Chunling Yi

A Tight Junction-Associated Merlin-Angiomotin Complex Mediates Merlin's Regulation of Mitogenic Signaling and Tumor Suppressive Functions

The Merlin/NF2 tumor suppressor restrains cell growth and tumorigenesis by controlling contact-dependent inhibition of proliferation. We have identified a tight-junction-associated protein complex comprising Merlin, Angiomotin, Patj, and Pals1. We demonstrate that Angiomotin functions downstream of Merlin and upstream of Rich1, a small GTPase Activating Protein, as a positive regulator of Rac1. Merlin, through competitive binding to Angiomotin, releases Rich1 from the Angiomotin-inhibitory complex, allowing Rich1 to inactivate Rac1, ultimately leading to attenuation of Rac1 and Ras-MAPK pathways. Patient-derived Merlin mutants show diminished binding capacities to Angiomotin and are unable to dissociate Rich1 from Angiomotin or inhibit MAPK signaling. Depletion of Angiomotin in Nf2(-/-) Schwann cells attenuates the Ras-MAPK signaling pathway, impedes cellular proliferation in vitro and tumorigenesis in vivo.

Targeting large kinase active site with rigid, bulky octahedral ruthenium complexes.

A strategy for targeting protein kinases with large ATP-binding sites by using bulky and rigid octahedral ruthenium complexes as structural scaffolds is presented. A highly potent and selective GSK3 and Pim1 half-sandwich complex NP309 was successfully converted into a PAK1 inhibitor by making use of the large octahedral compounds Lambda-FL172 and Lambda-FL411 in which the cyclopentadienyl moiety of NP309 is replaced by a chloride and sterically demanding diimine ligands. A 1.65 A cocrystal structure of PAK1 with Lambda-FL172 reveals how the large coordination sphere of the ruthenium complex matches the size of the active site and serves as a yardstick to discriminate between otherwise closely related binding sites.

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

Without the transcription regulator YAP, KRAS-mutant pancreatic neoplasia cannot progress into invasive cancer. Targeting YAP to Promote the Antitumor Response Pancreatic ductal adenocarcinoma is frequently associated with mutations in KRAS, but targeting RAS proteins is clinically challenging. From studies with genetically engineered mouse models and correlations in human samples, Zhang et al. determined that the protein YAP mediated the transition from pancreatic metaplasia to invasive adenocarcinoma by transcriptionally activating genes induced by KRAS signaling. Deleting Yap in Kras or Kras:Trp53 mutant mouse pancreatic epithelia prevented their proliferation, decreased the expression and secretion of KRAS-regulated proteins that mediate inflammation and migration, and promoted immune cell infiltration in the tumor microenvironment in mice. Because Yap deletion did not affect normal pancreatic development or function, the findings indicate that Yap may be a viable target for KRAS-mutant PDAC. 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 KrasG12D and KrasG12D:Trp53R172H 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.

The p130 Isoform of Angiomotin Is Required for Yap-Mediated Hepatic Epithelial Cell Proliferation and Tumorigenesis

The p130 isoform of Amot exerts two oncogenic activities that promote liver cancer. Oncogenic Times 2 Proteins in the angiomotin (Amot) family can either promote or inhibit tumorigenesis through their actions on the Hippo-Yap pathway. Yi et al. found that the p130 isoform of Amot enhanced the activity of the transcription factor Yap to promote liver tumorigenesis. Mice with a liver-specific deficiency in Amot exhibited normal liver development but had reduced incidence of liver tumors in two models of hepatic cancer. By binding Yap and inhibiting its phosphorylation, Amot-p130 increased the nuclear translocation of Yap. In the nucleus, Amot-p130 enhanced the transcriptional activity of Yap for a subset of target genes, including those associated with tumorigenesis. Thus, the p130 isoform of Amot performs dual oncogenic functions by promoting Yap nuclear translocation and augmenting the activity of Yap at cancer-associated genes. The Hippo-Yap signaling pathway regulates a number of developmental and adult cellular processes, including cell fate determination, tissue growth, and tumorigenesis. Members of the scaffold protein angiomotin (Amot) family interact with several Hippo pathway components, including Yap (Yes-associated protein), and either stimulate or inhibit Yap activity. We used a combination of genetic, biochemical, and transcriptional approaches to assess the functional consequences of the Amot-Yap interaction in mice and in human cells. Mice with a liver-specific Amot knockout exhibited reduced hepatic “oval cell” proliferation and tumorigenesis in response to toxin-induced injury or when crossed with mice lacking the tumor suppressor Nf2. Biochemical examination of the Amot-Yap interaction revealed that the p130 splicing isoform of Amot (Amot-p130) and Yap interacted in both the cytoplasm and nucleus, which involved binding of PPxY and LPxY motifs in Amot-p130 to WW domains of Yap. In the cytoplasm, Amot-p130 prevented the phosphorylation of Yap by blocking access of the WW domains to the kinase Lats1. Within the nucleus, Amot-p130 was associated with the transcriptional complex containing Yap and Teads (TEA domain family members) and contributed to the regulation of a subset of Yap target genes, many of which are associated with tumorigenesis. These findings indicated that Amot acts as a Yap cofactor, preventing Yap phosphorylation and augmenting its activity toward a specific set of genes that facilitate tumorigenesis.

Validation of the p21-Activated Kinases as Targets for Inhibition in Neurofibromatosis Type 2

Neurofibromatosis type 2 (NF2) is a dominantly inherited cancer disorder caused by mutations at the NF2 gene locus. Merlin, the protein product of the NF2 gene, has been shown to negatively regulate Rac1 signaling by inhibiting its downstream effector kinases, the p21-activated kinases (Pak). Given the implication of Paks in tumorigenesis, it is plausible that merlins tumor suppressive function might be mediated, at least in part, via inhibition of the Paks. We present data indicating this is indeed the case. First, analysis of primary schwannoma samples derived from NF2 patients showed that in a significant fraction of the tumors, the activity of Pak1 was highly elevated. Second, we used shRNAs to knockdown Pak1, 2, and 3 in NIH3T3 cells expressing a dominant-negative form of merlin, NF2(BBA) (NIH3T3/NF2(BBA)), and find that simultaneous knockdown of Pak1-3 in these cells significantly reduced their growth rates in vitro and inhibited their ability to form tumors in vivo. Finally, while attempting to silence Pak1 in rat schwannoma cells, we found that these cells were unable to tolerate long-term Pak1 inhibition and rapidly moved to restore Pak1 levels by shutting down Pak1 shRNA expression through a methylation-dependent mechanism. These data suggest that inhibiting Pak could be a beneficial approach for the development of therapeutics toward NF2. In addition, the finding that the shRNA-mediated Pak1 suppression was silenced rapidly by methylation raises questions about the future application of such technologies for the treatment of diseases such as cancer.

Development of small-molecule inhibitors of the group I p21-activated kinases, emerging therapeutic targets in cancer

The p21-activated kinases (PAKs), immediate downstream effectors of the small G-proteins of the Rac/cdc42 family, are critical mediators of signaling pathways regulating cellular behaviors and as such, have been implicated in pathological conditions including cancer. Recent studies have validated the requirement for PAKs in promoting tumorigenesis in breast carcinoma and neurofibromatosis. Thus, there has been considerable interest in the development of inhibitors to the PAKs, as biological markers and leads for the development of therapeutics. While initial approaches were based on screening for competitive organic inhibitors, more recent efforts have focused on the identification of allosteric inhibitors, organometallic ATP-competitive inhibitors and the use of PAK1/inhibitor crystal structures for inhibitor optimization. This has led to the identification of highly selective and potent inhibitors, which will serve as a basis for further development of inhibitors for therapeutic applications.

Loss of the Putative Tumor Suppressor Band 4.1B/Dal1 Gene Is Dispensable for Normal Development and Does Not Predispose to Cancer

ABSTRACT The band 4.1 proteins are cytoskeletal proteins, harboring a conserved FERM domain highly homologous to the N-terminal FERM domain of ezrin, radixin, moesin, and merlin. Recently, a truncated form of the 4.1B protein, termed Dal-1, was identified in a screen as down regulated in adenocarcinoma of the lung and was mapped to chromosome 18p11.3, which is lost in 38% of primary non-small cell lung carcinoma tumors. Analysis of several meningiomas has shown that Dal-1 expression was lost in 76% of the tumors. To further elucidate the function of the 4.1B/Dal-1 gene in development and tumorigenesis we generated mice deficient for this allele. The 4.1B/Dal-1 null mice develop normally and are fertile. Rates of cellular proliferation and apoptosis in brain, mammary, and lung tissues from the 4.1B/Dal-1 null mice were indistinguishable from those seen with wild-type mice. Aging studies indicate that these mice do not have a propensity to develop tumors. Analysis of fibroblasts from these mice demonstrated that the growth characteristics and kinetics of these cells were not different from those of cells from the wild-type mice. These findings indicate that the 4.1B gene is not required for normal development and that 4.1B/Dal-1 does not function as a tumor suppressor gene.

Merlin in organ size control and tumorigenesis: Hippo versus EGFR?

The role of the NF2 gene as a tumor suppressor has been well established. In this issue of Genes & Development, Benhamouche and colleagues (pp. 1718-1730) demonstrate that NF2 is also involved in the regulation of organ size control in mammals. Conditional knockout of Nf2 in the mouse liver results in massive organ enlargement and eventual tumor development, which is attributed to the specific expansion of oval cells. Here we discuss these findings and the proposed molecular mechanisms involved within the context of our current understanding of the pathways regulated by NF2.

Cell growth density modulates cancer cell vascular invasion via Hippo pathway activity and CXCR2 signaling

Metastasis of cancer cells involves multiple steps, including their dissociation from the primary tumor and invasion through the endothelial cell barrier to enter the circulation and finding their way to distant organ sites where they extravasate and establish metastatic lesions. Deficient contact inhibition is a hallmark of invasive cancer cells, yet surprisingly the vascular invasiveness of commonly studied cancer cell lines is regulated by the density at which cells are propagated in culture. Cells grown at high density were less effective at invading an endothelial monolayer than cells grown at low density. This phenotypic difference was also observed in a zebrafish model of vascular invasion of cancer cells after injection into the yolk sac and extravasation of cancer cells into tissues from the vasculature. The vascular invasive phenotypes were reversible. A kinome-wide RNA interference screen was used to identify drivers of vascular invasion by panning small hairpin RNA (shRNA) library-transduced noninvasive cancer cell populations on endothelial monolayers. The selection of invasive subpopulations showed enrichment of shRNAs targeting the large tumor suppressor 1 (LATS1) kinase that inhibits the activity of the transcriptional coactivator yes-associated protein (YAP) in the Hippo pathway. Depletion of LATS1 from noninvasive cancer cells restored the invasive phenotype. Complementary to this, inhibition or depletion of YAP inhibited invasion in vitro and in vivo. The vascular invasive phenotype was associated with a YAP-dependent upregulation of the cytokines IL6, IL8 and C-X-C motif ligand 1, 2 and 3. Antibody blockade of cytokine receptors inhibited invasion and confirmed that they are rate-limiting drivers that promote cancer cell vascular invasiveness and could provide therapeutic targets.

Yes-associated protein mediates immune reprogramming in pancreatic ductal adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is characterized by a high degree of inflammation and profound immune suppression. Here we identify Yes-associated protein (Yap) as a critical regulator of the immunosuppressive microenvironment in both mouse and human PDAC. Within Kras:p53 mutant pancreatic ductal cells, Yap drives the expression and secretion of multiple cytokines/chemokines, which in turn promote the differentiation and accumulation of myeloid-derived suppressor cells (MDSCs) both in vitro and in vivo. Pancreas-specific knockout of Yap or antibody-mediated depletion of MDSCs promoted macrophage reprogramming, reactivation of T cells, apoptosis of Kras mutant neoplastic ductal cells and pancreatic regeneration after acute pancreatitis. In primary human PDAC, YAP expression levels strongly correlate with an MDSC gene signature, and high expression of YAP or MDSC-related genes predicts decreased survival in PDAC patients. These results reveal multifaceted roles of YAP in PDAC pathogenesis and underscore its promise as a therapeutic target for this deadly disease.