-

tion for maintaining high cell viability in culture (Cotter and Al-Rubeai,

1995).

Animal serum, especially serum from bovine fetuses, plays an important

role in the growth and survival of animal cells in culture. Serum contains

growth factors, proteins, and other nutrients. The removal of serum has

been associated with apoptotic induction in different cells, including

hybridomas and Chinese hamster ovary (CHO) (Zanghi et al., 1999) and

plasmacytomas (Singh et al., 1994). The mechanism by which the removal

of growth factors, such as those contained in serum, induces apoptosis

seems to involve the expression of the proto-oncogene c-myc (Al-Rubeai,

1998). In CHO cells, apoptosis is induced by nutrient depletion due to

accelerated nutrient consumption in the absence of serum. On the other

hand, when serum or plasma is present at high concentrations, death by

apoptosis is also induced, but this phenomenon can be avoided by adding

supplements containing thiol groups, such as L-cysteine or L-tryptophan

(Al-Rubeai, 1998).

When cells are deprived of an energy source such as glucose or glutamine,

apoptosis can be induced. The effects of lack of glutamine on apoptosis

induction have been observed in hematopoietic cells, such as hybridomas

and plasmacytomas (Mercille and Massie, 1994a; Singh et al., 1994). Sanfeliu

and Stephanopoulos (1999) demonstrated that the lack of glutamine is the

major factor activating apoptosis in a CHO cell line producing interferon-

gamma, and that this effect could be attenuated by the expression of the

gene Bcl-2, which codes for an anti-apoptotic protein that participates in

the signaling pathway that is triggered in response to apoptotic stimuli. The

inﬂuence of glucose and glutamine on the apoptotic pathways is not yet

completely understood, but the concentrations of both compounds regulate

cell growth and the catabolic pathways. Their depletion can block DNA

synthesis and arrest the culture in the G

phase, causing apoptosis. How-

ever, this does not explain why cells in phases S, G

, and M also undergo

apoptosis. It is suggested that apoptosis can be induced when the energy

required for synthesis of the ATP, needed for the duplication of cellular

material, is decreased to a critical level, resulting in the activation of

regulatory proteins that promote apoptosis (Al-Rubeai, 1998).

Mechanisms of cell proliferation and cell death in animal cell culture

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153

The relationship between apoptosis induction and glutamine depletion

in batch cultures of hybridomas led to studies suggesting that apoptosis is

activated upon depletion of any amino acid. Although the lack of essential

amino acids is critical, the depletion of any amino acid can induce

apoptosis, since the synthesis of molecules involved in the regulation of

cell death is blocked (Simpson et al., 1998). There are two probable

explanations for this. According to one of them, the decrease in the

intracellular concentration of amino acids can lead to a depletion of tRNA

molecules, blocking translation (protein synthesis). According to the

second theory, transcription and translation can be impaired by the de-

crease in the intracellular ATP levels. Although not all amino acids

contribute to the energetic requirements of the cells, the absence of any

amino acid results in the induction of apoptosis. Experiments with murine

hepatocytes showed that protein synthesis is inhibited at the level of the

initiation of the peptide chain. This indicates that the role of amino acids

as protein precursors could be one of the most signiﬁcant factors inducing

death by apoptosis (Singh and Al-Rubeai, 1998).

7.4.2 Oxygen limitation

The apparent involvement of free radicals in apoptotic processes has led to

studies of the effects of dissolved oxygen tension on animal cells in culture.

Hydrogen peroxide was demonstrated to be a factor inducing apoptosis,

whereas antioxidants such as catalase and superoxide dismutase protect

cells. Formerly, it was believed that the Bcl-2 protein acted as an antiox-

idant, but today it is known that anoxic conditions can induce apoptosis

(Mercille and Massie, 1994a) and that Bcl-2 expression presents an anti-

apoptotic action also under limiting conditions of oxygen. These studies

are very important for large-scale cultivation processes and for processes

where oxygen may be limiting, since the supply of oxygen is a key factor

that limits the viable cell concentration attained in these culture systems.

The effects of Bcl-2 protection under anoxic conditions have already been

demonstrated for different cell lines. For cells derived from Burkitt’s

lymphoma transfected with the Bcl-2 gene, the resistance to anoxic

conditions increased, and cell viability could be maintained at high levels

for a longer period than with control cells (Singh et al., 1997). In the case

of a hybridoma, the expression of this gene also conferred a higher

viability and integrity of the cells when exposed to anaerobic conditions

(Simpson et al., 1997).

7.4.3 Susceptibility to shear stress

Animal cells do not have a cell wall and are, thus, highly susceptible to the

effects of shear stress. Cells respond to hydrodynamic stress within

minutes, altering their metabolism and the gene expression pattern (Nol-

lert et al., 1991). Under sub-lethal levels of shear stress, there is initially an

increase in passive transmembrane transport, simultaneously with damage

to surface receptors. The plasma membrane is generally the main site for

shear damage, and it may lose its capacity to mediate the transport of ions

and molecules, so that the cell loses its viability. It has been demonstrated

154 Animal Cell Technology

that the susceptibility of some cell lines to the effects of hydrodynamic

stress is related to cell size and cell cycle, with cells in S and G

phases

more prone to damage and smaller cells more resistant to injury. It has

also been observed that cell damage and the dominating death mechanism

(necrosis or apoptosis) depend on the intensity and exposure time to shear

stress (Al-Rubeai et al., 1995a, 1995b). Exposing hybridoma cells to

moderate agitation levels resulted in a decrease of viability and an increase

in cell death by apoptosis (Al-Rubeai et al., 1995b), whereas extreme shear

levels led to cell death by necrosis, with complete cell fragmentation.

The hydrodynamic environment in a stirred-tank bioreactor can be

stressful for animal cells. This arises from the high energy introduced into

the system to maintain homogeneity and cells in suspension. Furthermore,

gas bubbling results in cell damage, especially at the gas–liquid interface

near the headspace of the bioreactor. While knowledge of the effects of

hydrodynamic conditions has improved bioreactor design and operation

strategies, so as to decrease hydrodynamic stress, a compromise still exists

concerning the need for efﬁcient agitation and aeration and the suscept-

ibility of cells to shear. Certainly, if apoptosis is induced by hydrodynamic

stress, then the suppression of this death mechanism, e.g. by molecular

strategies, would allow enhanced agitation and aeration, improving mass

transfer with a decreased effect on cell viability (Singh and Al-Rubeai,

1998).

7.4.4 Osmolality

Culture medium osmolality has a large inﬂuence on animal cell culture.

Hyperosmotic conditions, achieved by using salts, CO

, or concentrated

media, has been shown as a low-cost method to increase speciﬁc produc-

tivity of cells. Hybridoma cultures, for instance, show improved produc-

tivity with an increase in osmolality (Oh et al., 1993). However, this

method is not very popular due to the negative effects on cell growth. The

drop in cell growth under hyperosmotic conditions occurs, probably, due

to cell death by apoptosis. Thus, expression of anti-apoptotic genes, such

as Bcl-2, could allow the use of hyperosmotic conditions to simulta-

neously limit cell death and increase cell productivity (Kim and Lee,

2002).

7.5 Methods of detection of cell death by apoptosis

Currently, there are many methods available for determining cell death by

apoptosis in cell cultures and tissues. These methods are based essentially

on changes that occur in apoptotic cells. During apoptosis, several

phenomena can be observed, such as DNA fragmentation, chromatin

condensation, nuclear fragmentation, cytoplasm acidiﬁcation, cytochrome

c release from the mitochondria, exposure of intracellular phospholipids

and the activation and breakdown of proteins. These apoptotic phenomena

can be detected by direct or indirect methods, on cell populations or on

individual cells that are representative of a population. The main principles

used by the different detection methods are:

Mechanisms of cell proliferation and cell death in animal cell culture

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155

(i) DNA fragmentation;

(ii) morphological changes;

(iii) changes in membrane asymmetry;

(iv) activation of apoptotic proteins;

(v) release of cytochrome c in the cytoplasm.

7.5.1 DNA fragmentation

During controlled cell death, the DNA is initially cleaved into 300–500 kb

fragments (Brown et al., 1993). Later on, these fragments are decreased to

multiples of 180–200 bp, which is the distance between histone proteins,

around which the condensed chromatin is wrapped. This phenomenon

occurs simultaneously with morphological changes, such as chromatin

condensation. The resulting fragments are known as LMW-DNA (low

molar mass DNA) fragments, which are able to pass through pores of the

nuclear membrane and reach the cytoplasm, and HMW-DNA (high molar

mass DNA) fragments, which remain in the nucleus. The main enzyme

involved in this process is a caspase-activated DNAse (CAD) (Nagata,

2000).

Agarose gel electrophoresis

The DNA fragments can be visualized in electrophoretic runs in agarose

gels showing a distinct band pattern (DNA laddering). The sizes of the

bands are multiples of the smallest nucleosomal fragments. Unlike those of

apoptosis, the DNA fragments arising from necrosis are of irregular size,

resulting in a smear in the sample lane. Although the gel electrophoresis

pattern is a biochemical indication of apoptosis, in some cases the classic

morphological features of apoptosis may be noted without DNA fragmen-

tation (Al-Rubeai, 1998; Nagata, 2000).

TUNEL

TUNEL stands for terminal deoxynucleotidyl transferase x-dUTP nick

end labeling. This assay is based on the detection of DNA fragments

marked by an enzyme that incorporates modiﬁed nucleotides to the 39-

OH ends of the fragments, which can be then speciﬁcally detected. The

enzyme is a deoxynucleotidyl transferase, which can act in absence of a

complementary strand. Among the nucleotides, there is one speciﬁcally

marked with a ﬂuorochrome, an enzyme, or an antigen. This allows

different methods of detection.

When the deoxynucleotide is associated with a ﬂuorochrome, the cells

can be observed under a ﬂuorescence microscope, whereby apoptotic cells

present an intense ﬂuorescence and, in advanced stages, nuclear fragmenta-

tion can be visualized. For a quantitative analysis, either a hemocytometer

in an optical microscope or a ﬂow cytometer can be used (Tinto et al.,

2002). Peroxidase-marked deoxynucleotides can be quantiﬁed by chromo-

genic tests that use the enzyme substrate. Indirect methods use antigens

linked to the nucleotide and recognized by labeled antibodies. However,

156 Animal Cell Technology

this is a less sensitive technique, which is used mainly in pre-ﬁxed

histological samples.

In the TUNEL test, it is necessary to permeabilize the cells to introduce

the enzyme and the deoxynucleotides, but the permeabilization is carried

out after weak ﬁxation with formaldehyde, so that low molar mass

fragments are not lost. Permeabilization is carried out in an ice bath,

followed by labeling with the reaction solution.

Propidium iodide, ethidium bromide, and DAPI

These methods use ﬂuorescent labels, such as propidium iodide, ethidium

bromide, or DAPI (49,69-diamidino-2-phenylindole), which are incorpo-

rated into the DNA, allowing chromatin condensation and nuclear frag-

mentation to be visualized under a microscope with the appropriate

ﬂuorescence ﬁlters. To allow ﬂuorochromes to enter the cells and reach

the nucleus, the cells need to be prepermeabilized, for example, with 70%

ethanol at –208C. LMW-DNA fragments may be lost by the permeabili-

zation, decreasing the amount of DNA inside the cells. The lower nucleic

acid concentration results in a lower ﬂuorescence intensity in apoptotic

cells, which can be detected by ﬂuorescence microscopy or ﬂow cytome-

try (Calle et al., 2001).

7.5.2 Morphological changes

Several methods are based on microscopy observations to detect morpho-

logical changes in the cells, mainly chromatin condensation. Different

markers can be used to differentiate between the stages of apoptosis.

To distinguish apoptotic from normal and necrotic cells, ﬂuorescent

dyes can be used to form complexes with DNA, such as acridine orange,

ethidium bromide, or propidium iodide (Mercille and Massie, 1994b).

Fluorescence microscopy following cell labeling clearly reveals chromatin

condensation and fragmentation in apoptotic cells. By combining the use

of ﬂuorescent dyes that are able to label the chromatin in intact cells (e.g.

acridine orange) with those that just label the chromatin in cells with a

damaged membrane (e.g. ethidium bromide and propidium iodide), it is

possible to identify different levels of cell viability (Mercille and Massie,

1994a).

Acridine orange is able to penetrate cells independently of the integrity

of the cell membrane. It intercalates the bases in double strands of DNA

and emits green ﬂuorescent light. It can also form complexes with RNA

and single-stranded DNA, but in this case it does not intercalate the bases

and emits red ﬂuorescent light. Thus, a viable cell can be visualized with a

green nucleus and, possibly, reddish spots in the cytoplasm.

Ethidium bromide is not able to permeate the plasma membrane, and

only penetrates non-viable cells that have lost the selective permeability of

the membrane. It can intercalate the bases of double-stranded DNA

molecules, emitting an orange ﬂuorescent light. Ethidium bromide also

forms weak complexes with RNA, resulting in red ﬂuorescence. Thus,

non-viable cells will present a strong orange ﬂuorescence, since ethidium

Mechanisms of cell proliferation and cell death in animal cell culture

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157

bromide labeling overlays acridine orange ﬂuorescence. If there is still

cytoplasmic material inside the cell, it will be labeled dark red.

As shown in Figure 7.3 (see color section), the morphologies observed

in a sample incubated with acridine orange and ethidium bromide allow

the identiﬁcation of four levels of cell viability, as described below:

(i) Viable nonapoptotic (VNA) cells show the nucleus uniformly colored

green, with no chromatin condensation. In this cell population, only

acridine orange is able to penetrate the cells and bind to the DNA.

(ii) Early apoptotic cells, also called viable apoptotic (VA), show brilliant

green granules in the nuclear region. These granules correspond to the

condensed and fragmented chromatin, which means that the cells

have already initiated the apoptotic process, but still keep the selective

permeability of the membrane. In this phase, the formation of

apoptotic bodies containing material that will be expelled by the cells

can be observed.

(iii) Late apoptotic cells, also known as non-viable apoptotic (NVA), are

colored reddish-orange, with granules in the nuclear region showing

brilliant orange ﬂuorescence. The cells have lost membrane integrity

and have become permeable to ethidium bromide. Another character-

istic of this phase is the decrease in cell size, due to the loss of cell

material as a result of elimination of apoptotic bodies.

(iv) Necrotic cells are totally colored orange, indicating loss of selective

permeability of the membrane. These cells do not show any conden-

sation or fragmentation of the chromatin, and present no decrease in

size. On the contrary, there is a tendency for an increase in cell

volume. No apoptotic bodies are formed. This stage is very short, and

cell lysis occurs within a short time.

7.5.3 Membrane asymmetry

Translocation of the phosphatidylserine from the inner to the outer leaﬂet

of the plasma membrane is an initial event related to the apoptotic process

and possibly serves as a signal for the removal of apoptotic bodies by

phagocytic cells (Martin et al., 1995). The exposure of this phospholipid

has been largely used as a speciﬁc apoptosis marker.

Membrane asymmetry changes can be detected by ﬂow cytometry using

a ﬂuorescent marker (e.g. ﬂuorescein, FITC) conjugated to annexin V, a

protein that has high afﬁnity for phosphatidylserine. When using a

ﬂuorescence microscope, this technique can be quantitative if a hemocyt-

ometer is used.

Unfortunately, annexin V also binds to phosphatidylserine residues in

the inner leaﬂet of the plasma membrane of necrotic cells due to the loss of

integrity of the membrane. However, the use of propidium iodide as a

counter-marker allows viable, apoptotic, and necrotic cells to be distin-

guished (Pla

sier et al., 1999).

158 Animal Cell Technology

7.5.4 Apoptotic proteins

There are a large number of genes related to apoptosis, and the corre-

sponding proteins modulate the cell death process. Tests that detect these

proteins or their activities are largely used to detect apoptosis in cell

culture. This group of apoptotic proteins is very large and, theoretically,

any key protein participating in apoptotic pathways, such as p53 protein

and caspases 3 and 7, can be used to detect apoptosis (Kim and Lee, 2002;

Arden and Betenbaugh, 2004).

7.5.5 Cytochrome c release

Alterations of the mitochondrial membrane may precede those that occur

in the nucleus. These changes can involve both the inner and the outer

membrane, leading to a dissipation of the transmembrane potential and/or

to the release of intermembrane proteins through the outer membrane.

The main group of proteins responsible for mitochondrial alterations

consist of the proteins known as Bax, which form pores in the outer

membrane, causing the release of cytochrome c to the cytoplasm (Loefﬂer

and Kroemer, 2000; Arden and Betenbaugh, 2004). Western blot techni-

ques can be used to speciﬁcally detect the presence of cytochrome c in the

cytoplasm of apoptotic cells. However, complex puriﬁcation protocols are

required, and there is the possibility of incomplete separation of

mitochondria from the cytoplasm; therefore, this technique is not very

popular.

7.6 Apoptosis suppression by molecular techniques

7.6.1 Molecular basis of apoptotic cell death

Much of the current knowledge of the genetic regulation of apoptosis

began with genetic studies of the nematode Caenorhabditis elegans,in

which 131 of 1090 somatic cells formed during adult development die by

apoptosis. This cell death is regulated by a group of approximately 13

genes (Horvitz et al., 1994). The apoptotic mechanism identiﬁed in C.

elegans seems to be highly evolutionarily conserved in vertebrates and

invertebrates, since homolog genes were also discovered in mammals,

although in a much greater amount.

The events involved in apoptosis can be divided into four phases:

initiation, signaling, effector, and degradation (Mastrangelo and Beten-

baugh, 1998). During the initiation phase, the cell is exposed to insults that

trigger the apoptotic cascade. The main insults found in cell cultures are:

nutrient depletion (such as glucose, glutamine, and other amino acids),

deprivation of growth and survival factors (e.g. factors in fetal bovine

serum), accumulation of toxic metabolites (ammonia and lactate), oxygen

concentration (hypoxia and hyperoxia), hydrodynamic stress, loss of cell

adhesion (e.g. during suspension adaptation), viral infection (e.g. when

viral vectors are used for recombinant DNA transfer), and protein over-

expression. Once apoptosis has been triggered (initiation), the message is

transmitted through the signaling cascade during the signaling phase. This

Mechanisms of cell proliferation and cell death in animal cell culture

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15 9

stage of apoptosis includes several parallel pathways that converge to a

few or even a single pathway of the effector phase. Each insult probably

activates a different signaling cascade pathway, ﬁnally leading to the

effector phase, which involves the activation of caspases, the proteases

responsible for cellular degradation (Dickson, 1998). During this last

phase, which is irreversible, it is possible to observe the speciﬁc morpholo-

gical changes of apoptosis, such as chromatin condensation, nuclear

fragmentation, cellular shrinkage, blebbing of plasma membrane, and

formation of apoptotic bodies. Different cell lines respond to different

insults with varying intensities, possibly as a result of activation of differ-

ent apoptotic pathways (Singh and Al-Rubeai, 1998).

The caspase family

The characteristic morphological features of apoptosis are due to the direct

or indirect action of caspases, a highly conserved protease group (Alnemri

et al., 1996). Caspases are cysteine-aspartate proteases, members of a

cysteine-protease family that speciﬁcally cleave their substrates after

aspartic acid residues. To date, at least 14 mammalian caspases have been

identiﬁed. Although they share a common structure, they can be divided

into two functional groups: those that participate in the apoptosis process

(caspases 2, 3, 6, 7, 8, 9, 10, and 12) and those that are responsible for

cytokine processing during immune response and are involved in the

inﬂammatory process (caspases 1, 4, 5, and 11) (Alnemri et al., 1996).

However, due to the lack of functional information on some of the

caspases (e.g. caspases 2, 4, 5, 7, 11, 12, and 13), the exact division between

the two groups is still ambiguous.

Caspases are expressed as zymogens or procaspases and require a

proteolytic cleavage at speciﬁc aspartic acid residues to become active.

These cleavages generate four fragments: the N-terminal prodomain (2–25

kDa), the large subunit (17–21 kDa), the C-terminal small subunit (10–13

kDa), and a small linker between large and small subunits (Figure 7.4).

Some caspases do not possess the small linker. An active caspase consists

of a heterotetramer containing two small and two large subunits, which

are derived from two procaspase molecules (Walker et al., 1994). The N-

terminal prodomain and the linker (if present) are removed from the

mature and active caspase. The observation that caspases, to become active,

undergo the same process as their substrates (they are cleaved at aspartic

acid residues) suggests a caspase proteolytic cascade, that is, an active

caspase can subsequently cleave and activate other procaspases (Thorn-

berry et al., 1997).

Caspases involved in the apoptotic process can be subclassiﬁed as

initiator or effector caspases, depending on the structure of their prodo-

main. This also reﬂects their different roles in the apoptotic cascade

(Earnshaw et al., 1999).

Initiator caspases (caspases 2, 8, 9, 10, and 12) possess a functional

prodomain. This prodomain is large and contains interaction domains

(structural motifs) where adaptor proteins, like Apaf-1 and FADD, can

bind. The interaction with adaptor proteins is important for the caspase

activation process. There are two types of interaction domains: DED

160 Animal Cell Technology

(death effector domain) and CARD (caspase recruitment domain) (Muzio

et al., 1996; Hofmann et al., 1997). The contact with CARD domains is

based on electrostatic interactions, while the contact with DEDs is based

on hydrophobic interactions. Caspases 2, 9, and 12 have CARDs and

caspases 8 and 10 have DEDs. Generally, initiator caspases act upstream of

the effector caspases and are responsible for their activation (Thornberry

et al., 1997; Earnshaw et al., 1999).

The main activation mechanism of initiator caspases is autoproteolytic,

which is dependent on procaspase aggregation. Caspases possess low

intrinsic enzymatic activity when in a zymogen form. Their catalytic

activity increases when they are cleaved and assembled into tetramers

(Yamin et al., 1996). It has been observed that procaspase aggregation or

oligomerization facilitates intermolecular proteolysis, which results in

autocatalysis (Srinivasula et al., 1998; Yang et al., 1998). Adaptor proteins

recruit and bring together initiator caspases into activating complexes,

which allows the autoactivation.

Effector caspases (caspases 3, 6, and 7) may possess a small inactive

prodomain or may lack it completely, since they do not need to form

Caspase 1

Caspase 2

Caspase 3

Caspase 4

Caspase 5

Caspase 6

Caspase 7

Caspase 8

Caspase 9

Caspase 10

Caspase 14

CARD

DED DED

316 331152 435

297 317119 404

17528 277

270 290 377

311 331 418

179 19423 293

19823 303

374 385

216

479

315 331 416

415

219

521

242

Procaspase domain structure

Preferred

substrate

sequence

WEHD

YEVD

DEHD

VDVAD

DEVD

DNQD

LEVD

(W/L)EHD

VEID

VEHD

DEVD

(I/L)ETD

LEHD

IEAD

Function

Inflammation

Apoptosis

Inflammation

Apoptosis

Keratinocytes

differentitation?

Figure 7.4

Caspase family. This scheme illustrates the domain structures, internal cleavage

sites, preferred peptide substrate sequences, and bio logical function of caspases.

Each procaspase consists of a large and small domain and may also possess DED

(death effector domain) and CARD (caspase r ecruitment domain) (adapted from

Hill et al., 2003 ).

Mechanisms of cell proliferation and cell death in animal cell culture

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