Elevated expression of the DNA damage response
proteins PARP1 and
poly(ADP-ribose) glycohydrolase (PARG) in
glioma stem cells (GSCs) suggests that
glioma may be a unique target for PARG inhibitors (PARGi). While PARGi-induced cell death is achieved when combined with ionizing radiation, as a single agent PARG inhibitors appear to be mostly
cytostatic. Supplementation with the NAD+ precursor
dihydronicotinamide riboside (NRH) rapidly increased NAD+ levels in GSCs and
glioma cells, inducing PARP1 activation and mild suppression of replication fork progression. Administration of NRH+PARGi triggers hyperaccumulation of
poly(ADP-ribose) (PAR), intra S-phase arrest and apoptosis in GSCs but minimal PAR induction or cytotoxicity in normal astrocytes. PAR accumulation is regulated by select PARP1- and PAR-interacting
proteins. The involvement of XRCC1 highlights the base excision repair pathway in responding to replication stress while enhanced interaction of PARP1 with
PCNA, RPA and ORC2 upon PAR accumulation implicates replication associated PARP1 activation and assembly with pre-replication complex
proteins upon initiation of replication arrest, the intra S-phase checkpoint and the onset of apoptosis.