Are poly(ADP-ribosyl)ation by PARP-1 and deacetylation by Sir2 linked?

Poly(ADP-ribose) polymerase-1 (PARP-1) safeguards genomic integrity by limiting sister chromatid exchanges. Overstimulation of PARP-1 by extensive DNA damage, however, can result in cell death, as prolonged PARP-1 activation depletes NAD⁺, a substrate, and elevates nicotinamide, a product. The decline of NAD⁺ and the rise of nicotinamide may downregulate the activity of Sir2, the NAD⁺-dependent deacetylases, because deacetylation by Sir2 is dependent on high concentration of NAD⁺ and inhibited by physiologic level of nicotinamide. The Sir2 deacetylase family has been implicated in mediating gene silencing, longevity and genome stability. It is conceivable that poly(ADP-ribosyl)ation by PARP-1, which is induced by DNA damage, could modulate protein deacetylation by Sir2 via the NAD⁺/ nicotinamide connection. The possible linkage of the two ancient pathways that mediate broad biological activities may spell profound evolutionary roles for the conserved PARP-1 and Sir2gene families in multicellular eukaryotes.