Michael E. Engel

Interdependent SMAD and JNK Signaling in Transforming Growth Factor-β-mediated Transcription

SMAD and JNK cascades are essential components of the transforming growth factor-β (TGF-β) signaling machinery and are implicated in common transcriptional responses. However, the relationship of these pathways to one another downstream of the TGF-β receptor complex is unknown. We show that JNK is rapidly activated by TGF-β in a SMAD-independent manner and phosphorylates Smad3 outside its -SSXS motif. Smad3 phosphorylation by JNK facilitates both its activation by the TGF-β receptor complex and its nuclear accumulation. JNK regulates SMAD- and TGF-β-mediated transcriptional responses, yet JNK activators only partially stimulate transcriptional responses characteristic of TGF-β without coincident SMAD pathway activation. These results suggest an interdependent relationship between the JNK and SMAD pathways in TGF-β-mediated transcription.

Signal transduction by transforming growth factor‐β: A cooperative paradigm with extensive negative regulation

Transforming growth factor‐β (TGF‐β) represents an evolutionarily conserved family of secreted factors that mobilize a complex signaling network to control cell fate by regulating proliferation, differentiation, motility, adhesion, and apoptosis. TGF‐β promotes the assembly of a cell surface receptor complex composed of type I (TβRI) and type II (TβRII) receptor serine/threonine kinases. In response to TGF‐β binding, TβRII recruits and activates TβRI through phosphorylation of the regulatory GS‐domain. Activated TβRI then initiates cytoplasmic signaling pathways to produce cellular responses. SMAD proteins together constitute a unique signaling pathway with key roles in signal transduction by TGF‐β and related factors. Pathway‐restricted SMADs are phosphorylated and activated by type I receptors in response to stimulation by ligand. Once activated, pathway‐restricted SMADs oligomerize with the common‐mediator Smad4 and subsequently translocate to the nucleus. Genetic analysis in Drosophila melanogaster and Caenorhabditis elegans, as well as TβRII and SMAD mutations in human tumors, emphasizes their importance in TGF‐β signaling. Mounting evidence indicates that SMADs cooperate with ubiquitous cytoplasmic signaling cascades and nuclear factors to produce the full spectrum of TGF‐β responses. Operating independently, these ubiquitous elements may influence the nature of cellular responses to TGF‐β. Additionally, a variety of regulatory schemes contribute temporal and/or spatial restriction to TGF‐β responses. This report reviews our current understanding of TGF‐β signal transduction and considers the importance of a cooperative signaling paradigm to TGF‐β‐mediated biological responses. J. Cell. Biochem. Suppls. 30/31:111–122, 1998.

Deletion of Mtg16, a Target of t(16;21), Alters Hematopoietic Progenitor Cell Proliferation and Lineage Allocation

ABSTRACT While a number of DNA binding transcription factors have been identified that control hematopoietic cell fate decisions, only a limited number of transcriptional corepressors (e.g., the retinoblastoma protein [pRB] and the nuclear hormone corepressor [N-CoR]) have been linked to these functions. Here, we show that the transcriptional corepressor Mtg16 (myeloid translocation gene on chromosome 16), which is targeted by t(16;21) in acute myeloid leukemia, is required for hematopoietic progenitor cell fate decisions and for early progenitor cell proliferation. Inactivation of Mtg16 skewed early myeloid progenitor cells toward the granulocytic/macrophage lineage while reducing the numbers of megakaryocyte-erythroid progenitor cells. In addition, inactivation of Mtg16 impaired the rapid expansion of short-term stem cells, multipotent progenitor cells, and megakaryocyte-erythroid progenitor cells that is required under hematopoietic stress/emergency. This impairment appears to be a failure to proliferate rather than an induction of cell death, as expression of c-Myc, but not Bcl2, complemented the Mtg16−/− defect.

Myeloid Translocation Gene 16 (MTG16) Interacts with Notch Transcription Complex Components To Integrate Notch Signaling in Hematopoietic Cell Fate Specification

ABSTRACT The Notch signaling pathway regulates gene expression programs to influence the specification of cell fate in diverse tissues. In response to ligand binding, the intracellular domain of the Notch receptor is cleaved by the γ-secretase complex and then translocates to the nucleus. There, it binds the transcriptional repressor CSL, triggering its conversion to an activator of Notch target gene expression. The events that control this conversion are poorly understood. We show that the transcriptional corepressor, MTG16, interacts with both CSL and the intracellular domains of Notch receptors, suggesting a pivotal role in regulation of the Notch transcription complex. The Notch1 intracellular domain disrupts the MTG16-CSL interaction. Ex vivo fate specification in response to Notch signal activation is impaired in Mtg16−/− hematopoietic progenitors, and restored by MTG16 expression. An MTG16 derivative lacking the binding site for the intracellular domain of Notch1 fails to restore Notch-dependent cell fate. These data suggest that MTG16 interfaces with critical components of the Notch transcription complex to affect Notch-dependent lineage allocation in hematopoiesis.

The enemy within: dormant retroviruses awaken

Mammalian genomes harbor regulatory elements from ancient retroviral infections. These retroviral remnants are normally silenced by DNA methylation—but this can change. Reactivation of one such element triggers the expression of a nearby oncogene during the development of Hodgkins lymphoma (571–579).

Acyclovir-resistant herpes simplex virus pneumonia post-unrelated stem cell transplantation: A word of caution

Abstract: HSV causes serious complications in immunocompromised patients, especially SCT recipients. Although ACV is an effective antiviral prophylaxis, the emergence of ACV resistance is a growing problem. The authors describe two cases of fatal ACV‐resistant HSV in two pediatric patients following unrelated donor SCT. Despite the in vitro sensitivity of the HSV isolates to foscarnet, both patients failed to respond to foscarnet therapy. Other antiviral therapies should be considered in those patients who fail to show rapid clinical improvement.

Kaiso Directs the Transcriptional Corepressor MTG16 to the Kaiso Binding Site in Target Promoters

Myeloid translocation genes (MTGs) are transcriptional corepressors originally identified in acute myelogenous leukemia that have recently been linked to epithelial malignancy with non-synonymous mutations identified in both MTG8 and MTG16 in colon, breast, and lung carcinoma in addition to functioning as negative regulators of WNT and Notch signaling. A yeast two-hybrid approach was used to discover novel MTG binding partners. This screen identified the Zinc fingers, C2H2 and BTB domain containing (ZBTB) family members ZBTB4 and ZBTB38 as MTG16 interacting proteins. ZBTB4 is downregulated in breast cancer and modulates p53 responses. Because ZBTB33 (Kaiso), like MTG16, modulates Wnt signaling at the level of TCF4, and its deletion suppresses intestinal tumorigenesis in the ApcMin mouse, we determined that Kaiso also interacted with MTG16 to modulate transcription. The zinc finger domains of Kaiso as well as ZBTB4 and ZBTB38 bound MTG16 and the association with Kaiso was confirmed using co-immunoprecipitation. MTG family members were required to efficiently repress both a heterologous reporter construct containing Kaiso binding sites (4×KBS) and the known Kaiso target, Matrix metalloproteinase-7 (MMP-7/Matrilysin). Moreover, chromatin immunoprecipitation studies placed MTG16 in a complex occupying the Kaiso binding site on the MMP-7 promoter. The presence of MTG16 in this complex, and its contributions to transcriptional repression both required Kaiso binding to its binding site on DNA, establishing MTG16-Kaiso binding as functionally relevant in Kaiso-dependent transcriptional repression. Examination of a large multi-stage CRC expression array dataset revealed patterns of Kaiso, MTG16, and MMP-7 expression supporting the hypothesis that loss of either Kaiso or MTG16 can de-regulate a target promoter such as that of MMP-7. These findings provide new insights into the mechanisms of transcriptional control by ZBTB family members and broaden the scope of co-repressor functions for the MTG family, suggesting coordinate regulation of transcription by Kaiso/MTG complexes in cancer.

Mtg16/Eto2 contributes to murine T-cell development

ABSTRACT Mtg16/Eto2 is a transcriptional corepressor that is disrupted by t(16;21) in acute myeloid leukemia. Using mice lacking Mtg16, we found that Mtg16 is a critical regulator of T-cell development. Deletion of Mtg16 led to reduced thymocyte development in vivo, and after competitive bone marrow transplantation, there was a nearly complete failure of Mtg16−/− cells to contribute to thymocyte development. This defect was recapitulated in vitro as Mtg16−/− Lineage−/Sca1+/c-Kit+ (LSK) cells of the bone marrow or DN1 cells of the thymus failed to produce CD4+/CD8+ cells in response to a Notch signal. Complementation of these defects by reexpressing Mtg16 showed that 3 highly conserved domains were somewhat dispensable for T-cell development but required the capacity of Mtg16 to suppress E2A-dependent transcriptional activation and to bind to the Notch intracellular domain. Thus, Mtg16 integrates the activities of signaling pathways and nuclear factors in the establishment of T-cell fate specification.

Matched unrelated bone marrow transplantation with reduced-intensity conditioning for leukocyte adhesion deficiency.

Matched unrelated bone marrow transplantation with reduced-intensity conditioning for leukocyte adhesion deficiency

Vincristine, irinotecan, and temozolomide for treatment of relapsed alveolar rhabdomyosarcoma.

Event-free survival for recurrent alveolar rhabdomyosarcoma (ARMS) is poor, and a consensus approach to treatment in the relapse setting has not been established. Recent studies suggest that a combination regimen of vincristine, irinotecan, and temozolomide (VITA) is active against recurrent sarcomas. We present our single-institution experience with this regimen for relapsed ARMS in heavily pretreated patients, including those with prior exposure to a combination regimen of vincristine and irinotecan. We observed a complete radiographic response in 1 of 4 patients who received VITA as a fifth attempted salvage regimen. Radiographic remission for the responsive patient was sustained for 27 weeks before disease recurrence. All therapies were administered in the outpatient setting and no grade III or grade IV toxicities were observed. These findings suggest that for patients with refractory ARMS, VITA in combination should be among the treatment options considered. They also reinforce the need for biological correlates to prospectively identify patients who may benefit from this treatment.