Tax oncoprotein of HTLV-1 binds to the human homologue of Drosophila discs large tumor suppressor protein, hDLG, and perturbs its function in cell growth control.
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HTLV-1 Tax oncoprotein interacts with various cellular factors and modulates transcription and the cell cycle. To identify more cellular targets, we employed the yeast two hybrid system with Tax using a human cDNA library, and isolated a cDNA encoding the human counterpart of Drosophila discs large tumor suppressor protein, hDLG. Tax binding to hDLG was confirmed in vitro and also in HTLV-1-infected T-cells. Furthermore, hDLG was found to be efficiently phosphorylated in Tax-transfected cells and HTLV-1-infected T-cells. The C-terminus of Tax and the PDZ domain of hDLG were responsible for the binding of Tax to hDLG. The C-terminal peptide of Tax prevented the binding of hDLG to APC tumor suppressor gene product, suggesting inhibition of hDLG function by Tax. Over-expression of hDLG in NIH3T3 cells by microinjection induced a reduction of BrdU incorporation into DNA, but co-expression of Tax suppressed this inhibitory effect of hDLG. These results suggest that hDLG arrested the cell cycle and that Tax canceled this inhibitory action of hDLG through targeting hDLG. Therefore, Tax affects this novel regulatory pathway of the cell cycle alteration, of which seems to play a role in the development of human cancer. (+info)
Adenomatous polyposis coli protein (APC)-independent regulation of beta-catenin/Tcf-4 mediated transcription in intestinal cells.
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Alterations in the transcriptional activity of the beta-catenin-Tcf complex have been associated with the earlier stages of colonic transformation. We show here that the activation of protein kinase C by the phorbol ester PMA in several intestinal cell lines increases the levels of beta-catenin detected in the nucleus and augments the transcriptional activity mediated by beta-catenin. The response to PMA was not related to modifications in the cytosolic levels of beta-catenin and was observed not only in cells with wild-type adenomatous polyposis coli protein (APC) but also in APC-deficient cells. Binding assays in vitro revealed that PMA facilitates the interaction of the beta-catenin with the nuclear structure. Our results therefore show that beta-catenin-mediated transcription can be regulated independently of the presence of APC. (+info)
Beta-catenin mutations are frequent in human hepatocellular carcinomas associated with hepatitis C virus infection.
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Hepatocellular carcinoma (HCC) is one of the most common fatal cancers worldwide. Hepatitis B virus and hepatitis C virus infections, exposure to aflatoxin, and excessive intake of alcohol have been identified as major risk factors. However, the molecular mechanisms underlying their development are still poorly understood. Recently, beta-catenin, one of the key components of the Wnt signaling pathway, has been found to be mutated in about 20% of HCCs, suggesting a role of the Wnt pathway in their development. In this study, we examined beta-catenin and APC mutations in 22 HCCs associated with HCV infection, using single-strand conformation polymorphism (SSCP) followed by direct DNA sequencing. beta-Catenin mutations were found in nine (41%) cases, but no APC mutations were found. beta-Catenin immunohistochemistry revealed nuclear accumulation of beta-catenin protein in all nine tumors with a beta-catenin mutation and two additional tumors without a mutation. These results suggest that activation of the Wnt signaling pathway by beta-catenin mutation contributes significantly to the hepatocellular carcinogenesis associated with HCV infection. (+info)
Dysregulated expression of beta-catenin marks early neoplastic change in Apc mutant mice, but not all lesions arising in Msh2 deficient mice.
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We have analysed the pattern of beta-catenin expression by immunohistochemistry in mice singly or multiply mutant for Apc, p53 and Msh2. We observed increased expression of beta-catenin in all intestinal lesions arising on an ApcMin+/- background. In all categories of lesion studied mosaic patterns of beta-catenin expression were observed, with the proportion of cells showing enhanced expression decreasing with increasing lesion size. p53 status did not alter these patterns. We also show that beta-catenin dysregulation marks pancreatic abnormalities occurring in ApcMin+/- and (ApcMin+/-, p53-/-) mice. In these mice both adenomas and adenocarcinomas of the pancreas arose and were characterized by increased expression of beta-catenin. We have extended these analyses to intestinal lesions arising in mice mutant for the mismatch repair gene Msh2. In these mice, increased expression of beta-catenin was again observed. However, in contrast with ApcMin+/- mice, a subset of lesions retained normal expression. Taken together, these findings show that increased expression of beta-catenin is an efficient marker of early neoplastic change in both murine intestine and pancreas in Apc mutant mice. However, we also show that dysregulation of beta-catenin is not an obligate step in the development of intestinal lesions, and therefore that genetic events other than the loss of Apc function may initiate the transition from normal to neoplastic epithelium. (+info)
EB3, a novel member of the EB1 family preferentially expressed in the central nervous system, binds to a CNS-specific APC homologue.
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APCL, a homologue of the adenomatous polyposis coli (APC) tumor suppressor, can deplete cytoplasmic beta-catenin like APC. However, as its biological function remains unclear, we have been using a yeast two-hybrid system to search for proteins that associate with its carboxyl region. Among several cDNA clones we isolated from a fetal-brain cDNA library as candidates, six included an identical sequence with significant homology to EB1, a protein known to bind to APC. The full-length cDNA of this novel homologue of EB1, named EB3, encoded a protein of 282 amino acids with 54% identity to EB1, and it was expressed preferentially in brain tissue on Northern blots. Confocal microscopy demonstrated that exogenous EB3, like EB1, is associated with the cytoplasmic microtubule network. Moreover, in these experiments EB3 and APCL appeared together in the perinucleus and the cytoplasmic microtubule network. Since APCL is also expressed highly and specifically in the central nervous system, APCL-EB3 interaction may be specific to the CNS, possibly involving stability and/or extension of microtubules during neuritogenesis. (+info)
APCL, a central nervous system-specific homologue of adenomatous polyposis coli tumor suppressor, binds to p53-binding protein 2 and translocates it to the perinucleus.
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APCL, a central nervous system-specific sequence homologue of the adenomatous polyposis coli tumor suppressor, can regulate the cytoplasmic level of beta-catenin as the adenomatous polyposis coli tumor suppressor does, but its overall biological function remains unclear. Using a yeast two-hybrid system, we attempted to isolate proteins that might associate with the unique COOH-terminus of APCL. Among 166 cDNA clones isolated from a human fetal-brain cDNA library as candidates for interaction with APCL, 32 encoded parts of p53-binding protein 2 (53BP2), a molecule that interacts with p53 and Bcl2. An in vitro binding assay indicated that the Src-homology-3 domain and the ankyrin-repeat domain of 53BP2 were both required for binding to the COOH-terminus of APCL. Confocal microscopy showed that APCL and 53BP2 proteins were localized together in the perinuclei of normal mammalian cells, but this was not the case in cells that expressed truncated APCL and 53BP2 proteins. These findings suggested that binding of the COOH-terminus of APCL to 53BP2 regulates the cytoplasmic location of 53BP2. Because 53BP2 also interacts with p53 and Bcl2 and regulates p53 function, our results suggest that APCL might be involved in the p53/Bcl2-linked pathway of cell-cycle progression and cell death. (+info)
Adenomatous polyposis coli (APC) protein moves along microtubules and concentrates at their growing ends in epithelial cells.
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Adenomatous polyposis coli (APC) tumor suppressor protein has been shown to be localized near the distal ends of microtubules (MTs) at the edges of migrating cells. We expressed green fluorescent protein (GFP)-fusion proteins with full-length and deletion mutants of Xenopus APC in Xenopus epithelial cells, and observed their dynamic behavior in live cells. During cell spreading and wound healing, GFP-tagged full-length APC was concentrated as granules at the tip regions of cellular extensions. At higher magnification, APC appeared to move along MTs and concentrate as granules at the growing plus ends. When MTs began to shorten, the APC granules dropped off from the MT ends. Immunoelectron microscopy revealed that fuzzy structures surrounding MTs were the ultrastructural counterparts for these GFP signals. The COOH-terminal region of APC was targeted to the growing MT ends without forming granular aggregates, and abruptly disappeared when MTs began to shorten. The APC lacking the COOH-terminal region formed granular aggregates that moved along MTs toward their plus ends in an ATP-dependent manner. These findings indicated that APC is a unique MT-associated protein that moves along selected MTs and concentrates at their growing plus ends through their multiple functional domains. (+info)
APC mutations in sporadic medulloblastomas.
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The cerebellar medulloblastoma (WHO Grade IV) is a highly malignant, invasive embryonal tumor with preferential manifestation in children. Several molecular alterations appear to be involved, including isochromosome 17q and the p53, PTCH, and beta-catenin gene mutations. In this study, 46 sporadic medulloblastomas were screened for the presence of mutations in genes of the Wnt signaling pathway (APC and beta-catenin). Single-strand conformational polymorphism (SSCP) analysis followed by direct DNA sequencing revealed 3 miscoding APC mutations in 2 (4.3%) medulloblastomas. One case contained a GCA-->GTA mutation at codon 1296 (Ala-->Val), and another case had double point mutations at codons 1472 (GTA-->ATA, Val-->Ile) and 1495 (AGT-->GGT, Ser-->Gly). Miscoding beta-catenin mutations were detected in 4 tumors (8.7%). Three of these were located at codon 33 (TCT -->TTT, Ser-->Phe) and another at codon 37 (TCT-->GCT, Ser-->Ala). Adenomatous polyposis coli (APC) gene and beta-catenin mutations were mutually exclusive and occurred in a total of 6 of 46 cases (13%). Although germline APC mutations are a well established cause of familial colon and brain tumors (Turcot syndrome), this study provides the first evidence that APC mutations are also operative in a subset of sporadic medulloblastomas. (+info)