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Impairment of tissue repair in pneumonia due to β-cell deficiency: role of endoplasmic reticulum stress in alveolar macrophages
BMC Research Notes volume 12, Article number: 160 (2019)
Diabetes mellitus (DM) patients are susceptible to delayed resolution of pneumonia. However, the pathogenesis of the impaired tissue repair in inflamed lungs in diabetic patients is unknown. We evaluated phagocytosis of apoptotic cells (efferocytosis), hepatocyte growth factor (HGF) production in bronchoalveolar lavage fluid (BALF), and lung histology in the resolution phase following acute lung injury in streptozotocin (STZ)-induced β-cell-depleted hyperglycemic mice. We also investigated efferocytosis and HGF production by macrophages under β-cell depletion condition ex vivo.
In β-cell-depleted mice, efferocytosis was not significantly different from that in control mice; however, the concentration of HGF in BALF was decreased. In addition, diminished HGF production by alveolar macrophages and DNA synthesis in the alveolar epithelium was observed by immunohistochemistry. Ex vivo experiments confirmed that HGF production by macrophages was impaired under β-cell depletion probably because of endoplasmic reticulum stress.
Diabetes mellitus (DM) is an independent risk factor for serious infectious diseases, including respiratory tract infections . Furthermore, diabetic patients are prone to chronic bacterial pneumonia and un-resolving pneumonia [2, 3].
The engulfment and removal of apoptotic cells by phagocytes, known as efferocytosis, is involved in the regulation of inflammatory responses and the maintenance of lung tissue regeneration [4, 5]. Efferocytosis is suggested to induce the increases of the production of growth factors to promote tissue repair [6, 7]. Hepatocyte growth factor (HGF) is one of the most important growth factors and has pleiotropic effects, including regenerative, protective, and angiogenic activities [6, 8, 9]. HGF is produced by alveolar macrophages (AMs) that phagocytosed apoptotic neutrophils among resolution phase of injured lung  and is considered to be a pivotal growth factor for bronchial epithelial cells and alveolar type II cells [10, 11].
The endoplasmic reticulum (ER), an intracellular organelle, is involved in the synthesis and folding of transmembrane proteins into their proper three-dimensional structures. When ER stress is detected by the three types of sensors on the ER membrane (PERK, IRE1α, and ATF 6), the unfolded protein response (UPR) is induced to protect cells . Once the UPR becomes impaired, the cell becomes apoptotic. Under diabetic conditions, ER stress is provoked in monocytes . Therefore, we hypothesized that ER stress is also induced in AMs under diabetic conditions, and leads to impaired efferocytosis and HGF production and subsequent delayed resolution of pneumonia.
Model of acute lung injury in diabetic mice
Experimental diabetes was induced in 5-week-old, specific pathogen-free, male Sic:ICR mice (commercial source: Charles River Agricultural Cooperative Association, Kanagawa, Japan) with a single intraperitoneal injection of streptozotocin (STZ; Sigma-Aldrich, St. Louis, MO, USA)  at 250 mg/kg of body weight in 0.1 M citrate buffer (pH 4.5). Control mice received an equal volume of citrate buffer. Blood glucose levels were measured with a Glutest Sensor (Sanwa Chemical Co., Nagoya, Japan) to confirm that STZ treatment successfully depleted β-cells. Confirmed blood glucose levels after 48 h from STZ injection were 403 ± 23.97 mg/dl. More than 250 mg/dl were considered hyperglycemic. Ten days after administration, mice were anesthetized with an intraperitoneal injection of sodium pentobarbital (60 mg/kg), and the trachea was cannulated with an 18G cannula. Then, lipopolysaccharide (LPS; 5.0 mg/kg body weight) (Sigma-Aldrich) was administered intratracheally. After the experiment, mice were euthanized using deep anesthesia.
BAL and phagocytosis assay
Bronchoalveolar lavage (BAL) was performed as previously described  in each group of five mice at 0, 3, 6, 12, 24, 36, and 48 h, and 3, 4, 5, and 7 days after LPS inoculation. To observe macrophage engulfment of apoptotic neutrophils, the cells were fixed to a glass slide with cytospins, and then myeloperoxidase (MPO) activity was examined with the DAB Substrate Kit (Sigma-Aldrich) to visualize ingested neutrophils .
ELISA for HGF
The concentration of murine HGF in bronchoalveolar lavage fluid (BALF) was determined as previously described . Briefly, we measured using a commercial rat HGF sandwich enzyme-linked immunosorbent assay (ELISA) kit (Institute of Immunology, Tokyo, Japan). We had already ascertained its usability for murine HGF in previous experiments .
At predesignated time points, mice were euthanized, and lungs were harvested. Paraffin-embedded specimens of whole lungs were prepared as previously described . The number of HGF-positive alveolar macrophages was qualitatively assessed by visual inspection.
Proliferating cells in the tissue sections were assessed by immunostaining for proliferating cell associated nuclear antigen (PCNA). The dewaxed sections were stained for PCNA using a polyclonal antibody against human PCNA (Abnova, Taipei, Taiwan).
One mouse and two mice from the control and LPS-treated groups, respectively, were prepared, and four fields per mouse were imaged at 400× for blind counting by visual inspection.
Phagocytosis assay and HGF measurement ex vivo
Aged neutrophils, suspended in 300 μl of DMEM (Wako Pure Chemical Co., Osaka, Japan), were placed on cultured AMs. Then, the un-ingested cells were washed away, and the AMs were stained for MPO. Aged neutrophils were obtained using a technique previously reported [6, 15]. Harvested fresh neutrophils from mouse peritoneal cavity were incubated overnight at 4 °C in Hanks solution (Wako Pure Chemical Co). Neutrophils aged by these techniques were routinely 95 to 97% viable and around 70% apoptotic .
The concentration of HGF in the culture supernatants was determined as stated above.
Real-time PCR analysis
RAW264.7 cells (mouse peritoneal macrophage cell line) were cultured at 1–2 × 106 in serum-free DMEM (standard glucose: 1 g/dl) containing with 3.5% albumin, not containing with insulin, in humidified 5% CO2 at 37 °C. Total RNA was extracted with the RNeasy Mini kit (QIAGEN, Gaithersburg, MD, USA). The extracted mRNA was quantified by measuring the absorbance, and was used as a template for RT-PCR to synthesize cDNA by SuperScript III (Invitrogen, Carlsbad, CA, USA).
The synthesized cDNA was then used as a template for real-time PCR with SsoFast EvaGreen Supermix and Low ROX (BIO-RAD, Hercules, CA, USA) as the enzyme. The primers used for CHOP, Bip, spliced XBP-1, total XBP-1, and β-actin have been previously described . The PCR program consisted of an initial denaturation step at 50 °C for 30 s, followed by 40 cycles of denaturation at 95 °C for 5 s and annealing at 53 °C for 30 s.
Determination of HGF concentration under tunicamycin stimulation
RAW264.7 cells were cultured in DMEM supplemented with 10% heat-inactivated fetal bovine serum (FBS), 100 μg/ml streptomycin, and 100 U/ml penicillin in 5% CO2 at 37 °C at 1 × 106 per well for 20 h. Tunicamycin (TM; Sigma-Aldrich) was added to the DMEM at 1 μg/ml, and DMSO was added to the control medium at 10 μl/ml. Apoptotic neutrophils [6, 15] suspended in 300 μl of DMEM without FBS, were added to the cultured RAW264.7 cells in triplicate and incubated for 20 h at 37 °C in 5% CO2. Culture supernatants were collected and centrifuged at 150×g for 5 min at 4 °C. Clarified supernatants were used to measure HGF concentrations.
Data are presented as mean standard error of the mean (SE). The statistical significance of differences between groups was determined by the unpaired (two-tailed) t test. One-way ANOVA was used for comparing several groups. A P value less than 0.05 was considered statistically significant.
Kinetics of inflammatory cells in lung injury
We investigated the kinetics of cell infiltration into the airway following LPS-induced inflammation. The cell numbers of BALF were not significantly different between β-cell-depleted hyperglycemic and control (Fig. 1a). Analysis of the cells in BALF at each time point revealed that the increase in cell numbers was due to neutrophils, and there was a negligible change in the number of AMs (Fig. 1b, c).
Neutrophil influx into the lungs of control mice was apparent at 24 h. Then increased, and reached a peak at 48 h. In β-cell-depleted mice, neutrophil influx was also detected at 24 h, reached a peak at 36 h, and then gradually decreased over a 7-day period. These results show that in control mice, inflammation was initiated within 24 h and reached a peak at 36 h. In the β-cell-depleted mice, the numbers of neutrophils at each time point were not significantly different, and the peak level was equivalent (Fig. 1b).
Neutrophil removal and HGF production in lung injury
The number of MPO-positive macrophages began to increase at 24 h (7.0 ± 6.0% of total AMs) in control mice, reached a maximum at day 3 (29 ± 15% of total AMs), and remained high up to day 5. The kinetics of MPO-positive AMs were similar among the groups (Additional file 1: Figure S1A). The number of AMs remained unchanged from baseline until 36 h, but started to increase at 48 h (Fig. 1c). After 48 h, the percentage of neutrophils gradually decreased until the end of the observation period (Fig. 1b). Taken together, these findings indicate that the efferocytosis activity was augmented following LPS administration and contributed to the resolution of acute inflammation, especially between 48 h and 5 days.
The concentration of HGF in BALF reached a peak on day 5 in both groups; however, it was significantly lower in β-cell-depleted mice (1.656 ± 0.714 ng/ml) than in control mice (4.093 ± 0.693 ng/ml). Furthermore, the HGF concentrations in control mice were continuously higher than those in β-cell-depleted mice from day 3 to day 5 (P < 0.05, Fig. 1d).
HGF production and DNA synthesis in the lung were attenuated in β-cell-depleted hyperglycemic mice
HGF-positive cells were present within bronchial epithelial cells and AMs at 5 days after LPS administration in both mouse groups (Fig. 2a). In β-cell-depleted mice, the number of HGF-positive AMs was much lower than that in control mice (P < 0.05, Fig. 2a).
To assess regenerative proliferation, cells undergoing DNA synthesis were identified by anti-PCNA polyclonal antibody. In both groups, the PCNA-positive cells were mainly alveolar epithelial cells. Consistent with the difference in HGF production, there were much fewer PCNA-positive alveolar epithelial cells in β-cell-depleted mice than in the control mice on day 5 (P < 0.05, Fig. 2b).
HGF production was suppressed in β-cell-depleted mouse macrophages
We examined the efferocytosis of AMs and measured the concentrations of HGF in culture supernatants. The percentage of MPO-positive AMs, was similar in the two groups (Additional file 1: Figure S1B); however, HGF production was significantly lower in the AMs from β-cell-depleted mice than in the AMs from control mice (P < 0.05, Fig. 3a).
mRNA expression of UPR was increased in insulin-starved macrophages.
At first, we compared mRNA expression between culture with standard glucose (1 g/dl) medium and culture with high glucose (4.5 g/dl) medium. After 5 days culture, however we found no significant differences between those conditions. At this point, we thought of the influence of insulin starvation. Then, we performed a time course analysis under insulin starvation. After 24 h, the mRNA expression of Bip and CHOP suddenly reached peak levels, whereas the mRNA expression of sXBP-1 increased in an earlier phase (Fig. 3b). These results suggested that diabetes causes ER stress in macrophages under insulin-starved or insulin-resistant conditions.
HGF production was suppressed under ER stress.
ER stress may play a role in suppressing HGF production by mouse AMs. We tested this hypothesis by applying a strong ER-stress inducer, TM, to the cells. In the absence of apoptotic neutrophils, the RAW264.7 did not produce HGF; however, in the presence of apoptotic neutrophils, the macrophages produced HGF. When we added TM to the RAW264.7, the HGF concentration in the culture supernatant decreased (P < 0.05, Fig. 3c).
Our data improve our understanding of the pathogenesis of delayed-resolving pneumonia in diabetic patients. HGF production during the resolution phase of inflammation was impaired in β-cell-depleted mice. Following impaired tissue repair may subsequently lead to poorer outcomes for patients with pneumonia. The mechanism of suppressed HGF production is unknown; however, we could speculate that it might be due to acceleration of the PERK-eIF2-α-ATF4-CHOP pathway or inhibition of endoplasmic-related degradation (ERAD) in the UPR . PI3K/AKT signal pathway is also suggested to regulate HGF expression [18, 19]. Considering the insulin signal that is mediated by PI3K/AKT pathway , we supposed that suppressed PI3K/AKT pathway activation due to insulin starvation additionally decreased HGF expression in this study.
Contrary to our hypothesis, ER stress had a negative effect on efferocytosis. The diabetic condition is characterized by hyperglycemia, acidemia, higher osmotic pressure, intracellular starvation, and ER stress. We postulate that these may have complicated effects on macrophage function. Further, adding to the differences between alveolar and peritoneal macrophages, in the resolution phase of lung inflammation, recruited macrophages uptake apoptotic cells more effectively .
First, the STZ-induced hyperglycemia model may more closely reflect Type1 diabetes because β-cell depletion is a feature of Type1 diabetes. Second, the blood sugar levels of the mice were very high, in the range of severe diabetes. Thus, our model does not accurately reflect common clinical diabetes. Third, our study included animals and ex vivo experiments; the alveolar macrophages in humans possibly express different features.
hepatocyte growth factor
- ER stress:
endoplasmic reticulum stress
unfolded protein response
bronchoalveolar lavage fluid
proliferating cell associated nuclear antigen
fetal bovine serum
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YY and RK mainly performed laboratory investigation, designed study methodology, analyzed the data, and prepared the manuscript. They were equally contributed. MT and MS helped laboratory investigation. TT helped laboratory investigation and revised the manuscript. KM and HA designed the study, helped laboratory investigation, and edited the manuscript. All authors read and approved the final manuscript.
The authors thank Dr. Norio Abiru for critical suggestion of the hypothesis about ER stress.
The authors declare that they have no competing interests.
Availability of data and materials
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.
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Ethics approval and consent to participate
The experimental protocols in this study were approved by the Ethics Review Committee for Animal Experimentation of Nagasaki University School of Medicine (No 1102170901, 7/2/2011).
This work was supported by JSPS KAKENHI Grant Numbers JP24591169, JP16k09545.
The funding body had no role in the design of the study and collection, analysis, and interpretation of data and in writing the manuscript.
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Yamashita, Y., Kuroki, R., Takaki, M. et al. Impairment of tissue repair in pneumonia due to β-cell deficiency: role of endoplasmic reticulum stress in alveolar macrophages. BMC Res Notes 12, 160 (2019). https://doi.org/10.1186/s13104-019-4209-0
- Diabetes mellitus
- Hepatocyte growth factor
- Endoplasmic reticulum stress