Shelli M. Morris

Disabled‐2 Colocalizes with the LDLR in Clathrin‐Coated Pits and Interacts with AP‐2

Disabled‐2 (Dab2) is a widely expressed relative of Disabled‐1, a neuron‐specific signal‐transduction protein that binds to and receives signals from members of the low‐density lipoprotein receptor (LDLR) family. Members of the LDLR family internalize through clathrin‐coated pits and vesicles to endosomes, from where they return to the cell surface through the secretory pathway. In this study, we show that the Dab2 phosphotyrosine‐binding domain binds peptides containing the sequence FXNPXY. This core sequence is found in the intracellular domains of LDLR family members and is important for receptor internalization. Dab2 transiently colocalizes with the LDLR in clathrin‐coated pits, but is absent from endosomes and lysosomes. Dab2 is alternatively spliced and its localization depends on a region of the protein that contains two DPF motifs that are present in the p96 Dab2 protein and absent in the p67 splice variant. This region is sufficient to confer Dab2 binding to the α‐adaptin subunit of the clathrin adaptor protein, AP‐2. Overexpression of p96 but not of p67 Dab2 disrupts the localization of AP‐2. These findings suggest that in addition to previously reported signal‐transduction functions, Dab2 could also act as an adaptor protein that may regulate protein trafficking.

Myosin VI Binds to and Localises with Dab2, Potentially Linking Receptor‐Mediated Endocytosis and the Actin Cytoskeleton

Myosin VI, an actin‐based motor protein, and Disabled 2 (Dab2), a molecule involved in endocytosis and cell signalling, have been found to bind together using yeast and mammalian two‐hybrid screens. In polarised epithelial cells, myosin VI is known to be associated with apical clathrin‐coated vesicles and is believed to move them towards the minus end of actin filaments, away from the plasma membrane and into the cell. Dab2 belongs to a group of signal transduction proteins that bind in vitro to the FXNPXY sequence found in the cytosolic tails of members of the low‐density lipoprotein receptor family. The central region of Dab2, containing two DPF motifs, binds to the clathrin adaptor protein AP‐2, whereas a C‐terminal region contains the binding site for myosin VI. This site is conserved in Dab1, the neuronal counterpart of Dab2. The interaction between Dab2 and myosin VI was confirmed by in vitro binding assays and coimmunoprecipitation and by their colocalisation in clathrin‐coated pits/vesicles concentrated at the apical domain of polarised cells. These results suggest that the myosin VI–Dab2 interaction may be one link between the actin cytoskeleton and receptors undergoing endocytosis.

Dual roles for the Dab2 adaptor protein in embryonic development and kidney transport

The Disabled‐2 (Dab2) gene has been proposed to act as a tumor suppressor. Cell culture studies have implicated Dab2 in signal transduction by mitogens, TGFβ and endocytosis of lipoprotein receptors. To identify in vivo functions of Dab2, targeted mutations were made in the mouse. In the absence of Dab2, embryos arrest prior to gastrulation with a phenotype reminiscent of those caused by deletion of some TGFβ signal transduction molecules involved in Nodal signaling. Dab2 is expressed in the extra‐embryonic visceral endoderm but not in the epiblast. Dab2 could be conditionally deleted from the embryo without affecting normal development, showing that Dab2 is required in the visceral endoderm but dispensable in the embryo proper. Conditionally mutant Dab2−/− mice are overtly normal, but have reduced clathrin‐coated pits in kidney proximal tubule cells and excrete specific plasma proteins in the urine, consistent with reduced transport by a lipoprotein receptor, megalin/gp330, in the proximal tubule. This evidence indicates that Dab2 is pleiotropic and regulates both visceral endoderm function and lipoprotein receptor trafficking in vivo.

Regulation of the Wnt signaling pathway by disabled-2 (Dab2)

The adaptor molecule Disabled‐2 (Dab2) has been shown to link cell surface receptors to downstream signaling pathways. Using a small‐pool cDNA screening strategy, we identify that the N‐terminal domain of Dab2 interacts with Dishevelled‐3 (Dvl‐3), a signaling mediator of the Wnt pathway. Ectopic expression of Dab2 in NIH‐3T3 mouse fibroblasts attenuates canonical Wnt/β‐catenin‐mediated signaling, including accumulation of β‐catenin, activation of β‐catenin/T‐cell‐specific factor/lymphoid enhancer‐binding factor 1‐dependent reporter constructs, and endogenous cyclin D1 induction. Wnt stimulation leads to a time‐dependent dissociation of endogenous Dab2–Dvl‐3 and Dvl‐3–axin interactions in NIH‐3T3 cells, while Dab2 overexpression leads to maintenance of Dab2–Dvl‐3 association and subsequent loss of Dvl‐3–axin interactions. In addition, we find that Dab2 can associate with axin in vitro and stabilize axin expression in vivo. Mouse embryo fibroblasts which lack Dab2 exhibit constitutive Wnt signaling as evidenced by increased levels of nuclear β‐catenin and cyclin D1 protein levels. Based on these results, we propose that Dab2 functions as a negative regulator of canonical Wnt signaling by stabilizing the β‐catenin degradation complex, which may contribute to its proposed role as a tumor suppressor.

Differences in DNA Methylation Signatures Reveal Multiple Pathways of Progression From Adenoma to Colorectal Cancer

BACKGROUND & AIMS Genetic and epigenetic alterations contribute to the pathogenesis of colorectal cancer (CRC). There is considerable molecular heterogeneity among colorectal tumors, which appears to arise as polyps progress to cancer. This heterogeneity results in different pathways to tumorigenesis. Although epigenetic and genetic alterations have been detected in conventional tubular adenomas, little is known about how these affect progression to CRC. We compared methylomes of normal colon mucosa, tubular adenomas, and colorectal cancers to determine how epigenetic alterations might contribute to cancer formation. METHODS We conducted genome-wide array-based studies and comprehensive data analyses of aberrantly methylated loci in 41 normal colon tissue, 42 colon adenomas, and 64 cancers using HumanMethylation450 arrays. RESULTS We found genome-wide alterations in DNA methylation in the nontumor colon mucosa and cancers. Three classes of cancers and 2 classes of adenomas were identified based on their DNA methylation patterns. The adenomas separated into classes of high-frequency methylation and low-frequency methylation. Within the high-frequency methylation adenoma class a subset of adenomas had mutant KRAS. Additionally, the high-frequency methylation adenoma class had DNA methylation signatures similar to those of cancers with low or intermediate levels of methylation, and the low-frequency methylation adenoma class had methylation signatures similar to that of nontumor colon tissue. The CpG sites that were differentially methylated in these signatures are located in intragenic and intergenic regions. CONCLUSIONS Genome-wide alterations in DNA methylation occur during early stages of progression of tubular adenomas to cancer. These findings reveal heterogeneity in the pathogenesis of colorectal cancer, even at the adenoma step of the process.

Inactivation of TGF-β signaling and loss of PTEN cooperate to induce colon cancer in vivo.

The accumulation of genetic and epigenetic alterations mediates colorectal cancer (CRC) formation by deregulating key signaling pathways in cancer cells. In CRC, one of the most commonly inactivated signaling pathways is the transforming growth factor-beta (TGF-β) signaling pathway, which is often inactivated by mutations of TGF-β type II receptor (TGFBR2). Another commonly deregulated pathway in CRC is the phosphoinositide-3-kinase (PI3K)-AKT pathway. Phosphatase and tensin homolog deleted on chromosome 10 (PTEN) is an important negative regulator of PI3K-AKT signaling and is silenced in ∼30% of CRC. The combination of TGFBR2 inactivation and loss of PTEN is particularly common in microsatellite-unstable CRCs. Consequently, we determined in vivo if deregulation of these two pathways cooperates to affect CRC formation by analyzing tumors arising in mice that lack Tgfbr2 and/or Pten specifically in the intestinal epithelium. We found that lack of Tgfbr2 (Tgfbr2IEKO) alone is not sufficient for intestinal tumor formation and lack of Pten (PtenIEKO) alone had a weak effect on intestinal tumor induction. However, the combination of Tgfbr2 inactivation with Pten loss (PtenIEKO;Tgfbr2IEKO) led to malignant tumors in both the small intestine and colon in 86% of the mice and to metastases in 8% of the tumor-bearing mice. Moreover, these tumors arose via a β-catenin-independent mechanism. Inactivation of TGF-β signaling and loss of Pten in the tumors led to increased cell proliferation, decreased apoptosis and decreased expression of cyclin-dependent kinase inhibitors. Thus, inactivation of TGF-β signaling and loss of PTEN cooperate to drive intestinal cancer formation and progression by suppressing cell cycle inhibitors.

Transforming growth factor-beta signaling promotes hepatocarcinogenesis induced by p53 loss†

Hepatocellular carcinoma (HCC) results from the accumulation of deregulated tumor suppressor genes and/or oncogenes in hepatocytes. Inactivation of TP53 and inhibition of transforming growth factor‐beta (TGF‐β) signaling are among the most common molecular events in human liver cancers. Thus, we assessed whether inactivation of TGF‐β signaling, by deletion of the TGF‐β receptor, type II (Tgfbr2), cooperates with Trp53 loss to drive HCC formation. Albumin‐cre transgenic mice were crossed with floxed Trp53 and/or floxed Tgfbr2 mice to generate mice lacking p53 and/or Tgfbr2 in the liver. Deletion of Trp53 alone (Trp53KO) resulted in liver tumors in approximately 41% of mice by 10 months of age, whereas inactivation of Tgfbr2 alone (Tgfbr2KO) did not induce liver tumors. Surprisingly, deletion of Tgfbr2 in the setting of p53 loss (Trp53KO;Tgfbr2KO) decreased the frequency of mice with liver tumors to around 17% and delayed the age of tumor onset. Interestingly, Trp53KO and Trp53KO;Tgfbr2KO mice develop both HCC and cholangiocarcinomas, suggesting that loss of p53, independent of TGF‐β, may affect liver tumor formation through effects on a common liver stem cell population. Assessment of potential mechanisms through which TGF‐β signaling may promote liver tumor formation in the setting of p53 loss revealed a subset of Trp53KO tumors that express increased levels of alpha‐fetoprotein. Furthermore, tumors from Trp53KO mice express increased TGF‐β1 levels compared with tumors from Trp53KO;Tgfbr2KO mice. Increased phosphorylated Smad3 and ERK1/2 expression was also detected in the tumors from Trp53KO mice and correlated with increased expression of the TGF‐β responsive genes, Pai1 and Ctgf. Conclusion: TGF‐β signaling paradoxically promotes the formation of liver tumors that arise in the setting of p53 inactivation. (HEPATOLOGY 2012)

A technology platform to assess multiple cancer agents simultaneously within a patient’s tumor

Simultaneous in vivo assessment of multiple cancer drugs and drug combinations using microinjection technology predicts systemic response in model tumors and has shown feasibility for assessment of drug efficacy in a pilot study in cancer patients. There’s no place like the human Animal models of human tumors and dish cultures of cancer cells are not sufficient to predict an individual patient’s response to therapy. In the emerging era of personalized medicine, why limit ourselves to rodent models and engineered in vitro tumor models when we can study a drug directly in the patient’s tumor? This question was answered by Klinghoffer et al. by creating a microinjection system called CIVO that delivers small doses of up to eight different drugs simultaneously, directly into the tumor. The tumors could then be removed and evaluated for various markers of cancer response; in short, the authors looked for markers of cell death and drug-related mechanisms of action. By using an injection-tracking dye, Klinghoffer and colleagues could see where the drug was deposited and then use an automated analyzer for quantitative image processing along the 6-mm injection tract. In mouse models of human lymphoma, the authors were able to correctly predict systemic responsiveness to doxorubicin or vincristine—or not, in the case of resistant lymphomas. They also uncovered unexpected drug sensitivities, which were not picked up by traditional cell culture, including to novel anticancer agents, and confirmed these in vivo. The authors pilot-tested the device in dog and human patients, demonstrating the ability of CIVO to inject and track local tumor response to chemotherapies. Ultimately, such a personalized approach to drug sensitivity testing will allow for rational selection of therapeutics while sparing patients the pain—and time—associated with ineffective treatments. A fundamental problem in cancer drug development is that antitumor efficacy in preclinical cancer models does not translate faithfully to patient outcomes. Much of early cancer drug discovery is performed under in vitro conditions in cell-based models that poorly represent actual malignancies. To address this inconsistency, we have developed a technology platform called CIVO, which enables simultaneous assessment of up to eight drugs or drug combinations within a single solid tumor in vivo. The platform is currently designed for use in animal models of cancer and patients with superficial tumors but can be modified for investigation of deeper-seated malignancies. In xenograft lymphoma models, CIVO microinjection of well-characterized anticancer agents (vincristine, doxorubicin, mafosfamide, and prednisolone) induced spatially defined cellular changes around sites of drug exposure, specific to the known mechanisms of action of each drug. The observed localized responses predicted responses to systemically delivered drugs in animals. In pair-matched lymphoma models, CIVO correctly demonstrated tumor resistance to doxorubicin and vincristine and an unexpected enhanced sensitivity to mafosfamide in multidrug-resistant lymphomas compared with chemotherapy-naïve lymphomas. A CIVO-enabled in vivo screen of 97 approved oncology agents revealed a novel mTOR (mammalian target of rapamycin) pathway inhibitor that exhibits significantly increased tumor-killing activity in the drug-resistant setting compared with chemotherapy-naïve tumors. Finally, feasibility studies to assess the use of CIVO in human and canine patients demonstrated that microinjection of drugs is toxicity-sparing while inducing robust, easily tracked, drug-specific responses in autochthonous tumors, setting the stage for further application of this technology in clinical trials.

NTRK3 is a potential tumor suppressor gene commonly inactivated by epigenetic mechanisms in colorectal cancer.

NTRK3 is a member of the neurotrophin receptor family and regulates cell survival. It appears to be a dependence receptor, and thus has the potential to act as an oncogene or as a tumor suppressor gene. NTRK3 is a receptor for NT-3 and when bound to NT-3 it induces cell survival, but when NT-3 free, it induces apoptosis. We identified aberrantly methylated NTRK3 in colorectal cancers through a genome-wide screen for hypermethylated genes. This discovery led us to assess whether NTRK3 could be a tumor suppressor gene in the colon. NTRK3 is methylated in 60% of colon adenomas and 67% of colon adenocarcinomas. NTRK3 methylation suppresses NTRK3 expression. Reconstitution of NTRK3 induces apoptosis in colorectal cancers, if NT-3 is absent. Furthermore, the loss of NTRK3 expression associates with neoplastic transformation in vitro and in vivo. We also found that a naturally occurring mutant NTRK3 found in human colorectal cancer inhibits the tumor suppressor activity of NTRK3. In summary, our findings suggest NTRK3 is a conditional tumor suppressor gene that is commonly inactivated in colorectal cancer by both epigenetic and genetic mechanisms whose function in the pathogenesis of colorectal cancer depends on the expression status of its ligand, NT-3.

TGF‐β inactivation and TGF‐α overexpression cooperate in an in vivo mouse model to induce hepatocellular carcinoma that recapitulates molecular features of human liver cancer

Hepatocellular carcinoma (HCC) results from the cumulative effects of deregulated tumor suppressor genes and oncogenes. The tumor suppressor and oncogenes commonly affected include growth factors, receptors and their downstream signaling pathway components. The overexpression of transforming growth factor alpha (TGF‐α) and the inhibition of TGF‐β signaling are especially common in human liver cancer. Thus, we assessed whether TGF‐α overexpression and TGF‐β signaling inactivation cooperate in hepatocarcinogenesis using an in vivo mouse model, MT1/TGFa;AlbCre/Tgfbr2flx/flx mice (“TGFa;Tgfbr2hepko”), which overexpresses TGF‐α and lacks a TGF‐β receptor in the liver. TGF‐β signaling inactivation did not alter the frequency or number of cancers in mice with overexpression of TGF‐α. However, the tumors in the TGFa;Tgfbr2hepko mice displayed increased proliferation and increased cdk2, cyclin E and cyclin A expression as well as decreased Cdkn1a/p21 expression compared to normal liver and compared to the cancers arising in the TGF‐α overexpressing mice with intact TGF‐β receptors. Increased phosphorylated ERK1/2 expression was also present in the tumors from the TGFa;Tgfbr2hepko mice and correlated with downregulated Raf kinase inhibitor protein expression, which is a common molecular event in human HCC. Thus, TGF‐β signaling inactivation appears to cooperate with TGF‐α in vivo to promote the formation of liver cancer that recapitulates molecular features of human HCC.