TGF-β/Smad signaling in renal fibrosis

TGF-β (transforming growth factor-β) is well identified as a central mediator in renal fibrosis. TGF-β initiates canonical and non-canonical pathways to exert multiple biological effects. Among them, Smad signaling is recognized as a major pathway of TGF-β signaling in progressive renal fibrosis. During fibrogenesis, Smad3 is highly activated, which is associated with the down-regulation of an inhibitory Smad7 via an ubiquitin E3-ligases-dependent degradation mechanism. The equilibrium shift between Smad3 and Smad7 leads to accumulation and activation of myofibroblasts, overproduction of ECM (extracellular matrix), and reduction in ECM degradation in the diseased kidney. Therefore, overexpression of Smad7 has been shown to be a therapeutic agent for renal fibrosis in various models of kidney diseases. In contrast, another downstream effecter of TGF-β/Smad signaling pathway, Smad2, exerts its renal protective role by counter-regulating the Smad3. Furthermore, recent studies demonstrated that Smad3 mediates renal fibrosis by down-regulating miR-29 and miR-200 but up-regulating miR-21 and miR-192. Thus, overexpression of miR-29 and miR-200 or down-regulation of miR-21 and miR-192 is capable of attenuating Smad3-mediated renal fibrosis in various mouse models of chronic kidney diseases (CKD). Taken together, TGF-β/Smad signaling plays an important role in renal fibrosis. Targeting TGF-β/Smad3 signaling may represent a specific and effective therapy for CKD associated with renal fibrosis.

Suppression of Smad-1 mRNA expression level by Smad-2 likely control dichotomy of NF-κB and Smads mediated activation

The aim of this study was to find out how NF-κB and Smad-mediated signaling influenced the expression of astrogliogenic versus neurogenic markers of brain development in U4C cells which were either enriched (Tg Jak-1) or deprived in Jak-1 molecule (Jak-1 KO). Genetically modified U4C cells were transfected with NF-kB reporter plasmid in order to follow its activation when cells were cotransfected with different combinations of Smads constructs. In wild type cells no significant activation of NF-κB was observed while genetically modified cells exhibited somewhat different pattern of NF-κB activation depending on the Smad constructs combination used. The absence of NF-κB activation in Jak-1 transgenic cells transfected with Smad-1 plus Smad-3 was accompanied by the appearance of apoptotic cells as revealed by DAPI staining. Smad-1 expression was undetectable in Jak-1 transgenic cells and was downregulated in wild type cells upon transfection with Smad-2. The absence of p65 nuclear translocation in Smad-2 transfected cells and the presence of Smad-4 in nucleus of the same cells indicates dichotomy in NF-κB and Smads mediated signaling pathways. The significance of this study is that helps to elucidate the point of collaboration among three different signaling pathways - Jak-1 mediated cytokine signaling, NF-κB and Smads mediated pathways.