Physiological and pathological functions of E3 ubiquitin ligases Smurfs

E3 泛素连接酶介导的受控蛋白质降解在调节广泛的细胞反应中发挥着至关重要的作用。泛素-蛋白酶体系统的失调通常伴随着肿瘤的发生和进展。我们发现 Smurf2 在某些乳腺癌组织和细胞中表达上调。 我们发现，在转移性乳腺癌细胞中，用特定的短干扰 RNA 减少 Smurf2 表达会诱导细胞变圆和肌动蛋白细胞骨架重组，这与活动性和侵袭性较低的表型相关。 Smurf2的过表达促进裸鼠模型中的转移并诱导乳腺癌细胞的上皮间质转化、迁移和侵袭。 此外，Smurf2 E3 连接酶缺陷突变体 Smurf2CG 的表达抑制了上述转移行为。 这些结果确立了 Smurf2 在乳腺癌进展中的重要作用，并表明 Smurf2 是乳腺癌细胞迁移和侵袭的新型调节因子。 为了解决 Smurfs 在 TGF-β 信号传导中的生理意义，我们培育了缺乏 Smurf1 或 Smurf2 的小鼠，并报告说，Smurf1 缺陷的小鼠围产期正常，但由于成骨细胞活性增强和对 BMP 的反应性增加，表现出年龄依赖性骨量增加。 令人惊讶的是，这种骨骼异常并不是由 Smad 介导的 TGF-β 或 BMP 信号传导改变引起的。 相反，Smurf1 的缺失会导致成骨细胞中磷酸化 MEKK2 的积累，并激活其下游 JNK 信号级联。 我们的结果揭示了 Smurf1 在调节成骨细胞生理学和骨稳态中的新功能，并为丝裂原激活蛋白激酶 (MAPK) 信号通路在形成对 TGF-β 细胞因子家族的特异性生物反应中的重要性提供了一个有趣的例子。 我们最近对 Smurf2 敲除小鼠进行表征的工作澄清了文献中关于 Smurf 调节 TGF-β 信号转导的相互矛盾的报道，表明 Smurf2 确实具有抑制作用，但它是通过单泛素化减弱 Smad3 活性而不是像之前报道的那样促进其降解。我们的研究还意外地发现了 Smurf2 作为肿瘤抑制因子的一种先前未被认识的功能，该功能通常通过 RNF20 控制组蛋白修饰的表观遗传景观来维持基因组稳定性。

Controlled protein degradation mediated by E3 ubiquitin ligases plays a crucial role in modulating a broad range of cellular responses. Dysregulation of the ubiquitin-proteasome system often accompanies tumorigenesis and progression. We found that Smurf2 is up-regulated in certain breast cancer tissues and cells. We show that reduction of Smurf2 expression with specific short interfering RNA in metastatic breast cancer cells induces cell rounding and reorganization of the actin cytoskeleton that is associated with a less motile and invasive phenotype. Overexpression of Smurf2 promotes metastasis in the nude mouse model and induces epithelial-to-mesenmchymal transition, migration and invasion of breast cancer cells. Moreover, expression of an E3 ligase-defective mutant of Smurf2, Smurf2CG, suppressed the above metastatic behaviors. These results establish an important role for Smurf2 in breast cancer progression and indicate that Smurf2 is a novel regulator of breast cancer cell migration and invasion. To address the physiological significance of Smurfs in TGF-beta signaling, we have generated mice lacking either Smurf1 or Smurf2, and reported that Smurf1-deficient mice are perinatally normal but exhibit an age-dependent increase of bone mass due to enhanced osteoblast activity and increased responsiveness to BMP. Surprisingly, this skeletal abnormality is not caused by alteration in Smad-mediated TGF-beta or BMP signaling. Instead, loss of Smurf1 results in accumulation of phosphorylated MEKK2 in osteoblasts and activation of its downstream JNK signaling cascade. Our results reveal a novel function of Smurf1 in the regulation of osteoblast physiology and bone homeostasis, and provide an interesting example for the importance of the mitogen-activated protein kinase (MAPK) signaling pathway in shaping specific biological response to the TGF-beta family of cytokines. Our recent work on characterizing Smurf2 knockout mice clarified contradictory reports in the literature about Smurf regulation of TGF-beta signaling by showing that Smurf2 indeed has an inhibitory role, but it does so by attenuating Smad3 activity through mono-ubiquitination rather than promoting its degradation as previously reported. Our study also led to an unexpected discovery of a previously unrecognized function of Smurf2 as a tumor suppressor that normally maintains genomic stability by controlling epigenetic landscape of histone modifications via RNF20.