Targeting antioxidant enzyme expression as a therapeutic strategy for ischemic stroke

Abstract

During ischemic stroke, neurons and glia are subjected to damage during the acute and neuroinflammatory phases of injury. Production of reactive oxygen species (ROS) from calcium dysregulation in neural cells and the invasion of activated immune cells are responsible for stroke-induced neurodegeneration. Scientists have failed thus far to identify antioxidant-based drugs that can enhance neural cell survival and improve recovery after stroke. However, several groups have demonstrated success in protecting against stroke by increasing expression of antioxidant enzymes in neural cells. These enzymes, which include but are not limited to enzymes in the glutathione peroxidase, catalase, and superoxide dismutase families, degrade ROS that otherwise damage cellular components such as DNA, proteins, and lipids. Several groups have identified cellular therapies including neural stem cells and human umbilical cord blood cells, which exert neuroprotective and oligoprotective effects through the release of pro-survival factors that activate PI3K/Akt signaling to upregulation of antioxidant enzymes. Other studies demonstrate that treatment with soluble factors released by these cells yield similar changes in enzyme expression after stroke. Treatment with the cytokine leukemia inhibitory factor increases the expression of peroxiredoxin IV and metallothionein III in glia and boosts expression of superoxide dismutase 3 in neurons. Through cell-specific upregulation of these enzymes, LIF and other Akt-inducing factors have the potential to protect multiple cell types against damage from ROS during the early and late phases of ischemic damage.

Keywords: Antioxidant enzymes; Ischemic stroke; Leukemia inhibitory factor; Neuroprotection; Oxidative stress.

Figure 1. ROS-Mediated Damage During the Early and Late Phases of Stroke Pathophysiology

(A) Acute energy failure is primarily responsible for oxidative damage during the cytotoxic phase of stroke pathophysiology. During the first few minutes to hours after the onset of ischemic stroke, neurons experience a shortage of oxygen and glucose, which interferes with ATP production. Without ATP to maintain the electrochemical gradient, excitotoxic neurotransmission increases and neurons experience an influx of calcium. Enzymes such as NAPDH oxidase and neuronal nitric oxide synthase are activated either directly or indirectly by calcium signaling. High levels of calcium trigger mitochondrial dysfunction, which contributes to ROS production and apoptosis. This phase primarily affects cells in the umbra, which are directly fed by the occluded vessel. (B) From approximately 18 to 96 h after the onset of stroke, the activation of microglia and peripheral leukocytes facilitates neural cell damage. Pro-inflammatory microglia and phagocytic cells (neutrophils/macrophages) produce ROS via enzymes such as inducible nitric oxide synthase, NADPH oxidase, and myeloperoxidase. This “respiratory burst” damages cells adjacent to the ischemic core (the penumbra) and increases the volume of the infarct.

Figure 2. The Synergistic Effects of Pro-Antioxidant Therapeutics

Cellular therapies, including HUCB cells and NSCs, exert their protective actions via the release of soluble factors which activate pro-survival signaling cascades. Through the release of these factors, NSCs are able to increase expression of SOD2 while HUCB cells are able to increase expression of Prdx4/Mt3 in oligodendrocytes and Prdx5 in neurons. LIF, and possibly other IL-6 family cytokines, exerts similar effects on pro-survival signaling and increases expression of SOD3 in neurons and Prdx4/Mt3 in oligodendrocytes. In addition to the direct effect on antioxidant expression, LIF reduces oxidative stress during ischemic stroke by promoting the self-renewal of NSCs and preventing the formation of peroxynitrite (ONOO-) by reacting with superoxide. By decreasing the formation of peroxynitrite, nitric oxide increases angiogenesis and promotes vasodilation in the infarct. Both of these processes increase perfusion of ischemic tissue and contribute to brain repair.