Xiurong Zhao

Intracerebral hemorrhage (ICH) is an often fatal type of stroke that kills approximately 30,000 people annually in the United States. If the patient survives the ictus, then the resulting hematoma within brain parenchyma triggers a series of adverse events causing secondary insults and severe neurological deficits. This article discusses selected aspects of secondary brain injury after ICH and outlines key mechanisms associated with hematoma toxicity, oxidative stress, and inflammation. Finally, this review discusses the relevance of hematoma resolution processes as a target for ICH therapy and presents potential clinically relevant molecular targets that could be harnessed to treat secondary injury associated with ICH injury.

OBJECTIVE: Phagocytosis is necessary to eliminate the hematoma after intracerebral hemorrhage (ICH); however, release of proinflammatory mediators and free radicals during phagocyte activation is toxic to neighboring cells, leading to secondary brain injury. Promotion of phagocytosis in a timely and efficient manner may limit the toxic effects of persistent blood products on surrounding tissue and may be important for recovery after ICH. METHODS: Intrastriatal blood injection in rodents and primary microglia in culture exposed to red blood cells were used to model ICH and to study mechanisms of hematoma resolution and phagocytosis regulation by peroxisome proliferator-activated receptor gamma (PPARgamma) in microglia/macrophages. RESULTS: Our study demonstrated that the PPARgamma agonist, rosiglitazone, promoted hematoma resolution, decreased neuronal damage, and improved functional recovery in a mouse ICH model. Microglia isolated from murine brains showed more efficient phagocytosis in response to PPARgamma activators. PPARgamma activators significantly increased PPARgamma-regulated gene (catalase and CD36) expression, whereas reducing proinflammatory gene (tumor necrosis factor-alpha, interleukin-1beta, matrix metalloproteinase-9, and inducible nitric oxide synthase) expression, extracellular H(2)O(2) level, and neuronal damage. Phagocytosis by microglia was significantly inhibited by PPARgamma gene knockdown or neutralizing anti-CD36 antibody, whereas it was enhanced by exogenous catalase. INTERPRETATION: PPARgamma in macrophages acts as an important factor in promoting hematoma absorption and protecting other brain cells from ICH-induced damage.

After ischemic stroke, various damage-associated molecules are released from the ischemic core and diffuse to the ischemic penumbra, activating microglia and promoting proinflammatory responses that may cause damage to the local tissue. Here we demonstrate using in vivo and in vitro models that, during sublethal ischemia, local neurons rapidly produce interleukin-4 (IL-4), a cytokine with potent anti-inflammatory properties. One such anti-inflammatory property includes its ability to polarize macrophages away from a proinflammatory M1 phenotype to a "healing" M2 phenotype. Using an IL-4 reporter mouse, we demonstrated that IL-4 expression was induced preferentially in neurons in the ischemic penumbra but not in the ischemic core or in brain regions that were spared from ischemia. When added to cultured microglia, IL-4 was able to induce expression of genes typifying the M2 phenotype and peroxisome proliferator activated receptor γ (PPARγ) activation. IL-4 also enhanced expression of the IL-4 receptor on microglia, facilitating a "feedforward" increase in (1) their expression of trophic factors and (2) PPARγ-dependent phagocytosis of apoptotic neurons. Parenteral administration of IL-4 resulted in augmented brain expression of M2- and PPARγ-related genes. Furthermore, IL-4 and PPARγ agonist administration improved functional recovery in a clinically relevant mouse stroke model, even if administered 24 h after the onset of ischemia. We propose that IL-4 is secreted by ischemic neurons as an endogenous defense mechanism, playing a vital role in the regulation of brain cleanup and repair after stroke. Modulation of IL-4 and its associated pathways could represent a potential target for ischemic stroke treatment. SIGNIFICANCE STATEMENT: Depending on the activation signal, microglia/macrophages (MΦ) can behave as "healing" (M2) or "harmful" (M1). In response to ischemia, damaged/necrotic brain cells discharge factors that polarize MΦ to a M1-like phenotype. This polarization emerges early after stroke and persists for days to weeks, driving secondary brain injury via proinflammatory mediators and oxidative damage. Our study demonstrates that, to offset this M1-like polarization process, sublethally ischemic neurons may instead secrete a potent M2 polarizing cytokine, interleukin-4 (IL-4). In the presence of IL-4 (including when IL-4 is administered exogenously), MΦ become more effective in the cleanup of ischemic debris and produce trophic factors that may promote brain repair. We propose that IL-4 could represent a potential target for ischemic stroke treatment/recovery.

Peroxisome proliferator-activated receptor-gamma (PPARgamma) is a transcription factor that regulates the expression of various gene products that are essential in lipid and glucose metabolism, as well as that of the peroxisome-enriched antioxidant enzyme, catalase. Activation of PPARgamma is linked to anti-inflammatory activities and is beneficial for cardiovascular diseases. However, little is known about its role in intracerebral hemorrhage (ICH). 15-Deoxy-Delta(12,14)-prostaglandin J2 (15d-PGJ2) acts as a physiologic agonist for PPARgamma. In this study, we found that injection of 15d-PGJ2 into the locus of striatal hematoma increased PPARgamma-deoxyribonucleic acid (DNA) binding activity and the expression of catalase messenger ribonucleic acid (mRNA) and protein in the perihemorrhagic area. Additionally, 15d-PGJ2 significantly reduced nuclear factor-kappaB (NF-kappaB) activation and prevented neutrophil infiltration measured by myeloperoxidase (MPO) immunoassay, and also reduced cell apoptosis measured by terminal deoxynucleotide transferase dUTP nick-end labeling (TUNEL). In addition, 15d-PGJ2 reduced behavioral dysfunction produced by the ICH. Altogether, our findings indicate that injection of 15d-PGJ2 at the onset of ICH is associated with activation of PPARgamma and elevation of catalase expression, suppression of NF-kappaB activity, and restricted neutrophil infiltration. All these events predicted reduced behavioral deficit and neuronal damage.

BACKGROUND AND PURPOSE: Intracerebral hemorrhage (ICH) remains a major medical problem for which there is no effective treatment. Oxidative and cytotoxic damage plays an important role in ICH pathogenesis and may represent a target for treatment of ICH. Recent studies have suggested that nuclear factor-erythroid 2-related factor 2 (Nrf2), a pleiotropic transcription factor, may play a key role in protecting cells from cytotoxic/oxidative damage. This study evaluated the role of Nrf2 in protecting the brain from ICH-mediated damage. METHODS: Sprague-Dawley rats and Nrf2-deficient or control mice received intracerebral injection of autologous blood to mimic ICH. Sulforaphane was used to activate Nrf2. Oxidative stress, the presence of myeloperoxidase-positive cells (neutrophils) in ICH-affected brains, and behavioral dysfunction were assessed to determine the extent of ICH-mediated damage. RESULTS: Sulforaphane activated Nrf2 in ICH-affected brain tissue and reduced neutrophil count, oxidative damage, and behavioral deficits caused by ICH. Nrf2-deficient mice demonstrated more severe neurologic deficits after ICH and did not benefit from the protective effect of sulforaphane. CONCLUSIONS: Nrf2 may represent a strategic target for ICH therapies.