Nucleotides induce higher order chromatin degradation

Higher order chromatin degradation (HOCD) is a stepwise dismantling of the genome through the excision of chromatin loops and their oligomers at matrix attachment regions (MARs) during the early stages of programmed cell death. Although HOCD ultimately leads to the inactivation of the genome and cell death, a partial HOCD in cells receiving sublethal signals may result in the loss of genetic stability leading to neoplasia, degeneration, and aging. The present study was undertaken to determine the role of protein poly(ADP-ribosyl)ation in HOCD. Nuclei isolated from rat glioma C6 cells were able to carry poly(ADP-ribosyl)ation as assessed by the incorporation of ³²P-NAD⁺ into TCA-insoluble fraction. Under the same experimental conditions, millimolar NAD⁺ induced rapid HOCD in nuclei. However, while poly(ADP-ribosyl)ation was totally abrogated by specific inhibitor, benzamide, NAD⁺-induced HOCD was unaffected. Benzamide also failed to inhibit HOCD induced by H₂O₂ exposure in intact cells. These results indicate that HOCD is not mediated through chromatin poly(ADP-ribosyl)ation, and that NAD⁺ activates MAR-associated endonuclease or facilitates the access of the enzyme to DNA by other mechanisms. Furthermore, other nucleotides including NADP⁺, ATP, UTP, GTP, and CTP were also found to induce HOCD in isolated nuclei indicating that HOCD is controlled by nucleotide-related ligands.