Discussion
In this study, we measured plasma FAA and carnitine levels in HM
patients and healthy pregnant women as a control group. The original
finding of the present study was that three amino acids
(i.e., alanine, arginine, and
valine) were significantly decreased, and three carnitines (i.e., C8DC,
C16:1, and C18) were significantly increased in the pregnant women with
HM.
The mean plasma total carnitine concentration in non-pregnant women has
been reported to be approximately 40 µmol/L [16]. It has been found
that plasma carnitine concentrations decreased during pregnancy and this
decrease was mostly in the first half of pregnancy [17]. The reasons
for these low plasma carnitine concentrations in pregnancy are currently
unknown, but several hypotheses have been asserted: Ringseis et al.
speculated that plasma carnitine concentrations may be low in pregnant
women due to reduced carnitine synthesis [18]. However, their study
had a clear limitation because the number of subjects was very small.
Cho and Cha [17] demonstrated that low carnitine levels were caused
by increased urinary loss of carnitine in Korean pregnant women. Grube
et al. [8] showed that increased expression of carnitine/organic
cation transporter 2 (OCTN2), a carnitine-transporting protein in human
placenta, can play an important role in carnitine supply of growing
fetus especially in early gestation. Grube et al. [8] detected that
this protein located in the membrane of syncytiotrophoblast, had a
dominant role for carnitine uptake from maternal circulation. However,
they have not simultanously measured the carnitine level changes in
maternal plasma. Bai et al. [19] have also investigated the reason
of carnitine reduction and concluded that OCTN2 mediated L-Car transfer
across placenta played a major role in maternal plasma L-Car reduction
during pregnancy. We found higher carnitine levels in plasma of pregnant
women with HM compared to the healthy pregnant women. This finding
suggests a defect in carnitine uptake by syncytiotrophoblasts due to the
absence of a healthy pregnancy in HM patients or decreased expression of
OCTN2 protein in HM cases. Although we did not study OCTN2 protein in
pathology specimens of HM patients, this speculation may be a hypothesis
for further studies.
Studies on amino acids have been previously published in several cancers
and obstetric diseases such as gestational diabetes [20-23].
Neoplastic cells need some amino acids, such as glutamine, glycine,
aspartic acid and serine, for the new vessel formation, DNA synthesis,
duplication of protein, and synthesis of hormones. In an in vitro study
on mice, Roux et al. [12] demonstrated that the glutamine amino acid
uptake of pancreatic adenocarcinoma cells increased compared to
non-pancreatic tumor cells, therefore, the plasma glutamine levels
decreased in the environment. Additionally, Saglik et al. [24] found
that glutamine amount was higher in pterygium tissue than in normal
conjunctival tissue. Both studies were designed at the tissue level,
unlike ours. In our study, circulating plasma glutamine levels were
found similar in both groups, however, alanine, arginine, and valine
levels were lower in HM group than in healthy pregnants. This
discrepancy may be due to different histopathogenesis of the diseases,
differences in method used, small sizes of our study, female gender or
younger age of our participants.
Liu and colleagues [25] pointed out changes in FAA concentrations in
both plasma and gastric fluid, including threonine, serine, alanine,
valine, methionine, isoleucine, leucine, tyrosine, phenylalanine,
lysine, and arginine in the diagnosis of gastric cancer. They found an
increase in gastric fluid FAA levels and a decrease in plasma FAA
concentrations in gasric cancer
patients compared to nongastric cancer patients. The inconsistency of
these metabolic phenotypes between plasma and gastric fluid in gastric
cancer patients (opposite FAA levels) was explained as abnormal
accumulation of several
metabolites in tumor microenvironment. Camelo et al. [13] showed
that high FAA levels in the intervillous space were compatible with
syncytiotrophoblast activity in healthy pregnant women. They thought
that these high levels occurred due to an asymmetric influx or active
transport from the trophoblast cells to the blood in the intervillous
space. However, there is no study researching the association between
the plasma FAAs and HM. As the first study in the literature, we found
decreased concentrations in several maternal plasma FAAs in patients
with HM. This finding may be caused by impaired active transport from
the intervillous space to the blood in patients with HM, or due to
excessive accumulation of FAAs in trophoblastic microenvironment,
because trophoblastic cells are highly proliferative and are similar to
cells in cancer tissue. Although molar pregnancy is not malignant tumor,
it has a risk for gestational trophoblastic neoplasia. However, we think
that if the tissue-supported studies are performed, it will contribute
to our results.
Miyagi et al. [20] suggested that plasma FAA profiling has an
important role for cancer screening and diagnosis in patients with
asymptomatic early-stage of the disease. Morever, they have demonstrated
that the differences in FAA metabolism did not releated with impaired
nutritional support of cancer patients. In contrast, some authors have
suggested that low plasma FAA levels were associated with anorexia,
malnutrition, and progressive weight loss in cancer patients. Thus, the
factors affecting plasma FAA levels in these subjects still remain
controversial and unclear [10,11]. Our HM patients had impaired
nutritional support due to hyperemesis, but there was no correlation
between the plasma FAA and urine ketone levels.
Several factors, such as obesity and food intake, may affect the serum
FAA and carnitine levels. Rigamonti et al. [26] showed that whey
consumption in obese female subjects increased some circulating amino
acids (alanine, arginine, asparagine, citrulline, glutamine,
hydroxyproline, isoleucine, histidine, leucine, lysine, methionine,
ornithine, phenylalanine, proline, serine, threonine, tyrosine and
valine). They reported that of these amino acids, isoleucine, leucine,
lysine, methionine, phenylalanine, proline, tyrosine, and valine
correlatated negatively with starvation and positively with satiety.
Additionally, Yamada et al. [27] speculated that deficiencies of
certain dietary substrats, such as folic acid, proteins, and vitamin
B-12, which are essential for nucleic acid formation in decidual cells,
may affect the risk of abortion releated with chromosomal disorders.
Although, we did not measure other circulating parameters such as folic
acid and vitamins, we showed that HM patients had different plasma FAA
and carnitine levels compared to the healthy pregnants. We excluded
obese pregnants, smokers, and women who use food supplements from the
study to avoid changes in plasma FAA and carnitine levels that may occur
due to these factors. However, further epidemiological and biochemical
studies are needed to obtain a more precise definition of specific
dietary correlation.
The limitations of this case-control study were that it was designed
only between pregnant women and only by measuring the maternal plasma.
However, this study is the first in the literature to investigate FAA
and carnitine profiling in HM. Although more research is needed to
support our results, including larger sample sizes and investigating
tissue samples, we think current results may be considered as references
for subsequent studies on HM.