Although p53 is a key modulator of cellular stress responses, the mechanism of p53-mediated apoptosis is ambiguous. p53 can mediate apoptosis in response to death stimuli by transcriptional activation of proapoptotic genes and transcriptional-independent mechanisms. Recent studies have shown that the p53 protein can directly induce permeabilization of the outer mitochondrial membrane by forming a inhibitory complex with a protective Bcl-2 family protein, resulting in cytochrome c release. However, how the mitochondrial p53 pathway mediates neuronal apoptosis after cerebral ischemia remains unclear. We examined the interaction between the mitochondrial p53 pathway and vulnerable hippocampal CA1 neurons in rats using a transient global cerebral ischemia (tGCI) model. Western blot analysis and immunofluorescent staining revealed mitochondrial p53 translocation after tGCI in the hippocampal CA1 neurons. Coimmunoprecipitation revealed that translocated p53 bound to Bcl-X L in the mitochondrial fraction. To examine the effect of a specific p53 inhibitor on the mitochondrial p53 pathway and apoptotic cell death after tGCI, we intravenously administered pifithrin-␣ (PFT). Mitochondrial p53 translocation and interaction between p53 and Bcl-X L were prevented by treatment with PFT. Moreover, cytochrome c release from mitochondria and subsequent apoptotic CA1 neuronal death were decreased with PFT treatment. These results suggest that the mitochondrial p53 pathway is one of the novel mechanisms mediating delayed death of vulnerable hippocampal CA1 neurons after tGCI.

It has been demonstrated by numerous studies that apoptotic cell death pathways are implicated in ischemic cerebral injury in ischemia models in vivo. Experimental ischemia and reperfusion models, such as transient focal/global ischemia in rodents, have been thoroughly studied and the numerous reports suggest the involvement of cell survival/death signaling pathways in the pathogenesis of apoptotic cell death in ischemic lesions. In these models, reoxygenation during reperfusion provides oxygen as a substrate for numerous enzymatic oxidation reactions and for mitochondrial oxidative phosphorylation to produce adenosine triphosphate. Oxygen radicals, the products of these biochemical and physiological reactions, are known to damage cellular lipids, proteins, and nucleic acids and to initiate cell signaling pathways after cerebral ischemia. Genetic manipulation of intrinsic antioxidants and factors in the signaling pathways has provided substantial understanding of the mechanisms involved in cell death/survival signaling pathways and the role of oxygen radicals in ischemic cerebral injury. Future studies of these pathways could provide novel therapeutic strategies in clinical stroke.

Recent studies have revealed that the phosphatidylinositol 3-kinase (PI3-K) pathway is involved in apoptotic cell death after experimental cerebral ischemia. The serine-threonine kinase, Akt, functions in the PI3-K pathway and prevents apoptosis by phosphorylation at Ser473 after a variety of cell death stimuli. After phosphorylation, activated Akt inactivates other apoptogenic factors, including glycogen synthase kinase-3b (GSK3b), thereby inhibiting cell death. However, the role of Akt/GSK3b signaling in the delayed death of hippocampal neurons in the CA1 subregion after transient global cerebral ischemia (tGCI) has not been clarified. Transient global cerebral ischemia for 5 mins was induced by bilateral common carotid artery occlusion combined with hypotension. Western blot analysis showed a significant increase in phospho-Akt (Ser473) and phospho-GSK3b (Ser9) in the hippocampal CA1 subregion after tGCI. Immunohistochemistry showed that expression of phospho-Akt (Ser473) and phospho-GSK3b (Ser9) was markedly increased in the vulnerable CA1 subregion, but not in the ischemic-tolerant CA3 subregion. Double staining with phospho-GSK3b (Ser9) and terminal deoxynucleotidyl transferase-mediated uridine 5 0 -triphosphate-biotin nick end labeling showed different cellular distributions in the CA1 subregion 3 days after tGCI. Phosphorylation of Akt and GSK3b was prevented by LY294002, a PI3-K inhibitor, which facilitated subsequent DNA fragmentation 3 days after tGCI. Moreover, transgenic rats that overexpress copper/zinc-superoxide dismutase, which is known to be neuroprotective against delayed hippocampal CA1 injury after tGCI, had enhanced and persistent phosphorylation of both Akt and GSK3b after tGCI. These findings suggest that activation of the Akt/GSK3b signaling pathway may mediate survival of vulnerable hippocampal CA1 neurons after tGCI.

Summary:The endoplasmic reticulum (ER), which plays important roles in apoptosis, is susceptible to oxidative stress. Because reactive oxygen species (ROS) are robustly produced in the ischemic brain, ER damage by ROS may be implicated in ischemic neuronal cell death. We induced global brain ischemia on wild-type and copper/zinc superoxide dismutase (SOD1) transgenic rats and compared ER stress and neuronal damage. Phosphorylated forms of eukaryotic initiation factor 2␣ (eIF2␣) and RNA-dependent protein kinase-like ER eIF2␣ kinase (PERK), both of which play active roles in apoptosis, were increased in hippocampal CA1 neurons after ischemia but to a lesser degree in the transgenic animals. This finding, together with the finding that the transgenic animals showed decreased neuronal degeneration, indicates that oxidative ER damage is involved in ischemic neuronal cell death. To elucidate the mechanisms of ER damage by ROS, we analyzed glucose-regulated protein 78 (GRP78) binding with PERK and oxidative ER protein modification. The proteins were oxidatively modified and stagnated in the ER lumen, and GRP78 was detached from PERK by ischemia, all of which were attenuated by SOD1 overexpression. We propose that ROS attack and modify ER proteins and elicit ER stress response, which results in neuronal cell death.

Proinflammatory cytokines and chemokines are quickly upregulated in response to ischemia/reperfusion (I/R) injury; however, the relationship between I/R-induced oxidative stress and cytokine/chemokine expression has not been elucidated. We investigated the temporal profile of cytokine and chemokine gene expression in transient focal cerebral ischemia using complementary DNA array technology. Among 96 genes studied, 10, 4, 11, and 5 genes were increased at 6, 12, 24, and 72 h of reperfusion, respectively, whereas, 4, 11, 8, and 21 genes, respectively, were decreased. To clarify the relationship between chemokines and oxidative stress, we compared the gene and protein expression of monocyte chemoattractant protein 1 (MCP-1) and macrophage inflammatory protein-1 alpha (MIP-1 alpha) in wild-type (WT) mice and copper/zinc-superoxide dismutase (SOD 1) transgenic (Tg) mice. Monocyte chemoattractant protein-1 and MIP-1 alpha mRNA were significantly upregulated at 6 to 12 h of reperfusion. In the SOD 1 Tg mice, however, MCP-1 and MIP-1 alpha mRNA expression was significantly decreased 12 h postinsult. In the WT mice, MCP-1 and MIP-1 alpha protein expression peaked 24 h after onset of reperfusion determined by immunohistochemistry. In the SOD 1 Tg mice, MCP-1 and MIP-1 alpha immunopositive cells were reduced, as were concentrations of these proteins (measured by enzyme-linked immunosorbent assay) at 24 h of reperfusion. Our results suggest that MCP-1 and MIP-1 alpha expression is influenced by I/R-induced oxidative stress after transient focal stroke.