The involvement of chronic poly(ADP-ribose) polymerase activation in ataxia-telangiectasia.

摘要

In the disease ataxia-telangiectasia (A-T), loss of the a&barbelow;taxia-t&barbelow;elangiectasia m&barbelow;utated gene (atm) leads to the inability of the cellular machinery to respond to single and double-stranded genomic lesions leading to a higher concentration of unrepaired DNA damage. With the pathways disrupted, the proximal response, activation of the nuclear enzyme poly (ADP-ribose) polymerase (PARP) leading to the generation of ADP-ribose polymers, persists indefinitely placing a metabolic burden on the cell. Since NAD+utilization in the PARP reaction results in the diversion of key cofactors from metabolic pathways, the need for regeneration comes at the cost of both the precursor molecules and energetic stores producing a net loss in the cycling of NAD+ and ATP. With this thesis, I examined the metabolic consequences of chronic PARP activity and the relationship between PARP inhibition, cellular proliferation and the loss of NAD+ and ATP.; Utilizing primary fibroblasts from A-T patients as the model system for chronic PARP activation, I demonstrated an increase in PARP activity with a concomitant loss in the steady-state concentration of NAD+. Upon inhibition, the proliferative capacity of the cells was restored and the steady-state NAD+ levels increased. Following supplementation of the cells with nicotinamide, the levels improved to nearly normal. Though the rate of oxygen consumption was nearly double in the A-T cells, the lower steady-state levels of ATP were not a result of mitochondrial dysfunction as measured by mitochondrial activity, oxidant production or membrane potential. The inhibition of PARP improved the steady-state levels of ATP in the A-T cells while only marginally affecting mitochondrial activity. The decreased ability of the A-T cells to detoxify lipid peroxides, a product of oxidative stress, was restored with PARP inhibition. Signal transduction in the A-T cells through the NF-kappaB pathway in response to ionizing radiation was also disrupted further suggesting an imbalance in the cells.; These results suggest that the secondary effect of chronic PARP activation in A-T cells contributes to a metabolic imbalance, while inhibition breaks the futile cycle of energy utilization thus restoring the metabolic capacity necessary for proliferation and the ability to prevent the accumulation of toxic oxidative metabolites.