miR-29c Inhibits Renal Interstitial Fibrotic Proliferative Properties through PI3K-AKT Pathway.

Renal fibrosis, in particular tubulointerstitial fibrosis, which is characterized by an increased extracellular matrix (ECM) formation and development in the interstitium, is the common end pathway for nearly all progressive kidney disorders. One of the sources for this matrix is the epithelial to mesenchymal transition (EMT) from the tabular epithelium. The driving force behind it is some profibrotic growth factors such as transforming growth factor-β (TGF-β) which is responsible for the formation of collagen in renal fibrosis. miR-29c, which is an antifibrotic microRNA, downregulates renal interstitial fibrosis by downregulating the TGF-β and collagen. However, it is not known whether miR-29c mediates the TGF-β1-driven PI3K-Akt pathway and Col-1 triggering within NRK-52E cultures. The main objective of this investigation was to examine the influence of miR-29c on the downregulation of the TGF-β1-driven PI3K-Akt pathway and Col-1 triggering in NRK-52E cultures. This study revealed that miR-29c inhibited TGF-β1 expression in NRK-52E cell cultures. Overexpression of miR-29c significantly inhibits NRK-52E culture proliferation mediated by TGF-β1. miR-29c inhibited the expression of Col-1 and decreased PI3K/Akt phosphorylation. These findings revealed a novel mechanism by which miR29c inhibits the proliferation of renal interstitial fibrotic cultures by downregulating the PI3k-Akt pathway, which is controlled by TGF-β1.

1. Introduction

Renal interstitial fibrosis (RIF) plays a significant role in chronic renal disorders and persists throughout the progression of the majority of kidney diseases [1]. Renal fibrosis has no effective treatment, yet its prevalence has gradually increased [2]. Novel treatment options for renal fibrosis may be discovered by studying more about the complex mechanisms and cellular factors that contribute to this disease [3-5]. Consequently, the preservation of the parenchymal functional/structural state is essential for protecting against organ fibrosis [6].

TGF-β has several cellular functions, including the pathogenesis of tissue fibrosis in RIF [7, 8]. TGF-β activates the classical TGF-β/Smad signaling pathway for fibroblast initiation [9, 10]. Furthermore, TGF-β triggers the TGF-β/PI3K-AKT signaling pathway, which promotes fibroblast proliferation in RIF [8, 10, 11]. Noncanonical (non-smad) TGF-β effectors, including c-Abl, PAK2, Akt, mTOR, and tuberin (TSC2), are triggered during early/rapid renal fibrogenesis in obstructive nephropathy. Treatment with mTOR or c-Abl inhibitors, rapamycin or imatinib mesylate, respectively, blocks noncanonical TGF-β pathways and lowers interstitial fibroblasts, myofibroblasts, and ECM protein accumulation. Combinatorial inhibition of critical regulators over these non-smad TGF-β networks is therefore an effective treatment for renal fibrogenesis [8].

MicroRNA is a small regulatory RNA that regulates genomic expression by posttranscriptional inhibition/targeted mRNA instability [12]. miRNAs are essential gene regulators implicated in numerous organ models of fibrosis, including cardiac [13, 14], renal [15], and hepatic fibrosis [16], according to findings from previous studies. miR-29, an antifibrotic microRNA, downregulates RIF in human and rat cells [17]. It is known that microRNA-29c downregulates collagen type 1 (Col-1)/type 3 (Col-3) in NRK-49F cultures and in murine with unilateral ureteral obstruction (UUO) [18]. miR-29c was significantly downregulated in UUO nephropathic renal tubular epithelial cultures or TGF-β1-exposed kidney tubular epithelial cells (NRK-52E) cultures. miR-29c knockdown can fully enhance the production of specificity protein 1 (Sp1), but an ectopic expression of miR-29c in NRK-52E cultures considerably reduces the expression of Sp1 triggered by TGF-β1 [19]. However, it is unknown whether miR-29c mediates the TGF-β1-driven PI3K-Akt pathway and the activation of Col-1 in NRK-52E cultures.

The influence of TGF-β1 on the miR-29c expression profile in NRK-52E cultures was examined in this study. Following that, we investigated if miR-29c affects the proliferation of NRK-52E cultures that is triggered by TGF-β1. Furthermore, we examined whether miR-29c influences the PI3K-AKT signaling pathway to regulate the Col-1 expression profile in NRK-52E cultures, to exert miR-29c's function in RIF cultured proliferative property/ECM synthesis.

2. Materials and Methods

2.1. Culture/TGF-β1 Assays

Kidney culture line (NRK-52E) was obtained through ATCC (USA). NRK-52E cultures were grown in Dulbecco's Modified Eagle Medium (DMEM) with 10% fetal bovine serum (FBS) and streptomycin (100 μg/ml; Invitrogen™) and penicillin (100 units/ml) at 37°C and CO2 (5%). Human recombinant TGF-β1 was acquired from PeproTech™. Cultures (40% confluence) were exposed to recombinant human TGF-β1 (10 ng/ml, R&D Systems™) or negative control in DMEM for as long as required.

2.2. Culture Transfection

Cultures were seeded in complete medium with 10% FBS in 6-well culture plates (24 h until 60-70% confluency). To synchronize culture development, cultures were incubated for one day in serum-free media 24 hours before exposure.

All primers for miR-29c detection assays were obtained from Shanghai GenePharma™ Co., Ltd. The transfection was carried out in accordance with the kit's protocols.

2.3. Real-Time Polymerase Chain Reaction (Real-Time PCR)

Total RNA was collected from cultures/tissue using TRIzol® (Invitrogen™, USA) according to kit protocols. Consequently, cDNAs were prepared from total RNA using the Prime-Script® RT reagent kit (TaKaRa™, Japan).

U6 was employed as a reference for detecting miR-29c. The outcomes of the dataset were evaluated employing the ΔCt methodology. All primers were fabricated using Invitrogen™. Complete dataset outcomes reflected the mean ± SD of three separate assay runs.

2.4. Western Blotting (WB)

Protein lysates were developed and exposed to sodium dodecyl sulfate–polyacrylamide gel electrophoresis (SDS/PAGE), transported over polyvinylidene difluoride (PVDF) membranes, and blotted through standardized techniques, employing anti-collagen I antibody (1 : 100) (Millipore, USA) or anti-PI3K, anti-AKT, anti-phospho-AKT antibodies, anti-phospho-PI3K (1 : 100) (Abzoom Biolabs, USA). Anti-GAPDH antibodies (1 : 1000) (Abzoom Biolabs, USA) were used as a normalization control to ensure equivalent proteomic loading levels.

2.5. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium Bromide (MTT) Assay

MTT assays were utilized to evaluate the effect of miR-29c/TGF-β1 (10 ng/ml) on cellular proliferation. Cultures (8 × 103 cultures/well) were plated in 96-well plates and maintained in DMEM with FBS (10%). The culture medium was withdrawn after 3 days of culturing. MTT reagent (Sigma, USA, 5 mg/ml PBS) was injected into individual wells, incubated for 2 hours for color development, and after that, the formazan crystals that had grown were dissolved with DMSO. At 570 nm, the absorbance was measured using a microplate reader. Individual assays were used to quantify the cellular content of three wells per cohort.

2.6. Soft Agar Assay

500 cells were placed into 0.5 ml of growth medium containing agar (0.35%) and layered across a 0.5% agar base to avoid anchorage-dependent culture growths. After layer solidification overlaid on 1 ml of standardized growth medium and refreshed every 48 hours. Viable colonies were considered as having more than 50 cultures or being greater than 100 mm in size. After imaging, qualified colony quantification was performed after 14-21 days.

2.7. Statistical Analyses

All of the analyses were performed using SPSS13.0® for Windows®. Student's t-tests were utilized to assess statistically significant differences between the control and study cohorts, and one-way ANOVA was employed to compare multiple cohorts. The post hoc Bonferroni test (post-ANOVA) was used to determine significant differences between cohorts. P values < 0.05 were deemed to confer statistical significance.

3. Results

3.1. TGF-β1 Treatment Downregulated miR-29c within NRK-52E Cultures

TGF-β1 is responsible for the epithelial to mesenchymal transition (EMT) from the tubular epithelium, and α-smooth muscle actin (α-SMA) is a typical protein marker for activated fibroblasts [20, 21]. Therefore, α-SMA was employed to verify the TGF-β1-driven EMT model. In the first place, this work demonstrated the appropriate concentration of TGF-β1. After a one-day stimulation with different concentrations of TGF-β1, the content of α-SMA protein and RNA that was present in NRK-52E cultures was analyzed. According to Figures 1(a) and 1(b), the expression of α-SMA increased gradually from 1 to 20 ng/ml of TGF-β1, demonstrating a significant dose-effect relationship (P < 0.01). However, at 20 ng/ml of TGF-β1, the expression increased slightly from the concentration of 10 ng/ml; therefore, the concentration of 10 ng/ml was chosen as the ideal TGF-β1 dose. The optimal TGF-β1 effect time was then determined. Figures 1(c) and 1(d) show a threefold increase in α-SMA in the 24 h group compared to the control cohort (P < 0.01), indicating that 24 h was chosen as the optimal TGF-β1 effect time. The ideal TGF-β1 dose and effect time for the EMT model was effectively produced by activating NRK-52E cells with TGF-β1 (10 ng/ml) for 24 hours. The expression of miR-29c was then evaluated in the TGF-β1-driven EMT model. Following one day of culture exposure to TGF-β1 at 10 ng/ml (Figure 1(e), P < 0.05), miR-29c exhibited downregulation of 50%.

Figure 1

TGF-β1 treatment downregulated miR-29c within NRK-52E cultures. (a) Selected the optimal TGF-β1 concentration (24 h stimulation) by WB. (b) Selected the optimal TGF-β1 concentration (24 h stimulation) by RT-PCR. (c) Selected the optimal effect time of TGF-β1 at 10 ng/ml by WB. (d) Selected the optimal effect time of TGF-β1 at 10 ng/ml by RT-PCR. (e) The miR-29c expression in the EMT model driven by TGF-β1. ∗∗P < 0.01 and ∗P < 0.05.

3.2. miR-29c Thwarted TGF-β1-Driven Proliferative Property for NRK-52E Cultures

MTT assays were performed to analyze the culture proliferation. TGF-β1 promotes proliferation rates in NRK-52E cells (P < 0.05) as depicted in Figure 2(a). In the meantime, miR-29c inhibited NRK-52E culture proliferation (P < 0.01). Thus, TGF-β1 stimulation was added to the cultures with miR-29c overexpression. Through miR-29c, proliferation rates of NRK-52E cells were reduced following TGF-β1 exposure (P < 0.01). It was demonstrated that miR-29c suppressed the proliferation of NRK-52E cells induced by TGF-β1.

Figure 2

MiR-29c inhibited TGF-β1-driven proliferation of NRK-52E cultures. NRK-52E cultures that were treated with negative controls (NRK-52E), miR-29c mimics (miR-29c), TGF-β1 (TGF-β1), or both (TGF-β1+miR-29c). (a) Cultures were introduced into 96-well plates and grown for 24, 48, or 72 hours, evaluated through MTT assays. (b) A soft agar medium was used to grow the cultures for 14-21 days before crystal violet staining/imaging was performed. The number of colonies reflected colony percentages. ∗P < 0.05 and ∗∗P < 0.01. (c) RT-qPCR dataset outcomes for miR-29c expression profiles in NRK-52E cultures transfected with miR-29c mimic. ∗P < 0.05.

Soft agar colony formation assays are commonly used to assess culture transformation in vitro [22]. This test was applied to confirm miR-29c's inhibitory activity in vitro. miR-29c levels were drastically higher in miR-29c overexpressing cultures (Figure 2(c)). NRK-52E cultures and miR-29 overexpressing cultures were plated onto soft agar plates and permitted to form colonies for 14-21 days with/without TGF-β1 exposure. The results matched the MTT assay perfectly. TGF-β1 stimulated the growth of NRK-52E colonies (Figure 2(b), P < 0.05). In the meantime, miR-29c inhibited the growth of NRK-52E colonies (Figure 2(b), P < 0.01). Then, after TGF-β1 treatment, miR-29c overexpressing cultures generated considerably fewer colonies than vector-transfected cultures, demonstrating the proliferation suppressor role of miR-29c (Figure 2(b), P < 0.01).

3.3. miR-29c Activated the PI3K-AKT Pathway in NRK-52E Cultures

miR-29 family activation/expression is intimately associated with TGF-β1-driven PI3K-Akt pathway activation in human lung fibroblasts, according to several previous studies [23, 24]. However, it remained unclear whether miR-29c mediates TGF-β1-driven PI3K-Akt pathway activation in NRK-52E cultures. The total protein and expression profiles of phosphorylated PI3K/Akt of overexpressing miR-29c cells and control were examined in this study after they were subjected to TGF-β1. As indicated in Figure 3, miR-29c overexpression inhibited TGF-β1-induced PI3K-AKT phosphorylation. P-PI3K transcriptomic and p-Akt proteomic levels were drastically downregulated, although the PI3K and Akt total protein levels were not altered in any of the samples.

Figure 3

miR-29c activated the PI3K-AKT pathway in NRK-52E cultures. (a) The negative control and overexpressing miR-29c cultures were exposed to TGF-β1, and then, the total protein and phosphorylation of PI3K/Akt expression profiles were evaluated through WB. (b) Negative control and overexpressed miR-29c cultures were exposed to TGF-β1, with PI3K relative expression subsequently evaluated through RT-qPCR. (c) Negative control and overexpressing miR-29ccultures were exposed to TGF-β1, with PI3K relative expression subsequently evaluated through RT-qPCR. (d) The negative control and overexpressing miR-29ccultures were exposed to TGF-β1, with AKT relative expression subsequently evaluated through RT-qPCR. (e) The negative control and overexpressing miR-29ccultures were exposed to TGF-β1, and then, AKT phosphorylation relative expression was subsequently evaluated through RT-qPCR. ∗P < 0.05 and ∗∗P < 0.01.

3.4. miR-29c Downregulated Col-1 through the PI3K-AKT Pathway

As one of the ECM components, Col-1 is the target gene of the miR-29 family in liver fibrosis [25] and cardiac fibrosis [14]. However, it remained unknown whether Col-1 is the target gene of miR-29c in NRK-52E cultures. Therefore, NRK-52E cultures were treated with or without LY294002 (a phosphorylation inhibitor of PI3K-Akt) in miR-29c-inhibited cultures. Transient transfection of miR-29c inhibitor into NRK-52E cell cultures to determine whether it was successfully transfected. miR-29c inhibitor significantly downregulates miR-29c, according to the results of a dataset (Figure 4(c)). Figures 4(a) and 4(b) demonstrate Col-1 upregulation detected in cellular lysates treated with miR-29c inhibitor (P < 0.01). The upregulation of Col-1 by miR-29c inhibitor decreased following treatment with LY294002 (P < 0.01). The results of these datasets revealed that miR-29c inhibited Col-1 via lowering PI3K-Akt phosphorylation levels, hence reducing the synthesis of ECM.

Figure 4

miR-29c reduced the expression of Col-1 through the PI3K-AKT pathway. (a) With/without LY294002 (a phosphorylation inhibitor of PI3K-Akt) treatment, NRK-52E cultures grown with a miR-29 inhibitor were analyzed using WB. (b) With/without LY294002 exposure, NRK-52E cultures were examined by RT-PCR after culturing with concomitant exposure to miR-29 inhibitor. ∗∗P < 0.01. (c) RT-qPCR dataset outcomes regarding profiles of miR-29c expression in miR-29c inhibitor-transfected NRK-52E cultures. ∗P < 0.05.

4. Discussion

The results of such datasets shed light on a novel molecular mechanism in which miR-29c is crucial in the induction of early-phase damage in renal tissue. Damaged epithelial cultures may exhibit more heterozygous phenotypes with incomplete EMT procedures in organ fibrosis [26]. Proliferation pressure and other related stress-inducing factors, such as macrophage-derived TGF-β1, are experienced by RIF cultures that have been damaged [27]. RIF cultures initiate host repair/regeneration responses in response to damage by activating the PI3K-Akt pathway. TGF-β1 triggered the PI3K-Akt pathway, which increased collagen I levels in human mesangial cultures [28]. This response results in basement membrane modification [27, 29]. In this investigation, it was discovered that miR-29c inhibits the activation pathway of PI3K-Akt, suppresses the expression of Col-1gene, prevents matrix reconstruction, and reduces the proliferation of cultured cells (Figure 5). Downregulation of miR-29c has been observed in TGF-β1-stimulated tubular epithelial cells and fibrotic kidneys. Furthermore, miR-29c overexpression inhibits the Col-1 expression caused by TGF-β1, whereas knockdown of miR-29c is capable of enhancing Col-1 expression [19]. Knockdown of miR-29c in UUO murine kidneys dramatically enhanced tubulointerstitial fibrosis intensity, as determined by α-SMA, whereas miR-29c overexpression could enhance the renal condition by inhibiting renal fibrosis development, according to previous studies [30]. Currently, animal studies and relevant pathological findings are being conducted in our laboratory. Preliminary findings from animal studies indicate that miR-29c can inhibit renal fibrosis.

Figure 5

A model with miR-29c implications in NRK-52E cultures. Macrophage-generated TGF-β1 inhibits the miR-29c expression and consequently activates the PI3K-Akt pathway, which then increases the Col-1 expression, which reconstructs the matrix and promotes the culture proliferation in RIF. Such a process plays a crucial part in the etiology of renal fibrosis and miR-29c suppresses this process.

TGF-β1 can downregulate miR-29c; however, this indicates that it affects miR-29c levels directly. In addition to the miR-29c-driven ECM formation via the PI3K-AKT pathway, additional pathways, including the Smad, Wnt, and MAPK pathways, are also connected with this mechanism and contribute to the human kidney fibroblasts proliferation. Thus, the disordered miR-29c expression may be a result of the disruption of numerous crucial signaling pathways associated with renal fibrosis. Additional investigations are required to determine the intermediate signaling pathway/s between ligand binding to TGF-β1 and miR-29c, as well as alternate pathways for miR-29c-mediated ECM production and matrix reconstruction.

miR-29a, miR-29b, and miR-29c were found to have antifibrotic properties. Through proteomic downregulation of the TGF-β pathway, this miRNA family ameliorates renal fibrosis. TGF-β-induced suppression of miR-29 expression is regulated by Smad3 signaling triggers. Smad3 binds to an SBE in the miR-29 promoter region and regulates its transcription rate in cultured proximal tubular epithelial cells (PTECs) and fibroblasts, as well as post-UUO murine kidneys in vivo. In contrast, Smad3-deficient mice exhibited miR-29 overexpression and ameliorated renal fibrosis in UUO murine models. According to the most recent studies, TGF-β1 regulates the expression of miR-29c via Wnt/β-catenin signaling. miR-29c could serve as a vital fibrosis-linked microRNA generated by fibroblasts in renal fibrosis induced by TGF-β1/Wnt/β-catenin [30-32]. Based on these dataset results, this study hypothesized that miR-29c could inhibit RIF induced by TGF-β1 via the PI3K-AKT signaling pathway, which is confirmed in culture studies.

5. Conclusion

In conclusion, our study reveals that TGF-β1 activated the matrix reconstruction by the PI3K-Akt pathway, and this activation was important for the proliferation of human renal fibroblasts. miR-29c plays a pivotal role in inhibiting the PI3K-AKT pathway and subsequent Col-1 expression, culture proliferation, and synthesis of ECM. The current study's findings suggest that miR-29c downregulation can induce an abnormal increase in RIF matrix reconstruction and play a key role in the pathogenesis of renal fibrosis. These dataset results further increase the potential value of tailored PI3K-Akt cascade inhibitors, as they may give favorable effects for avoiding pathogenic fibrosis in human renal fibroblasts.