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  • Considering that TGF plays a pivotal role in inflammation

    2021-10-13

    Considering that TGF-β1 plays a pivotal role in inflammation, renal fibrosis, cell growth, differentiation, and apoptosis, it has been viewed as a vital mediator in kidney disease [49,50]. Studies have demonstrated that TGF-β1 is upregulated in both human and experimental kidney diseases [51,52]. TGF-β1 also is up-regulated in glomerular diseases [53] and experimental and human diabetic nephropathy [54,55]. Increased carboxypeptidase of TGF-β1 and TGF-β1 receptors is a characteristics of virtually all human and experimental CKD [56,57]. In these studies, the authors have concluded that TGF-β1 has been implicated in the pathogenesis of renal fibrosis. The animal models involved include acute glomerulonephritis, diabetic nephropathy, and ureteral obstruction. Transgenic mice overexpressing active TGF-β1 develop progressive glomerulosclerosis and tubulointerstitial fibrosis, indicating that persistently increased TGF-β1 activity is sufficient to induce progressive renal disease in mice [[58], [59], [60]]. Consistent with the above findings, our results showed that the expression of TGF-β1 in the UUO mouse model was higher than in the control group. Moreover, treatment with FK228 showed downregulation of TGF-β1 compared to the untreated mice. These results strongly suggest that overexpression of TGF-β1 and renal fibrosis are closely related, which is consistent with previous studies [61,62]. Therefore, we examined the effect of FK228 on the TGF-β1 signaling pathways to further clarify the mechanism by which FK228 inhibits inflammatory cytokine and matrix protein expression. Inhibition of TGF-β1 signaling might serve as a novel mechanism by which FK228 attenuates renal fibrogenesis. TGF-β1 exhibits its biological and pathologic activities via Smad and non-Smad signaling pathways. Of these, Smad-dependent mechanisms have been well studied in many pathophysiological processes associated with TGF-β1 signaling pathways [63]. TGF-β1 activates its receptors, TGF-β receptor type I (TβRI) and TGF-β receptor II (TβRII), which transmits their signals from cell surface to nucleus. This results in the phosphorylation and activation of downstream signaling molecules Smad2/3. Phosphorylated Smad2/3 heterodimerizes with Smad 4, translocates to the nucleus and binds to target genes to regulate their transcription [64]. The present study have shown that diabetic Smad2 knockout mice displayed attenuated tubulointerstitial fibrosis, and decreased expression of Smad3 protein and TGF-β1. These findings desplay that the important role of Smad2 for pro-fibrotic TGF-β/Smad signaling during experimental diabetic nephropathy [65]. In addition to Smad pathways, TGF-β1 can also directly activate MAPKs [66]. The MAPK family comprises three main kinases: p38 MAPK, extracellular-signal regulated kinase (ERK) and c-Jun terminal kinase (JNK) [66]. MAP kinases interact with TGF-β/Smad signalling at several levels and these interactions are the most clearly established example of pathway crosstalk with TGF-β/Smad signalling. Several studies have reported that the ERK and P38 MAPK pathways are involved in renal tubular EMT, which is a considerable factor leading to renal fibrosis [67,68]. It has been reported that ERK participates in TGF-β1-induced EMT [67]. They have shown that epidermal growth factor (EGF) abrogates TGF-β1-induced collagen and α-SMA expression through EGFR/MEK/ERK activation in human tubular epithelial cells [67]. ERK activation can enhance TGF-β1-induced EMT in rat kidney epithelial cells, and ERK inhibition reduces the TGF-β1-induced EMT [69]. The ERK pathway also has a role in regulating cell proliferation, differentiation, survival, and apoptosis [70,71]. In addition, there is sufficient evidence that TGF-β1 signals through MAPKs and the activation of p38 MAPK is required in TGF-β1-induced EMT in human proximal tubular epithelial cells. It has been suggested that P38 MAPK functions as a component in the signaling of ECM induction by TGF-β1 [72]. The previous study has revealed that the physiologically optimal levels of Zn inhibit high glucose-induced EMT, most likely through inhibition of ROS, TGF-β1 production, and PI3K/Akt, ERK and p38 MAPK signaling pathways in NRK-52E cells [68]. A previous study has demonstrated that cyclosporin A activate JNK signaling in human renal epithelial cells and that JNK inhibition reduce the cyclosporin Ainduced E-cadherin downregulation and cell migration [73].