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起着至关重要的作用。泛素-蛋白酶体系统的失调常伴随肿瘤的发生和发展。我们发现Smurf2在某些乳腺癌组织和细胞中被上调。我们发现，在转移性乳腺癌细胞中，通过特异性短干扰RNA减少Smurf2的表达，可诱导细胞围圆和肌动蛋白细胞骨架的重组，这与移动性和侵袭性较低的表型相关。在裸鼠模型中，Smurf2过表达促进乳腺癌细胞转移，诱导乳腺癌细胞上皮向间质转移、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。这些结果证实了Smurf2在乳腺癌进展中的重要作用，并表明Smurf2是一种新的乳腺癌细胞迁移和侵袭调节因子。为了研究蓝精灵在tgf - β信号传导中的生理意义，我们培养了缺乏Smurf1或Smurf2的小鼠，并报道了缺乏Smurf1的小鼠在围生期正常，但由于成骨细胞活性增强和对BMP的反应性增强，其骨量表现出年龄依赖性增加。令人惊讶的是，这种骨骼异常不是由smad介导的tgf - β或BMP信号的改变引起的。相反，Smurf1的缺失会导致成骨细胞中磷酸化的MEKK2的积累，并激活其下游的JNK信号级联。我们的研究结果揭示了Smurf1在调节成骨细胞生理和骨稳态中的新功能，并为丝裂原活化蛋白激酶（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基因敲除小鼠的研究澄清了文献中关于Smurf调控tgf - β信号传导的矛盾报道，表明Smurf2确实具有抑制作用，但其作用是通过单泛素化降低Smad3活性，而不是像之前报道的那样促进其降解。我们的研究还意外发现了以前未被认识到的Smurf2作为肿瘤抑制因子的功能，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.