Bone morphogenetic protein 7 (BMP7) has been reported to confer renoprotective effects in acute and chronic kidney disease models, but its potential role in Type 2 diabetic nephropathy remains unknown. In cultured human proximal tubular epithelial cells (PTECs), exposure to advanced glycation end-products (AGEs) induced overexpression of intercellular adhesion molecule 1 (ICAM1), monocyte chemoattractant protein 1 (MCP1), interleukin 8 (IL-8) and interleukin 6 (IL-6), involving activation of p44/42 and p38 mitogen-activated protein kinase (MAPK) signalling. BMP7 dose-dependently attenuated AGE-induced up-regulation of ICAM1, MCP1, IL-8 and IL-6 at both mRNA and protein levels. Moreover, BMP7 suppressed AGE-induced p38 and p44/42 MAPK phosphorylation and reactive oxygen species production in PTECs. Compared with vehicle control, uninephrectomized db/db mice treated with BMP7 for 8 weeks had significantly lower urinary albumin-to-creatinine ratio (3549±816.2 μg/mg compared with 8612±2037 μg/mg, P=0.036), blood urea nitrogen (33.26±1.09 mg/dl compared with 37.49±0.89 mg/dl, P=0.006), and renal cortical expression of ICAM1 and MCP1 at both gene and protein levels. In addition, BMP7-treated animals had significantly less severe tubular damage, interstitial inflammatory cell infiltration, renal cortical p38 and p44/42 phosphorylation and lipid peroxidation. Our results demonstrate that BMP7 attenuates tubular pro-inflammatory responses in diabetic kidney disease by suppressing oxidative stress and multiple inflammatory signalling pathways including p38 and p44/42 MAPK. Its potential application as a therapeutic molecule in diabetic nephropathy warrants further investigation.

CLINICAL PERSPECTIVES

  • ESRD from T2DN imposes a heavy burden on most healthcare systems in the developed world, but therapeutic approaches to delay or prevent its occurrence are very limited. Diabetic tubulopathy has recently been shown to play a significant role in intrarenal inflammation during DN.

  • BMP7 ameliorates diabetic tubulopathy by suppressing inflammatory and oxidative stress responses both in cultured renal tubular cells exposed to AGEs and in diabetic db/db mice.

  • It therefore has the potential of being developed into an anti-inflammatory and antioxidative agent for the treatment of Type 2 diabetic kidney disease.

INTRODUCTION

Diabetic nephropathy (DN) is the leading cause of end-stage renal disease (ESRD) in most developed countries [1], and over 90% are due to Type 2 diabetes. Although the precise pathogenetic mechanisms of Type 2 diabetes have yet to be worked out, emerging evidence strongly suggests an inflammatory component involving activation of the circulating complement system and pro-inflammatory chemokines and cytokines [2], which are closely associated with the development and progression of DN [3]. Despite current recommendations of full renin–angiotensin system (RAS) blockade and stringent glycaemic, lipid and blood pressure control, the residual risk of renal progression in DN has remained high. Short of a specific protocol, it is conceivable that any therapeutic approach that intervene the downstream tubulotoxic effects of Type 2 DN (T2DN) is a promising alternative for renoprotection. Anti-inflammatory concepts open a new door for DN treatment, pre-clinical trials and clinical trials have been launched targeted specific cytokines, such as an interleukin (IL)-1β-specific antibody [4,5], a tumour necrosis factor (TNF) blocker [6], or modulating pro-inflammatory pathways [79]. Although the results of some of these clinical trials are promising, their overall clinical utilities are still limited [2]. Therefore, the development of additional therapeutic interventions remains a major priority.

Bone morphogenetic protein 7 (BMP7) is a homodimeric protein that belongs to the transforming growth factor β (TGF-β) superfamily. BMP7 is vital for kidney development and is only selectively expressed in several adult organs including kidney [10]. BMP7 has been reported to be down-regulated under several disease conditions [1113], and administration of exogenous BMP7 has been shown to inhibit injury in acute or chronic kidney disease [1416]. For DN, BMP7 has been reported to preserve podocytes [17] and rescue type 1 DN [1821]. However, most of these studies mainly focused on the anti-fibrotic role of BMP7. Gould et al. [22] reported that BMP7 suppressed TNF-α-induced pro-inflammatory cytokines in proximal tubular epithelial cells (PTECs), and Wang et al. [21] also reported that BMP7 reduced tubulointerstitium macrophage infiltration in streptozotocin (STZ)-induced DN, suggesting an anti-inflammatory role of BMP7.

Tubular damage is a major feature of DN [2327], and tubulointerstitium lesions are predictive of renal function and prognosis in DN patients [28]. We recently found that different kinds of advanced glycation and products (AGEs) caused differential effects on renal tubular cell inflammation [29,30]; however, whether BMP7 can prevent AGE-induced kidney injuries or preserve kidney function in T2DN remains unknown. Herein, we explored the potential anti-inflammatory role of BMP7 in AGE-induced tubular injury in vitro and in a murine model of T2DN.

Part of this work was presented at the American Society of Nephrology Renal Week held in Atlanta, GA, U.S.A., on 7–10 November 2013, and subsequently published in abstract form [30a].

MATERIALS AND METHODS

Cell culture

Primary human renal PTECs were purchased from Lonza. Cells were cultured in renal epithelial cell basal medium (REBM) with supplemented growth factors and maintained at 37°C in 5% CO2 and 95% air. In all experiments, cells (from passage 2 to 4) were grown to 70–80% confluency and were arrested in serum-free medium for 24 h before stimulation. Cells were stimulated with AGEs of human serum albumin (100 μg/ml; Sigma–Aldrich) with or without BMP7 (100 ng/ml; R&D Systems). Inhibitors of p38 (SB203580 10 μM), p44/42 mitogen-activated protein kinase (MAPK) (PD98059 25 μM) signalling pathways were added into the culture system, gene and protein level of different cytokines and chemokines were detected. Unless otherwise specified, PTECs were harvested at 3 or 24 h after stimulation for gene and protein expression study, respectively.

Animal model

For in vivo study, genetically diabetic male db/db mice and their non-diabetic db/m littermates (The Jackson Laboratory) were used. All work with mice was approved by the Committee on the Use of Live Animals in Teaching and Research of The University of Hong Kong, and was performed in accordance with the NIH Guide for the Care and Use of Laboratory Animal.

At the age of 9 weeks, mice underwent uninephrectomy (Unx) or sham operation as described previously [31]. Briefly, a dorsal–ventral incision was made on the right side of the abdominal cavity in anaesthetized mice, the right renal pedicle was identified and ligated followed by carefully dissected out of the kidney. Then the abdominal wall and skin was sutured. Sham-operated mice were also anaesthetized and had their abdominal cut and right renal pedicle manipulated without ligation. The animals were then warmed in the intensive care unit until full recovery from anesthesia. Two weeks after operation, Unx and sham-operated mice were randomized into four groups (n=10–11 each): db/db sham group, Unx db/db vehicle group, Unx db/db BMP7 group and Unx db/m vehicle group and received BMP7 (300 μg/kg of body weight) or equivalent volume of vehicle intraperitoneally every other day. After 8 weeks of treatment, all mice were killed and the kidney was harvested; half of the kidney was snap frozen in liquid nitrogen and stored at −70°C for mRNA and protein extraction. The other half was fixed with 10% formalin and embedded in paraffin for histological study. Systolic blood pressure was measured by using a computerized tail cuff system (Visitech Systems). Twenty-four hour urine samples were collected by metabolic cages. Serum was drawn and separated from the sub-mandibular vein at baseline and end point and stored at −70°C for further investigation.

Total RNA extraction and real-time PCR

Total RNA was extracted from the cultured cells and kidney cortical tissue using TRIzol reagent (Invitrogen) and was reverse transcribed to cDNA with High Capacity cDNA Reverse Transcription kits (Applied Biosystems). Real-time PCR analysis was performed using SYBR green Master Mix (Applied Biosystems) according to the manufacturer's instruction, the amplification was carried out in an ABI Prism 7500 sequence detection system (Applied Biosystems). The primer sequences used in both in vitro and in vivo studies were listed as Table 1. The relative quantification of cDNA in each sample was normalized to β-actin cDNA and the data were analysed by the SDS software (Applied Biosystems). Data are presented as the relative fold change compared with the control.

Table 1
Primer sequences for real-time PCR
GeneSpeciesAccession numberPrimer sequence
ICAM1 Homo sapiens NM_000201.2 Forward: 5′-GGCCTCAGTCAGTGTGA-3′ 
   Reverse: 5′-AACCCCATTCAGCGTCA-3′ 
IL-8  NM_000584.3 Forward: 5′-GTGCAGTTTTGCCAAGGAGT-3′ 
   Reverse: 5′-TAATTTCTGTGTTGGCGCAG-3′ 
MCP1  NM_002982.3 Forward: 5′-GATCTCAGTGCAGAGGCTCG-3′ 
   Reverse: 5′-TGCTTGTCCAGGTGGTCCAT-3′ 
IL-6  NM_000600.3 Forward: 5′-ATGAACTCCTTCTCCACAAG-3′ 
   Reverse: 5′-TGTCAATTCGTTCTGAAGAG-3′ 
β-Actin  NM_001101.3 Forward: 5′-TGACGTGGACATCCGCAAAG-3′ 
   Reverse: 5′-CTGGAAGGTGGACAGCGAGG-3′ 
ICAM1 Mus musculus NM_010493.2 Forward: 5′-TGGCCTGGGGGATGCACACT-3′ 
   Reverse: 5′-GGCTGTAGGTGGGTCCGGG-3′ 
MCP1  NM_011333.3 Forward: 5′-CTCTTCCTCCACCACCAT-3′ 
   Reverse: 5′-CTCTCCAGCCTACTCATTG-3′ 
β-Actin  NM_007393.3 Forward: 5′-TCCATCATGAAGTGTGACGT-3′ 
   Reverse: 5′-GAGCAATGATCTTGATCTTCAT-3′ 
GeneSpeciesAccession numberPrimer sequence
ICAM1 Homo sapiens NM_000201.2 Forward: 5′-GGCCTCAGTCAGTGTGA-3′ 
   Reverse: 5′-AACCCCATTCAGCGTCA-3′ 
IL-8  NM_000584.3 Forward: 5′-GTGCAGTTTTGCCAAGGAGT-3′ 
   Reverse: 5′-TAATTTCTGTGTTGGCGCAG-3′ 
MCP1  NM_002982.3 Forward: 5′-GATCTCAGTGCAGAGGCTCG-3′ 
   Reverse: 5′-TGCTTGTCCAGGTGGTCCAT-3′ 
IL-6  NM_000600.3 Forward: 5′-ATGAACTCCTTCTCCACAAG-3′ 
   Reverse: 5′-TGTCAATTCGTTCTGAAGAG-3′ 
β-Actin  NM_001101.3 Forward: 5′-TGACGTGGACATCCGCAAAG-3′ 
   Reverse: 5′-CTGGAAGGTGGACAGCGAGG-3′ 
ICAM1 Mus musculus NM_010493.2 Forward: 5′-TGGCCTGGGGGATGCACACT-3′ 
   Reverse: 5′-GGCTGTAGGTGGGTCCGGG-3′ 
MCP1  NM_011333.3 Forward: 5′-CTCTTCCTCCACCACCAT-3′ 
   Reverse: 5′-CTCTCCAGCCTACTCATTG-3′ 
β-Actin  NM_007393.3 Forward: 5′-TCCATCATGAAGTGTGACGT-3′ 
   Reverse: 5′-GAGCAATGATCTTGATCTTCAT-3′ 

ELISA

Pro-inflammatory cytokine expressions were measured in 24 h condition medium. Intercellular adhesion molecule 1 (ICAM1), monocyte chemoattractant protein 1 (MCP1), IL-8 and IL-6 protein levels were quantified using commercial kit (PeproTech) following the manufacturer's instructions. Renal cortical protein was extracted by using RIPA lysis buffer (Millipore), cortical MCP1 and ICAM1 levels were detected with mouse MCP1 and ICAM1 Duoset ELISA kits (R&D Systems), respectively.

Western blot analysis

Growth-arrested PTECs were stimulated with AGEs (100 μg/ml) with or without BMP7 (100 ng/ml) for 30 min. Cells were washed with PBS and then lysed with lysis buffer containing protease inhibitor (Sigma–Aldrich) as previously described [32]. Total protein concentrations were determined by Pierce BCA method (Thermo Scientific). Equal amount of proteins were dissolved in loading buffer and denatured by boiling at 95°C for 10 min. Protein samples were spearated by SDS/PAGE (12% gel) and transferred on to PVDF membrane (Millipore). Membrane was blocked with 5% non-fat milk and incubated overnight with primary antibodies against phospho-p38 MAPK (1:1000 dilution), p38 MAPK (1:2000 dilution), phospho-p44/42 (1:2000 dilution) and p44/42 (1:2000 dilution) (Cell Signaling Technology) in TBS with 0.1% Tween 20, and subsequently incubated with horseradish peroxidase (HRP)-conjugated secondary antibodies (Dako) for 2 h. Reactions were visualized by using ECL plus chemiluminescence (GE Healthcare) with ChemiDoc XRS+ system (Bio-Rad). Density of bands were quantified by bundled ChemiDoc software and normalized by actin level.

Detection of intracellular reactive oxygen species

Intracellular reactive oxygen species (ROS) were measured using OxiSelect™ Intracellular ROS Assay kit (Cellbiolabs) according to the manufacturer's instruction. Briefly, PTECs were pretreated with DCFH-DA (dissolved in Dulbecco's modified Eagle's medium) for 1 h, cells were washed and stimulated with plain medium or medium with AGEs, AGEs with BMP7 and H2O2 for 1 h, then were lysed for fluorescence measurement using FLUOStar Omega plate reader (BMG Labtech) for excitation at 485 nm and emission at 520 nm.

PTECs were seeded in eight-well chamber slides (Nunc Lab-Tek II; Thermo Scientific) and growth-arrested for 24 h until 80% confluence. Cells then treated with AGEs, AGEs with BMP7, H2O2 (500 μM) as positive control and plain medium as normal control. After 1 h incubation, cells were washed with PBS and incubated with an oxidative stress indicator CM-H2DCFDA (1 μg/ml; Invitrogen) for 45 min, then cells were washed with PBS and fixed with 4% paraformaldehyde for 15 min. Slides were mounted with Vectashield and 4′,6-diamidino-2-phenylindole (DAPI) (Vector Laboratories) and visualized with a fluorescence microscope.

Quantitation of malondialdehyde level in renal tissue

Lipid peroxidation of renal cortical tissue was determined by using OxiSelect™ TBARS Assay kit (Cell Biolabs) according to the manufacturer's instruction. Tissue lysate or malondialdehyde (MDA) standard was incubated with SDS lysis solution for 5 min at room temperature; thiobarbituric acid (TBA) was added to the mixture and incubated at 95°C for 60 min. Supernatant of the lysate was then transferred into 96-well plate and read for absorbance at 540 nM. Data are presented as fold change of MDA–TBA adduct compared with the db/db sham group.

Biochemical analysis of serum and urine specimens

Urine albumin concentration was determined by mouse albumin ELISA kit (Bethyl Laboratories), urine creatinine, serum blood urea nitrogen (BUN) and blood glucose concentrations were measured by enzymatic method using commercially available test kits (Stanbio Laboratory).

Histological examination and immunohistochemistry

Periodic acid–Schiff (PAS) staining (Sigma–Aldrich) was performed in 4-μm-thick paraffin sections following the manufacturer's protocol. Glomerular and tubulointerstitial damage of kidney were quantified by Image Pro Plus Software 5.0 (Media Cybernetics); 30 fields in the cortex were randomly selected to collect the integrated absorbance (‘optical density’) (IOD) of lesions and area of the field. Data were presented as the average IOD per area (IOD/area) value.

Immunohistochemistry was performed to detect macrophage cell marker F4/80. Four micrometers paraffin sections were deparaffinized and rehydrated through a descending ethanol gradient. Enzymatic digestion with protease protease K was used for antigen retrieval.

Sections were incubated with anti-F4/80 antibody (1:50 dilution; Serotec) at 4°C overnight and subsequently HRP-conjugated secondary antibodies. Positive staining was visualized as brown colour using DAB substrate (Dako). The number of F4/80+ cells was counted in 20 randomly selected high-power cortical fields per kidney section and expressed as the average number of cells per field (number/high-power field).

Statistical analysis

All data are presented as means±S.E.M. unless otherwise specified. Statistical analysis was performed using GraphPad Prism 5.0 for Windows (GraphPad Software). Difference between multiple groups was analysed by one-way ANOVA with Bonferroni's intergroup comparison. P<0.05 was considered statistically significant.

RESULTS

BMP7 attenuated AGE-induced pro-inflammatory response in PTECs

Real-time PCR showed that AGEs (100 μg/ml) superinduced expression of ICAM1 and IL-8 over equivalent doses of HSA in PTECs, and BMP7 dose-dependently attenuated this response. The optimal dosage of BMP7 at 100 ng/ml was used in the present study (Figure 1).

BMP7 dose-dependently reduced AGE-induced tubular inflammation

Figure 1
BMP7 dose-dependently reduced AGE-induced tubular inflammation

(A) PTECs stimulated with AGE (100 μg/ml) and co-incubated with increasing concentrations of BMP7 (0–200 ng/ml) for 3 h; (B) PTECs were stimulated with same concentration (100 μg/ml) of AGE and HSA, co-incubated with or without BMP7 for 3 h. mRNA expression was determined by real-time PCR. *P<0.05, **P<0.01, ***P<0.001.

Figure 1
BMP7 dose-dependently reduced AGE-induced tubular inflammation

(A) PTECs stimulated with AGE (100 μg/ml) and co-incubated with increasing concentrations of BMP7 (0–200 ng/ml) for 3 h; (B) PTECs were stimulated with same concentration (100 μg/ml) of AGE and HSA, co-incubated with or without BMP7 for 3 h. mRNA expression was determined by real-time PCR. *P<0.05, **P<0.01, ***P<0.001.

The anti-inflammatory effect of BMP7 was confirmed by the expression profile of other cytokines. Real-time PCR and ELISA results revealed that BMP7 reduced AGE-induced overexpression of MCP1, IL-8, ICAM1, and IL-6 in PTECs in both gene and protein levels (Figure 2).

BMP7 attenuated AGE-induced pro-inflammatory response in PTECs

Figure 2
BMP7 attenuated AGE-induced pro-inflammatory response in PTECs

(A) mRNA expression of MCP1, IL-8, ICAM1 and IL-6 in PTECs cultured with AGEs (100 μg/ml), AGEs plus BMP7 (100 ng/ml) or BMP7 alone. Nil denotes medium control. mRNA were collected after 3 h of treatment. (B) Secreted MCP1, ICAM1, IL-6 and IL-8 levels in culture supernatants, collected at 24 h and measured by ELISA respectively. *P<0.05, **P<0.01, ***P<0.001.

Figure 2
BMP7 attenuated AGE-induced pro-inflammatory response in PTECs

(A) mRNA expression of MCP1, IL-8, ICAM1 and IL-6 in PTECs cultured with AGEs (100 μg/ml), AGEs plus BMP7 (100 ng/ml) or BMP7 alone. Nil denotes medium control. mRNA were collected after 3 h of treatment. (B) Secreted MCP1, ICAM1, IL-6 and IL-8 levels in culture supernatants, collected at 24 h and measured by ELISA respectively. *P<0.05, **P<0.01, ***P<0.001.

AGEs induced pro-inflammatory responses through multiple signalling pathways in PTECs

Pretreatment of PTECs with MAPK inhibitors attenuated AGE-induced cytokine expressions. Inhibition of p38 signalling with SB203580 reduced overexpression of IL-8 and MCP1, and inhibition of p44/42 signalling with PD98059 reduced up-regulation of IL-8 and ICAM1 (Figure 3). This suggested that AGEs induced pro-inflammatory responses through multiple signalling pathways.

AGE-induced PTECs expression of pro-inflammatory factors is mediated through multiple pathways

Figure 3
AGE-induced PTECs expression of pro-inflammatory factors is mediated through multiple pathways

PTECs were pre-treated with p44/42 inhibitor (PD98059, 25 μM) or p38 inhibitor (SB203580, 10 μM) 30 min before adding AGEs into the culture medium. 3 h mRNA and 24 h supernatants were collected for measurement. (A) mRNA and protein changes in PTECs incubated with p44/42 inhibitor; (B) mRNA and protein changes in PTECs incubated with p38 inhibitor. *P<0.05, **P<0.01, ***P<0.001.

Figure 3
AGE-induced PTECs expression of pro-inflammatory factors is mediated through multiple pathways

PTECs were pre-treated with p44/42 inhibitor (PD98059, 25 μM) or p38 inhibitor (SB203580, 10 μM) 30 min before adding AGEs into the culture medium. 3 h mRNA and 24 h supernatants were collected for measurement. (A) mRNA and protein changes in PTECs incubated with p44/42 inhibitor; (B) mRNA and protein changes in PTECs incubated with p38 inhibitor. *P<0.05, **P<0.01, ***P<0.001.

BMP7 suppressed AGE-induced phosphorylation of p38 and p44/42 protein

To test whether anti-inflammatory effect of BMP7 is mediated through MAPK pathways, phosphorylation of p38 and p44/42 protein was determined by Western blot analysis. Figure 4 shows that AGEs increased p38 and p44/42 phosphorylation in PTECs, whereas co-incubation with BMP7 significantly suppressed their phosphorylation. This result suggests that BMP7 attenuates AGE-induced pro-inflammatory responses through modulating p38 and p44/42 MAPK signalling pathways.

BMP7 suppressed phosphorylation of p38 and p44/42 induced by AGEs

Figure 4
BMP7 suppressed phosphorylation of p38 and p44/42 induced by AGEs

PTECs were treated with AGEs (100 μg/ml) or AGEs plus BMP7 (100 ng/ml) for 30 min; nil denotes medium control. Bars are means±S.D. of three independent experiments. *P<0.05.

Figure 4
BMP7 suppressed phosphorylation of p38 and p44/42 induced by AGEs

PTECs were treated with AGEs (100 μg/ml) or AGEs plus BMP7 (100 ng/ml) for 30 min; nil denotes medium control. Bars are means±S.D. of three independent experiments. *P<0.05.

BMP7 attenuated AGE-induced ROS accumulation in PTECs

Considering that AGEs may also induce cellular oxidative stress and increased ROS production may lead to the activation of p44/42 or p38 MAPK [33], ROS formation was measured in AGE-stimulated PTECs. The immunofluorescence study showed that after stimulation with AGEs for 1 h, ROS accumulation was significantly higher than the medium control, AGEs co-incubated with BMP7 reduced ROS formation in PTECs. Quantitative intracellular ROS measurement by OxiSelect™ Intracellular ROS Assay kit also showed a 50% increase in ROS formation in AGE-stimulated cells and it was abolished by co-incubated with BMP7 (Figure 5).

BMP7 attenuated AGE-induced ROS accumulation in PTECs

Figure 5
BMP7 attenuated AGE-induced ROS accumulation in PTECs

PTECs were cultured in chamber slides with (A) plain medium, (B) H2O2 (500 μM as positive control), (C) AGEs (100 μg/ml) and (D) AGEs (100 μg/ml) plus BMP7 (100 ng/ml) for 1 h. ROS generation was detected by immunofluorescence signals using a Leica microscope (magnification: × 100); (E) quantitative data of green fluorescent signals. Results were obtained from three independent experiments, and data were presented as fold change of IOD compared with medium control, *P<0.05; (F) intracellular ROS quantified by OxiSelectTM Intracellular ROS Assay kit. Data were from three independent experiments, and results are presented as ROS concentration. **P<0.01.

Figure 5
BMP7 attenuated AGE-induced ROS accumulation in PTECs

PTECs were cultured in chamber slides with (A) plain medium, (B) H2O2 (500 μM as positive control), (C) AGEs (100 μg/ml) and (D) AGEs (100 μg/ml) plus BMP7 (100 ng/ml) for 1 h. ROS generation was detected by immunofluorescence signals using a Leica microscope (magnification: × 100); (E) quantitative data of green fluorescent signals. Results were obtained from three independent experiments, and data were presented as fold change of IOD compared with medium control, *P<0.05; (F) intracellular ROS quantified by OxiSelectTM Intracellular ROS Assay kit. Data were from three independent experiments, and results are presented as ROS concentration. **P<0.01.

BMP7 preserved kidney function and kidney morphology in diabetic kidney

At the age of 9 weeks, diabetic db/db mice have already developed hyperglycaemia (blood glucose), obesity (body weight), manifestation of albuminuria [urine albumin-to-creatinine ratio (UACR)] and reduction of kidney function (BUN) compared with non-diabetic db/m mice as shown in Table 2. BMP7 did not affected blood glucose level and systolic blood pressure throughout the experiment. Unx db/m mice showed a slight increase in systolic blood pressure and serum BUN (P<0.05) after 8 weeks of treatment; however, Unx did not show any effect on these parameters in db/db mice. As reported in our previous study [31], Unx markedly exaggerated UACR and serum BUN level in db/db mice compared with sham-operated db/db mice in this study. Average UACR of Unx-db/db vehicle group was 2-fold higher than Unx db/db mice receiving BMP7 injection (8612±2037 μg/mg compared with 3549±816.2 μg/mg, P=0.036). Serum BUN of Unx-db/db mice treated with BMP7 showed significantly lower level (33.26±1.09 mg/dl compared with 37.49±0.89 mg/dl, P=0.006) when compared with Unx-db/db vehicle group.

Table 2
Physical and biochemical parameters of experimental animals (n=10–11 per group)

*P<0.05 compared with db/db animals; P<0.05 compared with baseline; P<0.05 compared with Unx-db/db vehicle group; §P<0.05 compared with Unx-db/db vehicle group.

db/mdb/db
ParameterUnxSham + vehicleUnx + BMP7Unx + vehicle
Body weight (g)     
 Baseline 24.45±0.53* 41.1±0.92 40.16±0.85 39.77±0.45 
 8 weeks 26.95±0.43* 49.58±1.59 50.85±2.22 46.02±1.30 
Blood glucose (mM)     
 Baseline 9.86±0.31* 16.35±1.33 16.51±1.27 16.89±1.66 
 8 weeks 8.24±0.35* 26.59±0.90 26.48±2.36 27.44±1.70 
Systolic BP (mmHg)     
 Baseline 117.4±5.36 116.6±2.45 124.5±7.86 123.6±6.36 
 8 weeks 130.7±5.15 119.9±11.29 120.0±9.79 124.8±13.10 
UACR (μg/mg)     
 Baseline 437.9±26.44* 4220±356.6 4395±830.7 4068±397.5 
 8 weeks 404.3±57.14* 3371±1029 3549±816.2§ 8612±2037 
BUN (mg/dl)     
 Baseline 21.02±0.97* 31.82±1.99 33.32±2.17 35.14±1.35 
 8 weeks 32.69±0.89 28.98±1.93 33.26±1.09§ 37.49±0.89 
db/mdb/db
ParameterUnxSham + vehicleUnx + BMP7Unx + vehicle
Body weight (g)     
 Baseline 24.45±0.53* 41.1±0.92 40.16±0.85 39.77±0.45 
 8 weeks 26.95±0.43* 49.58±1.59 50.85±2.22 46.02±1.30 
Blood glucose (mM)     
 Baseline 9.86±0.31* 16.35±1.33 16.51±1.27 16.89±1.66 
 8 weeks 8.24±0.35* 26.59±0.90 26.48±2.36 27.44±1.70 
Systolic BP (mmHg)     
 Baseline 117.4±5.36 116.6±2.45 124.5±7.86 123.6±6.36 
 8 weeks 130.7±5.15 119.9±11.29 120.0±9.79 124.8±13.10 
UACR (μg/mg)     
 Baseline 437.9±26.44* 4220±356.6 4395±830.7 4068±397.5 
 8 weeks 404.3±57.14* 3371±1029 3549±816.2§ 8612±2037 
BUN (mg/dl)     
 Baseline 21.02±0.97* 31.82±1.99 33.32±2.17 35.14±1.35 
 8 weeks 32.69±0.89 28.98±1.93 33.26±1.09§ 37.49±0.89 

Morphologically, sham-operated db/db mice displayed mild glomerular and tubulointerstitial damage, while Unx-db/db mice developed glomerular hypertrophy, nodular glomerulosclerosis and severe tubular damage (tubular dilation and atrophy), and such glomerular and tubular damages were partially abrogated by BMP7 treatment (Figure 6).

BMP7 improved renal morphology in diabetic kidney

Figure 6
BMP7 improved renal morphology in diabetic kidney

Representative PAS staining of the renal cortical sections in Unx-db/m mice (A), sham-operated db/db mice (B), Unx-db/db vehicle-treated mice (C) and Unx-db/db BMP7-treated mice (D). Magnification × 400. (E) Quantification of glomerular tubulointerstitium damage using ImagePro plus, and expressed as IOD/area × 105. *P<0.05, **P<0.01.

Figure 6
BMP7 improved renal morphology in diabetic kidney

Representative PAS staining of the renal cortical sections in Unx-db/m mice (A), sham-operated db/db mice (B), Unx-db/db vehicle-treated mice (C) and Unx-db/db BMP7-treated mice (D). Magnification × 400. (E) Quantification of glomerular tubulointerstitium damage using ImagePro plus, and expressed as IOD/area × 105. *P<0.05, **P<0.01.

BMP7 attenuated renal cortical inflammation and macrophage infiltration in diabetic mice

To explore whether exogenously administration of BMP7 confers similar anti-inflammatory effect in diabetic kidney as shown in the in vitro study, cortical expression of pro-inflammatory cytokines ICAM1 and MCP1 were examined by real-time PCR and ELISA. Diabetes caused overexpression of MCP1 and ICAM1 in Unx-db/db mice in both gene and protein levels when compared with Unx-db/m control mice (Figure 7). A significant increase in macrophage infiltration in the tubulointerstitium was also observed in parallel with the up-regulation of MCP1 and ICAM1 in Unx diabetic kidney (Figure 8). Administration of BMP7 reduced renal cortical expression of MCP1 and ICAM1 at 40% and 16.7% respectively when compared with Unx-db/db vehicle group.

BMP7 attenuated renal cortical inflammation

Figure 7
BMP7 attenuated renal cortical inflammation

(A) Renal cortical ICAM1 and MCP1 mRNA expression, determined by real-time PCR. (B) Renal cortical protein levels of ICAM1 and MCP1, determined by ELISA and normalized by total protein levels of tissue lysates. *P<0.05, **P<0.01.

Figure 7
BMP7 attenuated renal cortical inflammation

(A) Renal cortical ICAM1 and MCP1 mRNA expression, determined by real-time PCR. (B) Renal cortical protein levels of ICAM1 and MCP1, determined by ELISA and normalized by total protein levels of tissue lysates. *P<0.05, **P<0.01.

BMP7 attenuated macrophage infiltration in diabetic mice

Figure 8
BMP7 attenuated macrophage infiltration in diabetic mice

Representative renal cortical sections of F4/80 immunostaining in Unx-db/m mice (A), sham-operated db/db mice (B), Unx-db/db vehicle-treated mice (C) and Unx-db/db BMP7-treated mice (D). Magnification × 400. (E) Quantification presented as number of F4/80+ cells (red arrows) per high power field. *P<0.05.

Figure 8
BMP7 attenuated macrophage infiltration in diabetic mice

Representative renal cortical sections of F4/80 immunostaining in Unx-db/m mice (A), sham-operated db/db mice (B), Unx-db/db vehicle-treated mice (C) and Unx-db/db BMP7-treated mice (D). Magnification × 400. (E) Quantification presented as number of F4/80+ cells (red arrows) per high power field. *P<0.05.

BMP7 suppressed p38 and p44/42 MAPK activation in DN

To explore the possible molecular mechanisms of BMP7 in modulating diabetic kidney inflammation, Western blot analysis demonstrated a significant increase in cortical p38 and p44/42 MAPK phosphorylation in Unx-db/db mice injected with vehicle than mice received sham-operation. Treatment of BMP7 significantly suppressed 40% of p38 and 60% of p44/42 MAPK phosphorylation, suggesting that BMP7 exerts an in vivo anti-inflammatory effect by modulating these MAPK pathways in diabetic kidney (Figure 9).

BMP7 suppressed p38 and p44/42 phosphorylation in diabetic mice

Figure 9
BMP7 suppressed p38 and p44/42 phosphorylation in diabetic mice

Renal cortical p38 and p44/42 phosphorylation was determined by Western blot analysis and quantified by ChemiDoc software. Samples were randomly picked up in each group (db/db sham: 3/10, Unx-db/db BMP7: 5/10, Unx-db/db vehicle: 5/11, respectively). *P<0.05.

Figure 9
BMP7 suppressed p38 and p44/42 phosphorylation in diabetic mice

Renal cortical p38 and p44/42 phosphorylation was determined by Western blot analysis and quantified by ChemiDoc software. Samples were randomly picked up in each group (db/db sham: 3/10, Unx-db/db BMP7: 5/10, Unx-db/db vehicle: 5/11, respectively). *P<0.05.

BMP7 reduced cortical lipid peroxidation in diabetic kidney

The degree of oxidative stress in renal cortex was determined by measuring the end product of lipid peroxidation. Aldehydic secondary products MDA in the kidney reacted with TBA to form the MDA–TBA adduct, which can be measured colorimetrically. Lipid peroxidation was significantly higher in Unx-db/db vehicle group than the other groups (db/db sham, Unx-db/db BMP7 and Unx-db/m groups, P<0.01). Administration of BMP7 reduced 20% of MDA formation in diabetic kidney (Figure 10).

BMP7 reduced cortical lipid peroxidation in diabetic kidney

Figure 10
BMP7 reduced cortical lipid peroxidation in diabetic kidney

Cortical levels of lipid peroxidation are determined using OxiSelect™ TBARS Assay kit, and data are presented as fold change of MDA-TBA adduct compared with the db/db sham groups. **P<0.01.

Figure 10
BMP7 reduced cortical lipid peroxidation in diabetic kidney

Cortical levels of lipid peroxidation are determined using OxiSelect™ TBARS Assay kit, and data are presented as fold change of MDA-TBA adduct compared with the db/db sham groups. **P<0.01.

DISCUSSION

BMP7 has been demonstrated to have profound anti-fibrotic effects in the diabetic kidney, as demonstrated mainly in STZ-induced animal models, reflecting Type 1 diabetes. Herein, we showed for the first time that BMP7 also possesses anti-inflammatory effects in renal tubular epithelial cells and in a model of T2DN.

In vitro, BMP7 attenuated AGE-induced tubular inflammation through suppressing multiple pathways including p38 and p44/42 MAPK. A strength of these in vitro data stems from the fact that they were derived using primary human PTECs that offer a good viable model for the proximal tubule epithelia. Although immortalized human transformed cell lines have been extensively used and are extremely useful for these purposes, they exhibit limitations in their phenotypic resemblance to primary PTECs in culture [34]. In vivo, we demonstrated that BMP7 restored renal function and abrogated cortical inflammation in T2DN. Finally, BMP7 exerted antioxidative effects both in vitro and in vivo.

Our findings are in line with previous observations in certain non-diabetes-related investigations. For instance, BMP7 repressed TNF-α induced pro-inflammatory chemokines and cytokines in primary human PTECs [22]. Lee et al. [35] found that BMP7 significantly inhibited constitutive and IL1-β-induced MCP1 expression in mesangial cells by modulating the c-Jun N-terminal kinase (JNK)/activator protein 1 (AP1) pathway. Administration of BMP7 decreased IL-1β production in an inflammatory arthritis rat model [36]. Our previous study also demonstrated that BMP7 suppressed polymeric IgA induced TNF-α overexpression [37]. In diabetes, BMP7 reduced monocyte/macrophage infiltration in an STZ-induced diabetic rat model [21]. All these data suggested BMP7 conferred anti-inflammatory effect both in vitro and in vivo. It is likely that AGEs induce pro-inflammatory chemokine and cytokine overexpression by activating multiple intracellular signalling pathways, as the inhibition of a single signal transduction pathway could only suppress the expression of certain pro-inflammatory factors or even induced the expression of other factors (data not shown). Our in vivo results using db/db animals also revealed inhibition of p38 and p44/42 signalling pathways and their downstream genes by exogenous BMP7. So we believe BMP7 reduced various pro-inflammatory factors in AGE-induced tubular inflammation, mostly because BMP7 simultaneously suppressed multiple signalling pathways including p38 and p44/42 MAPK.

In addition to its action on the renal tubules, BMP7 may also exert notable glomerular effects in the context of DN. In vitro, BMP7 ameliorated glucose-induced oxidative stress via inhibition of protein kinase C (PKC), JNK phosphorylation and c-Jun activation in rat mesangial cells [38], and reduced apoptosis by blocking high-glucose-mediated caspase-3 overactivity in podocytes [17]. In addition, treatment with BMP7 restored synaptopodin and podocin expression in mouse podocytes exposed to high glucose [39]. In vivo, transgenic expression of human BMP7 in STZ-induced diabetic mice reduced glomerulosclerosis, interstitial collagen, albuminuria, podocyte dropout and increased nephrin levels compared with wild-type animals [18]. In STZ-induced diabetic CD1 mice with renal lesions resembling human DN, BMP7 inhibited glomerular hypertrophy and glomerulosclerosis, although it was most effective in ameliorating tubular inflammation and tubulointerstitial fibrosis [19]. Also in STZ-induced diabetic C57Bl/6J connective tissue growth factor (CTGF)+/− mice, there was increased phosphorylation of the BMP signal transduction proteins Smad1/5 and expression of the BMP target gene leading to reduced glomerular basement membrane thickening and albuminuria compared with CTGF+/+ mice, suggesting a pathogenetic role of CTGF in BMP signalling in DN [20]. Finally, BMP7 partially restored renal function, reduced albuminuria and improved glomerular histology toward normal in STZ-induced diabetic rats [21].

Advanced glycation and oxidative stress are two key players in the pathogenesis of DN [40]. Oxidative stress is induced by multiple pathways in diabetes. Hyperglycaemia causes excessive ROS synthesis by direct autoxidation during the formation of AGEs and by inducing mitochondrial dysfunction [41]. Experimental studies showed that AGEs can further increase ROS generation through AGE receptor (RAGE) by stimulating NADPH oxidase activity [42] and that AGE-induced oxidative stress is attenuated by NADPH oxidase inhibitor in human endothelial cells [43]. Oxidative stress in diabetes may also be enhanced by a change in antioxidant defence. There is evidence suggesting that AGEs decrease the enzyme activities of SOD and catalase [44] and altered activities of these enzymes are also observed in diabetic animal models. Hence, persistent hyperglycaemia and oxidative stress enhance AGE formation, thus setting up a vicious cycle of auto-amplification culminating in cell injury. Indeed, BMP7 has been shown to attenuate ROS generation in high-glucose-stimulated rat mesangial cells [38] as stated above. Here, we have shown for the first time that BMP7 not only attenuated oxidative stress in high-glucose-exposed mesangial cells, but also reduced ROS formation in AGE-stimulated PTECs, in addition to its anti-inflammatory action. Moreover, BMP7 reduced lipid peroxidation in the diabetic kidney, substantiating the antioxidative effects of BMP7 in vivo. Mechanistically, we found BMP7 attenuated ROS formation and suppressed p38 and p44/42 phosphorylation. This is not surprising considering that ROS induced activation of extracellular-signal-regulated protein kinase (ERK) and p38 MAPK [33]. Whether BMP7 modulates the expression of pro-inflammatory cytokines directly or indirectly through ROS generation requires further investigation.

Although a large body of evidence points to an anti-fibrotic role of BMP7 [14,4547], mostly stating that BMP7 counteracts the pro-fibrotic effects of TGF-β in several disease models [46], there exists opposite views. For instance, Dudas et al. [48] reported that BMP7 failed to attenuate TGF-β-induced epithelial-to-mesenchymal transition in human PTECs, in which cells cultured with BMP7 alone displayed increased vimentin, CTGF or even TGF-β expression. Interestingly, we also found that PTECs stimulated with BMP7 had significantly increased CTGF and type I collagen and reduced E-cadherin gene expression. As our mouse model did not develop notable renal fibrosis, we did not observe significant differences in pro-fibrotic markers in different diabetic groups. In addition, Masson Trichrome staining or immunohistochemical staining of collagen I and V did not yield significant differences among the various diabetic groups (data not shown). Hence, although BMP7 may promote fibrosis in a single cell type in a culture system, it is possible that its overall effects in vivo are the results of interactions between different resident and infiltrating cells, which is beyond the scope of the present study. Nevertheless, the anti-fibrotic role of BMP7 in T2DN warrants further investigation.

Abbreviations

     
  • BMP7

    bone morphogenetic protein 7

  •  
  • AGE

    advanced glycation end-product

  •  
  • BUN

    blood urea nitrogen

  •  
  • HSA

    human serum albumin

  •  
  • HRP

    horseradish peroxidase

  •  
  • PTEC

    proximal tubular epithelial cell

  •  
  • DN

    diabetic nephropathy

  •  
  • T2DN

    Type 2 diabetic nephropathy

  •  
  • ESRD

    end-stage renal disease

  •  
  • TNF

    tumour necrosis factor

  •  
  • TGF

    transforming growth factor

  •  
  • IL

    interleukin

  •  
  • ICAM1

    intercellular adhesion molecule 1

  •  
  • MCP1

    monocyte chemoattractant protein 1

  •  
  • CTGF

    connective tissue growth factor

  •  
  • MAPK

    mitogen-activated protein kinase

  •  
  • ERK

    extracellular-signal-regulated protein kinase

  •  
  • Unx

    uninephrectomy

  •  
  • ROS

    reactive oxygen species

  •  
  • UACR

    urine albumin-to-creatinine ratio

  •  
  • STZ

    streptozotocin

  •  
  • REBM

    renal epithelial cell basal medium

  •  
  • TBA

    thiobarbituric acid

  •  
  • MDA

    malondialdehyde

  •  
  • PAS

    periodic acid–Schiff

  •  
  • IOD

    integrated absorbance (‘optical density’)

  •  
  • JNK

    c-Jun N-terminal kinase

AUTHOR CONTRIBUTION

Rui Xi Li, Wai Han Yiu and Sydney Tang designed and performed the experiments, acquired and analysed the data and drafted the paper. Hao Jia Wu, Dickson Wong, Loretta Chan, Miao Lin and Joseph Leung were involved in experiments, data collection and paper preparation. Kar Neng Lai and Sydney Tang conceived the study, and participated in its design and co-ordination, were involved in drafting the paper and revising it critically for important intellectual content and gave final approval of the version to be published. All authors have read and approved the final version of the paper.

We sincerely appreciate the time and effort from all who contributed to the present study.

FUNDING

The present study is supported by a General Research Fund of the Research Grants Council of Hong Kong [grant number HKU7770/09M], the National Basic Research Program of China 973 programme number 2012CB517600 [grant number 2012CB517606], an Endowment Fund established for the “Yu Professorship in Nephrology” (to S.C.W.T.), and the Hong Kong Society of Nephrology Research Grant 2012. W.H.Y. and H.J.W. are partially supported by Mr Winston Leung and by the Hong Kong Concrete and the Continental Cement and Gypsum Co. Ltd.

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