Poly(ADP-ribose) polymerase 1 (PARP1) is an important component of the base excision repair (BER) pathway as well as a regulator of homologous recombination (HR) and non-homologous end-joining (NHEJ). Previous studies have demonstrated that treatment of HR-deficient cells with PARP inhibitors results in stalled and collapsed replication forks. Consequently, HR-deficient cells are extremely sensitive to PARP inhibitors. Several explanations have been advanced to explain this so-called synthetic lethality between HR deficiency and PARP inhibition: (i) reduction of BER activity leading to enhanced DNA double-strand breaks, which accumulate in the absence of HR; (ii) trapping of inhibited PARP1 at sites of DNA damage, which prevents access of other repair proteins; (iii) failure to initiate HR by poly(ADP-ribose) polymer-dependent BRCA1 recruitment; and (iv) activation of the NHEJ pathway, which selectively induces error-prone repair in HR-deficient cells. Here we review evidence regarding these various explanations for the ability of PARP inhibitors to selectively kill HR-deficient cancer cells and discuss their potential implications.

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1 At the mouse neuromuscular junction, adenosine (AD) and the A 1 agonist 2-chloro-N 6 -cyclopentyl-adenosine (CCPA) induce presynaptic inhibition of spontaneous acetylcholine (ACh) release by activation of A 1 AD receptors through a mechanism that is still unknown. To evaluate whether the inhibition is mediated by modulation of the voltage-dependent calcium channels (VDCCs) associated with tonic secretion (L-and N-type VDCCs), we measured the miniature endplate potential (mepp) frequency in mouse diaphragm muscles. 2 Blockade of VDCCs by Cd 2 þ prevented the effect of the CCPA. Nitrendipine (an L-type VDCC antagonist) but not o-conotoxin GVIA (an N-type VDCC antagonist) blocked the action of CCPA, suggesting that the decrease in spontaneous mepp frequency by CCPA is associated with an action on L-type VDCCs only. 3 As A 1 receptors are coupled to a G i/o protein, we investigated whether the inhibition of PKA or the activation of PKC is involved in the presynaptic inhibition mechanism. Neither N-(2[p-bromocinnamylamino]-ethyl)-5-isoquinolinesulfonamide (H-89, a PKA inhibitor), nor 1-(5-isoquinolinesulfonyl)-2-methyl-piperazine (H-7, a PKC antagonist), nor phorbol 12-myristate 13-acetate (PHA, a PKC activator) modified CCPA-induced presynaptic inhibition, suggesting that these second messenger pathways are not involved. 4 The effect of CCPA was eliminated by the calmodulin antagonist N-(6-aminohexil)-5-chloro-1-naphthalenesulfonamide hydrochloride (W-7) and by ethylene glycol-bis(b-aminoethyl ether)-N,N,N 0 ,N 0 -tetraacetic acid-acetoxymethyl ester e6TD-BM, which suggests that the action of CCPA to modulate L-type VDCCs may involve Ca 2 þ -calmodulin. 5 To investigate the action of CCPA on diverse degrees of nerve terminal depolarization, we studied its effect at different external K þ concentrations. The effect of CCPA on ACh secretion evoked by 10 mM K þ was prevented by the P/Q-type VDCC antagonist o-agatoxin IVA. 6 CCPA failed to inhibit the increases in mepp frequency evoked by 15 and 20 mM K þ . We demonstrated that, at high K þ concentrations, endogenous AD occupies A1 receptors, impairing the action of CCPA, since incubation with 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, an A 1 receptor antagonist) and adenosine deaminase (ADA), which degrades AD into the inactive metabolite inosine, increased mepp frequency compared with that obtained in 15 and 20 mM K þ in the absence of the drugs. Moreover, CCPA was able to induce presynaptic inhibition in the presence of ADA. It is concluded that, at high K þ concentrations, the activation of A 1 receptors by endogenous AD prevents excessive neurotransmitter release.

Aim The aim of this is to identify the gene expression profiles indicative of reversible versus irreversible injury in experimental (murine) renal artery stenosis (RAS). Method Two‐kidney‐one‐cuff hypertension was established by placement of a polytetrafluoroethylene cuff on the right renal artery (N=18). Sham surgery was performed by manipulating the right renal artery without cuff placement (N=6). To determine potential reversibility, the cuff was removed at 7 days (7D cuff‐off, N=6) and 14 days (14D cuff‐off, N=6) after initial RAS surgery. In a control RAS group, the cuff was left in place for 28 days (28D RAS, N=6). Kidneys were harvested at 28 days for assessment of atrophy and fibrosis. Renal blood perfusion and oxygenation were measured using 16.4T MRI. RNASeq analysis was performed on all the groups at 28 days following initial surgery. Results Following cuff placement, intra‐renal perfusion was reduced by 65% (p<0.0001 vs. baseline). In the 7D cuff‐off group intra‐renal perfusion was restored to baseline values after cuff removal, whereas in the 14D cuff‐off group perfusion was not significantly improved following cuff removal. In the 28D RAS group the reduction in perfusion was maintained throughout the study period. The 7D cuff‐off group had a significantly lower % atrophy (7 ± 2.2%, p=0.0042) and % area of interstitial fibrosis (0.57 ± 0.08 %, p<0.0001) compared to 28D RAS group (atrophy 66 ± 13%, area of fibrosis 7.79 ± 0.89 %). The 14D cuff‐off group had severe atrophy (56 ± 16.6%) and interstitial fibrosis (5.05 ± 1.22%) similar to that seen in 28D RAS group. RNASeq analysis data were normalized to appropriate sham controls in all the groups and was performed at 28 days following initial surgery. Induction of greater than Log2 ratio >2 and p values < 0.00001 were considered significant (following adjustment for multiple comparisons). Fibrosis associated genes were differentially regulated in the 7D cuff‐off compared to the 14D cuff‐off and 28D RAS group (table ). Conclusion We conclude that cuff removal at 7 days is associated with relative preservation of renal structure, indicating that renal injury at this point is reversible. We have identified a number of fibrosis associated genes that are differentially regulated in the 7D cuff‐off group versus 14D cuff‐off group. Further analysis will determine the functional relevance of these differentially regulated genes. Support or Funding Information This project was funded by National Institutes of Health, NIAID R01 AI100911. Values of Log 2 Ratio Genes 7D Cuff‐off vs Sham 14D Cuff‐off vs Sham 28D RAS vs Sham COL1A1 0.96 2.62 2.23 COL1A2 0.79 2.07 1.91 COL3A1 1.26 2.60 2.35 MMP12 1.24 2.85 2.95 MMP14 1.34 2.70 2.55 MMP19 2.89 3.86 4.05 MMP2 1.55 3.49 3.22 MMP7 6.96 9.22 9.59 MTOR −0.46 −1.67 −2.06 SERPINE1 1.25 3.37 2.35 TGFB1i1 0.44 1.46 1.53 TGFB2 0.82 1.83 1.97 TGFB3 0.59 1.91 1.99 TGFBi 0.98 1.88 2.02 KLF15 −0.90 −1.76 −2.60 UBD 4.49 6.23 6.18 TIMP1 2.94 5.30 5.16 TIMP2 0.50 1.46 1.52 TIMP3 −0.35 −1.26 −1.36 HSPB1 0.16 2.00 2.13 HSPB8 1.51 ...