Poly(ADP-Ribose) Polymerase Inhibition in Acute Lung Injury. A Reemerging Concept

Abstract

PARP1, the major isoform of a family of ADP-ribosylating enzymes, has been implicated in the regulation of various biological processes including DNA repair, gene transcription, and cell death. The concept that PARP1 becomes activated in acute lung injury (ALI) and that pharmacological inhibition or genetic deletion of this enzyme can provide therapeutic benefits emerged over 20 years ago. The current article provides an overview of the cellular mechanisms involved in the pathogenetic roles of PARP1 in ALI and provides an overview of the preclinical data supporting the efficacy of PARP (poly[ADP-ribose] polymerase) inhibitors. In recent years, several ultrapotent PARP inhibitors have been approved for clinical use (for the therapy of various oncological diseases): these newly-approved PARP inhibitors were recently reported to show efficacy in animal models of ALI. These observations offer the possibility of therapeutic repurposing of these inhibitors for patients with ALI. The current article lays out a potential roadmap for such repurposing efforts. In addition, the article also overviews the scientific basis of potentially applying PARP inhibitors for the experimental therapy of viral ALI, such as coronavirus disease (COVID-19)-associated ALI.

Keywords: cell death; coronavirus; cytokines; inflammation; olaparib.

Figure 1.

Overview of key biological functions of PARP1. The top part depicts the various domains of PARP (poly[ADP-ribose] [PAR] polymerase), including its DNA-binding domain, with its zinc fingers (ZnI, ZnII, ZnIII), which are essential for recognition of DNA-strand breaks. This domain also contains the NLS. The automodification domain contains the conserved BRCT fold that serves an important protein–protein interaction module in DNA repair and cell signaling. This domain accepts PAR polymers in the context of auto-PARylation of PARP1. The catalytic domain contains the active site of the enzyme, where binding and cleavage of nicotinamide adenine dinucleotide (NAD⁺) takes place. It also contains the WGR domain, which is one of the domains involved in the RNA-dependent activation of PARP1. Below the domains, on the right side, the structure of NAD⁺ is presented, with the nicotinamide part highlighted. The middle part of the figure shows the sequences of the PARylation process catalyzed by PARP, starting with recognition of the DNA-strand breaks by the DNA-binding domain (gray ovals depicting the zinc fingers binding to the DNA breaks), followed by the catalytic activation of the enzyme and the cleavage of NAD⁺, the production of nicotinamide, and the generation of PAR polymers, which, in turn, PARylates various acceptor proteins as well as PARP itself. The consumption of NAD⁺ has metabolic and bioenergetic effects. PARP inhibitors prevent the binding of NAD⁺ to the active site of PARP and inhibit the catalytic activity of the enzyme. On the left side, the effect of PARG (PAR glycohydrolase) and ARH3 (ADP-ribosylhydrolase 3) is shown; these enzymes break down the PAR polymers, leading to the liberation of free PAR. Reprinted by permission from Reference . BRCT = BRCA1 C-terminal; NLS = nuclear localization signal; PARylation = poly–ADP-ribosylation; WGR = tryptophan-glycine–arginine-rich.

Figure 2.

Mechanisms responsible for the cytoprotective and antiinflammatory effects of PARP inhibitors in nononcological diseases. (A) PARP activation and consequent NAD⁺ depletion (the “Berger Hypothesis”). These processes can lead to a cellular energetic deficit and cell dysfunction; inhibition of PARP prevents these processes and exerts cytoprotective effects (inhibition of cell necrosis). (B) Role of PARP activation and free PAR polymers in inducing mitochondrial release of AIF (apoptosis-inducing factor), which, in turn induces cell death (parthanatos). Inhibition of PARP suppresses these processes and inhibits parthanatos. (C) The role of PARP in liberating free PAR polymers, which, on their own, exert cytotoxic effects; inhibition of PARP prevents free PAR polymer formation and suppresses cell death. (D) PARylation contributes to activation of the proteasome through an interaction with RNF146; PARP inhibitors suppress these processes. (E) Role of PARP in contributing to proinflammatory signal transduction via enhancing JNK-mediated (left sequence) and NF-κB–mediated (right sequence) activation of multiple genes and gene products. By inhibiting PARP, these processes are attenuated and inflammatory signaling can be attenuated. (F) PARP regulates the activation of the cytoprotective Akt pathway. Under normal conditions, PARylation anchors the ATM–NEMO complexes, which are retained in the nucleus. However, after PARP inhibition, the ATM–NEMO complex translocates to the cytoplasm, where Akt and mTOR are recruited to form the ATM–NEMO–Akt–mTOR cytoprotective signalosome, which, in turn, activates various mitochondrial protective and cell-survival pathways. Adapted by permission from Reference . ARH3 = ADP-ribosylhydrolase 3; ATM = ataxia telangiectasia mutated; NEMO = NF-κB essential modulator; P = phosphate group; PAAN = PARP-1–dependent AIF-associated nuclease; Ub = ubiquitin group; UPS = ubiquitin-proteasome system.

Figure 3.

Pathophysiological triggers of PARP activation and interacting pathways of injury. Various pathophysiological conditions lead to the formation of various reactive oxygen species from various sources (such as the mitochondria, xanthine oxidase, or reduced NAD⁺ phosphate [NADPH] oxidase). In inflammatory states, various proinflammatory pathways are stimulated in response to autoimmune responses and/or proinflammatory microbial components. The corresponding isoforms of NOS (nitric oxide [NO] synthase; brain NOS in the central nervous system, endothelial NOS in the cardiovascular system, and inducible NOS under inflammatory conditions) produce NO (but under conditions of l-arginine depletion, NOS can also produce superoxide). Under low-pH conditions (such as tissue hypoxia/acidosis), nitrite can also be converted to NO. Superoxide (which is produced from various cellular sources, including mitochondria) and NO react to yield peroxynitrite. Peroxynitrite and hydroxyl radical induce single-strand breaks in DNA, which, in turn, activate PARP. This can deplete the cellular NAD⁺ and ATP pools. Cellular energy exhaustion triggers the further production of reactive oxidants. PARP activation leads to cellular dysfunction via the energetic mechanism as well as via several other pathways outlined in Figure 2. Oxidative and nitrative stress can cause endothelial-cell dysfunction, at least in part though the depletion of NADPH concentrations, which, in turn, leads to reduced endothelial NO formation. The cellular dysfunction is further enhanced by the promotion of proinflammatory gene expression by PARP, through the promotion of NF-κB, AP1 (activator protein-1), and MAP (mitogen-activated protein) kinase activation. PARP can also promote complement activation. The oxidant-induced proinflammatory-molecule and adhesion-molecule expression, along with the endothelial dysfunction, induce neutrophil recruitment and activation, which initiates positive-feedback cycles of oxidant generation, PARP activation, and cellular injury. For instance, tissue-infiltrating mononuclear cells produce additional oxidants and free radicals. PARP is also involved in triggering the release of mitochondrial cell-death factors, such as AIF. There are many oxidative and nitrosative injury pathways that are triggered by oxygen- and nitrogen-centered oxidants and free radicals, which act in parallel or in synergy with PARP-mediated pathways of cell injury. Although most of the pathways shown in the figure have been demonstrated in acute lung injury (ALI), the relative contribution of cell necrosis versus inflammatory cell injury, as well as the relative role of the various pathways shown in the figure, depends on the specific form of ALI and the stage of the disease. Reprinted by permission from Reference .