Tanshinone IIA down-regulates -transforming growth factor beta 1 to relieve renal tubular epithelial cell inflammation and pyroptosis caused by high glucose.

Diabetic nephropathy (DN) is a microvascular disease caused by diabetes. Tanshinone IIA has been indicated to ameliorate streptozotocin-induced DN. This study explores the effect of tanshinone IIA on high glucose-induced renal tubular epithelial cell pyroptosis and inflammation. High glucose-stimulated HK-2 cells were used as the in-vitro model of DN and were treated with tanshinone IIA at concentrations of 1, 5, 10 μM for 24 h with the same doses of tolbutamide as the control. After tanshinone IIA treatment, HK-2 cells were transfected with pcDNA-transforming growth factor beta 1 (TGFB1) or sh-TGFB1 for 48 h. RT-qPCR was used to detect the mRNA levels of TNF-α, IL-6, IL-1β, and IL-18. Cell apoptosis and pyroptosis were detected by flow cytometry and cell immunofluorescence. Bioinformatics screening predicted that tanshinone IIA might be an effective component of Salvia miltiorrhiza Bunge (Labiatae) for the treatment of DN. Tanshinone IIA exerted a protective effect in the in-vitro model of DN by suppressing inflammation and pyroptosis via the TGFB1-dependent pathway. Tanshinone IIA inhibited high glucose-induced renal tubular epithelial cell inflammation and cell death through pyroptosis by regulating TGFB1, indicating the therapeutic potential of tanshinone IIA for DN treatment.

Highlights

Tanshinone IIA was an effective component of Salvia miltiorrhiza in DN.

High glucose induced renal tubular epithelial cell injury through pyroptosis.

Tanshinone IIA inhibited high glucose-induced pyroptosis by targeting TGFB1.

Introduction

Diabetic nephropathy (DN) is a microvascular disease caused by diabetes. It is clinically characterized by progressive renal dysfunction and is one of the major causes of end-stage renal disease [1]. Nearly 40% to 50% of patients with type 1 diabetes are diagnosed of DN, which eventually develops into end-stage renal disease. This number reaches 30% among patients with type 2 diabetes. Therefore, it is imperative to take effective measures to improve the early diagnosis and treatment for DN patients [2].

DN is a multifactorial progressive disease with extremely complex pathogenesis. Accumulating studies have shown that inflammation is a significant mechanism in the occurrence and progression of DN [3,4]. Changes in the hyperglycemia-induced mitogen-activated protein kinase-related signaling pathways can stimulate the renin-angiotensin aldosterone system by regulating gene transcription, induce the production of ECM, promote the increase of reactive oxygen species (ROS) and inflammatory mediators [5].

At present, it is believed that the release of pro-inflammatory factors mainly depends on a special type of pro-inflammatory death mode, pyroptosis, which is different from the classical apoptosis and necrosis [6]. Pyroptosis refers to the state that after cells are stimulated by inflammatory signals, inflammatory cysteine enzymes (caspase-1, caspase-11) cleave gasdermin family proteins and expose their active N-terminal. The latter form pores on the cell membrane, causing the release of inflammatory factors interleukin-1β (IL-1β) and IL-18 and promoting the inflammatory response [7,8]. Recently, it has been reported that this cysteine aspartate-dependent form of programmed cell death is associated with a large number of pro-inflammatory mediators and is related to the progression of diabetes and diabetic complications [9,10]. Moreover, pyroptosis is reported to regulate pancreatic β-cell survival in diabetic patients [11]. Punicalagin reduces DN by downregulating NOX4 and inhibiting pyroptosis through the TXNIP/NLRP3 pathway [12]. Caspase-11/4 and GADMD-mediated pyroptosis is activated to participate in the occurrence of DN [13].

Tanshinone IIA is mainly derived from Salvia miltiorrhiza Bunge (Labiatae). It has been reported that tanshinone IIA ameliorates streptozotocin-induced DN, partly by attenuating PERK pathway-induced fibrosis [14]. Mechanistically, the anti-fibrotic effects of tanshinone IIA are associated with suppression of TGF-β pathway signaling [15]. In this study, we aimed to investigate the effect and regulatory mechanism of Tanshinone IIA in DN progression in vitro using a high glucose-induced renal tubular epithelial cell injury model. We assumed that tanshinone IIA inhibited the high glucose induced inflammation and pyroptosis of renal tubular epithelial cells by targeting TGFB1. The findings of our study suggest TGFB1 as potential targets of tanshinone IIA for the DN treatment.

Materials and methods

Cell culture and treatment

Human renal tubular epithelial cell line (HK-2) (ATCC, Manassas, VA, USA) was cultured in Keratinocyte Serum Free Medium (K-SFM, Invitrogen, CA, USA) containing 10% fetal bovine serum (FBS; Gibco, Carlsbad, CA, USA), 100 U/mL penicillin and 100 U/mL streptomycin (Sigma, St. Louis, MO, USA), and maintained in a humidified chamber at 37°C with 5% CO2. HK-2 cells were incubated with 30 mM D-glucose for 48 h to establish a hyperglycemic model. HK-2 cells stimulated by high glucose were treated with tanshinone IIA (1, 5, 10 μM; Sigma-Aldrich) [16] and positive drug tolbutamide (Sigma, St. Louis, MO, USA) for 24 h under manufacturer’s instructions, and the cells were collected for subsequent experimental studies.

Cell transfection

The short hairpin RNAs against TGFB1 (sh-TGFB1) was designed and synthesized by GenePharma (Shanghai, China). The TGFB1 overexpression plasmid (pcDNA-TGFB1, GenePharma) (NC_000019.10) and its negative control (pcDNA-NC, GenePharma) were used to overexpress TGFB1. The cells were seeded into 6-well plates at the density of 5 × 105 cells/well. Then the cell transfection was performed using Lipofectamine 2000 transfection reagent (Invitrogen, Carlsbad, CA) under manufacturer’s instructions. Cells were collected after 48 h of transfection for subsequent experiments [17].

Reverse transcription‑quantitative PCR (RT-qPCR)

Total RNA was collected from the cells using TRIzol reagent (Invitrogen, Carlsbad, CA, USA) according to the manufacturer’s instructions, and the RNA was reverse transcribed into cDNA using the PrimeScript RT kit (Takara Biotechnology, China). The cDNA was used as template, and SYBR Green (Takara, Kusatsu, Japan) was used for PCR amplification. The conditions of PCR amplification were as follows: pre-denaturation at 95°C for 30 s; 95°C for 5 s; annealing at 60°C for 30 s, with total 35 cycles. The relative expression of the target genes was calculated by the 2−ΔΔCt method. The β-actin was used as an internal reference for mRNAs [18]. The primer sequences are as follows: TGFB1: F: 5’-AGCTGTACCAGAAATACAGCA-3’, R: 5’-ATAACCACTCTGGCGAGTC-3’; TNF-α: F: 5’-CAGAGGGAAGAGT CCCCCAG-3’, R: 5’-CCTTGGTCTGGTAGGAGACG-3’; IL-6: F: 5’-GGAGACTTGCCTGGTGAAA-3’, R: 5’-CTGGCTTGTTCCTCACTACTC-3’; IL-1β: F: 5’-CCTGTGGCCTTGGGCCTCAA-3’, R: 5’-GGTGCTGATGTACCAGTTGGG-3’; IL-18: F: 5’- GCAGTTTTGCCAAGGAGTGCT-3’, R: 5’- TTTCTGTGTTGGCGCAGTGTG-3’; GAPDH: F: 5’-TCATTTCCTGGTATGACAACGA-3’, R: 3’-GTCTTACTCCTTGGAGGCC-3’; U6: F: 5’-TGCTATCACTTCAGCAGCA-3’, R: 5’-GAGGTCATGCTAATCTTCTCTG-3’.

Flow cytometry analysis

Cells were collected and digested with 0.25% trypsin without EDTA, and the single cell suspension was prepared and washed 3 times with PBS. The cells were resuspended in pre-cooled 1 × Binding Buffer, repeatedly blown and mixed with pipette to achieve a cell density of about 1 × 106 cells/mL. Next, 5 μL of Annexin V-FITC was added gently and incubated at room temperature for 15 min in the dark. The cells were resuspended in 0.5 mL of pre-chilled 1× Binding Buffer. After adding 10 μL of propidium iodide (PI), cell apoptosis was detected by flow cytometry. The experiments were repeated three times and the average value was obtained [19].

Western blotting

The transfected cells were collected after 48 h, and RIPA cell lysate was added. Total cell proteins were extracted on ice, and then the protein concentration was detected by a BCA protein detection kit. The protein sample was heated and denaturized in a boiling water bath. The denaturized protein sample of the equal amount was added to the loading hole for SDS-PAGE gel electrophoresis. After the protein was separated, it was transferred to the PVDF membrane for electrotransfer. The transformed membrane was blocked with 5% skimmed milk at room temperature for 1 h. The primary antibodies including cleaved-caspase-3, cleaved-caspase-9, cleaved-caspase-1, cleaved-caspase-12, GRP78, CHOP, IL-1β, N-GSDMD, TGFB1, and β-actin (1:1000, Abcam, Cambridge, Britain) were diluted and incubated overnight at 4°C, and then the secondary antibody (1:3000, Abcam, Cambridge, Britain) was diluted and incubated for 30 min at room temperature, and washed 3 times with TBST. Proteins were detected by the chemiluminescence method, and the relative protein expression of each group was calculated by the ImageJ analysis software (Thermo Fisher Scientific Inc., USA). Analysis on each protein sample was repeated 3 times [19].

Cell immunofluorescence

The N-GSDMD and TUNEL immunofluorescence double staining method was used to detect renal tubular epithelial cell pyroptosis. After the renal tubular epithelial cells grew after three generations, they were dripped on the cover slips in the six-well plates and incubated in the cell incubator for 4 h. The six-well plates were taken out, washed with PBS for 15 min, and then added with 4% paraformaldehyde dropwise. Next, 50 μL of terminal deoxynucleotidyl transferase enzyme reaction solution and streptavidin-FITC labeled working solution were added in order, and the primary antibody (N-GSDMD: 1:50) was added and incubated at 4°C overnight. The secondary antibody (fluorescence labeled anti-rabbit IgG antibody: 1:100) was incubated at room temperature for 2 h. The nucleus was stained with DAPI working solution at 37°C, and the slides were mounted in a dark room with glycerol and then observed and imaged under a fluorescent microscope [20].

Active ingredient collection and target screening

The TCM System Pharmacology (TCMSP) database (http://lsp.nwu.edu.cn/tcmsp.php) is a unique TCM system pharmacology platform. The biological components of Chinese herbal medicine compound were searched and the effective components of Salvia miltiorrhiza were screened according to the required conditions: oral bioavailability (OB) ≥20%; drug likelihood (DL) ≥0.1; blood brain barrier (BBB) ≥0.3; half-life (HL) ≥8. Potential targets of each active ingredient were manually searched on the TCMSP platform. The DN-related targets were further predicted by the Comparative Toxicogenomics Database (CTD, http://ctdbase.org/) [21].

PPI network construction

Protein–protein interaction (PPI) networks are critically involved in the cellular function and biological processes. The common targets of Salvia miltiorrhiza and DN were submitted to the String11.0 database (https://string-db.org) to construct a PPI network model [21].

Statistical analysis

All data were expressed by the mean ± standard deviation, and SPSS 21.0 was used for statistical analysis. One-way ANOVA was used to analyze comparison among multiple groups, and the independent unpaired t-test was used for statistical analysis between the two groups. P < 0.05 was considered as statistically significant.

Results

In this study, we explored the effect and underlying mechanism of tanshinone IIA in DN progression in vitro. DN cell models were established using high glucose-stimulated HK-2 cells. Our findings revealed that tanshinone IIA inhibited the high glucose induced pyroptosis of renal tubular epithelial cells by downregulating TGFB1.

Prediction of tanshinone IIA as the effective component of salvia miltiorrhiza in DN and its structural formula

At first, we intended to predict a potentially active ingredient of Salvia miltiorrhiza in DN. It has been previously reported that tanshinone IIA ameliorates streptozotocin-induced DN [14]. Based on the TCMSP Database, 202 components of Salvia miltiorrhiza were preliminarily predicted, and 171 active components were further screened with 170 targets (Table 1). Through ctdbase prediction, 42 DN-related targets were screened out which intersect with the 170 targets of active components of Salvia miltiorrhiza, with a total of 9 intersections; among them, tanshinone IIA has 150 targets, which intersect with DN-related targets, with a total of 9 identical targets (Figure 1a). Based on bioinformatics analysis, we assumed that tanshinone IIA was a potentially active ingredient of Salvia miltiorrhiza in DN and its effect on DN was further explored. The chemical structure of tanshinone IIA is C19H18O3, which is obtained from PubChem analysis (Figure 1b).

Table 1.

Components of salvia miltiorrhiza were predicted by TCMSP database. OB: oral bioavailability; DL: drug likelihood; BBB: blood brain barrier; HL: half-life of the drug

NSC 122421	34.49	0.63	0.28	14.56
Miltirone	38.76	0.87	0.25	14.82
Isotanshinone II	49.92	0.45	0.4	24.73
Isocryptotanshi-none	54.98	0.34	0.39	31.92
Dihydrotanshinone I	45.04	0.43	0.36	18.32
Danshenspiroketallactone	50.43	0.51	0.31	15.19
Cryptotanshinone	52.34	0.51	0.4	17.3
Salvilenone	30.38	1.07	0.38	20.81
Methylenetanshinquinone	37.07	0.46	0.36	24.33
4-methylenemiltirone	34.35	0.87	0.23	14.6
2-isopropyl-8-methylphenanthrene-3,4-dione	40.86	0.81	0.23	14.89
5,6-dihydroxy-7-isopropyl-1,1-dimethyl-2,3-dihydroph enanthren-4-one	33.77	0.8	0.29	14.91
Dehydrotanshinone IIA	43.76	0.52	0.4	23.71
Sugiol	36.11	0.7	0.28	14.62
1 H-Cycloprop(e)azulen-7-ol,decahydro-1,1,7-trimethyl-4-methylene	82.33	1.49	0.12	12.04
1,2,5,6-tetrahydrotanshinone	38.75	0.39	0.36	18.05

Figure 1.

Prediction of tanshinone IIA as the effective component of salvia miltiorrhiza in DN and its structural formula. (a) The intersected targets of salvia miltiorrhiza/tanshinone IIA and DN. (b) pubChem analysis of the chemical structure of tanshinone IIA.

Prediction and validation of the effect of tanshinone IIA on TGFB1, the target of DN

The String database predicted the interaction of the targets and revealed that TGFB1 was in the regulatory node (Figure 2a). We further explored the relationship between the expression of TGFB1 and tanshinone IIA. Tanshinone IIA was demonstrated to downregulate the expression of TGFB1 (Figure 2b), indicating that tanshinone IIA may regulate DN progression by targeting TGFB1.

Figure 2.

Prediction and verification of the effect of tanshinone IIA on TGFB1. (a) String database predicted protein interactions. (b) Western blotting was used to detect the effect of tanshinone IIA on the expression of TGFB1 protein. **P< 0.01, ***P< 0.005.

High glucose induced renal tubular epithelial cell inflammation and cell death not through the apoptosis pathway

Next, we investigated the high glucose-induced injury on renal tubular epithelial cells. High glucose treatment significantly increased the mRNA levels of inflammatory cytokines TNF-α and IL-6 in HK-2 cells (Figure 3a). In addition, high glucose treatment significantly promoted HK-2 cell death (Figure 3b). We further explored the effect of high glucose on HK-2 cell apoptosis by detecting the activities of caspase-9 and caspase-3. There was no significant change in the activities of caspase-3, caspase-9, GRP78, CHOP, and cleaved caspase-12 in HK-2 cells after high glucose treatment (Figure 3c), suggesting that high glucose induces renal tubular epithelial cell death not through the apoptosis pathway.

Figure 3.

High glucose did not cause the apoptosis of renal tubular epithelial cells. (a) The levels of TNF-α, IL-6 were detected by RT-qPCR. (b) Flow cytometry was used to detect the effect of high glucose on HK-2 cell apoptosis. (c) Western blotting was used to detect the expression levels of apoptosis-related proteins in each group. **P< 0.01, ***P< 0.005.

High glucose induced renal tubular epithelial cell injury through pyroptosis

Since high glucose induced cell damage not through the apoptotic pathway, we speculated that other forms of cell death, for example, pyroptosis, plays a significant role. The production of pyrosomes causes the release of pro-inflammatory cytokines IL-1β and IL-18 dependent caspase-1, which leads to pyroptosis. From the results, we observed that the levels of IL-18 and IL-1β mRNA were significantly increased after high glucose treatment (Figure 4a). The protein expression of matured IL-1β was demonstrated to be significantly increased in high glucose-induced HK-2 cells, thereby promoting pyroptosis. Moreover, we also found the significant elevation in the protein levels of cleaved-caspase-1 and N-GSDMD in HK-2 cells treated with high glucose (Figure 4c). Pyrosome-activated GSDMD is known to be cleaved into N-GSDMD by caspase-1, resulting in an increase in N-GSDMD. According to the results of immunofluorescence, the expression of N-GSDMD showed evident elevation in HG treated HK-2 cells (Figure 4d), indicating that high glucose induced renal tubular epithelial cell pyroptosis.

Figure 4.

High glucose induced renal tubular epithelial cell damage by the pyroptosis pathway. (a) The levels of IL-1β and IL-18 were detected by RT-qPCR. (b) Western blotting was used to detect matured IL-1β expression level in each group. (c) Western blotting was used to detect the expression levels of caspase-1 and N-GSDMD proteins in each group. (d) The pyroptosis of renal tubular epithelial cells was detected by N-GSDMD and TUNEL immunofluorescence double staining. **P < 0.01, ***P< 0.005.

Tanshinone IIA inhibited the high glucose-stimulated damage of renal tubular epithelial cells

In order to explore the effect of tanshinone IIA on the high glucose induced damage of renal tubular epithelial cells, we used tanshinone IIA at concentrations of 1, 5, 10 μM to treat high glucose-cultured HK-2 cells, and tolbutamide was used as a positive control for its function in normal glucose regulation [22,23]. We found that tanshinone IIA significantly reduced cell death induced by high glucose and inhibited the mRNA expression of inflammatory factors TNF-α and IL-6 (Figures 5A-5b). Moreover, we also detected the expression of apoptosis-related proteins. The results of western blot showed that the protein expression of caspase-3, caspase-9, GRP78, CHOP, and cleaved caspase-12 in high glucose induced HK-2 cells was not significantly changed after the treatment of different concentrations of tanshinone IIA, which indicated that tanshinone IIA reversed the high glucose induced HK-2 cell death not via the regulation of apoptosis pathways (Figure 5c).

Figure 5.

The role of tanshinone IIA in hyperglycemia-induced HK-2 cell injury and its relationship with apoptosis pathway. High glucose-cultured HK-2 cells were treated with tanshinone IIA (1, 5, 10 μM) and tolbutamide. (a) The levels of TNF-α, IL-6 were detected by RT-qPCR. (b) Flow cytometry was used to detect the effect of tanshinone IIA on HK-2 cell apoptosis. (c) The protein expression of genes-related to cell apoptosis was detected using western blot. *P < 0.05, **P < 0.01, ***P< 0.005.

Tanshinone IIA inhibited the high glucose-induced damage of renal tubular epithelial cells through the pyroptosis pathway

We further explored the underlying mechanism of tanshinone IIA on high glucose-induced death of renal tubular epithelial cells. We found that higher concentration of tanshinone IIA had more inhibitory effect on the mRNA expression of IL-18 and IL-1β (Figures 6A). The matured protein levels of IL-1β was significantly decreased after the treatment of tanshinone IIA at 5 μM or 10 μM (Figure 6b). Tanshinone IIA also decreased the protein expression of cleaved-caspase-1 and N-GSDMD in a concentration-dependent way (Figure 6c). The results of immunofluorescence showed the decrease in N-GSDMD expression in high glucose-induced HK-2 cells (Figure 6d). These findings indicated that tanshinone IIA inhibited the high glucose-stimulated cell inflammation and cell death of renal tubular epithelial cells through the pyroptosis pathway.

Figure 6.

The role of tanshinone IIA in hyperglycemia-induced cell injury and its relationship with pyroptosis pathway. (a) The levels of IL-1β and IL-18 were detected by RT-qPCR. (b) Western blotting was used to detect matured IL-1β expression level. (c) Western blotting was used to detect the expression levels of caspase-1 and N-GSDMD proteins in each group. (d) The effect of tanshinone IIA on pyroptosis of HK-2 cells was detected by N-GSDMD and TUNEL immunofluorescence double staining. *P < 0.05, **P< 0.01, ***P< 0.005.

Tanshinone IIA downregulated TGFB1 to inhibit high glucose-induced cell damage through the pyroptosis pathway

Rescue assays were performed to explore the effect of TGFB1 overexpression or knockdown on tanshinone IIA treated DN cell models. By transfecting pcDNA-TGFB1 into cells, we further investigated whether TGFB1 was involved in the regulation of tanshinone IIA on high glucose-induced HK-2 cell damage. The results showed that TGFB1 overexpression significantly reversed the effect of tanshinone IIA on high glucose-injured HK-2 cells, while TGFB1 knockdown showed enhancement on the tanshinone IIA-induced inhibition on cell inflammation and cell death of high glucose-induced HK-2 cells (Figures 7A-c). The decreased expression levels of N-GSDMD induced by tanshinone IIA in high glucose-treated HK-2 cells were reversed by TGFB1 upregulation while enhanced by TGFB1 knockdown (Figure 7d). After transfection of pcDNA-TGFB1 or sh-TGFB1, the pyroptosis-related proteins changed significantly, while the apoptotic-related proteins did not show significant change (Figure 7e-f). TGFB1 is synthesized by tubular and epithelial cells, which can promote the ECM production through distinct intracellular signals [24]. The NF-κB pathway is involved in inflammatory response of HK-2 cells [25]. We further found that TGFB1 overexpression significantly reversed the effect of tanshinone IIA on ECM-related genes (FN and COL-1) and NF-κB pathway-related genes (p65, p-IKKβ, p-IκBα) in high glucose-treated HK-2 cells, while TGFB1 silencing exhibited opposite effects (Figure 7g-h). These results further indicated that tanshinone IIA targeted TGFB1 to inhibit high glucose-induced cell injury through pyroptosis pathway.

Figure 7.

The role of TGFB1 in the . (a-b) The levels of IL-1β, IL-18, TNF-α, IL-6 were detected by RT-qPCR. (c) Flow cytometry was used to detect the effect of TGFB1 on HK-2 cell apoptosis. (d) The effect of TGFB1 on pyroptosis of HK-2 cells was detected by N-GSDMD and TUNEL immunofluorescence double staining. (e) Western blotting was used to detect the expression levels of caspase-3 and caspase-9, caspase-1 and N-GSDMD proteins in each group. (f) The levels of FN and COL-1 were detected by RT-qPCR. (g) The levels of p65, p-IKKβ, p-IκBα in HK-2 cells under indicated transfections. **P< 0.01, ***P< 0.005, compared with HG group; ##P < 0.01, ###P < 0.005, compared with HG+tanshinone IIA+pcDNA-NC group; &P < 0.05 compared with the HG+tanshinone IIA+pcDNA-NC group.

Discussion

One of the most serious consequences of diabetes is diabetic angiopathy, which is characterized by microvascular and macrovascular complications [26,27]. As an acute microvascular complication, DN has become the extremely frequent predisposing factor of end-stage renal disease, increasing the public health burden all over the world [28]. DN is distinctive by proteinuria and progressive recession of renal function, with hyperglycemia as an independent cause. The pathophysiological changes of DN include inflammatory cell infiltration, renal tubular and glomerular hypertrophy, mesangial dilatation, fibrosis, extracellular matrix accumulation, cell dysfunction, and death [29-32].

Our study investigated the effective components of Salvia miltiorrhiza on DN. We found that tanshinone IIA may be the main active ingredient of Salvia miltiorrhiza for DN treatment. Next, we searched the targets of tanshinone IIA on DN and found that TGFB1 was on the regulatory node. TGFB1 was initially recognized as the TGF-β inducible gene [33] and involved in cell death of human renal tubular epithelial HK-2 cells [34]. Salvia Miltiorrhiza [35,36] or another component of Salvia Miltiorrhiza, Salvianolate [37], have been reported to inhibit TGFB1. In this study, we found that tanshinone IIA downregulated the expression of TGFB1 in HK-2 cells in a dose-dependent way, which was in consistent with the previous studies.

After renal tubular epithelial cells are injured, the inflammatory response is amplified mainly through the release of pro-inflammatory factors by cell pyroptosis [38,39]. In the classical pathway of pyroptosis, caspase-1 cleaves GSDMD and activates IL-1β and IL-18, while in the non-classical pathway, GSDMD is cleaved by caspase-11 [40]. Apoptosis of cells depends on caspase-3, caspase-8, and caspase-9 [41]. The difference is that the occurrence of pyroptosis is dependent on the activation of caspase-1, while apoptosis can occur in the absence of caspase-1. The classical pathway of pyroptosis is to activate caspase-1, which can induce cell membrane perforation and release intracellular substances, leading to inflammatory response [12,42]. Another feature of pyroptosis is cell rupture. After caspase-1 is activated, the released contents can cause inflammation. GSDMD is the direct and final executor of pyroptosis. After gene knocking out GSDMD, IL-1β, IL-18, and other pro-inflammatory mediators can hardly be secreted out of the cell [43,44]. GSDMD protein is composed of the N-terminal domain which can promote cell death and the C-terminal domain which can inhibit cell death. Under physiological conditions, GSDMD protein remains inactive due to the self-inhibition of C-terminal to N-terminal. Under the action of inflammatory signal, inflammatory caspase cleaves GSDMD, exposes its active N-terminal domain and triggers pyroptosis. Studies have revealed that after the GSDMD-N-terminal is exposed, the phospholipid structure inserted into the cell inner membrane forms small pores, and then water inflows, cell swells, and cell membrane ruptures, leading to the release of a great deal of pro-inflammatory mediators in the cell [45]. The previous studies have also revealed decreased GSDMD-N and caspase1 protein levels when cell pyroptosis was inhibited [46]. In this study, we also detected the up-regulation of caspase-1, and its expression trend is in compliance with N-GSDMD. However, the expression of caspase-3, caspase-9, GRP78, CHOP, and cleaved caspase-12 did not show significant change. Therefore, we speculated that GSDMD may induce pyroptosis under the regulation of caspase-1, which may mediate the occurrence and development of renal inflammation and eventually lead to renal function damage.

We further explored whether tanshinone IIA activates the immune response related to cell death. The result showed that compared with the control group, tanshinone IIA treatment did not change the apoptotic proteins, but significantly downregulated the expression of pyroptosis proteins caspase-1 and N-GSDMD. In addition, studies have shown that the release of mature IL-1β and IL-18 could cause inflammatory reaction in the process of pyroptosis [47,48]. In this study, after tanshinone IIA treatment, the expression levels of IL-1β, IL-18, caspase-1, and N-GSDMD were down-regulated. The above experimental results furnished strong demonstration to support that tanshinone IIA may play a role in HK-2 cells by regulating the pyroptosis.

Next, we investigated the underlying mechanism of tanshinone IIA to regulate HK-2 cell pyroptosis. We found that tanshinone IIA significantly reduced cell death and inhibited the expression of inflammatory factors TNF-α and IL-6. In pcDNA-TGFB1 transfected cells, the cleavage products of caspase-1, TNF-α, IL-6, IL-1β, IL-18 and GSDMD were upregulated, and the mRNA expression levels of ECM as well as NF-κB pathway-associated proteins were also upregulated, while in cells with silenced TGFB1, the expression levels of these genes showed opposite changes, which indicated that tanshinone IIA regulated TGFB1 to activate the pyroptosis pathway. Moreover, Tanshinone IIA has also been reported to inhibit the HK-2 fibrosis in a dose-dependent manner, and the dose of 50 µM showed the best treatment effect [49]. In our study, we found the dose-dependent inhibition of Tanshinone IIA on HK-2 cell inflammation and pyroptosis, and the dose at 10 µM exhibited best inhibitory effect in vitro.

However, although there are many promising preclinical studies, no toxicological studies and clinical trials report the protective effects of tanshinone IIA on DN. Further studies should be conducted based on the toxicological and pharmacokinetics aspects of tanshinone IIA on DN, which will assist in determining safe dosage and structural transformation of new drugs. Moreover, the biological effects of tanshinone IIA are expected to be explored in vivo in future, and mechanism underlying the interaction between tanshinone IIA and TGFB1 needs further research.

Conclusion

In conclusion, our research showed that tanshinone IIA inhibits high glucose-induced HK-2 cell pyroptosis and inflammation by downregulating TGFB1, and TGFB1 overexpression reverses the effect of tanshinone IIA. Tanshinone IIA is suggested as a feasible candidate for the treatment of DN. Our research proved the effect of tanshinone IIA on DN for the first time, which may provide a novel insight for the DN therapy.

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