Natural killer cells

Natural killer cells
Vitamin A defi ciency in rats reduced the absolute
number of natural killer (NK) cells in blood and to
a lesser extent in spleen. The activity per NK cell was
normal in blood and only slightly, but signifi cantly, reduced
in the spleen (Zhao et al., 1994). NK cytotoxicity
was increased in vitamin A-defi cient rats when their
production of cytokines was stimulated by polyinosinic
acid:cytidylic acid (Ross, 1996). When vitamin
A-defi cient rats were fed a retinoic acid supplement,
the number of NK cells equalled those of control rats
and NK cytotoxicity was signifi cantly elevated (Zhao
& Ross, 1995). Thus, as for antibody production, adequate
vitamin A status is required to ensure the correct
pattern of cytokine gene expression.
7.8.4 Neutrophils
The neutrophils of vitamin A-defi cient rats showed
impaired chemotaxis, lower rate of adhesion to pathogenic
organisms, and impaired respiratory burst
activity. Neutrophil function was restored to normal
8 days after vitamin A administration (Twining et al.,
1997).
7.8.5 Monocytes/macrophages
Retinoic acid increased the proliferation of
granulocyte/macrophage precursor cells even in the
presence of maximally stimulating concentrations
of colony-stimulating factor (Tobler et al., 1986).
Retinoids can modulate macrophage function by
enhancing phagocytosis and IL-1 production (Dillehay
et al., 1988). An increase in transglutaminase
activity during macrophage activation is thought to
contribute to the enhanced functional capacity of
activated macrophages. Moore et al. (1984) reported
that physiological concentrations of retinoic acid
specifi cally induced the synthesis of transglutaminase
by cultured mouse macrophages. Retinoic acid stimulated
the capacity of macrophages to phagocytose and
kill the unicellular parasite Trypanosoma cruzi (Wirth
& Kierszenbaum, 1986). Transglutaminase activity
appeared to be involved in the retinoid effect, because
inhibition of the enzyme activity cancelled the effect.
Retinoic acid at physiological concentrations does
not act alone in inducing IL-1β production in human
monocytes; rather it strongly enhances phorbol esterinduced
IL-1β production at the level of transcription
(Matikainen et al., 1991). This effect of phorbol ester
is mediated through the activation of its cellular receptor,
protein kinase C. Macrophages from normal
healthy donors are usually not cytotoxic to tumour
cells in vitro, but they can be rendered tumoricidal
by interaction in vitro with bacterial products, lymphokines
or vitamin A (retinyl palmitate) (Tachibana
et al., 1984). Macrophages from rats or mice receiving
very high intakes of retinyl palmitate also exhibit an
increased ability to kill tumour cells (Tachibana et al.,
1984; Moriguchi et al., 1985).
7.9 Role of vitamin A in bone
metabolism and embryonic development
7.9.1 Effects of vitamin A on bone
metabolism
The gene expression of the nuclear retinoid receptors
(RARα, RARβ, RARγ, RXRα, RXRβ but not RXRγ)
and one of the cellular retinoid-binding proteins
(CRBP-I) has been demonstrated in adult rat tibia
(Harada et al., 1995). Retinoic acid (either all-trans
or 9-cis) increased the mRNA levels of the RARβ and
CRBP-I genes. These observations indicate that, in
Vitamin A: retinoids and carotenoids 175
bone, the actions of vitamin A are exerted through
these nuclear receptors by regulating target gene
expression, and through CRBP-1 by modulating the
intracellular transport of vitamin A.
The regulatory effects of vitamin A on bone metabolism
appear somewhat contradictory. Oreffo et
al. (1988) reported that retinol and all-trans retinoic
acids stimulate bone resorption through direct effects
on osteoclasts. On the other hand, retinoic acid acts as
an inducer of osteocalcin (Oliva et al., 1993), matrix
Gla protein (Cancela & Price, 1992) and bone morphogenetic
proteins (Rogers et al., 1992).
7.9.2 Consequences of retinoid imbalance
during embryogenesis
All-trans retinoic acid, and also 9-cis retinoic acid, 4-
oxo-all-trans retinoic acid and 3,4-didehydro-all-trans
retinoic acid, are implicated in pattern formation, i.e.
the development of the embryo in a spatially organized
fashion. In particular, endogenous retinoids
play a role in the anterior–posterior development of
the central body axis and in limb development. The
concentrations of retinoic acid and nuclear retinoid
receptors in the developing embryo are highly regulated,
both spatially and temporally. Treatment of embryos
with exogenous retinoids produces teratogenic
effects, while maternal insuffi ciency of vitamin A during
pregnancy results in either fetal death or severe
congenital malformations.
After the formation of the primary tissue layers in
vertebrate embryogenesis (gastrulation), the anterior–
posterior axis is established with the formation
of the neural tube. Cranial neural crest cells arising
from the developing hindbrain migrate and differentiate
to form parts of the face, both jaws, periocular
tissues, bones of the ear, the thymus, and the septa of
the heart and its major arteries. Many of the malformations
resulting from retinoic acid excess occur in
tissues whose origins can be traced back to the early
development of the neural tube. Malformations may
arise through the premature induction of gene transcription
by retinoic acid or through death of many
neural crest cells by apoptosis. Abnormalities are frequently
found in craniofacial structures, the central
nervous system, the heart and the thymus.
The developing eye is the most sensitive organ to
vitamin A deprivation. The eye may be completely
absent; otherwise, the lens is reduced in diameter and
the lens cells fail to elongate and form the necessary
crystallin fi bres. Cardiovascular defects include failure
of closure of the interventricular septum in the
heart. In the developing central nervous system, vitamin
A defi ciency leads to incomplete formation of the
myelencephalon (part of the posterior hindbrain) and
a failure of the neural tube to extend neurites into the
periphery. Development of the lungs, diaphragm and
urinogenital tract is also impaired. The above effects
have been reported in animals. Vitamin A defi ciency
suffi cient to cause morphological abnormalities in
human embryonic development is rare, although
functional defects, especially of the lungs, are quite
common.
Boylan & Gudas (1992) showed that the level of
CRABP-I expression infl uences the metabolism
of retinoic acid in the cytoplasm of differentiating
embryonic stem cells: the higher the CRABP-I level,
the faster the metabolism of retinoic acid to inactive
metabolites. Retinoic acid not immediately metabolized
presumably moves to the nuclear receptors and
initiates the activation of responsive genes, leading to
differentiation. The level of CRABP-I expression also
infl uences the sensitivity of the stem cells to the addition
of retinoic acid: the higher the level of CRABP-I,
the greater the concentration of retinoic acid required
to initiate differentiation (Boylan & Gudas, 1991). It
appears, therefore, that the function of CRABP-I in
embryonic stem cells is to regulate the concentration
of intracellular retinoic acid. This function is very
important in the developing embryo where excessive
retinoic acid could lead to teratogenesis.
The distributions of the nuclear retinoid receptors
(RARs and RXRs) in the mammalian embryo
are shown in Table 7.2. The characteristic spatial
and temporal expression of the different receptor
subtypes correlates with pattern formation, suggesting
that they play a role in morphogenesis and other
developmental events (Underhill et al., 1995).
7.9.3 Anterior–posterior development of
the central body axis
Pattern formation along the central anterior–posterior
axis is determined by the expression patterns of
Hox genes in the developing embryo. Within the Hox
locus of the chromosome, Hox genes are arranged in
decreasing order of sensitivity to retinoic acid in the
3´ to 5´ direction. Similarly, Hox genes at the 3´ ends of
176 Vitamins: their role in the human body
the cluster are expressed earlier in embryonic development
and in more anterior regions, while Hox genes
closer to the 5´ ends of the cluster are expressed at later
times in development and in posterior regions of the
embryo. The order of induction will correspond to the
gene expression patterns (anterior to posterior) and
will be infl uenced by the concentration of retinoic
acid (Gudas, 1994).
7.9.4 Limb development
Three regions of the developing limb bud have been
identifi ed that profoundly infl uence limb development,
namely the zone of polarizing activity (ZPA),
the apical ectodermal ridge (AER) and the progress
zone. The ZPA regulates the anterior–posterior axis,
whereas the AER is required for proper formation
of structures along the anterior–posterior axis. The
progress zone receives signals from both the ZPA
and the AER and translates this information into
distinct cell identities (Johnson et al., 1994). Several
members of the fi broblast growth factor (FGF) family
are expressed in the AER and these may initiate
the proliferation of the mesenchyme in normal limb
development.
Transplantation of mesenchyme from the ZPA
results in mirror-image duplication of the limb along
the anterior–posterior axis, a property called polarizing
activity. A secreted protein called Sonic hedgehog
has been identifi ed as the likely mediator of polarizing
activity within the limb bud. The expression of Sonic
hedgehog is regulated by Hoxb-8, a primary response
gene whose expression is induced by retinoic acid
(Tabin, 1995). When a bead containing retinoic acid
is placed on the anterior of a limb bud, it causes polarizing
activity by inducing an ectopic ZPA adjacent
to the bead. This effect is due to the retinoic acid-induced
expression of Sonic hedgehog, which is localized
and does not extend around the bead.