Physiological and pathological functions of E3 ubiquitin ligases Smurfs

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Smad ubiquitin regulatory factors (Smurfs) are members of the HECT family of E3 ubiquitin ligases. Two Smurfs, Smurf1 and Smurf2, were identified based on their activities to modulate TGF-beta/BMP signaling by promoting ubiquitin modification on Smads; however, subsequent studies have expanded the repertoire of Smurf substrates to include proteins such as RhoA, Runx2 and MEKK2 outside the TGF-beta/BMP pathway. To address the physiological function of Smurfs, we have generated mice lacking either Smurf1 or Smurf2. We found that these two paralogous E3 ubiquitin ligases have both common and unique functions during embryogenesis and in maintaining adult physiological homeostasis. Our earlier work on characterizing Smurf1 knockout mice revealed a novel mechanism in regulating osteoblast function and bone homeostasis, suggesting that targeting Smurf1 may prove to be an effective strategy for treating age-related bone losses in osteoporosis. Our subsequent 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. Because loss of both Smurfs leads to embryonic lethality, we have created a conditional Smurf2 knockout allele, which enabled us to investigate physiological functions of Smurfs by completely removal of all Smurf activities in desired tissues using tissue-specific expression of cre recombinase and obtain conditional Smurf null cells from late stages embryos or adult tissues. Using Smurf double knockout cells, we identified Smurfs as the E3 ligases that control endocytic turnover of Sonic hedgehog (Shh) receptor Patched1 via ubiquitin modification. We demonstrated that this regulation is crucial for the activation of Shh signaling both in cell culture assays and in sustaining the Shh-dependent proliferation of cerebellar granule neuron precursors. Our study of Smurf2 knockout mice 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. We found that Smurf2 interacts with a group of proteins that regulate DNA replication, chromatin remodeling and histone modification. We are currently validating these findings using in vitro and in vivo approaches and to investigate the biological relevance of these interactions in genomic stability and tumorigenesis. Consistent with the tumor suppresser function of Smurf2, we found that Smurf2 expression is decreased in the nucleus but increased in the cytoplasm of breast cancer cells. Smurf1 expression is also upregulated in the cytoplasm of the breast cancer cells. Overexpression of Smurf1 and Smurf2 promotes metastasis in nude mouse models and induces epithelial-to-mesenmchymal transition, migration, and invasion of breast cancer cells, suggesting that Smurfs play important roles in breast cancer progression. We are currently studying the mechanism underpinning the role of Smurf1 and Smurf2 promoting cancer cell migration and invasion.