p53 shades of Hippo
The three p53 family members, p53, p63 and p73, are structurally similar and share many biochemical activities. Yet, along with their common fundamental role in protecting genomic fidelity, each has acquired distinct functions related to diverse cell autonomous and non-autonomous processes. Similar to the p53 family, the Hippo signaling pathway impacts a multitude of cellular processes, spanning from cell cycle and metabolism to development and tumor suppression. The core Hippo module consists of the tumor-suppressive MST-LATS kinases and oncogenic transcriptional co-effectors YAP and TAZ. A wealth of accumulated data suggests a complex and delicate regulatory network connecting the p53 and Hippo pathways, in a highly context-specific manner. This generates multiple layers of interaction, ranging from interdependent and collaborative signaling to apparent antagonistic activity. Furthermore, genetic and epigenetic alterations can disrupt this homeostatic network, paving the way to genomic instability and cancer. This strengthens the need to better understand the nuances that control the molecular function of each component and the cross-talk between the different components. Here, we review interactions between the p53 and Hippo pathways within a subset of physiological contexts, focusing on normal stem cells and development, as well as regulation of apoptosis, senescence and metabolism in transformed cells.
In approximately half of human cancers TP53 is mutated; in many of the remaining half, the function of the retained wild-type (wt) p53 protein is compromised by deregulation of upstream or downstream regulators.1 Functionally, p53 inhi- bits the proliferation of potentially tumorigenic cells, chiefly through transcriptionally instigating cell cycle arrest, differentiation, senescence or apoptosis.2 Cancer-associated muta- tions in p53 may directly disrupt p53–DNA interactions or drive conformational changes in the p53 protein, in both cases leading to loss of transcriptional activity and tumor-suppressor capabilities.3 Mutations in the TP53 gene may also convey an additional selective advantage for tumors, as the mutant p53 (mutp53) protein may acquire cancer-promoting activities, augmenting cell migration, invasion and tumorigenesis.3 Thismutp53 gain-of-function (GOF) is executed often by ‘piggy- backing’ on other transcription factors or signaling pathways and enables the mutp53 to deregulate metabolic pathways, increase metastasis and enhance chemotherapy resistance.4–8 Recently, pro-survival functions have been attributed also to wt p53, with some activities shared between the wt and mutant isoforms.3,9–13 This may reflect a physiological role for p53 in maintaining homeostasis in the face of transient or fluctuating stress, and may confer context- dependent selective advantages also in those tumors that retain wt p53.10The p53 family of transcription factors consists of three paralogs, p53, p63 and p73, which evolved from a common ancestor through two gene duplication events.14 Interestingly, in organisms, which possess a single p53 family gene, for example, C. Elegans and Drosophila, this gene is more similar to an ancestral p63/p73 hybrid gene and less to p53 itself.14 Duplication events enabled the family members to expand their function and engage in additional cellular processes and signaling pathways, while retaining some common activities.14 Similar to p53, p63 and p73 are nuclear proteins that bind to canonical p53 DNA-binding sites and can transactivate p53- responsive promoters.
Both are principally recognized for their role in development, with p63 necessary for skin development and p73 linked to the formation of the nervous system.15–17 On top of their developmental roles, recent studies have determined that p63 and p73 are critical for the maintenance of genomic integrity and response to DNA damage in adult organisms.18,19 Like TP53, also the p63 and p73 genes can be transcribed from two alternative promoters and are alterna- tively spliced,20 giving rise to a profusion of isoforms.21–23 Of relevance to this review, the p63 and p73 isoforms can be roughly categorized into two groups: transactivation (TA) domain isoforms, which structurally resemble full-length p53 and act as tumor suppressors, and the ΔN isoforms, which in some cases can bind to p53, TAp63 or TAp73 to inhibit their function and promote tumorigenesis.24Differential upstream signaling distinctly impacts p53 family members’ stability and activity. Upon DNA damage, p53 is phosphorylated by numerous kinases, whereas p73 is primarily phosphorylated by c-Abl.25–27 Similarly, MDM2 is the primary E3 ubiquitin ligase for p53 but not p73,28,29 whereas ITCH has been shown to control the stability of both p63 and p73 but does not promote the degradation of p53.30,31 Another protein that differentially binds members of the p53family is YAP. YAP binds p63, p73 and mutp53, but not wt p53.32,33YAP was originally identified as a protein interacting with the c-Yes tyrosine kinase.34 Subsequent studies demonstrated that YAP is a transcription cofactor without direct DNA-binding activity.35 In addition to its interactions with p53 family members, YAP modulates transcription by binding to other transcription factors.36,37 Importantly, TEAD family transcrip- tion factors, which bind directly to specific DNA sequences and anchor YAP to chromatin, mediate most of the cell proliferative and anti-apoptotic transcriptional output of YAP and its paralog TAZ.
YAP and TAZ are phosphorylated and inactivated by the serine/threonine kinases LATS1 and LATS2 within the Hippo pathway.40,41 In mammals, the core cascade of the canonicalHippo pathway involves the mammalian sterile 20-like kinases MST1/2 (MST), which can phosphorylate and activate the two large tumor suppressors, LATS1 and LATS2 (LATS) kinases.42,43 This kinase module is bolstered by two adaptor proteins, WW45 and MOB1 (shown schematically in Figure 1). A plethora of upstream signals modulate this kinase module, facilitating diverse biological outcomes.42 Although the core cascade is evolutionarily conserved,44,45 the wide array of biological processes modulated by each component suggests that they elicit their biological impact also through additional pathways.46–48 For instance, cytoskeleton dynamics and mechanical cues are prominent regulators of MST/LATS- independent49 YAP/TAZ activity, which may liberate the kinases for interactions with the p53 pathway or other signaling pathways.In this review, we examine the cross-talk between the Hippo and p53 pathways in an attempt to better understand mutual modulation and integration of function. The major interactions and biological outcomes are summarized in Table 1 and are detailed in the following sections. Although the specifics differ within different biological contexts, it is clear that the two pathways are strongly intertwined at multiple levels. Interest- ingly, in multiple cases, cross-talk between specific compo- nents may lead to different, sometimes contradicting, cellular outcomes. Similarly, deregulation of either pathway, as often occurs in cancer, has profound effects on the reciprocal pathway and, by extension, on cell fate.A robust network of molecular regulation maintains embryonic stem cell (ESC) homeostasis. Signaling through Hippo and p53 pathways has important roles in dictating the fine balance of proliferation and differentiation of stem cells. For example, LATS kinases participate in maintaining the balance between pluripotency and differentiation, through both p53-dependent and -independent mechanisms. Inhibition of Lats1 and Lats2 expression in early mouse embryos results in irreversibleembryonic development. The core Hippo kinases and p53 act in parallel also in development to restrict organ size.58–60 For instance, conditional embryonic ablation of Yap or over- expression of p53 in mouse kidneys similarly causes under- development and small organ size.61,62In adult organisms, stem cell maintenance is required to sustain long-term preservation of tissue homeostasis.
The mammalian epidermis is a rapidly regenerating epithelial tissue whose maintenance depends on the self-renewing ability of epidermal stem cells residing in the basal layer.63 Both p63 and YAP have essential roles in epidermal basal stem cell proliferation and epithelial stratification, and are considered epidermal stem cell markers.64–68 Accordingly, activation of YAP increases the number of cells expressing p63 and expands the epidermal stem cell compartment.66 One factor that may mediate YAP-p63 cross-talk is 14-3-3σ; downregulation of 14-3-3σ, which normally restrains YAP transcriptional activity by sequestration of phospho-Ser127 YAP in the cytoplasm,41 results in the expansion and immortalization of primary keratinocytes expressing p63.68,69 Interestingly, the ΔNp63 isoform can repress 14-3-3σ expression.70 Together, these data suggest mechanisms by which YAP and p63 might augment each other’s function.In the lung, YAP and ΔNp63 functionally interact to regulate airway epithelial stem cell homeostasis.71 YAP regulates differentiation of adult lung epithelium progenitors72 by binding ΔNp63 to drive the expression of common target genes, including ΔNp63 itself.71,72 Persistent YAP expres- sion is required to restrain adult airway basal stem cell differentiation and following injury YAP is upregulated in the regenerative tissue.73 Accordingly, loss of either YAP or p63 is associated with an excess of differentiated ciliated cells. p63 null mutants die at birth without basal progenitors in their airway epithelium.74 Similarly, conditional deletion of Yap from fetal lung epithelial progenitors reduces p63-positive cells and disrupts branching morphogenesis, resulting in lung hypoplasia.
Thus, proper regulation of YAP-p63 signaling maintains the steady-state stem cell pool through balanced rates of stem cell self-renewal and differentiation. Disparity of either of these processes can set the stage for tumorigenesis.Elevated expression of YAP may cooperate with loss of p53 to promote the emergence of cancers with an altered differentia- tion status, relative to their cell of origin. Thus, p53 deletion in mouse mammary gland luminal cells leads to clonal expan- sion without compromising luminal identity.75 However, sub- sequent mammary tumors often undergo luminal-to-basal transition in conjunction with amplification of the Yap gene.75 Additional studies also confirm that inactivation of p53, or combined loss of p53 and Rb, in mammary epithelium results in mammary carcinomas that bear recurrent Yap amplifications.76,77 These carcinomas become ‘addicted’ to YAP overexpression76 and harbor features of EMT and stem cell-like transcriptional signatures.77 In line with this notion, amplification of the Yap genomic region is among the top ranked amplification events in mouse models of prostate cancer driven by p53/PTEN loss.78Stem cell-like transcriptional signatures are often asso- ciated with the acquisition of stem-like properties, in that a sub- population of transformed cells attains the ability to self-renew and generate the diverse types of cells that comprise the tumor.79 Cancer stem cells (CSCs), or tumor-initiating cells (TICs), have been identified in a wide variety of solid tumors and, analogous to embryonic and adult stem cells, both p53 family members and the Hippo components have been implicated as central regulators. TAZ is required for self- renewal and tumor initiation capacities in breast CSCs.80 Similarly, mutation of p53 is sufficient to instigate a CSC-like phenotype.81 Recently, Escoll and colleagues82 have demon- strated that mutp53 enhances YAP/TAZ stability by regulating the WASP-interacting protein, WIP, to promote CSC properties in breast cancer and glioblastoma. This study provides one mechanism to explain the functional link between mutp53 and YAP/TAZ during tumorigenesis.
Similarly, YAP inhibition restores hepatocyte differentiation in advanced hepatocellular carcinoma (HCC).83 Interestingly, different post-translational modifications, which have been shown to govern YAP stability, may dictate the differentiation status of HCC.84,85 Although hyperactivity of the YAP–TEAD functional axis predicts poor prognosis and stemness char- acteristics in HCC, tumors in which YAP is modified to interact with p73 (p-Y357) display more apoptotic markers and have better prognosis.85 Thus, YAP can promote either tumor cell death or tumor-initiating properties, depending on its regula- tory interactions with different members of the p53 family. In the next sections, we will review in more detail the YAP-p73 pro-apoptotic module and try to delineate the mechanisms that modulate the apparently opposing cellular functions of YAP.p63 was also shown to engage with YAP to promote tumorigenesis. In head and neck squamous cell carcinoma (HNSCC), p63 collaborates with the chromatin-remodeling factor ACTL6A to block differentiation and drive regenerative proliferation in a YAP-dependent manner.86 Mechanistically, p63 represses the WWC1 (Kibra) promoter. As WWC1 encodes a cytosolic phosphoprotein that activates LATS kinases,87 p63-dependent silencing of WWC1 activates YAP. In line with this, upregulation of YAP transcriptional targets is correlated with poor prognosis of HNSCC.86 In addition to activation of YAP by p63, YAP binds ΔNp63, not only in developmental settings but also in a tumorigenic context.88,89 Here, YAP binding leads to ΔNp63 stabilization by competition with the ITCH E3 ligase.88 Furthermore, the co-protective YAP-ΔNp63 association promotes epidermal squamous cell carcinoma CSC survival and migratory phenotypes.89 Inter- estingly, this interaction, like the YAP–mutp53 functional interaction,82 is regulated by membrane-associated integrin signaling.89In contrast to the oncogenic ΔNp63 isoform, which in somecases commandeers YAP to augment CSC properties, TAp63 and Hippo signaling seem to collaborate as tumor suppres- sors. In healthy tissue, TAp63 restricts mammary stem cell potential by inhibiting TAZ activity.90 Mechanistically, LKB1, a transcriptional target of p63,91 phosphorylates the microtubule affinity-regulating kinase family to safeguard LATS kinase activity.92
In aggressive human mammary adenocarcinomas, loss of TAp63 depolarizes cells and activates TAZ (but notYAP), which endows self-renewal capacity to breast cancer cells.80In summary, oncogenic YAP/TAZ often exploit functional deficiencies in the p53 family of tumor suppressors to propel stem-like characteristics in cancers. Furthermore, tumors that harbor mutp53 or ΔNp63 rely on YAP/TAZ to reinforce the stem cell properties and elude death.Cells bearing potentially transforming genomic aberrations must be eliminated to forestall tumorigenesis. The extent to which the Hippo tumor suppressors realize their anticancer function is often dependent on p53 or p73 activity.Transcriptional control of MST-driven apoptosis is con- served from Drosophila to mammals, with p53 serving as a central mediator throughout evolution. In flies, Hpo (the MST ortholog) is activated by ionizing radiation in a Drosophila melanogaster p53 (Dmp53)-dependent manner (Figure 2). In fact, ectopic expression of Dmp53 is sufficient to activate Hpo and elicit cell death.93 Induction of apoptosis by both Hpo and Dmp53 relies on inhibition of the Drosophila inhibitor of apoptosis protein, DIAP1. In response to radiation, Dmp53 transactivates the Reaper locus, which encodes DIAP1 inhibitors such as Hid, grim and reaper itself.94 In addition, Hpo negatively regulates DIAP1 both by modulating the transactivation capacities of Yki (the fly YAP ortholog) and by directly phosphorylating and inducing the ubiquitin-dependent degradation of DIAP1.95,96In mammals, the MST kinases facilitate apoptosis predo- minantly through transcription-independent mechanisms. MST1 augments apoptosis by phosphorylating and inhibiting the deacetylase SIRT1, thus enhancing p53 acetylation and function97 (Figure 2). Interestingly, SIRT1 is regulated by multiple kinases, such as CK2,98 cyclin B/Cdk199 and DYRK1A,100 kinases regulated collectively by LATS kinases,101–103 implicating a multi-pronged strategy by which Hippo kinases impact p53 activity.In flies, as well as in mammals, p53 and MST share upstream regulatory factors. Overexpression of the Rasassociation domain family 5 (RASSF5) activates both the MST1-LATS1 and p53 axes.104 Another RASSF member, RASSF1A, promotes p53 family and MST activation in response to genotoxic stress.105–107 DNA damage-induced phosphorylation of RASFF1A by ATM promotes MST2-LATS1 activation and p73 stabilization.
Mechanistically, RASSF1A promotes MST2 activation by abrogating the RAF1–MST2 inhibitory interaction.106 Subsequently, RASSF1A, together with the activated MST2-LATS1 complex, mediates apoptosis by inducing p73 transcription.108–110 Although the RASSF1A- MST-WW45 (SAV1)111 complex can regulate p73 activity, it has also been suggested that RASSF1-WW45 can regulate p73 independently of the MST-LATS module.112 In addition to activating the MST and p73 pro-apoptotic arms, RASSF1A also stabilizes p53 by promoting MDM2 degradation.107 Indeed, polymorphisms of RASSF1, in the region encoding the recognition site for ATM phosphorylation, convey resis- tance to DNA damage-inducing agents and are associated with decreased p53 stability and poor patient prognosis.108,113 Surprisingly, wt p53 can bind and repress RASFF1A by facilitating its promoter hypermethylation114,115 (Figure 2). Whether this represents an inherent negative feedback loop between p53 and RASFF1A, or a pro-survival function that is conferred upon wt p53 in the course of tumor progression, remains to be determined.Upstream modulation by RASSF proteins suggest a general sensitivity of the Hippo and p53 tumor-suppressor pathways to RAS-related signals. Indeed, aberrant hyperproliferation of cells caused by RAS oncogenic activation is a robust trigger of the p53-Hippo functional axis. Mutant K-RAS-dependent activation of the MST2-LATS1 kinase cassette results in sequestration of MDM2 by LATS1 and subsequent stabiliza- tion and activation of p53116 (Figure 2). Similarly, H-RAS triggers LATS2 nuclear translocation and an increase in LATS2 protein levels, which leads to p53 activation.117 LATS2- p53-dependent apoptosis upon oncogenic stress is mediated by phosphorylation of the apoptosis-stimulating protein of P531 (ASPP1), which shunts p53 to the promoters of pro- apoptotic genes.118 In transformed tissue, MST pro- apoptotic function is subverted by the ability of mutantK-RAS to divert wt K-RAS signaling.116 Similarly, H-RAS- driven transformation entails a reduction of LATS2 levels in order to overcome p53-dependent apoptosis and senescence.117
Thus, during RAS-driven transformation there is a strong selective pressure to deregulate Hippo signaling in order to overcome p53-related tumor-suppressive functions.Similar to oncogenic RAS, high YAP activity may disrupt the Hippo-p53 tumor-suppressive arm. In both flies and mammals, YAP expression overrides the ability of LATS and ASPP to drive p53-dependent transcription of pro-apoptotic genes. In Drosophila, Yki activation facilitates tissue growth by inhibiting p53 and ASPP, thereby repressing the expression of the pro- apoptotic gene reaper during tissue growth.119 In mammals, binding of cytoplasmic ASPP1 to LATS1 releases YAP/TAZ from negative regulation.120 Subsequently, the increased nuclear activity of YAP/TAZ inhibits apoptosis under low serum conditions and protects cells from anoikis. Over- expression of YAP can also inhibit apoptosis by disrupting the LATS2-ASPP1-p53 pro-apoptotic cascade118 (Figure 2). Similarly, nuclear YAP can repress LATS2 expression to inhibit replication stress-induced senescence mediated by p53- dependent induction of p21.121 However, under different conditions, the YAP–TEAD complex can actually bind the LATS2 promoter to augment its transcription,122,123 suggest- ing that differential binding of YAP to various cofactors may differently modulate LATS2-p53 activity. Interestingly, this mode of regulation may be bidirectional, also enabling p53 to exert anti-YAP activities, as treatment with the YAP inhibitor Verteporfin causes sequestration of YAP in the cytoplasm, which is mediated by p53-dependent transcriptional induction of 14-3-3σ.124,125In contrast to the above-described antagonistic roles of YAP and p53, there are apoptotic conditions in which YAP and p53 family members cooperate. In highly malignant tumor- repopulating cells, which are enriched by growing cancer cells on a soft matrix, the expression of nuclear protein 1 (NURP1) is compromised by reduced nuclear localization of YAP.126 Reduction in NURP1 expression is followed by a reduction in p53 mRNA and protein levels, suggesting an anti- tumorigenic axis involving YAP-NUPR1-p53.
Similarly, YAP increases the sensitivity of hepatocellular and renal carcinoma cells to chemotherapy by increasing p53 expression and activity.127,128 Interestingly, p53 can also drive a positive feedback, to transcriptionally increase YAP expression; indeed, YAP was recently identified as a direct transcriptional target of p53 in a genome-wide characterization of putative p53 response elements129 (Figure 2). Finally, YAP can facilitate p53-dependent cell death by promoting the stabiliza- tion of the pro-apoptotic factor ASPP2.130In addition to modulation of p53 activity, YAP, as a transcriptional co-activator, can drive apoptosis by cooperat- ing with p73. This functional interaction may be strongly conserved over evolution. In Drosophila, Yki interacts with the ancestral single p53 gene, Dmp53, to induce apoptosis.131 As Dmp53 resembles a p63/p73 hybrid gene, this may be analogous to mammalian YAP binding to p73, protecting p73 from ITCH-dependent degradation and driving apoptosis.132,133 YAP also enhances p73 transcriptional function by promoting p300-mediated p73 acetylation and recruitment to the promoters of apoptotic genes.106,110,134 Ofnote, RASSF1 promotes binding of YAP to p73, in preference over other transcription factors such as TEAD and RUNX2.135 Thus, under specific regulatory cues, YAP has a central role in the cellular response to DNA damage by augmenting p73 stability and driving pro-apoptotic transcription. In tumors retaining wt p53, curtailing of the tumor-suppressive YAP-p73 axis may be particularly crucial. Supporting this notion, re- expression of YAP in YAP-deficient multiple myeloma cells drives apoptosis by increasing p73 (not p53) levels and activity.
Interestingly, YAP and p73 are inhibited, transcrip- tionally and post-translationally, by ΔNp63,22,137 delineating an additional layer of cross-regulation between the pathways.The role of apoptosis as a tumor-suppressive mechanism is intuitive: it irreversibly removes dangerously transformed cells from the replicative pool. However, in other instances, senescence is the cell’s method of choice to prevent proliferation of pre-malignant cells.138 One strong trigger for senescence is the genotoxic acquisition of aneuploidy,139 which can occur when chromosomes fail to properly segregate during mitosis.140 Numerical changes in whole chromosomes often precede transformation and are powerful inducers of p53.140,141 In response to mitotic stress, LATS2 binds and inhibits MDM2, to enable p53 activation and induce G1/S arrest. In turn, activated p53 transcriptionally induces LATS2 expression, creating a positive feedback loop to ensure that the cell maintains proper DNA content53,142 (Figure 2). The ability of LATS2 to sense extra chromosomes has been attributed to changes in the actin cytoskeleton, specifically to the reduction of RHOA activity.143 Indeed megakaryocytes, naturally occurring polyploid cells, in which polyploidization is needed for platelet formation, escape the p53-LATS axis by uncoupling RHOA activity from Hippo signaling.144 Thus, induction of LATS2 in response to ploidy aberrations enables simultaneous activation of a protective p53-dependent growth arrest and inactivation of YAP/TAZ pro-proliferative signals.LATS2 and p53 cooperate also in response to replication stress, to induce p21 expression and promote cellular senescence.117,121 This p53-LATS homeostatic axis is tar- geted by oncogenes to overcome tumor-suppressive check- points and increase tolerance to polyploidy. For instance, silencing of YAP/TAZ in cancer or stem cell lines facilitates p53 activation and growth arrest.145–148 Similarly, v-SRC, a viral mutant variant of the c-SRC cellular proto-oncogene, enables abnormal spindle formation and transformation by reducing p53 levels and promoting YAP nuclear localization by squelching LATS activity.149 An analogous situation occurs during H-RAS transformation, in which silencing of LATS2 permits proliferation of polyploid cells.
On the other hand, perhaps akin to the above-mentioned p53-dependent oncogene-induced senescence,150,151 hyper- activation of YAP/TAZ can also activate p53. Loss of WRN protein function, as seen in the premature aging Werner syndrome, results in cellular senescence.152 In cancer models, depletion of WRN decreases tumor growth and increases chemotherapy sensitivity.153 Mechanistically, the depletion of WRN induces ATM-dependent phosphorylation of YAP and p53, leading to YAP stabilization and p53-dependentsenescence.152 Similarly, in the liver, YAP/TAZ hyperactivation leads to DNA damage and the generation of polyploid cells, which induces p53 and leads to hepatocyte senescence.154The sterol regulatory element-binding protein (SREBP) transcription factors are master regulators of cholesterol and lipid homeostasis and are emerging as central factors in the cross-talk between p53 and the Hippo pathway. SREBP1 expression is transcriptionally repressed by wt p53.155 LATS2 acts in concert with p53 to inhibit SREBP function (Figure 2) by binding the ER-tethered precursors of SREBP1 and SREBP2, impeding their processing and quenching the subsequent transcriptional activity of the cleaved, nuclear SREBPs.156 Expression of hepatic Lats2 is required to avoid fatty liver disease and for induction of a p53 pro-apoptotic response when mice are challenged with excess dietary cholesterol.156 Conversely, forced expression of SREBP1 in mouse liver leads to p53 activation.157 Thus, together, p53 and LATS2 function to maintain appropriate levels of SREBP activity and cholesterol homeostasis within the liver.Activation of the mevalonate–cholesterol pathway promotes YAP/TAZ nuclear localization and transcriptional activity by activation of RHO GTPases.158 Contrary to wt p53 repression of SREBP, mutp53 cooperates with SREBP to promote the expression of sterol-related genes4 and increases YAP/TAZ- dependent gene expression158 (Figure 2). Accordingly, expression data from breast cancer patients show that both SREBP and YAP/TAZ-related genes are expressed at higher levels in mutp53 tumors compared with wt p53 tumors.158 Of note, in this case the regulation of YAP/TAZ by SREBP activation is LATS independent.In line with the epistatic regulation of YAP and mutp53, recently Di Agostino and colleagues33 demonstrated a physical interaction between YAP (but not TAZ) and mutp53 in breast cancer cell lines (Figure 2).
This interaction drives the expression of pro-proliferative genes such as cyclin A (CCNA), cyclin B (CCNB) and CDK1 to promote tumorigenesis. Mutp53 may also augment YAP/TAZ activity by deregulating upstream Hippo components. Indeed, the miRNA transcriptome asso- ciated with mutp53 substantially differs from that of wt p53, and is significantly enriched with miRNAs that putatively target Hippo pathway components.159Tumors must coordinate the rewiring of cell hyperprolifera- tion with the altered metabolic demands of oncogenesis. To unleash cells’ pro-proliferative potential, mutp53 and YAP converge on additional facets of cell metabolism, beyond the aforementioned mevalonate pathway. For instance, tumor- associated mutp53 stimulates the Warburg effect through promoting GLUT1 translocation to the plasma membrane, which is mediated by activated RHOA and its downstream effector ROCK.5 Not only is RHOA a well-documented upstream effector of the Hippo pathway,49 but also augmented glycolysis has been shown to be crucial to sustain YAP/TAZ transcriptional activity and pro-tumorigenic functions.160–163 Similarly, both YAP and mutp53 protein levels are dependent on nutrient availability. Intriguingly, glucose restriction selec- tively promotes both mutp53 and YAP degradation through autophagy.164,165 Moreover, the recent discovery that the MSTkinases sustain autophagosome formation by phosphorylat- ing LC3166 suggests that cross-talk between Hippo signaling, autophagy and mutp53 might function at multiple junctions.Ultimately, homeostasis represents a balance between stemness and differentiation, cell death and proliferation, and anabolism and catabolism. Given the important functions of the Hippo and p53 pathways in these processes, it is no wonder that they are profoundly intertwined.
Conclusions
The complexity of signaling pathways tends to increase over evolution, facilitated at least partially by gene duplications events, as seen for both the p53 family members and Hippo pathway components.14,44 Paralogs may provide a platform to increase and fine tune the integration of signaling pathways into a comprehensive and precise network that maintains homeostasis and fitness. In the case of the p53 and Hippo pathways, signaling integration is highly conserved, with multiple interaction nodes in organisms with no duplications of either TP53 or Hippo genes (i.e., Drosophila). Interestingly, a significant portion of the cross-talk between the two pathways is dedicated to maintaining homeostasis between stemness and differentiation. This may be related to the primordial and conserved role of Hippo components and p53 family members in germline protection and organismal development.14,167 Alterations in this balance may be hijacked by cancer cells to maintain stemness features and to increase their competitive advantage.
Coordination of the p53 and Hippo tumor-suppressive pathways occurs on multiple levels (simplified summary in Table 1). For example, LATS2 can bind directly to MDM2, the major negative regulator of p53, blocking the ability of MDM2 to target p53 for ubiquitin-dependent proteasomal degradation and resulting in p53 accumulation and activation.53 Through this capacity, LATS2 mediates p53 activation in response to a number of diverse types of stress signals, including mitotic apparatus dysfunction, expression of oncogenic RAS and cholesterol overload,53,117,118,156 and during ESC differentiation.52 Similarly, both YAP and TAZ collaborate with different p63 isoforms to modulate adult and CSCs,71,86,89,90 whereas MST and YAP cooperate to direct p73-dependent apoptosis.106,168 mutp53 physically interacts with YAP to coordinately regulate the expression of cancer-related genes.33 In turn, optimal activity of YAP and TAZ relies on products of the mevalonate pathway,158 which is regulated positively by mutp534 and negatively by LATS2.156 Impor- tantly, downregulation of LATS alters the properties of wt p53.
This is reflected by reduced phosphorylation, conformational changes and alteration of transcriptional targets, all of which contribute to a mutant-like phenotype of wt p53.These multiple points of regulatory interactions may also dictate selective forces during tumorigenesis that drive rewiring and alterations of the two pathways. Although the TP53 gene is highly mutated in diverse cancer types, pan- cancer TCGA data170 reveal that there are also tumor types with relatively low prevalence of such mutations, for example, kidney renal clear-cell carcinoma 3%; lymphoblastic acute myeloid leukemia (LAML) 8%; glioblastoma multiforme 29%, as opposed to cancers with frequent TP53 mutations (ovarian carcinoma 83%, lung squamous cell carcinoma 82%, HNSCC 71%). Comparing the expression levels of the core Hippo components in tumors with wt or mutp53 within each type of cancer reveals an interesting trend (Figure 3). In tumor types where the pressure to mutationally inactivate p53 is low (exemplified by LAML), the large proportion of tumors that retain wt p53 also preserve expression of the Hippo tumor- suppressive kinase module. Furthermore, in the relatively few tumors with p53 mutations, the YAP-mutp53 target CCNA is not upregulated. Interestingly in HNSCC in which there is a strong pressure to mutate TP53, tumors that retain wt p53 appear to compromise its function by downregulating the Hippo kinases. Furthermore, in this tumor type, CCNA is expressed at higher levels in mutp53 tumors, suggesting the existence of a transcriptionally active mutp53-YAP complex. Although only correlative, these observations may reflect the close-knit network of interactions between the two pathways, and its corruption in the course of different malignant contexts. There is growing evidence supporting a scenario in which incoming physiological signals dynamically dictate a variety of functions and conformational states for different proteins. These conformational states might generate a functional spectrum ranging from wt to mutant-like behavior (‘pseudo- mutant’), even in proteins that are wt by sequence. For instance, specific post-translational modifications trigger changes in the conformation of wt p53 that can mediate its ability to interact with binding partners and fortify its commit- ment to specific biological processes.171 External and internal signaling events, such as growth factors, oxygen availability and differential binding to distinct DNA sequences or molecular chaperones also have an impact on the conformation of p53.172–176
Similarly, differential binding of YAP to TEAD or p73 reflects the diametrically opposing roles of YAP. Whereas YAP association with TEAD leads to cell proliferation, epithelial- to-mesenchymal transition, inhibition of apoptosis and tumorigenesis,177 YAP binding to p73 promotes apoptosis and differentiation.106,110 Thus in different conditions, YAP can feature either as tumor suppressor or as oncogene.36
Both YAP and p53 might embody an inherent capacity to adopt pseudomutant status, which must be coordinated; perhaps by their common upstream regulator, LATS (Figure 4). Interaction of additional elements of the Hippo pathway beyond LATS,169 might also modulate p53 conforma- tion. In turn, pseudomutant wt p53 might mimic the impact of mutp53 on Hippo pathway function. In parallel, YAP itself might embody a spectrum of conformations that facilitate tumor- suppressive or oncogenic biological outcomes, depending on various signaling alterations including p53′s functional state as suggested recently by Ferraiuolo and colleagues.
Conceivably, YAP, which has been shown to bind mutp53, might bind wt p53 in conditions that induce pseudomutant conformation. This dynamic situation is at odds with our binary categor- ization of genes into ‘oncogenes’ or ‘tumor suppressors’. When proteins are thus labeled, our prediction of the range of their potential is restricted. For instance, we typically assume that tumor suppressors will inherently favor cell death. However, seemingly paradoxical observations made in tumor patients, as well as mouse models indicate that excessive apoptosis actually can aggravate tumorigenesis.179,180 In contrast, through their ability to enable metabolic adaptation, p53 and Hippo components can support cell survival. These cell-protective functions help avoid damage and thereby prevent cancer development,181 but these same functions may eventually by hijacked by cancer cells to assure their well- being and provide them with a competitive advantage in stressful conditions. Understanding the nuances that control context-dependent protein behavior and contrasting physio- logical fates is not merely a basic NIBR-LTSi science puzzle, but will be essential for efficient therapeutic implementations.