Background
Post-transplant diabetes mellitus (PTDM) is a common metabolic complication following solid organ transplantation that has been reported to have adverse impacts on the function and survival of grafts [
1]. PTDM was shown increase the risk of cardiovascular morbidity and mortality, inducing unfavorable outcomes [
2]. The main cause of PTDM is the universal use of immunosuppressive drugs following transplantation, which accounts for up to 74% of the risk of PTDM [
3]. Calcineurin inhibitors (CNIs), which are common immunosuppressive drugs, contribute to the development of PTDM [
4]. Tacrolimus (FK506), an important member of the CNIs, is more diabetogenic than other CNIs and can lead to reduced beta-cell mass, excessive insulin secretion, and insulin resistance [
4,
5]. However, the detailed mechanisms underlying this process are still unclear.
Kidney is the second most important organ in systemic glucose metabolism after liver and regulates glucose reabsorption and gluconeogenesis [
6]. Gluconeogenesis occurs exclusively in the liver and kidney, and the kidney accounts for 40% of glucose absorption in the fasting state [
7], indicating that renal injury or abnormal gene expression in the kidney is important in the development of diabetes mellitus and PTDM. Some experiments have demonstrated that treatment with tacrolimus after organ transplantation may induce progressive renal failure with striped interstitial fibrosis, tubular atrophy, inflammatory cell infiltration and hyalinosis of the afferent arterioles [
8], which are potentially implicated with PTDM. Hence, we speculate that rectifying glucose metabolism disturbance in the kidney in a timely manner can benefit PTDM treatment.
Farnesoid X receptor (FXR), a nuclear receptor, is expressed in several glucose-processing organs that synthesize, store and mobilize glucose according to the organism’s needs [
9]. In particular, FXR is highly expressed in the kidney, with expression detected in mesangial cells, podocytes, glomeruli and proximal tubular cells [
10]. FXR is embedded into a complex signaling network coordinating glucose uptake, usage and production. FXR
−/− mice showed elevated serum glucose, impaired glucose metabolism and induced insulin intolerance, suggesting the critical role of FXR in glucose homeostasis [
11,
12]. Zhao et al. [
13] confirmed that high expression of FXR in the kidney can significantly inhibit renal fibrosis. In addition, renal FXR activation downregulated the genes associated with fibrosis and lipogenesis and reversed some renal pathologic changes involving glomerulosclerosis and proteinuria [
14,
15]. However, in contrast to studies on primary diabetes mellitus, no studies have examined whether FXR is involved in PTDM in kidney. The mechanism of how FXR regulates tacrolimus-induced diabetes mellitus is unknown. The aim of our study was to reveal this mechanism and identify potential targets to prevent the occurrence of PTDM.
Materials and methods
Animal care and the experimental design
A total of 21 Male C57BL/6J mice (age 8–10 weeks; weight 18–20 g) were prepared for this experiment and were randomly divided into three groups (n = 7/group): the control group, the tacrolimus (FK506) group and the FK506 + GW4064 (a FXR agonist) group. The control group was given normal saline solution (1 mg/kg/day), the FK506 group received FK506 (1 mg/kg/day, Sigma-Aldrich, USA, No. F4679) in diluent (10% ethanol in sunflower oil) and the FK506 + GW4064 group was given FK506 (1 mg/kg/day) and the FXR agonist GW4064 (30 mg/kg/day, MedChemExpress China, Shanghai, China, No. HY-50108). All three groups were intragastrically administered the treatments once a day for 3 months. Body weights were measured once a month at 09:00 a.m., and fasting blood glucose concentrations were measured by glucometer every day at 4:00 p.m. Mice were sacrificed through rapid cervical dislocation. Blood samples were immediately collected by venipuncture from the portal vein in tubes with appropriate anticoagulant (ethylenediaminetetraacetic acid; EDTA) for plasma. The tissues were quickly dissected and washed with ice-cold saline solution. Then, they were frozen in liquid nitrogen and stored at − 80 °C for further analyses.
Cell experiment and study design
To test if the FXR is involved in tacrolimus-induced diabetes mellitus, we purchased human renal cortex proximal tubule epithelial (HK-2) cell lines (Stem Cell Bank of the Chinese Academy of Sciences, No. CRL-2190TM) and divided them into three groups: (1) the control group: HK-2 cells were cultured routinely with dimethyl sulphoxide (DMSO) solution (Sigma-Aldrich, USA, No. 34869-100ML); (2) the FK506 group: HK-2 cells were treated with 15 µmol/L FK506 (Sigma-Aldrich, USA, No. F4679) for 24, 48 and 72 h; and (3) the GW4064 group: HK-2 cells were treated with 4 μmol/L GW4064 for 24, 48 and 72 h. We collected all cell samples for mRNA and protein detection.
Construction of a mammalian expression plasmid of human FXR
Human FXR cDNA was generated by PCR using the RevertAid RT Reverse Transcription Kit (Thermo Scientific, USA, No. K1691). The primers were 5′-ATAAGAATGCGGCCGCATGGGATCAAAAATGAATCTCATTGA-3′ (forward) with a NotI site and 5′-CGCGGATCCCTGCACTGCCCAGATTTCACAGAGAAG-3′ (reverse) with a BamHI site. After the plasmid was cut by NotI and BamHI, the PCR product (1431 bp) containing the full-length FXR cDNA was subcloned into the pHAGE-puro (Addgene, No. 118692) plasmid vector, which was referred to as pHAGE-puro-FXR. HK-2 cells were transfected with pHAGE-puro-FXR for 48 h in transient transfection assays. Real-time PCR, western blot and immunofluorescence analyses were used to identify the expression of FXR. Its downstream genes were detected by real-time PCR and western blots.
RNA interference
The control non-specific small interfering RNA and FXR siRNAs were designed by Guangzhou RiboBio Co. Ltd. Transfection of siRNA was implemented according to the procedure suggested by the manufacturer. The cell line HK-2 was chosen for the RNA interference. The control group was dealt with non-specific siRNA and the FXR knockdown groups were disposed with FXR siRNA. The nucleotide sequences of FXR siRNAs were si-FXR primer:GAAGAGGUAUUGAAUGCUA. After transfection with 50 nM siRNA for 48 h, the HK-2 cells were collected and processed for quantitative real-time PCR.
Western blot analysis
Total protein lysates were prepared by kidney tissue homogenization using radio immunoprecipitation assay (RIPA) lysis buffer. Protein concentration was measured by bicinchonininc acid (BCA) kits (Biosharp Life Sciences Co., Ltd., No. BL521A). Equivalent amounts of protein (80 μg/lane) were separated by 8–12% sodium dodecyl sulfonate-polyacrylamide gel electrophoresis (SDS-PAGE) (SDS-PAGE kits, Wuhan Google Biotechnology Co., Ltd., No. G2003) for electrophoresis and then transferred to nitrocellulose membranes. After the membranes were blocked with 5% (w/v) nonfat milk powder in Tris-buffered saline, then membranes were incubated overnight at 4 °C with FXR antibody (Abcam. No. ab58559). Protein bands were visualized using an electrochemiluminescence (ECL) method (ECL Kits; Wuhan Servicebio Biotechnology). Quantification of protein bands was carried out with ImageJ software.
In addition, we extracted the nuclear and cytoplasmic proteins in the mouse kidney cells and used western blotting with the same procedure as above to detect the different protein levels of peroxisome proliferator activated receptor γ coactivator-1α (PGC1α) and forkhead box O1 (FOXO1) in the nucleus and cytoplasm (primary antibody: PGC1α: Proteintech, Wuhan, China, No. 66369-1-Ig; FOXO1: Boster Biological Technology, USA, No. BM4249).
Real-time PCR
Total RNA samples from mouse kidneys and human cells were extracted and the cDNA was synthesized. Additional file
1: Table S1 shows the primer and target sequences used in this study.
Immunofluorescence
Immunofluorescence was carried out on free floating sections cut on a freezing microtome at 40 µm using goat anti-FXR antibody (Abcam. No. ab58559), rabbit anti-FOXO1 antibody (Boster Biological Technology, USA, No. BM4249) and mouse anti-PGC-1α antibody (Proteintech, Wuhan, China, No. 66369-1-Ig). Stained slides were visualized using light microscopy and were photographed at 10× magnification. ImageJ software was used to quantify FOXO1 and PGC-1α content and represented the integrated density. Average integrated density values were calculated in square pixels and converted to square micrometers.
Statistical analysis
All statistical analyses were carried out using SPSS version 17.0. Data are presented as the mean ± standard deviation (SD). Differences within groups were evaluated with ANOVA followed by Bonferroni correction for Student’s t test. A value of P < 0.05 was considered statistically significant.
Discussion
CNIs lead to PTDM mainly by injuring pancreatic beta-cells and may affect glucagon synthesis by alpha-cells [
16]. Particularly, tacrolimus was also related to insulin resistance in various organs, such as liver and muscle [
17]. Besides, it is reported that FK506 can increase lipolysis, inhibit lipid stotage and decrease the expression of lipogenic genes in human adipose tissue [
18] and a randomized crossover trial shows that treatment with FK506 impairs insulin sensitivity [
19]. These researches demonstrate that tacrolimus can induced glucose increase, which is an independent risk factor of PTDM [
20]. However, few reports have shown that gluconeogenesis and glucose uptake also play an important role in maintaining blood glucose balance. Disruption of gluconeogenesis and glucose transport could be an underlying mechanism of PTDM. The kidney is an important organ in systemic glucose metabolism, except that in the liver. Therefore, in this study, we focused on gluconeogenesis and glucose uptake in the kidney under tacrolimus conditions. We suggest that the novel mechanism is unique to tacrolimus-induced dysglycemia: both in vitro and in vivo, tacrolimus inhibits the expression of FXR and then induces gluconeogenesis and prohibits glucose uptake by increasing PEPCK expression and downregulating GLUT2 expression.
FXR has been reported to play a significant role in nutritional metabolism [
21]. Zhang et al. [
12] confirmed that FXR knockout cause mild glucose intolerance and insulin insensitivity in mice. Other researches found that fasted FXR
−/− mice show an age-dependent growth in plasma glucose levels [
22]. Furthermore, bile acid taurochenodeoxycholate can induce insulin secretion with activation of FXR and FXR can regulate insulin transcription and secretion via FXR-Kruppel like factor 11 (KLF11) pathway [
23,
24]. These researches suggest that FXR have an essential role on glucose metabolism and may be a potential treatment target in PTDM. The kidney is one of the main organs that highly expresses FXR. In our study, activated FXR in renal cells increased the expression of PEPCK, which is a key enzyme in gluconeogenesis. Many studies have claimed that FXR represses gluconeogenesis via repression of PEPCK and G6PAse [
11]. However, PEPCK expression was reduced in FXR-deficient mice after fasting and refeeding in another study [
25]. The reasons for this discrepancy are not clear but probably depend on the model systems used and the nutritional status. Recent studies have indicated that FXR can regulate insulin signaling by inducing the relocation of GLUT2 in β-cells [
26]. Thus, we also measured the transcription and expression of glucose transporters in the kidney, such as GLUT2. GLUT2 expression was improved after treatment with GW4064 and FXR transfection in HK-2 cells. Furthermore, when we knock out the FXR, we found the expression of PEPCK was improved and GLUT2 was restrained, which is in accordance with the previous experiment. According to our results, we suggest that FXR has an effect on hypoglycemia by restraining renal gluconeogenesis and promoting glucose uptake.
FOXO1 and PGC-1α are two transcriptional components that are targets of insulin signaling and can activate gluconeogenesis [
27]. FOXO1 was reported to directly bind to the promoters of gluconeogenic genes, which can activate glucose production [
28,
29]. Nakae et al. [
30] found that FOXO1 is directly phosphorylated by Akt, which results in the removal of FOXO1 from the nucleus. PGC-1α is a coactivator and at physiological levels, it initiates gluconeogenesis [
31]. In addition, Sasaki et al. [
32] showed that both insulin signaling and glucose reabsorption inhibit gluconeogenic genes, such as PEPCK, by inactivation of FOXO1 and PGC-1α, respectively. As our study suggests that FXR suppresses gluconeogenesis and promotes glucose uptake via PEPCK and GLUT2, respectively, we speculate that FXR may inhibit FOXO1 and PGC-1α via inducing the expression of SHP-1 to bind with the promoter of gluconeogenic genes by inducing them to transfer to the cytoplasm, which restrains the transcription of gluconeogenic genes and has a hyperglycemic effect.
We observed that tacrolimus induced the expression of gluconeogenic genes in the kidneys of diabetic mice, which is consistent with many reports that tacrolimus can induce insulin resistance [
33] and that GLUT2 gene expression is also downregulated by tacrolimus. This finding suggests that the dysfunction of glucose reabsorption in the kidney induced by tacrolimus is also one of the causes of PTDM. A study claimed that the mRNA and protein levels of GLUT2 in the gut did not change significantly after tacrolimus administration. Thus, the researchers believed that the enhancement of intestinal glucose transport resulted from increased expression of the glucose transporter SGLT1 [
34]. This finding may be due to different organ functions, and the sensitivity to tacrolimus may be inconsistent in different organs. Lopes et al. [
35] observed a negative regulator of insulin in the liver of a cyclosporin-treated group, which indicates that gluconeogenesis was enhanced and insulin resistance was increased.
FXR is reported as a homeostat in glucose metabolism, whose hypoglycemic effect relies on repression of gluconeogenesis and promotion of glucose uptake. We upregulated FXR by a GW4064 agonist in vivo and showed that activating FXR in the kidney can antagonize the hyperglycemia of FK506. Furthermore, we analyzed the mechanism underlying the development of tacrolimus-induced PTDM and how FXR regulates glucose metabolism in the kidney. As shown by immunofluorescence images, tacrolimus suppressed FOXO1 and PGC-1α transport to the cytoplasm, and FXR can induce the translocation of FOXO1 and PGC-1α from the nucleus to the cytoplasm via activating the SHP-1, which can limit the genes related to gluconeogenesis transcription. These results can explain the regulation of gluconeogenesis. However, how FXR and tacrolimus regulate GLUT2 in the kidney still requires further research.
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