Role of intestinal trefoil factor in protecting intestinal epithelial cells from burn-induced injury
SCIenTIfIC REPORTS |
Role of intestinal trefoil factor in protecting intestinal epithelial cells from burn-induced injury
OPEN Published: xx xx xxxx Although intestinal trefoil factor (ITF) can alleviate the burn-induced intestinal mucosa injury, the underlying mechanisms remains elusive. In this study, we investigated if ITF alters glutamine transport on the brush border membrane vesicles (BBMVs) of the intestines in Sprague-Dawley rats inflicted with 30% TBSA and the underlying mechanisms. We found that ITF significantly stimulated intestinal glutamine transport in burned rats. Mechanistically, ITF enhanced autophagy, reduces endoplasmic reticulum stress (ERS), and alleviates the impaired PDI, ASCT2, and B0AT1 in IECs and BBMVs after burn injury likely through AMPK activation. Therefore, ITF may protect intestinal epithelial cells from burninduced injury through improving glutamine transport by alleviating ERS.
Extensive severe burns not only lead to major damage of the skin and subcutaneous tissues, but also cause varying
degrees of multiple organ injuries1. One of the most seriously postburn-induced injury of organs is the intestine,
mainly due to the hypoxia-caused ischemia. In this setting, abnormal energy metabolism aggravates intestinal
mucosal injury and restrains mucosal repair as well2,3.
Glutamine (Gln) is an important energy source for maintaining the structure and function of human
intestinal mucosa4,5. A series of studies have confirmed that Gln administration can significantly improve intestinal
energy metabolism and promote ATP production in the postburn animals and patients, thereby alleviating
intestinal mucosal injury and promoting mucosal repair6?9. Therefore, many treatment guidelines for critical care
and severe burns recommend the routine use of Gln preparations to the patients10,11. The transport of Gln from
the intestinal lumen to intestinal epithelial cells (IECs) is a crucial initial step in Gln metabolism in the
intestine. Many studies showed that intestinal Gln transport is decreased significantly in abdominal trauma, systemic
infections, and sepsis12?14. However, the underlying mechanisms remain unclear. Our previous work also found
that enteral administration of Gln to the burn patients could not remarkably increase the intestinal transport of
Gln compared to healthy subjects15, suggesting that burn-induced injury of the intestine could compromise its
abilities to transport and utilize Gln.
There are several specific Gln transporters on the brush border of IECs, among which the sodium-dependent
amino acid transporters ASCT2 and B0AT1 stand out16,17. ASCT2 and B0AT1 are special membrane transporters
that depend on precise post-translational modification and folding in the endoplasmic reticulum (ER) for proper
functioning18. Related studies have found that N-terminal glycosylation of ASCT2 in the ER after synthesis is
key to its function19?22. Numerous studies have shown that severe endoplasmic reticulum stress (ERS) occurs in
multiple organs after severe burns, resulting in ER dysfunction and impaired post-translational modifications23.
We speculate that ERS may affect post-translational modification and folding for proper function of ASCT2 and
B0AT1 in the ER. Therefore, the present study was undertaken to address these issues.
A variety of drugs can improve intestinal Gln transport under some pathological conditions. For example,
growth hormone can improve Gln utilization in the patients with short bowel syndrome by augmenting the
protein level of Gln transporters24. Epidermal growth factor (EGF) can effectively boost the protein level of ASCT2
and Gln transport in intestinal ischemia13. Insulin-like growth factor-I(IGF-I) can stimulate sodium-dependent
Gln transport in IECs in piglets25. Our previous study also revealed that intestinal trefoil factor (ITF), an
intestine-specific growth factor, can significantly improve the bioavailability of Gln when administered
intragastrically to the burned animals26. However, it remains unclear whether ITF exerts this effect by modifying
intestinal Gln transporters though ERS alleviation in IECs after burn injury.
ITF secreted by intestinal goblet cells can stabilize intestinal mucosa due to its special spatial structure27,28.
Recent studies revealed that ITF can maintain intestinal mucosal integrity and promote cell proliferation and
migration of IECs29,30. Loss of TFF1, a member of the trefoil peptide family, caused significant morphologic
changes in the ER31. Since ITF and TFF1 are the members of same protein family, we wonder if loss of ITF may
also alter the ER in IECs. Therefore, in the present study we investigated whether ITF can promote the
expression and function of ASCT2 and B0AT1 proteins after burn injury; and if so, what are the underlying molecular
Effect of ITF on the burn-induced change in Gln transport by BBMVs of IECs. The results showed
that the total transport abilities of Gln in BBMVs and IECs were significantly reduced after burn injury (P < 0.05)
(Fig.?1a,d), and that sodium-dependent Gln transport was especially attenuated (P < 0.05) (Fig.?1b,e). The uptake
of Gln by this transporter in BBMVs was decreased approximately 80% on day 1 and was still markedly lower on
day 7 after burn injury (P < 0.05) (Fig.?1b).
In contrast, non-sodium-dependent Gln transport was increased after the injury except on day 1 (P < 0.05)
(Fig.?1c,f ). Although the absorption of Gln mediated by BBMVs was 65% of the total Gln transport by IECs
before burn, this value was declined sharply on day 1 and was still markedly lower on day 7 after burn (P < 0.05)
(Fig.?1g). The proportion of Gln transport mediated by the sodium-dependent Gln transporter was decreased to
60% after burn injury, approximately 30% decline (P < 0.05) (Fig.?1h,i).
ITF significantly improved total Gln transport in IECs and BBMVs on days 3 to 7 after injury (P< 0.05)
(Fig.?1a,d). ITF enhanced the activity of the sodium-dependent Gln transporter and therefore increased
the proportion of Gln transport in IECs and BBMVs (P < 0.05) after burn injury (Fig.?1h,i) but not the
Effect of ITF on the burn-induced morphological change in BBMVs of IECs. Using electron
microscopy, we observed that BBMVs were structurally impaired in the intestine of burned rats. On days 1 to 3
after burn injury, the spatial structure of BBMVs was severely damaged, as manifested by the irregular shapes,
broken vesicles, collapsed vesicle walls, rough borders, and many broken vesicle pieces. On days 5 to 7 after
burn, the structure of BBMVs was gradually recovered with relatively clearly defined contours, but vesicle walls
remained collapsed and borders remained rough (Fig.?2).
ITF significantly improved the burn-induced morphological changes in BBMVs, which returned to almost
normal on day 7 after burn injury with intact vesicle membrane structure, round vesicles, and clearly defined
contours, but the borders remained rough (Fig.?2). Therefore, ITF can rescue the integrity of BBMVs after burn
Effect of ITF on the burn-induced changes in the expression of ASCT2 and B0AT1 in IECs. Since
ASCT2 and B0AT1 constitute main sodium-dependent Gln transporters in the intestine16,17, it is logical to test
their involvements. We found that their protein expression levels in IECs were decreased markedly on days 1 to 7
after burn injury (P < 0.05) (Fig.?3a). However, ITF significantly enhanced the protein expression levels of ASCT2
and B0AT1, both returning to almost normal levels on day 7 after burn (Fig.?3a). Nonetheless, the mRNA level of
ASCT2 in IECs was increased on day 5 (Fig.?3b), and the mRNA level of B0AT1 was decreased markedly only on
day 1 after burn (P < 0.05) (Fig.?3b). ITF did not significantly alter the mRNA levels of ASCT2 and B0AT1, except
rescuing B0AT1 mRNA on day 1 only after burn (P < 0.05) (Fig.?3b). Therefore, ITF can rescue the protein but
not mRNA expression levels of ASCT2 and B0AT1 in the intestine after burn injury.
Effect of ITF on the burn-induced change in ERS indicators of IECs. Since extensive ERS occurs in
the cells of several organs secondary to severe burns23, we tested the involvements of CHOP and GRP-78, ERS
marker proteins. CHOP protein in IECs was increased significantly on days 1 to 5 after burn injury, but dropped
to the baseline on day 7 (P < 0.05) (Fig.?4a). GRP-78 protein was also increased significantly on days 1 to 7 after
burn injury (P < 0.05) (Fig.?4a). The mRNA levels of CHOP and GRP-78 share similar patterns with their protein
levels (Fig.?4b). ITF markedly decreased both protein and mRNA levels of CHOP and GRP-78 (P < 0.05) (Fig.?4),
indicating that ITF can alleviate burn-induced ERS in IECs.
Effect of ITF on the burn-induced change in protein disulfide isomerase in IECs. Extensive ERS
affects the expression and activity of protein disulfide isomerase (PDI), an ER chaperone and protein-folding
catalyst32,33. We found both mRNA and protein expression levels of PDI were decreased significantly but almost
recovered to the baseline on day 7 after burn injury (Fig.?5a). ITF markedly enhanced the PDI protein and mRNA
levels (Fig.?5a). We also studied the PDI activity with an insulin aggregation assay, and found that the PDI activity
in IECs was significantly decreased on days 1 to 7 after burn injury (Fig.?5b). ITF markedly heightened the PDI
activity on day 5, suggesting that ITF can enhance both expression and activity of PDI in IECs after burn injury.
NO-mediated S-nitrosylation of PDI (SNO-PDI) can inhibit PDI activity and its beneficial effects34. We
found SNO-PDI level in IECs was significantly increased on days 1 to 7 after burn injury (Fig.?5b). ITF markedly
reduced the SNO-PDI level (Fig.?5b), further supporting our notion that ITF can rescue PDI in IECs after burn
Effect of ITF on the burn-induced changes in AMPK phosphorylation and autophagy in rat
IECs. AMPK is an important upstream regulator of autophagy, and its phosphorylation promotes
autophagy35?37. The present study showed that 1 to 5 days after burn injury, the AMPK phosphorylation level
decreased significantly (P < 0.05) (Fig.?6a). Compared with the burn group, ITF administration significantly
improved the level of AMPK phosphorylation, with statistical significance 1 to 7 days after ITF administration.
The LC3-II/LC3-I ratio, an autophagy marker, increased significantly (P< 0.05) (Fig.?6b). Moreover, we also
investigated another autophagy indicator, the p62 protein level, and we found that the p62 protein level in rat
IECs obviously decreased 1 to 7 days after bury injury (P < 0.05) (Fig.?6b). Compared with the burn group, the
increase in the LC3-II/LC3-I ratio was more pronounced 3 to 5 days after ITF administration, and ITF markedly
boosted the reduction of the p62 protein level (P < 0.05) (Fig.?6b). These results suggested that ITF can improve
the AMPK phosphorylation level and enhance autophagy in rat IECs after burn injury.
PDI inactivation on the biological activity of ITF. To verify whether PDI has an important role in the
mechanism by which ITF promotes the protein levels of ASCT2 and B0AT1 and improves Gln transport, this
study applied 16F16, a PDI inhibitor38, to observe changes in the biological activity of ITF after PDI suppression.
The results showed that after IEC-6 cells were co-cultured with burn serum for 12hours, the protein levels of
PDI, ASCT2, and B0AT1 decreased significantly (P < 0.05) (Fig.?7a), the CHOP protein level increased markedly
(P < 0.05) (Fig.?7a), while Gln transport by IEC-6 decreased substantially (P < 0.05) (Fig.?7b). Compared with the
burn serum group, ITF administration significantly reduced the protein level of CHOP (P < 0.05) (Fig.?7a),
promoted the protein levels of PDI, ASCT2, and B0AT1 (P < 0.05) (Fig.?7a), and enhanced Gln transport by IEC-6
(P < 0.05) (Fig.?7b).
Compared with ITF treatment, after administration of 16F16 (10 ?mol/L), the PDI protein level decreased
enormously (P < 0.05) (Fig.?7a), the CHOP protein level increased sharply (P < 0.05) (Fig.?7a), while the ASCT2
and B0AT1 protein level decreased significantly and Gln transport by IEC-6 decreased (P < 0.05) (Fig.?7a,b). The
results of this study showed that after the suppression of PDI, ERS was enhanced, the protein level of Gln
transporters was reduced, and Gln transport by IEC-6 was impaired, which indicates that ITF indeed improves
intestinal Gln transport by upregulating the protein level of PDI and promoting the protein levels of ASCT2 and B0AT1.
Inhibition of autophagy mitigates the biological effect of ITF on IEC-6 cells. It is known that
autophagy can alleviate ERS by removing abnormal proteins39,40. To confirm whether the regulation of autophagy is a
key step in the mechanism by which ITF alleviates post-burn ERS and increases the protein levels of PDI, ASCT2,
and B0AT1, this study applied 3-methyl adenine (3-MA), an autophagy inhibitor, to observe changes in the
biological activity of ITF. The results showed that after IEC-6 cells were co-cultured with burn serum for 12hours,
the protein levels of PDI, ASCT2, and B0AT1 decreased significantly (P < 0.05) (Fig.?8a), the CHOP protein level
increased markedly (P < 0.05) (Fig.?8a), the LC3-II/LC3-I ratio increased (P < 0.05) (Fig.?8a), while Gln transport
by IEC-6 decreased substantially (P < 0.05) (Fig.?8b). Compared with the burn serum group, ITF administration
significantly increased the LC3-II/LC3-I ratio sharply (P < 0.05) (Fig.?8a), reduced the protein level of CHOP
(P < 0.05) (Fig.?8a), promoted the protein levels of PDI, ASCT2, and B0AT1 (P < 0.05) (Fig.?8a), and enhanced
Gln transport by IEC-6 (P < 0.05) (Fig.?8b).
Compared with ITF treatment, after administration of 3-MA (5 mmol/L), the LC3-II/LC3-I ratio was
substantially reduced (P < 0.05) (Fig.?8a), the protein levels of PDI, ASCT2, B0AT1 decreased enormously (P < 0.05)
(Fig.?8a), the CHOP protein level increased sharply (P < 0.05) (Fig.?8a), and Gln transport by IEC-6 decreased
(P < 0.05) (Fig.?8,b). The results of this study showed that after the suppression of autophagy, ERS was enhanced,
the protein level of Gln transporters was reduced, and Gln transport by IEC-6 was impaired, which indicates that
ITF indeed improves intestinal Gln transport by upregulating the autophagy level and promoting the protein
levels of ASCT2 and B0AT1.
Glutamine in the intestinal lumen is transported to IECs mainly through microvilli that are neatly arranged
in the brush border42. Chris IC et al. revealed microvilli curled into vesicle-like structures called BBMVs43.
Under normal circumstances, BBMVs are shaped as spherical vesicles with a smooth surface and high
refractivity. BBMVs that contain proteolytic enzymes, membrane receptors and various transporters function as a
core part of the intestinal transport system32. The present study found that the structure of BBMVs was severely
impaired after burn injury, as shown by such pathological changes such as varied shapes, rough borders, and
vesicle collapse or even fractures. Structural damage further led to functional impairment of BBMVs in that
Gln transport by BBMVs was decreased significantly. Because BBMVs account for the bulk of Gln transport
in IECs, functional impairment of BBMVs may cause a decline in Gln transport in the entire intestine, which
depends on sodium-dependent and independent Gln transporters in IECs. We revealed a sharp decline in
sodium-dependent transporters but an increase in non-sodium-dependent transporters after burn injury. It is
well known that sodium-dependent transporters are the dominant type of Gln transporters in IECs, especially
in BBMVs. Consistently, we revealed that they account for about 75% and 85% of the total Gln transport in IECs
and BBMVs, respectively (Fig.?1h,i).
Gln transport by IECs and BBMVs was decreased significantly after burn injury, which is associated with
intestinal hypoxic-ischemic injury. Previous research has shown that intestinal blood perfusion decreases
markedly and reaches a minimum 12 to 24 hours after injury, at a level approximately 20% of that before injury44.
As time progresses after burn injury, the intestinal blood supply starts to improve, but it remains significantly
lower on day 7 after burn injury, which is comparable with the change in the intestinal Gln transport after burn
injury in our study. Tissue ischemia and hypoxia after burn injury disrupts energy metabolism and attenuates
Na+-K+-ATP activity45 and sodium-dependent Gln transporter activity. In contrast, the enhanced activity of
sodium-independent transporters may be a result of compensatory change. Because intestinal villi are more
sensitive to ischemia and hypoxia, burn injury exerts a more notable influence on Gln transport by BBMVs, which
leads to a direct decline in the ability of BBMVs to transport Gln and a lower percentage of Gln transported by
BBMVs in the whole intestinal tract.
We showed that ITF significantly advanced Gln transport by IECs, especially by BBMVs in burned
rats. Moreover, we found that ITF is more potent on the sodium-dependent transporters than on the
non-sodium-dependent transporters as sodium-dependent Gln transporters dominate in BBMVs.
It was reported previously that ITF could lessen the stress responses triggered by intestinal damage, improve
intestinal blood perfusion, and attenuate tissue damage after burn injury46. However, the underlying mechanisms
are largely unclear in detail. We elucidate the main mechanism whereby ITF improves intestinal Gln transport
after burn injury is that ITF alleviates hypoxic-ischemic damage to IECs and maintains the normal structure of
IECs, especially BBMVs.
We investigated the effects of ITF on the protein levels of ASCT2 and B0AT1, two major subtypes of
sodium-dependent Gln transporters. The present study found that the protein levels of ASCT2 and B0AT1 was
significantly reduced after burn injury and ITF significantly rescued the protein expression of both transporters.
However, our results showed that both mRNA levels of ASCT2 and B0AT1 in IECs kept nearly unchanged after
burn, and ITF barely altered the mRNA levels of ASCT2 and B0AT1. These findings suggest ITF increases ASCT2
and B0AT1 protein levels likely by promoting their translation and regulating post-translational modification.
ASCT2 and B0AT1 are cysteine-containing transporter proteins with special space structures18. Upon peptide
chain synthesis, the cysteine residues in the two transporters are catalyzed by PDI to generate disulfide bonds in
the ER to form a correct spatial structure. Thus, normal function of the ER has a direct bearing on the
modification of these transporter proteins. Our study reveals that severe burn-induced ERS in IECs because the expression
of CHOP and GRP-78, two ERS markers were increased but the expression and activity of PDI were reduced.
Previous studies reported the disturbance of redox equilibrium in the process of ERS. PDI, a chaperone in the
ER, sustains oxidative damage and inactivation, resulting in unfolded or misfolded proteins due to the failure
to form disulfide bonds after peptide chain synthesis and subsequent accumulation of abnormal proteins33,47.
Consistently, we found the expression and activity of PDI were decreased during the severe burn-induced ERS in
IECs, ultimately leading to the failure of post-translational modification on ASCT2 and B0AT1.
With a decrease in PDI activity after burn injury, large amounts of proteins in the ER cannot be properly
modified, resulting in ERS48,49. Marie PC et al.31 reported that TFF1, a member of the same protein family as
ITF, maintained the structure and function of the ER. We demonstrate that ITF significantly lessened ERS since
it decreased the expression of CHOP and GRP-78, but increased the expression and activity of PDI, which is
consistent with the changes in ASCT2 and B0AT1 and sodium-dependent Gln transport. To further test if ITF
promotes Gln transport through PDI, we applied 16F16 to inhibit the PDI activity and found that the effects of
ITF were significantly suppressed, indicating ITF indeed promotes Gln transport through activating PDI.
Autophagy is a catabolic process for the degradation of redundant or damaged cell components and alleviates
ERS by removing unfolded or misfolded proteins. ERS and autophagy interact and jointly maintain cell
homeostasis39,40,50. The present study found that autophagy was enhanced in IECs after burn injury, which may protect
IECs in response to stress. However, enhanced autophagy is insufficient to clear the accumulation of large
numbers of abnormal proteins caused by burn injury, and ERS remains serious. In the present study, ITF significantly
enhanced autophagy in IECs, while at the same time, ERS was decreased to the minimum, suggesting that the
enhanced autophagy inhibits ERS. The inhibition of autophagy using 3-methyl adenine (3-MA) significantly
suppressed the effect of ITF on the enhanced autophagy. Furthermore, 3-MA significantly increased ERS and
reduced protein levels of PDI, ASCT2, and B0AT1, ultimately leading to a sharp decline in the ability of ITF to
promote intestinal Gln transport. Therefore, we showed that autophagy is a key step to regulate ERS in IECs and
is closely related to the synthesis and activity of Gln transporters.
It is well known that AMPK induces autophagy by reducing the activity of the mammalian target of
rapamycin (mTOR)35?37. The present study showed that AMPK phosphorylation was reduced in rat IECs after burn
injury but ITF improved post-burn AMPK phosphorylation. ITF induced the parallel changes in the levels of
AMPK phosphorylation and autophagy in IECs, suggesting that it can enhance autophagy via AMPK pathway.
Activated AMPK can not only alleviate the damage by abnormal proteins in the ER by activating autophagy but
also reduce cell damage by maintaining energy metabolism homeostasis and ameliorating oxidative stress. T.
Yano et al. demonstrated that AMPK activation plays an important role in maintaining the structure of IECs and
the integrity of the intestinal mucosal barrier51. The present study showed that ITF activates AMPK, providing an
insight into the mechanism by which ITF enhances autophagy and reduces post-burn ERS to maintain intestinal
mucosal barrier by improving oxidative stress and enhancing epithelial tight junctions. In summary, we showed
that ITF can significantly enhance intestinal Gln transport in burned rats and improve the utilization of enteral
Gln supplementation. Mechanistically, ITF enhances autophagy, reduces ERS, and alleviates the impaired PDI,
ASCT2, and B0AT1 in IECs after burn injury likely through AMPK activation. Therefore, ITF may be
therapeutically useful to improve intestinal glutamine transport by alleviating burn-induced ERS.
Materials and Methods
Animal handling. Adult male Sprague-dawley rats were purchased from Third military medical university
animal experiment center and allowed to acclimate for one week prior to the experiment. Rats were housed in a
temperature-controlled cubicle with a 12-hour light/dark cycle with laboratory chow and water ad libitum. All
animal procedures were performed in adherence to the southwest hospital for the Care and Use of Laboratory
Animals, according to the protocol outlined in the Guide for the Care and Use of Laboratory Animals published
by the US National Institute of Health (NIH publication no. 85?23, revised 1996), and the experimental protocols
were performed in accordance with the approved guidelines.
A well-established method with minor modifications was applied to induce a full-thickness scald burn52. 120
Sprague-dawley rats were randomly divided into three groups, namely normal group (n = 40), burned group
(n = 40) and burned + ITF group (n = 40). Each group included four time points that are 1 day, 3days, 5days and
7days post burn or ITF administration. Every observation phase include 10 rats in per group. All rats were
anesthetized with 1% pentobarbital sodium (40 mg/kg body weight i.p) and buprenorphine (0.05 mg/kg body weight)
was administered for analgesia. The dorsal surfaces were shaved, rats were placed to a mold with an opening
providing a 30% total body surface area burn. The rats in control group were immersed in 37?C water. Rats from
other groups were then immersed in 97 ?C water for 18 seconds on the dorsal. Lactated Ringers solution (40 ml/kg
body weight i.p.) was then administered intraperitoneally for resuscitation. Rats were housed in separate cages. In
group burned + ITF, rats were accepted gavage ITF (1 mg/kg body weight), bid, rats in other groups were perfused
with the same volume of saline. All rats were sacrificed at 1 day, 3days, 5days, 7days after burn. Small intestine was
excised immediately for preparing Brush BBMVs and IECs.
IECs preparation. IECs were isolated from the small intestine by a calcium chelation technique53. Briefly, a
60 cm section of small intestine was separated and the half of the section was rinsed thoroughly with D-Hanks
water, then it was cut into tissue pieces and collected in a culture bottle containing 20 mL cell isolation buffer
(0.15 mM EDTA, 112 mM NaCl, 25 mM NaHCO3, 2.4 mM K2HPO4, 0.4 mM KH2PO4, 2.5 mM L-glutamine, and
0.5 mM dithiothreitol, pH 7.4) for 30 minutes and gently palpitated to facilitate cell dispersion at 37 ?C. The buffer
was then collected, phenylmethylsulfonyl fluoride was added, and the suspension was centrifuged at 100 g for
5 minutes. Cells were been used for uptake studies of [3H]-Gln and extraction of total protein (4?5 mg/ml).
BBMVs preparation. A half intestine section was flushed gently with 30 ml of ice-cold isotonic saline
containing 0.1 mM PMSF and placed on an ice-cold plate. The intestine was opened along its length, and the tissue
was laid flat. The mucosa was scraped off using an ice-cold glass slide and shaken in a homogenizer that contains
8 ml of collection solution A (300 mM mannitol, 5 mM EGTA, 12 mM Tris?Cl, pH 7.4). The mucosa suspension
was homogenized for 2 minutes and then for 5 minutes on ice. Then, 2ml of ice-cold 50 mM MgCl2 as added to
the homogenate. This solution was kept on ice for 15minutes and gently stirred continuously using a magnetic
stirrer43. Finally, the protein concentration of the BBMVs was determined using standard protocols. BBMVs were
used for uptake studies of [3H]-Gln and transmission electron experiments43.
Cell culture. Rat small intestinal epithelium cell line (IEC-6) was purchased from American Type Culture
Collection (ATCC Cell Biology, Catalog No. CRL-1592). Cells were grown in a humidified incubator at 37 ?C in
5% CO2/94% O2/1% N2 and were routinely grown in high-glucose Dulbecco?s modified Eagle medium (DMEM)
that contained 10% fetal bovine serum (FBS), 4 mmol/L glutamine, 100 IU/mL penicillin, and 100 ?g/mL
streptomycin. IEC-6 cells were dissociated with 0.05% trypsin and 0.02% ethylenediamine-tetraacetic acid (EDTA)
when cell confluence was over 80%. Cells were seeded in 6-well cluster Costar tissue culture plates at a density
of 100,000 cells per well for Western blot analysis and seeded in the 24-well cluster Costar tissue culture plates
at a density of 10,000 cells per well for the [3H]-Gln transport experiments. When the cell confluence was over
60%, the IEC-6 monolayer was then washed 2 times with PBS, and the culture medium was replaced by DMEM
high glucose medium that contained 10% rat serum or burn rat serum (burn serum was collected and isolated
from rats exposed to burns for 3 days), or burn rat serum and 10 ?g/mL ITF, or burn rat serum and 10 ?g/mL ITF
and compound C/3-MA/16F16, with 4 mmol/L glutamine. For 12 hours, cell lysate was collected, and protein
was extracted for Western blot analysis. Cells in the 24-well cluster Costar tissue culture plates were used for the
[3H]-Gln transport experiments.
Uptake studies of [3H]-Gln. Uptake buffer (37 ?C) was composed of 137 mM NaCl (or 137 mM choline
chloride), 4.7 mM KCl, 1.2 mM MgCl2, 1.2 mM KH2PO4, 2.5 mM CaCl2, and 10 mM HEPES/Tris buffer (pH 7.4).
Uptake studies in 100 ?L rat IECs or 40 ?L BBMVs suspension (4?5 mg/ml) were initiated by simultaneously
adding 1 mL of this buffer, which also contained [3H]-L-glutamine (0.5 ?Ci/mL, 20 ?mol/L), into each transport
plate for 1 minute; the rat IECs or BBMVs were washed and filtered on filter paper, and then the radio-activity
was assayed by liquid scintillation spectrometry. IEC-6 cells in 24-well cluster Costar tissue culture plates were
rinsed 2 times with uptake buffer. Transport was initiated by simultaneously adding 1mL of this buffer, which also
contained [3H]-L-glutamine (0.5 ?Ci/mL, 20 ?mol/L) into each of 24 wells in a transport plate. Cell culture plates
were continuously shaken by an orbital shaker (1 Hz) during the transport period. After 1 minute, the assay was
terminated by discarding the uptake buffer and rinsing the cells 3 times with ice-cold uptake buffer. The cells were
solubilized by 1 mL of 0.2-N NaOH/0.2% SDS solution. The radio-activity of the isotope extracted from the cells
was assayed by liquid scintillation spectrometry. Uptake rates are expressed as pmol of [3H]-glutamine per minute
per mg of cells or BBMVs protein. Sodium-dependent glutamine transport values were obtained by subtracting
the total glutamine transport measured in choline chloride buffer from that in NaCl buffer54.
Western Blotting. Protein level of ASCT2 (1:1000 Sigma), B0AT1 (1:200 Santa), p-AMPK (1:1000, Abcam,
UK), AMPK (1:1000, Abcam, UK), CHOP (1:1000 CST), GRP-78 (1:1000 CST), PDI (1:1000 CST), SNO-PDI
(1:1000 CST), LC3-II (1:1000 Sigma), p62 (1:1000 CST) in IECs from SD rats or IEC-6 cells that underwent
different treatments were analyzed by Western blotting using commercially available antibodies. The cell lysates
were subjected to sodium dodecyl sulfate (SDS) (Amresco, OH, USA)-PAGE and subsequently transferred to a
polyvinylidene difluorid (PVDF) membrane (Millipore Immobilon, USA). Following washes with TBST, protein
bands were detected using enhanced chemoluminescence ECL (Healthcare, Buckinghamshire, United Kingdom)
according to the manufacturer?s instructions.
Morphologic observation for BBMVs. A copper network with a membrane was placed into
homogeneous BBMV suspension and rested for 10 minutes, and then excess liquid around the copper network was absorbed
with filter paper, following dropping negative staining water on the network (potassium phosphotungstate),
waiting for 10 seconds, quick drying and observing by transmission electron microscope (Third military medical
university center lab).
Q-PCR. Total RNA was extracted and purified from the intestinal mucosa samples with the TRIzol Reagent
protocol (Invitrogen, Carlsbad, CA, USA). Total RNA was quantified using the BECKMAN COULTER DU800
nucleic acid/protein analyzer. The first-strand cDNA was synthesized by reverse transcription from 2?g of total
RNA with First Strand cDNA Synthesis Kit (TOYOBO, OSAKA JAPAN). The primers sets for GAPDH were
5?-GAA GGG CTC ATG ACC ACA GT-3? (forward) and 5?-GGA TGC AGG GAT GAT GTT CT-3? (reverse).
The primers sets for ASCT2 were 5?-CCT AGA CCT GGG ATC ACG GA-3? (forward) and 5?-CAG ATC
AGG ACG TAG CGG TC-3? (reverse). The primers sets for B0AT1 were 5?-CAT GTT CGT GTC CTT CAT
GG-3? (forward) and 5?-GTT CTG GTT GAG TGG GCA TT-3? (reverse). The primers sets for GRP-78 were
5?-CAAGAACCAACTCACGTCCA-3? (forward) and 5?-ACC ACC TTG AAT GGC AAG AA-3? (reverse). The
primers sets for CHOP were 5?-CTG GAA GCC TGG TAT GAG GA-3? (forward) and 5?-GGG ATG CAG GGT
CAA GAG TA-3? (reverse). The primers sets for PDI were 5?-CTG CCC AAG AGT GTG TCT GA-3? (forward)
and 5?-TAT GCG CTG GTT GTC AGT GT-3? (reverse). They were designed and synthesized by Sangon Biotech
(CN). The Q-PCR was performed using the SYBR Green Realtime PCR Master Mix (TOYOBO, OSAKA JAPAN)
on an Applied Biosystems 7500 Real-Time PCR System (Foster City, CA, USA). The delta cycle threshold (CT)
method was used to analyze the relative expression of the target gene.
PDI activity assay. The PDI activity was detected with insulin degradation, as described previously. Briefly,
rat intestinal mucosa samples lysates were mixed with buffer (1 mM DTT, 30 ?M insulin, and 3.0 mM sodium
EDTA, in a buffer that contained 100 mM sodium phosphate). The precipitation of the insulin B chain was
measured by an increase in absorbance at 610 nm.
PDI S-Nitrosylation assay. SNO-PDI was tested using the biotin-switch assay, as described previously. Briefly,
protein samples were added to a mix with 20 mM methylmethanethiosulfonate and 2.5% SDS in HEN buffer and
were incubated at 50 ?C for 1 h. Acetone was used to remove methylmethanethiosulfonate, and then, we used 1 mM
ascorbate to reduce the free thiols. The formed thiols were linked to the biotin-HPDP. The biotinylated proteins were
pulled down, and the SNO-PDI that remained in the precipitation was detected by western blotting.
Statistical analysis. All datas were expressed as means ? SD. Since our experimental design contain two
variates. One being the treatment factor and the other the time factor, two-way analyses of variance (Two-Way
ANOVAs) was performed on the datas. All statistical analysis were performed using SPSS 13.0. P < 0.05 was
considered statistically significant.
We are grateful to the Institute of burn in Third military medical university first affiliated hospital. In addition,
this work was supported by Natural Science Foundation of China (No. 81372049) and the Preclinical Innovation
Fund of TMMU (No. SWH2014 LC02, SWH2016HWHZ-03).
J.H. and Y.S. developed initial concept. J.H. and X.P. designed experiments. J.H., Y.S., C.W., H.W. and D.W.
performed experiments and analysed data. J.H. wrote the manuscript. X.P. supervised the study. All authors
discussed the results and commented on the manuscript.
Competing Interests: The authors declare no competing interests.
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