Transcriptional cross-modulation with the AP-1 pathway

Transcriptional cross-modulation
with the AP-1 pathway
Cross-modulation can take place between the retinoid
receptors and the AP-1 signalling pathways, allowing
one type of transcription factor to regulate the function
of the other and increasing the level of complexity.
Since, in the main, retinoids induce cell differentiation
and AP-1 induces cell proliferation, cross-modulation
provides a molecular switch, allowing a fi nely tuned
balance between these cellular activities.
Retinoic acid stimulates the production of several
protein components of the extracellular matrix,
including collagen, fi bronectin and laminin. The
extracellular matrix forms a substrate for cell attachment
and migration, directs cell form and function,
and forms a barrier which must be removed in order
for tumour invasion to occur. A controlled rate of
turnover of the extracellular matrix (remodelling)
must accompany the changes which occur during
development and other events such as wound healing.
Remodelling requires the action of extracellular
proteolytic enzymes such as the family of metalloproteases.
This family includes collagenase, which
degrades interstitial collagens (types I, II and III)
and stromelysin, which, less specifi cally, degrades
fi bronectin, collagen types III, IV and V, laminin and
proteoglycans. The ability to modulate the activity of
collagenase and stromelysin, in addition to regulating
the level of their gene transcription, underlines the
importance of these enzymes in controlling the rate
of extracellular matrix turnover. Uncontrolled degradation
of the extracellular matrix can have severe
pathological consequences. In rheumatoid arthritis,
for example, the cells (synoviocytes) in the synovial
fl uid lubricating the joints produce an excess of collagenase
and stromelysin which degrade the connective
tissue and cause impairment of the joints.
Transcription of the collagenase gene is stimulated
independently by interleukin-1 (a cytokine) and the
tumour-promoting TPA and inhibited by liganded
RAR (Lafyatis et al., 1990). Both positive and negative
regulatory effects are mediated by the AP-1 binding
site. The inhibitory effect of RAR on AP-1 activity is
thought to be responsible for the clinical effects of
retinoids as anti-neoplastic, anti-arthritic and immunosuppressive
agents. In the case of the stromelysin
gene, inhibition can occur in the absence of protein
synthesis, indicating a primary transcriptional response
(Nicholson et al., 1990). All three subtypes of
RAR (α, β and γ) can repress transcriptional induction
of the human collagenase gene (Schüle et al.,
1991). RXR in the presence of its ligand, 9-cis retinoic
acid, has also been shown to inhibit AP-1 activation
(Salbert et al., 1993). Multiple regions of RAR, including
the DNA-binding domain, are required for inhibitory
activity and both c-Jun homodimers and c-Jun/
c-Fos heterodimers can be repressed. The inhibitory
effect of retinoic acid can be removed by treatment
of cells with TPA and also by treatment with both
c-Jun and c-Fos (Yang-Yen et al., 1991). At least in
vitro, inhibition of AP-1 activity requires an excess of
ligand-activated RAR, whereas removal of inhibition
requires an excess of AP-1 protein. This reciprocal effect
is due to an interaction between RAR and AP-1
proteins that results in mutual loss of DNA-binding
activity. The interaction appears to be weak in that a
stable complex between c-Jun and RARβ could not
be detected in the absence of a chemical cross-linking
agent (Yang-Yen et al., 1991). In spite of the apparently
weak interaction, however, endogenous RAR
levels appear suffi cient to allow inhibition of AP-1
activity. The RARβ2 isoform is particularly strongly
up-regulated in many epithelial tissues by retinoic
acid and could thus be an important modulator of
AP-1 activity in these tissues.
Vitamin A: retinoids and carotenoids 171
The mechanism of nuclear receptor/AP-1 interaction
has not been established, but it does not involve
direct DNA contact. Most likely it involves indirect interaction
through cell-type specifi c transcription factors
that could stabilize the receptor/AP-1 complex.
Based on the demonstration by Kamei et al. (1996)
that CBP can act as a transcriptional coactivator for
nuclear receptors and AP-1, these authors suggested
that competition for limiting amounts of CBP could
account for the inhibitory action of liganded RAR
on AP-1 activation. Another hypothesis proposed
by Caelles et al. (1997) is that nuclear receptors inhibit
the activity of Jun amino-terminal kinase (JNK),
thereby interfering with the c-Jun activation step that
is required for recruitment of CBP. Zhou et al. (1999)
proposed a novel mechanism in which liganded retinoid
receptors are able to interfere with the homo- and
heterodimerization properties of c-Jun and c-Fos and
in this way prevent the formation of AP-1 complexes
capable of DNA binding. These authors showed that
human RARα disrupted in vivo c-Jun–c-Fos dimerization
in a ligand-dependent manner. Inhibition of
dimerization was cell specifi c, occurring only in those
cells that exhibit retinoic acid-induced repression of
AP-1 activity. Furthermore, the glucocorticoid receptor
did not affect dimerization, suggesting that this
mechanism is specifi c for the retinoid receptors.
As already discussed, the retinoid receptors mediate
two distinct types of action: (1) cell differentiation by
transcriptional activation through specifi c response
elements on the DNA and (2) anti-proliferative activity
by inhibition of AP-1. Because the two types of
action are mechanistically distinct, various laboratories
have investigated whether novel retinoids can
be synthesized that inhibit AP-1 but do not activate
transcription. The potential advantage of such ‘dissociated’
retinoids as receptor ligands is that they
could be therapeutically useful in the treatment of
hyperproliferative and infl ammatory diseases without
the side effects resulting from the activation of
retinoid-responsive genes. Synthetic retinoids having
this dissociation ability have been described (Fanjul
et al., 1994; Chen et al., 1995; Nagpal et al., 1995), but
they display distinct receptor selectivities and their
differential action is limited to only some of the RAR
subtypes or isoforms. The aim is to discover retinoids
that are completely specifi c for AP-1 antagonism
through all the retinoid receptors.
7.8 Effects of vitamin A on the immune
system
7.8.1 Introduction
Vitamin A defi ciency is strongly associated with depressed
immunity (Smith et al., 1987) and therefore
a combination of defi ciency and infectious disease
is potentially fatal, especially among infants and
children. Randomized trials in Sumatran (Tarwotjo
et al., 1987) and Indian (Rahmathullah et al., 1990)
preschool-age children showed that vitamin A supplementation
decreased mortality due to infection by
96% and 54%, respectively.
7.8.2 Lymphocytes
Retinoic acid was shown to stimulate, and retinaldehyde
to inhibit, the activity of protein kinase C derived
from T cells (Isakov, 1988). This enzyme has a major
role in transmembrane signal transduction and therefore
its regulation by retinoids could account for the
modulatory effects of vitamin A on T-cell function.
Retinoic acid induces the expression of the RARα
gene in unstimulated cloned mouse T cells and increases
proliferation of these cells in the presence of
antigen (Friedman et al., 1993). Cytotoxic T cells recognize
antigens in association with MHC class I molecules.
Retinoic acid mediates the regulation of MHC
class I gene expression via a nuclear hormone receptor
(Nagata et al., 1992). Expression of the intercellular
adhesion molecule-1 (ICAM-1) gene is up-regulated
by retinoic acid in a RARβ/RXRα-dependent fashion
(Aoudjit et al., 1994, 1995). Retinoic acid can enhance
the functional responses of human T lymphocytes via
transcriptional up-regulation of IL-2 receptors (Sidell
et al., 1993).
Retinol is essential for proliferation of activated
human B cells (Buck et al., 1990) and, moreover, is
a normal modulator of B-cell function (H. Blomhoff
et al., 1992). In a study of the effects of retinoids on
B-cell immune function in newborn infants (Ballow
et al., 1996), retinoic acid enhanced the synthesis
of IgM in antigen-stimulated umbilical cord blood
mononuclear cells, whereas in adult peripheral blood
mononuclear cells it enhanced the synthesis of IgG.
IgM is the fi rst antibody produced during a primary
immune response and is 15 times more effective than
172 Vitamins: their role in the human body
IgG in activating the complement cascade. Highly purifi
ed T cells pre-incubated with retinoic acid released
a factor (probably a cytokine) which acted on cord
blood B cells to augment IgM synthesis.
Antibody production in the vitamin A defi ciency
state
Infantile tetanus arising from infection with Clostridium
tetanii remains a signifi cant cause of morbidity
and mortality in parts of the developing world (Henderson
et al., 1988), even though it is preventable by
vaccination with tetanus toxoid (TT). Kinoshita et
al. (1991) reported that vitamin A defi ciency in the
rat led to a decreased production of TT-specifi c IgM
and IgG antibodies in both the primary and secondary
responses. However, levels of total plasma IgG (as
opposed to anti-TT IgG) were abnormally elevated.
When the rats were repleted with retinol 1 day after
primary immunization with tetanus toxoid, anti-TT
IgM and IgG concentrations in both the primary and
secondary responses reached vitamin A-suffi cient
control levels. When the defi cient rats were repleted
2 days before the booster immunization, the secondary
response was normal in terms of antibody concentrations
and class switching from anti-TT IgM to
IgG. These experiments showed that immunological
memory to TT could be established and maintained
in the vitamin A-defi cient rat. In the early retinol-repleted
rats, the increase in anti-TT IgG production
during the secondary response persisted for 26 days
without any additional retinol in the interim. This
strong secondary response was specifi c for the IgG2a
and IgG2b subclasses of IgG and refl ected the previous
development of subclass-specifi c memory B cells
(Kinoshita & Ross, 1993). These authors showed that
vitamin A defi ciency most likely impaired the process
of B cell differentiation, either from the naive B cell to
the antibody-secreting plasma cell during the primary
response and/or from the quiescent memory B cell to
the plasma cell during the secondary response. The
observation that the total plasma IgG of vitamin Adefi
cient rats is elevated above normal shows that the
B cells are at least capable of producing antibodies,
albeit not the specifi c antibodies required to deal with
a particular antigen. This would explain reports of decreased
host defence against certain types of infection
in human vitamin A defi ciency.