Monitoring Hippo Signaling Pathway Activity Using a Luciferase-based Large Tumor Suppressor (LATS) Biosensor

Role of Yes-Associated Protein in Psoriasis and Skin Tumor Pathogenesis

Psoriasis and skin tumors (such as basal cell carcinoma, squamous cell carcinoma, and melanoma) are chronic diseases that endanger physical and mental health, and yet the causes are largely unknown and treatment options limited. The development of targeted drugs requires a better understanding of the exact pathogenesis of these diseases, and Yes-associated protein (YAP), a member of the Hippo signaling pathway, is believed to play an important role. Psoriasis and skin tumors are characterized by excessive cell proliferation, abnormal differentiation, vasodilation, and proliferation. Here, we review the literature related to YAP-associated disease mechanisms and discuss the latest research. YAP regulates cell apoptosis, proliferation, and differentiation; inhibits cell density and intercellular contacts and angiogenesis; and maintains the three-dimensional structure of the skin. These mechanisms may be associated with the occurrence and development of psoriasis and skin tumors. The results of recent studies have shown that YAP expression is increased in psoriasis and skin tumors. High expression of YAP in psoriasis and skin tumors may indicate its positive functions in skin inflammation and malignancies and may play an important role in disease pathogenesis. The study of new drugs targeting YAP can provide novel approaches for the treatment of skin diseases.

Angiogenesis in Chronic Inflammatory Skin Disorders

Angiogenesis, the growth of new blood vessels from preexisting vessels, is associated with inflammation in various pathological conditions. Well-known angiogenetic factors include vascular endothelial growth factor (VEGF), angiopoietins, platelet-derived growth factor, transforming growth factor-β, and basic fibroblast growth factor. Yes-associated protein 1 (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ) have recently been added to an important angiogenic factor. Accumulating evidence indicates associations between angiogenesis and chronic inflammatory skin diseases. Angiogenesis is deeply involved in the pathogenesis of psoriasis. VEGF, angiopoietins, tumor necrosis factor-a, interleukin-8, and interleukin-17 are unregulated in psoriasis and induce angiogenesis. Angiogenesis may be involved in the pathogenesis of atopic dermatitis, and in particular, mast cells are a major source of VEGF expression. Angiogenesis is an essential process in rosacea, which is induced by LL-37 from a signal cascade by microorganisms, VEGF, and MMP-3 from mast cells. In addition, angiogenesis by increased VEGF has been reported in chronic urticaria and hidradenitis suppurativa. The finding that VEGF is expressed in inflammatory skin lesions indicates that inhibition of angiogenesis is a useful strategy for treatment of chronic, inflammatory skin disorders.

Hippo Signaling Pathway as a Central Mediator of Receptors Tyrosine Kinases (RTKs) in Tumorigenesis

The Hippo pathway plays a critical role in tissue and organ growth under normal physiological conditions, and its dysregulation in malignant growth has made it an attractive target for therapeutic intervention in the fight against cancer. To date, its complex signaling mechanisms have made it difficult to identify strong therapeutic candidates. Hippo signaling is largely carried out by two main activated signaling pathways involving receptor tyrosine kinases (RTKs)—the RTK/RAS/PI3K and the RTK-RAS-MAPK pathways. However, several RTKs have also been shown to regulate this pathway to engage downstream Hippo effectors and ultimately influence cell proliferation. In this text, we attempt to review the diverse RTK signaling pathways that influence Hippo signaling in the context of oncogenesis.

Targeting Hippo-Dependent and Hippo-Independent YAP1 Signaling for the Treatment of Childhood Rhabdomyosarcoma

Rhabdomyosarcoma is the most common childhood soft-tissue sarcoma, yet patients with metastatic or recurrent disease continue to do poorly, indicating a need for new treatments. The SRC family tyrosine kinase YES1 is upregulated in rhabdomyosarcoma and is necessary for growth, but clinical trials using single agent dasatinib, a SRC family kinase inhibitor, have failed in sarcomas. YAP1 (YES-associated protein) is highly expressed in rhabdomyosarcoma, driving growth and survival when the upstream Hippo tumor suppressor pathway is silenced, but efforts to pharmacologically inhibit YAP1 have been unsuccessful. Here we demonstrate that treatment of rhabdomyosarcoma with DNA methyltransferase inhibitor (DNMTi) upregulates Hippo activators RASSF1 and RASSF5 by promoter demethylation, activating canonical Hippo signaling and increasing inactivation of YAP1 by phosphorylation. Treatment with DNMTi decreased rhabdomyosarcoma cell growth and increased apoptosis and differentiation, an effect partially rescued by expression of constitutively active YAP (S127A), suggesting the effects of DNMTi treatment are, in part, due to Hippo-dependent inhibition of YAP1. In addition, YES1 and YAP1 interacted in the nucleus of rhabdomyosarcoma cells, and genetic or pharmacologic suppression of YES1 resulted in cytoplasmic retention of YAP1 and decreased YAP1 target gene expression, suggesting YES1 regulates YAP1 in a Hippo-independent manner. Combined treatment with DNMTi and dasatinib targeted both Hippo-dependent and Hippo-independent regulation of YAP1, ablating rhabdomyosarcoma cell growth in vitro and trending toward decreased tumor growth in vivo. These results show that the mechanisms regulating YAP1 in rhabdomyosarcoma can be inhibited by combinatorial therapy of DNMTi and dasatinib, laying the groundwork for future clinical investigations. SIGNIFICANCE: This study elucidates the signaling pathways that regulate the oncogenic protein YAP1 and identifies a combination therapy to target these pathways in the childhood tumor rhabdomyosarcoma.

Identification of Celastrol as a Novel YAP-TEAD Inhibitor for Cancer Therapy by High Throughput Screening with Ultrasensitive YAP/TAZ-TEAD Biosensors

The Hippo pathway has emerged as a key signaling pathway that regulates a broad range of biological functions, and dysregulation of the Hippo pathway is a feature of a variety of cancers. Given this, some have suggested that disrupting the interaction of the Hippo core component YAP and its paralog TAZ with transcriptional factor TEAD may be an effective strategy for cancer therapy. However, there are currently no clinically available drugs targeting the YAP/TAZ–TEAD interaction for cancer treatment. To facilitate screens for small molecule compounds that disrupt the YAP–TEAD interaction, we have developed the first ultra-bright NanoLuc biosensor to quantify YAP/TAZ–TEAD protein–protein interaction (PPI) both in living cells and also in vitro using biosensor fusion proteins purified from bacteria. Using this biosensor, we have performed an in vitro high throughput screen (HTS) of small molecule compounds and have identified and validated the drug Celastrol as a novel inhibitor of YAP/TAZ–TEAD interaction. We have also demonstrated that Celastrol can inhibit cancer cell proliferation, transformation, and cell migration. In this study, we describe a new inhibitor of the YAP/TAZ–TEAD interaction warranting further investigation and offer a novel biosensor tool for the discovery of other new Hippo-targeting drugs in future work.

A gain-of-functional screen identifies the Hippo pathway as a central mediator of receptor tyrosine kinases during tumorigenesis

The Hippo pathway has emerged as a key signaling pathway that regulates various biological functions. Dysregulation of the Hippo pathway has been implicated in a broad range of human cancer types. While a number of stimuli affecting the Hippo pathway have been reported, its upstream kinase and extracellular regulators remain largely unknown. Here we performed the first comprehensive gain-of-functional screen for receptor tyrosine kinases (RTKs) regulating the Hippo pathway using an RTK overexpression library and a Hippo signaling activity biosensor. Surprisingly, we found that the majority of RTKs could regulate the Hippo signaling activity. We further characterized several of these novel relationships [TAM family members (TYRO3, AXL, METRK), RET, and FGFR family members (FGFR1 and FGFR2)] and found that the Hippo effectors YAP/TAZ are central mediators of the tumorigenic phenotypes (e.g., increased cell proliferation, transformation, increased cell motility, and angiogenesis) induced by these RTKs and their extracellular ligands (Gas6, GDNF, and FGF) through either PI3K or MAPK signaling pathway. Significantly, we identify FGFR, RET, and MERTK as the first RTKs that can directly interact with and phosphorylate YAP/TAZ at multiple tyrosine residues independent of upstream Hippo signaling, thereby activating their functions in tumorigenesis. In conclusion, we have identified several novel kinases and extracellular stimuli regulating the Hippo pathway. Our findings also highlight the pivotal role of the Hippo pathway in mediating Gas6/GDNF/FGF-TAM/RET/FGFR-MAPK/PI3K signaling during tumorigenesis and provide a compelling rationale for targeting YAP/TAZ in RTK-driven cancers.

A kinome-wide screen using a NanoLuc LATS luminescent biosensor identifies ALK as a novel regulator of the Hippo pathway in tumorigenesis and immune evasion

The Hippo pathway is an emerging signaling pathway that plays important roles in organ size control, tissue homeostasis, tumorigenesis, metastasis, drug resistance, and immune response. Although many regulators of the Hippo pathway have been reported, the extracellular stimuli and kinase regulators of the Hippo pathway remain largely unknown. To identify novel regulars of the Hippo pathway, in this study we created the first ultra-bright NanoLuc biosensor (BS) to monitor the activity of large tumor suppressor (LATS) kinase 1, a central player of the Hippo pathway. We show that this NanoLuc BS achieves significantly advanced sensitivity and stability both in vitro using purified proteins and in vivo in living cells and mice. Using this BS, we perform the first kinome-wide screen and identify many kinases regulating LATS and its effectors yes-associated protein (YAP) and transcriptional co-activator with PDZ- binding motif (TAZ). We also show for the first time that activation of receptor tyrosine kinase anaplastic lymphoma kinase (ALK) by its extracellular ligand family with sequence similarity (FAM)150 activates Hippo effector YAP/TAZ by increasing their nuclear translocation. Significantly, we show that constitutively active ALK induces tumorigenic phenotypes, such as increased cancer cell proliferation/colony formation via YAP/TAZ and elevated immune evasion via YAP/TAZ-programmed death-ligand 1 in breast and lung cancer cells. In summary, we have developed a new LATS BS for cancer biology and therapeutics research and uncovered a novel ALK-LATS-YAP/TAZ signaling axis that may play important roles in cancer and possibly other biologic processes.-Nouri, K., Azad, T., Lightbody, E., Khanal, P., Nicol, C. J., Yang, X. A kinome-wide screen using a NanoLuc LATS luminescent biosensor identifies ALK as a novel regulator of the Hippo pathway in tumorigenesis and immune evasion.

The Role of YAP and TAZ in Angiogenesis and Vascular Mimicry

Angiogenesis, the formation of new blood vessels from pre-existing vasculature, is a physiological process that begins in utero and continues throughout life in both good health and disease. Understanding the underlying mechanism in angiogenesis could uncover a new therapeutic approach in pathological angiogenesis. Since its discovery, the Hippo signaling pathway has emerged as a key player in controlling organ size and tissue homeostasis. Recently, new studies have discovered that Hippo and two of its main effectors, Yes-associated protein (YAP) and its paralog transcription activator with PDZ binding motif (TAZ), play critical roles during angiogenesis. In this review, we summarize the mechanisms by which YAP/TAZ regulate endothelial cell shape, behavior, and function in angiogenesis. We further discuss how YAP/TAZ function as part of developmental and pathological angiogenesis. Finally, we review the role of YAP/TAZ in tumor vascular mimicry and propose directions for future work.