01. Gene therapy Boulikas.pdf - Gene therapy & Molecular Biology

Figure 24. TNF-α signaling via trimerization of its receptors
(TNF-αR1), is causing: (i) activation of IκB kinase and
activation of NF-κB, a pathway which prevents cell death via
activation of the manganese superoxide dismutase and of the
zinc finger protein A20. (ii) induction of apoptosis via second
message ceramide (see Figure 23) and (iii) activation of JNK
leading to AP-1 activation and up-regulation of cytokine genes.
E. Interleukin-1β converting enzyme (ICE)
and apoptosis
The human interleukin-1β converting enzyme (ICE) is
a cysteine-rich protease that can cleave the inactive 31
kDa precursor of IL-1β to generate the active cytokine; it
has similarities to the C. elegans CED-3 protein. This
protease plays a central role in apoptosis; the exact role
and the involvement of IL-1β have not been elucidated; it
is believed that signals from IL-1β, TNF-α, vitamin D3,
and interferon-γ, which induce an antiproliferative
response, converge on sphingomyelin of plasma
membrane activating a sphingomyelinase which generates
a ceramide second messenger (Figure 23); in S.
cerevisiae, this leads to signal transduction via activation
both of a cytoplasmic protein phosphatase 2A and a
protein kinase leading to down-regulation in c-myc
expression and induction of apoptosis as well as RBmediated
cell cycle arrest via EGFR activation (Nickels
and Broach, 1996).
At least 10 ICE-like proteases have been identified
which mediate apoptotic death after their induction by a
number of stimuli (see Martin and Green, 1995); these are
divided into three families: (i) the ICE/CED3 family,
including ICE itself; (ii) the CPP32/Yama family; and (iii)
the Ich-1/Nedd2 family; they all contain the conserved
QACRG pentapeptide in which the central cysteine
participates in proteolytic catalysis (see Jänicke et al,
1996). Activation of these proteases by induction of
apoptosis results in the cleavage of a large number of key
regulatory proteins including among others poly(ADPribose)
polymerase or PARP (Lazebnik et al, 1994), RB
(Jänicke et al, 1996), PKCd (Emoto et al, 1995), Gas2
affecting microfilament reorganization (Brancolini et al,
1995), the DNA-dependent protein kinase (Casciola-
Rosen et al, 1995), and the sterol regulatory element
binding proteins (SREBPs) catalyzed by CPP32 ICE-like
protease (PARP et al, 1996). Since cleavage of a single
protein has not been shown to cause cell death it is not
clear how many substrate protein molecules need to be
cleaved. In addition different apoptotic pathways may
exist and may operate in different cell types.
Expression of the murine ICE cDNA in Rat-1 cells
induced programmed cell death and this phenomenon
could be reversed by overexpression of the bcl-2 oncogene
(Miura et al, 1993). Expression of members of the family
of cysteine proteases related to ICE have been shown to be
necessary for programmed cell death in a number of
Boulikas: An overview on gene therapy
68
organisms (Yuan et al, 1993). Overexpression of murine
ICE or of the ICE-like proteases NEDD-2/ICH-1 and
Yama/apopain induced apoptosis (Miura et al, 1993).
Mice lacking ICE were resistant to apoptosis induced by
Fas antibody (Kuida et al, 1995).
F. Role of poly(ADP-ribose) polymerase
(PARP)
PARP is a central mediator of genome integrity and
transmits signals from DNA damage to recruit locally
DNA repair activities (Zardo et al, 1998; Quesada, 1998,
this volume). An additional role of PARP is its
involvement in apoptosis causing suppression of an
apoptotic endonuclease; PARP is cleaved by an ICE-like
protease during TNF-induced apoptosis (Lazebnik et al,
1994). Cleavage of PARP would abort these pathways
resulting in loss of recruitment of DNA repair enzymes at
damaged sites but also in loss in the inhibitory function of
poly(ADP-ribose) groups on key regulatory enzymes
(DNA ligase, topoisomerase). It is unlikely that PARP
proteolysis by an ICE-like protease is a primary event
since PARP-deficient mice show normal resistance to
DNA damaging agents (Wang et al, 1995).
G. Apoptosis in autoimmune disease and
ischemic heart disease
T cells are produced by bone marrow and then migrate
to the thymus gland where they mature. The cytotoxic or
killer T cells directed against foreign bodies are released
in the bloodstream; a specific apoptotic mechanism
eliminates T cells directed against specific antigens on
healthy cells. However, the body allows some mildly selfreactive
lymphocytes to circulate; although harmless,
exposure to a microbe or food antigen can stimulate them
causing an expansion in their proliferation and resulting in
a mild autoimmune disease. Such mild autoimmune
reactions usually disappear when the stimulating antigen is
cleared away; in more severe autoimmune disease,
however, these lymphocytes survive longer inducing
apoptosis and self-destruction in healthy cells in various
tissues.
A number of classical diseases may originate by
autoimmune mechanisms including initiation of
atherosclerosis by apoptotic death of the epithelial cells in
the arterial wall, diabetes by destruction of the pancreatic
cells, lupus erythematosus, rheumatoid arthritis, and
others. The mechanism via which T lymphocytes directed
against self antigens defy apoptosis is not known; the
mechanism might involve overexpression of the Bcl-2
gene in these lymphocytes or down-regulation of a gene
encoding for the Fas ligand that sends a death message to
the lymphocyte (Weih et al, 1996; reviewed by Duke et al,
1996).