Differential ubiquitination of Smad1 mediated by CHIP: implications in the regulation of the bone morphogenetic protein signaling pathway

Smad1, a downstream regulator of the bone morphogenetic protein (BMP) receptors, is tightly regulated by the ubiquitin-proteasomal degradation system. To dissect the mechanisms that underlie the regulation of Smad1, it is important to investigate the specific ubiquitination site(s) in Smad1. Here we report that the α-NH₂ group of the N terminus and the ε-NH₂ groups of internal lysine residues 116, 118 and 269 (K116, K118 and K269) of Smad1 are ubiquitin acceptor sites mediated by the carboxyl terminus of Hsc70-interacting protein (CHIP). The in vitro degradation assay indicates that ubiquitination at the N terminus partially contributes to the degradation of Smad1. Furthermore, we demonstrate that the ubiquitination level of pseudo-phosphorylated Smad1 by CHIP is stronger than that of wild-type Smad1 and can be strongly inhibited by a phosphorylated tail of Smad1, PIS(pS)V(pS). Third, our results indicate that Hsp70 facilitates CHIP-mediated poly-ubiquitination of Smad1 whereas it attenuates CHIP-meditated mono-ubiquitination of Smad1. Finally, consistent with the in vitro observation, we show that CHIP preferentially mediates the degradation of phospho-Smad1/5 in vivo. Taken together, these results provide us a hint that CHIP might preferentially regulate phosphorylated Smad1 and thus the BMP signaling.     

鳖甲育肝颗粒对复合因素所致肝纤维化大鼠TGF-β1/Smads通路的影响*

目的: 探讨鳖甲育肝颗粒对复合因素所致肝纤维化大鼠的治疗作用及其对TGF-β1/Smads信号通路的影响。方法: 将SD大鼠随机分为空白对照组、模型组、秋水仙碱组、鳖甲育肝颗粒各剂量组(1.85、3.70、7.40 g/kg, n=8),通过每天灌胃5%乙醇15 ml/kg的基础及每周皮下注射40%四氯化碳2次复制大鼠肝纤维化模型,连续42 d,观测鳖甲育肝颗粒对大鼠肝功能、肝指数及其含水量、血清肝纤维化相关指标,测定TGF-β1/Smads信号通路关键蛋白及基因表达的影响。结果: 与空白对照组比较,模型组大鼠血清ALT、AST、ALP、HA、PCⅢ、C-Ⅳ、LN活性,肝组织含水量及肝指数、TGF-β1、Smad3 mRNA与Smad7 mRNA表达均显著升高(P＜0.01)。与模型组比较,秋水仙碱不同剂量鳖甲育肝颗粒干预组大鼠上述血清肝功能及肝纤维化相关指标,肝组织含水量及肝指数,TGF-β1及Smad3 mRNA表达显著下降(P＜0.01),而Smad7 mRNA表达均显著升高,(P＜0.01)。结论: 鳖甲育肝颗粒具有明显的降酶保肝、抗肝纤维化的作用,而抑制TGF-β1/Smads信号通路是其抗肝纤维化的作用机制之一。     

Activin/Nodal signalling before implantation: setting the stage for embryo patterning

Activins and Nodal are members of the transforming growth factor beta (TGF-β) family of growth factors. Their Smad2/3-dependent signalling pathway is well known for its implication in the patterning of the embryo after implantation. Although this pathway is active early on at preimplantation stages, embryonic phenotypes for loss-of-function mutations of prominent components of the pathway are not detected before implantation. It is only fairly recently that an understanding of the role of the Activin/Nodal signalling pathway at these stages has started to emerge, notably from studies detailing how it controls the expression of target genes in embryonic stem cells. We review here what is currently known of the TGF-β-related ligands that determine the activity of Activin/Nodal signalling at preimplantation stages, and recent advances in the elucidation of the Smad2/3-dependent mechanisms underlying developmental progression.     

Antagonistic Effects of TGFβ1 and BMP-6 on Skin Keratinocyte Differentiation

Several members of the transforming growth factor β (TGFβ) superfamily are expressed in the developing murine epidermis. Among these are TGFβ1, which is found in the basal layer, and bone morphogenetic protein (BMP)-6, located in the suprabasal layers. Although the role of TGFβ in cell growth has been studied extensively, little is known about the effects of these factors on keratinocyte differentiation. This study demonstrates that BMP-6 acts to positively regulate the differentiation of primary skin keratinocytes grown in culture. In contrast, TGFβ1 antagonizes keratinocyte differentiation blocking the upregulation of keratin markers by BMP-6. We show that the effects of BMP-6 on expression of keratin 1 (K1), a marker of differentiation, requires signaling through the Smad pathway. In addition, regulation of K1 levels by BMP-6 is modulated by the SEK signaling pathway. This suggests that regulation of keratinocyte differentiation by BMP-6 involves multiple signaling systems.     

Augmenter of liver regeneration ameliorates renal fibrosis in rats with obstructive nephropathy

Renal fibrosis is a hallmark in CKD (chronic kidney disease) and is strongly correlated to the deterioration of renal function that is characterized by tubulointerstitial fibrosis, tubular atrophy, glomerulosclerosis and disruption of the normal architecture of the kidney. ALR (augmenter of liver regeneration) is a growth factor with biological functions similar to those of HGF (hepatocyte growth factor). In this study, our results indicate that endogenous ALR is involved in the pathological progression of renal fibrosis in UUO (unilateral ureteral obstruction) rat model. Moreover, we find that administration of rhALR (recombinant human ALR) significantly alleviates renal interstitial fibrosis and reduces renal-fibrosis-related proteins in UUO rats. Further investigation reveals that rhALR suppresses the up-regulated expression of TGF-β1 (transforming growth factor β1) induced by UUO operation in the obstructed kidney, and inhibits Smad2 and Smad3 phosphorylation activated by the UUO-induced injury in the animal model. Therefore we suggest that ALR is involved in the progression of renal fibrosis and administration of rhALR protects the kidney against renal fibrosis by inhibition of TGF-β/Smad activity.