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

Loss of YAP protein in prostate cancer is associated with Gleason score increase

AIMS AND BACKGROUND

To investigate the expression of YAP protein by immunohistochemistry in prostate cancer tissues and hyperplasia or normal prostate tissues adjacent to cancer, and establish the correlation of YAP expression with Gleason score.

METHODS AND STUDY DESIGN

The expression of YAP protein was evaluated in tissue microarray by immunohistochemistry. The samples included 66 radical prostatectomy specimens with clinically detected prostate cancer and 54 hyperplasia or normal prostate tissues adjacent to cancer.

RESULTS

YAP expression was present mainly in the nuclei of basal cells in both prostate cancer tissues and normal prostate or hyperplasia tissues adjacent to cancer. Cytoplasmic expression of YAP was weaker than nuclear expression in both malignant and nonmalignant luminal epithelial cells. YAP expression was decreased or lost in prostate cancer tissues; hyperplasia or normal prostate tissues adjacent to cancer exhibited stronger nuclear and cytoplasmic expression of YAP (p = 0.0001). Downregulation of YAP expression in prostate cancer samples correlated with Gleason score increase (p = 0.002).

CONCLUSIONS

This immunohistochemical study expands our knowledge of the expression and localization of YAP in prostate cancer tissue and nonmalignant prostate tissue adjacent to cancer. YAP might function as a tumor suppressor in prostate cancer. Such information may provide the foundation for the treatment of preneoplastic and neoplastic lesions of the prostate.

Expression of LATS family proteins in ovarian tumors and its significance

Epithelial ovarian cancer is composed of a diverse group of tumors that can be derived from the fallopian tube, endometrium, or ovary. In this study, we explored the expression levels of LATS family members in ovarian tumors using normal ovaries, fallopian tubes, and endometrium as controls. Immunohistochemistry studies of LATS1, LATS2, Pax8, and calretinin were performed on normal ovary, fallopian tube, normal endometrium, and ovarian tumor sections. Statistical analyses were conducted using the χ(2) test, Fisher exact test, or Kruskal-Wallis H test. Patient survival was analyzed using the Kaplan-Meier method. LATS1 was expressed in normal ovarian epithelia, endometrium, and fallopian tubes, whereas LATS2 expression was observed in the normal fallopian tubes and endometrium. High expressions of LATS1 and LATS2 in serous cystadenomas gradually decreased in borderline cystadenomas and carcinomas, respectively. However, an opposite expression pattern was observed in mucinous tumors. Low expressions of LATS1 and LATS2 were also detected in clear cell carcinoma. Both LATS1 and LATS2 expression levels significantly correlated with recurrence and stage; LATS1 levels were also related with tumor grades in serous carcinoma. However, univariate and multivariate Cox regression analyses revealed that high expression of LATS1 was associated with better prognosis in patients with serous carcinoma. Both LATS1 and LATS2 were not related with the clinical variables in mucinous and clear cell carcinoma. LATS1 expression levels might be a valuable survival indicator in ovarian serous carcinoma.

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.

A code for RanGDP binding in ankyrin repeats defines a nuclear import pathway

Regulation of nuclear import is fundamental to eukaryotic biology. The majority of nuclear import pathways are mediated by importin-cargo interactions. Yet not all nuclear proteins interact with importins, necessitating the identification of a general importin-independent nuclear import pathway. Here, we identify a code that determines importin-independent nuclear import of ankyrin repeats (ARs), a structural motif found in over 250 human proteins with diverse functions. AR-containing proteins (ARPs) with a hydrophobic residue at the 13th position of two consecutive ARs bind RanGDP efficiently, and consequently enter the nucleus. This code, experimentally tested in 17 ARPs, predicts the nuclear-cytoplasmic localization of over 150 annotated human ARPs with high accuracy and is acquired by the most common familial melanoma-associated CDKN2A mutation, leading to nuclear accumulation of mutant p16ink4a. The RaDAR (RanGDP/AR) pathway represents a general importin-independent nuclear import pathway and is frequently used by AR-containing transcriptional regulators, especially those regulating NF-κB/p53.

microRNA 31 functions as an endometrial cancer oncogene by suppressing Hippo tumor suppressor pathway

Background

We aimed to investigate whether MIR31 is an oncogene in human endometrial cancer and identify the target molecules associated with the malignant phenotype.

Methods

We investigated the growth potentials of MIR31-overexpressing HEC-50B cells in vitro and in vivo. In order to identify the target molecule of MIR31, a luciferase reporter assay was performed, and the corresponding downstream signaling pathway was examined using immunohistochemistry of human endometrial cancer tissues. We also investigated the MIR31 expression in 34 patients according to the postoperative risk of recurrence.

Results

The overexpression of MIR31 significantly promoted anchorage-independent growth in vitro and significantly increased the tumor forming potential in vivo. MIR31 significantly suppressed the luciferase activity of mRNA combined with the LATS2 3’-UTR and consequently promoted the translocation of YAP1, a key molecule in the Hippo pathway, into the nucleus. Meanwhile, the nuclear localization of YAP1 increased the transcription of CCND1. Furthermore, the expression levels of MIR31 were significantly increased (10.7-fold) in the patients (n = 27) with a high risk of recurrence compared to that observed in the low-risk patients (n = 7), and this higher expression correlated with a poor survival.

Conclusions

MIR31 functions as an oncogene in endometrial cancer by repressing the Hippo pathway. MIR31 is a potential new molecular marker for predicting the risk of recurrence and prognosis of endometrial cancer.

The role of key genes and pathways involved in the tumorigenesis of Malignant Mesothelioma

Malignant Mesothelioma (MM) is a very aggressive cancer with low survival rates and often diagnosed at an advanced stage. Several players have been implicated in the development of this cancer, such as asbestos, erionite and the simian virus 40 (SV40). Here, we have reviewed the involvement of erionite, SV40, as well as, the role of several genes (p16(INK4a), p14(ARF), NF2, LATS2, SAV, CTNNB1 and among others), the pathways (RAS, PI3K, Wnt, BCL and Hippo), and their respective roles in the development of MM.

Lats2 is critical for the pluripotency and proper differentiation of stem cells

Differentiation is a highly controlled process essential for embryonic and adult development. Moreover, disruption of proper differentiation is often associated with human diseases, including cancer. We analyzed the involvement of the tumor-suppressor Lats2 in mouse embryonic stem cell (mESC) pluripotency and differentiation, and report that mESCs lacking Lats2 are unable to sustain stemness and are not able to initiate and coordinate developmental transcriptional programs. Lats2-/- mESCs retain bivalent 'poised' chromatin marks on developmental genes and exhibit germ layer ambiguity both in vitro and in vivo. Importantly, in coordinating proper germ layer specification, Lats2 engages in a feedback loop with another tumor suppressor, p53.

LATS2 suppresses oncogenic Wnt signaling by disrupting β-catenin/BCL9 interaction

Abnormal activation of Wnt/β-catenin-mediated transcription is associated with a variety of human cancers. Here, we report that LATS2 inhibits oncogenic Wnt/β-catenin-mediated transcription by disrupting the β-catenin/BCL9 interaction. LATS2 directly interacts with β-catenin and is present on Wnt target gene promoters. Mechanistically, LATS2 inhibits the interaction between BCL9 and β-catenin and subsequent recruitment of BCL9, independent of LATS2 kinase activity. LATS2 is downregulated and inversely correlated with the levels of Wnt target genes in human colorectal cancers. Moreover, nocodazole, an antimicrotubule drug, potently induces LATS2 to suppress tumor growth in vivo by targeting β-catenin/BCL9. Our results suggest that LATS2 is not only a key tumor suppressor in human cancer but may also be an important target for anticancer therapy.

miR-661 downregulates both Mdm2 and Mdm4 to activate p53

The p53 pathway is pivotal in tumor suppression. Cellular p53 activity is subject to tight regulation, in which the two related proteins Mdm2 and Mdm4 have major roles. The delicate interplay between the levels of Mdm2, Mdm4 and p53 is crucial for maintaining proper cellular homeostasis. microRNAs (miRNAs) are short non-coding RNAs that downregulate the level and translatability of specific target mRNAs. We report that miR-661, a primate-specific miRNA, can target both Mdm2 and Mdm4 mRNA in a cell type-dependent manner. miR-661 interacts with Mdm2 and Mdm4 RNA within living cells. The inhibitory effect of miR-661 is more prevalent on Mdm2 than on Mdm4. Interestingly, the predicted miR-661 targets in both mRNAs reside mainly within Alu elements, suggesting a primate-specific mechanism for regulatory diversification during evolution. Downregulation of Mdm2 and Mdm4 by miR-661 augments p53 activity and inhibits cell cycle progression in p53-proficient cells. Correspondingly, low miR-661 expression correlates with bad outcome in breast cancers that typically express wild-type p53. In contrast, the miR-661 locus tends to be amplified in tumors harboring p53 mutations, and miR-661 promotes migration of cells derived from such tumors. Thus, miR-661 may either suppress or promote cancer aggressiveness, depending on p53 status.

Clonal immortalized human glial cell lines support varying levels of JC virus infection due to differences in cellular gene expression

JC virus (JCV) is a ubiquitous human polyomavirus that causes the demyelinating disease Progressive Multifocal Leukoencephalopathy (PML). JCV replicates in limited cell types in culture, predominantly in human glial cells. Following introduction of a replication defective SV40 mutant that expressed large T protein into a heterogeneous culture of human fetal brain cells, multiple phenotypes became immortalized (SVG cells). A subset of SVG cells could support JCV replication. In the current study, clonal cell lines were selected from the original SVG cell culture. The 5F4 clone showed low levels of viral growth. The 10B1 clone was highly permissive for JCV DNA replication and gene expression and supported persistent and stable JCV infection over months in culture. Microarray analysis revealed that viral infection did not significantly change gene expression in these cells. More resistant 5F4 cells expressed high levels of transcription factors known to inhibit JCV transcription. Interestingly, 5F4 cells expressed high levels of RNA of markers of radial glia and 10B1 cells had high expression of markers of immature glial cells and activation of transcription regulators important for stem/progenitor cell self-renewal. These SVG-derived clonal cell lines provide a biologically relevant model to investigate cell type differences in JCV host range and pathogenesis, as well as neural development. Several transcription regulators were identified which may be targets for therapeutic modulation of expression to abrogate JCV replication in PML patients. Additionally, these clonal cell lines can provide a consistent culture platform for testing therapies against JCV infection of the central nervous system.

The Hippo pathway kinase Lats2 prevents DNA damage-induced apoptosis through inhibition of the tyrosine kinase c-Abl

The Hippo pathway is an evolutionarily conserved pathway that controls cell proliferation, organ size, tissue regeneration and stem cell self-renewal. Here we show that it also regulates the DNA damage response. At high cell density, when the Hippo pathway is active, DNA damage-induced apoptosis and the activation of the tyrosine kinase c-Abl were suppressed. At low cell density, overexpression of the Hippo pathway kinase large tumor suppressor 2 (Lats2) inhibited c-Abl activity. This led to reduced phosphorylation of downstream c-Abl substrates, the transcription coactivator Yes-associated protein (Yap) and the tumor suppressor p73. Inhibition of c-Abl by Lats2 was mediated through Lats2 interaction with and phosphorylation of c-Abl. Lats2 knockdown, or expression of c-Abl mutants that escape inhibition by Lats2, enabled DNA damage-induced apoptosis of densely plated cells, while Lats2 overexpression inhibited apoptosis in sparse cells. These findings explain a long-standing enigma of why densely plated cells are radioresistant. Furthermore, they demonstrate that the Hippo pathway regulates cell fate decisions in response to DNA damage.

The Hippo pathway acts via p53 and microRNAs to control proliferation and proapoptotic gene expression during tissue growth

The Hippo pathway has a central role in coordinating tissue growth and apoptosis. Mutations that compromise Hippo pathway activity cause tissue overgrowth and have been causally linked to cancer. In Drosophila, the transcriptional coactivator Yorkie mediates Hippo pathway activity to control the expression of cyclin E and Myc to promote cell proliferation, as well as the expression of bantam miRNA and DIAP1 to inhibit cell death. Here we present evidence that the Hippo pathway acts via Yorkie and p53 to control the expression of the proapoptotic gene reaper. Yorkie further mediates reaper levels post-transcriptionally through regulation of members of the miR-2 microRNA family to prevent apoptosis. These findings provide evidence that the Hippo pathway acts via several distinct routes to limit proliferation-induced apoptosis.

Factor inhibiting HIF-1 (FIH-1) modulates protein interactions of apoptosis-stimulating p53 binding protein 2 (ASPP2)

The asparaginyl hydroxylase factor inhibiting HIF-1 (FIH-1) is an important suppressor of hypoxia-inducible factor (HIF) activity. In addition to HIF-α, FIH-1 was previously shown to hydroxylate other substrates within a highly conserved protein interaction domain, termed the ankyrin repeat domain (ARD). However, to date, the biological role of FIH-1-dependent ARD hydroxylation could not be clarified for any ARD-containing substrate. The apoptosis-stimulating p53-binding protein (ASPP) family members were initially identified as highly conserved regulators of the tumour suppressor p53. In addition, ASPP2 was shown to be important for the regulation of cell polarity through interaction with partitioning defective 3 homolog (Par-3). Using mass spectrometry we identified ASPP2 as a new substrate of FIH-1 but inhibitory ASPP (iASPP) was not hydroxylated. We demonstrated that ASPP2 asparagine 986 (N986) is a single hydroxylation site located within the ARD. ASPP2 protein levels and stability were not affected by depletion or inhibition of FIH-1. However, FIH-1 depletion did lead to impaired binding of Par-3 to ASPP2 while the interaction between ASPP2 and p53, apoptosis and proliferation of the cancer cells were not affected. Depletion of FIH-1 and incubation with the hydroxylase inhibitor dimethyloxalylglycine (DMOG) resulted in relocation of ASPP2 from cell-cell contacts to the cytosol. Our data thus demonstrate that protein interactions of ARD-containing substrates can be modified by FIH-1-dependent hydroxylation. The large cellular pool of ARD-containing proteins suggests that FIH-1 can affect a broad range of cellular functions and signalling pathways under certain conditions, for example, in response to severe hypoxia.

ASPP1/2 regulate p53-dependent death of retinal ganglion cells through PUMA and Fas/CD95 activation in vivo

The transcription factor p53 mediates neuronal death in a variety of stress-related and neurodegenerative conditions. The proapoptotic activity of p53 is tightly regulated by the apoptosis-stimulating proteins of p53 (ASPP) family members: ASPP1 and ASPP2. However, whether ASPP1/2 play a role in the regulation of p53-dependent neuronal death in the CNS is currently unknown. To address this, we asked whether ASPP1/2 contribute to the death of retinal ganglion cells (RGCs) using in vivo models of acute optic nerve damage in mice and rats. Here, we show that p53 is activated in RGCs soon after injury and that axotomy-induced RGC death is attenuated in p53 heterozygote and null mice. We demonstrate that ASPP1/2 proteins are abundantly expressed by injured RGCs, and that short interfering (si)RNA-based ASPP1 or ASPP2 knockdown promotes robust RGC survival. Comparative gene expression analysis revealed that siASPP-mediated downregulation of p53-upregulated-modulator-of-apoptosis (PUMA), Fas/CD95, and Noxa depends on p53 transcriptional activity. Furthermore, siRNA against PUMA or Fas/CD95 confers neuroprotection, demonstrating a functional role for these p53 targets in RGC death. Our study demonstrates a novel role for ASPP1 and ASPP2 in the death of RGCs and provides evidence that blockade of the ASPP-p53 pathway is beneficial for central neuron survival after axonal injury.

Molecular mechanisms underlying the interaction of protein phosphatase-1c with ASPP proteins

The serine/threonine PP-1c (protein phosphatase-1 catalytic subunit) is regulated by association with multiple regulatory subunits. Human ASPPs (apoptosis-stimulating proteins of p53) comprise three family members: ASPP1, ASPP2 and iASPP (inhibitory ASPP), which is uniquely overexpressed in many cancers. While ASPP2 and iASPP are known to bind PP-1c, we now identify novel and distinct molecular interactions that allow all three ASPPs to bind differentially to PP-1c isoforms and p53. iASPP lacks a PP-1c-binding RVXF motif; however, we show it interacts with PP-1c via a RARL sequence with a Kd value of 26 nM. Molecular modelling and mutagenesis of PP-1c-ASPP protein complexes identified two additional modes of interaction. First, two positively charged residues, Lys260 and Arg261 on PP-1c, interact with all ASPP family members. Secondly, the C-terminus of the PP-1c α, β and γ isoforms contain a type-2 SH3 (Src homology 3) poly-proline motif (PxxPxR), which binds directly to the SH3 domains of ASPP1, ASPP2 and iASPP. In PP-1cγ this comprises residues 309-314 (PVTPPR). When the Px(T)PxR motif is deleted or mutated via insertion of a phosphorylation site mimic (T311D), PP-1c fails to bind to all three ASPP proteins. Overall, we provide the first direct evidence for PP-1c binding via its C-terminus to an SH3 protein domain.

In vitro radiosensitization by pentoxifylline does not depend on p53 status

PURPOSE

The mode by which the xanthine derivative, pentoxifylline, induces a radiosensitizing effect in cell cultures is a key and controversial radiobiological issue and requires further elucidation.

MATERIALS AND METHODS

Six human glioblastoma cell lines were tested for the effect of pentoxifylline treatment at maximum G2/M block on the basis of cell survival, mitotic activity, and micronucleus formation after exposure to gamma radiation. Cell survival was measured by the colony-forming assay. Micronucleus formation (an indicator of DNA damage) and the proportion of binucleated cells (a representation of mitotic activity) were determined using the cytokinesis-block assay.

RESULTS

Remarkably, exposure to a single dose of 4 Gy produced strong G2/M blocks in both p53 mutant and wild-type cells. Addition of pentoxifylline at the peak of radiation-induced G2/M blocks resulted in a p53-independent reduction in cell survival in all cell lines. This radiosensitization was strongly correlated with the magnitude of the radiation-induced G2/M block. The changes observed in mitotic activity and micronucleus yield were also p53-independent.

CONCLUSIONS

These results are at variance with the view that pentoxifylline preferentially sensitizes p53 mutant cells, and that sensitization occurs only when cells are irradiated in the presence of the drug. The data suggest that the effectiveness of pentoxifylline as radiosensitizer depends on the proportion of cells that are arrested in the G2/M phase transition following exposure to ionizing radiation. These findings can assist in the identification of cancers that may benefit from therapies using G2/M checkpoint abrogators.

ASPP1 and ASPP2 bind active RAS, potentiate RAS signalling and enhance p53 activity in cancer cells

RAS mutations occur frequently in human cancer and activated RAS signalling contributes to tumour development and progression. Apart from its oncogenic effects on cell growth, active RAS has tumour-suppressive functions via its ability to induce cellular senescence and apoptosis. RAS is known to induce p53-dependent cell cycle arrest, yet its effect on p53-dependent apoptosis remains unclear. We report here that apoptosis-stimulating protein of p53 (ASPP) 1 and 2, two activators of p53, preferentially bind active RAS via their N-terminal RAS-association domains (RAD). Additionally, ASPP2 colocalises with and contributes to RAS cellular membrane localisation and potentiates RAS signalling. In cancer cells, ASPP1 and ASPP2 cooperate with oncogenic RAS to enhance the transcription and apoptotic function of p53. Thus, loss of ASPP1 and ASPP2 in human cancer cells may contribute to the full transforming property of RAS oncogene.

Lats2 phosphorylates p21 after UV irradiation and regulates apoptosis

Lats2 (Large tumor suppressor 2), a member of the conserved AGC Ser/Thr (S/T) kinase family, is a human tumor suppressor gene. Here we show that in response to ultraviolet radiation, Lats2 is phosphorylated by Chk1 at Ser835 (S835), which is located in the kinase domain of Lats2. This phosphorylation enhances Lats2 kinase activity. Subsequently, Lats2 phosphorylates p21 at S146. p21 is a cyclin-dependent kinase (CDK) inhibitor, which not only regulates cell cycle by CDK inhibition but also inhibits apoptosis by binding to procaspase-3 in the cytoplasm. Phosphorylation by Lats2 induces p21 degradation and promotes apoptosis. Accordingly, Lats2 overexpression induces p21 degradation, caspase-3/9 activation and apoptosis. These findings describe a novel Lats2-dependent mechanism for induction of cell death in response to severe DNA damage.

N-terminal truncation of Lats1 causes abnormal cell growth control and chromosomal instability

The tumor suppressors Lats1 and Lats2 are mediators of the Hippo pathway that regulates tissue growth and proliferation. Their N-terminal non-kinase regions are distinct except for Lats conserved domains 1 and 2 (LCD1 and LCD2), which may be important for Lats1/2-specific functions. Lats1 knockout mice were generated by disrupting the N-terminal region containing LCD1 (Lats1(ΔN/ΔN)). Some Lats1(ΔN/ΔN) mice were born safely and grew normally. However, mouse embryonic fibroblasts (MEFs) from Lats1(ΔN/ΔN) mice displayed mitotic defects, centrosomal overduplication, chromosomal misalignment, multipolar spindle formation, chromosomal bridging and cytokinesis failure. They also showed anchorage-independent growth and continued cell cycles and cell growth, bypassing cell-cell contact inhibition similar to tumor cells. Lats1(ΔN/ΔN) MEFs produced tumors in nude mice after subcutaneous injection, although the tumor growth rate was much slower than that of ordinary cancer cells. Yap, a key transcriptional coactivator of the Hippo pathway, was overexpressed and stably retained in Lats1(ΔN/ΔN) MEFs in a cell density independent manner, and Lats2 mRNA expression was downregulated. In conclusion, N-terminally truncated Lats1 induced Lats2 downregulation and Yap protein accumulation, leading to chromosomal instability and tumorigenesis.

Mechanism of folate deficiency-induced apoptosis in mouse embryonic stem cells: Cell cycle arrest/apoptosis in G1/G0 mediated by microRNA-302a and tumor suppressor gene Lats2

Deficiencies in maternal diet, such as inadequate intake of folate, can inhibit normal development and lead to developmental defects. MicroRNAs (miRNAs) may play a role in mediating the effects of folate deficiency in the growing mammalian embryo, although conclusive evidences to support that possibility are not yet available. The goal of the present study was to investigate whether and how folate deprivation alters the properties of mouse embryonic stem cells (mESCs) in culture. For this purpose, mESCs were cultured in folate-deficient or complete culture medium. The results show that folate-deficient mESCs have a significantly higher rate of apoptosis, accumulate in G0/G1 and fail to proliferate. Expression profiling revealed several miRs and many mRNAs are differently expressed in folate-deficient cells. RT-PCR data confirmed differential expressions of 12 miRNAs in folate-deficient cells. Furthermore, bioinformatics analyses and in vitro studies suggested that miR-302a plays a critical role in mediating the effects of folate on cell proliferation and cell cycle-specific apoptosis by targeting Lats2 gene. Together, these results suggest that the effects of folate deficiency on mammalian development may be mediated by miRNAs that regulate proliferation and/or cell cycle progression in ESCs.

RASSF Signalling and DNA Damage: Monitoring the Integrity of the Genome?

The RASSF family of proteins has been extensively studied in terms of their genetics, structure and function. One of the functions that has been increasingly studied is the role of the RASSF proteins in the DNA damage response. Surprisingly, this research, which encompasses both the classical and N-terminal RASSF proteins, has revealed an involvement of the RASSFs in oncogenic pathways as well as the more familiar tumour suppressor pathways usually associated with the RASSF family members. The most studied protein with respect to DNA damage is RASSF1A, which has been shown, not only to be activated by ATM, a major regulator of the DNA damage response, but also to bind to and activate a number of different pathways which all lead to and feedback from the guardian of the genome, p53. In this review we discuss the latest research linking the RASSF proteins to DNA damage signalling and maintenance of genomic integrity and look at how this knowledge is being utilised in the clinic to enhance the effectiveness of traditional cancer therapies such as radiotherapy.

YAP oncogene overexpression supercharges colon cancer proliferation

The transcriptional co-activator YAP is an evolutionarily conserved regulator of organ size and progenitor cell proliferation. YAP is overexpressed at high frequency in many common human cancers and can directly drive cancer development in mouse models. YAP abundance and nuclear localization are negatively regulated by the Hippo kinase cascade, which, in epithelia, is activated by physiological cell-cell contact. Recent work in intestinal epithelium has established that YAP is constitutively inhibited by the Hippo pathway and entirely dispensable for normal development and homeostasis. YAP serves only in a standby capacity; should cell-cell contact be abrogated, as after intestinal damage, the loss of Hippo input permits increased YAP abundance and nuclear residence. In turn, YAP cooperates with β-catenin to transactivate genes that promote stem cell expansion for epithelial repair. This interplay between overexpressed YAP and β-catenin also drives proliferation of colon cancer cells. The dispensability of YAP in normal intestine makes YAP's expression or outputs attractive targets for cancer therapy.

A RASSF1A polymorphism restricts p53/p73 activation and associates with poor survival and accelerated age of onset of soft tissue sarcoma

RASSF1A (Ras association domain containing family 1A), a tumor suppressor gene that is frequently inactivated in human cancers, is phosphorylated by ataxia telangiectasia mutated (ATM) on Ser131 upon DNA damage, leading to activation of a p73-dependent apoptotic response. A single-nucleotide polymorphism located in the region of the key ATM activation site of RASSF1A predicts the conversion of alanine (encoded by the major G allele) to serine (encoded by the minor T allele) at residue 133 of RASSF1A (p.Ala133Ser). Secondary protein structure prediction studies suggest that an alpha helix containing the ATM recognition site is disrupted in the serine isoform of RASSF1A (RASSF1A-p.133Ser). In this study, we observed a reduced ability of ATM to recruit and phosphorylate RASSF1A-p.133Ser upon DNA damage. RASSF1A-p.133Ser failed to activate the MST2/LATS pathway, which is required for YAP/p73-mediated apoptosis, and negatively affected the activation of p53, culminating in a defective cellular response to DNA damage. Consistent with a defective p53 response, we found that male soft tissue sarcoma patients carrying the minor T allele encoding RASSF1A-p.133Ser exhibited poorer tumor-specific survival and earlier age of onset compared with patients homozygous for the major G allele. Our findings propose a model that suggests a certain subset of the population have inherently weaker p73/p53 activation due to inefficient signaling through RASSF1A, which affects both cancer incidence and survival.

An indirect role for ASPP1 in limiting p53-dependent p21 expression and cellular senescence

In addition to acting as a transcriptional cofactor for p53, ASPP1 has been shown to function in the cytoplasm to regulate the nuclear localization and activity of YAP/TAZ. We show here that the ability of ASPP1 to activate YAP results in the decreased expression of LATS2, which lowers the ability of p53 to induce p21, cell-cycle arrest and senescence. ASPP1 expression peaks in S-phase, and down-regulation of ASPP1 leads to a reduction in DNA synthesis and enhanced senescence in response to drugs that impede DNA replication. These activities of cytoplasmic ASPP1 in opposing p53-mediated p21 expression are in contrast to the role of nuclear ASPP1 in cooperating with p53 to induce the expression of apoptotic target genes, and may help to dampen p53 activity in normal cells.

Lats2 kinase potentiates Snail1 activity by promoting nuclear retention upon phosphorylation

Snail1 is a central regulator of epithelial cell adhesion and movement in epithelial-to-mesenchymal transitions (EMTs) during embryo development; a process reactivated during cancer metastasis. While induction of Snail1 transcription precedes EMT induction, post-translational regulation of Snail1 is also critical for determining Snail1's protein level, subcellular localization, and capacity to induce EMT. To identify novel post-translational regulators of Snail1, we developed a live cell, bioluminescence-based screen. From a human kinome RNAi screen, we have identified Lats2 kinase as a novel regulator of Snail1 protein level, subcellular localization, and thus, activity. We show that Lats2 interacts with Snail1 and directly phosphorylates Snail1 at residue T203. This occurs in the nucleus and serves to retain Snail1 in the nucleus thereby enhancing its stability. Lats2 was found to positively influence cellular EMT and tumour cell invasion, in a Snail1-dependent manner. Indeed during TGFβ-induced EMT Lats2 is activated and Snail1 phosphorylated at T203. Analysis in mouse and zebrafish embryo development confirms that Lats2 acts as a positive modulator of Snail1 protein level and potentiates its in vivo EMT activity.

Integrating developmental signals: a Hippo in the (path)way

The Hippo pathway, a signaling cascade that controls cell cycle progression, apoptosis and cell differentiation, has emerged as a fundamental regulator of many physiological and pathological processes. Recent studies have revealed a complex network of interactions directing Hippo pathway activity, and have connected this pathway with other key signaling pathways. Such crosstalk has uncovered novel roles for Hippo signaling, including regulation of TGFβ/SMAD and WNT/β-catenin pathways. This review highlights some of the recent findings in the Hippo field with an emphasis on how the Hippo pathway is integrated with other pathways to mediate diverse processes.

p53: guardian of ploidy

Aneuploidy, often preceded by tetraploidy, is one of the hallmarks of solid tumors. Indeed, both aneuploidy and tetraploidy are oncogenic occurrences that are sufficient to drive neoplastic transformation and cancer progression. True to form, the tumor suppressor p53 obstructs propagation of these dangerous chromosomal events by either instigating irreversible cell cycle arrest or apoptosis. The tumor suppressor Lats2, along with other tumor inhibitory proteins such as BRCA1/2 and BubR1, are central to p53-dependent elimination of tetraploid cells. Not surprisingly, these proteins are frequently inactivated or downregulated in tumors, synergizing with p53 inactivation to establish an atmosphere of "tolerance" for a non-diploid state.

Regulation and function of the TAZ transcription co-activator

TAZ (WWTR1), identified as a 14-3-3 binding protein with a PDZ binding motif, is implicated in mesenchymal stem cell differentiation. TAZ has been shown to be negatively regulated by phosphorylation-dependent and phosphorylation-independent mechanisms. Coupled with ASPP2, PP1 dephosphorylates TAZ to activate TAZ. TEADs mediate TAZ function in promoting cell proliferation and epithelial-mesenchymal transition (EMT). TAZ senses different cellular signals such as cell density and the extracellular matrix stiffness. Significantly, TAZ is overexpressed in breast cancer samples and papillary thyroid carcinoma tissues. These results indicate that TAZ plays an important role in cancer development and presents a novel target for TAZ overexpressed cancer therapy.

Mammalian Hippo pathway: from development to cancer and beyond

The Hippo pathway was discovered as a signal transduction pathway that regulates organ size in Drosophila melanogaster. It is composed of three components: cell surface upstream regulators including cell adhesion molecules and cell polarity complexes; a kinase cascade comprising two serine-threonine kinases with regulators and adaptors; and a downstream target, a transcription coactivator. The coactivator mediates the transcription of cell proliferation-promoting and anti-apoptotic genes. The pathway negatively regulates the coactivator to restrict cell proliferation and to promote cell death. Thus, the pathway prevents tissue overgrowth and tumourigenesis. The framework of the pathway is conserved in mammals. A dysfunction of the pathway is frequently detected in human cancers and correlates with a poor prognosis. Recent works indicated that the Hippo pathway plays an important role in tissue homoeostasis through the regulation of stem cells, cell differentiation and tissue regeneration.

Cytoplasmic ASPP1 inhibits apoptosis through the control of YAP

The ASPP (apoptosis-stimulating protein of p53) family of proteins can function in the nucleus to modulate the transcriptional activity of p53, with ASPP1 and ASPP2 contributing to the expression of apoptotic target genes. In this study, we describe a new function for cytoplasmic ASPP1 in controlling YAP (Yes-associated protein)/TAZ. ASPP1 can inhibit the interaction of YAP with LATS1 (large tumor suppressor 1), a kinase that phosphorylates YAP/TAZ and promotes cytoplasmic sequestration and protein degradation. This function of ASPP1 therefore enhances nuclear accumulation of YAP/TAZ and YAP/TAZ-dependent transcriptional regulation. The consequence of YAP/TAZ activation by ASPP1 is to inhibit apoptosis, in part through the down-regulation of Bim expression, leading to resistance to anoikis and enhanced cell migration. These results reveal a potential oncogenic role for cytoplasmic ASPP1, in contrast to the tumor-suppressive activity described previously for nuclear ASPP1.

An identity crisis for a cancer gene: subcellular location determines ASPP1 function

Two recent papers in Genes and Development argue that the ASPP1 protein has distinct roles in cell survival depending upon its subcellular localization that is determined by a complex interplay with LATS kinase and the YAP transcriptional co-factor.