Bone is constantly resorbed and formed throughout life by coordinated actions of osteoclasts and osteoblasts. Here we show that Smurf1, a HECT domain ubiquitin ligase, has a specific physiological role in suppressing the osteogenic activity of osteoblasts. Smurf1-deficient mice are born normal but exhibit an age-dependent increase of bone mass. The cause of this increase can be traced to enhanced activities of osteoblasts, which become sensitized to bone morphogenesis protein (BMP) in the absence of Smurf1. However, loss of Smurf1 does not affect the canonical Smad-mediated intracellular TGFbeta or BMP signaling; instead, it leads to accumulation of phosphorylated MEKK2 and activation of the downstream JNK signaling cascade. We demonstrate that Smurf1 physically interacts with MEKK2 and promotes the ubiquitination and turnover of MEKK2. These results indicate that Smurf1 negatively regulates osteoblast activity and response to BMP through controlling MEKK2 degradation.

The Suppressor of Fused [Su(fu)] protein plays a conserved role in the regulation of Gli transcription factors of the hedgehog (Hh) signaling pathway that controls cell fate and tissue patterning during development. In both Drosophila and mammals, Su(fu) represses Gli-mediated transcription, but the mode of its action is not completely understood. Recent evidence suggests that Su(fu) physically interacts with the Gli proteins and, when overexpressed, sequesters Gli in the cytoplasm. However, Su(fu) also traverses into the nucleus under the influence of a serine-threonine kinase, Fused (Fu), and has the ability to form a DNA-binding complex with Gli, suggesting that it has a nuclear function. Here we report that the mouse homolog of Su(fu) [mSu(fu)] specifically interacts with SAP18, a component of the mSin3 and histone deacetylase complex. In addition, we demonstrate that mSu(fu) functionally cooperates with SAP18 to repress transcription by recruiting the SAP18-mSin3 complex to promoters containing the Gli-binding element. These results provide biochemical evidence that Su(fu) directly participates in modulating the transcriptional activity of Gli.

In addition to allelic mutations, cancers are known to harbor alterations in their chromatin landscape. Here, we show that genomic ablation of Smurf2, a HECT-domain E3 ubiquitin ligase, results in dysregulation of DNA damage response and genomic stability, culminating to increased susceptibility to various types of cancers in aged mice. We demonstrate that Smurf2 regulates histone H2B monoubiquitination as well as histone H3 tri-methylation at K4 and K79 by targeting RNF20 to proteasomal degradation in both mouse and human cells. We further show that Smurf2 and RNF20 are co-localized at the γ-H2AX foci of double-stranded DNA breaks in the nucleus. Thus, Smurf2 has a tumor suppression function that normally maintains genomic stability by controlling the epigenetic landscape of histone modifications through RNF20.