Targeting the Hippo/YAP/TAZ signalling pathway: Novel opportunities for therapeutic interventions into skin cancers

Abstract

Skin cancers are by far the most frequently diagnosed human cancers. The closely related transcriptional co-regulator proteins YAP and TAZ (WWTR1) have emerged as important drivers of tumour initiation, progression and metastasis in melanoma and non-melanoma skin cancers. YAP/TAZ serve as an essential signalling hub by integrating signals from multiple upstream pathways. In this review, we summarize the roles of YAP/TAZ in skin physiology and tumorigenesis and discuss recent efforts of therapeutic interventions that target YAP/TAZ in in both preclinical and clinical settings, as well as their prospects for use as skin cancer treatments.

Keywords: Hippo signalling; basal cell carcinoma; epidermis; melanoma; skin cancer; squamous cell carcinoma.

FIGURE 1

Morphology of the skin. The epidermis and the underlying dermis are separated by a basement membrane (BM). Multiple, spatially distinct stem cell populations have been identified in the interfollicular epidermis (IFE) and the bulge, isthmus, infundibulum and sebaceous gland (SG) parts of the hair follicle and are indicated by different colours. Two populations of fibroblasts populate the dermis: papillary fibroblasts are in proximity to the BM, while reticular fibroblasts are found in the central dermis. The hair follicle is depicted in the growth phase (anagen), when a transient population of stem cells in the hair germ create an inner root sheath (IRS) and hair shaft (HS, protruding out of the skin surface), while stem cells in the permanent bulge region of the hair follicle give rise to the outer root sheath (ORS). The hair germ rests above the dermal papilla (DP), a population of mesenchymal cells that provides inductive signalling for hair growth and modulates hair follicle regeneration. Pigment‐producing melanocytes are present in the hair follicle and the IFE. APM, arrector pili muscle. Figure graphics were created with BioRender.com.

FIGURE 2

Key signals regulating YAP/TAZ activity. The transcription co‐regulators Yes‐associated protein (YAP) and transcriptional co‐activator with PDZ‐binding motif (TAZ), are predominantly regulated by phosphorylation (serine phosphorylation, orange; tyrosine phosphorylation, pink). Serine‐phosphorylated YAP/TAZ are exported from the nucleus and are either degraded in the cytoplasm via the proteasome or sequestered in the cytoplasm via 14‐3‐3 proteins or at tight‐ and adherens junctions. In their non‐serine‐phosphorylated and tyrosine‐phosphorylated states, YAP/TAZ accumulate in the nucleus, where they bind to various transcription factors, most notably those of the TEA domain (TEAD) family, to control target gene expression. In the nucleus, vestigial‐like family member 4 (VGLL4) competes with YAP/TAZ in binding to TEADs, while WW domain binding protein‐2 (WBP2) enhances the co‐activator functions of YAP/TAZ. The core of the Hippo pathway (dotted pink box) is defined by a kinase cascade composed of MST1 and MST2 kinases, large tumour suppressor (LATS)1 and LATS2 kinases and their co‐factors SAV1 and MOB1A and MOB1B. Membrane‐associated signalling events causing Hippo pathway activation include high‐molecular‐weight hyaluronan‐mediated clustering of CD44 and cell–cell signalling via Dachsous cadherin‐related 1 (DCHS1)/FAT1. Hippo pathway activation involves the phosphorylation of the core Hippo kinases, MST1/2 and LATS1/2: MST1/2 are autophosphorylated and subsequently phosphorylate LATS1/2. MST1/2 are also activated by TAO kinases. Activation of LATS1/2 causes the serine‐phosphorylation of YAP and TAZ and inhibits their transcription co‐regulator functions. PP1, together with apoptosis‐stimulating protein of p53 2 (ASPP2), antagonizes Hippo pathway activity by de‐phosphorylating YAP/TAZ. In addition to MST1/2, various upstream effectors of the LATS1/2 have been identified, including the MAP4K and TAOK families of kinases, which phosphorylate and activate LATS1/2. Nuclear Dbf2‐related (NDR)1/2 kinases act in parallel to LATS1/2 in the Hippo pathway to inactivate YAP/TAZ. The activities of the core Hippo pathway components are regulated by several upstream mechanisms. These involve various scaffolding proteins such as angiomotin (AMOT), neurofibromin 2 (NF2; also known as Merlin), kidney and brain protein (KIBRA; also known as WWC1), the protocadherin FAT1 and zonula occludens (ZO) proteins at tight junctions. Cell polarity and adhesion regulators promote LATS1/2‐mediated regulation of YAP/TAZ by altering actin dynamics and by facilitating Hippo pathway effector association. G protein‐coupled receptors (GPCR) signalling, mechanical cues and signals transduced by the extracellular matrix and matrix‐binding integrins (through FAK and SRC family kinases (SFKs)) can inactivate LATS1/2 by promoting a contractile F‐actin‐myosin cytoskeleton. SFKs also directly regulate YAP/TAZ nuclear abundance, predominantly by controlling their nuclear export rate. Soluble growth factors bind to and activate receptor tyrosine kinases (RTKs) and inactivate the Hippo pathway by stimulating PI3K–PDK1 signalling. EGFR activation causes inhibitory tyrosine phosphorylation of MOB1A/B. RASSF1A is recruited to the activated TGF‐b receptor I and subsequently targeted for degradation by the co‐recruited E3ubiquitin ligase ITCH. RASSF1A degradation then permits YAP association with SMADs and subsequent nuclear translocation of receptor‐activated SMAD2. YAP/TAZ are also regulated by WNT signalling: such as β‐catenin, YAP/TAZ also incorporates into the destruction complex and are targeted for proteasomal degradation. Upon WNT stimulation, inactivation of the destruction complex then drives β‐catenin as well as YAP/TAZ nuclear translocation. YAP/TAZ also interact with the Notch pathway: in the nucleus, YAP/TAZ can induce the gene expression of Notch receptors and/or Notch ligands to regulate Notch signalling, while the transcriptionally active Notch intracellular domain (NICD) can activate YAP1 gene transcription. Activated HH signalling leads to increased nuclear YAP abundance. AKT, Ak strain transforming; AP, activator protein; APC, adenomatous polyposis coli; cAMP, 3′ 5′‐cyclic adenosine monophosphate; CK1, casein kinase 1δ/1ε; Crb, crumbs; DVL, dishevelled segment polarity protein; EGFR, epidermal growth factor receptor; FAK, focal adhesion kinase; FGFR, fibroblast growth factor receptor; FRMD, FERM and PDZ domain containing; GSK, glycogen synthase kinase; HH, hedgehog; KLF, Krüppel‐like factor; LRP, LDL receptor‐related protein; MAP4K, mitogen‐activated protein kinase kinase kinase kinase; MLC, myosin light; MLCK, myosin light chain kinase; MLCP, myosin light chain phosphatase; NECD, Notch extracellular domain; P, phosphorylation; PDK, pyruvate dehydrogenase kinase; PI3K, phosphoinositide 3‐kinase; PKA, protein kinase A; PP1, protein phosphatase 1; PTPN, protein tyrosine phosphatase non‐receptor type; RASSF, RAS association domain family; ROCK, Rho‐associated kinase; RUNX, Runt‐related transcription factor; SFK, SRC‐family kinase; SMAD, suppressor of mothers against decapentaplegic; SMO, smoothened; STAT, signal transducer and activators of transcription; TAO, thousand and one; TBX, T‐box transcription factor; TGF, transforming growth factor; Ub, ubiquitylation; VEGFR, vascular endothelial growth factor receptor. Dotted lines indicate post‐translational modification events. Figure graphics were created with BioRender.com.

FIGURE 3

Regulation of YAP/TAZ in normal and neoplastic epidermal cells. Speculative aspects of signalling pathways that are not yet supported by experimental data are indicated by faded graphical elements. Hippo signalling via MOB1A/MOB1B and large tumour suppressor (LATS)1/LATS2 inhibits Yes‐associated protein (YAP) and transcriptional co‐activator with PDZ‐binding motif (TAZ) via serine phosphorylation (orange) to promote cytoplasmic retention and/or proteasomal degradation. Integrin (ITG)–SRC signalling in the basal epidermal cell compartment promotes YAP/TAZ nuclear localization and TEA domain (TEAD) binding. Direct phosphorylation of YAP/TAZ on tyrosine residues (pink) by SRC promotes increased nuclear localization. In the nucleus, vestigial such as family member 4 (VGLL4) competes with YAP/TAZ in binding to TEADs, while WW domain binding protein‐2 (WBP2) enhances the co‐activator functions of YAP/TAZ. The contractile F‐actin‐myosin cytoskeleton stabilizes ITGβ1 adhesions and thus contributes to SRC activation. ITGβ4 adhesions are part of hemidesmosomal complexes that are anchored to keratin 5/14 intermediate filaments. Nuclear localization of YAP in basal epidermal cells is also negatively regulated by PTPN14. At adherens junctions, α‐catenin controls YAP/TAZ activity and phosphorylation by modulating its interaction with 14–3‐3. α‐catenin can also inhibit activation of SRC by ITGβ4. EGFR signalling inactivates the Hippo pathway through the stimulation of PI3K–PDK1. G protein‐coupled receptor (GPCR) signalling, involving Gαs and PKA, suppresses LATS1/2 activation, presumably via decreasing F‐actin‐myosin cytoskeletal contractility downstream of Rho/ROCK. During cSCC progression, loss of function of the protocadherin FAT1 activates a CAMK2‐CD44‐SRC axis that promotes nuclear translocation of YAP and this drives the expression of zinc finger E‐box binding homeobox 1 (ZEB1) that stimulates the mesenchymal state. FAT1 loss of function also inactivates enhancer of zeste homologue 2 (EZH2), promoting SRY‐box transcription factor 2 (SOX2) expression, which sustains the epithelial state. Together, these molecular events promote a hybrid epithelial‐to‐mesenchymal transition (EMT) phenotype. If FAT1 can directly activate the Hippo pathway, is currently not known. In BCC, fibroblast activation and ECM remodelling in papillary dermis as a consequence of increased HH signalling in the epidermis may indirectly activate epidermal ROCK signalling through mechano‐reciprocity. AKT, Ak strain transforming; AP, activator protein; cAMP, 3′ 5′‐cyclic adenosine monophosphate; CAMK, Ca²⁺/calmodulin‐dependent protein kinase; CK1, casein kinase 1δ/1ε; DCHS, dachsous; EGFR, epidermal growth factor receptor; FAK, focal adhesion kinase; HH, hedgehog; MLC, myosin light chain; MLCK, myosin light chain kinase; MLCP, myosin light chain phosphatase; P, phosphorylation; PDK, pyruvate dehydrogenase kinase; PI3K, phosphoinositide 3‐kinase; PKA, protein kinase A; PP1, protein phosphatase 1; PTPN, protein tyrosine phosphatase non‐receptor type; RASSF, RAS association domain family; ROCK, Rho‐associated kinase; SFK, SRC‐family kinase; SMO, smoothened; Ub, ubiquitylation. Dotted lines indicate post‐translational modification events. Figure graphics were created with BioRender.com.