The mechanical microenvironment of cells is largely determined by their neighbor cells and surrounding extracellular matrix (Discher et al., 2005; Paszek et al., 2005; Engler et al., 2006; Wells, 2008). The difference in ECM composition leads to unique three-dimensional networks in various tissues and determines the rigidity or elasticity of substrates where cells habituate and grow. This physical characteristic of the microenvironment profoundly influences cell phenotypes, not only their morphology but also internal cytoskeleton organization and trafficking (Myers et al., 2011; Wong et al., 2020). To investigate the effect of ECM stiffness on cell functions, engineered hydrogels interfacing with ECM proteins, such as fibronectin and collagens, have widely been used as substrates to grow cells in vitro. As a result, ECM stiffness has been shown to regulate cell growth, migration, and differentiation, among many other important cell behaviors.

It was first shown by the Piccolo group that increased ECM stiffness promotes nuclear localization of YAP and TAZ and upregulation of their target genes (Dupont et al., 2011). Though the underlying molecular mechanisms were not fully uncovered then, they demonstrated that actin cytoskeleton tension resulting from manipulations of cell spreading and substrate rigidity constitutes the key link between ECM stiffness and YAP/TAZ activation. High ECM stiffness promotes cell spreading and subsequently leads to actin cytoskeleton tension, which in turn results in nuclear translocation of YAP/TAZ with a to-be-defined mechanism. One key player that relays stiffness signals to YAP/TAZ is RhoA GTPase, which can sense ECM stiffness through focal adhesion and in turn promote actin polymerization and stress fiber formation. Actin cytoskeleton can regulate YAP and TAZ through both Hippo-dependent and -independent pathways (Dupont et al., 2011; Wada et al., 2011). On one hand, the Hippo pathway can be inactivated by actin polymerization and stress fiber formation at high ECM stiffness, and activated by relaxed actin cytoskeletons at low ECM stiffness. Phosphorylation and localization of YAP/TAZ are thus modulated by ECM stiffness through the Hippo pathway (Codelia et al., 2014; Li et al., 2018; Meng et al., 2018). On the other hand, the ARID1A-SWI/SNF complex binds to YAP/TAZ and thus prevents the interaction between YAP/TAZ and TEAD. Polymerized nuclear actin, under high ECM stiffness, binds to ARID1A-SWI/SNF complex, subsequently relieving its sequestration of YAP/TAZ (Chang et al., 2018).

To date, our mechanistic understanding of how ECM stiffness regulates YAP/TAZ remains incomplete as emerging players have been reported. ECM stiffness can be sensed by cells through focal adhesions, as cells use integrin to pull integrin ligands in the ECM. In addition to common integrin ligands such as collagens and fibronectin, an ECM proteoglycan, Agrin, also act as an extracellular mediator of matrix mechanotransduction to regulate YAP/TAZ via integrins, its specific membrane receptor LRP4, and the actin cytoskeleton (Chakraborty et al., 2017; Chakraborty et al., 2020). Similarly, matricellular protein thrombospondin-1, acts through integrin-1 and the Hippo pathway, but not actin cytoskeleton, to activate YAP/TAZ (Yamashiro et al., 2020). Besides signaling through integrin/focal adhesion, plasma membrane protein Caveolin-1 (CAV-1) also play a critical role in mechanoregulation of YAP/TAZ (Moreno-Vicente et al., 2018). CAV1 loss promotes the interaction between YAP and the 14-3-3 protein YWHAH in an F-actin-dependent but Hippo-independent manner.

Besides membrane proteins, signaling transducers like small GTPases and protein kinases also mediate stiffness signals to the Hippo pathway. Small GTPases, such as RhoA and RAC1, have been demonstrated for their roles as molecular switches for stiffness sensing (Bae et al., 2014). Another GTPase, RAP2, also senses stiffness cues from focal adhesions (Meng et al., 2018; Yamashiro et al., 2020), as ECM stiffness modulates the GDP/GTP loading status of RAP2. Active/GTP-loading RAP2 activates non-canonical Hippo kinases MAP4K4/6/7 by direct binding to their hydrophobic motif leading to LATS activation and YAP/TAZ inhibition. JNK is also an important mediator of stiffness sensing for YAP/TAZ regulation in epithelial cells (Codelia et al., 2014). Stiffness-activated JNK can phosphorylate LIMD1, which directly binds to LATS1/2 and reduces their kinases activities, eventually leading to activation of YAP/TAZ.

Almost all adherent cells, in vitro or in vivo, experience physical forces transmitted between cells and across tissues, and one of the most common forces in animals is tension caused by mechanical stretch. For example, when cells undergo tissue growth or disease progression, the increase in ECM stiffness generates a static tension that stretches cells. Such kind of stretch forces, as previously discussed, modulate cell behaviors via a YAP/TAZ-dependent mechanism. In addition to ECM stiffening, stretching forces from external sources, such as tissue mechanical strain and artificial cell stretching devices, are able to regulate cell functions through the interplay of the Hippo pathway (Driscoll et al., 2015; Fletcher et al., 2018). Moreover, recent research has linked YAP dysregulation to malfunction of the cardiovascular system caused by pressure overload (Byun et al., 2019), suggesting a critical regulatory role of the Hippo-YAP/TAZ pathway in cells in response to various magnitudes of stretch force.

Static stretch activates YAP/TAZ by suppressing capping and severing proteins of F-actins (Aragona et al., 2013). Additionally, the stretch force applied to the nucleus may decrease the mechanical restriction of nuclear pores, thus facilitating nuclear transport of YAP/TAZ (Elosegui-Artola et al., 2017). The linker of nucleoskeleton and cytoskeleton (LINC) complex, which transfers cytoskeletal strain to the nucleus, is a key player for mechanical stretch to activate YAP/TAZ in this process (Driscoll et al., 2015; Koushki et al., 2020).

Cyclic stretch has also been reported to activate YAP/TAZ by suppression of the Hippo pathway (Codelia et al., 2014), even in cells on soft matrices (Cui et al., 2015). In Drosophila, the physiological mechanical strain can drive activation and nuclear localization of YAP/TAZ homolog Yki in Drosophila follicular epithelium to promote cell proliferation, via inactivating the LATS1/2 homolog Warts (Fletcher et al., 2018). The novel Hippo kinase Msn plays a critical role in sensing mechanical stretch in Drosophila gut. Mechanical stretch dissociates Msn from the plasma membrane and thus prevents phosphorylation and activation of Msn by the Tao kinase, which in turn activates expansion of Drosphila gut stem cells and leads to intestinal hyperproliferation (Li et al., 2014, 2018).

In mammals, cyclic stretch is known to act through thrombospondin-1/RAP2 in blood vessel cells to activate YAP and promote vascular remodeling (Yamashiro et al., 2020). Thrombospondin-1 acts via integrin αvβ1 to form focal adhesions and promotes nuclear shuttling of YAP by inactivating the RAP2 GTPase, results in vascular remodeling in response to the pulsatile blood flow and pressure. It is worth mentioning that externally applied mechanical stretch and ECM stiffening shared many mechanosensing machineries, such as RAP2 and Msn/MAP4Ks, to regulate YAP/TAZ activities. It would be important to understand how these universal machineries work in concert with cell-specific mechanosensors (e.g., VEGFR2 in endothelial cells) to control mechanoresponses of cells in future.

Cell-cell contact inhibition is the phenomenon that cells avoid proliferating as they achieve convergence in monolayers. During carcinogenesis, transformed cells slowly lose this character (Hanahan and Weinberg, 2011). Loss of contact inhibition allows the transformed cells to overcome physical restraint and in many cases contributes to cancer cell aggressiveness. Contact-dependent signaling is one of the first extracellular stimuli discovered to regulate the Hippo pathway (Zhao et al., 2007Zhao et al., 2008). Direct contact between normal cells activates the Hippo kinases, thus leading to phosphorylation and cytoplasmic retention of YAP/TAZ, triggering cell cycle arrest and/or autophagy (Pavel et al., 2018). In contrast, hypophosphorylation and nuclear localization of YAP/TAZ have been associated with loss of contact inhibition of cancer cells resulting from somatic mutations (Zhao et al., 2007; Zhang et al., 2010; Tranchant et al., 2017; Frank et al., 2018; Ouyang et al., 2020).

Various mechanisms have been proposed to explain how cell-cell contact inhibition modulates activities and localization of YAP/TAZ. These mechanisms can be roughly grouped into two types. The first one usually involves cis-interaction of cell membrane proteins at high confluence. For example, for tight junctions (TJs) form between cells, angiomotin (AMOT) complex at TJs is activated and transmit the antiproliferative signal from TJs to YAP via two independent mechanisms: AMOT can directly bind to YAP and thus sequester YAP at TJs, and/or AMOT can activate Merlin/NF2 to trigger LATS1/2-dependent YAP phosphorylation (Li et al., 2015). In addition to TJs, adherens junctions (AJs) protein E-cadherin, in confluent cells, trans-dimerize and subsequently stimulate MST1/2-LATS1/2 kinase cascade to inhibit activities of YAP/TAZ (Kim et al., 2011). PTPN14, a protein tyrosine phosphatase, can inhibit YAP transactivation activity through a direct interaction in response to cell confluence (Wang et al., 2012; Liu et al., 2013). The other type of mechanisms involves cell geometry and actin cytoskeleton remodeling. High cell density, as well as low ECM stiffness, reduces adhesive area and alters cell shape, which leads to inactivation of RhoA and subsequent reduction of stress fiber of actin cytoskeleton, which can inactivate YAP/TAZ through both Hippo kinases-dependent and -independent mechanisms (Aragona et al., 2013; Meng et al., 2018; Chang et al., 2018). Spectrin has been recognized as a key cytoskeletal protein that restricts YAP/TAZ activity in response to mechanical cues, such as cell-cell contact inhibition, through remodeling actin cytoskeleton particularly at cortical areas of cells. Loss of Spectrin proteins results in hyperactivation of YAP/TAZ, likely by elevating cortical myosin II activity, leading to cell over-proliferation even when cells confluence is reached in both mammals and Drosophila (Deng et al., 2015; Fletcher et al., 2015; Wong et al., 2015; Deng et al., 2020).

Shear stress, a fluid frictional force, is another major mechanical stimulus maintaining tissue homeostasis. Indeed, one of the most studied cell types in mechanotransduction is vascular endothelial cells (ECs), lining in the innermost layer of blood vessels. ECs are constantly subjected to shear stress from blood flow, and it is well recognized that ECs are able to sense and respond to changes in flow direction, pulsatility, and magnitude of shear stress via mechanosensors and mechanosensitive signaling pathways. As a result, endothelial phenotypes are highly associated with local blood flow patterns and distinct in different regions of the vascular tree. Three recent studies independently confirmed that flow patterns modulate endothelial phenotypes through regulation of YAP/TAZ activities: unidirectional laminar flow suppresses YAP/TAZ activities to keep ECs quiescent and inert to inflammatory cells, while disturbed oscillatory flow activates YAP/TAZ to promote a pro-proliferative and -inflammatory EC phenotype (Wang K. et al., 2016; Wang L. et al., 2016; Xu et al., 2016).

Mechanistically, the integrin–Gα13–RhoA axis was first reported to mediate the flow regulation of YAP/TAZ activities in ECs (Wang L. et al., 2016) and two recent studies revealed that disturbed flow acts through integrin α5β1 to induce YAP nuclear translocation and promote the pro-atherogenic responses, via c-Abl kinase and phosphodiesterase 4D5, respectively (Li et al., 2019; Yun et al., 2019). However, the mechanisms regarding how the flow-activated integrin signaling cascades crosstalk with Hippo kinases to modulate YAP/TAZ activities remain to be studied. In addition to the integrin-mediated mechanisms, it has been shown that short-term unidirectional laminar flow (15 dyne/cm² for 10 min) increases the nuclear localization of YAP in a LATS1/2-independent but an angiomotin-regulated manner (Nakajima et al., 2017). Last but not the least, caveolae, the plasma membrane microdomain, is known to sense shear stress signals and have been shown to relay such mechanical cues through the Hippo pathway to facilitate mechanoregulation of YAP/TAZ (Rausch et al., 2019). However, whether the caveolae-dependent mechanism mediates the flow regulation of endothelial phenotypes remains to be determined.

In addition to ECs, many other cell types, such as metastatic tumor cells and mesenchymal stem cells, are known to perceive shear stress stimuli and transduce the resulting biochemical signals in regulating cellular functions (Lee et al., 2017, 2018; Qin et al., 2019). In fact, the connection between shear stress and YAP was first reported in mesenchymal stem cells, where it was shown that exposure of mesenchymal stem cells to shear stress enhances YAP expression to promote their differentiation into chondrocytes (Zhong et al., 2013). More in vitro and in vivo studies are warranted to validate the roles of YAP/TAZ as mechanotransducers in modulating biological functions in the above-mentioned cell types.

The cardiovascular system is subjected to continuously shifting mechanical signals, including stretch, compression, distortion, and shear. Mechanotransduction profoundly influences the development of the cardiovascular system and the regulation of physiological functions (Garoffolo and Pesce, 2019). The roles of YAP/TAZ in CVDs have been well summarized in a recent review (Yu et al., 2020). Aberrant activation of YAP/TAZ contributes to a variety of cardiovascular conditions: atherosclerosis, pulmonary hypertension, myocardial hypertrophy, angiogenesis, restenosis, and myocardial fibrosis, while hypoactivation of YAP/TAZ is associated with aortic aneurysms, aortic dissection, reperfusion of myocardial ischemia, and myocardial infarction. Our review will solely focus on YAP/TAZ dysregulation in mechanical cues resulting from/in pulmonary hypertension, atherosclerosis, and cardiac hypertrophy.

Inflammation and wound-healing process following tissue injury and/or an idiopathic disease induce profibrotic responses, including abnormal ECM synthesis and deposition by fibroblasts and tissue stiffening and thickening. Such changes in structural and mechanical properties of the tissue are usually irreversible and lead to the formation of scar tissue, also known as fibrosis. The physical features of fibrotic tissues, together with the unresolved inflammation, continuously stimulate fibroblasts, resulting in pathological accumulation of ECM components and permanent tissue damage (Rockey et al., 2015; Tschumperlin et al., 2018).

YAP/TAZ serve as critical mechanotransducers in fibroblasts, coordinating profibrotic responses in various tissues, such as hepatic fibrosis, pulmonary fibrosis, kidney fibrosis, cardiovascular fibrosis, and others (Liu et al., 2015; Kim et al., 2019). The initial increase in ECM stiffness activates YAP/TAZ in fibroblasts, encouraging the development of profibrotic mediators and excessive deposition of ECM components. This results in progressive tissue stiffening, thereby forming an activation feed-forward loop to promote tissue fibrosis (Bertero et al., 2015a; Liu et al., 2015). Moreover, YAP/TAZ act as a molecular link between fibrosis and cancer. Fibrotic ECM stimulates cell proliferation and changes cell polarity, thereby promoting tumor development and growth (Noguchi et al., 2018). Various studies have assessed the efficacy of inhibiting YAP/TAZ activity as a new therapeutic strategy to reverse fibrosis (Liang et al., 2017; Haak et al., 2019; Alsamman et al., 2020; Dey et al., 2020), and the results of those have unfolded a promise of YAP/TAZ-targeting therapies for organ fibrosis.

MSDs refers to diseases that affect the muscles, bones, and joints, which mainly tendinitis, carpal tunnel syndrome, osteoarthritis (OA), rheumatoid arthritis (RA), etc.

Mechanical load has been shown to activate YAP by increasing the expression of nuclear accumulation of YAP. Hyperactivation of YAP by sustained mechanical overload or YAP overexpression alone is sufficient to induce skeletal muscle hypertrophy (Goodman et al., 2015; Iyer et al., 2019; Owens et al., 2020). Besides responding to mechanical load, YAP/TAZ play a crucial role in muscle cell stemness and myogenesis (Figeac et al., 2019; Zhang L. et al., 2019). However, more studies are required as their atrophic roles of YAP/TAZ in muscle cells have also been reported (Gnimassou et al., 2017), and more details about the connection between mechanical stimuli and YAP/TAZ signaling would further improve our understanding of the roles YAP/TAZ in mechanotransduction and muscle homeostasis.

OA is mainly caused by mechanical overload (Glyn-Jones et al., 2015), and it has been reported that YAP is both necessary and adequate to preserve cartilage homeostasis in OA (Deng et al., 2018; Zhang Q. et al., 2019). A few recent studies demonstrated that suppressing YAP activity is effective in attenuating OA progression (Gong et al., 2019; Thorup et al., 2020; Zhang et al., 2020). However, research is still lacking on how the mechanical forces, especially compression and hydrostatic pressure, regulate Hippo-YAP/TAZ pathway in chondrocytes in healthy cartilage and during OA development.

The biophysical factors in the tumor microenvironment play a pivotal role in disease progression and treatment outcomes. Common physical features that hinder cancer treatment, including solid stresses, interstitial fluid pressure, stiffness, and tumor microarchitecture, have been nicely reviewed by Nia HT, Munn LL, and Jain RK very recently (Nia et al., 2020). It has long been known that the changes in tumor microenvironment trigger signaling pathways to fuel cancer progression, immune blockade, and cancer resistance to therapy. Among the mechanosensitive pathways, the dysregulation of the Hippo-YAP signaling plays a central role in tumor development as it has been connected to most of the physical features in tumors. Emerging efforts have been made to elucidate the molecular mechanisms by which mechanical properties of various tumor types act through YAP/TAZ to promote cancer pathology.

Loss of cell-cell contact inhibition and unchecked cell growth are hallmarks of cancer, and the Hippo pathway is known to mediate contact inhibition and cell proliferation. Indeed, dysregulation of the Hippo signaling and hyperactivation of YAP/TAZ not only abolish the cell cycle regulation, as a result of losing contact inhibition, but also promote the transformation of mammary epithelial cells (Zhao et al., 2007, 2008). As the tumor grows, the changes in mechanical properties of the microenvironment continuously contribute to cancer progression and malignancy. Numerous studies have documented that stiff substrates activate YAP/TAZ to increase primary cancer cell growth, migration, and chemotherapy resistance (Lin et al., 2015; Chakraborty et al., 2017; Foster et al., 2017; Meng et al., 2018; Santinon et al., 2018; Molina et al., 2019; Ghasemi et al., 2020; Liu et al., 2020; Qin et al., 2020). Furthermore, it has been shown that Ras signaling-mediated oncogenic transformation of normal cells requires a stiff and/or fibrotic microenvironment (Meng et al., 2018; Panciera et al., 2020).

Mechanistically, the ECM-activated YAP/TAZ regulates the expression of various cytoskeletal regulators in cancer-associated fibroblasts and increases intracellular isometric tension, thus forming a forward feedback loop to enhance the tumor microenvironmental rigidity and support cancer cell growth and invasion (Calvo et al., 2013; Foster et al., 2017). It is speculated that YAP/TAZ hyperactivation is required for cancer cells to overdrive the mechanical checkpoints for growth (Aragona et al., 2013). In addition to cancer cells and cancer-associated fibroblasts, ECM stiffening during tumor progression induces vascular cell growth and allows blood vessel infiltration, potentially through the Agrin-YAP-dependent mechanism (Chakraborty et al., 2020). Indeed, it has recently been shown that the increases in tissue rigidity at metastatic sites enhance cancer angiogenesis and elevate their resistance to anti-angiogenesis therapy (Shen et al., 2020).

Cancer cells migrate and metastasize in blood and lymphatic vessels. Therefore, it would be important to know the flow regulation of YAP/TAZ in cancer cells. It has been reported that fluid shear stress induces YAP/TAZ activation to promote cancer cell migration and proliferation (Lee et al., 2017, 2018; Qin et al., 2019), suggesting a causative role of YAP/TAZ activation in cancer metastasis. In this regard, modulation of Hippo-YAP/TAZ signaling may represent a novel strategy for anti-metastasis therapies. In addition, the role of YAP/TAZ-mediated mechanosensing in tumorigenesis may be more complex and multi-functions in context-dependent manners and therefore worth further exploring. For example, a recent study in mice showed that activation of the YAP/TAZ in peritumoral normal hepatocytes suppressed liver tumor growth via cell competition mechanism against tumor cells (Moya et al., 2019). It would be important to elucidate the exact roles of YAP-dependent mechanoresponses in physical interaction between normal and tumor cells in future studies.