Regulation of the MST1 kinase by autophosphorylation, by the growth inhibitory proteins, RASSF1 and NORE1, and by Ras

The Drosophila RASSF homolog antagonizes the hippo pathway

Correct organ size is determined by the balance between cell death and proliferation. Perturbation of this delicate balance leads to cancer formation [1]. Hippo (Hpo), the Drosophila ortholog of MST1 and MST2 (Mammalian Sterile 20-like 1 and 2) is a key regulator of a signaling pathway that controls both cell death and proliferation 2, 3. This pathway is so far composed of two Band 4.1 proteins, Expanded (Ex) and Merlin (Mer), two serine/threonine kinases, Hpo and Warts (Wts), the scaffold proteins Salvador (Sav) and Mats, and the transcriptional coactivator Yorkie (Yki). It has been proposed that Ex and Mer act upstream of Hpo, which in turn phosphorylates and activates Wts. Wts phosphorylates Yki and thus inhibits its activity and reduces expression of Yki target genes such as the caspase inhibitor DIAP1 and the micro RNA bantam4, 5, 6. However, the mechanisms leading to Hpo activation are still poorly understood. In mammalian cells, members of the Ras association family (RASSF) of tumor suppressors have been shown to bind to MST1 and modulate its activity [7]. In this study, we show that the Drosophila RASSF ortholog (dRASSF) restricts Hpo activity by competing with Sav for binding to Hpo. In addition, we observe that dRASSF also possesses a tumor-suppressor function.

The Salvador-Warts-Hippo pathway - an emerging tumour-suppressor network

Intense research over the past four years has led to the discovery and characterization of a novel signalling network, known as the Salvador-Warts-Hippo (SWH) pathway, involved in tissue growth control in Drosophila melanogaster. At present, eleven proteins have been implicated as members of this pathway, and several downstream effector genes have been characterized. The importance of this pathway is emphasized by its evolutionary conservation, and by increasing evidence that its deregulation occurs in human tumours. Here, we review the main findings from Drosophila and the implications that these have for tumorigenesis in mammals.

Prognostic significance of mammalian sterile20-like kinase 1 in colorectal cancer

Mammalian Sterile20-like kinase 1 (Mst1) is a member of the yeast Ste20-related kinase family known to be involved in the process of apoptosis initiated by a variety of stimuli. The aim of this study was to determine the prognostic significance of Mst1 expression in colorectal cancer. A series of 1197 mismatch-repair-proficient colorectal cancers retrieved from a tissue microarray were randomized into two study groups. On the first group (n=599) receiver operating characteristic curves were used to determine the most clinically useful cutoffs to describe Mst1 expression with respect to T stage, N stage, tumor grade, vascular invasion and overall survival. The association of Mst1 expression and each outcome was immunohistochemically evaluated on the second study group (n=598) as well as on a third study group comprising 141 mismatch-repair-deficient colorectal cancers. A univariate analysis in the second study group showed that loss of cytoplasmic Mst1 was associated with higher T stage (P=0.001), higher N stage (P=0.029), vascular invasion (P=0.017) and overall survival (P=0.014). Nuclear Mst1 expression was not significantly associated with N stage, T stage or vascular invasion but was associated with tumor grade. In mismatch-repair-deficient colorectal cancers, loss of cytoplasmic Mst1 was associated with higher N stage (P=0.019) and shortened survival (P=0.0001). In a multivariate analysis, loss of cytoplasmic Mst1 was an independent adverse prognostic factor in this group of patients. Methylation analysis on 32 cases showed that the loss of cytoplasmic Mst1 expression is not likely due to promoter methylation. In summary, loss of cytoplasmic Mst1 expression is a marker of tumor progression in mismatch-repair-proficient as well as mismatch-repair-deficient colorectal cancers. These results are suggestive of a tumor suppressor role for Mst1 in human colorectal cancer.

Nuclear transport of Ras-associated tumor suppressor proteins: different transport receptor binding specificities for arginine-rich nuclear targeting signals

Ras proteins regulate a wide range of biological processes by interacting with a variety of effector proteins. In addition to the known role in tumorigensis, the activated form of Ras exhibits growth-inhibitory effects by unknown mechanisms. Several Ras effector proteins identified as mediators of apoptosis and cell-cycle arrest also exhibit properties normally associated with tumor suppressor proteins. Here, we show that Ras effector RASSF5/NORE-1 binds strongly to K-Ras but weakly to both N-Ras and H-Ras. RASSF5 was found to localize both in the nucleus and the nucleolus in contrast to other Ras effector proteins, RASSF1C and RASSF2, which are localized in the nucleus and excluded from nucleolus. A 50 amino acid residue transferable arginine-rich nucleolar localization signal (NoLS) identified in RASSF5 is capable of interacting with importin-beta and transporting the cargo into the nucleolus. Surprisingly, similar arginine-rich signals identified in RASSF1C and RASSF2 interact with importin-alpha and transport the heterologous cytoplasmic proteins to the nucleus. Interestingly, mutation of arginine residues within these nuclear targeting signals prevented interaction of Ras effector proteins with respective transport receptors and abolished their nuclear translocation. These results provide evidence for the first time that arginine-rich signals are able to recognize different nuclear import receptors and transport the RASSF proteins into distinct sub-cellular compartments. In addition, our data suggest that the nuclear localization of RASSF5 is critical for its cell growth control activity. Together, these data suggest that the transport of Ras effector superfamily proteins into the nucleus/nucleolus may play a vital role in modulating Ras-mediated cell proliferation during tumorigenesis.

The integration of signaling by multiprotein complexes containing Raf kinases

In vivo, eukaryotic cells are subjected simultaneously to a broad array of signals ranging from mitogens and inflammatory inputs to environmental stresses and developmental cues. The combinatorial nature of cellular signaling necessitates that a cell integrate its signal transduction pathways so as to implement rapidly and efficiently an appropriate suite of responses. Emerging evidence indicates that, over the course of evolution, cells have developed multiprotein signaling complexes, or "signalosomes" that mediate the coordinate regulation of different signaling pathways. Such molecular signal integration contrasts with the classical notion of signaling complexes assembled by scaffold proteins-entities that function to segregate specific pathways from one another. This review will focus on two signal integrating multiprotein complexes that involve Raf family kinases: the MLK3-B-Raf-Raf-1 complex and the Raf-1-Mst-2 complex.

Inhibition of cell migration by autophosphorylated mammalian sterile 20-like kinase 3 (MST3) involves paxillin and protein-tyrosine phosphatase-PEST

MST3 is a member of the sterile-20 protein kinase family with a unique preference for manganese ion as a cofactor in vitro; however, its biological function is largely unknown. Suppression of endogenous MST3 by small interference RNA enhanced cellular migration in MCF-7 cells with reduced expression of E-cadherin at the edge of migrating cells. The alteration of cellular migration and protruding can be rescued by RNA interference-resistant MST3. The expression of surface integrin and Golgi apparatus was not altered, but phosphorylation on tyrosine 118 and tyrosine 31 of paxillin was attenuated by MST3 small interfering RNA (siRNA). Threonine 178 was determined to be one of the two main autophosphorylation sites of MST3 in vitro. Mutant T178A MST3, containing alanine instead of threonine at codon 178, lost autophosphorylation and kinase activities. Overexpression of wild type MST3, but not the T178A mutant MST3, inhibited migration and spreading in Madin-Darby canine kidney cells. MST3 could phosphorylate the protein-tyrosine phosphatase (PTP)-PEST and inhibit the tyrosine phosphatase activity of PTP-PEST. We conclude that MST3 inhibits cell migration in a fashion dependent on autophosphorylation and may regulate paxillin phosphorylation through tyrosine phosphatase PTP-PEST.

Extensive DNA methylation in normal colorectal mucosa in hyperplastic polyposis

BACKGROUND

Hyperplastic polyposis of the colorectum is a precancerous condition that has been linked with DNA methylation. The polyps in this condition have been distinguished from typical small hyperplastic polyps and renamed sessile serrated adenomas. Sessile serrated adenomas also occur sporadically and appear to be indistinguishable from their counterparts in hyperplastic polyposis.

AIMS AND METHODS

The existence of distinguishing molecular features was explored in a series of serrated polyps and matched normal mucosa from patients with and without hyperplastic polyposis by assessing mutation of BRAF, DNA methylation in 14 markers (MINTs 1, 2 and 31, p16, MGMT, MLH1, RASSF1, RASSF2, NORE1 (RASSF5), RKIP, MST1, DAPK, FAS, and CHFR), and immunoexpression of MLH1.

RESULTS

There was more extensive methylation in sessile serrated adenomas from subjects with hyperplastic polyposis (p<0.0001). A more clearcut difference in patients with hyperplastic polyposis was the finding of extensive DNA methylation in normal mucosa from the proximal colon.

CONCLUSIONS

A genetic predisposition may underlie at least some forms of hyperplastic polyposis in which the earliest manifestation may be hypermethylation of multiple gene promoters in normal colorectal mucosa. Additionally, some of the heterogeneity within hyperplastic polyposis may be explained by different propensities for MLH1 inactivation within polyps.

Dmp53 activates the Hippo pathway to promote cell death in response to DNA damage

Developmental and environmental signals control a precise program of growth, proliferation, and cell death. This program ensures that animals reach, but do not exceed, their typical size . Understanding how cells sense the limits of tissue size and respond accordingly by exiting the cell cycle or undergoing apoptosis has important implications for both developmental and cancer biology. The Hippo (Hpo) pathway comprises the kinases Hpo and Warts/Lats (Wts), the adaptors Salvador (Sav) and Mob1 as a tumor suppressor (Mats), the cytoskeletal proteins Expanded and Merlin, and the transcriptional cofactor Yorkie (Yki) . This pathway has been shown to restrict cell division and promote apoptosis. The caspase repressor DIAP1 appears to be a primary target of the Hpo pathway in cell-death control. Firstly, Hpo promotes DIAP1 phosphorylation, likely decreasing its stability. Secondly, Wts phosphorylates and inactivates Yki, decreasing DIAP1 transcription. Although we understand some of the events downstream of the Hpo kinase, its mode of activation remains mysterious. Here, we show that Hpo can be activated by Ionizing Radiations (IR) in a Dmp53 (Drosophila melanogaster p53)-dependent manner and that Hpo is required (though not absolutely) for the cell death response elicited by IR or Dmp53 ectopic expression.

Association of mammalian sterile twenty kinases, Mst1 and Mst2, with hSalvador via C-terminal coiled-coil domains, leads to its stabilization and phosphorylation

Genetic screens in Drosophila have revealed that the serine/threonine kinase Hippo (Hpo) and the scaffold protein Salvador participate in a pathway that controls cell proliferation and apoptosis. Hpo most closely resembles the pro-apoptotic mammalian sterile20 kinases 1 and 2 (Mst1 and 2), and Salvador (Sav) has a human orthologue hSav (also called hWW45). Here we show that Mst and hSav heterodimerize in an interaction requiring the conserved C-terminal coiled-coil domains of both proteins. hSav was also able to homodimerize, but this did not require its coiled-coil domain. Coexpression of Mst and hSav led to phosphorylation of hSav and also increased its abundance. In vitro phosphorylation experiments indicate that the phosphorylation of Sav by Mst is direct. The stabilizing effect of Mst was much greater on N-terminally truncated hSav mutants, as long as they retained the ability to bind Mst. Mst mutants that lacked the C-terminal coiled-coil domain and were unable to bind to hSav, also failed to stabilize or phosphorylate hSav, whereas catalytically inactive Mst mutants that retained the ability to bind to hSav were still able to increase its abundance, although they were no longer able to phosphorylate hSav. Together these results show that hSav can bind to, and be phosphorylated by, Mst, and that the stabilizing effect of Mst on hSav requires its interaction with hSav but is probably not due to phosphorylation of hSav by Mst.

Spatiotemporal regulation of the kinase Mst1 by binding protein RAPL is critical for lymphocyte polarity and adhesion

RAPL, a protein that binds the small GTPase Rap1, is required for efficient immune cell trafficking. Here we have identified the kinase Mst1 as a critical effector of RAPL. RAPL regulated the localization and kinase activity of Mst1. 'Knockdown' of the gene encoding Mst1 demonstrated its requirement for the induction of both a polarized morphology and integrin LFA-1 clustering and adhesion triggered by chemokines and T cell receptor ligation. RAPL and Mst1 localized to vesicular compartments and dynamically translocated with LFA-1 to the leading edge upon Rap1 activation, suggesting a regulatory function for the RAPL-Mst1 complex in intracellular transport of LFA-1. Our study demonstrates a previously unknown function for Mst1 of relaying the Rap1-RAPL signal to induce cell polarity and adhesion of lymphocytes.

GTP-Ras disrupts the intramolecular complex of C1 and RA domains of Nore1

The novel Ras effector mNore1, capable of inducing apoptosis, is a multidomain protein. It comprises a C1 domain homologous to PKC and an RA domain similar to the Ras effectors AF-6 and RalGDS. Here, we determine the affinity of these two domains to the active forms of Ras and Rap1 using isothermal calorimetric titration. The interaction of Ras/Rap1-GTP with the RA domain of mNore1 is weakened significantly by direct binding of the C1 domain to the RA domain. In order to analyze this observation in atomic detail, we solved the C1 solution structure by NMR. By determining chemical shifts and relaxation rates, we can show an intramolecular complex of C1-RA. GTP-Ras titration and binding to RA disrupts this complex and displaces the C1 domain. Once the C1 domain tumbles freely in solution, a lipid binding interface becomes accessible. Furthermore, we provide evidence of phosphatidylinositol 3-phosphate binding of the free C1 domain.

Spatiotemporal organization of Ras signaling: rasosomes and the galectin switch

1. Ras signaling and oncogenesis depend on the dynamic interplay of Ras with distinctive plasma membrane (PM) microdomains and various intracellular compartments. Such interaction is dictated by individual elements in the carboxy-terminal domain of the Ras proteins, including a farnesyl isoprenoid group, sequences in the hypervariable region (hvr)-linker, and palmitoyl groups in H/N-Ras isoforms. 2. The farnesyl group acts as a specific recognition unit that interacts with prenyl-binding pockets in galectin-1 (Gal-1), galectin-3 (Gal-3), and cGMP phosphodiesterase delta. This interaction appears to contribute to the prolongation of Ras signals in the PM, the determination of Ras effector usage, and perhaps also the transport of cytoplasmic Ras. Gal-1 promotes H-Ras signaling to Raf at the expense of phosphoinositide 3-kinase (PI3-K) and Ral guanine nucleotide exchange factor (RalGEF), while galectin-3 promotes K-Ras signaling to both Raf and PI3-K. 3. The hvr-linker and the palmitates of H-Ras and N-Ras determine the micro- and macro-localizations of these proteins in the PM and in the Golgi, as well as in 'rasosomes', randomly moving nanoparticles that carry palmitoylated Ras proteins and their signal through the cytoplasm.4. The dynamic compartmentalization of Ras proteins contributes to the spatial organization of Ras signaling, promotes redistribution of Ras, and provides an additional level of selectivity to the signal output of this regulatory GTPase.

Expression of RASSF1A, an epigenetically silenced tumor suppressor, overcomes resistance to apoptosis induction by interferons

Resistance of human renal cell carcinoma (RCC) and melanoma to the apoptosis-inducing effects of IFNs was postulated to result from epigenetic silencing of genes by DNA methylation, a common feature of human cancers. To reverse silencing, 5-AZA-deoxycytidine (5-AZA-dC) or selective depletion of DNA methyltransferase 1 (DNMT1) by phosphorothioate oligonucleotide antisense (DNMT1 AS) were employed in cells resistant (<5% terminal deoxynucleotidyl transferase-mediated nick-end labeling positive) to apoptosis induction by IFN-alpha2 and IFN-beta (ACHN, SK-RC-45, and A375). 5-AZA-dC and DNMT1 AS similarly depleted available DNMT1 protein and, at doses that did not cause apoptosis alone, resulted in apoptotic response to IFNs. The proapoptotic tumor suppressor RASSF1A was reactivated by DNMT1 inhibitors in all three cell lines. This was associated with demethylation of its promoter region. IFNs augmented RASSF1A protein expression after reactivation by DNMT1 inhibition. In IFN-sensitive WM9 melanoma cells, expression of RASSF1A was constitutive but also augmented by IFNs. RASSF1A small interfering RNA reduced IFN-induced apoptosis in WM9 cells and in DNMT1-depleted ACHN cells. Conversely, lentiviral expression of RASSF1A but not transduction with empty virus enabled IFN-induced apoptosis. IFN induced tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and TRAIL-neutralizing antibody inhibited apoptotic response to IFN in RASSF1A-expressing ACHN cells. Accordingly, RASSF1A markedly sensitized to recombinant TRAIL. Normal kidney epithelial cells, although expressing RASSF1A, did not undergo apoptosis in response to IFN or TRAIL but had >400-fold higher TRAIL decoy receptor 1 expression than transduced ACHN cells (real-time reverse transcription-PCR). Results identified RASSF1A as regulated by IFNs and participating in IFN-induced apoptosis at least in part by sensitization to TRAIL.

Role of the tumor suppressor RASSF1A in Mst1-mediated apoptosis

Mammalian sterile 20-like kinase 1 (Mst1) is activated by both caspase-mediated cleavage and phosphorylation in response to apoptotic stimuli, including Fas ligation. Here, we examined the possible role of the tumor suppressor RASSF1A in Mst1 activation and Mst1-mediated apoptosis induced by death receptor signaling. Immunoprecipitation and immunofluorescence analyses revealed that Mst1 was associated with RASSF1A in cultured mammalian cells, with both proteins colocalizing to microtubules throughout the cell cycle. Whereas purified recombinant RASSF1A inhibited the kinase activity of purified recombinant Mst1 in vitro, overexpression of RASSF1A increased the kinase activity of Mst1 in intact cells, suggesting that regulation of Mst1 by RASSF1A in vivo involves more than the simple association of the two proteins. Both the activation of Mst1 and the incidence of apoptosis induced by Fas ligation were markedly reduced in cells depleted of RASSF1A by RNA interference and were increased by restoration of RASSF1A expression in RASSF1A-deficient cells. Moreover, the stimulatory effect of RASSF1A overexpression on Fas-induced apoptosis was inhibited by depletion of Mst1. These findings indicate that RASSF1A facilitates Mst1 activation and thereby promotes apoptosis induced by death receptor signaling.

NDR kinases regulate essential cell processes from yeast to humans

Members of the NDR (nuclear Dbf2-related) protein-kinase family are essential components of pathways that control important cellular processes, such as morphological changes, mitotic exit, cytokinesis, cell proliferation and apoptosis. Recent progress has shed light on the mechanisms that underlie the regulation and function of the NDR family members. Combined data from yeast, worms, flies, mice and human cells now highlight the conserved and important roles of the different NDR kinases in distinct cellular processes.

Nore1B regulates TCR signaling via Ras and Carma1

Nore1A was originally identified as a potential Ras effector, and Nore1B is an alternatively spliced isoform. Both share a Ras/Rap association domain (RA domain) but only Nore1A contains sequence motifs that predict SH3 domain binding and diacylglycerol/phorbol ester binding in the amino-terminal region. Here we report that Carma1 binds to Nore1A and Nore1B through the RA domain and that Carma1 interacts with active Ras in the presence of Nore1B. RNA interference against Nore1B attenuates NF-kappaB activation induced by T cell receptor (TCR) ligation, but not NF-kappaB activation induced by TNFalpha or lipoteichoic acid. In addition, Nore1B is also required for KiRas GV12-mediated ERK1 activation and Elk1 reporter activity in T cells. We also provide evidence that knockdown of Nore1B also impairs polarized redistribution of Ras at the B cell-T cell immune interface. Together, these findings suggest that endogenous Nore1B recruits active Ras to the APC-T cell interface and mediates the interaction between Ras and Carma1.

The RASSF1A Tumor Suppressor Activates Bax via MOAP-1

The novel tumor suppressor RASSF1A is frequently inactivated during human tumorigenesis by promoter methylation. RASSF1A may serve as a node in the integration of signaling pathways controlling a range of critical cellular functions including cell cycle, genomic instability, and apoptosis. The mechanism of action of RASSF1A remains under investigation. We now identify a novel pathway connecting RASSF1A to Bax via the Bax binding protein MOAP-1. RASSF1A and MOAP-1 interact directly, and this interaction is enhanced by the presence of activated K-Ras. RASSF1A can activate Bax via MOAP-1. Moreover, activated K-Ras, RASSF1A, and MOAP-1 synergize to induce Bax activation and cell death. Analysis of a tumor-derived point mutant of RASSF1A showed that the mutant was defective for the MOAP-1 interaction and for Bax activation. Moreover, inhibition of RASSF1A by shRNA impaired the ability of K-Ras to activate Bax. Thus, we identify a novel pro-apoptotic pathway linking K-Ras, RASSF1A and Bax that is specifically impaired in some human tumors.

Regulation of NDR protein kinase by hydrophobic motif phosphorylation mediated by the mammalian Ste20-like kinase MST3

NDR protein kinases are involved in the regulation of cell cycle progression and morphology. NDR1/NDR2 protein kinase is activated by phosphorylation on the activation loop phosphorylation site Ser281/Ser282 and the hydrophobic motif phosphorylation site Thr444/Thr442. Autophosphorylation of NDR is responsible for phosphorylation on Ser281/Ser282, whereas Thr444/Thr442 is targeted by an upstream kinase. Here we show that MST3, a mammalian Ste20-like protein kinase, is able to phosphorylate NDR protein kinase at Thr444/Thr442. In vitro, MST3 selectively phosphorylated Thr442 of NDR2, resulting in a 10-fold stimulation of NDR activity. MOB1A (Mps one binder 1A) protein further increased the activity, leading to a fully active kinase. In vivo, Thr442 phosphorylation after okadaic acid stimulation was potently inhibited by MST3KR, a kinase-dead mutant of MST3. Knockdown of MST3 using short hairpin constructs abolished Thr442 hydrophobic motif phosphorylation of NDR in HEK293F cells. We conclude that activation of NDR is a multistep process involving phosphorylation of the hydrophobic motif site Thr444/2 by MST3, autophosphorylation of Ser281/2, and binding of MOB1A.

Nore1 and RASSF1 Regulation of Cell Proliferation and of the MST1/2 Kinases

The six human Nore1/RASSF genes encode a family of putative tumor suppressor proteins, each expressed as multiple mRNA splice variants. The predominant isoforms of these noncatalytic polypeptides are characterized by the presence in their carboxyterminal segments of a Ras-Association (RA) domain followed by a SARAH domain. The expression of the RASSF1A and Nore1A isoforms is extinguished selectively by gene loss and/or epigenetic mechanisms in a considerable fraction of epithelial cancers and cell lines derived therefrom, and reexpression usually suppresses the proliferation and tumorigenicity of these cells. RASSF1A/Nore1A can cause cell cycle delay in G1 and/or M and may promote apoptosis. The founding member, Nore1A, binds preferentially through its RA domain to the GTP-charged forms of Ras, Rap-1, and several other Ras subfamily GTPases with high affinity. By contrast, RASSF1, despite an RA domain 50% identical to Nore1, exhibits relatively low affinity for Ras-like GTPases but may associate with Ras-GTP indirectly. Each of the RASSF polypeptides, including the C. elegans ortholog encoded by T24F1.3, binds to the Ste20-related protein kinases MST1 and MST2 through the SARAH domains of each partner. The recombinant MST1/2 kinases, spontaneous dimers, autoactivate in vitro through an intradimer transphosphorylation of the activation loop, and the Nore1/RASSF1 polypeptides inhibit this process. Recombinant MST1 is strongly activated in vivo by recruitment to the membrane; the recombinant MST1 that is bound to RasG12V through Nore1A is activated; however, the bulk of MST1 is not. Endogenous complexes of MST1 with both Nore1A and RASSF1A are detectable, and Nore1A/MST1 can associate with endogenous Ras in response to serum addition. Nevertheless, the physiological functions of the Nore1/RASSF polypeptides in mammalian cells, as well as the role of the MST1/2 kinases in their growth-suppressive actions, remain to be established. The Drosophila MST1/2 ortholog hippo is a negative regulator of cell cycle progression and is necessary for developmental apoptosis. Overexpression of mammalian MST1 or MST2 promotes apoptosis, as does overexpression of mutant active Ki-Ras. Interference with the ability of endogenous MST1/2 to associate with the Nore1/RASSF polypeptides inhibits Ras-induced apoptosis. At present, however, the relevance of Ki-Ras-induced apoptosis to the physiological functions of c-Ras and to the growth-regulating actions of spontaneously occurring oncogenic Ras mutants is not known.

Tailoring Ras-pathway--inhibitor combinations for cancer therapy

Constitutive activation of Ras pathways plays a critical role in cancer development and maintenance. Inhibitors of such pathways are already in use for cancer therapy, with significant but as yet only partial success in the most deadly types of human cancers, against which even combinations of Ras-pathway inhibitors with classic cytotoxic drugs or irradiation are insufficient. Combinations of farnesyl transferase inhibitors (FTI's), inhibitors of Ras pathways, are now in use in clinical trials. In this review we analyze possible reasons for the limited efficacy--including the diverse and sometimes even contradictory effects of active Ras pathways in tumor cells--and propose possible alternative methods of tailoring Ras-pathway inhibitor combinations for cancer therapy. Such tailoring is now possible thanks to increased knowledge of the complexity of Ras pathways, their cooperation with other oncogenic pathways, and their "addictive" nature. We provide examples demonstrating that this knowledge can be translated into useful drug combinations that disrupt multiple oncogenic pathways and hit a weak point of a given tumor cell. One such example is combination treatment with a Ras inhibitor and a glycolysis blocker for pancreatic tumor cells. The future design of such potential drug combination therapies and the follow-up of their outcome will undoubtedly be facilitated by gene-expression profiling and proteomic methods.

Ras-association domain family 1 protein, RASSF1C, is an IGFBP-5 binding partner and a potential regulator of osteoblast cell proliferation

The goal of this study was to identify downstream signaling molecules involved in mediating the IGF-independent effects of IGFBP-5 in osteoblasts. We identified RASSF1C, a member of the RASSF1 gene products, as a IGFBP-5 binding partner and as a potential mediator of IGFBP-5 effects on ERK phosphorylation and cell proliferation.

INTRODUCTION

It has been predicted that the intrinsic growth factor action of insulin-like growth factor binding protein (IGFBP)-5 involves either the binding of IGFBP-5 to a putative receptor to induce downstream signaling pathways and/or intracellular translocation of IGFBP-5 to bind to potential signaling molecules involved in osteoblast cell regulation. This study reports the characterization of isoform C of the Ras association family 1 (RASSF1C) gene as an interacting partner of IGFBP-5.

MATERIALS AND METHODS

IGFBP-5 was used as bait in a yeast two-hybrid screen of a human osteosarcoma cDNA library. Expression levels of RASSF1C were measured by RT-PCR and/or Northern blot. IGFBP-5 effects on ERK phosphorylation were evaluated by immunoblot analysis. The effect of RASSF1C siRNA on cell proliferation was measured by the AlamarBlue assay.

RESULTS

One of the clones that interacted strongly with the bait under high stringency conditions corresponded to RASSF1C. The interaction between RASSF1C and IGFBP-5 was confirmed by in vitro co-immunoprecipitation studies. Northern blot and RT-PCR analysis showed that RASSF1C was expressed in a variety of osteoblast cell types that produce IGFBP-5. Addition of synthetic RASSF1C-specific small interfering (si) RNA duplex or use of a RASSF1C-specific si-hairpin plasmid caused a decrease in cell number and abolished IGFBP-5-induced extracellular signal-regulated kinase (ERK)-1/2 phosphorylation but had no effect on IGFBP-5-induced increases in alkaline phosphatase (ALP) activity.

CONCLUSIONS

We have shown a novel interaction between IGFBP-5 and RASSF1C. Our findings that silencing of RASSF1C results in the reduction of osteoblast cell proliferation and that IGFBP-5 treatment increases phosphorylation of ERK-1/2 raise the possibility that RASSF1C, a Ras effector, could, in part, contribute to mediating the effects of IGFBP-5 on ERK phosphorylation and, consequently, cell proliferation.

Recognizing and defining true Ras binding domains I: biochemical analysis

Common domain databases contain sequence motifs which belong to the ubiquitin fold family and are called Ras binding (RB) and Ras association (RalGDS/AF6 Ras associating) (RA) domains. The name implies that they bind to Ras (or Ras-like) GTP-binding proteins, and a few of them have been documented to qualify as true Ras effectors, defined as binding only to the activated GTP-bound form of Ras. Here we have expressed a large number of these domains and investigated their interaction with Ras, Rap and M-Ras. While their (albeit weak) sequence homology suggest that the domains adopt a common fold, not all of them bind to Ras proteins, irrespective of whether they are called RB or RA domains. We used fluorescence spectroscopy and isothermal titration calorimetry to show that the binding affinities vary over a large range, and are usually specific for either Ras or Rap. Moreover, the specificity is dictated by a set of key residues in the interface. Stopped-flow kinetic analysis showed that the association rate constants determine the different affinities of effector binding, while the dissociation rate constants are in a similar range. Manual sequence analysis allowed us to define positively charged sequence epitopes in certain secondary structure elements of the ubiquitin fold (beta1, beta2 and alpha1) which are located at similar positions and comprise the hot spots of the binding interface. These residues are important to qualify an RA/RB domain as a true candidate Ras or Rap effector.

CpG island promoter hypermethylation of a novel Ras-effector gene RASSF2A is an early event in colon carcinogenesis and correlates inversely with K-ras mutations

We report in silico identification and characterisation of a novel member of the ras association domain family 1 (RASSF1)/NORE1 family, namely, RASSF2, located at chromosomal region 20p13. It has three isoforms, all contain a ras association domain in the C-terminus. The longest isoform RASSF2A contains a 5' CpG island. RASSF2A was cloned from a brain cDNA library and directly sequenced, confirming the genomic gene structure. In previous reports, we and others have demonstrated that RASSF1A is epigenetically inactivated in a variety of cancers, including sporadic colorectal cancer (CRC). In the present report, we analysed the methylation status of RASSF2A promoter region CpG island in sporadic CRC and compared it to K-ras mutation status. RASSF2A promoter region CpG island was hypermethylated in a majority of colorectal tumour cell lines (89%) and in primary colorectal tumours (70%), while DNA from matched normal mucosa was found to be unmethylated (tumour-specific methylation). RASSF2A expression was reactivated in methylated tumour cell lines after treatment with 5-aza 2-deoxycytidine. RASSF2A methylation is an early event, detectable in 7/8 colon adenomas. Furthermore, 75% of colorectal tumours with RASSF2A methylation had no K-ras mutations (codons, 12 and 13) (P=0.048), Fisher's exact test). Our data demonstrate that RASSF2A is frequently inactivated in CRCs by CpG island promoter hypermethylation, and that epigenetic (RASSF2A) and genetic (K-ras) changes are mutually exclusive and provide alternative pathways for affecting Ras signalling.

Role of the Ras-association domain family 1 tumor suppressor gene in human cancers

In recent years, the list of tumor suppressor genes (or candidate TSG) that are inactivated frequently by epigenetic events rather than classic mutation/deletion events has been growing. Unlike mutational inactivation, methylation is reversible and demethylating agents and inhibitors of histone deacetylases are being used in clinical trails. Highly sensitive and quantitative assays have been developed to assess methylation in tumor samples, early lesions, and bodily fluids. Hence, gene silencing by promoter hypermethylation has potential clinical benefits in early cancer diagnosis, prognosis, treatment, and prevention. The hunt for a TSG located at 3p21.3 resulted in the identification of the RAS-association domain family 1, isoform A gene (RASSF1A). RASSF1A falls into the category of genes frequently inactivated by methylation rather than mutational events. This gene is silenced and frequently inactivated by promoter region hypermethylation in many adult and childhood cancers, including lung, breast, kidney, gastric, bladder, neuroblastoma, medulloblastoma, gliomas and it has homology to a mammalian Ras effector (i.e., Nore1). RASSF1A inhibits tumor growth in both in vitro and in vivo systems, further supporting its role as a TSG. We and others identified the gene in 2000, but already there are over a 150 publications demonstrating RASSF1A methylation in a large number of human cancers. Many laboratories including ours are actively investigating the biology of this novel protein family. Thus far, it has been shown to play important roles in cell cycle regulation, apoptosis, and microtubule stability. This review summarizes our current knowledge on genetic, epigenetic, and functional analysis of RASSF1A tumor suppressor gene and its homologues.

The Ste20-like kinase Mst2 activates the human large tumor suppressor kinase Lats1

Originally identified in Drosophila melanogaster, the Warts(Wts)/Lats protein kinase has been proposed to function with two other Drosophila proteins, Hippo (Hpo) and Salvador (Sav), in the regulation of cell cycle exit and apoptosis. In mammals, two candidate Warts/Lats homologs, termed Lats1 and Lats2, have been described, and the targeted disruption of LATS1 in mice increases tumor formation. Little, however, is known about the function and regulation of human Lats kinases. Here we report that human Mst2, a STE20-family member and purported Hpo ortholog, phosphorylates and activates both Lats1 and Lats2. Deletion analysis revealed that regulation of Lats1 occurs through the C-terminal, catalytic domain. Within this domain, two regulatory phosphorylation sites were identified by mass spectrometry. These sites, S909 in the activation loop and T1079 within a hydrophobic motif, have been highly conserved during evolution. Moreover, a direct interaction was observed between Mst2 and hWW45, a putative ortholog of Drosophila Sav. These results indicate that Mst2-like kinases regulate Lats kinase activities in an evolutionarily conserved regulatory pathway. Although the function of this pathway remains poorly understood in mammals, it is intriguing that, in Drosophila, it has been linked to development and tissue homeostasis.

Role of the Kinase MST2 in Suppression of Apoptosis by the Proto-Oncogene Product Raf-1

The ablation of the protein kinase Raf-1 renders cells hypersensitive to apoptosis despite normal regulation of extracellular signal-regulated kinases, which suggests that apoptosis protection is mediated by a distinct pathway. We used proteomic analysis of Raf-1 signaling complexes to show that Raf-1 counteracts apoptosis by suppressing the activation of mammalian sterile 20-like kinase (MST2). Raf-1 prevents dimerization and phosphorylation of the activation loop of MST2 independently of its protein kinase activity. Depletion of MST2 from Raf-1-/- mouse or human cells abrogated sensitivity to apoptosis, whereas overexpression of MST2 induced apoptosis. Conversely, depletion of Raf-1 from Raf-1+/+ mouse or human cells led to MST2 activation and apoptosis. The concomitant depletion of both Raf-1 and MST2 prevented apoptosis.

The centrosomal protein RAS association domain family protein 1A (RASSF1A)-binding protein 1 regulates mitotic progression by recruiting RASSF1A to spindle poles

The protein RAS association domain family protein 1A (RASSF1A), which is encoded by a gene that is frequently silenced in many types of sporadic tumor, functions in mitosis as a regulator of the anaphase-promoting complex (APC). With the use of a yeast two-hybrid screen, we identified a human protein, previously designated C19ORF5, that interacts with RASSF1A. This protein, here redesignated RASSF1A-binding protein 1 (RABP1), contains two microtubule-associated protein domains, and its association with RASSF1A was confirmed in mammalian cells by immunoprecipitation and immunofluorescence analyses. RABP1 was found to be localized to the centrosome throughout the cell cycle in a manner dependent on its microtubule-associated protein domains. Ectopic expression of RABP1 induced both stabilization of mitotic cyclins and mitotic arrest at prometaphase in a RASSF1A-dependent manner. It also increased the extent of association between RASSF1A and Cdc20. Conversely depletion of RABP1 by RNA interference prevented both the localization of RASSF1A to the spindle poles as well as its binding to Cdc20, resulting in premature destruction of mitotic cyclins and acceleration of mitotic progression. These findings indicate that RABP1 is required for the recruitment of RASSF1A to the spindle poles and for its inhibition of APC-Cdc20 activity during mitosis.