Typically, YAP and TAZ are regarded as the effectors of the Hippo cascade of kinases, the core components of which comprise the mammalian STE20-like protein 1/2 (MST1/2) kinase and large tumor suppressor 1/2 (LATS1/2) kinase (Figure 1) [54]. Activated MST1/2 induces the phosphorylation and activation of LATS1/2. In turn, LATS1/2 promotes the phosphorylation of YAP/TAZ, resulting in cytoplasmic retention by binding to 14-3-3 protein and proteasomal degradation [55,56,57]. Conversely, those YAP/TAZ proteins not phosphorylated can enter the nucleus and initiate gene transcriptions, predominantly depending on their interaction with the TEA domain (TEAD) family [58].

Although the main issues of YAP/TAZ signaling concentrate on cell proliferation and differentiation, increasing evidence indicates their key roles in hepatocyte metabolism (Figure 2) [16,59]. For example, YAP knockdown reduced lipid droplets deposition in liver tumor cells induced by anti-PD-1 treatment, suggesting YAP promoted lipid uptake and synthesis [60]. This is in line with a study using the dexamethasone-induced hepatomegaly model, where the pregnane X receptor/YAP pathway was activated together with enhanced uptake of fatty acids and suppressed lipolysis and fatty acid β-oxidation [61]. Furthermore, overexpressed YAP in HCC cells promoted lipid peroxides production by transcriptionally upregulating the arachidonate lipoxygenase 3, which sensitized HCC cells to ferroptosis [62]. Given that YAP inhibition in mammary epithelial cells or prostate cancer cells reduced free fatty acid contents but increased triglyceride and cholesterol levels within cells [63,64], YAP-mediated lipid metabolism may vary with cell types.

Glucose metabolism is a major energy source for supporting cellular activities in liver. As the exclusive pathway for initiating glucose metabolism, glycolysis was found to be prevalent in energy-consuming tumor cells, along with frequent YAP nuclear translocation [65]. Indeed, YAP activation increased ATP content, glucose consumption, lactic acid production, extracellular acidification rate, and glycolysis-related enzymes expression [66,67,68], indicating the accelerated glycolytic rate. Hexokinase 2, lactate dehydrogenase A, glucose transporter 1 (GLUT1), and 6-phosphofructo-2-kinase/fructose-2,6-biphosphatase 3/4 (PFKFB3/4) contribute to glucose uptake [69,70]. GLUT1 was identified as the direct transcriptional target of YAP/TEAD1 to promote the glycolysis process [66,71,72]. In addition, the hypoxic microenvironment also helped YAP to be liberated from cytoplasm and stabilized in the nucleus [73,74,75,76]. Glycolysis was enhanced when YAP bound the promoters of pyruvate kinase M2 or PFKFB3 with the assistance of hypoxia-inducible factor 1α or TEAD1, respectively [77,78]. When blood glucose is too low to meet body requirements, gluconeogenesis is initiated by glucagon. In a capping actin protein of a muscle Z-line (CAPZ) liver-specific knockout mouse, YAP hyperactivation was observed to be associated with decreased gene expressions of glucose-6-phosphatase, phosphoenolpyruvate carboxylase 1, and fructose-1,6-bisphosphatase 1, serving as the key genes required for gluconeogenesis [79]. Loss-and-gain functional experiments in mouse primary hepatocytes demonstrated that active YAP inhibited gluconeogenic gene expression induced by glucagon or dexamethasone treatment, through preventing peroxisome proliferator–activated receptor-gamma coactivator 1 from binding to the promoters of gluconeogenic targets [80]. Overall, YAP plays a key role in promoting glucose uptake and consumption while inhibiting gluconeogenesis and, hence, is essential for maintaining glucose homeostasis in the whole organism [81,82].

YAP/TAZ is also required for amino acid metabolism in liver. For instance, the serine/glycine-producing enzymes were increased by YAP overexpression [83]. Activated YAP/TAZ upregulated the amino acid transporters of solute carrier 38A1 (SLC38A1), SLC7A5, and SLC3A2 to supply sources of essential amino acid synthesis [84,85]. Glutamine is the most abundant nonessential amino acid and can be used as the complement of intermediate products in the tricarboxylic acid after deamination by glutaminase and subsequent conversion into α-ketoglutarate [86]. Knockdown of YAP or TAZ could reduce intracellular glutamate concentrations with different mechanisms, where YAP deletion reduced expression of SLC1A5 but not glutaminase, while TAZ deletion decreased both SLC1A5 and glutaminase expression. Experiments in zebrafish indicated that YAP reprogramed glutamine metabolism by upregulating the expression and activity of glutamine synthetase to drive nucleotide biosynthesis [87].

Furthermore, YAP/TAZ participates in other metabolic processes in liver. It directly regulated the expression of key enzymes involved in deoxynucleoside triphosphates (dNTP) biosynthesis required for DNA replication and dNTP precursor pools maintenance, enabling cells to resist chemotherapeutics targeting dNTP synthesis and to avoid oncogene-induced senescence phenotype [88]. In addition, activated YAP promoted choline metabolism by inducing expression of those genes involved in apical extrusion during cell competition [89]. Moreover, YAP overexpression promoted the synthesis of ECM components such as aggrecan, type II collagen, and sulfated glycosaminoglycan [90].

Evidently, YAP/TAZ is known to positively regulate lipogenesis, glucose metabolism, amino acid metabolism, glutaminolysis, dNTP synthesis, choline metabolism, and ECM synthesis, but to negatively regulate gluconeogenesis (Figure 2). Interestingly, liver metabolism after YAP/TAZ inhibition is consistent with those functional changes induced by microgravity. Therefore, it reasonably anticipates that YAP/TAZ may act as a key modulator when it reacts to microgravity stimuli.

Increasing knowledge suggests that YAP/TAZ can respond to microgravity, although such responses have been poorly understood in hepatocytes. When cardiovascular progenitor cells (CPCs) were cultured on the International Space Station or with 2D clinostat on the ground, both spaceflight and simulated effects of microgravity were able to induce gene expressions of YAP and its target, superoxide dismutase 2, a marker of cell survival [15]. However, this finding was limited to CPCs derived from adult humans, because the YAP gene was downregulated by spaceflight in neonatal CPCs [91]. In human colorectal cancer cell HCT116, simulated effects of microgravity using RWV promoted YAP nuclear localization, which was correlated with stemness marker expressions of octamer-binding transcription factor 4 (Oct4), SRY-box 2 (Sox2), homeobox protein nanog (Nanog), and NK2 homeobox 5 [92]. On the contrary, YAP nuclear entry was impaired in glioblastoma cells or mesenchymal stem cells (MSCs) when cells were cultured within RPM or clinostat, respectively [93,94]. By utilizing a 2D clinostat, the activation and expression of TAZ were hindered by depolymerizing F-actin, stemmed from the simulated effects of microgravity, which accounted for the inhibited osteogenic differentiation of MSCs [95,96]. Furthermore, human microvascular endothelial cells subjected to hypergravity by centrifuge (4 g for 15 min or 20 g for 15 min~6 h) presented a dose-dependent increase in phosphorylated focal adhesion kinase, YAP, and myosin fibers, as well as improved tube formation, indicating a seemingly opposite response to microgravity in the same cell type [97,98]. Taken together, the YAP/TAZ pathway is positively or negatively regulated by microgravity stimuli in an experimental condition- or cell type-specific manner. It is yet difficult to draw an unambiguous conclusion from those studies since YAP/TAZ activation has not been measured in different cell types upon identical microgravity exposure or in the identical cell type under various microgravity loading modes. Nevertheless, space microgravity is assumed to be sensed by the YAP/TAZ molecule at the cellular level, although the elaborative perception mechanism remains obscure. Further exploration of whether YAP/TAZ is affected and how it is exactly inhibited by microgravity loading in hepatocytes, finally inducing liver dysfunction, is still needed.