Extracellular matrix stiffness regulates degradation of MST2 via SCF βTrCP

Identification of a novel alternative splicing isoform of the Hippo kinase STK3/MST2 with impaired kinase and cell growth suppressing activities

Mammalian Ste-20-like Kinases 1 and 2 (MST1/2) are core serine-threonine kinases of the Hippo pathway regulating several cellular processes, including cell cycle arrest and cell death. Here, we discovered a novel alternative splicing variant of the MST2 encoding gene, STK3, in malignant cells and tumor datasets. This variant, named STK3∆7 or MST2∆7 (for mRNA or protein, respectively), resulted from the skipping of exon 7. MST2∆7 exhibited increased ubiquitylation and interaction with the E3 ubiquitin-protein ligase CHIP compared to the full-length protein (MST2FL). Exon 7 in STK3 encodes a segment within the kinase domain, and its exclusion compromised MST2 interaction with and phosphorylation of MOB, a major MST1/2 substrate. Nevertheless, MST2∆7 was capable of interacting with MST1 and MST2FL. Unlike MST2FL, overexpression of MST2∆7 did not lead to increased cell death and growth arrest. Strikingly, we observed the exclusion of STK3 exon 7 in 3.2-15% of tumor samples from patients of several types of cancer, while STK3∆7 was seldomly found in healthy tissues. Our study identified a novel STK3 splicing variant with loss of function and the potential to disturb tissue homeostasis by impacting on MST2 activities in the regulation of cell death and quiescence.

Cortical tension promotes Kibra degradation via Par-1

The Hippo pathway is an evolutionarily conserved regulator of tissue growth. Multiple Hippo signaling components are regulated via proteolytic degradation. However, how these degradation mechanisms are themselves modulated remains unexplored. Kibra is a key upstream pathway activator that promotes its own ubiquitin-mediated degradation upon assembling a Hippo signaling complex. Here, we demonstrate that Hippo complex-dependent Kibra degradation is modulated by cortical tension. Using classical genetic, osmotic, and pharmacological manipulations of myosin activity and cortical tension, we show that increasing cortical tension leads to Kibra degradation, whereas decreasing cortical tension increases Kibra abundance. Our study also implicates Par-1 in regulating Kib abundance downstream of cortical tension. We demonstrate that Par-1 promotes ubiquitin-mediated Kib degradation in a Hippo complex-dependent manner and is required for tension-induced Kib degradation. Collectively, our results reveal a previously unknown molecular mechanism by which cortical tension affects Hippo signaling and provide novel insights into the role of mechanical forces in growth control.

Smurf1 polyubiquitinates on K285/K282 of the kinases Mst1/2 to attenuate their tumor-suppressor functions

Sterile 20-like kinases Mst1 and Mst2 (Mst1/2) and large tumor suppressor 1/2 are core kinases to mediate Hippo signaling in maintaining tissue homeostasis. We have previously demonstrated that Smad ubiquitin (Ub) regulatory factor 1 (Smurf1), a HECT-type E3 ligase, ubiquitinates and in turn destabilizes large tumor suppressor 1/2 to induce the transcriptional output of Hippo signaling. Here, we unexpectedly find that Smurf1 interacts with and polyubiquitinates Mst1/2 by virtue of K27- and K29-linked Ub chains, resulting in the proteasomal degradation of Mst1/2 and attenuation of their tumor-suppressor functions. Among the potential Ub acceptor sites on Mst1/2, K285/K282 are conserved and essential for Smurf1-induced polyubiquitination and degradation of Mst1/2 as well as transcriptional output of Hippo signaling. As a result, K285R/K282R mutation of Mst1/2 not only negates the transcriptional output of Hippo signaling but enhances the tumor-suppressor functions of Mst1/2. Together, we demonstrate that Smurf1-mediated polyubiquitination on K285/K282 of Mst1/2 destabilizes Mst1/2 to attenuate their tumor-suppressor functions. Thus, the present study identifies Smurf1-mediated ubiquitination of Mst1/2 as a hitherto uncharacterized mechanism fine-tuning the Hippo signaling pathway and may provide additional targets for therapeutic intervention of diseases associated with this important pathway.

Beta-Transducin Repeats-Containing Proteins as an Anticancer Target

Beta-transducin repeat-containing proteins (β-TrCPs) are E3-ubiquitin-ligase-recognizing substrates and regulate proteasomal degradation. The degradation of β-TrCPs' substrates is tightly controlled by various external and internal signaling and confers diverse cellular processes, including cell cycle progression, apoptosis, and DNA damage response. In addition, β-TrCPs function to regulate transcriptional activity and stabilize a set of substrates by distinct mechanisms. Despite the association of β-TrCPs with tumorigenesis and tumor progression, studies on the mechanisms of the regulation of β-TrCPs' activity have been limited. In this review, we studied publications on the regulation of β-TrCPs themselves and analyzed the knowledge gaps to understand and modulate β-TrCPs' activity in the future.