The metallic compound cisplatin has been used for many years to treat various human cancers. Here, we describe the cytostatic and cytotoxic properties of a new class of organometallic compounds that contain a ruthenium (II) atom covalently linked to carbon and nitrogen atoms. We found that several ruthenium-derived compounds (RDCs) led to G 1 arrest and induced apoptosis in tumor cell lines derived from glioblastomas, neuroblastomas, and lymphoid tumors at least as efficiently as cisplatin. We further analyzed the signaling pathways underlying these effects, and we showed that both RDCs and cisplatin induced p53 and p73 protein levels but with different intensities and kinetics. This accumulation of p53 and p73 proteins correlated with an increase in p21 and Bax expression, two p53 target genes linked to cell growth arrest and apoptosis. However, in contrast to cisplatin-induced apoptosis, overexpression of ⌬Np73, a p53 and p73 dominant-negative isoform, only partly reduced RDC-induced apoptosis, suggesting p53-dependent and p53-independent modes of action. This observation was further confirmed by the ability of RDC to induce apoptosis in p53Ϫ/Ϫ cells. Altogether, this study highlights key cellular and molecular features of RDCs and suggests that further development of this new class of compounds may contribute to improve future chemotherapeutic protocols.

Excessive signaling via the Notch1 receptor inhibits apoptosis in T lymphocytes. Since several antiapoptotic proteins are cleaved by caspases during cell death, we investigated whether Notch1 was a caspase substrate. Results demonstrate that the intracellular domain of Notch1 (NICD) is cleaved into six fragments during apoptosis in Jurkat cells or peripheral T lymphocytes. Notch1 cleavage is prevented by the caspase inhibitors DEVD-fmk and VEID-fmk or by Bcl-2 expression. Caspase-3 and caspase-6 cleave the NICD into six fragments using sites located within the NF-jB binding domain, the ankyrin repeats and the transactivation domain. Notch1 cleavage correlates with the loss of HES-1 expression in apoptotic T cells. Notch1 fragments cannot inhibit activation-induced cell death in a T-cell hybridoma, confirming the abrogation of Notch1 antiapoptotic activity by caspases. The ability of the NICD but not the fragments to antagonize Nur77 activity supports a role for this factor in Notch1 antiapoptotic function.

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