Physiological and pathological functions of E3 ubiquitin ligases Smurfs

由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常伴随着肿瘤的发生和发展。我们发现Smurf 2在某些乳腺癌组织和细胞中上调。我们发现，在转移性乳腺癌细胞中用特异性短干扰RNA减少Smurf 2表达可诱导细胞变圆和肌动蛋白细胞骨架重组，这与运动性和侵袭性较低的表型相关。Smurf 2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向间充质转化、迁移和侵袭。此外，Smurf 2的E3连接酶缺陷型突变体Smurf 2CG的表达抑制了上述转移行为。这些结果确立了Smurf 2在乳腺癌进展中的重要作用，并表明Smurf 2是乳腺癌细胞迁移和侵袭的新型调节因子。 为了阐明Smurfs在TGF-β信号传导中的生理学意义，我们产生了缺乏Smurf 1或Smurf 2的小鼠，并报道了Smurf 1缺陷小鼠围产期正常，但由于成骨细胞活性增强和对BMP的反应性增加，表现出骨量的年龄依赖性增加。令人惊讶的是，这种骨骼异常不是由Smad介导的TGF-β或BMP信号转导的改变引起的。相反，Smurf 1的缺失导致成骨细胞中磷酸化MEKK 2的积累及其下游JNK信号级联的激活。我们的研究结果揭示了Smurf 1在成骨细胞生理学和骨稳态调节中的新功能，并为丝裂原活化蛋白激酶（MAPK）信号通路在形成对TGF-β家族细胞因子的特异性生物反应中的重要性提供了一个有趣的例子。目前，我们正在表征Smurf 2缺陷小鼠和Smurf 1/Smurf 2双缺陷小鼠的表型

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. Currently, we are characterizing the phenotypes of Smurf2 deficient mice and Smurf1/Smurf2 double deficient mice