Intracellular signaling of the TGF-beta superfamily by Smad proteins.

TGF-beta is a potent inhibitor of cell growth, and accumulating evidence suggests that perturbation of the TGF-beta signaling pathway leads to tumorigenesis. Smads are recently identified proteins that mediate intracellular signaling of the TGF-beta superfamily. Smads 2 and 3 are phosphorylated by the TGF-beta type I receptor. Smad4 was originally identified as a candidate tumor suppressor gene in pancreatic cancers. Smads 2 and 3 form complexes with Smad4 upon TGF-beta stimulation. The heteromeric Smad complexes translocate into the nucleus, where they activate expression of target genes. Our recent study demonstrated that Smads exist as monomers in the absence of TGF-beta. Smads 2 and 3 form homo- as well as hetero-oligomers with Smad4 upon ligand stimulation. Both homo-oligomers and hetero-oligomers directly bind to DNA, suggesting that the signaling pathway of Smads may be multiplex. Smads 2 and 3 associate with transcriptional coactivators such as p300 in a ligand-dependent manner, p300 enhances transactivation by TGF-beta, suggesting that coactivators link Smads to the basal transcriptional machinery. A missense mutation of Smad2 identified in colorectal and lung cancers was introduced to Smad3. The mutant, Smad3(DE), blocked the activation of wild-type Smad2 and Smad3. Thus, the missense mutation not only disrupts the function of the wild-type Smad but also creates a dominant-negative Smad, which could actively contribute to oncogenesis.

Interaction of Drosophila inhibitors of apoptosis with thick veins, a type I serine/threonine kinase receptor for decapentaplegic.

Decapentaplegic (Dpp) is a Drosophila member of bone morphogenetic proteins, which belong to the transforming growth factor-beta superfamily. Members of this family regulate a variety of biological processes such as cell proliferation, morphogenesis, immune response, and apoptosis. Dpp plays a critical role in many aspects of Drosophila development. Members of the transforming growth factor-beta superfamily bind to two different types of serine/threonine kinase receptors, termed type I and type II. Type I receptors act as downstream components of type II receptors in the receptor complexes. Therefore, intracellular proteins that interact with the type I receptors are likely to play important roles in signaling. Several proteins have been identified through protein-protein interaction screenings. We identified Drosophila inhibitor of apoptosis (DIAP) 1 as an interacting protein of a Dpp type I receptor, Thick veins (Tkv). DIAP1 associates with Tkv in vivo. The binding region in DIAP1 is mapped to its C-terminal RING finger region. DIAP2, another Drosophila member of the inhibitor of apoptosis protein family, also interacts with Tkv in vivo. These data suggest that DIAP1 and DIAP2 may be involved, possibly as negative regulators, in the Dpp signaling pathway, which leads to cell apoptosis.

TGF-beta receptor-mediated signalling through Smad2, Smad3 and Smad4.

Smad family members are newly identified essential intracellular signalling components of the transforming growth factor-beta (TGF-beta) superfamily. Smad2 and Smad3 are structurally highly similar and mediate TGF-beta signals. Smad4 is distantly related to Smads 2 and 3, and forms a heteromeric complex with Smad2 after TGF-beta or activin stimulation. Here we show that Smad2 and Smad3 interacted with the kinase-deficient TGF-beta type I receptor (TbetaR)-I after it was phosphorylated by TbetaR-II kinase. TGF-beta1 induced phosphorylation of Smad2 and Smad3 in Mv1Lu mink lung epithelial cells. Smad4 was found to be constitutively phosphorylated in Mv1Lu cells, the phosphorylation level remaining unchanged upon TGF-beta1 stimulation. Similar results were obtained using HSC4 cells, which are also growth-inhibited by TGF-beta. Smads 2 and 3 interacted with Smad4 after TbetaR activation in transfected COS cells. In addition, we observed TbetaR-activation-dependent interaction between Smad2 and Smad3. Smads 2, 3 and 4 accumulated in the nucleus upon TGF-beta1 treatment in Mv1Lu cells, and showed a synergistic effect in a transcriptional reporter assay using the TGF-beta-inducible plasminogen activator inhibitor-1 promoter. Dominant-negative Smad3 inhibited the transcriptional synergistic response by Smad2 and Smad4. These data suggest that TGF-beta induces heteromeric complexes of Smads 2, 3 and 4, and their concomitant translocation to the nucleus, which is required for efficient TGF-beta signal transduction.

Smad6 inhibits signalling by the TGF-beta superfamily.

SMAD proteins have been identified as signalling mediators of the TGF-beta superfamily, which is involved in a range of biological activities including cell growth, morphogenesis, development and immune responses. Smad1, Smad2, Smad3 and Smad5 are ligand-specific: Smadl and Smad5 transduce signals from bone morphogenetic proteins, and Smad2 and Smad3 mediate signalling by TGF-beta and activin, whereas Smad4 acts as a common signalling component. For example, Smad2 is phosphorylated by the TGF-beta type I receptor upon ligand binding, forms a heteromer with Smad4, and then translocates into the nucleus where it activates transcription. Here we report the isolation of Smad6 in the mouse. Smad6 is quite different in structure from the other SMAD proteins, and forms stable associations with type I receptors. Smad6 interferes with the phosphorylation of Smad2 and the subsequent heteromerization with Smad4, but does not inhibit the activity of Smad3. Smad6 also inhibits the phosphorylation of Smad1 that is induced by the bone morphogenetic protein type IB receptor. These data indicate that signals of the TGF-beta superfamily are regulated both positively and negatively by members of the SMAD family.

Characterization of the interaction of FKBP12 with the transforming growth factor-beta type I receptor in vivo.

The type I transforming growth factor-beta receptor (TbetaR-I) is the efferent component of the receptor complex, which presumably phosphorylates intracellular targets. FKBP12, a binding protein for FK506 and rapamycin, is shown to associate with the cytoplasmic region of TbetaR-I in vitro. In this report, we investigated the interaction of FKBP12 with TbetaR-I in vivo. FKBP12 interacts with TbetaR-I in mammalian cells as well as in yeast. Ligand addition does not affect the interaction, and both constitutively active and kinase-negative mutants of TbetaR-I bind FKBP12. FKBP12 dissociates from TbetaR-I in the presence of a high concentration of FK506. The juxtamembrane region of TbetaR-I, containing the major phosphorylation sites by the type II receptor, is required for the interaction. One of the deletion mutants in this region, which was shown to mediate transcriptional response, does not bind FKBP12, suggesting that FKBP12 is not directly involved in TGF-beta signaling. Furthermore TbetaR-I does not phosphorylate FKBP12 in vitro. FKBP12 may not be a direct substrate of TbetaR-I but possibly modulates the TbetaR-I function through its interaction with the regulatory domain of the kinase.

The levels of granulocyte colony-stimulating factor in the plasma of the bone marrow aspirate in various hematological disorders.

We developed a sensitive method of measurement of granulocyte colony-stimulating factor (G-CSF) by an enzyme-linked immunosorbent assay, which we applied in the plasma of the bone marrow aspirate in 70 patients with various hematological disorders. The lowest limit of detection by this method is 2 pg/ml. G-CSF was detected in all but two of the patients. Compared to the G-CSF level in normal healthy controls, those in non-Hodgkin's malignant lymphoma, aplastic anemia, agranulocytosis and multiple myeloma were significantly higher, while the level in refractory anemia was not different. The G-CSF level in acute myelogenous leukemia patients was either elevated or decreased regardless of the French-American-British subgroup. The level in acute lymphoblastic leukemia was not different from the normal value, as was that in refractory anemia with an excess of blasts, and that in chronic lymphocytic leukemia. A patient with chronic myelomonocytic leukemia showed initial elevation of G-CSF with normalization after entering complete remission. The G-CSF level in chronic myelogenous leukemia was significantly decreased, although one patient in hematological remission who was under alpha-interferon therapy showed normal levels. The level in polycythemia vera was not significantly different from the normal value. The G-CSF level for the entire group showed an inverse, although not statistically significant, correlation with the percentages of myeloid cells of the bone marrow (r = -0.174, p = 0.1703, n = 80). These results are thought to reflect the regulatory mechanism of granulopoiesis in the bone marrow in various hematological disorders, and it is concluded that this method may be of clinical use in the treatment of patients with these disorders and in the selection of candidates likely to benefit from G-CSF administration.

Fatal aplastic anemia caused by Epstein-Barr virus infection after autologous bone marrow transplantation for non-Hodgkin malignant lymphoma.

We report a case of severe and fatal aplastic anemia during an episode of infectious mononucleosis caused by Epstein-Barr (EB) virus infection. The 13-year-old female patient had shown normal hematological findings and had previously undergone repeated chemotherapy and autologous bone marrow transplantation for refractory non-Hodgkin malignant lymphoma (NHL). She was probably in an immuno-suppressed condition prior to this episode of infection. The possible causal relationship of the EB virus infection in the pathogenesis of aplastic anemia was documented by the clinical course, demonstration of EB virus genome in the bone marrow cells, and an elevated plasma interferon (IFN)-gamma level.

Capillary leak syndrome likely the result of granulocyte colony-stimulating factor after high-dose chemotherapy.

Two cases of malignant lymphoma complicated with capillary leak syndrome following super high-dose chemotherapy and administration of granulocyte colony-stimulating factor (G-CSF) are presented. Subsequent to the nadir of granulocytes, and at the stage of rapid increase of granulocytes, the symptoms of fever, hypotension, dyspnea, pleural effusion and edema appeared, and laboratory data revealed hypoxia, hypocapnia and hypoalbuminemia. In addition, an abscess-like lesion was observed in the liver in one patient. After the administration of G-CSF was ceased or decreased, and pulse therapy with methylprednisolone was initiated, these symptoms disappeared quickly.

Myelodysplastic syndrome associated with myelofibrosis, a report of 3 cases.

Three cases of primary myelodysplastic syndrome (MDS) associated with myelofibrosis were initially diagnosed as refractory anemia by the presence of bicytopenia or pancytopenia and having normo- or hypercellular marrow with dysplastic features. The bone marrow aspiration of these patients showed dry tap a few months after admission, or on admission. Their bone marrow biopsy specimens revealed various grades of increased formation of reticulin fibers. One patient entered into complete remission in response to metenolone, while the other two patients died of cerebral hemorrhage several months after admission. These results indicate that this disease should be classified as a distinct subgroup of MDS.