Inflammation and Oxidative Stress: Potential Targets for Improving Prognosis After Subarachnoid Hemorrhage

Abstract

Subarachnoid hemorrhage (SAH) has a high mortality rate and causes long-term disability in many patients, often associated with cognitive impairment. However, the pathogenesis of delayed brain dysfunction after SAH is not fully understood. A growing body of evidence suggests that neuroinflammation and oxidative stress play a negative role in neurofunctional deficits. Red blood cells and hemoglobin, immune cells, proinflammatory cytokines, and peroxidases are directly or indirectly involved in the regulation of neuroinflammation and oxidative stress in the central nervous system after SAH. This review explores the role of various cellular and acellular components in secondary inflammation and oxidative stress after SAH, and aims to provide new ideas for clinical treatment to improve the prognosis of SAH.

Keywords: anti-inflammatory; antioxidant; delayed ischemic neurological deficit; inflammation; oxidative stress; poor prognosis; subarachnoid hemorrhage.

FIGURE 1

After subarachnoid hemorrhage (SAH), blood components enter the subarachnoid space. RBC rupture hemoglobin and its metabolites, together with other damage-associated molecular patterns (DAMPs), act as inducers of the secondary inflammatory response after SAH to activate the innate immune cells (microglia and astrocytes) in central nervous system (CNS). Subsequently, immune cells such as neutrophils and macrophages in the peripheral circulation infiltrate into the injured site under the action of chemokine recruitment. These peripheral immune cells, together with innate immune cells in CNS, act as the carriers of secondary inflammation after SAH, releasing large amounts of pro-inflammatory cytokines and peroxides causing damage to neurons. Under the influence of these inflammatory products and peroxides, neurons gradually appear cell dysfunction and even apoptosis.

FIGURE 2

The Toll-like receptor (TLR) family plays a key role in recognizing antigens produced by microorganisms. To date, 13 TLR family members have been discovered. TLR1, 2, 4, 5, and 6 were expressed on the cell surface, while TLR3, 7, 8, and 9 were expressed on the endosome membrane. Toll-like receptors are membrane receptors composed of extracellular domains, single transmembrane helical domains and intracellular signaling domains, which can bind to different ligands (TLR2 and TLR1 or TLR6 complexes recognize lipoproteins or lipopeptides, TLR3 recognizes double-stranded RNA, TLR4 recognizes lipopolysaccharides (LPS), TLR5 recognizes bacterial flagellins, TLR7 or TLR8 recognizes single-stranded RNA, and TLR9 recognizes CpG rich in hypomethylated DNA). When the TLR binds to the respective ligands, different downstream signals can be activated to produce different biological effects.

FIGURE 3

Under normal physiological conditions, mitochondrial oxidative respiratory chain composed of complex I–IV can transfer electrons and H⁺ and produce ATP together with ATP synthase. When complex I–IV is dysfunctional, electron leakage occurs. This leads to the production of ROS and H₂O₂.

FIGURE 4

The NADPH oxidase (NOX) family consists of seven catalytic subunits (NOX1-5 and DUOX1-2), regulatory subunits p22Phox, P47Phox or Noxo1, P67phox or Noxa1, and P40phox and Rac. They are widely expressed in endothelial cells (EC), vascular smooth muscle cells (VSMC), macrophages and other cells. Specifically, NOX1, 2, 4, and 5 are highly expressed in cardiovascular tissues. NOX – mediated ROS production mainly occurs on catalytic subunit Nox or Duox. For Nox1 and Nox2, ROS production requires complex interactions of regulatory subunits in the cytoplasm. Nox4, on the other hand, requires protein termed δ-interacting protein 2 (Poldip2). In addition, the increase in intracellular calcium was sufficient to promote the activation of NOX5 and DUOX1-2.

FIGURE 5

Myeloperoxidase (MPO) is produced and secreted mainly by neutrophils. It can produce a variety of oxidation products through halogenation cycle and peroxidation cycle and then cause damage to tissues and cells. In halogen cycle, MPO catalyzes halogen to produce HOCl and other strong oxides. In the peroxidation cycle, MPO can react with oxidizable molecules (RH) to form free radical intermediates. In addition, MPO can react with NO generated by NOS to form NO2^–.

FIGURE 6

In mammals, there are three isotypes of nitric oxide synthase (NOS) (eNOS, iNOS, and nNOS). ENOS is the most important source of NO in endothelial cells. The shear stress of blood flow on the vascular wall is the main mechanism by which eNOS produces NO. INOS can be induced in a variety of cell types. LPS as a proinflammatory medium can induce the expression of iNOS. At the same time, the transcription factors NF-κB and STAT-1α are believed to be necessary for iNOS transcription in most cells. nNOS is highly expressed mainly in peripheral nerve fibers and is thought to have a protective effect on atherosclerosis. The release of L-Glutamate (L-Glu) from baroreceptors activates nNOS and promotes the production of NO. Although the expression mechanisms of the three isotypes are different, all of them can generate NO and L-Citrulline (L-CCP) as the substrate.

FIGURE 7

Extracellular hemoglobin can be metabolized into hemoglobin dimer as well as heme. The hemoglobin dimer can bind to the haptoglobin and enter the cell mediated by CD163. Soon afterward, the hemoglobin dimer that enters the cell will be decomposed into heme. On the other hand, extracellular heme can bind to hemopexin and enter the cell mediated by CD91. In quick succession, heme and oxygen in the cell are catalyzed by heme oxygenased (HO1/2) to produce Fe²⁺ ions, CO and bilirubin. In addition, abnormal red blood cells can also be cleared by phagocytosis mediated by the expression of type II scavenger receptor (CD36) on the phagocytic membrane.

FIGURE 8

The transcription factor nuclear factor erythroid-derived 2-like 2 (Nrf2) plays an important role in cellular antioxidant and other important physiological processes. Under physiological conditions, Nrf2 can be ubiquitinated and bind to Keap1. Subsequently, Nrf2 can be degraded by keAP1-dependent proteasome. In addition to ubiquitination, various post-translational modifications such as phosphorylation can affect the stability of Nrf2 structure. Under moderate oxidative stress and antioxidant stimulation, Nrf2 can enhance its stability through various post-translational modifications such as phosphorylation. At the same time, Nrf2 with enhanced stability can be translocated to the nucleus and combined with the cis-acting element ARE to activate the transcription of antioxidant genes.

FIGURE 9

There are many isotypes of SOD. Among them, SOD1, SOD2, and SOD3 play major roles in the cell. Both were able to reduce superoxide to produce H₂O₂ and O₂, but they worked in different places. SOD1 exists in the mitochondrial membrane space, SOD2 is distributed in the mitochondrial matrix, and SOD3 is distributed in the cell matrix.

FIGURE 10

The GPX mediated reduction of hydrogen peroxide and lipid hydroperoxides involves the formation of multiple intermediates with the assistance of glutathione (GSH). The selenol (–SeH) of GPX reacts with peroxides to form selenic acid (Se-OH). Then the selenic acid is reduced by GSH to form the Se-SG intermediate of GPX. GPX-Se-SG was reduced by the second GSH to form GSSG. GSSG can be reduced by glutathione reductase (GR) as reduced equivalent by NADPH.