Samuel Glady H, Bujor Andreea M, Nakerakanti Sashidhar S, Hant Faye N, Trojanowska Maria
Arthritis Center, Division of Rheumatology, Boston University Medical Campus, Boston, MA, USA.
Fibrogenesis Tissue Repair. 2010 Dec 6;3:25. doi: 10.1186/1755-1536-3-25.
During scleroderma (SSc) pathogenesis, fibroblasts acquire an activated phenotype characterized by enhanced production of extracellular matrix (ECM) and constitutive activation of several major signaling pathways including extracellular signal-related kinase (ERK1/2). Several studies have addressed the role of ERK1/2 in SSc fibrosis however the mechanism of its prolonged activation in SSc fibroblasts is still unknown. Protein phosphatase 2A (PP2A) is a key serine threonine phosphatase responsible for dephosphorylation of a wide array of signaling molecules. Recently published microarray data from cultured SSc fibroblasts suggests that the catalytic subunit (C-subunit) of PP2A is downregulated in SSc. In this study we examined the role and regulation of PP2A in SSc fibroblasts in the context of ERK1/2 phosphorylation and matrix production.
We show for the first time that PP2A mRNA and protein expression are significantly reduced in SSc fibroblasts and correlate with an increase in ERK1/2 phosphorylation and collagen expression. Furthermore, transforming growth factor β (TGFβ), a major profibrotic cytokine implicated in SSc fibrosis, downregulates PP2A expression in healthy fibroblasts. PP2A-specific small interfering RNA (siRNA) was utilized to confirm the role of PP2A in ERK1/2 dephosphorylation in dermal fibroblasts. Accordingly, blockade of autocrine TGFβ signaling in SSc fibroblasts using soluble recombinant TGFβ receptor II (SRII) restored PP2A levels and decreased ERK1/2 phosphorylation and collagen expression. In addition, we observed that inhibition of ERK1/2 in SSc fibroblasts increased PP2A expression suggesting that ERK1/2 phosphorylation also contributes to maintaining low levels of PP2A, leading to an even further amplification of ERK1/2 phosphorylation.
Taken together, these studies suggest that decreased PP2A levels in SSc is a result of constitutively activated autocrine TGFβ signaling and could contribute to enhanced phosphorylation of ERK1/2 and matrix production in SSc fibroblasts.
在硬皮病（SSc）发病机制中，成纤维细胞获得一种活化表型，其特征为细胞外基质（ECM）产生增加以及包括细胞外信号调节激酶（ERK1/2）在内的几种主要信号通路的组成性激活。多项研究探讨了ERK1/2在SSc纤维化中的作用，然而其在SSc成纤维细胞中持续激活的机制仍不清楚。蛋白磷酸酶2A（PP2A）是一种关键的丝氨酸苏氨酸磷酸酶，负责多种信号分子的去磷酸化。最近发表的来自培养的SSc成纤维细胞的微阵列数据表明，PP2A的催化亚基（C亚基）在SSc中下调。在本研究中，我们在ERK1/2磷酸化和基质产生的背景下研究了PP2A在SSc成纤维细胞中的作用和调节。
我们首次表明，PP2A mRNA和蛋白表达在SSc成纤维细胞中显著降低，且与ERK1/2磷酸化和胶原蛋白表达的增加相关。此外，转化生长因子β（TGFβ）是一种参与SSc纤维化的主要促纤维化细胞因子，可下调健康成纤维细胞中PP2A的表达。利用PP2A特异性小干扰RNA（siRNA）证实了PP2A在真皮成纤维细胞ERK1/2去磷酸化中的作用。因此，使用可溶性重组TGFβ受体II（SRII）阻断SSc成纤维细胞中的自分泌TGFβ信号可恢复PP2A水平，并降低ERK1/2磷酸化和胶原蛋白表达。此外，我们观察到抑制SSc成纤维细胞中的ERK1/2会增加PP2A表达，这表明ERK1/2磷酸化也有助于维持低水平的PPA，从而导致ERK1/2磷酸化进一步放大。
综上所述，这些研究表明，SSc中PP2A水平降低是自分泌TGFβ信号持续激活的结果，可能导致SSc成纤维细胞中ERK1/2磷酸化增强和基质产生增加。
