Discussion
Our current study suggested that inhalational anaesthetic sevoflurane
enhanced ovarian cancer cell viability, proliferation, migration, and
invasion. In contrast, intravenous anaesthetic propofol inhibited those
cellular activities. These phenotypic observations could be associated
with upregulated expressions of GLUT1, MPC1, GLUD1, p-Erk1/2, HIF-1α,
CXCL12 and CXCR4, and downregulated PEDF expression in the sevoflurane
group. Sevoflurane, but not propofol, promoted metabolism of ovarian
cancer cells by enhancing the uptake of the metabolic substrates such as
glucose and glutamine.
The lower concentration of glucose and higher concentration of pyruvate
in the media were found after sevoflurane exposure compared to the naïve
control, which were in agreement with observed upregulated expression
level of GLUT1. The enhanced activity of GLUT1 may transport more
glucose into the cytoplasm to convert to pyruvate, which explained the
metabolic changes in the media after sevoflurane administration. In
contrast, propofol downregulated GLUT1, which resulted in the less
uptake of glucose from media, and hence the concentration of glucose was
increased after propofol exposure. The cellular glucose uptakevia high level of GLUT1 expression was correlated with the
malignancy of cancers (Leung, 2004; Pezzuto et al., 2020) and the
overexpression of GLUT1 in cancer cells were essential for the high rate
of glycolysis (Wright, 2020). Propofol was reported to downregulateGLUT1 gene in macrophages (Tanaka et al., 2010) and this was in
line with the downregulation of GLUT1 proteins in the rat brain tissue
under hypoxic preconditioning (Xiao et al., 2020). This is quite similar
to cancer cell scenario, as, under high proliferative rate, cancer cells
had inadequate oxygen supply and were actually under hypoxic condition
(Mudassar, Shen, O’Neill, & Hau, 2020). These supported our findings
that the malignancy of ovarian cancer cells was related to the
expression of GLUT1 after anaesthetic administration.
The concentration of lactate was increased in both sevoflurane and
propofol treatments, which might be resulted from the increased pyruvate
in glycolysis and then generated more lactate. It was reported that
sevoflurane and propofol both increased the lactate level in the blood
of dogs (Söbbeler et al., 2018). Another study in mice also demonstrated
that sevoflurane increased pyruvate and lactate levels (Horn & Klein,
2010). All these reports were in line with the findings of our current
study.
It was found that both the expressions of MPC1 and GLUD1 were
upregulated after sevoflurane exposure but were downregulated after
propofol treatment. Besides, the concentration of glutamine in media was
decreased after sevoflurane administration. MPC1, a member of
mitochondrial carrier system, locates at the inner membrane of
mitochondria and transports pyruvate into mitochondria from cytoplasm
(Taylor, 2017). The expression of MPC1 is decreased in most tumour types
especially those under a high rate of proliferation as related to the
increased rate of glycolysis. This suggests that pyruvate likely shifts
from the mitochondrial TCA cycle to cytoplasm glycolysis, which does not
require oxygen supply as cancer cells are usually under hypoxic
condition (Rauckhorst & Taylor, 2016). In addition, glutaminolysis
compensates for the disturbed function of the TCA cycle due to less
pyruvate intake and cancer cells, in turn, uptake glutamine and convert
them into glutamate under the activation of GLUD1. The glutamate can be
used by TCA cycle to restore the survival of cancer cells as the
intermediates of TCA cycle are the source for synthesis of amino acids,
proteins, fatty acids, lipids, carbon skeleton, and nucleic acids (Yoo,
Yu, Sung, & Han, 2020). Evidence from other studies showed propofol
might disturb the mitochondrial respiratory chain, which was related to
TCA cycle (Berndt et al., 2018). It was also reported that unlike
inhibitory effects of propofol, sevoflurane preserved the function of
the mitochondrial respiratory chain in a myocardial ischaemic model
(Lotz, Stumpner, & Smul, 2020). Our data demonstrated that after
sevoflurane administration, the MPC1 and GLUD1 expressions were
upregulated, which might enhance the activity of the TCA cycle to meet
the demands of cancer survival and progression. The decreased
concentration of glutamine in media suggested that the utilisation of
glutamine was likely increased and glutaminolysis was then promoted.
With the disturbed function of the TCA cycle after propofol exposure,
amino acids that can be used in the TCA cycle were accumulated, such as
asparagine and arginine (Pasini et al., 2018), which was consistent with
our findings.
Except glucose and glutamine, another “mirror change” of metabolites
between sevoflurane and propofol administration was isopropanol. It was
reported that the level of isopropanol was increased in the exhaled
breath of lung cancer patients, and it had been regarded as a potential
biomarker for lung cancer diagnosis (Chien et al., 2017). It seemed the
level of isopropanol had some correlations with cancer malignancy, which
was consistent with the findings of this study that sevoflurane enhanced
the malignancy of ovarian cancer cells and increased the level of
isopropanol, while propofol inhibited the malignancy of ovarian cancer
cells and decreased the level of isopropanol. Isopropanol can be
reversibly converted to acetone (Beauchamp, Valento, & Kim, 2016; Li,
Liu, Liu, Cheng, & Duan, 2017), which may also contribute to the
increased level of acetone in the media of the propofol group.
The levels of glycerol and fatty acids were increased in the propofol
group. Through β-oxidation of fatty acids, acetyl-CoA is generated and
used in the TCA cycle (Y. Liu, 2006). Thus, the changes of glycerol and
fatty acids in the propofol group were another evidence that the
mitochondria function and TCA cycle was inhibited or disturbed by
propofol treatment. The glycerol and fatty acids might also come from
cell membrane degradation and phospholipids broke down into them. From
an earlier study, it was found that propofol affected the membrane
ultrastructure of HeLa cells that the surface roughness of cellular
membrane was decreased in a dose-dependent manner (Zhang et al., 2016).
In the current study, the expression level of PEDF was decreased after
sevoflurane administration but increased after propofol treatment. In
human retinal pigment epithelium, the expression of GLUT1 was increased
under hypoxia condition that resulted in the increased uptake of
glucose, which led to a decrease of PEDF expression (Calado, Alves,
Simão, & Silva, 2016). Another study also reported that the
overexpression of the PEDF gene in mice was related to the
reduction of glucose uptake and decreased expression of GLUT1 (Calado,
Diaz-Corrales, & Silva, 2016). These reports were consistent with our
findings that sevoflurane increased the GLUT1 expression and glucose
uptake, which led to a downregulated expression of PEDF. However, an
opposite effect was found with propofol treatment.
In the current study, Erk1/2 signalling pathway was induced after
sevoflurane exposure but inhibited after propofol exposure. There was
evidence that the increased expression of PEDF was related to the
inhibition of Erk1/2 signalling pathway in a diabetic model (Dong et
al., 2019), which was in line with our results. HIF-1α is a
transcriptional factor that can be regulated by a variety of signalling
pathways, and Erk1/2 pathway is one of them (R. M. Liu, Xu, Chen, Feng,
& Xie, 2020). In cancer cells, the HIF-1α is overexpressed, which
regulates tumour survival-related genes, such as CXCL12 andCXCR4 (Gola et al., 2020; Xue et al., 2020). It was in line with
the results of the current study that sevoflurane upregulated Erk1/2
signalling pathway, and HIF-1α, CXCL12 and CXCR4 expressions, while
propofol downregulated these molecular entities.
Our study has some limitations. Firstly, the causal relationship between
cellular signalling changes and metabolic alterations induced by
anaesthetics remains unknown. However, it is very likely that, for
example, sevoflurane promotes cancer cell survival and development due
to survival cellular signalling pathway activation whereby more energy
substrates use up for cell proliferation and growth. Secondly, our
cultured cell study may not be relevant to human. Therefore, the
implications of our current study may be limited. However, in some
clinical studies, breast, colonic and rectal cancer patients were
anaesthetised with inhalational anaesthetics sevoflurane or desflurane,
or intravenous anaesthetic propofol during surgery and the survival rate
of propofol anaesthetised patients were significantly higher than those
with inhalational anaesthesia (Enlund et al., 2014; Wu et al., 2018).
Laboratory data including the one reported here and retrospective
clinical data all point to that sevoflurane might be a risk factor for
cancer patients, while propofol may be beneficial to cancer patients for
their surgery. Therefore, clinical studies are urgently needed to
evaluate anaesthesia regimens for cancer patients to optimise the
surgical outcomes.