Induction of AP-1 by YAP/TAZ contributes to cell proliferation and organ growth.

Yes-associated protein (YAP) and its homolog transcriptional coactivator with PDZ-binding motif (TAZ) are key effectors of the Hippo pathway to control cell growth and organ size, of which dysregulation yields to tumorigenesis or hypertrophy. Upon activation, YAP/TAZ translocate into the nucleus and bind to TEAD transcription factors to promote transcriptional programs for proliferation or cell specification. Immediate early genes, represented by AP-1 complex, are rapidly induced and control later-phase transcriptional program to play key roles in tumorigenesis and organ maintenance. Here, we report that YAP/TAZ directly promote FOS transcription that in turn contributes to the biological function of YAP/TAZ. YAP/TAZ bind to the promoter region of FOS to stimulate its transcription. Deletion of YAP/TAZ blocks the induction of immediate early genes in response to mitogenic stimuli. FOS induction contributes to expression of YAP/TAZ downstream target genes. Genetic deletion or chemical inhibition of AP-1 suppresses growth of YAP-driven cancer cells, such as Lats1/2-deficient cancer cells as well as Gαq/11 mutated uveal melanoma. Furthermore, AP-1 inhibition almost completely abrogates the hepatomegaly induced by YAP overexpression. Our findings reveal a feed-forward interplay between immediate early transcription of AP-1 and Hippo pathway function.

Two Hippo signaling modules orchestrate liver size and tumorigenesis.

The Hippo pathway is a central regulator of organ size and tumorigenesis and is commonly depicted as a kinase cascade, with an increasing number of regulatory and adaptor proteins linked to its regulation over recent years. Here, we propose that two Hippo signaling modules, MST1/2-SAV1-WWC1-3 (HPO1) and MAP4K1-7-NF2 (HPO2), together regulate the activity of LATS1/2 kinases and YAP/TAZ transcriptional co-activators. In mouse livers, the genetic inactivation of either HPO1 or HPO2 module results in partial activation of YAP/TAZ, bile duct hyperplasia, and hepatocellular carcinoma (HCC). On the contrary, inactivation of both HPO1 and HPO2 modules results in full activation of YAP/TAZ, rapid development of intrahepatic cholangiocarcinoma (iCCA), and early lethality. Interestingly, HPO1 has a predominant role in regulating organ size. HPO1 inactivation causes a homogenous YAP/TAZ activation and cell proliferation across the whole liver, resulting in a proportional and rapid increase in liver size. Thus, this study has reconstructed the order of the Hippo signaling network and suggests that LATS1/2 and YAP/TAZ activities are finetuned by HPO1 and HPO2 modules to cause different cell fates, organ size changes, and tumorigenesis trajectories.

Airway secretory cell fate conversion via YAP-mTORC1-dependent essential amino acid metabolism.

Tissue homeostasis requires lineage fidelity of stem cells. Dysregulation of cell fate specification and differentiation leads to various diseases, yet the cellular and molecular mechanisms governing these processes remain elusive. We demonstrate that YAP/TAZ activation reprograms airway secretory cells, which subsequently lose their cellular identity and acquire squamous alveolar type 1 (AT1) fate in the lung. This cell fate conversion is mediated via distinctive transitional cell states of damage-associated transient progenitors (DATPs), recently shown to emerge during injury repair in mouse and human lungs. We further describe a YAP/TAZ signaling cascade to be integral for the fate conversion of secretory cells into AT1 fate, by modulating mTORC1/ATF4-mediated amino acid metabolism in vivo. Importantly, we observed aberrant activation of the YAP/TAZ-mTORC1-ATF4 axis in the altered airway epithelium of bronchiolitis obliterans syndrome, including substantial emergence of DATPs and AT1 cells with severe pulmonary fibrosis. Genetic and pharmacologic inhibition of mTORC1 activity suppresses lineage alteration and subepithelial fibrosis driven by YAP/TAZ activation, proposing a potential therapeutic target for human fibrotic lung diseases.

TRAF6-mediated ubiquitination of MST1/STK4 attenuates the TLR4-NF-κB signaling pathway in macrophages.

Pattern-recognition receptors including Toll-like receptors (TLRs) recognize invading pathogens and trigger an immune response in mammals. Here we show that mammalian ste20-like kinase 1/serine/threonine kinase 4 (MST1/STK4) functions as a negative regulator of lipopolysaccharide (LPS)-induced activation of the TLR4-NF-κB signaling pathway associated with inflammation. Myeloid-specific genetic ablation of MST1/STK4 increased the susceptibility of mice to LPS-induced septic shock. Ablation of MST1/STK4 also enhanced NF-κB activation triggered by LPS in bone marrow-derived macrophages (BMDMs), leading to increased production of proinflammatory cytokines by these cells. Furthermore, MST1/STK4 inhibited TRAF6 autoubiquitination as well as TRAF6-mediated downstream signaling induced by LPS. In addition, we found that TRAF6 mediates the LPS-induced activation of MST1/STK4 by catalyzing its ubiquitination, resulting in negative feedback regulation by MST1/STK4 of the LPS-induced pathway leading to cytokine production in macrophages. Together, our findings suggest that MST1/STK4 functions as a negative modulator of the LPS-induced NF-κB signaling pathway during macrophage activation.

Mechanical cue-induced YAP instructs Skp2-dependent cell cycle exit and oncogenic signaling.

Mechanical tensions are usually generated during development at spatially defined regions within tissues. Such physical cues dictate the cellular decisions of proliferation or cell cycle arrest. Yet, the mechanisms by which mechanical stress controls the cell cycle are not yet fully understood. Here, we report that mechanical cues function upstream of Skp2 transcription in human breast cancer cells. We found that YAP, the mechano-responsive oncogenic Hippo signaling effector, directly promotes Skp2 transcription. YAP inactivation induces cell cycle exit (G0) by down-regulating Skp2, causing p21/p27 to accumulate. Both Skp2 reconstitution and p21/p27 depletion can rescue the observed defect in cell cycle progression. In the context of a tissue-mimicking 3D culture system, Skp2 inactivation effectively suppresses YAP-driven oncogenesis and aberrant stiff 3D matrix-evoked epithelial tissue behaviors. Finally, we also found that the expression of Skp2 and YAP is positively correlated in breast cancer patients. Our results not only reveal the molecular mechanism by which mechanical cues induce Skp2 transcription, but also uncover a role for YAP-Skp2 oncogenic signaling in the relationship between tissue rigidity and cancer progression.

MRTF potentiates TEAD-YAP transcriptional activity causing metastasis.

Yes-associated protein (YAP) and myocardin-related transcription factor (MRTF) play similar roles and exhibit significant crosstalk in directing transcriptional responses to chemical and physical extracellular cues. The mechanism underlying this crosstalk, however, remains unclear. Here, we show MRTF family proteins bind YAP via a conserved PPXY motif that interacts with the YAP WW domain. This interaction allows MRTF to recruit NcoA3 to the TEAD-YAP transcriptional complex and potentiate its transcriptional activity. We show this interaction of MRTF and YAP is critical for LPA-induced cancer cell invasion in vitro and breast cancer metastasis to the lung in vivo We also demonstrate the significance of MRTF-YAP binding in regulation of YAP activity upon acute actin cytoskeletal damage. Acute actin disruption induces nucleo-cytoplasmic shuttling of MRTF, and this process underlies the LATS-independent regulation of YAP activity. Our results provide clear evidence of crosstalk between MRTF and YAP independent of the LATS kinases that normally act upstream of YAP signaling. Our results also suggest a mechanism by which extracellular stimuli can coordinate physiological events downstream of YAP.

YAP and TAZ Negatively Regulate Prox1 During Developmental and Pathologic Lymphangiogenesis.

RATIONALE: The Hippo pathway governs cellular differentiation, morphogenesis, and homeostasis, but how it regulates these processes in lymphatic vessels is unknown. OBJECTIVE: We aimed to reveal the role of the final effectors of the Hippo pathway, YAP (Yes-associated protein) and TAZ (transcriptional coactivator with PDZ-binding motif), in lymphatic endothelial cell (LEC) differentiation, morphogenesis, and homeostasis. METHODS AND RESULTS: During mouse embryonic development, LEC-specific depletion of Yap/Taz disturbed both plexus patterning and valve initiation with upregulated Prox1 (prospero homeobox 1). Conversely, LEC-specific YAP/TAZ hyperactivation impaired lymphatic specification and restricted lymphatic sprouting with profoundly downregulated Prox1. Notably, lymphatic YAP/TAZ depletion or hyperactivation aggravated or attenuated pathological lymphangiogenesis in mouse cornea. Mechanistically, VEGF (vascular endothelial growth factor)-C activated canonical Hippo signaling pathway in LECs. Indeed, repression of PROX1 transcription by YAP/TAZ hyperactivation was mediated by recruitment of NuRD (nucleosome remodeling and histone deacetylase) complex and endogenous binding activity of TEAD (TEA domain family members) to the PROX1 promoter. Furthermore, YAP/TAZ hyperactivation enhanced MYC signaling and inhibited CDKN1C, leading to cell cycle dysregulation and aberrant proliferation. CONCLUSIONS: We find that YAP and TAZ play promoting roles in remodeling lymphatic plexus patterning and postnatal lymphatic valve maintenance by negatively regulating Prox1 expression. We further show that YAP and TAZ act as plastic regulators of lymphatic identity and define the Hippo signaling-mediated PROX1 transcriptional programing as a novel dynamic checkpoint underlying LEC plasticity and pathophysiology.

Hippo Deficiency Leads to Cardiac Dysfunction Accompanied by Cardiomyocyte Dedifferentiation During Pressure Overload.

RATIONALE: The Hippo pathway plays an important role in determining organ size through regulation of cell proliferation and apoptosis. Hippo inactivation and consequent activation of YAP (Yes-associated protein), a transcription cofactor, have been proposed as a strategy to promote myocardial regeneration after myocardial infarction. However, the long-term effects of Hippo deficiency on cardiac function under stress remain unknown. OBJECTIVE: We investigated the long-term effect of Hippo deficiency on cardiac function in the presence of pressure overload (PO). METHODS AND RESULTS: We used mice with cardiac-specific homozygous knockout of WW45 (WW45cKO), in which activation of Mst1 (Mammalian sterile 20-like 1) and Lats2 (large tumor suppressor kinase 2), the upstream kinases of the Hippo pathway, is effectively suppressed because of the absence of the scaffolding protein. We used male mice at 3 to 4 month of age in all animal experiments. We subjected WW45cKO mice to transverse aortic constriction for up to 12 weeks. WW45cKO mice exhibited higher levels of nuclear YAP in cardiomyocytes during PO. Unexpectedly, the progression of cardiac dysfunction induced by PO was exacerbated in WW45cKO mice, despite decreased apoptosis and activated cardiomyocyte cell cycle reentry. WW45cKO mice exhibited cardiomyocyte sarcomere disarray and upregulation of TEAD1 (transcriptional enhancer factor) target genes involved in cardiomyocyte dedifferentiation during PO. Genetic and pharmacological inactivation of the YAP-TEAD1 pathway reduced the PO-induced cardiac dysfunction in WW45cKO mice and attenuated cardiomyocyte dedifferentiation. Furthermore, the YAP-TEAD1 pathway upregulated OSM (oncostatin M) and OSM receptors, which played an essential role in mediating cardiomyocyte dedifferentiation. OSM also upregulated YAP and TEAD1 and promoted cardiomyocyte dedifferentiation, indicating the existence of a positive feedback mechanism consisting of YAP, TEAD1, and OSM. CONCLUSIONS: Although activation of YAP promotes cardiomyocyte regeneration after cardiac injury, it induces cardiomyocyte dedifferentiation and heart failure in the long-term in the presence of PO through activation of the YAP-TEAD1-OSM positive feedback mechanism.

Hippo Pathway Kinase Mst1 Is Required for Long-Lived Humoral Immunity.

The protein kinase Mst1 is a key component of the evolutionarily conserved Hippo pathway that regulates cell survival, proliferation, differentiation, and migration. In humans, Mst1 deficiency causes primary immunodeficiency. Patients with MST1-null mutations show progressive loss of naive T cells but, paradoxically, mildly elevated serum Ab titers. Nonetheless, the role of Mst1 in humoral immunity remains poorly understood. In this study, we found that early T cell-dependent IgG1 responses in young adult Mst1-deficient mice were largely intact with signs of impaired affinity maturation. However, the established Ag-specific IgG1 titers in Mst1-deficient mice decayed more readily because of a loss of Ag-specific but not the overall bone marrow plasma cells. Despite the impaired affinity and longevity of Ag-specific Abs, Mst1-deficient mice produced plasma cells displaying apparently normal maturation markers with intact migratory and secretory capacities. Intriguingly, in immunized Mst1-deficient mice, T follicular helper cells were hyperactive, expressing higher levels of IL-21, IL-4, and surface CD40L. Accordingly, germinal center B cells progressed more rapidly into the plasma cell lineage, presumably forgoing rigorous affinity maturation processes. Importantly, Mst1-deficient mice had elevated levels of CD138+Blimp1+ splenic plasma cell populations, yet the size of the bone marrow plasma cell population remained normal. Thus, overproduced low-affinity plasma cells from dysregulated germinal centers seem to underlie humoral immune defects in Mst1-deficiency. Our findings imply that vaccination of Mst1-deficient human patients, even at the early stage of life, may fail to establish long-lived high-affinity humoral immunity and that prophylactic Ab replacement therapy can be beneficial to the patients.

Prostaglandin E2 Activates YAP and a Positive-Signaling Loop to Promote Colon Regeneration After Colitis but Also Carcinogenesis in Mice.

BACKGROUND & AIMS: Prostaglandin E2 (PGE2) is mediator of inflammation that regulates tissue regeneration, but its continual activation has been associated with carcinogenesis. Little is known about factors in the PGE2 signaling pathway that contribute to tumor formation. We investigated whether yes-associated protein 1 (YAP1), a transcriptional co-activator in the Hippo signaling pathway, mediates PGE2 function. METHODS: DLD-1 and SW480 colon cancer cell lines were transfected with vectors expressing transgenes or small hairpin RNAs and incubated with recombinant PGE2, with or without pharmacologic inhibitors of signaling proteins, and analyzed by immunoblot, immunofluorescence, quantitative reverse-transcription polymerase chain reaction, transcriptional reporter, and proliferation assays. Dextran sodium sulfate (DSS) was given to induce colitis in C57/BL6 (control) mice, as well as in mice with disruption of the hydroxyprostaglandin dehydrogenase 15 gene (15-PGDH-knockout mice), Yap1 gene (YAP-knockout mice), and double-knockout mice. Some mice also were given indomethacin to block PGE2 synthesis. 15-PGDH knockout mice were crossed with mice with intestine-specific disruption of the salvador family WW domain containing 1 gene (Sav1), which encodes an activator of Hippo signaling. We performed immunohistochemical analyses of colon biopsy samples from 26 patients with colitis-associated cancer and 51 age-and sex-matched patients with colorectal cancer (without colitis). RESULTS: Incubation of colon cancer cell lines with PGE2 led to phosphorylation of cyclic adenosine monophosphate-responsive element binding protein 1 and increased levels of YAP1 messenger RNA, protein, and YAP1 transcriptional activity. This led to increased transcription of the prostaglandin-endoperoxide synthase 2 gene (PTGS2 or cyclooxygenase 2) and prostaglandin E-receptor 4 gene (PTGER4 or EP4). Incubation with PGE2 promoted proliferation of colon cancer cell lines, but not cells with knockdown of YAP1. Control mice developed colitis after administration of DSS, but injection of PGE2 led to colon regeneration in these mice. However, YAP-knockout mice did not regenerate colon tissues and died soon after administration of DSS. 15-PGDH-knockout mice regenerated colon tissues more rapidly than control mice after withdrawal of DSS, and had faster recovery of body weight, colon length, and colitis histology scores. These effects were reversed by injection of indomethacin. SAV1-knockout or 15-PGDH-knockout mice did not develop spontaneous tumors after colitis induction, but SAV1/15-PGDH double-knockout mice developed polyps that eventually progressed to carcinoma in situ. Administration of indomethacin to these mice prevented spontaneous tumor formation. Levels of PGE2 correlated with those of YAP levels in human sporadic colorectal tumors and colitis-associated tumors. CONCLUSIONS: PGE2 signaling increases the expression and transcriptional activities of YAP1, leading to increased expression of cyclooxygenase 2 and EP4 to activate a positive signaling loop. This pathway promotes proliferation of colon cancer cell lines and colon tissue regeneration in mice with colitis. Constitutive activation of this pathway led to formation of polyps and colon tumors in mice.

SURF4 has oncogenic potential in NIH3T3 cells.

SURF4, which is located in the Surfeit gene cluster, encodes for a conserved integral membrane protein containing multiple putative transmembrane regions. However, the physiological role of SURF4 has not been determined. We found that SURF4 demonstrated aberrant amplification and increased expression in the tumor tissues of several human cancer patients. Overexpression of SURF4 led to increased cell proliferation, migration, and maintenance of anchorage-independent growth. In addition, NIH3T3 cells overexpressing SURF4 induced tumor growth in the mice. Collectively, our findings demonstrate that SURF4 has the potential for inducing cellular transformation and cell migration in vitro and has oncogenic transformation ability in vivo.

Role of Angiomotin-like 2 mono-ubiquitination on YAP inhibition.

LATS1/2 (large tumor suppressor) kinases and the Angiomotin family proteins are potent inhibitors of the YAP (yes-associated protein) oncoprotein, but the underlying molecular mechanism is not fully understood. Here, we report for the first time that USP9X is a deubiquitinase of Angiomotin-like 2 (AMOTL2) and that AMOTL2 mono-ubiquitination is required for YAP inhibition. USP9X knockdown increased the LATS-mediated phosphorylation of YAP and decreased the transcriptional output of YAP. Conversely, over-expression of USP9X reactivated YAP in densely cultured cells. Both genetic and biochemical approaches identified AMOTL2 as a target of USP9X. AMOTL2 was found to be ubiquitinated at K347 and K408, which both reside in the protein's coiled-coil domain. The AMOTL2 K347/408R mutant, which cannot be ubiquitinated, was impaired in its ability to inhibit YAP. Furthermore, ubiquitinated AMOTL2 can bind to the UBA domain of LATS kinase, and this domain is required for the function of LATS. Our results provide novel insights into the activation mechanisms of core Hippo pathway components.

Feeding and Fasting Signals Converge on the LKB1-SIK3 Pathway to Regulate Lipid Metabolism in Drosophila.

LKB1 plays important roles in governing energy homeostasis by regulating AMP-activated protein kinase (AMPK) and other AMPK-related kinases, including the salt-inducible kinases (SIKs). However, the roles and regulation of LKB1 in lipid metabolism are poorly understood. Here we show that Drosophila LKB1 mutants display decreased lipid storage and increased gene expression of brummer, the Drosophila homolog of adipose triglyceride lipase (ATGL). These phenotypes are consistent with those of SIK3 mutants and are rescued by expression of constitutively active SIK3 in the fat body, suggesting that SIK3 is a key downstream kinase of LKB1. Using genetic and biochemical analyses, we identify HDAC4, a class IIa histone deacetylase, as a lipolytic target of the LKB1-SIK3 pathway. Interestingly, we found that the LKB1-SIK3-HDAC4 signaling axis is modulated by dietary conditions. In short-term fasting, the adipokinetic hormone (AKH) pathway, related to the mammalian glucagon pathway, inhibits the kinase activity of LKB1 as shown by decreased SIK3 Thr196 phosphorylation, and consequently induces HDAC4 nuclear localization and brummer gene expression. However, under prolonged fasting conditions, AKH-independent signaling decreases the activity of the LKB1-SIK3 pathway to induce lipolytic responses. We also identify that the Drosophila insulin-like peptides (DILPs) pathway, related to mammalian insulin pathway, regulates SIK3 activity in feeding conditions independently of increasing LKB1 kinase activity. Overall, these data suggest that fasting stimuli specifically control the kinase activity of LKB1 and establish the LKB1-SIK3 pathway as a converging point between feeding and fasting signals to control lipid homeostasis in Drosophila.

cAMP/PKA signalling reinforces the LATS-YAP pathway to fully suppress YAP in response to actin cytoskeletal changes.

Actin cytoskeletal damage induces inactivation of the oncoprotein YAP (Yes-associated protein). It is known that the serine/threonine kinase LATS (large tumour suppressor) inactivates YAP by phosphorylating its Ser127 and Ser381 residues. However, the events downstream of actin cytoskeletal changes that are involved in the regulation of the LATS-YAP pathway and the mechanism by which LATS differentially phosphorylates YAP on Ser127 and Ser381 in vivo have remained elusive. Here, we show that cyclic AMP (cAMP)-dependent protein kinase (PKA) phosphorylates LATS and thereby enhances its activity sufficiently to phosphorylate YAP on Ser381. We also found that PKA activity is involved in all contexts previously reported to trigger the LATS-YAP pathway, including actin cytoskeletal damage, G-protein-coupled receptor activation, and engagement of the Hippo pathway. Inhibition of PKA and overexpression of YAP cooperate to transform normal cells and amplify neural progenitor pools in developing chick embryos. We also implicate neurofibromin 2 as an AKAP (A-kinase-anchoring protein) scaffold protein that facilitates the function of the cAMP/PKA-LATS-YAP pathway. Our study thus incorporates PKA as novel component of the Hippo pathway.

The novel YAP target gene, SGK1, upregulates TAZ activity by blocking GSK3ß-mediated TAZ destabilization.

YAP (Yes-associated protein) and TAZ (transcription activator with PDZ binding motif) are important in tissue regeneration and cancer development, highlighting the importance of discovering partners that regulate their oncogenicity. SGK1 (serum/glucocorticoid regulated kinase 1), initially identified as a homolog of Akt in phosphoinositide 3-kinase signaling, acts as a serine/threonine protein kinase in multiple oncogenic pathways. However, possible links between SGK1 and Hippo-YAP/TAZ signaling remain unexplored. Here, we reveal that SGK1 is a potential positive feedback regulator of YAP and TAZ, showing that the TEAD-YAP/TAZ complex directly activates SGK1 transcription by binding to the distal enhancer of SGK1, and SGK1, in turn, stabilizes YAP/TAZ. Moreover, we demonstrate that expression of YAP/TAZ target genes is positively regulated by SGK1. Mechanistically, SGK1 inhibits ubiquitin-mediated degradation of TAZ by inhibiting GSK3ß activity. These findings expand our understanding of YAP/TAZ regulation to include the novel downstream target of YAP, SGK1.

Hippo effector YAP directly regulates the expression of PD-L1 transcripts in EGFR-TKI-resistant lung adenocarcinoma.

Developments of EGFR-TKI and immunotherapy targeting the PD1/PD-L1 pathway are considered most important medical breakthroughs in lung cancer treatment. Nowadays, 3rd generation EGFR TKI is widely used for T790M positive 1st and 2nd EGFR-TKI resistant lung cancer patients. Immunotherapy is powerful option for lung cancer patients without drug targets and chemotherapy resistant patients. It also has changed the concept of conventional anti-cancer therapy in the point of regulating tumor microenvironment. There are many studies linking these two important pathways. Recent studies demonstrated that PD-L1 expression is significantly correlated to the mutation status of EGFR, and activation of EGFR signaling can also induce the expression of PD-L1. However, the real linker between PD-L1 and EGFR signaling remains to be revealed. Our previous study revealed that the Hippo pathway effector YAP confers EGFR-TKI resistance in lung adenocarcinoma, and inhibition of YAP restores sensitivity to EGFR-TKIs. Thus, we examined whether PD-L1 is relevant, in terms of conferring EGFR-TKI resistance and whether YAP directly regulates the expression of PD-L1 in this context. First, we compared the expression levels of PD-L1 and YAP between EGFR-TKI-resistant PC9 cells and the parental PC9 adenocarcinoma cells. The expression levels of both YAP and PD-L1 were markedly higher in the EGFR-TKI-resistant cells compared to the parental cells, suggesting differential expression pattern between two cell types. YAP knockdown significantly decreased the expression of PD-L1 in the EGFR-TKI-resistant cells, while YAP overexpression increased the expression of PD-L1 in the parental PC9 cells. Then, our results revealed that YAP regulates the transcription of PD-L1, and the YAP/TEAD complex binds to the PD-L1 promoter. Surprisingly, knockdown of PD-L1 was sufficient to decrease cell proliferation and wound healing in the EGFR-TKI-resistant PC9 cells. These data suggest a PD1-independent oncogenic function of PD-L1. The Hippo effector YAP plays a crucial role in linking the PD-L1 and EGFR-TKI resistance by directly regulating the expression of PD-L1 in lung cancer. Targeting PD-L1 directly or via YAP could provide an effective therapeutic strategy for EGFR-TKI-resistant lung adenocarcinoma.

Injury-Mediated Vascular Regeneration Requires Endothelial ER71/ETV2.

OBJECTIVE: Comprehensive understanding of the mechanisms regulating angiogenesis might provide new strategies for angiogenic therapies for treating diverse physiological and pathological ischemic conditions. The E-twenty six (ETS) factor Ets variant 2 (ETV2; aka Ets-related protein 71) is essential for the formation of hematopoietic and vascular systems. Despite its indispensable function in vessel development, ETV2 role in adult angiogenesis has not yet been addressed. We have therefore investigated the role of ETV2 in vascular regeneration. APPROACH AND RESULTS: We used endothelial Etv2 conditional knockout mice and ischemic injury models to assess the role of ETV2 in vascular regeneration. Although Etv2 expression was not detectable under steady-state conditions, its expression was readily observed in endothelial cells after injury. Mice lacking endothelial Etv2 displayed impaired neovascularization in response to eye injury, wounding, or hindlimb ischemic injury. Lentiviral Etv2 expression in ischemic hindlimbs led to improved recovery of blood perfusion with enhanced vessel formation. After injury, fetal liver kinase 1 (Flk1), aka VEGFR2, expression and neovascularization were significantly upregulated by Etv2, whereas Flk1 expression and vascular endothelial growth factor response were significantly blunted in Etv2-deficient endothelial cells. Conversely, enforced Etv2 expression enhanced vascular endothelial growth factor-mediated endothelial sprouting from embryoid bodies. Lentiviral Flk1 expression rescued angiogenesis defects in endothelial Etv2 conditional knockout mice after hindlimb ischemic injury. Furthermore, Etv2(+/-); Flk1(+/-) double heterozygous mice displayed a more severe hindlimb ischemic injury response compared with Etv2(+/-) or Flk1(+/-) heterozygous mice, revealing an epistatic interaction between ETV2 and FLK1 in vascular regeneration. CONCLUSIONS: Our study demonstrates a novel obligatory role for the ETV2 in postnatal vascular repair and regeneration.

Ablation of Rassf2 induces bone defects and subsequent haematopoietic anomalies in mice.

RASSF2 belongs to the Ras-association domain family (RASSF) of proteins, which may be involved in the Hippo signalling pathway. However, the role of RASSF2 in vivo is unknown. Here, we show that Rassf2 knockout mice manifest a multisystemic phenotype including haematopoietic anomalies and defects in bone remodelling. Bone marrow (BM) transplantation showed that Rassf2(-/-) BM cells had a normal haematopoietic reconstitution activity, indicating no intrinsic haematopoietic defects. Notably, in vitro differentiation studies revealed that ablation of Rassf2 suppressed osteoblastogenesis but promoted osteoclastogenesis. Co-culture experiments showed that an intrinsic defect in osteoblast differentiation from Rassf2(-/-) osteoblast precursors likely leads to both haematopoiesis and osteoclast defects in Rassf2(-/-) mice. Moreover, Rassf2 deficiency resulted in hyperactivation of nuclear factor (NF)-κB during both osteoclast and osteoblast differentiation. RASSF2 associated with IκB kinase (IKK) α and ß forms, and suppressed IKK activity. Introduction of either RASSF2 or a dominant-negative form of IKK into Rassf2(-/-) osteoclast or osteoblast precursors inhibited NF-κB hyperactivation and normalized osteoclast and osteoblast differentiation. These observations indicate that RASSF2 regulates osteoblast and osteoclast differentiation by inhibiting NF-κB signalling.

Hippo pathway effector YAP inhibition restores the sensitivity of EGFR-TKI in lung adenocarcinoma having primary or acquired EGFR-TKI resistance.

The efficacy of EGFR-tyrosine kinase inhibitors (TKIs) is significantly limited by various resistance mechanisms to those drugs. The resistance to EGFR-TKI is largely divided by two classes; acquired resistance after EGFR-TKI treatment, and primary resistance marked by cancer cell's dependence on other oncogene, such as KRAS. YAP has emerged as critical oncogene in conferring drug resistance against targeted therapy. In this study, we evaluated the role of YAP in primary and acquired EGFR-TKI resistance using gefitinib-resistant A549 and PC9 cells and their parental cell lines. Our study revealed that EGFR-TKI resistance is associated with enhanced YAP activity. Notably, YAP activation was independent of the Hippo pathway. We confirmed that AXL is a downstream target of YAP that confers EGFR-TKI resistance. And our results showed that YAP can induce ERK activation in lung adenocarcinoma. The combination of YAP inhibition with EGFR-TKI overcomes primary and acquired EGFR-TKI resistance. We also found increased YAP expression in human lung cancer after acquiring EGFR-TKI resistance. Collectively, we suggest a novel EGFR-TKI resistance mechanism involving YAP activation and suggest targeting YAP and EGFR simultaneously may be a breakthrough treatment of primary and acquired EGFR-TKI resistant lung cancer.