Chemical dissection of the cell cycle: probes for cell biology and anti-cancer drug development.

Cancer cell proliferation relies on the ability of cancer cells to grow, transition through the cell cycle, and divide. To identify novel chemical probes for dissecting the mechanisms governing cell cycle progression and cell division, and for developing new anti-cancer therapeutics, we developed and performed a novel cancer cell-based high-throughput chemical screen for cell cycle modulators. This approach identified novel G1, S, G2, and M-phase specific inhibitors with drug-like properties and diverse chemotypes likely targeting a broad array of processes. We further characterized the M-phase inhibitors and highlight the most potent M-phase inhibitor MI-181, which targets tubulin, inhibits tubulin polymerization, activates the spindle assembly checkpoint, arrests cells in mitosis, and triggers a fast apoptotic cell death. Importantly, MI-181 has broad anti-cancer activity, especially against BRAF(V600E) melanomas.

Global DNA methylation profiling reveals silencing of a secreted form of Epha7 in mouse and human germinal center B-cell lymphomas.

Most human lymphomas originate from transformed germinal center (GC) B lymphocytes. While activating mutations and translocations of MYC, BCL2 and BCL6 promote specific GC lymphoma subtypes, other genetic and epigenetic modifications that contribute to malignant progression in the GC remain poorly defined. Recently, aberrant expression of the TCL1 proto-oncogene was identified in major GC lymphoma subtypes. TCL1 transgenic mice offer unique models of both aggressive GC and marginal zone B-cell lymphomas, further supporting a role for TCL1 in B-cell transformation. Here, restriction landmark genomic scanning was employed to discover tumor-associated epigenetic alterations in malignant GC and marginal zone B-cells in TCL1 transgenic mice. Multiple genes were identified that underwent DNA hypermethylation and decreased expression in TCL1 transgenic tumors. Further, we identified a secreted isoform of EPHA7, a member of the Eph family of receptor tyrosine kinases that are able to influence tumor invasiveness, metastasis and neovascularization. EPHA7 was hypermethylated and repressed in both mouse and human GC B-cell non-Hodgkin lymphomas, with the potential to influence tumor progression and spread. These data provide the first set of hypermethylated genes with the potential to complement TCL1-mediated GC B-cell transformation and spread.

The COOH-terminal domain of FLI-1 is necessary for full tumorigenesis and transcriptional modulation by EWS/FLI-1.

More than 85% of Ewing's family tumors carry a specific chromosomal translocation that fuses the NH(2) terminus of the EWS gene to the COOH terminus of the FLI1 transcription factor. It has been shown previously that both the transactivation domain encoded by EWS and the DNA binding domain of FLI1 were necessary for transforming cells to anchorage independence. We now report that a COOH-terminal domain in addition to the FLI1 DNA binding domain is necessary to promote cellular transformation. NIH 3T3 cells expressing a COOH-terminal deletion mutant (EWS/FLI1 DeltaC) have a greatly reduced capability to form colonies in soft agar and tumors in severe combined immunodeficient mice. The rate of tumor formation for NIH 3T3 that express EWS/FLI1 DeltaC is 50 days, whereas EWS/FLI1 forms tumors within 22 days. In addition, cells expressing the EWS/FLI1 DeltaC mutant failed to completely demonstrate the round-cell histology that is seen in both Ewing's tumor cell lines and NIH 3T3 cells expressing full-length EWS/FLI1. Northern and microarray analyses were performed to assess the effect of loss of the FLI1 COOH terminus on transcriptional modulation of EWS/FLI1 target genes. We found that although EWS/FLI1 DeltaC up-regulates smaller numbers of genes (21 genes) compared with EWS/FLI1 (34 genes), 41% of the EWS/FLI1 targets were also up-regulated by EWS/FLI1 DeltaC. On the other hand, EWS/FLI1 DeltaC is unable to down-regulate genes (3 genes) as efficiently as EWS/FLI1 (39 genes) with only one target gene repressed by both fusion constructs. Our study indicates that the EWS/FLI1 transcription factor has strong transcriptional activating as well as repressing properties and suggests that transcriptional activation and repression of target genes may occur through biochemically different mechanisms.

TCL1 oncogene expression in B cell subsets from lymphoid hyperplasia and distinct classes of B cell lymphoma.

Activation of the TCL1 oncogene has been implicated in T cell leukemias/lymphomas and recently was associated with AIDS diffuse large B cell lymphomas (AIDS-DLBCL). Also, in nonmalignant lymphoid tissues, antibody staining has shown that mantle zone B cells expressed abundant Tcl1 protein, whereas germinal center (GC; centrocytes and centroblasts) B cells showed markedly reduced expression. Here, we analyze isolated B cell subsets from hyperplastic tonsil to determine a more precise pattern of Tcl1 expression with development. We also examine multiple B cell lines and B lymphoma patient samples to determine whether different tumor classes retain or alter the developmental pattern of expression. We show that TCL1 expression is not affected by Epstein-Barr virus (EBV) infection and is high in naïve B cells, reduced in GC B cells, and absent in memory B cells and plasma cells. Human herpesvirus-8 infected primary effusion lymphomas (PEL) and multiple myelomas are uniformly TCL1 negative, whereas all other transformed B cell lines tested express moderate to abundant TCL1. This observation supports the hypothesis that PEL, like myeloma, usually arise from post-GC stages of B cell development. Tcl1 protein is also detected in most naïve/GC-derived B lymphoma patient samples (23 of 27 [85%] positive), whereas most post-GC-derived B lymphomas lack expression (10 of 41 [24%] positive). These data indicate that the pattern of Tcl1 expression is distinct between naïve/GC and post-GC-derived B lymphomas (P < 0.001) and that the developmental pattern of expression is largely retained. However, post-GC-derived AIDS-DLBCL express TCL1 at a frequency equivalent to naïve/GC-derived B lymphomas in immune-competent individuals (7 of 9 [78%] positive), suggesting that TCL1 down-regulation is adversely affected by severe immune system dysfunction. These findings demonstrate that TCL1 expression in B cell lymphoma usually reflects the stage of B cell development from which they derive, except in AIDS-related lymphomas.

Divergent Ewing's sarcoma EWS/ETS fusions confer a common tumorigenic phenotype on NIH3T3 cells.

Ewing's sarcomas express chimeric transcription factors resulting from a fusion of the amino terminus of the EWS gene to the carboxyl terminus of one of five ETS proteins. While the majority of tumors express EWS/FLI1 fusions, some Ewing's tumors contain variant chimeras such as EWS/ETV1 that have divergent ETS DNA-binding domains. In spite of their structural differences, both EWS/ETS fusions up regulate EAT-2, a previously described EWS/FLI1 target gene. In contrast to EWS/FLI1, NIH3T3 cells expressing EWS/ETV1 cannot form colonies in soft agar though coexpression of a dominant negative truncated ETV1 construct attenuates EWS/FLI1 mediated anchorage independent growth. When EWS/ETV1 or EWS/FLI1 expressing NIH3T3 cells are injected into SCID mice, tumors form more often and faster than with NIH-3T3 cells with empty vector controls. The tumorigenic potency of each EWS/ETS fusion is linked to its C-terminal structure, with the FLI1 C-terminus confering a greater tumorigenic potential than the corresponding ETV1 domain. The resulting EWS/ETV1 and EWS/FLI1 tumors closely resemble each other at both a macroscopic and a microscopic level. These tumors differ greatly from tumors formed by NIH3T3 cells expressing activated RAS. These data indicate that in spite of their structural differences, EWS/ETV1 and EWS/FLI1 promote oncogenesis via similar biologic pathways.

EWS/ETS fusion genes induce epithelial and neuroectodermal differentiation in NIH 3T3 fibroblasts.

Ewing's sarcoma is the least differentiated member of the peripheral primitive neuroectodermal (pPNET) tumor family. Chromosomal translocations involving the EWS gene and five different Ets family transcription factor genes create fusion genes encoding aberrant transcription factors and are implicated in the vast majority of Ewing's sarcoma cases. Here, NIH 3T3 fibroblasts were infected with control (tk-neo or RAS) and two different EWS/ETS-expressing retroviruses. In vitro studies of established polyclonal lines expressing the two EWS/ETS genes, either EWS/FLI1 or EWS/ETV1, showed induction of cytokeratin 15 gene expression. Both fusion genes also caused characteristic gross morphologic, histologic, and ultrastructural changes in NIH 3T3 cells when transformed cell lines were injected into CB-17-scid mice. Native NIH 3T3 cells with a spindled cell morphology were converted to polygonal cells with high nucleo-cytoplasmic ratios that continued to express abundant cytokeratin. Extracellular collagen deposition was abolished, rough endoplasmic reticulum was markedly diminished, and rudimentary cell-cell attachments appeared. Most strikingly, neurosecretory-type dense core granules like those seen in pPNET were now evident. This murine model, created in mesenchyme-derived NIH 3T3 cells, demonstrated new characteristics of both neuroectodermal and epithelial differentiation and resembled small round cell tumors microscopically.