The Role of Photoactivated and Non-Photoactivated Verteporfin on Tumor.

The Hippo signaling pathway regulates tissue homeostasis and organ size development in mammals. YAP/TAZ is the core effector of this pathway, and regulates tumor cell proliferation, invasion, and chemoresistance through multiple transcription factors. The core kinase chain of the Hippo signaling pathway phosphorylates YAP and prevents its nuclear translocation. Upon inactivation of the upstream kinase cascade, unphosphorylated YAP is translocated to the nucleus where it functions as a co-transcriptional activator ( Figure 2 ). The human YAP gene is located on chromosome 11q22, and the YAP protein contains an N-terminal TEAD-binding domain (TBD), a 14-3-3 protein binding site, and one or two WW domains in the middle. In addition, some YAP isoforms contain the SH3 binding motif, while the C-terminal contains a transcriptional activation domain and a PDZ binding motif. According to the number of WW domains, YAP can be divided into two categories and eight subtypes. TAZ and YAP have similar domain composition except that TAZ lacks the second WW domain, SH3 binding motif, and proline-rich domain ( Zhao et al., 2010 ). Since YAP lacks a DNA binding domain, it needs to bind to transcription factors to activate the downstream genes. The TBD domain of YAP can bind to several transcription factors such as TEADs, Smads, and Klf4, of which the YAP-TEAD interaction is best characterized ( Vassilev, 2001 ). Regulation of YAP/TAZ by core components of Hippo signaling pathway. The oncogenic function of YAP is mainly mediated by its nuclear localization and interaction with TEAD transcription factors. The human TEAD proteins include four subtypes, each with an N-terminal TEA domain, DNA-binding domain, proline-rich region, and C-terminal YAP/TAZ-binding domain. TEAD-YAP binding sites that have been identified include hTEAD1 (209–426)-hYAP (50–171), hTEAD4 (217–434)-hYAP (60–100), hTEAD4 (210–427)-hYAP (47–85) ( Gibault et al., 2017 ). TEADs contain a DNA-binding domain but lack an activation domain, while YAP lacks a DNA-binding domain but contains an activation domain. The YAP-TEAD heterodimeric transcription factor activates proliferation, invasion and adhesion-related genes that promote cancer development and progression ( Santucci et al., 2015 ). There are two main states in the regulation of YAP/TAZ by the core kinase reaction chain of the Hippo signaling pathway. In the first state, the upstream pathway of Hippo pathway is activated, and the activation complex formed by Mammalian Sterile20-like kinase 1/2 (MST1/2) and its regulatory protein Sav1 can directly phosphorylate lats and Mob1. Moreover, Large tumor suppressor (LATS) and Mob1 activation complexes further phosphorylate the transcription co-activator YAP/TAZ, thus inhibiting YAP/TAZ nuclear entry and initiating downstream target gene expression. In the second state, the upstream Hippo pathway signal is out of order and the kinase cascade is inactivated. Unphosphorylated YAP is therefore transferred to the nucleus and acts as a co-activator of transcription after binding with transcription factors. Liu-Chittenden et al. found VP and protoporphyrin IX (PPIX) could inhibited the transcriptional activity of Gal4-TEAD4 from the Johns Hopkins drug library using the luciferase reporter method and co-IP assay in HEK293 cells. Both drugs blocked the interaction between GAL4-TEAD4 and Ha-YAP proteins and inhibited transcriptional activity of the complex. VP showed a significantly stronger inhibitory effect compared to PPIX at the dosage of 10 mM and colocalized with purified YAP protein in vitro . Furthermore, 20 mM VP significantly increased trypsin-mediated cleavage of YAP without affecting TEAD2 lysis, indicating that VP selectively binds to YAP and inhibits the YAP-TEAD complex in the absence of light activation ( Liu-Chittenden et al., 2012 ). YAP and TEADs are up-regulated in many cancer types, and knocking out either inhibits the proliferation, migration, epithelial mesenchymal transformation (EMT) and oncogenic transformation of cancer cells by blocking transcription of the YAP-TEAD downstream target genes ( Gibault et al., 2016 ; Lin et al., 2017 ). Given that the YAP-TEAD complex is the final step in the Hippo pathway ( Meng et al., 2016 ), its targeted blocking by VP on the upstream proteins and the potential side effects are smaller. In the following sections, we have summarized the pharmacological effects of non-photoactivated VP on cancer cells ( Table 1 ) and animal tumor models ( Table 2 ). The role of YAP inhibitor verteporfin in solid tumor cells. The role of YAP inhibitor verteporfin in animal models. The Hippo signaling pathway plays an important regulatory role in tumor genesis and development. YAP is abnormally activated in multiple tumors, and associated with increased tumor progression and metastasis, and poor prognosis. Therefore, recent studies have explored YAP as a potential target for tumor treatment ( Panciera et al., 2017 ). Consistent with this, VP inhibits the proliferation of pancreatic ductal carcinoma ( Wei et al., 2017 ), non-small cell lung cancer ( Zhao et al., 2019 ), myxoid liposarcoma ( Trautmann et al., 2019 ), and melanoma ( Yu et al., 2014 ) cells, as well as their xenografts in mice. Helicobacter pylori infection of gastric epithelial cells promotes the nuclear translocation of YAP, which induces EMT and eventually gastric cancer. Studies showed that VP inhibits H. pyroli-induced proliferation, invasion, and metastasis of gastric cancer cells by disrupting the YAP1/TEAD4- Connective Tissue Growth Factor (CTGF) axis and blocking transcription of EMT-related genes ( Kang et al., 2018 ; Li N. et al., 2018 ). In patients with dedifferentiated liposarcoma (DDLS), the conversion of Tissue Inhibitors Of Metalloproteinase 4 (TIMP-4) to TIMP-1 is associated with poor prognosis.TIMP-1 knockout, TIMP-4 overexpressing, or VP-mediated YAP/TAZ blockade can inhibit the proliferation and migration of DDLS cells ( Madhu et al., 2019 ). VP also mitigates YAP/TEAD-induced chemoresistance in cancer cells. For instance, VP induced apoptosis in paclitaxel-resistant colon cancer cell lines by downregulating YAP and cox-2, and inhibited growth of the xenografts in mice ( Li et al., 2017 ). In addition, the YAP-TEAD complex promotes transcription of the pro-angiogenic Cysteine-Rich Angiogenic Inducer 61 (CYR61) and the immunosuppressive programmed cell death-Ligand 1 (PD-L1) in colon cancer tissues and cell lines ( Xie et al., 2019 ; Zhang et al., 2019 ). Gene silencing or VP-mediated inhibition of YAP1 downregulated both factors in colon cancer cells. YAP1 is associated with the early relapse in paclitaxel-resistant patients, and nonlight-activated VP reversed YAP-induced paclitaxel resistance in the HCT-8/T liver cancer cells both in vitro and in vivo ( Pan et al., 2016 ). Furthermore, VP inhibited the growth of the paclitaxel-resistant breast cancer cell line MDA-MB-231 ( Li et al., 2019 ), and sensitized the HER-2 positive breast cancer cell line HCC1569 to lapatinib ( Lin et al., 2015 ). The clinical stage and multidrug resistance of non-small cell lung cancer (NSCLC) depends on the overexpression of WBP5. VP sensitized the WBP5-overexpressing H69 lung cancer cells to multiple chemotherapy drugs, and decreased their proliferation rate and invasive ability ( Tang et al., 2016 ). VP inhibited tumor cell proliferation in vitro and in vivo alone or in combination with doxorubicin and pan-RAF inhibitors, and the combination therapy showed greater effect against xenografts ( Zhao et al., 2017 ; Isfort et al., 2019 ). YAP1/TAZ-TEAD also plays an important role in maintaining cancer stem cells (CSCs), the major determinants of tumor recurrence, metastasis and chemoresistance, and modulates the expression of CSC markers ( Shibata and Hoque, 2019 ). VP and CA3 weakened spheroid formation, matrix invasion, and tumor formation of the mesothelioma stem cells by inhibiting YAP1/TEAD ( Kandasamy et al., 2020 ). Likewise, Yu et al. found that VP impaired the oncogenic properties of melanoma stem cells ( Yu et al., 2014 ). VP sensitized the OV6+ bladder cancer stem cells to cisplatin by inhibiting the YAP/TEAD1/PDGF-BB/PDGFR autocrine signaling pathway, which downregulated PDGFB and impaired PDGF-BB secretion ( Wang J. K. et al., 2019 ). VP also downregulated the stem cell markers SOX2, NANOG, and OCT4 in transitional cell carcinoma of the bladder when used in combination with COX inhibitor ( Ooki et al., 2018 ). Dissemination of circulating tumor cells is crucial for distant metastasis, and YAP1 increases the number of circulating tumor cells following activation by the chromatin remodeling protein ZNF367. VP downregulated YAP1 in the ZNF367-overexpressing breast cancer MDA-MB-231 and 4T1 cells, and significantly reduced lung metastases in mouse models. It also inhibited the expression of tumor stem-associated protein SOX2, CD44, and CD133 in drug-resistant breast cancer cells and upregulated anoikis-induced cell death in drug-resistant breast cancer and cholangiocarcinoma cell lines ( Li et al., 2019 ; Sugiura et al., 2019 ; Wu et al., 2020 ). Taken together, VP can sensitize cancer cells to several chemotherapeutic drugs, including cisplatin, paclitaxel, tyrosine kinase inhibitors (TKIs), and RAF inhibitors by targeting axis. The mechanisms underlying YAP/TEAD-dependent drug resistance need to be elucidated further to treat recalcitrant tumors with greater efficacy. The YAP-TEAD complex is the central effector of multiple intersecting pathways ( Figure 3 ), and is therefore a potential target for cancer treatment. The Hippo/YAP and PI3K pathways interact at multiple levels; for instance, the scaffolding protein GAB2 is a key target of YAP and also interacts with growth factors and the PI3K signaling pathway. VP inhibited GAB2-dependent PI3K/AKT signaling in endometrial cancer cells by inhibiting YAP and TAZ, downregulated p-mTOR and its target genes p-4EBP1 and p-S6 in endometrial and liver cancer cells, and inhibited the growth of the xenografts ( Wang et al., 2015a ; Zhou Y. et al., 2019 ). Insulin resistance is an important pathological mechanism of endometrial cancer and endometriosis. PI3K/Akt regulates insulin/IGF1 signaling, and IRS1/2 expression in patients with endometrial cancer is positively correlated with YAP/TAZ. The insulin sensitizer metformin competitively binds to the transcription factor IRF-1 to inhibit the expression of YAP in A549 lung cancer cells. VP augmented the effect of metformin on YAP in lung cancer cells, and reduced the number of tumors in xenograft-bearing mice ( Jin et al., 2018 ). In addition, the transcriptome of VP-treated endometrial cancer cells showed differential expression of 549 genes involved in TGFβ1 regulation, lipoprotein metabolism, cell adhesion, endoderm cell differentiation, and integrin-mediated signaling pathways relative to that in the control cells. YAP inhibitors or metformin alone only partially inhibited the function of insulin and IGF1 in endometrial cancer cells, while their combination completely blocked the effects of insulin ( Wang et al., 2016 ). YAP transcriptionally activates IL-6, and stimulate IL-11 by up-regulating p65. Targeted inhibition of YAP by VP inhibited the binding between YAP and the IL-6 promoter, and downregulated IL-6 and IL-11 in endometrial cancer cells, resulting in lower proliferation rates ( Wang J. et al., 2019 ), increased sensitivity to adriamycin, and 45.36% decrease in tumor weight in the treated mice ( Bang et al., 2019 ). Regulation of Hippo signaling pathway and potential inhibitors of YAP/TEAD. The RAS-RAF-MEK-ERK signaling pathway regulates tumor cell proliferation, differentiation, invasion, cell cycle, and other processes. The loss of tumor suppressor NF2 in thyroid cancer activates YAP-TEAD transcription through RAS signaling. VP blocked the transcription of KRAS, HRAS and NRAS in the NF2-knockout Cal62 and Hth83 thyroid cancer cells by inhibiting YAP-TEAD, which reduced the proliferation and significantly retarded xenograft growth in mice ( Garcia-Rendueles et al., 2015 ). KRAS/FSTL5 double mutations can desensitize KRAS mutant lung cancer cells to XPO1 inhibitors. Consistent with the fact that FSTL5 mutations are often accompanied by YAP1 activation, the combination of XPO1 inhibitor and VP significantly inhibited the proliferation of these resistant KRAS/FSTL5 double mutant cancer cells ( Kim et al., 2016 ). The mir-630/YAP1/erk feedback loop modulates the resistance of EGFR-mutated tumors to TKIs. Combination therapy with VP and gefitinib inhibited YAP1, p-ERK and ps75-bad in lung cancer cells, and suppressed PC9 mir-630 knockout lung cancer xenografts in nude mice ( Wu et al., 2018 ). Furthermore, VP-mediated inhibition of YAP downregulated FOXM1 and CTGF, and suppressed YAP/TEAD4/FOXM1-dependent activation of CIN-related genes in liver cancer and synovial sarcoma cells ( Chen J. et al., 2017 ; Weiler et al., 2017 ). In addition, VP also inhibited liver cancer progression by blocking the HMGB1-YAP pathway and inhibiting s1p-mediated YAP upregulation ( Zhao et al., 2017 ; Jung-Chien et al., 2018 ). ROCK1 and ROCK2 are the negative regulators of the Hippo tumor suppressor pathway, and while activation of the Rho/Rock signal can increase YAP activity in pleural mesothelioma, ROCK2 knockout decreased tumor volume in a mouse model of osteosarcoma. VP also inhibited the proliferation of osteosarcoma (U-2OS) and mesothelioma (211H, H2052) cell lines by targeting YAP and its target genes, which upregulated N-cadherin and β-catenin ( Zhang et al., 2017 ; Zucchini et al., 2019 ). Activated YAP is known to stimulate the secretion of FGF ligands, which bind to FGF receptors and activate downstream PI3K, YAP and MAPK pathways. The Hippo/YAP and FGF/FGFR pathways form a positive feedback loop that regulates the activity of advanced ovarian serosa cancer cells. Non-photoactivated VP inhibited YAP, FGF1, and FGF2 in the lats knockout ovarian cancer cells, which induced apoptosis and suppressed their proliferative and migration abilities ( Hua et al., 2015 ). Activation of p38 MAPK induces apoptosis in various tumor cells. Moujahed et al. found that VP inhibited the expression of YAP-TEAD, VEGFA and OCT-4 in the SNB19 and LN229 glioma cells without light activation, and up-regulated p38 MAPK ( Al-Moujahed et al., 2017 ). Taken together, VP inhibits tumor growth and improves chemoresistance by targeting multiple signal transduction pathways associated with YAP-TEAD. The Hippo/YAP and PI3K pathways interact at multiple levels. PI3K/Akt regulates insulin/IGF1 signal transduction, while RAS-RAF-MEK-ERK signal activates the YAP-teach transcription signal pathway, regulating tumor cell proliferation, differentiation, invasion, cell cycle, and other processes. The activation of Rho/rock signal increases the activity of YAP. Cpd3.1 and VP could directly inhibit YAP-teach interaction. Simvastatin could indirectly inhibit the function of nuclear YAP by acting on the upstream signal pathway of YAP. VP blocked the accumulation of autophagosomes in the breast cancer MCF-7 cells following 1μM (IC50) chloroquine (CQ) treatment or serum starvation ( Donohue et al., 2011 ). Increased autophagy is frequently observed in malignant tumors, and knocking out YAP and TAZ in HeLa cells decreased the number of autophagosomes via downregulation of LC3-II and Baf A1. In addition, YAP/TAZ also transcriptionally regulates the expression of F-actin cytoskeleton and myosin II that are critical for autophagosome formation. At high cell densities, Hippo signaling is activated and relocates YAP/TAZ from the nucleus to the cytoplasm, which decreases transcription of myosin II complex and other actin cytoskeleton-related genes, resulting in impaired autophagic protein transport and autophagosome production. Likewise, VP treatment also reduced the expression of myosin light chain 2 in HeLa cells ( Pavel et al., 2018 ). Inhibition of autophagy sensitizes tumor cells to chemotherapeutic drugs, while lysosomal isolation induces chemotherapy resistance. VP alkalizes the pH of lysosomes by reducing RAS expression, which disrupts lysosomal membrane permeabilization and autophagy flux, and increases the expression of HMW-p62 to augment the effect of sorafenib in HepG2 and HuH7 liver cancer cells ( Gavini et al., 2019 ). In the absence of special laser activation, VP inhibits autophagy via p62 cross-linkage, which could be a potential target for neutralizing drug resistance in cancer cells ( Figure 4 ). Hippo YAP and VP regulate tumor cell autophagy. YAP can increase autophagy flux by regulating the cytoskeleton of myosin II and F-actin, or in the condition of nutrient deprivation. VP inhibits autophagy by producing 1O2 and engaging in p62 oligomerization, reducing myosin II levels and increasing reactive oxygen species ROS. The FAK signaling pathway is an upstream regulator of YAP. VP downregulated FAK and p-FAK in hMSCs and human osteosarcoma-derived cells, along with ß1-integrin, paxillin, and zyxin. However, knocking down YAP in hMSCs and Saos cells had the opposite effect on the FAK pathway ( Husari et al., 2019 ). Similarly, YAP knockout in the TKKK cholangiocarcinoma cells did not affect the Akt/mTOR signaling pathway, while VP significantly upregulated p-Akt and p-mTOR in HuCCT1 and TKKK cells. Furthermore, genetic ablation or VP-mediated pharmacological inhibition of YAP inactivated the IL-6/STAT3 signaling and inhibited IL-6-dependent STAT3 phosphorylation. Interestingly, VP also inhibited the IL-6/STAT3 axis in the YAP-low expressing HuH-28 cells ( Wu et al., 2020 ). In addition, VP inhibited HCQ and bafilomycin A1-induced autophagy in PC-3 and LNCaP prostate cancer cells, which increased p62 oligomerization and ROS production, and downregulated Nrf2 (antioxidant) and Bcl-xl (anti-apoptotic) in prostate tumor cells and xenografts, while YAP1 overexpression had no effect on any of these factors ( Wang et al., 2018 ). Hypoxia is a known inducer of YAP in glioma cells. VP inhibited the proliferation of YAP/TAZ knockout U87 and U343 cells under hypoxic conditions, indicating a YAP-independent function ( Eales et al., 2018 ). The recurrence, metastasis and chemoresistance of gastric cancer is directly related to the cancer stem cells (CSCs). Non-light-activated VP downregulated YAP1/TAZ-TEAD in CD44+ gastric CSCs, which decreased their proliferation and spheroid forming ability in vitro , and inhibited growth of both patient-derived and CSC-derived xenografts ( Giraud et al., 2020 ). The low survival rate of gastric cancer is associated with the overexpression of Clusterin protein in gastric CSCs. Although knocking out YAP1 or YAP2 in gastric CSCs had no effect on Clusterin expression, VP significantly inhibited the latter and was more effective against the CSCs compared to the with gastric cancer cells (MGC-803, BGC-823 cells lines) ( Xiong et al., 2019 ). Similarly, VP downregulated YAP target genes in malignant pleural mesothelioma (MPM) cells and inhibited their proliferation independent of YAP1 knockout ( Tranchant et al., 2018 ). Kuramoto et al. found that VP reduced oxidative phosphorylation in glioma stem cells (GSCs) and decreased mitochondrial membrane potential (MMP) and ATP levels, leading to massive GSCs death. Furthermore, the cytotoxic effect of VP was specific to the GSCs, rather than normal human fibroblasts, mouse astrocytes or rat neural stem cells,independent of YAP and ROS ( Kuramoto et al., 2019 ). To summarize, repositioned VP can also regulate the AKT/mTOR, IL-6/STAT3 and FAK signaling pathways, and inhibit CSCs partially independent of YAP.

Verteporfin causes photochemical damage to the mitochondria via ROS accumulation when activated by a 690 nm laser, it is routinely used as a PS for treating AMD ( Mellish and Brown, 2001 ). Although PS was initially tested in anti-cancer PDT, the lack of effective tumor targeting limited its applications since the efficacy of PDT depends on the selective accumulation of photosensitizing drugs in tumor tissues ( Mccaughan, 1999 ), however, the development of laser technology and innovative drug-loaded nanomaterials have re-emphasized the role of VP in tumor photoablation ( Baskaran et al., 2018 ). In addition, VP exerts multiple YAP/TEAD pathway-dependent and independent effects on tissue regeneration, inflammation, and tumor development even in the absence of photoactivation ( Gibault et al., 2016 ; Hertig et al., 2017 ; Wang Y. et al., 2019 ). In this review, we have summarized the pharmacological effects of VP on tumors both in the presence and absence of photo-stimulation, in order to provide new insights into anti-cancer chemotherapeutic drug design and targeted therapy.

CW wrote this manuscript. XL revised the manuscript. All authors contributed to the article and approved the submitted version.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

To summarize, both photoactivated and non-photoactivated VP can inhibit tumor growth, albeit through different pathways. Apart from HIPPO/YAP signaling, VP also targets the Wnt, PI3K, Ras, mTOR and NF-kB signaling pathways. Furthermore, novel drug carriers have achieved selective tumor accumulation of VP and selective killing of tumor cells via its photothermal or photodynamic activation. Thus, repositioned VP is a highly promising photosensitizer or YAP-TEAD inhibitor for tumor therapy, adjuvant therapy, PDT, and tumor imaging.

YAP-TEAD plays a vital role in cancer development and progression. VP inhibits the interaction between these two factors and subsequent transcription of downstream genes in the absence of photoactivation. Therefore, it is a highly suitable candidate for targeted anti-cancer treatment. Multifunctional nanoparticle carriers loaded with VP can selectively kill tumor cells under photoexcitation, whereas non-photoactivated VP inhibits YAP, SPAK, and OSR1 by targeting their kinase domains in an ATP-dependent manner and the suppression is more obvious in the dark ( Alamri et al., 2018 ). Furthermore, non-photoactivated VP also inhibited IL-17A production by the AhR-RORγt complex by targeting GLK, which is suggestive of a potential therapeutic effect against Th17-mediated autoimmune diseases ( Chuang and Tan, 2019 ). There are several concerns regarding the photodynamic effects of VP on tumor cells. First, it is unclear whether laser activation of VP can alter YAP protein expression in tumor cells since singlet oxygen-induced protein cross-linking can only be observed under intense light. Bae et al. showed that only short-term exposure of VP induced protein cross-linking under incandescent light ( Bae et al., 2008 ). In addition, Donohue1 et al. found that low thermal energy can activate VP to generate singlet oxygen in the dark, which covalently cross-links p62 into oligomers (high-MW p62) through oxidative stress carbonylation. Furthermore, the inhibitory effect of VP on p62 and autophagy can be amplified by exposure to overhead laboratory light during cell lysis ( Donohue et al., 2014 ). However, some studies show that photoactivation of VP is necessary for p62 cross-linkage. In human uveal melanoma cells (MEL 270), human embryonic kidney cells (HEK), and breast cancer cells (MCF-7), VP-induced HMW-p62 requires the presence of light ( Konstantinou et al., 2017 ). Interestingly, VP upregulated the expression of 14-3-3σ protein in EFE184 endometrial cancer cells and isolated YAP in the cytoplasm, and this effect was not reversed by knocking out YAP ( Wang et al., 2015b ). The 14-3-3 family proteins are involved in regulating autophagy ( Wang et al., 2012 ), and the mechanism through which VP induces autophagy in the absence of light remains to be elucidated. Secondly, When the PS is exposed to light, PS in normal tissues will also be activated, resulting in phototoxicity. However, the half-life (2~3 h) of VP was low, it is also recommended that animals should be left in the dark for 4 h after VP injection to avoid photosensitivity ( Gibault et al., 2016 ). Donohue et al. kept animals in dark until the morning after VP treatment ( Donohue et al., 2013 ). Curry et al. kept all mice in dark for 24 h after VP injection. A particularly detailed toxicity research on VP in vivo experiment remains to be studied ( Curry et al., 2017 ). Thirdly, YAP/TEAD activity can be impaired by directly blocking the interaction between both proteins (VP and other small molecule inhibitors), or indirectly by inhibiting the upstream factors of YAP like Tankyrase inhibitor (XAV939) or its nuclear localization (dasatinib, pazopani and A35) ( Tang et al., 2019 ). Furthermore, the YAP-TEAD complex has three highly conserved TEAD-YAP binding domain (YBD) interfaces. Interface 1 is an anti-parallel β-sheet formed between the YAP 52-58 amino acids and the β-sandwich of TEAD, interface 2 consists of the LXXLF α-helix motif (YAP residues 61-73) fitted into the hydrophobic groove of TEAD 2, and the third interface comprises of a Ω-shaped side chain formed by YAP residues 86-100 that is inserted into TEAD ( Gibault et al., 2017 ). “Peptide17” (P17) ring YAP-like peptide competes with YAP to occupy the TEAD-YBD interface 3 and disrupts the YAP-TEAD complex. However, peptides are limited by low chemical and physical stability and short half-life in the plasma. Kaan et al. screened the Maybridge Ro3 fragment library through thermal displacement analysis and identified that fragment 1 can down-regulate TEAD luciferin reporter activity in HEK293 cells. X-ray crystallographic analysis revealed that fragment 1 ( Kaan et al., 2017 ) bound to mTEAD4. Fragment 2 ( Li Y. et al., 2018 ) was identified by NMR as the binding partner of the TEAD N-terminal omega loop region, and Patent-22 ( Zhou W. et al., 2019 ) can occupy the TEAD-YBD interface 3 and disrupt the YAP-TEAD complex. Using similar structural analysis methods, Hit-2 ( Gibault et al., 2018 ) and small molecule compounds (CPD3.1) were identified that disrupted the YAP-TEAD interaction. In addition, Hit-2 also inhibited the YAP target gene AXL, Cyr61, and CTGF in MDA-MB-231 cells, and CPD3.1 in HeLa cells, which reduced their proliferation and migration ( Smith et al., 2019 ). Celastrol inhibited the proliferation, migration and clonal expression of the H1299 lung cancer cells and triple-negative breast cancer MDA-MB-231 cells by targeting the YAP-TEAD interactions ( Nouri et al., 2019 ). The binding of the lipid pocket of TEAD to palmitoyl ligand is crucial for its folding, stability, and binding to YAP. Flufenamic acid and MGH-CP1 can bind to this lipid pocket and reduce the expression of YAP target genes ( Gibault et al., 2017 ). TED-347 forms a covalent bond with cysteine in the palmitate-binding pocket of TEAD, leading to allosteric inhibition of YAP-TEAD ( Bum-Erdene et al., 2018 ). This reveals a “pharmacological window” for VP action for maximum tumor growth inhibition by disrupting the YAP-TEAD complex ( Figure 5 ). Structural of YAP/TEAD small molecule inhibitors.

This work was supported by funds from the National Natural Science Foundation of China (No 81473687), Shandong Provincial Natural Science Foundation, China (No ZR2009CM039 and No ZR2013HM038). High level project cultivation program of Shandong First Medical University, China (No 2018GCC14). Academic promotion of Shandong First Medical University, China (grant no. 2019QL017).