Induction of apoptosis in malignant and camptothecin-resistant human cells

Characterization of Camptothecin-induced Genomic Changes in the Camptothecin-resistant T-ALL-derived Cell Line CPT-K5

Acquisition of resistance to topoisomerase I (TOP1)-targeting camptothecin (CPT) derivatives is a major clinical problem. Little is known about the underlying chromosomal and genomic mechanisms. We characterized the CPT-K5 cell line expressing mutant CPT-resistant TOP1 and its parental T-cell derived acute lymphoblastic leukemia CPT-sensitive RPMI-8402 cell line by karyotyping and molecular genetic methods, including subtractive oligo-based array comparative genomic hybridization (soaCGH) analysis. Karyotyping revealed that CPT-K5 cells had acquired additional structural aberrations and a reduced modal chromosomal number compared to RPMI-8402. soaCGH analysis identified vast copy number alterations and >200 unbalanced DNA breakpoints distributed unevenly across the chromosomal complement in CPT-K5. In addition, the short tandem repeat alleles were found to be highly different between CPT-K5 and its parental cell line. We identified copy number alterations affecting genes important for maintaining genome integrity and reducing CPT-induced DNA damage. We show for the first time that short tandem repeats are targets for TOP1 cleavage, that can be differentially stimulated by CPT.

Raf kinase inhibitor protein (RKIP) blocks signal transducer and activator of transcription 3 (STAT3) activation in breast and prostate cancer

Raf kinase inhibitor protein (RKIP) is a member of the phosphatidylethanolamine-binding-protein (PEBP) family that modulates the action of many kinases involved in cellular growth, apoptosis, epithelial to mesenchymal transition, motility, invasion and metastasis. Previously, we described an inverse association between RKIP and signal transducers and activators of transcription 3 (STAT3) expression in gastric adenocarcinoma patients. In this study, we elucidated the mechanism by which RKIP regulates STAT3 activity in breast and prostate cancer cell lines. RKIP over expression inhibited c-Src auto-phosphorylation and activation, as well as IL-6-, JAK1 and 2-, and activated Raf-mediated STAT3 tyrosine and serine phosphorylation and subsequent activation. In MDA-231 breast cancer cells that stably over express RKIP, IL-6 treatment blocked STAT3 phosphorylation and transcriptional activation. Conversely, in RKIP knockdown MDA-231 cells: STAT3 phosphorylation and activation increased in comparison to parental MDA-231 cells. RKIP over expression resulted in constitutive physical interaction with STAT3 and blocked c-Src and STAT3 association. The treatment of DU145 prostate, but not PC3 prostate or MDA-231 breast, cancer cell lines with ENMD-1198 or MKC-1 dramatically increased expression of RKIP. Overexpression of RKIP sensitized PC3 and MDA-231 cells to MTI-induced apoptosis. Moreover, MTI treatment resulted in a decrease in Src-mediated STAT3 tyrosine phosphorylation and activation, an effect that was significantly enhanced by RKIP over expression. In stable RKIP over expressing MDA-231 cells, tumor xenograft growth induced by activated STAT3 is inhibited. RKIP synergizes with MTIs to induce apoptosis and inhibit STAT3 activation of breast and prostate cancer cells. RKIP plays a critical role in opposing the effects of pro-oncogenic STAT3 activation.

Optimization of the lactam side chain of 7-azaindenoisoquinoline topoisomerase I inhibitors and mechanism of action studies in cancer cells

Optimization of the lactam ω-aminoalkyl substituents in a series of 7-azaindenoisoquinolines resulted in new anticancer agents with improved Top1 inhibitory potencies and cancer cell cytotoxicities. The new compounds 14-17 and 19 exhibited mean graph midpoint cytotoxicity (GI50) values of 21-71 nM in the NCI panel of 60 human cancer cell cultures. Ternary 7-azaindenoisoquinoline-DNA-Top1 cleavage complexes that persist for up to 6 h were detected in HCT116 colon cancer cells. Ternary complexes containing 7-azaindenoisoquinolines were significantly more stable than those in which camptothecin was incorporated. DNA content distribution histograms showed S-phase block 3 h after drug removal. Drug-induced DNA damage in HCT116 cells was revealed by induction of the histone γ-H2AX marker. The 7-azaindenoisoquinolines were able to partially overcome resistance in several drug-resistant cell lines, and they were not substrates for the ABCB1 drug efflux transporter. Molecular modeling studies indicate that the 7-azaindenoisoquinolines intercalate at the DNA cleavage site in DNA-Top1 covalent complexes with the lactam side chain projecting into the major groove. Overall, the results indicate that the 7-azaindenoisoquinolines are promising anticancer agents that merit further development.

Molecular and cellular pharmacology of the novel noncamptothecin topoisomerase I inhibitor Genz-644282

Camptothecin derivatives are powerful anticancer drugs because of their ability to trap topoisomerase I (Top1)-DNA cleavage complexes. However, they exhibit clinical limitations due to the instability of their α-hydroxylactone six-membered E-ring structure. In addition, they exhibit bone marrow and intestinal toxicity, especially in adults, and are drug efflux substrates. Here, we report a novel Top1 inhibitor, Genz-644282. We show that Genz-644282 and its metabolites induce Top1 cleavage at similar, as well as unique genomic positions, compared with camptothecin. The compound also induces protein-linked DNA breaks and Top1-DNA cleavage complexes that persist longer after compound removal than camptothecin. Concentration-dependent and persistent γH2AX formation was readily observed in cells treated with Genz-644282, and was present in greater than 50% of the cell population following 24 hours compound exposure. The compound shows partial cross-resistance in cell lines resistant to camptothecin. These cell lines include the human prostate DU145RC0.1 and the leukemic CEM/C2 cells. Limited cross-resistance to Genz-644282 was also found in the Top1 knockdown colon cancer (HCT116) and breast cancer (MCF7) cell lines and in human adenocarcinoma cells (KB31/KBV1) that overexpress (P-glycoprotein, ABCB1), a member of the ATP-binding cassette family of cell surface transport proteins known to confer MDR. Together, our results provide the first molecular and cellular characterization of Genz-644282 and its clinically relevant metabolites.

Sensitivity of human prostate cancer cells to chemotherapeutic drugs depends on EndoG expression regulated by promoter methylation

Analysis of promoter sequences of all known human cytotoxic endonucleases showed that endonuclease G (EndoG) is the only endonuclease that contains a CpG island, a segment of DNA with high G+C content and a site for methylation, in the promoter region. A comparison of three human prostate cancer cell lines showed that EndoG is highly expressed in 22Rv1 and LNCaP cells. In PC3 cells, EndoG was not expressed and the EndoG CpG island was hypermethylated. The expression of EndoG correlated positively with sensitivity to cisplatin and etoposide, and the silencing of EndoG by siRNA decreased the sensitivity of the cells to the chemotherapeutic agents in the two EndoG-expressing cell lines. 5-aza-2'-deoxycytidine caused hypomethylation of the EndoG promoter in PC3 cells, induced EndoG mRNA and protein expression, and made the cells sensitive to both cisplatin and etoposide. The acetylation of histones by trichostatin A, the histone deacetylase inhibitor, induced EndoG expression in 22Rv1 cells, while it had no such effect in PC3 cells. These data are the first indication that EndoG may be regulated by methylation of its gene promoter, and partially by histone acetylation, and that EndoG is essential for prostate cancer cell death in the used models.

RKIP sensitizes prostate and breast cancer cells to drug-induced apoptosis

Cancer cells are more susceptible to chemotherapeutic agent-induced apoptosis than their normal counterparts. Although it has been demonstrated that the increased sensitivity results from deregulation of oncoproteins during cancer development (Evan, G. I., and Vousden, K. H. (2001) Nature 411, 342-348; Green, D. R., and Evan, G. I. (2002) Cancer Cell 1, 19-30), little is known about the signaling pathways leading to changes in the apoptotic threshold in cancer cells. Here we show that low RKIP expression levels in tumorigenic human prostate and breast cancer cells are rapidly induced upon chemotherapeutic drug treatment, sensitizing the cells to apoptosis. We show that the maximal RKIP expression correlates perfectly with the onset of apoptosis. In cancer cells resistant to DNA-damaging agents, treatment with the drugs does not up-regulate RKIP expression. However, ectopic expression of RKIP resensitizes DNA-damaging agent-resistant cells to undergo apoptosis. This sensitization can be reversed by up-regulation of survival pathways. Down-regulation of endogenous RKIP by expression of antisense and small interfering RNA (siRNA) confers resistance on sensitive cancer cells to anticancer drug-induced apoptosis. Our studies suggest that RKIP may represent a novel effector of signal transduction pathways leading to apoptosis and a prognostic marker of the pathogenesis of human cancer cells and tumors after treatment with clinically relevant chemotherapeutic drugs.