Hippo signaling regulates differentiation and maintenance in the exocrine pancreas

The mammalian Hippo pathway: regulation and function of YAP1 and TAZ

The Hippo pathway was originally identified as the signaling that controls organ size in Drosophila, with the core architecture conserved in mammals. In the mammalian Hippo pathway, mammalian Ste20-like kinases (MST1/2) and large tumor suppressor kinases (LATS1/2) regulate transcriptional co-activators, Yes-associated protein (YAP1) and Transcriptional co-activator with a PDZ-binding motif (TAZ). The Hippo pathway was initially thought to be quite straightforward; however, the identification of additional components has revealed its inherent complexity. Regulation of YAP1 and TAZ is not always dependent on MST1/2 and LATS1/2. MST1/2 and LATS1/2 play various YAP1/TAZ-independent roles, while YAP1 and TAZ cross-talk with other signaling pathways. In this review we focus on YAP1 and TAZ and discuss their regulation, function, and the consequences of their dysregulation.

The biology of YAP/TAZ: hippo signaling and beyond

The transcriptional regulators YAP and TAZ are the focus of intense interest given their remarkable biological properties in development, tissue homeostasis and cancer. YAP and TAZ activity is key for the growth of whole organs, for amplification of tissue-specific progenitor cells during tissue renewal and regeneration, and for cell proliferation. In tumors, YAP/TAZ can reprogram cancer cells into cancer stem cells and incite tumor initiation, progression and metastasis. As such, YAP/TAZ are appealing therapeutic targets in cancer and regenerative medicine. Just like the function of YAP/TAZ offers a molecular entry point into the mysteries of tissue biology, their regulation by upstream cues is equally captivating. YAP/TAZ are well known for being the effectors of the Hippo signaling cascade, and mouse mutants in Hippo pathway components display remarkable phenotypes of organ overgrowth, enhanced stem cell content and reduced cellular differentiation. YAP/TAZ are primary sensors of the cell's physical nature, as defined by cell structure, shape and polarity. YAP/TAZ activation also reflects the cell "social" behavior, including cell adhesion and the mechanical signals that the cell receives from tissue architecture and surrounding extracellular matrix (ECM). At the same time, YAP/TAZ entertain relationships with morphogenetic signals, such as Wnt growth factors, and are also regulated by Rho, GPCRs and mevalonate metabolism. YAP/TAZ thus appear at the centerpiece of a signaling nexus by which cells take control of their behavior according to their own shape, spatial location and growth factor context.

Regeneration and repair of the exocrine pancreas

Pancreatitis is caused by inflammatory injury to the exocrine pancreas, from which both humans and animal models appear to recover via regeneration of digestive enzyme-producing acinar cells. This regenerative process involves transient phases of inflammation, metaplasia, and redifferentiation, driven by cell-cell interactions between acinar cells, leukocytes, and resident fibroblasts. The NFκB signaling pathway is a critical determinant of pancreatic inflammation and metaplasia, whereas a number of developmental signals and transcription factors are devoted to promoting acinar redifferentiation after injury. Imbalances between these proinflammatory and prodifferentiation pathways contribute to chronic pancreatitis, characterized by persistent inflammation, fibrosis, and acinar dedifferentiation. Loss of acinar cell differentiation also drives pancreatic cancer initiation, providing a mechanistic link between pancreatitis and cancer risk. Unraveling the molecular bases of exocrine regeneration may identify new therapeutic targets for treatment and prevention of both of these deadly diseases.

Spatiotemporal proteomic analyses during pancreas cancer progression identifies serine/threonine stress kinase 4 (STK4) as a novel candidate biomarker for early stage disease

Pancreas cancer, or pancreatic ductal adenocarcinoma, is the deadliest of solid tumors, with a five-year survival rate of <5%. Detection of resectable disease improves survival rates, but access to tissue and other biospecimens that could be used to develop early detection markers is confounded by the insidious nature of pancreas cancer. Mouse models that accurately recapitulate the human condition allow disease tracking from inception to invasion and can therefore be useful for studying early disease stages in which surgical resection is possible. Using a highly faithful mouse model of pancreas cancer in conjunction with a high-density antibody microarray containing ∼2500 antibodies, we interrogated the pancreatic tissue proteome at preinvasive and invasive stages of disease. The goal was to discover early stage tissue markers of pancreas cancer and follow them through histologically defined stages of disease using cohorts of mice lacking overt clinical signs and symptoms and those with end-stage metastatic disease, respectively. A panel of seven up-regulated proteins distinguishing pancreas cancer from normal pancreas was validated, and their levels were assessed in tissues collected at preinvasive, early invasive, and moribund stages of disease. Six of the seven markers also differentiated pancreas cancer from an experimental model of chronic pancreatitis. The levels of serine/threonine stress kinase 4 (STK4) increased between preinvasive and invasive stages, suggesting its potential as a tissue biomarker, and perhaps its involvement in progression from precursor pancreatic intraepithelial neoplasia to pancreatic ductal adenocarcinoma. Immunohistochemistry of STK4 at different stages of disease revealed a dynamic expression pattern further implicating it in early tumorigenic events. Immunohistochemistry of a panel of human pancreas cancers confirmed that STK4 levels were increased in tumor epithelia relative to normal tissue. Overall, this integrated approach yielded several tissue markers that could serve as signatures of disease stage, including early (resectable), and therefore clinically meaningful, stages.

The Hippo pathway in disease and therapy: cancer and beyond

The Hippo tumour suppressor pathway co-ordinates cell proliferation, cell death and cell differentiation to regulate tissue growth control. In mammals, a conserved core Hippo signalling module receives signal inputs on different levels to ensure the proper regulation of YAP/TAZ activities as transcriptional co-activators. While the core module members MST1/2, Salvador, LATS1/2 and MOB1 have been attributed tumour suppressive functions, YAP/TAZ have been mainly described to have oncogenic roles, although some reports provided evidence supporting growth suppressive roles of YAP/TAZ in certain cancer settings. Intriguingly, mammalian Hippo signalling is also implicated in non-cancer diseases and plays a role in tissue regeneration following injury. Cumulatively, these findings indicate that the pharmacological inhibition or activation of the Hippo pathway could be desirable depending on the disease context. In this review, we first summarise the functions of the mammalian Hippo pathway in tumour formation, and then discuss non-cancer diseases involving Hippo signalling core components with a specific focus on our current understanding of the non-cancer roles of MST1/2 and YAP/TAZ. In addition, the pros and cons of possible pharmacological interventions with Hippo signalling will be reviewed, with particular emphasis on anti-cancer drug development and regenerative medicine.

Yap1 activation enables bypass of oncogenic Kras addiction in pancreatic cancer

Activating mutations in KRAS are among the most frequent events in diverse human carcinomas and are particularly prominent in human pancreatic ductal adenocarcinoma (PDAC). An inducible Kras(G12D)-driven mouse model of PDAC has established a critical role for sustained Kras(G12D) expression in tumor maintenance, providing a model to determine the potential for and the underlying mechanisms of Kras(G12D)-independent PDAC recurrence. Here, we show that some tumors undergo spontaneous relapse and are devoid of Kras(G12D) expression and downstream canonical MAPK signaling and instead acquire amplification and overexpression of the transcriptional coactivator Yap1. Functional studies established the role of Yap1 and the transcriptional factor Tead2 in driving Kras(G12D)-independent tumor maintenance. The Yap1/Tead2 complex acts cooperatively with E2F transcription factors to activate a cell cycle and DNA replication program. Our studies, along with corroborating evidence from human PDAC models, portend a novel mechanism of escape from oncogenic Kras addiction in PDAC.

The Hippo pathway effectors TAZ and YAP in development, homeostasis and disease

Studies over the past 20 years have defined the Hippo signaling pathway as a major regulator of tissue growth and organ size. Diverse roles for the Hippo pathway have emerged, the majority of which in vertebrates are determined by the transcriptional regulators TAZ and YAP (TAZ/YAP). Key processes regulated by TAZ/YAP include the control of cell proliferation, apoptosis, movement and fate. Accurate control of the levels and localization of these factors is thus essential for early developmental events, as well as for tissue homeostasis, repair and regeneration. Recent studies have revealed that TAZ/YAP activity is regulated by mechanical and cytoskeletal cues as well as by various extracellular factors. Here, I provide an overview of these and other regulatory mechanisms and outline important developmental processes controlled by TAZ and YAP.

Downstream of mutant KRAS, the transcription regulator YAP is essential for neoplastic progression to pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is an aggressive cancer with poor survival rates and frequently carries oncogenic KRAS mutation. However, KRAS has thus far not been a viable therapeutic target. We found that the abundance of YAP mRNA, which encodes Yes-associated protein (YAP), a protein regulated by the Hippo pathway during tissue development and homeostasis, was increased in human PDAC tissue compared with that in normal pancreatic epithelia. In genetically engineered Kras(G12D) and Kras(G12D):Trp53(R172H) mouse models, pancreas-specific deletion of Yap halted the progression of early neoplastic lesions to PDAC without affecting normal pancreatic development and endocrine function. Although Yap was dispensable for acinar to ductal metaplasia (ADM), an initial step in the progression to PDAC, Yap was critically required for the proliferation of mutant Kras or Kras:Trp53 neoplastic pancreatic ductal cells in culture and for their growth and progression to invasive PDAC in mice. Yap functioned as a critical transcriptional switch downstream of the oncogenic KRAS-mitogen-activated protein kinase (MAPK) pathway, promoting the expression of genes encoding secretory factors that cumulatively sustained neoplastic proliferation, a tumorigenic stromal response in the tumor microenvironment, and PDAC progression in Kras and Kras:Trp53 mutant pancreas tissue. Together, our findings identified Yap as a critical oncogenic KRAS effector and a promising therapeutic target for PDAC and possibly other types of KRAS-mutant cancers.

Pdx1 maintains β cell identity and function by repressing an α cell program

Pdx1 is a homeobox-containing transcription factor that plays a key role in pancreatic development and adult β cell function. In this study, we traced the fate of adult β cells after Pdx1 deletion. As expected, β-cell-specific removal of Pdx1 resulted in severe hyperglycemia within days. Surprisingly, a large fraction of Pdx1-deleted cells rapidly acquired ultrastructural and physiological features of α cells, indicating that a robust cellular reprogramming had occurred. Reprogrammed cells exhibited a global transcriptional shift that included derepression of the α cell transcription factor MafB, resulting in a transcriptional profile that closely resembled that of α cells. These findings indicate that Pdx1 acts as a master regulator of β cell fate by simultaneously activating genes essential for β cell identity and repressing those associated with α cell identity. We discuss the significance of these findings in the context of the emerging notion that loss of β cell identity contributes to the pathogenesis of type 2 diabetes.

The two faces of Hippo: targeting the Hippo pathway for regenerative medicine and cancer treatment

The Hippo signalling pathway is an emerging growth control and tumour suppressor pathway that regulates cell proliferation and stem cell functions. Defects in Hippo signalling and hyperactivation of its downstream effectors Yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ) contribute to the development of cancer, which suggests that pharmacological inhibition of YAP and TAZ activity may be an effective anticancer strategy. Conversely, YAP and TAZ can also have beneficial roles in stimulating tissue repair and regeneration following injury, so their activation may be therapeutically useful in these contexts. A complex network of intracellular and extracellular signalling pathways that modulate YAP and TAZ activities have recently been identified. Here, we review the regulation of the Hippo signalling pathway, its functions in normal homeostasis and disease, and recent progress in the identification of small-molecule pathway modulators.

Significance of expression of Yes-associated protein in pancreatic cancer

AIM: To detect the expression of Yes-associated protein (YAP), a component of the Hippo signaling pathway, in human pancreatic cancer, and to explore the correlation between the expression of YAP and clinicopathologic features of pancreatic cancer.

METHODS: Immunohistochemistry was used to investigate the protein expression of YAP in 45 pancreatic cancer specimens and 15 normal pancreatic tissue specimens. The mRNA expression of YAP was detected by RT-PCR.

RESULTS: YAP was mainly expressed in pancreatic ductal epithelial cells. The positive rate of YAP expression was significantly higher in pancreatic cancer than in normal pancreatic tissue (93.33% vs 26.67%, P < 0.05). The mRNA expression of YAP in pancreatic cancer was significantly higher than that in normal pancreatic tissue (0.3682 ± 0.0783 vs 0.0394 ± 0.0091, P < 0.05). The expression of YAP was associated with degree of differentiation of pancreatic cancer but had no correlation with age, lymph node metastasis, clinical stage, smoking, drinking, or diabetes mellitus.

CONCLUSION: The expression of YAP may play an important role in the development of pancreatic cancer, and YAP may be a new target for therapy of pancreatic cancer.

Mst1 and Mst2 kinases: regulations and diseases

The Hippo signaling pathway has emerged as a critical regulator for organ size control. The serine/threonine protein kinases Mst1 and Mst2, mammalian homologs of the Hippo kinase from Drosophila, play the central roles in the Hippo pathway controlling the cell proliferation, differentiation, and apoptosis during development. Mst1/2 can be activated by cellular stressors and the activation of Mst1/2 might enforce a feedback stimulation system to regulate oxidant levels through several mechanisms, in which regulation of cellular redox state might represent a tumor suppressor function of Mst1/2. As in Drosophila, murine Mst1/Mst2, in a redundant manner, negatively regulate the Yorkie ortholog YAP in multiple organs, although considerable diversification in the pathway composition and regulation is observed in some of them. Generally, loss of both Mst1 and Mst2 results in hyperproliferation and tumorigenesis that can be largely negated by the reduction or elimination of YAP. The Hippo pathway integrates with other signaling pathways e.g. Wnt and Notch pathways and coordinates with them to impact on the tumor pathogenesis and development. Furthermore, Mst1/2 kinases also act as an important regulator in immune cell activation, adhesion, migration, growth, and apoptosis. This review will focus on the recent updates on those aspects for the roles of Mst1/2 kinases.