Arsenite delays progression through each cell cycle phase and induces apoptosis following G2/M arrest in U937 myeloid leukemia cells.

Arsenic is a well known toxicant and carcinogen that is also effective as a chemotherapeutic in the treatment of acute promyelocytic leukemia. Although its effects on humans are well documented, arsenic's mechanism of action is not well understood. Its ability to act as a carcinogen and as a chemotherapeutic seems paradoxical. However, cancer cell transformation and cancer cell destruction can both occur through perturbations of the cell cycle machinery, making cell cycle function a likely target of arsenic action. Arsenic has previously been shown to inhibit cancer cell cycle progression, but the targeted cell cycle phase has been debated. This study was designed to identify the cell cycle phase at which U937 cells are most sensitive to arsenite-induced growth inhibition. Centrifugal elutriation was used to divide asynchronous cell cultures into specific cell cycle phase-enriched fractions. These fractions were monitored for cell cycle phase progression in the presence and absence of sodium arsenite. We found an overall reduction in cell cycle progression rather than induction of arrest at one specific checkpoint. G(2)/M is the phase most sensitive to arsenite-induced apoptosis. However, arsenite profoundly affects U937 cell growth by increasing the length of time it takes cells to transit each phase of the cell cycle. Future study of cell cycle inhibition by arsenic should consider that the effect may not be mediated by the major cell cycle checkpoints. Arsenic's ability to inhibit growth in any cell cycle phase may increase its value as a chemotherapeutic used together with other, more phase-selective agents, such as camptothecin.

Inorganic mercury attenuates CD95-mediated apoptosis by interfering with formation of the death inducing signaling complex.

Inorganic mercury (Hg2+) modulates several lymphocyte signaling pathways and has been implicated as an environmental factor linked to autoimmune disease. From the standpoint that autoimmune diseases represent disorders of cell accumulation, in which dysregulated apoptosis may be one mechanism leading to the accumulation of autoreactive lymphocytes, we have been investigating the influences of Hg2+ on CD95-mediated apoptosis. We demonstrate here that low and noncytotoxic concentrations of Hg2+ impair CD95 agonist-induced apoptosis in representative Type-I and Type-II T cell lines. Hg2+ treatment blocks the CD95 agonist-induced activation of initiator and effector caspases as well as the association between CD95 and the signaling adaptor, FADD. CD95 multimerization does not appear to be affected by Hg2+. Thus, the Hg2+ sensitive step within the CD95 death pathway is localized to the level of the death inducing signaling complex (DISC). Disruption of proper DISC formation may be a biochemical mechanism whereby Hg2+ contributes to autoimmune disease.