The importance of bystander effects in radiation therapy in melanoma skin-cancer cells and umbilical-cord stromal stem cells.

PURPOSE: To examine direct and bystander radiation-induced effects in normal umbilical-cord stromal stem cell (HCSSC) lines and in human cancer cells. MATERIALS AND METHODS: The UCSSC lines used in this study were obtained in our laboratory. Two cell lines (UCSSC 35 and UCSSC 37) and two human melanoma skin-cancer cells (A375 and G361) were exposed to ionizing radiation to measure acute radiation-dosage cell-survival curves and radiation-induced bystander cell-death response. Normal cells, although extremely sensitive to ionizing radiation, were resistant to the bystander effect whilst tumor cells were sensitive to irradiated cell-conditioned media, showing a dose-response relationship that became saturated at relatively low doses. We applied a biophysical model to describe bystander cell-death through the binding of a ligand to the cells. This model allowed us to calculate the maximum cell death (χ(max)) produced by the bystander effect together with its association constant (K(By)) in terms of dose equivalence (Gy). The values obtained for K(By) in A375 and G361 cells were 0.23 and 0.29 Gy, respectively. CONCLUSION: Our findings help to understand how anticancer therapy could have an additional decisive effect in that the response of sub-lethally hit tumor cells to damage might be required for therapy to be successful because the survival of cells communicating with irradiated cells is reduced.

Roscovitine sensitizes breast cancer cells to TRAIL-induced apoptosis through a pleiotropic mechanism.

The tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL/APO2L) is a member of the TNF gene superfamily that induces apoptosis upon engagement of cognate death receptors. While TRAIL is relatively non-toxic to normal cells, it selectively induces apoptosis in many transformed cells. Nevertheless, breast tumor cells are particularly resistant to the effects of TRAIL. Here we report that, in combination with the cyclin-dependent kinase inhibitor roscovitine, exposure to TRAIL induced marked apoptosis in the majority of TRAIL-resistant breast cancer cell lines examined. Roscovitine facilitated TRAIL death-inducing signaling complex formation and the activation of caspase-8. The cFLIP(L) and cFLIP(S) FLICE-inhibitory proteins were significantly down-regulated following exposure to roscovitine and, indeed, the knockdown of cFLIP isoforms by siRNA sensitized breast tumor cells to TRAIL-induced apoptosis. In addition, we demonstrate that roscovitine strongly suppressed Mcl-1 expression and up-regulated E2F1 protein levels in breast tumor cells. Significantly, the silencing of Mcl-1 by siRNA sensitized breast tumor cells to TRAIL-induced apoptosis. Furthermore, the knockdown of E2F1 protein by siRNA reduced the sensitizing effect of roscovitine in TRAIL-induced apoptosis. In summary, our results reveal a pleitropic mechanism for the pro-apoptotic influence of roscovitine, highlighting its potential as an antitumor agent in breast cancer in combination with TRAIL.

The up-regulation of human caspase-8 by interferon-gamma in breast tumor cells requires the induction and action of the transcription factor interferon regulatory factor-1.

Treatment of human breast tumor cells with interferon-gamma (IFN-gamma) elevates caspase-8 expression and sensitizes these cells to death receptor-mediated apoptosis through the increased processing and activation of apical procaspase-8. We have characterized the human caspase-8 gene promoter and studied the transcriptional regulation of caspase-8 gene expression in MCF-7 breast tumor cells treated with IFN-gamma. Our findings show that IFN-gamma induces the up-regulation of caspase-8 mRNA expression through a protein synthesis-dependent mechanism involving the action of the IFN-gamma-inducible transcription factor interferon regulatory factor-1 (IRF-1) and without altering mRNA stability. The human caspase-8 gene promoter lacks recognizable TATA and CAAT boxes but contains a consensus Sp1 binding site. We have identified two major IFN-gamma-inducible transcriptional start sites in these cells by S1 nuclease mapping, confirmed by primer extension analysis. Deletion analysis of the promoter defined an 82-bp minimal region responsible for IFN-gamma-inducible promoter activity. In this region, we have identified an IFN-stimulated response element that is important for both the basal and IFN-gamma-enhanced transcriptional activities. Electrophoretic mobility shift assay analysis demonstrated that IFN-gamma induces a complex between an oligonucleotide probe containing the ISRE motif and IRF-1 over a similar time scale to the induction of caspase-8 mRNA. Exogenously expressed IRF-1 in MCF-7 cells up-regulated the activity of a luciferase reporter plasmid containing an 82-bp region of the caspase-8 promoter. These data define a new pathway through which IFN-gamma might control the sensitivity of tumor cell to death receptor-mediated apoptosis.

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) decoy receptor TRAIL-R3 is up-regulated by p53 in breast tumor cells through a mechanism involving an intronic p53-binding site.

Tumor necrosis factor-related apoptosis-inducing ligand receptor 3 (TRAIL-R3) is a decoy receptor for TRAIL, a member of the tumor necrosis factor family. In several cell types decoy receptors inhibit TRAIL-induced apoptosis by binding TRAIL and thus preventing its binding to proapoptotic TRAIL receptors. We studied the regulation of TRAIL-R3 gene expression in breast tumor cells treated with the genotoxic drug doxorubicin (DXR). The breast tumor cell line MCF-7 (p53 wild type) responded to DXR with a marked elevation of TRAIL-R3 expression at the mRNA, total protein, and cell surface levels. In contrast, in EVSA-T cells (p53 mutant) DXR did not induce increased expression of TRAIL-R3. In MCF-7 cells overexpressing the human papillomavirus protein E6, which causes p53 degradation, DXR-induced TRAIL-R3 expression was notably reduced. Furthermore, in MCF-7 cells overexpressing a temperature-sensitive p53 mutant (Val135), shifting the cultures to the permissive temperature was sufficient to induce the expression of TRAIL-R3. We also cloned and characterized a p53 consensus element located within the first intron of the human TRAIL-R3 gene. This element binds p53 and confers responsiveness to genotoxic damage to constructs of the TRAIL-R3 promoter in transient transfection experiments. Our results indicate that genotoxic treatments such as DXR, frequently used in cancer therapy, may also induce genes such as TRAIL-R3 that potentially have antiapoptotic actions and thus interfere with the TRAIL signaling system. This is particularly important in view of the proposed use of TRAIL in antitumor therapy.