Nitroxide tempo, a small molecule, induces apoptosis in prostate carcinoma cells and suppresses tumor growth in athymic mice

BACKGROUND. in previous studies, nitroxide tempo (2, 2, 6, 6-tetramethyl-piperidine-l-oxyl), a small molecule, induced cell death in cancer cells. The current study examined the antineoplastic properties of tempo in the human hormonedependent/hormone-independent prostate carcinoma models (LNCaP, DU-145, and PC-3). METHODS. The apoptotic effects of tempo were examined by the flow cytometric analysis of cells labeled with fluorescein isothiocyanate-conjugated annexin-V, and by electron microscopy. Enzymatic assays were performed to measure the activities of 2 cysteine proteases, i.e., caspase-9 and caspase-3, in tempo-treated cells. The effects of tempo on cell proliferation and on cell cycle distribution profiles were measured by the flow cytometric assay using immuno fluorescent staining of incorporated 5'-bromo-2'-deoxyuridine (BrdU) coupled with 7 - amino- actinomycin D (7-AAD) staining of total DNA. The number of proliferating cells was also determined independently by enzyme-linked immunosorbent assay using chemiluminescent detection of incorporated BrdU. Subcutaneous growth of human prostate carcinoma in athymic mice was monitored after intraturnoral administration of tempo into tumor-bearing mice. In addition, cell viability assays were performed to compare the cytotoxic effect of a combination of doxorubicin or mitoxantrone and tempo with single agents. RESULTS. Tempo treatment of prostate carcinoma cells caused a significant increase in the number of apoptotic cells compared with control groups (tempo, 2.5 mNI, 24 hours: DIJ-145, approximately 3.4-fold; PC-3, approximately 6-7-fold; tempo 1 mM, 24 hours: LNCaP, approximately 12-fold). Tempo-induced loss of cell viability was blocked partially or completely after pretreatment of cells with actinomycin-D or cvcloheximide, suggesting a de novo macromolecule synthesisdependent mechanism of cell death. Electron microscopy revealed aggregation and marginalization of chromatin in the nuclei of a large number of tempo-treated lLNCaP cells. Tempo treatment of LNCaP cells resulted in enhanced activities of caspase-9 (tempo, 5 mM, 15 hours: approximately 2-fold) and caspase-3 (tempo, 2.5 mM, 24 hours: approximately 12-fold). Tempo treatment also led to an enhanced number of cells in G2/M phase of the cell cycle (tempo, 5.0 mM, 24 flours: DU-145, approximately 1.6-fold; PC-3, approximately 1.5-fold; LNCaP, approximately 5.3-fold), and decreased BrdU incorporation indicative of a decline in the number of proliferating cells (tempo, 2.5 mM, 24 or 48 hours; DU-145, approxiinatelv 2-3-fold; PC-3, approximately 1.2-fold; LNCaP, approximately 5-10-fold). Administration of tempo into LNCaP tumor-bearing mice resulted in a significant inhibition of tumor growth (percent initial tumor volume [Day 30, n = 4]: vehicle, 845.35 +/- 272.83; tempo, 9.72 +/- 9.72; tempo vs. vehicle, P < 0.02). In hormonerefractorv prostate carcinoma cells, a combination of relatively low doses of tempo and doxorubicin or mitoxantrone caused enhanced cytotoxicity as compared with single agents.