Nuclear localization and pro-apoptotic signaling of YAP2 require intact PDZ-binding motif

Control of tissue growth and cell transformation by the Salvador/Warts/Hippo pathway

The Salvador-Warts-Hippo (SWH) pathway is an important regulator of tissue growth that is frequently subverted in human cancer. The key oncoprotein of the SWH pathway is the transcriptional co-activator, Yes-associated protein (YAP). YAP promotes tissue growth and transformation of cultured cells by interacting with transcriptional regulatory proteins via its WW domains, or, in the case of the TEAD1-4 transcription factors, an N-terminal binding domain. YAP possesses a putative transactivation domain in its C-terminus that is necessary to stimulate transcription factors in vitro, but its requirement for YAP function has not been investigated in detail. Interestingly, whilst the WW domains and TEAD-binding domain are highly conserved in the Drosophila melanogaster YAP orthologue, Yorkie, the majority of the C-terminal region of YAP is not present in Yorkie. To investigate this apparent conundrum, we assessed the functional roles of the YAP and Yorkie C-termini. We found that these regions were not required for Yorkie's ability to drive tissue growth in vivo, or YAP's ability to promote anchorage-independent growth or resistance to contact inhibition. However, the YAP transactivation domain was required for YAP's ability to induce cell migration and invasion. Moreover, a role for the YAP transactivation domain in cell transformation was uncovered when the YAP WW domains were mutated together with the transactivation domain. This shows that YAP can promote cell transformation in a flexible manner, presumably by contacting transcriptional regulatory proteins either via its WW domains or its transactivation domain.

miR-375 is activated by ASH1 and inhibits YAP1 in a lineage-dependent manner in lung cancer

Lung cancers with neuroendocrine (NE) features are often very aggressive but the underlying molecular mechanisms remain elusive. The transcription factor ASH1/ASCL1 is a master regulator of pulmonary NE cell development that is involved in the pathogenesis of lung cancers with NE features (NE-lung cancers). Here we report the definition of the microRNA miR-375 as a key downstream effector of ASH1 function in NE-lung cancer cells. miR-375 was markedly induced by ASH1 in lung cancer cells where it was sufficient to induce NE differentiation. miR-375 upregulation was a prerequisite for ASH1-mediated induction of NE features. The transcriptional coactivator YAP1 was determined to be a direct target of miR-375. YAP1 showed a negative correlation with miR-375 in a panel of lung cancer cell lines and growth inhibitory activities in NE-lung cancer cells. Our results elucidate an ASH1 effector axis in NE-lung cancers that is functionally pivotal in controlling NE features and the alleviation from YAP1-mediated growth inhibition.

Opposing roles of angiomotin-like-1 and zona occludens-2 on pro-apoptotic function of YAP

YAP (Yes-associated protein) oncogene has been found to form a stable complex with members of the Angiomotin (Amot) family of proteins, which bind WW domains of YAP and sequester the protein in the cytoplasm and junctional complexes. The Amot-mediated retention of YAP in the cytoplasm results in the inhibition of its proliferative function. Using apoptotic 'read-out' of YAP in HEK293 cells, we confirmed the molecular mode by which Amot regulates YAP. We showed that a representative member of the Amot family, AmotL1 (Angiomotin-like-1), uses its PPxY motifs to bind WW domains of YAP and inhibit YAP's nuclear translocation and pro-apoptotic function. Recently we also showed that YAP uses its PDZ-binding motif to interact with zona occludens-2 (ZO-2) protein, which promotes YAP's translocation to the nucleus. We also asked if AmotL1, YAP and ZO-2 signal together. We report here that AmotL1 and ZO-2 form a tripartite complex with YAP and regulate its function in HEK293 cells in opposite directions. AmotL1 inhibits pro-apoptotic function of YAP, whereas ZO-2 enhances it. As YAP is a potent oncogene, the identification and characterization of its regulators is important. AmotL1 and ZO-2 are two candidates that could be harnessed to control the oncogenic function of YAP.

The Hippo pathway in biological control and cancer development

The Hippo pathway is an evolutionally conserved protein kinase cascade involved in regulating organ size in vivo and cell contact inhibition in vitro by governing cell proliferation and apoptosis. Deregulation of the Hippo pathway is linked to cancer development. Its first core kinase Warts was identified in Drosophila more than 15 years ago, but it gained much attention when other core components of the pathway were identified 8 years later. Major discoveries of the pathway were made during past several years. The core kinase components Hippo, Salvador, Warts, and Mats in the fly and Mst1/2, WW45, Lats1/2, and Mob1 in mammals phosphorylate and inactivate downstream transcriptional co-activators Yorkie in the fly, Yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ) in mammals, respectively. Phosphorylated Yorkie, YAP, and TAZ are sequestered in the cytoplasm by interaction with 14-3-3 proteins. Here we review recent progresses of this pathway by focusing on how these proteins communicate with each other and how loss of regulation results in cancers.

The Lats2 tumor suppressor augments p53-mediated apoptosis by promoting the nuclear proapoptotic function of ASPP1

Apoptosis is an important mechanism to eliminate potentially tumorigenic cells. The tumor suppressor p53 plays a pivotal role in this process. Many tumors harbor mutant p53, but others evade its tumor-suppressive effects by altering the expression of proteins that regulate the p53 pathway. ASPP1 (apoptosis-stimulating protein of p53-1) is a key mediator of the nuclear p53 apoptotic response. Under basal conditions, ASPP1 is cytoplasmic. We report that, in response to oncogenic stress, the tumor suppressor Lats2 (large tumor suppressor 2) phosphorylates ASPP1 and drives its translocation into the nucleus. Together, Lats2 and ASPP1 shunt p53 to proapoptotic promoters and promote the death of polyploid cells. These effects are overridden by the Yap1 (Yes-associated protein 1) oncoprotein, which disrupts Lats2-ASPP1 binding and antagonizes the tumor-suppressing function of the Lats2/ASPP1/p53 axis.

Functional complexes between YAP2 and ZO-2 are PDZ domain-dependent, and regulate YAP2 nuclear localization and signalling

The Hippo pathway regulates the size of organs by controlling two opposing processes: proliferation and apoptosis. YAP2 (Yes kinase-associated protein 2), one of the three isoforms of YAP, is a WW domain-containing transcriptional co-activator that acts as the effector of the Hippo pathway in mammalian cells. In addition to WW domains, YAP2 has a PDZ-binding motif at its C-terminus. We reported previously that this motif was necessary for YAP2 localization in the nucleus and for promoting cell detachment and apoptosis. In the present study, we show that the tight junction protein ZO (zonula occludens)-2 uses its first PDZ domain to form a complex with YAP2. The endogenous ZO-2 and YAP2 proteins co-localize in the nucleus. We also found that ZO-2 facilitates the nuclear localization and pro-apoptotic function of YAP2, and that this activity of ZO-2 is PDZ-domain-dependent. The present paper is the first report on a PDZ-based nuclear translocation mechanism. Moreover, since the Hippo pathway acts as a tumour suppressor pathway, the YAP2-ZO-2 complex could represent a target for cancer therapy.

Hippo signaling: growth control and beyond

The Hippo pathway has emerged as a conserved signaling pathway that is essential for the proper regulation of organ growth in Drosophila and vertebrates. Although the mechanisms of signal transduction of the core kinases Hippo/Mst and Warts/Lats are relatively well understood, less is known about the upstream inputs of the pathway and about the downstream cellular and developmental outputs. Here, we review recently discovered mechanisms that contribute to the dynamic regulation of Hippo signaling during Drosophila and vertebrate development. We also discuss the expanding diversity of Hippo signaling functions during development, discoveries that shed light on a complex regulatory system and provide exciting new insights into the elusive mechanisms that regulate organ growth and regeneration.

Targeting YAP and Hippo signaling pathway in liver cancer

IMPORTANCE OF THE FIELD

The Hippo signaling pathway plays pivotal roles in controlling both cell growth and organ size, emerging as a new paradigm in tumor suppression. Yes-associated protein (YAP) functions as a potent transcription co-activator and is a major downstream target tightly regulated by the Hippo pathway. Inactivation of the Hippo signaling induces YAP-mediated activation of various target genes that functionally causes cellular proliferation and outgrowth of organ size. Recently, YAP has been implicated as a bona fide oncogene in solid tumors, but little is known about its exact molecular mechanism in carcinogenesis.

AREAS COVERED IN THIS REVIEW

We discuss the latest important findings in the Hippo signaling pathway and the possible means of developing potential cancer therapeutics by targeting multiple sites along the Hippo pathway.

WHAT THE READER WILL GAIN

An overview of the emerging roles of YAP and Hippo signaling in oncogenesis and the possible ways of developing cancer therapies against the pathway components, downstream targets or interconnected pathways.

TAKE HOME MESSAGE

YAP is a key oncogenic driver in liver carcinogenesis and deregulation of the Hippo pathway causes tumor formation and malignancy. Targeting YAP and cognate downstream signaling targets may have clinical utility in cancer therapies.

Effect of pressure overload-induced hypertrophy on the expression and localization of p38 MAP kinase isoforms in the mouse heart

p38 mitogen-activated protein kinases (MAPKs) are serine/threonine specific protein kinases that respond to cellular stress and regulate a broad range of cellular activities. There are four major isoforms of p38 MAPK: alpha, beta, gamma, and delta. To date, the prominent isoform in heart has been thought to be p38alpha. We examined the expression of each p38 isoform at both the mRNA and protein level in murine heart. mRNA for all four p38 isoforms was detected. p38gamma and p38delta were expressed at protein levels comparable to p38alpha and 38beta, respectively. In the early phase of pressure-overload hypertrophy (1-7 days after constriction of the transverse aorta), the abundance of p38beta, p38gamma and p38delta mRNA increased; however, no corresponding changes were detected at the protein level. Confocal immunofluorescence studies revealed p38alpha and p38gamma in both the cytoplasm and nucleus. In the established phase of hypertrophy induced by chronic pressure overload (7-28 days after constriction of the transverse aorta), p38gamma immunoreactivity accumulated in the nucleus whereas the distribution of p38alpha remained unaffected. Hence, both p38alpha and p38gamma are prominent p38 isoforms in heart and p38gamma may play a role in mediating the changes in gene expression associated with cardiac remodeling during pressure-overload hypertrophy.

Yorkie: the final destination of Hippo signaling

The Hippo signaling pathway is a key regulator of growth during animal development, whereas loss of normal Hippo pathway activity is associated with a wide range of cancers. Hippo signaling represses growth by inhibiting the activity of a transcriptional co-activator protein, known as Yorkie in Drosophila and Yap in vertebrates. In the 5 years since the first report linking Yorkie to Hippo signaling, intense interest in this pathway has led to rapid increases in our understanding of the action and regulation of Yorkie/Yap, which we review here. These studies have also emphasized the complexity of Yorkie/Yap regulation, including multiple, distinct mechanisms for repressing its transcriptional activity, and multiple DNA-binding partner proteins that can direct Yorkie to distinct downstream target genes.