Discussion
In the present study, we demonstrated for the first time that placental
11β-HSD2 dysfunction leads to hallmark of PE features, deficient
trophoblast invasion, abnormal placentation and reduced placental blood
flow in pregnant rats. Further, using cultured human placental cells, we
revealed that 11β-HSD2 downregulation suppresses migration and invasion
of EVTs and promotes sFlt1 release. These data strongly indicate that
dysregulation of placental 11β-HSD2 play critical roles in PE
development.
Prior studies have demonstrated that placental 11β-HSD2 expression and
activity are suppressed by systemic administration of CBX
(Lindsay et al., 1996;
Sanna et al., 2016). Similarity, we
showed that systemic administration and placenta-targeted delivery of
CBX resulted in a decrease in 11β-HSD2 expression and activity in
placentas. In accordance with that 11β-HSD2 converts corticosterone into
11-hydroxycorticosterone, an increase in corticosterone concentration in
placenta was exhibited in the pregnant rats with administration of CBX
(1.2mg/kg & 2.4mg/kg ) or CBX-CSA, indicating that inhibition of
11β-HSD2 lead to excess GC in placenta. As expected, systemic
administration of CBX could lead to increased corticosterone
concentration in maternal circulation because 11β-HSD2 in maternal
peripheral tissues besides placenta was inhibited. Of note, our data
also demonstrated that specific inhibition of placental 11β-HSD2 by
CBX-CSA could increase corticosterone level in maternal circulation in
late gestation. It indicates that a large amount of corticosterone is
metabolized by the placenta in late gestation, and confirms that
placental 11β-HSD2 constitutes a barrier for glucocorticoids across
placenta.
As mentioned, local GC concentration in tissues is controlled by
11β-HSD1 and 11β-HSD2(Hunter & Bailey,
2015). Some studies have demonstrated that 11β-HSD1 in fetal membranes
regulates prostaglandin synthesis and metabolism through control of
local GC concentration (Li et al., 2014;
Mirazi et al., 2004). Since no specific
GC receptor antagonist is commercially available, we investigated the
effect of DEX administration on pregnant outcome in order to mimic the
effects of excess GCs. PE-like features including hypertension,
nephropathy and increased level of sFlt1 in maternal circulation were
induced in the pregnant rats with DEX administration. In the cultured
human placental cells, reduced migration and invasion and increased
sFlt-1 secretion were induced by cortisol in the cells with 11β-HSD2
knockdown. Together, it strongly suggests that excess GCs mediate the
development of PE caused by placental 11β-HSD2 downregulation.
Maternal endothelial dysfunction in PE mainly results from the factor(s)
released by the ischemic placenta. Among them, imbalance release of
sFlt1 and PlGF plays critical roles in PE development
(Seki, 2014;
Szpera-Gozdziewicz & Breborowicz, 2014).
Many studies have shown that sFlt1/PlGF ratio is markedly increased in
PE patients and in women with pre-existing conditions predisposing or
mimicking PE (Hodel et al., 2019;
Saleh et al.,;2017;
Zeisler et al., 2016). In consistence
with these studies, we demonstrated that an increased circulatory
sFlt-1/PlGF ratio was found in the rats with PE-like features induced by
placental 11β-HSD2 downregulation. Moreover, we found that 11β-HSD2
downregulation promoted sFlt1 release but did not affect PlGF release,
indicating that increased sFlt1 secretion from placenta is one of the
mechanisms of the PE development induced by placental 11β-HSD2
dysfunction.
In the present study, we demonstrated that 11β-HSD2 modulates sFlt1
release through proteolytic process of Flt1 in placenta. Membrane
anchored metalloproteases of the ADAM family assume central functions in
the living cell by the controlled cleavage and release of biologically
active proteins and peptides from the membrane surface (Jones et al,
2016; Zunke &Rose-John,2017). ADAM10 and ADAM17, the well characterized
members of the ADAM family, have been shown to mediate the release of a
number of cytokines and various receptors from cell membrane in human
placentas (Hung et al., 2008;
Ma et al.,2011;
Yang et al., 2012;
Zhao et al., 2010). Our previous study
has shown that ADAM10 can mediate sFlt1 release in human placenta
(Hu et al., 2015). In the present study,
we confirmed that ADAM17 is also involved in sFlt1 secretion in
placenta. The regulatory mechanisms responsible for ADAM10 and ADAM17
expression in placentas remain to be elusive. In the present study, we
firstly proved that excess GCs caused by 11β-HSD2 dysfunction could
upregulate ADAM17 but not ADAM10 mRNA expression. As bioinformatic
analysis shows seven putative GREs in ADAM17 gene promoter, we
then demonstrated that these elements conferred GCs upregulation ofADAM17 gene transcription using the molecular biological
approaches. Of note, Yang et al (2012)
have proposed that increased expression of ADAM10 and ADAM17 is
associated with oxidative stress, a common phenomenon in PE placenta.
Interestingly, our previous study has shown that decreased
H2S production contributes to increased ADAM10
expression in PE placentas (Hu et al.,
2015). The present study demonstrated that excessive GCs could promote
ADAM17 expression in placentas. All together, it may indicate that
ADAM10 and ADAM17 are the final paths of various factors linked to PE
development.
As mentioned, the first stage of PE is attributed to impaired functions
of EVTs. In the cultured human cell lines of EVTs, we demonstrated that
11β-HSD2 regulates migration and invasion function of EVTs. In the
animal model, we showed that placental 11β-HSD2 maintains normal
interstitial trophoblast invasion, endovascular SA remodeling, placental
blood flow and placental morphology, indicating that placental 11β-HSD2
is crucial for normal placentation and placental development. Of note, a
number of prior studies have proposed that IUGR in rodents caused by
administration of CBX is attributed to excess active GCs because GCs
have been known to inhibit fetal growth
(Murphy et al., 2012;
Welberg et al., 2000). In human studies,
it has been assumed that downregulation of placental 11β-HSD2 is
associated with IUGR because GCs have detrimental effects on fetal
growth (Causevic & Mohaupt, 2007; Hofmann
et al, 2001). Our findings indicate that abnormal placentation and
reduced placental blood flow caused by excess GCs due to 1β-HSD2
dysfunction also contribute to IUGR.
The limitation of this study is that we did not elucidate the mechanisms
responsible for excessive GC suppression of migration and invasion
function in EVTs. It is known that invasion function is associated with
expression of MMPs in EVTs, in particular, MMP2 and MMP9 are crucial for
invasion function of EVTs (Peng et al.,
2016; Zhang et al., 2013).
Interestingly, some studies have shown that synthetic GCs can regulate
MMP2 and MMP9 expression in trophoblast cell lines
(Mandl et al., 2006;
Pavek et al., 2007). More recently,
Kisanga et al (2018) have shown that
synthetic GCs can suppress migration and invasion of EVTs and lead to
more than 3,000 genes changed, suggesting that GCs modulate EVT function
via various signaling pathways. Nevertheless, the mechanisms underlying
GCs regulation of trophoblast invasion remains to be further
investigated.
In conclusion, our study has demonstrated a previously unrecognized role
of 11β-HSD2 in placental development and function. Excessive active GCs
caused by placental 11β-HSD2 downregulation can lead to impaired
invasion function of EVTs, abnormal placentation and increased sFlt1
release from placenta via elevated ADAM17 expression. Thus, our data
provide in vivo evidence that dysregulation of placental 11β-HSD2
plays a critical role in pathogenesis of PE. It is therefore
valuable to develop strategies to treat PE by using specific activators
of 11β-HSD2 or specific inhibitor to block GCs in placenta.