Donna L. Coyle

Depletion of the 110-Kilodalton Isoform of Poly(ADP-Ribose) Glycohydrolase Increases Sensitivity to Genotoxic and Endotoxic Stress in Mice

ABSTRACT Poly(ADP-ribosylation) is rapidly stimulated in cells following DNA damage. This posttranslational modification is regulated by the synthesizing enzyme poly(ADP-ribose) polymerase 1 (PARP-1) and the degrading enzyme poly(ADP-ribose) glycohydrolase (PARG). Although the role of PARP-1 in response to DNA damage has been studied extensively, the function of PARG and the impact of poly(ADP-ribose) homeostasis in various cellular processes are largely unknown. Here we show that by gene targeting in embryonic stem cells and mice, we specifically deleted the 110-kDa PARG protein (PARG110) normally found in the nucleus and that depletion of PARG110 severely compromised the automodification of PARP-1 in vivo. PARG110-deficient mice were viable and fertile, but these mice were hypersensitive to alkylating agents and ionizing radiation. In addition, these mice were susceptible to streptozotocin-induced diabetes and endotoxic shock. These data indicate that PARG110 plays an important role in DNA damage responses and in pathological processes.

A topical lipophilic niacin derivative increases NAD, epidermal differentiation and barrier function in photodamaged skin

Abstract:  The effects of myristyl nicotinate (MN), a nicotinic acid derivative designed to deliver nicotinic acid to skin without vasodilatation, on subjects with photodamaged skin have been studied. MN increased skin cell nicotinamide adenine dinucleotide (NAD) by 25% (P = 0.001) demonstrating effective delivery of nicotinic acid to skin. Relative to placebo, MN treatment of photodamaged facial skin increased stratum corneum thickness by approximately 70% (P = 0.0001) and increased epidermal thickness by approximately 20% (P = 0.001). In two separate studies, MN treatment increased rates of epidermal renewal by 6% (P = 0.003) to 11% (P = 0.001) and increased the minimal erythemal dose by 8.9 (P = 0.07) and 10% (P = 0.05) relative to placebo. MN treatment resulted in reductions in the rates of transepidermal water loss (TEWL) of approximately 20% relative to placebo on cheeks (P = 0.012) and arms (P = 0.017) of study subjects. Results of a tape stripping challenge before and after MN treatment demonstrated a significant correlation (P = 0.03) between increased skin NAD content and resistance to changes in TEWL for MN treated but not placebo subjects. Rates of TEWL changed more rapidly and to a greater extent in atopic subjects compared with normal subjects. The results indicate that MN enhances epidermal differentiation and barrier function in skin, suggesting that this method of nicotinic acid delivery may prove useful in limiting progression of actinic skin damage and possibly in treating other conditions involving skin barrier impairment.

Discovery and structure-activity relationships of modified salicylanilides as cell permeable inhibitors of poly(ADP-ribose) glycohydrolase (PARG)

The metabolism of poly(ADP-ribose) (PAR) in response to DNA strand breaks, which involves the concerted activities of poly(ADP-ribose) polymerases (PARPs) and poly(ADP-ribose) glycohydrolase (PARG), modulates cell recovery or cell death depending upon the level of DNA damage. While PARP inhibitors show high promise in clinical trials because of their low toxicity and selectivity for BRCA related cancers, evaluation of the therapeutic potential of PARG is limited by the lack of well-validated cell permeable inhibitors. In this study, target-related affinity profiling (TRAP), an alternative to high-throughput screening, was used to identify a number of druglike compounds from several chemical classes that demonstrated PARG inhibition in the low-micromolar range. A number of analogues of one of the most active chemotypes were synthesized to explore the structure-activity relationship (SAR) for that series. This led to the discovery of a putative pharmacophore for PARG inhibition that contains a modified salicylanilide structure. Interestingly, these compounds also inhibit PARP-1, indicating strong homology in the active sites of PARG and PARP-1 and raising a new challenge for development of PARG specific inhibitors. The cellular activity of a lead inhibitor was demonstrated by the inhibition of both PARP and PARG activity in squamous cell carcinoma cells, although preferential inhibition of PARG relative to PARP was observed. The ability of inhibitors to modulate PAR metabolism via simultaneous effects on PARPs and PARG may represent a new approach for therapeutic development.

Effect of myristyl nicotinate on retinoic acid therapy for facial photodamage

Abstract:  Based on the hypothesis that skin barrier impairment is a contributor to side‐effects associated with retinoic acid therapy, a double‐blind, placebo‐controlled pilot study examined the combined use of retinoic acid with myristyl nicotinate (MN), a lipophilic derivative of niacin that enhances skin barrier function, in female subjects with mild to moderate facial photodamage. The study involved a 1‐month run‐in period with placebo or MN prior to initiation of retinoic acid therapy for 3 months. Analysis of skin biopsies revealed that retinoic acid therapy resulted in stratum corneum thinning of approximately 25% (P = 0.006 versus baseline) that was ameliorated by MN use (P < 0.005). Therapy resulted in an increased rate of transepidermal water loss (TEWL) of approximately 45% (P = 0.001 versus baseline) and use of MN protected against the increase in TEWL with the strongest protection provided by prior use of MN (P = 0.056 versus placebo). MN use reduced the incidence of side‐effects of the therapy and again prior use provided the greatest reduction of side‐effects. Subjects showed statistically significant clinical improvement (P < 0.05 versus baseline) during the study. MN use did not interfere with any clinical improvement parameters and improved effects on temple laxity (P = 0.01 versus placebo). Analysis of changes in epidermal thickness, Ki67‐positive cells and intensity of loricrin staining demonstrated that MN either improved or did not interfere with retinoic acid efficacy. These results show that prior and concurrent use of MN can mitigate barrier impairment and improve the tolerability of retinoic acid therapy for facial photodamage without interfering with efficacy.