First published July 1, 2008 - More info
Platelets, activated by various agonists, produce microparticles (MP) from the plasma membrane, which are released into the extracellular space. Although the mechanism of MP formation has been clarified, their biological importance remains ill defined. We have recently shown that platelet-derived MP influence platelet and endothelial cell function. In this study, we have further examined the mechanism of cellular activation by platelet MP. To address the possibility that they may influence monocyte-endothelial interactions, we used an in vitro assay to examine their effects on the adhesion of monocytes to human umbilical vein endothelial cells (HUVEC). Platelet MP increased the adhesion of monocytes to HUVEC in a time- and dose-dependent manner. Maximal adhesion of monocytes to resting HUVEC was observed after 24 h of stimulation with MP. Similar kinetics were observed with U-937 (human promonocytic leukemia) cells, used as a model for the blood-borne monocyte. Maximal adhesion of resting monocytes to MP-stimulated HUVEC was observed after 5 h of stimulation with MP. The EC50s for MP-induced increases in HUVEC, monocyte, and U-937 cell adhesion is 8.74, 43.41, and 10.83 microg/ml of MP protein, respectively. The induction of monocyte-endothelial adhesion was mimicked by arachidonic acid isolated from MP. The observed increased cellular adhesiveness correlated with MP-induced upregulation of cell adhesion molecules. MP-stimulated HUVEC increased intracellular cell adhesion molecule-1 (ICAM-1) but not vascular cell adhesion molecule-1 (VCAM-1), P-, or E-selectin expression. Monocyte and U-937 lymphocyte function-associated antigen-1 (CD11a/CD18) and macrophage antigen-1 (CD11b/ CD18, alpham/beta2) were both upregulated upon MP stimulation, but an increase in p150,95 (CD11c/CD18), very late antigen-1, or ICAM-1 expression was not observed. The functional importance of these changes was demonstrated with blocking antibodies. MP also induced the chemotaxis of U-937 cells in a dose-dependent manner with an EC50 of 4.40 microg/ml of MP protein. Similarly, arachidonic acid isolated from MP mimicked the chemotactic response. A role for PKC was implicated in both adhesion and chemotaxis. GF 109203X, a specific inhibitor of PKC, significantly reduced monocyte-endothelial adhesion, as well as U-937 chemotaxis. The demonstration that platelet MP may modulate important aspects of endothelial and monocyte function provides a novel mechanism by which platelets may interact with such cells in human atherosclerosis and inflammation.
O P Barry, D Praticò, R C Savani, G A FitzGerald
Although neutrophils are best known as one of the first types of cell to respond to invading microorganisms, they also contribute to noninfectious chronic inflammation. One mechanism by which neutrophils mediate host defense is by internalizing microorganisms and degrading them using neutrophil serine proteases (NSPs), but whether NSPs have a role in noninfectious chronic inflammation has not been clearly determined. However, using mice lacking two very similar NSPs, proteinase 3 (PR3) and neutrophil elastase (NE), Kessenbrock and colleagues have now shown that these two NSPs have a crucial role in noninfectious inflammation induced by the subcutaneous formation of antigen-antibody immune complexes (ICs) (pages 2438–2447). When compared with littermate controls, mice lacking NE and PR3 exhibited reduced neutrophil infiltration to subcutaneous sites of IC formation. This was not a generalized defect in neutrophil extravasation to sites of noninfectious inflammation, as neutrophil infiltration in response to application of phorbol esters to the skin was not impaired. Further analysis indicated that NE and PR3 cleaved the antiinflammatory molecule progranulin (PGRN) both in vitro and in vivo and that PGRN administration to wild-type mice inhibited neutrophil infiltration to subcutaneous sites of IC formation. These data led the authors to conclude that NE and PR3 mediate local proinflammatory effects by degrading, and thereby inactivating, PGRN.
Soon after an individual becomes infected with HIV, the virus infects cells in the CNS and, during the late stages of disease, can cause dementia and encephalitis. To identify biomarkers of HIV-induced CNS damage and gain insight into the mechanisms underlying this pathology, Wikoff and colleagues used a new global metabolomics approach to analyze how the levels of metabolites in the cerebrospinal fluid (CSF) of rhesus macaques are affected by SIV-induced CNS disease (pages 2661–2669). Using capillary reverse phase chromatography and electrospray ionization with accurate mass determination followed by novel, nonlinear data alignment and online database screening, the authors observed that the level of each of several metabolites was increased in the CSF after the manifestation of SIV-induced encephalitis. These data were then combined with DNA array transcriptomic results, revealing that the increased levels of free fatty acids and lysophospholipids correlated with increased amounts of two phospholipases in the encephalitic brain, one of which (phospholipase A2 group IVC) is known to release some of the fatty acids shown to be affected. The identification of metabolic biomarkers of SIV-induced CNS damage has provided insight into potential mechanisms underlying this pathology, and the authors hope that similar approaches could be used to provide new information about other neurodegenerative and neuropsychiatric disorders.
High levels of blood glucose when fasting are diagnostic for diabetes and have been implicated in the pathogenesis of type 2 diabetes. It is therefore hoped that identifying genetic variants that affect fasting glucose levels will provide insight into the pathogenesis of type 2 diabetes. As such, Chen and colleagues analyzed genome-wide association studies for SNPs associated with variation in fasting glucose levels in nondiabetic individuals (pages 2620–2628). One SNP, rs563694, was found to have a statistically significant association with fasting glucose levels in a test cohort of 5,088 nondiabetic individuals from Finland and Sardinia and in a follow-up cohort of 18,436 nondiabetic individuals of mixed European descent. Specifically, the A allele of rs563694 was associated with increased fasting glucose levels, and this increase accounted for approximately 1% of the variation in fasting glucose levels among individuals. Although rs563694 is located between the genes glucose-6-phosphatase catalytic subunit 2 (G6PC2) and ATP-binding cassette, subfamily B (MDR/TAP), member 11 (ABCB11), the authors speculate that G6PC2 is most likely to be responsible for the variation in fasting blood-glucose levels; this hypothesis is supported by their analysis of other SNPs and several recently published studies.
Infantile hemangioma (IH) is a benign vascular tumor that develops in 5%–10% of infants of mixed European descent. Although the life cycle of IH is well defined, with rapid postnatal growth of endothelium followed by slow spontaneous regression of the blood vessels to fatty tissue, the cellular origin of the tumor has remained elusive. In this issue (pages 2592–2599), Khan, Boscolo, and colleagues have identified a stem cell as the cellular origin of IH and developed the first animal model for this common tumor of infancy. Multipotential stem cells were isolated from proliferating human hemangioma tissue based on expression of the stem cell marker CD133. After implantation in immunodeficient mice, clonal populations expanded from single CD133+ cells generated human blood vessels that expressed markers of IH. Two months after implantation, the number of blood vessels diminished, and human adipocytes became evident. Confirmation of hemangioma-derived CD133+ stem cells as the cellular precursors of IH was provided by the observation that when these cells were marked with GFP and then implanted in immunodeficient mice, the blood vessels that developed contained GFP+ cells, as did the adipocytes that appeared later.