A purine scaffold Hsp90 inhibitor destabilizes BCL-6 and has specific antitumor activity in BCL-6-dependent B cell lymphomas.

We report that heat shock protein 90 (Hsp90) inhibitors selectively kill diffuse large B cell lymphomas (DLBCLs) that depend on the BCL-6 transcriptional repressor. We found that endogenous Hsp90 interacts with BCL-6 in DLBCL cells and can stabilize BCL-6 mRNA and protein. Hsp90 formed a complex with BCL-6 at its target promoters, and Hsp90 inhibitors derepressed BCL-6 target genes. A stable mutant of BCL-6 rescued DLBCL cells from Hsp90 inhibitor-induced apoptosis. BCL-6 and Hsp90 were almost invariantly coexpressed in the nuclei of primary DLBCL cells, suggesting that their interaction is relevant in this disease. We examined the pharmacokinetics, toxicity and efficacy of PU-H71, a recently developed purine-derived Hsp90 inhibitor. PU-H71 preferentially accumulated in lymphomas compared to normal tissues and selectively suppressed BCL-6-dependent DLBCLs in vivo, inducing reactivation of key BCL-6 target genes and apoptosis. PU-H71 also induced cell death in primary human DLBCL specimens.

Kaiso contributes to DNA methylation-dependent silencing of tumor suppressor genes in colon cancer cell lines.

Aberrant CpG methylation of tumor suppressor gene regulatory elements is associated with transcriptional silencing and contributes to malignant transformation of different tissues. It is presumed that methylated DNA sequences recruit repressor machinery to actively shutdown gene expression. The Kaiso protein is a transcriptional repressor expressed in human and murine colorectal tumors that can bind to methylated clusters of CpG dinucleotides. We show here that Kaiso represses methylated tumor suppressor genes and can bind in a methylation-dependent manner to the CDKN2A in human colon cancer cell lines. The contribution of Kaiso to epigenetic silencing was underlined by the fact that Kaiso depletion induced tumor suppressor gene expression without affecting DNA methylation levels. As a consequence, colon cancer cells became susceptible to cell cycle arrest and cell death mediated by chemotherapy. The data suggest that Kaiso is a methylation-dependent "opportunistic" oncogene that silences tumor suppressor genes when they become hypermethylated. Because Kaiso inactivation sensitized colon cancer cell lines to chemotherapy, it is possible that therapeutic targeting of Kaiso could improve the efficacy of current treatment regimens.

Inhibition of NF-kappaB activity by BAY 11-7082 increases apoptosis in multidrug resistant leukemic T-cell lines.

Multidrug resistance (MDR) is the main reason for failure of cancer therapy with resistance to apoptosis being one of the mechanisms involved. Constitutive NF-kappaB activity has been detected in many tumors contributing to oncogenesis and tumor survival whereas inhibition of NF-kappaB activity has proved to enhance cell death induced by chemotherapeutic agents. Consequently, the use of BAY 11-7082, an irreversible inhibitor of IkappaB-alpha phosphorylation, could be beneficial in the treatment of certain tumors. Although there are several reports which demonstrate a transient activation of NF-kappaB by cytotoxic drugs, little is known about the role of NF-kappaB activation in the development of a chemoresistant phenotype in leukemic cells. In this study, we analyzed the relationship between NF-kappaB and the survival of murine leukemic drug resistant cell lines. The modulation of this transcription factor by BAY 11-7082 and the chemotherapeutic agents vincristine and doxorubicin was evaluated. The effect of BAY 11-7082 on the expression of genes containing NF-kappaB-binding sites was also studied. We found that the cell lines LBR-V160 and LBR-D160 (resistant to vincristine and doxorubicin, respectively) presented higher constitutive NF-kappaB activity than the sensitive LBR- and the active complex contained both p50 and p65 subunits. BAY 11-7082 (3.5 microM) inhibited constitutive NF-kappaB activity in the three cell lines whereas the anticancer agents did not. Treatment with BAY 11-7082 induced a higher percentage of apoptosis in LBR-V160 and LBR-D160 than in LBR-. Cells treated with BAY 11-7082 displayed modulation of NF-kappaB-inducible genes such as IL-10, IL-15, TNF-alpha and TGF-beta. Taken together, these data suggest that suppression of constitutive NF-kappaB activity by BAY 11-7082 may be a useful treatment for MDR leukemias.

Multidrug resistance modulators PSC 833 and CsA show differential capacity to induce apoptosis in lymphoid leukemia cell lines independently of their MDR phenotype.

Among the mechanisms that induce multidrug resistance (MDR), one of those most frequent is over-expression of a phosphoglycoprotein (Pgp) encoded in the mouse by the mdr-1 and mdr-3 genes. We have demonstrated that cyclosporin-A (CsA) as well as its analogue PSC 833 were able to revert the MDR phenotype in murine cell lines resistant to vincristine (LBR-V160) or doxorubicin (LBR-D160). The aim of this work was to evaluate the ability of PSC 833 and CsA to modulate mdr-1, mdr-3 and mrp-1 genes as well as to induce apoptosis analyzing the mechanism involved in the above tumor cell lines. By semi-quantitative RT-PCR, we demonstrated that mdr-3 was over-expressed in both resistant lines while mdr-1 was over-expressed only in LBR-V160; in contrast, mrp-1 expression was not evidenced in any of the cell lines. After treatment with 0.1 microg ml(-1) of either PSC 833 or CsA, LBR-V160 showed no changes in mdr-1 but decreased mdr-3 expression, while LBR-D160 failed to display any modification in the expression of these genes. Apoptosis was evidenced by fluorescence microscopy, S minuscule accumulation and agarose gel electrophoresis. Our results demonstrated that CsA (1 microg ml(-1)) was able to induce apoptosis in all cell lines: 18.31% (+/-4.46) for LBR-, 25.96% (+/-5.24) for LBR-V160 and 27.36% (+/-4.12) for LBR-D160, while PSC 833 (1 microg ml(-1)) only induced apoptosis 21.51% (+/-5.73) in LBR-V160 cell line. The expression of Bcl-2 family proteins (Bcl-2, Bax and Bcl-x(L)) was analyzed by flow cytometry showing high expression of the three proteins which was not significantly modified after treatment with either PSC 833 or CsA on the sensitive as well as on the resistant cell lines. Single stranded conformation polymorphisms analysis of p53 (Trp53) gene in the cell lines showed no mutation in exons 5-8 of the tumor suppressor gene. We conclude that depending on the concentration used, PSC 833 and CsA may act either by modulating the mdr-3 gene (0.1 microg ml(-1)) or by direct impact on the cells through induction of apoptosis (1 microg ml(-1)), in the latter case through a mechanism that might act independent of the Bcl-2 family proteins.

Dissimilar invasive and metastatic behavior of vincristine and doxorubicin-resistant cell lines derived from a murine T cell lymphoid leukemia.

Multidrug resistance (MDR) lines from a murine T-cell lymphoid leukemia were selected in increasing vincristine (VCR) or doxorubicin (DOX) concentrations. Surface markers were determined by flow cytometry in both resistant (LBR-V 160 and LBR-D 160) and sensitive (LBR-) cell lines. Results obtained revealed similar expression of CD25, CD24, CD8, CD4, C18 and CD44, while differences in binding to hyaluronic acid (HA) were found. LBR- and LBR-D 160 bound to HA only after phorbol ester (PMA) activation, while LBR-V160 failed to bind HA even after PMA treatment. Histopathological analysis disclosed that LBR-V160 was less invasive than LBR- and LBR-D160 cell lines. In vitro growth of cell lines analyzed by sulforhodamine-B uptake showed that doubling time for the three lines was 10.24 h (LBR-), 16.75 h (LBR-V160) and 16.29 h (LBR-D160). Mortality rate was determined after i.p. injection of 10(4) cells. Mice inoculated with LBR- died at 23 2.11) days, while those inoculated with LBR-V160 or LBR-D160 died at 41 (+/- 9.53) or 41 (+/- 4.96) days, respectively. Our results demonstrated that leukemic murine T cells cultured in the long-term presence of VCR or DOX not only presented changes in the resistance phenotype but also variations in their growth and metastatic pattern.