Hyperactivation of YAP has been commonly associated with tumorigenesis, and emerging evidence hints at multilayered Hippo-independent regulations of YAP. In this study, we identified a new MST4–YAP axis, which acts as a noncanonical Hippo signaling pathway that limits stress-induced YAP activation. MST4 kinase directly phosphorylated YAP at Thr83 to block its binding with importin α, therefore leading to YAP cytoplasmic retention and inactivation. Due to a consequential interplay between MST4-mediated YAP phospho-Thr83 signaling and the classical YAP phospho-Ser127 signaling, the phosphorylation level of YAP at Thr83 was correlated to that at Ser127. Mutation of T83E mimicking MST4-mediated alternative signaling restrained the activity of both wild-type YAP and its S127A mutant mimicking loss of classical Hippo signal. Depletion of MST4 in mice promoted gastric tumorigenesis with diminished Thr83 phosphorylation and hyperactivation of YAP. Moreover, loss of MST4–YAP signaling was associated with poor prognosis of human gastric cancer. Collectively, our study uncovered a noncanonical MST4–YAP signaling axis essential for suppressing gastric tumorigenesis.

Receptor tyrosine kinase (RTK) anaplastic lymphoma kinase (ALK) serves a crucial role in brain development. ALK is located on the short arm of chromosome 2 (2p23) and exchange of chromosomal segments with other genes, including nucleophosmin (NPM), echinoderm microtubule-associated protein-like 4 (EML4) and Trk-fused gene (TFG), readily occurs. Such chromosomal translocation results in the formation of chimeric X-ALK fusion oncoproteins, which possess potential oncogenic functions due to constitutive activation of ALK kinase. These proteins contribute to the pathogenesis of various hematological malignancies and solid tumors, including lymphoma, lung cancer, inflammatory myofibroblastic tumors (IMTs), Spitz tumors, renal carcinoma, thyroid cancer, digestive tract cancer, breast cancer, leukemia and ovarian carcinoma. Targeting of ALK fusion oncoproteins exclusively, or in combination with ALK kinase inhibitors including crizotinib, is the most common therapeutic strategy. As is often the case for small-molecule tyrosine kinase inhibitors (TKIs), drug resistance eventually develops via an adaptive secondary mutation in the ALK fusion oncogene, or through engagement of alternative signaling mechanisms. The updated mechanisms of a variety of ALK fusions in tumorigenesis, proliferation and metastasis, in addition to targeted therapies are discussed below.

Oncogenic KIT or PDGFRA receptor tyrosine kinase (RTK) mutations are compelling therapeutic targets in gastrointestinal stromal tumors (GIST), and treatment with the KIT/PDGFRA inhibitor imatinib is the standard of care for patients with metastatic GIST. Most GISTs eventually acquire imatinib resistance due to secondary mutations in the KIT kinase domain, but it is unclear whether these genomic resistance mechanisms require other cellular adaptations to create a clinically meaningful imatinib-resistant state. Using phospho-RTK and immunoblot assays, we demonstrate activation of KIT and insulin receptor (IR) in imatinib-resistant GIST cell lines (GIST430 and GIST48) and biopsies with acquisition of secondary mutations, but not in imatinib-sensitive GIST cells (GIST882 and GIST-T1). Treatment with linsitinib, a specific IR inhibitor, inhibited IR and downstream intermediates AKT, MAPK, and S6 in GIST430 and GIST48, but not in GIST882, exerting minimal effect on KIT phosphorylation in these cell lines. Additive effects showing increased apoptosis, antiproliferative effects, cell-cycle arrest, and decreased pAKT and pS6 expression, tumor growth, migration, and invasiveness were observed in imatinib-resistant GIST cells with IR activation after coordinated inhibition of IR and KIT by linsitinib (or shRNA) and imatinib, respectively, compared with either intervention alone. IGF2 overexpression was responsible for IR activation in imatinib-resistant GIST cells, whereas IR activation did not result from amplification, mutation, or KIT phosphorylation. Our findings suggest that combinatorial inhibition of IR and KIT warrants clinical evaluation as a novel therapeutic strategy in imatinib-resistant GISTs. .

The striatin-interacting phosphatase and kinase (STRIPAK) complexes integrate extracellular stimuli to result in intracellular activities. Previously, we discovered STRIPAK to be a key machinery responsible for loss of the Hippo tumor suppressor signal in cancer. Here, we identified the Hippo-STRIPAK complex to be an essential player for the control of DNA double-strand break (DSB) repair and genomic stability. Specifically, the MST1/2 kinases were found, independent of the classical Hippo signaling, to directly phosphorylate ZMYND8 and hence result in suppression of DNA repair in the nucleus. In response to genotoxic stress, the cGAS-STING pathway was determined to relay nuclear DNA damage signals to the dynamic assembly of Hippo-STRIPAK via a TBK1-induced structural stabilization of the SIKE1-SLMAP arm. As such, STRIPAK-mediated MST1/2 inactivation was found to increase the DSB repair capacity of cancer cells and to endow these cells with resistance to radio/chemotherapy and PARP inhibition. Importantly, targeting the STRIPAK assembly with each of three distinct peptide inhibitors efficiently recovered the kinase activity of MST1/2 to suppress DNA repair and re-sensitize cancer cells to PARPi in both animal and patientderived tumor models. Overall, our findings not only uncovered a previously unrecognized role for STRIPAK in modulating DSB repair, but also provided translational implications of co-targeting STRIPAK and PARP for a new type of synthetic lethality anti-cancer therapy.