Role of PECAM-1 in the adherence of PMN to hypoxic endothelial cells

Metabolic modulation of cytokine-induced brain endothelial adhesion molecule expression

OBJECTIVE

Cytokines contribute to cerebro-vascular inflammatory and immune responses by inducing ECAMs' expression. Ischemic insults can be separated into aglycemic and hypoxic components. However, whether aglycemia, hypoxia or OGD plays a major role in dysregulating BBB or promotes immune cell infiltration via ECAMs' expression is not clear. We investigated how expression of ICAM-1, VCAM-1, MAdCAM-1, PECAM-1, E- and P-selectin in response to TNF-α, IL-1β and IFN-γ was altered by aglycemia (A), hypoxia (H) or combined oxygen glucose deprivation (OGD).

METHODS

A cell surface enzyme linked immunoabsorbent assay (cell surface ELISA) was used to analyze ECAM expression.

RESULTS

We observed that ICAM-1 and PECAM-1 expressions were insensitive to hypoxia, aglycemia or OGD. Conversely, VCAM-1 and E-selectin were increased by hypoxia, but not by aglycemia. MAdCAM-1 and P-selectin were induced by hypoxia, and decreased by aglycemia. Patterns of cytokine-regulated ECAMs' expression were also modified by metabolic conditions.

CONCLUSIONS

Our results indicate that patterns of inflammation-associated ECAMs represent cumulative influences from metabolic stressors, as well as cytokine activation. The expression of ECAMs following tissue injury reflects mechanistic interactions between metabolic disturbances, and alterations in tissue cytokines. Normalization of tissue metabolism, as well as cytokine profiles, may provide important targets for therapeutic treatment of inflammation.

Effects of repeated restraint stress on platelet endothelial cell adhesion molecule-1 immunoreactivity and protein levels in the gerbil hippocampus after transient cerebral ischemia

Stress has long been known to be a causative factor of various disease states. In this study, we investigated the effects of repeated restraint stress on platelet endothelial cell adhesion molecule-1 (PECAM-1), a very important mediator in inflammation, immunoreactivity and protein levels as well as neuronal damage, in the gerbil hippocampus after 5 minutes of transient cerebral ischemia. Transient ischemia-induced neuronal death was shown in CA1 pyramidal cells 4 days after ischemia/reperfusion. However, repeated restraint stress protected neuronal death induced by ischemic damage. In the ischemia-group, PECAM-1 immunoreactivity and its protein levels were significantly increased in all the hippocampal subregions 4 days after ischemia/reperfusion. However, PECAM-1 immunoreactivity and its protein levels did not change significantly in the hippocampus of the stress-ischemia-group compared to the sham-groups. These results indicate that repeated restraint stress protects neuronal damage induced by transient cerebral ischemia, and this may be associated with maintenance of PECAM-1levels.

Folic acid deficiency increases delayed neuronal death, DNA damage, platelet endothelial cell adhesion molecule-1 immunoreactivity, and gliosis in the hippocampus after transient cerebral ischemia

Folic acid deficiency increases stroke risk. In the present study, we examined whether folic acid deficiency enhances neuronal damage and gliosis via oxidative stress in the gerbil hippocampus after transient forebrain ischemia. Animals were exposed to a folic acid-deficient diet (FAD) for 3 months and then subjected to occlusion of both common carotid arteries for 5 min. Exposure to an FAD increased plasma homocysteine levels by five- to eightfold compared with those of animals fed with a control diet (CD). In CD-treated animals, most neurons were dead in the hippocampal CA1 region 4 days after ischemia/reperfusion, whereas, in FAD-treated animals, this occurred 3 days after ischemia/reperfusion. Immunostaining for 8-hydroxy-2'-deoxyguanosine (8-OHdG) was performed to examine DNA damage in CA1 neurons in both groups after ischemia, and it was found that 8-OHdG immunoreactivity in both FAD and CD groups peaked at 12 hr after reperfusion, although the immunoreactivity in the FAD group was much greater than that in the CD group. Platelet endothelial cell adhesion molecule-1 (PECAM-1; a final mediator of neutrophil transendothelial migration) immunoreactivity in both groups increased with time after ischemia/reperfusion: Its immunoreactivity in the FAD group was much higher than that in the CD group 3 days after ischemia/reperfusion. In addition, reactive gliosis in the ischemic CA1 region increased with time after ischemia in both groups, but astrocytosis and microgliosis in the FAD group were more severe than in the CD group at all times after ischemia. Our results suggest that folic acid deficiency enhances neuronal damage induced by ischemia.

Change in platelet endothelial cell adhesion molecule-1 immunoreactivity in the dentate gyrus in gerbils fed a folate-deficient diet

Folate deficiency increases stroke risk. We examined whether folate deficiency affects platelet endothelial cell adhesion molecule-1 (PECAM-1), which is an immunoglobulin-associated cell adhesion molecule and mediates the final common pathway of neutrophil transendothelial migration, in blood vessels in the gerbil dentate gyrus after transient forebrain ischemia. Gerbils were exposed to a folic acid-deficient diet (FAD) for 3 months and then subjected to common carotid artery occlusion for 5 min. In the control diet (CD)- and FAD-treated sham-operated groups, weak PECAM-1 immunoreactivity was detected in the blood vessels located in the dentate gyrus. PECAM-1 immunoreactivity in both groups was increased by 4 days after ischemic insult. PECAM-1 immunoreactivity in the FAD-treated group was twice as high that in the CD-treated-sham-operated group 4 days after ischemic insult. Western blot analyses showed that the change patterns in PECAM-1 protein levels in the dentate gyrus in both groups after ischemic insult were similar to changes in PECAM-1 immunohistochemistry in the ischemic dentate gyrus. Our results suggest that folate deficiency enhances PECAM-1 in the dentate gyrus induced by transient ischemia.

Gastrin-releasing peptide (GRP) induces angiogenesis and the specific GRP blocker 77427 inhibits tumor growth in vitro and in vivo

Angiogenesis is becoming a major target for antitumor therapies, and identifying new angiogenic factors and their specific inhibitors may provide new avenues for tumor management. Here we identify gastrin-releasing peptide (GRP) as a new angiogenic molecule that is secreted by tumors and acts directly upon GRP receptors in the endothelial cells. Addition of GRP increases endothelial cell migration and cord formation in vitro, and induces angiogenesis in an in vivo assay. We have recently identified a small molecule GRP blocker, compound 77427. This inhibitor significantly reduced endothelial cell cord formation in vitro and angiogenesis in vivo. Conversely, when applied to VEGF-induced angiogenesis, the small molecule did not have any effect, demonstrating its specificity. Furthermore, this GRP blocker was able to reduce lung tumor cell growth in vitro as demonstrated by MTT and clonogenic assays. When applied to a xenograft model with lung cancer cells, compound 77427 reduced tumor volume to undetectable sizes, although when the treatment was suspended, tumors began to grow again at normal rates. Our collective observations indicate that GRP is a new angiogenic peptide and that its inhibition offers an attractive tool to reduce tumor burden.

Ischemia-induced changes of platelet endothelial cell adhesion molecule-1 in the hippocampal CA1 region in gerbils

We observed chronological changes of platelet endothelial cell adhesion molecule-1 (PECAM-1), final mediator of neutrophil transendothelial migration, immunoreactivity, and protein level in the gerbil hippocampus proper after 5 min of transient ischemia. One day after ischemic insult, PECAM-1 immunoreactivity and protein level increased slightly in the hippocampus proper. Thereafter, PECAM-1 immunoreactivity and protein level increased significantly in the hippocampus proper by 4 days after ischemic insult. Especially, PECAM-1 in the hippocampal CA1 region was higher than that in the CA2/3 region. Five days after ischemic insult, PECAM-1 immunoreactivity decreased compared to the 4 days post-ischemic group. However, the RNA levels of PECAM-1 in the hippocampus proper were significantly decreased in the 4 days post-ischemic groups compared to that in the sham-operated group. This result suggests that the increase of PECAM-1 and decrease of PECAM-1 RNA in the CA1 region 4 days after ischemia may be associated with transmigration of neurotrophil.

PECAM-1-dependent neutrophil transmigration is independent of monolayer PECAM-1 signaling or localization

Platelet endothelial cell adhesion molecule-1 (PECAM-1/CD31), a tyrosine phosphoprotein highly expressed on endothelial cells and leukocytes, is an important component in the regulation of neutrophil transendothelial migration. Engagement of endothelial PECAM-1 activates tyrosine phosphorylation events and evokes prolonged calcium transients, while homophilic engagement of neutrophil PECAM-1 activates leukocyte beta-integrins. Although PECAM-1 modulates polymorphoneutrophil transmigration via homophilic PECAM-1-PECAM-1 interaction, the mechanisms underlying endothelial PECAM-1 function are unknown. Proposed mechanisms include (1) formation of a haptotactic gradient that "guides" neutrophils to the cell-cell border, (2) service as a "passive ligand" for neutrophil PECAM-1, ultimately mediating activation of neutrophil beta integrins, (3) regulation of endothelial calcium influx, and (4) mediation of SH2 protein association, and/or (5) catenin and non-SH2 protein interaction. Utilizing PECAM-1-null "model" endothelial cells (REN cells), we developed a neutrophil transmigration system to study PECAM-1 mutations that specifically disrupt PECAM-1-dependent signaling and/or PECAM-1 cell localization. We report that interleukin-1 beta (IL-1 beta) elicits PECAM-1-dependent transmigration that requires homophilic PECAM-PECAM-1 engagement, but not heterophilic neutrophil PECAM-1 interactions, and is intercellular adhesion molecule-1 dependent. Conversely, whereas IL-8 and leukotriene-B(4)-mediated transmigration is PECAM-1-independent, PECAM-1 and IL-8-dependent transmigration represent separable and additive components of cytokine-induced transmigration. Surprisingly, neither monolayer PECAM-1-regulated calcium signaling, cell border localization, nor the PECAM-1 cytoplasmic domain was required for monolayer PECAM-1 regulation of neutrophil transmigration. We conclude that monolayer (endothelial cell) PECAM-1 functions as a passive homophilic ligand for neutrophil PECAM-1, which after engagement leads to neutrophil signal transduction, integrin activation, and ultimately transmigration in a stimulus-specific manner.

The acute influence of tobacco smoking on adhesion molecule expression on monocytes and neutrophils and on circulating adhesion molecule levels in vivo

Soluble adhesion molecules have been reported as risk markers of a wide range of human diseases and specific adhesion molecules may play a direct role in pathological processes. Serum soluble intercellular adhesion molecule-1 (sICAM-1) is known to be significantly elevated in smokers compared to non-smokers. We examined the acute effects of smoking a standard 2R1 research cigarette on the serum concentration of sICAM-1 and other circulating adhesion molecules (sP-selectin, sE-selectin, sL-selectin, sVCAM-1 and sPECAM-1) in heavy smokers (serum cotinine >/= 100 ng/ml), light smokers (serum cotinine </= 60 ng/ml) and non-smokers (serum cotinine </= 10 ng/ml) by ELISA. Adhesion molecule expression on the cell surface of monocytes and neutrophils in peripheral blood was examined by flow cytometry. The sICAM-1 concentration directly correlated to serum cotinine concentration (p= 0.047) and nicotine load (p= 0.033) in smokers and was significantly elevated compared to non-smokers (p= 0.037). Other than a decrease in the concentration of sP-selectin over 1 hour regardless of smoking, no significant temporal alterations of any adhesion molecule were observed following the smoking experience or in the non-smoking control group. No significant difference in surface expression of ICAM-1, CD18, PECAM-1 or L-selectin on peripheral monocytes or neutrophils was observed over a 1-hour period following smoking. These data suggest that the elevated concentration of sICAM-1 in smokers is not due to an immediate effect of smoking.

Endothelial permeability and IL-6 production during hypoxia: role of ROS in signal transduction

Prolonged hypoxia produces reversible changes in endothelial permeability, but the mechanisms involved are not fully known. Previous studies have implicated reactive oxygen species (ROS) and cytokines in the regulation of permeability. We tested whether prolonged hypoxia alters permeability to increasing ROS generation, which amplifies cytokine production. Human umbilical vein endothelial cell (HUVEC) monolayers were exposed to hypoxia while secretion of tumor necrosis factor-alpha (TNF-alpha), interleukin (IL)-1alpha, IL-6, and IL-8 was measured. IL-6 and IL-8 secretion increased fourfold over 24 h in a pattern corresponding to changes in HUVEC permeability measured by transendothelial electrical resistance (TEER). Addition of exogenous IL-6 to normoxic HUVEC monolayers caused time-dependent changes in TEER that mimicked the hypoxic response. An antibody to IL-6 significantly attenuated the hypoxia-induced changes in TEER (86 +/- 4 vs. 63 +/- 3% with hypoxia alone at 18 h), whereas treatment with anti-IL-8 had no effect. To determine the role of hypoxia-induced ROS on this response, HUVEC monolayers were incubated with the antioxidants ebselen (50 microM) and N-acetyl-L-cysteine (NAC, 1 mM) before hypoxia. Antioxidants attenuated hypoxia-induced IL-6 secretion (13 +/- 2 pg/ml with ebselen and 19 +/- 3 pg/ml with NAC vs. 140 +/- 15 pg/ml with hypoxia). Ebselen and NAC prevented changes in TEER during hypoxia (94 +/- 2% with ebselen and 90 +/- 6% with NAC vs. 63 +/- 3% with hypoxia at 18 h). N-nitro-L-arginine (500 microM) did not decrease hypoxia-induced changes in dichlorofluorescin fluorescence, IL-6 secretion, or TEER. Thus ROS generated during hypoxia act as signaling elements, regulating secretion of the proinflammatory cytokines that lead to alterations of endothelial permeability.