Vitamin D deficiency and impaired placental function: potential regulation by glucocorticoids?

TLDR: It is proposed that glucocorticoids are an important consideration when understanding the impact of vitamin D deficiency on placental function and fetal development.

Abstract: Maternal vitamin D deficiency has been implicated in a range of pregnancy complications including preeclampsia, preterm birth and intrauterine growth restriction. Some of these adverse outcomes arise from alterations in placental function. Indeed, vitamin D appears critical for implantation, inflammation, immune function and angiogenesis in the placenta. Despite these associations, absence of the placental vitamin D receptor in mice provokes little effect. Thus, interactions between maternal and fetal compartments are likely crucial for instigating adverse placental changes. Indeed, maternal vitamin D deficiency elicits changes in glucocorticoid-related parameters in pregnancy, which increase placental and fetal glucocorticoid exposure. As in utero glucocorticoid excess has a well-established role in eliciting placental dysfunction and fetal growth restriction, this review proposes that glucocorticoids are an important consideration when understanding the impact of vitamin D deficiency on placental function and fetal development.

Full text

REPRODUCTION
© 2017 Society for Reproduction and Fertility DOI: 10.1530/REP-16-0647
ISSN 1470–1626 (paper) 1741–7899 (online) Online version via www.reproduction-online.org
REVIEW
Vitamin D deciency and impaired placental function:
potential regulation by glucocorticoids?
NathanaelYates
1
, Rachael CCrew
2
and Caitlin SWyrwoll
2
1
School of Animal Biology, and
2
School of Human Sciences, The University of Western Australia, Perth, Australia
Correspondence should be addressed to C S Wyrwoll; Email:
caitlin.wyrwoll@uwa.edu.au
Abstract
Maternal vitamin D deciency has been implicated in a range of pregnancy complications including preeclampsia, preterm birth and
intrauterine growth restriction. Some of these adverse outcomes arise from alterations in placental function. Indeed, vitamin D
appears critical for implantation, inammation, immune function and angiogenesis in the placenta. Despite these associations,
absence of the placental vitamin D receptor in mice provokes little effect. Thus, interactions between maternal and fetal
compartments are likely crucial for instigating adverse placental changes. Indeed, maternal vitamin D deciency elicits changes in
glucocorticoid-related parameters in pregnancy, which increase placental and fetal glucocorticoid exposure. As in utero
glucocorticoid excess has a well-established role in eliciting placental dysfunction and fetal growth restriction, this review proposes
that glucocorticoids are an important consideration when understanding the impact of vitamin D deciency on placental function
and fetal development.
Reproduction (2017) 153 R163–R171
Introduction
As the sole conduit between maternal and fetal systems,
the placenta mediates fetal exposure to nutritional,
hormonal and other physiological cues throughout
gestation. In addition to facilitating the direct exchange
of oxygen and nutrients between mother and fetus, the
placenta functions in a specialized endocrine capacity to
produce hormones essential for maternal physiological
adaptations to pregnancy (
Evain-Brion & Malassine
2003
). As such, a healthy placenta is vital for normal fetal
growth and development, whereas impaired placental
function disrupts fetal growth outcomes and may also
result in long-term health complications in offspring via
developmental programming mechanisms (
Burtonetal.
2016
, Sferruzzi-Perri & Camm 2016).
Maternal vitamin D deciency has become a
signicant problem in modern day obstetrics; the
rates of vitamin D deciency have increased in recent
decades (
Looker et al. 2008), and it is estimated that
between 18% and 84% of pregnant women worldwide
are vitamin D decient (
Dawodu & Wagner 2007).
Although vitamin D has traditionally been associated
with bone health through its regulation of calcium
and phosphate absorption, it also plays central roles
in cellular proliferation and differentiation (
Samuel
& Sitrin 2008
), vascular function (Tare et al. 2011,
Ni et al. 2014) and immune regulation (Prietl et al.
2013
). As a consequence of these pleiotropic functions,
vitamin D metabolism is important for a range of key
developmental events, including decidualization
(
Halhalietal. 1991), modulation of maternal immune
function (Tamblynetal. 2015) and fetal bone formation
(
Young et al. 2012). Accordingly, maternal vitamin D
deciency has been linked to pregnancy complications
such as preeclampsia (
Bodnaretal. 2007, Achkaretal.
2015
), bacterial vaginosis (Bodnaretal. 2009), preterm
birth (
Qinetal. 2016) and gestational diabetes mellitus
(Burris & Camargo 2014). Moreover, gestational vitamin
D deciency is associated with intrauterine growth
restriction (IUGR) in infants (
Gernand et al. 2014,
Chenetal. 2015a) and adverse postnatal health outcomes
in offspring, including increased rates of asthma
(
Zoskyetal. 2014), hypertension (Tareetal. 2011) and
impaired neurodevelopment (
Whitehouse et al. 2012,
Eyles et al. 2013, Hawes et al. 2015, Pet & Brouwer-
Brolsma 2016).
There are several forms of vitamin D, but vitamin
D
3
is the naturally occurring form in animals. To elicit
biological effects, vitamin D, which is obtained through
the diet or synthesized in the skin via UVB exposure must
be converted to its active form, 1α 25-dihydroxyvitamin
D (1,25(OH)
2
D). To achieve this, circulating
vitamin D is rst converted to 25-hydroxyvitamin
D (25(OH)D) by hepatic 25-hydroxylase, which is
then hydroxylated to 1,25(OH)
2
D via the enzymatic
activity of 1α-hydroxylase, encoded by the CYP27B1
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gene (
Norman 2008). Although the latter conversion
typically occurs in the kidney, the placenta also exhibits
abundant expression and activity of 1α-hydroxylase
(Zehnderetal. 2002), which likely indicates a functional
importance for local placental vitamin D metabolism
during gestation (Fig.1). Circulating 1,25(OH)
2
D elicits
genomic effects by binding to the vitamin D receptor
(VDR), which dimerizes with the retinoid X receptor
(RXR) and induces subsequent gene transcription
through vitamin D response element (VDRE) binding
to target gene promoter regions (Haussleretal. 2013).
However, as with many steroid hormones, vitamin D
can also exert rapid non-genomic effects, likely via VDR
located within the plasma membrane (Norman et al.
2002, Menegaz et al. 2011). VDR is also abundantly
expressed in placental tissue, thus allowing for localized
placental 1,25(OH)
2
D binding and subsequent gene
transcription (Shahbazi et al. 2011). Interestingly, the
precursor 25-(OH)D readily crosses the placenta;
however, 1,25(OH)
2
D is unable to be transported across
the placenta, and thus is found in relatively low levels in
fetal circulation (Kovacs 2008).
A recent study shows that placental VDR expression
is downregulated in pregnancies complicated by
fetal growth restriction (Nguyen et al. 2015b) and
is associated with abnormal trophoblast expression
of cell-cycle regulatory genes in vitro (Nguyen et al.
2015a). This indicates that placental vitamin D
metabolism regulates fetal growth and development;
however, relatively little is known of the mechanisms
behind such regulatory processes, and whether they
may be compromised during vitamin D-decient
pregnancies. Interestingly, recent evidence shows that
placental-specic ablation of Vdr expression does
not have overt effects on placental phenotype or fetal
growth outcomes (Wilson et al. 2015). This suggests
that vitamin D homeostasis during pregnancy likely
involves communication between maternal, placental
and fetal compartments. Indeed, recent research has
shown an interaction between vitamin D pathways and
glucocorticoids (‘stress’ hormones) in the context of the
placenta, brain, lung and other tissues (Obradovicetal.
2006, Hidalgo et al. 2011, Chambers et al. 2015,
Tesic et al. 2015, Kassi et al. 2016). The current
review aims to discuss the potential mechanisms by
which placental function may be implicated in the
adverse outcomes associated with vitamin D-decient
pregnancies. The key focus of this review is to propose
Figure1 Vitamin D metabolic pathways during pregnancy across maternal, fetal and placental compartments. Vitamin D is absorbed into
maternal circulation via UVB-catalyzed synthesis in the skin or dietary intake (1) and is then converted to 25(OH)D via hepatic 25-hydroxylase
(2). This is subsequently hydroxylated to the active form of the metabolite in various tissues, including the kidney and placenta (3). The precursor
25(OH)D can also directly cross the placenta (6), where it is hydroxylated to the active metabolite in the fetal kidney (4). The active 1,25(OH)D
then elicits genomic effects through binding to the vitamin D receptor (VDR) in target tissues, including the placenta (5). This induces the
transcription of target genes via vitamin D response element (VDRE), thus enabling the regulation of a range of important processes during
gestation (6).
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that the involvement of factors extrinsic to vitamin D
pathways, particularly the action of glucocorticoids, is
crucial for eliciting placentaleffects.
Vitamin D deciency and placental development
and function
Vitamin D metabolic pathways in the placenta may
facilitate the transport of calcium and phosphate across
the placenta, enabling fetal skeletal development.
Changes in fetal femoral development with maternal
vitamin D insufciency can be observed as early as
19-week gestation using 3D ultrasound (
Mahon et al.
2010) and such changes in bone structure may occur
well into early childhood (Javaidetal. 2006). Although
vitamin D may have a role in fetal skeletal development,
other dominant pregnancy hormones such as prolactin,
estradiol and lactogen are also key to this process
(Kirbyetal. 2013). There is evidence that the increased
maternal intestinal calcium absorption that occurs in
pregnancy and calcium supply to the fetus is regulated
by mechanisms independent of the typical vitamin
D pathway. Thus, maternal vitamin D deciency is
associated with reduced fetal cord blood calcium and
bone mineralization in some (Cockburn et al. 1980,
Maghboolietal. 2007), but not all (Javaidetal. 2006)
studies. Indeed, a systematic review of vitamin D
supplementation in pregnancy found limited evidence
for an association between maternal vitamin D status
and offspring birthweight, bone mass or serum calcium
concentrations (Harveyetal. 2014). Furthermore, mice
with genetic removal of Vdr (which therefore eliminates
1,25(OH)
2
D
3
signaling) do not exhibit differences in
placental calcium transfer or fetal skeletal mineralization
(Kovacsetal. 2005). This raises the possibility that the key
function of vitamin D in pregnancy is beyond calcium/
bone effects. Indeed, maternal vitamin D deciency
is implicated in pregnancy complications associated
with placental dysfunction such as preeclampsia,
preterm birth and IUGR. This, in conjunction with
the presence of a local vitamin D metabolic pathway,
implies that vitamin D has a critical role in placental
development and function. So what are the underlying
biological mechanisms that contribute to the effects
of vitamin D on placental development and function?
The key mechanisms include effects on implantation,
inammation, immune function and angiogenesis.
Implantation, inammation and immune function
Vitamin D has potent effects on immune responses
(Liu et al. 2006) and inuences both the innate
and adaptive arms of the immune system (Hewison
2011
). Immune adaptations are vital for successful
pregnancy outcome and vitamin D likely acts to
promote implantation due to its role in inammatory
pathways and immune function. Thus, administration
of 1,25(OH)
2
D
3
in rats promotes decidualization
(
Halhali et al. 1991), and vitamin D likely has a key
role to play in the immune function of the decidua (for
an extensive review see Tamblynetal. 2015), given its
important immunomodulatory effects on trophoblasts.
Indeed, vitamin D metabolites enhance extravillous
trophoblast invasion in vitro (Chanetal. 2015) and act
in a paracrine manner to suppress cytokine secretion
in uterine natural killer cells (Evans et al. 2006)
and villous trophoblast (Díaz et al. 2009, Noyola-
Martínezetal.2013).
In light of this, it seems that the anti-inammatory
actions of vitamin D likely play a critical role in the
prevention of placental pathologies. Primary human
trophoblast cell cultures treated with antiphospholipid
antibodies (as a model of antiphospholipid syndrome,
an autoimmune disorder associated with pregnancy
loss and preeclampsia) demonstrate a dampened
inammation response in the presence of vitamin D
(Gysleretal. 2015), whereas vitamin D treatment inhibits
trophoblast expression and secretion of IL-10 in placental
tissue obtained from both healthy and pre-eclamptic
pregnancies (Barreraetal. 2012). The actions of vitamin
D extend into enhancing antibacterial responses to
Escherichia coli in trophoblast culture (Liuetal. 2009).
Genetic ablation of Cyp27b1 stimulates placental
inammation when challenged with LPS; yet, in wild-
type placentas, the presence of vitamin D metabolites
suppresses the inammatory response to LPS (Liuetal.
2011). Placental LPS-induced inammation can be
inhibited by pre-administration of vitamin D, preventing
IUGR, which appears at least partly due to increasing
interactions of the VDR and the nuclear factor kappa B
p65 subunit (Chenetal. 2015b). Therefore, it appears
that vitamin D has generally positive effects in regulating
placental immune function and providing protective
effects against inammation and bacterial infection.
Placental angiogenesis and nutrient transport
There is some evidence for maternal vitamin D
deciency altering placental vascular development.
Thus, dietary vitamin D-restricted mice exhibited
decreased vascular diameter (Liu et al. 2013) and
decreased fetal capillary length and volume (Tesicetal.
2015) in the labyrinth zone, which is the highly
vascular zone of the rodent placenta responsible for
nutrient and waste exchange. Mechanistic evidence for
vitamin D effects on angiogenesis have been provided
by studies in endothelial colony-forming cells. Thus,
vitamin D promotes the formation of capillary-like
structures in conjunction with an upregulation of
angiogenic factor vascular endothelial growth factor
(Vegf) (
Grundmannetal. 2012). These observed changes
in Vegf are supported by the observation that maternal
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vitamin D deciency is associated with a reduction of
Vegf in the highly vascular labyrinth zone of mouse
placenta (
Tesicetal. 2015).
In addition to altered angiogenic pathways, there
is also an association between vitamin D status and
placental nutrient transport. Indeed, Cleal and coworkers
(Cleal et al. 2015) found that human placental gene
expression of amino acid transporters correlated with
maternal vitamin status as well as vitamin D-binding
protein. This implies that while placental vitamin D
is important for supplying amino acids to the fetus,
delivery of vitamin D to the placenta is also crucial for
this process (Clealetal. 2015). Supporting associations
have also been found in mechanistic studies using
primary human trophoblast cell culture, whereby
1,25-dihydroxy vitamin D
3
was shown to regulate
system A amino acid transporter activity via increased
expression of the SNAT2 amino acid transporter
(Chenetal. 2017).
The specic role of placental vitamin D receptor
So how many of these observed effects of vitamin
D on placental development and function are
mediated via binding of the vitamin D receptor in the
placenta? Initial studies in Vdr-knockout (Vdr
−/−
) dams
demonstrated decreased fertility and a reduction in fetal
weight in comparison to heterozygous (Vdr
−/−
) dams
(Kovacsetal. 2005); however, the role of the placenta
in this study was not assessed. Elegant investigations
of placentas from Vdr
+/−
matings have proven to
be more denitive in aiding our understanding of
the signicance of placental Vdr, as this eliminates
maternal pathophysiology and generates each of the
three possible placental–fetal genotypes within the
one pregnancy (Vdr
+/+
, Vdr
+/−
and Vdr
−/−
). Thus, in
response to an LPS challenge to simulate maternal
infection, inammatory markers were upregulated in
the Vdr
−/−
placentas in comparison to wild type (Vdr
+/+
)
(Liuetal. 2011), further implicating a role for vitamin
D as a regulator of placental inammation. Microarray
and morphology comparisons of heterozygous-bred
Vdr
+/+
and Vdr
−/−
placentas by Wilson and coworkers
(Wilsonetal. 2015) have revealed that although there
were some differences in gene expression, this did not
manifest in altered placental morphology or placental
function, as fetal and placental weights were unaltered.
Of those genes that did change, Cyp24a1, which
encodes the enzyme responsible for the catabolism of
1,25(OH)
2
D
3
, was robustly downregulated in Vdr
−/−
placentas, whereas other altered genes are implicated
in pathways such as autophagy, cell signaling and
cytoskeletal modications and mammalian target of
rapamycin (mTOR) signaling. It should be noted that,
in the studies mentioned (
Liuetal. 2011, Wilsonetal.
2015), whole placental gene expression was assessed
and therefore, whether there are differential gene
expression responses between the junctional and
labyrinth zone of Vdr
−/−
placentas is yet to be established.
On the whole, however, the lack of a robust placental
phenotype in heterozygous-bred Vdr
−/−
placentae
highlights that maternal vitamin D status and the
associated physiological response is critical for causing
the placental phenotype, as opposed to direct actions
of vitamin D on the placental vitaminD receptor.
The potential physiological responses to maternal
vitamin D deciency are myriad. Our group has
recently begun to focus on the potential interactions
between vitamin D and glucocorticoids during
pregnancy; however, there are many other key
candidates of interest. Given the multifaceted role
of vitamin D in immune pathways, there remains
much work to be done elucidating how maternal
vitamin D status inuences appropriate immune and
inammation response in pregnancy. Furthermore, the
observation that mTOR expression is altered in Vdr
−/−
placentas is of particular interest as placental mTOR
signaling plays a pivotal role as a maternal nutrient
sensor and thus is critically implicated in fetal growth
and development (for a review see
Dimasuay et al.
2016). Herein, we discuss how vitamin D deciency
alters glucocorticoid-related pathways in the maternal,
fetal and placental compartments and how this may
elicit the observed changes in placental development
and function.
Glucocorticoids and pregnancy
Glucocorticoids regulate many physiological pathways
including cellular differentiation, immunoregulation,
inammation and metabolism. During pregnancy,
activity of the hypothalamic–pituitary–adrenal
(HPA) axis markedly increases and accordingly, free
glucocorticoids (cortisol and corticosterone in humans
and rodents respectively) rise. This upregulation
of glucocorticoids enables the mother to meet the
metabolic demands of pregnancy (Seckl & Holmes
2007, Wharfe etal. 2016) and plays an essential role
in the maturation of fetal tissues in preparation for birth
(Coleetal. 1995a,b, Diazetal. 1998, Surbeketal. 2012).
The issue arises, however, when fetal glucocorticoid
exposure is excessive as this results in placental
dysfunction, fetal growth restriction and altered organ
development (Benediktssonet al. 1993, Lindsayetal.
1996a, Nyirendaetal. 1998, Ainetal. 2005, Hewittetal.
2006, Holmes et al. 2006, Wyrwoll et al. 2009,
Cuffe et al. 2011, Vaughan et al. 2012). Furthermore,
the effects of early-life glucocorticoid exposure on
development resonates throughout the lifespan with
adverse effects on adult health outcomes including
cardiometabolic and neuropsychiatric disorders
(
Benediktsson et al. 1993, Lindsay et al. 1996a,b,
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Nyirendaetal. 1998, Sugdenetal. 2001, Holmesetal.
2006, Wyrwolletal.2006).
Given the potentially detrimental effects of
glucocorticoid excess, it is critical that fetal exposure
to glucocorticoids is tightly regulated. Indeed, fetal
glucocorticoid levels remain up to 10-fold lower
than those of maternal levels during the majority of
gestation (Beitinsetal. 1973, Edwardsetal. 1993). This
marked differential arises largely due to the presence
of the enzyme 11β-hydroxysteroid dehydrogenase
type 2 (Hsd11b2) in placental and fetal tissues.
Hsd11b2 catalyzes the conversion of cortisol and
corticosterone into their inactive forms, cortisone and
11-dehydrocorticosterone respectively. In humans,
Hsd11b2 is highly expressed in the syncytiotrophoblast
(Stewartetal. 1995) at the interface between maternal
and fetal circulations. Similar patterns of Hsd11b2
expression are seen within the labyrinth zone of the
rodent placenta (Brownetal. 1996, Burtonetal. 1996).
Expression of placental Hsd11b2 decreases at the end of
gestation, presumably to facilitate fetal organ maturation
(Burtonetal. 1996).
Environmental perturbation in pregnancy can
increase fetal glucocorticoid exposure by altering
placental transfer of glucocorticoids. 10–20% of
maternal glucocorticoids reach the fetus unaltered
(Benediktsson et al. 1997), and therefore, stressors
(ie. psychological stress, dietary insults, inammation
and hypoxia) that increase maternal glucocorticoid
levels can increase fetal glucocorticoid exposure.
Furthermore, these stressors generally reduce placental
Hsd11b2 expression and/or activity and thus increase
fetal glucocorticoid exposure (Langley-Evansetal. 1996,
Mairesseetal. 2007, Seckl & Holmes 2007, Vieauetal.
2007). Therefore, given the high incidence of maternal
vitamin D deciency in pregnancy, could this act as
a physiological stressor and exert effects on pathways
involved in fetal glucocorticoid exposure?
Vitamin D and glucocorticoid interactions
The possibility of vitamin D and glucocorticoid
pathway interactions are a relatively unexplored
consideration; however, it seems likely from other
studies that there is scope for substantial cross-talk
between these two hormones. Indeed, vitamin D exerts
antagonistic effects to the exposure of the synthetic
glucocorticoid dexamethasone in the hippocampus.
Thus, dexamethasone increases viable cells, decreases
apoptotic cells and reduces neurite outgrowth in
hippocampal cell culture with each change mitigated by
concomitant administration of vitamin D (Obradovicetal.
2006). In addition, vitamin D appears to downregulate
glucocorticoid receptor levels (
Obradovicetal. 2006).
Similar observations have been made in a squamous
cell carcinoma model whereby dexamethasone
induces de novo transcription of the vitamin D receptor
in a glucocorticoid receptor-dependent manner
(
Hidalgo et al. 2011). Furthermore, vitamin D also
appears to have direct effects on the glucocorticoid
sensitivity of peripheral blood mononuclear cells, with
evidence of direct downregulation of glucocorticoid
receptor expression and inhibition of GR translocation
to the nucleus (Kassietal. 2016). Moreover, in patients
with steroid-resistant asthma, vitamin D administration
benecially alters the clinical response to glucocorticoids
(Chambersetal. 2015).
So what occurs to glucocorticoid-related parameters
in models of gestational vitamin D deciency? There
are multiple pathways that inuence fetal exposure
to glucocorticoids with vitamin D deciency (Fig. 2).
Maternal vitamin D deciency in a mouse model of
pregnancy elevates circulating maternal glucocorticoids
as well as adrenal weight, indicating possible chronic
Figure2 Proposed effects of developmental vitamin D deciency on
glucocorticoid metabolism and levels in the mother and fetus.
(A)Ingestational vitamin D deciency, maternal circulating active
glucocorticoids (GCs) and GC release in response to stress is
elevated, likely increasing placental GC exposure and transport.
(B)In the placenta, 11β-Hsd2 (a key enzyme that inactivates GCs) is
decreased by vitamin D deciency, which decreases the conversion
of active GCs to inactive forms. (C) In the fetus, the combination of
increased maternal GC levels and decreased GC inactivation due to
vitamin D deciency leads to increased fetal GC exposure.
(D)Ultimately, vitamin D-decient fetuses exhibit a likely increase in
GC exposure in the brain, as indicated by increases in the
GC-responsive gene Tsc22d3.
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