Temporal changes in chromatin, intracellular calcium, and poly(ADP-ribose) polymerase during Sindbis virus-induced apoptosis of neuroblastoma cells

Sindbis virus (SV) induces apoptosis in many vertebrate cells, but the mechanism is unknown. To gain insight into this mechanism, the nature and time course of intracellular changes related to programmed cell death were studied in SV-infected mouse neuroblastoma cells. New virus production began at 5 h after infection and reached a peak at 12 h. Hoechst 33342 staining of DNA analyzed by flow cytometry demonstrated changes in chromatin beginning 6 h after infection. These chromatin changes were cell cycle dependent, affecting cells in G₀/G₁ but not S phase. Apoptosis was not dependent on increases in intracellular Ca²⁺ and occurred more rapidly in the absence of extracellular Ca²⁺. Nuclear changes were accompanied by activation of the DNA repair enzyme poly(ADP-ribose) polymerase (PARP), resulting in increased consumption of NAD which was apparent by 10 h after infection. SV-induced apoptosis also involved the proteolytic cleavage of PARP. This cleavage was detectable at 16 h after infection approximately the same time that DNA fragmentation was apparent by agarose gel electrophoresis. We conclude that SV-induced apoptosis of neuroblastoma cells is dependent on viral replication, is not dependent on a rise in intracellular Ca²⁺, and is accompanied by activation of PARP and of a protease that cleaves PARP.